July 12, 1918. THE COLLIERY GUARDIAN. 71 ________________________________________________________________________________________________________________________________________________________________________ of pressure along the copper conductor and the re- actance will probably reduce the actual current flow- ing to 1,000 amps, or less, but still the pressure be- tween the armour and earth in the neighbourhood of the contact and for some distance outbye may be at a very dangerous figure. If the resistance of the earth path had been reduced to 0-05 ohm, the current flowing through the earth path, assuming the other conditions to be the same, would be increased, but the pressure available for shocks would be reduced to a safe figure. In collieries the difficulty is to obtain the equivalent of the ideal conductor outlined above, though a coal seam itself would furnish the ideal conductor if good contact could be obtained with it. According to measurements made some years ago, the electrical resistance of some kinds of coal is fairly low, and the total resistance of a portion of the seam between, say, the pit bottom and the neighbourhood of the coal face should be very low if good con- nection could be made with the body of the seam. Even in the case of comparatively thin seams the large mass of coal should offer a very low resistance, just as a large mass of water does, if one could make sure of delivering the current to it. The difficulty of securing the ideal earth is increased by the fact that strata of high resistance are often interposed between the surface and the coal seam. Such high- resisting strata may be interposed even between two seams working to the same shaft. The protection of cables in mines now includes the use of selective protective apparatus that will promptly disconnect a cable upon which a leakage fault has commenced to grow. For this purpose the iron armour, if its resistance is maintained at the low figure necessary to comply with the Home Office regu- lations, should be sufficient, but it is not sufficient to prevent shock if contact is made between a conductor and the armour or one of the metallic bodies referred to. To render the whole system safe from shock in the event of such contact taking place, an attempt must be made to get as near the ideal earth as possible. In the first place the very best earth obtainable must be provided on the surface, and at least two earth plates should be fixed in ground that is either natu- rally always moist, or can be rendered so by the pro- vision of a small stream of water. A method that is ___________________________ much in favour at present is to bury fairly large iron plates, or a number of iron pipes, a good depth in the ground to where it is always moist if possible, and if this cannot be arranged, filling in the pit in which the pipes or plate are buried with .coke, and arranging for a small stream of water to be constantly dribbling over the coke, etc. The writer would prefer substantial copper plates not less than i in. in thick- ness, though J in. would be better, and the connecting cable between the earth system and the earth plate should be of the same substantial dimensions as out- lined above. Furthermore, the copper conductor of the cable should be electrically welded to the copper plate, and the weld should embrace large surfaces, both of the cable and the copper plate, care being taken to prevent the cable from becoming brittle in the neighbourhood of the weld. The writer’s objection to iron plates or iron pipes is the probability that rust will form on their surfaces and steadily build up a resistance between the plate or pipe and the earth. There is also the probability of the connection between the earth wire and the earth plate, being destroyed by electrolytic action. At least two such plates should be fixed on the surface as far apart as possible, and if practicable at different depths below the surface of the ground. Earth plates should also be fixed at the pit bottom and at as many points along the roads and as near the working face as possible. The sump should make good earth at the pit bottom, and the guides and any pipes that are in the shaft should be bonded to it by substantial copper wire bonds. The bonding wires should be welded to the earth plate, and should be given good contact with the guides, pipes, etc., by the aid of large-headed coarse-pitch screws, or by wrapping. The object of bonding the earth plate at the pit bottom to the guides, etc., is to connect it by as low a. resistance as possible with the earth plates on the surface. The earth plates on the roads and in the workings should be arranged in somewhat the same manner as described for the earth plates on the surface, but where practicable they may be laid horizontally; the squeeze of the mine will help to make the contact with the seam good. It is probable that if a horizontal cut is made in the seam similar to that made by a coal-cutting machine, and a copper plate is laid