Precipitated silica production process

Abstract

A precipitated silica production process that includes a precipitation reaction between a silicate and an acid is described, in which the acid used in at least one of the steps of the process is a concentrated acid.

Claims

1. A process for preparing precipitated silica, the process comprising: precipitating a silicate and at least one acid using the following steps, whereby a suspension of precipitated silica is obtained: (i) forming an initial stock comprising a silicate and an electrolyte, wherein a concentration of silicate (expressed as SiO.sub.2) in said initial stock is less than 100 g/l and, optionally, wherein a concentration of electrolyte in said initial stock is less than 19 g/l, (ii) adding an acid to said stock to form a reaction medium, wherein said acid is added until a pH value of the reaction medium of at least 7.0 is obtained, wherein the acid used before reaching a gel point in the reaction medium is sulfuric acid having a concentration of less than 20% by weight and wherein the acid used after reaching a gel point in the reaction medium is sulfuric acid having a concentration of at least 80% by weight, (iii) simultaneously adding sulfuric acid having a concentration of at least 80% by weight and a silicate to the reaction medium, (iv) adding sulfuric acid having a concentration of at least 80% by weight to the reaction medium, optionally until a pH value of the reaction medium of between 3.0 and 6.5 is obtained, separating a precipitate from the reaction medium, and drying the precipitate.

2. The process as claimed in claim 1, wherein the sulfuric acid having a concentration of at least 80% by weight in step (ii) is used after x minutes counting from the start of said step, with x being between 15 and 25.

3. The process as claimed in claim 1, wherein the concentration of silicate (expressed as SiO.sub.2) in said initial stock is at least 80 g/l.

4. The process as claimed in claim 1, wherein the sulfuric acid having a concentration of at least 80% by weight used in at least one of steps (ii), (iii) and (iv) is sulfuric acid having a concentration of between 90% and 98% by weight.

5. The process as claimed in claim 1, wherein the separation comprises a filtration and a liquefaction of the cake resulting from the filtration, said liquefaction optionally being carried out in the presence of at least one aluminum compound.

6. The process as claimed in claim 1, wherein the drying is carried out by atomization.

7. The process as claimed in claim 1, wherein the acid is added to said stock in step (ii) until the pH of the reaction medium is between 7.0 and 8.5.

8. The process as claimed in claim 1, wherein the suspension of precipitated silica has a solids content of at most 24% by weight.

9. The process as claimed in claim 1, wherein the sulfuric acid having a concentration of at least 80% by weight used in step (ii) is sulfuric acid having a concentration of at least 90% by weight.

10. The process as claimed in claim 1, wherein the sulfuric acid having a concentration of at least 80% by weight used in step (iii) is sulfuric acid having a concentration of at least 90% by weight.

11. The process as claimed in claim 1, wherein the sulfuric acid having a concentration of at least 80% by weight used in step (iv) is sulfuric acid having a concentration of at least 90% by weight.

12. The process as claimed in claim 1, wherein the sulfuric acid having a concentration of at least 80% by weight used in all of steps (ii), (iii) and (iv) is sulfuric acid having a concentration of at least 90% by weight.

13. The process as claimed in claim 5, wherein the aluminum compound is an alkali metal aluminate.

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: 637 liters of water, 14.1 kg of Na.sub.2SO.sub.4 (electrolyte), 362 liters of aqueous sodium silicate, having an SiO.sub.2/Na.sub.2O weight ratio equal to 3.5 and a density at 20 C. equal to 1.230.

(2) The concentration of silicate (expressed as SiO.sub.2) in the stock is then 86 g/l. The mixture is brought to a temperature of 83 C. while keeping it stirred.

(3) Then 462 liters of dilute sulfuric acid having a density at 20 C. equal to 1.050 (sulfuric acid with a concentration by weight equal to 7.7%) are introduced therein. The dilute acid is introduced at a flow rate of 529 l/h for the first 20 minutes of the reaction; the flow rate is then increased to 1056 l/h until the pH of the reaction medium reaches a value (measured at its temperature) equal to 8.0.

