PROCESS FOR THE PURIFICATION OF LIQUID SULPHUR Field of the invention
The present invention relates to a process for a process for the purification of liquid sulphur by degassing said liquid sulphur. More particularly, the present invention relates to a process for purifying liquid sulphur by degassing the liquid sulphur to remove hydrogen sulphide and hydrogen polysulphides therefrom. The present invention also relates to a composition for use in the degassing of liquid sulphur to remove hydrogen sulphide and hydrogen polysulphide impurities therefrom comprising a synthetic synergistic composition comprising of urea and ammonium thiosulphate. Background of the invention
The Claus process is a well-known process for producing elemental sulphur from hydrogen sulphide by the reaction of hydrogen sulphide and sulphur dioxide to produce elemental sulphur and water. Typically, hydrogen sulphide contained in product gas from petroleum refinery operations is partially combusted in a thermal zone to produce sulphur dioxide, which then reacts with the unburned hydrogen sulphide to yield sulphur and water. The sulphur is then condensed and recovered. One or more catalytic zones are also provided in which the same reaction is further promoted by means of a suitable catalyst, causing additional sulphur to be recovered.
In Claus plants the hydrogen sulphide is converted to elementary sulphur vapor by oxidation with sulphur dioxide. The sulphur vapor is condensed in condensers and the liquid sulphur is stored temporarily in sulphur pits. During condensation of the sulphur vapor in the condensers, the liquid sulphur absorbs a considerable amount of hydrogen sulphide from the vapour phase. A part of the hydrogen sulphide reacts with liquid sulphur and forms hydrogen polysulphides. The concentration of the latter increases with the increase in the liquid sulphur temperature.

Unfortunately, the transportation and handling of degassed liquid sulphur is extremely difficult. Explosions and fires have been reported due to release of hydrogen sulphide to the atmosphere during handling of the undegassed liquid sulphur. It is reported that hydrogen sulphide is lethal at 600 ppm in air and explosive at 3.5% volume in air at 150°C.
Apart from the problem of explosion and toxicity, another problem of hydrogen sulphide contamination in liquid sulphur is that of odor. The stench due to hydrogen sulphide is a nuisance. It is therefore essential that in installations producing or processing sulphur, the amount of hydrogen sulphide and polysulphides must be reduced to below 10 ppm by weight.
While it is relatively easy to remove dissolved hydrogen sulphide from liquid sulphur by physical processes such as stirring, spraying, pumping or by passing gas or air through it, removal of the hydrogen polysulphides formed is a difficult problem. In order to remove the polysulphides, it is first essential that they be decomposed to hydrogen sulphide and elemental sulphur according to the following reaction
(Formula Removed)
before the hydrogen sulphide thus formed can be removed from the liquid sulphur by
degassing
(Formula Removed)

