Keywords

1 Introduction

UO2(NO3)2 and UF4 are significant intermediates that connect different chemical unit operations in uranium chemical industry, such as dissolution, evaporation, concentration, extraction and etc. Commonly, HNO3 was widely used to dissolve U3O8, UO3, UO2, or UF4, and HF was used to produce UF4 in uranium chemical industry. Herein, a ternary system UO2(NO3)2 + HNO3 + H2O widely existed in unit operations of the former mentioned, and ternary system UF4 + HF + H2O can be simplified in the unit process of the latter mentioned. There is a basic physicochemical question about phase equilibrium of two ternary systems mentioned above, it is crucial to understand the phase behavior and thermodynamic properties of them with the continuous development of uranium chemical industry.

For ternary system UO2(NO3)2 + HNO3 + H2O, research temperature mainly was focused on 298 K – 357 K, in literature [1], three crystalline phase regions can be found for ternary system UO2(NO3)2 + HNO3 + H2O, which corresponding to UO2(NO3)2·6H2O, UO2(NO3)2·3H2O and UO2(NO3)2·2H2O, respectively. It was suggested that the crystalline form of UO2(NO3)2 is not only related to temperature, but also to co-existing ion in aqueous solution. Recently, Maliutin et al. [2] adopted Pitzer-Simonson-Clegg model to simulate the solubility of quaternary system UO2(NO3)2 + Ca(NO3)2 + HNO3 + H2O at 263 K, 298 K, and 323 K, and corresponding model parameters were presented. In our precious study [3], the Pitzer thermodynamic model was used to simulate the solubility of ternary system UO2(NO3)2 +  + HNO3 + H2O with HNO3 concentration range of 0 – 8.5 mol·kg−1 at 298 K. For solid-liquid phase equilibrium of ternary system UF4 + HF + H2O, there are very few reports in the literatures [4, 5]. Meanwhile, there are relatively large differences in the only data available, for instance, the solubility of UF4 in pure water is 0.005% from literature [4], but it is 0.028% in literature [5]. Indeed, it is difficult to evaluate their reliability.

Based on this, ternary systems UO2(NO3)2 + HNO3 + H2O and UF4 + HF + H2O at 298 K were investigated in this work, experimental thermodynamic data of density and solubility were measured for two systems at 298 K. These thermodynamic data acquired by this work can provide theoretical basis for comprehensive utilization and treatment of wastewater resources produced by uranium chemical industry.

2 Experimental Section

2.1 Reagents

Double-deionized water (κ ≤ 5.5 × 10–6 S·m−1) obtained from UPT water purification system was used in the experiments. Nitric acid with mass fraction 65% – 68% and hydrofluoric acid with mass fraction 40% were purchased from the Shandong Zibo Haofeng Chemical Co., Ltd., which were used directly as received without further purification. Uranyl nitrate hexahydrate and uranium tetrafluoride obtained by laboratory synthesis were used in the experiments. The detailed information of all chemicals used in this work is listed in Table 1.

Table 1. The source and purity of the chemicals used in this experiments
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2.2 Apparatus

An analytical balance (Sartorius, Germany) with a precision of 0.0001 g was used to determine the weight of solution. The thermostat (GDJS-1009, China) with the temperature kept at 298 ± 0.2 K was used for the phase equilibrium experiments. The densities of the sample of the equilibrium liquid phase were determined by weighing bottle method with an uncertainty of 5.0 × 10–4 g·cm−3.

2.3 Experimental Method

Isothermal dissolution method was used to determine the solubilities of systems UO2(NO3)2 + HNO3 + H2O and UF4 + HF + H2O at 298 K. The specific experimental steps are as follows: 1) a series of stock solutions of HNO3 with the mass fraction of HNO3 0% – 65% in glass bottles (approximately 20 mL) were prepared, and teflon bottles were used to prepare the stock solution of HF (approximately 20 mL) with the mass fraction of HF 0% – 40% because of its’ strong corrosion. 2) Excess uranyl nitrate hexahydrate and uranium tetrafluoride were added to the stock solutions acquired by step 1), respectively. 3) the samples were placed in the thermostat at 298 ± 0.2 K with a 120 rpm stirring speed to promote equilibration. 4) When the equilibrium point of solution was reached (approximately 12 to 15 days), the samples were kept a static condition at working temperature for an additional 24 h, and then the clear liquid of each solution at equilibrium was collected by filtering to determine density, and composition.

