Magnetisation of Stainless Steel allows efficient Separation
Slag is a normal and necessary “byproduct” of the steel making process and contains debris as well as a certain amount of metals. Recovering this metal content reduces the need for virgin ores and saves part of the production process.
At the Krefeld site of Outokumpu Nirosta GmbH in Germany, approximately 13 000 to 15 000 t of stainless steel slag were formed each month until the closure of the meltshop in December 2013. ThyssenKrupp Mill Services & Systems (TKMSS) was charged with treating the slag, which contained valuable, reusable metal components.
After two years of production in the existing treatment plant with a BHS rotor centrifugal crusher of type RSMX, it has been proven that dry treatment of the slags is significantly less problematic and more economical than wet treatment. This also becomes apparent in comparison with other steel mills which operate plants with wet treatment.
The customer’s laboratory discovered that the treated and returned metallic material with a grain size of less than 22.4 mm has on average a level of purity of more than 80 percent metal. The slag sold as construction material comprises only less than 1.2 percent metal (measurement limit 0.5 mm), has a cubical grain shape and is earth-dry, thus satisfying the requirements of customers within the construction materials industry.
An important aspect of the cost-benefit analysis is that there are no costs for water treatment. Even when considering that the plants must be protected from the elements and that they require partial dust removal, dry treatment works more economically. Today, a resource-saving process is available which requires neither water treatment nor the storage of sludge in landfills.
Stainless Steel becomes magnetic
Two series-connected cone crushers for coarse crushing were available at the Krefeld plant. The BHS-Sonthofen rotor centrifugal crusher of type RSMX was put into operation for secondary crushing in January 2011: following treatment by the cone crushers, this machine was fed a mix of grain sizes ranging from 4 to 22.4 mm consisting of pure metal and metal with slag, which it then crushed selectively.
One advantage of the impact technology used by these machines is that there is a huge difference in the elastic modulus of metal and slag: the metal parts are disaggregated much better than with pressure grinding in cone or jaw crushers. In addition, impact crushers in all process stages create material with cubical grains, which is highly suitable as construction material.
It was another effect, however, that was decisive in the choice of machinery: when stainless steel collides with a solid wall at high speed, it becomes magnetized for a short time. After it has been crushed, the normally a magnetic or slightly magnetic steel, used at the Krefeld site, can thus be separated from mineral substances using conventional magnetic separators.
The rotor centrifugal crusher contains a compact rotor with two centrifugal chambers mounted on a vertical shaft. The input material is fed from above into the center of the crusher and passes into the two centrifugal chambers of the rotor. The circumferential speed of the rotor causes the material to be accelerated outward at high velocity and to rebound against the anvil ring of the outer housing. In the process, the slag is shattered mainly as a result of the optimized single impact of each feed grain against the fixed wall, thereby being separated from the metal parts. As mentioned before, the endproducts are of excellent cubical shape which is expelled from the machine’s discharge outlet.