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BROWN COAL AND LIGNITE.

MECHANICAL PREPARATION OF THE COAL.

The size of the particles of coal and its cleanness or freedom from extraneous matter are weighty factors in the manufacture of briquettes. If the coal be well mixed with the bond and subjected to strong pressure, the particles may have a diameter of five, or at the highest seven, millimeters (three to five-sixteenths of an inch). The most satisfactory product is from coal of an even size of one-eighth of an inch (three millimeters) or from the slime of slack.

"The cleaner the product the higher the value, and so much the more widespread will be its market." This principle, founded on experience, holds as well for briquettes as for coal and coke, of which the quality is judged, aside from its heating power and other characteristics, especially according to its cleanliness. It is therefore best to separate all slate, pyrites and other foreign materials as completely as possible by separation and sorting, and to complete this by washing the coal after it has been reduced to proper size. By this means the briquettes often contain much less ash than the coal from which they were made.

The mechanical preparation of coal for briquetting consists of three operations:

• Separation;
• Crushing;
• Cleaning.

These operations are so fully described in all the standard works on the subject that it is hardly necessary to do more than refer to them here.

Separation.—The coal as it comes from the mine is sorted by hand, or separated into different sizes by a system of sieves or other suitable apparatus, the lump coal passing to the crusher, and the fine coal to the washing apparatus.

Crushing.—The apparatus for crushing is either a breaker or a crusher of the ordinary type, or a disintegrator of special construction. The latter seems to be preferred in Belgium, France, Germany, and Austria, as it produces a fine coal of very uniform size and homogeneity.

Cleaning.—In order to secure the smallest possible amount of ash and sulphur in the briquettes, the fine coal is washed to rid it of its clay, sand and iron pyrites, just as in preparing it for coking. The great improvement