872 THE COLLIERY GUARDIAN. April 23, 1915. dust from - Agujita were tested by Prof. P. Phillips Bedson, the results indicating a not very sensitive dust, although tests from the Palau side of the field, where the coal is less volatile but much cleaner, would probably give less satisfactory results. The very heavy loss of life in proportion to labour employed which has occurred in this field from explo- sions originated by firedamp is undoubtedly due, in part, to an absence of proper Government regulations, and to the ignorance of the individual miner on the subject. Apart from this, however, there is no doubt that the ventilation has in the past been quite inadequate to deal with the situation, although there is evidence latterly that the importance of this point is being more fully recognised. Method of Working. The method of working is very similar throughout.the field (fig. 3). Main slopes are driven on the full dip of the seam from the outcrop, and about 1,093 to 1,640 yds. 1,000 to 1,500 m.) apart, parallel airways being driven on each side of these at about 50 ft. (15 m.) centres. Entries are turned off to the right and to the left at 295 Fig. 3.—Showing Method of Working at No. 1 Mine, El Menor, OF THE COMPANIA CARBONIFERA DEL NORTE. To to 328 ft. (90 to 100 m.) centres, and driven on approxi- mately the strike of the seam, with an airway on the low side connecting with one of the parallel slope air- ways. At 40 to 50 ft. (12 to 15 m.) centres rooms are turned to the full rise off the entries, and after being driven up a short distance narrow, are opened out to from 16J to 20 ft. (5 to 6 m.) wide, and continued at this width to their limit, which is from 20 to 23 ft. (6 to 7 m.) below the airway of the next entry above. Much coal has been lost in the past by not getting rooms up to their limit. Even when they are driven up to their limit, they must be left until the entry has been completed before the pillars can be robbed. The great stumbling block always is to get any working mine developed suffi- ciently ahead of its output requirements to drive the entries to their limit without turning rooms. All the coal is hand-holed, being as a rule fairly soft for holing. It is blasted down by dynamite or some form of safety explosive, the shot-firing in fiery mines being performed by special shot-firers when all the men are out of the mine. Compressed air coal cutting machines of the puncher type have been tried on a large scale where the bands of impurities in the seam are hard, but their use was eventually abandoned as unsatis- factory. The output per hewer per shift varies from two to three tons. Where the seam splits, longwall could probably be employed with advantage in part of this basin; the pioneers of the field, however, did not come from a longwall country, and the few attempts which have been made to introduce it have been unsuccessful. Hoisting arrangements in shafts differ. In this basin as a rule the tub is tipped in an automatic self-dumping cage, whereby on reaching the proper position at the surface one end of the cage floor is automatically elevated and the opposite end door of the tub freed, the coal running down a shoot into a hopper, whence it can be fed into railway wagons as required. This system, which is common practice in the United States of America, has the advantage of reducing tipple labour and tub handling arrangements at the surface; on the other hand, it means breakage of the coal (which in this field is of less importance than most), end doors to the tubs, and the headgear has to be extra high and very strongly constructed to stand the wear and tear. Fully developed mines, either slopes or shafts, yield an average output of 400 to 700 tons per day. Administration. Each working mine is under a “ mine boss,” who is, as a rule, a foreigner, either American or Spanish, and occasionally British, and has one or more Mexican assis- tants under him. Some of the smaller producing mines with easy conditions employ Mexican mine bosses with satisfactory results. The mine bosses are responsible to a superintendent, who is responsible for the technical department to the general manager. Both superin- tendents and general managers are drawn from several nationalities, including American, Mexican, Spanish, Austrian, German, and British. In the writer’s experi- ence, want of satisfactory underground supervision is one of the main difficulties in the successful working of this field. The customs of the country and the climate do not foster initiative, and a man has usually either lost his own initiative or gone home before overcoming the obstacles. Surface Plant. The surface plant at the mines is, as a rule, steam driven. Steam is generated by return tubular boilers about 18 ft. long, the gases of combustion passing under and to the back of the boiler and returning through a number of tubes to the smoke stack at the front, the working pressure being 90 to 1.10 lb. This type of boiler is very unsuitable for the kind of water available, as, unless special precautions with the feed water are adopted, the tubes have to be frequently taken out and cleaned of scale. . At two of the collieries Lancashire boilers were introduced with more satisfactory results. Waste heat from coke ovens is used for firing to a limited extent; as a rule, inferior washed coal, mine run coal, and cleanings up round the washeries and screens are burnt. ■ The preparation of coal for market and the coking arrangements are interesting. At Palau and Menor, for special reasons, the coal is not washed or coked, but is either sold in a mine run or in a screened form, in each case after being picked either on belts or in wagons. The comparative cleanliness of the coal on this side of the basin enables this to be done ; on the other side, however, and also in the eastern and workable portion of the Esperanzas Basin (where the seam appears to occur in a very’ similar condition to that at Agujita), some form of treatment is essential in order to produce a marketable product. . Taking Agujita as an example, the coal in the seam occurs in two main varieties—one a bright friable coal, containing about 11 per cent, of ash, and yielding a highly intumescent coke; and the other a heavier dull black coal, much harder than the bright coal, containing about 37 per cent, of ash, and not giving so good a coke. In regard to the treatment-of this coal, the following are the main considerations :— (1) There is almost always in normal times a good demand for coke at