The development of a one-dimensional quasi-steady state model for the desulphurisation process at Saldanha Steel

Scheepers, Emile (Stellenbosch : University of Stellenbosch, 2003-04)

Thesis (MScIng)--University of Stellenbosch, 2003.

Thesis

ENGLISH ABSTRACT: The pneumatic injection of reagent powder into molten iron has become the preferred way to carry out iron and steel desulphurisation. It is therefore essential to not only understand the thermodynamic implications, but also the kinetic principles that govern the desulphurisation process. Key variables that influence the kinetics of the procedure are the condition and composition of the top slag and the melt as well as the injection conditions. Notable injection parameters include reagent flowrate, injection-lance depth and carrier gas flowrate. Owing to sampling restrictions, the subsequent data from Saldanha Steel®, South Africa does not provide adequate insight into the kinetic behaviour of the desulphurisation process and it was therefore the focus of this research to provide an improved quantitive comprehension of the calcium carbide injection procedure at Saldanha Steel. For this purpose a one-dimensional quasi-steady state model for momentum, heat- and mass transfer in rising gas-liquid-powder plumes has been developed for conditions relevant to the Saldanha Steel refining process. Combined with a model predicting the contribution of the topslag to the process, the overall rate of desulphurisation as a function of time can be determined, thus affording the ability to quantitatively explore and analyse the influence of the afore-mentioned injection parameters, as well as the nature of both the topslag and the melt, on the kinetics of the desulphurisation process. Sensitivity analyses concluded that individual increases in the calcium carbide flowrate, the depth of injection and the amount of carry-over slag will result in a reduction in the injection time, while a decrease in the reagent particle diameter and the initial mass of iron in the ladle will have the same effect. Molten iron temperature losses brought about by prolonged injection needs to be electrically recovered within a steelmaking furnace at a high cost. Owing to the high cost of the desulphurising agent, any reduction in the required injection time, while still maintaining product specifications, will therefore result in diminishing overall production costs. Although all the results contained in this study is of particular interest to the Saldanha Steel scenario, it also provides invaluable information and insights into the important variables and parameters playing a role in injection desulphurisation processes in general, along with the influence that changing conditions can have on the end result of such a procedure.

AFRIKAANSE OPSOMMING: Die pneumatiese inspuiting van reagentpoeier is die populêrste ontswawelingsmetode in die yster- en staal bedryf. Dit is dus van groot belang dat die gepaardgaande termodinamiese en kinetiese beginsels betrokke by die ontswawelingsreaksies baie goed verstaan word. Die kondisie en samestelling van die bo-slak en die vloeibare yster, asook die inspuitingkondisies is twee van die belangrikste veranderlikes wat die kinetika van die ontswawelingsproses beïnvloed. Beperkte monsternemingsgeleenthede het veroorsaak dat die relevante data, soos voorsien deur Saldanha Staal®, nie die nodige kinetiese insig in verband met die ontswawelingreaksie weergee nie. Dit is dus die doel van hierdie werkstuk om ‘n verbeterde kwantitatiewe begrip van die ontswawelingsproses by Saldanha Staal daar te stel. Vir hierdie doeleinde is ‘n een-dimensionele, kwasi-gestadigde toestand model vir stygende gas-vloeistof pluime ontwikkel. Die model inkorporeer momentum-, hitte- en massaoordragsprinsiepe en is verteenwoordigend van die ontswawelingsproses by Saldanha Staal. ‘n Tweede model simuleer die bydrae wat die bo-slak tot die algehele ontswawelingsproses maak en saam gee hierdie twee modelle die algehele ontswawelingstempo weer as ‘n funksie van tyd. Die modelle word ook gebruik om die invloed van die bogenoemde inspuitingsveranderlikes op die proses te ondersoek. Deeglike sensitiwiteitsanalise het gewys dat ‘n verhoging in die kalsium karbied vloeitempo, asook die inspuitingsdiepte van die lans en die hoeveelheid slak wat vanaf die boogoond na die ontswawelingseenheid oorgedra word, ‘n vermindering in die vereisde inspuitingstyd te weeg bring. Verkleining in die kalsium kardied partikels se gemiddelde diameter en vermindering van die hoeveelheid yster in die torpedokarre aan die begin van die proses, het dieselfde uitwerking op die vereisde inspuitingstyd. Geweldig baie geld moet aan elektrisiteit spandeer word om die temperatuur wat verlore gaan as gevolg van onnodige lang inspuitingstye, in die staalmaakoonde te herwin. Gekombineerd met die feit dat die kalsium karbied reagent baie duur is, beteken dit dat reduksies in die vereisde ontswaweling inspuitingstyd groot besparings te weeg kan bring. Alhoewel die saamgevatte resultate van spesifieke belang is vir die Saldanha Staal proses, verskaf hierdie studie waardevolle informasie oor die belangrikheid van verskeie veranderlikes, asook die rol wat veranderende toestande op die eindresultate van die ontswawelingproses kan hê.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16424
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