Cement Kilns

The Original Johnson's Greenhithe 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 is a transcript of an anonymous article in Building News and Engineering Journal, 2/7/1880 pp 5-7, believed to be out of copyright. It describes the plant in essentially its initial state, a few years after it was commissioned. While not necessarily written by Johnson, the words are undoubtedly mostly his own, in terms he used elsewhere.

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.

AN IMPROVED CEMENT MANUFACTORY.

From the somewhat crude and incomplete factory of Aspdin (Note 1), established at Wakefield more than half a century ago (Note 2), there has been through all these years but indifferent and unsatisfactory progress in improvements, either to cheapen the cost or better the quality of Portland cement. While England, from its favourable position both as regards the command of raw materials (chalk and clay) and fuel at a low cost, assisted by cheap and ready means of transit to every quarter of the globe, secured a monopoly of the trade, but little anxiety or desire arose for changing the original system of manufacture (Note 3). In addition to those advantages named, a belief existed that Portland cement could only be made from chalk and clay, or mud from the river Medway, in Kent; and to such an extent did this idea prevail, that the early cement makers, in Germany especially, used Medway mud at a very high cost for their first essays in their desire to produce Portland cement. The eminent chemists, however, who soon took charge of the cement question on the Continent, were not long in disabusing the public mind on this point, and it was soon made evident that a good and reliable Portland cement could be produced from other minerals than chalk and clay. This knowledge, accompanied by equally cogent commercial reasons, led to the establishment of foreign Portland cement works, more especially in Germany; and the considerable trade hitherto done with that country by English manufacturers has in consequence dwindled down to comparatively insignificant proportions, and, indeed, we are already beginning to receive supplies of cement from German manufacturers. This somewhat unexpected competition, and the increasing and more intelligent requirements of the engineer and architect, has given an impetus to this great industry which has, as we have already observed, resulted in many useful and satisfactory improvements.

That our readers may better understand the nature and character of the more advanced cement manufacture, we purpose in this essay to give a description and particulars of what may be regarded as the most favourable outcome of recent invention and progress displayed at the manufactory of Messrs. I. C. Johnson and Co., Greenhithe, in Kent. We select these works from their comparative nearness to London, and also in some measure from the fact of Mr. Johnson being the oldest practical cement-maker in England, or indeed, anywhere else. At a recent discussion which took place at the Institution of Civil Engineers, Mr. Johnson stated that he had been a cement-maker for upwards of fifty-five years (Note 4). The more credit to him, therefore for having been the first to shake off the trammels of, we might almost say, antiquity, and enter upon a system of manufacture which has upset the cherished traditions of the past, and its associated absurdities.

The works at Greenhithe were established with the object of following the new lines of manufacture, and, therefore, it was a comparatively easy task to arrange the plan and machinery to meet the requirements of the altered character of the industry. Favourably circumstanced as regards site, the new works command an inexhaustible supply of the finest chalk, while they are connected by a short line of railway to a wharf on the Thames, so that all the advantages which a cement works should possess are thus secured. The original object of Mr. Johnson was to avoid the use of an extensive system of "back" or reservoir space, and so hasten the process of manufacture, besides dispensing with the cost of land and buildings involved in their construction and arrangement. At first the experiments in this direction were attended with some difficulty, but eventually they culminated in the unqualified adoption of the "Goreham process" of mixing or washing, and the "Johnson kiln" (Note 5). These works of Messrs. Johnson being placed in a locality near which are dwelling-houses, the production of the gases from the kilns was considered by the surrounding inhabitants as dangerous to health, legal action having been taken to prevent the manufacture of cement in this otherwise favourably situated locality. It was shown, however, that more than ordinary precautions had been taken to avoid the chance of any nuisance, and the result was that the works have been prosecuted not only with profit and credit to their owners, but without inflicting harm or annoyance on their neighbours. Although not far from what may be termed the great Northfleet zone of the cement industry, the works at Greenhithe are comparatively isolated, and one feels surprised that any action could have been taken for their suppression, unless some under-current of interested opposition felt annoyed at the success of a new rival in an industry which some manufacturers doubtless consider as an hereditary privilege (Note 6). Of course the very nature of the manufacture of cement indicates that in its prosecution a large amount of noxious gases must be eliminated from the raw materials and the fuel by which they are converted. One of the most important gaseous products realised is that of carbonic acid, and unless some effectual means are provided for its careful destruction or dispersion, a danger not only to vegetable but animal life would result (Note 7). It will be seen, however, from our description of the works at Greenhithe, that every device that means or ingenuity could suggest, has been adopted to secure perfect immunity from danger to the surrounding country or its inhabitants.

