When the thick liquor is burned under limited air intake, a melt of sodium carbonate (Na2CO3) and sodium sulphide (Na2S) is formed. Dissolving this in water gives "green liquor". The sodium carbonate is causticized with calcium hydroxide (slaked lime, Ca(OH)2) to sodium hydroxide solution. Filtration produces "white liquor" which is re-used in the process. The filtered calcium carbonate is burned in a rotary drum furnace to calcium oxide (burnt lime, CaO).
Air consists of 78 % nitrogen, 21 % oxygen, just less than 1 % argon and other gases. In combustion processes that use air as an oxidant, the high nitrogen content is a limiting factor, since a portion of the energy generated is only used for heating of the inert gas and is therefore lost to the process.The addition of oxygen to an air stream increases the oxygen concentration and thus at the same time improves the firing efficiency of the combustion. The technology of oxygen addition in combustion processes is flexible and can be applied to almost all types of furnaces, such as shaft furnaces, rotary drum ovens, grate firing ovens, fluidized bed ovens, deck ovens, etc.The use of Oxyfuel technology in the rotary kiln used to burn the lime substantially reduces the energy input and the resulting emissions.
The Oxial process is used to control the optimal oxygen demand of the Oxipyr burner technology from Messer.