China is one of the major manganese resource countries and manganese producing countries in the world, and manganese ore reserves rank among the top in the world. In the development of electrolytic manganese industry for more than 40 years, the output of electrolytic manganese, export volume and domestic consumption have increased rapidly, and these three indicators have ranked first in the world. By the end of 2003, China's electrolytic manganese production capacity has reached 450,000 tons. China has thus become the world's largest producer, exporter and consumer of electrolytic manganese. In the production process of electrolytic manganese, a large amount of waste residue is generated in the anode of the electrolytic cell, wherein the mass fraction of manganese dioxide is 42% to 48%. Due to its complicated composition, the electrolysis process seriously changes the nature of the compound, and the utilization is difficult. They are discarded as an industrial waste or sold cheaply. At the same time, a considerable part of the anode slag paste is washed out by water, which has not been well developed and comprehensively utilized, resulting in waste of resources and considerable environmental pollution, which destroys the ecological balance. In this study, roasting - acid leaching - Oxidation Process manganese metal anode slime electrolysis (referred to as anode slimes Mn) waste. First, the anode mud is calcined at a high temperature to convert MnO ₂ into Mn ₂ O ₃ . Then, it is leached with sulfuric acid to convert the calcined product into Mn ²⁺ . Finally, the disproportionation solution is oxidized with sodium chlorate to obtain active manganese dioxide. The process can not only solve the environmental pollution problem of the anode mud, but also change the waste into treasure, and generate certain economic benefits.

First, the experimental part

(1) Experimental instruments and drugs

1. Experimental equipment. Circulating water type multi-purpose vacuum pump, produced by Zhengzhou Great Wall Teke Industry and Trade Co., Ltd.; Ma boiling furnace, Tianjin Taisite Instrument Co., Ltd.; collector type thermostatic magnetic stirrer, produced by Yuecheng Lecheng Electric Appliance Factory, Zhejiang Province.

2. Experimental materials and pharmaceutical manganese anode mud, provided by an electrolytic manganese plant in Xiangxi Tujia and Miao Autonomous Prefecture, Hunan Province. Sodium chlorate, sulfuric acid, acetic acid, hydrochloric acid, ammonium chloride, triethanolamine, hydroxylamine hydrochloride, potassium permanganate, sodium oxalate, EDTA, etc. are all domestic analytical reagents.

(II) Analysis of raw material properties

The mass fraction of water in the anode mud produced during the production of the electrolytic manganese plant is about 3%. The anion is mainly sulfate ion. The manganese content was quantitatively analyzed by literature method, and other ions were quantitatively analyzed by atomic absorption spectrophotometer. The metal ion mass fraction is shown in Table 1.

Table 1 The percentage of major metal ions in the anode mud

(3) Process route design

Drying the electrolytic manganese anode mud, controlling the temperature to be baked for a certain period of time, then transferring the calcined solid into a beaker, soaking with different concentrations of H ₂ SO ₄ acid for a certain time, and then adding a certain amount of 1 mol/L NaClO _{3 in} batches. Oxidation, filtration and drying to obtain primary manganese dioxide, the specific process flow is shown in Figure 1.

Fig.1 Process flow of manganese anode slime calcination to obtain active manganese dioxide

(4) Product analysis

The product MnO ₂ content was determined by two-step continuous titration method. The determination of the specific gravity is to accurately weigh 4g MnO ₂ , pour it into a 10mL measuring cylinder, tap the cylinder wall 20 times, measure its volume, and divide the mass by the volume to obtain its apparent specific gravity.

Second, the results and discussion

(1) Effect of calcination temperature and time on conversion rate

The conversion of MnO ₂ in the anode mud to Mn ₂ O ₃ is different at different calcination temperatures, which makes the conversion rate different. In order to determine the optimum temperature for roasting conversion, the reaction time was taken for 2 h according to experience. The effect of calcination temperature on the conversion of MnO _{2 was} investigated in the temperature range of 600-800 °C. The experimental results are shown in Fig. 2. It can be seen from Fig. 2 that the conversion rate of MnO ₂ to Mn ₂ O ₃ increases with temperature when the temperature is lower than 700 ° C. When the temperature exceeds 700 ° C, the conversion rate remains basically the same or even decreases slightly. It is because a small amount of MnO _{2 is} converted into Mn ₃ O ₄ , and it can be seen that the conversion rate of MnO ₂ is the highest when the calcination temperature is 700 °C.

