Leblanc Process - Process Detail

Process Detail

The sodium chloride is initially mixed with sulfuric acid solution and the mixture exposed to low heat. The hydrogen chloride gas bubbles off and is discarded. This continues until all that is left is a fused mass. This mass still contains enough chloride to contaminate the later stages of the process. The mass is then exposed to direct flame, which evaporates nearly all of the remaining chloride.

The coal used in the next step must be low in nitrogen to avoid the formation of cyanide. The calcium carbonate, in the form of limestone or chalk, should be low in magnesia and silica. The weight ratio of the charge is 2:2:1 of salt cake, calcium carbonate, and carbon respectively. It is fired in a reverberatory furnace at about 1000°C.

The black-ash product of firing must be lixiviated right away to prevent oxidation of sulfides back to sulfate. In the lixiviation process, the black-ash is completely covered in water, again to prevent oxidation. To optimize the leaching of soluble material, the lixiviation is done in cascaded stages. That is, pure water is used on the black-ash that has already been through prior stages. The liquor from that stage is used to leach an earlier stage of the black-ash, and so on.

The final liquor is treated by blowing carbon dioxide through it. This precipitates dissolved calcium and other impurities. It also volatilizes the sulfide, which is carried off as H₂S gas. Any residual sulfide can be subsequently precipitated by adding zinc hydroxide. The liquor is separated from the precipitate and evaporated using waste heat from the reverberatory furnace. The resulting ash is then redissolved into concentrated solution in hot water. Solids that fail to dissolve are separated. The solution is then cooled to recrystallize nearly pure sodium carbonate decahydrate.