Study On The Behavior Of Sulfur During The Digestion Process Of High Sulfur-Containing Bauxite And On The Process Of Sulfur Removal

At present, China's alumina industry has a very limited supply of high-grade bauxite resources. In order to bridge the gap between the natural shortage and the high industrial demand for high quality bauxite, an alternative solution to solve the shortage of high-grade bauxite is to develop a process of desulfurization for high sulfur containing bauxite. Moreover, sulfur can affect the production of alumina, so it is necessary to develop an economical and practical method for the removal of the sulfur in high sulfur-containing bauxite. This thesis is composed of three parts. The first part is the study on sulfur phase in high sulfur-containing bauxite; the second part is related to the behavior of sulfur in aluminate solution after digestion process by Bayer process; the third part is concerned with desulfuration of high sulfur bauxite by oxidation roasting, and the tretment of Bayer liquor containing divalent sulfur ions via wet oxidation process.Firstly, I investigated sulfur-presence phases of the ore by means of both the XRD and a chemistry quantitative analysis.The results show that high sulfur-containing bauxite is in the main form of sulfide sulfur (pyrite) and sulfate sulfur, and the main sulfur form of bauxites from different regions is not the same. Using sodium carbonate solution washing technology to wash bauxite can effectively remove sulfate sulfur. After washing process, the content of the total sulfur in ore is lowered to below0.2%and can meet the production requirements as to the sulfur content. Identification of the main sulfur form of high sulfur-containing bauxite can provide theoretical instruction for choosing methods for the removal of sulfur from bauxite.Secondly, the behavior of sulfur during digestion process was studied. Guangxi bauxite containing low amount of sulfur was mixed with Guangxi high sulfur-containing bauxite and was then digested, influence of content of sulfur on conversion of sulfur, on the concentration of S2-and on concentration of iron in aluminate solution was researched. Through digestion experiment using high sulfur-containing bauxite and aluminate solution containing various forms of sulfur in autoclave, the forms and the conversion mechanism of sulfur and the path of sulfur in Bayer liquor were investigated. The results show high amount of sulfur in bauxite would lead to more sulfur entering solution, and with the increase of sulfur in bauxite, iron and S2-concentration in aluminate solution rise significantly. In addition, the sulfur of aluminum hydroxide can make aluminum hydroxide colored. After high sulfur-containing bauxite is digested, the sulfur of aluminate solution is a mixture with various forms of sulfur, and various forms of sulfur can be mutually transformed; some of sulfur in bauxite remains in solution, some is discharged along with desiliconization substance of the red mud, but SO42-is finally discharged with the red mud in the form of3CaO·Al2O3·CaSO4·12H2O and CaSO4.In the third part, using the ore from Henan province, the oxidation roasting process of the desulfuration from the diasporic high sulfur-containing bauxite was studied in the muffle roaster and the action of CaO was emphasized. The results show that the roasting results of bauxite from the different main sulfur phase is not the same, and the content of sulfide (mainly pyrite) reduce with the addition of CaO after being roasted; CaO played the role of fixing sulfur and reduce the release of sulfur in the air. When the raw ore is roasted in the condition of600℃and45min, the release of sulfur in the air (S%) is0.51%, but lowers to0.31%when1%CaO is added to the ores; when CaO is added to the ores during roasting, pyrite reacts and hematite forms, CaO plays the role of fixing sulfur, and the main product of reaction of sulfur fixation is CaSO4. It can dramatically lower S2-concentration in aluminate solution and improve digestibility by roasting; Using Henan bauxite with mainly sulfide sulfur (pyrite), the S2" concentration in aluminates solution is0.15g/L after being roasted under the conditions of600℃,which is about10%of S2-concentration (1.78g/L) in the solution of digested raw ore under the same digesting conditions of240℃,60min60min,,and the extraction rate of aluminum increases to91.21%from85.27%.Because the iron content in alumina products was associated with bivalent sulfur ions in aluminate solution, the wet oxidation had been used to remove the bivalent sulfur ions in high pressure autoclave. The influence of various conditions on the removal of S2-and the oxidation mechanism was investigated in this thesis. The results show it can effectively remove the bivalent sulfur ions in high pressure autoclave with oxygen as the oxidizer by wet oxidation, the removal efficiency of bivalent sulfur ions is affected by the oxygen pressure inside the reactor, reaction temperature, reaction time. The optimum conditions for wet oxidation is the reaction temperature200℃, the oxygen pressure3.0MPa, S2-concentration of1g/L, under these conditions, the removal rate of S2-is99%. At high temperature and pressure, the majority of S2-in aluminate solution is oxidized to SO42-, only a small part of S2-is oxidized to thiosulfate, sulfite and other forms of sulfur; Conversion Of S2-to SO42-is mainly affected by the oxygen pressure and reaction temperature. When the temperature is up to260℃, the oxygen pressure reaches1.OMPa, more than98%of S2-is oxidized to SO42-.