Thermodynamic and Kinetic Modeling of Thermal Decarbonization of Low-Grade Phosphate Ore

RÉSUMÉ: Le minerai de phosphate est une source potentielle de phosphore avec une large application dans de nombreux procédés industriels pour produire des produits commerciaux tels que l'acide phosphorique, les aliments, les engrais, les pesticides et les détergents. Le minerai de phosphate est une roche naturelle contenant principalement la fluorapatite carbonée, la dolomite, la calcite et le quartz. Le minerai de phosphate à faible teneur est bénéfique pour produire du gaz phosphore pur (P2) par fusion, car il contient une concentration importante de quartz qui joue le rôle d'un agent fondant. Le gaz phosphore peut être transformé en d'autres produits intermédiaires purs tels que le pentoxyde de phosphore (P2O5) et l'acide phosphorique (H3PO4), qui ont un large éventail d'applications industrielles. Par conséquent, le procédé thermique par fusion est une technique prometteuse pour remplacer le procédé à l'acide phosphorique humide qui souffre de nombreux problèmes techniques (acide phosphorique de faible teneur) et cause de graves problèmes environnementaux (génération de phosphogypse).----------ABSTRACT: Phosphate ore is a potential source of phosphorus with wide application in many industrial processes to produce market products such as phosphoric acid, food, fertilizers, pesticides, and detergents. It is a natural rock that contains mainly carbonate fluorapatite, dolomite, calcite, and quartz. Low-grade phosphate ore is beneficial to produce high pure phosphorus gas (P2) by smelting process because it contains a considerable concentration of quartz that plays the role of fluxing agent. Phosphorus gas can be transformed into other pure intermediate products such as phosphorus pentoxide (P2O5) and phosphoric acid (H3PO4), which have a wide range of industrial applications. Hence, the thermal process that includes smelting is a promising technique to replace the wet phosphoric acid process that suffers from many technical issues (low-grade phosphoric acid) and causes serious environmental problems (generation of phosphogypsum)

Copper-Bearing Magnetite and Delafossite in Copper Smelter Slags

The cooling paths and kinetics in the system Cu-Fe-O are investigated by the empirical micro- and nanoscale analysis of slags from the flash furnace smelter at Olympic Dam, South Australia. We aim to constrain the exsolution mechanism of delafossite (Cu1+Fe3+O2) from a spinel solid solution (magnetite, Fe3O4) and understand why cuprospinel (CuFe2O4) is never observed, even though, as a species isostructural with magnetite, it might be expected to form. Flash furnace slags produced in the direct-to-blister copper smelter at Olympic Dam contain four Cu-bearing phases: Cu-bearing magnetite, delafossite, metallic copper, and cuprite. Delafossite coexists with magnetite as rims and lamellar exsolutions, as well as bladed aggregates, associated with cuprite within Si-rich glass. The empirical compositions of magnetite and rim delafossite are (Fe2+6.89Cu2+0.86Co0.13Mg0.15Si0.02)8.05 (Fe3+15.52Al0.41Ti0.01Cr0.01)15.95O32, and (Cu1+0.993Co0.002Mg0.002)0.997(Fe3+0.957Al0.027Ti0.005Si0.004)0.993O2, respectively. The measured Cu content of magnetite represents a combination of a solid solution (~6 mol.% cuprospinel endmember) and exsolved delafossite lamellae. Atomic-resolution high-angle annular dark field scanning transmission electron microscope (HAADF STEM) imaging shows epitaxial relationships between delafossite lamellae and host magnetite. Defects promoting the formation of copper nanoparticles towards the lamellae margins suggest rapid kinetics. Dynamic crystallization under locally induced stress in a supercooled system (glass) is recognized from misorientation lamellae in delafossite formed outside magnetite grains. The observations are concordant with crystallization during the cooling of molten slag from 1300 °C to <1080 °C. Melt separation through an immiscibility gap below the solvus in the system Cu-Fe-O is invoked to form the two distinct delafossite associations: (i) melt-1 from which magnetite + delafossite form; and (ii) melt-2 from which delafossite + cuprite form. Such a path also corroborates the published data explaining the lack of cuprospinel as a discrete phase in the slag. Delafossite rims form on magnetite at a peritectic temperature of ~1150 °C via a reaction between the magnetite and copper incorporated in the oxide/Si-rich melt. The confirmation of such a reaction is supported by the observed misfit orientation (~10°) between the rim delafossite and magnetite. HAADF STEM imaging represents a hitherto underutilized tool for understanding pyrometallurgical processes, and offers a direct visualization of phase relationships at the smallest scale that can complement both experimental approaches and theoretical studies based on thermodynamic modelling