It is known to accelerate the decomposition of polysulphides by adding nitrogenous compounds such as ammonia, ammonium salts, organic nitrogen compounds (such as alkyl amines, alkanol amines or aromatic nitrogen compounds) or urea. These nitrogen compounds function as a catalyst and thus shorten the decomposition time and hence the time required for degassing.
The SNEA sulphur degassing process [developed by Societe Nationale des Petroles d' Aquitaine (SNPA) now known as SNEA] involves pumping round and spraying of sulphur, with ammonia being added as catalyst (French Patent No. 1,435,788). This process was subsequently improved from non-continuous mode to continuous mode where the sulphur is
circulated over two compartments and sprayed with the addition of ammonia as a catalyst. Further modifications of this process involve using liquid catalyst [The process is known as Aquisulph (Oil and Gas Journal Jul. 17, 1989, pp. 65-69)].
A process was developed by Exxon for the degassing of sulphur which involved adding a liquid catalyst in the sulphur pit or tank (CEP October 1985, pp. 42-44 and in Hydrocarbon Processing May 1981, pp. 102-103). In the Exxon process the sulphur is not circulated or agitated in any other way. While the process results in saving in terms of energy costs, the retention time is very high. To achieve substantial degassing, the retention time is from 3 to 4 days.
US Patents Nos. 3,807,141 and 3,920,424 to Texas Gulf disclose a sulphur degassing process wherein the liquid sulphur flows down a column over dishes and the sulphur is degassed countercurrently with air. The equipment modifications required are expensive in this process.
Dutch Patent 173,735 granted to Shell Internationale Research Maatschappij discloses a method consisting of a single process step in which air or a mixture of an inert gas and oxygen is passed through liquid sulphur in the presence of a catalyst, typically a nitrogen compound, in finely divided condition and thereafter the liquid sulphur and the used gas are separated from each other.
A comparable method is described in DD-A 292,635. According to this method, the treated sulphur, prior to the further processing, is subjected to a supplementary post-gassing. However, such a post-gassing has no effect or substantially no effect on the reduction of the sulphide content in the liquid sulphur.
A proprietary sulphur degassing process known as "HySpec" (Procor) consists of a number of gas-liquid contact mixers arranged in series (presented at the Sulphur '94 conference at Tampa Fla., Nov. 6-9, 1994; WO-A 95/06616). A catalyst is added to the contact mixers and finally in the last mixing stage the sulphur is stripped of the added catalyst
by passing air through it. Such a gas-liquid contact mixer consists of a mixer driven by an electric motor, which circulates the sulphur with drawn-in air over a perforated cylinder. A disadvantage of this method is the use of moving parts such as an agitator, which comes into contact with the liquid sulphur. In systems with liquid sulphur, there is a great chance that moving parts will jam.
While the size of sulphur degassing plants can be reduced by using a catalyst, additional disadvantages of contamination of the sulphur arise. Another problem is of clogging due to the formation of ammonium sulphate salt where ammonia or ammoniacal compounds are used as a catalyst. The ammonium sulphate salts thus formed also lead to corrosion problems in the reactor thereby resulting in additional costs of adaptation of sulphur degassing plants Another significant disadvantage is that such a process requires a much longer degassing time, entailing higher investment and involving higher energy consumption.
British Patent No. 1,067,815 discloses a degasification process for removal of hydrogen sulphide by atomizing the liquid sulphur containing hydrogen sulphide by forcing it through a jet or nozzle and then directing the resulting spray against an obstacle, thus promoting the removal of the gaseous HiS. It is also disclosed that the presence of ammonia (100 ppm) promoted the removal of t^S. In the absence of the use of ammonia the t^S reduction is extremely slow, involving long time spans. However, the use of ammonia inherently results in a contaminated product.
Alternate methods for removal of HiS reminiscent of the Claus reaction have been reported in U.S. Patent No. 3,447,903 and Canadian Patent No. 964,040. In U.S. Patent No. 3,447,903 a catalytic process for producing elemental sulphur from I-^S and SC>2 in liquid sulphur is disclosed. The catalyst involved is described generically as a basic nitrogen compound having a KB value (in water) greater than 10"10 and a solubility in molten sulphur of at least one part per million. This process, as taught, can be practiced for the purpose of

controlling purity of liquid sulphur containing small concentrations of H^S. Canadian Patent No. 964,040 involves injecting liquid SO2 and a nitrogen containing compound, which complexes with the SO2 to form an adduct, into the molten sulphur for the expressed purpose of having the SOa -nitrogen adduct react with the undesirable polysulphide dissolved in sulphur. Hence, it is known that certain nitrogen compounds in combination with 862 will catalytically reduce the H^S and HaSx concentration found in liquid sulphur. Such processes against inherently involve soluble nitrogen containing species being present in the sulphur after degradation of the sulphide and polysulphides; i.e., the processes merely replace one contaminant for another contaminant.
US Patent No. 6149887 discloses a method for removing hydrogen sulphide and hydrogen polysulphide compounds out of liquid sulphur by stripping with a gas, such as air. The method is conducted in an apparatus equipped with at least two degassing compartments and a sulphur collection pit wherein the degassing compartments are separated from each other by a first partition wall, the last degassing compartment is separated from the sulphur collection pit by a second partition wall. Each degassing compartment contains at least a first, and second sub-compartments separated from each other by a third partition wall and are open to each other at the top and the bottom. At least one first sub-compartment in each said degassing compartment is provided at the bottom with a plurality of stripping gas inlet nozzles and at least one second sub-compartment in each said degassing compartment is not provided with stripping gas inlet nozzles. The one first sub-compartment is constructed to allow flow of liquid sulphur over or through the first partition wall to a subsequent degassing compartment. The last degassing compartment is constructed to allow flow of liquid sulphur to the sulphur collection pit over the second partition wall and the apparatus is also provided with a provision for discharging gas comprising hydrogen sulphide. The apparatus is expensive.

