Aluminum mining waste can be a source of green steel

Minerals that form the basis of modern society also cause a number of problems. Separating the metals we want from other metals is often energy intensive and can leave behind large amounts of toxic waste. Obtaining it in pure form can often require a second and significant energy input, which increases the associated carbon emissions.

A team of researchers from Germany has now figured out how to deal with some of these issues for a specific class of mining waste generated during aluminum production. Their method relies on hydrogen and electricity, which can be obtained from renewable energy and extracting iron and possibly other metals from waste. What remains may still be toxic but is not harmful to the environment.

Get out of the mud

The first step in aluminum production is to isolate the aluminum oxide from other materials in the ore. This leaves behind a substance known as red clay; It is estimated that approximately 200 million tons are produced annually. While the red color comes from the iron oxides present, it contains many other substances, some of which can be toxic. The process of isolating aluminum oxide leaves the material with a very basic pH.

All of these features mean that red clay generally cannot (or at least should not) be returned to the environment. It is generally kept in containment ponds, estimated globally to contain 4 billion tons of red clay, and several containment ponds have exploded over the years.

Iron oxides can account for more than half the weight of red clay in some locations, making it a good source of iron. Traditional methods processed iron ores by reacting them with carbon, releasing carbon dioxide. But there have been efforts to develop the production of “green steel” in which this step is replaced by a reaction with hydrogen, leaving water as the primary byproduct. Since hydrogen can be made from water using renewable electricity, this has the potential to eliminate much of the carbon emissions associated with iron production.

The German team decided to test the method of producing green steel on red clay. They heated some materials in Electric arc furnace Under an atmosphere that was mostly argon (which doesn't react with anything) and hydrogen (10 percent of the mixture).

Pumping (out) iron

The reaction was remarkably quick. Within a few minutes, metallic iron nodules began to appear in the mixture. Iron production is largely complete in about 10 minutes. The iron was remarkably pure, with about 98 percent by weight of the material in the nodules being iron.

She started the process with a 15-gram sample of red clay, and then reduced it to 8.8 grams, as much of the oxygen in the material was released as water. (It's worth noting that this water can be recycled back to produce hydrogen, closing the loop on this aspect of the process.) Of the 8.8 grams, about 2.6 (30 percent) was in the form of iron.

The research found that there were also some small pieces of relatively pure titanium formed in the mixture. Therefore, there is an opportunity to use this to produce additional metals, although the process probably needs to be improved to increase the production of anything other than iron.

The good news is that there isn't much red clay to worry about after that. Depending on the source of the original aluminum-containing ore, some of this may include relatively high concentrations of valuable materials, such as rare earth metals. The downside is that any toxins in the original raw material will be significantly more concentrated.

As a small plus, the process also neutralizes the pH of the remaining residue. So, that's at least one less thing to worry about.

The downside is that the process is incredibly energy-intensive, both in producing the required hydrogen and operating the arc furnace. The cost of that energy makes things economically difficult. This is partly offset by lower processing costs, as the ore has already been obtained and is of relatively high purity.

But the main feature of this is its extremely low carbon emissions. Currently, there is no price for these products in most countries, making the economics of this process much more difficult.

Nature, 2024. DOI: 10.1038/s41586-023-06901-z (About digital IDs).