October 30, 1914. THE COLLIERY GUARDIAN. 933 Result. Moisture content of mixture per cent. 18'2 .. Flame travelled throughout. 20'0 21'0 .. 99 99 23'6 .. 130 ft. and th died out. 32'5 ,, 10 ft. only. 21'5 „ throughout. 27'6 ,, 30 ft. only. 22'0 .. ,, throughout. 25'5 .. „ 100 ft. only. 19'80 .. 80 „ 21'40 .. 30 „ 10'2 „ throughout. 15’2 ., „ 90 ft. only. and otherwise as those which had been made with mix- tures of coal and incombustible dusts. Fullers’ earth was used as the incombustible dust in the first series of experiments, which can be summarised as follows :— Ash content of mixture per cent. 5’1 5*1 5'1 51 5'1 12'5 12'5 233 23'3 33'6 33'6 43'8 43'8 A second series of experiments of the same character was made, using as the incombustible dust Altofts shale pulverised so as to be of about the same degree of fine- ness as the fuller’s earth. It was found that when small percentages of Altofts shale were employed, such as would form from 10 to 15 per cent, of the whole mixture, the amount of water required to make the mixture incombustible was the same as when fuller’s earth was used. Mixtures con- taining higher proportions of shale, however, required less water than the corresponding mixtures of fuller’s earth. Thus mixtures having the following ash and moisture contents were incapable of propagating flame. Ash content of Moisture content of mixture, per cent. mixture, per cent. 23 20 33 15 39 9 It was apparent when watering the mixtures for the experiments that the shale was moistened more readily than the fuller’s earth, and seemed to form with the water a binding medium which agglomerated the fine coal dust. Moreover, such mixtures after becoming dry remained caked. These experiments show that the larger the quantity of incombustible dust mixed with the coal dust the less is the quantity of water required to render the mixture incapable of propagating flame. Roughly, it can be stated that a coal dust containing up to 15 per cent, of ash must also contain between 25 and 30 per cent, of water to prevent propagation of flame, and that eA ery additional 10 per cent, of ash reduces the water required by about 5 per cent. It is impossible to lay down any exact formula determining the quantity of incombustible dust which is equivalent to a given quantity of water, but we are of opinion that when the mixed dusts contain an appreciable quantity, say, 15 per cent, or more by weight of moisture, that moisture is at least as beneficial as an equal weight of incombustible dust. Indeed, for several reasons it appears to us that a combination of the two methods of rendering coaldust less dangerous—“ watering ” and “ stone-dusting ”—is to be preferred to either method alone. A roadway that has sufficient incombustible dust to bring the ash-content of the mixed dusts up to, say, 40 per cent, would, in our opinion, be maintained in a safe state by systematic spraying, provided that 15 per cent, of moisture was constantly present in the mixed dusts. In practice it is difficult so to wet coal dust that the water and the dust shall form an intimate mixture, and although it was found by experiment that dust and water mixtures, prepared outside the gallery and spread by hand along its length, could not be ignited when they contained about 30 per cent, of water, it seemed doubtful whether the spraying of dry coal dust strewn along the gallery with the quantity of water required to form a 30 per cent, mixture would suffice to prevent ignition. A number of experiments were ' therefore tried to ascertain the quantity of water required and the most efficient method of applying it. It early became apparent that jets or “ rose ” sprays of water were not effective unless a large excess of water were used. The jets or sprays, whilst moistening some of the dust, swept the major portion from the shelves on which it was distributed on to the floor, where pools of water formed and much dry dust accumulated. Some of it floated on the water. “ Atomisers,” which delivered the water obtained from the ordinal y water supply in the form of a fine mist, driven out under pressure, were found to be more efficacious. The mist impinged upon the coal dust without disturbing it from its position, and appeared to mix intimately with it and to wet it throughout A number of experiments were therefore made using “ atomisers” to wet coal dust previously distributed on the shelves and floor of the large gallery. The cannon was charged with blasting powder and arranged as in the previous series of experiments. Five pounds of dry coal dust were distributed along the “ cannon- tube ” just before firing, the remainder of the gallery having been treated with coal dust and then sprayed. The length of flame given by this 5 lb, of dry coal dust in the cannon-tube, the remainder of the gallery being dry and free from dust, was 100 ft. The main results obtained were as follows:— Coal dust. Water. Result. 1 lb. per linear...! lb. per linear Flame propagated through- foot. foot. out the gallery. 