Zinc Precipitation Stage Of Gold Deposition

     Metal deposition method using Zn powder (Zinc precipitation) was first introduced by Sulman and Teed (1895). Basic usage of this method is the electron affinity of zinc metal is much higher than in metals gold and silver, then gold and silver metals will sink and be replaced by a soluble zinc. The Precipitate that occurs will be shaped cement sand. Although the use of activated carbon is used widely but many are also used primarily cementation in rocks that contain lots of silver because the cementation is a process very simple and very inexpensive.
     The use of zinc powder (zinc dust) is one effective way to a solution containing a very fine gold concentrations ranging from several microns to 50 microns. -200 Mesh zinc powder were added to the box containing the zinc-rich solution, will precipitate the metals gold and silver in the form of zinc binding, black gold. In addition to zinc powder (zinc dust), variants / other forms of zinc that can be used is zinc or a zinc foil noodle.
      Zinc powder is added into the solution will precipitate the silver and Gold Ore Minerals. The principle is based DERET CLENEL deposition, which is based on differences in the stacking sequence of electro-chemical activity of metal - metal cyanide in solution, namely magnesium (Mg), Aluminum (Al), Zinc (Zn), Copper (Cu), Gold (Au), Silver (Ag), Mercury (Hg), Lead (Pb), iron (Fe), platinum (Pt). Each metal is located on the left of the bond cyanide complexes can precipitate metal it replaces. So in fact not only can urge Zn Au and Ag, Cu and Al can also be used. But because the price is really expensive so it is better if using Zn. In the process of making gold and silver on rich aqueous solution by using Zn powder is also called the Merrill Crowe process.
Some stages of the cementation process is: 
  • Sift the gold complex solution becomes clear, and which typically use clarifying filter. 
  • Deaerasi, That draw oxygen from a solution that uses a vacuum pump. Because this is very important because the solubility of oxygen inhibits the process of deposition of gold. 
  • The addition of catalysts, namely by Lead Acetate Lead Nitrate or whose function is to increase the activity of galvanized, so the reaction will be faster. 
  • The addition of refined zinc metal. And from this it will be the addition of zinc obtained reaction   
NaAu 2Zn + 2 (CN) 2 +4 +2 NaCN + 2 H2O = 2Au Na2ZN NaOH + 2 (CN) 4 + H2
2 + 2 NaAg Zn (CN) 2 + 4 + 2H2Zn NaCN (CN) 4 + H2
  • From a solution that has completed the process done subsequent addition of sulfuric acid (H2SO4) in the sediment that will dissolve the zinc and gold leaves as the residue is then filtered using a filter press. And the results obtained can be directly Precipitated melted into bars of metal that is ready to be purified. To improve the recovery of gold from the Merrill Crowe process done in a way that melted gold mixed with borax and siliceous Fluxing Agent at a temperature of 1200 oC.
Although chemically Merrill Crowe process is straightforward, but in practice is complicated. Because the Merrill Crowe process efficiency is very dependent with:
  • Material should be pure zinc powder 
  • Decrease in the oxygen content in the solution 
  • Use of Lead Nitrate (PbNO3) 
  • Zinc powder mixing in the reactor. 
  • Excess cyanide can dissolve the zinc powder 
  • Excess zinc powder in the sediment may complicate the process of Smelting Of Mineral ores of gold and silver. 
  • Solids content in the solution of <3 ppm 
  • Content of Arsenic and Antimony <1ppm