Thiourea leaching

Such reagent losses need to be minimized to ensure that thiourea leaching finds widespread use. Leaching with chlorine in acidic media proceeds via a Au1 intermediate ... 

Murthy, D. S. R. Kumar, V. Rao, K. V. Extraction of gold from an Indian low-grade refractory gold ore through physical beneficiation and thiourea leaching. Hydrometallurgy 2003, 68, 125-130. 

Lacoste-Bouchet, P. Deschenes, G. Ghali, E. Thiourea leaching of a copper-gold ore using statistical design. Hydrometallurgy 1998, 47, 189—203. 

Bruckard, W. J. Sparrow, G. J. Woodcock, J. T. Gold and silver extraction from Hellyer lead-zinc flotation middlings using pressure oxidation and thiourea leaching. Hydrometallurgy 1993, 33, 17 41. 

Shade, C.W. Automated simultaneous analysis of monomethyl and mercuric Hg in biotic samples by Hg-thiourea complex liquid chromatography following acidic thiourea leaching. Environ. Set Technol. 2008, 42, 6604-6610. 

Salim OM, Mahir I, Mahmut Bayramo glu (2005) Leaching of silver from solid waste using ultrasound assisted thiourea method. Ultrason Sonochem 12(3) 237-242... 

Sulfidic matrices often require an oxidative pre-treatment prior to leaching the gold (see above). Bio-oxidative processes to achieve this usually generate acidic aqueous phases which must be neutralized prior to cyanidation. To avoid this neutralization step, lixiviants such as thiourea have been considered, which can be used in acidic media.57-67 The thiourea-based processes have a... 

The development of more benign alternatives to cyanide for gold-leaching (see Section 9.17.3.1) such as thiourea, thiocyanate, or thiosulfate, which form stable complexes in water has prompted research to identify suitable solvent extractants from these media. Cyanex 301, 302, 272, Ionquest 801, LIX 26, MEHPA, DEHPA, Alamine 300 (Table 5) have been evaluated as extractants for gold or silver from acidic thiourea solutions.347 Whilst the efficacy of Cyanex 301 and 302 was unaffected by the presence of thiourea in the aqueous feed, the loading of the other extractants is severely depressed. Formation of solvated complexes of gold and of an inner-sphere complex of silver has been proposed.347... 

Ubaldini, S. Fornari, P. Massidda, R. Abbruzzese, C. An innovative thiourea gold leaching process. Hydrometallurgy 1998, 48, 113-124. 

Orgul, S. Atalay, U. Reaction chemistry of gold leaching in thiourea solution for a Turkish gold ore. Hydrometallurgy 2002, 67, 71-77. 

Gonen, N. Leaching of finely disseminated gold ore with cyanide and thiourea solutions. Hydrometallurgy 2003,69,169-176. 

Almeida, M. F. Amarante, M. A. Leaching of a silver-bearing sulfide by-product with cyanide thiourea and chloride solutions. Miner. Eng. 1995, 8, 257-271. 

After the optimization of these conditions, by adding an azide to the input stream it was possible to synthesize a range of substituted triazoles in a heterogeneously catalysed three-component reaction (Scheme 18). After the CFC, the stream was passed through a column containing a resin-immobilized copper-based catalyst, which was used in a previous work by the same authors to successfully catalyze the formation of triazoles from alkynes and azides [44]. An immobilized thiourea-containing cartridge was subsequently used to remove any leached Cu catalyst. In a similar way as for the alkynes production, the series of resins was used to purify the product. 

Ion-Selective Electrodes based on Bis-Thiourea Receptors. Bis-thiourea derivatives 14, 15, and 17, which have a good membrane solubility, sufficient lipophilicity to prevent leaching into the aqueous sample solution, and a low tendency for self-aggregation in nonpolar solvents, were incorporated into PVC matrix liquid membranes for ISEs. While membrane electrodes based on the dibutyl derivative 14 gave a phosphate response almost identical to that of a conventional anion-exchanger electrode, a membrane electrode based on the phenyl-substituted bis-thiourea 15 exhibited a slightly improved phosphate response, which seems to be the result of improved complexation of phosphate in the sensor membrane. 

Extraction is rapid compared to that by cyanidation, and the gold surface is not subject to the passivation observed in alkaline cyanide solutions. At present, the main disadvantage of the process is the high cost of the reagents, which is compounded by the instability of thiourea with respect to hydrolysis and to oxidation under the conditions of the leach. 

As a result of increasingly stringent environmental regulations, much attention has been devoted in recent years to the possibility that gold and silver can be leached by ligands other than cyanide. Other considerations, for example the presence of excessive quantities of soluble copper in the ore or the occurrence of gold in refractory (to cyanidation) minerals such as arsenopyrite or stibnite, have also prompted the search for alternatives to cyanide. Of the various possibilities, the one most studied and applied is the use of thiourea in acidic solutions with iron(III) as the oxidant. The principal leaching reaction is... 

Van Deventer, J.S.J., Reuter, M.A., Lorenzen, L. Hoff, P.J. (1990). Galvanic interactions during the leaching of gold in cyanide and thiourea solutions. Minerals Engineering, 3(6), 589-597. 

Fig. 5.2 Comparative leaching of gold and silver from ore using aqueous H SO or [Bmim] HSO4 in presence of Fe2(S04)3 and thiourea (Reproduced from Ref. [7], with kind permission of The Royal Society of Chemistry)...

Fig. 5.4 Extraction of goid and siiver (a) and base metais (b) from Perseverance ore using thiourea in ionic iiquids with the [Bmim ] cation and different anions (50°C, 48 h leaching) (Reproduced from Ref. [24], with kind permission of Elsevier)...

Gold. Because of the extreme toxicity of cyanide, a number of alternative leaching processes have been devised for gold ore. The only one of these to be commercialised involves thiourea, which uses ferric sulphate as an oxidant ... 

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