EXECUTIVE SUMMARY
The mining industry has long recognized the potential value of open pits as a means to manage environmental liabilities with a focus on site closure. Typically, open pits have been used as a repository for mine wastes, specifically as a strategy to manage potentially reactive wastes. The use of open pits specifically within the context of water quality management for mine closure has received little attention despite water quality management representing a risk and therefore a focus of long-term care and maintenance activities at many mines globally. Moreover, water quality management can represent the greatest environmental liability for operations and the single highest cost factor associated with site activities. This is especially true in instances where water quality management must be achieved through traditional water treatment with a Water Treatment Plant (WTP).
In-pit treatment can represent a lower cost alternative with less infrastructure and more flexibility in water management that is aligned with more recent regulatory thinking on the approvability of perpetual water treatment via WTPs. In-pit treatment can be especially attractive for waters that marginally exceed discharge criteria with relatively small loadings of constituents. In such cases, the water treatment solids that precipitate as a by-product of treatment represent very small volumes and a pit can have a large capacity for retention of the treatment solids.
In pit treatment of various metals has been used at several mines with variable results. In-pit treatment of arsenic with ferric sulphate is an innovative approach to water treatment that was evaluated in this study. The overall objective of the investigation was to demonstrate that in pit batch treatment of arsenic represents a viable and economical alternative to traditional water treatment applications.
Phase 1 of this evaluation program included a laboratory or “bench-scale” study that was used to define the design criteria, including the attainable treatment efficiencies, the required reagent dosages and the sludge production rates. Phase 2 of this study included a field scale treatment trial that evaluated the practicality and efficacy of in-pit batch treatment of arsenic within the Night Hawk Lake Mine (NHLM) open pit, as informed by the results in Phase 1.
Phase 1 of this investigation included testing of ferric sulphate dosing rates on water from Newmont Goldcorp’s NHLM open Pit as well as the characterization of the water treatment solids that were formed. The molar ratio of iron (Fe) to arsenic (As) and pH were evaluated to assess optimal dosage requirements and resulting treatment solids stability. The results showed that total concentrations for both arsenic and iron were unaffected after ferric sulphate addition with only 5 minutes of reaction time; but were much lower after 24 hours of settling time. In contrast, dissolved concentrations of arsenic and iron were reduced within 5 minutes of reaction time after ferric sulphate addition; with little change after 24 hours of settling time.
There was a trend of decreasing concentrations for total and dissolved arsenic with increasing iron dosage in the tests after 24 hours settling time. The highest iron dosage of approximately 20:1 resulted in the most effective removal of arsenic from solution, to total and dissolved concentrations below 0.05 mg/L. Concentrations of cobalt and nickel were also examined in the ferric sulphate addition tests. The results showed that ferric sulphate addition increased concentrations of cobalt, initially, followed by lower concentrations after 24 hours of settling time. Final cobalt concentrations (total and dissolved fractions) did however exceed initial concentrations, likely due to the usage of reagent grade ferric sulphate that was associated with an elevated cobalt content. Total and dissolved nickel concentrations decreased below initial concentrations in all tests for all dosages. Cobalt and nickel were therefore also considered key parameters for assessment within the field trial.
A treatment solids settling test was completed to determine a minimum settling time and to generate enough solids for characterization. Based on total concentrations of arsenic and iron, the majority of settling occurred between 5 minutes and 30 minutes, after which concentrations were relatively constant up to 24 hours of settling time, and decreased slightly again between 24 and 120 hours of settling time. Results of the settling test also provide an indication of the relative stability of treatment solids. Concentrations over 120 hours were either stable or decreased during that time. These results imply that the solids in contact with the water are relatively stable and that the observed concentrations are representative of those that would occur in waters in contact with the treatment solids. These results are consistent with other studies of arsenic treatment solids formed during the addition of ferric sulphate.
Phase 2 of this evaluation program included a field scale study that evaluated practicality and efficacy of in-situ batch treatment of arsenic within the NHLM open pit, as informed by the results in Phase 1.
