矿物演化行为对垃圾焚烧飞灰熔融特性的影响

以焚烧飞灰为主要原料,采用灰熔点测试、差示扫描量热法、X射线衍射、扫描电子显微镜等方法,研究了掺加SiO₂(以w计)对焚烧飞灰熔融特性的影响,并利用CASTEP模块模拟计算生成矿物的反应活性. 结果表明,当w(SiO₂)为29.14%时,焚烧飞灰流动温度为1 355 ℃,比原灰降低了近200 ℃,熔融特征温度随w(SiO₂)增加而上升. 将焚烧飞灰熔融并热处理后得到微晶玻璃,其矿物质组成为硅灰石、假硅灰石、钙铝黄长石、钙铁榴石、硬石膏和三型钾霞石等. 分子模拟计算结果表明,假硅灰石、钙铝黄长石和钙铁榴石形成能高,属耐熔矿物,而硅灰石、硬石膏和三型钾霞石等助熔矿物的低共熔会导致灰熔融温度降低. SiO₂/CaO(质量比,下同)<1时,过量的Ca2+易与活性氧发生集聚反应,形成热稳定性好的假硅灰石；SiO₂/CaO接近于1时,生成以硅灰石为主晶相的助熔矿物,硅灰石形成能为-41.67 eV,低于其他矿物,并且晶体氧原子中活性氧比例达到77.78%；当SiO₂/CaO>1时,大量无定形SiO₂及方石英(非活性氧)的存在致使灰熔融温度升高. 硅酸盐矿物熔体中非活性氧(Si—O—Si)和活性氧(Si—O,自由氧)占氧原子比例的变化是焚烧飞灰熔融特性改变的内因. 

Effects of Mineral Change on Melting Characteristics of MSW Incineration Fly Ash

The ash melting test and the techniques of DSC, XRD and SEM were used to investigate the effects of SiO₂ additive on the melting behavior of MSW (municipal solid waste) incineration fly ash. Additionally, the reaction activities of produced minerals were analyzed by using the CASTEP model. The results showed that the ash flow temperature was 1355 ℃ when the blending ratio of SiO₂ (BRS) was 29.14%, approximately 200 ℃ lower than the original fly ash. The increase of BRS tended to enhance AMT. XRD analysis indicated that wollastonite, pseudowollastonite, gehlenite, andradite, anhydrite and trikalsilite occurred in glass-ceramics. Molecular simulation showed that pseudowollastonite, gehlenite and andradite were refractory minerals due to the high formation energy. Wollastonite, anhydrite and trikalsilite were fluxing minerals and led to the lower AMT. When the mass ratio of SiO₂/CaO<1, excess Ca2+ was prone to combine with reactive oxygen to form pseudowollastonite, which had better thermal stability. When the mass ratio of SiO₂/CaO was close to 1, wollastonite, which was the main crystalline phase in glass-ceramics, was considered as fluxing mineral to make AMT reduction. The simulated results indicated that the formation energy of wollastonite (-41.67 eV) was lower than that of other minerals. Moreover, the ratio of reactive oxygen in wollastonite was up to 77.78%. When the mass ratio of SiO₂/CaO>1, a large number of amorphous SiO₂ and cristobalite (non-reactive oxygen species) resulted with AMT rise. The percentage change between active oxygen and non-active oxygen in melts was the internal cause of AMT.