Studies of Vaporization of Chromium from Thin Slag Films at Steelmaking Temperatures in Oxidizing Atmosphere SESHADRI SEETHARAMAN, GALINA JELKINA ALBERTSSON, and PIOTR SCHELLER In the present work, the volatilization of chromium from thin chromium-containing slag film surfaces was studied in oxidizing atmosphere in the temperature range 1673 K to 1873 K (1400 °C to 1600 °C). The slag films on alumina rings were exposed to air or pure oxygen and the loss of Cr from the post-experiment sample films was examined by SEM/WDS analysis. The mass loss of the samples was also monitored during the heat-treatment. The results indicate that chromium loss increased with increase in temperature and oxygen partial pressure was found also to be relatively less as the sample thickness increased. The implications of chromium escape from slags during the tapping of stainless steel slags are discussed. DOI: 10.1007/s11663-013-9904-y Ó The Minerals, Metals & Materials Society and ASM International 2013
I.
INTRODUCTION
CHROMIUM is an important alloying element in stainless steel. During the production of stainless steel, the final slag after oxygen blowing has a chromium oxide content as high as 8 mass pct as is known in Swedish steelmaking practice.[1] In view of the carcinogenic effect of hexavalent chromium, efforts are made world-wide to stabilize chromium in the silicate matrix of the solidified slag before disposal with a view to avoid the leaching of the harmful oxide by acid rains over a time period. On the other hand, very little information is found in literature regarding the emission of CrO3 vapor from slags during tapping of the slag after making alloy steel grades containing chromium. The present work was aimed at examining the possibility of CrO3 emissions from Cr-containing slags at steelmaking temperatures and the extent of the same to evaluate the health hazard aspects on the shop floor. A thin film sample was used from which, the loss of Cr was followed in the temperature range 1673 K to 1873 K (1400 °C to 1600 °C). WDS analysis was used in the post-experiment samples to get the average Cr content of the slag films. The technique was originally developed at the Institute of Iron and Steel Technology, Technical University Bergakademie Freiberg (TUBAF), Germany by two of the present authors along with other coworkers[2] to study the vaporization of V2O5 from slags
SESHADRI SEETHARAMAN, formerly Mercator Visiting Professor with the Institute of Iron and Steel Technology, TU-Bergakademie, 09596 Freiberg Germany, is now Professor Emeritus with the Division of Materials Process Science, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden. GALINA JELKINA ALBERTSSON, Ph.D. Student, is with the Division of Materials Process Science, Royal Institute of Technology (KTH). Contact e-mail:
[email protected] PIOTR SCHELLER, Professor Dr.-Ing. habil., is with the TU Bergakdemie Freiberg, 09596 Freiberg, Germany. Manuscript submitted December 31, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
containing vanadium. The present studies were carried out with respect to the gaseous emission of CrO3 from chromium-containing slags under three different oxygen partial pressures, oxygen, air, and argon. A. Thermodynamic Analysis Chromium exhibits three stable valence states in slags, viz. Cr2+, Cr3+, and Cr6+ and the corresponding oxides species are CrO, Cr2O3, and CrO3. In the case of Cr-containing slags, Wang et al.[3] have shown that, at low oxygen partial pressures, Cr exists in two valence states, viz. Cr2+ and Cr3+. The latter was reported to be stable at higher basicities and higher oxygen partial pressures. Morita et al.,[4] in their studies on the solubility of MgO-Cr2O3 in CaO-MgO-Al2O3-SiO2 slags reported that, in MgO-CaO melts containing as much as 40 to 55 wt pct CrOx, more than 50 pct of chromium exists as Cr6+ in the high CaO and low MgO region. In a subsequent work, Morita et al.[5] had shown that, in the case of MgO-CrOx-SiO2 melts, at 1873 K (1600 °C), the activity coefficient of chromium oxide (cCrO1.5) shows a slight increase as the oxygen partial pressure increases. But, it is not certain whether CrO3 would escape from slag melts at steelmaking temperatures from the slag surface. Mausbach et al.[6] had investigated the slag systems where transition metal cations exist with variable valency, using a spectroscopic technique in the ultraviolet-visual region. In the case of a slag-containing 53 mass pct CaO and 47 mass pct Al2O3, doped with 0.48 mass pct Cr2O3, the ions Cr3+ and CrO42 could be detected by these authors when the slag was melted in air. This is in conformity with the results of Morita et al.[4] Okretic et al.[7] used the same technique in the case of the slag system CaO (mass pct 54.5) and SiO2 (mass pct 45.5), which was doped with 1.5 as well as 2 mass pct Cr2O3. According to the results of these authors, the slag which was melted at 1773 K (1500 °C) in air and quenched exhibited the spectrum of CrO42.
