Demand for Gold
With gold prices hovering at around $950 an ounce, gold refining companies are in demand as jewelers and pawn shops continue to buy gold from cash-strapped customers.
In 2007, the U.S. Geological Survey reported that our nation exported 519 metric tons of gold; worldwide production was 2,380 metric tons, and has been dropping since 1997. The demand for scrap, including jewelry, dental crowns and electronics, is rising as gold producers search for new mines.
Refining separates and purifies the gold from other metals, which are combined with gold to make jewelry stronger.
Scrap Gold Popular
Old scrap constitutes up to 18 percent of this country's gold supply.
Gold producers have some scrap, but because of tight controls over waste materials in precious metals plants, almost all of this scrap can be recovered.
In the U.S., about two-thirds of the scrap comes from manufacturing, and the rest comes from old scrap, including discarded jewelry, dental materials, used plating solutions and electronics.
The Department of Defense also recovers large quantities of gold from military scrap.
Refiners in New York, Rhode Island, California, Florida and Texas purify most scrap, but sometimes scrap dealers process it and ship the upgraded product to fabricators and refiners for more treatment.
Smelting Separates Metals
The Federal Trade Commission requires gold refiners to know the karat count and the chemical analysis of the alloys they buy. Refiners must separate the scrap to meet karat standards.
Refiners use smelting to separate precious metal from nonmetallic impurities.
Midwest Refineries, LLC, in Waterford, Michigan, owns an assay laboratory to test gold for purity.
Fire assay is the oldest and one of the most accurate methods of gold testing; the gold is melted, and a sample is removed. This sample is weighed and recorded, then wrapped in assay lead foil with some pure silver. The wrapped ball is placed in a furnace in a disposable metal container, or cupel, which absorbs all nonprecious metals. The gold forms a button inside the cupel.
Once the worker takes the cupel out of the furnace, he brushes the button to remove any leftover cupel bits. The button is hammered flat, rolled thin and heated in a porcelain container with a weak nitric acid solution that removes the silver, which is poured off and kept. Workers rinse the gold in distilled water and dried. This gold is at least 99.999 percent pure, or 24 karats.
Pure gold is so soft that 1 ounce of it can be pounded out to 300 square feet.
After the gold is re-weighed, the weight of the impure sample is divided into the weight of the now-pure sample.
Jewelry supply companies sell an electronic tester that will reveal the karat value of gold. These testers aren't nearly as accurate as a fire assay, but can show what an unknown metal is or isn't.
Initial Refining
When gold ore is extracted from a mine, it's refined using electrolysis, which involves chloride or cyanide solutions, according to Peter S. Shor of New York, who filed a U.S. patent for a process for refining and purifying gold. The gold recovered from cathodes, or negatively charged electrodes, such as those in batteries, is often deposited on foils.
Once removed from the cathodes, the gold is melted and cast into gold ingots, or metal casts. At this point, the gold is about 96 percent pure. It contains trace elements, such as copper, silver, arsenic and nickel.
How Mints Refine
Gold mints usually electrolyze the crude ingots as gold anodes in hot, acidic solutions, composed of about 8 percent gold chloride and 10 percent hydrochloric acid. The refined gold is electrodeposited onto foil or rolled-sheet, gold cathodes. This gold is mint grade, or 98.5 percent pure.
The electrolysis cells used at mints and assay offices are usually built with glazed porcelain or chemical stoneware. During mint electrolysis, platinum and palladium, another rare metal, stay in a solution inside the electrolyte.
Nylon bags surround the anodes to remove some heavy silver chloride and lighter scum, or dross, mostly mixed silicates, borates and arsenic and antimony particles. Antimony is a brittle, lustrous white metallic element used in alloys and medical compounds.
The dross can be skimmed off or separated before removing the silver.
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