稀有金属(英文版) 2015,34(10),744-751

收稿日期：14 May 2015

基金：financially supported by the National Natural Science Foundation of China(No.51204037);

Extracting B₂O₃ from calcined boron mud using molten sodium hydroxide

Zhi-Qiang Ning Yu-Chun Zhai Qiu-Shi Song

School of Materials and Metallurgy,Northeastern University

Abstract：

Extracting B2O3 from calcined boron mud(CBM) was studied. The effect of factors such as reaction temperature and NaO H-to-CBM mass ratio on B2O3 extraction efficiency was investigated. The results show that increasing reaction temperature and NaO H-to-CBM mass ratio increases B2O3 extraction efficiency. There are two stages for the B2O3 extracting process: 0–20 min is the first stage, which is rapid; 20–50 min is the second stage, which is slower than the first stage. The overall extracting process follows the shrinking core model, and the first and second stages are determined to obey the surface chemical reaction model and the diffusion through the products layer model,respectively. The activation energies of the first and second stages are calculated to be 41.74 and 15.43 kJ·mol-1,respectively. The B2O3 extracting kinetics equations of the first and second stages are also obtained.

Keyword：

Boron mud; Sodium hydroxide; Boron oxide; Kinetics;

Author： Zhi-Qiang Ning,e-mail:ningzq@sina.com;

Received： 14 May 2015

1 Introduction

Boron, which is applied in many fields such as chemistry, manufacturing, and metallurgy and materials engineering [1], is a very important element, and boron ore resources are rich in different parts of the world. The USA and Turkey are the world’s major producers of boron [2–7]. Boron ore in China is mainly associated with boromagnesite and ludwigite, and it contains only about from 8 % to 15 % (averaging about 8.4 %) boron oxide [8–11]. For many years, a solid waste that was called boron mud (BM) appeared because boron was leached from boromagnesite and ludwigite ore to produce borax. In fact, boron was not extracted totally according to the traditional production process of borax and some residual boron still remained in the BM solid waste. This mud contained boron oxide in concentration levels varying from 2 % to 6 % [12, 13]. BM provides a useful resource for boron to supplement ore resources that should not be wasted.

The main chemical composition of BM is magnesium oxide and silicon dioxide, and its specific composition depends on the conditions under which it is produced. The unprocessed BM occupies a large area lands and causes a serious environmental pollution problem because of its strong basic. At present, many researchers who investigated BM processing focus on leaching magnesia from BM to produce magnesia products such as magnesium sulfate, magnesium carbonate, magnesium hydroxide, and magnesium oxide [12–16]. Few researchers studied boron oxide removal from BM in molten sodium hydroxide media.

One new technology that the boron oxide was extracted and that the silicon dioxide was simultaneously removed from the calcined boron mud (CBM) in molten sodium hydroxide media was established. This technology can take full advantages of boron oxide and silicon dioxide. The boron oxide and the silicon dioxide are in the form of B4O7^2-and Si O4^4-, respectively, in aqueous solution after aqueous-leaching the roasted CBM that exposure in the molten sodium hydroxide.

In this article, the effect of factors such as reaction temperature and Na OH-to-CBM mass ratio on B₂O₃extraction efficiency was investigated to optimize the conditions and determine the B₂O₃extracting kinetics of CBM in molten sodium hydroxide media.

Table 1 Chemical compositions of BM and CBM (wt%)  下载原图

Table 1 Chemical compositions of BM and CBM (wt%)

Fig.1 XRD patterns of BM and CBM

2 Experimental

2.1 Materials

Table 1 shows the chemical analysis results of BM and CBM which were obtained from Liaoning Province in China and used in all the experiments, examined by inductively coupled plasma optical emission spectrometer (ICP–OES) [17].

The X-ray diffraction (XRD) patterns of BM and CBM under the condition of 700 °C for 4 h shown in Fig. 1 indicate that magnesium carbonate (Mg CO₃), magnesium orthosilicate (Mg₂Si O₄) and its heterogeneous isomorphism with Fe instead of Mg are major phases in BM, and magnesium carbonate decomposes into magnesium oxide and carbon oxide at high temperature. Magnesium borate can be detected for BM weight loss because of the evolution of carbon dioxide. Also, dissociative silicon dioxide appears in CBM.

The scanning electron microscopy (SEM) image of BM shown in Fig. 2 indicates that BM particles are very tiny and most of them are \10 lm. Thus, the effect of particles size on the B₂O₃extraction efficiency is not considered.

