United States Patent Office 32227@567 3,227,567 FHOSPIIATE BONDED BASIC REFP.-@-CTORY 'AARTICLES I-loward M. Dess, Niagara FaRs, N.Y., assigner to Union Carbide Corporation, a corporation of New York No Drav,,ing. Filed May 18, 1962, Ser. No. 195,993 I Claim. (C]. lOu-59) The present invention relates to chem,'.Cally bonded basic refractory bricks and more pariicularly to phosphate bonded basic re.Lractory bricks. In the manufacture of basic refractories for the stp-eliridust.-y, two main types of products are i-,sed which are known g,-nerally as bumed bricks and chemically bcnded bricks. Thesp- bricks while mainly used for the construction of open hearth furnaces, are also used in electric fl,rnaces, ladle I;nin.-s, and the like. Burned bricks are produced by pressin.- suitably sized, basic refractory @r-@in mixlures conta;ning 2 to 3 percent of some i-@icxpcnsive organic bonding agent and then firing Lhe rressod shapes at temperatures rangin@ from about 1406 to 1600- C. Adhesive or.-anic mate.-.;als, such as starches, su.-ars, Gr waste lignin sulfite, are typically used as the bondin,@ a.-ent and are applied as concentrated aqueous solutions. With chemically bonded bricks, the bondin2 agent for the -rain mix,ure is an inorganic substance such as ma,-nesiurri si,-Ifate, magnesium chloride, magnesium chromate, chromic acid, or other suitable inorganic compounds which are soluble in water and which provide bonding by the precipitation of a dense ir@terlocking network of fi-tie crystalline material from a concentrated aqueous sol-,jtion. Chemically bonded bricks are not fired prior to ins'allation and depend upon the first heat of the furnace tc) form a suitable sintered layer at the hot fa,-e. At the elevated temperatures involved in furnace operation, the chemical bond is decor@iposed and is believed to be absorbed by the surrounding refractory .-rains during the sinterinprocess. Thus, the inorgan;c residue that is left after firina becomes incorpc-rat,-d into the sintered -rain structure. By way of contrast, the organic bonds of burned bricks burn out complete'@Y at much lower temreratures and usually contribute nothin.@ to th.- final s:.n+ered strlcture of the product. Some of th-@ advanta.-es that have been cited for the chemically bonded bricks as compared with the bumed br;cks are lower thermal conductivity, lower porosity in the unburned state, and a degree of pyroplasticity at elevated temperatures that permits stress relief. These advantages n-iay be all related in some way to the overall observat;on that ch,-mioa'.Iy bonded bricks are less subject to sheet spalling than burned bricks. On the d,-bit side, however, probably the most serious problem with chemic-ally bonded bricks is that they develop weak zones in the interior. In oderation, th-- situatior@ is as follows: With a chemicaiiy bonded brick in a furnace wall, the hot face is well within the sintering ran,-e while the outer face is cool enaugh so that the oricinal chemical bond is still lar,-ely intact. Powelier, there is a region interior the brick ivhich is hot enou.-h so that the cheriical bond is destroyed or badly weakened bLt not hot e-@iou,ah for to occur. The rei'ractory -rains in t'-,iis intermediat@- temperature zone ,ire Lherefare held to,- ether mainly by mechanical interlockin.@ and consequently this regior, of the br-'@ck is quite iveak and proie to failt,.re. This highly undesirable wea@'-,-,nin.- of the chemical bond is ma@'.nly caused by dehydration of the oriainal c-.meiititous material wh@ch was deDos:@'Led as a hydrate from aqueous solution. As a means of overcomin.- this prob'@em, external steel Pa4l-ented Jan. 4, 1966 2 claddina has been widely adopted as a me@ans,of strengthening c'h,-mically bonded bricks, and intemal steel plates are also used by a number oimanufa,.turers. However, the use of this claddin.@ tecb.,iique or the introduction of iron plates into the interior of the brick causes other problems associated with the formation of iron oxide rich zones in th,- brick that are almost equally undesirable. Also, much research has be--n done in recent years in an attempt to overcome the drop in stren.-th of thle chem10 ical bond in the intermediate temperature zone without the use o4l@ i)lates or claddin-. However, up to the present, no completely satisfactory solut@'.on has yet been found with the ordipary types of bonding agents previou.sly mentioned. Further, based on present knowledge, the Qhances 15 appear slight that any substantial improvements can be made that wou-Id raise the previously used magjipsium st-ilf ate or ma.-nesium chromate chemical bonding systems much above the present performance levels. However, siiice ebemically bonded bricks are advan20 ta,@eous in many resp,-cts as compared to bur-iied bricks, it would be of coilsiderable b@-nefit if a chemically bonded brick would be provided having increased strength at all temperatures encountered durin.- normal furnace operation. 25 It is there'ore an object of the present invention to provide a high quality chem;cally bonded brick having increased strength throughout the ran-e of ternperatures encountered in ope.