Silico-aluminate containing aggregates for production of monolithic refractory compositions, their method of production and their use

Abstract

The present invention relates to a porous aggregate, comprising Al.sub.2O.sub.3, SiO.sub.2 and optionally Fe.sub.2O.sub.3, having a d.sub.50 of equivalent pore diameter between 1 ?m or more and 50 ?m or below and a total porosity between 20% and 60%, for use in the formation of monolithic refractories. Also part of the invention is a method of formation for said aggregates, their use in the formation of monolithic refractories and monolithic refractories comprising such aggregates.

Claims

1. A porous aggregate comprising from 70 to 95 wt.-% Al.sub.2O.sub.3, from 3 to 10 wt.-% SiO.sub.2 and optionally up to 5 wt.-% Fe.sub.2O.sub.3, having a d.sub.50 of equivalent pore diameter between 1 ?m or more and 50 ?m or below, a total porosity between 20% and 60%, and an apparent density of 2.5 g.Math.cm.sup.?3 or less.

2. A porous aggregate according to claim 1 having a d.sub.10 equivalent pore diameter of 30 ?m or below.

3. A porous aggregate according to claim 1 having an open porosity of 20% or more.

4. A porous aggregate according to claim 1, which is additionally coated with a polymers and which has an open porosity of 20% or less.

5. A porous aggregate according to claim 4, wherein said polymer is a petroleum by-product.

6. A porous aggregate according to claim 1 having corundum and mullite as primary phases.

7. A porous aggregate according to claim 1, which maintains a porosity of 20% or above and/or a d.sub.50 of equivalent pore diameter of 50 ?m or below through subsequent firings at a temperature of 1300? C. or below.

8. A method of preparation of a porous aggregate, comprising the steps of (a) providing a particulate mixture of (i) a pulverised alumina containing base material; (ii) a pore former; and (iii) metal hydroxide; (b) adding a binder to said particulate mixture in a pelletiser in order to obtain a pelletised mixture; and (c) firing the pelletised mixture obtained at the end of step (b) in order to obtain a porous aggregate; wherein the porous aggregate comprises from 70 to 95 wt.-% Al.sub.2O.sub.3, from 3 to 10 wt.-% SiO.sub.2 and optionally up to 5 wt.-% Fe.sub.2O.sub.3, having a d.sub.50 of equivalent pore diameter between 1 ?m or more and 50 ?m or below, a total porosity between 20% and 60%, and an apparent density of 2.5 g.Math.cm.sup.?3 or less.

9. A method according to claim 8, further comprising a step of (d) coating the fired pellets obtained at the end of step (c) with a hydrophobic polymer.

10. A method according to claim 9, wherein said polymer is a polymer selected from the group of thermosetting binding agents, thermo-hardening binding agents and multi-component reacting binding agents.

11. A method according to claim 8, wherein the pulverised alumina containing base material is selected from silico-aluminates, pure alumina, and mixtures thereof.

12. A method according to claim 8, wherein the pulverised alumina containing base material comprises a calcined bauxite having a d.sub.90 of 200 ?m and a d.sub.50 of 45 ?m or lower, and/or a raw bauxite having a d.sub.90 of 1000 ?m and a d.sub.50 of 500 ?m or lower and/or wherein the pulverised bauxite comprises greater than 80 wt.-% Al.sub.2O.sub.3, less than 10 wt.-% SiO.sub.2 and less than 5 wt.-% Fe.sub.2O.sub.3 after calcination.

13. A method according to claim 8, wherein the said pore former is selected from the group of carbohydrates, carbon black, polymer particles, cereal flour and mixtures thereof.

14. A method according to claim 8, wherein the pore former is an ultrafine monosaccharide or polysaccharide having a d.sub.90 of 150 ?m or less and a d.sub.50 of 45 ?m or less.

15. A method according to claim 8, wherein the metal hydroxide is selected from the group aluminium hydroxide, aluminium oxide hydroxide and hydrated alumina.

16. A method according to claim 8, wherein the metal hydroxide has a d.sub.90 of 60 ?m and a loss on ignition at 1000? C. of 40% or less.

17. A method according to claim 8, wherein the mixture of step (a) consists of from 65 to 90 wt.-% pulverised alumina containing base material, from 5 to 20 wt.-% pore former and from 5 to 20 wt.-% metal hydroxide, based on the total weight of the mixture.

