Gms.

15.8

0.75

0.207

2.31

None.

3.61

7.7

67.1

7.3

2

500 16.6

450 0.25

1.12

100

1.62

1.62

42.0

50.4

21.2

18.54

.12

1.54

3.96

None.

17.05

1.65

65.1

11.05

400

.05

.200

138

.82

3

1.14

500

16.6 10.0

53.0

22.54

5.3

.18

.27

3.26

None.

9.76

5.70

67.5

7.9

480

.15

4

500

16.6 21.8

.720

102

.87

.89

49.8

6.86

10.8

15

.208

4.6 None.

7.45

12.50

62.8

5.8

860

.115

.417

250

785

5

500

16.6 27.0 51.0

1.96

13.18

13.8

28

54

4.08

None.

12.95 4.69

65.2

9.45

430

6

.10

.43

124

95

500

16.6 29.65

1.18

55.7

16.5

20

234

2.32

None. 11.00 3.08

15.41

71.4

6.90

600

.05

.30

173

1.00

1.73

18.53

From this set of tests the following conclusions may be drawn:

1. It is possible to treat ores with calcium chloride and sulphuric acid to obtain zinc chloride solutions containing only small amounts of dissolved calcium sulphate.

2. These solutions can be purified of iron, aluminum, and manganese by the use of chlorine gas followed by the use of crushed lime rock, which causes hydrolysis of the highly oxidized compounds of these metals, the generation of calcium chloride during hydrolysis precipitating some of the calcium sulphate dissolved in the zinc chloride solution.

3. Precipitation with lime gives a hydrate of zinc and regenerates calcium chloride for use in the next cycle.

4. The main impurities of the zinc hydrate are excess calcium hydroxide, calcium sulphate, calcium chloride and zinc chloride. More than 8 hours' contact of solution with the lime is necessary for converting all the calcium hydroxide to the soluble calcium chloride. Careful washing will remove most of the calcium chloride and much of the zinc chloride. During ignition to the form of oxide, most of the remaining zinc chloride is volatilized. The content of calcium sulphate can be kept down by using solutions containing more than 5 per cent zinc and by adding an excess of calcium chloride to the zinc chloride solution.

5. About 6 per cent of the total zinc content in the zinc hydroxide precipitate is present as zinc chloride, which is volatilized during ignition to the oxide.

6. With 3.5 per cent zinc solutions the final zinc oxide product will average over 65 per cent zinc.

7. No filtration or thickening difficulties are encountered.

BISULPHITE SOLUTIONS.

The employment of bisulphite solutions as a means of recovering zinc has attracted some attention in times past, so that the literature is full of speculations and arguments as to the advantages of sulphurous acid leaching of zinc, but to date no one has been able to handle the precipitation problem satisfactorily.

It has been proposed to recover the zinc as the monosulphite from such solutions by heating them to a sufficient temperature to drive off the extra sulphur dioxide. This sulphur dioxide was to be used over again in the leaching step of the process. Tests of sulphurous acid leaching made at the Salt Lake City station and of sulphurous acid as a solvent are described in another part of this bulletin. Practically all of the zinc in the bisulphite solutions could be precipitated as insoluble zinc monosulphite by boiling, but the solution had to be boiled till more than half of the water had been evaporated. Further, the zinc bisulphite solutions oxidized rapidly, so that the solutions tested contained nearly 50 per cent of the zinc as sulphate.

As the small-scale laboratory tests probably afforded a better chance for air to act on the surface of the solution than would probably be found in practice, the tests were repeated, care being taken to prepare a fresh solution which had had a minimum contact with the air. Even then 33 per cent of the zinc oxidized to the sulphate condition before the precipitation tests by boiling could be performed. On that account, in any process involving the precipitation of zinc bisulphite solutions provision must also be made for precipitating zinc sulphate. This necessity has led some to propose evaporating the solution to dryness and recovering only zinc sulphate, which could be sold as such or calcined to form sulphur dioxide and zinc oxide. The cost of evaporation would be a serious objection to such a method, as not more than a 10 per cent zinc solution can be prepared and the amount of water to be evaporated is too large.

Also, tests were made of the method of precipitating such solutions by the addition of enough zinc oxide from a later stage of the process to neutralize the acidity due to sulphurous acid and to the bisulphite of zinc. This would form zinc monosulphite, which is insoluble and should precipitate. On filtering the precipitate from the solution and calcining it, the zinc monosulphite breaks up into sulphur dioxide and zinc oxide and no solution has to be evaporated. This method worked very well with the zinc bisulphite solutions, but zinc sulphate solutions would not precipitate. Consequently, this method would not meet the requirements mentioned of having a precipitant which would recover the zinc from both zinc bisulphite and zinc sulphate solutions, but was preferable to heating the solution of zinc bisulphite.

Ammonia will precipitate zinc from either sulphate or bisulphite solutions, after which the ammonia can be recovered from the precipitated solutions by boiling with lime. However, absorption of ammonium compounds by the precipitated zinc hydroxide, together with mechanical losses of the ammonia, might make this process unattractive. Sodium hydroxide will also precipitate the zinc from both forms, but is too expensive.

