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渣漿泵懸浮液固定混合液的特性
一、流變特性
懸浮液是固液混合物中特殊一類，在固體顆粒達到一定飽和的情況下，它已不是牛頓流體，而具有流變性質(zhì)，與一般固液混合物不同。流變特性主要在具有初始剪切阻力的情況下出現(xiàn)，只有在克服這種阻力時懸浮液才開始流動。
很多研究成果指出，例如由黏土，水泥、細粒煤、磁鐵礦、硅鐵、泥炭組成的懸浮液，是黏塑性流體或者所謂的賓漢塑性流體。在懸浮液中出現(xiàn)非牛頓特性，是由于結構形成過程所致(因此，這種懸浮液也叫結構懸浮液)，其原因如下。處在液體中具有很大總分界表面和細分散的固體顆粒，在分子引力作用下趨于結合，這種引力只有在顆粒之間距離不大時才會出現(xiàn)。結構形成與一定的固體顆粒接觸有關，在其濃度足夠大或者在重力作用下沉降時發(fā)生這種接觸。結果形成實心疏松結構一空間格子: 其強度用極限或者初始剪應力0表征，這時懸浮液失去平衡狀態(tài)并開始運動(流動)。 可以分為靜剪應力(B2)和動剪應力(0n), 在懸浮液沿流動方向法向n上流動速度u的導數(shù)與切應力之間關系曲線圖上，0。和0z值用線段OA (或者0A')和OA”表示(圖2-1-2). du/dn=f (z)關系曲線稱為懸浮液流變特性。
應力表征曲線線段開始段A'B,而應力表征流變特性曲線線段。對于多數(shù)懸浮液，這些參數(shù)實際上是相等的，因此在進一步描述懸浮液中發(fā)生的過程時，利用極限剪應力θ≈0o≈0π
在懸浮液中存在極限剪應力0時，切應力和法向任意速度之間的關系，就不能用代表牛頓流體方程式(rd描述。這時利用黏塑性流體方程式(什維多夫賓漢方程):
因方根據(jù)牛頓定律，黏性由附加切應力與波體法向任意流速之比來確定，方程式(2 1-1)可以表為下列形式，
二、流變特性的影響因素
懸浮液流變特性，通過實驗發(fā)現(xiàn)并對對同一類懸浮液隨著一系列因素不同可以在很大范圍內(nèi)變化。例如，對于煤這種懸浮液，流變特性首先由混合物含水量（水的質(zhì)量與礦物粒質(zhì)量之比）來確定。
煤漿黏塑性特行在P大于0.25~0.3和煤固體顆粒粒徑為50~70um占35%以上時特別明顯。
表2-1-5給出泥炭流變特性，他們是固體顆粒在固液混合物中的濃度的函數(shù)。
對于黏土泥漿（黏土漿液，黏土一白混合物），流變特性與固液混合物中密度的關系舉例。
重介質(zhì)懸浮液(硅鐵和磁鐵礦粒化的混合物)或稱重介質(zhì)，在其密度大于清水密度的漿體濃縮過程中采用。其流變特性也與很多因素有關:固體顆粒密度，分散等級，濃度等(圖2-1-3).
介質(zhì)溫度大于40°C時流變特性將有很大的變化。
一些懸浮液具有觸變性(變稠)，這種特性表現(xiàn)越強烈，懸浮液處于固定狀態(tài)的時間就越長。因此，抽送具有觸變性懸浮液，例如渣漿泵重介質(zhì)，在些情況下， 泵的起動就困難。
利用懸浮液代替作為載體介質(zhì)的清水輸送固體題粒，是利用懸浮液粘塑性特性，在很小流速時通過管道輸送在結構液體中的相當大的顆粒(與載體介質(zhì)是清水時的流速相比)。
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).
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