1992-04-02_REVISION - M1988112 - Laserfiche WebLink

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1 - Pape 3 1 high vdumes of low pressure air. In this application a reduction In total cyanide pt 92 percent was achieved by lowering the cyanide Concentration from 560 mg/I to 44 mg/I. - , ~ - The major problem associated wah the system was the buildup of gypsum and copper ~thlocyanate preclphates on th@ packing Internals, which required manually deaning every two to lour months. The problem arose due -b the very low pH end hlgFi sullate concentrations generated, two problems minlmtzed in modern adaptations of the process. The process was utilized for economic, not environmental reasons and was limbed to the stripping of cyanide'from clarified barren soutions, not slurries. Ourlnq the 1930'5 there were about seven cyanide regeneration plants in operation utilizing various designs, butionly limbed Inlormation was available concerning these facilities (4). The plants were utilized In con)unctioh whh silver recovery primarily and were located in remote regions of the world. From the mid-1930'5 Into the 1950'5, a full-scale cyanide regeneration or recovery process was utilized by ' . the Real Oel Monte Company In Pachuca, Hidalgo, Mexico (5). The stripping of free cyanide was conducted at an acklic pH, employing sulfuric acid generated on-site through the burning of sulfur. The Volatilization stage and absorber stages were combined alternately and operated in series. The system wAs enclosed ' and the air recycled at 100 percent humidity to minimize evaporation. A total cyanide recoveryiof about 93 percent was achieved with about a 60 percent removal realized through each of the three striper stages. The operation was originally designed to treat about 350 tonnes of soution per day and was latdr expanded ' to accommodate about 1,100 tonnes of solution per day. The tree cyanide was concentrated unfit a 5 percent soution was obtained and then recycled to the metallurgical circuh. However, the process was restricted to recovery of cyanide from clarified barren solutions only. ' In the 1970'5 the process was reinvestigated and modernized by CANMET and McNamara (6). The renewed interest In the process was due to the Increasing costs of both cyanide and hs treatment. In the CANMEZ study, abench-scale laboratory apparatus was employed to Investigate the recovery of cyanide from six actual barren waters. The study summarized for the first time the various chemical reactions associated whh the process. The process was referred to as aclditication, vdatilization, and reneutralization or the AVR process, which Is the current terminology for cyanide recovery and regeneration from soutions. The study advocated the use of single complete mix aeration basins about 1.5m in depth instead of stripping towers, to minimize the mechanical problems associated with scaling and plugging of media ¢r internals, _ arxi to minimize pressure drop and horsepower requirements. It was recommended that aeration in a full- , scale operation be accomplished using multiple rows of fine bubble diffusers. The AVR process was evaluated as a primary wastewater treatment alternative, utilizing pH values below 2.0 ' to lacilhate removal of total and iron complexed cyanides through precipitation, Greater than i99 percent removal of total cyanide was achieved In the studies, with treated ehluents containing total cyanides ranging Irom 0.10 to 4.0 mg/I. In addition, the Investigation of metals preciphation was undertaken, demonstrating ' excellent removal efficiencies were achievable follow)ng reneutralization whh lime. However, the process was confined to solution treatment as h was considered not feasible for treatment of tailings glurries. In addition, the use of basins and fine bubble diffusers Increased the horsepower requirements compared to ' towers, due to the Increased pressure losses encountered In the system. The AVR process was further Investigated as a primary alterna['rve for treatment of mine decant water from 1985-1987 (Mudder, unpublished results). The tests employed a small pilot plant operated In belch mode ' using barren waters obtained from a silver mining operation. The pilot plant design Involved two 0.3m diameter 2.5m pdyethylene columns connected In series. ' One cdumn served as the stripptng chamber, while the other served as the absorption chamber. About 200 L of actual decant solution were utilized per experiment in conjunction with coarse bubble dfitiusers, as h was found turbulence was a more Important lactor In stripping of cyanide than the size of the bubble. The