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Process Electrowinning For Gold Mining

Electrowinning is an electrochemical process used to reduce the metal cations to the cathode surface of an aqueous solution derived from the chemical leaching proces. Electrowinning or often referred to as electroextraction is the electrodeposition of metals from ore minerals that have dissolved into the liquid and will be processed using Electrorefining to remove impurities from the metal. Electrowinning an electrolytic process that is very old in the mining industry

The English chemist Humphry Davy first obtained sodium metal in the form of the element in 1807 by electrolysis of molten sodium hydroxide. And then in 1847 Maximilian, Duke of Leuchtenberg first to show an experiment about Electrorefining copper. Then James Elkington patented a commercial process in 1865 and successfully opened the first factory in Pembrey, Wales in 1870. The first commercial plant in the United States is Balbach and Sons Refining and smelting Company in Newark, New Jersey in 1883

     Electrowinning is applied to a variety of chemical solutions that are found in large mining industry. This process involves the use of alkaline cyanide solution as an electrolyte in a cell as the anode and cathode, among others, can use:

     ( - )   
99,99 %
( + )

Cell reaction that occurs is: Merkury dicoating Copper, Lead, Iron, Aluminum
Anode: 2OH- O2 + H2O + 2e-
Cathode: 2Au (CN) 2 - + 2e- 2Au + 4CN-
Overall: 2Au (CN) 2 - + 2OH- 2Au + O2 + H2O + 4CN-

        Metals most often processed in electrowinning techniques are copper, gold and silver. Most metals occur in nature in the form of oxidized ore and thus should be reduced to form a pure metal. Ore is dissolved into some preprocessing in aqueous electrolyte or in the molten salt and the resulting solution is electrolysis. Then deposited on the cathode metal (whether in solid or in liquid form), while the anodic reaction is usually oxygen evolution. Some metals are naturally present as metal sulfides; These include silver, tin, molybdenum, cadmium, nickel, silver, cobalt, and zinc. In addition, gold and platinum group metals associated with sulfidic base metal ores.  
Most metal sulfide or its salts, are electrically conductive and this allows for efficient electrochemical redox reaction occurs in the liquid state or in aqueous solution.Some metals, including arsenic and nickel did not come out, but still electrolyze the electrolyte solution. This is then reduced by a chemical reaction to improve the metal. Other metals, the metals during the processing of the target has been reduced but not deposited on the cathode, sank to the bottom of the electrolytic cell, where they form a substance called the anode anode mud or slime. Metals in the sludge can be removed with standard pyrorefining method.

         For practical purposes, the extent to which metals can be recovered by electrowinning depending on its position in the electromotive series. In general, metals that have a more positive electrode plate is the standard potential is more easily compared with less positive potentials. As an illustration, the metals are more noble, such as silver anda gold, can be removed from the solution to less than 1 mg / l by using a cathode while the flat plate with copper and tin, the concentration in the range of 0.5 to 1 g / L or more required to deposit a homogeneous metal. 

 Electrowinning This applies mainly to remove metal from a solution containing high concentrations of metal ions moderate (> 3000 mg / l). Under 1,000 to 2,000 mg / l of metal, the process of conventional electrowinning become very inefficient but otherwise the benefit is in recovering the precious metals that would otherwise be converted into a metal hydroxide sludge by the waste water treatment system. In the electrowinning process will release gases create H + pH down so risky produce HCN gas. This gas is extremely dangerous and corrosive to the anode, to the alkaline cyanide solution must be maintained at pH 12.5.

Activation Over potential For The Evolution Of Selected Gases On Various Electrode Materials At 25 °C
Platinum ( Platinized )
    - 0.07 V    + 0.77 V   + 0.08 V    
- 0.07 V+ 0.93 V
- 0.09 V+ 1.02 V
Iron - 0.15 V+ 0.75 V
Platinum ( Shiny )
- 0.16 V+ 0.95 V
Silver - 0.22 V+ 0.91 V
Nickel - 0.28 V+ 0.56 V
Graphite - 0.62 V+ 0.95 V + 0.12 V
Lead - 0.71 V+ 0.81 V
Zinc - 0.77 V

Mercury -0.85 V

      In elektrowinning, the current is passed from inert anode through a liquid solution containing the metal absorption so that the metal is extracted and then stored in the electroplating process to the cathode. In electrorefining, the anode consists of a metal that is not pure, and will then pass a current through the anode which corroded acid electrolyte into the solution so that the electroplating process deposits refined pure metal to the cathode. Electrowinning is different from other recovery technologies (eg, evaporation, ion exchange) in a metal element that recovered than metal bearing solutions. Metals recovered are usually not pure enough for use as an anode material in a plating process.  

Because metal deposition rates associated with the available surface area, the cathode must be able to work and this is very important. Two types of flat plate and reticulated cathodes are used, each having its own advantages. Flat-plate cathode can be cleaned and reused, recovered and plated metal. Reticulated cathode has a higher deposition rate compared to the flat-plate cathode. However, they are not reusable and must be sent for recycling. Alternatively, the cathode starter from a pre-refined metals can be used, which became an integral part of the metal so it is ready for rolling or further processed.

Read Also :

- Step By Step Copper Smelting Process In Palabora Mines  

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