China is a major magnesite resource country, but there are still many gaps in the utilization of magnesium resources in China compared with the world's advanced countries. Magnesite due to the higher fire resistance, excellent adhesion and other physical and chemical properties, are widely used in metallurgy, building materials, chemical industry, light industry, farming and magnesium metal refining and other fields. China's cryptocrystalline magnesite reserves are limited, so it has not been included in the development plan. Therefore, the use of cryptocrystalline magnesite to produce high-purity magnesium oxide has great practical significance.
The selection of cryptocrystalline magnesite ore can be carried out by mineral processing methods and chemical methods. Because the matrix of the cryptocrystalline magnesite ore is dense and hard, finely embedded, high in impurity content, and has a wide range of similarities, it is difficult to achieve mineral monomer dissociation, and the actual sorting is difficult, and chemical treatment is used. The acid consumption is large, the cost is high, and the loss of magnesium is caused; the beneficiation method is difficult to improve the grade, and the high-purity magnesium oxide cannot be prepared.
Chemical treatment of magnesite, the ore is first calcined to increase its surface activity and increase solubility, and then leaching with hydrochloric acid or nitric acid, sulfuric acid, ammonium salt, bicarbonate and other leaching agents, and then according to magnesium and Different levels of impurity leaching, different methods are used to precipitate and separate the impurities, and finally obtain high-purity MgO products. Because the carbonization method has the advantages of high selectivity, no corrosion and high recovery rate, the magnesite is purified by carbonization method.
  1. Mineral sample and test procedure
(1) Raw material preparation
The size of the lump ore sent from the mine to the laboratory is about 50mm. It enters the 150×200 jaw crusher for medium breakage, then enters the 60×100 jaw crusher for fine breakage, and then enters the XPS-Φ250×150 roll crusher. The sifter is broken. At this time, the ore from the discharge port has a particle size of about 0 to 2 mm, and can enter the vibration grinding ore to carry out monomer dissociation. The ore composition analysis is shown in Table 1.
Table 1 Ore composition analysis (mass score) /%
SiO 2 | Fe 2 O 3 | CaO | MgO | Al 2 O 3 | IL |
10.79 | 0.29 | 3.12 | 40.04 | 0.06 | 45.65 |
(2) Test procedure
The test procedure is shown in Figure 1.
Figure 1 Process
   Second, the test
(1) Calcination test
The citrate activity method was used to characterize the reactivity of light burned magnesia. The method is as follows: 100 g of magnesite powder is weighed, placed in a muffle furnace after being filled with corundum , and the temperature is started after the power is turned on. When the furnace temperature rises to the specified temperature, the timer starts. After the specified time, the power is turned off and naturally cooled to room temperature. Weigh 1.00 g of light-burning powder into a 100 mL beaker using an analytical balance, add 50 mL, 0.2 mol/L citric acid solution for neutralization, measure the reactivity, and record the time required to neutralize the citric acid solution with phenolphthalein as an indicator. The shortest neutralization time has the highest activity.
1. Effect of calcination temperature Magnesite and limestone were placed in a tube furnace at temperatures of 700, 750, 800, 850 and 900 °C, respectively, and calcined for 2.5 h. The activity of the solid product after calcination of magnesite at different temperatures was studied. In order to obtain a highly reactive magnesite light burning powder, the calcination conditions must be controlled. The relationship between the activity of magnesite calcined powder (in the neutralization time) and the calcination temperature is shown in Fig. 2, and it can be seen that the calcination temperature is 800 °C, and the activity is the highest.
Fig. 2 Relationship between calcination temperature and activity of magnesite calcined powder at calcination time of 2.5 h
2. Effect of calcination time The magnesite was placed in a tube furnace with a furnace temperature of 800 °C to study the activity of different calcination time on the calcined magnesite calcined magnesite after magnesium calcination (in the neutralization time).
As the calcination time increases, the specific surface area of ​​MgO becomes smaller; and after the calcination time is more than 1 h, the specific surface area of ​​MgO sharply decreases. Because the calcination time becomes longer, the small grains of MgO combine with each other to form dense large grains under the action of molecular cohesion. The MgO grains grow and the pores are surrounded by dense grains, so that the specific surface area of ​​MgO becomes smaller, and the calcination time is longer. Long, the bigger the change. It can be seen from Fig. 3 that the calcining temperature of magnesite should be controlled at 800 ° C and the light burning time is 2.5 h. The light burned MgO powder obtained under this condition has the highest activity.
Fig. 3 Relationship between calcination time and activity of magnesite calcined powder at 800 °C
(II) Influencing factors of mineralization carbonization process
1, the impact of digestion time Mineral sample processing conditions: vibration grinding sample time: 2.5min, ventilation: 8L / min, heavy magnesium water heating temperature: 150 ° C, CO 2 ventilation time: 3min, solid-liquid ratio: 60: 1.
Table 2 Effect of digestion time on MgO recovery rate and grade
Digestion time / min | βCaO/% | βMgO/% | γ yield /% | ε recovery rate /% |
0 10 20 30 60 | 0.58 0.51 0.44 0.31 0.32 | 98.53 98.61 98.67 99.41 99.40 | 34.85 34.85 40.92 45.47 45.88 | 46.60 46.64 54.79 61.34 61.56 |
It can be seen from Table 2 that the content of CaO in the product decreases with the increase of digestion time, and the content of MgO in the product increases with the increase of digestion time, because the dissolution of MgO increases with time, but due to the limited content of CaO, only It can dissolve a part, so the increase of MgO content makes the content of CaO relatively decrease. However, the digestion time exceeds 30 min, and the grade and yield of MgO do not change much. Therefore, it is of little significance to continue to increase the digestion time.
