Reduction of High-Chromium-Containing Wastewater in the Leaching of Pyritic Waste Rocks from Coal Mines

Abstract

Coal is an abundant resource which can be used to produce low-cost energy; however, its usage causes great environmental damage. Before mineral coal can be used, it must be processed to remove coal tailings. These tailings contain pyrite and accumulate in large dumps, presenting significant environmental liabilities, such as acid mine drainage. Another industry that generates environmental liabilities is the chrome-plating industry, mainly because it produces hexavalent chromium (Cr⁶⁺) waste. The main aim of this work was to evaluate Cr⁶⁺ as a reduction agent in trivalent chromium (Cr³⁺) conversion in the leaching of coal-mine waste containing pyrite. Cr³⁺ is about 100 times less toxic than Cr⁶⁺ and can be easily removed from industrial effluents by alkaline precipitation. There are several sources of effluents containing Cr⁶⁺—a compound which is known worldwide to be toxic, carcinogenic, and mutagenic. A leaching and treatment device was developed and tested for waste treatment. The results indicated that the developed treatment system reduced 100% of Cr⁶⁺ to Cr³⁺ through pyrite leaching in a Cr⁶⁺ wastewater sample from the electroplating industry. In addition, the chromium sludge resulting from the treatment process, after calcination, was tested in a ceramic glaze as a pigment and, when compared with an industrial pigment, showed similar mineralogical characteristics.

1. Introduction

Many authors have evaluated the environmental impact of using coal to provide electricity, steel, cement, and coal-to-liquid fuels worldwide in terms of waste generation [1,2,3,4,5], acid mine drainage (AMD), and soil, water, and air pollution, in addition to the damage to people’s lives [6,7,8,9].

In the Brazilian carboniferous regions of Santa Catarina and Paraná, after extraction, coal needs to be processed in order to meet the combustion standards of thermoelectric plants. As a result of the treatment requirements, approximately 60% of the total amount of coal produced becomes solid waste, of which between 10% and 65% consists of pyrite, which must be discharged in waste deposits [10].

Pyrite (FeS₂) is a major natural sulfate mineral associated with hydrothermal mineral formation, ore deposits, and sedimentary media in anoxic conditions [11,12,13]. Pyrite is considered an industrial waste because it is responsible for acid mine drainage (AMD), which occurs as a result of the natural oxidation of pyrite in contact with water and oxygen. AMD is recognized as one of the most serious environmental problems in the mining industry due to the high acidity of the drainage water (it has a pH less than 3.5) and its high contents of soluble metals and sulfates [14,15,16,17]. However, pyrite can be a valuable ore mineral in certain contexts [18].

The natural process of pyrite oxidation occurs in a number of stages. First, reaction (1) shows the oxidation of pyrite with molecular oxygen in the presence of excess water at neutral pH; in this case, Fe²⁺ is released and sulfate acidity leads to a pH decrease. Then, molecular oxygen or bacterial action convert the Fe²⁺ into Fe³⁺ (reaction (2)) [19,20,21]. The overall reaction for pyrite oxidation (reactions (1)–(3)) is summarized in reaction (4). Reaction (5) shows the complete oxidation of pyrite, in which iron acts as the oxidizing agent. In the presence of water and oxygen, global pyrite oxidation is represented by reaction (6).

$${\mathrm{FeS}}_2+\frac{7}{2}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{Fe}}^{2+}+2{({\mathrm{SO}}_4)}^{2-}+2{\mathrm{H}}^{+}$$

(1)

$${\mathrm{Fe}}^{2+}+\frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}^{+}\leftrightarrow {\mathrm{Fe}}^{3+}+\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{Fe}}^{3+}+3{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Fe}{\left(\mathrm{OH}\right)}_3+3{\mathrm{H}}^{+}$$

