Insights of Rake Arms in Classifiers.

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The miner processing plant is designed to beneficiate desirable mineral from the gangue, and for that, the ore should be crushed and ground up to the liberation size. Currently, the liberation size of most of the metal is below 200 microns. To benefit this ore, wet processing is in the heart of mineral processing, which caused lots of water content in the final products. For removal of liquid from the pulp or slimes, which is a mixture of solid particles and liquid are passed through the thickener.

 

Thickener is the device through which liquid gets separated. Earlier, this process is done by employing a cylinder to fill with the pulp and then rest for the sedimentation. Sedimentation cause settling of particle and then the water is removed, and solids get separate. However, this was the batch process and took a considerable time. Therefore, inventors tried to operate sedimentation in a continuous process. However, the continuous process is practically impossible to get perfect separation. The first thickener designed a method by Clevenger where thickeners are fed from the center, and then it is distributed. Nevertheless, this continuous process also demands a large area, and sedimentation time or retention time of particle was huge. To solve the mentioned problem, inventors made high rate thickeners where they used polymers to aggregate ultrafine and fine particles and then consecutively done settling, which reduced the sedimentation time substantially. HRT works on two principal:

• The solid particle should respond well to flocculation 
• That the process sustains a permissible amount of polymer in both underflow and overflow due to uncontrolled overdosage

In simple words, a conventional classifier in a mineral processing plant is circular conical equipment where sedimentation happens and after that liquid reported in an overflow side and solid (pulp with high solid concentration). Whereas in a high rate thickener polymers help to reduce the time for settling. The principal force on the particle to settle is:

1. Gravitational force
2. Mechanical transporting Force through Rake

 

Conventional Thickener

Things you should remember about the basic thickner.

1.  The feedweel function is to dissipate energy of movement in the feed stream to cause the feed to enter the tank in a relatively qunincent condition and to provide a means of introducing the slurry at an appropriate depth in the thickener

2. The inlet stream going to thickener is influent and overflow stream known as effluent. Underflow has many names: mud, slurry, pulp, & sludge.

3. Cone, or Trench, scrapers act like the Rake Arms so that underflow solids can be discharged.

4. The overflow launder collects clarified liquor to transport peripheral liquid.

5. The area of a thickener must provide sufficient retention time to allow the slowest settling particle to reach the unit's bottom.

6. The overflow rate should be at a minimum to prevent separation via settling.

7. The flocculation of the feed must be controlled and must be complete. 

8. Feed velocity to thickener should b as low as theoretically calculated. The feed velocity must be high enough to prevent solid material "Sands out" in the feed launder. However, it shouldn't be so high as to cause extra turbulence in the feed wall.

9. The rake arm serves three functions

• Move the settled solids towards the discharge point;
• Maintain a degree of fluidity in the thickener to ensure hydraulic removal
• It increases underflow solids concentration b providing for channels for water to escape from te thickening solids in the compression.

10. Profile Curve:

1. 

 

Now let's talk about insights of Rake.

Although the thickener, which is cylindrical with the conical bottom with a steep slope designed to give gravitational force towards the bottom, a substantial amount of slimes and solids remain on that conical shape, to remove this stick material and speed up the process rakes are employed. Rake is the moving arm that has blades to move the particles. Following this, we should know.

• Drag force: This drag force (referred to hereafter only as of the "drag") acts in the opposite direction to rake motion and is generally a function of radius, rake speed, and sediment rheology. Although the drag force in itself is not an efficiency measure, it is a significant quantity to consider for rake operation because it can be integrated along each rake arm to provide an estimate of the total rake torque. Here we calculate the drag for each blade alone although it is essential to keep in mind that the drag due to the arm can also be a significant contributor to torque 
• Underflow fraction: It is a ratio of horizontal contribution into the underflow region from raking to total volumetric underflow. Defined what was termed a "rake blade efficiency" as the ratio QR to QUF.
• If UFF increases above one, then the rakes are delivering more material than the underflow is removing, and thus, it also an indicator of over-delivery at the underflow. 
• Raking efficiency: The "raking effectiveness" (RF) is defined as the ratio of the average radial velocity (Vr) to the average circumferential velocity (Vθ).
• RF is an indication of how much inward motion is generated by raking compared to rotational motion. As RF decreases, the bed is rotating more, and the likelihood of donut formation increases. In such a case, the rake torque usually increases significantly, and because there is no relative motion between Rake and sediment, there can be no sediment transport toward the underflow. 
• Inflow Fraction: The inflow fraction is a function of radius and is defined to be the ratio of sediment moved inwards at the level of the rake arm and below divided by the total volume of material moving inwards at that radius,
• raking efficiency provides useful information on the detailed design of individual rake blades; raking effectiveness indicates the likelihood of donut formation; 
• As per the recent research, blade size should not be similar; otherwise, the torque near the spigot will increase, causing excessive turmoil. 
• Some studies said that efficiency increased as the blade height and length were increased and that the optimal blade angle was between 20 to 30
• Since the entire thickener operation depends on its ability to continuously move dense underflows (up to 70-80% solids w/w), the rake design needs to be extremely robust. One of the few wear parts in a thickener, replaced anywhere from every one to every five years, depending on the duty. 

3. The rake arm serves three functions

• Move the settled solids towards the discharge point;
• Maintain a degree of fluidity in the thickener to ensure hydraulic removal
• It increases underflow solids concentration b providing for channels for water to escape from te thickening solids in the compression.

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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)