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What is the whiteness of ceramic raw materials?

- Dec 03, 2018 -

In order to pursue whiteness, the ceramic industry has done a lot of work. From the labor costly manual sorting of materials, the environmental protection of dangerous pickling and iron removal, to the investment of high-strength magnetic de-ironing, and even the development of biological iron removal, with the purpose to fundamentally remove the color impurities in the ceramic raw materials and iron or to reduce the compound as much as possible. In the case that iron removal is impossible, even companies have added expensive zirconium silicate brighteners to their formulations. This is a way to whiten and make every effort to smash countless brain juices.


First of all, it is necessary to understand the concept of whiteness on ceramic raw materials.


When a beam of light is incident on the surface of the object, optical phenomena such as specular reflection, diffuse reflection, and transmission can occur simultaneously. Whiteness reflects the intensity of the diffuse reflection of incident light on the material. Therefore, if the whiteness of the raw material is desired to be high, it is desirable that the raw material is burned with a strong diffuse reflection, which is reflected in the weak absorption of the raw material cake after burning, the low light transmittance after burning, and the weak gloss after burning.


The raw cake after burning is weak, mainly reflected in two aspects:



First, the colored impurities of the raw materials themselves. Colored impurities absorb light, causing whiteness to drop. This is the main direction of whitening ceramic raw materials.

On the other hand, it is the influence of the organic matter in the raw material and its firing process. If the raw material contains organic matter, especially high-carbon chain length, that is, organic matter with high ignition point, such as tree roots, coal, and the like, and the raw material is mud raw material, the granularity of such raw materials is very fine, and the cake is burned. In the process of formation, the air is difficult to circulate inside. In the rapid process of building ceramics, carbon blackening phenomenon is likely to occur, resulting in a decrease in whiteness of raw materials.

The light transmittance after burning is low, which is related to the content of the flux element of the raw material and the mineral composition of the raw material. Undoubtedly, the more the content of the flux element in the raw material composition, such as lithium, potassium, sodium, calcium, etc., especially the sodium element, the lower the whiteness of the raw material cake after burning.
Since the glass phase is formed in the raw material after burning, the glass phase is light-transmitting, resulting in light loss, which reduces the test whiteness. Interesting is the mineral composition, if the same composition of raw materials, one is a natural raw material, one is the ingredients from the ingredients, especially the raw materials are mixed with refractory raw materials such as quartz sand, the author believes that the raw materials after burning whiteness and There are certain differences in natural raw materials. The main reason is that the uniform mixing degree of the raw materials has a certain influence on the firing performance during the firing process.


The glossiness and reflected light intensity are reflected in the surface flatness of the raw material-fired cake, and also related to the composition and particle size of the raw material.



On the one hand, the higher the flux element in the raw material component, the higher the high refractive index such as BaO, the better the smoothness of the surface after the cake is burned, the stronger the diffuse reflection, and the lower the whiteness.
On the other hand, when the content of the glass phase is small, the particle size of the raw material has a great influence on the whiteness. If the white material content is high and the material is very fine, then the raw material cake is not only the surface of the fine raw material particles, but also has a certain light reflection phenomenon to the inner shallow raw material particles. Of course, the surface Emission is the best way to increase whiteness. It is a low probability event to expect light to be reflected inside.
Therefore, in this case, the degree of fineness of the white material determines the whiteness of the material. For example, when zirconium silicate is whitened, we often see that the thinner the silicate, the finer the zirconium silicate is added to the raw material, and the stronger the whitening effect is. But things are also double-sided, the smaller the graininess, the greater the tendency to burn the glass phase. If it becomes glass, it will not have the whitening effect, but it will weaken.

In general, the maximum influencing factor of whiteness is also reflected in the coloring element content under the same degree of sintering. However, understanding the impurity content of raw materials, particle size may affect the direction of whiteness, but also has certain reference significance.



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