Cement Kilns

Three Descriptions of Johnson's Cement Plant

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I C Johnson was a pioneer of the cement industry, and, following his emulation of William Aspdin's original product at Swanscombe, he established plants in his own right: Crown (1851), which was the first on the Medway, then Cliffe Creek (1854), then on William Aspdin's bankruptcy in 1856, he took over the Gateshead plant. He re-located to Tyneside for the next 25 years, becoming Mayor of Gateshead, and concentrated his efforts on developing the Johnson Chamber Kiln there. The Gateshead plant grew fairly large by the standards of the day, and its initial raw material - waste chalk ballast - soon ran out. Johnson then sourced a reliable chalk supply in Kent, eventually buying the quarry at Stone, and using Tyne coal ships to bring the chalk as a "return load". The logic of making clinker at the chalk source soon became unanswerable, and he decided to build a plant there. Sensibly, he reasoned that the new plant should be large from the outset, and incorporate all the best practices that he had absorbed in his long career. The result was the Greenhithe plant which opened in 1877. Johnson, now aged 70, moved back to the Thames and ran his business from there for the rest of his life.

As an unusually well-conceived plant, Johnsons naturally remained a major producer, advancing prudently but effectively with each technological innovation. It was finally killed off in 1971 by the ill-fated Northfleet project. Its main chalk quarry is now occupied by the Bluewater Retail Park.

The following are transcripts of three anonymous articles that describe the plant at significant stages in its evolution. The first is from The Engineer, XCIII, February 7, 1902, pp 130-133, which describes the plant in its most developed form as a chamber kiln plant. The second is from The Engineer, CV, February 28, 1908, pp 211-213, 220, which describes the plant after installation of its first rotary kilns. The third is “Johnson’s Cement Works, Greenhithe”, Cement and Cement Manufacture, 2, 1929, pp 97-107, 161-170, and describes the plant after it had been expanded to what was essentially its final form. All three articles are believed to be out of copyright.

Read The Engineer at Grace's Guide.

Note on Imperial units of the time: 1 ton = 1.016047 tonnes: 1 ft = 0.304799 m: 1 in = 25.4 mm: 1 h.p. = 0.7457 kW: 145.037 psi = 1 MPa.

From its foundation in 1877, the plant developed in stages until it had 54 chamber kilns. The first article describes the plant at this stage early in 1902, before it installed its first rotary kilns.


To be successful commercially, a Portland cement works must not only be well managed, but it must be placed in a suitable position (Note 1); and there are a number of considerations which have to be taken into account in this connection. An ideal works would be situated where all the materials necessary to manufacture were on the site or close at hand, and where the finished material was also used on the spot; for under such conditions there would be no expenses connected with carriage. Unfortunately, such a combination of circumstances must remain ideal—at all events, in this country. Here the manufacturer has to put up with what he can get. Generally speaking, he founds his works where, at all events, he can obtain one of the raw materials necessary on the site. If he is fortunate, he gets two. Thus, he may have chalk and clay, but in this case he will not have coal or coke; or he may have coal and clay (Note 2), but in this case he will not have the chalk. In nearly every case he plans his works by the waterside; for the materials he uses—saving, of course, his fuel—are cheap, and will not bear a heavy freight. Moreover, the ability to get rid of the manufactured cement by water carriage is of supreme importance.

The works which we are about to describe—those of I. C. Johnson and Co., Limited, at Greenhithe—possess the great advantages that they have on the site an enormous quantity of easily get-at-able chalk, and an excellent approach to the river Thames. True it is that clay is not obtainable on the spot; but it has only to be brought from a comparatively short distance, the company owning a quantity of clay land on the Medway (Note 3). Having its own wharf on its own premises, the necessary fuel is obtained at minimum expense, while as the wharf is large enough, and has at all times at least 20ft. of water round it, 2000-ton ships can come alongside and either deliver material or take away cement with as little delay as may be. From this works the company's cement is sent away entirely by water. There is no use whatever made of the railway, although it traverses the works (Note 4). It is worthy of note, too, that the water necessary for carrying on the various operations of manufacture is present in ample quantity, and can be obtained with but little cost for pumping, since the water level lies but 3ft. or 4ft. from the surface in some parts of the site. In fact, it may well be said that the position chosen for the works has been selected and laid out with skill and ability.

The works were commenced in 1877. Previously to this time chalk had been shipped from this exact place; indeed, from evidences discovered when the company was constructing its wharf, there is reason to believe that this place was one of the oldest chalk-shipping centres on the river Thames. The land acquired by the company covers some 200 acres, and contains enough chalk to suffice for very many years to come for the requirements, not only of these works, but of those in Gateshead belonging to the same company, and to which, as we shall afterwards show, chalk is continually being sent. The accompanying plan, Fig.1, shows the central portion of the site, that containing the works proper.

johnsons 1902 plan View HD image in a new window.

It would be impossible to show, within reasonable dimensions, the whole extent of the property. It will be observed that on the plan each building, or group of buildings. is provided with a number. By referring to the numbered table, it may be discovered what each of these buildings is. As will be seen, railway lines are laid throughout the works. There are, in all, some 4½ miles of these, and the gauge is all 3ft. 9in. For taking the trucks over these six steam locomotives are employed. There are also some lighter lines, of much smaller gauge, on which small tip trucks can be wheeled about by hand. Chalk is at present being obtained from several parts of the site. It is taken in the trucks and deposited alongside the line near the wash mills, which are marked 5 on the plan. Close by is also shot the clay brought in trucks from the wharf, where the barges from the Medway are unloaded. The line leading to the wharf is shown on the right hand of the plan. We shall have occasion to allude to the wharf and its appliances later on.

The greatest care is taken with the weighing and proportioning of the chalk and clay as it is put into the wash mill (Note 5). The method of procedure employed is as follows:—The clay and the chalk are loaded separately into hand barrows of known weight. There are two weighbridges. that for the chalk being under the immediate supervision of a foreman weigher, the recording portion of the apparatus being contained in a glass windowed cabin. The clay is weighed on a weighbridge close at hand, and well within sight of the foreman weigher. The amount of clay weighed at each operation is kept constant. The workmen themselves do the weighing of this material, and it is wonderful to observe what excellent guesses they make at the weight, very little adjustment usually being required when the barrow is wheeled on to the bridge. Any variation in the proportions of the ingredients used is made in the amount of chalk. The foreman weigher himself weighs the barrows of this substance, from which, of course, the flints have been removed, and keeps a record, not only of these, but of the barrows of clay as well. He also has a further and most important duty, in that he has charge of the wash mills, and is responsible for the amount of chalk and clay which is put into them in order to obtain any required mixture. He has, in addition, to keep a record of this. His actions are controlled from the chemical department. Continuously during manufacture samples are being taken and analysed. Notice of any alteration to be made in the requisite proportions is at once conveyed from the laboratory to the foreman who makes the change. We are informed that it is no uncommon thing for an addition or subtraction of 1 lb. weight in a total of some 500 lb. to be required. In spite of the somewhat rough and ready method of tilling and emptying hand barrows, with no attempt at scraping out, it appears that even such small differences as 1 in 500 make themselves apparent in the final result (Note 6). It is this attendance to small details which makes for success in the manufacture of cement.

The wash mills employed are of the ordinary type, having revolving spider arms carrying depending stirrers. The mingled slurry finds its way through gratings into catch-pits placed at that side of the wash mills remote from the holes into which the clay and chalk are charged. From these pits the mixture is in each case lifted by means of a revolving bucket wheel, which takes it to such a height that it can flow down wooden shoots leading to horizontal stone mills. These are contained in the building marked 6 on the plan. There are in all six mills, and they are driven by bevel gearing from a shaft running under the floor of the building, which is raised off the ground. This shaft also drives the pump in the building marked 7, which forces the ground slurry as it comes from the mills through flanged pipes to every part of the works where it is required. This pump is a triple ram force pump, with plungers 12in. in diameter, and having a 14in. stroke. The engine for driving these grinders and pump contained in 6 and 7, and also the wash mills, &c., is contained in the building marked 8. It is a horizontal compound engine driving through gearing on to a horizontal shaft. It obtains its steam at 80 lb. pressure from a battery of elephant type boilers contained in the building 11. This engine runs condensing—as, indeed, do the other main engines in the works. The water necessary for this is obtained a few feet below the surface. In the next compartment—numbered 10—is a horizontal engine working by ropes on to a countershaft, from which is driven—also by ropes—a two-pole dynamo. This supplies current for the whole of the works, there being a combination of arc and incandescent lighting (Note 7). This engine obtains its steam from the elephant boilers already mentioned, which also supply steam to an engine driving a grinding mill hereafter to be described.

