passed through the heretofore sealed area. With the air current through this fire area established, observers may be stationed at intervals along but outside of the return at such points that a safety lamp may be pushed into the return airway periodically to test the air. These tests should be made at intervals. When no showing of gas appears in the return from the fire area it is time enough to make a thorough inspection of the section which has been sealed up. When this is found to be safe for working, all the stoppings may be removed, the place cleaned up if necessary, fallen or burned timbering renewed and work resumed.

Gob fires are due to the spontaneous ignition of coal, and are most likely to occur in pack walls and gobs where there is an insufficiency of air. Ample ventilation is the best preventive.

Spontaneous Combustion.-According to Prof. Able, Dr. Percy, and Prof. Lewes, the causes of the spontaneous ignition of coal are: First, and chiefly, the condensation and absorption of oxygen from the air by the coal, which of itself causes heating, and this promotes the chemical combination of the volatile hydrocarbons in the coal and some of the carbon itself with the condensed oxygen. This process may be described as self-stimulating, so that, with conditions favorable, sufficient heat may be generated to cause the ignition of portions of the coal. The favorable conditions are: A moderately high external temperature; a broken condition of the coal, affording the fresh surfaces for absorbing oxygen; a supply of air sufficient for the purpose, but not in the nature of a strong current adequate to remove the heat; a considerable percentage of volatile combustible matter or an ex

tremely divided condition. Second, moisture acting on sulphur in the form of iron pyrites. The heating effect of this second cause is very small, and it acts rather by breaking the coal and presenting fresh surfaces for the absorption of oxygen.

Coal Storage.-Prof. Lewes gives the following recommendations for the storage of coal: "The coal store should be well roofed in, and have an iron floor bedded in cement; all supports passing through and in contact with the coal should be of iron or brick; if hollow iron supports are used, they should be cast solid with cement. The coal must never be loaded or stored during wet weather, and the depth of coal in the store should not exceed 8 ft., and should only be 6 ft. where possible. Under no condition must a steam or exhaust pipe or flue be allowed in or near any wall of the store, nor must the store be within 20 ft. of any boiler, furnace, or bench of retorts. No coal should be stored or shipped to distant points until at least a month has elapsed since it was brought to the surface. Every care should be taken during loading or storing to prevent breaking or crushing of the coal, and on no account must a large accumulation of small coal be allowed. These precautions, if properly carried out, would amply suffice to entirely do away with spontaneous ignition in stored coal on land."

When the coal pile has ignited, the best way to extinguish the fire is to remove the coal, spread it out, and then use water on the burned part. The incandescent portion is invariably in the interior, and when the fire has gained any headway usually forms a crust that effectually prevents the water from acting efficiently.

THE PREPARATION OF COAL

CRUSHING MACHINERY

The object of crushing coal is usually to reduce it in size preliminary to washing, a better separation of impurities being secured in the jig if the particles are of nearly the same size. At the same time the coal is crushed the larger pieces of slate, sulphur, etc., are separated by the action of the machine from the lumps of

coal and are, also, more effectively removed in the subsequent washing. In the case of anthracite coal it is reduced in size to meet the requirements of trade which changes in its demands from time to time, requiring a maximum of stove sizes today and a maximum of nut sizes tomorrow, and at no time being able to consume the lump as it comes from the mines.

Jaw crushers, ball and tube mills, stamps, etc., commonly employed in metallurgical operations do not find a place in coal preparation. Coal is

almost invariably broken down in size by some form

of rolls, a few forms of which are:

FIG. 1

Cracking Rolls.-This is a general name applied to rolls having teeth, which are usually made separate and inserted. These rolls, Fig. 1, are employed for breaking the coal, the object being to break the material into angular pieces with the smallest possible production of very fine material. The principal field for cracking rolls is in the preparation of anthracite coal, and the exact style or design of the roll depends largely on the physical condition of the coal under treatment. In most cases the rolls are constructed with an iron cylinder having steel teeth inserted, the size, spacing, and form of the teeth depending on the size and physical condition of the material to be broken. Cracking rolls vary from 12 to 48 in. in diameter and from 24

to 36 in. in face width. The teeth of the larger sizes are from 3 to 3 in. high, and of the smaller 1 in. or less.

The average practice in the anthracite regions of Pennsylvania is to give the points of the teeth a speed of about 1,000 ft. per min., though the speed in different cases varies from 750 to 1,200 ft. per min. One of the largest anthracite companies has a standard roll speed of 97.5 R. P. M. for the main rolls and 124.5 R. P. M. for the pony rolls. The harder the coal the faster the rolls can be run.

