Prussian blue, ferro-cyanide or potassium (yellow prussiate), and cyanide of potash are now recovered by the Gas Light & Coke Co. from the purifying materials used. There are two methods of recovering the cyanogen compounds: the first by absorption in the scrubber, the second by a bsorption in the oxide purifiers. In the first method a scrubber is used containing soda or potash and some suspended oxide or hydrate of iron j the cyanogen in the gas combines with the iron and alkali to form ferrocyanide. If the iron is in excess the compound is insoluble (probably as Prussian blue), but if the iron is not in excess, then the compound is soluble. After a certain period the liquid is run off for concentration. In the second method the cyanogen is fixed in the oxide of iron purifiers as Prussian blue (ferric ferrocyanide). By leaving one oxide purifier as No. 1 in the series long after it has become saturated with sulphuretted hydrogen as much as 8 or 10 per cent, of Prussian blue has been obtained from it. The oxide of iron is exposed to air in the usual way to revivify, and the sulphur extracted by carbon bisulphide in closed vessels; the sulphur is recovered, and the carbon bisulphide used over and over again.

The spent oxide is boiled with lime and water, when the Prussian blue is decomposed and ferrocyanide of lime is produced. The clear solution is drawn off acidified, and a per and proto salt of iron added yielding a pure Prussian blue, which is allowed to settle, washed, collected in bags, filter pressed, and dried. From this pure ferrocyanide of potash is produced by boiling with the calculated equivalent of caustic potash. Cyanide of potash is formed by fusing Prussian blue or ferrocyanide of potash with the right proportion of carbonate of potash.