Gravity enrichment of gold-bearing ores

Currently, gravitational concentration of gold is widely used in gold extraction plants in all countries of the world, including those that are the main producers of this metal.

According to the nature of the processed raw materials, these factories are divided into 3 groups:

• quartz and quartz-sulfide ores containing precious metals mainly in cyanide-soluble form.
• pyrite and arsenic-pyrite ores with finely disseminated gold in sulfides that are resistant to cyanidation, as well as ores containing sorption-active carbonaceous matter.
• complex ores containing, along with gold and silver, heavy non-ferrous metals (copper, lead, zinc, antimony), as well as uranium.

Within each group, the number of enterprises using gravity, flotation enrichment and cyanidation processes is determined (Tables 1, 2).

Table 1. Scale of application of gravity, flotation and cyanidation

Table 2. Gravity enrichment of ores

Gravity enrichment of gold-bearing ores -

More than 1/3 of enterprises practice it, however without gravity combination with other processes is almost never used.

In recent years, great progress has been made in the technology of gravity enrichment of gold ore raw materials. This is manifested, first of all, in the creation of new devices capable of extracting not only large but also very small particles of metallic gold released during the process of ore grinding, such as centrifugal concentrators and centrifugal jigging machines, in which the intensity of separation of gold particles and other minerals with a lower grain density increases many times over.

In the vast majority of cases, gravity is used in combination with cyanidation, flotation, or both of these processes. For simple ores, the most typical schemes are gravity and gravity-flotation enrichment with cyanidation of flotation tailings, and in some cases, gravity concentrates. The main purpose of gravity in these variants is to remove large free gold from the ore into products (concentrates) processed in a metallurgical cycle separate from the main mass of the ore.

In addition to increasing (usually by 2-4% of total gold recovery), this helps prevent or at least significantly reduce gold accumulation in grinding and mixing equipment.

Flotation, like gravity enrichment, is a mechanical enrichment method, when the concentration and separation of mineral components is carried out without damaging their crystalline structure and chemical composition. Such methods may also include magnetic, electrical and radiometric separation (including photometric sorting), separation of minerals by particle shape and size, selective adhesion (capture by sticky surfaces) and some other processes. However, unlike the above methods, including gravity, flotation is based on the use of chemical reagents that perform a variety of functions.

The basis of flotation enrichment, which is usually carried out in an aqueous environment, is the principle of imparting hydrophobic properties to the grains of the extracted component, due to which they are not wetted by water and are “pushed” to the boundary of the liquid and gas phases, even if the density of these grains is many times greater than the density of water.

Hydrophobicity of mineral grains is imparted by reagents-collectors (collectors), introduced into the suspension and fixed on the surface of the extracted particles, for example, sulphides. The process of separating the latter from the rest of the ore mass (flotation tailings) is intensified by aerating the pulp with air, using special frothers and reagents that depress the flotation of waste rock minerals, and also by regulating the hydrogen index (pH), i.e. creating an acidic, alkaline or neutral pulp environment.

Due to the extremely wide range of flotation reagents, the total number of which is about 6-8 thousand, the possibilities of concentrating virtually any minerals by flotation have been created. On the same basis, principles and methods of separating (selecting) various mineral mixtures to obtain individual products (concentrates) that meet market requirements and the conditions of their subsequent use or chemical-metallurgical processing have been developed. In this regard, flotation, as a method of mechanical enrichment of mineral raw materials, has very large capabilities, which determines its wide use in various industries, including non-ferrous and ferrous metallurgy, the coal industry, in the production of diamonds, phosphorus, graphite, barite, magnesite, pure kaolin clays and other mineral products. Currently, flotation annually processes more than 2 billion tons of minerals, and this is the best characteristic of this technological process.

Flotation plays a rather important role in the enrichment of gold ore raw materials. However, one important circumstance is taken into account, which distinguishes the flotation capabilities of gold-bearing ores from most non-ferrous metal ores. The latter are characterized by a clear separation of the main technological stages: ore enrichment and metallurgical processing of concentrates. These stages are carried out at separate enterprises (enrichment plants, metallurgical plants), which are often part of various production associations. At the same time, the overwhelming majority of gold recovery plants operate according to schemes with a complete cycle of ore processing to the final marketable product - gold bars (Doré alloy). For this reason, ore processing at gold mining enterprises, as a rule, is carried out according to combined schemes combining gravity-flotation enrichment operations with cyanidation and other chemical-metallurgical operations (smelting, roasting, autoclave or biochemical oxidation, etc.).

