Rare earth elements are in high demand for consumer electronics and many other products. They are difficult both to mine and recycle. Food processing produces tons of agricultural waste. Scientists have found a way to use agricultural waste to feed bacteria that make safer chemicals for recycling rare earths.
Waste disposal is a cost. Whoever needs to get rid of waste has to pay someone to handle it. Recycling treats the same materials not as waste, but as a commodity. The economics changes.
Making, say, shirts from plastic bottles is good for the environment. Recycling agricultural waste instead of using hazardous chemicals to recycle rare earth elements packs an even greater environmental benefit.
What’s rare about rare earth elements?
Most people probably never think about elements like neodymium, praseodymium, cerium, or lanthanum if they’ve heard of them at all. But they’re among seventeen rare earth metals.
They aren’t rare in the sense of scarce. The least common of them is nearly 200 times more abundant than gold. But they don’t occur in their own ores like most metals. They seldom occur in high enough concentrations to make mining them economically viable.
It hardly mattered until the mid 1960s. No one had much use for them. Nowadays, however, they are critically important in products as diverse as rechargeable batteries, fluorescent lighting, catalytic converters, night vision goggles, and computer memory.
So even if you know nothing about rare earths, you depend on them for multiple devices you use every day. They also have national security implications. The US doesn’t produce a domestic supply of rare earth elements. China dominates the world market for them. That leaves American manufacturers who need them vulnerable to supply disruptions.
Why mining and recycling rare earth elements are so difficult
Industry in general uses about 85 different elements. Cell phones alone contain as many as 65, including rare earths.
It may be more difficult to extract any given element from a pile of used cell phones than from rocks. Rare earth elements make only a small part of our current crisis with high-tech trash.
Consumer electronics uses rare earths in small amounts. As our gadgets get smaller, so does the amount of rare earth elements in each one. It is also more difficult to disassemble newer cell phones than earlier ones.
Current processes both for mining and recycling rare earth elements rely on hydrometallurgy. The technique uses liquid to extract metals from a substance that contains them. It dissolves the metals and forms a precipitate. The metal-containing substance can be an ore, residual material from some manufacturing process, or electronic waste.
Inefficient traditional methods of hydrometallurgy typically use sulfuric acid, a hazardous substance. They also require heat and pressure. In some cases, recycling electronic waste may cause more environmental harm than mining virgin materials. High energy costs of the heat and pressure make obtaining rare earths expensive.
Because of the small amounts of rare earth elements used in products, separating them out for their own sake costs more than it’s worth. But these same products often use more valuable materials. Efficiently extracting gold, iridium, and palladium along with the rare earths could make recycling electronic waste economically viable, maybe even profitable.
Bacteria to the rescue
The US Department of Energy’s Critical Materials Institute (CMI) has sponsored research on recycling rare earths primarily through the Idaho National Laboratory (INL). Researchers with the Lawrence Livermore National Laboratory and Purdue University have also participated.
This research has developed a technique that depends on a bacterium called Gluconobacter oxydans, the same one that causes fruit to rot.
The microbes make a chemical soup that includes such organic acids as gluconic acid. Whatever else the mixture contains makes it work better than organic acids alone in extracting rare earths from, say, ground up magnets.
Using microbes to extract metal is a type of biohydrometallurgy called bioleaching. Bioleaching has fewer environmental impacts than conventional technologies and costs less.
The team at INL constructed a 20-50% efficient bioreactor. It also used a heap leaching process that adds the biochemicals via a drip system to material on a liner, with similar results. The amount of rare earth elements recovered depends on the concentration in the materials processed, which could be from 1.5-5%. Different rare earths bring different prices, ranging from $2 per kilogram to $400 per kilogram.
So far, the lab has succeeded in extracting small amounts of rare earths—less than a gram. It hopes to scale the process up to extracting kilograms, and eventually tons.
The process of recycling mercury from fluorescent bulbs also recovers phosphor powders, which contain rare earth elements. Waste from mining other elements also includes rare earth elements, often in trace amounts that make them not worth recovering with standard hydrometallurgy.
The role of recycling agricultural waste
Growing Gluconobacter oxydans in laboratory conditions is not as easy as one might think. It requires carefully controlled lab conditions and plenty of glucose.
Buying glucose from a supplier accounts for 44% of the bioleaching process’ expense. Fortunately, scientists can extract it more cheaply from agricultural waste, such as potato pulp, sugar beet pulp, or corn stover.
The microbes feed on the glucose and produce the chemical soup. Which, by the way, although acidic, is not corrosive or otherwise hazardous.
CMI, with headquarters in Ames, Iowa, has studied corn stover. Iowa is the world’s leading corn producer, and therefore generates more of this waste material than anywhere else. Turning it from a waste to a resource will benefit Iowa corn farmers.
Other agricultural wastes are abundant near the INL, including potato waste and water from processing apples. The greatest environmental benefit comes from recycling locally available wastes to make the glucose.
Recycling agricultural wastes to help recycle rare earth elements uses one problem to solve another. We need imagination at least as much as money to think of ways to do that.
Cornfields could play a role in recycling old electronics / Laura Millsaps, Phys.org,
March 15, 2018
INL interns use metal-digesting microbes to recycle electronics / Cory Hatch, Idaho National Laboratory. August 24, 2017
Metal-eating microbes are cost-effective for recycling rare earth elements / Homeland Security News Wire. March 7, 2018
REE—rare earth elements and their uses / Hobart M. King, Geology.com
Why rare earth recycling is rare and what we can do about it / Jessica Marshall, Ensia. April 7, 2014
Rare earth oxides. US Agricultural Research service via Wikimedia Commons
Neodymium magnet. Public domain from Wikimedia Commons
TVs and monitors. Photo by David Wright. Public domain from Wikimedia Commons
Biotechnology research. Some rights reserved by Idaho National Laboratory
Corn stover harvest. Image courtesy of Idaho National Laboratory (INL) Bioenergy Program, via Flickr