Characterization of waste sludge pigment from production of ZnCl2

minerals Article Characterization of Waste Sludge Pigment from Production of ZnCl2 Hana Ovˇcaˇcíková1,* , Marek Veliˇcka 1, Petra Maierová1, Jozef Vlˇcek 1, Jonáš Tokarský2,3 and Tomᚡ Cegan 4   Citation: Ovˇcaˇcíková, H.; Veliˇcka, M.; Maierová, P.; Vlˇcek, J.; Tokarský, J.; ˇ Cegan, T. Characterization of Waste Sludge Pigment from Production of ZnCl2.Minerals 2021,11, 313. https://doi.org/10.3390/min11030313 Academic Editor: Anna Candida Felici and Lucilla Pronti Received: 14 February 2021 Accepted: 15 March 2021 Published: 17 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Thermal Engineering, Faculty of Materials Science and Technology, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] (M.V.); [email protected] (P.M.); [email protected] (J.V.) 2Nanotechnology Centre, CEET, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] 3Institute of Environmental Technology, CEET, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic 4Department of Non-Ferrous Metals, Refining and Recycling, Faculty of Materials Science and Technology, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] *Correspondence: [email protected]; Tel.: +42-05-9732-1523 Abstract: This study is focused on the treatment of waste sludge from a zinc chloride production in order to prepare iron-rich pigments usable for a production of glazes. In galvanizing plants, yellow waste sludge containing significant amount of ZnO, Cl, and Fe 2 O 3 , is formed. This raw waste sludge cannot be used as a pigment in glaze. Therefore, three methods of treating this material were proposed: (a) washing with H 2 O, (b) calcination at 180 ◦ C and washing by H 2 O, and (c) calcination at 900 ◦ C and washing by H 2 O. These methods helped to reduce Zn and Cl content up to 97%. According to X-ray fluorescence analysis percentage of Fe 2 O 3 increased from ~41% to ~98%. X-ray power diffraction analysis confirmed the formation of α -Fe 2 O 3 (hematite) in the pigment prepared. Scanning electron microscopy with Energy dispersive X-ray analysis showed clusters of rounded particles, and also the change in size of particles after calcination was observed. Particle size, specific surface area, and density measurements together with thermogravimetric and differential thermal analyses were performed. Pigments prepared from the waste sludge were added to transparent glaze in amounts of 1, 5, 10, and 15 wt.%. Pigment-containing glazes were applied by spraying on fired ceramic tiles and then fired at 1060 ◦ C. Color of glazes was determined by (Commission Internationale de l’Eclairage) CIE L*a*b* coordinates as colorless, light brown shades, brown-red, brown-yellow, and deep red-brown. Comparison with colors of glazes prepared using commercial pigments was also performed. Waste sludge can be used to prepare pigments and glazes containing pigments as an alternative to commercial products. Keywords: pigment; Fe sludge; ZnCl2; calcination; glaze 1. Introduction Preparation of glaze and pigment is an excellent alternative for recycling materials. The global pigment consumption in the industry is expected to be 282.7 thousand t in 2023/$8 bn, and, in Europe, it is expected to increase to approximately 85 thousand t. Pigments are widely applied in numerous industrial sectors, such as civil engineering, production of paper, paints, dyes, ceramic materials (18 thousand t in 2023), plastic materials, etc. This is the consumption for example: of mixed metal oxide pigments (60 thousand t), pearlescent pigments (71 thousand t), organic pigments (44 thousand t), metallic pigments (94 thousand t), and specialty pigments (14 thousand t) [ 1 ]. Pigments can be classified as inorganic, organic, and synthetic. They must fulfill three main requirements: thermal stability, chemical stability, and high colorings power [2]. Minerals 2021,11, 313. https://doi.org/10.3390/min11030313 https://www.mdpi.com/journal/minerals Minerals 2021,11, 313 2 of 16 Inorganic pigments exhibit covering properties, coloring properties, or other special features, and can be divided as