Enhanced chromium recovery from tannery waste by acid-alkali reaction in China

Leather processing is an important industry in China, and the discharge of chromium waste has been severely circumscribed. Though traditional processes for treating chromium waste have been widely used in tannery plants, technology adopting high efficiency and economic simultaneously is deficient. In order to develop an economic treatment for tannery waste, a mixed alkali of sodium hydroxide and magnesium oxide and diluted mineral acids were tested to improve the sedimentation effect of trivalent chromium from tannery wastewater. The results showed that the most feasible alkali was a mixture of sodium hydroxide and magnesium oxide with a mass ratio of 5:1 to balance the cost performance. The high chromium removal as 99.1% was obtained with a mixed alkali dosage only 0.2 g/L at pH 9.5, with a very low Cr-sludge yield. The effects of various factors on the extraction of trivalent chromium from Cr-sludge using mineral acids were investigated further more, and the results indicated that the feasible condition for chromium recovery from wet chromium sludge was as follows: 4% sulfuric acid as the leaching acid, chromium sludge (dry weight) load 23.55 g/L, extraction time 60 min, and reaction temperature 30°C. A chromium recovery of 20.86 mg/g was obtained under the feasible condition with a recovery rate of 91.8%. The method developed in the present study provided an effective solution to balance the cost performance for tannery waste treatment.