Thixotropic Refractory Binder Based Upon Aluminum Phosphate Gelled Silica Sols

A highly thixotropic binder has been developed for refractory compositions. The binder is especially suited to vibration casting of monolithic shapes, but it may also be used for gunning, trowelling, ramming or manufacture of preformed shapes. The main advantages of the binder are the high strengths and good resistance to chemical attack which it imparts to the material.

An improved refractory binder comprising:

As indicated, the invention utilizes aqueous colloidal silica sols which have an average particle size of at least 35 millimicrons with a preferred particle size range being within the range of 50-70 millimicrons. The silica concentration of the sol expressed as SiO.sub.2 should be at least 40% and, preferably, about 50% by weight. While the pH is not believed to be critical, it is preferred that the sols be alkaline, e.g. have a pH greater than 8. A typical silica sol useful in the practice of the invention is sold by Nalco Chemical Company under the name, NALCOAG.RTM. 1060, and has the following general description:

The aqueous colloidal silica sols are well known materials and are further described in the textbook, The Colloidal Chemistry of Silica and Silicates, by Ralph K. Iler, Cornell University Press, 1955. The disclosure of this treatise is incorporated herein by reference.

As indicated, the amount of phosphoric acid used to prepare the binders of the invention should be an amount to provide a P.sub.2 O.sub.5 content within the range of 5-35%. It is preferred to use relatively concentrated phosphoric acid solutions. Hence, I prefer to use a phosphoric acid which contains at least 70% phosphoric acid and, most preferably, 80% or more.

The alumina may be any hydrated alumina which is used in an amount sufficient to provide at least 5-15% by weight to the finished binder composition. The alumina is conveniently obtained from the Bayer process which dissolves bauxite or other alumina-containing ore in caustic and then precipitates the alumina. This provides an alumina hydrate of good purity and quantity. While particle size is not critical, it is convenient to use an alumina having about 30 mesh Tyler sieve size.

While it is most preferred that pure alumina be used, it is possible to substitute therefor alumina-containing materials such as high alumina clays so as not more than about 50% of the alumina substitute is used.

The binder of the invention may be considered as an aluminum phosphate gelled colloidal silica. Aluminum phosphate in its various forms and species, as well as its method of manufacture, is set forth in U.S. Pat. No. 2,460,344, the disclosure of which is incorporated herein by reference. While both colloidal silica and aluminum phosphate per se have been used as refractory binders for many years, the combination of these two materials seems to produce a binder that is quite remarkable in that it is capable of binding a wide variety of alumina-containing refractories which produce superior products.

The binders of the invention can be prepared conveniently by adding the alumina and the phosphoric acid directly to the colloidal silica sol using good agitation and a slow rate of addition. Also, the alumina can be added to the phosphoric acid or by other suitable means combined therewith to pre-prepare an aluminum phosphate which then would be added to the silica sol to form the compositions of the invention.

The finished compositions of the invention range in physical properties from rubbery-like gels to thick viscous liquids resembling maple syrup on a cold day with respect to their viscosity characteristics. In all cases the preferred binders of the invention can be converted to flowable, easily handled liquids by the application of shear in the form of high speed stirring and the like. When the phosphoric acid, alumina and colloidal silica are mixed, the product may gel in periods of time ranging between several hours to several months. In other instances the product will not gel but merely will thicken. In using the invention as a binder for various refractories, it is also possible to mix the phosphoric acid, alumina and colloidal silica directly with the other components of the formula, allowing the compositions to pre-form in situ.

The binders of the invention are particularly useful in producing a large number of refractory products which contain large quantities of alumina, e.g. at least 40% by weight. As indicated, the binders are useful in producing monolithic shapes as binders and for gunning, troweling and ramming mixes of the type used in steel mills where refractories contact molten steel or iron.

To illustrate typical refractory products which utilize the binder compositions of the invention, typical compositions are presented below in Table I.

Listed below are the various uses for which the compositions of the invention would be suitable: