Research On Preparation And Properties Of Coal Gangue-Bauxite Proppant Materials

unconventional natural gas reservoirs are characterized by low porosity and low permeability,which can be realized commercially by means of hydraulic fracturing technology.Proppant is ceramic material that acts as a support in the fracture during hydraulic fracturing to maintain high conductivity channels,with the aim of achieving efficient extraction of unconventional gas deposites.At present,the raw material for preparing proppant material is mainly bauxite,and the main problems it faces are:China’s bauxite resource endowment is poor,and the contradiction between supply and demand has existed for a long time;excessive and destructive mining of bauxite resources causes waste of resource,which also aggravates the shortage of bauxite resources,leading to the rising cost of raw materials,and the search for low-cost partial replacement of bauxite for the preparation of proppant material has become an urgent problem to be solved.Coal gangue as one of the largest solid wastes stockpiles,the main chemical composition of Al₂O₃ and Si O₂,and the chemical composition of low-grade bauxite is simila,with the composition of the preparation of proppant material conditions.If coal gangue can be used to partially replace bauxite as the raw material of proppant,it will significantly reduce the production cost of the material,and also solve the environmental pollution of coal gangue,and open a new way for the resource utilization of coal gangue.Base on the above,this paper used coal gangue and bauxite as the main raw materials to prepare silicon-rich,mullite and aluminum-rich proppant materials with Al-Si ratio（mass ratio）of 2.1:1,2.55:1,and 3:1,respectively,by solid-phase sintering process.The effects of raw material gringding time,sintering temperature and Al-Si ratio on the microstructure and mechanical properties of proppant materials were investigated using orthogonal experiments;the mechanisms of CaO,MnO₂,K₂CO₃ enhancement and toughening in different series of proppant materials was studied.The main findings of this study are as follows:（1）The effects of sintering temperature,gringding time and Al-Si ratio on the mechanical properties of coal gangue-bauxite system proppant materials were investigated using orthogonal experimental methods,and it was found that the grinding time was the most sigificiant factor affecting the mechanical properties of the system.The raw materials particals with small d₅₀ and high surface energy after 9 h grinding can greatly improve the sintering driving force,promote nucleation and growth of crystalline phase,and accelerate the solid phase reaction and sintering,and the grinding time was determined as 9 h.The proppant materials with Al-Si ratio of 2.1:1,2.55:1,3:1 reached the maximum flexural strength at1200℃,which were 104.5 MPa,117.8 MPa and 124.4 MPa,respectively.（2）MnO₂ was added to different series of proppant materials,separately,and the effect of sintering temperature and addition amount on the properties was investigated to obtain the strengthening mechanism of silica-rich proppant materials.The results showed that the flexural strength of Al-Si ratio 2.1:1 series increased by 17%and 20.4%,respectively,compared with the maximum value without addition sintered at 1200℃and 1300℃,and the flexural strength of the Al-Si ratio 2.55:1 series increased by about 13.6%.It is found that Mn⁴⁺ions formed a finite solid solution with inequivalently substitution of Al³⁺ions,accompanied by the creation of Al³⁺icon vacancy,and the released Al³⁺ions formed mullite nucleation with amorphous Si O₂;the solid solution produced lattice distortion to active the lattice,which also promoted mullite nucleation.Meanwhile,the Al-Si ratio of 2.1:1、2.55:1 series proppant mteriasl have a liquid phase environment favorable to mullite grain growth,and the mullite nucleation and growth.The solid solution strengthening mechanism generated by the addition of MnO₂ enchances the mechanical properties of the Al-Si ratio of 2.1:1and 2.55:1 series.（3）CaO was added to different series of proppant materials,separately,and the effect of sintering temperature and addition amount on the properties was investigated to obtained the strengthening mechanism of alumium-rich proppant materials.Sintered at 1300℃,the Al-Si ratio of 2.55:1 and 3:1 series at the addition amount of 2 wt.%increased the flexural strength by 25.5%and 15%,respectively,compared with the maximum value without the addition.The results show that the rate at which Al₂O₃ reaches the critical value of nucleation concentration controls the growth rate of mullite,and the amount of liquid phase of Al-Si ratio 2.55:1 and 3:1 series materials is less than that of Al-Si ratio 2.1:1series,while the content of Al₂O₃ is more than that of Al-Si ratio 2.1:1 series,therefore,the rate of reaching the critical value increases,which promotes the anisotropic growth of mullite grains and plays a toughening and strengthening effect on mechanical properties effect.Therefore,the addition of CaO has a stronger strengthening effect on the flexural strength of Al-Si ratio 2.55:1 and 3:1series.（4）adding K₂CO₃ to different series of proppant materials,the nucleation of mullite depends on the amount of Al₂O₃ dissolved in the amorphous mass from the innovative perpectives of liquid phase amount,liquid phase viscosity and capillary force generated by the liquid phase,the amount of liquid phase increases,the dissolvable amount increases,mullite nuclei are formed in large quantities,the distance between grains is shortened under the capillary gravitational force generated by the liquid phase,which limits the space for mullite growth,and the refined mullite grains improve the mechanical properties of different series of proppant materials.