Tumbler screening is a mechanical screening technique used in many fields that deal with raw materials and building materials for process and reuse.[1] This technique can achieve 99% high efficiency with its circular movement.[2] Machines with tumbler screening techniques are commonly used because of its unique rush design, high life time utility, and the flexible angular velocity the machines can achieve.[3] However, this type of machine requires low feed flow rate, and the particle size for separation has to be controlled within a specific range.[4] Furthermore, the performance of the process also depends on the intensity of the vibration.[5] The water steam treatment is also an issue of this screening technique since this technique is applied worldwide.
Presently, the most effective and environment-friendly way for processing of the waste treatment is to classify the products. The products are usually split into four categories, including organic, inorganic, hazardous and recyclable.[6] Nowadays, scientific research continues for further development of the method.
Overview
The tumbler screening technique is a separation method that applies an elliptical movement in three dimensions, which helps to efficiently produce very fine materials. It can be considered an improvement to the vibration and linear screeners.[2] Originally patented by Allgaier Werke GMBhH,[7] the basic principle for the machines with tumbler screening technique is that the fine particles stay on the center of the screen desk while larger particles move towards to the edges. Higher-quality separation can be achieved by adding more desks and cleaning desks to the machine.[8]
Applications
The tumbler screening technique has many applications.
It is utilized in the chemical and food industry in order to produce high-quality powder with sensitive properties; this technique can also provide a moderate treatment for the powder.[9] This technique can also be used as a high-efficiency method of recycling high-quality raw materials.[10]
In food processing, simpler tumbler screening machines with one deck and screen sizes from 1 to 4 mm are used to remove foreign bodies and ensure the safety of the products.[11]
This technique is also applied in the metallurgy and ceramics industries.
Finally, this technique is popular in the mining industry because of the long operating life of the tumbler screening machines.[3]
Advantages and disadvantages
Advantages
Tumbler screening is an improved technique to the other screening techniques in which product on the screen is moved from the centre by radial inclination while the tangential inclination leads to circular motion of the product.[12]
Tumbler screening provides 99% screening efficiency with gentle screening of delicate particles to prevent particle destruction.[2] The complicated circular three dimensional screening action is adjustable to accommodate different material characteristics and is mainly independent of the material load, improving the separation process. This technique is usually performed after crushing/milling. It can be used for both dry and wet screening.
Tumbler screening technique is a multi-deck elliptical action screening technique in which speed can be adjusted such that it has a faster travel rate at the feed and is slow at the discharge side, which can help increase in screen efficiency, so that products can be screened at high capacity to very fine separations.[4]
Tumbler screening has the advantage of having a container barrel with different outlet doors at each of the sorting levels.[13] Thus, they can be used for sorting mixture of particles.
A tumbler screening machine has the ability to run in a range of angular velocities. A range of product specialized mesh cleaning systems are accessible with rapid screen replacement systems optionally available.[4]
Tumbler screening machines have higher output per meter square of screening than the other vibratory screening machines.[3] They require lower power consumption at different speeds at different levels and sorting at each level. It has a lower maintenance and is compact.
Disadvantages
Tumbler screening has a disadvantage of blocking of the mesh in case of batch sieving if the particles are fed immediately inside the machine barrel. Hence, the feeding rate shouldn't be very high.[4]
Another disadvantage of tumbler screening technique is that it would be ineffective for higher particle sizes, ranging from six inches to +20 mesh.
If a number of tumbler screening machines are operated in parallel in a building without building statics configured accordingly, dynamic residual forces can generate resonance in vibration which in worst case can eventually lead to connection elements to the screening machine shearing off. Particle–particle and machine boundary collisions in the tumbling motion of the rotating structure will dissipate kinetic energy due to normal and frictional contact losses.[14]
Characteristics
The screening process performance can be affected heavily by many factors, such as intensity of vibration, size fraction of the feed and feed rate. The process performance can be assessed by screening efficiency. It also refers to product yield. Screening efficiency is defined as the comparison of the undersized materials brought on the screening and discharged oversized product in contrast to the original amount of undersized materials in the feed stream. Tumbler screening technique allows separation efficiency to be as high as 99% due to the circular screening assembly's three-dimensional oscillations,.[9] which will increase the effective screening area four times greater than normal vibrating screen.[5]
Enormous studies have been conducted in the screening process kinetics. that screening process performance can be described as an exponential function of the process characteristics,[15] which include feed rate, screening area, screen length, critical size fraction in feed. The sections below will give a brief introduction of these process characteristics and how they affect the screening.
