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随着全球经济的快速发展,城市固体废物的排放也带来了越来越大的环境压力。据统计,我国2020年城市固体废物排放量为2.35×108 t[1]。焚烧是全球应用最广泛的固废处理方法之一,通过焚烧不仅能大幅降低固废的体积和质量[2],而且焚烧所产生的热能还可用于发电,其在中国城市生活垃圾处理中的占比已从2004年的5.55%急剧上升至2020年的62.13%。尽管焚烧在垃圾处理方面表现良好,但其会产生次级废物——垃圾焚烧飞灰。飞灰的成分复杂,且在不同时间、地点所产生的飞灰的成分差距较大[3],主要包括重金属盐、硫化物、硝酸盐、活性炭和二恶英[4]等。此外,在焚烧过程中通常需要添加熟石灰来吸附焚烧过程中的酸性气体和痕量有机物,这使飞灰具有较高的碱度[5]。飞灰中含有剧毒的二恶英和易浸出的重金属,因此为危险废物[6],必须加以谨慎处置。2021年12月10日,生态环境部等多部门联合印发了《“十四五”时期“无废城市”建设工作方案》[7],提出要强化固体废物综合利用水平和无害化处置能力,推进“无废城市”建设。研究飞灰的无害化处理和资源化利用变得尤为重要。
目前,飞灰的处理工艺和方法主要包括固化稳定化[8]、热处理[9]和分离浸出[10]。洗涤是飞灰处理常用的预处理方法,水洗可去除飞灰中高浓度的可溶性盐。HU等[11]的研究表明,在液固比为2.5~3时,用清水洗涤几乎能去除飞灰中所有的可溶性盐类,但水洗后重金属的浸出率有所升高;YANG等[12]在液固比为10时用去离子水洗涤,使飞灰中氯化物的比例由16.6%降为1.2%。LI等[13]的研究发现,洗涤能使飞灰的孔径和比表面积增大,并提高其吸附能力。水洗后,超过90%的Pb和Zn依然留存在水洗灰中,且赋存形态基本不变[14]。酸洗可溶解飞灰中的氢氧化物和碳酸化合物,从而破坏飞灰的固体结构[15]。ZHAO等[16]通过硝酸酸洗能去除飞灰中87.97%的Cd;林涛等[17]采用盐酸能洗出飞灰中95%以上的Zn、Pb、Cd和81.38%的Cu;HUANG等[18]采用水洗和酸洗联用技术,能去除飞灰中86%的Pb、98%的Zn、96%的Cd和62%的Cu;孙福成等[19]将飞灰与酸性废水联合处理,使飞灰中Pb和Cd的浸出浓度降低了90%以上。这些研究结果表明,酸洗能使飞灰中的大部分可溶性盐和重金属从固相转移至液相,便于后续的回收处理。
采用电沉积技术处理含重金属的洗涤废液,是通过废液中金属离子的电迁移,使其在阴极发生电化学还原反应而析出金属的过程,具有回收处理成本低、二次污染小的特点。彭腾等[20]采用柠檬酸浸出-电沉积联用技术处理废锂电池,使废电池中94.84%的钴得以回收;李子良等[21]应用电沉积技术处理酸性含汞废液,其中98%的汞能得到回收。陈熙等[22]认为,在电沉积过程中,阳极的氧化反应会产生氧气,溶液中氧气浓度的升高会腐蚀阴极表面沉积的重金属单质,造成重金属单质的返溶现象,从而影响重金属的处理效率。近年来,电沉积技术越来越多地被用于催化剂制备和高浓度金属废水处理,但涉及用于飞灰中重金属回收利用的研究却很少。
本研究采用酸洗-电沉积联用技术,用以去除并回收飞灰中的Zn、Pb、Cu和Cd 4种重金属。以硝酸作为酸洗试剂,确定合适的洗涤工艺,并分析飞灰酸洗前后的微观形貌、晶相、重金属存在形态等的变化。通过电沉积技术以回收酸洗废液中的Zn、Pb、Cu和Cd,进而评估酸洗-电沉积技术应用于飞灰中重金属回收的可行性,以期为飞灰无害化处理和资源回收提供参考。
酸洗-电沉积联用技术回收垃圾焚烧飞灰中的重金属
Recovery of heavy metals from municipal solid waste incineration fly ash by the combined acid washing and electrodeposition technology
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摘要: 垃圾焚烧飞灰中的重金属浸出能力强、毒性大,属于危险废物,若处置不当将会对环境造成严重污染。采用酸洗-电沉积联用技术,考察硝酸浓度、液固比和酸洗时间对重金属去除率的影响,研究电沉积电压和时间对酸洗废液中重金属回收率的影响,同时评估处理后飞灰的浸出毒性。结果表明,在浓度为2 mol·L−1、液固比为25的硝酸中浸渍洗涤60 min条件下,酸洗能成功去除飞灰中95.26%的Zn、83.06%的Pb、72.62%的Cu和97.85%的Cd;在电压为14 V,电沉积4 h的条件下可回收酸洗废液中95.80%的Zn、99.04%的Pb、79.95%的Cu和90.37%的Cd。对Zn、Pb、Cu、Cd的连续提取和浸出毒性测试表明,处理后飞灰易浸出组分占比降低,残余态物质占比提高,重金属浸出质量浓度符合《生活垃圾填埋场污染控制标准》(GB 16889-2008)要求,可直接进行填埋或作为非危废固体进行资源化利用。本研究结果可为飞灰中重金属脱除和回收利用提供参考。