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2004—2020年,我国城市生活垃圾焚烧处理量从4.5×106 t增长到1.46×108 t[1],焚烧发电已成为我国城市生活垃圾处理的主要方式。生活垃圾焚烧处理具有减量化和能源利用等优点[2],但同时也会产生二次污染物。飞灰是垃圾焚烧过程中产生的一种含有重金属、二恶英和大量可溶性氯盐的固体废物[3],对人体和环境有害,被全球列为危险废物[4]。飞灰产生量与垃圾焚烧炉的类型有关,我国炉排炉和流化床垃圾焚烧炉的飞灰产生量分别占垃圾焚烧量的3%~5%和10%~20%[5]。随着我国垃圾焚烧量的增加,飞灰的产生量也在逐年增加。
目前,常用的飞灰处理方式有2类。固化/稳定化是指通过技术手段使飞灰中的重金属具有化学惰性或被物理包裹,减少其向环境中迁移[6-7]。其中,水泥固化技术操作简单、成熟,但能耗和占地较大且固化后的产物只能填埋处置[7]。化学稳定化技术可以降低飞灰的浸出毒性并且有长期的稳定性[8],但化学稳定剂的价格相对较高,某些螯合剂还会导致飞灰填埋后产生的渗滤液中NH4+-N等物质的质量浓度升高,增加渗滤液的处理难度[9]。热处理(如熔融、烧结等)[10]技术可将飞灰中大部分有毒的有机化合物分解,并把重金属固定在无定形相中,以降低其浸出率[11]。但该方法能耗高,并且会使飞灰中一些熔点较低的金属氯化物挥发,产生毒性更大的二次飞灰。分离浸出是通过浸取剂(水、酸、碱或生物浸取剂等)分离飞灰中的重金属和盐分等污染物,或分离有价值的元素回收利用,分离后的飞灰被填埋或进行水泥窑协同处置[12-16]。我国新颁布的《生活垃圾焚烧飞灰污染控制技术规范(试行)》(HJ 1134-2020)[17]中规定,应通过预处理控制飞灰处理产物中的可溶性氯质量分数(应不超过2%,以不高于1%为宜)。因此,采用水洗预处理脱除飞灰中的可溶性氯已成为必不可少的步骤。
目前,飞灰水洗方式和工艺已有较多研究[18-20],也有了一些工程应用,但关于水洗液性质和处理的研究较少。飞灰水洗液中含有大量的氯盐(氯化钾、氯化钠和氯化钙等)和一些重金属离子。高盐废水的处理方式主要有超滤、纳滤、反渗透、电渗析[21-23]等。其中,电渗析技术具有浓缩倍率高、消耗药剂少等优点,已被广泛应用[24]。电渗析在飞灰处理方面的研究主要是针对飞灰悬浊液中的重金属去除,该方法对飞灰中Cd、Cu和Zn的去除率可达80%以上,但对Pb的去除率只有12%[25-26]。而且,在电渗析过程中会产生大量的氯气,经过电渗析处理后飞灰的浸出毒性仍然较高[27]。电渗析对垃圾填埋场渗滤液中盐的浓缩效果较好,通过5级电渗析可使渗滤液的TDS浓缩48倍,水的回用率为62.5%,且随着电渗析的时间增长,阴离子膜的离子交换容量会降低,但膜污染并不严重[28]。本研究采用循环式电渗析装置探索飞灰水洗液中盐的浓缩,通过比较电导率变化、盐质量浓度的浓缩倍数、淡液回用率和能耗等因素获得最佳的电渗析浓缩工况;依据K+//Cl−—H2O 三元水盐体系相图,通过蒸发结晶的方式回收电渗析浓缩液中的氯化钾工业盐和混盐,以期为电渗析浓缩飞灰水洗液并回收盐的工业应用提供参考。
利用电渗析浓缩飞灰水洗液并回收工业盐
Concentration of fly ash eluate by electrodialysis and recovery of industrial salt
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摘要: 为回收飞灰中含有的大量可溶性盐,并利于飞灰后续填埋、水泥窑协同处置或建材利用,采用不同的液固比水洗飞灰并利用循环式电渗析装置浓缩飞灰水洗液,依据Na+、K+//Cl−—H2O 三元水盐体系相图通过蒸发结晶回收浓缩液中的盐。结果表明,以4 L·kg−1的液固比二级水洗飞灰后,可将飞灰中的可溶性氯质量分数降到《生活垃圾焚烧飞灰污染控制技术规范(试行)》(HJ 1134-2020)规定的限值之下。将飞灰一级水洗液加入电渗析装置的淡液室和浓液室,对比淡液室∶浓液室=2∶1 (V/V) 1次电渗析浓缩和淡液室∶浓液室=1∶1 (V/V) 2次电渗析浓缩2种工况可知,后者的浓缩倍数和淡液回用率(2.28倍、62%)比前者(2.03倍、54%)略高,但后者能耗是前者的1.37倍。因此,1次电渗析更适合用于飞灰水洗液的浓缩提盐。提高飞灰水洗液中盐质量浓度,可以提高电渗析浓缩速率。浓缩液蒸发结晶可得到纯度为92.2%的工业氯化钾和融雪剂,并可将飞灰中洗出的68%的钾回收为工业氯化钾。本研究结果可为电渗析浓缩飞灰水洗液并回收盐的工业应用提供参考。Abstract: In order to recover the large amount of soluble salt contained in fly ash and benefit to the subsequent landfilling, co-disposal in cement kiln or utilization as building materials, the fly ash was washed with different liquid to solid (L/S) ratios, and the eluate was concentrated by a circulating electrodialysis device. Then the salt was recovered from the concentrated solution by evaporative crystallization according to the Na+, K+//Cl−—H2O phase diagram. It was found that the mass fraction of soluble chlorine in the fly ash can be reduced to below the limit value of 《Technical specification for pollution control of fly-ash from municipal solid waste incineration》(HJ 1134-2020) when two-stage water washing with a L/S ratio