CLEAN TECHNOLOGIES FOR THE TREATMENT OF COAL ACID MINING EFFLUENTS

Jorge Rubio and Renato Silva Universidade Federal do Rio Grande do Sul, Professor, Mining Engineering Department, CEP 91501-970, Porto Alegre-RS, Brazil, jrubio@ufrgs.br - Telephone: +55 (51) 3308-9479 ABSTRACT Coal mining activities are the major responsible for acid mining effluents generation. Main characteristics of this wastewater are high concentration of heavy metal ions, sulphate, high acidity (corrosive) and low pH. Many techniques have been applied to acid mine drainage (AMD) treatment, mainly adjusting medium pH, removing some, but not all, the heavy metal ions (as colloidal hydroxides precipitates). Manganese ions removal poses the problem of precipitation as hydroxide at pH 9 where the neutralization becomes costly, eespecially if the flow-rates are high. More, sulphate ions are difficult to precipitate and constitute a great challenge to coal producers worldwide; mainly because of the high solubility of its calcium salt (gypsum solubility of about 1800 mg.L-1) and its toxicity. Most of the known alternatives are expensive, inefficient and complex. The present work summarizes research efforts and provides a short overview of the actual situation in the treatment of coal acid mining drainages with and without water reuse, in South Brazil. A complete comparison between on-line flocculation (small foot print) and dissolved air flotation or lamella settling systems was carried out in recent years and the results showed greater advantages for lamella settling techniques as the solid-liquid separation, namely: low costs and energy consumption, easy operation, and same quality of the clarified water. Thus, this flocculation-lamellar settling is being applied massively in the new treatment plant. Studies in a AMD treatment plant the Mn ions were precipitated as hydroxide at pH 9 and sulphate anions insolubilized as ettringite (3CaO.3CaSO4.Al2O3.31H2O) formed with calcium and aluminum chemicals at pH > 11.5. Removal > 90% was obtained lowering the sulphate feed of 1200 mg.L-1 to less than 200 mg.L-1 (the limit for drinking is 250 mg.L-1). Due to the light weight (spongy) precipitates the removal with microbubbles by dissolved air flotation or by settling is not sustainable yet and cannot be directly transferred to the industry. Therefore, a new basis is currently being investigated whereby aluminum polymeric species and sulphate ions co-precipitate at pH 4.5. Because of the relevance of this new alternative, the work provides the theoretical mechanisms (species formed, aqueous chemistry, flocculation, flotation and/or lamellar settling) involved, followed by recent applied work to validate bench results (sulphate removal > 70% were obtained). Most of the equipment layout, techniques and treatment rigs are based on applied research, updated solution chemistry and optimization of operating parameters. It is believed that results appear to have a good potential for a sustainable clean technology for the treatment of sulphate bearing coal acid mining effluents. Keywords: Coal acid mining effluents, Dissolved air flotation, Lamella settling, Sulphate ions removal

INTRODUCTION

Acid drainage is generated by solubilisation of metal sulphides due to oxidation of tailings from coal and

some metal-bearing ores by water and oxygen. Main elements in acid mine drainage (AMD) are heavy

metal ions, sulphate and low pH and constitute one of the main challenges in the mining industry.

Sulphate ions, at concentrations > 250 mg.L-1, cause environmental problems, (eespecially as sodium salt)

in the mining, metallurgical, chemical and agricultural sectors (Silva et al, 2010).

AMDs from coal mines in South Brazil are eespecially rich in sulphate ions reaching levels between

1.000 up to 14.000 mg.L-1. AMD are disposed into superficial and subterranean waters, small streams and

rivers, posing severe problems in coal and ore-producing countries, eespecially the removal of sulphate

ions (Kontopoulos, 1998; Rubio et al., 2007, Silva et al, 2010).

Sulphate ions alter the taste of water, cause digestion troubles in animals and humans, soil acidification

(sodium salts) and corrosion of metals (Morris and Levy, 1983, Bowell, 2000). Maximum tolerable level

of sulphate in water of 500 mg.L-1 (WHO, 2004) is recommended in some countries but lower values of

250 mg.L-1 are required in standard emission laws in Brazil and in the USA (Ministry of Health in Brazil,

2004; USEPA, 1991).

