Balancing act - balancing mineral oxides in kaolin clay

Clay Technology magazine
11 Feb 2013

A splash of Mr Muscle always helps, but a glossy white porcelain surface doesn’t appear by magic. Scott Fess, Product Manager at Applied Rigaku Technologies in Texas, USA, explains the importance of balancing mineral oxides in kaolin clay.  

Clay obtained at the mine site can be a mixture of several clay varieties, soil and rock, before it is processed and separated into various types – such as earthenware clay, stoneware clay, fire clay, ball clay and kaolin clay. One of the main differences between these is the firing temperature when making pottery, which is largely dependent on the clay’s iron oxide (Fe2O3) content. Another classification of types is by colour, which can range from white to brown. A clay’s colour is mainly determined by the content of naturally occurring pigments – rusty red iron oxide and white titanium dioxide (TiO2).  

The kaolinite component of clay, Al2Si2O5(OH4), is desired for its pure white colour. This is partly determined by how the fine particles scatter light, and partly by the presence of TiO2 and Fe2O3. As such, knowing the precise amounts present in clay is important for its grading, classifi cation and use. Kaolin clay is frequently used in products requiring a pure white colour, such as porcelain sanitary ware and fi ne ceramic tableware – although its other interesting use is in coatings for paper, where it offers a smooth, bright and glossy surface while aiding the retention of inks and dyes. Wherever a pure white colour is desired, kaolin with low Fe2O3 content is sought as a starting material, so this and other colour-bearing minerals are removed during final processing. By precisely balancing levels of TiO2 and Fe2O3, various hues of red and brown can be produced, hence the clay is also a useful ingredient in cosmetics and powders.  

Kaolinite can be extracted from the clay mixture, or the mine itself might be naturally rich in pure kaolinite. Once the starting clay materials are sorted and ready for processing, kaolin clays can be mixed to achieve the desired mineral balance, or some minerals might be removed to attain the preferred shade. With kaolin clays offering such a broad spectrum of finishes, its mineral content can affect the pricing and sale of kaolin coming from the mine.  

Mineral measurement  
For the classification, purifying and mixing of clays, a means of quickly and precisely measuring these mineral oxides is important to industry. Analysis must be fast and simple, and the analyser rugged and reliable for use at the mine site as well as in the refining and production facilities. Energy dispersive X-ray fluorescence (EDXRF) is a popular technique in the clay industry for the measurement of titanium and iron oxides, as well as aluminium, silicon and other oxides and elements in clay. EDXRF is simple to operate, requires minimal sample preparation and produces fast analysis times on the order of one to two minutes per sample. Small, self-contained systems are transportable and rugged enough for use at the mine and at-line in production process control, meaning EDXRF is a technique that can be used even by non-technical operators.  

In EDXRF, source X-rays enter the sample and cause the constituent atoms to emit their own characteristic fl uorescent X-rays, which are collected and counted by an X-ray detector. The energy of an atom’s fluorescent X-ray is unique to that atom. The intensities of these characteristic X-rays are then related to concentration by comparison to a stored calibration model.  

Direct excitation EDXRF is a low-cost solution for measuring titanium and iron. In this type of system, X-rays from a tube are focused directly onto the sample, fi rst passing through filters that remove parts of the background noise. This creates a ‘quiet spot’ in the spectrum at the energies of titanium and iron, allowing for the measurement of the fluorescent X-rays from the sample. EDXRF systems are typically equipped with filter sets that automatically switch to allow measurement of different groups of elements and oxides in the sample.  

For optimum evaluation and measurement of trace elements in kaolin clay, indirect excitation is often employed. In this case, the source X-rays are first directed at a secondary target, which becomes a monochromatic source of X-rays to illuminate a sample.  

Sample preparation is reasonably straightforward. Typically around 200 mesh (<75μm particle size) is required to grind the clay to a dry, homogeneous powder, which is a suitable size for measuring titanium and iron.  

Let’s calibrate  
Calibration relates measured X-ray intensity to known assay concentration, and is made by measuring a suite of reference clay samples assayed for TiO2 and Fe2O3. For optimum results, calibration standards are developed at the mine site or production facility to model the particular geology of the clay being mined or the product being produced. Even when calibrations do not include quantification of silicon and aluminum oxides, these are normally measured to refine the matrix X-ray absorption model, and can be given quantified assay values.  

Typical calibration results are described in the table and graph below. Here, seven kaolin clay samples were used to develop empirical calibrations for TiO2 and Fe2O3. The EDXRF response and correlation indicate accuracy (which is largely dependent on the error given in the assay values) on the order of 1–2% relative. Demonstrating just how precise EDXRF analysis is, the tables above show typical calibration standards of two selected samples, each of which was subjected to 10 repeat analyses over 120 seconds. 

The empirical method can be used to determine the detection limits for TiO2 and Fe2O3. In this method, 10 repeat measurements of a blank sample (such as research-grade boric acid) are taken from a static position, and the standard deviation (σ) determined. The lower limit of detection (LLD) is defined as 3σ – typical LLD of a modern EXDRF analyser is around 30ppm for TiO2 and 68ppm for Fe2O3.  

With the ability to produce such accurate measurements of these important mineral oxides, EDXRF is an ideal technique for measuring titanium and iron content in kaolin clay. And with the compact, rugged and self-contained nature of modern systems, not only are they suitable for use at the mine site, but also along quality control checkpoints during the sorting, mixing and production of clay formulations. This, along with USB data logging and networking capabilities, means modern measurement systems can be used by non-technical operators and technicians.