Abrasion Resistant Refractory Composition

This invention relates to a refractory composition characterized by high abrasion resistance and relatively low thermal conductivity, and in particular, to such a composition which may be used as refractory castable and gunning mixes.

Refractory castables are hydraulic setting compositions. They comprise granular refractory aggregates and chemical binders. The refractory castables are shipped in dry form, and when mixed with water to the desired consistency, may be poured like concrete, tamped or rammed into place, troweled or applied with an air gun. Refractory castables take a strong hydraulic set at room temperatures and maintain good strength until the desired ceramic bond is developed as the temperature is increased. Castables are specially suited for furnace linings of irregular contours, for patching brick work and for casting special shapes which may be urgently required. Numerous castable compositions are known, with each of the known compositions having different properties, making each one useful for different applications.

One such application involves the use of refractory castables in lining transfer lines employed in fluid catalytic cracking and fluid coking units used in petrochemical processes. In such units, highly abrasive catalysts or coke travel at high speeds thereby creating extreme erosion potential throughout the catalytic cracking unit. In such units, early abrasion resistant linings were of a dual layer construction consisting of dense hydraulic phosphate bonded refractory facing with insulating back-up which required extensive anchoring and hand ramming to install. To reduce the expense of dual layer linings, the refining industry began using castables of an intermediate density with field additions of stainless steel fibers which required less anchoring in the metal shell, and which could be poured relatively quickly. Although the foregoing improved on the time and cost of installation, lower conductivity was desired.

Abrasion resistant refractory linings in petrochemical vessels are typically chemically hydraulic or phosphate bonded refractory compositions. Abrasion resistance is generally obtained by utilizing a strong, dense refractory grain such as calcined fireclay or tabular aluminum, bauxite, MgAl and a strong bond consisting of aluminum orthophosphate or calcium aluminate cement. In the case of cement, the abrasion resistant bond is achieved by using large amounts of cement, or a combination of fumed silica, cement in amounts less than ten percent, and a surface active agent which allows flow at low water contents. Improved density and subsequent high thermal conductivity values, which are achieved by casting at low water contents results in a highly abrasion resistant bond at low cement levels.

In petrochemical applications, it is desirable that the refractory linings have low thermal conductivity properties. Unfortunately, both density and thermal conductivity generally are directly proportional so that when high bulk density and abrasion resistance are achieved, thermal conductivity is undesirably high. Both density and thermal conductivity may be lowered by the substitution of light weight grain for some of the calcined fireclay, but this has an adverse affect on abrasion resistance.

In addition to cast applications, gunning mixes are used in situations of limited access or where they offer advantages in application efficiency and in associated costs. Dense, homogeneous monolithic linings can be gunned without the use of forms and with a marked savings in time. Gunning mixes used in lining components of fluid catalytic cracking and fluid coking units should possess the same properties of low thermal conductivity and abrasion resistance as the refractory castables.

Accordingly, it is an object of this invention to provide a refractory composition characterized by high abrasion resistance and low thermal conductivity. The foregoing objective is achieved in a refractory composition comprising 40-96% by weight amorphous silica, 2-50% by weight -65 mesh calcined high density refractory fines, and 2-40% by weight calcium aluminate cement.

The utilization of refractory castable compositions in highly abrasive environments requires the castables to have excellent abrasion resistance properties, so that unit operation is reliable and not subject to high maintenance. In addition, the refractories are also required to have relatively low thermal conductivity for certain applications to afford unit thermal efficiency. Previous work indicated that a composition including amorphous silica should be considered when developing a refractory castable requiring excellent abrasion resistance and relatively low thermal conductivity.

A first series of mixes was prepared (see Table I below). The mixes A, B, C, D and E included calcined fireclay, vitreous silica and casting grade cement in various proportions. The calcined fireclay used in mixes A through D was mined in Alabama and has essentially 50 weight percent alumina content. In mixes A, B and C, the weight percent of the calcined clay added to the refractory composition, comprised 35%, 25% and 15% respectively. In mixes D and E, 25% and 15%, by weight, of the calcined clay was included in the mix, but in the later two mixes, the fireclay calcines were made as fine as possible, while maintaining the desired screen analysis. This placed the silica in the coarser (+65 mesh) fractions. Bulk density, strength and abrasion resistance generally increased with increasing fireclay content. However, since thermal conductivity also increases with density, maximum abrasion resistance and minimum thermal conductivity could not be optimized simultaneously. Accordingly, the object of the test work was to obtain the lowest possible thermal conductivity at an abrasion resistance after 1500.degree. F. reheat of 14 cc maximum volume loss. Mixes C and E had the lowest densities while reaching the abrasion resistance goal. Thermal conductivity tests were conducted on mixes C and E, but not on A, B and D. In analyzing the abrasion test resistance results, it should be noted that mix E had a 4 cc volume loss after drying at 250.degree. F., whereas mix C had a 5.8 cc volume loss. Accordingly, it was concluded that mix E would provide a better quality commercial castable. Further, since mix E included substantially 100% of the calcined clay in the fines (-65 mesh) as compared to mix C, the production of mix E would be much easier when compared to the production of mix C. The constituents of each mix are provided on a weight percent basis.

