Prokof’ev V.Yu., Rot R.I., Klyueva A.V., Gordina N.E.

Ivanovo State University of Chemical and Technology, Russia

Three-Component Systems Based Alumina for Absorption of the Hydrochloric Acid Vapor

Chemisorbents based on compounds of aluminum, calcium, magnesium, and zinc have found wide application for fine purification of technological gases to remove the hydrochloric acid vapor. As a rule, these sorbents have the form of a complex-phase formulation of oxides, hydroxides, carbonates, and aluminates. These systems are produced by hydrothermal or sol-gel syntheses. The disadvantages of these methods are the comparative complexity of the processes and the large amount of the liquid phase. A promising way to obtain multicomponent formulations is by “soft” mechanochemical synthesis.

To prepare chemisorbents, we used the following materials: Al(OH)₃ (gibbsite), CaO, Zn₄CO₃(OH)₆ (basic zinc carbonate), ZnO, and MgO. We prepared the following formulations: ZnO–CaO–Al(OH)₃, Zn₄CO₃(OH)₆–CaO–Al(OH)₃, and MgO–CaO–Al(OH)₃ (in all cases, the molar ratio ZnO(MgO):CaO:Al₂O₃ = 1:1:1). Dry mixtures were subjected to a mechanochemical treatment (MT) in a VM-4 rollerring vibration mill. The MT duration was 10 min. After the MT, water was added to the mixture to the optimal molding moisture content, monitored by the method described in. The resulting homogeneous paste was molded in a piston extruder into grains 8 mm in diameter.

Preliminary studies of the granulated formulations prepared from the starting materials demonstrated their rather good sorption capacity for the hydrochloric acid vapor (Table). At the same, time the mechanical strength of the grains was unsatisfactorily low: the grains completely disintegrated when kept in the desiccator. This circumstance requires a preliminary MT of the ingredients.

The X-ray analysis data demonstrate that a new phase (Ca(Zn₂(OH)₆) – double calcium-zinc oxide) is only formed in the ZnO–CaO–Al(OH)₃ formulation [1]. In the rest of the mixtures, the intensity of the reflections of basic zinc carbonate and calcium and magnesium oxides decreases. At the same time, the intensity of the gibbsite reflections, by contrast, grows upon the MT in all cases. The IR spectroscopic data are in agreement with the results of X-ray diffraction studies.

Table Characterization of the sorbents

No	Formulation	MT time, min	Mechanical strength, MPa	Parameters of a sorbents on its being kept over an HCl solution (РHCl = 4.4·10^–5 mm Hg)
				Mechanical strength, MPa	Cl^– content, mg·g^–1
1a	ZnO/ CaO/Al(OH)3	—	< 0.5	—	0.30 ± 0.02
1b		10	3.7 ± 0.3	1.8 ± 0.2	0.21 ± 0.01
2a	Zn4CO3(OH)6/CaO/ Al(OH)3	—	< 0.5	—	0.50 ± 0.02
2b		10	2.3 ± 0.2	2.2 ± 0.3	0.34 ± 0.01
3a	MgO/CaO/Al(OH)3	—	< 0.5	—	0.10 ± 0.01
3b		10	3.5 ± 0.3	1.8 ± 0.2	0.04 ± 0.00

Tests of the systems for the absorption of the hydrochloric acid vapor demonstrated (Table 1) that the formulation containing basic zinc carbonates has the maximum sorption capacity and that with MgO, the minimum capacity. The X-ray phase analysis failed to reveal the formation of new crystalline phases because of their small amount, but a halo appears in the X-ray diffraction patterns at medium diffraction angles, which shows that new X-ray-amorphous structures appear. The decrease in the absorption capacity for the HCl vapor upon the MT, compared with the starting formulations, is in all probability due to the full disintegration of these grains to a powder-like state in the course of keeping in the desiccator. The state of this kind makes it possible to raise the surface area accessible to gases.

A study of the acid-base properties of the surface of the sorbents demonstrated that, upon the MT, the amount of basic centers in the formulations with basic zinc carbonate and magnesium oxide decreases by approximately 25%. In all the formulations subjected to MT, the surface basicity is the same (a similar leveling of the acid-base properties upon treatment in mills has also been observed for other systems [2, 3]). Consequently, the absorption capacity is determined in the case under consideration by the chemical nature of the chemisorbent and, in particular, presence of a carbonate group, rather than by the number of basic centers.

Upon keeping in water and acid vapors, the initial mechanical strength is retained only by the Zn₄CO₃(OH)₆–CaO–Al(OH)₃ formulation. In the case of zinc and magnesium oxides, the grain strength decreases by more than a factor of 2. This strength degradation is due to the structural transformation of the grains in absorption of water vapor by the oxides.

Reference:

1. Prokof’ev V.Yu. at al. // Rus. J. Appl. Chem. 2013. 86 (7). pp. 1022-1028. DOI: 10.1134/S1070427213070148

2. Prokof’ev V.Yu. at al. // Inorganic Materials. 2000. 36 (9). pp. 899-903. DOI: 10.1007/BF02758701

3. Prokof’ev V.Yu. at al. // Glass and Ceramics. 2009. 66 (3-4). pp. 147-150. DOI: 10.1007/s10717-009-9132-8