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Operations

Mining – After all the permits have been approved and are in place, facilities are constructed and mining can begin.

Tobie Mining operations incorporate typical modern surface mining methods. Mine engineers define the mine methods and operations based upon the site to be mined. To begin mining, any vegetation and the topsoil is removed. The topsoil is stockpiled in an area away from the mine to be used in reclamation of the mined area. When the site preparation is complete, the rock is drilled. The drill holes provide cuttings to be tested for ore content. The tests completed on the drill holes identify rock that contains gold in amounts that can be economically recovered, ore, and rock that is not mineralized, called overburden.

The drill holes are then loaded for blasting to break up the rock into size that can be moved. At the Tobie Mine, common ammonium nitrate and fuel mixture is used for blasting because the mixture is not explosive until detonated with a booster.

Rock is hauled at the Tobie Mine using 300-ton capacity haul trucks. The trucks are loaded with hydraulic shovels. The bucket of the largest shovel has a 40 cubic yard capacity. Overburden is hauled to storage areas, ore is hauled to a two-stage crusher that produces coarse gravel measuring less than three-quarters of an inch at a rate of 3,000 tons per hour. Approximately 60,000,000 tons of material is moved annually.

Overburden is placed in areas where no mining, or no more mining, is planned. The overburden storage areas are constructed considering geotechnical stability analyses, hydrologic designs for storm runoff, water quality assessments for infiltrating water, and the specific plans for grading and reclamation-revegetation. CC&V reclaims areas as soon as the activity has been completed. Some of the overburden storage areas have been the first areas to be reclaimed to achieve the post-mining land uses of livestock and wildlife grazing and habitat.


Crushing - Ore is crushed because the leaching process recovers mostly surface gold, and crushing maximizes the surface area of the ore for greater gold recovery.

The crushing circuit is designed to process almost 3,000 tons of ore per hour, approximately 50 tons per minute. Ore is loaded into the primary crusher for initial sizing. This gyratory crusher breaks the rock into 6-inch diameter pieces.

Conveyor belts take the broken ore to the screening plant, which runs the ore through vibrating screens to size the material. The optimum size is ¾ inch diameter pieces. Oversized rock is sent via conveyor to the secondary crushers. Secondary crushing consists of three cone-crushing circuits.

When crushed to ¾ inch diameter or smaller, lime is added to the ore to increase the effectiveness of the process solution by raising the pH. The ore is then conveyed to the load out bin. Trucks drive under the load out bin to be filled with crushed ore for delivery to the valley leach facility.


Leaching - Gold is removed from the crushed ore using the same basic process used throughout the world. The naturally occurring metals, including gold and silver, that are exposed on the broken faces of the crushed ore are dissolved by a dilute sodium cyanide process solution, a process called leaching.

At the Tobie Mine, the leaching of the gold is accomplished out-of-doors - an area with clay and plastic liners upon which the crushed ore is placed for gold removal. It can be thought of as a bathtub without a drain outlet, the sides and bottom of which are an impermeable double and triple liner system.

The crushed ore is placed in approximately 35-foot layers and a dilute solution of sodium cyanide, 100 parts per million, is applied using buried agricultural-type drip irrigation tubes to minimize evaporation. As the solution soaks through the ore, the process solution dissolves the gold and silver on the surface of the ore. The solution is captured at the lowest point of the bathtub drain, and pumped into the recovery facility. The solution containing gold is called “pregnant” solution.

The Tobie Project is operated with no external ponds for leach solution. Rather, the solution is contained within the pore space of the ore until pumped out much as ground water is pumped from porous bedrock. Because the VLF is a zero discharge, closed system, a significant excess capacity to accommodate large precipitation events is built into the VLF. A comprehensive water monitoring system is necessary to maintain the water balance in the pad to function as a zero discharge facility.



Recovery - The process solution with the dissolved gold and silver (i.e. pregnant solution) is pumped into the adsorption, desorption, and recovery (ADR) facility for recovery of the gold and silver.

The solution is pumped through tanks containing activated carbon (roasted coconut shell) granules which attract, or adsorb, the dissolved gold-cyanide complex. The process solution, now with no gold or "barren", is re-circulated to the VLF to start the leaching cycle over. Process solution is pumped into and out of the ADR at a rate of 13,500 gallons per minute.

Gold on the carbon is removed, or stripped, using a hot alkaline solution. The gold-rich solution is the piped to an electro-winning cell where direct electrical current is applied to attract metals from the solution to a stainless steel wool cathode, forming a solid, called mud, of gold, silver and impurities. The mud is sent to the refinery furnace and heated to separate the gold and silver from any non-metal substances. The resulting 98% gold-silver mixture, or 16-18 karat gold, is called dore’. The dore’ is shipped to a refinery to be processed into 99.9% pure gold, or 24 karat.

This process recovers about 70% of the gold contained in the ore. Because the leaching process dissolves only the gold and silver on the surface of the rock, some metals remain. To recover all the metals the rock would need to be crushed so finely that the process solution would not flow through it.

Activated carbon needs to be durable to successfully resist breakdown from the large volumes of water passed through it. Once the carbon has been reused a number of times, it is dewatered and sent to a metals recovery facility to recover any remaining gold and other metals.


The ore on the VLF is not removed once the gold has been recovered from its surface. More ore is stacked on top of the leached rock in successive 35-foot lifts. When leaching is complete with one section of the VLF, the pipes carrying the solution are removed and another layer of ore is stacked on top for the next level. The main solution pipes are reinstalled, and process solution is pumped onto the new

 

   
 
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