The amount of municipal solid waste (MSW) is steadily increasing leading to an urgent need for the effective treatment of these wastes. Incineration is one of the methods for the treatment of these solid wastes. The bottom ashes produced from the incineration process are very unstable at standard atmospheric conditions, so there is need for process to alleviate the ash problems. In this study, the bottom ashes were first converted into the slurry form and then the slurry was made to react with CO2 to produce the carbonates. This carbonate process by using bottom ashes and carbon dioxide will be source recovery technology from waste material and, moreover, will also help to reduce the amount of CO2 emissions. The aim of this study was to determine the optimum conditions for the precipitation of CaCO3 using Aspen plus modeling program. The temperature and pressure for the precipitation of CaCO3 process were varied 25 to 500oC and 1.05 bar to 90bar, respectively. For producing the slurry, the optimum ratio of H2O to calcium oxide was determined to be 10 : 1. And the optimum precipitating conditions for calcium carbonate process system were found to be at 35 bar - 210oC.