Process for preparing precipitated silicas

Abstract

A process for preparing precipitated silica comprising a precipitation reaction between silicate and an acid, in which the acid used in at least one of the steps 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 precipitated silica, the process comprising: reacting a silicate with at least one acid in the following manner such that a precipitate is formed: (i) forming an initial stock comprising at least a portion of the total amount of the silicate involved in the reaction and an electrolyte, the concentration of silicate expressed as SiO.sub.2 in said initial stock being between 50 and 70 g/l, (ii) adding an acid to said initial stock to form a reaction medium until a pH value of the reaction medium of between 7 and 8.5 is obtained, (iii) adding an acid to the reaction medium and optionally simultaneously adding the remaining amount of the total amount of the silicate involved in the reaction, and (iv) adding an acid to the reaction medium, until a pH value of the reaction medium of between 4 and 6 is obtained; filtering the precipitate and washing using a filter equipped with a compacting means, to obtain a suspension of precipitated silica; and drying the suspension of precipitated silica by atomization, wherein, step (ii) comprises adding, for x minutes, a dilute acid followed by adding, after x minutes, a concentrated acid, where x is between 15 and 20, and wherein the acid used in step (iii) is a concentrated acid, wherein the concentrated acid, independently in each of steps (ii) and (iii), 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 as claimed in claim 1, wherein in step (iii), the acid and silicate are added simultaneously to the reaction medium.

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

5. The process as claimed in claim 1, wherein said concentrated acid is sulfuric acid with a concentration of at least 80% by weight.

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

7. The process as claimed in claim 1, wherein said concentration of silicate expressed as SiO.sub.2 in said initial stock is between 50 and 65 g/l.

8. The process as claimed in claim 1, wherein said electrolyte is sodium sulfate, its concentration in the initial stock being between 12 and 20 g/l.

9. The process as claimed in claim 1, wherein said drying is performed using a nozzle atomizer.

10. The process as claimed in claim 1, wherein the separation comprises filtration, washing and then compacting, using a filter press.

11. The process as claimed in claim 1, wherein the drying step provides a dried product, and wherein said dried product is milled, and then optionally agglomerated.

12. The process as claimed in claim 11, wherein the dried product is then agglomerated.

13. The process as claimed in claim 1, wherein the obtained suspension of precipitated silica has a solids content of at most 22% by weight.

Description

EXAMPLE 1 (COMPARATIVE)

(1) The following are introduced into a stainless steel reactor equipped with an impeller stirring system and with live steam heating in the reaction medium: 869 liters of water, 16.5 kg of Na.sub.2SO.sub.4 (electrolyte), 302 liters of aqueous sodium silicate, having an SiO.sub.2/Na.sub.2O weight ratio equal to 3.46 and a density at 20° C. equal to 1.236.

(2) The concentration of silicate (expressed as SiO.sub.2) in the stock is then 64 g/l. The mixture is brought to a temperature of 82° C. with continued stirring.

(3) 348 liters of dilute sulfuric acid with a density at 20° C. equal to 1.053 (sulfuric acid with a weight concentration equal to 8.1%) are then introduced therein. The dilute acid is introduced at a flow rate of 522 l/h for 40 minutes until the pH of the reaction medium reaches a value (measured at its temperature) equal to 8.0.

(4) The reaction temperature is 82° C. for the first 27 minutes of the reaction; it is then brought from 82° C. to 90° C. over approximately 13 minutes, then held at 90° C. until the end of the reaction.

(5) Next, 94 liters of aqueous sodium silicate of the type described above and 120 liters of sulfuric acid, also of the type described above, are introduced together into the reaction medium, this simultaneous introduction of dilute acid and silicate being carried out so that the pH of the reaction medium, during this period of introduction, is always equal to 8.0±0.1.

(6) After introducing all of the silicate, addition of the dilute acid is continued, at a flow rate of 298 l/h, for 8 minutes.

(7) This additional introduction of acid then brings the pH of the reaction medium to a value equal to 4.4.

(8) The total duration of the reaction is 88 minutes.

(9) A slurry of precipitated silica is thus obtained, which is filtered and washed using a filter press so that a silica cake is finally recovered, the moisture content of which is 82% (therefore a solids content of 18% 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 0.32%). After this disintegrating operation, a pumpable cake having a pH equal to 6.7 is obtained, which is then atomized using a nozzle atomizer.

(10) The characteristics of the silica obtained (in the form of substantially spherical beads) are the following:

(11) TABLE-US-00001 BET surface area (m.sup.2/g) 201 CTAB surface area (m.sup.2/g) 199 Ø.sub.50 (μm) * 6.1 F.sub.D (ml) * 16.4 V2/V1 (%) 29.2 IF (Å) 98 V.sub.d1<1 μm (ml/g) 1.72 * after ultrasound deagglomeration

EXAMPLE 2

(12) The following are introduced into a stainless steel reactor equipped with an impeller stirring system and with live steam heating in the reaction medium: 1040 liters of water, 19.7 kg of Na.sub.2SO.sub.4 (electrolyte), 365 liters of aqueous sodium silicate, having an SiO.sub.2/Na.sub.2O weight ratio equal to 3.46 and a density at 20° C. equal to 1.236.

(13) The concentration of silicate (expressed as SiO.sub.2) in the stock is then 64 g/l. The mixture is brought to a temperature of 82° C. with continued stirring.

(14) 210 liters of dilute sulfuric acid with a density at 20° C. equal to 1.053 (sulfuric acid with a weight content equal to 8.1%) are then introduced therein over the first 20 minutes of the reaction, and 11 liters of concentrated sulfuric acid with a density at 20° C. equal to 1.83 (sulfuric acid with a weight content equal to 94%) are then introduced therein until the pH of the reaction medium reaches a value (measured at its temperature) equal to 8.0.

(15) The reaction temperature is 82° C. for the first 20 minutes of the reaction; it is then brought from 82° C. to 90° C. over approximately 13 minutes, then held at 90° C. until the end of the reaction.

(16) Next, 107 liters of aqueous sodium silicate of the type described above and 6.9 liters of concentrated sulfuric acid, of the type described above, are introduced together into the reaction medium, this simultaneous introduction of concentrated acid and silicate being carried out so that the pH of the reaction medium, during this period of introduction, is always equal to 8.0±0.1.

(17) After introducing all of the silicate, addition of the concentrated acid is continued, at a flow rate of 18.9 l/h, for 8 minutes.

(18) This additional introduction of acid then brings the pH of the medium to a value equal to 4.3.

(19) The total duration of the reaction is 88 minutes.

(20) 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 20%, a saving in the water consumption of the reaction of 17%, a saving in the consumption of energy (in the form of live steam) in the reaction of 16%.

(21) A slurry of precipitated silica is thus obtained, which is filtered and washed using a filter press so that a silica cake is finally recovered, the moisture content of which is 84% (therefore a solids content of 16% 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 0.33%). After this disintegrating operation, a pumpable cake having a pH equal to 6.2 is obtained, which is then atomized using a nozzle atomizer.

(22) The characteristics of the silica obtained (in the form of substantially spherical beads) are the following:

(23) TABLE-US-00002 BET surface area (m.sup.2/g) 209 CTAB surface area (m.sup.2/g) 201 Ø.sub.50 (μm) * 4.4 F.sub.D (ml) * 21.4 V2/V1 (%) 22.4 IF (Å) 94 V.sub.d1<1 μm (ml/g) 1.74 * after ultrasound deagglomeration