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粉煤灰是煤粉经炉膛高温燃烧后,烟气经除尘装置收集得到的粉状固体物质. 我国粉煤灰近年的产生量均在5亿t以上,2021年我国粉煤灰年产量达到7.9亿t,综合利用率在75%左右[1 − 4],主要用作建筑工程中,如生产水泥、混凝土和建筑材料等[2, 5];用作环保材料的粉煤灰陶粒也有大量研究并实现了工程应用. 然而,大量库存粉煤灰不仅存在占地面积大、长期管理成本高的问题,同时还有潜在环境污染风险.
从化学组成、矿物组成和反应性等方面了解粉煤灰性质,对发挥粉煤灰自身性质优势进行资源化有着不可忽视的作用[2]. 粉煤灰的主要化学成分为SiO2、Al2O3、Fe2O3和少量CaO、MgO等[2 − 4]. 粉煤灰的矿物组成主要包括铝硅酸盐玻璃体、晶体矿物(石英、石灰、磁铁矿、赤铁矿等)以及未燃尽的碳粒[2, 5]. 粉煤灰的化学组成和矿物组成赋予了粉煤灰一定的火山灰活性,即能够在水分存在的条件下与氢氧化钙等碱性金属氢氧化物发生反应,生成水硬胶凝性化合物[6 − 7]. 其次,粉煤灰具有疏松多孔、比表面积较大、表面能高、且存在着许多铝、硅等活性点以及具有较强的物理和化学吸附能力,因此将粉煤灰进行改性制备为吸附剂、絮凝剂等用于去除污水中重金属、COD、氨氮、磷酸盐等污染物也是粉煤灰资源化的研究热点[2, 5, 8].
由矿物相组成可知,硅铝玻璃体为粉煤灰的主要组成部分,硅铝玻璃体内含有大量的活性Al2O3和SiO2,但致密的结构抑制了活性成分发挥作用,导致粉煤灰对磷的吸附容量并不高[9 − 10],通过改性是提高粉煤灰吸附容量最为常用的办法,改性方式包括碱改性、酸改性和制成陶粒等.
碱改性可以通过OH-破坏粉煤灰玻璃体表面的Si—O和Al—O键而使其解构,增大比表面,释放玻璃体内部的活性Al2O3和活性SiO2,提升了除磷容量[11 − 12]. Pengthamkeerati利用氢氧化钠对粉煤灰改性后除磷能力大幅度提升,其主要原因是比表面积3.39 m2·g−1增加到35.38 m2·g−1,且玻璃体溶出的活性金属物质提升了粉煤灰絮凝沉淀除磷的能力[13]. 鉴于粉煤灰所具有的火山灰活性,在Ca2+等存在的条件下,粉煤灰可以发生水化反应产生水化硅酸钙、水合铝酸盐等凝胶态产物,提升粉煤灰颗粒的比表面积和吸附性能[14 − 15]. 但当前对于粉煤灰水化反应和除磷反应间的耦合反应机制还未见详细报道,两者之间的协同关系有待研究.
酸改性可以通过酸使粉煤灰玻璃体表面形成凹槽和孔洞,内部的Fe3+、Al3+溶出,提高了粉煤灰的粗糙度和比表面积,提升了对磷的去除效果[16]. 刘文辉等利用不同硫酸、盐酸、硫酸与盐酸混合酸对粉煤灰进行了改性处理,发现酸性改性剂可以激发粉煤灰活性,增加粉煤灰的比表面积,暴露大量的Al、Si等活性点,提高除磷容量. 但是随着酸改性剂浓度的增加,磷的去除率反而下降,可能是由于强酸改性剂的加入会使污水pH值急剧下降,影响了Al、Fe等絮凝性[17].
