Quantitative Studies of Alite and Belite in Clinker Formation with Recycling of Heavy Metal Containing Ashes and Sludges

Autor: Pai-Haung Shih, 施百鴻
Rok vydání: 2005
Druh dokumentu: 學位論文 ; thesis
Popis: 93
Feasibility of municipal solid waste incineration (MSWI) bottom ash and heavy metal containing sludges as raw materials for cement production is investigated in this study. Uniaxial compression strengths (UCS) of hydrated clinkers produced from wastes replaced raw material are tested to clarify the influence of interference introduce by wastes. Also, a crystalline quantification technique is developed in order to investigate the effects of heavy metal oxides on the formation of silicate phase in clinkers. In this study, the MSWI ash is pretreated with techniques including magnet separation, water washing, EDTA separation and dense media separation to remove the possible interferences such as iron and salts prior to addition to raw mix. The results show that when the replacement percentage is below 10%(w/w), the UCSs of hydrated cement are higher than 18MPa, which could meet the Chinese National Standard (CNS) criteria for ordinary Portland Type I cement. But when the replacement percentage is higher than 10%(w/w), a dramatic drop of the UCSs is observed due to the insufficiency in calcium oxide. By adjusting the chemical compositions of replaced raw mix according to the compositional parameters (lime saturation factor, silica ratio, aluminum ratio and hydration modulus), the UCS of sintered clinker is as strong as commercial cement. The heavy metal containing sludge is added to the raw material with compositional parameter fulfilled. The results of QXRD show that when the total heavy metal contents in raw material is smaller than 3%, a increase to 140% of the tricalcium silicate (C3S) phase could be observed. But when the amount of total heavy metals is greater than 3%, significant decreases of C3S intensity could be observed. When the total amount increase to 6%, the intensity of C3S is only 80% relative to commercial cement. In summary, utilization of MSWI ash and heavy metal containing sludge is applicable if appropriate control of compositional parameters and heavy metal amount is applied. By adding 10%(w/w) of α-Al2O3 as internal standard to the NIST certificated reference material, X-ray powder diffraction (XRD) analysis is performed with short (1 hour) and long (8 hour) scanning time. The following crystalline quantification analysis show that the XRD profiles with scanning range of 15˚-75˚ (2θ), 0.03˚ per step, and 2s/step (short scanning time) are good enough for the following refinement. The crystalline quantification is performed with software package designed on quantitative Rietveld method basis; combinations of polymorphs of C3S are selected as initial model input for the refinement. The results show that both the RM and the RMT (rhombohedral, monoclinic and triclinic) combinations could cover all the variety of cements characterized by the three kinds of standard reference material. The cumulative relative error between the refined and certificated values is in the range of 7-23%. The method is stable to be used as crystalline quantification. Also, the amorphous weight percentage could be obtained by inverse derivation from the contents of α-Al2O3. In summary, combining the XRD operating condition described previously and the RMT combination for initial model in refinement, an optimal, fast and reliable crystalline quantification method is established. The established method could be used for the quantification of crystalline in future experiments. At last, a full factorial experimental design is applied to three heavy metal oxides including CuO, NiO and ZnO. In the experiments three kinds of heavy metal oxide levels (1%, 3% and 5%) are tested. With adding heavy metal oxides, the simulated cement raw materials are sintered at 950, 1100, 1250 and 1400°C for 3 hours. The heavy metal oxide incorporation analyses of clinkers show that almost all the ZnO would evaporate during the sintering process. ZnO evaporated at 950-1100°C. On the other hand, 80-90% and 55-70% respectively of CuO and NiO would be retained in the clinkers. By means of crystalline quantification assisted with differential thermogravimetric analysis (TG/DTA), the addition of 3% heavy metal oxides could lower the liquid melting temperature and hence increase the amount of dicalcium silicate (C2S) formation. The ability of lowering liquid formation temperature is ZnO>CuO>NiO in sequence. NiO has almost no effect on the melting temperature lowering. The crystalline quantification analyses of 1400°C clinkers show that CuO and NiO would have statistically significant effect on the amount of the two major silicate phases (C2S and C3S) formed. Nevertheless, the total moles of silicates per unit weight of clinkers would not change. The total moles of silicates do not have statistically difference with respect to the addition of heavy metal oxide is added. The high temperature sintering system could be considered as in equilibrium, heavy metal oxides could not affect the partition ratio between C2S and C3S. The partition ratio in clinkers (C3S/C2S molar ratio) could be expressed as an empirical equation with amounts of CuO and NiO as parameters. In summary, the MSWI ash and heavy metal containing sludge could be utilized as alternative raw materials as long as appropriate controls on compositional adjustments and heavy metal thresholds are taken into consideration. If heavy metals were introduced into the raw material in low dosage, heavy metal would not affect the total amount of silicate phases. Instead the partition ratio between C2S and C3S would be altered. Empirical model developed in this study could be utilized as a reference for future modification of equation. Reuse of industrial inorganic wastes as alternative raw material for cement production could solve the dilemma in waste treatment and waste disposal.
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