April 5, 1918/ THE COLLIERY GUARDIAN. 693 These figures, though conservative, will give ample protection to the prospective manufacturer. A plant such as this represents, at the present market prices, an investment of 250,000 dols., exclusive of ground and tracks. The boulets find a ready market at 4 dols. per ton, leaving a comfortable margin of profit available. Discussion. In discussing this paper, Mr. Burke Baker con- tributed an account of a small briquetting plant built by the American Briquet Company primarily as a demonstration plant. Its output per hour is 4 tons of 2 oz. briquettes, which have been retailed in the vicinity during the year the plant has been in opera- tion, and have proved very acceptable for use in ranges, heaters and grates in homes, and in steam heating plants in apartment houses and hotels. They have not been sold for power purposes, although satis- factory tests have been made in this field. Anthracite silt is used and is shipped direct from breaker or washery, the only preparation it receives being the removal of excess moisture by means of a Ruggles- Coles rotary dryer. Care is taken, however, to pur- chase silt containing less than 18 per cent. ash. The method used is the Hite emulsion process. The binder is composed of 0*5 per cent, corn starch or wheat flour; 1 per cent, asphaltum or hydrolene, and 6-5 per cent, water, on the weight of coal dust. It is prepared in a tank equipped with paddle agitators and heated by steam jets. The starch and water are first made into a paste by being brought to the boiling point with live steam; the melted asphaltum is then introduced and thoroughly beaten into the paste by the agitators. The rapid stirring breaks the hot asphaltum into minute particles which are distributed through the paste, producing a smooth chemical emulsion. This emulsion, when dried, is not soluble in water. The binder is mixed with the dry coal dust in a horizontal paddle mixer, 8 per cent, binder to the weight of coal being used. The resulting plastic mass is then discharged into the press, which is of the simple roll type. Since the binder is a liquid, and is not sticky or gummy until partly dried, it is readily mixed with the coal dust, and a fairly uniform mixture is attained with ease. From the press, the briquettes are conveyed to a drying oven. Here they are distributed 6 in. deep over a broad screen-conveyor which passes through a tunnel constructed of hollow tile. Through this tunnel and up through the layer of briquettes, a current of air is drawn by an exhaust fan at one end of the dryer, the air having been heated to approximately 225 degs. Fahr, by a furnace at the other end. The speed of the screen-conveyor is such that the briquettes are in the dryer one hour before they are discharged into an elevator which carries them to the loading chutes. This dryer is similar in design to those used in the textile industry and in the drying of ores. The briquettes thus made constitute a thoroughly satisfactory fuel for use wherever the prepared sizes of anthracite are consumed. They are water-proof and weather-proof; they withstand rain, freezing and thawing, the hot sun, or the heat of a boiler room, or all in succession. They are hard and tough enough to withstand rough handling, and can be piled in large quantities without softening or sticking. They hold their shape in the fire until completely consumed, and do not soften or fuse at any time. They are practically free from odour or smoke, and are entirely free from fumes or gases that hurt the eyes or skin, or injure flues and grates. The emulsion process has certain distinct advan- tages, which may be outlined as follow:—The total cost of binder, of drying, and of interest and depre- ciation on additional equipment in the emulsion process, is 65c. per ton of briquettes. The cost of the asphaltum binder, estimating 7 per cent, to the weight of briquettes and $17-50 as the price of hydro- lene, is $1*22 per ton. Thus there is an actual saving in the cost of manufacture of 57c. per ton. The emulsion binder, with its 1*5 per cent, of outside raw materials, instead of 7 per cent., greatly reduces the dependence upon outside sources of supply, thus avoiding many shutdowns due to delayed deliveries of asphaltum. An asphaltum of a high melting point and low penetration is required in the oil process. This is produced by only one or two of the large refineries. In the emulsion process, any grade of asphalt from crude petroleum to that of the highest melting point can be used, thus permitting the purchase of the lowest-priced product from any refinery. The other raw material—J per cent, of corn starch or wheat flour—can be bought in the open market and from many sources. By using only 1 per cent., instead of 7 per cent, of oil, the objectionable features of smoke, odour, and soot, are correspondingly reduced. The amount of smoke or odour produced by the 1 per cent, of asphaltum in its emulsified form is almost negligible, and there is no soot whatever. The emulsion-binder briquettes do not soften under heat. They can therefore be shipped in hot weather and to any latitude without danger of sticking in the cars; they can be stored in large quantities in summer without softening, and they do not soften and fuse in the fire, even under forced draught. The emulsion process is not so simple as the asphaltum process, since the preparation of the binder and the drying of the briquettes, although simple enough in themselves, require additional equipment. But this is compensated by the greater ease of mixing the binder with the dust The masticator is unnecessary, and the need of keeping the flux at the proper temperature is obviated. On the other hand, the lower cost of production and the superior charac- ter of the product would seem to mark this as another forward step in the industry. The American Briquet Company, after having operated its small plant in Philadelphia for a year, and having thoroughly proved the quality of the product and the practicability of manufacture on this small but commercial scale, is now making con- tracts for the erection of a plant of 50 tons per hour at Lykens, Pa., where arrangements have been made with the Susquehanna Collieries Company for a supply of silt produced by the colliery at that point. SPONTANEOUS HEATING OF SLACK HEAPS.