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sodium salts. The major part of the sublimates consists of niobium aluminide NbAl3 with impurities of iron, silicon, chro- mium, calcium, and other components.
ISSN 0036-0295, Russian Metallurgy (Metally), Vol. 2007, No. 1, pp. 10–13. © Pleiades Publishing, Ltd., 2007. Original Russian Text © A.G. Upolovnikova, V.M. Chumarev, R.I. Gulyaeva, L.Yu. Udoeva, 2007, published in Metally, 2007, No. 1, pp. 14–17.

Effect of Sodium Carbonate on the Oxidation of Sublimates during Niobium Electron-Beam Remelting A. G. Upolovnikova, V. M. Chumarev, R. I. Gulyaeva, and L. Yu. Udoeva Institute of Metallurgy, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia e-mail: [email protected] Received November 13, 2006

Abstract—The effect of sodium carbonate additions on phase formation during the air oxidation of the sublimates of niobium electron-beam remelting is studied by derivatography, X-ray diffraction analysis, and electron-probe microanalysis. The composition of the oxidation products changes with the heating temperature. The formation of sodium niobates and aluminates is detected by X-ray diffraction analysis at temperatures higher than 600°C. The oxidation of the sublimates is completed by the formation of sodium metaniobate and aluminate. Sodium carbonate additions are shown to accelerate the oxidation of the sublimates. PACS numbers: 64.70.Dv, 81.65.Mq DOI: 10.1134/S0036029507010028

When the Nb2Al phase is completely consumed for AlNbO4 formation, a new Al2O3 layer begins to form under the oxide, and the cycle repeats, which results in a layer-to-layer arrangement of the products of reactions (1) and (2). As the aluminum content in Al–Nb alloys increases, the oxidation rate decreases due to the enhancement of outer aluminum diffusion from an alloy and the formation of a protective film consisting only of alumina. We have found no data on the oxidation of niobium aluminide NbAl3 in the presence of sodium carbonate, and only the results of studies of the interactions of sodium carbonate with niobium and aluminum oxides are known [3–5].1 The purpose of the present work is to study the effect of sodium carbonate on the oxidability of the sublimates of niobium electron-beam remelting during air heating and to determine the conditions for the complete transformation of the components into the oxidized form.

INTRODUCTION In niobium production, electron-beam melting is the main procedure for refining the crude metal formed upon the aluminothermic reduction of niobium pentoxide. This procedure results in the almost complete removal of both impurities (due to their evaporation from the surface of a melting metal) and a considerable portion of niobium. In a deep vacuum at high temperatures, metal vapors are condensed on the furnace walls to form sublimates. The chemical technology of sublimate processing implies a stage of their preliminary oxidizing roasting, including roasting in the presence of sodium salts. The major part of the sublimates consists of niobium aluminide NbAl3 with impurities of iron, silicon, chromium, calcium, and other components. The behavior of this alloy during air oxidation was studied in detail in [1, 2]. The mechanism of NbAl3 oxidation was found to consist of several stages. At temperatures above 1000°C, an aluminum oxide layer begins to form on the alloy surface, and the lower aluminide (Nb2Al) forms at the oxide–alloy interface via the reaction 4NbAl 3 + 7.5O 2 = 5Al 2 O 3 + 2Nb 2 Al.

EXPERIMENTAL Two types of sublimates of niobium electron-beam remelting were studied (%): 82.1 Nb, 11.8 Al, 1.1 Fe, 0.2 Cr, 0.7 Si, and 4.1 O (sample I) and 24.6 Nb, 50.7 Al, 2.0 Ca, 0.8 Si, 2.8 Cr, 2.9 Fe, 0.8 C, 12.0 O, and 3.4 other impurities (sample II). They were represented by the following phases: sample I, mainly by NbAl3 and metallic Nb; sample II, by NbAl3 and metallic Al.

(1)

The oxidation via reaction (1) continues until the diffusion rate of aluminum through the Nb2Al layer is high. As the Nb2Al layer grows, the aluminum mass transfer is retarded, resulting in the formation of rapidly growing oxide phases due to the oxidation of Nb2Al via the reaction 2Nb 2 Al + 6.5O 2 = Nb 2 O 5 + 2AlNbO 4 .

1 A.V.

Lapitskii, “Study of Niobates and Tantalates,” Extended Abstract of Doctoral Dissertation in Chemistry (Moscow State University, Moscow, 1957).

(2) 10

EFFECT OF SODIUM CARBONATE ON THE OXIDATION OF SUBLIMATES ∆G, mg 25

∆G, mg (a)

20

(b)

50

1

1

40

15

30

10 5

20

2

2

10

0 –5 300

11

500

700

900

1100 T, °C

0 300

500

700

900

1100 T, °C

Fig. 1. Variation of the weight during oxidation of samples (a) I and (b) II of Nb–Al sublimates (1) without additions and (2) in a mixture with sodium carbonate taken in the ratio 1 : 0.6.

