Process for the purification of sulfur

Details Process for the purification of sulfur Process for the purification of sulfur

2001-12-21

2004-03-16

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Sulfur or compound thereof

Elemental sulfur

Reexamination Certificate

active

06706250

ABSTRACT:

This invention relates to a discontinuous process for the purification of sulfur by washing with an organic solvent. The sulfur that is to be purified is actually contaminated by solid or liquid, organic or inorganic chemical radicals. The process according to the invention pertains to, for example, the sulfur that is obtained in a “redox” process for desulfurization of a gas that contains at least hydrogen sulfide. This type of process can use a catalytic solution that comprises at least one multivalent metal (Fe
3+
or V
5+
, for example) that may or may not be chelated by at least one chelating agent under suitable conditions for carrying out the oxidation of the hydrogen sulfide into elementary sulfur and the simultaneous reduction of the multivalent metal that may or may not be chelated from a higher degree of oxidation to a lower degree of oxidation. The gaseous effluent that is recovered is almost free of hydrogen sulfide. The catalytic solution is at least partially reduced and contains elementary sulfur. Sulfur can be removed from the catalytic solution before or after the regeneration stage of the catalytic solution. The sulfur that is collected at least in part is then purified according to the process of the invention.
Patents FR-A-2 700 713 and U.S. Pat. No. 4,664,902 illustrate the technological background.
The prior art describes numerous “redox” processes and related devices that make it possible to eliminate the hydrogen sulfide and to recover the elementary sulfur that is formed during the process.
By way of example, a desulfurization process that uses an iron chelate comprises, for example, the two oxidation-reduction stages below:
In a first stage (absorption of gas and oxidation-reduction reaction), the hydrogen sulfide that is present in the gas that is to be treated reacts with, for example, chelated ferric ions according to the invention:
H
2
S+2 Fe
3+
(
chel
.)→S+2H
+
+2 Fe
2+
(
chel
.)  (1)
in a second stage (regeneration), the ferrous ions are reoxidized by the oxygen of the air following the reaction:
2 Fe
2+
(
chel
.)+2 H
+
+½O
2
→2 Fe
3+
(
chel
.)+H
2
O  (2)
The catalytic solution may or may not be an aqueous solution of chelated iron, produced from ferrous or ferric iron salts such as sulfates, nitrates, thiosulfates, chlorides, acetates, oxalates and/or phosphates. The ferrous and ferric ions can be replaced respectively by vanadate ions IV and V. The catalytic solution can also contain sodium, potassium, and ammonium ions, carbonates, and/or anthraquinone disulfonates.
The chelating agents, used individually or in a mixture, can be organic compounds that are known for their complexing properties, for example acetylacetone, citric acid, salicylic acid, sulfosalicylic acid, tiron (catechodisulfonic acid), dimercapto-2,3-propanol and amino acids, such as, for example, EDTA (ethylenediamine tetraacetic acid), HEDTA (hydroxy-2-ethylenediamine triacetic acid), NTA (nitrilotriacetic acid), DCTA (diamino-1,2-cyclohexane tetraacetic acid), DPTA (diethylenetriamine pentaacetic acid), IDA (iminodiacetic acid) and ADA (N-(2-acetamido)iminodiacetic) acid).
The solid sulfur that is formed in the “redox” processes is in close contact with the catalytic solution. Most often, the sulfur recovery techniques that are used are mechanical: filtration, flotation or centrifuging. The amount of catalytic solution that is entrained physically with the sulfur is therefore significant. Consequently, the sulfur that is produced is of poor quality.
In contrast, during second stage (2), secondary reactions can arise, in particular the degradation of the chelating agent, which cause the formation of products that accumulate in the catalytic solution and that can precipitate. These products, organic or inorganic compounds, are also entrained with the sulfur and contribute to its poor quality.
Thus the sulfur, obtained in particular from “redox” processes, is not of adequate quality, in particular in the hypothesis of an application in the chemistry sector.
The prior art describes processes and devices that make it possible to purify the elementary sulfur that is formed in “redox” processes.
U.S. Pat. No. 5,122,351 describes a method where the sulfur is washed by suspending it in water, then it is melted to be separated from the aqueous solution. The washing solution is concentrated by evaporation and reinjected in the “redox” process in the H
2
S absorption stage. The evaporated water is recycled for washing the sulfur. The investment costs of such a process are significant. Among other things, a pressurized separator and an evaporator are required. In addition, this process imposes the use of costly materials for limiting the corrosion problems that are linked to the presence of water that are increased at high temperature. Furthermore, the water-insoluble products will not be eliminated.
U.S. Pat. No. 4,705,676 describes a method for purifying sulfur by melting sulfur in a phase separator under an inert atmosphere. The supernatant aqueous catalytic solution is reinjected in the absorber or in the regenerator. The liquid sulfur is filtered. This process requires a pressurized separator that can resist corrosion. In addition, the filtration of the liquid sulfur can be considered only with a large-pore filter to limit the clogging problems. Fine particles thus will remain mixed with the sulfur.
U.S. Pat. No. 4,517,170 describes a method for extraction of sulfur from the catalytic solution of a “redox” process by suspending solid sulfur in a mixture of aliphatic hydrocarbons that have 4 to 8 carbon atoms and recovery of the aqueous catalytic solution. The sulfur and the hydrocarbons can then be separated following different methods. If the solid sulfur is separated mechanically (filtration, centrifuging, . . . ), it will contain a significant amount of hydrocarbons that limits its purity.
According to a variant of this method, the sulfur suspension in the hydrocarbons can be heated in a separator to a sufficient temperature to allow the melting of the sulfur. The use of a pressurized separator to keep the hydrocarbons in liquid phase and to obtain a phase separation produces high costs. In addition, the sulfur that is thus obtained has a purity that is reduced by the solubility of the hydrocarbons in the liquid sulfur.
Finally, if the separator is at a pressure that is close to atmospheric pressure, the hydrocarbons are vaporized. This vaporization leads to residues that pollute the sulfur. This process therefore leads to sulfur of a reduced purity.
The object of the invention is to propose a new method for washing sulfur that offers in particular the advantage of resulting in a sulfur of very high purity. Actually, according to the invention, the sulfur is washed in the liquid state, which makes it possible to eliminate any product that would remain adsorbed on the sulfur in the solid state. In addition, the invention uses an organic solvent that is almost insoluble in the liquid sulfur and that makes it possible to eliminate any compound that is soluble or insoluble in the solutions that are now used in the “redox” processes. The finest particles are also eliminated by extraction in the washing solvent. The process that is described in the invention furthermore has lower investment costs than the above-mentioned processes of the prior art. Actually, the chamber for bringing the sulfur into contact with the solvent is used at a pressure that is close to atmospheric pressure. In addition, it may be made of carbon steel because washing the sulfur does not require any addition of water, and water, optionally present in the sulfur, is vaporized under suitable operating conditions.
Finally, the purification of the sulfur as it is carried out by this invention makes it possible, on the one hand, to upgrade it, and, on the other hand, to reduce the volumes for disposal in the case where the unpurified sulfur would be considered as waste.
This invention relates to a process fo

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