Metal reclamation from a spent iron-based fischer-tropsch catalyst

English: Spent wax-coated iron-based low temperature Fischer-Tropsch catalyst were contacted with nitric acid in order to dissolve the contained metals. Dissolution experiments with wax-coated spent catalysts in concentrated nitric acid at elevated temperatures recovered 75% of the iron into a metal nitrate solution. Dissolution experiments with wax-coated catalyst caused foaming and large volumes of NOx gasses during dissolution. Severe wax separation problems were encountered after metal dissolution. This caused incomplete separation between residual solid, liquid and waxy components. Wax removal techniques, before nitric acid dissolution, in the form of thermal oxidation, anoxic thermal cracking and solvent extraction were investigated. Thermal oxidation experiments at 500 DC and 900 DC in air and anoxic thermal cracking experiments at similar temperature ranges were performed. Wax removal by solvent extraction was performed with Cg- C11 paraffin. Iron oxide phase transformations during wax removal techniques were studied by Mëssbauer spectroscopy, X-Ray diffraction and BET surface area measurements. Spent waxcoated catalyst consisted of 71% ferrihydrite and 26% Hagg iron carbide. Hagg iron carbide were absent after all wax removal techniques. Temperature excursions during thermal oxidation were studied varying bed volume and height. Samples of bed heights of above 10 mm showed significant temperature deviations above the targeted heat treatment temperature. Samples generated from thermal oxidation at 500 DC contained 78% maghemite and 17% hematite, samples that were oxidized at 900 DC contained only 24 % maghemite but 72% hematite. Thermal cracking of the wax-covered spent catalyst 500 DC resulted in a catalyst residue containing 23% ferrihydrite and 66% maghemite which transformed to 49% and 65% hematite at 750 DC and 900 DC. A maghemite content of 39% was found in the catalyst residue after cracking at 750 DC which changed to 24% after wax cracking at 900 DC. Differences in iron oxide phases between thermal oxidation and thermal cracking were attributed to the less oxidizing environment for thermal cracking due to the absence of air. Dissolution experiments showed > 80% metal recovery for solvent extraction and thermal oxidation and cracking at temperatures up to 500 DC. Lower recoveries were obtained for treatments at higher temperatures and dissolution efficiencies were correlated to sample hematite content. Higher hematite content of low surface area correlated to less efficient dissolution. Pure commercially purchased hematite could be dissolved appreciably if the surface area of the sample obtained was high. Heat treatment of the pure hematite decreased the surface area as well as the amount of iron that could be recovered during nitric acid dissolution. Wax-coated catalyst was also de-waxed by solvent extraction with a C9-C11 paraffin fraction and submitted to heat treatments varying from 350-750 DC at different residence times. The resultant samples showed marked increased hematite content and decreasing surface area for the 600 DC samples over the 350 DC samples and very rapid conversion to hematite and decrease surface area for the 750 DC samples. Thus a higher content of hematite in the de-waxed spent catalyst indicates exposure to higher temperatures resulting in a drop of the surface area and lower metal recoveries. The overriding conclusion of this study is that the hematite phase is to be avoided. This is best achieved by low catalyst recovery temperatures. A high sample surface area also results in efficient dissolution and catalyst recovery in nitric acid. Resultant metal nitrate solutions were used to prepare a fresh catalyst that was tested for activity and selectivity and compared well to a standard commercially available Ruhrchemie type catalyst. This proved that a chemically viable metal reclamation technology was developed for spent wax-coated iron-based low temperature Fischer- Tropsch catalysts.