Removal of sulfur by adding zinc during the digestion process of high-sulfur bauxite

This paper proposes a novel approach to sulfur removal by adding zinc during the digestion process. The effects of zinc dosage on the concentrations of different valence sulfur in sodium aluminate solution were investigated at length to find that high-valence sulfur (S2O3 2−, SO3 2−, SO4 2−) concentration in sodium aluminate solution decreases, but the concentration of the S2− in the sodium aluminate solution increases as zinc dosage increases. This suggests that zinc can react with high-valence sulfur to generate S2− at digestion temperature, which is consistent with our thermodynamic calculation results. In this study, as zinc dosage increases, sulfur digestion rate decreases while sulfur content in red mud markedly increases when zinc dosage was below 4%; the digestion rates of sulfur and sulfur content in red mud remains stable when zinc dosage was above 4%; the alumina digestion rate, conversely, increased slightly throughout the experiment. This suggests that high-valence sulfur in sodium aluminate solution can be converted to S2− and then enter red mud to be removed completely by adding zinc during the digestion process.

causticization increase the production cost. Desulfurization efficiency of desulfurization by precipitators is high, but only a form of sulfur was removed.
Large swaths of China's high-sulfur bauxite reserves are unable to be developed or utilized due to the lack of appropriate deep desulfurization method for alumina production. This paper proposes a new method of sulfur removal in which zinc is supplied to prevent S 2− from oxidizing into S 2 O 3 2− , SO 3 2− , and SO 4 2− in the sodium aluminate solution, which forces S 2− into red mud to decrease the concentration of S 2− , S 2  , and total sulfur in the liquor. As opposed to other methods of sulfur removal, this method does not need discharge or causticization processes to remove the sulfate, and all forms of sulfur in liquor can be removed. As discussed below, removal of sulfur via the zinc addition in liquor was investigated to provide a theoretical and technical basis for the effective utilization of high-sulfur bauxite.

Results and Discussions
Effects of zinc dosage on the concentrations of different valence sulfur in liquor. Zinc dosage was varied between 0 and 5% through a series of digestion experiments at temperature of 533 K (260 °C), digestion time of 60 min, lime dosage of 13%, digestion liquor α k (molar ratio of Na 2 O k to Al 2 O 3 ) of 1.42. The effects of zinc dosage on the concentrations of different valence sulfur in sodium aluminate solution are shown in Fig. 1. Figure 1 shows that the concentration of the high-valence sulfur (S 2 O 3 2− , SO 3 2− , SO 4 2− ) in the sodium aluminate solution decreased as zinc dosage increased, while the calculated values of the S 2− concentration in the solution increased notably with the increase of zinc dosage. This suggests that zinc can react with high-valence sulfur to generate low-valence sulfur at the digestion temperature, which is consistent with our thermodynamic calculations (as shown in Fig. 2). Calculated values of the S 2− concentration are attained according to reactions as follows:  It also can be seen from Fig. 1 that in this experiment, as zinc dosage increased, the concentration of the S 2− in the sodium aluminate solution decreased notably when zinc dosage was below 3%; the concentration of the S 2− remains stable when zinc dosage was above 3%; the experimental values of the S 2− concentration is lower than calculated values throughout the experiment because the S 2− converted in solution entered into red mud; when zinc dosage was 5%, the S 2− and S 2 O 3 2− in liquor were removed completely, the SO 3 2− and SO 4 2− were removed ) was calculated using Factsage 7.0 software, the results are shown in Fig. 2. As can be seen from Fig. 2, the ΔG θ of the reactions of zinc with S 2 O 3 2− , SO 3 2− were all negative value in the temperature range of 298~573 K (25~300 °C), the ΔG θ of the reaction of zinc with SO 4 2− was negative value when the temperature above 448 K (175 °C). This suggests that zinc can react with high-valence sulfur to generate S 2− at digestion temperature, because digestion temperature of high-sulfur bauxite is higher than 513 K (240 °C) in industrial production.
The more negative the ΔG θ value, the more favorable the reaction is, so S 2 O 3 2− is easiest to be reduced by zinc, then SO 3 2− , SO 4 2− is most difficult to be reduced at temperature of 533 K (260 °C), this explains why S 2 O 3 2− in liquor were removed completely, the SO 3 2− and SO 4 2− were removed nearly completely when zinc dosage was 5% (as shown in Fig. 1).

Effects of zinc dosage on digestion rates of alumina and sulphur. Again, six different zinc dosages
(between 0% and 5%) were tested under the same other conditions described above. The Effects of zinc dosage on the digestion rates of alumina and sulfur are shown in Fig. 3.
It can be seen from Fig. 3 that as zinc dosage increased, the digestion rates of sulfur decreased obviously when zinc dosage was below 4%; when zinc dosage was above 4%, the digestion rates of sulfur remains stable. While the alumina digestion rate increased slightly throughout the experiment. Zinc in sodium aluminate solution exists mainly in the form of ZnO 2 2− , the solubility product constant of ZnS (2 × 10 −24 ) is very low, so it is difficult to decompose in sodium aluminate solution, the S 2− can react with Zn 2+ easily to generate ZnS 33,34 , the more S 2− in liquor, the easier ZnO 2 2− produce to Zn 2+ . As a reductant, the zinc reacted with high-valence sulfur to generate  S 2− , and then the S 2− reacted with Zn 2+ to generate ZnS went into red mud, so the sulfur in liquor was removed. The reactions are as follows: The sulfur content increased in red mud as zinc dosage increased (as shown in Fig. 4), while the digestion rates of sulfur decreased. When zinc dosage was above 4%, the sulfur in liquor was nearly removed.
Effects of zinc dosage on content of sulfur in red mud. The contents of sulfur in red mud described above are shown in Fig. 4. The X-ray diffraction pattern of red mud is shown in Fig. 5, where digestion temperature is 533 K (260 °C), digestion time is 60 min, and zinc dosage is 5%.
It can be seen from Fig. 4 that when zinc dosage was below 4%, the sulfur content increased substantially in the red mud as zinc dosage increased; the sulfur content remains stable when zinc dosage was above 4%.
The results shown in Figs 3 and 4 altogether indicate that sulfur in sodium aluminate solution can be effectively removed by adding zinc in the digestion process.
We also find ZnS in red mud, as shown in Fig. 5.

