International Journal of Mineral Processing 149 (2016) 1–8
Contents lists available at ScienceDirect
International Journal of Mineral Processing
journal homepage: www.elsevier.com/locate/ijminpro
Removal of sulfate ions by dissolved air flotation (DAF) following
precipitation and flocculation
J. Amaral Filho, A. Azevedo, R. Etchepare, J. Rubio ⁎
Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Campus do Vale, Prédio 43819-Setor 6, 91501-970, Porto Alegre-RS, Brazil
a r t i c l e
i n f o
Article history:
Received 16 May 2015
Received in revised form 12 December 2015
Accepted 22 January 2016
Available online 26 January 2016
Keywords:
Sulfate removal
Precipitation
Polyaluminum
Flocculation
Flotation
Micro and nanobubbles
a b s t r a c t
The removal of sulfate ions from wastewater is an environmental challenge faced by several industrial sectors,
such as the mining, metallurgical, chemical and petrochemical industries. Most existing options are inefficient
and costly, particularly for sulfate-bearing acid mine drainage (AMD; coal and metal sulfides). This work focused
on the precipitation of sulfate with polyaluminum chloride at pH 4.5 at the bench scale and their separation by
flotation. However, these hydrophilic precipitates did not float via dissolved air flotation (DAF) and needed to
be flocculated with cationic polyacrylamide. The best removal of sulfate-sodium salts from solution was obtained
−1
of polymer flocculant. The sulfate-bearing flocs were readily
using an Al/SO2−
4 molar ratio of 4:1 and 20 mg·L
removed from the aqueous solutions by microbubbles (MBs, 30–100 μm) and nanobubbles (NBs, 150–800 nm).
The separation was very rapid and followed a first-order flotation kinetics model with a high rate constant of
4.1 min−1. The results were validated using AMD generated by a coal mine with a sulfate concentration of
1753 mg·L−1, and the anion concentration was rapidly reduced to below the World Health Organization
(WHO) standard of 500 mg·L−1. Attempts to improve the removal efficiency with sodium oleate flotation collector and conditioning with NBs were not successful. The maximum removal percentage (80–82% of the feed content) appears to be limited by the efficiency of the DAF process and the chemical equilibrium of the precipitates,
which leaves some soluble sulfate in solution. Bubbles readily attach to the flocs and become entrained and/or
entrapped in the flocs, creating aerated flocs. Because all of these mechanisms operate simultaneously, the flotation of the flocs is very rapid, as indicated by the high kinetics rate constant. We concluded that the DAF of sulfateloaded flocs has potential for the treatment of voluminous sulfate-bearing effluents, including coal AMD, at high
superficial rates.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Sulfate ions are found in many surface and ocean waters and wastewaters worldwide. Furthermore, sulfate ions are believed to be the second most abundant anions in rivers and seawater (Lens and Pol, 2000).
Sulfate ions also occur naturally in numerous minerals, such as barite
(BaSO4), epsomite (MgSO4·7H2O) and gypsum (CaSO4·2H2O). Moreover, sulfate salts are used as chemicals in various forms, and they are
formed in the mining industry by the oxidation of metal sulfides in
aqueous systems, such as rock mining drainage (RMD) or acid mine
drainage (AMD).
Sulfate ions are thought to be responsible for severe environmental
disturbances in terms of excess mineralization of surface waters
(Maree and Strydom, 1985) and for health and industrial problems
(Lens et al., 1998; Silva et al., 2010; USEPA, 1999). In the coal mining
⁎ Corresponding author.
E-mail address: [email protected] (J. Rubio).
URL's: http://www.ufrgs.br/ltm (J. Amaral Filho), http://www.ufrgs.br/ltm
(A. Azevedo), http://www.ufrgs.br/ltm (R. Etchepare), http://www.ufrgs.br/ltm (J. Rubio).
http://dx.doi.org/10.1016/j.minpro.2016.01.012
0301-7516/© 2016 Elsevier B.V. All rights reserved.
industry, AMD is generated following the oxidation of pyrite (Akcil
and Koldas, 2006; Kontopoulos, 1998) in the presence of bacteria.
