Abstract: A new processing route for developing porous ceramic materials from waste ashes has been developed and it relies on the use of starch as a pore former by exploiting its gelling ability in water and burn off properties during sintering. Incinerator bottom ash (IBA) and pulverized fuel ash (PFA) were processed and sintered to form new ceramic materials using conventional ceramic technology involving milling, pelletization and sintering. PFA was added to IBA to control its sintering behavior. Starch was incorporated to a range of milled IBA/PFA mixes and its effect on the physical and microstructural properties of sintered pellets has been investigated. The efficiency of starch to engineer porosity in the sintered pellets is dependent on the IBA/PFA ratio, with minimal changes in density occurring for mixes containing high levels of IBA. A 40/60 (IBA/PFA) milled mix can form sintered pellets with properties controlled by the level of starch addition. Densities as low as 1.3g/cm3, an extensive volume of interconnected porosity, and water absorption capacities around 25% were obtained for pellets from 40/60 (IBA/PFA) mix containing 20 to 30% starch and sintered at 1120oC. Processing IBA and PFA to produce porous ceramics using starch as a pore forming additive represents a potentially attractive reuse application for these high-volume waste byproduct materials.
Introduction
In the past few decades, the rapid process of industrialization and urbanization has led to greatly increased generation of waste materials. In the meantime, increasingly stringent environmental control regulations together with a shortage of acceptable sites for new landfills have resulted in limitations on the waste disposal options. Incineration of municipal solid waste (MSW) is one of the possible alternatives, which provides a means of producing a reduced volume (up to 90%) of sterile, odourless and practically inert ash, while using the calorific power of waste to produce energy (vapour, electricity). The management of waste materials even after incineration presents a problem due to considerable amounts of solid residues produced.
The MSW incinerator bottom ashes are a multi-component system of slag fractions, glass, metals, other constituents (e.g. rock) and unburnt residues [1, 2]. The current relatively low level of IBA reuse combined with the potentially large amounts of IBA that are likely to be produced in the future makes the development of new applications for this material an important research area.
Previous work has investigated the production of dense ceramic materials from the fine fraction (<8mm) of IBA using conventional ceramic technology involving wet milling, drying, compaction and sintering [3, 4]. This demonstrated that IBA can be processed to produce ceramics with maximum density of 2.6 g/cm3, but sintering occurred over a relatively narrow temperature range before sample softening and bloating resulted in density reductions.
An attractive reuse application for mineral wastes is the fabrication of porous ceramic materials due to their wide field of utilization in lightweight structural materials, biomaterials or thermally insulating applications. The requirements for the ceramic matrix and pore structure depend on the intended application of the material and can be satisfied by using the appropriate production techniques.
"Starch consolidation" technique was developed by Lyckfeldt and Ferreira [5] for forming porous ceramics using starch as both consolidator/binder and pore former, and has applied by a number of researchers to produce porous materials [6, 7]. The fundamental principle of this technique is the possibility of forming complex-shaped components by transforming a powder suspension into a rigid structure due the water absorption and the swelling ability of starch granules in warm water [8, 9].
In the present work, starch was added to milled IBA to produce a porous material. However, pulverized fuel ash was also introduced to control the behavior of IBA during heating. The sintered properties of milled IBA/PFA pellets have been investigated. The effect of starch addition on the physical properties of sintered (IBA/PFA) pellets is reported, together with selected mineralogical and microstructural characterization data.
Experimental work
2.1. Materials
IBA was obtained from a major ash processing plant in SE England where it had been weathered for between six to eight weeks. The majority of the ferrous and non-ferrous metals were already removed and the ash with a particle size of <8mm was selected for use in these experiments. This size fraction represents approximately 45% by mass of the total weathered ash and was used because commercially viable outlets for this material have not so far been developed.
Low lime (CaO) PFA was collected from the 2,000 MW coal-fired power station at West Burton, Nottinghamshire, UK. PFA complied with the specification for PFA use as a cement replacement material in concrete [10].
