Kaolin, as an industrial mineral deposit with excellent functions, is widely used in ceramics, papermaking, rubber, plastics, building materials, paints, petrochemicals, etc., especially in the ceramic industry. Kaolin can be used not only as a ceramic blank, but also as a glaze.
Regardless of whether the ceramic industry is another industrial sector, there are certain requirements for the whiteness of kaolin. The natural whiteness of kaolin produced in nature is often affected by 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 function 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-particle iron-containing impurities. The chemical iron removal method can effectively remove this part of iron impurities.
1. The chemical iron removal method of
kaolin . At present , the chemical iron removal methods commonly used in kaolin include oxidation method, restoration method and Chineseization-restoration combined method, among which the restoration method is the most widely used. Which method is used in detail depends on the type of iron ore contained in kaolin.
(1) Oxidative iron removal method
When kaolin contains pyrite and organic matter, the mineral deposits are often gray. These materials are difficult to remove by pickling and restoration methods, and oxygen iron removal method is required for bleaching.
The oxidation iron removal method uses a strong oxidant to oxidize the pyrite in a restored state into water-soluble ferrous ions in an aqueous medium; together, it oxidizes the dark organic matter to make it colorless that can be washed away by water. Oxide. The oxidants used in the oxidation method include hydrogen peroxide, hydrogen peroxide, ozone, and so on.
The effect of oxidative iron removal is related to the pH value of the medium, and is also affected by factors such as ore characteristics, temperature, dosage of medicament, pulp concentration, and bleaching time.
1. The influence of pH value. Hypochlorite is a weak acid salt, and its oxidation ability is different under different pH values. It is stable in alkaline medium, but unstable in acidic and neutral medium, and differentiates quickly, generating strong oxidizing components. Under weakly acidic (Ph5～6) conditions, its activity is the highest, and its oxidizing ability is the strongest. At this moment, the divalent iron ion is relatively stable.
2. The influence of temperature. As the temperature rises, the hydrolysis rate of the bleaching agent accelerates, and the bleaching rate also accelerates, and the required bleaching time is shortened. However, when the temperature is too high, the heat consumption is large, and the differentiation speed of the medicine is too fast, which will ruin and pollute the environment. In practice, the expected effect can be achieved by increasing the dosage, adjusting the pH value, and extending the bleaching time at room temperature.
3. The influence of dosage of medicament. The optimal dosage of the agent is related to the characteristics of the original ore, the degree of oxidation of impurities, the reaction temperature, the time and the pH value, etc. Too much or too little dosage of the agent will affect the effect of iron removal.
4. The influence of slurry concentration. The dosage of the medicament must be punctual. Decreasing the concentration of the pulp will reduce the iron removal and bleaching effect; if the concentration is too high, because the product is not washed, too much residual medicament ion after filtration will also affect the product function.
5. The effect of bleaching time. The longer the time, the better the effect of iron removal. At the beginning, the response speed is very fast, and then it becomes slower and slower. Reasonable and economical bleaching time needs to be determined through experiments.
(2) Restoration method for iron removal
1. Bento powder restoration method The
most commonly used agent for iron removal from kaolin is also known as Bento powder in industry. Its molecular formula is Na2S2O4, which is a strong restoring agent. The trivalent iron present in kaolin The oxide is insoluble in water and hardly soluble in dilute acid, but in the presence of a stable powder, the trivalent iron in iron oxide can be restored to divalent iron. Because ferrous iron is soluble in water, it can be removed by filtering and scrubbing. The primary response of this process is as follows:
The primary factors that affect this response process are as follows:
(1) The effect of acidity
The response of the iron oxide recovery of the stable powder should not be carried out under alkaline conditions. However, the pH value of the bleaching response should not be too low, otherwise the stability of the powder will decrease and a differentiated response will occur. The experiment shows that when pH=0.8, only 2min at room temperature, the stable powder will differentiate by half.
