­­Dissolution behaviors of Ta₂O₅, Nb₂O₅ and their mixture in KOH and H₂O system

WANG Xiao-hui(王晓辉)^{1, 2}, ZHENG Shi-li(郑诗礼)¹, XU Hong-bin(徐红彬)¹, ZHANG Yi(张 懿)¹

1. National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,

Key Laboratory of Green Process and Engineering, Institute of Process Engineering,

Chinese Academy of Sciences, Beijing 100190, China;

2. Graduate School of Chinese Academy of Sciences, Beijing 100039, China

Received 6 July 2009; accepted 21 January 2010

Abstract: The dissolution behaviors of Ta₂O₅, Nb₂O₅ and their mixture in KOH and H₂O system were investigated. A L₉(3⁴) orthogonal design was used to study the effects of reaction temperature, mass ratio of KOH to Ta₂O₅, and reaction time on the dissolution rate of tantalum. It was found that the effect of reaction temperature on the dissolution rate of tantalum was much greater than that of the other factors. The results of factorial experiments showed that Ta₂O₅ was mainly transformed into insoluble potassium tantalate at low temperature (350 °C) and transformed into soluble potassium tantalate at high temperature (450 °C). The insoluble potassium tantalate was analyzed by XRD, which was proved to be KTaO₃. Differently, almost all Nb₂O₅ was transformed into soluble potassium niobate at 350-450 °C. As for the mixture of Ta₂O₅ and Nb₂O₅, the dissolution rate of tantalum increased and the dissolution rate of niobium decreased as an interaction existed between niobium and tantalum. And increasing the mole ratio of Nb₂O₅ to Ta₂O₅ in the mixture was beneficial to the dissolution of both Ta₂O₅ and Nb₂O₅. In addition, the mechanism of the interaction between niobium and tantalum was also investigated through phase and chemical analysis.

Key words: Ta₂O₅; Nb₂O₅; KOH; dissolution behavior; mechanism; solid-solution

1 Introduction

Tantalum and niobium are important rare refractory metals and are widely used in steel, electronic and other high-tech industries[1-3]. The decomposition of the ore is the key step in extracting niobium, tantalum and their compounds from niobium-tantalum ore. At present, the hydrofluoric acid method is widely used in the tantalum-niobium hydrometallurgical industry for the decomposition of the ores[4-6]. However, due to the strong volatility, about 6%-7% of the hydrofluoric acid is lost during the decomposition process, which is harmful to human beings and equipments. As well, a large amount of wastewater containing fluoride is generated which needs to be treated[7-9]. More importantly, this method is only appropriate for high-grade niobium-tantalum ores[10]. Although the resources of niobium-tantalum ores are abundant in China, most of them are in low-grade and difficult to decompose by hydrofluoric acid[11]. Therefore, it is imperative to develop a new and clean production process, so as to achieve optimum resource utilization.

Recently, a new process for the leaching of low-grade niobium-tantalum ores using a KOH roast-water leach system was proposed by the Institute of Process Engineering, Chinese Academy of Sciences, China, with the objective to eliminate fluorine pollution at the source[12]. In this new process, low-grade refractory niobium-tantalum ore is decomposed using KOH molten salt instead of highly concentrated hydrofluoric acid and then is leached by H₂O. The experimental results of the new process show that the decomposition rate for the low-grade refractory niobium-tantalum ore is almost 15% higher than that for the hydrofluoric acid process.

The new process of leaching niobium and tantalum from a low-grade niobium-tantalum ore is under development and there is a general lack of information. Although some studies have been performed on alkali fusion[13-14] and the phase equilibria of Nb₂O₅-K₂CO₃ system and Ta₂O₅-K₂CO₃ system have been given [15-16], no previous work has ever been reported on the fundamental dissolution behaviour of Nb₂O₅ and Ta₂O₅ in KOH molten salt and H₂O system. The aim of this work is to investigate the dissolution behavior of Nb₂O₅, Ta₂O₅ and their mixture in KOH molten salt and H₂O system. And the interaction between niobium and tantalum in the decomposition process was also investigated.

2 Experimental

2.1 Materials

All the chemical reagents employed were of analytical grade and deionized water was used in the corresponding procedures during the experiments. The Nb₂O₅ and Ta₂O₅ samples used for the present study were of reagent grade and were supplied by the Ningxia Orient Tantalum Industry Co., Ltd, China.

2.2 Equipment

The dissolution process was carried out in a 500 mL SUS316 stainless batch reactor equipped with a thermometer, a mechanical stirrer and a reflux condenser. The reactor was heated by immersing in a furnace to reach and maintain the desired temperature within ±2  °C.

