After coal, china clay is probably the most important of England's mineral resources. Moreover, the means of obtaining it form a most interesting and unusual side to the whole practice of mine and quarry engineering.
Any one who has travelled through the neighbourhood of St. Austell, in the heart of Cornwall, will be familiar with the outward appearance of china clay workings. They form weird landmarks, visible for miles, with their huge white tips rising in a series of miniature mountains, some elongated and flat-topped, others having an absolutely conical formation, according to the methods of tipping in force.
The workings themselves are far less prominent. There are no huge quarries or terraced mountainsides, as in some of the Welsh slate workings or in the open-cut iron mines of northern Sweden.
It is the waste-tip that gives to the china clay country its unmistakable skyline. As the visitor passes through the workings, he seems to be in a world that has been freshly whitewashed, for to the untutored eye the clay waste seems to be virtually as white as the clean, soapy material itself.
In its dry state, which does not occur naturally, china clay consists of a fine white powder. In a damp state, and when well kneaded, it forms a firm, sticky paste. It is whiter than pipeclay or ball clay, such as is found in considerable quantities in southern Dorset and often mistaken for china clay, but it is also less viscous and plastic. The great china clay beds have been formed by the decomposition of felspar in granite, this decomposition having been caused by the absorption of some acid solution.
In its natural state, subject to the continual absorption of water through climatic changes, the clay is normally of a soft consistency. Sometimes, though not often, it is hard, and occasionally so hard as to need blasting. Normally, however, blasting is not necessary. The recovery of china clay is effected by jets or currents of water, which wash it out. In Devonshire, beds occur in the south-western corner of Dartmoor, whence the clay is run down to Plymouth by rail. The deposits are not, however, of such great importance as some of those farther west, and horse haulage is sufficient for its removal in many places. Outside Plymouth may be seen the china clay wagons, horse-drawn on the mining tramroad, crossing at right angles the main line of the Great Western Railway.
Farther west in Cornwall is the great clay-bearing area known as the Hens-barrow district, lying to the north-west of St. Austell and Par. Great quantities of the clay from this part are shipped from the neighbouring port of Fowey to all parts of the world. Hensbarrow is the greatest of the English china clay districts. South of the mining town of Redruth lies another district, and in the "toe" of Cornwall there is a further china clay area.
China clay beds are much elongated, as a rule, following the direction of the regular veins in which decomposition of the rock has taken place. Such a clay bed may be as much as half a mile, or even more, in length, but relatively narrow. A rich area of china clay does not consist of a single mass, similar to a Welsh slate mountain, for instance. It takes the form of a series of parallel veins, or sometimes there will be two systems of veins, at an angle to each other. Though the width of a vein of clay is narrow, its depth may amount to some 300 feet in places.
The sides of the bed are bounded by hard, undecomposed granite. Sometimes the line of demarcation between the soft clay and the hard rock is sharply defined. Sometimes, on the other hand, it is gradual. The good clay-bearing soil in the middle of the vein will deteriorate on either side into an intermediate soft rock, bounded in its turn by the hard rock flanking the bed. This soft rock is poor in kaolin (the technical name for the pure china clay), but, as the workings expose it to the weather, a marked improvement will take place progressively, because of the hastening of decomposition. This intermediate soft rock, therefore, can be made to yield a considerable proportion of kaolin after some time has elapsed.
A section through a china clay pit reveals a profile similar to that of a truncated basin or funnel. If a rectangular, or beaker-shaped pit were excavated, the perpendicular walls, formed of this plastic, slippery material, would have a constant tendency to fall in, and a spell of wet weather would bo disastrous. Again, the funnel form of pit is most convenient for excavation.
The clay bed is overlaid by a layer of loose earth, gravel and boulders. Sometimes, too, a layer of peat-bog lies above this. Then the upper stratum of the clay itself is frequently so stained by iron oxide and other foreign matter as to be unfit for use. The "over-burden," as these top layers are called, shows a considerable variation in thickness, from 3 feet to as much as 60 feet. For all depths of any magnitude the miners bring mechanical excavators to their aid, great power-driven navvies that eat into the clayey mass as if it were butter.
In clay mining, as in other kinds of mining, the miners have to be preceded by prospectors and surveyors. Trial pits are dug where clay-bearing veins are known to exist, and these are supplemented at times by boreholes, from which samples of the underlying material are brought up for inspection. There are also alternative methods of winning the clay, choice of method depending on the nature of the locality.
Where the clay bed is situated on a hill, or at any rate on high ground at no great distance from a fairly deep valley, the engineers responsible for the layout of the works may carry an adit through the hillside from the pit. Down this adit the mixed kaolin, mica and other ingredients of the raw material are carried bodily by the water used in excavating them to settling and drying plant situated at the mouth of the conduit in the valley below. This carriage of kaolin and "sand," as the waste constituents are called collectively, is possible only where the adit is on a sharp downgrade. When only a more gentle gradient is possible, the same system can still, however, be carried out if the kaolin and the sand are separated beforehand, in the claypit itself. In these conditions, the adit will carry off the kaolin, well mixed with water, while the sand is raised over the side of the pit by cable-operated wagons, to be dumped or tipped on some site close to the pit.
