Chapter III: The Preparation of the Colour Earths

The preparation of the raw materials for the purpose of making earth colours is a very important matter, because many minerals or pigmentary earths merely require mechanical treatment to render them at once fit for use. The mechanical preparation differs considerably, in accordance with the raw material under treatment, substances that are found native in a finely powdered condition only needing, for the most part, to be levigated.

It rarely happens, however, that the raw material occurs in condition for use direct, an example of this kind being afforded by the finest clays or ochres. Whilst these are found in a state of extremely fine powder, they nearly always contain certain quantities of sandy ingredients or even large lumps of foreign minerals, and therefore require levigating. Sometimes they need crushing as well, the small particles cohering so strongly that mere treatment with water (levigation) is unable to separate them. Mechanical force is therefore necessary, a passage through grooved rollers being generally sufficient to crush the lumps; but in some cases stamps have to be used.

When solid materials have to be treated, mechanical appliances must always be used, their selection depending on the materials in question. Thus, gypsum, for example, can be crushed with ordinary rolls or mill stones, its degree of hardness being so very low (2) that it can be scratched with the finger-nail.

If, however, the material to be reduced is limestone, which belongs to the third degree of the scale of hardness (can only be scratched with an iron nail), or heavy spar (hardness 3–3·5), very powerful stamps or edge-runners must be employed to break it down into small lumps, which can then be further reduced, without any special difficulty, in an ordinary mill.

It is thus evident that a great variety of mechanical appliances are used in the manufacture of earth colours. Before going into their construction it is necessary to point out that, whatever the mechanical treatment employed, a considerable expenditure of mechanical force is entailed; and more power is needed when mixtures have to be prepared. It is therefore essential, in planning a factory for making earth colours on a large scale, to make provision for ample motive power.

This power may be supplied by a steam engine; but it must not be forgotten that the prime cost and running expenses of such an engine are considerable, and form an important item in view of the low value of most earth colours. Consequently, it is highly important to be able to generate motive power as cheaply as possible.

Now, the cheapest and most uniform source of power is water; and therefore, wherever the conditions allow of the erection of the colour works near a stream or river, which can supply the power to run the various machinery, the most favourable circumstances will have been secured, the power being obtained at minimum cost, whilst the upkeep of the motor cannot be very great. If there is sufficient head for the water to be run through a trough over the top of the levigation tanks, the conditions will be ideally favourable.

Wind power costs nothing, once the motor has been installed; but unfortunately, one is dependent on the weather, and sometimes there is not enough wind, for days together, to drive the sails at all, and therefore all the operations have to be stopped, including levigation, the water for which has to be raised by a windmill pump.

In districts where the winters are severe, water power may also fail and work have to be stopped; and consequently, even when water power is the prime source of energy, a steam engine must be installed as a stand-by, being, of course, only used when the main source of power gives out or proves insufficient.

The machines employed for preparing the raw materials in the manufacture of earth colours may be divided into the following groups:--

Machines operating entirely by pressure: crushers; machines acting by impact: stamps; those acting by impact and pressure: vertical mills (edge-runners), ball mills, centrifugal mills; and, finally, machines with a frictional action: grinding mills. Then there are the levigating machines, which do not reduce the material but separate the coarser particles from the finer. The construction of the foregoing machines is a matter for the machinery manufacturer rather than the maker of earth colours; but as the business of the latter is dependent on them, a short description is considered necessary. The selection depends, on the one hand, on the nature of the materials to be treated, and, on the other, on the size of the works, since a manufacturer who has to deal with large quantities of a given raw material will require different machines from those used on a small scale. The sole purpose of the following description is to indicate to the colour maker the way in which the reduction of the raw material can be accomplished.

CRUSHING MACHINERY

_Crushers and Breakers._--Crushers usually consist of grooved iron rollers revolving on horizontal axes. One of the rollers is fixed, the other being adjustable by screws, in order that lumps of different sizes may be treated in one and the same machine, which may be employed either to turn out a roughly crushed product, or to reduce it to a certain degree of fineness.

If several pairs of crushing rollers be mounted in series, and each set a little closer than its predecessor, the material can be reduced progressively from large lumps to a fairly fine powder.

Each pair of rollers is geared together by pinions, and is turned in such a way as to draw the material in between. If the gear pinions have the same number of teeth, the two rollers will revolve at the same speed and will then merely crush the material into lumps of a size depending on the distance at which the rollers are set apart.

Nevertheless, by simply altering the gear ratio of the pinions, the crushing action of the rollers can be supplemented by a grinding action, a much finer powder being then obtainable than otherwise, the one roller running at a higher speed than the other.

These crushers differ in strength of construction, very strongly built machines being required for dealing with large lumps of hard material, whereas substances of low crushing strength, such as clay or other earthy materials, can be treated in much lighter machines. In any case, however, it is advisable to have the machine stronger than is absolutely necessary for the work in view; for, although the prime cost is thus increased, the outlay on repairs will be reduced, and the machines can, if necessary, be used on harder material as well. The framework supporting the rollers should always consist of a strong iron casting; and the machine should be set up as close as possible to the engine or motor, to minimise the loss of power in transmission through long shafting, etc.

