Science on a summer’s evening 2017: Chemistry’s role in recovering metals from “urban mines”

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When we think about metals like iron and gold or aluminium and zinc, we think of them as familiar and part of our everyday lives and therefore, to some extent, we take them for granted. But what if we spoke about metals such as dysprosium or gadolinium? These metals are as much a part of our everyday life as many others, as we found out last week at the Playfair Library in the Old College at UoE, where urban mining was the theme of the evening’s lecture delivered by Professor Jason Love. For example, the mechanism by which a touchscreen on a smartphone or tablet works relies on an electrically active metal like dysprosium or indium, both elements we hear little about despite relying on them every day.

Many of the metals that we rely on are classed as rare earth metals, meaning their relative abundance on earth is low or the price of mining or extraction is high; in some cases, countries run monopolies on the metals we all need to live in the current day and age, thus hiking up the price. For instance, in 2011, dysprosium mines in the US had to close, as China released its stockpile of the elemental metal, sending the price into decline and rendering the mining of this element in the US unprofitable.

Metals can vary greatly in price, from copper at roughly £2 per kilogram, to gold at £40,100/kg, so when taking this into consideration, the mining cost needs to be balanced against the global price of the metal. Many metals are extracted from ore, rather than being mined as solid metal. The traditional extraction method involves smelting, followed by electrowinning, a process in which a counter metal ion is added to the solution, allowing there to be an electrical displacement and the metal ions to separate out based on their electrical activity, which then enables the elemental metal to be obtained.

This process works well with metal ores and also with the recycling of some metal-containing products. However, an improved cost and yield-efficient method is to dissolve the metal source in a strong, concentrated acid and perform a biphasic extraction. This process, dubbed urban mining, is being developed by Professor Love of the University of Edinburgh’s Science and Engineering department, who looks specifically at recovering gold ions from Waste Electronic and Electrical Equipment (WEEE) such as smartphones. Taking into consideration that from one tonne of ore only 1g of gold is acquired, whereas one tonne of WEEE can generate 300g of gold, we can see that this can be a profitable method given the price of this precious metal.

Dissolving a smartphone in a strong acid would give you a solution of its many constituent parts, which can be initially separated through a process known as biphasic extraction. This involves adding an organic solvent to the aqueous solution or vice versa, shaking the mixture, and allowing the metal ions to pass into their preferred medium. The addition of an amide into the solution forms spheres around the gold ion called micelles, which further aids extraction. Following this, the addition of an oxidising agent produces gold metal and the byproducts are recycled back into the system to be processed.

Although it is still at the basic research stage, this process by which we can recover expensive but essential metals from WEEE has serious benefits. With proposed regional recycling of WEEE goods, metals could be recovered locally and therefore the need for their long haul transportation would become unnecessary. In an age where we are environmentally aware, traditional mining generates a lot of waste. So having a system whereby we can recycle all the waste products not only has benefits environmentally, but also financially, as the clean up process of sometimes quite harmful material has a cost that has to be taken into consideration. This wonderful and ingenious research is still in the very early stages of development, and scaling this up to industrial level will involve collaboration with engineers not only to optimise the process, but also to develop a system by which we can ensure the constant recycling of reagents, thus further taking this chemistry into greener territory.


This article was written by Blair Donaldson and edited by Teodora Aldea.

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