Workers harvest hot metal from a pilot-scale
DC furnace at the Outotec Research Center
in Pori, Finland.

Mineral processing specialist Outotec believes its DC (direct current) smelting technology offers a cost-efficient and environmentally sustainable solution for ferroalloy processing. For example, it can be used for direct smelting of ores with no pretreatment and also offers a cost-efficient method for recycling challenging byproducts such as dust from steel production.

Typical applications for direct current smelting include ferrochrome, ferronickel, and ilmenite, and dust recycling in steel production, especially stainless steel. Highly flexible, DC smelting can be used for processing feed ores with very fine particle sizes, or those with highly variable chemical or mineralogical compositions. This flexibility opens up opportunities for making sustainable use of reserves that would have previously been considered unviable using conventional smelting technology.

“DC smelting offers a significant advantage over traditional methods in this regard,” said Lauri Närhi, Outotec’s technology director–ferroalloys technology. “As well as cheaper, lower-grade reductants such as anthracite or coal, it can also make use of carbon-neutral bioreductants produced from renewable sources like charcoal made from local wood. Using these types of bioreductants can help producers to cut the carbon emissions of their operations and reduce their reliance on increasingly scarce fossil-fuel resources.”

In DC smelting, the feed material is gravity-fed into the furnace, together with the reductant, via charging tubes. A continuous direct electrical current is supplied by a transformer and rectifier, with a central graphite electrode forming the negative cathode and the bottom of the furnace forming the positive anode. The smelting energy is supplied by an open plasma arc, which can be likened to a giant welding torch. The alloy sinks to the bottom and the slag floats at the top, while the off-gas produced is extracted and cleaned. The gas can be used as a combustible fuel elsewhere in the plant, helping to improve energy efficiency.

One of the biggest potential sustainability benefits of DC smelting technology lies in recycling. It can be used to process and recover value from fine materials such as the dust that results from the production of steel and other metals. A steel plant can produce tens of thousands of tons of fine dust every year, and this material is very difficult to treat, often ending up in landfill sites.

Complementing existing smelting operations with a DC furnace will significantly reduce waste volumes by enabling these fine materials to be recycled with no need for complex and expensive agglomeration or pelletizing equipment, according to Outotec. Furthermore, the dust resulting from steel production, for example, contains valuable materials such as iron and zinc, and these can be extracted using the DC process. For smaller-scale operations, Outotec DC smelting is a viable alternative to traditional smelting—the simpler layout and smaller furnace mean that the plant footprint is significantly smaller.

In the future, it is hoped that the technology will play an important role in supporting zero-waste production. “Zero waste is becoming an increasingly important goal, particularly in Europe,” Närhi pointed out. “This is where 100% of the materials entering the plant leave as either finished product or end up being reused, recycled or converted into energy— with none going to landfill. The ability to recycle and recover valuable metals from previously dumped material offers huge environmental and economic benefits for producers.”

The demonstration plant at the Outotec Research Center in Pori has played an essential role in developing the DC smelting process by allowing testing of a wide range of raw materials and reductants. The center’s unique capabilities cover the entire concept and product development chain, from mineralogical analysis of feed material to the scaled-up process and operating parameters. One key goal for future development will be to further improve the energy efficiency of the process, for example through pre-reduction of the feed material prior to smelting, which reduces the electricity consumption of the furnace.

“Without the ability to test different feeds and reductants in a pilot-scale DC furnace, the technology would not have moved on nearly as quickly,” Närhi said. “The Outotec Research Center is helping us realize this and many other sustainable processes for our customers within a much shorter timeframe.”

Outotec’s history with DC smelting began in the 1980s, when it was first researched by Outokumpu. The process was revisited in 2011 when it was decided that the required technology was sufficiently advanced to make the process commercially viable, and construction of a pilot-scale DC furnace at the Outotec Research Center in Pori, Finland, began.

“The ability to offer sustainable technologies and services to our customers with less harmful impact on the environment is a key driver of our R&D process, and the DC smelting process is a perfect example of this,” said Närhi.