Electrolysis of Alumina

Alumina is reduced to aluminium in an electrolytic cell. The inner coke lining of the cell acts as cathode and the carbon rods used inside the cell act as anode. The cell is charged with alumina and cryolite. Cryolite is mixture of sodium and aluminium fluoride. It is used to reduce the melting point of alumina. It also helps to conduct electricity through the alumina. The electrolytic cell is a steel container lying from inside with sheet of graphite. The graphite acts as cathode. Carbon rods are put inside the cell that act as anodes. Electric current is passed through the dissolved oxide. After the electrolysis, aluminium is produced on the surface of cathode and oxygen is separated at the carbon anode. Electrolytic cell is also known as a pot. It consists of steel sheets lined with refractory bricks. It is set up on a strong foundation. Carbon lining is made inside the cell. Several carbon rods are suspended from the top. These carbon rods act as anodes. Current to the anode is supplied by bars and metal rods. The carbon rods are partly dipped into the liquid bath of cryolite. Direct current is passed through the electrolyte. Due to electrolysis, decomposition of the liquid takes place and the metallic aluminium is collected at the bottom of the cell.

When the cell is put into operation, a small amount of pure molten alumina is added, so as to cover the surface. The lining of the bottom surface must have adequate strength and high electrical conductivity. Aluminium reduction cell operates at 4.5V-5.5V which is approximately three times greater than the decomposition voltage of alumina. So, out of the total energy input supply to the cell, it is only one-third of the energy is used for actual reduction. The remaining amount is transformed into heat. The heat generated due to electrical current maintains the electrolyte cell at the working temperature. It compensates the loss of heat due to electrolysis. It also compensates the heat loss due to radiation and continuous addition of cold raw materials. Oxygen evolves at the anode. It oxidizes the carbon of the anode. The temperature of electrolysis ranges from 900'C-1000'C. There is wear and tear of anodes due to high temperature condition and oxidation of carbon. So,the anodes are gradually lowered as they are damaged regularly. Depending upon the types of anode, electrolytic cells may be classified into two groups as the prebaked cell and Soderberg cell.

Prebaked electrolytic cell: This type of cell consists of round shaped anodes. Each one may weigh up to 100kgs. These anodes are made of petroleum, coke, coal and pitch pressed and baked for several hours. Individual anodes are raised or lowered to maintain the proper positions with respect to the bar. These electrodes are replaced due to regular consumption during electrolysis of alumina.

Soderberg electrolytic cell: This cell has anodes into which the electrodes paste is continuously passed and compressed. As the electrodes are consumed during the electrolysis, additional amount of paste is pushed downwards by compression of new electrode pastes. Two types of anodes are used in this cell. One is horizontal spike anode and the other one is vertical spike anode. It has less current efficiency in comparision to that of prebaked cell. The better current efficiency of the latter one is due to better current distribution and low heat radiation. Power consumption is comparatively less in prebaked cell. The anode carbon consumption is similar in both the cells. Consumption of electrolyte is more in this cell. Removal of waste gas from this cell has added new problems to the electrolysis of alumina In this process, the bauxite ore is crushed and washed properly. The powder ore is fused with sodium carbonate. The anode block size is increased to improve the efficiency of the cell. The cathode block is also improved by quality of carbon lining. Corrosion resistant alloys are used for constructing electrolytic cell. Refractory metals are used for electrical conductivity between the liquid and the cathode beds. New developments are taking place in the field of automation. Automation of alumina feeding and control of cell voltage needs improvement on a priority basis. The electrolyte preparation, electrode processing and thermal balance are to be improved for better performance of the electrolytic cell.

Solid cryolite is placed in the reduction cell. The anode is put into the cryolite bed and electric current is switched on. The cryolite offered resistance to electric current. Heat is generated and slowly melting of cryolite takes place. Additional amount of cryolite is added up to desired level. Sufficient amount of alumina is added to the molten cryolite. Current passes to the dissolved cryolite and electrolysis takes place. After the electrolysis, aluminium is deposited towards the cathode carbon lining. The oxygen is separated at the anode point. Carbon reacts with oxygen so that carbon monoxide and carbon dioxide gases are formed. These gases are allowed to pass of to the atmosphere. The electrolyte is kept in continuous agitation by the bubbling gases at the anode end. Magnetic effect is produced by the flow of current through the cell wall lining. The metallic aluminium goes down to the bottom of the cell. When sufficient amount of metal is collected, it is separated out by vacuum ladle or Siphon. Some amount of molten metal is left in the cell for continuing the electrolysis process.

The aluminium obtained from electrolytic cell has some impurities like iron, titanium and silicon. To obtain a pure metal, aluminium is refined to get such level of purity. Refining of aluminium is carried out by electrolysis of fused electrolytic salt. The refining cell has three layers. The bottom layer serves as anode. The middle layer is known as electrolyte layer. The top layer is cathode layer. At the beginning of the refining operation, aluminium is charged into the refining cell. Electrolyte is added through the graphite tubes. The electrolytic process starts and pure alumina is collected at the top. Pure aluminium is removed through graphite ladle. The purity of metal depends upon the difference in the difference of layers and the possibility of circulation of the electrolyte due to the magnetic effect. The voltage required to transfer aluminium from anode to cathode is of the order of a few milli volts. Pure conductivity of the electrolyte makes it necessary to maintain the cell voltage at about 5-7V across the cell. The refining cell is a heavy rectangular box opened at the top of the box. The cell is thermally insulated with carbon blocks on the bottom and magnesite bricks on the side walls. The current is introduced at the bottom of the cell through steel bars embedded in the carbon lining. The current flows from the carbon lining to the graphite anodes through electrolyte. The electro refining of aluminium is dependent upon the composition of electrolyte and the working temperature.

Electro refining process is carried out by three different methods used by the aluminium industry.

Hoops process: This process is developed by William Hoops in 1900 a.d. The electrolyte used in this process is a mixture of aluminium fluoride, sodium fluoride and Barium fluoride. The temperature varies between 900'C-1000'C. The purity of the metal produced by this process is around 99.98%.

Gadeau process: In this process electrolyte is so adjusted that the cell operates at a temperature of 720'C. Temperature is reduced by addition of barium chloride to the cryolite. The purity of the metal is up to 99.99%.

Aluminum Fluoride process: The electrolyte is a mixture of aluminium fluoride and sodium fluoride. The operating temperature is 740'C. The purity of the metal is 99.99%. The composition of electrolyte remains constant during the operation. The consumption of electrolyte is very less. The consumption of graphite is more in comparison to other processes. The current efficiency is low and the cell operates at lower temperature. The brick lining of the cell is made of magnesite bricks. After the refining is completed, the aluminium metal is converted into different products. The major consumption of aluminium is in the form of sheet, container, wire, utensils, foil and implement. Use of aluminium and its alloys are in increasing side in the industries like auto mobiles, air crafts, locomotives and chemicals. Availability of raw materials and power supply helped in the development of aluminium industry