Production of precipitated silica employing a fast blender

Abstract

A unique method for preparing precipitated silica entails reacting a silicate with an acidifying agent to obtain a suspension of precipitated silica, and separating and drying the suspension, and further wherein the precipitation includes contacting a silicate with an acidifying agent in an acidic medium in a fast blender.

Claims

1. A method for preparing precipitated silica product wherein the method comprises: (A) reacting a silicate with an acidifying agent to obtain a suspension of precipitated silica, wherein step (A) comprises: (i) forming a reaction medium by simultaneously adding silicate and acidifying agent to a fast blender, while maintaining the pH of the reaction medium (pH.sub.1) from 2 to 5.5, where the addition is performed without the addition of an electrolyte and the reaction medium has a residence time in said fast blender, (ii) introducing the reaction medium issuing from step (i) into a stirred reactor, the pH of the reaction medium (pH.sub.2) in said reactor being regulated from 2 to 5.5, optionally with pH.sub.2pH.sub.1, (iii) adding silicate to the reaction medium in said stirred reactor until the pH value of the reaction mixture ranges from 7 to 10, (iv) simultaneously adding silicate and acidifying agent to said reaction medium in the stirred reactor after step (iii), and maintaining the pH of said reaction medium from 7 to 10, (v) stopping the silicate addition in step (iv) while continuing to add acidifying agent to the reaction medium in the stirred reactor until the pH value of the reaction medium in the stirred reactor is less than 6 to produce a suspension of precipitated silica; and (B) separating said precipitated silica from said suspension and drying the separated precipitated silica to obtain a precipitated silica product, wherein step (i) is performed at a temperature of between 70 C. and 95 C. and wherein no initial bottoms are present in the reactor of step (ii) before the introduction of the reaction medium issuing from step (i).

2. The method as defined by claim 1, wherein the simultaneous addition of said silicate with said acidifying agent in step (i) is carried out in continuous mode.

3. The method as defined by claim 1, wherein said reactor operates in semi-continuous mode.

4. The method as defined by claim 1, wherein the simultaneous addition of said silicate with said acidifying agent in step (i) is carried out in a fast blender selected from the group consisting of symmetrical T or Y blenders or tubes, asymmetrical T or Y blenders or tubes, tangential jet blenders, Hartridge-Roughton blenders, vortex blenders, and rotor-stator blenders.

5. The method as defined by claim 1, wherein the simultaneous addition of said silicate with said acidifying agent in step (i) is carried in a tangential jet blender, a Hartridge-Roughton blender or a vortex blender which comprises a chamber having at least two tangential inlets via which the silicate and acidifying agent are separately introduced, and an axial outlet via which the reaction medium exits.

6. The method as defined by claim 5, wherein said at least two tangential inlets are located symmetrically, and in opposition, about the centerline of said chamber.

7. The method as defined by claim 1, wherein, in step (ii), to regulate the pH of the reaction medium (pH.sub.2) in the reactor from 2 to 5.5, acidifying agent or, optionally, silicate or a basifying agent, is added simultaneously to the reaction mixture issuing from step (i).

8. The method as defined by claim 1, wherein the pH.sub.1 and pH.sub.2 range from 2.5 to 5.

9. The method as defined by claim 1, wherein pH.sub.2pH.sub.1.

10. The method as defined by claim 1, wherein the entire precipitation is carried out at a temperature ranging from 70 C. to 95 C.

11. The method as defined by claim 1, wherein the temperature of the reaction medium ranges from 70 C. to 86 C. during steps (i) and (ii).

12. The method as defined by claim 1, wherein the temperature of the reaction medium is increased during step (iii) to a value of from 85 C. to 95 C., and then maintained at this value during steps (iv) and (v).

13. The method as defined by claim 1, wherein said fast blender is a tangential jet blender, a Hartridge-Roughton blender or a vortex blender, comprising a chamber having at least two tangential inlets via which the silicate and acidifying agent are separately but simultaneously introduced in step (i), and an axial outlet via which the reaction medium exits, the at least two tangential inlets optionally being located symmetrically about the centerline of said chamber.

14. The method as defined by claim 1, wherein the residence time through the fast blender is shorter than 1 second.

15. The method as defined by claim 1, wherein the drying is carried out by spray drying.

16. The method as defined by claim 1, wherein the separation comprises a filtration carried out employing a filter press.

17. The method as defined by claim 1, wherein the separation comprises a filtration carried out employing a nozzle spray dryer.

18. The method as defined by claim 1, wherein the separation comprises a filtration carried out employing a vacuum filter.

19. The method as defined by claim 1, wherein the separation comprises a filtration carried out employing a turbine spray dryer.

