Preventing Decomposition in Sodium Cyanide Compositions

Abstract

The present invention relates to a method for reducing the decomposition of an aqueous sodium cyanide solution by passing an inert gas through the solution to remove carbon dioxide from the solution and / or by storing the aqueous sodium cyanide solution under an inert gas or under vacuum, and to a method for reducing the decomposition of a sodium cyanide slurry, sodium cyanide paste, or solid sodium cyanide composition by storing the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide composition under an inert gas or under vacuum.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 337,231, filed May 2, 2022, the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for reducing the decomposition of aqueous sodium cyanide solutions.

[0003] The present invention also relates to a method for reducing the decomposition of a sodium cyanide slurry, sodium cyanide paste, or solid sodium cyanide composition. [Background technology]

[0004] Sodium cyanide is produced by the absorption of hydrogen cyanide gas into aqueous sodium hydroxide or by the reaction of liquefied hydrogen cyanide with aqueous sodium hydroxide. Sodium cyanide is a commodity chemical used in mining to extract gold and silver from ores. It is typically sold as an aqueous solution of 25-35% sodium cyanide by weight, or as a solid with a purity of 97-99%. Aqueous sodium cyanide slurries are also used. See, for example, U.S. Pat. No. 4,902,301. The formation of contaminants, including sodium carbonate and sodium formate, progresses over time and reduces the content of sodium cyanide in solution. These contaminants are believed to result from the reaction of cyanide in solution with dissolved carbon dioxide and / or carbon dioxide absorbed from the atmosphere.

[0005] In the production of sodium cyanide, contaminants such as sodium carbonate and sodium formate are known impurities. See U.S. Pat. No. 4,847,062. Other patents describe techniques for separating such impurities. See U.S. Pat. App. Pub. No. 2010 / 0296995 and U.S. Pat. No. 3,273,959. These techniques usually involve the production of sodium cyanide solids, since the freshly formed pure solution is used as the feedstock and can be immediately processed. However, such techniques are capital intensive and only temporary, since carbonate and formate contaminants continue to form after separation, whether the sodium cyanide is in the form of a solid, paste or slurry, or in solution.

[0006] The formation of these contaminants is not only problematic for the storage of solid sodium cyanide, but also impacts the supply of solutions, as customers see less cyanide than is discharged by the production facility. During storage and transportation, significant losses of sodium cyanide in solution occur. Also, decomposition of the product can generate ammonia, which is toxic and harmful to those exposed to the ammonia.

[0007] Therefore, there is a need for a method that reduces contaminant build-up in sodium cyanide compositions and reduces the amount of cyanide lost to decomposition, thereby enabling the production and delivery of high purity sodium cyanide products. Summary of the Invention

[0008] The present invention relates to a method for reducing the amount of sodium cyanide lost through decomposition. The method significantly reduces contaminants that build up in sodium cyanide compositions, allowing for the production and delivery of high purity sodium cyanide products. The present invention eliminates the need for capital intensive separation processes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The present invention relates to a method for reducing the decomposition of an aqueous sodium cyanide solution. The present invention also relates to a method for reducing cyanide decomposition in an aqueous sodium cyanide slurry, an aqueous sodium cyanide paste, or a solid sodium cyanide composition. The aqueous medium is preferably water itself. The method of the present invention reduces cyanide decomposition by more than 50%, more than 70%, more than 90%, or more than 95%, and preferably substantially prevents decomposition.

[0010] The method of the invention includes passing an inert gas through the aqueous sodium cyanide solution to remove carbon dioxide from the solution and / or storing the aqueous sodium cyanide solution under inert gas or under vacuum. In one embodiment of the invention, the inert gas is passed through the aqueous sodium cyanide solution by bubbling. If the solution is stored under vacuum, the pressure in the container is at a reduced pressure below atmospheric pressure, at least during storage.

[0011] The aqueous sodium cyanide solution or solid sodium cyanide may contain any amount of sodium cyanide. Commercial-scale aqueous sodium cyanide solutions contain about 15-45% sodium cyanide by weight, more typically about 25-35% sodium cyanide by weight. An inert gas is any gas that is generally recognized as inert in chemical reactions. Such gases include, but are not limited to, gases selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen, and mixtures thereof. Any gas that does not react with or cause decomposition of cyanide may also be an inert gas for purposes of the present invention.

