THE RECOVERY OF CYANOGEN
559
Fig. 348.— The Davis-Nbill Cyanide Pbocess.
resulting cream is tlien run into the first bay of any suitable washer-scrubber. On meeting with the gas the iron carbonate is converted into su.lph.ide by the action of the sulphuretted hydrogen in the crude gas, whilst the hydro-cyanic acid combines with the sulphid.e in the presence of sodium carbonate to form sodium ferrocyanide—
(a) FeCO3 + H2S = FeS + CO, + H2O.
(b) FeS +2Na2CO3 + 6HCN=Na1Fe(CN)(S
+ H2S +2 CO2 +2 H2O.
In addition to sodium ferrocyanide cer-tain quantities of other compounds, such as sodium and ammonium ferrous-ferrocyanide, are formed. For this reason the liquor under-goes further treatment, so that the whole of the cyanide may be recovered as a soluble prussiate. Leaving the washer-scrubber the liquor passes through a preliminary heater and th.cn enters a still in which. it meets with a strong solution of caustic soda. In this way the sodium ferrous-ferrocyanide is converted into sodium ferrocyanide as required—
Na2Fe2(CN)6 + 2 NaOH = Na4Fe(CN)e +
Fe(OH)2.
The liquor obtained is then pumped through filter-press No. 2, in which any mud (cliiefly consisting of iron sulphide) is retained. The cyanide solution, consisting in the main of prussiate of soda, but containing a small proportion of carbonate of soda, is then passed through an. evaporator, and is finally recovered as yellow prussiate crystals. These crystals are afterwards washed with mother liquor from the fi.nish.ed crystallizing tanks, for the purpose of removing any impurities such as soda and sulphocyanides. The washed crystals are then treated in a re-dissolver, where the liquor they form is boiled for about 15 horns, before being run off to the finished crystallizing
tanks. Slow cooling is desirable at this stage, otherwise small crystals will be formed.