Studies On Controllable Preparation Of Coal Slag-based Lithium Silicate And Adsorption Of CO₂

CO₂capture technology of high temperature could be used to capture CO₂in flue gas directly.It was of great significance to develop high temperature CO₂capture technology for reducing carbon emissions and mitigating greenhouse effect.As a high temperature CO₂adsorbent,Li₄Si O₄showed a good application prospect because of its high adsorption capacity and fast adsorption rate.However,the cost of Li₄Si O₄was too high and it was prone to sintering.Therefore,a new Li₄Si O₄adsorbent was prepared from solid waste coal-fired slag to solve the above problems.Coal slag was one of the largest solid waste emissions each year,but it contained a large number of Si elements,which could be effectively used as a silicon source for the preparation of Li₄Si O₄adsorbent.In this paper,the effects of preparation process,adsorption conditions and doping modification on the adsorption effect were investigated in detail.Li₄Si O₄adsorbent was prepared by using coal slag.The main research contents are as follows:Firstly,the silicon precursor was separated from coal slag by alkali extraction method,and the silicon precursor was determined to be pure Si O₂by a series of spectral characterization.The precursor of silicon was mixed with Li Ac,Li₂CO₃and Li NO₃by impregnation method,respectively.The thermodynamic calculation of the preparation reaction was carried out by HSC 6.0 software,and the reaction mechanism of Li₄Si O₄preparation from different lithium sources was analyzed by TG-DSC.The XRD pattern and CO₂adsorption amount were used as the evaluation criteria to determine the lithium source using Li Ac as the adsorbent.The optimum preparation conditions of Li₄Si O₄adsorbent（LAS）were determined by using CO₂adsorption capacity as evaluation standard:calcination temperature 750°C,heating rate 5°C/min,calcination time 120 min.Secondly,the optimal adsorption conditions of LAS at various CO₂concentrations were determined.The results showed that when the CO₂concentrations were 99.999%and 50%,and the temperature was 650°C for 1 h,the adsorption capacities of LAS were 356 mg/g and 353 mg/g,respectively,almost reaching the theoretical adsorption capacity（367 mg/g）.At low CO₂partial pressure（8%）,the adsorption reaction rate was relatively slow,but at 610°C for 2 h,the adsorption capacity could also reach 213 mg/g.At the same time,the bi-exponential model was used to fit the adsorption process under different CO₂partial pressures,and it was determined that the adsorption kinetics was mainly controlled by CO₂diffusion.The optimal adsorption conditions of LAS at various CO₂concentrations were tested.The results showed that LAS had high adsorption effect at high CO₂concentration,but its stability was insufficient.At low CO₂concentration,although the adsorption amount cannot reach the theoretical adsorption amount,it has excellent cycle stability.Finally,the cycle stability of coal slag composite Li₄SiO₄adsorbent under three CO₂concentrations was investigated.It was determined that 10 wt%addition was used at 99.999%and 50%CO₂concentrations,and no coal slag was doped at 8%CO₂concentration.The reason for the improvement of cyclic adsorption performance by doping slag under high CO₂concentration was proposed and explained,which was Si element in coal slag reacted with Li₂CO₃to form Li₂Si O₃and Li₄Si O₄,which promoted the transformation of Li⁺and CO₂.