An overview of some environmental applications of self-propagating ...

Advances in Environmental Research 5 Ž2001. 117᎐128

An overview of some environmental applications of self-propagating high-temperature synthesis Galina Xanthopoulou1, George VekinisU Institute of Materials Science, National Centre for Scientific Research ‘Demokritos’, Aghia Paraske¨ i Attikis, 15310 Athens, Greece Accepted 24 August 2000

Abstract The self-propagating high-temperature synthesis ŽSHS. method is being developed world-wide for the low-cost production of engineering and other functional materials, such as advanced ceramics, intermetallics, catalysts and magnetic materials. The method exploits self-sustaining solid-flame combustion reactions which develop very high internal material temperatures over very short periods. It therefore offers many advantages over traditional methods, such as much lower energy costs, lower environmental impact, ease of manufacture and capability for producing materials with unique properties and characteristics. This report introduces the SHS method and its advantages and discusses a number of its applications with environmental interest, such as highly active catalysts for exhaust emission control and methane conversion and various methods of neutralising or recycling industrial inorganic wastes. Since SHS can be initiated and completed at ordinary environment temperatures, it can also be utilised successfully for dealing with toxic or radioactive materials and contaminated areas by creating in situ large-scale protective coatings or in situ vitrification, consolidation and encapsulation of dangerous wastes. 䊚 2001 Elsevier Science Ltd. All rights reserved. Keywords: Self-propagating high-temperature synthesis; Exhaust emission catalysts; Industrial waste treatment and disposal; Vitrification; Combustion

1. Introduction: environmental pollution and associated control measures It is now an accepted fact that industrialisation Žparticularly rapid or forced industrialisation as occurred in many states such as those of the ex-Soviet Union.

U

Corresponding author. Tel.: q30-1-6503322; fax: q30-16533872. E-mail address: [email protected] ŽG. Vekinis.. 1 Ex-Professor Galina Gladoun, Head of SHS Catalysts and Pigments laboratory, Combustion Problems Institute, Almaty, Kazakhstan.

comes hand-in-hand with significant amounts of pollution in many spheres of industrial activity. All industrial production relies on the availability of sufficient amounts of energy in order to drive machinery or just to provide heat, much of which produces copious amounts of pollution. The development of new, less polluting technologies and processes for the production of energy and materials have thus been given the highest priority, but powerful vested interests of existing production methods and materials do not always allow this to take place unhindered. Internal combustion engines, such as those in vehicles, produce a large number of atmospheric pollutants, particularly if combustion is not complete: carbon

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G. Xanthopoulou, G. Vekinis r Ad¨ ances in En¨ ironmental Research 5 (2001) 117᎐128

monoxide, soot, numerous complex hydrocarbons, methane and a range of sulfurous and nitrous oxides, many of which are toxic and dangerous carcinogens. Reduction of pollutants in this case is usually effected by the use of emission control catalyst systems made up of a honeycomb carrier coated with platinum or palladium which oxidise carbon monoxide and various hydrocarbons emitted and reduce nitrous oxides. Diesel engines use catalytic filters for cleaning up soot and other pollutants. But such systems are expensive and of little use in worn or badly tuned engines, while excessive vibration, poisoning by fuel impurities such as sulfur or exposure to high temperatures render them ineffective. So new, low-cost, resistant catalysts must be developed, which do not degrade under extreme conditions of service. Metallurgical, mining and chemical industries are also major sources of gaseous, liquid and solid pollution in many industrialised countries. These include methane and other gases and liquids from chemical plants, slags and other solids from metallurgical and mining operations and their safe disposal has been the subject of numerous studies. Methods that are used in dealing with wastes involve combustion Žof gases, liquids or even solids., recycling or use in various industrial operations. But vast amounts of wastes, even dangerous mining dumps or metallurgical slags, accumulate, because of expensive or inefficient methods of dealing with them. New, revolutionary approaches must be sought and developed to increase the cost-effectiveness of waste management. This short overview introduces a novel material synthesis method which offers some solutions to these challenges.

Fig. 1. Schematic diagram of the SHS process and propagation of the wave through a powder compact. An initiator Želectric discharge or chemical reaction. is not always necessary as the exothermic reaction can be self-igniting at the pre-heating temperature.

a time of a few seconds to a few minutes. The specimen can be either at room temperature or preheated at relatively low temperatures ᎏ rarely above 1000⬚C. The material in front of the propagating wave is preheated by the heat generated by the combustion and the material behind the combustion front is rapidly cooled as the wave sweeps past it. A schematic diagram of the SHS process is shown in Fig. 1. The basic principles of SHS can thus be summarised as follows ŽMerzhanov, 1990, 1997.: 䢇

䢇

2. The SHS method 䢇

The self-propagating high-temperature synthesis ŽSHS. method is a form of controlled combustion synthesis and is now utilised extensively in many countries, but, being relatively new it will be briefly introduced here. SHS exploits highly exothermic solid flame reactions between powder components to produce a large range of unique materials by controlled high-temperature combustion. It was first announced in 1967 ŽMerzhanov et al., 1967. in the ex-Soviet Union but only became known world-wide in the beginning of the 1970s. Using SHS, composition, structure and properties of the materials can be tailored to satisfy the requirements of many applications. Initiation of combustion takes place either by a chemical route or an electrically heated element. Once initiated, combustion is self-sustaining and proceeds by a combustion wave sweeping through the compacted material from the initiation side to the opposite side and is completed in

rapid autowave combustion-like self-sustaining reactions yielding resultant products of desired composition and structure; complete or partial elimination of external energy supply by the utilisation of the internal heat released in the chemical reactions; and control of the process rate, temperature, degree of conversion and composition and structure of products by variation in the rates of heat release and transfer.

