US 3649315 A
Mineral wool, aluminum powder, hydrated lime, and plastic refractory clay are dry blended to form a homogenous mix, the homogenous mix blended with water and cast in molds, the molded mix vibrated and allowed to expand and set after which the molded article is dried and fired. In forming insulating refractories for higher temperature use requirements, pyrophyllite or alumina may be substituted for some of the refractory clay.
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Description (OCR text may contain errors)
United States Patent Booth  METHOD OF MANUFACTURING LOW DENSITY INSULTING REFRACTORIES  inventor: Alfred E. Booth, Lancaster, Pa,
 Assignee: Armstrong Cork Company, Lancaster, Pa.
22 Filed: May 27,1969 21 Appl.No.: 828,318
..l06/67, 106/40 [5 1] Int. Cl. l ..C04b 21/02  Field of Search ..106/40, 67; 264/43, 71
 References Cited UNITED STATES PATENTS 2,278,486 4/1942 Quigley et al 106/40 Mar. 14, 1972 Primary Examiner-James E. Poer Attorney-William G. Taylor  ABSTRACT Claims, lprawing Figure PAIENTEDMAR 14 I972 HYDRATED LIME PLASTIC REFRACTORY CLAY SIZING DUST MINERAL PYROPHYLL ITE FBER ALUMINA DRY MIXER WET BLENDER CAST IN MOLDS VIBRATE EXPAND AND SET REMOVE MOLDS POST- DRY CUT TO DESIRED SIZE KILN FIRING FINISHINGI INVENTOR ALFRED E- BOOTH ATTORNEY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for manufacturing insulating refractories, in particular insulating fire bricks.
2. Description of the Prior Art Insulating refractories have conventionally been made from mixtures of refractory clays and sawdust and such mixtures have included pyrophyllite and alumina where use at very high temperatures is required. The clay and sawdust are mixed with water in a pug mill after which the mixture is conventionally mechanically molded in a wooden frame. The wooden frame is ordinarily filled to overflow and pressed. At this stage, the wood form or frame is removed and the bricks are loaded onto pallets and delivered to a dryer where they are dried at about 300350 F. for 72 hours or more. These bricks are then loaded onto kiln cars which are moved continuously through a conventional brick kiln. The brick kiln has three zones and in the first zone, the sawdust is burned out to leave voids. In the second zone, the bricks are fired at elevated temperatures to effect the desired ceramic bonding and the third zone is the cooling zone in which the bricks are cooled at gradually decreasing temperatures. The overall time required in the kiln is generally about 72 hours.
A slightly different system has been developed for forming insulating fire brick wherein, in addition to the clay and sawdust mix conventionally used, an additional amount of plaster is added. In this process, the dry mix including the plaster, clay and sawdust, and where desired pyrophyllite and alumina, is mixed with water and the wet mix poured into molds. The mix readily sets or hardens in the mold and, after a short period of time, the mold can be removed by extruding the slabs directly onto a kiln car.
The loaded kiln car is charged directly into a high temperature dryer or preferably into a combination dryer and kiln wherein drying temperatures above l,OOO F. may be utilized. By the time the end of the preheating portion of the kiln is reached, all the sawdust has been burned out and the bricks are then fired to the desired maximum operating temperature. Again the kiln has a zone to provide for gradual cooling of the bricks.
Obviously, the burnout of the sawdust in the abovedescribed methods provides a problem and requires considerable processing time. In the second method described, the decomposition of the gypsum plaster provides added difficulties in processing, although some time saving is realized through the use of higher drying temperatures.
SUMMARY OF THE INVENTION I have discovered that insulating refractories of required low densities may be formed without the necessity of incorporating burnout material, such as sawdust, therein. In accordance with the invention, mineral wool, aluminum powder, hydrated lime, particles of plastic refractory clay, and alumina and/or pyrophyllite when desired, are dry blended, mixed with water to form a moldable mix which is poured or cast into molds and vibrated. The molded mix is allowed to expand and set after which it is dried to a moisture content of about to percent. At this point, the slab stock is readily removed from the molds. The slab stock is put on conventional conveying equipment and transferred to a kiln wherein it is fired at a temperature within the range of from about 2,200 to 2,850 F. for anywhere from about 2 to 6 hours dependent on end use temperature requirements and mix compositions.
