Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
pubs.acs.org/jced
MgF2 Solubility in MgF2 + Salt + H2O Systems (Salt = MgSO4, (NH4)2SO4, NH4Cl) at 298.15 K Hongliang Li,† Chao Xu,† Ze Tan,‡ and Dewen Zeng*,† †
College of Chemistry and Chemical Engineering, Central South University, 410083 Changsha, People’s Republic of China Guangdong Guanghua Sci-tech Co., Ltd., 515061 Shantou, People’s Republic of China
J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 12/15/18. For personal use only.
‡
ABSTRACT: MgSO4 and (NH4)2SO4 are the main impurities in the hydrometallurgical systems of zinc and manganese. To understand their influences on MgF2(s) solubility, which the fluoride in these systems is expected to remove, we measured MgF2(s) solubility in NH4Cl, (NH4)2SO4 and MgSO4 aqueous solutions at 298.15 K. It was found that the MgF2(s) solubility exhibits salt-in effect in the (NH4)2SO4(aq) and NH4Cl(aq) solutions, but salt-out and salt-in effects in the MgSO4(aq) solution. Furthermore, the MgF2(s) solubility in (NH4)2SO4(aq) is higher than in NH4Cl(aq) solution. Ionic association reactions have been derived to interpret the above solubility phenomena.
■
INTRODUCTION There is a need to remove fluoride from hydrometallurgical systems of zinc and manganese. As mentioned in our previous work,1 a possible approach is removing fluoride in electrolyte aqueous solution by precipitating it as sparingly soluble salts, such as MgF2 or CaF2. Determining the effectiveness of the approach requires knowing the solubility of the sparingly soluble salts in MgSO4 or (NH4)2SO4 aqueous solution, which commonly appear as impurities in zinc or manganese hydrometallurgical processes. However, their solubility behaviors in these systems are unknown until now. In this case, we plan to investigate MgF2(s) solubility in MgF2+ salt + H2O systems (salt = MgSO4, (NH4)2SO4) at 298.15 K. To gain a profound understanding of the effect of anion in solution to its solubility, we also investigated MgF2(s) solubility in a NH4Cl aqueous system.
Table 1. Purities of the Chemical Reagents Used in This Work purity (mass fraction) >0.98 >0.99 >0.99 >0.995 >0.99
Table 2. Impurity Content of the Final Obtained Magnesium Sulfate Producta
■
EXPERIMENTAL SECTION Materials. All reagents used in this work (MgF2, MgSO4· 7H2O, (NH4)2SO4, NH4Cl, and NaF) are commercial reagents delivered by Sinapharm Chemical Reagent Co., Ltd., China. Their original purities are listed in Table 1. Distilled water (conductivity 0, i.e., mMg(aq) > 1/k1, the MgF2(s) 2+ solubility increases with increasing mMg(aq) , exhibiting the saltin effect. The inflection point is located at 1/k1. Citing the value of k1 listed in Table 5, we have 1/k1 = 1/101.8 = 0.0158 mol kg−1. Based on the experimental inflection point located at ∼0.0126 mol kg−1, as shown in Table 6 and Figure 2, the calculation results and experiment data agree with each other quite well. This simplified calculation is helpful for us to understand the nature of the salt-in effect in the MgF2 + MgSO4 + H2O system, that is, the ionic association factors decrease the ionic activity coefficients of ion in solution. Furthermore, we tried to calculate the MgF2(s) solubility isotherm at 298.15 K under the above assumptions. First, we calculated the species (Mg2+, MgF−, MgF2, F−) concentration in the MgF2(s) saturated solution in pure water with m(MgF2) = 0.00136 mol·kg−1, and obtained = −18.231. Using the ln k value as a criterion, one can calculate the MgF2(s) solubility when extra Mg2+ ions are added to the aqueous solution; the calculated results are presented in Figure 4. When the
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dewen Zeng: 0000-0003-0858-8606 Funding
This work was supported by the National Natural Science Foundation of China (No. 21776316). Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Li, H.; Wang, S.; Zeng, D. Experimental measurement of the solid−liquid equilibrium of the systems MF2 + H2O (M = Mg, Ca, Zn) from 298.15 to 353.15 K. J. Chem. Eng. Data 2018, 63, 1733− 1737. (2) Rix, C. J.; Bond, A. M.; Smith, J. D. Direct determination of fluoride in sea water with a fluoride selective ion electrode by a method of standard additions. Anal. Chem. 1976, 48, 1236−1239. (3) Cai, D.-H.; Zeng, D.-W. Influence of total ionic strength adjustment buffer on the determination of fluorine in zinc hydrometallurgy system by selective electrode method. Metall. Anal. 2018, 38, 41−46. (4) Analytical Laboratory of Qinghai Institute of Salt Lakes at Chinese Academy of Sciences. Analytic Method of Brines and Salts; Science Press: Beijing, 1988. (5) Hefter, G.; Tromans, A.; May, P. M.; Königsberger, E. Solubility of Sodium Oxalate in Concentrated Electrolyte Solutions. J. Chem. Eng. Data 2018, 63, 542−552. (6) Arthur, E.; Martell, R. M. S. Critical Stability Constants; Springer: New York, 1976. (7) Fovet, Y.; Gal, J.-Y. Formation constants beta (2) of calcium and magnesium fluorides at 25 degrees C. Talanta 2000, 53, 617−626.
Figure 4. Change in MgF2(s) solubility when Mg2+ ions are added to the solution at 298.15 K. Triangle symbols (▲) represent the experimentally determined MgF2(s) solubility in the MgF2 + MgSO4 + H2O system; the curve represents the predicted results under the assumptions described in the text.
calculated results are compared with the experimental results, both agree with each other quite well at low Mg 2+ concentrations, especially at the stationary point. With increasing Mg2+ concentration, the predicted MgF2(s) solubility increases more than MgF2(s) solubility in pure water. This means that the ionic association interaction is the main D
DOI: 10.1021/acs.jced.8b00772 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
