Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G7/00—Compounds of gold
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B11/00—Obtaining noble metals
  • C22B11/04—Obtaining noble metals by wet processes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C1/00—Electrolytic production, recovery or refining of metals by electrolysis of solutions
  • C25C1/20—Electrolytic production, recovery or refining of metals by electrolysis of solutions of noble metals
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
  • E21B43/28—Dissolving minerals other than hydrocarbons, e.g. by an alkaline or acid leaching agent

Definitions

• This invention lies in the field of hydrometallurgy.
• it involves the simultaneous electrolytic recovery of gold and restoration of elemental iodine in spent lixiviant solutions used, for example, in in situ mining, heap-leach mining, or agitated-leach recovery processes.
• the object of the present invention is to provide an economic process for recovering gold from an iodine- containing lixiviant in a one-step process in which gold and iodine are electrolytically reduced simultaneously with the oxidation of iodide. Iodine is produced at a rate sufficient to provide for a recycle leaching of gold.
• a further object of the invention is to prevent iron fouling of the electrolysis.
• Fig. 1 is a schematic drawing of a laboratory set-up in which iodine/iodide lixiviant solution is sprinkled onto a column of crushed auriferous ore, and the column effluent is passed through an electrochemical cell for removal of gold and regeneration of iodine before being recycled back onto the ore column.
• An improved process for recovering gold from gold- containing materials including auriferous ores by iodide/iodine leaching wherein the pregnant lixiviant containing dissolved gold and iodine is treated in an electrolytic cell to reduce gold in solution to elemental gold for recovery and simultaneously reduce iodine to iodide at the cathode, so as to prevent iodine from interfering with subsequent gold recovery processes.
• iodide present at the anode is oxidized to elemental iodine to regenerate the leach solution to the desired iodide:iodine weight ratio, e.g. about 2:1 to about 10:1.
• Gold is precipitated in the cathode co - partment, and if desired, the cathode effluent may be treated for further removal of traces of gold before being passed to the anode compartment.
• a method for preventing iron contamination of the cathode is also provided comprising buffering the lixiviant solution to a pH of about 5.
• the process described herein involves the simultane ⁇ ous electrolytic reduction of iodine and reduction and precipitation of gold at a cathode, preferably a stain- less steel cathode, with concomitant reoxidation of iodide to iodine at the anode, preferably a carbon anode, of the same cell.
• the cathode effluent is subjected to a process for removal of gold, such as by being passed through a bed of activated carbon or anion exchange resin to remove traces of gold before passing into the anode compartment.
• iodine in the solution entering the ore zone is reduced to iodide either in the ore zone or in the cathode compartment of the electrolytic cell, and is then completely replenished by oxidation in the anode compartment of the cell. Since no iodine is present in the cathode effluent, no iodine is available for adsorption onto the carbon bed or anion exchange resin in the modified process.
• the current in the cell is adjusted to generate the desired concentration of elemental iodine in the anode effluent.
• the total combined iodine and iodide concen ⁇ tration depends on the characteristics of the material being leached (3 gpl is a typical total iodide-iodine concentration for use in in situ mining) .
• the iodide to iodine ratio in the regenerated lixiviant is also a function of the qualities of the feed material, however when too low an iodide to iodine ratio, i.e., too much iodine, is produced, the rate of solubility of iodine at the anode will not keep pace with iodine generation and cause iodine to crystallize on the anode and interfere with the process. If the iodide to iodine ratio is too high, i.e. , too little iodine, the iodine may be exhausted before the gold leaching is completed, causing gold to redeposit in the feed material.
• an iodide ratio of at least about 2:1 is desirable, and ratios up to about 10:1 are generally useful.
• the quantity of oxidized iodine in the cathode compartment will be insufficient to carry the total current used to regenerate the iodine in the anode compartment.
• the remainder of the current must then be carried by some other cathode half-reaction; usually this additional half-reaction will be the reduction of water to hydrogen gas (in addition to the small contribution from the reduction of gold complex to elemental gold) .
