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Thermodynamics of cyanide complexes

Solis, Jose S. (1995) Thermodynamics of cyanide complexes. PhD thesis, Murdoch University.

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The work described in this thesis was to study the thermodynamics of the binding of cyanide to metal ions of relevance to the hydrometallurgical processing of gold. In particular, to produce reliable and accurate thermodynamic data needed to develop reliable chemical speciation models for the gold-refining industry, especially with regard to the high saline conditions of many Australian operations.

The stability constants of binary and ternary Cu(I) systems were studied using potentiometric glass electrode titration and evaluation of results by the ESTA suite of computer programs. In particular, the study of the Cu(I)-CN- system produced four major binary species in the conditions studied. The stability constants for the three species, Cu(CN)2-, Cu(CN)32-, and Cu(CN)43- are reported. The Cu(I)-NH3 system was attempted but oxidation of Cu(I) in the presence of ammonia was observed to be fast even in the absence of air. The presence of a high Cl" concentration stabilizes Cu(I) and since it also complexes with the metal the ternary system of Cu(I)-NH3-Cl- was considered. This study has established the existence of three ternary species, Cu(NH3)Cl°, CuNH3Cl2- and Cu(NH3)2Cl° together with the binary Cu(NH3)+ in the conditions of the present study. The other ternary system, Cu(I)-NH3-CN-, was studied but no analysis could be made with the present evaluation method. Failure of these experiments may be due to the closeness of the protonation constants of cyanide and ammonia in this medium, thus making it difficult to detect the minor effects in the experimental data which may arise from the ternary complex formation. Also, the experiment is restricted from the variation of the free cyanide and ammonia concentration because of the oxidation of Cu(I), volatility of NH3 at high p[H] and the volatility of HCN at low p[H].

Measurements of heat of reactions of common metal-cyanide systems were calorimetrically determined by the isoperibol continuous titration calorimeter. Enthalpies of metal-cyanide complexation were determined from these calorimetric measurements. Included in this study was the measurement of the heat of ionization of water and HCN as they were relevant to the metal-cyanide heats of reaction studies. These two systems were studied at variable ionic strength.

The solubility of AgCN and CuCN in HCN aqueous solution was studied. Another method of deriving equilibrium constants is from solubility measurements. The apparent solubility product constant of both AgCN and CuCN were sucessfully measured using the pH variation method.

Heat capacity measurements were conducted with cyanide solution using the Picker-flow calorimeter. Measurement of the volumetric heat capacities required densities which were measured accurately using a Vibrating Tube densimeter, thus the apparent molar volumes were also reported for these cyanide solutions. From these heat capacity measurements and other relevant literature data the change in heat capacity in association to the ionization of HCN was derived. For the first time, such data were determined calorimetrically and reported. The present values were obtained from NaCN and KCN respectively, and are, -228.5 and -222.8 J K-1mo-1. The results are in very good agreement and provides a most reliable estimate of ∆Cp° for the ionization of HCN.

By establishing reliable equilibrium constants in this way, a better and accurate description of the chemistry of a given aqueous cyanide solution is achieved. These thermodynamic information obtained at high saline medium serve to develop more realistic models of such hydrometallurgical solutions in local processing industries.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Physical Sciences, Engineering and Technology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Hefter, Glenn and May, Peter
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