Dissolution Behavior of Sodium Phosphate in a Na₃PO₄–Na₂WO₄–NaOH Solution System

Abstract

Sodium hydroxide autoclaving is the main method for smelting scheelite in China. In this method, sodium phosphate is added as an additive to realize the highly efficient decomposition of scheelite, and a crude sodium tungstate solution containing sodium phosphate and sodium hydroxide is obtained. In the subsequent process of ion exchange, phosphorus ions in the solution compete with the resin adsorption of tungstate, which reduces the adsorption capacity of the resin and endangers the purity of the subsequent sodium tungstate solution. To remove the phosphorus from crude sodium tungstate solution, a chemical purification method is usually adopted. The principle of the chemical purification method is to use chemical reagents to react with impurities to form precipitates to achieve the purpose of impurity removal. Because of the advantages of simple industrial implementation and high impurity removal efficiency, it has been widely used in phosphorus removal from crude sodium tungstate solution. However, in the process of phosphate removal in a crude sodium phosphate solution, the chemical purification method has some disadvantages. First, the additional cost of chemical reagents is required, and other metal impurities from chemical reagents would be introduced to crude sodium tungstate solution. Second, phosphate impurity removed by the chemical precipitation method is usually sedimented in other forms but sodium phosphate, which makes the phosphate resource unable to be recycled for tungsten smelting. Therefore, a novel phosphorus removal method needs to be developed. The dissolution behavior of sodium phosphate in a Na₃PO₄–Na₂WO₄–NaOH system was investigated in this paper. The results showed that in binary or ternary solution systems of sodium phosphate, sodium tungstate, and sodium hydroxide, the common-ion effect and salt effect exist simultaneously. The common-ion effect decreases the solubility of sodium phosphate, while the salt effect increases the solubility of sodium phosphate. Increasing the concentration of sodium hydroxide or sodium tungstate and lowering the temperature of the solution can greatly reduce the phosphorus concentration in crude sodium tungstate solution, making the crude sodium tungstate solution meet industrial requirements of ion exchange. The results of the study lay a theoretical foundation for the development of new phosphorus removal methods.

1. Introduction

Tungsten is an important strategic metal with high hardness, a high density, a high melting point, and other characteristics. It is widely used in various fields, including national defense and aerospace, and is an important metal for national economies [1]. With the continuous consumption of tungsten resources, scheelite has become the main raw material for tungsten smelting in China [2,3,4,5,6]. From the early 1980s to the late 1990s, with the efforts of tungsten metallurgy researchers, the sodium hydroxide decomposition method on scheelite represented by hot ball milling (mechanical activation) and sodium hydroxide autoclaving achieved revolutionary breakthroughs [7,8,9,10,11]. Since then, almost all tungsten smelting enterprises in China have used the sodium hydroxide decomposition method to treat tungsten minerals [12,13].

As shown in Figure 1, sodium hydroxide autoclaving is the main method to decompose scheelite in China. To prevent the occurrence of “reforming calcium tungstate” in the crude sodium tungstate solution by generating Hydroxyapatite, sodium phosphate is usually added as an additive for the collaborative digestion of scheelite [9,14,15]. Thus, excess sodium phosphate will remain in the crude sodium tungstate solution, which will lead to adverse effects during the subsequent impurity removal and purification of the crude solution [16].

In industry, chemical precipitation is the most commonly used method to remove excess phosphate from crude sodium tungstate solution [17,18,19,20]. However, chemical precipitation methods have some problems, such as the large consumption of phosphorus removal reagents, the introduction of other metal impurities, and removal of unrecyclable phosphorus for subsequent scheelite smelting processes [21,22,23,24,25]. Therefore, a novel phosphorus removal method needs to be developed. Usually, to prevent other impurities from being introduced, the solubility differences of solutes will be manipulated by changing the concentration of a solute or the temperature of a solution containing several solutes, thereby allowing some or several solutes to be selectively separated [26,27,28,29,30,31].

