Bleaching was introduced in edible oil refining at the end of the 19th century to improve the colour of cottonseed oil. Originally, it was a batch process at atmospheric pressure, in which natural bleaching clay was added to hot oil with the sole objective of removing colouring pigments. Today this is no longer the case, and bleaching has become a critical process in edible oil refining to remove impurities (soaps, phospholipids, oxidation products, trace metals, contaminants etc.) from edible fats and oils prior to deodorisation. It usually follows neutralization, but it may also follow the degumming process in the case of physical refining. Bleaching is an old process; it is rather complicated and not easily understood. Although degumming and neutralization are strictly chemical processes and deodorization a physical process, bleaching is a combination of the two.
An understanding of the bleaching process requires a knowledge of the properties of the bleaching clay as well as the types of impurities present in the fats and oils.
Impurities in Fats and Oils
Impurities in vegetable oils have various sources. They can be minor ingredients naturally contained in the oil or degradation products produced with abuse of the seed, fruit, or extracted oil. Technically, some of the impurities are compounds produced by the oil plant and should be called by-products rather than impurities.
Some detrimental impurities develop due to degradation processes in the oil fruit or seed; some develop downstream during processing. The chemicals added during processing are another source for impurities in edible fats and oils.
Table 9.1 lists most of the common impurities and by-products contained in edible fats and oils. Table 9.2 defines and classifies the impurities by origin.
Typical Levels of Common Trace Constituents in Some Crude Vegetable Oils
The aim of the bleaching process is to remove the unwanted impurities listed in Table 9.1 without changing the triglyceride molecule. During bleaching, the oil is treated with a chemical and an adsorptive material to remove all of the undesirable impurities present except for those captured by the deodorization process. The five characteristics of activated bleaching earth outlined below make bleaching a versatile and highly effective tool to remove most of the undesirable impurities. However, the bleaching process must be controlled to prevent the development of other undesirable impurities.
Bleaching Earth – Introduction
Bleaching earth is made from naturally occurring minerals such as palygorskite, which is also known as attapulgite, sepiolite, bentonite, and other minerals that all belong to the aluminum silicate family. Bleaching earth, often called “fuller’s earth,” has been known and used for many years. The use of fuller’s earth for filtration and colour improvement of oils was identified and developed by W.B. Albright and H. Eckstein for N.K. Fairbanks & Company, Chicago, IL. Initially, in 1880, they developed a process to bleach cottonseed oil with fuller’s earth. Bleaching earths consist of natural clays, which have “bleaching activity,” and others that become active only after a specific treatment. The first activated bleaching earths were developed by Pfirschinger Mineralwerke, Kitzingen in 1907 and by Erdwerke Kronwinkl, Franz Schmidt & Co. in 1909.
The natural, neutral, or non-activated bleaching clays are derived from clay mineral deposits, which are merely dried, milled, and sieved to obtain a desired range of particle sizes. Natural weathering over a very long period has rendered the original mineral partly porous, and has given it some power of adsorption for pigments, soap, and so forth. Such material came into early use to cleanse fat from sheepskins (i.e. fulling); therefore, in England it became known as Fuller’s earth. This class of material is particularly mild in action, not promoting chemical changes in triglycerides, showing little tendency to split soap, and generally responding only modestly when bleached with acid to make it a more active adsorbent.
Throughout the world (with the exception of Antarctica), deposits of different forms of the clay mineral bentonite are found, usually near the surface and in seams up to a 15-feet thickness. The name “bentonite” is derived from the large deposits associated with Bentonite shale at Rock River, Montana. At least 85% of this material consists of forms of an aluminium silicate known as montmorillonite, which was identified as long ago as 1847 at Montmorillon, France. The activity of this form of bentonite changes little with acid leaching, and it is not the source of acid-activated bleaching earths.
A much more important variant, calcium montmorillonite or non-swelling bentonite [southern (United States) bentonite, Texas bentonite, etc.], lends itself very readily to leaching with mineral acid (HCl or H2SO4); its power of adsorption and catalysis is thereby dramatically increased.