4,5
P(CrO3) x 10-4 [Pa]
4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 1650
1700
1750
1800
1850
1900
1950
2000
T [K] Fig. 1—The partial pressure of CrO3(g) (Pa) as a function of temperature in Cr–O2 system at elevated temperatures. The partial pressure of O2(g) = 9 9 104 Pa.
The authors describe the time-resolved spectral behavior of the CrO42 on the basis of the formation of Cr2O72 ions and subsequent evaporation of CrO3 vapor. The reactions presented by the authors are:
A slow escape rate of CrO3 from the slag to the gas phase at high oxygen pressures prevailing at the surface of the slag can be compensated by diffusion of chromium ions from the bulk. Okretik et al.[7] had presented a value for the chemical diffusion coefficient, D(Cr3+/ Cr6+) as 3.5 9 107 cm2/s. On this basis, if the diffusion of Cr ions from the bulk to the surface is the ratecontrolling step, the loss or Cr from the bulk slag may be within the analytical error limits and the Cr loss from the slag may be detected in a conventional chemical analysis. Estimation of Rayleigh and Reynolds numbers of a thin film system of the type studied indicates that convective mass transfer in the slag film as well as in the surrounding gas phase may not be the rate-controlling steps and the evaporation rate may be controlled by the diffusion of CrO3 to the slag film from the bulk to the surface. In this respect, it was felt that it would be useful to apply the thin slag film technique developed by the present workers at Freiberg, Germany[2] to study the escape of Cr from the slag surface. This technique enables the evaporation of Cr from both sides of the film and the surface to volume ratio would be high promoting the vaporization process.
2 Cr3þ þ 3=2 O2 þ 8ðNSi-OÞ ! Cr2 O2 7 þ 4 ðNSi-O-SiNÞ ½1 Cr2 O2 7
þ 4ðNSi-O-SiNÞ ! 2 CrO3 ðgÞ þ 2ðNSi-OÞ
½2
Among the oxides of chromium, CrO3 has the lowest melting point, 497 K (197 °C). The boiling point of CrO3 is 524 K (251 °C). It would be expected that CrO3 would have a high vapor pressure at elevated temperatures. Okretic et al.[7] estimated the vapor pressure of CrO3 as 0.1 Pa at 1773 K (1500 °C). If the vapor pressure of CrO3 is as high as these authors have reported, it should have been observed by earlier workers and should have been reported in other earlier publications on Cr-containing slags. To the knowledge of the present authors, there has been no report on the emission of Cr6+ from Cr-containing slags. FACTSAGEä calculations[8] indicate that the CrO3 gas was formed in an oxidizing atmosphere (PO2 ¼ 9 104 Pa) at steelmaking temperatures. The vapor pressure of CrO3 as a function of temperature at steelmaking temperatures from the above-mentioned source is presented in Figure 1. It is seen that the vapor pressures according to FACT SAGEä are much lower than the value reported by Okretic et al.[7] If the FACT SAGE calculations are to be relied upon, the emission of CrO3 would be considerably less from Cr-containing slags at steelmaking temperatures. Attempts to extract a phase stability diagram from FACT SAGEä involved a certain degree of uncertainty as the calculated phase diagram indicates the presence of the 4-valent Cr, viz. CrO2 (solid) as a stable phase existing at steelmaking temperatures. On the other hand, it is known[9] that CrO2 is also a low-melting oxide [melting point: 648 K (375 °C)] and is reported to decompose above this temperature.
