生活垃圾焚烧飞灰差异性特征对脱氯除盐效果影响研究——以重庆和天津飞灰为例

随着生活垃圾焚烧技术的普及,垃圾焚烧飞灰的资源化利用迫在眉睫,然而飞灰中的高浓度氯元素严重阻碍了其资源化进程.水洗技术作为常用的脱氯除盐技术,得到越来越多的应用,但除氯效果受飞灰自身性质特征影响较大.本研究从重庆和天津采集了两种理化性质具有较大差异的垃圾焚烧飞灰,在对其粒径分布、化学组成、矿物组成、pH值和浸出毒性等基本特征进行分析表征的基础上,采用水洗技术和碳酸化水洗技术对两种飞灰进行了脱氯处理.结果表明,重庆飞灰氯元素的赋存形态包括NaCl、KCl等可溶性氯盐,还含有较高浓度的难溶氯盐(如CaClOH、Friedel盐和Ca₆(CO₃)₂(OH)₇Cl),因此,水洗技术难以达到理想的脱氯效果,氯盐去除率最高为88%.在水洗过程中通入CO₂可以显著降低反应体系的pH值,促进难溶盐的转化和溶解,进而提高飞灰氯盐的去除率.重庆飞灰采用加速碳酸化水洗技术,氯盐去除率最高可达94%.天津飞灰中氯元素主要以可溶性氯盐的形式存在,难溶盐占比很少,因此,水洗技术可以达到较高的脱盐率(96%).采用加速碳酸化技术对天津飞灰进行处理,发现氯盐去除率较纯水洗技术反而有所降低,分析其主要原因可能与加速碳酸化过程中碳酸钙等物质的再沉淀对氯盐的裹挟作用有关.由此可知,飞灰自身性特征尤其是氯盐的赋存形态对于脱氯除盐技术的选择有重要影响.因此,实际资源化利用过程中,可根据飞灰性质来决定预处理手段,从而最大化提升效率和节约成本.

Effect of intrinsic property of MSWI fly ash on dechlorination: Illustrated by two ash samples from Chongqing and Tianjin

With the rapid development and wide application of municipal solid waste incineration (MSWI) technology, recycling of MSWI fly ash becomes more and more important. However, the high content of chlorine in fly ash significantly inhibits its recycling as construction materials. Wet-washing has been adopted to remove the chloride salts from MSWI fly ash, but the removal efficiency is highly influenced by the intrinsic property of the ash samples. This research collected two ash samples from Chongqing and Tianjin incineration plants and analyzed their physic-chemical characteristics by performing particle size distribution, chemical and mineralogical composition, and leaching toxicity analysis. Based on this analysis, the ash samples were further treated by wet-washing and wet-carbonated-washing. The results revealed that the chlorine in Chongqing fly ash exists as both soluble (e.g. NaCl and KCl) and hardly soluble salts (including CaClOH, Friedel''s salts and Ca₆(CO₃)₂(OH)₇Cl). Wet-washing of the Chongqing fly ash gives a Cl removal ratio of only 88% due to existence of some hardly soluble salts. Wet-carbonated-washing of the Chongqing fly ash significantly improved the Cl removal ratio, reaching a maximum value of 94%. It is assumed that injection of CO₂ in the washing process decreased the solution pH, which consequently promoted the decomposition and solubilization of these hardly soluble chlorine-containing salts. Different from the Chongqing fly ash, a high Cl removal ratio (96%) was obtained for the Tianjin fly ash when treated by the wet-washing method. This may be ascribed to the relatively lower content of the hardly soluble chlorine-containing salts in the Tianjin fly ash. However, when treated by the wet-carbonated-washing method, the Cl removal ratio decreased by approximately 2%. This may be caused by the capture or capsulation effect of some newly formed precipitates (e.g. calcite). Based on these results, we conclude that the intrinsic property of the ash samples especially the chemical forms of the chlorine-containing salts and their fractions in the MSWI fly ash pose a significant influence on chloride removal. Therefore, MSWI fly ash dechlorination should be a property-oriented process in industrial-scale applications in order to make it cost-effective.