DESCRIPTION OF THE DRAWING The FIGURE of the drawing is a flow diagram setting forth schematically the process ofthis invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The description of the preferred embodiments is made with reference to the flow diagram.
Based on the total weight of dry ingredients, the basic mix contains from about I to 7.5 percent by weight inorganic fiber, about 0.2 to 1.0 percent by weight aluminum powder, about 1 to 10 percent by weight hydrated lime, and about 30 to 97 percent plastic refractory clay. Up to a maximum of 25 percent by weight of pyrophyllite may be substituted for part of the clay in the manufacture of 2,600 F. rated fire brick and up to a maximum of about percent by weight of alumina may be substituted for part of the clay in the manufacture of 2,800 F. rated fire brick.
Scrap from any of the steps, through drying the slab stock. may be used in the basic mix and the amount of scrap utilized is not critical since the ingredients on mixing and wet blending will be in essentially the form of the original dry ingredients although minor adjustments in the amount of aluminum powder and hydrated lime will have to be made where much scrap from this source is used in the mix.
In addition, fired sizing dust from the finishing operation may be utilized in the dry blended mix up to a maximum of percent by weight of the dry blend mix.
The mixture is initially dry blended to form a homogenous mix which is added directly to a wet blender, such as a J. Hv Day Co. Centri-Flo Mixer, at which point water is added to give a wet blend of an essentially creamy consistency. The amount of water to be added will vary with the plasticity of the blend, but generally the addition of about 1 part by weight of water to 1 part by weight ofdry blend is required. I have found that wet blends having an apparent viscosity in the range of about 12 to 23 poises, at a shear rate of sec., form mixes of the desired consistency. The mix formed at the wet blender is transferred directly to casting molds, bottomless molds carried on a pallet such as cement-asbestos board, and the molds are partially filled, usually to a maximum of about 50 percent of the volume of the mold. At this point, the molds are preferably vibrated to remove the entrapped air and the vibration also apparently aids the expansion and setting reaction of the mix. The wet blended mix in the molds goes through an exothermic reaction and the mix expands to essentially completely fill the mold and achieve an initial set.
The casting molds having the set mixed formed in slabs therein are then transferred to a dryer and control dried to approximately a 20 to 25 percent moisture content, usually at a temperature of about 500 F. Alternatively, the cast molds may be predried to remove some of the water and to further set the mix. At this point, the slabs have shrunk and the mold forms can be removed from the cast slabs. The slabs remain on the pallet and are then passed through a postdryer which removes the remaining water necessary to give a final moisture content of from 20 to 25 percent moisture by weight. In either instance, after drying, the slabs are in a condition such that they may be readily cut with saws or wires to any desired size.
The dried slabs, cut as desired, are next loaded onto a suitable conveyor, such as a conventional brick kiln car, and transported to a kiln where they are fired to the desired temperature, usually a temperature within the range of from about 2,200 F. to about 2,850 F. On removal from the kiln, they may be finished by any conventional brick finishing operation such as grinding or cutting with bandsaws.
In the practice of the invention, we prefer to utilize a commercial grade mineral wool although other inorganic fibers such as relatively fine glass fibers, asbestos fibers and refractory fibers such as Fiberfrax, a high alumina fiber produced by Carborundum Company, may be used equally as well. The use of fine fibers is preferred because the function of fibers is primarily to make the expanded product more dimensionally stable on first set.
As the plastic refractory clay, kaolin is preferred although other clays such as Pine Lake clay and various Fire clays may be utilized, especially for lower temperature uses. The clay used must have a degree of plasticity such that it entraps and retains the gas formed from the gas producing reaction after the ingredients have been wet blended. The clay must also possess the refractory properties necessary at the use temperature such that specified reheat shrinkage requirements are met by the fired product when tested by ASTM C 210.
