Method of production of a colloidal silica concentrate

1. A method of producing a colloidal silica concentrate from a geothermal fluid containing silica, the method comprising:

a. reducing the temperature of the fluid to a temperature sufficient for colloidal nucleation to occur;

b. settling the fluid to allow colloidal silica to form;

c. adding a dispersant to the settled fluid;

d. passing the fluid containing the colloidal silica through a first ultrafiltration (UF) unit comprising a spiral wound membrane to yield a first UF colloidal silica concentrate including between 0.1 mass % and 20 mass % silica;

e. treating the first UF concentrate using one or more further ultrafiltration steps to yield a further UF colloidal silica concentrate including greater than 20 mass % silica; and

wherein the colloidal silica concentrate is substantially monodisperse and wherein the substantially monodisperse concentrate comprises a standard deviation of range of particles sizes of the colloidal silica of less than 50 nm.

2. A method as claimed in claim 1 wherein the method further comprises curing the fluid for a period following colloid nucleation and prior to the fluid being received at the first ultrafiltration unit.

3. A method as claimed in claim 1 , wherein the method further comprises adjusting the pH of the geothermal fluid to between 7.0 and 9.0 prior to ultrafiltration.

4. A method as claimed in claim 1 wherein the concentration of silica in the geothermal fluid containing silica is at least 300 ppm.

5. A method as claimed in claim 1 wherein the geothermal fluid is reduced from a temperature of between about 90° C. and about 250° C.

6. A method as claimed in claim 1 wherein the temperature is reduced to a temperature of between 20° C. and 80° C.

7. A method as claimed in claim 1 wherein the settling time is at least 5 minutes.

8. A method as claimed in claim 1 wherein the spiral wound membrane of the first UF unit has a molecular weight cutoff (MWCO) of between 1000 daltons and 7000 daltons.

9. A method as claimed claim 1 wherein the colloidal silica retained by the first UF unit has an average size of from 5 nm to 100 nm.

10. A method as claimed in claim 1 wherein the first UF unit comprises two or more stages wherein each stage comprises multiple UF membranes in parallel where said stages are connected in series.

11. A method as claimed in claim 1 wherein the first UF unit produces a first UF concentrate wherein the silica concentration is between 1 mass % and 20 mass % silica.

12. A method as claimed in claim 1 wherein the first UF concentrate is adjusted to a pH of between 9.0 and 11.0.

13. A method as claimed in claim 1 wherein the one or more further ultrafiltration steps comprises ultrafiltration through a second ultrafiltration unit wherein the membrane of the second UF unit retains particles of greater than approximately 200000 daltons.

14. A method as claimed in claim 1 wherein the one or more further ultrafiltration steps comprises ultrafiltration through a second ultrafiltration unit wherein the membrane of the second UF unit retains particles of greater than approximately 200000 daltons and produces a UF concentrate of greater than 20 mass %.

15. A method as claimed in claim 1 wherein the first UF unit produces a first UF permeate which is passed to a reverse osmosis (RO) unit to yield an RO permeate and an RO concentrate.

16. A method as claimed in claim 1 wherein the one or more further ultrafiltration steps comprises ultrafiltration through a second ultrafiltration unit,

wherein diafiltration is carried out in the second ultrafiltration unit,

wherein the first UF unit produces a first UF permeate which is passed to a reverse osmosis (RO) unit to yield an RO permeate and an RO concentrate and at least a portion of the RO permeate is used to carry out the diafiltration in the second ultrafiltration unit.

17. A method as claimed in claim 16 wherein the diafiltration reduces the total dissolved salt concentration to less than approximately 1500 ppm.

18. A method as claimed in claim 1 wherein the geothermal fluid contains silica at greater than 400 ppm, the pH of the geothermal fluid is adjusted to between 7.5 and 8.5; the temperature of the geothermal fluid is reduced from between 100° C. and 130° C. to between 35° C. and 75° C.; the fluid is settled for at least 5 minutes; and the pH of the first UF concentrate is adjusted to between 9.0 and 11.0.