| Phosphogypsum(PG)is a by-product of the phosphate fertilizer industry.and has over 6 billion tonnes have accumulated worldwide,which also being added at a rate of 200 million tonnes per year.The huge quantity of PG causing not only serious environmental pollution but also a waste of resources.In addition to the physical usage method of PG,the chemical usage method by preparation of calcium carbonate through carbonation reactions of CO2 and PG can make full use of the calcium element in PG,and consume carbon dioxide becomes the focus of PG use.The chemical utilization of PG is limited by the high content of impurities in PG,which affects the efficiency of conversion of carbonation and the quality of the calcium carbonate product.Therefore,it is significant to study preparing high-purity nano calcium carbonate from PG.In this paper,the effect of impurities on the reaction process and product quality of calcium carbonate from PG was firstly investigated.Impurities of phosphorus,fluorine,magnesium and iron impurities with related high content in PG were selected as major cationic and anionic impurities to investigate content on the reaction conversion rate as well as the purity,crystal shape and morphology of calcium carbonate through single-factor experimental method.The results showed that the conversion of calcium sulfate reduced from 95.2%to 79.0%contain magnesium,phosphate and iron ions,and the conversion decreased with increasing impurity content for the same impurities.The presence of low concentrations of fluoride ions(1.0%)increased the conversion of calcium sulfate to 96.3%,but further reduced the conversion to 92.3%while increasing the concentration of fluoride ions to 2.0%.The calcium carbonate products prepared in the presence of impurities were also examined by SEM and XRD to investigate the effect of the presence of impurities on the morphology and crystalline shape of the calcium carbonate.The results showed that magnesium ions,phosphate and iron ions cause obvious changes in the morphology and crystalline shape of the calcium carbonate by adsorption on the crystal surface or by insertion into the crystal lattice.When the magnesium impurity content increases from 1.0%to 5.0%,the percentage of calcite in calcium carbonate decreases and the percentage of aragonite increases.The effect of iron ions on the crystalline shape of the product is similar to that of magnesium ions,while an increase in iron impurity content from 0.5%to 2.0%,morphology of the calcium carbonate become a clusters of rod-like particles.An increase in phosphate content from 1.0%to 15.0%results in the formation of spherical aragonite.The presence of fluorine impurities has less influence on the morphology of calcium carbonate,and when the content of fluorine impurities is 1.0%and 1.5%,the product is mainly calcite.The presence of impurities can all lead to a reduction in the purity of the calcium carbonate product,with phosphorus impurities having the greatest impact on purity.Secondly,the phase transfer method used and potassium chloride used as the phase transfer agent to leach and transfer calcium ions from PG into the solution,so as to achieve the purpose of impurity removal through solid-liquid separation.The effects of phase transfer agent concentration,phase transfer quantity,phase transfer time and phase transfer temperature on the leaching rate of calcium ion were also investigated.The experimental study showed that optimization operation condition was achieved at a KCl concentration of 3 mol·L-1,a liquid-solid mass ratio of 10:1 between the phase transfer agent and PG,a reaction time of 70 min and a reaction temperature of 60℃,with a leaching rate of 24.5%.The leaching solution was subjected to a carbonation reaction to obtain calcium carbonate with a purity of 99.4%,which was about 15%comtent higher compared to calcium carbonate prepared directly from PG.The mechanism of the phase transfer process was also explained based on the salt effect and ionic atmosphere theory,which suggests that the presence of the strong electrolyte potassium chloride causes the calcium and sulphate ions to be surrounded by a chloride ionic atmosphere and a potassium ionic atmosphere,thus inhibiting the combination of calcium ions with sulphate to form calcium sulphate precipitates and thus increasing the dissolution of calcium ions.Subsequently,this paper investigates the preparation of calcium carbonate nanoparticles by carbonation of leaching solutions.The effects of reaction temperature,CO2 flux and ammonia quantity on the reaction performance,the particle size and morphology of the calcium carbonate products were investigated.The results showed that the particle size of calcium carbonate increased with increasing temperature,and nano-sized calcium carbonate was obtained at low temperatures from 10℃ to 20℃.When the temperature was over 40℃,the calcium carbonate was micron-sized.Moderate CO2 flux and excess ammonia favoured the production of small size calcium carbonate.The calcium carbonate particle size first decreased and then increased with increasing CO2 flux and decreased to a range with increasing ammonia concentration.The optimized conditions obtained from the study were:temperature of 10℃～20℃,CO2 flux of 50 mL·min-1～100 mL·min-1 and ammonia concentration greater than 0.48 mol·L-1.At complete reaction,high purity calcium carbonate with particle size below 100 nm and purity of 99.4%was prepared.Finally,the reaction mechanism of the gas-liquid two-phase carbonation process was analyzed in this paper,and a simplified kinetic model of the gas-liquid two-phase reaction to produce nano calcium carbonate was obtained.Through experimental studies and data fitting,the crystallization kinetic equations were obtained for 283.15 K-313.15 K and CO2 fluxes of 50 mL·min-1.The nucleation factor and growth factor were 1.4 and 0.2,respectively,and the activation energies for nucleation and growth were 61.5 kJ·mol-1 and 66.7 kJ·mol-1,respectively.The fitted equations were also checked for errors in the crystallisation rate and met the crystallisation kinetic error requirements. |
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