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 electroplating 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 of this research is to study the leaching efficiency of nickel hydroxide sludge by sulfuric acid. The experiment in the shake flask was carried out using 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 respectively. 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 using 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 of nickel from nickel hydroxide sludge by Thiobacillus ferrooxidans and Thiobacillus thiooxidans. The experiment in the shake flask was carried out using nickel hydroxide sludge which containing 10 g/L of nickel ion in a different culture with bacteria. The adapted strain of both types of Thiobacillus spp. has 47% efficiency in leaching nickel which corresponds to the sulfuric acid concentration of 0.15N and is higher than for non-adapted bacteria and the sterile control, respectively. The experiment in a column was carried 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. thiooxidans 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 ions 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 conditions were at a flow rate of 15 ml/cm(2)-hr, with 20% inoculum and the concentration 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 efficiency 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 flow rate of 15 ml/cm(2)-hr, with 20%(v/v) inoculum, 250 g sludge per column and a controlled pH of 1.5-2.0 at the column inlet.