Reclamation of Cemented Carbides

Reclamation of Cemented Carbides

17 Reclamation of Cemented Carbides Reclamation of cemented carbide scraps is getting more popular because of economic needs. Figure 1 gives the stat...

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17 Reclamation of Cemented Carbides

Reclamation of cemented carbide scraps is getting more popular because of economic needs. Figure 1 gives the static resource life data of various raw materials required for production of cemented carbides, based on 1981 information.Eli The 'static resource life' indicates the ratio between presently known reserves and annual consumption and, because both of these quantities are changing with time, it is only a relative indicator. The figure illustrates that the life of tungsten reserves is less than the binder cobalt. Under such situations, the reclamation of cemented carbides is rather necessary, both from strategic and economical points of view. There are several types of cemented carbide scraps available. They are: 9 Hard scraps: worn out or discarded cutting tips, buttons, bits, hot rolls 9 Soft scraps: discarded presintered parts, collected powders from the machining of presintered parts, etc. 9 Grinding sludges 9 Dust collector fines 9 Floor sweeps 369

370 Cemented Tungsten Carbides

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RAW MATERIALS FOR CEMENTED CARBIDES

Figure I. Static resource life data of various cemented carbide raw materials.

Soft scraps are usually recycled directly by the carbide parts producers and thus not available to the open market. There are three major reclamation processes for cemented carbides, namely: ~ Cold stream process 9 Zinc process 9 Chemical processes At present, approximately 65% of the world' s production of tungsten goes into manufacturing of cemented carbides. Processing of this material with a physical process rather than a chemical process would be preferred for the economic viewpoint. Of course, the recovery of tungsten is maximum in the chemical processes.

Reclamation of Cemented Carbides 371 1.0

SORTING

Before actual reclamation, the scrap is sorted. The compositions of cemented carbide scraps fall in the following limits: TiC 0-20 wt% (Ta,Nb)C 0-22 wt% Co 3-15 wt% WC Balance Before the sorting operation, which is carried out on an automatic machine based on density measurement,E2] the preparation of the scrap is important. The purposes are: 9 Cleaning from oil, grease, dirt etc. 9 Elimination of scraps which do not contain tungsten 9 Elimination of braze and the coatings which contaminate some scraps 9 Size reduction by crushing (<15 mm for the zinc process) for attaining a high yield Figure 2 gives a flowsheet of the preparation of the cemented carbide scrap.E21 Figure 3 illustrates the device for the sorting of cemented carbide scrap based on density selection.

2.0

COLD STREAM PROCESS

The crushed raw material (-6 mesh) is fed into a blast chamber and drawn by vacuum into a primary classifier. The full charge is pressurized and metered into a high pressure, dried air system and accelerated through a venturi towards the blast chamber. The fragments are impinged at a speed of MACH 2 against a fixed target of cemented carbide. The air expands as it leaves the venturi, generating an adiabatic cooling, which lowers the possible oxidation of the particles. After blasting, the powder is transported to a primary classifier, a secondary classifier, or a fines collector, depending on the size. Oversize particles are returned to the blast chamber and the process is repeated. One of the major limitations of this process is that cemented carbides with a high cobalt content cannot be processed because of their rather high toughness. The other limitation is contamination by oxygen and elements

372 Cemented Tungsten Carbides of the blast chamber surface, i.e., iron. Oxygen contamination can be removed by sintering in hydrogen, but this tends to give carbon deficiency problems. The recovery yield of this process is 90-92% and particles as fine as 1.5-2.00 ~tm are produced. [3] ,

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3.0

THE ZINC PROCESS

The zinc process relies on the principle that molten zinc forms an alloy system with cobalt and disrupts the integrity of the cemented carbide. The zinc can then be removed by distillation, leaving a fragile mass of tungsten carbide and cobalt that is readily ground and resintered. The intermetaUic compound CoZn13 is responsible for the bloating of the scrap. The fusion is carried out under argon at 900~ and subsequently zinc is distilled at a pressure less than 7 kPa. The product is then milled to the desired particle size. This process was invented by Barnard et al. [4] who got the clue from another past patent awarded to Powder Alloys Ltd.[5]

Reclamation of Cemented Carbides 373

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Yamaguchi and Okadat111 suggested that the friability of Zn-treated cemented carbide is dependent on its microstructure, such that a lamellar structure favours friability. Cemented carbides with a coarse WC grain and/or high cobalt content did not produce regular lamellar structures. Large amounts of molten zinc and high temperatures were also not favourable for the formation of such microstructures. However, heating time did not affect the structure of the reaction layer and the interlamellar spacing. It was found that the expansion of a Zn-treated mass during cooling was more when the lamellar structure was regular. The zinc process is usually operated batchwise. Graphite crucibles are loaded with 14-22 kg of scrap cemented carbide and zinc in the amount of 1 to 1.3 times the weight of the carbide. Adequate zinc removal requires that the pressure be held for at least 5 hours. Distillation of zinc from the product requires a total of approximately 15 hours. Figure 4 is schematic diagram of the retort in which the process is carried out. Limitations of the process are: 9 Large pieces of cemented carbide must be treated more than once 9 Adequate zinc removal might require two distillations 9 The process has no purifying action

374 Cemented Tungsten Carbides 9 The process requires pure zinc since any non volatile impurity contained in the zinc will remain in the reclaimed powder 9 The process is most suited for straight WC-Co grades 9 Repeated use of process results in a progressive enrichment in impurities 9 Problems arise with grades containing modified binders and coatings such as alumina