SUMMARY.

Summarizing, the precipitation of zinc from its various solutions constitutes the major problem in the hydrometallurgy of zinc. Electrolytic precipitation is only being perfected and the installation costs of an electrolytic zinc plant are so great that such a process could be used only for the treatment of large bodies of ore or in a custom plant. Chemical precipitation of a zinc oxide product from most of the solutions that are capable of commercial preparation is difficult. Zinc chloride solutions are easily precipitated by lime, although the product is not of the highest grade. Sulphate solutions of zinc can not be precipitated economically by any known direct precipitation process, but can be precipitated after conversion to zinc chloride. The sulphite solutions of zinc deserve further study.

OXIDIZED ZINC ORES.

INTRODUCTION.

Beneficiation of low-grade oxidized ores of zinc has long been a baffling metallurgical problem, and satisfactory methods of treatment have still to be worked out. Most ores of this type are ocherous and earthy and can not be concentrated mechanically. On that account such ores are not worked for their zinc content alone unless they are of sufficiently high grade for direct shipment to the smelter. Throughout the intermountain western region are deposits of oxidized zinc ore containing as high as 25 per cent zinc which can not be concentrated by any known method and is rarely accepted by zinc smelters. For a time, shortly after the European war sent the price of zinc up, ores with as low as 20 per cent zinc content were shipped from the intermountain States to the zinc smelters in the gas belt, because the smelters did not have sufficient roaster capacity and wished oxidized ores in order to keep the distillation furnaces going at full capacity while the roasting equipment was being augmented.

CONCENTRATION PROCESSES.

GRAVITY CONCENTRATION.

As mentioned, the ores containing oxidized zinc minerals are usually too ocherous and clay-like to permit mechanical separation. A few ores are relatively "free" and can be concentrated by jigs, tables, and other gravity concentration machines. The calamine ores of Arkansas, which are being concentrated and shipped to zinc smelters, are an example of this type of "free" ore. Ores containing 2 to 10 per cent zinc are the ones usually mined and milled in this way, the Joplin type of wet mill being in favor. Needless to say, all of the zinc mineral in the more finely divided ore is lost, and the slime problem is important; flotation has not been satisfactory. Hence, recoveries by gravity concentration vary from 40 to 70 per cent, probably averaging about 60 per cent. The oxidized zinc ores of the intermountain western region are usually too intimately associated with the gangue material to be amenable to this type of treatment. In Arkansas and Missouri the oxidized zinc ores were formed from the oxidation, in place, of blende coarsely disseminated in a flint or chert gangue, whereas those of the intermountain region are from oxidation of contact metamorphic deposits or of replacement deposits and are usually contaminated with the oxidized minerals of iron, lime, magnesia, manganese, and other metals. Much of this

ore is so yellow with limonite that individual particles of smithsonite or of calamine are very difficult to distinguish. It is these ores that resist gravity concentration.

FLOTATION.a

As in most western oxidized zinc ores the minerals are finely intercrystallized, concentration of such ores by fine grinding and flotation seemed possible. Sulphidizing and flotation has become popular for the treatment of oxidized ores of lead and of copper, and it was thought the same method might be applied to oxidized zinc ores. However, in none of the ores tested could any apparent concentration be made of the zinc carbonate and zinc silicate minerals by the usual sulphidizing methods. In fact the flotation froth from the yellowish ocherous ores from Utah, Nevada, and Colorado was often slightly lower in zinc than the tailing. Sulphureted oils, ammonium sulphide, and other such reagents all failed.

Mr. Frank Bird, of Salt Lake City, reported that by sulphidizing with sodium sulphide followed by the use of oleic acid as a flotation oil, he was able to obtain a partial flotation of oxodized zinc minerals. By making up a synthetic ore of zinc carbonate and quartz it was possible to verify his tests, but when the method was applied to a number of natural ores, high in lime, the result was that the lime was floated and the zinc minerals were left behind with the other gangue minerals. Supposedly a superficial film of metallic oleates is formed in the presence of sodium hydroxide, and these oleates are easily "oiled" by the flotation oil. Possibly resinates could be used for the same purpose, as is probably being done at the plant of the Chino Copper Co. at Hurley, N. Mex., where copper carbonates are treated with a solution containing sodium sulphide, sodium resinate, etc. However, commercially valuable results in the flotation of zinc carbonate by any of these methods were not obtained in the tests at the Salt Lake City station.

Where a small amount of lead carbonate was present with the zinc carbonate, usuaily the lead carbonate could be sulphidized and floated in the presence of the zinc, providing no manganese dioxide, lead peroxide, basic sulphates of iron, or other elements deleterious to sulphidizing were present.

MAGNETIC SEPARATION OF LIMONITE AND OXIDIZED ZINC

MINERALS.

Magnetic separation of limonite in oxidized ores of zinc was deemed possible because a very slight reducing roast is reputed to render limonite "magnetic," so that it could be removed from the ore with an ordinary low intensity magnetic separator.

• Experimenters: R. W. Johnson and A. J. McChrystal.
Experimenters: S. S. Arentz and O. C. Ralston.