18. A method according to claim 8, wherein in step (b), the amount of binder added is no more than 15 wt.-%, on the basis of the total weight of the said particulate mixture, such as for example from 2 wt.-% to 10 wt.-%.

19. A method according to claim 8, wherein in step (c), the firing is carried out at a temperature of 1200? C. or more, for a duration of 2 hours or more.

20. A method of using the porous aggregate according to claim 1, in the preparation of a monolithic refractory composition.

21. A method according to claim 20, wherein the said porous aggregate forms 60 wt.-% or less of the said monolithic refractory composition.

22. A method according to claim 20, wherein the said monolithic refractory composition comprises 70 wt.-% or more Al.sub.2O.sub.3, 20 wt.-% or less SiO.sub.2 and 5 wt.-% or less CaO.

23. A method according to claim 20, wherein the said monolithic refractory composition has a density of less than 2.8 t.Math.m.sup.?3 at 800? C.

24. A method according to claim 20, wherein the said monolithic refractory composition has a thermal conductivity of 3 W.Math.m.sup.?1.Math.K.sup.?1 or less.

25. A monolithic refractory composition comprising the porous aggregate of claim 1.

26. A porous aggregate according to claim 4, wherein the polymer is selected from thermosetting binding agents, thermo-hardening binding agents, and multi-component reacting binding agents.

27. A porous aggregate according to claim 5, wherein the polymer is a petroleum by-product polymer that is a hydrocarbon containing from 20 to 40 carbon atoms per molecule.

Description

SHORT DESCRIPTION OF THE FIGURE

(1) The invention will be further illustrated by reference to the following figures:

(2) FIGS. 1 and 2 represent pore size distribution curves as measured in porous aggregate pellets according to the present invention.

(3) It is understood that the following description and references to the figures concern exemplary embodiments of the present invention and shall not be limiting the scope of the claims.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present invention according to the appended claims provides porous aggregates for use in the formation of refractory compositions, a method for forming said porous aggregates, a composition provided in said method, as well as the use of the porous aggregates in the formation of monolithic refractories.

(5) The porous aggregates are alumina based silica containing aggregates. They may comprise from 75 to 95 wt.-% Al.sub.2O.sub.3, such as from 78 to 92 wt.-% Al.sub.2O.sub.3, or from 80 to 90 wt.-% Al.sub.2O.sub.3, such as for example 85 wt.-% Al.sub.2O.sub.3 or more. They may further comprise from 3 to 10 wt.-% SiO.sub.2, such as between 4 to 8 wt.-% SiO.sub.2, or from 4.5 to 6 wt.-% SiO.sub.2, such as for example 5 wt.-% SiO.sub.2 or more. They may also optionally comprise other metal oxides, such as for example no more than 5 wt.-% Fe.sub.2O.sub.3, or no more than 3 wt.-% Fe.sub.2O.sub.3. According to one embodiment, the porous aggregates may further comprise no more than 5 wt.-% TiO.sub.2, or no more than 3 wt.-% TiO.sub.2.

(6) The porous aggregates according to the present invention may have a d.sub.50 of the equivalent pore diameter of 50 ?m or less, such as for example 30 ?m or less, or 15 ?m or less, or even 10 ?m or less. As used herein, the pore volume, pore volume distribution and equivalent pore diameter being measured by mercury intrusion porosimetry method (ASTM D4404-10).

(7) The porous aggregates according to the present invention may have an open porosity of 20% or more, such as 25% or more, and a total porosity of from 20% to 60%, for example from 25% to 50%, or from 30% to 45%, such as about 40% or more. As used herein, the open porosity as well as total porosity is measured by the Archimedes principle (ASTM C20).

(8) The porous aggregates according to the present invention may have an apparent density of 2.5 g.Math.cm.sup.?3 or less, such as for example 2.1 g.Math.cm.sup.?3 or less, or 2.0 g.Math.cm.sup.?3 or less, or even 1.8 g.Math.cm.sup.?3 or less. In one embodiment, the apparent density of the porous aggregates according to the present invention may be from 1.5 to 2.5 g/cm.sup.3, or from 1.0 to 2.2 g/cm.sup.3. As used herein, the apparent density is also measured by the Archimedes principle (ASTM C20).