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Reclamation of Cemented Carbides 375 The recovery of the process is better than the cold stream process and is around 97-98%. In the USA, the carbide industry uses approximately 812% zinc processed materials; in Europe this percentage is somewhat lower and in Japan is less than 2%. [6]

4.0

CHEMICAL PROCESSES

There are a variety of chemical processes of reclamation in which one or more of the constituents are altered chemically. The simplest of these is a leaching step to remove cobalt binder. Usually, the process is done in combination with milling. Aronsson and Pastor[ 1] have done an excellent review of various processes which are shown in various flowsheets (Figs. 5-8). The processes are: 9 Binder leach process (Fig. 5) 9 Nitrate process (Fig. 6) 9 Oxidation in a molten alkaline medium (Fig. 7) 9 Sodium hypochlorite process (Fig. 8) Kobayashi [9] describes a simple high temperature process for reclaiming cemented carbide scraps, in which the scrap is treated between 1800 and 2000~ in a carbon monoxide atmosphere. After treatment, the material is crushed, milled and recovered as a coarse, but immediately reusable, powder. Scrap with cobalt up to 20% can be recycled by this method, though the product is normally restricted to wear part applications. [1~ Some particle size control is possible by varying the crushing and milling conditions after heat treatment. Table 1 shows the changes in composition and particle size of the powder reclaimed by the high-temperature process in comparison with the zinc process. It can be seen that the carbon contents of the two powders are about the same, but the zinc process causes slightly greater decarburization. Impurities like Fe, SiO 2, and oxygen are higher in High-Temperature processed powder, as compared to the zinc process. The author [9] could get 1 ~tm size WC powders from the scrap used in the HT-process, while the zinc process gave rise to a somewhat larger particle size. Each of the chemical processes has a particular set of advantages in a given situation, but suffers from increased costs, decreased yield (but not always), and increased environmental problems compared to the more direct methods described earlier. [7] In chemical processing, all impurities

376 Cemented Tungsten Carbides are reduced to very low levels. The economics of process selection naturally depends on the complexity of the system. For example, the cost savings for zinc processed material are high with the added value of tungsten, cobalt, and tantalum in the system. However, the advantages of the zinc process could vanish should the cost of sorting of scrap increase significantly. One such example may be the removal of coating from the inserts to improve sorting and quality control of the final product. [8] T a b l e 1. Comparison Between HT-Process and Zinc Process on Recovered Powder Composition and Size. E9] Item

FSSS

Chemical contents (%)

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size, ~m

Total C

Co

Fe

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Si (ppm)

Ca

S

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5.41 5.43 5.48

15.75 15.71 15.01

0.13 0.09

0.47 0.28

46 10

9 14

4 1

Scraps HT-Pro. Zinc Pro.

1.02 1.30

5.89 5.90 5.85

5.18 5.20 5.15

0.05 0.08 0.04

0.33 0.27

58 10

15 9

2 2

Scraps HT-Pro. Zinc Pro.

1.00 1.07

5.65 5.65 5.63

10.02 9.58 9.45

0.07 0.09 0.03

0.39 0.34

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6 7

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0.87 1.10

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0.14 0.20 0.10

0.48 0.34

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Reclamation of Cemented Carbides 377

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The TiN and Ti(C,N) coated WC-Co cemented carbides can be reclaimed by the zinc process for use in high cobalt containing tough grades. [6] Contrary to the case of scrap reclaimed by direct methods, the processing cycle of scrap through the chemical method is akin to ore concentrate (artificial scheelite CaWO4). For the details of reduction, see Ch. 3. The details of impurities confronted in cemented carbide production at various processing stages have been adequately highlighted in previous chapters. All of these are equally important in reclaimed powder as well. Additional study is required to understand their role when they are present in combination with a group of impurities. Investigations are needed on the role of repeated recycling of a particular grade of cemented carbide on the nature of impurity pick up and its effect on ultimate end properties. It is expected that more use of consolidation processes like HIP and pressure sintering would be needed for such grades of powders.

Reclamation of Cemented Carbides 379 REFERENCES 1. Sarin, V. K., Advances in Powder Metallurgy, (G. Y. Chin, ed.), ASM International, Materials Park, Ohio, p. 283 (1982) 2. Aronsson, B., and Pastor, H., Sintering of Multiphase Metal and Ceramic System, (G. S. Upadhyaya, ed.), Sci-Tech. Publications, Vaduz, p. 397 (1990) 3. Walraedt, J., Powder Met. Int., Vol. 2, 1970, p. 77. 4. Barnard, P. G., Starliper, A. G., and Kenworthy, H., U.S. Patent, 1971, Patient No. 3, 95, 484 5. Trent, E. M., U.S. Patent, 1946, No. 2, 407, 752 6. Kieffer, B. F., and Lassner, E., Tungsten 1987, MPR Publishing Services, Shrewsbury, p. 59 (1988) 7. Kieffer, B. F., Tungsten 1982, Mining Journal Book Ltd., London, p. 102 (1982) 8. Borchers, P., Discussion on the paper of Kieffer and Lassner cited in Ref. 7, Tungsten 1982, MPR Publishing Services, Shrewsbury, p. 59 (1988) 9. Kobayashi, T., W-Ti-Re-Sb 1988, Vol. II, (F. Chongyue, ed.), Pergamon Press, Oxford, p. 645 (1989) 10. Brookes, K. J. A., Metal Powder Report, Vol. 39, p. 214 (1984) 11. Yamaguchi, T., and Okada, M., J. of the Am. Cer. Soc., Vol. 61, p. 529 (1978)