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.[1]

Contents

General categories

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 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 salable product based on a grade or a size range.

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

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.

Types of mechanical screening

There are generally three 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

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 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.

References

  1. ^ http://www.rotex.com/02applications/applications.aspx
  2. ^ Woven Wire Mesh Glossary of Terms
  3. ^ Woven wire
  4. ^ Soil Gradation
  5. ^ Screening
  6. ^ The Complete Wire Mesh Glossary of Terms
  7. ^ WS Tyler ยป F-Class
  8. ^ RHEWUM WA- The original
  9. ^ Engelsmann Separators and Screeners
  10. ^ Sweco - Vibratory Screener, Sifters, Separators, Round Screen, Vibratory Separator