The effect of surfactants on peat dewatering

The effect of cationic, anionic and neutral surfactants on the mechanical dewatering of a highly decomposed fuelgrade peat has been examined. Surfactant adsorption and zeta potential of the peat particles correlated with dewatering of peat samples. The cationic surfactants had large positive effects on dewatering, the anionic surfactants had a negative effect while the neutral surfactant had no effect. The effects of the surfactants could be explained by charge neutralization and double layer suppression. The cationic surfactants were superior to analogous organic salts because of enhanced interaction by the hydrophobic effect.     

The effect of charged additives and heat pretreatment on the dewatering of fuel-grade peat

The addition of salt, mineral acid or polyelectrolyte or the preheating of fuel-grade peat results in significant improvement of the amount of water released during mechanical pressing. The charged species all act by suppressing the charge on the colloidal particles and, for different reasons, organic salts, protons and polyelectrolytes are all more effective than simple inorganic salts. The cations with large organic portions, including surfactants, have an augmented adsorption to the particle surface because of the hydrophobic effect. The polyelectrolytes are strongly associated with the particles because of the chelate effect. Protons are very effective because they can actually neutralize the net charge on the particle surface by forming covalent bonds with the acid functions. The mechanism by which heat pretreatment improves dewatering is not caused by suppression of the surface charge. The effect could be due to melting or softening of peat waxes causing alterations of surface features which improve water release. It is also possible that heating provides sufficient energy to overcome the energy barrier keeping the particles apart.     

Biological additives for improved mechanical dewatering of fuel-grade peat

Several different biological compounds were found to be effective pretreatment agents for the removal of water from highly humified peat by mechanical pressing. The peat was pressed for 2.5 min at 1.96 MPa. The agents added were polymers or surfactants. Two compounds were particularly effective. The cationic polysaccharide, chitosan, increased the amount of water removed by 60% compared with the control. The surfactant from Bacillus subtilis increased the amount of water removed by 〉 50%. The mechanism of dewatering by the chitosan is shown to be coagulation of the colloidal particles of peat suspended in water. This allows efficient phase separation by pressing. It is suggested that the surfactant improves dewatering by increasing the spreading coefficient between the trapped interstitial water and the peat waxes.     

Influence of Impurities on Dispersion Properties of Bayer Alumina

Among the different impurities contained in Bayer alumina (Ca, Fe, Na, Si, Mg), calcium was found to greatly influence the dispersion of alumina particles in water. Up to 90% of calcium is dissolved at acidic pH, whereas calcium remains on the alumina surface in the basic pH range and screens the negative Al–O⁻ charges. The presence of calcium, through reducing repulsive interactions between particles, has a negative effect on the dispersion of alumina. The adsorption of the Na⁺ salt of poly(acrylic acid) (PAA-Na) is strongly influenced by Ca²⁺/PAA-Na interactions in suspension with an increase of the maximum adsorbed amount of PAA-Na in the presence of calcium. The amount of PAA-Na needed to reach a high electrostatic repulsion and a minimum of viscosity is 2 times higher in the presence of 400 pm calcium than for a low calcium content (∼80 ppm). Finally, with an appropriate amount of PAA-Na, a similar state of dispersion can be reached with or without the presence of calcium.     

Distinction between Electrostatic and Electrokinetic Effects on the Permeability of Colloidal Packed Beds

Electrostatic and electroviscous effects can significantly decrease the permeability of packed beds which consist of colloidal particles. This results in poor filterability of colloidal suspensions. Electrostatic effects refer to the dependency of the structure of the packed bed on the particles' tendency to agglomerate or disagglomerate. This tendency is influenced by the particle charge, and thus the pH and the ionic strength of the suspension. Electroviscous effects relate to the increased flow resistance of the pores due to a streaming potential being established when the electrochemical double layer of the particles is sheared off. It is difficult to distinguish electrostatic and electroviscous effects because they are interrelated. The comparison of permeability measurements of both TiO₂ and Al₂O₃ with a classical permeability model demonstrates the influence of the two different effects on permeability.     

