Low-grade Diatomite Purification And Preparation And Performance Of TiO₂/g-C₃N₄/Diatomite Photocatalyst

Diatomite is a non-metallic mineral resource formed after the death of ancient single-celled algae and radiolarians,whose remains have been accumulated for about 10,000 to 20,000 years,with large specific surface area,stable chemical properties and high porosity,and is widely used in chemical,light industry and building materials.At present,the processing and utilization of diatomite in China is still dominated byⅠandⅡhigh grade resources,and there are few researches on new purification technologies ofⅥlow-grade diatomite and new directions for the application of high value-added functional mineral materials,duing to the complexity of associated minerals of poor quality resources.In response to this situation,the reserch aims to identify a purification technology of VI low-grade diatomite and the preparation of purified diatomite concentrate into high-performance TiO₂/g-C₃N₄/diatomite photocatalyst;at the same time,the mechanism and scientific problems of each main process of purification and the relationship between the preparation process,structure and properties of the photocatalyst are studied in depth.The main results are as follows.（1）Reserch of process mineralogy shows that the SiO₂ content of diatomite from Heilongjiang is 59.22%,which is a VI low-grade diatomite,mainly composed of opal,kaolinite,montmorillonite,quartz,feldspar,mica,hematite,magnetite,rhodochrosite and other minerals,and the content of amorphous minerals is about 12.39%.The sieving yield of-0.025 mm particle size fraction is 79.47%,the particle size characteristic D₅₀ is 9.38μm,the particle size distribution characteristic D₉₇ is 65.37μm,the volume average diameter D_（4,3）is 15.10μm,the area average diameter D_（3,2）is 4.15μm,and the length average diameter D_（1,0）is 1.31μm.The mass of the raw diatomite changed by 6.42%during the heating process,which is attributed to the processes of free water（crystalline water）removal,metal oxidation,organic matter combustion,impurity decomposition,mineral phase change,etc.The main diatoms of the raw diatomite are round sieve algae and straight chain algae,with diameters of about 10.00～50.00μm,and the vein minerals cover the diatom surface non-uniformly or block the diatom pore channels,and the atomic percentages of Si,Al,and Fe are 65.74%,24.90%,and 9.36%,respectively.The N₂ adsorption-desorption isotherms are type IV H3 hysteresis loops,which are the typical mesoporous structure feature,where the surface area of-0.025 mm particle size fraction is 47.0258 m²·g^-1,the most available diameter is 3.8652 nm,the average pore diameter is 9.3405 nm,and the pore volume is0.1505 m L·g^-1.In addition,the surface roughness and spatial geometric complexity of the mesopores in the-0.025 mm particle size fraction are relatively high,while the geometric parameters of the macropores are relatively low.The pore structure of raw diatomite has different characteristic parameters in different particle size fractions,among which the mesopore structure is better characterized in the fine size fraction.（2）Reserch of low-grade diatomite purification experiments shows that the combined scrubbing-magnetic separation-acid leaching-roasting process is used with scrubbing concentrate of 25%,scrubbing speed of 1300 r·min^-1,scrubbing time of 30 min,settling time of 0.5 min,siphoning depth of 13 cm,dosage of 0.3%Na OH,dosage of 0.3%（Na PO₃）₆,magnetic field intensity of 1.91 T,sulfuric acid concentration of 26.63%,acid leaching temperature of 130℃,acid leaching solid-liquid ratio of 1:4,acid leaching speed of 150 r·min^-1,acid leaching time of 6 h,roasting temperature of 600℃ and roasting time of 3 h.The SiO₂ content of diatomite concentrate is 86.93%with a recovery of 64.51%,Fe₂O₃ content of 1.24%with a recovery of 9.98%,Al₂O₃content of 6.75%with a recovery of 17.87%.Compared with the raw diatomite,the SiO₂ content increased by 27.71%,the Al₂O₃ content decreased by 11.57%,and the Fe₂O₃ content decreased by5.61%.The diatomite concentrate is mainly composed of minerals such as opal and quartz,and the contents of feldspar,kaolinite,montmorillonite,mica,magnetite and rhodochrosite are significantly reduced;the N₂ adsorption-desorption isotherm of diatomite concentrate is type IV H3 hysteresis loop,the specific surface area is 77.2955 m²·g^-1,the most countable diameter is10.1449 nm,the average pore diameter is 12.2703 nm,and the pore volume is 0.3287 m L·g^-1;the diatom surface-covered and pore-clogged vein minerals in the diatomite concentrate are effectively separated,and the pore structure characteristics are significantly optimized.