ELECTROLYTIC RECOVERY OF CYANIDE AND HEAVY METALS FROM CYANIDE LIQUORS AND EFFLUENTS

 

R. L. C. Santos & L. G. S. Sobral*

CETEM-Centro de Tecnologia Mineral, Rua 4, Quadra D, Cidade Universitária, Rio de Janeiro, Brasil, CEP. 21941-590

Lsobral@cetem.gov.br

Phone: (021)560-7222, Fax: (021)260-9154

 

Abstract

 

In this technical contribution the efforts were  focused on designing a particular reaction system comprising a side-by-side cell with a cation exchange membrane (Nafionâ 417 –DuPont) separating the anolyte  from the catholyte to abate heavy metals from cyano-complexes, such as (Cu(CN)32-, Ag(CN)2-, Au(CN)2- etc), and recover free cyanide as such.

 

Introduction

 

Some industrial processes such as hydrometallurgical extraction of gold and metal plating use alkaline cyanides (NaCN or KCN) as main reagents. The liquid effluents from such processes contain free cyanide, heavy metal cyano-complexes and tiocyanate, that are toxic for the human being and aquatic organisms. Those chemicals present different stability as well as toxicity and treating modes, being recommended to be removed from those effluents before releasing them to the environment. The main commercially used treatment processes, are, according to Potter et all1, the oxidation with chlorine, ozone or hydrogen peroxide. The natural degradation, biodegradation and electrolytic treatment are also considered. Despite an extensive work carried out by Ingles and Scott2, the comparison among the methods, and the researches, in a pilot plant, of the Homestake Co.3, the using limits of such methods are not completely established, and the choice of the most suitable one will depend on each particular case.

 

In the conventional gold hydrometallurgy the leaching solutions contain free cyanide in the range of 0.1 to 0.3% w/v, while in the plating solutions such concentration goes up to 150 g/L, being common to find, in rinsing water, around  3 to 5 g/L. Considering the relative cost of cyanide, especially in plating industries, that use potassium cyanide rather than sodium cyanide, being, in general, 15 to 20% of the variable costs, the possibility of regenerating such chemical is becoming quite attractive. In addition, the degradation processes under practice, on such industrial sectors, do not allow neither the recovery of heavy metals nor the free cyanide from such liquid effluents.

 

Electrodeposition is an important and effective method for the recovery and recycling  of metals from mainly aqueous process streams. The range of implications is diverse with several commercial cell designs available4. Although many waste and process solutions contain mixtures of metal ions the majority of experimental studies have considered solutions containing single metal ions. Only few have considered aspects of electrodeposition of a single metal ion from a mixture of  two or more metal ions in solution.

Experimental

 

The solution used for deposition of the aforementioned metals contained 0.5 mol dm-3 K2SO4 (AnalaR) as a base electrolyte and the pH being adjusted to 11 by using sodium hydroxide, when needed, after being added the previously prepared complex metal cyanides . Solutions were made up using deionized water and de-aerated with oxygen-free nitrogen prior to the electrochemical experiments.

 

The deposition of the metals were previously studied at a rotating vitreous carbon (RVC) disk (area = 3.7 x 10-3 dm2) embedded in a TeflonŇ holder and attached to a rotating disk assembly by Sobral (1993)5.

 

The electrowinning reaction system (laboratory cell and flow system) is sketched in Fig.2.  The anode used was a platinized titanium mesh electrode and the current collector for the nickel foam cathode was a gold plated copper sheet. The nickel foams used were supplied by Sumitomo Electric Industries Ltd. with 17, 25 and 35 pores per inch (p.p.i.) with respective specific surface areas of 1000, 1700 and 2500 m-1. The anolyte and catholyte were separated by a NafionŇ 417 cation exchange membrane (DuPont). The anolyte and catholyte reservoirs were filled with 4 dm3 of electrolyte. Both reservoir were fitted with nitrogen bubblers to remove dissolved oxygen. With the exception of the glass reservoirs, the electrolyte flow system was entirely constructed from PVC pipework.

