Aluminium is extracted from a mixture of molten cryolite and Aluminium oxide, using a process called the Hall– Héroult process. Molten cryolite is used as the electrolytic bath for extraction of Aluminium as it has a lower melting point and readily dissolves in water. This cryolite is placed in a reduction cell, a steel case coated with graphite, and acts as the negative cathode. An anode made of carbon is partially dipped into the liquid and electricity is passed through the anode. The ions in the cryolite include Al and Oxygen ions. When electricity flows, the positive Al ions are attracted to the negative cathode and the negative Oxygen ions are attracted towards the positive anode in block form. The Aluminium that has sunk to the bottom is collected in liquid form through a valve. At the same time, the Oxygen picks up carbon from the anode forming carbon dioxide and evaporates.
Challenges due to gas bubbles in Aluminium Smelting:
During the electrolysis process, a large number of bubble accumulates at the bottom of the anode that could form a continuous gas layer through a process of bubble collision and coalescence. This process effectively reduces the contact area between the anode and the bath. This large scale bubble formation also reduces the conductivity of the reduction cell thereby reducing the efficiency of the electrolysis process.
Lyng Drilling employs its expertise in development of cutting edge technology for the efficient extraction of Aluminium using the electrolysis process. Lyng Drilling provides solutions for slotting anodes using patented steel saw blades. Slotting of anode increases the wet surface area and evaporation of carbon dioxide. While bubbles slow down the electrolysis process, adding slots increases the evaporation rate and introduces movement in the whole bath with new fresh ions stirred by the action. Independent research has confirmed that the average gas bubble removal rate increases from 36% to 63% with slotting. With slots, the largest size of bubbles formed is also significantly reduced as the opportunity for coalescing is reduced. In summary, slotting will significantly improve current efficiency, power efficiency, and stability.
Introducing slotting can increase production by up to 2.5% without expanding the footprint of existing facilities by enabling an increase of the amperage input to the process and thereby increasing process speed.
Slotting using coolants:
The challenge of slotting is to achieve narrow thin (10-12 mm wide) and deep slots (300-400 mm) in a dry environment without the use of coolants as the presence of moisture in the anode will result in major disadvantages for electrolysis. With the anode material being relatively porous, the presence of moisture will require a comprehensive cleaning and drying process. It is conceivable to process the anodes immediately after they have been calcinated in a furnace so that the residual heat is used to dry any coolant. However, such a process will also produce increased thermal load on the tool and anode bodies.
Based on years of experience of developing drill bits for drilling hard and abrasive formations, Lyng Drilling, in a collaboration project with Norsk Hydro, has developed a patented technology for development of a circular saw blade that incorporates cutting edges mounted on its periphery to achieve slotting in a safe manner. With the circular saw blade mounted on a spindle of a machining unit, the saw blade can be rotated and moved linearly along the path that corresponds to the extent and depth of the required slot. With the entire slotting process being conducted in a dry environment, the safety and environmental risks associated with coolant usage are completely removed.
Click on the video below to see advantages of slotted anodes versus anodes with pre-baked slots: