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How to Recover Refractory Ores

Refractory ores are unoxidized, primary ores of valuable minerals such as gold or silver that are mixed with other minerals such as sulfides and selenides. These other mineral constituents make the ore resistant to traditional mining methods such as cyanide leaching. Special processes, such as roasting, bio-oxidization, pressure oxidization or fine grinding, must be use to recover the valuable portions of the ores, most of which are expensive, environmentally unfriendly and typically not cost-effective. This article describes the process for recovering refractory ores through roasting, one of the most economical ways to recovering refractory ores because it works with sulfides, the most common associated mineral type.

Things You'll Need

External heating source
Refractory ore samples
Sifting Tray

Instructions

- 1
  
  Pyrite is typically associated with refractory ores.
  
  Identify mixtures of valuable ores with other minerals that are resistant to industry-standard leaching processes. The most common refractory minerals are the sulfides, including pyrite, galena, sphalerite and chalcopyrite. Most of these have a metallic luster and contain a large amount of sulfur. They are dense and heavy so they can be separated from mine tailings piles using sifting trays and vibrating tables.
- 2
  Expose the ores to extremely hot air of approximately 1,000 degrees Fahrenheit using a roasting oven. This process is generally applied to sulfide minerals with high amounts of organic material. Oxygen introduced to the system burns the organic carbon creating a fuel that burns the sulfur, which is the resistant mineral, off of the valuable ore. During roasting, the sulfide is converted to an oxide, and sulfur is released as sulfur dioxide in gas form.
- 3
  Capture the sulfur dioxide gas to employ as a byproduct. Sulfur dioxide is the base compound of sulfuric acid, which is used in car batteries and oil processing, among its many various uses. Once the sulfur has been removed from the mineral concentrate, the gold or silver is able to be extracted by traditional methods such as cyanide leaching.
- 4
  Use a sodium cyanide solution to dissolve the gold. The gold will be suspended in solution and can be recovered in two basic ways: the Merrill-Crowe zinc precipitation process or the adsorption of the gold onto activated carbon. The Merrill-Crowe process has been used the longest and works by removing the oxygen from the solution, then mixing a fine zinc powder with it. The gold is recovered as a very fine gold precipitate on a precoat filter. Adsorption of gold is very much the same as the Merrill-Crowe method. The gold molecules adhere as dissolved solids to a surface of carbon, rather than a filter.

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