The general conditions of gold dissolution at leaching by hashing

1. Owing to dispersiveness and high viscosity of pulp diffusion of oxygen is complicated. Continuous saturation of pulp by oxygen (aeration) is necessary condition. This is achieved by intensive hashing of pulp.

2. Gold is present in pulp as fine particles which are dissolved with the high rate at hashing. Therefore at the given way it is possible to use less concentrated cyanic solutions, than at extraction by infiltration.

3. Increase of pulp viscosity has the negative effect on dissolution rate of gold particles. On the other hand, dilution leads to increase in pulp volume, equipment volume, increase of cost of pulp and solutions processing. In practice the small dilution is used: at cyanidation of pulp with the big content of granular fraction the relation solid:liquid = 1, at processing of silt pulp this relation is 1:2,5-3, for sulphidic ores and concentrates – 1:2-6.

Leaching process is carried out by periodic and continuous ways. At periodic way pulp is pumped in parallel working tanks. Pulp is agitated during the certain time and drained in tank-collectors.

At continuous leaching pulp is given in a line of consistently connected tanks where it is agitated. Continuously working extraction system is connected to continuously working dehydration system.

In result of cyanidation the pulp consisting from gold-containing solution and cyanidation tails is formed. For separation of solution from tails decantation (clarification) and filtration are used.

At decantation the pulp is washed out by weak cyanic solution some times for full washing of dissolved gold. Rich gold-containing solutions are given on gold sedimentation. The poor solutions, received at the subsequent washings, are used as turnaround water for washing of next portion of cyanidation tails.

At filtration a firm phase is separated from gold-containing solution by means of porous partition and repeatedly washed out on a filter.

Sedimentation of precious metals from cyanic solutions

For allocation of gold and silver from cyanic solutions the next ways of sedimentation are used: by zinc, by aluminium, ion exchange, wood or activated coals, extraction.

The greatest distribution in practice the first way was received.

Sedimentation by zinc. Physical and chemical bases of process.

In electrochemical series of metals in cyanic solutions a potential of zinc is much more negative than potentials of gold and silver. Therefore metal zinc easily forces out precious metals from cyanic solutions:

2Au(CN)₂^- + Zn = 2Au + Zn(CN)₄^2- (1)

2Ag (CN)₂^- + Zn = 2Ag + Zn(CN)₄^2- (2)

The equilibrium constant of reaction (1) is equal 1,0 10²³, and reaction (2) – 1,4 10³². Hence, in the thermodynamic relation gold and silver can be settled from solution practically completely.

Alongside with the basic reactions by-reactions proceed also. Zinc, being a strong reducer, reduces molecules of water with allocation of gaseous hydrogen:

2H₂O = H₂ + 2OH^- - 2e (3)

Cyanic solutions always contain dissolved oxygen which is reduced with formation of hydroxyl ions:

O₂ + 2H₂O = 4OH^- - 4e (4)

Therefore at cementation the significant part of metal zinc is spent useless:

Zn + 4CN^- + 2H₂O = Zn(CN)₄^2- + H₂ + 2OH^- (5)

2Zn + 8CN^- + O₂ = 2Zn(CN)₄^2- + 4OH^- (6)

Theoretical charge of zinc on gold sedimentation on reaction (1) makes 0,19 g on 1 g gold. Practically owing to oxidation of zinc on reactions (5) and (6) its charge in tens times is higher.

According to the modern representations, cementation is the electrochemical process and it is similar to work of a galvanic microcell. At dipping of metal zinc in cyanic solution between zinc and solution an ion exchange begins. In result on anode sites of zinc there is its ionization, and on cathode sites - reduction of gold, oxygen and water. Electrons thus flow in metal from anode sites to cathode sites.

Process of gold sedimentation proceeds in diffusion area, and sedimentation rate is limited to diffusion rate of Au(CN)₂^- anions to a surface of cathodic sites. Therefore to acceleration of cementation the methods promoting increase of diffusion rate are applied – 1) increase in cathodic surface, 2) intensive hashing, 3) rise in temperature.

1. For increase in cathodic surface preliminary lead-covering of metal zinc is carried out. For this purpose metal zinc is processed by solution of any soluble lead salt (acetic or nitrate). On zinc surface the spongy friable deposit of metal lead is formed which has the very big specific surface. Application of lead-covering zinc considerably accelerates sedimentation process.

2. Intensive hashing gives double effect. On the one hand rate of gold sedimentation grows as diffusion of Au(CN)₂^- ions to the surface increases, on the other hand - reduction of oxygen increases that results to the significant useless charge of zinc. Therefore in practice cyanic solutions before sedimentation of precious metals from them are subjected to deaeration (removal of oxygen), and sedimentation is made by infiltration of anoxic solution through dispersed zinc layer.