Acid-activated clays were first produced in Germany around 1905; then several other countries around the world followed—this extension is continuing today. The approximate world production of 700,000 tons per year (2001) must be seen against a world production of total bentonite-type minerals of 6 million tons per year. The acid-activated clays are used in cleaning mineral oils as well as vegetable and animal fats; they are also used as catalysts in promoting organic reactions such as polymerization and isomerization, and may take the form of fixed beds or powdered additions.
Properties of Bleaching Earth
First, the bleaching clays must exhibit a certain cation exchange capacity. The other important parameters for natural bleaching clays are a high absorption capacity, particle size distribution, porosity, and large surface area. Natural bleaching clays can be slightly acidic to slightly basic.
Effective adsorption requires a large surface; for practical reasons, the high specific surface (m2/g) of a very porous solid is used. The channels by which molecules reach this surface must be negotiable by the molecules concerned. The nature of the surface must allow acceptably firm bonds, chemical or physical, between it and the adsorbate. In the case of bleaching or purification by adsorption, pigments or other components are selectively retained on the pore surface, and the triglyceride escapes. Gradually, the concentration of pigment on the available surface of the adsorbent and the concentration remaining in the oil come into balance, so further exchange is negligible. The best temperature for the oil/clay contact must be chosen, as must the duration of the contact; an excess of either will encourage undesirable side effects.
A helpful action is to remove from the oil any material like gum or soap which will compete for room on the clay surface. This leaves the surface much freer to work on remaining traces of gum or soaps as well as on an adsorbing pigment.
Bentonite minerals have limited sorptive properties in the natural state and require chemical treatment by acids to create the surface area and porosity needed for bleaching vegetable oils. Bleaching clays of this nature are commonly referred to as “acid” or “acid-activated” clays.
Attapulgite and sepiolite minerals have a naturally high affinity for adsorbing oil contaminants without any acid treatment. These natural clays can be acidified with mineral acids as well as used in conjunction with chelating acids such as citric or phosphoric acids to improve bleaching activity with respect to chlorophyll.
Bleaching Earth Acid Activation
Bleaching clays are activated to varying degrees via interaction with acids ranging from completely natural clays to highly acid-treated clays. Directionally the benefits to having increased adsorbent activity and/or impurity removal (i.e. bleaching efficiency) improve as the clay acidity of a base mineral is increased (pH lowers with limit approaching 2 pH).
The advantages for an activated bleaching earth include the following:
(i) the surface area of the starting material has increased by several hundred percent which results in a high adsorption capacity (250-350m2/g)
(ii) acid activation has provided the clay with some acid centres by formation of silanol groups and by replacing the Ca-ions between the layers with protons and Al-ions, both of which give the clay catalytic properties;
(iii) a third important parameter is related to the ion exchange capacity and is strongly related to the ion exchange capacity of the starting clay material
Natural bleaching clay contains a high proportion of complex aluminium silicate in which a number of other metals such as iron, magnesium, calcium, sodium, and potassium are present in varying degrees as impurities. With acid-activation, the aluminium, magnesium, and iron cations are leached by acid and gaps are created in the crystalline structure and a large increase occurs in internal surface area. The liberated, more acidic cations (Al+3, Mg+2, and Fe+2) now replace the cations Ca+2, Na+, and K+. Now an increase occurs in the concentration of sites of strong surface acidity—not merely total surface acidity. When pigment-bearing oil contacts activated clay, the cations at the strongly acidic sites are ready to donate a proton to the pigment molecules, which usually contain electrophilic bonds ready to accept it, and thus form a positively charged carbonium ion (i.e., organic cation). The pigment molecule is then held to the clay surface by electrostatic attraction. Evidently, this whole interaction is only able to occur if the channels or pores leading to the active sites within the clay are easily able to allow the passage of pigment molecules. Although the interaction on the outer surface of the clay particles may be the same, its contribution to the whole is small. Also, the chemisorption effect can lead to a reaction proceeding on the surface which radically alters the pigments: for example, the ferric ion (Fe+3) has the ability to convert orange-red carotenoid to a green colour.
