Process for preparing precipitated silica having specific morphology, particle size and porosity

Abstract

The invention relates to a process for the preparation of precipitated silica formed of aggregates of large primary silica particles, at the surface of which occur small primary silica particles, in which the acid used in at least one of the process steps is a concentrated acid.

Claims

1. A process for preparing precipitated silica, the process comprising: reacting-a silicate with at least one acid using the following steps, whereby a silica suspension is obtained: (i) preparing an aqueous precipitated silica suspension (1) forming an initial stock comprising a silicate and an electrolyte, the silicate concentration expressed as SiO.sub.2 in said initial stock being less than 100 g/l and, the electrolyte concentration in said initial stock being less than 19 g/l, (2) adding an acid to said stock to form a reaction medium until a value of the pH of the reaction medium of at least approximately 7 is obtained, (3) simultaneously adding an acid and a silicate to the reaction medium, and (4) stopping the addition of the silicate used in step (3) while continuing the addition of an acid to the reaction medium, until a value of the pH of the reaction medium of between 2.5 and 5.3 is obtained, whereby an aqueous precipitated silica suspension exhibiting a pH of between 2.5 and 5.3 is obtained, and (ii) bringing said aqueous precipitated silica suspension into contact with an acid and a silicate to form a reaction medium, so that the pH of the reaction medium is maintained between 2.5 and 5.3, separating precipitate from the silica suspension; and drying the precipitate, wherein step (2) comprises adding, for x minutes, a dilute acid followed by adding, after x minutes, a concentrated acid, wherein x is between 10 and 25, wherein said concentrated acid is selected from the group consisting of sulfuric acid with a concentration of at least 80% by weight, acetic acid with a concentration of at least 90% by weight, formic acid with a concentration of at least 90% by weight, nitric acid with a concentration of at least 60% by weight, phosphoric acid with a concentration of at least 75% by weight, and hydrochloric acid with a concentration of at least 30% by weight.

2. The process according to claim 1, wherein, after the aqueous precipitated silica suspension is brought into contact with said acid and said silicate used in step (ii), an alkaline agent is added to the reaction medium, so as to increase the pH of the reaction medium up to a value of between 4.7 and 6.3.

3. The process according to claim 1, wherein the acid used in said step (ii) is a concentrated acid selected from the group consisting of sulfuric acid with a concentration of at least 80% by weight, acetic acid with a concentration of at least 90% by weight, formic acid with a concentration of at least 90% by weight, nitric acid with a concentration of at least 60% by weight, phosphoric acid with a concentration of at least 75% by weight, and hydrochloric acid with a concentration of at least 30% by weight.

4. The process according to claim 1, wherein the acid used in step (3) is a concentrated acid selected from the group consisting of sulfuric acid with a concentration of at least 80% by weight, acetic acid with a concentration of at least 90% by weight, formic acid with a concentration of at least 90% by weight, nitric acid with a concentration of at least 60% by weight, phosphoric acid with a concentration of at least 75% by weight, and hydrochloric acid with a concentration of at least 30% by weight.

5. The process according to claim 4, wherein the acid used in said steps (3) and (4) is a concentrated acid selected from the group consisting of sulfuric acid with a concentration of at least 80% by weight, acetic acid with a concentration of at least 90% by weight, formic acid with a concentration of at least 90% by weight, nitric acid with a concentration of at least 60% by weight, phosphoric acid with a concentration of at least 75% by weight, and hydrochloric acid with a concentration of at least 30% by weight.

6. The process according to claim 1, wherein in said step (2), the concentrated acid is added after reaching a gel point in the reaction medium.

7. The process according to claim 1, wherein said concentrated acid is sulfuric acid with a concentration of at least 80% by weight.

8. The process according to claim 1, wherein said concentrated acid is sulfuric acid with a concentration of between 90 and 98% by weight.

9. The process according to claim 1, wherein said drying is carried out by atomization.

Description

EXAMPLE 1 (COMPARATIVE)

(1) The following are introduced into a 25-liter stainless steel reactor equipped with a system for stirring by propellers and with heating via a jacket: 7.91 liters of municipal water, 4285 grams of aqueous sodium silicate, with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and having a concentration equal to 235 g/l, 134 grams of sodium sulfate Na.sub.2SO.sub.4 (electrolyte).

(2) The silicate concentration (expressed as SiO.sub.2) in the stock is then 72 g/l.

