Browsing by Subject "Precipitation"

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    Crystal growth and nucleation kinetics of diethylenetriammonium hexachlororhodate (III) salt
    (2019) Engelbrecht,Edmund; Hagemann, Justin; Lewis, Alison
    At Anglo American Platinum's Precious Metal Refinery, rhodium separates from a base metal-rich solution by precipitation. Hexachlororhodate (III) ions and cationic protonated diethylenetriamine ions exchange to form diethylenetriammonium hexachlororhodate (III) crystals, a rhodium metal precursor. The objective of this work is to determine nucleation and growth kinetics of diethylenetriammonium hexachlororhodate (III) salt. Two reactor configurations, namely a transient continuous stirred tank reactor (CSTR) and a t-mixer plug flow reactor (PFR), were used to determine nucleation and growth rates. The objective of the configurations was to eliminate kinetic biases that may be caused by mixing at the mesoscale. Transient saturation in the CSTR ranged up to 43, and in the PFR saturation was varied between 2 and 64. Precipitation kinetic parameters were estimated through data fitting concentration and volume average crystal size profiles to a mass and population balance model. Temperature dependence of kinetic parameters was found to be universal between reactor configurations. Both growth through interfacial attachment and agglomeration, as defined in this work, were exothermic processes with activation energies of -192.9kJ/mol and -656.1kJ/mol respectively. Nucleation was found to be an endothermic process with an activation energy of 50.9kJ/mol in accord with the observed heat of crystallisation. No evidence of heterogeneous primary nucleation in the form of crystals adhering to the side walls of the reactor or the agitator blades was observed. The experiments were not explicitly designed to distinguish between primary and secondary nucleation mechanisms, therefore excluding one over the other is not justified. However, considering the presence of nucleated crystals in each system combined with the good model fit using a nucleation rate expression typically associated with secondary nucleation, it is likely that the dominating nucleation mechanism is secondary in nature. In the PFR configuration, nucleation and growth occur at a faster rate compared to the CSTR under the experimental conditions in this work. This observation is inferred from the fitted temperature independent kinetic parameters and is linked to a much higher mixing intensity achieved in the PFR relative to the CSTR. Flow conditions, described by the Reynolds number, can limit conversion in the PFR configuration by a mixing limitation at the micro- or mesoscale. Micro- and mesoscale mixing were represented by a characteristic length scale that was empirically related to temperature for the microscale and fluid velocity for the mesoscale. Under conditions where the Reynolds number is below the point where conversion is independent of the Reynolds number, either micromixing or mesomixing can become a rate limitation. At a sufficiently high Reynolds number neither micromixing nor mesomixing limits conversion and the system equilibrium becomes the limitation. In the CSTR, the system equilibrium limited reaction conversion as the micro- and mesoscale mixing zones were sufficiently small relative to the reactor volume. Parameters related to mixing were found to differ between the configurations, which was caused by different flow patterns within each configuration. Scanning electron microscopy (SEM) photographs suggest that crystals in the PFR configuration collide both in the radial and axial direction, giving rise to a feathery flat and elongated agglomerated crystal cluster. In contrast, the crystals in the CSTR configuration collide in a chaotic but consistent pattern, giving rise to a desert rose-like agglomerated crystal cluster. The derived model used to describe agglomeration is based on the agglomeration principles proposed by Von Smoluchowski coupled with Fick's law of diffusion and gives a good representation of crystal size. The PFR growth rate supersaturation exponent was 1.13, suggesting a Burton-Cabrera-Frank type growth model, and is indicative of crystal growth from screw dislocations that is limited either through mass transfer to the crystal surface, or surface integration. Thus, in this instance, the rate of aqueous hexachlororhodate (III) conversion to crystal would be responsive to mixing conditions on the micro- or mesoscale, as was experimentally found in the PFR configuration. In comparison, the CSTR growth rate supersaturation exponent was 2.31 and is more in line with polynuclear growth that appears to be limited by interfacial attachment kinetics, as the system equilibrates in the bulk. Lastly, a key finding of this work is the ability to manipulate the crystal morphology by changing reactor configuration. By creating elongated flat crystal structures in the PFR configuration as opposed to a desert rose crystal structure in the CSTR, it may be possible to reduce impurities within the crystal by entraining less mother liquor.