Figure 2 Relationship between calcination temperature and conversion rate

At the calcination temperature of 700 °C, the effect of different calcination time on the conversion rate was studied. The results are shown in Fig. 3. When the calcination time is lower than 3h, the conversion rate is increasing, reaching the highest at 3h, when the calcination time is greater than 3h. The conversion rate of MnO ₂ does not change much, and the longer the calcination time, the economical consideration is not preferable. Therefore, the optimal calcination time of electrolytic manganese anode slime is 3 h.

Figure 3 Relationship between roasting time and conversion rate

(2) Effect of sulfuric acid concentration and acid leaching time on conversion rate

The concentration of sulfuric acid has a great influence on the dissolution of Mn(III). The effect of sulfuric acid concentration on the conversion rate is investigated by treating the quantitative electrolytic manganese anode calcined material with the same volume of different mass concentration of H ₂ SO ₄ solution. The acid leaching temperature is 95 ° C, the liquid-solid ratio is 3:1, the acid immersion stirring time is 120 min, and the experimental results of the effect of sulfuric acid mass concentration on disproportionation are shown in Fig. 4. The results show that when the percentage of H ₂ SO ₄ is 10%, the dissolution of Mn(III) is the most. When the concentration of sulfuric acid is less than 10%, the amount of acid is insufficient, the disproportionation reaction is incomplete, and the conversion of MnO ₂ is decreased. When the acid concentration is more than 10%, the acid amount is too large, the acidity of the solution is enhanced, the stability of Mn ^{3+ is} enhanced, the degree of disproportionation reaction is increased in the reverse reaction direction, and the conversion rate of MnO ₂ is also lowered.

Figure 4 Relationship between volume percentage of sulfuric acid and conversion rate

Determine the mass concentration of H ₂ SO _{4 by} 10%, keep the other conditions unchanged, change the acid leaching time, and study the dissolution of Mn(III). The experimental results of the influence of reaction time on conversion rate are shown in Fig. 5. The results show that the conversion rate of MnO ₂ increases with the increase of reaction time. When the acid leaching time reaches 2h, the rate of increase of conversion rate slows down. It can be concluded that the optimum acid leaching time of electrolytic manganese anode mud is 2h.

Figure 5 Relationship between acid leaching time and conversion rate

(III) Effect of the addition amount of NaClO ₃ and the number of additions on the conversion rate

The acid leached solution was filtered, the pH of the filtrate was adjusted to 5.5-6.5, and the oxidation time was controlled to 6 h. The effect of NaOH sodium on the oxidation of Mn ^{2+ was} investigated by adding unequal concentration of 1 mol/L NaClO _{3 .} With the increase of the amount of oxidant, the conversion rate of MnO ₂ increases gradually. When the amount of oxidant exceeds the theoretical value of 120%, the conversion of MnO ₂ does not change. From this, it can be concluded that the amount of NaClO ₃ added is 120% of the theoretical value.

It was found through experiments that the number of additions of NaClO ₃ also directly affected the conversion rate of MnO ₂ . When the addition amount of NaClO ₃ was the same, the batch addition experiment was carried out, and it was found that the product MnO was added in two times (every 3 hours). The yield of ₂ is the largest, and its apparent specific gravity is also the largest, being 1.54 g/cm ³ . X-ray diffraction analysis (see Figure 6) shows that the main component in the sample is γ-MnO ₂ , and a small part is α-MnO ₂ . Studies have shown that MnO ₂ has five different crystal configurations. In terms of electrochemical performance, γ-MnO _{2 has the} highest activity and the best discharge performance. The γ type in this product mainly indicates that the obtained product has higher activity and better discharge performance.

Figure 6 product X-ray diffraction pattern

Third, the conclusion

Through a large number of parallel experiments and data analysis, the optimal combination conditions for the preparation of active manganese dioxide by manganese anode mud roasting-acid leaching-oxidation method are as follows: control roasting temperature at 700 ° C, calcination for 3 h, calcination of solids Move into the beaker, control the temperature at 95 ° C, the liquid-solid ratio is 3:1, dip with H ₂ SO ₄ with a mass concentration of 10% for ₂ h, add 2 times the theoretical value of 120% NaClO ₃ to the oxidation, which can be obtained. High yield and large apparent specific gravity manganese dioxide, the conversion rate reached 84.6%, and the apparent specific gravity was 1.54 g/cm ³ . The roasting acid leaching oxidation process can achieve the purpose of recycling and reusing the electrolytic manganese anode mud. The obtained chemical manganese dioxide can be used as a battery-grade chemical manganese dioxide after further treatment.

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