CHARACTERISATION OF INCINERATOR RESIDUES FOR THEIR POSSIBLE RE-USE

Transcription

1 CHARACTERISATION OF INCINERATOR RESIDUES FOR THEIR POSSIBLE RE-USE Luciano Morselli, Alfredo Riva, Lucia Ramponi, Fabrizio Passarini University of Bologna, Department of Chimica Industriale e dei Materiali; V.le Risorgimento, Bologna Italy Abstract MSW incineration produces a high quantity of residues (slags, fly ashes, sludges). Recycling suitability of residues is therefore an important issue requiring as a first step their physical and chemical characterisation (mass, granulometry distribution, merceology) and determination of their hazardousness (i.e. pollutant contents of heavy metals and chlorinated organic compounds). Characterisation of slags from a representative national plant has shown that an average concentration of pollutants very close to that in fly ashes is found only in the thinner fractions of these residues. Separation of these fractions may therefore be sufficient for obtaining recyclable inert residues. Introduction Municipal Solid Waste (MSW) incineration permits waste reduction by about 70% in weight and 90% in volume as well as the recovery of both energy and materials. In 1997 most of the MSW produced in Italy still finished up in landfills (79.9 %), only 6.6 % being incinerated [1]. This situation is slowly changing in order to comply with the most recent European and Italian laws governing waste disposal in the context of sustainable development, which lay down precise guidelines in order of priority for waste management, namely: prevention of waste production and of hazardous waste; recovery and recycling of matter from waste; recovery of energy from waste; safe landfilling of inert waste and inert residues derived from other waste treatments [2]. Italian law has consequently set a target of 35% for selective collection to be attained by the end of 2003 [3] so that national and local waste management is more and more addressing the issues of recycling, recovery and incineration. Incineration can aid in achieving this target on condition of adopting or optimising modern technologies in terms of combustion chamber, flue gas treatments and energy recovery efficiency. But another aspect of waste incineration should be considered and investigated. If it is considered that a high amount of solid residues derive from incineration (about 30% in weight of the input waste, made up of 27.9 % slags, 2.3% fly ashes and 0.9% sludges) entailing at a national level considerable mass flows which end up in landfills (slags t, fly ashes t, sludges t) [4], then one of the main problems confronting integrated waste management is the recycling of these residues. According to EU law [5] only fly ashes and sludges are classified as hazardous waste which till now has been subject to inertisation and disposed of by landfilling. Slags, not being classified as hazardous, are sent directly to special waste landfills without undergoing any prior treatment. This study analyses slags and fly ashes for the purpose of comparing the chemical composition of the two residues and assessing their effective toxicity as well as the possibility of recovering a part of them as inert material.

2 Experimental and Discussion The study, conducted on slags and fly ashes deriving from an incineration plant in Emilia Romagna, Italy, was aimed at determining the recycling suitability of these residues and entailed their characterisation in terms of mass, granulometry distribution, merceology and hazardousness. The plant investigated is fairly representative of general Italian incineration technology consisting of a grate furnace and flue gas depuration with lime, a fabric filter and a wet scrubber. Several samples of fly ashes and slags (separated from ferrous and inert materials) were analysed in order to determine the concentration of elements suitable for reuse (Ca, Fe, K, Mg, Na, Ni, Sb, Si, Ti, V, Zn) and of micropollutants such as heavy metals (As, Cd, Cr, Cu, Hg, Pb, Se) and chlorinated organic compounds (PCDD, PCDF), which are very important indexes of the hazardousness of residues. Such a characterisation is instrumental for determining the possibility of reusing the residues as, for instance, in cement production, ceramic tile production and glazing, chemical and physical processes for the removal and recovery of metals. Fig. 1 is a schematic representation of the procedure adopted, where d stands for particle diameter. Sample of fly ashes Sample of slags (100%) Separation of ferrous material (97.5%) Separation of inert material (d> 8 cm) Granulometry distribution Analytic determinations Heavy metals Metals suitable for reuse PCDD/PCDF Comparison (43%) Analytic determinations on thinner fractions (d< 9.5 mm) Heavy metals Metals suitable for reuse PCDD/PCDF Fig. 1 Schematic representation of the analytic procedure. Metal concentrations were determined by atomic absorption after fusion of the sample with LiBO 4 [6,7,8]; PCDD/F analysis was conducted according to a certified procedure in a private laboratory. Fly ashes were sampled by collecting about 30 Kg from 15 different sacks in which the content had been appropriately homogenised. Samples were then mixed so as to be perfectly homogenous. Tab. 1 shows the resulting metals concentrations while PCDD/F concentrations are given in Tab. 2.