(9) The porous aggregates according to the present invention may in one embodiment be coated with a polymer. Incorporation of aggregates with open porosity induces a higher requirement for casting water during placement of the refractory monolithic. This additional water leads to a rise in uncontrolled porosity and to a deterioration of the key monolithic properties (mechanical strength, abrasion resistance, infiltration resistance to molten slag/metal). The coating provided on the aggregates allows to avoid this drawback and to use same level of water than for castables comprising non porous aggregates. By coating the porous aggregates, this may be helpful in order to prevent water absorption into the aggregates. After coating the porous aggregates with a polymer, many previously open pores become closed or internal pores, and open porosity may be reduced to 20% or below, or to 10% or below, such as for example 5% or below. In one embodiment, the open porosity may be from 1 to 20%, or from 2 to 10%, or from 3 to 5%. The coating prevents water absorption during monolithic casting. This may avoid the need for providing additional water addition to reach equivalent flow and placement properties.

(10) The porous aggregates according to the present invention may be present in the shape of individually shaped pellets, such as pellets obtained from a pelletiser known to the skilled person in the art. The pellets may have a pellet diameter of 1 mm, or of 2 mm, or for example anywhere within the range from 0.5 mm to 20 mm, such as for example 5 mm.

(11) Unless otherwise stated, the equivalent particle diameters (d.sub.90, and d.sub.50-values) referred to herein are as measured in a well known manner by sedimentation of the particulate material in a fully dispersed condition in an aqueous medium using a Sedigraph 5100 machine as supplied by Micromeritics Instruments Corporation, Norcross, Ga., USA (web-site: www.micromeritics.com), referred to herein as a Micromeritics Sedigraph 5100 unit. Such a machine provides measurements and a plot of the cumulative percentage by weight of particles having a size, referred to in the art as the equivalent spherical diameter (esd), less than given esd values. The mean particle size d.sub.50 is the value determined in this way of the particle esd at which there are 50% by weight of the particles which have an equivalent spherical diameter less than that d.sub.50 value.

(12) According to one aspect of the present invention, the porous aggregates may be formed using the method of the invention. The invention therefore provides a method for preparing pelletised porous silico-alumina aggregates to form refractory compositions such as monolithic refractory compositions for use, for example, as linings of metallurgical vessels. The porous silico-alumina aggregates obtained according to the method of the present invention may be used such that they constitute the bulk of a formed monolithic refractory composition, or such that they constitute only a portion of a monolithic refractory composition comprising further constituents, such as for example 60 wt.-% or less of the said formed monolithic refractory composition, or 50 wt.-% or less of the formed said monolithic refractory composition, such as for example from 5 wt.-% to 40 wt.-% or from 20 wt.-% to 30 wt.-% of the said monolithic refractory composition.

(13) According to the present invention, it has been found that porous silico-alumina aggregates for forming refractory compositions may be obtained by providing a pulverised alumina containing base material, a pore former and metal hydroxide as components of a particulate mixture. A binder is then added to said particulate mixture in a pelletiser and it is subsequently pelletised in said pelletiser. The obtained pellets are then fired in order to obtain the porous aggregates according to the present invention.

(14) In one embodiment, the particulate mixture used in the method may comprise or consist of 65 wt.-% to 90 wt.-% pulverised alumina containing base material, 5 wt.-% to 20 wt.-% pore former and 5 wt.-% to 20 wt.-% metal hydroxide, based on the total weight of the mixture. Depending on the nature of the pulverised alumina containing base material, the amount used may vary from 60 wt.-% to 80 wt.-%, such as for example 70 wt.-% or 72 wt.-% pulverised alumina containing base material or more. The amount of pore former used may vary, depending on the nature of the pore former, and may vary from 8 wt.-% to 18 wt.-%, or from 10 wt.-% to 15 wt.-%, such as for example 14 wt.-% or more. The amount of metal hydroxide used may vary, depending on the nature of the pore former, and may vary from 8 wt.-% to 18 wt.-%, or from 10 wt.-% to 15 wt.-%, such as for example 14 wt.-% or more.

(15) The said alumina containing base material may be selected from the group consisting of silico-aluminates, of silico-aluminate compound mixtures, mixtures containing pure alumina, silico-aluminate compounds, and mixtures thereof. The silico-aluminates may be selected from the group of raw clays (such as kaolinite, montmorillonite smectite, pyrophyllite, halloysite, illite, vermiculite, palygorskite), calcined clays (such as metakaolin or chamotte), raw or synthetic aluminosilicates (such as kyanite, silimanite or andalusite), raw or calcined bauxite, mixtures of pure alumina with raw or synthetic siliceous compounds (such as quartzite, tridymite, cristobalite, fused silica, sand, fume silica, silicon carbide), and mixtures thereof.

