Effluent treatment processes at the Key Lake mine There are two water effluent discharges from the Key Lake mine:

(i) Effluent discharge from the dewatering system that was installed around the Gaertner and Deilmann open-pit mines; and

(ii) Treated mill effluent discharge, which includes water returned from the tailings facilities and contaminated water from the dewatering system.

The Deilmann/Gaertner dewatering system has evolved over time. About thirty dewatering wells were originally installed in the Gaertner dewatering system, and approximately one hundred dewatering wells were installed in and around the Deilmann pit between 1983 and 1995. In addition, about 120 horizontal drain holes were installed in the Deilmann pit to depressurize the pit slope in 1993, to compliment about 180 horizontal drains previously installed prior to 1993. This overall dewatering system was divided into two parts. A contaminated water system, which collected and transported water to the mill, and a clean water system, which was allowed to discharge directly to the environment (Horsefly Lake/Little McDonald Lake/McDonald Lake system), under site-specific, regulatory-controlled water quality objectives (Table 1).

Table I. Key Lake effluent water quality criteria

Contaminant Dewatering Effluent Quality Objectives*

* Allowable Maximum Monthly Arithmetic Mean Concentration.

** Maximum Monthly Arithmetic Mean Concentration (Also regulated by Grab and Composites sample concentration).

*** Provincial limit on total radium, Federal limit on dissolved radium.

**** Federal limits only specify minimums (6.0 monthly, 5.5 composite, 5.0 grab).

Following completion of a pre-mining surface drainage programme, it was estimated that approximately 72 million m³ of static groundwater needed to be removed from the mining area, and about 15.4 million m³ would need to be pumped to the surface on an annual basis.

Historically (1982–1998), discharge rates to the Horsefly Lake/McDonald Lake system have averaged 10.9 million m³/y, ranging from a peak of 15.7 million m³ in 1986 to a low of 7.8 million m³ in 1998. Over the period 1994–1998, about 84% of the overall volume of dewatering water was released to Horsefly Lake and 16% was sent to the mill. The clean dewatering water contributes 32-90% of the pre-mining flow in the Horsefly Lake/McDonald Lake drainage system, tending to produce more uniform flow conditions relative to pre-mining conditions. The first of these lakes (Horsefly) is very small, at 480 000 m³. The next lake, Little McDonald Lake is approximately 4.3 million m³, and McDonald Lake is approximately 25.6 million m³ in volume.

While the pit dewatering volumes have decreased with time, nickel concentrations and loadings began to generate interest in 1991 when the request for the first nickel-specific dewatering environmental assessment was received from our regulatory agencies.

Nickel loadings to Horsefly Lake started to increase, particularly starting in 1994, peaking in 1995. The peak annual loading of nickel was about 2800 kg/y, increasing from about 370 kg/y in 1982. In 1996, a reverse-osmosis (RO) plant was commissioned to reduce this nickel loading to the environment. Table 2 shows the history of nickel concentrations in the clean dewatering effluent discharge. As previously noted, studies into the environmental significance of this increased nickel loading (as a supplement to the initial intent to study the environmental significance of molybdenum discharges) and efforts to reduce the nickel loading formed a core of the work being documented in this CRP, as discussed in more detail later in this report. In 1999, efforts to flood the Deilmann and Gaertner pits, coupled with good operation of the RO plant generated much reduced flows to the Horsefly/McDonald Lake system (2.0 million m³), much lower nickel loadings (107 kg) and nickel concentrations (0.053 mg/L) which were equivalent to those seen in the early years 1982-85 (0.026, 0.054, 0.052, and 0.055 respectively). In 2000, even lower flows and loadings are expected because more dewatering water is being pumped back into the Deilmann pit to enhance slope stability during flooding of the pit. In fact, the main current reason to operate the RO plant is to provide clean industrial water for the mill (for fire water make-up, pump gland water, reagent mixing, and for water coolers in the Powerhouse which cannot be fouled, etc.).

The mill effluent treatment consists of four main sections: solvent extraction raffinate neutralization, radium removal, pH adjustment and tailings neutralization. A schematic is provided in Figure 4.

The raffinate neutralization circuit consists of four neutralization pachucas and a thickener.

The radium-removal circuit has three-mechanically agitated reactors, flash mix tanks and a thickener. The pH adjustment section consists of two small agitated tanks and a discharge launder system to four monitoring ponds (5000 m³ each). The tailings neutralization section consists of a small splitter box and two large agitated holding tanks connected to the tailing pumps.

