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optimal design and planning of heap leaching

optimal design and planning of heap leaching

5 per cent copper sulphate, 2 per cent sulphuric acid, 2 per cent copper sulphate or ferrous sulphate plus sulphuric acid, 2 per cant ferric sulphate, and distilled water have very nearly the same rates of penetration.

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When cuprite is leached, either in the presence or in the absence of oxygen, a layer of metallic copper forms practically as soon as the mineral comes in contact with sulphuric acid. This metallic copper forms a difficulty permeable layer on the surface of the particles that slows down the dissolution. The metallic copper may be converted to copper sulphate by the aid of an oxidizer. Atmospheric oxygen is a fairly good oxidizer but ferric sulphate is a much better one. On particles 100 mesh or smaller in size this metallic coating of copper does not markedly hinder the rate of dissolution, but it is very harmful for larger sizes.

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The rate of dissolution of bornite is markedly increased by increases in temperature. When minus 100 plus 200 mesh bornite was leached with acidified ferric sulphate 64 per cent of the copper was dissolved in 1 day at 50° C., in 4 days at 35° C., and in 14 days at 23° C. Eighty per cent of the copper was dissolved in 6 hours at boiling temperature. Bornite dissolves more rapidly in ferric chloride than in ferric sulphate. Sulphuric acid plus air attack bornite more slowly than ferric sulphate solutions.

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Chalcopyrite is frequently found in leaching ores, but it is not appreciably attacked by common solvents at ordinary temperatures.

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The foregoing data show that the rate of dissolution is faster when the mineral is more finely ground, that the rate increases with an increase in temperature, and that the rate of dissolution is more rapid in ferric chloride than in ferric sulphate.

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In heap leaching, a complete drying would be practically impossible. As a heap may contain several million tons of ore complete drying, even of the surface, could hardly be expected. Experiments have shown that the copper can be brought to the surface even though the particles of ore are only partly dried.

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In heap leaching, very short periods of alternate wetting and drying can not be maintained, but laboratory work has shown the advantage gained by keeping the cycles as short as possible. With 3 inch pieces of ore, an extraction of 80, per cent of the water soluble copper was obtained in 6 hours with a 0.5 hour period of drying and a 0.5 hour period of washing, whereas 25 hours was required for a 6.0 hour period of drying and a 2.0 hour period of washing. Any advocacy of shorter cycles in altennate wetting and drying presupposes that the heaps are porous and wall, aerated.

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The soluble copper can be removed by alternate wetting and drying in approximately 15 to 25 per cent of the time required to remove it by flood, washing, provided the washing and drying periods are as close to each other as possible but long enough to permit a fairly thorough drying of the charge and soaking in of the leaching solution. As an example, it took approximately 150 hours to remove 90 per cent of the water soluble copper by vat washing from the minus 1 plus ¾ inch size of a porphyry ore saturated with copper sulphate, where as only 31½ hours was required by alternate wetting and drying when the period of drying was 4.0 hours and the period of washing 0.5 hour.

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A rapid movement of air past the surface of the ore promotes rapid drying. Anything that interferes with the circulation of air slows down the rate of extraction, thus demonstrating the necessity of having an open heap where free circulation of air is possible. Slime or other material that will coat the surface would also hinder drying. The fate of extraction is also increased by an increase in temperature.

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