May 21, 1915. THE COLLIERY GUARDIAN. 1067 pressed air to the mines is normally between 100 lb. and 1101b. per sq. in., with a maximum of 120 lb.-and a minimum of 901b. per sq. in. At the higher pressure the pipe system-holds about 105 (short) tons, and at the lower pressure about 82 (short) tons of air. This means that without exceeding the contract limits of pressure, no less than about 23 (short) tons, or 46,000 lb. of air, can be drawn from the pipe system. In other words, the whole compressing plant could be shut down for several minutes -at the times of maximum demand, or one of the smaller compressors (3,500 kw.) for over half-an-hour, without materially affecting the supply. It was anticipated that one of the advantages which would be gained by the general use of power which was supplied through meters from a central source would be the exact information obtained of the relative costs of power for the various services on the different mines, and that economies would result from-such knowledge. While this anticipation has in the ease of electric power been realised to a certain extent, in the ease of com- pressed air power the result has been most marked, since, for the first time in engineering practice, reliable air meters have been used to measure the supply. Compressed air is used chiefly underground, and for (d) operating rock drilling machines, (b) working small winches, and (c) blowing out the workings after blast- ing has taken place. As most of the mines have many working levels, and the “stopes” (or working rock faces) are frequently at considerable distances from the shafts, it will be evident that many miles of air pipes are required in most of the mines, while the pipe joints will number many thousands. It is very difficult to keep the joints of underground air pipes tight, and when it is realised that at the barometer pressure on the Rand (24-5 in.) an orifice of only in. diameter will pass 450 lb. of air per hour at 1001b. pressure, or 16 air units, some idea of the cost of leakages will be obtained. By inserting air meters in the various pipe lines, and measuring the flow at times when it was known that no work was being done by the air, many hitherto unknown leaks were dis- covered, some of considerable magnitude. These were rectified, and now a systematic inspection of all air pipe lines and joints is periodically made, while the daily readings of the various air meters serve as a general check. Rock Drills. There are about 3,500 compressed air rock drilling machines in daily use in the mines of the group. They are all of the percussion type, as the rock is so hard that no rotatory drill has so far proved practicable, and are of two classes, viz., piston drills and hammer drills. Piston machine drills are generally much heavier than hammer machine drills, and each class has its own special qualities, which fit in for certain positions and types of work. They each have a similar property of wasting air unless they are well made and well looked after. From the character of their work, rock machine drills are subject to very rough usage, but will operate success- fully even with the pistons and cylinders badly worn, the only result, and one which does not appeal to the miner, being an abnormal consumption of air for the work done. Under these circumstances, and until means were available for accurately measuring the air, it was not surprising that the air consumption was very heavy. By developing a system of machine drill main- tenance, originally introduced at Village Main Reef Limited by Mr. W. Calder, the resident engineer, and applying it on all the mines of the group, very substan- tial savings have been made in the amount of air used by the drills, etc., which, coupled to the savings made by stopping leaks, etc., have had the effect of reducing the air units per equivalent work done from 100 per cent, in 1911 to about 63 per cent, in 1914. All new machine drills of, say, 3|in. standard bore, are now ordered with cylinders bored 3 in. diameter, and with pistons 3| in. diameter. As the cylinders and pistons wear, the former are bored out in steps of in. and the latter are ground down in similar steps. The new drills, with small cylinders, are first fitted with old pistons which have been reduced to the limiting minimum diameter, while the new pistons are first fitted into old cylinders which have been bored out to the limiting maximum diameter. The same procedure is adopted with the valve chests, valves, etc. Each drill, whether it apparently requires examina- tion or not, is brought to the surface in turn at short periods, and taken to the drill shop. It is there thoroughly overhauled, all defective parts are made good, and particular attention is paid to the cylinder, piston, and valve. The cylinder (or valve chest) of any drill in which the piston (or valve) is more than ^^in. slack is reamered out to the next standard size, and is fitted with a piston (or valve) which has been ground down to suit. Thus, as the cylinders and valve chests get larger and the pistons and valves smaller, they are changed about so as to work through the range of sizes, and by this means the parts have a maximum life, and once in about every three months are certain of fitting within yjp in. Before the drills are again passed for service they are tested at full air pressure with a meter in circuit, and any drill which takes more than a fixed quantity of air per hour is put back for further examination, and is not allowed to go into the mine until it is satisfactory. The drill fitter on each mine works under contract, and has to maintain all the drills up to a certain standard at a stated price for machine drill shift. The system has been in operation most successfully for over two years, to the advantage both of the drill fitter and the mine.