Multipolar Magnesium Cell

Abstract

Molten metal is continuously produced by electrolysis of a molten electrolyte which is denser than the metal in an electrolytic multipolar cell characterized by a high and stable current efficiency. Molten metal droplets are separated from the circulating electrolyte along a set of horizontal channels of gravity settlers disposed between the electrolysis chamber and the metal collecting chamber. Thereafter the metal rises to and floats on the surface of the electrolyte in the metal collection chamber, is conveyed to a metal collecting reservoir immersed in the electrolyte and periodically removed to maintain the cell in continuous operation. The coalescence of the metal droplets is enhanced by sealing the cell to prevent ingress of air into the chlorine room and into the front compartment. The sealing is obtained by lowering the joint between the covers and the cell walls to levels close to the electrolyte level and by using large impervious ceramic tiles in the lowered joint, in the barrier wall between the chlorine room and the front compartment and as a cladding of the walls of the cover of the electrolysis chamber and by cladding the exposed graphite surfaces with a gas barrier foil. The barrier wall and other consumable components are removable without emptying the cell, for an increased campaign life. Surplus heat is controllably and safely extracted from the electrolyte in a recoverable way by a set of evaporative heat extraction devices immersed in the electrolyte across the circulating stream.

1 . A process for the production of molten magnesium metal by electrolysis in a cell with an electrolysis chamber and a metal collection chamber, said process comprising electrolyzing in said electrolysis chamber an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal, and at least one electrode assembly that comprises a cathode defining within it a cavity, an anode disposed within the cavity and at least one intermediate bipolar electrode disposed between the anode and the cathode within said cavity,
 said electrolytic cell being characterized by a primary barrier wall that can be separately removed and replaced while the cell is in full operation.   
     
     
         2 . A process for the production of molten magnesium metal by electrolysis in a cell with an electrolysis chamber and a metal collection chamber, said process comprising electrolyzing in said electrolysis chamber an electrolyte containing a fused salt of said metal to produce said metal, said electrolyte having a greater density than said metal, and at least one electrode assembly that comprises a cathode defining within it a cavity, an anode disposed within the cavity and at least one intermediate bipolar electrode disposed between the anode and the cathode within said cavity
 said process being characterized by the use of gravity settlers as a means to increase metal collection.   
     
     
         3 . An electrolytic process as claimed in  claim 2 , wherein said gravity settler consists of a set of thin plates stacked vertically, said plates being disposed in horizontal or sub-horizontal planes to divide the flow of the electrolyte into horizontal or sub-horizontal channels disposed one upon the other to form uniform-velocity streams that operate in parallel, and being so designed that the retention times of the electrolyte between entry and exit of each channel are proportional to the height of the channel. 
     
     
         4 . An electrolytic process as claimed in  claim 3 , wherein the average retention time of the electrolyte in the gravity settler is between one and ten times the average retention time of the electrolyte in the inter-electrode spaces and the ratio between the height and the length of each channel is between 1/10 and 1/40. 
     
     
         5 . An electrolytic process as claimed in  claim 3 , wherein said set of plates are shaped, across the plane of electrolyte circulation, as a set of wide-angled chevrons, for the efficient collection of metal droplets under the lower faces of said plates along said channels. 
     
     
         6 . An electrolytic process as claimed in  claim 2 , wherein said gravity settlers are located on the back of said cathodes in said electrolysis chamber. 
     
     
         7 . An electrolytic process as claimed in  claim 2 , wherein said gravity settlers are located in said metal collection chamber continuous with the exit ports of said electrolysis chamber. 
     
     
         8 . An electrolytic process as claimed in  claim 2 , wherein said metal that separates in said gravity settler is allowed to rise to the surface of the electrolyte and to float as a pad in a corner of said metal collection chamber wherefrom it is conveyed to an open-bottom metal collecting reservoir immersed in the electrolyte. 
     
     
         9 . An electrolytic process as claimed in  claim 1 , characterized by increased cell efficiency obtained by sealing said cell to prevent any ingress of ambient air during normal cell operation into the chlorine room and into the front compartment. 
     
     
         10 . An electrolytic process as claimed in  claim 9 , wherein said sealing of said cell, includes locating the joints between the cell covers and the cell casing to an elevation, in the front compartment, above the maximum operating level of said electrolyte and, in the chlorine room, below the minimum operating level of said electrolyte, and the provision of a horizontal layer of large, impervious and corrosion-resistant ceramic tiles that lay flat on top of the cell walls to support the refractory-lined and steel-cased covers and to electrically insulate them from said steel casing, while sealing the outer rims of said steel casing and the bottom outer edges of said steel-cased covers. 
     
     
         11 . An electrolytic process as claimed in  claim 10 , wherein a primary barrier wall assembly, removable and replaceable while said cell is in operation, electrically insulates said covers from each other and reaches down below the minimum operating level of the electrolyte to form a stable sealing structure between the chlorine room and the front compartment. 
     
     
         12 . An electrolytic process as claimed in  claim 11 , wherein said removable barrier wall extends upwards above the covers and outwards towards said outer rim of said steel casing as a double layer of large, impervious, insulating and corrosion-resistant ceramic tiles disposed with staggered vertical joints and tightly bonded together by high temperature cement applied to their back-to-back faces. 
     
     
         13 . An electrolytic process as claimed in  claim 1 , wherein said primary barrier wall is backed up by a secondary barrier wall made of ceramic tiles attached and sealed to the cover of said electrolysis chamber, said cover being also clad by ceramic tiles around and inside the refractory walls of the chlorine room to provide added corrosion resistance and protection to the ingress of ambient air into the chlorine room. 
     
     
         14 . An electrolytic process as claimed in  claim 9 , wherein the steel-cased covers are provided with high temperature seals in all joints between the steel cases of said covers and the equipment permanently installed on them, to prevent ingress of ambient air into said cell, including the provision of high temperature foil, including household aluminum foil to clad the exposed surfaces of the anodes. 
     
     
         15 . An electrolytic process as claimed in  claim 2 , wherein power efficiency improvements include the provision of means for extracting surplus heat from said electrolyte by an energy recovery system based on evaporative/condensing heat transfer technology. 
     
     
         16 . An electrolytic process as claimed in  claim 15 , wherein said means for extracting surplus heat from the electrolyte include an array of evaporative heat extraction devices immersed in the electrolyte across the circulating stream in the metal collection chamber to transfer said surplus heat from the electrolyte to a condenser outside the cell. 
     
     
         17 . An electrolytic process as claimed in  claim 16 , wherein said array of heat extraction devices immersed in the electrolyte supply low pressure steam to a heat recovery system somewhere else in the plant and is frd with boiler-quality water via a pressure regulating valve responsive to a thermostat that senses and controls the temperature of the electrolyte. 
     
     
         18 . An electrolytic process as claimed in  claim 17 , wherein each said evaporative heat extraction device is provided with an individual level control valve that responds to a float located close to the bottom of the evaporator, said level control valve to avoid flooded conditions in said evaporator by its throttling action and therefore providing automatic protection against unsafe operation. 
     
     
         19 . An electrolytic cell apparatus that produces molten magnesium with the process of  claim 1 . 
     
     
         20 . An electrolytic cell apparatus that produces molten magnesium with the process of  claim 2 .