Patent Publication Number: US-8123928-B2

Title: Shut-down and start-up procedures of an electrolytic cell

Description:
FIELD OF THE INVENTION 
     The technical field relates to controlled shut-down and start-up procedures of an aluminium smelter and, more particularly, to the shut-down procedure of an electrolytic cell without anode removal. 
     BACKGROUND 
     An aluminium smelter may be shut-down for routine re-lining of the electrolytic cell over a period of five to eight years. During maintenance shut-down for re-lining, the maximum metal and often the bath are siphoned from a cell and the cell is allowed to cool down. The anodes are usually raised after the cell power is shut off to reduce sodium contamination. The frozen electrolyte, alumina cover, refractory lining, and cathode are subsequently dug out of the cell using tools such as jackhammers, and any of the anodes which can be recovered are removed for cleaning, a labour intensive process. 
     If a smelter is shut down as a result of labour conflicts, power constraints or economic downturns, it is generally desirable to shut down the electrolytic cell while keeping the cell intact unless the cell was already scheduled for re-lining. In unexpected temporary shut-downs, for example, because of electrical failure or accident, it may be possible to maintain an electrolytic cell in a “sleeping mode” with low energy applied in the cell and by lowering the anodes until they are submerged in liquid metal. Upon start-up, sufficient heat may be generated by raising the anodes close to the metal surface in order to gradually heat up the metal and sufficiently enough to pour liquid bath into the cell. Such a procedure may be dangerous, requires constant supervision and is not cost-effective if the shut-down is of prolonged duration because of the energy consumption during stoppage and the difficulty to raise the anode beam when the metal temperature is lower than 800° C. 
     The normal restart procedure after a complete classical stoppage requires that the electrolyte, the anodes, and alumina cover be removed in order to expose the frozen metal surface. About 20% of old anodes are generally scrapped and the good ones are cleaned and put back in the cell to be used for restart. After pre-heating the cathode to minimize the risk of explosions from moisture, molten electrolyte from so-called “donor cells” is added to cover the cathode to a depth sufficient to submerge the anodes in electrolyte. As power builds up, the anodes are raised away from the cathode to a predetermined anode-cathode distance. 
     BRIEF SUMMARY 
     It is therefore an aim in certain embodiments of the present invention to address the above mentioned issues and to provide controlled shut-down and start-up procedures for electrolytic cells which reestablish normal operations relatively quickly and with relatively low associated costs. 
     According to a general aspect, there is provided a process for shutting down an operating electrolytic cell for the production of aluminium, the electrolytic cell having a cathode block, a depth of molten aluminium layer covering the cathode block, and a depth of molten electrolytic bath covering the molten aluminium layer, a plurality of anodes disposed for vertical movement into and out of the electrolytic cell to vary an anode-cathode distance separating a bottom surface of the anodes from a top surface of the molten aluminium layer, electrolysis electric power being applied to the anodes to reduce alumina fed to the electrolytic cell and produce aluminium metal on the cathode block. The process includes a) gradually moving downwardly the anodes from an operating position where the bottom surfaces of the anodes are immersed in the electrolytic bath to a cooling position where the bottom surfaces are immersed in the molten aluminium layer; b) allowing the electrolytic cell to cool and moving up and down the anodes periodically on a short distance to break a peripheral crust of solidifying electrolytic bath forming at a periphery of the electrolytic cell during the cooling step; c) after the electrolytic bath is completely solidified into a cohesive mass, raising the anodes to create a space between a bottom surface of the solidified electrolytic bath and a top surface of molten aluminium layer; and d) allowing the electrolytic cell to cool until the aluminium layer solidifies. 
     In an embodiment, the electrolysis electric power to the electrolytic cell is cut off after step a). In another embodiment, step a) is performed within one hour of cutting off power to the electrolytic cell. 
     In an embodiment, in step a), the anodes are moved downwardly on a distance between 6 and 7 cm over a period of fifteen to twenty minutes. 
     In an embodiment, at least one pouring hole is formed and maintained in the solidifying electrolytic bath. 
