STUDYING ADSORPTION CHARACTERISTICS OF “NAVBAHOR” ALKALINE BENTONITE IN BLEACHING COTTON OIL

ABSTRACT

In this scientific work, it was studied the possibility of obtaining adsorbents that can be tested and used in the purification of vegetable oils from bentonite clay samples of some prospective mines in the republic. The chemical composition of Navbahor alkaline and alkaline earth bentonites was studied and compared with other clays available in the CIS countries. As a result of determining the high water consumption of Navbahor alkaline earth bentonite, it was concluded that it can only be activated with acid and that termal activation is sufficient for alkaline bentonite. Alkaline and alkaline earth bentonites were analyzed differential termically, and it was determined that alkaline bentonite has a huge amount of adsorbtion water. Experimental research indicated that the ideal temperature for activating alkaline bentonite is between 190 and 200 ℃, and the ideal activation time is two hours. It has been determined that the optimal conditions for bleaching cotton oil with a color of 11 red units using activated bentonite obtained in this manner are an oil temperature of 85 ℃ and a process time of 30 minutes.

АННОТАЦИЯ

В данной научной работе изучена возможность получения адсорбентов, позволяющих испытать и использовать образцы бентонитовой глины некоторых перспективных месторождений республики при очистке растительных масел. В исследованиях изучены химический состав щелочного и щелочноземельных глин Навбахорского месторождения и сопоставлены химическим составом глин других месторождений Государств СНГ. В результате определили, что щелочноземельная глина Навбахорского месторождения имеет высокую набухаемость в воде и пригодно только для кислотной активации, а для щелочной глины достаточно термическая активация в сравнительно низких температурах обработки. Путем дифференциально термического анализа шелочного и щелочноземельных бентонитов в составе щелочного бентонита определили достаточно большое содержание адсорбционной воды. Экспериментальным способом определена подходящая температура активации, которая составляет 190-210℃ при продолжительности обработки 2 часов. Оптимальными режимами отбелки хлопкового масла с цветностью 11 красных единиц, полученным вышеуказанным способом активированным бентонитом, являются технологические режимы – неизменная температура 85 ℃ в течение 30 мин. 

Ключевые слова: глина, бентонит, адсорбент, отбеливание, физико-химический состав, активация, цветность, адсорбционные свойства.

Keywords: clay, bentonite, adsorbent, bleaching, physicochemical composition, activation, color number, adsorption property.

Introduction. Factories are re-equipped, production of import-substituting and export-oriented products is increasing. Significantly, small businesses and private entrepreneurs now operate in an environment that is conducive to investment and has the necessary legal and regulatory support. This benefits the quick growth of the export potential. The need to develop import-substituting, export-focused, and locally focused production capacities, as well as the further acceleration of technical and technological re-equipment, are all issues that our nation faces on both an internal and external level. It is mentioned that it enables a stable position in the market.

Bentonite clay is currently one of the most significant commodities traded globally. This isn't because bentonite clay is scarce in some nations, but rather because of their quality. Each of the bentonite clay mines, or sometimes each section or layer of the mine, has special features that vary from one another in terms of dispersion, binding capacity, exchange activity, and other characteristics. The amount of each component in the clay as well as the types and proportions of exchangeable bases all play a role in this difference. Although this process has been used for so long, it is sometimes hard to produce a product that meets industry standards despite working on them. The needs for bentonite clays' quality change along with the fields in which they are used. As a result, developing new bentonite clay deposits requires doing technological and scientific research.

Literature analysis. It is known that pigments, which are formed in seeds either during the growth phase or during oil extraction, are related to the color of oils. Their quantity is not permanent and is directly influenced by the kind of oily raw materials used and technological advancements, or processing techniques. Carotenoids in palm oil have a maximum content of 25-40 mg/kg, and rapeseed oil has a maximum chlorophyll content of 5-20 mg/kg [1, 2].

Many vegetable oils contain carotenoids and xanthophylls, which melt well in the same solvents as them. Solvents, for instance, dissolve them along with the oil during the extraction process and make them pass into the oil's composition. As a result, the oil that is extracted has more carotenoids than forpressed oil.

Since most carotenoids are highly resistant to alkalis, neutralized oils are used to preserve them. By neutralizing carotenoids in potent alkaline solutions, they can be absorbed on the soapstock surface. Surfactant centers that form on the surface of solid adsorbents can also be used to carry out this process [2, 3].

