Drawing Of Tetrahedral Phase Diagram Of Al₂（SO₄）₃-AlCl₃-Al（OH）₃-H₂O System At 75℃ And Determination Of Several New Polyaluminum Compounds By Powder Electron Diffraction Method

The solution chemistry of aluminum,especially the morphological chemistry of the hydrolyzed polyaluminum,which is not only the key to revealing the mechanism of interaction between aluminum flocculants and colloidal particles thereby to select the efficient flocculant objectively in water pollution control chemistry,but also the key to exploring the migration and transformation law of aluminum thereby to prevent and control aluminum pollution effectively in environmental chemistry.The hydrolysis process of aluminum salt is very complicated,and there are many kinds of intermediates in different species between monomer Al（H₂O）₆³⁺and Al（OH）₃.To study the formation and transformation mechanism of polyaluminum species,pure polyaluminum compounds need to be separated as much as possible from complex and changeable hydrolysis solutions and their structures further well resolved.The solid-liquid equilibrium isothermal phase diagram of the aluminum salt hydrolysis system at different temperatures is an important basis for effectively controlling and separating various polyaluminous compounds.It is possible to control the composition of the solution falling into a single-phase crystalline region of a hydrolysis product in a certain temperature phase diagram so that it is precipitated separately.However,there are only a few kinds of polyaluminum compounds which can be separated from Al Cl₃-Al（OH）₃-H₂O ternary system.Although the addition of Na₂SO₄to precipitate the polyaluminum forms as sulfate or sulfate chloride is a widely used method in the literature,the Na⁺ions will bind to polyaluminum and interfere with the study of the formation and evolution of the polyaluminum forms themselves,and more troublesome is that the system will become a complex seven-component system of Na₂SO₄-Na Cl-Na OH-Al₂（SO₄）₃-Al Cl₃-Al（OH）₃-H₂O,which makes it more difficult to determine the phase diagram and control the synthesis.In this thesis,a solid-liquid equilibrium tetrahedral isothermal phase diagram of Al₂（SO₄）₃-Al Cl₃-Al（OH）₃-H₂O system at 75°C under atmospheric pressure was designed and measured.According to this diagram,Al₂（SO₄）₃was used to capture more new polyaluminum forms in the Al Cl₃-Al（OH）₃-H₂O system and crystallize them as a single phase,respectively.The method established in this thesis for determining and plotting the tetrahedral isothermal phase diagram of the Al₂（SO₄）₃-Al Cl₃-Al（OH）₃-H₂O system at 75°C under atmospheric pressure is described as follows:firstly,the required material ratios at each feeding point are calculated by selecting a suitable feeding cross section;The alkalinity of the aluminum chloride and aluminum sulfate solution was adjusted by Al powder in the low alkalinity zone and by Al（OH）₃in the high alkalinity zone during the hydrolysis reaction;After sufficient solid-liquid equilibrium,the total components,saturated mother liquor and wet solid samples were taken out,and the content of each element was determined by solution X-ray fluorescence spectrometry after depolymerizing the samples with KOH solution respectively;Finally,the tetrahedral coordinate system with the sum of mass fractions of four components equal to 1 was converted into a Cartesian coordinate for the calibration point connection to obtain the tetrahedral isothermal phase diagram.Three-dimensional electron diffraction structure analysis results show that the structuralformulaare[Al₄（OH）₈（H₂O）₆]（SO₄）₂·2H₂Oand[Al₄（OH）₈（H₂O）₆]（SO₄）₂·1.5H₂O.The structure of the polyaluminum specie[Al₄（OH）₈（H₂O）₆]⁴⁺（step─Al₄）in these two compounds are similar to that in aluminite mineral（[Al₄（OH）₈（H₂O）₆]（SO₄）₂·8H₂O）.Therefore,they are two new polyaluminum compounds,which can be regarded as two new members of the aluminite mineral family.It was confirmed that minerals can be synthesized under laboratory conditions as long as the chemical composition of the solution is controlled under the guidance of the phase diagram.It was again shown that the precipitation of all polyaluminum forms in solution follows the phase diagram pattern of their crystalline formation.The tetrahedral phase diagram creates a broader space for the precipitation of many aluminum salt hydrolysis products than the three-dimensional phase diagram,laying the foundation for the controlled synthesis of more polyaluminum compounds.Three-dimensional electron diffraction structure analysis result shows that the structural formula is[Al₈（OH）₁₄（H₂O）₁₈]（SO₄）₄Cl₂·2H₂O.The structure of polycation[Al₈（OH）₁₄（H₂O）₁₈]¹⁰⁺（P─Al₈）in this compound is similar to that in[Al₈（OH）₁₄（H₂O）₁₈]（SO₄）₄Cl₂·14H₂O reported by Zhang Xiaojie in 2018.The step─Al₄sulfate and P─Al₈sulfate chloride synthesized by this work were all captured from spontaneously hydrolyzed Al Cl₃-Al（OH）₃-H₂O system by adding Al₂（SO₄）₃as the fourth component under the guidance of tetrahedral phase diagram.A transformation from“extensive cultivation less harvest”to“catching a turtles in a jar”has been realized for polyaluminum species capturing.Finally,Al Cl₃solution was forcedly hydrolyzed with KOH solution and aged at50℃to capture the polyaluminum species.X-ray diffraction structure analysis shows that the structural formula is[Al_30.5O₈（OH）₆₀（H₂O）₂₄]（2,6-NDS）₆Cl_3.5·75H₂O.The structure of the polyaluminium form[Al_30.5O₈（OH）₆₀（H₂O）₂₄]^15.5+(Al_30.5)is that Al₃₀do not share a coordination with two occupancy 1/4 Al(Al_1/4（OH）₂[（H₂O）_1/4]₄)suspended from the points on either side of its waist to form the discrete existing Al_30.5morphology.Different from the chain-like Al_30.5discovered by our research group,it is a precursor for forming a chain-like structure.