in the cut, the plate having a substantial wire lead welded to it, good connection will soon be obtained with the body of the seam. To assist the matter small pieces of coke, or even of coal, might be packed round the plate, in the same manner as the coke is packed round the earth plates at present in vogue. All the ironwork underground should be bonded to the earthing system (wherever practicable), the girders upon which haulage or pumping plant is fixed, any iron or steel pit props and the rails, at as frequent intervals as possible. The object in view is to get as near to the ideal earth, outlined above, as possible. Carrying out the suggestions made will mean expense and more or less trouble, but will give a better chance of avoiding shock and saving life. The writer suggests that the copper plates, with connecting wires welded to them, should be got ready and should be connected to the seam when opportunity offers, as the working face moves forward. In certain cases they might be placed _____________________________ The University of Sheffield.—The matriculation exami- nation for the degree in mining will be held on Monday, September 9. For information apply to the Registrar before Saturday, August 24. The mining diploma (day) course commences on Wednesday, October 2, and the part- week mining course on Thursday, October 3. The cer- tificate (Saturday afternoon) course commences' on Sep- tember 21, and the mining teachers’ course on September 28. The courses in electricity applied to mining commence on Saturday, September 21. AIR-CARRIED POWDERED COAL* Powdered coal purverised and distributed by the Holbeck system is being used for heating 46-sheet and pair furnaces at the plant of the Standard Tin Plate Company, Canonsburg, Pa., an interesting feature of the installation being the arrangement of coal dust collectors in both the distributing and pulverising parts of the system. The pulverised coal is delivered to a main conveying duct, and is carried in suspension in a continuous current of air, with branch lines leading to the furnaces. The conveyor duct forms a closed system through a return duct leading to dust collectors, and thence to the air intake to the distributing blowers. Thus the only outlet is through the branch lines to the furnaces. The installation comprises two dis- tributing blowers with inter-connected dust collectors and four pulverisers, which deliver coal through indi- vidual dust collectors to two powdered coal storage bins. Two pulverisers supply a single bin. The screw exhauster 4= > TO FURWACE5 DISTRIBUTING BLOWER PULVERIZE* RETURN FROM FURNACES PULVERIZED COAL BIN . COAL DRIER DRIED COAL BLN I SCREW CONVEYOR Fig. 1. conveyors, placed in concrete trenches, receive the coal from the coal storage pocket and deliver it to an 18 in. belt conveyor. After passing a magnetic separator which removes all foreign matter the coal goes to a bucket elevator. This elevates and dis- charges it through a spout to an automatic registering scale which weighs and drops it to a drier. This drier, which is hand fired, removes the moisture from the coal so that only J per cent, remains. There Fig. 2. Blower, Mdm Coal Dus! Conveyor is not much danger of the coal taking fire, as the velocity of the gases is low, due to the large area of the drier. Also, the drier is provided with dust-tight rings at each end to prevent dust leakage. From the drier the coal is conveyed by a screw con- veyor, to the two dried coal bins, one being placed between two Bonnot pulverisers. Each pulveriser has a capacity for pulverising 2,500 lb. of coal per hour to a fineness of 95 per cent, through a 100 mesh screen, and 85 per cent, through a 200 mesh screen. After being pulverised, the fine coal dust is drawn through an air separator on the top of each pul- veriser and discharged into dust collectors, one for each pulveriser. From these the coal dust drops to the storage bins while the air is drawn back to the pulverisers, maintaining a partial vacuum in them. This, it is said, ensures an absolute quality of fineness of grinding and encloses the system, so there is no escape of coal dust to the atmosphere. Also, the combustible gases released by pulverising the coal are kept in the system and delivered to the suction piping of the distributing blowers. The powdered coal is fed to a cast iron high pressure distributing blower along with air in such proportion * Iron Age. that it is carried by suspension through pipes to the different furnaces at a velocity of approximately a mile a minute. The coal dust and air travelling through the pipe form a non-combustible mixture which requires additional air for combustion of the coal. This is furnished under a small blast pressure at the coal burner. The amount of coal dust and air fed to the dis- tributing blower is governed by a regulator, which in turn is governed by the volume of air flowing through the line, so that, irrespective of varying demands on the system, a constant mixture of coal and aii’ will