Study on the removals of calcium and magnesium from seawater using CO2

Abstract

Abstract: Researches on the precipitation of calcium and magnesium from seawater have been carried out using CO₂. The precipitation in the seawater，Mg-free seawater and Ca-free seawater system have been studied at pH value between 8.0 and 9.0. It was found that the removal rate of calcium can reach 90% when the pH value is between 8.0 and 9.0. Meanwhile，the removal rate of magnesium can reach 10% to 60%. In the Mg-free seawater system，the removal rate of calcium can reach 99% when the pH value is between 8.0 and 9.0. In the Ca-free seawater system，there is no precipitation between pH 8.0 and 8.3. The removal rate of magnesium can reach 60% when the pH value is between 8.5 and 9.0. The precipitated solids were characterized using X ray diffraction（XRD）analysis. The results show that they consist of CaCO₃·H₂O at pH 8.0，of CaCO₃·H₂O and MgCO₃·3H₂O between pH 8.3 and 9.0 in the seawater system. In the Mg-free seawater system，the precipitated solids consist of CaCO₃. In the Ca-free seawater system，the precipitated solids consist of MgCO₃·3H₂O.

Key words: carbon dioxide, seawater, calcium, magnesium, pH value

摘要： 对CO₂法脱除海水中的钙离子和镁离子进行了研究，分别考察了pH值因素对CO₂脱除海水、无钙海水和无镁海水中钙和镁的影响。结果表明，pH值在8.0～9.0时，钙离子的沉淀率均可以达到90%以上，同时镁离子的沉淀率从10%增长到60%；在无镁海水体系中，pH值在8.0～9.0时，钙离子的沉淀率均可以达到99%以上；在无钙海水体系中，pH值为8.0～8.3时，几乎没有沉淀产生；当pH值为8.5～9.0时，镁离子的沉淀率可以达到60%。将得到的固体进行XRD检测可知，在海水体系中，当pH值为8.0时，固相为CaCO₃·H₂O；当pH值为8.3～9.0时，固相为CaCO₃·H₂O 和MgCO₃·3H₂O的混合物；在无镁海水体系中，固相为无水CaCO₃；在无钙海水体系中，得到的固相为MgCO₃·3H₂O。

关键词: 二氧化碳, 海水, 钙, 镁, pH值

ZHANG Jijun1，2，YUAN Junsheng1，2，LI Xia1. Study on the removals of calcium and magnesium from seawater using CO2[J]. Chemical Industry and Engineering Progree, 2012, 31(03): 681-686.

张继军1，2，袁俊生1，2，李霞1. 利用CO₂脱除海水中的钙离子和镁离子[J]. 化工进展, 2012, 31(03): 681-686.

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