These works, we are informed, witnessed the birth of the Johnson chamber kiln (Note 8), and there are no less than 54 kilns of this kind, but of different types, in various parts of the works. Some of these are of the ordinary tank form, in which the gases go direct over the top of the slurry, when it is being dried, to the chimney. Of these there are some as much as 120ft. long. In the later designs the slurry is held on a floor of iron plates carried on brick arches. In these the heated gases generated by the combustion of the charge arc first led over the top of the slurry to the end remote from the kiln as in the original form; they then descend by two vertical flues to two horizontal flues running under the iron plates towards the kilns again, these flues being connected to a further horizontal flue running also under the iron plates back to the far end of the chamber and thence to the chimney. Then there are kilns with firing pits, and others—the latest (Note 9)—where these pits are done away with, the fire being carried on removable girders, the level on which the trolleys are taken for the purpose of removing the burnt clinker being the same as the surrounding ground. The various modifications have been carried out with the object of cheapening production and facilitating the removal of the clinker. At the present time there are no rotary kilns or other apparatuses of this kind. It will be observed from the plan that there are three sets of kilns. There is one row containing as many as 33 kilns, and two other rows containing 12 and 9 respectively. Taking them on the average, each kiln will produce 27 tons of clinker per "burn," and three " burns " are obtained front each kiln every 14 days (Note 10). The tanks are, of course, filled with slurry from the flange pipe already mentioned.

When the burning is finished and the charge has cooled, the clinker, after being carefully inspected and the underburnt portions removed. is taken in trolleys running on the narrow-gauge lines already mentioned to either one of the two grinding mills which are on the premises. The first of these—which is contained in the buildings marked 12 and 13—consists of three horizontal stone mills in conjunction with a tube mill. The clinker is first broken in a crusher, and is then elevated by means of a bucket conveyor to the stone mills, the delivery from which descends to a tube mill, which in its turn delivers on to an endless belt. This belt drops the cement into a conveyor which communicates with the various bins in the cement warehouse No. 15. The engine driving these mills and their accessories is contained in building No.14. It is a horizontal cross compound engine of about 300 horse-power, and it works in conjunction with a jet condenser. It obtains its steam from the elephant type boilers already alluded to. The second and more modern grinding mill is contained in compartment 18. Here there are ball mills in place of stone mills, though the finishing is done as in the first instance, by means of tube mills. The arrangements in this mill are very complete. Rope-driving is employed throughout—in fact, rope-driving is largely used in all parts of these works—and any one of the mills can be stopped or started at will, irrespective of the others. For taking the cement in its various stages of manufacture about from place to place, either trough conveyors with worms or endless belts are used. Indeed, we are informed that this company was the first to use belts for this purpose. They act extremely well, and in this particular instance. practically speaking, the cement is never touched by hand from the clinker stage until it is delivered, finished, in the storehouses marked 21. Here the delivery could be made, were it necessary to do it, direct into sacks or casks for despatch. the length of time it is on the belt serving practically to bring the temperature of the recently-ground cement down to that of the atmosphere. All the operations are automatic. If it is necessary to take the material to a higher level this is done by means of encased bucket conveyors, and everything has been done to ensure the even working of all the parts. Here, too, there is a complete dust-collecting plant, the dust being drawn into ducts placed in various positions, and all delivering into a settling room, where the dust collects and falls down shoots on-to a travelling belt at a lower level. The driving engine for this part of the works is contained in compartment No. 19, and obtains its steam from Lancashire boilers, contained in No. 20. The horsepower of the engine is about 225. The total storage available on the premises amounts to some 10,000 tons (Note 11). The storage bins can be subdivided, so that it would be quite possible to have ten 1000-ton lots entirely separated front one another. The weekly output of the works is about 1300 tons on the average.

We have before now described minutely the various operations carried out in cement works. It will not be necessary, therefore, to again do so in the present instance. We may, however, draw attention to the method of raising the coke to the charging level of the kilns. This is primitive, but apparently highly efficient. Scaffold poles of the requisite size are provided in the middle of their length with iron hooks. These hooks fasten into iron eyes built into the brickwork at the side of the kilns. The poles are used for lifting baskets of coke in very much the same way as water is lifted from rivers in Oriental countries. At each end of the pole is fastened a rope. One rope has a hook attached to it, on which can be hung the basket. The other rope may be manipulated by one man, and we are informed that calculations show that this method, crude as it nun seem, is cheaper and more expeditious than any mechanical device which might be employed. Certainly, the men were wonderfully expert as we saw them at work, and seemed without effort to manage the basket so that it was landed right into the charging door.

As already intimated, great attention is paid to the chemistry of the manufacture at these works. The chemical laboratory is well appointed and conveniently situated, and here a constant series of experiments is continually going on. Attached to it is a mechanical testing laboratory with all the necessary appliances for efficient testing. We had an opportunity of inspecting both these, and of examining the method of keeping the tests which, as regards the chemical composition, are carried out concurrently with the manufacture of all the cement which is made. The company makes a strong point of testing its cement at long dates. We are informed that the averages of three months' briquettes are :—Neat cement, 1100 lb. per square inch; 3 of sand, 1 of cement, 350 lb. per square inch. This is found most useful for comparison with some of the cements, which test high at seven days, but do not increase at a later date.

Naturally enough, there is a considerable amount of repair and constructive work in connection with a works of this class and size. In all cement works the repairs form a heavy item. The company does its own repairs, and a large part of its constructive work as well. There is a large engineering shop—numbered 28 on the plan—and here, besides the work of repairing, is done the over-hauling of the locomotives and the construction of the trucks used for carrying the chalk, clay, flints, &c. There are three smiths' fires, lathes, shearing, drilling, and screwing machines, &c., besides wood-working machinery. On the works, too, are made the casks in which the cement is sent away; and the sacks, too, are sorted and mended on the spot. These operations are carried out in the buildings marked 24 and 3 respectively. There is also a pattern shop and small brass foundry marked 9 on the plan.

About 300 men are employed on the works as a total. Many of these have been with the company ever since the works were started, and they mostly reside in the immediate neighbourhood. They are provided with a good size dining hall—number 4 on the plan—and they themselves manage the catering arrangements. The scheme works well, and is self-supporting. There is an arrangement by which, if the sales do not exceed the expenditure, each man pays a penny per week. We understand, however, that at the present time this subscription is not needed.

johnsons 1902 wharf

As we have already remarked, one of the necessary adjuncts to the success of a cement works is a cheap and ready outlet for the manufactured material. All the cement made at these works is sent away by boat, and for this purpose a wharf has had to be constructed in the river. This, together with the arrangement of the railway lines on it, are shown in Fig. 2. The wharf is large enough for three steamers and eight barges to be alongside at one time. Here are unloaded the barges from the Medway which bring the clay. Here, too, the steamers which take the chalk to the Gateshead works of the company are loaded up. Colliers which have brought coal to London from the northern port are chartered for this purpose As the quantity shipped is large, and as it is of the utmost importance that the loading should be carried out as quickly as possible, an ingenious tipping arrangement has been devised. It is the patent of Messrs. Watson and Langston, and is illustrated on page 133. One of these is already at work: the second is being erected. It will be unnecessary to remind our readers of the almost universal use of the old-fashioned ballast wagon worked by horses. Although this method is costly, owing to the excessive wear and tear of rolling stock and horseflesh, it would seem that for dealing with low-priced materials such as chalk, stones, ballast, or sand, very little, if any, advance has been made in the methods used until the production of the present arrangement. The apparatus used by Messrs. Johnsons consists of a projecting framework on the wharf capable of being hoisted up so as to allow of vessels being berthed beneath it, and then of being lowered horizontally over the hold. A glance at the engravings on page 133 will show that the platform is carried by chains, which run over pulleys on a wooden framework down under the floor of the wharf where they are attached to winches. On the projecting framework is pivoted a cradle, which when at rest is in a horizontal position, and upon which the metals on which the wagons run are laid. The line on the wharf leading to the tipper are laid on an incline towards it. The loaded truck, on a small impetus being given to it, will travel on to the cradle by gravity. Its motion is arrested by its wheels coming against bent up rails. There is a sufficient over-hang on the truck to allow it when loaded to tip up the cradle, so that the load is shot downwards into the hold of the vessel. The load having been got rid of, the tendency of the cradle is to revert to the horizontal position, which it does, carrying the truck with it. The slight shock caused by a stop arresting the further motion of the cradle when it has become horizontal is enough to send the truck backwards sufficiently to clear the cradle and to roll on to a return line laid with an incline away from the tipper. The result is that the whole action is practically automatic. The truck runs on to the cradle, gets rid of its load, and runs off again without being touched by hand. A. strong brake, actuated by a lever, is fixed to the apparatus, so that the cradle and truck can be held at any angle. It is quite possible with one of these appliances to deal with three-ton trucks at the rate of one a minute There is no reason whatever, though it is here used only for chalk, flints, &c., why this apparatus should not be equally well employed for coals, ores, &c., where large quantities are required to be shipped quickly.

The wharf is equipped with cranes, &c., for the unloading of barges, &c., and the loading of the steamers with cement. Separate portions of the wharf are devoted to each of these operations, so that they can proceed concurrently. In order that night work array be carried on, there are a number of arc lamps along the wharf, and plug boxes are provided for the attachment of flexible cables lighting portable clusters of lamps which can be hung anywhere—over or in ships' holds and over barges. The same principle is adopted in other parts of the works. Outside the kilns, for example, are further plug boxes, and the leads can be taken right inside the kilns, enabling the charges to be laid or removed equally well by night as by day; for work is carried out continuously night and day— Sundays, of course, excepted (Note 12) — by different shifts. The quarrying of the chalk even is carried out by night, and here also specially designed electric fittings are used with excellent effect.

