S. K. Roy

Improvement in the oxidation behavior of austenitic stainless steels by superficially applied, cerium oxide coatings

The influence of superficially applied CeO2 coatings on the isothermal-oxidation behavior, preceded by nonisothermal heating, as well as cyclic-oxidation behavior of three grades of austenitic stainless steel (AISI-316, −321, and −304), in dry air is reported. The superficial coating had a thickness of 2.1 μm. The linear heating rate employed was 6 K min−1 up to a maximum temperature of 1423 K, and the isothermal holding temperature was 1273 K. The results clearly depict that CeO2 coatings not only reduced the rates of scale growth for all three varieties of steel but also imparted improved scale adhesion to the respective alloy substrates, as evident from the fact that the coated steels could withstand a number of thermal cycles without scale rupture. In the bare condition, 321-grade steel exhibited the best performance. However, in the presence of the coating, the improved performances of 316 and 321 grades were almost identical, whereas the 304 variety showed improvement only in the first cycle of exposure. The kinetics results have been substantiated by postoxidation analyses of the alloy/scale combinations by SEM, EDS, EPMA, and XRD techniques to reveal the role of rare-earthoxide coatings on the observed behavior.

Particle characteristics and their influence on dewatering of kaolin, calcite and quartz suspensions

Abstract The effect of particle properties and dispersion on the filtration and dewatering behaviour of three different minerals namely kaolin, calcite and quartz in aqueous media have been examined. The primary particle properties such as particle size, size distribution, specific surface area, density, surface structure, dry bed porosities and sphericities of particles as well as their dispersion properties like solid concentration, pH and zeta potential have been considered. Quartz suspension shows higher settling rate and filterability (lower capillary suction time, CST) compared to calcite and kaolin at almost all solid concentrations and pH. The filterabilities of calcite and quartz are close to each other in the whole pH range and solid concentration. The settling and filterability of all three mineral suspensions showed dependence on slurry pH. The kaolin–water system showed strongest pH dependence in comparison to calcite and quartz–water system. Invariably, they all exhibited best settling and filterability characteristics at their respective point of zero charges (pzc). The vacuum filtration and dewatering characteristics of kaolin, calcite and quartz in aqueous suspensions in terms of cake permeability, specific resistance to filtration and cake saturation have been discussed on the basis of their particle size and its distribution, sphericity of the particles, bed porosity, water retaining capacities and the particle dispersion properties. Improved dewatering is observed near the pzc of each mineral.

Synthesis of stable nanocrystalline cubic zirconia

Abstract Nanocrystalline pure cubic zirconia was prepared by precipitation in polymer matrix. The average particle size obtained by this method was smaller than that obtained by aqueous coprecipitation. X-ray diffraction showed that the cubic form is stable in air up to 1173 K and partially stable in an inert argon atmosphere up to 1273 K. The stability of the cubic phase of ZrO 2 at such high temperatures was due to the presence of surface and bulk oxygen vacancies present in the nanocrystalline samples. Polymer decomposition during calcination increased oxygen vacancies already present in the nanocrystalline samples.

Direct selective laser sintering of iron-graphite powder mixture

In the present work, laser sintering of a powder mixture of iron and graphite (99.22 and 0.78%, respectively) was carried out using a pulsed Nd-YAG laser. The paper reports experimental details with evolution of the microstructures and evaluation of some of the physical and mechanical properties of the resultant sintered material. The obtained results indicate suitability of the laser sintered material for some special applications. # 2003 Elsevier Science B.V. All rights reserved.

Flocculation and dewatering of kaolin suspensions in the presence of polyacrylamide and surfactants

Flocculation, as a result of the interaction between non-ionic polyacrylamide polymer (PAM-N) and kaolin surface in aqueous suspension, has been discussed both in the absence and in the presence of surfactants namely, cationic cetyl trimethyl ammonium bromide (CTAB), anionic sodium dodecyl sulphate (SDS) and non-ionic TX 100. The results of separation properties have been discussed in the light of kaolin surface charge, PAM-N and surfactant adsorption including conformation of the adsorbed polymer and properties of solution due to mutual interaction of polymer and surfactants. n nThe kaolin settling rate improves by more than twentyfold through flocculation by PAM-N. Flocculation also reduces the specific resistance to filtration (SRF) from 7.8×1011 to 1.1×1011 m/kg. The high molecular weight polymer, however, entraps excess water in the flocs resulting in very high cake moisture content. Pretreatment with either of the surfactants reduces the adsorption of non-ionic PAM due to blocking of some surface sites by surfactant molecules. The polymer under these circumstances assumes different conformation favouring conditions for better flocculation by bridging and increases settling rate. Though flocculation of the surfactant pretreated kaolin does not reduce SRF, the cake moisture is reduced substantially. n nAddition of PAM-N from a mixture with surfactants leads to increase in PAM-N adsorption on kaolin, but it decreases settling rate as well as moisture content of the filter cake without any change in the SRF value.

Improvement of nonisothermal oxidation behavior of fe and fe-cr alloys by superficially applied reactive oxide coatings

The nonisothermal oxidation behavior of pure iron and a few iron-chromium alloys in dry air has been studied. The effects of a superficial coating of a reactive oxide, CeO2, on the oxidation behavior were studied. Linear heating rates of 3 K/min and 6 K/min were maintained up to a final temperature ranging from 1273–1473 K. Coatings were applied either from a slurry or an aqueous bath. The CeO2 coating has been found to be effective not only in decreasing the nonisothermal oxidation rate but also in improving the scale adherence. Moreover, the coated samples withstood a number of heating cycles without scale rupture. The mass gain of the samples as a function of temperature was recorded by means of a sensitive balance, and the scales have been characterized by SEM, EPMA, and x-ray diffraction analysis.

Stabilization of intrinsic defects at high temperatures in ZnO nanoparticles by Ag modification

The stabilization of defects in ZnO at high temperatures has been investigated. The properties of unmodified and modified ZnO nanoparticles (NPs) with 2 at.% of Ag prepared by microwave assisted combustion method, have been systematically studied using X-ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) and photocatalytic activity measurements. Though the XRD data shows a marginal shift in the ZnO peak position upon Ag addition, the amount of shift does not change with annealing temperatures. The PL data reveals that the defect mediated visible emission intensity of unmodified ZnO NPs increases with increase in the annealing temperature, whereas it remains almost unchanged in Ag-ZnO. This study clearly establishes that silver is an efficient stabilizer of intrinsic defects in ZnO at high temperatures. This is further supported by the core and valence band XPS spectra.

Polymer adsorption: its correlation with flocculation and dewatering of kaolin suspension in the presence and absence of surfactants

Abstract The adsorption characteristics of polyacrylamide flocculants on kaolin surface have been studied at 25 °C as a function of concentration in the presence and absence of surfactants. The adsorption density of flocculants corresponding to maximum settling rate (Γoptfloc) and minimum value of specific resistance of the cake to filtration (ΓminSRF) have been computed and compared with the adsorption density for monolayer coverage (Γ∞). It has been established in this study that the optimum flocculant concentration for the highest settling rate corresponds to about 50% coverage of the solid surface (i.e. Γoptfloc≈Γ∞/2) for untreated as well as surfactant-pretreated kaolin. Flocs suitable for filtration and dewatering are obtained by flocculation of either untreated or surfactant-pretreated kaolin. But in each case, the requirement of polymer concentration for achieving the minimum specific resistance to filtration (SRF) is sufficiently lower than that required for optimum flocculation. The adsorption density of polymer corresponding to minimum SRF is less than about 25% of the plateau adsorption (i.e. ΓminSRF≈0.25 Γ∞). This has been found to be valid for flocculation with any of the anionic, cationic or nonionic polyacrylamide flocculants used in this investigation.

Studies on flocculation and dewatering of kaolin suspensions by anionic polyacrylamide flocculant in the presence of some surfactants

Abstract This paper reports the results of studies on flocculation and dewatering of kaolin suspensions by anionic polyacrylamide (PAM-A) flocculant in the presence of surfactants. They were namely anionic Sodium Dodecyl Sulphate (SDS), cationic Cetyl Trimethyl Ammonium Bromide (CTAB) and a non-ionic surfactant of polyoxyethylenic ether with trade name Triton X-100. The adsorption of PAM-A on kaolin is governed by a combination of both physisorption and chemisorption. The optimum flocculation concentration of PAM-A for fresh kaolin as well as surfactant-pretreated kaolin corresponds to about 50% coverage of the surface (i.e. Γopt. floc≈Γ∞/2). Pretreatment of kaolin with all three surfactants form bigger flocs with PAM-A. The highest settling rate and the minimum specific resistance of the cake to filtration (SRF) are obtained by flocculating the SDS-pretreated kaolin. The flocs thus formed are suitable for filtration and reduce SRF substantially. The PAM-A concentration corresponding to the minimum SRF in all the cases is sufficiently lower than that required for optimum flocculation, and the adsorption density is about 25% of the plateau adsorption density (i.e. Γmin. SRF≈0.25Γ∞). The influence of simultaneous addition of PAM-A and surfactants from their mixtures on flocculation and dewatering of kaolin suspension is dependent on the nature of mutual interaction of PAM-A and surfactant. Simultaneous addition of PAM-A with either SDS or TX 100 increases settling rate, whereas it decreases when added from its mixture with CTAB. The cake moisture content when both polymer and surfactants are present is governed more by the effect of polymer than the surfactant.

Influence of surfactants on flocculation and dewatering of kaolin suspensions by cationic polyacrylamide (PAM-C) flocculant

Abstract This paper reports the result of studies on flocculation and dewatering of kaolin suspensions by cationic polyacrylamide (PAM-C) flocculants in presence of surfactants. The surfactants used were namely anionic sodium dodecyl sulphate (SDS), cationic cetyl trimethyl ammonium bromide (CTAB) and non-ionic TritonX 100, which is a polyoxyethylenic ether compound. The unflocculated kaolin has a very slow settling rate of about 0.03 cm/s and can be improved by more than ten times using PAM-C as a flocculating agent. PAM-C adsorbs on kaolin primarily through electrostatic attraction and the flocculation is governed mainly by charge neutralisation and bridging. Partial pre-coating of kaolin with all three surfactants leads to both physical adsorption as well as chemisorption at PAM-C. Pretreating the kaolin with surfactants can further increase or decrease settling rate depending on the type of surfactant used. However, the flocs thus formed shows better filtration and dewatering behaviour estimated in terms of reduction in specific resistance to filtration (SRF) and cake moisture. The minimum SRF occurs under conditions corresponding to far lower adsorption of PAM-C than that for best flocculation. Simultaneous addition of PAM-C and each of the three surfactants decreases settling rate and sediment volume markedly and does not reduce SRF any further but they all reduce cake moisture substantially. The reduction in cake moisture for different PAM-C: surfactant mixture is in the following order: PAM-C: SDS (1:1)>PAM-C: TX 100(1:1)>PAM-C: CTAB(1:1). Low cake moisture in comparison with PAM-C alone, may be a result of reduction in entrapment of excess water in the smaller flocs formed by simultaneous addition with surfactants and to some extent due to hydrophobicity caused by adsorption of surfactants.