Mechanism Research On CO₂ Carbonation For Preparation Of Precipitated Calcium Carbonate Based On Waste Carbide Slag

The serious problems caused by global warming have attracted the society’s high attention to human-caused carbon dioxide emissions.CO₂ mineralization is expected to achieve large-scale reduction of greenhouse gases and economic benefits and also obtains high value-added carbonate products.CO₂ indirect mineralization technology for the preparation of high-grade calcium carbonate replaces the traditional energy-intensive ore mining and calcination process,effectively reducing production energy consumption and environmental carbon emissions.Using calcium-based industrial solid waste as raw material,coupled leaching-mineralization process has the potential to significantly reduce the operating cost by introducing a new recycling mechanism.However,there are also problems such as low conversion rate,large water consumption and impurities affecting the quality of calcium carbonate products.In this work,from the perspective of strengthening the mechanism of calcium component leaching and mineralization transformation,we explored the kinetic mechanism of calcium component leaching and the shaping of microstructure of calcium carbonate by CO₂ mineralization.On this basis,we realized the synthesis and optimization of high-quality calcium carbonate by means of admixtures and in situ regulation technology.Furthermore,the macroscopic properties and environmental and economic benefits of light calcium carbonate products prepared by indirect mineralization of calcium-based solid waste CO₂ were further elucidated.In this work,we used waste carbide slag（WCS）as calcium-based raw material and ammonium sulfate（（NH₄）₂SO₄）as the calcium component leaching agent,and systematically analyzed the effects of raw material particle size,（NH₄）₂SO₄ solution concentration,temperature and solid-liquid ratio on the leaching conversion and phase transformation mechanism in of calcium component.The leaching system achieved a calcium leaching rate of nearly 90%in about 15 min,and the calcium was transformed from the irregular Ca（OH）₂crystalline phase to CaSO₄·2H₂O with a rod-like structure and a purity of about 93%.At the same time,surface coverage control kinetics mechanism is proposed to construct calcium component transformation in this leaching system and experimentally verified.The kinetic analysis showed that the initial rate-limiting step of the calcium leaching exhibited a surface chemical reaction of ions,accompanied by the crystalline of CaSO₄·2H₂O into a product island diffusion control mechanism.Based on this kinetic mechanism,to maximize the treatment of waste carbide slag,the calcium leaching was optimized by intensifying ion diffusion,and the leaching of calcium was activated by rehydration in the product island diffusion control phase,which could achieve a maximum leaching rate of about 90%of calcium at a high solid-liquid ratio（15%）and directed to obtain CaSO₄·2H₂O crystalline phase.The method of reverse adsorption of impurities based on froth flotation was proposed for the influence of insoluble materials such as residual carbon particles on the product quality of CaSO₄·2H₂O under（NH₄）₂SO₄ leaching system.TBP and PO were selected as residual carbon organic adsorbents for the separation and purification of the target product CaSO₄·2H₂O by reverse adsorption split-phase separation.When the O/S of TBP in the system was 1:2,the residual carbon removal rate could reach about 95%,and the purified CaSO₄·2H₂O with a whiteness of about 92%was also obtained.The residual carbon carboxyl groups form a relatively strong affinity with the organic solvent TBP at the P-O-C group position,and the adsorption behavior of TBP with carbon impurities at specific adsorption sites was calculated using density flooding theory simulations.The molecular electrostatic potential predicts that TBP showing electroneutrality centered on the P=O functional group has a tendency to adsorb the electroneutrality associated with[Fe（H₂O）₆]³⁺cations,reflecting the characteristic that TBP can synergistically remove residual carbon and Fe（III）to some extent under acidic systems.Based on the above leaching and separation of calcium from waste carbide slag,light calcium carbonate was further synthesized by CO₂ mineralization under ammonia medium system.The effects of reaction conditions such as mineralization reaction temperature,CO₂flow rate and ammonia content on the conversion of CaSO₄·2H₂O were systematically investigated.The CO₂ carbonation under relatively mild conditions undergoes a rapid growth period at the beginning and enters a stable plateau after 15 min,and finally achieves a maximum conversion of about 92%,with a corresponding product purity of about 95%of calcium carbonate.In addition,the evolution of the crystalline phase during the whole reaction was obtained by the CO₂ in situ mineralization reaction,and the calcium underwent the transformation of gypsum dihydrate phase-amorphous calcium carbonate-calcium carbonate crystalline phase,while the size of the particles was reduced from the initial micron level to nano-scale CaCO₃.The addition of the compound additive ASDP to the reaction system at a flow rate of 10 m L/min can change the ionic supersaturation to a certain extent,prolong the initial nucleation induction period of CaCO₃,inhibit the growth rate of CaCO₃crystallization,and help to regulate the distribution of CaCO₃ particles in the particle size range of 100-500 nm.Finally,the whole life cycle environmental benefits of three calcium carbonate products with different particle size specifications were evaluated by constructing a whole life cycle inventory and model for the preparation of light calcium carbonate by CO₂ indirect mineralization.The detailed impacts of raw materials,production steps,and transportation on environmental carbon emissions and economics were systematically evaluated.The higher the degree of calcium carbonate refinement,the higher the total life-cycle greenhouse gas emissions of calcium carbonate preparation.Compared with the GWP values of calcium carbonate prepared by carbonation of waste carbide slag by conventional ammonium chloride leaching,there was no significant increase in GWP values for micron-scale（1-5μm）micronized calcium carbonate prepared by the ammonium sulfate process in this study.For micronized calcium carbonate,the ammonium sulfate process also helped to reduce the total GWP of the calcium carbonate,achieving a reduction of about 3-4%.The conversion of the raw calcium to the calcium carbonate is accompanied by an effective CO₂ fixation（negative emissions）,which can fully offset the carbon impact on the raw material side and the production operation side.The potential green premium can be significantly reduced on the raw material side through the carbon trading incentive of recovering CO₂ and choosing a more flexible CO₂ gas source.