According to Kontopoulos, 1998, the oxidation of pyrite proceeds as follows:

Equation 1. 2FeS2(s) + 7O2(aq) + 2H2O → 2Fe+2 + 4SO4-2 + 4H+

Equation 2. 4Fe+2 + O2(aq) + 4H+ → 4Fe+3 + 2H2O

Equation 3. FeS2(s) + 14Fe+3 + 8H2O → 15Fe+2 + 2SO4-2 + 16H+

According to Equation 2 Fe+2 ions could be oxidized to Fe+3 if the oxidation potential is maintained and

the Fe+3 formed by Equation 3, will generate more sulphate ions. Fe+2 will be oxidised to Fe+3, according

to Equation 2 and will be oxidise pyrite once more (autocatalysis) (Kontopoulos, 1998).

Metal ions precipitation as hydroxide is usually carried out at specific level of pH, depending on the metal

ion. Then, flocculation with polymers of the formed colloidal precipitates takes place and separation of

flocs is performed by different techniques or directly in ponds.

Technologies used for removal of sulphate ions are either inefficient or highly expensive due to the high

solubility and stability of these anions in aqueous solutions. Precipitation with lime is employed

extensively for partial sulphate ions removal with a low practical efficiency due to the high solubility of

the produced CaSO4 (gypsum), about 2000 mg.L-1 SO4-2 (Rubio et al., 2009, Silva et al, 2010).

Processes for treating sulphate ion-bearing waters and wastewater also include biological degradation

(Maree and Strydom, 1985; Maree and Strydom, 1987); membrane filtration (mainly reverse osmosis)

(INAP, 2003); adsorption and/or ion exchange (Lopez, 2009); and chemical precipitation. Recently, the

ettringite formation has been discussed for many authors (INAP, 2003, Philippe et al., 2009) but the high

costs involved with pH adjustment to sulphate ion immobilization has been reported (Silva and Rubio,

2010).

A new basis for sulphate ions immobilization using aluminium-bearing salts at pH 4.5 is being

extensively investigated by our research group. This technique is related with aluminium species

distribution, eespecially due to polymeric species.

A review of the aluminium distribution in solution, namely the presence of monomeric, polymeric and

colloids species (Ala, Alb and Alc, respectively) was fully discussed in the first paper of this series (Silva

et al., 2010).

This work aimed to make a short review of techniques used worldwide for AMD treatment, especially

physico-chemical techniques used for sulphate ions immobilization, a great challenge. The focus of this

work was to discuss upcoming research showing recent results with using aluminium-bearing salts for

sulphate immobilization at pH 12 (ettringite generation-3CaO.3CaSO4.Al2O3.31H2O) and at pH 4.5. A

series of pilot trials were conducted in some plants installed in different places of southern Brazil during

the last 4 years. Process efficiency was measured by monitoring the final concentration of the sulphate

anions in the treated AMD.

A SHORT OVERVIEW

Metal hydroxides precipitation and flocculation

A comparative study between lamella settling and dissolved air flotation as techniques used for flocs of

metal hydroxides separation from water has been reported by Silveira et al., 2009. Special attention was

addressed to manganese ion removal due to high solubility of its hydroxides at neutral pH medium.

According to our knowledge, solubility of this metal decreased strongly only at pH 9. The costs involved

on the precipitation, flocculation and solid liquid separation are about US$ 0.25 or 0.4 US$ to treat one

cubic meter of AMD, using settling or flotation, respectively.

Calcium and barium sulphate precipitation

A great number of elements may interact with SO4-2 in aqueous solution to form insoluble salts over

different solubility. Table 1 shows the solubility in water of the most insoluble sulphate salts at 25 °C

(Handbook of Chemistry and Physics, 2008).