follows: (a) white pigments, (b) black pigments, and (c) colored pigments. Their other special features are the anticorrosion properties (ferrite pigments) [ 3 ], but pigments can be also metallic, nacreous, luminescent [ 4 ], magnetic [ 5 ], or anti-rust [ 6 ]. The quality of pigment depends on the optical and physical properties. Inorganic pigments are powder substances coloring the environment (binder) in which where they are dispersed, or coloring the surface on which they are applied. Ceramic pigments are compounds of inorganic origin with various crystal structures. Ceramic pigments are formed by a thermally stable host lattice (MgAl 2 O 4 , TiZn 2 O 4 , ZrSiO 4 , TiO 2 , etc.), which is colorless in its pure form, and a chromophore determining the color of the resulting pigment and refractive indices of individual compounds. This property is very important because high refractive index of the pigment particles relative to the refractive index of the medium where it is dispersed indicates the quality of the pigment. By changing the amount of the individual compounds, or changing the stoichiometric representation of the elements in the structure, it is possible to prepare pigment shades for coloring ceramic glaze. Varying calcination conditions can also be used to achieve different color shades. The most important property of the pigment is the color (the ability of pigmentation), and the ability to get dispersed in a matrix. The thermal stability of pigments in a ceramic material or engobe is determined to be between 1200 and 1300 ◦ C. The pigment in the glaze and the pigment for decoration must be stable at temperatures of 1000–1200 ◦ C and 625–775 ◦C, respectively [7–9]. Iron pigments, both natural and synthetic, have low cost of production and significant advantages for industrial purposes. The treatment of iron pigment can result in changes in color, such as black/magnetite, red/hematite, brown/maghemite, yellow/goethite, and orange/lepidocrocite [ 10 , 11 ]. The natural iron pigments comprise the following compounds: limonite (Fe 2 O 3 .xH 2 O) from yellow to red ocher, goethite (Fe 3+ O(OH))—redbrown, hematite ( α -Fe 2 O 3 )—metallic grey ferrous mica. Naturally-hydrated Fe 3+ oxides were used as pigments already in an Early Stone Age. Many inorganic pigments are prepared with metallic oxides or salts. Interesting is the preparation of ferrous pigments by chemical means from waste raw materials, using precipitation reactions, in the form of ferrous precursors, e.g., magnetite (Fe 3 O 4 ) [ 12 , 13 ], goethite α -FeO (OH), hematite ( α -Fe 2 O 3 ) [ 14 – 16 ], and maghemite γ -Fe 2 O 3 [ 17 , 18 ]. α -Fe 2 O 3 , which can be further prepared, e.g., by the hydrothermal approach [ 19 ], sol-gel process, chemical precipitation, high-temperature thermal oxidation, etc. [20]. The path to the preparation of iron-rich pigments begins with the processing of ZnCl 2 . The ZnCl 2 is a crystalline and hygroscopic powder exhibiting thermochromic properties. ZnCl 2 is anhydrous with a density of 1400–1700 kg/m 3 , with pH value of 5. The color of the powder is white or slightly greyish. Heated to a temperature of 500–700 ◦C, molten anhydrous ZnCl 2 dissolves metallic zinc. When this melt is rapidly cooled, yellow diamagnetic glass is formed. Raman spectroscopy analyses show that the yellow glass formed contains zinc ions. As the temperature increases, the color changes from white to yellow. This change in color is caused by lattice imperfections. Oxygen molecules are lost. The reaction of ZnO with acids proceeds as follows Equation (1) and in alkaline medium, and the following reaction occurs, as in Equation (2) [21]: ZnO + 2HCl = H2O + ZnCl2, (1) ZnO + 2NaOH + H2O→Na2[Zn(OH)4]]. (2) ZnCl 2 is used in industry for metal surface treatment and electroplating, as an electrolyte or ion exchanger, in chemical syntheses, etc. [ 22 – 25 ]. Industrially, ZnCl 2 is prepared from so-called overhaul acids, which are obtained in galvanizing plants by dissolving zinc coating on poorly galvanized objects (and aids and tools) in HCl. The Fe content Minerals 2021,11, 313 3 of 