Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
22772-1041
4~:057
The invention relates to a process for the preparation
of sulphur granules in which, in a granulation zone, a sulphur melt
is with the aid of at least one feeding device supplied in upward
direction into a bed of sulphur nuclei, which are kept separate
from each other and which are contacted with a gas.
A process of thls type is known from U.S. Patent Specif-
ication No. 3,231,413 and Canadian Patent Specification No.
689,442, describing the granulation of, arnong other things, sulphur,
by the so-called spouted-bed granulation method. In these known
processes, in an installation provided with a conical bottom a melt
is sprayed upwa-d through a bed of moving particles with the aid of
a very powerful gas stream, particles being blown from the bed and
falling back into the bed in a fountain the shape of an umbrella.
In this process, the particles grow by being covered with thin
layers of melt.
A disadvantage of this process is that in the bed of
sulphur nuclei a high temperature of 80-90C is maintained, so
that individual granules coalesce to form sulphur agglomerates.
As a result, blockages occur, and said agglomerates moreover have
to be crushed afterwards. In principle, a lower bed temperature
can be established, but this requires an unacceptably large amount
of energy-rich gas.
Another disadvantage of this known process is that for
spraying of the melt a large amount of energy-rich gas is required,
viz. 1.3-2 kg per kg of melt. Finally, the spouted-bed granula-
tion has the disadvantage that the capacity per bed is limited, so
that for the processing of a large amount of melt a number of
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~L~42~57
spouted beds have to be installed next to each other, which in-
volves a considerable investment.
An object of the present invention is tG provide a pro-
cess by application of which sulphur granules can be prepared
from a sulphur melt, with little or no agglomeration of granules
in the bed.
Another object of this invention is to provide a process
requiring only a small amount of energy-rich gas.
A further object of this invention is to provide a pro-
cess in which a relatively large amount of sulphur melt can be
processed per granulation zone.
According to the present invention there is provided
a process for preparing sulphur granules comprising feeding a
sulphur melt by means of a feeding device upwardly into a gran-
ulation zone having a fluidlzed bed of sulphur nuclei kept sepa-
rate from each other wherein the sulphur melt has a temperature
of at least 5C above the crystallization temperature, the sul-
phur nuclei have a temperature of 30-70C, the melt, after
leaving the feeding device, is contacted with a powerful gas
stream having a temperature about equal to the temperature of
the melt and a velocity of at least 100 m/sec, in such an amount
that the mass ratio of the gas stream to the sulphur melt is
between 0.1 : 1 and 0.6 : 1, and the sulphur granules formed
are continuously discharged from the granulation zone.
The sulphur melt may have been obtained in various ways,
for example by melting of solid sulphur, or directly by a so-
called Claus process. It is in general not necessary to pre-
I.
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~Z~2057 '
treat the melt. An additional advantaye of the present inven-
tion is that when a hydrogensulphide containing sulphur melt
is used (the so-called sour sulphur) it need no longer be de-
gassed beforehand. It may be important, though, to remove any
clay or sand components from the melt beforehand, to prevent
wear on the feeding devices.
The temperature of the sulphur melt should be at least 5C
higher than the crystallization temperature of the melt, for
it has been found that when a melt of lower temperature is used
sulphur accretion occurs around the outlet of the feeding device.
On the other hand, application of a sulphur melt wi-th a high
temperature has the disadvan-
-3- 12~
tage that the crystallization of the sulphur takes an undesirably long
time. Therefore, by preference a sulphur melt with a temperature of be
tween 125 and 140 C is used.
In the present process, the sulphur melt is with the aid of a
feeding device supplied in upward direction into a fluidized bed of
sulphur particles, the melt being contacted with a powerful gas stream.
As the feeding device, a hydraulic or pneumatic sprayer may be uaed, for
for example.A purely hydraulic sprayer has the advantage of a relatively
low energy consumption but the disadvantage of a rather high degree
of agglomeration in the bed, which disturbs the granulation. The latter
phenomenon will not occur when a pneumatic sprayer is used, or at least
to a much lesser extent. However, the energy consumption is rather high
in this case.
By preference, in the present process a sprayer i5 used in
which the melt is under hydraulic pressure supplied via the inner of
two concentric channels, and is very shortly after exiting this channel
contacted with a powerful gas stream supplied via the outer channel.
As the powerful gas stream, various gases can be used, for
example nitrogen. By preference, air is used.
In the present process the temperature of the gas stream
should be about equal to that of the sulphur melt. When a gas is used
which has a temperature below about 125 C, accretion of sulphur occurs
around the outlet of the feeding devices. When a gas of higher tem-
perature is used, the crystallization of the sulphur is found to take an
undesirably long time. By preference, the gas temperature i8 chosen be-
tween 130 and 140 C.
The amount of energy-rich gas to be used in the process
according to the invention should be between 0.1 and 0.6 parts by mass
per part by mass of sulphur melt. By preference, 0.2-0.4 parts by weight
are used per part by weight of sulphur melt. The velocity of this gas is
preferably 150-250 m/sec.