AMD contains high levels of sulfate, heavy metal ions and acidity and
constitutes one of the main challenges in this sector. Enhanced soil acidity is another pressing issue.
Because diarrhea is associated with the ingestion of water containing high levels of sulfate anions, regulatory agencies are becoming increasingly concerned with the rising concentrations of these ions in
drinking water. The World Health Organization (WHO) has established
a maximum tolerable level of sulfate in sources of drinking water:
500 mg·L−1 (WHO, 2011). Many countries have recommended stricter
maximum standard limits, such as 250 mg·L−1 and 150 mg·L− 1 in
Brazil and the Netherlands, respectively (Brazil, 2011; Staatsblad,
2011). Accordingly, there is an urgent need to develop more efficient
and sustainable treatment systems to comply with the standard limits.
However, the removal of sulfate ions from water and wastewater is
complex due to the high solubility and stability of these anions in aqueous solutions. The following methods represent the main processes
used to treat sulfate ion-bearing water: (1) biological degradation or
adsorption, (2) membrane filtration (primarily reverse osmosis),
2
J. Amaral Filho et al. / International Journal of Mineral Processing 149 (2016) 1–8
(3) adsorption and/or ion exchange in resins, and (4) chemical precipitation. Silva et al. (2010) summarizes several sulfate removal processes,
and Table 1 describes other upcoming techniques.
Some of these alternatives are costly in terms of investment, operation and maintenance, whereas others have a large footprint and/or
poor efficiency.
Many authors have hypothesized that the efficiency of the coagulation properties of polyaluminum chloride (PAC) depends on the distribution of Al species, the basicity and the Altotal concentration. For
example, Parker and Bertsch (1992), Wang et al. (2002) and Zhou
et al. (2006) showed that the soluble species that generate soluble complexes with sulfate ions, designated as Ala, primarily include monomeric
species. In contrast, Alb species include polynuclear Al species and possess structures that are fairly stable and withstand further hydrolysis,
resulting in higher coagulation efficiencies. The fraction of aluminum
that precipitates as hydroxide salt represents the colloidal or solid
phase Alc (Zhou et al., 2006). Silva et al. (2010) extended these studies and were able to form strong flocs of sulfate/Al-bearing coprecipitates with polyacrylamide, which readily settled in lamellar
settlers. The operating parameters and general costs were evaluated,
and the main drawbacks were found to be the slow separation kinetics and some design problems concerning the removal of the flocs
(Silva and Rubio, 2011).
Dissolved air flotation (DAF) is a well-known technology with potential in many solid–liquid separation processes, including the removal
of oil, colloidal suspensions, bacteria, algae, and viruses in water and
wastewater treatment (Edzwald and Haarhoff, 2011; Karhu et al.,
2014; Liu et al., 2012; Pernitsky and Guest, 2012; Veneu et al., 2012).
The main advantage of modern DAF is its high separation kinetics (9–
14 m·h− 1), which are superior to those of particle settling methods
(5–6 m·h−1) (Edzwald, 2010; Rodrigues and Rubio, 2007; Rubio et al.,
2002; Silva and Rubio, 2011; Silva et al., 2010; Zaneti et al., 2011, 2013).
The present work, which is a continuation of the work by Silva and
Rubio (2011) and Silva et al. (2010), investigates the separation of
PAC/SO24 − flocs via DAF with microbubbles (MBs) and nanobubbles
(NBs), focusing primarily on the separation kinetics of the flocs after
optimizing the chemical and operating parameters.
2. Materials and methods
2.1. Materials
Reagent grade anhydrous sodium sulfate (Na2SO4) was employed to
prepare feed solutions with 1800 mg·L−1 of SO24 −. Commercial PAC
(QGS Química do Brasil Ltd., Sapucaia do Sul/RS, Brazil) with an Al2O3
content of 18.2% (mass fraction) and a density of 1.31 g·cm− 3 was
used as the aluminum source for the sulfate co-precipitation, and sodium oleate (from Vetec) was used in some tests as a flotation collector
reagent. Flocculation was performed with a low-charge cationic polyacrylamide (Flonex 4115 SH – SNF Floerger®).