Table 1. Solubility of various sulphate bearing salts at 25 °C (Handbook of Chemistry and Physics, 2008)

Salt Kps (25 °C) Solubility, SO4-2 mg.L-1

BaSO4 1.08*10-10 2.45 PbSO4 1.6*10-8 42.5

SrSO4 3.8*10-7 135

CaSO4 2.4*10-5 2.1 x 103

Due to the high toxicity for humans of lead and strontium elements, precipitation of these salts is highly

needed. Solubility of gipsite (CaSO4.2H2O) is limited to 2000 mg.L-1 of sulphate ions, far from

appropriate limits for human consumption. Conversely, barium sulphate is much more insoluble and

barium hydroxides and/or sulphides have shown good capacity for sulphate precipitation in a wide pH

range (INAP, 2003), while barium carbonates shows limited efficiency at neutral, strongly acidic and

strongly basic medium. The problems related to barium sulphate precipitation are the high costs and

residual traces of barium in water. In Brazil, ministry of health limits the barium concentration in drinking

water at 0.7 mgL-1.

Membrane filtration

Recent developments in the membrane industry have reduced overall costs involved but still are not a

effective alternative for sulphate removal. Membranes are sensitive to scaling and to the presence of

solids in the feed solution. However pretreatment has to be considered. Membrane manufacturers have

developed a variety of membranes that permit the selective separation of specific ions from solution. An

example of this development is the use of reverse osmosis (RO) and nano-filtration (NF) applied to the

removal of sulphate ions from solution. Whereas RO has been used extensively for this application,

rejecting close to a 100% of all sulphate ions, NF offers different outcomes. Since the membrane pores of

the nano-filters are wider, they will permit smaller mono-valent ions, such as chloride, to pass through.

NF will typically reject 75% of the sulphate ions in solution. There are several equipment and technology

providers that are specific for complex sulphate removal applications using NF.

Compared to membrane technologies, the major disadvantages of IX is that it is a cyclical process,

meaning that for a continuous flow the design has to take into account the sequential operation of loading

and regeneration of the resin. Notwithstanding this operational disadvantage, IX systems are generally

very adaptable to solutions with high scaling potential and high solids content.

Adsorption of sulphate ions onto resins

Ion Exchange (IX) technology has been globally accepted as a feasible and reliable way to remove

scaling ions from water. The technology is widely applied in municipal water treatment systems and

potable water production.

The use of the IX process in sulphate removal from mine effluents has been studied e.g. in South Africa

using the Gyp-CIX process. It comprises a two stage treatment regime, where the first IX column

removes cations such as Ca+2 from solution, and the second column removes anions such as SO4-2 from

the solution. The sulphate is removed from the solution by a precipitation reaction which occurs when the

regenerants from both columns are contacted. Gyp-CIX has been taken over by BioteQ Environmental

Technologies and has been modified to what is now called Sulf-IX (Lopez, 2009). Although the sulphate

removal method is similar to the original Gyp-CIX process, the difference is manifested in that solid

gypsum is the only by-product. This overcomes the main disadvantage identified by Lorax in the Gyp-

CIX process, i.e., the generation of high quantities of gypsum sludge in the regeneration step.

CASE STUDIES

Methodology

Sulphate ions immobilization for aluminum bearing salts at pH 4.5

Sodium sulphate solutions (Na2SO4) with 700, 1000 and 1800 mg.L-1 of SO4-2 were used on the

precipitation of the aluminum colloids at the bench experiments. Solutions of NaOH 1 M and HNO3 1 M

were used on the pH adjustment.

Polyaluminum Chloride (PAC TE 1018 - Panamericana) and aluminum chloride (AlCl3.6H2O - Vetec)

were used as coagulants. Polyaluminum chloride has about 160.000 mg.L-1 of aluminum ions in solution,

while aluminum chloride solutions were prepared with 60.000 mg.L-1 of aluminum.

After aluminum salts conditioning at several pH, the solution was stirred for 10 minutes and vacuum

filtration was carried out for solid-liquid separation. Samples were collected for sulphate ion

determination. Additionally, surface charges values, in deionised water, of the aluminum PAC-species

colloids in the absence or presence of sulphate ions (1800 mg SO4-2.L-1) were determined as a function of

medium pH using a micro-electrophoresis equipment (model Zeta PlusTM - Brookhaven Instruments).