Table II. Average nickel concentration in Key Lake dewatering system effluent

Raffinate from the solvent extraction circuit is pumped into pachuca #1 at the head end of the neutralization train. The raffinate flows by gravity through each of the pachucas in series with lime being added to progressively raise the pH from one to seven. Each of the pachucas is connected by a launder and the lime is added to the launder. Slurry from either pachuca #3 or

#4 is pumped back to pachuca #2 to provide a seed material for precipitate formation. In 1995, airlifts were installed on each pachuca to remove oversize from the bottom of the vessels, thereby reducing plugging problems. In 1997, the raffinate line was re-routed from a central front-end feed box to flow directly to #1 pachuca while the recycle was directed from the front-end feed box to #2 pachuca. This permitted aeration of the raffinate prior to neutralization in order to improve arsenic and organic levels in the final effluent. Steps were also made to permit higher temperature operation to promote organic removal. In other modifications to help reduce final effluent toxicity, the overflow launders on the bulk neutralization thickener were resealed to trap entrained organics and a removal system was installed to facilitate the removal of these organics. Scrap steel may be dissolved in #1 pachuca to improve contaminant precipitation by increasing the iron concentration. The overflow from pachuca #4 is fed into the bulk neutralization thickener where the solids are separated and removed as a 25% solids slurry. The thickener underflow is pumped to the tailings splitter box for disposal at the DTMF. The overflow from the thickener flows to the radium-removal circuit.

Contaminated water from reservoir 1 or 2 that has to be treated over and above process water requirements and other sources of contaminated mine water are fed into the circuit in the bulk neutralization thickener feed well. This mixes with the slurry from neutralization pachuca #4 before going into the neutralization thickener.

Reservoirs 1 and 2 provide much of the storage capacity within this effluent treatment system.

Reservoir 1 tends to be used for non-sulphate saturated water storage (such as runoff, RO plant reject water) while reservoir #2 holds sulphate rich water (return water from the tailings management facilities and recycle effluent for re-treatment).

In the radium-removal circuit, barium chloride is added to the bulk neutralization thickener overflow stream before it flows into reactor #1. The discharge from reactor #1 is treated with lime in a small flash mix tank then fed into reactor #2. The pH is maintained at 10.5 to 11.0.

The overflow from reactor #2 goes through another flash mix tank to which further lime can be added if required to maintain the pH and then into reactor #3.

FIG. 4. Schematic of mill effluent treatment (bulk neutralization) circuit.

The discharge from reactor #3 goes into the radium-removal thickener for separation of the solids from the solution. The underflow solids are pumped to the tailings tanks and the solution flows to the effluent pH adjustment mixing tanks. Sulphuric acid is added in two stages to reduce the pH too slightly below neutral to control ammonia toxicity. It then flows by gravity through an open launder system to one of the four monitoring ponds located along the north side of the mill terrace.

The effluent is sampled while a pond is filling and the sample is analysed by the laboratory when the pond is full. The pond is held until the results are available at which time a decision is made by comparing the analysis results with the licence parameters. The pond is either released to Wolf Lake or recycled to reservoir 2. When a pond is batch released to Wolf Lake, a discharge composite sample is collected from the discharge pipeline. The discharge composite analysis results are reported as the final analysis of the monitoring pond’s quality.

In the tailings neutralization circuit, the tailings from the counter-current decantation (CCD) circuit and the underflow streams from the two bulk neutralization thickeners are fed to a mix box. The combined slurry flows to the two tailings holding tanks where it is adjusted to pH 10.5 to 11.0 with lime and pumped to the tailings thickener by either a three stage centrifugal pump installation or two positive displacement diaphragm pumps. Domestic sewage (including camp sewage since early 1998), raffinate from the molybdenum removal circuit and carbonate wash from solvent regeneration are also added to the tailings hold tanks.

In 1996, a new tailings thickener system was commissioned. Combined tailings slurry from bulk neutralization is thickened to 35-40% solids in a 30-metre diameter thickener. Underflow is transferred by a two-stage centrifugal pump installation or two positive displacement pumps to the DTMF. Raise water from the DTMF is combined with thickener overflow in a storage tank prior to being recycled to reservoir #2.

Because Key Lake mill circuit employs a conventional ammonia strip and precipitation process, effluent ammonia control is necessary. Control has been good, so the next contingent level of control has not been installed (zeolite adsorption or some combination of strong and weak acid cationic ion exchange on the ammonium sulphate vapour condensation system).

Also note that supplemental ferric sulphate addition is not required to control dissolved arsenic. Driving effluent to pH 10-10.5, with solids removal prior to final neutralization has effectively controlled effluent arsenic concentration. The switch to the arsenic/nickel-free McArthur River ore as prime mill feed further controls this contaminant.