A process and apparatus for preparing oxalic acid by calcium carbonate deposition method is disclosed. Said process includes such steps as reaction between calcium hydroxide and sodium oxalate to generate calcium oxalate deposit and aqueous solution of sodium hydroxide, concentrating said aqueous solution, returning it back to reactor, reacting on CO to synthesize sodium formate, dehydrogenating it to generate sodium oxalate, washing said calcium oxalate deposit with water, dissolving it in the mixture of water and cosolvent, filling CO2 gas in the solution to generate calcium carbonate deposit and the aqueous solution of oxalic acid, concentrating the aqueous solution, crystallizing and drying to obtain solid oxalic acid. |