Description of the plant in 1930
In 1930, just before it closed, the British Standard cement plant at Rainham, Kent was very typical of cement plants in the southeast of England. The following is intended to introduce the processes used at that time. The cement making process consists of a number of "unit processes" in which input materials are withdrawn from storage, put through a processing operation, and the output materials are then put into another store.
Picture: ©Historic England - NMR Aerofilms Collection. Catalogue number 32181. A high-definition version can be obtained from Historic England. This was taken on 28/5/1930, viewed from SSE and shows kiln A2 (nearer) operating, although the clinker store is full to overflowing. Trains of chalk from the quarry and clay from the quay are lined up at the washmills at bottom right. The stack to the left is that of the power house, with a cooling tower immediately to the left. In the background is the quay, with a group of Goldsmith's sailing barges.
Getting raw materials
The raw materials used at the plant were chalk and alluvial clay. Both materials are soft and very wet. The chalk contained 18-20% of water, and the clay 30-40%. The chalk also contained a substantial amount (~5%) of flint in the form of very hard slabs or nodules 5-20 cm in diameter. The chalk was obtained from the rising ground just to the south of the plant. Chalk was obtained from the face by hand using picks, allowing it to be obtained in lumps of 100 mm or less, and the more obvious flint layers could be rejected. The broken chalk was allowed to roll down the working face and was directed by portable chutes into tipper trucks on a 24" gauge tramway. The raw chalk was brought to the plant on the tramway. The clay was obtained from the Medway estuary in a manner unique to the area. Sailing barges beached at the extraction area on the falling tide. During low tide, the crew filled the barge with clay dug from the inter-tidal zone. When the high tide floated the barge off, it was sailed back to the plant and unloaded at the wharf, and transferred to a small working stockpile. From there it could be transferred to tramway tipper trucks.
Making rawmix slurry
In this stage the two lump-form raw materials were mixed and ground with added water to produce a liquid rawmix slurry with the consistency of paint. This was accomplished in washmills. There were two washmills, located at the far right in the above photograph. These were commonly installed in pairs, so that one could be operated while flint was being removed from the other. The tramway, accessing the quarry for chalk and the wharf for clay, passed close to the washmills. A wagon tippler bridging the track lifted the individual trucks and tipped their contents into a chute which fed either washmill. Water was run into the continuously running washmill: a certain amount was run down the raw material chute to keep it clear, and the rest was run in through a pipe which would discharge directly over the rotating harrows. The action of the mill ground the raw material, and the fine material, in suspension in water, flowed out of the mill through gratings in its sides.
The trucks of raw material were tipped into the mill periodically, at a rate suitable for the production rate of the washmill, and keeping the level of material in the mill reasonably constant. Trucks of chalk and clay were tipped in a ratio - probably three or four of chalk to one of clay - specified by the laboratory staff in order to get the required slurry composition. Appropriate numbers of trucks of both materials were kept on hand on the sidings so that the washmill could be kept running through the day.
The coarse slurry from the washmills discharged into a sump from which it was lifted by a bucket elevator into a set of screeners, located in the raised structure to be seen above the washmills in the picture. In these, the slurry was forced through sieves, typically with a mesh size of around 0.15 mm. Slurry passing through the sieves ran down into the slurry mixers. The muddy material retained on the sieves, mainly consisting of harder chalk particles, with some sand and flint fragments, dropped down into a tube mill in which it was ground with a little extra water and returned to the washmill for further sieving.
The finished slurry contained 42-48% water. The composition of the slurry running into the slurry mixers would vary continually, depending on the amount of clay in the mill at any time. The purpose of the mixer was to even out these variations. Samples of the slurry entering the mixer would be analysed, at least hourly, and a calculation of the average clay content of the mixer could be made. The laboratory staff would make periodic changes to the chalk/clay tipping ratio in order to achieve the desired calcium carbonate content - probably around 77% - on the average of the day's production. The aim of the blending operation was to maintain this composition very constant. There were two slurry mixers, each containing when full about 1400 tonnes of slurry - enough to keep two kilns running for 36 hours. Thus it was possible to feed the kilns with one mixer while perfecting the blend in the other during a day's operation. Alternatively, further blending could be achieved by pumping slurry from one mixer to the other.
The washmill would gradually fill up with flint, which, because it is very hard, is almost unaffected by the gentle grinding action of a washmill. At the end of the day, the mill was washed out, and the flints dug out during the next day.
