Recovery of Zinc from ash of galvanizing plant by hydrometallurgical route

Abstract

Zinc ash is a discarded material of galvanizing plant and consists mostly of oxidized zinc that forms on the surface of molten zinc metal in galvanizing pot. The ash is removed from the top of galvanizing baths by skimming or straining and discarded. Both hydrometallurgical and pyrometallurgical processes can be used for recovery of zinc from ash. But, hydrometallurgical methods are comparatively clean and can be adapted in small and medium scale industries. In this work, hydrometallurgical path is followed to recover zinc from discarded ash of galvanizing plant. The composition of the supplied ash was determined by XRF analysis and it was found that the ash consists of around 62% zinc along with other impurities like chlorine, iron, lead, aluminium, copper, etc. Chlorine cannot be tolerated in the subsequent electrowinning of zinc metal, as even a very small amount in the electrolyte is extremely damaging to the electrode and operator of the electrolysis. Two methods were tried to remove chlorine: calcinations and washing with distilled water. Calcination can remove chlorine, but because of high iron content in this sample, a stable complex of iron with zinc and oxygen was formed which is not soluble in diluted sulfuric acid. Washing with distilled water was tried and it was found to be satisfactory. The parameters of liquid/solid ratio, temperature and time were optimized to minimize energy required for washing. Washing with distilled water at liquid/solid ratio of 10:1 (ml/gm) for 30 minutes at 80oC removed 80% of chlorine present in the sample. It was found that zinc in minor level (around 2%) is also lost in distilled water. Zinc ash was then leached in dilute sulfuric acid. Leaching parameters of particle size, concentration of leaching reagent, liquid/solid ratio, time and temperature were optimized to obtain maximum amount of zinc in leach liquor. Optimum leaching parameters were found as: mesh size of -200+270 mesh, concentration of H2SO4 of 3.2 M (~17% H2SO4), liquid/solid ratio of 5, leaching time of 140 minutes and leaching temperature of 70oC. With these optimum conditions, the concentration of zinc in leach liquor was found as around 117 g/L (which is about 94% of the zinc present in the sample). The leach liquor was purified with respect to soluble impurities. Iron is released into the solution mostly as Fe2+ during leaching. For precipitation of iron, at first it was converted into Fe3+ by oxidation with sparging air and addition of oxidising agent MnO2. After oxidation, Fe3+ is precipitated by controlling the PH between 4.5 to 5.2 using Ca(OH)2 to adjust PH. The leach liquor is filtered to remove precipitates. After removal of iron and aluminum, other impurities are removed by simple cementation technique. The cementation technique was carried out in three different stages. First stage was carried out by the addition of zinc dust at ambient temperature. This stage removes copper and cadmium as cement. The second stage was carried out at high temperatures of 85oC in the presence of potassium antimony tartarate, which removes practically all of the impurities. In the third and final step, activated charcoal was added to the purified solution to adsorb other impurities and unwanted organics (if present). The third step is the polishing step. After each step the leach liquor is filtered to remove precipitated impurities. The electrowinning technique was applied for producing a pure zinc metal from the purified solution. Electrolysis was performed at 30oC with current densities of 30, 45 and 60 mA/cm2. The effects of zinc and free sulphuric acid concentrations and current density on the current efficiency were studied. The minimum zinc concentration at which electrowinning should be stopped was found to be in the range of 40 g/l with a sulphuric acid content of >150 g/l.