Spent Potlining (SPL) is a waste material generated in the primary aluminium smelting industry. Spent Potlining is also known as Spent Potliner and Spent Cell Liner.
Primary aluminium smelting is the process of extracting aluminium metal from aluminium oxide (also known as alumina). The process takes place in electrolytic cells that are known as pots. The pots are made up of steel shells with two linings, an outer insulating or refractory lining and an inner carbon lining that acts as the cathode of the electrolytic cell. During the operation of the cell, substances, including aluminium and fluorides, are absorbed into the cell lining. After some years of operation, the pot lining fails and is removed. The removed material is spent potlining (SPL). SPL was listed by the United States Environmental Protection Agency in 1988 as a hazardous waste. Hazardous properties of SPL are:Toxic fluoride and cyanide compounds that are leachable in water
Corrosive - exhibiting high pH due to alkali metals and oxides
Reactive with water - producing inflammable, toxic and explosive gases.
The toxic, corrosive and reactive nature of SPL means that particular care must be taken in its handling, transportation and storage. SPL from aluminium reduction cell cathodes is becoming one of the aluminium industry's major environmental concerns. On the other hand, it also represents a major recovery potential because of its fluoride and energy content.
Most SPL is currently stored at the aluminium smelter sites or placed in landfills. Dissolved fluorides and cyanides from SPL landfill leachate may have environmental impacts. It is not acceptable that leachate contaminated with these harmful compounds may escape from stockpiles. Other storage methods are secure landfills or permanent storage buildings. These are expensive solutions and may create future problems.
Production of primary aluminium metal with the Hall–Héroult process involves the electrolytic reduction of alumina in cells or pots. The electrolyte is made up of molten cryolite and other additives. The electrolyte is contained in a carbon and refractory lining in a steel potshell. The pots typically have a life of 2 to 6 years. Eventually the cell fails and the potlining (SPL) is removed and replaced. The SPL generated is listed by various environmental bodies as hazardous waste. Due to the concentrations of ﬂuorides and cyanides in spent potliner, and the tendency to leach in contact with water, the US Environmental Protection Agency (USEPA) listed the materials on 13 September 1988 (53 Fed. Reg. 35412) as a hazardous waste (K088) under 40 C.F.R., Part 261, Subpart D. International shipment of SPL is subject to the protocols of the Basel Convention on the Transboundary Movement of Hazardous Wastes and Their Disposal. As the environmental regulation agencies in an increasing number of countries define SPL as a hazardous material, the disposal costs can easily run to more than $1000 per tonne SPL. World production of primary aluminium is in the order of 40 million tonnes. The world’s smelters also produce about one million tonnes of toxic SPL waste. Past industry practice has been to landfill this waste. This must change if the aluminium industry wants to claim a reasonable degree of sustainability and environmentally tolerable emissions. Landfill of unreacted SPL is considered a practice of the past.
The primary aluminium industry has systematically worked to minimize the amount of SPL produced, by extending the lifetime of the lining in the smelter pots. Since the 1970s, SPL has been recognised as a valuable resource for other industries, including as a feedstock in the cement, mineral wool and steel production processes. The International Aluminium Institute has defined the following “International Aluminium Institute SPL Voluntary ObjectiveThe Aluminium Industry recognises that spent pot‐lining has properties that makes it a valuable material for use in other processes and will therefore strive to convert all spent pot lining into feed stocks for other industries, which include cement, steel, mineral wool and construction aggregate companies or to re‐use and or process all SPL in its own facilities.
Pending final deposition, the industry will endeavour to store all spent pot‐lining in secure, waterproof, ventilated buildings/containers that will maintain the spent pot‐lining in a dry state with no potential for the build up of noxious gases.”
There is variation in composition of SPL depending on such factors as the type of aluminium smelting technology used, the initial components of the cell lining and dismantling procedures. Indicative composition of SPL for three different technologies is shown in the following table.
SPL is hazardous due to:Toxicity from fluoride and cyanide compounds that are leachable in water
Corrosive - exhibiting high pH due to alkali metals and oxides
Reactive with water in a way that produces inflammable, toxic and explosive gases.