[0012] The inert gas can be bubbled into the aqueous sodium cyanide solution using means known in the art. In the method of the present invention, all or substantially all (e.g., >50%, >60%, >70%, >80%, >90%, >95%, >96%, >97%, >98%, >99%, >99.9%) of the carbon dioxide is preferably removed from the solution or the surrounding atmosphere. Enough carbon dioxide needs to be removed to prevent or at least minimize decomposition.

[0013] The inert gas may be bubbled into the sodium cyanide solution via a sparger / injector below the liquid level. This process may be completed using several locations throughout the sodium cyanide production process, including but not limited to the absorption tower, separation vessel, sodium cyanide recycle line, and sodium cyanide product pipeline for storage. The invention may also be practiced by sparging the inert gas through the solution while contained within the storage vessel, or while loaded into the transport vessel, or while in the transport vessel. When the dissolved carbon dioxide is removed from the solution, the carbon dioxide should preferably be removed from the interface between the solution and the gas, and the headspace within the vessel will retain the solution. This may be completed by creating a vacuum (via a steam ejector or other means) or by introducing the inert gas to cover the solution stored in the process equipment, storage vessel, and transport vessel.

[0014] When the aqueous sodium cyanide solution or solid sodium cyanide is stored under inert gas, the inert gas is introduced into the head space of the container containing the solution. This can be done by bubbling the inert gas into the solution or by introducing the inert gas directly into the head space. Sufficient inert gas should preferably be used to replace the carbon dioxide-containing gas in the head space or dissolved in the aqueous solution, or to reduce the concentration of carbon dioxide in the head space, so that the above-mentioned decomposition is prevented or minimized.

[0015] Removal of carbon dioxide from the aqueous sodium cyanide solution according to the method of the present invention results in an aqueous sodium cyanide solution that comprises an inert gas and is substantially free of carbon dioxide, or that has been stored under an inert gas, which are separate embodiments of the present invention. Preferably, the aqueous sodium cyanide solution comprises an inert gas, is substantially free of carbon dioxide, and is stored under an inert gas.

[0016] The present invention also provides methods for reducing the decomposition of sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide. Such methods store the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide under an inert gas or under vacuum. The inert gas used in these methods is the same as that described above. The sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide usually contains about 25-100% by weight sodium cyanide. Typically, the sodium cyanide paste or slurry is a water-based paste or slurry. Prior to forming the paste or slurry, carbon dioxide can be removed from the aqueous portion by bubbling with an inert gas and then mixed with the sodium cyanide. These methods of the present invention produce a sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide stored under an inert gas or under vacuum as described above, each being a separate embodiment of the present invention.

[0017] When the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide is stored under vacuum, the pressure in the container is at a reduced pressure below atmospheric pressure, at least during storage. Inert gas can be bubbled into the aqueous portion using means known in the art. In the method of the present invention, all or substantially all (e.g., >50%, >60%, >70%, >80%, >90%, >95%, >96%, >97%, >98%, >99%, >99.9%) of carbon dioxide is preferably removed from the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide or from the ambient atmosphere. Enough carbon dioxide needs to be removed to prevent or at least minimize decomposition. Carbon dioxide can be removed by creating a vacuum (via a steam ejector or other means) or by introducing an inert gas to blanket the solution stored in process equipment, storage vessels, and transport vessels.

[0018] Removal of carbon dioxide from sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide according to the method of the present invention results in a sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide that comprises an inert gas and is substantially free of carbon dioxide, or that has been stored under an inert gas, which are separate embodiments of the present invention. Preferably, the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide comprises an inert gas, is substantially free of carbon dioxide, and has been stored under an inert gas.

[0019] The present invention also relates to a method for preparing a slurry or paste of sodium cyanide in an aqueous system, the method comprising the steps of passing an inert gas through the aqueous solvent to remove carbon dioxide, combining sodium cyanide with the aqueous solvent from which carbon dioxide has been removed to form an aqueous slurry or paste of sodium cyanide, optionally dewatering the aqueous slurry or paste of sodium cyanide to remove water from the slurry or paste, and / or storing the aqueous sodium cyanide slurry or paste under an inert gas or under vacuum. In one embodiment of the present invention, the inert gas is passed through the aqueous sodium cyanide solution by bubbling. The aqueous solvent is preferably water itself. The inert gas may be bubbled into the aqueous system via a sparger / injector as described above for the aqueous solution of sodium cyanide.

