FR2471949A1 - Fine silica prepn. - by reaction of alkali metal and acid using sequestering agent to complex with impurity metallic ions(BR 7.7.81) - Google Patents

Abstract

Silica is prepd. by reaction of alkali metal silicate and an acid, using a silicate soln. free of metallic impurities. Alternatively, the silica may be ppted. in the presence of sequestering agent present in sufficient amt. to complex any remaining metallic impurities, esp. alkaline earth metals (Ca). The complexing agent, which may be EDTA, diethylene triamine pentacetic acid, nitrilotriacetic acid or alkali tripolyphosphate, is added to the initial silicate soln. A sudden dilution of the medium is effected after the gel period, by addition of water. Specifically, alkali silicate is added to the acid soln. continuously until opalescence appears. Acid addn. is then stopped and the soln. allowed to stand to equilibrate the reaction conditions, then further acid is added until the reaction is complete. The silica produced has a very fine particle size, i.e. the cohesion of the agglomerates when dry is weaker, esp. high surface area particles.

Description

PROCESS FOR MANUFACTURING FINE PRECIPITED SILICES
The subject of the present invention is a new process for producing silica.

 It is known that it is common in the field of silicas, to use air grinding, or micronisation, to obtain fine products. This technique is applied to the manufacture of silicas for uses such as thixotropic agents for plasticols, varnishes, reinforcing agents, etc.

 However, such a technique is applied after creation of the silica morphology which has a primary structure formed of aggregates and a secondary structure consisting of agglomerates. However, these during drying can form larger agglomerates that resist grinding and are found in the finished product. The latter then has very wide particle sizes with a percentage of coarse products, which even when it is low can be unacceptable for certain applications due to the initiation of vulcanizate rupture.

 The subject of the present invention is a process for obtaining so-called fine silicas, that is to say the degree of cohesion of the agglomerates of the dried product which is manifested by a great fineness of the ground products.

 The present invention is more particularly applicable to silicas with a high specific surface area for which the degree of agglomerate cohesion is greater.

 However, it has been observed that, unexpectedly, the fineness and friability of a precipitated silica can be checked by carrying out the precipitation with a solution of silicate that is substantially free of metallic impurities and in particular free alkaline earth metal and more especially calcium.

 According to the invention, it is possible to use a solution of pure silicate. But we know that the mode of preparation that involves pure sand, fresh water .... is then very expensive and can hardly be considered.

 Also advantageously, according to the invention, a precipitation silica is prepared from a solution of an alkali silicate containing as impurities at least one alkaline earth metal, and an acidifying agent, according to a process which is characterized by the fact that the precipitation is carried out in the presence of a sequestering agent, in an amount sufficient to complex at least partly the metallic impurities, especially calcium, and possibly magnesium, contained in the silicate, and preferably the screen.

 Advantageously, the complexing agent is added to the silicate solution used for precipitation.

 The precipitation of the silica according to the invention can be carried out by any known means and generally applies to the various families of silica preparation methods by the action of an acidifying agent and an alkali silicate solution and more particularly a strong acid solution such as phosphoric acid, and an aqueous solution of a sodium silicate of SiO 2 / Na 2 O molar ratio of between 1 and 4.

 In particular, it is possible, according to a first family of processes, to add an alkali silicate solution to an acid solution until the appearance of the opalescence, to stop the addition of acid at this point, to observe a period of maturation of the reaction mass. during a period of time sufficient to obtain an equilibrium of the reaction conditions, then add the acid remaining in the reaction medium until complete precipitation of the silica. According to this method the stabilization of the medium usually begins a few minutes after the appearance of the opalescence so that the ripening period is from 10 minutes to 2 hours, preferably 10 to 15 minutes.

 According to another family of methods of preparation, an alkali silicate solution is added to an acid solution until the polymers are maximally formed, and at least one stoppage of the acid addition is observed from this point. .

