ASSESSMENT OF THE LEACHING BEHAVIOUR OF FLY ASH FROM A COBALT SMELTER, ZAMBIA

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1 ASSESSMENT OF THE LEACHING BEHAVIOUR OF FLY ASH FROM A COBALT SMELTER, ZAMBIA M. VÍTKOVÁ*, V. ETTLER*, J. HYKS** AND T. ASTRUP * Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague - Albertov 6, Prague, Czech Republic ** DHI, Hørsholm, Denmark Department of Environmental Engineering, Technical University of Denmark, Lyngby, Denmark SUMMARY: The leaching characteristics of fly ash from a cobalt smelter were studied as a function of ph and time using the ph-static leaching test (CEN/TS 14997, ph range 5-12) coupled with mineralogical investigation and speciation-solubility modelling. Two sets of experiments with different equilibration time (48 h and 168 h) were performed. The leaching of metals showed typical U-shaped curves with the highest amounts released at ph 5 and increased concentrations at ph 12. Generally, no significant differences occurred between the 48-h and 168-h leaching. The main solubility-controlling phases in this system were CaCO 3 and CaSO 4 2H 2 O. This approach represents a useful tool for assessing the leaching behaviour of waste materials over a wide range of conditions. 1. INTRODUCTION Solid residues generated in large quantities during high-temperature processes represent a potential environmental risk considering the presence of hazardous components and their possible mobility (Ettler et al., 2008). Fly ashes containg high amounts of metals are of major concern with respect to further treatment or disposal of these materials (Meima and Comans, 1999; van der Sloot et al., 2001). For most developing countries, thermal ashes pose a serious environmental problem since waste treatment is not a priority. Dedicated solutions for different types of these residues need to be developed in relation to specific geological or climatic conditions. As a first step, the leaching behaviour of thermal ashes should be evaluated in terms of their geochemical characterisation. The release of major elements and metals is generally affected by changes in the ph associated with the dissolution/precipitation of various solid phases (Astrup et al., 2006; Vítková et al., 2009). The ph-dependent leaching is an important tool in the evaluation of the leachability of components at different ph values and may be used in the assessment of contaminant release in the long-term perspective (Dijkstra et al., 2006; van der Sloot et al., 2001). Detailed chemical and mineralogical investigations in combination with geochemical speciation modelling provide complex information about solubility controls and release mechanisms. Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste Venice, Italy; 8-11 November by CISA, Environmental Sanitary Engineering Centre, Italy

2 As a case study, fly ash (FA) from a cobalt smelter in Zambia has been investigated with special emphasis on methodology that is also applicable to other solid residues of different origins. The aim of this study was to evaluate the leaching behaviour of selected major elements and metals as a function of the ph and time in association with mineralogical/geochemical investigation of solubility-controlling phases. 2. FROM SMELTER TO LABORATORY 2.1 Smelting of slags The smelter at Chambishi (Zambia) was designed to recover cobalt through reprocessing of old Co-rich slags from the nearby Nkana slag dump. Extensive mining and smelting of copper at this site have resulted in about 20 million tons of slag (containing % Co) dumped in a square kilometre to a depth of about 30 m (Jones et al., 2002). Cobalt is present in oxidised form (CoO) in the slag; therefore, the recovery process requires sufficiently reducing conditions to obtain as much cobalt as possible. These conditions are achieved by addition of a reductant (carbon) to an arc furnace. As a result, various metallic elements are reduced and the separation process of valuable nonferrous metals from iron and the gangue slag constituents followed. The operating temperature is between 1500 and 1600 C, since it needs to be higher than the liquidus temperature of the alloy containing Co, Cu and Fe. Cobalt and iron recovery increases with increased carbon addition (Jones et al., 2002). On the other hand, the recycling of silicate slags produces fine dust particles, which are trapped by the filtering system at the end of the smelting process. This leads to another concern that should be dealt with. 2.2 Specific experimental conditions The interpretation of the laboratory data primarily depends on the test conditions and specific parameters that may have a significant effect on the final results. The equilibrium period depends strongly on the mineralogical composition of the material. Changes in element concentrations under different conditions (ph, time) are the result of dissolution/precipitation of mineral phases and sorption/desorption processes (Dijkstra et al., 2006). Kirby and Rimstidt (1994) distinguish five types of possible reactions of ash from municipal solid waste incineration (MSWI): complete dissolution of readily soluble phases partial dissolution of less soluble phases partial equilibrium between solid phases and the solution formation of secondary phases surface adsorption and desorption Rapid dissolution of soluble phases may be followed by slow release of ions from less soluble phases. In relation to this, test alternatives including initial washout of very soluble salts were performed (Hyks et al., 2007; Vítková et al., 2009) to identify the key phases controlling the release of contaminants. Otherwise, the leachates are dominated by ions released from the main soluble phases (Kirby and Rimstidt, 1994). Considering the limited time required for standardised leaching tests, there is no guarantee that the long-term leaching behaviour in a possible real scenario is appropriately predicted. The reaction time of 48 h used in the ph-static leaching test may not be sufficient to reach equilibrium conditions for all the solid phases. A prolonged equilibration period represents a useful test alternative to assess the leachability and solubility controlling phases more adequately (Dijkstra et al., 2006; Hyks et al., 2007).

