Study of Hydrometallurgical Treatment of Metal-Bearing Material from Environmental Burdens Containing Iron, Chromium, Nickel, and Cobalt

Abstract

In Slovakia, around 200 environmental burdens that contain a significant amount of usable raw materials were created by the extraction of ores or the dumping of industrial waste. In this research, the hydrometallurgical metal recovery method from a metal-bearing environmental landfill in Sereď was investigated. The analysis of a representative sample of waste obtained from this landfill proved the presence of significant amounts of metals (43.45% Fe; 1.3% Cr; 0.09% Co, and 0.23% Ni). A thermodynamic study of the metals’ (Fe, Cr, Ni, and Co) leaching probability confirmed the possibility of metal extraction in an acidic environment. Subsequently, the effect of the most important factors on the leaching process (stirring intensity, temperature, liquid-to-solid phase ratio, and acid concentration) was experimentally tested. The analysis of the results determined the optimal leaching conditions. The extraction of 90.35% Fe and 59.62% Cr was ensured at a stirring intensity of 400 rpm, a leaching temperature of 80 °C, a liquid-to-solid phase ratio of 40, and a H₂SO₄ concentration of 3 mol/dm³. Various techniques, including SEM, EDX, XRD, Eh-pH diagrams, and AAS analysis, were used to analyze samples and products after leaching. The possibility of precipitating metals/compounds from the leachate to obtain a marketable product was theoretically proposed and proven.

1. Introduction

When producing metals, the metallurgical industry often ignores mine tailing deposits and industrial waste dumps, which have significant raw material potential. Such mineral deposits and waste dumps are considered environmental burdens (EBs) and have considerable economic potential. In the country where an EB is located, it is possible to solve critical issues of ecological stability by designing innovative technologies for raw material processing.

In the 13th century, mining developed significantly in Central Europe. Primarily, iron ore, antimony, gold, and copper were mined here. Because of these long-term activities, heaps, tailing ponds, landfills, and drainage channels began to emerge, which today harm the environmental quality of the area. Most territories where raw materials were mined in the past are currently environmental burdens [1].

Slovakia was one of the locations where intensive mining took place in the past. Among the most significant environmental burdens in Slovakia with large potential to yield metal are the villages of Smolník, Rudňany, Slovinky, Krompachy, Istebné, Čučma, and Sereď. These environmental burdens contain iron, copper, zinc, chromium, nickel, cobalt, etc., and thus represent a new potential source of raw materials for metal production. Each EB has an identifier (ID) that is listed in the Information System of Environmental Burdens of Slovakia, published on the Enviroportal platform [2].

In Slovakia, the landfill Smolník (ID: SK/EZ/GL/237) is the most famous and richest EB in terms of iron and copper ores. Minerals such as pyrite (FeS₂) and chalcopyrite (CuFeS₂) are the most represented in this burden [3,4]. The EB in Rudňany (ID: SK/EZ/SN/899) contains waste from the mining and processing of minerals containing mercury, copper, and barite (BaSO₄) [5,6]. The Slovinky landfill (ID: SK/EZ/SN/900) is an EB with more than 4.8 million cubic meters of flotation sludge, originating from past copper production. The deposited material contains arsenic, cadmium, copper, zinc, lead, and manganese This landfill also contains industrial waste and slag from the former copper production. Industrial waste from the production of iron, steel, copper, and zinc sulfate, as well as sludge from the production of manganese and zinc, was deposited at the landfill EB near the town Krompachy, Halňa (ID: SK/EZ/SN/896), over an area of 10 ha and at a volume of 760,000 m³ [7,8]. An EB involving the slag heaps in the village of Istebné (ID: SK/EZ/DK/178) was created by depositing industrial waste, slag, and dust based on iron and chromium from the Orava Ferroalloy Plants [9]. The EB Čučma tailing pond (ID: SK/EZ/RV/777) with a volume of 166,692 m³ is located in the Rožňava district; it contains flotation waste from the former processing of antimony ore raw materials. Antimony ore contains senarmonite, valentinite, and stibikonite. Increased As, Ca, and Pb concentrations were detected in antimony oxides [10].

The landfill comprising metallurgical sludge (ID: SK/EZ/GA/222) from the production of nickel and cobalt near the city of Sereď mainly contains waste from these activities. EBs mainly contain solid leaching residue (SLR), the resulting by-product from the leaching of poor iron–nickel Albanian ore [11,12]. The tailing pond is composed of several elevations [13] and has a height ranging from 20 to 80 m, and in some areas even spans up to 100 m, as shown in Figure 1a. Nickel was produced using Albanian Ni ore containing 1% Ni, 0.06% Co, 49%–52% Fe, and 2.57% Cr. The ore was processed using the wet industrial metallurgical Caron process. The Ni ore was mechanically adjusted to a grain size of 0.1 mm, and then reduction-roasted at a temperature of 740–765 °C. The resulting roasted material was leached in a NH₃ + CO₂ solution. Subsequently, the resulting SLR was placed in a landfill. Overall, 1 ton of metal produced 120 tons of waste. Currently, the landfill contains up to 6 million tons of SLR, and, in addition to having a relatively high iron content, also contains metals such as nickel, cobalt, and chromium [14]. The fine granularity of the material represents a serious problem for the inhabitants of the city of Sereď and its surroundings due to its high dustiness, as shown in Figure 1b. Thus, the surrounding environment is also contaminated, while traces of dust from the landfill with a size of 0.01–0.05 mm can be found up to 50 km, Figure 1c, from the landfill [13].

