-
meerin
andss ss fou5, suH toh reexp
specied residual concentrations of iron or sulfate and the pH.
The data indicate that in particular, basic
lth toGold
ccountld are2012).
that forms under natural conditions when geologic strata
contain-ing pyrite or other sulde bearing minerals are exposed to
theatmosphere or oxidizing environments (Fripp et al., 2000;Gaikwad
and Gupta, 2008; Jennings et al., 2008; Taylor et al.,2005). These
waters typically pose a risk to the environmentbecause they contain
elevated concentrations of metals such asiron, aluminium and
manganese, and possibly other heavy metals
l., 2010). ApH valu
vated levels of heavy metals, sulfate and radioactive subssuch
as uranium. The impacts of low pH can be immediasevere hence AMD is
often detrimental to aquatic life (Joand Hallberg, 2003; Taylor et
al., 2005).
AMD has a signicant potential to have an impact on
theenvironment and the health of the people that are dependent
onthe water around the AMD polluted region (Chapman, 2011).AMD has
long-term environmental impacts that include revegeta-tion and
rehabilitation difculties (Taylor et al., 2005). This isbecause
soils contaminated with AMD have an imbalance of
Corresponding author. Tel.: +27 11 717 7592; fax: +27 86 211
8303.E-mail address: [email protected] (C. Sheridan).
Minerals Engineering 64 (2014) 1522
Contents lists availab
n
elstechniques to address the situation.AMD is acidic water laden
with iron, sulfate and other metals
committee on acid mine drainage (Coetzee et athe Witwatersrand
Basin is characterised by
lowhttp://dx.doi.org/10.1016/j.mineng.2014.03.0240892-6875/ 2014
Elsevier Ltd. All rights reserved.MD ines, ele-tanceste
andhnsonmining in the metropolitan area currently poses challenges
such asthe threat of groundwater and surface water contamination
arisingfrom Acid Mine Drainage (AMD). Clean water is universally
anessential resource and South Africa faces a threat to water
securityin the near future if the issue of AMD is not fully
addressed(Chapman, 2011; Cobbing, 2008). Considering the
environmentaland ecological threats this poses there is a need for
innovative
characteristic yellow-boy. Following this, the ferric iron that
doesnot precipitate from solution is able to oxidize additional
pyrite.
Acid Mine Drainage (AMD) is a growing problem in coal andgold
mines areas (Chapman, 2011; Taylor et al., 2005). In SouthAfrica,
the main focus of attention is to address the Witwatersrandgold
elds around Johannesburg as it has been listed as a problem-atic
area with respect to AMD according to the Inter-ministerial1.
Introduction
South Africa owes much of its weain areas such as the
Witwatersrandlanga Coal elds. Mining currently aof South Africas
GDP and coal and gorevenue generators (Statistics South,oxygen
furnace slag has signicant potential as a replacement reagent for
lime in treating acid minedrainage.
2014 Elsevier Ltd. All rights reserved.
its mineral riches foundelds and the Mpuma-s for approximately
9%two of the three largestHowever, the legacy of
such as uranium (Akcil and Koldas, 2006) and often have a low
pHand high sulfate concentrations. AMD is formed through a numberof
chemical reaction pathways, namely pyrite oxidation,
ferrousoxidation and iron hydrolysis (Akcil and Koldas, 2006;
Singer andStumm, 1970; Stumm and Morgan, 1996). The oxidation of
ferrousiron to ferric iron follows after which ferric iron
precipitates as ironhydroxide (Fe(OH)3) at pH above 3.5 (Tutu et
al., 2008) leading toAcid mine drainage maximum treatment capacity
of the slag. These experiments indicated that slag replacement
strategiesare wholly dependent on the strength of the acid mine
drainage, the required residence time and theRemediation of acid
mine drainage using
Tendai Name, Craig Sheridan Industrial and Mining Water Research
Unit, School of Chemical and Metallurgical Engin2050, South
Africa
a r t i c l e i n f o
Article history:Received 20 August 2013Accepted 24 March
2014Available online 19 April 2014
Keywords:Basic Oxygen Furnace (BOF) slagStainless Steel (SS)
slag
a b s t r a c t
In this study basic oxygenmine drainage. The stainleable. Basic
oxygen slag wamine water with a pH of 2.slag was able to increase
pexperiments. In these batcrapid process. Additional
Minerals E
journal homepage: www.etallurgical slags
g, University of the Witwatersrand, Johannesburg, Private Bag 3,
Wits, Johannesburg
stainless steel slag were both assessed for potential use in
treating acidteel slag was able to effect some pH change but was
found to not be suit-nd to have a signicant potential as a
remediating agent. For a model acidlfate concentration of 5000 mg/L
and iron concentration of 1000 mg/L, the12.1, reduce the soluble
iron by 99.7% and reduce sulfate by 75% in batch
actors most reaction was completed within 30 min indicating that
this is aeriments were conducted with continuous ow reactors to
assess the
le at ScienceDirect
gineering
evier .com/locate /mineng
-
a major problem as it shields the limestone from reacting with
the
ls Enecessary elements vital for plant growth. Detrimental
effectsposed by excess iron include interference with the uptake of
man-ganese which is important for plant growth; clogging of sh
gills;and build-up of iron and acid in animals internal organs can
resultin fatal consequences (Fripp et al., 2000). Buildings and
infrastruc-ture are also subject to degradation with time due to
the corrosiveeffects of AMD (Taylor et al., 2005).