johnsons 1880

Our woodcut gives a fair representation of the works of Messrs. Johnson and Co., and an examination of it will show the peculiarly favourable character of their position, and the originality and skill which have been displayed in their arrangement. What may be regarded as the most important feature in connection with the establishment is the chalk source, and in the quarry immediately behind the works, a face of upwards of 50ft. in depth, proves not only that the quality of that indispensable carbonate of lime ingredient is good, but the supply will be able to withstand the greatest possible run upon it for many years to come. The chalk is as nought, however, if the clay or mud (silica and alumina ingredient) cannot be secured. This has also due attention, and the connection with the river commands a supply from the Medway on the same equally favourable conditions and terms as all the other factories both in the Medway and the Thames. The command of coke (gas), coal, and every other subordinate want are all secured under the best and most convenient conditions. The works may be said to be built on the river level, and, therefore, at little cost, receives the raw materials and, with equal convenience, secures the ready despatch of the manufactured cement by locomotives, which, in the various branches or departments of the industry, are constantly at work.

The cement works at Greenhithe may be considered, at present, as from three to four hundred ton-power sic manufacture per week, which means the handling and moving of about two thousand tons weight in seven days, besides the water used in mixing the raw materials.

The first, and, we might almost say, the most important building on the works is that at the right-hand corner of the illustration, where the chalk and clay are mixed together, and in which is placed the wash-mill. This machinery of mixture is in duplicate, so that, in the event of accidents, no hindrance to the manufacture can arise. It will be well to explain at this point that the system adopted, and which we are about to describe, involves a continuous operation of washing, owing to the total abandonment of receptacles or backs of storage (Note 8), an inseparable adjunct of the wet, or old method. The distinctive term "semi-wet process" hardly conveys the meaning of the difference between the two systems, and we, therefore, at starting, give this explanation:-

The wash-mill is the beginning of the manufacture proper, for we cannot recognise the quarrying of chalk or digging of the clay as an operation where technical skill is required. The chalk is brought to the wash-mill by the locomotive, and it, together with the clay, is conveniently arranged so that the workmen readily put into the mill the regulated proportion of the one and the other. The speed of the rotating mill, with its series of iron cutters, is so arranged, and the supply of the materials favourably adjusted to secure a fair reduction or maceration of the now partially-combined chalk and clay. The quantity of water which enters the wash-mill varies from forty to fifty per cent. of the weight of the raw materials. The result is, the production of a thin pasty mass, which is sufficiently fluid to be readily elevated to the hoppers of the horizontal millstones placed in the adjoining building to the left. The act of elevating tends to further perfect the mixture, and after passing through the millstones, which renders the slurry more fluid still, it is pumped or forced to an elevation high enough to secure its flow, by gravitation, to all points of the flues, which form the salient feature in Mr. Johnson's patent kiln. Before entering on further description, we will point out the exact position of these flues on our accompanying woodcut. Adjoining the mill where the slurry from the wash-mill is operated upon are the engine and boiler-houses, and next to these are the cement grinding-mills, and warehouses for storing the cement. On the same level, and immediately behind and parallel to this line of building, at a distance of thirty feet, are the kilns, at present numbering fifteen. At the entrance-height of the kilns (all of which are covered in) are the drying-flues, segmental in form, and about 10ft. high. These flues receive the slurry, which is conveyed by pipes direct from the mixing-mill-stones, and inlets or holes in the arches, readily permit of its dropping down at any desired point. The flues have an inclination of 1 ft. in 100 ft. rising from the kiln, so that the thickest or deepest part of the wet slurry is at the point nearest the kiln where the greatest heat is produced. When the kiln is lighted the end of the flue is built up and made air-tight, and the flue itself connected to the main heat channel, joined to the main chimney 300ft. high, and thus secures not only a draught for the kiln, but a perfect method of exhausting all the gases which arise during the combustion of the kiln. The inventor of this kiln and its drying adjuncts claims for his system that the slurry being heated and dried from above, is more compact in character (Note 9), and, therefore, more susceptible to beneficial heat action when placed in the kiln. An objection has been raised by some critics that the heated gases in their passage over the slurry or slip are partially absorbed, and, by such absorption, introduce into the process a new element of distrust or danger. Mr. Johnson, however, to dispose of this objection, has had the following analysis made of the thin deposited scum, or film, resulting from the passage of the heated gases, from which it will be seen that no injurious result from that cause is likely to arise, or even possible.

