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Monday, July 4, 2011

The Chemistry of Gold Extraction: Part 1. Simple Methods

A flotation cell similar to that used in the agglomeration process for gold recovery
by Dhatfield



Due to the complexity of this subject we are going to have to present it in several parts.

There are as many ways of extracting gold from its ores as there are ores it all depends upon which method is the most commercially viable and efficient. If the ore is amenable all that may be necessary is to reduce it to fine particles and extract the gold by gravity separation in water like any other method like panning or using a sluice box. A more advanced system is by using a “shaker table” a device that is slightly tilted in two directions.  The gold bearing fines are added at the top corner of the table with the lighter materials being washed by water off the lower edge of the table leaving behind the gold to be recovered from the surface.  There are several other schemes that have been developed over the years for affecting this process including the widely used “spiral classifier.”

A gold panner using a gold pan the simplest form of gravity separation
Photo by Alan Souter 


Gold agglomeration is a technique that is used for the recovery of gold when it is impossible to use cyanide or mercury because of environmental concerns.  The use of a process called coal-gold agglomeration makes use of a slurry made from finely divided coal and oil that works on the principle that gold is a hydrophobic material that won’t be wetted by water but will be attracted to the slurry.  After the gold has combined with the slurry, the slurry is burned causing the gold to form larger particles that can be extracted using gravity methods using water.

In an actual test of this process gold ore was finely divided and mixed with powdered charcoal, oil and water then stirred briskly for an hour that formed agglomerates containing gold.  Once the gold had formed a suitable agglomerate a collector such as potassium amyl xanthate (PAX) was stirred with the agglomerate for an additional fifty minutes.  The gold bearing agglomerate and the residue gangue was then ashed in a muffle furnace and the gold recovered from the ash using aqua regia. 

Greater gold concentration was obtained by increasing the viscosity of the oil or the increased size of the coal particles.  The rate of gold recovery was also influenced by the rate of stirring.  Stopping before after the optimum amount of stirring decreased the amount of gold recovery. By recycling the agglomerate showed that the amount of gold recovered could be further increased resulting in lowering the costs of the process.

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