Cleaning filters

Residual catalyst is removed from sintered metal or ceramic filters 3 by an aqueous solution of hydrogen peroxide, which decomposes preferentially on the catalyst particles creating gas bubbles which flush the particles out. The filters 3 are used to remove catalyst from a solvent used in the catalytic hydrogenation of anthraquinone or a derivative thereof. For cleaning, the filters are transferred to vessel 4, 5 and initially flushed with the solvent and then dried with steam and flushed with hot water. The filters may then be flushed with warm alkali, water, hydrogen peroxide solution, water, hot nitric acid, water, more hydrogen peroxide solution and water in turn, and finally dried with steam and hot air.