The research conducted around the Witwatersrand Basin areahas
also found high levels of radioactive material like uraniumwhich
may pose cancer risks (Coetzee and Winde, 2006). TheDWAF
(Department of Water and Forestry, 1996) stated that
con-centrations of sulfate that are greater 600 ppm causes the
waterto taste bitter and may result in diarrhoea. Elevated sulfate
concen-trations also results in gypsum formation which degrades
concretestructures and causes scaling in pipes and lters
(Madzivire, 2009;Swanepoel, 2011).
Appropriate treatment methods need to be implemented toaddress
the threats posed by AMD. Historically, focus has mainlybeen on
minimisation and control as the best practice (Tayloret al., 2005).
However, the generation of AMD is in essenceunavoidable and it is
practically difcult to inhibit the formationof AMD at its source.
Various methods have thus been proposedto tackle problems posed by
AMD with mixed results to ensuretreated efuents meet threshold
values set by the government byremoving heavy and toxic metals and
maintaining acidity atacceptable levels. Feasible methods for
treating AMD were identi-ed and divided into active and passive
processes (Akcil andKoldas, 2006; Taylor et al., 2005) in the
literature. Active treatmentinvolves addition of alkaline chemicals
like limestone, lime, causticsoda and ammonia while passive
treatment involves developingnatural chemical and biological
systems that are self-operating likeconstructed wetlands (Gaikwad
and Gupta, 2008; Ochieng et al.,2010). Advantages with active
treatment systems are that theycan cope with higher ow rates, are
exible and have smaller foot-prints. Disadvantages include high
operating costs associated withalkaline chemicals, constant
monitoring and maintenance, skilledmanpower and production of
sludge outweigh the advantages.The advantage with passive treatment
system is they require littlemaintenance. This is critically
important in a country such as SouthAfrica which has a skills
shortage.
Passive treatment systems have been used for a number ofyears to
treat mine efuent of varying compositions and pH levels(Dvorak et
al., 1992). They have been argued to be the long termstrategy to
solving AMD problems and with further research theymay become more
widely used in future (Jennings et al., 2008).Passive treatment
methods used include primarily wetlands(Johnson and Hallberg, 2003;
Sheoran and Sheoran, 2006;Sheridan et al., 2013; Taylor et al.,
2005; Wallace and Knight,2006) and limestone-based beds or drains
(Kleinmann et al.,1998; Taylor et al., 2005). Ochieng et al. (2010)
explored the feasi-bility of aerobic and anaerobic wetlands which
are also currently inuse in South Africa. Although aerobic wetlands
were able toremove various metals from efuents, their main
disadvantage liesin the fact that they cannot handle typical AMD
efuents and theyrequire vast surface areas for their operation. A
further disadvan-tage is their inability to reach pH levels greater
than 8 (Tayloret al., 2005). The disadvantage with anaerobic
wetlands is therequirement for large area of land for effective
treatment. (Battyand Younger, 2004) are of the opinion that
vegetation is difcultto establish in AMD treatment applications in
both aerobic andanaerobic wetlands. The main disadvantage with
limestone bedsis the regular maintenance required to ensure maximum
life andeffectiveness. The porosity of the beds and that of organic
matter
16 T. Name, C. Sheridan /Minerais reduced as the systems get
blocked with treatment precipitates(Cravotta, 2003; Johnson and
Hallberg, 2003; Potgieter-Vermaaket al., 2006). Armouring of
limestone occurs when ferrous iron isAMD.
