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Ongoing Research:

Beneficial Reuse of Spent Potliner Waste

Summary: The Cement Industry Environmental Consortium (CIEC) has undertaken the evaluation and initiated specific research on spent potliner (SPL) from primary aluminum reduction for beneficial utilization in the production of portland cement. SPL is a Resource Conservation and Recovery Act (RCRA) listed waste (K088) generated from primary aluminum production. Volumes generated are variable, being based on economic factors, off shore aluminum production, and energy costs (primarily electricity). The Environmental Protection Agency (EPA) has estimated that 47,724 tons of SPL would be generated on an annual basis. Worldwide 650,000 tons of SPL are generated. SPL contains a variety of compounds having fuel or mineral value for cement manufacture. The hazardous waste designation relates to cyanide and fluoride concentrations that could pose environmental risk if not managed appropriately. Prior to SPL being designated a listed waste, significant volumes were recycled as alternate raw materials and/or alternate fuels at a number of cement plants. Historical use in cement manufacture utilized the SPL for its fuel value (averaging 8460 Btu/pound) and mineral constituents primarily alumina and silica.

The research funded by CIEC focused on developing a beneficial use strategy for the hazardous constituents, while retaining the ability to use the remaining compounds for augmenting fuel and raw material requirements. This approach was predicated on the known and accepted use of SPL for fuel and mineral content, and the CIEC’s prior work on cyanide bearing wastes from mining operations. The research undertaken at the University of Utah’s Department of Chemical and Fuels Engineering, found that the cyanide (and the associated ammonia component) could under specific conditions be beneficially used to reduce NOx compounds from flue gases. Further investigations by CIEC staff indicate that the fluoride compounds could beneficially be used as a traditional fluxing/mineralizing compound that reduces energy requirements in the formation of cement clinker. This research has lead CIEC to apply for two patents covering these specific beneficial applications.

Introduction of the SPL into the kiln system is envisioned to be as a finely divided dry particulate, placed into the preheater portion of the kiln. Placement location is based on specific temperature profile and chemical properties of the SPL as they relate to both ammonia and cyanide concentrations.

A perceived limitation to use of the CIEC technologies is the RCRA designation which effectively precludes recycling of a listed waste.

Use of Potliner Constituents: The primary use of SPL waste in cement manufacture would be for fuel (20-60% by weight and 8460 Btu/pound on average) and mineral components from the refractory portion (35-75%). The refractory materials consist of alumina, and silica, with some minor amounts of metallic aluminum. The fuel/refractory components have been commonly used in cement production, to the extent that its use is frequently found in various kiln operation handbooks. Deleterious components (to cement manufacture) include sodium and the difficulty in grinding, due to hardness of the aluminum/silica carbide materials. The refractory components replace clays and bauxite raw products. Advantages in using the refractories include low inherent moisture (especially in winter) when compared to clays.

Use of SPL as an alternate fuel for cement manufacture had been common practice prior to the regulatory listing as a RCRA waste. The SPL’s average fuel value is approximately 2/3 that of coal, 1/2 the equivalent of coke and about twice that of refuse derived fuel. Use rates of up to 10% total fuels requirement were reported from Giant Cement prior to SPL’s RCRA listing. Use limitation was reported as being the sodium concentration. Currently, limited use of SPL is reported from the Lonestar facility in Missouri. It is recognized that there is significant variability in fuel values among SPL generators, based on SPL handling and processing methods.

The refractory component of SPL is typical being primarily alumina/silica combinations that are either cast in place materials or brick. Refractory materials are one of the more routine products recycled by the cement industry, replacing purchased clay or bauxite products. Chemically similar alternate raw materials include fly ash, and certain foundry sands. A somewhat troublesome component to use of the SPL refractory component is the difficulty in grinding for the size reduction needed to introduce SPL into the kiln system. This problem is exasperated when SPL use is through the fuel system (coal mills) whose designs are not conducive to processing hard minerals.

