含硫酸钙微米晶铬渣的晶体生长和表界面作用调控及六价铬分离机制研究

The general treatment method of chromium-containing slag is reducing hexavalent chromate and then landfilling the residue. However, the chromium can not be recycled and thus may cause secondary pollution via such method. In the previous study, the applicant has extracted Cr(VI) from the nano-Mg(OH)2 and Cr(VI) containing slag through transforming the nano-Mg(OH)2 into bulk magnesium salt under the action of interfacial regulating agent, and investigated the mechanism of Cr(VI) desorption and separation. In this work, we will extract Cr(VI) from another type of Cr(VI)-containing slag which contains the calcium sulphate microcrystals, and study the mechanism of Cr(VI) desorption and separation. The pre-test results show that under the action of interfacial regulating agent, the CaSO4·2H2O would transform to CaSO4, and simultaneously the particle size and crystallinity would get changed accompanied by the desorption of large amount of Cr(VI) from the slag. In this study, we will investigate the phase transformation, crystal growth, and the changes in aggregation state and surface state of calcium sulphate，as well as the relevant changes in the modes of chromium occurrence, under the conditions of different interfacial regulating agents and temperature. We aim to summarize the relationships between the phase transformation and crystal growth of calcium sulphate and the adsorption/desorption of Cr(VI), to further study the interfacial interactions between Cr(VI) and solid, and to find out the key interfacial factors affecting Cr(VI) desorption. This work is expected to widen the applications of recycling heavy metals from solid waste by phase separation under the action of interface regulation, and to provide theoretical guidance for the recycling of chromium from calcium sulphate microcrystals and Cr(VI)-containing slag.

传统的铬渣处理方法是将六价铬还原后堆储，不仅未回收铬资源，而且易造成二次污染。申请人在先前的研究中，曾针对含纳米氢氧化镁的铬渣，研究了表界面调控剂促纳米氢氧化镁聚集生长成大颗粒的过程中，六价铬的分离机制。本工作拟针对另一类含硫酸钙微米晶的铬渣进行六价铬的分离机制研究。预实验表明，二水硫酸钙微米晶在外加表界面调控剂作用下，会转变为含不同结晶水的物相，晶粒尺寸及结晶度均出现改变，与此同时，六价铬大量脱附。本研究拟探讨在不同的表界面调控剂和温度等条件下，硫酸钙物相变化、晶体生长、聚集态和表面状态变化等，以及与此过程相关的铬赋存状态的改变。总结硫酸钙物相变化、晶体生长与六价铬的吸/脱附动力学之间的规律，进一步研究六价铬与硫酸钙颗粒的微观界面过程，找到调控铬脱附的关键界面因素。该工作有望拓展表界面调控剂调控重金属固废的分相回收原理的适用范围，为含硫酸钙的铬渣处理和铬资源回收提供理论指导。

铬渣的治理是世界性环保难题，其危害主要来自于迁移性强、毒性高、可致癌的六价铬。目前传统处理方法是还原固定后填埋，不仅铬资源得不到回收和利用，而且可能存在二次污染。因此，理想的解决思路是将铬渣中的六价铬深度提取并回用到工业，控制污染的同时收获资源。但是，目前仍没有彻底提取铬渣中六价铬的有效方法。本项目以氯酸盐行业含硫酸钙的铬渣为例，在深入研究渣中重金属铬的赋存状态的基础上，发展了利用表界面调控在水热条件下实现铬渣中六价铬深度提取回收的方法，系统研究了调控铬赋存关键矿物（如硫酸钙）的物相转变、晶体生长及六价铬有效分离的微观机理。研究结果表明，包夹态的六价铬（包括晶界间包夹和晶格内掺杂）是氯酸盐工业含硫酸钙铬渣中六价铬缓释及难以提取的根本原因。在此研究基础上，我们提出了水热条件下表界面调控物相转变协同铬形态转变的策略实现六价铬的彻底脱除(>99%)，进一步改进了表界面调控剂（包括过硫酸盐、硫酸和盐酸）并优化了处理条件。机理研究表明，水热条件调控不同硫酸钙物相间的转变，导致晶体溶解-重结晶过程，在此过程中吸附和包夹态Cr(VI)完全释放；而水表界面调控剂（如过硫酸盐、硫酸和盐酸）提供H+将所释放的CrO42-转化为不易再进入矿物晶格的Cr2O72-离子形态，而且可以降低二水硫酸钙到无水硫酸钙的相转变温度，进而促进硫酸钙晶体的生长和完善，从而促进六价铬完全分离。公斤级实验和中试、工程放大实验均达到了完全脱毒的目标，说明上述策略的可行性。该项目成果有望将表界面调控物相转变协同重金属形态变化策略拓展应用于含重金属危废的处理，为铬渣中六价铬的深度提取回收提供了重要的理论指导。