[0020] An inert gas is any gas that is generally recognized as inert in chemical reactions. Such gases include, but are not limited to, gases selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen, and mixtures thereof. Any gas that does not react with or cause the decomposition of cyanide may also be an inert gas for the purposes of the present invention. When the sodium cyanide paste or sodium cyanide slurry is stored under vacuum, the pressure in the container is reduced below atmospheric pressure, at least during storage.

[0021] The present invention also relates to an aqueous sodium cyanide solution that contains an inert gas and is substantially free of carbon dioxide. The aqueous sodium cyanide solution is stored under an inert gas or under vacuum.

[0022] The present invention also relates to an aqueous solution of sodium cyanide stored under an inert gas or under vacuum.

[0023] The present invention also relates to sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide stored under inert gas or under vacuum.

[0024] Exemplary embodiments of the present invention:

[0025] E1. A method for reducing the decomposition of an aqueous solution of sodium cyanide, comprising: and / or passing an inert gas through the aqueous sodium cyanide solution to remove carbon dioxide from the solution; 10. A method comprising storing the aqueous sodium cyanide solution under an inert gas or under vacuum.

[0026] E2. A method for reducing the decomposition of an aqueous solution of sodium cyanide, comprising: 10. A method comprising storing the aqueous sodium cyanide solution under an inert gas or under vacuum.

[0027] E3. The method of E1 or E2, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen and mixtures thereof.

[0028] E4. The method according to any one of E1 to E3, wherein the aqueous sodium cyanide solution contains 15 to 45% by weight of sodium cyanide, or the aqueous sodium cyanide solution contains 25 to 35% by weight of sodium cyanide.

[0029] E5. The method according to any one of E1 to E4, wherein the aqueous solvent of the aqueous sodium cyanide solution is water.

[0030] E6. The method of any one of E1-E5, wherein decomposition of the aqueous sodium cyanide solution is reduced by more than 50%, more than 70%, more than 90%, or more than 95%.

[0031] E7. The method of any one of E1-E5, wherein decomposition of the aqueous sodium cyanide solution is substantially prevented.

[0032] E8. The method of any one of E1-E7, wherein when the aqueous sodium cyanide solution is stored under vacuum, the pressure is less than atmospheric pressure.

[0033] E9. The method of any one of E1-E8, wherein all or substantially all (e.g., >50%, >60%, >70%, >80%, >90%, >95%, >96%, >97%, >98%, >99%, >99.9%) of the carbon dioxide is removed from the solution.

[0034] E10. The method of any one of E1-E9, wherein the inert gas is bubbled below the liquid level via a sparger or injector.

[0035] E11. The method of E10, wherein the bubbling is performed in an absorption tower (absorption column), a separation vessel, a sodium cyanide recycle line, or a storage sodium cyanide product pipeline.

[0036] E12. The method of E10, wherein the bubbling is performed in the storage vessel while on board or within the transport vessel.

[0037] E13. The method of any one of E1-E12, wherein carbon dioxide is removed from the interface between the solution and the gas.

[0038] E14. The method of any one of E1-E13, wherein carbon dioxide is removed using an inert gas or vacuum blanket.

[0039] E15. The method of any one of E1-E14, wherein sufficient inert gas is used to replace the carbon dioxide-containing gas in the headspace or dissolved in the aqueous sodium cyanide solution.

[0040] E16. The method of any one of E1-E14, wherein sufficient inert gas is used to reduce the concentration of carbon dioxide in the headspace.

[0041] E17. The method according to any one of E1 to E16, wherein the resulting aqueous sodium cyanide solution comprises an inert gas and is substantially free of carbon dioxide.

[0042] E18. The method according to any one of E1 to E16, wherein the resulting aqueous sodium cyanide solution is stored under inert gas.

[0043] E19. The method according to any one of E1 to E16, wherein the aqueous sodium cyanide solution obtained comprises an inert gas and is substantially free of carbon dioxide and is stored under inert gas.

[0044] E20. The method according to any one of E1 to E19, wherein an inert gas is passed through the aqueous sodium cyanide solution by bubbling.

[0045] E21. A method for reducing decomposition of sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide, comprising: 1. A method comprising the step of storing sodium cyanide paste, sodium cyanide slurry or solid sodium cyanide under an inert gas or under vacuum.

[0046] E22. The method of E21, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen and mixtures thereof.

[0047] E23. The method of E21 or E22, wherein the sodium cyanide paste, sodium cyanide slurry or solid sodium cyanide comprises 25-100% by weight sodium cyanide.

[0048] E24. The method of any one of E21 to E23, wherein the sodium cyanide paste is an aqueous sodium cyanide paste.

