Comparison of chemical leaching and bioleaching of nickel from nickel hydroxide sludge

Nickel hydroxide sludge samples used in this research were obtained from a sand drying bed of a chemical wastewater treatment plant for a nickel elect roplating process containing 43% nickel. Its pH was around 8. This research consists of two parts both in a shake flask and a column. The first part o f this research is to study the leaching efficiency of nickel hydroxide slu dge by sulfuric acid. The experiment in the shake flask was carried out usi ng nickel hydroxide sludge which containing nickel ion at 10 g/L by varying the concentration of sulfuric acid at 0.01, 0.05, 0.1, 0.15, 0.5, 1 and 5N . The result indicated that the concentration of sulfuric acid at 5, 1 and 0.5N can achieve 100% leaching efficiency within 24, 48 and 84 hours respec tively. The concentration of sulfuric acid at 0.5 N can consume 1.06g.H2SO4 per g sludge. The experiment in the column was carried out using 500 g of nickel hydroxide sludge in a column 5 cm in diameter with 50 cm height by u sing 1N H2SO4 at a now rate of 15 ml/cm(2)-hr. It can leach nickel at 13.56 % within 3.5 hours. The other part of this research is bacterial leaching o f nickel from nickel hydroxide sludge by Thiobacillus ferrooxidans and Thio bacillus thiooxidans. The experiment in the shake flask was carried out usi ng nickel hydroxide sludge which containing 10 g/L of nickel ion in a diffe rent culture with bacteria. The adapted strain of both types of Thiobacillu s spp. has 47% efficiency in leaching nickel which corresponds to the sulfu ric acid concentration of 0.15N and is higher than for non-adapted bacteria and the sterile control, respectively. The experiment in a column was carr ied out using nickel hydroxide sludge in the same size of column as in the first experiment using an adapted strain of T. ferrooxidans and T. thiooxid ans in 9K medium and thiomedium, respectively. The optimum condition for T. ferrooxidans was obtained by varying flow rate at 5, 10, 15 and 20 ml/cm(2 )/hr; the inoculum amount of T. ferrooxidans at 10% and 20%(v/v); ferrous i ons in 9K medium at 4, 10, 20, 30, 40 and 50 g/L; and quantity of sludge at 250 and 500 g per column. The results indicated that the optimum condition s were at a flow rate of 15 ml/cm(2)-hr, with 20% inoculum and the concentr ation of ferrous iron at 30 g/L, under which T. ferrooxidans can leach 250 g of nickel from nickel hydroxide sludge per column at 32% within 90 days. The controlled pH of the aforementioned step is between 2.5-3.0. The effici ency of nickel leaching from the sludge mixed with sulfur packed in columns with T. thiooxidans was higher, with 38% leaching within 102 days at a flo w rate of 15 ml/cm(2)-hr, with 20%(v/v) inoculum, 250 g sludge per column a nd a controlled pH of 1.5-2.0 at the column inlet.