Abstract: The municipal solid waste incineration fly ash is classified as hazardous waste and will cause serious pollution to the environment if disposed improperly, owing to its strong leaching ability and high toxicity of heavy metals. Herein, the combined acid washing and electrodeposition technology was used to study the effects of the nitric acid concentration, liquid-solid mass ratio, acid washing time on the removal rate of heavy metals, and the effects of electrodeposition voltage and time on the recovery rate of the heavy metals from the acid washing waste solution, as well as to evaluate the leaching toxicity of the treated fly ash. The results showed that 95.26% of Zn, 83.06% of Pb, 72.62% of Cu and 97.85% of Cd were successfully removed from fly ash after immersing and washing for 60 min in nitric acid with concentration of 2 mol·L−1 and liquid-to-solid ratio of 25. Meanwhile, 95.80% of Zn, 99.04% of Pb, 79.95% of Cu and 90.37% of Cd in the acid washing waste solution were recovered at a voltage of 14 V and 4 h of electrodeposition. The continuous extraction and leaching toxicity tests of Zn, Pb, Cu and Cd demonstrated that the content of easily leachable components of the washed fly ash decreased, the proportion of residual state materials increased, and the leaching mass concentration of heavy metals met the requirements of “Pollution Control Standards for Domestic Waste Landfills” (GB 16889-2008), as thus, the acid washed fly ash can be directly landfilled or used as a non-hazardous waste solid for resource utilization. This study can provide a reference for the removal and recovery of heavy metals from fly ash.
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表 1 垃圾焚烧飞灰的XRF分析结果
Table 1. XRF analysis results of the MSWI fly ash
% (质量分数) Ca Cl S K Si Na Fe Mg Zn Al Pb Ti 36.23 15.82 8.77 4.21 2.21 1.17 1.1 0.762 0.44 0.41 0.21 0.21 P Cr Br Cu Ba Mn Sb Sr Sn Rb Cd Ni 0.18 0.08 0.61 0.059 0.058 0.041 0.037 0.032 0.029 0.011 0.007 0.004 表 2 飞灰中重金属质量分数
Table 2. Mass fraction of heavy metals in MSWI fly ash
mg·kg−1 Zn Pb Cu Cd 2872 1166 588 107 表 3 酸洗前后飞灰浸出液的重金属质量浓度
Table 3. Mass concentrations of heavy metal in fly ash leaching solution before and after acid washing
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