Effect of iron endpoint during Peirce-Smith converting on matte mineralogy and downstream processing of base and platinum-group metals

[摘要] ENGLISH ABSTRACT: The process route for the production of base and platinum-group metals fromnatural sulfide ores commonly requires the conversion of high-iron furnace matteinto an iron-lean converter matte. This is followed by pre-treatment through coolingof the iron-lean molten matte, physical processing of the solidified matte andhydrometallurgical metal extraction. Lonmin is the third largest producer ofplatinum-group metals in the world and utilizes Peirce-Smith converters for blowinghigh-iron furnace matte with air to a final iron concentration or endpoint. Themolten matte is water granulated and solidification occurs via fast-cooling. Thesolidified matte is ground in a closed circuit ball mill with hydrocyclone classificationand subjected to first stage atmospheric leaching. The specification of an ideal ordesirable converter iron endpoint requires careful consideration. Most importantly,it must ensure the crystallization of converter matte with mineralogical qualities thatare within the setpoints of the downstream unit processes and techniques. Anadditional consideration is for the final blown converter matte to achieve anoptimum bulk concentration of the base metals Ni and Cu and platinum-groupmetals Pt, Pd, Rh, Ru and Ir.Mattes characteristic of variable iron endpoints were regularly produced at theLonmin converter plant section. Uncertainty by plant metallurgists in knowing thedesirable iron endpoint, particularly within the context of the Lonmin base metalrefinery, and poor control has had detrimental effects on the mineralogical quality ofthe final matte and hence on the processing characteristics of the solidified matteparticles downstream. A desirable iron endpoint required investigation, selectionand implementation at Lonmin. The primary focus of this study was therefore toquantify the effect of a specific iron endpoint on the mineralogy and mineralchemistry of solidified converter matte. A fundamental examination of thesolidification process upon cooling was regarded as critical to an in-depthunderstanding of the attained mineralogy and mineral chemistry as a function of aspecific iron endpoint. It became equally important to quantify the effect of the resultant mineralogy, and hence iron endpoint, on the physical property of mineralstructures in relation to downstream grinding, liberation and leaching characteristics.Despite considerable industry context, limited in-depth and coherent studies on theeffect of a specific iron endpoint on fast-cooled converter matte systems were foundin both industrial and scholarly literature. Previous findings in literature offered alimited quantitative understanding of the effect on mineralogy and mineralchemistry. Phase and cooling equilibria of multi-component, iron endpoint specificNi-Cu-S matte systems were also not fully available. These would have beenparticularly useful in understanding the complexities of converter mattesolidification as a function of iron endpoint. Physical property knowledge ofconverter matte mineral structures was hardly available and even less so in relationto grinding, liberation and leaching processes. A comprehensive investigation wastherefore required to address these extensive knowledge gaps with respect to fastcooledconverter matte systems in an industrial framework.Three Peirce-Smith converter production samples, representative of the extent invariability of iron endpoints attained at the converter plant, were used in asystematic investigation coupled to a novel combination of modern analyticaltechniques, computational thermochemistry and metallurgical testwork. Themodern analytical techniques included the application of high resolutiontransmission electron microscopy and focused ion beam scanning electronmicroscopy tomography. Computational thermochemistry was applied through theuse of MTDATA phase diagram software. Metallurgical testwork involved laboratorybatch grinding at various specific energies. Closely associated leach experimentswere also considered relevant to this wide-ranging investigation.The Peirce-Smith converter samples investigated were indicative of mattes thatattained specific endpoints of 5.17%, 0.99% and 0.15 weight% Fe. The highestcombined bulk concentration of the important base and platinum-group metals wasachieved in the matte which attained a specific iron endpoint of 0.99%. Themineralogy of all three converter mattes was dominated by nickel sulfide mineralstructures matched to the natural mineral of heazlewoodite. Mineral structures of copper sulfide, NiCu-alloy, spinel and OsRu-alloy were also constituents of thedifferent converter mattes. The attainment of a specific iron endpoint was found toresult in measurable mineralogical differences with respect to relative mineralabundances, external morphological characteristics and mineral chemistry. Themineralogical differences were particularly distinct between mineral structures ofthe high (5.17%) and low (0.99% and 0.15%) iron mattes. Subtle mineralogicaldifferences were evident between mineral structures of the low iron mattes.The 0.99% Fe matte was characteristic of a significantly higher NiCu-alloy relativeabundance, compared to the 5.17% Fe matte. The NiCu-alloy structures were foundto act as the primary collectors of the economically significant platinum-groupmetals. Mineralogical observations were used to develop an understanding of theunderlying mineralization mechanism of NiCu-alloy structures. High-fidelity color andgrayscale 3D reconstructions were produced of the resultant mineralized structures.It was shown theoretically that variations in iron endpoint specific startingcompositions of oxygen-free liquid matte systems alter the solidification pathwaytowards the eutectic. Moreover, a quantitative understanding of liquid phasesolidification of the high and low iron matte systems, including oxygen, wasdeveloped to within ±2.5 oC. Most of the specific energy available for grinding wasexpended breaking the nickel sulfide matrix, particularly of the high iron matte. Thebreakage rates of copper sulfide mineral structures in the 5.17% Fe matte werecalculated to be higher than in the 0.15% Fe matte at 25kWh/t specific energy. The degree of copper sulfide liberation was shown to be higher for the 5.17% Fe mattethan for the 0.15% Fe matte at the same specific energy of grinding. A higher degreeof Ni extraction and Cu cementation could be achieved when leaching low iron matteparticles. The production of converter matte attaining a specific iron endpoint of0.99% was found to be the most suitable with respect to endpoint selection criteria.A practical iron endpoint range of 1.6% to 1.0% was recommended for theproduction of converter matte with a resultant mineralogical quality within theconstraints of the Lonmin base metal refinery.This study offers an integrated understanding of base and platinum-group metalsproduction as a function of a desirable iron endpoint at Lonmin. This was notpreviously available in metal production literature. New technology for themonitoring and consistent control of such a practical iron endpoint range cansubsequently be implemented.

[发布机构] Stellenbosch University