All about enhanced gravity separator: Falcon and Knelson Concentrator.

Separation of minerals from gangue using gravity separation technology is not recent, and humans are using it since the last 2000 years without much change in the process. Although growing technology in the previous century found various forces that could be used to extract minerals, gravity separation is not obsoleted from major mineral processing plants. For some minerals, it is even at least the primary stage for beneficiation. Inventors use a phenomenon where particles can be separated using the differential setting rates, which happened due to specific gravity posses by each particle. In the market, we can found several types of equipment to serve the mentioned purpose. For example, jigs, shaking concentrators, and flowing film concentrators. These equipment have proven capability in the industrial scale.

Not only gravity separation is as comparable efficiency with other technology in mineral processing, but it also has some additional economic and environmental benefits. Gravity separation equipment required a lower cost of installation per tonne of throughput than flotation. It also energy effective and environmentally friendly because no reagent is used. 

However, the conventional gravity separation is mostly useful for coarser material, and fines are misplaced. Hence, inventors (four significant inventors out of which three are not from the mineral processing background and Mozley was a professional engineer) tried to separate desirable minerals from the finer section by increasing the amount of gravitational force on the particle which called as enhanced gravity separation. Now we can go up to -25micron using gravitation separation technology.

The scope of this blog is limited to the enhanced gravity separation and there working principle but not about modeling and product prediction. First, we will discuss the phenomenon of gravity separation and the behaviors of a particle in it. Secondly, we will discuss enhanced gravity separation and there working models. In the end, we will go through the comparison between the EGS. For the first, we will review only two separators: Falcon Concentrator(FC) and Knelson Concentrator (KC), in the second and final part of this article we will see rest of the concentrators and their working principle. Now, let's get-go deep down to gravity separation.

 What is gravity separation
Simply put, the separation of two or more minerals by gravity is the result of their relative movement in response to gravity and one or more other forces. The factors which are essential in determining the relative motion of a particle in the fluid consist of the specific gravity, weight, size, and particle shape in absolute terms and relative to all other particles in the system. It is a characteristic of gravity separation devices that, for the particles to move relative to one another, they must be held slightly apart from each other, or 'dilated' by the superimposed force. Four mechanisms primarily explain the operation of gravity concentration devices: density, stratification; flowing film; and horizontal shear.[Burts description]

However, not all mineral combinations are amenable to this type of beneficiation technique. A concentration criterion is a term coined to define which particular ore can be used or not. Burt draws graphs and specified a line. The concentration criterion number above the line can only be used for the gravity concentration.

Size Limit Curve for Gravity Separation

The separation of mineral by gravity separation relies on a particle settling rate in a fluid. The terminal velocity of spherical particle for coarser material can be described using Newton's law, and for the fine strokes, the law is applicable. For this case, fines are those materials whose Reynolds number is less than one, and coarser materials are those whose Re>1000. Both these equations consist of density as well as particle size. Some researchers also made dimensional parameters to show the effect of particle shape and angularity factor. 

Principle of Enhanced Gravity Separation and its requirement.

Ineffective separation of heavy minerals occurs at the fine particle size is a difficult problem in mineral processing and particularly in gravity separation. As the particle size decreases, the force associated with the water flow becomes dominant over the associated with gravity. Because of this reason, a substantially large part of the fine particles' desirable minerals proved to be impossible to recover using conventional technology for gravity separation. To mitigate this, various techniques had introduced, but the multi gravity separation came as an effective and reliable solution. The poor performance of gravity-based separators can be solved by increasing the settling rate of the fine particle. Settling velocity of a fine particle (2mm to -40micron)in a fluid can be varied from 1e-9 to 1e-4, which is not appropriate due to the requirement of higher time to achieved terminal velocity. Therefore to make the terminal velocity within a small duration of time, acceleration should be high enough. Now, the machines are designed to enhance the acceleration using centrifugal force, which is more than 50 to 300 times, and the resultant velocity of particles is 500 to 30000 times more. Some particles that might be coarser may not reach terminal velocity within short residence time in the machine. Therefore enhanced gravity separation is required.