In other conditions, the adit system has of necessity to be ruled out, and all the material won from the pit must be raised from it to a higher level. The procedure of the engineers is as follows. First they sink a vertical shaft into the rock at the side of the clay bed, and some distance away. The depth of this shaft is often considerable—sometimes as much as 240 feet—as it has to penetrate to a level well below the bottom of the clay bed. The bottom of this shaft forms a sump, and from a point a little way above the sump the miners drive a horizontal cross-gallery or drift in the direction of the bed. The end of the drift should be immediately below the centre of the nascent pit, and from it a perpendicular rise is taken up until the bottom of the pit has been penetrated. Into the rise is inserted a box pipe, square in section, and containing plugholes at intervals from near the bottom to the top.
Powerful Gravel Pumps
When work is proceeding the plughole most nearly on a level with the bottom of the pit is kept open, the others being closed. The clay, in a fluid condition, enters the box pipe through this hole, passes down it, then along the drift into the sump at the bottom of the shaft through the rock. Thence it is brought to the surface by an ordinary pumping engine. The size and capacity of the pumps vary according to the dimensions of the workings. The box pipe varies correspondingly, and may be anything from 4 in. to 9 in. across. Separation of kaolin and sand, in this type of working, takes place in the pit itself, only the clay-laden water passing down through the box pipe. The sand is conveyed to the edge of the pit, and thence to the tips, by an ordinary inclined cable tramway.
As the bottom of the pit deepens progressively, in the course of successive excavations, there comes a time when the plughole in the central box pipe is at too high a level to deal with the flow of kaolin-laden water. Then it is the turn of the next hole downwards. This has its plug opened and the old hole passes out of use. So the process goes on until the lowest hole is reached, and that particular pit exhausted.
In new and shallow pits, the miners may use a much simpler method for raising the kaolin, the pumping taking place within the pit itself, with delivery over the side. A centrifugal pump driven by a motor is mounted, in a small shed, on a bed-frame of timber near the bottom of the pit, as yet shallow. Frame, pump and pump-house descend easily with the bottom of the pit itself as the output of clay goes on. The engineers use powerful gravel pumps capable of raising the entire mixture produced by the miners working in the pit, even pebbles passing easily through. A strainer, however, needs to be rigged to prevent larger stones from entering and damaging the pump. These stones, occurring in the clay, are carried out by the usual wagon after the strainers have separated them from the sand-and-kaolin mixture.
A common arrangement comprises two centrifugal pumps on the one frame, with the driving motor between them. One of the pumps is connected to a suction pipe descending into the fluid material. This pump delivers it to its fellow, which in turn raises the mixture to the surface, where it goes through the separation process. This combination of pumps can work against a fair head, as much as 80 feet if necessary.
The primary breaking of the ground and removal of the clay are accomplished by washing out with water. A stream of water is directed over the surface of the clay, which is simultaneously attacked by men armed with chisel-edged picks and curious two-bladed hoes. Tbe efforts of these men, assisted by the constant flow of water, cause the formation of a galley, or "strake," as it is called. As the water flows, the sides of the strake are undermined, so that considerable pieces of the kaolin-laden clay are constantly falling into the water stream. Thus the stream, in due course, produces a dense fluid mass of mixed clay and water, which can be dealt with by the pumps and separators.
The modern development of this old but simple system involves the substitution of water jets, working under pressure, for the original running stream. In deep pits, the force of gravity alone is sufficient to produce a good pressure at the jets, but in newer, shallower pits a compression pump has to be brought into action, delivering water at a force varying from 50 lb. to 100 lb. per sq. in. These rapid jets of water are directed at the clay-bearing ground. The miners keep the jets in constant motion, so that, instead of merely drilling or scouring the clay, they tend to stir it up and to break up the big lumps they bring down.
The old strake method, however, has its advantages. For instance, it enables the men to locate easily any discoloured areas in the clay, and to extract them bodily before they have time to mix with and spoil the pure white kaolin washed down from another part of the working.
Advantages of the "Strake" Method
Hand washing, though it is slower and more expensive, is often retained where china clay of really high quality has to be produced. Even when working on the newer method, the high-pressure water jets need to be supplemented by a good deal of hand work unless the clay bed in question is particularly rich, having a kaolin content of 30 per cent or over.
