|The mill area is where Freeport breaks rock (ore) into a fine sand. Breaking the rock into sand allows the workers to separate the valuable copper and gold mineral grains from the rest of the common rock-forming minerals such as feldspar and quartz. The process of breaking rock into sand is called milling and the process of separating out the valuable mineral grains is called concentration.|
The mill is more than a thousand feet lower than the mine. The ore is transferred from the mine to the mill by dropping the rock down a big chute carved into the rock. Gravity does a lot of the work - imagine dropping rocks off a 100 story tall building!
|After the ore falls down the chute, it is shipped by underground conveyor belts to the mill. There are a few different kinds of mills, but they all work on the same basic principle. Most of the mills are large, hollow, steel cylinders tilted on their side like a big log floating on a river. They put ore rock inside the cylinders, along with some very hard, steel cannon balls, and then spin the cylinder. The cannon balls bounce around and smash the rock into sand.|
|These conveyor belts are stacking ground up rock for processing. Some of the pieces are fist sized because they've only been through the primary crushing process. Other piles are of finer-grained material.|
|The company then pours the ground-up rock sand into big vats that contain a special liquid that causes the valuable minerals to float to the top and spill out, like a beer that is poured too quickly spilling out over the lip of the glass. The sand made of valuable minerals is called concentrate and is transported down to the sea to be shipped to a smelter. The sand made of worthless common minerals is called tailings and is dumped into a river designated for slurrying the tailings to the flat lowlands.|
|This is a photograph of an underground haul truck. It's mainly used for moving small amounts of rock here and there. Note how it bends in the middle, allowing it to turn tight corners in narrow tunnels. In a smaller operation, miners might use haul trucks to move ore to the ore chute to the mill. The mine here, however, is much bigger scale and designed so that gravity breaks the rock and the rock naturally falls into the ore train (a process called block caving).|