(16) In one embodiment of the present invention, the alumina based starting material is a pulverised calcined bauxite or a mixture of alumina containing material and calcined bauxite having a d.sub.90 of 200 ?m or less and a d.sub.50 of 45 ?m and/or a raw bauxite having a d.sub.90 of 1000 ?m and a d.sub.50 of 500 ?m or lower, and/or wherein the pulverised bauxite comprises 80 wt.-% Al.sub.2O.sub.3 or more, 10 wt.-% SiO.sub.2 or less and 5 wt.-% Fe.sub.2O.sub.3 or less after calcination. In one embodiment, the pulverised bauxite comprises 82 wt.-% Al.sub.2O.sub.3 or more, such as for example 85 wt.-% Al.sub.2O.sub.3 or more, 9 wt.-% SiO.sub.2 or less, such as for example 8 wt.-% SiO.sub.2 or less and 5 wt.-% Fe.sub.2O.sub.3 or less, such as for example less than 4 wt.-% Fe.sub.2O.sub.3 or less, or 3 wt.-% Fe.sub.2O.sub.3 or less. In one embodiment, the pulverised bauxite comprises about 89 wt.-% Al.sub.2O.sub.3, about 6.5 wt.-% SiO.sub.2 and about 1.5 wt.-% Fe.sub.2O.sub.3.

(17) In order to obtain the porous silico-aluminate aggregates according to the present invention, the particulate mixture is placed into a pelletiser and mixed with a binder. By the action of the pelletiser, pellets of the particulate mixture are obtained. The amount of binder added to the particulate mixture may be 8 wt.-% or less, such as for example 6 wt.-% or less, or for example 5 wt.-% or less, based on the total weight of the particulate mixture.

(18) In a subsequent step, according to the invention the pellets are fired at a temperature of up to 1500? C., such as for example 1400? C. or less, or even at 1200? C. or less, such as for example about 1000? C. or about 1100? C., for a duration of up to 8 hours, such as from 3 to 7 hours or about 5 hours.

(19) In a further step, the fired pellets may optionally be coated with a polymer. This polymer may be selected from solid thermoplastic agents, thermohardening (thermo-setting) binding agents and liquid, optionally multi-component, binding agents. The binding agents may, for example, be selected from the group consisting of cellulose, cellulose of butyrate acetate, alkylds, phenolic binders, polyester binders (such as polycaprolactone or polyethylene terephthalate), vinyl-polymers (such as polybutadiene, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, styrene-butadiene-acrylonitrile, polyethylene or polypropylene), polyurethane binders, linear hydrocarbons having 20 or more carbon atoms, aromatic alkanes, glycols (such as PEG 1000), polylactic acids or polyimides. The liquid, optionally multi-component, binding agents may, for example, be selected from the group comprising alkyds (with possible addition of colbalt-derived catalysts for adjustment of reticulation speed), phenolics (with possible addition of catalysts), polyesters (with possible addition of catalysts), polyurethanes (polyisocianate reticulating due to the presence of moisture, or reticulating due to the addition of a second liquid component such as polyol and presence of a catalysts such as amine) or epoxy (reticulating due to the presence of a second liquid component such as amine). The polymer for coating may also be a hydrophobic polymer, such as for example a wax. Coating of the pellets with a hydrophobic polymer may avoid or reduce water absorption into the pellet aggregates. The obtained porous aggregate may represent the starting material for forming monolithic refractory compositions, for example as linings in metallurgical vessels.

(20) The step of coating of porous aggregates with polymers as described herein is also part of the invention as described herein. Both coated and uncoated pellets are considered part of the present invention.

(21) The coated porous aggregates can be obtained by simple application of a polymer to the aggregates while they are rotating in a special device such as pelletiser, mixer, heinrich mixer or concrete mixer. The rotation of the said aggregates in the mixer prevents any lump formation. The said mixer rotation is stopped when the coating is hard and homogeneous.

(22) The main constituent of the particulate mixture for forming the aggregates according to the present invention may be pulverised bauxite. It has been found that calcination of pulverised bauxite prior to use in the particulate mixture contributes to the stability of the aggregate's porosity, especially at high temperatures (?1400? C.). As mentioned above, it may represent from 25 wt.-% to 90 wt.-% of the total particulate mixture, such as for example from 50 wt.-% to 70 wt.-%.

