Influence of fly ash and steam on microstructure, and mechanical properties of oxide bonded porous SiC ceramics

Ceramic filters specially SiC filters are used in advanced coal combustion and gasification processes to remove fine dust particles from the fuel gas at high temperatures and high pressure for protection of turbine blades and other downstream components from corrosion and erosion and to meet the environmental regulations. Processing of corrosion resistant porous SiC ceramics at low temperature using a simple technique is still challenging. In this study oxide bonded porous SiC ceramics were synthesized by cost effective method. The corrosion behaviour of SiC ceramic filter materials in presence of steam, coal ash and both coal ash and steam was investigated at 1000 degrees C for 96-240 h. The apparent changes in mass, porosity and density with corrosion duration and environment were recorded. Finally SEM, XRD and mechanical tests of the corroded samples were carried out. The corrosion test results indicated water vapour is the perpetrator for strength degradation. (C) 2019 SECV. Published by Elsevier Espana, S.L.U

EFFECT OF SiC PARTICLE SIZE ON THE MATERIAL AND MECHANICAL PROPERTIES OF MULLITE BONDED SiC CERAMICS PROCESSED BY INFILTRATION TECHNIQUE

The influence of SiC particles size on the bonding phase content, microstructure, SiC oxidation degree, flexural strength, porosity and pore size distribution of mullitebonded porous SiC ceramics were studied and compared with oxidebonded porous SiC ceramics. The SEM morphologies and EDS elemental analysis results showed the presence of needle shaped crystals of mullite and fish scaled cristobalite in the bond phase. It was found that increase of SiC particle size effectively enhanced the porosity and decreased the strength. The porosity decreased as the size of SiC particles decreased from 36 vol. % at 99 μm to 25 vol. % at 4.47 μm. The oxidation degree of SiC was found to be reduced by infiltration of mullite precursor sol and enhanced with sintering temperature. Bimodal pore size distributions were obtained for mullite-bonded porous SiC ceramics and the average pore diameter varied in the range of 2 - 30 μm with variation of particle size

Studies on permeability properties and particle capture efficiencies of porous SiC ceramics processed by oxide bonding technique

Porous SiC ceramics bonded with mullite (MBS of fractional porosity (e) of 0.29-0.56, average pore size (dpore) of 5-11 μm, flexural strength (σ) of 9-34 MPa and elastic modulus (E) of 7-28GPa) and cordierite (CBS with e of 0.33-0.72, dpore of 6-50 μm, σ of 5-54 MPa and E of 6-42 GPa) were prepared by heating in air at 1350-1500°C compacts of desired amounts of SiC, Al2O3 and MgO powders and petroleum coke dust as the pore former. Air permeation behavior of well-characterized samples was studied with fluid superficial velocity (vs) from 0.08 to 1.0 m s-1and at RT to 750°C. The Darcian (k1) and non-Darcian (k2) permeability coefficients were evaluated by fitting the Forchheimer’s equation to experimental pressure drop-superficial velocity data. Porosity dependence of permeability coefficients was explained in terms of structural characteristics. Changes in pressure drop experienced by the porous ceramics at high temperatures were explained by temperature dependence of permeability coefficients and variation of fluid properties. Collection efficiency (η) of filter ceramics operating on removal of solid NaCl nanoaerosol particles (of 7-300 nm size) was determined from particle counts before and after filtration at vs = 0.05-0.10 m s-1. Experimental results showed variation of η from 96.7 to 99.9% for change of e from 0.56 to 0.68. The size-selective fractional collection efficiency at different porosity levels was derived using the well-known single-collector efficiency model considering some boundary conditions and the model data were validated with experimental results. The test results were used to examine the applicability of the filter ceramics in nanoparticle filtration processes

Permeability and nanoparticle filtration assessment of cordierite-bonded porous SiC ceramics

