Synthesis of submicron silver powder from scrap low-temperature co-fired ceramic an e-waste: Understanding the leaching kinetics and wet chemistry

Highlights

• Various waste component of LTCC was analyzed.

• Leaching kinetics for Ag was investigated and optimized.

• Selectively Ag was recovered by leaching-precipitation-wet chemical reduction.

• Ag-NPs was synthesized through wet-chemical reduction.

• A cost-effective and sustainable eco-efficient industrial valorization process.

Abstract

The current study focuses on the understanding of leaching kinetics of metal in the LTCC in general and silver leaching in particular along with wet chemical reduction involving silver nanoparticle synthesis. Followed by metal leaching, the silver was selectively precipitated using HCl as AgCl. The precipitated AgCl was dissolved in ammonium hydroxide and reduced to pure silver metal nanopowder (NPs) using hydrazine as a reductant. Polyvinylpyrrolidone (PVP) used as a stabilizer and Polyethylene glycol (PEG) used as reducing reagent as well as stabilizing reagent to control size and shape of the Ag NPs. An in-depth investigation indicated a first-order kinetics model fits well with high accuracy among all possible models. Activation energy required for the first order reaction was 21.242 kJ mol⁻¹ for Silver. PVP and PEG 1% each together provide better size control over silver nanoparticle synthesis using 0.4 M hydrazine as reductant, which provides relatively regular morphology in comparison to their individual application. The investigation revealed that the waste LTCC (an industrial e-waste) can be recycled through the reported process even in industrial scale. The novelty of reported recycling process is simplicity, versatile and eco-efficiency through which waste LTCC recycling can address various issues like; (i) industrial waste disposal (ii) synthesis of silver nanoparticles from waste LTCC (iii) circulate metal economy within a closed loop cycle in the industrial economies where resources are scarce, altogether.

Introduction

Because of light, compact, high-speed and better functionality low-temperature co-fired ceramic (LTCC) commonly used in multi-chip ceramic modules, radio frequency (RF) microwave circuits, various sensor and MEMs (Imanaka, 2006) (Gongora-Rubio et al., 1999, Gongora-Rubio et al., 2001). Its application also continuously expanding (Claeys et al., 2004). Global LTCC market by the end user is 60.74%, 12.62%, 10.57%, 10.23% and 5.84% for consumer electronics, home appliances, computer peripherals, automotive and others, respectively (Research, 2017). Asia-Pacific region is the largest producer of the LTCC product as well as LTCC industrial waste during manufacturing (marketreportsworld.com, 2017, QYResearchGroup, 2017). This LTCC waste contains a significant amount of Ag. Hence, not only because of a precious metal but also from environment prospective these wastes need to be recycled through the sustainable eco-efficient process. Numerous studies have been reported for recycling of e-waste through hydrometallurgy but report regarding LTCC is scarce.

Due to distinctive physical and chemical properties, like; high thermal and electrical conductivity, surface-enhanced Raman scattering, chemical stability, catalytic activity, and nonlinear optical behavior of Ag-NPs as compared to their bulk, the inorganic Ag-NPs has been used in nanomedicine, drug delivery and biomedical devices, cosmetics, electronics, energy sector, and environmental (Lubick, 2008, Zielińska et al., 2009, García-Barrasa et al., 2011). For above-listed application perspective, several authors have reported various route (chemical, physical, photochemical and biological routes) for the synthesis of Ag-NPs (Prabhu and Poulose, 2012, Desireddy et al., 2013, Quang Huy et al., 2013). Ag NPs synthesis through wet chemical reduction has been reported by several authors (Ajitha et al., 2016, Jacob et al., 2007, Silvert et al., 1996, Tejamaya et al., 2012, Zhang et al., 2000). Most of the process reported are mainly synthesized from a pure chemical which is essentially expensive processes and involves complex route.