B. Experimental In the preliminary experiments at TU-Bergakademie (TUBAF), Freiberg, Germany, thin slag films were formed on the Pt-30 pct Rh/Pt-6 pct Rh thermocouple in a Single Hot Thermocouple Technique (SHTT) unit. As this unit had no provision for the installation of flowrate control of the gas, the experiments were modified and the chromium loss was studied at the Royal Institute of Technology, Stockholm, Sweden in the thermogravimetric analysis (TGA) unit, where different gas atmospheres could be used and an accurate control of the gas flow rate could be achieved. C. Materials and Sample Preparation The slag samples were prepared from pure oxides. The purity as well as the suppliers are presented in Table I. The slag composition was chosen so that it was saturated with alumina. The composition in mass pct is given in Table II. The component oxide powders were mixed thoroughly in an agate mortar and pre-melted in an induction furnace. Alumina crucibles, with diameter of 16 mm, placed inside a conducting graphite crucible were used to pre-melt the samples. After the slags were melted and homogenized, an alumina tube with an inner diameter of 4 mm and an outer diameter of 6 mm was dipped into the slag melt and a column of the slag was sucked inside the tube. The alumina tube was then taken out and quenched at room temperature in argon atmosphere. The slag-containing alumina tube was then cut into thin slices of 0.5 and 1.0 mm thickness. The experiments were conducted above the solidus temperature 1619 K (1346 °C) of the slag of this composition, as computed by FACTSAGEä software (FACTSAGE 6.1), Thermfact Ltd. (Montreal, Canada) METALLURGICAL AND MATERIALS TRANSACTIONS B
Table I. Chemical Pt-wire Cr2O3 SiO2 MgO CaO Al2O3
Purity of the Chemicals Used, Weight Percent Purity (Percent)
Supplier
99.99 99.8 99 (reagent grade) 99 99 99.8
Alfa-Aesar, Germany Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich
Table II. Alumina-Saturated Synthetic Slag with a Slag Basicity (CaO/SiO2) Set to 1. The Slag Composition Weight Percent Al2O3
CaO
SiO2
MgO
Cr2O3
45.0
20.0
20.0
8.0
7.0
Table III. Amounts of Various Condensed Phases (Mass Percent) Present in Slag in Air According to FACTSAGE Software
ASlag-Liq#1 CaMg2Al16O27_Solid ASpinel
1673 K (1400 °C)
1773 K (1500 °C)
1873 K (1600 °C)
61.0 16.9 22.1
71.9 0.0 28.1
78.0 0.0 21.9
and GTT-technologies (Aachen, Germany). Databases chosen were Fact53 and FToxid. A phase analysis of the above slag in air at various temperatures according to FACTSAGEä calculations is given in Table III, where ASlag-liq#1 is the liquid slag and ASpinel is spinel phase solid solution, (FToxid database). It is seen that the phases present are liquid slag, solid CaMg2Al16O27, and spinel phase. The samples were heat-treated in air or oxygen at a pre-determined flow rate for a specified time interval in SETARAM TGA 24, (Setaram Instrumentation, Caluire, France) TGA instrument. During the oxidative heattreatment, the mass losses could be followed as a function of time. The instrument is provided with a dual furnace system having an accuracy of 1 lg. A detailed description of the instrument is given in an earlier publication.[10] The sample was suspended in the even temperature zone (±1 K) of the furnace by a Pt-wire inside the alumina reaction tube. An inert reference sample was positioned in the reference furnace. A Pt30 pct Rh/Pt-6 pct Rh thermocouple was used to monitor the temperature. It was placed as close to the sample as possible. The evaporation experiments were carried out under isothermal conditions. The sample was heated to the target temperature in the range of 1673 K to 1873 K (1400 °C to 1600 °C) at a heating rate of 30 K/min in argon atmosphere at a flow rate of 50 Nml/min. The oxidant gas was switched on when the system had attained steady target temperature. The starvation rate of gas flow was determined to be 300 Nml/min. In order to avoid any gas phase mass METALLURGICAL AND MATERIALS TRANSACTIONS B
transfer, the gas flow rate was kept above the starvation rate, viz. at 320 Nml/min. The samples were kept at the target temperatures for different time sequences. During the experiments, the mass and temperature changes of the sample were registered every 2 seconds by the software. The samples were cooled at the rate of 40 K/min after the experiment and then subjected to SEM/WDS analysis. Selected experiments were repeated to confirm the reproducibility.
II.