Cordierite bonded porous SiC ceramics having pore fractions (epsilon) between 0.33 and 0.72 and pore sizes of 6-50 mu m, flexural strength of 5-54 MPa, and elastic modulus of 6-42 GPa were prepared by oxide bonding at 1350 degrees C in air compacts of SiC, Al2O3 and MgO powders with petroleum coke (PC) as the sacrificial pore former. To test the applicability of the porous ceramics in the fluid flow field, air permeation behavior was studied with fluid superficial velocity from 0.083 to 0.90 m s(-1) and at 26-750 degrees C. The Darcian, k(1), and the non-Darcian, k(2), permeability coefficients were evaluated by fitting Forchheimer's equation to the experimental results. The temperature dependence of the permeability coefficients was explained from structural changes occurring during test conditions. The collection efficiency of filter ceramics (epsilon = 0.62-0.68) operating on removal of nanosized aerosol particles with sizes varying from 7 to 300 nm was determined by counting particles before and after filtration at a fluid superficial velocity of 0.1 m s(-1). Experimental results showed variation of collection efficiency from 96.7 to 99.9%. The size-selective fractional collection efficiency at different porosity levels was derived by using the well-known single-collector efficiency model considering some boundary conditions, and the model data were validated with experimental results. The test results were used for examination of the applicability of the filter ceramics in nanoparticle filtration processes

Quantifying uncertainty in the measurement of arsenic in suspended particulate matter by Atomic Absorption Spectrometry with hydride generator

Arsenic is the toxic element, which creates several problems in human being specially when inhaled through air. So the accurate and precise measurement of arsenic in suspended particulate matter (SPM) is of prime importance as it gives information about the level of toxicity in the environment, and preventive measures could be taken in the effective areas. Quality assurance is equally important in the measurement of arsenic in SPM samples before making any decision. The quality and reliability of the data of such volatile elements depends upon the measurement of uncertainty of each step involved from sampling to analysis. The analytical results quantifying uncertainty gives a measure of the confidence level of the concerned laboratory. So the main objective of this study was to determine arsenic content in SPM samples with uncertainty budget and to find out various potential sources of uncertainty, which affects the results. Keeping these facts, we have selected seven diverse sites of Delhi (National Capital of India) for quantification of arsenic content in SPM samples with uncertainty budget following sampling by HVS to analysis by Atomic Absorption Spectrometer-Hydride Generator (AAS-HG). In the measurement of arsenic in SPM samples so many steps are involved from sampling to final result and we have considered various potential sources of uncertainties. The calculation of uncertainty is based on ISO/IEC17025: 2005 document and EURACHEM guideline. It has been found that the final results mostly depend on the uncertainty in measurement mainly due to repeatability, final volume prepared for analysis, weighing balance and sampling by HVS. After the analysis of data of seven diverse sites of Delhi, it has been concluded that during the period from 31st Jan. 2008 to 7th Feb. 2008 the arsenic concentration varies from 1.44 ± 0.25 to 5.58 ± 0.55 ng/m3 with 95% confidence level (k = 2)

Stepwise complexometric determination of aluminium, titanium and iron concentrations in silica sand and allied materials

<p>Abstract</p> <p>Background</p> <p>This study aimed at measuring the quantities of Al, Ti and Fe in silica sand and allied materials employing a complexometric method in the same analyte and a stepwise indirect titration with EDTA. The method involves the complexation of Al, Ti and Fe with excess EDTA and the selective de-complexation of TiO-EDTA and Al-EDTA complexes with tartaric acid and NaF respectively. In addition to its simplicity, rapidity and accuracy, the proposed method does not require the use of a separation technique or any sophisticated instrumentation.</p> <p>Results</p> <p>Each of the test samples were analyzed five times using the proposed method. The method's accuracy was confirmed by analyzing the US National Institute of Standards and Technology's (NIST) Standard Reference Materials (SRM) 81a, 89 and IPT SRM 61 using the procedure proposed, in addition to analyzing Ti and Fe levels by spectrophotometry and that of Al by complexometry.</p> <p>Conclusion</p> <p>The study shows that there is good agreement between the proposed and existing methods. The standard deviations of the measurements were calculated by analyzing five replicates of each sample, and were found to be less than 1.5% in our method.</p