2.2 General extraction procedure

The mixture of CBM and sodium hydroxide were put into a stainless steel crucible according to a certain mass ratio, the ratio of Na OH-to-CBM varied from 1.00 to 1.75 in interval of 0.25. A wire type resistance furnace was used, and the temperature was controlled by a programmable temperature controller with a precision of ±2 °C. When the settingtemperature, which was set from 400 to 550 °C in interval of 50 °C, was reached, the stainless steel crucible with the mixture was put into the resistance furnace and timing began. Inpidual samples were taken every 5 min to cease the reaction from 0 to 50 min. Every sample was leached with water at 90 °C for 40 min at a speed of 450 r min^-1. After leaching, the dry solid residuals was obtained after dehydrating the solid residuals at 400 °C for 4 h, the boron oxide and silicon dioxide can be separated by carbonating leaching solution to accomplish the recovery of the silicon dioxide. The recovery of silicon dioxide can reach 88.24 % under the experiment conditions of the stirring speed of 450 r min^-1, temperature of 60 °C, sodium silicate concentration of leaching solution of 1 mol L^-1, the carbonation time of 6 h, and the unchanged content of B₂O₃in the solution after carbonating. The general process flow diagram is shown in Fig. 3.

Fig.2 SEM image of CBM

The content of B₂O₃in the leaching solution was determined by chemical titration [18], and the B₂O₃extraction efficiency was calculated as:

where a(B₂O₃) is B₂O₃extraction efficiency and m(B₂O₃) and m(B₂O₃) are the mass of B₂O₃in leaching solution and the mass of B₂O₃in CBM ores, respectively.

2.3 Chemicals and reagents

The sodium hydroxide used in B₂O₃extracting experiments were all for industrial grade. The reagents used in the chemical titration experiments were all for chemically pure. The water used in all experiments was deionized water.

Fig.3 General process flow diagram

2.4 Characterization

The phase composition of the experimental samples were determined by XRD (D/max-2500PC) with Cu Ka radiation (0.15405 nm) at 40 k V and 40 m A, continuous scanning, scanning range 2θ of 10°–90°, and scanning speed of 10 (°) min^-1. The morphology of the experimental samples was characterized by SEM (SSX-550). The elements of the experimental samples were determined by X-ray energydispersive spectroscopy (EDS) attached to SEM. The chemical compositions of the experimental samples were examined by ICP–OES (ICAP 7000).

3 Results and discussion

3.1 Chemical reaction

The main chemical reactions that occur between CBM and Na OH at high temperature are as follows:

The main chemical reaction that occurs during the leaching process at 90 °C for 40 min at stirring speed of 450 r min^-1is as follow:

Fig.4 Effect of reaction temperature on B2O3extraction efficiency

The main chemical reaction that occurs at 400 °C for drying the solid residuals is as follow:

3.2 Effect of reaction temperature

The influence of reaction temperature on B₂O₃extraction efficiency with Na OH-to-CBM mass ratio of 1.75 is shown in Fig. 4. The temperature has a noticeable influence on B₂O₃extraction efficiency. The B₂O₃extraction efficiency increases with the increase in the reaction temperature. Figure 4 also shows that the effect of reaction temperature on B₂O₃extraction efficiency is pided into two parts: 0–20 min is the first stage in which the extracting reaction is very rapid; 20–50 min is the second stage in which the extracting reaction is quite slow. When the reaction time is 20 min, the B₂O₃extraction efficiency changes from 41.24 % to 94.95 % with the reaction temperature changing from 400 to 550 °C; when the reaction time is 50 min, the B₂O₃extraction efficiency changes from 49.33 % to 96.59 % with the reaction temperature changing from 400 to 550 °C. At 400 °C, the increment of the B₂O₃extraction efficiency is 7.09 % (49.33 %– 41.24 %) in 20–50 min; while at 550 °C, the increment of the B₂O₃extraction efficiency is only 1.46 % (96.59 %– 94.95 %) for the same period of time. The reason is that B₂O₃in CBM tends to react completely. Therefore, it does not have a noticeable influence on the B₂O₃extraction efficiency to increase reaction time after 20 min at 550 °C, and the reaction temperature should not be higher than 550 °C for saving energy consumption.

