Title : Reducing the negative impact of heap gold leaching on the natural environment
Abstract:
As is known, the geotechnology of heap leaching of gold has the following disadvantages: long-term ore mining, Reduced quality with large volumes of simultaneously processed ore, the use of large volumes of irrigated and productive solutions, the presence of settling ponds, the risk of landslides. Significant areas of fertile land or forest lands are often alienated. To reduce these negative consequences, new approaches are presented to intensify mass transfer during heap leaching of gold – periodic irrigation, reduction of ore crushing size, activation of chemical cyanidation reactions (acidification, increased concentration of sodium cyanide, increased temperature of solutions, the use of ultraviolet radiation, special electrolysis preparation of solutions, etc.), which contribute to reducing the noted disadvantages. With the help of various solvents, it is possible to effectively transfer many minerals to a mobile state.
The mechanism of the leaching process is determined by the structure and composition of the dissolved mineral, the nature of the chemical bond in its crystal lattice, and the complex of physico-chemical properties of the solvent. The process in the kinetic domain is characterized by the independence of the dissolution rate from the solvent velocity, small absolute values of the dissolution rate coefficient, and an increase in the dissolution rate coefficient by an order of magnitude with an increase in solvent temperature by 10 °c. The dissolution of most salts proceeds according to diffusion kinetics, i.e. it is determined by the rate of diffusion processes. Particles of a substance dissolved in a liquid are carried away by the latter and transported along with it. The combination of these two processes is the convective diffusion of matter in a liquid. The rate of diffusion of reagents in rocks is estimated at approximately 1 cm/day. Therefore, with heap leaching of gold, the process should be completed in 1-5 days with a crushing size of -10 mm. In fact, with the classic version of gold leaching, the process can last more than 100 days. By increasing the mass transfer due to periodic irrigation, it was possible to increase the degree of gold leaching in 30 days from 20% to 40%. By reducing the fineness of ore crushing to 2 mm and increasing mass transfer by increasing the oxygen concentration from 8 mg/l to 39 mg/l, it is possible to increase the degree of gold leaching to 68%.in order to leach gold from crushed ore with a higher degree and with a larger crushing size, other options for the intensification of gold production are needed.
For this purpose, the chemical reactions of gold cyanidation are activated simultaneously in the entire volume of the stack. This is possible if the process is transferred to an area controlled not only by intradiffusion inhibition, but also by physico-chemical processes, for example, due to their activation by oxygen, increasing the effective concentration of sodium cyanide by more than an order of magnitude. Studies of the temperature dependence of the conditional rate constant at relatively high rates of gold leaching have shown that the e_act at 15-22 ° c is 60 kj/ mol, at 7-15 ° c - 80 kj/mol, which indicated an intermediate region of kinetics, when the degree of gold leaching is influenced by different forms of physico-chemical activation - of which currently time investigated: acidification of solutions; reagent-concentration; ion-hydrogen. This is not a complete list of activation of gold leaching from ore raw materials. Effective methods of activating gold leaching through the use of ultraviolet radiation and through special electrolysis preparation of an aqueous solution are being developed.
