Which of the following metals cannot be extracted via hydrometallurgy?

Iron

The correct answer is option is 3. Iron.

Hydrometallurgy is a method of extracting metals from their ores using aqueous solutions. While it's a versatile technique applicable to a wide range of metals, some metals are more suited for extraction via hydrometallurgy than others. Let us discuss each metal in the context of hydrometallurgy:

1. Copper: Copper is commonly extracted via hydrometallurgical methods such as heap leaching, solvent extraction, and electrowinning. Copper ores are often rich in sulfides or oxides, making them amenable to dissolution in acidic solutions for subsequent metal recovery.

2. Zinc: Zinc is another metal commonly extracted using hydrometallurgical techniques. The most common process for zinc extraction is the hydrometallurgical process called "roasting-leaching-electrowinning," where zinc sulfide concentrates are roasted to produce zinc oxide, which is then dissolved in sulfuric acid to form a zinc sulfate solution for electrolytic recovery.

3. Iron: Iron is primarily extracted via pyrometallurgical methods such as smelting, where iron ore (typically hematite or magnetite) is reduced with carbon to produce metallic iron. While there are some hydrometallurgical methods for extracting iron from certain ores, they are less common and usually involve complex processes.

4. Gold: Gold is typically extracted using a combination of pyrometallurgical and hydrometallurgical methods. While pyrometallurgical methods such as smelting are commonly used for initial concentration and refining of gold ores, hydrometallurgical methods such as cyanidation are extensively used for extracting gold from low-grade ores and concentrates.

While all four metals mentioned (copper, zinc, iron, and gold) can theoretically be extracted via hydrometallurgy to some extent, iron is primarily extracted using pyrometallurgical methods, making it less suitable for extraction via hydrometallurgy compared to copper, zinc, and gold.

Therefore, the correct answer is Iron.