* * For valuable information re rock drills, see paper on “ Air Consumption and Maintenance Costs of Rock Drills,” by Messrs. E G-. Izod and E. J. La-schinger, Proceedings of the South African Institute of Engineers, vol. 12, p. 81, 1913. Air is used in small quantities for a few winches and pumps, in places where it is not convenient to carry electric cables, and where pipes are already in existence, as well as for operating the doors of rock chutes, etc. For a short time each day fairly large quantities are used, after blasting, for blowing out the workings, particularly in dead ends and those parts of the mines where good ventilation is difficult. Much more atten- tion is now being paid to mine ventilation than in the past, and as the systems are made more perfect, less compressed air will be required for removing the blast- ing fumes. It must be remembered that the exhaust of every working machine drill is discharging fresh air into the mine. SETTLING TANKS FOR HYDRAULIC- PACKINC PLANT. Since pure sand is rarely employed in hydiaulic goaf packing, clarifying tanks are needed for the waste water draining out of the goaf ; and these tanks are preferably arranged underground, to obviate the wear and tear on the pumping plant occasioned by raising the effluent in its original condition. At the Ferdinand Colliery, Kattowitz, underground settling tanks of this kind have been in use for nearly 18 months in the 300-m. level, and have a capacity for clarifying 880 galls, of water per minute. The two tanks are each 20 m. long and 3 m. wide, covering a total area of about 130 sq. m. They are arranged side by side (being separated merely by a partition wall), and are topped by a vaulted roof on account of the considerable rock pressure. The plant, according to E. Steuer (Gliickauf), is intended to remove from the effluent all suspended matters likely to damage the pumps—namely, fine sand, clay and floating cinders (the goaf packing used being composed of clay, sand, broken pit waste and boiler ash and cinders). The crude effluent enters the tanks through two openings out of the gutter a. Near the inlet of each tank is a suspended sloping baffle b, under- neath which are several ribs, for the purpose of breaking the flow of the incoming water and distributing same uniformly across the surface. The baffle also serves to retain large floating impurities. As the effluent flows through the tanks at the low rate of about | in. per second, towards the outlet c, the solid impurities in Fig. 2.—Longitudinal Section of Tank During Clarification. Fig. 3.—Longitudinal Section of Tank During Evacuation of Sludge. Fig. 4.—Plan of Settling Tanks. suspension are deposited, according to their specific gravity, in more or less flat curves on the ridged bottom of the tank, the ridges having an angle of about 60 degs. and bordering two deep longitudinal channels d (fig. 5) in each tank. These channels •gradually fill up with thick sludge, and at certain intervals are closed at the top by lowering the suspended beams shown in figs. 3 and 5, thus shutting off the thick sludge from the clearer supernatant water, except at the discharge,end. At the front end, each channel d is connected to a pipe e, which rises, outside the tank (figs. 2 and 3), to about 5 ft. below the water-level, and is fitted with a quick-closing valve f. At the open end of the channels d is a cylindrical piston g, movable horizontally between guides and attached to a rope, which runs upwards and over a pulley h. When the valve f is opened, the pressure of water in the tank acts on the piston g, which in turn forces the sludge out of the channel d and through the 8 in. pipe e, and at the same time prevents the sludge from being thinned by the following water. When the rope has run off to a certain extent over the pulley, the cylinder will have reached the end of its travels, where- upon the valve f is closed. The circumstance that the greatest deposition of sludge takes place in the front end of the tank can be counteracted by shaping the channel and piston accordingly (see fig. 3). The beams for closing the channels are mounted in pairs on pulleys, so as to counterbalance each other and be easily raised and lowered by a hand winch. Since the expulsion of the sludge from any one channel takes only three to five minutes from start to finish, tbe work of clarification proceeds without interruption, and the water level only sinks about an inch, so that the contents of the tanks suffer very little disturbance ; and at the end of another minute the clarified water begins to overflow again. At present the sludge (about 60,000 galls, per 24 hours) is run either into a flushing pipe leading down to the bottom level or into an old goaf, but it is intended later- on to discharge into the tank i (figs. 2-4), from which it will be forced by pneumatic pressure to a distance or to a higher level by connecting the tank to the compressed air main supplying the pneumatic drills. The floating material caught by the suspended baffles b (mostly porous cinders) is taken out by a rotary skimmer fc, about 100 cu. ft. being recovered in 24 hours. The plant, which together with the vaulting cost about £2,700, is attended to by one man, a pensioner, J IW................ $• I i II JlF - Fig. 1.—View of Tank Gallery. Fig. 5.—Cross Section through Tanks. A__a who also puts in a portion of his time at other work. No cleaning out is required, and the upkeep for the whole of last year cost only a few shillings, whilst the saving on new pump parts alone was equivalent to about £200. It is therefore evident that the plant fulfils its purpose, especially since the thick muddy water entering the tanks runs off perfectly clear to the pumps.