Bimal P. Singh

The role of surfactant adsorption in the improved dewatering of fine coal

Abstract This paper describes the adsorption kinetics and isotherms for cationic and anionic surfactants on coal surfaces. Both types of surfactants adsorb significantly rapidly; although the cationic surfactant achieved much higher levels of adsorption than the anionic one. The adsorption isotherms are of typical Langmuir type and employed for calculation of free energies. The results indicate that adsorption occurs through hydrophobic interaction in addition to electrostatic attraction between the surfactant molecules and the coal surface. Surfactant adsorption is shown to be a significant factor in the improved dewatering of coal particles. These improvements have been attributed to the changes which surfactant adsorption causes in the wetting characteristics of the coal surfaces. The results obtained are evaluated in terms of surface effects and by reference Laplace–Young equation. Furthermore, this equation has been used to quantify dewatering phenomena.

Factorial design of experiments on the effect of surface charges on stability of aqueous colloidal ceramic suspension

The present investigation was aimed at studying the synergistic effects of different surface-active agents (SAA) on the stability of aqueous alumina suspensions. The SAA used were dibasic ammonium citrate as dispersant, albumin as binder and octanol-2 as an antifoaming agent. Stability of suspensions was assessed through measurement of the specific surface charge of the system, using the fact that the particles exhibit maximum repulsion between them at high magnitude of surface charge. The experiments were statistically designed to bring out the effect of each variable as well as the effect due to interactions. The results show that the strongest affects on stability of aqueous colloidal suspension is that of dibasic ammonium citrate and albumin. The effect of interactional parameters, though slightly favourable for increasing the stability of the suspension, is very small. Dibasic ammonium citrate and albumin, both highly negatively charged SAA, stabilise the suspension by increasing the overall surface charges of the ceramic particles which inturn increases the overall repulsive forces. On the other hand, the positive charge of octanol-2 neutralises/decreases the magnitude of negative surfaces charge and, consequently, decreases the stability of the suspension. It has been shown that factorial design of experiment approach is very helpful in defining conditions for an optimum performance in such complex systems.

Use of surfactants to aid the dewatering of fine clean coal

Vacuum filtration of clean coal slurries and flocculated slurries with cationic, anionic and nonionic surfactants has been investigated. Significant reductions in filter cake moisture content were obtained for all types of surfactant. The most effective surfactant was CTAB. The final moisture content of filter cakes was reduced from 21 to 11.7 wt% by using a moderate dosage of flocculant followed by surfactant pretreatment. Measurements of electrophoretic mobility, surface tension, adsorption, dewatering kinetics and kinetics of adsorption of surfactants were carried out. A direct correlation exists between the point of zero charge and residual moisture reduction in the filter cake. Test results show that surface tension by itself does not give much indication of the effectiveness of surfactants as dewatering agents. The enhancement in filtration dewatering characteristics has been attributed to the changes that surfactant adsorption causes in the wetting characteristics of the coal surface.

Influence of surface charge on maximizing solids loading in colloidal processing of alumina

Abstract Stability of concentrated aqueous colloidal alumina powder suspensions with and without dispersant (albumin and dibasic ammonium citrate (DAC)) has been investigated by measuring surface charge at different solids loading and pH values. The iso-electric point (iep) of alumina powder was found to be pH iep =8.5. The surface charge of alumina powder changed significantly with anionic polyelectrolyte and iep shifted towards more acidic pH range under different dispersion conditions. The study illustrated that albumin is more effective as dispersant than DAC at all solid loading conditions. The computed specific interaction energies −Δ G SP 0 of albumin (7.5) and DAC (3.5) indicate strong adsorption of albumin compared to DAC on alumina powder surface, leading to higher solid loading.

Influence of surfactants on filter cake parameters during vacuum filtration of flocculated iron ore sludge

Abstract The influence of certain surfactants on the dewatering of fine iron ore sludge flocculated with anionic flocculants has been investigated in a laboratory-scale vacuum filtration unit. The filtration data have been used in calculating the cake parameters from an integrated form of Darcys law. A narrow concentration range of surfactant enables reduction of the specific cake resistance and an enhanced rate of filtration. The non ionic surfactant polyethylene oxide (PEO) was most effective in reducing the moisture content of the cake. Filter cake with 12.6% moisture content has been obtained using this surfactant. The surface tension reduction mechanism is responsible to some extent for the enhanced dewatering, but does not account for all the effects. A complex surfactant adsorption mechanism in the presence of flocculants seems to play a greater role in enhancing dewatering.

