The properties of gold in ores from the standpoint of recovery are its extremely high specific gravity (15.5 to 19. 3 depending upon amount of alloying metal admixed); the fact that mercury wets it readily in the presence of water (amalgamation); its solubility in dilute aqueous solutions of alkaline cyanides to form relatively stable compounds of the form NaAu(CN)2; and its response, particularly as naturally alloyed, to flotation collectors.
Native Gold Ores
Free milling lode ores are those in which the gold is relatively coarse and amalgamable, the sulfide content is low and nonarsenical, oxidized compounds of bismuth and antimony are absent, and the gangue is substantially free from talc, clay and graphitic constituents. With these ores, there are advantages in extracting as much free gold as possible in the grinding circuit by gravity concentration. Concentration of free gold by gravity is a relatively simple method of recovery and when used in cyanide plants is applied ahead of cyanidation. On lode gold ores, launder traps, hydraulic traps or pulsating jigs are sometimes used in the grinding circuits for recovery of as much as 60 percent of the total gold in the mill feed. The jig hutch product may be continuously discharged onto a shaking riffled table with the concentrate fed in batches to barrel amalgamation. Homestake recovers 20 to 25 percent of the gold in launder traps. Other recovery methods have not been successful because of cable splinters, blasting wire, etc. , in the ore. Woolen blankets have long been used for trapping fine gold particles and particularly for tellurides. Blankets are generally laid overlapping on wide inclined tables.
From this practice of using blankets came the development of corduroy to entrap gold and a South African version of corduroy is sheet rubber having "V" shaped riffles molded into its surface. The Johnson concentrator, an inclined rotating cylinder, the plane tables, and belt concentrators are lined with this material.
Amalgamation depends upon the wetting and alloying of metallic gold with mercury. Direct amalgamation in which the entire ore stream flows over mercury-covered copper plates has now been generally abandoned to prevent stream pollution. It has been replaced by a concentration step which subjects only a relatively small quantity of high grade concentrate to barrel amalgamation. This method eliminates the tedious cleaning and recoating of the copper plates and reduces the chances for loss through theft. The gravity concentrate is ground for several hours in a small mill or barrel with steel balls or rods before the mercury is added. This form of amalgamation is the simplest and most common method of treating an enriched gold-bearing concentrate. Examples of free gold concentration and amalgamation are shown in the flowsheets of Dome, Homestake, Itogon-Suyoc Palidan, Kalgoorlie, Campbell Red Lake, Blyvooruitzicht, and Vaal Reefs.
Following the recovery of the coarser free gold particles by gravity and barrel amalgamation, the grinding of the ore in cyanide solution with ball or pebble mills is generally practiced. Separate cyanidation of sand and slimes has diminished with the development of closed-circuit fine grinding for "all-slime11 treatment by agitation.
Other Free Milling Ores
Gold mineralization in these ores may occur in a limey siltstone containing intermittent shale beds. Sulfides are seldom seen, but pyrite, galena, sphalerite, chalcopyrite, antimony, mercury and arsenic occur in minute amounts. Gold occurring in micron size is readily amenable to cyanidation as at Carlin and Cortez. Other free milling ores are Benguet, Camflo, Kinross, Kloof, and the new Pueblo Viejo.
Native Gold Ores
Free milling lode ores are those in which the gold is relatively coarse and amalgamable, the sulfide content is low and nonarsenical, oxidized compounds of bismuth and antimony are absent, and the gangue is substantially free from talc, clay and graphitic constituents. With these ores, there are advantages in extracting as much free gold as possible in the grinding circuit by gravity concentration. Concentration of free gold by gravity is a relatively simple method of recovery and when used in cyanide plants is applied ahead of cyanidation. On lode gold ores, launder traps, hydraulic traps or pulsating jigs are sometimes used in the grinding circuits for recovery of as much as 60 percent of the total gold in the mill feed. The jig hutch product may be continuously discharged onto a shaking riffled table with the concentrate fed in batches to barrel amalgamation. Homestake recovers 20 to 25 percent of the gold in launder traps. Other recovery methods have not been successful because of cable splinters, blasting wire, etc. , in the ore. Woolen blankets have long been used for trapping fine gold particles and particularly for tellurides. Blankets are generally laid overlapping on wide inclined tables.
From this practice of using blankets came the development of corduroy to entrap gold and a South African version of corduroy is sheet rubber having "V" shaped riffles molded into its surface. The Johnson concentrator, an inclined rotating cylinder, the plane tables, and belt concentrators are lined with this material.
Amalgamation depends upon the wetting and alloying of metallic gold with mercury. Direct amalgamation in which the entire ore stream flows over mercury-covered copper plates has now been generally abandoned to prevent stream pollution. It has been replaced by a concentration step which subjects only a relatively small quantity of high grade concentrate to barrel amalgamation. This method eliminates the tedious cleaning and recoating of the copper plates and reduces the chances for loss through theft. The gravity concentrate is ground for several hours in a small mill or barrel with steel balls or rods before the mercury is added. This form of amalgamation is the simplest and most common method of treating an enriched gold-bearing concentrate. Examples of free gold concentration and amalgamation are shown in the flowsheets of Dome, Homestake, Itogon-Suyoc Palidan, Kalgoorlie, Campbell Red Lake, Blyvooruitzicht, and Vaal Reefs.
Following the recovery of the coarser free gold particles by gravity and barrel amalgamation, the grinding of the ore in cyanide solution with ball or pebble mills is generally practiced. Separate cyanidation of sand and slimes has diminished with the development of closed-circuit fine grinding for "all-slime11 treatment by agitation.
Other Free Milling Ores
Gold mineralization in these ores may occur in a limey siltstone containing intermittent shale beds. Sulfides are seldom seen, but pyrite, galena, sphalerite, chalcopyrite, antimony, mercury and arsenic occur in minute amounts. Gold occurring in micron size is readily amenable to cyanidation as at Carlin and Cortez. Other free milling ores are Benguet, Camflo, Kinross, Kloof, and the new Pueblo Viejo.
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