Recovery of Copper and Cyanide from Copper-cyanide Solution by Direct Electrolysis

: Using direct electrolysis method, the recovery of copper and cyanide from high concentration copper cyanide solution was researched. The distribution of copper cyanide species was calculated by stability constants and balance equation. The results showed that the cupric ion was confirmed to generate at low potential (<0.4V) on the titanium electrode coated with iridium and tantalum oxides during the electrolysis process, which led to the oxidation of cyanide and the precipitate obtained on the anode was proved to be cupric hydroxide by XRD at high potential. The copper was recovered by direct electrowinning, the recovery of copper increased with increasing temperature, which reached 80% at 70℃; but the loss of cyanide was serious, the free cyanide concentration was significantly lower than original value.


Introduction
The cyanide is the most widely applied leaching reagent for gold minerals due to its high efficiency and low cost. However, as a common paragenetic element, amounts of copper can be dissolved during the cyanidation process, which significantly increased the consumption of cyanide. Furthermore, part of the dissolved copper ions became insoluble copper cyanide complexes and coated on the surface of gold-bearing minerals, which decreased the cyanide leaching speed. Since the cyanide solution containing copper can't be directly recovered for leaching, the separation-recovery of copper and cyanide respectively in the barren solution is the key point to solve the problem [1]. AVR (acidification, volatilization and regeneration) technique and some modifications thereof [2][3] have been widely used to treat the solution in the gold industry, but there is a hidden serious danger because the highly toxic of hydrocyanic acid produced in this process, while copper is recovered as complexingprecipitate which should be treated further to obtain metal and the natural degradation of residual cyanide in wastewater is necessary. Solvent extraction [4][5] before AVR has been investigated to recover copper and cyanide, the copper cyanide complexes can be effectively extracted and concentrated into higher concentration for further recovery; however, the high concentration copper cyanide solution is still treated by traditional method, therefore, the same danger still exists.
Direct electrowinning has been researched for a long time and considered to be a potential technique for recovery of copper and cyanide. Since the cyanide would be destroyed in electrowinning process [6], the membrane was chose to avoid cyanide oxidation on anode [7][8]. However, the expensive price and short lifetime of membrane seriously restrain its industrial application. Furthermore, most work [9][10][11] has been focused on direct electrowinning of dilute copper cyanide solution whose current efficiency is low. It has been demonstrated that the copper can catalyze cyanide decompose; However, the reaction mechanism of copper cyanide oxidation on anode are incomplete [12][13][14]. On the other hand, less investigation has been carried on the high concentration copper cyanide solution at present. It is obvious advantage that electrodepositon of concentrated solution because the current efficiency is greatly improved. It is necessary to pay attention to the oxidation of cyanide on anode, which is the key to achieve direct electrowinning concentrated copper cyanide solution.
In this work, direct electrowinning of high concentration copper cyanide solution was carried out to recover copper and cyanide. The distributions of copper cyanide species were calculated by stability constants at different temperatures and pH [15]. Cyclic voltammetry behaviors of copper cyanide complex were discussed on different electrodes. Correspondingly, the electrochemical reactions in this process were proposed.

Experimental
All solutions were prepared by analytical grade reagents and deionized water. The CuCN was dissolved into NaOH + NaCN solution and adding Na 2 SO 4 to make the final solution. The free cyanide concentration was measured using titration with silver nitrate, potassium iodide as indicator. The copper concentration of copper cyanide solution was analyzed by atomic absorption spectrometry (AAS). The current efficiency of cathode was determined by the initial and final concentration in the solution.
Copper cyanide dissolved into cyanide solution to form cyanocuprate ions, the concentration distributions are dependent on pH, temperature and the total concentration of copper and cyanide. The concentration of free cuprous is so low that it can be ignored. The equilibrium of copper cyanide complex can be represented by the reactions (1)~ (3): The respective stability constants of reactions at different temperatures were listed in Table 1. The pH value was measured by PHS-3C instrument. The mass balance of the copper and cyanocuprate species can be described by the following equation: By considering reactions (1)~(3) and solving the equation of the mass balance, the distribution of copper cyanide species can be calculated.
The cycle voltammetry of copper cyanide solution was detected by electrochemical workstation (Zahner_IM6e), the working electrode was titanium coated with iridium and tantalum oxides with an area of 1 × 1 cm 2 , cleaned by nitric acid and washed by deionized water. Titanium plate was used as counter electrode and saturated calomel electrode as the reference electrode.
Potentiostatic electrolysis was carried out to define the process of anodic reaction and sodium diethyldithiocarbamate was added at low potential, which could form cupric precipitate on the anode to prove the generation of Cu 2+ . The phase of anode product was identified by a D/max-2500PC X-ray diffractometer (Japan) with Cu K a radiation.
Direct electrowinning was used to recover copper with plexiglass cell at different temperature. The anode was titanium coated with iridium and tantalum oxides, while the titanium plate was cathode. As the constant current electrowinning method was used, the anode potential (vs. SCE) was measured by Agilent 34410A instrument. The current efficiency of cathode was determined by measuring the mass of cathode.