1.1. The use of metallurgical slags for treating AMD
Although various processes have been proposed for the treat-ment
of AMD using traditional passive treatment schemes, noneof these
methods is ideal as a long term solution due to high oper-ating
costs and technological failures (Barrie Johnson and
Hallberg.,2005). Slags are a highly alkaline by-product of the
smelting pro-cess for metals such as steel, copper etc. Slags are
highly alkalinebecause they are composed primarily of hydrated
amorphous sil-ica, calcium oxide and magnesium oxide (Ziemkiewicz
andSkousen, 1998) and often lime is used in the smelting process
asa ux. They are widely available in countries such as South
Africadue to its large minerals rening industry. The potential use
of slagfor the treatment of AMD has been studied and described by
vari-ous authors (Gaikwad and Gupta, 2008; Kruse et al., 2012;
Shenand Forssberg, 2003; Ziemkiewicz and Skousen, 1998). Researchby
Ziemkiewicz and Skousen (1998) suggested direct addition ofsteel
slag into streams affected by AMD as an alternative
treatmentmethod. Feng et al., 2004 further supported ideas by
Ziemkiewiczand Skousen (1998) by citing that slag can increase the
pH of acidmine water to almost neutral levels and remove heavy
metals.(Bowden et al. 2006) further discovered that rapid iron
removalwas possible using steel slag. In South Africa, the use of
Slag LeachBeds (SLBs) as a form of passive technology has not been
fullyinvestigated, but shows potential in being able to treat
AMD(Sheridan et al., 2013). Previous studies on SLBs have
mainlyfocussed on stormwater pollution (Taylor et al., 2005) and
AMDfrom disused coal mines and direct treatment of
water(Ziemkiewicz and Skousen, 1998). The motivation of the
presentstudy was to evaluate two common types of slags, which are
wasteproducts, to neutralise AMD as an alternative to using lime,
whichneeds to be mined. Hence, the use of slags has the potential
to pro-vide a cheaper alternative to lime, while at the same time
makinguse of a waste stream that is readily available in South
Africa.
1.2. The mechanism of slag in acid mine drainage remediation
The CaO found in slag reacts with water to form hydratedcalcium
hydroxide (Ca(OH)2). Dissolution of (Ca(OH)2) followsand creates
alkalinity. Different forms of iron react with hydroxide(OH) to
forms different products depending on the resulting pHvalues
obtained by the addition of slag. The removal of iron insolution by
slags is due to acid neutralising ability which leads
toprecipitation (Feng et al., 2004; Rose, 2010). The removal of
sulfatein AMD in slag can be attributed to the formation of
gypsum(CaSO42H2O) and other sulfate precipitates that can possibly
form.The reactions involved in pH increase, iron and sulfate
removal arepresented as Eqs. (1)(4).
CaOH2O! CaOH2 1
CaOH2 ! Ca2 OH 2
Fem mOH ! FeOHm 3
Ca2 SO24 2H2O! CaSO4 2H2O 4
1.3. Research objectivesoxidized and ferric hydroxide
precipitates on the limestone is also
ngineering 64 (2014) 1522Within this context, this study sought
to investigate the abilityof Basic Oxygen Furnace (BOF) and
Stainless Steel (SS) slags to
-
5 L with Solution A of AMD. The experimental volumes were 5times
greater than in Section 2.3 since many samples were takenfrom each
experiment and we did not wish to signicantly alterthe mass balance
by taking many samples. The masses of slag usedin these experiments
ranged from 100 g to 5000 g. The slagsamples were placed in
unstirred, open, 5 L glass beakers and5 mL aliquots were sampled
after every 30 min and stored in50 mL test tubes for analysis. The
sampled solutions were testedfor pH, iron and sulfate levels as
described above.
2.5. Testing the maximum capacity of the slag using continuous
AMDow
For this experiment, two packed bed reactors were
constructedusing 1500 mL beakers. 1200 mL of SS slag and BOF slag
wereplaced into the two beakers. Synthetic AMD solution B waspumped
(using a peristaltic pump) into the base of each beaker
assaying (Scroobys Laboratory Services). The composition of
the
C 1.06 0.76
ls Engineering 64 (2014) 1522 17reduce the acid, sulfate and
iron content of a typical Witwaters-rand gold basin AMD. In
addition it was sought to investigate thekinetics, determine the
rate of pH change and the amount of ironand sulfate removed by the
slag from the AMD, with a view tomaximising the lowering of iron
and sulfate concentration andacidity of AMD and to determine the
long-term capacity of the slagto treat AMD.
