Decolorization of edible oils is part of the oil refining process, which removes impurities that adversely affect the appearance and properties of triglycerides (triacylglycerol compounds). Generally, the degumming and alkali refining (neutralization) process is first carried out. The decolorization process removes the specific harmful impurities that cannot be removed in the previous process. The decolorization process is a process in which the adsorbent and the grease are mixed and filtered. The amount and characteristics of the adsorbent are related to success or failure as well as the efficiency of the decolorization operation.
I. Activated clay
Activated clay is a highly active adsorbent processed by bentonite as a raw material, and it is most widely used in the decolorization of the oil industry. Activated clay has strong adsorption capacity for pigments in oils, especially chlorophyll and other colloidal impurities, and has stronger adsorption capacity for basic atomic groups and polar atomic groups. After the oil is decolorized by the clay, a little bit of earthy smell remains, which can be removed during the deodorization process.
II. The composition
The components of the activated clay are mainly silica (SiO2), aluminum oxide (Al2O3), ferric oxide (Fe2O3) and other compositions.
III. Performance
The main properties and usage of activated clay are:
1. It has wide decolorization adsorption and detergency. It has good adsorption and decolorization effect in the application of oil decolorization process, and has strong adsorption to pigments and impurities. The decolorization rate of various pigments in vegetable oil reaches 85~ 95%.
2. It has a specific adsorption, can remove all kinds of oil dark pigment and impurities, such as chlorophyll, lutein and carotene, the color of the oil after decolorization and purification is light, clear, shiny, and has good stability and low acid value. .
3. It has the ability to adsorb and remove aflatoxin and benzopyrene in oil.
4. As a processing aid, the activated clay is completely separated from the oil after strict adsorption by adsorbing the above-mentioned pigments, impurities and harmful substances. This filtering technology and management methods are mature technologies in China.
5. The shelf life of activated clay is generally 12 months. Although the expired activated clay does not affect the quality of the oil, it will adsorb a certain amount of moisture in the air, which will reduce the decolorizing force and increase the loss of oil, resulting in an increase in production and processing costs.
IV. The production process
The active clay processing and production methods using bentonite as raw materials mainly include all-wet method, full-dry method and semi-wet method.
The conventional all-wet method is a method in which bentonite, sulfuric acid, and water are mixed and heated to 100 ° C for a period of time after being stirred, washed, dried, and crushed to obtain the finished product. The biggest disadvantage of this method is that it consumes a large amount of acid, which results in a large amount of washing water, serious environmental pollution, and high cost.
The traditional all-dry process is to mix a small amount of sulfuric acid and water with bentonite, stir well, leave it for a period of time, then dry and crush to obtain the finished product. Although this method reduces the amount of acid used and does not produce environmental pollution such as waste water discharge, its product quality is low, which greatly limits its use.
The semi-wet production process is to mix a small amount of sulfuric acid and water with bentonite, stir well, activate (place) for a certain period of time at a certain temperature, and then add acid and water for the second step of activation. The semi-wet production process solves the problems of the full wet method and the full dry method, and has the advantages of simple process, low cost and high product quality.
In general, the semi-wet process reduces the acid consumption by half to two-thirds (1/2 to 2/3) compared to the conventional process, and the water consumption is also reduced by 1/2 to 2/3. The treatment does not produce environmental pollution, and the cost is low. Under the same circumstances, the product quality is 30% higher than the traditional method.
V. Application in the decolorization of oil refining
Activated clay has a strong selective adsorption effect on pigments, and absorbs pigments and other impurities in oils under certain conditions to achieve the purpose of decolorization. The oil treated by the adsorbent not only achieves the purpose of improving oil color and removing gum, but also effectively removes some trace metal ions in the oil and some substances which can cause poisoning of the hydrogenation catalyst, thereby providing good conditions for further purification of the oil.
Activated clay has large specific surface area, strong adsorption capacity, low activity, and does not chemically interact with oils and other chemicals. It has become the most widely used edible oil decolorizer. The amount of clay added in the process of decolorization is based on the color of oil and fat. The requirements for the standard of refined oil are determined. For example, the internal control standard for small package oil is higher than the standard for bulk color of externally sold oil. The amount of clay used in the process of decolorization is also slightly higher. However, the addition of too much white soil will also reduce the yield of refined oil, because the clay will adsorb some oil while adsorbing the pigment, which increases the refining and cost. Generally, the oil content of waste white clay in the refinery is 20%-25%. At present, a few companies use the extraction method to separate the oil from the activated waste clay, but the color of the oil after separation is black due to the action of clay, and it is impossible to decolorize the oil to good color. Therefore, domestic oil refining manufacturers regard the oil content of waste clay as an important indicator for the selection of activated clay manufacturers.
