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Spent potliner (SPL) is produced by the Aluminum Industry, worldwide, at an estimated rate of 650,000 tonnes per year. This material is generally comprised of a "first cut" (which is the carbon containing fraction) and the "second cut" (which is the refractory containing portion).
The "first cut" has been identified as a hazardous waste in many countries because it contains concentrations of cyanide and fluoride. These contaminants are known to "leach" into the surrounding soils and groundwater during both short term and long term storage and can cause potential contamination of drinking water reserves.
Because of this concern, this fraction of the potliners should be managed and disposed of in an environmentally acceptable manner.
Worldwide Environmental Disposal Attitudes
Throughout the world, the environmental regulating agencies have adopted a disposal hierarchy. The waste generator is to address the generation of waste materials as follows:
An example of this is the US EPA's suggestion that the waste generation be minimized by increasing the potlife expectancy through the "use of stronger steel shells that limit the deformation of the cathode." They also state that other "factors that can affect the potlife include cell preheat procedures, use of high quality cathode blocks, use of sidewall blocks with higher thermal conductivity, use of ramming pastes with better physical and mechanical characteristics and balancing of the magnetic field in the cell."
However, no matter what efforts are made to increase the potlife, ultimately there will be material that will need to be managed upon removal of the potliner. With that in mind, we need to look for the best environmental alternative that fits into the suggested disposal hierarchy.
Over the years, many disposal options have been studied. In general, however, the primary studies have been focused on the "first cut". Since the "second cut" is not currently regulated as a hazardous waste, it has been disposed of in landfills.
The disposal options considered for the "first cut" range from straight landfilling of the material, macro encapsulation and landfilling, mixing with "anti-agglomeration" agents incinerated and then landfilled (the Reynolds Process) and high temperature oxidation and then landfilling the resulting slag (the Ausmelt Process).
Recycling options include vitrification (the Ormet/Vortec Process) and use as a supplemental fuel in an industrial furnace (ERAtech's proposed Process). The only option which has the technical possibility to recycle or reuse both cuts, is the use in a cement kiln. Here, the first cut may be used as a fuel and the second cut as a raw materials substitute.
The "disposal options" all rely on the placement of a majority of the SPL stream into a landfill, either directly without prior treatment or subsequent to treatment. In either case disposal by these methods should be considered as the least desirable alternative.
Of the "recycling options" only the use of the SPL in a cement kiln addresses the entire problem in a cost effective manner. Vitrification requires immense energy expenditure. Using the SPL as fuel and raw materials in cement kilns, however, conserves fossil fuels and utilizes the mineral components in the product.
Test studies in the US have shown that 99.9+% of the cyanide contained in the first cut is destroyed when the material is used as a supplemental fuel in a cement kiln. Other research studies performed for the US EPA by Battelle Laboratories have also shown complete combustion (99.9+%) and destruction of cyanide at temperatures of 760 °C (1400 °F), a temperature routinely exceeded in the cement kiln.
During these same studies, it was also documented that 99.9% of the fluoride was removed through absorption in the clinker and the cement kiln dust (CKD).
Overall, a minimum of five cement operations tested the use of "first cut" SPL and the results found there were "no adverse environmental problems" associated with its use. Furthermore, the cement manufacturers had experienced no product quality problems, yet still had the benefit of energy recovery with the supplemental fuel.
Other hidden benefits experienced while using this material is that fluoride acts as a "fluxing agent" which, in the correct proportions, can reduce the temperature requirements for the calcining reaction to take place. Also the presence of sodium can elevate the alkali level in low alkali clinker.
An environmental challenge exists for the best method to manage and dispose of spent potliners from the Aluminum Industry.
There is proven technology that creates a win-win situation where both the needs of regulatory agencies worldwide and the needs for supplemental fuels and feed replacement for the cement industry are met. The need to properly review the operations and plant chemistry before implementing such a plan at a cement facility is critical. With the correct experience and technology, the proper supplemental fuel and feed substitution specifications can be developed. With this important information, feed rates can be implemented that will not impact the environmental performance, safety or clinker quality at the facility or the surrounding community.
This technology can be successfully implemented in a cost effective manner to assist the Aluminum Industry in properly managing their spent materials while reducing the use of nonrenewable resources by the cement industry. It also eliminates any future liabilities due to both short and long term storage through the disposal of this resource in the land.