Researchers are testing a process that extracts iron from red mud
Rare earths, lithium, copper and iron: Metals like these form the basis of modern society, but also cause a number of problems. Separating the metals we want from other minerals is often energy intensive and can leave behind large amounts of toxic waste. Obtaining them in pure form often requires a second, significant energy expenditure, driving up associated carbon emissions.
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A team from the Max Planck Institute for Iron Research has now figured out how to solve some of these problems for a specific class of mining waste produced during aluminum production. Their method relies on hydrogen and electricity, both of which can be obtained from renewable energy, and extracts iron and possibly other metals from the waste. What remains is still toxic, but no longer as harmful to the environment. They published the corresponding study in “Nature”.
Red mud with toxic substances
The first step in aluminum production is isolating the aluminum oxide from the other components of the ore. What remains is a material known as red mud; It is estimated that almost 200 million tons of it are produced annually. The red color comes from the iron oxides present, but the mud contains many other substances, some of which can be toxic. And the process of isolating the aluminum oxide leaves it with a very basic pH.
All of these properties mean that the red mud generally cannot (or at least should not) be returned to the environment. It is typically stored in catch basins, which hold an estimated 4 billion tons of red mud worldwide. And many catch basins have burst over the years.
The iron oxides can make up more than half the weight of red mud in some places, which could make it a good source of iron. Traditional processing of iron ores involves reacting them with carbon, releasing carbon dioxide. However, there are efforts to develop “green steel production” in which this step is replaced by a reaction with hydrogen, leaving water as the main byproduct. Because hydrogen can be produced from water using renewable electricity, this has the potential to eliminate much of the carbon emissions associated with iron production.
But back to the Max Planck team: The researchers wanted to test a method for producing green steel on red mud. They heated some of the material in an electric arc furnace under an atmosphere at 900 mbar consisting mostly of argon (which does not react with anything) and hydrogen (which makes up 10 percent of the mixture).
Pump (out) iron
The reaction was remarkably quick. Within a few minutes, metallic iron nodules formed in the mixture. Iron production was largely complete after about 10 minutes. The iron was remarkably pure, with about 98 percent by weight of the material in the nodules being iron.
Starting from a 15-gram sample of red mud, the process reduced this amount to 8.8 grams because much of the oxygen in the material was released in the form of water. (This water could be recycled into hydrogen production, closing the loop in that aspect of the process). Of these 8.8 grams, approximately 2.6 grams (30 percent) were in the form of iron.
The researchers found that there were also some small pieces of relatively pure titanium in the mixture. So there is a possibility that this mixture could be used to produce other metals, although the process would probably need to be optimized to increase the yield of metals other than iron.
Good news and disadvantages
The good news is that there is a lot less red mud to worry about afterwards. Depending on where the original aluminum-containing ore comes from, some of it may contain relatively high concentrations of valuable substances, such as rare earths. The disadvantage is that all toxic substances are much more concentrated in the original ore.
As a small plus, the process also neutralizes the pH of the remaining residues. So at least you have one less thing to worry about.
The downside is that the process is incredibly energy intensive, both in producing the required hydrogen and in operating the arc furnace. The cost of this energy makes things economically difficult. This is partially offset by the lower processing costs – the ore has already been mined and has a relatively high degree of purity.
The most important feature, however, is the extremely low carbon emissions. Currently, in most countries there is no price for these emissions, making the economics of this process much more difficult.
The article originally appeared on Ars Technica.
(jl)
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