TABLE 26.-Roasting and leaching partly oxidized zinc ore, Lower Mammoth No. 2.

With neither sample did it seem possible to roast the ore to advantage under the conditions that usually give good results in the treatment of zinc sulphide. The oxidized zinc minerals were so intimately associated with the oxidized iron minerals that the formation of ferrites in roasting was suspected; therefore a series of roasts were made to determine whether more careful roasting would take care of this difficulty. A quantity of ore was placed on a roasting dish and put into a cold muffle furnace. The temperature was gradually raised and the ore was rabbled intermittently. Grab samples of the calcines were taken at intervals, and these were treated with dilute sulphuric acid to determine the total amount of soluble zinc.

The effect of temperature on the solubility of the zinc is shown by the upper curve in figure 8, where the gradual temperature rise is plotted on one axis and the percentages of soluble zinc, as determined by the analysis of the solutions from the grab samples, on the other. At the same time, observations were made of the amounts of soluble iron. The sharp peak in the solubility curve for zinc illustrates how extremely sensitive this ore is to heat treatment. Heating the ore much above 700° C. seemed to cause the formation of insoluble zinc compounds, probably ferrites. The great amount of work, by other investigators, on the formation of zinc ferrites points to their being the cause of this decrease in solubility. All past experience has shown that in roasting such ore, zinc ferrites commence to form at a temperature slightly above 700° C. The increase in solubility up to that point is doubtless due to the desulphurization of the zinc sulphide present. This ore roasts rather slowly until a temperature above 600° C. is reached, and then, later, slight overheating above 700° C.

may spoil the calcine for leaching purposes. This is the most prominent instance of the formation of ferrites with which the writers are familiar.

The solubility curve for the iron is even more interesting. As is well known, pyrite becomes more soluble in acids on heating to a

FIGURE 8.-Curves showing effects of roasting on solubility of zinc and iron
in semioxidized zinc ore from Lower Mammoth mine.

temperature hot enough to drive off some of the sulphur. Thus the rising solubility of the iron may be due either to this heating or to the partial desulphurization of the pyrite. However, the greater proportion of the iron in this ore is present as oxidized minerals, and these are rendered less soluble in dilute acids by heating. The downward trend of the curve at temperatures above 400° C. is

doubtless due to this cause. The sharp up-turn beyond 850° C. was unexpected and the writers have been unable to explain it. However, as 850° C. is much above the permissible temperature for roasting zinc calcine, this latter part of the iron curve is of little practical importance.

MARMATITE ORE FROM WASA MINE.

Further tests were made with the marmatite ore from the Wasa mine, near Hall, Mont., by roasting and leaching with sulphuric acid. This ore, referred to in previous pages, contained a small amount of "rosin-jack," but the greater proportion of the zinc sulphide was black. The black zinc sulphide appeared homogeneous to the eye, even under the microscope, yet the purest crystal that could be selected contained 55 per cent Zn and 11.5 per cent Fe. Most of the crystals contained more than 16 per cent Fe. An analysis of this ore was as follows: Zinc, 12.9 per cent; iron, 20.72 per cent-insoluble, 38.10 per cent; lime (CaO), 1 per cent; copper, 0.20 per cent; and lead, 0.30 per cent. Another analysis has been given in Table 19 (p. 69). The excess of iron is present chiefly as pyrite, with a small amount of pyrrhotite. As marmatite is supposed to be a solid solution of iron sulphide in zinc sulphide, the assumption may be made that roasting the zinc sulphide to the oxide without some zinc ferrite being formed along with the iron oxide would be a difficult matter.

The results of a series of roasts and leaches made in the laboratory at the Salt Lake City station by Walter Neal, representing McKeever Bros., under a cooperative agreement with the Bureau of Mines, are given in Table 27. For some reason the analysis of the solution for zinc gave unusually high titrations; hence, the recoveries, as calculated from the weight of zinc in the solutions, are disregarded. The tailing assays show that not more than about 75 per cent of the zinc could be recovered. However, this recovery was higher than was expected from a marmatite ore. On the assumption that the formula of zinc ferrite is ZnFe,O,, every unit of iron is combined with 0.58 unit of zinc. The flotation tests of this ore show that the purest zinc concentrate that could be made by roasting the pyrite to a nonflotative form contained 49 per cent Zn and 20 per cent Fe. Hence there is enough iron in the average marmatite particles to account for 23 per cent of the total zinc in the ore, provided that all of the marmatite iron is converted into ferrite iron by roasting, and that the ferrite of zinc is insoluble in the dilute acid solutions used,

The highest recovery of zinc, under the best conditions of roasting and leaching, was 74.7 per cent, or 25.3 per cent of the total zinc was insoluble. This latter figure compares closely with the 23 per cent calculated as ferrite. Consequently, the writers feel justified in stating that when marmatite ore is roasted, most of the iron present