3 Metal Unit Value Al % 2.3 Ca % 29.0 Fe % 0.8 K % 2.1 Mg % 1.0 Na % 2.5 Si % 4.4 As ppm 19 Cd ppm 277 Cr tot ppm 448 Cu ppm 1344 Hg ppm 9 Mn ppm 747 Ni ppm 136 Pb ppm 6981 Sb ppm 804 V ppm 61 Zn ppm Tab. 1 Metal concentrations in fly ashes Pollutant mg/kg I-TEF (ng TE/Kg) Σ PCDD Σ PCDF Σ (PCDD+PCDF) Tab. 2 Determination of PCDD and PCDF- 2,3,7,8 substituted isomers - in fly ashes A homogenous sample of slags was collected from the bottom ash dump and bigger pieces of ferrous and inert materials (diameter > 8 cm) were manually selected so as to determine the relative composition of the sample (Tab. 3). Fractions Weight (Kg) % in weight of the initial sample Ferrous material Inert material Residual material Total Tab. 3 Composition of the sample of slags collected. A sample of the residual material ( Kg) was dried at 105±5 C and weighed until constant weight ( Kg). The dried slags were subsequently subdivided by size to determine their granulometry distribution (Tab. 4). The thinner material with d< 9.5 mm (57.2 % of the dried slags sample) was divided

4 into four different fractions: A (φ < 1 mm), B (φ = 1 2 mm), C (φ = 2 4 mm) and D (φ = mm), and metal and organic pollutant concentrations were determined for each fraction (Tab. 5). Diameter % of dried sample Weight of fraction (Kg) d > 16 mm d < 16 mm d < 9.5 mm d < 4 mm d < 2 mm d < 1 mm Tab. 4 Granulometry distribution of slags Fractions A B C D Diameter of particles (mm) < Mass (Kg) Parameter Unit Al % Ca % Fe % K % Mg % Na % Si % Ti % Parameter Unit A B C D As ppm Cd ppm Cr tot ppm Cu ppm Hg ppm Mn ppm Ni ppm Pb ppm Sb ppm Σ PCDD µg/kg < 5 Σ PCDF µg/kg < 5 Tab. 5 Analytic determination of metals and PCDD/F on the thinner fractions of slags Comparison of the four different fractions of slags reveals a substantial homogeneity of concentration for a number of elements, such as Ca, Fe, Mg, K, Hg, Sb, the average concentration of the whole fraction of which (A+B+C+D) (see Tab. 6) is in line with that of the single classes. The values for other parameters, especially heavy metals, were found to be more fluctuating, the average proportion between minimum and maximum values being 2.2. Concentration peaks were found in only