DEVELOPMENT AND TESTING OF THE INTERNAL ROTARY COMPRESSION PRESS

The Internal Rotary Compression (IRC) press is based on a concept patented by Anderson Metal Industries, Inc. (AMI) in which one dewatering roll is configured internally and eccentrically to the other. The press can simultaneously apply severe mechanical compression and shear forces to the material being dewatered. This design permits a larger no-slip entry volume for dewatering than that obtainable from conventional external roll presses of the same size.

Bench-scale tests of the IRC press have been conducted at IGT with several different materials including peat, wood fines, and paper waste. Significant dewatering was achieved for each material tested. Dewatered peat moisture contents in the range of 40 to 50 weight percent have been achieved. These results represent a significant improvement over those achievable by conventional continuous presses.

Tests have also been conducted in an IRC prototype press designed to produce 45 kg/h of dewatered peat. Tests with moderately and slightly decomposed peats have achieved pressed peat moisture contents as low as 50 weight percent. This paper presents the results of tests conducted with peat in the bench-scale and IRC prototype presses.     

Dewatering of Biomaterials by Mechanical Thermal Expression

Dewatering by mechanical thermal expression (MTE) for a range of materials is explored using a laboratory-scale MTE compression-permeability cell. It is shown that MTE can be used to effectively dewater a range of biomaterials including lignite, biosolids, and bagasse. The underlying dewatering mechanisms relevant to MTE, namely (1) filtration of water expelled due to thermal dewatering, (2) consolidation, and (3) flash evaporation, are discussed. At lower temperatures, the dominating dewatering mechanism is consolidation, but with increasing temperature, thermal dewatering becomes more important. A major focus is an investigation of the effects of processing parameters, including temperature (20 to 200°C) and pressure (1.5 to 24 MPa), on material permeability, a fundamental dewatering parameter. It is illustrated that permeability is particularly dependent on the processing temperature, owing to changes in both the material structure and the water properties. In addition, a comparison of permeability in the direction of applied force (axial) and perpendicular to the direction of applied force (radial) is presented. It is shown that, due to alignment of particles under the applied force, the permeability and, hence, rate of water removal in the radial direction is greater than in the axial direction. SEM micrographs are presented to illustrate the particle alignment.     

Dewatering of Biomaterials by Mechanical Thermal Expression

Dewatering by mechanical thermal expression (MTE) for a range of materials is explored using a laboratory-scale MTE compression-permeability cell. It is shown that MTE can be used to effectively dewater a range of biomaterials including lignite, biosolids, and bagasse. The underlying dewatering mechanisms relevant to MTE, namely (1) filtration of water expelled due to thermal dewatering, (2) consolidation, and (3) flash evaporation, are discussed. At lower temperatures, the dominating dewatering mechanism is consolidation, but with increasing temperature, thermal dewatering becomes more important. A major focus is an investigation of the effects of processing parameters, including temperature (20 to 200°C) and pressure (1.5 to 24 MPa), on material permeability, a fundamental dewatering parameter. It is illustrated that permeability is particularly dependent on the processing temperature, owing to changes in both the material structure and the water properties. In addition, a comparison of permeability in the direction of applied force (axial) and perpendicular to the direction of applied force (radial) is presented. It is shown that, due to alignment of particles under the applied force, the permeability and, hence, rate of water removal in the radial direction is greater than in the axial direction. SEM micrographs are presented to illustrate the particle alignment.     

Polymer particle adsorption at textile/liquid interfaces: a simple approach for a new functionalization route

The adsorption of amino‐functionalized poly(methyl methacrylate) particles prepared by emulsion polymerization onto polyamide/elastane fibres was investigated. The influence of several physicochemical parameters (pH, ionic strength) on the colloidal stability and surface properties of the cationic latex particles was first examined. Then, particle adsorption was studied as a function of particle concentration, particle size, time, pH and salinity. The results obtained demonstrated that the adsorption process was mainly governed by electrostatic interactions and the adsorption isotherms were analysed on the basis of a Langmuir model. Finally, the effect of washing steps on the adsorbed particles was investigated. The results obtained indicated strong interactions between particles and textile fibres. Copyright © 2012 Society of Chemical Industry     

A chemical and thermal pretreatment process for the dewatering of fuel-grade peat

Using a batch heating system raw peat was heated to temperatures below 200°C before mechanical dewatering. Variations were made in heat up time, residence time and helding temperature. After heat treatment, dewatering could be further enhanced by the addition of chitosan, a polycationic biopolymer.     