Pure kaolin is extracted from the sand by a separation process. The pumps deliver the viscous stream of mixed clay and sand into hopper boxes, each of which is equipped with a sliding door in the bottom. The sand is heavier than the kaolin, and it settles in these hoppers, the cleaned kaolin passing on with the main stream of water. The hopper boxes work alternately in pairs. One box is filled up while the other is discharging its load of arrested sand through the bottom doors into a waiting skip or tramway wagon. Where gravel pumps are not in use, separation takes place at the bottom of the pit by means of shallow sandpits equipped with slatted fronts, which allow the water, with the kaolin in it, to pass through, leaving the sand and other impurities behind.
Methods of disposing of the waste products also vary. The incline which carries the sand wagon away from the pit may have a fixed top. When the wagon has reached this by cable haulage in the normal way, it is detached from the cable and run off along a branch track, slightly downhill, to the tipping place. The tip thus assumes a constantly extending, elongated form, which grows steadily in extent, though not in height, as long as tipping continues.
Another method, much practised in Cornwall, involves the use of an extending top to the tramway incline, and this causes the conical formation so familiar in the Hensbarrow distrct. The end of the railed inclines is built up of sliding beams and guys, and by this means the incline is lengthened as the tip grows in size. This arrangement is known as the "sky-tip." In the sky-tip the sand wagon always consists of a hopper skip with automatic discharge doors at the bottom. Before the kaolin is ready for drying and marketing, it has first to pass through a more or less elaborate process of purification. On arrival at the surface the clay-laden water, after separation from the heavier impurities, runs in a wide, shallow stream through a set of long, narrow channels. There are two sets, containing anything from six to thirty channels approximately 20 feet long, 2 feet wide and 9 in. deep. The first set of channels is known as the "drags." The stream passes through these drags at a speed sufficient to keep most of the clay mixture in suspension, only the heavier particles dropping out. The second set of channels is known as the "micas." Here the flow runs much more slowdy, and the channels receive a deposit containing flakes of mica (hence their name) and particles of the finest sand previously held in suspension.
The flow through the micas is interrupted at intervals for the purpose of scouring. They yield a residue of clay of an inferior grade, called mica clay. At the head, and often at the ends, of the micas, wire screens are fitted for the purpose of straining out chips of wood, pieces of grass and other kinds of vegetable matter which have found their way into the flow. Sometimes these strainers take the form of revolving screens, sometimes they are fixed screens fitted with revolving brushes.
From the micas, the stream of purified clay-water flows on through a system of pipes, sometimes of stoneware and sometimes of wood, into the settling pits. These are partly dug out of the ground and partly built up above the natural surface. Settling pits are almost invariably circular in shape, and lined with masonry. In depth they vary from 8 to 10 feet; in diameter they may measure anything from 25 to 40 feet. In each settling pit, the floor slopes slightly downwards from the inflow towards a "hatch " or sluice on the opposite side. In this hatch there are plugholes, normally kept closed with the exception of those at the top. Through these the clear water flows out of the pit while the clay remains deposited inside.
Into the Drying Kilns
Each pit gradually fills up with clay as the inflow of clay-water and the outflow of clear water continues. When the pit is almost full, that is to say, when the clay deposit reaches to within about a foot of the top, the entering stream is shut off. The clay at this stage consists of 60 to 70 per cent water, and has been compared, in appearance and in consistency, to clotted cream. "Slurry" is the name given to it.
The settling pit being full, the hatch is raised, and the whole creamy mass of the slurry pours out, to be conducted into storage tanks. These tanks are of considerable size. Average dimensions are 140 feet long, 50 feet wide and 7 ft. 6 in. deep. Sometimes they are roofed over to prevent dust and other foreign bodies from contaminating the slurry. Further settlement takes place in the storage tanks, more water being run off from the clay before it is finally passed, either through pipes or by wagons, into the drying kilns, where it enters the last stage of its preparation.
A drying kiln consists of a long roofed-over shed with furnaces at one end, standing adjacent to a railway siding, or, failing that, a road. Each drying kiln is divided into two sections, one called the "pan" and the other the "linhay." The pan may be as much as 200 to 350 feet long and 12 to 18 feet wide. It is lined with fireclay tiles, these resting in their turn on walls of firebrick. The walls themselves are hollow, and form the flues of the drying furnaces, which are generally three in number and burn coal. A tall smokestack rises at the opposite end of the kiln. The wet clay is spread out on the floor of the kiln in a huge layer, about a foot thick at the furnace end of the pan, decreasing to about half that thickness at the cooler chimney end.
After the drying process this will have shrunk to about 9 in. and 4-1/2 in. at furnace and. chimney end respectively. At the hot end of the pan, drying will have been completed in about twenty-four hours ; at the other end several days may elapse before the kaolin is ready.
During the drying process, the clay is cut into blocks, partly for convenience of carriage and partly to prevent it from cracking as it dries. When the blocks of clay are completely dried they are stored in the adjacent linhay, whence they may be transferred to waiting railway wagons or lorries for transport as they are required.