Purification and Crystallization of Manganese Sulfate Products Prepared by Leaching Pyrolusite with Sulfur Dioxide

Soft manganese ore leaching with sulfur dioxide due to the utilization of both the poor manganese ore resources, but also reduces exhaust sulfur dioxide pollution to the environment, which is increasingly attach importance to the majority of scientists. Literature (1) reported the kinetics of sulfur dioxide leaching of pyrolusite. Although there are many reports on the purification and crystallization process of manganese sulfate, the leaching solution obtained by leaching soft manganese ore with sulfur dioxide has the characteristics of low impurity iron concentration and pH value ≥2, and the base purification treatment process is more convenient and economical. In response to this feature, a feed-grade standard manganese sulfate product can be obtained by a simpler method of purifying and crystallizing the leachate.

I. Theoretical analysis of leachate purification and crystallization process

(a) in addition to Fe and Al

In the process of SO ₂ leaching of pyrolusite, in addition to the leaching of manganese, it is accompanied by the leaching of impurities such as Fe, Al, Ca, Pb, and As. The leaching reaction formula can be expressed as:

Wherein Me represents a metal Fe, Al, Ca, Pb and the like. Due to the excess of SO ₂ during the reaction, after the iron leaching, the following reaction occurs between the remaining SO ₂ and Fe ³⁺ :

Thereby, Fe ^{3+ is} reduced to Fe ²⁺ , and thus the iron in the leachate is mainly present in the form of Fe ²⁺ .

In order to obtain a certain quality of MnSO ₄ , impurities therein must be removed. Usually, the neutralization method is used to increase the pH of the solution, and a part of the metal impurities such as iron and aluminum are hydrolyzed to form a precipitate and removed. The precipitation of metal ions, the pH at the start of precipitation, and the pH at the time of complete ([Me]<10 ^-5 mol/L) are shown in Table 1. The ion concentration in the leachate is the actual concentration or the largest possible ion in the solution. concentration).

Table 1 Precipitation reaction of main metal ions in leachate and related data

It can be seen from Table 1 that in order to make c(Fe ²⁺ )<10 ^-5 mol/L (ie, iron precipitation is complete) in the solution, the pH of the solution must be adjusted to >9.18, however, this value has exceeded Mn ²⁺ The initial pH of the precipitate causes a large amount of Mn ^{2+ to} precipitate and is lost, so it is not preferable to directly remove the ferrous iron by the neutralization hydrolysis method. It can also be seen from Table 1 that the pH of the Fe ³⁺ precipitate is 3.27, and Mn ²⁺ does not hydrolyze. For this reason, to remove Fe ^2+, Fe ²⁺ can be first oxidized to Fe ^3+, and then the PH value of the solution was adjusted to 5, at this time, only complete precipitation of iron, aluminum precipitate also completely, can be removed simultaneously. Since ferromanganese is itself an oxidant, it oxidizes with Fe ²⁺ :

2Fe ²⁺ +MnO ₂ +4H ⁺ =Mn ²⁺ +2Fe ³⁺ +2H ₂ O (2)

The Fe ^{2+ is} oxidized to Fe ³⁺ ; at the same time, it can react with the remaining SO _{2 in the} solution to form MnSO ₄ without introducing harmful ions. Therefore, pyrolusite is used as an oxidant in the test. Since the reaction (2) is an endothermic reaction, heating is required during the test to promote the reaction.

(2) Excluding heavy metals

After the above-mentioned impurity removal process, heavy metal impurities such as Pb are present in the leachate and need to be removed. Generally, these metals are used to remove soluble sulfides, such as ammonium sulfide, but this method is easy to produce flocculated amorphous precipitates, which are difficult to filter; the amount of sulfides is not well controlled, and the manganese is lost, and the impurities are eliminated. At the same time, because the mass fraction of Pb and As in the raw materials used in the test is very low, the remaining heavy metal ions such as Pb can be removed by using the precipitant SDD (diethylamine thioformate).

(3) In addition to colloidal impurities

After the impurities are removed, a portion of CaSO ₄ , a small amount of MgSO ₄ , and some unprecipitated H ₂ SiO ₃ colloids, small particles of Fe(OH) ₃ and Al(OH) ₃ are present in the manganese sulfate leachate. These colloidal impurities can be removed by standing.

(4) Manganese sulfate crystal

MnSO ₄ is a rosy crystal, and depending on the temperature at the time of crystallization, a crystal product of various roses containing different crystal waters can be formed. The temperature is below 90 ° C to form MnSO ₄ ·H ₂ O; above 200 ° C, MnSO ₄ is formed. Industrial products refer to MnSO ₄ ·H ₂ O.

The solubility of MnSO ₄ decreases with increasing temperature. At 0 ° C, the mass concentration of MnSO ₄ in the saturated aqueous solution of manganese sulfate is 374 g / L, 265 g / L at 100 ° C, 7 g / L at 200 ° C, because the desired product is MnSO ₄ · H ₂ O, so when crystallizing The temperature should be controlled below 90 °C. Between 0 and 100 ° C, the solubility of MnSO ₄ is large and decreases slightly with the increase of temperature. Therefore, the method of evaporating the solvent is used to crystallize manganese sulfate in the solution into MnSO ₄ ·H ₂ O product. In the test, when the solution was crystallized, the temperature was controlled between 80 and 90 ° C, and after crystallization, it was subjected to hot filtration.

