According to the homologous linear rule applied widely in the chemistry area, the linear equation between the formation reaction Gibbs free energy of calcium and magnesium composite compounds was fitted out. The formation reaction Gibbs free energies of Mg3(BO3)2and Mg2B2O5 are obtained from the formation reaction of Ca3(BO3)2 and Ca2B2O5 at different temperatures. Then, the thermodynamic calculation models for the formation reaction of Mg3(BO3)2 and Mg2B2O5 were deduced based on the approximate linear rule between the reaction Gibbs free energy and temperature, and it was verified by the laboratory tests. The results of thermodynamic calculation and XRD phase identification indicate that ascharite in the iron concentrate will be transformed into suanite firstly, and then into kotoite by reacting with the serpentine decomposition product of serpentine during the roasting process.%依据广泛应用的同族化合物线性递变规律，通过拟合钙、镁化合物生成反应标准吉布斯自由能之间的线性关系，由不同温度条件下 Ca3(BO3)2和 Ca2B2O5生成反应的标准吉布斯自由能求得相应温度条件下 Mg3(BO3)2和Mg2B2O5生成反应的标准吉布斯自由能，进而基于化学反应的标准吉布斯自由能与温度之间的近似线性关系，推导Mg3(BO3)2和Mg2B2O5生成反应标准吉布斯自由能，并通过试验进行了验证与探讨。热力学计算及XRD物相分析结果表明：在还原焙烧过程中，含硼铁精矿中的硼镁石(Mg2[B2O4(OH)](OH))分解为遂安石(Mg2B2O5)，遂安石进一步与蛇纹石的分解产物镁橄榄石或顽火辉石反应最终转变成小藤石(Mg3(BO3)2)。
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