Verification of Possibility of Molten Steels Decopperization with ZnAl₂O₄

Abstract

In the previous research works, ZnAl₂O₄ material was considered as one of the solutions for the decopperization process of molten steels; up to 33% of decopperization efficiency was reported by utilising the ZnAl₂O₄ filter. In order to verify the decopperization possibility of ZnAl₂O₄ materials, iron-based alloys with various copper and carbon contents were interacted with ZnAl₂O₄ substrates in a heating microscope under an argon gas atmosphere at 1600 °C. Fe-Cu alloys were found to react with the ZnAl₂O₄ substrate during the interaction process, and a reaction layer with a complex composition around the alloy droplet was formed; however, Cu was not detected in the reaction layer. Cu was later found infiltrated inside of the ZnAl₂O₄ substrates. Furthermore, the Cu-Zn compounds were detected when the copper content in Fe-Cu alloys was 10 wt% Cu. After interaction experiments, copper was decreased in all cases. Thereby, the copper evaporation and infiltration into the ZnAl₂O₄ substrate were considered as the reasons for copper loss. Moreover, oxygen dissolved in melt was found to have a great effect on the copper evaporation process.

1. Introduction

After centuries of industry development, steel scrap’s stock has become a very prospective raw material resource for steel reproduction. Steel scrap is a cheap but abundant raw material with a high energy content. Every ton of recycled steel scrap saves 1134 kg of iron ore, 635 kg of coal, and 54 kg of lime [1]. However, this new material supplier requires more advanced recycling technology and treatment due to the residual elements contained in the steel scraps.

Copper, as a common tramp element in steel scrap, strongly prevents the effective use of steel scrap, i.e., during the heat treatment process, liquid copper forms and penetrates the grain boundaries, causing cracking [2,3]. As reported in the literature [2], the hot shortness is exhibited when the copper concentration is greater than 0.2 wt%. Copper dissolved in steel melts could not be oxidized in the presence of iron due to its lower affinity for oxygen than iron. Through a common pyrometallurgical method, copper could not be removed from molten steel. In order to apply steel scrap, dilution with virgin pig iron or direct reduced iron (DRI) became the industry solution.

Daehn et al. [4] reported the huge negative influence of copper in constraining future global steel recycling. A variety of methods during the last decades for the removal of copper from steel scrap were suggested based on the unique properties of copper, including that copper has a good affinity to sulfur, a high evaporation rate at high temperatures, and forms volatile gas species with Cl. Thereafter, a series of decopperization methods were reported: (1) treatment by reaction with sulfur-containing slag [5,6,7,8]; (2) vacuum distillation [2,9,10,11,12]; (3) treatment by reaction with chlorine-containing slag [13]; (4) filtration [2]. More decopperization methods and investigations have been reported and summarized in reviews by Sandig et al. [14] and Daehn et al. [15]. Besides the above-mentioned methods, a reactive filtration method was reported in earlier literature [16,17,18]. Wieliczko et al. [16] filtrated copper-containing liquid alloy by applying an Al- and Zn-based ceramic spinel (ZnAl₂O₄); 30–33% copper was removed. Moreover, a 70% of decopperization rate was achieved by blowing fine-grain ZnAl₂O₄ spinel powder into the steel under laboratory conditions. Li et al. [17] stated that intermetallic Cu-Zn compound would be formed by adding Al₂O₃-ZnO-C materials in liquid Fe-Cu alloy. Namely, the carbon could reduce ZnO (as shown in Equation (1)), then the released Zn would react with Cu and forming Cu-Zn compound. Later, the formed intermetallic Cu-Zn compound would deposit on the surface of Al₂O₃. Through this process, copper could be successfully removed from the liquid Fe-Cu alloys. The proposed mechanism is illustrated in Figure 1. However, these researchers did not present the in-detail morphology of the Cu-Zn compound formation on Al₂O₃ ceramic with corresponded chemical analysis. Furthermore, the formation of intermetallic compound Cu-Zn was also stated during the welding process between Zn-coated steel and Cu electrodes by Kim et al. [19].

$$\mathrm{ZnO}+\mathrm{C}=\mathrm{Zn}+\mathrm{CO}$$

(1)

The unclarified copper-filtration possibility of ZnAl₂O₄ filter and the urgency of finding an efficient decopperization method led the present work aim to verify the possibility of ZnAl₂O₄ as a prospective solution for decopperization. Therefore, in the present work, various contents of copper-/carbon-bearing iron were interacted with ZnAl₂O₄ substrate. For the comparison, pure alumina was also interacted with alloy samples.

