CLEANER MANUFACTURING OF CRITICAL METALS

Layer Growth in Co(W)-Zn Systems at Hard Metal Recycling

NADINE KOERBLER,1,5 TAMARA EBNER,1 STEFAN LUIDOLD,1

HELMUT ANTREKOWITSCH,2 CHRISTOPH CZETTL,3

CHRISTIAN STORF,3 AMY CLARKE,4 and KESTER CLARKE4

1.—CD-Laboratory for Extractive Metallurgy of Technological Metals, Nonferrous Metallurgy,Montanuniversitaet Leoben, Franz-Josef-St. 18, 8700 Leoben, Austria. 2.—Chair of Nonferrous-metallurgy, Montanuniversitaet Leoben, Franz-Josef-St. 18, 8700 Leoben, Austria. 3.—Ceratizit,Metallwerk-Plansee-Straße 71, 6600 Breitenwang, Austria. 4.—Metallurgical and MaterialsEngineering, Colorado School of Mines, 1500 Illinois St., Golden, CO, USA. 5.—e-mail:nadine.koerbler@unileoben.ac.at

The influence of W content on the Co(W)Zn system and the behavior of indi-vidual elements was investigated. Cobalt is a so-called critical element and isone of the main parts in hard metal products. Because of its toxicity, it re-quires special care in handling. Further optimization of hard metal recycling,especially by the zinc process, is needed to use scrap as a secondary resource.In this work, Co(W) materials were put in contact with liquid zinc and held atspecific temperatures, giving insight into the formation of intermetallic Co-Znphases by varying the temperature, duration, and W content. Lower tem-peratures led to a minimum layer width, and increasing temperatures and Wcontents resulted in greater thicknesses of the intermetallic phases. The ele-ment distribution analyses show that W diffuses from the base materialthrough the intermetallic Co-Zn layer and accumulates at detached Co-Znphases, forming thin, enriched filaments. This can be attributed to the factthat the intermetallic Co-Zn phases and Zn have no solubility in W. Conse-quently, the latter is located on the outside of these phases. The combinationof practical experiments and statistical analyses will ultimately provide apossibility for improving recycling processes.

INTRODUCTION

The increasing demand for critical materials andelements affects ecologic and social consequences, sothe use of scrap as a source of raw materials istimely and important. It is essential to act carefullyand apply environmentally friendly and resource-efficient methods, especially in handling and pro-cessing critical metals. This enables conservation ofprimary resources to ensure constant raw materialflow. Because of the toxicity of some critical ele-ments, special care is required to maintain ahealthy environment. Frequently, these elementsare mined in politically unstable countries, withlittle to no environmental standards. Therefore, theaim is to use different recycling methods to recovercritical elements from scrap to protect naturalresources and to minimize the dependence on theraw material supply by other countries.1,2

Critical metals include cobalt and tungsten,which are the main constituents in the powdermetallurgical production of hard metals. Co is usedas a tough binder, and W serves in the form oftungsten carbide (WC) as the hard phase. Duringthe production of various hard metal products, suchas cutting tools, a compaction of the metal powderby liquid-phase sintering at high temperaturestakes place. In this case, some W is dissolved inCo, which does not completely precipitate at lowertemperatures, so W remains in the binder phase.This can negatively influence the recycling of hardmetals. For hard metal recycling, different methodsare applied: the indirect, semi-direct, and directroutes. The direct processes are the most environ-mentally friendly and include the zinc process, inwhich solid hard metal scrap is placed in contactwith liquid zinc.3,4 Figure 1 illustrates the schemaof the zinc process.

JOM, Vol. 72, No. 2, 2020

https://doi.org/10.1007/s11837-019-03929-3� 2019 The Author(s)

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Co and Zn react at the interface, where inter-metallic Co-Zn phases are formed. As a result ofincreasing amounts of Co-Zn phases relative to Co,the WC-Co separates, with the WC detaching in alamellar manner into the liquid zinc. A subsequentvacuum distillation removes the zinc, and a poroussponge of solid WC and Co remains. After a specialtreatment and some compensation of losses, thismaterial can be reintegrated into the manufactur-ing process.4,6–10

Investigations of the zinc process are commonlyperformed by using a range of simplified laboratory-scale experiments. The current tests combine prac-tical measurements at a small scale, with theoret-ical statistical analyses. Such experiments arenecessary for further improvements and optimiza-tion of already existing recycling processes. Thefocus of this study is the evaluation of the influenceof W on the formed intermetallic phases and thebehavior of the Co(W) system in a Zn matrix.Therefore, liquid Zn was cast onto solid Co(W) cubesin crucibles, which were kept at defined tempera-tures (600�C, 675�C, 750�C) and durations (0.5 h,4.25 h, 8 h) in a nitrogen atmosphere furnace.

