Na^(+),K^(+),Mg^(2+),Ca^(2+)∥Cl^(-),SO_(4)^(2-)H_(2)O六元体系中杂卤石形成条件的再认识

Revisiting the crystallization field of polyhalite in the Na^(+),K^(+),Mg^(2+),Ca^(2+)∥Cl^(-),SO_(4)^(2-)H_(2)O hexary system

研究海水体系(即Na^(+),K^(+),Mg^(2+),Ca^(2+)∥Cl^(-),SO_(4)^(2-)H_(2)O六元体系)中杂卤石的形成条件不仅有助于理解海相蒸发盐成因,对开发杂卤石钾资源利用技术也具有重要的指导作用。但杂卤石以及其他含有硫酸钙矿物(如二水石膏、半水石膏、无水石膏、钾石膏、多钙钾石膏、钙芒硝和水钙芒硝)的相平衡,无论是在实验研究方面还是在热力学模拟方面都仍然存在很多争议。由于缺乏杂卤石在复杂水溶液体系中可靠的相平衡数据,使人们对认识杂卤石成因和利用杂卤石钾资源带来巨大障碍。本工作对海水体系中杂卤石的形成条件进行了热力学模拟和实验研究。超过一年的长时间固液平衡实验表明25℃下Na^(+),K^(+),Mg^(2+),Ca^(2+)∥Cl^(-),SO_(4)^(2-)H_(2)O六元体系中杂卤石的形成区域极为可观,且较前人实验结果均大数倍;同时证实了热力学模型预测结果的可靠性。这些结果为讨论盐矿床中杂卤石的形成条件提供了物理化学依据。25℃下可靠的热力学信息表明:杂卤石与其他盐类矿物的共生情况丰富多样,并且与前人看法不同,杂卤石的形成并不需要极高的钾、镁浓度。这给以杂卤石为指示寻找可溶性固体钾盐带来了挑战,但却指示了以杂卤石为线索更容易找到钾、镁盐未饱和的富钾卤水。

Studying the crystallization field of polyhalite in the Na^(+),K^(+),Mg^(2+),Ca^(2+)∥Cl^(-),SO_(4)^(2-)H_(2)O hexary seawater system is essential for understanding the origin of polyhalite evaporites.It is also helpful for developing potassium extraction techniques for polyhalite resources utilization.However,the phase equilibrium relations between polyhalite and the more generally CaSO_(4)-based salts(e.g.,gypsum,anhydrite,hemihydrates,syngenite,gorgeyite,glauberite and hydroglauberite)are far from clear in terms of both experimental evidence and thermodynamic models.The lack of reliable equilibrium data between polyhalite and multicomponent brines has created huge obstacle to constraining the polyhalite origin conditions and utilizing polyhalite resources.In this study,we re-examined the crystallization field of polyhalite in theNa^(+),K^(+),Mg^(2+),Ca^(2+)∥Cl^(-),SO_(4)^(2-)H_(2)O hexary system at 25℃,both experimentally and by thermodynamic simulation.Our new experimental results using long equilibration time indicate the thermodynamically stable crystallization field of polyhalite is very large even at 25℃and several times larger than the widely adopted experimental results.Moreover,our experimental results well support the predictions by the temperature dependent thermodynamic models.These more reliable phase equilibrium data provided solid physiochemical constrains to the origin of polyhalite evaporites,indicating a diverse stable mineral assemblage of polyhalite in the evaporite basin.Moreover,the potassium/magnesium concentration needed for polyhalite formation is not as high as previously suggested based on phase diagram.The improved knowledge implies that polyhalite is more likely to be an indicator for potassium-rich brines unsaturated with sylvine,rather than for large-scale soluble potassium salts(e.g.,sylvine,carnallite,kainite and picromerite).