Tratamento da lama fina gerada na lavagem de gases do convertedor AOD-L, na produção de aço inoxidável, visando à recuperação de cromo e níquel

Abstract

In 2004, around 24 thousand t of AOD-L sludge were generated by ACESITA S/A in the stainless steel making process. The fine fraction of this material, around 15 thousand t, constitutes an environmental liability containing Fe, Cr and Ni. Techniques such as briquetting and magnetic separation aiming to recycle this material have not shown satisfactory results. Therefore, based on Its granulometry and also on its content of Ni and Cr, the use of hydrometallurgy was investigated in this study in order to treat the fine fraction of the AOD-L sludge. The study included the characterization and leaching of the sludge. The sludge is constituted by a fine powder (mean diameter of 1 nm) containing (33 ± 2) % w/w ofmoisture content and it was classified as hazardous waste (class I). X-Ray diffraction analysis has identified the following crystalline phases: chromium-bearing magnetite (Cr-FeaO^), hematite (Fe203), calcite (CaCOs) and chromite (FeCr204), this latter a spinel oxide that isvery difficult to dissolve in aqueous solutions. Scanning Electron Microscopy with Energy-Dispersive Spectrometry has pointed out that Ni is present in very fine particles, possibly in the metallic form or associated with Fe and O elements. The sludge presented the followingchemical composition (in % w/w): (33,9 ± 1,6) of Fe; (10,2 ± 0,9) of Cr; (1,4 ± 0,1) of Ni, and other elements such as Ca, Mg, Mn and Si. The leaching was carried out in two steps: Leaching LI, aiming to reduce the Fe content with consequent Cr and Ni concentration in the sludge; and Leaching L2, aiming to maximize Cr and Ni extraction. For LI, the following operating factors were investigated: type of leaching agent (H2SO4 and HCI), temperature (25, 40, 55 and 70°C) and acid concentration (3, 6 and 12 % v/v). In this operation, a higher extraction of Fe and Ni were obtained, in comparison to the extraction of Cr. The best separation factors and Fe extraction levels were obtained in hydrochloric media. For the LI operation, the content of Fe in the sludge was reduced around 50 % w/w while Cr was concentrated approximately 100 % w/w. The main disadvantages of LI are the high extraction of Ni (around 40 % w/w) and the excessive consumption of acid. For L2, the tests were carried out at 140°C and atmospheric pressure condition. In the experiments, the following operating factors were investigated: H2SO4 concentration (60 and 65 % v/v) and HCI04/sludge ratio (0 and 4 mL/g). Higher extractions of Cr and Ni associated to lower extractions of Fe were obtained. The addition of HCIO4 favors the chromite dissolution and also the selectivity to Cr and Ni in detriment of Fe. The increase on the H2SO4 concentration, on the other hand, has not favored the selectivity of operation and resulted on the decrease of the metal extractions. However, the technique limitation of the experimental apparatus used for the L2 tests requires the investigation of this step at high pressure conditions.