 Finally, it is possible to use a family of silica preparation methods according to which a stoppage of acid addition is not employed, whereby an acid solution and a solution are introduced simultaneously. of alkali silicate in a silicate solution.

 In all these processes, the medium can be diluted rapidly after the freezing period.

 It is also possible to introduce a large number of variants, in particular, it is possible to use a mode of preparation with interruption of the addition of acid and then simultaneous addition of an acid solution in such a way that the pH of the medium of The reaction is kept substantially constant until the precipitation of the pigment is complete.

 Finally, it is possible to resort to post-addition treatments such as, for example, blocking the micro-pores by postaddition of a silicate solution, advantageously corresponding to 15 to 30 parts by weight of silica for 85 to 70 parts by weight. silica added initially, or by performing a final heating around 1000C at a pH of the order of 9 to 7.

 The silicates used are especially sodium silicate solutions with a molar ratio Rm of between 2.6 and 4 and whose SiO2 concentration varies from 50 to 150 g / l.

 The reaction temperature is between 50 and 950C and the precipitation start pHs vary between 9.5 and 11.

 The addition times of the acidifying agent can generally vary between 45 min. and 2 hours, these times being understood without the possible stops.

 The sequestering agents according to the invention must have a sufficient conditional constant (according to G. Schwarzenbach Complexometry (Titration, Interscience, New York 1957).

 It should generally be greater than about 3 (counting the log of the stability constant), and preferably about 5. If this constant is not high enough, an excess of sequestrant should be added.

 It is also necessary to take into account the complexing power of the added products, with respect to other metals contained in the reaction medium.

Among the complexing agents that may be mentioned
- diethylenediaminetetraacetic acid (EDTA)
diethylene triamine pentaacetic acid (DTPA)
nitrilotriacetic acid (NTA)
alkaline tripolyphosphates (TPP), in particular of sodium,
whose conditional stability constants (defined by
G. SCHWARZENBACH Complexometric Titrations-Interscience -New-York 1957) (at pH 10) are weak. The values of these conditional constants are calculated using the method described by
RINGBOM (Complexes in Analytical Chemistry - DUNOD-Paris 1967), from the values of the constants found in the tables (for example: 1) STABILITY CONSTANT-LG SILLEN and AE MARTELS London-The Chemical Society-1964
2) RINGBOM
3) The publications of G.SCHWARZENBACH ...)
Of course, the present invention is not limited to its sole sequestering agents but extends to any sequestering agent that meets the aforementioned requirements, as well as to all types of metallic impurities.

 According to the invention, in order to demonstrate the fineness and friability of the product obtained was used a measure that consists in subjecting the product to drying and then grinding and measuring the apparent density under slight pressure.

 This method of determination appeared to be more representative of the characteristic that it was desired to show, in the matter of coherence of the agglomerates, than the methods usually used for particle size distribution by counting the particles through a calibrated orifice, which is insignificant when the product is thin (<2y>) or packed density which also reflects the handling experienced by the product after grinding.

Practically in the examples this determination was carried out as follows
In a matrix of internal diameter equal to 25 mm and height equal to 80 mm is added 3 g of ground silica having a maximum of 7% moisture at 1900C for 2 hours.

 This silica mass has a piston of 5 cm 2 of surface on which is added a weight determined so as to exert on the product a pressure of 4 kg / cm 2 and the specific volume Vo is measured as expressed in cm3 / g.

 According to the test, a difference of 0.2 points can be considered significant.

 The products according to the invention are preferably with a large external surface, but of course the invention is not limited to a single type of product which may have all the general applications of silicas.

 But, the present invention will be more easily understood using the following examples given for illustrative but not limiting.

 In the examples, the BET specific surface area is determined according to the method of Brurauer-Emmett and Teller described in Journal of American Chemical Society Vol 60 p. 309 (1938).

 The CTAB surface is determined by adsorption of cetyltrimethylammonium bromide at pH 9 according to the method of Jay-Janzen Kraus (Rubber Chemistry and Technology 44 (1971) 1287-1296).