3.3 Effect of Na OH-to-CBM mass ratio

The influence of Na OH-to-CBM mass ratio on B₂O₃extraction efficiency under the conditions of 550 °C is shown in Fig. 5. As shown in Fig. 5, the B₂O₃extraction efficiency is improved with the increase in Na OH-to-CBMmass ratio. Excess sodium hydroxide is necessary to make sodium hydroxide have full contact with CBM and ensure sufficient reaction. Figure 5 also shows that extracting reaction is rapid from 0 to 20 min, while that is slow from 20 to 50 min. Therefore, the effect of Na OH-to-CBM mass ratio on B₂O₃extraction is also pided into two parts.

Fig.5 Effect of Na OH-to-CBM mass ratio on B2O3extraction efficiency

3.4 Kinetics analysis

The B₂O₃extracting process from CBM using molten sodium hydroxide is a typical liquid–solid system. B₂O₃and Si O₂in CBM react with sodium hydroxide and generate soluble sodium borate and sodium magnesium silicate. Mg in CBM generates magnesium hydroxide in molten sodium hydroxide. Also sodium magnesium silicate and magnesium hydroxide product can be attached to the particles surfaces, and reaction can be analyzed with the shrinking core model [19]. According to the model, the rate between solid particle and reaction reagent may be controlled by one of the following steps: diffusion through the fluid (outer diffusion), diffusion through the product layer (inner diffusion), and the chemical reaction at the surface. As we know, temperature has few influences on the leaching rate when the diffusion through the fluid is the rate-controlling step. But on the one hand, temperature has a noticeable influence on B₂O₃extraction efficiency. On the other hand, reaction is controlled by inner diffusion when the reaction occurs at high temperature and generates solid product. Therefore, it can be concluded that the diffusion through fluid is not the rate-controlling step in the process of extracting B₂O₃from CBM in molten sodium hydroxide.

The following analysis can determine the kinetic parameters and rate-controlling step for extracting B₂O₃from the CBM. If the surface chemical reaction is the ratecontrolling step, then the following expression of the shrinking core model can be used to describe the B₂O₃extracting kinetics of the process.

where a is extraction efficiency, k_ris the apparent rate constant of the chemical reaction at the surface, and t is the reaction time. Similarly, if the diffusion through the product layer is the rate-controlling step, then the following expression of the shrinking core model can be used to describe the B₂O₃extracting kinetics.

where k_dis the apparent rate constant of the diffusion through the product layer.

The experimental data presented in Figs. 4 and 5 were used to fit Eqs. (7) and (8), and the analysis results are shown in Figs. 6, 7, 8 and 9, respectively. In Table 2, the apparent rate constants and correlation coefficient values are given for surface chemical reaction and diffusion through the product layer. Furthermore, the linear relationship between 1-(1-a)^1/3and the first and the secondstage reaction time can be seen in Figs. 6 and 8 as a function of the reaction temperature and the Na OH-toCBM mass ratio, respectively. While the relationship between 1-3(1-a)^2/3? 2(1-a) and the first stage reaction time for the reaction temperature and the Na OHto-CBM mass ratio is not linear, it is linear for the second stage reaction time, as seen in Figs. 7 and 9, respectively.

Fig.6 Plots of 1-(1-α)1/3vs time at different temperatures: a the first stage and b the second stage

Fig.7 Plots of 1-3(1-α)2/3+ 2(1-α) versus time at different temperatures: a the first stage and b the second stage

Fig.8 Plots of 1-(1-α)1/3versus time at different Na OH-to-CBM mass ratios: a the first stage and b the second stage

According to Arrhenius equation, it is represented as [20]:

where k is the reaction rate constant, k₀is the pre-exponential factor, E is the reaction activation energy, R is the mole gas constant (8.314 k J mol^-1), and T is the thermodynamic temperature.

The apparent rate constants (R²) are shown in Table 2, which are determined from the straight lines in Figs. 6 and 7b (dot line) and Figs. 8 and 9b (dot line) and were used to calculate the activation energy to make the plot of lnk versus T^-1. The Arrhenius plots of surface chemical reaction and diffusion through the product layer are shown in Figs. 10 and 11, respectively. The activation energies are listed in Table 3.

The apparent activation energy of the first stage process is calculated to be 41.74 k J mol^-1, and the apparent activation energy of the second stage process is calculated to be 10.25 and 15.43 k J mol^-1for surface chemical reaction and diffusion through the product layer, respectively. As we know, the apparent activation energy is usually [40 k J mol^-1for a chemically controlled process and the activation energy of diffusion controlled reactions is usually between 8 and 20 k J mol^-1[21–24]. In accordance with these results, the first stage process and the second stage process are determined to obey the surface chemical reaction model and the diffusion through the product layer model, respectively.