3 3. EXPERIMENTAL STUDY 3.1 Fly ash Fly ash (FA) was sampled at the Chambishi smelter situated in the Zambian part of the Copperbelt Province. The main constituents of the FA were as follows: SiO 2 (22%), FeO (12.7%), MgO (7%), CaO (10.7%), K 2 O (4.7%), C (11.3%), S (2.6%) with relatively high contents of metals and other contaminants (see Table 1). The principal phases were calcite (CaCO 3 ); low crystalline Ca-Fe-Mg silicates; silicon dioxide (SiO 2 ), both amorphous and crystalline; and zinc sulphide (ZnS). A significant portion of amorphous phases was present in the sample. 3.2 Experimental procedure The fly ash was subjected to a standardised ph-static leaching test according to CEN/TS All the experiments were performed in triplicate with procedural blanks. Determination of acid/base consumptions (ANC-acid neutralisation capacity, BNC-base neutralisation capacity) was carried out prior to the leaching experiments. Two sets of experiments with different equilibration time (48 h and 168 h) were performed, both at L/S = 10. A mass of 15 g (dry matter) was placed in a glass bottle and the corresponding volume of demineralised water was added. The ph values 5-12 (5, 6, 7, 8, 9, 10, 11, 12) were selected to cover the range of possible scenarios for a 48-h test. Subsequently, selected ph values (5, 7, 9, 11) were used for a long-term (168-h) experiment to assess the influence of time on leaching. Acid (HNO 3 ) or base (NaOH) was added to adjust the ph using manual titration in the initial stage of the test and continuous ph control and acid/base titration followed. An experiment at the natural ph ~ 9.5 (i.e., without addition of acid/base) was included in both cases. After the experiment, the physico-chemical parameters (ph, Eh and specific conductivity) were measured. The leachate was filtered (0.45 µm) and prepared for further analyses of cations and anions (ICP-OES, HPLC). The solid residues were dried at 40 C and prepared for XRD analysis. The analytical results were coupled with geochemical modelling (PHREEQC) to determine the speciation in the leachate and possible solubility-controlling phases. Table 1 - Chemical composition of the fly ash.

