Discussion on the existence mode and bleaching process of coloring matter in kaolin
Depending on the types and causes of kaolin, iron components and contents contained also different, in addition to containing quartz, feldspar, mica, except that there is usually a number of different metal oxides (mainly iron Southern Oxide), organic matter and carbonaceous matter, the former is the main dyeing substance that affects the whiteness of kaolin.
Kaolin is a widely used non-metallic material, the application relates to the field of paper, ceramics, petrochemicals, paints, refractory materials and the like. The whiteness of kaolin is an important indicator affecting its application, but the quality of high-altitude resources is less and less, and most of it needs to be treated by iron removal. Depending on the origin and type of kaolin, the composition and content of iron are also different. In addition to quartz, feldspar, mica, etc., there are usually a number of different metal oxides (the oxidation of iron in the south is mainly Matter), organic matter and carbonaceous matter, the former is the main dyeing substance that affects the whiteness of kaolin.
The physical sorting method (re-election, magnetic separation) is often used for pre-enrichment of kaolin raw materials or pre-separation of harmful substances, sorting out most impurity minerals, and purifying kaolin. However, the physical sorting method has certain limitations. It is difficult to effectively separate the impurity minerals with fine particle size and low content. It is impossible to separate the lattice iron in kaolin. It is difficult to obtain high whiteness by physical sorting alone. High-quality kaolin, therefore, high-quality kaolin needs to be bleached. The coloring materials in kaolin are mainly iron minerals, carbonaceous and organic matter. Titanium minerals are less in southern materials (currently difficult to handle). The manner in which the coloring matter is present and the appropriate method of bleaching.
1 The existence form of iron and its bleaching method
Iron minerals are the most common coloring materials in kaolin. Each kaolin has different degrees of distribution. Common iron minerals include: pyrite (FeS2), limonite (Fe2O3·nH2O), hematite (Fe2O3). ), siderite (FeCO3), magnetite (Fe3O4), etc., among which trivalent iron is the most common. These iron minerals will cause kaolin to exhibit different degrees of gray, brown, pink, etc., which will reduce the whiteness of kaolin. Due to the different nature of the iron minerals, different bleaching processes can be used.
From the domestic and international data, the cost of bleach is relatively high. Usually, after removing a large amount of impurities, various chemical bleaching treatment methods are adopted. Domestic washing and grading methods are generally used to remove sandy minerals (quartz, feldspar) and heavy minerals (iron, titanium oxide). For high-iron kaolin, strong magnetic separation bleaching, reduction bleaching, redox bleaching, Acid leaching bleaching and reduction complex bleaching, wherein reduction complex bleaching is most effective.
2.1 Removal of iron from pyrite-type kaolin - oxidative bleaching
Generally use strong oxidants (sodium hypochlorite, hydrogen peroxide, potassium permanganate, etc.), oxidation of the pyrite water-soluble ferrous salt is then removed by washing in water, the reaction can be expressed as:
FeS2 + 8NaClO → Fe2+ + 8Na+ + 2SO42- + 8Cl- (1)
In order to prevent the subsequent Fe2+ from being reoxidized, the addition of complexing agent (oxalic acid) is an effective method. The process parameters are generally: pH = 5~6; the amount of bleaching agent is about 1.5%; the bleaching time is 20~35 °C. It is 2~3h.
2.2 Fe2O3 (iron oxide) type iron removal
Iron oxide is one of the most common impurities in kaolin. It is usually in the form of trivalent oxide or iron oxide water. It is insoluble in water and difficult to react in dilute acid. Iron removal methods including acid leaching, reduction, acid leaching reduction, etc. are often employed.
2.2.1 Acid leaching and iron bleaching
Iron is often removed by removing iron and oxalic acid from hydrochloric acid. The reaction equation is:
Fe2O3 + 6HCl → 2FeCl3 + 3H2O (2)
3C2O42- + Fe3+ → [Fe(C2O4)]3- (3)
Experiments have shown that when the pH is 5~7, the iron is basically insoluble, and it must be easily removed in a strong acidic environment (pH<2), and the effect of adding hydrochloric acid to hydrochloric acid is better than that of single addition. This is because the complexation of oxalic acid contributes to the dissolution of iron. Although the method is simple, it cannot obtain high-quality kaolin with whiteness greater than 90°.
2.2.2 reduction iron bleaching method
The reducing agent is used to reduce the insoluble ferric iron in the kaolin into divalent iron (dissolved in water), thereby removing the iron oxide. The sodium dithionite is usually used to reduce Fe3+ to Fe2+ at a pH of 2 to 4 and at normal temperature. After washing and removing iron by filtration, the specific reaction is as follows:
Fe2O3+Na2S2O4+H2SO4=Na2SO4+2FeSO3+H2O (4)
Since sodium dithionite in acidic environments or high temperature environments decompose into SO2, H2S and the like, to reduce the bleaching effect, and the working environment is poor, a very strong irritating odor, add some suitable metallic zinc helps to iron powder, which process The parameters are: pH=2~4; the amount of sodium dithionite added is 7%~8%.
2.2.3 Acid leaching reduction combined with iron removal
Hydrochloric acid or sulfuric acid is often added to the active metal zinc to achieve a better iron removal effect. The reaction is as shown in formula (5) to formula (7):
6H++Fe2O3→2Fe3++3H2O (5)
Zn+2H+→Zn2+ +H2 (6)
Fe3++H2→Fe2++2H+ (7)
In order to prevent the subsequent oxidation of Fe2+, the addition of the complexing agent oxalic acid is also an effective auxiliary means. The process parameters are: hydrochloric acid 10%; pH 2~3; zinc powder 1‰; oxalic acid 2%.
2.3 redox combined iron removal
When the presence of iron in kaolin is complex, including pyrite, organic matter, and iron oxide ore, redox combined bleaching is required to remove different impurities. For example, sodium hypochlorite and hydrogen peroxide are used as oxidants, and sodium dithionite is used as a reducing agent for iron removal, which can achieve better results.
3 Conclusion
The removal of iron from kaolin is an important part of kaolin processing. The iron removal process depends on the type and existence of iron in the raw materials. Iron oxide is the most common harmful impurity in kaolin. The use of dithionite for iron removal and acid leaching can have certain effects. Reductive addition or acid leaching plus complexation will make the iron removal effect better. It should be pointed out that in addition to considering the type and existence of iron, the choice of iron removal process for kaolin raw materials should also consider environmental, economic and technical analysis, and select a safe, reliable and economical method for removing iron.
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