Starch is considered as a condensation polymer of glycose, composed of carbon, hydrogen and oxygen with a general molecular formula (C6H10O5)n. It consists of linear amylose and highly branched amylopectin polysaccharides [8, 9]. The glucose units expose a large number of hydroxyl groups that confer a strong hydrophilic character, facilitating dispersion of granules in aqueous media. Starch granules are insoluble in water below 50oC but when heated to a temperature between 55 and 80oC they absorb water and increase in volume. The starch granules undergo a rapid and irreversible swelling by water absorption, causing the particles to stick together and, consequently, consolidate into a solid body [9, 11]. A commercially available grade of potato starch was used in these experiments (Robin starch by Reckitt and Colman Products Ltd.).
2.2. Chemical and mineralogical characterization
The chemical compositions of milled IBA and as-received PFA were determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Perkin Elmer 3580B) following digestion using lithium metaborate and tetraborate fusion at 1000oC and acid dissolution [12].
Major crystalline phases in milled IBA and PFA were characterized by X-ray diffraction (XRD, Phillips PW1830 fitted with PW 1820 goniometer), on samples ground to less than 150µm, using Cu K? radiation at an accelerating voltage of 40 kV and current of 40 mA.
2.3. Production of sintered IBA/PFA pellets
500g batches of IBA and PFA were wet-milled for 8 hours in a 3 litre polypropylene mill containing high-density alumina milling media rotating at approximately 50rpm. Batches were made with IBA/PFA ratios of 100/0, 80/20, 60/40, 40/60, 20/80 and 0/100. The milled slurries were passed through a 355µm sieve to remove oversize material prior to de-watering by pressure filtration using a stainless steel extraction vessel (Whatman GF/C filter paper). The filter cakes were formed into spherical pellets and were then fired in an electric furnace at temperatures between 1080 and 1220oC, using a ramp rate of 6oC/min and a dwell time of 1 hour.
2.4. Production of sintered pellets using starch
Additions of up to 30% by weight of <150µm granular starch were thoroughly mixed with milled IBA/PFA dried powders, and water added to give a water to solids ratio of 0.82. This was stirred for 2 hours and the mix then dried at 80oC until it had a suitable consistency for forming spherical pellets of similar size to those made directly from the filter cake that did not contain starch.
The pellets were fired at temperatures between 1080 and 1200oC with a 1-hour dwell, using a ramp rate of 1oC/min to 500oC, to allow effective starch burnout, followed by a 5oC/min ramp up to the sintering temperature.
2.5. Thermal behaviour of starch and starch-containing milled material
Starch and 40/60 (IBA/PFA) milled powders containing 0 and 30% starch were subjected to thermogravimetric analysis (TGA) in an air atmosphere from 25 to 900oC using a Stanton Redcroft thermal analyzer (STA-1500 Series) at a heating rate of 6oC/min in air, using calcined alumina as reference material.
2.6 Characterization of sintered material
The density of sintered pellets was determined using Archimedes' method and the water absorption capacity determined from the weight increase of “surface dry” samples after immersion in water for 24 hours. Selected sintered pellets were crushed and ground to <150?m to determine the crystalline phases present by XRD. Polished surfaces of sintered pellets were examined by Scanning Electron Microscopy (SEM-Phillips XL40).
Results
3.1. Material characterization
The chemical compositions of milled IBA and as-received PFA are given in Table 1. Milled IBA contained high levels of SiO2 and CaO, with moderate levels of Al2O3 and Fe2O3, also found in other IBA characterisation studies [1, 2]. However, the CaO content in PFA was very low (<2%) and SiO2, Al2O3 and Fe2O3 were the major oxides present, representing 86% in weight of the samples, classifying the ash as ASTM Type F [13].
