Talc attack: Fast hydrothermal synthesis

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  • Published: Jul 1, 2016
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Talc attack: Fast hydrothermal synthesis

Au naturel

Natural talc, hydrated magnesium silicate, forms underground over geological time periods, which is fine if you have time to wait and access to a deposit of the clay mineral. Now, X-ray diffraction reveals that a hydrothermal synthesis can be used to make the same substance in a matter of seconds.

Natural talc, hydrated magnesium silicate, forms underground over geological time periods, which is fine if you have time to wait and access to a deposit of the clay mineral. Now, X-ray diffraction reveals that a hydrothermal synthesis can be used to make the same substance in a matter of seconds.

A synthetic form of talc offers several advantages over the natural mined mineral, such as high purity and tuneable properties. Moreover, the rapid process reported in the journal Angewandte Chemie by a French team produces nanocrystals that unique properties of potential utility in various applications. The team comprises Angela Dumas, Cédric Slostowski, Guillaume Aubert, and Cyril Aymonier, of the Institut of Condensed Matter Chemistry of Bordeaux (ICMCB-CNRS) at the University of Bordeaux, in Pessac, Marie Claverie and Christel Careme of Imerys Talc, Toulouse, and Christophe Le Roux, Pierre Micoud, and François Martin of Geosciences Environnement Toulouse (GET).

Mineral mix

Talc is a non-toxic clay mineral widely and famously used in the cosmetics and personal hygiene industries, although inhalation can cause pneumonia or granuloma and paediatricians generally recommend the use of corn starch powder rather than talcum powder these days. It is the softest mineral (defined as 1 on Moh's scale, where diamond, the hardest is 10). And its various physical characteristics make it useful in paper making, plastic, paint and coatings, rubber, food, electric cable, pharmaceuticals, ceramics, and the aforementioned cosmetics.

The material is, chemically speaking, Mg3Si4O10(OH)2 and it is mined from deposits around the world. For instance, the talc deposit at Trimouns, Luzenac, in the French Pyrenees is the result of 14 million years of continuous or episodic circulation of hydrothermal fluids. It is this timescale and the complex geochemistry that means the precise composition of talc from different sources can vary. Indeed, mined talc is not necessarily pure nor homogeneous and for some applications that is a significant problem so Aymonier and Martin hoped to address this issue in developing a scalable synthesis of the mineral that would allow them to take back control of talc. Their hydrothermal approach uses supercritical water - pressurised and heated water above its critical point existing in a hybrid phase between liquid and gas.

"Water properties under supercritical conditions can be finely tuned from gas-like to liquid-like," the authors explain. And while the natural process to talc, which is metamorphosis under bedrock, leads to micrometre-sized crystalline particles that are rather impure and hydrophobic, the supercritical hydrothermal flow synthesis produces relatively pristine nanocrystals within seconds.

High-speed talc

The team also points out that their technology also offers access to the unique characteristics of talc nanocrystalline structures. The researchers point out that nanoscopic mineral particles have long been overlooked in natural ores, although they can affect the properties of the product. Even in small amounts, nanominerals can induce defects in ore-grade material through their reactivity, the authors write, highlighting the importance of studying the growth behaviour and characteristics of the nanominerals, which are now possible with the high-speed hydrothermal approach to talc production.

The team used X-ray diffraction to study the products of their hydrothermal synthesis carried out with different pressure and temperature settings. "Two sets of samples were synthesized to investigate the role of temperature and the influence of reaction time under supercritical conditions at 400 degrees Celsius," they explain. The XRD patterns of samples obtained at 25 megapascals over a 20 second reaction time show reflections characteristic of a talc structure. At lower temperature no talc was formed. Silicon-29 nuclear magnetic resonance (NMR) spectroscopy was also used in the work to study the products.

"From an industrial point of view, the decrease of the synthesis time for the production of nano-talc from a few hours in batch reactor to few tens of second by using a continuous process represents a major technical breakthrough," the team writes. The researchers add that their hydrothermally synthesized talc nanocrystals are, in total contrast to their natural microscopic counterparts, hydrophilic. This characteristic opens up new opportunities for industrial applications.

Related Links

Angew Chem Int Engl Edn 2016, online: "Fast-Geomimicking using Chemistry in Supercritical Water"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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