6. The tailing settles through brine in about three times the time necessary for it to settle in water. Hence ample capacity for thickening the slimes is necessary. Filtering brine likewise requires more filter area than filtering water.

7. The precipitate obtained by using lime settles much faster than the ore being treated and is more easily filtered. Hence much smaller thickeners and filters are satisfactory for such precipitate.

8. The precipitate obtained by using lime may be reduced at the plant or shipped to the smelter direct. Direct reduction of such a high-grade oxidized product of lead is very simple.

CYCLIC LEACHING AND ELECTROLYTIC PRECIPITATION.

ROASTING AND LEACHING TEST AT SALT LAKE STATION.

The second series of laboratory leaches was carried out with electrolytic precipitation instead of precipitation with lime, in order to determine whether the impurities entering the solution under these conditions would detrimentally affect the leaching power of the solutions in any way. Electrolytic precipitation was studied thoroughly before these tests were begun. It was found possible to use iron anodes without affecting the properties of the precipitated lead. The lead always tends to precipitate as sponge lead. This spongy lead will oxidize during drying, and the dried material is readily converted to metallic lead when melted in a reducing atmosphere. Analyses of the dried precipitate revealed as much as 4 per cent of iron in some samples. In melting this material the iron oxide formed a dross, which could be skimmed off, leaving lead of exceptional purity.

The low proportions of lead in the dried precipitate, ranging from 68.4 to 91.7 per cent lead, is due to oxidation of lead and to the presence of small amounts of entrained iron, which later was drossed off. The lead extractions were not determined for all of the tests, as the amount of acid found best in the previous series of leaches (11 pounds per ton) was used. In those determined the extraction was slightly more than 70 per cent. Sixty-two per cent of the silver was extracted by the brine, which accounts for the silver in the final product. The sponge lead could be produced free of silver by precipitating the silver, before electrolysis, on scrap iron, the lead remaining in solution. The silver might also be removed by use of a small amount of the sponge lead, which will slowly displace silver from its solutions. The silver precipitate would form only a small quantity of base bullion when melted. The lead deposited from the desilverized solution would be ready for marketing as soon as melted into bars.

USE OF IRON ANODE.

In this series of tests it was thought that the use of an iron anode, although permitting a very low operating voltage through anodic solution of the iron, might contaminate the solution too much with ferrous salts to permit good leaching of the lead. The first three cycles were run during very cold weather and at night the solutions often cooled nearly to freezing temperature. Whenever this happened, sodium sulphate and other substances crystallized from the solution, so that the content of iron left in the solution after precipitation decreased for a while. The later cycles were run under warmer conditions, and the iron built up to some extent, although a considerable amount must have been left in the ore, from the barren solution at each leach, as the iron left in the solution should be equivalent to the lead removed. Roughly, the iron content of the solution should have increased one-third of 1 per cent on each cycle. The iron was undoubtedly building up during the last tests, as was the sulphur. The material being tested contained some oxidized zinc and it was anticipated that the zinc would likewise build up, but it did not. At the Bunker Hill & Sullivan test plant purifying the solution, after electrolysis, with caustic lime, leaving a precipitate of ferrous and ferric hydrates and calcium sulphate, was tested.

The voltage necessary to obtain a fairly large current density was remarkably low, due, of course, to the use of the soluble iron anode. A poor connection would run up the voltage necessary to operate the cell, as shown by two sets of duplicate conditions in which this effect was studied. The difference in the yield of lead per kilowatt-hour resulting from a slightly higher voltage is very great. All of the data on the solution analyses and the average electrical conditions are given in Table 20. More detailed data on these tests are given in Tables 14 to 16.

TABLE 20.-Results of cyclic leaching and electrolysis of Horn Silver tailings a [8 liters saturated brine used per 1,000 grams of material containing 7.4 per cent Pb.]

It is remarkable that such high current densities could be employed with a solution so dilute in metal, and still permit of such high current efficiencies as were obtained. The solution being strongly impregnated with sodium chloride accounts for the high electrical conductivity.

TESTS AT BUNKER HILL & SULLIVAN PLANT.