     In an embodiment, in step b), the anodes are moved upwardly a height of up to 1.5 cm and downwardly a height of up to 1.5 cm once every hour. In a particular embodiment, the anodes are moved upwardly and downwardly within approximately one to five minutes. 
     In an embodiment, step c) is performed after the molten aluminium layer has reached a temperature of 825° C. 
     According to another general aspect, there is provided a process of restarting an electrolytic cell that is shut down as described above. The process for restarting the cell comprises: adding molten electrolytic bath to the electrolytic cell and into the space defined between the solidified electrolytic bath and the solidified aluminium layer; applying electrolysis electric power to the electrolytic cell following the addition of molten electrolytic bath to the electrolytic cell; then, adding additional molten electrolytic bath to the electrolytic cell and simultaneously raising the anodes until the anode-cathode distance is between seventeen and twenty centimeters. 
     In an embodiment, the additional molten electrolytic bath is superheated above the liquidus temperature of the electrolytic bath before being added to the electrolytic cell. 
     In an embodiment, the anode-cathode distance is maintained between 17 to 20 cm until the previously solidified aluminium layer metal has completely melted; and then siphoning excess electrolytic bath from the electrolytic cell and restoring the anode-cathode distance to the operating position. 
     According to another general aspect, there is provided a process for shutting down an operating electrolytic cell for the production of aluminium and having vertically displaceable anodes. The process comprises: lowering the anodes until a bottom surface of the anodes is immersed in an aluminium layer of the electrolytic cell in a molten state; allowing the aluminium layer and an electrolyte bath in a molten state to cool down with the bottom surface of the anodes immersed in the aluminium layer, the electrolyte bath covering the aluminium layer; determining if the electrolyte bath is solidified, if the electrolyte bath is solidified, raising the anodes before solidification of the aluminium layer to create a space between a bottom surface of the solidified electrolyte bath and the bottom surface of the anodes and a top surface of the aluminium layer. 
     In an embodiment, the cooling down step further comprises periodically moving up and down the anodes to break a peripheral crust of the electrolyte bath at a periphery of the electrolytic cell. 
     In an embodiment, the bottom surface of the anodes remains immersed in the aluminium layer during the periodically moving up and down step. 
     In an embodiment, the electrolysis electric power to the electrolytic cell is cut after the anode lowering step. In another embodiment, the anode lowering step is performed within one hour of cutting electrolysis electric power to the electrolytic cell. 
     In an embodiment, the anode lowering step comprises moving downwardly the anodes along a distance of approximately five to seven centimeters over a period of at least ten minutes. 
     In an embodiment, wherein the anode raising step is performed after the aluminium layer has reached a temperature below approximately 825° C. 
     In an embodiment, the determining step further comprises monitoring a temperature of the aluminium layer and wherein the anode raising step is performed after the temperature of the aluminium layer is below approximately 825° C. 
     In an embodiment, the anode raising step is performed before the aluminium layer reaches approximately 660° C. 
     According to a further general aspect, there is provided a process for shutting down an operating electrolytic cell for the production of aluminium; the process comprising: moving downwardly anodes of the electrolytic cell from an operating position where bottom surfaces of the anodes are immersed in an electrolytic bath in a molten state to a cooling position where the bottom surfaces of the anodes are immersed in an aluminium layer in a molten state, the aluminium layer being covered by the electrolytic bath; allowing the electrolytic cell to cool and periodically moving up and down the anodes with the bottom surfaces of the anodes remaining in the aluminium layer to break a peripheral crust of the electrolytic bath forming at a periphery of the electrolytic cell; monitoring a state of the electrolytic bath; after the electrolytic bath has completely solidified into a cohesive mass, raising the anodes to create a space between a bottom surface of the electrolytic bath in a solid state and a top surface of the aluminium layer in the molten state; and allowing the electrolytic cell to cool until the aluminium layer solidifies. 