(3)

$${\mathrm{FeS}}_2+\frac{15}{2}{\mathrm{O}}_2+\frac{7}{2}{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Fe}{\left(\mathrm{OH}\right)}_3+2{({\mathrm{SO}}_4)}^{2-}+4{\mathrm{H}}^{+}$$

(4)

$${\mathrm{FeS}}_2+14{\mathrm{Fe}}^{3+}+8{\mathrm{H}}_2\mathrm{O}\leftrightarrow 15{\mathrm{Fe}}^{2+}+2{({\mathrm{SO}}_4)}^{2-}+16{\mathrm{H}}^{+}$$

(5)

$${\mathrm{FeS}}_2+\frac{15}{4}{\mathrm{O}}_2+\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{Fe}}^{3+}+2{({\mathrm{SO}}_4)}^{2-}+{\mathrm{H}}^{+}$$

(6)

Pyrite oxidation is a complex reaction; if a gap exists between the top of the valence band and the bottom of the conduction band, this is referred to simply as the band gap [22]. Rimstidt [23] describes pyrite oxidation in three distinct steps: in the first step, a cathodic reaction is generated by the transfer of electrons from pyrite surfaces to aqueous oxidant species, usually O₂ or Fe³⁺; in the second step, charge is transported from the anodic site of the reaction to replace the electrons lost from the cathodic site; in the last step, at the anodic site, the oxygen atoms of water molecules interact with sulfur atoms to create oxidized sulfoxide species [24]. Metal sulfides can be used as cathodes or anodes, especially natural FeS₂ ore (pyrite), because of their high electron-change capacities (4e⁻/FeS₂) and low costs (∼USD 0.5 per kg) [25,26].

Several researchers have already studied the reduction of hexavalent chromium (Cr⁶⁺) under analytical and/or industrial conditions in the leaching of pyrite [27,28,29,30,31]. Chromium reduction occurs through the action of pyrite, which acts as a reducing agent, transforming hexavalent chromium into trivalent chromium. This process can be used to treat industrial effluents with hexavalent chromium; it is a treatment technique that is simple, effective, and economical for the treatment of wastewater.

Benincasa et al. [28] and Kantar et al. [32] studied the reaction between Cr⁶⁺ and pyrite. The researchers evaluated the effects of pH and different Cr⁶⁺ concentrations on chromium-reduction behavior. The results demonstrated that Cr⁶⁺ reduction by pyrite decreased with increasing solution pH and Cr⁶⁺ concentration. The decrease in Cr⁶⁺ removal with increasing Cr⁶⁺ concentration indicated that there was a limited number of surface reactive regions in the pyrite; that is, with excess surface regions, Cr⁶⁺ adsorption onto pyrite surfaces regulated the overall rate of Cr removal, leading to first-order reaction kinetics. It can also be stated that Cr⁶⁺ oxidatively dissolved the pyrite surface, releasing ferrous iron and sulfates into the reaction solution as the reaction progressed [31]. Cr⁶⁺ reduction is significant at pH < 2.3, and the reduction of Cr⁶⁺ to Cr³⁺ involves the oxidation of Fe and S₂, according to reaction (7). Finally, all pyrite particles at acidic pH show variable Cr contents on their surfaces.

$$2{\mathrm{FeS}}_2+5{\left({\mathrm{Cr}}_2{\mathrm{O}}_7\right)}^{2-}+32{\mathrm{H}}^{+}\leftrightarrow 2\mathrm{Fe}{\left(\mathrm{OH}\right)}_3+4{({\mathrm{SO}}_4)}^{2-}+10{\mathrm{Cr}}^{3+}+13{\mathrm{H}}_2\mathrm{O}$$

(7)

Chromium has special importance as the fifth metallic chemical element, after iron, aluminum, manganese, and copper. It is the main constituent of chromite, which is considered one of the most important industrial minerals in the world, its applications covering a range of industrial processes, metallic and non-metallic [33]. Resources of chromium derive from around 12 billion tons of chromite—a quantity sufficient to meet conceivable demand for centuries. These resources are concentrated in Kazakhstan, Southern Africa, Russia, and India [34,35,36]. Chromium occurs in 82 different types of and exclusively in ultramafic igneous rocks, but only the chromite type is mined commercially. Chromite varies widely in composition according to the chemical formula (Mg, Fe²⁺)(Cr, Al, Fe³⁺)2O₄, the varying proportions depending on the deposits [37].

