诱导结晶同步除硫铁晶种制备及其结晶行为调控

Rational utilization of the high-sulfur bauxite is one of the most important measures to resolve the tight supply of bauxite resource. But there still exist problems such as excess impurities in the alumina product caused by the precipitation of sulfur and iron, which dues to lack of regulation for the crystallization behavior of sulfur and iron in the sodium aluminate solution. Previous research of our team showed that simultaneous removal of sulfur and iron in sodium aluminate solution can be effectively achieved before the process of seed precipitation by induced crystallization. But it is difficult to quickly and efficiently obtain the seed and this seed is unstable, which becomes one of the major bottlenecks for the simultaneous removal of sulfur and iron by induced crystallization. Regarding preparation of sulfur and iron seed from sodium hydroxide as a subject, combining chemical composition quantitative analysis with the thermodynamics is adopted to confirm the existed states of sulfur and iron in Na-S-Fe-H2O system, and quantum chemistry calculation is performed to analyze the possible reaction pathways of seed preparation. The seed formation mechanism is revealed combining with the crystallization kinetics and TEM, SEM, XRD analysis, etc. A method of regulating crystal growth is used to obtain good performance sulfur and iron seed and realize simultaneous and efficient removal of the sulfur and iron by induced crystallization for the digestion solution of high-sulfur bauxite. The result is expected to reveal the mechanism of sulfur and iron precipitated from the sodium hydroxide solution, regulate the crystallization behavior of sulfur and iron and establish the regulation mechanism for the simultaneous and efficient removal of the sulfur and iron by induced crystallization. It is also meaningful to develop continuously, stably and healthily for alumina industry in China.

高硫铝土矿合理利用是缓解铝土矿资源供应紧张局面的一项重要措施。但现有生产工艺尚存在硫铁析出而引起的产品杂质超标等难题，其根源在于未能有效调控铝酸钠溶液中硫铁结晶行为。项目组前期研究表明，诱导结晶可调控铝酸钠溶液中硫铁在晶种分解工序前快速定点析出，但该晶种难以快速有效获得且不稳定，成为诱导结晶同步除硫铁技术的主要瓶颈之一。本项目以氢氧化钠溶液中硫铁晶种的制备为研究对象，拟采用热力学计算结合化学定量分析明确Na-S-Fe-H2O系中硫铁的赋存状态，通过量子化学计算探讨晶种制备的可能反应途径，结合硫铁结晶动力学及TEM、SEM和XRD等分析检测揭示晶种形成机理，通过调控晶体生长的方法制备性能优良的晶种，实现诱导结晶同步高效除硫铁的目的。本项目预期可揭示氢氧化钠溶液中硫铁析出机理，调控溶液中硫铁结晶行为，建立高硫铝土矿溶出液诱导结晶同步高效除硫铁的调控机制，促进我国氧化铝工业持续、稳定、健康发展。

构建了Na-S-Fe-H2O系热力学图，分析了拜耳法生产过程中硫和铁的行为。溶出后，铁最可能以Fe3O4和Fe2O3的形式进入赤泥，部分铁以Fe(OH)3-、HFeO2-、Fe(OH)4-和Fe(OH)42-的形式转入液相。硫的优势物种是S2-，而铁主要以Fe(II)形态存在。热力学分析结果与实验测得的溶液中铁和硫的形态分布一致。当温度降低时，硫和铁会结合为FeS2、FeS、NaFeS2、NaFeS2·2H2O等硫铁化合物析出。控制低电位和降低温度均有利于将硫和铁从溶液中脱除。XRD图谱表明，NaFeS2·2H2O、FeS和FeS2广泛存在于赤泥以及黄铁矿和高硫铝土矿溶出液的析出物中。热力学分析可用于指导拜耳法同步除硫和铁。. 通过GuassView5.0对S2-、SO32-、HFeO2-、FeO等分子/离子进行建模，采用B3LYP/6-31G（d, p）基组，对优化的结构进行能量计算，获得了各物质的结合能（A）、最高占有轨道能（EHOMO）、最低空轨道能（ELUMO）以及能量差ΔE(LUMO-HOMO)。通过对比各种物质的能量差，发现可能存在的硫离子形式中能量差最大的为S2-，可能存在的铁离子形式中能量差最大的是HFeO2-，硫铁分子可能存在的形式中能量最低的为FeS。通常，能量差越大越稳定。因此，可推断氢氧化钠溶液中，硫的主要存在形式为S2-，铁的主要存在形式为HFeO2-，硫铁分子的主要存在形式为FeS，这与化学定量分析及XRD分析基本相符，说明硫铁分子可以以FeS形式脱除，阐明了硫、铁与苛性碱液作用的可能机理。.采用高压溶出制备了含硫铁晶种，研究了硫铁晶种制备行为。结果表明：搅拌转速以及温度对硫、铁结晶影响显著。Avrami方程可以描述硫和铁的结晶过程，并计算了它们的表观活化能。研究发现，硫和铁的Avrami指数n在1.5和2之间，表明晶体以一维针状和二维片状结构生长，颗粒尺寸较小，与实验照片及SEM图像相符。XRD分析表明，该晶种的主要成分为Fe2O3, FeS2, NaFeS2·2H2O, Fe(OH)3和NaFeO2。在除硫铁实验中，这些组分可以起到诱导结晶和反应结晶的双重作用，促进更多的水钠硫铁矿转移到了残留物中，并且形成了硫化亚铁，而对铝的损失几乎没有影响。