     In an embodiment, the electrolysis electric power to the electrolytic cell is cut after the anode moving downwardly step. In another embodiment, the anode moving downwardly step is performed within one hour of cutting electrolysis electric power to the electrolytic cell. 
     In an embodiment, the bottom surface of the anodes is located above the top surface of the aluminium layer after the anode raising step and during the cooling step. 
     According to a still another general aspect, there is provided a process of restarting an electrolytic cell for the production of aluminium, the electrolytic cell having a solidified aluminium layer, an electrolytic bath solidified around anodes and spaced apart from the aluminium layer by a seven to twelve centimeter anode-cathode distance separating an bottom surface of the anodes from a top surface of the solidified aluminium layer; the process comprising: adding molten electrolytic bath to the electrolytic cell and into a space defined between the solidified electrolytic bath and the solidified aluminium layer; applying electrolysis electric power to the electrolytic cell following addition of molten electrolytic bath to the electrolytic cell; then, adding additional molten electrolytic bath to the electrolytic cell and simultaneously raising the anodes until the anode-cathode distance is between seventeen and twenty centimeters. 
     In an embodiment, the anode-cathode distance is maintained between 17 to 20 cm until the previously solidified aluminium layer metal has completely melted; and then excess electrolytic bath is siphoned from the electrolytic cell and the anode-cathode distance is restored to an operating position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an electrolytic cell in normal operation; 
         FIG. 2  is a flowchart showing the various steps in shut-down and start-up procedures of the electrolytic cell in accordance with an embodiment and corresponding to the schematic drawings of  FIGS. 1 , and  3  to  6 . 
         FIG. 3  is a schematic cross-sectional view of the electrolytic cell of  FIG. 1  during an initial shut-down phase in accordance with an embodiment; 
         FIG. 4  is a schematic cross-sectional view of the electrolytic cell of  FIG. 1  during a shut-down cooling phase in accordance with an embodiment; 
         FIG. 5  is a schematic cross-sectional view of the electrolytic cell of  FIG. 1  when the electrolytic cell shut-down has been completed in accordance with an embodiment; and 
         FIG. 6  is a schematic cross-sectional view of the electrolytic cell of  FIG. 1  during cell start-up in accordance with an embodiment. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown an electrolytic cell  20  for aluminium production. The cell  20  has an outer shell  22  containing an internal lining  24  and a cathode block  26  located in a lower section of the cell  20 . Anodes  28  are shown with a lower portion  44  immersed in a molten electrolytic bath  30  that lies over a molten aluminium metal layer  32 . The molten aluminium layer covers the cathode block  26 . 
     In an embodiment, the outer shell  22  is made of metal such as steel, the internal lining  24  generally includes blocks of refractory material, refractory lining paste and/or solidified bath, the cathode block  26  is a carbothermic cathode block, and the anodes  28  are made of carbonaceous material. The electrolytic bath  30  is cryolite-based and includes dissolved alumina. A thin, solid electrolyte and alumina-based crust  34  usually forms a top layer, covers the electrolytic bath  30 , and surrounds the anodes  28 . The aluminium layer  32  often will have a depth that varies between twelve to twenty centimeters (12-20 cm) and the electrolytic bath  30  has a depth that typically varies between sixteen to nineteen centimeters (16-19 cm). The depth of the aluminium layer  32  varies over time. It increases as liquid metal accumulates at the bottom of the crucible and it decreases when liquid metal is removed from the cell  20 . 
     The anodes  28  are connected to an anode beam (not shown) through attachment means  38  and an anode frame  39 . The anode frames  39  are adapted to lower and raise the anodes  28  within the electrolytic cell  20 . In an embodiment, electric power to lower and raise the anodes  28  within the electrolytic cell  20  is independent from the electric power for electrolysis of alumina, i.e. the anode frames  39  are powered separately from the very high voltage power supplied to the electrolysis cell  20 . The electrolysis power can be turned off while the electric power for moving the anodes  28  can be functional. The anode frame  39  can thereby be kept free to control the anode position in the cell  20 . 