16 in overhaul acids thus prepared is usually >1.5%. At the beginning of production, Fe is neutralized by the so-called zinc ash, which is also a waste product from galvanizing plants. Subsequently, the iron ions are oxidized by H 2 O 2 . Because Fe 2+ ions hydrolyze at higher pH than Zn 2+ ions, it is necessary to achieve oxidation of all Fe to the oxidation state III. Hydrolytic precipitation is based on the different stability of the hydroxide precipitates depending on the pH of the solution. The hydrolytic precipitation of Fe can be described as follows in Equation (3): Fe3+ +3H2O→Fe(OH)3+ 3H+. (3) The precipitate formed is filtered on a sludge filter press. Subsequently, metals nobler than Zn are removed from ZnCl 2 solution by cementation process. Zn powder is used as a cementing agent. The efficiency and kinetics of the cementation process increase with increasing temperature, and the solution pH required lies in the range of 4 to 5. The reaction can be described by the following general mechanism in Equation (4): Zn + Me2+ →Me + Zn2+. (4) This study is focused on the treatment of waste sludge from the ZnCl 2 production in order to prepare iron pigments usable for a production of glazes. The waste sludge primarily contains ZnO, Cl, and Fe 2 O 3 . In its original form, it is yellow, due to the high percentage of Cl. To obtain pigment of good quality, the waste sludge calcination at 180 ◦ C and 900 ◦ C, and subsequent washing in water was proposed. The choice of temperatures is based on previous experiments performed by our research group. The materials studied in this work were characterized by a wide range of methods (X-ray powder diffraction (XRPD), thermogravimetric analysis (TG), differential thermal analysis (DTA), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), X-ray fluorescence analysis (XRF), Brunauer-Emmett-Teller (BET) surface area analysis). Particle size distribution and density were also determined. Iron-rich pigments successfully prepared from the waste sludge were added to a transparent glaze and sprayed on ceramic tiles. The firing was performed at 1060 ◦ C. The described procedure led to obtaining glazes with different color and intensity, with a compact appearance, gloss and without visible defects on the surface. Colorimetric analysis was used to compare colors of glazes prepared using our pigments with colors of glazes prepared using commercial pigments. This study shows how the waste sludge can be used to prepare products—pigments and pigment-containing glazes—alternative to commercial products. 2. Materials and Methods 2.1. Materials All materials were obtained from the Czech Republic. Ceramic white slurry (labeled as CS) was obtained from Pávek Keramika, spol. s r.o. (Doubravice nad Svitavou, Czech Republic). Transparent glaze (labeled as TG) is a commercial white powder produced by Glazura s.r.o. company (Dobˇrín, Czech Republic). In the experiment, three types of commercial pigment (labeled as SP1, SP2, SP3) with visible different color SP1 (red), SP2 (light brown), and SP3 (dark brown) were used (Figure 1). Materials described above were used for recipe of glazes shown in Table 1, and chemical composition of these raw materials is presented in Table 2, part 3.1. These pigments were applied in their original form. These are synthetic inert pigments, without any other additives (fillers), very finely ground, with very good colorability, opacity, and particles distribution, resistant to weathering and chemicals, non-toxic, and thermally stable up to 800 ◦ C. Pigments are determined according to ˇ CSN EN 12878—Pigments for the colouring of building materials based on cement and/or lime. (European Standard specifying requirements and testing methods for pigment intended to use in the coloring of building materials based on cement and cement lime combinations. List of relevant pigments for this application is available in European Standards EN 459 1 and EN 459 2). Minerals 2021,11, 313 4 of 16 Pigments typically belong to one of the following classes: synthetic oxides, natural oxides, or iron hydroxides. Minerals 2021, 11, 313 4 of 16 Figure 1. Commercial pigments in powder form used for preparation of glazes. Table 1. Recipe of glazes prepared from waste Fe sludge (WFS) and commercial pigment (SP, standard pigment). Recipe Samples Amount of the Materials (wt.