-i hearth furnaces and similar opf--ra@ tions. -@O Other objects will be apparent from the followin.@ descriotion and claim. A chemically bonded basic refractory brick and I'@ke article in accordance with the present invention conprises a refractory mass formed bycompressing a mixture of 35 acid coated basic r--fractory particles with a concentrated acid phosphate bor@din.a solution. In a particular embodiment of the present invention, a refractory brick or like article is produced by contacting particles of basic refractory material with concentrated 40 sulfuric ac,d to -rrovide the particles with an acid sulfale coatin-; mixing the acid sulfate coated particles with concentrated pliosphoric acid; and compacting the mix into shapes si-iitable for use in fiirnace constrlction. In the pract;ce of the above-described and all other 45 embodiments of the present invention, the particulated basic refractory material can be an-jr reiractory material stiitab,e for use in the manufaciure of basic refractory bricks. Such materials include chrome ore, magnesite, periclase, blirned or fused dolomite, dead barned lime, 50 mixtures of chrome ore and ma.-nesia, and particles derived fi-om crushed fused lpixtures of chrome ore and ma.-ne,,i-a. When mixtiires of ebrome ore and magnesia are lised, the preferred proportion of magnesia ranges from 50 percent to 80 percent. 5-5 As regards th-. sulfuric acid treatment of the basic refractory parti-.Ies, it -@s important that the sulfuric acid employed have a concentration of between about 70 and 98 percent and that the amount of acid used be between -,ibout I and 6 percent of the weight o'L the refractory ma60 terial being treated. It has been found that if the acid concentrat:@on or amount is less than that specified, the subesquent reaction of the basic refractory material with phosphoric acid 's excessively rapid and the manufacture of a--rticles such as bricks and the like cannot be satisfact<)ri65 ly accomplished. If a greater a:mount of sulfuric acid is employed, it as been found that the bricks and like articles ultimately produced from the thiis treated material have a tendency to crack during use and are otberwise undesirable. 70 The manner of contacting the particles of basic refractory material with the conceitrated sulfuric acid is

3 imp6rtant in the present invention since it is necessary that substantially all of the basic refractory particles be provided with a substantially complete acid sulfate coating or shell before slbsequent mixing and processing with phosphoric acid. If such a coatin-. is not provided, the reaction between the phosphoric acid and baSiC Tefractorv material will be so rapid as to mak-e impossible th@- proper mlxing and pressing necessary for the manufacture of bricks and the like. It is also important in the manufacture of br.,cks and like articles in accordance with the present invention that the sulfuric acid be applied in a manner which avoids agglomeration of the refractory particles since ag,-Iomerated particles do not receive the proper coatin.- and the properties of the bricks ultimatelv produced are adversely affected. In this regard, contactin.- the particles with a fine spray of sulfuric acid is preferred; the addition of sulfuric acid by pouring or sprinkling with large drops is to be avoided. The acid sulfate coatin.- required in the present invention can be provided by spraying the concentrated sulfuric acid on the refractory particles using a device such a DeVilbiss comprmive air sprayer. I It has also been found that agitation of the refractory particles during the sulfate coating procedure is necessary to prevent the treated particles from sticking together, and in order to expose fresh surface to incoming spray droplets in@a.continuous and rapid faslhon to provide the required acid coatin- on the particles. In this re,,-ard, the agitation is continued until the particles no longer appear wet and no ]on.-er have a tendency to a-.glomera@te. When this condition has been reached, the refractory material is acid sulfate-coated in accordance with this invention and is free-flowing, easily handled, and can be stored for Ion-. pe@iods, up to weeks if kept dry. Immediately after the aforede-scribed coating treatment the refractoiy particles are quite warm due to heat devcloped by the reaction of sulfuric acid with the refi-actory material. Consequently, the acid siilfate coated refractory material is allowed to cool before being further processed so that the reactions involved will not be inconveniently rapid. In this respect it is important, before the treatment with phosphoric acid, that the sult'ate coated material be at a temperature below about 40' C. and preferably below 30' C. When the sulfate coated refractory particles have been cooled to a proper temperature, as aforementioned, a mix or raw batch comprising the acid sulfate coated refractory particles and concentr@ted phosphoric acid is prepared. It is important that the concentration of the phosphoric acid employed in the mixin.