There was an effort to process this waste, but several attempts were unsuccessful due to the fineness of the SLR and the chromium content. Since SLR has a high iron content, it could be used as a secondary raw material input for pyrometallurgical iron production. However, the problem is the chromium content, as only 50 kg of SLR can be used for one melting without affecting the pig iron quality. SLR could also be used as an additive in building materials. However, strict legislative requirements for chromium content do not allow it to be used in the construction industry [14].

Nevertheless, the fact remains that this landfill is a potential source of raw materials for producing metals such as nickel, cobalt, iron, and chromium. There are several metallurgical processes used to obtain metals from waste. The pyrometallurgical processing method is one of them, but this process is environmentally demanding [15,16]. On the other hand, the hydrometallurgical method is more acceptable, not only from an environmental but also from a technological point of view [17]. The hydrometallurgical processing of materials includes classic and intensification leaching [18,19]. It is possible to use ultrasound, ozone microwave radiation, etc., during intensification leaching [20,21,22]. Subsequently, the metals can be obtained from the leachate in pure metallic form or the form of compounds by using metal extraction methods (cementation, solvent extraction, chemical precipitation, and so on).

Figure 1. A view of the height of the landfill embankments (a); the dustiness of the landfill in Sereď (Source: www.seredonline.sk, accessed on 6 March 2020); (b); the landfill location, including the distance to the nearest residential area (c) [2,23].

Figure 1. A view of the height of the landfill embankments (a); the dustiness of the landfill in Sereď (Source: www.seredonline.sk, accessed on 6 March 2020); (b); the landfill location, including the distance to the nearest residential area (c) [2,23].

Recently, several studies have dealt with the leaching of waste or mineral raw materials containing iron, chromium, etc., in acidic media. The authors of [24] proposed a process for obtaining iron from a flotation slag (a secondary source of iron ore with a high recycling value) using hydrogen-based suspended magnetization roasting and magnetic separation.

In [25], the authors compared the efficiency of two precipitation methods, goethite precipitation (the process results in a FeO.OH precipitate) and jarosite precipitation (precipitation results in a M[Fe₃(SO₄)₂(OH)₆], where M represents any of the ions Na⁺ or NH₄⁺), used to remove iron from a solution after electric arc furnace (EAF) dust leaching in an acidic leaching medium. They used CaCO₃, NaOH, and an aqueous solution of Ca(OH)₂. The authors proved that a temperature of 80 °C, the use of Ca(OH)₂ as a precipitating agent, and a pH of 3.5 are optimal for Fe precipitation using both precipitating methods, with an efficiency of 99.98% iron removal from the solution. Via 12 h of precipitation, the authors Dutrizac and Chen [26] precipitated hematite (Fe₂O₃) directly from ferrous sulfate solutions by seeding with hematite, working at 100 °C, applying atmospheric pressure, and working with a pH in the range of 1.4 to 1.6.

A study [27] by Chang et al. removed iron from lateritic nickel ore leachate by precipitating goethite with magnesium carbonate. As oxidizing agents, they used oxygen and copper ions to catalyze the oxidation of ferrous ions. They found that, at pH 2.5, it was possible to limit nickel loss to below 5%. The work [27] compared different ion exchangers for the selective extraction of Fe using the ion exchange method. Blais et al. reported that metal hydroxides of Co, Cd, Cu, Mn, Ni, Pb, and Zn precipitate at pH values higher than 5, which provides the possibility of the separation of ferrous ions Fe³⁺, which precipitate at pH values lower than 4 [28,29]. Kuruc et al. found that chromium obtained from EAF slag is leached better in an acidic leaching medium compared to an alkaline medium; it was also found that it is necessary to use acids with a higher concentration, and the pretreatment of the material by melting is required [30]. The authors Wang et al. and Bai et al. focused on obtaining Cr₂O₃ from slag by leaching it in sulfuric acid, with subsequent precipitation by NaOH. Subsequently, they obtained Cr(OH)₃, and Cr₂O₃ by calcination [30,31]. Another study investigated the removal of chromium from wastewater by precipitation using NaOH to form a Cr(OH)₃ precipitate [30,32]. According to the authors, ion exchange can also be applied to remove Cr from the solution. Rengaraj et al. [33] used cathexis resins, called IRN77 and SKNI, to remove chromium from wastewater; thus, 95% of chromium was removed. Electrochemical processes such as electrocoagulation are also used to remove chromium from solutions. The authors used an aluminum electrode, where Cr(VI) was reduced to Cr(III) upon contact with the electrode. Afterward, they used hydroxide for Cr(III) precipitation from the solution. Chandravanshi et al. [34] used NaOH, Ca(OH)₂, and MgO to remove chromium and compared the effectiveness of these agents.