(23) The bauxite for use in the method of the present invention may have a grain size distribution such that the d.sub.90 is 200 ?m or less, such as for example 100 ?m or less, or even 80 ?m or less. Moreover, the grain size distribution may be such that 50 wt.-% of the particles have a particle diameter below 45 ?m, or 60 wt.-% of the particles have a particle diameter below 45 ?m, or even 70 wt.-% of the particles have a particle diameter below 45 ?m.

(24) In order to form a refractory material with low temperature conductivity, the density of the material is preferably low, since high thermal conductivity is an inherent property of dense materials. Several approaches to reduce density of a refractory material have been previously proposed, for example by incorporation lightweight aggregates type such as light chamotte or vermiculite. However, this method confers low refractoriness and low resistance to abrasion when hot, the said aggregates having a low melting point. Moreover the pore size distribution of such aggregates presents a d.sub.10 with high value. This high value indicates porosity with pores with a high diameter leading to low mechanical strength and low infiltration resistance if in contact with slag or liquid steel. According to the invention, the provision of a pore former in the particulate mixture, such as for example carbohydrates, allows the generation of binding strength during the initial agglomeration stage and the controlled formation of ultrafine porosity. The pore former may represent from 5 wt.-% to 20 wt.-% of the total particulate mixture. After pelletisation, the particulate mixture is fired at a temperature of 1500? C. or less, for a period of 2 hours or longer. The firing temperature results in the volatilization of the pore former, by which empty spaces (pores) of known size and distribution are generated. The porous silico-aluminate aggregate obtained by the method of the present invention may display an average pore size of 20 ?m or less, such as 15 ?m or less, or even 10 ?m or less.

(25) Another parameter that influences the mechanical properties of a porous aggregate is the porosity, which indicates the percentage of void spaces within the material. By means of addition of the pore former to the particulate mixture, the density of the obtained pellets, and therefore the final refractory lining can be influenced, as the amount and grain size distribution of the pore former such as carbohydrates is known.

(26) The grain size distribution d.sub.90 of a carbohydrate that may be used as a pore former according to the invention may be such that the d.sub.90 is 100 ?m or less, such as 80 ?m or less. Moreover, the grain size distribution may be such that 50 wt.-% of the particles have a particle diameter below 45 ?m, or 60 wt.-% of the particles have a particle diameter below 45 ?m, or even 70 wt.-% of the particles have a particle diameter below 45 ?m. According of the method of the invention, the pore former may have a loss on ignition of 99.9% at 1000? C.

(27) A metal hydroxide is provided as part of the said particulate mixture in the method according to the invention. It contributes to the generation of ultrafine porosity. Additionally, it provides strength during the agglomeration and firing steps. The metal hydroxide may be selected from the group consisting of brucite (magnesium hydroxide), aluminium hydroxide, hydrated lime (calcium hydroxide), aluminium oxide hydroxide and hydrated alumina (such as hydratable alumina or ?-alumina), and mixtures thereof. The said aluminium hydroxide may, for example, be selected from raw or synthetic gibbsite, bayerite, nordstrandite, doyleite. The aluminium oxide hydroxide may, for example, be selected from diaspore, boehmite, akdalaite. The metal hydroxide may represent from 5 wt.-% to 20 wt.-% of the particulate mixture used in the method according to the invention. The grain size distribution of the metal hydroxide for use in the method according to present invention is such that a d.sub.90 is 60 ?m or less, such as for example 50 ?m or less. The ultrafine metal hydroxide of the invention may have a loss on ignition of 30% or more at 1000? C., such as for example 34% or more. Furthermore, in some embodiments, particulate metal carbonates such as CaCO.sub.3 or MgCO.sub.3 may be added to the metal hydroxide according to the present invention.

(28) The porous silico-aluminate aggregates obtained according to the present method may have open porosity values of 20% or higher, such as for example 30% or higher, or for example 35% or higher, or for example 40% or higher. In one embodiment, the open porosity may be from 20 to 60%, such as from 30 to 50%.