[0049] E25. The method of any one of E21-E23, wherein the sodium cyanide slurry is an aqueous sodium cyanide slurry.

[0050] E26. The method of any one of E24 or E25, wherein the aqueous solvent of the aqueous sodium cyanide paste and the aqueous sodium cyanide slurry is water.

[0051] E27. The method according to any one of E24 to E26, wherein prior to forming the aqueous sodium cyanide paste or aqueous sodium cyanide slurry, carbon dioxide is removed from the aqueous portion by bubbling with an inert gas and then mixed with the sodium cyanide.

[0052] E28. The method of any one of E21-E27, wherein decomposition of the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide is reduced by more than 50%, more than 70%, more than 90%, or more than 95%.

[0053] E29. The method of any one of E21-E27, wherein decomposition of the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide is substantially prevented.

[0054] E30. The method of any one of E21-E29, wherein the sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide is stored under vacuum and the pressure is less than atmospheric pressure.

[0055] E31. The method of any one of E21-E30, wherein all or substantially all (e.g., >50%, >60%, >70%, >80%, >90%, >95%, >96%, >97%, >98%, >99%, >99.9%) of the carbon dioxide is removed from the sodium cyanide paste, sodium cyanide slurry or solid sodium cyanide.

[0056] E32. The method of any one of E21-E31, wherein the carbon dioxide is removed using an inert gas or vacuum blanket.

[0057] E33. The method of any one of E21 to E32, wherein the resulting sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide comprises an inert gas and is substantially free of carbon dioxide.

[0058] E34. The method of any one of E21-E32, wherein the resulting sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide is stored under inert gas.

[0059] E35. The method of any one of E21-E32, wherein the resulting sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide comprises an inert gas and is substantially free of carbon dioxide, and is stored under inert gas.

[0060] E36. A method for preparing a slurry or paste of sodium cyanide in an aqueous system, comprising: passing an inert gas through the aqueous solvent to remove carbon dioxide; combining sodium cyanide with the aqueous solvent from which the carbon dioxide has been removed to form an aqueous slurry or paste of sodium cyanide; Optionally, dewatering the aqueous slurry or paste of sodium cyanide to remove water from the slurry or paste; and / or storing the aqueous sodium cyanide slurry or aqueous sodium cyanide paste under inert gas or vacuum; A method comprising:

[0061] E37. The method of E36, wherein the aqueous solvent is water.

[0062] E38. The method of E36 or E37, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen and mixtures thereof.

[0063] E39. The method of any one of E36 to E38, wherein when the aqueous sodium cyanide slurry or aqueous sodium cyanide paste is stored under vacuum, the pressure is less than atmospheric pressure.

[0064] E40. The method of any one of E36-E39, wherein all or substantially all (e.g., >50%, >60%, >70%, >80%, >90%, >95%, >96%, >97%, >98%, >99%, >99.9%) of the carbon dioxide is removed from the aqueous system.

[0065] E41. The method of any one of E36-E40, wherein the inert gas is bubbled below the liquid level via a sparger or injector.

[0066] E42. The method of any one of E36-E41, wherein carbon dioxide is removed from the interface between the solution and the gas.

[0067] E43. The method of any one of E36 to E42, wherein carbon dioxide is removed using an inert gas or vacuum blanket.

[0068] E44. The method according to any one of E36 to E43, wherein the resulting aqueous sodium cyanide slurry or aqueous sodium cyanide paste comprises an inert gas and is substantially free of carbon dioxide.

[0069] E45. The method according to any one of E36 to E43, wherein the resulting aqueous sodium cyanide slurry or aqueous sodium cyanide paste is stored under inert gas.

[0070] E46. The method of any one of E36 to E43, wherein the resulting aqueous sodium cyanide slurry or aqueous sodium cyanide paste comprises an inert gas and is substantially free of carbon dioxide and is stored under inert gas.

[0071] E47. The method according to any one of E36 to E46, wherein an inert gas is passed through the aqueous sodium cyanide solution by bubbling.

[0072] E48. An aqueous solution of sodium cyanide containing inert gas and substantially free of carbon dioxide.

[0073] E49. An aqueous solution of sodium cyanide as described in E48, stored under inert gas or under vacuum.

[0074] E50. Aqueous solution of sodium cyanide stored under inert gas or vacuum.

