An experimental study of slag foaming

Stadler, S. A. C. (Susanna Aletta Carolina) (2002-12)

Thesis (MScEng)--University of Stellenbosch, 2002.

Thesis

ENGLISH ABSTRACT: Slag foaming occurs in several pyrometallurgical processes. These processes include steelmaking in basic oxygen steelmaking furnaces and electric arc furnaces (EAF) as well as various non-ferrous operations like sulphide smelting/converting and base metal slag cleaning. Although slag foaming in steelmaking processes has been extensively researched, little attention has been given to slag foaming in non-ferrous operations. Slag foaming phenomena are complex because often the system consists of three or more phases. The objectives of this study is to review the published work on slag foaming, to obtain through physical modelling an understanding of the principles governing foaming and to investigate slag foaming phenomena through pyrometallurgical experiments. To obtain these objectives, experiments were carried out with aqueous mixtures at different column sizes, different pore sizes for gas injection and varying liquid depths, and also for high temperature metallurgical slags with varying composition and at different temperatures. Through gas injection, foaming conditions were simulated and the equilibrium foam height was measured for different gas velocities. The following conclusions were drawn: 1. For physical modelling of slag foaming in 3-phase systems the average foam index increases with increasing amounts of solids present in the system. The effect of additional solids in the system is independent of the system geometry. 2. The following conclusions were reached by determining coefficients for an empirical dimensional model fitted to aqueous mixtures: Higher liquid density leads to lower foam index values. The influence of the liquid viscosity is dependent on the system investigated and may have a positive or negative result on foaming. The empirical model should only be applied to the property range and geometric set-up for which it was derived, as coefficients may vary greatly for different systems. 3. Influence of solid precipitates on slag foaming can be summarised by noting that small amounts of magnetite stabilise slag foaming, while precipitates of wollastonite and anorthite decreased foaming. The influence of solid precipitates is thought to be related to the density, morphology and degree of surface activity of the solid precipitates. 4. The foam index decreases with increasing basicity due to the lowering of the slag viscosity. This continues until the precipitation of solids starts and the foam index once again increases. 5. For increasing "FeO" concentration the foam index will decrease due to lower viscosity, but higher surface tension depression may lead to increased foam index values at high "FeO" concentrations. 6. Higher foam index values were obtained for slags with lower densities. The _1 empirical relationship observed is L IX: P 3 . 7. Higher foam index values were obtained for slags with higher viscosity. The empirical relationship observed is L IX: f1 . 8. Higher foam index values were obtained for slags with lower surface tensions. The empirical relationship observed is L IX: U-I. 9. Models derived for the foaming of basic steelmaking slags does not satisfactorily describe the foaming behaviour of acidic slags. 10. The physical properties of the slag influence the foam stabilisation mechanism.

AFRIKAANSE OPSOMMING: Slak skuiming kom in verskeie pirometallurgiese prosesse voor. Hierdie prosesse sluit die maak van staal in basiese staalmaakoonde en elektriese boogoonde in, sowel as verskeie nie-yster prosesse soos sulfied smelting/omsetting en die skoonmaak van basis metaal slakke. Alhoewel slak skuiming in staalmaak slakke reeds intensief nagevors is, is min aandag gegee aan slak skuiming in nie-yster prosesse. Slak skuiming verskynsels is kompleks, want dikwels betaan die sisteem uit drie of meer fases. Die doelwitte van hierdie studie is om werk wat reeds oor slakskuiming gepubliseer is te hersien, om deur fisiese modellering 'n begrip van die prinsiepe waarop skuiming berus te bekom en om slak skuiming verskynsels na te vors deur piromettallurgiese eksperimente te beplan en uit te voer. Ten einde hierdie doelwitte te bereik, is eksperimente uitgevoer met water mengsels in verskillende kolom groottes, verskillende porie groottes vir gas inspuiting en by verskillende vloeistof hoogtes, en ook vir hoë temperatuur metallurgiese slakke met variërende samestelling en by verskillende temperature. Skuimingskondisies is deur gas inspuiting gesimuleer en die ewewig skuimhoogte is gemeet by verskillende gas snelhede. Die volgende gevolgtrekkings is bereik: 1. Tydens fisiese (koue) modellering van slak skuiming in 3-fase sisteme styg die gemiddelde skuim indeks met toenemende hoeveelhede vastestof in die sisteem. Die uitwerking van addisionele vastestof in die sisteem is onafhanklik van die sisteem geometrie. 2. Die volgende gevolgtrekkings is gemaak deur die koëffisiente vir 'n empiriese dimensionale model te pas op waterige mengsels. Hoër vloeistof digtheid lei na laer skuim indeks waardes. Die invloed van die vloeistof viskositeit is afhanklik van die sisteem ondersoek en mag 'n positiewe of 'n negatiewe resultaat op skuiming hê. Die empiriese model moet slegs toegepas word op die eienskap omvang en geometriese opstelling waarvoor dit bepaal is, omdat koëffisiënte grootliks kan varieer vir verskillende sisteme. 3. Die invloed van vastestof partikels op slak skuiming kan opgesom word deur die waarneming dat klein hoeveelhede magnetiet slakskuiming stabiliseer terwyl neerslag van wollastoniet en anhortiet skuiming verminder. Die invloed van vastestof neerslag hou vermoedelik verband met die digtheid, vorm en graad van oppervlak aktiviteit van die vastestof partikels. 4. Die skuim indeks verminder met toenemende basisiteit as gevolg van die afname in die slak viskositeit. Dit geld tot die neerslag van vastestof begin en die skuim indeks weereens toeneem. 5. Vir toenemende 'FeO' konsentrasie sal die skuim indeks afneem as gevolg van laer viskositeit, maar hoër oppervlakspanning onderdrukking kan lei na hoër skuim indeks waardes by hoër 'FeO' konsentrasies. 6. Hoër skuim indeks waardes is verkry vir slakke met laer digthede. Die empiriese _1 verhouding is bepaal as L cx: p 3 . 7. Hoër skuim indeks waardes is verkry vir slakke met hoër viskositeit. Die empiriese verhouding is bepaal as L cx: Jl . 8. Hoër skuim indeks waardes is verkry vir slakke met laer oppervlakspannings. Die empiriese verhouding is bepaal as L cx: 0--1 • 9. Modelle afgelei vir die skuiming van basiese staalmaak slakke beskryf nie die skuimings gedrag van suur slakke bevredigend nie. 10. Die fisiese eienskappe van die slak beïnvloed die skuim stabiliseerings meganisme.

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