Addendum to "Process development for elemental recovery from PGM tailings by thermochemical treatment: Preliminary major element extraction studies using ammonium sulphate as extracting agent" [Waste Manage. 50 (2016) 334-345].

In a recent paper, a promising two-stage process for the extraction of major elements from Platinum Group Metals (PGM) was presented (Mohamed et al., 2016). This process involved solid-solid thermochemical treatment of tailings with ammonium sulphate (stage one), followed by an optimised acid dissolution step (stage two). The inclusion of a control experiment for the optimal dissolution procedure was however overlooked. The new set of data delivered by the control experiment reveals that (i) most of the silicon and calcium are extracted via the acid leaching step, (ii) aluminium and magnesium are extracted at each stage of the process, and (iii) the thermochemical step is the main contributor to chromium and iron extraction. The conclusion of the previous paper (Mohamed et al., 2016), whereby thermochemical treatment with ammonium sulphate represents a promising technology for extracting valuable elements from South African PGM tailings, withstands.

Process development for elemental recovery from PGM tailings by thermochemical treatment: Preliminary major element extraction studies using ammonium sulphate as extracting agent.

Mine tailings can represent untapped secondary resources of non-ferrous, ferrous, precious, rare and trace metals. Continuous research is conducted to identify opportunities for the utilisation of these materials. This preliminary study investigated the possibility of extracting major elements from South African tailings associated with the mining of Platinum Group Metals (PGM) at the Two Rivers mine operations. These PGM tailings typically contain four major elements (11% Al2O3; 12% MgO; 22% Fe2O3; 34% Cr2O3), with lesser amounts of SiO2 (18%) and CaO (2%). Extraction was achieved via thermochemical treatment followed by aqueous dissolution, as an alternative to conventional hydrometallurgical processes. The thermochemical treatment step used ammonium sulphate, a widely available, low-cost, recyclable chemical agent. Quantification of the efficiency of the thermochemical process required the development and optimisation of the dissolution technique. Dissolution in water promoted the formation of secondary iron precipitates, which could be prevented by leaching thermochemically-treated tailings in 0.6M HNO3 solution. The best extraction efficiencies were achieved for aluminium (ca. 60%) and calcium (ca. 80%). 35% iron and 32% silicon were also extracted, alongside chromium (27%) and magnesium (25%). Thermochemical treatment using ammonium sulphate may therefore represent a promising technology for extracting valuable elements from PGM tailings, which could be subsequently converted to value-added products. However, it is not element-selective, and major elements were found to compete with the reagent to form water-soluble sulphate-metal species. Further development of this integrated process, which aims at achieving the full potential of utilisation of PGM tailings, is currently underway.

Comparison of CO2 capture by ex-situ accelerated carbonation and in in-situ naturally weathered coal fly ash.

Natural weathering at coal power plants ash dams occurs via processes such as carbonation, dissolution, co-precipitation and fluid transport mechanisms which are responsible for the long-term chemical, physical and geochemical changes in the ash. Very little information is available on the natural carbon capture potential of wet or dry ash dams. This study investigated the extent of carbon capture in a wet-dumped ash dam and the mineralogical changes promoting CO2 capture, comparing this natural phenomenon with accelerated ex-situ mineral carbonation of fresh fly ash (FA). Significant levels of trace elements of Sr, Ba and Zr were present in both fresh and weathered ash. However Nb, Y, Sr, Th and Ba were found to be enriched in weathered ash compared to fresh ash. Mineralogically, fresh ash is made up of quartz, mullite, hematite, magnetite and lime while weathered and carbonated ashes contained additional phases such as calcite and aragonite. Up to 6.5 wt % CO2 was captured by the fresh FA with a 60% conversion of calcium to CaCO3 via accelerated carbonation (carried out at 2 h, 4Mpa, 90 °C, bulk ash and a S/L ratio of 1). On the other hand 6.8 wt % CO2 was found to have been captured by natural carbonation over a period of 20 years of wet disposed ash. Thus natural carbonation in the ash dumps is significant and may be effective in capturing CO2.