Wu Xiaoqin et al. [31] determined the solubility of calcium sulfate dihydrate in a CaCl₂–MgCl₂–KCl ternary solution system. The results showed that lowering the temperature had a positive effect on promoting calcium sulfate crystallization. The calcium ions in the solution showed a common-ion effect on calcium sulfate, which benefitted the crystallization effect of calcium sulfate. Shifeng Jiang et al. [32] investigated the extraction of sodium bicarbonate in a binary NaHCO₃–Na₂CO₃ solution system, and the results showed that increasing the concentration of sodium carbonate could promote the common-ion effect in solution, which enhanced the crystallization effect of sodium bicarbonate in the subsequent cooling process. Xuewen Wang et al. [33] investigated the solubility of sodium fluoride in a NaNO₃–NaCl–Na₂CO₃–Na₂SO₄–NaOH solution system, and research showed that increasing the solute concentration containing Na⁺ promoted the common-ion effect in solution; thus, the sodium fluoride crystal can be recovered by cooling crystallization without evaporation.

From the above, the common-ion effect caused by the competitive dissolution of different solutes is widely present in the solution system. The addition of a strong electrolyte containing the same ions to the solution of the insoluble precipitate easily produces the common-ion effect and shifts the chemical equilibrium toward the formation of insoluble matter. Similarly, crude sodium tungstate solutions are typical multisolute solution systems containing sodium phosphate, sodium tungstate, and sodium hydroxide. Interactions among the heterogeneous solutes may have complex effects on solute solubility. As the concentration of the more soluble sodium hydroxide increases, the less soluble sodium phosphate may precipitate from the solution by the effect of the common-ion effect, thereby solving the problems of the chemical precipitation method. Therefore, to study the influence of WO₄²⁻ and OH⁻ on the solubility of sodium phosphate, the article studied the dissolution behavior of sodium phosphate in a multicomponent Na₃PO₄–Na₂WO₄–NaOH system.

2. Materials and Methods

2.1. Materials

The experimental raw materials of sodium hydroxide (analytical grade), sodium phosphate (analytical grade), and sodium tungstate (>98%) were purchased from Macklin Scientific Co., Ltd., Shanghai, China. Ammonium molybdate and stannic chloride were added.

2.2. Experimental Apparatus and Procedures

The experimental procedures were mainly composed of three steps, and the detailed experimental flow diagram is shown in Figure 2.

A certain amount of sodium hydroxide/sodium tungstate (or a mixture of both) was put into deionized water, mixed, and stirred for more than 10 min, until no obvious undissolved hydroxide/sodium tungstate solids could be visually observed. Then, using the saturated solubility of sodium phosphate in the aqueous solution as a reference, excess sodium phosphate was added to the solution (the initial amount of sodium phosphate added in the mixed solution was 300 g per liter), and the mixed solution was set to a constant volume of 500 mL.

Then, the mixed solution was placed into the three-necked bottle equipped with a cooling tube. The condensing tubes were used to prevent errors due to the volatilization of the solution. The three-necked bottle was connected with a thermostat water bath device (ZNCL-GS240*150, Yuming Instrument, Co., Ltd., Shanghai, China). The thermostat water bath device was equipped with a magnetic stirring function and the temperature error of the water bath heater was within ±0.1 K. After that, the heating temperature of the heater was set, the mixed solution was heated, and the magnetic stirring speed was kept constant while stirring during the heating process.

During the mixing and solution heating process, the magnetic stirring device was stopped at 30 min intervals until the mix solution was stable. One mL supernatant liquid of the mixed solution was diluted to 100 mL as a solution to be tested. As for the preparation of the spectrophotometer test, the 3 mL ammonium molybdate and 1 mL SnCl₂ were added into the solution as phosphorus chromogenic agents. After that, a spectrophotometer (UV-1100, Mapada, Shanghai, China) was used to measure the content of phosphorus in the solution. The above experiment was repeated every 30 min until the measured data of phosphorus concentration trended to a constant which, in this case, can be considered as the sodium phosphate in the solution reaching the saturated solution state.

