Methods of cyanidation

I. Leaching by infiltration (percolation). This method is used for processing of ores in which the cyanic solution has easy access to gold particles at rather coarse grinding. Such crushing of ore allows to carry out infiltration through significant ore layer. A main parameter of this method is a total of solutions filtering through sand during the certain time (infiltration rate, sm/hour). The parameter depends from granulometric characteristic of material and a way of its loading in a tank. The good infiltration rate - 5 sm/hour. Infiltration rate is characterized by Darsi equation:

Q = - K · A · p/ h · μ

where K – a constant of permeability of a material;

A – the area of section of loading column;

P – pressure drop at leaking through a layer of material;

H – height of loading column;

μ – viscosity of solution.

Permeability constant changes over the wide range; it is determined experimentally in each concrete case.

For impregnation of ore pieces their structure (cracks, capillary channels) and a way of packing of particles have the great value. So, at cubic packing porosity makes 47,64 %, and at rhombic arrangement of particles - 25,96 %.

Wetting of ore particles by solution is improved at addition of hydrophilic surface-active substances in solution.

Cyanidation by infiltration is based on contact of cyanic solutions to particles of ore and gold. During process there is dissolution of gold particles and the natural filtration of solution through a layer of crushed ore (sand). For this purpose gold-containing sand is loaded in extracting tank with a filtering bottom and filled in cyanic solution. After the sand is washed out by cyanic solution of lower concentration, then it is washed out by water. Gold-containing solutions are passed through a sand layer and a filter, followed through a crane and given on sedimentation of gold. Dehydrated sand is unloaded in dump.

Extraction by infiltration is the most simple and cheap way of cyanidation. It demands the very simple equipment and small energy expenses.

Extracting tank is made from wood or sheet steel. At the enterprises with small productivity tank capacity makes 75-100 tons of sand, at the large enterprises the capacity of tanks achieves 800 tons of sand. Height of a tank - 2-2,5 meters, sometimes up to 4 meters. Diameter of a tank changes over a wide range. Usually a tank has a false bottom which serves as a filter. Duration of full processing of one lot of sand - 4-8 day. The average charge of reagents: 0,25-0,75 kg cyanide (on NaCN) and 1-2 kg lime (protective alkali prevents hydrolysis of cyanides) on 1 ton of dry sand.

II. Leaching by hashing of pulp. It is more effective process in comparison with infiltration. At the identical degree of ore crushing this process gives higher degree of gold extraction, than at extraction by infiltration. As a rule, to fuller opening of gold before extraction by hashing thinner crushing of ores is applied. The very liquid pulp left from mills (solid:liquid = 1:5), therefore the great value has operations of dehydration, thickening and filtration.

Thickening consists in partial dehydration of pulp by settling (sedimentation). Firm particles settle on a bottom of tank-thickener, and clarified solution is drained. After thickening a ratio solid:liquid = 1:1.