%) Transparent Glaze (TG) Waste Fe Sludge (WFS) Standard Pigments (SP) WFS/0 WFS/180 WFS/900 SP1 SP2 SP3 R1 GP01 99 1 GP02 95 5 GP03 90 10 GP04 85 15 GPX 90 10 GOKP 90 10 R2 GPF1 90 10 GPF2 90 10 GPF3 90 10 Table 2. Chemical composition of materials. Amount of Oxides in Raw Materials (wt.%) Oxides Ceramic Slurry (CS) Waste Fe Sludge (WFS) Waste Fe Sludge (WFS/0) Waste Fe Sludge (WFS/180) Transparent Glaze (TG) SP3 (Dark Brown) SP2 (Light Brown) SP1 (Red) Na 2 O 1.22 5.61 4.61 5.50 MgO 2.83 1.12 0.18 <0.3 0.58 0.44 <0.001 Al 2 O 3 12.42 0.20 0.74 0.49 9.10 <0.001 0.44 <0.001 SiO 2 69.57 80.10 0.16 0.80 <0.001 P 2 O 5 <0.0012 0.06 0.16 <0.01 0.022 0.073 <0.001 SO 3 0.19 0.15 0.19 0.71 0.12 0.27 0.33 0.20 K 2 O 2.08 0.88 <0.001 <0.001 <0.001 CaO 4.52 0.02 <0.001 0.30 0.05 0.31 0.01 TiO 2 0.56 <0.001 0.13 0.67 0.59 1.311 MnO 0.07 <0.001 0.006 1.06 0.75 0.13 Fe 2 O 3 4.09 40.92 90.09 97.56 0.22 96.97 95.22 97.807 BaO 0.05 0.05 Cl 22.08 2.47 0.49 0.006 0.017 <0.001 ZnO 29.87 1.52 0.24 0.15 0.13 0.03 PbO 0.05 0.12 <0.001 V 2 O 5 <0.001 0.03 <0.001 Cr 2 O 3 0.14 <0.001 0.12 <0.001 NiO 0.028 0.10 <0.001 CuO <0.001 0.047 <0.001 These pigments were applied in their original form. These are synthetic inert pigments, without any other additives (fillers), very finely ground, with very good colorability, opacity, and particles distribution, resistant to weathering and chemicals, non-toxic, and thermally stable up to 800 °C. Pigments are determined according to ČSN EN 12878— Pigments for the colouring of building materials based on cement and/or lime. (European Figure 1. Commercial pigments in powder form used for preparation of glazes. Table 1. Recipe of glazes prepared from waste Fe sludge (WFS) and commercial pigment (SP, standard pigment). Recipe Samples Amount of the Materials (wt.%) Transparent Glaze (TG) Waste Fe Sludge (WFS) Standard Pigments (SP) WFS/0 WFS/180 WFS/900 SP1 SP2 SP3 R1 GP01 99 1 GP02 95 5 GP03 90 10 GP04 85 15 GPX 90 10 GOKP 90 10 R2 GPF1 90 10 GPF2 90 10 GPF3 90 10 Table 2. Chemical composition of materials. Amount of Oxides in Raw Materials (wt.%) Oxides Ceramic Slurry (CS) Waste Fe Sludge(WFS) Waste Fe Sludge (WFS/0) Waste Fe Sludge (WFS/180) Transparent Glaze (TG) SP3 (Dark Brown) SP2 (Light Brown) SP1 (Red) Na2O 1.22 5.61 4.61 5.50 MgO 2.83 1.12 0.18 <0.3 0.58 0.44 <0.001 Al2O312.42 0.20 0.74 0.49 9.10 <0.001 0.44 <0.001 SiO269.57 80.10 0.16 0.80 <0.001 P2O5<0.0012 0.06 0.16 <0.01 0.022 0.073 <0.001 SO30.19 0.15 0.19 0.71 0.12 0.27 0.33 0.20 K2O 2.08 0.88 <0.001 <0.001 <0.001 CaO 4.52 0.02 <0.001 0.30 0.05 0.31 0.01 TiO20.56 <0.001 0.13 0.67 0.59 1.311 MnO 0.07 <0.001 0.006 1.06 0.75 0.13 Fe2O34.09 40.92 90.09 97.56 0.22 96.97 95.22 97.807 BaO 0.05 0.05 Cl 22.08 2.47 0.49 0.006 0.017 <0.001 ZnO 29.87 1.52 0.24 0.15 0.13 0.03 PbO 0.05 0.12 <0.001 V2O5<0.001 0.03 <0.001 Cr2O30.14 <0.001 0.12 <0.001 NiO 0.028 0.10 <0.001 CuO <0.001 0.047 <0.001 Minerals 2021,11, 313 5 of 16 The material studied was a waste Fe sludge obtained as a secondary product during production of ZnCl 2 . Untreated form of the waste sludge (labeled as WFS) from the production of zinc oxide (see Figure 5, yellow color samples) is not suitable as a pigment. Therefore, three methods have been proposed to prepare a pigment applicable to glazes: (a) waste Fe sludge washed by H 2 O (labeled as WFS/0 and in an experiment called as the original form), (b) waste Fe sludge washed by H 2 O and calcined at 180 ◦ C (labeled as WFS/180), and (c) waste Fe sludge washed by H 2 O and calcined at 900 ◦ C (labeled as WFS/900). 2.2. Sample Preparation Two recipes of prepared glazes (Table 1) were mixed. Recipe 1 contains 6 different mixtures of glazes. As a pigment, 1, 5, 10, or 15 wt.% of the waste Fe sludge was added into the transparent glaze (Table 2). Recipe 2 can be called “comparative” because of mixing transparent glaze with 10 wt.