 The pH is measured by a suspension of 5% silica in water.

 In addition, by flash drying is meant drying as described in French Patent 2,257,326.

 By micronisation is meant micronization carried out by means of a device of Jet-0-Mizer type and others described in Chemical Engineers' Handbook - 5th edition 8 - 43.

Example 1
In a strongly stirred reactor is added 100 liters of silicate of Rm 3.5, concentration of SiO2 80 g / l. This silicate containing 220 mg of Ca / l is added 140 g of 100% DTPANa and heated to 680C. While maintaining this temperature is added sulfuric acid to 366 g / l at 130 cc / min.

 We stop at the onset of opalescence and wait 10 minutes after opalescence. 62 L of water and acid are then added to pH 8 at a rate of 130 cc / min while raising the temperature to 85-90 ° C and maintained at this temperature for 30 minutes.

The pH is lowered to 3.5 and washed with water at pH 5. Dry in a flash apparatus described in French Patent 2,257,326 and micronise.

 In this example, the conditional constant log Kca'L 'is equal to 9.9 (L = DTPA).

 (G. SCHWARZENBACH and SANDERA - Helv Chim Acta 36 p 10891953).

 Note: The conditional constant KCa'L 'takes into account the secondary reactions of Ca 2+ and L with the H + and / or OH ions.

Example 2
We work in the same way as in Example 1 but without adding complexing agent.

 This example 2 shows the effect of the addition of complexing agent in a mode according to addition of acid in the silicate, stopping at opalescence and addition of water after stabilization of the medium, the drying mode being identical. For this mode we see the important effect of the complexing agent.

Examples 3, 4, 5, 6
Example 3 is compared with Example 1 but is precipitated while maintaining the temperature at 580C until opalescence (instead of 680C) so as to have a high CTAB surface silica, we see that the fineness decreases.

 Example 4 is identical to Example 3 except that it works without complexing. There is a drop in fineness substantially identical to that observed between Examples 1 and 2.

In Examples 5 and 6, the temperature is raised to 820 ° C. for the first part of the precipitation so as to have a surface
CTAB low, the presence of complexing improves finesse.

Example 7
Example 7 uses a different drying mode (Turbine atomizer such as that in Masters Spray Drying John Wiley & Sons - the dried cake is identical to that of Example 3.

Example 8-19
In the examples which follow, and up to Example 20, processes with post-addition, that is to say addition of silicate at the end of the reaction, are used. We note in Example 8 a high CTAB surface and great finesse,
in Examples 9 and 10, the effect of the complexing agent,
- Example 11 can be compared to Examples 5 and 6 but highlights the influence of the concentration which is low.

 Example 12 is a comparative example without complexing agent.

 In Examples 15 and 16 the nature of the complexing agent is changed.

 Examples 17 and 18 are made with pure silicate. On the one hand, good fineness is observed and, on the other hand, the action of the complexing agent is substantially zero.

 The purpose of Example 19 is to show the influence of pH.

Example 8
In a reactor equipped with a recycling device such as that described in French Patent No. 1,160,762, 100 liters of silicate Rm 3.5 are added to 100 g / l of SiO 2 +, the stoichiometric equivalent corresponding to the calcium of the silicate used. initially in the reactor, that is to say 150 g of DTPANa. Sulfuric acid (366 g / l) is added at a rate of 130 cc / min until the appearance of the opalescence at 680C.

 Stopping for 15 minutes then adding 62 l of water and continuing the addition of acid to pH 7.5. After which at 850C is added silicate (30 1) and the acid while maintaining a constant pH and this for 40 min.On terminates at pH 3.5, filtered, washed and dried flash and micronise.

 In this example, the conditional constant Log Kca'L 'is equal to 9.9 (L = DTPA).

