渣浆泵悬浮液固定混合液的特性

一、流变特性

   悬浮液是固液混合物中特殊一类，在固体颗粒达到一定饱和的情况下，它已不是牛顿流体，而具有流变性质，与一般固液混合物不同。流变特性主要在具有初始剪切阻力的情况下出现，只有在克服这种阻力时悬浮液才开始流动。

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。
    应力表征曲线线段开始段A'B,而应力表征流变特性曲线线段。对于多数悬浮液，这些参数实际上是相等的，因此在进一步描述悬浮液中发生的过程时，利用极限剪应力θ≈0o≈0π 
    在悬浮液中存在极限剪应力0时，切应力和法向任意速度之间的关系，就不能用代表牛顿流体方程式(rd描述。这时利用黏塑性流体方程式(什维多夫宾汉方程):
因方根据牛顿定律，黏性由附加切应力与波体法向任意流速之比来确定，方程式(2 1-1)可以表为下列形式，
二、流变特性的影响因素

悬浮液流变特性，通过实验发现并对对同一类悬浮液随着一系列因素不同可以在很大范围内变化。例如，对于煤这种悬浮液，流变特性首先由混合物含水量（水的质量与矿物粒质量之比）来确定。

煤浆黏塑性特行在P大于0.25~0.3和煤固体颗粒粒径为50~70um占35%以上时特别明显。

表2-1-5给出泥炭流变特性，他们是固体颗粒在固液混合物中的浓度的函数。

对于黏土泥浆（黏土浆液，黏土一白混合物），流变特性与固液混合物中密度的关系举例。

 重介质悬浮液(硅铁和磁铁矿粒化的混合物)或称重介质，在其密度大于清水密度的浆体浓缩过程中采用。其流变特性也与很多因素有关:固体颗粒密度，分散等级，浓度等(图2-1-3).
  介质温度大于40°C时流变特性将有很大的变化。

一些悬浮液具有触变性(变稠)，这种特性表现越强烈，悬浮液处于固定状态的时间就越长。因此，抽送具有触变性悬浮液，例如渣浆泵重介质，在些情况下， 泵的起动就困难。
  利用悬浮液代替作为载体介质的清水输送固体题粒，是利用悬浮液粘塑性特性，在很小流速时通过管道输送在结构液体中的相当大的颗粒(与载体介质是清水时的流速相比)。

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).