2.3 Procedure

All the experiments were conducted in batches. For each run, the required amounts of solid KOH were transferred to the reactor and then were heated to the desired temperature. When the temperature reached the pre-set value and kept stable, the mechanical stirrer was started and a certain amount of Nb₂O₅ (Ta₂O₅ or their mixture) was added to the reactor. The mixture was stirred at a constant speed under an atmospheric pressure. When the reaction time was reached, the products were cooled to room temperature quickly using cold airflow. Under the ambient conditions, the products were leached with a certain amount of water and filtered to obtain a solution and a solid residue. The resulting leaching solution and the residue were analyzed for Nb and Ta by ICP-OES. The dissolution rate (K) of the elements was calculated using the following expression:

K=[1-(m_r/m_o)]×100%

where m_r and m_o are the mass of the element calculated in the residue obtained in the leaching step and the mass of the element calculated in the Nb₂O₅(Ta₂O₅ or their mixture), respectively. The leaching residues were examined by X-ray diffraction analysis (XRD, using Phillips PW223/30).

3 Results and discussion

When Nb₂O₅ (Ta₂O₅) reacts with potash, K₃NbO₄ (K₃TaO₄) or KNbO₃ (KTaO₃) forms. When the mole ratio of K₂O to Nb₂O₅(Ta₂O₅)≤1:1, the reaction product is mainly in the form of KNbO₃ (KTaO₃). And when the mole ratio of K₂O to Nb₂O₅(Ta₂O₅)≥4:3, the reaction product is mainly in the form of K₃NbO₄ (K₃TaO₄). The KNbO₃ (KTaO₃) is insoluble and cannot be leached by water. By contrast, the K₃NbO₄ (K₃TaO₄) will hydrolyze to soluble K₈Nb₆O₁₉ (K₈Ta₆O₁₉) and then be leached in the water leaching process. The purpose of our research is to find the optium reaction conditions under which the highest dissolution rate of niobium and tantalum can be obtained. Therefore, the experiments below are all conducted under the condition of mole ratio of K₂O to Nb₂O₅(Ta₂O₅)>4:3.

3.1 Dissolution behavior of Ta₂O₅

3.1.1 Effect of leaching parameters on dissolution of Ta₂O₅ using L₉(3⁴) orthogonal design

Through our preliminary experiments, we found that the reaction temperature, mass ratio of KOH to Ta₂O₅ and reaction time are the main parameters affecting the dissolution of Ta₂O₅. Thus, an L₉(3⁴) orthogonal design was used to investigate the effect of reaction temperature (T_r), mass ratio of KOH to Ta₂O₅ (R_a) and reaction time (t) on the dissolution rate of Ta₂O₅ in KOH and H₂O system. The variable assignment and the level settings are listed in Table 1. The results of L₉(3⁴) orthogonal experiments are presented in Table 2.

Table 1 Experimental factors and levels

In Table 2, K₁, K₂ and K₃ represent the sum of dissolution rate of Ta₂O₅ of level 1, level 2 and level 3 of a factor, respectively. K₁/3, K₂/3 and K₃/3 represent the average of K₁, K₂ and K₃, respectively. R represents the maximum difference value among K₁, K₂ and K₃. The orthogonal experiment results of variance analysis are shown in Table 3.

Table 2 and Table 3 show that the most significant factor is reaction temperature, which is statistically significant at the 99% confidence level. In the selected range, the mass ratio of KOH to Ta₂O₅ and reaction time

Table 2 Results of L₉(3⁴) orthogonal experiments

Table 3 Variance analysis of orthogonal experiment results

have no significant influence on the dissolution of Ta₂O₅.

3.1.2 Effect of reaction temperature

According to the results of the orthogonal experiments, reaction temperature has much more significant influence on the dissolution of Ta₂O₅ than other factors do. Thus, the effect of reaction temperature was further investigated by factorial experiments under the conditions of reaction time of 1 h and mass ratio of KOH to Ta₂O₅ of 5:1. The results are shown in Fig. 1.

