Study On The Process And Kinetics Of Alumina Extraction From Coal Gangue By Pressurized Acid Leaching

Coal gangue(CG)is one of the largest industrial solid wastes in China.The accumulation of a large amount of CG will not only cause serious environmental problems,but also waste the alumina resources associated with it.At the same time,Bauxite resources are scarce in our country and its dependence on foreign countries is very high.Therefore,the development of extracting alumina technology from CG is of great significance for alleviating the“three wastes problems”caused by CG storage,reducing the degree of external dependence on Bauxite,and achieving sustainable economic and social development.The CG in Panzhou,Guizhou was used as raw material in this thesis.The basic properties of CG have been analyzed,such as industrial analysis,mineral composition,chemical composition,and theoretical acid consumptiontand thermal power.Based on these analyses,direct pressure acid leaching process for extracting alumina from CG was proposed.At the same time,the process was optimized and the characterization of acid leaching residue was studied.Based on the experimental results,the neutralization-pressure acid leaching process for extracting alumina from CG was proposed.The process optimization and characterization of leaching residue of the two-step leaching process have also been systematically studied.The results showed that this method can significantly reduce acid consumption,reaction temperature and free acid concentration in aluminum extraction solution.Finally,the kinetics of the extraction of alumina from CG in direct pressure acid leaching process and neutralization residue pressure acid leaching process were studied.The main conclusions of this paper were as follows:(1)The CG used in this experiment is a typical low-calorie CG,the mineral composition of it are kaolinite,quartz,carbonate,hematite and anatase.The SiO2,Al2O3,Fe2O3,TiO2,CaO and MgO of its chemical composition content are respectively 39.41%,18.41%,12.38%,4.43%,2.92%,2.06%,and the theoretical acid anhydride ratio is 1:1(mass ratio of 98%concentrated sulfuric acid to CG).(2)The direct pressure acid leaching process was optimized.The leaching rate of alumina reaches 99.32%at reaction temperature of 150℃,reaction pressure of 0.38Mpa,reaction time of 130 min,acid gangue ratio of 1.3:1,liquid-solid ratio of 4:1(sulfuric acid solution and solid Mass ratio,in g/g).And the main components of the acid leaching residue obtained under this condition are SiO2 and TiO2,the total content of the two is>98.00%.Compared with the atmospheric acid leaching process,this method has the advantages of less acid consumption,lower reaction temperature and shorter reaction time,which greatly improves the production efficiency of extracting alumina from CG.Howrvre,CG contains carbonate minerals,which react violently with sulfuric acid solution,it has the disadvantages of unsafe.At the same time,the concentration of free acid in the aluminum extraction solution is high,which is not conducive to product separation.Therefore,the neutralization-pressure acid leaching process to extract alumina from CG was proposed.(3)The minimum acid consumption of carbonate during the neutralization reaction is 0.2 times of the theoretical acid consumption.Based on this experiment,the process of neutralization reaction of CG was optimized.The results showed that the better process at room temperature has an acid anhydride ratio of 1:0.2,a reaction time of 120 min,and a liquid-solid ratio of 3:1.The carbonates can not be found in the neutralized slag obtained under this condition,and the iron oxide content is reduced to 7.73%,the calcium oxide content is reduced to 1.94%,the magnesium oxide content is reduced to 1.26%.At the same time,the neutralization process is accompanied by the dissolution of a small amount of amorphous aluminum-containing minerals.(4)Under the basis of theoretical acid consumption of CG,the optimal conditions of the neutralization slag pressure acid leaching process are:reaction time 120 min,reaction temperature 150℃,reaction pressure 0.38 Mpa,liquid-solid ratio 3.5:1.Under this condition,the alumina leaching rate reaches 95.77%.Under the optimal process conditions of neutralization,add fresh CG to the acid leaching solution of neutralization residue to carry out cyclic neutralization reaction.The results showed that compared with the direct pressure acid leaching process,the content of alumina and iron oxide in the aluminum extraction solution increased from 45.22 g/L to 62.20 g/L and 31.38 g/L to 57.95 g/L,the acid consumption is reduced from 1.3 times of the theoretical acid consumption to 1.2 times,and the residual free acid concentration is reduced from 10.72%to 3.78%.This method reduces acid consumption and improves acid utilization,which is beneficial to the subsequent separation and purification of alumina products.(5)The XRD,FTIR and EDS analysis results of the direct pressure acid leaching residue showed that kaolinite,carbonate and other minerals were effectively dissolved during the acid leaching process,and the SEM analysis results showed that there was no dense material formed on the surface of the acid leaching residue.Under the better technological condition,the main phases in the acid leaching residue are quartz,anatase.(6)The XRD analysis results of the neutralization slag showed that the characteristic diffraction peak of calcite gradually disappeared when the neutralization reaction time reached 40 minutes,and the characteristic peak of siderite gradually disappeared when the reaction time reached 80 minutes.When the reaction time reaches 120 minutes,the carbonates can not be found in the neutralization slag.At this time,the absorption peak of CO32-functional group can not be founed in the FTIR spectrum.The SEM spectrum showed that the microscopic morphology of the neutralization slag is fractured and microporous.(7)The XRD analysis results of the pressure acid leaching residue of the neutralization residue showed that the characteristic diffraction peak of hematite significantly weakened when the reaction temperature was raised to 130℃.When the reaction temperature was raised to 140℃,the characteristic diffraction peaks of hematite gradually disappear,and some characteristic diffraction peaks of kaolinite also significantly weakened.When the reaction temperature was raised to 150℃,the characteristic diffraction peaks of kaolinite basically not observed in the acid leaching residue,only the characteristic diffraction peaks of quartz and titanium dioxide were found.At this time,the absorption peaks of Al-O-H and Si-O-Al functional groups can not be found in the FTIR spectrum,and the needle-like morphology can not be found in the SEM spectrum.The results showed that hematite dissolves before kaolinite in the pressure acid leaching process.(8)The direct pressure acid leaching of CG and the pressure acid leaching of neutralization residue to extract alumina are typical liquid-solid phase non-catalytic reactions.The kinetic models are all unreacted nucleus reduction models,and the kinetic control types are all internal diffusion control of solid product layer(residual layer),can be fitted to surge dynamics equation:1-2/3X-(1-X)2/3=kt.The kinetic equations of the alumina leaching process were:1-2/3X-(1-X)2/3=8.6063e-30620/(RT)t and 1-2/3X-(1-X)2/3=14.93e 31543/(RT)t.The activation energies of Arrhenius were 30.62 KJ/mol and 31.54 KJ/mol,and the pre-index factors were 8.6063 and 14.93.