Based upon the above test results, mix E was considered an excellent candidate for a refractory castable mix. While each of the mixes tested comprised 35 weight percent cement, and the preferred mix comprised 15 weight percent ball milled calcined clay, and 50 weight percent vitreous silica, the range of each of the constituents of the mix may vary as follows: 40-96 weight percent +65 mesh amorphous silica, 2-50 weight percent -65 mesh calcined high density refractory aggregate fines of at least 140 lb/ft.sup.3, such as fireclay, tabular alumina, bauxite, MgAl or mixtures thereof, and 2-40 weight percent calcium alumina cement.

Additional work was conducted in an attempt to utilize mix E composition for producing both refractory castables and gunning mixes. In addition, fused silica was substituted for vitreous silica, in an attempt to determine if amorphous forms of silica, other than vitreous silica, could also be utilized. In reviewing the data developed in Table II below, mixes A through F included fused silica whereas, mix G included for comparison purposes, vitreous silica. The additions of various additives to the mixes were as illustrated. Mix C gunned well and shows superior properties compared to mix A which did not contain the gunning additive. Mix C had properties similar to mix E of Table I. In mixes B and D, a vinyl chloride acetate additive used in some mixes to improve spalling resistance was added; however, the resultant mixes showed some loss of strength and abrasion resistance. The foregoing was particularly evidenced in mix D which contained the gunning additive calcium chloride dihydrate as well as the vinyl chloride acetate additive. Mixes E and F included stainless steel fibers; these mixes had respectable properties. In comparing mixes C and F, it should be noted that the addition of steel fibers to mix F had a minor adverse affect on properties. In comparing mixes C and G (fused silica versus vitreous silica) it should be noted that the fused silica based mix had superior properties.

In addition to the properties of high abrasion resistance and low thermal conductivity, certain applications require compositions to have good flowability and long working times. Such properties are required in lining transfer lines of fluid catalytic cracking units. Mix H identified below, was formed in an attempt to obtain all the desired properties in one mix. Mix H is based upon mix E previously discussed. In addition, Mix H includes volatilized silica and -325 mesh alumina. The -325 mesh alumina is provided to improve the abrasion resistance, while both the volatilized silica and alumina are provided to improve the flowability.

Tests disclosed in co-pending application U.S. Ser. No. 748,260, filed 6/24/85, now abandoned, entitled "Abrasion Resistant Refractory Composition, Thomas Russell Kleeb, inventor, it was determined that the preferred ranges for the volatilized silica and -325 mesh alumina, are about 0.5 to 5.0 weight percent, and about 3.0 to 15 weight percent, respectively.

In the preferred embodiment, ALCOA's A-17 reactive alumina was used to obtain the -325 mesh alumina of the mix. The A-17 reactive alumina is almost entirely composed of fine, sintered corundum (alpha-alumina) crystals. Their high surface area and small crystal size makes them thermally reactive, that is, they will further sinter or react with other compounds at relatively low temperatures. Tabular alumina and calcined alumina may also be used as the alumina constituent of the mix.

The composition of the present invention provides a refractory castable or gunning mix which may be used in applications requiring high abrasion resistance and low thermal conductivity properties; such properties are required in transfer lines of fluid catalytic cracking and fluid coking units.

The utility of the present invention in energy savings may be seen from the following example in which it is assumed that conventional prior art material has a thermal conductivity of ##EQU1## and the presently claimed material has a thermal conductivity of only ##EQU2##

Heat Saving--A comparison of prior art and patent composition E (as per Table I) ##EQU3## Assume 5" lining thickness with 1350.degree. F. hot face. Assume 80.degree. F. still air ambient.

Assume heat value of $5 per MBTU.

In the present specification, all mesh sizes have been determined in accordance with Taylor Standard Series.

While a preferred embodiment of the present invention has been described and illustrated, the invention should not be limited thereto but may be otherwise embodied within the scope of the following claims.