Removal of metals

from OFI June 2020

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Bleaching earths are well known for their pigment removal properties, hence the name bleaching earth.

They are also an important absorbent for many other impurities in oils including primary and secondary oxidation products, residual gums, soaps and metals.

Each of these impurities, if not removed, would adversely affect the properties of the oil.

This article focuses on the importance of bleaching earths in the removal of metals from edible oils. The presence of metals in fully refined oils leads to colour reversion and oxidation of the oil during storage and use, resulting in the development of off-odours, off-flavours, rancidity and a shorter shelf life. Some metals, if accumulated in the body, may also be toxic.

Metal sources

There are two main sources of metals in edible oils – endogenous and exogenous.

Endogenous sources of metals are from where the plant grows, such as the soil, water, pesticides and fertiliser. Exogenous sources of metals are from the handling and processing of the oil crop, including transport, storage, crushing, extraction, refining and hydrogenation processes.

Endogenous sources are responsible for the presence of those metals that are beneficial to the growth and development of plants and humans, such as sodium, potassium, calcium, magnesium, iron, manganese, zinc and copper. However, some potentially harmful heavy metals such as lead, cadmium, chromium, cobalt, nickel and copper may also be present as pollutants in the environment, such as in ground waters. Cadmium and lead contaminants accumulate in the body as they have long half-lives and can be present from fuel and industrial emissions.

Exogenous sources of iron and copper are present due to corrosion and erosion of processing and handling equipment. Rust from mild steel and copper from brass or bronze fittings are common sources.

Iron and copper are catalysts for the oxidation of the oil. Iron catalyses the formation of hydroperoxides, and copper catalyses the decomposition of the hydroperoxides to secondary oxidation products. Iron and copper together synergically promote rapid oxidation of the oil. It should be noted that no copper or copper containing metals should come into contact with the oil at any stage, from harvesting until final use.

Residual sodium in the form of soap may be present in chemically neutralised oil. Transition metals, such as nickel, copper and chromium, are present in post

The presence of metals in fully refined oils leads to colour reversion and oxidation, resulting in off-odours, off-flavours, rancidity and a shorter shelf life. Bleaching earths play an important role in removing metal impurities from edible oils Patrick Howes

refined hydrogenated oils, resulting from the use of hydrogenation catalyst.

Removal and reduction of metals

All metals present in the oil, regardless of their source, need to be removed or reduced, in order to produce wholesome and stable refined oil.

The reduction of the concentration of metals take place at various stages in the refining of the crude oil. Washing of the oil will remove some of the metals.

In chemical refining, the caustic neutralisation and washing steps will help greatly in the removal of metals. Degumming also helps reduce the concentration of metals.

In physical refining, the caustic treatment and washing stages are eliminated, and the degummed oil normally goes directly to the bleacher. The demands on the bleaching stage in physical refining are therefore much higher than for chemical refining. Therefore, the bleaching earth consumption is higher for physical refining compared with chemical refining.

As the degummed and optionally caustic-treated and washed oil proceeds to the bleacher, there remains traces of metals that need to be removed. The metals are present in several inorganic and organic forms, such as as gums and soaps. Some pigments carry metals, such as chlorophyll which contains magnesium.

There are various mechanisms involved in the removal of the different forms of the metals present.

Metals in different forms

Metals such as calcium and magnesium can be present as salts of phosphatidic

Endogenous sources of metals are from where the plant grows, such as the soil, water, pesti cides and ferti liser

Removal of metals

acid (PA/M++) and of phosphati dyl ethanolamine (PE/M++), which are oil soluble.

If not removed at the degumming stage, the remaining traces of these metal-containing gums can be removed by bleaching earths. It has been proposed that the phosphorus in the gums bonds at the octahedral alumina sites in smecti te bleaching earths, with additi onal interacti ons with the silica hydroxyl groups of the silica fronds in acid-leached bentonites.

The cati on exchange capacity (CEC) has also been linked with the ability of bleaching earths to remove metals.

Natural clays such as att apulgite and sepiolite have a CEC of below 40meq/100g, whereas bentonite typically has a much higher CEC of about 70 to 130meq/100. Acid-leached bentonites (acid-acti vated bleaching earths) will have a lower CEC, than natural bentonites, due to the removal of part of the structurally charge-generati ng isomorphically substi tuted octahedral layer during the acid-leaching process. Overall, the ranking of CEC is bentonite>acid-leached bentonite>sepiolite>att apulgite.

Residual traces of soaps are removed by bleaching earths by two main mechanisms; by absorpti on and soap splitti ng/ion-exchange.

Soap is absorbed on the surface of bleaching earths, thus removing the metal present in soap form. However, if the residual soap level is too high, there is a resulti ng reducti on in the bleaching performance of soap-coated bleaching earth parti cles.

Splitti ng occurs when soap is in contact with acid-acti vated bleaching earths, resulti ng in ion-exchange of the metal with the acidic site in the bleaching earths, and the free fatt y acid (FFA) part of the soap remains in the oil, as seen from the resulti ng rise in FFA of the oil. This is why FFA rise occurs in chemically-refi ned oils that have not been washed free of soaps.

Moisture must be removed from the

oil before fi ltrati on of the spent bleaching earth, otherwise the fi lters will clog. However, the presence of moisture in the oil during bleaching has been shown to be benefi cial for the improved removal of trace metals, and some pigments.

The presence of 1% moisture in soyabean and palm oils during the bleaching process has been shown to improve the removal of copper by about 10%, and improves the removal of iron by about 30%.

The design of the bleacher system must be such that the moisture is removed before the oil is sent for fi ltrati on of the spent bleaching earth. Live dry-steam agitated multi -compartment bleachers work well with wet bleaching.

Hydrogenati on of refi ned oils uti lises transiti on metal catalysts. The catalyst is removed from the hydrogenated oil by fi ltrati on, but traces of catalyst parti cles and soluble transiti on-metal soaps remain in the hydrogenated oil.

Treati ng the oil with bleaching earth, followed by removal of the spent bleaching by fi ltrati on can remove the residual catalyst and transiti on metal soaps.

A further deodorisati on of the hydrogenated oil may be required. However, the use of acti vated carbon blended bleaching earths may remove the need for post-hydrogenati on deodorisati on. 