Self-heating of sulphide ores: transition from stage A to B and evaluating reliability of U.N. test protocol

Some materials when exposed to the atmosphere can exhibit a rise in temperature without requiring an external heat source. These materials are generally referred to as pyrophoric or self-heating substances. The handling, storage, and transportation of these products has been, and continues to be, a challenge. Reliably assessing the self-heating potential of these products is an essential first step. Over the years, a number of test methodologies have been developed and successfully used to "raise a red flag" when a material exhibits self-heating properties. A widely accepted test method recommended by United Nations is routinely used to certify the safety of shipping pyrophoric goods. Test work conducted at McGill University has shown that the U.N. approved test and other single stage testing methods can yield diametrically opposed test results when evaluating the self-heating properties of sulphides. Depending on the degree of oxidation (weathering), an active sulphide can exhibit a level of self-heating values ranging from less than 5J/g to 50J/g. Under the currently accepted analysis, a sulphide concentrate could be tested and shown to be negative (exhibiting minimal heating) which would be considered safe for shipping. If, however, that concentrate is stored over an extended period of time in a weathering favorable environment, the concentrate could oxidize and present a handling, storage, or shipping problem. To make the U.N. and other single stage self-heating procedure more reliable when testing of sulphides, it is suggested that a pre-weathering step is incorporated with the self-heating testing.The second study focuses on the transition of sulphide containing material from heating in Stage A (low temperature heating) to Stage B (mid-high temperature heating). With increasing sample temperature, sample moisture decreased leading to elimination of water which is a key reactant in stage A. Stage B reactions are independent from Stage A reactions and moisture is not required. It has been hypothesized that transition zone results from orthorhombic sulphur (stable below 95.3C) transforming into monoclinic (beta) sulphur (stable from 95.3C to 119.6C). FR-test technique was used to determine this transition zone. Past studies by F. Rosenblum have shown that transition from Stage A to Stage B is unclear. A study was undertaken to examine the relationship between the Stage transition and various factors including weathering duration, ambient temperature and initial element sulphur content of a sample. Results suggest that self-heating capacity measurements do not indicate whether a sample will progress to Stage B.

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