2. Experimental methods
In order to complete the research objectives, three
experimentswere conducted. These included a test to determine the
optimumratio of slag to AMD, a test to understand the rate at which
theAMD reacted with the slag and a test using continuous ow
todetermine the point of slag saturation.
2.1. Analytical techniques
Iron and sulfate were analysed using a Merck SpectroquantPharo
300 spectrophotometer and Merck reagent kits. Sulfate andiron ion
concentrations were analysed using the Merck test kitsNo. 114791
and No. 114761 respectively. The pH of all sampleswas measured
using a Metrohm 744 pH meter and calibrated withMetrohm buffer
solutions at pH = 4, 7 and 9 at frequencies accord-ing to the
manufacturers specications. After use, the electrodewas washed with
distilled water and then dried to prevent con-tamination of
subsequent samples measured. Data was analysedusing MS Excel.
2.2. Preparation of an articial AMD
A typical Witwatersrand gold basin AMD with low pH andelevated
concentration of metals and sulfate was simulated inthe laboratory.
The synthetic AMD was created according torecommendations by
Potgieter-Vermaak et al. (2006). Twodifferent solutions of AMD were
synthesised; Solution A had acomposition of 600 mg/L Fe, 4800 mg/L
SO42 and pH of 2.5 repre-senting a low-strength AMDwhilst Solution
B had a composition of1000 mg/L Fe, 5000 mg/L SO42 and pH of 2.25
representing a high-strength AMD. The synthetic AMD was prepared by
dissolvingindustrial (95% pure) hydrated ferrous sulfate
(FeSO47H2O) andsulfuric acid (H2SO4) in distilled water the desired
iron and sulfateconcentration. pH was adjusted using analytically
pure NaOH fromMerck Chemicals. Solution loss through sampling and
evaporationduring the course of experimentation was compensated by
theaddition of distilled water and the mass balance was
correctedfor these small losses.
2.3. Testing the ratio of AMD to slag
Samples were prepared to give slag to AMD ratios of 20, 40,
60,80, 100, 120 and 140 g/L (grams of slag per litre of AMD) by
addingan appropriate amount of slag with AMD from a prepared
stocksolution. The masses of slag used in the experiments ranged
from20 g to 140 g and the solution volume used was 1 L. The
sampleswere placed in unstirred, open, 2 L glass beakers and S left
for fourhours to allow for sufcient time for the reaction to take
place. Thesamples were ltered from the slag residue and each sample
wasthen analysed for acidity, iron and sulfate content. The pH
wasmeasured with a digital pH meter.
2.4. Testing the optimum time of contact between the AMD and
slag
T. Name, C. Sheridan /MineraThis experiment was designed to test
slag to AMD ratios of 20,40, 60, 80, 100, 120 and 140 g/L by adding
the mass and topping toS 0.34 0.13MnO 1.42 1.27P2O5 0.46 0.05SiO2
15.2 26.8Cr2O3 0.31 1.91NiO 60.01 0.31CuO 0.19 0.07Al2O3 5.52
5.87V2O5 0.48 0.05TiO2 4.02 0.68CoO 60.01 60.01slags is shown in
Table 1. The samples were pulverised to less than75 lm. Parallel
samples were prepared by measuring a knownmass, fusing with sodium
peroxide and then leaching with HCl,HNO3 and water. Samples were
analysed by ICP a Thermo ScienticiCap 6300 radial utilising iTeva
software.
The slags are mainly comprised of calcium oxide or lime
(CaO),silicon dioxide (SiO2), iron (II) oxide (FeO), magnesium
oxide(MgO) and aluminium oxide (Al2O3). The SiO2 was 11.6% more
inSS slag than in BOF slag. The BOF slag contained more Fe andCaO
than the SS slag.
Table 1Compositions of BOF and SS slag as determined by SLS-ICP
(ScienticLaboratory Supplies Inductively Coupled Plasma).
Analysis Composition of BOF slag(mass %)
Composition of SS slag(mass %)using a peristaltic pump and this
owed slowly up through themass of slag prior to overowing from a
overow nozzle. Flowrates of 4 mL/min, 8 mL/min, 12 mL/min and 16
mL/min were usedwhich translated to 2, 1, 0.5 and 0.25 h of
residence time. Resi-dence time is dened as the liquid volume
divided by the volumet-ric owrate, and is given by Eq. (6) in this
paper. Porosity of theslag was measure at approximately 40%. Each
test was completedtwice to check reproducibility. Samples of the
outow of the SLBswere taken every 2 h during operating days and
samples were ana-lysed for sulfate, iron and pH.
