Research On Extraction Of Alumina From Coal Gangue And Preparation Of Functional Materials

Coal gangue is a industrial by product discharged during the process of coal exploitation and processing,and is often considered as a kind of industrial solid waste.China’s energy consumption is dominated by coal,causing massive coal gangue emissions.Due to low utilization,the accumulated storage of coal gangue has exceeded 5 billion tons,resulting in serious environmental problems in China.At present,the technologies for the large-scale utilization of coal gangue is mainly concentrated in the fields of brick making,power generation,road paving,backfilling and other low added value uses.Coal gangue in many areas of China contains many valuable minerals and elements.To achieve high added value utilization of valuable elements and minerals existing in coal gangue,based on the previous research of our group,this paper puts forward the research ideas of extracting alumina from coal gangue by the sub molten salt method and preparation of functional materials using existing minerals in coal gangue.Hence,the thermodynamic research,leaching process and phase transformation law during the alumina extraction process were first studied in this paper.Then on the base of these,in order to realize the high value utilization of the leached residue,the technology and mechanism on preparation of mesoporous calcium silicate were studied.And finally,nano-photocatalytic functional materials were synthesized by using bentonite and kaolin rich in coal gangue,the effect and mechanism of photo-oxidation of As（Ⅲ）were also studied.The main results and progresses are as follows:（1）The thermodynamic characteristics of coal gangue reaction in the NaOH sub molten salt medium were systematically analyzed.The results indicated that kaolinite,quartz,hematite and rutile existing in gangue,can be decomposed and converted into corresponding sodium salts during the studied temperature range.When calcium hydroxide is added to the reaction system,the main silicon-containing phase in gangue tends to transform into NaCaHSiO₄phase,which is beneficial for alumina recovery.（2）The influences of leaching temperature,calcium-silicon ratio,NaOH concentration,and alkali-ore ratio on alumina extraction rate were investigated via orthogonal experiment.The results confirmed that the temperature had a remarkable effect on the recovery of alumina.Based on a series of single factor experiments,the optimal process conditions for the extraction of alumina from coal gangue by sub-molten salt method were determined as NaOH concentration of 47.5%,alkali-ore ratio of 6,temperature of 260℃,CaO to SiO₂mass ratio of 1.2:1,stirring speed of650rpm and reaction time of 120min.Under these conditions,the concentration of Al₂O₃in the leached liquor increased significantly,meanwhile,the caustic ratio of the leached liquor decreased to about 12.50.The content of Al₂O₃and alumina-silica ratio of the leached residue fell to 1.64%and 0.04,respectively.The main phase of leached residue was confirmed as NaCaHSiO₄.（3）Through XRD,SEM,ICP-OES and other analytical methods,the phase transformation law during the alumina extraction process was studied.The results showed that various factors such as leaching temperature,calcium-silicon ratio,NaOH concentration,and alkali-ore ratio can affect the phase transformation during the alumina extraction process.When the reaction conditions are insufficient,the alumina-containingphasesintheleachedresiduemaybe1.2Na₂O·0.8CaO·Al₂O₃·2SiO₂·H₂O,Na₈(Al₆Si₆O₂₄)（OH）₂（H₂O）₂and Ca₂Al₂SiO₆（OH）₂,which is the main reason for alumina loss in the alumina extraction process.By controlling the reaction conditions,the formation of alumina containing phases can be avoided,which helps to recover more alumina from coal gangue.（4）Based on phase control research,a method for preparing mesoporous calcium silicate by hydrothermal method using alumina leached residue was proposed.The effects of factors such as hydrothermal temperature,NaOH concentration,liquid-solid ratio,and reaction time were investigated systematically;meanwhile,the kinetics of the reaction process for preparing mesoporous calcium silicate was also analyzed.The results confirmed that under the conditions of hydrothermal temperature of 190℃,NaOH concentration of 30 g/L,liquid-solid ratio of 8 and reaction time of 300min,the leached residues can transform to calcium silicate effectively.The calculated activation energy of the reaction was 23.11k J/mol.The interface chemical reaction was determined as the major control step and diffusion-controlled as the minor control step.（5）The prepared mesoporous calcium silicate materials were systematically analyzed,and the adsorption ability,adsorption kinetics and thermodynamics of mesoporous calcium silicate were studied in detail.The results confirmed that the alumina leached residue had transformed to porous calcium silicate after hydrothermal treatment,and the pore size mainly concentrated between 2-20nm.Compared with leached residues,the specific surface area and pore volume of mesoporous calcium silicate both increased significantly.The adsorption process of methylene blue by mesoporous calcium silicate can be described by the quasi-second-order adsorption reaction model.The adsorption standard enthalpy changeΔH was 56.45k J/mol,which indicated that chemical absorption played an important role during the adsorption process.（6）Bi₂WO₆/bentonite（BWO/BENT）and g-C₃N₄/coal series kaolin（g-C₃N₄/CK）composite photocatalysis functional materials were prepared by hydrothermal and thermal polymerization method using gangue containing bentonite and coal series kaolin as carriers,and effect and mechanism for photo-oxidation of As（Ⅲ）were studied.The properties of the prepared samples were characterized through X-ray diffraction,transmission and scanning electron microscopy,UV-visible diffuse reflectance spectroscopy,X-ray photoelectron spectroscopy,and photoluminescence spectroscopy.The results showed that BWO/BENT and g-C₃N₄/CK composites had improved ability for photocatalytic oxidation of As（Ⅲ）.Such a synergistic effect could be benefit from decreased band gap,reduced aggregation and enhanced electron–hole separation efficiency.