TREATMENT OF COAL ACID MINE DRAINAGE IN BRAZIL - RECENT TRENDS by itlpw9937

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									XXVIII Convención Minera Internacional, AIMMGM AC, Veracruz, Ver., 28 al 31 de octubre de 2009




TREATMENT OF COAL ACID MINE DRAINAGE                                                                                      IN
BRAZIL - RECENT TRENDS
RENATO SILVA, ALINE NOGUEIRA DA SILVEIRA y JORGE RUBIO1*
Departamento de Engenharia de Minas, Laboratório de Tecnologia Mineral e Ambiental (LTM), Universidade
Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500-Porto Alegre-RS -91501-970, Brazil.
(*jrubio@ufrgs.br)


                                                       ABSTRACT
        Conventional AMD treatment techniques are combinations of neutralization and precipitation (usually
        with lime), followed by settling of the precipitates in ponds. Herein, an AMD from an extinct coal mine
                                              3 -1
        was treated, in a pilot plant (1-1.3 m h ), by neutralization, with lime, flocculation of the precipitates and
        flocs/liquid separation by flotation with microbubbles or by lamellar settling (LS). A comparison to a
        performance in a heavy loaded AMD by LS was conducted. The treated water was characterized for its
        quality in terms of inorganic or organic elements, suspended and dissolved solids, among others, to
        determine its useability for recycling. After neutralization of the AMD aerated flocs formed (within
                                                                                                                     -1
        seconds) entered into contact with microbubbles under high shearing and raised-up at rates > 120 mh
                                                                               3  -2  -1
        allowing a rapid solid-liquid separation by flotation at about 13 m m h loading capacity. Conversely,
                                                              -1
        the flocs settled in the LS at a rate of 5-6 m.h . Both AMD treatment techniques showed similar
        efficiencies (removal of ions > 90 %) but the separation by lamella settling presented advantages,
        namely less reagents (no flotation collector required), lower power requirements and process simplicity.
                                                                                             -3                   -3
        Operating costs of the AMD treatment by LS at pH 9 reaches about 0.3 US$.m against 0.6 US$.m for
        the flotation process and the treated water was nearly free of heavy metals ions, namely, Fe, Al, Mn;
        presented low BOD (biological oxygen demand) and TOC (total organic content); low solids content and
        no fecal coli forms, making it useable for irrigation, and other purposes. It is concluded that this
        research will contribute in the discussion of this old and complex problem in acid mining effluents
        worldwide.




INTRODUCTION
Natural aqueous drainage (superficial or subterranean) of low pH value pass through surface and deep
mine coal mining activities resulting in contamination by metals ions producing Acid Mine Drainage
problem. The Acid Mine Drainage (AMD) eventually migrates into streams and rivers and impact
negatively the quality of water bodies. This creates a serious problem in Brazil and in other coal and
mineral producing countries (Rubio et al, 2007).

Conventional AMD treatment in South Brazil involves neutralization with lime followed by flocculation
and flocs settling either in ponds or in lamellar settlers. Recently, flocs could also be separated by
flotation using dissolved air flotation-DAF (Rubio et al, 2007) as in the USA. DAF is a well proven
technology for separation of suspended solids, fibers and dispersed oils from water. Bubbles in the
suspension are formed by a reducing pressure of the water pre-saturated with air at high pressures (3
to 6 atm) (Rodrigues and Rubio, 2007). The DAF unit in the USA, integrated into an AMD treatment
system, is operating since 1999, and provides an environmentally acceptable stream prior to
discharging into a local river. The flow rate of DAF is about 80 m3.h-1, and system occupied an area of
17 m2. In Brazil, the first flotation plant capable of processing about 10-15 m3/h of the AMD generated
in a coal embarkation site is in operation since 2005. The unit uses lime for neutralization and sodium
oleate as collector for the precipitated metal ions and also to assist in the generation of small bubbles
and a polyacrylamide to form the flocs to be floated (Rubio et al, 2007).