20. The method as defined by claim 1, wherein the precipitated silica product has: a CTAB specific surface area (S.sub.CTAB) of from 40 to 525 m.sup.2/g, a BET specific surface area (S.sub.BET) of from 45 to 550 m.sup.2/g, a width Ld ((d84d16)/d50) of object size distribution measured by XDC grain-size analysis after ultrasonic disaggregation of at least 0.91, and a pore volume distribution such that the ratio V.sub.(d5-d50)/V.sub.(d5-d100) is at least 0.66.

21. The method as defined by claim 1, wherein the precipitated silica product has: a CTAB specific surface area (S.sub.CTAB) of from 40 to 525 m.sup.2/g, a BET specific surface area (S.sub.BET) of from 45 and 550 m.sup.2/g, and a pore distribution width ldp higher than 0.70.

22. The method as defined by claim 1, wherein the precipitated silica product has: a CTAB specific surface area (S.sub.CTAB) of from 40 and 525 m.sup.2/g, a BET specific surface area (S.sub.BET) of from 45 and 550 m.sup.2/g, a width Ld ((d84d16)/d50) of object size distribution lower than 500 nm, measured by XDC grain size analysis after ultrasonic disaggregation of at least 0.95, and a pore volume distribution such that the ratio V.sub.(d5-d50)/V.sub.(d5-d100) is at least 0.71.

23. The method as defined by claim 1, wherein the precipitated silica product has: a CTAB specific surface area (S.sub.CTAB) of from 40 to 525 m.sup.2/g, a BET specific surface area (S.sub.BET) of from 45 to 550 m.sup.2/g, a width Ld ((d84d16)/d50) of object size distribution lower than 500 nm, measured by XDC grain size analysis after ultrasonic disaggregation of at least 0.90, and a pore volume distribution such that the ratio V.sub.(ds-d50)/V.sub.(d5-d100) is at least 0.71.

24. The method as defined by claim 1, wherein the precipitated silica product has an object size such that the grain size mode measured by XDC grain-size analysis after ultrasonic disaggregation meets the following condition:
Mode XDC (nm)(5320/S.sub.CTAB (m.sup.2/g))+8.

25. The method as defined by claim 1, wherein the precipitated silica product has: a CTAB specific surface area (S.sub.CTAB) of from 60 to 330 m.sup.2/g, and a BET specific surface area (S.sub.BET) of from 70 to 350 m.sup.2/g.

26. The method as defined by claim 1, wherein the precipitated silica product has a CTAB specific surface area (S.sub.CTAB) of from 90 to 230 m.sup.2/g and a BET specific surface area (S.sub.BET) of from 110 to 270 m.sup.2/g.

27. The method as defined by claim 1, wherein the precipitated product comprises substantially spherical beads having an average particle size of at least 80 m.

28. The method as defined by claim 1, wherein the precipitated product comprises a powder having an average particle size of at least 15 m.

29. The method as defined by claim 1, wherein the precipitated product comprises granules having a particle size of at least 1 mm.

30. A method for preparing precipitated silica product wherein the method comprises: (A) reacting a silicate with an acidifying agent to obtain a suspension of precipitated silica, wherein step (A) comprises: (i) forming a reaction medium by simultaneously adding silicate and acidifying agent in a zone of turbulent flow, while maintaining the pH of the reaction medium (pH.sub.1) from 2 to 5.5, where the addition is performed without the addition of an electrolyte and the reaction medium has a residence time in the zone of turbulent flow, (ii) introducing the reaction medium issuing from step (i) into a stirred reactor, the pH of the reaction medium (pH.sub.2) in said reactor being regulated from 2 to 5.5, optionally with pH.sub.2pH.sub.1, (iii) adding silicate to the reaction medium in said stirred reactor until the pH value of the reaction mixture ranges from 7 to 10, (iv) simultaneously adding silicate and acidifying agent to said reaction medium in the stirred reactor after step (iii), and maintaining the pH of said reaction medium from 7 to 10, (v) stopping the silicate addition in step (iv) while continuing to add acidifying agent to the reaction medium in the stirred reactor until the pH value of the reaction medium in the stirred reactor is less than 6 to produce a suspension of precipitated silica; and (B) separating said precipitated silica from said suspension and drying the separated precipitated silica to obtain a precipitated silica product, wherein step (i) is performed at a temperature of between 70 C. and 95 C. and wherein no initial bottoms are present in the reactor of step (ii) before the introduction of the reaction medium issuing from step (i).

31. The method as defined by claim 30, wherein the residence time in the zone of turbulent flow is shorter than 1 second.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a tangential jet blender comprising a chamber, with an inside diameter equal to 4 d, having two tangential inlet tubes, with an inside diameter d, and an axial outlet tube, with an inside diameter of 2 d.

