水泥窑协同处置固废过程中重金属离子的转化迁移机制研究

当前我国基建规模大，水泥消耗量大，水泥行业正面临着原材料紧缺、燃料消耗严重等节能减排方面的难题。同时，我国每年产生大量固体废物，如城市生活垃圾产生量已经超过2.3亿吨，固体废物中所含的Ca、Si、Al、Fe等无机组分和一些有机组分可作为水泥熟料生产的替代原料和燃料，从而实现固体废物资源化处理。因此，水泥窑协同处置固体废物被认为是极具发展前景的技术，但是固体废物中含有重金属元素，它们会导致水泥熟料的性能发生改变，且带来环境污染的风险。本论文根据我国生活垃圾、市政污泥和工业矿渣等固体废物中的重金属种类和含量特征，选取了四种典型的重金属：Cu、Cr、Zn、Pb作为实验对象，系统研究了重金属对水泥熟料煅烧过程的影响规律，初步建立了重金属在水泥熟化生产过程中的迁移转化机理，得到如下结论： 首先，研究了重金属对水泥生料易烧性的影响规律，研究表明按照重金属对水泥熟料中游离氧化钙含量的影响规律将其分为两类，一类是重金属掺量不超过0.1wt.%的情况下可降低游离氧化钙含量，促进熟料烧成，但超过1.0 wt.%的限值后会提高游离氧化钙含量，增大幅度高达211 %，大大降低了水泥生料易烧性，比如Cr；另一类是重金属掺量不超过0.1 wt.%的时候也可降低游离氧化钙的含量，但随着掺量增加，重金属对水泥生料的易烧性影响不显著，如Cu、Zn和Pb等。 其次，系统研究了重金属在水泥熟料中的分布规律。研究表明，水泥熟料里面的Cr元素以正三价和正六价的形式同时存在。从水泥熟料中检测到的硅酸三钙和硅酸二钙被统称为硅酸盐相，铝酸三钙和铁铝酸四钙被统称为中间相。进一步研究表明，Cr元素主要分布于硅酸盐相中，而Cu、Zn、Pb元素主要赋存在中间相。 最后，研究了重金属在水泥熟料中的固化效果和浸出毒性。根据实验结果可知，在相同掺量条件下，水泥熟料对Cr元素的固化率最高，基本都在60 %以上，Cu和Zn元素的固化率次之，Pb元素的固化率相对最低。在浸出实验中，除了Cr元素外，Pb、Zn、Cu元素的浸出浓度都没有超出国家标准，因此，我们需要控制水泥生料中Cr元素的掺量不超过0.5 wt.%，这将对实际水泥窑协同处置固体废物的过程具有一定指导意义。

At present, China's infrastructure scale and cement consumption is large. The cement industry is facing problems in energy conservation and emission reduction, such as shortage of raw materials and serious fuel consumption. At the same time, our country produces a large amount of solid waste every year, such as municipal solid waste production has exceeded 230 million tons, solid waste contained in the inorganic components such as Ca, Si, Al, Fe and some organic components can be used as the cement clinker production of alternative raw materials and fuel, so as to realize the solid waste resource treatment. Therefore, the synergistic disposal of solid waste by cement kilns is considered to be a promising technology, but solid waste contains heavy metals, which can change the performance of cement clinker and bring the risk of environmental pollution. In this paper, according to the types and content characteristics of heavy metals in solid wastes such as domestic waste, municipal sludge and industrial slag, four typical heavy metals were selected: Cu, Cr, Zn and Pb were taken as experimental subjects to systematically study the influence of heavy metals on the calcination process, and preliminarily establish the migration and transformation mechanism of heavy metals in the cement curing production process. The following conclusions were drawn: Firstly, the influence law of heavy metals on the burnability of raw cement was studied. According to the influence law of heavy metals on the content of free calcium oxide in cement clinker, it could be divided into two categories. One is that the content of free calcium oxide can be reduced and the burning of clinker can be promoted when the content of heavy metals is not more than 0.1 wt.%. However, when the limit value of 1.0wt.% is exceeded, the content of free calcium oxide will increase by up to 211 %, which greatly reduces the burnability of cement raw materials, such as Cr. The other is that when the content of heavy metals is not more than 0.1 wt. %, the content of free calcium oxide can also be reduced. But with the increase of the content, the influence of heavy metals on the burnability of cement raw materials is not significant, such as Cu, Zn and Pb. Secondly, the distribution of heavy metals in cement clinker was systematically studied. The research shows that the Cr element in cement clinker exists in the form of positive trivalent and positive hexavalent simultaneously. Tricalcium silicate and dicalcium silicate detected from cement clinker are collectively referred to as silicate phase, tricalcium aluminate and tetracalcium aluminoferrite are collectively referred to as mesophase. Further study shows that Cr is mainly distributed in the silicate phase, while Cu, Zn and Pb are mainly distributed in the mesophase. Finally, the curing effect and leaching toxicity of heavy metals in cement clinker were studied. According to the experimental results, under the same dosage condition, cement clinker has the highest curing rate of Cr element, which is basically above 60 %, followed by Cu and Zn elements, and Pb element curing rate is relatively the lowest. In the leaching experiment, except for Cr, the leaching concentration of Pb, Zn and Cu elements did not exceed the national standard. Therefore, we need to control the content of Cr element in cement raw material is not more than 0.5 wt.%, which will have a certain guiding significance for the actual cooperative disposal of solid waste in cement kilns.