 

The anolyte was 0.5 mol dm-3 sodium hydroxide plus a mixture of  0.125 mol dm-3 K4Fe(CN)6 and 0.0025 mol dm-3 K3Fe(CN)6. The anodic process was thus the oxidation of potassium ferrocyanide to ferricyanide, as follows:

 

 

The high ratio of K4Fe(CN)6:K3Fe(CN)6 in the anolyte enables oxygen evolution at the anode to be avoided:

 

 

The catholyte was prepared by using analytical reagents and distilled water. Standard solutions were prepared by dissolving KAu(CN)2, which the deposition mechanis was well studied by Sobral (1993)5, in water and/or potassium sulfate which served as supporting electrolyte. The pH of the solution was adjusted with sodium hydroxide. The concentration of gold in solution was 0.25 mol m-3. All electrochemical experiments were controlled with a potentiostat (EG&G-Princeton Applied Research, Universal Programmer, model 175). The applied potentials and the resulting currents were stored in a PC computer using data acquisition software (Labtech Notebook) and subsequently analysed. The analysis of metals in the catholyte was carried out by atomic absorption spectroscopy and the free cyanide analyzed by potentiometric titration.

 

 

Fig. 2- Reactor system (electrowinning cell and solution flow).

 

The catholyte was prepared by using analytical reagents and distilled water. Standard solutions were prepared by dissolving KAu(CN)2, which the deposition mechanis was well studied by Sobral (1993)5, in water and/or potassium sulfate which served as supporting electrolyte. The pH of the solution was adjusted with sodium hydroxide. The concentration of gold in solution was 0.25 mol m-3. All electrochemical experiments were controlled with a potentiostat (EG&G-Princeton Applied Research, Universal Programmer, model 175). The applied potentials and the resulting currents were stored in a PC computer using data acquisition software (Labtech Notebook) and subsequently analysed. The analysis of metals in the catholyte was carried out by atomic absorption spectroscopy and the free cyanide analyzed by potentiometric titration.

 

Results and Discussions

 

Observing the curves of Figure 6, for the potential sweep toward the reduction of Au(CN)2-  ions, passing the solution once through the cell, of the reaction system of Figure 2, the current rises rapidly owing to the onset of the reduction of those ions transported to the cathode surface by convective diffusion. A well defined limiting current region is observed at different flow velocities. The same trend was observed for the reduction of the other cyanide complexes even using different nickel foam grades and flow velocities.

Fig.3- Current-potential curves for the reduction of 0.25 mol m-3 gold in nitrogen purged 500 mol m-3 K2SO4, pH 11 solution. Cathode Ni foam 17 p.p.i., 10 cm of length, and a potential sweep rate of  0.010 V s-1.

 

The maximum yield for gold electrowinning was obtained at -1.150 V(SCE), where the reaction system of the Figure 2 behaves as a continuously stirred tank reactor under mass transport control in a batch recycle mode. While running the gold electrowinning it was also observed that the free cyanide content increased with the electrolysis time in the batch recycle mode.

 

Conclusions

 

Reticulated nickel foam, a recently developed form of three-dimensional electrode, can be successfully used as an electrode material to electrodeposit heavy metals like the ones used in this study.

It was possible to electrowin the metals either simultaneously or separately at the surface of the metallic foam passing the catholyte solution once through the cell releasing an outlet electrolyte with higher free cyanide concentrations suitable to be recycled to the leaching process or to the plating tanks, depending on the origin of them. In addition, no matter what sort of electrolyte was used, the current efficiencies for the metals electrodeposition were enhanced as the dissolved oxygen was previously remove. This means that in a commercial scale the electrolyte can be de-aerated through a de-aeration tower before entering the cell.

 

References.

 

1)      G. M. Potter, Regeneration of Cyanide. A case Study. Pilot Cyanidation of a High-Copper Gold Ore from Punitaqui, Chile. Conference on Cyanide and The Environment, Tucson, Arizona , December 1984, page 457.

2)      Ingles, J. C. and Scott, J. S., Overview of Cyanide treatment Methods. Proceedings of Seminar, Cyanide in Gold Seminar, Otawa, January 1981.

3)      Withlock, J. L. and Muddler, T. I., The Homestake Treatment process. Proceeding of International Symp. On Biohydrometallurgy, 6, Vancouver, August 1985.

4)      D. Pletcher and F.C. Walsh, Industrial Electrochemistry 2nd edn. Chapman and Hall (1990).

5)      L. Sobral, The Electrowinning of Gold from Dilute Cyanide Liquors Using a three-dimensional Nickel Foam, PhD. Thesis at Imperial College, London, UK, 1993.