Dephosphorization of chromium-rich ashes from thermally-treated leather waste for utilization in pyrometallurgical process

Abstract

Acid leaching was investigated to remove phosphorus from the ashes of thermally-treated (gasification and combustion process) footwear leather waste in order to obtain a good quality raw material for the production of a ferrochromium alloy. The ash contained high amounts of chromium (55wt.% Cr2O3) and phosphorus (1.42wt.% P). The following key factors were selected for study: temperature, H2SO4 concentration in the leaching solution, acid solution/ash ratio and the particle size of the ash. The response was the phosphorus removal percentage and, under some experimental conditions, chromium loss. The strategy was based on a 24 full factorial design with a central point followed by the addition of star points in a face-centered central composite design. The maximum chromium loss was 1.1wt.%. The significant effects (according to variance analysis) were used for a proposed second-order response surface model. The model was useful for finding the optimal operating conditions in the experimental region by the maximization of phosphorus leaching. 99.2±2.5% phosphorus removal from the ash was achieved under the following conditions: 60°C, 13.9wt.% of H2SO4 in the leaching solution, 4.80 (ml of acid solution)(g of ash)−1, 0.0370±0.0370mm particle size. Mass balance was used as a tool to indicate that the remaining phosphorus content was acceptable for producing ferrochromium alloys.