Fig. 1 represents a breaker (made by the Badische Maschinenfabrik, Durlach), suitable for the rough crushing of clayey materials supplied in large lumps. It can, however, also crush shale, lime, chalk, as well as hard, sticky masses which would clog up a stone-breaker.

The material fed into this breaker is gripped at once by the powerful projecting teeth, which are connected together by sharp-edged ridges, and is crushed in such a way that it can be easily reduced still further by a succeeding pair of smooth rollers.

The granulator (Fig. 2), made by the same firm, is an example of a machine for crushing harder materials. It is similar in construction to a stone-breaker, but differs in the movement of the jaws, and combines the properties of breaker and grinder, inasmuch as it tears the material as well as crushes it. The figure shows the machine adapted for direct electric drive. If necessary, these granulators can be fitted with classifying jig screens.

_Stamps._--Stamps or stamping-mills have been used from prehistoric times, and were probably employed for reducing hard materials long before the introduction of grinding-mills. The underlying principle of the stamping-mill is very simple. The material to be reduced is placed in a trough or mortar, and the ram or head, which is of considerable weight, is raised by a mechanical device and then allowed to fall freely, from a certain height, on to the material underneath, which it crushes. The heavier the head and the greater the height of fall, the greater the effect produced. As a rule, a large number of stamps are mounted together, and in such a way that half of them are being lifted while the other half are falling. Either a separate mortar or trough is arranged under each stamp, or else the whole drop into a common trough charged with the material under treatment. Sometimes a lateral movement is imparted to the material in the trough, so as to bring it under the action of all the stamps in succession.

Although the construction of stamping-mills in general appears simple, various modifications are employed for different purposes.

As a rule, a single passage through a stamping-mill is not sufficient to reduce the material completely to the desired fineness, the first product always containing large and coarse fragments of various sizes, as well as fine powder.

If the latter were left in with the larger pieces for the second stamping it would impede the work, and the stamping-mill should therefore be provided with means for classifying the material discharged from the trough, to separate the fine from the coarse and grade the latter into sizes. This is usually effected by means of a grading-screen.

Stamping-mills are chiefly used for reducing brittle materials. A number of stamps arranged in a row are alternately lifted, by means of cams mounted on a common shaft, and then let fall on to the material lying on a solid plate, or else on a grating through which the crushings fall. Fig. 3 is a stamping-mill constructed by H. F. Stollberg, Offenbach.

These mills are very strongly built, as independent units, the frame being of cast-iron and the rams of best wrought-iron with interchangeable chill-cast heads. In some mills the stamps are rotated during the up-stroke, in order to equalise the wear on the heads, and also to economise power.

The grating or trough holding the material is perforated with holes, the diameter of which varies with the material under treatment and the desired degree of fineness in the product. To increase the efficiency of the mill, the grating or trough is adapted to move while the mill is running, in order to clean itself automatically. These mills are made in different sizes, with 2, 4, 6, or 8 heads.

_Edge-runners._--This type of crusher is highly suitable for reducing earth colours in large works. The special feature of the type is that both stones are mounted vertically and turn on a common shaft in the same way that a cart wheel does on its axle. These runners are particularly useful for reducing clay, chalk and other earth colours, which have to be dealt with in large quantities. They will also crush fairly large lumps, and they can therefore be used for the further reduction of materials roughly crushed in a breaker, etc. The material may be treated in either the wet or dry state, only slight alteration being needed to change from one method to the other.

There are numerous different patterns of edge-runner, but all of them can be divided into two groups, viz.: mills with stationary troughs, whilst the shaft carrying the runners rotates; and those in which the trough revolves, and the stones merely turn on the stationary horizontal shaft.

Comparison of the efficiency of the two types has shown that the revolving-trough type is the better, giving a larger output per unit time with a reduced consumption of power. Figs. 4 and 5 show a vertical section and plan respectively of this type of edge-runner. The trough _G_ is turned by means of a toothed crown gearing with the bevel pinion _O_ mounted on an overhead shaft _C_ driven by a belt pulley _N_.

The bearings of the vertical shaft _J_ of the trough are situated at _L_ and _M_. The runners _H_ are loosely mounted on the fixed horizontal shaft _E_ and revolve in consequence of the friction between them and the material in the trough. As the latter revolves, the material is continuously pushed aside by the runners, and is again brought under them by the action of scrapers.

The great advantages afforded by edge-runners, in consequence of their simplicity, easy management and low wear in comparison with other grinding appliances, have led to their reintroduction on a large scale. It should, however, be borne in mind that the edge-runner mill must be of a pattern suitable to the materials it will have to treat. The method of drive usually depends on local conditions. The revolving-trough type is chiefly useful for mixing, on account of the ease with which the materials can be charged.

The capacity of edge-runner mills depends on the nature of the material, the diameter and weight of the runners, the speed at which they are run, and also on the rate at which the reduced material is discharged in order to give place to fresh portions of the charge. This is effected by means of two sets of scrapers, the individual members of which can be adjusted in any direction. Their ploughing action also greatly assists the mixing effect.

