Because most copper is found in microscopic amounts in rocks, it must be separated from the rest of the rock before it is usable. Since ancient times man has known that copper will separate from the other materials in a rock if the rock is heated enough. Copper's melting point is 1,981oF (1,083o C), and even wood fires get hot enough to do the job. Learning that copper would melt so that it could be used in many ways was a big step for man.
Understanding the effect of heat on copper-bearing rocks ushered in the Copper Age. Now, people were able to make a wide variety of useful objects including metal axes and swords. People who knew how to process copper and make tools from the metal had a distinct advantage over those who did not. However, there were still impurities in the copper that caused it to fail in performance at times. Through trial and error, man learned that by adding certain substances to the melting copper and by increasing the heat used to melt it, a stronger metal alloy called bronze or brass could be made.
Early smelting techniques were used for centuries. As man's understanding about the properties of metals grew, and the demand for these metals also increased, new and better ways to process copper were developed. Today, copper processing can be viewed as a simple procedure based on complex scientific knowledge. The two types of copper minerals begin the processing story.
Sulfide copper minerals are a combination of substances including the elements copper and sulfur. If enough copper minerals are present in rock to make it economically feasible to mine, process, and refine, it is called ore. To collect the copper from this ore, the sulfur and other substances must be removed. For many sulfide copper minerals this is accomplished ultimately with fire, so it is known as a pyrometallurgical process.
First, the ore containing copper sulfides must be blasted with explosives to break it into manageable pieces. Next it is passed through crushers to reduce it to rocks that average 8 inches in diameter. From the crusher, conveyors feed the ore to huge, horizontal cylinders called mills. The ore is put into the mills with either steel balls, long steel rods, or sometimes just the ore itself, along with liquid (mostly water). The mills rotate causing the ore, (and balls or rods), and liquid to crash against one another, breaking the ore into smaller pieces. The ore is milled until it is so small that, mixed with liquid, it looks like thin, watery mud. This is called slurry.
The slurry is now ready for the next step, flotation. Liquids called reagents (one to attract copper and another to make bubbles) are added to the slurry, which is placed into large oblong, metal containers. Air is introduced into the containers, and agitators mix the slurry causing bubbles to form. The copper and reagents are attracted to each other, and the copper (and other metals) clings to the bubbles. As more and more slurry is added, the lighter bubbles, with their load of copper, overflow the containers and are carried to big tanks called thickeners. Here as the bubbles pop, the heavy copper and other metals settle to the bottom of the tank while the lighter liquid that contains the water and reagents rests at the top of the tank and is drained away and recycled back into the flotation system. |
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The solid material at the bottom of the tank is then transported to filters where a vacuum removes most of the remaining liquid. The result is a mixture of copper and other metals called copper concentrate that is between 20% and 30% copper.
The material, called tailings, which is left in the flotation cell, has had most of the valuable metals removed. Tailings are drained from the flotation tanks and placed in thickeners where some of the liquid is drained away. The tailings are then spread over a large area called a tailings impoundment or tailings dam.
Copper concentrate still has impurities like iron and sulfur that must be removed, so it is sent to a smelter. Here, the concentrate is heated together with a flux (a silica rock). When it is hot enough, the iron separates from the concentrate as iron silica glass or slag and is skimmed off. The sulfur, released as sulfur dioxide gas, is captured and used to make sulfuric acid.
The molten copper is poured into molds and cast into huge slabs called anodes that can weigh as much as 750 lbs. each. Even though the anodes are 99% pure copper, they still contain too many impurities for the strict demands of manufacturers. Therefore, they are sent to a refinery to remove the impurities. At the refinery the copper anodes are placed between cathode starter sheets in tanks filled with a solution of copper sulfate and sulfuric acid. Electric current is passed through the solution, and copper from the positively charged anodes is deposited in pure form on the negatively charged starter sheets, which act as cathodes. The impurities that were in the anodes settle to the bottom of the tank and are later removed to recover any precious metals such as gold, silver, and platinum. In approximately 21 days the starter sheets have gathered enough copper to make cathodes that weigh up to 300 pounds and are 99.99% pure. The cathodes are taken from the tank and sent to rod plants and various other manufacturers to be used to make wire and cable, to be alloyed with other metals, to be used as copper sheet and bars, and to be manufactured into many products we need.
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