CA1242057A - Process for preparing sulphur granules - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present 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 are contacted with a gas, the process being characterized in that a sulphur melt having a temperature of at least 5°C above the crystallization temperature is introduced into a bed of sulphur nuclei having a temperature of 30-70°C, which bed is fluidized with a gas, the melt, after having left the feeding device, being 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 process provides a more energy efficient production of sulphur granules having improved properties and reduces agglomeration of granules in the bed.

Description

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 ~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.

-2a-~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 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 -5a-~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 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.

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 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.

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l ~4 . . . or X O O _~ Us O

1-1 Cal U) Us O
. . Us f Us . .
X 9 _~ Cal _~ ED O
It O ED
a us us us cr H ~4 . . . or) us O .
K l O a _~ Ir) O
It O Ul It . . . l I) Ul ED 0:) JO . .
O O _1 ED Us O O
f O Us Us X . . . I) U) I') O . .
O O

H l O O ED ~0 l O
P.~ O _~ Cal l l O O
_1 l I O Us tl It :~
H O to I l l ;1 l O O
i O O l l I n H to . . . l l O O .
P O I C`J ,1 l Us O Us O
O
ED O. 1'') O _~ O ,_1 :~ O Cal _l O
O Cal O ED _l It ::' N . . . to l us _~ us .
H Pi O CN CN _I _~ ED l _I us O l l O `:S O Us _l Us H . . . i it if O Ir) 1~1 .
1_1 ~4 O O _l _l _l _~ Cal H l O us us O

O A I i or) It I) O tY~

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_l Lo I_ rl ~J V I_ V C N V no o V O

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no _~ l ta v . _~ O O V V C O O
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trl JO 3 a e e c ~J 0 g o x pa a o En CY: '1 ~4 Ed En O 01 54 us o us -lo- 20~
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g PHC .-1 O O us Ill O O O O I) us i O
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X O . Us O O O O O _~ Ox O
H _I I to l _~ a C`l I

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H _I l l l l l to C`l l I') v Us . Us O O O O O C`l CS~ O
H _~ _~ C`l _I l I try C`l or ) I

_~ f l U r r v a ~J it _~ C ~~ C`
J- 00 00 04--~ C `- Gl _~ So .. I. _, I, o ,1 E
o so g 0 co a\ so ,a . O Pi J- en c c 'I to Jo g pa 0 0 C C ~0~:~ C)_ O O V- ~0 ~Oz o C\
l tO ta l r f l to CJ :~
l i3 ,~ c 0 al a Go 0 0 JO U U
¢ % O IJ~ tO l h l ~3 1: C
E-l l ~3 P O tll O O ta H to H ta H l ¢ ¢ O ¢ --U'~ O Us o Us

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing sulphur granules comprising feeding a sulphur melt by means of a feeding device upwardly into a granulation zone having a fluidized bed of sulphur nuclei kept separate from each other wherein the sulphur melt has a temperature of at least 5°C above the crystallization tempera-ture, the sulphur nuclei have a temperature of 30-70°C, 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.

2. A process according to claim 1, wherein the temperature of the sulphur melt is between 125 and 140°C, the powerful gas stream has a temperature of between 130 and 140°C and a velocity of 150-250 m/sec, and the mass ratio of gas stream to sulphur melt is 0.2 : 1 to 0.4 : 1.

3. A process according to claim 1, wherein the bed of sulphur nuclei is fluidized by passing therethrough a gas which is of ambient temperature and has a superficial velocity of 1.8-2.5 m/sec.

4. A process according to claim 1, 2 or 3, wherein the sulphur particles in the bed have an average diameter of between 1.0 and 2.0 mm, and the bed is fluidized by passing a gas there-through with a superficial velocity of 2.0-2.3 m/sec.

5. A process according to claim 1, 2 or 3, wherein the bed temperature is between 40 and 65°C.