DEVELOPMENT OF TERMOCHEMICAL CARBON ADSORBENTS BASED ON FRUIT SEEDS AND APPLICATION IN SORPTION OF RARE METALS

ABSTRACT

Introduction Currently, the demand for the production of selective adsorbents is increasing due to the rapid development of industrial sectors of the national economy in our Republic and the world and the expansion of the fields of application of activated carbon sorbents. One of the urgent problems is the creation of effective new technologies that replace imports based on local products. It is necessary to develop an energy-efficient technology for obtaining selective carbon adsorbents. Based on this, obtaining cheap activated carbon that replaces imports is one of the urgent problems.

АННОТАЦИЯ

В настоящее время потребность в производстве селективных сорбентов возрастает в связи с бурным развитием промышленных отраслей народного хозяйства нашей Республики и мира и расширением областей применения активированных угольных сорбентов. Одной из актуальных проблем является создание эффективных новых технологий, замещающих импорт, на основе местной продукции. Необходима разработка энергоэфективной технологии получения селективных углеродных адсорбентов. Исходя из этого, получения дешевого активированного угля, замещающего импорт, является одной из актуальных проблем.

Keywords: adsorbent, activated carbon, organic substances, composition, structure, energy-efficient.

Ключевые слова: адсорбент, активированный уголь, органические вещества, композицион, структура, энерго-эфективной.

Several methods are used to increase the adsorption properties of coal adsorbents. These include thermal, steam, and chemical activation methods. [1].

In our country, mainly on the adsorption of organic substances on natural mineral compounds (bentonites) [2], and also on the adsorption of organic and inorganic substances on synthetic zeolites [3, 4, 5, 6, 7.], scientific researches have been carried out. The change in the adsorption volume due to the temperature difference of thermal treatment of carbon adsorbents obtained on the basis of fruit grains has not been fully studied. For this purpose, it is necessary to conduct a fundamental study of the adsorption of vapors of organic substances on activated carbon adsorbents with the help of thermal and water vapor at different temperatures.

The study of the change in the adsorption volume due to the difference in thermal treatment temperature of carbon adsorbents, including fruit grain-based coal adsorbents, is of great importance in adsorption processes. For this purpose, charcoal adsorbents were obtained by activating thermal (400-500^oC) and water vapor (800-850^oC) for 3 hours on the basis of the waste of walnut, apricot, peach and bitter almond fruit grains of fruit trees growing in the territory of our Republic. Adsorption of the obtained adsorbents with benzene vapors was studied.

Benzene obtained as adsorbate was purified and dried under vacuum conditions before use in sorption, its vapor pressure was first frozen and then heated until its vapor pressure was the same as the vapor pressure data given in the tables for pure benzene.

This can be seen from the adsorption isotherms of activated carbon adsorbents with benzene vapors at AU-O'K-74.5 at 800-850oC (Fig. 1) in a 10% aqueous solution of thermochemical ZnCl₂ based on fruit grain waste. -800-850, AU-O'K- The adsorption of benzene was studied on adsorbents 136-800-850 and AU-JU-136-800-850, activated carbon adsorbent of the foreign brand JACOBI, and the other two adsorbents (AU-JU-136- 800-850) shows a lack of adsorption in the initial. states.

Figure 1. Adsorption isotherms of carbon adsorbents obtained as a result of thermochemical activation at a temperature of 800-850^oC with benzene vapor based on waste of fruit grains

Specific surface area (S) of the coals of fruit grain waste, thermochemically activated at 800-850^oC: AU-OK-136 1397.11 m²/g, AU-YoK-74.5 1294.33 m²/g, AU-JU-136 985.09 m2/g, as well as saturation volumes (V_s): AU-OK-136 0.2514057 sm³/g, AU-YoK-74.5 0.2059116 sm³/g, AU-JU-136 0.1962319 sm³/g was found to be equal.

Adsorption of coal adsorbents activated thermally, i.e. with water vapor at 800-850^oC, with benzene vapors was studied (Fig. 2).

Figure 2. Adsorption isotherms of carbon adsorbents thermally activated at 800-850^oC based on waste of fruit grains with benzene vapor: (1) AU-OK, (2) AU-NO (3) AU-JU

Compared to the results of coal adsorbents activated thermochemically at 800-850^oC from the adsorption isotherms in the studied systems, the amount of benzene adsorption on carbon adsorbents activated by thermal water vapor at 800-850^oC: 1.28 times in AU-YoK-74.5, 1.28 times in AU-O'K-136 It was found to be 1.3 times lower than 1.26 times in AU-JU-136. The above-mentioned increase in adsorption properties indicates that changes have occurred in the structural structure of thermally and thermochemically activated carbon adsorbents at the same temperatures (800-850^oC).