* By Prof. George Knox, F.G.S., M.LM.E. Owing to the difficulty experienced at the present time in the South Wales and Monmouthshire coal field in finding a market for small coal, large heaps of this material have had to be laid down at tbe collieries. As the space available in the vicinity of collieries for such accumulation is usually very limited, many of the stacks have reached considerable heights—resulting in subsequent heating and the possible spontaneous ignition of the whole stack. Our attention at the School of Mines having been called to several instances of spontaneous heating in these heaps throughout the coal field, in both the house and steam coal areas, it has been suggested that, instead of issuing a separate report to each of the various collieries concerned, it would be advisable to issue a general brief report on this matter, as a guide to others who may have to commence stacking coal in the near future. To understand the nature of the changes which take place in the coal when stacked in heaps on the surface, it is necessary to briefly refer to the origin and com- position of coal. Origin of Coal. Coal represents the accumulation of solar energy over immense periods of time, through the action of the sun’s rays on the chlorophyll contained in plant cells— producing great forest growths, particularly in carboni- ferous times. The regenerative influence of the rays of the sun on chlorophyll enables the latter to absorb carbon dioxide and water vapour from the atmosphere, together with moisture and mineral salts from the roots. The interactions which follow result in the formation of starch granules and sugars, which ultimately form the food of the plant and are incorporated in the woody fibre as cellulose. This may be approximately expressed as follows:— Carbon *W ater dioxide, vapour. 6CO2 + 5H2O + Cellulose. Oxygen. Sun’s energy = CeHloO4 + 6O2 Although coal contains only very small quantities of slightly altered wood fibre in its composition, the relationship between vegetable matter and coal is well known, and is quite apparent from the following table:— Carbon. Hydrogen. Oxygen and nitrogen. Wood fibre 50 6’0 44*0 Peat 60 5*8 34’2 Lignite 66 5*2 28*8 Bituminous coal.. 88 5*6 6*0 Anthracite 94 3*4 2*6 If the carbon is kept as a fixed quantity, the change between vegetable matter and coal is still more notice- able, although it does not necessarily follow that these substances have been in another:— Carbon. Wood ........... 100 Peat ............ 100 Lignite ......... 100 Bituminous coal.. 100 Anthracite ..... 100 turn derived from one Hydrogen. Oxygen and nitrogen. .. 12*18 ... 88*07 9’85 ... 55*67 8*37 ... 42*42 6*12 ... 21*23 475 ... 5*28 Process of Change. Although the process of change from vegetable matter to coal is not yet thoroughly understood, the latest researches in this direction show that coal consists chiefly of altered leaves, small twigs, spores and fruits, together with globules of resinous matter. A large per- centage of the original vegetable matter was converted into a plastic vegetable mud, in which the fruits, spores, etc., were embedded. The change has probably taken place by a process of decay, through bacterial action, in the absence of air, subsequently assisted by a gradual increase of temperature and pressure as the vegetable matter descended under the accumulated mass of debris laid on top of it. Peat and cellulose have been artificially converted into coal in eight hours at a temperature of 340 degs. Cent, under 100 atmospheres pressure. At 310 degs. Cent, it took 60 hours In tbe conversion, the vegetable matter loses weight to the extent of 75 per cent., which indicates that coal seams are only a small volume of the original vegetable material from which they were derived. Microscopical and chemical analysis shows that coals are composed chiefly of :— (a) Humus bodies (having high oxygen content, and carrying much hygroscopic water). (ft) Resinous bodies (with medium oxygen contents, but melting at 300 degs. Cent.). (c) Carbon (very dry). With a varying percentage of hydrocarbons, moisture, and mineral matter (ash). Coal has the power of absorbing gases. Some coals will absorb three times their own volume of oxygen. In doing so, the absorbed gas becomes very active, and rapidly combines with the carbon and hydrogen of the resinous bodies, converting them into carbon dioxide and moisture. This chemical action (oxidation) increases the temperature of the coal; and as the temperature increases, the process of oxidation becomes still more rapid. Coal being an excellent non-conductor, the heat thus generated in a large heap of loose coal will increase steadily, and with increasing rapidity, until the ignition point of the coal is reached. * The Journal of the Monmouthshire Colliery Officials’ Association. Ignition Point. Although the ignition point of coal is generally stated as being about 370 degs. Cent, for cannel, 408 degs. Cent, for bituminous, 450 degs. Cent, for lignite, and 477 degs. Cent, for Welsh steam coal, this will depend on the state