G, mg (a)

(b)

60

40

20

DTA

DTA TG

TG

0

300

500

700

900 T, °C

300

500

700

900 T, °C

Fig. 2. Thermograms of the interaction of sodium carbonate with samples (a) I and (b) II of Nb–Al sublimates recorded at a heating rate of 10 K/min.

The interaction of the sublimates (particles powdered to a size less than 200 µm) with sodium carbonate (reagent grade) was studied in air under isothermal and dynamic conditions. In the former case, a charge was heated in a muffle furnace; in the latter case, experiments were carried out on a Q1500D Derivatograph thermogravimetric analyzer. Electron-probe microanalysis (EPMA) of the oxidation products was performed on a Cameca analyzer. Xray diffraction analysis was conducted on a DRON-2 automated diffractometer using CoKα radiation.

panied by an insignificant weight loss in the low-temperature range followed by an increase in weight at temperatures above 320°C.

RESULTS AND DISCUSSION The results of thermogravimetric (TG) analysis shows that air heating of a mixture of the sublimates and sodium carbonate at a rate of 10 K/min is accom-

In the DTA (differential thermal analysis) curve of a mixture of sample I and soda taken in a ratio of 1 : 0.6 (Fig. 2a), the endothermic effect at 90–120°C is caused by moisture removal. A further increase in the temper-

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Below 720°C, the effect of Na2CO3 on weight change ∆G is insignificant: curves 2 and 1 in Fig. 1 for the sublimates with Na2CO3 and without it [1] are virtually the same. A further increase in the temperature results in a higher rate of decrease of the weight of the samples with sodium carbonate additions. In the temperature range from 800 to 880°C, the T–∆G curve decreases sharply; however, further heating again results in an increase in the weight.

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UPOLOVNIKOVA et al.

Table 1. Effect of the 180-min roasting temperature on the phase composition of the oxidation products of Nb–Al sublimates with sodium carbonate (sublimates : soda = 1 : 0.6) Sample Troast , °C I

580 780 900

NbAl3 ; Nb2O5 ; Na2CO3 NbAl3 ; NaNbO3 ; Na3NbO4 (small) NaNbO3 ; Na3NbO4 (small); Al2O3 (small) NaNbO3 ; NaAlO2 (small) NbAl3 ; Al; Na2CO3 ; Al2O3 ; Nb2O5 NbAl3 ; Al; Al2O3 ; NaNbO3 ; Na2CO3 ; NaAlO2 NbAl3 ; Al2O3 ; NaNbO3 ; NaAlO2 NbAl3 ; Al2O3 ; NaNbO3 ; NaAlO2

1000 580 780

II

Phase composition of the product

900 1000

ature leads to an exothermic effect of oxide formation with a maximum at 605°C and an endothermic effect at 852°C corresponding to sodium carbonate melting. The exothermic effect at 820°C is related to the onset of the interaction of sodium carbonate with Nb2O5 and Al2O3. For sample II, the DTA curve (Fig. 2b) contains an exothermic effect with a maximum at 673°C associated with oxide formation and two endothermic effects with maxima at 650 and 874°C. The former effect indicates aluminum melting, and the latter peak shows sodium carbonate melting. For sample I, oxidation is completed below 1000°C, and the oxidation of sample II continues due to the presence of metallic aluminum in the sublimates. It should be noted that, for lean sublimates, the I, % 100

1

Table 2. Effect of Na2CO3 consumption on the phase composition of the products of oxidation of rich Nb–Al sublimates at 900°C for 60 min Content in the mixture, % sublimate

Na2CO3

80 60 40 35 25

20 40 60 65 75

2 60

40 3 20 4 0

50

100

150

200

250 t, min

Fig. 3. Dynamics of the relative X-ray reflection intensities of the phase components of the interaction products of Nb– Al sublimates with sodium carbonate: (1) NaAlO2, (2) NaNbO3, (3) Al2O3, and (4) Al3Nb.

NaNbO3 ; Nb2O5 ; Al2O3 NaNbO3 ; Na3NbO4 ; NaAlO2 NaNbO3 ; Na3NbO4 ; NaAlO2 Na2CO3 ; Na3NbO4 ; NaAlO2 Na2CO3 ; Na3NbO4 ; NaAlO2

maxima of the thermal effects are shifted to high temperatures. The DTA results indicate that, before the onset of the interaction involving sodium carbonate, the major parts of aluminum, niobium, and impurity metals are oxidized. It can be assumed that the further course of sublimate oxidation is determined by the formation of the sodium salts nNa 2 CO 3 + mM x O y

nNa 2 O ⋅ mM x O y + nCO 2 .

