T itle: Methods to Improve Anode Rod Preheat Energy

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MPLUS No.: MC-03-030


itle: Methods to Improve Anode Rod Preheat Energy

User Organization: Century Aluminum of West Virginia, Inc.

Richard Love from Century Aluminum of West Virginia, Inc.

Ravenswood, WV 26164
User Contacts: Richard O. Love, 304-273-6562

ORNL R&D Staff: Craig A. Blue, 865-574-4351

Puja Kadolkar, 865-574-9956

Relevance to ITP: Anode rods are used in the process of changing carbon anodes during aluminum reduction process. During this process, the anode rod assembly is required to be connected to the carbon anode by casting iron around the stub area. Heating of the stub area prior to the pouring of cast iron is carried out to achieve a strong and low-resistant connection between the rod assembly and the carbon anode. A low-resistant stub-carbon cast iron interface will allow efficient transfer of low voltage electrical current from the hanger to the anode. The current heating techniques used at Century Aluminum of West Virginia, consists of a 24-ft-long stub drying unit operated by gas burners. Heating of stubs using this technique was found to be slow, inefficient and non-uniform. The objective of this project was to investigate new and efficient techniques for heating the stub area of an anode rod in a controlled manner. The outcome of this proposed work has strong potential for yielding an efficient process for rapid change of graphite anode to cast irons. Such a process will minimize the delay in continuing the aluminum reduction process and make it more energy efficient.
Objective: The objective of this project was to investigate infrared heating as a means to heat the stub area of an anode rod in a controlled manner.
Results: In this present study, a flat bed infrared furnace was used to heat the anode rod assembly. The flat-bed furnace is a 88-kW unit consisting of two halves, the upper half carrying an array of tungsten halogen quartz lamps and the lower half with a drop-down arrangement used to load specimens into the furnace. The main body of the upper and the lower halves is made of water-cooled stainless steel plates lined with fire brick. Figure 1 shows the anode rod assembly, and Fig. 2 shows the assembly loaded in to the furnace. Thermocouples were connected at four locations (one top and one bottom on each stub) over the surface of the stub area (see Fig. 3) to record temperatures while the heating was performed.

Figure 1: Anode Rod Assembly. Figure 2: Anode Rod Assembly loaded

in the flat bed furnace.

Figure 3. Location of thermocouples.

Preliminary heating trial at ORNL was performed to simulate the heating conditions employed at Century Aluminum. Figure 4 shows the flow diagram for the stub and the stub hole drying process used at Century Aluminum.

Anode Conveyor

Graphite Dip Tank

Stub Hole Dryer

Indexing Station

Assembly Drop Zone

Pouring Area Station

Stub Dryer

Overhead Chain Conveyor for Assemblies

  1. Temperature of the stub entering the stub dryer is 20C.

  2. Average time for which the stubs are in stub dryer = 7.5 min. Average temperature of the stub at exit = 110C.

  3. Time required for the assemblies to enter the stub hole dryer = 80 s + 60 s = 140 s = 2 min and 20 s.

  4. Time for which the assemblies are in the stub hole dryer = 3 min.

  5. Time to travel from stub hole dryer to the indexing station = 37 s. At the indexing station, temperature of the stub is anywhere from 80 to 190C. Time for the stub assembly to spin and drop in to the anode = 28 s.

  6. Expected temperature for the stub prior to pouring the cast iron = 135C ± 10C.

Figure 4: Flow diagram for the stub and the stub hole drying process.

Based on the flow diagram, the time taken by the stubs to travel from the exit of the stub dryer to the indexing station just prior to the pouring of cast iron is 80 + 60 + 180 + 37 + 28 s = 6 min and 25 s. It is assumed that no significant increase in the stub temperature occurs while the assemblies pass through the stub hole dryer. The expected temperature of the stubs just prior to the pouring of cast iron is 135C ± 10C. Figure 5 shows the heating of the anode assembly using the flat bed infrared furnace whereas Fig. 6 shows the heating profile acquired during the preliminary test.

Figure 5: Anode assembly being heated using flat-bed infrared furnace.

Figure 6: Temperature profile for anode stub preheating.

The furnace was set to heat the assemblies up to 150C in 5 min followed by a hold time of 2 min at 150C. After the hold time, the furnace was turned off and the anode rod was allowed to cool. The cooling of the anode was performed to simulate the traveling of the assemblies from the stub dryer to the indexing station, which takes approximately 6.5 min (390 s).
Figure 6 clearly indicates the heating and cooling segments of the experiment. The difference in the T1 and B1 (and similarly T2 and B2) was due the cross section of the stub and one-sided heating of the furnace. At the end of the cooling cycle (just prior to pouring of the cast iron), the temperature of the entire stub assembly equalized to 135C ± 3C. The higher temperatures reached by the top surfaces and the overall heating time can be reduced using double-sided heating system for this kind of application.
Based on the preliminary results, additional tests were also carried out using the same one-sided infrared heating unit, to heat the anode stubs as fast as possible or in the shortest time possible. The furnace was operated with varying outputs viz. 60%, 70% and 80%, considering the fact that a full-scale production-based conveyor-type infrared heating unit would be less than 80 to 90% efficient. Temperature profiles recorded from thermocouples T1 and B1 for different outputs are shown in Fig. 7.

Figure 7: Heating of the stub using the infrared furnace with varying outputs.

It is interesting to note that heating of the stub is insensitive to the output of the furnace. It can also be noted that unlike conventional heating which takes 7.5 min, infrared heating takes only 2.5 min with only 60% output to heat the surface of stubs to the desired temperature. Based on the above results, a double-sided heating furnace (as shown in Fig. 8) as a replacement to the existing stub hole dryer is proposed for uniform and efficient heating of the entire stub surface in 2.5 to 3min.

Figure 8: Replacement of stub hole dryer.

In summary, infrared heating provides uniform and controlled heating of stubs. Replacement of the conventional heating furnace with a double-sided conveyor-based infrared furnace has been proposed. The furnace can be designed such that the same

unit can be used to heat the anode stubs as well as carbon anodes simultaneously. Such replacement indicates a huge potential for energy savings in the anode rod preheating process.

Craig A. Blue, Puja Kadolkar, and Richard Love, “Methods to Improve Anode Rod Preheat Energy Requirements,” Final MPLUS report, June 2004.

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