Industrial uses of chromium include ferrous and nonferrous alloy production, chromium plating, corrosion control, metal finishing, and incorporation into pigments, tanning compounds, wood preservatives, cement kilns, and glass-tank regenerators, among other uses [38,39,40,41,42].

Generation of residues is the main problem regarding the industrial application of chromium. Chromium is a transition metal with atomic number 24 and can be found in several oxidation states; however, the most common states found in natural environments are Cr⁶⁺ and Cr³⁺ [43,44]. Cr³⁺ is a stable form and 100 times less toxic than Cr⁶⁺ and is essential to humans in the metabolism of insulin. On the other hand, Cr⁶⁺ is a strong oxidizing agent and a skin and mucous membrane irritant; it also produces allergic reactions, such as eczema, in addition to respiratory disorders, kidney circulation problems, lung cancer, and gastrointestinal distress [43,44,45,46].

Wastewater-containing Cr⁶⁺ is produced by different industrial processes, such as metal plating, pigment and dye production, surface coating, leather tanning, and corrosion inhibitor manufacturing [32,47,48,49]. The reduction of Cr⁶⁺ to Cr³⁺ and the formation of insoluble Cr³⁺ precipitates are essential steps in the treatment of water and soil contaminated with Cr⁶⁺ [18]. Generally, the treatment of Cr⁶⁺ effluent (reduction of Cr⁶⁺ to Cr³⁺) is carried out using iron salts, sulfur compounds, metal particles, and precipitation by pH adjustment. At the end of the treatment process, an insoluble chromium hydroxide [Cr(OH)₃] is produced [50,51,52,53]. However, in all cases, treatment produces large amounts of sludge that presents difficulties in terms of management, transportation, and costs associated with final disposal in landfill, in addition to the potential leaching of hazardous waste during storage [54,55,56].

The development of pigments with low-cost raw materials has been a longstanding industrial aim and is required by the ceramic sector to increase market competitiveness [57]. Additionally, it has been reported by Pelino [58] and Garcia-Valles et al. [59] that the inerting of hazardous waste in ceramic matrices seems more suitable due to the technologies of production, with high-temperature vitrification converting hazardous waste into non-hazardous glass or glass–ceramic materials.

Many surveys have been developed regarding the recovery of chromium for use as a ceramic pigment from the waste produced in the chrome-plating process. The solid waste residues from a metallurgical plant containing high amounts of Cr³⁺ have been applied in ceramic glazes [60]. Costa et al. [61] developed a spinel pigment by a combination of two different sludges generated from Cr/Ni plating and Fe-rich galvanizing sludge generated during steel wiredrawing. Berry et al. [62] evaluated the possibility of producing chromium commercial pigment using Cr³⁺ tanned-leather shavings as a source of chromium.

In the present study, the aim was to evaluate the possibility of treating two hazardous materials (high-chromium-containing wastewater from the electroplating industry and pyrite derived from coal mining) together to generate a new product (chromium pigment).

The chromium reduction system was evaluated according to variations in oxidation–reduction potential (ORP), Cr⁶⁺ concentration, and changes in pyrite particle size. After calcination, the solid material resulting from the filtration process (chromium sludge) was characterized by X-ray diffraction (XRD), X-ray fluorescence (FRX), and particle size and colorimetric parameters and applied in a ceramic glaze as a pigment. The advantages of the chromium pigment generated from the waste are its low cost compared to industrial chromium pigment and the significantly minimized environmental impacts and costs of the generators required for the chromium and pyrite waste processing. Regarding the disadvantages of this pigment, it can be said that adjustments to pigment composition must be carried out to obtain different colors, the calcination process can oxidize Cr³⁺ to Cr⁶⁺ [63], and a large amount of chromium waste is needed to obtain a small amount of pigment (for example, 100 L of high-chromium-containing wastewater generates approximately 350 g of pigment).