Fluoride compounds (usually sodium fluoride) are used in aluminum manufacture as a fluxing material and may make up 5-10% of the total dry weight of the potliner. Unconfirmed reports of SPL use at the Giant Cement Plant indicated that sodium of up to 20% by weight in delivered SPL was one of the reasons for abandoning SPL use. Interest in use of fluoride fluxes and mineralizers in cement manufacture has grown in recent years as energy costs have increased and as a possible means to reduce emissions of atmospheric pollutants (as measured on a per ton of cement produced). Heidelberger Zement AG (Heidelberger) has championed use of fluxing compounds as a method for reducing kiln temperatures needed for chemical reactions to initiate and proceed to completion. The primary effect is related to a complex series of reactions between calcium oxide, silica and alumina constituents. The resultant product from the reactions is termed clinker. Ground clinker (with gypsum added) is portland cement. An alternate view of flux use is in increasing clinker production at similar fuel consumption rates. The fluoride compounds are entrained into the clinker and are integrally incorporated the finished product. Flux addition rates of not more then 0.5% of raw feed (measured as fluoride) are suggested. Addition rates greater then 0.5% may have a deleterious effect on product quality. Common fluxes in cement production include fluorspar and a chemically similar material derived from neutralization of fluoric acid. The flux is typically added as a fraction of the raw feed. Currently the TXI-Riverside Cement Plant in Crestmore California utilizes fluorspar to facilitate chemical reactions for the production of white cement. Since use of the flux at the TXI facility relates to cement performance criteria, quantification of energy savings is impractical. Flux use at a number of European plants operated by Heidelberger has permitted quantification of energy and production criteria. In one example a 5.0% increase in production rates were achieved along with a 2.3% decrease in energy consumption with 0.3% calcium fluoride addition to the raw feed. No change in air emissions were reported when the flux was used. Further literature review and CIEC staff discussions with industry representatives has led to the opinion that the fluoride compounds found in SPL will work as a fluxing material in cement manufacture. Provisional patent applications were filed by CIEC in 2001, with formal US and international patent applications filed in August 2002 covering use of potliner in cement manufacture. This application specifically identifies use of fluoride compounds from potliners as a viable fluxing agent in cement manufacture.

Cyanide and ammonia compounds are also found in SPL waste, and are formed as consequence of the electrolytic process used in aluminum reduction which cause reactions with atmospheric nitrogen, water and carbon dioxide and/or carbon from the electrolytic cell. The concentration of these compounds vary significantly depending upon reductive system used and length of service for the potliner. Cyanide compounds such as those found in SPL have been identified as potentially applicable in reduction of NOx in flue gases. CIEC sponsored research undertaken by the University of Utah, used both SPL and cyanide bearing mining wastes in demonstrating this application. Patent applications filed by CIEC in August 2002 specifically address use of cyanide compounds for NOx reduction. The ammonia component as a NOx reductant is also addressed in the patent application.

The mechanism proposed for use of SPL in NOx reduction, is based on proven and commercially utilized Select Non-Catalytic Reduction (SNCR) and Select Catalytic Reduction (SCR) methods for NOx control. Both processes introduce a reductant (usually ammonia or urea) into a NOx containing air stream at a specific temperature. The added chemicals react with NOx compounds to form N₂ and water. The SCR uses a catalyst material and operates at a slightly lower temperature then the SNCR. The ammonia compounds in SPL can be a direct replacement for the ammonia or urea of the SCR/SNCR applications..

The cyanide compounds were found to have a slightly different but overlapping temperature window when compared to ammonia, but having the same reducing effect on NOx containing combustion by-products. A noted advantage in cyanide use is in the degree of reactivity with NOx compounds such as to have no slip (reductant added in excess of NOx concentration) residue.

An interesting finding of the CIEC research was that the evolution of both ammonia and cyanide compounds could be enhanced with the addition of some moisture. The mechanism for this activation is not known, but has been confirmed in discussions with SPL generators. The rapid evolution of NOx reducing compounds prior to use is advantageous allowing for dry processing (grinding) and storage of the SPL for some period without significant loss of efficacy. Another aspect of ammonia/cyanide generation is that evolution is temperature modulated, in that higher concentrations are evolved at higher temperatures.

