Question:

Sodium cyanide is added as a depressant in the froth floatation process when the ore contains a mixture of $ {ZnS}$ and $ {PbS}$. This is because

Updated On: May 12, 2024
  • $\ce{Pb(CN)_2}$ gets precipitated without any effect on $\ce{ZnS}$
  • $\ce{ZnS}$ forms soluble complex while $\ce{PbS}$ forms froth
  • $\ce{PbS}$ forms soluble complex while $\ce{ZnS}$ forms froth
  • $\ce{Zn(CN)_2}$ gets precipitated without any effect on $\ce{PbS}$.
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The Correct Option is B

Solution and Explanation

$ {NaCN}$ forms a soluble complex with $ {ZnS}$ thus, it selectively prevents $ {ZnS}$ from coming to the froth.but allows $ {PbS}$ to come with the froth.
$ {4NaCN + ZnS -> $\underset{\text{ (Soluble complex). }}{ {Na_2 [Zn(CN)_4 ] }}$+ Na_2 S }$
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Concepts Used:

Concentration of Ore

The concentration of ore, also known as ore enrichment, is the process of removing gangue particles from ore in order to enhance the percentage of metal in the ore. The difference in physical or chemical properties of the ore and the impurities determines the procedures used to remove impurities from the ores. 

Methods of Concentration of Ore:

  • Hand-picking - It was the traditional system of directly concentrating ore with the hands. With the use of a hammer, the gangue or adhering solid matrix is removed from the ore in this procedure. The separation and identification of gangue are based on colour or lump form differences.
  • Hydraulic Washing - On passing the ore via an upward stream of water, the lighter gangue particles are separated from the heavier metal ore. This is a gravity separation method.This process is used to enrich ores that are heavier than the gangue particles that are present in them. A stream of water is run through crushed and finely powdered ore in this procedure. Water washes away the lighter gangue particles, leaving the larger ore particles behind.
  • Froth Floatation Process - The purpose of this process is to extract gangue from sulfide ores. The ore is pulverized and mixed with water to form a suspension. Collectors and Froth Stabilizers are added to this. Collectors like pine oils, fatty acids, etc. boost the metal part of the ore’s non-wettability, allowing it to form a froth.