Sustained Treatment of AMD Containing Al and Fe 3+ with Limestone Aggregate Neil Wolfe and Robert Hedin Hedin Environmental West Virginia Mine Drainage.

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