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.
This practice occurs in a variety of industries such as mining and mineral processing, agriculture, pharmaceutical, food, plastics, and recycling.
A method of separating solid particles according to size alone is called screening.
Screening falls under two general categories: dry screening, and wet screening. From these categories, screening separates a flow of material into grades, these grades are then either further processed to an intermediary product or a finished product. Additionally, the machines can be categorized into a moving screen and static screen machines, as well as by whether the screens are horizontal or inclined.
The mining and mineral processing industry uses screening for a variety of processing applications. For example, after mining the minerals, the material is transported to a primary crusher. Before crushing large boulder are scalped on a shaker with 0.25 in (6.4 mm) thick shielding screening. Further down stream after crushing the material can pass through screens with openings or slots that continue to become smaller. Finally, screening is used to make a final separation to produce saleable products based on a grade or a size range.
A screening machine consist of a drive that induces vibration, a screen media that causes particle separation, and a deck which holds the screen media and the drive and is the mode of transport for the vibration.
There are physical factors that makes screening practical. For example, vibration, g force, bed density, and material shape all facilitate the rate or cut. Electrostatic forces can also hinder screening efficiency in way of water attraction causing sticking or plugging, or very dry material generate a charge that causes it to attract to the screen itself.
As with any industrial process there is a group of terms that identify and define what screening is. Terms like blinding, contamination, frequency, amplitude, and others describe the basic characteristics of screening, and those characteristics in turn shape the overall method of dry or wet screening.
In addition, the way a deck is vibrated differentiates screens. Different types of motion have their advantages and disadvantages. In addition media types also have their different properties that lead to advantages and disadvantages.
Finally, there are issues and problems associated with screening. Screen tearing, contamination, blinding, and dampening all affect screening efficiency.
Like any mechanical and physical entity there are scientific, industrial, and layman terminology. The following is a partial list of terms that are associated with mechanical screening.
There are a number of types of mechanical screening equipment that cause segregation. These types are based on the motion of the machine through its motor drive.
An improvement on vibration, vibratory, and linear screeners, a tumbler screener uses elliptical action which aids in screening of even very fine material. As like panning for gold, the fine particles tend to stay towards the center and the larger go to the outside. It allows for segregation and unloads the screen surface so that it can effectively do its job. With the addition of multiple decks and ball cleaning decks, even difficult products can be screened at high capacity to very fine separations. [8]
Circle-Throw Vibrating Equipment is a shaker or a series of shakers as to where the drive causes the whole structure to move. The structure extends to a maximum throw or length and then contracts to a base state. A pattern of springs are situated below the structure to where there is vibration and shock absorption as the structure returns to the base state.
This type of equipment is used for very large particles, sizes that range from pebble size on up to boulder size material. It is also designed for high volume output. As a scalper, this shaker will allow oversize material to pass over and fall into a crusher such a cone crusher, jaw crusher, or hammer mill. The material that passes the screen by-passes the crusher and is conveyed and combined with the crush material.
Also this equipment is used in washing processes, as material passes under spray bars, finer material and foreign material is washed through the screen. This is one example of wet screening.
High-frequency vibrating screening equipment is a shaker whose frame is fixed and the drive vibrates only the screen cloth. High frequency vibration equipment is for particles that are in this particle size range of an 1/8 in (3 mm) down to a +150 mesh. Traditional shaker screeners have a difficult time making separations at sizes like 44 microns. At the same time, other high energy sieves like the Elcan Industries' advanced screening technology allow for much finer separations down to as fine as 10um and 5um, respectively.
These shakers usually make a secondary cut for further processing or make a finished product cut.
These shakers are usually set at a steep angle relative to the horizontal level plane. Angles range from 25 to 45 degrees relative to the horizontal level plane.
This type of equipment has an eccentric drive or weights that causes the shaker to travel in an orbital path. The material rolls over the screen and falls with the induction of gravity and directional shifts. Rubber balls and trays provide an additional mechanical means to cause the material to fall through. The balls also provide a throwing action for the material to find an open slot to fall through.
The shaker is set a shallow angle relative to the horizontal level plane. Usually, no more than 2 to 5 degrees relative to the horizontal level plane.
These types of shakers are used for very clean cuts. Generally, a final material cut will not contain any oversize or any fines contamination.
These shakers are designed for the highest attainable quality at the cost of a reduced feed rate.
Trommel screens have a rotating drum on a shallow angle with screen panels around the diameter of the drum. The feed material always sits at the bottom of the drum and, as the drum rotates, always comes into contact with clean screen. The oversize travels to the end of the drum as it does not pass through the screen, while the undersize passes through the screen into a launder below.
There are many ways to install screen media into a screen box deck (shaker deck). Also, the type of attachment system has an influence on the dimensions of the media.
