Restraining Sodium Volatilization in the Ferric Bauxite Direct Reduction System

Direct reduction is an emerging utilization technology of ferric bauxite. However, it requires much more sodium carbonate than ordinary bauxite does. The volatilization is one of the most significant parts of sodium carbonate consumption, as reported in previous studies. Based on the new direct reduction method for utilization of ferric bauxite, this paper has systematically investigated factors including heating temperature, heating time, and sodium carbonate dosage influencing sodium volatilization. For the purpose of reducing sodium volatilization, the Box–Benhken design was employed, and the possibility of separating iron and sodium after direct reduction was also investigated.


Introduction
Ferric bauxite is a highly valuable refractory bauxite that is widely distributed in China [1,2], Laos [3], and Tanzania [3], but is improperly used because it contains much more iron than ordinary bauxite.The possible utilization technologies of ferric bauxite include the extraction of alumina before iron [4,5], extraction of iron before alumina [6,7], pretreatment before Bayer processing [8], and biological treatment [9,10].Another technology that has recently become a popular topic of research is direct reduction [11][12][13].The addition of sodium carbonate facilitates the conversion of diaspore and boehmite into sodium aluminate (water soluble), which is then separated from the solution and used for alumina production.Hematite and goethite can be reduced to iron powder by carbothermal reaction and then separated from the magnetic concentrate.Iron powder may then be used for steel and casting production after agglomeration.However, the direct reduction technology of ferric bauxite at normal atmospheric pressure requires much more sodium carbonate than ordinary bauxite, mainly because ferric bauxite contains more iron than ordinary bauxite, which in turn increases the generated amorphous sodium ferrite and renders sodium volatile [14,15].Hence, volatilization is one of the most significant parts of sodium carbonate consumption [14].Our previous research [14] indicated that heating temperature, heating time, and sodium carbonate dosage are significant factors that influence sodium volatilization; however, the values of their effects have not yet been quantified.Box-Behnken design is an independent, rotatable quadratic design with no embedded factorial or fractional factorial points where the variable combinations are at the midpoints of the edges of the variable space and at the center, and is typically adopted to conduct quantitative research by determining the regression model.In this study, the Box-Behnken design is used to investigate the effects of experimental factors on the sodium volatilization ratio (R) for the purpose of reducing R.However, sodium is a harmful element in iron powder; a decrease in R increases the remaining sodium in the solid phase.Hence, the possibility of separating iron and sodium must necessarily be investigated to ensure that the restraining process can be conducted harmlessly.X-ray diffraction (XRD) [16] and scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) [17] analysis are performed to determine the mineral phase of sodium in iron powder and tailing.The findings can aid in reducing R and in the subsequent sodium separation after direct reduction and may be beneficial in reducing the sodium carbonate consumption of current alumina production processes.

Experimental Materials
The ore used was supplied by the Guangxi Zhuang Autonomous Region of China.Its chemical composition, XRD pattern, and SEM-EDS images are shown in Table 1, Figures 1 and 2 respectively.Table 1 indicates the alumina-silica ratio (A/S) of the ferric bauxite is only 2.7, an extremely low value.Figure 1 shows five main crystalline mineral phases in raw ferric bauxite, namely, hematite, boehmite, diaspore, kaolinite, and goethite.Figure 2 reveals that the minerals in ferric bauxite are finely disseminated and symbiotic with one another.Suggested from the EDS, Figure 2b is diaspore or boehmite, Figure 2c is hematite, Figure 2d is goethite symbiotic with kaolinite, Figure 2e is goethite (darker than hematite), Figure 2f is kaolinite symbiotic with goethite, and Figure 2g is diaspore symbiotic with goethite.conduct quantitative research by determining the regression model.In this study, the Box-Behnken design is used to investigate the effects of experimental factors on the sodium volatilization ratio (R) for the purpose of reducing R.However, sodium is a harmful element in iron powder; a decrease in R increases the remaining sodium in the solid phase.Hence, the possibility of separating iron and sodium must necessarily be investigated to ensure that the restraining process can be conducted harmlessly.X-ray diffraction (XRD) [16] and scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) [17] analysis are performed to determine the mineral phase of sodium in iron powder and tailing.The findings can aid in reducing R and in the subsequent sodium separation after direct reduction and may be beneficial in reducing the sodium carbonate consumption of current alumina production processes.

