I. Introduction Iron steel led to the rapid development of China's development of electrolytic manganese industry, as of June 2006, China's manganese production enterprises 158, with a total production capacity of 1,175,000 t. In 2005, the actual production of 576,000 tons, accounting for more than 95% of the world's total output, has become a veritable electrolytic manganese production country. In the near term, the price of electrolytic manganese has doubled over last year due to the surge in nickel prices, reaching more than 20,000 yuan / t, which will accelerate the development of China's electrolytic manganese industry. Manganese in the production process, the metal impurities contained in the raw material manganese great influence on the quality of manganese, lead, zinc, cobalt, nickel, copper, manganese production process is necessary to remove the harmful elements, magnesium and calcium will Affects current efficiency during electrolysis and purity of electrolytic manganese products. The production process of electrolytic manganese metal is shown in Figure 1. Figure 1 Flow chart of the principle of producing electrolytic manganese It can be seen from Fig. 1 that the electrolyzed anolyte (containing impurity calcium and magnesium) is used together with sulfuric acid for leaching. The calcium and magnesium in the neutralization process will not be removed, so the electrolytic manganese anolyte is recycled in the middle of Mg 2+ . Accumulation of impurities such as Ca 2+ gradually increases the energy consumption during the electrolysis process and affects the quality of the electrolytic manganese product. Methods for removing Ca and Mg Currently, there are mainly chemical precipitation methods, extraction methods, and concentrated standing methods. According to the literature, the chemical precipitation method is the main method for removing Ca and Mg, but it often brings some new impurity ions. However, the extraction of Ca and Mg ions from manganese sulfate has not yet found a very effective extractant; It is difficult to meet the requirements. We used the manganese fluoride chemical precipitation method to carry out experiments, avoiding the introduction of new impurity ions, and achieved satisfactory results. Second, the experimental part The raw materials used in the test were low-grade manganese oxide ore in a certain area of ​​Guilin, Guangxi (see Table 1 for composition analysis). The manganese sulfate solution was obtained by reduction roasting and sulfuric acid leaching. In the test, according to the actual composition of the manganese sulfate leachate, the precipitation and removal of calcium and magnesium were carried out. Table 1 Analysis of the composition of a manganese oxide ore in Guangxi The leaching process uses reduction roasting to reduce manganese oxide ore (mainly in the form of MnO 2 ) to MnO, leaching with concentrated H 2 SO 4 , sulfuric acid addition amount (acid/mine weight ratio = 0.55), leaching humidity 60 ° C, in liquid The mixture was leached at a solid ratio of 5:1 for 1 hour, and filtered to obtain a filtrate as a raw material liquid. The concentration of each ion in the raw material liquid was manganese ÏMn=43.80 g/L, magnesium ÏMg=0.72 g/L, and calcium ÏCa=0.12 g∕L. However, in the actual production, the electrolyte is subjected to multiple closed cycles, and the cumulative concentration of Mg 2+ usually reaches 20 g/L or more, so the concentration of Mg 2+ in the artificially prepared solution is 20 g/L, at this time, SO 4 2 - The concentration was 1.63 mol/L. At room temperature, the solubility product of CaSO 4 is Ksp = 1.9 × 10 -4 , and the calcium ion concentration is lowered to ÏCa = 4.66 × 10 -3 g / L due to the same ion effect. According to the thermodynamic data, the solubility products of CaF 2 and MgF 2 are relatively low, while the solubility product of MnF 2 is relatively large, so fluoride is a good precipitant for removing Ca and Mg. The number of grams dissolved per 100 g of water at 20 ° C was: MnF 2 1.06; CaF 2 0.015; MgF 2 0.013. During the test, the pH of the raw material liquid was adjusted by adding calcium oxide, and NH 4 F and MnF were respectively added to form a fluoride precipitate in Ca and Mg. The MnF precipitating agent is obtained by reacting manganese sulfate with sodium fluoride, filtering the precipitate out, washing the filter cake, and drying. Manganese was titrated and calcium and magnesium were analyzed by flame atomic absorption spectrophotometry. Third, the results and discussion (1) Selection of precipitant There are two balances in the precipitation generation process: Different metal fluorides have different equilibrium constants Ksp. In order to complete the precipitation and reduce the calcium and magnesium ion content in the manganese sulfate solution to meet the requirements of electrolysis, a sufficient concentration of F - must be obtained. And [F - ] is directly related to [H + ]. If the pH is too small (less than 3), the fluoride ion in the precipitant will generate HF, and the HF dissociation constant is relatively small (K=7.4×10 -4 ) On the one hand, it seriously corrodes the equipment, on the other hand, it reduces the F - number involved in the reaction and reduces the effect of removing calcium and magnesium. This test selects a suitable precipitant by investigating the relationship between the impurity removal effect of different metal fluorides and the pH of the raw material liquid. 