Stoping Width

The relation between the width of a vein and the stoping width necessary to extract the ore, should receive careful attention in the examination of narrow veins. The minimum stoping width for machine drills is usually from 4 to 5 ft. ; ihe new air-hammer drills require less. For hand drilling, 30 in., or sometimes even less, is required.

The sloping width and the amount of waste broken with the ore varies with the relative hardness of the ore and wall rocks, and also according to the firmness of the hanging wall, which, if loose, will contribute waste by caving. In some veins the waste may be shot down first and the ore broken down clean afterward. In the consideration of narrow vems the average amount and grade of ore must be dertemined as it stands, and the values, if any, that are carried by the wall rock; then, upon the assumption of a stoping width, the average value of the broken ore may be calculated. This average should be further corrected by the amount and value of the waste that may economically be sorted out, the final result being the grade that may be expected for the ore to be hoisted or shipped. These figures, in the case of an operating mine, may be checked by records of past production, where the volumes of material stoped can be determined, or where the amount and grade of the waste sorted out is known.

Hand Picking.—The results indicated by sampling are almost always diminished by slabs of wall rock that unavoidably become mixed with the ore. In deposits that carry heavy sulphides unevenly disseminated through the gangue, and in ores through which the values are irregularly distributed in such a manner that the richer portions are distinguishable to the unaided eye, the question of hand picking is of prime importance.

Hand picking is a very effective process, where properly arranged for and carried out on clean ore from which the fine material has been screened. Not only is the waste sorted out in hand picking, but clean mineral is saved as a high-grade product. The process yields two clean products, finished ore and clean waste, in one operation, and thereby saves not only mill capacity, and the cost of treating the waste, but also greatly reduces the metallurgical losses that the clean mineral would otherwise suffer. An inexpensive installation to permit efficient sorting will often result in the recovery of a payable product from an ore that could not otherwise be considered an asset.

Metallurgical Losses.—The question of metallurgical losses is it should receive careful attention, and in formal examinations an exhaustive series of tests may be necessary. The mineralogical character of the ore is the criterion in the field work, unless laboratory tests can be made. Before a plant is installed or a developed property purchased, however, large scale tests are advisable.