ANALYSIS. (Note 10)
Water16.30
Sand7.50
Alumina0.91
Oxide of Iron0.72
Sulphate of potash47.16
Sulphate of soda7.66
Chloride of sodium10.66
Sulphate of lime8.28
Sulphate of magnesia0.84
Total100.03

This arrangement of utilising the waste heat of the kiln under the circumstances we have described has almost, if not quite, revolutionised the system of cement manufacture. The proportions of kiln and flue capacity require careful adjustment, for the best advantage can be derived only when they are symmetrical in their measures. Thus it would be unprofitable, and, indeed, inconvenient, were too much kiln-room provided, and thereby require its being lighted when only partially filled. Again, too much flue-space, which would produce more dried slip or slurry than the kiln would contain, would also be a disadvantage (Note 11). Experience, however, has now arrived at the exact proportion of washing power, flue accommodation, and kiln capacity, so as to prevent the possibility of derangement in the continuous and regular manufacture under the new system. The whole of the processes, too, have another great advantage over the old-fashioned wet system, and that is the immunity from delay or stoppage by unfavourable weather, whether arising from rain or frost. Each kiln has its own carefully-covered and weather-proof flue; and thus, when the contents of the burnt-out kiln are taken away to be ground, the process of refilling may be begun, and so soon as that operation is complete the slurry is speedily allowed to cover the flue floor again for the next charge of the kiln. Under ordinary circumstances the routine of such a process becomes almost mechanical in character and unfluctuating in its conditions, because there is no possibility of intervening error arising to derange its uniformity or accuracy of result.

Securing the unvarying products from the kilns with such regularity permits of adjusting the means of reducing and grinding the clinker, so as to prevent any delay in emptying the kilns, which would practically lead to stoppage of the whole work. The clinker, on its withdrawal from the kilns, is wheeled across to the grinding mills, on the ground floor of which is placed a powerful Blake's stone-crusher, and after being cracked or crushed by its agency, is raised by elevators to the hopper-floors of the cement-grinding millstones, from which it issues in the required condition of fineness. Much difference of opinion at present prevails as to the exact quality of the powdered cement; but to meet the requirements of the most exigent demands, a sifting apparatus is provided, so that almost any degree of fineness can be secured. This, however, involves increased cost, which the advanced or intelligent consumer does not hesitate to meet by paying a higher price for cement so prepared.

The high chimney, so prominent a figure in the view of Messrs. Johnson's cement works, may be considered the leading agent in the industrial efforts we have described. All smoke, from whatever source, is either economically absorbed by its powerful influence or dissipated by its agency, and thus all waste or danger from noxious or dangerous gases is avoided.

In thus hastily describing the various points of interest attaching to the manufacture of Portland cement by the new process, it must not be assumed by our readers that the conversion of such simple materials into so valuable a constructive agent is unattended with anxiety and care. In contrast, however, with the old wet system of manufacture, it may be characterised as simplicity itself, for there are no risks of derangement of mixture when once the true proportions have been combined in the washmill (Note 12). Neither does the slurry, when it enters the drying flues, encounter any danger of disturbance of its parts, and thus it enters into the finishing stage of the kiln free from any further or damaging influence of any kind whatever. The semi-wet process involves the necessity of a more regular and accurate weighing of the raw materials which, under the old system is generally performed in a haphazard manner by the washmill-men, who are trained up to a belief that on their manipulation, dexterity, and occult astuteness, the whole success of cement-making depends. It is fortunate for the cement-makers operating within the district covered by our essays that the chalks and clays are so uniform in chemical and mechanical qualities, and, in consequence, the cement manufacturer, and those working under his authority, are saved a great amount of anxiety. It is probably this which has in a great degree hindered progress in cement-making, because the task, at first sight, seems an easy one to mix chalk and clay together, and it was seldom that much more intelligence was forthcoming than such as was capable of performing this simple task. Modern science, however, has thrown light on much that was in the old time obscure, and no rule-of-thumb practice is now tolerated, thereby increasing the comfort and confidence of cement-maker, and cement-consumer.

We ought not to forget a reference to one department of the works at Greenhithe, which may be said, in its reformed shape, to be the unavoidable outcome of much improvement all round, and that is the testing or challenge house. The testing-house is so arranged and controlled that a continuous system of testing the cement produced is daily—if not hourly—performed. The machine used is that invented by Mr. Michele, and is very simple in character, giving as uniform results as are desirable, which are daily recorded, and the briquettes, when fractured, put carefully away, in case they may be required for future reference. The section broken is two and a quarter square inches, being the original size adopted by English engineers from France.

The average breaking-strain of the year 1879 was 1,160 lb. per 2¼ sq. inch (Note 13), as stated by Mr. Johnson at the Institution of Civil Engineers.

There are several interesting points in connection with these cement works of Messrs. Johnson outside of their manufacturing value, such as the following.

