Asymmetric distribution in twin screw granulation.

Positron Emission Particle Tracking (PEPT) was successfully employed to validate measured transverse asymmetry in material distribution in the conveying zones of a Twin Screw Granulator (TSG). Flow asymmetry was established to be a property of the granulator geometry and dependent on fill level. The liquid distribution of granules as a function of fill level was determined. High flow asymmetry at low fill level negatively affects granule nucleation leading to high variance in final uniformity. Wetting of material during nucleation was identified as a critical parameter in determining final granule uniformity and fill level is highlighted as a crucial control factor in achieving this. Flow asymmetry of dry material in conveying zones upstream of binder fluid injection leads to poor non-uniform wetting at nucleation and results in heterogeneous final product. The granule formation mechanism of 60°F kneading blocks is suggested to be primarily breakage of agglomerates formed during nucleation. Optimisation of screw configuration would be required to provide secondary growth. This work shows how fill dependent flow regimes affect granulation mechanisms.

Thiol modification of silicon-substituted hydroxyapatite nanocrystals facilitates fluorescent labelling and visualisation of cellular internalisation.

Calcium phosphates are used widely as orthopaedic implants and in nanocrystalline form to enable the transfer of genetic material into cells. Despite widespread use, little is known about their fate after they have crossed the cell membrane. Here we present a method of surface modification of silicon-substituted hydroxyapatite (SiHA) through a silane group, which enables the engraftment of a fluorescent dye to facilitate real-time biological tracking. Surface modification of the nanocrystal surface was undertaken using (3-mercaptopropyl)trimethoxysilane (MPTS), which presented a thiol for the further attachment of a fluorophore. Successful modification of the surface was demonstrated using zeta potential measurements and fluorescence microscopy and the number of thiol groups at the surface was quantified using Ellman's reagent. In vitro experiments using the fluorescently modified particles enabled the discrimination of the calcium phosphate particulate from other biological debris following internalisation by a population of MC3T3 (pre-osteoblast) cells and the particles were shown to maintain fluorescence for 24 hours without quenching. The successful modification of the surface of SiHA with thiol groups offers the tantalising possibility of the intracellular growth factor delivery.

Twin screw wet granulation: the study of a continuous twin screw granulator using Positron Emission Particle Tracking (PEPT) technique.

In this paper, Positron Emission Particle Tracking (PEPT) techniques are utilised to track the trajectory of single particles through the mixing and conveying zones of a Twin Screw Granulator (TSG). A TSG consisting of conveying zones and mixing zones is used in this study. The mixing zones are arranged with kneading discs at an angle of 30°, 60° or 90°. Experiments were carried out using different mixing configurations with various screw speed and total mass flow rate. The PEPT data obtained were then utilised to obtain the residence time distribution (RTD) and the Peclet number in an attempt to gain some insight into the mixing of the process. The fill level of the granulator was also estimated to study the mechanism of granulation. As might be expected, it was shown that the residence time of the granulation process increases with decreasing screw speed. It also increases with increasing angle of the arrangement of kneading blocks in the mixing zones, but will decreases when powder feed rate is increased. The fill level of the mixing zone in particular increases when the screw speed decreases or when powder feed rate increases. Furthermore, the fill level of the granulator will increase when the mixing zone configuration changes from 30° to 90°. It is shown that the granulator is never fully filled, even using 90° mixer elements implying limited compaction which may explain why the granules produced are porous compared with those from a high shear mixer. Interestingly, the RTD analysis reveals that the extent of axial mixing in the mixing zone of the granulator does not change significantly for different configurations and process conditions. There is evidence of a tail in the RTD which implies some material hold up and channelling.

Modification of ilmenite surface chemistry for enhancing surfactants adsorption and bubble attachment.

In this study, microwave irradiation is used to modify ilmenite surface chemistry to enhance the adsorption of surfactants and the air bubble attachment. The results indicate that microwave irradiation can increase ilmenite flotation recovery by 20%. A positron emission particle tracking technique is used to study the dynamic behaviour of ilmenite particles in a Denver cell. The data shows that the poor flotation recovery of ilmenite is not only due to the reduce probability of ilmenite being captured by air bubbles, but also the short residence time of the particles remaining in the froth phase. The ilmenite particles can be frequently captured by air bubbles, but dropped to the bulk liquid from the froth phase, normally over 15 s. Microwave irradiation changes the ilmenite flow pattern in the Denver cell. The average time of ilmenite remaining in froth phase is increased from 11.5 to 29.1 s.

Laboratory scale bioremediation of acid mine water drainage from a disused tin mine.

Real acidic mine-water drainage was seeded with Acidithiobacillus ferrooxidans to catalyse the removal of iron contained therein. The addition of At. ferrooxidans increased metal precipitation kinetics and decreased the water iron content by approximately 70%. Supplementing non-sterile mine water with a bacterial growth medium accelerated metal removal by indigenous micro-organisms both at the 500 ml shake-flask and 5 l bioreactor scale.

Bioreduction and biocrystallization of palladium by Desulfovibrio desulfuricans NCIMB 8307.

The reduction of Pd(II) to Pd(0) was accelerated by using the sulfate-reducing bacterium Desulfovibrio desulfuricans NCIMB 8307 at the expense of formate or H(2) as electron donors at pH 2-7. With formate no reduction occurred at pH 2, but with H(2) 50% of the activity was retained at pH 2, with the maximum rate (1.3-1.4 micromol min(-1) mg dry cells(-1)) seen at pH 3-7, which was similar to the rate with formate at neutral pH. Excess nitrate was inhibitory to Pd(II) reduction using formate, but not H(2). Chloride ion was inhibitory as low as 100 mM using formate but with H(2) only ca. 25% inhibition was observed at 500 mM Cl(-) and H(2) was concluded to be the electron donor of choice for the potential remediation of industrial wastes. Deposited Pd was visible on the cells using transmission and scanning electron microscopy and analysis by energy dispersive X-ray microanalysis (EDAX) identified the deposit as Pd, confirmed as Pd(0) by X-ray powder diffraction analysis (XRD). The crystal size of the biodeposited Pd(0) was determined to be only 50% of the size of Pd(0) crystals manufactured chemically from Pd(II) at the expense of H(2) and, unlike the chemically manufactured material, the biocrystal size was independent of the pH. The "biological" Pd(0) functioned as a superior chemical catalyst in a test reaction which liberated hydrogen from hypophosphite. Pd, and also Pt and Rh, could be recovered by resting cell suspensions under H(2) from an industrial processing wastewater, suggesting a possible future application of bioprocessing technology for precious metals.