If run slow and overcrowded, the rolls will make more culm than when driven at a proper speed. One advantage of comparatively fast driven rolls is that the higher speed has a tendency to free the rolls by throwing out, by centrifugal force, any material lodged between the teeth. In one test it was found that less fine coal was produced at 800 ft. per min., but that the rolls blocked at this speed and hence had to be driven 1,000 ft. per min.

In one case a pair of main rolls 24 in. in diameter, 36 in. face, running at 1,000 ft. per min., handled 2,500 T. of coal in 24 hr. A pair of 19 in. X 24 in. main rolls run at 1,000 ft. per min. handled 300 T. mine run in 10 hr. A well-known maker of rolls for crushing bituminous coal gives a speed of 100 to 150 R. P. M., according to the output required, for rolls 24 in. in diameter and 33 in. long. As a rule, cracking rolls are never run up to their full capacity, as is the case with crushing rolls. The form of the teeth varies greatly, but, as a rule, the larger rolls have straight pointed teeth of the sparrow-bill or some similar form, Fig. 2a. The old curved, or hawk-billed, teeth, Fig. 2b, have now gone almost wholly out of use.

(a)

FIG. 2

(b)

On small sized rolls, rectangular teeth with a height equal to one side of the square base are frequently employed, and these may be cast in segments of manganese or chrome steel.

Corrugated rolls have teeth or corrugations extending their entire length. They were first introduced by Mr. E. B. Coxe, at Drifton, Pa., but they have not come into general use owing to the fact that, while they break some coal fairly well, in most cases it has been found that a continuous edge causes too much disintegration along its length, while a point splits the coal into three or four pieces only, all the cracks radiating from the place where the point strikes, thus producing very much less culm. Another advantage possessed by the toothed rolls is that if anything hard passes through the corrugated roll and breaks out a piece of the corrugation, the entire roll is ruined, while, in the case of the toothed rolls, any one of the teeth may be replaced.

Disintegrating rolls and pulverizers are sometimes used to reduce coking coal to the size of corn or rice before introducing it into the ovens. One roll is driven at double the speed of the other, the slower roll acting as a feed-roll, and the other as a disintegrator. The slower roll is commonly driven at from 1,800 to 2,000 ft. per min. peripheral speed, and the faster roll at from 3,600 to 4,000 ft. per min. The teeth are always fine,

FIG. 3

rarely being over in. high. In some cases, the inner roll is provided with a series of saw teeth from in. to in. high and having about -in. pitch, the individual teeth being set so as to form a slight spiral about the body of the roll. The other roll is provided with teeth having their greatest dimension in the direction of rotation, so that they tend to cross the teeth on the opposite roll. These teeth are also set so as to form a slight spiral, and thus prevent blocking. In other cases, the teeth on both rolls are set in the form of a steep spiral. Hammers.For the reduction of coal, crushers employing hammers have been used, Fig. 3. The crushing chamber is usually of a circular or barrel

form, and the crushing is done by means of hammers pivoted about a central shaft. These swing out by centrifugal force and strike blows upon the coal to be broken. When it is reduced sufficiently fine, it is discharged through bars or gratings at the lower portion of the machine. This style of machinery is usually employed in preparing coal for coke ovens, thus occupying the same field as the disintegrating rolls. A No. 3 pulverizer of this type will crush 50 to 75 T. per hr. run of mine, down to in., or it will crush 100 T. per hr. of slack. Such a machine occupies about 8 sq. ft. of floor space and requires 25 to 30 H. P. to run it.

Miscellaneous Forms of Crushers.-Most crushers can be classed under one of the previous heads, but there are some forms that depend on the material itself to do the crushing. For instance, in the preparation of coal for coke ovens, there has been a combined crusher and separator invented that may be described as follows: A large horizontal drum or cylinder, provided with screen openings around its periphery, is mounted in a horizontal position. The coal to be separated is fed into one end and is caught by shelves or plates projecting radially into the cylinder. These lift the material to the upper side, from which it falls by gravity and strikes the bottom, thus crushing the softer parts. The sulphur and slate, being harder than the coal, are not crushed by the same height of fall, and hence, by a proper adjustment of the diameter of the cylinder, the coal may be crushed and discharged through the screen while the slate and sulphur will pass out at the opposite end of the cylinder.

