Nitridation in the Processing and Preparation of Metals and Ceramics

Home , Niobium nitride, Subnitride, Tantalum nitride, Vanadium nitride

O. Addemir, A. Tekin and C.K. Gupta*

Faculty of Chemistry and Metallurgy, Istanbul Technical University, 80286 Maslak, Istanbul, Turkey; *Materials Group, Bhabha Atomic Research Centre, Bombay 400 085, India

ABSTRACT 1. INTRODUCTION

The formation and separation of metal nitrides from Most of the elements in the periodic table combine a variety of materials including ferroalloys, oxides, with nitrogen to form nitrogen compounds or nitrides. natural materials and minerals is an important com- Based on their structure and bonding characteristics, ponent in current materials processing practice. they have been divided into four groups and among Ferroalloys like ferroniobium and ferrovanadium these the transition metal nitrides and the diamond-like undergo nitridation by ammonia yielding a mixture of nitrides exhibit more readily usable properties. Among iron nitrides and the refractory metal nitride. Acid the many properties of the nitrides, the high melting leaching effectively separates the iron nitride from the point, high hardness and resistance to corrosion are mixture. Niobium nitride thus obtained is used to very attractive. However, nitrides are also brittle, produce niobium metal by pyrovacuum treatment. readily oxidise at high temperatures and many of them Vanadium nitride converts to metal when pyrovacuum thermally decompose. While some of these limitations treatment is followed by electrorefining. Nitride have been partially overcome in novel compositions suitable for decomposition to metal can be obtained like sialon, the extensive use of the majority of nitrides from niobium pentoxide by reacting ammonia or by is restricted by these limitations. In recent times, the reacting a mixture of carbon and nitrogen. Rice husk is special combination of nitride properties has been a ready-made intimate mixture of silica and carbon. It exploited in devising some useful processes. These can be converted to silicon nitride or to a mixture of processes, which finally result in the production of silicon carbide and silicon nitride by caibonitrothermic metals, use nitrides as the intermediates. These involve reduction, β sialon can be directly prepared from preparation of nitrides, starting from ferroalloys, pure kaolinite by carbonitrothermic reduction. Ilmenite is compounds and also minerals by easily executable another mineral amenable to nitridation processing. It processes. Nitrides of silicon have a special status as yields after reaction with nitrogen in presence of end-product nitrides on account of their unique carbon, a separable titanium nitride-iron mixture. All combination of properties. Preparation of these nitrides these attractive features of nitridation have been from new types of raw materials, therefore, remains an discussed in this paper. The possibility of incorporating important area of research. Both these aspects, nitridation into the process flowsheets has been pertaining, respectively, to preparation and use of highlighted. nitrides as intermediates and to preparation of nitrides as end products have been discussed in this paper.

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2. PREPARATION OF NITRIDES 2.1. J. Ferroniobium Iron can be nitrided both in nitrogen and in There are many general methods for the preparation ammonia. The nitridation takes place to a greater of nitrides. A convenient method is extent with ammonia at temperatures exceeding 400°C. Similarly, the nitridation of pure niobium has been investigated in both nitrogen and ammonia 161. M+—N 22 = MNn 2 However, no published information was available /4/ on the nitridation of ferroniobium for the formation of This method is based on the fact that nitride is thermo- nitrides. Some predictions can be made on the basis of dynamically stable at temperatures at which significant the phase diagrams of the Fe-Nb, Fe-N and Nb-N reaction rates between metal and nitrogen at about 1 systems 111. At room temperature, the stable phases are atm pressure are established. Many metals which do FeNb and Fe2Nb in the Fe-Nb system at 35% Fe; in the not undergo the above reaction or undergo that reaction Fe-N system they are Fe2N, Fe4N and Fe2Ni.x, and in only at high nitrogen pressures can be nitrided by the Nb-N system they are NbN and Nt^N. Nitrided reaction with ammonia ferroniobium would, therefore, contain a mixture of iron and niobium nitride phases.

