铝酸钙水泥结合含铬刚玉耐火浇注料中六价铬形成机理及其浸出行为研究

Corundum based castable bonded with calcium aluminate cement is one of the most important refractory castables. Cr2O3 is commonly introduced to corundum castables in order to improve the corrosion resistance of the refractory material, thereby extending its service life. However, Cr2O3 could react with calcium aluminate cement into toxic hexavalent chromium which can cause environmental pollution. In view of this problem, equilibrium experiments on the Cr2O3-Al2O3-CaO system at high temperatures would be conducted in order to investigate the physicochemical conditions of hexavalent chromium formation. In addition, SiO2 would be introduced into the ternary system for inhibiting the formation of hexavalent chromium, and then the release of hexavalent chromium from spent castables to the natural environment could be avoided. In the present project, isothermal sections of the phase diagram on the Cr2O3-Al2O3-CaO system would be experimentally determined, as well as the formation mechanism of hexavalent chromium in this system. Moreover, the distribution of chromium-containing phases on the Cr2O3-Al2O3-CaO-SiO2 system would be determined, as well as the inhibition mechanism of SiO2 on hexavalent chromium in the ternary system. At last, the influence of external conditions such as pH value and CO32- concentration in leach solution on the leach behavior of hexavalent chromium in spent chromium-containing corundum castables bonded with calcium aluminate cement would be studied. The safety of the spent castables under the conditions of acid rain could be guaranteed.

铝酸钙水泥结合刚玉质浇注料是重要的耐火浇注料品种，通常在其中引入一定量的三氧化二铬以提高其抗熔渣侵蚀性，从而延长浇注料的使用寿命。然而，该浇注料中三氧化二铬能与铝酸钙水泥反应生成有毒的六价铬，导致使用后的浇注料污染环境。针对此问题，本项目拟采用高温平衡实验法研究Cr2O3-Al2O3-CaO体系中六价铬形成的物理化学条件，并通过在上述体系中引入一定量的二氧化硅，抑制浇注料中六价铬的形成，避免使用后的浇注料中六价铬溶出。本项目研究内容包括：研究并完善Cr2O3-Al2O3-CaO三元系等温平衡相图，揭示该体系中六价铬形成机理；研究Cr2O3-Al2O3-CaO-SiO2体系中含铬相分布，探究二氧化硅对上述三元体系中六价铬形成的抑制机理；研究外部条件（如浸出液的pH值和碳酸根离子浓度）对用后铝酸钙水泥结合含铬刚玉浇注料中六价铬浸出行为的影响，保障该体系用后浇注料在酸雨条件下的安全性。

含铬耐火材料因其具有优异的抗熔渣侵蚀性能，广泛用于服役条件苛刻的领域例如超高温垃圾焚烧发电熔融炉。然而，用后含铬耐火浇注料中一旦含有六价铬将会面临处置困难，并且对环境构成严重威胁。针对此问题，本项目研究了铝酸钙水泥结合含铬耐火浇注料中六价铬形成与浸出行为，首先，研究了添加Cr2O3对耐火浇注料抗侵蚀的影响机制，开展了三元体系Al2O3-CaO-Cr2O3中六价铬相形成机理及其热稳定性研究，其次开展了四元体系Al2O3-CaO-Cr2O3-SiO2及浇注料基质体系中含铬相分布与六价铬抑制效果，研究了铝铬固溶体及镁铝铬尖晶石在铝酸钙水泥相存在条件下的高温热稳定性，最后研究了添加剂对含铬耐火浇注料中六价铬的抑制效果以及六价铬浸出机制。研究重要发现与结果如下：（1）三元体系中不是含铬高区域生成六价铬越高，六价铬形成取决于其组分配比及温度与气氛；（2）在世界上首次发现了一个新化合物CaAl2Cr2O7，并对其首次进行了结构解析；（3）六价铬相CaCrO4在1100oC左右易于分解，而六价铬相Ca4Al6CrO16在高达1500oC依然稳定存在；（4）MgO可有效抑制四元体系Al2O3-CaO-Cr2O3-MgO中六价铬形成，铬稳定存在于尖晶石Mg(Al,Cr)2O4中不易浸出；（5）在四元体系中Al2O3-CaO-Cr2O3-SiO2中SiO2可优先与CaO及Al2O3反应，从而抑制六价铬化合物CaCrO4和Ca4Al6CrO16的形成；（6）(Al,Cr)2O3 固溶体在1100-1300°C与铝酸钙水泥会反应生成六价铬相Ca4Al6CrO16，而在1500°C反应生成三价铬相Ca(Al,Cr)12O19；镁铝铬尖晶石Mg(Al,Cr)2O4与自由氧化钙、C12A7和CA相反应生成Ca4Al6CrO16，而与自由氧化钙反应生成六价铬和四价格的相Ca5Cr3O12,其中Cr4+也可被浸出；（7）铬的氧化状态随温度的升高按以下顺序变化：Cr(III)→Cr(VI)→Cr(III)；（8）耐火浇注料中铬的浸出不仅与铬的赋存形态有关，还于外部浸出条件有关。以上研究发现，深入地探究了含铬耐火浇注料体系中含铬相分布及铬的氧化转变机理，这将为环保型含铬耐火浇注料成分设计与制备奠定了坚实的基础。此外，本项目的基础研究成果也为不锈钢渣、尘和污泥等其它领域中六价铬控制提供理论指导与借鉴。