Process for preparing precipitated silicas

Abstract

A silica production method comprising reacting a silicate with at least one acid, in which the acid used in at least one of the steps of the production method is a concentrated acid, preferably selected from the group consisting of sulfuric acid having a concentration of at least 80% by weight, in particular at least 90% by weight, acetic acid having a concentration of at least 90% by weight, formic acid having a concentration of at least 90% by weight, nitric acid having a concentration of at least 60% by weight, phosphoric acid having a concentration of at least 75% by weight, and hydrochloric acid having a concentration of at least 30% by weight.

Claims

1. A process for preparing silica, the process comprising: reacting a silicate with at least one acid according to the following successive steps such that a precipitate is formed: (i) forming an aqueous stock with a pH of between 2 and 5, (ii) simultaneously adding silicate and an acid to said aqueous stock to form a reaction medium, such that the pH of the reaction medium is maintained between 2 and 5, (iii) stopping the addition of the acid used in step (ii) while continuing the addition of silicate to the reaction medium until a pH value in the reaction medium of between 7 and 10 is obtained, (iv) simultaneously adding silicate and an acid to the reaction medium, such that the pH of the reaction medium is maintained between 7 and 10, and (v) stopping the addition of the silicate used in step (iv) while continuing the addition of the acid used in step (iv) to the reaction medium until a pH value of the reaction medium of less than 6 is obtained; separating the precipitate, to obtain a suspension of precipitated silica; and drying the suspension of precipitated silica, wherein, in step (ii), a dilute acid is added for x minutes, followed by the addition of 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, wherein x is between 10 and 25.

2. The process as claimed in claim 1, wherein, in step (ii), the concentrated acid is used after reaching a gel point in the reaction medium.

3. The process as claimed in claim 1, wherein x is between 15 and 25.

4. The process as claimed in claim 1, wherein the acid used in at least one of the steps (iv) and (v) 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 as claimed in claim 1, wherein the acid used in steps (iv) and (v) 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 as claimed in claim 1, wherein said concentrated acid is sulfuric acid with a concentration of at least 80% by weight.

7. The process as claimed in claim 1, wherein said concentrated acid is sulfuric acid with a concentration of between 90% and 98% by weight.

8. The process as claimed in claim 1, wherein a maturation step is performed between step (iii) and step (iv).

9. The process as claimed in claim 1, wherein a maturation step is performed after step (v).

10. The process as claimed in claim 1, wherein, in step (v), the addition of the silicate is stopped while continuing the addition of the acid to the reaction medium until a pH value in the reaction medium of between 3 and 5.5 is obtained.

11. The process as claimed in claim 1, wherein, between step (iii) and step (iv), an acid is added to the reaction medium, the pH of the reaction medium after this addition being between 7 and 9.5.

12. The process as claimed in claim 1, wherein the entire reaction is performed between 70 and 95 C.

13. The process as claimed in claim 1, wherein the entire reaction is performed at a constant temperature.

14. The process as claimed in claim 1, wherein step (i) comprises the addition of an acid to water so as to obtain a pH value in the aqueous stock thus formed of between 2 and 5.

15. The process as claimed in claim 1, wherein step (i) comprises adding an acid to a water+silicate mixture so as to obtain a pH value said aqueous stock thus formed of between 2 and 5.

16. The process as claimed in claim 1, wherein step (i) comprises adding an acid to a stock containing preformed silica particles at a pH above 7, so as to obtain a pH value in said aqueous stock thus formed of between 2 and 5.

17. The process as claimed in claim 1, wherein said drying is performed by atomization.

Description

EXAMPLE 1 (COMPARATIVE)

(1) The following are introduced into a stainless-steel reactor equipped with an impeller stirring system and a heating jacket: 97 kg of water, 1.51 kg of Na.sub.2SO.sub.4 (electrolyte).

(2) The solution is brought to 92 C. The entire reaction is performed at this temperature. With stirring, dilute sulfuric acid with a density at 20 C. equal to 1.050 (sulfuric acid with a weight content equal to 7.7%) is introduced until the pH reaches a value of 3.7.

(3) A sodium silicate solution with an SiO.sub.2/Na.sub.2O weight ratio equal to 3.39 and with a density at 20 C. equal to 1.229 is introduced into the reactor over 25 minutes at a rate of 668 g/minute along with simultaneous introduction of sulfuric acid of the type described above at a regulated rate so as to bring the pH of the reaction medium to a value of 4.4 and then maintain it at said value.