1. NH 4 F precipitation calcium and magnesium test After the raw material liquid was adjusted to different pH values, NH 4 F was added to precipitate calcium and magnesium, and the amount was 1.5 times of the total molar amount of calcium and magnesium. After stirring for 1 hour, the flocculant polyacrylamide was added and filtered to determine the content of calcium and magnesium in the filtrate. The precipitation rate was calculated and the experimental results shown in Fig. 1 were obtained. It can be seen from the experimental results that the precipitation of calcium and magnesium with NH 4 F has a considerable relationship with the pH of the raw material solution, and the effect of precipitating calcium and magnesium is relatively poor when the acidity is high. In the range of pH=1～5, the precipitation rate of calcium and magnesium increased significantly with the increase of pH value; when the pH≥6.5, the precipitation of calcium and magnesium in the raw material solution was almost complete. Therefore, NH 4 F as a precipitant to precipitate calcium and magnesium should be controlled between pH = 6 ~ 7, in the actual production of manganese ore leaching solution pH between 2 ~ 3, which requires a large amount of calcium oxide to adjust The pH of the raw material liquid, and the introduction of ammonium ions into the solution, it is not suitable to remove the calcium and magnesium impurities in the manganese ore leaching solution by using NH 4 F as a precipitating agent. 2. MnF precipitation calcium and magnesium test After adjusting the raw material liquid to different pH values, MnF solid precipitated calcium magnesium was added. MnF was prepared by reacting MnSO 4 with NH 4 F in the laboratory. The molar amount of MnF was 1.5 times of the total molar amount of calcium and magnesium, and the mixture was stirred for 1 hour. Thereafter, the flocculant polyacrylamide was added, filtered, and the calcium and magnesium contents in the filtrate were measured, and the precipitation rate was calculated to obtain the experimental results shown in FIG. Fig. 2 Effect of NH4F on precipitation rate of calcium and magnesium at different pH values From the results, it can be seen that by using MnF to precipitate calcium and magnesium at a pH of 4, more than 90% of the calcium and magnesium in the raw material liquid can be precipitated, and the excess MnF is filtered out of the solution together with the precipitation of calcium and magnesium, and no new one is introduced. Impurity ions or other precipitated substances, the pH range required for precipitation is close to the pH value of the leachate (2.5 to 3), which is easy to achieve in actual production. Therefore, MnF removes a suitable precipitant for calcium and magnesium in the manganese-containing solution. (2) The influence of reaction stirring time The stirring time affects both the uniformity of the solid fluoride manganese particles in the reaction system and the mass transfer rate of the reactants in the solution. The effect of stirring time on the precipitation rate of calcium and magnesium was investigated under the conditions of a dosage coefficient of 1.5, a pH of 4, and room temperature. The results are shown in Fig. 3. Fig. 3 Effect of MnF on precipitation rate of calcium and magnesium at different pH values It can be seen from Fig. 4 that when the stirring time is above 50 min, the precipitation rate of calcium and magnesium is almost constant, and the stirring time is too short, the solid fluoride manganese particles are not completely dissolved and sink into the bottom of the reactor, resulting in the precipitation rate of calcium and magnesium. decline. At the same time, this will increase the amount of manganese adsorbed by the solids in the system and directly affect the manganese loss rate of the manganese sulfate solution. The prolongation of the stirring time will lower the ability of the calcium fluoride magnesium fluoride to adsorb manganese, thereby increasing the manganese recovery rate. Therefore, it is best to select 1h for mixing time. Figure 4 Effect of stirring time on precipitation rate of calcium and magnesium (3) The influence of the amount of precipitant (dosage coefficient) At room temperature, the pH of the raw material liquid was 4, and the calcium and magnesium were precipitated with different amounts of manganese fluoride, and stirred for 1 hour. The effect of the amount coefficient (ratio of the number of moles of manganese fluoride to the total moles of calcium and magnesium in the system) on the precipitation rate was examined. The results are shown in Fig. 4. It can be seen from Fig. 5 that the precipitation rate of calcium and magnesium increases linearly with the increase of R in the case of insufficient precipitant, and when the amount coefficient is 1, the precipitation of calcium and magnesium is not