Metallurgical distillation
Metallurgy is a branch of science and technology concerned with the properties of metals and their production and purification.
Metallurgy is a broad discipline that deals with separating out and treating metals. This is usually achieved through pyrometallurgy (using fire), hydrometallurgy (using liquid solutions) or a hybrid metallurgy that combines these or introduces other tools as well.
Pyrometallurgy employs high temperatures to carry out smelting and refining operations to extract metals from their minerals. Hydrometallurgy uses aqueous solutions to separate the desired metals. Which method is most appropriate depends on several factors, such as the available funding, energy requirements, environmental constraints, rate of particular target chemical reactions, and the required purity of the output metals produced.
Pyrometallurgical
Pyrometallurgy is the use of high-temperature processes to reclaim specific metals, typically from the black mass produced in earlier steps. Thermal techniques are also used in pre-treatment, though at lower temperatures, to remove organic materials before metal recovery begins. Within the practice of pyrometallurgy there are several different types of processes and reactions that are typically carried out, depending on the specific target metals and their compound states.
Distillation is the thermal treatment process of separating different metals by evaporation and condensation of the resulting vapours back into a solid / liquid state. It is carried out in a vacuum in order to ensure all vapours are collected and condensed, and to improve efficiency as less heat is required to evaporate metals under a vacuum.
Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere - a controlled environment containing inert gases such as nitrogen (commonly used) or argon in place of oxygen. It is carried out in a pyrolysis chamber and involves a change of chemical composition.
Refining is a broad term for purifying a target material by eliminating any other elements that may be contaminating it. Metals like cadmium, once recovered from nickel cadmium batteries, usually require some method of refining such as distillation to attain a viable end product of acceptable purity.
Roasting is a further purifying process. It is a method for separating unwanted sulphur and carbon from metal ore. This is carried out on sulphide ores, heating them to high temperatures in the presence of air (as opposed to pyrolysis).
Reduction is a means of separating a metal-oxide or ore into its parts - oxygen and metal. The metal oxide is heated to a molten state along with a reducing agent (also known as a reductant), which helps free the oxygen in the material in the form of carbon dioxide, leaving a refined metal behind.
Outputs:
357°C
Various combinations of these techniques are used to recycle the black mass of spent batteries. In the case of everyday alkaline batteries, a common approach is pyrolysis, carried out in a Waelz-rotary kiln to first evaporate off water and the trace amounts of mercury in the black mass, then zinc, and lastly the manganese in the mix is reduced to manganese-oxide, resulting in three output streams - zinc, mercury, manganese-oxide enriched slag.
Pyrometallurgical routes have traditionally been more popular, but they have their downsides. Namely, pyrometallurgy requires a lot of energy, has a relatively high carbon footprint, and often produces a process waste stream.
Input: Black mass and various metal compounds are commonly treated with pyrometallurgy.
Outputs:
Hydrometallurgical
Hydrometallurgy focuses on extracting metals using various liquid solutions that facilitate chemical reactions. It is a relatively new discipline, but is gaining popularity in modern battery recycling methods, particularly with regard to lithium-ion. In many instances it appears to offer many benefits over pyrometallurgical routes, such as purer output streams and lower emissions of hazardous byproducts.
Hydrometallurgical processes are also sometimes carried out in combination with pyrometallurgical ones, for example pyrolysis and water-leaching can be combined to recover lithium manganese oxide from lithium-ion batteries.
Hydrometallurgical approaches exist and are in practice in various parts of the world for alkaline, zinc-carbon, nickel metal hydride, nickel metal cadmium, and lithium batteries. There are pros and cons to be considered between hydrometallurgical and pyrometallurgical methods depending on the specific target materials and priorities of the recycling process.
Some companies use a mix of pyrometallurgy and hydrometallurgy to optimise their recycling methods for lithium-ion and nickel metal hydride batteries. Pyrometallurgy is used to produce slag, which is then used by the construction industry, and a metal alloy containing cobalt, nickel and copper, which is treated hydrometallurgically to produce electrode elements for new batteries.
Leaching is the process of extracting metals from solid materials that contain traces of various metals, such as the ores and compounds found in used batteries. The metals are leached out of the solid material and converted into soluble salts by the active ingredients in the solution, which can then be dried out and processed to produce pure metals.
Battery leaching is usually carried out in a relatively simple setting such as a vat or autoclave, using various acids as solvents.
Solvent extraction is effectively the same as leaching except that it is carried out on liquid input materials as opposed to solids. In this process one or more components are separated from a liquid through their interaction with a second immiscible liquid, commonly referred to as the solvent.
Chemical precipitation is essentially the creation of a solid from a solution. It is the conversion of a solution into a solid by converting the substance into an insoluble form or by making the solution supersaturated. Depending on the chemicals in question, precipitation may aim to separate and solidify metals or impurities from the mix. Precipitation occurs when evaporation or a change in pH or temperature leads a particular target material to the limit of its solubility.