The enhanced gravity concentrators that operate continuously and are commercially available include the Multi-Gravity Separator (MGS), the Kelsey Jig, the Knelson Concentrator, and the Falcon Concentrator. This multi gravity separator is used not only in mineral separation but also for dewatering, thickening, and regeneration of reagents used for flotation.

EGS is used in many forms, and it is still in continuous research. Down below, we will talk about the equipment that uses EGS and its advantages and drawbacks one by one, and in the end, we will make a qualitative and quantitative comparison.

Falcon Concentrator

Falcon Concentrator based on the enhanced gravity separation to decreases the sedimentation time of particle and so that the differential settling increases and particles get separated based on the density. It consists of a fast-spinning bowl. When feed is introduced from the bottom center, it uses centrifugal force to drain the slurry in a flowing film at its wall. Spring of the bowl can artificially increase the unit gravity force on the particle to 600 times. It is found that in the bowl, various mechanisms found for separation. The separation process within nelson and falcon concentration bowls based on two primary mechanisms:

1. Differential settling of the particle in the flowing film along the inside wall of the bowl
2. The selective reorganization of the particles in the retention zone through fluidization

The primary one is predominant in the falcon concentrator.

Based on the particle trapped in the retention zone, mainly three types of FC are present.

1. Falcon SB (Semi-batch)
2. Falcon C (Continuous)
3. Falcon UF (Ultrafine)

Falcon SB series use fluidized annular grooves upstream of the bowl outlet to avoid compaction and adjust the retention capacity by injection of counter-pressure water through the concentrate ridges. Falcon C series are operated continuously without any water addition due to subdivisions into hoppers with air operated valves to control the flow in the retention zone. Falcon UF concentrator series use a smooth bowl with a retention zone delimited by a slight reduction in diameter at the outlet, specifically designed to recover ultrafine particles (−5 µm). The bowl can be equipped with a variable lip in the retention zone to adjust its capacity. No fluidization counter-pressure is applied in these concentrators to prevent flushing out fine particles. Both Falcon SB and UF are operated in semi-batch mode and must be stopped before saturation of the bowl to avoid concentration losses by erosion or by unselective separation. Thanks to its design-oriented towards ultrafine particles recovery, Falcon UF has been successfully employed to recover both metal-bearing heavy minerals (tin, tungsten, tantalum, chrome, and cobalt) and native metals (gold, silver) with particle sizes down to 3 µm

In the continuous mode of operation, feed is fed from the bottom, and the impeller is employing to make a homogenous mixture in a cone. The bowl spins with very high acceleration and so that the more massive dense particle moves towards the wall and the light particle goes above. Once the more heavy particle gets towards the wall, the bed is formed in flowing film. The heavy concentrate moves and drained through the small outlet. Moreover, some particle gets stuck in the bowl. If the feed tonnage increase, then percentage concentrate also increases, but at one point, it gets constant.

 As per the recent studies, researchers divided the FC process into four stages.

1. When the bed is not formed, the separation starts by a desliming-only phase that is controlled by differential settling in the flowing film;
2. As the concentrate bed grows, the flowing film gets thinner, and shear increases in the flow, which makes particles at the surface layer of the bed undergo a lift force that can resuspend them into the tailings stream. The balance between this lift force and the enhanced gravity tends to suspend coarser and less dense particles first. During this phase, separation evolves from a differential settling driven selection to a resuspension driven selection;
3. When the bed is fully formed, the flowing film is fragile and highly sheared so that selective erosion of the bed is sturdy and concentrate grade continues to increase although the mass of concentrate remains constant. During this stage, dense and fine particles enter the retention zone as coarser and less dense particles make room for them while being resuspended. This leads to the emergence of furrows of higher-grade material at the surface of the bed;
4. As the surface layer of the bed contains higher-grade material and less gangue, the resuspension mechanism ejects more dense particles, until it balances the stream of particles settling at the bed surface and separation finally drops as the bed is saturated.

