New cathode heating unit: more efficient, cleaner and safer

A technological quantum leap at TRIMET

TRIMET Aluminium SE has commissioned a new cathode heating unit in the Essen aluminium smelter with four cathode furnaces and connected ladle and mold firing. This customized plant increases energy efficiency and enhances occupational safety. In addition, working steps have been reduced and are more ergonomic.

The new heating unit is the product of four years of planning. Designed to meet local requirements, the plant was initially tested as a prototype. Following 17 weeks of construction and installation, it commenced operation in January 2022 in the cathode repair workshop. Electrolysis cells for aluminium production were relined for this purpose. This involved casting of cathode ingots into cathode blocks using cast iron. Eleven of these blocks form the floor of an electrolysis cell in each case.

“Modernization of cathode treatment marks a further stage in the process of making our production locations fit for the future. We are increasing energy efficiency, lowering CO2 emissions and enhancing occupational safety,” says Dr. Andreas Lützerath, a member of the Executive Board of TRIMET Aluminium SE.

The cathode blocks, which consist of graphite, are heated to about 480 °C in the heating unit to minimize the temperature shock when pouring the 1,450 °C hot cast iron. However, the preheating temperature should not exceed 500 °C to prevent the blocks from igniting. Simultaneously, the cathode ingots placed in should achieve a temperature exceeding 600 °C. This technological challenge is solved by flameless line burners in the new plant that heat the cathode iron from above. Intelligent control of hot combustion gases means that additional undergrate firing can be dispensed with.

Modernization of the cathode heating station at TRIMET in Essen, Germany

The new heating station is the result of intensive planning. The system, which is tailored to on-site requirements, was first tested as a prototype before going into operation in the cathode repair shop in January 2022 after a 17-week construction and assembly period. Here, the electrolytic cells are relined for aluminum production. In the process, the cathode ingots required for current conduction are cast into the cathode blocks using cast iron. Eleven of these blocks then form the bottom of each electrolytic cell.

More efficient processes ensure greater occupational safety.

The line burners are integrated in the hydraulically actuated roof of the cathode furnaces, meaning the furnaces can remain completely closed during the cathode block heating phase. This innovative design drastically reduces heat losses. Outer sheath temperatures of only 40 °C mean that personnel are only exposed to low volumes of heat radiation. Additionally, the energy requirement is rapidly lowered. Whereas the gas nozzle burners previously used required more than 1,500 cubic meters of natural gas for each heating process, the new plant needs less than 500 cubic meters. The new plant reduces CO2 emissions to the same degree (by around two thirds). A programmable logic controller enables continuous modulation or cycle control of the output of the new burners. In addition, the new plant is considerably quieter than its predecessor, as the noise level has been reduced from over 90 decibels to around 75.

Aside from the core functions of the cathode heating unit, TRIMET has also renewed ancillary equipment, adjusting it to suit the new system. This simplifies handling and operation. Configuration of a combined conveying and casting ladle enables multiple filling of molten cast iron during the casting process. Insulation of the ladle protects personnel from excessive heat and enhances energy efficiency. The sheath temperature of the ladle was reduced from about 525 °C to around 100 to 270 °C. The insulation provided prevents the cast iron from cooling rapidly and, as a result, reducing its flowability. Preheating of the ladle occurs in close proximity to cathode heating. A finger-shaped heating element is lowered into the ladle for this purpose. A hot air system dries the residual casting mold, thus preventing water explosions caused by the sudden evaporation of condensate.

The four new cathode furnaces and the two ladle and mold heaters are equipped with separate exhaust gas systems. They convey combustion gases to a free-standing 17 meter chimney outside the building, preventing emissions within the production facility. TRIMET exploited the reconstruction of the cathode heating unit to optimize the factory infrastructure in line with occupational safety standards. All media and energy lines and the exhaust gas lines for the cathode furnaces are laid beneath the floor. This has eliminated any existing stumbling risks and made the workplace even safer.