Editing Leblanc process (section)

== Process detail ==
The sodium chloride is initially mixed with concentrated sulfuric acid and the mixture exposed to low heat. The hydrogen chloride gas bubbles off and was discarded to atmosphere before gas absorption towers were introduced. 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.<ref name="lenn2">{{Cite web|url=http://www.lenntech.com/Chemistry/Hydrochloric-Acid-Sodium-Sulphate.htm|publisher=Lenntech| title=Hydrochloric Acid and Sodium Sulphate|accessdate=22 April 2007}}</ref><ref name="EB1911">{{Cite EB1911 |wstitle=Alkali Manufacture|volume=1|pages=674–685}}</ref>

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&nbsp;°C.<ref name="lenn1">{{Cite web|url=http://www.lenntech.com/Chemistry/Soda-%20industries.htm|publisher=Lenntech|title=The Soda Industries|accessdate=22 April 2007}}</ref> Sometimes the reverberatory furnace rotated and thus was called a "revolver".<ref>{{Cite book|last=Museum|first=Victoria and Albert|url=http://worldcat.org/oclc/608086021|title=Catalogue of the mechanical engineering collection in the Science Division of the Victoria and Albert Museum, South Kensington, with descriptive and historical notes.|date=1908|publisher=H.M.S.O|volume=2|pages=107|oclc=608086021}}</ref>

The black-ash product of firing must be [[Leaching (chemistry)|lixiviated]] right away to prevent oxidation of sulfides back to sulfate.<ref name="lenn1" /> 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.<ref name="lenn1" />

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 H<sub>2</sub>S 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.<ref name="lenn1" />