(b) Malonic acid concentration and contact time: The interaction between high malonic acid concentration and prolonged contact time yielded a positive influence on leaching efficiency (Figure ). The leaching efficiency increased at low malonic acid concentration and short contact time and the increase continued until a maximum of about 50% was attained at a malonic acid concentration of 0.5 M and a contact time of 120 min. At high malonic acid concentration and short contact time, a negative influence on leaching efficiency was observed.