Amalgamation

Amalgamation is one of the oldest methods of extracting gold from its ores. The amalgamation of gold possibly dates from Roman times. The extraction of silver by amalgamation is generally attributed to the Spanish in Latin America in the late sixteenth century even though Biringuccio describes the fundamental principles of the “patio process” about a half a century earlier.The amalgamation of gold ores as described by Agricola in De Re Metallica is essentially the same as modern practice. The only changes are in the equipment employed and the sources of power to grind, mix, and separate. Probably, the chemical and physical processes are more clearly understood.Mercury is a heavy metal with specific gravity of 13.5, is liquid and boils at 357 oC. Its vapor pressure is low and can be in vapor state easily. Amalgam is an alloy between gold and mercury. There are three types of amalgams to be formed, AuHg2, Au2hg and Au3Hg. The content of mercury in the amalgams depends on two factors, gold feed grade and squeezing efficiency. In this way, a regular amalgam contains 60-65% of gold, but an amalgam treated by filtration can contain 80-85% of gold.

Basically, the amalgamation process comprises the following stages, crushing and grinding the ore until get a K80 of approximately 35 mesh. Finer values work and the final particle size is dependent on the mineralogy. The next stage is to pass the ore in the form of a thin pulp over a mercury covered copper or silver coated plate. The third stage is to remove the gold-mercury amalgam at regular intervals and re-dressing the plates with new mercury. Finally, it will necessary to eliminate the mercury by distilling the gold amalgam (retorting), thereby producing nearly pure gold and recovering the mercury for reuse on the plates. Many years ago, particle size reduction was done by gravity stamps and years later, was common to employ crushers such as jaw or cone crushers, and ball mills to reduce to whatever mesh is desired.

Ores can be treated successfully by amalgamation are called free milling ores. It is practically never possible to obtain high extraction of gold by amalgamation alone. This is du two reasons, fine gold that fails to settle upon and make contact with the plates, and a type of gold occurrence in which gold particles are brown lusterless. This may be due to coatings of oxide of iron, manganese or tellurium. Free milling ores seldom produce more than 70% recovery. This is the explanation of the existence of tailings piles about some countries assaying interesting gold contents. Many of them have been retreated by cyanidation and/or flotation.Amalgamation is used to recover relatively coarse gold and for this reason has been very important in gold recovery systems. Cyanide acts too slowly on coarse gold, but readily dissolves fine gold. Flotation concentration is not effective on coarse gold. The small miners who wish to treat a small tonnage of high grade ore and want a quick return rather than for recoveries near to 100%, probably still find the amalgamation process the best available recovery method. The final product grade depends on the gold ore treated. This means that high head grades will allow getting a final product with high gold content. As example, in places where there are important contents of silver, the final product can content 75 to 85% of gold.

Amalgamation is a very efficient process for recovering coarse free gold, but there is problem related to the hazards of working with mercury. Like most things in our environment it is safe enough if it is treated with due respect and reasonable precautions are observed. However, the effects of mercuric poisoning are cumulative and it can do considerable damage. Mercury can be absorbed through the skin or inhaled in vapor form. Gloves should be worn whenever it is handled. Be sure that you are in a well ventilated area if there is any possibility of vapors forming. Avoid breathing any of the vapor at all costs.

Schematic view of a retort

South America is place where the amalgamation process has been practiced for a long time. Basically the Jungle has been traditionally the favorite. Small miners employ this process due to alluvial deposits contain free gold, the process is fast and efficient, the final product has a high gold grade and recoveries are between 80 to 95%, commercialization is fast and simple without brokers, the process is simple and is not necessary a big investment or very special technical knowledge, the mercury can be reused and its losses are minimized, the final product weight is small and can be transported easily from the process plant to the commercialization place, the incomes can be distributed easily without much trouble, and the key reagent is mercury and can be obtained in many places without special authorizations.

There are some disadvantages with this process. Mercury and the final product are toxic and contaminant, the process produces environment problems due to earth and water receive the tailings, when is performed the gold-mercury separation by retorting the air and the operator receives gases containing mercury. With the time the retorting process evolved, but they are not totally efficient and the contamination problem is only solved partially. Other disadvantage is the lack of mechanical equipment and consequently the operator has to employ his hands. In the jungle is common to find other values valuable minerals (e.g. rutile, zirconium) that are not recovered by amalgamation.

If the amalgamation feed can be a gravimetric concentrate, the process will be more efficient and the material to be handled will be reduced and consequently gold losses. In order words the process must work as a secondary process. The main parameters that affect the amalgamation process are lack of liberation, gold surface contamination, and mercury degraded.

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