% of commercial (standard) pigments. Individually prepared mixtures (WFS/0; WFS/180; WFS/900 + TG) and (SP1; SP2; SP3; + TG) were homogenized in ball mill device with contain the content of 50% alumina balls by wet milling (5% H 2 O) for 30 min. The wet mixture was sieved through a 0.2 mm sieve. The obtained suspension was mixed with water to reach the composition expressed by weight of glaze. The range of litter weight was determined to be from 1442 to 1550 g.L −1 . The liter gravity of glaze was determined in a 1 L graduated cylinder. First, the weight of the empty container was determined. Subsequently, the container with the glaze in it was weighed and the weight of the empty container was subtracted. Ceramic tiles were prepared by casting into gypsum form having dimensions 10 ×10 cm2 (fireclay slurry with a water content of approximately 34%). The ceramic tiles were dried at 105 ◦ C and fired in an electric resistance furnace (LAC M125/13HTCeramic, CZ) at a maximum temperature of 900 ◦ C for 60 min. The glazes were applied by spraying method to ensure of compact layers on the surfaces of tiles. In this experiment, the final firing was gradually specified. Firing I: This was originally designed for a temperature of 1060 ◦ C, at a rate of 3 ◦ C/min, and it was without a dwell at this temperature. The samples glazed in this way showed craters on the surface. Craters are usually formed due to gas leakage from the glaze. In this case, the reason might have been the presence of ZnO in the pigment. Firing II: The optimal firing curve for this type of pigment was with two dwells: 30 min/600 ◦C and 60 min/1060 ◦C. 2.3. Characterization Techniques • Chemical composition of raw materials was determined using energy dispersive fluorescence spectrometer SPECTRO XEPO (SPECTRO Analytical Instruments GmbH, Kleve, Germany), equipped with 50 Watt Pd X-ray tube. • The phase composition of samples was characterized using a Bruker D8 Advance Xray powder diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). The diffraction patterns in the range of 5 ◦ to 70 ◦ 2 θ were recorded under CoK α ( λ = 1.78897 Å, U = 35 kV , I = 25 mA) irradiation with scanning rate 2 ◦ /min using fast positionsensitive detector VÅNTEC1. • The morphology of the particles was characterized using the scanning electron microscope (SEM) QUANTA 450 FEG, (FEI, Hillsboro, OR, USA); the images were collected using a secondary electron detector. • The particle size distribution (PSD) was analyzed using Mastersizer (Malvern Panalytical Ltd., Malvern, UK). Measurements were performed in an aquatic environment, and ultrasound was used for homogenization of the suspension. • The characterization of the thermal behavior was performed on TG/DTAanalyzer STA504 (TA Instruments, New Castle, Delaware, USA). The sample of scale placed in alumina crucible was analyzed in a temperature range from 21 to 1100 ◦ C in the dynamic atmosphere of N2(5 L·h−1), and the heating rate was 10 K·min−1. Minerals 2021,11, 313 6 of 16 • The reflectance spectra of the final glazes in the spectral range 400–700 nm were obtained using MiniScan EZ0828 spectrometer (HunterLab, Reston, VA, USA), model 45 ◦ /0 ◦ , small observation area. The color of the glazes was expressed using CIE L*a*b* coordinates calculated for 10◦observer and D65 illuminant. • Specific surface area was determined by BET method-equipment (Quantachrome NovaWin Instrument-Acquisition and Reduction for NOVA instrument, analysis gas nitrogen, Graz, Austria). • Density of samples was determined by Pycnomatic and Pycnomatic ATC (Helium pycnometer) POROTEC GmbH, Coconut Creek, FL, USA. 3. Results and Discussion 3.1. Chemical Composition Chemical composition of original materials (CS, WFS, FWS/0, WFS/180, and TG) used in experiments is provided in Table 2. The main oxides in the ceramic slurry (CS) are SiO 2 and Al 2 O 3 ( over 83 wt.%), and ~4 wt.% of Fe 2 O 3 and CaO was also found. Transparent glaze (TG) contains ~80 wt.% of SiO 2 , 9 wt.% of Al 2 O 3 and 5 wt.% of Na 2 O. The waste Fe sludge (WFS) in original form (presented as yellow color in Figure 5) contains 41 wt.