Electrochemical Aspects of Electroosmotic Dewatering of Clay Suspensions

ABSTRACT

An attempt is made to elucidate the electrochemical aspects of the electroosmotic dewatering (EOD) of clays as reported in some recent work, especially that on interrupted DC power electroosmotic dewatering published by Rabie, Mujumdar and Weber (2). These authors showed that the dewatering by EOD stops after the DC power has been on for several minutes or hours; on interruption of their power and on short-circuiting of the electrodes, conditions can be created again for some further dewatering by DC power EOD. This discovery, of Rabie et al. is interpreted as a fuel cell effect and it is shown that it affords clues to several other electrochemical strategies for the possible enhancement of the efficiency of the EOD by DC power.

Further, the open circuit potentials observed by Rabie et al. (2) on the interruption of DC power are given an electrochemical interpretation which leads to quantitative estimates in agreement with the experimental values.     

SOME OBSERVATIONS ON ELECTROOSMOTIC DEWATERING (EOD) UNDER A.C. ELECTRIC FIELD CONDITIONS

ABSTRACT

The electroosmotic dewatering (EOD) reported by Yoshida et al. (3) under low-frequency A.C. conditions is shown to be equivalent to EOD carried out under D.C. conditions with periodic polarity reversal. The interpretation of their results is carried out here in terms of electrochemical considerations (e.g., pH gradients, zeta potentials) with lesser role assigned to factors considered paramount by Yoshida, e.g., heating effect in the dry region near the anode.

More importantly, it is pointed out that the practical advantages of EOD under A.C. conditions claimed by Yoshida et al. are based on an invalid comparison of their results with EOD under continuous D.C. conditions; a comparison under equivalent conditions would be between low-frequency A.C. conditions and those under D.C. field but with periodic polarity reversal — the enhanced dewatering claimed under “A.C. conditions” by Yoshida would disappear when EOD under D.C. conditions with periodic interruption and/or polarity reversal is considered.     

Electrode Kinetic and Electro-Kinetic Effects in Electroosmotic Dewatering of Clay Suspensions

Lockhart's remarks (1) on our previous interpretation (2) of the electrochemical aspects of the electroosmotic dewatering (EOD) of clay suspensions are analyzed to provide some further clarification. Based on Lockhan's excellent work (1, 3-8), we put forward here novel electrochemical interpretations of some features of the following experimental observations:

(i) Galvani dewatering;

(ii) The dewatering efficiency:

(iii) High voltage needed for dewatering Al-kaolinite and aluminium electrode effect.     

A New Theoretical Approach To Electroosmotic Dewatering (Eod) Based On Non-Equilibrium Thermodynamics☆

ABSTRACT

A novel theoretical approach to electroosmotic dewatering (EOD), with or without a pressure gradient, of clays, sludges and other colloidal suspensions is proposed. The treatment is based on nan-equilibrium thermodynamics as developed in the work of Overbeek, De Groot and others. The interpretation of electrokinetic phenomena in terms of the cancepts of irreversible thermodynamics when combined with Onsager's relations, it has been shown by Overbeek, provides a complete framework for understanding all electrokinetic phenomena.

We have applied this approach here to the electroosmotic dewatering. both in the presence and absence of applied hydrostatic pressure.

The approach provides much clarification on the nature and significance of currents and fluxes observed during EOD: these are composed of three components, during combined pressure electroosmotic dewatering: (i) electrochemicavelectrical current; (ii) hydrodynamic flux: (iii) electroosmotic current.

We have also shown the manner in which the proposed new approach to EOD based on irreversible thermodynamics can be connected to the conventional approach based on the Helmholzu-Smoluchowski equation.     

ELECTROOSMOTIC DEWATERING (EOD) OF CLAYS AND SUSPENSIONS: COMPONENTS OF VOLTAGE IN AN ELECTROOSMOTIC CELL

In the electroosmotic dewatering (EOD) of clays, suspensions and fines, a continuous or interrupted DC voltage, V, is applied between the anode and the cathode. In the previous literature on this subject the components of V have not been completely clarified.

