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.
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)|
|Oxide of Iron||0.72|
|Sulphate of potash||47.16|
|Sulphate of soda||7.66|
|Chloride of sodium||10.66|
|Sulphate of lime||8.28|
|Sulphate of magnesia||0.84|
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.