Yerel Bir Çinko Karbonat Cevherinin Sodyum Hidroksit Liçiyle Değerlendirilmesi

Abstract

In the first step of this thesis study, following characterization, the dissolution behaviour of a smithsonite (ZnCO3) ore sample, including 29.16% ZnO, 26.51% Fe2O3 and 7.37% CaO, and having goethite and calcite as main gangue minerals, from Yahyalı/Kayseri (Turkey) region was investigated comprehensively in sodium hydroxide (NaOH) solutions at different concentrations using various analyses (chemical, XRD, TG/DTA, FT-IR and SEM-EDS) methods. It was observed that the leaching ratio of zinc increased continuously with the increase of NaOH concentration of the leaching solution from 1 to 4 M according to the general dissolution reaction of ZnCO3 + 4OH− → Zn(OH)42− + CO32−, and the highest zinc leaching ratios of 70.9% and 57.1% were reached after leaching for 1 hour in 4 M NaOH solutions at 25°C and 90°C, respectively. However, when the temperature of 3 M NaOH solution, which was selected as the optimum concentration, was increased from 25°C to 90°C, the leaching ratio of zinc decreased from 70.1% to 40.5% due to the lowered stability of the dissolved zincate species and their conversion into solid zinc oxide (ZnO) according to the Zn(OH)42− → ZnO + 2OH− + H2O reaction. On the other hand, the leaching ratio of zinc increased from 57.8% to 70.1% by leaching in 3 M NaOH solution at 25°C when the stirring time was extended from 0.25 to 1 hour, while extension of the stirring time to 4 hours lowered the leaching ratio of zinc to 60.6% because of the formation of solid hydrated calcium zincate (CaZn2(OH)6·2H2O). The chemical, XRD, TG/DTA, FT-IR and SEM-EDS analyses applied to the ore sample and the undissolved residues obtained after leaching showed that the smithsonite phase in the ore sample is selectively dissolved against the impurities present in the ore sample. Within the scope of this thesis study, besides the effects of NaOH concentration and stirring time, the effect of solid/liquid ratio (0.075, 0.10 and 0.15 g/mL) at different NaOH concentrations (2 and 3 M) were also investigated. In addition, the leaching behaviour of lead component in the sample (2.07% PbO) was also determined and the leaching ratios obtained for lead were also presented in the thesis.

In the last step of the thesis study, 'direct conversion' experiments were performed for the removal/recovery of the dissolved carbonate (CO32−) in the pregnant solutions, having both dissolved zinc and carbonate, obtained after leaching of the ore sample in sodium hydroxide solutions by using reagent grade (r-) and natural (n-) materials containing the alkaline earth elements (Mg, Ca, Sr and Ba). In the first place, the conversion behaviours of reagent grade strontium sulfate (r-SrSO4) and celestine concentrate (n-SrSO4) in pregnant solutions containing dissolved carbonate were investigated in accordance with the direct conversion reaction of SrSO4 + CO32− → SrCO3 + SO42−, and the XRD pattern of the converted solid obtained by addition of solid r-SrSO4 at r-SrSO4:CO32− mole ratio of 1:2 to the pregnant solution obtained after leaching of the ore sample in 3 M NaOH solution indicated a near-total conversion of r-SrSO4 by the interaction of dissolved CO32− to SrCO3 following 2 hours of stirring. Under the same experimental conditions, solid n-SrSO4 did not completely convert to SrCO3 due to its coarser particle size. The XRD patterns of the converted solids obtained by addition of gypsum or reagent grade calcium hydroxide at n-CaSO4·2H2O:CO32− or r-Ca(OH)2:CO32− mole ratios of 1:2 to pregnant solutions obtained by using initial NaOH solution concentrations of 3 M and 4 M indicated the formation of dominant CaZn2(OH)6·2H2O phase, in place of CaCO3, following 1 hour of stirring according to Ca(OH)2 + 2Zn(OH)42− + 2H2O → CaZn2(OH)6·2H2O + 4OH− reaction, while CaCO3 and Ca(OH)2 phases were formed predominantly by using initial NaOH solution concentration of 8 M. The purest CaZn2(OH)6·2H2O formation was observed at the initial NaOH solution concentration of 4 M, and the dissolved zinc could be recovered from the solution phase as hydrated calcium zincate at recoveries over 85%. Finally, for reagent grade magnesium hydroxide (r-Mg(OH)2) and for reagent grade barium sulfate (r-BaSO4) used cases, as expected, no interaction with the carbonate ion in the pregnant solution was observed, while reagent grade barium hydroxide octahydrate (r-Ba(OH)2∙8H2O) converted to BaCO3. The results obtained after direct conversion experiments showed that the pregnant solutions obtained after leaching of smithsonite ores in sodium hydroxide solutions could be evaluated by new and different routes that are not found in the related literature.