Ind. Eng. Chem. Fundam. 1984, 23, 436-440
436
thickness profiles of Carreau fluids smoothed out to uniform films after a sufficient spinning time. Power-law fluids exhibited steep gradients in their film thickness profiles and a spike at the origin even after long spinning time. The power-law model of non-Newtonian fluids was shown to be inherently unsuitable for the analysis of axisymmetric free surface flow on a flat rotating disk. The results also showed that the film thickness and its uniformity are determined chiefly by the rheological properties of the fluids and not by the initial profile prior to spinning. Nomenclature D e = dimensionless number, eq 19 h = film thickness, m ho = scaling parameter for film thickness, m H = dimensionless film thickness, h/ho K = power-law model parameter n = power-law index q = volumetric flow rate per unit circumference, eq 3 Q = dimensionlessvolumetric flow rate per unit circumference, eq 15c r = radial spatial coordinate, m ro = scaling parameter for radius, m R = dimensionless radial spatial coordinate, r / r O
= R(1-n)fn t = spinning time, s to = time constant, eq 9f T = dimensionless spinning time, eq 9e u, = radial component of velocity, m/s u8 = azimuthal component of velocity, rw V = dimensionless radial component of velocity, eq 15b x = dimensionless variable, eq 9d xo = initial value of x: y = dimensionless variable y1 = initial value of y ~f
z,z ' = axial spatial coordinate, m Z = dimensionless axial spatial coordinate, z / h o
Greek Letters = zero-shear viscosity, Pa s 0 = aximuthal spatial coordinate, rad X = time constant of a Carreau fluid, s qo
vo = kinematic viscosity, q o / p (m2/s) p =
density, kg/m3
= shear stress in rz plane, Pa w = angular velocity of disk, rad/s T,
Literature Cited Acrivos, A.; Shah, M. J.; Petersen, E. E. J. Appl. Phys. 1980, 37,963-968. Bird, R. B.; Armstrong, R. C.; Hassager, 0, "Dynamics of Polymeric Liquids". Voi. 1; "Fluid Mechanisms": Wiley: New York, 1977. Dahiquist, G.; Bjork, A. "Numerical Methods"; Prentice-Hall: Englewood Cliff, NJ, 1974; p 2. Daughton, W. J.; O'Hagan, P.; Givens, F. L. "Proceedings of Kodak Seminar on Microelectronics"; Kodak: Rochester, NY, 1978; p 15-20. Emslii, A. G.; Bonner, F. T.; Peck, L. G. J. Appl. Phys. 1958, 2 9 , 858-862. Givens, F. L.; Daughton, W. J. J. Electrochem. Sac. 1979, 726, 269-272. Jenekhe, S . A. Polym. Eng. Sci. 1983, 23, 830-034; 713-718. Jenekhe, S. A. Ind. Eng. Chem. Fundam. 1984, companion paper in this issue. John, F. "Partial Differential Equations"; Applied Mathematical Sciences, Voi. 1, Springer-Veriag: New York, 1971. Matsumoto, S.; Takashima, Y. "Proceedings of International Conference of Liquid Atomization and Spray System", Tokyo, 1978; p 145. Matsumoto, S.; Takashima, Y.; Kamiya, T.; Kayano, A.; Ohta, Y. Ind. Eng. Chem. Fundam. 1982, 21, 198-202. Meyerhofer, D. J. Appl. Phys. 1978: 4 9 , 3993-3997. O'Hagan, P.; Daughton, W. J. Proceedings of Kodak Seminar on Microelectronics", Kodak: Rochester, NY, 1977; pp 95-103. Perry, R. H.; Chilton, C. H., Ed. "Chemical Engineer's Handbook", 5th ed.; McGraw Hill: New York, 1973; pp 21-24, 18-63. Ukiistyi, A. E.; Tyabin, N. V.; Ryabchuk, G. V.; Lepekhin, G. I. Chem. Pet. Eng. (Engl. Transl.) 1978, 72, 519-521. Washo, B. D. IBM J. Res. Dev. 1977, 21, 190-198.
