Screen scroll centrifuge

Last updated

Screen/Scroll centrifuge is a filtering or screen centrifuge which is also known as worm screen or conveyor discharge centrifuge. This centrifuge was first introduced in the midst of 19th century. After developing new technologies over the decades, it is now one of the widely used processes in many industries for the separation of crystalline, granular or fibrous materials from a solid-liquid mixture. Also, this process is considered to dry the solid material. This process has been some of the most frequently seen within, especially, coal preparation industry. Moreover, it can be found in other industries such as chemical, environmental, food and other mining fields.

Contents

Fundamentals

Screen scroll centrifuge is a filtering centrifuge which separates solids and liquid from a solid-liquid mixture. This type of centrifuge is commonly used with a continuous process in which slurry containing both solid and liquid is continuously fed into and continuously discharged from the centrifuge. In a typical screen scroll centrifuge, the basic principle is that entering feed is separated into liquid and solids as two products. The feed is transported from small to larger diameter end of frustoconical basket by the inclination of the screen basket and slightly different speed of the scraper worm. [1] The solid material retained on the screen is moved along the cone via an internal screw conveyor while the liquid output is obtained due to centrifugal force causes the feed slurry to pass through the screen openings. [2] Furthermore, screen scroll centrifuge may rotate either in horizontal or vertical position.

Range of applications

The use of screen scroll centrifuge has been seen in numerous process engineering industries. One of the most noticeable applications is within coal preparation industry. In addition to that, this centrifuge is also employed in the dewatering of potash, gilsonite, in salt processes and in dewatering various sands. [3] Moreover, it is also designed for use in the food processing industry, for instant, dairy production, and cocoa butter equivalents and other confectionery fats. [4]

Designs available

Schematic of 3 stage Screen/Srcoll centrifuge Schematic of 3 stage S&S centrifuge.png
Schematic of 3 stage Screen/Srcoll centrifuge

Screen scroll centrifuges, which are also known as worm screen or the conveyor discharge, instigate the solids to move along the cone through an internal screw conveyor. The conveyor in the centrifuge spins at a differential speed to the conical screen and centrifugal forces approximately 1800g - 2600g facilitate reasonable throughputs. [2] [5] Some of the screen scroll centrifuges are available with up to four separate stages for improved performance. The first stage is used to de-liquor the feed which is followed by a washing stage, with the final stage being used for drying. In an advanced screen scroll centrifuge with four stages, two separate washes are employed in order to segregate the wash liquors. [5]

The two most common types of screen/scroll centrifuge used in many industrial applications are vertical screen/scroll centrifuge and horizontal screen/scroll centrifuge.

a typical structure of a vertical screen scroll centrifuge Vertical SS Centrifugre figure.png
a typical structure of a vertical screen scroll centrifuge
Horizontal Centrifuge figure Horizontal Centrifuge figure.png
Horizontal Centrifuge figure

Vertical screen scroll centrifuge

Vertical screen scroll is built with the main components of screen, scroll, basket, housing, and helical screw. Feed containing liquid and solid materials is introduced into vertical screen scroll centrifuge from the top. This is sped up by centrifugal acceleration produced from the rotating parts contacted. As such, centrifugal force slings liquids through the openings, while solids are held on the screen surface as they cannot pass through because of granular particles larger than the screen pores or due to agglomeration. Movement of solids across the screen surface is manipulated by flights. Liquids that have gone through screen are obtained and discharged through effluent outlet from the side of machine, while solids collected from the screen fall by gravity through the bottom discharge of the machine. [3]

Some of the available vertical screen scroll centrifuges are CMI model EBR and CMI model EBW which are manufactured by Centrifugal & Mechanical Industries (CMI). The former can dewater coarser particles size ranging from 1.5 in to 28 mesh whereas the latter can dewater finer particles size ranging from 1 mm to 150 mesh. [3]

Horizontal screen scroll centrifuge

Similar to a vertical screen scroll centrifuge, a horizontal screen scroll centrifuge is constructed of several main parts: screen, scroll, basket, housing, and helical screw. The screen and the basket with frustoconical geometry are assembled into the housing in a horizontal axis. Inside the frustoconical structure there is a tubular wall. Inside the tubular wall there is a cylinder of helical screw which flight on scroll pass. The tubular wall will have a slightly different angular speed to the helical screw. [6]

