Classifying – separating – and assorting solids – Fluid suspension – Gaseous
Reexamination Certificate
1998-12-16
2001-02-13
Nguyen, Tuan N. (Department: 3653)
Classifying, separating, and assorting solids
Fluid suspension
Gaseous
C209S142000, C209S154000, C209S590000, C073S646000
Reexamination Certificate
active
06186334
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of German Patent Application, Serial No. 197 56 099.7, filed Dec. 17, 1997, the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a method of and apparatus for screening bulk material.
It is known to use pneumatic sifters for screening granulate bulk material. These pneumatic sifter are available in a wide variety of different designs, such as counterflow sifters, deflection sifters etc., and can be configured also as zigzag sifters. While in counterflow sifters, the sifting gas is conducted in opposition to the bulk material being screened, in deflection sifters the gas flow is typically conducted transversely to the flow direction of bulk material. Regardless of the type of pneumatic sifter being used, all designs require a very precise setting of the energy of gas, e.g. air, to be supplied. If the air energy is too weak, the separation of the fraction of fine particles will not be satisfactory. On the other hand, if the gas energy is excessive, the amount of coarse particles being entrained together with the fraction of fine particles is too high. This causes a problems, in particular, when different mixtures of bulk material are repeatedly screened, so that a correct adjustment of the air energy can normally be carried out only on the basis of empirical observation and only approximated.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide an improved method and apparatus for screening bulk material, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved method and apparatus for screening bulk material by which the gas energy (quantity, velocity) is precisely adjusted in a simple and rapid manner, even when screening varying mixtures of bulk material.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by conducting a gas in counterflow to a direction of movement of incoming granulate bulk material for separating a fraction of fine particles from a fraction of coarse particles, determining the amount of coarse particles carried with the fraction of fine particles, comparing the determined amount with a desired value, and changing the energy of gas in dependence on the comparison between the determined amount and the desired value so as to match the determined amount with the desired value.
The determination of the amount of coarse particles contained in the fraction of fine particles can be carried out at any point after separation of the fraction of fine particles, and depends on the demands at hand, e.g. speed of control system. Preferred is a location, for example, immediately after the sifter, or a location in or after a separator positioned downstream of the sifter.
In principle, any process that is suitable to determine the amount of coarse particles in the outgoing fraction of fine particles may be applicable within the scope of the present invention. An example includes screening of cleaned granulate bulk material and subsequent weighing (or determination of volume) of the individual fractions. This process is, however, time consuming, which may be of secondary considerations when fairly even mixtures of bulk material are involved. However, when less homogenous mixtures are used, such as plastic granulates, and quantities of fine particles are encountered in the sifter, the gas energy must be correctly adjusted in a relatively rapid manner.
Therefore, according to a preferred embodiment of the present invention, the determination of the amount of coarse particles entrained together with the exiting fraction of fine particles is realized by having the outgoing granulate bulk material stream, which is carried by the gas stream, impact on a sound
3
emanating surface, and analyzing the structure-borne noise. Sound analysis has been applied, for example, for observing the operation of ball mills or of die casting machines. Examples of sound analysis processes are described in German Pat. Nos. DE-C-10 70 478, or DE-C-24 14 819, or DE-A-39 40 560, or DE-A-33 00 327, or European Pat. Nos. EP-A-0 000 827, or EP-A-0 112 619.
Preferably, the sound-emanating surface is formed by at least one impact plate which projects into the flow path of the outgoing gas stream.
In accordance with the present invention, an apparatus for screening granulate bulk material includes a pneumatic sifter having a gas inlet and a gas outlet, a sound-emanating area incorporated in the gas outlet and generating noise upon impact of granule-laden gas stream, a microphone actuated by sound waves radiating from the sound-emanating area upon impact of the granule-laden gas stream, and delivering an output signal, a sound analyzer having a comparator for comparing the output signal with a desired value and delivering an output signal, and an actuator operated by the sound analyzer for adjusting a gas energy of the gas stream in dependence of the output signal of the sound analyzer.
REFERENCES:
patent: 4222859 (1980-09-01), Medlock
patent: 4576286 (1986-03-01), Buckley et al.
patent: 4602716 (1986-07-01), Barla-Szabo et al.
patent: 5152604 (1992-10-01), Paul
patent: 5351832 (1994-10-01), Abbott et al.
patent: 5458245 (1995-10-01), Heckel et al.
patent: 10 70 478 (1959-12-01), None
patent: 24 14 819 C3 (1975-10-01), None
patent: 33 00 327 C2 (1984-07-01), None
patent: 39 40 560 A1 (1990-06-01), None
patent: 0 000 827 (1979-02-01), None
patent: 0 112 619 (1984-07-01), None
patent: 6-186069 (1994-07-01), None
Dohmann Heinrich
Westendarp Hans-Heinrich
Feiereisen Henry M.
Nguyen Tuan N.
Waeschle GmbH
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