Which of the reaction is suitable for concentrating ore by leaching process?

Leaching is a chemical method of concentration of ore where the ore is selectively dissolved in a suitable reagent, leaving behind impurities.

Let us examine each reaction:

Option A: $$2Cu_2S + 3O_2 \to 2Cu_2O + 2SO_2$$

This is a roasting reaction (heating a sulphide ore in the presence of air), not leaching.

Option B: $$Fe_3O_4 + CO \to 3FeO + CO_2$$

This is a reduction reaction in a blast furnace, not leaching.

Option C: $$Al_2O_3 + 2NaOH + 3H_2O \to 2Na[Al(OH)_4]$$

This is the Bayer's process, where bauxite ($$Al_2O_3$$) is dissolved in hot concentrated NaOH solution. The alumina dissolves to form soluble sodium aluminate $$Na[Al(OH)_4]$$, while impurities like $$Fe_2O_3$$, $$SiO_2$$, and $$TiO_2$$ remain undissolved. This is a classic example of leaching.

Option D: $$Al_2O_3 + 6Mg \to 6MgO + 4Al$$

This is a thermite-type reduction reaction, not leaching.

Therefore, the correct answer is Option C.