Fig. 6 illustrates an edge-runner mill with revolving trough and overhead drive; and Fig. 7 one with stationary trough and bottom drive; both made by the Badische Maschinenfabrik, Durlach. The runners are of grey cast-iron, chill-castings or cast-steel being used for crushing hard materials. The trough in all cases is lined with detachable chill-cast plates. Special attention is bestowed on the lubrication of all the moving parts, and all the lubricators are easily accessible.

The main shafts of the fixed-trough machines have forged cranks, and the metal crank bearings are provided with dust caps. All the shaft journals run in detachable metal bushes.

A special advantage attaching to this type is the automatic screening device and the returning of the screen residue. In some cases, complicated appliances are employed to return the coarse residue from the screen, bucket elevators, worm conveyors, etc., all entailing increased motive power, not inconsiderable wear, and a higher prime cost; but in this instance the object is achieved, without extra power or wear, by very simple means. The dust-proof shell enclosing the runners and screen is provided with large doors and charging hoppers.

The motive power required to drive edge-runner mills depends on the dimensions of the mill and on the class of material to be treated; the larger the mill and the coarser the material, the more power needed to drive it.

This type is the more suitable for raw materials that are of an earthy character, so that all that is necessary is to destroy the cohesion of the particles, as is the case, for example, with clay and all earthy minerals.

The wet method of crushing with edge runners is particularly suitable as a preliminary to levigation. A machine arranged for this purpose is shown in Fig. 8. It consists of two sets of edge runners, one with fixed, and the other with revolving trough. The material is introduced by hand, or by suitable charging mechanism, into the upper, fixed-trough machine, where it is continuously sprinkled with water and kneaded by the one runner, and is passed thence to the second roller which forces it through the slotted bed into the bed of the lower set. The slotted beds of the upper and lower set are offset; and the chief function of the lower set, with rotating bed, is to secure intimate admixture of the material which, in most cases, is already sufficiently reduced.

_Ball Mills._--Ball mills are generally used for crushing dry materials to fine powder. The mill shown in Fig. 9 is a typical form of grinding drum enclosed in a dust-proof casing, the latter being provided, at the top, with an opening connected to the dust exhaust pipe. The discharge outlet at the bottom can be closed by a slide.

The drum is provided with two strong lateral shields or cheeks (Fig. 10), one of which carries the interchangeable cross-arm and the charging hopper. Both cheeks are lined with detachable chill-cast plates, to take up the wear. The bed is formed of heavy steel bars (which can be turned round), between which are arranged adjustable slits for the discharge of the reduced material. Guard sieves are mounted all round, and close to, the bed, and interchangeable fine screens surround these in turn. The mesh of the fine screens determines the fineness of the product, and the residue falls down on to a plate which returns it to the interior of the drum. The reduction of the charge is effected by a number of very hard, forged steel balls of various sizes.

The mill must be run in the direction marked by the arrow on the outer shell, so that the residue on the screens can be returned to the drum by the plate provided for that purpose; and the prescribed working speed must be maintained. The mill must not be overloaded. The impact of the balls should be mild, but distinctly audible. Overloading reduces the output. Idle running causes the most wear, since the balls then roll directly on the bed, which, of course, should be prevented as far as possible. The feed is continuous; and, of course, only dry material should be introduced.

When the balls have lost size and weight through wear, they must be replaced by a fresh set.

_Pulverisers._--Pulverisers are the best form of crusher for tough and not over-hard materials. They are simple and strong in construction, of high capacity with comparatively small consumption of power, and furnish a good, uniform product, the size of which ranges from fine powder to coarse granules, according to the screens used and the class of material treated.

The crushing is effected by a cross-arm beater, composed of four to six radial steel arms on a divided, cast-steel hub, keyed on to the horizontal shaft. The arms are hardened, and are adjustably and detachably mounted on the hub.

The beating action of the arms, which run at high speed, forces the material against the studded surface of the hardened cheeks of the machine and also against the hardened square steel bars forming the periphery, the repeated impact of the material on itself, as well as against the arms and bars, progressively reducing it until small enough to fall through the screen on the under half of the casing, into a closed receptacle below. The screen mesh varies according to the degree of fineness required.

The peripheral bars are mounted in a very simple manner, and in such a way that when one edge of the bars is worn, a quarter turn brings a fresh, sharp edge into operation, so that all four edges of each bar can be utilised.

To prevent the escape of dust, the machine is provided with an air-circulation chamber, which maintains the flow of air in continuous circulation, the resulting strong draught also drawing the fine material through the screen and keeping the meshes open. By this means the capacity of the pulveriser is considerably increased. The interchange of the crushing organs and screens, and also the cleaning of the machine, can be effected without difficulty or loss of time.

The charge is introduced through a feed hopper at the side, and may vary, according to the size of the machine, from nut size to lumps twice as large as a man’s fist. If necessary, suitable mechanical feed devices can be applied.

_Disintegrators_ (Figs. 12 and 13).--This type of machine is used for reducing medium-hard or soft materials, especially where it is desired to obtain a comparatively large output of a gritty product.