The goal was to determine the optimal leaching conditions in terms of achieving high extraction rates of Fe from the solution and a minimal content of impure elements (Cr, Ni, and Co) remaining in the solid residue. Various analytical, spectroscopic, and microscopic techniques, such as SEM, EDX, XRD, Eh-pH diagrams, and AAS analysis, were used to analyze SLR and products after leaching.

2. Materials and Methods

The sample used in the laboratory experiments came from the SLR environmental burden located in Sereď. A gross sample was obtained via homogenization from the unit samples taken from the SLR from EBs. Subsequently, a representative sample was obtained from the gross sample according to the scheme shown in Figure 2. From representative sample A, one sample was taken for moisture determination, and from representative sample B, five analytical samples were subsequently taken for chemical analysis [35].

The following devices were used for crushing/milling and drying: a laboratory hot air dryer (HS122A/1982, Chirana Brno, Brno, Czech Republic) and a laboratory vibrating disk mill (Model 1982, OPS Přerov, Přerov, Czech Republic).

The chemical composition of the SLR samples was determined using the atomic absorption spectrometry (AAS) method and a Varian Spectrophotometer AA20+ device, with a detection limit of 0.3–6 ppb (Varian, Belrose, Australia).

A scanning electron microscope, MIRA3 FE-SEM (TESCAN, Prague, Czech Republic), was used to observe the morphology and determine the particle sizes; SEM also enabled semi-quantitative elemental analysis via the EDX method. The EDX data were processed by the AZtec software, v5.0. (Oxford Instruments, Oxford, UK).

The solid input SLR material and solid leaching residues, after washing and drying (104 °C, 24 h), were used for qualitative phase analysis via XRD using a Philips X’Pert PRO MRD (Co-Kα), a range of measuring (10–120° 2theta), and a scan step (0.0170°) X-ray diffractometer (Philips, Amsterdam, The Netherlands). The phases were identified using X’Per HighScore plus software, v3.0a (3.0.1).

A Dino-Lite ProAM4113T optical stereo microscope (AnMo Electronics Corporation, Hsinch, Taiwan, with magnifications under 100×) was used to analyze the morphology of the particles. The initial particle size analysis of SLR was performed using the laser diffraction method on a Malvern Mastersizer 2000E (Malvern Instruments, Great Malvern, UK, precision: ±1%), with a Scirocco2000M dry sample feeder.

A classic leaching apparatus was used for leaching [36], where the glass reactor with the leaching medium was immersed in a laboratory water bath (Memmert WNB 10, Germany). The temperature was controlled by a thermometer with a digital output. The temperature distribution in the bath was ±0.2 °C. Mixing was ensured by an IKA IUROSTAR 60 stirrer with a digital control unit and a fixed glass stirrer with two fixed arms.

The pH values of leachate and solutions before and after precipitation were measured with a digital tabletop multi-parameter meter with an IDS probe (inoLab^® Multi 9620, Weilheim, Germany).

All chemicals used in the experiments (H₂SO₄, NaOH, and H₂O₂) were purchased from Centralchem, Bratislava, Slovakia. Solutions were prepared by diluting the chemicals with distilled water. The results analysis was processed using common mathematical and statistical methods and by computer technology such as Microsoft Office Excel, HSC Chem-master (ver. 6.1), and Sigma Plot 10.0. Flowchart diagrams were created using the following program: Visio Professional 2013.

3. Results and Discussion

The chemical analysis of the SLR sample from the Sereď location was performed using the average value of the measurement of five analytical samples (S1–S5),

Table 1. The chemical composition of the SLR analytical samples from Sereď.

Monitored Elements Sample	Fe	Cr	Ni	Co	Ti	Si	Al	Mg	Ca	Mn
	Content (wt.%)
S1	49.25	1.2	0.22	0.09	0.16	0.53	1.97	0.9	1.14	0.18
S2	44.68	1.43	0.23	0.09	0.12	0.3	1.94	1.03	1.28	0.16
S3	42.33	1.17	0.24	0.09	0.15	0.39	2.03	1.10	1.31	0.17
S4	38.70	1.01	0.24	0.09	0.14	0.27	1.96	1.02	1.69	0.16
S5	42.28	1.66	0.23	0.10	0.12	0.36	1.96	1.04	1.28	0.17
Average	43.45	1.3	0.23	0.09	0.14	0.37	1.97	1.02	1.34	0.17
SD	3.470	0.23	0.008	0.004	0.016	0.09	0.03	0.07	0.19	0.007
Variance	12.07	0.053	5.6 × 10−5	1.6 × 10−5	0.0003	0.0082	0.0009	0.004	0.03	5.6 × 10−5

. The value of the standard deviation of the measurements was below 4% for five repeated measurements, which was acceptable for results of the AAS analysis. The moisture content of the tested sample, SLR from Sereď, was 18.36%. From the results of the analysis, it is clear that the sample contains an average of 43.45 wt.% of iron, 0.23 wt.% of nickel, 0.09 wt.% of cobalt, and 1.3 wt.% of chromium, which proves that SLR from the Sereď site is an important source of metals, especially iron.

