Process and system for producing sodium hydrosulfite crystals

1. A process for producing sodium hydrosulfite crystals, comprising:

providing a first water-based suspension containing sodium hydrosulfite in a first concentration;

separating at least a portion of the sodium hydrosulfite crystals from a remainder of the first water-based suspension to form a second water-based suspension containing the separated sodium hydrosulfite crystals, the second water-based suspension having a second sodium hydrosulfite concentration greater than the first concentration; and

cooling the second water-based suspension containing the separated sodium hydrosulfite crystals to a stable temperature.

2. The process of claim 1 , wherein providing the first water-based suspension comprises:

raising a pH of a water-based solution containing sodium hydrosulfite; and

lowering a temperature of the water-based solution to form the first water-based suspension.

3. The process of claim 2 , further comprising filtering a slurry containing sodium hydrosulfite to provide the water-based solution.

4. The process of claim 2 , wherein raising the pH of the water-based solution comprises adding sodium hydroxide to the water-based solution.

5. The process of claim 2 , wherein the pH of the water-based solution is raised before lowering the temperature of the water-based solution.

6. The process of claim 2 , wherein the pH of the water-based solution is raised to a range between 12 and 14.

7. The process of claim 2 , wherein the temperature of the water-based solution is lowered in an agitated cooling tank or in a cooling crystallizer to form the first water-based suspension.

8. The process of claim 2 , wherein the temperature of the water-based solution is lowered to a range between −6° C. and 10° C.

9. The process of claim 1 , wherein the sodium hydrosulfite crystals are concentrated by their separation from the remainder of the first water-based suspension.

10. The process of claim 9 , wherein the sodium hydrosulfite crystals are separated from the remainder of the first water-based suspension using a separator selected from a decanter centrifuge, a filter-press, a Nutsche filter, a disc filter, and a belt filter.

11. The process of claim 1 , wherein the stable temperature of the sodium hydrosulfite crystals is −12° C. or lower.

12. The process of claim 1 , further comprising collecting the remainder of the first water-based suspension as a liquid mixture containing an unseparated portion of sodium hydrosulfite initially contained in the first water-based suspension.

13. The process of claim 1 , wherein a major fraction of the sodium hydrosulfite crystals are dihydrate sodium hydrosulfite crystals.

14. The process of claim 1 , further comprising adding stabilizers to the sodium hydrosulfite crystal after separation from the remainder of the first water-based suspension.

15. The process of claim 1 , wherein cooling the second water-based suspension containing the separated sodium hydrosulfite crystals to the stable temperature comprises freezing the second water-based suspension containing the separated sodium hydrosulfite crystals.

16. A system for producing sodium hydrosulfite crystals, comprising:

a storage adapted to provide a first water-based suspension containing sodium hydrosulfite in a first concentration;

a separator adapted to receive the first water-based suspension from the storage and to separate at least a portion of the sodium hydrosulfite crystals from a remainder of the first water-based suspension to form a second water-based suspension containing the separated sodium hydrosulfite crystals, the second water-based suspension having a second sodium hydrosulfite concentration greater than the first concentration; and

a chiller adapted to receive the second water-based suspension containing the separated sodium hydrosulfite crystals from the separator and to cool the second water-based suspension containing the separated sodium hydrosulfite crystals to a stable temperature.

17. The system of claim 16 , wherein the storage comprises a cooling tank adapted to receive a water-based solution containing sodium hydrosulfite and to lower a temperature of the water-based solution, the system further comprising:

a first feeder operatively connected to the cooling tank and adapted to add a pH raising compound to the water-based solution; and

a second feeder operatively connected to the cooling tank and adapted to add a nucleating agent to the water-based solution to cause a partial precipitation of the sodium hydrosulfite to form the first water-based suspension.

18. The system of claim 16 , wherein the chiller is selected from a scraped surface heat exchanger, a screw heat exchanger, a cryogenic freezer, a Nutsche filter, a N 2 chiller and a NH 3 chiller.

19. The system of claim 16 , further comprising:

a valve connecting the storage to the separator;

a controller operatively connected to the storage, the valve, the separator and to the chiller, the controller being configured to:

cause an opening of the valve to allow a transfer of the first water-based suspension from the storage to the separator;

initiate an operation of the separator when the first water-based suspension is received at least in part of the separator; and

initiate an operation of the chiller when separated sodium hydrosulfite crystals are received at least in part in the chiller.

20. The system of claim 19 , further comprising a first sensor of a level of the first water-based suspension in the storage and a second sensor of a temperature of the first water-based suspension in the storage, the controller being configured to cause an opening of the valve to allow a transfer of the first water-based suspension from the storage to the separator when, concurrently, the first sensor indicates that there is a sufficient amount of first water-based suspension in the storage and the second sensor indicates that the first water-based suspension is at a sufficiently low temperature.