The Calcination of Kaolin Clay

This article was co-authored by:

Alex Ebben
Process and Sales Engineer

Carrie Carlson
Technical Writer

Kaolin clay, also known as China clay, is an essential industrial mineral primarily made up of the mineral kaolinite – a hydrous aluminum silicate. Kaolin can be found in applications all around us, from consumer products, to advanced industrial processing. Most notably, kaolin is lauded for its use in paper products, both as a filler and as a coating. It is also widely used in refractories, technical ceramics, paints, pigments, and more.

A recent market analysis by Grand View Research estimated that the kaolin market will exhibit a compound annual growth rate (CAGR) of 8.8% from 2018 to 2025. The firm notes that calcined kaolin and metakaolin, both produced in the calcination process, are seeing significant demand in the marketplace.

Used throughout a variety of industries to cause physical changes and chemical reactions within a material, calcination is recognized as one of the best ways to improve the natural properties of kaolin, making it whiter and more chemically inert.1

About Calcined Kaolin

While kaolin clay is widely used in its “raw” form, heat treating kaolin to produce metakaolin and other forms of calcined kaolin further increases its usability and creates an engineered product.

Calcination can enhance or alter a variety of the mineral’s properties to produce a material with characteristics suited to a given application. This might include improving the hydrophobicity and abrasiveness of the material, or even enhancing optical or electrical characteristics.2

How Calcination Changes Kaolin

Calcination causes kaolin to move through several reactions as described below, though generally, the free moisture removal, dehydroxylation, and mullite phases are the most widely recognized.1

Free Moisture Removal

Between 100° – 150°C, free moisture is driven off.


After free moisture is removed, chemically bound moisture is driven off in a process known as dehydroxylation. This produces a product known as metakaolin and typically happens between 400° – 600°C.

At this point, the reaction may be stopped if metakaolin is the intended product. Metakaolin is widely used as a supplementary cementitious material (SCM), allowing it to replace a portion of the clinker used in cement.

Exothermic Re-crystallization

Beyond dehydroxylation, exothermic re-crystallization* causes the metakaolin to transform to the spinel phase. *The exact reaction occurring here is still up for debate.

Mullite Formation

After the spinel phase has been reached, crystals of mullite begin to form from the spinels. This typically occurs at temperatures beyond 1050°C.

Cristobalite Formation

If the temperature is allowed to exceed 1100°C, a carcinogenic material known as cristobalite will form.

Kaolin calcination is most commonly carried out in a rotary kiln, often referred to as a calciner. This thermal processing device is incredibly flexible and can be configured to meet a wide variety of processing conditions.

3D Model of a FEECO Rotary Kiln/Calciner


In processing kaolin, the rotary kiln is of the direct-fired configuration, typically with a counter current air flow.

Controlling Properties of Kaolin Products Through Calcination

Calcination can be used to control a variety of end product characteristics when working with kaolin. Some of the most common characteristics targeted during calcination include:

  • Abrasivity
  • Brightneess
  • Color/whiteness
  • Opacity
  • Density
  • Surface area
  • Particle size distribution (PSD)
  • Refractoriness

As is the case in most thermal processing applications, the end product is primarily a result of the raw material feedstock characteristics, combined with the parameters chosen for calcination.

Feedstock Characteristics

Kaolin clay sources can vary significantly, with a number of accompanying impurities possible. Similarly, chemical and physical properties of kaolin sources vary as well. As a result, varying sources of kaolin often respond to calcination in different ways, making it somewhat unpredictable.

Calcination Parameters

Both time and temperature impact the properties of kaolin and can therefore be used to control the end product characteristics.

For example, varying retention time can be used during the metakaolin phase to produce a metakaolin with different properties. The authors of a recent study stressing the need for improved online monitoring of the kaolin calcination process give an example: Once kaolin has transformed to metakaolin, which exhibits excellent pozzolanic reactivity, the pozzolanic reactivity will decline if the material is allowed to stay in the kiln. In addition to this reduced reactivity, however, the metakaolin will also simultaneously improve in whiteness, while abrasiveness remains low – characteristics that lend it to use in the pharmaceutical industry. Further processing into the mullite phase will produce an even whiter, but much more abrasive kaolin.

According to the authors, the ability to monitor the calcination reaction in real time is becoming increasingly more important in creating kaolin products with optimal characteristics.


The variability exhibited by kaolin, combined with the mineral’s broad range of possibilities when calcined, and the litany of potential end uses, gives producers a flexibility to create a wide range of kaolin products. This flexibility also makes testing in a facility such as the FEECO Innovation Center an important part of developing a calcined kaolin that meets its intended application.  

The Innovation Center has a number of test kilns for conducting kaolin calcination tests at both batch and continuous pilot scale under varying process conditions. The data gathered during testing can then be used to scale up the process to engineer a commercial-scale, custom unit, tailored to the precise requirements of the application at hand.

The facility also boasts an automation control system that allows real-time data to be collected and trended for analysis, or even adjusted in real time from a user interface, providing unparalleled process transparency. Real-time data points collected include:

  • Feed and product rates
  • Temperatures (feed end, internal, thermal oxidizer, product, and exhaust gas)
  • Natural gas flow rates
  • System pressures
  • Gas sampling & analysis
  • And more…

Thermal testing in the Innovation Center is further complemented by comprehensive agglomeration and dryer testing capabilities.  


Calcined kaolin materials are important in many products and industrial processing endeavors, as is exhibited by their growing market demand. Calcination backed by expertise is essential in controlling and reaching the desired end product characteristics of a given kaolin material. And while calcination is a relatively established thermal technique, the variability of raw kaolin materials, combined with its temperamental response to calcination and the many specific requirements of the market, demand testing to produce a product of precise characteristics.  

FEECO is a leader in advanced thermal treatment and calcination processes and equipment. The expertise and comprehensive testing capabilities offered by the FEECO Innovation Center provide an unmatched process and product development experience – complemented by our ability to use the acquired data to design a custom rotary kiln of the highest quality. For more information, on our kaolin capabilities, contact us today!

About the Authors . . .

Alex Ebben is a Process Sales Engineer and thermal processing expert.

More About Alex

Carrie Carlson is a technical writer and visual designer.

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