The acid-activation process of the earth is as follows. The clay is separated from foreign material such as limestone at the mine site, and then transported to the activation plant, where it is crushed to convenient size and, when necessary, excess moisture is dried off. The granulated clay is then slurried with a predetermined proportion of water plus acid, usually HCl or H2SO4. The agitated slurry is then heated near its boiling point for a few hours to achieve the desired quality of product from that particular bentonite. Next follows a washing of the clay held in a filter until the appropriate residual acid is reached, then follow drying, grinding, sizing, and packing. The proportion of acid used and the duration of the extraction are important factors in the success of the process. Sizing to a particular distribution of particle sizes may also reduce yield and contribute to the finished cost. Nevertheless, the activated clays are so much more effective for many purposes than the neutral ones that many bleaching and purifying steps are rendered more economical, practical, and convenient by their use.
The 4 critical factors that determine the quality of the activated earth are as follows:
1) The quality of the raw clay as mined is the first and most obvious influence.
2) Next comes the degree of acid activation and the recognition that one can pursue this beyond the optimum.
3) Third, once the activated clay is washed free of surplus acid, it must be dried to the moisture level at which it operates best.
4) Finally, the activated clay must not be ground so fine that it creates difficulty in filtration after use
The 5 factors that determine the efficiency of a bleaching earth
1) Adsorption capacity
2) acid properties
3) catalytic properties,
4) ion exchange capacity
5) and particle size distribution
On the basis of these characteristics, it is clear that “activated bleaching earth” does much more than simply remove colour. The same is partially true for natural bleaching clays, but nature provides some limitations that are eliminated with acid activation.
Issues with Acid-Activated Earth Usage
Employing acid-activated clays can have detrimental side effects in the oil. The acidic properties are responsible for a number of desirable reactions but also present some that are undesirable. A desirable reaction is the splitting of soap according to the formalized equation shown below:
H-bentonite + Na-soap = Na bentonite + fatty acid
An undesirable reaction is the splitting of the triglyceride according to the formalized equation shown below:
triglyceride + water = diglyceride + fatty acid
These include increased levels of free fatty acids and the formation of undesirable 3-Monochloropropane-1-2-diol esters (MCPD).
Natural clays are often preferred by refineries that are sensitive to these contaminants. Furthermore, natural clays are the only bleaching clay option that meets the “Organic” oil certification because their manufacture does not involve any restricted chemical agent. In some situations, “organic” approved natural acids such as citric acid can further enhance the bleaching effectiveness of natural clays and still meet “organic” labelling requirements for chlorophyll-rich oils. This acid can either be added to the oil being bleached or acid treated clay can be prepared in advance.
Bleaching Mechanism – How Bleaching Earth Works
During the bleaching process adsorption occurs via many different mechanisms involving various physical and chemical interactions; most of them improve the quality of the oil, but some of them may reduce it. These mechanisms include the following:
Mechanism by which the sorbent binds a contaminant. This can occur in three different ways:
a) physically through surface attraction involving Van der Waals’ forces
b) chemically by “chemisorption” by electrochemical bonding to the surface of the clay
c) by molecular sieves which trap contaminants under pressure inside the pores of the clay during filtration
Mechanism by which the intra-granular pores are filled with some fluid—mainly oil — and in turn whatever contaminants came along with it. Oil retention is reported in two ways: as weight loss by Soxhlet extraction (with hexane being used as solvent), and as total organic matter determined by ashing.
Total oil retention depends on a number of variables including clay dosage levels, clay characteristics (e.g. particle size distribution and mineral type), permeability of the filter bed, incoming feedstock quality, cleanliness of the filter screen and the conditions used to purge the filter before disposal of the “spent” filter cake. Excessive oil retention increases the cost of running the process; oil loss through spent earth can typically range up to 35 weight percent solvent extractable and 50 weight percent for total organic matter of the earth used
Mechanism of trapping or physically removing suspended contaminants. The physical act of filtering out the suspended clay that simultaneously removes the minor contaminants adsorbed to the clay particles. Filters used in the bleaching process include namely
1) Processing filters (horizontal and vertical leaf filters) and
2) Polishing filters (bag, cartridge, paper).