Experimental Materials
The ore used was supplied by the Guangxi Zhuang Autonomous Region of China.Its chemical composition, XRD pattern, and SEM-EDS images are shown in Table 1, Figures 1 and 2, respectively.Table 1 indicates the alumina-silica ratio (A/S) of the ferric bauxite is only 2.7, an extremely low value.Figure 1 shows five main crystalline mineral phases in raw ferric bauxite, namely, hematite, boehmite, diaspore, kaolinite, and goethite.Figure 2 reveals that the minerals in ferric bauxite are finely disseminated and symbiotic with one another.Suggested from the EDS, Figure 2b is diaspore or boehmite, Figure 2c is hematite, Figure 2d is goethite symbiotic with kaolinite, Figure 2e is goethite (darker than hematite), Figure 2f is kaolinite symbiotic with goethite, and Figure 2g is diaspore symbiotic with goethite.conduct quantitative research by determining the regression model.In this study, the Box-Behnken design is used to investigate the effects of experimental factors on the sodium volatilization ratio (R) for the purpose of reducing R.However, sodium is a harmful element in iron powder; a decrease in R increases the remaining sodium in the solid phase.Hence, the possibility of separating iron and sodium must necessarily be investigated to ensure that the restraining process can be conducted harmlessly.X-ray diffraction (XRD) [16] and scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) [17] analysis are performed to determine the mineral phase of sodium in iron powder and tailing.The findings can aid in reducing R and in the subsequent sodium separation after direct reduction and may be beneficial in reducing the sodium carbonate consumption of current alumina production processes.

Experimental Materials
The ore used was supplied by the Guangxi Zhuang Autonomous Region of China.Its chemical composition, XRD pattern, and SEM-EDS images are shown in Table 1, Figures 1 and 2, respectively.Table 1 indicates the alumina-silica ratio (A/S) of the ferric bauxite is only 2.7, an extremely low value.Figure 1 shows five main crystalline mineral phases in raw ferric bauxite, namely, hematite, boehmite, diaspore, kaolinite, and goethite.Figure 2 reveals that the minerals in ferric bauxite are finely disseminated and symbiotic with one another.Suggested from the EDS, Figure 2b is diaspore or boehmite, Figure 2c is hematite, Figure 2d is goethite symbiotic with kaolinite, Figure 2e is goethite (darker than hematite), Figure 2f is kaolinite symbiotic with goethite, and Figure 2g is diaspore symbiotic with goethite.The proximate analysis is utilized to determine the weight percent moisture, volatile matter, fixed carbon, and ash content of the coal adopted.The proximate analysis results and the chemical composition of coal ash are shown in Tables 2 and 3, respectively.

Experimental Instruments
The direct reduction, grinding, and leaching experiments were performed with a muffle furnace (SX2-10-13, INCH, Beijing, China), a rod mill (XMB-70, Hengcheng, Ganzhou, China), and a stirrer (JJ-3, Guohua, Changzhou, China) with 1000 mL beaker, respectively.The other instruments  The proximate analysis is utilized to determine the weight percent moisture, volatile matter, fixed carbon, and ash content of the coal adopted.The proximate analysis results and the chemical composition of coal ash are shown in Tables 2 and 3 respectively.
The chemical composition of the samples was analyzed using an atomic absorption spectrophotometer (UV-9600, Rayleigh, Beijing, China).The mineral composition of ore was determined by XRD analysis (TTRIII, Rigaku, Tokyo, Japan).The morphology and microzone chemical composition of the sample were examined using an electron microscope (EVO 18, ZEISS, Jena, Germany) equipped with an energy-dispersive spectrometer.

Experimental Methods
The experiment flow chart is shown in Figure 3.