Cationic latex: Colloidal behavior and interaction with anionic pulp fibers

Styrene–butadiene latexes were prepared in the absence of an emulsifier using a redox initiator Fe(NO₃)₃/H₂O₂. Their positive charge was supplied by comonomer N,N‐diethyl aminoethyl methacrylate. At a given styrene–butadiene ratio (60/40) the particle size depends on the concentration of the comonomer and initiator. The latexes are stabilized by electrostatic repulsion, and remain positively charged and stable up to pH 8. At a higher pH, they acquire a negative charge and restabilize. The positively charged particles deposit readily on negatively charged fibers dispersed in water, thus covering the fiber surface. Upon dewatering and drying, the particles coalesce, and the fibers become covered by a polymeric film, which improves the interfiber bonding and, consequently, the mechanical properties of the fiber assembly. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1677–1682, 2000     

Influence of the electrokinetic properties of cellulose on the morphology of iron(III) oxide upon template synthesis

Microsized tubes and fibers of iron(III) oxide are obtained by the sol-gel method using cotton cellulose as template. The influence of the electrokinetic properties of the surface of cotton cellulose and sols of nanoparticles, as well as the calcination temperature on morphology and the properties of ceramic fibers, is studied. It is noted that the use of sol with a strongly acidic dispersion medium leads to the formation of microtubes, fully repeating the features of the structure of the original cellulose fiber. The mechanism of the formation of fibers and tubes based on electrostatic interactions is proposed. With an increase of the calcination temperature from 600 to 1200°C, there is an increase in the size of the α-Fe₂O₃ particles, a reduction of the inner diameter of the tubes, and an increase of their mechanical strength.     

Application of Enhanced Vacuum Filtration to Dewatering of Fine Coal Refuse

Abstract

It is difficult to reduce the moisture content of fine coal refuse to a satisfactory level because of the high mineral content and the large capillary forces associated with small particle sizes. An experimental investigation of important operating variables on dewatering of fine coal refuse is reported. The cake permeability, cake formation time and final moisture content are used to measure the efficiency of moisture removal. Factors that were studied are the addition of coarse particles, level of vacuum, pH and the use of coagulants, flocculants and surfactants as additives. Addition of a flocculant was the most effective single means of improving dewatering and the permeability could be increased by more than an order of magnitude and the moisture content lowered by as much as 0.05 kg water/kg dry cake. It was found that the ionic nature and molecular weight of the flocculant, the flocculant dosage, the mixing time and the mixing intensity must be carefully studied to obtain optimal performance.     

A power-law model of electrostatic interactions of colloidal particles in cake layer on membrane surface

The permeability of cake on membrane surface is a crucial factor affecting the efficiency of low pressure membrane process. In this study, a power-law model was suggested for better understanding of the effect of solution chemistry on cake permeability. The model equation was derived from the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and the dimensional analysis of parameters affecting the chemical interactions of colloidal particles. Experimental results from unstirred filtration of submicron latex particles at various ionic strengths were used for model application. The permeate flux as well as specific cake resistance and cake porosity was sensitive to ionic strengths. Different behaviors of cake permeability were observed for non-aggregated and aggregated particles. The proposed model aids to interpret complicated relations of cake permeability in terms of the characteristic ratio, which is defined as the ratio of electrostatic repulsion to van der Waals attraction.     

