Centrifugal mineral processing

Centrifugal concentration is a gravity separation method that utilizes the difference in inertial centrifugal force generated by mineral particles in a rotating slurry flow to separate particles according to density. In a gravitational field, as the particle size decreases, the settling force and settling velocity decrease rapidly, and the separation efficiency and equipment processing capacity also decrease accordingly. The inertial centrifugal force generated by the rotating slurry flow is tens to hundreds of times greater than gravity, thus greatly increasing the stratification rate of mineral particles and improving the processing capacity of the equipment.

Centrifugal concentrators have a large processing capacity and low separation cost, making them suitable for processing fine-grained ores, but their enrichment ratio is low, and they are generally used for roughing.

Centrifugal concentrators mainly fall into two categories: those that generate centrifugal force by rotating the machine body and those that generate centrifugal force by the rapid rotating flow of the slurry. The former includes horizontal and vertical centrifugal concentrators, while the latter includes short cone hydrocyclones, etc.

2.7.2 Centrifugal Concentrator Equipment

2.7.2.1 Horizontal Centrifugal Concentrator

Its structure is shown in Figure 2-40. It was successfully developed by Yunnan Tin Industry Company in my country in 1964. It consists of two parts: the main unit and the control mechanism. The main unit is a horizontally placed truncated conical drum. During operation, the slurry is fed to the drum wall tangentially through a flat feed nozzle. While rotating with the drum, it flows along the axial inclined surface of the drum wall, causing stratification. The heavier mineral particles at the bottom layer deposit on the drum wall, while the lighter mineral particles at the top layer are discharged with the slurry at the tail end of the drum as tailings. When the heavy mineral particles reach a certain thickness, feeding is stopped, and high-pressure water is introduced through the flushing nozzle to flush the heavy mineral particles down to form concentrate. Feeding, stopping the slurry, flushing to obtain tailings, and concentrate are all periodically completed by the control mechanism.

Figure 2-40 Structure of φ800×600 Yunnan Tin Centrifugal Concentrator

1 – Feed hopper; 2 – Flushing nozzle; 3 – Upper feed nozzle; 4 – Rotary drum; 5 – Chassis; 6 – Receiving trough; 7 – Protective cover; 8 – Separator; 9 – Diaphragm valve; 10 – Three-way valve; 11 – Motor; 12 – Lower feed nozzle; 13 – Washing water flat nozzle

Centrifugal concentrators utilize centrifugal force instead of gravity in planar sluices for mineral separation, enhancing the film separation process. Horizontal centrifugal concentrators are used to separate tungsten, tin, and iron ores with a particle size of 0.15–0.01 mm, achieving enrichment ratios of 1.5–3. Iron ore can be processed to obtain a final concentrate, while tungsten and tin require further fine-tuning with additional equipment to achieve the final concentrate.

Jet centrifugal concentrators are a newly developed type of horizontal centrifugal concentrator, successfully developed by the Beijing General Research Institute of Mining and Metallurgy in the early 1990s. Their structure, as shown in Figure 2-41, consists of a rotating drum, distribution plate, feed pipe, water pipe, and jet injector. The slurry is fed directly to the inside of the drum, while low-pressure clean water is fed to the distribution plate. The clean water overflows from the distribution plate, propelling the upper mineral particles along the longitudinal slope of the drum towards the discharge end. The varying point of impact of the jetting water creates a hydraulic weir around the circumference of the drum, causing the bed to alternate between loosening and compaction, thus enhancing the shearing and stratification effect of the Baynoh force. The φ1200 mm drum rotates at a high speed of 600–700 rpm, generating a centrifugal force 240–326 times stronger than gravity, which allows extremely fine useful mineral particles to deposit on the drum. The jetting water loosens the bed, pushing the heavier minerals at the bottom towards the inside of the drum, and finally discharging them as concentrate from the discharge port. The particle size recovery limit for treating fine tin ore slime can reach 0.003 mm. Besides treating tin ore slime, it can also be used to treat difficult-to-process fine-grained slimes such as gold, tungsten, tantalum, niobium, and rare earth elements.