Lamellar settling (LS) of the flocs is a solid-liquid separation alternative employed for the treatment of
acid mine drainage and it is becoming popular. Bone, B. (2003), claim successful of removal of more
than 95% of the metal ions from the AMD. The introduction of inclined plates with high elevation within
conventional tanks (Culp, 1968), named lamella settler (Demir, 1995). It performs the same function as a
conventional clarifier however; it occupies only a fraction of the space. The effective settling area is equal to a




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                                    XXVIII Convención Minera Internacional, AIMMGM AC, Veracruz, Ver., 28 al 31 de octubre de 2009
                                                                 

 projected area of each plate multiplied by the number of plates. Hence, the area and costs involved can be
 reduced by matching the clarifying and thickening requirements.

 In Brazil the employment of LS is getting popular because of savings in energy consumption, less reagents (no
 collector needed) and process simplicity. The aim of this work is to summarize recent research in AMD treatment
 evaluating two different processes, namely NF-DAF (neutralization, flocculation and flotation) and NF-LS. Rapid
 flocculation of precipitated were performed in a special designed flocculator (RGF), an helical hydrodynamic
 equipment (Rubio and Carissimi, 2004, Carissimi, 2007). Main targets were iron, manganese, aluminum
 and sulfate ions.

 METHODOLOGY

 Materials and Reagents
 Figure 1 shows photographs of two distinct AMD; the first emerging from an abandoned coal mine (SS-
                                                          3
 16 site) and a coal plant basin (approximately 20,000 m - COMIN company) in South Brazil (Criciúma
 city). These AMD are composed mainly of iron, aluminum and manganese ions, pH 3±0.5 and the flow
                                                 3 -1        3 -1
 rate in the SS-16 site varies from 60 – 90 m .h (85 m .h , as the mean value). Other physical-
 chemical characteristics of these acid mine water are high values of sulfate ions, dissolved solids, color
 and conductivity (mainly at COMIN basin). The physical chemical characteristics from the two AMDs
 studied are shown in Table 1.

 Lime was employed for the AMD neutralization and metal ions precipitation and Flonex 9045, a
                                                        ®
 cationic polyacrylamide supplied by the SNF/Floerger , and Qemifloc (non-ionic polymer) were used to
 flocculate the colloidal precipitates. Aluminum salts (aluminum polychloride-PAC and potassium
 aluminate-Alupan K) were utilized to precipitate sulfate ions at ettringite form. All reagents used were of
 commercial grade and solutions were prepared with local tap water.




Figure 1. AMD (SS-16 site) from an abandoned coal mine (left); Highly loaded AMD (COMIN) from a coal
                                              tailings
                                       processing plant (right).
                                                                 
    Table 1. AMD physico-chemical characteristics of both DAM´s
               Parameters                      Diluted AMD (SS-16 site)                    Loaded AMD (COMIN)
                     pH                                      3 ± 0.5                                 2.5 ± 0.5
                   -2      -1
               SO4 , mg.L                                     800                                     12.000
                                 -1
          Dissolved solids, mg.L                             1.280                                    18.000
                           -1
               Iron, mg.L                                      3                                       1.600
                           -1
            Aluminum, mg.L                                     34                                      1.150
                            -1
            Manganese, mg.L                                    2                                        12
                Color, Hz                                     19                                      20.000
                               -1
           Conductivity, µS.cm                               1.153                                    8.700




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Equipments and Procedures
NF-Neutralization-flocculation. Neutralization was done in stirred tanks and flocculation of precipitates
was conducted in a flocculation system composed by a contactor tube, serpentine like, to disperse the
flocculant, followed by an specially designed flocculator named FGR® (see

Figure 2). Main characteristics of the flocculator are in-line mixing reactor, plug flow (less short circuits
and dead zones), low volume/retention times and low foot-print. The helical reactor with a capacity of
            3 -1           -1                                                                               ®
about 1.2 m .h (20 L.min ), was made of industrial flexible PVC pipe (model Spiraflex, Goodyear
brand). NF-LS (

Figure 4) studies were carried out over a period of about one year covering summer and winter
seasons. A composite sample was always collected over six hours time period for determining metal
                                                                 ®
ions concentrations using atomic absorption (SpectrAA 110, Varian ). Studies were done at pH 9 with
                      3 -1                            -1
a flowrate of 1-1.3 m .h , using 250 and 9.300 mg.L of lime (in both, SS-16 and COMIN sites,
                        -1
respectively) and 5 mg.L of Flonex 9045.

