CRYSTALLIZATION FOULING BEHAVIOUR OF URANIUM OXIDE AND ALUMNOSILICATE SCALE IN HIGH LEVEL NUCLEAR WASTE MEDIUM

During high level nuclear waste (HLNW) liquor processing in evaporators operating in the temperature range 30 - 140 °C, dissolved silica, alumina, sodium hydroxide, uranium-235 and transuranic species (e.g. plutonium-238) invariably become concentrated. As the liquor evaporation proceeds, the sodium aluminosilicate (SAS) and radionuclides may exceed their solubility limits and co-precipitate, fouling the tubes and walls of the evaporator. If the fouling process is not effectively controlled or mitigated, radionuclide scale accumulation exceeding the critical mass necessary for self-sustaining nuclear fission reaction may proceed at an alarming rate, posing a serious criticality concern. To probe the mechanisms underpinning uranium oxide-sodium aluminosilicate co-crystallization fouling, fundamental studies simulating the process were undertaken. New knowledge and greater understanding gleaned from the present work comprise crystallo-chemical structure characteristics, solubility and the fouling mechanisms involved in the mixed oxides scale deposition. The implications of the findings with regards to uraniumbased scale formation in HLNW plants are highlighted

Sodium Aluminosilicate Solid Phase Specific Fouling Behaviour

Process heat transfer equipment fouling due to sodium aluminosilicate precipitation is a serious problem that confronts high-level nuclear waste liquor and Bayer process alumina processing plants. The fouling of 316 stainless steel substrate by thermodynamically stable and unstable sodium aluminosilicate polytypes: amorphous solid, zeolite A, sodalite and cancrinite crystals, been has studied in an isothermal, batch precipitation system at 65 °C. Fouling invariably occurred via heterogeneous nucleation, crystal growth and particulate adsorption processes, accompanied by solution-mediated, phase transformation and morphological changes. For the thermodynamically stable cancrinite fouling, the amount of scale deposited increased systematically with increasing crystallization time before levelling off as a result of depleted supersaturation. Where the deposited scale involved a less stable phase (e.g., amorphous, zeolite A), transformation to a more stable phases (e.g., sodalite/cancrinite) occurred. The scale layer coverage/growth characteristically increased and then decreased in a periodic manner. The periodicity of this unusual behaviour appeared to be directly and kinetically related to polytypic phase transformation of amorphous to zeolite A, zeolite A to sodalite and sodalite to cancrinite

Functionalized diatom silica microparticles for removal of mercury ions

Diatom silica microparticles were chemically modified with self-assembled monolayers of 3-mercaptopropyl-trimethoxysilane (MPTMS), 3-aminopropyl-trimethoxysilane (APTES) and n-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AEAPTMS), and their application for the adsorption of mercury ions (Hg(II)) is demonstrated. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy analyses revealed that the functional groups (–SH or –NH2) were successfully grafted onto the diatom silica surface. The kinetics and efficiency of Hg(II) adsorption were markedly improved by the chemical functionalization of diatom microparticles. The relationship among the type of functional groups, pH and adsorption efficiency of mercury ions was established. The Hg(II) adsorption reached equilibrium within 60 min with maximum adsorption capacities of 185.2, 131.7 and 169.5 mg g-1 for particles functionalized with MPTMS, APTES and AEAPTMS, respectively. The adsorption behavior followed a pseudo-second-order reaction model and Langmuirian isotherm. These results show that mercapto- or amino-functionalized diatom microparticles are promising natural, cost-effective and environmentally benign adsorbents suitable for the removal of mercury ions from aqueous solutions.Yang Yu, Jonas Addai-Mensah and Dusan Losi

Rheological behaviour of clay-rich gangue mineral dispersions during hydrometallurgical treatment

are discussed.

