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equal for both small and large mines; while the small mine is favored with regard to depreciation of equipment and interest on capital invested. If the mine is a shallow one, the fuel account is also in favor of the small mine—as a rule, not because of any great effort on the operator's part.
In the small mine the principal savings are in labor, supplies, equipment, and capital, because the working places are closer together, thus concentrating the working force. Fewer mine cars, less mine trackage, fewer entries and air courses are required and need be maintained, and the surface equipment is cheaper.
Concentration of the working force brings the mine labor under close supervision; and where fewer men are employed, all work to better advantage. The movement of cars is rapid and the miners load a good turn, producing a greater tonnage per unit of area opened. Concentration and close supervision result in the operation of the mine on a very small capital investment per ton of production, the charge for interest and depreciation of equipment being proportionately low.
Under close supervision supplies are not wasted, misused or lost, in many instances materials being used over and over again. Entries are usually worked out during the life of one set of timbers so that the expense of retimbering is obviated.
The operator of a small mine has a keen regard for the inroad on his profits that even one extra man on the payroll would produce. He realizes that much valuable time is wasted in the mine—some of it due to laziness, but more to misdirected energies and lack of or improper planning, so that the men are obliged to wait for material, tools, or cars. The chances are, in a small mine, that if a place is to be timbered, a fall is to be cleared up, or a room switch to be put down, all of the material is on the spot before the man arrives to do the work. The different working places are close together, therefore the workman does not waste much time in going from one place to another. In this way one tracklayer can do the work of two men who move long distances between jobs, and must look over the place, order their own material, and probably wait for its arrival.
At the rate of $3 per day, and working 200 days in a year, a man receives $600 per year; in 10 years he has received $6,000. During that time he has been a liability, because of the possibility, through carelessness on his or some other miner's part, of an accident or fatal injury. Any mechanical device, which might supplant a man, the first cost and upkeep of which would be less than $6,000 in 10 years, would be a good investment. The mechanical device will not lay off or quit; this precludes the necessity for training new men on the job, often a costly operation. The machine does not come under any workman's compensation act; it does not strike nor get out of repair as often as the human machine (provided it is properly designed and cared for).
Some of the comparisons made are unfair, but inasmuch as the small mine usually has a limited acreage and frequently shallow coal, it follows that the mine will never have many long entries, that the haul will be short and the work, while it lasts, will be concentrated.
Reverting to basic principles, the economies in a small coal mine result almost solely from close concentration of the working places and a still closer supervision of the labor employed.
Metallurgical Practice in the Witwatersrand District, South Africa
Discussion of the paper of F. L. Bosqui, presented at the San Francisco meeting,
September, 1915, and printed in Bulletin No. 101, May, 1915, pp. 997 to 1033.
H. A. WHITE, Springs, Transvaal (communication to the Secretary *). -It is, as the author points out in his valuable paper, a fact, that in many plants the "trommel washings” are allowed to become eyesores, but the simpler system of pumping the entire product direct to the tubemill circuit will avoid the extra expense of separation and small-scale treatment when one considers the relative distance of the points and the proportionately small amount of water.
It is a matter of record that in stamp batteries the increase of weight has been developed concurrently with the increased use of tube mills, but independently and on its own merits. The satisfactory duty, excellent running time, and freedom from breakages in the newer mills, such as the Modder Deep, with 2,000-lb. stamps, will probably go far in removing some of the conservative prejudices mentioned by Mr. Bosqui.
The success of the City Deep trial of the Nissen stamp cannot be entirely attributed to better feed distribution and discharge effects (published experiments on two-face discharge mortar boxes showed no advantages). Attention is directed to the treatment costs given by the author both before and after the increased capacity resulting from the addition of 16 Nissen stamps at Modder B. It will be observed that the only item of cost resisting the tendency to reduction caused by increased tonnage is that of milling, which has risen 1.3c.
With reference to the provision for amalgamation in the new plans, it seems obvious that a great loss of head would be eliminated by passing the tube cone overflow over a separate top series of plates; three at the top and three at the bottom will effect the maximum amalgamation. This plan is followed at the Princess Estate and Gold Mining Co., Ltd., Modderfontein Deep Levels, Ltd., and Geduld Proprietary Mines, Ltd., but elsewhere reliance is placed upon ample cone capacity and the underflow only is passed over three plates after going through the tube mill, while the overflow is passed direct to the cyanide works; both methods avoid the largely unnecessary elevation of considerable quantities of water, fine sand and slime involved in Mr. Bosqui's scheme.
The most economical use of the tube mill is still receiving considerable attention though many points have been eliminated from the field of controversy. The running speed favored by Mr. Bosqui (28 r.p.m. for the 5 ft. 6-in. tube) is very close to the point of maximum production of – 90 grade per horsepower-hour consumed. There is, however, another interesting maximum, which is that of capacity or greatest production of -90 material per tube. This speed, 31 to 32 r.p.m., is the more likely to be adopted as it involves but little waste of power, which is, after all, not the only item of cost.
* Received Nov. 17, 1915.
The latest Osborne bar liner coming largely into use has the upright bar held in position on top of instead of between the flat bars, as illustrated in this paper. This allows longer life for the more expensive radial steel bars and the reduction in crevice capacity is reflected in the saving of amalgam caught therein. We are still looking for the ideal liner which must be cheap and durable, have a good grip on the pebble load, be easily and quickly renewed, and cause no locking up of gold in the form of amalgam, in crevices.
The tube-mill scoop has now passed the experimental stage and is giving satisfaction, but it would be interesting to know whence the author derives the “sloping line of discharge” from inlet to scoop. The level of pulp is surely as horizontal as in the old tubes, though the height is considerably reduced. The "whole hog” peripheral discharge had only one defender at the early date referred to.
The scheme presented for collection and treatment of sand in fulfilment of the author's undertaking to "evolve a simpler method than the filter table” suffers from even graver disadvantages than the other well-known efforts to eliminate the table itself (the patented feature) from the whole system used under that title.
Mr. Bosqui's idea of treating a charge with cyanide solution in the same vat in which it is collected by water, re-introduces the identical defect which finally caused the abandonment of double treatment on the Rand, referred to by Mr. Bosqui. In that case also, the consequent contamination of the mill water by cyanide, and the inevitable, continuous loss of gold was aggravated by the occasional accidental heavy losses caused by allowing the cyanide solution to run into a vat still being filled by water-borne sand. These losses were doubtless better known than advertised; nevertheless the re-introduction of such possibilities is a retrograde step in defiance of the more modern practice which encourages only one method of getting cyanide solution into the mill water-stream, i.e., with a bucket. The new plants of the Barnato group now working, such as the Van Ryn Deep, Ltd., keep the collecting and treatment vats separate and double wash the sand to free it from colloidal slime, getting a better effect from the added water than can be possible in the single-stage hydraulic separation advocated in this paper.
Why this conservatism with regard to the sand filter table? True,
1 Journal of Chemical, Metallurgical and Mining Society of South Africa, vol. vii, p. 371 (1906–1907).