Mechanism by which contaminants are degraded by interaction with the surface of the clay. The best known and by far the most important reaction is the decomposition of hydroperoxides. For example, peroxides are effectively reduced (polymerized and/or decomposed into volatile oxidation by-products) by clay/oil interaction. A higher catalytic activity is equated with a lower peroxide value after bleaching. A number of secondary oxidation products such as aldehydes and ketones may form when the hydroperoxides decompose. Additionally, conjugated fatty acids may develop; these are much more sensitive to oxygen than nonconjugated unsaturated oils. Therefore, it is very important that contact with air be avoided during bleaching with the use of a vacuum and with nitrogen protection during filtering and storage.
With excessive heat and oxidation, pigments can form colour compounds that are difficult to remove or said to be “fixed.” In the event of colour fixation, red colour is more difficult to remove by bleaching clays alone and resistant to thermal degradation leading to higher red colour after deodorization.
Decomposition of peroxides with highly activated bleaching earth
Effect of Bleaching on Oil Quality
Bleaching is the physical and chemical interaction of an oil or fat with bleaching earth to improve its quality. However, with respect to oil specifications, it is quite variable and depends on the product and market we are dealing with.
To produce good quality and stable refined oil, the bleached oil should have the following parameters:
a) Phosphorus content : 3 ppm max
b) Moisture and impurities : 0.1% max
c) Bleached oil colour : 20R Lovibond max
Part of the carotenes are also adsorbed on the bleaching earth, but for palm oil the colour after dry degumming alone is not so critical as the majority of the carotenoids are thermally bleached under the conditions of steam refining/deodorization.
d) Peroxide value : 0.1 max
e) Bleaching earth : < 5ppm
f) Trace metal content
Fe < 0.1 ppm,
Cu < 0.1 ppm
Ni < 0.1 ppm
Recommended Purity Criteria for Oils Before Bleaching and Expected Purity After Bleaching.
Process Conditions for Bleaching
Colour reduction and the removal of other undesirable impurities are strongly influenced by the reaction conditions. At higher temperatures, exposure to oxygen will cause colour fixation and a drastically reduced oil stability, which is even more pronounced with the use of a highly activated bleaching earth. This explains why natural bleaching earths apparently have better colour reduction with atmospheric conditions than do activated bleaching earths. Conversely, with vacuum bleaching conditions, activated bleaching earths provided better colour reduction than did the natural bleaching earths.
Influence of Temperature
If bleaching earths were simply adsorbents, the best colour reduction would be expected at low temperatures. The adsorption equilibrium would be expected to shift toward desorption with high temperatures, and some of the adsorbed molecules would dissolve back into the oil. However, this is not observed. Decolouration improves as the temperature is increased, which indicates that bleaching earth is more than simply an adsorbent.
Chemical reactions take place on the surface of the bleaching earth. According to the rule of Van’t Hoff, the speed of reaction doubles with each temperature increase of 10°C. This rule is valid for all reactions, both the wanted and the unwanted.
Consequently, there must be an optimum temperature. This optimum temperature depends upon the type of oil, the by-products, and the impurities. The figure below shows the influence of temperature on the colour of bleached and deodorized palm oil. It can be observed that when the optimum temperature is exceeded, colour fixation occurs. As a practical rule of thumb, oils should be bleached at the lowest reasonable temperature. This temperature must be high enough to provide a low oil viscosity. It must be low enough to avoid undesirable side reactions, which would damage the oil and reduce the quality and shelf life. Most oils are bleached at temperatures between 90 and 100°C. Difficult-to-bleach oils may require a temperature as high as 120°C. It has also been found that less active bleaching earths require somewhat higher temperatures.