As nuclei for the fluidized bed, sulphur granules obtained
during screening and/or crushing of the granulate obtained from the bed
may be used, for example. Also, sulphur prills obtained by prilling of a
sulphur melt may be used for this purpose. The diameter of the sulphur
nuclei used may vary, partly in dependence on the desired grain size of
the product. In general, in the bed fresh sulphur nuclei with an average
057
diameter of between 1.0 and 2.0 mm are used.
The quantity of sulphur nuclei to be introduced may vary. It
has been found that for obtaining a satisfactory granulation it suffices
to use an amount equal to the amount of melt supplied. Preferably, the
total amount of particles used is such that the mass ratio of the par-
ticles introduced to the melt is about 1 : 2.5 to 3 : 2.
According to the invention, the bed is fluidized with the ald
of a gas, in particular air. To ensure that the bed is kept totally
fluidizedl this gas should have a minimum superficial velocity. On the
other hand, this must not be so high that the dust emission increases
intolerably. In general, a fluidization gas with a superficial velocity
of 1.8-2.5 m/sec, in particular 2.0-2.3 m/sec is used.
The temperature of the ~luidization gas should be so chosen,
in dependence on the temperaturP of the sulphur melt, the spraying gas
and the supplied sulphur particles, that the temperature in the bed is
between 30 and 70 C. Preferably, air of ambient temperature is used.
The average height of the bed may vary within wide limits, for
example 40-100 cm.
One of the essential features of the present invention is a
relatively low bed temperature of 30-70 C. This wholly or substantially
prevents agglomeration in the bed, and enables the use of a fluidization
gas which has hardly been preheated, if at all. Preferably, a bed tem-
perature of between 40 and 65 C is used.
The granules obtained in the bed are continuously discharged
for example via an overflow weir or via a drain in the wall or in the
bottom of the granulation zone. Preferably, the granules are discharged
via a discharge opening in the bottom.
The granulate discharged is subsequently screened into a frac-
tion of the desired particle size, for example 1.5-5, preferably 2.5-4.5
mm, and a coarser and a finer fraction. The finer fraction is returned
to the granulation bed. The coarser fraction may at least in part be
crushed, preferably after removal of fine dust, and the crushed granules
may be returned to the bed. It is also possible to melt this fraction
and return the obtained melt straightaway or convert it to prills and
use these prills as nuclei for the bed.
The air issuing from the fluidized bed, which contains an
amount of sulphur dust and, possibly, hydrogen sulphide, may be purified
124~05~
in a known manner, for example with the aid of a filter or a
cyclone. It is also possible to return a portion of the off
gases and use it as fluidization gas after mixing with cold gas,
so that the amount of dust to be removed is decreased.
The product granulate obtained (~50 of 2.5-4.5mm) has a
crushing strength (30-35 bar) and an impact resistance (30~)
which are better than in the case of commercially produced sul-
phur prills. Also the angle of repose (30), the angle of fric-
tion on steel (31) and the angle of rupture (39) are better
than in the case of prills, while the development of dust during
storage and transport is lower than with prills and, for example,
flakes.
The invention will be explained in detail with reference
to the accompanying drawing, which represents a schematic mode
of realization of the process.
A sulphur melt is supplied to fluidized-bed granulator A
via line 1, and solid sulphur particles obtained during screen-
ing of the granulate are supplied via lines 4 and 5. In this
operation, the sulphur melt is introduced into the bed in upward
direction with the aid of a feeding device which has been in-
stalled in or just above the perforated bottom plate. In addi-
tion, via line 2 a powerful gas stream is supplied. The bed
is kept fluidized with the aid of a gas supplied via line 3.
Via line 6, a sulphur-dust-containing off gas is discharged and
sent to a purification unit which has been omitted from the
drawing. The sulphur dust so recovered may be melted, for exam-
ple, and returned to A. Via line 7, the granules obtained are
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~24~ 57
carried to screening section B. 'rhe fine fraction obtained here
is via lines 8 and 5 returned to A, while via line 9 the product
fraction is discharged, for example to a polishing drum and/or
cooler, and subsequently to a storage or transhipment room which
have been omitted from the drawing. From B, the coarse fraction
is via line 10 led to tne crusher C, from where the crushed
granulate is via line 11 led to screen D. From D, the screened,
crushed material is via lines 12 and 4 returned to A, while the
dust obtained in D is discharged via line 13. This dust may
be melted and then returned to A.