The aluminum powder preferred is a very fine particle size, nonleafing aluminum powder sold by Alcan Metal Powders, Inc. (Grade 3,100).
As stated above, enough water is added at the wet blending step to form a mix of creamy consistency. The amount of water required will vary with the amount and plasticity of the clay in the mix. Generally, based on parts by weight of dry blend, the amount of water required will be in the range of from about 7 to parts by weight. The higher amounts of water are required for mixes utilizing relatively large amounts ofthe more plastic clays.
In order to speed the reaction but yet control it to the desired extent, the water added is preferably at a temperature of about 120 F. Obviously, cooler water may be used but this increases the dwell time necessary to expand and set the mix after it has been poured into the molds. With water temperatures greater than 140 F. the reaction is speeded up to the point where it is difficult to control and the cells formed may become too large.
A measured amount of wet mix is put into the mold so that the slab formed on expansion fills the mold cavity. Usually the wet mix required for forming a slab which fills the mold occupies about to 50 percent of the volume of the mold, the amount of expansion depending on mix composition. The mold, on filling, is vibrated, preferably with a high frequency low amplitude vibration, which acts to remove entrapped air from the mix and also has been found to aid the reaction and expansion of the mix in the mold. Usually, expansion is complete within 2 to 5 minutes of filling the molds and the mix sets or hardens to a desired degree of hardness in about 20 minutes. The degree to which the mix sets is not a critical limitation but does affect the ease of handleability of the filled molds.
Preferably the drying temperature utilized is about 500 F. although the use of higher temperatures (up to a maximum of about 800 F.) appears feasible. During drying, the slab stock in the molds is dried to a moisture content of about 20 to 25 percent although this range could be extended to from about 0 percent moisture to not more than percent by weight moisture. The lower the moisture content, the more friable the slab. Generally at 500 F. a drying time of about 1% hours is required. although this will vary slightly depending upon the particular mix used for forming the slab stock and the final moisture content. In a preferred embodiment, the molded slab stock is predried only to the extent necessary to shrink the slab stock away from the mold. At this point, the molds are easily removed from the slab stock. The slab stock is then put on a pallet and cycled through a postdrying cycle where the remainder of moisture is removed to give a slab stock of preferably 20 to 25 percent moisture content. This alternative method results in an advantage in that iii-process mold usage is minimized.
After drying, the slab may be cut to any desired size and all the scrap at this point may be recycled to the basic mixing operation and may be used in any proportions in the basic mix. Sizing dust from the final finishing operation and after the firing cycle is also used in the original dry mixing but in limited quantity. The sizing dust after firing is refractory and only up to 50 percent by weight ofthe dry blend mix may be utilized.
The dried slab stock is next loaded onto a suitable conveyor such as a conventional brick kiln car and transported to the firing kiln. A conventional brick firing kiln is divided into a heat up zone, a firing zone and a cooling zone. For brick rated at 2,000 F. or 2,300 F. the slab stock is fired within a temperature range of 2,200 to 2,400 E, usually at a minimum of about 2 hours for the 2,000" rated brick and about 4 hours for the 2,300 F. rated brick. For 2,600 F. rated brick, a minimum firing period of about 4 hours at a temperature within the range of 2,450 to 2,650 F. is required and for the 2,800 F. rated brick, a minimum firing time of about 6 hours at a temperature within the range of 2,650 to 2,850 F. is required.
The heat up time required is usually about onehalf hour and a cooling period ofabout 2 hours is usually required to insure against thermal shock.
The following table describes dry mix formulation for lightweight fire bricks rated for varying use temperatures.
TABLE 1 Parts by Weight For each of bricks (A), (B) and (C), the given materials were dry-blended to form a homogenous mix and the homogenous dry mix was then thoroughly blended with about F. water to form a wet blend of creamy consistency which was immediately cast into molds, vibrated and allowed to expand and set. To form a blend of the desired consistency. a ratio of approximately l.5 part by weight water to l part by weight dry mix was used for (A), a ratio of approximately 1 part by weight water to 1 part by weight dry mix was used for (B) and a ratio of approximately 4 parts by weight water to 5 parts by weight dry mix was used for (C). The wet blends had initial apparent viscosities of. respectively, (A) about 20 poises at a shear rate of 55 sec.', and (B) and (C) about 15 poises at a shear rate of 55 see". The amount of wet blend added to the molds varied from about 25 percent by volume for (A) to about 50 percent by volume for (C) and the time required for expansion and achieving initial set varied from a maximum of about 20 minutes for (A) to a minimum of about 5 minutes for (C).
