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TABLE 13.-Results of cyclic leaching of material from Horn Silver tailing dump with acidified brine and precipitation with lime. [Tests by M. J. Udy. Lead content of material, 7.98 per cent.]

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710

0.58 0.865 0.176

821

35

600

.76

880

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P. ct. P. ct. P. ct. Grams. P. ct. P. ct. P. ct. Grams.
None 0.233 0.252
12.20

P. ct.

P. ct. Grms.

C. c.

C. c. Grms."

105

1.75

88.5

200

1,000

24

21.22

140

2.04

290 14.00

Four agitation vessels used in series.

75

200

.96

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1,000

400 14.00

600

100 1.56

.93

90.2 200 1,000

14.00

75 1.46

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270 1.80 73

32.00

40.80

160

1.75

112 1.74 87.8 200 1,000

93.1 200 ,000 200 1,000

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60.86

177 2.09

1,000

51.34

1,000

220

56.55

000

53.05

1,000

57.13

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NOTE.-Average assay of precipitate in Nos. 14 to 24-Ag, 2.40 oz. per ton; Pb, 70.54 per cent; Zn, 3.15 per cent; S, 1.09 per cent; CaO, 5.42 per cent.

160 2.33 76.6
175 2.44 73.1
150 2.33 77.8

150 2.04 75.0

82.5 200
76.8 200
160 2.33 76.7 220
168 2.04 78.0 200
155 1.53 84.0 200
170 2.33 75.1 200
1,000
132 2.33 80.6 200 1,000
160 2.33
77.6 200 1,000
180 2.33 77.7 200 1,000
70 1.13 2.5 grams of CaCl, added.
165 2.91 70.0 200 1,000 150 1.13
180 2.62
71.0 200
1,000 130
1.13
187 2.33 73.6 200 1,000 90 1.13
137 2.04 82.1 200 1,000 400 1.13
180 2.04 77.0 200 1,000 310 1.13
200
1,000 340 1.70
200 1,000 270
1.70
200 1,000 200 1.70
200 1,000 100 1.70

410 3.39

Average of double precipitation.
Do.

450 3.39

Do.

1.70

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60 1.13

300 1.13 290 1.13

Fourth vessel removed at end of this
test.

These leaches were applied in a countercurrent decantation system devised by the experimenter, M. J. Udy. A number of leaching vessels were made by cutting off the bottom and lower part of 2.5-liter acid bottles of the ordinary type. The cut bottle, on being inverted, resembled the ordinary Pachuca tank, common in cyanide work. A rubber stopper with a glass tube was placed in the bottle neck; a jet of compressed air through this tube kept the charge of material and solution agitated. No central air lift for agitation, as is commonly seen in Pachuca tanks, was used, but the conical sloping shoulder of the vessel returned to the center all material that tended to settle. Periodically the agitation was stopped, the contents were allowed to settle, and a large proportion of the clear solution was siphoned off. In part of the tests four vessels and in the other tests three vessels were used in series. The thickened pulp was drawn through into the vessel below, while the solution that had been decanted was added to the pulp in the vessel above. In this way nearly barren solution entered the bottom vessel and pregnant solution was decanted from the top vessel. Only fresh material was placed in the top vessel and the material, after passing in turn through each vessel, was finaliy taken from the bottom one as a thick pulp. This pulp was filtered in a Buechner funnel to recover as much of the brine as possible. New brine to make up for the amount retained in the residue was added from time to time.

DISCUSSION OF RESULTS OF TESTS.

In the material from the Horn silver dump some of the lead is present as sulphide, some as sulphate, and some as carbonate. The determination of the sulphide lead (see Table 2, p. 15) shows that 11 per cent of the total lead is lead sulphide. By determining the percentage of sulphur soluble in ammonium acetate solution and the percentage soluble in water it was possible to estimate the proportion of sulphate lead in the ore, which was about 45 per cent of the total lead content. The proportion of lead that can be recovered as sulphide by use of a flotation machine indicates that the proportion of sulphide lead is nearer 25 per cent than 11 per cent of the total lead content. Evidently the ammonium acetate dissolves some of the sulphide lead in this material. Hence, after it was found that the first leaches contained too much acid, the acid concentration was progressively reduced in successive leaches. The improved grade of the precipitate in the later cycles is evident.

In tests 6, 7, and 8 the amount of lime necessary to precipitate all the lead was added in two portions to determine whether a very rich precipitate could be obtained in one of these fractional precipitations. The results were not encouraging and the method of adding at one time all the lime needed was again resorted to.

Some solution and some pulp were lost by splashing out of the agitation jars; hence the extractions were calculated from the assay of the residue rather than from the assay of the solutions.

In tests 15 to 24 the solutions and the precipitate were assayed for Pb, CaO, and S, and the residue was assayed for lead. In tests 1 to 14 four agitation vessels in series were used. In tests 15 to 17 the fourth vessel was removed as its use seemed to be unnecessary. In fact, all of the acid was used up in the first vessel and the other vessels were merely recovering lead sulphate and giving countercurrent washing of the material.

In test 18 the lead content in the solution had fallen to 0.99 per cent, so a fourth bottle was added again. This brought the lead content of the solution up to 1.74 per cent immediately, although about the same extraction was being maintained. A considerable amount of water evaporated during the tests, the amount varying with the time of agitation.

