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Sulfur phase and sulfur removal in high sulfur-containing bauxite HU Xiao-lian1, 2, CHEN Wen-mi1, XIE Qiao-ling1 1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China; 2. School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China Received 4 August 2010; accepted 3 November 2010 Abstract The sulfur phase in high sulfur-containing bauxite was studied by an X-ray diffraction analysis and a chemistry quantitative analysis. The s for the removal of different shaped sulfur were also discussed. The results show that sulfur phases in high sulfur-containing bauxites exist in the main of sulfide sulfur pyrite or sulfate sulfur, and the main sulfur s of bauxites from different regions are not the same. Through a combination of an X-ray diffraction analysis and a chemistry quantitative analysis, the sulfur phases of high sulfur-containing bauxite could be accurately investigated. Deciding the main sulfur of high sulfur-containing bauxite could provide theoretical instruction for choosing s for the removal of sulfur from bauxite, and an oxidizing-roasting process is an effective way to remove sulfide sulfur from high sulfur-containing bauxite, the content of S2− in crude ore in the digestion liquor is above 1.7 g/L, but in the roasted ore digestion liquor, it is below 0.18 g/L. Using the sodium carbonate solution washing technology to wash bauxite can effectively remove sulfate sulfur, the content of the total sulfur in ore is lowered to below 0.2 and can meet the production requirements for the sulfur content. Key words high sulfur-containing bauxite; sulfur phase; oxidizing-roasting; sodium carbonate solution washing technology 1 Introduction China is rich in bauxite, the reserve of which has been proved to reach 2.3109 t [1]. The amount of high sulfur-containing diasporic bauxite has reached 1.5108 t [2]. These ores are mainly composed of aluminum of middle-high proportion, silicon of middle-low proportion, sulphur of high proportion and middle-high Al/Si ratio A/S ores. Most of the ores are of high-grade alumina, but they can only be used after the removal of sulfur from the high sulfur-containing bauxite. Therefore， developing an economical and practical for the removal of the sulfur is of great importance for industry production. Moreover, in the process of alumina production, the sulfur in the ore can cause not only the loss of Na2O, but also leads to the corrosion of steel material and the rise in the iron concentration of the solution as a result of increasing the concentrate of S2− of the solution. For example, when the sulfur content of the bauxite exceeds 0.8, it will result in the decline of alumina quality for the existence of Fe, the damage of the equipment in the evaporation process and decomposition process for the corrosion of the steel equipment. It can even decrease the digestibility of the alumina [3]. The removal of sulfur from bauxite has attracted considerable attention in recent years with the rapid development of the alumina industry [3−7]. There are two basic processes for extracting alumina from bauxite, namely, the sintering process and the Bayer process. The disadvantage of the sintering process is its low efficiency low to 33 or less. Due to its low cost, the Bayer process is the most commonly-used for extracting alumina from bauxite [8−10]. In the Bayer process, the research area of the removal of sulfur from bauxite is mainly the removal of sulfur from sodium aluminate solutions or Bayer liquor. The s are 1 the removal of sulfur by means of wet air oxidation and 2 adding a desulfurizing agent. But by er this problems for attention are that it should avoid generating thiosulfate and increasing corrosion of the equipment during air oxidation, and it also has a certain risk by means of wet air oxidation. Foundation item Project 20971041 supported by the National Natural Science Foundation of China; Project 09B032 supported by Scientific Research Fund of Hunan Provincial Education Department, China Corresponding author HU Xiao- lian; Tel 86-13107120683; E-mail huxiaolian71 DOI 10.1016/S1003-63261160908-4 HU Xiao-lian, et al/Trans. Nonferrous Met. Soc. China 212011 1641−1647 1642 It has been found that the removal of sulfur is achieved by adding a de-sulfurizing agent, which is mainly zinc oxide or barium oxide, to the liquor, but the basic principles of both s are not the same. In terms of the er process, the removal of sulfur is capable of separating divalent sulfur ions from the liquor by ing insoluble zinc sulfide and the latter concerns the removal of sulfur from the Bayer process liquors by precipitation with barium sulfate. However, in order to improve the pertinent choice of de-sulfurizing agent, the phase of the sulphur should be known first. To resolve this issue, it is necessary