With this possibility in view, some of the ores were treated with dilute solutions of hydrochloric acid, followed by washing with hot

60 TEMPERATURE,°C.

water. Only small quantities of ore were treated in these tests, 35 grams being the usual amount. It was found that the lead could be extracted in this way. The next step was

to use brine to which sulphuric acid had been added in order to generate hydrochloric acid, although it was felt that this would be a failure, as the sodium sulphate resulting from the reaction of sulphuric acid on sodium chloride should precipitate lead sulphate, if no soluble double salts are

FIGURE 1.-Curve showing solubility of lead chloride (PbCl2). formed. However, the

After Landolt and Börnstein.

brines leached out the lead

in spite of their containing sulphates in solution. As the final leaching solution to be adopted was a saturated solution of sodium chloride, the results of these tests are briefly summarized in Table 7 following. The ore treated was from the May Day mine of the Tintic district, Utah, and contained about 5 per cent lead, and some silver. In the first series of tests shown in this table two variables are involved the concentration of the acid used, and the amount of acid relative to the weight of ore. The time of contact with the material was of sufficient length to obliterate any effect of the acid concentration. Thus the results of this series of tests show the effect of the relative amount of acid on the extraction of the lead and on the acid efficiency. By acid efficiency is meant the amount of acid theoretically needed to convert all of the lead to chloride, compared

to the amount of acid actually consumed. In other words, if one equivalent of lead is extracted by an expenditure of acid equivalent to twice that amount of lead, the acid efficiency is 50 per cent. The acid efficiency was calcuated in each test by observing the number of grams of lead extracted, and also the number of grams of acid used up by the ore. One gram of lead theoretically requires 0.3125 gram of HCl to convert it to chloride. If twice that amount of acid was consumed by the ore for every gram of lead extracted, the acid efficiency was (0.3125÷0.6150) x 100 = 50 per cent.

TABLE 7.-Results of leaching oxidized lead ore from May Day mine with dilute solutions of hydrochloric acid.

The first series of tests (series A) showed that hardly enough acid had been used. Series B was run with slightly more acid, and the time of contact of the acid with the ore was varied from 1 hour to 48 hours. The results of this series of tests shows that a fairly good extraction can be obtained in a few hours, but that the acid efficiency increases slowly. This latter result is difficult to interpret, as it seems to indicate that after 48 hours more acid is left in the solution than after 24 hours from identical charges. Regeneration of acid is not probable. There is no method of determining accurately the acid content of a solution containing metals such as iron and lead, and it is probable that in the longer tests more of the material that affected the end point in titrating the acid had gone into solution. As much more important and more nearly commercial results were obtained shortly afterwards with the use of brine for leaching, this point was not investigated further.

In the third series of tests, series C, the same weight of acid relative to the weight of ore was used, but was diluted with different amounts of water and the concentration of the acid varied accordingly. Seem

ingly the more dilute acid converted a greater proportion of the lead to chloride and with a greater efficiency of acid.

The discovery that the brine solutions used in one or more tests were more efficient solvents of lead chloride was coincident with several other lines of evidence learned of at about the same time. The mills of the Holt-Dern Co. in Park City, the Knight-Christensen Co. in Silver City, and of the Bunker Hill and Sullivan Co. in Kellogg, Idaho, were visited. In each of them the use of brine solutions for other purposes had revealed the fact that the lead chloride formed in the particular processes used was soluble in brine solutions to a greater extent than in water. At the Bunker Hill plant it had been

FIGURE 2.-Curves showing solubility of lead chloride in brine.

found that sulphate of lead was likewise soluble in brine. About the same time Demassieux a published the results of measurements of the solubility of lead chloride in solutions of salt. His results are given in the curves of figure 2.