The morphologies of the particles before leaching (gross sample), magnified with an optical stereo magnifier, are shown in Figure 3a. The sample contains two fractions, namely, coarse and fine spherical particles. Figure 3b shows the particle size distribution of the gross sample. The predominant portion of the SLR gross sample was below 100 µm. A small particle size is beneficial in hydrometallurgical processing.

From the diffraction phase analysis (XRD) results shown in Figure 4a, it follows that Fe is present in the form of magnetite (Fe₃O₄), Wϋstite (FeO), and iron silicate (Fe₂SiO₄). The presence of a certain amount of Cr and Ni in the form of nickel chromate (NiO.Cr₂O₃) was also proven, and Mn, Ca, and Al were found in the sample in the form of calcium manganese silicate ((Ca,Mn)₂Si₂O₆) and calcium aluminum oxide (CaAl₂O₄).

A gross sample was also evaluated using SEM and EDX analysis. Figure 4b shows the morphology of the sample; it is clear that the sample contains particles of different sizes, as was also proven by optical microscopy. EDX analysis, as shown in Figure 4c, confirmed the results of the AAS analysis and demonstrated the presence of elements such as Fe, Cr, Ca, Al, Si, Mg, Mn, and Ni in the investigated sample.

3.1. Thermodynamic Study of SLR Leaching in Sulfuric Acid

A thermodynamic study makes it possible to predict whether a given chemical heterogeneous reaction will take place spontaneously to form products due to changes in external conditions, such as temperature, pressure, the concentration of reagents, etc. [37]. The simplest way to determine the direction of the reaction is to calculate the change in the standard Gibbs energy (∆G⁰). It is commonly known that in the SLR samples, heavy metals such as Ni, Co, and Cr are presented in oxide forms [13]. For this reason, the Equations (1)–(5) of these metal oxides in sulfuric acid H₂SO₄ were used to calculate ∆G⁰, as shown in

Table 2. Chemical reactions of Fe, Co, Ni, and Cr oxides with a sulfuric acid leaching medium at 20 and 80 °C.

Chemical Reaction	ΔG0 (kJ)		N°
	20 °C	80 °C
${Cr}_2{O}_{3_{\left(s\right)}}+{3\mathrm{H}}_2{SO}_{4_{\left(aq\right)}}={{{Cr}_2\left(S{O}_4\right)}_3}_{\left(aq\right)}+{3H}_{2}O$	−319.225	−93.738	(1)
${Fe}_3{O}_{4_{\left(s\right)}}+{4\mathrm{H}}_2{SO}_{4_{\left(aq\right)}}={{{Fe}_2\left(S{O}_4\right)}_3}_{\left(aq\right)}+FeS{O}_{4_{\left(aq\right)}}+{4H}_{2}O$	−27.639	6.282	(2)
$Fe{O}_{\left(s\right)}+{\mathrm{H}}_2{SO}_{4_{\left(aq\right)}}=FeS{O}_{4_{\left(aq\right)}}+H_{2}O$	−128.722	−88.569	(3)
$Ni{O}_{\left(s\right)}+{\mathrm{H}}_2{SO}_{4_{\left(aq\right)}}=NiS{O}_{4_{\left(aq\right)}}+H_{2}O$	−71.509	−66.73	(4)
$Co{O}_{\left(s\right)}+{\mathrm{H}}_2{SO}_{4_{\left(aq\right)}}=CoS{O}_{4_{\left(aq\right)}}+H_{2}O$	−78.045	−73.324	(5)

. Sulfuric acid appears to be the most suitable leaching medium; it is also often used in practice and is an economically and technologically more acceptable choice compared to other acids [38].

From the results of ∆G⁰ calculations for different temperatures, shown in Table 2, it can be seen that, regardless of the temperature, all reactions take place in a way that leads to product formation. At the same time, it is obvious that the values of ∆G⁰ at a temperature of 20 °C are lower, which proves that a temperature of 20 °C is sufficient for the successful course of reactions. If, in the case of a chemical reaction taking place at different temperatures, it is true that ${{∆}_{R}G}_{20°\mathrm{C}}^0<{{∆}_{R}G}_{80°\mathrm{C}}^0$, then the stability of the oxide decreases with increasing temperature.

Potential–pH diagrams (Eh-pH) (Pourbaix diagrams) are widely used tools of thermodynamic study, providing insight not only into the probability of the occurrence of given reactions in real time under certain conditions of temperature and pressure but also allowing us to determine the thermodynamic stability areas of the resulting compounds and ions [37,38].

Figure 5 shows the course of E—pH diagrams for individual elements and the leaching medium (sulfuric acid solution H₂SO₄) at 20 °C and 80 °C. The pH of the leaching medium was set in the range of 0–14, the molality of the main and minor elements was 1 mol/kg H₂O, and the pressure was set at 1 bar (100 kPa). The form of elements (Fe, Cr, Ni, and Co) presented in the water stability area was determined from the constructed Eh-pH diagrams.

