Robert Zeisler, age 8, of Lansing, Michigan, for his question:
How do they get gold from ore?
Gold is just about the prettiest metal in the world and people have been collecting it for ages. But there is not very much of it in the earth and there was never enough for everybody to have all they wanted. So gold became The Most Precious Metal ¬and people tried their hardest to get all they could. Many were carried away with gold fever and forgot all the other good things in the world. In time, most people got over the feverish yen for gold. Nowadays, we mine the precious metal for more sensible reasons.
The earth has more silver than gold and a lot more shiny nickel. Some of the gold is in lumps called nuggets, maybe strewn on or in the ground. Some is tucked into rocky pockets or washed along in gravelly streams. Some of it; is in streaky veins, hidden inside slabs of solid quartz and other minerals. More gold comes in flakes and tiny specks mixed in ores with a lot of other minerals and metals. But most of the earth's gold is mixed with sea water in fragments too small to be seen.
Most of the solid nuggets were found by gold hunters long ago. Nowadays, we get most of our gold from those fine fragments mixed in mineral ores. The problem is to separate the flakes, flecks and specks of gold from all the other ingredients in the ores. As a rule, this is done by a process called milling and there are three effective ways to do it. Usually the milling is done right there at the mine. But sometimes the rocky ore is chomped into chunks and sent to a mill somewhere else.
The milling starts by grinding the ore to fine powder. This is mixed with water and various chemicals. One milling.process is called flotation because it floats the specks of gold to the top of a bubbly brew. A certain chemical coaxes the gold fragments to stick onto the bubbles and another chemical stops other ingredients from sticking with the gold. Air is squirted into the mixture to make the bubbles float to the top. When this oily foam is skimmed off, the f ragmdnts of gold come with it.
Another milling process is called amalgamation. It works because gold sticks to silvery mercury. First the soupy mixture of ore and water is washed through tiny traps. This separates some of the heavy gold from the lighter ingredients. The remaining mixture is washed over copper plates coated with mercury. The gold clings to the mercury. When these two metals are heated, the mercury evaporates and the pure gold stays behind.
A process called cyanidation depends on the chemical cyanide and the metal zinc. The powdered ore goes into a tank of water mixed with a little cyanide. The heavy gold tends to sink down. Silvery white zinc coaxes the sunken treasure to separate from the other ingredients.
Most gold ores contain quartz, calcite and other fairly useful minerals. But many also contain other valuable metals besides gold. Often the same ore has lots of copper or lead. Sometimes an ore is mined to extract the copper and the smaller amounts of gold add a little something extra.