以黏土、页岩、污泥等为原材料制备的陶粒有较好的除磷效果,而粉煤灰的化学组成与上述材料的化学组成较为相似,因此利用粉煤灰制备陶粒是可行的[18]. Cheng等以粉煤灰、污泥和牡蛎壳为原材料在
1050 ℃下烧结8 min得到烧结陶粒作为湿地填料进行除磷,其最大吸附量可达到4.51 mg·g−1,化学吸附是其除磷的主要机制[19]. 常规粉煤灰陶粒制备方式主要有烧结法和免烧法两种,烧结陶粒和免烧颗粒孔隙较少且封闭,影响污染物去除容量,同时限制其内部有效成分的污染物去除效用[20 − 22].利用粉煤灰火山灰活性开发污染控制技术中的环保材料具有巨大资源化潜力,但目前国内外对于粉煤灰基除磷材料的研究依旧存在颗粒内部反应机制研究不充分、吸附容量小、能耗成本高等问题,限制了粉煤灰作为环保除磷材料的应用. 本研究利用生石灰作为弱碱激发剂激发粉煤灰活性,添加秸秆纤维煅烧造孔,开发了具备空间网络多孔结构的秸秆粉煤灰基除磷颗粒填料,并验证了除磷过程粉煤灰活性Al2O3激发水化与除磷反应相互耦合的机理.
秸秆粉煤灰基颗粒填料激发水化与除磷耦合机理研究
Coupling mechanism of induced hydration and phosphorus removal with straw fly ash-based granular fillers
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摘要: 粉煤灰添加水泥、生石灰和秸秆等辅料利用团粒、喷水养护并结合煅烧成孔的方式制备弱碱激发颗粒填料. 颗粒填料在间歇循环流态化完全混合反应器中进行除磷实验,通过比较除磷组和对照组(超纯水)中颗粒填料的抗压强度、结合水含量以及粉煤灰反应程度研究除磷过程对粉煤灰水化进程的影响,反应产物(凝胶态)Al含量、未反应粉煤灰(残余态)Al含量以及磷形态及含量研究粉煤灰玻璃体中Al的激发规律以及激发水化与除磷过程耦合机理. 研究结果表明,粉煤灰激发水化和除磷反应同步进行,难溶磷酸钙的形成有利于Ca(OH)2溶解释放OH-,促进粉煤灰玻璃体解构释放活性Al2O3和SiO2等除磷和水化活性物质,所释放的活性物质可以在Ca(OH)2存在的情况下发生火山灰反应,生成水化硅酸钙和水化铝酸钙等水化产物,进而实现长期有效除磷. 同时活性Al2O3可以发挥较大的除磷作用,Al-P含量占填料内总磷含量的比例可达40%左右.Abstract: The fly ash is added with auxiliary materials such as cement, quicklime and straw, and the weak alkali excited particle filler is prepared by agglomeration, water spray curing and combined with calcination to form pores. The granular packing is subjected to phosphorus removal experiments in a batch cyclic fluidized complete mixing reactor. The effect of phosphorus removal process on fly ash hydration process was studied by comparing the compressive strength, bound water content and fly ash reaction degree of granular fillers in phosphorus removal group and control group (ultrapure water). The activated law of Al in fly ash vitreous and the coupling mechanism of activated hydration and phosphorus removal processes were studied by comparing the Al content of reaction products (gel state), unreacted fly ash (residual state) Al content, and phosphorus speciation and content. The results show that the activated hydration and phosphorus removal of fly ash are synchronized. In this process, the formation of insoluble calcium phosphate is conducive to the electrolytic release of OH- from Ca(OH)2 ,then promote the release of active phosphorus removal and hydration active substances such as Al2O3 and SiO2 from fly ash glass phase, the released active material can undergo volcanic ash reaction in the presence of Ca(OH)2 to generate hydrated products such as calcium silicate hydrate and calcium aluminate hydrate, thereby achieving long-term effective phosphorus removal. The active Al2O3 can play a large phosphorus removal effect, and the proportion of Al-P content in the total phosphorus content in the filler can reach about 40%.
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Key words:
- fly ash /
- hydration reaction /
- phosphorus removal /
- coupling mechanism.
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表 1 粉煤灰和水泥的化学组成(%)
Table 1. Chemical composition of fly ash and cement
原料名称
Raw material nameSiO2 Al2O3 CaO MgO Fe2O3 粉煤灰 45.10 24.20 5.60 1.50 4.63 水泥 21.76 5.78 64.70 1.32 3.87 表 2 秸秆粉煤灰基颗粒填料物理性质
Table 2. Physical properties of straw fly ash based particle filler
粒径/mm Particle size 比表面积/(m2·g−1)
Specific surface area表观密度/(g·cm−3)
Apparent density堆积密度/(g·cm−3)
Bulk density孔隙率/%
Porosity破碎率与磨碎率之和/%
Sum of crushing rate and grinding rate2.00—5.00 16.25 2.10 0.74 64.80 2.30 -
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