- step be quite high, i.e. between about 50 percent and 95 percent, preferably between 75 and 95 percent, and that the amount of phosphoric acid used be between about I and 8 percent of the weight of the acid sulfate coated refractory material in the mixture. It has been found that if the concentration of the phosphoric acid is below about 50 percent the reaction with the acid sulfate coated material is excessively rapid and the manufacture of articles such as bricks and the like cannot be satisfactorily accomplished due to the very rapid hardeiiing of the mixture. If the amount of concentrated phosphoric acid in the mixture is greater than about 8 percent, the bricks ultimately produced from the mixture have a tendency to crack in use and have other undesirable characteristics. On the other hand, when amounts of phosphoric acid less than about 1.0 percent are used, satisfaCtOTy bonding between the refractory particles is generally not obtained. In preparin- the mix, the concentrated phos phoric acid can be added to the acid sulfate coated particles in any convenient manner, it bein.- necessary only to ensure that a substantially hornogeneous mixture be provided. The mix of concentrated phosphoric acid and sulfate coated refractory particles prepared in the manner aforedescribed is initially plastic and remains in this condition for at least several hours and usually for as long as 24 hours or more. 3,227,567 4 When a plastic mix of phosphoric acid and acid sulfate coated refractory particles has been prepared in accorda-@ice with this invention, the resulting material can be pressed or otherwise forrred into biicks or the like in the usual manner and by conventional techniques. In general, it is best to press bricks from the plastic mix within about 2 to 3 hours of its formation in order to achieve optimum properties in the final article. Hovvever, pressing can b-- delaye-d for longer periods and highly satis10 factory results are obtained so long as the mix is plastic. Ordinarily, a mix haviilg a relatively long working life is desired and under these circumstances the, hi-,her concentrations of phosphoric acid are used. However, if rapid settling is desired, for example where the mix is to 15 be used for patching, using a gun or similar device, lower concentrations of phosphoric acid are used. The phosphate bonded bricks produced in this manner arecharacterized by high strength and can be handled and transported routinely. Moreover, when used in opert 20 hearth fumace construction, tliese bricks have superior high temperature properties as compared to other chemically bonded bricks. Further, the strength of phosphate bonded bricks of this invention is substantially greater than that of @other che@nically bonded bricks in the tem25 perature range of 25' F. to 1400' F. The stren.-th of the present phosphate bonded bricks is at least equal to that of otber chemically bonded bricks at the other temperatures encountered in open hearth furnace operation. The followin-, examples are provided to further illus30 trate the present invention. EXAMPLE I A fused mixture of 40 percent chrome ore and 60 percent low calcined "caustic" sea water mao,nesia was pre35 pared and subsequently was crushed to a particle size distribi@.tion providing the M-hest bulk density. The crished material was introduced into a laboratory cement mixer apparatus and the particles were agitated and concurrently sprayed by means of a compressed air 40 sprayer with 98 percent sulfuric acid. The total amount of acid sprayed on the particles was 4 percent of the weight of the particles. i-Lhe material in the mixer was continuously agitated during spraying, and agitation was continued until the particles appeared dry and no lon,@er tended to stick to45 -ether. In this condition the particles were acid sulfate coated. The sulfuric acid treated particles were permitted to cool in the mixer to a temperature of about 30' C. Phosphoric acid amounting to 4 percent of the weiaht 50 of the acir sulfate coated particles and having a concentrat;on of 86 percent was then sprinkled into the material in the mixer and the material was agitated to thoroughly mix the acid and the acid sulfate coated particles. The phosphoric acid-containin.- mix was subsequently 55 removed from the mixer and pressed in a steel die at a pressure of 10,000 p.s.i. to provide a brick 9 inches x 41/2 inches x 21/2 inches. The brick was air aged ovemight and dried at about 100' C. to 110' C. to ensure completion of the phosphate bonding reaction be60 tween the acid and basic material in the particles. The brick thus obtained was strong and of good apPearaice and exhibited superior spalling resistance under conditions encountered in open hearth furnace operat;ons. The brick also had superior strength in the tem65 perature ran,@e of 25- C. to 1300- C. EXAMPLE 11 In order to determine the effect of the use of diiferent amounts oj' acid on the ag,'.ng properties of wet mixes 70 prepared in accordance with this invention, separate portions of the er-ushed basic refractory material of Example I were reacted with various amounts of phosphoric and sulftiric acid. The acid concentrations and the manner of treatin@,the refractory particles wexp, tjiq @amQ a,% 75 in Example I.