Color of bleached and deacidified palm oil as dependent on temperature. Conditions: 1.5% Tonsil Supr.11 OFF, oil deacidified at 240°C
Influence of Pressure
In the early days of fats and oils processing, bleaching was generally performed under atmospheric conditions. However, the detrimental effect of oxygen on the quality or oxidative stability has been recognized since at least 1929. Eicke reported on the differences experienced with atmospheric and vacuum bleaching of beef tallow at the DGF conference in Regensburg. The table below shows that atmospheric bleaching of tallow at 90°C provided a lower colour than vacuum bleaching.
However, a high increase in peroxide value (PV) was observed with atmospheric bleaching conditions, whereas the vacuum bleaching conditions had considerably lower PV increases. These results confirm that hydroperoxides decompose at least partially to conjugated fatty acids, which are more sensitive to oxidation.
Bleaching of Tallow: Atmospheric vs. Vacuum
Bleaching efficiency improves when operating pressure in the bleacher is run between 50 to 125 mmHg (absolute). Reduced pressure allows for a smooth water evaporation rate resulting in increased efficiency for phospholipids, chlorophylloids, and some red pigments removal. Reduced pressure also minimizes interaction of oil and air resulting in lower peroxide values, anisidine values, and bleached oil color.
Atmospheric bleaching provides ideal conditions for oxidation, i.e., exposure to oxygen at high temperatures. Obviously, vacuum bleaching offers better protection from oxidation than atmospheric bleaching. In general, fats and oils must be protected from exposure to oxygen during processing to maintain a bland or pleasing flavour with good stability or shelf life
Influence of Moisture
Moisture in the bleaching operation may arise from three sources. First, it may arise from the moisture naturally present in the clay as it is used; secondly, from the oil as it is brought forward to the bleaching operation; and thirdly, it may be deliberately added in certain bleaching procedures of technical-grade fats.
Bleaching efficiency is highly dependent on the interaction between vacuum, moisture and temperature. Bleaching clay has an attraction/affinity for polar compounds including water. Maintaining adequate moisture levels in the bleacher throughout the bleaching process affords better removal of soaps, phospholipids and chlorophyll due to bleaching clay’s affinity for these compounds. More specifically, phospholipids and soaps are more readily removed from fats and oils when properly hydrated. The absolute pressure of the vacuum works in conjunction with temperature to drive moisture out of the oil.
Depending on the incoming moisture levels, too strong a vacuum can be detrimental for bleaching efficacy in that it will pull moisture out of the system too quickly and drive moisture to below the optimal range (0.06 to 0.1%) to reap its benefits (reportedly as much as 30% improved activity). An indirect way to determine if moisture levels are in balance with vacuum and temperature is to monitor the moisture content of the oil coming out of the bleaching filter.
Influence of Time
The time necessary for maximum colour removal depends on the quality of the bleaching earth and the bleaching temperature. Colour removal increases as the temperature and/or time is increased; however, longer oil contact time with a bleaching earth can cause colour reversion, which also increases as the temperature increases.
The contact time between the oil and the bleaching earth refers to the total time that the bleaching clay is in contact with the oil going from the slurry tank through the filter press. Times typically range from 15 to 45 minutes, with 20 to 30 minutes being most common. Shorter times are recommended when higher bleaching temperatures are utilized. Long exposure times with bleaching earth and high temperatures damage the oxidative stability or quality of edible fats and oils.
It is generally accepted and understood that bleaching clays have a limited number of active sites or “parking places” for contaminants. The filling of these sites progresses well provided the amount of contaminant is matched by adequate pore space or volume and surface area.
As the proverbial parking garage nears capacity, the level of residual contaminants is so low that it is not worth spending additional production time to find a place to park the remaining residual contaminants. In contrast, excessive contact times may result in darkening of the deodorized oil, which is most likely due to oxidation and reduction reactions that are known to occur on the active surface of the bleaching clay.
Influence of Dosage
The amount of bleaching earth used depends on the type of absorbent used and the type of refined oil, as well as the adsorption of colour bodies and other impurities required. The percentage of clays used varies in a wide range from 0.15 to 3.0%, and only in extreme cases are higher quantities used. Use of acid-treated or activated earths far exceeds that of natural clays due to the higher bleaching efficiency, particularly with dark or high chlorophyll oils. On the basis of adsorbent activity, the acid-activated clays are generally 1.5 to 2 times more effective as bleaching agents than are the natural earths. The efficiency of an absorbent is measured by the minimum dose required to reduce the concentration of adsorbent to the required level.
