Mechanical screening

A screening machine is a machine for screening, ie, the separation ( separation) of solid mixtures according to particle size (from about 0.1 mm).

Introduction

The generation and determination of particle size and particle size distribution in dispersed solids is very important in many areas. The first examined the scientific, Georgius Agricola was. As a physician and scientist Universal, he wrote twelve books of metallurgy (published in 1556 ). In these he already scientifically describes the first seven and various screens next to other equipment of mineral processing technology. He died a year before the publication of his works, which are valid until today.

The following product features are dependent on the particle size distribution (selection):

• Surface reaction
• Taste
• Miscibility
• Filtration capacity
• Conductivity

Screens can be divided into three fundamentally different ways:

• After application: production sieves and laboratory sieves
• After driving: electromagnetically or by unbalanced motors or gearboxes.

For these species and their combination for the most part, the applications of the machine result.

Vibrating

Vibrating differ fundamentally in their propulsion systems and its dependent operation.

Freischwingersiebmaschine

This is the simplest and most common propulsion system: The cantilever machine belongs to the series of circular motion. It is driven by a (sometimes two) central shaft, which is usually equipped with adjustable centrifugal weights. By changing the weights or the speed you can quite easily affect the machine parameters. This type of drive is called load-dependent: the resonant circuit and thus the acceleration is reduced with increasing feed rate or caking. The version can be used for simple to moderate tasks ( sieved to about 250 mm).

Ellipsensiebmaschine

This Siebmaschinenart is also driven by a (sometimes two) central main shaft, which produces an oscillating circuit just like a cantilever drive. By a further, synchronized wave of the resonant circuit produced is transformed into a defined depending on the task ellipse. The second wave the resonant circuit to a distorted ellipse. These machines are like the cantilever for simple to moderate tasks used. These machines have the great advantage that they can work without inclination or with very low slope.

Exzentersiebmaschinen

The Exzentersiebmaschinen are driven by an eccentric drive shaft. The shaft is displaced with two roller bearings in the screen box and with two bearings on the rotating support frame. The resonant circuit is generated in this Antriebsvarinante not centrifugal force but through the eccentric shaft. This has the consequence that operates this drive system independent of the load. Thus, the system of forces is balanced, two circumferential counterweights are used. The resonant circuit is not reduced by fluctuating feed rates or caking. This drive system is used for moderate to severe operating conditions. These machines are well suited for classification tasks in addition to almost all Vorbrecherentlastung.

Linearsiebmaschinen

Linear transducers are driven for example by two counter-rotating shafts with centrifugal weights. Other versions of the drives are germ cells, two counter-rotating unbalanced motors or solenoid actuators. Work linear oscillator with a linear vibration that is directed toward machines focus. These machines can be used without any inclination. As Dewatering they have the great advantage that they can also promote increasing and thus ensure optimum drainage. These machines are used for simple to moderate tasks. Disadvantage is the lack of horizontal component of a resonant circuit. In difficult screenings are not uncommon blockages.

Sifters

In sifters is transported by a feeding screw into a cylindrical screen and whirled by a rotor with vortex strips through the screen. In this process, the fine material is screened and the coarse material (impurities) automatically deposited.

Laborsiebmaschinen

In the laboratory sector, screening machines can be divided into four types:

• Vibratory sieve shakers
• Gyratory
• Klopfsiebmaschinen
• Air Jet Sieve Shakers

Vibratory sieve shakers

Here, vertical union movements overlap with slight rotary motion. This leads to the fact that the sample material across the entire surface of the sieve and the particles distributed at the same time, an acceleration in the vertical direction to view ( to be thrown ). In the air they can perform free rotations and compared when falling back on the sieve with the mesh of the screen mesh. If the particles are smaller than this, so they pass the screen, they are larger, they are thrown up again. The rotational movement is making sure that they on the mesh have a different orientation at the next impact and so perhaps pass through a mesh.

Modern sieve shakers work with an electromagnetic drive, in which a spring -mass system is set in motion and transmits the resulting vibrations on the sieve stack. Oscillation amplitude (amplitude) and sieving should be set digitally and are continuously monitored by an integrated control unit - just as are screening results reproducible and accurate ( important prerequisites for a meaningful analysis ). The flexible adjustment of the parameters such as amplitude and sieving allows to optimize the degree of sieving for each sample individually. Therefore, this type of screening is the most common in the laboratory.

Gyratory

In this type of screening the sieve stack performs a horizontal circular motion in a plane. This keep the particles on the sieve mesh mostly in their orientation. Gyratory are preferably used for needle -, platelet-shaped, elongated or fibrous screening materials used ( ° considerably from a spherical geometry different), in which a high throw of the sample material is not necessarily an advantage. As can be employed with a large diameter of up to 400 mm in this process test sieves, it is also suitable for large task sets as they occur in the grain analysis of construction materials and aggregates for example.

A special type is the multi deck that can fractionate several particle sizes simultaneously.

Klopfsiebmaschinen

Here, a horizontal circular is superposed with a vertical movement which is initiated by a knock pulse. This characteristic of the sieving motions reach a higher sieving as they can be reached during screening with vibratory sieve shakers for heavier particles (eg abrasives).

Air Jet Sieve Shakers

The device is connected to a standard vacuum cleaner, in which the rate of passage subsequently retained. A small amount of the product to be analyzed is weighed on an analytical, which is then sealed airtight with a lid. Once the relatively short sieving 3-5 minutes and pressing the start button the screening begins. The rotating nozzle vanes, the product is dispersed and simultaneously blows the mesh from the bottom clean. The vacuum produced by the vacuum cleaner draws the fine product through the screen and is deposited in the vacuum cleaner bag from the air. Through the built-in three-way valve of the desired vacuum is not always exactly correct and fast. Then the sieve residue is weighed.

It guarantees a high, reproducible screening accuracy even in difficult to screen products and short sieving.

Mesh fabric are made in various mesh sizes ( from 20 microns ) and different diameters. Screens are available in various designs, depending on what demands are placed on the products. Interpretation of data in general, the required screening performance, the desired purity, the yield required, the feed material with its bulk density, and the available space. From these data, the optimal screening machine can be selected.

Besides Laborsiebmaschinen you also differs Industriesiebmaschinen.

• Apparatus (process engineering )
• Classification (process engineering )
• Particle Measurement
• Machine