| The suspension which containing a large number of fine particles and/or colloids would be mechanically compressed to a filter cake with extremely-low hydraulic permeability,and the pore water within the cake would not be drainage in a short time.As a results,the quality of the filer cake and/or filtrate recycled finally would be negatively influenced.Owing to the surface of the particles always have some electrical charges,the electroosmosis which do not depended on hydraulic permeability of the filter cake could be considered as a combined technology with mechanical press for deeply dewatering.Based on the self developed experimental equipment,the recessed chamber filter press（RCF press）was selected as the background on the application of combined technology,pressure-electroosmosis combined effects during slurry dewatering process were discussed according to theoretical analysis and laboratory experiments,with considering equipment and process characteristics of the RCF press.The slurry dehydrated in experiments were prepared by mixing low-liquid limit clay and municiple water in laboratory.The main conclusions are as follows:（1）Based on the non-equilibrium thermodynamic theory,the mass transfer process in the clay slurry system was analyzed by combining the process characteristics and drainage conditions of the RCF press,and the macroscopic water flow motion was simplified into hydraulic flow driven by pressure field and electro-osmotic flow driven by electric field.A mechanical model of the dewatering process of the slurry in the filter chamber was constructed,and the water flow motion laws driven by pressure field and electric field under different boundary conditions were compared and analyzed.Under the single-sided drainage condition with the anode closed and the cathode open,the hydraulic flow driven by the pressure field moves in the same direction as the electroosmotic flow driven by the electric field,and the electroosmotic flow will reduce the pore water pressure on the anode side.While under the double-sided drainage condition with both the anode and the cathode open,the hydraulic flow in the area on the anode side is opposite to the electroosmotic flow with the center of the filter chamber as the symmetry axis;the hydraulic flow in the area on the cathode side moves in the same direction as the electroosmotic flow.（2）The mathematical models of dewatering in the filtration stage and the consolidation stage were established for the constant-pressure filtration process under the action of double-sided drainage and mechanical pressure alone,respectively.Using the self-developed pressure-electroosmotic synergistic dewatering experimental device,constant-pressure filtration experiments were carried out at 500,700 and 900 k Pa for clay slurries with mass concentrations of 0.33 and 0.455,respectively.The variations of drainage rate and accumulated drainage volume during the constant-pressure filtration process were revealed.The time threshold points for the transition from the filtration stage to the consolidation stage during the press filtration process with different mechanical pressures were obtained for clay slurries of two mass concentrations by combining theoretical analysis and experimental observations.The mathematical models constructed for the filtration stage and the consolidation stage were solved separately under the boundary conditions of two-sided drainage based on the functional relationship between the pressure on the particles and the average porosity of the filter cake and the average mass specific resistance of the filter cake obtained from the compression-permeability experiments.The pore water pressure in the filtration stage was distributed sinusoidally along the thickness of the filter cake which could be selected as the initial condition of the consolidation stage.The spatial and temporal evolution of pore water pressure in the filter cake during the consolidation stage was deduced.The mathematical models were verified by comparing with the experimental observations in the filtering and compacting stages.（3）For clay slurry with a mass concentration of 0.33,pressure-electroosmosis collaborative dehydration experiments were carried out at 500 k Pa,900 k Pa,10 V and 20 V voltages,respectively.Through the analysis of the height of the sample,the drainage rate of the cathode and anode and the change law of the drainage volume during the experiment,the concept and quantification method of the manifestation point of electroosmotic synergy effect were proposed.On this basis,the effects of electric field application time,pressure and voltage variations on the dewatering mechanism of slurry in the filtration stage and the pressure-density stage were discussed by comparing and analyzing the relevant experimental results with those of mechanical pressure alone,revealing the synergistic dewatering effect of electroosmosis in the mechanical filtration process.The results show that under the condition of mechanical press filtration with simultaneous application of electric field,the hydraulic flow driven by pressure is the main dewatering mechanism in the filtration stage,while the electroosmotic flow driven by electric potential is the main dewatering mechanism in the pressure-dense stage.When the mechanical pressure was 500 k Pa,applying 10 V and 20 V on both sides of the filter chamber at the beginning of filtration press could improve the total drainage rate and could further reduce the dry basis water content of the filter cake by 6.5%and 11%in the same time;compared with the mechanical pressure alone at 900 k Pa,it could further reduce the dry basis water content of the filter cake by 2.4%and 6.9%,indicating that the dewatering effect could be obtained at low mechanical pressure using electroosmotic synergy than that at high mechanical pressure alone.The synergistic dehydration effect of electroosmosis requires a certain initiation time,and delaying the application of the electric field would cause a synchronous lag in the time of manifestation of the synergistic dehydration effect of electroosmosis.When the electric field application time was the same,increasing the supply voltage from 10 V to 20 V can advance the electroosmotic synergistic effect from 90 min to 55 min.The specific energy consumption during electroosmotic co-dewatering would increase with the percentage of dry matter mass in the filter cake.For the clay slurry configured in this paper,the required specific energy consumption is about 0.43～0.45 k W-h/kg-DSwhen the dry matter content is 80%,and about 0.8～0.85 k W-h/kg-DSwhen the dry matter content reaches 90%.Delaying the application time of the electric field reduces the specific energy consumption,but after the dry matter mass content of the filter cake exceeds 80%,whether the application time of the electric field is delayed or not has little effect on the specific energy consumption.（4）The analytical solutions of the pressure-electroosmotic synergistic dewatering in the filtration and pressure-density stages were derived based on the linear superposition principle,and the rationality of the models was verified by comparing with the experimental results.The paper has 53 figures,16 tables,and 134 references. |
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