With complete extraction of the lead such a leaching solution would contain only 0.5 per cent of lead.

Two series of tests with decreasing amounts of brine were triedone with solutions containing 2 equivalents of acid and the other with solutions containing 1.5. The leaches containing less brine, therefore, had higher concentrations of acid, although the total amount of acid was either 2 or 1.5 equivalents in every test. As the results of the previous series of tests had shown little or no effect due to acid concentration, any effects shown by these two series were probably due to causes other than acid concentration. Leaches were successful down to the point where 400 c.c. of brine to 100 grams of tailing was used, when the lead extraction fell off slightly in the series containing 1.5 equivalents of acid. The acid efficiency was not affected. These solutions contained almost 1.2 per cent lead. Therefore 1 per cent lead was chosen as the probable maximum in practical work at temperatures of about 20° C., and many of the later experiments were so planned that there would always be 1 volume of cold brine for each 1 per cent of lead in the ore. Thus the Wilbert tailing, containing 5.5 per cent lead, could be leached with about 550 c. c. of brine for every 100 grams of ore. As the brine had a specific gravity of about 1.2 at room temperatures this rule will give, roughly, solutions containing somewhat less than 1 per cent of lead. Hence a pulp ratio slightly less than that given by the rule will still have a large factor of safety. In countercurrent decantation it would probably be safe to use pulp ratios that would give solutions more nearly saturated with lead.

TESTS OF MATERIAL FROM SCRANTON MINE.

The results of a series of tests on the Scranton ore, similar to the series on the Wilbert tailing, are recorded in Table 11. Unlike the results with Wilbert tailing, as good an extraction of the lead was made in one-half hour as in 24 hours, and the efficiency of the acid was the same throughout the series. This indicates that practically all of the chemical reactions liable to take place had been completed during the first half hour. As there are few types of modern agitating or leaching machinery that will receive and discharge the ore to be leached in less than a half hour, no tests were made for a shorter length of time. The acid requirements of this ore are such that a solution containing more than 1.5 equivalents of acid must be used before the maximum extraction of lead is possible. The brine requirements of the ore are shown in the third series of tests to be about 700 c. c. of brine to every 100 grams of ore. As the ore contains 8.65 per cent lead, the leaching solutions could be built up to a strength of 1.03 per cent lead under conditions that would give a maximum extraction of lead.

TABLE 11.-Results of leaching Scranton ore with acidified brine.

EFFECT OF SULPHATES IN BRINES.

If any leaching process is employed for removing lead by the use of saturated brine acidified with sulphuric acid, the building up of sodium sulphate in the solution as the result of the action of sulphuric acid on sodium chloride might reach a point where the `solubility of the lead chloride in the brine would be seriously affected.

PRELIMINARY TESTS.

In order to determine how much sodium sulphate would do this, a series of rough solubility tests were made in the laboratory of the Bunker Hill & Sullivan Mining & Concentrating Co. at Kellogg, Idaho, by W. J. Winninghoff. The results of these determinations are given in Table 12 following.

TABLE 12.—Results showing effect of sodium sulphate on solubility of lead sulphate in brine solutions.

The table shows that the solubility of the lead increased with additions of sodium sulphate ranging up to about 3 per cent to brines at ordinary room temperature and then fell off. The maximum solubility of lead in heated solutions (96.5° C.) was in brines uncontaminated with sodium sulphate. Tests at ordinary temperatures by the writers once resulted in a solution containing 1.8 per cent lead, which is very close to Winninghoff's figure, 1.75 per cent lead, when the amount of sulphur in the solution was about 0.646 per cent (equivalent to 1.94 per cent H2SO).

As small proportions of sodium sulphate in the brine do not impair the solubility of the lead compounds, sulphuric acid can be used for acidifying the brine, provided the sodium sulphate is not permitted to build up too high in the solution. All of the various tables of the solubility of sodium sulphate in brines that are available in the literature show that in saturated solutions of NaCl it is possible to dissolve only a small amount of sodium sulphate, and in order to bring any serious amount of sodium sulphate into solution more water must be added. This would handicap the millman by necessitating the use of saturated solutions of NaCl in order to keep the solutions from fouling with sodium sulphate. Most mills using such a solution for treating lead ores would have to precipitate the lead from the solution and use the solution cyclicly, and any building up of sodium sulphate would prove fatal to the process. This objection would not apply to mills on the shores of the Great Salt Lake because the lake brine is nearly saturated with salt, and a constant supply of brine would be available at low cost. At mills situated in isolated localities the tailing could be washed and salt solution recovered, unless the recovered salt proved to be of too little value to pay for the trouble of carrying a wash solution which was gradually being built up to sufficient concentration to be used as leaching solution. These considerations are discussed further in another part of this bulletin.

OTHER TESTS.