US Patent No. 5080695 discloses a process for the degasification of liquid sulphur produced by the Claus process to remove hydrogen sulphide. The liquid sulphur is caused to flow continuously through a vessel where it is contacted by a counter-flowing inert gas such as nitrogen or air thereby stripping hydrogen sulphide from the sulphur. However the disadvantage of this process is that the portion of the conduit which is common to the flow of both liquid sulphur and gas should be long enough to ensure sufficient intimate contact between the gas and the liquid sulphur so that adequate stripping of hydrogen sulphide takes place. Further, the annular space between the gas tubing and the conduit should be large enough so as not to restrict the flow of sulphur through the vessel as required by the output of the manufacturing process, but small enough to allow the desired intimate contact. The pressure of the stripping gas must be sufficient to overcome the pressure of the liquid sulphur and the pressure head at the bottom of the tube in order to escape from the tube. These physical parameters render the process equipment expensive. Another disadvantage is that the reduction in ppm level of hydrogen sulphide is only to the level of 50%. The efficiency of the process also depends on the residence time, which in turn depends on the above-mentioned physical equipment requirements of tube length.
US Patent No. 5935548 discloses a process where hydrogen sulphide is removed from a molten sulphur steam containing at least one of hydrogen sulphide and hydrogen polysulphides by mixing the molten sulphur stream with a degassed molten sulphur in an eductor to form a mixture, contacting the mixture with a finely dispersed gaseous oxidant such as sulphur dioxide, oxygen enriched air or air, separating hydrogen sulphide from the mixture and recovering molten sulphur having a reduced hydrogen sulphide content. The process suffers from a singular disadvantage that the level of removal of hydrogen sulphide is dependant on the residence time and therefore for higher level of removal of hydrogen sulphide, longer tubes, have to be provided to increase the residence time.

US Patent No. 4844720 discloses a process for catalytic degradation of hydrogen polysulphide to hydrogen sulphide in liquid sulphur and removal of hydrogen sulphide from the liquid sulphur by contacting the liquid sulphur containing the hydrogen sulphide and hydrogen polysulphide with a solid degradation catalyst selected from alumina and alumina impregnated with a cobalt-molybdenum desulphurization catalyst at a temperature of from 250°F to 320°F and simultaneously purging the liquid sulphur in contact with solid catalyst with a gas selected from air or oxygen enriched air. It is claimed that this removes the hydrogen sulphide and hydrogen polysulphide from the liquid sulphur at a rate greater than previously achieved. However, again the residence times are still not suitable for large scale commercial operations. Additionally, the use of a solid catalyst apart from the use of air or oxygen enriched air results in a complex and expensive process.
German Patent No. 15 67 791 discloses a process where the liquid sulphur is sprayed into a chamber at a temperature of 125 to 145°C. Ammonia (used as a degasification accelerator) is added to the sulphur before it is sprayed. The degasification accelerator serves mainly to convert the polysulphide to liquid hydrogen sulphide. British Patent No. 1,433,822 teaches the use of air or diethanolamine as a degasification accelerator. The process in accordance with the British patent uses a vessel having two chambers and an overflow over which the treated sulphur flows from the first chamber into the second.
US Patent No. 4612020 discloses a process wherein the liquid sulphur containing the HaS is sprayed in a steel vessel and the liberated gases, which are rich in HaS, are withdrawn. A degasification accelerator comprising 40 - 100% nitrogen is added to the liquid sulphur. The steel vessel contains at least two chambers, which communicate with each other and are connected in series. Sulphur at temperatures in the range from 140 to 160°C is supplied to the first chamber. Sulphur is withdrawn from the second chamber and is cooled to temperatures of 120 to 135°C outside the vessel. The liquid sulphur, which has been cooled, is sprayed in
the gas space of the first chamber. The sulphur is caused to remain in the steel vessel for a

