When brought to the surface coal has to be prepared for the market by the removal of any impurities which it may contain. Coal contains two classes of impurities: those that form part of its composition, and those that are intimately mixed with it. So far as the former are concerned they consist for the most part of ash, sulphur, and phosphorus, and these cannot be removed by any mechanical process of cleaning. The impurities that are found mixed in the coal, but which do not form part of its composition are shale, mixed coal and shale, and iron pyrites. These substances can be removed by some process of cleaning.

The operations of preparing coal for the market include (1) Screening and Picking, (2) Washing, (3) Coking, and (4) Manufacture of Briquettes.

Screening. — When coal is brought to the surface it passes direct to the screens, where it is separated into two sizes by passing it over a screen. The round or lump coal passes from the screen on to a picking belt, while the small coal or dross passes through and is carried away for further sizing and treatment. If marketed as unwashed dross it is loaded direct.

In emptying the tubs on to the screen, suitable tipplers must be used to prevent the coal from being broken, as lump coal is generally the most valuable. These tipplers are usually of the rotary type, the tub turning through half a circle and returning when empty, or going completely round emptying its contents on the way, and finally passing to its original starting position. A plate is fixed so as to arrest the fall of the coal as it leaves the tub, and cause it to run quietly on to the screen. A tippler of this class is shown in Fig. 107. Screens are of four classes : (a) Fixed Bar Screens; (b) Movable Bar Screens; (c) Jigging or Shaking Screens; (d) Revolving Screens.

Fixed bar screens are used in a number of cases, and are generally inclined from 27 to 30 degrees to the horizontal, the spaces between the bars being from to 2 inches, depending on the class of coal that has to be dealt with. The chief objection to screens of this class is that they deliver the coal more or less intermittently upon the picking belt, and often at such a velocity as to cause breakage, especially when the coal is soft.

Movable bar screens are so arranged that the lower end of the screen bars are attached to eccentrics, and rock around a centre at the top. The eccentrics of adjoining bars are placed 180 degrees apart. This gives an up and down movement to alternate sets of bars, and thus throws the coal forward, while at the same time shaking the small stuff out from among the lumps. Screens of this type do not require to be set with as high an inclination as those having fixed bars.

The jigger or shaking screen is made with wire mesh, or more usually perforated steel plates forming the bottom of a tray into which the coal passes from the tippler. The tray is suspended by means of iron rods and inclined about 10 to 12 degrees. A to-and-fro movement is imparted to it by means of an eccentric fixed upon a revolving shaft, and connected to the frame of the tray by means of a suitable connecting-rod. These screens are given from 90 to 120 shakes per minute, and are very effective in separating the small from the lump coal. They have large capacities, one shaking screen being able to deal with four times as much coal as a fixed screen would. Where wet coal which has a tendency to stick together has to be separated, the shaking screen gives extremely good results. Their chief drawback is the amount of vibration that they impart to the whole of the supporting structure, unless it is of the most rigid construction. The general arrangement of a shaking screen delivering sized coal into trucks is shown in Fig. 108.

The revolving screen is generally used for the classification of small coal, the various classes of which in the descending order of sizes are cobbles, treble nuts, double nuts, single nuts, pearls, and duff. The screen consists of a long cylindrical tube formed from perforated plates, with different sizes of perforations. The finest size is at the top and the other sizes increase in order towards the lower end. The screen is inclined at such an angle that rotation causes the coal to pass down from the entry to the discharge end, the various sizes being separated when they reach the necessary width of perforation. A screen of this class is shown in Fig. 109.

Picking Belts. — After passing over the screens the coal is discharged on to a travelling belt, along which it is carried. Attendants are placed at intervals along each side of the belt, their work being to pick out all useless material from the coal. The product thus cleaned is carried along by the belt and discharged over its end into a wagon.

Various types of picking belts are used, but all others are gradually giving place to the plate belt. In this case the band, which is generally about 36 inches wide, is formed from a series of narrow steel plates, 9 to 12 inches broad, and jointed together. The band passes round hexagonal or octagonal pulleys at each end. The pulley next the screens is driven by power and thus communicates motion to the belt. The belt is supported along its entire length in an angle iron channel provided with rollers. The speed of the belt may be from 30 to 60 feet per minute, and such a belt may under favourable circumstances pass 200 tons of coal per day. A belt of this description is shown in Fig. 110.

Circular picking tables which revolve in a horizontal plane are also used, but are not so popular as the plate belt type.

Washing. — In many cases the smaller sizes of coal are washed in order to remove pyrites, slate, fireclay, and other incombustible or detrimental material. The term washing is a misnomer as it suggests tbe application of a purely mechanical process, whereas the separation of the coal from the useless material depends upon the difference in specific gravities of the coal and the useless material and is therefore a physical process.

There are several types of coal washers, some comparatively simple and others complicated, but the efficiency of all of them depends on the difference between the specific gravities of the coal and the foreign material to be removed. In general the best method of separation is first to classify the coal, and then treat the different sizes separately. There is, however, one type of washer that has found considerable favour of late where the coal is first washed and afterwards sized.

