Thesis (MScEng)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: During the production of pure Ti02 for the pigment industry, ilmenite, containing 35 -
60 % Ti02, is reduced to high titania slag, containing 85 - 95 % Ti02 and pig iron.
These ilmenite smelters are operated in very tight operating windows. Over reducing
the slag may lead to the formation of TiC and reducing much of the Ti02 to Th03.
According to Namakwa Sands furnace operators, this does not only affect the grade of
the product, but it can cause slag foaming and furnace eruptions. In under reducing
conditions, the liquid slag is fluxed by the FeO and may corrode the furnace lining
and consequently lead to run-outs. The reducing conditions in the furnace are not
only controlled by carbon addition, but also by temperature. Standard practise in
industrial ilmenite smelters is to operate the furnace with a slag freeze lining to
protect the refractory lining from chemical and physical attack by the slag. It is
therefore clear that it is of great importance to be able to predict the slag liquidus
temperature at different compositions. This can help the operator to avoid dangerous
operating conditions.
Over the past few decades, a number of solution models have been developed to
describe non-ideal solutions. With the rapid increase in computer power, these
models became more valuable and practical to use in advanced control and decisionsupport.
In this study, some of the better-known models are discussed and evaluated
for the Ti02 - Th03 - FeO system, based on a critical review of properties and
measurements published in literature.
Two of these models, the "modified quasi-chemical" model and the "cell" model were
chosen to be applied to the high-titania slag system. Both these models are based on
statistical thermodynamics with some differences in the initial assumptions. In this
study, the model parameters for the cell model were regressed from experimental
data.
The high-titania slag produced, consists mainly of titanium in different oxidation
states and FeO, placing its composition inside the Ti02 - Th03 - FeO ternary system.
Reliable experimental data for this system are very limited. All three binary systems contained in the Ti02 - ThO) - FeO system were considered, namely FeO - Ti02,
Ti02 - ThO) and FeO - ThO). Only liquidus data for these three binaries were used
to regress the model parameters. Accuracy of the models was determined by
calculating the root mean square (RMS) error between the experimental data point
and the value calculated using the model and the newly determined model parameters.
These errors corresponded weil with the reported experimental error of the datasets
for both the models and all the binary systems.
Due to the fact that this study focussed on the liquidus surface of the system, the
results were also plotted in the form of binary phase diagrams and ternary liquidus
isotherms. The cell model uses only binary interaction parameters to describe the
ternary system. These parameters are not expanded to higher order polynomials,
which makes this model more robust, but also less accurate than other models such as
the modified quasi-chemical model. / AFRIKAANSE OPSOMMING: Tydens die produksie van suiwer Ti02 vir die pigmentbedryf, word ilmeniet, wat 35
tot 60 % Ti02 bevat, gereduseer tot 'n hoë titaan slak, met 'n Ti02 inhoud van 85 tot
95 % Ti02, en potyster. Hierdie ilmeniet smeltoonde word binne baie nou
bedryfskondisies beheer. Oor-redusering van die slak kan lei tot the formasie van TiC
en die redusering van Ti02 tot Th03. Dit affekteer nie net die produk se kwaliteit nie,
maar kan volgens Namakwa Sands oond operateurs ook slak skuiming en ontploffings
tot gevolg hê. Gedurende onder-reduserende omstandighede in die oond, word die
vloeibaarheid van die slak verhoog deur die hoër FeO inhoud in die slak. Dit maak
die slak meer korrosief en kan lei tot faling van die vuurvaste stene. Die mate van
redusering in die oond word nie net bepaal deur die toevoeging van koolstof nie, maar
ook deur die temperatuur van die slak. Dit is 'n standaard praktyk van die industrie
om die oond te bedryf met 'n gevriesde slak laag om sodoende die vuurvaste stene te
beskerm teen chemiese en fisiese aanval van die slak. Dit is dus duidelik dat dit baie
belangrik is om die slak se smeltpunt by verskillende samestellings te kan voorspel.
Dit kan die operateur help om die oond binne veilige bedryfskondisies te hou.
'n Hele aantaloplossingsmodelle is oor die afgelope paar dekades ontwikkel vir die
beskrywing van nie-ideale oplossings. Hierdie modelle het oor die afgelope paar jaar
baie toegeneem in praktiese waarde as gevolg van die snelle toename in
rekenaarkapasiteit en -spoed. Dit het veral groot waarde in gevorderde beheerstelsels
en besluitneming steun. Sommige van die meer bekende modelle word in hierdie
studie bespreek en ge-evalueer vir die Ti02 - Th03 - FeO stelsel, gebaseer op 'n
kritiese evaluasie van eienskappe en eksperimentele data gepubliseer in die literatuur.
Twee van hierdie modelle, die "gemodifiseerde kwasi-chemiese" model en die "sel"
model, is gebruik om die hoë titaan slak stelsel te beskryf. Beide hierdie modelle is
gebaseer op statistiese termodinamika en het klein verskille m.b.t. die aanvanklike
aannames. Die model veranderlikes vir die sel model is in hierdie studie afgelei vanaf
die eksperimentele data. Die hoë titaan slak wat tydens hierdie proses geproduseer word, bestaan hoofsaaklik
uit FeO en titaan in sy verskillende oksidasie toestande. Dit plaas die samestelling
van die slak reg binne die Ti02 - Th03 - FeO temêre stelsel. Betroubare
eksperimentele data vir hierdie stelsel is baie beperk. In hierdie studie word daar
gekyk al drie binêre stelsels binne die Ti02 - Th03 - FeO temêre stelsel, naamlik:
FeO - Ti02, Ti02 - Th03 en FeO - Th03. Slegs die smeltpunt temperatuur data vir
hierdie twee binêre is gebruik in die afskatting van die model veranderlikes. Die
akkuraatheid van die modelle is bepaal deur die wortel van die gemiddelde kwadraat
van die fout tussen die eksperimentele waardes en die berekende waardes te bepaal.
Albei die modelle het 'n relatiewe klein fout in vergelyking met die geraporteerde
eksprimentele fout gehad vir al die binêre stelsels.
Hierdie studie het gefokus op die smeltpunt temperatuur van die slak en die resultate
is daarom ook in die vorm van binêre fasediagramme en isoterme projeksies op die
temêre fasediagramme gestip. Die "sel" model gebruik slegs binêre interaksie
parameters om die temêre stelsel te beskryf Hierdie parameters word vir die "sel"
model nie uitgebrei tot hoër order polinome en dit maak die "sel" model meer robuust,
maar minder akkuraat as ander modelle soos byvoorbeeld die "kwasi-chemiese"
model.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/50125 |
| Date | 12 1900 |
| Creators | Fourie, David Johannes |
| Contributors | Eksteen, J. J., Zietsman, J. H., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 159 p. : ill. |
| Rights | Stellenbosch University |
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