Laxmidhar Besra

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.

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. The 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. Addition 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.

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.

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.