September 22, 1916. THE COLLIERY GUARDIAN. 549 that so great a percentage of this stone dust is carried away from the road. A. thin seam with coal in the rippings was also selected for examination. The results showed that, although the upper coal does affect the road dust, a single stone dust- ing is sufficient for a period of well over three months. In the case of a thick seam (especially where the colliers set 9 ft. timber at the coal face) it is necessary to increase the quantity of stone dust, as the stall roads are of large area, and there is also a large surface of exposed coal on the sides of the .roads. It would appear that in such a road, a dusting of 33 lb. per yard is insuffi- cient. Pieces of coal are continually falling from the sides in these iroads, and the floor dust rapidly loses its ash content, while the dust on the roof and aiders main- tains a high ash percentage. It is not uncommon in any seam to find a road near the coal face which presents a dark colour on the floor and a lighter colour on the sides—the dark colour gradu- ally diminishing towards the roof—until in many cases the highest parts of the roads are white. A few weeks after stone dusting the coal dust film is quite distinct on the sides of the road, up to a height of about 4 ft. Doubtless owing to low air velocity, it dees not rise so readily to the higher parts. From experiments detailed in the paper, the writer recommends, for roads of average area (say, 40 to 50 sq.ft.), an original application of 401b. per yd., with subsequent applications, at intervals not exceeding four months’ duration, 201b. per yd. being ample the second time, and 12 1b. per yd., in many cases, sufficient after- wards. It does not seem to be necessary to clean the road more frequently than is generally done for con- venience in working. The quantity of stone dust required over 12 months will be from 60 to 70 lb. per yd. of roadway, in a district which has been previously stone dusted. Horse Haulage Roads. The roadways, extending from the stall road area back to the haulage double parting, might be expected to require more frequent treatment, but this does not seem to be the case. No doubt the reason is that a road of this description requires greatly more repairs than a stall MW ttwmrpegvpv. tv as fWAoe Tower tvwfvatg. I $ $ I 1 a Haulage Road. The box (a) is kept full of stone dust. The angle iron (b) is moved out and in by the passing trams. The angle iron (b) is connected by means of levers to the sluice valve (c). The sluice valve (c) liberates a small quantity of stone dust as each tram passes. road. Even if the seam is thick, it is the general prac- tice to raise the road in the course of repairing work. The constant repairing work, entailing the handling of considerable quantities of rubbish, causes a good deal of stone dust to be deposited. Main Haulage Roads. Haulage roads are coal dust accumulators, and the safety measures depend more on the location than on the quantity of the deposit. A roadway with a good roof and littletimber, where the dust can be located, seen, and examined at will, presents dangers that are known, that can be measured, and there is no doubt that such a roadway can be brought within safe limits. Then there is the road with cavities of unknown dimensions, and unknown dust contents above the timber, which dust is quite distinct from the observed dust found on the floor, roof, and sides. It not only provides the material for a possible inflammation zone, but it also falls into the roadway on a commotion taking place. A stretch of dry roadway, extending 680 yds. from the downcast winding shaft, was kept under observa- tion. The floor, sides, and roof were cleaned by hand. The average of four samples taken .after cleaning gave 43 per cent, of ash, showing the good effect of previous .stone dusting. Stone dust was then applied at the rate of 10-3 lb. per yard (except on one part where the road was of large area, and where the quantity laid down wa,s 161b. per yard). The result of this application was to increase the average of samples taken over the whole length to 67*6 per cent, of ash. Light and frequent dressings of stone dust were applied with the object of ascertaining the source of the dust which is found in .suspension in the atmosphere. This dry and dusty haulage road was kept under observation for four months, and 14 different examina- tions had been made. The following is a summary of the results :— Ash percentage in dust taken from roof and sides before the test was commenced ... 43 per cent. Quantity of stone dust applied in four months 60-4lb. per yd. [Note.—100 yds. of roadway of large area near the pit bottom received an application of 70 lb. per yd. in the four months.] Number of sprays in use .................. 3 Number of stone dust distributors in use ... 1 Average ash result from 77 samples of sus- pension dust ........................ 31-5 per cent. Average ash results from 78 samples of dust taken from roof and sides ........... 63-2 per cent. Average ash results from 7 samples of floor dust ................................ 44-60 per cent. Average weight of deposit of suspension'dust in oz. per sq. ft. per week, taken for 77 samples ............................. 