How to prepare potassium cyanide from potassium ferrocyanide

Question

How do we prepare potassium cyanide from potassium ferrocyanide?

It is something which I am curious about, but cannot find the answer. I am studying inorganic chemistry but this reaction cannot be found.

Answer 1

Until about 1900, potassium cyanide was produced by melting dry potassium carbonate with dry potassium hexacyanidoferrate(II): $$\ce{K4[Fe(CN)6] + K2CO3 -> 6KCN + FeO + CO2}$$ However, the product contains potassium cyanate, which is approximately formed as follows: $$\ce{K4[Fe(CN)6] + K2CO3 -> 5KCN + KOCN + Fe + CO2}$$

In a small scale, dry potassium hexacyanidoferrate(II) is melted in a covered porcelain crucible. It is heated as long as nitrogen escapes. $$\ce{K4[Fe(CN)6] -> 4KCN + N2 +FeC2}$$ The iron carbide sinks to the bottom and the molten potassium cyanide can be poured off.

Answer 2

In addition to Loong's excellent answer, I'm going to shed some light on the last process i.e. the decomposition reaction of potassium ferrocyanide.

Actual reaction is this:

$$\ce{3K4[Fe(CN)6] ->[\Delta]12KCN + Fe3C + 5C + 3N2}$$

The thermal decomposition of potassium hexacyanoferrate(II) to produce potassium cyanide, iron carbide, carbon and nitrogen. This reaction takes place at a temperature near 650°C.(source)

As always nitrogen gas escapes, iron carbide and carbon sinks to the bottom and the molten potassium cyanide can be poured off.

There is a paper which discuss the decomposition of potassium ferrocyanide trihydrate($\ce{K4[Fe(CN)6].3H2O}$) to give anhydrous potassium ferrocyanide. Further increase in heat gives various decomposition products like $\ce{α-Fe2O3, Fe3O4, Fe3C, Fe, FeO, KFeO2,\beta-FeOOH, KOCN, K2CO3}$ and $\ce{KCN}$.