2. Materials and Methods

2.1. ZnAl₂O₄ Substrates’ Preparation

To prepare the used ZnAl₂O₄-substrates, pure ZnO (90/RS Carl Jäger, Hilgert, Germany) and Al₂O₃ (Martoxid^®MR70, Martinswerk, Bergheim, Germany) were used. The raw materials were mixed in a molar ratio of 48 mol% Al₂O₃ and 52 mol% ZnO. A water-based slurry containing the oxides as well as 0.4 wt% Dispex^®A40 (CIBA, Basel, Switzerland) as a wetting agent and 0.2 wt% Optapix^®AC170 (Zschimmer&Schwarz, Lahnstein, Germany) as a pressing agent were mixed with 25 wt% distilled water (related to dry mass of oxides) in a PET container with alumina balls. The container was rotated for 12 h to homogenize the slurry. After that, the mixture was dried at 110 °C to less than 3% water. The dried raw material was crushed and sieved to less than <1 mm maximum grain size. The prepared granules were now 120 MPa uniaxial pressed to substrates with a diameter of 50 mm and a thickness of 5 mm and sintered in an electric-heated furnace under an oxidizing atmosphere at 1600 °C with a heating rate of 2 K/min and a dwell time of 5 h. After the sintering step, the substrates were grinded and polished on one side to ensure a flat surface for the wetting experiments. XRD (in Figure 2) investigations show the conversion of 96.6 wt% ZnAl₂O₄ and 3.4 wt% ZnO reaction product.

2.2. Fe-Cu Alloys’ Preparation and Experimental Setup

A cold crucible induction melter KIT (CCIM KIT) (LINN High Therm GmbH, Eschenfelden, Germany) was applied to produce Fe-Cu alloys. The in-detailed description about CCIM is reported elsewhere [20]. Armco iron with the composition as shown in

Table 1. Chemical compositions of Armco-iron (ppm).

Armco Iron	Fe (wt%)	C	Mn	Si	Al	O
	99.8	40	30	262	11	92

, pure copper (99.999%), and graphite were used as primary materials for the sample preparation. The prepared Fe-Cu alloys are listed in

Table 2. The conducted interaction experiments.

Fe-Cu (wt%)	ZnAl2O4	Al2O3
0.5% Cu, 40 ppm C	90 min	90 min
0.5% Cu, 0.5% C	90 min	90 min
1% Cu, 40 ppm C	90 min	90 min
1% Cu, 0.5% C	90 min	90 min
10% Cu, 40 ppm C	90 min	-

. The Fe-Cu alloy samples were prepared into a cylindrical form with a height of 8 mm and diameter of 7.5 mm, then etched with HCl and H₂O (HCl/H₂O = 1:1) mixed solution.

The experimental interaction was conducted in a heating microscope under argon with a flow rate of 0.3 L/min (2–3 ppm oxygen) at 1600 °C. Both the heating and cooling rate were kept at 40 K/min. The interaction process was monitored by a CCD camera (IMAGINGSOURCE GmbH, Bremen, Germany). A simplified schematic illustration of the experimental setup is shown in Figure 3. An in-detail description was presented in the previous publication [21]. The conducted experiments are summarized in Table 2.