3. Results and discussion
3.1. Slag characterisation
The BOF and the SS slag were sent to an external laboratory
forCaO 38.7 36.0MgO 6.80 13.0FeO 16.5 5.54
-
18 T. Name, C. Sheridan /Minerals E3.2.1. The pH changes of acid
mine drainageIn Fig. 1 the increase in pH with increased slag to
AMD ratios is
shown. The pH increased from 2.5 to 6.0 for SS slag and to 12.1
forBOF after four hours of contact time. A ratio of 100 g/L of AMD
wasobserved to be the maximum ratio as an increase in slag had
noappreciable increase in pH, especially for the SS slag.
Themechanism of pH increase is hypothesised to be from
dissolutionof available CaO present within the slag. Ca is reported
as CaObut may also occur in a CaOSi phase. This could also
explainwhy the SS slag was less reactive and had less alkalinity in
solution.
It can clearly be seen in Fig. 1 that BOF slag had higher
alkalinitythan SS slag. This is due to the fact that BOF slag had
more calciumoxide content than SS slag as shown by the data in
Table 1. It mayalso be that because SS slag had higher silicon
dioxide content thanBOF slag, the CaO present was not available for
dissolution as itwas locked in an insoluble glassy matrix. This was
not, howevertested although the results agree with Shen and
Forssberg (2003)who suggested that high silicon dioxide content in
compoundstend to make them less alkaline.
3.2.2. Reduction of iron and sulfateThe % removal of iron and
sulfate at different slag to AMD ratio3.2. Determining the effect
of the ratio of AMD to slag
Fig. 1. The pH changes at different slag to AMD ratios for
different slags after fourhours (pHo = 2.5).is presented in Fig. 2.
As seen from the gure, iron and sulfateremoval increased with an
increase in the slag to AMD ratio for
Fig. 2. Reduction of iron and Sulfate at different slag to AMD
ratio for different slagsafter four hours (Feo = 600 mg/L; SO42o =
4800 mg/L).both slags used. The highest percentage iron removal
recordedwas 63.6% for SS slag while 99.7% iron removal was recorded
forBOF slag. The highest iron removal was recorded at pH values
of6.0 and 12.1 for SS and BOF respectively. The maximum
sulfateremoval percentage recorded was 40% for SS slag while 75%
sulfateremoval was recorded with the use of BOF slag.
It was also noted that maximum sulfate removal was achievedat
the optimum slag to AMD ratio (100 g/L), where pH values of11.3 and
5.9 were recorded for BOF and SS slag respectively. Fromthe graph
above it is evident that iron was reduced by almost 100%at slag to
AMD ratios of 100, 120 and 140 g/L for BOF slag. The pHvalues of
11.3, 11.3 and 12.1 were recorded at those ratios.
Almost all the soluble iron was removed using BOF slag and
thiswas attributed to the formation of various precipitates. A
Pourbaixdiagram for FeSH2O System at 298 K (Rose, 2010) indicates
thatprecipitates such as Fe(OH)3 and Fe(OH)2 are formed, while
Rose(1989) claimed that other iron precipitates such as
(FeOOH),(Fe2O3) and (Fe5O8H4H2O) could also be formed at various pH
val-ues. The type of iron precipitate generated was not assessed in
thisstudy.
The low iron removal values obtained for SS slag are likely
dueto the fact that lower pH values were found. The EhpH
diagramindicates that below pH values of 9, the stable forms of
iron areFe2+, Fe3+ and FeSO4+. Thus SS slag caused less iron
removal thanBOF slag because of its inability to increase the pH
above 9 whichwould result in the formation of precipitates.
Neither of the slags used in this study contained barium or
leadin their composition; hence, the mechanism of sulfate
removalwould primarily be due to the formation of gypsum which is
onlysparingly soluble. As discussed, analysis of the sludge was not
con-ducted. The BOF slag caused signicantly greater removal of
sulfatethan SS slag.