In the present example, cylindrical pellets having Ys inch diameter and 1 inch in len.-th were pressed from the wet mixes at different intervals after formation of the mix and the compressive strength of the pellets was measured to provide an indication of the effect of a.@in.@ on the stren.@th of the pellets. All pellets were formed by pressing at 16,600 p.s.i. The compressive strengths of the peli'ets for various amounts of acid and for different pressing intervals are shown in Table 1. Table I Amount of Acid Compressive S'@rengths of Pellets Forrned at Used in Treatment, Different Intervals 1 after Formation of Percent Tvlix H2SO4 H3PO4 10 30 1 hr. 2 br. 3 br. (98%) (86-87%) 4 4 10,200 9. SOO 6,500 7 200 9, SOO 4 6 12,500 12,500 15,000 12:500 14,500 6 4 10,200 10,700 11.200 8 800 10,200 6 6 13,300 16,500 16,000 16:500 14,500 I -@Nleasured as elapsed time after substantiauy coinplete mixiug of H3PO4 and sulfate coated particles is obtained. It can be seen from the foregoing data that delays of up to 3 hours before pressin- do not sig-iificantly affect the strength of the pressed articles. EY-AMPLE IH The procedure ol' Example 11 was repeated using 6 percent of st@.1-'Luric acid (98% conc.) and 4 percent ol phosphoric acid in all@ instances. In this example the concentration of th-- phosphoric acid was varied to deLermine its effect on the aging properties of the mix. The results are shown in Tablc II. Table 11 Concentration of Compressive Strength of Pellets Fon-ned at Acid Used in Treat- Differetit Intervals I after Formatiou of NRX ment, PereeLit 10 30 1 hr. 2 hr. 3 hr. T2SO4 IE13PO4 98 95 12,700 io 700 11,700 14,000 14,000 98 86 10,200 io:700 11,100 8, 900 10, 200 98 65 12,000 91200 8,400 6,300 6,100 I iNIeasired as elapsed time after substa@.itially complete mixing of H3PO4 and sulfate coated particles is obtained. The foregoing data indicates that the aging properties of the mix improve-with the use of hi.-her concentrations of phosp'@ioric acid. EY-AMPLE IV The pro-zediire of Exami)le 11 was repeated using 98 percent conc. sulfi-iric acid and 86 percent conc. phosphoric acid i-n all instances. In this example the amounts of sullur-@c acid and phosphoric acid were varied and all pellets were pressed ,tt an agin.- time of 10 mintites. The pellets were heated to var-lous elevated temperatures after which the compressive stren.-ths were measured at room temperature. The results are set forth in Table III. To b le III Amount of Compressive Strength of Pellets Acid Used in after Heiting to Indicated Treatment, Ternporatures 1 -Percent H2SO4 lI3po4 100, 800, 1,100- 1,400- (98%) (86%) C. 0. C. C. 4 4 10,200 11,700 7,600 9,500 4 6 12,500 18,300 13,000 12,600 6 4 10,200 11,700 7,000 6,700 6. 6 .13,200 11,700 7,700 7,400 I inleasured at room temperature. 3)227,567 6 While th@ kof6goin- description has be6n directed specifically to the use of concentrated sulfuric acid for acid coating particles of basic refractory material and concentrated phosphoric acid as the bonding ag--nt, other materals can be used in the manner aforedescribed to provide substant,.ally equivalent results. For example, concentrated solutions of acid phosphates can be -Lised in place of pbosphoric acid as the bo-@idin.a a.@ent and in some instances concentrated phosphoric acid can be 10 used for acid coating th@- basic refractory material. i,i -eneral, the conc--ntrated acid uqed for acid coating the refractory material must be at least of equal strength and have at least as hi.-h a boiling point as the parent acid of the bonding solution. 15 Specific combinations of acid coating mate@-ials and bonding a-ents within the scope of the present invention are shown in Table IV. Table IY 20 Acid Coating Boiiding Agent Solution Material Con@.H2SOI------ COnC,HIP04. Cone. H3PO4 -------- Cone. H ' P04. 2,5 Cone. H2SO,------ Concentrated Solutions of NH4+, Mg++, Al+++, Fe@, Fe+-14 ,ind Cr@+ acid 3hospliates. CONC- H3PO4-------- Concentr,,,ted Solutions of NH4+, Mg++, Al-+, Fe++, Fo@+, alid Cr@ ,@cid phosphates. In --Ineral, the concentrations and amounts of the 30 acid coating rpaterial and bonding age I nt solution and the method of applying these materials should be as described in connection with the sulfuric acid-phosphoric acid embodiment of this inne@ition. However, in order to obtain optimum bondir,,-, the concentrations and 35 amounls of the materials, particularly of the bonding solution, may vary somewhat with the different bonding agents employed. As can be seen from the fore,-Oing Table 1, a variety of combinations of materials can be used in the practice 40 of the present invention including concentrated phosphoric acid for both the acid coating and bondin.