1992-04-02_REVISION - M1988112 - Laserfiche WebLink

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'' Pape 4 coarse bubble diffusers slmplitled the process, while maintaining process performance. Total cyanide levels were routinely reduced from an average of 330 mg/I. to less than 2.0 mg/I. Also in the' mkt-1980's, Investigations were conducted at the Noranda Research Center aimed et deslgning ' a cyanide recovery. process employing packed towers and plastic, media (7). Stripping towers) signAicantly reduced the surface area required for the AVN process, as well as the horsepower requlremenis due to the reduced pressure losses through the system. Employing {ntormatlon derived In a bench-soafe study, a continuous flow pilot plant system was designed and operated. The data obtained In the plldt study was ' utiliied In the development of a design for afull,-scale stripping system to treat about 2,000 m'I/d of barren sdution, utilizing an air to liquid ratio of 330. ' The pilot plant evaluations Indicated the total cyanide levels could be reduced from about 4,D00 mg/I to about 120 mg/I, for an overall removal efficiency of 97 percent. The stripped cyanide was reabsorbed In a second packed tower using a counter current caustic sdution. The process employed a pIN of 2.5 and conskierable precipitation of copper thlocyanate and calcium sulfate was realized. It was ndted that the precipitated sdids would eventually reduce the stripping efficiency, and mechanical cleaning pi the media would be necessary and difficult. The study demonstrated that use of modern mass transfer technology and materials could be employed In the removal and recovery of cyanide from clarified barren sollrtlons. A Lull-scale packed tower cyanide recovery system was designed and operated from 1985-1987 qy Golcorxfa Engineering and Mining Services at the Beaconsfield Gold Mine In Tasmania (8). The process was termed ' the C.R.P. or Cyanide Regeneration Process. Initially, a continuous flow pilot plant operation wet conducted to establish the appropriate air to liquid ratio, stripping time, and caustic requirements for the absorber. The full-scale facility was designed to treat 1,200 tonnes/d of clar'rfied barren water using a systenp of muitipie packed aeration towers. A schematic of the full-scale facility Is presented In Figure 1. The plant was designed using the most modern mass transfer technology, plastic media, and cyanide monRoring equipment. The system was designed for maximum safety incorporating an enclosed negative pressure system. Fan exhausts were monitored routinely for HCN with typical values of <1.D m~/m', which were 1/10 of the B-hour 10.0 mg/m' TLV for Iree cyanide. Cyanide recoveries of nearly 95 percent were reported in the full-scale system. The various laboratory, pilot plant, and full-scale cyanide recovery facilities described have demonstrated ~. that application of mass transfer technology coupled with state-of-the-art monitoring equipment and engineering design approaches can be employed In the construction and operation of full-scale cyanide ' recovery systems. However, it has been widely believed that the cyanide recovery process can not be extended to slurries, due to the problems associated with the chemical and physical properties of low pH slurries and the lack of adequate materials and equipment. Since 1985, an Intensive research effort has been undertaken to expand the capabilities of they process to slurries, while minlmlzing the chemical and mechanical problems associated with past systems. The work ' involved both detailed laboratory and continuous flow pilot plant evaluations conducted for Cyprus Gold of New Zealand, to develop and design the first full-scale commercial cyanide recovery systems for tailings slurries to be Installed at the proposed Golden Cross Mine near Walhi, New Zealand. Based on the test results, afull-scale cyanide recovery system utilizing packed towers would be guilt to treat 200 m'/hr of a 35 percent fallings slurry from a WAD cyanide level of 200 mg/I to Tess than 10 mg/I. The commercial cyanide recovery process for which patent applications have been tiled and a patent Is pending ' is known as the Cyanlsorb Process.