The solid-liquid mixture is fed into the closed rearward portion of the scroll. The rotation movement of the scroll, screen, and basket allows the liquid to pass through from the openings on the screen (via centrifugal force). The solid remains will be separated according to size due to the difference of the angular velocity of the helical screw and the basket. The helical screw pushes the solid material to be discharged to the forward end of the scroll. The processing time depends on helical screw pitch and the angular velocity difference. It may also be influenced by the design of the scroll feed opening. The solid particles exiting are usually collected via a conveyor in the collection unit. [6]

Main process characteristics and its assessment

The performance and output efficiency of the screen scroll centrifuge can be affected by several factors, such as particle size and feed concentration, flow rate of feed and screen mesh size of the centrifuge.

Particle size and feed solids

Particle size in the feed is one of the most important parameters to be taken into account since the choice of slot and screen holes size of screen scroll centrifuge or different types of process depends on feed contents. Non-uniform particles size in the feed can cause partial blockage on the screen due to the small size solids blocking the holes besides normal and larger particles. So, liquids flow over the screen instead of passing through it. As such, it requires higher solids content in the feed in order to obtain good and reasonable results - normally greater than 15% and up to 60% w/w. [5] Nevertheless, the flow rate of the feed can be monitored to overcome this setback. Another possible method is to carry out pre-treatment on the feed to be used for screen scroll centrifuge, for example, by the filtration process. Particle size, thereafter, can be analysed and the selection of particular screen size can be determined. However, it increases the total operating cost.

Typical operating range of particle size and feed concentration for screen scroll centrifuges are 100 – 20,000 µm and 3 – 90% mass of the solids in the feed. [7] In general, slot and screen holes size range 40 - 200 µm with open areas from 5 - 15%. [5] Nevertheless, recent products are claimed to be able to handle the particle size as low as 50 µm. [3] Screens are generally metallic foil or wedge wire and more recently metallic and composite screens perforated with micro-waterjet cutting.

Feed flow rate

As mentioned in the previous section, feed flow rate is one of the crucial parameters to be controlled to achieve highly efficient output. Centrifuge performance is sensitive to feed flow rate. Even though increasing the feed flow rate can prevent from blocking the screens, it is mentioned that wetter solids is achieved. [8] This is due to increase in hydraulic load on the centrifuge when higher feed rate is applied, while differential rotation speed between the cone and scroll, and retention time within dewatering zone of the basket are fixed. In addition, higher feed rate leads to a surge in the effective thickness of the bed since it is dragged down by the scroll.

Basket geometry and its material

The material variations for constructing and the design of main components of centrifuge such as the screen plate, helical screw and basket could actually improve the longer life term of the machine. Another important factor is the conical basket size and its angle within the centrifuge. Different basket size and angle between basket and helical screw can vary the angular speed; as a result, the quality of the product is affected. Moreover, the shape of the helical screw is also important since it optimizes the transportation of cake. [3] A selection of typical screen scroll centrifuge with different basket sizes found in the market is presented in the following Table 1. The helical scroll and conical basket sections are commonly built at the angle of 10°, 15° and 20°. [5]

Table 1 A selection of screen scroll centrifuge sizes

Basket Diameter, mm2504005508001000
Maximum rpm38003000200015001200
Maximum Centrifugal G at basket lip2000200015001000800
Maximum throughput t.h−13.510255070

Advantages and limitations over competitive processes

The screen scroll centrifuge has an advantage of having a driven scroll helical conveyor which gives a small differential speed relative to the conical basket. The helical conveyor is installed in the centrifuge to control the transport of the incoming feed, allowing the residence time of the solids in the basket to be increased giving enhanced process performance. [5] Moreover, the helical conveyor and conical basket sections are designed in certain angle of 10°, 15° and 20° being common such that solid particles are dragged on the conveyor along the cone towards the discharge point. As a result, there is no formation of even solids layer but form piles of triangular section in front of the blades of the conveyor. The residence time within screen scroll centrifuge is typically about 4 to 15 seconds which is longer than normal simpler conical basket centrifuge. This permits a sufficient interaction time between wash liquids and cake. However, the presence of the conveyor causes crystals breakage and abrasion problem as well as the formation of uneven solids layer which can lead to poor washing. This can be controlled by conveyor speed. [5]