Therefore, the kind and amount of earth or carbon used need only be enough to clean up the oil preparatory to hydrogenation or deodorization and to remove any undesirable impurities and pigments that will not be removed in later processing. The minimum required bleach is usually best as over-bleaching increases oil losses and can lead to flavour, oxidative, and even colour instability. The removal of colour pigments is a common, simple visual guide, often used to gauge the overall performance and adjust levels required of a bleaching earth; however, the ability to remove other undesirable impurities is less readily apparent.
Palm Oil Bleaching Process
The crude oil feedstock is heated and mixed with food grade phosphoric acid 85% concentration with an amount of 0.08% to 0.1% of crude oil quantity at a temperature range of 80–120°C of in a dynamic mixer. The mixture is fed into a degumming vessel with a retention time of 5 -15 minutes. After the degumming vessel, the oil and precipitated materials are fed directly to bleacher. Clay adsorbent is added to the bleacher simultaneously. Bleaching clay quantity depends on the crude oil quantity and typically in the range of 0.5% to 2% of incoming crude oil quantity. The bleaching process is normally carried out under vacuum (50 -100 mm Hg) to reduce the presence of oxidative reactants and to control moisture levels.
The bleaching process takes 30–45 min in a temperature range of 80–120°C. Although increased temperature increases the adsorption efficiency, bleaching at very high temperatures is not recommended since undesirable reactions also increase. The temperature should be high enough to maintain a low oil viscosity, which improves diffusion and mass transfer rates.
Finally, the bleaching earth is removed by filtration. An increase in FFA of less than 0.2% should be expected, but the final phosphorus content must be reduced to less than 5 ppm.
Typical Bleach Conditions (Time/Temperature/Dosage) for Some Common Oils
Canola oil: Effect of bleach time Bleach conditions: Clarion 470 (1.5 wt% dosage/vacuum/110 C bleach temperature)
Spent Bleaching Clay
The spent bleaching earth removed from the bleached oil with filters represents a substantial amount of waste material.
After its use, bleaching earth is loaded with a number of different impurities adsorbed from the oil, depending upon the type and condition of the oil bleached. The most common handling procedure is to discard spent bleaching earth directly from the filters to a landfill. The spent bleaching earth oxidizes rapidly when exposed to the air to develop a strong odour, and spontaneous combustion easily occurs, especially with oils high in polyunsaturates. Therefore, the spent bleaching earth must be covered with soil or sand soon after dumping.
Reducing oil losses in spent bleaching earth is very important as it will both directly increase the bleached oil yield and reduce the quantity of spent bleaching earth that must be disposed of.
The oil content of the spent bleaching earth may range from 25 to 75% of the weight of the earth. Oil retention is affected by the type of filters, the type of refined oil bleached, and the degree of colour reduction. It is important to recover as much of this oil as possible. Oil can be recovered by several methods, some performed on the cake while it is still in the filter and others after it has been removed from the filter. Some of the procedures for oil recovery include:
- Cake steaming: Blowing steam through the cake in the filter can reduce the oil content to as low as 20%
- Solvent extraction: Organic solvents can be used to extract the oil from the filter cake in certain enclosed filters as a separate process. Hexane, a nonpolar solvent, has performed well, but strong polar solvents, such as acetone or trichloroethylene, may also recover the impurities separated from the refined oil. Solvent extraction provides oil yields of over 95% with a quality comparable to the originally filtered oil.
Further utilization of the spent bleaching earth will depend upon the type of by-products adsorbed with consideration of ecological as well as economical aspects. The important parameters that help to determine the further use of a spent bleaching earth are as follows:
biological decomposition of organic substances,
soluble heavy metals content,
non-bentonitic material content (activated carbon, filter aid),
Potential Uses of Spent Bleaching Earth
In most cases, spent bleaching earth is discarded in a land fill. However, investigations have been underway for quite some time evaluating the possible use of the energy or oil portion of spent bleaching earth as well as the inorganic component.
