Title page for ETD etd-10202008-123653


Document Type Doctoral Thesis
Author Van der Vyver, Wilhelmina Fredrika
Email mientjie.vandervyver@exxaro.com
URN etd-10202008-123653
Document Title Determination of factors influencing the degree of reduction disintegration in Northern Cape lump ore and the role of gangue minerals in the propagation of cracks
Degree PhD
Department Materials Science and Metallurgical Engineering
Supervisor
Advisor Name Title
Prof P C Pistorius Supervisor
Keywords
  • reduction disintegration
  • gang minerals
  • iron oxides
  • cracks
Date 2008-09-02
Availability unrestricted
Abstract
The fundamental cause of low temperature breakdown (reduction disintegration) is reduction of hematite to magnetite, resulting in a volume expansion and stress relief through the formation of cracks. Serious reduction disintegration causes poor gas permeability, high flue dust production and scaffolding, poor gas distribution, higher fuel consumption and lower productivity.

Northern Cape iron ore generally performs well when tested for reduction disintegration properties both for blast furnaces and Corex; nevertheless, significant breakdown is experienced when used in the Corex process (at Saldanha Steel).

This study was hence conducted to determine the effects of the following on reduction disintegration:

  • different ore types (from Northern Cape)
  • initial particle size
  • temperature range
  • reduction gas composition

Although disintegration is clearly triggered by reduction, no direct correlation could be established between the percentage reduction and the percentage fines generated.

The results indicated that the presence of gangue minerals alone does not cause fractures to form, but does influence the direction and intensity of fractures to some extent. In many cases cracks form randomly, with no specific preference for either gangue minerals or iron oxides. For most of the samples, an incubation period was observed before the first cracks formed. No crack propagation was observed after initial cracking.

This study indicates that the degree of reduction disintegration depends mostly on furnace conditions. Reduction disintegration increased with higher hydrogen percentages (>5%), higher temperatures (in the 500C-700C range) and longer exposure. Disintegration of the samples decreased at temperatures higher than 750C.

For particles smaller than 16 mm an inverse relationship was found between the average particle size and the percentage of fines generated, in line with the observation that most of the disintegration is due to spalling from particle edges rather than particles breaking into smaller clumps.

The results indicate that it is important to manage the temperature in the top of the blast furnace and the COREX shaft, and the time spent at temperatures below 750C, to minimize the amount of fines generated.

2008 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.

Please cite as follows:

Van der Vyver, WF 2008, Determination of factors influencing the degree of reduction disintegration in Northern Cape lump ore and the role of gangue minerals in the propagation of cracks, PhD thesis, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-10202008-123653/ >

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