In palm, flax, cotton, soybean, sunflower, and corn oils, you can find carotenoids, which are provitamins. It has been discovered that soybean oil contains xanthophylls, particularly lutein. As a result of carotenoids' high alkali resistance, neutralized oils frequently contain them as well. This property serves as the foundation for oil purification technology [3, 4]. Carotenoids are actively adsorbed on the surface of solid adsorbents. Chlorophyll gives vegetable oils their green hue and easily dissolves in oils and oil-based solvents.

Due to inadequate cleaning of seeds before they are turned into oils, chlorophyll occasionally passes through organic impurities. Depending on their ratios, the yellow (orange) color of the carotenoids in a mixture, the green color of chlorophyll, and the color of oils are all determined. [3, 4]. Unlike carotenoids, chlorophylls interact with alkali; therefore, chlorophyll salts are formed, but alkaline treatment is unable to completely remove chlorophylls from oil. [4, 5]. The color of unrefined cottonseed oil is typically dark red to dark brown. Gossypol serves as the primary colorant in cottonseed oil.

During oil refining (cleaning), a sequence of different methods is used according to the composition of substances to be removed: phospholipids during hydration, free fatty acids during chemical neutralization, pigments during adsorption purification, waxy substances during winterization, and odorants during deodorization; substances are expelled [2, 6, 7].

Coloring substances are removed from oil by treatment with various adsorbents at the stage of adsorption purification.

Bentonite bleaching soils mainly consist of pure silicic acid (infusorite) or aluminosilicates containing varying amounts of silicic acid and calcium, magnesium and iron oxide alumina with a small amount of water. [8, 9].

Adsorption of coloring substances contained in vegetable oils takes place in the process of chemosorption, and hydrogen bonds are of great importance in this. The hydrogen bond is formed not only by the aluminosilicate structure but also by substances absorbed on the surface of the adsorbent, primarily water. Therefore, the humidity of the adsorbent is of great importance [10, 11].

 In order to provide the required technological properties, for example, high activity, high adsorption capacity, and directed selectivity, natural adsorbents are modified. Types of thermal, mechanical, and chemical activation for the modification of natural adsorbents are used [8, 12, and 17].

In the literature, the properties of the reactions corresponding to the laws of the Langmuir, Freundlich, and Van der Waals equations in the purification of vegetable oils with adsorbents have also been studied [18–20].

Adsorption refining is highly effective when it is performed after hydration, neutralization, washing, and drying of oils [2–7, 22–24].

Bentonite clay is used as an adsorbent in the oil industry. Among the clay substances, "bentonites" play an important role due to their valuable properties and, therefore, their wide use in many sectors of the national economy.

Pigments are one of the additional substances that need to be separated from vegetable oils at the stage of adsorption purification. The nature and structure of coloring substances in vegetable oils are different, but they all have a certain degree of polarization; therefore, polar adsorbents with sufficient selectivity and activity are usually used for adsorptive purification of oils.

For this, special adsorbents are used; traditionally, bleaching earths obtained from natural bentonite soils (aluminosilicates), activated with mineral acids, and a small amount of activated carbon were used [2, 23].

Based on the results of the previous physical and chemical analysis, it was concluded that the alkaline bentonite of the Navbahor mine contains a large amount of sodium and, accordingly, a large amount of adsorbed bound water. Based on its dispersity and high colloidity, its adsorption properties, it was concluded that they can be increased as a result of thermal activation [25].

Research methods. The methods of physico-chemical analysis recommended by VNIIJ and presented in O’zDSt 816:2015 were used for conducting experiments [26].

Oil color was measured on a Lovibond tintometer F series with dark oil in a 1 cm thick cuvette at a constant 70 yellow units. The color of pale oils was measured in a 13.5 cm-thick cuvette at a constant 35 yellow units and 35 red units.

The acid number of oil according to GOST 52110-2003 [27], the residual amount of soap according to GOST 5480-99 [28], the anisidine number according to GOST 31756-2012 [29], and the peroxide number according to GOST 51487-99 [30] are determined.

The filtration capacity of bleaching soils was determined depending on the level of filtration of specially mixed oil per unit of time.

