Lecture 2 Forms of speciation of precious metals in ores
Plan of lecture:
Geochemical data of formation of gold-ore deposits.
Location of gold deposits on Kazakhstan territory.
Gold minerals.
Silver minerals.
Preparation of ores for extraction of precious metals (crushing and grinding of gold-containing ores, sorting and concentration).
Gravitational methods of extraction of precious metals from ores.
Geochemical data of formation of gold-ore deposits. The content of gold in the earth's crust makes approximately 5·10-7 % (about 5 mg/t), i.e. gold - rather rare element; in comparison with silver its content in the earth's crust in 20 times, and in comparison with mercury - in 200 times is less. Distribution of gold in the earth's crust is non-uniformly. It is established, that the basic rocks contain insignificant quantity of gold. It is considered, that all primary gold deposits are connected with magmas intrusions and their differentiation during gravity sedimentation, i.e. gold is brought in the earth's crust by underground eruptions.
Gold deposits are not a complex of minerals which were allocated one-time and at the identical temperature. They represent the association of enclosing rocks and minerals consistently formed during the long time intervals and the big temperature intervals. It is considered, that gold was allocated in most cases in last stage of formation of deposits from the cold hydrothermal solutions. This process proceeded in some stages. Thus gold was allocated on anyone minerals: on quartz, on various sulfides, more often on pyrite and arsenopyrite; and not only on a surface of crystals, but also inside them.
Under certain conditions gold could be allocated in the form of fine crystals and disperse inclusions. Under favorable conditions the large crystals or aggregates-nuggets could be formed.
Depending on conditions of formation and structure of hydrothermal solutions could be formed:
- Quartz gold-ore deposits (enclosing rock - quartz);
- Quartz-sulphidic deposits (enclosing rocks - quartz and sulfides);
- Sulphidic gold deposits.
After formation the primary gold deposits could be exposed to erosion under influence of atmosphere and superficial waters. This process carries out to concentration of part of gold which is taking place in veins, and to dispersion of other part. The destroyed material is transported, precipitated in other places and covered by deposits of various thicknesses. So placer deposits are formed.
Location (lecture on discipline «The mineral raw material of Kazakhstan).
Minerals of gold. Gold in ores is present basically as particles of the native gold with the various form and sizes. Gold, as a rule, chemically is not connected to other elements, frequently forms the native alloys with non-uniform composition and structure. From chemical compounds of gold as minerals there are only tellurides and sometimes selenides. Usual impurities in the native gold - silver, copper, iron. Arsenic, bismuth, tellurium, nickel, manganese and other metals presence in small quantities.
Under numerous analyses of native gold from various deposits the gold content in the allocated grains makes 75-90 %, the silver content - 1-10 %, the iron content - from traces up to 1 %, the copper content - is not higher than 0,2 %. Only in separate samples of cupreous gold (for example, Karabash in Ural) the copper content makes 19-20 %. Palladeous, rhodeous and platinous gold meets in copper-nickel deposits.
The nature of gold minerals long time represented a riddle. Sometimes came out with assumptions, that minerals of gold are true alloys of silver with gold of high-temperature formation.
However it has been shown later, that natural minerals of gold are not alloys, they were formed by sedimentation from hydrothermal solutions. The average chemical composition of gold grains has the great importance for kinetics of processes of gold extraction by amalgamation and leaching, but the composition of superficial layers on gold particles is especially important. The surface of particles of native metals is covered by oxide films which complicate process of gold extraction.
It is possible to classify surface films as follows:
- The films, formed by sulphidic minerals (arsenopyrite, galenite);
- The dense shells of iron oxides, complicating extraction of gold;
- The black shells from argentite (Ag2S);
- The color films on gold;
- The dark oxide films.
Alongside with a chemical composition of gold particles and their surfaces for hydrometallurgy the great value has their form and size. Gold grains in ores have usually the developed surface, that positively influences on dissolution of gold at its extraction by hydrometallurgical ways. The wrong form of particles facilitates their catching on concentrating devices. The size of gold particles determines a degree of ore crushing at processing. In placers and primary ores quite often find the large nuggets, but the basic mass of gold in primary ores is submitted by the fine particles which can be found out only under a microscope at the big increase.
Minerals of silver. Silver meets in the native kind and as part of gold minerals. However as against gold silver present in ores as minerals of silver which represent chemical compounds (more often with sulfur).
As against gold the surface of native silver is exposed to significant changes. Under influence of light and oxidizers it is covered by a thin black film from silver oxide and hydrate of silver oxide or disperse metal silver which is formed at decomposition of chemical compounds. Sometimes this film is very thin and gives to silver a yellowish, golden shade.
The most widespread minerals of silver:
1. Horn silver or cerargyrite AgCl (native silver chloride) meets in the oxidized kind;
2. Silver glance or argentite Ag2S meets in sulphidic ores; frequently contains Cu2S;
3. Gessite meets in hydrothermal deposits;
4. Antimony-silver and arsenic-silver sulphidic minerals: stefanite 5Ag2S·Sb2S3, pyrargyrite 3Ag2S·Sb2S3, prustite 3Ag2S·As2S3, discrazite Ag3Sb2 are widely distributed in polymetallic ores;
5. Polybazite 9(Ag,Cu)2S · (Sb,As)2S3 and silver-containing versions of tetraedrite 3(Cu, Ag)2S · Sb2S3 (freybergite and argentotennantite);
6. Argentoyarozite AgFe3(OH)6(SO4)2 meets in ores of secondary origin (iron hats, etc.).