(4) After 25 minutes of simultaneous addition, the introduction of sulfuric acid of the type described above is stopped and the rate of the silicate solution is increased to 880 g/minute until a pH value equal to 8 is reached within about 2 minutes.

(5) A new simultaneous addition is performed over 18 minutes with a sodium silicate rate of 1075 g/minute (same sodium silicate as for the first simultaneous addition) and a rate of sulfuric acid of the type described above, with a weight content equal to 7.7%, regulated so as to maintain the pH of the reaction medium at a value of 8.

(6) After this simultaneous addition, the reaction medium is brought to a pH of 4.6 with sulfuric acid with a weight content equal to 7.7% at a rate equal to 515 g/minute and over about 4 minutes.

(7) The total duration of the reaction is 48 minutes.

(8) A slurry of precipitated silica is thus obtained, which is filtered and washed using a filter press so as finally to recover a silica cake whose moisture content is 80% (and thus a solids content of 20% by weight). This cake is then fluidized by mechanical and chemical action (addition of an amount of sodium aluminate corresponding to an Al/SiO.sub.2 weight ratio of 0.30%). After this liquefaction, a pumpable cake with a pH equal to 5.8 is obtained, which is then atomized using a nozzle atomizer.

(9) The characteristics of the silica obtained (in the form of substantially spherical beads) are the following: CTAB specific surface area: 190 m.sup.2/g BET specific surface area: 214 m.sup.2/g V.sub.(d5-d50)/V.sub.(d5-d100): 0.73 Width Dw (XDC): 1.66 Pore distribution width Pdw: 1.85

EXAMPLE 2

(10) The following are introduced into a stainless-steel reactor equipped with an impeller stirring system and a double heating jacket: 112 kg of water, 1.75 kg of Na.sub.2SO.sub.4 (electrolyte).

(11) The solution is brought to 92 C. The entire reaction is performed at this temperature. With stirring, dilute sulfuric acid with a density at 20 C. equal to 1.050 (sulfuric acid with a weight content equal to 7.6%) is introduced until the pH reaches a value of 3.7.

(12) A sodium silicate solution with an SiO.sub.2/Na.sub.2O weight ratio equal to 3.41 and with a density at 20 C. equal to 1.226 is introduced into the reactor over 15 minutes at a rate of 774 g/minute along with simultaneous introduction of sulfuric acid of the type described above at a regulated rate so as to bring the pH of the reaction medium to a value of 4.2 and then maintain it at said value. After the 15th minute, the addition of sulfuric acid of the type described above is stopped and concentrated sulfuric acid with a density at 20 C. equal to 1.83 (sulfuric acid with a weight content equal to 95%) is added simultaneously with the sodium silicate solution over 10 minutes at a regulated rate so as to maintain the pH of the reaction medium at 4.2.

(13) After 25 minutes of simultaneous addition, the introduction of concentrated acid is stopped and the rate of the silicate solution is increased to 847 g/minute until a pH value equal to 8 is reached within about 2 minutes.

(14) A new simultaneous addition is performed over 18 minutes with a sodium silicate rate of 1225 g/minute (same sodium silicate as for the first simultaneous addition) and a rate of concentrated sulfuric acid, with a weight content equal to 95%, regulated so as to maintain the pH of the reaction medium at a value of 8.

(15) After this simultaneous addition, the reaction medium is brought to a pH of 4.5 with sulfuric acid with a weight content equal to 95% at a rate equal to 93 g/minute and over about 2 minutes.

(16) The total duration of the reaction is 48 minutes.

(17) Compared to example 1, the following are observed: a gain in reaction productivity (as regards the final concentration expressed as SiO.sub.2 of the reaction medium) of 22%, a saving in the water consumption of the reaction of 18%, a saving in the consumption of energy (in the form of live steam) in the reaction of 24%.

(18) A slurry of precipitated silica is thus obtained, which is filtered and washed using a filter press so as finally to recover a silica cake whose moisture content is 79% (and thus a solids content of 21% by weight). This cake is then fluidized by mechanical and chemical action (addition of an amount of sodium aluminate corresponding to an Al/SiO.sub.2 weight ratio of 0.30%). After this liquefaction operation, a pumpable cake having a pH equal to 6.0 is obtained, which is then atomized using a nozzle atomizer.

(19) The characteristics of the silica obtained (in the form of substantially spherical beads) are the following: CTAB specific surface area: 190 m.sup.2/g BET specific surface area: 214 m.sup.2/g V.sub.(d5-d50)/V.sub.(d5-d100): 0.74 Width Dw (XDC): 1.68 Pore distribution width Pdw: 2.03