It can be seen from Fig. 1 that the dissolution rate of Ta₂O₅ increases significantly with the increasing reaction temperature. To investigate this phenomenon, the residues obtained after Ta₂O₅ is decomposed by KOH molten salt and dissolved by water were dried at 120 °C for 10 h and then analyzed by XRD. The results are shown in Fig.2. It can be seen from Fig.2 that all diffraction peaks of the insoluble residue are attributable to KTaO₃. The XRD pattern is consistent with that reported in JCPDS No.01-077-0917. In order to prove that the KTaO₃ is not formed in the water dissolution procedure, we used ethanol instead of water in the water dissolution procedure. There is also KTaO₃ in the residue as shown in Fig.2. This indicates that besides converting into K₃TaO₄, a part of tantalum directly converts into insoluble KTaO₃ in the KOH decomposition procedure. And this is the reason for the low dissolution rate of Ta₂O₅ under low reaction temperature. From Fig.1 we can also find that higher temperature results in lower conversion rate of insoluble KTaO₃. Therefore, increasing the reaction temperature is beneficial to the dissolution of Ta₂O₅. But when the reaction temperature is higher than 540 °C, the dissolution rate of Ta₂O₅ does not change significantly.

Fig.1 Effect of reaction temperature on dissolution rate of Ta₂O₅ (Reaction conditions: reaction time 1 h and mass ratio of KOH to Ta₂O₅ of 5:1)

Fig.2 XRD pattern of residue obtained after Ta₂O₅ being decomposed by KOH molten salt and leached by water (a) and ethanol (b)

3.2 Dissolution behavior of Nb₂O₅

The effects of reaction temperature, mass ratio of KOH to Nb₂O₅ and reaction time on the dissolution rate of Nb₂O₅ were examined. The results are listed in Table 4.

Table 4 Effects of reaction temperature, alkaline-to-ore mass ratio and reaction time on dissolution rate of Nb₂O₅

Table 4 shows that the dissolution rate of Nb₂O₅ is almost 100% in the selected range of reaction conditions, which indicates that most Nb₂O₅ is converted into K₃NbO₄ in the KOH decomposition procedure and then is dissolved by water. This also indicates that the dissolution behavior of Nb₂O₅ in KOH and H₂O system is different from that of Ta₂O₅.

3.3 Dissolution behavior of mixture of Ta₂O₅ and Nb₂O₅

According to the results of the above experiments, the dissolution behaviors of Ta₂O₅ and Nb₂O₅ in KOH and H₂O system are different. Ta₂O₅ converts into K₃TaO₄ and KTaO₃ while Nb₂O₅ converts only into K₃NbO₄. As we know, tantalum and niobium often coexist in minerals with the similar properties. Therefore, it is necessary to investigate the dissolution behavior of the mixture of Ta₂O₅ and Nb₂O₅. As the dissolution behaviors of Ta₂O₅ and Nb₂O₅ in KOH and H₂O system are different, when they are mixed together, there may be interaction between them. Therefore, we emphatically investigated the effect of mass ratio of Nb₂O₅ to Ta₂O₅ on the dissolution behavior of the mixture of Nb₂O₅ and Ta₂O₅. The Nb₂O₅ and Ta₂O₅ mixture was obtained through ball-mill mixing of pure Nb₂O₅ and Ta₂O₅. We use mass fraction of Nb₂O₅ to represent the mass ratio of niobium to tantalum in the mixture. The results are presented in Fig.3.

Fig.3 Dissolution behavior of mixture of Ta₂O₅ and Nb₂O₅ in KOH and water system at 350 °C (a) and 400 °C (b) (Reaction conditions: reaction time 1 h and KOH-to- Ta₂O₅(Nb₂O₅) mass ratio 5:1)

From Fig.3. we can see that when the Nb₂O₅ and Ta₂O₅ mixture reacts with KOH and water system, in case that the reaction is carried out at 350 °C and with increasing the mass fraction of Nb₂O₅, the dissolution rate of tantalum increases slowly at first and then rapidly, while the dissolution rate of niobium increases rapidly at first and then slowly. And in case that the reaction is carried out at 400 °C, the dissolution rates of tantalum increase significantly with increasing the mass fraction of Nb₂O₅ in the mixture while the increase of niobium is rather small. This indicates that an interaction exists between niobium and tantalum when Nb₂O₅ and Ta₂O₅ mixture reacts with KOH and H₂O system. The existence of niobium can promote the dissolution of tantalum, while the existence of tantalum can inhibit the dissolution of niobium. To investigate the mechanism of the interaction between niobium and tantalum, phase analysis and component analysis of the residues obtained were made. The results are shown in Fig.4 and Table 5.

