Practice of process

Two ways of precious metals sedimentation by zinc are used: sedimentation by zinc shaving and sedimentation by zinc dust.

1 stage - clarification of cyanic solutions. Solutions, giving on sedimentation by zinc, should be absolutely transparent, as the suspended particles settle on zinc surface, prevent its contact to solution and interfere with gold sedimentation. Besides the suspended particles pollute gold deposit that complicates it’s further processing. Therefore gold-containing solutions are subjected to clarification. Sand filters, leaf vacuum-filters, leaf filter-presses are applied to clarification.

2 stage - sedimentation of gold by zinc shaving or zinc dust. Sedimentation by zinc shaving is carried out in extractor. The extractor represents a long low box of rectangular section. The box is divided into some sections. The solution is given into each section only from below. For slow flowing of solution the box is established with a small bias. In each section of extractor the frames with iron grid are located on which zinc shaving is stacked as friable layer.

Solution in extractor goes in zigzag in kind of slow continuous jet through shaving. Precious metals are settled on shaving surface. At sedimentation in ex-tractors zinc shaving usually is not lead-covered. If it is necessary, lead-covering is made by dipping of zinc shaving in 5-10 % solution of lead nitrate or lead acetate for 1-2 minutes.

Unloading of gold-zinc deposit from extractor is made on the average with 1-2 times a month.

Sedimentation by zinc dust - the basic way of sedimentation of precious metals from cyanic solutions. Essence of this way: the clarified solution 1) is subjected to deaeration, 2) mixed with zinc dust and 3) filtered for separation of gold-zinc deposit from gold-free solution at simultaneous sedimentation of gold.

Deaeration is carried out in a vacuum-receiver (deaerator). A vacuum-receiver represents a hollow steel cylinder in which top part a lattice from wooden bars is located. Falling from above on this lattice, the solution is broken into fine drops. It promotes fast allocation of dissolved gases under action of pressure created by a vacuum pump. Deaeration of solutions allows to lower considerably a charge of zinc, to increase completeness and speed of gold sedimentation, to improve quality of gold deposits.

Mixing with zinc dust is carried out in an amalgamator, then solution is given in settler with vacuum-frames. A level of solution in settler should be higher than a top part of settling frames. Under action of pressure gold-free solution is passed through frames, and gold slime remains on a surface of filtering fabric as cake. Lead-covering of zinc dust is made by submission of lead acetate or lead nitrate solution in amalgamator or clarificator. A quantity of lead salts makes approximately 10 % from weight of loaded zinc dust.

Device unloading is made with 2-3 times a month. Gold deposit is pumped over in a filter-press where it is filtered and washed out. The dehydrated deposit is given for further processing.

In comparison with sedimentation by zinc shaving process of sedimentation by zinc dust has the following important advantages:

1) Zinc dust is cheaper than zinc shaving;

2) A charge of zinc dust is lower, than a charge of zinc shaving;

3) Fuller sedimentation of gold;

4) Reduction of cyanide charge;

5) Absence of turnaround gold and zinc;

6) Considerably higher quality of deposits that facilitates their further processing;

7) Compactness of equipment;

8) Full mechanization and automation of process.

Control questions:

1. The purpose and essence of cyanidation process.

2. Which kind of gold is extracted by cyanidation?

3. Write the chemical reaction of gold dissolution in cyanic solutions.

4. Stages of gold dissolution in cyanic solutions.

5. Kinetics of process. Which stage defines the general rate of cyanidation process?

6. Explain the phenomenon of gold passivating.

7. Which factors confirm the electrochemical nature of gold dissolution?

8. Call the factors raising the rate of gold dissolution in cyanic solutions.

9. The essence of extraction by infiltration. The definition of process rate, Darsi’s equation. The factors influencing on percolation rate of cyanic solution.

10. Construction of extracting tank.

11. The favourable conditions for extraction by hashing of a pulp.

12. The schemes of periodic and continuous extraction.

13. Which methods are used for precipitation of gold from cyanic solutions?

14. Write the chemical reactions of precipitation by zinc.

15. Which methods are used for increasing of diffusion rate at precipitation by zinc?

16. The purpose of lead-covering of metal zinc.

17. How hashing of a pulp influences on rate of gold precipitation?

18. Stages of gold precipitation by zinc dust.

19. Advantages of this way.