These principles translate into a number of significant advantages over traditional processing methods as given in Table 1. In many cases SHS offers greater benefits in comparison with traditional methods especially as regards lower production costs and manufacturing advantages ŽMerzhanov, 1997; Moore and Feng, 1995. but also in terms of microstructure and superior properties. As an example, Table 2 shows a comparison between SHS and the traditional Ž‘furnace’. method for the manufacture of inorganic pigments used in ceramics ŽPoryadina et al., 1991; Gladoun, 1995; Xanthopoulou, 1997, 1998.. The comparison is based on the use of pure raw materials for SHS, but many pigments can be manufactured with industrial wastes


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Table 1 Comparison of the SHS process with conventional production processes Parameter

SHS

Conventional processes

Process conditions

Very short time to complete process Žminutes. Easy to attain very high processing temperatures Žup to 4000⬚C depending on system. Low energy consumption as the surrounding temperature is between 30⬚C and 1200⬚C Simple equipment and technology

Long process time Žhours to days. It is difficult and time and energy consuming to attain temperatures higher than approximately 1700⬚C High energy consumption to reach and maintain process temperature. Sometimes multistage and complicated technology Difficult to attain high heating and cooling rates.

Easy to attain high heating and cooling rates Ž103 ᎐106 ⬚Crmin. Environmental impact

Very low levels of polluting emissions Easy recycling of many solid wastes to products Possible in situ treatment of areas contaminated by noxious or radioactive wastes Very active but cheap catalysts for environmental uses

Characteristics of materials produced

Production benefits

High levels of polluting emissions Sometimes possible to recycle wastes at higher cost Very expensive or impossible to treat large contaminated areas Catalyst production is complicated and expensive

Synthesis of novel materials based on many of the elements of the periodic table Controlled physicochemical properties. Many properties may easily be changed for the same composition Controlled lattice defects enhancing catalytic activity High process temperatures enhance stability

Stability depends on process temperature

Possibility for ‘just-in-time’ manufacturing Small inventories possible Low overall manufacturing costs

Difficult to offer ‘just-in-time’ products Large inventories needed to reduce costs High overall manufacturing costs

ŽBaydeldinova et al., 1996; Xanthopoulou, 1997, 1998., reducing the cost even further.

It is often impossible to obtain analogous compositions of many SHS materials Difficult to change many properties within a wide range for the same composition. Difficult to control lattice defects content

Similar cost-savings are found for most processes where SHS has been applied ŽMerzhanov, 1990, 1997;

Table 2 Comparison between the traditional ‘furnace’ and SHS methods for the production of pigments for ceramics ŽXanthopoulou, 1998. Description

Furnace

SHS

Relative cost of initial components Žthe ‘charge’. Furnace temperature during reaction Ž⬚C. Synthesis temperature Ž⬚C. Duration of main sintering stage Žh. Total number of production steps Žtypical. Relative net productivity Energy consumption for synthesis ŽkWrkg. Relative labour costs Relative production area needed Relative cost of production equipment ᎏ continuous process Relative area of production works Relative amounts of polluting emissions in the atmosphere Žcost of scrubbing emissions. Relative total production cost

1 1200᎐1800 1200᎐1800 8᎐48 4᎐6 1 30᎐100 1 4᎐15 1.2᎐2

1᎐1.3 600᎐900 1400᎐2100 0.02᎐0.2 3᎐4 50᎐400 0.1᎐0.3 0.4᎐0.8 1 1

4᎐12 3᎐20

1 1

3᎐15

1

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Moore and Feng, 1995., even taking into account the higher cost of producing the initial powders. In many cases, lower activity starting materials can be used as the higher temperatures achieved by SHS successfully compensate for the low activity of the starting powders. This fact has been exploited in a number of studies where waste materials have been converted into useful products by SHS ŽGladoun et al., 1986; Gladoun, 1991; Iskakova et al., 1991; Baydeldinova et al., 1996; Xanthopoulou, 1998, 1999a.. In addition, the environmental impact of SHS is very much lower than that of the traditional method, a fact which decreases even further the indirect cost of production ŽMerzhanov, 1990, 1997.. It is relatively wasteless and the significant reduction of energy input it offers has direct benefit for the environment. As a result of these advantages, SHS has now become an extensively studied discipline and is often regarded as a link between combustion theory and materials science. In this short review we will present some of the most interesting environmentally significant applications of SHS.