Experimental Methods
The experiment flow chart is shown in Figure 3.In each experiment, ore, sodium carbonate, and coal used were crushed to 100% passing 2 mm and were mixed.The mixed ratio of ore and coal was 1:0.25, while the amount of sodium carbonate changed.The amount of sodium carbonate was presented in the form of its relative mass percentage to ore.In each unit experiment, 41 g of the mixed materials was transferred to a lidded 100 mL graphite clay crucible.The crucible was then placed in a muffle furnace with an uncontrolled gaseous atmosphere at 1100 to 1150 °C.A distance of more than 30 mm was placed between the crucible lid and the powder mixture in the crucible to prevent them from making contact with each other.After cooling, the material was ground to 95% passing 0.074 mm at a grinding density of 50% and was then leached in a 1000 mL beaker at a water-solid ratio of 15:1, stirring speed of 360 r/min, leaching temperature of 75 °C, and leaching time of 30 min.The leaching yield of alumina (ηA) was then calculated from Equation (1): ηA, the leaching yield of alumina, %; c, the aluminum oxide concentration of pregnant leach solution, g/L; Vol, the volume of pregnant leach solution, L; M, the mass of ferric bauxite adopted in each unit experiment, g; ω, the mass fraction of Al2O3 in ferric bauxite, %.
The leaching residues were transferred to a magnetic separator, and the recycle of iron powder from tailing was conducted with a magnetic separation of 0.14 T.
The sodium content in the crucible and the material mixture were measured before and after heating, and the sodium content difference was attributed to sodium volatilization.R was then calculated from Equation (2): In each experiment, ore, sodium carbonate, and coal used were crushed to 100% passing 2 mm and were mixed.The mixed ratio of ore and coal was 1:0.25, while the amount of sodium carbonate changed.The amount of sodium carbonate was presented in the form of its relative mass percentage to ore.In each unit experiment, 41 g of the mixed materials was transferred to a lidded 100 mL graphite clay crucible.The crucible was then placed in a muffle furnace with an uncontrolled gaseous atmosphere at 1100 to 1150 ˝C.A distance of more than 30 mm was placed between the crucible lid and the powder mixture in the crucible to prevent them from making contact with each other.After cooling, the material was ground to 95% passing 0.074 mm at a grinding density of 50% and was then leached in a 1000 mL beaker at a water-solid ratio of 15:1, stirring speed of 360 r/min, leaching temperature of 75 ˝C, and leaching time of 30 min.The leaching yield of alumina (η A ) was then calculated from Equation (1): η A , the leaching yield of alumina, %; c, the aluminum oxide concentration of pregnant leach solution, g/L; V ol , the volume of pregnant leach solution, L; M, the mass of ferric bauxite adopted in each unit experiment, g; ω, the mass fraction of Al 2 O 3 in ferric bauxite, %.
The leaching residues were transferred to a magnetic separator, and the recycle of iron powder from tailing was conducted with a magnetic separation of 0.14 T.
The sodium content in the crucible and the material mixture were measured before and after heating, and the sodium content difference was attributed to sodium volatilization.R was then calculated from Equation (2): R, the sodium volatilization ratio, %; Q 1 , the evaporated sodium infiltrated into crucible, g; Q, the sodium content in material mixture, g; S 1 , the area of crucible inside surfaces contacted with the material, cm 2 ; S 2 , the area of crucible inside surfaces non-direct contacted with the material, cm 2 .
The form of sodium in iron powder and tailing was determined by SEM-EDS.The mineral composition of iron powder and tailing were examined by XRD.