Silver recovery from various e-waste resources has been reported are mostly leaching followed by silver metal recovery. Lim et al. have reported chemical durability of LTCC using 10 vol % of HCl, H₂SO_4, and KOH with respect to room temperature and 80 °C (Lim et al., 2009). Lim et al. indicated that LTCC has better potential to resists KOH even at 80 °C. Even 10 vol % of HCl and H₂SO₄ can leached hardly around 40% prove to be poor lixiviant (Lim et al., 2009). Petter et al. have reported gold and silver leaching from printed circuit board of mobile phones using ammonium thiosulfate ((NH₄)₂S₂O₃), ammonium hydroxide (NH₄OH), copper (II) sulfate (CuSO₄) and hydrogen peroxide (H₂O₂), which is lacking the leaching kinetics understanding (Petter et al., 2015). Li et al. have also reported 92% gold and 90% silver leaching from E-waste by (NH₄)₂S₂O₃, NH₄OH, CuSO₄ and H₂O₂ (Li and Huang, 2010). Bhat et al. have reviewed gold and silver leaching from e-waste by hydrometallurgical route and revealed that mostly leaching studies were focused on cyanide leaching or thiosulfate leaching (Bhat et al., 2012). Swain et al. have reported selective recovery of silver from waste LTCC but leaching kinetics aspect of the investigation never been reported (Swain et al., 2017). Silver leaching from e-waste followed by synthesis of Ag-NPs from potential waste can be a novel approach which can circulate the metal and close the loop in the material cycle. Also, Ag-NPs synthesis from e-waste ultimately can extensively support the green economy. Although material synthesis from e-waste through waste recycling is not uncommon, reports regarding waste LTCC recycling and their leaching kinetics from valorization perspective never been reported. Understanding of leaching behavior and its kinetics can address two important aspects of the application, i.e., (i) cost-effective, eco-efficient recovery process development, and (ii) understanding the chemical durability of LTCC towards the HNO₃ acid atmosphere. Hence, current research explores leaching kinetics of waste LTCC, selective recovery of silver though hydrometallurgy and wet chemistry for the synthesis of submicron silver powder from LTCC.

As leaching is the primary and essential stage for recovery of the metals, these wastes were leached using mineral acid, to develop eco-efficient process. To develop feasible eco-efficient process, understanding of the leaching kinetics and wet chemistry of silver reduction need to be understood well. Hence, the current study focuses on the understanding of leaching kinetics of metal in the LTCC in general and silver leaching in particular. Followed by leached silver was selectively precipitated using HCl and the precipitated AgCl was reduced to pure Ag-NPs using hydrazine through dissolution in ammonium hydroxide. Reduction chemistry for Ag-NPs also analyzed. For NPs synthesis commonly the Polyvinylpyrrolidone (PVP) used as a stabilizer and for Ag reduction Polyethylene glycol (PEG) used as reducing reagent as well as stabilizing reagent to control size and shape. Along with hydrazine either PVP or(and), PEG was used as a stabilizing agent and controlled reduction in the Ag-NPs synthesis from AgCl precipitate (García-Barrasa et al., 2011). The importance and novelty of the reported leaching-precipitation-wet chemical reduction process for the synthesis of Ag-NPs process for waste LTCC can be summarized below.

• Reported recycling process signifies simplicity, versatile and eco-efficient process, through which various issues like; (i) waste disposal through valorizations of silver as NPs (ii) circularize the economy by bringing back the material to production stream, and (iii) sustainable green economy and the futuristic carbon economy, can address simultaneously.

• Through the case study, a total recycling process has been developed for all kind of LTCC waste, which can easily recycle all component of the industrial LTCC waste.

• Without any special facilities or specialty chemicals, through the small-scale industrial facility, the waste can be converted to product/raw material for the same industry.

• Current process synthesizes the Ag-NPs from the industrial waste resources in contrary to from the pure chemicals as reported by numerous author in the literature.

• Understanding of fundamental characteristics like leaching kinetics and wet chemistry involve can provide a better avenue for an understanding of the waste recycling by hydrometallurgy and can be applied to the diversified waste contains Ag.