Understanding Lateritic Ore Agglomeration Behaviour as a Precursor to Enhanced Heap Leaching

Although nickel (Ni) laterite ores constitute the majority of Ni mineralization resource world-wide, in contrast to Ni sulphides, their processing via conventional beneficiation (e.g. multi-gravity and flotation) and hydrometallurgical routes is intractable as they are predominantly low grade and complex, both mineralogically and chemically. Due to their physico-chemical character-istics, low grade lateritic ores require more aggressive but costly chemical and hydrometallurgical techniques (e.g., leaching in high pressurized tanks) for value metal (Ni and Co) extraction. Processing such ores through cost-competitive heap (4-10 m high) leaching as an alternative, requires successful agglomeration of the feed into robust and porous granules. To date, produc-ing of granules with desirable attributes poses a major geotechnical challenge to industry. In the present work, we investigate agglomeration behaviour of siliceous goethite Ni laterite ore and selected oxides and clay minerals (hematite, quartz and kao-linite) which constitute the predominant host gangue phases of typical low grade Ni laterite ores. Fundamental knowledge and understanding of the agglomeration mechanisms and kinetics which are essential for producing robust real ore granules, and pivotal to the subsequent heap leaching process, are gleaned. Isothermal, batch agglomeration tests involving 30 and 44 % w/w sulphuric acid solution as a binder indicated that 5 – 40 mm granules of differing roughness and morphologies were produced in 8-14 min. The results showed feed characteristics (e.g., mineralogy and particle size distribution) and binder content (15-25 wt.%) dependent agglomeration behaviour. Slow agglomerate nucleation and growth were displayed by the kaolinite clay min-eral whilst the oxides exhibited faster agglomeration kinetics. Siliceous goethite feed ore fine/coarse ratio, H2SO4 binder dosage and acid content, product drying temperature and aging conditions, all showed significant impact on agglomeration mecha-nisms (e.g., particle wetting, nucleation and growth processes) and granule attributes (e.g., size and strength). Agglomerates strength increased with increasing fine/coarse particle ratio

Understanding the Mechanism of Arsenic Mobilisation and Behaviour in Tailings Dams

This study was carried out on leaching of tailings at 30 ᵒC and 40 ᵒC. The mineralogical and chemical composition of the tailings material were determined by Quantitative X-Ray Diffractometry (QXRD) and Scanning Electron Microscopy combined with Energy Dispersive Spectroscopy (SEM-EDAX). The study revealed that the tailings contain sulphides (arsenopyrite and pyrite) which can leach to produce arsenic (As) and other ions in solution. The acid released during leaching depends on the temperature of leaching. More acid was produced at higher temperature (40 ᵒC) than lower temperature (30 ᵒC). It was established that arsenic precipitation from solution was higher at higher temperature (40 ᵒC) than lower temperature (30 ᵒC). Mimicking the study in a typical tailings environment, it could be proposed that As mobilisation will be enhanced at lower temperature (30 ᵒC) than at higher temperature (40 ᵒC). Keywords: Tailings, Leaching, Arsenopyrite, Heavy metals and Temperatur

Acquired immune responses to three malaria vaccine candidates and their relationship to invasion inhibition in two populations naturally exposed to malaria

Background: Malaria still represents a major cause of morbidity and mortality predominantly in several developing countries, and remains a priority in many public health programmes. Despite the enormous gains made in control and prevention the development of an effective vaccine represents a persisting challenge. Although several para site antigens including pre-erythrocytic antigens and blood stage antigens have been thoroughly investigated, the identification of solid immune correlates of protection against infection by Plasmodium falciparum or clinical malaria remains a major hurdle. In this study, an immuno-epidemiological survey was carried out between two populations naturally exposed to P. falciparum malaria to determine the immune correlates of protection. Methods: Plasma samples of immune adults from two countries (Ghana and Madagascar) were tested for their reactivity against the merozoite surface proteins MSP1-19, MSP3 and AMA1 by ELISA. The antigens had been selected on the basis of cumulative evidence of their role in anti-malarial immunity. Additionally, reactivity against crude P. falciparum lysate was investigated. Purified IgG from these samples were furthermore tested in an invasion inhibition assay for their antiparasitic activity. Results: Significant intra- and inter- population variation of the reactivity of the samples to the tested antigens were found, as well as a significant positive correlation between MSP1-19 reactivity and invasion inhibition (p < 0.05). Interestingly, male donors showed a significantly higher antibody response to all tested antigens than their female counterparts. In vitro invasion inhibition assays comparing the purified antibodies from the donors from Ghana and Madagascar did not show any statistically significant difference. Although in vitro invasion inhibition increased with breadth of antibody response, the increase was not statistically significant. Conclusions: The findings support the fact that the development of semi-immunity to malaria is probably con tingent on the development of antibodies to not only one, but a range of antigens and that invasion inhibition in immune adults may be a function of antibodies to various antigens. This supports strategies of vaccination including multicomponent vaccines as well as passive vaccination strategies with antibody cocktails

