Number of ores concentrated by Levigation (hydraulic washing)

Levigation, also known as hydraulic washing, is a method used to separate the gangue from the ore by using the difference in their densities. It is a gravity separation technique that is commonly used for the concentration of ores. The process involves the addition of water to the powdered ore, which is then passed through a sloping channel. The lighter gangue particles are washed away, leaving behind the heavier ore particles.

Among the given ores - pitche blende, tinstone, chromite, galena, copper pyrites, and argentite - the number of ores that can be concentrated by levigation is '3'. Let's analyze each ore and understand why only three of them can be concentrated using this method.

Pitche Blende:
Pitche blende is an ore of uranium and contains a mixture of uranium oxides. It is a heavy ore and can be easily concentrated by levigation. The gangue materials present with pitche blende, which are lighter in density, can be washed away using water, leaving behind the concentrated ore.

Tinstone:
Tinstone, also known as cassiterite, is an ore of tin. It has a high density and can be concentrated by levigation. The lighter gangue materials can be separated from tinstone using the hydraulic washing technique.

Chromite:
Chromite is an ore of chromium and is a heavy ore. It can be concentrated by levigation as the gangue materials can be washed away, leaving behind the concentrated ore.

Galena:
Galena is an ore of lead and has a relatively high density. However, it cannot be concentrated by levigation as it is a sulfide ore and forms a colloidal solution with water. The galena particles do not settle down in the sloping channel and hence cannot be separated using this method.

Copper Pyrites:
Copper pyrites, also known as chalcopyrite, is an ore of copper. It is a sulfide ore and does not settle down in the sloping channel during levigation. Therefore, it cannot be concentrated using hydraulic washing.

Argentite:
Argentite is an ore of silver and is a sulfide ore. Similar to copper pyrites, it cannot be concentrated by levigation due to its colloidal nature.

In conclusion, out of the given ores, the three ores that can be concentrated by levigation (hydraulic washing) are pitche blende, tinstone, and chromite.