Recycling rare earth metals from electronic waste

BY MARIA BURKE

With China dominating the mining and refining of rare earth metals, other countries are increasingly looking to alternative sources such as electronic waste. Maria Burke reports

Rare earth metals are indispensable in smartphones, computers, screens and batteries. They are also key ingredients in many renewable technologies, such as wind and solar energy. According to the International Energy Agency (IEA), demand for rare earth elements is expected to grow by three to seven times, compared with current levels, by 2040. [1] But supply is challenging. They can be mined directly from the earth, recovered from secondary sources, such as end-of-life electronics, or extracted from unconventional sources like industrial wastes. China is by far the dominant market player, a concern to the US and the rest of the world. The IEA predicts that China will undertake 77% of all global refining of rare earth elements and 54% of all mining by 2030.

Rare earth metals always occur in compound form in natural ores, but they are difficult to separate. Traditional separation processes are very chemical- and energy-intensive, requiring several extraction steps. This makes extraction and purification expensive, time-consuming and extremely harmful to the environment. As a result, research is increasingly focusing on recycling and recovering them. Electronic waste is an important, but as-yet underutilised source.

‘Rare earth metals are hardly ever recycled in Europe,’ says Victor Mougel, an inorganic chemist at ETH Zurich, Switzerland. The recovery rate of rare earth elements in the EU is still below 1%, he says. ‘There is an urgent need for sustainable and uncomplicated methods for separating and recovering these strategic raw materials from various sources.’

For example, fluorescent lamp waste contains 17 times more rare earth metals than natural ores, particularly europium. Recycling methods for europium are no longer economically viable now that fluorescent lamps are being phased out, Mougel says. Switzerland sends most of this waste abroad for landfill disposal.

Mougel’s team recently reported on a simple method for efficiently separating and recovering europium from complex mixtures including other rare earth metals. [2] Existing methods are based on hundreds of liquid-liquid extraction steps and are inefficient. In their study, they show how a simple inorganic reagent allows them to obtain europium in a few simple steps and in quantities that are at least 50 times higher than with previous separation methods.

The method relies on tetrathiometallates, small inorganic molecules featuring four sulfur atoms around tungsten or molybdenum. They are the binding site for metals in natural enzymes and are used as active substances against cancer and copper metabolism disorders. Mougel’s team uses tungsten tetrathiolate (WS42−) ligands. It reduces europium from its trivalent state to an unusual divalent state which simplifies separation from the other trivalent rare earth metals.

The team reports that its strategy allows selective europium recovery with relatively high separation efficiency (separation factor of over 1000); and a recovery efficiency as high as 99% without waste pre-treatment.

‘Our recycling approach is significantly more environmentally friendly than all conventional methods for extracting rare earth metals from mineral ores,’ says Mougel.

The researchers have patented their technology and are founding a start-up called REEcover to commercialise it. They are currently working on adapting the separation process for other rare earth metals such as neodymium and dysprosium.

More metals

Meanwhile, researchers from the US Department of Energy’s Pacific Northwest National Laboratory (PNNL) have been looking at recovering a wider range of metals including manganese, magnesium, dysprosium and neodymium.

‘Our goal is to develop an environmentally friendly and scalable separation process to recover valuable minerals from electronic waste,’ says PNNL researcher Qingpu Wang. ‘[Our recent work] showed that we can spatially separate and recover nearly pure rare earth elements without complex, expensive reagents or time-consuming processes.’

The team’s method is based on a simple mixed salt water-based solution flowing continuously in reaction chambers, which induces non-equilibrium conditions. [3] Different metals behave differently when placed in the reaction chamber as two liquids flow together continuously. They form solids at different rates over time allowing the researchers to separate and purify them.

The research team first looked at separating neodymium and dysprosium, used to manufacture permanent magnets found in computer hard drives and wind turbines, among other uses. They reported that it took four hours for the two elements to separate and form pure solids in the reaction chamber, compared with 30 hours using conventional separation methods. Such methods include solvent extraction and organic ligand-based selective precipitation, which are chemical- and energy-intensive as well as time consuming. Their single-step method does not require membranes, solvents, or adsorbents, making it easy to scale-up and implement, the team says.

The team studied a mixed solution of neodymium chloride and dysprosium chloride in various metal ratios and sodium dibutyl phosphate as the reactant. Using a three-inlet microfluidic channel allowed them to visualise two precipitation processes simultaneously, helping them to understand the differences between the two metals at different concentrations. They used this information to identify concentration conditions for selective precipitation of dysprosium. The next step, they say, is to modify the reactor design to recover a larger amount of product efficiently.

Using a complementary technique, Wang and his colleague Elias Nakouzi, showed that they can recover nearly pure manganese (>96%) from a solution that mimics dissolved lithium-ion battery waste [4]. Battery-grade manganese is produced by a handful of companies globally and is used primarily in the battery cathode. In this study, the researchers used a gel-based system to separate the materials based on the different transport and reactivity rates of the metals in the sample. They created a model feedstock solution simulating dissolved battery electrodes, which they placed on top of a hydrogel loaded with sodium hydroxide as a precipitating agent. As the lithium, manganese, cobalt, and nickel ions diffused into the gel, a gradient of precipitates formed along the length of the reaction chamber.