2.4 Analysis Method

The mass fraction of UO2(NO3)2 and UF4 in equilibrium solution was acquired by determining the concentration of UO22+ and U4+, which can be obtained by the methods of dichromate titration and ammonium thiocyanate spectrophotometric with an uncertainty of 0.5%, respectively. The mass fraction of HNO3 and HF in equilibrium solution was acquired by determining the concentration of H+, which can be obtained by the method of sodium hydroxide titrate with an uncertainty of 0.5%. Schreinemakers’ wet residue method [6] and X-ray diffraction were used to determine the compositions of equilibrium solid phase for two systems.

3 Results and Discussion

3.1 Synthesis of UO2(NO3)2·6H2O and UF4

Synthesis of UO2(NO3)2·6H2O.

Take U3O8 and HNO3 with the concentration 6.0 mol·L−1 as raw material. First, adding 100 g U3O8 into 130 mL HNO3 solution and keep stirring with a 120 rpm stirring speed to make U3O8 dissolve sufficiently at 333 K – 363 K, the stirring was stopped after the solution was clarified, uranyl nitrate solution was obtained by filtration. Then, UO2(NO3)2 solution was evaporated until the volume of the solution was reduced to half of the original, and the heating was stopped and slowly cooled to room temperature to crystallize out UO2(NO3)2·6H2O crystals, After filtration, UO2(NO3)2·6H2O crystal with purity meeting the requirements of this experiment can be obtained.

Synthesis of UF4.

Taking the UO3 and NH4HF2 as raw material, according to the mass ratio of 1:1.5, UO3 and NH4HF2 were mixed evenly in a self-made fluorination reactor, which was then placed in muffle furnace. Setting temperature of muffle furnace at 723 K for 6 h, UF4 with purity meeting the experimental requirements can be obtained.

3.2 Phase Equilibrium for Ternary System UO2(NO3)2 + HNO3 + H2O at 298 K

The composition of saturation point measured by this work was compared with literatures for binary system UO2(NO3)2 + H2O at 298 K as shown in Fig. 1, the solubility of UO2(NO3)2 in pure water obtained by this work is 56.15%, and the solubility of UO2(NO3)2 in pure water obtained by literatures [7, 8] are 56.15% and 55.85%, respectively. The relative deviation below 0.005, which suggests that the experimental method is reliable.

Fig. 1.
figure 1

Solubilities of UO2(NO3)2 in pure water at 298 K

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The data of densities, compositions of equilibrium liquid phase and the compositions of wet-solid phase for ternary system UO2(NO3)2 + HNO3 + H2O at 298 K were presented in Table 2.

Table 2. Experimental phase equilibrium data for ternary system UO2(NO3)2(1) + HNO3(2) + H2O(3) at 298 Ka
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Fig. 2.
figure 2

Equilibrium phase diagram(a) and diagram of density-composition(b) for ternary system UO2(NO3)2 + NO3 H2O at 298 K

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According to the experimental phase equilibrium data of system UO2(NO3)2 + HNO3 + H2O at 298 K, the diagrams of solubility and density-compositions were plotted, and presented in Fig. 2. As shown in Fig. 2(a), the equilibrium phase diagram was composed of one univariant isothermals and one crystalline phase zone corresponding UO2(NO3)2·6H2O for ternary system UO2(NO3)2 + HNO3 + H2O at 298 K. In univariant isothermals, the solubilities of UO2(NO3)2 present a trend that decreases first and then increases with the increase of mass percentage of HNO3 in equilibrium liquid phase. This phenomenon may be due to 1) when the mass fraction of HNO3 in equilibrium solution is low, the dissociation of HNO3 molecules is relatively complete, it exists as H+ and NO3−, the common-ion effect of NO3− cause the solubility of UO2(NO3)2 decreases. 2) However, with the increase of mass fraction of HNO3 in solution, the common-ion effect get more weaker because of the lower degree of dissociation of HNO3 molecules, thus the solubility of UO2(NO3)2 increases slightly. Meanwhile, comparing the solubility of UO2(NO3)2 in mixed solution HNO3-H2O with literatures [3, 9], it can be shown from Fig. 2(a) that the experimental solubility measured by this work are in good agreement with literatures.

