Chemical removal of iron impurities in kaolin

Kaolin as a good performance industrial minerals, widely used in ceramics, paper, rubber, plastics, building materials, paints, and other petrochemicals, in particular using a lot in the ceramic industry. Kaolin can be used both as a ceramic blank and as a glaze.

Regardless of the ceramic industry or other industrial sectors, there is a certain requirement for the whiteness of kaolin. The kaolin produced in nature often affects its natural whiteness due to minerals containing some organic matter and elements such as iron , titanium and manganese . The iron impurities in kaolin not only affect the burnt color of ceramic products, but also seriously affect the dielectric properties and chemical stability of ceramic products. The conventional physical beneficiation method has no obvious effect on the removal of weak magnetic minerals such as pyrite and fine iron particles. The chemical removal of iron can effectively remove this part of the iron impurities.

1. Chemical de-ironing method of kaolin

At present, the chemical de-ironing methods commonly used in kaolin are oxidation method, reduction method and Chinese-reduction combined reduction method, among which the reduction method is the most widely used. Which method is specifically adapted depends on the type of iron minerals contained in the kaolin.

1.1 Oxidation and iron removal method

When kaolin contains pyrite and organic matter, the mineral is often grayish. These materials are difficult to remove by acid washing and reduction, and need to be bleached by oxygen removal. Oxidation and iron removal method uses a strong oxidant to oxidize pyrite or the like in a reduced state into a water-soluble ferrous ion in an aqueous medium; at the same time, the dark organic matter is oxidized to make it colorless to be washed by water. Oxide. The oxidizing agents used in the oxidation method are sodium hypochlorite, hydrogen peroxide, potassium permanganate, chlorine, ozone, and the like. The effect of oxidation and iron removal is related to the pH value of the medium, and is also affected by factors such as ore characteristics, temperature, dosage, slurry concentration, and bleaching time. (1) The effect of pH. Hypochlorite is a weak acid salt with different oxidizing power at different pH values. It is stable in alkaline media, unstable in acidic and neutral media, and decomposes rapidly, producing strong oxidizing components. Under weakly acidic (Ph5~6) conditions, the activity is the largest and the oxidizing ability is the strongest. At this time, the ferrous ions are relatively stable. (2) The effect of temperature. As the temperature increases, the rate of hydrolysis of the bleaching agent increases, the rate of bleaching increases, and the bleaching time required is reduced. However, when the temperature is too high, the heat consumption is large, and the decomposition rate of the drug is too fast, which causes waste and pollutes the environment. In actual operation, the expected effect can be achieved by increasing the amount of the drug, adjusting the pH value, and prolonging the bleaching time at normal temperature. (3) The effect of dosage. The optimum dosage is related to the characteristics of the ore, the degree of oxidation of the impurities, the reaction temperature, the time and the pH. The excessive or too small amount of the agent affects the iron removal effect. (4) The influence of slurry concentration. When the dosage of the agent is certain, the concentration of the slurry is reduced, and the effect of removing the iron is decreased. If the concentration is too high, since the product is not washed, excessive residual reagent ions after filtration may affect the performance of the product. (5) The effect of bleaching time. The longer the time, the better the iron removal effect, the faster the reaction at the beginning, and then the slower and slower, reasonable and economical bleaching time needs to be determined by experiment.