Flotation enrichment of ores at gold extraction plants

According to flotation activity in ores, gold-bearing minerals can be arranged in the following sequence (in descending order):

- intergrowths of metallic gold with iron sulfides (pyrite, arsenopyrite) and sulfides of heavy non-ferrous metals (chalcopyrite, galena, etc.);

- gold-containing sulfides, in which gold is present in the form of thin metallic inclusions;

- free grains of gold and natural alloys of gold with silver (electrum, kuestellite, etc.)

The greatest effect from the use of flotation is ensured when extracting gold from ores with predominantly sulphide mineralization. Flotation is rarely used for oxidized gold-bearing ores, since it does not provide satisfactory indicators of metal extraction into concentrates, being much inferior in this respect to the process of direct cyanidation of ore. However, the use of flotation is very useful in the process of mineralogical studies for the extraction of fine grains of free gold from oxidized ores for their subsequent microscopic examination in order to establish the size and morphology of gold grains. As a rule, the flotation process of gold-bearing ores is carried out in a slightly alkaline environment (pH = 7-9). To create such an environment, soda or lime is used (the latter is used less often, since it has a weakly expressed depressing property in relation to gold-bearing pyrite, and to some extent to native gold).

Ethyl or butyl xanthates are used as collectors. Pine oil or cresol are usually used as foaming agents. Copper sulfate is added to the pulp (during grinding) to activate pyrite.

Depression of gangue minerals, including clays, is carried out by silicate and (less frequently) sodium sulfide. The latter is also used for sulfiding the surface of oxidized mineral particles (malachite, azurite, cerussite, anglesite, smithsonite, etc.) in order to impart flotation activity to them.

For flotation of gold- and silver-containing ores, depending on their material composition, a variety of devices are used: multi-chamber mechanical, pneumatic-mechanical, pneumatic, as well as large-volume (vat) flotation machines. In recent years, flotation columns have been developed and are successfully operating at a number of gold mining enterprises, designed for enrichment of finely ground and slimy ores for concentration of native gold and coarse-grained gold-containing sulfides in ore grinding cycles. Flash flotation is considered as an alternative to gravity methods of gold extraction from "freshly ground" ores and is effectively used at factories.

Flotation is used as the only technological process extremely rarely. These are mainly enterprises processing complex ores, which, along with gold and silver, contain other non-ferrous metals (copper, lead, zinc, antimony) in concentrations and mineral forms that allow the possibility and economic feasibility of the associated extraction of these metals into liquid marketable products. Implementation of flotation in a special reagent mode allows the extraction of copper, lead, zinc and antimony concentrates of acceptable composition from gold-containing ores, which are sent for subsequent processing to specialized metallurgical plants. During flotation, a significant portion of the noble metals present in the original raw material also passes into these concentrates. The possibilities of their subsequent extraction are determined by the technology of the main metallurgical production.

The main strategy of gold mining enterprises engaged in the complex processing of polymetallic ores, in addition to obtaining conditioned concentrates of non-ferrous metals during flotation, is to ensure the maximum possible extraction of gold on site using other technological processes, in particular gravity enrichment and cyanidation. Most enterprises practice this type of combined gravity-flotation-cyanide technology in the processing of complex ores.

Favorable objects for the use of flotation are technologically refractory ores, in which gold is closely associated with iron sulfides and cannot be extracted by cyanidation without the use of rather complex and expensive preparatory processes: oxidative roasting, autoclave or biochemical oxidation of sulfides.

Flotation allows not only to concentrate gold-containing sulphides (pyrite, arsenopyrite) in a small volume of concentrate sent for metallurgical processing, but also to separate these sulphides, for example pyrite and arsenopyrite or pyrites of different generations, differing in gold content.

