OVERVIEW OF ELECTROWINNING PROCESS
The Allglass Australia Electrowinning Cell is used to recover precious metal, usually gold and silver, from solution (eluates) produced in the elution of carbon.
The eluate can be derived from either of two different elution systems, AARL or ZADRA, each system produces an eluate with very different properties, as shown in Table 1.
The Allglass Australia electrowinning cell can be operated in conjunction with either system.
AARL (ANGLO) ELUTION
The carbon is first pretreated with a concentrated caustic cyanide solution (1.3% NaOH and 3-5% NaCN) followed by elution with fresh water containing minimal dissolved solids. Elution temperatures range from 95-120 degrees Celsius. The solutions are passed only once through the carbon and collected in a storage tank. The combined electrolyte is then circulated through the electrowinning cells for 12-18 hours until an acceptable barren level, typically 3ppm Au, is reached.
A variation of the basic Anglo method is the split elution system whereby the second part of the eluate is recycled to the first half of the next elution. A smaller volume of electrolyte and a more concentrated solution is produced in this case.
ZADRA ELUTION
A caustic/cyanide solution is made from potable quality water for use as an eluate. Elution temperatures vary from 90 to 95 degrees Celsius for atmospheric ZADRA to 140 degrees Celsius for pressure ZADRA. The eluate is circulated through heaters to the elution column and then to the electrowinning cell. In the case of the pressure ZADRA elution, the eluate leaving the column is cooled to 90-95 degrees Celsius to pass through the electrowinning cell.
This recirculation is repeated until an acceptable barren level is reached.
|
PROPERTY |
AARL |
ZADRA |
|
NaOH(%) |
0.1-0.3 |
1 |
|
NaCN(%) |
0.2 |
0.2 |
|
pH |
12.5 |
13 |
|
Temp (Degree C) |
40-60 |
80-95 |
|
Au (ppm) |
100-750 |
5-50 |
|
Conductivity (mScm -1) |
20 |
40 |
|
Elution times (hrs) |
3-6 |
12-36 |
|
Typical cell efficiency (per pass at 7.5m3/h) |
70 |
90 |
Table 1; Typical Electrolyte Solutions from the AARL and ZADRA Elution methods
ELECTROWINNING
The electrowinning cell consists of a series of Anode (positive) and Cathode (negative) electrodes. The reactions that occur in the cell at the surface of these electrodes are heterogeneous, involving electron exchanges.
The metal (M) is deposited at the cathode according to the following equation:
M(CN) y-x + ye -> M + xCN-
Hydrogen is also evolved at the cathode:
2H2O + 2e -> H2 + 20H-
The major reaction at the anode is the oxidation of water to oxygen:
2H2O -> 4H+ + O2 + 4e
The recovery per pass achieved in practice will depend on many factors, including electrolyte temperature, ion concentrations and flow rates. Typical values are given in Table 1 above.