De-oiling of used bleaching earth salvages a portion of the oil and facilitates dumping of the spent bleaching clay. The oil extracted can be used as biodiesel plant feedstock as well as for production of industrial grease and lubricants.
Addition to Animal Feed.
Spent bleaching earth, whether used in the edible oil industry or for nonedible oils, contains on the basis of its triglycerides, predominately biologically degradable compounds with a high energy value. Accordingly, there are many possibilities for further utilization. Tests with pigs, poultry, and cattle showed that 3% spent bleaching earth may be added to the feed without any problem. However, this practice became prohibited in a growing number of countries due to stricter feed safety regulations.
Use in Biogas Plants.
The fine distribution of the fats or oils on the surface of the bleaching earth guarantees a fast decomposition and a high degree of utilization for its biological application. Utilization of spent bleaching earth in biogas facilities is environmentally friendly when certain rules are observed. Essentially, the entire organic content is converted to biogas, thus reducing the increase of CO, in the earth’s atmosphere.
Soil improvement is also an important consideration. When the sludge of fermented sewage obtained from anaerobic fermentation of fat is spread on agricultural fields, bleaching clay contributes to an improvement of the soil. Bentonite, the natural mineral used to produce bleaching earths, is present in all fertile soils and gives them good water adsorption and water retention capacity. In addition, due to its natural ion exchange capacity, bentonite retains trace elements in the soil, thus improving its fertility.
Cement or Brick Production.
Another potential application for spent bleaching earth is its use in the production of cement in which the organic compound serves as an energy source. As mentioned earlier, used bleaching earth has an energy content that corresponds to lignite or coal. The inorganic residue is an aluminium silicate, and thus a raw material in cement production. In this application, traces of heavy metal, such as nickel, are not a concern because they are immobilized at the high temperatures in the cementation furnace. The same is true for the use of spent bleaching earth in the production of bricks. These applications are the most environmentally friendly means of utilizing spent bleaching earth because the whole material is used, either as an energy source or as mass in the finished product. In this application, it is of no importance whether the used bleaching earth contains heavy metals, activated carbon, edible oil, or mineral acid.
Finally, another possibility for recycling bleaching earth is the regeneration of used bleaching earth; several patents have been issued in the United States and Europe for bleaching earth regeneration. Under ideal conditions, regenerated material with an activity of 90-95% of the original bleaching earth can be produced. The spent bleaching earth to be regenerated must not contain any filter aids, activated carbon, phosphorus compounds, or alkali. Unfortunately, all of the currently known procedures are so expensive that fresh bleaching earth is more cost effective.
Use of Activated Carbon in Bleaching
Another adsorbent that has long been used in the bleaching of edible oils is activated carbon. In 1855, Poll suggested its use for bleaching oils, and in 1899, Bornemann suggested charcoal for the same purpose. Early this century, R.V. Ostreyko invented the activation of carbon. He showed firstly that a highly adsorbent product could be obtained by roasting charcoal or other carbonaceous raw material in steam, carbon dioxide, or other gas which made oxygen available to give it its typical structure with a high surface area (up to 1500m2/g). Secondly, that conditioning the raw material with reagents such as zinc chloride, alkali carbonates or others, prior to carbonization, also led to greatly enhanced activity. Regarded popularly as “physical” and “chemical” activation, respectively, these two processes provided the basis of the industry which developed rapidly at the time of World War I. Coconut shell or hard-fruit kernel were found to yield a charcoal very suitable for military respirators or other gas-adsorption duties. By the 1930s, other activated carbon was developed for the recovery of volatile organic solvents
The traditional use of activated carbon in edible oil refining is in effective bleaching (colour removal), as a complementary (lipophilic) adsorbent to bleaching clay. However, as more efficient (activated) bleaching earths have become available, the use of activated carbon for bleaching has been substantially reduced. Today, activated carbon is mainly used for the removal of heavy polycyclic aromatic hydrocarbons (PAHs) from vegetable oils (coconut oil, palm kernel oil, olive pomace oil etc.) that have been contaminated by smoke drying or direct heating of the raw materials. At the end of the 1990s, it was also introduced in fish oil refining for the adsorption of dioxins and polychlorinated biphenyls. More recently, activated carbon treatment has begun to be applied more systematically as ‘best proven practice’ for the decontamination of edible oils (like sunflower and rapeseed oil). This practice arose with the growing attention paid to the removal of contaminants from edible oils imposed by stricter legislation and driven by stricter trading specifications.
