Keywords:
Abstract. Presently, the preparation of a highly pure manganese sulfate solution shows a long and
complex process with low efficiency from the low-grade manganese ore. The complete removal of
calcium and magnesium ions coordinating with the high recovery rate of manganese becomes one of the
challenges for the conventional preparation of manganese sulfate. In the paper, the general removal
methods of Ca2+ and Mg2+ including crystallization, electrolysis, extraction, and chemical
precipitation were reviewed at first. Moreover, a novel approach was proposed herein with the aid of
theoretical calculation, in which Ba(OH)2 was used to adjust the pH value of manganese sulfate
solution and BaF2 was used to completely remove Ca2+ and Mg2+. The initial experimental results
show that the calcium and magnesium removal rate reached 95.11% and 97.93%, respectively, and
the recovery rate of manganese keeps 96%, which shows the prospect of industrial application.
Introduction
With the development of energy and electronic materials, the demand for manganese products
with high purity is growing significantly such as battery-grade manganese sulfate. Moreover,
manganese sulfate is also the intermediate product for the other high pure manganese compounds
and manganese salt [1]. So far, manganese sulfate is usually prepared from the low-grade manganese ore
through leaching by sulfuric acid and subsequent purifying of the leach liquor. With a constant
decrease of manganese ore grade, the complex mineral composition results in a cumbersome
purification process and low economic benefit. A large amount of manganese residual was abandoned,
which causes the waste of manganese resources and the potential contamination to the environment.
Furthermore, in the purification process, Ca2+ and Mg2+ are easy to form saturated solutions and
generate crystals, which leads to pipe blockage. So The complete removal of Ca2+ and Mg2+
coordinating with a high manganese recovery rate in purification is of great importance to improve the
economic and environmental benefits. In the paper, the commonly accepted removal methods of Ca2+
and Mg2+ will be compared at first, including crystallization, electrolysis and extraction, and chemical
precipitation method. And with the aid of the theoretical calculation analysis, a novel approach was
proposed, i.e., Ba(OH)2 was employed to adjust the pH value of manganese sulfate solution for
avoiding the introduction of new impurity, and then BaF2 was applied for removal of Ca2+ and Mg2+.
The recovery rate of manganese and the effect of the impurity removal were characterized via chemical
titration and ICP test, respectively.
State-of-the-art of purification technology for Ca and Mg removal from
manganese sulfate solution
Crystallization
Based on the precipitation-solution equilibrium for insoluble balance system, the addition of the
ions causes the fluctuation of the balance and changes the solubility of the insoluble electrolyte such as
the dissociation equilibrium of the weak electrolyte solution [2]. Benrath V A. [3] studied the effect of
MgSO4 and MnSO4 on their solubility, which shows that the solubility of the substance will reduce
with the increasing of the solubility of the other substance. Helen E.Farrah [4] also studied the solubility of CaSO4 in MnSO4 solution, in which it is found that when the temperature is lower than
80℃, the solubility of CaSO4 increases with the increased concentration of the sulfuric acid solution, but
decreases with the increased concentration of manganese ions. Therefore, it is feasible theoretically to
decrease the solubility of CaSO4 and MgSO4 by increasing the concentration of MnSO4 in the solution,
which provides a route to the removal of magnesium and calcium. But unfortunately, the purification
should be repeated for the manganese sulfate solution with a high concentration of magnesium and
calcium, which leads to a sharp decrease in the manganese recovery rate. Moreover, MnSO4 and MgSO4
will generate eutectic or mixed crystal, which contributes to the fluctuation of the solubility of Ca2+
and Mg2+. Initially, our research group has tried to improve the manganese concentration by adding
some manganese sulfate in manganese ore leaching solution to reduce the relative
concentration of calcium and magnesium impurity. But the experimental results show that the
concentration of calcium and magnesium ions did not significantly reduce, which indicated that the
crystallization method to remove calcium and magnesium ions from manganese sulfate should be
further optimized.
