Thesis (MScEng)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Can salts present in an aqueous industrial effluent be retained by the [me ash that is
produced as a by-product of gasification or by power stations utilising coal as the raw
material? In order to answer this question, the actual chemistry that occurs during the
mixing and settling process, needs to be understood.
At the Sasol Secunda petrochemical plants in South Africa, ash is produced as a byproduct
from the gasification of coal, and by the coal-fired power stations (steam plants).
The [me portion of the ash (± 50J.lm in diameter) is disposed of through the use of a
closed loop wet ash disposal system. The ash is transported as a slurry to the disposal
sites (ash darns). The industrial effluent used to transport the ash consists mainly of the
recycled ash effluent, known as clear ash effluent (CAE), as well as a variety of process
waste streams containing high concentrations of salts. This mixture of ash and water is
pumped to ash dams, where the ash is allowed to settle and is therefore separated from
the effluent. From the ash darns the effluent flows into evaporation dams, and finally into
CAE dams before being returned to the ash plant in Sasol 2 and 3 to be mixed once again
with the ash. During this contact time of the ash with the water certain chemical
reactions may occur. If one understands what chemical reactions occur during this
process, and under what conditions they occur, then it will be possible to utilise the ash
disposal system to its full potential, possibly enhancing the salt retention ability. An
investigation was thus conducted into what processes actually occurs during the entire
ash water contact period. The overall aim of the project was to obtain an understanding of
the functioning of the [me ash disposal system so that its efficiency can be improved
upon, and furthermore, so that the ash darns can be utilised more effectively in retaining
salts.
This investigation focussed on the chemical reactions that occur when an industrial
effluent is mixed with fine ash, and consisted of four main aspects: • A literature survey on related issues.
• An analysis and evaluation of the changes that occur in the actual disposal system.
• Laboratory column experiments to investigate, in more detail, the different chemical
reactions, which occur during the different stages of the disposal process.
• The drilling of boreholes into the ash dams to obtain core material at a variety of
depths and locations for analysis purposes.
From this investigation it was concluded that salts are retained in the ash dams; based on
the results obtained from the laboratory column experiments and the production rate of
the fine ash from Sasol 2 and Sasol 3, the potential amount of salts that can be removed
from the system (either due to precipitation or water retention in the ash dams) is
approximately 95 tons/day. The salts that were found to be most pertinent to the wet ash
disposal system utilised at Sasol, Secunda, are Ca, S04, Na, and Cl. Of these, Ca, S04
and Na were identified in literature to be the components most commonly associated with
fly ash leachate. The Ca chemistry, which occurs in the ash disposal system, was
explored extensively. Is was found that Ca, which is initially present in the fresh fine ash
as lime, is leached from the ash into the effluent, where it reacts with carbon dioxide in
the atmosphere, and is therefore removed from the system due to the precipitation of
calcite. Sodium, S04, and Cl were all found to be retained in the ash; the S04 appears to
be retained in a stable form within the ash, not merely due to hydraulic retention, which
suggests that the ash system has the potential to act as a salt sink for S04 ions.
The mechanism of salt retention in the ash darns was found to be predominantly by
means of hydraulic retention, and therefore the salts have the potential to be flushed out
of the ash dams into the underlying soil material. However, results from the core drilling
exercise revealed that there doesn't appear to be a significant seepage of elements from
the ash fill material into the underlying vertisol material. Some components (AI, Fe, Na,
K, Mg, Cr, P, Ti and V) from the older, and inactive ash dam, do appear to have
percolated into the underlying material. However, a significant amount of water, and
therefore salts, are still retained in the ash dam. In terms of the mineralogical composition of the ash dams, a significant difference was
observed between the mineral phases present in the ash fill material of an active and an
inactive ash dam. Ettringite was detected throughout the borehole drilled into the
inactive ash darn, and was not evident at all in the core material from the two boreholes
drilled into the active dam, which suggests that this mineral is formed in the ash darns
over a long time period. The minerals quartz and mullite were found in the fresh [me ash
as well as in most of the core material obtained from the drilling exercise. The
mineralogical composition of the ash fill samples, from the boreholes drilled into the
centre of the active and inactive ash darns, was found to be very consistent with depth.
This finding, combined with the fact that the chemical composition of the core samples
varied more significantly in the borehole located near the edge of the active fine ash darn,
indicated that the lateral position of the ash in the ash dam influences the chemical
reactions that occur.
Overall, from this investigation it was concluded that although the chemistry, involved in
the mixing of an industrial effluent with fine ash, is extremely complex and site-specific,
it is possible to determine the most significant changes which occur within a wet ash
disposal system. Besides providing one with a better understanding of the working of the
Secunda ash disposal system, the results of this investigation have also provided the
framework for future research on this topic and related issues, i.e. the construction of a
pilot scale ash darn set-up; further column experiments to investigate the extent to which
S04 ions can be removed from the system; the influence of the addition of CO2 to the
system; and more extensive core drilling in the vicinity of the ash darns. / AFRIKAANSE OPSOMMING: Kan soute teenwoordig in 'n industriële uitvloeisel teruggehou word in fynas geproduseer
as neweproduk van steenkoolkragsentrales? Om 'n antwoord op hierdie vraag te kry,
moet die chemiese reaksies wat gebeur tydens die meng en wegdoening van die as en
aswater verstaan word.