Received for review April 15, 1983 Revised manuscript received February 15, 1984 Accepted April 24, 1984
Compatible Phases in the System ZnO-CuO-Cu-Cr,O, Ahmed M. Gadalla Chemical Engineering Department, Texas A&M University, College Station, Texas 77843
Zinc chromite exists over a wide composition range due to dissolving copper oxides as well as excess ZnO and Cr,O3. While ZnO c a n take both Cr203and Cu,O in solid solution, Cr,O:, can dissolve ZnO but not Cu,O. The limits of solubility depend on temperature and firing atmosphere and were established at 1100 O C in air by X-ray diffraction. Mixtures of ZnO and Cuz0.Cr,03 react to form ZnO.Cr,O, solid solution and Cu,O. The established tie lines were used to construct a tentative quaternary diagram. The ultimate phases expected to exist in industrial catalysts composed of oxides of copper, zinc, and chromium after use for long periods are concluded.
Introduction Various patents deal with the art of preparing catalytic phases, which lie in this system. Zinc-chromium oxide catalyst was used in industry for methanol synthesis from synthetic gas under high pressures but is now being replaced by low-pressure catalysts based on Cu-Zn oxide solid solution and Cr,03 or Al2O3. Catalysts containing oxides of Cu, Zn, and Cr are produced by several companies such as Badische Anilin und Soda Fabrik (BASF), Imperial Chemical Industries (ICI), Metall. Gesellschaft (MG), Power Gas Corporation (PGC), and Japan Gas and Chemical Co. (J.G.C.). The latter catalysts are now at-
tractive for the water gas shift reaction since they can operate at low temperature where the equilibrium is more favorable. An extensive range of compositions has been set forth and it should be noted that for each composition, different phases will form with various rates during the heat treatment process. Intermediate phases with variable states of oxidation can exist as noncrystalline phases, solid solutions, or fine crystallites. Accordingly, the activity and selectivity of each catalyst are very sensitive to operating conditions and in the literature one finds conflicting results, speculative conclusions, and claims that a certain
0196-4313/84/1023-0436$01.50/00 1984 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984
catalyst is orders of magnitude more active and more selective. This work determines the equilibrium phases existing for various compositions and helps in assessing the stability of various catalysts by indicating the possible reactions occurring during service.
Previous Work Oxides of Copper and Zinc. Chapple and Stone (1964) reported that ZnO at lo00 OC can take up to 10 mol % CuO in solid solution, accompanied by a change in its color from pale green to deep green. Using X-ray diffraction and electron microscopy, Mehta et al. (1979), Herman et al. (1979), and Bulko et al. (1979) showed that ZnO can dissolve 4-8% CuO at 350 OC in air. Schiavello et al. (1974) tabulated the change of lattice parameters with composition and concluded that the formation of the solid solution is accompanied by a slight deformation of the zinc oxide structure. They stated that the limit of solubility in air at 1000 OC is only 4 at. % copper. Using a reducing atmosphere containing 2% H2 and 98% Nz,Bdko et al. (1979) found that 11-12% copper was dissolved in ZnO at 250 “C. In one mixture, which initially contains copper and zinc distributed on the atomic scale as (Cu,Zn),(OH),C03, a solubility limit above 17% was reported. It is