Removal of the unwanted materials (e.g., sand, clays, etc.) from the ore is known as concentration, dressing or benefaction. Before proceeding for concentration, ores are graded and crushed to reasonable size. Concentration of ores involves several steps and selection of these steps depends upon the differences in physical properties of the compound of the metal present and that of the gangue. The type of the metal, the available facilities and the environmental factors are also taken into consideration. Some of the important procedures for concentration of ore are:

(i) Hydraulic washing

(ii) Magnetic separat

(iii)Froth-floatation process

(iv) Leaching

Which of the cases is suitable for concentration by hand picking?

When the impurities can be distinguished from the ore by the naked eye

The correct answer is option 3. When the impurities can be distinguished from the ore by the naked eye.

The case suitable for concentration by hand picking is: When the impurities can be distinguished from the ore by the naked eye.

Hand picking, also known as manual sorting, is a method of concentration where the ore particles are visually inspected, and the valuable minerals are separated from the gangue (waste material) by hand. This method is most effective when the impurities can be easily distinguished from the ore using the naked eye.

In this case:

• The ore and impurities have distinct visual characteristics that allow them to be readily identified by appearance alone.
• The valuable minerals may have a different color, texture, or luster compared to the gangue minerals, making them easy to recognize.
• Hand picking is particularly suitable when the valuable minerals are present in relatively large particles or aggregates, making them easier to handle and separate manually.

Therefore, when the impurities can be distinguished from the ore by the naked eye, hand picking is a suitable method for concentration.