Dissolution behavior of cobalt from WC–Co hard metal scraps by oxidation and wet milling process

doi:10.1016/j.hydromet.2014.01.004

Hydrometallurgy

Volume 143, March 2014, Pages 28-33

https://doi.org/10.1016/j.hydromet.2014.01.004 Get rights and content

Highlights

•
Efficient Co dissolution from WC–Co hard metal scrap discarded after industrial use
•
Environmentally less harmful wet milling process for Co recovery in acid solution
•
Optimization of chemical dissolution conditions and ball-milling process
•
Dissolution of Co from the mixture of WO₃ and CoWO₄ in H₂O₂-containing acid solution

Abstract

Wet milling was fundamentally investigated as a process for dissolution of Co from WC–Co hard metal scrap in this study. The development of a relatively fast and low acid-concentration recycling method of Co from WC–Co hard metal scrap is significant, considering the rare and uneven deposition of Co worldwide. WC–Co scraps were fully oxidized, resulting in a mixture of WO₃ and CoWO₄. CoWO₄ was readily decomposed to produce soluble CoSO₄ and insoluble tungstic acid in sulfuric acid solution of pH 2 or less. Dissolution conditions such as the concentration, temperature, and stirring rate of the solution were varied to determine changes in Co dissolution efficiency with dissolution time. Adding 2 vol.% hydrogen peroxide to the solution augmented the Co dissolution rate considerably. Co dissolution by disruption of the tungstic acid layer via wet milling in 1 M sulfuric acid solution with hydrogen peroxide was four times faster than simple chemical dissolution of Co in 1 M sulfuric acid solution. Wet milling is a time-efficient process to recycle Co from WC–Co scraps which uses a relatively low concentration acid solution.

Introduction

WC–Co hard metals have been used for machining tools which require a high degree of wear resistance, mechanical strength, and toughness for proper performance (Brookes, 1996). W and Co are rare metals that have very limited deposits worldwide. There have been many efforts to recycle discarded parts and tools made of WC–Co hard metal. Recycling methods of WC–Co scraps can be divided into direct and indirect processes. Direct recycling methods include the cold stream process and the Zn process. Hard scraps of WC–Co are pulverized using jet milling in the cold stream process, while in the Zn process, WC–Co scraps are immersed in molten Zn bath to selectively react Co with Zn, producing porous scrap residue which can be easily milled. Recovered Co-containing carbide powders are sintered for manufacturing hard metal tools. Indirect recycling methods such as chlorination, oxidation–alkaline leaching, and oxidation–reduction are composed of sequential steps of dry and wet processes. However, most of the conventional recycling processes, whether they are direct or indirect, restore WC–Co hard metal scraps to a mixed powder of WC particles and Co binder.

Selective elemental recovery of W and Co from hard metal scrap has been investigated based on hydrometallurgy or hydrothermal treatment methods. The melt bath technique was introduced to convert W scrap to WC powder (Venkateswaran et al., 1996). Selective removal of Co binder from WC–Co scraps has been attempted using an additive-involving acid solution (Lin et al., 1996), inorganic acid leachants (Gurmen, 2005, Malyshev and Hab, 2004, Malyshev and Hab, 2007), an acetic acid solution (Edtmaier et al., 2005), aqua regia treatment (Kim et al., 2010, Lee et al., 2011) and a hydrothermal extraction process (Kojima et al., 2005). Electrochemical recovery of W and Co, using hard metal scrap as an anode material in nitric acid solution, has also been reported (Madhavi Latha and Venkatachalam, 1989). The molten Sn process has been proposed to replace the Zn process to selectively recover W or WC by forming a Sn–Co intermetallic compound (Kamada and Nakamura, 2001). Researchers reported that the oxidation of WC or WC–Co scraps as a preliminary treatment to selective acid leaching is effective in increasing the dissolution or leaching rate of W or Co (Andersson and Bergstrom, 2000, Jia et al., 2011). However, they focused mostly on the separation of W from scraps, and, moreover, argued that Co is oxidized to CoO during scrap oxidation.

In this research, we propose the selective dissolution of Co from oxidized WC–Co hard metal tool tip scraps by an efficient wet milling process in sulfuric acid solution. Previously, we fundamentally studied the thermal oxidation behavior of WC–Co scrap to determine the preferential oxide phases and oxidation mechanism of WC and Co (Gu et al., 2012). Our results regarding the oxidation of WC–Co scrap corresponded to previous reports concerning the nature of oxidized phases of WC–Co hard metal at high temperatures (Basu and Sarin, 1996, Bhaumik et al., 1992, Casas et al., 2001, Lofaj and Kaganovskii, 1995, Voitovich et al., 1996). Based on the current understanding of WC–Co oxidation, in the present research we investigated the dissolution behavior of Co from oxidized WC–Co according to various dissolution conditions such as concentration, dissolution time, temperature, and stirring rate in the sulfuric acid solution. The effect of hydrogen peroxide addition on Co dissolution efficiency was also examined. Lastly, wet milling of oxidized WC–Co is suggested to dissolve Co efficiently by combining the optimized dissolution process with mechanical ball milling.

Section snippets

Oxidation of WC–Co scrap

WC–Co scrap samples from insert-tip-type cutting tools were provided by an industry after sufficient usage, when they became useless for cutting metals or alloys. The chemical composition of the WC–Co insert tip scrap was WC–9% Co-less than 0.5% TiNbC. The WC–Co insert tip scrap samples were covered with a CrAlN coating, which increases the wear-resistance and hardness of WC–Co insert tips. WC–Co scrap samples were isothermally maintained in an oxygen atmosphere at 900 °C for 3 h after heating

Oxidation of WC–Co hard metal tool tips

Fig. 1a shows a WC–Co hard metal tool tip discarded after use in a cutting machine. The edges of the WC–Co tool tip were seriously worn under repetitive cutting operations. Bare WC–Co surfaces were exposed by the local loss of CrAlN coating. The CrAlN coating is highly resistant to oxidation at 900 °C (Chim et al., 2009, Gu et al., 2012); thus, locally damaged edges and corners of the WC–Co tool tip can be the most vulnerable to WC–Co oxidation. The CrAlN coating is physically separated as

Conclusions

Co was selectively dissolved from discarded WC–Co hard metal tool tips using the wet milling process, which combines ball milling and chemical dissolution, after WC–Co tool tips were fully oxidized to the mixture of WO₃ and CoWO₄ particles. The efficiency of Co dissolution from oxidized WC–Co powder in sulfuric acid solutions was dependent on the acid concentration, dissolution time, solution temperature, and solution stirring rate. Soluble CoSO₄ and insoluble tungstic acid were produced when

Acknowledgment

This work was supported by a grant from the Fundamental R&D Programs for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea [100371-841].

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Dissolution behavior of cobalt from WC–Co hard metal scraps by oxidation and wet milling process

Highlights

Abstract

Introduction

Section snippets

Oxidation of WC–Co scrap

Oxidation of WC–Co hard metal tool tips

Conclusions

Acknowledgment

Int. J. Refract. Met. Hard Mater.

Mater. Sci. Eng. A

Int. J. Refract. Met. Hard Mater.

Thin Solid Films

J. Alloy Compd.

Hydrometallurgy

J. Mater. Process. Technol.

J. Alloy Compd.

Int. J. Refract. Met. Hard Mater.

Hydrometallurgy

Hydrometallurgy

J. Mater. Process. Technol.

Int. J. Refract. Met. Hard Mater.

Int. J. Refract. Met. H.

Determination of pH, Theory and Practice