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SURVEY OF U.S. MINERAL AND METAL PROCESS - page 8 / 12

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Zinc is produced electrolytically from dilute aqueous solutions of zinc sulfate in sulfuric acid although it can also be produced electrolytically from molten salts. The aqueous solution route is adopted in industry. Like the thermal processes of zinc production, the industrial electrolysis of zinc uses oxide starting-materials. The most important natural raw material, zinc blende, still needs to be roasted so it is converted to oxide. Using typical starting materials, the electrolytic process of zinc production consists of roasting, leaching, liquor purification, electrolysis, and finally melting and casting. The main problem in leaching and liquor purification is Zn-Fe separation. High yield of zinc can be achieved only with Fe concentrations of Because the Fe interferes with the electrolytic process even at low concentrations, it must be precipitated from the zinc sulfate solution. Liquor produced by zinc

~3%.

electrolysiscontains

25-30%

zinc.

Mineral formation residue

Mineral formation has replaced the precipitation of iron hydroxide worldwide to enhance zinc

production. In the jarosite process, an Fe

  • -

    where X represents

or

(3+) NHa+,

H30f, Na+, K+,

compound of the type

X[Fe@O&(OH)6],

is precipitated by adding alkali metal or

ammonium ions. These compounds correspond to the mineral jarosite. Precipitation of jarosite begins at and is complete at 1.5. The jarosite process can achieve zinc yields of 96- 98%. Iron can also be removed from electrolytic solutions by the goethite process. Goethite is

pH 4

pH

FeO(OH)

formed at

pH

2-3.5 and temperatures from 70 to 90°C. The goethite process has the

advantage of generating fewer residues than the jarosite process, and the zinc yield is

comparable to the jarosite process.

The hematite process was developed to enable Fe-containing residues from zinc production to

be disposed of at moderate

cost-and

without ecological problems. It differs from the other

processes in that the residues are subjected to reductive leaching in which the reducing agent is an excess of zinc concentrate. However, the cost and complexity of the hematite processing technology restricts more widespread use of the process As recognized by the National Research Council, mineral formation processes could potentially be improved by developing

[8].

methods to suppress the solubility of undesirable elements in leach solutions.

Thermal processing residue

Thermal processing produces dust, fine particles and slag that can-contain

8-

12% zinc.

Evaluation of by-product recovery economics

Once a process residue is identified as having by-product recovery potential, researchers can evaluate methods to separate impurities from valuable elements (e.g., separate mercury from copper-bearing process residue). Criteria such as favorable economics and reduced environmental impact are used to evaluate these opportunities. However, data are not readily available to describe the market potential and profitability of new technologies. Data on the value of process residue before separating impurities, after separation, and the cost of separation are all necessary in order to evaluate a technology’s market potential (Table 2). Additional data are required on the amount of process residue that is available for processing. For instance, consider the cost to recover, by-products from acid plant sludge generated as part of copper smelting operations and the value of those by-products. This process residue (acid plant sludge) has some initial value before impurities (mercury) are separated (Table 2, Row A). This value may be negative because the residue may be designated as “waste” if it cannot

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