4 4. RESULTS AND DISCUSSION 4.1 Leaching of major elements and metals The leaching trends of selected components are depicted in Figure 1. The largest amounts of Ca, Mg and K were leached under acid conditions with a gradual decrease towards ph 12 (Ca ranged from mg kg -1 to 40 mg kg -1 after 48 h). The highest concentrations of Co, Cu, Pb and Zn were released at ph 5 (yielding 720/830 mg kg -1, 1870/2080 mg kg -1, 260/270 mg kg -1 and 10.5/11.6 g kg -1 after 48/168 hours, respectively) followed by a rapid decrease towards the natural ph ~ 9.5 and finally another increase. This U-shaped leaching of metals is typically observed for incineration ashes (Astrup et al., 2006; Dijkstra et al., 2006; Hyks et al., 2007). Silicon followed the leaching trend of metals. In contrast, low concentrations of Fe (with maximum 2.5 mg kg -1 at ph 5) were detected in the leachates. The influence of a prolonged equilibration period was not as significant as expected. Generally, the concentrations of constituents were very similar for both experiments or slightly higher after 168 h. In contrast, the release of Cu dropped substantially after 168 h in the range of ph 5-11 (Figure 1) including the natural ph (not shown). Somewhat more effective long-term leaching (with higher concentrations) was observed for Co, K, Mg, S and Si. This could be explained by slow reactions after the dissolution of more soluble compounds (Kirby and Rimstidt, 1994). Similar to Dijkstra et al. (2006), the time-dependent leaching behaviour varied among elements and the ph. There was no divergence in the general leaching trends. Figure 1. Leaching of Ca, Co, Cu and Zn as a function of the ph and time.

5 4.2 Solubility-controlling phases Two main solubility-controlling phases were determined based on both geochemical modelling results and XRD data. Gypsum (CaSO 4 2H 2 O) precipitated under acidic conditions (ph 5-7), while calcite CaCO 3 dominated over the range of ph Several previous studies reported that gypsum in MSWI residues (Astrup et al., 2006; Dijkstra et al., 2006; Kirby and Rimstidt, 1994) is usually responsible for solubility control at ph values below 9. The diffraction data showed a certain portion of SiO 2 over the ph range selected, as well as clinopyroxene or low crystalline Ca-Fe-Mg silicates. The PHREEQC calculations confirmed the presence of SiO 2 in the leached samples over the ph range 5-7. Moreover, Zn sulphide was observed in all the samples. The mentioned phases were relevant for both (48-h and 168-h) experiments. After long-term leaching, the Fe (hydr)oxides such as ferrihydrite were assumed to be precipitated based on the geochemical modelling. Surface complexation to these secondary phases can describe the leaching of Cu. Sorption of metals onto (hydr)oxide phases can control their leachate concentrations, resulting in lower concentrations with increasing ph (Dijkstra et al., 2006). 5. CONCLUSIONS The leaching of metals and other components from the FA showed a strong dependence on the ph. The highest amounts of metals and major constituents were released under acidic conditions. The leaching results as well as the diffraction data were very similar for both 48-h and 168-h experiments. Despite some differences, we can assume that a standard period of 48 h was sufficient to reach equilibrium in the fly ash - water system. The geochemical calculations showed the presence of gypsum (CaSO 4 2H 2 O) over ph 5-7 and calcite (CaCO 3 ) over ph These results were in good accordance with the diffraction data. The ph-static leaching test proved useful to identify general leaching trends and, in combination with a complex geochemical approach, applicable to assessment of the leaching behaviour of ashes from a cobalt recovering process. ACKNOWLEDGEMENTS This work was funded by DTU Environment and the Czech Science Foundation (GAČR 205/08/0321). A student project was supported by the Grant Agency of Charles University (GAUK 53009) and University Student Project No. SVV The authors would like to thank the laboratory staff, specifically Sinh Hy Nguyen, for the analytical work. REFERENCES Astrup T., Dijkstra J.J., Comans R.N.J., van der Sloot H.A. and Christensen T.H. (2006) Geochemical modeling of leaching from MSWI air-pollution-control residues. Environ. Sci. Technol., 40, CEN/TS (2006) Characterization of waste - Leaching behaviour tests - Influence of ph on leaching with continuous ph-control. CEN 2006, Brussels, Belgium. Dijkstra J.J., van der Sloot H.A. and Comans R.N.J. (2006) The leaching of major and trace elements from MSWI bottom ash as a function of ph and time. Appl. Geochem., 21,

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