uncertain if the mixture is a saturated or a supersaturated solid solution. On the other hand, Schiavello et al. (1974) claimed that reducing zinc oxide solid solutions, containing initially 3 mol % CuO, precipitates Cu20. This conclusion contradicts the above results which indicate higher solubility in reducing atmospheres. This conclusion, however, was not based on detecting Cu20 but was based on the observation of Schiavello et al. that the lattice parameters decreased slightly after using the catalyst in a reducing atmosphere. Oxides of Zinc and Chromium. In view of earlier work by Ol’Shanskii and Shlepov (1953), Kubota (19611, and Johnson and Muan (1968) and in view of recent results by Abadir and Gadalla (1976), equilibrium relationships in the Cr-0 system were established. Conflicting data exist in the literature regarding the phases appearing during catalyst preparation. Puri et al. (1976) reported the formation of an unidentified phase on the dissociation of hydroxy zinc chromates. While this phase was considered by Goroshko et al. (1975) to be a metastable phase dissociating to the spinel “ZnCr,Oc and ZnO, it was considered by Shirashi (1981) to be a stable phase soluble in ammoniacal solution. Williams and Cunningham (1974) identified this phase as zinc dichromate ZnCr20, and stated that heating ZnCrO, will produce the dichromate and ZnO. The dichromate was found to dissociate in air or Nz below 525 OC to spinel and oxygen. Since the chromates are unstable, they will dissociate during preparing the catalyst or in the reactor under the highly reducing conditions. Thus they are not included in the present study. Zinc chromite, on the other hand, is stable and was found by Williams and Cunningham to take zinc oxide in solid solution with an increase in its lattice parameters. The limit of solubility was not determined. ZnO can also take Cr203in solid solution causing a slight shift in its X-ray diffraction pattern. Mehta et al. (1979) found that the solubility limits of CuzO and Cz03 when added together to ZnO, as a charge coupled substitution, are much higher than the solubility limits of the individual oxides. The charge deficiency due to replacing divalent Zn2+ with trivalent Cr3+ is made up by adding Cu’ to substitute another Zn2+without introducing any vacancies in the ZnO lattice. Under reducing conditions in the
437
Z
Z = c = C’ = Cu= K =
ZnO CUO Cue0
ZK = CK = C’K =
ZnOCrp03 CuOCr,O, Cu,oCr&
Copper
CrtO,
Figure 1. Tentative diagram for the quaternary system ZnO-Cr203-CuO-Cu (ignoring solid solutions). While solid tie lines were added in view of previous and present results, dashed tie lines were deduced.
presence of Cr2O3,16% copper oxide can be included in ZnO solid solution. Oxides of Copper and Chromium. Phase relationships in the system Cu0-Cu20-Cr203 were studied by Gadalla and White (1964) as a function of temperature at various oxygen partial pressures. Cu0.Cr203 reacts with excess CuO giving Cu20-Cr203with isothermal oxygen loss. The equilibrium temperature decreases with reduction in the oxygen partial pressure and in air it occurs at 890 OC. Any unreacted CuO dissociates to CuzO and the dissociation temperature increases with oxygen potential (in air it occurs at 1026 OC). Cu0.Crz03dissociates in the absence of free copper oxide to Cuz0.Crz03and Crz03with isothermal oxygen loss. In air this reaction occurs at 1100 OC. Both cupric and cuprous chromites were found to take a limited amount of CuO in solid solution.
