INDUSTRIAL AND ENGINEERING CHEMISTRY
68 A
Vol. 40, No. 18
Chemical Plants Get New Reactions, Speed Others with Help of Norton Pure Oxide Refractories I. Pure Oxide Refractories T h e refractories industry is under constant demand from users for refractories of better quality t o withstand higher temperatures and more severe operating conditions. Development of new industries, e.g., synthetic gasoBine, atomic power, and jet propulsion, and the use of low-cost oxygen will require refractories superior t o those now being supplied. To meet these demands, a group of special refractories, termed the " pure oxides, " have been developed. Although made of the same general types of refractory materials t h a t have been in use for years, these "pure oxide" refractories have superior qualities, in great part due t o their freedom from fluxes. They are characterized by being monocrystalline and self-bonded as compared with conventional glass-crystal-bonded refractories of the fireclay or " super-refractory'' types. The refractory oxides of interest are, in order of increasing cost per unit volume, alumina, magnesia, zirconia, beryllia, and thoria. For light refractories, such as crucibles and tubes, all of these oxides have been commercially developed. Some of these shapes are illustrated in Figure 1. For heavy refractory ware, only the three pure oxides reasonable in cost have been used at present: alumina, magnesia, and zirconia.
11. Properties (I)
Description
T h e refractories whose properties are to be described have three common properties: (a) They are of high purity, consisting of a minimum of 97oj', alumina, or magnesia, or stabilized zirconia (zirconia plus stabilizer), (b) they are composed principally of electrically-fused grain, and (c) they have been fired to pyrometric cone 35 (177.5" C. or 3230" F.).
Fig. 1-Group o f Pure Oxide Refractories
( A ) A l u m i n a Products
RA 1190-35. A dense pure alumina product of 14 mesh and finer sizing. A1,0, = 99 +%. LA 63A-35. A pure alumina insulating product of relatively low bulk density and having grain size eight mesh and finer. A1203 =99 +yG. LA 63C-35. Similar in composition to LA 6311-35 but of higher bulk density. LA 63D-35. Similar in composition to LA 63C-35 but of still higher bulk density.
( B ) iwagnesia Products
LM 171-35. A dense periclase product, six mesh and finer in size. NTgO =98 +yo. ( C ) Stabilized Zirconia Products
LZO 190-35. A dense lime-stabilized zirconia product, eight mesh and finer in size. ZrOz plus CaO =9801,. LZO 172A-35. A dense lime-stabilized zirconia product, six mesh and finer in size. ZrOz plus CaO =99 +%. LZO 148A-35. A dense lime-stabilized zirconia product, 14 mesh and finer in size. ZrOz plus CaO =98 +yo.
LZO 187-35. A lime-stabilized zirconia insulating product, eight mesh and finer in size. ZrOz plus CaO =98TC. 111. Uses ( I ) H e a t Transfer and Heat Storage of PUW Oxide Refractory IVaZZs The great variation in bulk density, thermal conductivity and specific heat of pure oxide heavy refractories requires careful consideration in predicting the characteristics of furnace walls. To illustrate these variations, the heat transfer and heat storage of 4% inch thick pure oxide refractory walls backed up with 4% inches of insulating firebrick were calculated. T h e hot face was assumed as 2550" E., although much below the useful temperature of these refractories. T h e calculated values are shown in Figure 2.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1948
FIG. 2-HEAT
TRANSFER A N D STORAGE OF PURE OXIDE REFRACTORY WALLS
I I
Pure Oxide Refractory
Al2Os Insulating A1203 Magnesia Stabilized Z r 0 2 Insulating Stab. ZrOz I
69 A
RA 1190-35 . . . . . . . . . . . . . . . . . . LA 63A-35.. . . . . . . . . . . . . . . . . . LM 171-35 . . . . . . . . . . . . . . . . . . . LZO 148A-35. . . . . . . . . . . . . . . . LZO 187-35. . . . . . . . . . . . . . . . . .
