Heavy media mineral processing

2.2.1 Principle of Heavy Media Mineral Processing

Heavy media mineral processing is a gravity separation method that separates minerals in a medium (heavy medium) with a density greater than that of water. During mineral separation, the density of the medium is greater than the density of the lighter minerals to be separated but less than the density of the heavier minerals. Heavy media mineral processing is primarily based on particle density, with particle size and shape having relatively little influence. From the gravity separation performance index, it can be seen that the E value increases with increasing medium density, indicating that the separation accuracy of heavy media mineral processing is higher than that of water media, effectively separating two types of mineral particles with a density difference of 50–100 kg/m³. Heavy media are divided into two categories: heavy liquids and heavy suspensions. The former consists of some high-density organic liquids or inorganic brine solutions. The latter is a suspension of a weighting agent (finely ground heavy minerals or alloys) mixed with water. Heavy liquids are toxic and expensive, and are only used in laboratories. Heavy suspensions are inexpensive, readily available, and non-toxic, and are widely used in industry. The upper limit for feed particle size in heavy media beneficiation is 300 mm, and the lower limit is 0.5 mm, with a separation density of 1300–3800 kg/m³. Gravity analysis and the plotting of separation performance curves are commonly used methods to determine the feed particle size, separation density, and achievable separation parameters for the ore in heavy media beneficiation.

There are many types of heavy media separators, mainly including five categories: cone-shaped heavy media separators, drum-shaped heavy media separators, heavy media vibrating sluices, heavy media hydrocyclones, and heavy media vortex hydrocyclones. The first two are static separation equipment, and the latter three are dynamic separation equipment. Static separation is characterized by slow media movement speed, using a relatively stable fine-particle suspension as the medium, and the medium density is close to the actual separation density. Dynamic separation is characterized by the medium being subjected to external forces, resulting in high-speed rotational motion or vertical pulsation, using a relatively coarse-particle unstable suspension as the medium, and the actual separation density is greater than the feed medium density, allowing the use of lower-density weighting media.

2.2.2 Heavy Media Separator

2.2.2.1 Conical Heavy Media Separator

The structure of a conical heavy media separator is shown in Figure 2-1. There are two types: internal lifting and external lifting. Its shape is a conical trough, with slowly rotating (4-5 rpm) agitator blades inside to maintain the stability of the heavy suspension. During operation, the feed ore is fed from above the liquid surface, and the heavy media suspension is fed from the feed point and different depths inside the cone. Light products overflow through the overflow weir around the cone, while heavy products are discharged from the bottom via an air lift or pump. The conical heavy media separator is a deep-trough static separator. Due to the large and stable volume of the separator trough, the mineral particles remain in the trough for a long time, resulting in high precision in separating fine particles. It is suitable for processing materials with high light product yields. The disadvantage is the large volume of media and the large volume of circulating media. Generally, the feed particle size is 50-3 mm, the cone diameter is 2-6 meters, and the processing capacity is 25~150 tons/hour.

a— External lifting type                           b— Internal lifting type

Figure 2-1 Schematic diagram of a cone-shaped heavy media separator

1— Feed; 2— Medium; 3— Light product; 4— Heavy product; 5— Pump; 6— Air lifter

2.2.2.2 Drum-type Heavy Media Separator

Its structure is shown in Figure 2-2. There are single-chamber and double-chamber versions. The single-chamber version uses only one medium, while the double-chamber version uses two media with different densities. During operation, the suspension and ore are fed into the drum from one end, while the lighter product is discharged from the other end. The heavier product settles to the bottom and is lifted by lifting plates mounted on the drum wall into the discharge chute for discharge. The drum-type heavy media separator is a shallow-trough static separator. The suspension depth is not large; the stirring action of the drum stabilizes the suspension density. The discharge chute is relatively wide, allowing for the discharge of coarser ore particles. However, the strong stirring affects the settling of fine particles, making it unsuitable for processing fine ore particles. It is suitable for applications with high heavy product yields, and the medium volume and circulation volume are less than those of the cone-type heavy media separator. Generally, the feed particle size is 150~6 mm, the drum diameter is 1.5~3 meters, and the processing capacity is 20~100 tons/hour.

a— Single-chamber two-product type              b— Double-chamber three-product type

Figure 2-2 Schematic diagram of a drum-type heavy media separator

1— Feed; 2— Medium; 3— Light product; 4— Heavy product; 5— High-density medium;

6— Low-density medium; 7— Intermediate product; 8— Low-density chamber; 9— High-density chamber.

