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Sustained Treatment of AMD Containing Al and Fe3+ with Limestone Aggregate
Neil Wolfe and Robert HedinHedin Environmental
West Virginia Mine Drainage Task Force MeetingMarch 2011
Limestone Advantages
• Limited solubility– cannot over-treat– Can install years of treatment capacity
• Can store on site in open piles without loss• Not hazardous• Less than 20% the cost of lime and caustic
alternatives
Limestone Disadvantage
• When exposed to acidic water containing Al and Fe3+, solids quickly form
• Solids foul limestone aggregate, greatly decreasing its effectiveness
Project GoalsUnderstand why the effectiveness of limestone treatment is reduced by Al and Fe solids.
Develop a treatment approach that can overcome these limitations and provide sustainable treatment of low pH AMD with limestone aggregate
pH 2.9Acidity, 290 mg/LAl, 29 mg/LFe, 8 mg/LMn, 15 mg/L
Two parallel 27 m3 roll off containers filled with ~30 tons limestone
Construction of Experimental Flush Units
Agri Drain SDS
“Small” limestone
East Box
West Box
Experimental
• Measure performance of limestone units– Influent and effluent samples
• pH, alkalinity, acidity, sulfate• Total and dissolved Al, Fe, and Mn
• Vary operational parameters– Flow rate– Water level in limestone bed– Drainage trigger (time, water level, chemistry)
Solids
Solids form when alkaline conditions arising from calcite dissolution cause:
• Al3+ Al(OH)3
• Fe3+ Fe(OH)3
Solids accumulate in the aggregate in two manners
Solids accumulate in pores
Solids form scale on limestone
Scale flakes off revealing clean LS surface
What happens when a limestone bed is drained empty?
• Solids in aggregate pores are flushed away– porosity is restored
• Some scales are dislodged, exposing clean limestone surface– Alkalinity generation is restored
Treatment Goal
• Identify the retention time necessary to achieve treatment goals
• Identify the bed drainage schedule that maintains treatment effectiveness
Long-term alkalinity generation
Location pH Acid Fe Al Mn
Influent 3.0 239 8.9 25.6 14.1
Effluent 6.9 -58 0.6* 2.9* 3.0**
Metals are mg/L and acidity is mg/L CaCO3 *Suspended solid; **dissolved
Average chemistry since change to 1 gpm and 1/wk drainage (28 months)
Mn removal, flow 1 gpm, drain 1/wk
Solids accounting for the West Box
Fe flush, 40.7%
Fe Retain, 49.0%
Fe Discharge,
10.3%
West Box Fe Accounting
Al Flush, 34.8%
Al Retain, 51.8%
Al Discharge,
13.4%
West Box Al Accounting
Solids accumulation will eventually affect calcite dissolution and bed performance:
• Limestone must be either replaced or cleaned
Cleaning limestone bed with an excavator and AMD flow
Cleaned limestone aggregate
• Cleaning cost: $5/ton• New limestone costs
$22/ton
Effluent pH of Mitchell East Box Before and After Limestone Cleaning
4
4.5
5
5.5
6
6.5
7
210 230 250 270 290 310 330 350 370 390 410 430
Days of Operation
pH
East Box
Limestone Cleaned
West box performance as of March 2011
• 3.2 ton CaCO3 net alkalinity generation
• 10% of installed limestone has dissolved
West box performance as of March 2011
• At $40/ton LS installed (with settling pond)– $408/ton CaCO3 net alkalinity
• Comparative costs– Anna S VFP system, $400-550/ton CaCO3
– NaOH system, $1,500 – 2,500/ton CaCO3
– lime plant, $800-1000/ton CaCO3
– Stream dosing (lime), $175-291/ton CaCO3
Summary
1. Effective treatment of acidic Al and Fe3+
contaminated mine water can be achieved with limestone aggregate as long as the bed is regularly drained empty.
2. Fouled limestone aggregate can be cleaned and its alkalinity generation restored.
3. Sustainable Mn removal can be achieved at pH levels between 6.5 and 7.0.
Scootac system (2010)
• Renovation of existing VFP– 735 tons LS aggregate and 223 CY organic
substrate
• Installation of parallel DLB– 1,000 tons LS aggregate– AgriDrain SDS
• Polishing pond
• Flow distribution
Scootac system, Nov 2010 – Mar 2011
Flow pH Acid Alk Al Mn Fe SO4
gpm mg/L CaCO3 mg/L
Raw 85 3.9 88 0 9.9 25.2 0.3 961
Drainable LS Bed 44 7.4 -219 243 <0.1 0.9 0.1 952
Vertical Flow Pond 41 7.1 -137 164 <0.1 21.3 1.0 923
Polishing Pond na 7.3 -163 193 0.1 11.7 0.8 992
Scootac system (on line Nov 2010)
Questions?
• Insert good photo or figure