In the patterns shown, the main shaft is of steel, with dust- and dirt-proof red-brass bearings with pad or ring lubrication. The spindle case draws out to facilitate cleaning. Mechanical feeding attachments can be provided.

According to local conditions, the disintegrator can be mounted either on a brick foundation, with lateral discharge outlet into a storage bin, or on a raised grating of iron joists.

If the product is to be conveyed to a distance, it is advisable to have a hopper-shaped collector leading directly to a worm conveyor or bucket elevator.

The arrangement shown in Fig. 13, in which the disintegrator is mounted on a dust-proof cast-iron collector, has been found very suitable for colour works of various kinds (aniline, lead, mineral and other colours), particularly on account of the suppression of dust; whilst the automatic charging worm greatly increases the capacity as compared with charging by hand.

LEVIGATION

The effect of levigation is based on the circumstance that bodies of greater density than water remain longer in suspension in that medium in proportion as the fineness of their particles increases. This treatment consequently enables the finer portions of a substance to be mechanically separated from the coarser. Levigation is extensively practised in colour works because it furnishes powder of finer grain than can be obtained by even the most careful grinding.

The appliances used for levigation may be of a very simple character, consisting only of several tubs or tanks, mounted in such a way that the liquid contained in one can be run off into the one next below. With this primitive plant, the material to be levigated is stirred up in the water in the uppermost tub and left to settle until the coarsest particles may be assumed to have settled down, whereupon the turbid water is drawn off into another tub, in which it is left to settle completely. When the clear liquid has been carefully drawn off, a fine sludge is left in the bottom of the tub, consisting of the fine particles of material mixed with water.

When a particularly fine powder is required, a single levigation does not always suffice, but the liquid in the second tub must be left to settle for a short time only, and then run into a third for complete subsidence.

A well-designed levigator for treating large quantities of powder is illustrated in Fig. 14. A stirrer _R_, driven by cone gearing, is arranged in a wooden or stone vat _G_. The levigating water enters close to the bottom of the vat, through the pipe _W_. When _G_ is half full of water, the stirrer is set running, and the substance to be levigated is added. After a while, the water laden with the levigated powder begins to run off at _A_ into the long narrow trough _T_{1} provided, at the opposite end from _A_, with a number of perforations through which the water runs into the trough _T_{2}. From this it escapes through the perforations into the trough _T_{3} and thence successively into _T_{4} and _T_{5}, finally discharging into the large tank _S_.

The coarsest and heaviest of the water-borne particles deposit in the trough _T_{1} finer particles settling down in _T_{2}, and so on in succession, until the water reaching the tank _S_ contains only the very finest of all in suspension, these taking a long time to settle down to the bottom. The deposit in the upper troughs can be returned to the vat, whilst that in the lower ones will be fine enough to dry as it is. The residue in the vat is discharged through _Z_ when the operation is finished.

It will be evident that the fineness of the product depends on the number and length of the troughs _T_, the larger these factors the more delicate will be the particles remaining in prolonged suspension in the liquid.

Many earth colours require no treatment beyond levigation to fit them for use in paints. This is the case with, _e. g._, the white clays; and certain grades of ferric oxide, which occur native in the state of fine powder, may also be included in this category. In many cases, however, if large quantities of a finely pulverulent mineral be stirred up with water and left to stand, the deposited solid matter forms such a highly coherent mass that it can only be distributed in water with difficulty, the fine particles adhering so firmly together that it is hardly possible to stir them up again completely in the liquid by means of a paddle.

Nevertheless, this can be easily effected by using a special appliance of the kind employed by starch manufacturers for a similar purpose, viz. the levigation of starch. This apparatus is designed in such a way that the pulpy charge of material is gradually and completely disseminated in the introduced liquid.

Fig. 15 shows a device of this kind, consisting of a circular vessel provided with a step bearing for a vertical shaft driven by cone pinions. The lower part of the shaft is provided with a thread, on which a nut is adapted to travel up and down. By means of rods, this nut is connected to a wooden cross-bar provided with stiff bristles on its lower face. A horizontal handle is attached to the nut. The water is admitted through the pipe on the right.

In working the apparatus, the shaft is rotated and the handle held firmly, thus causing the nut and attached cross-bar to rise to the limit of its travel. The levigating liquid, mixed with the material under treatment, is then admitted, until the vessel is full, and when the solids have completely subsided, the clear liquid is drawn off, and the operation is repeated until a thick layer of sediment has accumulated on the bottom of the vessel.

To levigate this, the cross-arm carrying the bristles is lowered until it just touches the surface of the deposit, and a continuous stream of water is admitted through the pipe at the side. The bristles gradually disseminate the upper layers of the sediment in the water, which becomes turbid and is then drawn off into another vessel, cement-lined pits being used in the case of large quantities. When the brushes no longer encounter any of the sludge, the cross-arm is lowered sufficiently to stir up another layer; and in this way, large quantities of solid matter can be distributed in water. If the cross-arm is rotated at low enough speed, the coarser particles of material keep on settling down again, and the collecting vessels will receive only the finest particles.

In addition to the mechanical separation of coarse and fine particles, levigation accomplishes another purpose, namely that the prolonged contact of the treated material with water dissolves out any admixed soluble constituents which might affect the quality of the colour, the latter being left in a purified condition.

