Aluminium is produced by the electrolysis of alumina dissolved in molten cryolite. The electrolytic cell known as the Hall-Heroult cell, uses anodes consisting of a compacted mixture of petroleum coke and pitch. The cathodes are made from carbon blocks which are predominantly anthracite, metallurgical coke, graphite and pitch and form the cell bottom and the cell walls.
Although significant technological refinements in the cell design and construction have been made, the basic process of producing aluminium still remains substantially similar to what it was one hundred years ago. Carbonaceous materials are still the main lining material used today in the cell.
The carbon anode blocks are consumed during electrolysis and must be replaced every four or five weeks of operation in standard cells. The oxygen resulting from the decomposition of alumina burns the carbon anode at a theoretical rate of approximately 330 kg per ton of aluminium produced, but in practice the carbon consumption is about 450 kg per ton of aluminium due to side reactions. This results in the emission of carbon oxides, sulfur oxides and other undesirable gases which are now being recognized as major atmosphere polluants, but such emissions are considerably less dangerous and less polluting than those produced during fabrication of the carbon anodes with pitch as a binder.
At present, the method of producing carbon blocks to be used as anodes and cathodes in aluminium production cells consists of mixing petroleum coke with pitch for the anode and anthracite and other carbonaceous materials with pitch for the cathode, followed by compacting and calcining. Calcining designates a baking process in which volatiles are driven off at high temperatures without fusing the material.
The fabrication of the carbon anodes and cathodes involves the use of pitch. During fabrication, gases are emitted especially from the pitch during the long period required for calcining the blocks. These gases are polluting and very dangerous to the environment and are recognized as a major hazard to the health of workers involved in the production.
The pitch serves as the binder for the dry mixture of carbonaceous materials. Unfortunately, the pitch binder presents a series of serious hazards for health and for the environment.
Both solid and liquid pitch is used. The utilization of solid pitch results in unsatisfactory working conditions for the workmen, such as irritation and itching of the skin and eyes, and special precautions must be taken in order to protect the workmen during all operations where pitch is involved.
Additional problems result from the utilization of liquid pitch, particularly in regard to the storage and transportation to the utilization plant.
During the calcining of the carbon blocks, which is required to eliminate the volatile components and stabilize the blocks, there is an emission of aromatic polycyclic hydrocarbons (PAH), which are very dangerous to the health and special equipment is required to absorb these products. However, the residual products after absorption are also difficult to eliminate and the cost of disposal is high.
Utilization of pitch as a binder requires the mixing operations with carbonaceous materials to be carried out at about 150.degree.-200.degree. C. and this creates operating complications and high operating costs.
The calcining process is complicated and costly and large furnaces are required which are difficult to operate, are polluting and expensive because of the high energy consumption. On account of the requirement of low thermal gradients during heating of these blocks to over 1000.degree. C. and later cooling, the calcining operation normally takes as long as 2 to 4 weeks.
An additional disadvantage of the pitch is due to the fact that when the blocks are calcined the pitch is transformed mainly into a form of carbon which oxidizes more rapidly than the petroleum coke. This leads to disintegration of the anode block with formation of unutilized carbon powder which is detrimental to the operation of the electrolytic cell and increases the carbon consumption.
It would therefore be extremely advantageous for the aluminium industry to produce carbon blocks fabricated with a non-polluting binder without the necessity of handling pitch or like dangerous materials, avoiding the emission of polluting substances. Additionally, there is a need to develop compositions and methods which eliminate the high temperature fabrication and the long calcining times required to form the carbon blocks following the current state of the art.