Example 9
In a strongly stirred reactor is added 1.64 liter of silicate of Em 3.5 at 80 g / l of SiO 2 + 180 g of DTPANa corresponding to the stoichiometric amount of calcium contained in the silicate at the start. The procedure is as above but without adding water after the frost period.

 In this example the conditional constant log KCa'L 'is equal to 9.9 (L = DTPA).

Example 10
Example according to the above without addition of complexing agent.

Example 11
In a strongly stirred reactor, 73 liters of silicate Rm 3.5 at 120 g / l SiO2 and 210 g of DTPANa corresponding to the calcium contained in the starting silicate are added. At 63, sulfuric acid is added at 92.5 g / l for 160 minutes. At this time the pH is 8.0. It is heated to 82 ° C. and silicate and acid are added while maintaining the pH at 8 and this for 30 minutes. The pH is lowered to 4.5, filtered, washed, dried and micronized.

 In this example the conditional constant log KCa'L 'is equal to 9.9 (L = DTPA).

Example 12
The procedure is as in Example 11 but without addition of complexing agent.

Example 13
The cake of Example 9 is dried in an oven at 120 ° C. and then micronized.

Example 14
The procedure is the same with the washed cake from Example 10.

Example 15
In a strongly stirred reactor, 8.25 l of silicate of Rm 3.5 and 130 g / l SiO2 and containing 13 g of Ca are added. 150 g of TPP are added and then at 600 ° C. they are neutralized with sulfuric acid. at 40% at 12.3 cc / min. Opalescence is stopped for 12 minutes and then 7 liters of water are added and the neutralization is continued to pH 8. At this time, 850 g of silicate is added to 850 ° C. at 84 cc / min for 30 minutes. acid to maintain the pH at 7.5. It ends at pH 3.5, filtered, washed, dried and micronized.

 In this example the conditional constant log KCa'L 'is equal to 5 (L = TPP) (RINGBOM already mentioned the same remark as in example 1).

Example 16
The procedure is the same but the TPP is replaced with 20 g of NTANa.

 In this example the conditional constant log KCa'L 'is equal to 6.2. (L = NTA) (G.SCHWARZENBACH and SANDERA, already mentioned).

Example 17
Mode identical to the previous mode without complexing but with silicate not containing calcium or complexing agent.

Example 18
Mode identical to that of Example 15 with silicate without calcium but with 20 g of NTANa.

 In this example the conditional constant log KCa'L 'is equal to 6.2 (L = NTA).

Example 19
The procedure is as above but the reaction is terminated at pH 6.

This example shows that the degree of fineness also depends on other factors but that despite all the effect of the complexation is maintained.

 Examples 20 and 21 correspond to two so-called simultaneous addition modes without stopping at opalescence. There is a very sensitive effect of the means according to the invention.

Example 20
In a strongly stirred reactor is added 30 1 of water + 8.75 1 of silicate Rm 3.5 at 80 g / l of SiO 2 + 125 g of NTANa corresponding to the stoichiometric amount of calcium contained in all the silicate added in the reactor at the end of precipitation. Keeping the pH at 10 and the temperature at 850 ° C., the silicate is added at 1250 cc / min and the sulfuric acid at 310 cc / min for 100 min. The pH is lowered to 8 and the temperature is maintained at 950 ° C. for 20 minutes. We go down to pH 3.5, filter, wash and dry.

 In this example the conditional constant log KCa'L 'is equal to 6.2 (L = NTA).

Example 21
It operates as above but without adding complexing.

 These examples clearly show the interest of the process according to the invention and the effectiveness of the treatment of silicates with a view to precipitation. In order to clearly demonstrate these various advantages, Tables 1 and 2 summarize firstly the process steps and the main characteristics of the product obtained.