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    Ecotypes of wild rooibos ( (Burm. F) Dahlg. Fabaceae) are ecologically distinct
    (Elsevier, 2011) Hawkins, H-J; Malgas, R; Bienabe, E
    Aspalathus linearis (Burm. F) Dahlg., Fabaceae is cultivated by small- and large-scale commercial farmers of the Cederberg and Bokkeveld Plateau in the Cape Floristic Region of South Africa, for the production of an herbal beverage called 'rooibos' or 'rooibos tea'. Small-scale farmers also harvest A. linearis from the wild and market the tea as an organic and fair-trade certified product. However, little is known about the apparent ecotypes of wild A. linearis. We hypothesized that 1) rooibos ecotypes are ecologically distinct (occurring in different plant communities defined by environmental variables); 2) rooibos ecotypes are functionally distinct due to variance in water use efficiency; and 3) rooibos ecotypes are associated with threatened vegetation types/species, making populations of wild rooibos ecotypes worthy of conservation. Ecotypes of wild rooibos were identified based on plant habit and local knowledge. Plant communities were classified using Twinspan, environmental factors were tested as predictors of ecotype distribution and water use efficiency of ecotypes growing across a rainfall gradient was determined from foliar natural abundance of 13C. Wild rooibos was not generally associated with endangered vegetation types but was associated with plant species having endangered status. Wild rooibos occurred in four plant communities and comprised five wild rooibos ecotypes: shrub, tree, upright, salignus and prostrate types. Although some ecotypes clearly co-occurred, evidence is provided for habitat preference between the ecotypes: Prostrate and upright ecotypes occurred at higher elevations (>400–600 m). Shrub ecotypes occurred at lower rainfall sites (<200 mm p.a.) and the salignus ecotype occurred at higher rainfall sites (>500 mm p.a.). Foliar 13C indicated greater water use efficiencies by ecotypes in relatively drier areas. The extent to which this is a plastic or inherent response requires further investigation. Considering that wild rooibos ecotypes differ both ecologically and possibly also functionally and genetically, it is concluded that populations of wild rooibos ecotypes should be considered as distinct and worthy of conservation. This distinctness should be considered when farmers apply for both harvesting and ploughing rights on land with wild rooibos ecotypes.
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    The effect of a morphology modifier on the precipitation behaviour of nickel powder
    (Elsevier, 2006) Ntuli, Freeman; Lewis, Alison Emslie
    The effect of a morphology modifier on the precipitation process of nickel powder was investigated in order to develop an understanding of its mechanism of action. Experiments were conducted on a pilot-plant scale using a 75-L autoclave with modifier dosages in the range of 0.25–5 vol%. Samples were collected from each successive batch reduction within a cycle and the powder was separated from the liquor before being washed and dried for subsequent analysis. The active particle rate processes were identified by transforming the particle size distribution (PSD) data into moments and from the change in surface area as measured by the BET method. Scanning Electron Microscopy (SEM) micrographs of the powder were used to observe the powder morphology and to validate the proposed particle rate processes and mechanism of action of the modifier. Evolution of the first moment (m0) and third moment (m3), equivalent to the total number of particles and volume, respectively, indicated that growth and aggregation were the major particle rate processes at a modifier dosage of 0.25 vol%. Breakage became apparent at dosage levels above 0.25 vol%. A decrease in BET surface area was noted in each cycle, indicating the presence of aggregation. The magnitude of decrease in the surface area indicated that the extent of aggregation decreased with increasing modifier dosage. SEM micrographs revealed that the powder was compact and aggregated at modifier dosages between 0.25 and 3 vol% and that loose porous powder was produced at 5 vol%. The modifier was found to inhibit growth, resulting in the formation of weaker agglomerate bridges leading to shear-induced breakage. This led to an increase in the surface area available for reduction. However, the effect of increased surface area in promoting reduction was outweighed by growth inhibition above a modifier dosage of 1 vol%. Thus, the number of attainable batch reductions increased when the modifier dosage was increased from 0.25 to 1 vol% and decreased with further increases in modifier dosage.
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    Metal sulphides from wastewater: Assessing the impact of supersaturation control strategies
    (IWA Publishing, 2012) Mokone, T P; van Hille, R P; Lewis, A E
    Metal sulphide precipitation forms an important component of acid mine drainage remediation systems based on bacterial sulphate reduction. However, the precipitation reaction is inherently driven by very high levels of supersaturation with the generation of small particles with poor solid–liquid separation characteristics. In this study, the effect of strategies used to manage supersaturation was investigated during copper and zinc sulphide precipitation reactions. Initial batch studies showed the origin of sulphide (biological or chemical) had no significant effect on the efficiency of zinc sulphide precipitation. For copper, low metal removal efficiency was obtained at metal to sulphide molar ratios below 1.6 in the synthetic sulphide system. This was improved in the biogenic sulphide system, due to the presence of residual volatile fatty acids, but the presence or absence of particulate organic matter had no effect on recovery. Subsequent studies, conducted using synthetic sulphide solutions in a seeded fluidised bed reactor with multiple reagent feed points (2FP and 6FP) and different recirculation flow rates (300 and 120 mL min−1) showed efficient zinc sulphide precipitation, but limited (<10%) deposition on the seeds. Increasing the number of sulphide feed points (2–6) reduced precipitate loss as fines by approximately 10%. Zinc sulphide fines could be effectively recovered from suspension by settling under quiescent conditions. In the copper system, metal recovery was low (ca 40%) due to the formation of very small copper sulphide particles (mean particle size of ca 0.01 μm). Increasing the number of reagent feed points did not affect supersaturation to the extent of altering particle characteristics. The copper sulphide fines could not be recovered by settling, remaining in a stable colloidal suspension due to their highly charged surfaces (zeta potential −50 mV). The change in recirculation flow rate had a limited effect (ca 5% improvement) on process efficiency. The results show that the extremely high supersaturation prevalent during metal sulphide precipitation is difficult to control using conventional approaches and suggest that the seeded fluidised bed reactor is not suitable for this application.