M + nNH3 = MNn+ |-nH2 It is known from the nitriding behaviour of pure metals that, for efficient reaction, the metal should be The nascent nitrogen produced during the dissociation used in the form of a fine powder. A similar considera- of ammonia is very reaction and nitrides the metal. tion applies to the nitridation of a ferroalloy. Thus Other important reactions useful for the formation of ferroniobium powder was used as the starting material the nitride are carbothermic reduction of an oxide IM. The powder was contained in an alumina boat and under nitrogen or ammonia. charged into a tube reactor. The charge was heated to the experimental temperature in the range of 500 to 1050°C and then maintained at the reaction tempera- M02n + —nC + —N 22 =MN+—CO Y 3 2 5 ture for a predetermined duration. Ammonia flow was maintained in the reactor during heating, soaking and The temperature required for the occurrence of all these cooling. The product was characterised by measuring reactions is in the range of 800 to 1200°C. Metal weight changes and by X-ray diffraction. The results nitridation may be initially inhibited by a protective are summarised in Table 1. surface oxide film. To achieve a practically useful The results in the table indicate that the weight gain conversion rate, it is advantageous to work with increased with increase of temperature up to 950°C to powders of <10 μηι size. These general methods of 1000°C and then remained steady. Nitriding appears to nitridation are application to a variety of materials, be very sensitive to the particle size of the powder; very including ferroalloys, oxides and also minerals. poor nitriding was observed when powder coarser than 270 mesh was used. X-ray diffraction pattern showed the presence of discrete nitride phases of iron and 2.1. Nitriding of Ferroalloys niobium and also a small amount of FeNbN phase. The processing of ferroalloys by nitridation begins with nitriding the alloy to obtain the mixed-iron alloy constituent nitride. Kirby and Fray /1-3/ were the first 2.1.2. Ferrovanadium to investigate a process for upgrading ferrochromium Even though vanadium nitride could be obtained by by a route involving nitriding. Subsequently, Suri, heating vanadium metal powder at 1250°C under a Singh and Gupta /4,5/ investigated processes for nitrogen or ammonia atmosphere 161, there is no upgrading ferroniobium and ferrovanadium by a mention in the literature of the conditions for nitriding similar route. ferrovanadium to obtain the nitrides of the ferroalloy

274 Ο. AdJemir, Α. Tekin and C.K. Gupta. High Temperature Materials and Processes

Table 1 Nitriding of ferroniobium by ammonia

Particle size of ferroalloy Nitriding Weight gain, Nitrogen % powder, mesh size temperature, °C content, wt%

-270 750 4 -

-270 800 7 - -270 900 11.5 5.76 -270 950 10.5 7.20 -270 1000 11.5 7.34

-140 950 5 -

-270 950 10.2 8.50

-270 1000 12.17 - -325 1000 12.8 8.60

Nitriding time : 4.5 to 5 h, Sample weight: 3 g, Ammonia flowrate : 200 ml/min. constituents. According to the phase diagrams of the The nitridation of ferrovanadium was carried out 151 Fe-N and V-N systems 111, iron and vanadium each by loading a known quantity (3, 20 or 80 g) of form a number of nitrides. The nitrides of iron were ferroalloy powder in an inconel boat in a silica tube mentioned earlier. Vanadium forms VN].X and V2Ni-y reactor. The charge was heated under ammonia flow to phases in addition to the vanadium-nitrogen solid temperatures ranging from 900 to 1000°C. Nitrided solution. Nitrided ferrovanadium can be expected to samples were cooled and weight gain was measured. yield a mixture of iron nitride and vanadium nitride The results are summarised in Table 2. phases.

Table 2 Nitriding of ferrovanadium by ammonia

Temperature, °C Duration, h Weight gain, % jj 900 5 8.5 900 6 9.6 950 5 10.3 950 6 11.3 950 7 11.2 950 8 13.0 1000 7 13.0

Sample weight: 3 g, Powder size : -270 mesh, Ammonia flowrate : 200 ml/min.

