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to operate the rest of the plant intermittently, with batch. charges. This made the working of the plant somewhat different from the proposed commercial process, but permitted complete weighing and measuring of all the products formed during a cycle. The filtering period consumed 5 to 6 hours, owing to the imperfect

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filter, and the other steps of the process were dependent on the filtering, as it was the slowest step.

The filtered and decanted pregnant solution was treated with the proper proportion of slacked lime to precipitate the lead, the precipitate allowed to settle, and the clarified barren brine siphoned off with a hose, while the thickened sludge was filtered on a smaller suction filter. The barren brine was returned by air lift through an iron pipe to the solution storage at the head of the system. A compressor and a suction pump operated the plant.

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As some sulphuric acid would be used in the flotation of the zinc sulphide (sphalerite) from the mate

rial being treated it was not known how much acid would be left to extract the lead, so a series of preliminary tests of small charges were run to determine how much acid was necessary. It was found that about 15 per cent of the lead was soluble in brine alone. The original material contained 6.5 per cent lead. By the use of about 60 pounds of sulphuric acid per ton of ore, a 72 per cent extraction was possible, leaving 1.84 per cent lead in the tailing, supposedly as galena, as it was insoluble in the acid brine. The results of these preliminary tests are shown in Table 14.

TABLE 14.-Results of preliminary tests to determine proportion of acid needed.

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The results of the large-scale leaches are recorded in Table 15. In each leach 36 pounds of material, computed on the dry basis from the moisture sample, was used. The material itself was not dried, being taken from the barrel in which it had been shipped in the wet condition. On that account the weight of wet ore is also. recorded. Table 16 shows the complete assays that were made on the various products in each of the tests.

TABLE 15.-General results of large-scale tests of cyclic leaching.

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TABLE 15.-General results of large-scale tests of cyclic leaching—Continued.

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TABLE 16.-Results of assays of precipitate, tailing, and solution from cyclic leaches.

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The results were disappointing in two respects (1) the precipitate was of very low grade as compared with that obtained in the smaller leaches in which all the products could be handled more efficiently; (2), the extractions of lead were rather low.

PURIFYING THE PRECIPITATE.

The first attempts to remedy the low grade of precipitate were made during the large-scale tests, thinking that the trouble was due to the difficulty of mixing the lime with the solution as efficiently on the larger scale as in the smaller laboratory tests.

PRECIPITATION WITH LIME.

In test 1 (Tables 15 and 16), 3.4 pounds of lime assaying 54 per cent CaO and about 40 per cent MgO was used to precipitate the lead in the solution. A precipitate containing 18.8 per cent Pb and 25 per cent CaO was formed. This precipitate was very dark when fresh but turned white on exposure to air. In the second test the quantity of lime was reduced to 1.5 pounds. This lowered the lime content in the precipitate to 7.74 per cent but raised the lead content to only 25.6 per cent. A considerable amount of sodium chloride was left in this precipitate. In test 3 the lime was reduced to 0.75 pound, giving a precipitate containing 8.36 per cent CaO and 20.8 per cent Pb. The solution after precipitation was barren. In test 4 the lime was cut to 0.5 pound; the grade of the precipitate was 47.5 per cent Pb, which was encouraging. Therefore only 0.5 pound of lime was used in test 5, but the precipitate contained only 34.1 per cent Pb. In test 6 mechanical troubles arose, so that the figures obtained are of no importance.

In these six tests the lime had been slaked in a bucketful of brine and stirred with a stick. In the subsequent tests the lime was slaked in a ball mill in order to break up all lumps and obtain a smooth paste. Also less leaching solution was used in order to make a richer solution for precipitation, the hope being entertained that the grade of the precipitate would also rise under these conditions. In test 7 about half of the amount of solution used in former tests and 0.875 pound of lime were used. The precipitate assayed only 21.71 per cent Pb and 16.47 per cent CaO. The barren solution still contained some lead and it was thought that the reaction had been incomplete. Test 8 was made under similar conditions and more care was exercised, but the resulting product was of about the same grade. In test 9 the lime was cut to 0.5 pound again but the precipitate still contained 11.7 per cent CaO, although it ran 35.11 per cent Pb. However, the solution still contained 1.04 grams Pb per liter after precipitation. In test 10 the lime was further cut to 0.375 pound, with the result that the precipitate assayed 41.3 per cent Pb and 6.08 per cent CaO, but the solution still contained 0.88 gram Pb per liter. An attempt to duplicate these results in test 11 gave a precipitate with 12.5 per cent CaO and 26.3 per cent Pb.

These experiences are very similar to those met with by the Bunker Hill & Sullivan Co. during one stage of its tests. It seemed that too many substances enter the precipitate to permit preparing a product desirable for direct reduction to lead. Calcining the precipitate and leaching with hydrochloric acid failed to remove the lime and other contaminating elements. Enough salt was present to volatilize much of the lead when the precipitate was calcined at a red heat in an effort to raise its grade.

PRECIPITATION ON SCRAP IRON.

This led to an attempt to develop other precipitants for the lead. Scrap iron was tried in an effort to prepare sponge lead. Some of the first iron used was slightly galvanized and the zinc caused quick precipitation of an excellent grade of lead sponge. However, as soon as the zinc was gone, or if pure iron or cast iron was used, the lead did not precipitate.

PRECIPITATION WITH SODIUM SULPHIDE AND HYDROGEN SULPHIDE.

Sodium sulphide and hydrogen sulphide were tested as precipitants. The precipitates from both of these settled slowly and were difficult to filter. Very often the orange-colored sulphochloride of lead was thrown down and the grade of the precipitate was only about 35 per cent Pb. On roasting this material part of the lead was volatilized and the grade of the residue was raised to only 42 per cent Pb.

PRECIPITATION WITH SODIUM CARBONATE.

Sodium carbonate gave quick precipitation and a precipitate containing 69.89 per cent Pb, which was very easy to filter, was obtained. On calcining at 800° C. the grade was raised to 77.14 per cent Pb. It was thought that it might be possible to utilize crude "trona" from the alkaline lakes of Nevada and California, but on learning that it would cost nearly 1 cent per pound when delivered in Utah, further search for a cheap precipitant was deemed necessary.

EFFECT OF USING LIMESTONE.

In nearly every test the solution, when ready for precipitation, was slightly acid, and it was thought best to neutralize it with limestone in place of using slaked lime. Ground limestone is used in purifying some of the zinc sulphate solutions in modern electrolytic zinc work, the result being that most of the iron, aluminum, and other metal compounds capable of quick hydrolysis at ordinary temperatures are precipitated. The brine solutions of lead were likewise purified, also the calcium chloride resulting from the hydrolysis precipitated most of the sulphates still in solution. After purification

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