NF-DAF-Dissolved air flotation with micro-bubbles, smaller than 150 µm was studied for 6 months (SS-
16 site) to remove heavy metal and sulfate ions. Sulfate removal was not studied in lamella settling due
the light difficult-to-settle sludge and DAF studies were not carried out in COMIN plant due to the lower
costs (50 % reduction) found in the SS-16 work. NF-DAF process (see Error! Reference source not
found.) with 30 % recycle rate was studied to precipitate sulfate removal keeping an strict
                              +3    +2       -2
stoichiometry between Al , Ca and SO4 ions to form ettringite (3CaO.3CaSO4.Al2O3.31H2O) at pH
                                                                          -1
12 (Cadorin, 2007). Best sulfate removal was obtained using 1 mL.L of aluminum polychloride, 0.5
      -1                               -1                                                               -1
mL.L of Alupan K and 1.900 mg.L of commercial lime per cubic meter of wastewater. More, 5 mg.L
                                                                                            -1
of non-ionic polymer (Qemifloc 1020) was used for ettringite flocculation and 30 mg.L of oleic acid
(collector) was used to enhance the precipitates collection by bubbles.The reactor’s design features,
the residence times, together with other characteristics and operating data, are listed in Table 2.




                                            Figure 2. FGR. Flocs Generator Reactor




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                                      XXVIII Convención Minera Internacional, AIMMGM AC, Veracruz, Ver., 28 al 31 de octubre de 2009
                                                                   


    Table 2. NF-Lamellar Settling and NF-Dissolved Air Flotation. Main operating parameters and
    equipment design data.
                    Processes                             Parameters (operating)                             Values
                                                                   Tank volume                             2000 Liters
            Neutralization/Precipitation                       Mechanical agitator                         1500 Watts
                                                          Residence time (conditioning)                       30 min
                                                                 Number of rings                                19
                                                                Diameter of pipe                             0.025 m
               Flocs formation - FGR                                  Length                                   12 m
                                                                                                                    -3   3
                                                                      Volume                              5.9 x 10 m
                                                                 Residence time                              0.3 min
                                                                   Tank Volume                              330 Liters
                                                                 Residence time                               19 min
                                                                                                                 3   -2 -1
                                                            Superficial liquid velocity                   4.5 m .m .h
                  Lamellar settling                                                                                    2
                                                                       Area                                  0.22 m
                                                             Inclination of the tubes                           60°
                                                              Diameter of the tubes                          0.05 m
                                                                  Cell Volume                               160 Liters
                                                                Residence time                               9.6 min
                                                                                                                3  -2 -1
         Dissolved Air Flotation High Rate                   Superficial liquid velocity                   10 m .m .h
                                                                                                                  3 -1
                                                                  Air flow rate                             0.3 m .h
                                                                                                                     2
                                                                       Area                                  0.11 m


All samples (feed and treated AMD) were preserved following directions of the Standard Methods for
the Examination of Water and Wastewater (APHA, 2005). Process efficiencies were measured by
monitoring the residual metal ions concentration in duplicates. The water turbidity and apparent color
were monitored using a Hach Turbidity Meter Model 2100 N and Merck Model SQ 118, respectively.

