Dr Wolff employs purifiers in which the gas is washed with water containing calcium chloride, and then passed through bleaching-powder solution or other oxidizing material.
A large quantity of the salt is now prepared from the "spent oxide" of the gas works, the cyanogen compounds formed in the manufacture of the gas combining with the ferric oxide in the purifiers to form insoluble iron ferrocyanides.
It is now prepared from the calcium ferrocyanide formed in gas purifiers (see above) by decomposition with ferrous' sulphate.
Not fewer than four lime purifiers were employed, and as the one which was first in the series became exhausted, i.e.
It will be noticed that in the earlier stages the quantity of sulphur impurities is actually increased between the purifiers - in fact, the greater amount of sulphiding procures the ready removal of the carbon disulphide, - but it is the carbon dioxide in the gas that is the disturbing element, inasmuch as it decomposes the combinations of sulphur and calcium; consequently it is a paramount object in this system to prevent this latter impurity finding its way through the first box of the series.