% of Fe 2 O 3 and high content of chlorine (22 wt.%) and zinc oxide (up to 29 wt.%). After washing the waste sludge by H 2 O (WFS/0), the amount of chlorine and ZnO significantly decreased to 2.4 wt.% and 1.5 wt.%, respectively (Table 2). On the contrary, the amount of iron oxide reached 90 wt.% at sample WFS/0 and more than 97 wt.% at sample WFS/180 (Table 2). The differences in the composition of the WFS/180 and WFS/900 samples did not exceed 0.5 wt.%. Commercial pigments are prepared by synthetic ways. Large amount of Fe 2 O 3 expected in these pigments was confirmed by chemical analysis (Table 2). The red color pigment (SP1) contains over 97 wt.% of Fe 2 O 3 . The light brown pigment SP2 contains 95 wt.% of Fe2O3, and the last one, dark brown pigment SP3, contains 96 wt.% of Fe2O3. 3.2. Phase Composition of Commercial Pigments X-ray diffraction analysis showed (Figure 2) showed the most intense reflections of hematite ( α -Fe 2 O 3 ) [ 26 – 28 ] in all three pigments. Moreover, magnetite was detected in samples SP2 (Figure 2b) and SP3 (Figure 2c). Only one phase was found in the pigment SP1 (Figure 2a), i.e., hematite with the most intense reflection at the position of 38 ◦ 2 θ . In the case of light brown pigment SP2, the presence of three phases was found: α -hematite and magnetite at position 42 ◦ 2 θ , and maghemite ( γ -Fe 2 O 3 ) at position 13 ◦ 2 θ . The color of maghemite corresponds to the color of the SP2 pigment. In the case of the dark brown pigment SP3, dominant reflections of hematite and magnetite at the position 42–44 ◦ 2 θ are present. In addition, reflections of wüstite (FeO) can be observed at the following positions: 42◦, 50◦, and 74◦2θ. 3.3. Granulometry of Commercial Pigments The synthetic pigments SP1, SP2, and SP3 are very fine powders. All curves on ( Figure 3 ) reveal very similar grain size. Red pigments SP1 have grained size Dv(10) = 0.95 µm , Dv(50) = 9.38 µm , and Dv (90) = 66.7 µ m. The parameters of light brow pigment SP2 are Dv (10) = 0.47 µ m, Dv (50) = 5.83 µ m, and Dv (90) = 19.4 µ m, while the parameters of the distribution curve of dark brown pigment SP3 are Dv (10) = 0.43 µ m, Dv (50) = 7.02 µ m, and Dv(90) = 28.4 µm . The optimum size of pigment particles for most applications is 0.1–10 µm . From the measured data, it can be concluded that the synthetic pigments SP1, SP2, SP3 have the properties declared by the manufacturer. 3.4. SEM Analysis of Commercial Pigments Commercial pigments show exhibit a considerable heterogeneity of the particles, as shown by SEM analysis (Figure 4). The particles of these pigments have an irregular shape (see Figure 4B,C-1, with scale 20 µ m). Image with scale 200 µ m (Figure 4C-2) also Minerals 2021,11, 313 7 of 16 do not show the presence of spherical grains. Based on SEM analysis, it can be stated that, in terms of particle shape (affecting pigment dispersibility), commercial pigments SP1 (Figure 4A) SP2 (Figure 4B), and SP3 (Figure 4C-1,C-2) are not ideal powder systems. Minerals 2021, 11, 313 7 of 16 Figure 2. XRD patterns of (a) SP1 pigment (red color); (b) SP2 pigment (light brown color); and (c) SP3 pigment (dark brown color). 3.3. Granulometry of Commercial Pigments The synthetic pigments SP1, SP2, and SP3 are very fine powders. All curves on (Figure 3) reveal very similar grain size. Red pigments SP1 have grained size Dv (10) = 0.95 µm, Dv (50) = 9.38 µm, and Dv (90) = 66.7 µm. The parameters of light brow pigment SP2 are Dv (10) = 0.47 µm, Dv (50) = 5.83 µm, and Dv (90) = 19.4 µm, while the parameters of the distribution curve of dark brown pigment SP3 are Dv (10) = 0.43 µm, Dv (50) = 7.02 µm, and Dv (90) = 28.4 µm. The optimum size of pigment particles for most applications is 0.1–10 µm. From the measured data, it can be concluded that the synthetic pigments SP1, SP2, SP3 have the properties declared by the manufacturer. Figure 3. Grain size distribution of pigment: 1) SP1 (red); 2) SP2 (light brown); 3) SP3 (dark brown). 