Sluice box mineral processing

2.5.1 Sluice Box Mineral Processing Principle

Mineral particles are separated in a sluice box under the combined action of gravity, friction, and water flow. The water flow in the sluice box is turbulent, and its movement includes not only inclined flow parallel to the bottom of the sluice box, but also vortices and hydraulic jumps perpendicular to the bottom. These two types of upward flow not only loosen the bed but also help the mineral particles stratify according to density. The result of stratification is that denser, coarser particles are located at the bottom layer, and less dense, finer particles are located at the top layer. Driven by the inclined water flow, the mineral particles settle at different locations from the feed point. Coarser, denser particles settle first closer to the feed point and become the bottom layer of the bed at that location. Finer, less dense particles settle furthest from the feed point and become the top layer of the bed at that location. After settling at the bottom of the sluice box, the mineral particles continue to move forward along the bottom of the sluice box under the impetus of the water flow. During the process, fine mineral particles in the upper layer, especially those with high density, will pass through the gaps between coarse particles and move to the lower layer under the influence of gravity. The gaps between mineral particles are larger when in motion than when stationary, making the segregation and stratification more pronounced. If the segregation is too strong, even finer particles with lower density will move to the lower layer, reducing separation efficiency. As the mineral particles move forward in the chute, frictional resistance will occur between the particles and the chute bottom, as well as between the particles themselves. Due to differences in density and particle size, the coefficient of friction also differs, resulting in velocity differences between the particles. Therefore, the denser mineral particles at the bottom experience less water flow force and greater frictional force, moving slowly or not at all along the chute bottom; the less dense mineral particles at the top experience greater water flow force and less frictional force, moving faster. Coarse mineral particles experience a much greater water flow force than fine particles, and their movement speed is greater than that of fine particles.

Sluice box beneficiation is widely used for processing rare metal ores such as gold, platinum, tin, iron, and tungsten. It is a common beneficiation method for processing low-grade placer deposits.

2.5.2 Sluice Boxes

There are many types of sluice boxes, which can be classified according to the particle size of the ore they process: (1) Coarse sand sluice boxes, which process ores with a particle size greater than 2-3 mm, with a maximum feed particle size of 100-200 mm. These mainly include fixed sluice boxes, Ross sluice boxes, and belt sluice boxes with grids; (2) Fine particle sluice boxes, which process materials with a particle size of 2-0.074 mm. These mainly include converging sluice boxes and cone concentrators;

(3) Micro-fine particle sluice boxes (sludge sluice boxes), which process materials with a feed particle size less than 0.074 mm, with an effective recovery particle size limit of 0.01 mm. These mainly include Moritz concentrators (40-layer shaking tilting bed), sludge belt sluice boxes, vibrating belt sluice boxes, and crossflow belt sluice boxes. 2.5.2.1 Coarse Grain Sluice Box

This is typically a long sluice box made of wood or iron plate (an endless belt conveyor can also be used). The bottom of the sluice box is equipped with baffles or other rough surfaces to create strong vortex currents and collect heavy mineral particles. Mineral particles larger than 2-3 mm are loosened and stratified under the action of the inclined water flow. Heavy mineral particles are concentrated in the depressions of the baffles at the bottom of the sluice box or on the rough surface, while light mineral particles are located in the upper layer and carried into the tailings by the water flow. It is mainly used for separating placer gold, platinum, tin, and other rare metal placer deposits. The maximum feed particle size can reach 100-200 mm, with a lower limit for recoverable particle size of 0.074 mm. Commonly used types include fixed sluice boxes, Ross sluice boxes, and belt conveyor sluice boxes.

A. Fixed Sluice Box

This is a long sluice box with a uniform width and a fixed bottom. It is an important roughing device for placer gold. It is divided into land-based sluice boxes and draughtsman sluice boxes. Land-based sluices are typically 15 meters long, 0.5-0.6 meters wide, and have a slope of 5°-6°. When processing high-grade placer gold, a set of auxiliary sluices is added at the tail of the main sluice, with a total width 5-10 times that of the main sluice. These are divided into several channels, each 0.7-0.8 meters wide and 6-12 meters long, to supplement the recovery of fine gold particles. On gold-dredging vessels, sluices are generally located on both sides of the cylindrical washing screen (as shown in Figure 2-25), perpendicular to the ship’s axis, with each channel being 0.6-0.8 meters wide. The undersize product from the cylindrical washing screen enters the auxiliary sluice, while the tailings from the auxiliary sluice enter the longitudinal sluice for scavenging to supplement the recovery of gold particles and other heavy minerals. The slope of the auxiliary sluice is 5°-7°, while the slope of the longitudinal sluice is 0.5°-1° less.

