Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a balling process. It is
especially suitable for the control of a balling process
producing green balls from finely crushed iron ore and water.
Balling is a process which can be applied ~o the
p~elletizing of iron ore to form a product suitable as a
blast furnace feed material. In a typical pelletizing
plant an ore or a mixture of ores is dried and then ground
to the required fineness. A very small proportion of a binding
additive (e g. bentonite~ is added to improve its adhesion
properties. A finely controlled amount of water is added.
The product, now referred to as concentrate, is then
charged to several balling drums into which a further small
proportion of water is sprayed. In each rotating balling
drum the damp materil agglomerates to form green balls.
The product of each drum is screened: undersize and
crushed oversize green balls are recirculated, while
the acceptable product is charged to a moving grate on
which the green balls are dried and preheated before being
fired to form the required pellets.
A balling drum rotates about its longitudinal axis
which is slightly inclined to the horizontal. The higher
end is the charge end, the lower end the discharge end.
A conveyor is provided to return the undersize green balls
from the discharge end to the charge end.
The mechanism of balling is complex but two significant
stages in green ball growth are the initial formation of
small porous stable balls known as seeds and a subsequent
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growth phase in which balls grow by taking up either raw
concentrate or the material of smaller balls.
In the balling process new concentrate at a
constant rate and returned undersized green balls are
S supplied to the charge end of the balling drum. In the
flrst fe~ metres of the drum the existing balls grow and
new seeds are formed from the concentrate. In the remainder
of the drum the larger balls grow at the expense of the
smaller ones. At the discharge end a new distribution of
green balls emerges of which the undersized are recycled
to the charge end and pass through the drum again.
We have performed a theoretical analysis on the
balling drum circuit and have found that the balling -
process is essentially a non-linear feedback system, with
non-linear components due to the drum and the screens, and
an effective linear part consisting of the net loop time
dealy. Such loops are a feature ln control systems analysis
and, depending upon the nature of the loop components, the
system may exhibit a periodic non-linear, limit cycle,
oscillation. This behaviour is continuously observed in
practice on balling circuits and is known colloquially
as 'surging' i.e. a self-sustaining oscillation in product
size distribution. If a large number of undersized green
balls are discharged and returned to the charge end of the
drum they will act as seeds; and during their next pass
through the drum they will reduce the number of new seeds
formed by initially taking up more of the available raw
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material and subsequently crushing more of the seeds that do
form to grow further. This will result in a small number of
undersized balls being discharged, and correspondingly a small
number of seeds being recycled. In the next pass of these
seed~l through the drum new seed formation will be less inhibited
and the product will once again contain a large number of
undersized green balls. In the normal operating region to
ohtain acceptable ball quality surging is inevitable in the
conventional balling circuit.
Surging in the balling drum results in cyclical
variations in the number of green balls formed of satisfactory
size. These variations must be contained to a small proportion
of product throughput to enable satisfactory performance to be
achieved in the subsequent stages of the pelletizing process.
We are aware of no practical and economic way which
has yet been found to elirninate surging but various control
methods have been proposed. These include variation in the
binding additive addition and in the water spray addition in
the balling drum. Each method has its own disadvantages.
According to the present invention, as herein claimed,
there is provided a process for balling particulate material com-
prising the steps of: improving the adhesion properties of the
particulate material by adding water and a binding medium;
balling said particulate material in a balling drum to produce
green balls; discharging said balls from said balling drum;
separating balls having a size other than predetermined size
from balls having said predetermined size after their discharge
from the said balling drum; and continuously returning said
separated balls to said balling drum for balllng with adde~
particulate material, in which process the recurrence of dis-
charged separated balls identically as those balls recharged to
the balling drum after a constant time interval is modified by
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varying the normally constant mass relationship between the
discharged separated and recharged balls so as to reduce the
system gain.
The recurrence of discharged separated balls identi-
call~ as those balls recharged to the balling drum after a
constant time interval may be modified by feeding discharged
separated balls to a reservoir and recharging separated balls to
the balling drum from the reservoir at a more constant rate than
that at which they are discharged.
Discharged separated balls of different sizes may be
fed to separate reservoirs and balls may be recharged to the
balling drum from each reservoir at a more constant rate than ;~
that at which they are fed to that reservoir. The average rate
of feeding the separated balls to the or each reservoir may be
equal to the average rate of removing the balls from that
reservoir.