A sample of AMD containing 1700–1800 mg·L−1 of SO2−
4 , obtained
from the coal region of Santa Catarina State, South Brazil, was employed
as the model wastewater used to validate the proposed treatment
scheme at the bench scale.
Reagent grade hydrochloric acid (HCl) and sodium hydroxide
(NaOH), both obtained from Merck®, were used to adjust the pH of
Table 1
Upcoming sulfate ion removal techniques and comments.
Process employed/references
Biological processes
Low pH sulfidogenic bioreactors
(Ňancucheo and Johnson, 2014)
Upflow anaerobic sludge blanket
(UASB)–submerged aerated biofilter
(SAB) (Amaral et al., 2014)
Anaerobic sequencing batch biofilm
reactor (Sarti and Zaiat, 2011)
Anaerobic sequencing batch biofilm
reactor (Sarti et al., 2009)
Chemical precipitation
Precipitation with
hydrolyzed aluminum
(Silva and Rubio, 2011;
Silva et al., 2010)
Ettringite precipitation (Cadorin, 2008)
Adsorption and/or ion exchange
Polystyrene weak base ion exchange resin
(Amberlyst) (Guimarães and Leão, 2014)
Anion exchange resin
(Haghsheno et al., 2009)
Main features
Main conclusions
- Treatment of two synthetic mine waters (pH 1.3 to 3.0)
in an upflow biofilm sulfidogenic bioreactor
- Pilot-scale studies with real textile effluent
- Equalized effluent was fed into the UASB and SAB by gravity
- Glycerol acts as an electron donor. The sulfate content was
reduced by approximately 98%
- COD/SO2−
ratios were 2.4–2.8 for the UASB influent
4
- Sulfate removal reached 41–54%
-
Full-scale study
High sulfate loading rates (580–5200 mg·L−1 of SO2−
4 )
Reactor operated for 110 cycles (48 h each)
Pilot-scale study
Industrial effluent containing high sulfate concentrations
(250 to 3000 mg·L−1 of SO2−
4 )
- Domestic sewage was used to dilute the sulfate-rich
industrial wastewater, providing some organic matter
for sulfate reduction
- Ethanol was added as a supplementary source of electrons
- After 71 cycles, the sulfate removal efficiency was 99%
- Biofilm formation followed two different patterns: one at
the beginning of the colonization and one in the mature biofilm
- The process combined removal of sulfate (86%) and organic
matter (70%) in a feed with sulfate concentrations
N2000 mg·L−1 of SO2−
4
- Formation of high concentrations of reduced sulfur
compounds and residual COD was observed
- Sulfate-bearing precipitates are formed with aluminum
salts (AlCl3 and polyaluminum chloride — PAC)
- Laboratory and pilot studies using lamella settling in
batch and continuous studies. Studies of the settling of
the formed flocs were successful (N80% sulfate removal)
- Bench-scale and pilot trials with AMD
(680–1800 mg·L−1 of SO2−
4 )
- Precipitation of sulfate by ettringite formation using
lime and aluminum salts (PAC and Alupan) at pH 12
- Precipitation of sulfate depended on pH (optimal at 4.5), the
Al-reagent/sulfate ions mass ratio (optimal at 7:1) and time
(10 min)
- Amberlyst resin, a weak base (tertiary amine)
macroporous polystyrene resin with an exchange
capacity of 1.25 eq·L−1
- Laboratory-scale studies with a copper mine wastewater
- Sulfate ions concentration in feed = 500–900 mg·L−1
(pH 9)
- Adsorption experiments on a Lewatit K6362 resin
in a fixed bed column
- Best results (77% sulfate removal) at bench-scale were
obtained with a [PAC]:[Alu]:[SO−2
4 ] mass ratio of 2:1.2:1
- At the pilot-scale, up to 88% sulfate removal was attained
- In acidic conditions, the process was fast (k = 1.67 ×
10−4 s−1) and exothermic (H = −25.06 kJ mol−1).