YuanMingliang [5] analyzed the MnSO4-MgSO4-H2O phase diagram proposed by Benrath. The
residual manganese sulfate in the solution was controlled to be 5% by adjusting the evaporation capacity
of the water. Li Xuanhai [6] studied the effects of the concentration of the feeding liquid, operating
temperature, keeping time, and stirring intensity on the crystallization rate of manganese sulfate in
high-temperature crystallization methods. It is found that when the temperature is below 27℃, the
solubility of manganese sulfate increases with the increase of temperature, while it will gradually
decrease with an increase of temperature above 27℃, especially higher than 100℃. The solubility
curves of manganese sulfate at high temperatures are shown in Fig.1. In cases, precipitate at high
temperature occurs for the manganese sulfate solution with a low concentration of Mn, which suggests
a new way to purification with saving energy and reducing consumption.
Fig.1 The temperature-dependence curve of MnSO4 solubility
Electrolysis
Manganese sulfate solution with Ca2+ and Mg2+ impurities is electrolyzed and is transformed
into the anodic product (manganese oxide) or cathode product [7] (electrolytic manganese metal), and
then the as-obtained product is washed and dissolved in sulfuric acid, finally, further removal of
calcium and magnesium is performed to prepare manganese sulfate solution with high purity. In
comparison, the anode product method can significantly improve the current efficiency and reduce
energy consumption than the Cathode product method, the calcium and magnesium content can be
reduced to 3.0μg / g. However, the electrolysis method to prepare high purity manganese sulfate presents
long process flow, low efficiency, and high energy consumption. And the high concentration of heavy
metals and calcium and magnesium impurities in manganese sulfate solution will reduce the hydrogen
evolution potential and lessen the purity of the electrolytic products.
Chemical precipitation
Some chemical agents are used to precipitate with calcium and magnesium ions, which is based
on the principle of the different ionic compounds with varieties of solubility in the solution. The
difficulty of the process is to ensure that the precipitation agent and the precipitate rarely react with or
absorb the manganese ions.
Chen Lijuan [8] employed MnF2 to remove Ca and Mg, and the removal rates of Ca and Mg
impurities were 97% and 98.3%, respectively. Bao Xinjun [9] has done the process optimization. The
optimal technical parameter and procedures are as follows: the MnSO4 solution concentration is
increased to 600g/L, the amount of MnF2 addition is 1.2 times the theoretical one. MnF2 was added
to the solution by the separated two lots at 90℃. The Experimental results show that the
precipitation rate of Ca2+ is higher than 95%, and the precipitation rate of Mg2+ reaches 87.04%.
Moreover, it is feasible to use MnF2 as the precipitant to remove Ca and Mg from the Manganese sulfate
solution due to its pH value close to the one of the manganese sulfate solution. Li Junqi [10] used
a chelating agent to precipitate calcium and magnesium ions as MgY and CaY (Y is chelating agent
ion), respectively. The purification depends on the stirring intensity, pH value, the amount of
chelating agent, and the difference of adding ways. While ZhengWenjun [11] used NH4F to remove Ca
and Mg ions before the carbonized treatment of the solution. The results showed that the residual Ca
reaches 1 mg/L, but the concentration of Mg2+ had a little change before and after the carbonized
treatment. It indicated that most magnesium ions did not participate in the precipitation reaction
and were still kept in the mother liquor. Therefore, it is necessary to further optimize the conditions of
carbonization to prepare manganese sulfate with high purity.
The chemical precipitation method to remove Ca and impurities shows the advantages of simple
operation, short duration, and high removal effectivity of calcium and magnesium ions. But little
attention is attracted to the manganese recovery rate in the purification process. In our paper, the chemical
precipitation method was considered to obtain the complete removal of calcium and magnesium ions
coordinating with the high recovery rate of manganese, and the process parameters were optimized with
the aid of theoretical calculation analysis.
Extraction
Liu Honggang [12] reported the extraction method i.e. the roasting-leaching-P507 extracting to
remove calcium and magnesium ions from GuangXi low-grade manganese ore. The results showed
that the extraction rates of Mg and Ca were 72.06% and 48.97% respectively, and the loss rate of Mn
was 6.35% when 30% P507 and 70% sulfonated kerosene were mixed as extraction agents. Moreover,
it avoids the problems in the traditional fluoride precipitation method. But the purity of the production
can not meet the requirements of battery-grade manganese sulfate.
Study of a novel method to remove Ca and Mg by BaF2
In the paper, BaF2 was employed as a precipitating agent to remove Ca and Mg from manganese
sulfate solution. The appropriate reaction conditions were determined through theoretical calculation
using system ionization and dissolution equilibrium to ensure the high recovery rate of manganese.