0-109 oz. Another road was selected over which a smaller quantity of coal passed, in which the air velocity was low, and the gradient less heavy. This road was cleaned and dusted by means of the ejector. Analysis of Results. Quantity of stone dust applied at com- mencement of test ................... 31 lb. per yd. Quantity of stone dust applied during test ... nil. Quantity of coal passed over road ........ 16,476 tons. Number of sprays in use .................. 3 Average ash of 10 samples from roof and sides over whole period ............. 80-5 per cent. Average ash of 11 samples of suspension dust taken over whole period .....;....... 40'9 per cent. Average weight of deposit in oz. per sq. ft. per week ............................ 0-036 oz. Number of days over which the test extended 92 days. Average ash of 3 samples of suspension dust at the end of 92 days' .............. 33-9 per cent. Average ash of 3 samples taken from roof and sides over whole road at end of 92 days ................................ 75-2 per cent. Average ash of samples taken from floor of whole stretch of road at end of 92 days 32-0 per cent. The deposit of suspension dust, weekly, was only one-third of that in the more dusty road. The ash con- tent of the deposit over the whole period was 8*8 per cent, higher. Effect of Watering the Road. A general survey of haulage roads showed that in roads where the floor was watered the dust did not accumu- late so quickly on the timber and sides. In order to obtain reliable information on this point, a road which has a wet floor (owing to a small stream of water running continually over it) was kept under observation for one month. Analysis of Results.—Average weight of deposit per square foot per week, 0-027oz.; ash of samples of sus- pension dust, 26-8 per cent. These deposits were small in quantity, except in one case, where the heavier deposit was due to high speed of haulage. The general conditions, however, would favour a low deposit, the area being large, the gradient small, and the speed of haulage moderate. Doubtless the wet floor is largely instrumental in keeping down the dust in this road. There is also evidence that highspeed of haulage may treble the dust deposit locally. The following figures are quite representative of a road which is inclined to be wet in parts, and which has also the favourable conditions of low speed of haulage, low air velocity, and a £mall quantity of coal passing. The maximum speed of haulage is six miles per hour, the velocity of the air in no place exceeds 400 ft. per minute, and the quantity of coal averages 130 tons per day. A dry stretch of this road 400 yds. in length required only 49,6001b. of stone dust, equivalent to 1241b. per yard, to maintain an average of 67-5 per cent, of ash in 71 samples taken over a period of 21 months. It was further noted that a single stone dusting of 34 lb. per yard was sufficient to maintain an average of 64-4 per cent, of -ash over a period of four months. Having obtained some information as to the source and deposition of dust, it is necessary to form an esti- mation of the areas which receive the deposit. A haulage road, 55sq.ft, in area, will in many cases present 36 sq.ft, of horizontal surface, suitable for dust accumulation, on the roof and sides alone. If 0-1 oz. be taken as a likely deposit per square foot per week (and this is a moderate figure for a dusty road), then the dust deposited on a roadway 50sq. ft. in area amounts to 3-6 oz. per foot run per week—equivalent to 7*2 oz. per 100 cu. ft. of air space. In the Fifth Report of the Explosions in Mines Com- mittee, it is stated that “ lib. of coal dust to 100 cu. ft. of air seems to be the least quantity to be practically effective to develop an explosion.” It is certain that there are many haulage roads on which the dust in suspension in the atmosphere is suffi- cient to form a deposit of 1 lb. per 100 cu. ft. of airspace in the course of a fortnight, and such roads may be passing less than 400 tons of coal per day. It is also the case that there are roads passing large quantities of coal, which, under favourable circum- stances of gradient and air velocity, may provide the deposit necessary for the lower limit of propagation in one week, or even four days. It is reasonable to draw the following conclusions from the experiments :— (1) On many haulage roads, the dust film which is formed by particles carried in suspension in the atmo- sphere may accumulate in sufficient quantity to be dangerous in the period of 14 days. On parts of excep- tionally dusty iroads this accumulation may assume dangerous proportions in four days. (2) All the dust raised in suspension in the atmo- sphere on a stone-dusted road (in the ordinary course of working) will pass through a 60 sieve; 95 per cent, of it will pass through a 90 sieve; and 77 per cent, of it will pass through a 180 sieve. It is therefore certain that only the finest of the stone dust comes into suspension under normal conditions. (3) The greatest accumulations occur where the velo- city of the air is high and where the speed of haulage