Research articleEvaluation on chemical stability of lead blast furnace (LBF) and imperial smelting furnace (ISF) slags
Introduction
Metallurgical slags were considered to be chemically inert in the 1980s because metals are associated with glass and silicate; however, many recent studies have pointed out that the leaching from slag dumping sites have deleterious effects on the environment and led to the contamination of soils, sediments, and streams (Douay et al., 2009, Ettler et al., 2005; Seignez et al., 2008). Several literature reports have been focused on quantifying pollutant mobility resulting from the leaching of metallurgical slags either in batch (Ettler et al., 2002, Ettler et al., 2003; van Hullebusch et al., in press; Yin et al., 2014) or in open flow experiments (Seignez et al., 2006), in order to approximate their fate under landfill conditions (Mahé-Le Carlier et al., 2000) and in soils or soil-like environments (Ettler et al., 2004, Sobanska et al., 2000).
When slags are subjected to weathering and leaching, the release of major elements and metals is controlled by mineralogical phases present in the slags (Piatak et al., 2004, Piatak et al., 2015), and affected by changes in the pH associated with dissolution/precipitation of a number of solid phases (Vítková et al., 2013). The pH-stat leaching tests have been widely applied for the evaluation of municipal solid waste fly ash (Quina et al., 2009), contaminated soils and sediments (Cappuyns and Swennen, 2008), in many environmental samples (Rigol et al., 2009), and secondary lead smelter fly ash (Vitkova et al., 2009). However, only a few studies have applied pH-stat leaching tests on metallurgical slags such as arsenic metallurgical slags (Ganne et al., 2006) and primary lead smelting slags (de Andrade Lima and Bernardez, 2013).
Thus, this paper focused on the pH-stat leaching of lead blast furnace (LBF) and imperial smelting furnace (ISF) slags and further addressed the chemical stability and leaching behaviors of these slags as a function of liquid bulk pH. The objectives of this study were (i) to investigate the leaching behavior of both slags as a function of pH (from 4 to 10), (ii) to evaluate the effect of short term leaching (1 day) as far-from equilibrium condition, (iii) to evaluate the effect of long term leaching (9 days) as near-equilibrium condition, (iv) to couple with the geochemical model to identify possible speciation-solubility controlling phases and (v) to study the changes in surface morphology and newly formed secondary phases after being subjected to weathering.
Section snippets
Lead blast furnace (LBF) and imperial smelting furnace (ISF) slags
Lead blast furnace (LBF) and imperial smelting furnace (ISF) slags used in this study were obtained from lead and zinc smelters located in the industrial basin of Nord-Pas-de-Calais, northern France. Both slags are granulated wastes where the size distribution is log normal with a 500 μm dominant class and the particle size distribution varies from 100 μm to 5 mm (Sobanska et al., 2000). However, the particle size distribution of both LBF and ISF used in this study varies from 100 μm to 2 mm
Leaching behavior of LBF and ISF as a function of pH under open air atmosphere
Leaching of major elements (Ca, Fe, Si, Zn and Pb) from LBF and ISF slags can be observed as a function of pH as well as leaching time: short-term leaching (1 day) and long-term leaching (9 days). Fig. 1 illustrates the different leaching patterns of major elements during pH-dependent leaching of LBF slag. All elements (Ca, Fe, Zn, Mg except Si and Pb) were found to be relatively dependent on the pH where highest concentrations can be found at pH 4 and a sharp decrease or lowest concentrations
Dissolution kinetics of both slags
During far-from equilibrium leaching condition under open air atmosphere, the release of elements from both LBF and ISF slags are controlled by dissolution kinetics and the primary mineral phases present in each slag. The leaching rates of Ca, Si and Zn from LBF slags are two times faster than those from ISF slags at pH 4, suggesting that LBF slags are prone to weathering under acidic conditions where leaching of two major-matrix elements Ca and Si consequently enhanced the leaching of Zn
Conclusions and comments
The present study showed that the environmental stability of LBF and ISF slag depends on a wide range of parameters: the changes in pHs, the primary mineral phases present in the slags, the influence of the atmospheres, and slag-water interaction time.
Acknowledgements
The authors thank the EU for providing financial support through the Erasmus Mundus Joint Doctorate Programme ETeCoS3 (Environmental Technologies for Contaminated Solids, Soils and Sediments, grant agreement FPA n°2010-0009). The authors are thankful to Dr Nicolas Seignez and Dr Arnaud Gauthier for providing slag samples, and Dr Gaëlle Charron for SEM analysis.
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