4. The calcium chloride solution added should be saturated wi h sodium chloride in order to prevent precipitation of a basic chloride of lead by dilution of the brine.

5. The precipitation of lead from the brine by lime takes place quickly both in cold and in warm solutions.

6. The lime should be slaked in brine in order to prevent dilution of the leaching solution.

7. Any sodium chloride carried down with the lead precipitate can be washed out with water.

8. Some chlorine is left in the precipitate, according to the efficiency of washing, and it is probable that this would cause volatilization of lead chloride during smelting of the precipitate.

LEACHING WITHOUT CALCIUM CHLORIDE.

The second series of cyclic leaches was performed on a sample of calcines from a shaft roaster (Holt-Dern) of semicommercial size used at the Bunker Hill & Sullivan test plant. In this series of tests the cycle consisted of leaching with neutral brine followed by precipitation with lime. Calcium chloride was not added to any of the leaches, in order that sodium sulphate might build up in the solution if it would. The numerical data are contained in Table 45. In cycle 11 the solution was evaporated to a smaller volume after precipitation. During the evaporation a scum of brown material separated but gave a negative assay for lead. Evaporation was necessary because the washing with water diluted the solution and more sodium chloride had to be added to keep the solution saturated. The accumulation of solution was undoubtedly greater than could be allowed in practice and to retain all impurities resort was had to evaporation of the solution to reduce its volume.

In cycle 13 the lead-bearing brine had a yellowish green color, probably due to manganese and a small content of iron. The lead hydrate precipitate was colored brown and the average assay of all the precipitates shows about 1 per cent of manganese.

In cycle 14 large volumes of water were used to wash the sodium chloride out of the precipitate.

In cycle 15, during the analysis for lime, considerable iron and manganese was noted.

In cycle 16 an excess of sodium chloride was added to the saturated brine before precipitation. The lead seemed to be thrown down more rapidly than in previous cycles, precipitation being practically complete in a few minutes.

In cycle 17 an excess of sodium chloride was added to the saturated brine, but the precipitation was difficult and the precipitate held sodium chloride tenaciously.

With these points in mind, a series of cyclic leaches of blast-waster calcine was made with neutral brine by Larson in the company's laboratory. Forty liters of saturated brine was prepared as a stock solution and about 10 pounds of ore per charge was leached in earthenware jars. The brine solutions were precipitated with lime and filtered. Any alkalinity of the filtrate was neutralized with hydrochloric acid. In the first leach about 4 tons of solution per ton of ore were used, followed by one or two more brine leaches, and finally a water wash to recover sodium chloride from the tailing. The wash water from the tailing and from the lead hydrate precipitate was evaporated to saturation and added to the stock solution. In spite of this precaution the amount of solution decreased markedly in the 18 cycles, and might well have been replaced by new brine if the specific purpose of these tests had not been to determine the effect of fouling when no new brine could be introduced.

The first nine cycles were on material that had received a preliminary washing with water and did not deteriorate very fast. After that unwashed material was used to make the fouling agents build up faster. At the completion of cycle 17 the solution had so deteriorated that only about 8 grams of lead per liter could be dissolved, hence the solution was treated with calcium chloride. The chlorine content before treatment with calcium chloride had fallen to 165 grams per liter. After precipitation it rose to 182 grams per liter and the solubility of lead in the solution increased in about the same proportion, as was evidenced by the first runnings of the next leach.

CONCLUSIONS FROM RESULTS OF TESTS.

Larson's conclusions on the cyclic operations, are as follows:

1. Neutral brine, used cyclically, extracts about 99 per cent of the. soluble lead formed in blast roasting (which is 85 to 99 per cent of the lead in the ore) within certain limits of fouling. In other words, with fouling taken care of, the lead extraction of the neutral brine process depends entirely on the sulphating efficiency of the roasting step of the process, and it has been shown that this is 85 to 99 per

cent.

2. Neutral brine gives erratic results with silver. In general the only conclusion that can be definitely drawn is that the better the sulphating of the lead the better is the extraction of the silver. Acidified brines are more reliable. A high content of calcium chloride in the solution causes lower extractions of silver.

3. Considerable manganese was soluble in the brine but did not build up, owing to complete precipitation by the lime. The manganese does, however, lower the grade of the precipitate markedly.