In fixed-bed columns, the maximum adsorption capacity
−1
was 11.6 mgSO2−
4 ·mLresin
- 100% resin elution was achieved by increasing the pH to 10
and 12 with NaOH
- The removal of sulfate ions was dependent on the mass
concentration of the adsorbent, flow rate and bed height
- Maximum removal (approximately 100%) occurred with
1000 mg·100 mLresin−1
J. Amaral Filho et al. / International Journal of Mineral Processing 149 (2016) 1–8
3
the solutions. Deionized (DI) water at room temperature (23 °C ± 1)
with a conductivity of 3 μS·cm−1, a surface tension of 72.5 ±
0.1 mN·m−1 and a natural equilibrium pH of 5.5 was used to prepare
all of the aqueous solutions used in the experiments. The DI water was
prepared from tap water using a reverse osmosis system (Fisher
Scientific®) in our laboratory.
2.2. Methods
2.2.1. Precipitation of sulfate ions with PAC
The first process aimed to determine the best Al/SO2−
4 molar ratio for
reaching the standard emission values for sulfate ions in drinking water
sources (500 mg·L−1, WHO, 2011).
The precipitation of sulfate ions was performed using PAC in a 0.5-L
beaker subjected to magnetic stirring at pH 4.5 to simulate the ideal
conditions for sulfate adsorption investigated in our previous studies
(Silva and Rubio, 2011; Silva et al., 2010).
Precipitates formed after 5 min were filtered using acetate cellulosic
membranes (0.45 μm pore diameters) before residual sulfate analysis.
The effect of the Al/SO2−
4 molar ratio (from 1:1 to 5:1) on the precipitation of the anion was studied using a 0.019 M Na2SO4 solution.
The zeta potential of the precipitates formed using PAC at different
Al/SO24 − ratios at pH 4.5 was measured by a ZetaSizer Nano ZS
(ZEN3600 — Malvern®).
2.2.2. Flocculation of co-precipitates with a cationic polyacrylamide
A high-molecular-weight low-charge cationic polyacrylamide,
Flonex 4115 SH (Floerger®), was selected as the polymer flocculant
after preliminary jar-tests with several polymer types and concentrations (data not shown). Flocculation of the precipitates was performed
under stirring at 120 rpm for 60 s to allow dispersion and flocculant adsorption, followed by slow stirring at 40 rpm for 180 s.
The efficiency of the flocculation was assessed visually (size and settling velocity of the flocs). After flotation, the efficiency was measured in
terms of residual sulfate concentration in the treated water as a function
of time and polymer flocculant concentration.
2.2.3. Study of micro- and nanobubble DAF of sulfate-bearing precipitates
2.2.3.1. Synthetic solutions of sodium sulfate. The DAF setup, shown in
Fig. 1, consisted of a mechanical stirrer (Fisatom 713-D) for reagent
conditioning, a flotation cell (2-L capacity — inner diameter of 9 cm
and height of 33 cm) and a saturation vessel. The latter, which was
employed for the saturation of air in water, was a steel vessel equipped
with an internal glass container with a height of 15 cm, an inner diameter of 12 cm and a wall thickness of 1 cm. Compressed air was fed to the
saturation vessel, and a constant internal pressure of 5 atm was maintained for 30 min for the air saturation of water.
MBs and NBs were generated by depressurizing the air-saturated
water solutions at a high flow velocity through a needle valve (Globo
012 — Santi®, composed of steel, 2 mm internal diameter) into to the
flotation cell.
The size distribution of MBs were previously measured in our laboratory (Rodrigues and Rubio, 2003), and the D32 (Sauter mean diameter) of 33–37.5 μm was recorded. The average bubble size distribution
of the NBs was measured via dynamic light scattering (DLS) technique
using a ZEN3600 instrument.
The pH of the aqueous solutions was determined using a pH meter
(Analion, AM 608), and the concentration of residual sulfate ions was
measured using the turbidimetry method (Eaton and Franson, 2005)
with a nephelometer (HACH® 2100 N).
All experiments were performed in triplicate at room temperature
(25 ± 2 °C).
2.2.3.2. Flotation kinetics. The effect of the polymer flocculant concentration on flotation rates was studied at 10 and 20 mg·L−1. The treated
Fig. 1. Experimental setup for the flotation of sulfate-bearing flocs. (a) Saturation vessel;
(b) needle valve; (c) conditioning/flotation cell.
water samples were collected at different flotation times to measure
the residual sulfate contents. The results were expressed as sulfate removal percentages. The flotation kinetics were calculated by fitting
the data to a first-order kinetics model for simple batch flotation
(Zuñiga, 1935). The kinetics rate constant k was obtained using Eq. (1).