TABLE 1

<tb> @@@@ <SEP> @@ <SEP> en
<tb><SEP> Ex <SEP> n <SEP> Drying <SEP> pH
<tb><SEP> CTAB <SEP> cm3 / g
<tb> 1 <SEP> flash <SEP> 192 <SEP> 4.5 <SEP> 4.31
<tb>: <SEP> 2 <SEP>: <SEP>"<SEP>:<SEQ> 205 <SEP>: <SEP> 4,7 <SEP>: <SEQ> 4,12
<tb>: <SEP> 3 <SEP>: <SEP>"<SEP>:<SEP> 285 <SEP>: <SEP> 5.5 <SEP>: <SEP> 4.14
<tb>: <SEP> 4 <SEP>: <SEP>"<SEP>:<SEP> 280 <SEP>: <SEP> 5.0 <SEP>: <SEP> 3.92
<tb>: <SEP> 5 <SEP>: <SEP>"<SEP>:<SEP> 150 <SEP>: <SEP> 5.2 <SEP>: <SEP> 4.08
<tb>: <SEP> 6 <SEP>: <SEP>"<SEP>:<SEP> 155 <SEP>: <SEP> 5.5 <SEP>: <SEP> 3.83
<tb>: <SEP> 7 <SEP>: <SEP> atomic. <SEP>: <SEP> 282 <SEP>: <SEP> 5.0 <SEP>: <SEP> 4.04
<tb>: <SEP> 8 <SEP>: <SEP> flash <SEP>: <SEP> 271 <SEP>: <SEP> 4.3 <SEP>: <SEP> 5.01
<tb>: <SEP> 9 <SEP>: <SEP>"<SEP>:<SEP> 190 <SEP>: <SEP> 4.5 <SEP>: <SEP> 4.50
<tb>: <SEP> 10 <SEP>: <SEP>"<SEP>:<SEQ> 185 <SEP>: <SEP> 4.3 <SEP>: <SEP> 4.33
<tb>: <SEP> 11 <SEP>: <SEP>"<SEP>:<SEP> 221 <SEP>: <SEP> 5.3 <SEP>: <SEP> 4.56
<tb>: <SEP> 12 <SEP>: <SEP>"<SEP>:<SEP> 211 <SEP>: <SEP> 4.5 <SEP>: <SEP> 4.25
<tb>: <SEP> 13 <SEP>: <SEP> oven <SEP>: <SEP> 187 <SEP>: <SEP> 4,5 <SEP>: <SEP> 3,81
<tb>: <SEP> 14 <SEP>: <SEP>"<SEP>:<SEP> 180 <SEP>: <SEP> 4.8 <SEP>: <SEP> 3.52
<tb>: <SEP> 15 <SEP>: <SEP> flash <SEP>: <SEP> 210 <SEP>: <SEP> 5 <SEP>: <SEP> 4,79
<tb>: <SEP> 16 <SEP>: <SEP>"<SEP>:<SEP> 205 <SEP>: <SEP> 5.1 <SEP>: <SEP> 4.65
<tb>: <SEP> 17 <SEP>: <SEP>"<SEP>:<SEP><SEP> 221 <SEP>: <SEP> 4.8 <SEP>: <SEQ> 4.82
<tb>: <SEP> 18 <SEP>: <SEP>"<SEP>:<SEP> 212 <SEP>: <SEP> 4,7 <SEP>: <SEQ> 4,78
<tb>: <SEP> 19 <SEP>: <SEP>"<SEP>:<SEP> 215 <SEP>: <SEP> 7.5 <SEP>: <SEP> 4.32
<tb>: <SEP> 20 <SEP>: <SEP>"<SEP>:<SEP> 192 <SEP>: <SEP> 4.8 <SEP>: <SEP> 4.58
<tb>: <SEP> 21 <SEP>: <SEP>"<SEP>:<SEP> 120 <SEP>: <SEP> 4.8 <SEP>: <SEP> 4.18
<Tb>
TABLE 2