Figure 2-25 Sluice configuration on a gold dredger: 1—Cylindrical washing screen; 2—Distributor; 3—Transverse sluice; 4—Longitudinal sluice

Fixed sluice gates come in three types: straight baffles, horizontal baffles, and grid baffles, as shown in Figure 2-26. Straight baffles are made of round or square timber, while horizontal baffles are mostly made of angle steel. These two types of baffles are relatively tall, creating larger eddies suitable for capturing coarser gold and platinum particles, and also have a scrubbing effect. Grid baffles are made of woven wire or by punching slits in iron plates and then stretching them; they are used in gold dredger sluice gates. A rough surface layer is laid beneath the baffles. Common surface materials include embossed rubber mats, reed mats, felt, and plush, used to capture fine gold particles.

Chute baffle types

a – Straight strip baffle; b – Horizontal strip baffle; c – Grid baffle

Fixed sluices operate intermittently, with the cleaning cycle depending on the gold content and other heavy mineral content of the ore. Generally, land sluices are cleaned every 5-10 days, while transverse sluices on gold dredgers are cleaned daily, and longitudinal sluices every 5 days. Each cleaning session lasts approximately 2-8 hours. Cleaning begins with initial rinsing with clean water, followed by removal of baffles and a final concentrated rinse.

The recovery rate of land sluices is generally 60%-70%, with a throughput of 0.5-1.25 m³/(m²·hour). On gold dredgers, single-layer sluices have a throughput of 0.3-0.4 m³/(m²·hour), and double-layer sluices have a throughput of 0.2-0.25 m³/(m²·hour).

B. Ross Sluice

Its structure is shown in Figure 2-27. During operation, ore is dumped into the feed trough at the head of the sluice using a truck or bulldozer, and high-pressure water jets are used to wash the ore, breaking up the sludge. After washing, the ore is flushed onto a perforated screen for sieving. The undersize fine particles enter a side sluice for further separation, while the oversize coarse particles are processed through a central sluice. Both sluices are equipped with baffles to capture gold particles. The entire sluice ranges in length from 4.3 to 12.2 meters and width from 1.8 to 9.7 meters, and is available in six different sizes. The largest size has a processing capacity of 750 cubic meters per hour and a water consumption of 75,000 liters per minute.

Figure 2-27 Ross Sluice

1—Feed trough; 2—Water spray pipe; 3—Perforated screen; 4—Central sluice;

5—First section baffle; 6—Second section baffle; 7—Side sluice

C. Belt Conveyor Sluice

Its structure is shown in Figure 2-28. The sluice itself is an endless rubber belt, installed at a 9° angle, moving upwards at a speed of 0.6 m/s. The surface of the belt is pressed into square grids, with a higher transverse groove at regular intervals, creating eddies in the flowing slurry, loosening and stratifying the mineral particles. Heavy minerals remain in the grids, while light minerals flow with the slurry to the end and are discharged as tailings. The belt has sidewalls to impede the slurry. This type of sluice operates continuously, is easy to operate, and has low labor intensity, and is used on gold dredges for collecting gold nuggets. The chute, with dimensions (width × length) of 0.716 meters × 5 meters, has a processing capacity of 6.4 cubic meters per hour and a maximum feed particle size of 16 millimeters.

Figure 2-28 Belt Conveyor Sluice

2.5.2.2 Fine-Grain Sluice

Used for processing ores with a particle size of 2–0.074 mm. Compared to coarse-grain sluices, fine-grain sluices have a smooth bottom, no baffles or rough paving, a shorter sluice length and a steeper slope, resulting in a weakly turbulent flow of the slurry. Both light and heavy minerals move along the bottom of the sluice and are continuously discharged as concentrate and tailings. Fine-grain sluices include tapered sluices, cone concentrators, inclined disc concentrators, and Johnson washing drums.