An example of the potential consequences of SPL reaction with water is the death of two workers and reported damage costs of $30 million due to an explosion of flammable gases from SPL in the hold of a cargo ship.
The leachable fluorides in SPL come from the cryolite (Na3AlF6) and sodium fluoride (NaF) that are used as a flux in the smelting process.
Cyanide compounds form in the pot lining when nitrogen from air reacts with other substances. For example nitrogen reacting with sodium and carbon according to the equation -
1.5N2 + 3Na + 3C → 3NaCN.
Aluminium carbide forms in the potlining from the reaction of aluminium metal and carbon according to the equation –
4Al + 3C → Al4C3.
Aluminium nitride forms from a number of reactions including the reaction of cryolite with nitrogen and sodium according to the equation -
Na3AlF6 + 0.5N2 + 3Na → AlN + 6NaF
Gases are generated from reactions of water with compounds such as un-oxidised aluminium metal, un-oxidised sodium metal, aluminium carbide and aluminium nitride. Typical gases from the reaction of SPL with water are:Hydrogen from aluminium metal and water – 2Al + 3H20 → 3H2 + Al2O3
Hydrogen from sodium metal and water – 2Na + 2H20 → H2 + 2NaOH
Methane from aluminium carbide and water - Al4C3 + 6H20 → 3CH4 + 2Al2O3
Ammonia from aluminium nitride and water – 2AlN + 3H20 → 2NH3 + Al2O3n
Past practices for dealing with Spent Potlining (SPL) include dumping it in rivers or in the sea or storing it in open dumps or landfilling. These methods are not environmentally acceptable because of the leachability of cyanides and fluorides. More recently SPL has been stored in secure landfills where it is placed on an impermeable base and covered with an impermeable cap. The amount of detailed information available on the quality of percolate from existing SPL landfills is very limited.
A 2004 study of a landfill containing SPL located in North America identified four chemical species as priority contaminants: cyanide, fluoride, iron and aluminium. Life-cycle assessment and ground water transport modelling were used to provide an understanding of the situation identifying environmental issues and significant ecotoxilogical potential impacts. The study observed that, while assumptions that the confinement of soil and waste was assumed to be perfect, in fact these sites could themselves become sources of contamination. The study states that the most advantageous option is the total destruction of the SPL fraction if concerns about the quality of long term confinement are considered. The major objection to the sealed type of disposal is that it will need to be monitored indefinitely. There is, therefore, a real need to find safe, acceptable alternative ways to landfill disposal.
SPL was dumped by previous owners in an unlined waste repository at the Kurri Kurri smelter in Australia resulting in contamination of the local groundwater aquifer with high levels of fluoride, cyanide, sodium sulphate and chloride.
An Interim Action conducted under Agreed Order No. DE-5698 between the Port of Tacoma and the Washington State Department of Ecology addresses the removal, through excavation and offsite disposal, of SPL zone material and associated contaminated soil at an old aluminium smelter site. The background to this situation is that from 1941 to 1947, the US Department of Defense built and operated an aluminum smelter at the Site. In 1947, Kaiser Aluminum & Chemical Corporation (Kaiser Aluminum) purchased the Site and operated the aluminum production facility until 2001. In 2002, Kaiser Aluminum closed the plant and, in 2003, the Port of Tacoma purchased the smelter property from Kaiser Aluminum for redevelopment.
A number of alternatives have been proposed for treatment of SPL. The alternatives can be classified as follows:disposal techniques where all or part of the SPL is destroyed or utilized by another industry including:
combustion for power generation
slag additives in iron and steel industry
fuel and mineral supplement in cement manufacture
red brick industry
conversion to inert landfill materials
recovery or recycling techniques where some of the SPL can be recovered for use in primary aluminium smelting:
fluoride recovery from leaching processes
cathode carbon additives
anode carbon additives
selective recovery of aluminium metal.
Recycling through other industries is an attractive and proven option; however, classiﬁcation of SPL as a hazardous waste has greatly discouraged other industries from utilizing SPL, due to the burdensome and expensive environmental regulations. The Arkansas Pollution Control and Ecology Commission noted that treated SPL used to construct roads was recovered and placed in secure landfill.