Mineral processing and metal extraction require a lot of energy and create large amounts of solid waste and wastewater. These materials also need energy for treatment and disposal. As the need for metals grows, the metallurgical industry must use materials with lower metal content, such as primary sources like mineral ores or secondary sources like waste from mining or household garbage. Because of this, mining and recycling must develop better, more efficient, and less harmful methods for processing minerals and metals.

Mineral processing is used to separate valuable minerals from unwanted materials in raw resources. This process uses about 30 gigajoules of energy per tonne of metal, which makes up about 29% of the total energy used in mining in the United States. Pyrometallurgy, which uses high heat, produces greenhouse gases and harmful dust. Hydrometallurgy uses large amounts of chemicals like sulfuric acid, hydrochloric acid, and cyanide, which are not very selective in extracting metals. Even though some countries restrict its use due to environmental risks, cyanidation remains the main method for recovering gold from ores. In less developed countries, some miners still use mercury to extract gold and silver, even though it is dangerous. Bio-hydro-metallurgy uses living organisms like bacteria and fungi. While this method only needs oxygen and carbon dioxide from the air, it requires low ratios of solid to liquid and long processing times, which reduces the efficiency of the process.

Ionometallurgy uses special non-aqueous solvents, such as ionic liquids and deep eutectic solvents, to create closed-loop systems that recover metals efficiently. This method allows metals to be processed at moderate temperatures in non-aqueous environments, which helps control metal forms, handle impurities, and achieve good solubility and energy efficiency. This simplifies traditional processing methods and reduces the size of metal processing plants.

Metal extraction with ionic fluids

Deep Eutectic Solvents (DESs) are liquids made from two or three inexpensive and safe materials that can combine with each other, often through hydrogen bonds, to form mixtures that melt at a lower temperature than each individual material. DESs are usually liquid below 100 °C and have similar physical and chemical properties to traditional ionic liquids, but they are much cheaper and better for the environment. Most DESs are made by mixing choline chloride with a hydrogen-bond donor, such as urea, ethylene glycol, or malonic acid. Other choline salts, like acetate or nitrate, are more expensive or require special preparation, and the DESs made from them are often thicker and have higher conductivity. This can lead to slower metal plating and less even results, which is why DESs based on choline chloride are still preferred. For example, Reline (a 1:2 mix of choline chloride and urea) has been used to recover zinc and lead from a mixture of metal oxides. Similarly, Ethaline (a 1:2 mix of choline chloride and ethylene glycol) helps dissolve metals during the electropolishing of steel. DESs have also been used to recover metals like copper, zinc, and gallium from complex mixtures, as well as precious metals from minerals. It has been shown that metals can be recovered from complex mixtures using electrocatalysis with DESs as lixiviants and an oxidizing agent, while metal ions can be separated from the solution through electrowinning.

Precious metals are rare, naturally occurring elements with high economic value. Chemically, they are less reactive than most other elements. Examples include gold, silver, and the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum. Extracting these metals from their host minerals usually involves pyrometallurgy (e.g., roasting) or hydrometallurgy (e.g., cyanidation), or a combination of both. Studies have shown that gold dissolves in Ethaline at a rate similar to the cyanidation method, which can be improved by adding iodine as an oxidizing agent. In industrial processes, iodine could act as an electrocatalyst, being continuously recovered through electrochemical oxidation at the anode of an electrochemical cell. Metals can be selectively deposited at the cathode by adjusting the electrode potential. This method also allows better selectivity because unwanted minerals, such as pyrite, tend to dissolve more slowly.

Sperrylite (PtAs₂) and moncheite (PtTe₂), which are common platinum minerals in many deposits, do not react under the same conditions in Ethaline as other minerals. This is because they are disulfide (pyrite), diarsenide (sperrylite), or ditelluride (calaverite and moncheite) minerals, which are resistant to iodine oxidation. The exact mechanism by which platinum minerals dissolve in Ethaline is still being studied.

Metal sulfides, such as pyrite (FeS₂), arsenopyrite (FeAsS), and chalcopyrite (CuFeS₂), are typically processed through chemical oxidation in aqueous solutions or at high temperatures. Most base metals, like aluminum and chromium, require (electro)chemical reduction at high temperatures, which demands significant energy and may produce large amounts of aqueous waste. In aqueous media, chalcopyrite is harder to dissolve than covellite or chalcocite due to surface effects, such as the formation of polysulfide species. The presence of chloride ions has been suggested to change the surface structure of sulfides, making them easier to leach by preventing passivation. DESs provide high chloride ion concentrations and low water content, reducing the need for additional salt or acid, and avoiding oxide chemistry. Thus, the electrodissolution of sulfide minerals in DESs has shown promising results, with metal ions released into the solution for recovery.

When copper is extracted from sulfide minerals using Ethaline, chalcocite (Cu₂S) and covellite (CuS) produce a yellow solution, indicating the formation of [CuCl₄] complexes. In contrast, solutions from chalcopyrite contain both Cu and Cu species due to the generation of reducing Fe species at the cathode. The best selective recovery of copper (>97%) from chalcopyrite is achieved using a mixed DES of 20 wt.% ChCl-oxalic acid and 80 wt.% Ethaline.

Recovery of metals from oxide matrices is typically done using mineral acids. However, electrochemical dissolution of metal oxides in DESs can increase dissolution rates by more than 10,000 times in pH-neutral solutions.

Studies show that ionic oxides like ZnO are highly soluble in ChCl:malonic acid, ChCl:urea, and Ethaline, similar to their solubility in aqueous acidic solutions like HCl. Covalent oxides like TiO₂, however, are almost insoluble. The electrochemical dissolution of metal oxides depends on proton activity from the hydrogen-bond donor (HBD), which determines the ability of protons to accept oxygen, and on temperature. It has been reported that eutectic ionic fluids with lower pH values, such as ChCl:oxalic acid and ChCl:lactic acid, allow better solubility than those with higher pH (e.g., ChCl:acetic acid). Therefore, different solubilities can be achieved by using various carboxylic acids as HBDs.

Outlook

Currently, it is not well understood how stable most ionic liquids are when used in real-world situations involving electricity. The choice of ionic fluid is often based on trial and error because there is very little information about how metal ions behave in these liquids, which is needed to predict how they dissolve or interact. There are no diagrams that show how metals react under different conditions, no standard measurements for how easily substances gain or lose electrons, and little knowledge about how these liquids interact with water or their chemical makeup. It is important to note that most processes involving ionic liquids described in scientific studies are still in early stages of development, such as proving an idea works in the lab. However, ionometallurgy has the potential to recover metals in a more targeted and environmentally friendly way by using solvents that are less harmful, reducing pollution, and avoiding the use of dangerous chemicals.