Pilot trials. Acid mine drainage from a coal tailing basin was used as a typical effluent containing high

levels of sulphate ions.

Figure 1 shows a photograph of the AMD surrounding a coal tailing deposit (Capão da Roça site) in

South Brazil. Main physical-chemical characteristics of this acid mine water are shown in Table 2.

Figure 1. Acid mine drainage (pH = 3.1) generated in a coal tailing deposit in Capão da Roça site- South

Brazil.

Table 2. Acid Mine Drainage, main chemical and physicochemical parameters.

Parameter Acid Water pH

SO4-2, mgL-1

Mn, mgL-1 Fe, mgL-1 Al, mgL-1

Total solids, mgL-1 Dissolved solids, mgL-1

Turbidity, NTU Color, Hz

Conductivity, µScm-1

3.0 1400±90

3 57 21

1600 1580 0.7 600 1753

The layout of a small pilot rig, treating about 1-1.2 m3h-1, is detailed in the Figure 2. Acid mine drainage

(AMD) was conditioned with varying PAC concentrations at pH 4.5 in a tank (2.000 L) and flocculation

of precipitates was conducted with 15 mg.L-1 Flonex 4115, in a special “serpentine like flocculant

conditioner” (SF in Figure 2) followed by a flocs generator reactor named FGR® (Rubio and Carissimi,

2005). This FGR® was developed at our laboratory and main characteristics and advantages reported of

this in-line mixing reactor, compared to stirring tanks are: no need of moving parts; plug flow (less short

circuits and dead zones); low volume/retention times and low foot-print (Rubio and Carissimi, 2005,

Carissimi, 2007).

This helical reactor has a capacity of 1.2 m3.h-1 (20 L.min-1) and was made of industrial flexible PVC pipe

(model Spiraflex, Goodyear® brand). The reactor’s constructive characteristics and residence time,

together with other ancillary equipments used are shown in Table 3.

Table 3. PF-SL pilot plant studies: Main operating and design parameters.

Processes Parameters Values

Precipitation at pH 4.5 Tank volume Mechanical agitator Residence time (conditioning)

2000 L 1500 Watts 20 min

Flocs formation – FGR

Number of rings Diameter of pipe Length Volume Residence time

19 0.025 m 12 m 5.9 x 10-3 m3 0.3 min

Lamellar settling

Tank Volume Residence time Superficial liquid velocity

Area Inclination of the tubes Diameter of the tubes Height

330 L 19 min 4.5 m3.m-2.h-1 0.22 m2 60° 0.05 m 2.3 m

Figure 2. Precipitation-flocculation-lamella settler-PF-SL rig (1-1.2 m3h-1).

PF-SL studies were carried over a period of 6 months, covering summer and winter. The flow exiting

from the equipment (LS) fed tanks whereby samples were collected (composites of at least 2 hours) for

determining sulphate and metal ions concentrations. All samples were preserved following directions

from Standard Methods for the Examination of Water and Wastewater (APHA, 2005). Process

efficiencies were measured by monitoring the residual sulphate ions concentration in duplicates and

results were expressed by means of all values obtained.

Ion Chromatograph was employed for chloride and sulphate anions determination (Advanced Compact IC

- Metrohm®). Metal ions were determined by atomic absorption (SpectrAA 110, Varian®) and results are

expressed as mg L-1.

Ettringite generation at alkaline medium – precipitation, flocculation and flotation pilot unit

This work was conducted in a small pilot unit treating about 1-1.5 m3h-1 of AMD (Figure 3). Figure 4

shows a photograph of the AMD emerging from an abandoned coal mine (SS-16 site) in South Brazil.

This AMD is composed mainly of Fe, Al, Mn and SO4-2 ions and low pH 3±0.5. The physical-chemical

characteristics of this acid mine water are shown in Table 4.