As shown in Fig. 2(b), the change trend of densities of equilibrium liquid phase has a same regulation that the densities of equilibrium solution decreases first and then increases with the increase of mass fraction of HNO3 for ternary system UO2(NO3)2 + HNO3 + H2O at 298 K.

3.3 Phase Equilibrium for Ternary System UF4(1) + HF(2) + H2O(3) at 298 K

The data of densities, compositions of equilibrium liquid phase for ternary system UF4 + HF + H2O at 298 K were presented in Table 3.

The diagrams of solubility and density-compositions were plotted according to the experimental phase equilibrium data of system UF4 + HF + H2O at 298 K and presented in Fig. 3. As shown in Fig. 3(a), the solubility of UF4 in pure water is 0.31%, and it decreases sharply with the increase of mass fraction of HF when the mass fraction of HF is between 0% - 10.5%. However, the solubility of UF4 presents a trend of slow increase with the increase of mass fraction of HF when the mass fraction of HF beyond 10.5%. Likewise, this change trend is same as that in ternary system UO2(NO3)2 + HNO3 + H2O at 298 K, which is also due to the co-ionic effect caused by partial dissociation of HF in aqueous solution. Meanwhile, the solubility of UF4 in water at 298 K was compared with literatures [3, 4] in this work, it can be shown that there are tremendous difference for the solubility of UF4 in water at 298 K, which is 0.005% measured by lit. [3], 0.028% obtained by lit [4]. and 0.31% acquired by this work. And from Fig. 3(a), the solubility of UF4 in HF + H2O at 298 K obtained by lit [3]. is slightly higher than the experimental value measured by this wok. It may be due to the low purity of UF4 synthesized by this work, where a small amount of UO2F2 existing in UF4. However, these thermodynamic data is still of great value for guiding the comprehensive utilization of waste liquid resources from the process of UF4 production.

Table 3. Experimental phase equilibrium data for ternary system UF4(1) + HF(2) + H2O(3) at 298 Ka
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Fig. 3.
figure 3

Equilibrium phase diagram(a) and diagram of density-composition(b) for ternary system UF4 + HF + H2O at 298 K

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From Fig. 3(b), it is shown that the density of equilibrium solution increases with the mass fraction of HF for ternary system UF4 + HF + H2O at 298 K, which suggests that the density of equilibrium solution is dominated by HF.

For ternary system UF4 + HF + H2O at 298 K, the composition of wet-solid phase was not given in this work because of its low solubility of UF4 in pure water. Thus, the X-ray diffraction method was used to identify the equilibrium solid phase for this system. As shown in Fig. 4, the X-ray diffraction of wet-solid phase of point 12 for system UF4 + HF + H2O at 298 K can be consistent well with the characteristic peak of the standard card of PDF#11–0623, which suggests that the equilibrium solid phase is UF4·2.5H2O.

Fig. 4.
figure 4

X-ray diffraction pattern of equilibrium solid phase of point 12 for ternary system UF4 + HF + H2O at 298 K

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4 Conclusions

In this paper, solid-liquid phase equilibria of ternary systems UO2(NO3)2 + HNO3 + H2O and UF4 + HF + H2O at 298 K were investigated by isothermal dissolution method, the compositions and densities of equilibria liquid phase were measured experimentally. Phase diagrams and diagrams of density-composition were constructed for systems UO2(NO3)2 + HNO3 + H2O and UF4 + HF + H2O at 298 K. There are unique isothermal dissolution curve for two ternary systems, and corresponding crystallization zone are UO2(NO3)2·6H2O and UF4·2.5H2O, respectively. In univariant isothermals, the solubilities of UO2(NO3)2 or UF4 present a trend that decreases first and then increases slightly with the increase of mass percentage of HNO3 or HF in equilibrium liquid phase.