Classifying – separating – and assorting solids – Special applications
Reexamination Certificate
2000-08-09
2001-11-20
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Special applications
C209S003000, C209S215000, C209S225000, C209S218000, C209S228000, C505S400000
Reexamination Certificate
active
06318558
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for separating non-ferrous particles of different electrical conductivity, in particular of waste materials, and to a device for carrying out the method.
When separating useful materials, in particular waste materials, it is possible to separate ferromagnetic materials, i.e. in particular iron, without any problems by means of simple magnetic methods. Because of their different electrical conductivity, non-ferrous metals can be further separated from one another and from plastics materials following the removal of the ferromagnetic materials by means of eddy-current separation. In the eddy-current separator a current is induced and thus a force produced in an inducing magnetic field in the particles to be separated by the latter, irrespective of their conductivity, and this current and force expel the particles from the magnetic field. The deflection of non-ferrous metals in the eddy-current separator is in this case determined by the electrical conductivity &sgr; and the density &rgr; (relative density) of the materials to be separated. With the density the same, increasing conductivity is accompanied by increasing eddy currents and a corresponding increase in the force that expels the particles from the inducing magnetic field. The force that is to be applied for the same quantitative effect increases with the density. &sgr;/&rgr; is therefore a suitable characteristic quantity for the qualitative assessment of the separating capacity.
Eddy-current separators of this kind are known in various configurations. For example, EP 0 305 881 A1 describes a method and a device for sorting non-ferrous metal particles by means of eddy-current separation. A conveyor belt runs around a rotating magnet system and the different particles are thrown off in different trajectory parabolas and can thus be sorted to a certain degree. As an improved version, EP 0 339 195 B1 describes a magnetic separator with a conveyor belt, which is guided over a belt drum consisting of non-electrically conductive material, to transport a fraction of particles of greater or lesser electrical conductivity which is to be sorted, with a magnet system which is driven so as to rotate in the belt drum at a higher rotational speed than that of the belt drum, and with a collecting vessel disposed in the material discharge zone of the belt drum for the separated electrically conductive particles. This publication indicates in particular how damage to the belt drum due to particles, in particular iron particles, coming between the conveyor belt and the belt drum can be prevented. This is achieved by a certain geometrical structure.
However the disadvantage of the known eddy-current separators lies in the fact that different non-ferrous metals can only be separated from one another with difficulty and subject to error. This is due in particular to the fact that the physical properties determining the separating capacity only differ slightly.
The object is therefore to improve the separation of non-ferrous metals from one another when using eddy-current separation.
SUMMARY OF THE INVENTION
This object is solved by a method for separating non-ferrous particles of different electrical conductivity, in particular of waste materials, in which the supplied particles to be separated are cooled and then subjected to eddy-current separation in the cooled state. A device for carrying out this method is characterised in that a cooling chamber through which the particles are guided is provided, and that an eddy-current separator (magnet system) is provided, to which the still cooled particles are fed in a transport stream.
As the electrical conductivity of non-ferrous metals increases as temperatures drop, and the density does not change significantly at the same time, it becomes easier to separate the different materials. The eddy currents induced in the particles increase superproportionally, so that the force acting on the particles is augmented accordingly. As a consequence, it is therefore also possible to separate different non-ferrous metals practically error-free with an eddy-current separator otherwise unchanged.
For example, the ratio &sgr;/&rgr; in the temperature range of 100-300 K differs for aluminium, magnesium, copper and zinc, as indicated in the graph represented in FIG.
1
. The values are taken from: CRC Handbook of Chemistry and Physics, Editor: David R. Lide, Vol. 1992-93, 73rd issue, published by CRC Press, Boca Raton, etc.
The graph shows that there is an increase in both &sgr;/&rgr; for each element in absolute terms and &Dgr;(&sgr;/&rgr;) for each two elements as temperatures drop. A higher yield and a more accurate separation, especially below 150 K, can therefore be expected when separating waste.
DE 196 00 647 proposes a method for utilizing cable sleeves by means of cryogenics. In this case the cable sleeves are to be successively cooled to temperatures of around −85° C., so that they embrittle. This embrittlement enables them to be shattered in a hammer mill and the individual components of a cable sleeve thus made accessible to further sorting. After passing through the hammer mill, the particles resulting from the shattering process are by no means cooled, but rather heated, and there is no intention of separating non-ferrous metals.
The eddy-current separation should take place directly after cooling in order to make optimum use of the increased conductivity at the cooled particles.
As can be seen from the graph in
FIG. 1
, an increased separating capacity is to be detected in particular below 150 K. It is therefore preferable to cool the particles to 100-150 K. It is, moreover, sufficient to cool at least the surfaces of the particles to the desired temperature, as the eddy currents produced by the inducing magnetic fields essentially flow at the surface of the particles.
If liquid nitrogen is used to cool the particles, the latter are cooled simply and effectively. As the boiling point of nitrogen is approximately 80 K, the preferred temperature range can be reached at least at the surfaces of the particles. The nitrogen has no further influence on the process.
The different materials also have different coefficients of thermal conductivity; they therefore react to the cooling at different speeds and with different intensity. As this cooling process takes place over a finite time and the separation closely follows the cooling in terms of time, the temperature of the particles to be sorted differs, in spite of an identically operating cooling plant.
However this effect, felt to be disturbing on the first impression, may also be utilised: Since the thermal conductivity of a material to be sorted or separated is also a material constant and the plant-specific cooling also operates in a reproducible fashion, it is even possible to improve the separation by a suitable choice of parameters as a result of the electrical conductivity of one non-ferrous metal at a certain temperature specifically entering into competition with the electrical conductivity of the other non-ferrous metal at a completely different temperature and thus making separation easier. This effect can be detected experimentally according to the plant, as well as calculated beforehand theoretically and specifically used when separating certain compositions of the total transport stream.
In order to minimise unwanted heat absorption, the cooling chamber is formed as a closed channel with a feed opening and a delivery opening for the particles that are to be separated. The coolant introduced into the closed channel, for example liquid nitrogen, can be economically metered. The particles are fed through the channel by forming the channel as a chute or shaker conveyor. As the channel has an essentially rectangular cross section, there is no possibility of the particles to be separated agglomerating. The channel is preferably of the width of the downstream conveyor belt to the eddy-current separator. A conveyor belt of electrically non-conductive material has in particular
Barlow Josephs & Holmes, Ltd.
Jones David A.
Walsh Donald P.
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