A step by step investigation of Cr(III) recovery from tannery waste

Tanneries produce significant quantities of hazardous wastewater and according to international environmental organizations, its further exploitation in the context of the circular economy consider being matador. Such a case study is the recovery of Cr(III), as it is an essential reagent in the treatment of the leather, but a significant portion ends up in wastewater. According to the literature, Cr(III) can be recovered directly from the wastewater and also from the corresponding sludge, which delivered by applying physicochemical and biological methods. Aim of this study is the optimization of Cr(III) hydrometallurgical recovery from tannery sludge, reporting a thorough investigation into; how experimental conditions affect the efficiency at all steps of the process, namely leaching and precipitation. The reference sample was air dried sludge, which obtained from the tannery wastewater treatment plant located in the industrial area of Thessaloniki in Northern Greece. The chemical characterization revealed that it contained high amounts of Cr(III) (14.1%), Ca (14.8%) and organic matter (22%). Τhe extraction of Cr(III) was examined by applying various acids (H₂SO₄, HNO₃, HCl) with the following experimental conditions: concentration range 0.02-2 N, contact time 60 min, temperature at 25^o C, and liquid-to-solid ratio (L/S) 25. According to the results, the highest selectivity was obtained for H₂SO₄, since the insoluble CaSO₄ was formed, and also the highest efficiency, due to the formation of the soluble CrSO₄⁺ (93% by the use of 1 N H₂SO₄). Regarding Cr(III) precipitation from the leaching solution, by increasing its equilibrium pH in the range 6-9.5, as alkaline reagents was examined NaOH and Ca(OH)₂. A higher purity Cr(OH)₃ precipitate was obtained by the application of NaOH (70 %). On the contrary, the use of Ca(OH)₂ also led to the insoluble CaSO₄ formation, but its efficiency was limited with respect to the Cr(III) content (16.5 %).