Study On Resource Utilizations Of Low-grade And Boron Mud

In our country, there are abundant magnesium resources. With the rapid development of national economy, the consumption of magnesium resources, especial mineral magnesium resources, is great more and more, the reserve of high-quality resources reduses rapidly, so it becomes increasingly urgent in the study for utilization of low-grade magnesite and other waster materials of MgO. In this dissertation, a green new technology, ammonium sulfate calcination method, was designed to realize the comprehensive utilization of low-grade magnisite and boron mud. The process flows were gotten through, the processing parameters were optimized, the chemical, (NH4)2SO4, realized recycling, the reaction kinetics was studied, the products of MgSO4·7H2O, Mg(OH)2 and MgO were prepared. Taking self-made MgO and CaO as raw materials, MgO-CaO clinker was prepared; taking nano-Mg(OH)2 as raw materials, fireproof paint was prepared. The following conclusions were obtained.(1) Compound of magnesium was prepared through ammonium sulfate calcination method from low-grade magnesite. The technological conditions were optimized through single factor experiments and orthogonal experiments. The results indicate that the conversion rate of Mg increases with the increasing of calcination temperature first then decreases, increases with the increasing of calcination time and mol ratio of (NH4)2SO4 and MgO. At the optimum technological conditions of alcination temperature 475℃, calcinations time 4 h, n((NH4)2SO4):n(MgO) 1.1:1, the conversion rate of Mg is 91.4%. The content of Mg in final product of MgSO4·7H2O is 9.72%, which meets feed-grade standard.(2) TG-DTA analysis was used to study the reaction mechanism of light-burned magnesia and ammonium sulfate. The results indicate there are three exothermic peaks, which are corresponding to three reactions (3) The apparent activation energies of three reactions of light-burned magnesia and ammonium sulfate were calculated through Kissinger method and Doyle-Ozawa method. The results are shown in following table.The reaction rate equations are First peak: Second peak: Third peak:(4) Compound of magnesium was prepared through ammonium sulfate calcination method from boron mud. The experiment results indicate that the conversion rate of Mg increases with the increasing of calcination temperature first then decreases, increases with the increasing of calcination time and mol ratio of (NH4)2SO4 and MgO. At the optimum technological conditions of alcination temperature 475℃, calcinations time 4 h, n((NH4)2SO4):n(MgO) 1.1:1, the conversion rate of Mg is 84.0%.(5) TG-DTA analysis was used to study the reaction mechanism of MgSiO3 and ammonium sulfate. The results indicate there are three exothermic peaks, which are corresponding to four reactions (6) The apparent activation energies of first and second reactions of MgSiO3 and ammonium sulfate were calculated through Kissinger method and Doyle-Ozawa method. The results are shown in following table.(7) Micro-Mg(OH)2 was prepared through ammonia precipitation method from self-made MgSO4 solution. The studies show that the precipitation rate of Mg increases with the increasing of initial concentration of Mg2+, reaction time, pH value and ageing time, increases first then decreases with the increasing of reaction temperature and ageing temperature. The optimum technological conditions obtained through experiments are initial concentration of Mg2+ 2.0 mol·L-1, reaction temperature 60℃, pH value 11, reaction time 60 min, at that time, the precipitation rate of Mg is 90.1%. XRD analysis indicates the product is Mg(OH)2 and well crystallized. SEM test indicates the grains are flower-like spherical, the granularity is uniform, the average grain size is about 2.5μm, the dispersivity is good. TG-DTA analysis indicates the decomposition of Mg(OH)2 occures at 310～390℃.(8) The studies of Mg(OH)2 crystal growth kinetics of ammonia precipitation method indicate the mass of Mg(OH)2, i.e. the precipitation rate of Mg, and the average grain size of crystal increase exponentially with the reaction time. With going on of precipitation reaction, the growth rate of crystal decreases gradually. The relations of m-t and D-t are(9) Taking self-made high-purity micro-MgO and CaO powder as raw materials, high-prity MgO-CaO clinker was prepared through one step sintering technics. The bulk density can be up to 3.31 g·cm-3, the apparent porosity is 1.08%. Because the impurities in self-made MgO-CaO clinker are few, the sintering character and hydration resistance are better than those of saled clinker. Adding TiO2 micro-powder in material can promotes the sinter properties of clinker, the optimum added amount is 1%, at that time, the hydration resistance improves greatly, after that the improvement is not significant.(10) Nano-Mg(OH)2 powder was prepared taking the mixed alkaline solution of ammonia and NaOH as precipitator. The studies indicate that the average particle size of powder decreases first then increases with the increasing of initial concentration of Mg2+ and reaction temperature, increases with the prolonging of reaction time and ageing time, decreases with the increasing of stirring rate. When initial concentration of Mg2+ is 2 mol·L-1, reaction temperature is 50℃, reaction time is 60 min, stirring rate is 900 r·min-1, ageing time is 90 min, the morphology of Mg(OH)2 particles is hexagonal flake, the grain size is 50～140 nm, the average diameter is 91.4 nm.(11) Taking self-made nano-Mg(OH)2 powder as fire retardant, fireproof paint was prepared. According to the appearance properties, dehydration times and resistance properties of fireproof coat, the optimum formulation was confirmed. This paint meets national first class technical standard, which and TG-DTA results indicate that self-made nano-Mg(OH)2 powder has good flame retardancy.