RESULTS
SEM/EDX analysis of the post-experiments samples from the experiments carried out at the TUBAF, Germany indicated total Cr loss after the exposure to air for nearly 24 hours at steelmaking temperatures [1673 K to 1873 K (1400 °C to 1600 °C)]. These samples were nearly colorless and transparent. Ocular inspection of the cross sections of the slag films from SHTT measurements showed a lens-shaped convex cross section. This was very similar to the cross section of the slag film in the case of vanadium evaporation.[2] This shape is attributed to the competing effects of wetting of the Ptwire and the surface tension of the slag. The convex shape indicates that the wetting of Pt-wire by the slag was poor. If the surface tension cSl-G decreases (e.g., due to evaporation) and wetting angle remains constant, the convex shape is likely to increase. The convex shape facilitates the evaporation with a larger surface area exposed to the atmosphere facilitating the escape of molecules from the surface. Isothermal evaporation curves presented in the experiments carried out in the TGA unit in Stockholm are plotted as dimensionless mass changes against time. The dimensionless mass loss r represents the ratio of mass loss to the initial mass. In the present series of experiments, the mass loss from the slag sample could not be attributed only to the loss of Cr from the slag. Both EDS analysis of the postexperiment samples as well as preliminary experiments conducted in an induction furnace indicated the loss of silicon from the slag. The probability of Si lost as SiO (g) in the oxidizing atmosphere was considered less on thermodynamic considerations. On the other hand, the reasons for Si loss were not clear. Thus, the results were analyzed with emphasis on the scanning electron microscope (SEM) images coupled with wavelength dispersion spectroscopic (WDS) analysis of the post-experiment samples. This method enabled accurate elemental mapping and the Cr content of the thin film samples was considered as the conclusive basis to determine the Cr loss. In the present experiments, there was a possibility of Pt from the hanging wire evaporating in the oxidizing atmosphere as PtOx as reported by Alcock and Hooper[11]: In order to confirm this, two calibration experiments were carried out, one with a blank alumina ring without the slag, hung in the balance using a Pt-wire, very similar to the experiments with Cr-containing slag. The exact mass loss recorded could be taken into account in the mass loss experiments with
TGA,1873K 0.00 0
2000
4000
6000
-0.01
0.5mm O2
-0.02
σ
1.0mm Air 0.5mm Air
-0.03 -0.04 -0.05
t [s] Fig. 2—Dimensionless mass loss curves for slag samples corresponding to sample thickness 1 mm and to samples thickness 0.5 mm.
Fig. 4—SEM micrograph of the sample 24 h at 1873 K (1600 °C) in oxygen.
Fig. 3—Samples before (to the left) and after (to the right) the heat treatment at 1873 K (1600 °C) for 24 h in pure O2.
Cr-slag. In case of some slag samples, examined after the heat-treatment, some Pt was found dispersed in the slag, which is surmised to be due to the evaporated PtOx mixing with the slag matrix; but the mass change was found negligible as observed in reference trials. It should be mentioned that the examination of the Pt hanging wire after the experiments showed that no chromium was detectable in Pt in the case of all the experiments. Post-experiment visual observation of the slag films in the alumina rings showed that the slag film had a relatively flat surface in contrast to those in Pt rings. This effect is attributed to the relatively better wetting of the alumina wall by the slag. It was difficult to get microscopic pictures as the films were very brittle and cracked easily. The mass loss curve (dimensionless and normalized as Dwt/wtinitial) for slag samples corresponding to sample thicknesses 1 and 0.5 mm are shown in Figure 2. The samples were soaked for 2 hours at 1873 K (1600 °C), in air as well as pure oxygen. The mass loss values were compensated for PtOx loss. The mass loss rate is lower in air as compared to oxygen treatment and decreases with sample thickness. While the dissolution of the alumina ring in the slag could be negligible as the slag is saturated with Al2O3, Cr2O3 from the slag could dissolve in the alumina ring. Figure 3 shows the slag samples in alumina rings before
