Patent GB-938111-A (GB)
Title: Improvements in or relating to the electrolysis of sodium sulphate
Inventor: KERTI JOSZEF
IPC: C01B17/74
Filed: 1961-08-11
Abstract: 938,111. Electrolysis of sodium sulphate. CHEMOLIMPEX MAGYAR VEGYIARU KULKERESKEDELMI VALLALAT. Aug. 11, 1961, No. 29141/61. Class 41. In a process for producing sulphuric acid and sodium hydroxide by the electrolysis of aqueous sodium sulphate solution using a lead anode and a<SP> </SP>mercury cathode to form lead sulphate at the anode and sodium amalgam at the cathode, the sulphated lead anode is subsequently submitted to electrolytic reduction to restore the lead with production of sulphuric acid, and conversion of the sodium amalgam to mercury and sodium hydroxide. The electrolytic cell comprises an upper mercury cathode, an intermediate inert electrode and a lower lead electrode serving as anode, the surface of the sodium sulphate solution being above the mercury electrode during the electrolysis of the sodium sulphate solution. The electrolysis is preferably carried out on a concentrated sodium sulphate solution (about 44% by weight of sodium sulphate at saturation) at a temperature of from 40-70‹ C. and continues until the concentration reaches a figure not less than about 25% by weight. The diluted sodium sulphate solution is then replaced by a dilute solution of sulphuric acid, the surface level of the acid being above 'the intermediate electrode but below the mercury electrode, which acid solution is then electrolysed with the sulphated lead electrode as cathode and the intermediate electrode as anode until the concentration of the acid increases to a value not exceeding about 40%, by reduction of the sulphated lead to metallic lead at the cathode and evolution of oxygen at the inert anode. The sodium amalgam is converted to sodium hydroxide and mercury in a separate conversion zone. In a cell 1 of sheet iron coated with hard rubber the active porous lead powder 3, alternately acting as anode then cathode, is mixed with 1% of barium sulphate powder acting as an anti-aggregation agent to prevent sintering of the lead grains and is enclosed by inert mesh 4 secured in position by synthetic resin mesh 5. The mercury cathode flows in an iron channel 6 provided with an insulating coating and circulates alternately between electrolytic cells and amalgam conversion cells in order that the amalgam can be kept at a low value for sodium, e.g. below 0.1%, permitting sodium amalgam formation with a good current yield. The apparatus comprises a bank of electrolytic cells connected in series with the sodium sulphate solution losing strength as it passes through successive cells in alternation with the sulphuric acid solution which increases in concentration as it circulates through a series of the cells. The process yields caustic soda solution of 50-70% concentration and sulphuric acid of 30-40% concentration with a high current yield. Both the electrolysis of sodium sulphate solution and of sulphuric acid operate with a potential difference of 2-2.5 volts ; and current densities of about 500 amps. per sq. metre at the lead electrode and about 1400 amps. per sq. metre at the mercury electrode have been found suitable. The sodium hydroxide solution is passed through a series of conversion cells in parallel with the electrolytic cells so that its concentration is increased stepwise.