Toxic Metal Reduction

NEUTRA-MILL WATER TEATMENT SYSTEM – CASE STUDY

TOXIC METAL REDUCTION – THE AGRICOLA GOLD MINE

• pH raised
• Access to the site is difficult
• Remote site with no infrastructure
• Aqueous copper concentrations lowered
• Treatment precipitates deposited in opencut
• Achieved discharge standards for release into a waterway within a National Park

LOCATION

South-eastern Queensland, Australia. 150 kilometres north-west of Brisbane (Latitude: 26° 18′, Longitude: 152° 36′).

SETTING AND CLIMATE

The Agricola Gold Mine is an abandoned site located in steep terrain amongst wet and dry sclerophyll forests and rare temperate rainforest of the Conondale Ranges National Park (left). It is set at an elevation of 610 m above sea level, and runoff from the site drains into the Mary River.

CLIENT

The Queensland Department of Mines and Energy and the Queensland Department of Environment.

BACKGROUND

Gold mineralisation occurred in association with broad pyritic lodes in schistose Palaeozoic metasediments and metavolcanics. Operators of the Agricola mine commenced opencut mining during 1987. They ran into financial difficulties in 1989 and abandoned the site. Their legacy included an open cyanide-bearing sulphidic tailings dam, a large opencut and a body of polluted water created by the unintentional damming of a local waterway.

The key reactive minerals within the waste rock and ore include primary pyrite and supergene copper carbonates (eg. malachite). Pyrite oxidation was responsible for acid generation, and dissolution of copper carbonates resulted in partial acid neutralisation and the mobilisation of copper into the water.

WORK PROGRAM

Earth Systems was commissioned to conduct the detoxification and discharge of pit waters from two acidic and copper-rich water bodies. This task involved:

• geochemical modelling,
• designing a work program for managing the polluted water bodies,
• developing and testing an effective treatment methodology,
• treating the pit waters,
• coordinating the disposal / entombment of treatment sludge,
• discharging the treated water to a local waterway within a National Park under strict discharge parameters, and
• monitoring the treatment and discharge process. The on-site portion of the program was conducted in collaboration with Golder Associates.

SITE ISSUES

Comprehensive minesite rehabilitation, including earthworks and revegetation, was dependent on demineralisation and discharge of acidic, copper-rich water stored in mining pits.

A series of biological monitoring stations (employing local crustaceans) was installed in the receiving creek to ensure minimal impact from the discharge process.

The mine site is in a relatively remote location with limited access via unsurfaced forestry tracks. No power was available at the site. Chemical treatment of the pit water was identified as the most cost-effective remedial strategy. A portable chemical dosing system with low power requirements was required. A portable Multi Purpose Neutra-Mill was an ideal solution for this acid water issue.

RESULTS

The results tabulated below indicate the changes in water chemistry achieved after combining bodies of acid water into a single opencut and treating with hydrated lime. Treatment with lime was completed in four days.

Treated water was discharged over land, and entered a 100m mixing zone in the local creek at a dilution ratio of 1:1. Chemical and biological monitoring was conducted throughout the discharge process, and no impacts were recorded at the biological monitoring stations.

Water treatment and discharge formed an integral part of successful rehabilitation of the Agricola minesite.