Working from the Eh-pH diagram for the Cr-S-H₂O system, shown in Figure 5a, it follows that, in the upper and lower stability limits of water (shown in the diagrams by blue lines), chromium occurs in the ionic form of Cr³⁺ ions at pH values ranging from 0 to 2, but only in a narrow potential interval from 0 to 0.35 V. Chromium can also be found in an ionic form when increasing the voltage in the interval from 1 to 1.3 V. When the temperature is increased to 80 °C, as shown in Figure 5b, the interval of the appearance of Cr³⁺ ions increases, but the pH value at which these ions occur also drops significantly.

In the Fe-S-H₂O system, shown Figure 5c, in the water stability area at pH values from 0 to 6 iron presents as Fe²⁺ ions at a voltage from 0 to 0.7 V; however, with an increasing pH, this area narrows. More significant narrowing (pH just to 5) can be observed at 80 °C, as seen in Figure 5d.

At a temperature of 20 °C, the area of Ni²⁺ ion occurrence significantly depends on the voltage, as shown in Figure 5e. Nickel occurs here in an ionic form at voltages from 0 to 0.4, and then from 0.6 to 1.3 V. For both voltage intervals at 20 °C, Ni is soluble up to a pH of 5. On the other hand, at a temperature of 80 °C, the area of Ni²⁺ ion occurrence is continuous, as shown in Figure 5f, but only up to a pH value of 3.5.

Cobalt occurs in the Co-S-H₂O system, as shown in Figure 5g, in ionic form as the Co²⁺ ion at pH values ranging from 0 to 6 and voltages ranging from 0 to 1.5 V for a temperature of 20 °C. However, upon increasing the temperature to 80 °C, this area becomes narrower and Co²⁺ ions cannot be observed at pH values higher than 5, as shown in Figure 5h.

A thermodynamic study based on the calculation of ∆G⁰ and based on constructed Eh-pH diagrams confirmed that all mentioned SLR components are leachable in H₂SO₄ solutions. It was proven that leaching at an ambient temperature was more advantageous, not only from an economic point of view (no heating was necessary) but also from a processing point of view. It is clear that in the water stability area, the occurrence of metals in ionic form at a temperature of 20 °C is possible in a wider range of pH values compared to a temperature of 80 °C. At 20 °C, metals such as Cr, Fe, Ni, and Co occur in ionic form up to pH values of 2, 6, 5, and 6, while at 80 °C, they appear at 1, 5, 3.5, and 5, respectively.

It must be said that the stability area of phases/ions changes not only with the temperature change but also with the change in the concentration of the components. With the number of components, the number of interactions and difficulty in constructing the Eh-pH diagram increase. Heterogeneous solutions created after the leaching of waste also contain secondary metals in an ionic state, and these can affect the leaching of the main monitored components in the solution. Eh-pH diagrams depict the dissolution of metals in an ideal state and indicate the formation of new phases. It is therefore important to experimentally test the leaching of metal compounds under real conditions.

Among the most important factors influencing the leaching process are the stirring intensity, temperature, the ratio of liquid to solid phase (L:S), and the concentration ($c_{H_2{SO}_4}$) of the leaching medium. Therefore, leaching conditions were proposed, under which it was assumed that the highest rate of recovery of Fe metals into the solution would occur. The results from testing the parameters of the laboratory experiments are shown in

Table 3. Conditions for experimental leaching.

Parameter	Influence of Stirring Intensity	Influence of Temperature	Influence of L:S Ratio	Influence of Concentration of Leaching Medium
Stirring intensity (rpm)	100; 200; 300; 400	400	400	400
Temperature (°C)	60	20; 40; 60; 80	80	80
Liquid-to-solid ratio (L:S)	40	40	5; 10; 20; 40	40
$c_{H_2{SO}_4}$ (mol/dm3)	1	1	1	0.25; 0.5; 0.75; 1; 2; 3

.

3.2. Effect of Stirring Intensity on Leaching

Overall, 200 mL of H₂SO₄ solution with a 1 mol/dm³ concentration was poured into a glass reactor and placed in a water thermostat. The leaching temperature (60 °C) was determined based on a thermodynamic study of Eh-pH diagrams and from the conclusions of the studied literature. Subsequently, 5 g of the weighed SLR sample was poured into the solution (at a ratio of L:S of 40, according to the authors [39]), and the leaching time was 60 min. Stirring was performed using a glass stirrer immersed in the solution. In this set of experiments, stirring intensities of 100, 200, 300, and 400 rpm were analyzed. After adding the sample to the solution, at the specified time intervals (5, 10, 15, 30, and 60 min), 6 mL of solution was taken for chemical analysis to determine the Cr and Fe elements using the AAS method. The conditions of the experiments are shown in Table 3.

Based on the graph, shown in Figure 6a,b, it is possible to determine that the highest extractions of Fe and Cr were 63.42% and 41.45%, respectively. Based on the results, the optimal stirring intensity of 400 rpm was determined. When the mixing intensity was increased to 500 rpm, technical problems occurred during leaching (the effect of centrifugation may occur); therefore, such conditions were not evaluated. A value of 400 rpm was set as a constant for the following series of experiments.