Effects of zinc dosage on the concentrations of iron in sodium aluminate solution.
Again, six different zinc dosages (between 0% and 5%) were tested under the same other conditions described above. The Effects of zinc dosage on the concentrations of iron in sodium aluminate solution are shown in Fig. 6. It can be seen from Fig. 6 that when zinc dosage was below 4%, the concentration of Fe 2 O 3 decreased substantially as zinc dosage increased; the concentration of Fe 2 O 3 remains stable when zinc dosage was above 4%. So iron  in sodium aluminate solution was also removed, when sulfur in liquor was removed by adding zinc during the digestion process.

Effects of zinc dosage on absorbance of liquor. Again, six different zinc dosages (between 0% and 5%)
were tested under the same other conditions described above. The effects of zinc dosage on absorbance of liquor are shown in Fig. 7. Sulfur removal rate is measured by observing the absorbance change of liquor, represented by color change. Absorbance was measured at 578 nm in a 4 cm cell in this study.
It can be seen from Fig. 7 that the absorbance of liquor decreased as zinc dosage increased. This suggests that the color of sodium aluminate liquor fades notably.
It can be seen visually from Fig. 7 that the colours of digestion liquors change from opaque to transparency with the increase of zinc dosage, the colours of digestion liquor is the same as that of normal digestion liquor in the Bayer process when zinc dosage is 5%, this means that the addition of zinc in liquor can make the colour of sodium aluminate solution fade noticeably.
The higher the sulfur concentration in sodium aluminate solution, the deeper the colour of solution 35 . So, it can be seen from Fig. 7 that the sulfur in sodium aluminate solution can be effectively removed by adding zinc in the digestion process.
Mechanism of sulfur removal. Based the above result and discussion, we propose the following mechanism of sulfur removal (shown in Fig. 8). As a reductant, the zinc reacted with high-valence sulfur (S 2  , the S 2− promotes ZnO 2 2− to produce Zn 2+ , and then the S 2− reacted with Zn 2+ to generate ZnS went into red mud. As a reductant and precipitator, the zinc can remove sulfur completely in sodium aluminate solution during the digestion process.

Conclusions
The high-valence sulfur in sodium aluminate solution can be converted to S 2− and entered into red mud in the form of ZnS by adding zinc during the digestion process. In this study, as zinc dosage increases, sulfur digestion rate decreases while sulfur content in red mud markedly increases when zinc dosage was below 4%; the digestion rates of sulfur and sulfur content in red mud remains stable when zinc dosage was above 4%; the alumina digestion rate, conversely, increased slightly throughout the experiment. Considering both production cost and desulfurization efficiency, the optimum zinc dosage was determined to be 4%. So the sulfur in sodium aluminate solution can be effectively removed by adding zinc in the digestion process, which provide a theoretical basis for the effective removal of sulfur in alumina production process.

Materials and Method
Materials. The high-sulfur bauxite used in this experiment was obtained from Zunyi mining area in China.
The chemical components of mineral samples are shown in Table 1. The X-ray diffraction pattern of the mineral is shown in Fig. 9. The QEMSCAN image of the mineral is shown in Fig. 10.
As shown in Figs 9 and 10, the primary sulfur-bearing mineral is pyrite. Figure 10 also demonstrates that pyrite was a granular aggregate distribution, particle size is 10~100 μm, and goethite was infection distribution in pyrite around. During the Bayer process of alumina production, the sulfur in high-sulfur bauxite first enters the solution in the form of S 2− , then the S 2− is gradually oxidized into various forms of S 2 Table 2.

Method.
Digestion experiments on high-sulfur bauxite were conducted in a XYF-6 digester, as shown in Fig. 11, which heated by molten salts.
The mineral sample and evaporation spent liquor according to a certain proportion were placed in a 100 ml stee1 bomb which was sealed in the experiment. The digester was heated to 533 K (260 °C) and held for 5 min,   36 . The dried red mud was sampled and observed with an X-ray fluorescence analyzer and carbon-sulfur analyzer.
The digestion rate of alumina (η A ) was calculated as follows: A ore m ud ore (A/S) ore Mass ratio of alumina to silica in raw ore (A/S) mud Mass ratio of alumina to silica in mud The digestion rate of sulfur (η S ) was calculated as follows:    S ore F F mud ore ore mud S ore Mass percentage of sulfur in raw ore (%) S mud Mass percentage of sulfur in mud (%) F ore Mass percentage of iron in raw ore (%) F mud Mass percentage of iron in mud (%)