Device and method for sorting a particulate stream
The invention relates to a device for continuously sorting a particulate stream, comprising: a) a screw conveyor ( 1 ) for conveying the particulate stream to a separation device, b) a sieve ( 7 ) for separating the particulate stream ac cording to size, characterized in that it further comprises c) a pre-sieve ( 3 ) comprising elongated sieve openings formed by fingers extending towards an end portion of the pre-sieve with the tips of the fingers not being connected in that end portion, the pre-sieve ( 3 ) being located between the conveyor outlet and the sieve, d) wherein the pre-sieve ( 3 ) encloses part of the circumference of the conveyor screw in an axial end portion of the conveyor screw, and e) the radial distance between the conveyor screw ( 1 ) and the pre-sieve ( 3 ) is less than 10%, preferably less than 5% of the conveyor screw diameter. The pre-sieve allows to separate elongated objects like threads or wires from the particulate stream which would otherwise clog or block subsequent separation devices including the sieve.
1 . Device for continuously sorting a particulate stream, comprising:
a) a screw conveyor ( 1 ) for conveying the particulate stream to a separation device,
b) a sieve ( 7 ) for separating the particulate stream according to size,
c) a pre-sieve ( 3 ) comprising elongated sieve openings formed by fingers extending towards an end portion of the pre-sieve with the tips of the fingers not being connected in that end portion, the pre-sieve ( 3 ) being located between the conveyor outlet and the sieve,
d) wherein the pre-sieve ( 3 ) encloses part of the circumference of the screw conveyor in an axial end portion of the screw conveyor, and
e) the radial distance between the screw conveyor ( 1 ) and the pre-sieve ( 3 ) is less than 10% of the screw conveyor diameter.
2 . Device according to claim 1 , characterized in that the pre-sieve ( 3 ) comprises elongated sieve openings including an angle of −40° to 40° with the conveying direction of the screw conveyor.
3 . Device according to claim 1 , characterized in that the width of the elongate openings increases from the base to the tip of the fingers.
4 . Device according to claim 3 , characterized in that the increase in width is by a factor of 2 to 6.
5 . Device according to claim 4 , characterized in that the increase in width is by a factor of 3 to 5.
6 . Device according to claim 1 , comprising at least one of:
a) the width of the elongate openings at the base is between 1 and 5 mm;
b) the width of the elongate openings at the tip of the fingers is between 4 and 20 mm;
c) the length of the fingers from base to tip is between 100 and 500 mm;
d) the mesh size of the sieve ( 7 ) is between 200 and 1,000 μm.
7 . Device according to claim 6 , wherein
a) the width of the elongate openings at the base is between 2 and 4 mm;
b) the width of the elongate openings at the tip of the fingers is between 10 and 14 mm;
c) the length of the fingers from base to tip is between 150 and 250 mm; and
d) the mesh size of the sieve ( 7 ) is between 300 and 800 μm.
8 . Device according to claim 1 , characterized in that it further comprises a mechanical impact device for providing mechanical impacts to the pre-sieve ( 3 ).
9 . Device according to claim 8 , characterized in that the mechanical impact device is a hammer or piston-vibrator.
10 . Device according to claim 8 , characterized in that the impacting force and impacting frequency of the mechanical impact device is controllable.
11 . Device according to claim 8 , characterized in that the mechanical impact device provides impacts in the area of the base of the fingers of the pre-sieve ( 3 ).
12 . The use of a device of claim 1 for the separation of a particulate ash stream from a fluidized bed boiler.
13 . A method for operating a fluidized bed boiler, comprising the steps of:
a) carrying out a fluidized bed combustion process;
b) removing at least one ash stream from the fluidized bed boiler;
c) separating the ash stream into at least two fractions, wherein the separation includes a separation step using a device according to claim 1 ;
d) recirculating a separated particle fraction into the bed of the fluidized bed boiler.
14 . The method of claim 13 , characterized in that the boiler is a circulating fluidized bed boiler (CFB) or a bubbling fluidized bed boiler (BFB).
15 . The method of claim 13 , characterized in that the fluidized bed and the ash stream from the fluidized bed comprise ilmenite particles and that the separated recirculated particle fraction is enriched in ilmenite.
16 . The method of 13 , characterized in that the separation includes a step of using a magnetic separator ( 12 ) comprising a field strength of 2,000 Gauss or more.
17 . The method of 14 , wherein the field strength is 4,500 Gauss or more.
18 . The method of claim 15 , characterized in that the fraction of ilmenite in the bed material is 25 wt. % or more.
19 . The method of claim 18 , wherein the fraction of ilmenite in the bed material is 30 wt. % or more.
20 . Device according to claim 1 , wherein the radial distance between the screw conveyor ( 1 ) and the pre-sieve ( 3 ) is less than 5% of the screw conveyor diameter.