4 lb. per linear 1 lb perlinear ..Flame travelled 150ft. and foot. foot. then died out. 2 lb. per linear..2 lb. per linear... Flame travelled 100 ft, and foot. foot. then died out. It would appear, therefore, that when dry coal dust is damped with an equal weight of water in the form of a fine mist, it is no longer capable of propagating flame beyond a limited distance even when the source of ignition is the flame from a charge of 24 oz. of blasting powder extended by burning coal dust to a length of 100 ft. It was found, as might be expected, that if the coal dust in the cannon-tube, that is to say immediately in front of the “ shot-hole,” were also treated with water, less water was required to render the dust non-ignitible. Thus, experiments in which as much as 201b. of dust were placed in the cannon tube, and in which the remainder of the gallery contained coal dust at the rate of 2 lb. per linear foot, only resulted in flames measuring 45 ft. from the cannon when the whole of the dust, including that in the cannon-tube, was treated with half its weight of water. Provided, therefore, that the dust for a considerable distance round a shot-hole is kept wet, there is no danger of its ignition by any flame arising from the explosive. More difficult to guard against is the chance of an ignition of firedamp being transmitted to coal dust. We. therefore, made experiments to see whether dry coal dust could he so treated with water as to render it incapable of “ extending ” a gas explosion. The arrangement of the galleries was altered so that a gas explosion in the 3 ft. gallery could be caused to travel in»o the large gallery, to wThich it was joined by a right-angle bend. The large gallery was strewn with coal dust and watered. One end of the large gallery, 50 ft. from the entrance to the 3 ft. gallery, was closed by an iron plate; the other end, 400 ft. from the entrance to the 3 ft. gallery, was open. To obtain the gas explosions, either coal gas or fire- damp was allowed to stream in at the closed end of the 3-ft. gallery, which was 150 ft. distant from'its junction with the large gallery, the capacity of the smaller gallery being about 1,000 cubic feet. One hundred cubic feet of gas were allowed to enter, 30 seconds being the time taken for its entry into the tube, and, after two minutes had been allowed to elapse for the partial diffusion of the gas into the air, an electric fuse was fired at a point 25 ft. from the closed end. By these means a flame 70 ft. long was projected from the 3 ft. gallery into the large gallery. No means weie employed to shut off the 3 ft. gallery from the large gallery during the making of the explosive mixture, the idea being to reproduce an occurrence possible in a mine, namely, a “feeder” of gas at the end of a gallery opening on to a main road. Preliminary trials showed that this method of making an explosive atmosphere allowed explosions to be obtained of moderate violence and speed of travel; and that, provided the rate of passage of gas into the gallery and the time that was allowed to elapse before ignition were the same in each case, each explosion, with the particular combustible gas used, was of the same order of magnitude. As was to be expected, the coal gas explosions were much more violent than the firedamp explosions. Either the coal gas or the fire- damp explosions when they entered the large gallery were found to be capable of stirring up and igniting dry coaldust strewn therein. Comparative experiments showing the effect of coal gas explosions on (1) dry coal dust and (2) coal dust mist-sptayed with an epial weight of water, the dust- strewn “ zone ” being 100 ft. long in each case, and the dust present at the rate of 2 lb. per linear foot, gave the following results:— Length of flame, measured from beginning of dust-strewn zone. Feet. C Dry coal dust (1st experiment)..... 400 I Dry coal dust (2nd experiment) ...... 320 < Mist-sprayed coal dust (1st experiment) 100 t Mist-sprayed coal dust (2nd experiment) 90 It has been shown that a great measure of safety can be obtained if water is intimately mixed with coal dust so that the mixture contains over 30 per cent, of water. It would appear that sufficiently intimate mixing of the dust and water can be obtained by the use of “ mist ” sprayers, but in order to get the same results as were obtained by 30 per cent, of water intimately mixed with the coal dust before it was placed in the gallery, it was necessary where sprayers were employed to put on a larger quantity, namely, about an equal quantity to the weight of coal dust. This probably was due to the fact that the whole of the water applied by the sprayers was not uniformly distributed through the whole of the coal dust. These experiments appear, therefore, to show that immediately after the application of the water and before any evaporation has occurred, the protection obtained by mist-spraying coal dust with an equal weight of water is sufficient to prevent the propagation of a flame started by a coal gas explosion of some violence. In this respect water is more effective than an equal weight of dry incombustible dust, probably because it prevents the coal dust from being raised in suspension in the air. We found the flame arising from coal gas explosions, 1 roduced in the 3 ft. gallery in the manner described above, was propagated throughout the large gallery when the latter was strewn with either a 1:1 or a 1J : 1 mixture of dry incombustible dust and coal dust. A 2:1 mixture extinguished the flame after it had travelled 370 ft. If, however, a 1:1 mixture of incombustible dust and coal dust, after being strewn in the large gallery, was mist-sprayed so that the mixture contained 15 per cent, of water, it was no longer capable of propagating the flame of the coal gas explosion that entered from the 3 ft. gallery, but the flame died out after it had travelled 140 ft. Explosions of firedamp and air, however, produced in the same manner as the coal gas explosions, were found on repeated trial to be incapable of igniting a dry 1 : 1 mixture of coal dust and incombustible dust. Comparative experiments with coal gas and firedamp in our one-foot