Classifying – separating – and assorting solids – Sifting – Plane sifters
Patent
1997-02-11
1999-06-01
Young, Karen M.
Classifying, separating, and assorting solids
Sifting
Plane sifters
209352, 209250, B65G 5360
Patent
active
059081155
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for the classification of solid particles. More particularly it relates to a classification method which comprises sending solid particles with a diameter generally less than 200 .mu.m suspended in a flow of transporting gas through a screening means and in which the cut-off value of the screening may be as low as 20-25 .mu.m.
It is well known that in the preparation of the catalyst supports as well as of the solid components it is necessary to control carefully the solid granulometry, particularly in the case in which the particle size distribution (PSD) of the catalyst directly influences the morphology of the final product. This is the case, for example, of the modern gas phase processes for the polymerisation of olefins, in which supported catalysts of the Ziegler/Natta type are used. The latest technology developments in the preparation of said catalysts allow to produce extremely active catalyst components, the size and morphology of which strongly influence the characteristics and morphology of the obtained polymer. In particular, the use of catalysts supported on magnesium chloride, which allow a remarkable simplification in the process for the production of polyolefins (polyethylene, polypropylene, EPR, EPDM, etc.), is well known. In fact, it is possible to obtain catalysts in the form of spherical particles which are suitable for producing polymers that duplicate the spherical form of the catalysts; said polymers have good morphological characteristics (fluidity and bulk density) and do not require final extrusion and pelletization steps which, as is well known, are expensive in terms of apparatus and energy required. The support of such catalysts is normally obtained by reacting magnesium chloride with an alcohol in an inert hydrocarbon. The particles of the obtained MgCl.sub.2 /alcohol adduct are subsequently de-alcoholated and treated with titanium halide, thus obtaining a catalytic component to be used in the polymerisation reaction. Examples of such catalysts are given, for example, in patents U.S. Pat. No. 4,399,054 and EP 395083.
It is clear that, in view of the above mentioned mechanism of replication of the initial form of the catalyst component particles, it is extremely important to avoid the introduction of very small particles in the reaction system, since said small particles bring about the formation of fines in the final polymer with consequent problems in the plant operation. Therefore, an essential step in the preparation of the support or the catalytic component is the screening, which should be carried out in such a way to obtain particles having diameters falling within the desired range. The known methods of classification (by gravity or centrifugal force in liquid or gas) have the drawback that a clear-cut screening is not possible. In other words, when particles having a diameter lower than a particular cut-off value have to be removed, the above mentioned known processes bring about removal of a portion of the particles having a useful dimension together with the fines. This is due to the fact that it is necessary to eliminate the "grey area" around the cut-off value, which is a serious drawback in the case where the substance to be classified is expensive.
Other classification systems are based on calibrated filtering grids which, however, create problems both in terms of low productivity and in terms of filtering grid fouling, especially in the case of reduced mesh opening size (for example between 20 and 25 .mu.m). Another disadvantage of such a system derives from the mechanical energy transmitted from the vibrating filtering grid to the particles, which can cause breakage (with the consequent lowering of overall efficiency in the classification operation) of fragile particles, such as those comprising magnesium chloride/alcohol adducts, or can damage the particles with subsequent problems during the polymerisation (generation of fines). One of these systems is illustrated in WO92/19392 where a circular vibratory
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Arletti Arrigo
Govoni Gabriele
Hess Douglas
Montell North America Inc.
Young Karen M.
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