The general chemical composition of iron alloys was analyzed by the spark spectrometer Foundry-Master UV (Oxford Instruments, Abingdon, England). Moreover, Bruker G4 Ikarus and Bruker G8 Galileo combustion analyzers were applied to estimate the O, C, and S contents. After the interaction experiments, metal droplet and ZnAl₂O₄ substrates were embedded into epoxy resin and perpendicularly cut for cross-sectional analysis. Scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX) was used for morphology and chemical determination (Ultra55, Zeiss NTS GmbH, Heidelberg, Germany).

3. Results and Discussion

3.1. Experimental Observation

A melting mechanism of Fe-Cu alloys on ZnAl₂O₄ substrate is illustrated in Figure 4a. As shown in Figure 4a, Fe-Cu alloys start to melt when the temperature is high enough (melting temperature of iron 1538 °C). Later, a symmetrical liquid droplet is formed. With a longer interaction period, the Fe-Cu alloys liquid spread on the ZnAl₂O₄ substrate. Moreover, during all the interaction experiments, the formation of gas bubbles inside the molten Fe-Cu alloys was observed; with the addition of 0.5 wt% C, the gas formation was much heavier than 40 ppm C.

Figure 4b indicates an image of liquid Fe-Cu (1 wt% Cu) alloys spread on the ZnAl₂O₄ substrate. Furthermore, after interaction with the ZnAl₂O₄ substrate, a reactive layer and plenty of powder were found in the experimental chamber; according to the chemical analysis, this powder consisted of ZnO. It indicates that the ZnO in the ZnAl₂O₄ substrate was reduced by the carbon contained in the Fe-Cu alloys, then re-oxidized by the oxygen contained in the protective gas. On the other hand, a perfect symmetrical Fe-Cu alloy liquid droplet was presented on the Al₂O₃ substrate, as shown in Figure 4c. No reaction was observed.

3.2. Interfacial and Cross Sectional Analysis

Figure 5a shows an image of Fe-1% Cu alloy interacted with ZnAl₂O₄ substrate; a reaction layer was formed around iron sample. According to the SEM/EDX analysis, the reaction layer consists of complex elements including O, Al, Fe, Zn, Mn, and Si without Cu. (see Figure 5b,d)). Elements such as Si, Mn, and Fe are transferred from the side of the iron sample. At the cross section (see Figure 5c), only some Fe was detected.

Figure 6a indicates the cross section between Fe-1% Cu-0.5% C alloy and ZnAl₂O₄ substrate. The cross section was divided into four different subareas; the copper content was EDX measured at each subarea; the measured copper content is depicted in Figure 6c. The copper content decreases from the ZnAl₂O₄ substrate interface (Figure 6a, subarea 4) to inside of the substrate. Figure 6b indicates the Fe-Cu alloy interaction area, according to the measured point 1 and 2 (Figure 6b), the detected ratio of Cu and Fe in ZnAl₂O₄ substrate is similar to with Fe-1% Cu alloy. It indicates that Cu was infiltrated into the ZnAl₂O₄ substrate along with liquid Fe-Cu alloy; the formation of intermetallic compound Cu and Zn was not detected.

After the interaction between iron alloy with 10 wt% Cu and ZnAl₂O₄ substrate, metallic Cu was detected on the bottom of the newly formed reactive layer as shown in Figure 7a. Figure 7b is the cross-section area between alloy and substrate, it shows that the compounds with bright colour were scattered in this area. EDX analysis revealed that point 3 (86.84 wt% Cu) is pure copper, which is transferred from the Fe-10% Cu sample side. Furthermore, point 4 consists mostly of Al and Fe. Finally, the element’s ratio (Cu/Zn = 0.6) at point 5 indicates that it is a Cu₅Zn₈ compound.

Fe-Cu alloy is known as an alloy with a metastable miscibility gap. During the solidification process, if a single-phase liquid is undercooled into the metastable miscibility gap, it will divide into two liquids, iron and copper [22]. According to Brillo and Egry [23], the energy of mixing ∆G is strongly positive for Fe-Cu. Consequently, Fe-Cu does not form a homogeneous solution at all temperatures and compositions. Since copper has a greater density than iron [24], it could accumulate at the bottom of the alloys.

