Study On Removing Impurities And Decolorizing From Hanshan Anhydrite Mineral Powder

Natural anhydrite mineral powder from Hanshan region in Anhui Province wasutilized for impurity removal and decolorizing research after being processed bytraditional technology in this paper. On the basis of identifying impurities that leadto low whiteness of anhydrite, methods for removing impurities includingcalcination, chloridizing-calcination, acid leaching, calcination-leaching andbioleaching were investigated and parameters of the processes optimized. Chemicalthermodynamics and reaction kinetics were also studied. The economic feasibilitiesof different methods were compared. The quality and whiteness of the anhydritemineral powder were improved effectively through these methods. The results inthis paper will provide some basis for developing process for anhydrite whitening.Carbonaceous substance and iron minerals were found main impurities in theanhydrite after a series of characterization and analyses. For carbonaceoussubstance, graphite and organic substance played an important role. For ironminerals, free iron oxides presenting in Fe3+influenced the whiteness mainly.Carbonates and structural iron had little effect on whiteness.The optimal conditions of direct calcination were as follows: calcinationtemperature800℃; calcination time2h. Then the whiteness could be raised from86.8%to96.2%. NH4Cl was selected as a chloride additive of the method ofchloridizing-calcination. Under the optimal conditions, the whiteness could be over97%with5%NH4Cl. Research on chemical thermodynamics showed a chloridingreaction happened between NH4Cl and irony substances.Oxalic acid was employed in leaching and the optimal conditions were asfollows: pulp concentration50g·L^-1, oxalic acid concentration0.6mol·L^-1,leaching temperature70℃,leaching time60min and stirring speed400r·min^-1, thewhiteness could rise to91%. The process follows a chemical reaction-controlledmodel at low temperature region and at high temperature region it reforms to adiffusion-controlled model. Based on the optimum conditions of calcination andleaching, a new process of calcination-leaching for removing impurities andwhitening was developed. The effect of the process was better than the simple sumof the two processes. Calcinations and leaching had synergism in the united processby which the whiteness could be improved to100.8%. The processes of chloridizing-calcination and calcinations-leaching are superior to leaching withoxalic acid at removing impurities according to the feasibility in economy.Bioleaching processes were explored in the research. Aspergillus-niger,penicillium and a kind of mould got from anhydrite serosity all had strong ability atcreating acids. But some colored substances would be created in the solution whenmoulds were fermenting and both ways of in-situ bioleaching and two-stagebioleaching didn’t get expected result.