During all four stages, the deposition of particles onto the bed is always governed by differential settling, which does not depend on the flowing film thickness, so that desliming of fine particles stays the same. In literature, we can found that Kroll-Rubutin did the prediction model for FC recovery of the desirable mineral. From the last five decades, FC is continuously using for the beneficiation of iron, tin, titanium, gold, coal, and silver.

Knelson Concentrator (KC)

Knelson is similar equipment like a falcon concentrator where gravitational force artificially enhances by centrifugal force to beneficiate economic mineral from finer size classes with a very high enrichment ratio (1000:1). Unlikely to FC, its KC is a semi-continuous batched process. The KC was invented in the late '70s and was commercialized in 1980. This equipment consists of 4 cones in a similar axis in which rings are made for the film to form in this ring holes are drilled to flow fluid inward at a specific pressure. The original gravity separators were accurate for coarser material, but the fine particles get misplaced in the conventional method. To solve this problem, Knelson comes up with the idea that by injecting water into the bowl under pressure to counter the centrifugal force and create fluidization, then we can separate heavier fine particles using stratification. It could be used in the range of 0-180g

Design of KC

The concentrate bowl is the core component of a KC, and its structure has been iterated four times. The first-generation bowl comprises an inner cone with an inclination of 30° and an outer cylindrical cone, and the pressure water holes are drilled through the vertical cone wall. Both the inner and outer second bowls are cylindrical, and the depth of the ring gradually increases from the top to the bottom, which makes the water pressure required to fluidize the bottom rings much more significant than that in the top rings. The third unit was designed with a stepped inner cone bowl and a conical-shaped outer bowl; moreover, tangential backpressure water injection was introduced. A further important innovation was launched in 1984 in the form of a "V" shaped ring. The fourth and latest unit was designed with a wedge-shaped profile for the inner bowl rings, and the inner bowl rings were molded in polyurethane to a conical stainless steel casing.

This equipment is mainly divided into two subparts based on its discharge method for concentrate.

1. Semi-Continuous discharge concentrator 
2. Manual discharge series
3. Extended duty series 
4. Center discharge series
5. Quantum series
6. Continuous variable discharge concentrator. 
7. A centrifugal accelerating motion through the dilute zone of the inner bowl
8. Proclamation or migration through the separation zone toward the concentrate bed 

The fundamental separation principle of a KC is based on the difference in the settling velocity of mineral particles in the centrifugal field under the action of fluidization water. As soon as the feed material enters the conical bowl and descends onto the base plate at the bottom of the rotating conical bowl as slurry through the central feed pipe, the centrifuged mineral particles move immediately toward the conical wall of the concentrate bed at different rates depending on their sizes and specific gravities. At the same time, fluidization water at high pressure is tangentially injected into the fluidization holes opposite to the bowl rotation direction, which can prevent compaction, creating a fluidized concentrate bed. Under the effect of the centrifugal force and water fluidization, dense particles settle in the rings as a concentrate. In contrast, the lighter gangue particles are transported out of the bowl by the upward flow slurry as tailings.

The movement of a particle in an inner bowl of the KC can be divided into two distinct stages based on the structure of the flowing slurry.

While designing KC, rings are made to give area to form the fluidized bed, which helps to possible separation mechanism. The rings are gradually filled with solid, dense, light particles from the outer to the inner and completely fluidized at the beginning of the test. Further, as soon as the concentrate bed builds-up, the selective recovery begins on the surface of concentrate bed; there is little or no mass transfer between the material subsequently recovered and solids that are already in the outer ring, even fine dense particles. Thus, the densest minerals are mainly recovered in the inner groove sections, and the outer rings retain their coarse size and low grade. Moreover, consolidation trickling was proposed by Huang to explain the selective recovery of the fine high-density particles.

