96

1500 kilogrammes to the square centimetre. Small coal mixed with only 5 per cent of the magnesia cement (of which 2½ per cent remains as ash) will produce hard, firm briquettes. They require a pressure of 150 to 250 kilogrammes per square centimetre, and the briquettes require twenty-four hours to become completely dry and hard, and should not be exposed to rain for at least three days. The amount of ash, and the fact that it does not increase the calorific power of the material, would not recommend it for use with brown coal except under very special conditions, despite the advantages offered in cheapness of plant required for using it.

As stated previously, however, pitch is the bond most extensively used, either by itself or sometimes with the addition of a small percentage of coal tar. Hard pitch is the variety in most general use, although machines of the Evrard system, which use only soft pitch, are also in use in considerable numbers.

In Russia the use of bitumen is reported, but whether the material is of natural occurrence or the name is simply used for a manufactured article, does not appear.

MIXING COAL AND BOND.

In the manufacture of briquettes it is of the highest importance that there should be the most intimate mixture possible of the coal and its bond, and to accomplish this there are as many processes as there are bonds, if not more. Certain bonds, such as Irish moss, dextrine, silicate of soda, clay, magnesia, cement, etc., can be mixed with the coal without the aid of heat. By Saltery's patent the molasses is dissolved in warm water and then mixed with the coal in a mixing cylinder, which resembles a clay-cutting machine. While the processes require a less costly plant than those employing heat, the briquettes produced must be dried after manufacture, either in the air or by artificial heat, before being ready for market.

The methods which give the best results are those which heat the coal and bond and press them while in that condition.

Hard pitch may be powdered and mixed with the fine coal, and the mixture passed through pulverizers of suitable construction to reduce both to the requisite fineness, and the mixture then heated to secure the plasticity necessary for perfect agglomeration, or the pulverized and