We must not omit to mention a matter which is proving of considerable pecuniary benefit to the company. On a portion of its site the chalk is overlaid by a rich bed of excellent gravel held in sand and loam (Note 13). This would have had in any case to be removed somehow in order to get at the chalk. As a fact, it is being quarried hydraulically by a subsidiary company, which pays to royalty for the privilege. Water, we have already said, is readily obtainable. The gravel is washed down by this, the process having the advantage, besides removing the material, of separating it from the sand and loam. The washing water is led over depositing beds, where the loam falls down in a slurry. This, when mixed with a certain amount of waste from the chalk pits, was found to be an excellent material for making bricks. A brick factory was therefore added, and bricks are manufactured on the site, kilns for this purpose having been erected. The sand, gravel, and larger stones find a ready market, and, as a fact, some 1000 yards of stones for road repairs are sent out per week to various local authorities in London and the neighbourhood. The method of hydraulically working and washing the gravel and sand is, we understand, the subject of a patent, and is similar to the processes at work in the diamond mines at Kimberley. We may also mention that attached to the works there is a large concrete slab factory The company is certainly to be congratulated, not only upon the design and management of its works, but also upon the enterprise exhibited in developing to the full the various advantages possessed by the site.

- o - O - o -

The second article (February 1908) describes the plant after they have taken the plunge and installed rotary kilns. Having waited for many of their competitors to work through the "teething" errors of the new technology, a fairly substantial rotary plant was installed, although it will be seen that the technology was still at a transitional stage, and a lot more had to be learned. The company, still independent of APCM, had, like the other "independents", expanded considerably.

The article, like the others, is anonymous, but it is pretty certainly the work of Bertram Blount. Blount was a professional consultant, and knew that a combination of double-line-spacing and extreme verbosity could turn a thin offering into a substantial-looking report.

In order to match the order of the text, the figures have been re-arranged, although the numbering is kept. Note on the photographs: neither my own copy nor that of Grace's Guide have very good quality photographs. The photographs are part of a set owned by Johnsons, and now held by the Blue Circle Archive. I have only one of these, of much better quality, and I have replaced Fig. 8 with this.


In our issue of February 7th, 1902, we gave a description of the cement works at Greenhithe of I. C. Johnson and Co., Limited. We have recently had an opportunity of seeing in operation a new rotary kiln plant which this firm has now had at work for something under a year. This plant is not intended, at present at all events, to oust the old chamber kilns, for they are now working side by side (Note 14). The output of the original works was, it may be remembered, about 1200 tons a week (Note 15). The new plant will make very nearly an equal amount, and hence the firm's capacity for producing cement has been practically doubled. British firms are frequently being upbraided for their backwardness in adopting new methods, but Messrs. Johnsons certainly cannot be accused of this (Note 16), for not only have they laid down the very latest type of rotary kilns, but they have installed a new power plant, which consists of suction gas engines direct coupled to continuous-current dynamos. The whole forms a most interesting arrangement, and we propose to describe it in the following article.

The efficient use of the rotary kiln requires considerable knowledge and skill, but it may be said that when these are brought to bear the results obtained are certainly equal to, if, indeed, they do not surpass, those which have for long been associated with the chamber and other forms of kilns. The same initial mixture of chalk and clay will, however, not do for both types of kiln. If a certain mixture will produce a good result in one, it is by no means necessarily the case that it will do so in the other (Note 17). Some of the failures which have been experienced with rotaries, if we may for shortness call them so, have been due to the fact that this point has not been sufficiently realised. Mixtures which have for years been used in ordinary chamber kilns have, without alteration, been fed into rotaries, and the two types of kilns have not produced similar results.

Messrs. Johnsons have always been particular as to weighing exactly the proportions of the raw materials put into their wash-mills. They do not believe in rule-of-thumb methods, though we must admit that we have known some wonderfully even results to be obtained in works where rule-of-thumb methods of mixing were in vogue. Messrs. Johnsons prefer to know exactly where they are from the very beginning, and this care in weighing is exercised also in the case of the chalk and clay fed into the new wash mills which have been constructed to prepare the slurry for the rotaries. For the mixture to be absolutely right is one of the great secrets of successful cement manufacture, and another equally important thing when using rotary kilns is that the ingredients must not only be most thoroughly mixed, but they must be must completely disintegrated; there must be no lumps. By "lumps" we mean something which, when a dried pat of slurry is examined, can be detected by the unaided eye. With ordinary kilns this fineness of division is not nearly so important. Bits of chalk the size of a pin's head, or even larger, may find their way into the kilns, and the resulting cement may not be affected in any way. The reason for the difference in the action in the two kind of kilns has not, we believe, been satisfactorily explained, but it has been suggested that possibly the prolonged stay of the materials in the ordinary kilns has something to do with the matter. It is easy to understand that this might very well be so, since the period of burning in an ordinary kiln is about a week, while with rotaries, using the wet process, the time occupied between the throwing of the chalk and clay into the wash mills and the final storing in the bins of the finished cement, is only between three and four hours (Note 18). Having regard to the relative speeds in the two cases, it can well be imagined what a considerable saving of ground space there is for a given output of cement when rotaries are used. This point of saving space becomes more and more important each year, for the specifications of consulting engineers get more and more stringent as time goes on. The finished cement has to be stored, perhaps, in layers not deeper than say, 3ft. 6in.; it must be turned so often, and it must remain for such and such a time under the influence of air before being despatched (Note 19). Hence it has become increasingly necessary for the cement manufacturer, who has to fulfil all these stipulations and many more, to make his cement as cheaply as he can, compatibly, of course, with good quality; and it is undoubtedly cheaper to employ the rotary kiln than the ordinary chamber kiln.

The questions then arise, is the material turned out by rotaries worse than, equal to, or better than cement from ordinary kilns? The reply is that both in appearance and quality it is equal to the best, cement which can be produced, providing, of course, that the plant has been properly designed in the first instance. and that it is properly worked. We are informed by Messrs. Johnsons that even experts cannot tell either by tests or by appearance the method by which a given sample of their cement has been manufactured (Note 20). Still, although all this may be so, it is undoubtedly a plucky thing for any firm, after years of working with one process, to adopt not only another which is entirely different, but, in addition, to make a radical change in the method of driving the machinery. We may here repeat, however, that it does not appear to be Messrs. Johnson's intention to discard, at all events at present, their numerous chamber kilns, but to use them concurrently with their rotaries.

With so much preliminary we may proceed to discuss the actual plant, and to do this it will be convenient to start at the commencement of operations, or, in other words, with the prime movers. We believe that this the first instance of the employment of gas engines with suction producers for motive power in a cement works in this country. The adoption of this form of power was not made without the most careful inquiries and investigations, The final decision was not uninfluenced by the facts that a mixture of coal and coke can be used in the producers, that, the works being on the banks of the Thames, coke is readily obtained from the large London gas companies at a moderate price, and that coal can be brought to the site by water.

johnsons 1908 fig 2

The Power House. Somewhat irritatingly, the right-hand part of the front elevation is actually the side elevation.

We need not go again into the question of the layout of the works, having discussed this in our former article. It will suffice to say that the coal and coke are brought in trucks drawn by steam locomotives from the company's wharf to the rear of the new engine-house. At this point they are shot on to the charging platform of the Dowson gas producers, of which there are four. A glance at the accompanying engraving—Fig. 2—show how these and the mains into which they deliver are arranged. There are in the engine-house—which is a simple brick building with its roof formed by a water tank, and with one end closed in at temporary manner so as to permit of easy future extension—three double-cylinder gas engines made by the National Gas Engine Company. These engines each drive a direct-current compound-wound dynamo made by Messrs. Siemens Brothers. The gas engines are of the horizontal type, each cylinder being governed and fired by magneto from a separate shaft. The engines and dynamos are coupled direct through flexible couplings, and the foundations of all the machines and engines are formed of one solid mass of concrete. We carefully noticed in various positions to see whether there was any vibration, and we found that there was practically none, and the engines which were running at a speed of about 170 revolutions per minute, were working very sweetly and well. We were informed that it was customary to run each engine for a period of fourteen days without stopping night and day, and that this practice had been in operation for some ten months, with no stoppage due to valve or other similar trouble. The rated brake horse-power of the engines is something over 200, and the dynamos are each designed for an ordinary output of 155 kilowatts at 220 volts at 170 to 175 revolutions per minute, though both engines and dynamos arc capable of taking heavy overloads for long periods. Two of the engines are always at work, the third being used as a standby. A switch-board is fitted at one end of the engine-room, and is fixed well away from the wall, so that easy access may be had to the back connections. The field windings of the dynamos are connected to equalising switches on the board, and we may say that the dynamos work in parallel perfectly, there being practically no motion of the main voltmeter needle, though very heavy alterations in load may be taking place. The current is conveyed by over-head cables to the various points where it is used, and these will be referred to in due course. Views of the engine house and switchboard are given in Figs. 3 and 4.

johnsons 1908 fig 3

Figure 3: inside the Power House, showing No 3 gas engine in the foreground, with the flywheel and dynamo behind.

johnsons 1908 fig 4

Figure 4: The switchboard, which is at the far end of the room in Fig. 3.

