ES2296521A1 - Clay monoliths for the treatment of contaminant gaseous effluents - Google Patents

Abstract

The clays wherefrom said monoliths are formed are of useful environmental application because of their coincident abundance, low cost, extrudability and ability to adsorb volatile organic compounds. The present invention has the objective of increasing the value of an abundant economic natural resource in the north of Morocco, which to date has solely been used for the manufacture of traditional pottery objects, through its use in environmental protection technologies. The Moroccan clays studied may be employed in controlling contaminant emissions into the atmosphere from various types of industry, both in the form of powder and of monoliths having a honeycomb structure. This design having recognised advantages may be achieved with said materials without the need for additives other than water and thus without making the process of preparation expensive.

Description

Clay monoliths for the treatment of gaseous pollutant effluents.

Mastery of the technique

The invention is applicable in development commercial filters, both in the form of sieves and monolithic, constituted by clays as protection technology environmental, mainly in the treatment of effluents contaminated soda

Prior state of the art

It is well known that natural clays are minerals consisting of small crystalline particles whose structure consists of layers of tetrahedra formed by ions silicon coordinated with four oxygen, alternated with other layers in which aluminum or magnesium ions are coordinated octahedrally with six oxygens or hydroxyl groups. During the process of forming a clay the aluminum can replace the silicon in the layers of tetrahedra, while iron, magnesium, manganese and other cations of similar size may replace the aluminum in the octahedral layers (see, for example, Grim, R.E. in Clay Mineralogy, Mc Graw Hill, New York, 1968). These aluminosilicates are very abundant in nature to be the result of multiple aging processes, mostly with the participation of water, which the constituents suffer soil primitives (see, for example, Sposito, G. in The Chemistry of Soils, Oxford University Press, New York, 1989). its abundance associated with a relatively low cost and its interesting physicochemical properties justify its traditional use in ceramics, paper, paints, plastics, as chemical support, in ion exchange membranes, in fluid discoloration, etc., and even more recently, in other more advanced applications such as catalysis (see, for example, Ciullo, P.A. in Industrial Minerals and their uses: a handbook and formulary; William Andrew Inc., New Jersey, 1996). TO in spite of all this background of experience and knowledge around the clays are still announced improvements in the techniques that are used for extraction and processing (see, for example, Murray, H. in Applied Clay Science; vol. 17, p. 207-221, 2000).

The north of Morocco is a geographical area in the abundant minerals of clay nature, which It represents a huge potential for the development of the economy local. However, it is noteworthy that until now you are clays are only being used as raw material for manufacture of traditional ceramic objects, of low value in the market and zero technological interest.

This situation contrasts with the existence in the area or in its surroundings of different industries that emit various pollutants into the air, among them Volatile Organic Compounds (VOCs), substances of very varied origin and recognized toxicity and that, therefore, have been selected in this study to check the performance of the material elaborated on a laboratory scale. The emission of VOCs from chemical and petrochemical industries has become for years, not only in Morocco but worldwide, in one of the most worrisome forms of air pollution due to the multiple harmful effects they cause both in humans as in the environment (see, for example, Heck, RM et al . in Catalytic air pollution control: Commercial technology; p. 281-305; John Wiley, New York, 2002). In many cases, these emissions come from industrial processes that use solvents, polymers and resins such as those involved in painting and coating operations (see, for example, Kiely, G. in Environmental Engineering, McGraw Hill, Berkshire, UK, 1997) . Among the very diverse technologies developed to eliminate VOCs in polluted air, adsorption is perhaps one of the most used (see, for example, Hocking, MB in Handbook of chemical technology and pollution control; Academia Press, San Diego, 1998; and Khan , FI and Ghoshal, AK in Journal of Loss prevention in the Process Industries, vol. 13, p.
527-545, 2000).

In this sense, many diverse materials such as active carbons, zeolites, aluminas, silicas and even polymers have been successfully tested (see, for example, Ruthven, DM in Principles of adsorption processes; John Wiley, New York, 1984; Gélin et al . in Microporous Materials; vol. 4, p. 283-290, 1995; Takeuchi et al . in Separations Technology; vol. 5, p. 23-24, 1995; Carsten et al . in Microporous and Mesoporous Materials; vol. 35, pp. 349-365, 2000; and Marsh, H. in Activated carbon compendium; Elsevier, Amsterdam, 2001). However, there is still a continuous effort to find adsorbents that associate efficiency with low cost, as evidenced by our own recent research in which we have compared the yield of diatomaceous earths with that of commercial silica (see, for example, Zaitan, H and Chafik, T. in Comptes Rendus Chimie; vol. 8, p. 1701-1709, 2005), or studied the possibilities as adsorbents of VOCs of both natural carbon-based monoliths activated after extrusion (see, for example, Gatica, JM et al . In Comptes Rendus Chimie; in press) as various clay minerals from Morocco in powder form (see, for example, Harti, S. et al . In Applied Clay Science; sent).

