Relationships Between Smelter Grade Alumina Characteristics and Strength Determined by Nanoindentation and Ultrasound-Mediated Particle Breakage

The mechanical strength of smelter grade alumina (SGA) is of considerable practical significance for the aluminum reduction process. Attrition of alumina during transportation and handling generates an increased level of fines. This results in generation of dust, poor flow properties, and silo segregation that interfere with alumina feeding systems. These lead to process instabilities which in turn result in current efficiency losses that are costly. Here we are concerned with developing a fundamental understanding of SGA strength in terms of its microstructure. Nanoindentation and ultrasound-mediated particle breakage tests have been conducted to study the strength. Strength of SGA samples both industry calcined and laboratory prepared, decrease with increasing α-alumina (corundum) content contrary to expectation. The reducing strength of alumina with increasing degree of calcination is attributed to the development of a macroporous and abrasion-prone microstructure resulting from the ‘pseudomorphic’ transformation of precursor gibbsite during the calcination process.

     

Anthocyanin-rich sweet potato lacto-pickle: production, nutritional and proximate composition

Anthocyanin pigment-rich sweet potato (SP) cubes were pickled by lactic fermentation by brining the cut and blanched cubes in common salt (NaCl, 2–10%) solution. They were then inoculated with a strain of Lactobacillus plantarum (MTCC 1407) and incubated for 28 days. Treatment with 8–10% brine solution was found to be organoleptically most acceptable. The final product with 8% and 10% brine solutions had a pH (2.5–2.8), titratable acidity (TA) (1.5–1.7 g kg⁻¹), lactic acid (LA) (1.0–1.3 g kg⁻¹), starch (56–58 g kg⁻¹) and anthocyanin content (390 mg kg⁻¹) on fresh weight basis. Sensory evaluation rated the anthocyanin-rich SP lacto-pickle acceptable based on texture, taste, aroma, flavour and after taste. Principal component analyses reduced the eleven original analytical and proximate variables (pH, TA, LA, starch, total sugar, anthocyanin, organic mater, ash, fat, protein and calories) to three independent components (factors), which accounted for 91% of the total variations.     

Investigation of the Ledge Structure in Aluminum Smelting Cells

In aluminum smelting cells, ledges freeze on to cell walls from the cryolitic bath when the temperature drops below the bath liquidus point. Modern cell design and control cause a suitable ledge profile to form and be maintained, in order to protect the cell walls from corrosive liquids (molten salts and Al metal) and ensure efficient current distribution and cell heat balance. During cell operation, a significant ledge, freezing and melting does occur following heat balance changes due to batch operations. The ledge formation mechanism has been studied at the laboratory scale in our previous work. It shows a linkage between the rate and directional nature of ledge growth and its structure as affected through a superheat change. An open ledge structure can dominate the laboratory ledge material growth or melt it out quickly when the superheat either decreases or increases, respectively. This paper begins the investigation of industrial ledge samples, in terms of structure and composition, primarily to identify whether the same ledge formation mechanism exists in industrial cells. In this study, as expected, the industrial ledge shows more complexity than the laboratory ledge; the open structure is different compared to the laboratory ledge due to the inclusion of carbon dust, a large thermal gradient across the ledge, and sufficient aging of the ledge in the cell. The comparison between the laboratory ledge and the industrial ledge has provided insight into the ledge growth mechanism in aluminum smelting cells.

     

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.     

Pulsed galvanostatic control of solid-state polymeric ion-selective electrodes

We report on galvanostatically controlled solid-state reversible ion-selective sensors for cationic analytes utilizing a conducting polymer as a transduction layer between the polymeric membrane and electron-conductive substrate. The instrumental control of polymeric membrane ion-selective electrodes based on electrochemically induced periodic ion extraction in alternating galvanostatic/potentiostatic mode was introduced recently creating exciting possibilities to detect clinically relevant polyions such as heparin and protamine and drastically improve the sensitivity of ion-selective sensors limited by the Nernst equation. The present study forms the basis for development of reliable, robust, and possibly maintenance-free sensors that can be fabricated using screen-printing technology. Various aspects of the development of solid-contact galvanostatically controlled ion-selective electrodes with a conducting polymer as a transduction layer are considered in the present work on the example of a model system based on a sodium-selective membrane. The protamine-selective solid-contact sensor was fabricated and characterized, which represents the next step toward commercially viable polyion sensing technology. A substantial improvement of a low detection limit (0.03 mg L-1) was achieved. A simplified diffusion-based theoretical model is discussed predicting the polarization at the interface of the conducting polymer and the membrane, which can cause the disruption of the sensor response function at relatively small current densities.