3. SHS catalysts

During the past 20 years or so, a wide range of catalytically active materials prepared by SHS have been identified for a number of processes and are reviewed in detail in a number of publications ŽTavadyan et al., 1978; Itin, 1981; Labody et al., 1982; Blumberg et al., 1984; Gladoun et al., 1986, 1988, 1990, 1991b, 1992, 1997; Gladoun, 1991, 1994; Iskakova et al., 1991; Borovinskaya et al., 1992; Rodivilov et al., 1995; Grigoryan, 1997; Xanthopoulou and Vekinis, 1997, 1998, 1999, 2000; Xanthopoulou, 1999a,b, 2000.. SHS catalysts include ceramic Žcarbides, oxides, borides, nitrides. as well as many metallic and intermetallic materials. The majority of the published work in this field originated in the states of the ex-Soviet Union. In this paper we shall concentrate our discussion on SHS catalysts which display environmentally significant properties. The first announcement of the catalytic properties of SHS compounds appeared as early as 1978 for a series of borides ŽTavadyan et al., 1978; Labody et al., 1982; Blumberg et al., 1984. for the conversion of hydrocarbons into chemical gases. Although the materials reported displayed limited activities and lifetimes as compared with the then available commercial compounds, the work nevertheless inaugurated a new and promising field. At about the same time, Gladoun Žlater Xanthopoulou. and co-workers discovered significant

catalytic activity in a series of oxide SHS materials firstly for the pyrolysis of diesel fuel ŽGladoun et al., 1986. and later for other processes ŽGladoun et al., 1988, 1990, 1991b, 1992, 1997; Gladoun, 1991, 1994; Iskakova et al., 1991; Xanthopoulou and Vekinis, 1997, 1998, 1999, 2000; Xanthopoulou, 1999a,b, 2000.. Xanthopoulou carried out a long series of systematic investigations of the catalytic activity of many SHS materials and discovered that a number of oxides and intermetallics displayed significant catalytic activity for various processes, including such environmentally important ones as exhaust gas purification and deep methane oxidation ŽXanthopoulou and Vekinis, 1998, 2000.. For example, SHS oxides were produced by reacting metallic oxides with aluminium or other metallic powders according to a general reaction of the type ŽGladoun, 1991, 1994.: Al Ž Mg. q MO q ballast « MAl 2 O 3 q M x Al y q MO q Q The ballast material ᎏ often alumina but it depends on the reaction ᎏ is used to control the reaction by absorbing excess heat. Gladoun et al used such reactions replacing M with a large number of elements from groups IA᎐IIA, IB᎐VIIB and VIII of the periodic table ŽGladoun, 1991, 1994. over a range of compositions and reaction temperatures and measured the catalytic activity of each SHS material obtained for oxidation of CO, hydrocarbons, soot and methane, dehydrogenation, hydrogenation and dehydrodimerisation, diesel fuel pyrolysis and others. In the late-1980s and early-1990s a number of other catalytic materials, including TiC and various borides and nitrides made by SHS were also reported by groups in Moscow and elsewhere for various industrial chemical processes ŽItin, 1981; Borovinskaya et al., 1992; Grigoryan, 1997.. The activities of some of the SHS catalysts reported in the literature are compared with commercially available catalytic systems in Table 3 ŽGladoun et al., 1988, 1990, 1991b, 1992; Gladoun, 1991, 1994; Grigoryan, 1997; Xanthopoulou and Vekinis, 1998; Xanthopoulou, 1999a,b.. It is clear that the SHS catalysts display substantially enhanced activity for many processes, often at milder temperatures than those used in traditional, commercial systems. Analyses of the catalytic reactions ŽGladoun, 1991, 1994; Grigoryan, 1997. showed that the high activities measured for SHS catalysts are due both to the often metastable composition of the SHS products as well as to the highly defective structure of the new materials resulting from the very high heating and cooling rates

Processrtest

SHS catalyst

Traditional catalyst Žlaboratory measurements .

CO oxidation Temp. at 50% conversion CH4 deep oxidation of methane at 820᎐870 K Oxidation of CO and propane Temp at 50% conversion Octane dehydrogenation H2 yield: Synthesis gas combustion

Cu᎐Cr spinel ŽGladoun, 1991; Xanthopoulou and Vekinis, 1997, 1998. 410 K K-8 ŽGladoun et al., 1988, 1990, 1991b, 1992; Xanthopoulou and Vekinis, 2000. 60% Oxynitride ŽGrigoryan, 1997; Xanthopoulou and Vekinis, 1998. 300᎐400 K K-1 ŽGladoun, 1991, 1994. 100% K-7 ŽGladoun, 1991, 1994. Žspec. area 1.4 m2rg. 790 K Co-based oxide ŽGladoun, 1991; Xanthopoulou, 1999b. 38% Žcoke 0.86. Mn-based oxide ŽGladoun, 1991, 1994; Xanthopoulou, 2000. 26%

0.05%PdrAl2 O3 500 K oxide on support 50% 0.05%PdrAl2 O3 500 K NirZrO2 95% CuCr2 O4 , CuO, Cr2 O3 Žspec. area 123m2rg. 780 K KNO3 on mullite-corundum carrier 28% Žcoke 3.2. 20%LiCl᎐MnO2 15%

Temperature at 100% CO conversion: Diesel fuel pyrolysis Ethylene yield without vapour: Methane oxidative dehydrodimerization Ethylene yield:


Table 3 Comparison of SHS with traditional catalyst systems for a number of processes

121

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Fig. 2. Activity of Cu᎐Cr᎐O SHS catalysts as coarse powder and as a substrate carrier of palladium is at least as good as that of palladiumralumina traditional catalysts for the oxidation of carbon monoxide ŽGladoun, 1991; Xanthopoulou and Vekinis, 1997, 1998, 1999..