For successful levigation it is essential that the charge should be in a sufficiently fine condition at the outset. Clayey raw materials require no preliminary treatment other, perhaps, than passing them through a disintegrator, whereas hard, crystalline substances must first be ground in a wet mill, such as an edge-runner mill with stationary bed, into which the materials are fed with an admixture of water, provision being made for keeping the charge under the runners all the time. The crushed material is screened previous to levigation.

In the levigation process a few vessels of large size are preferable to a number of small ones. The nature of the material will determine whether any stirrers are required or not, these being unnecessary in the case of the pigmentary earths, which naturally remain a long time in suspension and therefore do not require stirring up.

The pulpy levigated material is taken out of the tubs, etc., drained (if necessary) and dried. The draining may be effected in bags, or--in large plants--filter presses or hydro-extractors. In these latter instances, pumps will be provided for feeding the sludge direct to the presses, and conveyors for delivering the pressed material to the drying-plant.

DRAINING AND DRYING

The levigated colour earths form a stiff pulp containing a large quantity of water, which can be eliminated in various ways. Usually, the mass is dried by spreading it out thinly on boards and leaving it exposed to the air until it has become solid; or else it is only left long enough to acquire the consistence of a thick paste, which is then shaped into cones or blocks, which are allowed to dry completely in an airy place. If the colours are to be sold in the form of powder, the dried lumps are crushed.

To accelerate drying, the pulp may be put through a hydro-extractor, or dried in hot-air stoves or rooms. As, however, this last method entails special appliances and also expenditure, this acceleration is only resorted to when rendered necessary by special conditions.

_The Hydro-extractor._--When a substance is set in rapid rotation, it tends to fly away from the centre at which the rotational force is applied. The centrifugal force thus coming into action increases with the velocity of rotation and with the distance of the substance from the axis of rotation.

The centrifugal hydro-extractor consists, therefore, of a vessel in rapid rotation; and if a liquid be introduced into such vessel, it is projected with considerable force against the peripheral walls. If the peripheral surface be perforated, the liquid portion of a charge consisting of liquid and solid matters will be ejected through the perforations, while the solid matter remains inside. As a rule, a few minutes’ treatment in a hydro-extractor is sufficient to separate the water from a thin pulp so completely that the solid residue is in an almost completely dry state. A hydro-extractor which, though of an old pattern, is well adapted for the purposes of the colour-maker, is shown in Fig. 16.

The drum A, which revolves easily on a vertical axis, is of metal, and is provided with a large number of fine perforations on its peripheral surface. It can be rotated at high speed by means of the crank _f_ and pinions _d_, _e_, or by the fast-and-loose pulley _a b_ connected with a source of power. To prevent any of the charge from being projected over the rim of the drum, the upper edge is turned over so as to leave only a comparatively small opening at the top. The lower end of the drum shaft carries a strong steel spindle, which must be carefully machined and enable the drum to revolve as easily as possible. This is essential, because even small machines require a comparatively large amount of motive power--which is not surprising in view of the high speed at which the drum has to revolve in order to perform its functions.

The drum is enclosed in a casing of somewhat larger diameter, which may be of any convenient material. The bottom of the casing is preferably tapered slightly downward, and is covered, at its lowest part--below the bearing of the drum--with a sieve communicating with a pipe through which the ejected liquid is drained off.

When a liquid, containing solid matter, is fed into the drum, which is already running at high speed, the liquid is thrown, by the centrifugal force, against the peripheral surface of the drum and escapes through the perforations, leaving the solid matter behind. Where large crystals are in question, as for instance in sugar factories, the centrifugal machine can be employed without any additional precautions, the liquid being expelled and the crystals being practically dried by keeping the machine running a short time longer. In the case of the pulp obtained by levigating colours, however, this procedure would result in failure, because the fine solid particles would be ejected along with the liquid and the drum would be left quite empty.

In this case it is therefore necessary to provide means for retaining the solid matter in the drum, and allow only the water to escape, with which object the drum is lined with a bag of closely woven fabric, open at the top and fitting snugly against the inner surface of the drum. When the drum is first started, the ejected liquid is milky, no fabric being sufficiently close to retain all the extremely fine solid particles present. In a very short time, however, the liquid will begin to run away perfectly clear, this occurring as soon as the pores in the fabric have become so far obstructed by the projected solids as to allow water alone to pass through. The milky water is then returned to the feed tank and run slowly into the machine. The water is very quickly expelled, and the colour remains in the drum as a stiff paste, of sufficient consistence to be moulded into lumps of any desired shape. The use of the hydro-extractor may be particularly recommended when ample motive power is available and accelerated draining is desirable.

Fig. 17 illustrates a modern type of hydro-extractor with bottom discharge and suspended drum, the shaft of which is coupled directly to an electro-motor.

