Sustained Treatment of AMD Containing Al and Fe 3+ with Limestone Aggregate

1. Sustained Treatment of AMD Containing Al and Fe3+ with Limestone Aggregate Neil Wolfe and Robert Hedin Hedin Environmental West Virginia Mine Drainage Task Force Meeting March 2011

3. 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

4. Limestone Disadvantage • When exposed to acidic water containing Al and Fe3+, solids quickly form • Solids foul limestone aggregate, greatly decreasing its effectiveness

5. Project Goals Understand 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

6. Construction of Experimental Flush Units pH 2.9 Acidity, 290 mg/L Al, 29 mg/L Fe, 8 mg/L Mn, 15 mg/L Two parallel 27 m3 roll off containers filled with ~30 tons limestone Agri Drain SDS East Box West Box “Small” limestone

7. 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)

8. 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

9. Solids accumulate in pores

10. Solids form scale on limestone

11. Scale flakes off revealing clean LS surface

12. 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

13. Treatment Goal • Identify the retention time necessary to achieve treatment goals • Identify the bed drainage schedule that maintains treatment effectiveness

14. Long-term alkalinity generation

15. Average chemistry since change to 1 gpm and 1/wk drainage (28 months) Metals are mg/L and acidity is mg/L CaCO3 *Suspended solid; **dissolved

16. Mn removal, flow 1 gpm, drain 1/wk

17. Solids accounting for the West Box

18. Solids accumulation will eventually affect calcite dissolution and bed performance: • Limestone must be either replaced or cleaned

19. Cleaning limestone bed with an excavator and AMD flow

20. Cleaned limestone aggregate • Cleaning cost: $5/ton • New limestone costs $22/ton

22. West box performance as of March 2011 • 3.2 ton CaCO3 net alkalinity generation • 10% of installed limestone has dissolved

23. 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

24. Summary • 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. • Fouled limestone aggregate can be cleaned and its alkalinity generation restored. • Sustainable Mn removal can be achieved at pH levels between 6.5 and 7.0.

25. 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

27. Scootac system, Nov 2010 – Mar 2011

28. Scootac system (on line Nov 2010)

29. Questions? • Insert good photo or figure