Question

What is the role of zinc metal in the extraction of silver?

Answer 1

Argentite ($A{g_2}S$) ore of silver undergoes various processes to extract pure silver from it. It is treated with NaCN forming a complex, which is treated with Zn dust. Remember zinc is more electropositive than silver.

Complete step by step answer:
-Silver is usually found in combined forms with sulphur, chlorine, antimony and arsenic. Argentite ($A{g_2}S$), chlorargyrite (AgCl, also known as “horn silver”) and galena (an ore of lead and silver) are some important ores of silver.
-The extraction of silver from ore is done by the process of smelting or chemical leaching. Electrolytic refining of copper and the Parkes process on lead metal obtained from lead ores which contains small quantities of silver also produces silver ,When we extract silver from Argentite ore ($A{g_2}S$) zinc dust is used in its purification. We will see this process in a little detail now.

To extract silver from argentite we use the Mac Arthur and Forrest’s cyanide process. Let us see this process in a stepwise manner:
(1)Concentration: The ore is crushed and then concentrated by the process of froth floatation.

(2)Ore treatment with NaCN: The concentrated ore formed in the previous process is treated with a solution of 0.4 – 0.6 % sodium cyanide (NaCN) for some hours. We then pass an air current into the formed solution. This agitates the solution and converts the silver present in it into soluble sodium argento complex ($Na[Ag{(CN)_2}]$.
This step can be shown in the form of following reaction:
$A{g_2}S + 4NaCN \to 2Na[Ag{(CN)_2}] + N{a_2}S$

(3) Silver precipitation: Here we filter the solution of sodium argento complex to remove the insoluble impurities. The filtrate is then treated with zinc dust, which precipitates out the silver present in it.
The reaction form of this step is:
$2Na[Ag{(CN)_2}] + Zn \to N{a_2}[Zn{(CN)_4}] + 2Ag \downarrow$
This happens because Zn is more electropositive than Ag and has the ability to displace Ag from the complex.

(4)Electrolytic reaping: The impure silver precipitate is then subjected to electrolytic refining for further purification. Here we take a cathode of pure silver sheet and the impure silver as the anode. Silver nitrate acidified with 1% nitric acid is taken as the electrolyte. On passing electricity to this pure silver gets deposited at the cathode.
This is how we purify silver from argentite ($A{g_2}S$) using sodium cyanide (NaCN) and zinc dust (Zn).
-The commercial grade fine silver has a purity of silver of 99.9%. Silver with a purity of more than 99.99% can also be extracted.

Note: Silver nitrate is used as an electrolyte to enrich the solution in silver ions for deposition at the cathode. Also zinc dust is used because it being more electropositive in nature than silver it acts as a better reducing agent and can thus displace silver in the complex.