to be aware of the phase of the sulphur and its content of bauxite so that the appropriate s can be chosen for the removal of the sulfur. So far, studies on the phase of sulfur of high-sulfur bauxite have seldom been reported, and although there are many kinds on the research s for the phases, the for the study of the bauxite phase is the use of an X-ray diffraction analysis and a scanning electron microscopy [11−12]. ZHANG et al [11] investigated the existence of sulfur mineral in high-sulfur bauxite from Guizhou Province, China, by an X-ray diffraction analysis and a scanning electron microscopy. Due to the limitations of the instrument, the X-ray diffraction phase analysis is imperfect in many aspects. Firstly, it mainly analyzes crystal, and is not suitable for the analysis of non-crystal, which means that the diffraction peak of sulfate, which does not crystallize well, cannot be shown in the X-ray diffraction pattern, and cannot demonstrate whether or not sulfate is present in the ore. Secondly, XRD cannot quantitatively and accurately measure the content of sulfur at various phases, and a scanning electron microscopy can only per a topographical observation and cannot be used as a of the phase. Therefore, using seven kinds of high sulfur-containing bauxites from three provinces, namely, Henan province, Guizhou province and Guangxi province, China, by means of both XRD and a chemistry quantitative analysis, the sulfur-presence phase of the ore was shown by X-ray diffractometry, the total sulfur content was measured by chemical analysis, and a quantitative measurement of the content of various phase sulfur was carried out. On the basis of these, the further law of the sulfur phase of high sulfur-containing bauxite was explored. Moreover, different s for the removal of different phase sulfur were also investigated. 2 Experimental 2.1 High-sulfur bauxite Experiment materials included seven types of high- sulfur bauxites, namely, Guizhou ores A and B, Henan ores A, B and C, Guangxi ore A and B. Those ores were ground using a vibrating grinding mill. The main chemical compositions of these bauxites are shown in Table 1. Table 1 Chemical compositions of seven bauxites Sample wAl2O3/wSiO2/ A/S wST/ Guizhou ore A60.65 11.74 5.17 0.36 Guizhou ore B66.58 14.52 4.59 0.63 Henan ore A61.62 12.65 4.87 0.97 Henan ore B70.27 6.32 11.12 0.85 Henan ore C80.18 7.21 11.12 0.78 Guangxi ore A65.16 4.33 15.05 2.28 Guangxi ore B60.11 4.20 14.31 0.10 It can be seen from Table 1 that the sulfur contents of Henan bauxite samples A, B, C and Guangxi bauxite sample A are relatively high, respectively 0.97, 0.85, 0.78 and 2.28. When the sulfur content of the bauxite exceeds 0.8, it will result in the decline of the quality of the alumina for the pollution of Fe, the damage of the equipment in evaporation process and the decomposition process, as a result of the corrosion of the steel equipment [3]. In order to meet this specification, it is necessary to remove the sulfur from the high-sulfur bauxite. It can be seen from aluminum silicon ratio that Guizhou ores A, B and Henan ore A belong to the middle-grade bauxites A/S of 4−7, but Henan ores B, C and Guangxi ore are high-grade bauxites A/S 11. 2.2 Pregnant liquor The pregnant liquor in the digestion experiment came from an aluminate solution of one alumina factory by the Bayer process in China. The main chemical composition is shown in Table 2. It is composed of total alkali NT, caustic alkali NK and Al2O3, and the concentration ratio of Al2O3 to caustic alkali RP was 0.61. Table 2 Chemical composition of aluminate solution ρNT/gL−1 ρNK/gL−1 ρAl2O3/gL−1RP 248 228 140 0.61 2.3 s 2.3.1 Analysis of main chemical composition of solid phase and ores The main chemical compositions, Al2O3 and SiO2, of the solid phase and ores were analyzed by a wavelength dispersive−X-ray fluorescence spectrometer, and the total sulfur of bauxite was analyzed by the national standard “GB3257.18 82”, combustion− iodimetry of bauxite chemical analysis . 2.3.2 Analysis of main chemical composition of liquid phase HU Xiao-lian, et al/Trans. Nonferrous Met. Soc. China 212011 1641−1647 1643 After the digestion process was finished, the liquid phase was separated from the solid phase by filtration. The composition of liquid phase Al2O3 was analyzed by an EDTA compleximetry and the caustic alkali was analyzed by acid-base titration. The concentration of S2− in the aluminate solution was analyzed according to the literature [13]. The sample was firstly acid-decomposed by adding the SnCl2 hydrochloric acid solution and releasing H2S, and the sulphur of H2S was determined. 