complete. Calcium has substantially precipitated completely when R is about 1.25; when R is about 1.5, magnesium is also substantially precipitated completely. When the dosage coefficient is increased, there is no obvious increase in the removal effect of calcium and magnesium ions. Li will only waste the precipitant, and too much precipitant will adsorb more manganese, which will increase the loss of manganese and increase the difficulty of filtration. Figure 5 Effect of dosage coefficient on precipitation rate (4) The influence of reaction temperature The effect of temperature on the precipitation rate of calcium and magnesium was investigated under the conditions of a dosage coefficient of 1.5, a pH of 4, and stirring for 1 h. The results are shown in Fig. 6. Figure 6 Effect of temperature on precipitation rate of calcium and magnesium As can be seen from Fig. 6, the relationship between the temperature and the concentration of calcium and magnesium in the filtrate after precipitation of calcium and magnesium in the temperature range of 40 to 100 °C. When the dosage coefficient is 1.5 and the pH value is 4, the precipitation rate of Ca and Mg ions increases with the increase of temperature, and the removal effect is better. Since the formation of both CaF 2 and MgF 2 requires absorption of heat, heating facilitates its formation. The Ca ion concentration tends to be balanced at around 60 ° C; the Mg ions are balanced at around 90 ° C. On the other hand, the temperature of the reaction system has a great influence on the pressure filtration performance of calcium fluoride and magnesium fluoride slag. The high reaction temperature is favorable for the dissolution of solid manganese fluoride, accelerates the reaction rate, shortens the production time, and improves the filter press performance of calcium fluoride and magnesium fluoride slag. At the same time, the flocculation effect of calcium fluoride and magnesium fluoride is better at higher temperatures. On the contrary, if the temperature is too low, calcium fluoride and magnesium fluoride tend to be gelatinous, and the ability to adsorb manganese ions is rapidly increased, and the difficulty of pressure filtration is also rapidly increased. However, too high a temperature will increase energy consumption. Considering the above factors, it is reasonable to control the reaction temperature at 90 °C. (5) Relationship between rest time and sedimentation rate Another factor closely related to impurity removal in actual production is the sedimentation rate of calcium fluoride and magnesium. The calcium fluoride and magnesium fluoride formed by the combination of the fluoride precipitant and the calcium and magnesium in the feed liquid are easy to form a colloid. Therefore, it is necessary to add a flocculant polyacrylamide to the system (about 20 g per t leaching solution), at this time, fluorine The sedimentation rate of calcium and magnesium restricts the progress of the entire impurity removal process. Under the condition of pH 4, dosage coefficient of 1.5 and temperature of 90 °C, the relationship between sedimentation rate and standing time of different concentrations of calcium and magnesium was observed in the experiment as shown in Fig. 7 and Fig. 8. Figure 7 Settling velocity after precipitation of different concentrations of magnesium ions Figure 8 Settling velocity after precipitation of different concentrations of calcium ions It can be seen from Fig. 7 that for magnesium, as the ion concentration increases, the sedimentation of the flocculent magnesium fluoride becomes more and more difficult. When the concentration of magnesium ions is concentrated to 40 g/L, only about 40% of magnesium fluoride can be precipitated, and the rest is in the form of colloids. When the magnesium ion concentration is the original concentration in the leaching solution (about 0.7g/L), the sedimentation effect is preferably more than 90%. It can be seen from Fig. 8 that for calcium ions, the concentration has little effect on the sedimentation, and the precipitation rate is 70% to 80%. However, the time required for calcium and magnesium to settle statically is above 15h. In summary, the removal of calcium and magnesium by manganese fluoride should be carried out as far as possible without the enrichment of magnesium ions, and the filter residue is separated by centrifugation and pressure filtration. Fourth, the conclusion By removing the calcium and magnesium from the manganese ore leaching solution by fluoride precipitation, the following results are obtained: MnF is used as a precipitating agent to remove calcium and magnesium, and no new impurity ions are introduced into the system compared with NH 4 F. The pH value is 4, which is close to the manganese leaching liquid phase, so it is a suitable precipitant for removing calcium and magnesium impurities in the manganese-containing solution. When the temperature is 90 ° C, the dosage coefficient is 1.5, the stirring time is 1 h, and the pH value of the raw material liquid is 4, the precipitation rates of calcium and magnesium are 96% and 99%, respectively. 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