Electroosmotic dewatering and consolidation of mineral waste tailings

Effective dewatering of mineral waste tailings and subsequent disposal are important issues which confront the mining and mineral processing industry world-wide. Conventional, ore treatment and hydrometallurgical processes used in the extraction of value metals from low grade ores involve the production of fine particles via grinding for desired liberation.They require the utilisation of voluminous amounts of water, generating equivalently large volumes of intractable waste tails of low solid density. For clay slimes, flocculation and thickening operations used for dewatering produce fast settling rate but low sediment consolidation, even after several years of impoundment in dams; warranting alternative, cost-effective methods for enhanced dewaterability. In the present work, electroosmotic dewatering and consolidation of model and real kaolinite and smectite clay mineral pulps were investigated. The study focused on probing the specific influence of variables such as the dispersion pH, supernatant (electrolyte) concentration, voltage input, and current reversal/intermittence on electrical energy consumption during electroosmosis and establishing optimum physico-chemical conditions conducive to the electroosmotic dewatering process. Remarkable efficacy in electroosmotic consolidation, reflecting a dramatic increase in clay slimes sediment loading by 20-30 wt per cent to 'spadeable' pulps was readily achieved at acceptable power consumption rates under laboratory scale conditions.

Population balance based modelling of clay and oxide minerals agglomeration behavior

Agglomeration of fine mineral particles as a precursor to heap leaching is an important means of enhancing leaching rates and metal recoveries, particularly in processing low grade laterite ores. To fully understand the underlying mechanisms and kinetics of agglomeration, it is necessary to establish a useful, predictive model based on feed and product characteristics (e.g., size and structure analyses), for better design and control of the agglomeration processes. Useful rate parameters of the mechanisms and kinetics may be extracted from appropriate agglomeration experiments and used for the optimization and scale-up and also the benchmarking of our understanding on real ore agglomeration processes. In this paper, the modelling of the batch drum agglomeration process of selected clay and oxide mineral ores (hematite, quartz, kaolinite and smectite) using a population balance model is explored. The coalescence kernels which are linked to batch granulator operating conditions are reviewed. One-dimensional population balance modelling approach is developed based on the results of single minerals. The use of a physically based coalescence kernel shows a great promise for the modelling of the granule size distribution of single minerals and both the fundamental material properties and the operating conditions are linked to the model

Interfacial-chemistry mediated behavior of colloidal talc dispersions

Interfacial chemistry and rheological behavior of talc suspensions as a function of pH, talc solid content, and type and concentration of ions have been investigated using electrokinetic measurements, rheology, settling behavior, and solution analysis. Zeta potential measurements show a strong dependence on the pH history of the talc suspension that only occurs when the surface area (solid content) of the talc is high. Particle interactions measured through dispersion yield stress measurements show a similar dependence. Talc is a magnesium silicate mineral and the dependence seen in the electrokinetic properties in this study has been attributed to Mg(II) dissolution at low pH, and has been confirmed by solution analysis. At high solid content (>20 wt%), pronounced Mg(II) ion dissolution occurs at low pH values. Formation and adsorption of electropositive Mg(II) hydrolysis products occurs at high pH (>9), and these lead to zeta potential reduction and, at high solid contents, charge reversal. Particle interactions reflect the surface chemistry behavior. Consequently, for a freshly prepared suspension at high pH, the yield stress is lower compared to after the pH has been taken to 5 and subsequently increased.Kristen E. Bremmell, and Jonas Addai-Mensahhttp://www.elsevier.com/wps/find/journaldescription.cws_home/622861/description#descriptio

Particle and pore dynamics under column leaching of goethitic and saprolitic nickel laterite agglomerates

In an agglomerated particle heap leach process, the pore network structure within the agglomerates and the particle bed is a critical determinant of efficient lixiviant flow. During the leaching period a degree of slumping can occur, changing the structure of the inter-particle void spaces and the porosity of the particle bed. The ability to characterise the agglomerate pore network structure and measure its changes over the leach period is essential for the understanding and improvement of lixiviant flow and the ultimate performance of the heap. Three dimensional (3D) non-destructive X-ray micro-tomography (XMT) offers a great opportunity to provide detailed spatially resolved structural information of these multiphase particulate systems. The aims of this paper are to compare and contrast the dynamic pore network structure of saprolitic (SAP) and goethitic (G) Ni laterite mineral agglomerate beds undergoing acid leaching by periodic scanning with XMT, as well as to extract quantitative porosity information from this data and correlate it with/to other measures of bed stability, specifically mass loss and particle bed slump. The resulting XMT analyses were able to reliably discriminate between solid and gas phases, enabling the estimation of porosity from the segmented images which in turn agreed well with the measured mass loss and slump. The SAP particle bed has consistently higher porosity and leach stability compared to the G particle bed and this is consistent with the higher HL processability of SAP ores compared to G ores observed in other studies. © 2016 The Society of Powder Technology Japa