Optimisation of performance of dispersants in aqueous plasma dissociated zircon suspension

Abstract Plasma dissociated zircon (PDZ) is a unique product obtained during in-flight dissociation of zircon sand in a plasma reactor. In PDZ, submicron zirconia crystallites are intimately mixed with glassy silica. This paper describes the selection and optimisation of dispersants in controlling stability of an aqueous suspension of PDZ. It has been observed that inorganic dispersants and organic additives give clearly distinct behaviour. In both cases, optimum dispersant dosages can be obtained from the streaming potential characteristics. It is shown that there is a critical concentration of each dispersants, which gives the highest streaming-potential/surface charge and hence optimum performance. Studies showed that the isoelectric point of PDZ particulate is at pH 2.8. Additions of dispersant increase the charge in magnitude and modified the surface to have more negative charge in the pH range 2.8–9.4. An assessment of particle/additive interactions, overall stability, and optimised chemical additive required to obtain homogeneous, well-stabilized suspension can be derived from the streaming potential characteristics.

Stabilization of Aqueous Silicon Nitride Suspension with Dolapix A88

The dispersing phenomenon of silicon nitride suspension has been investigated systematically by measuring surface charge density, sediment volume, and turbidity of the suspension. Dolapix A88 has been used as dispersant to improve the stability of the suspension, and assessed through measurement of the specific surface charge of the Si3N4 system. The isoelectric point (IEP) of Si3N4 powder has been found to be at pHiep=6.91, and there was no shift in IEP due to the presence of dispersant, indicating that the interaction between the powder and the dispersant Dolapix A88 is purely physisorbing in nature. Good agreement has been observed among dispersion characteristics, surface charge, and streaming potential inflection. The article also describes the selection and optimization of the dispersant in controlling the stability of an aqueous suspension of Si3N4. It can be concluded from this investigation that the PCD technique for measurement of specific surface charge can be conveniently used for the assessment of dispersibility of ceramic suspensions in dilute as well as in concentrated suspensions.

Characterization and Dispersion of Multiwalled Carbon Nanotubes (MWCNTs) in Aqueous Suspensions: Surface Chemistry Aspects

Dispersability of multiwalled carbon nanotubes (MWCNTs) in aqueous suspensions was investigated with gum arabic GA (anionic) and polydiallyldimethylammonium chloride (PDADMAC) cationic dispersants. The isoelectric point (iep) of MWCNTs was at pH 4.5. The surface charge of MWCNTs changed to highly negative and iep shifted to pH < 1.5 with GA, whereas with PDADMAC highly positive and iep is shifted to pH 10.2. The interaction energy of MWCNTs with GA and PDADMAC was 9.79 and 10.35 RT unit, respectively. The higher interaction energy with PDADMAC shows more favorable interaction with them. Stabilization mechanism with GA is attributed mainly by electrostatic repulsive forces, whereas with PDADMAC, electrostatic and steric effect.

Performance Evaluation of Dispersants Through Streaming Potential Measurements

Stability of 5% (w/v) colloidal alumina suspension was examined in the presence of three organic dispersants; Dispex N‐40, Darvan C, and tribasic ammonium citrate (TAC), and three inorganic dispersants; sodium tri‐polyphosphate, sodium silicate, and Calgon (sodium hexameta phosphate). The performance and the required dosage for the maximum dispersibility were evaluated by streaming potential measurement. It was shown that there is a critical concentration of each dispersant indicated by the attainment of the highest streaming potential for maximum stability conditions of the suspension. Dispersion quality of the suspension in the presence of dispersant Darvan C was investigated in detail using the sedimentation technique. It was observed that the magnitude of the charge developed on a particle surface at the dosage indicated by the inflection point is the minimum dosage required for the maximum stability of the suspension.

Investigation on the Effect of Ultrasonic Pretreatment on Selective Separation of Iron Values from Iron Ore Tailings by Flocculation

Abstract Selective flocculation studies were carried out on Barsuan iron ore tailings having 50.5% iron, 7.2% alumina, and 7.8% silica in the absence and presence of ultrasonication at different experimental conditions using sodium hexametaphosphate as a dispersant and starch as a flocculant. The imposition of ultrasonication resulted in a marked improvement in grade as well as recovery. The results indicate that ultrasonication is able to mitigate the adverse effects of gangue materials. A concentrate assaying 59% iron with 75% recovery could be obtained without ultrasonication, while the application of ultrasonication led to an improvement in concentrate grade to 65% with a corresponding increase in recovery to 91%. The improvement is attributed to the disruption caused at a solid/liquid interface which results in favorable modification of the mineral surface, facilitating selective adsorption of flocculant on the iron ore fines. This, in turn, leads to enhancement in selectivity and recovery. In additi...