Dispersion properties of alumina powders in silica sol

The dispersion of alumina powders in silica sol has been investigated by zeta potential, sedimentation, and rheological measurements. Zeta potential of alumina in silica sol changes significantly in comparison with that of alumina in deionized water. This is caused by the absorption of silica colloidal particles with negative charge on the surface of alumina particles. Sol-dispersed alumina slurry shows a minimum in sedimentation volume and viscosity around pH 10. The viscosity depends strongly on the silica sol concentration and reaches a minimum in 10–15 wt.% silica sols. It is proposed that the dispersion and stabilization of alumina particles in silica sol are attributed to the electrostatic and steric effects of the colloidal particles absorption. Effects of pH, solids content and silica sol concentration on the rheological behavior of sol-dispersed alumina slurries are discussed in detail.     

Investigation of the role of interfacial chemistry on particle interactions, sedimentation and electroosmotic dewatering of model kaolinite dispersions

The influence of pulp chemistry on particle interactions and dewatering behaviour of colloidal kaolinite dispersions has been investigated under coagulation conditions. The dispersion shear yield stress, settling rate and consolidation showed strong dependence upon pH and ionic strength, indicating a maximum at ∼pH 3.2 which was established as the isoelectric point (iep) by particle zeta potential analysis. A “gel point” solid concentration at which the dispersion began to be significantly networked and gravity-driven consolidation of the pulp was completely suppressed, occurred at 13 vol.% (∼28 wt.%). The dewatering rates due to coagulation were significantly lower than those commonly achieved by polymeric flocculation, however the sediment consolidation was ∼25% higher when compared with flocculated pulps. Electroosmosis was found to be effective in consolidating pre-sedimented pulps to spadeable pastes (∼30 vol.%) at pH values away from the iep where zeta potential was higher and ionic strength low (10^− 3 M). This pulp consistency or markedly improved consolidation behavior is not achievable under coagulation and/or flocculation conditions.     

木质素磺酸钠在TiO2/水界面的吸附特性

木质素磺酸钠在固体表面的吸附特性决定了其应用性能,利用红外和紫外分光光度仪,采用剩余质量分数法研究了温度、pH值、无机盐和氢键破坏剂脲对木质素磺酸钠在TiO₂/水界面吸附动力学和等温吸附性能的影响,初步探讨了其在固液界面的吸附作用机理。结果表明,该吸附为单层多点式吸附,随着温度升高和pH值减小,木质素磺酸钠在TiO₂/水界面的吸附速率常数和饱和吸附量均增大,而离子强度的增大和脲的加入却使吸附速率常数减小;木质素磺酸钠在TiO₂/水界面的吸附驱动力为静电、疏水和氢键作用,疏水作用力可显著增加其吸附量。     

Prediction of the air consumption when dewatering a filter cake obtained by pressure filtration

In this communication, we report some data for the dewatering of a coal flotation concentrate in which very fine particles occur Dewatering is carried out by pressure filtration (up to 6 bar) followed by blowing to partially dry the cake (the filtration and dewatering phase respectively). For the systems studied, a reasonable prediction of the air consumption in either a lab filter, or a one-element test filter, or a full-scale continuous filter is possible, based on measurement of the liquid permeability for saturated flow in a lab filter cake. Together with scaling-up characteristics of the filtration step (not reported here), this gives the possibility of a reliable prediction of the required size and energy consumption of a large-scale continuous-pressure filter based on bench-scale test data.     

Anwendung von Tensiden bei der mechanischen Flüssigkeitsabtrennung

Use of Surfactants for Mechanical Liquid Separation . Mechanical dewatering of finegrained solid materials can be improved by the addition of surfactants. As interface-active substances surfactants reduce surface tension and consequently the capillary forces within the bed of solid particles. A hydrophobization of the solid surface due to the adsorption of surfactants may contribute to further reduction of moisture as well. The development of non-foaming surfactants enables the use without distrubing other ongoing processes by the formation of foam. Investigations on the interaction between the adsorption behaviour, type of material surface, and contact time during dewatering processes support the optimization of dewatering agents. The use of surfactants is an attractive means of improving or replacing other methods of mechanical dewatering.