What is Kaolin?
Kaolin, or “China clay,” is a common name for a group of minerals that are composed entirely or mainly of kaolinite or aluminum silicate. Raw kaolin that has been ground and classified may contain small amounts of other silicates, such as mica, illite, chlorite, smectite, and quartz.
This material is primarily used in the coating industry, where impurities are removed by washing with water. Although kaolin has a clay-like structure, its value in the coating industry is not mainly due to its optical properties, unlike talc and mica. In this article from the Journal of Physical Chemistry, we will delve into more details, so stay with us.
Kaolin Structure
The structure of kaolinite can be described as layers of silica rings bonded to layers of alumina octahedra through shared oxygen atoms. These layers naturally group together, exhibiting varying degrees of order in their arrangement.
Because each kaolinite particle has an oxygen surface on one side and a hydroxyl surface on the other, strong hydrogen bonding occurs between the top and bottom layers. This feature makes the exfoliation of kaolinite more difficult than other silicate fillers such as talc and mica. Kaolinite is hydrophilic and easily disperses in water. For non-aqueous applications, surface treatment can improve compatibility with the matrix.
Geological Origin of Kaolin
Kaolin forms when anhydrous aluminum silicate found in feldspathic rocks, like granite, undergoes weathering or thermal processes. The process that transforms hard granite into the soft mass found in kaolin pits is called “kaolinization.”
During this process, quartz and mica mostly remain unchanged, while feldspar transforms into kaolinite. In some deposits, smectite may also form in small quantities. Processing and refining the softened granite yield primarily kaolinite along with some mica, feldspar, and quartz, and depending on its origin, organic materials or heavy minerals.
Production Methods for Kaolin Grades
Coating-grade kaolins are divided into two main categories: hydrated and calcined clays. Hydrated clays retain their structural integrity and the surface hydroxyls, known as crystallization water. Most hydrated clays used in coatings are washed with water.
For producing water-washed grades, the clay is slurried in water, and impurities are removed by centrifugation or hydrocycloning to yield a specific particle size fraction. The refined slurry is then either dewatered (to reduce soluble impurities) and dried or concentrated to 70% and sold as slurry. To enhance brightness, these water-washed clays are chemically bleached or undergo high-intensity magnetic separation to remove iron and titanium impurities.
Flake clay is produced through the attrition grinding of coarse clay derived from the water-washing of soft clay. This process converts kaolinite into wide, separate plates, improving brightness, opacity, and barrier properties.
A notable point: Airfloat clay is a type of dry hydrated kaolin that has been separated to reduce impurities and control particle size distribution. Due to greater abrasion caused by mineral impurities and poorer color, the coating industry generally uses less of this clay compared to water-washed grades.
Calcined Clay: Calcined clay is produced through thermal processing of water-washed, bleached kaolin. At low calcination temperatures, between 650°C and 700°C, structural hydroxyls are removed, resulting in the formation of amorphous metakaolin. Specific gravity decreases from 2.58 to around 2.50, while hardness, porosity, brightness, opacity, and absorption properties increase.
Fully Calcined Clays: These clays, with the highest brightness and opacity, are produced at temperatures between 1000°C and 1150°C. At this temperature, the amorphous structure of the clay completely collapses, and the specific gravity increases to 2.6–2.7 without the formation of mullite (with a specific gravity of 3.2 and hardness of 6–7). The balance between opacity and brightness in calcined clays can be adjusted by controlling the temperature, heating rate, and fluxes used during calcination.
Functional Properties of Kaolin
Due to kaolin’s significant impact on the optical properties of coatings, this clay is offered in various grades to provide a range of features such as crystallinity, formability, controlled particle size, brightness, and opacity.
Kaolin clays produced through chemical modification are the most widely used. These grades have dispersibility coatings to enhance water dispersion and benefit from surface treatments with stearates or silanes to improve compatibility with organic matrices.
Structured Clay Products: In some cases, these products are referred to as SAMS (Synthetic Alkali Alumino-Silicates). They are produced by reacting kaolin with alkalis, such as alkali silicate, under high temperature and pressure conditions. The final product includes kaolin plates with reactive amorphous edges. This reaction and subsequent aggregation help produce products with controlled increases in porosity, oil absorption, brightness, opacity, tint strength, and leveling characteristics.
Applications of Kaolin in Industries
Kaolin is used in various industries due to its unique physical and chemical properties. Factors such as shape, particle size, color, softness, and non-abrasiveness are important physical properties of kaolin. Chemically, features such as low base exchange capacity, surface chemistry, and relative insolubility play key roles in many applications.
In the 1960s, over 2 million tons of kaolin were consumed in the United States, with more than half used in the paper industry. Kaolin’s physical and chemical composition significantly influences its use in various industries, including paper, rubber, ceramics, plastics, ink, catalysts, insecticides, and others.
Kaolin Deposits
Although kaolin is found in many parts of the world, deposits with desirable characteristics like high whiteness, suitable viscosity, and other qualities are rare. Interestingly, the best kaolin deposits are not located in China. Instead, the United States, particularly in the southeastern states, boasts high-quality deposits. This mineral has been mined in Georgia and South Carolina since colonial times. Other valuable deposits are found in Brazil and the United Kingdom.
Application of Kaolin in the Paper Industry
One of the primary uses of kaolin is in the paper industry. Kaolin acts as a paper coating, improving the brightness, smoothness, gloss, and printability of the paper. It is also used as a filler in the paper industry, helping to reduce costs and conserve tree resources.
Application in Porcelain and Ceramics
Contrary to popular belief that kaolin is only used for producing porcelain, its use in the paper industry far surpasses that in other industries. However, kaolin remains a valuable component in producing porcelain and ceramics. Its color, gloss, and hardness are desirable features for manufacturing these products.
Application in Cosmetics and Personal Care
Kaolin has numerous benefits for the skin. It can be used alone or combined with water and other natural ingredients to create facial cleansers, masks, scrubs, and other skincare products, eliminating the need for chemicals.
This compound can gently cleanse the skin, reduce oiliness, soothe sensitive skin, act as an effective acne treatment, remove dead skin cells and dryness, improve skin elasticity, and delay the signs of aging. Additionally, kaolin is beneficial for reducing skin irritations such as insect bites, rashes, and reactions to poisonous plants.
