Kaolin iron removal technology

Iron minerals are very common coloring substances in kaolin, and each type of kaolin has different degrees of distribution. Common iron minerals include pyrite, limonite, hematite, siderite, magnetite, etc. These iron minerals will make kaolin appear gray, brown, pink, etc. to varying degrees, reducing the whiteness of kaolin. Due to the different properties of iron minerals, different iron removal processes can be used. Let's learn about it together.

011. Iron removal of pyrite-type kaolin

Generally, strong oxidants (sodium hypochlorite, hydrogen peroxide, potassium permanganate, etc.) are used to oxidize pyrite into water-soluble ferrous salts in water, and then removed by washing.

To prevent subsequent re-oxidation of Fe²⁺, the pH value can be adjusted to 5~6 by adding a complexing agent (oxalic acid), and the amount of bleaching agent is about 1.5%; the bleaching time is 2~3h at 20~35℃, which can improve the re-oxidation problem.

022. Iron removal from hematite-type kaolin

Iron oxide is one of the common impurities in kaolin. It often exists in the form of trivalent oxides or iron oxide hydrates. It is insoluble in water and difficult to react in dilute acid. Commonly used iron removal methods include acid leaching, reduction, and acid leaching and reduction combined methods.

Acid leaching iron removal method: hydrochloric acid and oxalic acid are often used for iron removal. When the pH is 5-7, iron is basically insoluble and can only be easily removed in a strong acidic environment (pH < 2), and the effect of hydrochloric acid plus oxalic acid is better than adding them separately. This is because the complexing effect of oxalic acid helps the dissolution of iron. Although the method is simple, it is impossible to obtain high-quality kaolin with a whiteness greater than 90°.

Reduction iron removal method: Use a reducing agent to reduce the insoluble trivalent iron in kaolin to divalent iron (soluble in water), so as to remove iron oxide. Sodium dithionite is often used to reduce Fe³⁺ to Fe²⁺ at pH 2~4 and room temperature. After filtering and washing for multiple times, iron is removed. Since sodium dithionite decomposes into SO₂, H₂S, etc. in an acidic environment or a high temperature environment, the bleaching effect is reduced, and the working environment is poor and the irritating odor is very strong. Adding some metal zinc powder appropriately helps to remove iron.

Acid leaching reduction combined iron removal method: Hydrochloric acid or sulfuric acid is often used to add active metal zinc, which can achieve a relatively good iron removal effect. In order to prevent the subsequent oxidation of Fe²⁺, adding a complexing agent oxalic acid is also an effective auxiliary means.

033. Complex iron type kaolin iron removal method

When the iron in kaolin exists in a complex form (including pyrite, organic matter, and iron oxide), it is necessary to use a redox combined method to remove iron to remove different iron impurities separately. For example, using sodium hypochlorite and hydrogen peroxide as oxidants and sodium dithionite as a reducing agent for iron removal can achieve relatively good results.

The above is an introduction to the iron removal methods of different iron impurities in kaolin. In actual mineral processing, the iron impurities in kaolin vary. It cannot be judged only based on historical cases. It is necessary to determine its impurity characteristics based on analysis and finally design a suitable kaolin iron removal method for it.