BRIQUETTING. When coal is hewn and transported, much of it is unavoidably reduced to small pieces and dust, the supply of which frequently exceeds the demand. The material tends to accumulate, a loss to the coal mining industry and an obstacle to its further development. One of the most effective methods of overcoming this difficulty is a process in which the small coal is mixed with some binding material, such as pitch, and then com pressed into blocks or briquettes (also known as "patent fuel") of various convenient shapes and sizes. Some ingenious house holder probably first arrived at this particular solution, mixing his excess of small coal with clay and moulding it by hand into balls; a process which has been in vogue in various parts of the world for many years. But commercial briquettes must possess clearly defined properties, which are lacking when made with inorganic binders like clay. They should be relatively free from ash and clinker-forming ingredients, water—proof, and sufficiently hard to stand transportation with a minimum of breakage. They should not crumble unduly during combustion, and the binder must not be too expensive.
Many materials have been tried; inorganic substances such as Portland cement, silicate of soda, lime and clay; and organic, such as resin, pitch, molasses, mucilage and starch. None of these so nearly approaches the ideal binder as "medium-soft coal-tar pitch," itself a by-product of coal when the latter is carbonized or baked in closed retorts or ovens. The pitch and various coals are blended in proportions depending on the purpose for which the fuel is intended. A good fuel for domestic use, yielding a high degree of radiant heat, can be made from a mixture of 8o% anthracite, I I % bituminous coal, and g% pitch. For steam-raising purposes a long flame is required, which cannot be obtained from anthracite. Steam coal therefore forms the chief ingredient ; standard practice approximating to 65% steam coal, io% bitumi nous coal, 16% anthracite, and 9% pitch. The price of pitch makes it imperative to use as little as possible consistent with adequate cohesion. In both types of fuel the pitch burns off rapidly, bringing the coking effect of the bituminous coal into play, thus keeping the briquetted material coherent long of ter the pitch itself has been consumed.
Making the Blocks.—The actual processes of manufacture are simple. The coal is washed to reduce the ash content, then dried, of ter which both coal and pitch are ground to a powder and fed by conveyors to a closed vessel known as a pug-mill or kneader. Here the materials are thoroughly mixed by means of rotating blades. Superheated steam is also admitted to the vessel, melting the pitch and reducing the mixture to a plastic mass. Domestic briquettes are most frequently egg-shaped, known as "ovoids" or "boulets," and are produced by passing the plastic material be tween rolls having semi-ovoid recesses in their faces. Into these recesses the material is compressed, emerging as briquettes weigh ing, in general, from one to one and a half ounces. Steam-raising briquettes are usually rectangular in shape, made in blocks weigh ing from 1 o to 281bs. The mixture falls into suitable moulds or recesses in a rotating table, the motion of which is intermittent. During each pause a stamp is forced into one of the moulds, compressing the material to the required shape and size.
The briquetting of lignite or brown coal, as practised in Ger many and elsewhere, follows somewhat different lines. Lignite does not usually require any binder other than that contained within itself in the form of bitumen and water. Again, lignite may contain as much as 6o% water, necessitating much more elaborate drying equipment. Owing to the highly inflammable and explosive nature of dry lignite dust it is essential that steam dryers only should be used.
Briquette manufacture originated in Germany, but has since been developed in many countries, among which may be mentioned Great Britain (South Wales), France, Belgium, Australia, the United States and Canada. The increasing demand for pitch for road surfacing and other purposes has reacted unfavourably on the black coal briquetting industry.
BIBLIOGRAPHY.—Literature on the subject is scanty. Reference may Bibliography.—Literature on the subject is scanty. Reference may be made to vol. i. of Franke's Handbook of Briquetting (London) ; and Stillman's Briquetting (New York, 1923). Much information lies buried in the proceedings and publications of various British, European and American engineering and research societies and institutions.
(H. P. V.)