Cyanide complexes process

The differences between the individual iron-cyanide complex processes stem from the type of complex selected and the method of regeneration. In two processes, that of the Gesellschaft fur Kohlentechnik and the Fischer process, alkaline aqueous solutions of potassium ferricyanide and ferrocyanide are used, and regeneration is carried out by contact with air and electrolysis, respectively. The other two processes of this category, the Staatsmijnen-Otto and the Autopurification processes, employ suspensions of complexed ferric-ferro-cyanide compounds in alkaline solutions that are regenerated by air contact. The latter two processes are essentially identical, although they were developed independently in 1945 in the Netherlands and in England, respectively. 

Sulfur Complexes. Silver compounds other than sulfide dissolve in excess thiosulfate. Stable silver complexes are also formed with thiourea. Except for the cyanide complexes, these sulfur complexes of silver are the most stable. In photography, solutions of sodium or ammonium thiosulfate fixers are used to solubilize silver hahdes present in processed photographic emulsions. When insoluble silver thiosulfate is dissolved in excess thiosulfate, various silver complexes form. At low thiosulfate concentrations, the principal silver species is Ag2(S203) 2j high thiosulfate concentrations, species such as Ag2(S203) 3 are present. Silver sulfide dissolves in alkaline sulfide solutions to form complex ions such as Ag(S 2 Ag(HS) 4. These ions are... 

Thus, co-deposition of silver and copper can take place only when the silver concentration in the electrolyte falls to a very low level. This clearly indicates that the electrolytic process can, instead, be used for separating copper from silver. When both silver and copper ions are present, the initial deposition will mainly be of silver and the deposition of copper will take place only when the concentration of silver becomes very low. Another example worth considering here is the co-deposition of copper and zinc. Under normal conditions, the co-deposition of copper and zinc from an electrolyte containing copper and zinc sulfates is not feasible because of the large difference in the electrode potentials. If, however, an excess of alkali cyanides is added to the solution, both the metals form complex cyanides the cuprocyanide complex is much more stable than the zinc cyanide complex and thus the concentration of the free copper ions available for deposition is considerably reduced. As a result of this, the deposition potentials for copper and zinc become very close and their co-deposition can take place to form alloys. 

Cyanide complexes have a venerable history (see CCC S )),1 and find utilization in many industrial processes including as synthetic catalysts e.g., Co cyanides on inorganic supports catalyze alkylene oxide polymerization,187 molecular magnetic materials, in electroplating, and in mining. Their pharmacology and toxicology is well explored... 

The above, together with the fact that a fourfold variation of [CN ] for the [WO(OH)(CN)4]3 complex showed zero-order dependence on free cyanide concentration further points to a dissociative activation for the cyanide exchange process, also in the case of the mono oxo (classic 16-electron) [MO(X)(CN)4]m species. 

The cyanide exchange processes are in most cases the slowest. However, it is interesting to note that in the case of the Mo(IV) center, previously unexplained more-rapid-than-expected oxygen exchange at pH values lower than 6 (Fig. 19b) might be associated with the cyanide exchange (which was found to be of comparable rate). It is quite possible that the rapid decomposition observed for the aqua oxo complex of Mo(IV) at these pH values is due to hydrolysis stimulated by the relative rapid cyanide exchange at this acidity. 

In the cyanidation process the ore is crushed and roasted with sodium chloride and then treated with a solution of sodium cyanide. Silver forms a stable silver cyanide complex, [Ag(CN)2]. Adding metallic zinc to this complex solution precipitates sdver. 

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

In the extraction process, the aurocyanide complex plus other metal cyanide complexes that are common in cyanide leach liquors (e.g. Fe(CN) 4-, Fe(CN)63-, Zn(CN)42-, Ni(CN)42-, Cu(CN)43-and Co(CN)63-) load onto the resin by simple ion exchange ... 

As discussed above, the ligands that have been typically utilized for the preparation of chromium nitrides are multidentate. Consequently, ligand exchange reactions of such complexes are difficult and rare. Wieghardt and co-workers have reported such a process, however, for the synthesis of a nitrido chromium cyanide complex 43 (Eq. (13)) [18]. Thus, treatment of CrN(salen) 42 with excess sodium cyanide and tetramethyl ammonium chloride results in the formation of a six-coordinate penta-cyano chromium nitride [21]. 

Another example of the effect of a change of concentration upon the cathodic process can be found in electrolysis of a solution of salts of copper and bismuth. As the respective deposition potentials, which practically equal the equilibrium potentials are fairly close (7c( — 0.34 V, 71 it, = 0.23 V) the two metals cannot he separated from each other electrolytically. On the addition of cyanide, however, Cu++ ions are converted into cupricyanide ions from which copper cannot be deposited prior the cathode reaches the potential Ttt u equalling to about — 1.0 V. As bismuth does not form cyanide complexes the resulting difference in potentials, 7ti — 7Cou — 1.23 V is a sufficient guarantee that during electrolysis only bismuth will be, preferentially deposited. 

A similar consideration can be applied to the cathodic processes. In a solution of mercuric nitrate bivalent mercury will bo reduced to univalent until the ratio of the respective activity of the mercurous salt formed and tho mercuric salt still remaining reaches the equilibrium value. During the course of further reaction the ratio of activities of both ions in the solution will not change any longer, and metallic mercury will be deposited. Therefrom, it is evident that mercuric nitrate cannot be quantitatively reduced to mercurous salt. Bivalent mercury can be reduced practically completely to univalent in the case of mercuric chloride. As the solubility of the mercurous chloride formed by the reduction and consequently also the concentration of Hg2+ ion is very small the equilibrium between the ions in the solution will be attained only then, when nearly all Hg++ ions will be reduced to univalent ones. On the other hand on reduction of the very slightly dissociated cyanide complex Hg(CN) the equilibrium between mercurous and mercuric ions is reached at the very beginning of electrolysis as soon as a hardly noticeable amount of Hg++ ions has been formed from that moment on metallic mercury will be deposited at the cathode with practically 100 p. o. yield. 

The oxidation of thiocyanate is a rather complex process, which may be dealt with in two separate steps. First, sulphate and cyanide ions are formed ... 

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