Fig.9 Plots of 1-3(1-α)2/3+ 2(1-α) versus time at different Na OH-to-CBM mass ratios: a the first stage and b the second stage

Table 2 Apparent rate constant and correlation coefficient values  下载原图

Table 2 Apparent rate constant and correlation coefficient values

3.5 Characterization of dry solid residuals

The dry solid residuals were obtained from the testing at 550 °C for 20 min under the conditions of Na OH-to-CBM mass ratio of 1.75.

The XRD patterns of the CBM after exposure in molten sodium hydroxide and the dry solid residuals are shown in Fig. 12. It can be seen that there are Na₄B₂O₇, Na₂Mg Si O₄, Na₄Si O₄, and Mg O phases in CBM after exposure in molten sodium hydroxide and there are only magnesium oxide and a small amount of magnesium hydroxide phases in the dry solid residuals, indicating that B₂O₃and Si O₂removals during extracting reaction are efficient at high temperature because they dissolve into leaching solution and cannot be found in the dry solid residuals.

The SEM image of the dry solid residuals is presented in Fig. 13. It can be seen that the dry residual particles of Si O₂and B₂O₃extracted and leached slag are almost completely broken after reaction in molten sodium hydroxide system.

The EDS results of the CBM and the dry solid residuals are presented Fig. 14. It can be seen that although there are several elements (B, O, Na, Mg, Al, Si, Ca, and Fe) in CBM and dry solid residuals, the intensity of B and Si in dry solid residuals is much weaker than that in CBM, indicating that most of B and Si are extracted from CBM.

The chemical compositions of the dry solid residuals examined by ICP–OES are shown in Table 4. Compared with Table 1, it can be seen that the content of B₂O₃is lowered, the contents of magnesium and iron are improved, and the content of magnesium oxide is 92.04 %. The reason is that 94.95 % B₂O₃and 91.90 % Si O₂are extracted from CBM at 550 °C for 20 min under the conditions of Na OH-to-CBM mass ratio of 1.75 and dissolve into the leaching solution in B4O7^2-and Si O4^4-after aqueous leaching. Although the content of iron is improved, thecontents of iron dioxide and silicon dioxide are still low, and they cannot usually be accurately evaluated by XRD. Therefore, it is possible that they transfer to amorphousphase after exposure in molten sodium hydroxide and aqueous leaching according to the comparison of Figs. 12 and 14.

Fig.10 Arrhenius plots of surface chemical reaction controlling: a the first stage and b the second stage

Fig.11 Arrhenius plot of diffusion through product layer controlling (the second stage)

Table 3 Activation energies and B₂O₃extracting kinetic equations from CBM  下载原图

Table 3 Activation energies and B₂O₃extracting kinetic equations from CBM

Fig.12 XRD patterns of CBM after exposure in molten sodium hydroxide and dry solid residuals after aqueous leaching

Fig.13 SEM image of dry solid residuals after aqueous leaching

Fig.14 EDS results of CBM and dry solid residuals after aqueous leaching

Table 4 Chemical compositions of dry solid residuals (wt%)  下载原图

Table 4 Chemical compositions of dry solid residuals (wt%)

4 Conclusion

The effect of reaction parameters on the B₂O₃extraction efficiency of CBM was examined. The results show that the B₂O₃extraction efficiency increases with the increase in reaction temperature, reaction time and Na OH-to-CBM mass ratio. The B₂O₃extraction efficiency is up to 96.59 % under the conditions of the reaction temperature of 550 °C, reaction time of 50 min, and mass ratio of Na OH-to-CBM of 1.75. Fe and Mg elements are enriched in the solid residual, and the content of magnesium oxide is 92.04 %. The solid residual may be used as raw material to produce high-purity magnesium products.

The B₂O₃extracting process of CBM using molten sodium hydroxide includes two stages and follows the shrinking core model. The first and second stages of the process are determined to obey the surface chemical reaction model and the diffusion through the product layer model, respectively. The activation energies of the first and secondstagesarecalculatedtobe41.74and 15.43 k J mol^-1, respectively. The B₂O₃leaching kinetics equation of the first and second stages can be expressed as respectively.