(2) The effect of temperature is the
same as that of most chemical reactions. The reaction of the stable iron and iron oxide accelerates with the increase of temperature, but the stability of the stable powder decreases greatly with the increase of temperature. In practice, other conditions are well controlled, and bleaching at room temperature can also achieve better effects.
(3) The influence of safe powder dosage
Theoretically, based on the amount of iron oxide contained in kaolin, the most suitable powder can be calculated, but the actual dosage far exceeds the theoretical dosage.
The amount of safe powder generally needs to be determined through experiments. In addition, the Fe2O3 content in the kaolin to be de-ironed and bleached should not be too high (generally less than 1%), otherwise the excessive amount of safe powder will increase the cost of de-ironing.
(4) The influence of other elements The
reaction time has a greater impact on the effect of iron removal, and the time is too short to reach the desired whiteness; the time is too long to spoil the agent, and even the de novo oxidation of the air oxidation of ferrous iron will also cause the whiteness of the product to decrease. . It is generally believed that the response time should be 40min to 2h, and the response should be washed and filtered immediately after the response, otherwise it will show the phenomenon of yellowing, that is, the de novo oxidation of divalent iron will reduce the whiteness of kaolin; although the pulp concentration has little effect on the bleaching itself, However, when the concentration is too high, the viscosity of the slurry increases, making it difficult to respond. Generally, the slurry concentration should be controlled below 15%.
In addition to the commonly used restoration agents, the former is very unstable in comparison. The latter is much more stable. However, when bleaching is used, the concentration of zinc ions in the wastewater will be too high, which will pollute the river water. For this reason, the restoration method can be used.
In practice, this method is bleaching by reacting with other agents during the bleaching process. The detailed process is as follows:
Under the condition of pH 7.0～10.0, mix a certain amount of NaOH with ore pulp, and then pass in SO2 gas. Adjust the pH value between 6 and 7, this pH value is conducive to the most occurrences in the slurry. Then adjust the pH to 2.5-4 with H2SO3 or SO2, and the bleaching response can occur at this moment. The resulting response is as follows:
The essence of this method is still to restore the bleaching effect, but when the pH is 6 to 7, many of the generated are very stable.
When the pH value drops afterwards, it reacts immediately with the iron oxide in the kaolin slurry, and is used in time, and then the loss of differentiation is avoided.
3. Recovery complexation method for removing iron
As mentioned above, after the trivalent iron in kaolin is restored to divalent iron, if it is filtered and washed improperly, the product will turn to yellow. A more useful way to deal with this problem is to add a complexing agent, so that the divalent iron ions can be complexed instead of simply being oxidized. There are many agents that can be used to complex iron, such as phosphoric acid, polyvinyl alcohol, hydroxylamine, hydroxylamine salt, oxalic acid, polyphosphate, ethylenediamine acetate, citric acid and so on.
(3) Oxidation-recovery combined iron removal method
Some kaolins can not achieve satisfactory effects by using oxidation or recovery iron removal method alone. In this case, oxidation-recovery combined iron removal method is required for bleaching. The process is to oxidize and remove the dyeing organic matter and pyrite in the kaolin with a strong oxidizer and hydrogen peroxide, and then use it for restoration and bleaching to make the remaining iron oxides in the kaolin such as Fe2O3, FeOOH, etc. It is restored to soluble divalent iron and removed, so that this type of kaolin can be bleached.
The fine processing of kaolin must go through a series of processes such as purification and classification, stripping, magnetic separation, and chemical bleaching. Because the chemical bleaching method has relatively high chemical cost, it is necessary to make full use of the previous steps in industrial production to beneficiate kaolin, remove iron, and then chemically bleach to minimize the amount of pulp processed by the bleaching process. The amount of bleach.
With the development of science and technology, the ceramic industry and other industrial sectors have higher and higher requirements for the whiteness of kaolin, and chemical iron removal and bleaching methods will be more widely used.
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