Second, the test part

(1) Test procedure

Take 500 mL of the leaching solution obtained by leaching the pyrolusite with sulfur dioxide in a 1000 mL beaker, and place it in a constant temperature water bath to control the reaction temperature between 90 and 100 ° C. Electric stirring, adding a certain amount of pure MnO ₂ , adjusting the pH of the solution to 3.5, and reacting for 1 hour. After that, lime water was added to adjust the pH to 6, and reacted for 1 hour. The SDD was then added dropwise with stirring. The SDD addition volume was 0.1% of the volume of the leachate. After reacting for a certain period of time, it was filtered, and the filtrate was concentrated by evaporation to form a saturated solution of MnSO ₄ . The saturated solution was allowed to stand for 24 hours, and then filtered, and the filtrate was evaporated to crystallize between 80 and 90 ° C and dried to obtain MnSO ₄ ·H ₂ O.

(two) analytical methods

Analysis of Mn: In the presence of H ₃ PO ₄ , the sample solution is heated to 220-240 ° C, Mn ^{2+ is} oxidized to Mn ³⁺ with NH ₄ NO ₃ , and after cooling, the solution is reduced with a standard ammonium ferrous sulfate solution. fair. The indicator used a 2 g/L solution of phenylanthranilic acid.

Third, the test results and discussion

(1) The effect of temperature on iron removal

Since the temperature has a great influence on the process of complete oxidation of Fe ²⁺ to Fe ³⁺ , when other conditions are the same, the mass fraction of MnSO ₄ ·H ₂ O in the crystallized product obtained by neutralization and purification at different temperatures and its oxidation and removal are eliminated. The temperature of iron is closely related. The test results are shown in Table 2.

Table 2 Relationship between oxidation neutralization temperature and product quality

It can be seen from Table 2 that the temperature of the product increases in the mass fraction of MnSO ₄ ·H ₂ O in the product. This is because the reaction is an endothermic reaction, and raising the temperature is beneficial for the oxidation reaction to proceed more thoroughly. However, if the temperature is too high, the evaporation amount of the solution is large, and some manganese sulfate products are precipitated, so that the product mixture is lost in the iron slag; when the temperature is greater than 90 ° C, the mass fraction of MnSO ₄ ·H ₂ O in the product changes little. . In the actual process, the temperature of each iron removal can be between 90 and 100 °C.

(2) The effect of standing on product quality

In order to investigate the effect of rest time on product quality, the relationship between the static liquid removal and product quality was studied. The results are shown in Table 3.

Table 3 Effect of rest time on product quality

It can be seen from Table 3 that after standing for 24 hours, the mass fraction of MnSO ₄ ·H ₂ O in the product is increased by 2%, which is mainly due to the fact that the impurities can be further removed by standing. The filtrate was concentrated by evaporation, SO ₄ ^2- concentration is increased, since the common ion effect, CaSO _4, MgSO ₄ solubility greatly reduced, most of the CaSO ₄ and MgSO ₄ can settle down; the same time, due to prolonged boiling destroyed H ₂ The colloidal structure of SiO ₃ , Fe(OH) ₃ and Al(OH) ₃ is allowed to stand for 24 hours, so that H ₂ SiO ₃ , Fe(OH) ₃ and Al(OH) ₃ can be precipitated and removed by filtration. The mass fraction of impurities in the product is greatly reduced, and the mass fraction of MnSO ₄ ·H ₂ O is increased.

(3) Product quality

The leachate obtained by leaching the pyrolusite with SO ₂ is purified, filtered, concentrated, allowed to stand, filtered, and crystallized to obtain MnSO ₄ ·H ₂ O. The chemical analysis method was used to analyze Fe, As, and Pb by Mn atomic absorption spectrometry and compared with the national standard GB1622-86 industrial grade manganese sulfate standard and GB8213-87 feed grade manganese sulfate standard. The results are shown in Table 4.

Table 4 Quality of manganese sulfate products

It can be seen from Table 4 that the quality of the manganese sulfate ore produced by the method exceeds the GB1622-86 industrial grade and the GB1622-87 feed grade manganese sulfate standard.

Fourth, the conclusion

(1) For the leachate obtained by leaching the soft manganese ore with SO ₂ , firstly, between 90 and 100 ° C, the Fe ^{2+ is} oxidized to Fe ³⁺ with pure MnO ₂ , and the pH is adjusted to 6 by adding lime water, and the reaction is 1 h. The iron is removed and the impurity aluminum is also removed.

(2) The heavy metal in the leachate can be removed by adding SDD dropwise under stirring.

(3) Whether it is left or not, it has a certain impact on the quality of manganese sulfate products. In practice, the anti-mixing liquid should be allowed to stand for 24 hours to achieve the required quality of the manganese sulfate product.

(4) The purified liquid is concentrated and crystallized between 80 and 90 ° C, and then dried to obtain a MnSO ₄ ·H ₂ O product. After analysis, the quality of the manganese sulfate product obtained by this method reaches the GB8213-87 feed grade manganese sulfate standard.

(5) The purification process of the leachate obtained by leaching the pyrolusite with sulfur dioxide is convenient and economical, and the method is simpler and more economical than other methods for preparing manganese sulfate by using pyrolusite.

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