Distribution of copper cyanide species
The concentration of Cu(CN)-2 was obtained by the following equation: The same method was also used to calculate Cu(CN)2-3and Cu(CN)3-4 concentration. The results of distribution showed the Cu(CN)-2concentration was less than 10 -5 mol/L at 25℃, Cu(CN)2-3and Cu(CN)3-4 were dominant in copper cyanide solution at pH 12, copper concentration was 0.125 mol/L, free cyanide was 0.096 mol/L.  Fig. 1 shows the relationship between distribution of copper cyanides and pH at 25℃. As can be seen, the Cu(CN)3-4 complex ions were the main existence form of copper cyanide complexes in solution, and the proportion of Cu(CN)3-4 concentration improved with the pH increased and tended to be stable when the pH exceeded 11. The relation of Cu(CN)2-3concentration vs. pH was contrary to Cu(CN)3-4. The distribution of copper cyanide species was depended on the ratio of copper and free cyanide from reaction (5) at certain temperature. Since the dissociation equilibrium of HCN was affected by pH, the content of free cyanide increased with increasing of pH, which led to more cyanide chelated with copper.  Fig. 2 shows the distributions of copper cyanide species under different temperature at pH 12. As can be seen, the temperature was a significant factor of the distribution of copper cyanide species, because the stability constants was relate to temperature. The content of Cu(CN)2-3 increased with the temperature increasing and Cu(CN)3-4concentration decreased gradually. The Cu(CN)2-3 was dominant in the distribution as temperature exceeding 60 o C. Fig. 3 showed the cyclic voltammetry curve of copper cyanide solution at 25 o C on a platinum anode. The anodic reaction of copper cyanide solution can be divided into three parts. The first region was about 0.35~0.47 V (vs. SCE), the precipitate was not found on anode by potentiostatic electrolysis at 0.4 V, the conversion of Cu(I)/Cu(II) was assumed in the region which was proved in subsequent work. The current increased and reached a limiting value, then it decreased with increasing potential from about 0.5 to 0.58 V. The free or complex cyanide was oxidized in the range which led to current increased sharply, and the precipitate was formed on the anode which was the reason of current decay. When the potential exceeded 0.58 V, the current increased with increasing potential, the bubble was observed on the surface of anode. The similar results were obtained in Fig. 4 by titanium coated with iridium and tantalum oxides. Polarization measurements were carried out at different temperature, with the results shown in Fig. 5. The current increased with increasing temperature because the Cu(CN)2-3 was easier oxidized than Cu(CN)3-4 and the distribution of copper cyanide species was changed. The limit current disappeared when the temperature reached 50 o C however the precipitation still existed on the anode, which showed that the formation of precipitation can been inhibited at a higher temperature but not been avoided.  Fig. 6 shows the XRD pattern of anodic precipitate after potentiostatic electrolysis. As can be seen, the production of Cu 2+ was proved by adding sodium diethyldithiocarbamate at low potential (0.4 V), which indicated that the reaction of Cu + oxidation occurred. Cu(CN)3-4 being supposed predominant, the anode reactions were represented by the follow steps:

Analysis of anode products
2Cu 2+ + 10CN -→ 2Cu(CN)3-4 + (CN) 2 (7)  Fig. 7 showed the XRD pattern of anode after electrolysis at 0.5 V. As can be seen, the precipitation on the anode was cupric hydroxide. When a large number of Cu 2+ generated on the anode, there were not enough cyanide to reduce cupric ions on the surface of anode, which led to cupric hydroxide formed. The cupric hydroxide have been proved to catalyze the oxidation of cyanide, the cyanide was oxidized directly on the anode coated Cu(OH) 2 . Thus, high anodic potential accelerated the oxidation of cyanide to reduce the recovery of cyanide. The concentration-time relationships of copper and cyanide at different temperature were shown in Fig. 8 and Fig. 9 respectively, the anode potential was controlled to less than 0.5 V. The reaction rate of electrodepositon increased slightly at 30~60 o C but the rate was accelerated significantly at 70 o C. The final copper concentration decreased with decreasing temperature but there was little difference at 50~70 o C, the recovery of copper reached 80 wt%. The temperature changed the distribution of copper cyanide species, the Cu(CN)2-3 was the main complex ion at 70 o C, the reaction occurred more easily because the deposition potential of Cu(CN)2-3 was more lower than Cu(CN)3-4. Fig.9 showed the change of the free cyanide concentration with time, the effect of temperature was significant. At 70 o C, the concentration of cyanide increased sharply on initial stage because the coppercyanide complex dissociated and free cyanide was released, then decreased further with increase in time.

Copper recovery by electrolysis
The decrease of current efficiency caused the releasable cyanide couldn't counteract the loss of oxidation on anode, the final value of free cyanide decreased significantly compared to the original concentration, the cyanide wasn't effectively recovered.

Conclusions
1) The copper cyanide species mainly existed as Cu(CN)2-3 and Cu(CN)3-4, the latter was usually dominant at normal temperature. The distribution was significantly controlled by temperature and Cu(CN)2-3changed to primary complex at 60 o C.
2) The formation of cupric ion was confirmed by experiment with adding sodium diethyldithiocarbamate at low potential and the cupric ion oxidization was the main cause of the cyanide loss. The precipitate on the anode at high potential was identified as cupric hydroxide.
3) The copper can be recovered effectively by direct electrowinning but the cyanide was damaged.