RECENT DEVELOPMENT IN PROCESSING OF OXIDE AND REFRACTORY GOLD ORES

摘要

After more than 100 years, alkaline cyanide leaching of gold and silver ores, either directly in the case of oxidized ores, or after pretreatment of refractory ores by roasting, pressure leaching or bacterial leaching, remains the dominant leaching method to extract the precious metals from ores. Leaching with halides (chlorine, bromine and iodine) and with thiourea and thiosulfates is less common but practiced in some cases to dissolve the precious metals both from ores and jewelry and electronic scrap. Cyanide solutions can contain from 1 ppm Au to 30,000 ppm Au. According to the source, i.e., agitated tank leaching, heap leaching, eluates from stripping activated carbon or waste electroplating solutions. Similar variations occur with silver which can have a profound influence on the recovery method. The presence of other elements in the leach solutions, particularly mercury and copper, likewise play an important role in the procedures required to recover the gold and silver values from solution. The same circumstances are found when lixivants other than cyanide are used to dissolve the precious metals. For decades, the preferred method of recovering gold and silver values from alkaline cyanide leach solutions was by zinc dust precipitation according to the Merrill Crowe process developed in the late 19th century. Finely ground gold and silver ores or concentrates were either leached directly or after roasting, in agitated tanks or Pachucas and subjected to solid/liquid separation by filtering or by treatment in counter-current-decantation washing thickeners (CCD). This solid/liquid separation step was often very difficult especially when ores containing high quantities of clay were treated which caused filtration or thickening problems. Following testing and development by the U.S. Bureau of Mines (USBM) in the late 60s, the introduction of carbon-in-leach (CIL) or carbon-in-pulp (CIP) plants to recover gold and silver from leach pulps was one of the greatest operational and cost saving improvements made in the industry in the 20th century. It was first used by Homestake in 1973 in South Dakota and quickly spread around the world to be the most common process to recover gold and silver from cyanide solutions. Another great leap forward in the industry, also developed by USBM some 20 years ago, was the advent of heap leaching in which either run-of-mine (ROM) or crushed ores are stacked on impervious pads covered with high density polyethylene material (HDPE) and leached by percolation of weak cyanide solution through the heaps. Almost 200 million tons of ore were treated in this fashion in the United States in 1998. In this case, carbon-in-solution (CIS) is widely used to recover the gold and silver values from the leach solution but the Merrill Crowe process is also extensively used. The Russians have long used ion-exchange resin to recover gold and silver from cyanide pulps at several plants including the giant Muruntau Plant in Uzbekistan which is one of the largest gold plants in the world. Ion-exchange resin was also successfully used at the Golden Jubilee plant in South Africa for several years and a great deal of testing has been carried out in many universities and private laboratories world-wide, especially at the University of Utah, MINTEK and Newmont's metallurgical laboratories. Solvent extraction, which is widely used in the copper and uranium industries to recover these metals from weak leach solutions, has also been extensively tested at several universities and private laboratories to recover gold and silver from cyanide solutions. This method has yet to be used commercially. The use of halides for leaching gold ores which was widely used before the introduction of cyanide leaching (especially chlorine), is not practiced commercially at this time, but laboratory and pilot plant work continues at several sites throughout the world and commercial silver leaching is practiced at Itos in Bolivia. Activated carbo