THE RECOVERY OF CYANOGEN
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calcium sulphate. In this way a means is provided for separating out the cyanogen as hydrocyanic acid and recovering it as such.
The Bartlett Hayward Process
This process, which has recently been introduoed in America, is based on the well-known Bueb method. Explained briefly, the various stages are as foliows :—
1) The crude gas, completely freed from tar, is treated in a centrifugal washer with. sulphate of iron, the reactions being:—
(« ) In upper bays of washer—
2FeSO4,7HaO +2H2S +4NH3 = 2FeS +2(NH4)2S04.
(6 ) In lower bays of washer—
2 FeS + 2 NH3 + 6 HCN = 2 H2S + (NH4)2Fe2(CN)6
ammonium ferrous-ferrocyanide.
(NH4)2Fea(CN)0 + H2S + 2NII3 = FeS + (NH4)4Fe(CN)6
ammonium ferrocyanide.
2. The liquor flowing from the washer is treated in a still when. part of the ammonium ferrocyanide is reconverted into the ferrous-ferrocyanide :—
(NH4)4Fe(CN)8 + FeS= (NH4)2S + (NH4)2Fe2(CN)6.
3. The ammonium sulphide is Condensed and run into a precipitating tank, while the cyanide liquor—still containing some ferrocyanide—is run to a neutralizing tank where it is treated with a small quantity of sulphuric acid :—
(NH4)4I%(€N)6 + FeS + H2SO4 = (NH4)2Fe2(CN)6 + H2S + (NH4)2 SO4.
Thus the end products are an insoluble double ferrocyanide and sulphate of ammonia.
4. The liquor is then filter-pressed, and the cyanide cake recovered thereby is finally washed for the removal of sulphate of ammonia. The sludge is again passed through a filter-press, while the sulphate liquor is evaporated to crystallization. The sulphate of ammonia passes from the evaporator to a centrifugal drier.
Modifications of Feld Process
Feld 1 has recently introduoed a series of variations in the methods of absorbing cyanogen compoimds from coal gas. In one method. he filis the washer with. a solution of a ferrous salt free from ferric, and adds so much milk of lime that each. mole-cule of ferrous salt encounters three of calcium hydrate :—
FeSO4 + 3 CaO + 6 HCN = CaSO4 + Ca2Fe(CN)(i + 3 H»O.
The ferroso-calcic cyanide thus obtained remains in solution.
In another method he uses salts the oxides of which are displaceable by ammonia, of which process the following reaction is typical:—
MgCl2 + 4 NH3 + 6 HC5N + Fe(OH) 2 = MgFe(CN)0 + 4 NH4C1 + 2 H2O.
2 See also Chap. XVI, p. 542.
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