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Guideline on the Basics of Gravity Separation

Author : Jordan | Last Updated :

1Equivalent diameter

The diameter of a sphere with the same properties in some aspects as the mineral particle is used to represent the diameter of the mineral particle.

2Gravity separation

Gravity separation is a process in which the density difference between the ore particles is used to sort or separate the ore particles under the action of external force. The size and shape of the particles also affect the accuracy of sorting by density.

3Separation size

The separation size refers to the average particle size of the extremely narrow size gradation whose distribution rate each accounts for 50% in the setting sand and overflow.


It refers to the volume occupied by the liquid in the unit volume of the suspension.

5Equal settling ratio

In the equal settling particles, the ratio of the particle size of small density ores to that of large density ores is called equal settling ratio.

6Equal settlement phenomenon

In the process of settlement, some ore particles with large size and small density usually settle at the same speed as those with small size and large density, which is called equal settlement phenomenon.

7Equal settling particles

Particles with different densities but the same terminal velocities in the same medium are called equal settling particles.

8Equal settling ratio

The ratio of the particle size of the low-density ore particles to that of high-density ore particles in the equal settling particles.

9Significance of equal settling ratio

When gravity separation is adopted depends on density difference, the influence of particle size on settlement should be considered. The larger the equal settling ratio, the larger the range of particle sizes can be beneficiated.

In the particle group, when the particle size ratio of the largest particle to the smallest particle does not exceed the equal settling ratio, the light and heavy materials can be separated by the difference of fall velocity.

10Factors influencing the equal settling ratio

  1. 1 The influence of medium density: The equal settling ratio increases with the increase of the medium density.
  2. 2 The influence of equal settling velocity: The equal settling ratio is related to the resistance coefficient during the settlement of ore particles.
  3. 3 The influence of particle shape: The equal settling particles with different shapes have a large equal settling ratio.


The selected material is fed to the jig screen to form a dense material layer, which is called the bed.

12Suction effect

The effect that enables particles with small particle sizes to continue to move downward toward the gradually shrinking bed gap is called the suction effect.

13Medium solid

The heavy suspension is a two-phase fluid composed of finely crushed dense solid particles and water.

High-density particles play a role in increasing the density of the medium, so it is also called medium solid. The main medium solids used in ore dressing are ferrosilicon, magnetite, galena and pyrite.

14Medium resistance

The resistance of the separating medium acting on the ore particles; Mechanical resistance: The resistance caused by the friction and collision between ore particles and other surrounding objects and the walls of the machine.

15Free setting

It refers to the settlement of a single particle in an infinitely medium.

16Hindered settling

In a container, when the particles are large or there are many particles in the medium, interference will occur between the particles, the container wall and other particles, which is called hindered settling.

17Types of hindered settling velocity

  1. 1 The particles settle in particle groups of uniform density and particle size.
  2. 2 The particles settle in particle groups of the same size but different density.
  3. 3 The particles settle in particle groups of different sizes, densities and shapes.
  4. 4 Coarse particles settle in a finely dispersed suspension.

18Is the hindered settling velocity less than the free settling velocity?

  1. 1 When the particle settles, its relative velocity with the medium will increase, which leads to an increase in the resistance of the medium.
  2. 2 When the particle size is too wide, the buoyancy effect of the particle is greater than that of water.
  3. 3 The generation of mechanical resistance.
  4. 4 The viscosity of the medium increases. Due to the existence of particle clusters, the resistance to particle sedimentation will increase, so the hindered settling velocity is less than the free settling velocity.

19Pressure resistance

Due to the inertia of the medium, the flow state and dynamic pressure of the medium before and after moving are different. The resistance caused by the pressure difference is called pressure resistance.

20Solid volume concentration

It refers to the volume content of solid particles in the medium.

21Classification size

It refers to the particle size of the critical particles that separate the two products calculated according to the settling velocity.

22Dense-media process

It is to put the ore crushed to a certain size into a fluid with a density greater than water (ie, heavy medium).

According to the buoyant effect, the ore particles with a density less than the medium will float, while those with a density greater than the medium will sink. The dense-media process will be achieved by obtaining two products respectively.

23Dense-media ore dressing process

According to the nature of the operation, the dense-media ore dressing process can be divided into the preparation operation, separation operation, product processing operation, medium recovery and reuse.

24Judging whether the water flow characteristics of a jigging cycle are reasonable

  1. 1 Whether it is beneficial for the bed to be loosened as soon as possible.
  2. 2 The effect of bed separation according to density.
  3. 3 The impact on suction effect according to the characteristics of different raw materials.

25What are the factors influencing the jigging beneficiation process?

  1. 1 The impact of stroke and jig frequency.
  2. 2 The influence of supplementary water under the sieve and the water to the mine.
  3. 3 The influence of bed thickness and artificial bed.
  4. 4 The influence of the properties and amount of the ore.

26Hydraulic classification

Hydraulic classification refers to the process of classifying ore particles with wide granularity into several narrow-grade products according to the different settling velocity of ore particles in moving medium.

27Application of hydraulic classification in mineral processing

  1. 1 Before some gravity separation operations (such as shaking table beneficiation, spiral chute beneficiation, etc.), the selected raw materials are divided into narrow-grained grade.
  2. 2 It forms the closed-circuit work with the grinding operation and timely separates qualified particle size products to reduce overgrinding.
  3. 3 Desliming and dehydration of raw ores or separated products.
  4. 4 Determine the particle size composition of fine materials (mostly -0.075 mm), that is, conduct hydraulic analysis.

28Hydraulic analysis (elutriation for short)

It is a method of indirect measurement of particle size composition by measuring the settling rate of particles.

Range: It is often used for the determination of the particle size composition of materials less than 0.1mm.