The next portion of the new plant to be considered is the wash mills. There is nothing special in the design or construction of these. There are four of them placed in juxtaposition and arranged in pairs so that one of each pair feeds the other, and so that the materials get a double treatment by the revolving arms and beaters. Each mill is 18ft. in diameter, and the motor working them and other mills, which we shall mention immediately, is a Siemens machine of 120 horse-power, driving by ropes on to shafting. A special siding and platform have been arranged for the new mills. The chalk obtained from the site, and the clay coming from the company's property on the Medway, are brought to the siding and shot at different ends of the platform. Thence they are taken in hand barrows to the weighing house, where the weight of each barrow load is most carefully taken and recorded. There is one weighing machine for chalk and another for clay. After being weighed the correct number of loads of each substance is shot into one or other of the first two wash mills, into which there is always flowing the regulation quantity of water. After passing through this mill, and getting broken up by the beaters, the mixed materials flow by gravity into the next mill, where they are subjected to further beating.

For the manufacture of cement in ordinary kilns, the resultant slurry would then be taken to horizontal stone mills, but for rotaries a different course is pursued, largely, we believe, because it is necessary to remove all traces of flint, as well as to rid the mixture of the larger particles of chalk. In the present case the slurry is lifted by a bucket wheel from the catch pit into which the second of each pair of wash mills delivers, and is taken into a series of three smaller mills of special construction. Each of these mills consists of a circular tank some 6ft. or so in diameter. Inside this there is a further tank a foot or more less in diameter, so that an annular space is left between it and the outside tank. The slurry raised from the washmill sump is delivered into the inside tank, which is provided with a number of vertical openings some 6in. or so wide, arranged at equal distances round its periphery, and some 2in. or 3in. apart. These openings are covered with sheets of metal gauze, having openings ½ mm diameter (Note 21). Inside the inner tank is a special form of beater, which can stir the slurry up and throw it outwards by centrifugal action against the gauze-covered openings. The result is that a certain portion of the slurry is forced through the gauze and finds its way into the annular space above mentioned. This slurry is then in a condition to be burnt, and is led away by gravity to two reservoir mixers, which will shortly be referred to.

There is a certain amount of residuum left in these mills, and this flows away by gravity to a fourth mill of similar construction, saving that the openings in the gauze are ¾ mm, instead of ½ mm diameter. Here a considerable quantity of water is added, and the matter which finds its way through the gauze is led back to the wash mills to be further broken up, and to be returned in due course to the three smaller mills. What remains inside the fourth mill consists almost entirely of small particles of flint. These are removed, and form at present a waste product.

It will thus be seen that the cycle of operations is continuous, and that an exceedingly fine-grained and well mixed slurry is always being produced. The delivery from the three smaller mills might be taken at once to the kilns, but so as to provide for stoppages on Sundays, when the mills are not worked, though the kilns are, the two reservoir mixers have been constructed. These are placed side by side. They are each 30ft. in diameter, and are provided with revolving stirrers, so as to keep the slurry in proper condition for use (Note 22). When one reservoir is full the slurry overflows from it into the second. From the sump into which these two mixers deliver the slurry is raised by pumps to a platform above the feeding ends of the rotaries, the lift being some 36ft. An interior view of the pump house is given in Fig. 5.

johnsons 1908 fig 5

Figure 5: The Pump House, with two sets of 3-throw pumps.

For this purpose two sets of pumps have been installed, one set being sufficiently powerful to deal with the full amount of slurry required, the other set acting as stand-by. Each pump has three vertical rams 8in. in diameter. They were made by Taylor, of Rochester, as also, we may mention, was the mixing arrangement. The pumps are driven through double reduction rope gearing from a 40 horse-power Siemens motor. It is to be noticed that rope driving has been much used throughout the new works, and it appears to be answering extremely well, the absence of noise being particularly noticeable. The slurry is delivered on the kiln-charging platform into a small stirring tank 12ft. in diameter, in which a revolving stirrer is continually at work. Above the end of each kiln there is a wooden measuring tank. These tanks are charged from the mixer just mentioned, and at stated intervals the contents are let down into the kilns by the withdrawal of the plug leading to the pipe A in Fig. 1 (Note 23).

The Rotary Kiln House. Notice that all dimensions, including civils, are in millimetres.

johnsons 1908 fig 1View HD image in a new window.

There are three rotary kilns (Note 24), each being 110ft. long. They are cylindrical in form, and 7ft. in outside diameter (Note 25). They are entirely lined with fire-brick, the thickness of which is, at one end, 9in. They are driven by a 190 horse-power motor (Note 26), which is fixed in a room by itself, so as to protect it from dust and grit as much as possible. This motor drives a rope pulley, and from this ropes are taken to a shaft running across the kiln house. The shaft carries three pulleys for each kiln. Two of these in each case are for giving motion to the kiln, and they are of different diameters, so that a speed of either one or two revolutions per minute may be given to the kiln, depending on which pulley is in use. There are fast and loose pulleys on the gearing, so that either belt can be used at will. There are three reductions between the driving pulley and the kiln, the first reduction being by bevel gearing. The third pulley mentioned as being on the cross shaft is for revolving the clinker cooler, reference to which will shortly be made.

It will be remembered that the slurry is discharged from the measuring tanks through the pipe marked A at the higher end of the kiln. In its passage to the lower end of the kiln the slurry is first of all dried and disintegrated, and is then burnt. The firing is done front the end remote from that into which the wet slurry is discharged, and hence the temperature gradually rises as the lower end is reached, the maximum temperature being in the burning zone, which occupies the space between about 6ft. and 20ft. from the lower end. In this space the temperature rises to something like 3000 deg. Fah., while the gases escaping to the chimney only have a temperature of some 536 deg. Fah.

The kilns are revolved by means of toothed wheels arranged approximately at the centre of their length. These toothed wheels are not connected directly to the shells of the kilns, but by means of tangential brackets, the object being to allow for expansion, and so to prevent the rupture of the toothed rings. The weight of each kiln is taken on three sets of rollers arranged as shown in the engraving—Fig. 1. Rings for bearing on these rollers are loosely held in position round the kiln by means of a series of brackets on each side of the rings. The latter are a very loose fit on the bodies of the kilns, and there is no fear in this case of breakage by reason of expansion, as there is with the driving ring. As a matter of fact, there has been no trouble with breakage of these driving rings, the precaution against damage by expansion, as above mentioned, having proved perfectly efficacious.

The slurry by the time it reaches the lower end of the kiln has been burnt to clinker, and we may say, that we carefully examined a good deal of it, not only as it came from the kilns, but by observing a large heap of it and found that it was exceedingly well and regularly burnt.

The kilns are fired with powdered coal, which is reduced to such a state of fineness that there is not more than 10 per cent. residue on a sieve of 32,400 openings to the square inch (Note 27). The process of grinding the coal requires considerable care and attention, one of the chief points to be observed being that it must be thoroughly dried before the actual grinding process takes place. Before going on to describe the method of drying employed it will be necessary to say a word or two concerning the way in which the clinker coming from the kilns is cooled, because it is the heat extracted from the clinker which is used in drying the coal. The cooler consists of two cylinders, B and C—Fig. 1—which are of different diameters. The smaller cylinder B, as may be seen in the engraving, does not go quite to the end of C, and one end of C is closed. The clinker as it gradually comes away from the kiln by the revolving action of the latter falls down a shoot D into the right-hand end of the cylinder B. The clinker is, of course, very hot, and the method of cooling it employed is the drawing of large volumes of cold air over it as it travels, by means of the rotating motion of B and C, first to the left-hand end of B, and then back again to right-hand end of C. The fan employed for this purpose is mounted above the firing platform of the kilns, and is indicated by the letter P on the drawings (Note 28). It has another function in addition to that already detailed, and to this we refer later on. The air, heated by its passage, as shown by the arrows, over the glowing cement clinker, passes first of all into a chamber E. In this chamber much of the fine dust, brought over with the rush of the air, is deposited. There are two possible exits from chamber E; one is by the trunk F, which connects with the chamber H, and the other through the duct G, which is connected directly to the suction of the fan P. The latter is only used when for any reason it is desired not to use the chamber H. In the latter there is a revolving drier I. Into this the coal, after passing through a crusher, is deposited by means of arrangements which are not shown on the drawing. As the coal passes from end to end of this drier it is, of course, subjected to a high temperature by reason of the heated air in the chamber. The result is that it is completely dried. In this condition it falls into a hopper and is raised by the elevator LL and deposited into the hopper J. The delivery from this is into a Kominor grinding mill and thence into the tube mill K, in which the coal is ground to the requisite fineness, and in this condition it falls into a hopper and is lifted by the elevator L1L1 to such a height that it can fall by gravity into the hopper N, from which the supply for firing the kiln is drawn. The firing is brought about by forcing into the end of the kiln R a mixture of hot air and powdered coal. The hot air is driven into the kiln through a pipe Q by the fan P, and obtains its heat from the clinker; as already explained. The coal is fed by a worm-feeding apparatus into the pipe O, which is in communication with the discharge pipe of a smaller fan S. The result is that a continuous stream of finely powdered coal, together with a large volume of heated air, is forced into the kiln R through the nozzle Q1. The amounts of coal and air can be regulated to a nicety, the former by altering the position of a friction wheel on the friction disc driving the worm-feeding mechanism. The man in charge of the burning can tell by observing with blue glasses through peep holes in the ends of the kilns exactly how the combustion of the coal is going on, and how the burning is proceeding, and he can make his adjustments accordingly. The coolers are, as we have said, revolved by belts from pulleys on the main shaft, which also drives the kilns themselves.

