Corrosion problems of copper waffle coolers are experienced in Platinum Group Metals (PGM’s) smelting furnaces. The copper cooler wear mechanism was studied through a post-mortem analysis of the refractory corrosion products that were removed from a PGM smelter. Post-mortem samples were characterised using Scanning Electron Microscopy (SEM), X-Ray Fluorescence Spectroscopy (XRF) and X-Ray Powder Diffraction (XRD). On visual inspection of the refractory wall it was observed that at the slag-feed interface the front refractory (mag-chrome) brick was completely corroded and only the freeze lining (frozen slag) formed a barrier between the copper cooler and the feed. At the bottom section of the slag zone the front refractory brick was still intact. Base metal sulphides and element sulphur were the major phases observed at the copper cooler-freeze lining interface while at the copper cooler-front brick interface only covellite (CuS) and element sulphur were observed. It was concluded that wear proceeded through two mechanisms: Reaction of copper with base metal sulphides which infiltrated the freeze lining and gaseous attach of copper by sulphur forming covellite. Front mag-chrome refractory bricks are replaced by graphite blocks in the latest furnace wall designs. A post-mortem graphite block was analysed with SEM, XRD and Inductively Coupled Plasma (ICP) to determine the phases associated with copper cooler corrosion. Base metal sulphides were observed at the copper cooler-graphite block (cold face) interface. Good agreement was found between the phases in the graphite block and the phases in the post-mortem sample where the refractory brick was used. Laboratory experiments were carried out to determine the effect of corrosive gas composition and copper cooler surface temperature on the corrosion rate and morphology of the corrosion products. Tests were performed on copper foils at temperatures from 80°C to 140°C. Corrosive gases included H2S, S2 and S2 with HCl. It was found that when a copper foil is exposed to sulphur the sulphides that form are covellite at 80°C, covellite and yarrowite (Cu9S8) at 110°C, yarrowite and digenite (Cu1.8S) at 140°C. Linear corrosion rate behaviour was observed between 80°C and 110°C since the sulphide scales are not passivating and they poorly adhere to the copper foil. Average corrosion rates of copper foil by sulphur vapour was 54 mm/y at 80°C and 80 mm/y at 110°C, above 112°C the corrosion rate decreased to 5 mm/y. Additions of HCl enhance the corrosion rate at temperatures above the melting point of sulphur (112°C). Chalcocite (Cu2S) forms when copper is exposed to H2S. It was concluded that the corrosion rate and the morphology of the corrosion product are functions of temperature and the corrosive gas composition. Copyright / Dissertation (MSc)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/27993 |
| Date | 17 September 2010 |
| Creators | Thethwayo, Bongephiwe Mpilonhle |
| Contributors | Prof A M Garbers-Craig, s27558933@tuks.co.za |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Dissertation |
| Rights | © 2010, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
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