Removing impurities from an electrolyte

Abstract

It is disclosed a purifier assembly and method for removing impurities from an electrolytic bath before using the same with an electrolytic cell for making a metal, such as aluminum or aluminium. The assembly comprises a purification tank, located upstream the cell, for containing the bath; and at least one row, preferably at least two rows, of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes. The rows of vertically oriented cathodes and anodes are configured in size to be inserted into the tank. The purifier assembly is configured to maintain an anode-to-cathode distance (ACD) between the cathodes and anodes. The purifier is particularly adapted for removing sulfur, phosphorus, iron, and/or gallium from cryolite for the eco-friendly production of aluminum with a cell using oxygen-evolving or inert anodes, which preferably requires a purer bath.

1 . A purifier assembly for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal, the purifier assembly comprising:
 a purification tank located upstream an electrolytic cell and configured to contain at least a portion of the electrolytic bath; and   at least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes,   wherein the at least one row of vertically oriented cathodes and anodes is configured in size to be inserted into the purification tank; and   wherein the purifier assembly is configured to provide and maintain an anode-to-cathode distance (ACD) between each of said vertically oriented cathodes or anodes.   
     
     
         2 . The purifier assembly of  claim 1 , comprising at least two parallel rows of alternating vertically oriented cathodes and anodes, each anode or cathode of one row being adjacent respectively to another anode or cathode of adjacent parallel rows, forming as such an array of alternating columns of vertically oriented cathodes and anodes. 
     
     
         3 . The purifier assembly of  claim 1 , further comprising:
 at least one anode connection rail configured to be operatively connected to the power supply, each of the anode connection rail being configured to support one row of vertically oriented anodes and to electrically connect the anodes one to each other in a parallel arrangement; and   at least one a cathode connection rail configured to be operatively connected to the power supply, each cathode connection rail being configured to support one row of vertically oriented cathodes and to electrically connect the cathodes one to each other in a parallel arrangement.   
     
     
         4 . The purifier assembly of  claim 3 , wherein the at least one anode and cathode connection rails are configured to be independently moved at a desired position relative to the electrolytic bath in the purification tank. 
     
     
         5 . The purifier assembly of  claim 4 , wherein the at least one cathode connection rail is configured to entirely plunge the cathodes below a bath-vapor interface of the electrolytic bath. 
     
     
         6 . The purifier assembly of  claim 3 , wherein each of the anodes or cathodes comprises a longitudinal stem having one end connected to an anode body or a cathode plate, and an opposite end configured to be operatively connected to the anode or cathode connection rails respectively. 
     
     
         7 . The purifier assembly of  claim 6 , wherein each of the anode bodies and cathode plates have a top extremity connected to their respective stems, the top extremities of the cathodes being located below the top extremities of the anodes when the row of electrodes is plunged into the electrolytic bath in order to have the cathode plates entirely plunged into the bath. 
     
     
         8 . The purifier assembly of  claim 7 , wherein the anode body comprises carbon or graphite, the cathode plates comprises steel or stainless steel, and the longitudinal stem comprises steel or stainless steel. 
     
     
         9 . The purifier assembly of  claim 7 , wherein each of the anodes and cathodes further comprises a protective sleeve around the longitudinal stem for protecting the longitudinal stem from corrosion. 
     
     
         10 . The purifier assembly of  claim 9 , wherein the longitudinal sleeve comprises:
 a metal oxide of the metal to be produced, the metal oxide being aluminum oxide when the metal to be produced is aluminum;   a semi-noble metal, such as copper; or   silicon carbide (SiC).   
     
     
         11 . The purifier assembly of  claim 3 , further comprising a supporting structure for securing the at least one row of alternating vertically oriented cathodes and anodes at a position relative to the purification tank. 
     
     
         12 . The purifier assembly of  claim 11 , wherein the connection rails are secured to the supporting structure for reinforcing and stabilizing the position of anodes and cathodes. 
     
     
         13 . The purifier assembly of  claim 11  or  12 , wherein the supporting structure is configured in size to close a top opening of the purification tank and seal the tank. 
     
