1. Introduction
One of the greatest challenges in the mining and processing industries is waste management and waste copper slag is defined as one such waste [
1]. Waste copper slag (WCS) is a by-product produced in the copper pyrometallurgical process. It is estimated that 30 million tons of WCS are generated and dumped globally every year [
2,
3]. In addition, the slag contains a lot of harmful elements, such as Cu, Pb and Zn, which cause pollution to the environment. These elements are not only non-biodegradable and toxic but also accumulate in human organisms, causing some physical disorders and health concerns [
4,
5,
6]. Therefore, WCS is classified as hazardous waste [
7,
8,
9]. Unfortunately, more than 80% of them are directly dumped without treatment, which may cause potential pollution [
10,
11].
In order to comply with strict environmental regulations, numerous studies have been conducted to upgrade WCS, including building material [
12]. However, this method usually has new contamination, such as the leachability of zinc, arsenic and antimony, and can only accommodate a very small fraction [
13,
14].
Regarding the recovery of valuable metals from WCS, previous work has mainly focused on recycling elements from WCS using beneficiation, leaching and pyrometallurgical processes. These methods mainly include flotation [
15,
16], acid leaching [
17,
18], roasting followed by water leaching [
19], bio-leaching [
20] and direct reduction–magnetic separation or smelting reduction [
21,
22,
23]. Roy et al. [
15] adopted flotation to recover copper sulfide from copper smelter slag. The results show that under the optimal conditions, the recovery rate of copper sulfide reached 84.82%. There is no doubt that flotation can recover copper sulfide to a certain extent. However, the copper oxide within it cannot be recovered. Muhammad et al. [
18] employed sulfuric acid to extract metals from copper-rich converter slag. A copper precipitation rate of 90% can be obtained. At the same time, the iron extraction rate is only slightly higher than 60%. Mikoda et al. [
20] used acidithiobacillus thiooxidans to leach valuable metals from copper metallurgical slag. The extraction rate of rare earth element (REE) and Co can reach more than 90%. However, the process duration is too long. Physical beneficiation and hydrometallurgical process can efficiently extract Cu, Pb, REE, etc. However, these processes cannot effectively recover iron as the main element in copper slag.
The pyrometallurgical process is generally used to recover iron from copper slag [
24]. Smelting reduction is a deep reduction process; during this process, FeO is reduced to pig iron by a reductant at a high temperature above 1450 °C [
1]. It can obtain a high iron recovery rate, but shortens the use time of the kiln. It is effective to recover iron from copper slag (CS) by direct reduction–magnetic separation. By reducing CS at 1200–1300 °C, fayalite and copper sulfide can be transformed into metallic iron and copper, and then Fe-Cu alloy phase can be formed [
25]. Subsequently, a magnetic separation process is used to recover the Fe-Cu alloy. However, magnetic separation tailings have not been effectively used.
In this paper, a comprehensive process for recovering valuable metals from WCS is developed. The main steps of the proposed process include flotation to enrich Cu and direct reducing–magnetic separation to produce crude Fe-Cu alloy, and subsequent non-magnetic tailings to replace clinker to produce common Portland cement. Meanwhile, lead and zinc were recovered as dust through this process. No tailings were produced during the entire process, indicating that this is a clean and green process.
2. Materials and Methods
2.1. Materials
The WCS used in this study was supplied by Anhui Province, China. The chemical composition of WCS is shown in
Table 1. The WCS contains 39.54% Fe, 0.81% Cu and 32.11% SiO
2. Meanwhile, the contents of Pb and Zn are 0.12% and 2.35%, respectively, which can be volatilized from the reduction process. As shown in
Figure 1, the main mineral phases of copper slag are magnetite (Fe
3O
4) and fayalite (Fe
2SiO
4). The microstructure of the copper slag was observed by SEM-EDS, and the result is shown in
Figure 2. It can be seen from
Figure 2 that iron mainly exists in the form of fayalite (Fe
2SiO
4) and magnetite (Fe
3O
4), while copper mainly exists in the form of matte (Cu
2S). Meanwhile, iron minerals and copper minerals exist as independent particles, which facilitates the recovery of copper through the flotation process.
2.2. Methods
The flow chart of the proposed WCS treatment process is presented in
Figure 3, which was developed based on the basic understanding of the following steps: (1) the WCS is enriched with Cu by a flotation process; (2) direct reduction–magnetic separation of flotation tailing pellets to prepare Fe-Cu alloy and to remove Pb and Zn; (3) non-magnetic tailing to prepare Portland cement to achieve zero tailings. The experimental details of each step are described as follows.