The results of the phosphorus concentration under different experimental conditions were drawn into a three-dimensional data point chart, and the trend of the change in the phosphorus concentration was determined by surface fitting. Then, the surface fitting results were analyzed. In particular, the surface fitting of the experimental data was obtained by fitting a finite number of experimental data points. In the process of fitting 3D points to a 3D surface, the fitting situation between adjacent experimental data points may have errors compared with the actual solubility, and these errors were more obvious at the boundary of the experimental conditions. Therefore, surface fitting was only used as a reference to assist in judging the trend of changes in phosphorus concentration.

3. Results and Discussion

3.1. Dissolution Behavior of Sodium Phosphate in a Binary Na₃PO₄–Na₂WO₄ Solution System

The solubility of sodium phosphate in a binary Na₃PO₄–Na₂WO₄ solution system was investigated at temperatures of 275.15–353.15 K and sodium tungstate concentrations of 20–100 g/L; furthermore, the solubility of Na₃PO₄ in water was compared with that of sodium phosphate [34]. The results are shown in Figure 3.

From Figure 3 and Figure 4, at 275.15 K, as the concentration of sodium tungstate increased from 0 g/L to 100 g/L, and the concentration of saturated phosphorus in the solution increased from 8.45 g/L to 10.00 g/L before decreasing to 7.80 g/L. Thus, the solubility of sodium phosphate showed a trend of first increasing and then decreasing. At temperatures from 293.15 K to 353.15 K, as the concentration of sodium tungstate was increased from 0 g/L to 100 g/L, the concentration of saturated phosphorus in the solution decreased from 113.90 g/L to 55.02 g/L. Thus, the saturated phosphorus concentration in the solution showed a downward trend.

The above results show that a significant difference existed in the trend of changes in the saturated phosphorus concentration at temperatures of 275.15 K and 293.15~353.15 K with increasing sodium tungstate concentration. These changes may be caused by the competition between the common-ion effect and salt effect, as shown in Figure 5.

From Figure 5, sodium phosphate can ionize PO₄³⁻ and Na⁺ ions in solution. Due to the influence of the interionic Coulomb force, an ionic atmosphere that attracts hydroelectric ions is easily generated around PO₄³⁻ ions. If additional Na⁺ ions are introduced into the solution (or a solute containing Na⁺), the concentration of Na⁺ in the solution increases, and the possibility of Na⁺ and PO₄³⁻ adsorption is significantly increased, resulting in a decrease in the saturated solubility of the sodium phosphate solute. Correspondingly, the dissolution of sodium tungstate and the introduction of WO₄²⁻ into a solution system will also affect the saturation solubility of sodium phosphate. Among them, WO₄²⁻ can produce an ionic atmosphere that attracts Na⁺, which promotes a competitive relationship between WO₄²⁻ and PO₄³⁻ in attracting Na⁺. Therefore, the addition of sodium tungstate to a solution will have both a common-ion effect and a salt effect on PO₄³⁻ ions in the solution. The common-ion effect decreases the saturation solubility of sodium phosphate, while the salt effect increases the saturation solubility of sodium phosphate.

At a temperature of 275.15 K, the temperature of the solution is relatively low. Therefore, the thermal movement and degree of diffusion of ions are relatively weak, and the Coulomb force effect between ions is relatively strong, which promotes the ability of the ionic atmosphere of WO₄²⁻ to attract Na⁺ and makes the salt effect dominant. As the sodium tungstate concentration is increased, the common-ion effect caused by the increase in Na⁺ concentration gradually becomes dominant. The ability of the PO₄³⁻ ion to attract Na⁺ is relatively enhanced, which makes the saturated phosphorus concentration in solution first increase and then decrease with increasing sodium tungstate input. At temperatures of 293.15 K to 353.15 K, the thermal movement and degree of diffusion of ions are relatively high. The salt effect on Na⁺ attraction is relatively weakened, and the increase in the Na⁺ concentration in solution leads to the common-ion effect becoming dominant, which promotes the saturated phosphorus concentration in solution to show an overall downward trend with increasing sodium tungstate input.