Factors Affecting Sodium Volatilization
Previous research [14] indicates that the four significant factors that affect sodium volatilization are heating temperature, heating time, coal dosage, and sodium carbonate dosage.Coal dosage is mainly determined by the hematite and goethite content of ferric bauxite.Hence, it cannot be changed to reduce sodium carbonate consumption.Thus, the effects of heating temperature, heating time, and sodium carbonate dosage on sodium volatilization were measured in this study with a constant coal dosage.The parameter scales were given according to the optimum conditions obtained by the previous research [18] because carbonate consumption reduction must be achieved on the premise of synthetically recovering iron and aluminum.In the Box-Behnken design, the fluctuation range of each parameter scale is 10%.The experimental results are shown in Table 4, the variance analysis of the regression models is shown in Table 5.The confidence analysis of the quadratic model is shown in Table 6.Where, df is the degree of freedom (a non-dimensional number); F is the homogeneity test of variance (a non-dimensional number); P is the probability of obtaining a result that is at least as extreme as the actually observed result, given that the null hypothesis is true (a non-dimensional number); VIF is the variance inflation factor (a non-dimensional number).
The variance analysis of the regression models (Table 5) shows that the quadratic model has a satisfactory fitting effect.The confidence analysis (Table 6) shows that the quadratic model should be the chosen model.The response surfaces of the time-sodium carbonate dosage plot and their corresponding contours are shown in Figure 4.  Where, df is the degree of freedom (a non-dimensional number); F is the homogeneity test of variance (a non-dimensional number); P is the probability of obtaining a result that is at least as extreme as the actually observed result, given that the null hypothesis is true (a non-dimensional number); VIF is the variance inflation factor (a non-dimensional number).
The variance analysis of the regression models (Table 5) shows that the quadratic model has a satisfactory fitting effect.The confidence analysis (Table 6) shows that the quadratic model should be the chosen model.The response surfaces of the time-sodium carbonate dosage plot and their corresponding contours are shown in Figure 4.The response surfaces of the temperature-sodium carbonate dosage plot and their corresponding contours are shown in Figure 5.
The response surfaces of the temperature-sodium carbonate dosage plot and their corresponding contours are shown in Figure 5.The response surfaces of the temperature-time plot and their corresponding contours are shown in Figure 6.
Figure 4 shows that the effect of heating time on sodium volatilization is more significant than that of sodium carbonate dosage.Figure 5 shows that the effect of heating temperature is more significant than that of sodium carbonate dosage.Figure 6 shows that the effect of heating temperature on sodium volatilization is more significant than that of heating time.The descending order of their magnitude is as follows: heating temperature > heating time > sodium carbonate dosage.
Figure 6 shows that R initially decreases and then increases with an increase in heating temperature and that the minimum R is reached at 1150 °C.Some verification experiments were conducted to determine the cause of this minimum R.
Under the reaction conditions explained in Section 2.3, the curve between ηA and heating temperature is shown in Figure 7. ηA initially decreases and then increases with an increase in heating temperature and reaches its peak at 1100 °C.After reacting with an aluminum-bearing mineral, most sodium carbonate molecules are converted into crystal sodium aluminate.These crystals crystallize better and become more stable at higher temperature, hence the minimum R is reached at 1150 °C instead of 1100 °C.The response surfaces of the temperature-time plot and their corresponding contours are shown in Figure 6.
Figure 4 shows that the effect of heating time on sodium volatilization is more significant than that of sodium carbonate dosage.Figure 5 shows that the effect of heating temperature is more significant than that of sodium carbonate dosage.Figure 6 shows that the effect of heating temperature on sodium volatilization is more significant than that of heating time.The descending order of their magnitude is as follows: heating temperature > heating time > sodium carbonate dosage.
Figure 6 shows that R initially decreases and then increases with an increase in heating temperature and that the minimum R is reached at 1150 ˝C.Some verification experiments were conducted to determine the cause of this minimum R.
Under the reaction conditions explained in Section 2.3, the curve between η A and heating temperature is shown in Figure 7. η A initially decreases and then increases with an increase in heating temperature and reaches its peak at 1100 ˝C.After reacting with an aluminum-bearing mineral, most sodium carbonate molecules are converted into crystal sodium aluminate.These crystals crystallize better and become more stable at higher temperature, hence the minimum R is reached at 1150 ˝C instead of 1100 ˝C.Table 6 shows that the quadratic model of the effect of heating temperature, heating time, and sodium carbonate dosage on R (sodium volatilization ratio) can be described by Equation (3).Table 6 shows that the quadratic model of the effect of heating temperature, heating time, and sodium carbonate dosage on R (sodium volatilization ratio) can be described by Equation (3).Table 6 shows that the quadratic model of the effect of heating temperature, heating time, and sodium carbonate dosage on R (sodium volatilization ratio) can be described by Equation (3).R " 2.44A 2 `0.81B 2 `0.23C 2 `0.89AB `0.36AC `0.025BC `0.88A ´0.47B `0.26C `7.58 (3) The changing rate of R with A, B, and C can be determined by calculating the partial derivative of R with respect to heating temperature (A), heating time (B), and sodium carbonate dosage (C) as described by Equations ( 4)- (9).BR BA " 4.88A `0.89B `0.36C `0.88 BR BB " 1.62B `0.89A `0.025C ´0.47 BR BC " 0.46C `0.36A `0.025B `0.26 Analysis shows that BC 2 " 0.46 ą 0, which means that all these curves are concave and have no flex point.Hence, the larger the second derivative becomes, the more rapidly R changes.Given that the second derivative of heating temperature is 3.01 and 10.61 times larger than that of heating time and sodium carbonate dosage, respectively, heating temperature has a much stronger dependence on R than the other two factors, or its effect is more significant than that of the other two factors.Furthermore, the reaction between bauxite and sodium carbonate accelerates as temperature increases and is conducted more thoroughly as heating time increases.The interactions between A, B, and C are also presented in Table 6.
Our previous research [14,15] indicates that η A and R increase with an increase in sodium carbonate dosage.However, given that the effect of heating temperature is larger than that of sodium carbonate dosage, the effect of sodium carbonate dosage on R can be ignored in some cases.Hence, at a low temperature and high sodium carbonate dosage, a high η A and a low R may be simultaneously achieved.