(4) The reaction temperature is 83 C. for the first 20 minutes of the reaction; it is then brought from 83 C. to 92 C. over approximately 15 minutes, then held at 92 C. until the end of the reaction.

(5) Next, 87.8 liters of aqueous sodium silicate of the type described above and 119 liters of sulfuric acid, also of the type 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 8.00.1.

(6) After introducing all of the silicate, the dilute acid continues to be introduced, at a flow rate of 432 l/h, for 6 minutes.

(7) This additional introduction of acid then brings the pH of the reaction medium to a value equal to 4.8.

(8) The total duration of the reaction is 60 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 77% (therefore a solids content of 23% 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 0.22%). After this disintegrating operation, a pumpable cake having a pH equal to 6.9 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) 158 CTAB surface area (m.sup.2/g) 155 .sub.50 (m).sup.* 2.1 F.sub.D (ml).sup.* 16.9 V2/V1 (%) 57 .sup.*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: 700 liters of water, 15.5 kg of Na.sub.2SO.sub.4 (electrolyte), 402 liters of aqueous sodium silicate, having an SiO.sub.2/Na.sub.2O weight ratio equal to 3.5 and a density at 20 C. equal to 1.230.

(13) The concentration of silicate (expressed as SiO.sub.2) in the stock is then 86 g/l. The mixture is brought to a temperature of 83 C. while keeping it stirred.

(14) Then 489 liters of dilute sulfuric acid having a density at 20 C. equal to 1.050 (sulfuric acid with a concentration by weight equal to 7.7%) are introduced therein. The dilute acid is introduced at a flow rate of 546 l/h for the first 20 minutes of the reaction; the flow rate is then increased to 1228 l/h until the pH of the reaction medium reaches a pH value (measured at its temperature) equal to 8.0.

(15) The reaction temperature is 83 C. for the first 20 minutes of the reaction; it is then brought from 83 C. to 92 C. over approximately 15 minutes, then held at 92 C. until the end of the reaction.

(16) Next, 98.0 liters of aqueous sodium silicate of the type described above and 6.5 liters of concentrated sulfuric acid having a density at 20 C. equal to 1.83 (sulfuric acid with a concentration by weight equal to 95%) 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 8.00.1.

(17) After introducing all of the silicate, the concentrated acid continues to be introduced, at a flow rate of 201/h, for 6 minutes.

(18) This additional introduction of acid then brings the pH of the reaction medium to a value equal to 4.8.

(19) The total duration of the reaction is 60 minutes.

(20) Compared to example 1, the following are observed: a gain in reaction productivity (relating to the final concentration, expressed as SiO.sub.2, of the reaction medium and taking into account the reaction time) of 11%, a saving in the reaction water consumption of 10%, a saving in the reaction energy consumption (in the form of live steam) of 10%.

(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 78% (therefore a solids content of 22% 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 0.21%). After this disintegrating operation, a pumpable cake having a pH equal to 6.6 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) 157 CTAB surface area (m.sup.2/g) 155 .sub.50 (m).sup.* 2.5 F.sub.D (ml).sup.* 19.3 V2/V1 (%) 60 .sup.*after ultrasound deagglomeration

EXAMPLE 3

(24) The following are introduced into a stainless steel reactor equipped with an impeller stirring system and with live steam heating in the reaction medium: 860 liters of water, 19.0 kg of Na.sub.2SO.sub.4 (electrolyte), 492 liters of aqueous sodium silicate, having an SiO.sub.2/Na.sub.2O weight ratio equal to 3.5 and a density at 20 C. equal to 1.230.

(25) The concentration of silicate (expressed as SiO.sub.2) in the stock is then 86 g/l. The mixture is brought to a temperature of 82 C. while keeping it stirred.

(26) Then 232 liters of dilute sulfuric acid having a density at 20 C. equal to 1.050 (sulfuric acid with a concentration by weight equal to 7.7%) are introduced therein over the first 20 minutes of the reaction, and then 17 liters of concentrated sulfuric acid having a density at 20 C. equal to 1.83 (sulfuric acid with a concentration by weight equal to 95%) are introduced therein until the pH of the reaction medium reaches a value (measured at its temperature) equal to 8.0.

