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In the two particle suspensions, the large particle size d 1 group is located in the lower layer, and the small particle size d 2 group is in the upper layer; The two groups of particles, the dense δ 2 group are in the lower layer. δ 1 group in the upper layer; Two groups of particles (free sedimentation ratio, see post), the density of the δ 2 group is in the lower layer, and the δ 1 group is in the upper layer; The two types of granules, there are three cases of suspension stratification, namely When the situation is reversed, the density is higher and the density is lower; When the two groups are mixed, they are not suspended.
The sedimentation law of particles in the medium (2)
Second, the interference settlement of particles in the medium
1. Interference settlement of uniform grain groups
A group of particles consisting of particles of the same or similar density, particle size and shape. When they settle in a medium, the mechanical resistance caused by mutual friction, collision, etc. depends mainly on the volume concentration or looseness of the particle group in the medium. Many formulas for calculating the interference velocity of uniform particle group interference have been proposed. Among them, the representative and widely used formula is the Ryushenko formula, namely
Where υ CT and υ 0 — interference settlement and free settling velocity of particles, m/s;
λ and θ—volume concentration and looseness of the population, decimal;
n — Index, related to the particle size and shape of the particles (see Tables 7 and 8).
According to a study by Moreland et al., the value of n is also related to the Reynolds number, and the relationship is shown in Fig. 4. The Reynolds number is calculated as d v Ï… 0 Ï/μ, and Ï… 0 is the free settling velocity.
Table 7 Relationship between n value and granularity (shape is polygonal)
Â
Average particle size, mm
2.0
1.4
0.9
0.5
0.3
0.2
0.15
0.08
n value
2.7
3.2
3.8
4.6
5.4
6.0
6.6
7.5
Table 8 Relationship between n value and shape (particle size 1mm)
Particle shape
Round shape
Polygon
rectangle
n value
2.5
3.5
4.5
Formula (26) is suitable for the case where the particle volume concentration is small, and the ratio of the maximum and minimum particle sizes is less than 1.5.
Another method of CT is to treat the settled group as a filter medium, in which the formula proposed from the bottom to the top is suitable for a large concentration of dissolution or a small degree of looseness (θ ≤0.8). The formula is as follows:
When N≤7 υ CT =1.8BN (4-27)
7<N≤17 υ CT =2.4BN 5/6 (4-28)
17<N≤750 υ CT =3.6BN 2/3 (4-29)
750<N≤5000 υ CT =5.7BN 3/5 (4-30)
5000<N≤130000 υ CT =7.2BN 4/7 (4-31)[next]
Figure 3-4-4 Relationship between n value and Reynolds number Re
1. spherical particles; 2. quartz ; 3. coal particles
N — dimensionless parameter;
Ar — Archimedes's number.
2. Interference settlement of non-uniform grains
Variety of particle size, density and shape of particles of different populations make interference with settling in the medium, it is prevalent in mineral form of ore particles motion, there is a very important role in the classification and beneficiation. However, this form of interference settlement is complicated due to many influencing factors. In theory, there is no suitable formula for calculating the sedimentation velocity.
However, the study of the suspension stratification of non-uniform grains in the ascending water flow indirectly reveals some rules of interference settlement. It is known that if a mixture of two different particle groups is in suspension in a rising water stream, the suspended particles will undergo stratification which is different in nature. It has the following conditions:
(1)
(2)
(3)
(4)
1 When the rising water velocity (critical rising water velocity) of the particle suspension is suspended, the δ 1 group with a small density is in the upper layer, and the δ 2 group with a high density is in the upper layer;
2
3
The above situation and the critical rising water velocity were confirmed by experiments. But there are different opinions on the reasons for explaining these phenomena. [next]
Riashenko believes that the suspension is layered according to the relative density of each group, the relatively dense particles are below, and the relative density is lower, according to this point of view.
Where υ 01 and υ 02 —corresponding to the rate at which δ 1 and δ 2 particles are free to settle;
n - index, see (4-26).
Scholars from China Mining Institute and Central South University of Technology have made researches and proposed different formulas for critically rising water speed. The following formulas are close to the measured values, namely
The fact that the granule suspension is stratified and the above arguments have certain practical significance for the loose stratification of the ore particles during grading and re-election.
3. Equal drop ratio of particle sedimentation
The ratio of the particle size of the small-density particles to the particle size of the large-density particles equal to the end-to-end velocity is called the isodolite ratio of the particles, and the value is greater than 1. It can be obtained from the relationship of equal settlement speeds.
In the case of free settlement, the equivalence ratio e 0 is a constant within a certain range of Reynolds number, ie
Where d v1 and d v2 are equal to the particle size of the light and heavy ore particles;
P 1 and P 2 - shape correction coefficients of light and heavy ore particles;
δ 1 and δ 2 — the density of light and heavy ore particles;
N—index, between 0.5 and 1, varies with Reynolds number. When Re≤0.5, n=1:rE=3000~100000, n=0.5: Re=0.5~3000, 0.5<n<1 .
Interference sedimentation equal reduction ratio e CT , in terms of uniform particle size,
Where θ 1 and θ 2 are the degrees of looseness;
n 1 and n 2 - indices, see Table 3-4-7.
If the mineral particles of different natures are interfering with each other in the medium, the formula for the interference settlement rate in this case has not been calculated yet, and the ratio of interference and sedimentation cannot be obtained.