TEMA engineers, specialist in centrifuges, claims that horizontal screen scroll centrifuge can achieve higher overall recovery of fines up to 99% can be achieved, combining with very low product moisture. Furthermore, it is recommended that operating with the feed containing more than 40% solids with minimal size of 100 µm achieve the best results. [9] The use of the screen scroll centrifuge with horizontal orientation is more economical as its capacity is 40% more tonnage than that of vertical orientation of the same size for the same energy cost. In addition, maintenance of the horizontal screen scroll centrifuge can be carried out easily since total disassembly is not needed. [10] Nowadays, screen scroll centrifuges are equipped with CIP-cleaning system for the purpose of self-cleaning within the centrifuge.

On the other hand, it has a downside of possible blockage to the screen due to the feed slurry containing small crystals besides large and normal solids crystals. Consequently, this causes the screen to become less permeable so the liquids flow over the screen rather than passing through the screen mesh. This problem, however, can be overcome by reducing the flow rate of feed. [5]

Possible heuristics to be used during design of the process

The basket, helical screw, screen filter, and other parts are designed to meet up the process input and certain performance. Most of the parts are made from metal to be able to handle the separation process. The bigger the bowl could contain more input but at the same time could increase the process and residence time. The helical screw is made to be able to hold and move the particle around to be able to control the cake movement. The screen filter is made to be able to sieve the particle and the water. The cleaning process for this type of machine could be difficult compare to other separation model. The design mostly being optimized with low maintenance feature and provided with good sealing to prevent the leaking and breakup of the construction. [3] [8]

Necessary post-treatment systems

After removing liquids from the slurry to form a cake of solids in the centrifuge, further or post-treatment is required to completely dry the solids. Drying is the most common process used in the industry. Another post-treatment system is to treat the products with another stage of deliquoring process. [11]

New development

The modern screen/scroll centrifuge has been modified in several ways from the original design:

Related Research Articles

Filtration Process that separates solids from fluids

Filtration is a physical separation process that separates solid matter and fluid from a mixture using a filter medium that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter medium are described as oversize and the fluid that passes through is called the filtrate. Oversize particles may form a filter cake on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known as blinding. The size of the largest particles that can successfully pass through a filter is called the effective pore size of that filter. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles. Filtration occurs both in nature and in engineered systems; there are biological, geological, and industrial forms.

Centrifuge Device using centrifugal force to separate fluids

A centrifuge is a device that uses centrifugal force to separate various components of a fluid. This is achieved by spinning the fluid at high speed within a container, thereby separating fluids of different densities or liquids from solids. It works by causing denser substances and particles to move outward in the radial direction. At the same time, objects that are less dense are displaced and move to the centre. In a laboratory centrifuge that uses sample tubes, the radial acceleration causes denser particles to settle to the bottom of the tube, while low-density substances rise to the top. A centrifuge can be a very effective filter that separates contaminants from the main body of fluid.

Centrifugation

Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate away from the axis of the centrifuge, while the less dense components of the mixture migrate towards the axis. Chemists and biologists may increase the effective gravitational force of the test tube so that the precipitate (pellet) will travel quickly and fully to the bottom of the tube. The remaining liquid that lies above the precipitate is called a supernatant or supernate.

Cyclonic separation Method of removing particulates from a fluid stream through vortex speration

Cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydrocyclone is used; while from gas, a gas cyclone is used. Rotational effects and gravity are used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from a gaseous stream.

Dust collector

A dust collector is a system used to enhance the quality of air released from industrial and commercial processes by collecting dust and other impurities from air or gas. Designed to handle high-volume dust loads, a dust collector system consists of a blower, dust filter, a filter-cleaning system, and a dust receptacle or dust removal system. It is distinguished from air purifiers, which use disposable filters to remove dust.