Isothermal studies were carried out at temperatures 580–1000°C for 180 min. The phase compositions of the oxidation products are presented in Table 1. The oxidation of the niobium-rich sublimates (sample I) in air at 580°C leads to the formation of Nb2O5. The complete transformation of Nb2O5 into sodium meta- and orthoniobates was detected at 780°C. Above 900°C, X-ray diffraction patterns show weak reflections of alumina. As the temperature increases to 1000°C, niobium is represented in the oxidation products only by sodium metaniobate, which is evidently related to the transformation of orthoniobate by the reaction [6] Na 3 NbO 3 + Al 2 O 3

80

Phase composition of the products

NaNbO 3 + 2NaAlO 2 .

Roasting of sample II with a low niobium content at 580°C results in niobium and aluminum oxides (see Table 1). Sodium aluminate and sodium niobate appear in the oxidation products at 780°C. The presence of the main component of the sublimates, the intermetallic compound NbAl3, is observed up to 1000°C. Some amount of Al2O3 unbound to sodium is also retained in the cake, which can be due to a Na2CO3 deficiency. The study of the effect of the Na2CO3 consumption on the phase composition of the oxidation products of the niobium-rich sublimates (sample I) at 900°C shows (Table 2) that sodium metaniobate and niobium pentoxide are formed at 25% sodium carbonate in the charge. An increase in the Na2CO3 content to 40–60% results in the appearance of an additional phase (sodium orthoniobate), which completely replaces NaNbO3 as the consumption of sodium carbonate increases. When the charge contains more than 65% Na2CO3, excess sodium carbonate is observed in the roasting products. RUSSIAN METALLURGY (METALLY)

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EFFECT OF SODIUM CARBONATE ON THE OXIDATION OF SUBLIMATES

The study of the effect of the roasting duration at 1000°C on phase formation in a mixture of lean sublimates and sodium carbonate (ratio 1.5 : 1) shows that the initial intermetallic compound NbAl3 is identified in the oxidation products up to 200-min holding (Fig. 3). The reflections of NaNbO3 and NaAlO2 become dominant after 80-min roasting. For complete oxidation of the NbAl3 intermetallic compound at the sublimate particle size under study, it is necessary to prolong the roasting–caking process to 240 min. The EPMA data for the cake of the sublimates with soda obtained at 1000°C in air (holding 240 min, ratio 1 : 1.5) confirm the absence of the metallic NbAl3 phase. The presence of alumina along with sodium aluminate and meta- and orthoniobates indicates that soda consumption higher than the stoichiometric value is required to transform all components of the sublimates into sodium salts. CONCLUSIONS The chemical and phase compositions of Nb–Al sublimates was found to affect their oxidability. We established the following sequence of chemical transformations in the sodium carbonate–sublimate system on air heating: niobium and aluminum oxides first form and then transform into sodium niobates and aluminates. The efficiency of this transformation increases above the melting temperature of sodium carbonate (above 850°C). For complete oxidation of the components of the sublimates with a low niobium content, it is necessary to provide a 1.5 soda excess, a temperature of at least

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1000°C, and a roasting duration of 240 min. Rich sublimates can be transformed into the oxidized form at lower temperatures (900°C) and sodium carbonate consumptions. ACKNOWLEDGMENTS We are grateful to V.P. Mar’evich and A.A. Pankratov for the X-ray diffraction analysis and electronprobe microanalysis. This work was supported by the President’s Grant for Scientific Schools, project no. NSh-5566.2006.3. REFERENCES 1. V. M. Chumarev, R. I. Gulyaeva, V. P. Mar’evich, et al., “Chemistry and Kinetics of Oxidation of Sublimates Formed upon Electron-Beam Remelting of Niobium,” Izv. Ross. Akad. Nauk, Ser. Met., No. 6, 3–7 (2003) [Russian Metallurgy (Metally), No. 6, 485–499 (2003)]. 2. R. A. Perkins, K. T. Chiang, and G. H. Meier, “Formation of Alumina in Nb–Al Alloys,” Scr. Metallurgica 22 (3), 419–424 (1988). 3. Z. I. Shapiro, V. K. Trunov, and V. V. Shishov, Methods of Preparation of Alkaline Metal Niobates: Review, Ser. “Reagents and Extra-High-Purity Substances” (NIITEKhIM, Moscow, 1978) [in Russian]. 4. V. Ya. Abramov and N. I. Eremin, Leaching of Aluminate Cakes (Metallurgiya, Moscow, 1976) [in Russian]. 5. A. Reisman, F. Holtzberg, and E. Banks, “Reaction of the Group VB Pentoxides with Oxides and Carbonates. VII. Heterogeneous Equilibria in the System Na2O or Na2CO3–Nb2O5,” J. Am. Chem. Soc. 80 (1), 37–42 (1958).