Ln
R∞
R∞ R
¼ k:t
ð1Þ
The García-Zuñiga model of flotation kinetics is given by Eq. (2),
R ¼ R∞ 1 ek:t
ð2Þ
where
R
R∞
k
t
Calculated removal of sulfate ions (%);
Removal of sulfate ions under steady state conditions (%);
Separation rate constant (min−1);
Flotation time (s).
2.2.3.3. Studies of DAF assisted by NBs. NBs aqueous dispersions were selectively formed according to the technique recently described by
Calgaroto et al. (2015), Calgaroto et al. (2014). In this study, the effects
of NBs injection on sulfate-loaded flocs before the addition of the MBs
and NBs formed in the DAF were studied. The conditions were as follows: 20 mg·L− 1 PAC; pH 4.5; sodium oleate as a hydrophobizing
agent at concentrations of 0, 7.5, 15 or 30 mg·L−1; duration of 2 min.
First, 0.5 L of NBs at pH 4.5 was added to the flotation cell and conditioned for 5 min. Next, MBs and NBs were injected at a 30% recycle
ratio (0.3 L) through the previously described steel needle valve. The
size distribution of the NBs was measured using a ZetaSizer Nano ZS
(ZEN3600 — Malvern®).
After 5 min of flotation, 50 mL aliquots of treated water were
sampled from the bottom of the flotation cell, and the residual sulfate
concentration was measured. Conventional DAF tests were performed
using 0.5 L of DI water at pH 4.5 without NBs conditioning. To account
for the volume of liquid employed in the NBs (0.5 L) and the airsaturated water solutions (0.3 L), the residual sulfate concentration
values were multiplied by a 2.6 dilution factor. This calculation is a common procedure employed in DAF bench-scale studies.
4
J. Amaral Filho et al. / International Journal of Mineral Processing 149 (2016) 1–8
Table 2
Coal acid mine drainage from Santa Catarina State, South Brazil.
Main parameters, chemical elements and concentrations.
Parameter
Values
Al, mg·L−1
Ca, mg·L−1
−1
SO2−
4 , mg·L
Conductivity, mS·cm−1
Hardness, mg·L−1
Fe, mg·L−1
P, mg·L−1
Mg, mg·L−1
Mn mg·L−1
pH
K, mg·L−1
Zn, mg·L−1
Na, mg·L−1
30
363
2100
4.1
1058
108
b0.01
37
6
2.3
13
1.7
320
2.2.4. DAF of sulfate-loaded flocs formed from coal mine AMD
Validation studies were conducted using a sample of typical
Brazilian AMD (Table 2). The AMD was pretreated at pH 8.5 using
lime to remove metal ions through precipitation. Next, DAF was performed with or without NB assistance for sulfate removal. The sludge
resulting from this treatment was sampled for characterization in
terms of elemental Fe, Mn, Zn, Al, Cl and S (Eaton and Franson, 2005).
3. Results
3.1. Precipitation of sulfate ions with PAC
Fig. 2 shows the effect of the Al/SO2−
4 molar ratio on the precipitation
of sulfate from its sodium salt solution and its residual sulfate concentration. Fig. 3 presents the zeta potentials of the precipitates. The results
(Fig. 2) indicate that sulfate ion removal increased as a function of the
Al/SO24 − molar ratio according to previously reported chemical and
physical reactions (Silva and Rubio, 2011; Silva et al., 2010).
Although the highest sulfate removal efficiency was attained at a
5:1 M ratio, the best practical condition for an initial sulfate ion concentration of 1800 mg·L−1, typical for coal AMD in Brazil, was a 4:1 M ratio.
This condition guarantees a sulfate content below the established limit
(500 mg·L−1) while minimizing reagent consumption.
