Leaching of Ni-Cu-Fe-S Peirce Smith converter matte : effects of the Fe-endpoint and leaching conditions on kinetics and mineralogy.

[摘要] ENGLISH ABSTRACT: In a first stage atmospheric leach at the Lonmin Marikana base metals refinery,nickel-copper-iron-sulphur Peirce Smith converter matte is leached in recycledelectrolyte from the electrowinning section. The electrolyte contains sulphuric acid,copper and nickel sulphates, and a small amount of iron sulphate. The convertermatte contains mostly nickel, copper and sulphur (typically 48 %, 28 % and 23 %,respectively), but also minor amounts (<5 %) iron and cobalt. The matte alsocontains platinum group elements (PGEs) and other precious metals totalling 0.2 –0.7 % (platinum, palladium, iridium, rhodium, ruthenium, osmium and some gold).The predominant mineral phases are heazlewoodite, chalcocite and a nickel-copperalloy phase, as well as some entrained slag and spinel minerals. The purpose of thefirst stage leach is to extract nickel, while simultaneously precipitating copper andPGEs contained in the recycled electrolyte. Nickel, cobalt and iron are leached byacid and oxygen. Copper is precipitated by a redox reaction in which copper ionsoxidise nickel from the matte. The purpose of this study was to determine the effectsof key variables on the performance of the first stage leach (specifically on theremoval of PGEs and copper from solution and the overall extraction of nickel) andto improve fundamental understanding of these effects.Batch leaching tests were carried out to investigate the effects of the followingfactors: availability of oxygen, initial acid concentration, initial copper concentration,iron endpoint (iron content of the matte), solids/liquid ratio and stirring rate. Liquidsamples were analysed with Atomic Absorption Spectroscopy (AA) to determineleaching kinetics. Characterisation of solid samples from leach tests by quantitativeX-Ray diffraction (XRD) and scanning electron microscopy with an energy dispersivesystem (SEM-EDS) helped to improve understanding of the leaching mechanism.The oxidative leaching mechanism entails an initial period in which the alloy phase isleached by acid and oxygen, while copper reacts with the nickel-copper-alloy andheazlewoodite phases (which react galvanically with each other) to form a chalcociteprecipitate. In a second reaction period, heazlewoodite was transformed to milleriteby acid leaching and the particle structure became more porous. The rate of copper precipitation and nickel extraction were faster during the second reaction period thanthe first reaction period. Some copper leaching occurred once the leachable nickel(60 – 70 %) had been dissolved, provided that the solution was strongly acidic (pH < 2).The non-oxidative leaching mechanism entails a galvanic interaction, between thenickel-copper-alloy and heazlewoodite phases, in which nickel is leached from bothphases and copper is precipitated as chalcocite. Leaching by acid was negligible inmost non-oxidative tests. An initial fast period of copper precipitation was followedby a second slower period. The decrease in reaction rate can probably be linked tothe decreasing availability of the nickel-copper-alloy phase. During non-oxidativeleaching, the particle structure remained mostly intact. Copper precipitation kineticsunder non-oxidative conditions was found to be slower than under oxidativeconditions. The faster copper precipitation kinetics under oxidative conditions ismost likely caused by an increase in porosity and reaction area as nickel is leachedfrom the matte by acid and oxygen.The initial acid concentration, solids/liquid ratio and Fe-endpoint were the mostimportant factors determining reaction kinetics under oxidative conditions. Low initialacid concentrations (37 g/L) and a high solids/liquid ratio improved the extent ofcopper precipitation. Nickel extraction was enhanced by low solids/liquid ratios andhigh initial acid concentrations (74 g/L). Nickel extraction was significantly less(56 % less in one instance) when leaching high iron mattes (5.7 % Fe) rather thanlow iron mattes (< 1 % Fe). Copper precipitation was initially faster when leaching ahigh iron matte, but slower nickel leaching from high iron mattes led to an excess ofavailable acid, which resulted in copper being leached. The results suggest that highiron mattes will lead to poor copper and PGE precipitation in the first stage leach andalso to lower nickel extractions. Consequently, Peirce Smith converting at the plantmust be carefully controlled to avoid high iron mattes.Under non-oxidative conditions, the solids/liquid ratio and Fe-endpoint were the mostimportant factors. The rate of copper precipitation was faster when a high iron mattewas leached, so that a higher percentage copper was precipitated and more nickelwas extracted from the matte.

[发布机构] Stellenbosch University