Overview of the process of removing iron and whitening in kaolin (2)

Iron is the primary dyeing element of kaolin. Iron-bearing mineral deposits will turn into Fe2O3 when calcined at high temperature, which makes the kaolin mineral yellow or brick red. In order to efficiently remove iron-containing impurities in kaolin ore, it is necessary to know the occurrence status of iron-containing impurities in kaolin so as to adopt corresponding iron removal methods for different irons to achieve the effect of removing iron and whitening kaolin. Scholars at home and abroad have done a lot of research work on the occurrence of iron in kaolin.
Iron is the primary dyeing element of kaolin, and iron-bearing deposits will turn into Fe2O3 when calcined at high temperatures, which makes the kaolin ore yellow or brick-red. In order to efficiently remove iron-containing impurities in kaolin ore, it is necessary to know the occurrence status of iron-containing impurities in kaolin so as to adopt corresponding iron removal methods for different irons to achieve the effect of removing iron and whitening kaolin. Scholars at home and abroad have done a lot of research work on the occurrence of iron in kaolin.
Table 1 Requirements for the whiteness of kaolin in various industrial sectors

1 Oxidative bleaching method for removing iron
The oxidative bleaching iron removal method is mainly used to treat pyrite-type kaolin. It is difficult to bleach the pyrite in the kaolin ore by the recovery bleaching method or the pickling method. Oxidative bleaching method uses strong oxidants as bleaching agents to oxidize pyrite in kaolin to form soluble Fe2+ in the kaolin pulp system. At the same time, other dark organic matter can also be oxidized into colorless oxides and can be bleached. The strong oxidants used are generally H2O2, NaClO, O3 gas, Cl2 gas, KMnO4 and so on.
In a strong acid solution medium, Fe2+ is stable. But when the pH value is higher, Fe2+ may be oxidized to insoluble Fe3+, and the solubility decreases. In addition to the influence of pH value, the oxidative bleaching process is also affected by factors such as the characteristics of kaolin mineral deposits, the temperature of the slurry system, the time of bleaching, the amount of oxidant, and the concentration of kaolin slurry.
Huang Mingli et al. used NaClO as an oxidant to carry out oxidative bleaching and iron removal from a pyrite-bearing kaolin ore in the west of Gusuyang Mountain. The calcined whiteness of the kaolin ore is 66-68%. The content of disseminated pyrite is 1 to 3%. After oxidizing and bleaching, the calcined whiteness can reach 84.5%. The optimal process conditions are 12-15% pulp concentration, temperature control at 30-40℃, and pH The value is controlled at 5-6, the amount of bleaching agent is 1.5-2.5%, the bleaching time is 1 to 3 hours, and the final filter is washed.
Because the oxidative bleaching method for removing iron is mainly aimed at bleaching pyrite-containing kaolin, it has little effect on removing iron and whitening kaolin containing other types of dyed metals, and does not have the conditions for widespread use.
2 Oxidation restoration combined iron removal method
In the clay mineral deposits, there is a kind of gray (such as a certain kaolin produced in Georgia, USA), which is different from the pink and beige clay. The above-mentioned restoration bleaching method does not improve its whiteness and brightness, and the oxidation method is used. The effect of bleaching is not very good. Therefore, the oxidation-recovery combined bleaching method for removing iron is presented. Take the oxidation-recovery composite bleaching method of gray kaolin in Georgia, USA as an example. The dyeing organic matter and pyrite and other impurities of kaolin are removed; then sodium bisodium is used as a restoring agent for restoration and bleaching, so that the iron oxides remaining in the kaolin, such as Fe2O3, FeOOH, etc., are restored to soluble divalent iron. The kaolin is then bleached.
The detailed process is as follows: the kaolin ore is prepared into a slurry with a kaolin concentration of 30%, and the mixture is mixed with about 1.2kg/t of anhydrous sodium carbonate and tripolyphosphate. The mass ratio of the two agents in the solution is about 1:2 , Mix thoroughly and evenly, adjust the pH value to 6.4; after removing the coarse particles on the 100 mesh sieve, add 1.7kg/t NaClO in the slurry, mix it evenly, and then add 0.66kg/t H2O2, mix it evenly, and centrifuge for classification. 2μm occupies 90% of the fine-grain grade, add 10% sulfuric acid and aluminum sulfate (50% each) solution, adjust the pH to 3, then add 2.72kg/t Na2S2O4 to bleach again, then filter and dry, after bleaching The whiteness value of kaolin and the comparison results with no oxidizing agent and restoring agent are shown in Table 2.
Table 2 The effect of high-territory oxidation-recovery combined bleaching method