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The results in the table indicate that the weight to the nitrides was accomplished by Krishanamurthy, gain, which is indirectly a measure of uptake of Venkataramani and Garg /10/ by carbonitrothermic nitrogen by the alloy, depends on temperature and reduction. A compacted mixture of niobium pentoxide duration of the experiment. The maximum weight gain and carbon was heated to 1500°C under flowing of 12 to 13% is obtained at 950°C to 1000°C in 6 to 7 nitrogen. The oxide-carbon mixture converted h. X-ray diffraction pattern of the nitrided samples completely to the nitride in less than 2 h. Residual showed the presence of Fe2.5N, Fe3N and 5VNi.x carbon and oxygen contents in the nitrides thus phases. Chemical analysis of the nitride product gives obtained were 0.2 and 0.4%, respectively. The residual 36.4% Fe, 48.8% V and 11.49% N. impurity contents tend to be higher in the case of tantalum nitride. While the presence of significant quantities of residual carbon and oxygen impurities 2.2. Nitriding of Oxides may be considered as a disadvantage of the process as The conversion of oxide to nitride as a step in the far as nitride preparation per se is considered, it is not a production of pure metal from oxide was pioneered by serious limitation if the nitride formation is considered Guidotti, Atkinson and Kesterke /8,9/. They converted as a step in the overall sequence involving the oxides of vanadium, niobium and tantalum to the formation of the nitride and its decomposition to the respective nitrides by heating them in anhydrous metal. ammonia at temperatures between 600 and 1300°C. Even though the conversion of a metal oxide to its Lower reaction temperatures result in the formation of nitride by carbonitrothermic reduction is a technique oxynitride while the formation of the mononitride known for a long time /11/, an interesting innovation as phases is favoured at higher temperatures. The most regards the raw material has been attracting attention rapid and complete reaction occurred at above 1200°C. in recent times. The usual practice in carbonitrothermic The reaction time is considerably reduced by reduction is to add the required quantity of carbon to conducting the nitridation as a two-step operation: the oxide externally before it is loaded in a furnace for formation of oxynitride by reaction between pentoxide conversion to the nitride. The innovation is the use of and ammonia at 700 to 800°C followed by reaction of starting materials which require no external addition of the oxynitride with ammonia at 1100 to 1200 °C to carbon. Pyrolysed rice husk (PRH) is a unique raw obtain the nitride which contains £ 0.1% residual material in the sense that it contains both carbon and oxygen. silica in the right proportion. This is illustrated in Conversion of the oxides of niobium and tantalum Table 1 which gives the chemical composition of PRH

Table 3 The chemical composition of pyrolysed rice husk (PRH) powder

Component Composition, wt%

Si02 47.91

Fe203 0.701 MnO 0.102

Na20 0.161

K20 0.256 CaO 0.174 MgO 0.270 C 50.0

276 Ο. Addemir, Α. Tekin and C.K. Gupta. High Temperature Materials and Processes powder from the Trakya region of Turkey. The silica 1500°C, the silica-carbon mixture converts to the and carbon are present in PRH in an intimate and silicon carbide-carbon mixture or silicon nitride-carbon homogeneous mixture which is highly desirable for a mixture at Pco = 1 atm. Whether carbide or nitride good carbothermic or carbonitrothermic reduction forms is determined by the partial pressure of nitrogen. process to yield silicon carbide or silicon nitride. At a given temperature and Pco value, carbide The overall reaction for the formation of silicon formation is favoured at low nitrogen pressures and nitride from silica may be represented by nitride formation is favoured at high nitrogen pressures. For nitride to form above about 1475°C, the

3Si02 + 6C + 2N2 = Si3N4 + 6CO required partial pressure of nitrogen must exceed 1 atm. The required maximum partial pressure of and the formation of silicon carbide by nitrogen increases with further increase in temperature. An experimental arrangement involving nitrogen flow over a silica carbon mixture at temperatures in the Si02 + 3C = SiC + 2CO range of 1350 to 1500°C can provide a low value of Pco Silicon nitride can transform to silicon carbide, in the and a high value of PNj. Pure nitride can be antici- presence of carbon, by pated at low temperatures and an increasing amount of carbide at high temperatures.

S13N4 + 3C = 3 SiC + 2N2 The results of carbothermic/carbonitrothermic reduction of PRH powder are shown in Fig. 1. The

The conditions for the occurrence of each of the above product is Si3N4 at 1400°C, a mixture of Si3N4 and SiC reactions have been calculated by Ishii, Sano and Imaiy at 1450°C and only SiC at 1500°C. At 1450°C, the

/12/. The minimum temperature at which a Si02 + C proportions of carbide and nitride in the product mixture can be converted to Si3N4 or SiC at a carbon depend strongly on the reaction time. The conversion of monoxide partial pressure (Pco) of 0.01 atm is 1200°. If Si3N4 to SiC increases slowly at first, up to 4 h of the Pco value is higher, then the carbide or nitride can reaction time, but the increase is rather sharp between 4 be formed only at higher temperatures. For example, at h and 6 h of reaction time. Excess carbon in the charge

100 Reaction Temperature —- + a • ο 80 -

(υ -*- I400°C > tz 60 τ ο Ο -+-|450°C 40 - CO h • I500°C 20

0 m,— a 1 1 1 4 6 8 10 Reaction Time (h)

Fig. 1: The effect of reaction time on the proportions of SiC and Si3N4 in the product of carbonitrothermic reduction of PRH at 1450°C.