Alumina Leaching and Sodium Volatilization
To verify the effects of restraining method on η A and the total sodium volatilization amount per unit mass ferric bauxite (V), some verification tests were conducted as shown in Table 7.Under the conditions of heating temperature of 1100 ˝C, heating time of 35 min, sodium carbonate dosage of 120%, the obtained η A is 76.02%, which is higher than 75.92%, the value obtained under previous optimum conditions.However, when sodium carbonate dosage is 100% or 110%, η A would be only 73.53% or 74.91%, respectively.That means η A will not increase until sodium carbonate dosage rise enough (e.g., from 85% to 120%).Analogously, V would reduce only when the sodium carbonate dosage achieve 120%.If not, it would be increased.

Tailing
Tailing is the residue of magnetic separation.The XRD pattern of tailing obtained with the restraining method is shown in Figure 8, and the SEM-EDS images are shown in Figure 9.
Minerals 2016, 6, 31 10 of 14 under previous optimum conditions.However, when sodium carbonate dosage is 100% or 110%, ηA would be only 73.53% or 74.91%, respectively.That means ηA will not increase until sodium carbonate dosage rise enough (e.g., from 85% to 120%).Analogously, V would reduce only when the sodium carbonate dosage achieve 120%.If not, it would be increased.

Tailing
Tailing is the residue of magnetic separation.The XRD pattern of tailing obtained with the restraining method is shown in Figure 8, and the SEM-EDS images are shown in Figure 9.  under previous optimum conditions.However, when sodium carbonate dosage is 100% or 110%, ηA would be only 73.53% or 74.91%, respectively.That means ηA will not increase until sodium carbonate dosage rise enough (e.g., from 85% to 120%).Analogously, V would reduce only when the sodium carbonate dosage achieve 120%.If not, it would be increased.