     An electrolysis electrical current flows through the aluminium electrolytic cell  20 . The electrical current enters the cell  20  through the anodes  28  via the anode beam, the anode frame  39 , and the attachment means  38 , passes through the electrolytic bath  30  and the molten aluminium layer  32 . The electrical current then enters the cathode block  26  and is carried out of the cell  20  by cathode bars  40 . The cathode bars  40  are typically made of steel and electrical conductors  42  are attached thereto to route the electrolysis current. 
     During cell operation, the electrolytic bath  30  and the molten aluminium layer  32  are contained within a crucible including the internal lining  24  and the cathode block  26 . 
     During normal operation, an inferior portion  44  of the anodes  28  is immersed in the electrolyte bath  30  as shown in  FIG. 1 , without being in contact with the molten aluminium layer  32 . Metal aluminium produced during electrolysis accumulates at the bottom of the crucible and a relatively clear interface is established between the molten aluminium layer  32  and the molten electrolyte bath  30 . The anodes  28  are immersed to penetrate into the bath  30  and maintain an approximate anode-cathode distance “d” between the bottom surface  46  of the anodes  28  and the top surface  48  of the aluminium layer  32  of approximately four centimeters (4 cm), i.e. the distance between the bottom surface  46  of the anodes  28  and the interface between the aluminium layer  32  and the electrolyte bath  30 . It is appreciated that the anode-cathode distance “d” can range between three to four centimeters (3-4 cm) in alternative embodiments. 
     Referring now to  FIG. 2 , the various steps of one embodiment of a process for shutting down and starting the cell  20  are shown in a flowchart. More particularly, the left flowchart shows the steps for shutting down the cell  20  while the right flowchart shows the steps for starting the cell  20  following the cell shut down. 
     The cell shut down can be planned or it can be unintended, such as during a power failure. Block  60  represents the cells  20  in normal operation wherein the anodes  28  are immersed to penetrate into the electrolyte bath  30  and maintain an anode-cathode distance “d” of approximately four centimeters (4 cm). 
     In preparation for shut-down, the anodes  28  are lowered gradually through the bath  30  so as that their inferior portions  44  are immersed in the aluminium layer  32  as shown in  FIG. 3  and represented in block  62 . Typically, the anodes  28  will be lowered to a distance of about six (6) cm over a fifteen to twenty minute period. The anodes  28  are gradually lowered to reduce bath spillage. Bath will tend to flow over the crust of the smallest anode butts that are lower than the sidewall. In case of a planned stoppage, a few holes may be made around the smallest butts. The descent of the anodes  28  into liquid metal  32  is controlled at a rate intended to cause any displaced liquid bath  30  to flow over the top crust  34  thereby minimizing any bath spill over the sidewall. The lowering speed is related to the free space observed under the sidewall. 
     Following the lowering step, the anodes  28  can be partially submerged in aluminium metal by as much as four to nine centimeters (4-9 cm) because the anodes  28  displace aluminium metal  32  and the aluminium metal  32  level increases. Thus, following step  62 , the bottom surface  46  of the anodes  28  is located approximately between four to nine centimeters (4-9 cm) below the top surface  48  of the aluminium layer  32 , i.e. four to nine centimeters (4-9 cm) below the interface between the aluminium layer  32  and the electrolyte bath  30 . 
     In a planned shut-down, step  62  can be performed while the electrolysis cell power is still on. In a non-intentional shut down, step  62  should be performed as soon as possible while the electrolyte bath  30  and the aluminium metal  32  temperatures are still relatively high. For instance, step  62  could be performed within approximately one hour of the electrolysis cell power being cut off. If step  62  is not performed early enough following the cut off of the electrolysis cell power, the electrolyte bath  30  can become too viscous and the submerged inferior portions  44  of the anodes  28  may entrain electrolytic bath  30  which could interfere with a following start-up procedure, which will be described in more details below. By immersing the anodes  28  in the metal layer  32  while the electrolytic bath  30  is still substantially fluid, a substantially clean anode surface can be submerged in the metal layer  32 . 
