Technology for obtaining gold, silver and platinum at home.
Gold is obtained by amalgamation or leaching. During amalgamation, gold is dissolved in mercury (Hg), forming amalgam, which, when the Au content is more than 15%, becomes solid. From the obtained amalgam, mercury is distilled off in condensing devices, leaving Au and Ag (silver) as nodules. Cleaning Au from silver is discussed below.
For leaching, a solution of sodium or potassium cyanide — NaCN or KCN (concentrated solution in distilled water) is used, in which Au dissolves in the presence of oxygen (blowing air) to form complex anions [Au (CN) 2] - - dicyto-zolate potassium: 4Au + 8CN- + O2 + 2H2O = 4 [Au (CN) 2] - + 4OH-.
From the resulting solution, gold is isolated with metallic zinc (sawdust Zn) in a colloidal sediment: 2 [Au (CN) 2] - + Zn = [Zn (CN) 4] 2- + 2Au. The precipitated gold is treated to separate zinc from it with dilute sulfuric acid; washed and dried.
Further purification of Au from impurities (mainly silver) is carried out by treating it with hot concentrated sulfuric acid H2SO4 or electrolysis is needed. Au also dissolves easily in aqua regia (a mixture of hydrochloric HCl and nitric acid HNO3 acids 1: 4), forming the complex hydrochloric acid H [AuCl4], which crystallizes upon evaporation in the form of light yellow needles of the composition H [AuCl4] • 4H2O.
It is also easy for Au to dissolve in "chlorine water" and in aerated ones, i.e. air purged alkali metal cyanide solutions.
All Au compounds decompose readily when heated to release metallic Au. Due to its softness, Au is used in alloys, usually with silver or copper. These alloys are also used for electrical contacts. Dealing with these reagents requires GREAT CARE since they are all deadly poisonous. The presence of Au in radio details is determined by the passport of the part and by the characteristic color of the coatings.
Silver is obtained by amalgamation or leaching. When leaching, silver is dissolved by the action of a concentrated solution of potassium cyanide KCN or NaCN in distilled water. From the obtained complex cyanide compound K [Ag (CN) 2], Na [Ag (CN) 2], metallic silver is precipitated by passing an electric current ([Ag (CN) 2] - Ag + 2CN-) through a solution of a metal-ceramic crystalline layer the cathode.
Hydrochloric acid and dilute sulfuric acid do not affect Ag. Silver is well dissolved in nitric acid HNO3, therefore nitric acid is used in the separation of Ag from gold Au, which does not dissolve in HNO3: Ag + 2HNO3 = AgNO3 + NO2 + H2O.
All Ag compounds under the action of reducing agents (for example, formalin, hydrosin) are easily reduced with the release of metallic Ag. The amalgamation process is similar to Au gold. A large amount of Ag is used for electrical contacts and in silver-zinc batteries.
The release of platinum is similar to gold. When platinum is dissolved in aqua regia, it produces hexachloroplatinic acid H2 [PtCl6], which upon evaporation of the solution is released as red-brown crystals of composition H2 [PtCl6] • 6H2O, which, upon further heating, decompose with the release of Pt.