Coal washers may be classified as follows : — (1) Trough Washers, (2) Rotary Washers, and (3) Jigging Washers. Whatever the type of washer used, a plentiful supply of clean water is needed if the best results are desired.

The Elliott Washer (Fig. 111) is a typical example of the trough class. In this washer a long stationary trough is provided, along the bottom of which a scraper chain moves. The trough is slightly inclined, and the coal to be treated is fed in at the upper end. The coal is suspended in the water and passes with it to the lower end of the trough, while the dirt falls to the bottom and is carried uphill by the scrapers and delivered at the upper end. By careful regulation of the inclination of the trough and the speed of the scraper chain, a very effective separation can be obtained. Separate troughs are required for each size of coal treated.

The Robinson Washer (Fig. 112) is an example of the rotary class. It consists of a conical chamber fitted with stirring arms. The unwashed coal is fed in at the top and the water at the bottom, the whole being kept in a state of agitation by the stirrers. The heavy dirt settles to the bottom and is drawn off at intervals, while the lighter coal is carried up with the stream of water and discharged over the top.

Jiggers. — This class of washer is extensively used, and is represented by the Luhrig, Humbolt, Evance Coppee and others. The principles embodied in this class are those embodied in the jigs originally used for dressing metalliferous ores. The jigging part consists of a large rectangular tank divided into two compartments by a division that comes part of the way down, leaving a connection between the two spaces at the bottom. In one of the compartments a piston, which is made to reciprocate by means of a crank or eccentric, agitates the water. Near the top of the other compartment a perforated metal screen is fixed. The unwashed coal is fed on to this perforated screen, and the pulsations of the water produced by the action of the piston causes the dirt to separate and settle on the plate. The finest portion of the dirt passes through the perforations in the plate, and is drawn off from the sloping bottom of the tank at intervals. The coarse dirt is run off from time to time by opening the dirt sluice, which is situated at the level of the perforated plate. The washed coal and the excess of water passes off at the top. When dealing with fine sizes a bed of orthoclase felspar is placed on the perforated plate. The specific gravity of felspar is between that of dirt and coal, hence it allows the dirt to pass through while the washed coal is drawn off from the top. The length of stroke of the piston and the number of pulsations per minute have to be regulated for the various sizes of coal treated. A jig or washing tank, such as is used with washers of this class, is shown in Fig. 113.

Coking. — Certain kinds of coal when heated become pasty, and if the heating be carried on so as to partially or wholly exclude air, a hard, porous mass is formed. This mass consists of the fixed carbon and ash of the original coal, all the volatile constituents being driven off by the heat, and is known commercially as coke. Coking is carried out in firebrick chambers called coke ovens.

The original form of coke oven was the beehive, so called because of its shape. In this class of oven sufficient air is admitted to burn the escaping gases during the process of coking.

Beehive ovens produce a good quality of coke, but this system of coking involves a serious loss of heat and also of valuable by-products. The modern type of coke oven, frequently called a by-product oven, or sometimes a retort oven, consists of a long, narrow chamber, wherein the coal is heated without access of air. In ovens of this class the by-products, consisting of hydrocarbon gases, tar, and ammonia, which are wasted in the beehive oven, are all recovered. It is now a common practice to charge these by-products ovens by means of a mechanical arrangement, which compresses the small coal as it is put into the oven into a solid cake. It also pushes the coke out of the oven once the process of coking has been completed.

The chief classes of coke are furnace, steel, and foundry. Furnace and steel cokes are hard and porous and should break into columnar pieces when drawn from the oven. They should be as free from sulphur as it is possible to get them, and the percentage of ash and moisture should be low.

Foundry coke should be hard, compact, non-porous, and capable of giving an intense heat when used in the cupola, or melting furnace.

Briquettes. — The finer sizes of coal, especially those of the non-coking varieties, and coal dust may be manufactured into a class of fuel that is known as briquettes. Machines for making these briquettes are divided into two classes : (1) Those that make small ovoids about the size of eggs, and (2) those that make bricks. The first class of machine consists of a pair of rolls fitted with semi-ovoid recesses, which come opposite each other as the rolls revolve. The prepared material is fed between these rolls and crushed into the openings, dropping out on the delivery side in the form of small ovoid-shaped pellets. In the brick machine the material is fed into a die or mould and pressed into shape by a plunger, as in the manufacture of ordinary building bricks. In both cases the product when discharged from the machine is ready for the market.

With practically all classes of British coal it is necessary to use a binding material before the small coal can he made to adhere. This binding material is usually pitch, which is ground fine and incorporated with the small coal by means of a worm conveyor or mixer working in a steam jacketed pan, the heat from which renders the pitch plastic. The material after mixing is carried forward, while still hot, to the rolls or brick making presses. Briquettes have not come into general use in this country, largely because of the amount of better-class fuel available at moderate prices, and also because of the quantity of fine ash they produce when used for ordinary household purposes. Up till now there has been no serious attempt made in this country to manufacture briquettes of small sizes suitable for industrial purposes. Perhaps the large increase of tar distillation, due to the stoppage of the export of this material to Germany, and the consequent increase in the quantity of pitch that will be available, may in the near future lead to further development in the manufacture of briquettes.