Polyaluminum chloride was conditioned in a tank to reach a mass ratio between aluminum and sulphate

about 1.5 to 4.0:1. Lime was employed for the AMD pH adjustment up to pH 12. Flonex 9045 (a high

molecular weight cationic polymer supplied by SNF/Floerger®) was utilized to generate flocs. Sodium

oleate was employed to enhance the flocs hydrophobicity and the flotation kinetic. All reagents used are

commercial products and solutions were prepared with local tap water.

High Rate Flotation (HRF) studies were carried out at room temperature over a period of 8 months,

covering summer and winter. The flow exiting from the equipments (HRF) fed tanks whereby samples

were collected (composites of at least 2 hours) for determining metal and sulphate ions concentrations

using atomic absorption (SpectrAA 110, Varian®) and chromatograph ion (Advanced Compact IC -

Metrohm®). All samples were preserved following directions from Standard Methods for the Examination

of Water and Wastewater (APHA, 2005).

Figure 3. Unit pilot scale to treatment of acid water – Criciúma/SC (~ 1 m3h-1). Conditions [1] lime; [2]

reagents (flocculants and collector); [3] AMD neutralization – metals ions precipitation; [4]

FGR (flocs generator reactor); [5] Needle valves for bubbles formation; [6] multistage (three

phase) pump for in line, air-water saturation, air compressor pump; [7] MB (micro-bubbles) -

HR (high rate)-flotation unit;

Figure 4. Fresh AMD from an abandoned coal mine in South Brazil (SS-16 site)

A centrifugal pump multiphase (water/air), Edur®, was employed to disperse, dissolve and saturate air in

water. The pump receives the air at the inlet (suction), them shears it within the impeller and rapidly

reaches an operating pressure of about 8 kgf.cm-2 (784 kPa). The air saturated water flows downstream

through a connecting tube passing through a 1/4" needle valve where depressurization takes place to form

microbubbles by cavitation and nucleation of the dissolved air.

Table 4. Acid Mine Drainage (SS-16 site), main parameters

Parameter Acid Water pH

SO4-2, mgL-1

Mn, mgL-1 Fe, mgL-1 Al, mgL-1

Total solids, mgL-1 Dissolved solids, mgL-1

Turbidity, NTU Color, Hz

Conductivity, µScm-1

3.0 800±20

2.1 3.0

33.7 1279 1279 0.32 19

1153

Results and Discussion

Ettringite generation at alkaline medium using calcium and aluminum salts-SS-16 site case

Table 5 shows the effect of pH in the ettringite precipitation after AMD treatment in a flotation pilot unit

(PF-DAF or precipitation-flotation by DAF-dissolved air flotation). 70 % of sulphate ions could be

removed from AMD by DAF system at pH 12 and the final sulphate ion concentration was adequate to

drinking limits (250 mg.L-1). However, only 50% of sulphate was removed at pH 11.5. These results

show the strong pH dependence to ettringite formation.

Table 5. Effect of pH on the removal of sulphate ions from AMD-SS-16 site. [SO4-2] initial = 847 mg.L-1.

Process PF-DAF operating at 1 m3.h-1. PAC:SO4-2 mass ratio = 2.5:1 mg.mg-1.

Parameter PAC:SO4-2, mg.mg-1

2.5:1 2.5:1 2.5:1 pH 11 11.5 12 Sulfato, mg.L-1 678 412 236

The effect of PAC concentration on the ettringite precipitation is shown in Table 6. 2500 mg.L-1 of PAC

(PAC:SO4-2 = 2.9:1) was employed for sulphate ions to match drinking water limits, however the residual

chloride concentration increased at 483 mg.L-1. Other reagents should be investigated in future works to

reduce chloride ions concentration in treated water.

Table 6. Effect of PAC:SO4-2 mass ratio on the removal of sulphate ions from AMD-SS-16 site. [SO4

-2]

initial = 847 mg.L-1. PF-DAF process operating at 1 m3.h-1.

