Gold Processing at Boddington Mine

Gold processing plant at Boddington mine

The Boddington Gold Mine is a gold and copper mine the largest gold mine in the country, located about 130km south-east of Perth in Western Australia and 17 km northwest of Boddington

The Boddington deposits were previously mined between 1987-2001 for the production of 6.1 Moz of gold. The current mining operation is of significantly larger scale, producing 696 000 ounces of gold in 2014, along with 69 million pounds of copper.

Built on the footprint of the original Boddington gold mine, the operation involves open cut mining from two large pits. The process flow sheet at Newmont Boddington Gold (NBG) comprises primary crushing, closed circuit secondary and tertiary crushing using High Pressure Grinding Rolls (HPGR), ball milling and hydro-cyclone classification to generate a milled product with a P80 of 150 ┬Ám. The ground ore is then floated to produce a gold rich copper concentrate for filtration and sale to overseas smelters. Flotation tailings are then leached for further gold recovery. The ore processing rate is approximately 35 million tonnes per annum. Typical head grade is approximately 0.8 gram per tonne gold and 0.11 per cent copper.

The BGM is located within the Saddleback greenstone belt (SGB), a fault-bounded sliver of Archaean volcanic and shallow level intrusive rocks, surrounded by granitic and gneissic rocks.

The SGB produced more than 6Moz of gold and is a highly prospective exploration area for further gold mineralisation in both large tonnage stock-work gold resources and high-grade lode-type gold resources.

The Boddington deposit is not a single, amorphous zone of gold and copper mineralisation. It is essentially a complex mineral district, composed of geologically distinct, structurally variable, high-grade vein, lens and stockwork occurrences, set within an enormous halo of background, low-grade mineralisation.

In the bedrock the copper is present primarily as chalcopyrite with cubanite, bornite, chalcocite and small quantities of diginite. Of the remaining sulphide minerals, pyrrhotite and pyrite are the most common but the proportions of each vary throughout the resource. Small amounts of arsenopyrite are found in discrete siliceous shear zones.

Molybdenite is also present as a minor accessory. Native gold and electrum are the main gold occurrences and the median gold particle size is around 11 microns. Some gold is also associated with native bismuth and bismuthinite and the iron sulphides in the deposit (Dunne et al, 2007).

Gold Processing at Newmont Boddington Gold

Ore processing stage in gold processing plant at Boddington mine through the stages of  crushing . The Boddington primary crushing circuit comprises two 60” x 113” XHD primary gyratory crushers located at the edge of the mine with a nominal capacity of 3,670 t/h each at an OSS (open side setting) of 175mm. The crushed product (nominal P80 of 165mm) is conveyed 2.5km to the coarse ore stockpile (COS) located at the plant facility.   The COS has a live capacity of 40,000 tonnes and a total capacity of nearly 400,000 tonnes, when bulldozed to the side.

Coarse ore is reclaimed via any one of three reclaim feeders onto the secondary crusher feed conveyor. The secondary crushing circuit comprises six secondary crusher feed surge bins and six secondary (MP1000) cone crushers. Un-scalped ore drawn from the COS is fed to the secondary crushers which are closed circuit with four 3.6m by 7.3m single deck banana screens with an aperture of approximately 50mm. The secondary crusher closing screen undersize is sent to the tertiary crushing circuit.

Boddington primary crushing circuit
The tertiary crushing circuit comprises four feed bins and four 2.4m diameter by 1.65m wide HPGRs each fitted with two 2.8MW variable speed drives. The HPGR product is conveyed to and stored in four fine ore bins with a combined capacity of 20,000 tonnes . The four HPGRs operate in closed circuit with eight 3.66 m x 7.93 m wet screens (two per mill) with an aperture of 9mm. Wet screen oversize is returned to the HPGR circuit and the wet screen undersize reports to the grinding circuit.

Mineral ore is then inserted into SAG (Semi Autogenous Grinding) to make smaller.  mineral rocks . To make  very fine particles less than one-fifth of the diameter of one millimeter, the ore is put into the Ball Mills which look like huge steel drums rotating, adding water to do  to make the mineral ore become a pulp.

SAG (Semi Autogenous Grinding)
There are four parallel ball mills, each consisting of a 7.9 m x 13.4 m (26’ by 44’) ball mill with twin 8MW fixed speed motors in closed circuit with twelve 26 inch cyclones. Wet screen undersize gravitates to the mill discharge hopper where it is combined with the ball mill discharge and pumped to hydro-cyclones for classification. The cyclone underflow is reground to the final primary grind size of 80 per cent passing 150 microns.

Boddington has flash flotation facilities which can treat a portion of the cyclone underflow stream. The flash concentrate is treated through a gravity concentrator, with the gravity tailings sent to cleaner flotation and the concentrate treated by intensive cyanidation. The leach liquor is pumped to the electro-winning circuit while the “residue” is delivered to the concentrate thickener. This circuit is currently offline pending further economic analysis.

The milled product (cyclone overflow) is treated in a flotation circuit that produces a copper-gold concentrate for export. Treatment of the concentrate includes regrinding to a P80 of 25 microns and 3 stages of cleaning to achieve an acceptable grade. The final concentrate is thickened and filtered, then trucked to a port located in Bunbury, Western Australia. The concentrate grade typically ranges from 15 to 20 per cent copper and 60 to 100 grams per tonne gold and contains 75 to 85 percent of the copper and 50 to 60 per cent of the gold.

Process Froth Flotation
The cleaner scavenger tailings stream is thickened and leached in an elevated cyanide leach circuit, and the scavenger tailings are thickened and leached in a conventional leach/adsorption circuit. The leached slurry from the cleaner scavenger tailings leach circuit is delivered to the scavenger tailings circuit for further leaching and the recovery of gold onto activated carbon. The scavenger tailings leach circuit consists of two parallel trains of 12 leach tanks. Cyanide and oxygen are added to the slurry to leach the gold, and carbon is pumped in a counter-current direction to adsorb the leached gold. Activated carbon added to the scavenger tailings adsorption circuit is recovered and treated by conventional split-AARL (Anglo-American Research Laboratory) elution and reactivated in horizontal reactivation kilns. Gold recovery from the split-AARL pregnant electrolyte solution is by electro-winning, cathode sludge filtration, drying and smelting.

Process Carbon In pulp

Leach tailings (residue) is pumped to the residue disposal area (tailings dam) about 5km from the processing plant, and any residual weak acid dissociable cyanide present in the leach residue stream is maintained below a targeted level by a Caro’s acid cyanide destruction plant . The Boddington Process Plant is fully compliant with the International Cyanide Management Code and has recently undergone a re-certification audit.

Leach tailings (residue)