Extractive metallurgy
Extractive metallurgy is a branch of metallurgical engineering wherein process and methods of extraction of metals from their natural mineral deposits are studied. The field is a materials science, covering all aspects of the types of ore, washing, concentration, separation, chemical processes and extraction of pure metal and their alloying to suit various applications, sometimes for direct use as a finished product, but more often in a form that requires further working to achieve the given properties to suit the applications.
The field of ferrous and non-ferrous extractive metallurgy have specialties that are generically grouped into the categories of mineral processing, hydrometallurgy, pyrometallurgy, and electrometallurgy based on the process adopted to extract the metal. Several processes are used for extraction of the same metal depending on occurrence and chemical requirements.
Mineral processing
begins with beneficiation, consisting of initially breaking down the ore to required sizes depending on the concentration process to be followed, by crushing, grinding, sieving etc. Thereafter, the ore is physically separated from any unwanted impurity, depending on the form of occurrence and or further process involved. Separation processes take advantage of physical properties of the materials. These physical properties can include density, particle size and shape, electrical and magnetic properties, and surface properties. Major physical and chemical methods include gravitational or magnetic separation, froth flotation etc., whereby the impurities and unwanted materials are removed from the ore and the base ore of the metal is concentrated, meaning the percentage of metal in the ore is increased. This concentrate is then either processed to remove moisture or else used as is for extraction of the metal or made into shapes and forms that can undergo further processing, with ease of handling.Ore bodies often contain more than one valuable metal. Tailings of a previous process may be used as a feed in another process to extract a secondary product from the original ore. Additionally, a concentrate may contain more than one valuable metal. That concentrate would then be processed to separate the valuable metals into individual constituents.
Hydrometallurgy
is concerned with processes involving aqueous solutions to extract metals from ores. The first step in the hydrometallurgical process is leaching, which involves dissolution of the valuable metals into the aqueous solution and or a suitable solvent. After the solution is separated from the ore solids, the extract is often subjected to various processes of purification and concentration before the valuable metal is recovered either in its metallic state or as a chemical compound. This may include precipitation, distillation, adsorption, and solvent extraction. The final recovery step may involve precipitation, cementation, or an electrometallurgical process. Sometimes, hydrometallurgical processes may be carried out directly on the ore material without any pretreatment steps. More often, the ore must be pretreated by various mineral processing steps, and sometimes by pyrometallurgical processes.Pyrometallurgy
involves high temperature processes where chemical reactions take place among gases, solids, and molten materials. Solids containing valuable metals are treated to form intermediate compounds for further processing or converted into their elemental or metallic state. Pyrometallurgical processes that involve gases and solids are typified by calcining and roasting operations. Processes that produce molten products are collectively referred to as smelting operations. The energy required to sustain the high temperature pyrometallurgical processes may derive from the exothermic nature of the chemical reactions taking place. Typically, these reactions are oxidation, e.g. of sulfide to sulfur dioxide. Often, however, energy must be added to the process by combustion of fuel or, in the case of some smelting processes, by the direct application of electrical energy.Ellingham diagrams are a useful way of analysing the possible reactions, and so predicting their outcome.