Theoretical Discussion for the Quaternary System After dehydration and dissociation of chromates, the various compositions of Cu-Zn-Cr oxide catalysts lie inside the quaternary system Zn0-Cr,03-Cu0-Cu which is constructed and shown tentatively in Figure 1. This is a simplified diagram showing the relation between stoichiometric phases (i.e., by ignoring solid solubility). In view of the phase rule it can be stated that, at constant oxygen partial pressure and at the same temperature, the maximum number of phases which can coexist in equilibrium is four. Such four phases are joined by a tie-tetrahedron. The quaternary is shown to consist of the tetrahedra which are formed from the tie lines established in the ternary systems. The reaction path for any mixture will be a straight line extending from the face Zn0-Crz03-Cu0 (Z-K-C) to the face ZnO-Cr203-Cu(Z-K-Cu) and must be parallel to the base Crz03-CuO-Cu (K-C-Cu) since during oxidationreduction reactions the ratio ZnO/Crz03is kept constant. A t low temperatures and/or at high oxygen partial pressures the compositions will be near the face ZnO-Crz03-Cu0 (Z-K-C). During use (under reducing conditions) the copper exists as free copper metal and has catalytic property for methanation. As a first approximation the remaining phases lie near the ternary system ZnO-Crz03-Cu20, and if the solid solution phases contain both
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Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984
o
Cne Phase TwoPhoses Three Phoses
A
Industrial Catolyst
a
- Binary SS SS
Solid Solution
JGC P
Yo 0 Cr,O,
maximum solubility of 4 molar % CupOin ZnO was found to occur under the experimental conditions. Results obtained on firing ZnO-Crz03 mixtures are shown on the ZnO-Crz03edge. Cr,03 can take up to 20% molar ZnO in solution. Comparing the ionic radii of Zn2+ and Cr3+it could be concluded that Zn2+can substitute for Cr3+in the Cr203structure. A possible mechanism for the solubility of ZnO in Crz03can be expressed by the equation 2Zn0
Cr20,
2Zn'c,
+ 2 0 0 + V0
(1)
Similarly, ZnO can take up to 7% molar Crz03in its lattice. A possible mechanism is Cr203
cu
Figure 2. Compatible phases in the system Cu20-ZnOCr203in air a t 1100 OC.
Cu2+and Cu+ ions they will be slightly shifted toward the left.
Experimental Techniques Fisher certified ACS grade CuO and ZnO and Fisher certified grade Cr203were used in this study. In view of previous work by Gadalla and White (1964), the plane Zn0-Cr,03-Cuz0 can be achieved in air at 1100 "C and at lower temperatures under reducing atmospheres. Accordingly, mixtures of ZnO, CuO, and Cr203were prepared, mixed with intermediate grinding in an agate mortar, pressed to pellets (to increase the contact and enhance solid state reactions), fired with a rate of 10 "C/min up to 1100 "C and held at that temperature for 2 h. The pellets were crushed, ground, compacted again, and fired. The process was repeated until microscopic and X-ray examinations indicated the absence of any further change. The phases are reported and plotted on the ternary diagram Zn0-Cu20-Cr203 shown in Figure 2. During this process weight changes were followed and showed that losses due to volatilization were not detectable. Previous investigations have shown that at such high temperature bulk diffusion occurs and homogeneous phases are produced. The rate of reaction was found to be high around 1026 "C, the temperature at which CuO dissociates to CuzO in air. The disadvantage of this technique for producing catalysts is that the resulting phases are coarse with low surface areas and are not active as catalysts. On the other hand, this technique will produce sharp X-ray peaks with minimum broadening and facilitates identification of phases. This technique was used to establish the compatible phases and the limits of solubilities in the system CuzO-ZnO-Crz03 in air at 1100 "C. Results and Discussion The Binary Systems. Mixtures of CuO and ZnO were fired at 900 "C to avoid dissociation to CuzO (with intermediate grinding to promote reaction and homogeneity as explained earlier). No binary compound was formed and only the diffraction patterns for ZnO solid solution and CuO were found to exist. To investigate the system CuzO-ZnO in air, firing was carried out at 1100 "C, above the dissociation temperature of CuO. As indicated in Figure 2, ZnO does not dissolve to any appreciable extent in Cu20. On the other hand a
