Lecture 4 Cyanidation (cyanide process) of gold-containing ores

Plan of lecture:

1. The essence of cyanidation process.

2. Thermodynamics and kinetics of cyanidation process.

3. The electrochemical nature of precious metals dissolution.

4. The factors accelerating diffusion as the probable ways of intensification of cyanidation process.

5. Interaction of cyanic solutions with accompanying minerals.

6. Methods of cyanidation (extraction by infiltration, extraction by hashing of a pulp).

7. Sedimentation of precious metals from cyanic solutions.

8. Sedimentation by zinc. Physical and chemical bases and practice of process.

The considered above methods of gravitational concentration and amalgamation allow to extract only relatively coarse gold from ores. However the overwhelming majority of gold-containing ores alongside with coarse gold contains significant and sometimes prevailing quantity of fine gold which cannot be taken by these methods. The basic method of fine gold extraction is cyanidation.

Cyanidation process consists in extraction of precious metals by diluted solutions of cyanic salts of alkaline or alkali-earth metals (KCN, NaCN, Ca (CN)₂). The received gold-containing solutions are separated from a firm phase (dump tails) by thickening (condensation) or filtration and directed on sedimentation of precious metals by metal zinc. A deposit of precious metals is given on refining for preparation of pure gold and silver.

Thermodynamics of cyanidation process. According to Elsner (1846) dissolution of gold in cyanic solutions at presence of oxygen proceeds on the following reaction:

4Au + 8KCN + O₂ + 2H₂O = 4KAu (CN)₂ + 4KOH

On 1896 y. Bodlender has shown, that at dissolution of gold in cyanic solutions one more product is formed – hydrogen peroxide:

2Au + 4CN^- + 2H₂O + O₂ = 2Au(CN)₂^- + 2OH^- + H₂O₂

which then in part can be spent for the further oxidation of gold

2Au + 4CN^- + H₂O₂ = 2Au(CN)₂^- + 2OH^-

and in part to decompose

2H₂O₂ = 2H₂O + O₂

or to collect in the solution. Similarly there is silver dissolution.

From the equations it is visible, that gold is oxidized by oxygen or hydrogen peroxide up to monovalent condition and passes in the solution as complex anion Au(CN)₂^-.

Thermodynamic probability of cyanidation reactions (to study independently).

Kinetics of cyanidation process. Interaction of precious metals with cyanic solutions proceeds on phase boundary of two phases - firm and liquid. Therefore, the cyanidation process is the typically heterogeneous process and it’s speed is determined according to the general laws for heterogeneous processes.

Let's imagine a particle of gold or silver, placed in cyanic solution which is in contact to gaseous oxygen or air. In the result of chemical interaction proceeding on the metal surface, cyanide ions and oxygen molecules are spent and their concentration in the nearby to gold layers is reduced. The difference of reagents concentration near to phase boundary and in depth of solution leads to diffusion of ions СN^- and oxygen molecules from volume of solution to surface of gold particle. In process of oxygen concentration reduction in solution new portions of oxygen will pass from gaseous phase in liquid phase, filling thus its loss.

Thus, process of gold dissolution in cyanic solution will consist of four stages:

1. absorption (dissolution) of oxygen by cyanic solution;

2. transport of СN^- ions and oxygen molecules from solution volume to a surface of metal;

3. chemical reaction on the metal surface;

4. transport of soluble products of reaction (ions Ag(CN)₂^- and OH^- or molecules Н₂О₂) from the metal surface in volume of solution.

Each stage is characterized by individual speed, and anyone of them can be the slowest (limiting), i.e. will define the general speed of process.

The numerous experiments are shown, that at hashing the process of dissolution of precious metals proceeds in diffusion mode and depending on concentration of СN^- ions and molecules of dissolved oxygen it is controlled either diffusion of cyanide, or diffusion of oxygen. In the field of low concentration of cyanide diffusion of dissolved oxygen molecules proceeds quickly and does not limit a rate of metal dissolution. Diffusion of СN^- ions opposite proceeds with small rate. In the field of high concentrations of СN^- ions a rate of cyanide diffusion becomes higher than a rate of oxygen diffusion. For increase a rate of oxygen diffusion it is necessary to raise the partial (fractional) oxygen pressure above the solution.

The mechanism of dissolution of gold and silver is identical, the distinctive feature of gold - propensity to passivating. It is shown that in the beginning at increase in hashing intensity a speed of gold dissolution increases, and then it starts to reduce or remains constant. The phenomenon of gold passivating is known for a long time however the nature of process till now is not established. It is supposed that at excessive intensity of hashing the molecules of dissolved oxygen are adsorbed on gold surface and inhibit the dissolution process. Introduction of small amounts of surface-active substances (spirits, ethers) in solution leads to that these substances displace adsorbed oxygen on the metal surface therefore rate of gold dissolution grows.