Abstract
The conventional processes for recovery of silver from cyanide leach solutions are the carbon adsorption, the Merrill-Crowe zinc dust cementation, the Ion Exchange, and Solvent Extraction processes; among other available options for recovery of precious metals from cyanide solutions, Electrocoagulation (EC) is a very promising electrochemical process that does not require high concentrations of silver in cyanide solutions to yield excellent results and neither pretreatment of cyanide solutions like Merrill-Crowe process (deoxygenating and clarification). The present study has been done for the recovery of silver contained in pregnant solution from the cyanidation process using the electrocoagulation technology with iron electrodes, and therefore develops an alternative technology for Merril-Crowe process. The average silver content in pregnant solution was of 52 ppm, recovery was obtained of 99% of silver, with this optimum operating parameters, pH = 8, residence time = 20 minutes and conductivity by addition of sodium chloride = 4 grs/L. Finally the characterization of the solid products formed during the EC process with X-ray Diffraction and Scanning Electronic Microscope was performed, results suggest that magnetite particles and amorphous iron oxyhydroxides are present (Lepidocrocite).
Highlights
Cyanidation processes are especially suitable for treatment of silver-bearing sulphidic materials
The metal cations react with the OH-ions produced at the cathode (see Equation (6)) during the evolution of hydrogen, to yield both soluble and insoluble hydroxides that will react with or adsorb silver, from the cyanide solution and contribute to coagulation by neutralizing the negatively charged colloidal particles that may be present at neutral or alkaline pH
With this parameters the EC process gave the results shown in Table 4, Table 5 and Table 6 and Figure 5 shows the pH behavior vs. Time. From this results we can determine that, when residence time increases from 10 to 15 minutes the recoveries of silver increase respectively from 37% to 77%, for 2 grs of NaCl, from 40% to 73.33% for 3 grs of NaCl and from 48.88% to 80% up to 99% for 4 grs of NaCl, this occurs in the pH range from 9 to 10 approximately, that coincides with the production of magnetic iron, Fe3O4, which has magnetic properties that accelerates the process of adsorption of precious metals
Summary
Cyanidation processes are especially suitable for treatment of silver-bearing sulphidic materials. The sodium cyanide concentration (molecular weight of NaCN = 49) should be equal to 4 × 0.27 × 10−3 × 49 = 0.05 g/L or approximately 0.01% This was confirmed in practice at room temperature by a very dilute solution of NaCN of 0.01% - 0.5% for ores and for concentrates rich in silver 0.5% - 5% [1]. The two conventional processes for silver recovery from cyanide leach solution are the carbon adsorption process and the Merrill-Crowe zinc dust cementation process. The EC process is a very promising technique in the recovery of precious metals such as silver and gold: EC needs no chemical reagents, does not generate toxic materials of cyanide requiring special disposal and this makes it an ecologically viable technique. Other method that has been tried for the recovery of gold and silver is adsorption on a chemically modified chitosan with magnetic properties [5]
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