Adsorption of 10% solution of thermochemical H₂SO₄ at 800-850^oC with benzene vapors of carbon adsorbents activated by fruit grain waste was studied (Fig. 3).

Figure 3. Adsorption isotherms of carbon adsorbents with benzene vapor obtained by steam activation of 10% solution of thermochemical H₂SO₄ at 800-850^oC on the basis of waste of fruit grains

Compared to the results of thermochemically activated coal adsorbents at 800-850^oC from the adsorption isotherms in the studied systems, the amount of benzene adsorption on carbon adsorbents activated in a 10% solution of H₂SO₄: 1.32 times in AU-YoK-74.5, 1.25 times in AU-O'K-136 times, it was found to be 1.36 times less in AU-JU-136. The above-mentioned increase in adsorption properties indicates that changes have occurred in the structural structure of thermally and thermochemically activated carbon adsorbents at the same temperatures (800-850^oC).

In order to study the composition and structure of coal adsorbents obtained on the basis of fruit grain waste, the elemental composition and internal structure are shown in the electron microscope using a scanning electron microscope.

Figure 4. Analysis (SEM) of AU-JU-136 activated carbon sample

It can be seen from the adsorption isotherms in the mentioned systems that the amount of adsorption increases sharply from the zero value of relative specific pressure to R/Rs≈0.4, then the adsorption slowly increases and approaches the state of saturation. The sharp appearance of adsorption isotherms at such a low relative pressure (R/Rs≈0.4) is the basis for saying that benzene vapors are adsorbed on surfaces with a high adsorption potential in the initial fillings.

At low relative (P/Ps=0.1-0.2) pressures in the studied systems, it can be seen that the adsorption isotherms are steep due to the large absorption of benzene vapors. Adsorption isotherms of these samples with benzene vapor were found to belong to type I of the classification of adsorption isotherms proposed by Brunauer. Adsorbents that produce type I isotherms are microporous adsorbents. This type of isotherms is characterized by the fact that they form an almost right angle to the P/Ps=1 axis as a result of their sharp rise.

The specific surface area (S) of the adsorbents was determined from the structural adsorption parameters using the equation of Brunauer, Emmet, Teller (BET) theory. If the ordinate is R/Rs/a(1 - R/Rs) and the values of R/Rs are placed on the abscissa axis, straight line coordinates are obtained.

The specific surface of adsorbents was calculated using the following formula:

                                                                          (3.1)

Here S-relative specific surface area (m2/g);

am-monomolecular layer (mol/kg);

NA-Avagadro number;

ω- surface occupied by one molecule (nm2)

Based on isotherms of adsorption of benzene vapors on coal adsorbents, monolayer capacity am, saturation volume Vs (or adsorption as) and their relative surfaces S were calculated from the important indicators of adsorbents. The obtained results are presented in Table 1.

Table 1.

Structure - sorption indicators of benzene vapor adsorption of fruit grains activated with the help of thermal and water vapor

It was found that the specific surface area (S) and saturation volume (a_s) of coal adsorbents increase during thermochemical activation when the temperature of thermochemical and water vapor activation is the same in all studied adsorbents. In the case of fruit seed pod charcoal, the specific surface area of ​​the thermochemically activated adsorbent is much larger than that of the adsorbent activated by steam, as shown in the table below (Table 1) [8]. Activation under these conditions leads to the opening of additional pores in the coal adsorbent layers due to the release of various gases and tars in the coal. Based on the isotherms of benzene vapor adsorption on coal adsorbents and the equation of micropore volumetric saturation theory (MHTN), adsorbent micropores (W₀), adsorption volumes (V_s) and mesopores for saturated states

W_me= V_s- W₀

determined using the formula.

The ability of adsorbents activated by heat and steam at high temperatures to absorb benzene molecules is related to the size, structure and potential of the pores in coal, and on the other hand, it is related to the specificity of the interaction of non-polar benzene molecules with activated carbon adsorbents. Due to the fact that the composition, structure, nature of the obtained adsorbents are almost similar to each other, the amount of benzene vapor adsorption does not differ much.

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