The electrochemical nature of precious metals dissolution

Till now the process of dissolution of precious metals in cyanic solutions was considered as only the chemical process (connection of metal with molecules of dissolved oxygen and cyanide ions). However, according to the modern representations, this process is electrochemical and submits to the general laws of electrochemical corrosion of metals. According to it dissolution of gold and silver in cyanic solutions can be considered as the result of action of short-circuited galvanic cell, in which one electrode is the gold particle, and another electrode – any electrical conducting mineral in which this particle is impregnated. Julian and Smart have evidently shown on experience the opportunity of such mechanism of dissolution.

Solution containing CN^- ions is poured into the vessel, divided into two parts by a porous partition. A gold plate is immersed in one of vessel section, pyrite piece – in another section. Gold and pyrite are connected through galvanometer. At the moment of short circuit of electrical network the pointer of galvanometer deviates from zero, showing passage of current in external circuit from gold to pyrite. However already through the short time the current in the circuit starts to decrease and soon practically disappears. If now through the solution in which pyrite was immersed to pass an air or oxygen bubbles, in external circuit of element again there is a current which will proceed until air is passed.

Results of this experience well explain a role of oxygen. On the surface of gold electrode there is releasing of electrons as a result of gold oxidation with formation of complex anion Au(CN)₂^-. For steady course of this process it is necessary to use the depolarizer, absorbing these electrons. Oxygen reducing on pyrite surface up to hydrogen peroxide acts as depolarizer.

In this case the mechanism of gold dissolution is electrochemical.

The basic difference of electrochemical mechanism from only chemical mechanism lies in the fact that the general reaction of interaction of metal with reagents at electrochemical corrosion is divided on two independent processes:

1) Anode process - transition of metal as ions in solution with leaving of equivalent quantity of electrons in metal;

2) Cathodic process - assimilation appeared in metal superfluous electrons by any depolarizer (for example, oxygen).

The opportunity of such division of the general reaction of corrosion is explained the electronic conductivity of metal and ionic conductivity of solution. This circumstance allows anode and cathode processes to proceed as well territorially separately - on various sites of interface metal - solution. Space division of anode and cathode process is not necessarily, and in some cases both processes proceed on one surface, alternating in time. However in most cases the territorial division of anode and cathode processes is more favourably in the power relation as each of them can be located on those sites where their course is facilitated. Therefore division of metal surface into anode and cathode sites - the second important difference of electrochemical mechanism of corrosion from only chemical mechanism. This division is explained the heterogeneity of surface in electrochemical relation. The reasons of heterogeneity can be various: electroconductivity of micro- and macro inclusions, deformations and internal stress in metal, etc. It is considered, that the surface of even very pure metal nevertheless is non-uniform owing to a crystal lattice of metal always has defects.

It is known, that speed of electrochemical process depends on potential of a surface on which this process proceeds. Displacement of potential in the positive side accelerates the course of anode process and brakes cathode process. Displacement of potential in the negative side brakes anode process and accelerates cathode. Presence of connection between speed of dissolution process and potential of metal is the additional attribute of electrochemical mechanism.

Experimental conditions of research of cyanidation process are easier, than the real conditions. Particles of gold in ore have various dispersiveness and structure whereas in experiences the samples are made from chemically pure gold and silver and have the correct geometrical form. Used in experiences cyanic solutions are absolutely pure whereas the industrial cyanic solutions contain the significant amount of impurity. Besides used ores contain the foreign minerals, capable to cooperate with cyanic solutions that causes numerous by-effects. Nevertheless, the most important and basic moments of cyanidation process are well explained by results of laboratory experiments.

The analysis of work of gold-extracting factories has shown, as in real industrial conditions process of dissolution of precious metals has diffusion character. In this connection, all factors accelerating diffusion are considered as probable ways of intensification of cyanidation process. To them concern:

1. Diffusion rate grows with increase of hashing rate. Therefore, using the intensive hashing it is possible to achieve substantial growth of dissolution rate irrespective of the fact which kind of diffusion (CN^- ions or molecules of dissolved oxygen) is limiting. Therefore application of devices with high hashing intensity is expedient.

2. Optimum cyanide concentration will be such, at which rate of diffusion of CN^- ions and oxygen become equal. The further increase in cyanic solution concentration is inexpedient and will not lead to increase in dissolution rate. Experimentally established optimum concentration of cyanide - 0,02-0,10 %.

3. The structure of gold-containing ores includes the various accompanying minerals, capable to be oxidized with the big rate. The secondary oxidation reactions reduce oxygen concentration and rate of gold dissolution. Hence, one more important factor – intensive aeration of a pulp.

4. The extraction of precious metals at high partial pressure of oxygen allows to increase rate of process in tens times.

5. Rate of precious metals dissolution rather poorly grows with rise in temperature. At the same time, the increase in temperature considerably accelerates the secondary reactions of interaction of cyanide with accompanying minerals, hydrolysis and decomposition of cyanic solutions. In practice the temperature of solutions is 15-20⁰C.

6. Diffusion rate depends on a size of surface through which diffusion proceeds. Therefore, a size of gold particles is one of the major factors determining dissolution rate. The specific surface of fine gold particles is more, than specific surface of coarse ones, therefore they are dissolved faster. Full dissolution of coarse grains demands a lot of time (3-4 hours). During the crushing the sizes of gold particles, possessing the big flexibility, essentially does not vary. Therefore factories usually carry out the gravitational concentration or amalgamation for extraction of coarse gold. It is necessary to take into account however, that extremely fine-dispersed gold also complicates the processing of gold-containing ores. It is very difficult to open such fine gold even at the most intensive crushing. Therefore ores with thin gold refer to category of refractory ores.

7. The specific surface of particles is defined not only their sizes, but also their form. Therefore a form of gold particles also influences on cyanidation rate. At equal mass of gold particles rate of dissolution of spherical particles will be less, than cubic ones, and cubic particles - it is less, than lamellar, etc.

8. Diffusion coefficient is inversely proportional viscosity of pulp. As against quartz ores, the clay and ochreous ores form pulps with the high viscosity owing to swelling of colloid particles (slimes). Dissolution rate of gold in such pulps is considerably reduced also process is necessary to carry out at high degrees of deliquation. However it leads to increase in a volume of equipment and high charge of reagents. Therefore ores with the high content of slimes refer to category of refractory ores.

9. Dissolution rate of metals depends from ligature structure of gold particles and their contact to other electroconducting materials. For example, presence of copper and silver accelerates transition of gold in solution. On the contrary tellurium complicates gold dissolution. Rate of natural tellurides dissolution is much lower, than rate of pure gold dissolution. Native platinum in cyanic solutions practically is not dissolved and leaves in tails. If platinum forms the firm solutions with silver and gold it can be dissolved in part, but thus rate of gold dissolution is sharply reduced.

10. At cyanidation of amalgamation tails a pulp contain a plenty of mercury and amalgam. Mercury and amalgam gold are dissolved extremely slowly and the basic part of gold is lost with tails.

Interaction of cyanic solutions with accompanying minerals (the section to study independently).