3.2. Dissolution Behavior of Sodium Phosphate in a Binary Na₃PO₄–NaOH Solution System

The solubilities of sodium phosphate and sodium hydroxide in water are significantly different; additionally, sodium hydroxide also contains Na⁺. Under the same temperature conditions, the solubility of sodium hydroxide is greater than that of sodium phosphate, thereby providing the basis to produce the common-ion effect on the concentration of sodium phosphate. Based on this, the saturated phosphorus concentration in binary Na₃PO₄–NaOH solution systems at temperatures of 275.15–353.15 K and sodium hydroxide concentrations of 0–360 g/L were studied, and the solubility of sodium phosphate in water was compared with that of sodium phosphate, as shown in Figure 6 and Figure 7.

From Figure 6 and Figure 7, under the condition that the concentration of sodium hydroxide remains unchanged, the precipitation of sodium phosphate can be promoted by lowering the temperature of the solution. At temperatures of 275.15–353.15 K and sodium hydroxide concentrations of 0–360 g/L, the saturated phosphorus concentration in solution showed an overall downward trend with increasing sodium hydroxide input. As the sodium hydroxide concentration in the solution was increased, at 353.15 K, the saturated phosphorus concentration decreased from 113.90 g/L to 38.41 g/L. At 275.15 K, the saturated phosphorus concentration decreased from 8.45 g/L to 0.14 g/L. This result is different from the saturated phosphorus concentration in the binary Na₃PO₄–Na₂WO₄ solution system because, in the binary Na₃PO₄–NaOH solution system, the saturated phosphorus concentration in the solution showed a downward trend in the temperature range of 293.15–353.15 K.

In the binary Na₃PO₄–NaOH solution system, at 275.15 K, with increasing sodium phosphate concentration, the saturated phosphorus concentration showed a downward trend, which is different from that of the binary Na₃PO₄–Na₂WO₄ solution system. The ion behavior in the binary Na₃PO₄–NaOH solution system is shown in Figure 8.

From Figure 8, the reason may be that the degree of ionization of sodium hydroxide in solution is stronger than that of sodium tungstate. Thus, the ability of OH⁻ to attract Na⁺ is weaker than that of WO₄²⁻ because the common-ion effect is always stronger than that of the salt effect. As the Na⁺ concentration in the solution increased, the saturated phosphorus concentration in the solution decreased.

It can be concluded that the stronger the ionization ability of the solute with the Na⁺ ion component that is added to the solution, the weaker the salt effect on PO₄³⁻. Therefore, compared with the addition of sodium tungstate, the addition of sodium hydroxide to sodium phosphate solution is more beneficial to reduce the saturated phosphorus concentration.

3.3. Dissolution Behavior of Sodium Phosphate in a Na₃PO₄–Na₂WO₄–NaOH Ternary Solution System

Based on the above studies on the solubility of sodium phosphate in the binary Na₃PO₄–Na₂WO₄ and Na₃PO₄–NaOH solution systems, it can be seen that when sodium tungstate and sodium hydroxide are added into the solution system, the interaction between different ions has a significant difference on the phosphorus concentration in solution.

On the one hand, it is necessary to take into account the influence of the interaction between WO₄²⁻ and OH⁻ on the solubility of sodium phosphate to provide a reference for the variation in phosphorus concentration in complex solution systems. On the other hand, considering that the composition of crude sodium tungstate solution during actual tungsten smelting is usually an alkaline solution containing sodium phosphate and sodium tungstate, it is necessary to comprehensively explore the dissolution law of sodium phosphate in a ternary system of Na₃PO₄–Na₂WO₄–NaOH to guide subsequent research on the removal of phosphate from crude sodium tungstate solution.