RESULTS AND DISCUSSION
Figures 5, 6 and 7 show results obtained treating both AMD´s at pH 9. Figures 5 and 6 show that the
removal of Al and Fe ions increased for the loaded AMD (COMIN) but the residual concentration,
despite being low, is higher than the diluted AMD (SS-16). This phenomenon can be explained in
terms of the complete ions precipitation well above the solubility product constants yielding more and
clear (noticeable) colloidal precipitates, and not a haze observed in diluted systems. Furthermore, the
complete removal of manganese ions was attained at pH 9 (Figure 7) due to the complete precipitation
of manganese hydroxide (Mn (OH)2), in this condition.




                                             3 -1
Figure 4. LS unit for AMD treatment 1-1.5 m h : SS-16 COMIN sites in South Brazil: [1] LS (Lamella Settler – inclined tubes); [2]
Decanted sludge; [3] Treated water; [4] Pump system to settled sludge; [5] FGR (Flocs Generator Reactor); [6] SF (Serpentine
Contactor).



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                                                                          3 -1
Figure 5. Flotation (high rate) pilot plant unit for AMD treatment (1-1.3 m h ). Conditions [1] Lime; [2] Reagents (flocculants and
collector); [3] AMD neutralization – metals ions precipitation; [4] SF (Serpentine Contactor); [5] FGR (flocs generator reactor); [6]
MB (micro-bubbles) - HR (high rate)-flotation unit; [7] Multistage (three phase) pump for in line air-water saturation; [8] Air
compressor; [9] Needle valves for bubbles formation.




       Figure 5. AMD treatment (iron removal) by NF-DAF (SS-
                                                                          Figure 6. AMD treatment (aluminum removal) by NF-
       16 site) and NF-LS (SS-16 and COMIN) at pH 9.
                                                                          DAF (SS-16 site) and NF-LS (SS-16 and COMIN) at
       Comparisons between processes efficiencies, showing
                                                                          pH 9. Comparisons between processes efficiencies,
       removal % and residual concentration. For the feed iron
                                                                          showing removal % and residual concentration. For
       concentration, see Table 1.
                                                                          the feed aluminum concentration, see Table 1. 




Figure 7. AMD treatment (Manganese removal) by NF-DAF (SS-16 site) and NF-LS (SS-16 and COMIN) at pH 9. Comparisons
between processes efficiencies, showing removal % and residual concentration. For the feed manganese concentration, see
Table 1.




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                                 XXVIII Convención Minera Internacional, AIMMGM AC, Veracruz, Ver., 28 al 31 de octubre de 2009
                                                                 

The characteristics of sludge generated after AMD (SS-16) treatment by NF-DAF and NF-LS are
shown in       Table . This table also shows similar removal of sulfate ions with both processes, the
                                                                                                    -1
extent being limited by the solubility of the CaSO4 and hydrates species, at about 1800-2000 mg.L
                                                                              -1
sulfate ions. In the case of the AMD-COMIN (initial sulfate = 12.000 mg.L ) the sulfate removal
reached to 80% removal but, again, the residual concentration was limited by the solubility of the
CaSO4. Figure 8 shows that because of the heavy metal ions concentrations, the sludge volume
generated after flocculation of the precipitates in the loaded COMIN AMD is much higher and browner
(iron hydroxide) than that found in the SS-16 site.

      Table 3. Solids concentration in sludge, generated after AMD SS-16 treatment, at pH 9.
                                                                                                                       -1
               Techniques                          Sulfate removal, %                     Solids concentration, g.L
                   NF-DAF                                       20                                      20,0
                    NF-LS                                       20                                      14.6


Error! Not a valid bookmark self-reference.Table 4 shows approximate operating costs for the
performance of NF- LS and NF-DAF treating the SS-16 and COMIN AMD´s at pH 9. Values obtained
show that NF-LS in COMIN AMD is more costly than SS-16 AMD mainly because of the neutralization
                                   -1                        -1
stage, the acidity being 6,000 mg.L CaCO3 against 210 mg.L found for the diluted SS-16 AMD. Also,
this table proves that NF-DAF is less expensive than NF-LS in treating SS-16 AMD, mainly because of
energy and reagents usage.