_Filter-presses._--Whereas the hydro-extractor is only used in particular cases for the purpose of the earth-colour manufacturer, the filter-press enjoys more extensive application. Every filter-press is composed of a number of closely fitting press frames, held together by the pressure of a screw. These frames, when assembled, form chambers provided with inlet and outlet openings. Suitably shaped and stitched filter-cloths are secured inside the chambers, and the sludge to be filtered is run into the press from a high-level tank. The water passes through the filter-cloths and runs off, whilst the colour earth gradually fills the chambers. When draining is completed, the press is taken apart and emptied. In this way the earths are obtained in the form of more or less dry cakes, which are then put through further treatment or dried.

Fig. 18 shows a Dehne filter-press suitable for the earth-colour manufacturer. Wood internal fittings are often used, because wood does not affect the shade of the colours; but, wherever the nature of the materials admits, iron presses are to be preferred on account of their greater durability and the certainty of the joints continuing tight. The finer the grain of the levigated colour, the more difficult the expulsion of the water; but as a rule, a pressure of 115–195 inches, water-gauge, will be sufficient.

If the sludge be run into the press from a tank at sufficient height, two charges can be worked in a day, but the cakes will not be as firm as butter of medium hardness. It is better to pump the charge into the press by means of a special diaphragm pump. The drainage is then incomparably quicker, the cakes will be formed in about an hour and will also be drier. A good deal, however, depends, naturally, on the nature of the earth colour.

If the colour contains acid, alkali or salts, the filter-cloths can be washed by flushing the press with water under pressure. The cloths are made of specially fine cotton fabric. The press-runnings, which are never quite clear, are collected in a clarifying tank, where they are treated with lime and kieserite, whereby gypsum is formed, and the mass is put through a filter-press, which retains the solids and leaves the effluent clear.

Filter-cloths which have become choked by use are spread on a table and scrubbed with water, or else washed in a special machine (Fig. 19), consisting of a rotary drum, with belt drive, the rotation circulating the water in the interior trough and enabling it to extract the dirt from the cloths. The flow and discharge of the water are controlled by valves, and the water may be warmed by admitting steam into the machine. The size of the washer depends on that of the filter-cloths.

From the press, the cakes of colour are conveyed to the drying-plant, usually by the aid of automatic machinery.

_Drying Appliances._--The stiff paste or cakes from the hydro-extractor or filter-press can be shaped, but require to be dried before they are put on the market. Drying is a wearisome operation, the finely divided material taking a very long time to dry completely, even during the summer months, whilst in winter it is almost impossible to get certain colours--such as ferric oxide colours and levigated clay--quite dry in the air, the inside of the lumps remaining soft and pasty after lying for months.

The only way in which this troublesome delay in the completion of the operation can be overcome is by artificial drying; but as the employment of artificial heat entails expense, it is necessary to carry on the process with the smallest possible outlay, in view of the low commercial value of most earth colours.

Long experience has convinced the author that the arrangement of the drying-rooms in many colour works is based on entirely wrong principles, and that a great portion of the heat furnished by the fuel is wasted. For this reason the description of a properly arranged drying-room will be welcomed by a number of readers.

It is a well-known fact that hot air is lighter than cold. Consequently, when a room is artificially heated, the highest temperature will be found just under the roof or ceiling, and articles placed in that part of a heated room will dry much faster than those near the floor. If the drying-room is heated by an ordinary stove, articles placed on a fairly low level will only dry very slowly, because the hot air flowing from the stove tends to ascend.

In order, therefore, to utilise the entire space of the drying-room, it is necessary to place the heating apparatus in such a position that the whole of the room will be warmed as uniformly as possible. The stove should therefore be situated in a chamber underneath the drying-room proper.

Because air that is already saturated with moisture cannot take up any further quantity, care must be taken to remove the damp air continuously from the drying-room, and to replace it by dry air. This may be effected by suitably designed ventilation, on the lines shown in Fig. 20, which represents a drying-room arranged in such a way as to provide for all the above-mentioned contingencies, and ensure continuous drying.

The heating apparatus is located in the cellar, and consists preferably of a slow-combustion stove comprising a cast-iron cylinder, with an air inlet _A_ (with sliding regulator _T_), for the air of combustion, and a shoot _F_ at the top, through which the stove is fed with fuel--preferably coke, on account of its great heating power.

The stove is surrounded by an iron or brick shell _M_, having two flues _R_ and _R1_ leading to the chambers I and II, where they terminate in register cowls _K_, which can be adjusted, by turning the handles _h_, so that when the slots _o_ in _K_ coincide with corresponding slots in the end of the pipe, the maximum amount of hot air from the stove is delivered into the drying-chambers; and, by suitably adjusting the cowls and the draught through the fire-door _T_, it is possible to regulate the temperature of the chambers to within one degree of the thermometer scale. When only one of the drying-chambers is required to be heated, the register in the other is closed, and the whole of the hot air is delivered to the first one. With this arrangement none of the heat is wasted, and the contents of one chamber can be dried while those of the other are being removed and replaced.

The moisture-laden air from the drying-chambers can be led direct into the stove chimney. When coke is used, the flue gases consist almost entirely of carbon dioxide. If the vent pipes are led from the top of the drying-chambers into the chimney, the hot gases ascending the latter induce a strong draught in the chambers and carry off the moist air into the open. These pipes, also, are fitted with registers, which, when suitably adjusted, assist in the maintenance of a uniform drying temperature.