2. Materials and Methods

2.1. Materials

Pyrite concentrate was obtained by the jigging of coal tailings from Cambuí Mine, Paraná, Brazil—the same deposits that were characterized by Menezes et al. [64]. The material is composed of 65% FeS₂, with around 35% pyritic sulfur compounds that can be used as reductant agents. Furthermore, this material was analyzed for specific density, apparent density, particle size, and pore space. Figure 1 shows the fractions of pyrite that were used as reductant agents in the lixiviation column system.

A high-chromium-containing wastewater sample was collected from the tank used in the current treatment carried out in the electroplating industry. Nowadays, this small industry produces bolts, clamps, metal clips, and other materials used in agricultural machines and generates around 300 L/week of chromium effluent. After proper treatment, the metallic sludge is finally discharged at a high cost, besides the considerable wastage of material. The sample obtained contained a Cr⁶⁺ concentration of 0.0096 M and had a pH of 2.39.

2.2. Experimental Equipment and Procedure

The leaching column system consisted of a column in a polyvinyl chloride tube that was 700 mm in height and 150 mm in diameter, and the base of the column was installed with a stopcock for flow control. Ninety percent of the column was used as a leaching bed and was filled up with pyrite waste, 30% of this volume being empty space that was covered with effluent and 10% of the leaching bed not being covered to permit contact between pyrite surfaces and improve pyrite oxidation. The retention time between the wastewater and the leaching bed was 3.4 min. The maximum capacities for the leaching device developed were 15 kg of pyrite added to the leach cell and 100 L of high-chromium-containing wastewater in the reservoir.

Recirculation was carried out with a submerged pump at a flow rate of 72 L/h. One-hundred liters (L) of chromium wastewater was percolated through the system until the total reduction of Cr⁺⁶ to Cr³⁺. The initial pH value was 2.7 ± 0.1 and the solid–liquid ratio was 150 g/L. A schematic drawing of the leaching column system is shown in Figure 2a. After 36 h of recirculation, the pH of the solution was adjusted to 8.5–8.7 with sodium hydroxide (99% of purity) to form the sludge (Figure 2c), which was removed by filtration. All experiments were performed at room temperature (25 °C), and three repetitions of the experiments were performed.

To evaluate the pyrite-reduction capacity, oxidation–reduction potential (ORP) and Cr⁶⁺ concentration were measured. The wastewater solution had a pH between 2.0 and 2.7, which was naturally maintained in this range by pyrite dissolution, because this reaction generates sulfuric acid by sulfur oxidation. Figure 2b shows the wastewater color change after 36 h of the reduction process. Furthermore, changes in pyrite particle size were determined to evaluate the degradation of pyrite by leaching.

As a result of the filtration process, 390 g of solid material was dried, ground, and calcined at 1100 °C for 2 h, following the procedure used by Ozel and Turan [65]. A calcined sample was characterized by X-ray diffraction (XRD), X-ray fluorescence (FRX), particle size, and colorimetric parameters. A powder with particles of sizes less than 75 µm was called Chromium Sludge Pigment (ChSlPi) and was compared with Industrial Pigment (InPi) in terms of its granulometric, mineralogical, and chemical characteristics. Both pigments were treated under the same conditions and sequentially applied in ceramic glazes. The hexavalent chromium wastewater, before and after pyrite treatment, was characterized according to environmental legal rules, and the metal contents were evaluated by colorimetric methods [66]. All analyses followed the procedures described in the Standard Method for the Examination of Water and Wastewater [67].