3.2. Flocculation-DAF of sulfate-bearing flocs generated from the sodium
sulfate solution
Fig. 3. Effect of Al/SO2−
4 molar ratio on the zeta potentials of co-precipitates formed by Al/
−1
SO2−
; Error
4 interaction in DI water. Conditions: pH = 4.5; sulfate in feed = 1800 mg·L
bars = ± Standard deviations.
flocculated precipitates do not float properly, leading to a fairly low
sulfate removal (1800 to 1100 mg·L−1). These precipitates were too hydrophilic to attach to the bubbles. However, once these precipitates
were flocculated with 10–20 mg·L−1 of polymer flocculant, the
resulting flocs were amenable to colliding with and adhering to the
rising bubbles, decreasing the feed sulfate concentration to final concentrations on the order of 350–400 mg·L− 1, which are below the
500 mg·L−1 WHO standard. The mechanisms involved include the formation of large spongy-fluffy flocs, which allow the entrainment of MBs
and NBs, leading to the formation of the so-called aerated flocs (Oliveira
and Rubio, 2012). The polymer chains also conferred sufficient hydrophobicity to enhance the probability of bubble-particle attachment
and flotation (Ren et al., 2007; Zaslavsky et al., 1984). The best concentrations were 10–20 mg·L−1 after 5 min of flotation using a 4:1 Al/SO2−
4
molar ratio. The formed flocs were large (a few cm) and withstood the
shear induced by the rising bubbles.
The precipitates produced via the flocculation mechanism with a
cationic polyacrylamide were also positively charged (Fig. 3: +16 mV
at a 4:1 Al/SO2−
molar ratio). Thus, other mechanisms, such as hydro4
gen bonding, might play a role in the adsorption of polymer molecules.
However, this is typical in flocculation using commercial reagents due
The effect of the polymer flocculant concentration on the flotation of
sulfate-loaded flocs is shown in Fig. 4. It was observed that non-
Fig. 2. Precipitation of sulfate ions with PAC at pH 4.5 and filtration of residual sulfate
concentration. Effect of the Al/SO2−
molar ratio on the residual sulfate concentration.
4
Conditions: sulfate in feed = 0.019 M (1800 mg·L−1). Precipitates were filtered using
acetate cellulosic membranes (0.45 μm pore diameter); Error bars = ± standard deviation.
Fig. 4. DAF of sulfate-bearing flocs: The effect of polymer flocculant concentration on
residual sulfate concentration. Conditions: pH = 4.5; sulfate in feed =1800 mg L−1;
Psat = 5 atm; [Al/SO2−
molar ratio] = 4:1; conditioning time = 10 min; flotation
4
time = 5 min; Error bars = ± Standard deviations.
J. Amaral Filho et al. / International Journal of Mineral Processing 149 (2016) 1–8
5
Fig. 5 shows the kinetics of DAF of flocs for 10 and 20 mg·L− 1 of
polymer flocculant. The use of higher concentrations generates better
results. Flotation (and thereby sulfate ion removal) with polymer flocculant at 20 mg·L−1 was very fast (30 s), reducing the sulfate concentration in the feed to approximately 380 mg·L−1, which is below the
WHO standard. Furthermore, the flotation rate followed a first-order
model with kinetics constants of 0.87 and 4.03 min− 1 for 10 and
20 mg·L− 1 of polymer flocculant, respectively. Table 3 summarizes
the experimental and calculated sulfate removal data and the corresponding correlation coefficients.
3.3. Flocculation and DAF of sulfate-bearing flocs generated from AMD
Fig. 5. DAF kinetics of sulfate-bearing flocs and residual sulfate. Conditions: pH 4.5; Psat =
5 atm; sulfate in feed = 1800 mg·L−1; [Al/SO2−
4 ] = 4:1; DAF recycle ratio = 30%;
conditioning time = 10 min.
to the presence of other short macromolecules or monomers, even with
different surface charges (Kitchener, 1978, Kitchener, 1972).
It is believed that a complete removal of the feed sulfate is practically
impossible considering the process inefficiency and the chemical equilibrium between the residual sulfate and the co-precipitates. Thus, 80–
82% sulfate removal has been considered the maximum achievable removal efficiency in practice (Silva and Rubio, 2011; Silva et al., 2010).