Further microstructural examinations includedscanning electron microscopy (SEM) and thicknessmeasurements by light optical microscopy (LOM).

MATERIALS AND METHODS

Design of Experiments

The design of experiments, which was compiledby the statistical software MODDE 12, included 27experiments that varied parameter durations,including the temperature and desired W contentof the base material with associated layer thick-nesses and four investigations (14-1 to 14-4) withthe same conditions to check the representativenessof the tests (seen Table I).

Materials

The applied recycling method depends upon thereaction between Co and Zn, along with the impactof several alloying elements, especially tungsten.To investigate the influence of tungsten on thereaction between Co and Zn alloys of Co(W),nominal W contents of 0 wt.%, 5 wt.%, and10 wt.% were considered. After the processing,the measured W contents (SEM) were 0 wt.%,3.77 wt.%, and 4.01 wt.%. The powders used weremelted in an induction furnace at a temperature of1530�C, with a holding duration of 3 h. Then, thesolidified and compacted material was sliced intocubes with an edge length of 1 cm, which werepositioned in a quartz crucible. The crucibles werefilled with liquid 99.999% pure zinc (SEM) andheld in a nitrogen atmosphere furnace at definedtemperatures (600�C, 675�C, or 750�C) and holdingdurations (0.5 h, 4.25 h, or 8 h). The solid Co(W)interacted with the liquid zinc and formed inter-metallic Co-Zn phases. After holding for specifiedtimes, the Co(W)-Zn samples were air-cooled andmachined. The designation of each sample, forexample, 675 4.25 5 W, contains the temperature(600�C, 675�C, or 750�C), holding duration (0.5 h,4.25 h, or 8 h), and desired W content (0 wt.%,5 wt.%, or 10 wt.%).

Sample Preparation and Analysis

A smooth sample surface resulted from metallur-gical preparation in water. The polishing utilized adiamond suspension (1 lm) followed by Eposalmicro-etching.

Figure 2 shows an example of the received sam-ples, including the Co(W) cubes. The area aroundthe cubes shows the intermetallic Co-Zn phasesformed, surrounded by zinc.

The microstructure of the Co(W)-Zn surface wasanalyzed by an SEM (JEOL JSM IT-300 LV)equipped with an EDS analyzer. The layer thick-nesses of the intermetallic phases were measuredwith a light optical microscope.

Fig. 1. Flow sheet of hard metal recycling applying the zinc process.Adapted from Ref. 5.

Koerbler, Ebner, Luidold, Antrekowitsch, Czettl, Storf, A. Clarke, and K. Clarke848

Table I. Design of experiments, including sample designation, temperature, holding duration, W contents ofthe base material, and associated average layer thicknesses (measured by light optical microscopy)

Sampledesignation

Temperature(�C)

Target value(%)

Actual value(%)

Duration(h)

Layer thickness(lm)

1 600 0 0 0.5 56.02 675 0 0 0.5 69.03 750 0 0 0.5 93.04 600 0 0 4.25 2075 675 0 0 4.25 1546 750 0 0 4.25 97.07 600 0 0 8 2128 675 0 0 8 1339 750 0 0 8 94.010 600 5 3.77 0.5 43.011 675 5 3.77 0.5 85.012 750 5 3.77 0.5 96.013 600 5 3.77 4.25 54.014-1 675 5 3.77 4.25 11215 750 5 3.77 4.25 45.016 600 5 3.77 8 19017 675 5 3.77 8 12118 750 5 3.77 8 47.019 600 10 4.01 0.5 63.020 675 10 4.01 0.5 65.021 750 10 4.01 0.5 11522 600 10 4.01 4.25 76.023 675 10 4.01 4.25 13024 750 10 4.01 4.25 10525 600 10 4.01 8 10126 675 10 4.01 8 11227 750 10 4.01 8 18014-2 675 5 3.77 4.25 21114-3 675 5 3.77 4.25 19214-4 675 5 3.77 4.25 213

Fig. 2. Zinc casting sample containing the Co(W) cube withintermetallic Co-Zn phases around the cube.