2, the impact of carbonization time Mineral processing conditions: vibration grinding sample time: 2.5min, ventilation: 8L / min, heavy magnesium water heating temperature: 150 ° C, digestion time: 30min, solid-liquid ratio: 60:1.
Table 3 Effect of carbonization time on MgO recovery and grade
Digestion time / min | βCaO/% | βMgO/% | γ yield /% | ε recovery rate /% |
2 3 4 5 | 0.80 0.31 0.44 0.62 | 98.22 99.41 98.67 98.35 | 33.33 45.45 48.48 48.48 | 44.44 61.34 64.94 61.73 |
It can be seen from Table 3 that the carbonization time has a certain influence on the content of CaO, the content of MgO and the yield, and the carbonization time is too short, resulting in incomplete digestion of MgO and low grade of MgO. When the carbonization time is too long, the dissolved CaO increases, so that the grade of MgO is relatively lowered, and the requirement for preparing high-purity magnesium cannot be achieved. The carbonization time is preferably 3 minutes.
3, the influence of liquid-solid ratio Condition: vibration grinding sample time: 2.5min, ventilation: 8L / min, heavy magnesium water heating temperature: 150 ° C, CO 2 ventilation time: 3min, digestion time: 30min.
Table 4 Effect of liquid-solid ratio on MgO recovery and grade
Digestion time / min | βCaO/% | βMgO/% | γ yield /% | ε recovery rate /% |
40:1 50:1 60:1 | 3.0 3.5 0.31 | 98.55 98.61 99.41 | 66.67 60.61 45.45 | 89.18 81.12 61.34 |
The solid-liquid ratio also has a certain influence on the content of CaO, the content and the yield of MgO. If the liquid-solid ratio is too large, the carbonization and filtration load will increase, and a large amount of waste liquid will be generated, and the energy consumption will increase; the liquid-solid ratio is too small. The digestion is not complete, the residue rate is high, and the MgO ash is wasted. According to the data, the MgO content in a typical solution is 7 to 9 g/L.
(3) Secondary carbonization
The purpose of carbonization is to introduce CO 2 into the digested slurry, convert Mg(OH) 2 and Ca(OH) 2 into CaCO 3 and Mg(HCO 3 ) 2 by carbonization, and filter and filter CaCO 3 and Mg ( HCO 3 ) 2 to complete the effective separation of calcium and magnesium, the main influencing factors are carbonization temperature, carbonization time and final pH value, and the final pH value is a key indicator to determine whether calcium and magnesium are effectively separated. Since Ca(OH) 2 is slightly more basic than Mg(OH) 2 and CaCO 3 is less soluble than Ca(OH) 2 , Mg(OH) 2 is less soluble than MgCO 3 , and Ca(OH) 2 should absorb CO 2 first. When carbonization is CaCO 3 , when the carbonization is complete, the pH value is reduced from 12 in the initial state to 9.5, and then Mg(OH) 2 starts to carbonize. The carbonization is carried out in two steps. When the pH value falls to 7.1, the following occurs. reaction:
Mg(OH) 2 +CO 2 +2H 2 O=MgCO 3 +3H 2 O
MgCO 3 +3H 2 O=Mg(HCO 3 ) 2 +2H 2 O
Mg(OH) 2 is almost completely converted to Mg(HCO 3 ) 2 . The final pH is neither too high nor too low. If the pH is too high, the carbonization of Mg(OH) 2 will be incomplete, and the MgO yield will be low. If the pH is too low, the side reaction CaCO 3 +H 2 O+CO 2 =Ca(HCO 3 ) 2 will occur, which will affect the effective separation of calcium and magnesium. The quality of MgO is lowered and the CaO content is exceeded, so the final pH value of carbonization is preferably 7.0 to 7.1.
The magnesium carbonate precipitate obtained by the first carbonization was added with water for the second carbonization. Table 5 shows the changes in pH and calcium and magnesium contents during the second carbonization.
Table 5 Changes in pH and calcium and magnesium contents of secondary carbonization
Time /a | pH value | CaO/(g·L -1 ) | MgO/(g·L -1 ) |
20 40 60 80 | 8 7 6.8 6.5 | 0.043 0.032 0.02 0.012 | 4.09 5.97 6.22 6.97 |
The charge at the time of secondary carbonization is relatively pure MgCO 3 , so the carbonization time is short. As long as the Mg 2 + content in the carbonized solution is substantially constant, the carbonization can be stopped to ensure the quality of the high-purity magnesium oxide.
 Third, the conclusion
The experimental results show that the optimum process conditions for extracting MgO are: calcination temperature: 800 ° C, light burning time 2.5 h, vibration grinding time: 2.5 min, ventilation: 8 L / min, heavy magnesium water heating temperature: 150 ° C Digestion time: 30 min, CO 2 ventilation time: 3 min, solid-liquid ratio: 60:1, pH of secondary carbonization was 7.0. A high-purity active product with a MgO grade greater than 99.41% at zero ignition was obtained, and the MgO recovery rate was 61.34%.
The carbonization method has the advantages of strong selectivity, no corrosion, easy control, and the like, and the process is relatively simple, the product quality is stable, and it is suitable for industrial production, and has great development prospects.
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