A number of cyclic leaches were made at the Bunker Hill & Sullivan plant with electrolytic precipitation, the cell holding 5 tons of solution. When working on the large scale some difficulty was encountered in washing the brine out of the sponge lead, but compressing the sponge with a hydraulic press remedied this trouble. The cartridges of compressed sponge were in convenient form to place in the melting furnace.

Treatment of the barren brine with lime to remove the iron that had entered the solution during electrolysis was also initiated at this plant, and it was said that no difficulty was experienced in removing the iron. Some calcium sulphate precipitated with the hydrates of iron and made the settling of the precipitate rapid and filtration relatively easy.

More silver was extracted in the tests in which electrolytic precipitation was used than in the series where precipitation with lime was used. This result was believed to be due to the presence of ferric chloride in the leaching solution from electrolytic precipitation. The iron in the solution tended to be reduced at the cathode, so that a preponderance of ferrous iron was present, but there was always some ferric iron present, owing to oxidation by air. The solubility of all compounds of silver in solutions containing ferric iron is well known, and is the basis of many proposed methods for treatment of silver ores, but none have been in recent use.

LEACHING UNROASTED MATERIAL WITH BRINE CONTAINING FERRIC

CHLORIDE.

Further work along the lines of leaching the raw ores with brines containing ferric chloride was initiated, but has not been carried to any great extent. A few of the samples at hand were leached with acid brines containing the amounts of iron that were introduced by the electrolysis of the brines in the tests previously described. The electrolyte, after most of the lead has been removed, might be passed through a few cells with carbon anodes in order to oxidize the ferrous iron to the ferric state before being again used as the leaching agent. These leaches were carried on in parallel with other leaches that were identical, except that no iron was present in the solution. Two series of tests were run. In one the time of contact of solution with ore was 16 hours and in the other series 3 hours. The brines containing

ferric chloride were so proportioned that 1 per cent of iron, in the ferric state, was present in the solution. This proportion of iron is rather low, but is only slightly lower than the proportion that was left in the solution after electrolysis. During the leaching with acidified brine the basic minerals of the ore consume the acid and the iron chlorides tend to hydrolyze and be thrown out. Only ferrous chloride survives the leaching operation. The results of these leaches are presented in Table 21. They show that the small amount of iron used caused very little more silver to go into solution than does the use of acid brine alone. Sixteen hours of contact gave lower extractions of the silver than three hours. Most of the materials tested contain some zinc sulphide and other minerals, and the equilibrium conditions would be for the silver to be thrown out of solution and the zinc to pass into solution. Hence it is not surprising that a longer time of contact should give lower extractions of silver.

TABLE 21.-Data on leaching tests made with and without iron in the solution.

[Experiments made by G. J. Holt and C. E. Sims.]

CHARACTER OF LEACHES IN WHICH IRON WAS ADDED TO SOLUTION.

TABLE 21.-Data on leaching tests made with and without iron in the solution-Contd.

If an oxidized lead ore contains silver in such form that it is easily soluble in acidified brine, there is no need of roasting to make the silver soluble, hence the process of extracting the lead and the silver by leaching would be extremely simple. To test out a considerable range of materials of this type for simultaneous leaching of the lead and the silver, and also any zinc that might tend to dissolve, a series of tests were made on 13 different ores and tailings with varying amounts of sulphuric acid, as calculated for the theoretical requirements of the lead in the material. The results are given in Table 22, which requires little explanation. The amount of acid used is stated in equivalents of the lead and with most of the ores leaches containing 1, 2, 5, and 10 equivalents of acid were used. As some of these materials contained gold, the analyses of the various products for that metal are given. The percentages of gold extraction were not calculated.

Some of the tests in which 10 equivalents of acid were used show lower extractions of lead than those with 5. This is probably due to the large excess of sodium sulphate, formed by action of the sulphuric acid on the sodium chloride. A large excess of sodium sulphate depresses the solubility of the lead compounds in a brine. The large extractions of silver in some of the materials tested indicates that this method could be used to advantage for their beneficiation. With the other materials other methods, as outlined further on in this report, would be more desirable.