3.4. SEM Analysis of Commercial Pigments Commercial pigments show exhibit a considerable heterogeneity of the particles, as shown by SEM analysis (Figure 4). The particles of these pigments have an irregular shape (see Figures 4B and 4C-1, with scale 20 µm). Image with scale 200 µm (Figure 4C-2) also do not show the presence of spherical grains. Based on SEM analysis, it can be stated that, in terms of particle shape (affecting pigment dispersibility), commercial pigments SP1 (Figure 4A) SP2 (Figure 4B), and SP3 (Figure 4C-1 and 4C-2) are not ideal powder systems. Figure 2. XRD patterns of ( a ) SP1 pigment (red color); ( b ) SP2 pigment (light brown color); and ( c ) SP3 pigment (dark brown color). Minerals 2021, 11, 313 7 of 16 Figure 2. XRD patterns of (a) SP1 pigment (red color); (b) SP2 pigment (light brown color); and (c) SP3 pigment (dark brown color). 3.3. Granulometry of Commercial Pigments The synthetic pigments SP1, SP2, and SP3 are very fine powders. All curves on (Figure 3) reveal very similar grain size. Red pigments SP1 have grained size Dv (10) = 0.95 µm, Dv (50) = 9.38 µm, and Dv (90) = 66.7 µm. The parameters of light brow pigment SP2 are Dv (10) = 0.47 µm, Dv (50) = 5.83 µm, and Dv (90) = 19.4 µm, while the parameters of the distribution curve of dark brown pigment SP3 are Dv (10) = 0.43 µm, Dv (50) = 7.02 µm, and Dv (90) = 28.4 µm. The optimum size of pigment particles for most applications is 0.1–10 µm. From the measured data, it can be concluded that the synthetic pigments SP1, SP2, SP3 have the properties declared by the manufacturer. Figure 3. Grain size distribution of pigment: 1) SP1 (red); 2) SP2 (light brown); 3) SP3 (dark brown). 3.4. SEM Analysis of Commercial Pigments Commercial pigments show exhibit a considerable heterogeneity of the particles, as shown by SEM analysis (Figure 4). The particles of these pigments have an irregular shape (see Figures 4B and 4C-1, with scale 20 µm). Image with scale 200 µm (Figure 4C-2) also do not show the presence of spherical grains. Based on SEM analysis, it can be stated that, in terms of particle shape (affecting pigment dispersibility), commercial pigments SP1 (Figure 4A) SP2 (Figure 4B), and SP3 (Figure 4C-1 and 4C-2) are not ideal powder systems. Figure 3. Grain size distribution of pigment: (1) SP1 (red); (2) SP2 (light brown); (3) SP3 (dark brown). Minerals 2021, 11, 313 8 of 16 Figure 4. SEM images of commercial pigments (A) red pigment SP1 with scale 20 µm; (B) light brown pigment SP2 with scale 20µm; (C-1) dark brow pigment SP3 with scale 20 µm and (C-2) dark brown pigment SP3 with scale 200 µm.. 3.5. The Treatment of Waste Fe Sludge. The waste Fe sludge (Figure 5, labeled as WFS/0) was delivered in piece form containing over 23% of moisture. The aim was not only to achieve as much iron content in the mixture as possible but also to eliminate chlorine content. Three methods were designed for this process, with the resulting color effect of individual pigments (see Figure 5): 1. WFS/0—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS, 2. WFS/180—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS and calcined at 180 °C at the mode of 15 °C/min for 10 h/180 °C, and 3. WFS/900—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS and calcined at 900 °C at the mode of 15 °C/min for 3 h/900 °C. Figure 5. Form and color of the untreated waste Fe sludge (WFS), waste Fe sludge washed in water (WFS/0), waste Fe sludge calcined at 180 °C (WFS/180), and waste Fe sludge calcined at 900 °C (WFS/900). Figure 5 show untreated (labeled as WFS) and treated pigments (labeled as WFS/0, WFS/180, and WFS/900). Washing the waste with water removes soluble impurities. Washing and calcination lead to a partial reduction of the Zn and Cl amount contained in the sludge. After calcination, the amount of H 2 O decreased. WFS/180 and WFS/900 contained 2.2 wt.% and 0.2 wt.