4. Sulphur builds up steadily with the continued use of the brine. No maximum of sulphate sulphur seemed to be reached, the highest concentration, in cycle 17, being 11.08 grams per liter. Long before this amount was reached it was seen that the solution was deteriorating in its ability for dissolving lead. However, the sulphates need only be eliminated intermittently and can be eliminated very well by the use of crushed solid calcium chloride. One pound of precipitated calcium sulphate is obtained by the addition of every pound of calcium chloride. It is possible that some of this could be sold for gypsum.

5. The use of a preliminary water wash greatly prolongs the efficient life of the lixiviant and removes manganese from the system, resulting in a higher grade of precipitate.

6. Practically complete precipitation of lead is obtained by the use of lime. Manganese and zinc are also completely precipitated. The silver, however, is not completely precipitated and the brines usually carried 4 to 6 milligrams of silver per liter after lime precipitation.

7. The precipitate, if not washed, contained 55 to 60 per cent lead and after washing with water, 65 to 70 per cent lead.

8. Washing the precipitate was difficult, owing to the tendency of the filter cake to crack quickly. Repulping of precipitate with fresh water, followed by filtration, was found necessary in order to remove the sodium chloride from the cake. With the larger amounts of precipitate handled in these tests, it was difficult to obtain a cake containing less than 5 per cent of chlorine. The rest of the chlorine seems to be locked up in a complex lime-lead chloride. It was more difficult to bring the lime and solution into efficient contact in these tests and in the test plant, and this may account for the higher amount of insoluble chlorine in the precipitate as compared with the work performed at the Salt Lake City station.

9. Washing the tailing with water and evaporating the washings in order to recover sodium chloride from the tailing is feasible, if the salt is worth the trouble. Such washing tends, however, to return to the system some of the sodium sulphate left in the tailing.

10. No advantage is apparent in conducting the operations at temperatures other than the ordinary mill temperature. Heating of solution in a mill is generally by introduction of exhaust steam into the solution, which is undesirable in this process because the solution should always be saturated with sodium chloride. In this connection it was noted that in test 12, when the temperature dropped very low over night, sodium sulphate crystallized out of the solution.

Larson's conclusions on fouling meet the authors' approval although they doubt the advisability of using a preliminary water

wash to remove soluble fouling agents. The water left in the pulp will tend to dilute the brine later applied, also as much moisture must leave the leaching tanks as was in the water-washed pulp before leaching. These conditions do not permit partial recovery of salt by washing with water after leaching with brine. Where the brine is applied to dry ore without washing with water, it will be necessary to add water at the end of the cycle to keep up the volume of mill solution. As it has been definitely shown that the use of calcium chloride is sufficient to prevent fouling, and that the chlorine content of the solution must be kept up near the point corresponding to saturation with sodium chloride, these are the two conditions that would have to be watched in large-scale work. The company is now testing out this and other improved processes. It is to be hoped that the results of the work will be published at an early date.

Since the time of writing this bulletin some of the results have been described by Larson a in an article published in the Mining and Scientific Press.

As acidified brine must be used in order to get satisfactory extractions of the silver, and sulphuric acid would probably be the acid used, on account of its cheapness, the only disadvantage would be the more frequent additions of calcium chloride necessary in order to keep the resulting sodium sulphate from building up too fast.

There is considerable difficulty in getting good extractions of silver from this type of ore. Later experiments have led the writers to think that much of the difficulty is due to the reprecipitation of silver from the solution on unroasted zinc sulphide. The lead is often roasted completely to the sulphate or oxide form before the zinc sulphide is much affected, and when working on complex sulphides it has been found that the unroasted sphalerite could precipitate large amounts of silver from brine solutions. Consequently the longer the time of contact of the leaching solution with an ore containing a small amount of zinc sulphide, the more silver will be precipitated and the lower the extraction. On that account a roaster that will efficiently roast the sphalerite, as well as the galena, is desirable. Amalgamation, cyanidation, hyposulphite lixiviation, leaching with acid brine, brine containing cupric chloride, brine containing ferric chloride, and other methods were tried in an effort to extract silver from badly roasted ore. In all tests it was found difficult to get extractions that would even parallel those obtained with acid brine leaches. By roasting the ore properly and leaching with acid brine it was found that as much as 80 per cent of the silver could be extracted, and that usually the lead was practically all converted to the sulphate in these good roasts.

a Larson, C. L., Hydrometallurgy of lead-silver at the Bunker Hill smelter: Min. and Sci. Press, Aug. 25, 1917, pp. 275-279.