3.3. Effect of Temperature on Leaching

The next set of experiments aimed to determine the effect of temperature on the leaching of the SLR sample. The sulfuric acid concentration was set to 1 mol/dm³, the slurry stirring intensity was 400 rpm, the L:S ratio was 40, and the duration of the experiment was 60 min. The conditions of the experiments are shown in Table 3. Experiments were performed at 20, 40, 60, and 80 °C. At 5, 10, 15, 30, and 60 min, a 6 mL sample was taken for chemical analysis via the AAS method.

Working from the graphs in Figure 7a,b, it follows that after 30 min of leaching at a temperature of 80 °C, the highest extraction rates of 65.65% and 49.77% for Fe and Cr, respectively, were achieved. For this reason, the temperature was fixed at 80 °C for further experiments.

3.4. Effect of Ratio Liquid to Solid on Leaching

The next set of experiments aimed to determine the effect of the ratio of liquid to solid phase (L:S) on the SLR sample leaching. To set the L:S ratios (40, 20, 10, and 5), the SLR samples of 5, 10, 20, and 40 g were weighted and an acid volume of 200 mL was used. The concentration of sulfuric acid used for SLR leaching was 1 mol/dm³, the slurry stirring intensity was 400 rpm, and the temperature was 80 °C. Each experiment lasted for 60 min. The conditions of the experiments are shown in Table 3. At 5, 10, 15, 30, and 60 min, 6 mL of samples was taken for chemical analysis by the AAS method.

From the graphs in Figure 8a,b, it can be seen that the highest extractions of 65.07% of Fe and 49.77% of Cr were achieved at an S:L ratio of 40. The results with the S:L ratio 40, which was already determined in the first experiment according to the study [39], confirmed the correctness of the setting of the test conditions. For this reason, the L:S ratio was fixed at 40 in further experiments when monitoring the influence of the concentration of the leaching medium.

3.5. Effect of Concentration of H₂SO₄ on Leaching

The last set of leaching experiments aimed to determine the effect of the sulfuric acid concentration on the leaching of the SLR sample. SLR samples of 5 g were added to sulfuric acid solutions with concentrations of 0.25, 0.5, 0.75, 1, 2, and 3 mol/dm³. The slurry stirring intensity was 400 rpm, the temperature was 80 °C, the L:S ratio was 40, and each experiment lasted for 60 min, as shown in Table 3. From the graphs in Figure 9a,b, it follows that the highest extraction level, 90.35% of Fe, was achieved at a concentration of 3 mol/dm³ at 30 min of leaching. We achieved 59.62% extraction Cr after 60 min. For this reason, a concentration of 3 mol/dm³ was determined to be optimal.

It is obvious that leaching extraction significantly depends on the chosen process conditions. Based on the laboratory experimental results, the optimal conditions for SLR leaching were as follows: a mixing intensity of 400 rpm, a K:P ratio of 40, a temperature of 80 °C, and a leaching medium concentration of 3 mol/dm³ H₂SO₄. The intensity of mixing and the concentration of the leaching medium significantly influence the speed of dissolution (leaching). This indicates that the leaching process of SLR by sulfuric acid takes place via the diffusion route. Diffusion is a process occurring at the molecular level that depends on temperature; the effect of temperature can be expressed using the Arrhenius equation. By applying the Arrhenius equation in logarithmic form, i.e., lnk = lnA − E_a/RT (where k is the rate constant, A is the frequency factor, T is temperature in Kelvin (K), and R is the universal gas constant), the value of the apparent activation energy can be calculated as E_a (kJ.mol⁻¹). This, according to Zelikman et al., indicates that in the temperature range from 20 to 80 °C chemical reactions (1)–(5) probably take place in the diffusion zone [40,41].

However, further increasing the mixing intensity (more than 400 rpm) is technically unfeasible (the so-called centrifugation effect may occur). Increasing the concentration of the leaching medium, the temperature, and the L:S ratio is not advantageous from an economic and environmental point of view (due to a high rate of consumption of acid and the high cost of energy).

The results of SLR sample leaching under optimal conditions are summarized in Figure 9c. It is obvious that the highest level of extraction of Fe was achieved (already standing at 90.35% for 30 min). The extraction of Cr reached 40% for 30 min and 50% for 60 min. In the case of Ni and Co, the maximum extraction was reached in 15 min. This was the limit of reaching the saturated solution, equilibrium was established, and further extension of the leaching time did not contribute to the improvement of Ni and Co extraction.

Figure 9d shows the comparison of metal extraction after 30 and 60 min of leaching under optimal conditions. It is obvious that it is advisable to end the leaching process after 30 min, where the extraction of Cr is lower (40%) than that seen after 60 min. The high content of Cr is undesirable due to the possible formation of a coprecipitate of Fe and Cr. The content of Ni and Co remains unchanged, which is advantageous during precipitation.

The change in the composition of the sample before and after leaching was also confirmed by X-ray analysis, shown in Figure 10a. During the leaching of SLR in an acidic medium, iron, chromium, and cobalt oxides decompose in sulfuric acid. It is clear from Figure 10a that there was significant dissolution of FeO and significant removal of Fe₃O₄, as experimentally proven in Figure 9c. Other compounds (containing Co, Ni, and Cr) were partially dissolved.