The colours to be dried are spread on trays laid on suitable racks in the drying-chambers; and, by carefully planning out the available space, a very large quantity of colour can be quickly and completely dried in a comparatively small plant. The cost of the fuel is so small as to be more than counterbalanced by the saving of time.

The heating arrangements in drying-rooms are capable of improvement in many respects, especially where steam is at disposal; and in such cases, it is better to substitute steam heating for a fire. It will then be necessary to put in a good fan, or other device, to ensure the removal of the moist air. An excessive room temperature--above, say, 50° C. (122° F.)--is not only superfluous, but in many cases injurious, because, apart from the fact that some colours change in shade when over-warmed, an unduly high temperature causes the surface layers to dry very quickly and form a crust which prevents the escape of water vapour from the interior of the material.

Another form of drying-plant for earth colours is the drying-floor, a large room with a rammed concrete or stone floor, intersected with brick flues (about one foot square), covered with iron or concrete slabs and conveying hot flue gases from a furnace. These floors are particularly suitable where there is a possibility of utilising an existing supply of hot flue gases.

Drying-tunnels are specially adapted where large amounts of material have to be dried. The tunnels are built of brick and provided with a rail track on which the trucks carrying a series of trays laden with colour are run. As the trucks move slowly forward, they are met by a current of hot air which dries the charge. The tunnel is kept filled with laden trucks, each fresh one introduced pushing a finished one out at the further end.

In many cases, drying troughs are also useful. These are long, semicircular, jacketed troughs of boiler plate, hot air or steam being passed through the jacket space. A worm conveyor keeps the contents moved forward, turned over and mixed to facilitate drying.

Mention may finally be made of vacuum drying-cupboards, which are heated, air-tight chambers, for the material, in which the air is partially exhausted, thus increasing the rate of evaporation of the water and causing the materials to dry quickly at a much lower temperature than otherwise.

CRUSHING AND SIFTING

The distributing and covering power of the earth colours depends--apart from their special properties--on the fineness of their particles. For this reason, all the means adopted for the purpose of pulverisation are of particular interest. The most important crushing and powdering devices have already been described, and may be referred to, all that needs mention in addition being the fact that stone mills also are used for fine grinding.

The ground products, however, are not entirely homogeneous, always containing, in addition to the very finest particles, those of a coarser nature which must be removed by sifting.

Sifting machines are essentially sieves through which the colour is passed. The sieves are made of wire gauze or bolting-cloth, stretched on prismatic frames which are rotated (centrifugal sieves), or superposed on the flat and reciprocated. In centrifugal sieves, the material is projected against the sieve, and the whole apparatus is in a state of vibration, or else beaters are provided to keep the fine orifices in the sieve from choking up.

Nowadays there are numerous types of sifting devices, none of which, however, can be considered as the best for all purposes, since each type of earth colour behaves differently and requires special treatment. The proportion of moisture in the material, also, has an important influence on the method of treatment required.

A typical flat sifting-machine, with eccentric jig motion, is illustrated in Fig. 21. The machine is fed through a hopper provided with feed rollers, the rate of feed being adjustable. The screened product is discharged through a shoot at one side of the machine, and the residue at the opposite side, into boxes, etc., placed underneath.

For materials that give off a large amount of dust, the machine can be enclosed in a dust-proof casing, in which event the product and residue are delivered into drawers. The machine is easily cleaned and the sieves quickly changed, and is well adapted for dealing with a succession of different materials. The hopper can be fitted with a pair of adjustable crushing rollers.

Fig. 22 is a drum sifter, which is fed by means of a hopper and worm; and the drum can be covered with wire or silk gauze. The sifted product falls into a worm conveyor in the bottom of the casing and is discharged at the side. This may be replaced by a series of mouths for discharging direct into bags, or the machine can be adapted to deliver into an elevator, worm conveyor or other means of transport to a distance.

The screenings are discharged through a shoot at the back of the machine, and can be handled in various ways. A beater is provided to clear the drum and increase the output.

Fig. 23 illustrates a centrifugal sifting-machine for producing very fine powder in large quantities without any escape of dust. It contains a screening drum, the frames of which are detachable and facilitate changing the sieves. A beater revolving inside the drum projects the powder against the sieves, such portions as pass through being taken up and discharged by a worm conveyor; this, however, can be replaced by a bagging device, etc.

CALCINING

Colour earths are sometimes calcined at a high temperature in order to modify their structure and shade, the operation being accompanied, in some cases, by the destruction of organic admixtures and the expulsion of volatile constituents.

An important feature of calcining is that it improves the covering power of many colours, especially heavy spar and certain ferric oxide pigments. This alteration is probably due to the heat causing the finest particles to cohere, and also to the expulsion of chemically-combined water, etc.

The change of shade, which is often dependent on the degree and duration of the heating, is probably also connected with cohesion; but in many instances it is attributable to chemical modifications produced by the treatment; ferric hydroxide, for example, losing its water of hydration when heated and becoming transformed into ferric oxide.

The details of the calcination process vary with the nature of the material, and will therefore be described, together with the appliances used, when we deal with the colours which require to be put through this treatment.

