Method for regulating an electrolytic cell for aluminum production

Abstract

The invention concerns a method for regulating an electrolytic cell designed for aluminium production by fused bath electrolysis enabling accurate and reliable control, in time and optionally in space, of the amount of alumina dissolved in the electrolytic solution ( 13 ). The inventive regulating method is characterized in that it comprises: a) controlling the addition of a specific amount Qo of alumina in the solution ( 13 ); b) determining the value of an indicator A of the added amount Q of alumina which is rapidly dissolved in the solution ( 13 ); c) determining the amount ΔQ of added alumina which is not rapidly dissolved in the solution ( 13 ); d) adjusting at least one setting means and/or one control operation and/or at least an intervention on the cell, on the basis of the value obtained for the amount ΔQ, so as to maintain it or bring it down to a value lower than a reference value S. In an advantageous embodiment of the invention, the indicator A is determined by analyzing at least one electrical measurement on the cell.

1 . Method for regulating an electrolytic cell for production of aluminum by electrolytic reduction of alumina dissolved in an electrolyte bath based on cryolite, the cell comprising a pot, at least one anode, at least one cathode element, the pot comprising internal side walls and being capable of containing a bath of liquid electrolyte, the cell also comprising at least one means of adjustment of the cell, the cell being capable of carrying an electrolytic current in the bath, the current having an intensity I, the method comprising adding alumina to the bath and further comprising: 
 a) controlling an addition of a specific amount Q o  of alumina in the bath;    b) determining a value of an indicator A of an added amount Q of alumina which is rapidly dissolved in the bath;    c) determining an amount ΔQ of added alumina which is not rapidly dissolved in the bath;    d) adjusting at least one adjustment means and/or at least one control operation, and/or at least one intervention on the cell, as a function of the value obtained for the amount ΔQ, so as to maintain said value or bring said value to a second value lower than a reference value S.    
     
     
         2 . Method according to  claim 1 , wherein the amount Q corresponds to a fraction of alumina that dissolves within a time less than or equal to a determined time threshold T, and the amount ΔQ corresponds to a fraction of alumina that dissolves within a time greater than the threshold T.  
     
     
         3 . Method according to  claim 2 , wherein the time threshold T is between 100 and 1000 seconds.  
     
     
         4 . Method according to of  claim 1 , wherein the indicator A is determined starting from at least one electrical measurement on the cell capable of detecting variations of electric characteristics of the bath caused by the fraction of added alumina that is dissolved in the bath.  
     
     
         5 . Method according to  claim 4 , wherein the indicator A is determined from an analysis of a voltage U and/or an intensity I measured on the cell.  
     
     
         6 . Method according to  claim 5 , wherein the method includes signal processing.  
     
     
         7 . Method according to  claim 4 , wherein the indicator A is determined from a measurement of the resistivity of the bath.  
     
     
         8 . Method according to  claim 4 , wherein, in order to determine a local nature of dissolution of alumina, the at least one electrical measurement is performed at at least two different locations of the cell.  
     
     
         9 . Method according to any one of  claim 1 , wherein the indicator A is partly or wholly determined by sampling or using probes.  
     
     
         10 . Method according to any one of  claim 1 , wherein the amount Q is determined by calibrating the indicator A.  
     
     
         11 . Method according to  claim 10 , wherein the calibrating is done by modelling and/or making statistical measurements on at least one cell operating under similar conditions as the cell of said method.  
     
     
         12 . Method according to  claim 1 , wherein the amount ΔQ of alumina that does not dissolve in the bath is determined by subtracting quantities Q o  and Q, such that ΔQ=Q o −Q.  
     
     
         13 . Method according to  claim 1 , wherein the adjusting comprises at least a modification to the amount Q o  of alumina added per unit time.  
     
     
         14 . Method according to  claim 1 , wherein the adjusting includes at least one modification to the position of the at least one anode, so as to modify a distance between the at least one anode and the cathode element.  
     
     
         15 . Method according to  claim 1 , wherein the at least one control operation includes adding a determined amount of AlF 3  into the said electrolyte bath.  
     
     
         16 . Method according to any one of  claim 1 , wherein the at least one intervention includes fast displacement of the at least one anode to modify interface conditions between the at least one anodes and the bath, and/or to eliminate gas bubbles present under the surface of the at least one anode.  
     
     
         17 . Method according to any one of  claim 1 , wherein the at least one anode is selected from the group consisting of anodes made of a carbonaceous material and non-consumable anodes.  
     
     
         18 . Method according to  claim 17 , wherein the non-consumable anodes comprise metallic anodes, coated anodes and/or anodes of a ceramic—metal composite.