The curves show that the solubility of lead chloride in water is at first decreased by small additions of sodium chloride, owing to the well-known "common ion" effect, but that after about 5 per cent of sodium chloride has been added, the solubility of the lead chloride begins to increase within the range of temperatures shown (0° to 100° C.), to the point of saturation of the solution with both PbCl and NaCl, and finally falls to zero again with a solution saturated with NaCl. The increase in solubility in such solutions is usually

a Demassieux, N., Equilibrium between lead chloride and sodium chloride in aqueous solutions; Chem. Abs., vol. 8, 1914, p. 1899.

ascribed to the formation of a double salt of the two substances involved. No attempt was made to determine the formula of this double salt, as the increased solubility is the important fact in these experiments, the character of the compound formed being chiefly of scientific interest.

The curves in figure 2 show that at any temperature between 0° and 100° C., the solubility of the lead chloride is greatest in the solutions that are more concentrated in sodium chloride. Consequently all brines used as leaching solutions in further experiments were made nearly saturated.

USE OF ACIDIFIED BRINE FOR LEACHING.

FIRST SERIES OF PRELIMINARY TESTS.

To verify the fact that lead chloride was soluble in saturated brine, three different carbonate-of-lead ores were treated with brine to which had been added either sulphuric or hydrochloric acid. Five tests of each ore were made. In the first three tests of each ore increasing amounts of sulphuric acid were added to the brine; the other two tests were with hydrochloric acid. Sulphuric acid reacts ⚫ with the brine to form hydrochloric acid and sodium sulphate, and it was thought that the sodium sulphate would decrease the solubility of the lead chloride by tending to precipitate insoluble lead sulphate. It was found that sodium sulphate has no such effect unless a considerable proportion is present, and later tests of the solubility of lead sulphate in brines showed that as a rule the addition of sodium sulphate in proportions ranging up to about 2 per cent of the weight of the solution tended to increase the solubility of the lead sulphate. This is a curious chemical anomaly.

As all of the ores tested contained more or less silver, and some of it was known to be present as chloride, the leaches were tested for that metal. The results showed that some silver was being leached. In each charge 100 grams of 10-mesh ore and 1,000 c. c. of saturated brine (1,210 grams), into which the acid was measured, were used. The charges were agitated in 2.5-liter acid bottles for 24 hours. In the first three tests with each ore the theoretical, three times the theoretical, and five times the theoretical amount of sulphuric acid, respectively, needed to convert the lead carbonate to sulphate were used. In the fourth and the fifth tests the theoretical and five times the theoretical amount of hydrochloric acid needed, respectively, to convert the lead carbonate to lead chloride were used.

The results are shown in Table 8. Excellent extraction of the lead is possible if sufficient acid is applied, but only part of the silver is extracted. The efficiency of the acid is lowest with the ores that contain the largest amount of soluble gangue materials, such as lime,

alumina, and iron. The best acid efficiency obtained with the Chief Consolidated ore was 24.6 per cent, that is, of all the acid consumed only 24.6 per cent did useful work in leaching lead, the rest of the acid being wasted on gangue materials.

As regards the action of hydrochloric acid and that of sulphuric acid, there did not seem to be any great difference except in a few instances. Thus tests Nos. 1 and 4 on the ore from the May Day mine apparently showed that a high extraction of lead was possible with one equivalent of hydrochloric acid, whereas one equivalent of sulphuric acid gave only a low extraction. With five equivalents of acid present the action seemed to be the same. Considerable care was taken in making up the samples, but it is possible that some error crept into one of these determinations, as there is no such marked discrepancy in the other results. Former experiments by Ralston and Gartside a had shown definitely that there was no difference in the action of the two acids in the leaching of zinc from oxidized ores. Most of the data in Table 8 tend to show the same result for the lead carbonate ores, although there seem to be some minor differences, probably due to errors in the analytical work.

TABLE 8.-Results of leaching lead carbonate ores in acidified brine.

a Ralston, O. C., and Gartside, A. E., Leaching a zinc-lime ore with acids: Met. and Chem. Eng., vol. 13, 1915, pp. 151-155.

In tests 1, 2, and 3 of each series the amount of sulphuric acid used was, respectively, the theoretical, 3 times the theoretical, and 5 times the theoretical amount; in tests 4 and 5 the amount of hydrochloric acid used was the theoretical and 5 times the theoretical.