A. Tapered Sluice

Its structure is shown in Figure 2-29. It is a sluice with a bottom slope of 16°–20° and a width that gradually narrows from the feed end to the discharge end, also known as a fan-shaped sluice. During operation, a slurry with a concentration of 50%–60% is fed from the top and flows towards the tapered discharge end. The mineral particles stratify according to density, with the lower layer of heavy minerals flowing slowly and the upper layer of light minerals flowing quickly. Upon reaching the discharge end, heavy products, intermediate products, and light products are separated using the discharge slots at the bottom of the trough or the cutting plate at the tail of the trough. The length of the trough is generally 600–1200 mm, the width at the feed end is 125–400 mm, the width at the discharge end is 10–25 mm, and the ratio of the widths at both ends (contraction ratio) is 10–20.

During operation, the feed concentration should be strictly controlled, with a fluctuation range of ±2%. The effective recovered particle size is 2.5–0.038 mm. The processing capacity of a single constriction sluice is 0.9 tons/hour. Multiple sluices are commonly used in combination, with three combination methods: circular combination, parallel arrangement combination, and multi-layer multi-stage combination. Constriction sluices are generally used as roughing equipment for separating gold, titanium, zirconium, tantalum, and niobium from coastal placer deposits. They can also be used to process gold, tungsten, tin, titanium, zirconium ores, and hematite ores.

Figure 2-29 Working principle of a tapered chute

1—Chute; 2—Fan-shaped surface; 3—Light product;

4—Heavy product; 5—Intermediate product; 6—Cutting plate

B. Cone Cone Distiller

Its structure is shown in Figure 2-30. Developed by E. Reichert of Australia in the 1950s, it is also known as the Reichert Cone Distiller. It is an inverted cone with a 17° slope, called the separating cone, with a slurry distribution cone positioned directly above it. During operation, a slurry with a concentration of 55%–65% is evenly fed into the separating cone through the periphery of the distribution cone, then flows towards the center. The concentrate is discharged through an annular slit in the central section, while the tailings are discharged through the central tailings pipe. Its separation principle is the same as that of a converging sluice, but without the sidewall vortex effect.

A cone constiller consists of several vertically arranged separating sections composed of multiple separating cones. There are two types of sorting cones: single-layer and double-layer. Figure 2-30 shows a sorting section consisting of a double-layer sorting cone and a single-layer sorting cone, which completes one roughing and one cleaning operation. The concentrate obtained from this sorting section can be further cleaned using a converging sluice, and the tailings are sent to the next sorting section for scavenging.

Figure 2-30 Schematic diagram of cone concentrator operation

1—Distribution cone; 2—Double-layer separation cone; 3—Single-layer separation cone

Both the separating cone and the distribution cone are made of fiberglass, making them lightweight and sturdy, and are mounted on the frame. A cone concentrator typically consists of 3 to 9 separating cones forming 2 to 4 separation stages, with various combinations available.

The cone concentrator has a diameter of 2 meters and a processing capacity of 60-75 tons/hour, effectively recovering particles of 2-0.03 mm. In the 1980s, Australia developed cone concentrators with diameters of 3 meters and 3.5 meters, with processing capacities of 200-300 tons/hour.

Cone concentrators have high processing capacity and low production costs, and are widely used as roughing equipment for placer and vein ores.

C. Inclined Disc Concentrator

Its structure is shown in Figure 2-31. It consists of a rotating disc mounted at an angle of 32°-35°, with an Archimedean spiral pattern on its surface. During operation, the slurry is fed into the disc from one side. In the settling zone at the bottom of the disc, particles settle according to density and size. The settled particles rise to the other side as the disc rotates, where they are washed by water and then flow downwards with the wash water. The material rotates within approximately one-third of the disc’s area and stratifies according to density. Lighter minerals move to the surface and overflow from the disc with the slurry, becoming tailings. Heavier minerals move to the lower layers, falling into the Archimedes’ spiral grooves and being carried along the grooves to the center of the disc, where they are discharged as concentrate.

Figure 2-31 Principle of Inclined Disc Separator

1—Concentrate Discharge Hole; 2—Flow Stabilizer; 3—Disc Rotation Direction;

A—Feeding Zone; B—Settling Zone; C—Washing Zone

The disc is made of fiberglass, and the disc surface is molded with epoxy resin mixed with corundum (-150 mesh). It is lightweight, durable, and easy to move, suitable for dispersing small placer deposits and placer gold deposits. An 800 mm diameter inclined disc separator has a processing capacity of 1.5–3 tons/hour, with a particle size of -6 mm.