In relation to the design of these adsorbents, compared to the more conventional ones such as those based on particle beds, monolithic structures in the form of honeycomb offer proven advantages such as low pressure drop in gaseous flows with high spatial velocity, as well as lower weight and space requirements, which makes them especially attractive in environmental applications (see, for example, Heck, RM in Catalytic air pollution control: Commercial technology; p. 11-24, John Wiley, New York, 2002). However, for a paste to be extrudable, it must have very specific rheological properties that allow it to be shaped with a rigid structure such as that of a monolith (see, for example, Avila, P. et al . In Chemical Engineering Journal, vol. 109, p. 11-36, 2005). This makes the extrusion process often tedious and requires a large number of tests using various additives that improve plasticity (see, for example, Gatica, JM et al . In Carbon; vol. 42, page 3251-3254, 2004 ; and Mohino, F. et al . in Applied Clay Science; vol. 29, p. 125-136, 2005). In this sense, the use of different additives that provide binder, plasticizer, lubricant and dispersant properties to the base materials (see, for example, Gatica, JM et al . In Carbon; vol. 42, p. 3251-3254) entails an obvious economic cost both for its use and for the subsequent heat treatment that must be carried out for its elimination and the necessary reactivation of the adsorption capacity. It can also induce negative effects on the chemical-structural properties of the base material both by direct interaction and by the subsequent treatment indicated (see, for example, Velde, B. in Origin and Mineralogy of Clays: Clays and Environment; Springer, Berlin , nineteen ninety five).

With the background exposed, it is clear that it would be very interesting to get an abundant and cheap material as are the natural clays that abound in the north of Morocco could also be used as an adsorbent of compounds volatile organic, without prejudice that this application could extrapolate to the clearance of other contaminants that they are usually present in the gaseous effluents generated for any type of human activity. This would give added value. to the existing product while contributing a new material competitive compared to those already used in applications of environmental decontamination. It would also be of great interest get that from these clays filters could be obtained or monolithic supports and, even more so, that the methodology of manufacturing does not involve the use of additives to facilitate the extrusion process

Explanation of the invention.

Two clays are proposed specifically from deposits located in the Tetouan area, although they are very abundant in all the north of Morocco. Clay called FERA contains various minerals according to the X-ray diffraction analysis: quartz, muscovite, vermiculite, kaolinite and albite. For its part, the analysis of the other clay, called TEFA, it indicates that it is composed of quartz, calcite, Muscovite and Clinochlor. From the elementary point of view, the Chemical analysis using Dispersive Energy Spectroscopy (EDS) reveals that the average clay composition (% by weight) is, in the case of clay FERA, 0 (39.3%), Si (23.4%), Fe (16.3%), Al (13.2%), K (5.3%), Mg (0.8%), Ca (0.6%), C (0.4%), Ti (0.4%) and Na (0.3%); and in the case of TEFA clay: 0 (44.4%), Si (21.3%), Al (9.2%), Fe (8.9%), Ca (5.8%), C (5.1%), K (2.8%), Mg (1.9%), Ti (0.2%), Cl (0.2%) and S (0.2%).

Before any use, clays in the form of powder have been ground and sieved to an average grain size of 180 microns