Analysis of XRD data for the milled IBA and as-received PFA showed that the main crystalline phases present in milled IBA were quartz (SiO2) and calcite (CaCO3), together with smaller amounts of gehlenite (Ca2Al2SiO7) and some hematite (Fe2O3). Major crystalline phases present in PFA were quartz and mullite (Al6Si2O13), with smaller amounts of hematite and possibly some anorthite (CaAl2Si2O8). These are in general agreement with mineralogy reported previously in IBA and PFA [2, 14, 15, 16].
3.2 Properties of sintered IBA/PFA pellets
3.2.1. Physical properties
The effect of firing temperature and PFA addition on the density of IBA/PFA pellets is shown in Figure 1. It is clear that pellets containing different IBA/PFA concentrations behave differently with firing temperature. IBA pellets sinter between 1080 and 1100oC with a maximum density of 2.3g/cm3 at 1100oC, but higher temperatures result in sample deformation and bloating associated with reduced density and pore formation [3, 4, 17]. Increasing PFA concentration to 60% in the mixes delays sintering and the samples reach a maximum density of 2.4g/cm3 at 1190oC. Further increase to 100% PFA results in pellets achieving densities between 2.3 and 2.5g/cm3 in the temperature range 1080 to 1220oC.
IBA contains high concentrations of Ca-containing phases, oxides, carbonates, silicates and sulphates that reduce the melting point of the glassy phase [18]. The low viscosity, high mobility liquid formed that dissolve the remaining refractory minerals is responsible for the accelerated densification of IBA at relatively low temperatures. Increasing PFA concentration in IBA/PFA pellets reduces the amount of fluxing minerals, and the high viscosity and low mobility flow of the melts delays densification and results in a wider temperature interval over which pellets sinter.
Figure 2 shows the effect of firing temperature and PFA addition on the water absorption of sintered pellets, demonstrating the reduction that occurs with increasing sintering temperature. Pellets containing high levels of IBA exhibit a more rapid reduction in water absorption with increasing sintering temperature. 100% PFA-containing sintered pellets are relatively non-porous at the temperatures investigated due to the high densities attained.
3.2.2. Microstructure
Figures 3(a), (b) and (c) show SEM micrographs of polished surfaces of pellets of 60/40 (IBA/PFA) mix with 30% starch and 40/60 (IBA/PFA) mix with 0 and 30% starch, fired at 1120oC. The main difference between the 60/40 and the 40/60 (IBA/PFA) pellet microstructure is the level of densification of the glassy phase achieved. The polished surfaces of 60/40 (IBA/PFA) pellets appear to be well densified with some residual porosity, where the 40/60 (IBA/PFA) pellets show higher levels of a smaller pore network, where the pores are interconnected.
3.3. Effect of starch on properties of sintered pellets
3.3.1. Thermal behaviour of starch and starch-containing milled material
TG data for starch alone and 40/60 (IBA/PFA) milled powder containing 0 and 30% starch is shown in Figure 4. According to the TG curve starch begins decomposing around 200oC and is completely oxidized by approximately 450oC. This result is consistent with the thermal analysis studied reported previously [5, 19]. The 40/60 (IBA/PFA) mix containing 30% starch confirms starch burn off by a 26% weight loss between 200 and 450oC. This is followed by a secondary weight loss, accounting for 2.7% of the original sample weight, between 600 and 750oC and this is associated with the decomposition of CaCO3 to CaO and CO2 [20].
3.3.2 Physical properties
The effect of firing temperature and starch addition on the density and water absorption of 60/40 and 40/60 IBA/PFA pellets fired at temperatures between 1100 and 1200oC is shown in Table 2. Starch addition reduced the densities of 40/60 IBA/PFA pellets fired at 1100?C from 1.55g/cm3 to 1.16g/cm3 for a 30% starch addition. The corresponding reduction in density for the 60/40 IBA/PFA pellets was from 1.58g/cm3 to 1.46g/cm3, indicating reduced effect of starch in mixes containing higher IBA contents. The effect of starch addition reduced in pellets sintered at higher temperatures. Water absorption significantly increased with starch addition although the extent of increase was lower in mixes containing higher levels of IBA.