The heat transfer of a zirconia lining would be expected t o be lower than alumina or magnesia. However, Figure 2 indicates that, despite the greater bulk density, a zirconia wall would store less heat because of low specific heat.
Alumina Uses and Limitations In general, pure oxide alumina refractories will withstand up to 1900" C. in oxidizing or reducing atmospheres. A high temperature laboratory kiln has been used to 1950" C. with an RA 1190-35 lining with only slight glazing of the liningwhere the burner flame impinged. Alumina has been heated t o 1800" C. in contact with graphite with no destructive effect. A twelve foot high molybdenum wound furnace lined with RA 1190-35 and insulated with insulating alumina LA 63A-35 has been operated successfully for long periods a t a 1900" C. winding temperature in a dry hydrogen atmosphere. The chamber temperature is low (1500° C.) because of high end heat losses encountered in its particular use. Refractories containing appreciable quantities of Si02cannot be used near the interior of this type of furnace because of reduction of the Si02. The high electrical resistivity of alumina is demonstrated by this furnace. The high thermal conductivity of dense, pure alumina refractories should be considered in application but this may be offset by insulating with low density, pure alumina refractories. (2)
(3) Magnesia Uses and Limitations Pure magnesia refractories can be used to 2300" C. in oxidizing atmospheres as demonstrated by the reheat test.* At high temperatures (above 1700" C.), magnesia cannot be used in highly reducing atmospheres. In high temperature use, ordinary magnesia refractories such as magnesite have been limited to furnace sections subjected to little or no load. This limitation is no longer necessary for the pure periclase mixture LM 171-35 as demonstrated by the high temperature load tests.*
I
1 I
Heat Transfer Btu./hr./ sq. ft.
Heat Storage in Pure Oxide Btu./sq. f t . of Wall
2290 1970 2305 2020 1925
920 730 930 765 7 00
46,000 20,000 45,000 35,000 24,000
I
I
Specific Heat
Cold Face Temp,. O F.
0.271 0.269 0.288 0.169 0.168
360 325 365 330 320
Interface
T:$p.
I
The high thermal conductivity must be considered in furnace design. This may be offset in some applications by backing up with insulating zirconia but the magnesia-zirconia interface temperature must be held below 2000" C. (3630" F.). Above this temperature and because of the eutectic between magnesia and zirconia, a low viscosity liquid is formed. A laboratory load-test furnace was lined with a H'' wall magnesia tube backed u p with insulating zirconia. This furnace was heated to 2200" C. and held for 10 minutes. The furnace bottom was covered by a one inch thickness of the liquid, which crystallized in cooling, and the magnesia tube had almost entirely disappeared.
(4) Stabilized Zirconia Uses and Limitations Stabilized zirconia refractories may be used up t o 2400" C. in oxidizing and moderately reducing atmospheres as shown by the reheat data and will withstand loads up to high temperatures.* Zirconia is an excellent high temperature insulator. The laboratory loadtest furnace, when constructed entirely of stabilized zirconia, has been heated easily to 2435" C. (4415" F.). Difficulties in reaching temperatures over 2100" C. have been encountered with the same burners when other materials have been used as the lining. When in c o n t k t with carbon a t 2200" C. (3990" F.) in vacuum, or in hydrogen or nitrogen atmospheres, zirconia will decompose to carbide, nitride, and hydride. However, graphite tube furnaces have been heated to 2300" C. using insulating stabilized zirconia as insulation whereathe outer part of the insulation is exposed to air with no detectable carbide formation in the zirconia. Zirconia has very low electrical resistivity at high temperatures so that i t cannot be used as a core in high temperature wire-wound resistor furnaces. *For detailed article on "Pure Oxide Refractories" by 0 . 1 . Whittemore, Jr., Norton Research Laboratories, write to: .
NORTON COMPANY
WORCESTER 6, MASS.
