Study On The Leaching Silver Feom The Slag Of Zinc-Smelting Containing Pyrite By Modified Thiourea Method

Silver is used in many areas. The cyanidation method and thiocarbamide method have been used to leachng silver. However, the cyanidation method is environmental pollution, and the thiocarbamide method is not suitable for the raw materials containing high content pyrite. In this work, a modified thiourea method was used to leach silver form the zinc-smelting slag containing high content pyrite, and its leaching kinetics was also investigated.The effects of the processes parameters, including the stirring speed, the leaching temperture, the leaching time, thiocarbamide concentration, hydrogen peroxide concentration and initial nitric acid concentration were investigated by single factor experiment design and response surface methodology. The leaching ratio of silver first increases and then keeps constant, when the stirring speed and the leaching time increase. The leaching ratio of silver increases at first and then decreases, when the other parameters increase. The interaction between hydrogen peroxide concentration and thiocarbamide concentration, hydrogen peroxide concentration and temperature, thiocarbamide concentration and temperature, initial nitric acid concentration and temperature are significant. The optimal conditions of leaching silver from the slag of zinc-smelting containing pyrite are as follows, the ratio of liquid to solid6:1, stirring speed200r·min-1, hydrogen peroxide concentration0.68mol·L-1, nitric acid concentration4.91mol·L-1, thiocarbamide concentration4.35mol·L-1, temperature45℃, leaching time1h, leaching ratio of silver92%.The leaching reaction of silver follows the shrinking core model and the leaching rate is controlled by the internal diffusion. The apparent activation energy is64.34kJ·mol-1. The apparent order of the reaction are2.75,1.08and0.74for nitric acid, thiocarbamide and hydrogen peroxide solution. The kinetic equationof the leaching process is as follows1-2/3x-(1-x)2/3=39118.64-r20C12.75C21.08C30.74exp(-63340/RT)t Where C1, C2and C3is initial nitric acid concentration (mol·L-1), thiocarbamide concentration(mol·L-1) and hydrogen peroxide concentration(mol·L-1), respectively.