3.3.3 Mineralogy and microstructure
The starch-free and starch-containing 40/60 (IBA/PFA) pellets sintered at 1120oC, had identical X-ray diffraction patterns, and therefore mineralogical compositions. Sanidine (KAlSi3O8) was identified as the major crystalline phase present together with quartz, albite (NaAlSi3O8) and hematite. Sanidine is a common constituent in extrusive igneous rocks such as rhyolites, where the rock is rapidly cooled. It is the high temperature form of potassium feldspars or K-spar. At temperatures below 900oC microcline and orthoclase are the stable forms of KAlSi3O8, while at higher temperatures sanidine is stable [16]. It may form from solid-phase reactions between aluminosilicates, such as mullite, and liberated K oxides or other compounds such as sulphates during sintering. The development of feldspar minerals has previously been reported in sintered mixes containing coal fly ash [21].
Polished surfaces of 40/60 (IBA/PFA) pellets sintered at 1120?C containing 0 and 30% starch are shown in Figure 2. These contain high levels of porosity with the addition of starch increasing the level and connectivity of the pores.
Discussion
IBA contains significant levels of network modifiers and particularly Ca- containing phases that reduce the melting point of the residual glassy phase in IBA. As a result sintering only occurs over a narrow temperature range before the samples soften and deform. This makes the use of carbonaceous additives to control porosity in IBA problematic, since the pores created by the thermal decomposition of organic particles at lower temperatures are effectively filled with glassy phase, resulting in negligible reductions in sintered density.
The incorporation of low lime PFA in IBA intends to increase the concentrations of refractory silicate mineral phases and reduce the overall concentration of fluxing minerals. As a result IBA/PFA samples containing higher PFA concentrations have a higher viscosity glassy phase at a given temperature causing a wider temperature interval between initial sintering and sample melting. Pores produced as a result of thermal decomposition at low temperatures remain in the sintered material during heating.
The efficiency of starch to control the porosity of IBA/PFA pellets depends on the IBA/PFA ratio. Starch addition has a very significant effect on reducing the densities of mixes with high PFA concentrations, while minimal changes in density occur for mixes containing high levels of IBA. Water absorption increased with PFA content and starch addition. Pellets produced from 40/60 (IBA/PFA) mix sintered at 1100oC achieved densities as low as 1.16g/cm3 for 30% starch addition, while pellets sintered at 1120oC achieved densities around 1.3g/cm3 for 20 to 30% starch additions. These showed an extensive volume of interconnected porosity and water absorption capacities around 25%.
Conclusions
1. A procedure has been outlined for the production of porous ceramic materials from IBA with PFA mixes using starch as a pore former by exploiting its gelling ability and burn off properties upon sintering.
2. The control of density of sintered IBA using organic additives is problematic because of the narrow sintering range due to the presence of high concentrations of fluxing minerals in the material.
3. The addition of low lime PFA to IBA is a potential way to control the sintering behavior of IBA by altering its composition. PFA addition reduces the overall concentration of Ca-containing compounds and results in material sintering over a wider temperature range prior to sample melting.
4. PFA addition also controls the efficiency of a carbonaceous material to produce porosity. Sintered pellets from 40/60 (IBA/PFA) mix using starch achieved significantly lower densities, higher water absorptions and more porous microstructures than 60/40 (IBA/PFA) pellets.
5. It is generally feasible to produce porous ceramics from IBA, by using PFA to control the sintering behavior of IBA and starch to control the porosity of the final product. However, the use of starch has limited application in large scale production due to the requirement for water-based processing. Additional work is needed on the selection of appropriate carbonaceous additive and development of the optimum pore structure.
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