dwelling time of 12 to 32 hours. The gas spaces of the chambers are scavenged with an oxygen-free inert gas. The sulphur is withdrawn from the vessel and sprayed in the gas space of one of the chambers about 30 to 50 times until the dwelling time of the sulphur in the vessel has expired. The disadvantage of this process is that the residence time is too long rendering the process unsuitable for large-scale commercial operations.
US Patent 5030438 discloses a process for the degassification of liquid sulphur using a catalyst system comprising of a heterocyclic compound. Examples of the heterocyclic compound include quinoline, isoquinoline, acridine, benzacridine, benzoquinoline, quinoxaline and the like. However, the commercial viability of the process on commercial scale has not been demonstrated.
US Patent 5004591 discloses an improved catalytic process for removing Ir^S and sulfanes from liquid sulfur and a catalyst system for carrying out the process. The catalyst comprises a basic component and optionally a surfactant component The surfactant when employed comprises at least one compound selected from the group consisting of fatty amines, fatty alkylene diamines, salts of fatty amines, salts of fatty diamines, oxyalkylated derivatives of fatty amines, oxyalkylated derivatives of fatty diamines, salts of oxyalkylated fatty amines, salts of oxyalkylated fatty diamines, fatty quaternary ammonium compounds and benzalconium salts. The base used comprises heterocyclic compounds such as quinoline, isoquinoline, benzoquinoline, acridine, benzacridine, quinoxaline, quinazoline, phenazine, phenantridine, phenantrolines, naphthyridines, bipyridyls. As used in this disclosure, the catalyst system can also be monocomponent comprising a single compound having both basic and surfactant properties. As such, in the two component system, the purpose of the surfactant is only to ease the operation of the continuous process.
As disclosed above, it is known in the art to remove the hydrogen sulphide and hydrogen polysulphides by physical separation. The release of hydrogen sulphide from the liquid sulphur accelerates the rate of decomposition of the hydrogen polysulphides to

hydrogen sulphide and elemental sulphur. However, the major disadvantage of modern commercial degassing processes is that they require large, complex and expensive equipment. For example, the Shell process takes place in a sulphur pit or storage tank where liquid sulphur is vigorously agitated by bubbling air at atmospheric pressure. The stripping columns are open at the top and bottom in order to allow the sulphur to circulate along with the sweep air to displace the hydrogen sulphide produced from vapour phase.
Another disadvantage of the prior art processes is that a large retention time is required in the sulphur pit. For example, the Shell process requires the liquid sulphur to be circulated through the stripping column for about 20 - 24 hour, and the SNEA process typically requires 24 - 32 hours.
In an effort to overcome the disadvantages of the prior art, the applicants' own co-pending application No. Del/2002 teaches a process for the purification of liquid sulphur by the removal of hydrogen sulphide and hydrogen polysulphide present therein comprising adding an effective amount of a degassing agent comprising a non-ammonia heteroaromatic nitrogen bearing compound such as herein described and an alkyl amine to a solution of liquid sulphur in a degassing chamber, sweeping the vapor phase of hydrogen sulphide out of the degassing chamber, separating out the substantially pure liquid sulphur.
However, there is always a scope for improvement in sulphur degassing processes and thus there is a definite need for a liquid sulphur degassing process that not only efficiently reduces the hydrogen sulphide concentration in the liquid sulphur but also requires a relatively short residence time to achieve the desired liquid sulphur degassing, a minimum space area and reduced cost. Objects of the invention
It is the principal object of the present invention to degassify the liquid sulphur reliably at low cost while precluding the risk of explosion or fire.

Another object is to effect the method such that an undesired temperature rise of the liquid sulphur to be treated is avoided.
It is another object of the invention to provide a method for the degasification of liquid sulphur where that is economical and efficient.
It is a further object of the invention to provide a process for the degasification of liquid sulphur by the removal of hydrogen sulphide impurities that is safe, requires less time and is easy to operate and results in a final pure product with minimal contamination. Summary of the invention
The present invention is based on the use of a combination of agents which act synergistically to provide an effective degassing agents for reducing the concentration of hydrogen sulphide and hydrogen polysulphide present in liquid sulphur. The invention is based on the surprising interaction and results obtained by the use of a synergistic composition comprising of a synthetic mixture of urea and ammonium thiosulphate. It has been now surprisingly found that this composition when contacted with liquid sulphur containing hydrogen sulphide and hydrogen polysulphide as impurities reduces the concentration of the contaminants to less than 10 ppm while requiring less residence time. Another significant advantage of using the synergistic mixture of the invention in degassing of liquid sulphur is that the level of equipment modification is low thereby reducing the costs of the process. Thus, the process of the invention is suitable for large scale commercial operations. Since the components of the mixture are readily available, the net costs of the process are further reduced.
The mixture of urea and ammonium thiosulphate is neither a simple admixture nor a product of a chemical reaction but a synergistic composition showing improved and unexpected properties.
Accordingly the present invention provides a process for the purification of liquid sulphur by the removal of hydrogen sulphide and hydrogen polysulphide present therein

