Kaolin is a non-metallic mineral material having a wide range of applications, kaolinite group mineral as a main component of clay mineral was set having unique properties, field of application relates to paper, ceramics, petrochemicals, paints, clinker, Rubber, chemical fiber, refractory materials and other industries. The whiteness of kaolin is an important indicator to determine its application value. According to its use, it has different requirements on the whiteness of kaolin. In particular, the papermaking and ceramic industries have high requirements on the whiteness of kaolin. However, the genesis and types of kaolin in nature are different, and the dyeing substances contained in them are also different. In addition to non-clay mineral quartz , feldspar , mica, etc., there are some metal oxides, organic matter and carbon, which become the main dyeing substances affecting the whiteness of kaolin.
Traditional methods for improving the whiteness of kaolin include physical methods (including hand selection, magnetic separation, flotation, etc.); chemical method, redox method, and microbial iron removal method, and organic acid iron removal method. Although some of the variegated material is removed, it is difficult to remove the coloring matter due to the extremely fine particle size and complicated symbiosis. Therefore, the search for non-traditional kaolin removal methods contributes to the deep processing of kaolin and the improvement of economic value. This paper focuses on the analysis of the occurrence of various dyeing substances in kaolin and the development status of iron removal and whitening technology.
1 the form of dyeing substances
1.1 The form of iron
The most common dyeing materials in kaolin are iron ore, including pyrite (FeS2), limonite (Fe2O3•H2O), hematite (Fe2O3), jarosite [(K, Na, H)Fe3 ( SO4) 2 (OH) 6] and the like. Due to their presence, the kaolin is gray, green, brown, pink, etc., resulting in a decrease in whiteness.
1.2 Carbon and organic matter
Kaolin other major coloring substances carbon and organic matter, especially in the coal-based kaolin in common, appear dark gray, brown and so on. The carbonaceous material is mainly organic carbon and carbonaceous materials (coal, carbon, graphite ); the organic matter mainly exists in the form of lignin-humic acid, which contains various organic functional groups. High temperature calcination is the most effective bleaching method for carbonaceous and organic kaolin.
1.3 The form of titanium
In addition to iron, carbon and organic matter, some kaolins also contain a small amount of titanium, mainly in the form of rutile TiO2, anatase or brookite-type TiO2. Often appearing tan, light yellow, brownish black or dark brown, usually removed by mechanical sorting or oxidative calcination.
2 Status of iron removal technology
2.1 Insurance powder reduction method
Using sodium dithionite as a reducing agent, in an acidic medium, the reaction formula is:
According to the oxidation-reduction reaction formula, it can be concluded that:
It can be seen that the electrode potential of the SO42-/S2O42-electric pair increases as the pH of the solution decreases. When the acidity of the solution is sufficiently high, the electrode potential of Fe3+/Fe2+ is independent of pH. However, when the pH of the solution gradually increases, it is due to Fe3+. Hydrolysis will occur, at which point the ferroelectric pair is converted to Fe(OH)3, /Fe2+, and its [Fe(III)/Fe(Il)] is:
From the above formula, the relationship between the electrode potential and the pH value of the solution can be obtained. When pH=1.53, the maximum value is reached, and when pH=6.45, it is the smallest. When the pH value is more than 6.45, since the reduced product of iron is Fe(OH)2:, it cannot be removed by filtration. Therefore, when dithionite is used as the reducing agent, the acidity of the reaction medium should be at pH=1.53-6.45. The acidity is too small, the electrode potential difference between the two is small, the reaction speed is slow and incomplete, the acidity is too large, the acid consumption is large, and the stability of the dithionite is lowered, and the easy separation is released. The pH value of the solution is controlled at about 3 should. In addition, since the insurance powder is easily decomposed and its reducing ability is reduced, the reaction is as follows:
These side reactions waste both reagents and product quality, and if not washed in time, the product will turn yellow. To solve the above drawbacks, an appropriate amount of a chelating agent is usually added, such as oxalic acid, citric acid, EDTA, poly-phosphate, hydroxylamine salts, chelating ion may be dissolved in water, with the exclusion of the filtrate, bleaching with fast speed, the advantages of good effect.
2.2 Acid immersion hydrogen reduction method
This is a combined pickling and reducing bleaching process, the basic principle is to use hydrochloric acid, sulfuric acid, oxalic acid medium, zinc powder or aluminum powder as the reducing agent, the use of active metal in an acidic solution continuously generated continuously displaced H2 :, The H2 changes the colored insoluble in the kaolin to soluble Fe2+ and is removed with the filtrate. The reaction formula is as follows:
For coal-based kaolin with low whiteness, iron removal by acid immersion hydrogen reduction method and sulfur removal by calcination method can be used to maximize the whiteness of the product.
2.3 flotation method
Kaolin pulping is added with lime powder as adsorbent, and Fe2O3 is adsorbed from the slurry solution onto the lime carrier. The carrier can adhere to the bubble by its own hydrophobicity and the hydrophobicity caused by the reliable collector to obtain the iron-containing carrier. Foam products and in-tank products containing kaolin concentrates to separate Fe2O3 from kaolin. Due to the effects of carrier adsorption, absorption, crystal mixing, enthalpy, agglomeration, etc., as well as the influence of the pH of the medium, the addition time and location of the carrier on the adsorption flotation, the Fe2O3 in the kaolin can be reduced by 0.72% by adsorption flotation. Up to 0.5% or less.
2.4 oxidation method
Pyrite and organic matter contained in kaolin sometimes require oxidation by bleaching. The ferrite in a reduced state is oxidized to a ferrous ion dissolved in water by a strong oxidizing agent in an aqueous medium; at the same time, the dark organic matter is oxidized to become a colorless oxide which can be washed away by water. Commonly used oxidants are sodium hypochlorite, hydrogen peroxide, potassium permanganate, chlorine, ozone and the like.
2.5 Microbial iron removal and whitening method
Thiobacillus ferrooxidans is one of the most commonly used bacteria in mineral microbial processing, which oxidizes pyrite and other sulfide minerals. The low cost, low environmental pollution, and no influence on the physicochemical properties of kaolin are a new whitening method for kaolin.
3 conclusions
The choice of the kaolin bleaching process depends on the type and mode of occurrence of the dyeing material in the raw material. Iron is the most common dyeing material in kaolin. The effect of reducing the complexation with sodium dithionite is better. The carbon and organic dyeing materials are treated by high temperature calcination. However, for the bleaching of complex coal-based kaolin, further research is needed to determine a reliable and economically sound method.
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