The Controllable Growth Of MgCO₃·3H₂O In The Process Of CO₂ Mineralization

Recently,the global CO₂ emissions continue to increase,however,the excessive emissions of CO₂ will destroy the earth's climate and ecosystem balance,bring a great threat to human environment,so the comprehensive utilization is the essential method to control CO₂ pollution,the key of this method to utilize CO₂ is to find a raw material with a low cost and abundant reserves.CO₂ mineralization is a comprehensive utilization way for reducing CO₂ emissions in recent years,it mainly uses natural ores or calcium/magneniusm salt to store CO₂ as insoluble solid carbonates?such as calcium carbonate or magnesium carbonate?,and some high-value chemical products are produced simutaneously.Using MgCl₂ as raw material to mineralize CO₂ can not only reduce CO₂ but also avoid the waste of magnesium salt,which is of great significance for the development and utilization of magnesium resources and the reduction of CO₂ in China.In this paper,the MgCl₂·6H₂O and NH₃·H₂O solution were transformed into Mg?OH?₂,then the formed highly reactive Mg?OH?₂ was reacted with CO₂,transforming Mg?OH?₂ into MgCO₃·3H₂O crystals.The precipitation of MgCO₃·3H₂O in Mg?OH?₂–CO₂–H₂O system was investigated,the main contents are as follows:?1?MgCO₃·3H₂O was prepared by precipitation method with Mg?OH?₂as intermediate.The effects of reaction temperature?20⁴0oC?,the initial concentration of MgCl₂(0.1¹ mol·L^-1)on the composition and morphology of MgCO₃·3H₂O were investigated.The results showed that the concentration of MgCl₂ was 0.1 mol·L^-1,the Mg?OH?₂ was not reacted with CO₂ at 20°C,the obtained crystals were amorphous Mg?OH?₂,with a diameter of 150²00?m.When the temperature was 30⁴0°C,the obtained crystals were bundle-like MgCO₃·3H₂O,with a diameter of 30⁶0?m.Meanwhile,the initial concentration of MgCl₂ also has an obvious influence on the composition and morphology of MgCO₃·3H₂O crystals.The optimum condition of the MgCO₃·3H₂O crystal was 40 ^oC,and the concentration of the MgCl₂ was 0.5 mol·L^-1,the obtained MgCO₃·3H₂O crystals with a length of 50⁵5?m and a diameter of 15?m.?2?In the Mg?OH?₂–CO₂–H₂O system,MgCO₃·3H₂O was prepared by precipitation method,the concentration of Mg²⁺and pH value were measured in the liquid phase,the composition of solid phase were characterized by XRD,FTIR and TG in the mineralization process.With the increase of reaction time,the phase transformation tend to change from Mg?OH?₂ to a mixture of Mg?OH?₂ and MgCO₃·3H₂O to pure MgCO₃·3H₂O crystals.When the adding order of MgCl₂ and NH₃·H₂O was changed,the Mg?OH?₂ crystals with different morphology could be obtained,which is due to the preferred orientation of different plans of Mg?OH?₂,the growth mechanism of MgCO₃·3H₂O was dissolution–recrystalliation.?3?In the Mg?OH?₂–CO₂–H₂O system,MgCO₃·3H₂O was prepared by precipitation method.The effects of CO₂ concentration?16.7¹00%?and impurity?NaCl?MgSO₄?on the quality of the obtained crystals and the carbon sequestration rate of MgCl₂ were investigated.The results showed that the Mg?OH?₂ was transformed into the MgCO₃·3H₂O phase with the increase of CO₂ concentration,the carbon sequestration rate of MgCl₂ was increased with the increase of CO₂ concentration.When the NaCl was added to the system,all of the products were rod–like MgCO₃·3H₂O crystals.The diameter of MgCO₃·3H₂O and the carbon sequestration rate were decreased with the increase of NaCl concentration.The MgSO₄ was added to the system,all of the products were rod–like MgCO₃·3H₂O crystals.The diameter of MgCO₃·3H₂O was increased with the increase of MgSO₄concentration,and the carbon sequestration rate of MgCl₂ was decreased.Adding NaCl and MgSO₄ impurities would both reduce the purity of the products,but the purity of the products was still above 98%.