3ZnO
2Cr'z,
+ 300 +
(2) The above equations are written using Kroger-Vink notation which is widely used in defect chemistry. The equations are written so as to conserve electrical neutrality, mass balance, and site relations. Znlc, indicates a Zn2+ occupying the normal lattice site of Cr3+,thus disturbing the electrical neutrality, and is equivalent to adding a local negative charge; 00 indicates oxygen ion occupying a normal 0" site. V o indicates a vacant 0" site, which corresponds to two missing electrons or two localized positive charges. Similarly, Cr'& indicated C P in the Zn2+ position with a localized positive charge and is a vacancy in the Zn2+ site corresponding to two excess localized electrons. For more details on these notations, reference is made to Kroger (1964). The spinel was found to exist over a wide range of compositions including the stoichiometric composition Zn0.Cr203which can take either excess Zn2+or excess C P in solid solution. In the first case, oxygen vacancies can be created (reaction 1) and in the second case cation vacancies in zinc sites are created (reaction 2). In view of the present results, the previously published behavior of mixtures of Crz03and ZnO can be explained. Ishi et al. (1977) used a constant heating rate of 10 OC/min and obtained DTA curves with two exothermic peaks in various atmospheres. The first peak was found to occur at temperatures depending on the oxygen partial pressure. While in nitrogen it occurred at 800 "C, in oxygen it occurred at 700 OC. They attributed this peak to catalytic oxidation of Cr,03 to Cr03 in the presence of ZnO. According to Abadir and Gadalla (1976), Cr03 is unstable under the above operating conditions. To exclude the possibility of oxidation at low temperature to ZnCr2C, ?r ZnCrO,, the present author heated ZnO and Crz03in air at 450 "C for 24 h and the diffraction pattern was examined. A temperature of 450 "C was selected since Williams and Cunningham (1974) stated that the dichromate dissociates in air below 525 "C. The only intermediate phase which was found to form was the spinel "ZnO-Crz03". Accordingly, the above-mentioned peak can be attributed to the formation of ZnCr,O,. The shift in peak temperature to lower values at higher oxygen pressures is due to decreasing the number of available oxygen vacancies as the oxygen pressure increases. Since the product of the vacancy concentration is fixed by the equilibrium equation, concentration of Cr3+vacancies increases causing an increase in the rate of diffusion of Cr3+which is the limiting factor in forming the spinel. The second exothermic peak, which was not explained by Ishi et al., can be attributed to the solution of the unreacted oxides in the spinel solid solution. The behavior of commercial zinc chromite catalysts for methanol synthesis can also be explained in view of the present results. The catalyst supplied by the Harshaw
Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984
Chemical Company (Zn0312, T 1/4, Batch 69), was found to consist of ZnCrO,.Zn(OH),, ZnCrO,, Zn(OHI2,ZnCr204 solid solution, and Cr(N03)3.9H20. The diffraction patterns of the first three phases indicate that they exist as very fine crystallites since their peaks were present but were broad. Values for d-spacings for the spinel solid solution were close to those published for stoichiometric ZnCr,04. In addition to the above crystalline phases a noncrystalline phase (possibly the binder) caused the background to form a broad diffraction band at low angles. This pattern indicates that the effective catalytic phase(s) will be formed in the reactor. The following dehydration and reduction reactions are expected to occur even in air 2 [ZnCrO,.Zn( OH)2] = ZnCr204+ 3Zn0 + 3/202 + 2H20 (3)
+ H20 BZnCrO, = ZnCr204+ ZnO + 3/202 ZII(OH)~= ZnO
(4) (5)