be delivered to the distributing duct. The regulator controls the speed of a variable speed motor which drives the screw feeding coal to the blower. A float controls the free -air admitted to the blower with the coal dust. The distributing pipe foi’ the 46 sheet and pair furnaces is 1,480 ft. long, and is carried up and over and through the roof trusses with branches dropping down at each furnace to supply the burners. The method of dropping a branch pipe to the burner, and also the manner in which the supply of coal and secondary air are controlled by the furnace operator, are illustrated. Approximately 600 lb. of coal, it is said, were re- quired per ton of steel heated with hand firing, while with powdered coal this was found to be reduced to 280 lb. per ton. The following notes will be of assistance in inter- preting the illustrations: Fig. 1. Dried coal after pulverisation is delivered through an air separator to the main collector A, from which it falls to the coal storage bin. A screw con- veyor carries it from here to a distributing blower, which takes in air through the air intake and delivers the mixture to the main coal dust conveying duct. Coal dust not taken by the furnace continues through the conveying dust, which returns to dust collectors B and C. These are open to the air intake, but the coal settles to the bin. Fig. 2. A branch line from the coal dust conveyor and one from the secondary air supply unite at the burner at rear of furnace. Provision for regulation by the operator and the elevator blower for the secondary air line are shown. SILICA BRICKS.* By M. Bied. Recent experiments with the various fluxes employed in the manufacture of silica bricks have revealed the quite unexpected fact that notable quantities of iron oxide do not sensibly lower the fusing point of silica, even when lime is present. Flat, cylindrical test pieces were used, 50 mm. in diameter and 30 mm. in height, formed by pressure in a cast iron mould, and baked at a temperature fixed by comparison with Seger cones. By the advice of M. Le Chatelier, the first fluxes tried were sodium salts, then alkaline clays—particularly glauconite and ferri-potassic silicate. It was in substitu- ting mixtures of iron oxide and alkalies, then of iron oxide and lime for glauconite, that the author noticed the very small influence of iron on the fusing point of the bricks. In the first experiments, a mixture of 75 parts of Piolenc sand and 25 parts of roasted pyrites gave, after an hour’s baking at 1,500 degs. Cent., perfectly hard test pieces, with a 4 per cent, expansion; but the same mixture, with sodium silicate solution (43 degs. Baume) as binding medium and baked under the same conditions, gave test pieces which fused completely. Assuming the fusing point of Piolenc sand to be 1,750 degs. Cent., it is evident that this temperature is not lowered 10 degs. for every 1 per cent, of iron oxide added.. In order to ascertain whether lime had as mischievous an action as the alkalies in presence of iron, the author compared the two following mixtures, in one of which was incorporated about 2 per cent, of CaO, and in the other the same proportion of Na2O. Piolenc sand.......... 91 ... 91 Roasted pyrites _____ 9 ... 9 Hydraulic lime (Teil) ... 4 ... — Sodium silicate ....... — ... 10 After an hour’s baking at 1,500 degs., the first mixture furnished a perfectly sound piece with sharp edges, and an expansion of 3*8 per cent. The second mixture, on the contrary, showed incipient vitrification, and the expansion was only 0’8 per cent. Six new series were then made with varying propor- tions of iron oxide and of lime as follow:— Souvigny quartz... 100 ... 100 ... 100 ... 100 ... ICO ... 100 Roasted pyrites ... 3... 3... 4... 4... 5... 5 Teil lime.......... 0... 2... 0... 2... 0... 4 Results after firing at 1,450 degs. Hardness ........No ... Yes ... No ... Yes ... No ... Yes Expansion........ 3’8 ... 3’0 ... 4’0 ... 2’0 ... 4’0 ... 3’0 After firing at 1,700 degs.:— Hardness ........No ... Yes ... No ... Yes ... No ... Yes Supplementary ex- pansion ........ 4’0 ... 2’8 ... 1’8 ... 4’4 ... 1’8 ... 3’0 Iron alone is therefore not a binding agent, as it does not allow of the formation of a tridymite network; so it is necessary to add lime as well. Determinations were afterwards made of the influence of the iron and lime mixture on the fusing point, with the following results:— Souvigny quartz....... 100 ... 100 ............ Roasted iron pyrites... 3 ... — Teil lime ............ 3 ... 2 Fusing point.......... 1,725 degs. C. ... 1,730degs. C. Under the same conditions of heating, a good German brick (the Stella brand) crumbled at 1,730 degs. Cent. These experiments prove that an addition of 3 per cent, of iron oxide lowers the fusing point by only 5 degs. Cent., that is, to an extent hardly exceeding the limit of experimental error. A brick containing iron when exposed for several days in the arch of a crucible furnace, i.e., at about * Comptes Rendus de FAcademie des Sciences.