Behaviour and fate of uranium in a high-level nuclear waste processing system

Effective control and management of high-level nuclear waste (HLNW) during its hydrothermal processing is crucial to the US Department of Energy's Savannah River tank farm operations. Approximately 130 million dm3 of this 'cold war' legacy waste liquor are stored at site. During processing, sodium aluminosilicate (SAS) polytypes (e.g., amorphous, zeolite, sodalite, and cancrinite) and sodium diuranate waste products enriched in fissionable uranium (U) crystallize, invariably, leading to evaporator fouling and major criticality concern. Fissionable product fouling and its mitigation pose intractable challenges, warranting greater understanding and new knowledge of the underpinning mechanisms. In this paper, some of the tactical investigations into uranium behaviour and interactions with SAS phases in HLNW carried out are reported. It is shown that SAS-mediated co-crystallization of sodium diuranate phase from HLNW liquor at high ionic strength and pH, rather than U ion complex adsorption, largely accounts for the crystallographic destination and presence of U in the plant solid foulant.J. Addai-Mensah and H. Musiyarir

Assessing the impact of preload on pyrite-rich sediment and groundwater quality

Pyrite-rich sediments would, invariably, undergo redox reactions which would lead to acidic aqueous environment containing solubilized toxic metal species. When such sediments are subjected to preload, a technique employed by geotechnical engineers to improve the load-bearing capacity of highly compressible formation, transient flow of pore water, accompanied by acidity transfer, would occur as a response. Despite the concomitant environmental and socio-economic significance, to date, there has been limited interdisciplinary research on the underpinning geotechnical engineering and geo-environmental science issues for pyrite-rich sediments under preload. In this study, we investigate the effect of pyrite-rich sediment pore water transfer under preload surcharge on the receiving environment and the impact on the groundwater speciation and quality. Sediment samples were obtained at close depth intervals from boreholes established within pristine areas and those subjected to the preload application. Soil and pore water samples were subjected to solid/solution speciation, moisture contents, soil pH and the Atterberg Limits' analyses using standard analytical techniques and methods. Standpipes were also installed in the boreholes for groundwater sampling and in situ monitoring of water quality parameters. It is shown that the imposition of preload surcharge over pyritic sediment created a reducing environment rich in SO42-, iron oxide minerals and organic matter. This reducing environment fostered organic carbon catabolism to generate excess pyrite and bicarbonate alkalinity, which would invariably impact adversely on soil quality and plant growth. These were accompanied by increase in pH, dissolved Al, Ca, Mg and K species beneath the surcharge.Ohene Karikari-Yeboah and Jonas Addai-Mensa

Atmospheric acid leaching of siliceous goethitic Ni laterite ore: effect of solid loading and temperature

