Reduction of Environmental Impact in Hardmetal Technologies

Metal Powder Report Volume 72, Number 4 July/August 2017 metal-powder.net Reduction of environmental impact in hardmetal technologies SPECIAL FEATURE Pankaj K. Mehrotra Kennametal Inc., Latrobe, PA, USA As energy, environmental remediation and raw material costs for hardmetal production escalate with increasing demand, the need for developing and implementing technologies for reducing environmental impact becomes more urgent, while, at the same time, reducing material cost. A range of technologies can be employed for recycling hard and soft hardmetal scrap which consume less energy and require lower capital investment than for virgin powder. Performance and processing targets with recycled materials must be equivalent to those from virgin material. Conventional solvent based processing can be replaced by water based processing of hardmetal compositions to reduce environmentally damaging emissions and associated costs. This will be particularly useful as government regulations move to reduce or eliminate emission of organic solvents. Raw material consumption can be reduced by employing near-net-shape or net-shape technologies, since many hardmetal parts are finished by hard grinding, which generates grinding sludge. Partial or complete replacement of WC/Co with other hardmetals is another strategy worthy of consideration in reducing the impact of hardmetal production on the environment. Hardmetals play a critical role in myriad industrial processes 30% increase over the same period [2], although recent trends are including manufacturing, energy production, construction, wear flat, possibly due to developments in oil shale exploration. resistant applications, etc. As a result, sustainable hardmetal pro- Furthermore, the availability of W bearing ores has become duction is of paramount importance in supporting the modern more restrictive due to non-uniform geographical distribution industrial base, especially as it expands with global population (Fig. 3). Over 2/3 of world reserves of W are in China and Russia growth and elevated standards of living. Concurrently, reduction [3]. Although tungsten production and demand are in balance, of environmental impact has become more urgent as harmful about 90% of production is located in the same two countries emissions remediation costs have escalated, or has been complete- [4]. ly outlawed. As shown in Fig. 1, a majority of W production is used Therefore, it is important that strategies be developed to reduce to meet the demand for hardmetals, so considerations for W environmental impact of hardmetal production, while simulta- supply chain are critical to hardmetal production. neously increase supply and lower cost. This must be done without Starting in 2003, W precursor prices underwent dramatic esca- sacrificing form and function of the end product. lation showing an eight times increase by 2011. Fig. 2 [1] shows Hardmetal manufacturing process is schematically depicted in this trend since 2005, although, prices have moderated recently Fig. 4, starting from ore/concentrates to hard shaping. This paper due to global reduction in industrial activity. At the same time, will examine technologies for reducing cost and environmental industrial electric costs (as a measure of energy costs) have seen a impact, and increasing W availability during each of the processes in the manufacturing chain. However, change in one process may require modification in a subsequent process to ensure process and product performance remain unchanged. Therefore, a holistic E-mail address: [email protected]. 0026-0657/ß 2016 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mprp.2016.02.052 267 SPECIAL FEATURE Metal Powder Report Volume 72, Number 4 July/August 2017 SPECIAL FEATURE FIGURE 1 Applications of tungsten. approach is required which considers the entire process chain for benefits balanced by costs. FIGURE 3 Raw material production Global W deposit reserves (3 million tons). Greater recycling of hardmetal scrap in raw material production is one way to simultaneously reduce cost, energy consumption and environmental impact, while concurrently increasing W availability. By one estimate, only 35% of W used is recycled [4], however, approach is the development of a separate value stream for addi- recent trend is for greater recycling. A range of technologies can be tional recoverable materials which accompany the scrap, such as employed for recycling hard and soft hardmetal scrap, which Co, Ti, Ta and Nb. consume less energy and require lower capital investment than Uncontaminated soft scrap can be recycled by certification and by using ore concentrates. As stated before, performance and limited additions to the milling process. Composition control and processing targets with recycled materials must be equivalent to grain size classification are key control parameters in this regard. those from virgin material. Both ‘soft’ and ‘hard’ hardmetal scrap Attention must be paid to carbon level to counter oxidation during can be cleaned and oxidized to provide high W content feedstock powder handling, and microstructural features, such as porosity, for ammonium paratungstate production, a common precursor uncrushed granules and contamination caused by improper pow- for WO3 and virgin W metal powder. A consideration for this der classification. FIGURE 2 WO3 cost escalation over 10 years. 268 Metal Powder Report Volume 72, Number 4 July/August 2017 SPECIAL FEATURE FIGURE 4 Overview of hardmetal manufacturing process. SPECIAL FEATURE Hard scrap can also be recycled by other, lower cost, means, characteristics which do not affect the green shaping processes. however with limitations in quality and use [5]. In the cold stream This includes powder flow, Scott density and powder compaction process, hard scrap is crushed and screened to generate powder behaviors. Furthermore, subsequent delubing and sintering pro- which can be used to make grade powder. This process leaves cesses, such as sintering characteristics, including shrinkage, also chemistry and grain size unchanged, but may increase some need to remain unaffected when using water processed powder. impurities, such as Fe, as a result of the crushing process. The zinc Development of microstructure, carbon control and sintering process is an established method to convert sintered hardmetal distortion are some of the properties which should be considered scrap into powder by first bloating it by making a Co-Zn amalgam, in this regard. Partial or full replacement of WC with other hard- followed by Zn evaporation, crushing and screening [6]. The metals, such as TiC, TiCN, NbC and Mo2C, and of Co with Fe and resulting powder can be subsequently milled to produce grade Ni, are additional technologies which can be used to tailor cost, powder. WC particle size and impurities/alloying additions remain availability and environmental impact. unchanged, and therefore, its use is limited to compositions with similar grain size and chemistry as the starting scrap material. Also, Green shaping Zn contamination needs to be controlled. High temperature bloat- Raw material consumption can be reduced by employing near-net- ing, crushing and screening of sintered hardmetal scrap also yields shape or net-shape technologies, since many hardmetal parts are powders which can be used in making hardmetal powders. Chem- finished by hard grinding, which generates grinding sludge [7]. istry of hardmetal in this process remains unchanged, but a Fig. 5 conceptually shows the relationship between pressing pres- relatively large grain size results due to the high temperature used. sure, sintering shrinkage and microstructure, which can be used Also, intensive energy consumption maybe a concern. Selective such that near-net or net shape parts can be pressed and sintered dissolution of Co in an electrochemical bath is another process to with acceptable microstructure. obtain WC powder from hard scrap, where relatively pure WC can Optimization of uniaxial pressing, use of multi-axial pressing, be obtained with original grain size. However, consideration must and side pressing are green shaping technologies which can be be made for recovery of Co and other additives and disposition of used for making complex, net or near-net shape parts. Fig. 6 solvents. schematically illustrates these technologies. They become partic- Considerations for powders produced by recycling hard scrap ularly useful as complexities of parts increase to meet the ever include carbon control and microstructure, since some of these increasing demand for performance. processes may cause oxidation of powder, and pressing and sintering behavior may be different for recycled powders than those made with virgin powders. Grade powder production Conventional processes to manufacture grade powder involve solvents as transient additives to facilitate powder milling and spray drying. Organic solvents have been traditionally used for this purpose, which require not only expensive building and equip- ment to handle flammable liquids and vapors, but also entail high disposal costs. Solvent based powder processing can be replaced by water based processing of hardmetal compositions to reduce environmentally damaging emissions and associated costs. This will be particularly useful as government regulations move to reduce or eliminate emission of organic solvents in the industry. Process development and choice of additives to optimize powder milling FIGURE 5 and drying are required. Powder, thus produced, should

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