Refining Intensity And Dynamics Of The Pulp Refining Process Conducted By The Isometric Straight-Bar Refiner Plates

The pulp refining process is the key operation unit for the fiber separation and fiber morphology change,and it is a critical step in the modification of fiber and pulp properties to meet the requirements of the targeted paper.Disc refiner isone of the most widely used refining equipments,and the bar profile of refiner plates will directly affect the energy consumption,pulp,and fiber properties during the refining process.At present,isometric straight-bar refiner plates are one of the most widely used refining plates,with adjustable bar profiles and a wide range of applications,however,there is no suitable theory to characterize the refining process and to guide the optimal design of the refiner plates.And the unclear relationship between the changes of pulp and fiber properties,and the bar profile of refiner plate or the control parameters,limits the optimization of the refining process and makes it difficult to reduce the energy consumption and enhance of pulp and fiber quality.Based on these problems,this study investigates the low consistency refining process,in terms of the isometric straight bar refiner plate,in the following aspects:（1）The effective refining intensities,applicable to the strength characterization of the pulp refining process conducted by the isometric straight-bar refiner plates,were investigated from a theoretical point of view.The physical meaning of the cutting edge length of the refiner plate was explained in terms of the equivalent total bar length and the total bar sliding length of the rotor,and the relationships between the bar width,groove width,bar angle,and sector angle of an isometric straight-bar refiner plate and different comprehensive characterization parameters of refiner plate was analyzed.And the net tangential force per bar crossing zone f_t^net),Modified Edge Load（MEL）,and Kerekes’ modified Specific Edge Load(SEL_kerekes)were found to have the potential to describe all pulp refining processes conducted by the isometric straight-bar refiner plates,while the widely used Specific Edge Load is only applicable to the pulp refining process conducted by the isometric straight-bar refiner plates with the same angular parameters.By considering the effects of different bar heights,dam numbers,dam height,and pseudo-configuration of the refiner plate on the average bar crossing angle and the equivalent bar crossing number,a general model of net tangential force(f_t^net)and normal force(f_n^net)per bar crossing zone for characterizing the refining intensity of the low consistency pulp process conducted by isometric straight-bar refiner plates was constructed.（2）Based on the net tangential force per bar crossing zone(f_t^net)during the pulp refining process,a design method of isometric straight-bar refiner plate,with total bar length in one sector as the core,was proposed.By considering the pulp refining process for a specific type of pulp,a reasonable target value of f_t^net can be selected,and the total equivalent bar length and the bar length of one sector can be obtained.Similarly,for the isometric straight-bar refiner plate with dams,the equation for the dam number in one sector was derived by assigning the equivalent bar length to the bars and dams,and finally optimizing the dam number and bar parameters.Through the establishment of the isometric straight-bar refiner plate design method,the design of bar parameters for an isometric straight-bar refiner plate can be obtained theoretically.（3）An experimental platform was established to study the refining characteristics of different plates during low consistency pulp refining,and the characterization performance of different refining intensities on the variation of pulp Schopper Riegler（SR）and average fiber length was verified by refining trials with different plates.It was found that the net tangential force per bar crossing zone(f_t^net)is the only refining intensity with physically meaning that can effectively characterize the refining process conducted by all isometric straight-bar refiner plates,in addition to the empirical Modified Edge Load（MEL）that can effectively guarantee this type of refining process.However,the characterization performance of the effective refining intensity on the pulp SR and the average fiber length in the low consistency refining is different,the fiber cutting effect of the low consistency refining increases with the increase of the effective refining intensity for the same specific refining energy;For the variation of the pulp SR,the effective refining intensity has a better characterization on the variation of the pulp SR during the refining processes with different plates under the same refining conditions,while for the same refiner plate with different refining conditions,the specific refining energy is the key parameter to control the variation of pulp SR,rather than the effective refining intensity.（4）A dynamic model was developed to investigate the variation of pulp SR and fiber average length during the refining process of the mixed pulp of softwood and hardwood pulp.It was shown that the variation curves of pulp SR and average fiber length with specific energy consumption were S-shaped and inverse S-shaped during the low consistency refining process of the mixed pulp of softwood and hardwood pulp,respectively.And the variation of both with specific energy consumption can be expressed as a cubic function.Meanwhile,a power function relationship between pulp SR,or average fiber length,and specific energy consumption was established based on the cubic function.Andthe characteristics parameters,such as characteristic energy consumption,the characteristic pulp SR（or the average fiber length）,and so on,of different processes performed by different refiner plates were different.The comparison showed that the pulp SR and the average fiber length calculated by the characterization model have a good correlation with the experimental data,especially for the characterization of the evolution of pulp SR.（5）The effects of bar parameters such as the average bar crossing angle and the equivalent dam number on paper tensile and tear index,and bulk were investigated,and the optimization of bar parameters of isometric straight-bar refiner plate for low consistency pulp refining was achieved.The specific energy consumption and the average number of bar crossing zones were introduced into the empirical power function model for the paper tensile index,tear index,and paper bulk,and it was found that the introduction of these two parameters can help to improve the characterization performance of the power function model.Under the same refining energy,there is a critical average bar crossing angle which makes the maximum value of paper tensile and tear index,and the range of them are 30°～50°,350～60°.While the paper bulk decreases with the increase of bar crossing angle.Meanwhile,with the increase of equivalent dam number,the paper tensile index decreases,while the tear index and paper bulk increase under the same refining energy.The physical meaning of the cutting edge length of the straight bar refiner plates was explained from two perspectives,as well as the meaning of the Specific Edge Load（SEL）and the limitations of its application were clarified.A common refining intensity,applicable to the characterization of the low consistency refining process conducted by the isometric straight-bar refining plates,and the design method for straight-bar refiner plates related to the proposed refining intensity were modeled.And it provided a theoretical basis for the characterization of the low consistency pulp refining and the refiner plate design.The study of the dynamic model of pulp and fiber properties’ changes and the characterization model of paper properties,as well as the optimization of the angular parameters and dam parameters in the refining process,can provide new ideas for the parametric design of refiner plate and the optimization of the pulp refining by considering the energy consumption and the paper properties.