Copper cyanidation-precipitation

A silver (or copper) catalyst suitable for the oxidation of methanol may also be prepared by heating silver or copper cyanide or a mixture of these in the presence of air to the point where puffing occurs. By incorporating a fervo- or ferri-cyanide, e.g., bismuth ferro-cyanide, bismuth ferri-cyanide, calcium cerium ferro-cyanide, cerium cobalt ferro-cyanide, vanadium or molybdenum ferro-cyanide with the starting material, an activated product may be obtained. The silver or copper cyanides are prepared by precipitating a soluble cyanide with silver nitrate or cupric chloride respectively.30... 

Addition of aqueous cyanide ion to a copper(II) solution gives a brown precipitate of copper(II) cyanide, soluble in excess cyanide to give the tetracyanocuprate(II) complex [Cu(CN)4] . However, copper(II) cyanide rapidly decomposes at room temperature, to give copper(I) cyanide and cyanogen(CN)2 (cf. the similar decomposition of copper(II) iodide, below) excess cyanide then gives the tetracyanocuprate(I) [Cu(CN)4] . 

Copperil) cyanide. CuCN (and copperil) thiocyanate), are similarly obtained as white precipitates on adding cyanide and thiocyanate ions (not in excess) respectively to copper(II) salts ... 

The concentration of cyanide, CN, in a copper electroplating bath can be determined by a complexometric titration with Ag+, forming the soluble Ag(CN)2 complex. In a typical analysis a 5.00-mL sample from an electroplating bath is transferred to a 250-mL Erlenmeyer flask, and treated with 100 mL of H2O, 5 mL of 20% w/v NaOH, and 5 mL of 10% w/v Kl. The sample is titrated with 0.1012 M AgN03, requiring 27.36 mL to reach the end point as signaled by the formation of a yellow precipitate of Agl. Report the concentration of cyanide as parts per million of NaCN. 

The chlorination process, introduced in Europe in 1843, roasted ore with chlorides, followed by a hot brine leach and subsequent precipitation of the silver on copper. In 1887 it was discovered that gold and silver can be recovered by sodium cyanide, and this process displaced the dangerous chlorination process. By 1907 the cyanide process, where a cyanide solution is mixed with 2inc dust to precipitate the silver, was universally in use. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

Iodide ions reduce Cu to Cu , and attempts to prepare copper(ll) iodide therefore result in the formation of Cul. (In a quite analogous way attempts to prepare copper(ll) cyanide yield CuCN instead.) In fact it is the electronegative fluorine which fails to form a salt with copper(l), the other 3 halides being white insoluble compounds precipitated from aqueous solutions by the reduction of the Cu halide. By contrast, silver(l) provides (for the only time in this triad) 4 well-characterized halides. All except Agl have the rock-salt structure (p. 242). Increasing covalency from chloride to iodide is reflected in the deepening colour white yellow, as the... 

Determination of silver as chloride Discussion. The theory of the process is given under Chloride (Section 11.57). Lead, copper(I), palladium)II), mercury)I), and thallium)I) ions interfere, as do cyanides and thiosulphates. If a mercury(I) [or copper(I) or thallium(I)] salt is present, it must be oxidised with concentrated nitric acid before the precipitation of silver this process also destroys cyanides and thiosulphates. If lead is present, the solution must be diluted so that it contains not more than 0.25 g of the substance in 200 mL, and the hydrochloric acid must be added very slowly. Compounds of bismuth and antimony that hydrolyse in the dilute acid medium used for the complete precipitation of silver must be absent. For possible errors in the weight of silver chloride due to the action of light, see Section 11.57. 

An interesting application of these results is to the direct quantitative separation of copper and cadmium. The copper is first deposited in acid solution the solution is then made slightly alkaline with pure aqueous sodium hydroxide, potassium cyanide is added until the initial precipitate just re-dissolves, and the cadmium is deposited electrolytically. 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

Copper forms practically aU its stable compounds in -i-l and +2 valence states. The metal oxidizes readily to -i-l state in the presence of various com-plexing or precipitating reactants. However, in aqueous solutions +2 state is more stable than -i-l. Only in the presence of ammonia, cyanide ion, chloride ion, or some other complexing group in aqueous solution, is the +1 valence state (cuprous form) more stable then the +2 (cupric form). Water-soluble copper compounds are, therefore, mostly cupric unless complexing ions or molecules are present in the system. The conversion of cuprous to cupric state and metalhc copper in aqueous media (ionic reaction, 2Cu+ — Cu° -i- Cu2+) has a Kvalue of 1.2x106 at 25°C. 

Copper(l) cyanide is a precipitate obtained by adding potassium cyanide solution to an aqueous solution of Cu2+ salt ... 

The redispersion of a sol which has been precipitated by the addition of an electrolyte may also occasionally be effected by the removal of the precipitating agent through washing thus certain precipitates such as silver chloride, zinc sulphide and copper ferro-cyanide are readily redispersed in water where the precipitating salts are removed by thorough agitation and filtration. 

System IV is a pretreatment technology for water containing cyanide and heavy metals including chromium, nickel, zinc, lead, cadmium, and copper. The technology precipitates a range of heavy metals there is no need to install separate pieces of equipment for individual metals. A cyanide treatment system expansion option is available for waste streams that also contain cyanide. System IV is not offered commercially. 

Sometimes, when tho process is conducted in tho manner just described, cyanide of iodine is produced, and is found in the third receiver in white needle-shaped crystals. The residue In the retort still retains some iodine In the form of iodide of lead and sodium. This can bo recovered by tho method proposed by Soubetean—namely, the addition of sulphate of copper, by which a subiodido of copper is formed. The precipitate is filtered, and the filtrate, which still retains some iodine, treated with a further quantity of tbo cupreous salt and iron filings, when another interchange takes place, and a further precipitation of iodide of copper occurs. Tho first of these changes may be expressed thus... 

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