     Following step  62 , in a planned shut-down, the electrolysis power to the cell  20  is shut off. 
     Once the power to the cell  20  is shut off, the cell  20  is allowed to cool down as shown in block  64 . The electrolytic bath has a higher melting point than aluminium metal. Thus, during the cooling step  64 , the electrolytic bath  30  and the metal layer  32  cool down until the entire bath  30  solidifies from the top down forming a cohesive mass. The cooling and solidification process may take up to one day. 
     During the cooling step shown in FIG.  4 ., the anodes  28  are moved every hour up to approximately 1.5 cm then down to approximately 1.5 cm, as shown in block  66 . It is appreciated that the anodes  28  can be moved upwardly and downwardly to a height of up to about 1.5 cm. For instance, the anodes are moved up and then down every hour. It is appreciated that the anodes  28  can be moved upwardly and downwardly within one to five minutes. As mentioned above, the anode frame  39  is powered separately from the very high voltage power supply to the cell  20  and is thereby kept free to control the anode position in the cell  20 . The back and forth (or up and down) movements of the anodes  28  gently break the freezing electrolyte crust forming on the cell perimeter, i.e. cracks are formed in the crust. The up and down movements of the anodes  28  destroy the bond between the electrolyte crust  30  and the internal lining  24  of the cell  20 . During the back and forth motion of the anodes  28 , the bottom surface  46  of the anodes  28  remains below the top surface  48  of the aluminium layer  32 , i.e. below the interface between the aluminium layer  32  and the electrolyte bath  30 . The back and forth motion of the anodes  28  is not a vigorous movement and the electrolytic bath  30  still freezes around the anodes  28 . 
     A pouring hole  50  is created and maintained in the freezing electrolytic bath  30  at one end of the cell  20  through which molten bath from donor cells may be added to the cell  20  on start-up. The pouring hole is maintained open during bath cool down with a sufficient width to pour in liquid bath for an eventual start-up. This activity will be explained in more details below with reference to  FIG. 6 . It is appreciated that a second hole may be created in the freezing electrolytic bath  30  in order to see the liquid bath from the other end of the pot during restart. 
     During the cooling step, the temperature of the molten metal  32  is monitored, as shown in decision block  68 , and when the molten metal has cooled to approximately 825° C., the electrolytic bath  30 , which has a melting point of about 900° C., will have completely solidified. It is appreciated that the electrolytic bath  30  can be solidified at a temperature around 825° C. For instance, the temperature in the decision block  68  can range between 800 and 830° C. 
     Well before the aluminium metal solidifies at about 660° C., the anode frame  39  is lifted to raise the anodes  28 , as shown in block  70 , by a height of about ten to twelve centimeters (10-12 cm) thereby forming a space  52  that extends from the bottom surface  54  of the frozen electrolytic bath  30  to the top surface  48  of the metal layer  32 . Therefore, a space exists between the bottom surface  46  of the cleaned anodes  28  and the top surface  48  of the metal layer  32 , as illustrated in  FIG. 5 . 
     Because the metal layer  32  is still liquid, the anodes  28  will lift cleanly out of the metal and present a conductive surface for enabling start-up. The top surface  48  of the metal layer  32  will likewise be clean and free of any solid bath because all electrolyte material will freeze as a cohesive mass  30  between the anodes  28 . It will also be understood that the metal level will decrease as the anodes  28  are withdrawn thereby increasing the separation between the molten metal  32  level and the frozen bath  30 . The bottom surface  46  of the cleaned anodes  28  is spaced apart from the top surface  48  of the metal layer  32 . 
     Finally, the metal  32  is allowed to cool, as shown in block  72 , until it too has frozen. Once the metal temperature has reached approximately 660° C., as shown in decision block  74 , the aluminium layer  32  is completely frozen and the shut-down is completed, as shown in block  76 . For safety reasons, precautions are taken to ensure that there is no water contamination into the cells  20  at any step during the shut-down procedure. During the metal solidification, the distance between the bottom surface  46  of the anodes  28  and the top surface  48  of the metal layer  32  increases due to metal contraction. 