The bathymetry survey showed that the water volume in the NHLM open pit is approximately 100,000 m3 with an average depth of about 10 m and a maximum depth of 22 m. On average, water quality monitoring results exhibit measured arsenic concentrations of 0.6 mg/L representing a total of 60 kg of arsenic in the pit water. The Phase 1 lab testing indicated that a 20:1 molar ratio of iron to arsenic (Fe:As) will result in treated arsenic concentrations of 0.05 mg/L or less and that was the target ratio selected for dosing of the pit water. Therefore, about 900 kg of iron (Fe) was required to dose the entire pit. This represents about 6 totes of commercially available ferric sulphate solution (50% w/w) each containing 1,350 kg of solution with 12.25% Fe, or 165 kg of Fe each.
The field trial was designed to include circulation of about 10% of the pit water volume. The concept was to pump the minimum volume that allows mixing of the required ferric sulphate dose with pit water by dispersing the reagent with the pit water over a large area of the water surface. This was accomplished by the use of two water cannons stationed around the perimeter of the pit.
The pH, dissolved oxygen (D.O.), temperature and specific conductance were measured in the pit water profile to a maximum depth of 22 m, before and after ferric sulphate addition. The results show that the pH was typically in the range of 7.5 to 8.5 and overall values were slightly lower after treatment than before. The oxygen saturation values varied between about 100% near the water surface to less than 10% at depths greater than 16 m, with very little difference before and after treatment. The temperature of the water varied between 4 and 5° prior to treatment and 3 to 5° after treatment with near surface temperatures higher than those at depth. The specific conductance values in November before and after treatment, were in the range of 450 to 575 µS/cm, exhibiting lower values near the surface with abrupt increases at a depth of about 12 m below surface. The specific conductance values were slightly higher after treatment than before. The specific conductance profiles in November suggest that there may be a density layering at a depth below 12 m from surface with higher TDS water in the bottom layer.
The total and dissolved arsenic concentrations at three (3) depths within the pit were compared for the sampling event immediately prior to treatment and three (3) sampling events after treatment. The
pre-treatment samples exhibited slightly lower dissolved concentrations at depth than the total concentrations. Immediately post-treatment, November 2018, the total arsenic concentration at the surface at NHP1 decreased from an original value of 0.6 mg/L total arsenic, on average, to a value of 0.01 mg/L total arsenic and the dissolved concentration was less than 0.002 mg/L. These results translate into an approximate treatment efficiency of 98% within the surface depths. The mid-depth samples for the same sampling event were approximately 0.03 and 0.01 mg/L for the total and dissolved arsenic concentrations, respectively. The arsenic concentrations at depth remained close to the initial concentrations prior to treatment. The arsenic concentrations in subsequent sampling events, post-treatment, exhibited similar trends with depth showing the lowest concentrations near the surface and that the highest concentrations at depth.
The study showed effective treatment of arsenic in the upper 10 m of the pit providing a positive effect that can be used for the management of water in similar pits in temperate climates. These results also suggest that pits having natural outlets for flow may allow for the management of treated waters, with near surface waters released to the environment at the values observed after this treatment trial.
Considerations of the practical implications of in-situ batch treatment were also explored as part of this investigation. An evaluation of the required frequency for treatment was completed using a quantitative modelling approach, for NHLM open pit. The post treatment monitoring data provided a basis for assessing the potential need for additional treatment in the future. MineModTM, a proprietary water quality modelling tool, was used to assess the need for and timing of in-pit treatment events.
A similar model was also developed for a much larger scale pit, using the McEwen Mining Inc. Black Fox Mine, near Matheson Ontario, as an example in order to compare the requirements for in-situ treatment at this scale. A comparison of capital and operating expenditures was also completed for this example, in order to evaluate the application of in-situ treatment versus the construction of a water treatment plant.
Modelling showed that after an initial treatment at the time of the pit filling, periodic in-pit treatment can maintain arsenic concentrations below a selected discharge limit of 0.15 mg/L with treatment on the order of every 7 years for the NHLM open Pit, and 32 years for the Black Fox Pit. This low frequency of treatment potentially represents substantial savings in operating costs compared to ongoing and continual treatment with a conventional water treatment plant.