and after the exposure to pure oxygen at 1873 K (1600 °C) for 24 hours. It is seen that the sample after the heat treatment is nearly colorless indicating the loss of chromium oxide from the slag. This is in conformity with the observations using the SHTT apparatus. Similar observations were made by Okretic et al.[7] The alumina ring used as a sample holder in the present case was slightly colored by the dissolved Cr ions. The alumina ring contained up to 0.13 at. pct of dissolved Cr according to WDS analysis. An approximate mass balance was carried out to account for Cr dissolved in the alumina ring before estimating the Cr loss. (The original Cr content in the sample was 2.0 at. pct. Volume of the alumina ring wall is: 15.7 mm3. Volume of the slag sample is: 12.5 mm3.) Figure 4 shows the SEM micrograph of the thin film slag sample after treatment in pure oxygen for 24 hours. The sample consisted of MgAl2O4 spinel grains and amorphous matrix. The WDS analysis of the sample after 24 hours at 1873 K (1600 °C) in oxygen is given in the Table IV. It is seen that the slag sample, after the oxidation treatment, consisted of MgAl2O4 spinel phase with a chromium content of 0.03 at. pct and a glassy phase with 0.0 at. pct Cr. A. Effect of Temperature The effect of temperature on Cr evaporation was studied at three different temperatures, viz. 1673 K, 1773 K, and 1873 K (1400 °C, 1500 °C, and 1600 °C). The samples were treated in pure oxygen for 6 hours. METALLURGICAL AND MATERIALS TRANSACTIONS B
Table IV. WDS Analysis of the Sample After 24 h at 1873 K in Oxygen (Atomic Percent)
1.80
O
Mg
Al
Si
Ca
Cr
1.60
2h O2
61.82 66.59
8.36 3.07
29.65 17.84
0.10 8.03
0.04 4.47
0.03 0.00
1.40
6h O2
6h in oxygen
Cr [at%]
Spinel Matrix
2.00
1.60
1.20 1.00 0.80 0.60
1.40
0.40
Cr [at%]
1.20
0.20
1.00
0.00
0.80
1673
0.60
1773
T[K]
0.40 0.20 0.00 1650
1873
Fig. 6—Average Cr content in the slag after 2 and 6 h in oxygen. 1700
1750
1800
1850
1900
T [K] TGA, 1873K
Fig. 5—Average Cr content in the slag as a function of temperature after 6 h in oxygen. The sample thickness was 0.5 mm.
0 0
2000
4000
6000
8000
-0.01 -0.02 -0.03
σ
The results are presented in Figure 5. The sample thickness was considered to be uniform and the same as the that of the alumina ring, viz. 0.5 mm. It is seen that the Cr content decreases during the same treatment time and reaches zero value at 1873 K (1600 °C) after 6 hours treatment, while the loss is about 50 pct for the same time period at 1773 K (1500 °C).
The effect of soaking time on the average Cr content is presented in Figure 6. It is seen in this figure that longer soaking time leads to decrease in the Cr content in the samples.
1873K Air
-0.05
1873K Ar
-0.06 -0.07 -0.08
B. Effect of Soaking Time
1873K O2
-0.04
t [s]
Fig. 7—Dimensionless weight loss. TGA curves, isothermal oxidation of 0.5 mm slag films for 2 h.
C. Effect of Oxygen Partial Pressure In order to see the effect of oxygen partial pressure on the chromium loss from the thin slag film samples, experiments were conducted by soaking the samples in air, oxygen, and argon gases. The mass loss curves obtained at 1873 K (1600 °C) after 2 hours isothermal oxidation of 0.5 mm slag films for 2 hours is presented in Figure 7. The evaporation rate was found to increase with increasing oxygen partial pressure. The evaporation rate in argon for the 0.5 m sample is more than that in air with 1 mm sample as seen in Figure 2. The rate in air is about 0.4 times that in oxygen. This seems to correspond approximately to square root of PO2 . In view of the uncertainties in the mass loss measurements, as mentioned earlier, any reaction mechanism postulated on the basis of these curves might be misleading. A photo of the sample after treatment at 1673 K (1400 °C) for 2 hours is presented in Figure 8. METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 8—The polished sample surface after treatment at 2 h at 1673 K (1400 °C).
It is seen that there is a variation of the color of the sample. The color shift is due to the differences in the valence state of Cr-oxide. Surface was reddish most likely due to the presence of Cr6+. The sesquioxide, Cr2O3 is more stable at 1673 K (1400 °C) and may be present in the interior of the thin film. Samples that were soaked at higher temperatures for shorter time intervals were red-colored. Okretic et al.[7] reported the change of the sample color in the chromium-containing slags under the different oxygen partial pressures. The change was found to be reversible and associated with the valence change of the chromium ions.
