Bleaching

Adsorption, not chemistry

Despite the name, bleaching is adsorption: fine clay particles present an enormous surface that binds color pigments (carotenoids, chlorophyll), trace metals, residual soaps left from neutralization, and oxidation products. The oil is heated and mixed with the adsorbent under vacuum (to exclude oxygen and drive off moisture), held briefly, then the spent solids are filtered out. The result is a lighter, cleaner, more oxidatively stable oil ready for deodorization.

Bleaching earth, carbon and dosage

The workhorse adsorbent is bleaching earth — natural or acid-activated clay — sometimes with a small co-dose of activated carbon for specific contaminants such as certain pigments or polycyclic aromatic hydrocarbons. The central trade-off is that spent earth retains oil (a meaningful fraction of its own weight), so every extra kilogram both costs money and loses oil. Refiners therefore optimize the dose to hit the color and quality target with the least earth, matching grade and amount to the specific, well-degummed and neutralized feed.

Filtration, spent earth and best practice

After the adsorption step the oil is filtered — often with filter aids — to remove the spent earth, which leaves as an oily cake that must be handled carefully (spent earth can be prone to self-heating). Bleaching works best on clean feed: it cannot efficiently fix poor upstream degumming or neutralization, so it is not a catch-all. Done well on properly prepared oil, bleaching delivers the color and purity that let deodorization finish the oil into a bland, stable retail product.

Vacuum, temperature and time

The bleaching step runs under conditions chosen to maximize adsorption while protecting the oil. Vacuum excludes oxygen (preventing oxidation while the oil is hot) and helps dry the oil and earth. Temperature is high enough to make the adsorbent effective but controlled to avoid heat damage. Contact time is long enough for adsorption equilibrium but no longer, since extended hot contact risks oxidation and colour reversion. Getting these conditions right lets a given dose of earth do its most work, which is part of controlling the cost-and-loss trade-off that defines the stage.

Spent earth handling and safety

The spent bleaching earth leaving the filter is oily and deserves careful handling: because it combines finely divided material with adsorbed oil and oxidation products, it can be prone to self-heating and even spontaneous ignition if piled warm, so mills manage it accordingly — cooling, prompt removal, and appropriate storage. Beyond safety, the retained oil in spent earth represents the stage's yield loss, and some operations recover a portion of that oil. Responsible spent-earth handling is thus both a safety and an economic matter, and it is one more reason bleaching-earth dosage is minimized to what the color target actually requires.

Colour reversion and getting the most from earth

A subtlety of bleaching is colour reversion — oil that looked well-bleached can darken slightly later if oxidation products or trace pro-oxidants remain, or if the oil was over-heated. Good practice therefore targets not just immediate colour but stable colour: proper vacuum and controlled temperature during bleaching, adequate removal of soaps and metals, and clean handling afterward. Getting the most from a given earth dose — the right grade, well dispersed, at the right conditions, on clean feed — is how refiners hold both colour and cost, keeping the stage's characteristic oil-in-spent-earth loss to the minimum the quality target allows.

Typical conditions (indicative)

Equipment & materials

Refining machinesBleaching earthActivated carbonFilter aids

Other refining stages

DegummingNeutralizationDewaxingDeodorization

FAQ