     Thus, the electrolytic cell  20  is shut-down with the bottom surfaces  46  of anodes  28  located approximately nine centimeters (9 cm) above the frozen aluminium layer  32 , as shown in block  78 . It is appreciated that the bottom surface  46  of the anodes  28  can be spaced apart from the top surface  48  of the metal layer  32  by a distance ranging approximately between eight to ten centimeters (7-12 cm). 
     Before start-up, donor cells are heated up to superheat electrolytic bath approximately twenty to forty degrees (20° to 40°) above the liquidus of the electrolytic bath. Superheated liquid bath  56  is poured down the pre-formed pouring hole  50  to fill the space  52  (shown in  FIG. 5 ) of a shut-down cell, as shown in block  80 , so as to ensure that the inferior portions of the anodes  28  which extend through the frozen electrolytic bath  30  are completely wetted. The superheated liquid bath  56  is allowed to rise in the pouring hole  50  and up the refractory lining  24  on the perimeter of the cell  20  to a height sufficient to submerge the anodes  28  by seven to ten centimeters (7 to 10 cm) and to fill the space  52  between the top surface  48  of the frozen metal  32  and the frozen bath  30 . As mentioned above, the initial anode-cathode distance “d”, in the start-up procedure, as shown in  FIG. 6  corresponds to the anode-cathode distance during shut-down and is, in a non-limitative embodiment, approximately between seven and twelve centimeters (7 and 12 cm) and, in a particular embodiment, nine centimeters (9 cm). 
     Then, power is subsequently applied to the cell  20  using as low voltage as possible which, in an embodiment, does not exceed 50 volts, as shown in block  82 . 
     After the electrolysis power is back on, additional liquid bath is poured in the electrolysis cell  20  and the anodes  28  are raised at the same time by keeping a substantially similar immersion until the anode-cathode distance “d” is between seventeen to twenty centimeters (17-20 cm). 
     Typically, six to twelve donor cells must be preheated to provide sufficient superheated bath  56  to start a new cell  20 . While the volume of liquid superheated bath  56  that is required for a start-up will vary considerably, a typical cell  20  may require (ten to twenty) tonnes of liquid superheated bath  56  to build up the depth of the molten electrolyte until the anode-cathode distance “d” reaches seventeen to twenty centimeters (17-20 cm), as shown in block  84 . In conventional restart procedures, the anode-cathode distance is typically between seven and twelve centimeters (7 to 12 cm). Increasing the anode-cathode distance for the restart procedure is a safety precaution to avoid projection of the molten metal located at the surface of the cathode outside the cell  20 . Metal projections could injure any operators in the vicinity. Before the metal in the cell completely remelts, the feeding of any alumina should be avoided since it could otherwise cause undesirable hard deposits on the frozen metal pad. 
     As the cell  20  heats up, the previously frozen metal pad  32  becomes molten, as shown in decision block  86 , and excess bath can be siphoned, as shown in block  88 , for starting up other cells. As the start-up progresses, the anode cathode-distance “d” is restored to approximately three to four centimeters (3 to 4 cm), as shown in block  90 , thereby restoring normal operating conditions, as shown in  FIG. 1  and block  92 . 
     It will be appreciated that the processes described herein provide a shut-down procedure which allows an electrolytic cell to be started up in a much more efficient, economical, and safe manner. One of the primary reasons for economy is that the time and labour associated with the normal removal and refurbishing of anodes is eliminated. In addition, there is no labour involved in removing any bath from the cell and the metal left in the cell does not require any surface preparation for start-up. Accordingly, because there is less interference, physical damage to the structure of existing cells is limited and the prospects for faster and successful start-ups can be vastly improved. Conveniently, a safer operation procedure can also be provided, resulting in a significant reduction of the costs associated with cell start-ups. 
     The embodiments of the invention described above are intended to be exemplary only. For instance, it is appreciated that the electrolyte bath and aluminium layer temperatures and the anode-cathode distances for the various steps can vary from the ones described above. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.