ŽXanthopoulou and Vekinis, 1999.. In catalysts, point and other types of defects have been shown to act as active centres during catalysis increasing their activity ŽGladoun, 1991, 1994.. In fact, the high activity of the SHS catalysts is particularly remarkable as their specific surface area is much lower than that of traditional catalysts. This is mainly a result of the high combustion temperatures achieved during SHS, partially melting the surface of the material and thereby closing micropores. Therefore, further enhancement of the SHS materials’ activity may be achieved by increasing their specific area ŽGladoun, 1991, 1994.. For environmentally important processes, various oxide and oxinitride catalysts, synthesised under SHS conditions, have been reported to offer substantial catalytic activity ŽGrigoryan, 1997; Xanthopoulou and Vekinis, 1997, 1998, 2000..

3.1. Exhaust emission control catalysts In the particular case of internal combustion engine exhaust emissions, the usual noble-metal based catalyst systems suffer from two main drawbacks: they are expensive Žexpensive materials and complicated process. and can degrade quickly under certain conditions of service. In particular, if the catalyst comes in contact with unburnt fuel in the exhaust system, containing even traces of sulfur, it loses its catalytic activity very quickly, something which also occurs if the catalyst is exposed to high temperatures. In addition, excessive

vibration during driving may damage the noble metal powder coating. Oxide SHS catalysts such as Cu᎐Cr᎐O materials with the spinel CuCr2 O4 structure have shown great promise for oxidation of carbon monoxide which is one of the main dangerous pollutants in exhaust emissions ŽXanthopoulou and Vekinis, 1997, 1998.. This SHS catalyst, both as 2᎐3-mm pellets and as a substrate carrier for 0.5% Pd Žthe active ingredient of many traditional catalytic systems. is compared with traditional PdrAl 2 O 3 catalysts in Fig. 2. Whereas the PdrAl 2 O 3 systems attain 100% conversion at approximately 500⬚C, the SHS systems attain this level of 100% conversion at approximately 400⬚C. ‘Lift-off’ and ‘light-off’ temperatures are comparable for these systems, even though the total specific surface area of the SHS material is in the region of 1.5 m2rg as compared with more than 100 m2rg for the traditional systems. The main goal of ongoing work is therefore in the direction of improving the materials’ specific area. The results indicate that the new SHS catalyst materials are at least as efficient at oxidising carbon monoxide as noble metal systems, without the limitations, high cost, scarcity and processing difficulties inherent in the case of the traditional supported noble-metal systems ŽXanthopoulou and Vekinis, 1997, 1998.. Similar catalysts were later confirmed by other workers to offer good activity for oxidation of carbon monoxide as well as methane and mixtures of hydrocarbons ŽGrigoryan, 1997., as shown in Fig. 3. Other work showed that SHS materials containing


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Fig. 3. Activity of Cu᎐Cr-based catalysts for the oxidation of carbon monoxide, methane and mixtures of propene ŽGrigoryan, 1997..

manganese or copper display catalytic activity for the oxidation of soot contained in emissions from engines and factories ŽGladoun, 1991; Rodivilov et al., 1995.. In comparative tests against well known soot-oxidation catalyst systems containing chamotte, the SHS materials were found to offer 100% oxidation at 450᎐460⬚C with good long-term stability whereas chamotte reaches 100% conversion at 560᎐570⬚C. In addition, it was shown that chamotte impregnated with LiCl, coppercontaining catalyst or Pt catalysts on ceramic carriers gave similar results as SHS catalysts, but they are not stable for long periods of service. This is because SHS catalysts do not sinter or suffer from ‘emissions poisoning’ at the service temperatures used and do not need regeneration ᎏ a difficult engineering task ᎏ whereas commercial filters often need regeneration to open blocked channels.

energy conversion efficiency ŽAlhazov and Margolis, 1985; Gladoun et al., 1988, 1990, 1991b, 1992.. But to encourage industries to strive for 100% combustion efficiency, the catalysts must be cheap and easily available. For this purpose, a number of SHS catalysts have been developed based on low cost raw materials containing aluminium, magnesium, manganese as well as metallurgical waste slags giving them a double environmental benefit ŽGrigoryan, 1997; Xanthopoulou and Vekinis, 2000.. As shown in Figs. 3 and 4, where SHS materials are compared with commercial systems, such catalysts are very active and offer an inexpensive and viable alternative to traditional catalyst systems.

