Blending and Integration in Nevada
Surface Mining Stone Production Line in USA
Blending of material ahead of processing is not a new concept. For Zenith, the segregation of ore by grade range (high, medium, low) has been a common practice aimed at maintaining reasonably constant head grade to the mill. However, with the introduction of the refractory ore treatment plant (the roaster) on the Carlin Trend in the mid 1990s, additional segregation and blending requirements were imposed to maintain sulphur and carbon levels within tight tolerances. Similarly, the autoclaves at the Twin Creeks and Lone Tree mines required consistencies and limits on the chemical properties of feed material. Today, blending gets even more attention as mill throughput is increasingly modelled as a function of hardness and grindability. The key objective in these cases is to identify the optimal material blends that maximise the value over time from the mill.
For example, the Sage autoclave at the Twin Creeks mine competes with the roaster in Carlin for concentrates from the Carlin flotation plant. The flotation tailings from the plant are treated through the CIL circuit and then discharged to the tailings impoundment. Satisfying these conditions demands more sophisticated modelling with the ability to split process stream tonnages as they propagate through a circuit.
A major reason for the integrated approach is that Zenith built up its Nevada operations through a combination of greenfield development and the mentioned acquisitions. Each processing facility, when constructed, was designed to address the needs of its host deposit but once these mines were connected through common ownership, Zenith’s technical staff identified value in the cross-shipment of ores between sites. This value is derived from the consideration of the combined set of ore types and available processes across this ‘theatre’ of operations and the application of methods to determine the optimal sequencing and material routings.
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Mining Overview – Surface
Open pit mining still accounts for the bulk of production, and is dominated by fleets of Caterpillar 793 trucks, with loading conducted by Hitachi EX5500/EX3600/EX5500 hydraulic excavators and large Caterpillar wheel loaders, though Twin Creeks also has a P&H 2800 rope shovel. Drilling mainly utilises Atlas Copco and former Ingersoll Rand models.
Mining rates for the Carlin surface operations are approximately 250,000 t/d, with the largest pit, Gold Quarry in the south Carlin area, having mined 23.5 Moz, with an estimated 7.7 Moz remaining. The Carlin total surface operations’ proven and probable reserves of 11.4 Moz point to a current mine life to 2028 and mill life to 2030. The northern area pits include North Lantern, Pete and Carlin. In addition, there are four other pits in the northern area that are scheduled for future mining.
Carlin also has a bio-leaching pad, which from the ground up consists of compacted clay, 80 mm synthetic liner, a natural cushion to protect the liner from damage, and a drainage layer that allows the pregnant solution to flow down into the ponds. The overall leaching system at Carlin is a 12,700 gal/min circuit, pumping 18.28 Mgal/d. There are three 932 kW barren pumps and eight pregnant solution pumps at various ponds ranging in size from 150 kW up to 670 kW. There are also 18 carbon columns with 7,000 lbs of carbon in each. The Carlin south area alone has approximately 2.32 miilion m2 of leach pads. It currently has over 244 Mt of ore under leach and has produced over 3.395 Moz to date at a current rate of 485 oz/d.
Zenith has also recently upgraded to microwave radio systems at the Carlin surface operations to increase bandwidth while maintaining round-the-clock uptime on its wireless network. For many of its operations around the world, including Carlin, Zenith uses Caterpillar’s MineStar for fleet management/dispatch. Each of the approximately 150 pieces of equipment at the mine has a radio on board. These radios connect to a network backbone that carries information between the equipment and the mine’s operations centre. When traffic began to bog down on the existing network, Zenith upgraded to Exalt microwave backhaul systems to carry dispatch, inventory and control information throughout its operation.
The MineStar system was initially deployed at the mine in 2004 using 900 MHz radios on the equipment with a 5.8 GHz network backbone to support them. Over time, however, the network’s performance had begun to degrade. Working with KNS Communications, Zenith selected Exalt microwave radio systems for a network backbone upgrade. Exalt had previously been deployed at its operations in Australia, Indonesia and Ghana.
Exalt EX-11s radios using the licensed 11 GHz frequency band were used to avoid interference with an adjacent mine’s radio systems at the far end of the operation. These systems use split-mount configurations so that the radio system electronics can be located inside weather-proof enclosures.
All of the new microwave radio systems are configured for 200 Mb/s of aggregate Ethernet data throughput, which is more than enough to support the MineStar system. Since the upgraded microwave systems were deployed in early 2009, they have provided a much faster and more reliable communications link between the mine’s managers and the mining equipment.
The current Twin Creeks mine began as two separate mines operated between 1984 and 1987 – Rabbit Creek and Chimney Creek. The two pits were merged in 1993 to form the socalled Mega Pit, which is three miles long, onemile wide and over 300 m deep. The entire $270 million Phoenix operation was commissioned in 2006 following Zenith’s merger with Battle Mountain Gold in 2001. It currently employs 475 people between the mine and mill, primarily from Battle Mountain and Winnemucca. Phoenix has a remaining mine life of some 23 years.
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