Similarly, the oxidation restoration combined iron removal is mainly aimed at kaolin similar to a certain kaolin produced in Georgia, USA. It has little effect on removing iron and whitening from kaolin ore containing other dyed metal substances, and does not have the conditions for widespread use. .
3 Iron removal method for chlorination roasting process
Zheng Shuilin and Liu Wenzhong used the chlorinated roasting method to discuss the iron removal and whitening process of coal-based kaolin. Zheng Shuilin’s research group used NaCl as the chlorinating agent, adjusted the atmosphere with CO2 gas, and used a corundum tube furnace at a heating rate of 4 to 5 °C/min, a constant temperature of 860 to 950 °C, a constant temperature of >1.5h, and a CO2 gas flow rate of 1.5 to 2.5 Under L/min conditions, calcined kaolin products with higher brightness and lower Fe2O3 content can be produced from coal-based kaolin.
In the process of chlorination and roasting of coal-based kaolin, iron oxides take FeO to participate in the chlorination reaction (Fe2O3 is first restored to FeO), and the chlorination product is mainly FeCl2, and there is also a small amount of FeCl3 at low temperatures. The study of the process mechanism indicated that when the temperature of the kaolin mine rises to 550°C, NaCl will differentiate under the induction of SiO2, H2O and other components in the system. Fe2O3 in the kaolin system is first restored to FeO under the action of the recovery atmosphere generated by the gasification of carbon in the coal-based kaolin, and then forms FeCl2 gas under the action of chlorination; part of FeCl2 can also react with Cl2 to produce FeCl3 .
A small amount of chloride (FeCl2 and FeCl3) occurs between the system temperature of 550~700℃. After the temperature continues to rise, a lot of FeCl2 evaporates. The gaseous FeCl2 escapes from the surface of the solid particles and is carried by the CO2 gas flow. Walk out. The iron removal method has a good prospect from the perspective of removing iron from kaolin and developing calcined kaolin products. However, the emission of solid particles of ferrous chloride in the process is now the most important environmental problem (ie PM2.5 problem). ), it is not easy to use industrially.
4 Microbial iron removal method
Microbial iron removal technology has obvious characteristics of low investment, low cost, low energy consumption, and low environmental pollution in the iron removal technology of kaolin, which has been paid attention to by relevant researchers. At present, there have been studies at home and abroad on the use of Thiobacillus ferrooxidans, heterotrophic microorganisms and dissimilar iron restoration bacteria GS-15 to produce biological effects and organic acids generated by fermentation to remove iron and whiten kaolin. Take the iron removal method of Thiobacillus ferrooxidans and the iron removal method of dissimilation iron restoration bacteria GS-15 as examples.
Yuan Xin’s research team selected kaolin ore, which mainly contains pyrite impurities, to oxidize and remove iron from this type of kaolin with the innovative effects of Thiobacillus ferrooxidans. Thiobacillus ferrooxidans (T.f. bacteria for short) is one of the most commonly used bacteria in the microbial processing technology of mineral deposits. The study showed that in the process of Thiobacillus ferrooxidans oxidizing the pyrite in kaolin, Tf bacteria oxidizes FeS2 in kaolin ore to ensure satisfactory nutrients and concentration for its growth, and then separate Fe2+ from kaolin, and then achieve progress. The intention of whiteness.
The research results indicated that the kaolin containing FeS2, after 35 days of oxidation by T.f. bacteria, the calcined whiteness increased from 73.6% to 84.9%, and the iron removal rate reached 71.98%. The Guo Minrong research group used the innovating effects of heterotrophic iron restoring bacteria to carry out in-depth research on removing iron from Fe2O3 kaolin. The six influencing factors of treatment time, carbon source and its dosage, system pH value, inoculation amount of heterotrophic iron-restoring bacteria, and system temperature on the iron removal effect of kaolin were discussed.
The research results indicate that under anaerobic conditions, the amount of sucrose required per gram of kaolin is 0.04g, the amount of heterotrophic iron restoration bacteria is 0.5mL, the system pH is 6.0, and the system temperature is 30°C. The iron-restoring bacteria have the best iron removal effect. After 16 days of cultivation, the mass fraction of Fe2O3 in the kaolin with a slurry mass concentration of 100g/L decreased from 0.59% to 0.52%, and the natural whiteness of kaolin increased from 75% to 80%. %.
Generally speaking, microbiological iron removal methods do have many advantages in the methods of removing iron from kaolin. However, bacterial strains generally have strict requirements on the system environment, and are difficult to manipulate in industrial production, and there are many technical problems. Therefore, the method of microbial iron removal needs to be further studied.
5 Magnetic separation and iron removal method
The magnetic separation method for iron removal has the advantages of energy saving and low environmental pollution, and has become the preferred method for iron removal by kaolin manufacturers. The basic principle of magnetic separation and separation of iron is to magnetize and separate the iron-bearing ore deposits in kaolin in a magnetic field to eradicate it. The basis for the separation is that the quality of the susceptibility of the mineral is different, that is, the magnetic field has different magnetic field effects on different iron-bearing minerals.
The mass magnetic susceptibility χ of ferromagnetic mineral deposits is greater than 3.8*10-4 m3/kg. The iron-bearing mineral deposits in this type of mineral deposits mainly include magnetite, maghemite (γ-hematite), titanomagnetite, and zinc. Iron spinel and pyrrhotite, etc., are the easiest and most important magnetic separation targets for magnetic separation. For such strong magnetic minerals, only a weak magnetic field separator with a magnetic induction intensity of 0.12-0.15T can be used to complete the magnetic separation. leave. The mass specific magnetic susceptibility χ of weak magnetic deposits is 7.5-0.126*10-4 m3/kg/kg. The iron-bearing deposits in this type of deposits mainly include hematite, limonite, siderite, ilmenite, and chromium. For iron ore, for such weakly magnetic minerals, a strong magnetic separator with a magnetic induction intensity of 1 to 2 T is required to complete the magnetic separation of kaolin.
Ordinary magnetic separation is more useful for strong magnetic minerals, but for weak magnetic minerals, a high-gradient strong magnetic field magnetic separation method is required. The high-gradient strong magnetic field magnetic separation method has a good magnetic separation and iron removal effect on the weakly magnetic fine particles and even the colloidal particles in kaolin, and the whiteness of kaolin has been improved significantly. However, because of the current equipment and technical constraints, and the diversity of iron content in kaolin, the iron in kaolin is removed through magnetic separation. The quality of the product is not very ambitious and cannot satisfy all needs.

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