A PROCESS TO PREPARE ELEMENTAL SULPHUR

Abstract

The invention is directed to a process to prepare elemental sulphur by (i) contacting an aqueous solution comprising bisulphide with oxidised sulphide-oxidising bacteria under anaerobic conditions wherein elemental sulphur is produced and a reduced sulphide-oxidising bacteria is obtained and (ii) wherein the reduced sulphide-oxidising bacteria are oxidised by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised sulphide-oxidising bacteria.

Claims

1. A process to prepare elemental sulphur by (i) contacting an aqueous solution comprising bisulphide with oxidised sulphide-oxidising bacteria under anaerobic conditions wherein elemental sulphur is produced and a reduced sulphide-oxidising bacteria is obtained and isolating elemental sulphur from the aqueous solution and (ii) wherein the reduced sulphide-oxidising bacteria are oxidised by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised sulphide-oxidising bacteria.

2. The process according to claim 1, wherein the electrochemical cell comprises a cathode which transfers electrons to a compound having a more positive electrode potential than the electrode potential of the reduced sulphide-oxidising bacteria and wherein as a result of this difference in potential between anode and cathode an electric current between said electrodes results.

3. The process according to claim 2, wherein the compound having a higher electrode potential than the electrode potential of the reduced sulphide-oxidising bacteria is oxygen or nitrate.

4. The process according to claim 1, wherein the electrochemical cell comprises a cathode which transfers electrons to a compound having a more negative electrode potential than the electrode potential of the reduced sulphide-oxidising bacteria and wherein a potential is provided between anode and cathode such that the transfer of electrons can take place.

5. The process according to claim 4, wherein the compound having a lower electrode potential than the electrode potential of the reduced sulphide-oxidising bacteria is the hydronium ion and wherein hydrogen is produced.

6. (canceled)

7. The process according to claim 1, wherein the contacting in (i) and the oxidation of the reduced sulphide-oxidising bacteria in (ii) take place in separate steps.

8. (canceled)

9. The process according to claim 1, wherein the anaerobic conditions are defined as a concentration of molecular oxygen in the aqueous solution of at most 0.1 M.

10. The process according to claim 1, wherein the pH of the aqueous solution is between 7.5 and 9.5.

11. The process according to claim 1, wherein the salinity of the aqueous solution expressed as the molar concentration of the total of sodium and/or potassium cations is between 0.5 and 1.5 M.

12. The process according to claim 1, wherein in (ii) a potential of between 0.6 to 0.4 V is applied to the anode versus a Ag/AgCl reference electrode.

13. The process according to claim 1, wherein aqueous solution comprising bisulphide is obtained by contacting an alkaline absorbent with a sour gas comprising sulphur compounds including hydrogen sulphide.

14. The process according to claim 13, wherein the alkaline absorbent comprises oxidised sulphide-oxidising bacteria as obtained in step (ii).

15. A process to improve the effectiveness to absorb hydrogen sulphide of an aqueous solution comprising oxidised and reduced sulphide-oxidising bacteria in a hydrogen sulphide absorption process by oxidising the reduced sulphide-oxidising bacteria by transfer of electrons to an anode of an electrochemical cell.

16. A process to treat a hydrogen sulphide comprising gas comprising the following steps: (a) contacting the hydrogen sulphide comprising gas with an aqueous solution comprising oxidised sulphide-oxidising bacteria thereby obtaining a loaded aqueous solution comprising bisulphide and oxidised sulphide-oxidising bacteria and a gas having a lower content of hydrogen sulphide, (b) contacting the loaded aqueous solution comprising bisulphide with oxidised sulphide-oxidising bacteria under anaerobic conditions wherein elemental sulphur is produced and a reduced sulphide-oxidising bacteria is obtained, and (c) oxidising the reduced sulphide-oxidising bacteria by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised sulphide-oxidising bacteria for reuse in steps (a) and (d) isolating elemental sulphur from the aqueous solution obtained in any of steps (a)-(c).

17. The process according to claim 16, wherein elemental sulphur is isolated from the aqueous solution obtained in step (b) and from the aqueous solution obtained step (c) in a combined sedimentation step to obtain elemental sulphur and an aqueous solution poor in elemental sulphur and comprising oxidised sulphide-oxidising bacteria.

18. The process according to claim 16, wherein steps (b), (c) and optional step (e) are performed according to a process comprising (i) contacting an aqueous solution comprising bisulphide with oxidised sulphide-oxidising bacteria under anaerobic conditions wherein elemental sulphur is produced and a reduced sulphide-oxidising bacteria is obtained and isolating elemental sulphur from the aqueous solution and (iii) wherein the reduced sulphide-oxidising bacteria are oxidised by transfer of electrons to an anode of an electrochemical cell to obtain the oxidised sulphide-oxidising bacteria.

Description

EXAMPLE 1

[0043] Three experiments were performed in a three electrode setup 100 ml of an aqueous solution with a biomass concentration of 80 grN/L of reduced sulphide-oxidising bacteria (bac.sup.) as obtained in an anaerobic reactor of a process as described in WO2015/114069 was contacted with a graphite rod as anode at different anode potentials of 0.1V, 0V, and 0.1V vs Ag/AgCl in a so-called three electrode set up. The sulphide-oxidising bacteria in the aqueous solution were contacted with bisulphide and most of the sulphide-oxidising bacteria were reduced sulphide-oxidising bacteria (bac.sup.).

[0044] The experiment was repeated for the medium alone. The medium was obtained by separating the bacteria from the aqueous solution by means of centrifugation at 10000 rpm for 10 min.

[0045] The result for 0.1 V vs Ag/AgCl is shown in FIG. 3. FIG. 3 shows that the aqueous 1c) solution with the sulphide-oxidising bacteria (line a) resulted in higher recovered charge than the medium alone (line b), revealing electron storage inside the sulphide-oxidising bacteria. A small current was produced from the medium alone, which may be the result of traces of bisulfide (electrochemical oxidation). The total charge that was harvested from the solution with microorganisms was 112 mC, while the same volume of medium produced only 13 mC. This means that 99 mC of charge was stored inside the sulphide-oxidising bacteria.