Bench-scale study of calcined metal hydroxide sludge as flux in AOD converter process

Metal hydroxide sludge is a metallurgical residue from neutralisation of spent pickling baths in stainless steel plants. Metal hydroxide sludge mainly contains metal hydroxides, calcium, fluoride and water and has up until now been deposited in landfills in Sweden. This thesis shows that hydroxide sludge is fully possible to recycle back into the AOD converter during slag reduction stage. Its calcium fluoride contents are utilized as flux. Prior to reintroduction, the hydroxide sludge is dried and calcined to drive off all water, included chemically bonded water. Calcined metal hydroxide sludge is called hydroflux. Metal content in hydroflux is present as metal oxides.

In a bench-scale AOD converter seven successful reduction tests, with three different varieties of hydrofluxes, type A, B and C were performed. The reduction tests are conducted with same basicity and CaO/CaF2 ratio as a full scale AOD converter. Chemical analysis and X-ray diffraction analysis show that near identical final slag chemistry is achieved as planned. Some differences in metal retrieval, depending on hydroflux type used, are noticed. Briquetting as agglomeration method is performed on one hydroflux. Three types of strength tests are performed on the hydrofluxes and results are compared to toughness of mineral fluorspar. Theoretical calculation results show how final slag as well as additives will increase with use of hydroflux as calcium fluoride provider in the AOD converter during reduction procedure.

The pros and cons for every hydroflux are weighed against each other and one type of hydroflux is recommended for further full-scale usage

Author: Hällsten, Siv

Source: Lulea University of Technology

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