     
         14 . The purifier assembly of  claim 13 , wherein the supporting structure is configured to provide insulation, to be resistant to corrosion and to prevent gas from escaping the purification tank when the top opening is closed, the purification tank being then equipped with a gas outlet for safely collecting anode gas. 
     
     
         15 . The purifier assembly of  claim 13 , wherein the purification tank belongs to a melter, the melter being used for melting a dry bath of said electrolytic bath. 
     
     
         16 . The purifier assembly of  claim 1 , wherein the at least one row of alternating vertically oriented cathodes and anodes forms a compact array with an ACD ranging from about 1 cm to about 5 cm. 
     
     
         17 . The purifier assembly of  claim 16 , wherein the ACD is about 2.5 cm. 
     
     
         18 . The purifier assembly of  claim 1 , wherein the power supply comprises a DC rectifier. 
     
     
         19 . The purifier assembly of  claim 1 , wherein the metal to produce by the electrolytic cell is aluminum, the electrolytic bath then comprising cryolite and the impurities to remove comprising sulfur, phosphorus, iron, nickel, chromium, copper, gallium or mixture thereof. 
     
     
         20 . A method for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal, the method comprising the steps of:
 injecting at least a portion the electrolytic bath to purify into a purification tank located upstream an electrolytic cell;   positioning into the purification tank at least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes, the at least one row of alternating vertically oriented cathodes and anodes being configured in size to be inserted into the purification tank and to provide and maintain an anode-to-cathode distance (ACD) between each of the vertically oriented cathodes or anodes; and   applying the electric current between the anodes and cathodes for a period of time to remove impurities from the electrolytic bath.   
     
     
         21 . The method of  claim 20 , wherein each of the cathodes comprises a cathode plate, the cathodes being positioned in the electrolytic bath so as to be entirely submerged in the electrolytic bath. 
     
     
         22 . The method of  claim 20 , wherein when the electrolytic bath to purify is a dry electrolytic bath, the method further comprises the step of:
 melting the dry bath before injecting at least the portion the electrolytic bath into the purification tank; or   melting the dry bath directly in the purification tank.   
     
     
         23 . The method of  claim 20 , wherein the electric current is a direct current applied using a DC rectifier. 
     
     
         24 . The method of  claim 20 , wherein applying the electric current comprises measuring an amount of impurities present in the bath before adapting a total electric charge passing through a purifier assembly comprising said purification tank and said at least one row of alternating vertically oriented cathodes and anodes. 
     
     
         25 . The method of  claim 24 , wherein the total charge is about 0.1 to about Ampere-hours (Ah) per kilogram of electrolytic bath to purify. 
     
     
         26 . The method of  claim 25 , wherein the total charge is about 0.3 to about 4.0 Ampere-hours (Ah) per kilogram of electrolytic bath to purify. 
     
     
         27 . The method of  claim 20 , wherein a current density of the cathodes is about 0.004 to about 0.4 A/cm 2 . 
     
     
         28 . The method of  claim 27 , wherein the current density of the cathodes is about 0.1 to about 0.3 A/cm 2 . 
     
     
         29 . The method of  claim 20 , wherein the voltage across a purifier assembly comprising the row of alternating vertically oriented cathodes and anodes and the purification tank is between about 0.5 V and about 2.5 V. 
     
     
         30 . The method of  claim 20 , wherein the period of time is between about 1 to about 150 hours. 
     
     
         31 . The method of  claim 30 , wherein the period of time is between about 24 to about 96 hours. 
     
     
         32 . The method of  claim 20 , further comprising increasing current density for either increasing a specific electrical charge (Ah/kg bath), reducing the period of time of purification, or a combination thereof. 
     
     
         33 . The method of  claim 20 , wherein the method further comprises removing the cathodes from the purification tank and removing solid impurities accumulated on the cathodes. 
     
     
         34 . The method of  claim 20 , wherein the metal to produce by the electrolytic cell is aluminum, the bath then comprising cryolite and additives, the impurities to remove comprising sulfur, phosphorus, iron, nickel, chromium, copper, gallium or mixture thereof. 
     
     
         35 . The method of  claim 34 , wherein the additives comprises sodium fluoride, aluminum fluoride, calcium fluoride, and/or aluminum oxide.