(27) The reaction temperature is 82 C. for the first 20 minutes of the reaction; it is then brought from 82 C. to 92 C. over approximately 15 minutes, then held at 92 C. until the end of the reaction.

(28) Next, 120 liters of aqueous sodium silicate of the type described above and 7.6 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 8.00.1.

(29) After introducing all of the silicate, the concentrated acid continues to be introduced, at a flow rate of 23 l/h, for 6 minutes.

(30) This additional introduction of acid then brings the pH of the medium to a value equal to 4.8.

(31) The total duration of the reaction is 65 minutes.

(32) Compared to example 1, the following are observed: a gain in reaction productivity (relating to the final concentration, expressed as SiO.sub.2, of the reaction medium and taking into account the reaction time) of 25%, a saving in the reaction water consumption of 26%, a saving in the reaction energy consumption (in the form of live steam) of 26%.

(33) 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 77% (therefore a solids content of 23% 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 0.29%). After this disintegrating operation, a pumpable cake having a pH equal to 6.6 is obtained, which is then atomized using a nozzle atomizer.

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

(35) TABLE-US-00003 BET surface area (m.sup.2/g) 163 CTAB surface area (m.sup.2/g) 160 .sub.50 (m).sup.* 2.5 F.sub.D (ml).sup.* 18.3 V2/V1 (%) 61 .sup.*after ultrasound deagglomeration

EXAMPLE 4

(36) The following are introduced into a stainless steel reactor equipped with an impeller stirring system and with a heating jacket: 86.4 liters of water, 0.77 kg of Na.sub.2SO.sub.4 (electrolyte), 44.2 liters of aqueous sodium silicate, having an SiO.sub.2/Na.sub.2O weight ratio equal to 3.5 and a density at 20 C. equal to 1.230.

(37) The concentration of silicate (expressed as SiO.sub.2) in the stock is then 80 g/l. The mixture is brought to a temperature of 87 C. while keeping it stirred. The temperature is maintained at this value throughout the whole reaction.

(38) Then 22.1 liters of dilute sulfuric acid having a density at 20 C. equal to 1.050 (sulfuric acid with a concentration by weight equal to 7.7%) are introduced therein over the first 20 minutes of the reaction, and then 1.46 liters of concentrated sulfuric acid having a density at 20 C. equal to 1.85 (sulfuric acid with a concentration by weight equal to 98%) are introduced therein until the pH of the reaction medium reaches a value (measured at its temperature) equal to 8.0.

(39) Next, 10.7 liters of aqueous sodium silicate of the type described above and 0.69 liter 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 8.00.1.

(40) After introducing all of the silicate, the concentrated acid continues to be introduced, at a flow rate of 2.01/h, for 6 minutes.

(41) This additional introduction of acid then brings the pH of the reaction medium to a value equal to 4.8.

(42) The total duration of the reaction is 60 minutes.

(43) Compared to example 1, the following are observed: a gain in reaction productivity (relating to the final concentration, expressed as SiO.sub.2, of the reaction medium and taking into account the reaction time) of 37%, a saving in the reaction water consumption of 27%, a saving in the reaction energy consumption of 32%.

(44) A slurry of precipitated silica is thus obtained, which is filtered and washed using a vacuum filter so that a silica cake is finally recovered, the moisture content of which is 85% (therefore a solids content of 15% 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 0.27%). After this disintegrating operation, a pumpable cake having a pH equal to 6.8 is obtained, which is then atomized.

(45) The characteristics of the silica obtained (in powder form) are the following:

(46) TABLE-US-00004 BET surface area (m.sup.2/g) 155 CTAB surface area (m.sup.2/g) 164 .sub.50 (m).sup.* 2.8 F.sub.D (ml).sup.* 12.6 V2/V1 (%) 58 .sup.*after ultrasound deagglomeration