Predicting in vivo starch digestibility coefficients in newly weaned piglets from in vitro assessment of diets using multivariate analysis.

The study was based on correlating a dataset of in vivo mean starch digestibility coefficients obtained in the immediate post-weaning phase of piglets with a range of dietary in vitro variables. The paper presents a model that predicts (R2 0.71) in vivo average starch digestibility coefficients in the 0.5 small-intestinal region of newly weaned piglets fed cereal-based diets using seven in vitro variables describing starch properties that are fundamentally associated with the quality of feed materials, i.e. hydration, structure and amylolytic digestion. The variables were: Rapid Visco Analyser (RVA; measures the viscosity of materials when sheared under defined hydration and temperature regimens); RVA end viscosity; RVA (gelatinisation) peak viscosity; DeltaH (gelatinisation enthalpy that provides an estimate of helical order or degree of crystallinity in starch); water solubility index (WSI; that denotes the amount of soluble polysaccharides released from starch granules to the aqueous phase); grain endogenous amylase (concentration of endogenous alpha-amylase in cereals, assessed by pasting cereal flours in 25 g of AgNO3, an amylase inhibitor v. water using RVA).

Visualisation of natural aquatic colloids and particles -- a comparison of conventional high vacuum and environmental scanning electron microscopy.

The applicability of environmental scanning electron microscopy (ESEM; imaging of hydrated samples) and conventional high vacuum scanning electron microscopy (SEM; imaging of dried samples at high vacuum) for the observation of natural aquatic colloids and particles was explored and compared. Specific attention was given to the advantages and limitations of these two techniques when used to assess the sizes and morphologies of complex and heterogeneous environmental systems. The observation of specimens using SEM involved drying and coating, whereas ESEM permitted their examination in hydrated form without prior sample preparation or conductive coating. The two techniques provided significantly different micrographs of the same sample. SEM provided sharper images, lower resolution limits (10 nm or lower), but more densely packed particles, suggesting aggregation, and different morphological features than ESEM, suggesting artefacts due to drying. ESEM produced less easily visualised materials, more complex interpretation, slightly higher resolution limits (30-50 nm), but these limitations were more than compensated for by the fact that ESEM samples retained, at least to some extent, their morphological integrity. The results in this paper show that SEM and ESEM should be regarded as complementary techniques for the study of aquatic colloids and particles and that ESEM should be more widely applied to aquatic environmental systems than hitherto.

Assessment of cross-flow filtration for the size fractionation of freshwater colloids and particles.

This research has evaluated the ability of cross-flow filtration (CFF) to perform correct size fractionation of natural aquatic colloids (materials from 1nm to 1mum in size) and particles (>1mum) using scanning electron microscopy (SEM) combined with atomic force microscopy (AFM). SEM provided very clear images at high lateral resolution (ca. 2-5nm), whereas AFM offered extremely low resolution limits (sub-nanometer) and was consequently most useful for studying very small material. Both SEM and AFM were consistent in demonstrating the presence of colloids smaller than 50nm in all fractions including the retentates (i.e. the fractions retained by the CFF membrane), showing that CFF fractionation is not fully quantitative and not based on size alone. This finding suggests that previous studies that investigated trace element partitioning between dissolved, colloidal and particulate fractions using CFF may need to be re-visited as the importance of particles and large colloids may have been over-estimated. The observation that ultra-fine colloidal material strongly interacted with and completely coated a mica substrate to form a thin film has important potential implications for our understanding of the behaviour of trace elements in aquatic systems. The results suggest that clean, 'pure' surfaces are unlikely to exist in the natural environment. As surface binding of trace elements is of great importance, the nature of this sorbed layer may dominate trace element partitioning, rather than the nature of the bulk particle.