Carbon in pulp

Leached pulp and carbon are transferred in a countercurrent flow arrangement involving a series of tanks, usually numbering 4 to 6. In the final tank, fresh or barren carbon is put in contact with low grade or tailings solution. At this tank the fresh carbon has a high affinity for gold and can remove trace amounts of gold (to levels below 0.01 mg/L Au in solution). As it moves up the train, the carbon loads to higher and higher concentrations of gold, as it comes in contact with higher grade solutions. Typically concentrations as high as 4000 to 8000 grams of gold per tonne of carbon (g/t Au) can be achieved on the final loaded carbon, as it comes in contact with freshly leached ore and pregnant leach solution (PLS). ==Removal of gold from carbon==

The final loaded carbon is removed from the machinery and extracted with a hot alkaline solution of cyanide. The elute solution passes through an electrowinning cell where the gold metal is deposited. The solution then passes back through the loaded carbon, extracting more gold and other metals. This process continues until the carbon has been stripped of its metals. The cathodes (wire wool, now plated with gold and other metals) are removed and placed in sulfuric, hydrochloric, or nitric acid. The acid burns off the wire wool and other metals such as copper, and leaves a sediment of gold and a solution of acid and dissolved silver. The acid and silver are drained off, after which the gold sediment is washed with water numerous times. ==See also==