Fig. 6 depicts sulfate-bearing flocs after formation in AMD and after
flotation. These flocs appear as compact and dense aerated flocs with
a diameter of approximately 1–5 mm.
During additional DAF tests of the formed flocs, the main chemical
and operating parameters were kept the same as previously described,
and the study focused on the effects of the addition of sodium oleate
and conditioning with NBs.
Fig. 7 shows the bubble size distribution (in intensity) of NBs at
pH 4.5. A log-normal mono-modal curve was observed. The average
(triplicate) diameter of this distribution was 440 nm.
Fig. 8 shows the results of sulfate removal at pH 4.5 for DAF with and
without NBs conditioning at varying sodium oleate dosages (7.5 to
30 mg·L−1). Although the residual sulfate ion concentration is always
below the 500 mg·L− 1 WHO standard, the addition of the collector
Table 3
DAF kinetics of sulfate-bearing flocs. Experimental sulfate removal data and correlation coefficients relative to values calculated using a first-order flotation kinetics model. Conditions: R∞
[10 mg·L−1 polymer flocculant] = 82%; R∞ [20 mg·L−1 polymer flocculant] = 84.5%; k [10 mg·L−1 polymer flocculant] = 0.87 min−1; k [20 mg·L−1 polymer flocculant] = 4.03 min−1;
pH 4.5; Psat = 5 atm; sulfate in feed =1800 mg·L−1; [Al/SO2−
4 ] = 4:1; DAF recycle ratio = 30%; conditioning time = 10 min.
Time
0
30
60
180
300
600
Polymer flocculant — 10 mg·L−1
Polymer flocculant — 20 mg·L−1
R experimental, %
R calculated, %
Correlation coefficient
R experimental, %
R calculated, %
Correlation coefficient
0
2
60
74
81
80
0
30
48
76
81
82
1
0.07
1.26
0.97
1.00
0.98
0
73
83
81
82
82
0.0
73.2
83.0
84.5
84.5
84.5
1
1.00
1.00
0.96
0.97
0.97
Fig. 6. Flocs formed after precipitation of sulfate ions in AMD with PAC and flocculation: (a) Flocculated precipitates; (b) Flocs after 60 s of flotation and sludge formation.
6
J. Amaral Filho et al. / International Journal of Mineral Processing 149 (2016) 1–8
Table 4
Chemical characterization of the sludge produced by sulfateloaded DAF floc concentrates. Conditions: AMD = 1750 mg L-1
sulfate; Psat =5 atm; pH = 4.5; [Al/SO42-] = 4:1; [polymer flocculant] = 20 mg·L-1; total conditioning time = 10 min; flotation
time = 5 min.
Fig. 7. Average bubble size distribution measured by light scattering of NBs water
dispersions using a ZEN3600. Conditions: pH = 4.5, Psat = 5 atm, duration = 30 min;
Error bars = ± Standard deviations.
did not enhance the flotation of the flocs, and the sulfate removal
remained essentially constant at all collector levels. This observation
suggests a high degree of hydrophobicity, which might be conferred
by the low quantity of sodium oleate relative to that of the polyacrylamide molecules.
Nevertheless, conditioning with NBs slightly enhanced the removal
rate without the addition of sodium oleate, lowering the sulfate feed
concentration from 1753 mg·L−1 to approximately 350 mg·L−1.
This removal of approximately 78–80% of the feed content was less
than the 80–82% removal obtained with the synthetic sulfate solution.
However, this behavior is quite common in real-world systems and
may be attributed to the influence of the presence of other ions,
i.e., the matrix effect, on the overall treatment (Silva and Rubio, 2011;
Silva et al., 2010). Again, the results were not far from the previously
discussed maximum achievable removal. However, additional fundamental studies are underway in our laboratory to identify the optimal
NBs concentration (still unknown) and the superficial bubble area flux
(Sb) to improve the overall DAF performance.
Table 4 shows the results of the quantitative analysis of elemental Fe,
Mn, Zn, Al, Cl and S contents of the sludge generated by the DAF of
sulfate-loaded flocs. The sludge contained high concentrations of aluminum, sulfate and chloride. This sludge might be recovered or recycled
and used as a raw material, such as in the production of coagulants or
as an ancillary pozzolanic material in the cement industry (Babatunde
and Zhao, 2007; Xu et al., 2009).
