of the solution in this way, the addition of either slaked lime, sodium carbonate, sodium sulphide, or hydrogen sulphide to the solution resulted in a precipitate containing about 70 or more per cent lead. Roasting the precipitate raised the grade to between 75 and 80 per cent Pb.

The reactions involved in the purification are probably as follows, iron chloride being taken as one of the representative impurities precipitated:

2FeCl3+6H2O =2 Fe(OH)3+6 HCl.

6HC1+3CaCO 3 CaCl2+3H2O+3 CO2.
3CaCl2+3Na2SO4=3 CaSO1+6 NaCl.

The precipitation results in these tests were checked a number of times with freshly prepared solutions and with solutions that had become foul from cyclic leaching with practically identical results. In a few tests calcium chloride was added to the foul solution; calcium sulphate was precipitated. This precipitate was tested for lead and was usually found to be barren, or nearly so, hence apprehension of losses from inclusion of lead in the calcium sulphate formed during purification of the solution was set aside. Adding the limestone to the pulp before filtration left the calcium sulphate and the hydrolyzed iron and aluminum hydroxides in the tailing, so that only one filtration was necessary. This simplified the process.

TESTS TO DETERMINE BEST CONDITIONS FOR MAXIMUM EXTRACTION.

EFFECT OF VOLUME OF BRINE USED.

After the troubles in precipitation had been solved by finding how to purify the solutions, the question of obtaining the maximum extraction by discovering the best conditions for solution of the lead was next attacked. A series of tests was designed to see what effect different volumes of brine in relation to the weight of the ore would have. It was thought that an excess of brine would be a better solvent than a minimum of brine. As solutions containing not more than 15 grams of lead per liter of solution can be prepared at ordinary temperatures, and it was thought best not to proportion the brine to the ore in a ratio that would give a solution too near the saturation point, a number of ratios calculated to give weaker solutions were used in these tests. The results of the tests are recorded in Table 17. The best results were obtained with the lowest ratio of solution to ore, ratios ranging from 3.25:1 to 20:1 being tested. The concentration of acid was greatest in the pulp having the solution ratio of 3.25:1; hence strength of the acid solution was evidently the determining factor. A method of leaching that would permit the application of all the acid in a very thick pulp, followed by dilution with brine in

order to extract the soluble lead compounds, is indicated. The refractory lead compound that required this special treatment was known to be galena, as all the carbonate of lead and the sulphate of lead dissolve with ease.

TABLE 17.-Results of tests showing the effects of the ratio of solution to ore on the extraction obtained.

Another series of tests were then made to determine whether better extractions could be obtained by heating the pulp. The results, presented in Table 18, show that practically no benefit was derived from heating. The heating of a saturated brine is difficult, because dilution is not permissible. Heating of solutions with exhaust steam would, therefore, necessarily have to be by use of coils of pipe immersed in a tank of brine, which would make the heating expensive. Direct heating with a fire would also be difficult.

TABLE 18.-Results of tests showing effect of heating pulp, on the extraction of lead. [Tests by G. J. Holt and F. G. Moses.]

a Ratio of volume of solution (c. c.) to weight (grams) of material tested.

NOTE.-Leaches 1 and 2 were boiled for an hour on the hot plate; Nos. 3, 4, and 5 were leached cold to compare this method with hot leaching; Nos. 6 to 11 were boiled 15 minutes on the hot plate.

EFFECT OF ADDING CHLORIDE OF LIME.

The results of a series of tests to determine whether adding chloride of lime during the leaching would provide chlorine to attack the undissolved lead sulphide are shown in Table 19. It was found that the use of 10 pounds of bleaching powder per ton of material treated increased the extraction of the lead by about 9 pounds per ton of material treated. The price of chloride of lime and that of lead would help determine whether this reagent could be used at a profit. No other methods of increasing the extraction were tested.

TABLE 19.-Results of tests made to improve leach tailings.

[Tests by G. J. Holt and F. G. Moses.]