277 Vol. 15, No. 4, 1996 Nitridation in the Processing and Preparation of Metals and Ceramics brings about the decomposition of the nitride to the (vanadium or niobium) nitride could be readily carbide. When relatively pure nitride is desired, after separated from each other. Iron nitride is soluble in nitride formation is complete, excess carbon in the acidic media whereas the nitrides of niobium or product can be burnt off in air at 700°C in 3 h, and vanadium are not. Thus, acid leaching of the nitrided then unreacted silica can be removed as sodium silicate ferroalloy results in the selective leaching of the iron by boiling with sodium hydroxide solution. nitride, leaving the refractory metal nitride as the residue /4,5/. The nitrides of vanadium, niobium and tantalum 2.3. Nitriding of Minerals decompose at above 1500°C under high vacuum. In certain instances naturally occurring minerals Decomposition of the mononitride proceeds through the can be directly used, instead of pure oxides obtained by subnitride V2N, Nt^N or Ta2N and results in the metal- mineral beneficiation, for obtaining the nitrides. An nitrogen solid solution. The feasibility of obtaining the interesting example is the preparation of the important pure metal from the nitride depends on whether these ceramic β sialon from a variety of naturally occurring solid solutions could be degassed to pure metal. While materials like clay /13/, volcanic ash /14/, rice husk the solid solutions of nitrogen in niobium and tantalum /15/ and kaolin /16/. The overall reaction for the could be degassed completely (to <100 ppm N) at preparation of β sialon from kaolin by carbo- 2200-2500 under high vacuum, vanadium nitride nitrothermic reduction may be written as: cannot be similarly processed because of the high vapour pressure of vanadium. Several works on

3(2Si02.Al2P3.2H20) + 15C + N2 electrorefining of vanadium /18/ have established that

= 2S13AI3O3N5 + 26CO + 2HzO nitrogen can be effectively removed from vanadium by fused salt electrorefining. This way pure metal could be As is obvious from the reaction, the purity of the sialon obtained from decomposed vanadium-nitrogen solid obtained strongly depends on the purity of the kaolin. solution. Two varieties of Turkish kaolin were converted to β Oxygen impurity present in niobium and tantalum sialon by carbonitrothermic reduction at 1475°C. The nitrides gets removed dining pyrovacuum processing by product could be milled to micron size powder and evaporation of the suboxide NbO or TaO 1191. When fabricated by powder metallurgy methods. both carbon and oxygen are present as impurities, their Another mineral which can be processed by nitrida- removal occurs predominantly as CO and to a smaller tion is ilmenite /17/. Carbonitrothermic reduction of extent as NbO or TaO /20/. ilmenite at ~1450°C yields a product consisting of The many possibilities of processing alloys and titanium nitride and iron. Much of the iron is separated intermediate compounds by nitridation are shown in from the brittle nitride by a simple process comprising Figs. 2, 3 and 4, the process flowsheets for treating dry grinding and sieving. ferroniobium and ferrovanadium and niobium pentoxide. As regards the beneficiation of the mineral ilmenite by nitridation, the product containing iron and 3. PROCESSING OF NITRTOED PRODUCT brittle titanium nitride can be readily separated to a significant extent by grinding and sieving. Further The objectives of nitridation as a materials separation may be achieved by acid leaching. preparation technique are clear: to obtain the product of required purity in a readily fabricable form. When used as a processing step, nitridation should lead to a 4. CONCLUSION product that can be separated and/or processed further to yield the pure metal. Nitridation is a useful technique for the preparation The attractive feature of using nitridation for treat- of nitrides as end products as well as for obtaining ing ferroalloys is that iron nitride and the metal nitrides as interprocess intermediates, starting from a

278 Ο. Addemir, Α. Tekin and C.K. Gupta. High Temperature Materials and Processes

Ferroniobium

Crushing and grinding

NH3 Nitriding 950°C, 5h

NitridesjofFe, Nb

HNFLI Leachine Leach liquor HCl 25°C, 20 min (discard)

(Vanadium nitride, Leach residue Fe as impurity) (Niobium nitride, Fe as impurity)