(29) In one embodiment, the method according to the present invention may comprise an additional coating step. After the firing of the pelletised mixture, the obtained pellets may be coated with a hydrophobic polymer, such as for example with a petroleum by-product polymer, such as a wax. This step may be useful in order to prevent water absorption by the aggregates. Water absorption by the aggregates during mixing is responsible of many drawbacks such as water casting increasing during the monolithic casting leading to increasing the porosity of the said monolithic after drying and thus leading to mechanical strength decreasing and/or infiltration resistance decreasing.

(30) The particulate mixtures described above as employed in the method for forming porous aggregates also form part of the present invention.

(31) The porous alumina aggregates according to the present invention may be used in the preparation of monolithic refractory compositions. In one embodiment, the porous aggregates may be used in such amount that they form 40 wt.-% or more of a final monolithic refractory compositions, such as for example 50 wt.-% or more, or 60 wt.-% or more of the total refractory composition. In one embodiment, the porous aggregates may be used in such an amount that they form the bulk of the final monolithic refractory composition.

(32) The preparation of monolithic refractory compositions may require the addition of from 0.5 wt.-% to 25 wt.-% cementitious binder, such as for example, 5 wt.-% to 15 wt.-% or about 10 wt.-% calcium aluminate and/or calcium silicate cement. Further optional additions are colloidal alumina suspensions and/or colloidal silica suspensions used as liquid addition for preparation of an installable product, permitting the stiffening and setting of the castable mixture once installed by destabilisation of the colloidal dispersion and gellification; acids such as phosphoric acid which react with oxides or hydroxides such as magnesia and alumina or other impurities leading to cross reticulation; sodium silicate, reacting either with acids (causing setting by gellification of hydroxysilicates), salts (increasing viscosity of silicate solution and gel formation) or alkaline earth metal hydroxides (causing coagulation); aluminium phosphates hardening at a temperature greater than 100? C. or reacting at lower temperature with oxides such as magnesia forming a bond by creation of a Mg and/or P hydrates network; polysaccharide-based water soluble polymers; species which would cause the reticulation, polymerization or co-polymerization of organic components, when present, which are capable of being reticulated, polymerized or co-polymerized in the presence of said species; and hydration of a reactive alumina substantially free of calcium oxides known as hydratable alumina or ?-alumina. All these additives help to achieve adequate handling and strength for castable materials. According to the present invention, the amount of cement used is such that the CaO-content in the final monolithic composition is 5 wt.-% CaO or less, such as for example 3 wt.-% CaO or less, or even 2 wt.-% CaO or less, such as for example from 0.1 to 5 wt.-% CaO or from 1 to 4 wt.-% CaO, or even about 2.5 wt.-% CaO, based on the total weight of the obtained monolithic refractory composition.

(33) In order to form a monolithic refractory with the aggregates according to the present invention, a calcium alumina cement is admixed thereto in order to obtain an ultra-low cement formulation as known by the skilled person of the art. During the mixing, water is added in order to obtain a castable composition. Then, the said castable composition is cast in order to form a monolith, such as a vessel lining. After hardening, and an optional step of drying, the monolithic can be put in service. The placement can be done by casting as well as by gunning, shotcreting, spraycasting, ramming, troweling, self-flowing, or rodding.

(34) There is a directly proportional correlation between a decrease in density and a reduction in thermal conductivity. The refractory material obtained according to the present invention may have a density of 2.8 t.Math.m.sup.?3 or below, such as 2.7 t.Math.m.sup.?3 or below, or even 2.6 t.Math.m.sup.?3 or below at 800? C. The method of the invention is advantageous since, despite having low density, the refractory composition according to the invention presents physical properties no more than 40% weaker when compared to the original composition, i.e. without addition of porous silico-aluminate aggregates.

EXAMPLES

Example 1

(35) Porous alumina containing aggregates for use in monolithic refractory compositions according to the present invention were prepared. The porous alumina aggregates were prepared by mixing together the components as shown in Table I.

(36) TABLE-US-00001 TABLE I Component Amount wt-% size (d.sub.50/diam.) LOI Rotary 720 kg 72 25 ?m 0 to 0.15% kiln 0.09 mm bauxite grade IMP IIA Indal 140 kg 14 4.5 ?m 99.5% < 34.5% alumina 45 ?m hydrate RPF 14 Indian 140 kg 14 45 to 50 ?m 99.5% < 99.9% sugar 120 ?m

(37) The bauxite used had the following chemical composition: SiO.sub.2: 5.8 wt.-%; Al.sub.2O.sub.3: 87.0 wt.-%; TiO.sub.2: 3.7 wt.-%; Fe.sub.2O.sub.3: 1.9 wt.-%. LOI in Table I stands for loss on ignition.