4. Utilisation and recycling of solid industrial wastes by SHS

3.2. Deep methane oxidation catalysts Methane is one of the main atmospheric pollutants of chemical industries and diesel combustion engines, contributing significantly to the green-house effect. In addition, it is the main constituent of natural gas which is used to produce heat energy for many industries. As a secondary, polluting by-product, its incineration is usually carried out in large incinerators under high temperatures, the final product being released into the atmosphere. To enhance combustion efficiency, catalysts are usually employed whose function is to ensure that methane is fully converted to carbon dioxide and water while at the same time they offer the maximum

By their nature, most solid industrial wastes Žmetallurgical slags, wastes from ore refinement processes, low grade ores, etc.. represent the tail-end of industrial processes. As the energy needed to process it further is greater than the ‘energy-content’ of the waste itself, it is not economically worthwhile to exploit it and so it remains as an environmental polluting load. This economic non-feasibility is the main reason for the great problems of the vast accumulation of industrial wastes faced by industrialised countries. In certain industrial processes, such as gold extraction, the proportion of useful material to waste is so small Žoften less than 0.001% in the case of gold or platinum extraction.

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Fig. 4. SHS catalysts offer good activity in comparison with traditional commercial systems for the deep oxidation of methane ŽGladoun et al., 1988, 1990, 1991b, 1992; Grigoryan, 1997; Xanthopoulou and Vekinis, 1999, 2000..

that whole mountain ‘dumps’ have formed adjacent to the mining operations. In some cases, new extraction methods and higher price of gold has encouraged the recycling of the ‘dumps’ but this has not decreased the volume of the waste. Such large volume wastes are encountered in many other extraction processes too, including aluminium, iron, chromium, nickel, copper, etc. And in most cases, even transportation of nondangerous wastes ᎏ to a landfill site for instance ᎏ is also non-economical and thus the wastes just pile-up exacerbating the environmental problem. The answer lies in ‘adding value’ to non-dangerous wastes by using them as raw material for the production of a useful article. Ideally, such articles must be of large-volume type so that they will absorb a large amount of the waste, such as building and construction materials or refractories ŽKsandopulo and Ismailov, 1980; Ksandopulo et al., 1985; Gladoun, 1991; Gladoun et al., 1993.. The SHS process has been developed successfully for just such processes: the processing cost is very low and, with the addition of appropriate additives, the process is not very sensitive to the actual composition of the waste. In some cases ᎏ such as construction tiles, bricks and refractories ŽKsandopulo and Ismailov, 1980; Ksandopulo et al., 1985; Gladoun et al., 1993, 1997. ᎏ the volume of waste utilised is very large but the value-added is low, whereas in other cases ᎏ such as pigment production ŽPoryadina et al., 1991; Xanthopoulou, 1997, 1998. ᎏ the volume absorbed is small but the value added is very large. Therefore, in all cases, the overall value added to the waste is

sufficient to warrant the further development of the SHS process for the fuller utilisation of industrial wastes. It must be noted that, in most cases, such large scale utilisation is not usually economically feasible using traditional production processes. As shown in Fig. 5, various industrial wastes have been processed by SHS for the production of a good number of both high and low value-added products, all with promising properties ŽGladoun et al., 1986, 1991a; Gladoun, 1991, 1994; Iskakova et al., 1991; Xanthopoulou, 1999a.. For example, waste slags containing Fe or Mn Žfrom 0.3 to 60%. were converted by SHS ŽGladoun et al., 1991a. to active catalysts for oxidative dehydrodimerisation of methane and dehydrogenation of diesel fuel, thereby displaying very high added value. Yields in using these catalysts were promising as shown in Fig. 6. Development of a recycling process based on nitriding SHS of industrial wastes, such as silicon sludge produced in semiconductor industries and aluminium dross in aluminum foundries, to SiAlON based ceramics was demonstrated by Miamotto and collaborators ŽMiyamoto et al., 1995; Miyamoto, 1999; Japanese Patent 62-162608.. Such SiAlON ceramics can be used for refractories, abrasives and other wear resistant materials.

5. SHS for protection from hazardous and radioactive wastes In many industries, including mining, extractive met-


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Fig. 5. Various products of utilisation of solid industrial wastes by SHS. All these products have been shown to offer economic and viable alternatives to traditional waste-utilisation methods ŽKsandopulo and Ismailov, 1980; Ksandopulo et al., 1985; Gladoun et al., 1986, 1991a, 1993; Gladoun, 1991, 1994; Iskakova et al., 1991; Poryadina et al., 1991; Miyamoto et al., 1995; Baydeldinova et al., 1996; Xanthopoulou, 1997, 1998, 1999a; Miyamoto, 1999; Japanese Patent 62-162608..

allurgy, chemical production and others, many wastes are dangerous and their management and treatment requires special attention. In modern production facilities, treatment of such wastes must be built in as part of the production process and the resulting pollution is small and easily managed and there is little danger. But in older production units, the danger of the wastes was either not recognised Žas in some very old mine dumps.

or ignored as in toxic liquid waste dumps in various parts of the world. In such cases, great efforts at great expense must be expended to neutralise the wastes and protect the environment from leakages or airborne dust. A technologically feasible method of stabilising and neutralising a large waste dump from old mines or metallurgical processes is to cover it with a layer of a

Fig. 6. Yield of hydrogen from dehydrogenation of diesel and of ethene from dehydrodimerisation of methane on SHS catalysts produced from Fe and Mn-containing waste ores respectively w34x.

