Research On The Rheological Characteristics And The Mechanism Of Shear Action During Paste Mixing

The cemented paste backfill technology(CPB)carries out thickenning process of low concentration tailings slurry,and then the thickened tailings will be added to the mixer together with the binding material,mixed into a toothpaste-like paste,and transported to the goaf for filling,which effectively reduces the risk of disasters in the tailings storage facility and the goaf.Mixing is the core part of CPB.Its purpose is to disperse,soak and homogenize the materials,such as binding materials,tailings and additives,to form a stable and transportable paste and improve the strength of the filling body.It can be seen that the mixing process is crucial for improving the transport performance of the paste and the mechanical properties of the backfill body.However,previous researches on paste backfill rarely involved mixing techniques.The research takes the shear action of the mixing process as the point of view.Combined with physical experiments,theoretical analysis and numerical simulation,the influence mechanism of mixing shear on the rheology of the paste was studied,the mixing shear effect of the rheological characteristics of the paste was proposed and the parameters of the mixer blades was optimized.Finally,the results were applied to engineering practice,and measures were proposed for the characterization and monitoring of the mixing quality on site.The main research results of the thesis are as follows:(1)A model of the relationship between the mixing shear and the microstructure of the paste was established.The essential difference between paste mixing and concrete mixing is the size of the particles,and shear is the key to agglomerating fine particles in the dispersed paste.The importance of the shear action in the mixing process was quantified by the dimensionless method,and the research results of chemical powder mixing were used for reference.A model for the relationship between the mixing shear rate and the time and the aggregation size in the microstructure of the paste was established.(2)The effect of shear rates on the microstructure and chemical properties of the paste was studied,and the microstructure response mechanism of the paste under shear was revealed.By testing the average chord length of aggregates under different mixing shear rates,the influence of mixing shear on the microstnucture was analyzed.The results show that when the shear rate is less than 480s-1,the average chord length increases with the shear rate decreasing,which is consistent with the model;however,when the shear rate exceeds 480s-1,high shears promote particles at rest aggregating,and the model and test results are quite different.Combined with ion concentration and hydration heat test results,the cause for the particle aggregating under high shear was analyzed from the chemical view and the influence of mixing shear on the microstructure was discussed based on the electric double layer theory.(3)The effect of shear rates on rheological characteristics of the paste was studied,and the mixing shear effect of it was proposed.Tests of mixing shear on the rheological behavior of the paste were carried out,and the results showed that when the shear rate exceeded the first threshold(240?360s-1),remarkable shear thinning occurred.When the shear rate exceeded the second threshold(480?600s-1),shear thickening happened.And the trend of change is consistent with the results of microstructure and chemical properties test.Based on the experimental results and taking 45?m as the critical particle size,the micro-structural changes of shear thickening and thinning were studied,and the rheological shear response coefficient of the mixing process was obtained.Thus,the equations of rheological and shear rate and time were established and the effect of mixing on the rheology of the paste is defined as the mixing shear effect.(4)The shear performance of the blade was studied by a self-made paste experimental mixer.On the basis of theoretical research,the relationship equation between torque and rheological parameters was constructed,and the physical significance of mixing stability time was analyzed.Based on the blade shear performance test,the results showed that the optimal width-diameter ratio of the blade is 0.3 and the aspect ratio is about 2,and the optimum mixing time of the paste is 4.5 min under the experimental ratio.(5)The paste mixing was simulated using a multiphase flow model.According to the characteristics of paste shear thinning,the water and tailings(including cement)were regarded as the initial phase and quasi-fluid phase respectively.Using the multi-phase flow model of ANSYS Fluent software,the paste mixing was simulated,and the effect of the blade parameters on flow velocity and tracer concentration distribution were analyzed,which is consistent with the results of physical experiments.This indicates that the fine particles such as tailings are regarded as a simulation of the quasi-fluid phase suitable for the paste mixing process.(6)The process and equipment parameters of a copper mine paste mixing system was optimized,and a method for characterizing the paste mixing quality was proposed.Combined with the engineering background of a 180m3/h paste mixing system of a copper mine,the mixing process and equipment parameters were optimized by applying the research results of this paper,and the mixing quality characterization method and its monitoring and control measures were put forward.