The effective recovery particle size limit is 0.074 mm, and the enrichment ratio can reach 20–35. D. Johnson Concentrator

The structure is shown in Figure 2-32. Originally used in South Africa, it is a slowly rotating, tilted sorting cylinder, 3.6 meters long and 0.75 meters in diameter. The inner wall is lined with grooved rubber plates, the grooves being 3 mm deep and 6 mm wide, with a serrated cross-section. The cylinder rotates at 0.1–0.3 rpm. A slurry with a concentration of 30%–60% (usually 55%) is fed into the cylinder. Driven by the rotation, the slurry flows along the cylinder wall, creating shearing action within the cylinder. This causes stratification according to density; lighter minerals are transferred to the surface and carried away by the slurry, while heavier minerals are deposited in the grooves and carried to the top by the rotation, where they are washed away by flushing water and fall into the concentrate bin. It is mainly used in grinding circuits to recover individual gold particles, with a processing capacity of 5–20 tons/hour.

Figure 2-32 Johnson Concentrator

1—Separation Cylinder; 2—Cylinder Rotation Direction; 3—Groove Rubber Plate; 4—Concentrate Trough; 5—Tailorage Trough

2.5.2.3 Fine Particle (Slime) Sluice Box

Used for processing materials with a particle size less than 0.074 mm, widely used for separating tungsten, tin, tantalum, niobium, gold, and platinum slime. It utilizes inclined flow (thin film) for separation. The film thickness is approximately 1 mm, and the flow velocity is relatively slow, essentially laminar. Mineral particles are mainly loosely suspended by the Begno force generated by shear motion. The concentration of the slurry in the upper layer of the film is low, and the mineral particles stratify according to interference settling velocity. The concentration of the slurry in the lower part of the film is very high, and the mineral particles cannot be suspended, moving in layers and stratifying according to segregation. Therefore, heavy mineral particles aggregate in the lower layer, and light minerals in the upper layer. Light minerals in the upper layer are carried out by the water flow as tailings, while heavy minerals in the lower layer are deposited at the bottom of the sluice and are either continuously carried out by the counter-moving sluice surface or periodically discharged as concentrate after the feed stops.

According to the operation method, fine particle sluices are divided into intermittent working sluices and continuous working sluices. According to the separation principle, they are divided into sluices with simple film flow and sluices combining film flow and shaking action. The former includes slime belt sluices, while the latter includes cross-flow belt sluices, vibrating belt sluices, Mozley concentrators, and double-unit transverse shaking tilting tables. The combination of multiple force fields is the development direction of slime sluices.

A. Slime Belt Sluice

Its structure is shown in Figure 2-33. It is an inclined, endless belt that moves against the direction of slurry flow. Mineral separation takes place on the belt surface, which is 3 meters long and 1 meter wide. Slurry and wash water are fed onto the belt surface from above via a distribution plate. The area below the belt is the coarsening zone, approximately 2.4 meters long. Above the conveyor belt is the refining zone, approximately 0.6 meters long. The slurry flows downwards in a thin layer along the belt surface and stratifies according to density. The upper layer of lighter minerals is carried by the water flow over the tail pulley and discharged into the tailings trough. The lower layer of heavier minerals is deposited on the belt surface, carried by the conveyor belt to the refining zone to be washed by the flushing water, and finally discharged into the concentrate trough around the first pulley. The belt surface slope is 13°~17°, and the belt speed is 0.03 m/s. The feed concentration is 25%~35%, and the feed particle size is 0.074~0.01 mm. The throughput is 1.2~3 tons/hour, and the enrichment ratio is 4~7. It is widely used for the refining of tungsten and tin ore slime.

Figure 2-33 Slime Belt Conveyor Chute

1—Belt surface; 2—Feed distribution plate; 3—Water distribution plate; 4—Concentrate trough; 5—Tailure trough

B. Crossflow Belt Conveyor Chute

Its structure is shown in Figure 2-34. It is an endless belt conveyor, 1.2 meters wide and 2.75 meters long, installed horizontally in the longitudinal direction and inclined at 0°~3° in the transverse direction. The entire belt and brackets are suspended from the frame by four steel wire ropes.