SIZING AND CLASSIFYING APPARATUS

Stationary Screens, Grizzlies, Head-Bars, or Platform Bars.-These are the various names given to an inclined screen employed for removing the fine material from the run of mine so that only the coarse portion will be passed to the crushers. At concentrating works always and sometimes at anthracite coal breakers, the term grizzly is employed, and a common form is shown in Fig. 4. This is composed of flat bars held apart by cast-iron washers through which the bar bolts are passed to hold the entire frame together. Grizzlies are usually placed at an angle of from 45° to 55°, and ordinarily they are from 3 to 6 ft. wide and from 8 to 12 ft. long, the amount of space between the bars depending on the size of the run-of-mine material and on its subsequent treatment.

FIG. 4

In the anthracite coal breakers, the terms platform bars or head-bars are usually employed, and these bars are made of 11-in. to 2-in. round iron placed

FIG. 5

at an inclination of 5 in. to 1 ft., the spacing depending on the size of coal it is desired to make in the breaker.

A standard size for a bituminous lump screen (the bars are called a screen) for Ohio, Pennsylvania, Indiana, and Illinois is 12 ft. long and 6 ft. wide over the screen surface. The screen consists of 6 bearing bars 4 in. by in. of soft steel and 39 steel screen bars, Fig. 5, with 1 in. clear space between bars. In Iowa, the same sized bar is used, but the space between the bars is 1 in. In the other Western and Southern States there is apparently no standard.

Adjustable Bars.-The top of the bar is cylindrical and projects beyond the web which supports it, so that any lump which passes through the upper part will fall freely without jamming. The two ends of the bar are V-shaped and fit into similarly shaped grooves, so that the bars can be set at distances from each other varying with the sum of the width of the bases of the triangles, the usual opening being about 4 in. These bars are generally 4 ft. long, but they can be of any size.

Finger bars are screen bars that are fixed at one end only, and the bars are narrower at the lower end than at the top, so that the spaces between them are wider at the bottom than at the top, thus giving less tendency for pieces of material to become wedged between the bars.

Movable or oscillating bars are screen bars that are attached to eccentrics at their lower ends, the eccentrics of adjoining bars being placed 180° apart.

This movement throws the material forward and the bars do not, therefore require nearly the same inclination as fixed bars.

Shaking screens have an advantage in that the entire area of the screen is available for sizing, and hence a greater capacity can be obtained from a given area of screening surface. They also occupy less vertical height than a revolving screen. In coal breakers they are particularly applicable where the coal is wet and has a tendency to stick together. The principal disadvantage of the shaking screen is that the reciprocating motion imparts a vibration to the framing of the building. For anthracite coal, the screens usually have an angle or pitch of from in. to 2 in. per ft., the average being aboutin. per ft. These screens are run at from 90 to 280 shakes per min., the average being about 200 shakes per min. or 100 rev. per min. for the camshaft. The throw of the eccentric or cam varies from 2 in. to 5 in.

The capacities of shaking screens operating on anthracite coal have been given as follows. The parties giving these figures advise the use of 140 R. P. M. for the camshaft.

For broken and egg coal, sq. ft. per T. for 10 hr.

For stove and chestnut coal, sq. ft. per T. for 10 hr.
For pea and buckwheat coal, sq. ft. per T. for 10 hr.
For birdseye and rice, 11 sq. ft. per T. for 10 hr.

Size of Mesh.-The following perforations have been adopted by two of the largest anthracite coal companies as the dimensions for the holes in shaking screens to produce sizes equivalent to those produced by revolving

Revolving Screens, or Trommels.-The screen is placed about the periphery of a cylinder or frustum of a cone. The material to be sized is introduced at one end; the small size passes through the screen, and the other size is discharged from the other end. If the form is cylindrical, it is necessary to place the supporting shaft on an incline so that the material will advance towards the discharge end. The inclination of the shaft determines the rapidity with which the material will be carried through the screen. The advantage of the conical screen is that the shaft is horizontal and hence the bearings are simpler. This is a very decided advantage in many plants where the machinery must of necessity be crowded into a minimum space and be hard to get at.

Revolving screens are frequently jacketed, that is, two or more screens are placed concentrically about the same shaft, the inmost one being the coarsest, and each succeeding screen serving to make additional separations. This method reduces the space necessary for a given amount of sizing machinery. In other cases, a long cylindrical screen has a coarse mesh near its discharge end and finer mesh near the entrance end, thus making two or more through products as well as the overproduct. The disadvantage of jacketed screens is that the necessarily slow speed of the inmost screen reduces the capacity of the entire combination, so that if rapid work is essential, it is better to use fairly large-diameter screens placed one after the other in place of jacketed screens. Another disadvantage is that, to renew the inner jackets, it is often necessary to remove the outer ones.