MIXING AND IMPROVING

It is very important that the maker of earth colours should always be able to turn out his products uniform in shade, and since the raw materials are liable to vary in character, and the composition of the earths from one and the same deposit is not invariable, the desired shade has to be obtained by mixing. For this purpose, standard samples must be prepared, for comparison in matching.

Mixing is a highly important operation, on the proper performance of which oftentimes depends the sale of certain colours and the reputation of the maker. It may be effected in various ways, such as shovelling the ingredients together or by combining the work with grinding in edge-runner mills, ball mills, etc. Another method is the mixing barrel shown in Fig. 24, a strong cask mounted on an axial shaft driven by a motor, etc. The barrel is filled about two-thirds full of the materials to be mixed, and, after closing the feed door, is slowly rotated, since, if run at excessive speed, the contents are merely projected against the sides of the barrel by centrifugal force, and it can then be turned for hours without result. The mixing effect can be considerably increased by mounting the barrel so that the shaft is offset from the longitudinal axis of the barrel by an angle of about 30°, the contents being then moved from side to side at each revolution and thus more intimately intermixed by the twofold motion.

In addition to such home-made appliances, there are mixing-machines of the type illustrated in Fig. 25, the body of which is fitted with a distributing worm at the top, and a pair of rollers at the bottom. Below the rollers, which are covered by plates that can be adjusted at a convenient angle, is a worm conveyor delivering into an elevator, outside the machine casing, which connects the two worms. One or more discharging-doors, according to the size of the machine, are provided under the worm conveyor at the end next the elevator. The feed hopper can be arranged on the elevator or on top of the machine, according to local conditions.

In working this machine, the elevator and distributing worm are started and the full charge is fed into the hopper. When it has all passed through the distributor and is lodged on the sloping plates and bottom rollers, the latter and the worm conveyor are set in motion, the material being then carried through by the rotation of the rollers and dropping on to the conveyor, which delivers it to the elevator, to be returned to the distributor. In this way the charge is kept in continuous circulation, and the finely divided particles are repeatedly intermingled, a uniform mixture being obtained. The effect is heightened by the grinding action of the rollers as the material passes between them.

The serial order of the various ingredients, their physical condition (granular or powder), and their density, are all immaterial, the mixing being effected so intimately that when, for example, a colour is shaded with aniline dyes, the ingredients are so completely blended in less than an hour that even the smallest sample then taken will perfectly represent the bulk.

These machines are made in various sizes, are entirely automatic, both in charging, discharging and mixing, and are quite dust-proof, the consumption of power being also small. If necessary, they can be combined with a crusher or sifter feeding direct into the hopper.

A simple means of ascertaining whether the mixing is completed, and one that can also be employed for judging the character of ground materials, consists in placing a sample of the product on a sheet of white paper and spreading it out, under gentle pressure, with a steel or horn spatula. No irregularities, streaks, spots or granules should then be discernible either by the unaided eye or under a magnifier.

Improving, which consists in staining earth colours with other (usually organic) colouring agents, to improve the shade, is an operation which is generally resorted to only in case of need, because it means extra expense, and is of no value unless light-proof colours are used. No permanent effect can be obtained by merely mixing-in coal-tar dyes at random. In addition to certain organic dyestuffs, artificially prepared mineral colours and colour lakes--artificial preparations of an organic dyestuff with an inorganic substratum--are also used for improving.

Another way of improving earth colours is by precipitating certain coal-tar dyes on them, in presence of a fixing agent. Of course the dyes used must not only be fast to light, but also inert towards the substratum and to any other ingredient, such as lime, that is subsequently added to the earth colours.

The following dyestuffs (Höchst) are suitable for direct precipitation on siliceous colours (green earths, clay, ochres, etc.).

Auramine, conc. O, I, II; new phosphine extra; chrysoidine A cryst., B cryst., C extra; Vesuvine (all marks); cachou brown D, G; dark brown M, MB; safranine G, GS conc., B conc.; rhodamine O extra, B, B extra; fuchsine (all marks); fuchsine acetate; cerise G, R; grenadine O, R, RR; maroon O extra; new fuchsine O, P; methylene violet (all marks); peacock blue P; Victoria blue B, R; thionine blue GO; methylene blue (all marks); malachite green (all marks); brilliant green (all marks); coal black O, I, II.

MOULDING

The colour pulp can be made into tablets by moulding it in dry boxes divided into a number of compartments. The colour shrinks in drying, and the tablets will then easily fall out of the moulds. Cones are obtained by placing the pulp in a box, the bottom of which is perforated with numerous holes of uniform size, the box being then tapped against the surface of a stone table. At each stroke, a certain amount of colour is forced, in the shape of small cones, through the perforations, on to a sheet of paper underneath. The cones are then dried.

Some colours are moulded into blocks by forcing the partly dried paste into suitable moulds--preferably of metal, so that they may be engraved with the maker’s name, or other imprint--and left to dry slowly and without cracking. The cakes may be prevented from crumbling by incorporating a small quantity of adhesive, such as a weak solution of dextrin, with the water in which the colour is suspended.