Recovery of Phosphorus from Florida Phosphatic Waste Clay

This MS thesis examines the recovery of phosphorus (P) from Florida waste clay (WC). A comprehensive literature review revealed that: (i)-The most important values being lost to WC are P and rare earth elements (REEs). For the recovery of these values from WC, two crucial attempts are the removal of extremely fine-sized clays, followed by the recovery of phosphate content, which can pave the path for the recovery of REEs; (ii)-Any scientific/ technological solution should, at the same time, be economically and environmentally attractive to the industry. As such, moving from mostly chemical separation processes to the primarily physical/ physicochemical processes seems to be the ultimate solution. De-sliming followed by froth flotation for the recovery of P and enrichment of REEs from WC showed that the direct flotation of apatite using both fatty acid and hydroxamates, even in the presence of various regulating agents, does not offer a solution for the recovery of P from WC. Hydroxamic acid collectors, despite their capability in the selective flotation of apatite and REE-bearing minerals, do not produce a successful selective separation of phosphate minerals from WC. The double-stage reverse flotation process in which the cationic removal of silicates is followed by the anionic rejection of carbonates both at fairly acidic conditions, becomes less attractive from the economic stance when the reagent consumption is taken as a determining factor. Added to this is the significant loss of REEs with this approach. The single-stage process where only a cationic removal of silicates following the cyclone-assisted removal of clays are employed, offers a more economically viable route for the conditional recovery of P from WC. This superiority also owes to its relative capability to avoid the loss of REEs as the other paramount value contained in WC. The removal of clays, silicates, and carbonates through the de-sliming followed by the single-stage reverse flotation process meets the specifications of the medium-grade phosphate rock that is routinely mixed with the high-grade rock to be sold to the fertilizer production industry. Removal of the associated gangues significantly alleviates the acid consumption in the subsequent chemical recovery of REEs from WC, which has long been alluded to by the industry and researchers as the limiting factor. The results obtained from the multiscale investigation on the effect of Al3+ and Mg2+ ions on the flotation of apatite using fatty- and hydroxamic-acid collectors showed that fatty acid and hydroxamates adsorb on the bare mineral through a bidentate Ca—O bond along with a Na+ mediated O—O bonding, and a mono-dentate Ca—O bond along with a hydrogen bonding, respectively. On the Al3+-treated surface, fatty acid adsorbs through the bidentate interaction with both adsorbed Al3+ and lattice Ca2+ ions. The Al3+ site is preferred over Ca2+ lattice site by OHA to interact, resulting in stronger adsorption, and thus enhanced flotation. For BHA, however, Al3+ doesn’t benefit collector adsorption, and thereby leads to a declined flotation. Mg2+ presence leads to a stronger bidentate/ tridentate interaction with both adsorbed Mg2+, and lattice Ca2+ ions for both collector types. Fatty acid establishes a stronger interaction with the bare and Al3+/Mg2+-treated mineral compared to the hydroxamates. Both Al3+ and Mg2+ ions enhance the adsorption of fatty acid, and thereby its flotation. Mg2+ is more favorable than Al3+ in apatite flotation using either fatty acid or hydroxamates. Interestingly, even though Al3+ deters the functionality of BHA on apatite surface, it contributes to a stronger mineral-BHA bonding. This could possibly be because of rendered surface charge due to precipitation of Al3+ species. As well, the inconsistency between the two hydroxamates towards Al3+ could be attributed to the type of hydrocarbon group. Overall, to develop a more industrially applicable knowledge, the effects of the metal ions on the collector consumption by the known gangue minerals, and thereupon on the flotation selectivity, should also be studied in future endeavors