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Slag cleaning furnace

The electric furnace technology for the cleaning of copper and nickel slags is a well-established method. Low investment costs, small footprint combined with continuous process development and equipment improvement make electric slag cleaning an attractive choice for modern smelters.

In the electric Outotec® Slag Cleaning Furnace, cleaning of the Flash Smelting Furnace slag is carried out by surface coke reduction. The Slag Cleaning Furnace is operated batch-wise in a circular electric furnace equipped with three Soderberg electrodes.  

The slag is tapped periodically from the Flash Smelting Furnace along water-cooled slag launders through the already charged coke layer into the furnace. Reduction of metal oxides starts during tapping and continues until the end for the entire time that the slag is in the furnace.  

Reduced metal droplets and mechanically trapped matte or metal droplets are settled through the slag layer at the bottom of the electric furnace to form the matte or metal alloy layer. Immiscible slag components with a much lower density remain at the top of the bath and form the cleaned slag layer.

Electric power is used for compensation of the heat needed for the endothermic reduction reactions, furnace heat losses and the melting of the solid material fed into the furnace. Concentrate or coke can be injected into the electric furnace in order to raise the sulfur content of metal alloy and thus lower liquidus temperature, or to enhance reduction kinetics. Concentrate injection is typically used when cleaning nickel slags from the Outotec® Direct Nickel Flash Smelting Process (DON). It is also applied when cleaning cobalt containing copper slags obtained from the Outotec® Direct Blister Flash Smelting Process.

The reduced slag from the Slag Cleaning Furnace is tapped through the slag tapping holes via launders into granulation or to additional treatment. The matte or metal alloy is tapped and transported to further refining.

The furnace bottom is cooled with hearth cooling air fans to prevent matte or metal alloy from penetrating the refractory layer.

The off-gas from the furnace is first post-combusted and cooled in the off-gas incinerator. For this purpose, air and ventilation gases are typically used.

The post-combusted and diluted off-gas is led further for additional cooling by air or water spraying. Draft control is necessary for the operation of the Slag Cleaning Furnace. Normally a slight negative pressure is maintained in the furnace to avoid dust and fugitive gas emissions into the working atmosphere. Temperature control is needed if the off-gas is to be cleaned in a bag filter or some other cleaning system before venting into the atmosphere.