Valorization of MSWI bottom ash through ceramic glazing process: a new technology

Abstract

In this study a ceramic glaze composed by relevant amounts (>80 wt.%) of a frit obtained from vitrified Municipal Solid Waste Incineration (MSWI) bottom ash was produced. This glaze was applied by a conventional wet technique (already in use in the ceramic industry) and by an innovative one, i.e. the plasma-spraying technique. Potential advantages of plasma spraying over conventional wet glazing techniques are lower raw material wastage, no production of glazing wastewater and wet sludge and therefore a simpler process management. The technical characteristics (microstructure, aesthetic appearance, dry particle abrasion resistance and acid resistance) of glazes applied by both methods were compared. The wet-deposited glaze, after firing at 1200 °C, was dense and completely amorphous. In the as-deposited condition, the plasma-sprayed glaze was also completely amorphous, but its technical usefulness was limited by the presence of pores and defects. Through heat treatments at different temperatures, it was possible to induce crystallisation of the plasma-sprayed glaze and to modify its colour; in all cases, however, its aesthetic appearance (quantitatively determined by colorometry) was significantly different from that of the wet-deposited glaze. Complete densification and extensive crystallisation could be achieved after heat treatment at 1200 °C. The wet-deposited glaze and the plasma-sprayed glaze heat-treated at 1200 °C had similar abrasion resistance; however, this latter exhibited significantly better resistance to acids.

Introduction

The generation of municipal solid waste (MSW) in Italy in 2008 was about 32.5 millions tons. 44.9% of this amount (16 millions tons) has been landfilled without pre-treatment, while only the 10.9% of MSW (4.1 millions tons) have been incinerated (MSWI) (ISPRA, 2009). Incineration with energy recovery is a waste-to-energy (WTE) technology that involves the combustion of the waste in order to obtain heat, which can in turn be used to produce electric power. Municipal Solid Waste Incinerators (MSWIs) reduce the volume of the original waste by 95–96%, depending on its composition and on the degree of pre-treatment (Ramboll, 2006). By-products of waste incineration are flue gases (which need to be cleaned of pollutants before they are dispersed in the atmosphere) and residual solids (ashes and particulates).

More in detail, the solid outputs of waste incineration are represented by fly ashes (≈3–5 wt.% of the original quantity of waste) and bottom ashes (≈20–35 wt.%) (European Commission, 2004, Lapa et al., 2002). Many efforts have been made to improve the environmental quality of these solid residues from waste incineration and to recycle or utilise at least part of them. Both in-process and post-treatment techniques are applied. In-process measures aim to change the incineration parameters in order to improve burnout or to shift the metal distribution over the various residues. Post-treatment techniques include: ageing, mechanical treatment, washing, thermal treatment and stabilisation (European Commission, 2006). In particular, vitrification is one of the most promising technological options for the transformation of MSWI residues into inert materials: it aims to generate a product that can be reused or, at least, deposited in standard landfills with minimised risk. The high temperatures involved in the process (the first melt-producing reaction occurs at temperatures up to 1000 °C, and the subsequent pyro-reactions, needed to develop a homogeneous glass melt (Volf, 1984), require T > 1300 °C) (Barbieri et al., 2001) indeed lead to the complete destruction of organic pollutants (e.g. dioxins). Moreover, heavy metals can be incorporated in the glassy network. Because of these advantages, several companies build and install vitrification facilities around the world (especially in Japan).

The authors performed a number of investigations concerning the valorisation of MSWI bottom ash, particularly focussing on its vitrification and subsequent reuse in the ceramic field (Ferraris et al.,, Saccani et al., 2001, Andreola et al., 2010a, Andreola et al., 2002, Barbieri and Lancellotti, 2004, Schabbach et al., 2011a, Andreola et al., 2008, Schabbach et al., 2011b). The inert vitreous product can indeed, in principle, be utilized in various ways: as road-base material, in embankments, as blasting medium, as a partial replacement of sand in concrete, and in the production of construction and decorative materials, such as water-permeable blocks, tiles, pavement bricks and decorative stones for gardens.(Abe et al., 1996, Nishigaki, 2000) However, since vitrification is an energy-intensive process, involving relatively high costs, its use can only be fully justified if a high-quality product with optimised properties can be fabricated, which can compete with other current materials used, for example, for construction, architectural or insulation applications.

For instance, in the ceramic tile industry, the glazing process employs appreciably high amounts of glass, so that its partial or complete replacement with vitrified bottom ash can, in principle, be economically/environmentally viable. Ceramic glazes, which serve both an aesthetic purpose (providing a glossy, colourful surface, otherwise difficult to achieve) and a technical purpose (making the tile surface waterproof, facilitating the removal of dirt and stains), indeed consist of mixtures of various raw materials, among which frits, i.e. glasses obtained from mixtures of silicates and carbonates, melted and rapidly cooled in water (Weiand et al., 1992), have a particular technical and quantitative relevance.

Recently attention have been focused in the study of ceramic glazes obtained from wastes. The objective is to obtain the environmental benefices as the reduction of the storage of some industrial residues and the reduction of the raw material used (Romero et al., 2002, Romero et al., 2003). The environmental sustainability of the use of vitrified MSWI bottom ash or CRT glass as constituent of ceramic tile glazes has recently been examined by the authors using the Life Cycle Assessment (LCA) methodology (Andreola et al., 2007, Barberio et al., 2010). In particular, the goal of the LCA study was to assess and compare the environmental impacts of two end-of-life scenarios of MSWI bottom ash: landfill disposal (conventional scenario) and bottom ash recovery for glaze frit production (innovative scenario). The results demonstrated that the innovative scenario is environmentally advantageous for all of the selected impact categories. The principal advantages of the proposed system are connected with the use of secondary materials from waste instead of primary raw material in the frit production, and with the avoidance of landfill disposal (Barberio et al., 2010).