The intensity of the vibration
The intensity of the vibration refers to the amplitude and the frequency of the vibration. It can affect the screening performance in many aspects. Amplitude is defined as the distance travelled by the screen in one circle. In the Tumbler Screening case, the screening is moving in three dimensions. The movement of the particles is determined by the vibration. Inappropriate vibration speed will lead to segregation, resulting in uneven distribution of the particles on the screen. Small particles will be over oversized particles and not being sieved. This will decrease the screening efficiency significantly.
Another reason why vibration intensity is important is that it will affect the contacts number of particles with screening. If the vibrating speed is too fast, the particle will go forward along the screen, reducing the possibility that the particles will pass through. But a high number of contact attempts will increase the screening effect as the residual time of the particles will be reduced. When the screen is moving too slow, it will cause fluctuation, which will decrease the efficiency. The common speed range is 180 to 280min-1,[10] and the common amplitude applied to the screener is 60 to 80mm horizontally and 5 to 40mm vertically.[10]
Feed rate
Feed rate affects the screening performance of the tumbler screener as well. The amount of material presented to a screen for processing is called feed rate. It is often related to the amplitude and frequency of the vibration. When feeding to a screener in a high rate, the capacity of the screening will be pushed to a limit very fast, causing stratification. In this case, the intensity of the vibration needs to be increased to process more materials in a certain period of time. However, when feeding in a low feed rate, the efficiency of the screener will be at low state as well. The particles will stay in a loose state, which will decrease the effectiveness.
Material input to a tumbler screening technique should be constant and uniform throughout the running. Otherwise, the particles will not be evenly distributed on the screen and it may cause fluctuation. And it will lead to a poor performance of the tumbler screening. Feed rate should be more constant and steady when dealing with finer particles. The common feed rate range of a tumbler screening machine is 1kg/h to 50 to/h.[16]
Particle size distribution
Particle size is the most important parameter among others. It affects the screening technique directly. For a screening system, the particle size can be summarized as oversize particles, which is particles larger than the screen opening; undersized particles, also called fine particles, which is smaller than the screen opening. The undersized particles will be produced as product streams in screening operation.
Another important particle size is the near-mesh size particles. The near-size particles often are close to the size of the screening opening. The increasing number of near size particles will cause blinding effect. This phenomenon will decrease the travelling velocity of the processed materials and hence reduce the efficiency of the screening. It has been shown that near-size particles which will cause the blind of the screen have a maximum size of 1.1 times opening.[17] Feed stream should be treated to decrease the existence of near size particles before screening.
Other characteristics
Other characteristics such as moisture content of the feed; open area of the screen deck; shape of the particles and the screen length also affect the process. High percentage of moisture may lead to aggregation of the materials and increase oversize materials which can block the passage of the undersize materials.
Waste stream
Waste stream post-treatment flowsheet
Through tumbler screening is applied in a variety of processing, the waste stream treatment for tumbler screening becomes very important in industry. At present, the most effective and environmental way to treat the waste stream is classification processing. Just as the figure shows, the waste is being separated at beginning to classify the waste material into recyclable material, hazardous waste, organic matter and inorganic matter. Then transferring the recyclable material to recycle bin for reuse, collecting and composting the non-toxic organic matter as refuse-derived fuel (RDF) and send the hazardous waste and unusable inorganic matter to landfill. In addition, it is also possible to convert some waste into useful material by chemical process.[6] All these activities aim to minimize the amount of waste to landfill; this will contribute to reducing the environmental damage of waste and increase the resource recovery of source and energy.
Generally, there are two ways to separate the waste material into different classes which includes mechanical separation and manual separation.
Mechanical separation aims to prepare the waste stream for manual separation which could separate metals, plastics, glasses and some other components easily and fast by many different types of sorting method and machines.