5 one fraction, the others showed similar (Na, Si, Cu, Pb) or very different (Cr tot, Ni) values. Moreover, from one granulometric fraction to the other, increasing (As) or decreasing (Al, Ti, Cd) concentration trends were observed, which may be accounted for by enrichment phenomena due to the physical and chemical characteristics of the elements (volatility). Tab. 6 compares the average metal concentrations in the A,B,C,D fraction of slags and in fly ashes. Average A,B,C,D Slags Fly ashes Parameter Concentration (%) Concentration (%) AL Ca Fe K Mg Na Si Ti Concentration (ppm) Concentration (ppm) As Cd Cr tot Cu Hg Mn Ni Pb Sb Tab. 6 Comparison between the thinner fraction of slags and fly ashes. Ca concentration in fly ashes was found to be higher than in the thinner fractions of slags as emission neutralisation had been made with Ca(OH) 2. Cd, Hg, Pb and Sb concentrations were considerably higher in fly ashes (30 times for Cd, 2 for Hg, 2.5 for Pb, 5 for Sb). The concentration of other metals was however lower in fly ashes than in thin slags (Al, Fe, Mg, Si, Cu, Ni), while no substantial differences in concentration between slags and fly ashes were reported for other elements (Na, Mn, Cr tot). Considering the average concentrations of heavy metals, it may be concluded that the characteristics of the thinner fraction of slags (d < 9.5 mm) are similar to those of fly ashes and, therefore, that most of the pollutant load of the slags is held in their thinner fraction. Slag decontamination can therefore be made simply through a physical process of granulometric separation so as to be able to reuse the fraction with d > 9.5 mm as inert material without any further treatment, thus having to submit only the thin fraction (about 43% of the initial sample) to decontamination treatments. Conclusions Emilia Romagna is one of the most advanced regions in Italy as far as waste management is concerned. In 1997, 12% of its waste was selectively collected and 25.8 % incinerated (32.4 % of MSW incineration in

6 Italy took place in this region) [1]. Throughout Northern Italy selective collection and incineration rates are quickly approximating those of the most advanced European countries (with recycling at 25% and incineration at 30 % on average [9]). The problem of solid residues and of their minimisation, recovery and inertisation will become urgent issues at a national level when similar rates of incineration will be attained also in Southern Italy. This study reveals that only a small fraction (about 43 %) of bottom ashes are hazardous so that the separation of the thinner fraction may be sufficient to obtain an inert residue which can be recycled without the need for further treatments. It should also be borne in mind that the chemical composition of slags strongly depends on the type of incinerator input waste. So far MSW has been made up of undifferentiated waste with, over the last few years, an increase in it of plastic materials and a decrease of organic matter content, resulting in a higher calorific value. This is a fundamental parameter for the optimisation of plant running conditions. Over the next few years a change in the average composition of MSW can be expected as a consequence of the increased selective collection of materials. This will result in a change in both waste calorific value and output residue composition. The elimination of materials such as paper, wood, plastics, metals and organic matter could lead to a concentration of the remaining hazardous substances in MSW and thus in solid residues. Further studies for determining the chemical and physical characteristics of the residues originating from different selective collection options are therefore required. Bibliography [1] ANPA, Secondo Rapporto sui Rifiuti Urbani e sugli Imballaggi e Rifiuti di Imballaggio, February, 1999 [2] Comission of the European Community, Communication from the Commission on the review of the Community Strategy for Waste Management, Brussels, 07/30/96. [3] D. Lgs. 22/97 Attuazione delle direttive 91/156/CEE sui rifiuti, 91/689/CEE sui rifiuti pericolosi e 94/62/CEE sugli imballaggi e sui rifiuti di imballaggio. [4] L incenerimento dei rifiuti. Ricognizione sulla realtà regionale, Regione Emilia Romagna università degli Studi di Bologna Dipartimento di Chimica Industriale e dei Materiali, a cura di Luciano Morselli, 1999 [5] Council Directive on Waste 75/442/EEC as amended by 91/156/EEC and adapted by 96/350/EC; Council Directive on Hazardous Waste 91/689/EEC as adapted by 94/31/EC. [6] Metodi analitici per i fanghi, Quaderno n. 64, CNR Istituto di ricerca sulle acque. [7] L. Morselli, M. Cecchini, E. Grandi, A. Iannuccilli, L. Barilli, P. Olivieri, Heavy Metals in Atmospheric Surrogate Dry Deposition, Chemosphere, 38, , [8] L. Morselli, L. Barilli, P. Olivieri, M. Cecchini, R. Aromolo, V. Di Carlo, R. Francaviglia, L. Gataleta, Heavy Metals Determination in Surrogate Dry Deposition. Characterisation of an Urban and a Natural Site, Annali di Chimica, (in press). [9] J. Hannequart, C. Allen, B. Dewulf, The Actions of the Member States of the European Union regarding Waste, First European Forum on Waste, 11/17/97, Brussels.