Thesis (MScAgric)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Coal mining in South Africa is estimated to produce 200 Ml of acid mine drainage
(AMD) per day in the Pretoria-Witwatersrand-Vereeniging (PWV) area alone, while
electricity production resulted in approximately 27 Mt of ash in 2001. A large number
of collieries in South Africa are tied to power stations where these two waste streams,
acid mine drainage and fly ash, have the capacity to neutralize each other and provide
an opportunity for co-disposal.
The aim of this study was to investigate the reactions that occur during the co-disposal
of fly ash leachate (FAL) and AMD and to examine the precipitates that result from
the neutralisation reactions. Potentiometric titration was employed to investigate the
neutralisation of Al-Fe salt solutions, simulating acid mine drainage (AMD), with
alkaline solutions of Ca or Na hydroxide as well as fresh alkaline leachate from fly
ash (FAL). The effectiveness of fly ash in removing metals and other salts from acid
mine water was examined by analysing the neutralised water and modelling the
results thermodynamically. Precipitates, prepared from large scale synthetic AMD
and FAL co-disposal at various pH levels and Fe:Al ratios, were characterised
according to composition, mineralogy and surface properties.
The experimental neutralisation of synthetic acid mine drainage was achieved through
titrating the components of SAMD (Fe and Al salt solutions) and solutions of various
Fe:Al mole ratios with different bases in air and N2, and comparing the SAMD-FAL
system with these simple acids and bases. The FAL used in all experiments was
produced from fresh fly ash collected at Arnot power station. The SAMD was
prepared as a solution with a pH of 2.5 and containing 12.7 mmol/L Al, 10.9 mmol/L
Fe and 40.8 mmol/L SO4. The characterisation of reaction solids was achieved by
collecting the precipitates formed from the co-disposal of FAL and SAMD with Fe:Al
ratios of 7.3, 0.8 and 2.5.
From the titration experiments it was found that upscale potentiometric titrations of
SAMD show buffer zones at pH values of 3.5, 4, 6 and 10 corresponding to Fe(III)precipitation, Al precipitation, Fe(II) hydrolysis and oxidation, and Al redissolution,
respectively, while downscale potentiometric titrations with SAMD show buffer
zones at pH values 12 – 11, 9 and 4.5, which correspond to Fe oxidation and
precipitation, Al precipitation and Al re-dissolution, respectively. A high
concentration of Al in the simulated AMD inhibited the crystallinity of the
precipitates and resulted in a large quantity of SO4 being removed from solution,
which suggests that an aluminium sulphate phase is precipitating, but it is not
crystalline and cannot be identified by XRD. Titrations performed up-scale by adding
FAL to AMD showed near-complete metal and substantial SO4 removal from
solution.
The characterisation of reaction solids by x-ray diffraction, infrared spectroscopy,
thermal gravimetric and differential thermal analysis revealed that the precipitates
consist of poorly crystalline, highly Al-substituted goethite and ferrihydrite with large
amounts of SO4 included in the structure. Poorly crystalline bayerite appears at a high
pH and high Al concentration, and calcite is present in precipitates made by adding
SAMD to FAL. High surface charges of between 330 cmolc/kg positive and 550
cmolc/kg negative charge and potentially large specific surface areas between 7 and
236 m2/g suggest a strong potential for the precipitates to function as low-grade
adsorbents in wastewater treatment. The similarity of these ochre precipitates to soil
minerals implies that land disposal of the neutralised solids is also viable. / AFRIKAANSE OPSOMMING: Steenkool mynbou in SA produseer na benaming 200 ML suur mynwater per dag in
die PWV area alleenlik, terwyl opwekking van elektrisiteit naastenby 27 Mt vliegas
geproduseer het in 2001. ’n Groot aantal steenkoolmyne in SA word verbind met
kragsentrales, waar hierdie twee strome afval, suur mynwater en vliegas, die
kapasiteit het om mekaar te neutraliseer en die weg te baan vir gesamentlike
wegdoening.