2.2.2.3 Heavy Medium Vibrating Sluice Box

Its structure is shown in Figure 2-3. It is a long vibrating sluice box with a perforated screen at the bottom and a water chamber beneath the screen. During operation, the ore and heavy medium suspension are fed in from the feed end. Due to the oscillation of the sluice bottom and the rising water flow beneath the screen, the medium forms a high-density bed with good flowability in the sluice box. The ore stratifies according to density and moves along the sluice box, finally being discharged separately as light and heavy products through the separation baffle at the discharge end. Its advantages include the ability to utilize coarse-grained (-2 mm) weighting media, achieving a solid volume concentration as high as 60%, and achieving high separation density even with lower-density weighting media (such as magnetite). The recovery and purification of the medium are simple. The feed particle size is 75–6 mm, the sluice box width is 0.4–1.0 m, and the processing capacity is 25–80 tons/hour. Due to the large vibration of the equipment, it is now rarely used.

Figure 2-3 Vibrating chute for heavy media

1—Chute body; 2—Supporting spring plate; 3—Screen plate; 4—Water chamber; 5—Separation baffle plate

2.2.2.4 Heavy Medium Hydrocyclone

Its structure is the same as that of a conventional hydrocyclone, often installed at an angle, with its axis at an angle of 18°~30° to the horizontal. During operation, ore and heavy suspension are fed into the hydrocyclone at a certain ratio (1:(4～6)) and pressure (0.08～0.2 MPa). Heavy products are discharged through the underflow port, and light products are discharged through the overflow pipe. When the heavy suspension rotates at high speed in the hydrocyclone, it will produce a concentration effect. The density of the suspension near the overflow pipe and the central axis is low, while the density of the suspension near the underflow port and the wall is high. Therefore, the actual separation density is greater than the actual density of the fed suspension. The separation density is affected by the hydrocyclone’s structural parameters; increasing the cone angle and the diameter ratio of the overflow pipe to the underflow port can both increase the separation density. Its characteristics are simple structure, small footprint, and the ability to use lower density weighting media. The lower limit of feed particle size can reach 0.5 mm, and the general feed particle size is 20–2 mm, which is widely used. The processing capacity of a φ430 mm heavy media hydrocyclone is 45 tons/hour.

2.2.2.5 Heavy Media Vortex Hydrocyclone

Its structure is shown in Figure 2-4. There are three types of heavy media vortex hydrocyclones: Tagawa type, Diner type, and three-flow type. Its working principle is the same as that of a heavy media hydrocyclone, but during installation, the underflow inlet faces upward and the overflow pipe faces downward, which can increase the underflow inlet diameter to improve the feed ore particle size. The feed particle size is generally 40–2 mm.

a—Tagawa type                  b—Dinar type                c—Three-stream type

Figure 2-4 Schematic diagram of a heavy medium vortex cyclone separator

1—Feed; 2—Feed medium; 3—Light product; 4—Heavy product; 5—Air duct; 6—Intermediate product

The Tagawa-type vortex hydrocyclone is a vertically inverted hydrocyclone. An air duct is inserted at the heavy product discharge port to stabilize the air column inside the hydrocyclone. The yield of light and heavy products can be adjusted by changing the vertical position of the air duct or by changing the diameter ratio of the underflow port to the overflow pipe. The hydrocyclone diameter is 300–500 mm, the feed particle size is 30–75 mm, and the processing capacity is 20–80 tons/hour.

The DINA-type vortex hydrocyclone is a cylindrical hydrocyclone installed at a 25° angle to the horizontal. The central outlet at the top and bottom ends of the cylinder serves as the feed inlet and the light product outlet. During operation, the suspension is fed tangentially from the inlet near the bottom of the cylinder, and the heavy product is discharged tangentially from the outlet near the top of the cylinder. Its advantages include separate feed of ore and suspension, making it easy to control the density of the suspension. Simultaneously, the suspension acts as a protective layer for the vessel wall, resulting in minimal wear on the cylinder. The yield ratio of heavy product to light product can be adjusted over a wide range (from 9:1 to 1:9). The feed particle size is 40–2 mm. The cylinder diameter is 225–400 mm, and the processing capacity is 10–80 tons/hour.

The three-flow vortex hydrocyclone is a two-stage DINA hydrocyclone, with each stage having a separate medium inlet and heavy product outlet. The feed enters the first stage, the light product from the first stage enters the second stage, and the final light product is discharged from the second stage. Suspensions of the same or different densities can be fed into the two stages. The former is used to separate the heavy product, light product, and intermediate product to be processed. The latter is used to separate two heavy products of different properties and one light product. Its main feature is high separation efficiency. The cylinder diameter is 250–500 mm, and the processing capacity is 15–90 tons/hour.