3.3. Determining the optimum contact time between the AMD and
slag
The concentration curves for pH, iron and sulfate for SS and
forBOF slag are shown in Fig. 3. The results of this experiment
showedthat the pH of AMD increased with an increase in contact
timebetween AMD and both slags. The increase in pH was also
depen-dent on the slag to AMD ratio. The pH increased rapidly in
the rst30 min and thereafter reached steady state for all slag to
AMD ratiocombinations. The pH increase was higher for BOF slag
comparedto SS slag. The maximum pH reached for BOF was 11.3 while
5.9was reached for SS. The concentration of iron decreased much
fas-ter in BOF slag than SS slag and the increase was also shown to
be afunction of the ratio of slag to AMD for both slags. The iron
concen-tration was lowest after 3 h with BOF slag for a slag to AMD
ratio of100 g/L compared to a residual concentration of 220 mg/L of
ironfor the SS slag. The rate of removal was most rapid in the
rst30 min. For sulfate, similar trends were observed, the rate
ofdecrease of concentration was most rapid in the rst 30 min andthe
rate was also a function of the slag to AMD ratio with higherratios
showing the highest rate.
These experiments indicate that in particular, BOF slag could
bedirectly added to AMD as a form of remediation provided
heavymetals are not leached from the slag by the AMD (this was
nottested in this study). These results agree with those found by
oth-ers (Ziemkiewicz and Skousen, 1998; Zurbuch, 1996) where
theirtests indicated that AMD could be treated by directly
applyingalkaline products into the mine discharge.
3.4. Testing the maximum treatment capacity of SS and BOF
slagsusing continuous ow reactors
ngineering 64 (2014) 1522Further experiments were carried out in
a continuous process inorder to inform for design or practical
purposes in this experiment,
-
ls Engineering 64 (2014) 1522 19T. Name, C. Sheridan
/MineraSolution B was used such that the results would be
representworst-case scenario slag-treatment schedules. The
experimentswere conducted with ow rates of 4, 8, 12 and 16 mL/min
translat-ing to residence times of 2, 1, 0.5, 0.25 h. Given that SS
slag wasfound to be less effective than BOF slag, it was not tested
anyfurther and thus only results for BOF are presented.
3.4.1. Measuring the effect of ow rate on pHThe pH changes for
treated efuent at ow rates of 4, 8, 12 and
16 mL/min in leach bed occupying a volume of 1.2 L for a period
of12hrs continuous process using BOF slag are presented in Fig.
4.The feed pH of the simulated AMDwas 2.25. The maximum pH val-ues
measured in the outlet were 13.2, 11.0, 9.9 and 7.9 for owrates of
4, 8, 12 and 16 mL/min respectively. Fig. 3 also shows thatpH
increased rapidly in the rst two hours of sampling before
grad-ually declining. This is primarily a function of contact time
andindicates that the alkalinity is being consumed. As expected,
thepH increased with an increase in residence time (or decreasingow
rate). This is primarily a function of contact time.
3.4.2. Effect of Flow rate on reducing iron concentration in
acid minedrainage
The removal of iron at ow rates of 4, 8, 12 and 16 mL/min overa
period of 12 h in the continuous process using BOF slag is
shownFig. 5. The removal in iron concentration was higher at low
ow
Fig. 3. Concentration curves for sulfate, irorates and decreased
as the ow rate was increased. The feedconcentration of iron was
1000 mg/L. Iron was decreased to non-detectable concentrations as
shown in the Figure for feed ow rateof 4 mL/min and 8 mL/min. The
concentrations began to increaseduring the course of the experiment
which is related to the pointin the experiment when the pH values
began to drop.
n and pH for SS slag and for BOF slag.
Fig. 4. Effect of ow rate on reduction of acid for BOF slag for
a period of 12 h.
-
Precipitates of iron were thought to have been formed at highpH
values. These precipitates of iron therefore ensured that nosoluble
iron was detected. At low ow rates, these precipitateswould have
been ltered by the slag. At higher ow rates, less reac-tion time
was afforded to the slag to react and bring about high pHvalues
necessary for formation of precipitates.
3.4.3. The effect of ow rate on sulfate concentrationIn Fig. 6
the sulfate concentration at AMD ow rates of 4, 8, 12
and 16 mL/min over a period of 12 h in continuous process
usingBOF slag is presented. The removal of sulfate was also higher
atlow ow rate and decreased as the ow rate was increased. Thefeed
composition sulfate was 5000 mg/L and was decreased to aminimum
concentration of approximately 740 mg/L for a ow rateof 4 mL/min.
That minimum concentration achieved was stillabove the DWAF general
limit for wastewater disposal into a waterresource (Department of
Water and Forestry, 1996). Sulfatereduction was lower for high feed
ow rates.