Selective masking and demasking for the stepwise complexometric determination of aluminium, lead and zinc from the same solution

Background: A complexometric method based on selective masking and de-masking has been developed for the rapid determination of aluminium, lead and zinc from the same solution in glass and glass frit samples. The determination is carried out using potassium cyanide to mask zinc, and excess disodium salt of EDTA to mask lead and aluminium. The excess EDTA was titrated with standard Mn(II)SO(4) solution using Erichrome Black-T as the indicator. Subsequently selective de-masking agents - triethanolamine, 2,3-dimercaptopropanol and a formaldehyde/acetone mixture - were used to determine quantities of aluminium, lead and zinc in a stepwise and selective manner. Results: The accuracy of the method was established by analysing glass certified reference material NBS 1412. The standard deviation of the measurements, calculated by analysing five replicates of each sample, was found to be less than 1.5% for the method proposed. Conclusion: The novelty of the method lies in its simplicity and accuracy afforded by there not being a need for a prior separation or instrumentation. The proposed method was found to be highly selective for the precise determination of aluminum, zinc and lead in the routine analysis of glass batch and allied materials

Thermal shock and chemical corrosion resistance of oxide bonded porous SiC ceramics prepared by infiltration technique

Using the water- and air-quenching technique, the thermal shock resistance to cooling was evaluated as a function of quenching temperatures and quenching cycles for oxide bonded porous SiC ceramics prepared by infiltration technique. It was observed that the residual strength of the quenched samples decreases with increase in the quenching temperature. However residual strength becomes independent of quenching cycles after certain cycles since the thermal shock produced by repeated cycles remained almost constant. The hot corrosions of SiC samples exposed to Na2SO4 salt were performed at 1000 degrees C. The weight loss, strength reduction and morphology evolution of the SiC specimens during corrosion were revealed and explained. The chemical and thermal shock resistance results suggest a potential advantage of porous SiC ceramics prepared by infiltration technique for several industrial applications. (C) 2018 Elsevier B.V. All rights reserved

Permeability and dust filtration behaviour of porous SiC ceramic candle filter

Porous SiC candle filter (similar to 660 mm L and 75 mm OD) were fabricated by ramming process using commercially available SiC powder (d(50) = 212 mu m), with and without alumina and small amount (3 wt %) of clay as the major binder phase additives, following heat treatment at 1400 degrees C in air. Depending on the composition, porosity of the candle filters varied from 36 to 39 vol% and C-ring strength varied from 15 to 23 MPa. The air permeability and dust filtration efficiency of the candle filter were evaluated using laboratory made test set up. At room temperature, Darcian (k(1)) and non-Darcian (k(2)) permeability parameters varied from 1.9 to 2.2 x 10(-12) m(2) and 5.4-9.72 x 10(-8) m, respectively. Airborne fly ash particle filtration tests showed good performance of SiC candle filter with filtration efficiency of >97%. (C) 2020 Elsevier Ltd. All rights reserved

Dispersion of SiC powder suspension in mullite sol and influence on properties of sintered ceramics

The dispersion behavior of SiC powders in various concentration of mullite sol was studied by zeta potential, rheological, and sedimentation measurements. Effect of pH, sol concentration, solid loading on the rheological properties are discussed in detail. The lowest viscosity and sedimentation was obtained for 45 wt% SiC powders dispersed in 15 wt% mullite sol, at pH similar to 2. The adsorption isotherms belong to Langmuir type and optimum concentration of mullite for maximum dispersion was found to be similar to 0.036 g/m(2). The stabilization mechanism of SiC powder into mullite precursor sol was explained by Derjaguin-Landau-Vervey-Overbeek (DLVO) theory. Coating was confirmed by transmittance electron microscopy and energy-dispersive X-ray spectroscopy analysis. Further sintering of the mullite-coated SiC powder compact at 1300 degrees C produced porous SiC ceramic with porosity similar to 40 vol% and pore diameter similar to 3 mu m. The corrosion behavior of the ceramics was studied in strong acid medium at 90 degrees C and compared with the ceramics obtained from uncoated SiC powder