Mining Our Way to a Greener Future Ram Parthiban

In the transition to a “greener future”, the demand for raw materials required to make essential appliances and technologies which decrease greenhouse gas emissions will skyrocket. Solar panels, electric vehicle batteries, wind turbines and so on, are all composed of a wide range of critical metals like lithium, cobalt, nickel, and rare earth elements. While these elements are found in very small quantities throughout the earth’s crust, economic concentrations are very limited and are often hard to extract. Therefore, to progress to a greener future, a dependable supply of critical metals must be achieved through recycling and developing environmentally safe extraction techniques.

Now, one might wonder how we can make the extraction of critical metals more environmentally friendly to reduce the detrimental impact of the mining industry. In order to achieve this, a substantial amount of research has been done to produce novel extraction methods. Some of the new, environmentally safe extraction techniques include:

1. Bioleaching – The use of microorganisms to extract critical metals by eating away unwanted waste material, thereby leaving only the metal of interest. This can be used to extract copper, gold, silver, lead and zinc, to name just a few.

2. Phytomining – An extraction technique that involves growing plants on an ore body so that they preferentially absorb the metal particles. They are then harvested and processed to produce a purified metal, like lithium.

3. Solvent and Supercritical Fluid Extraction – This method involves using non-toxic and nonflammable solvents and supercritical fluids (e.g., CO 2) to extract metals from their ores. Metals that can be extracted using these methods include platinum group elements, rare earth elements, and nickel.

4. Electrolysis – A method that involves passing an electric current through a molten ore that is usually mixed with a flux (a chemical compound used to remove impurities) to produce positive metal cations: for example, aluminium.

5. Modern Pyrometallurgy – An updated pyrometallurgical technique using electric arc furnaces or microwave-based furnaces to produce high temperatures to melt an ore rather than the conventional fossil-fuel based furnaces. This is then combined with oxygen-enriched air to reduce the use of carbon. Examples include iron and tin.

6. Ion Exchange and Solid Phase Extraction – Selective exchange and extraction of metal ions or waste material from an ore solution (thereby purifying it) using solid compounds and mixtures composed of various salts and fluxes, for example, cadmium, chromium, arsenic, and antimony.

Much of the current research focuses on implementing a combination of all these processes to improve existing extraction methods. For example, one common problem is how to extract vanadium, useful for producing superconducting magnets and medicines treating high cholesterol and heart disease, from bauxite, the primary ore for aluminium. Traditional methods of vanadium extraction use highly corrosive acids, which are unsafe and environmentally damaging.

Instead, the bauxite can first be treated with sodium hydroxide (NaOH) at 170-180°C to form sodium aluminate (Na[Al(OH)4]). To increase the vanadium concentration, aluminium oxide (Al 2O3) is removed and sent off to undergo electrolysis, producing aluminium metal. The remaining solution is crystallised, producing sodium vanadate (NaVO 3). This is finally reduced through a reaction with a supercritical gas mixture of carbon monoxide (CO) and carbon dioxide (CO 2) to produce vanadium. The best bit of this process is that the carbon monoxide and carbon dioxide are produced as byproducts of the aluminium electrolysis, generating a much more efficient, safe and environmentally friendly process.

While these processes work in some areas, widespread adoption is not feasible yet. Despite their promise, these processes need to be optimised to obtain consistent results and metal yields throughout the entire process. In theory, they work, but when we try to implement them on a larger scale, a lot can go wrong. For example, we still don’t know how each microorganism works, making bioleaching optimisation difficult, and there may be better, safer chemical mixtures that we can use for solid-phase extraction just waiting to be found. Another drawback is the cost of implementation for some of these techniques. On average it takes about $30 to $50 million USD to build an electric arc furnace while microwave-assisted heating furnaces can cost much more. The faster these processes are implemented, the sooner our “‘greener future’” arrives, thus making additional research and commercial optimisation imperative. Without these critical minerals, the technology we’re relying upon to save us will be unable to function. So while these techniques may not be perfect yet, they are most certainly a significant and exciting step in the right direction.

by Jasmin Small