The separated balls discharged from one balling drum
may be recharged to another balling drum. The separated balls
discharged from one balling drum may be exchangedwith separated
balls of different sizes discharged from another balling drum
before being recharged to the balling drums.
Discharged balls under a predetermined size may be
separated from those balls discharged from the balling drum.
A predetermined proportion of these discharged under-size balls
may be removed from those returned to the balling drum. The
balling process may be adapted for the production of green
balls from iron ore, water and a binding additive.
The invention is illustrated by way of example in the
accompanying drawings which show diagrammatically in Figures 1
to 5 five balling processes in which surging is controlled.
In Figure 1 a source 11 of raw material for the
balling process steadily supplies a concentrate of ground iron
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ore, water and binding medium to a balling drum 12. Green
balls are formed in the balling drum. At the discharge end
of the drum is a screen 13 at which correctly sized and
oversize green balls are directed towards a grate 14 from
which they continue in the pelletizi.ng process, with the pos-
sibility that oversize balls are instead crushed and recycled.
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Undersize green balls are separated at the screen 13 and
conveyed back to a device 15 from which they are fed at a
controlled rate back to the charge end of the balling drum
together with fresh concentrate from the source 11.
In the process illustrated in Figure 2 a source 21
of concentrate supplies two bslling drums 22a and 22b, having
at their discharge ends screens 23a and 23_ feeding correctly
sized and oversize green balls to grate 24.
The undersize balls are normally returned to the
charge ends of the drums in which they were formed but
cross-conveyors 26a and 26b are provided whereby some or
all of the undersize green balls from drum 22a and/or 22b
can for a period of time be switched to the charge end of
the other drum 22b and/or 22_ respectively.
Although only two balling circuits are shown for the
sake of simplicity, in practice five or s~ might be
interconnected in a similar manner.
In Figure 3 a source of concentrate 31 supplies the
charge end of one balling drum 32 which is provided with
screen 33 at its discharge end to feed correctly sized and
oversize green balls to a grate 34. The undersize green
balls separated at the screen pass to a further screen 37
in the return circuit. This screen 37 is selected, and can
be changed, to remove from the return circuit balls either
greater or smaller than a chosen size. These balls are
conveyed to a crusher 38 from which they are recycled and
their material in due course recurs in the feed concentrate.
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The remaining balls are returned to the charge end of the
balling dru~ as in the previous ex~mples.
It is possible to replace the screen 37 with a simple
sampling device to extract randqm green balls from the
return circuit at a controllable rate, the extracted balls
likewise being crushed before being recycled.
In Figure 4 a source 41 feeds concentrate to a single
balling drum 42 provided with a discharge screen 43
feeding a grate 44 as before. The undersize green balls
from the screen 43 are conveyed to another screen 47a at
which the largest balls are taken out and passed to a surge
hopper 45a. The remaining balls pass through the screen
47_ to a further screen 47b at which the larger balls are
taken out and passed to a second surge hopper 45_. The
smaller balls pass through the screen 47b to a third
surge hopper 45c . The three sizes of undersize green
balls in the three surge hoppers 45a, 45b and 45c are
thence conveyed each at a constant rate to be recharged
into the balling drum 42.
In Figure 5 a source 51 feeds concentrate to two
balling circuits which respectively have balling drums 52_
and 52_ and discharge screens 53_ and 53b feeding grate 54
in a similar manner to the balling circuits previously
described. The undersize green balls from the screens 53_
and 53b go to further screens 57a and 57b to be separated
` into larger and smaller balls. These are normally returned
to the charge ends of the drums in which they were formed but
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cross-conveyors 56a and 56b are provided whereby some
or all of the smaller or the larger green balls from
balling drum 52a and/or 52b can for a period of time be
switched to the charge end of the other drum.
As in the case of the system illustrated in Figure 2,
more than two balllng circuits may well be interconnected
in a similar manner.
It will be appreciated that other control methods and
combinations of control methods will be possible to modify
the fundamental direct relationship between discharged -
undersize balls and those recharged to the balling operation
to reduce surging.
In the above examples reference is made to passing
oversize green balls to the grate. This may be a tolerable
practice when, as is often the case, such balls form only
about five per cent of the product,
