Metals, which are the foundation of modern society, also pose various problems. Separating the required metals from other minerals is often energy-intensive and can leave behind large amounts of toxic waste. Obtaining them in their pure form often requires a second large energy input, with an associated increase in carbon emissions.
A team of German researchers has discovered a way to address some of these problems with certain types of mining waste generated during aluminum production. Their method relies on hydrogen and electricity, both of which are powered by renewable electricity, and has the potential to extract iron and other metals from waste. What’s left may still be toxic, but it won’t harm the environment.
out of the mud
The first step in aluminum production is separating aluminum oxide from other materials within the ore. This leaves behind a substance known as red mud. It is estimated that about 200 million tons are produced annually. The red color comes from the iron oxide present, but it also contains many other substances, some of which are toxic. Additionally, the process of separating aluminum oxide leaves the material with a very basic pH.
All of these characteristics mean that red mud generally cannot (or at least should not) be returned to the environment. It is commonly stored in containment ponds, which are estimated to hold 4 billion tons of red mud worldwide, and many containment pods have ruptured over the years.
In some places iron oxides can account for more than half of the weight of red mud, making it a potentially good source of iron. Traditional methods process iron ore by reacting it with carbon, causing the release of carbon dioxide. However, efforts have been made to replace this step with a reaction with hydrogen and develop “green steel” production, leaving water as the main by-product. Hydrogen can be produced from water using renewable electricity, potentially eliminating many of the carbon emissions associated with steel production.
A German team decided to test a method of producing green steel on red mud.they heated some of the ingredients Electric furnace Under an atmosphere consisting mostly of argon (does not react with anything) and hydrogen (10 percent of the mixture).
Pumping (draining) iron
The reaction was surprisingly fast. Within minutes, chunks of metallic iron began to appear in the mixture. Iron production was almost complete by about 10 minutes. The iron was surprisingly pure, with about 98 percent of the mass of the mass being iron.
A process that started with a 15-gram red mud sample reduced this to 8.8 grams, as much of the oxygen in the material was released in the form of water. (It’s worth noting that this water can be recycled into hydrogen production, closing the loop on this aspect of the process.) Of that 8.8 grams, about 2.6 grams (30 percent) was in the form of iron.
The study found that some small pieces of relatively pure titanium formed in the mixture. Therefore, this could potentially be used to produce additional metals, but the process would need to be optimized to increase the yield of anything other than iron.
The good news is there’s a lot less red mud to worry about after this. Depending on the source of the original aluminum-bearing ore, some of it may contain relatively high concentrations of valuable materials such as rare earth minerals. The disadvantage is that the toxic substances contained in the original ore are greatly concentrated.
As a small plus, this process also neutralizes the pH of the remaining residue. That means at least one less thing to worry about.
The downside is that this process is incredibly energy intensive, both in producing the necessary hydrogen and in operating the arc furnace. The cost of that energy makes it economically difficult. This is partially offset by lower processing costs. That is, because the ore is already obtained and relatively pure.
However, the key feature of this product is its extremely low carbon footprint. At the moment, most countries do not have a set price, making the economics of this process much more difficult.
Nature, 2024. DOI: 10.1038/s41586-023-06901-z (About DOI).