The present research evaluates the technical characteristics (microstructure, aesthetic appearance, dry particle abrasion resistance and acid resistance) of a glaze composed by relevant amounts (>80 wt.%) of a frit obtained from vitrified MSWI bottom ash. The glaze was applied onto fired tile bodies both by a conventional wet technique (already in use in the ceramic industry) and by an innovative one, i.e. the plasma-spraying technique.

Conventionally, glazes are applied onto ceramic tiles as slurries: the various constituents of a glaze mixture (including the frit) are dispersed in an aqueous suspension, which is deposited onto the surface of the fired or unfired (“green”) tile and is subsequently dried and fired. The wet technique generates glazing wastewater that requires purification, resulting in the production of glazing sludge (estimated sludge production in the Modena and Reggio Emilia ceramic district: about 35,000 ton/year) (Andreola et al., 2010b). Glazing sludge, whose composition is not constant but it is strictly dependent on the type of manufactured glaze (which may also contain heavy metals such as Pb, Cd, Cu, Zn, etc.) and chemical compounds used for depuration (inorganic/organic coagulants, flocculants, etc) is codified in Europe as special waste and classified as hazardous or not hazardous (European Waste Code 10.12.11 or 10.12.12) depending on the content of heavy metals (Andreola et al., 2010). The management and recycling of this type waste is difficult (Andreola et al., 2010).

In this study, one of the most common methods for the deposition of this slurry, the waterfall application, was chosen and compared to the dry technique of plasma spray. In the waterfall application, the tile passes through a curtain of glaze in aqueous suspension, so that a layer of glaze and water remains on the upper surface of the tile. The uniform thickness of the suspension curtain and its constant flow rate guarantee a uniform film thickness on the tile (ACIMAC, 2000). After firing, this results in a smooth layer of glaze of some hundreds of microns of thickness.

By contrast, atmospheric plasma spraying (APS) is a dry deposition technique, belonging to the group of thermal spray technologies. In a direct-current (DC) plasma torch, an electric arc is struck between a tungsten cathode and a copper anode with a central nozzle as a gas mixture (typically Ar + H₂) flows through them. The gas, ionised by the electric arc, is converted to thermal plasma as it emerges into the atmosphere from the anode nozzle, and the coating material, in dry powder form, is radially injected into it. The plasma, on account of its high temperatures (up to 15000 °C at the nozzle exit) (Herman et al., 2000) heats the powder particles up to their melting point and above, resulting in a stream of molten droplets directed toward the substrate. Upon impact onto the substrate, the molten droplets flatten and solidify in a few microseconds, assuming a typical lamellar (or splat-like) morphology.

Potential advantages of plasma spraying over conventional wet glazing techniques (such waterfall application) are no wastewater glazing to be managed, simpler waste management with no wet sludge to be disposed of on the production line, and excellent flexibility (the same technique can be adapted to a vast range of glazing materials, without the need to adjust the suspension formulation). Solid residues coming from APS processing of ceramic mixtures, which consist of overspray powders collected in the spray booth and in the filter (Davis, 2004), are indeed in dry powder form and in significantly lower amount and hazardousness than sludge from traditional wet glazing. Moreover, the thermal plasma processing of ceramic mixtures generally results in vitrified inert wastes, easier to manage; accordingly, the plasma spray process itself has been proposed as a waste inertisation technique (Kuo et al., 2010).

Disadvantages might include the higher investment and operating costs of plasma-spraying equipment compared to conventional waterfall glazing equipment, and possible initial difficulties in adapting such equipment to ceramic tile production lines.

Although plasma spraying has seldom been tested with glasses, its suitability for this purpose was shown by previous research with bioactive glasses deposited onto titanium alloys (Gabbi et al., 1995, Schrooten and Helsen, 2000, Miola et al., 2009, Cannillo et al.,). Other studies considered metal-glass composite coatings as possible oxidation-resistant layers in Thermal Barrier Coating (TBC) systems for the protection of Ni superalloy turbine blades (Mack et al., 2006), plasma-sprayed BSAS (BaO–SrO–Al₂O₃–SiO₂) glass as Environmental Barrier Coating (EBC) for the protection of SiC-based composites (Harry and Linsey, 2002), functionally Graded Coatings on sintered alumina with a CaO–ZrO₂–SiO₂ glass system mixed with alumina (100% alumina at the interface with the substrate, 100% glass as top layer) (Cannillo et al., 2007). Some studies also considered plasma-sprayed glasses deposited onto traditional ceramics, e.g. for the decoration of wine glasses (Bessmertnyi, 2001, Min’ko et al., 2002) and for the functionalisation of porcelain stoneware tiles (Bolelli et al., 2005a, Bolelli et al., 2005b, Bolelli et al., 2007). The APS technique has also been employed to deposit composite glazes consisting of recycled Cathodic Ray Tube glass mixed with alumina (Barbieri et al., 2005). These latter studies showed that coatings of some hundreds of micrometres could be obtained by multiple scans of the plasma torch in front of the substrate. As-deposited coatings exhibited good cohesive strength and could be employed in the as-deposited condition, without firing, but they were generally somewhat defective (pores, inter- and intralamellar cracks): good densification was obtained by heat treating the coatings at different temperatures (Bolelli et al., 2005a, Bolelli et al., 2005b, Bolelli et al., 2007, Barbieri et al., 2005).