3.3. The Composition of Fe-Cu Alloys

The measured chemical compositions of Fe-Cu alloys after the interaction are listed in

Table 3. Chemical composition of Fe-Cu samples after experiments (ppm).

wt%	C	Si	Mn	Al	O	Cu Wt%	Cu Loss (%)
0.5% Cu Al2O3	40	58	16	36	72	0.382	23.4
0.5% Cu ZnAl2O4	15	138	13	41	423	0.411	17.6
1% Cu Al2O3	40	91	15	55	80	0.718	28.2
1% Cu ZnAl2O4	13	77	13	28	559	0.908	9.2
0.5% C 0.5% Cu Al2O3	0.516 wt%	50	27	38	51	0.35	25.0
0.5% C 0.5% Cu ZnAl2O4	14	50	13	38	252	0.321	31.3
0.5% C 1% Cu Al2O3	0.457 wt%	50	15	45	62	0.757	24.3
0.5% C 1% Cu ZnAl2O4	18	66	13	46	301	0.722	27.8

. The copper content was found to be decreased in all cases. For the Al₂O₃ substrate cases, the copper losses at various cases were close to each other; the copper loss was considered as mainly contributing to the copper evaporation. Moreover, the oxygen content was slightly decreased for Al₂O₃ substrate cases. In the heating microscope, a graphite tube is installed for the heating process; it could react with the oxygen contained in the protective gas forming CO gas. Hence, the formed carbon monoxide (CO) in the experimental atmosphere might decrease a fraction of oxygen in the liquid samples. Furthermore, when liquid Fe-Cu alloy interacts with ZnAl₂O₄ substrates, more copper was found vanished with the addition of 0.5% carbon. For ZnAl₂O₄ substrates, copper evaporation into the atmosphere and transfer into the ZnAl₂O₄ substrates were the reasons for the copper loss. Thereby, with an addition of carbon content (0.5 wt% C) in the Fe-Cu alloys, more ZnO was reduced into Zn and O; it increased the porosity of the ZnAl₂O₄ substrates. Furthermore, due to the positive mixing energy ∆G for molten Fe-Cu and high density of copper [23], more copper would accumulate at the liquid sample’s bottom. Therefore, more copper would have infiltrated into the ZnAl₂O₄ substrates along with liquid Fe-Cu samples.

In the meantime, the reduction of ZnO in ZnAl₂O₄ substrates supplied the oxygen into Fe-Cu alloys; consequently, the oxygen content after interaction with ZnAl₂O₄ substrates was increased. Besides this, aluminium was found to slightly dissolve into the iron sample and after interaction with Al₂O₃ and ZnAl₂O₄ substrates. According to the previous literature [21,25], under present low-oxygen partial pressure and high temperature, Al₂O₃ would dissolve into the liquid iron. The carbon content was strongly decreased after interacting with ZnAl₂O₄ substrates. However, the carbon content after making contact with the Al₂O₃ substrates kept the same.

3.4. Copper Evaporation Mechanism

In the present investigation, copper was mainly decreased due to its high evaporation rate. According to the literature related to the copper evaporation [10,26,27,28,29], the copper evaporation process from liquid iron can be divided into three main stages: (1) copper transfer from the bulk of the liquid phase to the interface; (2) copper evaporation from the liquid metal surface; (3) transfer of the copper vapours from the interface to the core of the gaseous phase, as illustrated in Figure 8.

