TABLE 2.- Mineralogical association of lead in materials tested.

The sample from the Horn Silver dump was the only one that contained any noteworthy proportion of lead sulphate; hence the percentage of total lead present as sulphate was not tabulated. In this particular sample the percentage of the lead present as sulphate was 45 per cent. The figures on sulphide lead are based on the proportion of lead insoluble in acidified ammonium acetate solution. According to this criterion a saturated solution of salt, acidified with sulphuric acid, will dissolve considerable quantities of lead sulphide. Small amounts of lead sulphide were present in nearly all the ores tested, although they are classed as carbonate ores.

The amount of this low-grade material available is hard to estimate, because, as a rule, much low-grade ore has been left underground in mining the higher grade ore, but is known to be very large. Each of the tailing dumps mentioned contains many thousands of tons and there are known to be many similar dumps in all of the western base-metal mining States. The amount of low-grade ore left underground in the Tintic district of Utah is very large, as the zone of oxidation extends to a depth of at least 1,500 feet, and is known to be much deeper in many mines.

As the lead carbonate is often so intimately associated with the oxidized gangue minerals that it is practically impossible to separate the particles of lead carbonate from the gangue except by fine grinding, with loss of lead in the slimes, and as many lead carbonate ores refuse to respond to flotation processes that are adapted to slime treatment, hydrometallurgical or pyrometallurgical methods are the only ones that can be applied to many of the carbonate ores. Where sulphidizing and flotation can be applied, it will be of great value in the treatment of these ores, as many of them yield only a small proportion of their gross value to gravity methods of concentration.

METHODS OF TREATMENT.

Hydrometallurgical, pyrometallurgical, and flotation methods have been worked out for the treatment of the low-grade ores, slimes, and tailings, and are included in this report.

The hydrometallurgical method depends upon the fact, previously hinted at, that lead chloride and lead sulphate are soluble in a saturated solution of sodium chloride, and that lead carbonate and lead oxide also can be dissolved if the brine is acidified with either sulphuric or hydrochloric acid." Acid-brine solutions could be applied to any material that does not contain too much acid-consuming gangue to prevent practical application of the method. From such solutions it has been found practicable to recover the lead by electrolytic precipitation, as metallic lead, or by the use of lime, as hydroxide of lead. These facts, which seemingly permit a simple process for the recovery of lead from such low-grade ores, form the basis of one of the proposed processses.

If such an ore contains silver in forms other than the chloride the brine solution will not dissolve the silver and some other step will have to be added to the treatment, such as a chloridizing roast to precede the leaching. This point has not been worked out to the same extent that the leaching of the lead has, for the reason that the methods of chloridizing silver are already well known. Only a few tests have been run to see whether the lead and the silver could be leached simultaneously after a chloridizing roast for the silver.

The flotation of argentiferous lead carbonate ores has been extensively tested, and on the ores to which it is adapted the results are encouraging. In order to make the particles of lead carbonate float from a pulp of the ground ore it has been found necessary to form a film of lead sulphide over them by introducing soluble sulphides such as hydrogen sulphide or sodium sulphide into the water in which the ground ore is suspended. The lead carbonate particles, by interaction with this material, are superficially converted into sulphide of lead. These particles can then be caused to enter the froth of a frothing-flotation process. For the ores containing silver in forms that are insoluble in brine, flotation is also a logical procedure. Most of the insoluble silver minerals are sulphides and are especially amenable to flotation. Furthermore, if the lead is only partly oxidized and is partly soluble in brine the natural sulphides of lead might be floated with the artificially filmed particles. Hence, there seems to be a definite field for each of the methods described.

a For a preliminary discussion of this subject, see Salt in the metallurgy of lead, by O. C. Ralston, C. E. Williams, M. J. Udy, and G. J. Holt, Am. Inst. Min. Eng. Bull. 128, August, 1917, pp. 1205-16.

SCREEN ANALYSES OF MATERIALS CONTAINING LEAD CARBONATE.

Returning to a consideration of the properties of the materials in question, the results of a number of screen analyses are given in Tables 3, 4, and 5. It is easy to see why the statement is often made that such materials tend to slime too much, because the percentage of lead in the fine sizes is very large compared to that in the coarser sizes. The tailing from the Wilbert dump contains some lead in all the sizes, indicating that the degree of crushing necessary to liberate all the lead has not been attained.

39094°-18-Bull. 157-2

TABLE 4.-Results of screen analysis of material from Horn Silver dump. a

TABLE 5.—Results of screen analysis of material from Wilbert dump. a

At the time the experimental work on lead carbonate ores was initiated little was known of the solubility of lead chloride or of lead sulphate in strong brines, and hence the first work done was to endeavor to convert the lead of the ore into lead chloride, which is known to be soluble in hot water.

The solubility of lead chloride, as given by Landolt and Börnstein," is shown in Table 6. A curve plotted from the figures given in Table 6 is shown in figure 1.

If a cheap method of conversion of lead carbonate into lead. chloride can be found, as well as a cheap source of heat for heating water, the number of tons of solution necessary per ton of ore to leach out the lead from most of the ores tabulated would not be large

• Experiments by C. Y. Pfoutz, C. L. Larson, M. J. Udy, H. C. Neeld, C. E. Sims, G. J. Holt, F. G Moses, J. F. Cullen, C. E. Williams, and O). C. Ralston.

Landolt, H. H., Landolt-Börnstein, physikalische-chemische Tabellen, 1905, 3d ed., p. 564,