The manual separation divides into positive sorting and negative sorting, positive sorting aims to collect the recyclable material from waste stream and negative sorting aims to remove the hazardous and unusable waste out of waste stream.[18] Positive sorting requires a higher quality but low collection rate; on the other hand, negative sorting allows a high collection rate but low quality. It is possible increase the effectiveness with a good controlling of manual separation. A good separation of waste contributes to further treatment of the waste stream and increases the efficiency of the waste treatment system.
Process design
Tumbler screening machines are composed of a screen box, screen, vibrators, damping spring device, a chassis, and other components. The screen box is vibrated by using cylinder-type eccentric shaft vibration exciter with consistent amplitude vibrator mounted on the side panel screen box and V-belt driven by a motor, which produces centrifugal force.[19] The screen side plate is made of high quality steel plate with bolt or rivet connections on side panels, beams, ring groove and chassis. Furthermore, the vibrators mounted on panel screen box are rotated by the motor through the coupling, provided a centrifugal inertial force to vibrate the sieve. The materials on the inclined surface of the sieve screen would be vibrated by the screen box in a continuous throwing motion. As this happens, particles smaller than the s
Unfortunately, a certain percentage of fine particles usually remain on the above screen after the oscillation. Moreover, without good maintenance, long-term usage may cause wearing of the screen surface, which may lead to oversize particles fall through screen. The efficiency of tumbler screening machine η can be expressed as: η = Mp÷MF, where Mp is the mass of fine particles which pass through the screen surface during the operation of tumbler screening machine, and MF is the mass of fine particles which in the initial feed of material to the tumbler screening machine.[20]
The efficiency of tumbler screening machine is affected by following factors:
Machine factors (e.g. the material of machine, the width and length of machine, the amplitude of vibration, the speed of revolution)
Screen surface factors (e.g. type and size of screen pore, the inclined angle of screen surface)
Initial feed (e.g. the size and distribution of initial feed material)
21st-century developments
The segmented screening inserts have one round central segmental with six to eight outer segments. This technique has the advantage that it can withstand significantly higher loadings, gives longer service life, and maintenance and spare parts costs are low, as the entire screen insert is not replaced for every screen tear. Less area is required for changing screen.[21]
Out-feed from a tumbler screening machine can be monitored by proximity-type signal transmitter, which detects overload, blockage or screen breakage, by a change in the out-feed layer height which shows deviation from the optimum operating condition. Faulty production can be easily avoided.
The fine balancing procedure called multi-balance aids in almost eliminating the residual imbalances in tumbler screening machines, thus leading to less risk of vibration and resonance effects in steel fabrications or concrete buildings.[21] The highly flexible docking system uses a pneumatically-operated lifting system with lifting rods, and puts no additional weight on machine. It accelerates the process of inspection/ changing of screen significantly.
Another significant technical advancement in Tumbler screening includes the combination of ultrasound and air to clean extremely small openings for better screening efficiency.[2]
Related Research Articles
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.
A sieve, fine mesh strainer, or sift, is a device for separating wanted elements from unwanted material or for controlling the particle size distribution of a sample, using a screen such as a woven mesh or net or perforated sheet material. The word sift derives from sieve.
In the field of extractive metallurgy, mineral processing is the process of separating commercially valuable minerals from their ores. Depending on the processes used in each instance, it is often also known as ore dressing or ore milling.
A drywasher is a common desert mining tool for gold mining.
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.
A vibrator is a mechanical device to generate vibrations. The vibration is often generated by an electric motor with an unbalanced mass on its driveshaft.
Mechanical screening, often just called screening, is the practice of taking granulated or crushed ore material and separating it into multiple grades by particle size.
Gravity separation is an industrial method of separating two components, either a suspension, or dry granular mixture where separating the components with gravity is sufficiently practical: i.e. the components of the mixture have different specific weight. All of the gravitational methods are common in the sense that they all use gravity as the dominant force. One type of gravity separator lifts the material by vacuum over an inclined vibrating screen covered deck. This results in the material being suspended in air while the heavier impurities are left behind on the screen and are discharged from the stone outlet. Gravity separation is used in a wide variety of industries, and can be most simply differentiated by the characteristics of the mixture to be separated - principally that of 'wet' i.e. - a suspension versus 'dry' -a mixture of granular product. Often other methods are applied to make the separation faster and more efficient, such as flocculation, coagulation and suction. The most notable advantages of the gravitational methods are their cost effectiveness and in some cases excellent reduction. Gravity separation is an attractive unit operation as it generally has low capital and operating costs, uses few if any chemicals that might cause environmental concerns and the recent development of new equipment enhances the range of separations possible.
