Two key characteristics determine kaolin’s economic value for diverse commercial and industrial applications. The first is kaolin brightness and the second is kaolin particle size (fine and controllable ones).
The papermaking industry accounts for one of the largest users of this white clay mineral as kaolin whiteness is one of the primary properties for different paper grades. We will discuss one of these kaolin industrial applications after we outline kaolin physical analysis.
Kaolin Properties – The Physical Analysis
Kaolin is a clay mineral principally composed of hydrated aluminosilicate (kaolinites). Mostly found in masses called clay beds, kaolinite mineral shows and has the following kaolin physical properties, including kaolin brightness.
1. The Texture
Kaolin has an earthy clay-like texture. As a friable kaolinite mineral, it is earthy and uneven in fractures that are easy to break or crumble. As a result, you can easily shape or mold it, particularly when you make it moistened or wet.
2. The Appearance
It has a luster appearance – showing the light reflected by the mineral. Many frequently describe kaolin as earthy, waxy, dull, or pearly in look.
3. The Odor
It might be odorless with a soft consistency. However, when moisturized, it develops a clay-like odor.
4. The Colour
In its natural condition, kaolin is usually in white and soft powder form. You can find them in typical bright white color to cream and pale yellow.
Some of the minerals show a distinct rust hue mainly due to the impurities of iron oxide pigmentations. The tints are red to pink to orange or light orange. Other colour mixtures or staining are usual browns to light browns, tans, or sometimes shades of blue.
5. The Transparency
It is opaque or translucent.
As mentioned earlier, kaolin brightness and particle size will determine the commercial value of this white china clay.
Other essential kaolin physical properties are flat particle shape, soft and non-abrasive texture, and chemical inertness. The fine and controllable particle size will affect strength, fluidity, plasticity, abrasiveness, color, and ease of dispersal.
In the ceramics industry, kaolin has a main function to impart brightness or whiteness.
Other kaolin physical analysis parameters used are as follows.
- Kaolin brightness
- pH (acidity)
- Viscosity (25%, 60 rpm, 30oC)
- Moisture Content (MC)
- Retained on 325 Mesh
- Passing 325 Mesh
- Loss On Ignition (LOI)
- Particle Size Distribution
Kaolin in Paper Industry
The major paper-making companies are looking for various grades of kaolin and kaolin brightness for the following targets.
- As a filler, to improve the paper sheets look good in color, brightness, smoothness, and opaqueness.
- As a coating, to make the paper sheet print well (printability). It includes ink absorption, ink pigment holdout, roughness, low linting tendency, and more.
Consequently, many papermaking manufacturers have blended this white china clay with other pigments or key minerals such as talc, calcium carbonate, satin white, titanium dioxide, and the like. The main purpose is to enhance kaolin brightness and to obtain the expected effect of the final paper products.
According to Stratum Resources (2001), there are several typical paper kaolin grades related to the kaolin brightness and other key characteristics.
1. Filler Grade
The key characteristics for this grade are brightness or whiteness and particle size distribution. This low-cost kaolin is good and high in brightness. Indonesia has dominated the market for this kaolin grade.
2. Delaminated Grade
Whiteness, aspect ratio, surface area, and lesser extent of particle size distribution are the key characteristics for this grade. There has been a growing demand for lightly coated paper in Asia regions, in which Brazilian kaolin producers have offered products in high brightness and low viscosity.
3. Calcined Grade
This kaolin grade offers brightness and high-level opacity. It is one of the highest value china clay as brightness standards keep on increasing.
4. No. 1 Coating Grade
It has three main characteristics: kaolin brightness (>90 % to >91%GE), particle size distribution (from quite fine to coarse), and viscosity.
5. No. 2 Coating Grade
It has similar key characteristics with the above no. 1 coating grade. However, its brightness has a range of two to three points below No. 1 grades.
Using clay minerals as one of the important ingredients for papermaking has been a trend, especially in the Asian regions. Kaolin is indeed a less costly alternative compared to other wood paper fillers since the decreasing of wood supply and the increasing of paper demand.
To sum up, kaolin brightness determines the commercial value of this china clay. This physical property plays a significant role in enhancing paper appearances, like gloss degree, whiteness, smoothness, opacity, and printability.