To measure the bleaching ability of bleaching soil, 100 g of oil was poured into a 200 cm³ laboratory beaker and mixed with 1–3 g of bleaching soil samples while stirring in a MM-5 heating mixer and heated at 85 ℃ for 30 minutes. acquitted in the case. Then, after stopping the stirrer, the filter was poured into a funnel lined with filter paper and filtered in a drying cabinet at a constant temperature of 45 ℃.

To measure the oil capacity of the bleaching soil, the residue left on the filter paper was sprayed in an air stream for 10 minutes. The oil capacity of the bleaching soil was determined by calculating the difference between the weights of the oil-saturated and pre-weighed filter paper and the combined weight of the filter paper and the oil-saturated bleaching soil using the following formula:

X = [P ₁ -(P ₂ +P)] × 100/P+[P ₁– (P ₂ +P)]

Here: P₁– combined weight of the funnel with oil-saturated filter paper and oil-saturated bleaching earth, g;

P₂– the weight of the funnel together with oil-saturated paper, g;

P– amount of bleaching earth taken for mixing with oil, g.

The following formula was used to determine the bleaching ability of bleaching soil:

A = [(C₁– C ₂) × 100] / C₁

Here: A-activity of bleaching soil, %;

C₁– starting oil color, red unit;

C₂– the color of refined oil, red unit.

Results and their discussion Laboratory experiments on obtaining bleaching soil used for bleaching vegetable oils on the basis of Navbahor alkaline bentonite clay were carried out at Urganch State University and Tashkent Institute of Chemical Technology.

In our experiments, the composition of raw materials available in local mines, including the composition of alkaline and alkaline earth clays from the Navbahor mine, was analyzed, and the possibilities of obtaining activated clays for use as bleaching earth in the oil industry were studied.

The natural chemical parameters of Navbahor alkaline and alkaline earth bentonites used in our research are presented in Table 1.

Table 1.

Average indicators of alkaline and alkaline earth bentonites of the Navbahor mine

As can be seen from Table 1, Navbahor alkaline bentonites are distinguished from other bentonites by their high sodium content, and, accordingly, the colloidal and swelling properties (nabukhaemost) of these bentonites are high compared to other bentonites. According to the chemical analysis, Navbahor alkaline bentonite contains sodium, aluminum, and calcination losses, compared to alkaline earth bentonite, which is much higher. The content of calcium, magnesium, and silicon is lower.

 Table 2 shows the results of comparing the exchangeable complex composition and colloid properties of Navbahor alkaline and alkaline earth bentonites with clays obtained from the main bentonite deposits in the CIS countries.

Table 2.

Composition and colloid of exchangeable cations of ordinary “Navbahor” alkaline and alkaline earth bentonites (in comparison with the main bentonite deposits in the CIS countries)

As can be seen from Table 2, Navbahor alkaline and alkaline earth bentonites belong to the group of clays with high exchangeable cations. However, the colloid of Navbahor alkaline bentonite is much higher than the colloid of alkaline earth bentonite, although it is slightly less than that of Oglanli and Askangel bentonites, which have a very high colloid. In addition, the swelling of Navbahor alkaline bentonite differs sharply from the swelling of alkaline earth bentonite.

Navbahor alkaline bentonite swells when mixed with water to form a viscous, mobile slurry. When mixed tightly, it becomes fluid (thixotropy). Alkaline earth bentonite does not exhibit thixotropic properties when mixed with water, but its coarsely dispersed suspension quickly separates into layers and sinks.

In order to determine the mineralogical composition of alkaline and alkaline earth bentonites from the Navbahor mine, X-ray analysis was carried out (Fig. 1). Accordingly, alkaline and alkaline earth bentonites consist of minerals such as montmorillonite, quartz, calcite, dolomite, limonite, illite, and kaolinite. Alkaline bentonite contains up to 70% montmorillonite mineral. In the alkaline earth bentonite sample, the amount of montmorillonite mineral is relatively low, and the amount of remaining quartz, calcite, and dolomite, as well as illite and kaolinite, is somewhat higher.

Figure 1. Diffractogram of alkaline (a) and alkaline earth (b) bentonites of the Navbahor mine

In order to determine how the alkaline and alkaline earth bentonites of the Navbahor mine change under the influence of temperature, differential thermal analysis of their samples was carried out (Fig. 2). According to it, it was determined that Navbahor alkaline bentonite has a large amount of adsorbed bound water compared to alkaline earth bentonite, and the formation of the first endoeffect in alkaline earth bentonite at a relatively low temperature is due to the abundance of calcium and magnesium cations in its exchange complex.