(2) The following examples illustrate the invention but without limiting its scope.

EXAMPLE 1

(3) A method comprising the following steps is implemented;

(4) a precipitation reacting using sodium silicate and sulfuric acid,

(5) a filtration-washing step, using a filter press,

(6) drying by means of a turbine spray dryer.

(7) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(8) A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 77 C., at a flow rate of 377 L/h, is added to the fast blender for 8 minutes and 30 seconds, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 77 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.0.

(9) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.2. The temperature is increased to 84 C.

(10) After 8 minutes and 30 seconds of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(11) A new simultaneous addition of sodium silicate and sulfuric acid is carried out for 40 minutes in the stirred vessel, with a sodium silicate flow rate of 35 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(12) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 3.9 in 5 minutes by sulfuric acid having a concentration of 80 g/L.

(13) The slurry obtained is filtered and washed on a filter press (dry extract of the cake 20%). After dilution, the cake obtained is mechanically disintegrated, adding sodium aluminate (Na.sub.2O/Al.sub.2O.sub.3 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until a pH of 6.5 is reached. The resulting slurry (having a dry extract of 11%) is spray dried using a turbine spray dryer.

EXAMPLE 2

(14) A method comprising the following steps is implemented:

(15) a precipitation reacting using sodium silicate and sulfuric acid,

(16) a filtration-washing step, using a filter press,

(17) drying by means of a turbine spray dryer.

(18) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(19) A sodium silicate solution (SiO.sub.2Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 79 C., at a flow rate of 458 L/h, is added to the fast blender for 7 minutes, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 79 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.6.

(20) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.3. The temperature is increased to 81 C.

(21) After 7 minutes of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(22) A new simultaneous addition of sodium silicate and sulfuric acid is carried out for 40 minutes in the stirred vessel, with a sodium silicate flow rate of 34 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(23) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 3.9 in 5 minutes by sulfuric acid having a concentration of 80 g/L.

(24) The slurry obtained is filtered and washed on a filter press (dry extract of the cake 20%). After dilution, the cake obtained is mechanically disintegrated, adding sodium aluminate (Na.sub.2O/Al.sub.2O.sub.3 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until a pH of 6.5 is reached. The resulting slurry (having a dry extract of 11%) is spray dried using a turbine spray dryer.

EXAMPLE 3

(25) A method comprising the following steps is implemented:

(26) a precipitation reacting using sodium silicate and sulfuric acid,

(27) a filtration-washing step, using a filter press,

(28) drying by means of a turbine spray dryer.

(29) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(30) A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 82 C., at a flow rate of 382 L/h, is added to the fast blender for 8 minutes and 30 seconds, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 82 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 3.1.

(31) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.3. The temperature is increased to 82 C.

(32) After 8 minutes and 30 seconds of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(33) A new simultaneous addition of sodium silicate and sulfuric acid is carried out for 40 minutes in the stirred vessel, with a sodium silicate flow rate of 34 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(34) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 3.9 in 5 minutes by sulfuric acid having concentration of 80 g/L.

(35) The slurry obtained is filtered and washed on a filter press (dry extract of the cake 21%). After dilution, the cake obtained is mechanically disintegrated, adding sodium aluminate (Na.sub.2/Al.sub.2O.sub.3 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until, a pH of 6.5 is reached. The resulting slurry (having a dry extract of 11%) is spray dried using a turbine spray dryer.

EXAMPLE 4

(36) A method comprising the following steps is implemented:

(37) a precipitation reacting using sodium silicate and sulfuric acid,

(38) a filtration-washing step, using a filter press,

(39) drying by means of a turbine spray dryer.

(40) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(41) A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 79 C., at a flow rate of 321 L/h is added to the fast blender for 10 minutes, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 79 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.6.

(42) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.4. The temperature is increased to 81 C.

(43) After 10 minutes of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(44) A new simultaneous addition of sodium silicate and sulfuric acid is carried out for 40 minutes in the stirred vessel, with a sodium silicate flow rate of 34 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(45) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 3.9 in 5 minutes by sulfuric acid having a concentration of 80 g/L.

(46) The slurry obtained is filtered and washed on a filter press (dry extract of the cake 20%). After dilution, the cake obtained is mechanically disintegrated, adding sodium aluminate (Na.sub.2O/Al.sub.2O.sub.3 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until a pH of 6.5 is reached. The resulting slurry (having a dry extract of 11%) is spray dried using a turbine spray dryer.

