UBC Theses and Dissertations
Iron precipitation and associated metal loss from simulated process solutions Javed, Tasawar
Ferric iron precipitation is an important integral step of the hydrometallurgical processes. The precipitation product is often amorphous and leads to a significant amount of valuable metal loss to the residue along with residue disposal issues. Characterization of the leach residue samples from CESL and Vale medium temperature (150 °C) hydrometallurgical processes revealed that the precipitation of iron in the form of amorphous iron oxide phases results in approximately 2-4 times higher metal loss compared to the crystalline phases. To address the issue, simulated process solutions were used to study the effect of process parameters and their relative importance on batch precipitation conditions with the aim of obtaining a stable iron oxide phase i.e. hematite, while minimising associated metal loss to the precipitation product. It was found that the factors: initial ferric, H2SO4 and seed concentrations play an important role in the iron precipitation step. Mathematical models were developed for the iron precipitation and metal loss to the precipitates using statistical data analysis techniques. Results from this study show that the presence of low ferric or high acid concentrations and moderate amounts of seed are required to minimize metal loss to the precipitation product with moderate to high levels of iron precipitation. The supersaturation and the nucleation to growth ratios were found to determine the final product quality i.e. the particle size and associated metal loss. The presence of various anions or cations was also found to play an important role on the iron precipitation rate and product quality/nature. For example, the presence of chloride in the solution accelerated precipitation kinetics. The sulphate salts of the metals such as Mg and Cu increased the extent of precipitation, while aluminum sulphate decreased the extent of precipitation. Presence of the sodium ion in the system accelerated the precipitation kinetics but changed the nature of the product to sodium jarosite. The presence of low levels of arsenic (As:Fe ≤ 0.08) in the system were found to severely retard the precipitation rate. Adsorption of sulphate and incorporation of OH‒ into the hematite structure were responsible to produce a poorly crystalline hematite product.
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