Recycling of aluminum metal matrix composite using ionic liquids:

Abstract

Recycling of aluminum metal matrix composite via electrolysis in ionic liquids at low-temperature was investigated. The electrolytic melt comprised of 1-butyl-3-methylimidazolium chloride (BMIC) and anhydrous AlCl₃. Aluminum metal matrix composite (Duralcan^®, Al-380, 20 vol.% SiC) was electrochemically dissolved at the anode, and pure aluminum (>98%) was deposited on a copper cathode. The influence of experimental parameters such as concentration of electrolyte and applied cell voltage on the efficiency of aluminum metal matrix composite recycling was studied at 103 ± 2 °C. High applied voltages and concentration of AlCl₃ yielded high current densities. Current densities obtained during this process were in the range of 200–500 A/m² and current efficiencies in the range of 70–90%. The deposits were characterized by scanning electron microscope, X-ray diffractometer, mass spectrometer, and atomic absorption spectrophotometer. Characteristics of the deposited microstructure ranging from columnar to spherical were obtained. Energy consumption was in the range of 3.2–6.7 kWh/kg-Al for the experimental conditions studied. The optimum conditions obtained in the present investigation for maximum current efficiency and least energy consumption with uniform deposit microstructure were low applied voltage and intermediate electrolyte concentration. Low energy consumption and no emission of pollutants are the two main advantages of this process compared to the current recycling processes.

Introduction

Metal matrix composites offer numerous beneficial properties such as high strength, stiffness, fatigue, and thermal properties. An enormous interest in the manufacture of metal matrix composites was triggered by their application in structural, automotive, and defense applications [1]. The advancement of metal matrix composites in the automotive market is still hampered by the low-volume usage of these materials due to their high cost in comparison with aluminum alloys. High demand and cost of production led to the recycling of these composites. Recycling, a significant factor in the supply of many of the metals used in daily life provides environmental benefits in terms of energy saving, reduced volumes of waste, and reduced pollutants emission [2].

Metal matrix composites are recycled by melting composite scrap in furnaces such as induction furnace, reverbatory melter, hearth furnace, and rotary barrel furnace. The scrap from aerospace and commercial applications is melted and cast into ingots. Only a slight change in the tensile properties of composite was observed after several recycling steps [3]. Salt fluxing technique is commonly used in the reclamation of ceramic-particulate-reinforced metal matrix composites [3], [4]. This process is based on the principle of effective de-wetting of ceramic particles from aluminum matrix using molten salts. The molten salts are thermodynamically stable in the presence of liquid aluminum and can effectively remove ceramic particles by wetting them, but the salt itself is not wetted by the liquid aluminum. The salts used for separating reinforcement from aluminum matrix are a mixture of halides of sodium and potassium, because surface energy of these salts is lower than that of aluminum [5]. This phenomenon occurs at high temperatures, normally above the melting temperature of aluminum.

In the present study, aluminum metal matrix composite is reclaimed via electrorefining in ionic liquid at low-temperature. Reddy and co-workers [6], [7], [8], [9] have reported electrorefining of aluminum alloys using chloroaluminate ionic liquids, which yielded high-purity aluminum deposits with low energy consumption and no emission of pollutants. Ionic liquids are low melting salts with negligible vapor pressure, high electrical conductivity, and wide electrochemical window of about 4.0 V. These favorable properties render chloroaluminate ionic liquids as potential electrolytes for low-temperature aluminum production [10]. The concept of extraction and refining of aluminum using ionic liquids originated when ionic liquids were used for electrodeposition of metals [11], [12], [13], [14]. Ionic liquids are non-volatile and recyclable which make them environmentally benign electrolytes for recycling metal matrix composites.

Chloroaluminate ionic liquids are prepared by mixing alkyl-imidazolium chlorides with aluminum chlorides [15]. Chloroaluminate ionic liquids are liquids at room-temperature and hence are also termed as room-temperature ionic liquids [16]. Here we take an example of 1-butyl-3-methylimidazolium chloride (BMIC) and AlCl₃ ionic liquid. In these melts several anionic species are often present in equilibrium, which depend on the ratio of the two components BMIC and AlCl₃ given by Eq. (1):

BMIC + AlCl₃ ↔ [BMI]⁺ + [AlCl₄]⁻

With an excess of the Lewis acid AlCl₃, additional anion species can be formed from further acid–base reactions given by Eqs. (2), (3):

[AlCl₄]⁻ + AlCl₃ ↔ [Al₂Cl₇]⁻

[Al₂Cl₇]⁻ + AlCl₃ ↔ [Al₃Cl₁₀]⁻

When BMIC is present in molar excess over AlCl₃, i.e., [AlCl₃:BMIC] < 1.0, the ionic liquid is termed as basic ionic liquid and only equilibrium equation (1) need to be considered. In an equimolar solution, i.e., [AlCl₃:BMIC] = 1.0, which is termed as neutral melt, AlCl₄⁻ is the only anion present in the liquid. When AlCl₃ is present in molar excess over BMIC, i.e., [AlCl₃:BMIC] > 1.0, the ionic liquid is termed as acidic ionic liquid and equilibria given by Eqs. (2), (3) predominate in the solution [17].

This paper discusses the effect of process variables such as electrolyte concentration and applied voltage on current efficiency and deposit microstructure during the recycling of an aluminum metal matrix composite in a BMIC–AlCl₃ ionic liquid electrolyte.