| dc.description.abstract | It is beneficial to recover Cr and Pb from chromium plating sludge (referred to as CrPS) since it is primarily composed of PbCrO4 and PbO. This study investigated a novel technology for recovering chromium and lead from CrPS, by using thermite reaction between chromium oxide and aluminum (i.e., waste, parings and scrap). Sludge from Chromium wastewater treatment process (referred to as CrWS) , mainly composing of chromium oxide (Cr2O3) and silicon dioxide (SiO2), was also added in the recovering process to function as a glass-former that facilitate the separation of metal and slag and improves the quality of slag. In this study, the recovery process was divided into 3 stages: refinement of the CrPS, recovery of lead form the CrPS, and again, recovery of chromium as Cr-Fe alloy from the resultant slag.
During the refinement of the starting CrPS, the sludge was heated at a temperature ranging from 100℃ to 600℃. It was found that the major species identified as PbCrO4, PbO, Cr3O4, and CrO were not changed; the total concentration of crystal phases, however, increased at 500℃. This suggests that the loss on ignition (i.e., 8.53 w/w% for CrPS) and the oxidized degree of thermit oxides might affect. The thermite reaction between chromium oxide and aluminum may proceed in a one-step path: CrO3+2Al→Al2O3+Cr, or in a two-step path: 2CrO3+2Al→Cr2O3+Al2O3 and CrO3+2Al→2Cr+Al2O3. The PbCrO4 in the starting CrPS might decompose into PbO and CrO3 when heated, which led to the activation of thermite reactions between PbO and Al, as well as between CrO3 and Al. However, the species, the oxidized degree, the reaction path, and the stoichiometry of Al were not clear. Therefore, the actual stoichiometry between CrPS and Al has to be determined experimentally by properly assuming what the thermit oxides are in the starting material. Several mixes of CrPS and Al, with Al in excess of its stoichiometry were tested.
Thus, the highest reaction temperature developed would then determine the optimum mix of CrPS and Al, reflecting a mix under the balanced effects of factors such as species, oxidized degree, reaction path, impurities, non- reactive oxides, Al stoichiometry, and heat loss from the reactor wall. In this study, a mix ratio of CrPS:Al=4.53:1 showed the optimum mix for the CrPs and Al thermite. However, in this case, the recovered alloy and slag were not separated.
In the recovering of Pb by thermite reactions, it was found that one kg of the above optimum mix, when added with 0.72 kg of CrWS, would result in a maximum recovery of lead alloy (18 w/w%), with 95.61% of Pb purity. The Pb recovery rate reached 76.79% as compared to initial Pb in the inputs. The 69 % of slag by weight of inputs was also recovered that retained most of the chromium oxides.
In the subsequent recovery of chromium as Cr-Fe alloy, the above slag was further pulverized and mixed with Fe2O3-Al thermite with proper amount of Al stoichiometry. Optimum recovery of Cr-Fe alloy was achieved with Cr:Fe ranging from 4.61:1 to 1.55:1. Chromium recovery rate reached 80.78% as compared to the Cr input at second phase. This process also recovered 62.3 w/w% of slag compared to the inputs at second phase. The recovered slag was vitrified and stable.
The results of this work suggest that to recover Pb and Cr from CrPS and CrWS using thermite reactions is a promising technology not only energy conservative but also recycling-beneficial.
| en_US |