Following a brief outline of the EOD that emphasizes the central role of the electrochemical double layer, we provide an analysis of the components of V, Three case are distinguished

(1)Threshold voltage V-n, for initiating the EOD and its connection to the so-called open-circuit voltage;

(2)EOD at very low voltages viz. around IV;

(3)EOD at very high rates for prolonged times where electrodes such as Al can form barrier oxides and some “caking” is also observed near the a node, This attempt to define the components of V under different conditions of EOD is aimed to identify parameters and processes that arc involved in the effect of V on EOD; RT for example, it is generally not recognized that —pH can form a significant part of V F under some conditions of EOD     

A review of “ MODERN COATING AND DRYING TECHNOLOGY” Edward Cohen and Edgar Gutoff VCH Publishers,Inc New York NY 1992, 310 p

ABSTRACT

Recently some workers in the field of electrostatics have proposed a “new” method of electroosmotic dewatering (EOD) in which a third electrode (called the “gate” electrode) is placed between the anode and the cathode to enhance the EOD. These authors present a conceptual analysis in terms of the notions of a field-effect transistor (FET). We show here that the proposed method is simply a variation of the multistage EOD already practiced by the workers in this field. It is further demonstrated that an analysis based on the FET model is not applicable to the phenomena involved in the proposed EOD method.     

ELECTROOSMOTIC DEWATERING BY A “NEW” METHOD USING A “GATE” ELECTRODE: FIELD EFFECT TRANSISTOR (FET) MODEL OR SIMPLY A MULTISTAGE DEWATERING?

Recently some workers in the field of electrostatics have proposed a “new” method of electroosmotic dewatering (EOD) in which a third electrode (called the “gate” electrode) is placed between the anode and the cathode to enhance the EOD. These authors present a conceptual analysis in terms of the notions of a field-effect transistor (FET). We show here that the proposed method is simply a variation of the multistage EOD already practiced by the workers in this field. It is further demonstrated that an analysis based on the FET model is not applicable to the phenomena involved in the proposed EOD method.     

ELECTROOSMOTIC DEWATERING (EOD) OF CLAYS AND SUSPENSIONS: COMPONENTS OF VOLTAGE IN AN ELECTROOSMOTIC CELL

ABSTRACT

In the electroosmotic dewatering (EOD) of clays, suspensions and fines, a continuous or interrupted DC voltage, V, is applied between the anode and the cathode. In the previous literature on this subject the components of V have not been completely clarified.

Following a brief outline of the EOD that emphasizes the central role of the electrochemical double layer, we provide an analysis of the components of V, Three case are distinguished

(1)Threshold voltage V-n, for initiating the EOD and its connection to the so-called open-circuit voltage;

(2)EOD at very low voltages viz. around IV;

(3)EOD at very high rates for prolonged times where electrodes such as A𝓁 can form barrier oxides and some “caking” is also observed near the a node, This attempt to define the components of V under different conditions of EOD is aimed to identify parameters and processes that arc involved in the effect of V on EOD; RT for example, it is generally not recognized that —pH can form a significant part of V F under some conditions of EOD     

ELECTROCHEMICAL TREATMENT OF TUMORS (ECT): ELECTROOSMOTIC DEWATERING (EOD) AS THE PRIMARY MECHANISM

ABSTRACT

A recent series of experimental papers on the electrochemical treatment (ECT) of tumors provide empirical data and observations amenable to a theoretical analysis of this phenomenon from an electrochemical point of view. An attempt is made here to develop the idea that ECT is a case of electroosmotic dewatering (EOD) of the tumor tissue with the consequent changes in pH, with the concomitant role of reactions at the electrodes. Some speculative suggestions are made to enhance the efficacy of the ECT process; in particular, a multi-electrode system is proposed to achieve complete necrosis of a large tumor     

ELECTROOSMOTIC DEWATERING PROCESS AND DESIGN EQUATIONS

This paper deals with electroosmotic dewatering that can be very effective for sludge which is difficult to dewater by conventional mechanical techniques. First, the dewatering mechanism of sludge due to electroosmosis is analyzed theoretically based on an electroosmotic flow through a compressible packed bed of particles, and then its processes under the conditions of both constant electric current and constant voltage are discussed both theoretically and experimentally. Further theoretical expressions for the dewatering rate and the electric power consumption are proposed using a simple model in which the sludge bed consists of two parts: a dewatering bed and a dewatered bed. The equations obtained theoretically are demonstrated to be applicable to the design of electroosmotic dewatering equipment.     

高盐高有机制药废水污泥电渗透高干脱水

为研究高盐高有机制药废水污泥的电渗透脱水效果,深入认识化学污泥的电脱水过程,本文采用电渗透高干脱水技术对经抽滤脱水的高盐高有机制药废水化学污泥进行深度脱水,考察了泥饼初始pH的改变对污泥电渗透高干脱水过程中阴阳极污泥的含水率、电流、电导率、pH、zeta电位与能耗的影响,验证了对高盐高有机制药废水污泥实行电渗透高干脱水的可行性,解析了化学污泥电渗透脱水过程的机制。结果表明,泥饼pH为2、3、4时,zeta电位为正值,电渗流反向流动,无法脱水;pH增至5时,zeta电位为负值,电渗流从阴极脱除,污泥含水率从53.2%降至44.8%,脱水效果最好;但pH增至6时,脱水量有所降低。污泥电导率随pH的增加而降低。pH为5时初始电流最大。脱水15min时,即污泥含水率降至45.5%时,能源利用率最高。     

