Minerals Engineering, Vol.135, 9-12, 2019
Conversion behaviours of Sr- and Ca-containing solids in dissolved carbonate containing alkaline pregnant zinc leaching solutions
The alkaline dissolution of smithsonite (ZnCO3) zinc ores in sodium hydroxide solutions at 298 K results in dissolved carbonate containing pregnant zinc leaching solutions. This study investigated the conversion behaviours of celestine (natural SrSO4), reagent grade strontium sulfate, gypsum (natural CaSO4 center dot 2H(2)O) and reagent grade calcium hydroxide with these pregnant solutions using XRD, DTA and chemical analyses to understand the utilization of dissolved carbonate contents of the pregnant solutions with these different solids. Firstly, the conversion behaviours of celestine and reagent grade SrSO4 to SrCO3 were investigated in the pregnant solution and it was found that a near-total conversion of reagent grade SrSO4 to SrCO3 solids was achievable with a SrSO4:CO32- mole ratio of 1:2 and a conversion time of 2 h. The percentage of zinc lost from the pregnant solution during conversion to SrCO3 solids was 0.78%. When celestine was used under the same experimental conditions, the conversion to SrCO3 was partial, probably due to the coarser particle size of celestine in comparison to reagent grade SrSO4. Secondly, the conversion behaviours of gypsum and reagent grade Ca(OH)(2) were tested under the similar conditions and completely different behaviour was observed with respect to Sr -containing solids, i.e. the conversion solids mainly contained hydrated calcium zincate (CaZn2(OH)(6)center dot 2H(2)O), verified by both XRD and DTA analyses, in place of expected CaCO3. In conclusion, the dissolved carbonate content of the pregnant solutions could be utilized as SrCO3 in an efficient way with the use of Sr-containing solids, and on the other hand, the dissolved zinc in the pregnant solutions may also be separated as CaZn2(OH)(6)center dot 2H(2)O with the use of Ca -containing solids.