Question

In the extraction of which of the following metals, the ore is concentrated by leaching process?
I) Ag
II) Au
III) Zn
IV) Al

• A. I& III only
• B. II& III only
• C. III& IV only
• D. I, II& IV only

Hint:

Leaching is a chemical method of ore concentration. Key examples include: \begin{itemize} \item Cyanide process for noble metals (Au, Ag). \item Bayer process for bauxite (Al). \end{itemize}

Answer 1

The Correct Option is D

Step 1: Understand the Leaching Process.
Leaching is a chemical concentration method used for ores where the ore is soluble in a suitable solvent, but the impurities are not. The metal is then recovered from the solution. This process is particularly effective for precious metals (like gold and silver) and for aluminum. Step 2: Analyze each metal for concentration by leaching. \begin{itemize} \item I) Ag (Silver): Silver ores (like argentite, Ag\(_2\)S) are concentrated by cyanide leaching (MacArthur-Forrest cyanide process). The ore is treated with a dilute solution of sodium or potassium cyanide in the presence of air (oxygen). Silver forms a soluble dicyanoargentate(I) complex. \[ \text{Ag}_2\text{S} + 4\text{NaCN} \xrightarrow{\text{O}_2/\text{H}_2\text{O}} 2\text{Na}[\text{Ag(CN)}_2] + \text{Na}_2\text{S} \] So, silver is concentrated by leaching. \item II) Au (Gold): Gold is also extracted by cyanide leaching. Gold reacts with oxygen and cyanide ions to form a soluble dicyanoaurate(I) complex. \[ 4\text{Au} + 8\text{CN}^- + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4[\text{Au(CN)}_2]^- + 4\text{OH}^- \] So, gold is concentrated by leaching. \item III) Zn (Zinc): Zinc ores (like sphalerite, ZnS) are primarily concentrated by froth flotation, followed by roasting. While there can be hydrometallurgical routes involving leaching for zinc, the primary and most common concentration method is froth flotation for sulfide ores. For oxide ores, sometimes acid leaching is used, but it's not typically the \textit{initial} concentration step for the ore itself in the way cyanide leaching is for Ag/Au or Bayer process for Al. Therefore, it's generally not considered among the metals \textit{primarily} concentrated by leaching in this context. \item IV) Al (Aluminum): Aluminum ore (bauxite, Al\(_2\)O\(_3\).nH\(_2\)O) is concentrated by the Bayer process, which is a leaching process. Bauxite is digested with a hot concentrated solution of NaOH, which dissolves alumina to form soluble sodium meta-aluminate, while impurities like iron oxides remain insoluble. \[ \text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O} + 2\text{NaOH} \xrightarrow{\text{Heat}} 2\text{Na}[\text{Al(OH)}_4] + (x-1)\text{H}_2\text{O} \] So, aluminum is concentrated by leaching. \end{itemize} Step 3: Identify the correct combination.
From the analysis, Ag (I), Au (II), and Al (IV) are metals whose ores are concentrated by leaching. Zinc (III) is generally not. Step 4: Match with the given options.
The combination I, II& IV only matches option (4).