2[Cr(N03),.9H20] = Cr2O3 + 18Hz0 + 6N02 + 3/202 (6) Cr203+ ZnO = ZnCr204 (7) The reactions are more complicated and the phases produced will be nonstoichiometric due to formation of solid solutions. In the presence of hydrogen, water will be produced instead of oxygen, and reactions 3,5, and 6 may tend to be exothermic. This implies that in all catalysts containing zinc chromite, the chromate, the hydrate, and the hydroxy-chromate peaks should disappear and the amount of ZnO solid solution should increase. This statement was found to be correct when the catalyst was heated in air at 450 OC for 2 h. It should be noted that if this catalyst is heated rapidly in the reactor, the sudden evolution of gases may shatter it to fine particles. The Ternary Cu20-Zn0-Cr203 It is evident from Figure 2 that the binary spinel solid solution can also dissolve copper oxides and the compositions reached in air at 1100 “C are projected on Figure 2. What appears here as a ternary solid solution (S.S.) may be actually a quaternary S.S. containing both Cu2+and Cu+ which lies in the space between this plane and the face ZnO-CuO-Cr203 (Figure 1). The existence of maximum solubility of copper oxide in spinel on the line joining the compositions corresponding to ZnOCr203and the projection of Cu0.Cr203 (66.7% Cr203and 3.3% Cu20)may be due to the solubility of Cu0.Cr203 in Zn0.Cr203. They both have spinel structure and the two phases may form a complete series of solid solutions at low temperatures when the mixtures lie on the face Zn0-Cu0-Cr,03. As the temperature increases or as the oxygen pressure is reduced, the solubility limit is expected to shrink; i.e., Zn0.Cr203 can stabilize Cu0.Crz03 to a certain degree by taking it in solution. It is also evident that ZnO takes both Cu+ and Cr3+in solid solution, as a charge coupled substitution, as proved earlier by Herman et al. (1973) Mehta et al. (1979), and Bulko et al. (1979). This can be expressed by the equation
42110
Cu20 + Cr& BCU’~,+ 2Cr’2n+ 400 (8) It seems also that the solution of ZnO in Cr203is limited to the binary edge and Cr203does not take copper oxides in solid solution. This is in harmony with previous work carried out by Gadalla and White (1964) in the system Cr203-CuO-Cu20. It is also clear that Cu20does not take any appreciable amount of Cr3+or Zn2+in solid solution. The existence of a wide range of compositions consisting of spinel phase and Cu20 indicates that the tie line ZnOCuz0.Crz03does not exist and the following reaction has
439
a negative free energy change. ZnO(S.S.)+ Cu20’Cr203(SS.)=ZnO*Cr203(S,S.) + CUZO (9) The tie lines Zn0-CrzO3-Cu20(ZK-C’) and Cu20.Cr203ZnOCr203(C’K-ZK) were used to construct the quaternary systems shown in Figure 1. The intersection of the reaction pathes of some industrial catalysts with the plane Zn0-Cr203-Cuz0 are indicated by points. In the absence of information on the limits of solubilities under the operating conditions for methanol synthesis and in view of statements of Mehta et al. (1979) and Herman et al. (1979) that reducing atmospheres produced higher solubility limits in ZnO, it could be assumed that BASF catalyst consists of ZnO (S.S.) and copper. The effective phases in the catalyst of Power Gas Corporation and Japanese Gas Chem. Co. are ZnO (S.S.) and a spinel S.S. These phases when fully oxidized consist of a spinel solid solution, ZnO solid solution, and CuO. The Quaternary Zn0-Cu0-Cu-Cr203 A tentative quaternary system was constructed (Figure 1)in view of previous and present work after ignoring the solid solutions and plotting only stoichiometric compositions. Those lines in the base triangle Cr203-Cu0-Cu20 (KC-C’) were previously established by Gadalla and White (1964), while those in the system Zn0-Cr203-Cuz0 were established during the present study. Tie lines Cu20. Crz03-Cu (C’K-Cu), ZnOCr203-Cu (ZK-Cu), and ZnO. Cr203-Cu0Cr,03 (ZK-CK) are inevitable. The first two tie lines were not reported before and are shown dashed. While the time line ZnO-CuO-Cr203(Z-CK) does not exist, the tie line Zn0Cr203-Cu0 (ZK-C) does exist. This conclusion was based on the fact that the second tie line is the only one in harmony with the above established tie lines and divides the whole quaternary to small tetrahedra. Accordingly, the deduced tie line ZK-C was shown dashed. The small tetrahedra thus formed determine the compatible phases, which are 1. Cr203-Zn0~Cr,03-Cu0~Cr203-Cu20~Cr203 (K-ZK-CK-C’K) 2. Zn0~Cr203-Cu0~Cr,03-Cu20~Cr203-Cu0 (ZK-CK-C’K-C)
3. Zn0~Cr203-Cuz0~Cr203-CuO-Cu20 (ZK-C’K-C-C‘) 4.
Zn0-Zn0-Cr203-Cu0-Cu20
5.
