Men who worked in the mine during its previous periods of operation are usually more than willing to reminisce. Their stories are always worth a hearing even though they seldom lose glamour in the telling. No sooner does a mine close down than rumors begin circulating and the ore becomes richer with every year of aging legend.

Tin

The principal ore mineral is cassiterite (SnOs) although many of the Bolivian ores contain accessory tin-bearing sulphides such as stannite (Cu2FeSnS4), teallite (PbS.SnS) and franckeite (Pb5Sn3Sb2O14). In a few mines teallite is the principal ore mineral and from time to time handpicked teallite ore has been marketed in England. However, tin occurring in any of the sulphide minerals (known as "acid-soluble" tin) is not welcome at most smelters.

In a new prospect, costs are not a matter of record as they would be in an operating mine. In forecasting them there are two avenues of approach; one is by detailed estimate, the other is by comparison.

The amount that the producer receives from the smelter in payment for ore is likely to be very different from the gross value of the contained metal, a fact which many a shipper has learned only too late. The liquidation value ranges from as little as 30% of the gross in certain zinc ores carrying high freight charges to 75% or 85% for some copper and silver ores. Rarely, though, does it exceed 80%. The smelter must be compensated for costs and metallurgical losses, and it must earn a profit on its operations.

(by S. A. Tyler)

Throughout some districts the bottoms of the lowest oreshoots on the veins are points on an imaginary gently rolling surface; where the depths of some of such orebodies are known, the approximate positions of the bottoms of others can be predicted by projecting this surface. Thus in the Parral district, México, the orebodies (except those in the immediate vicinity of the city) bottom at a roughly constant elevation of 1700 meters, which means 50 to 300 meters below their outcrops.

Recovering a metal from its ore involves two kinds of processes. One is purely physical—the separation of the metallic mineral from its gangue. The other is chemical—breaking down the metallic mineral itself to recover the metal and get rid of the other elements that are combined with it. Both processes are illustrated by the usual steps in treating a galena ore that occurs in calcite gangue.

Limestone is used as a flux in at least small amounts in smelter charges of almost all 'types and in large proportions when the smelter charge would otherwise contain an excess of silica. Limestone for this purpose should carry as little silica as possible.

For iron furnaces, the sulphur and phosphorus content should be low; S should not exceed 0.5% although few limestones carry this much; P should be under 0.1% for basic iron and under 0.01% for low-phosphorus iron.

Purposes