The influence of reducing conditions on the dissolution of a Mn-rich slag from pyrometallurgical recycling of alkaline batteries Priscilla Pareuil, Hakim Hamdoun, François Bordas * , Emmanuel Joussein, Jean-Claude Bollinger Université de Limoges, Groupement de Recherche Eau-Sol-Environnement (GRESE) EA 4330, Faculté des Sciences & Techniques,123 avenue Albert Thomas, 87 060 Limoges, France article info Article history: Received 9 June 2009 Received in revised form 16 July 2010 Accepted 20 August 2010 Available online 15 September 2010 Keywords: Slag Reductive conditions Sodium ascorbate Nitrogen bubbling Manganese abstract The redox potential (Eh) is a key parameter for controlling the release of elements from solid materials. Nevertheless, this parameter is seldom taken into account during risk assessment studies within any regulatory framework. We studied the incidence of redox changes to the solid materials using two batch procedures: i) a gradient of redox conditions obtained using sodium ascorbate solutions at various concentrations; ii) N 2 bubbling in water. These experiments were performed on two Mn-rich slag samples coming from a pyrometallurgical plant that recycles alkaline batteries. Both samples differed slightly in their chemical composition and solid characterization (i.e. presence of Mn oxide) and pre- sented different behaviours. The present study focused on the release of the main slag elements (i.e. Mn and Si) chosen as indicators of the dissolution of primary silicate phases. Solid phase analyses (SEM-EDS and XRD) were coupled with the monitoring of elements in leachates in order to understand their behaviour and the mechanisms involved. The results indicated that the solid composition plays an important role in the release mechanisms. The presence of Mn oxide enhanced the mobilization of Mn in the greatest reducing conditions (320  5 mV/SHE), to the extent that 42% of the total Mn was leached. This demonstrated the significance in studying the solid phases (using SEM-EDS and XRD) before and after any leaching experiment. From a laboratory practice point of view, it was easier to use sodium ascorbate and allowed, in our case, greater reducing conditions to be reached. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Slags constitute a by-product from pyrometallurgy arising from (i) the concentration of impurities in molten metals and (ii) the oxidation of metals on their contact surface by air. The liquid hot slags are separated from the molten metals, and then cooled to ambient temperature. Significant volumes of slags are produced yearly throughout the world, either from ferrous or non-ferrous metallurgy production. In 2008, the world production of ferrous slag was approximately 240e290 Mt and the output of steel slag represented about 115e170 Mt (data from http://minerals.usgs.gov/ minerals/pubs/commodity/iron_&_steel_slag/mcs-2009-fesla.pdf). Slags are either dumped or reused as a secondary material in order to preserve natural resources and their after-life is an important economical and ecological concern. In Europe, around 50% of steel slags are reused in road construction and only 11% are dumped (http://www.ctpl.info). In all cases, environmental safety has to be investigated because of the potential mobility of their constituents such as metallic elements. To assess the risk of contamination, various standardized leaching procedures are available depending on the country’s legislation, the origin of solid materials and the way they will be reused. These tests allow the influence of a parameter in controlled conditions to be easily studied, however the results are often difficult to translate to “in- situ” environmental conditions. Most of them are performed with high-purity water (i.e. EN 12457-1, 2002). Some are carried out by studying the effect of pH conditions (i.e. XP CEN/TS 14997, 2006) but none takes into account the influence of the redox potential (Eh) on the release of elements, despite its role in the control of the speciation and/or the mobility of elements. Nevertheless, the Dutch standard NVN 7348 (Draft) (1994) proposed to determine the redox properties of a material by evaluating its reducing capacity using a batch leaching experiment and monitoring the redox potential, such as that used by Mizutani et al. (2000) on municipal solid waste incineration residues. Redox potential is now recognized to be, like pH, a key factor in element release mechanisms (Fällman and Aurell, 1996; Quevauviller et al., 1996; van der Sloot et al., 1996; Wahlström, 1996; Cappuyns and Swennen, 2008; Davranche and Bollinger, 2000; Fällman, 2000). Nevertheless, during environ- mental risk assessments of slags, its influence has not been studied until now. * Corresponding author. Tel.: þ33 555 457 485; fax: þ33 555 457 203. E-mail address: francois.bordas@unilim.fr (F. Bordas). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman 0301-4797/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2010.08.019 Journal of Environmental Management 92 (2011) 102e111