（3）Reserch on the purification mechanism of low-grade diatomite shows that D_F1=2.4364,D_F2=2.6006,D_M1=3.6000 and D_M2=8.0992 of diatomite concentrate are smaller than that the fractal dimension of raw diatomite.The surface roughness and spatial geometric complexity of the mesopore and macropore are significantly improved,which is favorable to the interaction between diatomite and other substances in the process of adsorption and diffusion.Fe and Al are not completely controlled by a certain model during acid leaching,and the internal diffusion contraction nucleation model 1-2/3X-（1-X）^2/3 with R²≥0.8277 is applicable,and the degree of applicability is internal diffusion>chemical reaction>external diffusion.The two thermal reaction stages from 120℃ to 315℃ and 315℃ to 550℃ in the roasting process correspond to the same thermal reaction model,the random nucleation and subsequent growth model[-ln（1-α）]²with R²≥0.9056 is applicable,and the degree of applicability is the random nucleation and subsequent growth>3D diffusion>tertiary chemical reaction;the apparent activation energies of the two stages are 17.36 k J·mol^-1 and 92.59 k J·mol^-1.Leaching concentrate is easy to thermochemical reactions in the low and medium temperature stage（below 315℃）,consuming large amounts of combustible masses,while the remaining materials have lower thermal reaction activity and require higher energy excitation.（4）Reserch of TiO₂/g-C₃N₄/diatomite photocatalyst preparation and performance shows that the composite has a small forbidden band width and the best photocatalytic performance for methylene blue（MB）solution with 99.51%degradation rate when the ratio of active factor is 5:1,the ratio of carrier is 1:0.1,the polymerization temperature is 550℃and the polymerization time is 2.0 h.The interaction effects of carrier ratio,polymerization temperature and polymerization time on the photocatalytic performance of the composites are significant and all reach a significant level,with the degree of influence being carrier ratio>polymerization temperature>polymerization time.Among them,the influence of carrier ratio on the degradation efficiency of MB solution is the most significant.The best degradation effect of the composite on MB solution is 98.77%in the 1st time;the degradation effects are similar in the 2nd and 3rd times,with the degradation rate of 90.59%and 89.16%,respectively,and the degradation effect tends to be stable;after three photocatalytic cycles,the composite still has a high degradation rate on MB solution and can be recycled in actual production.（5）Reserch on the reaction mechanism of TiO₂/g-C₃N₄/diatomite photocatalyst shows that the degradation process of the composite for MB solution is applicable to the one-level kinetic model,and controlled by a combination of diffusion effects.Benzoquinone（BQ）has the most obvious inhibitory effect on MB solution photocatalytic reaction,followed by disodium ethylenediaminetetraacetate（EDTA-2Na）and tert-butanol（TBA）,and the degradation rates are89.56%,94.99%and 96.82%respectively;under the synergistic effect of TiO₂,g-C₃N₄ and diatomite,O₂^-·、h⁺、·OH can oxidize with MB molecule,so that the strong chromophore（-C=N-）within the MB molecule is attacked,the benzene ring is opended by oxidation,and the large conjugated structure（nitrogen-sulfur heterocycle）is destroyed and degraded to inorganic small molecules,H₂O and CO₂,where O₂^-·mainly controls the photocatalytic process of the composite for MB solution.The loading of diatomite with TiO₂/g-C₃N₄ increases the possibility of visible light photocatalytic reaction on the TiO₂ surface and extends spectral response range of TiO₂;it also increases the quantum efficiency and the number of active sites on the surface of g-C₃N₄.There are 92 figures,32 tables and 228 references in this dissertation.