For this purpose the crude distillate is redistilled over sulphuric acid and then fractionated.
The fluorene is separated from this by placing it in a freezing mixture, and is then redistilled or crystallized from glacial acetic acid, or purified by means of its picrate.
The product so obtained is then redistilled under diminished pressure and finally distilled again from a sealed and evacuated apparatus (V.
This is neutralized with lime and redistilled in order to remove the acetic acid.
A redistilled zinc, from an ordinarily pure commercial zinc, is often called chemically pure, but redistillation is seldom practised except for the recovery of zinc from galvanizer's dross and from the skimmings and bottoms of the melting furnaces of zinc rolling mills.