Abstract: The reaction mechanisms of carbon dioxide carbonation and ammonium bicarbonate+carbon dioxide composite carbonation were discussed in the pilot plant with the ammonia chloride leaching solution of calcium carbide slag that was calcium chloride?鄄ammonia water system as the research object.The carbon dioxide carbonization process was long and slow，and the particles were coarse and agglomeration was serious.The root cause was that pH of the calcium chloride?鄄ammonia system was low，and the reaction rate of carbon dioxide absorption was slow.It was um carbonate nucleus at the initial stage of carbonization，which was not conducive to ultrafine particles. On the contrary，the complex carbonization process was rooted in the fact that the complex carbonization of ammonium bicarbonate could form a large number of calcium carbonate nucleus in a short time，which was conducive to the ultrafine particles.Ammonia was a key minority in either carbon dioxide carbonation process or ammonium bicarbonate+carbon dioxide composite carbonation process.It was difficult for the system to achieve ammonia equilibrium.The dissociation of free ammonia was a slow reaction and control step.impossible to form a large amount of calci?鄄

Key words: calcium chloride-ammonia water system；nano calcium carbonate；composite carbonization mechanism