Tailing
Tailing is the residue of magnetic separation.The XRD pattern of tailing obtained with the restraining method is shown in Figure 8, and the SEM-EDS images are shown in Figure 9.  Figure 8 shows that nepheline is the main sodium-bearing mineral in tailing.Combined with XRD pattern, Figure 9b is the iron powders lost in tailing, and Figure 9c,d are the nephelines and Figure 9 shows that the sodium in tailing is mainly symbiotic with aluminum and silicium.A comparison of Figures 8 and 9 proves that the gangue granules in the SEM images are mainly nepheline.Given that the A/S of the ferric bauxite adopted is only 2.7, the generation a finite amount of nepheline is acceptable.Kaolinite is the main silicium-bearing mineral in ferric bauxite ore, and sodium carbonate is the main composition of the material.Hence, nepheline is inevitably generated from the reaction between kaolinite and sodium carbonate.The carbon black shown in Figure 8 is the residue of coal.

Iron Powder
Iron powder is the concentrate of magnetic separation.The XRD and SEM-EDS patterns of iron powder obtained with the restraining method are shown in Figures 10 and 11 respectively.
Figure 8 shows that nepheline is the main sodium-bearing mineral in tailing.Combined with XRD pattern, Figure 9b is the iron powders lost in tailing, and Figures 9c,d are the nephelines and Figure 9 shows that the sodium in tailing is mainly symbiotic with aluminum and silicium.A comparison of Figures 8 and 9 proves that the gangue granules in the SEM images are mainly nepheline.Given that the A/S of the ferric bauxite adopted is only 2.7, the generation a finite amount of nepheline is acceptable.Kaolinite is the main silicium-bearing mineral in ferric bauxite ore, and sodium carbonate is the main composition of the material.Hence, nepheline is inevitably generated from the reaction between kaolinite and sodium carbonate.The carbon black shown in Figure 8 is the residue of coal.

Iron Powder
Iron powder is the concentrate of magnetic separation.The XRD and SEM-EDS patterns of iron powder obtained with the restraining method are shown in Figures 10 and 11, respectively.Figure 10 shows that nepheline is the main gangue mineral in iron powder.Combined with XRD pattern, Figures 11b,c are nepheline and iron powder, respectively.Figures 11b,c show that most of the granules in iron powder are large enough and achieve monomeric liberation, which supposedly indicate the removing possibility of nepheline.If nepheline can be removed by beneficiation, then the quality of iron powder is not affected.Otherwise, the remaining sodium inevitably becomes a harmful component.Figure 10 shows that nepheline is the main gangue mineral in iron powder.Combined with XRD pattern, Figure 11b,c are nepheline and iron powder, respectively.Figure 11b,c show that most of the granules in iron powder are large enough and achieve monomeric liberation, which supposedly indicate the removing possibility of nepheline.If nepheline can be removed by beneficiation, then the quality of iron powder is not affected.Otherwise, the remaining sodium inevitably becomes a harmful component.Figure 8 shows that nepheline is the main sodium-bearing mineral in tailing.Combined with XRD pattern, Figure 9b is the iron powders lost in tailing, and Figures 9c,d are the nephelines and Figure 9 shows that the sodium in tailing is mainly symbiotic with aluminum and silicium.A comparison of Figures 8 and 9 proves that the gangue granules in the SEM images are mainly nepheline.Given that the A/S of the ferric bauxite adopted is only 2.7, the generation a finite amount of nepheline is acceptable.Kaolinite is the main silicium-bearing mineral in ferric bauxite ore, and sodium carbonate is the main composition of the material.Hence, nepheline is inevitably generated from the reaction between kaolinite and sodium carbonate.The carbon black shown in Figure 8 is the residue of coal.

Iron Powder
Iron powder is the concentrate of magnetic separation.The XRD and SEM-EDS patterns of iron powder obtained with the restraining method are shown in Figures 10 and 11, respectively.

Conclusions
The effects of heating temperature, heating time, and sodium carbonate dosage on sodium volatilization were investigated.Results show that the descending order of their magnitude is as follows: heating temperature > heating time > sodium carbonate dosage.Thus, R can be reduced by setting the heating temperature to the minimum possible value, and ηA can be increased by appropriately increasing the sodium carbonate dosage.In other words, a low R and a high ηA can be simultaneously achieved at a low heating temperature and high sodium carbonate dosage.Actually, it is verified that ηA would increase only when sodium carbonate dosage rise enough (e.g., from 85% to 120%).
With sodium volatilization restrained, more sodium is distributed in the solid phase.Nepheline is the main sodium-containing mineral in iron powder, and almost all of its granules are large enough and achieve monomeric liberation, which indicates the removing possibility of nepheline.The restraining process can theoretically be conducted harmlessly.

