Impact of Sludge Towards Stabilization of the Fire Road Mine

Transcription

1 Impact of Sludge Towards Stabilization of the Fire Road Mine + M. Coleman 1, K.D.Phinney NB Power 2 Consulting Chemist./Chemical Engineer

2 Mine Location

3 Background Mined ; 25 acre site (1 ha). AMD since mid-8 s. Waste rock contoured Hydrated lime treatment 4 one -acre sludge ponds/yr Long term solution-sludge disposal into waste rock

4 Initial Dredging Rationale Disposing of lime neutralization sludge into acid generating rock could provide several benefits including: utilization of the excess alkalinity final disposal area for sludge minimize additional land disturbance decrease personal liability reduce accessibility of oxygen into the waste rock.

5 Equivalent Lime Consumption and Mine Water Pond Acidity 35 Mg/L as CaCO3 Tonnes of CaCO CaCO3 equiv of lime consumed Mine water pond Acidity (mg/l) Years

6 Decreasing Acidity in Mine Water from Monitoring Wells Acidity Scale (mg/l) Pink >1 Orange 2-1 Gray 15-2 Green <15

7 Factors That May be Impacting the Change in Acid Generation 1. Waste rock weathering. 2. Vegetation coverage increasing. 3. Putting the sludge back onto the waste rock. 4. ph of precipitation 26, m 3 sludge, late fall 214

8 Objectives We are noticing a decrease in mine water acidity. Why? Changes in in-situ neutralization? Impact of neutral ph groundwater or precip recharge? Waste rock weathering into smaller particles? Is AMD burning out? Can you measure or calculate the impact of the sludge on the acidity of the mine water? Are there short term impacts that can be identified after each dredging period?

9 Groundwater Chemistry Evaluation Generated acidity (initial acidity) Calculated acidity as a result of oxidation of sulfide minerals Equivalent to SO -2 4 concentration (original H 2 SO 4 ) Residual acidity Calculated acidity due to mineral acidity (H + ) and conc. of metallic ions (Al, Fe, Mn, Cu and Zn) Equivalent to CaCO 3 demand for neutralization % Acidity removed in-situ in the waste rock Difference between (initial and residual)/initial)

10 Compare Consumed CaCO 3 versus Calculated Residual Acidity Demand Equivalent to CaCO 3 demand for neutralization

11 Comparison of Calculated Residual Acidity versus Measured Acidity Apr-14 Jun-14 Aug-14 Oct-14 Dec-14 Feb-15 Apr-15 Jun-15 Aug-15 Oct-15 Dec-15 Feb-16 Apr-16 Jun-16 Aug-16 Oct-16 Dec-16 Mg/L as CaCO3 Residual Acidity Acidity Measured in Grab Sample

12 Groundwater Monitoring

13 Initial and Residual Acidity in Mine Water Well 26S, 26D and 27 mg/l as CaCO MW 26S MW26D % Reacted

14 Initial and Residual Acidity in Mine Water Wells 17S, 17D and 16D MW 17S MW17D MW 17D % acid removed Initial Generated Acidity (mg/l) residual acidty (mg/l) Jun MW16D Jun

15 Comparison of Initial and Residual Acidity in Mine Water Pond Acidity (mg/l as CaCO3) Jun-96 Jun-97 Jun-98 Jun-99 Jun- Jun-1 Jun-2 Jun-3 Jun-4 Jun-5 Jun-6 Jun-7 Jun-8 Jun-9 Jun-1 Jun-11 Jun-12 Jun-13 Jun-14 Jun-15 Jun-16 Year Initial Acidity Pond Residual Acidity Pond

16 Comparison of Initial and Residual Acidity in Mine Water Pond- Impact of Dredging 12 Initial Acidity Pond 1 Residual Acidity Pond mg/l as CaCO Jun-1 Jun-11 Jun-12 Jun-13 Jun-14 Jun-15 Jun-16 Years

17 Percent Acidity Removed In-situ Mine Water Pond -% Acidity Removed In-situ Apr-1 Aug-1 Dec-1 Apr-11 Aug-11 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14 Dec-14 Apr-15 Aug-15 Dec-15 Apr-16 Aug-16 Dec-16 Year

18 Estimated Lime Savings after Dredging Acidity (mg/l) tonnes of Ca(OH) Ave Residual Acidity Prior to Dredging Ave Residual Acidity After Dredging Lime Savings at Ca(OH)2 (tonnes) Lime Savings ($) Year of Dredging Lime Savings (thousands of dollars)

19 Conclusions Decreases in initial mine water acidity a result of acid generation burning out -less acid production. SO 4 decreasing, (example well 26 above the sludge influence) mine water ph increasing lower Al conc so partial dissolution of feldspars and mica? In-situ neutralization is occurring. Residual acidity is much less than initial acidity We saw in well 16D that we can have initial acidity but have it mostly neutralized in-situ.

20 Conclusions The percentage of acidity removed in-situ is increasing. Waste rock weathering (smaller particle size increased surface area)? Trend will most likely continue but those sites at higher percentages seem to taper off and not hit 1% Based at what we saw at well 16D, we may not need to hit 1%. At what point will the residual acidity be negligible at the mine water pond but spot neutralization may be required at specific areas within the mine site? Look at surface amendments to increase alkalinity?

21 Conclusions There are some measurements that can be made on the impact of the sludge on the acidity of the mine water. The effect of dredging can be monitored by acidity and ph changes as a plug or plume flow in the mine water pond chemistry. Almost $6K in estimated lime savings from last 4 dredging episodes Propose to look at potential cost savings to recycle sludge directly back into waste rock during low groundwater flow periods to reduce dredging costs.

22 Impact of Sludge Towards Stabilization of the Fire Road Mine In memory of K.D.Phinney February NB Power 2 Consulting Chemist./Chemical Engineer