Zn0-Zn0.Cr203-Cu20-Cu
(Z-ZK-C-C’) (Z-ZK-C’-Cu)
(ZK-C’K-C-Cu) 6. ZnO~Cr203-Cu20~Cr203-Cu20-Cu 7. Cr2O3-Zn0.Cr2O3-Cu2O.CrzO3-Cu (K-ZK-C‘K-CU) It should be noted that under operating conditions for methanol synthesis lower temperatures and reducing atmospheres are used. The limits of solubility will vary, but the present information indicates the tendency for any change and the possible phases that may coexit together on use. Conclusions While no intermediate compound was found to exist between ZnO and CuO or between ZnO and Cu20, the spinel Zn0.Cr,03 was found to exist over a wide range of compositions. It takes excess ZnO or excess Crz03in its lattice forming a binary solid solution. The spinel can also dissolve copper oxide forming a ternary or quaternary S.S. While Cr203dissolves a large amount of ZnO, Cu20.Cr2O3 dissolves a limited amount of ZnO. The limits of solubility
440
Ind. Eng. Chem. Fundam. 1984, 23, 440-446
depend on the firing atmosphere and temperature, and the solubility limits indicated in Figure 2 are determined in air at 1100 "C. ZnO and Cu20.Cr203are not compatible and on firing they form Zn0.Cr203 and Cu20. The tie lines determine the compatible tetrahedra in the quaternary system ZnOCuO-Cu-Cr203. Registry No. ZnO, 1314-13-2; Crz03,1308-38-9; CuO, 1317methanol, 67-56-1. 38-0;CuzO, 1317-39-1; Literature Cited Abadk, M. F.; Qadalla, A. M. Trans. J. Br. Ceram. Soc. 1976, 75(4), 74-77. Bulko. J. P.; Herman, R. G.; Kller, K.; Simmons, G. W.J. Phys. Chem. 1979, 83, 3118-22. Chapple, F. H.; Stone, F. S. Proc. 8r. Ceram. Soc. 1964, 7 , 45-58. Gadalla, A. M.; White. J. Trans. Br. Ceram. SOC. 1984, 63(10), 535-52. Goroshko. 0. N.; Lavrova, V. V.; Rusov, M. T.; Ryzhak, I. A. Katal. Katal. 1975, 73,92.
Herman. R. G.; Kller, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. B. J. Catal. 1978, 56, 407-29. Ishil, T.; Furruichl. R.; b r a . Y. J. Thennal Anal. 1877, 7 7 , 71-80. Johnson, R. E.; Muan, A. J . Am. &ram. Soc. 1888, 57, 432. Kr-r, F. A. "The Chemistry of Imperfect Crystals", North-Holland Publlshlng Company: Amsterdam, 1964. Kubota, 8.J. Am. Ceram. Soc. 1861, 44(5), 247. Mehta, S.; Simmons, G. W.;Klier, K.; Herman, R. G. J. Catal. 1979, 57, 339-60. Ol'shanskll, Y. I.; Shlepov, V. K. Wl.Akad. Naukk SSSR 1953, 97, 583. Purl, V. K.; Awasthl, R. B.; Choudhary. R. L.; Qangull, N. C.; Ghosh, S. K.; Sen, S. P. fertilzer Techno/. ( I d i a ) 1976, 73(2-3), 158-61. Schiavelio, M.; Pepe, F.; DeRosi, S. Z . Phys. Chem. (frankfurt am Main) 1874, 92, 109-24. Shirashi, T. "ham Kogyokoto Semmon Gekko Kuo, Rikogaku Hen 1981, 17. 45.