(3) The mixture is homogenized by stirring and brought to 95 C. The entire reaction is carried out with stirring (300 rev/min, propeller stirring).

(4) Sulfuric acid with a concentration equal to 80 g/l is introduced into the mixture over 15 minutes at a flow rate of 146 g/min (i.e., 2192 grams of sulfuric acid). Sulfuric acid with a concentration equal to 80 g/l is subsequently introduced into the mixture over approximately 5 minutes at a flow rate of 438 g/min (i.e., 2153 grams of sulfuric acid).

(5) Once acidification is complete, the simultaneous introduction is carried out, into the reaction medium, over 10 minutes, of a sodium silicate solution (with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and with a density d20 equal to 1.230+/0.006) having a concentration of 235 g/l, at a flow rate of 80 g/min, and sulfuric acid with a concentration equal to 80 g/l, at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 8.

(6) At the end of the 10 minutes of simultaneous addition, sulfuric acid with a concentration equal to 80 g/l is introduced at a flow rate of 80 g/min until the pH of the reaction medium reaches a value of 4.7 (i.e., 414 grams of sulfuric acid in 5 minutes).

(7) The simultaneous introduction is subsequently carried out, into the reaction medium, over 29 minutes, of a sodium silicate solution (with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and with a density d20 equal to 1.230+/0.006) having a concentration of 235 g/l, at a flow rate of 60 g/min, and sulfuric acid with a concentration equal to 80 g/l, at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 4.7.

(8) On conclusion of the reaction, a precipitated silica reaction slurry is obtained and is kept stirred at a temperature of 95 C. for 5 minutes. After this maturing, the precipitated silica slurry is recovered by emptying the reactor.

(9) The slurry is filtered and washed under vacuum. The obtained filtration cake is washed 4 times with 5 liters of municipal water. It is subsequently resuspended by mechanical liquefaction in the presence of water and sodium aluminate (Al/SiO.sub.2 weight ratio of 0.3%). The resulting slurry (solids content of 10% by weight) is subsequently dried by atomization using a rotary atomizer.

(10) The characteristics of the precipitated silica obtained in the powder form are then as follows: CTAB specific surface: 158 m.sup.2/g Median size d50 of aggregates: 98 nm V.sub.(d5-d50)/V.sub.(d5-d100): 0.83 Mode (Hg porosimetry): 33 nm

(11) It is found, in particular by TEM, that the precipitated silica is formed of aggregates of large primary silica particles (Ipp), at the surface of which occur small primary silica particles (spp).

EXAMPLE 2

(12) The following are introduced into a 25-liter stainless steel reactor equipped with a system for stirring by propellers and with heating via a jacket: 7.91 liters of municipal water, 4286 grams of aqueous sodium silicate, with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and a density d20 equal to 1.230+/0.006 and having a concentration of 235 g/l, 134 grams of sodium sulfate Na.sub.2SO.sub.4 (electrolyte).

(13) The silicate concentration (expressed as SiO.sub.2) in the stock is then 72 g/l.

(14) The mixture is homogenized by stirring and brought to 95 C. The entire reaction is carried out with stirring (300 rev/min, propeller stirring).

(15) Sulfuric acid with a concentration equal to 80 g/l is introduced into the mixture over 14 minutes at a flow rate of 157.8 g/min (i.e., 2191 grams of sulfuric acid). 95% Sulfuric acid is subsequently introduced into the mixture until the pH of the reaction medium reaches a value of 8 (i.e., 34.5 grams of 95% sulfuric acid in 5 minutes).

(16) Once acidification is complete, the simultaneous introduction is carried out, into the reaction medium, over 10 minutes, of a sodium silicate solution (with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and with a density d20 equal to 1.230+/0.006) having a concentration of 235 g/l, at a flow rate of 80 g/min, and 95% sulfuric acid, at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 8.

(17) At the end of the 10 minutes of simultaneous addition, 95% sulfuric acid is introduced until the pH of the reaction medium reaches a value of 4.8 (i.e., 30 grams of sulfuric acid in approximately 8 minutes).

(18) The simultaneous introduction is subsequently carried out, into the reaction medium, over 27 minutes, of a sodium silicate solution (with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and with a density d20 equal to 1.230+/0.006) having a concentration of 235 g/l, at a flow rate of 62.8 g/min, and 95% sulfuric acid, at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 4.8.