It should be said that each kiln is kept entirely separate from the others, not only as regards its feeding and burning arrangements, but also as regards its chimneys. Messrs. Johnsons have gone to the expense of building a separate chimney for each kiln, being fully persuaded that only in this way can the draught be correctly regulated and the best results obtained. It may be mentioned that there is also a separate dust chamber at the base of each chimney, with doors for the ready removal of the considerable quantities of dust which collect (Note 29). Our readers will have no difficulty in seeing from the engraving how all the various parts we have mentioned are driven. In every case the motion is obtained by means of electric motors. The whole of the rotary kiln plant was supplied by Messrs. F. L. Smidth and Co., of Copenhagen, Messrs. Johnsons being unable, at the time the plant was put down, to obtain what they required in this country (Note 30). It must be said that the work throughout has been carried out in excellent fashion. Views of the kilns from the firing and charging ends are given in Figs. 7 and 8, on page 220.

johnsons 1908 fig 7

Figure 7: east-facing view of the kiln house from the firing floor.

johnsons 1908 fig 8

Figure 8: a higher-definition version of this from the Blue Circle Archive. West-facing view from the cold end of the kilns.

In our description the point has been reached in the process of manufacture when finished clinker is being produced in a regular stream, It now remains to grind and to store it. As the clinker comes away from the coolers it drops upon a horizontal plate conveyor running across the kiln-house. On its way it passes over a Denison automatic weighing machine—Fig. 9—which records the amount of material carried on the conveyor. So accurately is this weighing carried out that the records are taken as a basis for paying the men, who accept them readily (Note 31).

johnsons 1908 fig 9

Figure 9: a view of the clinker conveyor carrying all three kilns' product, with a Denison weigher left foreground. The conveyor ran between the first and second piers, and No 3 concentric rotary cooler can be seen above, with the hot end towards the left.

The conveyor delivers the weighed clinker into a hopper, from which it is raised by the aid of an elevator to another conveyor. The latter, which is also of the plate type, takes it to the grinding plant, which is in a neighbouring building. An illustration of this plant is given in Fig. 10, and in referring to it we may call attention to the conveyor staging, which is visible outside the building The clinker descends first of all into a ball tube mill 6ft. 2in. in diameter, and from it to a flint mill of the same size. The former is charged with steel balls varying in size from 4in. to 2in. in diameter. The size of the flint balls in the second mill is from 2in. to 1in. in diameter. From the latter the cement is discharged in a finished condition, and is led away by band conveyors to the bins in the store-house, at one end of which the grinding mills are situated. The grinding tube mills were supplied by Messrs. E. Newell, of Misterton, Gainsborough. The motor driving them is of 260 horse-power. A view of it is given in Fig. 6, which may be taken as typical of all the motors employed, and we may add that there are six of them, aggregating in all some 650 horse-power employed throughout the works (Note 32).

johnsons 1908 fig 10

Figure 10: the new ball- and tube-mill set for finish grinding. This, I think, is an eastward view, and the overhead clinker conveyor from the kiln house can be seen through the door.

johnsons 1908 fig 6

Figure 6: A 260 HP (194 kW) finish mill motor.

The whole of the plant has been exceedingly well laid out and erected. It has now been in full operation with only one break of four days during the last Christmas holidays for some ten months. There has never been any other entire stoppage. When in full operation, as we saw it, the plant produces some 1000 to 1100 tons of cement per week.

The various engravings which we are enabled, by the courtesy of Messrs. Johnsons, to reproduce, give a good idea of the new installation, and will not require any further reference than that which has been already made.

We may add that since our last article on these works a large cooperage has been put to work. The firm does a large export trade, and a large quantity of the whole amount of cement shipped is sent away in wooden casks. A complete equipment for making these casks has been laid down. There are machines for cutting, shaping, and grooving, and tongueing the staves; for forcing the hoops on to them, and for making and putting in place the end pieces. The result appears to be the production of wonderfully serviceable casks, which are made all the stronger by the fact that the hoops are fashioned from band steel specially corrugated on the spot. This corrugation, of course, makes the bands very much stiffer than they would otherwise be, so that the casks are all the better able to withstand rough usage.

A special feature of these casks is that the staves from which they are made, before being shaped, grooved, and tongued, are thoroughly dried by being passed through a chamber heated by hot air to a temperature of 180 deg. Fah., the transit through the chamber occupying six days. This thorough drying of the wood has been adopted to minimise the possibility of any subsequent shrinkage after the casks are made and to render them particularly suitable for exportation to hot climates.

It may be of interest if we give the following particulars regarding the cost of power in the new plant. We are informed by Messrs. Johnsons that during a continuous six months' run the total production of electrical energy was 672,159 Board of Trade units, this figure being obtained from watt-hour meters in the engine-room. Including all stand-bys, loss, and stoppages, the total fuel used was 2.056lb. per unit, the cost being 0.255d. per unit. The fuel used was a mixture of Welsh anthracite coal and gas coke (Note 33).

- o - O - o -

The third article is from Cement and Cement Manufacture which in 1929 was in its second year of publication, and had already emerged as a platform for publicising the new developments in the industry, and particularly those of Blue Circle. The new kilns 6 and 7 had been installed the previous year, and were Britain's largest kilns, remaining so until overtaken by West Thurrock No 6 in 1934. The general uprate of the plant accompanying the new kilns gave an opportunity to demonstrate what was considered the state-of-art design features of the time. Because Johnsons underwent only minor modifications in the remaining 42 years of its life, this is a description of its more-or-less final form, as one of Blue Circle's five large base-load Thames-side plants, each of which made about half a million tonnes a year.

Johnson's Cement Works, Greenhithe.


The Johnson's Works of The British Portland Cement Manufacturers, Ltd., situated on the southern bank of the Thames at Greenhithe in Kent, are now approaching the completion of an extensive reconstruction and enlargement. The reconstructed works will have an annual output of about 400,000 tons of cement (Note 34).

The chalk quarry adjoins the works. Clay is obtained from two sources, one a London clay deposit two and a half miles distant, where the clay will be washed and pumped to the works; and the other from land near the works.

Chalk Quarry.

The output of chalk required is 120 tons per hour throughout a working week of 140 hours. This material will be handled by an electrically-driven navvy. This machine, which weighs 92 tons, travels on a caterpillar track and has a bucket capacity of 3½ cubic yards. It is connected by means of a trailing cable to a substation of the outdoor type, at which the electricity is transformed from 3,000 to 500 volts.

The quarry face, which is about 100 ft. to water level, is at present being worked in 30/35-ft. stages. The overburden is stripped by means of an oil-driven navvy. The top stage is operated by a steam navvy, and the development of the second stage by a similar machine. The chalk is dumped into 10-ton railway trucks, and hauled in rakes of twelve trucks to the washmill by steam locomotives.


The loaded trucks after arrival from the quarry gravitate down an inclined siding into a side discharge wagon tippler. The chalk is tipped into a hopper, from which, by means of a two-way valve plate, it is fed into either of two wash-mills. These two mills are of standard design, being 30 ft. in diameter. Each mill is driven by a 225-h.p. motor geared through a single-reduction totally-enclosed gear to the Countershaft. For cleaning out purposes the mill is driven by the same motor through a liquid starting resistance, by means of which the mill can be "inched" round as required. Each motor and control gear is housed so that the operator has a clear view of the interior of the mill at all times.

The slurry gravitates from each washmill to three central discharge elevators, delivering through a feed-trough to eight vibrating screens. The screened slurry passes through into a distributing trough fitted with weir-type control outlets which feed it into four 6 ft. diameter by 30 ft. long peripheral discharge wet tubemills. Each tubemill is driven by a 350-h.p. motor through a set of double reduction gears. These gears are totally enclosed and run in oil. The final drive is by a cardan shaft fitted at each end with flexible couplings connecting to the trunnion end of the mill. Each mill motor, as with all motors of 100-h.p. and over, is fed by 3,000-volt A.C. current, the control gear comprising iron-clad draw-out type trucks.

The rejects from the vibrating screens are conveyed by a reversible belt-conveyor into either of two 20 ft. diameter grit mills on the first floor of the washmill, the fine slurry from these mills gravitating back to the washmills and the flint grit concentrates discharging direct into 10-ton trucks below. Each grit mill is driven by a 30-h.p. motor through an enclosed-worm reduction gear to the countershaft. All elevators and conveyor drives in the mill are driven through similar D.B.S. worm gears direct-coupled to the motors.

