to the furnace which generates the dusts or another steel producing furnace whilst permitting removal of calcium zinc and heavy metals from the system thus avoiding the production of byproducts suitable only as landfill the acetic acid solution is treated with hydrogen sulphide in a step 6 in which zinc cadmium copper lead and other heavy metals are precipitated as insoluble sulphides which are recovered providing a sulphide concentrate very acceptable for zinc smelters if desired step 6 may be preceded by cementation using zinc dust to cement out metallic lead and copper prior to sulphide precipitation the quantity of hydra sulphide added during the step 6 is maintained at a sufficient level to ensure that to all of the calcium remains in solution and in a following step 8 the calcium is precipitated by the addition of sulphuric acid sufficient only to precipitate most of the calcium as calcium sulphate whilst minimizing the residual sulphate ions in solution this step regenerates the acetic acid which is subsequently returned to step 4 together with sufficient make up acetic acid to maintain a desired acetic acid concentration in the leaching step the content of sulphate ions in the recycled acetic acid should be minimized to avoid sulphate interaction with the.