Calcined Kaolin

As a company, let us refresh our knowledge about the uses, production and benefits of Calcined Kaolin DG80, an important product in our product portfolio. First, I would like to discuss the stages in the transformation of raw kaolin mineral into calcined form. Kaolin, or as it is commonly called 'china clay', is a hydrated aluminium silicate crystalline mineral formed over millions of years through hydrothermal weathering of granite rocks. Hydrated kaolin is characterized by fine particle size, plate-like or layered particle shape and chemical inertness. Metakaolin is produced by calcination of kaolin to create an amorphous pozzolanic white mineral additive for use in cement-based products. Calcined Kaolin is an anhydrous aluminium silicate produced by heating ultra-fine natural kaolin in a furnace at high temperatures. The calcination process increases whiteness and hardness, improves electrical properties and changes the size and shape of kaolin particles.

What is Calcined Kaolin and why should we use it in our paints, papers, plastics, rubber and ceramics?

Naturally occurring kaolin is known as "hydrated kaolin" because it contains water within its crystal lattice. This water cannot be removed simply by drying the kaolin. Calcining a mineral means heating it to a temperature at which a phase transition occurs, generally involving the loss of a volatile component. In the case of kaolin, it loses crystallisation water. When heating kaolin to 800ºC, to produce Metakaolin, water is lost in the form of hydroxyl (-OH) groups: 2 Al2Si2O5(OH)4 -› 2 Al2Si2O7 + 4H2O Metakaolin is an interesting material. When added to concrete, it acts as a pozzolan. In this respect, it means it increases the strength of concrete, a useful property in the construction sector, particularly in oil well cementing. To produce calcined kaolin, you need to continue heating to 1050ºC. On the way there, two different reactions occur. First, spinel formation at 950C: 2 Al2Si2O7 -› Si3Al4O12 + SiO2 Then the spinel phase transforms into mineral mullite plus a high degree of crystalline cristobalite SiO2: 3 Si3Al4O12 -› 2 Si2Al6O13 + 5 SiO2 Here, the calcination process improves opacity and whiteness because it causes the kaolin structure to collapse and become denser. The particles become sharper and harder, but become more porous due to the voids created where the –OH groups were present. Flash calcination is a method in which kaolin is heated very rapidly. This increases the number of pores compared to conventional calcination methods. These changes give calcined kaolin some useful properties in various applications. In particular, I would like to describe its role in the paint sector, where we are also very active.

Kaolin mineral and calcined kaolin particle appearances under SEM Microscope (Scanning Electron Microscope)

[caption id="attachment_155212" align="aligncenter"] Calcined Kaolin Mineral Kaolin[/caption]  

Paints

Better opacity and whiteness make calcined kaolin an excellent filler that can work in cooperation with titanium dioxide. Hard calcined kaolin particles help improve the structural strength of a coating and also the scrub resistance of both water and oil-based paints. It also imparts corrosion resistance and fire retardant properties to paint. The calcination and subsequent grinding of hard calcined particles requires a great deal of energy. This makes calcined kaolin approximately three times more expensive than hydrated kaolin. However, it is still less than half the price of titanium dioxide. Therefore, it is a very good option, especially in paints with high CPVC (critical pigment volume concentration). The irregular nature of its particles positively develops wear resistance in traffic paints, adhesion and scrub resistance of interior flat paints. However, due to the micropores of calcined kaolin, staining properties increase. The removal of stains can be problematic in paints with high PVC values. Additionally, oil absorption is higher than that of normal hydrated clays. The amount of use is determined according to the properties of pigments and other fillers in the formulation. When kaolin is subjected to calcination, it produces high gloss. They also contain small amounts of water-soluble salts. It is an important raw material for the stability of emulsion paints. Calcined kaolin with very small particle size has better covering power than calcined kaolins with larger particle structure and imparts greater gloss to the paint. This is satisfactory for semi-gloss paints, but flat paint formulation does not give the desired result unless modified with a high surface-smoothing pigment such as diatomaceous silica. The price of calcined kaolin is higher than normal clay and is used in the production of paints with specific advantages. For this reason, its use and calculation in paints should be done very carefully.

