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How bleaching in oil refining works

Modern oil bleaching processes remove a wide range of impurities, including various pigments. Removing these requires advanced processes that use combinations of different bleaching agents to bind specific impurities, which are subsequently filtered out with the bleaching agents. Efficient bleaching makes it possible to remove pigments such as:

• carotenoids and chlorophyll
• oxidation products through decomposition and partial removal
• contaminants such as soaps and trace metals
• traces of phosphatides
• polycyclic aromatic hydrocarbons
• other pollutants

To ensure the product is commercially viable, it is necessary to remove these substances because they may adversely affect the quality and stability of the final product and impact the performance of subsequent processes such as phosphatides for double distilled water (DDW).

The oil bleaching process

Typically bleaching is a process which contains oil mixing with bleaching agent followed by separation using filters. Upon saturation, the filters shift to regeneration while fresh filters take on the filtration duty. Saturated filters contain clay (a bleaching agent) which still has some bleaching power. The technique of passing feed oil through a saturated filter before mixing it with fresh clay is called prefiltration. By applying this technique, it is possible to reduce clay consumption. However, prefiltration only works with dry oils; it is therefore not always possible to use this option.

Two-stage bleaching of edible fats and oils

Using a two-stage bleaching technique, such as mixing oil with different agents under different conditions (like temperature and moisture), often optimizes the bleaching process, enabling the deepest extraction of components such as chlorophyll. Two-stage bleaching typically has two filtration steps, which ensure the best outcome for delicate oils with lowest colour and/or highest purity (like coconut or grapeseed). To optimize investment costs, two-stage bleaching can be used with single-step filtration, which can deliver significant bleaching clay savings for setups such as for palm or rapeseed oil processing.

The bleaching agents generally used include:

• natural bleaching earths, particularly useful in treating animal fats although they have a limited colour-reducing effect
• acid-activated bleaching earths, widely used to remove all types of impurities
• activated carbon for removal of polyaromatic hydrocarbons as well as a wide range of specific pollutants
• synthetic amorphous silica, used mainly in wet bleaching with a focus on selectively removing phosphatides, trace metals and soaps