The Study On The Influence Factors Of Hydration And Hydrothermal Processes To A Series Of Magnesium Compound Products

Hydromagnesite?4MgCO₃·Mg?OH?₂·4H₂O?is a natural carbonate mineral with abundant reserves and low CaO content,so it is a high-quality mineral raw material for preparing active magnesium oxide.However,due to the limitations of many factors such as geography,transportation and technology,it is pretty difficult to hydromagnesite.Up to now,there has been little research about hydromagnesite in China.Therefore,it is the biggest problem to develop and utilize hydromagnesite resources for a series of high-quality magnesium compounds.Magnesium hydroxide is one of the important magnesium compounds.Because of its advantages of flame retardant,safety,non-toxicity and smoke suppression,it has become a research hotspot in the field of new inorganic flame retardants.The preparation methods of magnesium hydroxide include physical processing and chemical synthesis.It is an important subject to product magnesium hydroxide by light burned magnesia obtained from magnesium ore,which is needed to be further studied.And it is of great significance for the comprehensive utilization of magnesite resources.Using hydromagnesite as the raw material,the activity of magnesium oxide was studied under different calcination conditions.During the calcination process,hydromagnesite lost crystalline water and hydroxide group,and then released carbon dioxide molecule while left magnesium oxide powder.The magnesium oxide obtained under different calcination conditions had different reactivity,resulting in different hydratability.The activity of calcined product was measured by citric acid method and hydration method.The results showed that the calcination temperature and time had great effects on the activity of magnesium oxide,followed by the heating rate.The high-activity magnesium oxide was produced under the optimum conditions that the calcination temperature was 600?,time was 0.5 h and heating rate was 10?·min^-1.Then the hydration kinetics of the high-activity magnesium oxide was studied.The hydration curves of magnesium oxide were made under 25°C,45°C and 65°C respectively,and the data of hydration rate were fitted with different dynamic equations.The results showed that the hydration process of active magnesium oxide was controlled by chemical reaction first,and gradually changed into internal diffusion control in the last period.According to the Arrhenius equation,the activation energies of the hydration reaction of active magnesium oxide were 27.18 kJ·mol^-1 and 6.076kJ·mol^-1,respectively.With the light burned magnesia as raw material and acetic acid as hydration agent,several experimental factors were investigated by single factor method at normal pressure,including reaction temperature,reaction time,stirring speed,solid to liquid ratio,acetic acid concentration and MgO particle size.In the MgO-CH₃COOH solution system,CH₃COOH decomposed into CH₃COO^-and H⁺,then CH₃COO^-adsorbed on the surface of MgO and combined with Mg²⁺to form the intermediate complex CH₃COOMg⁺,thereby promoting the hydration reaction process and generating more magnesium hydroxide,there was no other substances introduced into the hydration product.The optimum reaction conditions for the magnesium oxide hydration were determined as follows:the reaction temperature of 80°C,reaction time was 2 h,stirring speed was 1400 r·min^-1,acetic acid concentration was 0.07 mol·L^-1,magnesium oxide was solution ratio was 1:30,magnesium oxide particles was 500 mesh.Under these conditions,the hydration rate was 93.81%.The hydration products were characterized by XRD,SEM,TG-DTG and FT-IR.The results indicated that the particles were"card room",and acetic acid had no effect on the structures.Since the magnesium oxide could not be completely transformed into Mg?OH?₂under normal pressure,and the morphology of hydration products were irregular,agglomerative severely and the crystallization performance was unsatisfactory,the modification studies of Mg?OH?₂ precursor were carried out under high pressure.The nonionic surface modifiers of PEG6000 and PVP were used as dispersants,and the modification effects of single and complex dispersants were investigated.It was found that the effect of single PEG6000 was better than that of PVP and composite modification,so PEG6000 was chosen as the modifier.Then the effects of PEG6000dosage,Mg?OH?₂ dosage,KOH mineralizer concentration,hydrothermal temperature and time on the morphology,structure and dispersibility of hydrothermal products were also studied.Compared with atmospheric hydration,the product only contained magnesium hydroxide under the effect of PEG-KOH.It was found that the growth direction of the crystal had changed,the characteristic diffraction peak intensities I₀₀₁/I₁₀₁01 and I₀₀₁/I₁₁₀ value had increased,the morphology of the product was changed from"card room"agglomerates to regular hexagonal sheet-like structures,the thickness increased obviously and the dispersibility was greatly improved.The BET value decreased from 71.1 m²/g to 21.3 m²/g...