2.3. XRD Methodology

In order to identify the crystalline phases by XRD, a SHIMADZO XRD 6100 diffractometer was used, with CuKa (l = 1.5418 Å) radiation, operating at 40 kV and 30 mA. The data collection was carried out with a step interval of 0.02° and a step time duration of 0.6 (s), in the 2θ range of 4 to 80°. The data collections were identified using HighScore Plus software for ICDD-pattern indexing. Amounts of crystalline phases were determined by the Rietveld method.

2.4. XRF Methodology

The chemical compositions of the materials (ChSlPi and InPi) were analyzed by X-ray fluorescence with dispersive energy (EDXRF) using a Shimadzu EDX-7000 spectrometer. The analysis was performed by the Solid Fuels Laboratory (SATC—Criciúma/SC).

2.5. Particle Size

The particle-size analyses were carried out by LASER diffraction, in water, according to the Fraunhofer method, using a Bettersizer S2-WD device. Particle-size analysis was performed by the Laboratório Central de Equipamentos Multiusuários—Cemulti, Passo Fundo/RS.

2.6. Colorimetric Parameters

The colors of the sample (ChSlPi) and industrial pigment (InPi) were characterized by differential colorimetry (DL*a*b) using the colorimetric parameters L*a*b. Figure 3a show the brown–red Industrial Pigment (Industrial Pigment—InPi) and Figure 3b shows the brown pigment (Chromium Sludge Pigment—ChSlPi). The pastille sample was made with 2 g of sample compacted with 74 N/cm². The color was evaluated with a Minolta 2600d spectropolarimeter in the Mineral Processing Laboratory (LAPROM), Federal University of Rio Grande do Sul, Brazil.

3. Results and Discussion

The chromium reduction system was evaluated for wastewater effluent to analyze the changes in the sizes of particle and chemicals variations in ORP potential and Cr⁶⁺ concentration. The leaching promotes the consumption of fine particles of less than 2.36 mm. After cycle tests, the particle sizes of pyrite showing soft density increased by around 3% and porous space increased by around 8.5%, in addition to a decrease in the apparent density of around 10%, probably as a result of the dissolution of fine particles. These results are shown in Figure 4.

Another important result obtained with this controlled system was an ORP evaluation for chromium reduction by pyrite oxidation. The ORP value for the Cr⁶⁺ solution before leaching was 624 mV, and this was reduced to 355 mV after 36 h of recirculation. Figure 5 show the reduction percentages of Cr⁶⁺ and Cr³⁺ as a function of leaching time. It can be observed that, over approximately 480 min of leaching time, the Cr⁶⁺ and Cr³⁺ concentrations were practically the same and, as expected, when the amount of Cr⁶⁺ decreased, the amount of Cr³⁺ increased. In addition, the total amount of chromium remained constant.

The behavior of chromium reduction kinetics can be described by fourth-order polynomial regression for the leaching time interval and the conditions used in the experiment. Equations (8) and (9) show the regressions for the reduction of Cr⁶⁺ and Cr³⁺, respectively:

$$C{r}^{6+}reduction \left(\%\right)=-2·{10}^{-11}·x^4+{10}^{-7}·x^3-0.0002·x^2+0.0187·x+66.4$$

(8)

$$C{r}^{3+}reduction \left(\%\right)=2·{10}^{-11}·x^4-{10}^{-7}·x^3+0.0002·x^2-0.02·x+33.7$$

(9)

where x is the leaching time. The coefficients of determination for Equations (8) and (9) were R² = 0.975 and R² = 0.9776, respectively.

The color change of solutions can be seen in Figure 6, in which the orange solution (Cr⁶⁺) turned green (Cr³⁺). It is very important to report that the chromium reduction by pyrite leaching depends on the surface oxidation of pyrite and on the pH of the solution [68].