Conclusions
The effects of heating temperature, heating time, and sodium carbonate dosage on sodium volatilization were investigated.Results show that the descending order of their magnitude is as follows: heating temperature > heating time > sodium carbonate dosage.Thus, R can be reduced by setting the heating temperature to the minimum possible value, and η A can be increased by appropriately increasing the sodium carbonate dosage.In other words, a low R and a high η A can be simultaneously achieved at a low heating temperature and high sodium carbonate dosage.Actually, it is verified that η A would increase only when sodium carbonate dosage rise enough (e.g., from 85% to 120%).
With sodium volatilization restrained, more sodium is distributed in the solid phase.Nepheline is the main sodium-containing mineral in iron powder, and almost all of its granules are large enough and achieve monomeric liberation, which indicates the removing possibility of nepheline.The restraining process can theoretically be conducted harmlessly.

Figure 2 .
Figure 2. (a) SEM image of ferric bauxite; (b) EDS image of point 1; (c) EDS image of point 2; (d) EDS image of point 3; (e) EDS image of point 4; (f) EDS image of point 5 and (g) EDS image of point 6.

Figure 2 .
Figure 2. (a) SEM image of ferric bauxite; (b) EDS image of point 1; (c) EDS image of point 2; (d) EDS image of point 3; (e) EDS image of point 4; (f) EDS image of point 5 and (g) EDS image of point 6.

Figure 4 .
Figure 4. (a) Response surfaces of heating time-sodium carbonate dosage plot and (b) contour of heating time-sodium carbonate dosage plot.

Figure 4 .
Figure 4. (a) Response surfaces of heating time-sodium carbonate dosage plot and (b) contour of heating time-sodium carbonate dosage plot.

Figure 5 .
Figure 5. (a) Response surfaces of heating temperature-sodium carbonate dosage and (b) contour of heating temperature-sodium carbonate dosage plot.

Figure 5 .
Figure 5. (a) Response surfaces of heating temperature-sodium carbonate dosage and (b) contour of heating temperature-sodium carbonate dosage plot.

Figure 6 .
Figure 6.(a) Response surfaces of heating temperature-time and (b) contour of heating temperature-time plot.

Figure 6 .Figure 6 .
Figure 6.(a) Response surfaces of heating temperature-time and (b) contour of heating temperature-time plot.

Figure 7 .
Figure 7. Plot between η A and temperature.

Figure 9 .
Figure 9. (a) SEM image of tailing; (b) EDS image of point 1; (c) EDS image of point 2 and (d) EDS image of point 3.

Figure 10
Figure10shows that nepheline is the main gangue mineral in iron powder.Combined with XRD pattern, Figures11b,care nepheline and iron powder, respectively.Figures11b,cshow that most of the granules in iron powder are large enough and achieve monomeric liberation, which supposedly indicate the removing possibility of nepheline.If nepheline can be removed by beneficiation, then the quality of iron powder is not affected.Otherwise, the remaining sodium inevitably becomes a harmful component.

Figure 11 .
Figure 11.(a) SEM image of iron powder; (b) EDS image of point 1 and (c) EDS image of point 2.

Figure 11 .
Figure 11.(a) SEM image of iron powder; (b) EDS image of point 1 and (c) EDS image of point 2.

Table 1 .
Chemical composition of ferric bauxite.

Table 3 .
Chemical composition of coal ash.

Table 2 .
Proximate analysis of the coal used.

Table 3 .
Chemical composition of coal ash.

Table 4 .
Box-Behnken design and experimental results.

Table 5 .
Variance analysis of regression models.

Table 6 .
Confidence analysis of the quadratic model.

Table 6 .
Confidence analysis of the quadratic model.

Table 7 .
Verification of restraining method on sodium volatilization and alumina leaching.