VI. Acceptance criteria of bleaching clay used for vegetable oil refining
Color: white or gray, light pink
Appearance: powdered solid
Decolorization rate: ≥90%
Particle size (over 200 mesh screen, %): ≥90%
Moisture content: ≤10%
Free acid (% by H2SO4): ≤0.3
The national standard GB25571-2011 specifies the physical and chemical indicators of activated clay as follows:
VII. Other decolorizing agents for decolorization of vegetable oil
China's bentonite is rich in resources and is the world's largest producer of bentonite. China's annual consumption of bentonite is about 5 million tons, of which 40% is used in the iron ore pellet market, 30% is used in the foundry market, and activated clay accounts for 10%, mainly used for the refining of edible oils and fats. It is mainly used in the petroleum industry for the refining, decolorization and purification of petroleum, oil paraffin, wax oil, kerosene and other minerals. It is also used in food and pharmaceutical industries.
The annual demand for activated white clay for oil refining is about 200,000 tons. In 2016, the consumption of edible oil and fat in China has reached a record 30 million tons, and the demand for decolorizing agents has increased year by year. However, there are many manufacturers producing activated clay, and the quality is uneven. In addition, since the price of activated clay is higher than that of attapulgite, the application of activated clay in the oil refining industry is not only fierce in the industry, but also always faces attapulgite and other challenge of bleaching materials.
1) Activated carbon
Activated carbon is formed by carbonization of wood chips, bagasse, chaff, and hard nut shells, and then chemically or physically activated. Its main component - carbon content reaches up to 90 ~ 98%, density is 1.9 ~ 2.1 tons / M3, bulk density is 0.08 ~ 0.45 tons / M3, it has loose pores, large specific surface area, high decolorization coefficient, and hydrophobic; it is capable of adsorbing high-molecular substances, and is particularly effective for the removal of blue and green pigments. It also removes the trace oil from the mineral oil. In addition, it has strong adsorption capacity for gases, polycyclic aromatic hydrocarbons and pesticide residues. Due to its high price and high oil absorption rate, it is often used together with bleaching soil or activated clay in the decolorization operation of oil, and the mixing ratio is usually 1:10-20. Mixed use can significantly improve the decolorization ability, and can remove the smell of floating soil.
2) Attapulgite
The attapulgite is a magnesium-rich fibrous mineral and its main component is silica, which is fine and has a blue-gray or gray-white appearance. The farmers in the place of origin used it as a feed additive and found no animal lesions. The attapulgite has a good decolorization effect, and compared with the activated clay, the amount of decolorization is small, the oil loss is small, and the price is low. The problem is that filtration is more difficult, and the fineness of the soil should be appropriately reduced.
3) Natural bleaching clay
Natural bleaching clay, scientific name of which is bentonite. Its main component is (Al4Si8O20(OH)4•nH2O), and a small amount of Ca, Mg, Fe, Na, K and other components are mixed. The suspension has a pH of 5-6 and is acidic, so it is also called acid clay. Natural bleaching clay is processed from mining to final grinding and is only physically treated. Its structure is microporous crystal or amorphous, its specific surface area is much larger than other clays, and it has certain activity, but its decolorization coefficient is low (refers to the ratio of oil column height before and after decolorization of the same batch of oil). The oil absorption rate is also high, so it is gradually replaced by activated clay.
4) Zeolite
Zeolite belongs to the multi-cycle of acidic volcanic lava and clastic sedimentary interlayer, the sedimentation of multi-mineral lake basin, and the melting or hydrolysis of multi-line volcanic glass to form the clinoptilolite deposit. After mining, screening, milling and sieving, it is obtained. Zeolite adsorbent. Its chemical composition is mainly silica, followed by alumina. Zeolite has a good decolorizing effect. It can also reduce the acid value and moisture of oil when it is decolored, and the price is cheaper than that of activated clay. It is a new material for decolorization of oil.
5) Diatomaceous earth
Diatomaceous earth is formed by the single-cell potassium silicate shell remains under the action of natural forces. The diatomaceous earth with good purity is white, generally light gray or reddish brown. The main chemical component is silica, which has a certain adsorption capacity for pigments, but the decolorization coefficient is low and the oil absorption rate is high. It is often used as a filter aid in the production of oils and fats.
6) Other bleaching agents
Adsorbents used for oil decolorization also include activated alumina and alumina treated with sulfurous acid.
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