of 4 L·kg−1 was adopted in each stage. The fly ash eluate from first stage was added into the dilute and concentration chamber in the electrodialysis device. The two working conditions were compared, i.e., one-stage electrodialysis with dosage of the fly ash eluate in dilute chamber to concentration chamber = 2:1(V/V), and two-stage of electrodialysis with dosage in dilute chamber to concentration chamber = 1:1(V/V). The results showed that the concentration ratio and the rate of diluted water reuse of the latter (2.28 times and 62%) were slightly higher than those of the former (2.03 times and 54%), but the energy consumption of the latter was 1.37 times that of the former, so one-stage electrodialysis was more suitable for the concentration of fly ash eluate. The speed of concentration by electrodialysis can be increased by increasing the mass concentration of salt in the fly ash eluate. The industrial grade potassium chloride with purity of 92.2% and deicing agent can be obtained by evaporative crystallization, in which 68% of the potassium washed from the fly ash can be recovered as industrial grade potassium chloride. The results of this study can provide a reference for the industrial application of concentration of fly ash eluate by electrodialysis and recovery salt from the concentrate.
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Key words:
- fly ash /
- water-washing /
- chloride /
- electrodialysis /
- industrial salt /
- potassium chloride
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图 8 30 ℃时Na+、K+//Cl-—H2O 三元水盐体系相图[39]及回收工业盐过程中各类盐的组成点
Figure 8. Phase diagram of Na+、K+//Cl-—H2O ternary water salt system at 30 ℃ and composition points of various salts in the process of industrial salt recovery
表 1 液固比为4 L·kg−1的飞灰水洗液中各元素的质量浓度
Table 1. Mass concentrations of elements in the fly ash eluates by washing at a liquid to solid ratio of 4 L·kg−1
Cr/(mg·L−1) Cu/(mg·L−1) K/(g·L−1) Mg/(mg·L−1) Na/(g·L−1) Pb/(mg·L−1) Ba/(mg·L−1) Ca/(g·L−1) Cl/(g·L−1) 0.55±0.01 0.05±0 10.10±0.12 0.05±0 6.48±0.21 17.30±1.67 4.51±0.10 9.71±0.03 30.40±0.32 表 2 电渗析前后物料质量浓度
Table 2. Mass concentration of material before ang after electrodialysis
考察项目 液体量/L 含盐量/(g·L−1) 钾/(g·L−1) 钠/(g·L−1) 预处理后飞灰水洗液 — 62.7 ± 0.9 24.4 ± 0.3 5.68 ± 0.18 淡液(工况1) 1.62 3.62 ± 0.66 0.94 ± 0.03 0.62 ± 0.03 浓液(工况1) 1.38 127 ± 1(*2.03倍) 48.0 ± 0.2(*1.97倍) 15.5 ± 0.1(*2.73倍) 1次电渗析淡液(工况2) 1.65 3.44 ± 0.39 0.96 ± 0.02 0.64 ± 0.06 1次电渗析浓液(工况2) 2.35 101 ± 1(*1.62倍) 34.2 ± 0.4(*1.4倍) 9.20 ± 0.08(*1.17倍) 2次电渗析淡液(工况2) 0.7 3.96 ± 0.22 1.12 ± 0.01 0.71 ± 0.01 2次电渗析浓液(工况2) 1.3 143 ± 1(*2.28倍) 52.4 ± 0.4(*2.14倍) 15.2 ± 0.1(*2.68倍) 注:*为浓缩倍数;浓缩倍数=浓液中该物质的质量浓度/预处理后飞灰水洗液中该物质的质量浓度。 -
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