The flints from the washmills discharge into a flint excavator situated between the two mills, and discharge through a hopper on the grit-mill floor into the grit-trucks below. The flint wash-water is handled by a two-throw drainage pump situated on the washmill discharge level. The fine slurry leaving the tubemills flows to three slurry elevators of the same type and capacity as the coarse slurry elevators, and the discharge flows through a launder to a distributing box situated centrally between four slurry mixers. These air-agitated reinforced concrete slurry mixers are 66 ft. in diameter by 11 ft. 4 in. high, the capacity of each mixed being about 600 clinker tons (Note 35).

All four mixers are interconnected by valves and pipes for equalising purposes, and a second set of suction and delivery pipes connected to one of the pumps in the pump house serves for conditioning purposes.

Clay Supply.

At the London clay deposit the clay is dug by a steam shovel, deposited into trucks, and hauled by steam locos to a washing plant. The clay slurry is pumped by electrically-driven pumps through a 6-in. diameter main feeding into 66 ft. diameter slurry mixers with "sun-and-planet" stirring gear.

The clay deposit on the land adjoining the works (Note 36) is being excavated by means of a floating dredger. An electrically-driven grab at the forward end deposits the clay into a clay hopper which feeds into a washmill at the rear end of the dredger, and the clay slurry from the screens flows into a sump in the centre. From this sump it is pumped by a centrifugal pump through a 7-in. diameter floating delivery main to a 20-ft. diameter storage tank on the shore. From this tank the slurry gravitates to one of two slurry pumps in an adjoining pump house. The power for these two electrically-driven pumps and also for the floating dredger is brought by underground cables to an outdoor-type transformer, in which it is stepped down from 3,000 volts to 500 volts, the supply to the dredger being carried by means of floating cables.

The clay slurry is pumped through a 6-in. diameter pipe-line back to a 66-ft. diameter "sun-and-planet" mixer, two mixers acting as storage supplies for the two clay slurries.

Adjoining these two storage supplies is a 48-ft. diameter air-agitated clay mixing tank, into which clay slurry from the two storage tanks is led and mixed to form a constantly regulated mixture. These two slurries gravitate from each of their respective storage tanks through regulating valves into an existing sump equipped with a bucket-wheel elevator, the mixed slurry flowing from the elevator delivery through a trough into the 48-ft. air mixer.

The mixed clay slurry then flows into a small pump house, in which are situated four electrically-driven slurry pumps which pump into delivery mains delivering clay slurry to a measuring tank at the washmill. This tank, situated above the grit-mill floor, holds an adjustable supply of clay slurry. Part of this clay supply leads direct to the grit mills and the remainder to the washmills, the operation of the tank being controlled by the man who works the chalk tippler.

Pump House.

The slurry from the four 66 ft. air-agitated storage mixers is led by control valves and a 15-in. diameter suction pipe into the main pump house nearby. This building, which was formerly one of the cement warehouses and is 120 ft. long by 92 ft. wide, is divided into three equal bays. The northern bay, which contains band and circular saws, slotting machine, and other tools, is the wagon repair shop. The central bay is the main pump house, and is equipped with six sets of slurry pumps. All the slurry pumps — thirteen in number — in use on the plant are of the one design, three-throw plungers, 12-in. diameter and 15-in. stroke. Each of these six pumps is connected by stop valves to the one 15-in. diameter suction main, and each delivers, again by means of suitable stop valves, to delivery mains, 8-in. diameter, carried on an overhead gantry leading the slurry to the four kilns.

These six pumps, which extend down the centre line of the building and over which runs a 7½-ton travelling crane, are driven through an overhead countershaft at the side of the bay by independent belts, the countershaft being driven by a 150-h.p. motor in the centre of the southern bay. This bay is divided into three sections; one is a storeroom; the centre section is the motor house; the end section contains another motor-driven slurry pump connected by a common suction and delivery main to the four 66-ft. slurry mixers, and is used for transferring slurry from any one mixer to any other for conditioning purposes (Note 37).

Rotary Kilns.

The completed plant will consist of four rotary kilns (Note 38); two existing kilns, called respectively No. 4 and No. 5, each 202 ft. long, 9 ft. diameter, with an enlarged burning zone 10-ft. diameter, will give a combined output of 2,700 tons of clinker per week. Each kiln is provided with a rotary cooler set at right angles to the axis of the kiln (Note 39). The kilns are driven through the ordinary gearing and belt drive by totally-enclosed A.C. commutator motors (65/22 h.p.), having a speed variation ratio 700/230 r.p.m., 3-phase, 500 volts, 50 cycles. The two new kilns, No. 6 and No. 7, are each 317 ft. long overall with a burning zone 46 ft. and 12 ft. diameter.

The clinker coolers are of the recuperator type; twelve cylinders, each 4 ft diameter and 20 ft. long, are placed round the discharge end of the kiln, the centre line of the cylinders lying parallel to the centre line of the kiln. The hot clinker leaves the kiln shell at the extreme end, discharging into twelve holes on its periphery and thus entering into each of the twelve cooler cylinders. In each of these, through the medium of spiral guides, the clinker is slowly transferred to the discharge end where it falls on to a shaker conveyor (Note 40). These kilns are supported on six roller bedplates and are driven through four sets of reduction gears. The first and second reduction gears are totally-enclosed and run in oil under a system of forced lubrication to the bearings and sprayed on the teeth. The motors for these drives are of the variable-speed commutator type of 160/47 h.p., 650/225 r.p.m., 500 volts, 3-phase, 50 cycles, totally enclosed and water-cooled. The revolutions of the kiln can be varied from a maximum of 1 in 70 seconds to a minimum of 1 in 210 seconds (Note 41).

Slurry Feed.

The slurry leaving the pump-house is conveyed through an 8-in. diameter main to the feed end of the kilns, leading in succession to the spoon feeds above kilns Nos. 7, 6, 5 and 4, the delivery to each spoon feed being controlled by a valve on a tee-piece connecting to the slurry main. The existing spoon feeds on kilns Nos. 4 and 5 are belt-driven by a belt passing round the shell of the kiln (Note 42). The feeds on kilns Nos. 6 and 7 are electrically driven by variable-speed D.C, motors geared to the drive-shaft of the spoon feed. The variation in feed is controlled and governed by an adjustable variable-shunt resistance on the kiln firing-floor.

The overflow (Note 43) from the four spoon-feeds is taken by a separate pipe-line to an existing 66-ft. diameter mixing storage tank situated behind kilns Nos. 4 and 5. The accumulated slurry in this mixer flows by a suction pipe to three 3-throw slurry pumps near the feed end of No. 6 kiln, and is thence pumped back to the 8-in, slurry main to feed the four kilns.

Coal Supply.

The coal arrives at the works siding in standard 10-ton colliery wagons (Note 44). The loaded wagons are placed in rakes of seven at a time on to an incline, from which they gravitate one at a time over a side discharge tippler (a duplicate of that at the washmills) and discharge into a double-compartment bin, each compartment discharging on to a rotating feed-table which delivers the coal into skips holding 1¾ tons each. These two skips alternately lift to the top of the storage silo and discharge the coal into a top hopper provided with distributing chutes. This storage silo, which is of reinforced concrete construction, has a storage capacity of 1,000 tons in two compartments, one holding 750 tons of slack and the other 250 tons of duff (Note 45).

The operation of this plant, which has a handling capacity of 100 tons per hour, is practically automatic and can be run by one man who places the loaded wagons on the tippler. The tippler is operated by one push button, which elevates, tips, and returns the empty wagon. As each empty skip reaches the level of the rotating feed-table the latter automatically begins to discharge and fill the skip. When the skip is practically full its increasing weight operates a switch which shuts off the feed-table, and after a few seconds the full skip is hoisted to the top of the silo into the tip, and the same cycle of operations begins on the descending empty skip.

The coal discharges through openings provided with regulating feeds in the bottom of the silo on to a travelling belt. The slack and duff in any desired proportions feed into a cross-conveyor mixing-trough, which feeds through a roll crusher on to an inclined belt-conveyor which passes over and feeds into the coal hoppers of the four kilns on the firing floor platform. The coal from each hopper passes through a cubimeter into a pulverizer, and an accurate record is thus kept of the coal consumption of each kiln.

On leaving the kilns the exit gases pass through dust-precipitation chambers (Note 46) on their way to the chimneys. The precipitation chamber for kilns Nos. 4 and 5 is a reinforced concrete structure lined with firebrick. It is divided into two separate compartments, one for each kiln; each compartment is hopper-bottomed and fitted with the necessary appliances for dust precipitation. The precipitated dust is discharged from the hopper bottom through self-dosing valves into a trough provided with a scraper chain, discharging into a small mixing tank supplied with slurry. The mixed product is then pumped back into the 66-ft. storage mixer (Note 47). The precipitation chamber directly adjoins the chimney leading into it by means of two rotary damper-doors so that either compartment can be isolated while the other is working. The chamber connects with the outlets of the two kilns by a flue carried on concrete piers. The dust chamber for kilns No. 6 and 7 is similar in design, and is connected direct to the kiln outlets by short reinforced concrete flues.

