[1],united States Patent Office 31716,493 3,716,493 PROCESS FOR MAKING FINE SIZED LOW DENSITY SILICA Ellsworth G. Acker, Baltimore, and Milton Edward Winyall, Ellicott City, Md., assignors to W. R. Grace & Co., New York, N.Y. No Drawing. Continuation of abandoned application Ser. No. 731,266, May 22, 1968. This appucation Feb. 26, 1971, Ser. No. 119,407 Int. Cl. BOIJ 13100; COlb 33116 U.S. Cl. 252-317 4 Claims ABSTRACT OF THE, DISCLOSURE A solution of alkali metal silicate is partially neutralized with about 6% sulfuric acid to ,ibout pH 10.6-10.9, stirred vigorously and completely neutralized with more sulftiric acid. After neutralizatiorl- is complete, ammonia is added until a pH of greater than 7 is obtained. The thus formed slurry is then aged at about 80' C. for about I hour and filtered. The residue is washed with about 0. 1 % ammonium carbonate solution at about 80' C. and the thus washed solids are sized and dried. The resultant silica gel has a bulk density of about 0.03-0.10 -./cc., and a surface area of about 200-400 M.2/g. This application is a co-@itinuation of application Ser. No. 731,266, filed May 22, 1969, and now abandoned. This invention relates to a process for making silica. More specifically, this invention relates to a process for making fine s@'zed, relatively low density silica. Silica gel @as found many commercial and industrial applications, and its low cost and high utility have led to widespread production 14-owever, several areas in which silica gel could be very useful have not been sufficiently exploited. Provided that a silica gel having a very low bulk density and at the same time an intermediate to relatively high surface area were conveniently and cheaply available, such a gel coul@d be used extremely well as a paint flatting agent and in general as a resin thickening agent. Several processes have heretofore enjoyed widespread use for making silica gel. Foremost of these has been the method for preparing acid-set silica gel. This process comprises neutralizing an aqueous alkali metal silicate solution with a stron,@ mineral acid until a pH of about 1.5 has been reached. The thus acidified mixture then sets to a gel which is subsequently crushed to the desired particle size, washed with ammonia to rernove the entrapped salts, and dried. Fine sized gels resulting from such a process generally have a bulk density of about 0.25 g./ce. and a surface area of about 300 M.2/g. These gels are also very hard and hence dffficult to size into fine particles. it would therefore be desirable and advanta.@eous to be able to obtain readily a silica gel having a bulk density of about 0.03 to 0.10 g./cc. and a surface area of about 200 to 400 M.2/g., and which lends itself readily to sizing into particles of about 0.5 to 20 microns. It is therefore an object of this invention to provide an improved process for making silica gel. A further object of this invention is to provide a process for making a silica gel having a relatively low bulk density, intermediate to relatively large surface area, and which is easily sized into fine particles In summary, this inven+ion is a process for making silica gel havir@g a bulk density oj' about 0.03-0.10 g./ce. a surface area of about 200-400 m-2/g., and which can be sized to particles of about 0.5-20 microns. The process comprises: (1) Preparing anaqueous alkali metal silicate solution; Patented Feb. 13, 1973 2 (2) PartialIy neutralizing said solution to a pH of about 10.6- 10.9 to obtain an alkaline silica hydrosol; (3) Permitting said bydrosol to set to a gelatinotis mass of aikaline-set hydrogel; k'4) Agitating said gelatinous mass to obtain a slurry of finely divided particles of alkalive-set hydrogel; (5) Adding additional acid to said slurry of alkaline-set hydrogel to completely neutralize said alkali metal silicate in said slurry; 10 (6) Making the thus neutralized slurry alkaline by adding aqueous ammonia; (7) Aging said ammoniacal slurry to obtain a slurry of aged silica hydrogel particles; (8) Washing said aged hydro,-el particles to obtain salt15 free silica hydrogel particles; and (9) Drying said salt-free particles to obtain silica gel having a bulk density of about 003-0.10 g./cc., a surface area of about 200-400 M.2/g., and which can be easily sized to about 0.5-20 microns. 20 Any alkali metal silicate may be used as the silica source for this inverition, but preferably sodium silicate is employed. The aqueous silicate solutions contemplated may be about 15-24% SiO2 by weight and are typically about 20% SiO2 by weight. 25 The initial nelitralization is accomdlished by adding a strori- mineral acid to the aforementioned silicate solution until said solution has a pH of about 10.6-10.9. Typically, enou,-h 4-9% sulfuric acid is added to the solution to result in a pH of about 10.8. 30 This alkaline silica hydrosol will then set to a hydrogel. Before said hydro.-el becomes hard, and while it is in an easily dispersible, gelatinous state, it should be vi-orously physically agitated, thereby disintegratin-, it into v'ery fine - elatinous particles and dispersing same to form a slurry. 35 gaid phvsical agitation is best performed by any means of a rapid-stirring apparatus. When a fine slurry of gel particles has been formed, enough mineral acid is added to at least neutralize all of the alkali metal silicate initially present, and preferably, a 40 20% excess of said acid is added. Typically.. enough about 6% aqueous sulfuric acid is added to brin-, the pH of the resultant slurry to about 0.5-4. The thus neutralized silica -el slurry is made alkaline 45 by adding enou,ah aqueous ammonia to cause a pH greater than 7.0. In a preferred embodiment of this invention, an aqueous ammonia solution of about 20% NH3 by weight is added until the resulting slurry has a pH of about 8-10.5. 