Refining metals with melanin
Khai Trung Le talks to associate professor Jean-Philip Lumb about replacing chlorine and hydrochloric acid with less energy- and risk-intensive reagents with a skin-deep process.
The one-two punch of a less energy-intensive means of processing metals without the use of toxic solvents and reagents could help reduce the environmental impact of metals production. A collaboration between McGill and Western Universities, Canada, is looking at perhaps an unlikely source – melanin.
Associate professor of chemistry at McGill, Jean-Philip Lumb, has previously explored the use of melanin as a means of replacing toxic reagents such as chlorine and hydrochloric acid without forming metal oxides, which are typically inert, have high melting points and lattice energies and strong bonds. ‘Melanin is a biopolymer that has inspired a lot of uses, including the production of adhesive pigments. These pigments sequester heavy metals.’ Lumb’s team produced active portions of melanin polymer oxidants called ortho quinones that can be used to oxidise a metal in a benign fashion.
‘We’re able to transfer the potential chemical energy stored in O2 to the ortho quinone. A small molecule inserts itself between the metal and oxygen, known as a redox relay, using the energy in O2 to manipulate the metal. You’re not using the O2 directly on the metal, but rather the organic cofactor.’
The joint McGill/Western team focused on germanium, most commonly acquired by burning lignite collected by strip or open coal pit mining, resulting in germanium oxide- and zinc oxide-rich ash. Typically, the germanium is leeched from the mixture with hydrochloric acid to create germanium tetrachloride, which is further processed to increase its purity. However, the method derived by Lumb’s team is more straightforward – ‘We don’t use bulk quantities of solvents. We take all of our reagents and put them into stainless steel jars with ball bearings and shake them back and forth.’ Rather than resulting in germanium tetrachloride, this process creates an air- and moisture-stable Ge(IV)-catecholate (reduced form of the ortho quinone) platform that is able to be converted to high-purity GeH4. The catecholate is also recoverable, and can be processed back into an ortho quinone state.
Much of the drive behind Lumb’s work is his desire to move away from energy-intensive processes that use dangerous reagents. ‘The use of chlorine can incur a great deal of risk, yet it is something that is used at massive scales.’
While it may be more of a struggle to see industrial application – ‘chlorine is dirt cheap, as is hydrochloric acid. The challenge is facing processes that are in place in huge scales and that penetrate so many different markets’ – Lumb foresees more opportunities in metals recycling. ‘The volumes are typically smaller and the separations are trickier, and we think we could have the biggest short-term impact in selective separation chemistries.’
Lumb will also need to convince stakeholders that the process is competitive with existing industrial processes. ‘At room temperature, ball milling has an infinite advantage. But if you’re talking about several hundred degrees, it becomes a little more difficult to compare. I would say we’re extremely competitive in terms of rates of reactivity and efficiency relative to standard metallurgical processing, which most of the time requires lots of energy.’
The team claims the method can be used with other metals including zinc, copper, manganese and cobalt, and Lumb is targeting more difficult separations that are handled in greater quantities, including aluminium, iron and titanium.
To read the paper, A chlorine-free protocol for processing germanium, visit bit.ly/2teS0OY