Coal preparation plant

A coal preparation plant is a facility that washes coal of soil and rock, crushes it into graded sized chunks (sorting), stockpiles grades preparing it for transport to market, and more often than not, also loads coal into rail cars, barges, or ships.

Screw conveyor

A screw conveyor or auger conveyor is a mechanism that uses a rotating helical screw blade, called a "flighting", usually within a tube, to move liquid or granular materials. They are used in many bulk handling industries. Screw conveyors in modern industry are often used horizontally or at a slight incline as an efficient way to move semi-solid materials, including food waste, wood chips, aggregates, cereal grains, animal feed, boiler ash, meat, and bone meal, municipal solid waste, and many others. The first type of screw conveyor was the Archimedes' screw, used since ancient times to pump irrigation water.

Spiral separator

The term spiral separator can refer to either a device for separating slurry components by density, or for a device for sorting particles by shape.

A vacuum ceramic filter is designed to separate liquids from solids for dewatering purposes. The device consists of a rotator, slurry tank, ceramic filter plate, distributor, discharge scraper, cleaning device, frame, agitating device, pipe system, vacuum system, automatic acid dosing system, automatic lubricating system, valve and discharge chute. The operation and construction principle of vacuum ceramic filter resemble those of a conventional disc filter, but the filter medium is replaced by a finely porous ceramic disc. The disc material is inert, has a long operational life and is resistant to almost all chemicals. Performance can be optimized by taking into account all those factors which affect the overall efficiency of the separation process. Some of the variables affecting the performance of a vacuum ceramic filter include the solid concentration, speed rotation of the disc, slurry level in the feed basin, temperature of the feed slurry, and the pressure during dewatering stages and filter cake formation.

A slurry pump is a type of pump designed for pumping liquid containing solid particles. Slurry pumps changes in design and construction to adjust to multiple type of slurry which varies in concentration of solids, size of solid particles, shape of solid particles, and composition of solution. Slurry pump are more robust than liquid pumps; they have added sacrificial material and replaceable wear parts to withstand wear due to abrasion.

Centrifugal extractor

A centrifugal extractor—also known as a centrifugal contactor or annular centrifugal contactor—uses the rotation of the rotor inside a centrifuge to mix two immiscible liquids outside the rotor and to separate the liquids in the field of gravity inside the rotor. This way, a centrifugal extractor generates a continuous extraction from one liquid phase into another liquid phase.

High-density solids pumps are hydrostatically operating machines which displace the medium being pumped and thus create a flow.

Dewatering screw press

A dewatering screw press is a screw press that separates liquids from solids. A screw press can be used in place of a belt press, centrifuge, or filter paper. It is a simple, slow moving device that accomplishes dewatering by continuous gravitational drainage. Screw presses are often used for materials that are difficult to press, for example those that tend to pack together. The screw press squeezes the material against a screen or filter and the liquid is collected through the screen for collection and use.

A separation process is a method that converts a mixture or solution of chemical substances into two or more distinct product mixtures. At least one product mixture of the separation is enriched in one or more of the source mixture's constituents. In some cases, a separation may fully divide the mixture into pure constituents. Separations exploit differences in chemical properties or physical properties between the constituents of a mixture.

A pusher centrifuge is a type of filtration technique that offers continuous operation to de-water and wash materials such as relatively in-compressible feed solids, free-draining crystalline, polymers and fibrous substances. It consists of a constant speed rotor and is fixed to one of several baskets. This assembly is applied with centrifugal force that is generated mechanically for smaller units and hydraulically for larger units to enable separation.

The peeler centrifuge is a device that performs by rotating filtration basket in an axis. A centrifuge follows on the principle of centrifugal force to separate solids from liquids by density difference. High rotation speed provides high centrifugal force that allows the suspended solid in feed to settle on the inner surface of basket. There are three kinds of centrifuge, horizontal, vertical peeler centrifuge and siphon peeler centrifuge. These classes of instrument apply to various areas such as fertilisers, pharmaceutical, plastics and food including artificial sweetener and modified starch.