In order to test its application potential in the treatment of contaminated gaseous effluents, its ability to adsorb volatile organic compounds has been studied and, in particular, o-xylene has been chosen as a model because it is one of the most common emissions in many industries from the area The tests have been carried out under dynamic conditions at atmospheric pressure in an instrumental device already described in the literature (see, for example, Zaitan, H. and Chafik, T. in Comptes Rendus Chimie; vol. 8, p. 1701-1709) . Nitrogen has been used as a carrier gas for o-xylene vapors, preparing a mixture containing 3600 ppm of contaminant by means of a saturator associated with a condenser that is immersed in a thermostatic bath to maintain the temperature at 10 ± 0.5 ° C. The total flow of the mixture during the tests was 100 cm 3 min -1. The adsorption tests have been carried out at 27 ° C in a quartz reactor with 1 gram of the clay, which has previously undergone an in situ flow treatment of 100 cm 3 min -1 of N_ {2} at 210 ° C for 30 minutes to clean its surface of previously adsorbed species. Once the sample saturation is reached, a flow of pure N2 (100 cm3 min -1) is passed through to induce thermal desorption at 27 ° C until the concentration of o-xylene at the outlet of the reactor becomes zero. A Programmed Thermal Desorption (DTP) experiment is then performed using a sample heating rate of 5 ° C min -1. The composition of the gas mixture at the outlet of the reactor has been monitored by an FTIR spectrophotometer (Jasco 410 model) recording infrared spectra with an analysis resolution of 4 cm -1, using a cell built in Pirex glass with CaF 2 windows and integrating the FTIR bands characteristic of o-xylene in the region between 2600 and 3200 cm -1. The quantitative analysis of these bands has been carried out based on a previous calibration performed with mixtures of known o-xylene concentration.

Following the numerical data treatment described in the literature (see, for example, Gatica, JM et al . In Comptes Rendus Chimie, in press) it has been found that, under the indicated experimental conditions, the FERA and TEFA clays have a capacity to total adsorption of o-xylene at 27 ° C of 157 and 156 µg g -1, respectively. These quantities, although lower than those of other adsorbents in the market, are comparable to those of other materials studied in the literature (see for example, Zaitan, H. and Chafik, T. in Comptes Rendus Chimie; vol. 8, p. 1701-1709, 2005) and can be considered acceptable if one takes into account that no effort has been made to improve the texture of the clays, which according to our previous studies proved to correspond in both cases to that of a non-microporous material with a surface around 40 m 2 g -1. Furthermore, it has been observed that most of the adsorbed amount (81% in the case of FERA and 74% in the case of TEFA) corresponds to a reversible adsorption type (removable at room temperature) and that both clays can be completely regenerated at low temperature (65ºC and 100ºC respectively) which adds more attractiveness to the results obtained. It should not be forgotten that the clays studied are very economical when compared to other materials. Its estimated price is 30 euros per ton (excluding transport costs) which, together with the results mentioned, makes them a very competitive adsorbent potential.

Additionally, the possibility of extrusion of the two clays in the form of honeycomb monoliths has been studied following the methodology previously described in the literature (see, for example, Gatica, JM et al . In Carbon; vol. 42, p. 3251-3254, 2004). The result of this study has shown that both clays meet the necessary requirements to be extruded by having a liquid limit (LL) and an adequate plasticity index (IP): LL = 49.2% and IP = 28.8% in the case of clay called FERA, and LL = 51.2% and IP = 28.8% in the case of TEFA.

The predictions have been confirmed at achieve extrusion in an extruder machine capable of shaping Honeycomb square section monoliths with a density of 4 cm-2 cells, in 2x2 and 4x4 configurations, and with a wall thickness of 1.3 mm. To prepare the paste to extrude no more additives than water have been required, added in amounts of 0.325 ml and 0.4 ml per gram used of FERA and TEFA clay, respectively. After obtaining the monoliths, they were dried in stove at 90 ° C overnight, obtaining samples such as those from the Figure 1.

The mechanical resistance of the monoliths has determined by compression tests performed on a Universal Shimadzu Mechanical Testing Machine AG-IS capable of working at a maximum pressure of 100 kN. This study has revealed that FERA clay monoliths and TEFA resist pressures exerted in the direction of the channels (longitudinal) of 2.53 MPa and 1.62 MPa respectively.

Claims (5)

1. Clay monoliths for the treatment of polluting gaseous effluents, characterized by being constructed by natural clays from northern Morocco, whose structural composition contains minerals such as quartz, muscovite, vermiculite, kaolinite, albite, calcite and clinochlor, with a chemical composition of 39-45% of 0, 21-23% of Si, 9-13% of Al, 9-16% of Fe and a lower content of alkali and alkaline earth metals and other elements. 2. Clay monoliths for the treatment of gaseous pollutant effluents, according to claim 1, characterized by being extruded without the need for additives other than water. 3. Clay monoliths for the treatment of gaseous pollutant effluents, according to claims 1 and 2, characterized by their honeycomb shape. 4. Use of clay monoliths for treatment of polluting gaseous effluents, according to claims 1 to 3 as adsorption filters of o-xylene and similar volatile compounds, regenerable at low temperature. 5. Use of clay monoliths for treatment of polluting gaseous effluents, according to claims 1 to 4, as support for active phases for environmental catalysis