(38) The porous alumina aggregate was obtained as follows. Rotary kiln bauxite grade IMP IIA170 mesh (72 wt.-%), Indal Alumina hydrate RPF 14 (14 wt.-%) and ultrafine sugar (14 wt.-%) were mixed together for 3 minutes at low speed. Special care was taken in that no formation of sugar lumps had taken place. Sugar lumps form when the sugar is in storage for a long time or in a humid atmosphere. Short grinding in a bowl mill for 3 minutes was sufficient to remove any sugar lumps.

(39) At the initial step of pelletisation, around 40% of the mixture was placed into a pan pelletiser. For this purpose, a pan pelletiser with a diameter of 2 meters was used at a speed about 100 rev/minute. The total capacity of the pan pelletiser used was 400 kg per shift. Once the pan pelletiser was activated, water was slowly added to the mixture. Sequentially small amounts of powder were added to the mixer, followed by a proportional amount of water (in total 8 wt.-% based on the whole weight of the particulate mixture). This stage was carried out entirely at room temperature. At the end of this process, small round pellets having a size in the range of 0.1 mm to 50 mm were formed. Next, the pellets were fired in a rotary kiln at a maximum temperature of 1500? C. for 3 hours, but were maintained at the maximum temperature for no more than 15 minutes.

(40) Following granulation, the composition of the porous silico-aluminate aggregates were analysed, revealing the proportion of the every component, namely 89 wt.-% Al.sub.2O.sub.3, 5.25 wt.-% SiO.sub.2, 1.86 wt.-% Fe.sub.2O.sub.3 and 2.83 wt.-% TiO.sub.2.

(41) FIG. 1 shows the pore size distribution of the obtained pellets. Measurements were made using the mercury intrusion porosimetry method according to ASTM D4404-10. The d.sub.50 equivalent pore diameter measured was 4.6 ?m, and the d.sub.10 equivalent pore diameter measured was 13.7 ?m. The apparent density of the obtained pellets was 2.09 g.Math.cm.sup.?3, and the open porosity 40.7%.

(42) In order to prevent water absorption by the previously prepared pellets, coating with wax was carried out as follows. The pellets were heated up in oven at 250? C. for 10 hours. The hot aggregate pellets were then transferred to a rotating concrete mixer and 5 wt.-% to 10 wt.-% wax on the basis of the total weight of the pellets added. 6 wt.-% of wax were added to 3 to 6 mm and to 5 to 10 mm grade pellets respectively. Pellets of 1 to 3 mm grade required 7% wt.-% wax. Rotation of the mixer continued until complete cooling down of the pellets. In this form, the pellets could be stored for longer periods without atmospheric water absorption.

Example 2

(43) Porous silico-aluminate aggregates were prepared for use in monolithic refractory compositions according to the present invention were prepared. The porous alumina aggregates were prepared by mixing together the components as shown in Table II:

(44) TABLE-US-00002 TABLE II Component Amount wt-% size (d.sub.50/diam.) LOI Gyana rasc 720 kg 72 <45 ?m <0.1 mm 0.15% bauxite pulverised Aluminium 140 kg 14 1.2 ?m <0.01 mm 34.5% hydroxide Martin OL-107 Ultrafine 140 kg 14 20 ?m <0.05 mm 99.9% icing sugar PS

(45) At the initial step of pelletisation, around 40% of the mixture was placed in a pan pelletiser with a capacity of approximatively 50 kg. Once the pan pelletiser was activated, water was slowly added to the mixture. Sequentially small amounts of powder were added to the mixer, followed by a proportional amount of water (in total 8 wt. % based on the whole weight of the particulate mixture). This stage was carried out entirely at room temperature. At the end of this process, small round pellets having a size in the range of 0.1 mm to 30 mm were formed. Next, the pellets were fired in a lab electric furnace at a maximum temperature of 1500? C. for 5 hours.

(46) FIG. 2 shows the pore size distribution of the obtained pellets. Measurements were made using the mercury intrusion porosimetry method according to ASTM D4404-10. The d.sub.50 equivalent pore diameter measured was 4.8 ?m, and the d.sub.10 equivalent pore diameter measured was 23 ?m. The apparent density of the obtained pellets was 1.96 g.Math.cm.sup.?3, and the open porosity was 46.2%.