Flux-Calcined Diatomaceous Earth

What is Flux-Calcined Diatomaceous Earth, and is it allowed for use in organic production?

By Gwynn Sawyer Ostrom

Diatomaceous earth (DE) is the fossilized remains (skeletons) of single-celled aquatic algae called diatoms. A mineral compound, DE is found in exposed ancient lake and ocean beds, and can be mined and processed in several ways. It is typically classified into three grades: a natural grade, a calcined grade, and a flux-calcined grade. Once graded, DE can be used in a variety of ways including as a filtration aid, as an active ingredient in a pesticide, or as an absorbent for industrial spills. For organic processing, the National Organic Program (NOP) regulations allow the use of nonsynthetic diatomaceous earth at §205.605(a), with the annotation that it can be used for food filtering only. It may be used to filter wine, or as a clarifying agent for other processed foods like juices.

The grades of DE are defined by how the material is treated after collection. Natural grade products are dried at relatively low temperatures (less than 500°F), and consist primarily of amorphous silica, with less than 3% naturally occurring crystalline silica. When DE is heated to high temperatures, typically around 1800°F (1000°C), calcination occurs. Calcination can be defined as thermal treatment of ores with a limited supply of air or oxygen to bring about a thermal decomposition. At this temperature, calcining causes a structural change to the diatomaceous earth, whereby some of the amorphous silica may undergo a physical mineralogical transformation to form crystalline silica. As a result, calcined DE may contain up to 40% crystalline silica.

Flux-calcined DE grades are also calcined at high temperatures, but in this case the calcination occurs in the presence of a fluxing agent such as soda ash (sodium carbonate), which reduces the temperature at which calcination occurs. This helps fuse the diatoms together, increasing the particle size and thereby producing better filter aid permeability. Some DE deposits contain natural fluxing agents. As with the calcined grade, some of the amorphous silica is transformed to crystalline silica during flux calcination. Flux-calcined grades may contain up to 70% crystalline silica. 

OMRI considers this calcining process for DE to be nonsynthetic, with the understanding that calcination of DE involves a structural, not chemical, change to the material. OMRI reviews natural, calcined and flux-calcined Diatomaceous Earth as nonsynthetic, and thus allowed at §205.605(a) as a food filtering aid for use in organic processing as nonsynthetic, nonagricultural (nonorganic) substances allowed as ingredients in or on processed products labeled as “organic” or “made with organic (specified ingredients or food group(s)).”  If any fluxing agents remain in the DE product, the fluxing agents must be individually reviewed for compliance.  

This article was originally published in the Spring 2018 edition of the OMRI Materials Review newsletter.