Method to remove carbonate from a caustic scrubber waste stream

1. A method to recover caustic from a caustic waste stream comprising: reducing the temperature of a caustic waste stream comprising dissolved Na 2 CO 3 to a temperature less than or equal to the temperature at which Na 2 CO 3 precipitates from the caustic waste stream to form a caustic waste stream comprising precipitated Na 2 CO 3 , and separating the precipitated Na 2 CO 3 from the caustic waste stream by a solid-liquid separation system, wherein the caustic waste stream comprises dissolved Na 2 CO 3 has Na 2 CO 3 present in an amount of 8 weight percent, NaOH present in an amount of 2 to 10 weight percent, and NaCl present in an amount of 3 weight percent, based on the total weight of the caustic waste stream.

2. The method of claim 1 , wherein the solid-liquid separation system comprises centrifugation, filtration, or a combination of centrifugation and filtration.

3. The method of claim 1 , wherein the temperature is reduced with one or more chiller systems.

4. The method of claim 1 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is concentrated prior to reducing the temperature of the caustic waste stream comprising dissolved Na 2 CO 3 .

5. The method of claim 4 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is concentrated using a concentration method selected from the group consisting of reverse osmosis, evaporation, membrane filtration, pervaporation, and combinations of two or more of the foregoing concentration methods.

6. The method of claim 5 , wherein the concentration method comprises evaporation, membrane filtration, or a combination thereof.

7. The method of claim 6 , wherein the membrane filtration method comprises reverse osmosis, nanofiltration or a combination thereof and is in fluid communication with the caustic waste stream.

8. The method of claim 5 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is concentrated using pervaporation.

9. The method of claim 1 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is reduced to a temperature of 4 to 10° C.

10. The method of claim 1 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is reduced to a temperature below 20° C.

11. The method of claim 1 , wherein the caustic waste stream is chilled to a temperature less than or equal to 13° C.

12. The method of claim 1 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is reduced in temperature using chilled water, a refrigerant selected from the group consisting of ammonia, carbon dioxide, fluorocarbons, and hydrofluorocarbons, or a combination of the foregoing chilling methods.

13. The method of claim 12 , wherein the caustic waste stream comprising dissolved Na 2 CO 3 is reduced in temperature using a refrigerant selected from the group consisting of ammonia, carbon dioxide, fluorocarbons, hydrofluorocarbons and combinations thereof.

14. The method of claim 1 , wherein said caustic waste stream comprising dissolved Na 2 CO 3 is derived from a neutralization reaction between a caustic and a phosgene containing waste stream.

15. The method of claim 1 , wherein said caustic waste stream is derived from a polycarbonate resin production process.

16. A method to recover caustic from a caustic waste stream comprising: reducing the temperature of a caustic waste stream comprising dissolved Na 2 CO 3 to a temperature less than 20° C. to form a caustic waste stream comprising precipitated Na 2 CO 3 , wherein the caustic waste stream is derived from a polycarbonate production process, and separating the precipitated Na 2 CO 3 from the caustic waste stream by a solid-liquid separation system.

17. The method of claim 16 , further comprising concentrating the caustic waste stream comprising dissolved Na 2 CO 3 prior to reducing the temperature.