<tb> T.
<Tb>

<SEP> A <SEP> B <SEP> C <SEP> D <SEP> E <SEP> F <SEP> G
<tb> Ex.
<Tb>

1 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP> +
<tb> 2 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP>
<tb> 3 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP> +
<tb> 4 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP>
<tb><SEP> 5 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP> +
<tb> 6 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP> +
<tb><SEP> 7 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> + <SEP> - <SEP> +
<tb><SEP> 8 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> - <SEP> + <SEP> +
<tb> 9 <SEP> + <SEP> - <SEP> + <SEP> - <SEP> - <SEP> + <SEP> +
<tb> 10 <SEP> + <SEP> - <SEP> + <SEP> - <SEP> - <SEP> + <SEP> +
<tb> 11 <SEP> + <SEP> - <SEP> - <SEP> - <SEP> - <SEP> + <SEP> +
<tb><SEP> 12 <SEP> + <SEP> - <SEP> - <SEP> - <SEP> - <SEP> + <SEP>
<tb> 13 <SEP> + <SEP> - <SEP> + <SEP> - <SEP> - <SEP> + <SEP> +
<tb> 14 <SEP> + <SEP> - <SEP> + <SEP> - <SEP> - <SEP> + <SEP> +
<tb><SEP> 15 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> - <SEP> + <SEP> +
<tb> 16 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> - <SEP> + <SEP> +
<tb> 17 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> - <SEP> + <SEP>
<tb> 18 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> - <SEP> + <SEP> +
<tb> 19 <SEP> + <SEP> - <SEP> + <SEP> + <SEP> - <SEP> + <SEP> +
<tb><SEP> 20 <SEP> - <SEP> + <SEP> - <SEP> - <SEP> + <SEP> - <SEP> +
<tb> 21 <SEP> - <SEP> + <SEP> - <SEP> - <SEP> + <SEP> - <SEP>
T. = treatment
A = acid in silicate
B = simultaneous addition
C = Opalescence stop
D = Water at stabilization
E - Heat treatment
F = Post-addition
G = Complexing

Claims (12)

 1) Process for obtaining a precipitation silica by the action of an alkali silicate solution. and an acidifying agent characterized in that the alkali silicate solution is free of metallic impurities. 2) Process according to 1, characterized in that the precipitation is carried out in the presence of a sequestering agent in an amount sufficient to complex at least partly the metal impurities. 3) Method according to one of claims 1 or 2, characterized in that the metal impurities consist of alkaline earth metals and in particular calcium. 4) Method according to one of claims 1 to 3, characterized in that the complexing agent is added in the initial solution of silicate. 5) Method according to one of claims 1 to 4, characterized in that the sequestering agent is of the group of the ethylene diamine tetracetic acid (EDTA) diethylenetriamine pentacetique (DTPA) nitrilotriacétique (NTA) and alkaline tripolyphosphate (TPP). 6) Method according to one of claims 1 to 5, characterized in that it causes a sudden dilution of the medium after the period of freezing. 7) Process according to claim 6, characterized in that the water is added to the reaction medium after the period of freezing. 8) Method according to one of claims 1 to 7, characterized in that an alkali silicate solution is added to an acid solution until opalescence appears, that one stops the addition at the point of opalescence, a period of ripening is observed to achieve a balance of reaction conditions, and the acid addition of the reaction medium is continued until the precipitation is complete. complete, and finally separating the silica obtained from the reaction medium. 9) Method according to one of claims 1 to 7, characterized in that an alkali silicate solution is added to an acid solution until maximum polymer formation and that there is observed at least one stop the addition of acid from this point. 10) Method according to one of claims 8 and 9, characterized in that beyond the point of interruption of addition of acid, is simultaneously added an alkali silicate solution and a solution of acid of such that the pH of the reaction medium is kept substantially constant until precipitation of the pigment is complete. 11) Method according to one of claims 1 to 7, characterized in that is simultaneously introduced an alkali silicate solution and an acid solution in a water glass solution prepared in advance. 12) New silica characterized in that it is obtained by implementing the method according to one of claims 1 to 11.