Parameter PAC:SO4-2, mg.mg-1

1.5:1 2.9:1 4.0:1 pH 12 11,8 12 Cloreto, mg.L-1 247 483 712 Sulfato, mg.L-1 425 218 47

Sulphate ions immobilization at pH 4.5 using aluminum bearing salts-Capão da Roça case

An extensive characterization including EDS-SEM, zeta potential and size distribution of sulphate-

bearing colloids has been done and published recently by Silva et al., 2010.

Figure 5 shows the effect of pH on sulphate ions uptake by aluminum colloids using PAC, followed by

filtration. Best results were obtained at pH 4.5, which matches with the maximum positively charge of the

Al-poly-species colloids studied by Silva et al., 2010. Similarly, Xu et al., 2004 have found that between

pH 4.5 to 5.0 there is a predominance of the Alb species, which subsequently strongly react with sulphate

ions, as proven in the first part of this series (Figure 6 taken, with permission of Silva et al, 2010).

2 4 6 8 10 120

400

800

1200

1600

2000

2400

Con

cent

raçã

o fin

al d

os ío

ns su

lfato

, mg.

L-1

pH

PAC TE 1018 - 1800 mg.L-1 * PAC TE 1018 - 1000 mg.L-1 * PAC TE 1018 - 700 mg.L-1 *

Figure 5. Effect of pH on co-precipitation of sulphate ions-PAC and separation of colloids with (0.45 μm)

membrane (with permission of Silva et al, 2010). Na2SO4. Al:SO4-2 ratio = 0,5:1. [SO4

-2]inicial =

1800, 1000 and 700 mg.L-1.

Pilot studies. Table 7 shows results of the removal of ions by precipitation, flocculation and lamella

settling process-PF-LS as a function of aluminum:sulphate ratio. The final concentration of sulphate ions

decreased while the aluminum-sulphate ratio increased and the aluminum concentration decreased when

PAC dosage increased. Results may be explained by the interaction between sulphate ions and the Al

species and by thermodynamics phenomena (Kps or solubility products).

Table 7. Precipitation, flocculation and lamella settling process-PF-LS. Physico-chemical values of the

treated samples. Effect of aluminum:sulphate ratio. [SO4-2]initial = 1364 mg.L-1 on process

efficiency.

Parameters Final concentrations as a function of aluminum:sulphate ratio (mg.mg-1)

0.3:1 0.7:1 1.2:1 pH 4.5 4.5 4.5

Fe, mg.L-1 0.7 1.3 0.3 Mn, mg.L-1 2.9 2.9 2.1 Al, mg.L-1 133 34 5.8 Cl, mg.L-1 328 857 1288

SO4-2, mg.L-1 1266 811 612

Aluminum and manganese residual concentration could be reduced on the overflow by a secondary stage

of treatment at pH 9 due to solubility of the hydroxides of these cations. It is believed that due to the good

quality of the water reached in this work the PF-SL process appears to be a good alternative for many

miscellaneous applications to remove sulphate ions, a main task eespecially for the mining industry

worldwide. Another issue found in the process was the sludge generation, produced at pH 4.5 and

containing a high concentration of aluminum. One technical option might be the recovery (maybe

leaching) and be used as raw material for other applications (coagulants, ceramic materials, among

others) or recycled to the process decreasing the global costs of the treatment. This technical way-out is

been studied separately in our University.

CONCLUSIONS AMD sustained treatment is available nowadays by upcoming alternatives including Mn and Sulphate

removal, two of the most difficult-to-remove ions. In all cases, operating costs are fairly high, and efforts

are being made to reduce them by optimizing the solution chemistry and by searching for new and more

effective reagents. This work illustrated efforts made at bench and pilot scale to treat coal acid wastewater

in South Brazil. Special emphasis was addressed to the removal of sulphate by co-precipitation with Al-

bearing salts at pH 4.5 where removal % were always near 80-85 %.

ACKNOWLEDGEMENTS

The authors wish to thanks all the students and colleagues from LTM-Laboratório de Tecnologia Mineral e Ambiental (32 years in 2011!!) and to all institutions supporting research in Brazil, in particular to CNPq. Acknowledgements are extended to Copelmi Co. and SIECESC for contributions on supporting the pilot plant experiments.

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