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' Page 5 <br />' The fdlowing sections of the paper locus on the chemistry, performance, applications, and economics of <br />the cyanide recovery from sdutlons end slurries, wrih emphasis on the cyanide recovery systtjm proposed <br />for the Gdden Cross Mine. <br />' PROCESS DESC731PTION AND CHEMISTRY <br />' Cyanide recovery processes utilize the vdatllriy of HCN at a lowered pH (I.e. Figure 2) to striplfree cyanide <br />from sdutton or slurry and recover ri In usable lorm (I.e. NaCN or Ca(CN),). The vdatilriy of fr6e mdecular <br />cyanide (HCN) is related to ris Henry's Lew Constant, which Is affected by severai factors Including viscosfry, <br />hydrogen bonding, pH, and temperature (7). The transfer of hydrogen cyanide from a sdutloh or slurry to <br />' air Is IiquW film Ilmried, and high volumes of low pressure air (I.e. <0.5 psi) are required tlo create the <br />necessary air to liquid contact to facliitate its removal. <br />There ere several factors which affect the rate and extent of HCN removal from slurries or sdutions through <br />air stripping. These factors Include: <br />' 1. <br />2. The pH of the soution <br />The form of cyanide <br /> 3. The concentration of cyanide <br /> 4. The temperature of the slurry or solution <br />' 5. The pressure maintained within the recovery system <br /> 6. The air to liquid ratio <br /> 7. The mechanical dispersion equipment <br /> 8. The viscosity of the solution or slurry <br />' 9. The liquid to air contact period <br />' A great deal of Information Is available regarding the effects of these design parameters on the recovery of <br />cyanide from clarified solutions, but Iimried data Is available concerning the treatment of tailings slurries. <br />The current cyanide recovery processes are upgraded versions of the original Mills Crdwe Process <br />previously discussed. A schematic of the generalized cyanide recovery process is presented pn Figure 3. <br />~.... The process Is conducted In three stages. The first stage known as acid'rfication, Invdves IoWering of the <br />' wastewater pH in the range of 1.5-8.5 wrih the use of concentrated mineral acid. The acid empIoyed most <br />' ~ commonly Is sulfuric acid, due to ris relatively low cost and ease of availabilriy. In addition, the handling, <br />storage, and feeding of sulfuric acid Is quite common in Industry. The potential problems associated with <br />sulfuric acid Include an Increase in sulfate and total dlssdved solids concentrations and the potential for <br />preciptation of calcium sulfate or gypsum In oversaturated solutions. The pH'of the soution or slurry Is <br />monriored In-situ continuously. The acidification step must be enclosed to prevent escape of HCN gas and <br />requires about 10 to 20 minutes to complete. The reduction of HCN during the~ackfitication step Is about <br />1D l0 15 percent of the total cyanide concentration. <br />The pH of a soution, slurry, or sludge Is lowered In accordance wrih the stability of the particular complexes <br />from which the cyankfe will be recovered. For the recovery of free cyanide Irom simple cyanid ' (I.e. NaCN <br />' and Zn(CN),), a pH In the range of 4.5 to 8.5 Is employed. For recovery of weak acid dlsso~ bte (VJAD) <br />cyanide, the pH Is lowered to about 4.0. For removal of Iron complexed cyanide the pH Is IoWered to less <br />than 2.0. In this case, an insduble metal Iron cyanide complex forms, and precipitates from splutlon. The <br />' best approach Is to utilize a near neutral or basic pH when possible to minimize precipitation problems and <br />an Increase In total d(ssdved solids. <br />From the acidification stage, the acidified solution or slurry containing HCN passes Into the cyanide stripping <br />' or vdatillzatlon stage, which consists of either open or packed towers, a series of complete rolx reactors, <br />