COAL-BREAKERS. Coal-breakers and the machinery used in them for the preparation of anthracite coal for the market have been ably described by Mr. Eckley B. C'oxe, in the Trans actions of the American Institute of Alining Engineers, six, 398, of which this article is largely an abstract. Anthracite coal as it comes from the mines is not marketable. The "run of mine" can not. as in the ease of bituminous coal, be sold. Anthracite, being very compact and practically free from volatile combustible matter, burns only at the surface. and it is, therefore, deemed important to have the limps as nearly of a uniform size as possible, so that between them a large amount of surface will remain exposed to the action of the air without checking the draft too much or allowing enough air to rates to cool the coal below the ignition-point. In other words, if the pieces of coal of the size of a chestnut and smaller are mixed with lumps of the size of an egg. they fill the air-passages and prevent a free draft. It has long been recognized. therefore, that one of the most important points in preparation is to have a uniform sizing, and also to make as large a number of different sizes as can be produced without too great expense. It. is also essential to remove all the dust, which is of little or no use at present, and depreciates the value of coal in the market.
Mixed with the pure coal, large amounts of slate, "slate-coal" and "bony coal " generally occur. The term "slate-coal" is commonly used to designate lumps composed partly of coal and partly of slate, in which the pure coal occurs in such large masses Butt, by rebreaking, pieces of pure coal of marketable sizes can be obtained economically; arid "bony coal " to designate Minns in which the coal and slate are so interst ratified that they can not be sepa rated economically by mechanical preparation ; also coal in which the impurities are present in such high percentages as to destroy or greatly diminish its market value. In other words, slate-coal is coal from which, by breaking and preparation, as certain amount of pure coal earn be obtained : bony coal is coal which can not be economically rendered more pure by mechani cal preparation. although it may be used for certain purposes in its crude condition.
The problem is, to remove the impurities as completely as possible. Of course. when the slate occurs in separate pieces, it should be eliminated without further lu•ealcing. lint the slate-cod must be broken into smaller pieces to separate the slaty portion from the coal. It is generally impossible to sell all the larger lumps which come from the mines, and machinery must be provided for breaking them up into suet] sizes as the market. requires.
The coal coining from the mines should be Ilivided into its various sizes, and the free slate in each size should be removed, before any breaking is done. This can be done either by hand l:Ilanr or by mechanical moans. In the first case the coal is passed along Antes. on the sides of which men mud boys are placed who pH; out the slate, and in some cases the bony and slate-coal, and allow the pure coal to ',ass into the pockets. The mechanical slating of the coal depends upon one or more of three physical charneteristies of the coal and slate: the difference in thenr specific gravity; the difference of the forms in which they break ; and the difference of their angle of friction, or, in other words, the difference in the angle of a chute. lined with stone or iron, down which the coal or slate will slide without any increase of velocity. As a rule, slate will not slide down a chute which will carry coal.
Machinery for Sizing Coal.—This may be divided into two classes : fixed or movable bars, and fixed or movable screens. In the first, the openings through which the coal falls are much longer than they are wide, while in the second the ratio of the length to the width of openings does not generally vary much from unity. In special cases the first class may be used to take out dust or fine coal; otherwise. they arc seldom employed, except for large coal, unless when exact sizing is not important. The reason is. that long, flat pieces fall out with the cubical pieces of much smaller dimensions, rendering the coal thus sized unsightly, incon venient to handle in the furnace, etc. There are three types of the first class now in common use : 1. The adjustable bars, supported at both ends. 2. The finger-bars, supported at one end. 3. The oscillating bars.