After autoclaving scheelite with sodium hydroxide, in general, the concentrations of sodium tungstate and residual alkali in solution are approximately 200 g/L and 80 g/L, respectively. Based on the above observation, the experimental conditions of the Na₃PO₄–Na₂WO₄–NaOH ternary solution system were as follows: sodium tungstate concentrations of 150 g/L to 250 g/L and sodium hydroxide concentrations of 80 g/L to 120 g/L. The concentration of saturated phosphorus in crude sodium tungstate solution was investigated, as shown in Figure 9, Figure 10 and Figure 11.

From Figure 9 and Figure 10, under the conditions of sodium hydroxide concentration of 80 g/L and the solution temperature being lowered from 323.15 K to 283.15 K, when the sodium tungstate concentration was 150 g/L, the concentration of saturated phosphorus concentration decreased from 0.4247 g/L to 0.0091 g/L. When the sodium tungstate concentration was 200 g/L, the saturated phosphorus concentration decreased from 0.3967 g/L to 0.0069 g/L. Finally, when the sodium tungstate concentration was 250 g/L, the saturated phosphorus concentration decreased from 0.3602 g/L to 0.0062 g/L, respectively. Thus, the saturated phosphorus concentration in solution showed a downward trend, and the trend slowed down with decreasing solution temperature. Therefore, increasing the sodium tungstate concentration is beneficial to decrease the saturated phosphorus concentration; however, the effect is generally small.

From Figure 9, Figure 10 and Figure 11, when the sodium hydroxide concentration was 120 g/L and the solution temperature was lowered from 323.15 K to 283.15 K, the variation in the saturated phosphorus concentration in the solution was similar but less than that found with the sodium hydroxide concentration of 80 g/L. Therefore, the common-ion effect is very significant when sodium hydroxide and sodium tungstate coexist in sodium phosphate solution.

In the Na₃PO₄–Na₂WO₄–NaOH ternary solution system, the saturated phosphorus concentration decreases by increasing the concentration of sodium hydroxide or sodium tungstate in the solution. Regarding the actual situation of phosphorus removal from crude sodium tungstate solution, by adding sodium hydroxide to purify phosphorous, on the one hand, no new impurities will be introduced; on the other hand, sodium phosphate can precipitate out from the solution and be recycled for the decomposition of scheelite, which can fully solve the problems existing in the traditional chemical purification method for phosphorus removal.

4. Conclusions

Through the study of the saturated phosphorus concentration in binary Na₃PO₄–Na₂WO₄ and Na₃PO₄–NaOH solution systems and a ternary Na₃PO₄–Na₂WO₄–NaOH solution system, we obtained the following conclusions:

• The addition of sodium hydroxide/sodium tungstate into the sodium phosphate solution will produce the common-ion effect and the salt effect, both of which will affect the saturated phosphorus concentration in the solution. The common-ion effect decreases the saturated phosphorus concentration, while the salt effect increases the saturated phosphorus concentration. In most cases, the common-ion effect is stronger than the salt effect.
• In binary or ternary solution systems containing sodium phosphate, sodium tungstate, and sodium hydroxide, increasing the concentration of sodium hydroxide or sodium tungstate and decreasing the solution temperature can significantly decrease the saturated phosphorus concentration in the solution.
• Regarding the actual situation of tungsten smelting, the phosphorus concentration in the solution can be reduced by increasing the sodium hydroxide concentration and lowering the temperature in the ternary Na₃PO₄–Na₂WO₄–NaOH solution system. In this way, no additional impurities are introduced, and the phosphorus removal effect is significant, meeting the requirements of industrial production. Furthermore, the recovered sodium phosphate can be recycled as an additive for subsequent scheelite decomposition processes.