Sulphide ores are generally concentrated by

Which method is used for the concentration of the ore containing iron chromate $(FeCrO_4)$?

Observe the following statements regarding the purification of bauxite:
$I$. During Hall's process,silica is removed as $Si$ (vapour).
$II$. Bauxite ore contaminated with $Fe_2O_3$ is purified in Baeyer's process.
$III$. During Serpeck's process,$AlN$ is formed.
The correct answer is:

During the extraction of sodium aluminate from bauxite,the compounds of ....... are insoluble and precipitate out.

The correct statements regarding the froth floatation method in metallurgy are:
$I$. Used for the purification of sulphide ores
$II$. Used for the roasting of sulphide ores
$III$. It is based on the relative densities of gangue and ore particles
$IV$. It is based on the difference in the wetting properties of gangue and ore particles in frothing agent and water

Collectors are the substances which help in attachment of an ore particle to air bubble in froth. $A$ popular collector used industrially is