Elemental analysis showed the enrichment of nickel near the gel-solution interface, followed by the formation of an almost pure manganese product further along the reactor. The team worked out that a sodium hydroxide concentration of 10mM and a gel/solution volume ratio of 2:1 produced the most efficient separations.

‘The beauty in this process is its simplicity,’ says Nakouzi. ‘Rather than relying on high-cost or speciality materials, we pared things back to thinking about the basics of ion behaviour. And that’s where we found inspiration.’

As well as recovering minerals from e-waste, Nakouzi expects their approach will be relevant to chemical separations from complex feed streams and diverse chemistries such as recovering magnesium from sea water and mining waste.

Biomethods

Taking a different approach, researchers from South Korea’s Pohang University of Science and Technology (Postech) have assessed the economic viability of biological methods to extract tungsten from semiconductor waste. Tungsten is widely used in electronics, semiconductors, aviation, and automotive industries. Given its rarity and the limited number of countries where it can be mined, research into recovering tungsten from industrial waste, particularly wastewater, has become increasingly important. It offers a solution both to a costly disposal problem and an environmental hazard. Industrial wastewater, if not properly treated, can severely impact water quality and soil, making this field of research a promising solution for both resource recovery and environmental protection.

In this study, the research team used bioleaching to recover tungsten from wastewater generated by the semiconductor manufacturing industry. [5] Bioleaching uses microorganisms and their metabolites to extract metals from waste. Compared with conventional hydrometallurgical and pyrometallurgical methods, the researchers say using microorganisms has a lower environmental impact and can extract metals at relatively low energy and cost. Here the team used the common fungus Penicillium simplicissimum, which metabolises glucose or sucrose to produce organic acids like gluconic, citric, and oxalic acids that dissolve metals in waste. Following bioleaching, they recovered tungsten from the solution using two purification processes: activated carbon-based adsorption-desorption and ammonium paratungstate precipitation. Analysis revealed the adsorption-desorption process was about 7% cheaper than precipitation.

Bioleaching accounted for more than 85% of total cost in both cases, mainly because it needs multiple reactors for extended processing times. The next steps will involve looking at adapting microbial strains for quicker processing and more efficient returns. ‘Our study demonstrates the economic and industrial feasibility of an eco-friendly bioleaching process for tungsten recovery,’ says team leader Jeehoon Han. ‘We aim to enhance the economic viability of this process by developing high-efficiency microbial strains.’

Also using bio-methods, at Cornell University, US, researchers have shown that genetically engineering a bacterium could improve the efficiency for the purification of rare metals and minerals compared with using solvent-heavy methods. Rather than extracting rare earth elements from waste product, they are working on improving ways to recover them where they occur naturally. ‘Traditional thermochemical methods for separating lanthanides [rare earth metals] are environmentally horrible,’ says Buz Barstow, an environmental engineer at Cornell. ‘It’s difficult to refine these elements. That’s why we send rare earth elements offshore – generally to China – to process them.’

Adsorption, or biosorption, of rare earth elements onto bacterial cell membranes offers a sustainable alternative to traditional solvent extraction methods, which often require high temperatures and harsh chemicals, he says. Certain bacteria will bind to metal ions in relatively high numbers and could potentially be reused over multiple biosorption and desorption cycles. Both Gram-negative and Gram-positive bacteria have already been found to have binding sites for rare earth elements and provide an effective separation method. But for biosorption-based purification to compete economically, researchers are looking to enhance the capacity and specificity of biosorption sites.

The problem is that the variety and complexity of bacterial membrane surface sites make targeted genetic engineering difficult.

Barstow’s team used multiple rounds of in vivo random mutagenesis induced by a plasmid called MP6 to change the genome of the bacteria Vibrio natriegens. [6] MP6 introduces errors into the microbe’s genome, producing mutants that the team then screened to select the ones with the best capacity and selectivity for biosorbing rare earth elements.

‘Given the ease of finding significant biosorption mutants, these results highlight just how many genes likely contribute to biosorption as well as the power of random mutagenesis in identifying genes of interest and optimising a biological system for a task,’ Barstow says.

The best performing engineered strain of Vibrio natriegens was capable of biosorbing 210% more dysprosium, compared with the wild-type bacteria, with selectivity improvements of up to 50% for rare earth elements between the lightest (lanthanum) and heaviest (lutetium).

Vibrio natriegens – and a growing array of bacterial tools – offer a way to safely bring rare earth elements and minerals processing back to the US, Barstow believes. For example, biological processing at the Mountain Pass mine in California – the only active rare earth element mine in the US – could help bring it back to robust productivity.

‘This new work gives us a shot to leapfrog thermochemical methods,’ Barstow says. ‘We can engineer this and other bacterium and, because we don’t need to purify proteins, we can operate this kind of system much more cheaply than competing biological processes.’

The US no longer has expertise in thermochemical processing methods, he adds. ‘For purifying rare earth elements, we’re now left with competing green methods. So even if we wanted to use old thermochemical methods, we probably couldn’t. We no longer know how to do it. We are being forced to innovate our way out of this problem.’