Table 4. Comparative operating costs for NF-DAF and NF-LS, treating two AMD having different
acidities and heavy metals concentrations, at pH 9.
                                                                    -3                           -3                         -3
  Process (place)         Energy consumption, U$.m                         Reagents, U$.m              Total Costs, U$.m
      LS (SS-16)                             0.2                                   0.1                          0.3
     DAF (SS-16)                             0.4                                   0.15                         0.55
     LS (COMIN)                              0.2                                   1.6                          1.8


Removal of Sulfate by Dissolved Air Flotation of the Ettringite, at pH 12. This technique employs
polyaluminum chloride (PAC) and lime at pH >12, whereby the sulfate ions are rapidly precipitated as
                                                    -2
ettringite (3CaO.3CaSO4.Al2O3.31H2O) at PAC:SO4 =2:1 mass ratio. The solids produced are
amenable for separation by DAF, showing advantages over flocs settling regarding process kinetics
                                                                 3  -2 -1
and treated water quality. Best results were obtained at 13 m .m .h loading capacity yielding a
                                                                                -2
residual turbidity of 0.5 NTU. Process efficiency was about 82%, with a SO4 final concentration
                  -1
around 150 mgL , meeting drinking water standards of the Brazilian Health Ministry’s Order 518/2004.

It is believed that this alternative has some potential in the treatment of sulfate bearing effluents also
                                               -3
considering that the treatment cost (US$m 3.20), appear to be low when compared to other
alternatives (see Table 5).

   Table 5. Sulfate removal efficiencies, from AMD´s, by different treatment techniques
                                                           -2                 -1            -2             -1
                   Process                           [SO4 ]initial, mg.L            [SO4 ]treated, mg.L          Efficiency, %
    Precipitation with BaS (Cadorin, 2007)                          2190                         190                   91
           SAVMIN (Cadorin, 2007)                                   649                           69                   89
            CESR (Cadorin, 2007)                                    1690                         190                   89
       This work (ettringite formation)                             815                          148                   82


Quality of Treated Water and Reuse
The biological and physical-chemical quality of the treated water was determined and compared with
some international guides and laws. A large research of international guides for water reuse was
made, comparing the physical-chemical quality of treated water for NF-DAF and LS processes at pH 9.
Results are shown in Table 6I.


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       Table 6I. Comparative quality parameters between values obtained for the treated AMD either by
       NF-DAF and NF-LS and guides/laws around the world.
                              Average
         Parameters            values               1           2            3           4           5            6          7
                            (NF-DAF/LS)
      pH                           9              6-9.5         5-9         6-8         6-9          -          6.5-8.5    6-8.1
                 -1                  (a)
      BOD, mg.L                   ND                -            -           -          ≤10          -            10        10
      Fecal Coliforms/100
                                   ND                -           -         <200         ND          100           10        2.2
      mL
                  -1                                                                   500-        500-
      TDS, mg.L                   1052             1000          -         <200                                    -         -
                                                                                       2000        3500
                  -1
      TSS, mg.L                    ND                -           -           -          ≤5           -            10         10
      Color. Hazen                 9                15           -           -           -           -             -          -
                    -1
      Cadmium, mg.L              <0.002              -          0.2          -         0.05       0.0051         0.01       0.01
                 -1
      Lead, mg.L                  0.02               -          0.5          -          10          0.2          0.1        0.1
                       -1                                                                                 (b)
  Chromium, mg.L                   <0.004            -          0.5         -           1        0.0049         0.1         0.01
(a)                 (b)                III VI               (1)                                                                   (2)
   Non-detectable;      Values to Cr . Cr = 0,008 mg/L ;        Drinking water - Brazil Health Ministry, law number 518/2004;
                                                                       (3)
Effluents emission limits – Brazil, law number 357/2005 (CONAMA); Class 1 to reuse (car washing and uses with direct contact
                                                                      (4)
between users and water, including fountain) NBR 13.969 - Brazil; Environmental Protection Agency (US) – Reuse to clothes
                         (5)                                                  (6)
and vehicles washing;        Agriculture uses in Canada (Font: CCME, 2005);       Unrestricted irrigation – Israel (Font: INBAR, Y.,
       (7)
2007); Unrestricted irrigation – Saudi Arabia (Font: ABU-RIZAIZA, O.S.,1999).



