Whitehorse Copper

Once an ore deposit has been defined during advanced-stage exploration, a different team of professionals (including environmental scientists, economists, engineers, etc.) will look at the financial and technical feasibility of mine development, plus its environmental and socio-economic impacts. A project that is still prospective given those considerations will be submitted to government permitting bodies, and only after extensive review and consultation may a project be approved for mine construction. The construction phase itself will often take many years!

Mining can take place either as surface (open-pit) mining or underground mining. Open-pit mines are often developed where the orebody is large, low-grade, and relatively close to the surface. These are often called "bulk tonnage" targets. Underground mining is significantly more expensive than surface mining, and is generally restricted to cases where the ore body is high-grade, deep, and/or mineralization is in discrete lenses or veins.

Most of the ore production at Whitehorse Copper came from underground mining. An underground portal (access point) led to a decline or ramp that gradually descended to the main Little Chief ore body, directly under the Little Chief open pit. While miners and equipment typically used the decline for access, ore was removed using a hoist system in a vertical excavation known as a shaft.

Mining engineers will design the underground workings of a mine to maximize the amount of ore that can be recovered, while still leaving enough rock in place to prevent collapse. The zones where rock is extracted are known as stopes, and the support zones are known as pillars. The mine design is continually refined as underground access enables production geologists and mining engineers to get a better understanding of ore distribution and rock stability.

To extract rock from a stope, underground drill rigs drill a series of holes on the rock face. The layout of these holes, and their depth, is optimized to achieve a certain size of fragmented material after the blast, and minimize the damage to the surrounding rock. Blast holes are loaded with explosives, the area is vacated, and then a complex blasting sequence is initiated. Not all charges detonate at once - the order and configuration are also important to optimize the blast.

Once a blast is complete, underground miners will stabilize the area with required rock support. The blasted rock (or muck) is then scooped up from the muck pile and transported either 1) directly to the surface, 2) to a waiting underground haul truck, or 3) to an underground jaw crusher to further reduce the size of the rock fragments. Typically, this initial movement of rock material is done by a loader known as a load-haul dump (or LHD).

The first step in ore processing is crushing. In an underground mine, ore is first transported via haul truck or LHD to a piece of equipment called a grizzly. Ore is dumped onto the grizzly screen, and pieces of ore that are smaller than the aperture size on the screen pass through. Pieces that are larger than the screen aperture are caught in the screen and need to be manually broken. The goal of blast design is to minimize the volume of rock caught in the grizzly.

Material that passes through the grizzly goes to the primary crusher to reduce the grain size of the material, typically to less than 15 cm in diameter. For surface mines, these primary crushers are typically  gyratory crushers , but for underground mines an underground  jaw crusher  may first be employed. Such was the case at Whitehorse Copper beginning in 1974. The ore was then transported to the surface using equipment known as a skip hoist. Once on the surface, it was conveyed to a gyratory crusher for additional grain size reduction.

In a mine like Whitehorse Copper, crushed ore is conveyed to a mill building (concentrator), where it is first subjected to additional grain size reduction in a grinding circuit. A grinding circuit typically involves 1-2 types of grinding mills, huge metal drums partly filled with metal balls that grind the rock down to a flour-like size (typically less than 0.2 mm in diameter). Semi-autogenous (SAG) mills are typically used first, and have 5-20% steel ball content by volume. These mills rely largely on the impact of rock pieces themselves on surrounding rock to pulverize material. These are followed by ball mills, which have a higher percentage of steel balls (up to 40%) and achieve a finer target grain size.

Ground material is transported to flotation tanks for separation of the valuable ore minerals from the unwanted waste materials (gangue minerals or tails). The flotation tanks are aerated, with bubbles rising to the top of the tanks. Two types of chemicals (or reagents) are added to the slurry of water and ground rock: 1) frothers help stimulate bubble production, and 2) collectors make target minerals aerophilic (or hydrophobic) and preferentially attach themselves to the bubbles. The bubble-rich froth at the top of the tank can be skimmed off - it is highly concentrated in ore minerals.

Following flotation, the concentrate slurry (or pulp) is thickened in a thickening tank. Additional chemicals are added that cause the minerals to settle to the bottom (sedimentation), leaving a clarified H ₂ O-rich fluid on the top that can be decanted and reused. The thickened pulp can now be dried and dewatered.

This mill product, known as the concentrate, is packaged and transported offsite. The milling on-site only separates ore minerals from gangue; it doesn't isolate the individual metals (e.g. copper) within those minerals. That is the role of smelting. Whitehorse Copper concentrate was transported on the White Pass & Yukon Railway to Skagway, Alaska, and then shipped south to the Port of Vancouver, British Columbia. It was then transported by rail to a smelter in Flin Flon, Manitoba, where the copper was extracted and refined.