of division of the coal. Fayol was able to get spontaneous ignition in coals at much lower tempera- tures, according to the fineness of the particles of the coal. Powdered gas-making coal took fire in five hours at 100 degs. Cent.; in two hours, at 150 degs. Cent.; in 40 minutes at 200 degs. Cent., and in a little over one minute at 400 degs. Cent. Powdered anthracite took fire in four hours at 200 degs. Cent., and in little over half-an-hour at 400 degs. Cent. Though the degree of inflammability of coal cannot be directly deduced from the chemical composition, it is well known that spontaneous heating is more common in coals with a high volatile content. This spontaneous ignition of coal in bulk has been for many years a source of great trouble in shipping. It was a noticeable fact that the larger the cargo, and the longer the voyage, the greater was the risk. It has been found that with cargoes of 500 tons of coal, cases of spontaneous combustion amounted only to 0*25 per cent., whereas with cargoes of 2,000 tons the amount is increased to 9 per cent. It was also noted that tbe length of the journey was the determining factor in the number of cases of spontaneous ignition—showing that a certain time-limit is necessary to enable the oxidation to generate sufficient heat to produce spontaneous ignition. Cases of heating in heaps of coal stored on the surface were also very common; and coking coals thus stored for long periods lose their coking properties. The heating appears to be directly related to :—(1) The quantity of oxygen contained in the coal, (2) the amount of resinous bodies present. While gas-making coals and coking coals are most likely to ignite spontaneously, it is not safe to stack large masses of the driest steam coal to heights of more than 10 ft. or 12 ft., unless all the slack under 1-in. mesh has been taken out. Storage of Coal. Large coal free from slack may be safely stored up to a height of 20 ft. to 30 ft., but unscreened coal or slack should never be piled up in large heaps. Slack or “through” coal containing “rashings” or other bituminous shale bands may ignite more readily than clean slack. Washed (sized) slack does not heat so readily as unwashed slack. This is probably due to the better ventilation of the heap, and the more rapid radiation of heat through the interspaces between the even-sized pieces of coal. Where coal has to be stacked in a limited space, it should be laid out in layers of 3 ft. to 4 ft. in height. The oxidation of the fresh raw coal in the bottom layer, and the heat resulting from it, will have been dissipated before the second layer is laid down. As coal heaps under 7 ft. in height have not been known to ignite spontaneously, the length of time the lower layers are exposed to the atmosphere before the succeeding layers are put down will enable fairly large stacks being made with comparative safety. Heaps between 9 ft. and 13 ft. thick usually show a steady rise of temperature to 60 degs. to 70 degs. Cent., then finally fall again. This, it should be noted, will render a coking coal unsuitable for coking. Stacks of coal over 13 ft. in height are always liable to spontaneous heating if tipped in one layer. Long after heating has commenced, the exterior surface will not show the slightest change in temperature. The maximum heating usually takes place at a depth of. from 13 ft. to 15 ft. from the surface. This heated area is surrounded by a zone of considerably lower tempera- ture, which separates it from the normal temperature zone of the outside. After about three months heating, water vapour (steam) begins to issue from the heap. This is succeeded by the issue of a colourless gas that smells strongly of paraffin or petrol, and later smoke will appear, when the temperature of the hot zone has reached 120 degs. to 150 degs. Cent. At about 200 degs. Cent., yellowish fumes are given off, and the tempera- ture suddenly rises from 200 degs. to 300 degs. Cent., when ignition takes place. Treatment of Heating* Heaps. Once heating has started in a slack heap over 13 ft. in depth, there are two possible methods of stopping it:—(1) To exclude it from oxygen and prevent further oxidation; (2) to provide for the heat being radiated as rapidly as it is generated by oxidation. While the former method is possible with slack heaps left in the mine, it cannot be easily accomplished where the heaps have been laid down on the surface. To allow for rapid radiation of the heat generated under the second method, the slack heap may either be reduced in height by taking away the upper layers, or ventilation pipes may be inserted at short intervals throughout the whole mass. If the temperature has gone beyond the “ steaming ” stage, and there is a danger of rapid oxidation suddenly increasing the temperature to the ignition point, a number of small pits may be sunk in the mass to the region of greatest temperature, and these kept flooded with water, which will gradually filter through the mass, absorbing a large percentage of the heat. The top layer to a depth of 10 to 15 ft. should then be taken off and deposited elsewhere. If the coal stack has been stored behind any wood barricade (or other wood structure), this greatly increases the danger of spontaneous ignition, as the ignition point of timber is much lower than that of coal. If tbe wood barricade is a temporary erection, it should be at once removed; but if it is a permanent structure, the coal should be immediately moved from behind it. Prevention of Heating*. 1. Wherever possible, size the coal before stacking it, and put each size in a separate heap. 2. Never stack heaps of “through” coal, or mixed small coals to a height of more than 13 ft.