Reduction in Treatment Costs

NEUTRA-MILL WATER TEATMENT SYSTEM – CASE STUDY

ACID ROCK DRAINAGE – GOLD MINE, NEVADA, U.S.A.

• pH raised
• Acidity reduced
• Significant potential for reagent cost savings
• Treatment precipitates deposited in tailings dam

LOCATION

South-western U.S.A.. Near Carlin, Nevada (Latitude: 40°42N’, Longitude: 116° 06’W).

SETTING AND CLIMATE

The minesite is located in the grassy, undulating hills of north central Nevada. The area has low rainfall with precipitation occurring mainly in winter as snow.

Geothermal springs are not uncommon throughout the area. The Carlin area is known for epithermal ore deposits including the large Carlin North and South mines.

CLIENT

Gold Mining Group, Nevada

BACKGROUND

Surface mining of the opencut area at this site has ceased with limited underground mining still continuing. Mining at this site had been conducted for 6-8 years previously.

Gold is still being extracted via heap leach with activities onsite continuing with minimal staff. Rehabilitation works have been conducted over much of the site including revegetation of the waste rock pile.

SITE ISSUES

Acidic water at the site is stored within a 340 million litre (90 million gallon) tailings dam. The main source of acid water at the site is from collection ponds at the toe of the waste rock pile. Springs located within the waste rock pile produce acid water all year round. Water flow from the waste rock pile increases significantly with the onset of the Spring thaw.

Water pH in the tailings dam is managed at the site through the addition of 50% sodium hydroxide solution at an average rate of approximately 40,000 litres per month. ‘Snow makers’ are used in the drier months to enhance evaporation and increase available storage volumes. Limited amounts of tailings dam water are re-used within the process plant.

WORK PROGRAM

Site environmental personnel had previously been introduced to the Neutra-Mill Technology and were interested in its potential to reduce reagent costs. A test case was established to determine the efficiency of reagent and potential cost benefits of using a Multi Purpose Neutra-Mill and hydrated lime for treating acidic water. The trial was conducted over 28 days with water quality monitoring conducted by Shepherd Miller Inc., a US distributor of the Neutra-Mill.

CONDUCT OF TREATMENT TRIALS

A portable Multi Purpose Neutra-Mill was deployed directly into the tailings dam without requiring any site modifications and electrical power was supplied via a single phase generator. Approximately 1.5 tonnes of hydrated lime was dispensed daily into the tailings dam via the Neutra-Mill. The hydrated lime was manually loaded by two personnel. This required about one hour per day of their time. An automated storage and delivery system would be utilised in longer term treatment operations.

TREATMENT RESULTS

The key results from the trial are:

• 36.5 tonnes of hydrated lime {Ca(OH)2} were added via the Multi Purpose Neutra-Mill (49.3 tonnes CaCO3 equivalent),
• Total acidity over the trial was reduced by 41.7 tonnes CaCO3 equivalent,
• Sludge paste pH value of 6.5 indicated absence of unreacted hydrated lime in sludge, and
• pH value of 12.5 in suspension around the Multi Purpose Neutra-Mill, indicated saturation with respect to hydrated lime.

Based on the results the Neutra-Mill achieved an efficiency of 85% of the available alkalinity of the hydrated lime, with potential for further improvement.

BOTTOM LINE

Significant cost savings are achievable by using the Neutra-Mill for routine management of acid drainage on the site. Key cost benefit analysis figures are:

• Reduction in annual reagent costs by using hydrated lime instead of sodium hydroxide solution – approximately US$400,000.
• Capital costs for an automated Multi Purpose Neutra-Mill dosing system with storage silo – approximately US$70,000 (paid for by reduction in reagent costs over first quarter).
• Potential further reduction in reagent costs by substituting quicklime (CaO) for hydrated lime – another US$40,000 annually.

The Multi Purpose Neutra-Mill shown in the pictures is easily transported between sites using a custom built trailer, and can therefore be used at several sites saving further capital outlay for treatment facilities.

The Multi Purpose Neutra-Mill was manually loaded in this short term trial.

Close up of the suspension plume created by the Multi Purpose Neutra-Mill.