comprising adding an effective amount of a degassing agent comprising a synergistic mixture of urea and ammonium thiosulphate to a solution of liquid sulphur in a degassing reactor, sweeping the vapor phase of hydrogen sulphide out of the degassing chamber, and separating out the substantially pure liquid sulphur.
In one embodiment of the invention, the degassing agent is added to the liquid sulphur at the rate of 10 to 15 ppm by weight.
In another embodiment of the invention, the sweep to remove the hydrogen sulphide gas released from the liquid sulphur is carried out in vacuum of about 0.02 bar.
In yet another embodiment of the invention, the residence time for the liquid sulphur in the sulphur pit is in the range of 3 to 4 hours.
In a further embodiment of the invention, the temperature in the sulphur pit is maintained in the range of 125 to 140°C.
In yet another embodiment of the invention, the temperature in the sulphur pit is maintained by means of LP steam coil.
In another embodiment of the invention, the concentration of the hydrogen sulphide and hydrogen polysulphides in the final product is between 4-10 ppm.
The invention also provides a degassing agent for use in degassing of liquid sulphur comprising a synergistic mixture of urea and ammonium thiosulphate, in a ratio of 4:1 to 19:1.
The invention also relates to a process for preparing a catalyst composition for use as a degassing agent which comprising mixing in any conventional manner urea and ammonium thiosulphate in a ratio of 4:1 to 19:1. Brief description of the accompanying drawing
The present invention will now be described in greater detail with reference to the sole figure of the accompanying drawing which shows a schematic representation of the liquid sulphur degassing system for use in the process of the present invention.

Detailed description of the invention
The present invention involves the use of a synthetic mixture of a synergistic mixture of urea and ammonium thiosulphate as a degassing agent for the degassing of liquid sulphur to reduce the concentration of hydrogen sulphide and hydrogen polysulphide. The solution of the degassing agent is metered into the sulphur pit of the Claus process by a chemical injection pump. In the process of the invention, the residence time of the liquid sulphur in the sulphur pit required to reduce the concentration of hydrogen sulphide and hydrogen polysulphide to less than 10 ppm is reduced to 3 - 4 hours. The process also involves the use of air sweep to remove the gaseous hydrogen sulphide released from the liquid sulphur, liquid sulphur circulation and spray in the sulphur pit.
In order to illustrate the invention better, reference is made to the sole figure of accompanying drawing showing schematically, the process flow of the invention using the degassing agent of the invention.
As shown in the drawings, Liquid sulphur is shown at 1, heated by an electrical heating means 2. Hydrogen sulphide gas is produced in a conventional Kipps apparatus by reacting iron sulphide with concentrated Sulphuric acid. The hydrogen sulphide so produced is sent the vessel 1 containing molten sulphur so that the molten sulphur is sufficiently gassified with HaS. Thereafter, the valve 7 is closed to prevent further gassification and the vessel 1 is heated by heating means 2. At the same time, the molten sulphur in vessel 1 is dosed with a catalyst though 3. The sulphur collected at 4 is substantially free of HaS and hydrogen polysulphides.
The present invention will now be illustrated by the following non-limiting examples. Example 1
To liquid sulphur in a degassing reactor was added, an effective amount of a degassing agent, a degassing catalyst comprising of quinoline and an alkyl amine (such as
ethylamine, dipropylamine, butylamine). The reactor was heated. The vapor phase of

hydrogen sulphide was swept out of the degassing chamber, and substantially pure liquid sulphur was separated. This process, referred to as CATDEGAS Process was used as a control.
The process described above was repeated except that the catalyst employed was a mixture of urea and ammonium thiosulphate in accordance with the present invention (conveniently referred to as CATDEGASS -I). The comparison between the two processes are shown in Table 1 which clearly demonstrates that the present invention is superior in terms of performance and result.
Example 2
Example 1 was repeated three times with the catalyst employed being urea alone, ammonium thiosulphate alone and a combination of both. The results are shown in Table II. As can be seen, a combination of urea and ammonium thiosulphate synergistically interact to remove maximum amount of hydrogen sulphide in the shortest possible time.
Example 3
The process of the Example 1 was repeated with varying ratios of the two components of the catalyst of the present invention. The results depicted in Table III show that the ratio of the two components are critical for optimum results, thereby establishing that the catalyst composition is synergistic in nature.
Example 4
The process of Example 1 was repeated three times with the catalyst used being Urea alone (A), ammonium thiosulphate alone (B) and the a combination of the two (CATDEGAS-I) under air purging. The results are shown in Table 4. As can be seen from this Table, the catalyst composition of the present invention results in the removal of maximum amount of hydrogen sulphide from the liquid sulphur in shortest possible time
under air purging.

