1992-04-02_REVISION - M1988112 - Laserfiche WebLink

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1 ..- ' Pape 6 or a shallow aeration basin. The vdatillzatlon system Is sealed to prevent escape of HCN laden air and to ' allow efficient recovery of cyenlde. All configurations are eppllcaWe to either slurries or Garffled soutions. A stripping efficiency In the range of 90-99 percent Is posslde. In the case of complete mb4 reactors or aeratbn basins, coarse bubble diffusers ere employed In the bottom of the reactors, and the reactors are sealed to.ellminate release of HCN gas to the atmosphere. The use of diffusers provides air for agitation ' of the solution as well as stripping of the tree cyenlde. Mechanical agitation does not provide the necessary turbulence to tacllftate stripping of HCN: •!: ' The~free cyenlde produced through volatilization Is entrained In an air stream, passed upflow through a packed tower, and reabsorbed Into a caustic soution moving counter current to the air flow. The caustic sdution Is then returned to the metallurgical clrcuft for reuse. The pH of the causiic sdulion Is maintained in the range of 10.5-11.5. Once the soution or slurry Is tree of recoverable free cyenlde, fl enters the reneutralization 6tage of the process. In this stage, the pH Is readJusted to 9.0-10.5 with lime, to precipitate residual metals and to add buffedng tapacrty. With cyanide removed, the metals ere released In free lorm Into sdutipn, allowing precipitation to low levels as their stable and Insoluble carbonate and hydroxide complexes. the addition ' of buffering capachy Is Important in the event a slurry being treated exhibits the potential for aciQ generation through sulfide oxklation. The lime may be supplied as thickened sludge from other treatment processes fl present on site. The simplified chemistry of the process Is presented In the fdlowing reactimns: (1) Ca(CN), + H,SO. = 2HCN + CaSO~ (acidification) (2) M(CN), + H,SO~ = 2HCN + M"SO.- (3) HCN/H,0 =HCN/Air (volatilization) ' (4) HCN + NaOH = NaCN + H,O (absorption) (5) M" + 20H- = M(OH)r (preciphation/reneutralization) (6) Na~Fe(CN)e + 2CuS0, = Cu,Fe(CN), (red/brown) + 2NazS0~ (iron cyanide removal) ' The process chemistry Indicates that the Inftial phase of recovery Invdves conversion of CN- fo HCN and inhiation of breakdown and of precipRatlon of neutral metal cyanide complexes. The eXlent of the conversion dependent on the pH chosen for recovery and Corms of cyenlde being recovered. At very low pH values, precipitation of Iron complexed cyanide occurs as the copper-iron cyanide complex, which ~:: accounts for the removal of this portion of the total cyanide. ' PROCESS PERFORMANCE As discussed previously, excellent overall stripping and recovery efficiencies of total and WAD cyanides Irom barren and decant solutions are achievable by any of the recovery processes mentioned. In they case of the ' three full-scale systems described above, overall removal efficiencies ranged from 92 to 99 percent. In the case of the Flin Flon Mine, the total cyanide level was reduced from 560 mg/I to 44 mg/I. The performance of the Gdconda C.R.P. facility is presented In Table 2. The Inhial total and free cyanide levels of 200 mgJl ' and 10-30 mg/I were reduced to <5.0 mg/I and <0.50 mg/I, respecWely. The residual cyanidedevets were reduced further once the treated effluent was passed through a carbon adsorption column prior to discharge. t In addition, the copper, nickel, and zinc concentrations were all reduced to < 1.0 mg/I in the treated effluent. The level of treatment obtained was consistent wtih that echlevaWe by any chemical and/or biological treatment process currently used In the mining Industry. The cyanide regeneration process also enhanced the recovery of gdd from solution through carbon adsorption. 1