Electrometallurgy
involves metallurgical processes that take place in some form of electrolytic cell. The most common types of electrometallurgical processes are electrowinning and electro-refining. Electrowinning is an electrolysis process used to recover metals in aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes. The metal of interest is plated onto the cathode, while the anode is an inert electrical conductor. Electro-refining is used to dissolve an impure metallic anode and produce a high purity cathode. Fused salt electrolysis is another electrometallurgical process whereby the valuable metal has been dissolved into a molten salt which acts as the electrolyte, and the valuable metal collects on the cathode of the cell. The fused salt electrolysis process is conducted at temperatures sufficient to keep both the electrolyte and the metal being produced in the molten state. The scope of electrometallurgy has significant overlap with the areas of hydrometallurgy and pyrometallurgy. Additionally, electrochemical phenomena play a considerable role in many mineral processing and hydrometallurgical processes.Ionometallurgy
Mineral processing and extraction of metals are very energy-intensive processes, which are not exempted of producing large volumes of solid residues and wastewater, which also require energy to be further treated and disposed. Moreover, as the demand for metals increases, the metallurgical industry must rely on sources of materials with lower metal contents both from a primary and/or secondary raw materials. Consequently, mining activities and waste recycling must evolve towards the development of more selective, efficient and environmentally friendly mineral and metal processing routes.Mineral processing operations are needed firstly to concentrate the mineral phases of interest and reject the unwanted material physical or chemically associated to a defined raw material. The process, however, demand about 30 GJ/tonne of metal, which accounts about 29% of the total energy spent on mining in the USA. Meanwhile, pyrometallurgy is a significant producer of greenhouse gas emissions and harmful flue dust. Hydrometallurgy entails the consumption of large volumes of lixiviants such as H2SO4, HCl, KCN, NaCN which have poor selectivity. Moreover, despite the environmental concern and the use restriction imposed by some countries, cyanidation is still considered the prime process technology to recover gold from ores. Mercury is also used by artisanal miners in less economically developed countries to concentrate gold and silver from minerals, despite its obvious toxicity. Bio-hydro-metallurgy make use of living organisms, such as bacteria and fungi, and although this method demands only the input of and from the atmosphere, it requires low solid-to-liquid ratios and long contact times, which significantly reduces space-time yields.
Ionometallurgy makes use of non-aqueous ionic solvents such ionic liquids and deep eutectic solvents, which allows the development of closed-loop flow sheet to effectively recover metals by, for instance, integrating the metallurgical unit operations of leaching and electrowinning. It allows to process metals at moderate temperatures in a non-aqueous environment which allows controlling metal speciation, tolerates impurities and at the same time exhibits suitable solubilities and current efficiencies. This simplify conventional processing routes and allows a substantial reduction in the size of a metal processing plant.
Metal extraction with ionic fluids">Ionic liquid">ionic fluids
DESs are fluids generally composed of two or three cheap and safe components that are capable of self-association, often through hydrogen bond interactions, to form eutectic mixtures with a melting point lower than that of each individual component. DESs are generally liquid at temperatures lower than 100 °C, and they exhibit similar physico-chemical properties to traditional ILs, while being much cheaper and environmentally friendlier. Most of them are mixtures of choline chloride and a hydrogen-bond donor or mixtures of choline chloride with a hydrated metal salt. Other choline salts have a much higher costs or need to be synthesised, and the DES formulated from these anions are typically much more viscous and can have higher conductivities than for choline chloride. This results in lower plating rates and poorer throwing power and for this reason chloride-based DES systems are still favoured. For instance, Reline has been used to selectively recover Zn and Pb from a mixed metal oxide matrix. Similarly, Ethaline facilitates metal dissolution in electropolishing of steels. DESs have also demonstrated promising results to recover metals from complex mixtures such Cu/Zn and Ga/As, and precious metals from minerals. It has also been demonstrated that metals can be recovered from complex mixtures by electrocatalysis using a combination of DESs as lixiviants and an oxidising agent, while metal ions can be simultaneously separated from the solution by electrowinning.Recovery of precious metals by ionometallurgy
Precious metals are rare, naturally occurring metallic chemical elements of high economic value. Chemically, the precious metals tend to be less reactive than most elements. They include gold and silver, but also the so-called platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum. Extraction of these metals from their corresponding hosting minerals would typically require pyrometallurgy, hydrometallurgy, or both as processing routes.Early studies have demonstrated that gold dissolution rate in Ethaline compares very favourably to the cyanidation method, which is further enhanced by the addition of iodine as an oxidising agent. In an industrial process the iodine has the potential to be employed as an electrocatalyst, whereby it is continuously recovered in situ from the reduced iodide by electrochemical oxidation at the anode of an electrochemical cell. Dissolved metals can be selectively deposited at the cathode by adjusting the electrode potential. The method also allows better selectivity as part of the gangue tend to be dissolved more slowly.
Sperrylite and moncheite, which are typically the more abundant platinum minerals in many orthomagmatic deposits, do not react under the same conditions in Ethaline because they are disulphide, diarsenide or ditellurides minerals, which are particularly resistant to iodine oxidation. The reaction mechanism by which dissolution of platinum minerals is taking place is still under investigation.