Mechanical screening

Mechanical screening, often just called screening, is the practice of taking granulated 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.

General categories

Screening fall 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 categorised into moving screen and static screen machines, as well as by whether the screens are horizontal or inclined.

Applications

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.

Process

Model of Screening Process

A screening machine consist of a drive that induces vibration, a screen cloth that causes particle separation, and a deck which holds the screen cloth 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 cloth 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.

Physical principles

• Vibration - either sinusoidal vibration or gyratory vibration.
  • Sinusoidal Vibration occurs at an angled plane relative to the horizontal. The vibration is in a wave pattern determined by frequency and amplitude.
  • Gyratory Vibration occurs at near level plane at low angles in a reciprocating side to side motion.
• Gravity - This physical interaction is after material is thrown from the screen causing it to fall to a lower level. Gravity also pulls the particles through the screen cloth.
• Density - The density of the material relates to material stratification.
• Electrostatic Force - This force applies to screening when particles are extremely dry or is wet.

Screening terminology

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.

• Amplitude - This is a measurement of the screen cloth as it vertically peaks to its tallest height and troughs to its lowest point. Measured in multiples of the acceleration constant g (g-force).
• Acceleration - Applied Acceleration to the screen mesh in order to overcome the van der waal forces
• Blinding - When material plugs into the open slots of the screen cloth and inhibits overflowing material from falling through.^[1]
• Brushing - This procedure is performed by an operator who uses a brush to brush over the screen cloth to dislodged blinded opening.
• Cloth, screening cloth - it is the material defined by mesh size, which can be made of any type of material such steel, stainless steel, rubber compounds, brass, etc.^[2]
• Contamination - This is unwanted material in a given grade. This occurs when there is oversize or fine size material relative to the cut or grade. Another type of contamination is foreign body contamination.
  • Oversize contamination occurs when there is a hole in the screen such that the hole is larger than the mesh size of the screen. Other instances where oversize occurs is material overflow falling into the grade from overhead, or there is the wrong mesh size screen in place.
  • Fines contamination is when large sections of the screen cloth is blinded over, and material flowing over the screen does not fall through. The fines are then retained in the grade.
  • Foreign body contamination is unwanted material that differs from the virgin material going over and through the screen. It can be anything ranging from tree twigs, grass, metal slag to other mineral types and composition. This contamination occurs when there is a hole in the scalping screen or a foreign material's mineralogy or chemical composition differs from the virgin material.
• Deck - a deck is frame or apparatus that holds the screen cloth in place. It also contains the screening drive. It can contain multiple sections as the material travels from the feed end to the discharge end. Multiple decks are screen decks placed in a configuration where there are a series of decks attached vertically and lean at the same angle as it preceding and exceeding decks. Multiple decks are often referred to as single deck, double deck, triple deck, etc.
• Frequency - Measured in hertz (Hz) or revolutions per minute (RPM). Frequency is the number of times the screen cloth sinusoidally peaks and troughs within a second. As for a gyratory screening motion it is the number of revolutions the screens or screen deck takes in a time interval, such as revolution per minute (RPM).
• Gradation, grading - Also called "cut" or "cutting." Given a feed material in an initial state, the material can be defined to a have a particle size distribution. Grading is removing the maximum size material and minimum size material by way of mesh selection.^[3]
• Shaker - A generic term that refers to the whole assembly of any type mechanical screening machine.
• Stratification - This phenomenon occurs as vibration is passed through a bed of material. This causes coarse (larger) material to rise and finer (smaller) material to descend within the bed. The material in contact with screen cloth either falls through a slot or blinds the slot or contacts the cloth material and is thrown from the cloth to fall to the next lower level.^[4]
• Mesh - Mesh refers to the number of open slots per linear inch. Mesh is arranged in multiple configuration. Mesh can be a square pattern, long-slotted rectangular pattern, circular pattern, or diamond pattern.^[5]
• Scalp, scalping - this is the very first cut of the incoming material with the sum of all its grades. Scalping refers to removing the largest size particles. This includes enormously large particles relative to the other particle's sizes. Scalping also cleans the incoming material from foreign body contamination such as twigs, trash, glass, or other unwanted oversize material.

Types of 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.

• Circle-throw vibrating equipment - This type of equipment has an eccentric shaft that causes the frame of the shaker to lurch at a given angle. This lurching action literally throws the material forward and up. As the machine returns to its base state the material falls by gravity to physically lower level. This type of screening is used also in mining operations for large material with sizes that range from six inches to +20 mesh.^[6]
• High frequency vibrating equipment - This type of equipment drives the screen cloth only. Unlike above the frame of the equipment is fixed and only the screen vibrates. However, this equipment is similar to the above such that it still throws material off of it and allows the particles to cascade down the screen cloth. These screens are for sizes smaller than 1/8 of an inch to +150 mesh.^[7]
• Gyratory equipment - This type of equipment differs from the above two such that the machine gyrates in a circular motion at a near level plane at low angles. The drive is an eccentric gear box or eccentric weights.^[8][9]
  

  Chemical Tumbler Screener
• Trommel screens - Does not require vibrations, instead, material is fed into a horizontal rotating drum with screen panels around the diameter of the drum.

Tumbler screening technique

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. ^[10]

Circle-throw vibrating equipment

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 equipment

High frequency vibrating 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.

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.

Gyratory Screener

Gyratory equipment

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

Trommel screens have a rotating drum with screen panels around the diameter of the drum and is on a shall angle. The feed material always sits at the bottom of the drum and as it 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.

References