Solid NaCN Production from Caustic Soda, Ammonia and Natural Gas

The process under analysis comprises three major sections: (1) HCN Production; (2) Neutralization; and (3) Purification.

HCN Production. Hydrogen cyanide is produced through a direct synthesis method similar to the Andrussow process. Natural gas, ammonia, and air are fed to a reactor in the presence of a platinum-rhodium catalyst, where HCN is formed through a catalytic reaction. The product stream is subjected to subsequent purification steps, to produce a 99.5% HCN stream.

Neutralization. First, caustic soda is fed to an absorption column to absorb hydrogen cyanide from the vapor stream that exits the neutralization reactor. The sodium hydroxide solution is then fed to the neutralization reactor alongside hydrogen cyanide from the previous section. While the reaction is exothermic, the reaction heat is removed by means of circulation through an external heat-exchanger, in such a way that the solution temperature is kept below 50 °C. Yields greater than 98% are achieved based on HCN. At the end of this step, a salt solution of NaCN (around 45 wt%) is obtained.

Purification. The salt solution is fed to evaporative crystallizers, in which water is removed to reach the saturation concentration and begin the crystallization. The sludge from the crystallizer is then fed to a set of centrifuges to separate the Sodium Cyanide crystals. The mother liquor recovered from the centrifuges still contains about 40 wt% in Sodium Cyanide, so it is recycled to the crystallizers. Finally, residual moisture in the crystals is removed in a spin flash dryer using hot air. The dried solid powder is then compacted in hydraulic roll presses into briquettes for safer handling, which are then packaged into storage bins and directed to storage.