~'~42057
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Example I
To a circular fluidized-bed granulator having a diameter of
45 cm and being provided with a perforated bottom plate (aperture
diameter 2 mm), which contained a bed of sulphur particles (average
diameter approx. 3 mm) with a height of approx. 50 cm, a sulphur melt
was continuously supplied at a rate of 150 kg/h. The melt, which had a
temperature of 135 C, had been obtained by melting rod-shaped solid
sulphur and subsequent filtration over a metal filter (aperture size
approx. 0.5 mm). The melt was at a feed pressure of 3 bar brought into
the bed in upward direction via the central channel (diameter 3 mm) of a
hollow conical sprayer mounted in the bottom plate. Via a channel
concentrically arranged around this central channel, which first channel
on the outlet side had an area of 84 mm2 and an annulus with a width of
1.3 mm, under a feed pressure of 1.73 bar an air stream was supplied
which had a temperature of 135 C. The velocity of this air stream upon
exit from the sprayer was about 180 m/sec, and the quantity about 55.5
kg/h (mass ratio of air to melt 0.37 : 1).
Also, 150 kg/h of solid sulphur particles, with an average
diameter of 1.0-2.0 mm and a temperature of 36 C, which had been
obtained in screening and crushing operations on the granulate from the
bed, were supplied to the bed. The bed of particles had a temperature of
approx. 45 C and was fluidized with the aid of an upward stream of air
(1800 m3/h, with a temperature of about 20 C and a superficial velocity
of 2.0 m/sec).
Via an overflow, granules were continuously discharged from
the bed to a screening section provided with flat Engelman sieves with
woven wire cloth of aperture size 2.5 mm and 4.5 mm.
The fine fraction thus obtained (approx. 110 kg/h) was returned to the
bed, while the obtained coarse fraction (approx. 39 kg/h) was crushed to
an average size of 1.0-1.5 mm, after which the crushed material was
screened over a sieve with an aperture size of 860 em. The dust obtained
in this operation was melted and returned to the granulator, and the
remaining crushed material was returned to the bed. The fraction
obtained during screening which had a diameter of 2.5-4.5 mm (approx.
145 kg/h) was discharged as product. The properties of the product gra-
nules are represented in Table II.
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The stream of air issuing from the granulation bed, which had
a dust content of about 250 mg/m3 of air, was led to a cyclone. The
sulphur dust caught here (233 g/h, with a ~50 of 35 ~nm), was melted and
returned to the granulator. The air issuing from the cyclone had a dust
content of 17 mg/m3 of air.
Example II
In the same way as in Example I, a sulphur melt and solid
sulphur particles were contimlously supplied to an oblong fluidized-bed
granulator with a length of 2 m and a width of 1 m, provided with a per-
forated bottom plate in which 17 sprayers of the type described inExample I had been mounted at intervals of 34-35 cm. The amount of
sulphur melt supplied was about 3 tonnes per hour, also about 3 tonnes
of sulphur partlcles being supplied per hour. The bed, the bottom plate
of which had been mounted at an angle of about 3 , was at its lowest
point provided with a discharge unit in the form of a downcomer with a
control valve. The spraying devices were at the outlet opening provided
with a ring of tungsten carbide.
The other process conditions were virtually the same as those
in Example I;
sulphur melt: temp. 135 C;
feed pressure 4 bar.
spraying air: temp. 135 C
velocity 180 m/sec
feed pressure 1.74 bar
air to melt mass ratio 0.36 : 1
fluidization air: temp. 20 C
superficial velocity 2.0 m/sec
quantity 30,000 m3/h
bed: temp. 45 C.
The granules discharged from the bed via the bottom were by
screenlng separated into a fraction smaller than 2.5 mm (approx. 37 %),
a fraction of 2.5-4.5 mm (approx. 50 %) and a fraction larger than 4.5
mm (approx. 13 %), after which the latter fraction was crushed, and this
crushed fraction and the fraction smaller than 2.5 mm were returned to
57
--8--
the bed. As product fraction about 70 tonnes/day of sulphur granules
were obtained, having virtually the fiame properties as described in
Example I.
The stream of air issuing from the granulation bed, which con-
tained about 250 mg of sulphur dust per m3 of air, was purified with theaid of a dust filter, yieldlng a purified stream of air, which contained
2 mg of sulphur dust per m3 of air. The sulphur dust thus obtained, as
well as the dust obtained during crushing, was melted and returned to
the granulator.
Example III-XIII
In the manner described in Rxample I, a sulphur melt was con-
verted to granules. The process conditions applied are summarized in
Table I, the product properties in Table II.
N _
1. In the 'Raw Material' row, PR refers to molten prills, PZ to molten
rod-shaped sulphur and EP to molten fluidized-bed granules.
2. 'Fines percentage' is the fraction 2.5 mm, expressed as a percen-
tage.
3. 'Coarse percentage' is the fraction 4.5 mm, expressed as a percen-
tage.
4. 'Percentage of product granules' is the fraction 2.5-4.5 mm,
expressed as a percentage.
5. In examples XII and XIII, the granulate was discharged from the bed
with the aid of a conveyor screw mounted in the bottom plate.
6. In Table II the product properties of commercially available sulphur
prills have been included for comparison.
~20~57
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