is high. (4) Most of the dust which enters the mine from the .surface is deposited within 80 yds. of the downcast shaft, probably because the area of the road near the pit bottom is generally large and the air velocity corre- spondingly low. (5) If the journeys of coal are thoroughly sprayed before leaving double partings, and if the empty trams are similarly treated before coming down the pit, the dust raised from the tops of the trams (and in the case of the empty trams, from the inside of the trams) is less in quantity than that raised from the floor. (6) The floor dust on a dry haulage road loses its ash rapidly owing to leakage of coal dust from tram doors, and to the grinding of pieces of coal by the tram wheels. A very pure coal dust is found in the groove cut by the flange of the wheel on each side of the road. This dust ‘ is raised into the atmosphere in large quantities by every passing journey. Floor dust will generally become reduced in ash at the rate of 5 per cent, per week on roads dealing with 300 to 400 tons of coal per day if the gradient is 3 to 4in. per yd,, and’the speed of haulage is high. (7) If the floor of a haulage road be kept moist, and the roof and sides stone dusted, and if all full and empty journeys be sprayed, it is possible under average condi- tions to reduce the rate of accumulation of suspension dust to about 0-027 oz. per sq. ft. of road surface per week, even where the quantity of coal passing amounts to 300 tons per day. The suspension dust will not, however, maintain a high ash percentage over a long period, as when the floor is wet, the deposit is formed chiefly of dust blown from the tops of the loaded trams. It will probably carry 27 per cent, of ash at the end of three months, being diluted to this extent by stone dust raised into the air from the roof and sides by the vibra- tion and increased air velocity caused by passing journeys. (8) If the floor of a very dusty haulage road cannot be kept moist, but all journeys of trams are sprayed, the rate of accumulation of suspension dust will in many cases be 0-109 oz. per sq. ft. per week, where the quantity of coal passing amounts to 300 to 400 tons per clay. When, on a road of this description, stone dust is laid in sufficient quantity to bring about a mixture on roof, sides, and floor of 60 per cent., incombustible dust to coal dust, it will generally be found that the ash con- tent of the suspension dust collected one week after stone dusting will average 40 per cent. If no further stone dust be laid, the ash content of the suspension dust at the end of four -weeks will be reduced to 20 per cent., although the mixture on the roof, sides, and floor may still exceed 50 per cent, incombustible dust to coal dust. This serious decrease is due largely to the daily supply of fresh coal dust ground up by the tram wheels. It accumulates alongside of the rail, exactly in the same way as dust accumulates in the groove alongside of a tram rail in a street. In order to abate the dust nuisance in towns, this groove is swept daily. In a colliery the dust lodging in this groove can only be diluted (if water cannot be used), and by applying stone dust to the floor, close to each rail, at the rate of 1 lb. per yd. of roadway per day, it is possible to maintain the ash percentage of the suspension dust at about 28 per cent, for eight weeks after the road was stone- dusted. (9) On haulage roads, where the velocity of the air is low, the speed of haulage moderate, the gradient not exceeding 3 in. per yard, and where the conditions generally are against a heavy “ make ” of dust, the average deposit of suspension dust per square foot of road surface per week may be as low 'as 0-036 oz. An application of stone-dust at the rate of 301b. per yard on a road of this description has been found -to maintain the dust on the roof and sides at 75 per cent, ash, and the suspension dust at 34 per cent, ash, after three months 'had elapsed from the time the stone dust was applied. The floor dust, -however, showed only 32 per cent, ash at the end of the three months. After examining the recommendations of the Explo- sions in Mines Committee, the author proceeds to deal with the various classes of haulage roads and the best methods of stone dusting them. Haulage roads can be divided into four clashes :— (1) The very dusty road, where conditions will not permit of the floor being watered. If the quantity of coal passing does not exceed 400 tons per day, and the gradient does not exceed 4 in. per yard, the quantity of stone dust required per annum is 1801b. per yard of roadway of 55 sq.ft, area. The quantity used on each application depends on the floor dust. It is likely that the floor dust will drop 5 per cent, of its ash per week. It will probably be necessary to apply 101b. per yard to the roof, sides and floor at the begin- ning of each month, and 51b. per yard to the floor only during the third and fourth weeks of each month. Stone du-st distributors should be placed 400 yds. apart. The dust applied to the floor can best be distri- buted by a tram attached to a journey. When there are cavities above the timber, ttie ejector should be used.