This invention from Sherrit Gordon describes a process to produce ammonium carbonate from gaseous ammonia, carbon dioxide and water by condensing these under the right conditions.
This process can be part of a hydrometallurgical process in which material containing a desired metal, such as nickel, is leached in an ammoniacal ammonium carbonate system to produce a leach liquor comprising dissolved metal values in  ammoniacal ammonium carbonate solution.