3.5. Calculating the effect of residence time on reducing iron
and
The removal of sulfate in the AMD is shown in Fig. 10. It can
beseen that the feed composition of sulfate was 5000 mg/L and
thatthe sulfate concentration was 693 mg/L after 1 h, and outlet
ofthe leach bed had a continuously rising sulfate concentration to
a
20 T. Name, C. Sheridan /Minerals Engineering 64 (2014)
1522sulfate in AMD
The experiments carried out were able to reduce iron to
levelsbelow DWAF general limit for disposal of wastewater into a
waterresource, but sulfate levels were still above the limit
(Departmentof Water and Forestry, 1996). It was decided to design
the contin-uous process in way that would hopefully be to reduce
sulfateconcentration to below 400 mg/L. Concentration of sulfate
and ironwere thus plotted against residence time and a correlation
wasestablished. The residence time capable of reducing sulfate
concen-tration below 400 mg/L was tabulated from the equation
relatingconcentration to residence time.
In Fig. 7 data is presented which shows iron and sulfate
removalin AMD as a result of different residence times for ow rates
of 4, 8,12 and 16 mL/min. From the Figure, it can be seen that the
removalof iron and sulfate was greatest at a residence time of 2 h.
This nd-ing is in agreement with those found by others (Kruse et
al., 2012;Skousen and Ziemkiewicz, 2005) whose experiments found
thatslag leach beds required one to three hours of residence time
fortheir design.
Sulfate removal in synthetic AMD using BOF was correlated totime
and the resulting function is given in Eq. (5):Fig. 5. Effect of ow
rate on reduction of iron after treatment with BOF slag for aperiod
of 12 h.Sulfate concentration 1077 lnresidence time 1457:6 5where y
was the sulfate concentration and x, was the residencetime.
According to the equation a residence time of 2.67 h wouldbe
required to achieve a residual sulfate concentration of 400
mg/L.Thus, we used a ow rate of 3 mL/min of AMD to the slag
reactorto achieve that concentration according to the calibration
curve.That ow rate was fed to the slag bed for 2 days to ascertain
howacid, iron and sulfate concentration changed with time.
The pH changes of AMD for the designed residence time of2.67 h
using BOF slag over a period of two days are shown inFig. 8. The pH
changed from 2.25 to a maximum of 13.3 in the rsttwo hours and
decreased thereafter throughout the course of theexperiment. Data
was collected between at 8 am and at 8 pmand hence there is no data
between 12 h and 24 h. The data from24 h onwards followed the same
trend, however.
For iron, the feed composition iron was 1000 mg/L and this
wasreduced to below detection limit as shown in the graph in the
rst1012 h. After 12 h the concentration of iron began to
increase.The average concentration of iron between 012 h, 1224 h
and2436 h is also presented in Fig. 9.
Fig. 6. Effect of ow rate on sulfate reduction after treatment
with BOF slag for aperiod of 12 h.Fig. 7. Iron and sulfate
reduction at different residence times.
-
maximum of 3700 after 36 h. The experiment was designed to
havean initial sulfate removal to below 400 mg/L; however, this is
notpossible due to solubility constraints. We chose this
value,however so as to be conservative in our experimental
design.
4. Predicting slag requirements for treating different
quantitiesof different strength AMD streams
For design purposes, it would be necessary to understand
thereplacement strategy of the slag treating different strength
AMD.The residence time s, void fraction e, volume V, feed ow rate
Qand mass of slag Vslag were all related according to the
followingequation.
sVslagQ
s 6
We have chosen to use Vslag for this calculation since the mass
ofFig. 9. Reduction of iron for a continuous process using BOF slag
for duration of2 days.
Fig. 10. Reduction of sulfate for a continuous process using BOF
slag for duration of2 days.
T. Name, C. Sheridan /Minerals Engineering 64 (2014) 1522 21slag
heaps is unknown, however there volumes can reasonablyaccurately be
estimated. The slag has a bulk density of approxi-mately 1200 kg/m3
which was used for conversion from experi-mental data for this
scale-up exercise. In Fig. 11 the data is usedto illustrate the
amount of slag required for a set of different feedow rates to
achieve a specied set of varying outletconcentrations.
As can be seen, the amount of slag required for a xed AMD owrate
will increases with an increase in residence time. For example,if
it was required to treat 12 mL/d of AMD and the target
residualconcentrations of iron and sulfate were 1.3 and 844 mg/L
respec-tively, approximately 3300 m3 of slag would be needed with
aresidence time of 2.67 h.
The volume of decant, or efuent, in the Witwatersrand Basin
isexpected to range between 12 and 20 mL/d (Coetzee et al.,
2010).The target iron and sulfate concentration need to be
consideredbefore predicting the amount of slag required to treat
the AMD.Under these conditions, i.e. treating these ow rates, the
slagwould have to be replaced after the residence time specied.