gallery showed that it is impossible to obtain with the most explosive mixture of methane with air any approach to the speed and violence of coal gas explosions. The 1 :,1 mixture of coal dust and incom- bustible dust can, we believe, be regarded as proof against ignition by the flame of the most violent firedamp explosion. In order to obtain the protection above-mentioned by means of water, it must be understood that the moisture content of any portion of the dust must never fall below the minimum indicated by our experiments. An average of, say, 30 per cent, of moisture, maintained in a gallery by alternate patches of wet mud (or even pools of water) and half-damped dust, might not pre- vent the transmission of an explosion—for the drier dust only might be raised and ignited. How fast the wetted dust would dry and Low often therefore water would have to be applied must depend on the conditions prevailing in each individual pit. • It would be possible to institute a series of experi- ments upon the rate of drying of various sorts of dusts under different conditions of air current, of tempera- ture, of hygrometric conditions, of quality of water, &c., but we do not see that they could possibly replace the necessity for actual trial in each pit. We have already expressed the opinion that the treatment of mines by placing incombustible dust in barriers or in zones is not sufficient, for the flame may pass by a barrier and may ti av rse zones of considerable extent. And if the spaces between the zones are of considerable width, thej e is a danger of the untreated coal dust in the inter-zonal spaces being ignited. It is therefore in our opinion more simple, and certainly more safe, to have incombustible dust throughout the roads. We hold the same view as to watering. Comparative Experiments With Various Incombustible Dusts. As a result of the trials that have been made with a number of dusts of diffeient kinds that have been submitted to us, the general statement can be made that the fineness of an incombustible dust rather than its chemical composition affords a measure of its probable effectiveness in preventing the ignition of cofll dust with which it may be mixed. The dusts that we have tried, each of which was of about the same degree of fineness, can be described as follows:— (1) Fuller’s earth. (2) Fuller’s earth substitute. (3) Shale dust (pulverised in a disintegrator). (4) Dolomite dust. (“ Extra superfine.”) (5) Oolitic stone dust (containing 97 per cent, of calcium carbonate). (6) “Chance Mud Lime.” In each case the whole of the dust could pass through a safety lamp gauze (28 x 28 sieve) and between 80 and 90 per cent, by weight could pass through a 200 X 200 sieve. In order to test the various dusts, mixtures of each were made with coaldust of a like degree of fineness, and the mixture spread in the large gallery. The cannon (charged with 24 oz. of blasting powder), and “ cannon-tube” were employed as the means of ignition. With each dust it was found that an equal propor- tion by weight with coal dust rendered the mixture incapable of propagating the flame of the cannon-shot beyond a limited distance ; whereas mixtures containing 40 per cent, by weight only of any one of the incom- bustible dusts propagated flame. Of the six dusts enumerated above, three (Nos. 4, 5, and 6) consist mainly of carbonates. It has been suggested that such dusts, by virtue of the carbonic acid gas that they liberate when strongly heated, should be particularly effective in preventing the ignition of coal dust. Our large scale experiments do not bear out this supposition, which, moreover, laboratory experiments, described in our Second Report, had already discounted. It is true that a carbonate of which the temperature of decomposition is very low—such as sodium bicarbonate—is more effective than other incombustible dusts, whether on account of the carbonic acid gas evolved, or by reason of the steam simultaneously liberated, it is difficult to decide. But with the majority of the carbonates the decomposition temperature is too high, and the rate of evolution of carbon dioxide consequently too slow, for them to perform any function other than that of “ inert materials. It will be understood that if carbonate dusts are used, the “incineration test” for determining the percentage of incombustible dust mixed with the coal dust on the mine roadways, which we proposed to apply to shales and similar forms of incombustible dusts, is unsuitable. It becomes necessary to make separate deteiminations, with the dried dusts, of (1) the total loss of weight of the coal dust and carbonate dust mixture on heating; and (2) the loss of weight of the mixture when treated with dilute acid to decompose the carbonate, whereby carboni<--acid gas is evolved. For the first operation the high temperature of a blowpipe flame is required, and the heating has to be continued during about an hour to ensure complete decomposition of the carbonate. The loss of weight of the mixture is that due to combustible carbonaceous matter together with that due to carbonic acid gas from the carbonate. The second operation requires to be carried out in a special apparatus, of which there are several forms on the market. As already stated, the loss of weight due to carbonic-acid ga3 froiii the carbonate is thereby obtained; this loss subtracted from the total loss enables the percentage of combustible matter in the mixed dusts to be determined. The degree of fineness of the incombustible dust is of importance. In general it can be stated that the finer the dust the better.