The recovery of tungsten from heavy alloy scrap has been explored oxidation of tungsten heavy alloy using oxygen and its subsequent reduction with hydrogen was studied. The oxidation behaviour of the scrap was investigated by simultaneous TG/DTA. Oxidation studies showed a weight gain of 26% and the enthalpy of oxidation was found to be -625 kJ/mol of tungsten. The X-ray diffraction of the oxidised powder has shown the formation of WO3, Fe2WO6 and NiWO4 phases. The reduction of the oxidized material in hydrogen atmosphere at 1000 C yielded a pure tungsten phase in powder form with an approximate weight loss of 25%

Puvvada, G V K

Srikanth, S

English

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NML Tecarrrlcal Joujr tal, Vol . 41, No. 1, January -March , 1999.1"p. 21-29Printed it? India, C' NML. ISSN 0027- 6839Recycling of tungsten based heavy alloy scrapG. V. K. PUVVADA and S. SRIKANTH'National Metallurgical Laboratory. Jarnshcedpur - 831 007, India~NML Madras Complex, Taramani , Madras - GUI) 113, IndiaAbstract : The recovery of tungsten from heavy alloy scarp has been explored. Theoxidation of tungsten heavy alloy using oxygen and its subsequent reduction withhydrogen was studied . The oxidation behaviour of the scrap was investigated bysimultaneous TG/DTA. Oxidation studies showed a weight gain of 26%n and theenthalpy of oxidation was found to be - 625 kJ/ mol of tungsten . The X-ray diffractionof Itic oxidized powder has shown the formation of WO-l' Fe2WO5 and W011 phases.The reduction of the oxidized material in hydrogen atmosphere at 1000°C yielded apure tungsten phase in powder form with an approximate weight loss of 25%.Keywords: 7 'tuzgsten alloy, Thermal atutlysis, Oridation-Reduction, RecyclingINTRODUCTIONAmongst. the tungsten based scrap, the two major sources that are being generatedcontinuously are cemented tungsten carbide (CTC) scrap which is widely used in the hardmetal industry for the manufacturing of cutting tools, drilling tools and high wear resistantparts and the liquid phase sintered W-Fe-Ni or W-Ni-Cu heavy alloy kinetic energypenetrators which are used in the armament industry . The tungsten heavy alloys offers theconcurrent properties of high density, relatively good corrosion resistance , high mechanicalproperties and high hardness. This combined set of properties has caused these alloys to beused as components in inertial systems, electrical contacts, boring bars, radiation shieldingand kinetic energy penetrators' I.Tungsten heavy alloys typically contain 90-97% W in a binder consisting of somecombination of Ni, Fe, Cu and traces of other alloying elements . This composition has adensity . in the range of 17.1 to 18.6 g/cm' 1't. With an appropriate thermo-mrchanicalprocessing such alloys are capable of yielding ultimate tensile strengths in excess (it' 1500MPa while retaining good ductility. Alloys within the 90 93% W compositional range areviewed txs ideal for kinetic energy penetration as they offer the greatest number of optionsin fabrication with the. retention of properties critical for good ballistic performance.These alloys are typically produced by a liquid phase sintering process from either co-reduced oxides and/or blended elemental powders in a reduced atmosphere. The W-Fc-Niis a classic system for liquid phase sintering. Prior studies on these alloys have beenestablished that maximum alloy ductility is realized with a nickel- iron binder ratio of 7:3respectively, and virtually all current commercial W-Fc-Ni heavy alloy formulations em-ploy this ratiol'1.The scrap that is generated during the manufacture of these alloys is of three typesnamely powder scrap, machining scrap and defective penetrators. Out of these three, therejected penetrators are a major source of the scrap. Generation, of such scrap in India isapproximately 150 tonnes per year and requires to be recycled 121. Therefore, studies were21F _11111:1 ----7l l-I