Utilization of the ores

The minerals that are saved as ore concentrates or that are recognized in milling operations as considerable constituents of the waste are not numerous. The ore minerals are franklinite, willemite, and zincite, and the chief waste minerals are calcite, garnet, fowlerite, tephroite, zinc schefferite, and sphalerite.

Franklinite, the most abundant mineral in the ore, is removed from the crushed ore by magnetic separators, and the concentrate, shipped to the smelting works as franklinite, constitutes about 36 percent of the crude mineral ore. The concentrate from the Franklin mine contains 14 to 18 percent of zinc, 34 to 40 percent of iron, and 12 to 16 percent of manganese. The franklinite from Sterling Hill has about the same composition, but some of it runs higher in both zinc and iron, and it is generally lower in manganese. The silica rarely exceeds 3 percent. The variation in the composition of the concentrate is due partly to the wide range in the composition of franklinite itself and partly to tiny particles of various silicates that may still be attached to the grains of franklinite after the crushing. Franklinite is used in the manufacture of zinc oxide and of spiegeleisen.

Willemite is the most abundant constituent of the nonmagnetic part of the ore. It is separated from the waste minerals by treatment in jigs for the coarser material and on tables for the finer material. The willemite concentrate from the Franklin mine, which constitutes about 18 percent of the crude milled ore, contains 44 to 49 percent of zinc, 1.2 to 3 percent of iron, 4 to 6 percent of manganese, and 3 to 7 percent of lime. From willemite is made a high-grade spelter.

Zincite constitutes less than 1 percent of the crude ore and in the milling is separated with the willemite.

Besides these definite mineral concentrates, about 25 percent of the crude ore is comprised in products known as half-and-half, dust fines, and slimes. The first is an intermediate magnetic product consisting of mixed grains of franklinite and silicates; the other two are mill products and are, of course, not of simple mineral composition.

The minerals separated as waste in milling constitute at the Franklin mine about 20 percent of the crude milled ore. Calcite, more or less manganiferous, forms 92 to 96 percent of the tailings, and next in abundance is andradite garnet, which sometimes is scarcely appreciable and at other times forms as much as 2 percent of the crude ore. Part of it is separated with the magnetic fraction and part with the willemite concentrate, in both of which it is an undesirable impurity. Fowlerite and tephroite are rarely present in large amounts. Both are separated mainly with the willemite and are undesirable because they increase the manganese content of the concentrate. Zinc schefferite is rare at Franklin, but at Sterling Hill it is in places sufficiently abundant to lower the grade of the willemite concentrate. Sphalerite is rare at Franklin but is common at Sterling Hill, both disseminated in the ore and as a vein filling. It may at times amount to nearly 1 percent of the willemite concentrate, in which it is separated. As it is not reduced in the smelting process it can be regarded only as a waste product.

Another element of economic importance in the mineralogy of the ore deposits is the presence of minute amounts of compounds of lead and arsenic, as even a trace of lead in the concentrate is objectionable. Besides its sparse occurrence as native lead and in galena, lead is found in the very rare silicates margarosanite, barysilite, nasonite, calcium larsenite, larsenite, and roeblingite and in the arsenate hedyphane. Of more importance is its persistent presence in hardystonite, as that mineral is much less rare than the true lead compounds. The arsenic compounds, though rather numerous, are rare and local. They include the arsenides chloanthite, lollingite, and niccolite; the arsenates allactite, arseniosiderite, cahnite, chlorophoenicite, hedyphane, holdenite, and svabite; and the arseniosilicates schallerite and ferroschallerite and some of the friedelite.

All the minerals listed in the preceding paragraph are removed from the crude ore wherever possible, both in the mine and on the picking table. Their total amount in the ore as mined is inconsiderable, and such are the pains taken to check the lead content of the concentrate before smelting that the zinc manufactured from the ore is practically lead free.


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	This page created: January 12, 2001 7:20 PM