1992-04-02_REVISION - M1988112 - Laserfiche WebLink

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.' Page 2 ' The excess soutions end sdlds are normally disposed of In an Impoundment. Several potent181 problems may result from disposal including contamination of groundwater, exposure of wildlHe and water fowl to elevated cyanide concentrations, and Impacts on surlace water environments tdlowing discharge of treated or untreated effluents. Theie' Is a need to mRlgate these problems.and to minimize potential adverse environmental Impacts. The ' conventlonai approach Invdves treatment of mill tailings or decant sdutlons and lining of Imppundments, while Ignoring the options associated wrih'recycle of process soutions and reagents. The bapital and operating costs of wastewater treatment and tailings disposal ere substantial and oRen dictate th~ economic ' viability of a mining protect. Treatment costs are Increasing sharply due to more stringent en Ironmentai regulations and the need for more sophisticated and advanced techndogies. In conjunction wRh treatment and solid waste disposal costs, are the rapidly Increasing costs of water and reagents, particularly cyanide. The cost of cyanide has risen sharply due to world-wide defrtands and shortages. The spot-market price for cyanide in certain Instances Is several times higher than the average price. The cost of cyanide Is a primary concern in remote areas where cyanide availability is Ilmfted. ' As a result, every attempt must be made to reduce reagent consumption through recycle and recovery to minimize the costs of production and of environmental protection. One excellent approach Invdves recovery • and recyGe of cyankle from process soutions and slurries. Historically the process, termed acidification, vdatilization, and reneutralization or AVR, has been employed In the recovery of cyanide frdm clarHied barren or decant solutions. ' Although cyanide recovery has been employed In the full-scale recovery of cyanide Irom solutions, only recently has considerable Interest In the process resurfaced, particularly for application with tailings slurries. This paper describes In detail the history, chemistry, applications, advantages, and costs of various cyanide ' recovery processes. Emphasis is placed on the development of the Cyanisorb Process for recovery of cyanide from slurries. This ' process has been tested and proven through an extensNe pilot plant evaluation for application at Cyprus Gdd's proposed Gdden Cross Mine In New Zealand, and would represent the first Lull-scale application of cyanide recovery from slurries. ~ HISTORY OF CYANIDE RECOVERY PROCESSES ' The recovery of cyanide Irom mining solutions through acidification, air stripping, and reabsotption was originally known as the Mille Crows Process (1). The process Involved acidifying clarified barreln solution, then stripping the HCN formed and reabsorbing h from the air stream wRh a caNstic or milk of lime spray. ' The process was employed et the Flin Flon Mine operated by Hudson Bay Smelting and Mining, Company Irom about 1930-1975 (2,3). The Canadian mining operation Invdved cyanidation of gold bearing tailings originating from the flotation of acopper-zinc ore. To condition the barren solution for stripping, the pH was first adjusted downward to 2.5 with sulfuric acid supplied from the zinc electrolytic plant. The acWHied sdutlon was then passed through a series of four stripping towers packed with wooden grids. The towers were constructed of concrete and measured about Sm x 2m x 7.Sm In height. Th@ stripping ' towers were operated In a series, each connected to one of four concrete absorption tunnels by means of closed loop recycled air streams. ' The air and liquid vdumes In each stripping loop were about 56,000 m'/hr and 107 m'/hr, resulting In an air to liquid ratio of about 525/1. The HCN entrained in the air stream was reabsorbed by contacting the air stream with lime slurry sprays and recollected In a central storage tank. The stripping process required