Ore Preparation and Chemical Enrichment Methods

Ore preparation is an engineering discipline that provides the raw material requirements needed by industry by obtaining high-value products from primary and secondary sources. Within this discipline, the physical, physicochemical, chemical and mineralogical properties of all raw material sources to be contributed to the economy are taken as a basis. The enrichment process is achieved as a result of separating minerals and structures that make up the structure of the ore or secondary source to be enriched from each other according to these property differences. Regardless of which property difference is used for enrichment, the ore or material to be enriched must first be reduced below a certain size. Following the liberation of minerals, the ore or material is enriched by using the most appropriate enrichment method (Figure 1). [caption id="attachment_103759" align="aligncenter"] Figure 1. Process from ore to metal[/caption]   [caption id="attachment_103760" align="aligncenter"] Figure 2. Ore preparation- Pyrometallurgy-Hydrometallurgy[/caption]   In addition to enrichment processes carried out according to physical properties such as specific gravity, magnetic susceptibility, electrical conductivity, brittleness, color-shape, fragility, or by the flotation method where mineral surface properties and physicochemical properties are effective, chemical enrichment methods are also among the most commonly used methods in ore preparation processes.

In the chemical enrichment process, a difference is created by disrupting the chemical structure of the minerals in the ore structure. This chemical structure is usually changed through a thermal process (pyrometallurgy), with a solvent (hydrometallurgy) or with bacteria (biometallurgy).

Pyrometallurgy: In processes for recovering valuable metals from primary or secondary sources, a series of thermal treatments are applied to the ore, altering the chemical and physical properties of the material. Ores, concentrates, intermediate products, fuels, additives and oxygen in air form the raw materials of pyro-metallurgical operations. The products of pyrometallurgy are matte, metals, alloys and similar materials. Pyrometallurgical methods are generally used in the production of Fe-steel and non-ferrous metals (Zn, Pb, Cu, Sb, Sn, etc.) as well as ferroalloys, and in refractory material production (MgO, silicon carbide or other carbides, SiO2 and CaO). Pyrometallurgical methods include preliminary treatments (drying, calcination, roasting, sintering, agglomeration) and applications such as smelting, distillation, converting, fire refining and casting.

Hydrometallurgy:

Hydrometallurgy, which involves the use of water and solvents instead of pyrometallurgical methods requiring high temperatures, can be used not only in metal production but also in obtaining pure products by removing impurities from industrial minerals. The main purpose in hydrometallurgy (leaching) is to ensure that the phases to be separated pass into solution while others remain as solids. The emergence of the cyanidation method at the end of the 19th century for dissolving gold from ores using dilute sodium cyanide solution and the introduction of the Bayer process in alumina production involving the extraction of bauxite with NaOH under high temperature and pressure are considered the beginning of hydrometallurgical processes. Hydrometallurgical methods can be applied in the recovery of gold and silver, as well as aluminum, uranium, nickel, copper, zinc, titanium and platinum metals, and in the recovery of rare earth elements, which are very important for today. Generally, a hydrometallurgical process consists of the following steps: a) ore preparation b) dissolution (leaching) c) solid-liquid separation and solution purification d) recovery of metallic values from the solution.

In a leaching operation, depending on the characteristics of the ore, one of the following methods is applied: a) in-situ leaching, b) heap leaching, c) percolation or vat leaching d) agitation leaching and e) pressure leaching.

Generally, in-situ and heap leaching operations are applied to low-grade metallic ores, while agitation tank leaching is applied to higher-grade ores. In particular, in the leaching processes of sulfide structures, some preliminary treatments such as roasting are used to ensure oxidation, and oxidation conditions are created with certain chemicals, allowing leaching operations to be performed. Additionally, recovery of these ores is possible through pressure leaching. Pressure dissolution can be performed in both acidic and basic environments. In the pressure leaching process, while it is a particularly suitable process for ores that require complete oxidation such as high-tenor and sulfide minerals, the initial investment cost of the process is quite high due to the requirement for autoclaves and oxygen facilities used. The selection of the leaching agent (solvent) used in the leaching operation depends on several factors: a) The chemical and physical character of the material to be subjected to leaching b) The cost of the solvent involved in the leaching operation c) The corrosiveness of the leaching agent d) Selectivity for the component desired to be leached e) Reusability. In addition to solvents such as water, ferric sulfate, sodium cyanide, sodium sulfide, sodium thiosulfate and aqueous chloride, certain acids (sulfuric, nitric, hydrochloric acid, etc.) and bases (sodium carbonate, sodium hydroxide, etc.) are also used in leaching operations.

For example, cyanide used in gold and silver production; sulfuric acid used in leaching copper, nickel, cobalt ores; NaOH used in aluminum production in the Bayer process are among the most important solvents.

After the leaching operation is completed, solid-liquid separation is performed to separate the dissolved and undissolved materials from each other, and the obtained metal-loaded solution is first purified. As a result of leaching, other metals present in the ore may have been dissolved along with the metal. At this stage, the aim is to remove unwanted metals from the solution. There are certain methods that can be applied for purification. Some of these are solvent extraction (SX), cementation and ion exchange methods. After this stage, as the final step, the metal in the liquid solution is transferred back to the solid phase for production. The metal obtained after this step can be used as raw material as well as in further refining processes. Some of the methods that can be used for this purpose are: a) electrowinning b) gas reduction and c) precipitation with metal.

Biometallurgy:

Biometallurgy, one of the chemical recovery methods in which bacteria are used for the purpose of oxidizing the structure in the leaching processes of sulfide ores, has recently become one of the most discussed topics. In the direct bioleaching mechanism, bacteria attach to the mineral surface and directly oxidize the mineral without any intermediate process. This method is a simple, effective and environmentally compatible technology for processing sulfide ores. It has been successfully applied on an industrial scale for the past 25 years in the recovery of copper, gold and uranium. The economics and efficiency of the bio-leaching process largely depend on the activity of bacteria, the mineralogical composition and chemical composition of the ore.