1.2 reduction iron removal method

1.2.1 Insurance powder reduction method

The most commonly used agent for the reduction of iron in kaolin is sodium dithionite, which is also known as insurance powder in the industry. Its molecular formula is Na2S2O4, which is a strong reducing agent. The oxide of ferric iron present in kaolin is insoluble in water and difficult to dissolve. In the presence of dilute acid, the ferric iron in the iron oxide can be reduced to divalent iron in the presence of the insurance powder. Since the ferrous iron is soluble in water, it can be removed by filtration and washing. The main reactions of this process are as follows: Fe2O3+Na2SO4+H2S2O4=Na2SO4+2FeSO3+H2O The main factors affecting this reaction process are as follows: (1) Effect of acidity The reaction of reducing iron oxide by insurance powder should not be carried out under alkaline conditions. However, the pH of the bleaching reaction should not be too low, otherwise the stability of the powder will decrease and a decomposition reaction will occur. Tests have shown that at pH = 0.8, the proof powder will decompose by half at room temperature for 2 minutes. (2) The effect of temperature As with most chemical reactions, the reaction between the insurance and iron oxide increases with increasing temperature, but the stability of the powder decreases greatly with increasing temperature. Other conditions are controlled in actual production, and bleaching at normal temperature can also achieve good results. (3) Influence of the amount of insurance powder In theory, according to the amount of iron oxide contained in kaolin, the insurance powder can be used most, but the actual dosage far exceeds the theoretical dosage. The amount of powdered powder is generally determined by experiment. In addition, the Fe2O3 content of the kaolinite to be de-iron bleached should not be too high (generally less than 1%), otherwise excessive use of the insurance powder will lead to an increase in the cost of iron removal. (4) Influence of other factors The reaction time has a great influence on the iron removal effect, and the time is too short to reach the ideal whiteness; the time is too long to waste the agent, and even the reoxidation of the ferrous iron by air oxidation also causes the whiteness of the product to decrease. . It is generally believed that the reaction time should be 40min to 2h. After the reaction is completed, it should be washed and filtered immediately. Otherwise, the phenomenon of yellowing will occur, that is, the re-oxidation of divalent iron will reduce the whiteness of kaolin; although the concentration of pulp has little effect on bleaching itself, However, when the concentration is too high, the viscosity of the slurry increases, making the reaction difficult to carry out. Generally, the concentration of the slurry should be controlled below 15%.

1.2.2 sodium borohydride reduction method

In addition to sodium dithionite, the commonly used reducing agent has zinc dithionite, which is quite unstable compared to the latter. The latter is much more stable. However, when the zinc dithionite is bleached, the concentration of zinc ions in the wastewater is too high, which causes pollution to the river water. For this purpose, sodium borohydride reduction can be employed. This method actually performs bleaching by reacting sodium borohydride with other agents to form sodium dithionite during the bleaching process. The specific process is: Mixing a certain amount of sodium borohydride and NaOH with the slurry at a pH of 7.0 to 10.0, and then introducing SO2 gas. Adjust the pH to 6-7, which is good for producing the largest amount of sodium dithionite in the slurry. The pH is adjusted to 2.5 to 4 by H2SO3 or SO2, and a bleaching reaction occurs. The reaction for producing sodium dithionite is as follows: NaBH4+9NaOH+9SO2=4Na2S2O4+NaBO2+NaHSO3+6H2O The essence of this method is still the reduction bleaching effect of sodium dithionite, but at pH 6-7, a large amount of sodium dithionite is formed. stable. When the subsequent pH is lowered, the sodium dithionite reacts with the iron oxide in the kaolin slurry immediately, and is used in time, thereby avoiding the decomposition loss of sodium dithionite.

1.2.3 reduction complexation iron removal method

As mentioned above, after the trivalent iron in kaolin is reduced to divalent iron by sodium dithionite, if it is not immediately filtered and washed, the product will return to yellow. A more effective way to solve this problem is to add a complexing agent to make the ferrous ions complex and not easily oxidized. Many agents can be used to complex the iron, phosphorus acid, polyethylene alcohol, hydroxylamine, hydroxylamine salt, oxalic acid, polyphosphates, salts of ethylenediaminetetraacetic acid, and citric acid.

1.3 oxidation-reduction combined iron removal method

Some kaolins cannot achieve satisfactory results by using the oxidative iron removal method or the reduction iron removal method alone. At this time, the oxidation-reduction combined iron removal method is required for bleaching. The process firstly oxidizes the dyed organic matter and pyrite in the kaolin with a strong oxidizing agent sodium hypochlorite and hydrogen peroxide, and then uses the sodium dithionite for reduction bleaching to make the remaining iron oxides in the kaolin such as Fe2O3 and FeOOH. The like is removed by reduction to soluble ferrous iron to bleach this type of kaolin.

2, the conclusion

The finishing of kaolin should undergo a series of processes such as purification and classification, stripping, magnetic separation and chemical bleaching. Because the chemical bleaching method has relatively high cost of chemicals, in the industrial production, the kaolin should be fully used for beneficiation, iron removal, and then chemical bleaching in order to minimize the amount of pulp processed by the bleaching process and reduce The amount of bleach used. With the development of science and technology, the ceramic industry and other industrial sectors are increasingly demanding the whiteness of kaolin, and the chemical iron removal bleaching method will be more widely used.

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