Demetallising used, highly acidic electropolishing baths

摘要

A process for demetallising mixts. contg. mainly phosphoric and sulphuric acid comprises electrolysis in a cell with a partition between the anode and cathode compartments, so that the Fe (III) ions are reduced to Fe (II) ions and pptd. as FeSO4 on reaching the solubility limit. The ppte. is then removed. Preferably, in the rinsing process subsequent to electropolishing, the rinsing water is in circulation, and the electrolyte obtd. from the circulation is returned. In an example, stainless steel was electropolished at 45-80 deg C and 5-25 A/dm<2>, using an electrolyte contg. 60 wt% 85% H3PO4, 36 wt% 96% H2SO4, 1 wt% morpholinomethane- di-phosphoric acid, 0.5 wt% diethanolamine and 2.5 wt% water, with a closed rinse water circulation (distn. and recycling). The used electrolyte (density 1.760; 35.1 wt% H2SO4, 37.8 wt% H3PO4, 4.5 wt% Fe, 82 g/l) was demetallised for 20 hrs. at 60 deg C, 3 V and 1.5 A/l in a 10-l cell with carbon rods and stainless steel plates as electrodes, sepd. by a porous ceramic partition with a pore size of 1 micron. The filtrate from this process had a density of 1.675, acid contents of 31 wt% H2SO4 and 38 wt% H3PO4, and a Fe content of 2.5 wt% (41 g/l). After replacing the H2SO4 which had been used and adjusting the density to the required value, this soln. was suitable for immediate re-use as an electrolyte for electropolishing.