- agenl. The fact that the use of concentrated phosphoric acid alone can be used to provid.- a mix having a long workin.@ liie which can be used in the manufaclure of re45 fractory articles such as bricks and the lil-,e ha-,,ing superior high temperature properties is considered to be an important feature of this invention. As to tlie bondin.- agents other than phos-phoric acid which can be used, in general all acid phosphates havin@ 50 significant solubility in water can be used, however thos'e containin,@ Na+ and Ca++ as secondary cations are to be avol 'ded inasmjich as the refractoriiiess of the final product is adve.-sely affec' 'ed by th--se mat--rials. As can be seen frc)m tho foregoing description, an im55 portant adva-@itage of the present invention is that by acid coating basic re,ractory material, the subsequent reaction w:lth acid phosphate bondin- material is slowed down so that phosphate bo-ilded bricks a-@ld other refractory shapes having superior properties can be conveniently produced. 60 Without intendin.- to limit the invention or being bound by the ex-olanation, the following hypothesis directed to a particular embodiment of this inventiorl is beli-,ved to d-,scribe thisphenomena: Basic refractory material contains magnesia in a form 65 which reacts initially in a very rapid manner with th-. appli@-d concentrated sulfuric acid to form ma,@nesium sulfat,- and/or -maenesium acid sulfate as a coating on the surface of the individual basic refractory particles. These coatings t--nd to hinder further reaction of the 70 sulfuric acid which either remains entrapp-@d physically in the surface coatings or is presert in the form of acid sulfates. In any event, it has been established that th-coating on the air-dried grain is definitely acidic. For example, th-, pH of a water slurry of particles of a fused 75 40/60-chrome ore/ma.-nesite mixture was between 10

7 and I 1. The initial pH of a ivater slurry of the same material after sulfuric acid coating treatment was between 1.9 and 2.5. When concentrated phosphoric acid or acid phosdhate solution is added to the uncoated basic refractory particles, -i.e. untreated with sulfuric acid, the reaction is extremely rapid and exothermic and yields various ma.niesum phosphates. Consequently, the reaction proceeds so quickly that there is insufficient time for proper mixin.or pressing. With the H2SO4 treated acid coated material, however, the follow:ng equilibrium reaction is @hypothesized as the first step: M.-SO.i(particle surface)+H3PO@--M_@HP04+H2SO4 The equilibrium lies predominantly on the left side of this equation because H2SO4 is a stronger acid than H3PO4. Bondin.- occurs as a result of further reaction of the;se acids with the underlyin.- magnesia substrate., T-he rate of this reactio@i is limited, however, by the presence of the protective coating Of MgSO4 formed during the precoating. ThLs, in unpressed mixes, the charge stays moist ard workable for hours. With subsequent pressing, hoviever, bonding rapidly occurs since the comers and edges of the refractory particles, forced in close co-@itact wil]i each other, tend to cut through the protective MgSO4 coatin-,, thereby exposin.- fresh magnesia surface and permittin-, the followin.- bonding reactions to rapidly take place: H2SO4+M.-O--> MgSO,,+H2,0 2MgHP04+M-0-->Mg3(PO4)2+H20 As the phosphoric acid is used up, the liquid phase disappears and th-- solid reaction products are obtained as new crystalline phases (possibly gelatinous initially) in intimate contact with the basic refractory particle surfaces znd serve to c-.ment together the refractory particles. 3,227,567 8 From the foregoin- description it can be seen that the present invention constitutes a substantial industrial benefit by providing a convenientmethgd for producing phosphate bonded basic refractory bricks which have superior stren,@th in the as-produced or over-dried state and at temperatures up to the sintering temperature of the basic brick material. What is