In metallurgical processes, gold and silver are commonly extracted from their low grade ores by which of the following methods?

Introduction / Context:
This question belongs to the topic of metallurgy in chemistry. Different metals are extracted from their ores using specific processes based on their reactivity and ore type. Gold and silver are precious metals often found in low grade ores. Their extraction method is distinctive and is frequently tested in examinations. Knowing the correct method helps you understand how selective dissolution and complex formation can be used in metallurgy.

Given Data / Assumptions:
- The metals considered are gold and silver.
- The ores are often low in metal content and require a process that can selectively dissolve the metal.
- Options include smelting, roasting, leaching and a broad hydrometallurgical approach.
- We assume familiarity with the cyanide process, also called MacArthur Forrest process, used for these metals.

Concept / Approach:
Gold and silver are typically extracted using leaching with a dilute solution of sodium cyanide or potassium cyanide in the presence of air. The metals form soluble complexes such as [Au(CN)2]^- or [Ag(CN)2]^-. These complexes are then treated to recover the metal, often by displacement with zinc. This method is a form of hydrometallurgy but is specifically known as the cyanide leaching process. Smelting and roasting are more suitable for sulphide ores of base metals like copper, zinc and iron, not for gold and silver in low grade ores.

Step-by-Step Solution:
Step 1: Recall that gold and silver occur in native form and in ores that may not respond efficiently to high temperature processes alone. Step 2: Remember that cyanide leaching is the standard industrial method for extracting gold and silver from crushed ore by forming soluble cyanide complexes. Step 3: Leaching involves selectively dissolving the desired metal into solution, a characteristic hydrometallurgical technique. Step 4: Smelting involves melting and reduction at high temperatures and is not the primary modern method for these precious metals from low grade ores. Step 5: Roasting is mainly used to convert sulphide ores to oxides or to remove volatile impurities and is not itself the extraction step for gold and silver. Step 6: Therefore, the correct choice is leaching by the cyanide process.

Verification / Alternative check:
To verify, you can recall the key reactions for gold extraction. In the cyanide process, gold reacts with sodium cyanide, oxygen and water to form a soluble complex: 4Au + 8NaCN + O2 + 2H2O produces 4Na[Au(CN)2] and 4NaOH. A similar reaction occurs for silver. These complexes are then treated with zinc dust, which displaces gold or silver from solution. This sequence of dissolution and recovery is a hallmark of leaching. Smelting and roasting, in contrast, do not rely on forming such complexes and are used in different contexts.

Why Other Options Are Wrong:
Smelting: Involves high temperature reduction but is not typically used in the first step to extract gold and silver from low grade ores.
Roasting: Mainly serves as a preparatory step for sulphide ores, not as the primary extraction process for gold and silver.
Hydrometallurgy without leaching: Hydrometallurgy is a broad term that includes leaching, but the most specific and correct answer among the options is leaching by the cyanide process.

Common Pitfalls:
Students sometimes select hydrometallurgy as a general term without specifying leaching or confuse roasting and smelting because they both involve heat. Another pitfall is forgetting that gold and silver are noble metals and are usually extracted by processes that exploit their ability to form stable complexes, not by simple high temperature reduction. Always link gold and silver extraction with the cyanide leaching process in exam contexts.

Final Answer:
Gold and silver are commonly extracted from their ores by leaching using the cyanide process.