Mössbauer quantification of pyrrhotite in relation to self-heating A. Navarra a, * , J.T. Graham b , S. Somot c,1 , D.H. Ryan b , J.A. Finch c a École Polytechnique de Montréal, 2500 chemin de Polytechnique, Montreal, QC, Canada H3T 1J4 b Physics Department, McGill University, 3600 University St., Montreal, QC, Canada H3A 2T8 c Mining and Materials Engineering Department, McGill University, 3610 University St., Montreal, QC, Canada H3A 2B2 article info Article history: Received 30 December 2009 Accepted 31 March 2010 Available online 18 April 2010 Keywords: Analytical methods Mass balancing Sulphide ores Ore handling Ore mineralogy abstract Pyrrhotite (Po) is widely considered the most significant mineral in the self-heating of sulphide ores and concentrates. It is therefore desirable to determine the amount of Po in a sample. This is commonly accomplished through X-ray diffraction (XRD) or quantitative evaluation of materials by scanning elec- tron microscopy (QEM–SEM), neither of which are suited to oxidizing samples such as Po. The sample preparation for these methods often requires heating and drying, which may alter the composition. This paper introduces another quantitative method based on Mössbauer analysis, whose sample preparation is limited, and not as likely to alter the sample. The technique is tested using binary and ternary mixtures containing pyrrhotite with pentlandite (Pn), pyrite (Py) and sphalerite (Sp). A detection limit of 2 wt.% Po is easily obtained in the Po–Pn binary mixtures. The Po is also successfully measured in Pn–Po–Py and Sp–Po–Py mixtures; however, the presence of sphalerite increases the time required for an accurate mea- surement. The time required to measure Po in complex samples may be optimized through future work. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Significance of pyrrhotite Although pyrrhotite (Po) is believed to be the most significant mineral in the self-heating of sulphides (Rosenblum and Spira, 2001; Somot and Finch, 2006; Sudbury and Petkovich, 1961), there is debate regarding the mechanisms of self-heating, which may in- deed vary with the nature of the samples. Even without a complete understanding, industry has adopted a rule of thumb: stockpiles containing more than 10 wt.% Po are at high risk of self-heating and/or self-igniting (Rosenblum and Spira, 2001). The self-heating phenomenon is often accompanied by the release of noxious gasses (H 2 S, SO, SO 2 ) and acid drainage. Although the rule of thumb is supported by case studies, it is clearly insufficient: Bayah et al. (1984) observed pyrrhotite driven self-heating with only 7 wt.% Po, and it has been shown that the phenomenon may occur with less than 4.5 wt.% Po (Somot and Finch, 2006). The 10 wt.% limit is especially questionable because of the high quantification limits (ca. 9 wt.%) inherent to the methodology of Rosenblum and Spira (2001). To refine the industrial tolerances for Po content, an improved understanding of the self-heating phenomenon and more reliable quantification techniques are essential. A promising hypothesis was introduced by Somot and Finch (2006), suggesting that Po mobilizes the sulphur within the stockpiles. The sulphur may then be transported to oxygen-rich zones for the exothermic production of SO, SO 2 , and H 2 SO 4 . The catalyzing effect of pyrrhotite may be related to its variable S/Fe atomic ratio. Pyrrhotite is defined to be the group of minerals having the NiAs substructure, and the general formula Fe 1x S, with x between 0 and 0.13 (Wang and Salveson, 2005). The sulphur poor Po (0.00 < x < 0.10) exhibits hexagonal crystal structures; the end- member having x = 0 is referred to as troilite. The sulphur rich Po (0.09 < x < 0.13) exhibits a monoclinic phase that is weakly mag- netic; this monoclinic/magnetic phase co-exists with the hexago- nal phase when 0.09 < x < 0.12, and dominates for 0.12 < x< 0.13. Po supports several stoichiometries, each with different sulphur content: FeS (x = 0), Fe 11 S 12 (x = 0.08), Fe 10 S 11 (x = 0.09), Fe 9 S 10 (x = 0.10), Fe 7 S 8 (x = 0.13). Furthermore, these compounds can be blended into non-stoichiometric masses (Murch et al., 1974; Belzile et al., 2004; Wang and Salveson, 2005). By contrast, pyrite (Py) is presumed stoichiometric unless otherwise specified (FeS 2 , x = 0.50), hence has a much narrower definition; it is not confused with its neighbour greigite (Fe 3 S 4 , x = 0.25), because of significant differences in physical properties. The structure of pyrrhotite is of longstanding interest, because of its importance in geology and mineral processing (Belzile et al., 2004; Wang and Salveson, 2005). There have been several X-ray and Mössbauer spectrographic studies (for a review, see Wang and Salveson, 2005). However, the relationship between Po transformation and the kinetics of self-heating is not well known; 0892-6875/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2010.03.022 * Corresponding author. Tel.: +1 514 567 0636. E-mail address: alessandro.navarra@gmail.com (A. Navarra). 1 Present address: COREM, 1180, rue de la Minéralogie, Quebec, QC, Canada G1N 1X7. Minerals Engineering 23 (2010) 652–658 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng