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

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