By die Sasol petrochemiese aanlegte in Secunda, Suid Afrika, word fynas geproduseer as
'n neweproduk in die vergassing en die stoomopwekkingprosesse. Die fynas (50)lm
diameter) word weggedoen deur 'n geslote nat asstelsel. Die industriële uitvloeisel wat
gebruik word vir die vervoer van die as bestaan hoofsaaklik uit hergebruikte aswater
(genoem CAE - clear ash effluent), asook 'n verskeidenheid ander prosesafvalstrome wat
hoë konsentrasies soute bevat. Die mengsel van as en aswater word in 'n asflodder
gepomp na die asdamme, waar die as besink en sodoende geskei word van die waterfase
(aswater). Vanaf die asdamme vloei die aswater na verdampingsdamme, en daarna na die
CAE damme, vanwaar die CAE weer na die Sasol aanleg teruggepomp word om weer
met as gemeng te word.
Gedurende die kontak tussen die CAE en as gebeur sekere chemiese reaksies. Indien
hierdie reaksies verstaan word, en onder watter toestande dit plaasvind, kan die
asdamstelsel tot volle kapasiteit benut word deur moontlik die soutretensie binne die
asdam te verhoog. 'n Ondersoek is gedoen om te bepaal watter prosesse plaasvind
gedurende kontak tussen die as en water. Die doel van die ondersoek was om 'n beter
begrip te kry oor die funksionering van die fynas-wegdoeningstelsel en om te bepaal of
die asdamme meer effektiefbedryfkan word om moontlik meer soute te akkommodeer.
Die ondersoek het uit vier hoofaspekte bestaan:
• Literatuuroorsig,
• 'n Analise en evaluering van die veranderinge wat plaasvind oor die asdamstelsel, • Laboratoriumskaal kolomeksperimente om in meer besonderhede die chemiese
reaksies wat 'n rol in die aswaterstelsel speel, te bepaal, en
• Die boor van toetsgate op die bestaande asdamme om boorkerne te ontleed by
bepaalde dieptes en liggings.
Uit die ondersoek is bevind dat soute wel in die asdamme behou word. As die
kolomtoetse as basis gebruik word, en die produksietempo van fynas vanaf Sasol 2 en 3,
dan kan daar 'n potensiële 95 ton soute per dag deur die asstelsel verwyder word (deur
hoofsaaklik waterretensie en presipitasie van soute). Die mees prominente soute wat in
die Sasol asstelsel voorkom is Ca, S04, Na, en Cl. Vanhierdie soute, is Ca, S04, en Na
deur die literatuur geïdentifiseer as komponente wat met vliegas loog geassosieer word.
Die Ca chemie, wat in die asstelsel plaasvind, is in besonderhede ontleed. Dit is bevind
dat Ca, teenwoordig in die vars fynas as kalk, vanuit die as in die aswater geloog word,
waar dit dan met atmosferiese CO2 reageer en dan vanuit die stelselverwyder word deur
die presipitasie van kalsiet. Natrium, S04 en Cl word in die as teruggehou. Dit wil
voorkom asof die S04 in 'n stabiele vorm in die as teruugehou word, nie net deur die
hidrouliese retensie nie en dat die asstelsel dalk as 'n potensiële sink vir S04 kan optree.
Die meganisme van soutretensie in die asdamme is hoofsaaklik deur hidrouliese retensie,
met die gevolg dat die soute potensieel in die onderliggende grond uitgewas kan word.
Die resultate van die boorkernondersoek wys egter dat daar nie beduidende uitwassing
van hierdie soute in die grond is nie. Dit wil voorkom of sekere komponente (Al, Fe, Na,
K, Mg, Cr, P, Ti en V) van die ou en onaktiewe asdam in die grond geloog is.
'n Beduidende verskil was gevind tussen die minerale fases in die asmateriaal van die
aktiewe en onaktiewe asdamme. Ettringiet was teenwoordig deur die hele diepte van die
boorkern van die onaktiewe dam, maar was nie teenwoordig in beide boorkerns van die
gate op die aktiewe asdam nie. Dit impliseer dat hierdie mineraaloor 'n langer tyd
gevorm word. Kwarts en mulliet was deurentyd in al die boorkerne teenwoordig. Die
mineralogie van die boorkern van die middel van die aktiewe asdam was baie konstant
met diepte (in teenstelling met dié van die boorkern op die kant van die asdam) wat daarop dui dat die laterale posisie van die as in die asdam die chemiese reaksies wat mag
plaasvind kan beïnvloed.
Die ondersoek bevestig dat alhoewel die chemiese reaksies betrokke in die aswaterstelsel
baie kompleks en liggingspesifiek is, die mees beduidende veranderinge wat in die
asstelsel plaasvind, wel bepaal kan word. Die ondersoek het benewens 'n beter begrip
van hoe die asdamme reageer, ook 'n raamwerk gegee vir verdere navorsing in hierdie
veld, bv. die bou van 'n loodsskaal-asdam, verdere kolomtoetse om die vermoë van die
asstelsel om S04 ione te verwyder te bepaal en die invloed van gemanipuleerde kalsiet
presipitasie deur die byvoeging van CO2.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52681 |
| Date | 12 1900 |
| Creators | Koch, Emma Wendy |
| Contributors | Lorenzen, Leon, Schutte, B., 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 | English |
| Type | Thesis |
| Format | 322 p. : ill. |
| Rights | Stellenbosch University |
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