Phase Transformation Mechanism Of Aluminosilicate Minerals In Coal Wastes Calcined With Sodium Carbonate

Coal wastes,mainly including coal gangue?CG?and coal fly ash?CFA?,generated from coal mining,processing and utilization.Generally,CG and CFA are dumped in lands which pose a serious threat to the environment and ecology.It is of great significance to develop a comprehensive utilization of CG and CFA for the sustainable development of coal-based areas.CG and CFA,as the typical aluminosilicate wastes,contains about 17⁵0%?Al₂O₃?and 34⁷0%?SiO₂?.The preparation of aluminum or/and silicon products from CG and CFA is one of the most important value-added utilization ways.Mullite?Mul?and kaolinite?Kln?with the stable structure and poor activity are generally the main aluminosilicate minerals in CG and CFA,respectively.It is important to activate the reactive activity of these aluminosilicate for the utilization of coal wastes.Thermal activation and mechanical activation are the common methods by which CG or CFA can be partly activated,but it has been confronted with low activation efficiency of aluminosilicate minerals;in comparison,sodium carbonate?Na₂CO₃?is widely used as a good additive for the completely activation of CG and CFA.However,this method was blocked in its industrial application due to the large consumption of Na₂CO₃.A clear understanding of the mineral compositions of CG and CFA and the phase transformation mechanism of each mineral calcined with Na₂CO₃ is of great significance for the further process optimization.In this thesis,the relatively content of each crystalline and amorphous phase in coal wastes was measured firstly;then the phase transformation mechanism of Mul or Kln calcined with Na₂CO₃ were investigated respectively;the effect of silica on the aluminosilicates calcined with Na₂CO₃ were also investigated;based on the above results,the activation process of coal wastes calcined with Na₂CO₃was optimized.The main results obtained in this thesis are as follows:?1?Quantitative analysis of crystalline and amorphous phases in coal wastes was performed.The presence of organic matter has interference to the quantitative analysis of crystalline minerals in CG,which led to a reduction in the precision and accuracy.The interference increased with the increasing of the organic matter content.An approximate and efficient method for the quantitative analysis of crystalline minerals in CG was established without the separating of organic matter and crystalline minerals when the organic matter content in CG is<30 wt.%.On the other hand,a comprehensive quantitative analysis of crystalline and amorphous phases in CFA was also performed combining with the alkali separation technology of amorphous aluminum-silicon matrix?AASM?and amorphous silicon?AS?.The results indicated that the AS in CFA can be removed under certain alkali leaching conditions.The terminating point of the removal of AS was marked by the forming of hydroxysodalite in desilicated CFA.?2?Thermal behavior,phase transformation and microstructure change for the mixture of Kln-Na₂CO₃ and Mul-Na₂CO₃ at different temperatures were investigated through experimental investigation and quantitative calculation,respectively.The results showed that the[AlO₆]octahedron in Kln gradually transformed into the[AlO₄]tetrahedron by dehydroxylation at 300⁶00°C,and the bridging oxygen between[AlO₄]and[SiO₄]layers together with the bridging oxygen in[AlO₄]and[SiO₄]layer further were broken with the participation of Na₂CO₃.Subsequently,all the obtained[AlO₄]and[SiO₄]as well as Na⁺ion could be recombined in a specific manner to transform into sodium aluminates,sodium silicates and sodium aluminosilicates.For the Mul-Na₂CO₃,the phase transformation mechanism could also be interpreted in terms of the combination change of different oxygen coordination.The Na₂O decomposed from Na₂CO₃ would enter in the structure of Mul through filling oxygen vacancies and then react with the oxygen atoms in[AlO₆],resulting in the formation of[AlO₄]at 500⁸00°C.With the temperature increasing,the residual AlO₆ originated from Mul was further transformed into[AlO₄].Then the phenomenon of the breaking of bridging oxygen bonds and the recombining of[AlO₄],[SiO₄]and Na⁺ion could also be happened.Especially for the SAS,nepheline and zeolite,all the generated[AlO₄]and[SiO₄]alternately recombined and formed an anion frame-like structure by sharing the oxygen atom owing to the limitation of Pauling's electrostatic valence rule.Meanwhile,Na⁺ion was filled in the anion frame-like structure of SAS,nepheline and zeolite to further maintain their charge neutrality and structural stability.?4?Effect of silica?quartz?Qtz?and AS?on the aluminosilicates calcined with Na₂CO₃ was investigated;and the results were further compared with the thermodynamic calculation of Na₂O-SiO₂-Al₂O₃.The results showed that the mass percentage of Na₂CO₃ played a key role in the phase transformation of the reaction system of Kln,Qtz and Na₂CO₃ at 850°C,determining the Na:Al:Si molar ratio of the formed sodium aluminum silicate.Compared with the reaction system of Kln-Na₂CO₃,the existence of Qtz inhibited the formation of these sodium aluminum silicates in the calcined products.The aluminosilicates with high Al:Si molar ratio can react with silica?Qtz/AS?with the addition of Na₂CO₃.However,an excess amount of silica addition in aluminosilicates can form a series of sodium silicates,which results in the excess consumption of Na₂CO₃.The thermodynamic calculation results were consistent with the experimental results that the Al:Si molar ratio of sodium aluminosilicates in the calcined products decreased with the adding of silica in certain amount,and then formed a series of sodium silicates.?5?Process optimization and mechanism analysis of coal wastes calcined with sodium carbonate was studied.The results indicated that the addition of red mud?RM?reduced the consumption of Na₂CO₃ during the activation of coal wastes.When the Na:Al:Si molar ratio of CG/CFA-RM-Na₂CO₃ system was within 1:1:1¹.5:1:1,the aluminosilicates in CG/CFA can be activated effectively.Compared with CG/CFA calcined with Na₂CO₃,the addition of RM reduced the consumption of Na₂CO₃ by about 30⁴6%.The main reason is that the RM could be used to adjust the Al:Si molar ratio of coal wastes to the appropriate value,avoiding the formation of sodium silicates from the subsidiary reaction of CG/CFA and Na₂CO₃.