While it is possible to speculate that any volatile compounds in the SPL could also be used to reduce NOx through a re-burn technique, no confirming research has been proposed.

The University researchers were limited in budget, material availability and test equipment. Their recommendation that additional confirmation testing at practical commercially viable introduction rates are noted. However based on research undertaken on behalf of CIEC (SPL and cyanide bearing mining waste), an optimum scenario for NOx reduction (maximum observed cyanide/ammonia generation rates) was projected, using a 5.0% SPL addition as raw feed and assuming a 600 ppm NOx in the flue gas stream. This scenario would see an overall NOx reduction of nearly 80%, of which 12% would be due to the cyanide compounds being present. The presented scenario while reported as part of the research effort, is not considered a realistic application due to SPL availability. A typical one million ton/year (clinker) plant would need 50,000 tons SPL which is more then EPA estimates is annually generated in the United States. A second issue is that a 5.0% feed rate would introduce fluoride compounds at concentrations that might affect cement quality.

Processing, Storage and Introduction: Processing of the SPL is straightforward, requiring size reduction (200 minus mesh) to increase surface area that is required for the raw material reactions to commence (cement production) and to achieve greater contact area for the NOx reduction to proceed. The SPL fuel component would be combusted a bit later in the firing process as kiln temperatures increased permitting burning of the carbon compounds. Particle size reduction to the 200 mesh range would generally match both raw feed and coal size requirements.

Delivery and conveyance of processed (ground and dry) SPL would be identical to other finely divided raw materials received at cement plants. Delivery by rail, truck, or barge is possible. Storage in enclosed segregated silos/bins is envisioned, though silo size and design will vary depending on preheater style and plant configuration. Segregated storage requirements are due to both the SPL constituents and unique location limitations for material introduction. If pneumatic conveyance systems are used, dry compressed air would assure that no moisture is added to the processed SPL.

The introduction of the processed SPL into the preheater portion of the cement kiln system, at specific temperatures are the basis for the CIEC’s patent claims for NOx reduction. A temperature window of 1400° to 1600°F appears to be the optimum temperature for NOx reduction from the ammonia/cyanide components. These temperatures are found within the preheater tower section of the kiln system, with the exact location determined based on preheater tower design. Optimally introduction would be via gravity or mechanical placement as opposed to use of pneumatics which would introduce tramp air and potentially reduce production rates. Once introduced no further handling or processing is needed. Immediately prior to SPL introduction a small amount of water would be added to accelerate the evolution of ammonia and cyanide.

The internal environment of the preheater kiln section consists of a high concentration of uniformly sized particles falling through a series of cyclones towards the kiln. Heated air from the kiln moves at high velocity counter to the direction of particle movement Air movement is induced by fan placement. The net effect is that as the solid particles which are partially suspended within the turbulence and pressure of the air stream move downward they become heated to the point that chemical reactions commence, while the counter moving air cools, as it moves towards the system exit. Within this section of the kiln system a temperature profile can be reliably identified. The residence time within a typical preheater tower is 20-30 seconds.

Regulatory conundrum: While the use of SPL as a raw material/fuel in the manufacture of portland cement is a known and historically a industry accepted practice, the current regulatory status essentially preclude its use. Options developed by CIEC to beneficially use those identified constituents of concern in reducing conventional a air pollutant and reducing fuel requirements do not appear to lessen the regulatory constricts to use of SPL. This conundrum appears to be a unique aspect of the RCRA regulations concerning generation, treatment, processing and disposal/delisting of categorical wastes such as SPL.

It is the desire of CIEC to further develop, demonstrate and commercialize use of this technology. It is believed that a regulatory mechanism can be identified that will allow use of the technology, once proven, to be utilized by cement companies in proximity to aluminum manufacturers without the necessity of obtaining RCRA Treatment, Storage, Disposal Facility permits (Part B). CIEC staff understands that for EPA (Agency) to allow beneficial utilization of SPL it will be critical to demonstrate the claimed efficacy of the technology. To this end CIEC staff hopes that EPA will provide input, guidance and direction on additional research needed to permit the Agency to allow SPL to be beneficially used in the manufacture of portland cement.

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