2.3.3 Study on phase of sulfur The study on the sulfur phase of high-sulfur bauxite used two s, namely, XRD and chemistry phase analysis of sulfur. The sulfur in bauxite mainly exists in the of pyrite, with maybe a small quantity of sulfate. Therefore, the determination of the sulfur was mainly composed of the determination of the total sulfur, the contents of sulfide sulfur and sulfate sulfur. The analysis of the total sulfur in the bauxite was based on the national standard “GB3257.18 82”, combustion- iodimetry of bauxite chemical analysis . The sample with a fusing agent was burned at 1 30020 C with oxygen, and SO2 was released, which was absorbed into H2SO3 in water. The sulphur of H2SO3 in water was determined by a titration of standard iodine with starch as an indicator. To determine the content of the sulfate sulfur and sulfide sulfur [14], the ore was washed with 10 sodium carbonate solution, and 10 BaCl2 solution was added to the filtrate after it was separated from the filter cakes by filtration. Then, the barium sulfate was determined by means of barium sulfate gravimetry and was converted into sulfur, which was the content of the sulfate sulfur. The sulfur in the dried filtrate cake, namely, sulfide sulfur was determined by burning iodimetry. 2.3.4 Roasting experiment and digestion experiment The roasting experiment was carried out in a muffle furnace SX2-5-12, with a temperature control accuracy of 5 C. After spreading the powder-like bauxite in a porcelain dish, it was quickly removed into a muffle when the furnace reached the specified temperature, and then quickly taken out and cooled down after pre-determined roasting time to begin to carry out the digestion experiment. The digestion experiment was carried out on a molten salt furnace, using an automatic temperature control device, with a temperature accuracy of 1 C. The digestion experiment temperature was adjusted according to the experimental requirements, the dissolution time was 60 min and the stirring speed was 60 r/min. The mineral powder and the pregnant liquor were placed in a 100 mL-steel bomb, which was sealed in the experiment. When the furnace reached the specified temperature for 5 min, the steel bomb was placed in it and the dissolution process was completed within the predetermined time. When the experiment was over, the sodium aluminate solution was filtered. Then the contents of Al2O3 and SiO2 in the dried filter cake were sampled and analyzed, and the contents of Al2O3, caustic alkali and S2− in filtrate were also analyzed. 3 Results and discussion 3.1 XRD analysis According to Ref. [15], the sulfur in bauxite is mainly FeS2 and most of it exists in the of pyrite. Since the content of sulfur in bauxite is very low, generally below 3, and most of it appears in the of pyrite, other types of sulfur mineral scatter, such as sulfate, and they are difficult to crystal. It can be predicted that no sulfate exists in the XRD patterns of bauxite. According to the standard XRD pattern of pyrite, a strong diffraction peak appears at 2θ33.08. Moreover, such a diffraction peak does not overlap with the other diffraction peak of the bauxite. In addition, there are some weak peaks, but such weak peaks often overlap with the other diffraction peak of the bauxite. Therefore, it can be basically judged whether or not the sulphur stays in the of pyrite according to whether or not a diffraction peak of pyrite emerges, and the content of pyrite can be judged according to the intensity of the diffraction peak. The XRD pattern of high sulfur bauxite is shown in Fig. 1. As shown in Fig. 1, Henan ore A and Guangxi ore A have obvious characteristic diffraction peaks of pyrite crystal at 2θ33.08 and no other phase of S appears in the XRD pattern. This indicates that the primary phases of sulfur in two types of high-sulfur bauxites are pyrite. In addition, Guizhou ore B only has a weak diffraction peak at 2θ33.08, while other ores have no peaks of pyrite crystal at all. This indicates that there is very small content of pyrite in these bauxites. Although Henan bauxites B and C have very high content of total sulphur, respectively 0.78 and 0.85, there is no obvious characteristic diffraction peaks at 2θ33.08, and this may be due to the small content of pyrite. In addition, XRD verifies that these seven bauxites are all diaspore β-AlOOH and contain a different amount of titanium-containing minerals, such as TiO2. 3.2 Chemical phase analysis of sulfur The contents of the total sulfur, sulfide sulfur and sulfate sulfur in high-sulfur bauxite were determined by a chemistry phase analysis of sulfur, and the results are listed in Table 3. As shown in Table 3, the main existing of sulfur is sulfide sulfur, which accounts for above 80 of the total sulfur in both the Guangxi bauxite A and Henan HU Xiao-lian, et al/Trans. Nonferrous Met. Soc. China 212011 1641−1647 1644 Fig. 1 X-ray diffraction patterns of high-sulfur bauxite Table 3 Sulfur phase analytic result
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