CONCLUSIONS
Results obtained in this work showed that both AMD treatment techniques studied showed similar
                                                                                           -3
efficiencies (removal of ions > 90%) at pH 9. LS presented lower costs (0.3 US$.m ) than flotation
                       -3
process (0.6 US$.m ), mainly because less reagents (no flotation collector required), lower power
                                                                                                           -
requirements and process simplicity. Yet, flotation is more efficient in terms of loading capacity (13 m.h
1       3   2                                                          -1
  or m /m /h), while lamellar settling reaches levels of only 5-6 m.h . The treated water presented low
heavy metals ions, BOD (biological oxygen demand) and other organic parameters making it useable
for irrigation, and other purposes

ACKNOWLEDGEMENTS
The authors wish to thank students and colleagues at the LTM-Universidade Federal do Rio Grande do
Sul and to all institutions supporting research in Brazil, mainly to CAPES and CNPq.

REFERENCES
Abu-Rizaiza, O.S. (1999); Modification of the standards of wastewater reuse in Saudi Arabia. Water
  Research: Grã Bretanha, v. 33, n 11, p. 2601-2608.
APHA, 2005. Standard Methods for the Examination of Water and Wastewater. 21th ed. American
  Public Health Association, Washington.
Bone, B. 2003. Remediation schemes to mitigate the impacts of abandoned mines. Environmental
  Agency for England and Wales. Cornwall, England.
Cadorin, L.M, (2007). Desenvolvimento de técnicas para o tratamento de efluentes ácidos de minas
  por precipitação química e flotação por ar dissolvido. Dissertação de mestrado. PPGEM-UFRGS,
  83 p.
Canadian Council of Minister of the Environment. Canadian Environmental Quality Guidelines for the
  protection agricultural water uses-Summary Table, 7 p., 1999, updated 2005.
Carissimi, E. 2007. "Desenvolvimento do Reator Gerador de Flocos (RGF): Aspectos Teóricos e
  Aplicações no Tratamento e Reúso de Águas e Efluentes". PhD Thesis. DEMIN, UFRGS, Porto
  Alegre-Brazil. (in portuguese)




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Resolução n° 357 (2005). Classificação dos corpos de água e padrões de lançamentos de efluentes.
   Ministério do Meio Ambiente; Conselho Nacional do Meio Ambiente - CONAMA, 23 p., (as from
   17th July).
Culp, G. L., Hansen, S. D., Richardson, G., 1968. High-Rate Sedimentation in Water Treatment Works.
   JAWWA, v. 60, n. 6, p. 681-689.
Demir, A., 1995. Determination of settling efficiency and optimum plate angle for plated settling tanks.
   In.: Water Research. v. 29, n. 2, p. 611-616.
EPA (2004). Guidelines for water reuse. US. Environmental Protection Agency, Municipal Support
   Division, Washington, DC, EPA/625/R-04/108, 449 p.
Inbar, Y (2007). New standards for treated wastewater reuse in Israel. In: Zaidi, M.K.(ed). Wastewater
   Reuse –Risk Assessment, Decision-Making and Environmental Security, Session 8, pp. 291–296.
Rodrigues, R.T., Rubio, J., 2007. DAF - Dissolved Air Flotation: Potential Applications in the mining
   and mineral processing industry. International Journal of Mineral Processing, v. 82, p. 1-13.
Rubio, J.; Carissimi, E (2004). Reator Gerador de Flocos - RGF. Patent- 16-09-2004 via EITT-UFRGS:
   PI 0406106-3, (in portuguese).
Rubio, J., Carissimi, E., Rosa, J.J., 2007. Flotation in water and wastewater treatment and reuse:
   Recent trends in Brazil. International Journal of Environment and Pollution, v. 30, p. 193-208.
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          
                                                          



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