Question

Name the important ores of silver. Describe the extraction of silver from these ores.

Name the important ore of silver. Write all the steps and reactions involved in the Cyanide process for the extraction of silver from its ore.

Answer 1

The important ores of silver are:

1. Argentite (Silver glance): Ag₂S
2. Pyrargyrite (Ruby silver): 3Ag₂S.Sb₂S₃ or Ag₃SbS₃
3. Hom silver (Chlorargyrite): AgCl

Extraction from its Ores (Cyanide Process): The ore argentite is the primary source from which silver is extracted. The MacArthur-Forrest cyanide process is the method used to extract silver from its ores. The basis for this procedure is the fact that silver dissolves in a diluted sodium cyanide solution to produce sodium argentocyanide, a soluble compound. Scrap zinc can be added to this compound to extract silver. The following steps are used to carry out the process.

1. Concentration: The sulphide ore is crushed, powdered, and then concentrated by the froth floatation process.
2. Treatment with sodium cyanide: A dilute sodium cyanide solution (0.4-0.7%) is applied to the concentrated ore, and the combination is constantly stirred by an air current for many hours. Silver from the ore enters the solution as the soluble complex sodium argentocyanide.
   \[\ce{Ag2S + 4NaCN <=> \underset{(or sodium argentocyanide)}{\underset{Sodium dicyanoargentate (I)}{2Na[Ag(CN)2]}} + Na2S}\]
   Na₂S is oxidized into Na₂SO₄ by the air blow. As a result, the reaction’s reversibility is confirmed, and it moves forward.
   \[\ce{2Na2S + 2O2 + H2O -> Na2S2O3 + 2NaOH}\]
   \[\ce{Na2S2O3 + 2NaOH + 2O2 -> 2Na2SO4 + H2O}\]
3. Precipitation of silver: After filtering, the solution from the previous stage is treated with leftover zinc. Because zinc is more electropositive, it precipitates silver out of the complex’s solution.
   \[\ce{2Na[Ag(CN)2] + Zn -> \underset{zincate (II)}{\underset{Sodium tetracyano}{Na2[Zn(CN)4]}} + 2Ag \downarrow}\]
   Silver thus obtained is in the form of a dark amorphous mass.
4. Fusion: When a bright compact mass is produced, the precipitated silver is filtered, crushed, and dried, and fused with borax.
5. Purification: Zinc, copper, and gold impurities are typically present in the resulting silver. The electrolytic refining method is used to purify this. In an electrolytic tank, a thin sheet of pure silver serves as the cathode and the impure silver block as the anode. The electrolyte is a silver nitrate solution with 1% nitric acid. Pure silver deposits itself on the cathode when an electric current flows through it. Zinc and copper impurities enter the solution, and any gold that may be present is removed as anode mud.

Extraction from Argentiferrous Lead: About 1% of Ag is present in the lead that is extracted from argentiferrous galena. Silver is extracted from this argentiferrous lead using a number of methods. The two most significant of these are Parke's and Pattinson’s procedures, which are covered below.

1. Parke’s process: Parkes process is based on the following facts:
   1. Zinc and molten lead are almost incompatible. The upper layer is formed by lighter molten zinc, while the lower layer is formed by heavier lead.
   2. Silver is more soluble in molten zinc than in molten lead.
   3. Zinc-silver alloy solidifies before molten lead.
   4. Since zinc is volatile, distillation can be used to separate it from silver.
2. Pattinson’s process: This procedure raises the silver content of the argentiferrous lead to roughly 2.6%; it is therefore employed when the silver content of the argentiferrous lead is low (less than 2.6%).
   The method is based on the observation that at 576 K (303°C), the silver-lead system forms a eutectic mixture with 2.6% silver by mass. Lead separates out of a molten lead-silver alloy when it cools down till the alloy’s silver content reaches 2.6% and all of the mass solidifies.
   The argentiferrous lead is melted and then cooled in Pattinson’s procedure. Lead crystals split during the cooling process. Preforated ladles are used to extract them. Until the entire material hardens, the melting and chilling procedure is repeated. To recover the silver, the resulting alloy is put through a cupellation process.