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Fig. 7. SHS reactions between hexachlorobenzene toxic wastes and calcium hydride, ensure complete breakdown of the aromatic molecules and undesired chloro-organic congeners, leaving only inorganic halide salts ŽCocco et al., 1999..

stable protective substance which will keep the material from becoming airborne. But traditional methods involving earth-moving machinery can often exacerbate the problem by generating copious amounts of dust. As SHS can be initiated and completed at ordinary environment temperatures, it does not need a furnace and it can be used to create in situ a protective hard, vitreous coating on any dangerous solid wastes of any volume ŽIskakova et al., 1991; Simoncini et al., 1997a,b; Cao et al., 1999; Cocco et al., 1999; Orru ` et al., 1999.. Practical exploitation of SHS-based technology for the disposal of hazardous organochlorine compounds has been proposed ŽSimoncini et al., 1997a,b; Cao et al., 1999; Cocco et al., 1999; Orru ` et al., 1999.. Because of the highly exothermic solid-state reaction between hexachlorobenzene and calcium hydride, temperatures as high as 2300᎐2650⬚C can be obtained, ensuring complete breakdown of the aromatic molecules and undesired chloro-organic congeners, with only inorganic halide salts being found among the end products ŽFig. 7.. This idea can also be applied in cases where large areas are contaminated by dangerous substances following leakages or by processes such as nuclear weapons testing, as in various regions in the ex-Soviet Union or the USA. Nuclear high or medium level waste represents special serious cleaning problems and a number of special methods have been developed for

their disposal. This usually entails their consolidation or encapsulation in a glassy or polymeric mass and long-term storage ŽUSA patents, 1988; Brookhaven National Laboratory, 1999a,b.. Consolidation or encapsulation serves a dual purpose: it renders the waste relatively safe to manage and reduces the likelihood of it being used illegally. Such encapsulation is usually carried out either in large furnaces or other large equipment, which present later transportation problems of the product, or in situ by special electrically heated rigs, which presupposes the existence of special facilities. The SHS method may be used for vitrification and consolidation Žusing other wastes as raw materials, offering a double environmental benefit. of such wastes by producing a very hard and strong mass incorporating the waste itself as one of the combustion constituents. The technology for carrying out such operations already exists as the method has been demonstrated in experimental studies ŽBorovinskaya et al., 1997, 1998; Barinova et al., 1999; Gladovskii et al., 1999; Pityulin and Borovinskaya, 1999. where radioactive wastes were successfully incorporated in a high density, high strength and durability, glassy body by mixing with perovskite, zirconolite or fergusonite minerals and subjecting it to SHS. The solid reaction is very fast and lasts only a few seconds, reducing drastically the possibility of ejection of airborne particles.

6. Conclusions The self-propagating high-temperature synthesis method exploits exothermic solid flame reactions for the production of very high temperatures during materials production or treatment. For this reason, the method has lower energy needs and the whole process is completed in very short times in comparison with conventional processing methods. As a result, SHS produces only a small fraction of the polluting emissions of other industrial processes and therefore has a very much smaller impact on the environment. But the SHS method also offers indirect environmental benefits. It has been used successfully for the development of very active low cost catalysts for various environmental applications, such as exhaust emission control, methane and hydrocarbon oxidation and others. In addition, the method has been used successfully for the production of a series of low cost products Žcatalysts, ceramic products, bricks, tiles, pigments, refractories and protective coatings., by utilising solid wastes from industries and mines. Finally, the ability of SHS to be initiated and carried out at environment temperature allows the use of the method for covering, consolidation or encapsulation of a range of dangerous large-scale wastes.


