The correct statement is:

• Leaching of bauxite using concentrated NaOH solution gives sodium aluminate and sodium silicate
• The blistered appearance of copper during the metallurgical process is due to the evolution of CO2
• Pig iron is obtained from cast iron
• The Hall-Heroult process is used for the production of aluminium and iron.

The correct statements are:

• Leaching of bauxite using concentrated NaOH solution gives sodium aluminate and sodium silicate: This is the Bayer process, a crucial step in aluminum production. Bauxite, the primary ore of aluminum, is treated with a concentrated NaOH solution to dissolve the aluminum oxide (Al2O3) as sodium aluminate (NaAl(OH)4). Silicon dioxide (SiO2), a common impurity, also reacts to form sodium silicate (Na2SiO3).

• The blistered appearance of copper during the metallurgical process is due to the evolution of CO2: This is incorrect. The blistered appearance is due to the evolution of SO2 (sulfur dioxide), not CO2 (carbon dioxide). This happens during the process of converting copper matte into blister copper.

• Pig iron is obtained from cast iron: This is incorrect. Pig iron is an intermediate product in iron production, obtained from a blast furnace. It is then further processed to produce cast iron (by remelting and adding carbon, silicon, etc.).

• The Hall-Heroult process is used for the production of aluminum and iron: This is partially incorrect. The Hall-Heroult process is specifically used for the production of aluminum, not iron. Iron production utilizes different processes like the blast furnace method.

Therefore, only the statement about leaching bauxite is entirely correct.