Tensioned screen cloth is typically 4 feet by the width or the length of the screening machine depending on whether the deck is side or end tensioned. Screen cloth for tensioned decks can be made with hooks and are attached with clamp rails bolted on both sides of the screen box. When the clamp rail bolts are tightened, the cloth is tensioned or even stretched in the case of some types of self-cleaning screen media. To ensure that the center of the cloth does not tap repeatedly on the deck due to the vibrating shaker and that the cloth stays tensioned, support bars are positioned at different heights on the deck to create a crown curve from hook to hook on the cloth. [9] Tensioned screen cloth is available in various materials: stainless steel, high carbon steel and oil tempered steel wires, as well as moulded rubber or polyurethane and hybrid screens (a self-cleaning screen cloth made of rubber or polyurethane and metal wires).
Commonly, vibratory-type screening equipment employs rigid, circular sieve frames to which woven wire mesh is attached. Conventional methods of producing tensioned meshed screens has given way in recent years to bonding, whereby the mesh is no longer tensioned and trapped between a sieve frame body and clamping ring; instead, developments in modern adhesive technologies has allowed the industry to adopt high strength structural adhesives to bond tensioned mesh directly to frames. [10]
Modular screen media is typically 1 foot large by 1 or 2 feet long [11] (4 feet long for ISEPREN WS 85 [12] ) steel reinforced polyurethane or rubber panels. They are installed on a flat deck (no crown) that normally has a larger surface than a tensioned deck. This larger surface design compensates for the fact that rubber and polyurethane modular screen media offers less open area than wire cloth. Over the years, numerous ways have been developed to attach modular panels to the screen deck stringers (girders). [13] Some of these attachment systems have been or are currently patented. [14] Self-cleaning screen media is also available on this modular system. [15]
There are several types of screen media manufactured with different types of material that use the two common types of screen media attachment systems, tensioned and modular.
Woven wire cloth, typically produced from stainless steel, is commonly employed as a filtration medium for sieving in a wide range of industries. Most often woven with a plain weave, or a twill weave for the lightest of meshes, apertures can be produced from a few microns upwards (e.g. 25 microns), employing wires with diameters from as little as 25 microns. A twill weave allows a mesh to be woven when the wire diameter is too thick in proportion to the aperture. Other, less commonplace, weaves, such as Dutch/Hollander, allow the production of meshes that are stronger and/or having smaller apertures.
Today wire cloth is woven to strict international standards, e.g. ISO1944:1999, [16] which dictates acceptable tolerance regarding nominal mesh count and blemishes. The nominal mesh count, to which mesh is generally defined is a measure of the number of openings per lineal inch, determined by counting the number of openings from the centre of one wire to the centre of another wire one lineal inch away. [17] For example, a 2 mesh woven with a wire of 1.6mm wire diameter has an aperture of 11.1mm (see picture below of a 2 mesh with an intermediate crimp). The formula for calculating the aperture of a mesh, with a known mesh count and wire diameter, is as follows:
where a = aperture, b = mesh count and c = wire diameter.
Other calculations regarding woven wire cloth/mesh can be made including weight and open area determination. [18] Of note, wire diameters are often referred to by their standard wire gauge (swg); e.g. a 1.6mm wire is a 16 swg.
Traditionally, screen cloth was made with metal wires woven with a loom. [19] [20] [21] Today, woven cloth is still widely used primarily because they are less expensive than other types of screen media. Over the years, different weaving techniques have been developed; either to increase the open area percentage or add wear-life. Slotted opening woven cloth [22] is used where product shape is not a priority and where users need a higher open area percentage. Flat-top woven cloth [23] is used when the consumer wants to increase wear-life. On regular woven wire, the crimps (knuckles on woven wires) wear out faster than the rest of the cloth resulting in premature breakage. On flat-top woven wire, the cloth wears out equally until half of the wire diameter is worn, resulting in a longer wear life. Unfortunately flat-top woven wire cloth is not widely used because of the lack of crimps that causes a pronounced reduction of passing fines resulting in premature wear of con crushers.
On a crushing and screening plant, punch plates or perforated plates [24] are mostly used on scalper vibrating screens, after raw products pass on grizzly bars. [25] Most likely installed on a tensioned deck, punch plates offer excellent wear life for high-impact and high material flow applications.