Fig.4 XRD patterns of residues obtained under different reaction conditions: (a) 350 °C, w(Nb₂O₅)=0.75; (b) 350 °C, w(Nb₂O₅)=0.5; (c) 350 °C, w(Nb₂O₅)=0.25; (d) 350 °C, w(Nb₂O₅)=0.1; (e) 400 °C,  w(Nb₂O₅)=0.5; (f) 400 °C, w(Nb₂O₅)=0.25

Mole ratio of Ta to Nb in residue   Table 5 Composition of residues obtained under different reaction conditions

From Fig.4, we can find that the XRD patterns of the residues are very close to the XRD pattern of KTa_0.77Nb_0.23O₃ (JCPDS No.70-2011), which is a solid solution of KTaO₃ and KNbO₃. But from Table 5, we can find that the mole ratio of Ta to Nb in the residues are all different from 0.77:0.23, which indicates that the residues are not KTa_0.77Nb_0.23O₃. As we know, the Nb⁵⁺ and Ta⁵⁺ are quite similar in chemical properties and ionic radius and there may be isomorphism replacement between Nb⁵⁺ and Ta⁵⁺. KTaO₃ and KNbO₃ can form continuous solid solution by isomorphism replacement between Nb⁵⁺ and Ta⁵⁺. Thus, we conjecture that the residues of Nb₂O₅ and Ta₂O₅ mixture are KTaO₃-KNbO₃ solid solutions.

According to Vegard’s law[17], the lattice parameter of continuous solid solution has a linear relationship with the mole fraction of one component in it. Therefore, the line of lattice parameter to mole fraction of niobium in the residues is plotted. The results is shown in Fig.5. From Fig.5. we can see that, the relationship between the lattice parameter and mole fraction of niobium is accorded with Vegard’s law. This result indicates that the leaching residues of Nb₂O₅ and Ta₂O₅ mixture are really KTaO₃ and KNbO₃ solid solutions. This result also indicates that when Nb₂O₅ and Ta₂O₅ mixture reacts with KOH, by isomorphism replacement between Nb⁵⁺ and Ta⁵⁺, a part of Nb⁵⁺ ions enter the crystal lattice of KTaO₃, forming KTaO₃- KNbO₃ solid solution. This is why the leaching rate of Nb₂O₅ decreases when it is mixed with Ta₂O₅. And when the mole fraction of Ta₂O₅ increases (the mole fraction of Nb₂O₅ decreases), more Nb⁵⁺ ions enter the KTaO₃ lattice. Therefore, the leaching rate of Nb₂O₅ decreases with increasing the mole fraction of Ta₂O₅. Similarly, when Nb₂O₅ and Ta₂O₅ mixture reacts with KOH, there may be a part of Ta⁵⁺ ions entering the lattice of K₃NbO₄, forming K₃NbO₄-K₃TaO₄ solid solution and then may be leached. Therefore, the leaching rate of Ta₂O₅ increases with the increase of Nb₂O₅ mole fraction in the mixture. But as the K₃NbO₄ and K₃TaO₄ are difficult to obtain, further investigation is needed for an in-depth explanation.

Fig.5 Relation between lattice parameter and mole fraction of Nb

In short, increasing the niobium to tantalum ratio in Nb₂O₅ and Ta₂O₅ mixture is effective for increasing the dissolution rate of niobium and tantalum.

4 Conclusions

1) The dissolution behavior of Ta₂O₅, Nb₂O₅ and their mixture in KOH and H₂O system was investigated. Under the different reaction temperatures, Ta₂O₅ will be partly converted into K₃TaO₄ and then be dissolved and partly converted into insoluble KTaO₃. Increasing the reaction temperature is beneficial to the dissolution of Ta₂O₅.

2) Under the same reaction conditions, Nb₂O₅ will be almost 100% converted into K₃NbO₄ and then be dissolved.

3) When the mixture of Ta₂O₅ and Nb₂O₅ reacts in KOH and H₂O system, by the formation of KTaO₃- KNbO₃ and K₃NbO₄-K₃TaO₄ solid solutions, the dissolution rate of Ta₂O₅ increases while the dissolution rate of Nb₂O₅ decreases. And increasing the mole ratio of Nb₂O₅ to Ta₂O₅ in the mixture is beneficial to the dissolution of both Ta₂O₅ and Nb₂O₅.

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(Edited by LI Xiang-qun)

Foundation item: Project(2009AA06Z103) supported by the National High-tech Research and Development Program of China; Project(51004094) supported by the National Natural Science Foundation of China; Project(2007CB613501) supported by the National Basic Research Program of China

Corresponding author: WANG Xiao-hui; Tel/Fax: +86-10-82544856; E-mail: wangxh@home.ipe.ac.cn

DOI: 10.1016/S1003-6326(09)60409-X