Figure 2. Differential thermal analysis of alkaline (a) and alkaline earth (b) bentonites of the Navbahor mine

Based on the results of the complex physico-chemical analysis above, it can be concluded that the alkaline bentonite of the Navbahor mine contains a large amount of sodium and, accordingly, a large amount of adsorbed bound water. Its high dispersity and colloid also contribute to its adsorption properties. It means that it can be increased by thermal activation, but it is difficult to activate alkaline earth bentonite in this way.

In order to determine the optimal parameters of the thermal activation of Navbahor alkaline bentonite, we measured 100 g samples on a technical scale, the dimensions of which are 2-3 cm at most, in a laboratory drying cabinet at different temperatures (between 40 ℃) and dried them for different time durations (interval of 1 hour). Then, the influence of the dispersion level of the adsorbent samples on the sorbent properties was studied in the samples sieved until the residue on the sieve no. 0056 (10000 meshes per cm²) was 20%.

The indicators of the obtained adsorbents are presented in Table 3.

Table 3.

Indicators of the adsorbent obtained as a result of thermal activation

In the study of the effect of thermal activation on the adsorption properties of the adsorbent, the results obtained directly from the cotton oil bleaching were checked in the Central Laboratory of "Urganch yog’-moy" JSC.

The whitening of cottonseed oil was carried out in a water bath at a temperature of 85 ℃ and stirring for 30 min according to the traditionally accepted method [1]. All the following experimental tests conducted to determine the adsorption capacity of treated sorbents were neutralized with caustic soda from the refinery of "Urganch yog’-moy” JSC, washed with water, and then passed through the drying stage (the initial color is different). These tests were carried out on cottonseed oil samples.

Table 4 shows the results of testing the effect of thermal activation on Navbahor alkaline bentonite in a cottonseed oil sample with an initial color of 11 red units.

Table 4.

Effect of thermal activation on the adsorption properties of Navbahor alkaline bentonite

As can be seen in Table 4, it was observed that the adsorption capacity of Navbahor alkaline bentonite significantly increased as a result of thermal treatment, and the drying temperature was 190–210 ℃ and the drying time was 2 hours as an optimal parameter. In order to create a method of activating Navbahor alkaline bentonite, further research was conducted on a sample of Navbahor alkaline bentonite thermally activated under optimal conditions with the highest degree of whitening, a drying time of 2 hours, and a drying temperature of 190–210 ℃.

This sample was prepared by grinding the residue on sieves with a dispersion level of 10,000 mesh per cm² to 0,5-50%. Taking into account that the fraction smaller than 0,001 mm has a high adsorption capacity [8], and at the same time, this fraction can increase the filtration time by 1,5 times, it is considered an acceptable indicator to grind the sample dispersion level to 20% of the residual remaining on the 10000 mesh/cm² sieve.

Figure 3. Influence of mixing time of thermally activated alkaline bentonite Navbahor on the bleaching rate of cottonseed oil

Also studied was the effect of mixing time of the adsorbent and cottonseed oil at a constant temperature (100 ℃) on the degree of bleaching of cottonseed oil with an initial color equal to 11 red units in a thermally activated sample of Navbahor alkaline bentonite under optimal conditions. The results are presented in Figure3. 

As can be seen from Figure 3, the optimum mixing time is 30 minutes. Further processing leads to partial desorption of the oil color.

Summary. It was found that the use of the adsorbent obtained by the mentioned method in an amount of no more than 1 % is effective in bleaching oils with a relatively high initial color. In this case, due to the reduction of the amount of alkali used for refining, it is possible to leave the color of the oil higher and reduce the cost of whitening vegetable oils due to the use of this adsorbent. 

In conclusion, an adsorbent that has enough adsorption properties can be produced with the help of thermal activation of Navbahor alkaline bentonite under optimal conditions. However, compared to conventional adsorbents made using acid, this adsorbent only partially purifies vegetable oils. Acid processing is a harmful process for the environment and, on the one hand, raises the price of the product. Based on these, we concluded that Navbahor alkaline bentonite can only be sufficiently activated thermally. However, after checking the degree of purification of vegetable oils in the company's laboratory and then utilizing it in the conditions and amounts that produce good results, this inexpensive and environmentally friendly adsorbent obtained only by thermal activation has a high efficiency.

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