Thegure shows that the amount of slag required would increase
withan increase in the feed ow rate. It also shows that more slag
isrequired with an increase in residence time for a xed AMD owrate.
This implies that if one were to treat 20 mL/d, after 2.67 habout
5500 m3 of slag would need replacing, and the nal concen-tration
would be approximately 1.3 ppm Fe and 844 ppm sulfate. Ifone were
to treat 20 mL/d after 0.5 h, about 200 m3 of slag wouldneed
replacing, and the nal concentration would beFig. 8. Reduction of
acid for a continuous process using BOF slag for duration of2
days.
Fig. 11. A prediction of the amount of slag required to treat
different feed ow ratesof AMD.
-
22 T. Name, C. Sheridan /Minerals Eapproximately 188 ppm Fe and
3190 ppm sulfate. Most likely anal design would require several
parallel systems such thatreplacement could occur whilst others
remain operational.
5. Conclusions and recommendations
Both slags used were able to reduce acid, iron and sulfate
con-centration. However, the BOF slag was signicantly better than
theSS slag in reducing acid, iron and sulfate concentration. It
wasshown that acid, iron and sulfate removal depended on the
amountof slag added per 1L of synthetic AMD, the contact time
betweenslag and AMD and ow rate of synthetic AMD fed to the
SLBs.The ratio tests showed that acid, iron and sulfate
removalincreased with an increase in slag to AMD ratio. A ratio of
100 gslag to 1 L of AMD was found to be the optimum at which
maxi-mum removal was achieved for both BOF and SS slag in the
batchexperiments. At that ratio 63.6% iron removal was found for SS
slagcompared to 99.7% iron removal for BOF slag. A 39.8%
sulfateremoval was found for SS slag compared to 75% sulfate for
BOF slagat the same ratio. SS slag managed to increase the pH of
syntheticAMD from 2.5 to 6.0 compared to 12.1 for BOF slag.
Acid, iron and sulfate removal was found to be very rapid in
therst hour of contact between slags with AMD in the batch
pro-cesses. Remediation of AMD was also successful in a
continuousow process at low lower ow rates. For design purposes,
theamount of slag required to treat different feed ow rates of
AMDfor target iron and sulfate concentration and that any design
wouldbe need to understand the strength of the AMD, the ow rate of
theAMD and also the desired residual concentration, particularly
ofsulfate.
From this study, future research to be conducted includes
focus-ing on establishing the toxicity of any trace metals or
elements thatmay leach into AMD during the removal of acid, iron
and sulfate.Further research could also be carried out to ascertain
the compo-sition of the slag residue after pH neutralisation and
iron andsulfate removal. This will enable us to know if it can be
usedfurther for other purposes such as road construction or as
anaggregate in cement. The slag could also potentially be used
inconjunction with constructed wetlands as a more efcienttreatment
process.
Acknowledgements
We gratefully acknowledge the nancial support from the Glo-bal
Change and Sustainability Research Institute of the Universityof
the Witwatersrand, Johannesburg. We also extend our thanksto Harsco
Metals and Minerals, South Africa for the provision ofthe
metallurgical slags.
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ngineering 64 (2014) 1522
Remediation of acid mine drainage using metallurgical slags1
Introduction1.1 The use of metallurgical slags for treating AMD1.2
The mechanism of slag in acid mine drainage remediation1.3 Research
objectives
2 Experimental methods2.1 Analytical techniques2.2 Preparation
of an artificial AMD2.3 Testing the ratio of AMD to slag2.4 Testing
the optimum time of contact between the AMD and slag2.5 Testing the
maximum capacity of the slag using continuous AMD flow
3 Results and discussion3.1 Slag characterisation3.2 Determining
the effect of the ratio of AMD to slag3.2.1 The pH changes of acid
mine drainage3.2.2 Reduction of iron and sulfate
3.3 Determining the optimum contact time between the AMD and
slag3.4 Testing the maximum treatment capacity of SS and BOF slags
using continuous flow reactors3.4.1 Measuring the effect of flow
rate on pH3.4.2 Effect of Flow rate on reducing iron concentration
in acid mine drainage3.4.3 The effect of flow rate on sulfate
concentration
3.5 Calculating the effect of residence time on reducing iron
and sulfate in AMD
4 Predicting slag requirements for treating different quantities
of different strength AMD streams5 Conclusions and
recommendationsAcknowledgementsReferences