Decomposition 1825°C,2x 10"3 torr

NiobiumI spong e

Electron beam melting

Niobium Vanadium dendrites (99.9% pure) (99.4% pure) Fig. 2: Flowsheet for the processing of ferroniobium by nitridation. Consolidation

NbiOs Vanadium ingot Fig. 3: Flowsheet for the processing of ferrovanadium by nitridation.

variety of raw materials such as ferroalloys, oxides, minerals and also unique materials like rice husk. Nitriding agents are generally ammonia or nitrogen and carbon. Ferroniobium and ferrovanadium could be nitrided by ammonia at about 1000°C to yield products containing iron nitrides and niobium nitride or iron

Niobium nitrides and vanadium nitride. Iron nitride could be (~99.Tj/o pure) selectively leached out in an acid solution. Oxides of the group V refractory metals vanadium, Electron beam melting -N2 -CO, NbO niobium and tantalum could be converted to the respec- I tive nitrides by reaction with ammonia or by Niobium carbonitrothermic reduction. Pyrolysed rice husk (>99.9% pure) contains a homogeneous ready-made mixture of silica Fig. 4: Process for the production of niobium and carbon. It readily nitrides to silicon nitride or through the nitride intermediate. converts to a silicon carbide-nitride mixture.

279 Vol. 15, No. 4, 1996 Nitridation in the Processing and Preparation of Metals and Ceramics

The mineral kaolin could be converted to β sialon 9. R.A. Guidotti, G.B. Atkinson and D.G. Kesterke, by carbonitrothermic reduction. Ilmenite converts to a Bureau of Mines Report of Investigations 8103, readily separable mixture of iron and titanium nitride 1976. on carbonitrothermic reduction. 10. N. Krishnamurthy, R Venkataramani and S.P. Nitrides are veiy useful interprocess intermediates, Garg, Int. J. Ref. Hard Met., March, 1984, p. 41. not only because they are readily produced from more 11. E. Friederich and L. Sittig, Z. Anorg. Chem., 143, easily available raw materials, but also because they can 293 (1925). be converted to pure metal by pyrovacuum 12. T. Ishii, A. Sano and I. Imai, in: S. Somiya, M. decomposition and, in the case of vanadium, by fused Mitomo and M. Yoshimura (eds.), Silicon Nitride salt electrorefining. I, English edition, Elsevier Applied Science, London, 1990; p. 59. 13. J.G. Lee and I.B. Cutler, Am. Ceram. Soc. Bull., REFERENCES 58, 869 (1979). 14. S. Umebayashi, in: F.L. Ritcey (ed.), Nitrogen 1. A.W. Kirby and D.J. Fray, Trans. Inst. Min. Met. Ceramics, NATO Advanced Study Institutes Sec. C, 98, C33-40 (1989). Series, Applied Sciences, NoordhofF International, 2. A.W. Kirby and D.J. Fray, Trans. Inst. Min. Met. Leyden, The Netherlands, 1977; p. 323. Sec. C, 98, C89-95 (1989). 15. K.H. Jack, in: M.B. Bever (ed.), Encyclopaedia of 3. A.W. Kirby and D.J. Fray, Metall. Trans. Β, 20B, Materials Science and Engineering, Vol. 6, 219-226(1989). Pergamon Press, Oxford, 1986; p. 4385. 4. A.K. Suri, K. Singh and C.K. Gupta, Metall. 16. I. Higgins and A. Hendry, Br. Ceram. Trans. J., Trans. Β., 23B, 437-442 (1992). 85, 161 (1985). 5. K. Singh, A.K. Suri and C.K. Gupta (to be 17. N. Krishnamurthy, Ph.D. Thesis, University of published). Bombay, 1991. 6. P. Schwarzkopf and R. Kieffer, Refractory Hard 18. C.K. Gupta and N. Krishnamurthy, Extractive Metals, Macmillan, New York, 1953. Metallurgy of Vanadium, Elsevier, Amsterdam, 7. ASM Metals Handbook, Vol. 4 American Society 1992. for Metals, Materials Park, Ohio, 1992. 19. S.P. Garg and C.V. Sundaram, BARC Report 777, 8. R.A. Guidotti, G.B. Atkinson and D.G. Kesterke, Bhabha Atomic Research Centre, Bombay, 1974. Bureau of Mines Report of Investigations 8079, 20. S.P. Garg and C.V. Sundaram, BARC Report, 1975. Bhabha Atomic Research Centre, Bombay, 1975.

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