Die doel van hierdie studie was om die reaksies wat plaasvind gedurende
gesamentlike wegdoening van vliegas loog (VAL) en suur mynwater (SMW) te
ondersoek, asook die neerslae wat mag vorm as gevolg van neutralisasie reaksies.
Potensiometriese titrasies was gebruik om die neutralisering tussen Al:Fe-sout
oplossings te ondersoek as nabootsing van SMW met gebruikmaking van alkaliese
oplossings van Ca of Na hidroksied asook vars loog van VA. Die effektiwiteit van
VA om metale en soute uit SMW te verwyder was getoets deur outleding van die
geneutraliseerde water en modellering van die termodinamika. Neerslae berei uit
groot-skaal sintetiese SMW en VAL en met gelyktydige storting by verskeie pH
vlakke en Fe:Al verhoudings, was gekarakteriseer volgens samestelling, mineralogie
en oppervlak eienskappe.
Die eksperimentele neutralisering van sintetiese suur mynwater (SSMW) was gedoen
deur titrering van die SSMW komponente en oplossings van verskeie Fe:Al molêre
verhoudings met verskillende basisse in lug en N2, en vergelyking van SSMW-VAL
sisteem met hierdie eenvoudige sure en basisse. Die VAL gebruik in alle
eksperimente was geproduseer van vliegas verkry van die Arnot kragsentrale. Die
SSMW was berei as ’n oplossing met ’n pH van 2.5 en bevat 12.7 mmol/L Al, 10.9
mmol/L Fe en 40.8 mmol/L SO4. Die karakterisering van vastestowwe uit die reaksie
was gedoen deur die bemonstering van neerslae gevorm as gevolg van die gelyktydige
wegdoening van VAL en SSMW met Al:Fe verhoudings van 7.3, 0.8 en 2.5.Die was waargeneem in die titrasie eksperimente dat hoër-skaal potensiometriese
titrasie van SSMW buffersones, by pH waardes 3.5, 4, 6 en 10, ooreenstem met
Fe(III) presipitasie, Al presipitasie, Fe(II) hidrolise en oksidasie, en Al her oplossing,
terwyl laer skaal potensiometriese titrasie met SSMW buffer sones by pH waardes 12
- 11, 9 en 4.5 ooreenstem met Fe(III) presipitasie en oksidasie, Al presipitasie en
heroplossing respektiewelik. ’n Hoë konsentrasie Al in die sintetiese SMW het
kristalliniteit van die neerslae geïnhibeer en veroorsaak dat ’n hoeveelheid SO4 uit
oplossing verwyder is, wat suggereer dat die AlSO4 fase neerslaan maar nie kristallyn
is en gevolglik nie opgetel word met x-straal diffraksie nie. Titrasies gedoen by hoërskaal
deur byvoeging van VAL tot SSMW, het feitlik volledige metaal en SO4
verwydering uit oplossing getoon.
Die karakterisering deur x-straal diffraksie, infrarooispektroskopie, termies
gravimetries en differentiële termiese analise, het getoon dat die presipitate bestaan uit
swak kristallyne, hoë Al-gesubstitueerde goethiet en ferrihidriet met groot
hoeveelhede SO4 vasgevang in die struktuur. Swak kristallyne bayeriet verskyn by
hoë pH en hoë Al-konsentrasies en kalsiet is teenwoordig in neerslae gevorm deur
byvoeging van SSMW tot VAL. Hoë oppervlakladings van tussen 330 cmolc/kg
positief en 550 cmolc/kg negatiewe lading en ook potensieel groot spesifieke
oppervlak van tussen 7 en 236 m2/g, dui op ’n sterk potensiaal vir neerslae om as
laegraadse adsorbeermiddels in afvalwaterbehandeling gebruik te word. Die
ooreenstemming in hierdie geelbruin neerslae met grond minerale, impliseer dat die
land storting van geneutraliseerde vastestowwe ook lewensvatbaar is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16266 |
| Date | 04 1900 |
| Creators | Burgers, C. L. (Colleen Lucie) |
| Contributors | Fey, M. V., University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xiv, 110 leaves : ill. |
| Rights | University of Stellenbosch |
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