Ir-7-lri ]r uullmunG V. K. PUVVADA and S . SR[KANTHconducted to produce the heavy alloy in powder from the defective penetrator rods byoxidizing a piece of rod in oxygen atmosphere and then reducing the oxidized heavy alloypowder in hydrogen atmosphere. This paper reports the observed oxidation-reductionbehaviour of the scrap using thermal analysis, X-ray diffraction and scanning electronmicroscopy.In developing the various metal oxidation theories, it has generally been assumed thatthe rate of growth of oxide layer is conducted by the transport of one or more chargedspecies across the layer and the movement of these charged species is brought about by thejoint action of a chemical and electrostatic potential gradient''. The overall oxidation ic-action of any metal can be. expressed as:aM+(h/2) 0, (g)= MO, (I)The oxidation of Ni-Cr-10W ternary alloy was previously studied by Espcvik ct aIHL4I,where oxidation was conducted in the presence of air at 1200°C and has resulted in theformation of NiO, NiWO4 and NiCr,O4 phases. The studies conducted by Aliprando andSatyanska"? on the oxidation of directionally solidified cobalt-tungsten (Cu-45W) eutecticalloys have shown the formation of CoO and CnWO4 phases, when oxidized at 150'c for1000 min in the presence of pure oxygen. In their study, the weight gain of pure Co andpure W were found to he higher compared with that of the Co-45W alloy at all the threedifferent temperatures studied. This could he due to the formation of intermetallic com-pounds between Co and W, which are not readily oxidized. They have also reported thatthe oxidized product was always found to consist of an yellow WO, outer layer with aninner oxide layer of blue W„,,OSR. The studies conducted by Espevik et all", on the oxidationof Co-Cr-W alloys have shown that increasing the tungsten content from 3 to 10 wt%results in an increase in the oxidation resistance of the alloy in the temperature range of1000-1250°C in air. The formation of CoWO4 beneath Cr,O; layer was noticed in thisinvestigation. From these studies it is evident that whenever binary or ternary alloys wereoxidized, the formation of intermetaliic compounds inhibit the complete oxidation of thealloy.EXPERIMENTALThe heavy alloy rods having the composition given in the Table I were supplied by M/s. Heavy Alloy Penetrator Project, Tiruchirapalli. High purity oxygen and hydrogen wereprocured from commercial sources. The oxidation behaviour was studied using a simulta-neous TG/DTA (SEIKO Model No. 320) and the metallography of the sample was doneon a Nikon optical microscope. The SEM studies were carried out using JOEL JSM 840Ascanning electron microscope-RESULTS AND DISCUSSIONMaterial characterizationThe as received heavy alloy rods were cut into smaller pieces using a laboratory sizesilicon carbide cutting wheel. The cut alloy pieces were etched in hydrofluoric acid andafter washing with dilute HCl and water, the samples were observed under the opticalmicroscope. The microstructure has revealed spheroidal grains of tungsten embedded iniron and nickel binder, which is typical of a liquid phase sintered tungsten based heavyalloy. The microstructure obtained from one of the samples is shown in Fig. I. Similarmicrostructures were also reported by Caldwell'". The X-ray analysis conducted on the2212ECYCI.ING OF TUNGSTEN BASED HEAVY ALLOY SCRAPreceived sample indicates the presence ()I'y(Fe-Ni) phase (Fig. 2), and this indicates that thespheroidal tun( stcn grains are enibcdclcd in this phase. The chemical composition of thematerial was also analyzed by the standard wet chemical methods and the results obtainedarc. given in Table I.Table I : Wet cheritirnl aidalYsis of the heai'r alloy sampleElement Wt (%)W 93.10Ni 2.85Co 0.75Fe 3.30Fr, I M( 1. ^nrrrr^^ 4 o/ ir, Fru^ ^^rvr lrr..