Studies On Activation Mechanism Of Silicate By Composite Organic Acids And Its Application In Ceramics Manufacture

The phenomenon of massive silicon absorption during the growth process of silicon-absorbing plants shows that the composite low-molecular-weight organic acids can effectively break the core Si-O-Si linkage in the silicate crystal structure under normal temperature and pressure,destroy the integrity of the mineral structure,and increase the mineral reactivity.The use of activated silicate raw materials in the preparation of ceramics or other materials that require high sintering temperature can effectively reduce the firing temperature,save energy and reduce emissions.In this paper,the liquid phase was analyzed by ²⁷Al NMR and AAS tests through static experiments.The solid phase was characterized by XRD,SEM,TEM,and FTIR.The activation effect of low molecular weight organic acids on frame,layered and chain silicates minerals was systematically studies.The the atom dissolution kinetics,form of atoms,the mineral composition and lattice changes,and mineral micro-morphological changes was explored during the destruction of silicone backbone of silicate mineral by organic acids.In addition,the synergy mechanism of the efficient destruction of silicate minerals by complex organic acids was revealed.At the same time,the theoretical calculation of the dissolution process was carried out using density functional theory.The effect of organic acids on the Si-O and Al-O bonds in silicate crystals was discussed at the molecular level.On this basis,the formation of mullite whiskers from organic acid-activated kaolin was studied,and basic research on the application of composite organic acid-activated silicate minerals in ceramic production was conducted.The results are as follows:?1?The dissolution rate of Si from quartz in composite acid?oxalic acid and citric acid?is larger than that of the single acid,and the dissolution data fits well with the Elovich equation?C=a+bInt?.The surface reaction mechanism is followed by the reaction of the composite organic acids and the framework aluminosilicate mineral?alkali feldspar?.Oxalic acid dissolves Si-O in sillimanite more easily than citric acid,and citric acid dissolves Al-O more easily than oxalic acid.Under the synergetic action of oxalic acid and citric acid,alkali feldspar had the most reduction in the intensity of XRD diffraction peaks on the?040?,and the shape of the corrosion pits on the surface of the alkali feldspar increased with time:amygdala,diamond,corrugated channels,and corroded grooves on the edges.?2?In the presence of the organic acids,the activation process of layered silicate mineral?kaolinite?also follows the surface reaction mechanism.The dissolution ability of citric acid to Si in kaolinite is stronger than that of oxalic acid,and the dissolution ability of oxalic acid to Al in kaolinite is stronger than that of oxalic acid.The order of crystallinity of kaolinite after treatment with different solutions is as follows:blank>citric acid>oxalic acid>complex acid.In the composite organic acids,oxalic acid with a small size,enters the interlaminar layer along the c-axis against the?001?surface,and citric acid destroy kaolinite along the a-axis or b.The two organic acids attacked kaolinite alternately and promoted each other,so that the kaolinite mineral structure was destroyed effectively and part of the lattice lines became distorted.?3?The dissolution process of wollastonite and sillimanite in the organic acids are controlled by the diffusion step and the surface reaction step,respectively.For wollastonite oxalic acid was more effective than citric acid to attack?002?with higher Ca surface density,while citric acid is effective than oxalic acid to attack?200?with higher Si surface density.Thus,the crystal structure of wollastonite is more severely damaged in composite organic acids than that in the single acid.However,as the reaction time increases,the reaction product accumulates on the surface of the mineral,hindering the diffusion of the substance and lowering the reaction rate.For sillimanite,oxalic acid is easier to dissolve Si than citric acid,citric acid is easier to dissolve Al than oxalic acid;oxalic acid is easier to attack?210?than citric acid,while citric acid is easier to attack?120?than oxalic acid.Upon the composite organic acids,oxalic acid and citric acid alternately attack and synergize,and the sillimanite mineral dissolves faster.?4?The reaction energy of Si and Al sites in organic acids was calculated by the density functional theory,the activation energy of Al-O and Si-O bonds are reduced under the oxalic acid and citric acid molecules.The activation energies of the reaction sites under acidolysis mechanism are lower than that under the catalytic mechanism.The activation energy of Al-O bond under oxalic acid was lower than that of citric acid,while the activation energy of Si-O bond under citric acid was lower than that of oxalic acid.The carboxyl organic acid is more stable than the monodentate chelates with Si and Al.?5?TG-DSC analysis reveals that the exothermic peak of kaolinite activated with complex organic acid at the temperature of 996.6°C is reduced to 923.6°C.The activated kaolinite at940°C gained a large amount of glass phase.The introduction of calcium phosphate into activated kaolin at 1220°C obtained the longest aspect ratio of mullite whikers?⁸0?.The increase of P content is beneficial to the increase of liquid phase content,which contributes to the crystallization of mullite in accordance with its crystal habits grow along the c-axis.However,when the content of Ca₃?PO₄?₂ exceeded 4.5 wt%,the polymerization degree of P in the glass increased,resulting in the reduction of the aspect ratio of mullite whiskers.?6?The ceramic raw materials doped with kaolinite activated by composite acid obtained the best performance,the flexural strength increase from 78.23 MPa to 92.67 MPa,and needle-like mullite cross-linked with each other has appeared in the sample.After activated kaolin and calcium phosphate were both introduced into the raw materials,the sample sintering temperature was reduced from 1330°C to 1240°C.The microstructure and crystal phase composition of the sample were optimized,and the flexural strength increased to 95.31 MPa.According to thermodynamic calculations,we found that the anorthite crystal may transform to mullite crystal during calcination.