Available online 14 September 2014In this study, atmospheric acid leaching behaviour of siliceous goethitic nickel (Ni) laterite ore is investigated. Specifically, the effect of -200 μm feed solid loading (30 vs. 45 wt.%) and temperature (70 vs. 90 °C) on leach kinetics, acid consumption capacity and Ni and cobalt (Co) extraction was studied under isothermal, batch (4 h) leaching conditions at pH 1. Incongruent leaching was observed for constituent elements reflecting slow but steady release of value (Ni and Co) and some of gangue metals such as Fe, Mg and Al accompanied by faster and sharp release of Na and Si. Higher temperature and lower pulp solid loading, both led to a 40-50% increase in overall Ni and/or Co extraction and higher acid consumption. At 70 °C and 45 wt.% solid loading, Ni/Co extraction after 4 h was the lowest (∼14/16%) whilst the highest extraction (∼67/56%) was observed at 90 °C and 30 wt.% solid loading. Temperature appeared to have dramatic influence on Ni/Co and other impurity metals' extractions revealing the chemical reaction controlled nature of the leaching. Higher solid loading and longer leaching time also both slowed down the leach kinetics. A two-stage chemical reactions-controlled leaching mechanism involving a faster initial leaching kinetics followed by a slower leaching at lower rate constants and higher activation energies was established for release of Ni, Co, Fe and Mg. The mechanism reflects the fast leaching of reactive host mineral phases (e.g., clays and Mg-silicates) during first 30 min followed by slow leaching of more refractory mineral phases (e.g., goethite and quartz) during the rest of leaching period. The findings provide a greater understanding for enhanced atmospheric acid leaching process of siliceous goethitic laterite ores.Jennifer MacCarthy, Jonas Addai-Mensah, Ataollah Nosrat

Self-nucleation and kinetic behavior of nanocolloidal sodalite particles in highly caustic liquors

Sodium aluminosilicate (SAS) crystallization from supersaturated caustic liquors is of high industrial significance with regards to intractable heat exchanger precipitation fouling issues in Bayer process alumina refining and high level nuclear waste processing. In the present work, isothermal (65 °C) homogeneous nucleation behavior of nanocolloidal SAS particles in optically clear solutions as a function of time was investigated by dynamic light scattering (DLS). At a high SiO₂ relative supersaturation (σ) of 12, the solution was perennially metastable, reflecting a long nucleation induction time of 12 h. Upon ephemeral preheating at 100 °C, and/or a 25− 40% increase in σ, the DLS analysis showed that rapid nucleation and moderate particle growth occurred in the optically clear solutions. Furthermore, evolution uni-, bi-, and trimodal particle size distributions in the range of 20−1800 nm with time was observed, accompanied by significant time-dependent distribution broadening effects. For growth mechanisms, both nanoparticle aggregation and surface integration of ionic growth units are revealed, manifesting in polydispersed, low mass density-contrast agglomerates in optically clear liquors. The SAS solid product observed after prolonged crystallization was high carbonatesodalite crystals, comprising agglomerates of nanoparticles. The pivotal roles played by SiO₂ supersaturation and temperature in the early stages of sodalite nucleation and growth are demonstrated by the results.Jonas Addai-Mensah, Tatiana Khmeleva and John C. Thoma

Critical copper concentration in sphalerite flotation: effect of temperature and collector

Available online 1 December 2015Sphalerite flotation and separation from other sulphide minerals are strongly dependent on pulp solution conditions such as pH and copper concentration but also temperature. The surface of sphalerite needs to be activated with copper ions to enable collector adsorption and therefore sphalerite flotation. It was found that the collectorless flotation recovery and rate of sphalerite increased with increasing copper concentration up to a maximum value before decreasing with further copper addition independently of temperature. This increase in recovery is attributed to the copper-activation of sphalerite with formation of the hydrophobic species of polysulphide/elemental sulphur. Several complementary solution and surface analytical studies have shown that the maximum recovery corresponds to the maximum number of surface zinc ions that can be replaced with copper ions (copper-activation mechanism). The decrease in recovery is not the result of further adsorption of copper hydroxide in molecular form at the sphalerite surface but it is attributed to copper hydroxide precipitation covering the hydrophobic surface species. Addition of xanthate collector restores the flotation recovery and rate of copper-activated sphalerite. It was also found that the flotation recovery and rate of copper-activated sphalerite at pH 10.5 are reduced when temperature is below 12°C in the absence or presence of collector, which is in good agreement with observations made at several flotation plants that zinc recovery is on average less during winter than in summer. Possible causes for this decrease in sphalerite flotation at lower temperatures are discussed.T.W.J. Albrecht, J. Addai-Mensah, D. Fornasier