^^ ali, r ) /00)20000 2L000 )4000 44 .000 31000.c.ooo M.000 7000 k 000 f2 000Fig. 2 : A-rav /if/ rue Io,; ,cnhI of rhr rr.s Ferro r-J nnrg slrir heuv.r alloy23G. V. K. PUVVADA and S. SRIKANTHOxidation studiesThe oxidation mechanism of the heavy alloy scrap using TG/DTA by placing a pieceof alloy in a flowing stream of oxygen. The experimental conditions adapted dul'ing thethermal analysis study are given in "fable 2.Table 2 . Experlnteirtcrl eolyditions adaptecl for the TGIDTA stir-liesSample Weight (Initial)Sample Wcight (Final)Reference MaterialFurnace AtmosphereThermocouplesHeating Rate63.174 In-79.60 mgtx-AI,O,C).„ 400 ml/min1'1-11% Rh5°C/minThe oxidation reaction was found to commence around 700°C and was completedaround 1000°C and the weight gain of the material was found to be 261%. Thethermogravimeiry and the DTA results are shown in Figs. 3 and 4 respectively, where theenthalpy of oxidation was obtained from the peak area of the DTA curve using the expres-Sionl7lAH° = K x Peak Areawhere,K = lnstrulllent COnStant in Amperes (Determinedconducted by melting pure melts)AH° = Ch,uvc in Enihalpv... (2)from calibration experiments130127.5115117 5500 650 800TEMP ° C CHEATING)950 1100100Fig. !- Weighi guin nhtuulcr( rlttrirr,t; the o.ricletiotl Of llrc t[ulgsrerl (real r (Wov24RECYCLING OF TUNGSTEN BASED HEAVY ALLOY SCRAPso20O -500 650-665 u/_frynye00 - 950 11 00TEIP_*C IME*T1Ma)Fig. 4 : The UTA peak and its area o/luiucd chu•irtg the oxiclat n o / r/rc' heaiw a/loYThe peak area measured by the instrument was found to he -888 µv.s/mg from whichthe enthalpy of reaction was calculated to he -625 kJ/mole. The details of the calculationsare given in the Appendix. The enthalpy of formation of the WO, phase given in theliterature is -825 kJ/mole at 900°0"1. Determination of the enthalpy of reaction by differ-ential thermal analysis is associated with some uncertainities due to the unccrtainitiesinvolved in the determination of the instrument constant K. The theoritical weight gain forthe complete oxidation of W to WOs corresponds to 26.1% and this close agreementbetween the theoritical and experimental values indicates complete oxidation of tungsten totungsten trioxide.The oxidation of the heavy alloy sample was also carried out in a laboratory size tubefurnace by placing 20g of the material into it in the presence of oxygen at a flow rate of600 ml/min at 1000°C for 4h. Under these conditions the oxidized material was found tocontain WO,1 Fe2WO6 and NiWO4 phases determined by conducting the X-ray diffractionanalysis, which is shown in Fig. 5. The formation of NiWO4 , phase was reported previouslyby Espevik et all` land the combination of WO, and W,00,, phases were reported by Aliprandoand Satyanskal5j. In the latter case, oxidation was conducted over a longer period of time,which is little over 15 h. The oxidized heavy alloy scrap collected in the powder form wasobserved under SEM where the formation of WO, grains were revealed (Fig. 6). Thematerial is found to detach along the grain boundaries and one would expect the formationof iron and nickel tungstates along these boundaries.Reduction studiesThe powder obtained from the oxidation of the heavy alloy was subjected to reductionin a tube furnace at 1000°C in the presence of H, for I h, and the weight loss after reductionwas found to he close to 25%. The reduced powder was collected and subjected to XRD,which has indicated the formation of W as shown in Fig. 7. The minor phases of iron,nickel and cobalt were not detected by XRD and by comparing this with the XRD obtained25G. V. K. PIJVVADA and S . SRiKANTHfrom the oxidized alloy scrap (Fig. 5), it is evident that the peaks related to NiWO4 andFe,WO< are absent and therefore it could he presumed that those minor metals arc in theirelemental form. The SEM photomicro`,raph obtained from the reduced powder is shown inFig. 