Hazards and optimal utilization of iron in low-carbon recycled aluminum alloys

Abstract

Driven by the proposed new circular economy goals and “dual carbon” strategy (carbon peak and carbon neutrality), the inherent recyclability of aluminum and its alloys makes their secondary utilization critical for green and sustainable development. Owing to its controllable source, scrap aluminum produced by manufacturing industries, such as the automobile, aerospace, and electronics industries, represents a high-value resource that will be critical for the global supply of aluminum and its alloys. However, large amounts of impurity elements (Fe, Si, Mg, Cu, and others) are introduced into scrap aluminum during the recycling process, among which Fe is the most harmful. Consequently, recycled aluminum is largely restricted to lower-grade applications, precluding its comprehensive substitution of primary aluminum. This article reviews the detrimental effects of Fe and the resulting Fe-rich phases (FRPs) on aluminum alloys, summarizes existing Fe removal and deterioration mitigation methods, and evaluates the industrial feasibility of these methods to provide comprehensive theoretical guidance for future FRP control technology. Moreover, this review provides guidance for resolving the impediments to the grade preservation and subsequent use of industrial recycled aluminum.

1. Introduction

The accelerating modernization and development of the automobile industry has positioned the weight reduction of automobiles through the use of aluminum alloys as a key measure to achieve energy conservation and reduce environmental pollution [1–2]. The implementation of this measure has resulted in unprecedented pressure on the production capacity of the aluminum industry, especially in the high-end aluminum industry, which requires the acceleration of capacity expansion and upgrading of technology. At present, the sources of aluminum alloys in the world can be divided into two categories: electrolytic aluminum and recycled aluminum [3]. Fig. 1(a) illustrates the production process for these two types of aluminum alloys. The production of electrolytic aluminum results in high carbon emissions, and its long-term use is limited by factors such as high energy consumption, corrosion loss, pollution problems, and resource consumption. As an alternative source of aluminum alloys, the production of recycled aluminum is significantly more efficient than that of electrolytic aluminum, consuming less than 5% of the energy necessary for primary electrolytic aluminum production [4–7]. Fig. 1(b) highlights the significant energy-saving and emission-reduction effect of employing recycled aluminum instead of electrolytic aluminum. Meanwhile, the equipment required for the production of recycled aluminum is relatively simple, the raw materials are widely available, and the corresponding investment and cost are considerably low, resulting in significant economic benefits. Therefore, scrap aluminum recycling can significantly reduce the overall energy consumption within the aluminum alloy industry [8–12].

According to International Aluminum Institute forecasts, the production capacity trends for the next few decades indicate that recycled aluminum will progressively become the dominant source, surpassing electrolytic aluminum (Fig. 1(c)). Moreover, some researchers have compared the current status of domestic and international production of recycled aluminum, highlighting the potential and necessity of developing secondary recycled aluminum in China [13–17].

However, recycled aluminum contains a significantly higher Fe content than primary aluminum, considerably limiting the high-quality utilization of recycled aluminum. The accumulation of Fe is attributable to two factors [18–20]: (i) inadequate sorting of scrap aluminum used in daily life, which often contains Fe-based components; and (ii) Fe pickup from the utensils and operating