It might be expected that when lime is used all of the sodium sulphate would react with lime to form calcium sulphate, which would accompany the precipitate of lead hydroxide. Calcium sulphate has a definite solubility and hence all of the sulphur is not precipitated in this way. However, it was noticed that whenever lime was added in small excess of the amount theoretically needed, the precipitate contained much more sulphur, presumably as calcium sulphate. In test 21 the excess of lime was cut out, with the result that the grade of the precipitate was improved. In test 22 an excess of lime was again added. The grade of the precipitate was 10 per cent lower, whereas the sulphur content of the precipitate increased again. Solubility relations are also such that a considerable proportion of either the calcium sulphate or the sodium sulphate must be thrown down during leaching. Hence although one might expect an equivalent of sodium sulphate in the pregnant solution for every molecule of lead removed, this condition was usually not realized. Likewise, if all the sulphate sulphur left in solution were to react with the lime to form calcium sulphate, one chemical equivalent of CaSO, should be precipitated for every equivalent of lead hydroxide, and the highest grade of precipitate in the combined mixture of Pb(OH), and CaSO, would contain 55 per cent lead. It is possible to obtain a precipitate containing as much as 80 per cent lead in leaching material that does not contain large amounts of impurities, although the highest grade of precipitate obtained from this particular series of leaches was 73.46 per cent lead. This result was due to leaving most of the sodium sulphate and most of the calcium sulphate in the ore, although calcium sulphate is soluble in saturated brine to some extent (0.572 parts CaSO, per 100 c. c. at 26° C.) and tends to precipitate to some extent on addition of lime.

CONCLUSIONS FROM LABORATORY TESTS.

The general conclusions that are to be drawn from this series of leaches are as follows:

1. It is possible to recover about 80 per cent of the lead in the material tested in the form of a hydrate precipitate containing 70 per cent lead. That is, from each ton of material 128 pounds of lead can be extracted, giving about 175 pounds of precipitate. 2. About 12 to 20 pounds of sulphuric acid and 30 pounds of lime would be required to precipitate the lead from a ton of this material. 3. About one-fifth of the solution was lost in the small-scale tests, or one ton of solution to one ton of material treated. The loss could doubtless be made much less in large-scale work with modern filters and agitating machinery. One ton of solution to one ton of material is about the density of the pulp discharged from a Dorr thickener. With an Oliver filter the proportion of solution left in the tailing ought to be about 20 per cent of the weight of the original material. As the solution would consist of 26.8 per cent NaCl by weight, the salt loss in the tailing would be about 5 per cent of the weight of the original material, or 100 pounds per ton when the pulp was not washed on the filter in order to recover salt.

4. The material in this tailing dump, being already ground to fine consistence, would not require grinding for treatment by the leaching process.

5. Sulphates do not build up in the solution to an appreciable extent, nor are they found to any extent in the precipitate. Hence they largely remain in the material being treated. If the tailing were given a final wash in order to recover brine, the sodium sulphate would be taken back into solution. This fact would seem to make washing of the tailing inadvisable.

6. In case sulphates do build up in the solution to any extent, work on other ores has shown that the sulphate can be precipitated by adding calcium chloride to the solution.

7. Excess of lime is to be avoided as the excess lime not only contaminates the precipitate but carries down more of the sulphate sulphur (in the form of calcium sulphate) that would otherwise stay in solution.

LARGE-SCALE CYCLIC LEACHING.

The results of the experiments just described were so encouraging that steps were taken to interest the company that was then trying to work the material of the Horn Silver dump by flotation. The zinc being present as sulphide, the company was having some success in floating that mineral, together with a small amount of sulphide of lead and some sulphide of silver. Most of the lead and a small

amount of silver were left in the tailing from this plant, and the problem of recovering the lead had not been solved. This company, known as the Caldo Mining Co., sent F.

G. Moses to cooperate with the bureau in some large-scale tests.

PLANT USED AND METHOD OF OPERATION.

In these tests a plant of the general form shown in figure 6 was used, each charge of material that entered the system weighing 25 to 50 pounds. This amount was large enough to permit estimating the mechanical difficulties in handling the various products, and also to permit conducting the tests under practically the same conditions as in practice, although the operations tended to be intermittent, whereas similar machinery on a commercial scale could be used continuously. A more representative sample of the dump was taken than the one used in the smallscale tests, in order that the results would have greater value. In each test the brine was siphoned from a storage tank at the head of the plant into the Pachuca agitator. Then the compressed air for agitation was turned on, and the calculated

3

4

6

1, iron storage tank, 4 by 3 feet; 2, iron Pachuca tank, 2.5 by 6 feet; 3, iron settling tank, 4 by 4 by 4 feet; 4, suction filter, 3.5 by 3.5 by 1 foot; 5, solution receiver; 6, precipitation tank, 3 by 2 feet; 7, precipitate filter; 8, sump; 9, air lift; 10, vacuum receiver; 11, vacuum pump; 12, compressed-air receiver; 13, air compressor. All iron parts protected with acid-proof paint.

amount of sulphuric acid was added. The FIGURE 6.-Flow-sheet of test plant. charge of material, after being softened. with a little brine, was then added, and the mixture was agitated for 2 to 3 hours to permit complete solution of the soluble lead. The raw material, as received, contained a large proportion of moisture and therefore enough solid salt was added to saturate this water, in order that the leaching solution would not be diluted.

It was found that practically all of the lead went into solution in the first 15 minutes. The charge was then drained into a settling tank, allowed to settle, the clear solution decanted to the precipitation tanks, and the thickened pulp filtered on a suction filter. Usually the settling period was overnight, although most of the material settled in 2 to 3 hours. The filter (fig. 7), was a "homemade" filter of the intermittent type, which made it necessary

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