8, where grains of 1-2 micron size were observed and the EDAX has also con brinedthe presence of tungsten, nickel, iron and cobalt. The cut heavy alloy piece and the powderobtained after reduction are shown in Fig. 9- The alloy powder thus obtained can herecycled hack into the industry for making the kinetic energy penetrators or tungsten canbe sepcrated from the rest of the metals by selective leaching.Fitt. 5 : SLM Irltutornicro rcrfrlr of the n.tidr,.cd ttitl slett Itcal" 1v allay poll-derFig. 6 : X-ray difractograrn of the oxidized langsten liecnvy alloy26RECYCLING OF TUNGSTEN BASED HEAVY ALLOY SCRAP7 : SEM P/rotonricrngraph of the reduced powderIII33aiom Sacco 90000 arm" 'n amrL. 5 ^l{%/ izrc 'rr>,^ rcr»^ r>J' rlrr c'chrc c cf r>c>>i'C/eFig. 9 : The tungsten heavy alloy before and after oxidation-reduction27G. V. K. PUVVADA and S . SR[KANTHCONCLUSIONSThe oxidation of W-Fe-Ni with minor amounts of cnhalt was studied by thermal analy-sis in the presence of oxygen flow. The oxidation reaction was found to commence ataround 700°C and was found to complete at around 1000°C. The weight gain correspondsto the formation of WO, and the enthalpy of oxidation was determined to he -625 kJ/iuoleof tungsten. The heavy alloy was oxidized in a tube furnace for 4 h in the presencc ofoxygen flow at 1000°C and the XRD obtained from this material has revealed the formationof• WO;, Fc,WO and NiWO4 phases. The oxidized fraction was subjected to reduction inthe presence of hydrogen atmosphere at 1000°C for I h and the reduced fraction was foundto contain pure tungsten metal powder along with minor fractions of iron, nickel and cobalt.This material can he recycled hack into the industry as it is or tungsten can he scpcratcdfrom the other metallic elements by known methods such as leaching.REFERENCES1. Steven G. Caldwell., Microhardness variations in tungsten based heavy alloys as function ofcomposition and processing variables , Progress in Powder Metallurgy; 1987 Annual PowderMetallurgy Conference Proceedings , 1987; 43:123-138.2. Private communication with M /S Heavy Alloy Penetrator Project, Tiruchirapalli, TN, India,1998.3. Bailey. M. and Ritchie , 1. M.. Metal Oxidation : An electrochemical perspective. Oxidation ofMetals, 1988; 30:405-418.4. Espevik, S. Rapp. R. A, David, P. L and birth, J. P., Oxidation of Ni-Cr-W ternary alloys,Oxidation of Metals, 1980; 14:85-108.5. Aliprando , J. J. and Satyanska, S. R.. The oxidation of ternary Co-Cr-W alloys, Oxidation ofMetals, 1981; 15:455-469.6. Espcvik, S. Rapp. R. A, David, P. L. and Hirth, J. P., Oxidation of ternary Co-Cr-W alloys.Oxidation of Metals, 1983; 20:37-65.7. Smykatz Kloss, W., Differential thermal analysis, Applications and results in Mineralogy, Mineralsand Rocks, 1974; I I:p:20, Springer-Verlag Publishers.8. lhsan Barin, Thermochemical data of pure substances, hind Edition, 1995, VCH Publishers.Weinheim.28RECYCLING OF TUNGSTEN BASED HEAVY ALLOY SCRAPAPPENDIXThe calculation of the enthalpy of oxidation using equation 2 is described here. Thesimultaneous TG/DTA's instrument constant K is first calculated by melting pure gold inthe instrument's furnace- The peak area obtained from the heat of melting of gold is foundto he 18 hv.s/mg. Having known the heat of melting of gold from the literature,", theinstrument constant K is calculated as shown belowMolecular Weight of Gold = 196.97 g° =AHm12.55 kJ/mole0.064 J/mgPeak Area Obtained = 18 p.v.s/mgK = AH°/Peak AreaAs Volt is equal to J/A.S where, J is Joule, A is Ampere and S is SecondK = 3555.5 AThis instrument constant of 3555.5 amperes is used in calculating the enthalpy of haspalloy oxidation and the step wise calculations involved are shown below:Molecular weight of Tungsten = 183.85 gPeak Area Obtained = -888 µv.s/mg (Fig. 4)K = 35.56 AAH° = K x Peak Area = -3.16 J/maAttributing the entire liberation of heat to the oxidation of W alone,AH° = (-3.16 x 183850) - 0.93 = -624.7 J/mg29

Recycling of tungsten based heavy alloy scrap

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Recycling of tungsten based heavy alloy scrap

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