New Developments in Treatment Options for Spent Aluminum Potliner
David Gossman, Gossman Consulting, Inc.
Over the last 25 years there have been reoccurring attempts to use spent aluminum potliner (SPL) in cement kilns. There are two major difficulties to overcome relative to the use of SPL,
other than the off and on again approach to the issue that the aluminum industry has had on the issue. The first of these is regulatory and perception related – SPL has cyanide. While cyanide is certainly toxic it will also
clearly be destroyed in a cement kiln, and unless mixed with acid or blown as powder into the air is unlikely to present a significant safety concern to employees in a plant. Nevertheless, the combination of the perception of problems
with this issue and the fact that it is the cyanide that renders this waste hazardous in the eyes of the USEPA and other regulators around the world makes this a difficult problem to overcome. Only a handful of cement plants already
permitted to burn hazardous waste are in a position to deal effectively with this issue in order to use SPL as a fuel, mineralizer and alternative raw material.
The other issue is not as well known but is of far more concern. SPL is highly water reactive generating ammonia, methane, hydrogen and heat on contact with as little water as can be found in humid air! There have been fires and
explosions in enclosed shipping containers. The finer the material is ground, as is done prior to injecting it into a cement kiln, the greater the potential for hazardous gas release. Any cement plant using SPL, even one used to dealing
with other hazardous wastes, needs to take extra precautions when handling this material.
All of that said, imagine my pleasant surprise during a recent visit to a cement plant in Australia to find that a company had developed a process to both destroy the cyanide in SPL and the reactivity, rendering the SPL non-hazardous.
The plant I was visiting, as well as two others in southern Australia, are now using SPL as easily as most plants use coal or coke – no special permitting, no special handling. Trucks are pneumatically unloaded into a storage tank
and from there fed to the kiln like any other solid powdered fuel. The plant gets not only the fuel value but also the mineralizing benefits of the fluoride present in SPL and the material value from the aluminum.
The process for rendering hazardous waste SPL non-hazardous has been developed and patented by a company called Regain Services Pty Ltd (Regain). Regain has worked for years to refine and develop the process into a truly elegant
solution – speaking as a chemist, of course. I cannot go into details because of the confidential nature of what I have seen but the process is certainly ready for prime time. While not mobile it is easily set up in a relatively
small amount of space right at the aluminum plant generating the SPL. The process results in a blended product with a relatively consistent quality that gets tested prior to shipment to the cement plants. The process is currently
operating at two aluminum plants (soon to be a third) and provides nonhazardous SPL to three cement plants.
The development of the Regain process opens up all kinds of possibilities for both further processing and other end uses. For cement plants that might be limited in their potential use of SPL because of the sodium content it would
certainly be possible to leach a portion or most of the sodium fluoride from the SPL prior to use in the kiln. The aqueous based sodium fluoride solution could then be used at a different wet process plant where the mineralizing
properties are not the only benefit. Sodium fluoride acts as a viscosity reducing agent in wet plant slurry tanks reducing the percent of water required in the slurry and thus reducing overall energy consumption. Without the issue of
cyanide and reactivity other industries may also find treated SPL to be a valuable additive for its fluxing action and fuel savings. The brick industry is one that comes to mind.
The folks at Regain have proven that innovation is alive and well in the hazardous waste treatment and recycling industry. It will be interesting to see how the USEPA embraces this new option for treating and reusing SPL.