WIIIlams,~R.J. J.; Cunningham, R. E. Ind. f n g . Chem. Rod. Res. Dev. 1974, 73,49-60.
Receiued for review May 5, 1983 Accepted April 9,1984
The Quaternary System Cr,O,-MgO-CuO-Cu,O in Air and Its Bearing on the Performance of Copper Melting Furnaces Ahmed M. Gadalla" and Nalla A. L. Mansour Chemical Engineering Department, Texas A&M University, college Station, Texas 77843
The quaternary system was studied by construcing four sections, three of constant magnesia content and the fourth having a MgO/Cr203ratio of 1: 1. The existing conjugation tetrahedra were flxed and the corresponding reactions were established. MgOCr,O, dissolves copper oxldes and excess MgO and Cr203forming a wide range of solid solutions. Cu2Mg03 takes Cr2O3 in its lattice and its stability drops. I n the presence of Cr203, the solubility of decreases the stabili copper oxides in periclase decreases. CuO-C~O,enters into the spinel phase. Cu20-Cr203 of guggenite giving Cu20 and spinel at 1035 C. Partial melting with isothermal oxygen pickup occurs at about 1120 OC in mixtures poor in Cr2O3. Increasing the Cr2O3 content in chrome-magnesite bricks increases the a refractoriness due to the formation of solid solutions, but above saturation refractoriness. Magnesite bricks have h the liquid phase is formed and it penetrates the perlclase grains.
Introduction Studying the attack of refractories used in copper melting and refining furnaces is not as difficult as that used in the extraction process. While in copper melting furnaces the slag consists primarily of copper oxides, in copper converters consideration must be given to modes of attack both by ferrous silicate and by copper oxides. To throw light on the corrosion problem in copper melting furnaces, phase relationship in relevant systems should be studied. Previous work by Gadalla et al. (1963) and Gadalla and White (1964a*b*C) on the systems CuO-Cu20-Si02, CuOCU~O-A~~ CuO-Cu20-Mg0, O~, and CuO-Cu20-Cr203explains the reason for the superiority of refractories containing MgO or Cr203 over silica and aluminosilicate refractories in melting copper and they recommended using chrome-magnesia or magnesia-chrome refractories. The present study is carried out to determine the optimum composition which can resist fluxing by copper oxides. Previous Work The System Cu0-Cu20-Mg0. Trojer (1958) and Riby and Hamilton (1961) reported the formation of a binary compound in basic refractory bricks and named it 'guggenite". They assumed that it has the composition 0198-4313/84/1023-0440$01.50/0
CuMg02. Gadalla and White (1964b) proved that guggenite is an intermediate binary phase of composition near to Cu2Mg0,. Later Drenkhahn and Mueller-Bouchbaum (1975) confirmed the existence of Cu2Mg03and reported that it is orthorhombic. Gadalla and White (1964b) found that this compound is more stable than CuO and dissociates in air isothermally at 1060 "C with loss of oxygen giving CuzO and magnesia. They proved that contrarily to silica and alumina, where a liquid phase is formed at 1060 "C and at 1230 "C, respectively, as soon as copper oxide is added, no liquid is formed with magnesia unless the copper oxide content (taken as solid solution by magnesia) exceeds 35.7 wt %. The System Cu0-Cu20-CrzOp During service in copper refineries, Rigby and Hamilton (1961, 1962) and Ust'yantsev et al. (1971) reported the existence of copper chromite which is formed from the reaction of copper oxide with free Cr203 or with Cr203in the spinel phase that contains mainly magnesium chromite. The latter was found to decompose in the presence of copper oxides and silica. Gadalla and White (1964c), however, investigated the system at different oxygen partial pressures. In air, in the presence of excess CuO, the spinel Cu0.Cr203has a low stability and forms Cuz0.Cr2O3with oxygen loss at about 890 "C but in the presence of Cr2O3,CuOCr203 0 1984 American Chemical Society
