Article by Dario Gozzi
Electronic waste, particularly electronic boards or PCBAs, contains countless metals such as gold, silver, platinum, copper, aluminum, nickel, tin, cobalt, indium, antimony, zinc, and iron. They have an interesting economic as well as strategic value.
Metals can be recovered through mechanical, thermal, chemical and biological processes.
Mechanical treatment
Electrical and electronic waste treatment plants consist of a sequence of machines that allow progressive crushing of the incoming materials. This facilitate their separation.
In the treatment of WEEE, the first step is always dismantling to separate and isolate the hazardous materials.
A typical recycling plant consists of one or more pre–grinders that perform primary crushing. Downstream of the primary pre-grinders, there may be hammer mills. They further refine the fractions, preparing them to facilitate the separation phase performed by dedicated lines consisting of a set of multiple densimetric tables.
Pyrometallurgical process
There are two main approaches to metal recovery: pyrometallurgy and hydrometallurgy.
Pyrometallurgy is a conventional heating process for the recovery of metals in general, including precious metals, from end-of-life electronic boards.
These processes, in addition to being energy–intensive, produce a lot of slag, sludge, ash and other waste products resulting from incineration, smelting and sintering. The slag may contain toxic elements, making it necessary to handle it in safe conditions.
The scrap is melted in furnaces in the presence of oxygen and other elements such as calcium oxide and silicon dioxide. Compared to other recovery techniques, pyrometallurgy causes air pollution.
Hydrometallurgical process
The hydrometallurgical process is a process that uses chemicals to separate metallic elements from the context. It uses reagents in aqueous solution. This process has the advantage, compared to other techniques, of being selective, economical and with a low environmental impact. It is low impact because it involves the extraction of metals through solvents in the aqueous phase and then their separation by electrolysis.
Hydrometallurgy occurs in three phases: leaching, concentration and purification of the solution, metal recovery. Leaching involves the use of acidic or basic aqueous solutions that are brought into contact with the material containing the precious metal.
Acid leaching has the advantage of requiring relatively low temperatures (usually less than 200 °C), less pretreatment than alkaline leaching and less time. The acid that is used is typically sulfuric acid or hydrochloric acid. Alkaline leaching has the advantage of being more selective and therefore the solution will contain fewer impurities. Sodium carbonate and sodium hydrogen carbonate are used.
After leaching the solution must be concentrated and unwanted ions must be removed by processes such as precipitation. Recovery of the metal is the last stage of the process which can be done by electrolysis, reduction or precipitation.
Biohydrometallurgy
This technology uses microorganisms in an aqueous medium, for the recovery of metals. Biohydrometallurgy is also known as bioleaching in which microorganisms play a significant role and is the emerging technology for the extraction of metals (mainly precious metals) from electronic waste.
Direct bioleaching is a process by which solid metals are converted into water-soluble forms through the action of microorganisms. Indirect bioleaching is facilitated by the supply of oxidants by microorganisms.
During bacterial leaching, metal ions remain in solution at pH values between 1.5 and 3 due to the acidic environment. The ability of the microorganisms involved in these transformations to work on metals by adapting to a highly acidic environment, brings this technology to a higher level than others. The only drawback is that the bacteria need a long period of time to operate.
