很多研究成果指出，例如由黏土，水泥、细粒煤、磁铁矿、硅铁、泥炭组成的悬浮液，是黏塑性流体或者所谓的宾汉塑性流体。在悬浮液中出现非牛顿特性，是由于结构形成过程所致(因此，这种悬浮液也叫结构悬浮液)，其原因如下。处在液体中具有很大总分界表面和细分散的固体颗粒，在分子引力作用下趋于结合，这种引力只有在颗粒之间距离不大时才会出现。结构形成与一定的固体颗粒接触有关，在其浓度足够大或者在重力作用下沉降时发生这种接触。结果形成实心疏松结构一空间格子: 其强度用极限或者初始剪应力0表征，这时悬浮液失去平衡状态并开始运动(流动)。 可以分为静剪应力(B2)和动剪应力(0n), 在悬浮液沿流动方向法向n上流动速度u的导数与切应力之间关系曲线图上，0。和0z值用线段OA (或者0A')和OA”表示(图2-1-2). du/dn=f (z)关系曲线称为悬浮液流变特性。
Characteristics of Suspension Fixed Mixture for Slurry Pump
I. Rheological properties
Suspension is a special kind of solid-liquid mixtures. When solid particles reach a certain saturation, it is no longer a Newtonian fluid, but has rheological properties, which is different from general solid-liquid mixtures. Rheological properties occur mainly in the case of initial shear resistance, and only when this resistance is overcome can the suspension begin to flow.
Many studies have pointed out that suspensions consisting of clay, cement, fine coal, magnetite, ferrosilicon and peat are viscoplastic fluids or so-called Bingham plastic fluids. The appearance of non-Newtonian characteristics in suspension is due to the formation of structure (hence, this suspension is also called structural suspension), and the reasons are as follows. Solid particles with large total boundary surface and fine dispersion in liquid tend to combine under the action of molecular gravity, which occurs only when the distance between particles is not large. Structural formation is related to the contact of certain solid particles, which occurs when their concentration is large enough or when they settle under the action of gravity. As a result, a solid porous structure, a spatial lattice, is formed: its strength is characterized by limit or initial shear stress 0, when the suspension loses its equilibrium state and begins to move (flow). It can be divided into static shear stress (B2) and dynamic shear stress (0n). On the curve of the relationship between the derivative of velocity u and shear stress, 0. The values of 0 and 0 Z are represented by line segment OA (or 0 A') and OA (Fig. 2-1-2). The du/dn=f(z) curve is called rheological property of suspension.
The stress characterizes the beginning section of curve A'B, while the stress characterizes the rheological characteristic curve section. For most suspensions, these parameters are essentially the same. Therefore, in further describing the process of suspension occurrence, the ultimate shear stress theta_0o_0pi is used.
When the ultimate shear stress is zero in suspension, the relationship between shear stress and normal arbitrary velocity can not be described by the Newtonian fluid equation (rd). In this case, the viscoplastic fluid equation (Shdov-Bingham equation) is used.
According to Newton's law, the viscosity is determined by the ratio of the additional shear stress to the normal arbitrary velocity of the wave body. The equation (21-1) can be expressed as follows.
Influencing factors of rheological properties
The rheological properties of suspensions are found by experiments and can vary in a wide range with a series of factors for the same kind of suspensions. For example, for coal suspension, rheological properties are first determined by the water content of the mixture (the ratio of water mass to mineral particle mass).
The viscoplastic behavior of coal slurry is especially evident when P is greater than 0.25~0.3 and the particle size of coal solid is 50~70um, which accounts for more than 35%.
Table 2-1-5 gives the rheological properties of peats as a function of the concentration of solid particles in solid-liquid mixtures.
For clay slurry (clay slurry, clay-white mixture), the relationship between rheological properties and density of solid-liquid mixture is illustrated.
Heavy medium suspension (a mixture of ferrosilicon and magnetite granulation) or weighing medium is used in the concentration of slurry whose density is greater than that of clear water. Its rheological properties are also related to many factors: solid particle density, dispersion grade, concentration, etc. (Figure 2-1-3).
The rheological properties will change greatly when the medium temperature is higher than 40 degree C.
Some suspensions have thixotropy (thixotropy). The stronger this characteristic is, the longer the suspension stays in a fixed state. Therefore, pumping thixotropic suspensions, such as heavy media, is difficult to start in some cases.
Using suspension instead of clear water as carrier medium to transport solid particles is to use the viscoplastic characteristics of suspension to transport considerable particles in structural liquids through pipeline at a small flow rate (compared with the velocity when carrier medium is clear water).