The clinker from kilns Nos. 6 and 7 delivers from the recuperators into either of two cross shaking conveyors, which in turn deliver into either of two main shaking conveyors running parallel with the kilns. The clinker from the coolers of kilns Nos. 4 and 5 also discharges directly over and into these conveyors.

The combined clinker output from the four kilns delivers into either of two clinker elevators housed in a tower just outside the end of the kiln building. The capacity of each of these elevators is 65 tons per hour.

Grinding Mills.

The clinker is ground in six compound tube mills. Each mill is 36 ft. long by 7 ft. 2 in. in diameter and divided by diaphragm plates into four grinding compartments. The capacity of each mill is 20 tons of clinker per hour (Note 48). The mills are driven through a spur-wheel and pinion drive, each being connected by a coupling to a 750-h.p. motor. These motors, which run at 158 r.p.m., are of the self-starting auto-synchronous type working on a 3-phase 3,000 volt supply and arranged for unity power factor at full load.

The mills are fed from an overhead bin running the full length of the building and divided into six compound compartments, one to each mill; each clinker compartment holds 200 tons and each adjoining gypsum compartment holds 50 tons. At the kiln end of the overhead clinker bins is a reserve clinker storage with a capacity of 4,000 tons (Note 49). This is a reinforced concrete structure with retaining sides, and the floor on the ground level is divided along its centre by a partition wall separating the store into two equal compartments.

Clinker leaving the shaking conveyors at the coolers is elevated to the top of the elevator tower and delivered into either of two 20-in. wide troughed belt-conveyors (Note 50). These run at the same level by an overhead gantry across to the top of the reserve clinker store and thence along the full length of the overhead clinker bins. Each belt is provided with a travelling tripper-gear running the full length of the bins, which deposits the clinker into any of the bins and both sides of the reserve store. The stored clinker from the latter is withdrawn through two chutes which deliver into an elevator discharging on to either of the conveyor belts and thence into the mill bunkers.

The grinding mill building is 140 ft. long by 114 ft. wide. One-half the building houses the six grinding mills (and storage hoppers) and the other half, separated by a dust-proof concrete partition, is again divided into two divisions; one section houses the six 750-h.p. driving motors and the other is further sub-divided into a main substation equipped with distribution panels and switches, a substation power supply, and two air-washing chambers.

Adjoining the firing end of the kiln building is a reinforced concrete gypsum building having a storage capacity of 800 tons. Gypsum, which arrives by boat at the jetty, is loaded into standard 10-ton wagons, and after passing over a weighbridge passes by an elevated railway to the top of the gypsum store and unloads direct into the store beneath. The outlet, which is on the ground level, leads to a crusher. The crushed material falls direct from the crusher into either of the two clinker elevators and thence to the duplicate 20-in, belt conveyor which carries it to the gypsum bins over the grinding mills. Gypsum and clinker in the desired proportions are fed to the grinding mills by rotating feed-tables in the usual manner.

The output of ground cement from the six tube-mills discharges into either of two 18-in, diameter screw conveyors running under the mill floor. After elevation the cement discharges on to two 24-in, flat-belt conveyors housed in an inclined steel gantry, which extends from the tower, over the top of the grinding mill building, to the top of No. 1 and No. 5 silos. Here the two conveyors discharge into two cross conveyors, one running along the top of each line of silos (Note 51).

Silos and Warehouses.

The silos, eight in number, each hold 2,000 tons of cement, giving a total storage capacity of 16,000 tons (Note 52). The diameters are 32 ft. 6 in. inside by 96 ft. high from ground level. They are constructed of reinforced concrete and are arranged in two lines of four, each line having a tunnel 13 ft. wide to allow of the bulk wagons entering for filling (Note 53). Each silo is divided by vertical walls into four compartments, one comprising half the total capacity, the other half being sub-divided into three. Platforms are arranged every 12 ft. outside the silos to allow of samples being taken of the contents.

The silos are filled from the cross-belt-conveyors through feed hatches in the top, separate feed hatches being provided for each compartment. Each compartment discharges through flexible pipe connections to bulk wagons in the tunnel below, and a compressed-air supply is arranged to facilitate the discharge. The bulk wagons, three in number, are of steel construction and totally enclosed, with a capacity of 25 tons each. They are electrically-driven from an overhead line, and supported on two bogies. The leading bogie is driven on each axle by standard tramway-type single-geared motors, the necessary controls being carried in an enclosed cab. The trailing bogie is not driven.

The two existing cement warehouses, capable of storing 6,000 tons of cement, will remain and be used for road and lorry trade and, in case of emergencies, to supplement the output of the packing plant on the jetty. Cement is delivered to these warehouses by two 24-in. wide flat-belt conveyors, which in turn are fed from the conveyors over the silos. These conveyors, housed in an overhead steel gantry, discharge on to two existing 14-in. flat-belt conveyors running the whole length of the two warehouses which discharge on to the flat floor. By means of two screw conveyors below floor level the stored cement is then conveyed to a 2-valve packer and packed in sacks and casks. The packed material delivers on the one hand direct into a loading bay connected by a concrete road to the main road and on the other to a railway siding. The steam and petrol lorries and trailers used for road delivery are housed in one bay of a new building, the other bay housing the railway locomotives.

Details of the jetty will be given in a later number (Note 54).


Note 1. The preamble on the location of cement plants is written very much from the perspective of Johnson's, comparing and contrasting plants on the Thames and on the Tyne, and emphasising the considerations that caused them to progressively relocate their business to the South.

Note 2. As at Gateshead. In efficient modern practice, location of a plant at the source of fuel would seem an odd idea, but the coal and coke for an early plant constituted a major mass-flow. It is perhaps significant to the later history of the British industry that the Johnson kiln - which was for too long considered the standard equipment in Britain - was developed on Tyneside, where coal was so cheap as to scarcely warrant a mention on the balance sheet.

Note 3. This implies a specific land holding, rather than the more usual ad hoc working of the foreshore. The clay was evidently supplied in a relatively dry condition. I don't know exactly where their land was - any ideas? This source was later (1920) replaced with London Clay.

Note 4. With the further expansion of the plant, a rail link was established within the next few years.

Note 5. As mentioned in the section on Raw Material Preparation, the final composition of the rawmix depends on its chemistry and not the proportions of the individual mineral components, which are themselves variable in composition. The exacting approach to weighing described here was mainly designed to inculcate a meticulous approach in the operatives.

Note 6. With the raw materials in question, a 1-in-500 control would be a bare minimum for acceptable chemical consistency. Bear in mind that the plant had little slurry blending capacity, and the slurry feed to the kilns was essentially that leaving the mills.

Note 7. Lighting was the only use for electricity at this stage.

Note 8. This statement ought to be definitive, but seems suspect all the same. The Johnson chamber kiln patent dates from 1872, five years before the Greenhithe plant was established, and there were certainly chamber kilns at the Gateshead plant before 1877. However, these were probably the first on the Thames.

Note 9. This is the block at the top (NW) of the plan, built into the worked out chalk quarry and connected to the main stack at the top of the quarry face.

Note 10. This implies that the plant capacity was 2187 tons per week. Later the "average" output is stated to be 1300 tons per week, indicating that the average kiln turn-around time was actually 7.85 days (including maintenance time) and not the "three times in 14 days" mentioned. The latter might be regarded as a "flat-out" performance only resorted to - with extra labour - in very busy periods. Batch kilns are normally rated on the assumption that they achieve one burn per week.

Note 11. This represents about 7-8 weeks' throughput.

Note 12. Like many of the cement manufacturers, Johnson came from a Nonconformist, Sunday-observing tradition. Everything except a minimal supervision of the kilns ceased on Sundays, and this largely dictated the natural seven-day cycle of cement plants. This regime began to break down as rotary kilns were adopted.

Note 13. These are glacial "head" deposits associated with the downland valley that runs north into the River Thames at Greenhithe and overlay the chalk on the eastern edge of the plant's lands.

Note 14. Although larger rotary kilns were installed from 1913, some of the chamber kilns remained in operation - off and on - into the mid 1920s.

Note 15. Actually the first article said 1300 tons per week. It is likely that, with the increasing demand for finer cement, the clinker grinding plants became a constraint on capacity.

Note 16. On the other hand, they weren't exactly early-adopters. It was the eighteenth plant to install rotary kilns.

Note 17. The most commonly identified cause of this was the lack of absorption of fuel ash and sulfur in the early rotary kilns, resulting in over-limed clinkers.

Note 18. At first sight this sounds like a daft statement, but it becomes clear that this plant had no storage for slurry or clinker, and materials did indeed pass through the plant at this rate. The Bureau of Mines Equation indicates that the residence time in the kilns, running flat out at 2 rpm, was about 40 minutes.

Note 19. Particularly after rotary kilns began to be installed, high free-lime clinker was common, and this was a way of letting the cement's unsoundness "die down". In the case of Johnsons, it also gave a last-ditch opportunity to do a bit of blending.

Note 20. As the lady said, "They would say that, wouldn't they?"

Note 21. This is an early "screening mill". Because the slurry hits the screen at an acute angle, the "cut size" obtained is somewhat smaller than the 500 µm sieve opening.

