Kaolin is a famous Chinese white clay that is unique in its chemical and physical properties. It is also distinct in mineralogy and morphology, making it versatile for various commercial and industrial uses. How kaolin is sterilized will determine the quality level of this raw mineral.
In this article, we will explore more about kaolin uses in brief, why kaolin must be sterilized, and methods and materials of how to sterilize this clay mineral.
Brief Usage of Kaolin
Kaolin is a soft white clay essential for industrial clay ingredients, especially for different manufacturers. People also call it china clay, named after the Kao-ling hill in China, where people have mined it for centuries.
In its natural state, this white clay principally contains kaolinite – (Al2Si2O5(OH)4) – a hydrated aluminum silicate mineral. Other minerals in kaolin are, among others, dickites, nacrites, and halloysites.
Why and how kaolin is sterilized is closely related to its industrial and commercial uses. The common long-history application of kaolin is in the making of ceramic, china, or porcelain.
Later on, due to its unique specifications, it has been a versatile raw material for papermaking (coating and fillers), rubber and plastic, fiberglass, and paint industries. Other usages are in medicines, cosmetics, fertilizers, pharmaceuticals, pesticides, detergents, ink, crayons, and other products.
Why Kaolin Must be Sterilized
Before answering the question of how kaolin is sterilized, it is better to know the reasons behind why we must sterilize this clay mineral. The most important factor relates to its commercial price.
The crude kaolin found in nature commonly comprises various amounts of other minerals, like quartz, feldspar, muscovite, and anatase. Besides, the presence of iron oxide/hydroxide pigments often adversely stains this raw clay mineral.
The ferric iron pigments can be red (hematite), reddish-brown (maghemite), brownish-yellow (goethite), orange (lepidocrocite), brownish red (ferrihydrite), and more.
Even a small amount of oxide, hydroxide, or other hydrated oxides (0.4%) can cause red to yellow pigmentation to the kaolin deposits. This stained pigmentation may decrease kaolin’s brightness.
Similarly, the existence of iron ores may pose adverse contamination to kaolin and reduce its refractoriness (resistance to heat and other treatments). For example, the hematite can create problems while operating with blast ovens or kilns (furnaces).
All of them might result in a dramatic drop in its commercial price as high-quality kaolin has a low level of iron-bearing minerals.
Therefore, it is frequently necessary to chemically bleach the clay to eliminate the iron oxide pigments. Also, there is the need to refine the clay by washing it with water to remove other minerals. That way, kaolin is more appropriate for commercial uses.
But, how kaolin is sterilized? What methods and materials do we need?
How Kaolin is Sterilized – Methods and Materials
As previously stated, whiteness or brightness is one of the most significant factors that determine kaolin’s economic value and commercial applications. So, the next question is how kaolin is sterilized. The following are some methods and materials to refine or remove impurities from this white clay.
1. Physical Separation
We can lessen the impurities in kaolin by performing flotation or magnetic separation.
2. Chemical Leaching Process
These processes use chemicals, like Na2SO3, HCL, C12, dithionite, and the like. The extraction can yield 60 to 70 percent. However, it is low in iron removal efficiency.
3. Bioleaching Process
Some researchers conduct the latest process by using what-so-called heterotrophic bacteria and fungi. Unfortunately, the method still poses several unresolved issues regarding its industrial use.
4. Hydro Sulfites
Many industries have used hydro sulfites (for example, sodium hydrosulfite and zinc hydrosulfite) as reductive bleaching agents. The purpose is to increase the kaolin’s brightness level.
This method has several weaknesses. Kaolin bleaching by using hydro sulfites requires specific acidic conditions. It is, unfortunately, asking for relatively high operational costs and causes environmental effects. Also, considered unstable chemically, hydrosulfites may result in a decrease in process effectiveness.
Widely used in photography, thiourea (NH2CSNH2) is a fixing agent to remove stains from negatives. Having good chemical stability, thiourea is also a strong reducing agent, for example, in the wet refinement for valuable metals like gold (Au) and silver (Ag). For kaolin, thiourea acts as a bleaching agent for removing the iron with an extraction yield of about 94%.
Those are some methods and materials of how kaolin is sterilized. Some of them are better than others in giving good extraction yields. Why must we sterilize this clay mineral? It is to remove the impurities from other minerals or even iron ferric pigments thus obtaining high-quality kaolin products.