One option for enriching low-grade gold ores (Au 2.2 g/t) is a combined gravity-flotation technology. A special activator of metallic gold and gold intergrowths with pyrite is used in the flotation process. In combination with potassium amyl xanthate (pyrite collector) and soda carbonate introduced into the pulp to maintain an optimal pH of 8.4-8.6, the reagent allows extracting 85% of gold into concentrate while preserving about 75% of pyrite in the flotation tailings, represented mainly by fractions that do not contain gold. Taking into account gravity, the total gold extraction into concentrates at the plant is more than 90% - with a concentrate yield of only 1.9% of the ore.

When processing carbonaceous sulphide ores, improvement of quality and reduction of the yield of gold-containing concentrates is achieved by preliminary flotation removal of waste coal fractions in terms of gold content from the ore or by sequential flotation of carbon and sulphides with careful selection of the reagent regime at each stage.

In the case of the simultaneous presence of refractory (in sulphides) and easily cyanidable gold in the ores, flotation enrichment is supplemented by a cyanidation operation, to which either the initial ores are subjected before flotation or the tailings of flotation enrichment. The pyrite and arsenopyrite concentrates obtained during flotation are also processed on-site by the cyanidation method, but only after preliminary chemical, thermochemical or biochemical opening of the gold-bearing sulphides.

At enterprises processing simple ores with relatively easily cyanided gold, flotation is used only if it ensures the production of gold-rich tailings and if this significantly reduces the costs of hydrometallurgical processing, since not the entire mass of ore is subjected to cyanidation, but only the flotation concentrates.

Flotation has become an extremely diverse process in terms of the reagents used and the equipment used, which allows it to be used much more widely than before, including on poor and complex ores. Due to flotation, it is possible to increase gold extraction and ensure acceptable profitability of deposit development. At the same time, the multivariance of the process requires comprehensive and thorough laboratory and technological studies of ores, as well as extensive experience and knowledge in order to find exactly the option that will provide the best effect for specific conditions.

The basis of modern technology for extracting gold, as well as silver from ores of primary deposits, is cyanidation, which consists of selective leaching of precious metals with aqueous solutions of alkaline cyanides: sodium, potassium, calcium. Then the dissolved metals are separated from the solutions by various methods, ultimately obtaining high-quality commercial products - metal ingots (Doré metal), sent to refining plants. In some cases, gold and silver refining is carried out directly on site, i.e. in the conditions of a gold mining enterprise.

It should be noted that in the past, cyanidation of gravity concentrates containing large particles of gold and other heavy minerals (in particular sulfides) in tank-type devices (mechanical and pneumatic-mechanical agitators) was considered unacceptable due to the low rate of gold dissolution and the difficulty of maintaining the suspension in a suspended state, which resulted in the sedimentation of heavy fractions at the bottom of the devices. At present, these problems are solved by using horizontal drum mixers, as well as devices with forced circulation of cyanide solutions and cone reactors. These devices make it possible to process gold-bearing gravity concentrates with virtually any granulometric characteristics by cyanidation. Thus, the traditional technology of gravity concentration of gold with deep finishing of primary concentrates to rich “gold heads” suitable for smelting into gold-silver alloy (Doré metal) is supplemented by an alternative method of hydrometallurgical processing of concentrates with moderate metal content, after their single or double re-cleaning on concentration tables or other finishing devices.

The efficiency of this option increases even more if not only gravity concentrates but also gravity enrichment tailings of ore (using a “softer” leaching mode) are subjected to cyanidation, since in this case it is possible to direct the solid residues of the “concentrate” cycle into the general hydrometallurgical process, ultimately obtaining a single commercial product.

The history of the world mining and metallurgical industry most likely does not know other examples of such dynamic development and development of technological processes as cyanide leaching of gold. This is evidenced, for example, by the following figures. The cyanide leaching process was patented in October 1887. The following year, 1888, a demonstration semi-industrial installation was created, and in 1889, the world's first factory with cyanide leaching of gold-bearing ores was built. A year later, the second industrial cyanide leaching installation was put into operation, gold production at which increased in 4 years from 9 kg (1890) to 9 tons (1893), i.e. a thousand times. The subsequent rapid development of cyanidation technology led to the process very quickly taking a leading position in the overall world production of gold from ore raw materials, which increased from 200 to 2500 tons per year over 110 years (1890-2000). Over the past 20 years, 92% of gold has been obtained in the world using cyanidation from ores of primary deposits (the remaining 8% is accounted for by the metal extracted incidentally from ores of heavy non-ferrous metals: copper, lead, antimony, etc.).

