The Flocculation Behaviour Of Pulp Fiber Suspensions And The Effect Of Yield Stress Coupling On Paper Formation

The papermaking industry consists of multiple technological processes,which involve the cooking of raw materials,the pumping and storage of sizing materials,the beating and removal of impurities,the online copying and so on,and these processes are closely related to the rheological properties of pulp.The pulp fiber suspension is a multi-phase fluid with fiber as solid phase,its flocculation and yield characteristics ensure good paper properties and quality,which is one of the key factors for stable operation of each operation unit in the papermaking process.It can also provide a theoretical basis for the research and development of new papermaking equipments and the operation of the papermaking process with high efficiency energy saving.The length of pulp fiber is generally 1-3mm,which makes it easy to form flocs due to the interlacing and winding.Whether the fibers can be dispersed evenly in the suspension to reduce the flocculation plays an important role in the production of high quality paper sheets.In order to study the effect of flocculation and yield characteristics of fibers on paper evenness.In this paper,from the perspective of pulp rheology,the gel crowding number,compressive and shear yield stress and Froude number of different pulp fiber suspensions were measured.The effects of fiber size,mechanical beating,surface charge and crowding number on the yield characteristics of suspensions were studied;In addition,the effect of yield stress coupling on paper evenness was also investigated.After experiments,the main research results of this paper are as follows:(1)The gel point of bleached spruce wood pulp fiber suspension before and after classification ranged from 1.97 to 5.51 g/L,and the gel crowding number,N_gel,ranged from26.1 to 37.8,and the linear relationship between N_geland fiber aspect ratio,A,can be expressed:N_gel=11.26+0.22A.The gel point of the fiber suspensions were between 3.89 g/L and 5.98 g/L before and after the mechanical beating,and the N_gelwas between 8.7 and 21.6.After the mechanical beating,the gel point and the N_gelof the suspension were significantly lower than those of the suspensions without mechanical beating.When the aspect ratio is about 60,the gel points of the fiber suspensions of the bleached acacia wood pulp,the bleached needle wood pulp and the bleached bagasse pulp ranged from 3.97 to 7.45 g/L,and the values of the gel crowding number,N_gel,ranged from 19.3 to 22.9.(2)The compressive yield stress,P_y,and shear yield stress,?_y,of bleached spruce wood pulp,bleached acacia wood pulp,bleached bagasse pulp and bleached softwood pulp suspensions all increased exponentially with the increase of the crowding number,and the relationship can be expressed respectively as_yP(28)a N^b andt_y(28)m Nⁿ.Under the same crowding number,the compressive and shear yield stress of spruce pulp fiber suspensions increased with the increase of fiber aspect ratio,the shear yield stress of bleaching acacia wood pulp suspension also increases with the increase of beating degree,while the compressiove yield stress increases first and then decreases.(3)The Froude numbers of the fiber suspentions of the bleached spruce pulp,the bleached acacia wood pulp,the bleached bagasse and the bleached softwood pulp all decreased with the increase of crowding number,and finally tended to be stable.At the same crowding number,the smaller the fiber aspect ratio was,the smaller the Froude number of the bleached spruce pulp fiber suspension was.There was a quantitative functional relationship between the Froude number and N/N_gelbefore and after mechanical beating treatment,which is y=1.46+3.29×10^-7e^-14.8*N/Ngel.(4)When the aspect ratio is about 60,the formation index of the bleached acacia wood pulp,the bleached bagasse and the bleached softwood pulp were all decreased with the increase of the forming concentration,and that also shows the same decreasing trend with the increase of the compressive and shear yield stress.In addition,the relationship between formation index and Froude number satisfies the exponential function of y=ax^b.