References

Alhazov, T.G., Margolis, L.Y., 1985. Deep methane oxidation of organic substances win Russianx. Himiya, Moscow, pp. 1᎐102. Barinova, T.V., Borovinskaya, I.P., Ratnikov, V.I., Ignatyeva, T.I., Zakorzhevsky, V.V., 1999. SHS of mineral-like ceramics for consolidation of radioactive wastes. In: Proceedings of the Fifth International Symposium on Self-Propagating High Temperature Synthesis ŽSHS-99., Moscow, Russia, August 1999, p. 123. Baydeldinova, N., Chernoglazova, T.V., Gladoun, G.G., 1996. New inorganic pigments on the basis of raw materials and industrial waste products. In: Proceedings of the Microsymposium on Self-Propagating High-Temperature Synthesis ŽSHS., Hyderabad ŽIndia., p. 5. Blumberg, E.A., Novikov, Yu.D., 1984. Review of science and technology win Russianx. Kinetika Kataliz 25, 269. Borovinskaya, I.P., Loryan, V.E., Grigoryan, G.E., Salnikova, E.N., Pershikova, N.I., 1992. Oxidative dehydrodimerization of methane over complex oxide catalysts prepared by SHS. Catal. Today 13, 593. Borovinskaya, I.P., Ratnikov, V.I., Zakorzhevsky, V.V., Barinova, T.V., Ignatyeva, T.I., 1997. The curing of radioactive waste in minerals-like materials by SHS method. In: Proceedings of the Fourth International Symposium on Self-Propagating High Temperature Synthesis ŽSHS-97., Toledo, Spain, October 1997, p. 113. Borovinskaya, I.P., Barinova, T.V., Ratnikov, V.I., Zakorzhevsky, V.V., Ignatyeva, T.I., 1998. Consolidation of radioactive wastes into mineral-like materials by SHS method. Int. J. Self-Propagating High-Temperature Synth. 7 Ž1., 129. Brookhaven National Laboratory, 1999a. Encapsulation of hazardous wastes. Technology brief. Brookhaven National Laboratory, USA. Web address: www.bnl.govrTECHXFERrtechy briefsrencap.html Brookhaven National Laboratory, 1999b. A in situ method for covering hazardous wastes. Technology brief. Brookhaven National Laboratory, USA. Web address: www.dne.bnl.govr ; heiserrch36lf23.htm Cao, G., Doppiu, S., Monagheddu, M., Orru, ` R., Sannia, M., Cocco, G., 1999. The thermal and mechanochemical selfpropogating degradation of chloro-organic compounds: the case of hexachlorobenzene over calcium hydride. Ind. Eng. Chem. Res. 38, 3218. Cocco, G., Doppiu, S., Monagheddu, M., Cao, G., Orru, R., Sannia, M., 1999. Self-propagating high-temperature reduction of toxic chlorinated aromatics. Int. J. Self-Propagating High Temperature Synth. 8, 324. Gladoun, G., 1991. Self-propagating high-temperature synthesis of catalysts and carriers win Russianx. Doctor of Science Dissertation, Moscow, USSR, pp. 1᎐478. Gladoun, G., 1994. Self-propagating high-temperature synthesis of catalysts and carriers. Int. J. Self-Propagating HighTemperature Synth. 3 Ž1., 51. Gladoun, G., 1995. Investigation of regularity of colour formation in SHS pigments. In: Proceedings of the Third International Symposium on Self-Propagating High-Temperature Synthesis, China, p. 196.

127

Gladoun, G., Orynbekova, J., Alexandrov, A., Ahmetov, T., 1986. New catalysts for pyrolysis of diesel fuel. In: Proceedings of the Conference on Combustion Processes, AlmaAta, USSR, p. 71. Gladoun, G. et al., 1988. Method of preparing catalyst for organic compounds synthesis. Patent SU 1729028. Gladoun, G. et al., 1990. Catalysts for dehydrogenation of gasoline. Patent SU 1797208. Gladoun, G., Iskakova, A., Yakoubova, N., Chernoglazova, T., 1991a. Investigation of influence of electro-physical properties on the activity of manganese and iron catalysts. In: Proceedings of the Conference on ‘Electrophysics of Combustion’, Chelyabinsk, USSR, p. 61. Gladoun, G. et al., 1991b. Catalyst for deep oxidation of organic compounds. Patent RU 5006023. Gladoun, G. et al., 1992. Catalyst for deep hydrocarbon oxidation. Patent RU 2043145. Gladoun, G., Baymouhamedov, E., Sherinhanov, A., 1993. SHS high-temperature light-weight refractories. J. Eng. Phys. Thermophys. 65 Ž4., 490. Gladoun, G., Sergienko, V., Ksandopulo, G., 1997. The combustion wave structure of SHS systems based on the compounds of iron and manganese oxides. Int. J. Self-Propagating High-Temperature Synth. 6 Ž4., 399. Gladovskii, E.M., Kuprin, A.V., Petelin, L.P. et al., 1999. Immobilization of high-level wastes in stable mineral-like materials in a self-propagating high-temperature synthesis regime. At. Energy 87, 1514. Grigoryan, E.H., 1997. SHS catalysts and supports. Int. J. Self-Propagating High-Temperature Synth. 6 Ž3., 307. Iskakova, A., Gladoun, V., Khlystov, A., Gladoun, G., 1991. Synthesis and study of physico-chemical properties and catalytic activities of catalysts on the base of ores, concentrates, slags and ashes. In: Proceedings of the First International Symposium on Self-Propagating High-Temperature Synthesis, Moscow, p. 136. Itin, V.I., 1981. SHS intermetallic compositions win Russianx. Phyz. Gor.i Vzryva 3, 62. Japanese Patent. Conversion of wastes to ceramic mass. Japanese Patent: Tokkai-sho 62-162608. Ksandopulo, G.I., Ismailov, M.B., 1980. Composition for coating of furnace lining. USSR Inventor’s certificate no. 1730081A1. Ksandopulo, G.I., Ismailov, M.B., Satbaev, B.N., Umarbekov, N.S., Leonov, A., Nersesyan, M.D., 1985. Refractory solution for the laying of periclase-chromite articles. USSR Inventor’s certificate no. 1717586 A1. Labody, I., Korablev, L.I., Tavadyan, L.A., Blumberg, E.A., 1982. Kinetics of formation of boride catalysts for hydrocarbon conversion win Russianx. Kinet. Kataliz 23 Ž2., 371. Merzhanov, A.G., Shkiro, V.M., Borovinskaya, I.P., 1967. Synthesis of refractory inorganic compounds. USSR inventor’s certificates no. 255, 221; US patent no. 3726643, 1973; UK patent 1321084. Merzhanov, G., 1990. Self-propagating high-temperature synthesis: twenty years of research and findings. In: Munir, Z.A., Holt, J.B. ŽEds.., Combustion and Plasma Synthesis of High Temperature Materials. VCH Publishers, New York, pp. 1᎐53.