Note 22. The tanks probably held together around 300 m3 of slurry, equivalent to 150 tonnes of clinker, so only 25 hours run for three kilns making 2 t/h each. So the statement that these are only to see the plant through Sundays is the literal truth. In fact, apart from Sundays, the plant has no blending capacity at all - just as was the case with the chamber kilns. The chemical control system, although conducted with a degree of OCD, must have been extremely variable. No doubt the plant's management soon learned some hard lessons.

Note 23. Unfortunately, the article does not elaborate on this, and the drawing gives no clue. It may have been an automated process, but most likely was performed manually. From the drawing, the "measuring tank" appears to hold about 600 L of slurry, equivalent to about 300 kg of clinker, so a kiln making 2 t/h would take a 600 L slug of feed every 9 minutes. Really?

Note 24. These were the first of F L Smidth's "second generation" kilns, the first being only 18 m long. This description is also applicable to the similar kilns installed at Newhaven and Premier in the next two years, after which the design was modified by extension to 43-45 m with an enlarged (2.4 m) burning zone, examples being installed at Harefield, Wouldham and (belatedly) Kirton Lindsey.

Note 25. Like all FLS kilns, they were metric throughout, and were 34 m long (when cold) and 2.1 m diameter inside the shell. With a shell thickness of 20 mm, the outside diameter was 2.14 m. Blue Circle drawings give 111'6½" and 7'0¼". I believe that these vague dimensions are quoted by The Engineer, not because of incompetence, but rather in order to give an air of bluff, no-nonsense Englishness to a design which was in fact continental.

Note 26. This appears to be grossly oversized, since each kiln should need at most 20 HP at full speed, and each cooler 10 HP at the most - a total of 90 HP. It is scarcely believable that the equipment required another 100 HP for bearing friction and drive losses.

Note 27. i.e. about 85 μm.

Note 28. Thus the air is drawn through the cooler by the firing fan, and not by the kiln suction. The cooler throat was very narrow - only wide enough for normal-sized clinker to trickle through. FLS at this time still held to the philosophy that all the combustion air should go through the firing pipe, and if there was any secondary air at all, it consisted only of cold leakage around the hood. For the same reason, the firing pipe (see Fig. 7) was very large. This probably gave rise to incomplete combustion and poor burning zone heat exchange. To prevent the flame from propagating back up the pipe and causing an explosion or setting fire to the coal mill, cold air would have to be bled into the firing loop, further reducing efficiency. It is perhaps significant that anthracite was being used, since this would be less "firey". Anthracite was not usually burned by Thames-side plants.

Note 29. Given that the drop-out chamber can only have been about 70% efficient at best, clearly the quantity of dust emitted from the stack was also considerable. The short (24 m) stacks would have ensured that most of this landed within the plant perimeter, but Greenhithe High Street must have got a fair bit.

Note 30. Newells were the first British kiln suppliers, and installed their first (Kirtlington) about the same time.

Note 31. Presumably a brick on the weigh-frame would give everyone a bonus.

Note 32. This is again transitional technology: the old system was a central steam engine with every piece of equipment belt-driven from a lay-shaft. Electrification allowed each piece of equipment to have its own motor. But here there are a limited number of motors, with several items sharing each.

Note 33. A "Board of Trade Unit" is 1 kWh. The fuel cost was 23s.2d. per ton. Guessing a calorific value of 29 MJ/kg for the fuel, the thermal efficiency is 13.6%! It's around 11p per kWh in modern (2014) money.

Note 34. In fact, developments in kiln chain heat exchangers in the early 1930s extended the clinker capacity of the plant to about 500,000 tonnes a year.

Note 35. The four kilns together made (eventually) about 60 tons an hour, so each mixer can hold about ten hours' run. The 66-ft air-agitated mixer became the standard Blue Circle design.

Note 36. This was in fact the Kent plant clay quarry, which was being shared as a temporary expedient during the modification of the London Clay quarry at Bean.

Note 37. A rather second-rate expedient - two more mixers were added in the 1930s as a preliminary blending stage.

Note 38. This implies that the new kilns were not yet running, and a photograph shows them partially constructed. But although the date is uncertain, they started in 1928: the article's publication must have been delayed.

Note 39. Kilns 4 and 5 (made by Newells and Vickers respectively) originally had their coolers under the kiln, in a northwest direction. As part of the uprate, the coolers were modified and rotated to a southwest direction, purely in order to avoid installing conveyors to plumb the old kilns into the new clinker handling system. Because Kiln 4 was furthest from the new kilns, this resulted in a very long cooler - 120 ft for a 200 ft kiln. Other examples exist of kilns and coolers being used as glorified conveyors.

Note 40. FLS had introduced their "Unax" planetary cooler in 1923. Various German manufacturers produced their own versions, some of them "reflex". However, these were Vickers' first, produced under the "Recuperator" trade name. The advantage of the "reflex" design was that it avoided the use of the "tunnel" approach to the kiln hood. On the other hand it had distinct disadvantages: it was necessary to "screw" the clinker uphill, and the support of the outlet end, and the clinker receiver, were wrapped around the hottest part of the kiln. The tendency of the cooler tubes to fall off led to the eventual abandonment of the design, and in the case of the Johnsons kilns, they were replaced with Fuller coolers in 1959-1960.

Note 41. This represented a considerable advance on previous designs and became a Blue Circle standard.

Note 42. This ensures that the feed rate is proportional to the kiln speed. This was a practice dating back to the Hurry and Seaman patents, but there is no evidence of this device on "sister kilns" installed elsewhere, and this was probably a local modification.

Note 43. It was necessary to supply an excess of slurry to spoon feeders in order to ensure a constant level in their reservoirs. This therefore involved a considerable recirculation of slurry. The large recirculation meant that the old mixer was effectively the kiln feed mixer for the plant. This arrangement was forced on the plant by the fact that the kilns had been located at a position leaving no room for the new washmill plant at the feed end - the logical choice - so that slurry had to be pumped a long distance across the plant.

Note 44. Note that although the plant now had a rail link, the coal still came in by water, and the "colliery wagons" were loaded at the wharf. The cement was similarly double-handled.

Note 45. Slack was small coal with a maximum size of 15-20 mm, and duff was fine coal below 3 mm. Both were wet - the duff being practically mud - and because of their unfavourable handling characteristics, they were cheap enough to offset the cost of drying.

Note 46. These were simple drop-out boxes, in which the exhaust gas velocity was reduced sufficiently for the larger dust particles to drop to the bottom. The large capacity of the kilns, combined with the inefficiency of this system, must have caused a nuisance in the neighbourhood, and in 1933 the kilns were fitted with electrostatic precipitators - the first to be installed on ordinary British kilns.

Note 47. Recycling of the entire kiln dust in this manner causes a thickening of the slurry, and necessitates an overall higher slurry moisture, partially or completely offsetting the thermal advantage of recycling. However, the dust from the drop-out chambers was probably low in sulfate and alkalis (these having gone out in the stack emission) so return of the coarse dust to the slurry probably worked in this case.

Note 48. This represents an energy consumption of about 26 kWh/tonne. This would be feasible at a typical cement fineness of 300 m2/kg.

Note 49. This is less than three days' capacity and was far too small. In practice, the main clinker stoarage was in the open.

Note 50. The use of rubber conveyor belts for clinker relies upon the clinker remaining cool - even synthetic rubber will not stand clinker much over 120°C. Even efficient coolers will not deliver cool clinker all the time.

Note 51. Rubber belt conveyors for cement are cheap to run, but have many problems. Sloping belts have to ascend at a shallow angle, as hot cement easily slides back downhill, particularly at transfer points. There is continuous spillage, particularly at the discharge end, because a certain amount of cement sticks to the belt, and gradually falls off the return strand. Bevans, a few years earlier had been provided with Fuller-Kinyon pumps, but this was not tried again until the 1950s.

Note 52. With the warehouse described later, this gives a storage capacity of 22,000 tons, or about 2 weeks' run.

Note 53. Note that cement was delivered in bulk until the 1950s. The "bulk wagons" are "torpedoes" used for shuttling cement by rail to the wharf - the large majority of cement was despatched by water. The small amount of rail loading of packed cement took place at the old packing plant fed by the old warehouse.

Note 54. The wharf had its own, much larger packing plant.

Johnsons pic
Britain from Above features some of the oldest and most valuable images of the Aerofilms Collection, a unique and important archive of aerial photographs. You can download images, share memories, and add information. By the end of the project in 2014, 95,000 images taken between 1919 and 1953 will be available online.
This was taken on 27/2/1939 and shows the plant from the southwest. Kilns 6 & 7 are running. Kilns 4 & 5 are in the open, beyond the kiln house. In front of the kiln house, and perpendicular to it, the remains of the main chamber kiln bank can still be seen, while the third block is visible in the quarry to the left of the stack. The washmill plant is lower right. Lines of chalk wagons are to be seen at the end of the track that went under London Road (foreground) into the Stone Castle quarry. In the background can be seen the wharf, connected to the plant by rail. Between the houses on Charles Street and the river, the by-now worked out alluvial clay quarry can be seen. Zoom in on the plant in High Definition.

Original content © Dylan Moore 2014: last edit 01/06/16.

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