Fig. 3. Layer thickness measurements of an etched sample 17 (6758 5 W) with Co(W) material, intermetallic layer, and liquid zincdetached intermetallic phases (light optical image).

Layer Growth in Co(W)-Zn Systems at Hard Metal Recycling 849

LOM Layer Thickness Measurements

The statistical analysis of the total thickness ofthe intermetallic Co-Zn phases formed at the inter-face between Co(W) and Zn with MODDE 12constitutes the main part of this research. Theparameters used ranged from 0 wt.% W to10 wt.% W and 0.5–8 h of holding time at thetemperatures 600–750�C. Temperature, duration,and W content were found to influence the thick-nesses of the intermetallic Co-Zn layers, which weremeasured in a light optical microscope. Figure 3demonstrates the measurement process on sample17 (675 8 5 W).

A list of the average layer thicknesses with theassociated temperature, treatment duration, and Wcontent is provided in Table I.

The statistical program MODDE 12 PRO servedfor the calculation of diagrams with different tem-peratures, holding durations, and W contents. Fig-ure 4 shows the evolution of layer thickness withtemperatures from 600�C to 750�C for 0 wt.% W.The highest layer thickness of approximately175 lm appears at the temperature 650�C. Increas-ing temperature has a negative effect and leads tothinner layers.

Figure 5 shows the formation of intermetallicphases at treatment durations ranging from 0.5 hto 8 h at a W content of 0 wt.%. At a reactionduration of 5.5 h, a maximum of approximately160 lm occurs. Shorter treatment durations resultin smaller layer thicknesses as well as durations> 6.5 h.

Figure 6 highlights the influence of the W contentranging from 0 wt.% W to 5 wt.% W for a treatmentduration of 4.25 h. Low temperatures of 600�C and650�C show a negative effect on the layer growth.The higher temperature of 750�C results in amaximum thickness of approximately 150 lm for5 wt.% of W. Decreasing W content results inthinner intermetallic layers at 750�C.

SEM Analysis

The distribution of different elements such astungsten, cobalt, and zinc in the Co(W)Zn systemwas analyzed by an SEM via EDS. Figure 7 showsthe location of tungsten (A), cobalt (B), and zinc (C)in sample 14-1 (675 4.25 5 W). The colors in thesefigures indicate the concentration of the elements.W is homogeneously distributed in the base

Fig. 4. MODDE evaluation of the layer thicknesses at 0 wt.% W,depending on duration and temperature.

Fig. 5. MODDE evaluation of layer thicknesses at 0 wt.% W,depending on duration and temperature.

Fig. 6. MODDE evaluation of the layer thicknesses at 4.25 h,depending on the W content and temperature.

Koerbler, Ebner, Luidold, Antrekowitsch, Czettl, Storf, A. Clarke, and K. Clarke850

material and hardly exists in the intermetalliclayer. Accumulations of W appear in the outerlayer. Comparison of Co and Zn illustrates atransition of the Co(W) material via the intermetal-lic Co-Zn phases to the area where Zn dominates.Some regions show higher contents of Co and Zn,which represent the previously mentioned detachedintermetallic Co-Zn phases. Figure 7 (D) presents asummary of all of the elements measured with EDS.

Figure 8 depicts accumulations of tungstenaround the intermetallic Co-Zn phases in sample13 (600 4.25 5 W) analyzed by SEM/EDS.

The bright areas are enrichments of tungstenbetween intermetallic Co-Zn phases. The resultsfrom three selected spectra include the liquid zincphase, an intermetallic phase, and a W-rich phase.The bright regions (A) consist of approximately 53%W and 47% Zn. The angular phases are 98.9% zincwith 1.1% cobalt (B). The gray regions (C) corre-spond to 100% zinc.

RESULTS AND DISCUSSION

The results of the layer thickness measurements,shown in Figs. 4, 5 and 6, illustrate the influence ofthe investigated parameters. At varying tempera-tures (Fig. 4), a maximum appears at mediumtemperatures of 675�C, duration of 4.25 h, and Wcontent of 0 wt.%. At higher values of 750�C, anegative effect occurs because WC detachmentproceeds faster than the formation of Co-Zn phases.A rising W content shifts the maximum thicknessesto higher temperatures. Low temperatures lead tothinner layers.

The same is shown in Fig. 5, where increasingtreatment durations lead to lower layer thicknesses,caused by the dominant detachment of intermetallicphases. A maximum occurs at medium durations of5.5 h, medium temperature of 675�C, and W contentof 0 wt.%.