There are three commonly used elutriation methods: gravity setting method, upward flow method and centrifugal sedimentation method.

29Locus of zero vertical velocity (LZVV)

Since the fluid motion modes of the outer vortex and inner vortex are different, and the inner vortex is formed by the gradual inward migration of the outer vortex during its movement, there must be a tracepoint where the axial velocity is equal to zero.

During the normal separation process in a cyclone, the locus where the axial velocity is zero is called the locus of zero vertical velocity.

30Segregation phenomenon

The fine particles will drill through the pores of the bed by their gravity, making themselves located below the coarse particles, which is called the segregation phenomenon.

31Working principle of spiral classifier

The finely ground pulp is fed into the water tank from the feed inlet located in the middle of the settlement zone. The spiral rotates at a low speed to agitate the pulp so that light and fine particles can be suspended on top and overflows to enter the next procedure.

The coarse and heavy particles settle to the bottom of the tank and are conveyed by the spiral to the discharge port to be discharged.

If the spiral classifier and the grinder form a closed circuit, the setting sand from the classifier enters the grinder through the chute for regrinding, and the setting sand sent back to the grinder is called "returned sand".

32The classification process of spiral classifier

After the slurry is fed from the side near the lower end of the tank, a grading liquid level is formed in the lower part of the tank.

The coarse particles sink to the bottom and are discharged after being screwed to the top. The fine particles failed to settle were discharged with the slurry flow through the overflow weir.

A lifting device is arranged on the upper beam of the frame to adjust the distance between the spiral blades and the bottom of the groove and to lift the spiral shaft when the machine is stopped to prevent the spiral blade from being buried by ore deposits.

33What are the main factors affecting the process effect of the spiral classifier?

1 The nature of the ore

Density: The greater the density of the ore, the higher the production capacity (almost proportional).

Particle size composition and mud content: As the slurry viscosity increases, the settling rate of the ore particles decrease and the processing capacity and classification accuracy are reduced. Therefore, when the mud content is high, a lower classification concentration is often used, but this in turn leads to a reduction in capacity.

2 The structure of the spiral classifier

Classification area: It affects the processing capacity of the spiral classifier and determines the classification size. As the area of classifier increases, its processing capacity increases and the classification accuracy becomes finer.

The rotation speed of the spiral: The classification of coarse particles can be agitated greatly without affecting the settlement of particles. For fine particle classification, strong agitation should be avoided, and the spiral rotation speed is just sufficient to transport the returned sand along with the outer tank.

3 Feeding concentration

It is often adjusted by adding water at the discharging port of the mill. In actual production, controlling the feeding concentration of the classifier is an effective method to control the overflow particle size of the classifier. At the same time, it restricts the processing capacity.

34The classification principle of hydrocyclone

Under the action of 0.4~3.5 atmospheres, the slurry enters into the cyclone along the tangent direction from the inlet pipe to make the high-speed rotation.

Under the action of centrifugal force and gravity, the coarse particles, being thrown to the wall, moves downward in a spiral motion to be discharged from the discharge port at the bottom.

The fine grains move at a small speed toward the wall of the device and are carried away from the overflow pipe by the fluid flowing in an eddy current toward the center.

35The working principle of the hydrocyclone

Under pressure, the pulp is fed into the hydrocyclone along the feed pipe, and then it makes a rotary motion under the restriction of the cylindrical wall.

The outer slurry that moves downwards in the rotation is called the outer spiral flow. As the space section of the downward flow decreases, the inner slurry that forced to flow upward is called internal spiral flow.

Coarse particles are thrown to the wall due to the large inertial centrifugal force, and gradually flow downwards to be discharged to become settling sand; fine particles are driven by the liquid flowing toward the center to become overflow being discharged from the central overflow pipe.

36What are the factors influencing hydrocyclones?

  1. 1 Diameter. The hydrocyclones with a larger diameter are often used for coarse classification; For fine classification, a small diameter cyclone is used.
  2. 2 The diameter of the feeding pipe. The size of the feeding port has a certain impact on the processing capacity, separation particle size and classification efficiency.
  3. 3 The diameter of the overflow pipe. If the diameter of the overflow pipe increases, the overflow flow increases, the overflow particle becomes coarser and the sand settling concentration increases.
  4. 4 The diameter of the apex diameter. The discharging area of the apex diameter often increases due to wear, so the settling sand yield increases with lower concentration.
  5. 5 The effect of the taper angle on the hydrocyclone. The smaller taper angle is used for fine classification or dehydration; the large taper angle for coarse grading or concentration.
  6. 6 The influence of feed pressure on the hydrocyclone. The feed pressure directly affects the processing capacity, but has little effect on the separation size.
  7. 7 The influence of the nature of the ore on the hydrocyclone. The higher the feed concentration, the coarser the grading particle size and the lower the grading efficiency.

37The advantages, disadvantages and internal speed distribution of hydrocyclones


  1. 1 Simple structure, light and flexible, no transmission parts.
  2. 2 Low equipment cost, easy disassembly and assembly, convenient maintenance, easy manufacturing, small floor space, low capital construction cost.
  3. 3 Fine classification size, especially for fine (<0.1 mm) materials.
  4. 4 High efficiency of classification, sometimes up to 80%.


  1. 1 Fast wear.
  2. 2 The power consumption of the pump is large, so the maintenance cost is more.
  3. 3 It is difficult to determine and maintain its best working conditions because there are so many factors that affect the process effect of the cyclone.
  4. 4 The internal speed of the hydrocyclone is three-dimensional space velocity, namely tangential speed, radial velocity and axial velocity.
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Author : Jordan Jordan is the writer of the blog with a broad knowledge of this industry. Most importantly, he hopes to help you in your projects sincerely.


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