(19) On conclusion of the reaction, a precipitated silica reaction slurry is obtained and is kept stirred at a temperature of 95 C. for 5 minutes. After this maturing, the precipitated silica slurry is recovered by emptying the reactor.

(20) The slurry is filtered and washed under vacuum. The obtained filtration cake is washed 4 times with 5 liters of municipal water. It is subsequently resuspended by mechanical liquefaction in the presence of water and sodium aluminate (Al/SiO.sub.2 weight ratio of 0.3%). The resulting slurry (solids content of 10% by weight) is subsequently dried by atomization using a rotary atomizer.

(21) In comparison with example 1, the following are found: a gain in productivity in the reaction (regarding the final concentration, expressed as SiO.sub.2, of the reaction medium and taking into account the duration of the reaction) of 28%, a saving in water consumption in the reaction of 25%, a saving in energy consumption in the reaction of 28%.

(22) The characteristics of the precipitated silica obtained in the powder form are then as follows: CTAB specific surface: 173 m.sup.2/g Median size d50 of aggregates: 101 nm V.sub.(d5-d50)/V.sub.(d5-d100): 0.84 Mode (Hg porosimetry): 28 nm

(23) It is found, in particular by TEM, that the precipitated silica is formed of aggregates of large primary silica particles (lpp), at the surface of which occur small primary silica particles (spp).

EXAMPLE 3

(24) The following are introduced into a 25-liter stainless steel reactor equipped with a system for stirring by propellers and with heating via a jacket: 9.93 liters of municipal water, 4286 grams of aqueous sodium silicate, with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and having a concentration equal to 235 g/l, 134 grams of sodium sulfate Na.sub.2SO.sub.4 (electrolyte).

(25) The silicate concentration (expressed as SiO.sub.2) in the stock is then 61 g/l.

(26) The mixture is homogenized by stirring and brought to 95 C. The entire reaction is carried out with stirring (300 rev/min, propeller stirring).

(27) 95% Sulfuric acid (with a density d20 equal to 1.65) is introduced into the mixture over 15 minutes at a flow rate of 10.33 g/min. 95% Sulfuric acid is subsequently introduced into the mixture until the pH of the reaction medium reaches a value of 8 (i.e., 195 grams of sulfuric acid in 5 minutes).

(28) Once acidification is complete, the simultaneous introduction is carried out, into the reaction medium, over 10 minutes, of a sodium silicate solution (with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and with a density d20 equal to 1.230+/0.006) having a concentration of 235 g/l, at a flow rate of 80 g/min, and 95% sulfuric acid, at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 8.

(29) At the end of the 10 minutes of simultaneous addition, 95% sulfuric acid is introduced until the pH of the reaction medium reaches a value of 4.8 (i.e., 29.6 grams of sulfuric acid in 7 minutes).

(30) The simultaneous introduction is subsequently carried out, into the reaction medium, over 27 minutes, of a sodium silicate solution (with a SiO.sub.2/Na.sub.2O weight ratio equal to 3.55+/0.12 and with a density d20 equal to 1.230+/0.006) having a concentration of 235 g/l, at a flow rate of 62.8 g/min, and 95% sulfuric acid, at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 4.8.

(31) On conclusion of the reaction, a precipitated silica reaction slurry is obtained and is kept stirred at a temperature of 95 C. for 5 minutes. After this maturing, the precipitated silica slurry is recovered by emptying the reactor.

(32) The slurry is filtered and washed under vacuum. The obtained filtration cake is washed 4 times with 5 liters of municipal water. It is subsequently resuspended by mechanical liquefaction in the presence of water and sodium aluminate (Al/SiO.sub.2 weight ratio of 0.3%). The resulting slurry (solids content of 10% by weight) is subsequently dried by atomization using a rotary atomizer.

(33) Compared to example 1, the following are observed: a gain in productivity in the reaction (regarding the final concentration, expressed as SiO.sub.2, of the reaction medium and taking into account the duration of the reaction) of 28%, a saving in water consumption in the reaction of 25%, a saving in energy consumption in the reaction of 26%.

(34) The characteristics of the precipitated silica obtained in the powder form are then as follows: CTAB specific surface: 147 m.sup.2/g Median size d50 of aggregates: 100 nm V.sub.(d5-d50)/V.sub.(d5-d100): 0.84 Mode (Hg porosimetry): 31 nm

(35) It is found, in particular by TEM, that the precipitated silica is formed of aggregates of large primary silica particles (lpp), at the surface of which occur small primary silica particles (spp).