III.
DISCUSSION
The present results are summarized in Figure 9. It is seen that longer time of soaking and higher oxygen levels in the oxidant gas contributes to higher chromium loss from thin slag films. After 6 hours in oxygen, chromium was mainly distributed in the solid solution phase with spinel, MgAl2O4. In the last case, the chromium amount was very close to the noise level of WDS. The solid chromates were only found at higher concentrations of Cr when the treatment time was shorter than 6 hours in oxygen or when treated at lower oxygen partial pressures and temperatures. The near linear mass loss curve, in spite of the fact that Cr content drops from 2 at. pct to 0.27 at. pct [in 2 hours at 1873 K (1600 °C) and pure oxygen], seems to indicate that the vapor pressure of the CrOx species, and therefore its activity in the liquid remains constant. If so, the slag should have some solid compound of Cr even at 0.27 at. pct Cr. Based on the diffusion coefficient value reported by Okretic et al.,[7] it can be surmised that the Cr-loss reaction is likely to be diffusion controlled as mentioned earlier. It is important to look into the implications of the present results on carcinogenic Cr emission during stainless steelmaking, where the slag may contain as much as 8 mass pct Cr2O3 during oxygen blowing process (as found in Swedish practice[1]). Some emission
1873 K Cr original
2,00
2h Ar
1,62
of Cr6+ above the slag surface could be possible. The present results indicate that this emission is relatively small in air. It has been pointed out by Fruehan[12] that the rate of cooling of the slag after tapping would be quite fast and it would take only a few minutes for the temperature to fall below the solidus temperature. Thus, CrO3 emission may be insignificant during this period. On the other hand, the cumulative effect of Cr emission over a longer period of alloy steelmaking may have to be looked into in order to evaluate the possible health hazard to the plant operators. In this connection, it can be noted that the emission of Cr from the slags is very low in an argon atmosphere. One solution to solve the plant situation may be to tap the slag under a ‘‘carpet’’ of argon and allow it to solidify under inert atmosphere. It is admitted that such a modification will have consequences on the project economy.
IV.
CONCLUSIONS
The evaporation of Cr oxide from the aluminasaturated synthetic slag films in oxidizing atmospheres (air and pure O2) in the temperature range 1673 K to 1873 K (1400 °C to 1600 °C) was investigated by a thin film technique. Post-experiment samples were also analyzed by WDS to get the average Cr content of the samples. The present results show that there is a loss of Cr from the slag films and the loss increases with increasing temperature, soaking time as well as oxygen partial pressure. The implications of Cr emission on the plant practice in stainless steel making are discussed.
ACKNOWLEDGMENTS The authors express their sincere thanks to Professor Bo Bjo¨rkman of Lulea˚ Technical University, Sweden and Mr. Go¨ran Andersson of Swedish Steel Producers Association (‘‘Jernkontoret’’) for their valuable comments during this work. The authors acknowledge the support received from Institute for Iron and Steel Technology (IEST), TU Bergakademie, Freiberg (TUBAF) for making the SHTT unit available in the early stages of this study. Our special thanks are due to Ms. C. Schro¨der (TUBAF) for her valuable help with SEM analysis. The work was carried out with the financial support from The Swedish Foundation for Strategic Environmental Research (MISTRA) through Jernkontoret.
4h Air
1,25
2h O2 6h O2 0.27 0.04
0,02
24h O2
Fig. 9—Average Cr content (atomic percent) in the slag after soaking at 1873 K (1600 °C). The original Cr content in the sample was 2.0 (atomic percent).
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METALLURGICAL AND MATERIALS TRANSACTIONS B
9. D.R. Lide: Handbook of Chemistry and Physics, 87 ed., CRC Press, Boca Raton, FL, 1998, pp. 4–53. ISBN 0-84930594-2. 10. A. Semykina, V. Shatokha, M. Iwase, and S. Seetharaman: Metall. Mater. Trans. B, 2010, vol. 41B, pp. 1230–39. 11. C.B. Alcock and G.W. Hooper: Proc. R. Soc. Lond., 1960, vol. 254, p. 551. 12. R.J. Fruehan: Private communication, Dec. 2012.