Figure 10b shows the SEM image of the sample after leaching under optimal conditions. It is clear that the portion of the fine fraction decreased compared to the gross sample. According to the EDX analysis, shown in Figure 10c, it can be seen that the iron content was much lower (~10%) than it was in the input gross sample, shown in Figure 4c. It follows that the residual iron in the solid fraction after leaching is in the form of magnetite, which does not leach at 80 °C, which was also proven by ∆G⁰ = 6.282 kJ from the leaching Equation (2).

3.6. Proposal for Treatment of Leachate

The next step in the hydrometallurgical processing of SLR is to obtain a final marketable product of the highest possible purity from the leachate. The solution after SLR acid leaching contains mainly Fe, and also Cr, Ni, and Co. It is possible to obtain iron from the solution by applying the basic extraction method of chemical precipitation. There are several methods of iron precipitation.

The hematite process of Fe precipitation from a sulfate solution is a cheap and simple method. The advantage of this method is the purity of the resulting product, which can be further processed without major problems. Products obtained in this way can be further used in the production of cement or as inputs in the production of pig iron or pigment in the chemical industry.

Sodium hydroxide is most often used as a precipitating agent. It is also possible to use the Goethe process, which is carried out at pH 2–2.5 and a temperature of 70–90 °C. However, its disadvantage is that the emerging goethite also contains a small number of impurities (sulfates), which makes it impossible to use goethite as an input raw material in the production of iron in a blast furnace. During the precipitation itself, it is important to monitor the temperature and the pH value, which can also affect the precipitation of Cr, Ni, and Co in the form of hydroxides,

Table 4. Predicted chemical reactions of precipitation of acid leaching products by NaOH solution at 20 and 80 °C.

Chemical Reaction	ΔG0 (kJ)		N°
	20 °C	80 °C
${Fe}_2{(SO}_4{)}_{3_{\left(aq\right)}}+6NaO{H}_{\left(aq\right)}=2Fe(OH{)}_{3_{\left(s\right)}}+3N{a}_{2}S{O}_{4_{\left(aq\right)}}$	−461.554	−517.163	(6)
$FeS{O}_{4_{\left(aq\right)}}+2NaO{H}_{\left(aq\right)}=Fe(OH{)}_{2_{\left(s\right)}}+N{a}_{2}S{O}_{4_{\left(aq\right)}}$	−100.007	−115.321	(7)
${Cr}_2{(SO}_4{)}_{3_{\left(aq\right)}}+6NaO{H}_{\left(aq\right)}=2Cr(OH{)}_{3_{\left(s\right)}}+3N{a}_{2}S{O}_{4_{\left(aq\right)}}$	−305.515	−347.134	(8)
$NiS{O}_{4_{\left(aq\right)}}+2NaO{H}_{\left(aq\right)}=Ni(OH{)}_{2_{\left(s\right)}}+N{a}_{2}S{O}_{4_{\left(aq\right)}}$	−88.38	−103.104	(9)
$CoS{O}_{4_{\left(aq\right)}}+2NaO{H}_{\left(aq\right)}=Co(OH{)}_{2_{\left(s\right)}}+N{a}_{2}S{O}_{4_{\left(aq\right)}}$	−108.241	−108.684	(10)

.

From a theoretical point of view, it is possible to verify the probability of usable product formation during the precipitation of the acid leachate obtained after the leaching of the SLR sample. Table 4 shows the chemical precipitation reactions (6)–(10) of the compounds formed during acid leaching, which are shown in Table 2.

∆G⁰ was calculated to determine the probability of the course of the reactions and the formation of the desired poorly soluble metal precipitates in the leachate. The negative value of the standard Gibbs energy indicates that these reactions are thermodynamically feasible and spontaneous.

Based on Figure 5c, after acid leaching, iron is presented as Fe₂(SO₄)₃ and FeSO₄; therefore, the oxidation of Fe²⁺ ions to Fe³⁺ ions is required. The oxidation of Fe²⁺ ions by hydrogen peroxide, as shown in

Table 5. Predicted chemical reactions of Fe²⁺ to Fe³⁺ ion oxidation at 20 and 80 °C.

Chemical Reaction	ΔG0 (kJ)		N°
	20 °C	80 °C
$2FeS{O}_{4_{\left(aq\right)}}+H_2{O}_{2_{\left(aq\right)}}+{\mathrm{H}}_2{SO}_{4_{\left(aq\right)}}=F{e}_{2}S{O}_{3_{\left(aq\right)}}+{2H}_{2}O$	−193.916	−173.676	(11)

, is based on the negative standard Gibbs energy (at both temperatures) and should probably be accomplished to form products. Based on the thermodynamic study of precipitation reactions, it is clear that a temperature of 20 °C is sufficient for precipitate formation.

An important factor in the precipitation process is the stability of the resulting products.

Table 6. Solubility product constant near 20 °C.