EXAMPLE 5

(47) A method comprising the following steps is implemented:

(48) a precipitation reacting using sodium silicate and sulfuric acid,

(49) a filtration-washing step, using a filter press,

(50) drying by means of a nozzle spray dryer.

(51) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(52) A sodium silicate solution (SiO.sub.2Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 82 C., at a flow rate of 384 L/h, is added to the fast blender for 8 minutes and 30 seconds, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 82 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.0.

(53) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rare regulated in order to maintain the pH of the reaction medium at a value of 4.4. The temperature is increased to 82 C.

(54) After 8 minutes and 30 seconds of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(55) A new simultaneous addition of sodium silicate and sulfuric acid, is carried out for 40 minutes in the stirred vessel, with a sodium silicate, flow rate of 35 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(56) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 4.0 in 5 minutes by sulfuric acid having a concentration of 80 g/L.

(57) The slurry obtained is filtered and washed on a filter press (dry extract of the cake 20%). After dilution, the cake obtained is mechanically disintegrated adding sodium aluminate (Na.sub.2O/Al.sub.2O.sub.3 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until a pH of 6.5 is reached. The resulting slurry (having a dry extract of 18.6%) is spray dried by means of a nozzle spray dryer.

EXAMPLE 6

(58) A method comprising the following steps is implemented:

(59) a precipitation reacting using sodium silicate and sulfuric acid,

(60) a filtration-washing step, using a filter press,

(61) drying by means of a turbine spray dryer.

(62) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(63) A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 78 C., at a flow rate of 376 L/h, is added to the fast blender for 8 minutes and 30 seconds, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 78 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 3.9.

(64) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.1. The temperature is increased to 85 C.

(65) After 8 minutes and 30 seconds of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(66) A new simultaneous addition of sodium silicate and sulfuric acid is carried out for 40 minutes in the stirred vessel, with a sodium silicate flow rate of 33 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(67) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 3.9 in 5 minutes by sulfuric acid having a concentration of 80 g/L.

(68) The slurry obtained is filtered and washed on filter press (dry extract of the cake 20%). After dilution, the cake obtained is mechanically disintegrated, adding sodium aluminate (Na.sub.2O/Al.sub.2O.sub.2 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until a pH of 6.5 is reached. The resulting slurry (having a dry extract of 11%) is spray dried using a turbine spray dryer.

EXAMPLE 7

(69) A method comprising the following steps is implemented:

(70) a precipitation reacting using sodium silicate and sulfuric acid,

(71) a filtration-washing step, using a filter press,

(72) drying by means of a turbine spray dryer.

(73) The silica precipitation reaction is carried out by sequencing a Hartridge-Roughton type stainless steel fast blender, with an inlet diameter d of 5 mm (FIGURE), and a stirred (160 rpm) stainless steel vessel having a volume of 170 L, in the following steps.

(74) A sodium silicate solution SiO.sub.2/Na.sub.2O weight ratio 3.45), having a concentration of 50 g/L and a temperature of 86 C., at a flow rate of 376 L/h, is added to the fast blender for 8 minutes and 30 seconds, simultaneously with sulfuric acid, having a concentration of 21 g/L and a temperature of 86 C., at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 3.9.

(75) This reaction medium is accommodated in the stirred vessel, into which a sodium silicate solution is introduced simultaneously at a flow rate regulated in order to maintain the pH of the reaction medium at a value of 4.1. The temperature is increased to 86 C.

(76) After 8 minutes and 30 seconds of simultaneous addition, the feeds of the fast blender are interrupted and a sodium silicate solution is introduced into the stirred vessel until pH value of 8.0 is obtained. The temperature is increased to 92 C. and maintained at this level until the end of the reaction.

(77) A new simultaneous addition of sodium silicate and sulfuric acid is carried out for 40 minutes in the stirred vessel, with a sodium silicate flow rate of 32 L/h, in a concentration of 235 g/L, and a sulfuric acid flow rate, having a concentration of 80 g/L, regulated in order to maintain the pH of the reaction medium at a value of 8.0.

(78) On completion of this second simultaneous addition, the reaction medium is adjusted to a pH of 4.0 in 5 minutes by sulfuric acid having a concentration of 80 g/L.

(79) The slurry obtained is filtered and washed on a filter press (dry extract of the cake 20%). After dilution, the cake obtained is mechanically disintegrated, adding sodium aluminate (Na.sub.2/Al.sub.2O.sub.3 weight ratio 0.8), in an Al/SiO.sub.2 ratio of 0.3% and sulfuric acid until a pH of 6.5 is reached. The resulting slurry (having a dry extract of 11%) is spray dried using a turbine spray dryer.

(80) The methods exemplified above have a very satisfactory silica productivity.