Steel production: bauxite waste as iron ore | nd-aktuell.de

Aluminum is the most common metal in the earth’s crust. However, the base light metal is practically only found chemically bound. This is one reason why the metal was only discovered in 1825. The most important starting mineral for aluminum production is bauxite, which contains around 60 percent aluminum compounds. At the end of the 19th century, an economical process called fused electrolysis was invented, using so-called alumina (pure aluminum oxide Al₂O₃) to produce aluminum. This clay is produced on an industrial scale from bauxite using caustic soda. What is left over – the so-called red mud – is currently being more or less safely deposited, and some of the caustic soda is recovered. Because the sludge is highly alkaline, sealing the settling tanks is not that easy. Concrete, for example, decomposes easily. In addition, the landfills often overflow during heavy rainfall and contaminate their surroundings (see basement). Per tonne of Al₂O₃ One to one and a half tons of red mud are produced. The red mud owes its eponymous color to the iron compounds. It contains up to 60 percent iron (III) oxide and iron (III) hydroxide. One could also view the bauxite waste as a type of iron ore.

At least that’s what scientists at the Max Planck Institute for Iron Research in Düsseldorf apparently thought. You recently published it in the specialist journal »Nature« presented a process that kills two birds with one stone: On the one hand, it turns the corrosive waste from bauxite processing into crude steel, and on the other hand, the other mineral residues become useful building materials. And the whole thing can also contribute to the climate-neutral transition of the steel industry. In order to reduce the iron oxide in the red mud to iron, the process should use so-called green hydrogen, i.e. hydrogen that is produced using excess renewable electricity.

Currently, the vast majority of steel is produced in several steps from iron ore – iron oxide or iron carbonate. Some of the processes used are centuries old, although they are constantly being perfected. To extract iron, the oxygen must be removed from the oxides in the ore. In today’s integrated steelworks, this happens in the blast furnace. There, the oxygen is removed from the processed iron oxides using the carbon from hard coal coke and the entire material is melted at temperatures above 1500 degrees Celsius. The coke serves as a heating material and the carbon monoxide produced during combustion serves as a reducing agent. But where carbon is burned, the greenhouse gas CO becomes₂ free, which is around 1.8 tonnes of CO per tonne of crude steel in blast furnaces₂. With a view to climate change, new processes for producing crude steel are being researched around the world. The focus is on reduction with “green” hydrogen. As part of EU-funded projects, several corporations and research institutions in Western Europe are currently working on processes for direct reduction with hydrogen, which is to be produced using renewable energies.

The first plant of this kind is planned for Luleå in northern Sweden. The HYBRIT (Hydrogen Breakthrough Ironmaking Technology) project is intended to support the Swedish CO₂-Reduce emissions by up to ten percent. The aim is to establish the world’s first industrial process for fossil-free steel production from iron ore in Sweden by 2035. The advantage in Sweden: The iron ore is located locally, hydrogen can also be produced locally using hydro and wind power and transported and stored without high pressures. However, the resulting iron still has to be further processed into steel in electric arc furnaces.

In general, however, with this development path, the costs of hydrogen from electrolysis are just as much a problem as the quantities of hydrogen required. While hydrogen with an energy content of 55 terawatt hours (TWh) is currently consumed annually in material applications in Germany, the German hydrogen strategy assumes a need of 80 TWh for the conversion of the steel industry alone by 2050. And, unlike currently, this hydrogen would have to be produced in a completely climate-neutral way. But according to the company Swiss Steel, electrolyzers with an output of just 200 MW are currently running in Europe.

In Austria, where they are also working on producing crude steel using hydrogen, they are looking for a way that uses less hydrogen and, if possible, gets from iron ore to steel in one step. A test facility for the smelting reduction of ore using a hydrogen plasma was put into operation at the Montanuniversität Leoben in cooperation with the steel company Voestalpine. Less hydrogen is needed to reduce the iron oxide because part of the reduction energy comes from the electrons in the arc. The gas ionized by the high temperatures in the arc is far more reactive than molecular hydrogen.

And this is where the Austrian research meets the new concept for red mud utilization from Düsseldorf: They also work with an electric arc furnace with a hydrogen plasma. However, there are a few things that are different in terms of the practical setup of the Max Planck researchers’ previous test facility. While in Leoben iron ore and hydrogen are continuously fed into the arc furnace via a hollow graphite electrode, in Düsseldorf they work with a tungsten electrode that is used solely to generate the arc and thus also the hydrogen plasma. The hydrogen is supplied separately in a mixture with the noble gas argon. The decisive advantage is that there is no need for complex pre-processing of the red mud. In the electric arc furnace, the red mud initially becomes a viscous melt, the iron oxide is reduced by the hydrogen plasma and iron drops are formed, which can be separated from the slag due to their higher density. As the authors around Matic Jovičević-Klug and Dierk Raabe write in “Nature”, after about ten minutes in the arc furnace, 70 percent of the iron contained is converted into metal. In a laboratory test, the processing of 15 grams of red mud produced 2.6 grams of high-purity metallic iron and neutral residual slag, which can be used as a building material. Indian metallurgy expert Chenna Rao Borra says in a “Nature” commentary on the study that hydrogen plasma reduction might also be suitable for low-grade iron ores and some waste from previous iron ore processing. However, whether the process from Düsseldorf is really economical on an industrial scale can only be assessed once it has been tested in a pilot plant.