Comparison of Actual and Calculated Lag Times in Humidity Cell Tests Kelly Sexsmith, Dylan MacGregor, Andrew Barnes

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1 Comparison of Actual and Calculated Lag Times in Humidity Cell Tests Kelly Sexsmith, Dylan MacGregor, Andrew Barnes

2 Introduction Lag Time is the amount of time for acidic conditions to develop Needed to assess when management plans should be in place for preventing or mitigating ARD Typically defined through kinetic testing Observe lag time (rare) Calculate lag time (subject to uncertainty)

3 What we aimed to do Explored the relationships between lag time in HCT and different sample parameters: ABA parameters (TIC, NP, AP) Calculated lag times Mineralogy Rate of acidification

4 SRK Dataset Results for 229 tests from 33 sites were reviewed Test durations ranged from 80 to 520 weeks! 13 tests still operating Compiled data for tests where: acidic conditions developed following a distinct lag time (>20 weeks) acidic conditions should have developed but did not develop despite an extensive period of testing 30 tests from 14 sites met the criteria

5 Calculating Lag Time Time to NP depletion ( Ca NP * + Mg) release = (eqn 1) rate or Time to NP depletion = SO 4 NP * release rate (eqn 2) Where: NP* is the available NP in units of kg CaCO 3 eq/t (Ca+Mg) and SO 4 release rates are the steady state non-acidic release rate from the humidity cell test in units of kg CaCO 3 eq/t/year.

6 Available NP Available NP (NP*) was determined using 4 methids: Method 1 - Site specific unavailable NP was estimated based on the TIC or NP found in samples with acidic paste ph values, and was subtracted from the TIC or NP. Method 2 - TIC or NP was divided by a factor representing the relative availability of NP/AP (NP/AP crit ) established through kinetic testing. (NP/AP crit ) is typically a value between 1 and 1.5. Method 3 - For sites where there was no specific information as to whether TIC or NP was more appropriate for a site, or where site specific correction factors could not be established, the lesser of the TIC or NP value was selected and this was divided by an NP/AP crit factor of 1.5. Method 4 Available NP measured directly by undertaking scaled up NP titration on sub sample of HCT material without size reduction.

7 Defining Actual Lag Time

8 General Characteristics of these Tests

9 ph Profiles (comparison)

10 Calculated versus Actual Lag Times predicted lag time NP * = 0.74 (eqn 3) SO release rate 4

11 Effects of Other Variables Secondary relationships examined through residual analysis. Specifically, the difference between actual and predicted lag times (i.e. residuals from eqn 3) plotted against the following other variables: Rate of acidification (ph change) Difference between NP and TIC (Ca+Mg)/SO4 release rates Sulphur content Sulphate production rates. Qualitative approach used for mineralogy

12 Rate of Acidification J2: 2 ph units in ~190 weeks = slow B2: 2 ph units in ~8 weeks = rapid

13 Lag Time versus Rate of Acidification

14 Lag Time in Fast Tests

15 Lag Time in Slow Tests

16 Mineralogy Some indications Samples with relatively low carbonate, but with favorable silicate minerals tended to have longer than expected lag times Samples with iron carbonates tended to have longer than expected lag times. Several exceptions and no clear patterns evident. Samples with shorter than expected lag times often had NP measurements that were inconsistent with mineralogy or TIC Blinding by precipitates did not seem to be a factor No relationships of faster than expected lag times in samples with high TIC or high SO4 production.

17 Conclusions Tests where lag time can be measured are relatively rare (30/229 tests) All of these tests had low NP and high sulphide content and were clearly PAG Calculated lag times were typically longer than actual lag times, but were correlated (r = 0.67) Estimates of lag time can be improved by selecting the most appropriate NP (usually the lesser of TIC or NP), and by applying correction factors to account for availability/reactivity

18 Conclusions Tests with the longer than expected lag times showed a relatively gradual development of acidic conditions and slow rates of acidification The results emphasize the value of running some tests for an extended period of time either to determine actual lag times, or demonstrate that there is a prolonged period of neutral ph conditions in situations where the lag times are expected to last for an impractically long time.

19 Acknowledgements SRK would like to thank Our clients for supporting these programs and allowing us to use the data. SRK geochemists that helped to prepare the sample inventory and identify candidate tests for this work.

20 Thank you for listening Please see our poster regarding available NP Deportment of sulfide and Neutralization Potential by Particle Size in Limited Buffering Mine Wastes

Comparison of Actual and Calculated Lag Times in Humidity Cell Tests

Comparison of Actual and Calculated Lag Times in Humidity Cell Tests Kelly Sexsmith 1, Dylan MacGregor 1 and Andrew Barnes 2 1. SRK Consulting, Canada 2. SRK Consulting, United Kingdom ABSTRACT It is common