The chimney is 300ft. above the level of its base, and equal to 350ft. above river level. The base of the chimney is 25 ft. square, and at the top it is 11 ft. in diameter. It cost £2,500, and its gross weight is 2,500 tons. Five hundred tons of sand were used in its construction. There were 600,000 gault clay bricks used in building it, and the mortar was composed of one of grey lime (Note 14) and three of clear sharp pit-sand, found on the premises. The works, or rather the buildings of the works, cover about an acre of ground only, and it is in this direction that they form a remarkable contrast to works in which the wet process is carried on. The total horse-power now in use is somewhere about 150 (Note 15).

The sum expended in the erection of the works we have described was about £20,000 (Note 16), exclusive of land. The ground belonging to the Company is about 75 acres.

The proprietors of these works provide for the comfort of their workmen, and have a building on the premises in which coffee and other refreshments can be obtained at any time. There is also a reading-room, wherein are a plentiful supply of daily, weekly, and monthly papers, and magazines. Messrs. Johnson have other cement works pretty nearly conducted on the same system at Cliffe, on the Thames, and Gateshead-on-Tyne.

NOTES

Note 1. Johnson was not above insulting his rivals, and did so more as he got older. Despite the unreferenced assertions in the cretinous and illiterate article on Johnson in Wikipedia, William Aspdin - the inventor of "Portland cement as we know it" - had little or nothing to say about Johnson. And Johnson, of course, did not support Newcastle United - he supported Chelsea.

Note 2. Joseph Aspdin moved to Kirkgate, Wakefield in 1825.

Note 3. Essentially the same could be said of the industry in the first three-quarters of the 20th century.

Note 4. Johnson started making Portland cement in 1845: his previous experience was with "Roman" cement and "British" cement.

Note 5. The nomenclature used in the article needs explanation. The earlier process is referred to here as the "wet process" and involved making a slurry containing 60-90% by mass of water, and allowing this to settle over a long period of time. The newer process - patented by Goreham of Swanscombe - is referred to as the "semi-wet process" and involved making a slurry with 40-50% by mass of water, re-grinding it, and burning it immediately. The terms took on new meanings in the 20th century, and the two processes are better described as the "thin slurry process" and the "thick slurry process".

Note 6. Here's Johnson kicking off again, having a go at those who nurtured him. He was probably right, though. As for hereditary privilege, Johnson's eldest son became a "professional" big game hunter. So much for the virtues of primogeniture.

Note 7. Plus ça change.

Note 8. The abolition of the huge capacity of slurry backs reduced not only the plant area required, but also reduced the manufacturing time from 3-6 months to a bit more than a week. In fact slurry was no longer stored at all. This meant that slurry was produced "on demand" whenever a kiln chamber was empty, and inevitably increased the batch-to-batch variability of the product, particularly if the chalk and clay were being proportioned only by weight. The plant was turning around 2-3 kilns per day, so each charge represented a 4-5 hour run on the raw mill system.

Note 9. The traditional means of drying the settled slurry was the "drying flat", consisting of iron drying plates heated from below. The slurry closest to the heating surface was superheated, and as a result the dried material had a "foamed" structure. Heat from above, on the other hand, simply evaporated water bleeding to the surface, and produced a dense slab of dry material with few voids.

Note 10. This could be the earliest-ever analysis of "alkali build-up". Some features are a little suspect, but the overall picture is familiar enough. The material derives largely from salts that evaporate in the hottest part of the kiln and re-condense in the cooler drying chamber. In a conventional bottle kiln, most of this would escape into the atmosphere.

Note 11. Later kilns were deliberately designed with excess drying space (since there was always ample excess heat available to dry more slurry) and the excess dryings were burned in a separate continuous shaft kiln.

Note 12. In slurry backs, there was a tendency of the chalk and clay components, depending on their relative particle sizes, to settle out at different rates, so that the settled material was not homogenous.

Note 13. i.e. 3.5 MPa: presumably this was on neat cement paste.

Note 14. From the Medway or Dorking.

Note 15. This seems rather a low value: if 60% of it was used for cement grinding, it is sufficient for 2.5 ton/hr at most, and 350 tons weekly clinker production would take 140 hours, implying round-the-clock working, which is unlikely. However, another explanation is that, as one suspects, early cement plant operation was highly seasonal, and it was acceptable to lay on extra shifts on the rare occasions when the plant ran flat out.

Note 16. £20,000 in 1877 = £1,885,000 in 2016, or £106 per annual tonne.

Note 17.

Note 18.

Note 19.

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 in September 1924 and shows the plant from the east. 44 years after the writing of the article, little of the original plant is in use, but much of it is still there, the original kiln bank dominating the centre of the plant. The old finish mill feed hopper tower is also visible. Zoom in on the plant in High Definition.
Original content © Dylan Moore 2016: commenced 17/02/16: last edit 01/08/16.
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