The technological advantages of cyanidation carried out using solutions with a very low cyanide concentration (0.3-1 g/l and below) are, first of all, that it is carried out in a slightly alkaline environment (pH = 9.5~11.5) at normal (“room”) temperature and atmospheric pressure, which determines the high economic efficiency of cyanidation of gold ores.

An important role was played by the developments of the US Bureau of Mines (US BM) on the adsorption extraction of gold from cyanide media using granular activated carbons (1952) and heap cyanide leaching (HCL) of large-piece ores and ore dumps (1969).

The first commercial enterprise of heap leaching of gold with carbon adsorption was created in 1974 for waste rock dumps containing less than 2.5 g/t gold, which at that time made their processing by conventional factory technology unprofitable. In the 80s of the last century, the KB process was extremely widespread in the gold mining industry of the USA, and then in other countries. This was facilitated by the next development of USBM for preliminary agglomeration of finely crushed and slimy ores before KB (1979). In Russia over the past 10 years, about 20 industrial enterprises have been created that carry out heap leaching of gold ore raw materials, with a total processing volume of more than 5 million tons per year.

As a rule, heap leaching is used for open-pit mined ores with a gold content of 0.5 to 1.5 g/t, from which 50 to 80% of the metal is extracted by cyanidation. This ensures the profitable operation of enterprises of various sizes: from 0.5 to 15 million tons of ore per year. Sometimes combinations of heap and dam leaching operations are used.

The bulk of the ore is subjected to heap leaching after preliminary crushing to 65 mm and agglomeration of crushed ore with lime and cyanide solution. Processing of poor ores (Au less than 0.5 g/t) is carried out without crushing and agglomeration by the dam leaching method. Gold recovery into solutions is 70%, including 80% with heap leaching and 65% with dam leaching.

Another direction for increasing the efficiency of the hydrometallurgical process is the integration of heap and dam leaching operations with factory cyanidation technology.

The dam leaching process is carried out on ore of "face" size without preliminary crushing. Gold is extracted from solutions in a separate unit. Gold-saturated coals from both leaching cycles are combined and eluted using standard technology. The total gold recovery is 90%, including 95% in the factory technology cycle and 73% in dam leaching.

The possibility of profitable processing of low-grade gold ore materials by cyanidation is confirmed by the practice of enterprises that perform additional gold extraction from old tailings of enrichment of previous years. This issue, given its importance (including for the Russian gold mining industry), deserves special consideration in a separate publication. Here it should only be noted that, given the minimal costs of developing this type of "technological" gold deposits and preparing old tailings for subsequent hydrometallurgical processing (cyanidation using factory technology), the profitability of the process is ensured with gold extraction at a level of 0.4-0.5 g / t of the original raw material.

The objects of application of cyanidation are not only poor, but also fairly rich gold-containing materials, in particular, concentrates from flotation and gravity enrichment of ores.

As for gravity gold-bearing concentrates, until recently the only acceptable method of their processing was considered to be deep finishing (re-cleaning) with subsequent smelting of the resulting "gold heads" into metal ingots. However, special devices have now been created that allow leaching large grains of metallic gold with cyanide solutions.

An important area of ​​using the cyanide process is the processing of refractory ores and concentrates. These include materials containing dispersed inclusions of gold in dense and cyanide-insoluble grains of iron sulfides: pyrite and arsenopyrite. The possibility of processing such materials by "cyanide-free" hydro- or pyrometallurgical methods has been studied for a long time. However, positive results, from an economic point of view, have not been obtained. Therefore, almost all currently operating gold mining enterprises extract gold from refractory pyrite and arsenopyrite ores (concentrates) by the same cyanide process, but only after additional mechanical (fine and ultrafine grinding), chemical (autoclave oxidation), thermochemical (roasting) or biochemical opening of gold-containing sulfides. As a rule, these preparatory operations are significantly more expensive than the cyanidation itself. However, taken together they ensure high gold extraction in the final commercial product and overall economic efficiency of the technological process.

Cyanidation also plays a significant role in the processing of complex gold ores containing copper, lead, antimony, zinc and other heavy non-ferrous metals, the associated extraction of which seems technologically possible and economically feasible.