| With the decrease of bauxite grade in China,the cost advantage of Bayer process is weakened obviously.It will be difficult to treat low-grade bauxite with bauxite-silicon ratio less than 5.On the contrary,the advantages of the sintering method for processing low-grade aluminum-containing resources are increasingly prominent,but the factors such as high energy consumption,large slag amount,and serious secondary reaction restrict the development of the sintering method.Therefore,exploring innovative sintering methods with low consumption and high yield is great significance to the utilization of low-grade aluminum resources in China and the sustainable development of the alumina industry.In this paper,the pure reagents of CaCO3,SiO2,Na2CO3,MgO were used as raw materials to simulate the low-sodium lime sintering method,and modern analytical methods such as XRD,SEM-EDS,FT-IR,Raman,TG-DSC and liquid phase analysis were used.The formation and transformation mechanism of the CaO-SiO2-Na2O system and the CaO-SiO2-Na2O system with MgO,the microscopic morphology,the self-powder properties and the stability of calcium silicate in sodium aluminate solution were studied.The main conclusions are as follows:The phase type,crystal structure,self-powder property,micro-morphology and stability of sintered products were studied when C/S was 2.0（CaO to SiO2 molar ratio）and N/S（NaO2 to SiO2 molar ratio）were 0-0.2.During the sintering reaction,Na2O acts as a fluxing agent,reducing the decomposition temperature of CaCO3 from 785℃ to 764℃.When the sintering temperature is 1400℃,as the N/S increases from 0 to 0.2,the γ-2CaO·SiO2 content in the sintered product decreases from 79%to 32%,and theβ-2CaO·SiO2 content increases from 0 to 46%,and the self-powder performance deteriorates severely.The reaction rate of the dicalcium silicate in sodium aluminate solution increased from 1.48%to 8.86%,and the stability decreased significantly.When the sintering temperature is 1300℃-1400℃ and the N/S is 0.1,the main phase of the sintered product is y-2CaO·SiO2,β-2CaO·SiO2 and a small amount of free CaO.Increasing the sintering temperature is favorable for the formation of γ-2CaO·SiO2 and the crystallinity of the sintered product is increased.When the sintering temperature is 1350℃ and the N/S is 0.1,the holding time is extended from 0 min to 90 min,and the crystallinity of the sintered product is increased from 75%to 89%.The reaction rate of dicalcium silicate in sodium aluminate solution was reduced from 12.55%to 7.77%,and the stability of the sintered product was improved.The effect of MgO particle size and content on the mineral phase transformation,crystal structure,self-powder properties and microstructure of CaO-SiO2-Na2O system was studied when the N/S ratio was 0.05～0.2.In the presence of Na2O,the addition of MgO is beneficial to promote the conversion of β-2CaO·SiO2 to γ-2CaO·SiO2 in the sintered product,and improve the self-powder property of the sintered product.When N/S is 0.05,MgO addition of 3%can completely eliminate the negative effect of Na2O on the sintering product.When N/S is 0.2,MgO can reduce the negative effect of Na2O production,and the self-powder rate of the sintered product can be increased by about 50%.When N/S is not more than 0.2,decreasing the MgO particle size,increasing the contact area of the sintered product during the reaction,While promotes the formation ofγ-2CaO·SiO2,and improves the self-powder property of the sintered product.According to the elemental analysis of the scanning electron microscopy,the C/S ratio ofγ-2CaO·SiO2 is as low as 1.4,and β-2CaO·SiO2 is closer to the saturation coefficient of 2CaO·SiO2. |
