The ignition properties of pyrite, pyrrhotite pentlandite and violarite

Abstract

The oxidation and ignition behaviour of the four major sulfide minerals present in the nickel concentrates smelted at the Kalgoorlie Nickel Smelter (KNS) has been established. These minerals are pyrite (FeS2), pyrrhotite (Fe1-nS, where n = 0 to 0.125), pentlandite ((FeNi)9S8) and violarite ( Ni2FeS4 ).The characteristic behaviour of these sulfides has been examined using Thermogravimetry-Differential Thermal Analysis (TG-DTA) under normal oxidation conditions ( l0ºC/min, air atmosphere). By increasing the heating rate to 40ºC/min and using an oxygen atmosphere, the tendency of the sulfides to ignite was established. Ignition was characterised by a highly exothermic reaction which occurred in association with a rapid mass loss over a short time span. Significant overheating of the samples beyond the temperature of the surroundings was observed. Pyrite, pyrrhotite and violarite all exhibited ignition behaviour while pentlandite did not.Using Isothermal Thermogravimetry (TG) the sulfides were subjected to shock heating conditions (heating rate = 3000-5000ºC/min, oxygen atmosphere) analogous to those which exist in an industrial flash smelter. The order of reactivity of the sulfides agreed with that observed during TG-DTA ignition trials. Even under these more intensely oxidising conditions pentlandite did not ignite. The effect of particle size on the ignition temperature was determined, larger particles igniting at a higher temperature. The magnitude of this effect varied according to the characteristics of the minerals.Products collected during Isothermal TG were examined by optical microscopy, Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA). Using these techniques it was possible to establish the morphology of the products and hence, to propose mechanisms for the reaction of the four sulfide minerals under ignition conditions.In order to simulate the thermal environment which exists in the KNS, a pilot scale model of the reaction shaft was used. Nickel sulfide concentrates of varying mineralogy and particle size distribution were smelted under various conditions. The effect of larger particle size and increasing oxygen partial pressure on the reactivity of these concentrates was established.The products were quenched at the base of the shaft and collected for examination by optical microscopy, SEM and EPMA. Products ranged from unreacted to completely oxidised particles. The morphology and composition of these species were identified. Approximately 30 particles in each of 26 samples were examined with a view to establishing the frequency of occurrence of the particular product types in concentrates of differing mineralogy and particle size. This allowed proposals to be made regarding the fate of the individual sulfide minerals during ignition in the pilot scale flash reactor.