Coeur Alaska's Kensington gold mine, a historic underground mine 72km north of Juneau, was discovered in the late 1800s and operated into the early 1900s. Coeur acquired ownership of the mine in 1995 and launched extensive development efforts before resuming commercial production in July 2010.
The underground mine produces mine-water discharge stemming from snow and rain infiltration. The Kensington mine is situated in the middle of a temperate rainforest and the site receives an annual average of 190 cm of rain making for an abundance of stormwater and crea ting unique mine-water treatment challenges.
Kensington's first mine-water treatment plant was built as a pilot plant and consisted of a lamella clarifier followed by sand filtration. In 2007, this was replaced with a 5.7m3/min (1,500 gpm) treatment plant that includesa 21m-diameter solids contacting clarifier and a four-plex multi media filter system. The plant utilizes a conventional coagulant and polymer feed system for clarification in the solids contacting clarifier, followed by multimedia filtration.
Mine water is directed through a series of ditches, sumps and pipelines out through an adit and into a 1.5 million litre (0.4 million gallons) flow equalization pond. Water from the pond is pumped to the site's Comet Beach Mine Water Treatment Plant, where it is treated to remove soluble and nonsoluble etals and suspended solids.
Kensington's mine-water treatment plant had been performing well and consistently meeting the Environmental Protection Agency's (EPA's) National Pollutant Discharge Elimination System (NPDES) effluent limits.
However, Coeur determined that additional capacity would be necessary, as continued mine development and record snowfall levels would ultimately increase the treatment capacity needs of the mine.
Coeur determined that it needed to double the capacity of the plant in order to meet the projected demand. The company contracted Veolia Water Technologies to design the expansion for an additional treatment capacity of 5.7m3/min..
Since 2005, Veolia has worked closely with the mine in providing water and waste-water treatment engineering design services, systems and technologies. It designed a mine-water treatment plant expansion that utilises the same treatment processes as the existing plant, but high-rate, sand-ballasted flocculation and clarification is used rather than a conventional clarifier.
An ACTIFLO® clarifier and four-plex multimedia filter system were installed to work in tandem with the existing mine-water treatment plant. Coeur bought the proprietary equipment from Veolia, along with start-up, commissioning, controls design, and civil engineering services. Coeur executed the civil engineering work and building expansion using its local mechanical and electrical contractors and its local controls contractor to integrate the new plant's controls into the existing plant.
Because the ACTIFLO process allows for high overflow rates and short retention times, it has the same capacity but provides a significantly smaller footprint than the conventional clarifier system serving Comet Beach Plant #1. The new system (Comet Beach Plant #2) is small enough to be housed inside the mine's expanded equipment building, protected from the harsh winter weather and the danger of freezing.
Mine water is pumped from the equalization pond to a conditioning tank, where hydrochloric acid and coagulant are added before the water enters the ACTIFLO clarifier. Typically, pH adjust ment through acid addition is not necessary. The pH adjustment is necessary only when grouting operations are in process in the mine, because the grout raises the pH of the mine water.
The new plant's flocculant feed system (also serving Comet Beach Plant #1), consists of two 5.7m3 tanks that makedown powdered flocculant to a liquid for the ACTIFLO clarifier. The high-rate, ballasted, flocculation process includes:
- injection tank
- maturation tank
- settling tank
- recirculation pump
Raw water is first mixed with a coagulant in a high-shear environment, where it is retained for two minutes.
In the next tank, the water is injected with a polymer along with microsand and mixed aggressively for about another two minutes. Flocculant feed flow is proportional to the flow of water into the clarifier system. The water then enters a maturation zone, where gentle shear is applied for an additional six minutes.
Seeding the Floc
The microsand serves as a seed for floc formation, providing surface area that enhances flocculation and also acts as a ballast to aid a rapid settlement, resulting in very high rate of settling.
The microsand-ballasted flocs increase in size, trapping smaller flocs before the water enters the sedimentation tank where the large flocs immediately begin to settle. The clarified water at this stage then counter-flows upward through settling tubes to collection troughs.
The microsand and other solids in the ballasted flocs that settle in the base of the tank are pumped to a hydro cyclone centrifuge. Sludge and micro sand are separated at the hydrocyclones by creating a vortex effect and exerting centrifugal force on the particles. Clean microsand falls back into the injection tank and the solids and metals precipitated in the clarifier are removed as sludge and sent to a holding tank.
Clarifier effluent proceeds to the system's clarified water tank, where it is pumped to four parallel multimedia filters for final polishing. Filter media includes garnet and anthracite. The filter system operates with all units in service and each unit is equipped with an air scour to ensure adequate backwashing and removal of particulates. Effluent from the multimedia filters is discharged to the Sherman Creek NPDES Outfall.
Filter backwash operates sequentially. When a filter's differential pressure exceeds the user adjustable set-point, it goes through a backwash cycle to remove the trapped particulate matter. Backwashing is performed with clean filtered water from the other filters in service.
Filter backwash is recycled through the treatment process and the sludge produced in the clarifier is de-watered and disposed of in the mine.
Comet Beach Mine Water Treatment Plant #2 was started up and commissioned during the March quarter of 2011. With the added capacity brought online by the new plant expansion, the mine has sufficient treatment capacity to meet projected flows.
After the new plant initially came online, operators reported that it was not achieving the required level of water quality expected. Upon investigation, the technology provider determined that the chemistry of the mine water was interfering with the chemistry in the ACTIFLO treatment process. Once the mine rectified this, the clarification system immediately began producing crystal-clear water as per the design specifications.
Kensington mine's Comet Beach Mine Water Treatment Plant #2 is today meeting all regulatory requirements, and its efficiency and flexibility ensure the plant's performance over a range of operating conditions. The short 10-minute hydraulic residence time in the clarifier enables the operator to quickly see the effects of process changes made to the system, thereby enabling quick system optimization and adjustments.
The addition of the new mine-water treatment plant benefits the Kensington mine in several ways: it has provided an innovative solution for expanding capacity to meet current and projected needs, while minimising total installation and long-term operating costs.
In addition, Coeur Alaska has an excellent reputation for environmental protection. With the safety and well-being of the land, waters and population in mind, Coeur Alaska is committed to operating an ultra-safe and environmentally sound mining operation. The newly expanded mine-water treatment plant is just one more example of that.
About the Authors
Chris Howell is global director of mining & primary metals for Veolia and Paul Hoeferlin is a senior sales engineer for Veolia Water Technologies.