How Metals are Mined
The first step in mining is to break the rocks into pieces by carefully placing explosive charges into holes drilled in the rock. Blasting is carefully done by highly experienced technicians. This is an example of blasting at the Red Dog mine in northwest Alaska.
Blasting breaks up two kinds of rock. The rocks with enough valuable minerals - the ore - are hauled to a processing facility called a mill. In many cases the ore is hauled in large trucks, but in some mines the ore is crushed on site and hauled to the mill on large conveyor belts.
Ore processing - separating the valuable minerals (such as gold) from the rocks - takes place in the mill facilities, usually as close as possible to the mining site to reduce hauling costs.
While there are many ways to process different kinds of ore, generally the mill starts by grinding ore into a powder. This big machine, called a "Ball Mill" contains large steel balls. As the machine rotates steel balls smash and grind the ore into a powder. In the case of a gold mine, the ground-up ore is mixed with water to make a slurry that is pumped to flotation cells.
These are flotation cells - they are the machines that separate valuable minerals from other minerals that are in the ore. The flotation cells mix in reagents that make special bubbles. You may think of them as a special kind of soap bubble. The flotation cells have large paddles that agitate the slurry and help the bubbles form. As the bubbles float upward through the slurry, the valuable minerals attach to the bubble surfaces. The bubbles float off of the top of the cell taking the valuable minerals with them. This is a very effective process of extracting valuable minerals from the ore.
Valuable minerals concentrated by flotation cells need further processing in order to recover metals such as gold. In some cases, the next step in the process is to oxidize the ore with high pressure steam in a machine called an autoclave. You may think of it as a giant pressure cooker. This is a picture of the inside of an autoclave. Notice the figure of a worker at the far end. The emissions from autoclaves is tightly regulated and monitoring and controlling them is an important part of protecting air quality.
The metals in the material that comes out of the autoclave can be dissolved with chemical solutions and then separated from carbon in large tanks. Because these chemicals can be hazardous in some forms, they are handled carefully and within strict parameters specified in the mine's permitting process.
In the case of gold, electrical processes are used to precipitate the gold from the chemical solution. The chemicals are then recycled and used again.
Finally the precipitated gold is melted and poured into molds to make gold bars. This is the mine product.
Valuable metals are extracted using chemicals, electricity and heat
Gold is heavy but it does not take up much space. It can be shipped by airplanes or even snow machines if needed. Gold mines do not need special port facilities or large ships to transport the metal they produce. In this way, gold mines can be successfully run in a much wider variety of locations than many other metal mines which can only feasibly be built near ports or the road system.
Images Courtesy United States Geological Survey; Image source: Earth Science World Image Bank