Figure 6 includes varying W contents and demon-strates that low concentrations have negativeeffects on the layer growth. It is seen that low andmedium temperatures obtain a trend to decreasinglayer thicknesses with increasing W contents. The

Fig. 7. Element distribution of (A) tungsten, (B) cobalt, and (C) zinc in sample 14-1 (675 4.25 5 W) and (D) a summary of all of the elements(EDS).

Fig. 8. SEM/EDS mapping of the outer area representing zinc,detached Co-Zn phases, and W enrichments in sample 13 (600 4.255 W); analyzed areas: (A) W-rich regions; (B) intermetallic Co-Znphases; (C) pure zinc.

Layer Growth in Co(W)-Zn Systems at Hard Metal Recycling 851

highest temperature of 750�C and W content of5 wt.% resulted in a maximum of intermetallic layerthicknesses at medium holding durations of 4.25 h.

The chemical SEM/EDS mapping (Fig. 7) illus-trates the distribution of the individual elementssuch as Co, W, and Zn. W is only detectable in thebase material and in the area around the detachedintermetallic Co-Zn phases, but not in the Co-Znphases. Because of the insolubility of W in Zn, it isdisplaced to the outer regions and forms enrich-ments at already detached intermetallic phases.

Figure 8 shows a sample with 5 wt.% W, whichwas treated at 600�C with a duration of 4.25 h. Thespots A, B, and C were measured with an SEM/EDS.It is seen that there are compact W enrichments to aconcentration of 53% W enclosing Co-Zn phases (A),which are surrounded by pure Zn (C). The inter-metallic phases consist of approximately 98% Znand 2% Co (B). Due to naturally occurring convec-tions in the liquid zinc, caused by temperaturegradients in the melt and crucibles, solid W agglom-erates to visible enrichments.

CONCLUSION

For the optimization of individual recycling pro-cesses of critical metals such as Co and W for hardmetal commodities, an understanding of the influ-ence of W content on layer growth in the Co(W)Zn-system is essential. Temperature, duration, and Wcontent were examined to determine the effects onlayer thickness. Combined investigation concepts,measuring layer thicknesses with distribution anal-yses, were applied.

To recycle hard metal, several methods areapplied, including indirect, semi-direct, and directprocesses. A direct method, the zinc process, repre-sents the most environmentally friendly process. Inthis method, solid hard metal scrap is put in contactwith liquid zinc. Reaction between Co and Zn formsCo-Zn phases. Because of the increasing volume, theconnection between Co and WC breaks. A subse-quent vacuum distillation removes the zinc and aporous sponge of WC and Co remains, which can bereintegrated into the manufacturing process.

To better understand and optimize the existingzinc process, further investigations concerning thebehavior of W in the first stage of the zinc processare needed. For simplicity, Co(W) served as the basematerial, which contacted the liquid zinc. By vary-ing the parameters duration, temperature, and Wcontent, the formation of the intermetallic Co-Znphases was analyzed (LOM).

The results indicated that a maximum total layerthickness occurs at medium durations for varyingtemperatures. In addition, the results of a variableduration show that a maximum can be achieved atmedium temperatures and low W contents. Withincreasing W concentration, the maximum shifts to

higher temperatures. By varying the W content, itcan be seen that a maximum appears at mediumholding durations and high temperatures.

Chemical mapping, which was done by SEM/EDS,illustrates the location of the individual elementssuch as tungsten, cobalt, and zinc after the exper-iments. The results show that W diffuses throughthe intermetallic Co-Zn layer and accumulates atthe surface of the already detached intermetallicCo-Zn phases.

Finally, the results of this study lead to theconclusion that pure W enrichments will be presentin practice after a vacuum distillation of zinc next tothe porous WC and Co. Accumulations lead to Wlosses in WC, which is necessary for hard metalproduction, and have a negative influence on recy-cling processes and the quality of secondaryresources.

By combining the results of the layer thicknessmeasurements and element distributions, themicrostructural evolution during the zinc processwas investigated to develop improvements. Futureexperiments will deal with the decomposition ofhard metals to gain further optimization.

ACKNOWLEDGEMENTS

Open access funding provided by Mon-tanuniversitaet Leoben. The financial support bythe Austrian Federal Ministry of Digital and Eco-nomic Affairs and the National Foundation for Re-search, Technology and Development is gratefullyacknowledged.

OPEN ACCESS

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