Sustainable Method for Recovery of Copper Naphthenate from Used Railroad Ties

Recovery of the wood preservative copper naphthenate (CN) from end-of-life railroad ties was investigated to promote the secondary use of this biomass as a fuel source and mitigate the negative impacts of landfilling and other alternative tie disposal methods. Thermal desorption of naphthenic acids from CN-treated ties preceded inorganic extraction with ethylenediaminetetraacetic acid (EDTA) and biodegradable chelating agents, 2,6-pyridine dicarboxylic acid (PDA) and 1-hydroxy ethylidene-1,1-disphosphonic acid (HEDP). Organic preservative components were recovered from thermal pretreatment at temperatures between 225 and 300 °C. This thermal desorption also mirrored torrefaction by increasing the ties’ higher heating value and carbon content, in addition to lowering the moisture content within the biomass. These effects improved the biomass properties for use as a thermochemical conversion feedstock. Preservative component recovery was assessed at micro-, bench-, and pilot-scales, demonstrating scalability of the proposed process. Extraction efficiency of copper and other naturally present inorganics with PDA after thermal desorption was the most effective of those tested, further upgrading the properties of the biomass for pyrolysis. The ash content of the biomass was decreased by up to 64 % using PDA, and 100 % of copper was removed from both the raw sample and that desorbed at 225 °C specifically. Estimations of the bio-oil yields that would be generated from subsequent fast pyrolysis of the ties were completed based on a review of literature. The sample that has undergone both pretreatment steps is expected to generate the highest quantity of bio-oil from subsequent fast pyrolysis, as well as the highest quality.