Synthetic screen media is used where wear life is an issue. Large producers such as mines or huge quarries use them to reduce the frequency of having to stop the plant for screen deck maintenance. Rubber is also used as a very resistant high-impact screen media material used on the top deck of a scalper screen. To compete with rubber screen media fabrication, polyurethane manufacturers developed screen media with lower Shore Hardness. To compete with self-cleaning screen media that is still primarily available in tensioned cloth, synthetic screen media manufacturers also developed membrane screen panels, slotted opening panels and diamond opening panels. Due to the 7-degree demoulding angle, polyurethane screen media users can experience granulometry changes of product during the wear life of the panel. [26]
Self-cleaning screen media was initially engineered to resolve screen cloth blinding, clogging and pegging problems. The idea was to place crimped wires side by side on a flat surface, creating openings and then, in some way, holding them together over the support bars (crown bars or bucker bars). This would allow the wires to be free to vibrate between the support bars, preventing blinding, clogging and pegging of the cloth. Initially, crimped longitudinal wires on self-cleaning cloth were held together over support bars with woven wire. [27] In the 50s, some manufacturers started to cover the woven cross wires with caulking or rubber to prevent premature wear of the crimps (knuckles on woven wires). One of the pioneer products in this category was ONDAP GOMME made by the French manufacturer Giron. [28] During the mid 90s, Major Wire Industries Ltd., a Quebec manufacturer, developed a “hybrid” self-cleaning screen cloth called Flex-Mat, without woven cross wires. [29] In this product, the crimped longitudinal wires are held in place by polyurethane strips. Rather than locking (impeding) vibration over the support bars due to woven cross wires, polyurethane strips reduce vibration of longitudinal wires over the support bars, thus allowing vibration from hook to hook. [30] Major Wire quickly started to promote this product as a high-performance screen that helped producers screen more in-specification material for less cost and not simply a problem solver. [31] They claimed that the independent vibrating wires helped produce more product compared to a woven wire cloth with the same opening (aperture) and wire diameter. This higher throughput would be a direct result of the higher vibration frequency of each independent wire of the screen cloth (calculated in hertz) compared to the shaker vibration (calculated in RPM), accelerating the stratification of the material bed. Another benefit that helped the throughput increase is that hybrid self-cleaning screen media offered a better open area percentage than woven wire screen media. Due to its flat surface (no knuckles), hybrid self-cleaning screen media can use a smaller wire diameter for the same aperture than woven wire and still lasts as long, resulting in a greater opening percentage.
A sieve, fine mesh strainer, or sift, is a tool used 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.
A mat is a hard floor covering that generally is placed on a floor or other flat surface. Mats serve a range of purposes including:
A mesh is a barrier made of interlaced strands of metal, fiber or other flexible or ductile materials. A mesh is similar to a web or a net in that it has many interwoven strands.
Mineral processing is the process of separating commercially valuable minerals from their ores in the field of extractive metallurgy. Depending on the processes used in each instance, it is often referred to as ore dressing or ore milling.
Strapping, also known as bundling and banding, is the process of applying a strap to an item to combine, stabilize, hold, reinforce, or fasten it. A strap may also be referred to as strapping. Strapping is most commonly used in the packaging industry.
A window screen is designed to cover the opening of a window. It is usually a mesh made of metal, fibreglass, plastic wire, or other pieces of plastic and stretched in a frame of wood or metal. It serves to keep leaves, debris, bugs, birds, and other animals from entering a building or a screened structure such as a porch, without blocking fresh air-flow.
Mesh is a measurement of particle size often used in determining the particle-size distribution of a granular material. For example, a sample from a truckload of peanuts may be placed atop a mesh with 5 mm openings. When the mesh is shaken, small broken pieces and dust pass through the mesh while whole peanuts are retained on the mesh. A commercial peanut buyer might use a test like this to determine if a batch of peanuts has too many broken pieces. This type of test is common in some industries, and, to facilitate uniform testing methods, several standardized mesh series have been established.
Shale shakers are components of drilling equipment used in many industries, such as coal cleaning, mining, oil and gas drilling.They are the first phase of a solids control system on a drilling rig, and are used to remove large solids (cuttings) from the drilling fluid ("mud").
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 sieve analysis is a practice or procedure used in civil engineering and chemical engineering to assess the particle size distribution of a granular material by allowing the material to pass through a series of sieves of progressively smaller mesh size and weighing the amount of material that is stopped by each sieve as a fraction of the whole mass.
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.
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. Every gravitational method uses gravity as the primary force for separation. 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.
Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of the Earth's crust. In industry, it is an important unit operation in mineral processing, ceramics, electronics, and other fields, accomplished with many types of mill. In dentistry, it is the result of mastication of food. In general medicine, it is one of the most traumatic forms of bone fracture.
A baghouse, also known as a baghouse filter, bag filter, or fabric filter is an air pollution control device and dust collector that removes particulates or gas released from commercial processes out of the air. Power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers and other industrial companies often use baghouses to control emission of air pollutants. Baghouses came into widespread use in the late 1970s after the invention of high-temperature fabrics capable of withstanding temperatures over 350 °F (177 °C).
Tumbler screening is a separation method that uses three-dimensional elliptical movement to separate very fine particles from larger ones.
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.
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.
NIAflow is simulation software for mineral processing plants. Based on a flowsheet interface, it calculates the material flow through a variety of processing machinery.