Figure 2-34 Schematic diagram of cross-flow conveyor belt chute

A – Roughing zone; B – Middlings zone; C – Cleaning zone;

1 – Feed trough; 2 – Middlings return trough;

3 – Horizontal wash trough; 4 – Longitudinal wash trough;

5 – Belt movement direction; 6 – Counterweight;

7 – Concentrate; 8 – Middlings; 9 – Tailings

A rotating unbalanced weight is installed below the belt near the first pulley end. The belt surface moves longitudinally at a speed of 0.01 m/s, while simultaneously undergoing planar gyratory motion driven by the unbalanced weight. During operation, the slurry is evenly fed onto the belt surface through the feed trough. Under the combined action of the shear force generated by the transverse inclined water flow and the gyratory motion, the mineral particles are loosened and stratified according to density. The stratified mineral particles are distributed in a fan shape on the belt surface and are finally carried out by the transverse water flow and the longitudinally moving belt, becoming concentrate, middlings, and tailings.

The equipment accepts ore particles of 0.1~0.005 mm, has a processing capacity of 4~5 tons/hour, an effective recovery particle size limit of 0.01~0.005 mm, and an enrichment ratio as high as 10~50, making it particularly suitable for the beneficiation of mineral slime.

C. Vibrating Belt Conveyor Chute

Its structure is shown in Figure 2-35. The working face is an infinitely curved belt with a longitudinal slope of 1°~4°. The belt moves upward at a speed of 0.035 m/s, and through a swinging and rocking mechanism, it generates left-right swinging and asymmetrical reciprocating motion along the longitudinal direction. During operation, under the action of water flow and combined rocking motion, the mineral particles on the belt surface rapidly stratify. Fine heavy mineral particles are distributed on both sides of the belt, while coarser heavy mineral particles are deposited in the lower layer in the center of the belt surface. These are conveyed by the belt to the beneficiation zone and finally discharged as concentrate. Light mineral particles are distributed in the upper layer in the center of the belt surface and flow with the water to the tailings end as tailings.

Figure 2-35 Vibrating Belt Conveyor Chute

1—Sorting belt; 2—Feed trough; 3—Water feed trough; 4—Shaking mechanism; 5—Swinging mechanism; 6—Tailor pipe; 7—Concentrate pipe

A single unit with dimensions of 800 mm × 2500 mm has a processing capacity of 71–85 kg/hour, an effective recovery particle size limit of 0.02 mm, and an enrichment ratio of approximately 10. It is suitable for slime beneficiation, achieving a recovery rate of over 70%.

D. Mozley Concentrator

Its structure is shown in Figure 2-36. Developed by R.H. Mozley in 1967, it is a fine-particle sluice with a rotating shearing motion on the bed surface. The bed surface is made of fiberglass, 1525 mm long and 1220 mm wide, with 40 layers housed in two frames, also known as a 40-layer rotating tilting bed. A rotating unbalanced weight is installed between the two frames, causing the bed surface to perform planar rotating shearing motion. The frames are suspended by steel wire ropes and periodically rotated by a pneumatic device to discharge concentrate. Its most significant feature is the use of the Begno force to enhance the separation of fine particles. The effective recovery particle size limit is 0.01–0.005 mm (based on cassiterite), the total bed area is 74.4 m², the processing capacity is 2.1–3.1 tons/hour, the enrichment ratio is 3–6, and the operating recovery rate is 65%–75%. It is suitable for roughing of tungsten, tin, tantalum, niobium, gold, and platinum ore slimes.

E’ Double-unit transverse shaking tilting bed

Figure 2-36, 40-layer shaking tilting bed

a—Separation stage; b—Concentrate discharge stage

1—Frame; 2, 3—Upper and lower bed surfaces; 4—Wire rope; 5—Pneumatic device; 6—Feed

Its structure is shown in Figure 2-37. The bed surface moves symmetrically back and forth along the longitudinal direction, and its working principle is similar to that of a shaking table. During separation, only tailings are discharged, while heavy minerals remain on the bed surface.

Figure 2-37 Double-unit transverse rocking tilting table

1—Separation stage table surface; 2—Concentrate discharge stage table surface;

3—Feed distributor; 4—Frame; 5—Concentrate trough

The concentrate is discharged after the feed is stopped. Each unit has two sets of 1-4 bed surfaces, which operate alternately. The bed surface is 2.7 meters wide and 1.2 meters long. The slurry is fed onto one set of bed surfaces along the entire width. After separation, the bed surface rotates 50°, and water is sprayed to collect the concentrate. The transverse slope of the bed surface is 1.3°~2.5°, the stroke is 40~180 mm, and the stroke rate is 90~120 rpm. The feed particle size is -0.3 mm, the processing capacity is 5 tons/hour, and the enrichment ratio is 20~500. It is suitable for the separation of tungsten, tin, tantalum, niobium, gold, and platinum minerals.