A centrifuge is a device that employs a high rotational speed to separate components of different densities. This becomes relevant in the majority of industrial jobs where solids, liquids and gases are merged into a single mixture and the separation of these different phases is necessary. A decanter centrifuge separates continuously solid materials from liquids in the slurry, and therefore plays an important role in the wastewater treatment, chemical, oil, and food processing industries. There are several factors that affect the performance of a decanter centrifuge, and some design heuristics are to be followed which are dependent upon given applications.

Trommel screen

A trommel screen, also known as a rotary screen, is a mechanical screening machine used to separate materials, mainly in the mineral and solid-waste processing industries. It consists of a perforated cylindrical drum that is normally elevated at an angle at the feed end. Physical size separation is achieved as the feed material spirals down the rotating drum, where the undersized material smaller than the screen apertures passes through the screen, while the oversized material exits at the other end of the drum.

A solid bowl centrifuge is a type of centrifuge that uses the principle of sedimentation. A centrifuge is used to separate a mixture that consists of two substances with different densities by using the centrifugal force resulting from continuous rotation. It is normally used to separate solid-liquid, liquid-liquid, and solid-solid mixtures. Solid bowl centrifuges are widely used in various industrial applications, such as wastewater treatment, coal manufacturing, and polymer manufacturing. One advantage of solid bowl centrifuges for industrial uses is the simplicity of installation compared to other types of centrifuge. There are three design types of solid bowl centrifuge, which are conical, cylindrical, and conical-cylindrical.

A conical plate centrifuge is a type of centrifuge that has a series of conical discs which provides a parallel configuration of centrifugation spaces.

References

  1. ↑ B. J. Anderson and D. S. Arand, “Dual-stream juice processing for recovering juice solids from extractor core material”. United States Patent EP0414964, 14 July 1993.
  2. 1 2 S. Tarleton and R. Wakeman, Filtration: Equipment Selection, Modelling and Process Simulation, 1st ed., Elsevier, 1999.
  3. 1 2 3 4 5 6 “CMI Screen Scroll Centrifuges,” Elgin Equipment Group, 2010. [Online]. Available: http://www.elginindustries.com/equipment_group/centrifugal_and_mechanical_industries/products.aspx. [Accessed October 2013].
  4. ↑ F. D. Gunstone, J. L. Harwood and A. J. Dijkstra, The Lipid Handbook, 3rd ed., Taylor & Francis Group LLC, 2007.
  5. 1 2 3 4 5 6 7 8 S. Tarleton and R. Wakeman, Solid/Liquid Separation: Scale-up of Industrial Equipment, 1st ed., Elsevier, 2005.
  6. 1 2 M. T. Mullins and J. R. Rodebush, “Horizontal Scroll Screen Centrifuge”. United States Patent US6736968 B2, 18 May 2004.
  7. ↑ D. B. Purchas, “Solid/Liquid Separation Technology,” Uplands Press, Croydon, 1981.
  8. 1 2 M. Thomas, C. Veal and J. Smitham, “Novel basket design for fine coal centrifuges,” The Journal of The South African Institute of Mining and Metallurgy, pp. 47-52, 2001.
  9. ↑ “Siebtechnik Centrifuges,” 2002. [Online]. Available: http://www.lochtec.dk/produkter/centrifuger/centrifuger.html#Conturbex Centrifuge. [Accessed October 2013].
  10. ↑ “Tema Engineers,” Tema Engineers, An Agent of Siebtechnik, Specialist in centrifuges, 2008. [Online]. Available: http://www.tema.com.au/CENTRIFUGES.html. [Accessed October 2013].
  11. ↑ G. G. Chase, “Solid/Liquid Separations,” The University of Akron.
  12. ↑ C. &. M. Industries, “Dramatic Operating Cost Reduction Through Innovative Engineering,” [Online]. Available: http://www.elginindustries.com/~/media/Images/Equipment/Brochures/LLPP%20Brochure.ashx. [Accessed October 2013].
  13. ↑ "Waterjet Centrifuge Screens for Screen Scroll / Conturbex Centrifuges" . Retrieved 23 December 2013.
  14. ↑ “Scroll Screen Centrifuges,” Broadbent, 2009. [Online]. Available: http://www.broadbent.co.uk/en/ipd-products-scrollscreencentrifuges. [Accessed October 2013].
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.