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Secondary Aluminum Processing Waste: Salt Cake Characterization and Reactivity
Tolaymat, T. AND X. Huang. Secondary Aluminum Processing Waste: Salt Cake Characterization and Reactivity. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-15/109, 2015.
The specific objectives of the study were to identify and characterize the metal constituents in salt cake, investigate the leaching behavior of metals from salt cake following its reactions with water, identify the dominant crystalline mineral phases in salt cake, evaluate factors that impact the reactivity of salt cake, and evaluate salt cake reactivity with water.
Thirty-nine salt cake samples were collected from 10 SAP facilities across the U.S. The facilities were identified by the Aluminum Association to cover a wide range of processes. Results suggest that while the percent metal leached from the salt cake was relatively low, the leachable metal content may still pose a contamination concern and potential human and ecological exposure if uncontrollably released to the environment. As a result, salt cake should always be managed at facilities that utilize synthetic liner systems with leachate collection (the salt content of the leachate will increase the hydraulic conductivity of clay liners within a few years of installation). The mineral phase analysis showed that various species of aluminum are present in the salt cake samples with a large degree of variability. The relative abundance of various aluminum species was evaluated but it is noted that the method used is a semi-quantitative method and as a result there is a limitation for the data use. The analysis only showed a few aluminum species present in salt cake which does not exclude the presence of other crystalline species especially in light of the variability observed in the samples. Results presented in this document are of particular importance when trying to understand concerns associated with the disposal of salt cake in MSW landfills. From the end-of-life management perspective, data presented here suggest that salt cake should not be size reduced before disposal. Also, care should be taken not to size reduce it after disposal by compacting it with heavy bulldozers. The MSW decomposition process is exothermic and as a result MSW landfill temperatures are typically greater than 37oC and may reach 50oC or greater. The elevated temperatures present in most MSW landfills can be conducive and could initiate salt cake reactions that otherwise may not take place. Furthermore, elevated temperatures can increase the rate of the salt cake reaction which further propagate the salt cake reaction. Even outside of MSW landfills, once a salt cake reaction starts, it may self-propagate and increase the rate at which the reaction occurs. While the heat generation potential per gram of salt cake sample is a sample specific parameter, the results presented herein suggest that higher environmental temperatures tend to intensify the salt cake reaction. At elevated temperatures encountered in MSW landfills, elevated levels of H2 gas may be generated as a result of salt cake reactivity and will be of concern. Hydrogen is an explosive gas and can potentially cause fires. Hydrogen is even of greater concern if generated in MSW landfills. Apart from being a fire hazard when mixed with CH4, landfill gas collection systems are not designed to handle H2 since MSW landfill gas generally consists of CH4 and CO2 at ~ 50/50 ratio. Like hydrogen, methane is an explosive gas and care must be taken if salt cake reacts with liquids. But unlike hydrogen, in an MSW landfill, the gas collection system is design to handle methane gas which is a green house gas generated during the anaerobic decomposition of MSW. The level of NH4+ resulting from the salt cake reaction is significantly lower as compared to the levels found in landfills and as a result the ammonia generated from salt cake should not have a detrimental impact on the performance of MSW landfills. From a human health perspective, the concentration of ammonia gas resulting from the salt cake reaction was relatively high and can potentially be of concern.