Ceramics

It is also useful in ceramics because it provides good whiteness. However, the most useful property for the ceramicist is that calcined kaolin is already calcined. Therefore, it will have no plasticity and there will be no expansion or shrinkage in this material. It gives mechanical strength and a fine white surface texture. Calcined kaolin is easy to distinguish from hydrated kaolin by looking at data sheets. Just look at the LOI (Loss on Ignition) value. For clay washed with water it is around 13%, but for calcined kaolin it will be zero. The LOI test essentially calcines the kaolin. Therefore, LOI is the percentage of mass lost due to the loss of crystallisation water.

Rubber and Plastics

The use of calcined kaolin as a filler in rubber and plastics helps impart tensile strength. Therefore, it is a reinforcing filler rather than a non-reinforcing one like hydrated kaolin. The fire-retardant property of calcined kaolin plus its electrical insulation property makes it particularly useful in the plastic coating of electrical cables. It also provides UV resistance, therefore it is commonly added to greenhouse films and garden furniture.

Agricultural Chemicals (Pesticides)

Interestingly, calcined kaolin is used as a pesticide. When sprayed onto fruit, the fine sharp calcined kaolin particles deter insects by entering their joints and irritating them. It also reflects sunlight and acts as a sunscreen. Calcined kaolin has many important applications such as its use in paper coating and petrochemical catalysts. It is necessary to point out how important a raw material it is in many fields. In the meantime, if we look at the work being done for titanium savings in the paint sector; the general recommendation when working on formulation to reduce the ratio of kaolin to titanium dioxide is that you always need to change the amount of titanium dioxide on a volume basis, since the densities are different. For example; instead of 10 kg of titanium dioxide, you want to use approximately 6.6 kg of calcined kaolin. It may be necessary to calculate the cost savings on a volume basis and then calculate the weight savings. That is, the higher the titanium dioxide content for glossy and semi-gloss paints, the more cost-effectively we can modify this ratio with calcined kaolin. Typically, we can replace 10% of titanium dioxide on a volume basis. Titanium dioxide has a high refractive index and strong absorption in UV, the light spectrum region and strong reflection property give it very high luminosity in the visible spectrum. Since calcined kaolin also exhibits similar properties, it can be used as an alternative filler along with titanium dioxide as a pigment. These properties give them gloss and opacity. Summarizing once more by going through it briefly, mineral kaolin is calcined through controlled heat treatment to decompose the bound hydroxyl groups in its chemical structure, transforming its flat particle structure into an amorphous (porous) state. After the calcination process, whiteness, hardness and reflectivity values increase, electrical conductivity decreases. It undergoes a series of refining processes such as washing, magnetic separation and bleaching to be freed from free silica and other impurities, with important parameters such as particle distribution and whiteness kept under strict control. Thus, calcined kaolin becomes a functional filler that can be used in many sectors. That is, it becomes a functional filler with aluminium silicate properties that provides high whiteness used in water and solvent-based paints. While providing TiO2 savings, it increases the opacity and color resistance of the paint. It is an ideal filler for water-based semi-matt and matt paints. With its fine, flat particle structure, it fills the voids between TiO2 particles, providing excellent covering power. It is possible to achieve savings of up to 20% of the TiO2 used. It is used to enhance the covering effect of TiO2 and is superior to nodular calcium carbonate and talc with the same particle size distribution as itself due to its flat particle structure. It also improves the rheological and thixotropic properties of the paint, provides settling resistance during storage and improves the mechanical properties of the resulting film. Ultrafine kaolin, with the advantage of fine particle distribution, is used to achieve partial savings on carbon black and precipitated silicas, provide volume in rubber applications, and increase tensile, tear and elongation strength. It is used in automobile tyres, bicycle inner tubes, latex work gloves, automobile windscreen wipers, door seals, non-black rubber products. Technological growth and development in calcined kaolin production, which is both versatile and impressive, has led to an increase in calcined kaolin sales worldwide. Increasing use of recycled paper for production and increasing cardboard applications predict growth in the calcined kaolin market with each passing time. Finally, looking at the topic from a geographical perspective, it is expected that in the future, Asia will dominate the calcined kaolin market globally and have the maximum share of revenue. East Asia and South Asia are also expected to grow at high CAGR (compound annual growth rate). Europe is the second largest market in the calcined kaolin market due to the presence of a leading producer in the region.     Nigar Cangönül Technical Sales Manager Günkem