Research On The Technology And Reaction Process Of Preparing Calcium Carbonate From Phosphogypsum

Phosphogypsum is a kind of solid waste produced in the wet process of phosphoric acid production with calcium sulfate dihydrate as the main component.In addition to calcium sulfate dihydrate,it also contains a certain amount of quartz,a small amount of soluble phosphorus,soluble fluorine,heavy metals and other impurities.In some areas,phosphogypsum is radioactive.Because of its complex chemical components,simple stacking treatment may cause serious damage to the surrounding ecological environment.On the other hand,the main component of phosphogypsum is calcium sulfate,which is rich in calcium ions and is a potential resource.Using phosphogypsum as raw material to prepare calcium carbonate can not only realize the resource utilization of phosphogypsum,but also save precious natural limestone and marble.According to the academic thought of transformation,using phosphogypsum as raw material to prepare calcium carbonate with high purity was studied.The effects of technological conditions on leaching rate of calcium ion in phosphogypsum and carbonation ration of calcium ion were systematically discussed.The influence of technological process on the crystal structure and morphology of carbonation product,and the reaction mechanism was proposed.The recycling of auxiliary in the preparation of calcium carbonate was explored.The technology of preparing calcium carbonate from phosphogypsum was comprehensively evaluated in terms of environmental benefit and product value.The results show that:under the action of sodium hydroxide,calcium sulfate in PG could all be converted into calcium hydroxide.The main phases of the product were calcium hydroxide and quartz.The optimum technological conditions were as follows:PG 20 g,sodium hydroxide concentration 2.0 mol/L,liquid-solid ratio 8:1 m L/g,reaction temperature room temperature and reaction time 10 min.Under the action of hydrochloric acid,calcium hydroxide in the alkali leaching product of PG could be completely converted into calcium chloride.The main component of solid product was silica.The optimum technological conditions were hydrochloric acid concentration 3 mol/L,reaction temperature 60°C,liquid-solid ratio 10:1 m L/g and reaction time 60 min.Under the optimized conditions,the leaching rate of calcium ion was 99.6%,and the phase of solid product was quartz.Vaterite was successfully prepared from calcium chloride solution.The optimum technological conditions were 20 m L of ammonia water,100 m L/min of carbon dioxide flow rate,30°C of reaction temperature and 60 min of reaction time.Calcium carbonate with vaterite or vaterite and calcite or vaterita,calcite and aragonite was successfully realized by adjusting the technological conditions during the preparation of calcium carbonate.The basic physical properties of the products met the standard requirements of precipitated calcium carbonate for general industry（HG/T 2226-2010）.Calcium sulfate in phosphogypsum could be dissolved by ammonium acetate,sodium acetate,sodium chloride or ammonium chloride.Under optimized conditions,the leaching rate of calcium ion in phosphogypsum was 98.1%,85.1%,49.42%and 58.64%,respectively.Ammonium chloride was the optimal additive for dissolving calcium sulfate in phosphogypsum considering the economic cost of the additives and the complexity of the whole calcium carbonate preparation system.The optimal conditions for the preparation of calcium carbonate from calcium sulfate leaching solution in phosphogypsum were as follows:ammonia dosage 10 m L,CO₂flow rate 50 m L/min,reaction temperature 30°C and reaction time 60 min.The phase of calcium carbonate obtained under optimized process conditions was rhomboid calcite.The content of calcium carbonate in the product was 99.8%,and the whiteness of the product was 97.6%.The basic properties of the product met the requirements of related indexes in the standard HG/T 2226-2010.The product composed of rhomboid calcite or spherical aragonite or mixed with calcite and aragonite could be realized by adjusting the amount of ammonia,the flow rate of CO₂,reaction temperature,reaction time and the number of recycling of auxiliary.In the process of preparing calcium carbonate from phosphogypsum,ammonium chloride could be recycled for 9 times,and the reaction efficiency was more than 50%.