Color change is related to a change in 3d orbital electronic levels and ligand field splitting, hence the change in absorption spectra and the resulting color. If the ligands change, usually the light–matter interaction energy also changes (∆E_elec). In case of Cr³⁺ color change, the excitation of an electron from the lower 3d orbital to a higher 3d orbital in the hexaamminechromium (III) complex ion occurs. Regarding Cr⁶⁺, the orange dichromate (VI) ions form an equilibrium and one species dominates, depending on the pH of the solution. Change in the pH of the solution (Cr⁶⁺) does not result in a change of oxidation state, but the electronic environments of the central chromium ions are changed such that they have different UV–visible absorption spectra and different field-splitting ∆E_elec values, resulting in color variations in the Cr⁶⁺ solution [69].

According to the ASTM D7348-21 standard [70], the pigment had a 3.8% of loss ignition. The sample calcined (Figure 6b) was analyzed to identify the chemical composition and the mineral phases formed. Figure 7 and Figure 8 show the diffractograms for the InPi and ChSlPi pigments, respectively.

The diffraction patterns confirmed zincochromite and chromite as majority phases in the two samples. InPi showed a more defined phase than the treatment with coal waste concentrate (pyrite), but the treatment system was effective in converting two wastes into one material with mineralogical phases very similar to the raw industrial material. On the other hand, the chemical composition (

Table 1. Chemical compositions of the materials.

Composition	Cr2O3	Fe2O3	ZnO	SiO2	SO3	CaO	CuO	Others
ChSlPi	50.45	13.64	30.19	0.36	3.46	1.38	0.06	0.46
InPi	29.32	28.99	40.89	0.35	0.32	0.06	0.06	0.02

) between the two samples (InPi and ChSlPi) showed differences, mainly concerning chromium, iron, and zinc contents. It can be said that the chromium and zinc came from the wastewater galvanization and that the iron came from the pyrite dissolution.

Large amounts of chromium could be observed in both pigments. According to Liao et al. [71] and Li et al. [72], the amount of chromium contained in a pigment, when applied in the sintering of ceramic glaze, has no influence on toxicity, because glass–ceramic materials form spinel structures and residual glass successfully immobilizes Cr.

To evaluate the differences in particle-size distribution between InPi and ChSlPi samples, granulometric analyses were performed. The results indicated that ChSlPi had bigger particles than InPi, which can result in poor dispersion in pigment applications in ceramics. This dispersion problem can be solved with wet grinding media to achieve a suitable particle size. The results are shown in Figure 9.

The two pigment samples were compared with respect to their color behavior. The colorimetric behavior of the two samples pigments obtained from wastewater treatment were evaluated (

Table 2. Colorimetric parameters.

Pigment Type	L*	a*	b*	DEL*a*b*
InPi	30.61	16.50	10.80	0.00
ChSlPi	28.29	10.10	12.83	25.24

).

Results for CIE L*a*b* and DE_L*a*b* represent color differences; thus, the InPi is lighter in color. Parameter a* reveals that it is redder; b* reveals that it is less yellow. The global color difference is strong (DE_L*a*b*); however, the color differences above 1.5 can be seen by the human eye [73,74]. On the other hand, the reflection of the light demonstrates the similarity of InPi and ChSlPi when the curve shapes are compared. The percentages of reflectance are in the same region of the spectrum, around 650 nm. Figure 10 shows the percentage reflectance as a function of wavelength in the visible range for InPi and ChSlPi. An adjustment to the chemical composition could be made to increase the similarity between InPi and ChSlPi.

The brown–red pigment derived from sludge was submitted to industrial testing in a ceramic factory (Figure 11). Firstly, the brown–red pigment was wet-ground for 10 min with a ball ceramic mill and, to improve the dispersion powder, a dispersant solution was added. The pigment was mixed with 1 Wt% and 5 Wt% with transparent glazes as flux. The mix was homogenized in an eccentric mill for 5 min and applied to ceramic tiles as a surface glaze. The tiles were heated inside a kiln at 1050 °C for 5 min. The methods used to color the ceramic material and determine the composition of the industrial pigment were described by Marcello et al. [75]. It is important to note that the procedure used to implement ChSlPi in the ceramic material was the same as that used to implement the commercial pigment.

