CHAPTER XV.
Ratio of Output to the Mine.
| DETERMINATION OF THE POSSIBLE MAXIMUM; LIMITING FACTORS; COST OF EQUIPMENT; LIFE OF THE MINE; MECHANICAL INEFFICIENCY OF PATCHWORK PLANT; OVERPRODUCTION OF BASE METAL; SECURITY OF INVESTMENT. |
The output obtainable from a given mine is obviously dependent not only on the size of the deposit, but also on the equipment provided,—in which equipment means the whole working appliances, surface and underground.
A rough and ready idea of output possibilities of inclined deposits can be secured by calculating the tonnage available per foot of depth from the horizontal cross-section of the ore-bodies exposed and assuming an annual depth of exhaustion, or in horizontal deposits from an assumption of a given area of exhaustion. Few mines, at the time of initial equipment, are developed to an extent from which their possibilities in production are evident, for wise finance usually leads to the erection of some equipment and production before development has been advanced to a point that warrants a large or final installation. Moreover, even were the full possibilities of the mine known, the limitations of finance usually necessitate a less plant to start with than is finally contemplated. Therefore output and equipment are usually growing possibilities during the early life of a mine.
There is no better instance in mine engineering where pure theory must give way to practical necessities of finance than in the determination of the size of equipment and therefore output. Moreover, where finance even is no obstruction, there are other limitations of a very practical order which must dominate the question of the size of plant giving the greatest technical economy. It is, however, useful to state the theoretical considerations in determining the ultimate volume of output and therefore the size of equipments, for the theory will serve to illuminate the practical limitations. The discussion will also again demonstrate that all engineering is a series of compromises with natural and economic forces.
Output giving Least Production Cost.—As one of the most important objectives is to work the ore at the least cost per ton, it is not difficult to demonstrate that the minimum working costs can be obtained only by the most intensive production. To prove this, it need only be remembered that the working expenses of a mine are of two sorts: one is a factor of the tonnage handled, such as stoping and ore-dressing; the other is wholly or partially dependent upon time. A large number of items are of this last order. Pumping and head-office expenses are almost entirely charges independent of the tonnage handled. Superintendence and staff salaries and the like are in a large proportion dependent upon time. Many other elements of expense, such as the number of engine-drivers, etc., do not increase proportionately to increase in tonnage. These charges, or the part of them dependent upon time apart from tonnage, may be termed the "fixed charges."
There is another fixed charge more obscure yet no less certain. Ore standing in a mine is like money in a bank drawing no interest, and this item of interest may be considered a "fixed charge," for if the ore were realized earlier, this loss could be partially saved. This subject is further referred to under "Amortization."
If, therefore, the time required to exhaust the mine be prolonged by the failure to maintain the maximum output, the total cost of working it will be greater by the fixed charges over such an increased period. Conversely, by equipping on a larger scale, the mine will be exhausted more quickly, a saving in total cost can be made, and the ultimate profit can be increased by an amount corresponding to the time saved from the ravages of fixed charges. In fine, the working costs may be reduced by larger operations, and therefore the value of the mine increased.
The problem in practice usually takes the form of the relative superiority of more or of fewer units of plant, and it can be considered in more detail if the production be supposed to consist of units averaging say 100 tons per day each. The advantage of more units over less will be that the extra ones can be produced free of fixed charges, for these are an expense already involved in the lesser units. This extra production will also enjoy the interest which can be earned over the period of its earlier production. Moreover, operations on a larger scale result in various minor economies throughout the whole production, not entirely included in the type of expenditure mentioned as "fixed charges." We may call these various advantages the "saving of fixed charges" due to larger-scale operations. The saving of fixed charges amounts to very considerable sums. In general the items of working cost alone, mentioned above, which do not increase proportionately to the tonnage, aggregate from 10 to 25% of the total costs. Where much pumping is involved, the percentage will become even greater.