Hydrometallurgical treatment of zinc silicate ore from Thailand

In recent years much emphasis has been placed on the leaching of zinc silicate ores by sulphuric acid, because these are the next best source for zinc after sulphide ores. An attempt has been made to assess the suitability of sulphuric acid as leachant for the silicate ore found in Paedang (Thailand) and also to study the effect of a new flocculating agent on the coagulation of silicic acid. The influence of parameters, such as H2SO4 concentration, time of leaching, temperature and solid:liquid weight ratio, have been studied. It was found that a maximum zinc extraction (95% Zn) was obtained at a 4.5 M concentration of sulphuric acid in 3 h leaching at 70°C, keeping the solid:liquid weight ratio at 1:5 Iron and magnesium dissolution was 30.0% and 65.0%, respectively.

During leaching some silica was also dissolved (maximum 25%), which forms silicic acid. The silicic acid polymerises to larger size colloidal particles, forming a gel, and gives problems during filtration. To overcome the filtration difficulty, a new flocculating agent, Magnafloc 156, was tried and it was found that 0.5 g was required for coagulation of slurry containing 1 kg of ore. This quantity was very small compared to the other flocculating agents used. Leaching of low grade ore (5.7% Zn) gave a lower zinc recovery (80%) compared to the high grade ore. This may be due to the presence of very large quantity of silica in the ore (77.6% SiO2).