Effect of Operating Parameters on the Effectiveness of Electric Field-Enhanced Separations

The purpose of this work is to summarize our latest research results about constant-current electroosmotic dewatering (EOD). The Terzaghi-Voigt combined model is used for the analysis of EOD of various solid-liquid systems by considering the creep deformation of the material. Bentonite clay, KC-flock, zinc oxide, and some of their mixtures are used as experimental materials. It is assumed that the electroosmotic pressure gradient E _pg and modified consolidation coefficient C _e of the materials are constant. The relation between the ratio of creep deformation to total deformation B and the modified consolidation coefficient C _e is determined and compared with previous observations for the case of mechanical expression. The effectiveness of dewatering is described depending on operating parameters such as preconsolidation pressure, material composition, electric current density, and total solid volume per unit cross-sectional area. The model is verified by comparing the calculations with experimental data.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used.

The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.     

阴极滤布对活性污泥电渗透脱水的影响(英文)

The mechanical dewatering of activated sludge is troublesome due to its high compressibility of solids. The dewatering can be enhanced dramatically by the use of electroosmosis,in which an electric field is applied to the sludge cake.In this study,the influence of filter cloth on the cathode on the dewatering of activated sludge was investigated.It was found that thicker filter cloth led to lower water removal from the sludge cake,so a stainless steel cathode net with small pore size instead of filter cloth was applied,which improves the dewatering efficiency and reduces the electric power consumption.Moreover,water absorbent materials were helpful to remove the water from the sludge cake.For the electroosmotic dewatering at 7 kPa and 24 V·cm-1,the water content in the sludge cake decreased to 60%(by mass) with the average 0.075 kW·h·kg-1of water removed by using the cathode net.     

Evaluation of Suitable Material Properties of Sludge for Electroosmotic Dewatering

To estimate suitable material properties of sludge for electroosmotic dewatering, we experimentally measured the electrical and physical properties of a sludge bed. The use of an upper electrode that had a smaller area than the cross-sectional area of the bed was also investigated with the aim of reducing the electrical contact resistance between the electrode and the bed caused by dewatering. We found that sludge with a lower specific electrical resistance and a higher hydrodynamic compressibility in cake filtration provided greater electroosmotic dewatering performance. However, such a sludge was not necessarily available for using a smaller-area upper electrode.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE *

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used. The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE*

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used. The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.

     

微波能对化学污泥脱水性能的影响

研究大豆蛋白污水化学污泥的脱水性能，考察微波辐射的功率及时间对污泥脱水性能的影响，确定功率和时间的最佳条件。研究120s内污泥沉降、过滤脱水及比阻的变化，并探讨污泥胞外聚合物含量和污泥颗粒粒径的变化相关机理。结果表明：适宜的微波辐射可明显改善污泥的脱水性，800W、100W和60W微波改善污泥脱水性能的最适宜时间分别为60s、120s和120s，上清液中多糖的含量介于460～470mg／L时污泥脱水性能最佳。适宜的微波辐射能够改善污泥的脱水性能，过量的微波辐射则使污泥脱水性恶化。     

ELECTROOSMOTIC DEWATERING PROCESS AND DESIGN EQUATIONS

This paper deals with electroosmotic dewatering that can be very effective for sludge which is difficult to dewater by conventional mechanical techniques. First, the dewatering mechanism of sludge due to electroosmosis is analyzed theoretically based on an electroosmotic flow through a compressible packed bed of particles, and then its processes under the conditions of both constant electric current and constant voltage are discussed both theoretically and experimentally. Further theoretical expressions for the dewatering rate and the electric power consumption are proposed using a simple model in which the sludge bed consists of two parts: a dewatering bed and a dewatered bed. The equations obtained theoretically are demonstrated to be applicable to the design of electroosmotic dewatering equipment.     

Electroosmotic Dewatering of Tomato Paste Suspension Under AC Electric Field

Laboratory tests have demonstrated electroosmosis under an alternate current (AC) to be effective for dewatering tomato paste suspension. Application of an alternate current (AC) field provided more promising results than those of direct current (DC) electrical field. The results show that the electroosmotic dewatering is influenced by the wave form (sine, rectangular, triangular). The decrease in the wave frequency resulted in an increase in the dewatering rate, whereas increasing the initial bed height increased the dewatering rate.