Fischer et al. [30] stated that the apparent evaporation rate constant $k_{Cu}$ can be expressed by Equation (2). As shown in Equation (2), $k_{Cu}^L$ is liquid phase mass transfer coefficient (m·s⁻¹), $k_{Cu}^e$ is the free evaporation rate constant of Cu (m·s⁻¹), and $k_{Cu}^G$ is the gas phase mass transfer coefficient (m·s⁻¹).

$$k_{Cu}=\frac{1}{\frac{1}{k_{Cu}^L}+\frac{1}{k_{Cu}^e}+\frac{1}{k_{Cu}^G}}$$

(2)

3.5. Effect of Oxygen on Copper Evaporation

Oxygen is known as a surface-active element; it accumulates on the liquid Fe-Cu alloy surface and blocks the available sites for metal–gas reactions. Consequently, oxygen prevented the copper evaporation to atmosphere, as illustrated in Figure 9. Furthermore, a few ppm of oxygen can significantly reduce the surface tension value [31]. Thereby, its effect on the free evaporation rate constant of copper from liquid iron can be expressed by assuming Langmuir’s ideal adsorption isotherm [32,33].

$$k_{Cu}^e=k_{Cu}^{eo}\left(1-{\mathsf{\theta }}_O\right)$$

(3)

where $k_{Cu}^e$ is the free evaporation rate constant of Cu on the surface of liquid iron (m·s⁻¹), $k_{Cu}^{eo}$ is the free evaporation rate constant of Cu on the surface of oxygen free liquid iron (m·s⁻¹) and ${\mathsf{\theta }}_O$ is the degree of surface coverage by absorbed oxygen.

$$K_O^{Cu}·a_O=\frac{{\mathsf{\theta }}_O}{\left(1-{\mathsf{\theta }}_O\right)}$$

(4)

where $K_O^{Cu}$ is the adsorption equilibrium constant of oxygen on the surface of liquid iron and $a_O$ is the activity of oxygen in liquid iron.

$$k_{Cu}^e=\frac{k_{Cu}^{eo}}{\left(1+K_O^{Cu}·a_O\right)}$$

(5)

The value of $K_O^{Cu}$ was reported as 110 in the Equation (6) by Lee [34].

$${{\rm Y}}_{Fe-O}=1.890-0.299In\left(1+110 a_O\right)$$

(6)

Therefore, high oxygen content greatly prevented the copper evaporation in the Fe-Cu alloys. Carbon addition would decrease the oxygen content in the melts and enhance the copper evaporation.

Moreover, oxygen reduces the activity coefficient of copper ${\gamma }_{cu}$ (as shown in (7)) in the liquid iron, due to the negative interaction coefficient between copper and oxygen, $e_{Cu}^o=-0.05$ at 1600 °C [35].

$$log {\gamma }_{cu}={\displaystyle \sum }_i{e}_{cu}^i\left[mass\% i\right]=e_{cu}^i·\left[mass\% i\right]+e_{cu}^O·\left[mass\% O\right]$$

(7)

where $i$ is the elements in the Fe-Cu alloy and $e_{cu}^i$ is interaction coefficient of Cu in the liquid iron.

4. Conclusions

Both ZnAl₂O₄ and Al₂O₃ substrates have interacted with liquid Fe-Cu alloys containing carbon to verify the decopperization possibility of ZnAl₂O₄ materials. After experiments, copper was found to decrease with both ZnAl₂O₄ and Al₂O₃ substrates. The copper evaporation and its infiltration along with liquid Fe-Cu alloys into the ZnAl₂O₄ substrate were the explanations for the copper loss. For the Fe-Cu (0.5–1.0 wt% Cu) alloys, no metallic Cu and Zn compounds were detected with ZnAl₂O₄ substrates. The intermetallic compound Cu₅Zn₈ was only detected with Fe-Cu (10 wt% Cu) alloy. According to the obtained experimental results, it can be concluded that the decopperization of liquid steels with ZnAl₂O₄ filter is implausible.

Besides this, the oxygen contents after interacting with ZnAl₂O₄ substrates were strongly increased with the decompose of ZnO. The oxygen dissolved into the liquid Fe-Cu alloys noticeably affected the copper evaporation, which led to a lower copper loss. Based on the experimental results, ZnAl₂O₄ materials showed a higher decopperization efficiency when the Fe-Cu alloys contained more carbon. However, this result was caused by the infiltration of Fe-Cu alloys into the ZnAl₂O₄ substrate with enhanced porosity.