Selective separation process

1. A method of selectively separating multivalent ions from monovalent ions, the method comprising:

(a) providing a reverse osmosis (RO) separation arrangement having an RO membrane, an input of said RO membrane being connected to a feed brine line and outputs of said RO membrane being connected to a permeate solution line and to a salt reject solution line, said RO membrane having a standard salt rejection ratio greater than 90%;

(b) introducing a feed brine, via said feed brine line, to said RO membrane, said feed brine containing multivalent cations, alkali cations and halide anions, a total concentration of said alkali cations and halide anions being at least 4%, by weight of said feed brine;

(c) applying a sub-osmotic pressure to said feed brine, to drive a first portion of said feed brine through said RO membrane to produce a permeate solution in said permeate solution line, a remainder of said feed brine being rejected by said RO membrane and being discharged as a reject solution via said salt reject solution line;

said sub-osmotic pressure being applied so as to differentiatively distribute said multivalent cations to said reject solution, with respect to said permeate solution;

said sub-osmotic pressure being less than 0.9 times a measured osmotic pressure of said feed brine, said measured osmotic pressure of said feed brine having been measured under actual process conditions;

wherein said sub-osmotic pressure is selected to achieve a separation coefficient (K separation ) of at least 1.1, said separation coefficient being defined as:

K separation =C alkali /C multivalent ,

wherein:

C alkali =[X p ] Mono /[X f ] Mono, [X p ] Mono being a total concentration of alkali cations in said permeate solution, and [X f ] Mono being a total concentration of said alkali cations in said feed brine; and

C multivalent =[X p ] Multi /[X f ] Multi, [X p ] Multi being a total concentration of said multivalent cations in said permeate solution, and [X f ] Multi being a total concentration of said multivalent cations in said feed brine.

2. The method of claim 1 , said standard salt rejection ratio being at least 98%.

3. The method of claim 1 , said sub-osmotic pressure being at most 200 bar, said sub-osmotic pressure being less than 0.9 times a theoretical osmotic pressure calculated according to:

π

=

-

RT

⁢

⁢

ln

⁢

⁢

(

x

s

)

V

wherein:

π is the osmotic pressure in bar;

R is the gas constant 0.083145 L·bar/moles·® K;

T is temperature (degrees Kelvin);

V is the molar volume of the solvent;

x s is the molar fraction of the solvent.

4. The method of claim 3 , wherein a first pressure coefficient (Kp) is defined by said sub-osmotic pressure divided by said theoretical osmotic pressure, and wherein Kp is at most 0.60.

5. The method of claim 1 , wherein a concentration of said alkali cations within said feed brine is at least 5%, by weight.

6. The method of claim 1 , wherein a combined concentration of calcium ions and magnesium ions within said feed brine is at least 0.05%, by weight.

7. The method of claim 1 , wherein a combined concentration of calcium ions and magnesium ions within said feed brine is within a range of 0.05%-20%, by weight.

8. The method of claim 7 , wherein said combined concentration of calcium ions and magnesium ions within said feed brine is at least 0.2%, by weight.

9. The method of claim 1 , wherein a measured pressure coefficient (Kpm) is defined by said sub-osmotic pressure divided by said measured osmotic pressure of said feed brine, and wherein Kpm is at most 0.75.

10. The method of claim 1 , wherein said alkali cations within said feed brine include lithium ions in a concentration of at least 150 ppm.

11. The method of claim 1 , further comprising collecting said permeate solution.

12. The method of claim 1 , wherein said sub-osmotic pressure is at least 10 bar and at most 200 bar.

13. The method of claim 1 , wherein a sulfate concentration within said feed brine is within a range of 0.4% to 10%, by weight.

14. The method of claim 1 , wherein said alkali cations within said feed brine include lithium ions in a concentration of at least 300 ppm.

15. The method of claim 1 , wherein a combined concentration of calcium ions and magnesium ions within said feed brine is within a range of 0.05%-5%, by weight, and wherein said alkali cations within said feed brine include lithium ions in a concentration of at least 150 ppm.

16. A method of selectively separating ions, the method comprising:

(a) providing a reverse osmosis (RO) separation arrangement having an RO membrane, an input of said RO membrane being connected to a feed brine line and outputs of said RO membrane being connected to a permeate solution line and to a salt reject solution line, said RO membrane having a standard salt rejection ratio greater than 90%;

(b) introducing a feed brine, via said feed brine line, to said RO membrane, said feed brine containing multivalent cations, alkali cations and halide anions, a total concentration of said alkali cations and halide anions being at least 8%, by weight of said feed brine;

(c) applying a sub-osmotic pressure to said feed brine, to drive a first portion of said feed brine through said RO membrane to produce a permeate solution in said permeate solution line, a remainder of said feed brine being rejected by said RO membrane and being discharged as a reject solution via said salt reject solution line;

said sub-osmotic pressure being applied so as to differentiatively distribute said multivalent cations to said reject solution, with respect to said permeate solution;

said sub-osmotic pressure being less than 0.9 times a measured osmotic pressure of said feed brine, said measured osmotic pressure of said feed brine having been measured under actual process conditions;

wherein said sub-osmotic pressure is at least 10 bar and at most 200 bar;

and wherein a combined concentration of calcium ions and magnesium ions within said feed brine is at least 0.05%, by weight;

wherein said sub-osmotic pressure is selected to achieve a separation coefficient (K separation ) of at least 1.1, said separation coefficient being defined as:

K separation =C alkali /C multivalent ,

wherein:

C alkali =[X p ] Mono /[X f ] Mono, [X p ] Mono being a total concentration of alkali cations in said permeate solution, and [X f ] Mono being a total concentration of said alkali cations in said feed brine; and

C multivalent =[X p ] Multi /[X f ] Multi, [X p ] Multi being a total concentration of said multivalent cations in said permeate solution, and [X f ] Multi being a total concentration of said multivalent cations in said feed brine.

17. The method of claim 16 , wherein said total concentration of said alkali cations and halide anions is at least 12%, by weight of said feed brine.

18. The method of claim 16 , wherein said combined concentration of calcium ions and magnesium ions within said feed brine is within a range of 0.20%-20%, by weight.

19. The method of claim 16 , wherein said combined concentration of calcium ions and magnesium ions within said feed brine is within a range of 0.05%-2%, by weight.