Ultrasonic machining is a subtractive manufacturing process that removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. The tool travels vertically or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm. The fine abrasive grains are mixed with water to form a slurry that is distributed across the part and the tip of the tool. Typical grain sizes of the abrasive material range from 100 to 1000, where smaller grains produce smoother surface finishes.
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.
Sensor-based sorting, is an umbrella term for all applications in which particles are detected using a sensor technique and rejected by an amplified mechanical, hydraulic or pneumatic process.
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.
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.
A circle-throw vibrating machine is a screening machine employed in processes involving particle separation. In particle processes screening refers to separation of larger from smaller particles in a given feed, using only the materials' physical properties. Circle throw machines have simple structure with high screening efficiency and volume. However it has limitations on the types of feed that can be processed smoothly. Some characteristics of circle-throw machines, such as frequency, vibration amplitude and angle of incline deck also affect output.
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.
Vibratory Fluidized Bed (VFB) is a type of fluidized bed where the mechanical vibration enhances the performance of fluidization process. Since the first discovery of vibratory fluidized bed, its vibration properties proves to be more efficient in dealing with fine particles which appears to be very difficult to achieve with normal fluidized bed. Even though numerous publications and its popularity in industrial applications, the knowledge about vibratory dynamics and properties are very limited. Future research and development are needed to further improve this technology to bring it to another level.
Gyratory equipment, used in mechanical screening and sieving is based on a circular motion of the machine. Unlike other methods, gyratory screen operates in a gentler manner and is more suited to handle fragile things, enabling it to produce finer products. This method is applicable for both wet and dry screening.
High-frequency vibrating screens are the most important screening machines primarily utilised in the mineral processing industry. They are used to separate feeds containing solid and crushed ores down to less than 200 μm in size, and are applicable to both perfectly wetted and dried feed. The frequency of the screen is mainly controlled by an electromagnetic vibrator which is mounted above and directly connected to the screening surface. Its high-frequency characteristics differentiate it from a normal vibrating screen. High-frequency vibrating screens usually operate at an inclined angle, traditionally varying between 0° and 25° and can go up to a maximum of 45°. They should operate with a low stroke and have a frequency ranging from 1500 to 9000 RPM. Frequency in High frequency screen can be fixed or variable. Variable High Frequency screen is more versatile to tackle varied material condition like particle size distribution, moisture and have higher efficiency due to incremental increase in frequency. G force plays important role in determining specific screening capacity of screen in terms of TPH per sqm. G force increases exponentially with frequency.
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.
NIAflow is simulation software for mineral processing plants. Based on a flowsheet interface, it calculates the material flow through a variety of processing machinery.
References
↑ The experts for separation process, Screening technique June 2004
1 2 Bilitewski B .2005, Thermal treatment of waste – State of the art. In: Sardinia 2005 Tenth International Waste Management and Landfill Symposium, Editors Raffaello Cossu, Rainer Stegmann
↑ Brereton, T & Dymott, K.R. 1973, Some Factors which influence screen performance, 10th international mineral processing congress, London, pp. 181-190
↑ Tsakalakis, K 2001, ‘ Some Basic Factors Affecting Screen Performance in Horizontal Vibrating Screens’, the European Journal of Mineral Processing and Environmental Protection, vol.1303, pp. 42-54
↑ Asian Institute of Technology 2004: Municipal Solid Waste Management in Asia. Asian Regional Research Program on Environmental Technology (ARRPET)
↑ Hosoda, Hiroyuki 2000, Development of Fluidised bed Pyrolysis and Melting System for Municipal Solid Waste, Proc. Int. Conf. Combustion, Incineration/Pyrolysis and Emission Controls (ICIPEC), Seoul, Korea
↑ Seibtechnik, G., Kemutec Specialists in Powder Processing Technology. pp. 1–16
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