Parameter
Value (mg·kg−1)
Aluminum
Chloride
Total iron
Manganese
Sulfate
Total zinc
1699
10,305
70
20
4983
9
4. Final considerations
In South Brazil, DAF is used to remove precipitates from the metal
ions present in coal AMD treated with lime. However, the sulfate ions
are not completely removed, and the concentrations generally correspond to those given by the Ksp of CaSO4, normally approximately
1800–2000 mg·L−1. Unfortunately, this water remains inadequate for
reuse in agricultural activities or as industrial water, and we need to
find a sustainable process for controlling soluble sulfate ions in coal
wastewater treatment and metal sulfide processing. The salinity
reduces the water available to plants by substantially reducing the
osmotic potential of the soil. Thus, the removal of these sulfate ions
from AMD is crucial if the treated water is to be reused.
The DAF process involving Al/sulfate-bearing flocs should be considered in the near future. Lamellar settling of the same flocs was studied in
our group (Silva and Rubio, 2011; Silva et al., 2010), but we encountered
kinetics and operational problems that were difficult to overcome,
namely, high water losses in the overflow (sludge) stream. Conversely,
DAF is a proven technology. In particular, the superficial loading capacity of modern column flotation cells (approximately 15 m·h−1) is at
least three times higher than that of lamellar settling (approximately
5 m·h−1), and the column flotation cells also provide a higher water recovery (Edzwald and Haarhoff, 2011; Kiuru and Vahala, 2001; Zaneti
et al., 2011, 2013). However, the floated solids always require a thickening stage (centrifugal separation or filtration) to enhance the water
recovery and permit their treatment, reuse or disposal.
Fig. 9 presents a possible layout for a plant to treat coal AMD, including pre-treatment (precipitation of metal ions at pH 8.5 with lime), precipitation of sulfate with PAC and lime at pH 4.5, flocculation using a
floc-generating reactor (FGR — Carissimi and Rubio, 2005) and column
flotation using a centrifugal multiphasic pump (for MBs and NBs generation) for the sulfate-bearing flocs.
5. Conclusions
Fig. 8. DAF of sulfate-bearing flocs in AMD and the effect of sodium oleate concentration
with or without NBs on residual sulfate content. Conditions: pH = 4.5; Psat = 5 atm;
initial sulfate ion concentration = 1753 mg·L−1; [Al/SO2−
4 ] = 4:1; conditioning time =
10 min; flotation time = 5 min; Error bars = ± Standard deviations.
The results show that the removal of sulfate ions by dissolved air flotation with micro- and nanobubbles strongly depends on their precipitation with polyaluminum chloride and on flocculation with a cationic
polyacrylamide. The best results were obtained with a polymer flocculant concentration of 20 mg·L−1, saturation pressure of 5 atm, pH of
4.5 and Al/SO24 − molar ratio of 4:1. The kinetics data were modeled
using a first-order reaction and showed that the flotation of flocs was
extremely rapid, with a rate constant of 4 min−1. The use of this approach to remove sulfate ions from a sample of coal-produced acid
mine drainage (sulfate in feed = 1753 mg·L−1) rapidly reduced the
anion concentration to below the 500 mg·L−1 level of the WHO standard. The main mechanisms include the attachment of bubbles to
flocs, the entrainment and entrapment of flocs and the formation of
aerated flocs that rise rapidly. The flotation of sulfate-bearing flocs
using micro- and nanobubbles has potential for the treatment of
voluminous sulfate-loaded industrial wastewaters at a high superficial
loading capacity.
J. Amaral Filho et al. / International Journal of Mineral Processing 149 (2016) 1–8
7
Fig. 9. Schematic diagram of the proposed sulfate-loaded wastewater treatment plant.
Acknowledgements
The authors would like to thank all the Brazilian institutes
supporting this research, namely, CNPq (485605/2012-0), FAPERGS
(10/0828-3 PQG), and UFRGS (scholarships). Special thanks go to all
the students at the laboratory (LTM) for their help with the experiments
and to Dr. William LaBarre for his kind revision of the English and helpful comments.
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