Activated carbon is still mostly added together with bleaching earth. It is used for
- removal of colour pigments, the bleaching earth–activated carbon ratio is typically 80:20 or 90:10.
- removal of dioxin/PCB from fish oil; dosage 1–3 kg/tonne
- removal of PAH from oil; i.e for coconut oil dosage can be 5 kg/tonne or higher. A particularly useful feature of superior grades of activated carbon is their capacity to adsorb polycyclic aromatic hydrocarbons (PAHs) from oil; activated earths are not similarly effective. Noteworthy is that while steam deodorization is capable of stripping out a substantial proportion of the so-called lighter PAHs (i.e., those whose molecules hold three or four rings), the heavy PAHs of five, six, and seven rings in the molecule persist throughout deodorization. It is, therefore, necessary to remove these heavy PAHs, some of which are known to be active carcinogens, by adsorbing them on activated carbon of established efficacy as part of the bleaching procedure, normally prior to deodorization.
- removal of traces of mineral-oil contamination; which would give rise to the appearance of bloom on vegetable oil and fat products, a carbon treatment was found to be most effective.
- removal of colour from coconut and Palm Kernel oils, high-class tallows, and lard
It is also an efficient adsorber of soap traces, and is free from the criticism that was at one time levelled against some bleaching earths—imparting a musty flavour and odour to oils treated by them. Also of practical importance is to recognize that activated carbon very readily adsorbs mucilage, gums, and so forth therefore, in any application, if one is relying on carbon to exert a final and vigorous removal of colour or odour, the oil should already have been degummed.
Apart from its higher cost compared to bleaching earth, the main disadvantages of activated carbon are its higher oil retention (it retains at least its own weight of fats, whereas a neutral earth may retain only 30% of its own weight and activated clay will hold about 70%) and poor filtration characteristics. To overcome the latter, activated carbon is mostly filtered together with bleaching earth.
Use of Silica in Bleaching
The removal from the crude oil of material which would compete with pigment for space on the surface of bleaching clay or carbon is most beneficial and often more important than the removal of pigment itself. With the correct form of amorphous silica, an agent was developed during the 1980s which is eminently effective in adsorbing soaps, phospholipids, and iron. The preliminary use of such silica clears the way for a greatly enhanced performance by activated bleaching clays, which then not only take up any remaining traces of these unwanted impurities, but also decompose peroxides and adsorb products of secondary oxidation, thus leading to higher Rancimat-stability test results. The simple addition of special silica just minutes prior to clay, then finally filtering both together, is referred to as dual addition. The continuous addition of silica to the continuous oil stream, followed by the passage of the silica-bearing oil first through a vacuum bleacher then through a filter already precoated with activated clay, gives the clay a still better opportunity to exert its maximal effect.
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- Fats and Oils: Formulating and Processing for Application (3rd Edition), CRC Press (2009)
- Introduction to Oil & Fats Technology, AOCS Press (2000)
- Green Vegetable Oil Processing, AOCS Press (2012)
- Edible Oil Processing (2nd Edition), AOCS Press (2013)
- Physical Properties of Lipid, Marcel Dekker (2002)
- Food Lipids (2nd Edition , Marcel Dekker (2002)
- The Lipid Handbook (3rd Edition), CRC Press (2007)
- Bleaching and Purifying Fats and Oils (2nd Edition), AOCS Press (2009)
- The AOCS Lipid Library : Enzymatic Degumming
- The AOCS Lipid Library : Wet Degumming