Regarding compliance with the environmental rules, the chromium wastewater and treated wastewater were characterized in terms of metal contents, pH, total solids, turbidity, colour haze, and conductivity (

Table 3. Chemical compositions before and after wastewater treatment.

Parameter	Wastewater	Treated Wastewater	Brazilian Standards CONAMA 430/2011
Trivalent chromium (mg/L)	236.90	0.80	1.00
Hexavalent chromium (mg/L)	500.00	ND	0.10
Total iron (mg/L)	16.20	1.80	15.00
Zinc (mg/L)	210.50	ND	5.00
Manganese (mg/L)	3.40	0.30	1.00
Total solids (mg/L)	11,240.00	0.30	-
Turbidity (mg/L)	6.00	8.79	5.00
Conductivity (mg/L)	600.00	9.57	-
pH	2.39	6.70	5.00–9.00

). The results showed that the leaching system treatment was able to reduce all of the Cr⁶⁺ to Cr³⁺ given the replacement of the conventional reduction system with sodium metabisulfite; in addition, it was not necessary to change the subsequent steps with pH adjustment and solid–liquid separation. After treatment with pyrite, the chromium wastewater was found to satisfy the Brazilian environmental legislation for wastewater disposal.

It was important make sure that the pH value after the reduction process and before neutralization was 2.78. In addition, after application, pyritic waste was leached with water for 24 h in the same system and kept at rest inside the column for 7 days to promote new oxidation on the pyrite surfaces and provide a new use for the chromium treatment process.

4. Conclusions

Pyrite oxidation is a natural reaction that involves water, oxygen, sulfur oxidation, and a species that can be reduced to iron. Reduction in the pyrite system is well-known and is responsible for AMD generation, causing environmental damage. However, if the system is applied in a controlled manner, pyrite can be used as an industrial reagent to treat hexavalent chromium wastewater.

The system reduction development was based on natural reactions, where a pyrite concentrate produced in coal mining was “dissolved” by hexavalent chromium, based on electrochemical interactions on the surface of pyrite with metallic ions in solution. As the electrochemical potential of hexavalent chromium is stronger than iron, the chromium is reduced first and released into the solution as iron, which acts as a chemical coagulant to promote the reduction of the metallic contents in the wastewater.

The sludge obtained by the treatment system, after being dried, ground, and calcined, had mineralogical characteristics and colour properties similar to pigments applied in the ceramic industry. However, the chemical compositions and particle sizes were different because sources of chromium, iron, and zinc were not controlled and the milling process was less effective, both of which factors have great impacts on colour and dispersion in ceramic applications.

The treated effluent showed total neutralization of hexavalent chromium, i.e., the concentration of the main hazardous component (Cr⁶⁺) was below the amount detectable by standard methods for the examination of water and wastewater [63]. Trivalent chromium was found to have a concentration of 0.8 mg/L in the effluent—less than the 1.0 mg/L maximum limit required by environmental legislation (CONAMA 430/2011). Additionally, the effluent generated had characteristics of low turbidity, colourlessness, and a pH close to neutral (pH: 6.70). Thus, the effluent could be discharged into the environment without causing damage [76].

One of the disadvantages of the process is that the pyrite loses its reduction efficiency, requiring time to breathe; that is, after the treatment of a certain amount of effluent, the pyrite saturates, losing its power of reduction, and has to be exposed to the air for natural oxidation to occur. Future process optimization is needed, in which the pyrite saturation times and characterizations of the same must be determined; the influence of parameters (effluent flow rate, pH variation, Cr concentration variations, among others) on pigment generation must be determined; the cost of the process should be evaluated in more detail; and, finally, a business plan must be developed and scaled up.