Figure 2-41 Working principle diagram of a jet centrifugal concentrator

1—Drum; 2—Clear water distribution plate; 3—Feed pipe; 4—Jet water jet; 5—Concentrate outlet; 6—Tailles outlet

Vertical centrifugal concentrators consist of a vertically mounted sorting tank that rotates around its axis. Common types include centrifugal gold cone separators and centrifugal disc separators.

The structure of a centrifugal gold cone separator is shown in Figure 2-42. The sorting tank is an inverted truncated cone mounted on a lower vertical shaft. The surface of the cone has annular grooves. The slurry is fed to the lower part of the sorting cone disc via a feed hopper. As the cone disc rotates, the slurry flows upward along the inner wall, and the mineral particles are stratified according to density. Heavier particles move towards the disc wall and enter the grooves, while lighter particles flow upward with the slurry and are discharged from the top into the tailings trough. When the heavy particles fill the grooves, feeding is stopped and the machine is shut down. The bottom valve is manually opened, and high-pressure water is used to flush the heavy particles into the concentrate trough. This equipment is mainly used for the beneficiation of placer gold deposits and the roughing of small vein gold deposits. The feed particle size is 0–10 mm. The equipment has a simple structure, is easy to operate, has a high separation enrichment ratio, and can achieve a gold recovery rate of over 90%, while consuming relatively little water.

Figure 2-42 Centrifugal Cone Separator

1—Feed pipe; 2—Top cover; 3—Rubber grid; 4—Cone disc; 5—Slurry distribution disc; 6—Water-throwing disc; 7—Upper bearing seat; 8—Pulley; 9—Frame; 10—Lower bearing seat; 11—Hollow shaft; 12—Motor; 13—Outer casing

The structure of the centrifugal disc separator is shown in Figure 2-43, and it is basically the same as that of the centrifugal gold concentrator. The separating disc is a hemispherical disc with annular grooves on its inner wall. The separating process and operation method are the same as those of the centrifugal gold concentrator. It is used to separate placer gold and vein gold ores of 0-10 mm. It features high enrichment ratio and high recovery rate. However, the hemispherical shape of the separating disc makes it more difficult to manufacture.

Figure 2-43 Centrifugal Disc Concentrator

1—Sandproof Cover; 2—Tailings Tank; 3—Hemispherical Concentrator Disc; 4—Motor; 5—Horizontal Shaft; 6—Motor Frame; 7—Frame

Figure 2-44 φ300 Short Cone Hydrocyclone

1—Feed Pipe; 2—Overflow Pipe; 3—Bottom Flow Port

2.7.2.3 Short Cone Hydrocyclone

Its structure is shown in Figure 2-44. The difference from a conventional hydrocyclone is the larger cone angle. The cone angle of the short cone hydrocyclone is 90°~120°, and the cone is particularly short. During operation, the slurry enters the cylinder tangentially from a sand pump or high-level pressure tank. The slurry moves downwards in a spiral motion, and the mineral particles are arranged radially according to their settling velocities. Lighter or finer particles with lower settling velocities are discharged through the central overflow pipe, while coarser or heavier particles move towards the vessel wall and transfer downwards. Due to the increased cone angle, the slurry flowing downwards along the vessel wall is obstructed, thus forming a rotating bed at the bottom of the cone. Within the hydrocyclone bed, mineral particles are loosened and stratified through rotational shearing motion. Lighter minerals are at the top and carried by the upward-flowing liquid to the overflow pipe for discharge. Heavier minerals are at the bottom and discharged from the underflow outlet.

A slightly modified water-medium composite hydrocyclone is also a type of short-cone hydrocyclone. Its characteristic feature is that it consists of three sections of cones with different tapers, the taper gradually increasing from top to bottom.

The bottom of the short-cone hydrocyclone can also be arc-shaped, with annular grooves on the inner surface, which can improve the enrichment ratio during gold beneficiation and enhance the separation effect of the short-cone hydrocyclone.

Short-cone hydrocyclones are mainly used for roughing placer gold and also for coal preparation. For separating -1 mm placer gold, the gold recovery rate can reach 95% at an enrichment ratio of ten.