highly remunerative prices, but the coal from which this coke is made should not exceed for beehive ovens 12 per cent, of ash. (2) The demand for high ash nut and pea coal is limited, and the higher the ash, the more limited is the market. It is most important, therefore, to obtain the highest possible output of coal under 12 per cent, of ash from the gross product, and render sufficient of the balance saleable without sending an excessive proportion to the refuse heaps. At Agujita there are two separate washers, a Luhrig washer for fine coal, and a Stewart washer for washing the larger sizes. The Luhrig washer is well known in this country. The Stewart washer consists of a basket or box with a perforated bottom, into which the material to be washed, is fed, and over which the current of water carries it. The entire box is suspended in a tank of water from eccentric suspension rods, which impart to it an upward and downward movement. This forces the water alternately backward and forward through the perforated bottom, lifting the coal, and allowing it to be carried away by the stream of water flowing from the top, while the heavier material or refuse settles on the screen plate, from which it works forward and off into the water tank through a valve set at a suitable height. This tank in which the jig works has a hoppered bottom, which directs the falling refuse into an elevator, by which it is removed. The coal-from the mines is brought in tubs to the tipple, where it is weighed and tipped. It is first passed through a rotary drum screened with Jin. square per- forations, 50 per cent, of the coal passing through this screen and 50 per cent, going over. The 50 per cent, which goes through is re-screened in a 1 in. rotary screen; 44 per cent, goes through, and 6 per cent, goes over; the latter, together with the 50 per cen£. over the J in. screen, is diverted to the Stewart side. The 44 per cent, is treated in a Luhrig type washer over felspar beds. Out of this 44 per cent., 38 per cent, is recovered as washed coking fines, and contains after treatment 11 to 12 per cent, of ash. The balance of 6 per cent, is rejected as refuse. The 50 per cent, which goes over the J in. mesh in the rotary screen is first passed over bars spaced 4 in. apart; 48 per cent, goes through, and 2, per cent, goes over. The latter is bone and refuse, and is rejected. The 48 per cent, is then elevated to a Bradford breaker, which consists of a trommel 6 ft. in diameter and 9 ft. long, with a perforated cover, the perforations being 1 in. in diameter. The trommel makes 25 revolutions a minute, and is fitted with shelves designed to throw the contents about and to detach the purer and more friable coal from the harder and tougher bone. The effect is to select 9 per cent, of the 48 per cent, which goes to the refuse dump. The remaining 39 per cent., which passes through the perforations, is passed on to the Stewart jigs, together with the 6 per cent, from the Luhrig side already mentioned, bringing the percentage at this point up to 45 per cent, of the total. The Stewart jigs reject 17 per cent, as refuse, leaving 28 per cent, of washed coal. This is divided by riddle as follows :— Ash-contents. Per cent. 13*0 per cent, fines for second-class coke through Tso in. mesh.................................. 19 to 20 9*5 per cent, of peas through A to f in. mesh ... 17 ,, 18 5*5 per cent, of nuts through f to 1 in. mesh.. 18 ,. 19 28*0 per cent. A summary of the results is as follows :— Per cent. Best coking coal ....................... 38 Inferior coal .......................... 28 Refuse.................................. 34 Total............................... 100 Fig. 4 illustrates diagrammatically the foregoing description. The two striking features of this separation process, when it is compared with current practice in the majority of other coalfields, are :—(1) The best and purest coal is obtained in the fines by screening off the larger sizes; and (2) 11 per cent, of the total output extracted in the form of refuse by mechanical dry separation. The separation process as described was only arrived at after a number of modifications, and with the present plant and facilities can probably not be much improved upon. The majority of the fixed carbon lost in the refuse is in the bone, but a certain amount of good coal also gets away. The Stewart washer could probably be replaced with advantage by more sensitive jigs, and the coal' on this side sized before washing. Experiments were made in England on pulverising the bony coal and subjecting it to air blast separation; the results, however, were unsatisfactory, due to the fine dissemination of the impurities. GROSS OUTPUT 100 PER CENT SCREEN THROUGH -J-INCH 50 PER CENT 4-INCH SCREEN OVER i-'NCH 50 PER CENT 4-INCH BAR-SCREEN THROUGH 4-INCH 48 PER CENT THROUGH 4-INCH 44 PER CENT j BREAKER WITH 39 PER CENT.,. 1 -INCH HOLES LUHRIG WASHER 45 PER CENT stewart[washers 28 PER CENT. Fig. 4.—Diagrammatic Illustration showing Method of Treatment. Analyses made of the refuse from the Stewart and Luhrig jigs resulted as follow :— Calorific value of dried refuse . Do. as delivered Stewart refuse. B.T.U. .. 7,047 .. .. 6,906 .. Luhrig refuse. B.T.U. . 8,513 . 8,345 Per cent. Per cent. Moisture... 1*99 .. 1*97 Ash .. 55*40 .. .. 46’94 Volatile matter 14*16 .. . 14*97 Fixed carbon .. 28*45 .. .. 36*12 Total .... .. 100*00 .. . 100*00 Sulphur 2*16 .. 3*1 Preliminary tests were made on samples of these fuels in Babcock and Wilcox chain grate stokers, with the view of considering the question of generating cheap electrical power from pit refuse on a large scale at the collieries. The results obtained, however, while incon- clusive, were not particularly encouraging. The washed fines from the Luhrig washer are coked in beehive ovens, of which there are altogether 400, 60 of them being arranged with a waste heat flue, and the others discharg- ing their waste heat into the atmosphere. The coke yield per ton of coal charged is about 64 per cent., and gives on the average a proximate analysis as follows :— Per cent. Moisture ................................... 1*85 Volatile matter .......................... 1*70 Fixed carbon .............................. 82*25 Ash ......................................... 14*70 Total ......;.................................. 100*00 Sulphur.......................................... 0*91 The conditions at the Cloete Mines, where there are between 200 and 300 beehive ovens, are much the same as at Agujita.