Example 5
The process of Example 1 was repeated with the catalyst of the present invention. In the first case, process employed air purging while in the second case, the porcess employed Sweep air. The comparative results are shown in Table 5. As can be seen from this Table, air purging results in the removal of maximum amount of hydrogen sulphide in a shortest possible time.
Example 6
The process of Example 1 was repeated three times without aeration. In the first case, no catalyst was employed. In the second case the catalyst of CATEGAS was employed and in the third case, the catalyst of the present invention was employed. The results are shown in Table 6. The comparative results show that the catalyst of the present invention show highly superior results, even in the absence of aeration as compared to CATEGAS process. Non use of any catalyst show poor results.
Example 7
The process of Example 1 was repeated with the catalyst of the present invention under different process kinetics such as varying dosing rate rates and aeration duration. The results for varying kinetics are depicted in Table 7.
ANNEXURE-I
COMPARISON STUDY BETWEEN CATDEGAS &
CATDEGAS-I
(Table Removed)

ANNEXURE-II
LABORATORY SCALE EXPERIMENTAL RESULTS OF CATDEGAS-I
Table - 2-: Comparison among degassing by catalyst A, B & CATDEGAS-I
with air purging
(Table Removed)

Table - 3 Comparison among various compositions (A :B) of CATDEGAS-I
with air purging
(Table Removed)

ANNEXURE . II
LABORATORY SCALE EXPERIMENTAL RESULTS OF CATDEGAS-I
Table-4: Comparison among catalyst A, B & CATDEGAS-I for air purging
in liquid sulphur
(Table Removed)

Table - 5 Comparison between air purging in liquid sulphur and sweep air over liquid sulphur for CATDEGAS-I
(Table Removed)

Table -6: Comparison among aeration without catalyst, usage of CATDEGAS & CATDEGAS-I with air purging in liquid sulphur
(Table Removed)

Table -7. Experimental results for kinetics study using CATDEGAS-I
(Table Removed)

We claim:
1. A process for the purification of liquid sulphur by the removal of hydrogen sulphide and
hydrogen polysulphide present therein comprising adding to said liquid sulphur in a
degassing reactor, an effective amount of a degassing agent comprising of a synergistic
mixture of urea and ammonium thiosulphate, the ratio of urea to ammonium thiosulphate
being in the range of 4:1 to 19:1, sweeping the vapor phase of hydrogen sulphide out of
the degassing chamber, separating out the substantially pure liquid sulphur.
2. A process as claimed in claim 1 wherein the degassing agent is added to the liquid
sulphur at the rate of 10 to 15 ppm by weight.
3. A process as claimed in claims 1 or 2 wherein the sweep to remove the hydrogen sulphide
gas released from the liquid sulphur is carried out by a blower at a pressure of about 1.2
bar.
4. A process as claimed in any preceding claim wherein the residence time for the liquid
sulphur in the sulphur pit is in the range of 3 - 4 hours.
5. A process as claimed in any preceding claim wherein the temperature in the sulphur pit is
maintained in the range of 125 to 140°C.
6. A process as claimed in claim 8 wherein the temperature in the sulphur pit is maintained
by electrical heating.
7. A process as claimed in any preceding claim wherein the concentration of the hydrogen
sulphide and hydrogen polysulphides in the final product is less than about 10 ppm by
weight
8. A catalyst composition for use as a degassing agent in degassing of liquid sulphur
comprising a synergistic mixture of urea and ammonium thiosulphate in a ratio of 4:1 to
19:1.

9. A process for preparing a catalyst composition for use as a degassing agent in degassing
liquid sulphur which comprising mixing in any conventional manner urea and ammonium
thiosulphate in a ratio of 4:1 to 19:1.
10. A process for the purification of liquid sulphur substantially as hereinbefore described
with reference to and as illustrated in the accompanying drawing.
11. A process for the purification of liquid sulphur substantially as hereinbefore described
and with reference to the forgoing Examples
12. A catalyst composition for use as a degassing agent in degassing of liquid sulphur
substantially as described hereinbefore and with reference to the foregoing example.
13. A process for preparing a catalyst composition for use as a degassing agent in degassing
of liquid sulphur substantially as described herein.