The concentration of Ca2+ and Mg2+ in the manganese sulfate solution is shown in Table 1.
Table 1 The concentration of Ca and Mg in the manganese sulfate solution
Impurity ions Ca2+ Mg2+
concentration/[mmol/L] 3.354 16.658
The impurity ions of Ca2+ and Mg2+can be precipitated by fluoride, and their fluoride
precipitation in the solution reaches the dissolution equilibrium as shown in Eq. 1-4.
Ca2++2F-=CaF2 （1）
Mg2++2F-=MgF2 （2）
Mn2++2F-=MnF2 （3）
[ 2 ] [F ]2 sp K = M + × – （4）
To ensure that the Mn2+ does not react with the precipitation, the residual F- in solution should
not precipitate in the form of MnF2 as a by-product. Since MnF2 saturation concentration in water is
1.05g/100g, the equilibrium concentration of F- can be obtained as [F-]aq=0.284mol/L according to the
Eq. 4. In this case, considering the solubility product constant of CaF2 and MgF2[13], the equilibrium
concentrations in solution were calculated and shown in Table 2.
Table 2 The theoretical removal rate of Ca and Mg
Impurity ions Ca2+ Mg2+
Equilibrium concentration[ mol/L] 3.35*10-10 8.06*10-8
Removal rate% 99.999990 99.999516
The concentration of F+ required for precipitation is:
[F-]s = S n*([Mn+]-[Mn+]Ө)=0.040mol/L （5）
The F+ is easy to hydrolyze in the water as shown in Eq. 6. So part of the fluorine is ready for the
precipitation of calcium and magnesium, and the other part maintains the equilibrium of ions in the
solution. The concentration of F+ except for precipitation is represented as Δ[F-] as shown in Eq. 8.
H++F-=HF （6）
K = [H+ ]×[F- ] / [HF] （7）
Δ[F- ]= [F- ] a q+[HF] （8）
Δ[F-]=[F-]aq+[H+][F-]/K =0.284×(1+10-pH+3.17) （9）
According to Eq. 5 and Eq. 9, the concentration of fluoride ion for the system was calculated to
guarantee the removal of Ca and Mg under different pH values and shown in Table 3.
Table 3 The theoretical F- concentration to remove Ca and Mg under different pH value
PH 1.0 1.5 2.0 2.5 3.0 3.5 4.0
[F-][mol/L] 42.331 13.608 4.525 1.652 0.744 0.457 0.366
Taking into account the operability and economical efficiency, the pH value of the solution is
determined to be between 3 to 4. In the study, Ba(OH)2 was used to adjust the pH value of manganese
sulfate solution, then BaF2 was introduced for the complete removal of Ca and ions. It is important
to note that Ba2+ will react with sulfuric acid, which appears in high concentration in the leaching
solution. So the use of the precipitation agent can not bring impurities to the system. Moreover,
the precipitant of barium sulfate shows large particles, which is easy to filter without side reaction.
The experimental results showed that the removal rate of Mg and Ca reached 97.93% and 95.11%
respectively, and the recovery rate of Mn is above 96%. The effect of temperature, stirring intensity, and duration of the process will be further studied in our following work.
Conclusion
The paper summarizes the generally accepted methods of removal of Ca and Mg for the high
pure manganese sulfate. The chemical precipitation method has the advantages of simple operation,
rapid response, and high removal efficiency of Ca2+ and Mg2+. Furthermore, a novel method is
proposed to adjust the pH value using Ba(OH)2 and completely remove impurities using BaF2. The
process parameters such as the pH value and the amount of BaF2 were determined through the
theoretical calculation analysis. The results show that the proposed method not only improves the
removal rate ofMg and Ca 97.93% and 95.11% respectively, but also guarantees the high manganese
recovery of more than 96%. The research indicates that it is feasible to realize the clean produce of
high pure manganese sulfate from low-grade manganese ore with a short process, which promotes the
sustainable development of the manganese resource industry. In our following work, we will focus on
process optimization to explore the efficient utilization of manganese resources.
Acknowledgments
This work was financially supported by the National Science and Technology Pillar Program
(2015BAB01B02).