50 Aging of the thus formed ammoniacal slurry is accomplished by stirring said slurry for about 0.5-1.5 hours at a temperature of about 60-95' C. Typically aging is done in a closed container to preclude the evaporation of water, althou.- h this technique is not mandatory for successful 55 a.-ing. Any suitable means may be employed to separate the suspended solid phase from the ammoniacal liquid phase of the slurry. Some such means which are practical are filtration, centrifugation, and decantation. We have found 60 filtration especially applicable to this invention because the separated solids can th-,n easily be washed free of residual salts while said solids are still retained on the filter. It is important that the thus separated silica -el particles 65 be washed free of residual salts. Said salts are primarily those formed during the aforementioned neutralization process. In the case where sulfuric acid is the mineral acid employed for neutralization, said salts consist essentially of alkali r@ietal sulfate. Salt removal can be effectively 70 achieved by washing with hot (about 60-95' C.) water and preferably with hot (about 60-95' C.) about 1.5% aqueous ammonium sulfate solution. A particularly effec-
[2]3,716,493 3 tive washing technique is initially washing with hot aqueous ammonia sulfate, as stated supra, followed by washing with hot (about 60-95' C.), about 1.5% aqueous ammonium carbonate solution. Drying of the desalted silica gel may be accomplished at any temperature sufficient to remove moisture therefrom. Preferably, the temperature should be high enou,,Ii to decompose any residual ammonium carbonate, i.e. -.reater than 58' C.; and ideally said dryiiig is performed at about 100-250' C. It is important that said drying operation be carried out as rapidly as possible, for example, within about three minutes. Slow dryin-, for example i@i an oven at a temperature of about 100-150' C., requires about 15-20 minutes to complete drying. Drying of the silica gel in this manner dobs not give the desired relatively low density and relatively high surface area@ Typical of mellhods employed to achieve said rapid drying, and hence to achieve said relatively low density and said relative high surface area: (1) Spray drying (requiring about 3 seconds). (2) Fluid energy milling (reqtiires about I second), Hence, by rapidly drying the gel is meant subjecting @aid gel to a drying process requiring not longer than about 3 minutes to achieve complete drying. The silica gel resultin@ from the hereinbefore described process is characterized by haviiig a bulk density of about 0.03-0. 1 0 g. /cc., and a surface area of about 200400 M.2/g. Said silica ael may be further characterized as having a pore volume of about 1.5-2.5 cc./g. By sizing of the silica gel encompassed by the instant invention is meant any means of changin.- the average size of the particles resulting from the aforementior@ed process. For example, silica gel made by the process herein described can be easily ground in a fluid energy mill to an average particle size of about 0.5-1.5 microns. If particles larger than those resulting from this process are desired, said larger particles rnay be prepared by spray drying. Using this technique, particles up to about 20 microns can be prepared. The following examples are provided to further describe and point out the various aspects of this invention. They are not intended to in any way limit or narrow the scope of said invention. EXAMPLE I A solution of 2500 ml. of 40' B6 sodium silicate (about 28% SiO2) was diluted with 1500 ml. of water to form a solution of about 20% SiO2. A solution of sulfuric acid, prepared by diluting 100 ml. concentrated (about 35.9 N) with 2900 ml. H20, was added to the stirred sodium silicate solution to form an alkaline silica hydrosol having a pH of about 10.8. After all the acid was added ' the sol was allowed to gel. After about 15 minutes, a soft gel had set arld further stirridg dispersed said -el into a slurry of very-fine gel particles. A second solution of sulfuric acid, prepared by diluting 250 inl. concentrated H2SO4 (about 35.9 N) with 6000 ml. water, was then added to the alkaline silica gel slurry and the rnixture was heated to about 80' C. The resultant acidic hydrogel slurry had a pH of about 2.0. Next, 400 ml. concentrated aqueous ammonia (30% NH3) was added to said acidic hydrogel slurry to form an alkaline hydro.@el slurry having a pH of about 9.0. This slurry was then aged at about 80' C. for about I hour and filtered on a Buchner funnel using partial vacuum. The collected solids were then washed with a solution of about 0.5% ammonium carbonate at about 80' C. The thus washed silica gel particles were then mixed with enou@h water to form a slurr y having about a 7% by wei,@ht silica content and spray dried at 165' C. EXAMPLEZ The procedure of Example I was repeated except the filtered particles were washed with about 0. I % ammoni4 um sulfate solution at about 80' C., then washed with about 0.1% ammonium carbonate solution at about 80' C., and dried in a fluid energy mill. EXAMPLE 3 The spray dried silica @el of Example I had a particle size of about 1-40 microns, a surface area of about 215 M.2/g., a pore volume of about 2.5 cc./g., and a bulk density of about 0.04 g./cc. 10 The fluid energy milled silica gel of Example 2 had an average particle size of about 1 micron, a surface area of about 250 M.2/ g., a pore volume of about 1.8 cc./g., and a bulk density of about 0.03 g./cc. Because of the properties unique to the silica gel pre15 pared in this invention (i.e. very low bulk density,- intermediate to relatively hi,-h surface area, and ease of sizing) said gel lends itself readily to many useful applications. It is particularly adaptable to use as a paint flatting agent, as an anticaking additive in comestible preparations 20 such as dry cake mixes, and as an anti-offset agent in the printing industry. Other uses include as resin filler and thickening agents. What is
