The complete guide

Bleaching earths and clays are vital for the edible oil industry. Oil producers use these crucial minerals to “bleach” or purify vegetable oils by allowing the bleaching clays to soak up impurities from the processed oil. Without them, our vegetable oils would simply not be fit for consumption.

Yet, despite their immense significance, bleaching earths can be poorly understood within the edible oils industry beyond their basic purpose. That is not due to any failure on industry professionals’ part. Bleaching earth mining, processing, and distribution are complex topics, but knowing how these allimportant minerals make their way from the ground into the oil vat can help edible oil producers improve their processes.

Multiple types of bleaching earth are utilised across various industries but the oils and fats sector is the largest user, and will continue to lead as population growth drives further consumption.

The three basic types of bleaching earth – bentonite, attapulgite and sepiolite –can be used as natural products but they are more commonly refined further for specific industries and uses.

Main types of bleaching clays

Bleaching earths are clay minerals that –when added to vegetable, animal or fish oil – have the ability to bind and absorb unwanted impurities and contaminants, such as trace metals, oxidation products, chlorophyll and carotenoids. Once the

The edible oil industry is the main consumer of bleaching earths worldwide. What are the main types of bleaching clays, how are they produced and utilised, what is their relationship to other adsorbents, which contaminants must they address and what are the environmental and other challenges facing producers?

Ile Kauppila

minerals are removed from the oil, they take the contaminants with them, thus cleaning or “bleaching” it.

All bleaching earths and clays are able to bind contaminants due to their specific physical properties, such as large surface area, porosity and particle size. However, different types of bleaching earths possess distinct characteristics that affect their overall performance, making them more or less suitable for different oils or processes. In general, three different clay minerals are used as bleaching earths: bentonite, attapulgite and sepiolite.

Jorge Bello, technical support manager at US Silica, says that each of these clays belongs to a group called 2:1 clays.

These clays get their name from their internal structures, which consist of one octahedral sheet sandwiched between two tetrahedral sheets, in the case of bentonite (also called montmorillonite). Attapulgite and sepiolite also have similar structures, but their internal “windows” or areas between the sheets are larger.

“Bentonite is a phyllosilicate, with layers like a pack of cards,” Pat Howes, technical and marketing director at Malaysia’s Natural Bleach Sdn Bhd, explains. “The space between the layers can be accessed from all four faces of the edges of the stacked layers.”

Bentonite removes impurities by absorbing them between the “cards” or mineral layers through capillary action. Due to the wider absorptive area, Howes notes that bentonite’s capillary effects are weaker than those of attapulgite or sepiolite.

“Attapulgite and sepiolite have a needlelike morphology. Their structures resemble a bundle of straws and, like straws, their interior absorptive area can only be accessed via either of the two ends.” The straw-like structure also gives attapulgite and sepiolite stronger capillary action.

It could be assumed that higher capillary attraction would make attapulgite and sepiolite more effective at removing impurities, but the situation is actually the opposite. In addition to impurities, attapulgite and sepiolite absorb large amounts of the oil product, which is naturally undesirable for edible oil producers.

“Bentonites result in about 15-20% oil loss, whereas attapulgites and sepiolites cause about 30-35% oil loss. That oil loss is a very important contributor to the total cost of vegetable oil refining,” says Howes.

Attapulgite and sepiolite can also pose a potential health hazard to oil refinery staff. If a worker were to inhale airborne powdered attapulgite or sepiolite, their needle-like fibrous structure could cause fine material particles to become trapped in the lungs – not unlike asbestos fibres.

Prolonged exposure to the powdered clays may result in lung damage, medical conditions such as chronic bronchitis or tuberculosis, and potentially even cancer, according to research by Purdue University. “Inhalation of these materials must be avoided,” Howes stresses.

Due to the lower oil loss and health risks associated with bentonite, these minerals are more popular as bleaching earths. However, attapulgite and sepiolite also have their uses, as long as correct safety precautions are followed.

Plentiful global supplies

Bleaching clays are naturally-occurring minerals that are found in huge deposits all around the world where ancient volcanoes were once active.

“Wherever there was volcanic activity, there is a potential to find a bleaching clay deposit of any type,” says Bello. “Volcanic ashes are the parent materials. The ashes have experienced rearrangement and evolution in their structure to become clay. Deposits are all over the world but ‘good’ clay deposits are only located in specific areas of the globe.”

One example of such deposits is the Deccan Plateau in India, which arose around 66-60M years ago. According to Howes, the super volcanic activity at the time deposited roughly 200,000 to 1M km3 of ash in the area. “We can compare that to the eruption from Mount St Helen’s in 1980, which was estimated at 4km3. Bentonites, attapulgites, other closely related minerals and intergrowth materials can be found in the Deccan Plateau,” Howes adds.

Similarly, there is a significant bleaching clay deposit in Europe, running from the area of Redhill, UK, across the English Channel, all the way to France, Spain and northern Africa. Other major deposits are found in Australia, Brazil, China, Iran, Nigeria and South Africa, with bleaching earths being mined from each of them.

However, certain bleaching earths are concentrated in specific areas. According