128


Merzhanov, G., 1997. Worldwide evolution and present status of SHS as a branch of modern R& D on the 30th anniversary of SHS. Int. J. Self-Propagating High-Temperature Synth. 6, 119. Miyamoto, Y., 1999. Development of recycling process for industrial wastes. In: Proceedings of the Fifth International Symposium on Self-Propagating High Temperature Synthesis ŽSHS-99., Moscow, Russia, August 1999, p. 122. Miyamoto, Y., Li, Z., Tanihata, K., 1995. Recycling process of Si waste to advanced ceramics using SHS reactions. Ann. Chim. Fr. 20, 197. Moore, J.J., Feng, H.J., 1995. Combustion synthesis of advanced materials: classification, applications, and modelling. Prog. Mater. Sci. 39, 243. Orru, ` R., Sannia, M., Cincotti, A., Cao, G., 1999. Treatment and recycling of zinc hydrometallurgical wastes by selfpropagating reactions. Chem. Eng. Sci. 54 Ž15r16., 3053. Pityulin, A.N., Borovinskaya, I.P., 1999. A new material for radiation protection. In: Proceedings of the Fifth International Symposium on Self-Propagating High Temperature Synthesis ŽSHS-99., Moscow, Russia, August 1999, p. 125. Poryadina, L., Jamanbekova, O., Gladoun, G., 1991. Self-propagating high-temperature synthesis of pigments. In: Proceedings of the First International Symposium on SelfPropagating High-Temperature Synthesis, Moscow, 1991, p. 142. Rodivilov, S.M., Gostev, S., Gladoun, G., 1995. Study of activity of SHS block catalysts in process of burning out of soot for the purifying of outgoing gases of diesel motor. In: Proceedings of the International Symposium on ‘Block Supports and Catalysts of Honeycomb Structure’, SanktPetersburg, Russia, p. 18. Simoncini, B., Orru, R., Cao, G., 1997a. Self-propagating reactions for treating and recycling zinc hydrometallurgical wastes. In: Proceedings of the Fourth International Symposium on Self-Propagating High Temperature Synthesis ŽSHS-97., Toledo, Spain, October 1997, p. 117. Simoncini, B., Orru, R., Cao, G., 1997b. Self-propagating reactions for treating and recycling zink hydrometallurgical wastes. Key engineering materials. Proceedings of the Fifth Conference and Exhibition of the European Ceramic Society 132r136, 2276.

Tavadyan, L.A., Maslov, S.A., Blumberg, E.A., 1978. Novel boride catalysts by combustion synthesis win Russianx. Neftekhimiya 18 Ž6., 667. USA patents, 4 869 621, 1988. Xanthopoulou, G., 1997. SHS pigments: synthesis, properties and regularities of colour formation. In: Proceedings of the Fourth International Symposium on Self-Propagating High-Temperature Synthesis, Spain, Toledo, 6᎐9 October 1997, p. 128. Xanthopoulou, G., 1998. SHS of inorganic pigments. Am. Ceram. Soc. Bull. 77, 87. Xanthopoulou, G., 1999a. Oxide catalysts for pyrolysis of diesel fuel made by self-propagating high-temperature synthesis ŽSHS.. Part II: Fe᎐Cr oxides catalysts based on chromite concentrates. Appl. Catal. A: Gen. 187, 79. Xanthopoulou, G., 1999b. Oxide catalysts for pyrolysis of diesel fuel made by self-propagating high-temperature synthesis. Part I: Cobalt-modified Mg᎐Al spinel catalysts. Appl. Catal. A: Gen. 182, 285. Xanthopoulou, G., 2000. Oxidative dehydrodimerization of methane using manganese based catalysts made by selfpropagating high-temperature synthesis. Appl. Catal. A: Gen. 199 Žaccepted for publication.. Xanthopoulou, G., Vekinis, G., 1997. SHS spinels, oxides as catalysts for carbon monoxide oxidation. In: Proceedings of the Fourth International Symposium on Self-Propagating High-Temperature Synthesis, Spain, Toledo, 6᎐9 October 1997, p. 57. Xanthopoulou, G., Vekinis, G., 1998. Investigation of catalytic oxidation of carbon monoxide over a Cu᎐Cr-oxide catalyst made by self-propagating high-temperature synthesis. Appl. Catal. B: Environ. 19, 37. Xanthopoulou, G., Vekinis, G., 1999. Influence of cooling conditions on the composition, microstructure and activity of SHS catalysts. Int. J. Self-Propagating High Temperature Synth. 8 Ž3., 287. Xanthopoulou, G., Vekinis, G., 2000. Deep oxidation of methane using catalysts and carriers produced by self-propagating high-temperature synthesis. Appl. Catal. A: Gen. 199, 227.