• the cell voltage necessary to be applied to effect the gold- recovery/iodine regeneration is then the difference in half-cell potentials between the iodide/iodine oxidation and the reduction of water at-the cathode pH, plus any cathode overvoltage for the water reduction plus the ohmic drop in the cell. Algebraically, this may be expressed as:
• E app E l " /I 2 + E H 2 0/H 2 + IR cell + E cath,over
• lixiviant solution When lixiviant solution first contacts a pyrite ore zone, it is common for extensive interaction between the iodine and the pyrite to take place rapidly because of the presence in the ore of finely divided pyrite parti ⁇ cles.
• the pH can drop rapidly to about three or less and a considerable quantity of iron can be solubilized. The high concen- trations of iron thus produced will then be transported to the cathode where undesirable electrode reactions or chemical precipitations can occur.
• the lixiviant solution may be buf ⁇ fered with a non-reducing buffer, preferably sodium acetate/acetic acid, to a pH sufficient to prevent iron dissolution, preferably at least about 5; this keeps the iron concentration to a low enough level that it inter ⁇ feres very little with the operation of the electrolysis cell. Buffering the lixiviant also retards dissolution of the stainless steel cathode, the degradation of the cathode being a hundred times faster at pH 3 than at pH 5.
• a non-reducing buffer preferably sodium acetate/acetic acid
• Fig. 1 depicting a preferred embodiment of this invention, shows original or regenerated lixiviant containing an optimum iodide:iodine ratio at reservoir R, being pumped into an ore column, from which pregnant lixiviant containing dissolved gold and reduced iodine, i.e. a higher iodide:iodine ratio than present in the original or regenerated lixiviant, flows into reservoir R 2 ⁇
• the pregnant lixiviant is conducted to the stainless steel mesh cathode of an electrolytic cell equipped with a conventional power supply where gold is reduced to elemental gold and precipitated onto the cathode, and substantially all the iodine present in the lixiviant is reduced to iodide.
• the cell is shown with a Nafion (Du Pont Co.) membrane separating the anode and cathode compartments.
• This membrane is a sheet of cation exchange material which is impervious to water and anions but permits passage of cations from the anode compartment to the cathode compartment.
• This arrangement prevents mixing of anolyte and catholyte while still permitting maintenance of electro-neutrality in the cell compart ⁇ ments as the electrode reactions take place.
• the gold- barren cathode effluent containing virtually no iodine is collected at reservoir R 3 from whence it may be conducted in the modified path to an activated carbon bed for removal of traces of gold, and then be conducted to the graphite rod anode compartment of the electrolytic cell.
• the gold- and iodine-barren lixiviant may be conducted from 3 via the primary path directly to the anode compartment.
• iodide is oxidized to iodine in an amount sufficient to provide a regenerated lixiviant for reuse, and the regenerated lixiviant is conducted to R,' and recycled.
• Example 1 In a typical laboratory-scale embodiment of this process, unbuffered solution containing 9 grams/liter ionic iodide and 1 gram/liter elemental iodine was pumped from a reservoir at 75 ml./min. into a glass column 23 cm. wide by 30 cm. long containing approximately 15 kg. of pyritic gold ore. Effluent from the column was depleted somewhat in iodine and contained gold ranging in concentration 10 to 0.1 mg./liter. This effluent solution was pumped into the cathode compartment of the electroly ⁇ sis cell and reduced at 1.0 amp of current using an applied voltage of 2.0 volts. No gold or elemental iodine was detected in the cathode effluent.
• the cathode effluent was then pumped into the anode compartment of the same electrolysis cell wherein iodide was oxidized to elemental iodine at such a rate that the anode effluent was replenished to 1 gram/liter iodine. A cell current efficiency of 95 percent was achieved.

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• Chemical & Material Sciences (AREA)
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• Geochemistry & Mineralogy (AREA)
• Physics & Mathematics (AREA)
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• General Life Sciences & Earth Sciences (AREA)
• Fluid Mechanics (AREA)
• Environmental & Geological Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Mechanical Engineering (AREA)
• Inorganic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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Citations (4)

• 1985
  • 1985-12-06 WO PCT/US1985/002426 patent/WO1987003623A1/en unknown
  • 1985-12-06 JP JP61500200A patent/JPS63502358A/ja active Granted
  • 1985-12-06 AU AU52071/86A patent/AU577173B2/en not_active Ceased

Patent Citations (5)

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