To further test the effect of sodium sulphate on the solubility of lead sulphate in the brines, C. L. Larson, in charge of the experimental work at the Bunker Hill and Sullivan mine, performed the following tests. A saturated solution of salt, prepared at a temperature of 17° C., was found to contain 192 grams of lead per liter and 0.36 gram of sulphur. To this solution was added an excess of PbSO and the whole agitated for 3 hours at a constant temperature of 17° C. The resultant solution contained 171 grams of chlorine, 7.56 grams of sulphur, and 14.95 grams of lead per liter. This test showed that at least part of the lead sulphate had reacted with salt to form lead chloride and sodium sulphate, and that the excess lead chloride

and sodium sulphate were left on the bottom of the container. A similar procedure, carried on at 8° C., gave a resultant solution containing 176 grams of chlorine, 7.35 grams of sulphur, and 13.34 grams of lead per liter. The results of these tests show that the solubility of the lead compounds decreases with decrease of temperature and that the estimate of 1 per cent lead being the proper maximum to use in practical work is probably correct.

Larson found, on treating a solution with excess of sodium sulphate and of lead chloride, that the resultant solutions contained 15.80 and 7.98 grams of lead per liter at 17° and 8° C., respectively. These figures show fair agreement with the results of the tests in which an excess of lead sulphate alone was added, as the excess of lead sulphate probably reacted with sodium chloride to form sodium sulphate. The sodium sulphate undoubtedly lowers the solubility of the lead compounds in the solution, when conditions are such that sodium sulphate is formed and thereby displaces sodium chloride in the solution. Subsequent tests showed that as long as the proper concentration of chlorine in the solution could be maintained the solubility of the lead compounds was not lowered, irrespective of the proportion of sulphur present. In this regard it would be of scientific interest to determine the solubility of the reciprocal salt pair, PbSo1-PbCl2-2NaCl-Na2SO1.

Several times solutions left out on a cold night lost their sodium sulphate by crystallization at the low temperatures. This happened both in experiments at the Bunker Hill & Sullivan laboratory and in the Bureau of Mines laboratory at Salt Lake City. This illustrates the Hoepfner method of removing sodium sulphate by refrigeration, which Höepfner proposed to use in his process for leaching zinc ores. No attempt was made to apply this fact in the experimental work on lead ores, but it suggests a possible method of removing sodium sulphate from the brine solution whenever it builds up to a dangerous proportion. The water of the Great Salt Lake is continually ridding itself of its burden of sodium sulphate in this way. In wintertime this "winter salt" crystallizes out and is driven on shore by the wind, where it quickly dries and is blown away. However, F. G. Cottrell, chief metallurgist of the Bureau of Mines, suggested that the best way to clear the brines of sulphates would be to add calcium chloride to the solution in order to precipitate the sulphur as insoluble calcium sulphate. Calcium chloride is a by-product in a number of the heavy chemical industries and can be bought at a low price or prepared rather cheaply in almost any locality. This step proved to be so easy in manipulation that the problem of refrigeration was not taken up.

39094°-18-Bull. 157—3

CYCLIC LEACHING.

To determine whether solutions would foul in cyclic work, two series of cyclic leaches were designed. In one of them it was proposed to precipitate the lead from solution as a basic hydroxide by the use of lime or milk of lime. In the other it was proposed to recover the lead by electrolysis. Much of the preliminary work in developing the processes that were worked out was on solutions prepared from roasted sulphide ores. A description of those experiments will be found under "Sulphide ores of lead," on pages 125-137. The beginnings of the completed idea were worked out at the experimental plant of the Bunker Hill & Sullivan company at Kellogg, Idaho, by F. G. Cottrell, chief metallurgist of the Bureau of Mines, and R. S. Handy, superintendent of the mill. This plant had been built for testing the Bunker Hill ore with the Malm dry-chlorination process. This process proved not to be adapted for the high-lead, low-zinc ore, but certain chemical facts were noticed that led to proposed processes whose modification in the laboratory at the Salt Lake City station resulted in the process that was ultimately worked

out.

LABORATORY LEACHING TESTS.

Material from the Horn Silver tailing dump, Frisco, Utah, was chosen for cyclic leaching tests, as in the solubility tests this material had promised high yields of lead with good acid efficiency, and it contained enough acid-soluble impurities to foul the solution provided the impurities of a tailing, slime, or ore could do so. The sample was a grab sample taken from the surface of the dump.

PROCEDURE AND RESULTS.

Two separate series of small laboratory leaches were made. In one the intent was to precipitate the lead by means of calcium hydroxide; in the other, to precipitate the lead by electrolysis. Later, some larger scale tests were made in which precipitation with lime was used.

The results obtained by precipitation of the lead as hydroxide are shown in Table 13. The work covered 24 complete cycles of leaching a charge of material with brine and precipitation of the lead from the brine with lime.

Too much acid was added to the brine in the first leaches so that the solutions were never quite neutral and the amount of impurities dissolved was excessive. By reducing the amount, of acid used, the lead went into solution better and a much better grade of precipitate was obtained. The lime for precipitation was always slaked in brine in order to prevent dilution of the brine leaching solution.