Drying of the respective slab stock thus formed was carried out in a dryer at 500 F. for about 1 /2 hours at which time, on removal from the dryer, the molded slab stock had a moisture content of about 25 percent by weight.
The slabs were removed from the mold and loaded onto a conventional brick kiln car and the slabs cycled through a conventional brick kiln.
The time and firing temperatures required in the kiln vary depending on the mix used and the use temperature requirements of the product. For each. the heat up time prior to firing is about one-half hour and the cooling down period required is about 2 hours. The gradual heating up of the dried slabs in the kiln and the gradual cooling down of the fired product is necessary in avoiding the possibility of thermal shock occurring in the slab stock during processing.
Mix (A) set forth in Table l is designed for fire brick rated at 2,000 F. and 2,300 F. The firing time at temperature required for 2,000 F. brick was about 2 hours at about 2,300" F. and the firing time required for 2300 F. brick was about 4 hours at about 2,300 F. For the 2,600 F. brick [Mix (8)] a firing time of about 4 hours at about 2,550 F. was required and for the 2,800 F. brick [Mix (C)] a firing time of about 6 hours at about 2,750 F. was required.
All of the bricks produced in accordance with the above were of low density, good strength and uniform pore structure as compared with a ragged pore structure for fire brick produced using a burnout technique. Physical properties for the various brick are set forth in Table II below.
TABLE ll A\erage (A) (B) (C) 2000 F. and 2300 F. 2600 F. 2800 F.
Brick equivalent weight 1.5 to 1.65 2.6 2.8 Reheat shrinkage 0.1192 less than less than (24 hr. at I950F.) 0.5%(a) l.0%(b) Crushing strength I40 psi more than more than 200 psi 200 psi Brick equivalent weight is the weight in pounds ofa brick measuring 21z 4% 9.
a. 24 hours at 2550 F.
.24 hours at 2750 F.
B. blending said mixture with sufficient water to provide a wet blend having an apparent viscosity in the range of about 12 to 23 poises at a shear rate of55 secf";
C. partially filling a mold with said wet blend;
D. vibrating said molds to remove entrapped air and allowing the vibrated mix to expand and set into a slab;
E. drying said slab to a maximum of about 35 percent by weight moisture; and
F. firing at a temperature within the range of about 2.200
to 2,850 F.
2. A method in accordance with claim I wherein up to 50 percent by weight of a fired sizing dust of the composition of the refractory produced in accordance with claim 1 is dryblended with the ingredients recited in claim 1.
3. A method in accordance with claim 2 wherein the slab is dried to a moisture content of 20 to 25 percent by weight.
4. A method in accordance with claim 2 wherein said inorganic fiber is mineral wool and wherein said aluminum powder is of extremely fine, nonleafing particle size.
5. A method in accordance with claim 4 wherein the water to dry mix ratio is about 7 to 15 parts water to 10 parts dry mix and wherein the water temperature at addition is between about F. and F.
6. A method in accordance with claim 5 wherein kaolin clay is utilized and wherein the slab is fired at a temperature in the range of about 2,200" to 2,400" F. for a minimum firing time of about 2 hours.
7. A method in accordance with claim 5 wherein a mixture of kaolin clay and pyrophyllite is utilized and wherein the slab is fired at a temperature in the range of about 2,450 to 2,650 F. for a minimum firing time ofabout 4 hours.
8. A method in accordance with claim 5 wherein a mixture of kaolin clay and alumina is utilized and wherein the slab is fired at a temperature in the range of about 2,650 to 2,850 F. for a minimum firing time of about 6 hours.