Elsevier

Journal of Environmental Management

Volume 180, 15 September 2016, Pages 310-323
Journal of Environmental Management

Research article
Evaluation on chemical stability of lead blast furnace (LBF) and imperial smelting furnace (ISF) slags

https://doi.org/10.1016/j.jenvman.2016.05.052Get rights and content

Highlights

  • Low environmental pH can destabilize slags faster.

  • Presence of oxygen enhance oxidative weathering and faster leaching rate.

  • Slag-water interaction time plays a great role in leaching of slags.

  • Oxide and carbonates are formed as secondary phases.

Abstract

The leaching behavior of Pb and Zn from lead blast furnace (LBF) and imperial smelting furnace (ISF) slags sampled in the North of France was studied as a function of pHs and under two atmospheres (open air and nitrogen). The leaching of major elements from the slags was monitored as a function of pH (4, 5.5, 7, 8.5 and 10) under both atmospheres for different slag-water interaction times (1 day and 9 days). The leaching results were coupled with a geochemical model; Visual MINTEQ version 3.0, and a detailed morphological and mineralogical analysis was performed on the leached slags by scanning and transmission electron microscopy (SEM and TEM). Significant amounts of Ca, Fe and Zn were released under acidic conditions (pH 4) with a decrease towards the neutral to alkaline conditions (pH 7 and 10) for both LBF and ISF slags. On the other hand, Fe leachability was limited at neutral to alkaline pH for both slags. The concentrations of all elements increased gradually after 216 h compared to initial 24 h of leaching period. The presence of oxygen under open-air atmosphere not only enhanced oxidative weathering but also encouraged formation of secondary oxide and carbonate phases. Formation of carbonates and clay minerals was suggested by Visual MINTEQ which was further confirmed by SEM & TEM. The hydration and partial dissolution of hardystonite, as well as the destabilization of amorphous glassy matrix mainly contributed to the release of major elements, whereas the spinel related oxides were resistant against pH changes and atmospheres within the time frame concerned for both LBF and ISF slags. The total amount of Pb leached out at pH 7 under both atmospheres suggested that both LBF and ISF slags are prone to weathering even at neutral environmental conditions.

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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