Chemical Formula	${\mathbf{F}\mathbf{e}\left(\mathbf{O}\mathbf{H}\right)}_3$	${\mathbf{C}\mathbf{r}\left(\mathbf{O}\mathbf{H}\right)}_3$	${\mathbf{C}\mathbf{o}\left(\mathbf{O}\mathbf{H}\right)}_2$	${\mathbf{N}\mathbf{i}\left(\mathbf{O}\mathbf{H}\right)}_2$
Ksp	6.3 × 10−38	6.31 × 10−31	2.5 × 10−16	2.8 × 10−15
pH	4.26	6.56	9.04	11.81

shows the Solubility Product Constant (Ksp) values of individual predicted precipitates at a temperature of 20 °C [42]. Based on the Ksp values, it can be said that Fe(OH)₃ is the least soluble in the obtained solution (leachate) and Ni(OH)₂ is the most soluble.

Sodium hydroxide promotes an alkaline medium and shifts the pH of an acidic solution into the alkaline range. The neutralization of a solution initiates the precipitation of metal contaminants in the form of insoluble compounds. It can be assumed that the change in pH is a key factor in the precipitation process and enables the selective precipitation of individual phases. Based on the pH values, it is possible to determine which substance/compound is the least soluble and which is the most soluble.

Based on the interactions, the hypothetical pH values were calculated, as shown in Table 6, at which iron (Fe³⁺), chromium (Cr³⁺), cobalt (Co²⁺) and nickel (Ni²⁺) ions can precipitate from an acidic solution.

It can be assumed that Fe³⁺ ions should start to precipitate at pH 4.26, Cr³⁺ should start to precipitate at pH 6.569, cobalt ions should start to precipitate at pH 9.04, and nickel should start to precipitate at pH 11.81. Based on this, the possibility of selective precipitation is assumed to be in the following order: Fe(OH)₃, Cr(OH)₃, Co(OH)₂, and then Ni(OH)₂. During the precipitation of Fe, some metals may be lost from the solution by absorption by the surface of the formed precipitate or by the formation of insoluble precipitates of the given metals. However, there are studies that prove that these losses are not significant if the appropriate conditions (pH, temperature, keeping to the exact extraction procedure) are chosen. For example, in [43] the authors achieved more than 90% Fe recovery using a one-step precipitation process (pH 4); by analyzing the composition of the final solution, they confirmed that there was only 21 and 17% loss of nickel and cobalt.

Subsequently, the precipitated Fe(OH)₃ can be filtered off, dried, and calcined to give the final product Fe₂O₃, which is marketable. The calcination reaction is shown in

Table 7. Predicted chemical reactions of calcination at 500 °C.

Chemical Reaction	ΔG0 (kJ)	N°
$2Fe(OH{)}_{3_{\left(aq\right)}}=F{e}_2{O}_{3_{\left(s\right)}}+{3H}_{2}O$	−106.687	(12)

. After filtering off the precipitate, Ni²⁺ and Co²⁺ ions remain in the solution and can be obtained as metals or compounds by suitable extraction methods. Solvent extraction is very common. This is followed by crystallization to create the final single-metal compound as a marketable powder product. Most of the earlier research on the solvent extraction and separation of Ni²⁺ and Co²⁺ used amine extractants from hydrochloric acid solutions or organic phosphorus extractants (Cyanex 272, D2EHPA, and PC-88A) from acidic sulfate media.

4. Conclusions

Old environmental burdens with metal-bearing content can be considered a rich, alternative source of valuable secondary raw materials. The content of metals in EBs often exceeds the content of metals in primary raw materials; moreover, the processing of EBs would subsequently lead to the remediation of EBs, which can be considered an indisputable advantage from environmental and ecological points of view. Environmental burdens are the result of the industrial production of metals or the mining of mineral raw materials, which results in different qualitative and quantitative types of waste, affecting the choice of processing technology.

In this work, we first theoretically verified, using a thermodynamic study, the possibility of Fe, Cr, Ni, and Co oxide leaching. A thermodynamic study confirmed the possibility of the acid-based extraction of metals at both 20 and 80 °C. Next, we experimentally extracted the required metals (Fe, Cr, Ni, and Co) from the SLR sample into a solution under different leaching conditions. Experiments showed that the best yields concerning technical and process results were achieved with the following combination of conditions: a stirring intensity of 400 rpm, an L:S ratio of 40, a temperature of 80 °C, and a leaching medium concentration of 3 mol/dm³. Under these conditions, we achieved the highest rate of iron extraction, at 90.35% (after 30 min of leaching), and of chromium extraction, at 59.62% (after 60 min of the leaching). The nickel content (49.79%) and cobalt (41.56%) in the leachate were low under these leaching conditions. Increasing the extraction of Ni and Co could be achieved by re-leaching the solid residue.

We proposed the theoretical possibility of obtaining a final product based on Fe that could be sold to the steel industry and which would not contain a harmful admixture—Cr. In the next study, we will focus on the experimental determination of the parameters of the selective extraction (precipitation method) of Fe and Cr. The added value of the proposed hydrometallurgical process is the presence of metal ions of nickel and cobalt (Ni²⁺ and Co²⁺) in the leachate. Using suitable extraction procedures (liquid extraction or precipitation), there is the possibility of extracting them and obtaining a marketable product as a result.