Furthermore, it expanded on the recovery mechanism of the KC by investigating the percolation or migration of dense particles in a gangue bed using a fluidized bed column in the gravitational field. This study showed that the percolation of dense particles was maximized at an intermediate fluidization flow in the gravitational field. Insufficient voidage would inhibit or limit the percolation of all but the coarsest dense particles below this flow magnitude. Above this flow magnitude, the fine, dense particles would be prevented from percolating because of the drag force of the ascending fluidization flow.

Similarly, Settling tests in the gravitational field were conducted in coarse and fine gangue beds. The results showed that dense particle recovery for the coarse gangue bed is high for coarser fractions when the bed is partially fluidized. Whereas, for the fine bed, most of the dense mineral percolated through the fine gangue bed easily, indicating that the resistance of the gangue bed to the percolation of dense particles is a function of bed voidage, particle size and density of the gangue bed. However, limited by the feed mass of each test and selection of other experimental parameters, those studies explored only one type of separation mechanism that could be defined as "surface plating."

Distinct Factors about the KC every operator should know:

1. During the separation process, most of the heavy particles are collected at the lower three rings and the bottom of the bowl in comparison with the upper two rings.
2. Feed type has a significant effect on both separation performance indices, i.e., grade and recovery. A considerable difference between densities of heavy and light particles will lead to a higher concentrate grade and a lower total recovery.
3. Increasing feed grade will enhance the concentrate grade and decrease the total recovery.
4. When feed particles get more substantial in size, the concentration drops sharply, whereas the total recovery gradually grows.
5. When feed particle population increases from 5000 to 100,000, both separation responses increase, but with further increase in feed particle population the concentrate grade remains unchanged, and the total recovery reduces
6. By enhancing the KC bowl rotational speed, the concentrate grade declines linearly due to the collection of more light particles within the bowl by a higher RCF while the total recovery increases.
7. When the feeding rate increases, the concentrate grade has no distinct trend, but the total recovery decreases because the retention time of particles inside the bowl decreases.
8. By increasing the concentration-time from one to four seconds, both the concentrate grade and the total recovery increase until two seconds, but after that, a further rise in concentration-time does not affect both separation indices.

The efficiency of KC is affected by many factors, such as:

1. Material properties (Density, particle size, shape)

1. Operating parameters (Fluidisation flowrate, rotational speed, feed rate, pulp density, enrichment cycle) 

In the KC, mainly two problems occur.

Unbalanced water pressure can make change the trajectory of the particle. When the pressure gradient is more than the centrifugal force, the fine particle will move towards the center. In the contradictory situation, where the pressure gradient is substantially low, then the particle will remain stratified. 

KC employed in various mineral processing plants. Mainly it is used to beneficiate gold and platinum. Additionally, it is using to separate rare earth mineral, tin, chromite, coal, etc.,

In the second part of this article, readers will get to know about the detailed review of Kelsay Jig and Mozley Multi-gravity separator. The second part will be mostly dedicated to comparing the EGS equipment and selection criteria of a particular ore. The CVD was designed for the base metal and coal industries. The current industrial application of CVD is mainly a scavenger to recover valuable metals in flotation tailings; meanwhile, it has considerable application potential for removing impurities and enriching fine coal

 --------------------------------------------------------------------------------------------------------------About the Author

Sachin Urade is a recent graduate student from IIT(ISM) Dhanbad, major in Mineral processing. He is dedicated to spreading his knowledge and skills to society. Writing blogs on mineral processing and extractive metallurgy is a freelancing work for him. He is open to the opportunity to be part of magazines and journals as a freelance editor and writer.  

Declaration: The author is taking some of the images from some research papers that are not here to make any money, and the author is not claiming these images. He took these images to make readers visible to what he is talking about. He also wants to show gratitude for to there work(credits are given to each one)