Grinding coal
Coal was probably obtained from merchants on the Thames or Medway, who obtained coal mainly by sea from the Northeastern coalfields. The cheapest grades of coal were usually used - "smalls" and "duff". The coal was received at the wharf by barge and stockpiled. Some of the "as received " wet coal was also used in the power plant boiler furnaces, but most of it was used to heat the kilns. This was dried by passing through a rotary drier heated with a small coal furnace. The dried coal was then ground to a fine powder in the coal mill. The type of mill used is uncertain, but at similar installations from the same equipment supplier Griffin mills were used. The resulting "fine coal" was then elevated to a hopper situated high up in the kiln house. The fine coal hopper was contained in the small enclosed tower with a pitched roof visible in the picture. Only small amounts of fine coal were kept, and special precautions were observed in its handling, because of its explosive nature.
Making clinker
In the rotary kilns, slurry was converted into clinker. The kilns, visible in the centre of the photograph view, were slightly inclined (probably 1 in 24) cylinders rotating on their axis. The kilns were lined with firebrick. The slurry was fed in at the upper end (right hand end in the picture), and gradually progressed down the kiln. The fine coal (see above) was used to heat the kiln, by injecting it into the lower end (left hand end in the picture) with a jet of air, through the "firing pipe". This acted like a Bunsen burner, producing a flame about 10 m long (about one sixth the length of the 60 m kiln). Thus, the lower end of the kiln is the "hot end". The slurry, entering the kiln at the "cold end", was rapidly heated up by the hot exhaust gases passing up the kiln. The temperature of the gases leaving the kiln was probably about 400°C. The water in the slurry was all evaporated in the first 10-15 m of the kiln, resulting in a crumbly mixture of chalk and clay of a "biscuit" consistency.
This dry material gradually moved on down the kiln, encouraged to flow by the kiln's rotation. In the middle third of the kiln, carbon dioxide was driven off, and finally the material moved into the "burning zone", under the flame, where partial melting and chemical reactions occurred, and dark-coloured clinker formed at a temperature of around 1400°C.
The white-hot clinker fell from the end of the kiln into the rotary cooler, situated underneath the kiln. The air that was used to burn the coal was drawn into the system through the cooler, and the function of the cooler was to transfer the heat in the clinker to the combustion air. The result was that clinker emerged from the cooler at around 200°C, while the combustion air entering the kiln was heated to around 500°C.
The clinker was moved in a steel tray conveyor to a bucket elevator which lifted it to a further conveyor that dropped the clinker into the clinker store - the large open shed seen behind the stacks in the picture.
Unlike other equipment on the cement plant, the kilns operated all the time, with only one or two major stoppages in a year for maintenance. Thus the kilns define the capacity of the plant: with two kilns operating, the plant could make around 100,000 tonnes of clinker per year. To make 100,000 tonnes of clinker, 165,000 tonnes of chalk and 50,000 tonnes of clay would have been used, together with 30-35,000 tonnes of kiln coal. However, it is doubtful whether the Rainham plant ever made its full output.
Making cement
Cement was made by feeding clinker, together with 4-5% of gypsum, into a "finish mill" system. The system at Rainham probably consisted of ball-and-tube mill sets, reducing the mixture to the fine grey powder that is Portland cement. The ball mills would grind the lump mixture down to a grit of around 1 mm, then the tube mills would finely grind the grit to a powder mostly finer than 0.1 mm. The function of the gypsum is to delay the setting of the cement when water is added. The cement was conveyed through screw conveyors and an elevator to the three cement silos. In addition to "ordinary" cement, Red Triangle's "Vitocrete" brand rapid-hardening cement was probably also made in small quantities, by grinding the clinker more finely.
Packing cement
Cement was extracted from the silos and packed for sale. Cement was normally packed in returnable jute sacks, containing usually 112 lb. Cement was originally packed by hand, but after 1923 automatic packers were used, filling bags to a predetermined weight directly from the silo. Cement for export was usually packed in barrels, holding 376 lb: the Rainham plant exported very little cement. The modern practice of selling cement in bulk did not begin until the 1950s. Most of the bagged cement produced by the plant was delivered to riverside depots by barge. Every plant had a large shop in which returned sacks were cleaned and repaired. This was probably among the buildings beside the wharf. The ordinary cement from Rainham was sold under the "Capstan" brand, mainly in the London market.
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