The haul trucks carry the ore out of the pit along a haulage road with a slope of no more than about nine percent. They dump the ore into a gyratory crusher which reduces the ore to eight inches or less — about the size of soccer balls. The primary crusher may send the crushed ore on to a secondary crusher or pile it directly onto the coarse ore stockpile.TOUR NOTE:
On your tour from the Mineral Discovery Center, you may get to see the haul trucks dumping ore into the crusher.
The ore is ground into a fine powder by large rotating mills. The two types used in the Mission South Mill are called SAG (semi-autogenous grinding) mills and ball mills.
SAG mills use larger pieces of ore to break up the smaller pieces (autogenous — does it by itself). The larger pieces break down as well. To help the process along, eight-inch-diameter steel balls are added to the rocks as they tumble inside the rotating mill (semi-autogenous — gets some help from the steel balls). The two SAG mills in the Mission South Mill each have two 3,000 horsepower electric motors. They can rotate in either direction which helps even out the wear on the steel liners inside the mill.
When the rocks are about 3/8-inch or smaller, they are fed as a slurry into the two ball mills. Each ball mill is turned by a single 3,000 horsepower electric motor. These mills contain literally hundreds of thousands of three-inch diameter steel balls that pulverize the ore until it is like fine sand or face powder. Only then are the copper minerals broken free of the rest of the rock to be separated by flotation.
The slurry of water and pulverized ore is mixed with milk of lime to raise the pH and small amounts of special reagents: a frother to make bubbles, and a collector chemical that causes the copper minerals to stick to those bubbles.
Air is blown into the tank and the mixture is vigorously agitated like a high-speed blender. Rising bubbles carry the copper minerals up and over the edge of the flotation tank. The bubbles break soon after they flow over the edge. The copper minerals are then ground up even finer and purified by another flotation process.
The dried copper concentrate of about 28 percent copper is shipped to the smelter. It represents less than one percent of the material removed from the mine. Concentrate is just a fine powder of the mineral chalcopyrite which is a naturally occurring compound of copper, iron, and sulfur.
The material that sinks in the first flotation cell goes on to two more flotation cells to recover as much copper as possible. What doesn’t float is called tailings because it goes out the “tail end” of the flotation circuit. About 80 percent of the water used in the milling process is reclaimed and re-used. The rest is used to keep the tailings damp and to prevent wind-blown dust.