Classifications

• • 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
  • C22B11/06—Chloridising

Definitions

• This invention relates to the recovery of silver from metallurgical intermediate materials which also contain one or more other precious metals.
• base metal ores such as those of copper, nickel, zinc, and lead also contain metals of the platinum group together with silver, gold, selenium, tellurium and other constituents. Such ores form important sources of the valuable metals which are known colloquially as "precious metals", that is to say the six platinum group metals, gold and silver.
• metals occur in the various metal ores in small amounts and become concentrated during the working up of the base metals in the form of various metallurgical intermediates, including anode sludges, leach residues and cements.
• the compositionsof these precious metal-containing materials thus vary widely depending upon the nature of the ore. Despite the differences in composition these materials tend to lend themselves to a more or less common scheme of treatment and tend to contain largely the same ingredients, although the proportions of valuable metals therein can vary.
• metals which may be present in the precious metals intermediate products include all six members of the platinum group, gold, silver, selenium, tellurium, lead, arsenic, antimony, tin, bismuth, copper, nickel, zinc, iron and sulphur.
• One known method can involve decopperizing of the precious metal intermediate by leaching with sulphuric acid and then smelting in a Dor6 furnace in which silver may be recovered in the form of DorG metal. Such a procedure is expensive and can produce harmful emissions of selenium, arsenic, lead and other heavy metals.
• the present method provides a hydrometallurgical method for working up precious-metal-containing intermediate materials to give selective and efficient separation of silver therefrom.
• silver present in a metallurgical intermediate material that also contains one or more other precious metals is concentrated and recovered by slurrying the material with water, chlorinating the aqueous slurry to dissolve platinum group metals and gold present and to produce a residue enriched in silver, dissolving silver from the residue with an aqueous thiosulphate solution and recovering silver from the resulting solution.
• the silver-containing precious metal intermediates to be treated should be slurried with water in the proportion of from 5 to 50% by weight solids.
• the resulting slurry is then chlorinated for a time sufficient to convert the silver content to silver chloride and to chlorinate most of the remaining metal values as chlorides.
• hydrochloric acid can be formed by reactions between chlorine and elements such as sulphur, selenium, tellurium, and arsenic or compounds thereof.
• a convenient temperature range for chlorination is from 60 to 80°C. Since chlorination is usually exothermic, cooling of the solution may sometimes be required, and the rate of addition of chlorine should be controlled to avoid possible overheating and/or excessive chlorine consumption.
• the silver-containing residue can then be leached, preferably at ambient temperature, with an aqueous thiosulphate solution, e.g. sodium thiosulphate, to dissolve silver selectively with regard to impurities such as silica and ferrites which may also be present in the silver-containing residue.
• an aqueous thiosulphate solution e.g. sodium thiosulphate
• the thiosulphate leach may be conducted at a temperature of from 10 to 80°C.
• Thiosulphate should be used in approximately the proportions of two to four mols of thiosulphate for each mol of silver to be leached.
• The'pH of the thiosulphate solution should be at least 2,and when lead is present in the chlorine leach residue to an extent requiring selective removal of the silver, the pH of the thiosulphate leach solution should be at least 7 and more preferably ⁇ pH 9 or pH 10.
• the thiosulphate leach is highly selective for silver as compared with lead in the precipitate being dissolved.
• a further advantage of maintaining the thiosulphate solution basic is that the stability of the solution is thereby increased. Small amounts of sulphite, e.g. sodium sulphite, added to the thiosulphate leach solution will also improve its stability.
• the thiosulphate leach solution containing silver from the residue may be treated in a variety of ways to recover the silver.
• cementation with metals such as iron, zinc or magnesium at ambient temperature produces cements analyzing about 90% silver.
• Organic reducing agents e.g. fructose, dextrose and lactose, can be used to produce silver precipitates of high purity, e.g. containing at least 90% of silver.
• Electrolytic recovery means may also be used.
• the composition of the silver-containing precious metal intermediates to be treated in accordance with the invention can vary widely depending upon the ore from which they are obtained, and it may be convenient to subject them to preliminary treatment.
• the sludge may be given a decopperizing leach in sulphuric acid prior to treatment in accordance to the invention.
• a lead removal step may be employed prior to treatment of the intermediate in accordance with the invention.
• the chlorine leach residue was then leached with thiosulphate as follows: 33.8 litres of water was added to 15.5 kg of the above chlorine leach residue, which already contained 39% moisture. NaOH was added to the agitated slurry to bring the pH to 10.0 at 22°C. Then 3.75 kg of Na 2 S 2 0 3 was added (3.9 kg Na 2 S 2 0 3 /kg Ag) and the slurry was agitated for 30 minutes at a pH of 10 (22°C). The residue was filtered off and washed with one cake displacement of water.
• the leach residue contained a large quantity of filter-aid, which tends to trap large amounts of leach liquor
• the wet residue was subjected to a repulp leach using 11 litres of water and 0.4 kg of Na 2 S 2 0 3 .
• Leaching was again conducted for 30 minutes at 22°C (pH 10).
• the leach residue was separated from the solution by filtration.
• the leach , residue analyzed 0.11% Ag and 5.2% Pb. 99.1% of the silver was extracted with only 0.8% of the lead.
• This example shows the effect of varying the chlorine leaching conditions.
• This example shows the effect of acidity on the extraction of silver by a thiosulphate solution.
• the leach conditions were similar to Example 1 and are listed in Table 2, together with the results of duplicate tests.
• the extraction of silver was lowest ( ⁇ 97%) at a pH of 2.0, and was generally 99% or higher between pH 4 and 12.
• the dissolution of lead was strongly influenced by the pH. At pH 6 and below, more than 75% of the Pb was dissolved, whereas at pH 8 less than 20% Pb was extracted and at a pH of 10 the dissolution of Pb was only ⁇ 1%.
• compositions of five other precious metal-containing materials susceptible to treatment in accordance with the invention are shown in Table 3. Each of these materials was chlorine leached under the conditions and for the times shown in Table 4. It will be seen from Table 4 that excellent extractions of platinum-group metals and gold were achieved with all the materials treated although the compositions thereof varied widely. On the other hand, extractions of silver were low.
• the compositions, in weight per cent, of the chlorine leach residues are shown in Table 5. All of the chlorine leach residues were susceptible to thiosulphate leaching, in accordance with the invention, to dissolve silver.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacture And Refinement Of Metals (AREA)

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Cited By (7)

Citations (6)

• 1982
  • 1982-03-11 CA CA000398157A patent/CA1188522A/en not_active Expired
• 1983
  • 1983-03-10 EP EP83301318A patent/EP0089184A1/de not_active Withdrawn

Patent Citations (6)

Non-Patent Citations (1)

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