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

[摘要] English: Spent wax-coated iron-based low temperature Fischer-Tropsch catalyst were contacted with nitricacid in order to dissolve the contained metals. Dissolution experiments with wax-coated spentcatalysts in concentrated nitric acid at elevated temperatures recovered 75% of the iron into ametal nitrate solution. Dissolution experiments with wax-coated catalyst caused foaming and largevolumes of NOx gasses during dissolution. Severe wax separation problems were encounteredafter metal dissolution. This caused incomplete separation between residual solid, liquid and waxycomponents. Wax removal techniques, before nitric acid dissolution, in the form of thermaloxidation, anoxic thermal cracking and solvent extraction were investigated. Thermal oxidationexperiments at 500 DC and 900 DC in air and anoxic thermal cracking experiments at similartemperature ranges were performed. Wax removal by solvent extraction was performed with Cg-C11 paraffin. Iron oxide phase transformations during wax removal techniques were studied byMëssbauer spectroscopy, X-Ray diffraction and BET surface area measurements. Spent waxcoatedcatalyst consisted of 71% ferrihydrite and 26% Hagg iron carbide. Hagg iron carbide wereabsent after all wax removal techniques. Temperature excursions during thermal oxidation werestudied varying bed volume and height. Samples of bed heights of above 10 mm showedsignificant temperature deviations above the targeted heat treatment temperature. Samplesgenerated 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. Thermalcracking 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 900DC. A maghemite content of 39% was found in the catalyst residue after cracking at 750 DC whichchanged to 24% after wax cracking at 900 DC. Differences in iron oxide phases between thermaloxidation and thermal cracking were attributed to the less oxidizing environment for thermalcracking due to the absence of air. Dissolution experiments showed > 80% metal recovery forsolvent extraction and thermal oxidation and cracking at temperatures up to 500 DC. Lowerrecoveries were obtained for treatments at higher temperatures and dissolution efficiencies werecorrelated to sample hematite content. Higher hematite content of low surface area correlated toless efficient dissolution. Pure commercially purchased hematite could be dissolved appreciably ifthe surface area of the sample obtained was high. Heat treatment of the pure hematite decreasedthe 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 andsubmitted to heat treatments varying from 350-750 DC at different residence times. The resultantsamples showed marked increased hematite content and decreasing surface area for the 600 DCsamples over the 350 DC samples and very rapid conversion to hematite and decrease surfacearea 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 metalrecoveries. The overriding conclusion of this study is that the hematite phase is to be avoided. Thisis best achieved by low catalyst recovery temperatures. A high sample surface area also results inefficient 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 viablemetal reclamation technology was developed for spent wax-coated iron-based low temperatureFischer- Tropsch catalysts.

[发布机构] University of the Free State