[0075] E51. Sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide stored under inert gas or under vacuum. EXAMPLES

[0076] example

[0077] Example 1

[0078] Nitrogen gas was bubbled through approximately 250 ml of sodium cyanide solution ("Test") in a closed, but not completely sealed, beaker for 30 minutes, after which a nitrogen blanket was applied. The solution was then subjected to a heat load and maintained at a temperature of 100°C (or about 100°C) for 24 hours. This was compared to a baseline control solution ("Control") that was subjected to the same heat load, but without the nitrogen gas bubbling and blanketing steps.

[0079] The results, shown in Table 1 below, showed a reduction in cyanide loss of 74%. At an initial concentration of 340 g / L of the solution, the loss in the control sample was equivalent to 34%. This is significantly greater than the 9% loss experienced in the test sample. [Table 1]

[0080] Example 2: Effect of solution strength

[0081] A second solution of about 250 ml with a reduced concentration of sodium cyanide ("low concentration") in a closed but not sealed sealed system was subjected to a similar process as the control, where a heat load was applied for 24 hours at about 100° C. This was compared to a control solution ("control") that was subjected to the same heat load, but without the bubbling and blanketing process.

[0082] The results, shown in Table 2 below, showed comparable losses of cyanide in both the high concentration solution ("Control") and the low concentration solution ("Low Concentration"), which leads to the conclusion that changes in cyanide strength have little effect on the loss of sodium cyanide due to its decomposition process. [Table 2]

Claims

1. 1. A method for reducing decomposition of an aqueous sodium cyanide solution, comprising: and / or passing an inert gas through the aqueous sodium cyanide solution to remove carbon dioxide from the solution; and / or storing said aqueous sodium cyanide solution under inert gas or under vacuum. A method comprising:

2. 1. A method for reducing decomposition of an aqueous sodium cyanide solution, comprising: storing said aqueous sodium cyanide solution under an inert gas or under vacuum.

3. 2. The method of claim 1, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen, and mixtures thereof.

4. 3. The method of claim 2, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen, and mixtures thereof.

5. 2. The method of claim 1, wherein the aqueous sodium cyanide solution comprises 15 to 45% by weight sodium cyanide, or wherein the aqueous sodium cyanide solution comprises 25 to 35% by weight sodium cyanide.

6. 3. The method of claim 2, wherein the aqueous sodium cyanide solution comprises 15 to 45% by weight sodium cyanide, or wherein the aqueous sodium cyanide solution comprises 25 to 35% by weight sodium cyanide.

7. 2. The method of claim 1, wherein decomposition of the aqueous sodium cyanide solution is reduced by more than 90%.

8. 3. The method of claim 2, wherein decomposition of the aqueous sodium cyanide solution is reduced by more than 90%.

9. The method of claim 1 , wherein all or substantially all of the carbon dioxide is removed from the solution.

10. The method of claim 2 , wherein all or substantially all of the carbon dioxide is removed from the solution.

11. 1. A method for reducing decomposition of sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide, comprising: storing said sodium cyanide paste, said sodium cyanide slurry or said solid sodium cyanide under an inert gas or under vacuum.

12. 12. The method of claim 11, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen, and mixtures thereof.

13. 12. The method of claim 11, wherein the sodium cyanide paste, the sodium cyanide slurry, or the solid sodium cyanide comprises 25 to 100% by weight sodium cyanide.

14. 12. The method of claim 11, wherein decomposition of the aqueous sodium cyanide solution is reduced by more than 90%.

15. 1. A process for preparing a slurry or paste of sodium cyanide in an aqueous system comprising the steps of: passing an inert gas through the aqueous solvent to remove carbon dioxide; combining sodium cyanide with the aqueous solvent from which carbon dioxide has been removed to form an aqueous slurry or paste of sodium cyanide; Optionally, dewatering the aqueous slurry or paste of sodium cyanide to remove water from the slurry or paste; and / or storing the aqueous sodium cyanide slurry or the aqueous sodium cyanide paste under inert gas or under vacuum. The method includes:

16. 16. The method of claim 15, wherein the inert gas is selected from the group consisting of nitrogen, argon, methane, natural gas, hydrogen, steam, helium, ammonia, oxygen, and mixtures thereof.

17. An aqueous solution of sodium cyanide containing an inert gas and substantially free of carbon dioxide.

18. 18. The aqueous sodium cyanide solution of claim 17 stored under an inert gas or under vacuum.

19. Aqueous solutions of sodium cyanide stored under inert gas or vacuum.

20. Sodium cyanide paste, sodium cyanide slurry, or solid sodium cyanide stored under inert gas or vacuum.

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