2.2.1. Flotation Process to Recover Cu
The flotation tests were carried out in a 0.75 L XFD laboratory flotation cell (FGC5-35, Yunhao Mining and Metallurgical Equipment Co., Ltd., Wuhan, China). In all flotation tests, a 500 g sample was used, the impeller speed was set to 600 rpm and the aeration rate was 0.2 dm
3/min. The specific operation of the flotation tests is outlined in
Figure 4. In the flotation process, analytical grade Butyl xanthate (C
5H
10OS
2) (g/t) and terpene oil (C
10H
18O) (g/t) were used as collector and frother, respectively [
7]. The recovery rate of copper is calculated as follows:
where
is the recovery rate of copper;
w1 and
w0 are the copper grades in the flotation concentrate and raw material, respectively; y is the yield of the flotation concentrate.
2.2.2. Preparation of Crude Fe-Cu Alloy by Direct Reduction–Magnetic Separation
The flotation tailings and flux were fully mixed, and then the mixture was pressed into cylindrical briquettes (height: 10 mm and diameter: 10 mm). The briquettes were dried at 110 °C for 4 h.
The direct reduction tests were performed in a muffle furnace at 1250 °C for 80 min. For the reduction test, 60 g briquettes were mixed with 120 g soft coal (ash content: 4.49%; production location: Shenfu) (1–5 mm) and then encased into a corundum crucible with a depth of 80 mm and diameter of 80 mm. [
21]. After reduction, the reduced briquettes were cooled to room temperature for the subsequent magnetic separation process. The specific operation process of magnetic separation is outlined in reference [
26]. The metal recovery rate (iron or copper) is calculated as follows:
where
is the metal recovery rate (iron or copper);
w′ and
w are the iron (or copper) grades in the magnetic separation concentrate and reduced pellets, respectively; and
y1 is the yield of the concentrate.
The volatilization rate of zinc (or lead) is calculated as follows:
where
is the volatilization rate of zinc (or lead);
w2 and
w3 are the content of zinc (or lead) in the reduced pellets and flotation, respectively; and
y2 is the yield of reduced pellets.
2.2.3. Production of Common Portland Cement
A certain proportion of non-magnetic material (NMT), clinker and gypsum were thoroughly mixed. Ingredients of Portland cement are shown in
Table 3.
The mixtures were cast into steel molds (cuboid) in three layers and compacted using a vibrating table [
27]. All samples were stored in the ambient environment under a plastic sheet, and then the samples were demolded and wet-cured until the test date. The samples were cured at room temperature (18–22 ℃) for 3 and 28 days separately. Concrete cuboids with a size of 40 × 40 × 160 mm were cast for the compressive strength test (Equipment: Universal testing machine, KY-D2305, Jilin Province Jilin Test Technology Co., Ltd., Jilin, China).
2.3. Characterization of Raw Materials and Products
The chemical compositions of WCS, flotation slag, Fe-Cu alloy and non-magnetical material tailings were measured by X-ray fluorescence spectroscopy (XRF, PANalytical Axios mAX, PANalytical B, V., Almelo, The Netherlands). The microstructure of reduced briquettes was observed and characterized by scanning electron microscopy (SEM) (TESCAN, MIRA3, LMH, FEI, Eindhoven, The Netherlands) and energy-dispersive spectrometry (EDS) (XMAX20, FEI, Eindhovern, The Netherlands). The mineral phases of WCS, flotation tailing, direct reduction dust and reduced briquettes were measured by X-ray powder diffraction (XRD, RIGAKU, D/Max-2500, Bruker corporation, Madison, WI, USA) with a 2θ scan range from 10° to 80°. The compressive strength of the Portland cement was measured according to the national standard (GB/T 175-2007).
4. Conclusions
The waste copper slag containing 39.54% Fe and 0.81% Cu was used as a raw material for recovering valuable elements through stepwise extraction. Under the optimal conditions, four products were obtained, including flotation concentrate, magnetic separation concentrate, zinc-bearing dust and Portland cement. The overall Cy recovery rate of the flotation concentrate is 77.78%, which can reach a copper content of 21.50%, which can be used as a burden for copper smelting. The magnetite concentrate contains 90.21% of Fe and 0.4% of Cu with 91.34% of iron recovery and 83.14% of copper recovery, which meets the requirements of weather steelmaking. Meanwhile, the zinc-bearing dust contains 65.17% of ZnO and 2.66% of PbO with an overall zinc and lead volatilization of 98.89% and 89.6%, which can be used as a raw material for preparing nano ZnO. The strength of common Portland cement meets the 42.5 strength level standard. The proposed process can provide an alternative method for the effective and green utilization of waste copper slag.