Patent GB-925378-A (GB)
Title: Producing manganese by fused salt electrolysis, and apparatus therefor
Inventor: WALSH JAY Y
IPC: C25C3/30
Filed: 1961-11-13
Abstract: 925,378. Fused salt electrolysis; manganese. MANGANESE CHEMICALS CORPORATION. Nov. 13, 1961, No. 40486/61. Class 41. Low-carbon low-silicon manganese is obtained in cast ingot form from a fused electrolyte bath consisting of at least 50% calcium fluoride, together with manganous oxide and a mixture of refractory inorganic oxides, both basic and acidic, e.g. acidic-SiO 2 , B 2 O 3 , Al 2 O 3 ; basic- MgO, CaO, BaO. The fused bath having a melting range from about 1150‹ C. to about 1300‹ C. contains at least 50% but not more than about 90% calcium fluoride and from 0.5 to 10.0% manganese as manganous oxide with additional inorganic oxides having an overall acidic character sufficiently acidic to facilitate the dissolution of basic manganous oxide but having a sufficient alkalinity to prevent the substantial electrolytic reduction of silica and silicon contamination of the metal product. Both the acidic and the basic inorganic oxides have free energy change of reaction values with manganese at 1327‹ C. to form MnO which are greater than zero. The bath composition is stable against decomposition and volatilization within the aforementioned fusion range and the molten electrolyte 9 is confined within a solid skull of its own composition 9<SP>1</SP> above the cathodic pool of molten manganese or manganese alloy 8. The steel shell 1 of the cell is lined with heatinsulating refractory 2 with the bath banks 7, in contact with solid electrolyte fluxes, being of high alumina or magnesia and the well, wherein there is only a pool of molten metal to support, comprising carbon-free electrically non-conductive refractory, e.g. corundal ramming mix, which is inert to molten manganese and its alloys produced. The electrically conductive metal cathode bar 4 of iron or steel contacts the pool of molten metal 8; the cylindrical carbonaceous anode 15, 20 to 30 inches in diameter, is preferably of petroleum coke carbon. Metal is removed from the cell by the tapping port 5 closed by an iron rod 6 which insert is removed by electrically melting. A cylindrical hood 10 is supported over the bath so that its lower edge is embedded and sealed in the solid skull mass 9<SP>1</SP>; it provides for feeding of charge through orifice 11/12 and maintains a small positive pressure of reducing gas, carbon monoxide, about the exposed anode to protect it from oxidation. To maintain a suitable flux skull around the fused pool of electrolyte its melting-point should not be more than 100-150‹ C. below the melting- point of the product metal, e.g. manganese at 1260 ‹ C. ; the bath is maintained at such a temperature above the metal melting-point as to maintain molten the central exposed pool of metal. The process yields metallic material comprising not less than 96.7% of the product, the balance comprising impurities primarily of carbon, silicon and aluminium, said metallic material being of manganese or an alloy thereof contaminated with a proportion of iron, varying from 1.5 to 15.5%, obtained by alloying with the iron cathode bar particularly during early casts before the cathode bar is protected by a manganese alloy shield. Admixture with the feed of suitable oxides, e.g. B 2 O 3  and/or simultaneous feeding with an anode bar of metal, e.g. of Cu or Ni, at a controlled rate produces a manganese alloy product, e.g. Mn-B, Mn-Ni, Mn-Cu. Other alloys which can be produced are those of manganese with iron, antimony, arsenic, chromium, cobalt, tin and zinc. Normal commercial anode current densities of 1000-3000 amps. per sq. ft. are reported; the cathode current density is not critical. The operating voltage is reported to be within the range 5.0-6.5 volts. The cell is brought up to operating temperature and temperature equilibrium by the passage of alternating current; it is replaced by direct current for the electrolysis. The development of solid phases in the fused electrolyte, usually refractory silicates, raises the electrical resistance of the cell and can be counteracted by the addition of 2% by wt. of sodium borate, which also serves to promote the solution of manganous oxide in the feed. Typical feed material is reduced pyrolusite ore, the manganese dioxide, MnO 2 , of said ore being fully reduced to manganous oxide for satisfactory current efficiency.