Having determined the conditions under which a plant for leaching lead carbonate ores should operate, it is possible to draw some conclusions as to the commercial application of the process. A proposed flow sheet for such a plant is shown in figure 8. . Practically all the machinery would be of standard make and each machine would be of the continuous operating type, these being the only differences between the test plant and the proposed commercial plant. A scrapiron launder might be used for removing any silver not precipitated with the lead by lime precipitation, as the silver tends to stay in solution. If it should be desired to remove the silver before the lead, the scrap-iron launder can be used ahead of the lime precipitation tank; then the iron passing into solution would be precipitated with

the lead. However, the proportion of this iron in comparison with the lead need not be large, and probably would be negligible. (See "Sulphide ores of lead," pp. 123 to 169.)

MATERIALS USED IN PLANT CONSTRUCTION.

The question of refractory materials for plant construction also received some attention. Iron construction can be used only for barren neutral brine. Acid brine will attack iron, and if the brine contains silver, the silver will be precipitated. On that account Pachuca tanks, thickeners, and filters used for leaching the ore would

solution.

FIGURE 8.-Proposed flow sheet for lead-leaching plant. material being treated. - precipitate. 1, from dump; 2, acid storage; 3, barren solution storage; 4, acid feeder; 5, wooden Pachuca tanks; 6, ground limestone; 7, Dorr thickener of wooden construction; 8, continuous suction filter (acid-proof); 9, wash water; 10, tailing; 11, brine to slake lime; 12, lime mixer and feeder; 13, precipitation Pachuca tank; 14, Dorr thickener; 15, continuous suction filter; 16, pump to return solution to storage; 17, precipitated basic lead hydroxide.

have to be of wooden construction. Lead pipe used in the construction of the Anaconda Copper Co. copper-leaching plant does not precipitate much silver from the dilute brines of that plant. Hence a lead air lift was used in the Pachuca tank of the testing plant at the Salt Lake City station with seeming success, although the silver extractions were not investigated thoroughly. The ability to use lead in the construction of an acid-proof continuous-suction filter solves that constructional problem, as such filters are already on the market. Also several special irons were tested for their ability to withstand the action of the solutions, and both "duriron" and "corrosiron" were found to be inert toward brines containing silver and lead in solution,

although they are very slightly acted on by the acid. Concrete tanks resist the brine fairly well, and were used for neutral brine in the leaching tests at the Bunker Hill & Sullivan testing plant. Acid brines attack concrete only slowly. Therefore there seems to be no difficulty as to obtaining cheap refractory materials for plant construction. In the testing plant at Salt Lake City some iron tanks, protected with an acid-proof paint were used, but the paint required frequent renewal.

CHARACTER OF SAMPLE TESTED.

Attention should be called to the fact that the sample used for the earlier, small-scale tests was a grab sample of the surface of the old dump, whereas the later large-scale tests were on a representative sample of the tailing from a flotation mill that was treating the material represented by the first sample. The large content of lead sulphate in the first sample was the cause of the low acid consumption in the tests made on it, but the second sample required more acid, for two reasons. The proportion of lead sulphate was less, and 5 to 10 pounds of lime per ton had been used in settling the slime from the tailing water of the flotation plant in order that the water could be used over again. This lime, of course, required an equivalent amount of acid to neutralize it.

CONCLUSIONS FROM RESULTS OF TESTS.

The conclusions drawn by the writers from the results of these experiments are as follows:

1. On a larger quasicommercial scale, the preparation of a solution from which a high-grade precipitate of lead can be thrown down is difficult unless resort is had to purification of the solution with lime

stone.

2. From the material tested, 83 pounds of lead and 2 ounces of silver per ton could be extracted by the use of 50 pounds sulphuric acid, 20 pounds of lime, with salt losses varying from 20 to 50 pounds, depending on the care used in handling and washing the material and the precipitate.

3. The precipitate obtained with the use of lime will average about 65 per cent lead, or more, and settles and filters rapidly.

4. Sodium carbonate and sodium sulphide give higher grades of precipitate but cost more than lime. The precipitate from the use of sodium sulphide is hard to filter, but the precipitate made by use of sodium carbonate settles rapidly and filters easily.

5. The time required for dissolving the lead need not exceed onehalf hour; the time for purification, about one-half hour; and the time for precipitation, 10 minutes. These facts permit the use of small Pachuca tanks for leaching and precipitatation.

39094°-18-Bull. 157-4