% of H 2 O, respectively. Density of WFS/0, WFS/180, and WFS/900 was 3.85 g/cm 3 , 4.95 g/cm 3 , and 4.46 g/cm 3 , respectively. During calcination of WFS/180, iron oxyhydroxides dehydrated to Fe 2 O 3 . This sludge must be well washed to remove most of the ZnCl 2 contained in the sludge. As mentioned before, the higher the calcination temperature, the more significant dehydration occurred. By calcination at 900 °C, the washed sludge was stabilized. The prepared pigments WFS/180 and WFS/900 are very fine powders. This is proven by their specific surface area (SSA) and better absorption of loose moisture compared to the WFS/0 sample, which was slightly coarser. Cl − bound in the structure and α-FeOOH were released during heat treatments as HCl. At the same time, the amount of Zn is reduced by up to 97%. During calcination of the WFS/900 sample, the Cl − in the mixture was significantly eliminated (by almost 99%). While the original waste Fe sludge WFS/0 contained ~41 wt.% of Fe 2 O 3 , usable waste sludge containing up to 90 wt.% of α-Fe 2 O 3 was obtained by the calcination. Figure 4. SEM images of commercial pigments ( A ) red pigment SP1 with scale 20 µ m; ( B ) light brown pigment SP2 with scale 20µm; (C-1) dark brow pigment SP3 with scale 20 µm and (C-2) dark brown pigment SP3 with scale 200 µm. 3.5. The Treatment of Waste Fe Sludge The waste Fe sludge (Figure 5, labeled as WFS/0) was delivered in piece form containing over 23% of moisture. The aim was not only to achieve as much iron content in the mixture as possible but also to eliminate chlorine content. Three methods were designed for this process, with the resulting color effect of individual pigments (see Figure 5): 1. WFS/0—Waste Fe sludge washed by H2Oby1LH2O/300 g of WFS, 2. WFS/180—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS and calcined at 180 ◦C at the mode of 15 ◦C/min for 10 h/180 ◦C, and Minerals 2021,11, 313 8 of 16 3. WFS/900—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS and calcined at 900 ◦C at the mode of 15 ◦C/min for 3 h/900 ◦C. Minerals 2021, 11, 313 8 of 16 Figure 4. SEM images of commercial pigments (A) red pigment SP1 with scale 20 µm; (B) light brown pigment SP2 with scale 20µm; (C-1) dark brow pigment SP3 with scale 20 µm and (C-2) dark brown pigment SP3 with scale 200 µm.. 3.5. The Treatment of Waste Fe Sludge. The waste Fe sludge (Figure 5, labeled as WFS/0) was delivered in piece form containing over 23% of moisture. The aim was not only to achieve as much iron content in the mixture as possible but also to eliminate chlorine content. Three methods were designed for this process, with the resulting color effect of individual pigments (see Figure 5): 1. WFS/0—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS, 2. WFS/180—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS and calcined at 180 °C at the mode of 15 °C/min for 10 h/180 °C, and 3. WFS/900—Waste Fe sludge washed by H 2 O by 1 L H 2 O/300 g of WFS and calcined at 900 °C at the mode of 15 °C/min for 3 h/900 °C. Figure 5. Form and color of the untreated waste Fe sludge (WFS), waste Fe sludge washed in water (WFS/0), waste Fe sludge calcined at 180 °C (WFS/180), and waste Fe sludge calcined at 900 °C (WFS/900). Figure 5 show untreated (labeled as WFS) and treated pigments (labeled as WFS/0, WFS/180, and WFS/900). Washing the waste with water removes soluble impurities. Washing and calcination lead to a partial reduction of the Zn and Cl amount contained in the sludge. After calcination, the amount of H 2 O decreased. WFS/180 and WFS/900 contained 2.2 wt.% and 0.2 wt.% of H 2 O, respectively. Density of WFS/0, WFS/180, and WFS/900 was 3.85 g/cm 3 , 4.95 g/cm 3 , and 4.46 g/cm 3 , respectively. During calcination of WFS/180, iron oxyhydroxides dehydrated to Fe 2 O 3 . This sludge must be well washed to remove most of the ZnCl 2 contained in the sludge. As mentioned before, the higher the calcination temperature, the more significant dehydration occurred. By calcination at 900 °C, the washed sludge was stabilized. The prepared pigments WFS/180 and WFS/900 are very fine powders. This is proven by their specific surface area (SSA) and better absorption of loose moisture compared to the WFS/0 sample, which was slightly coarser. Cl − bound in the structure and α-FeOOH were released during heat treatments as HCl. At the same time, the amount of Zn is reduced by up to 97%. During calcination of the WFS/900 sample, the Cl − in the mixture was significantly eliminated (by almost 99%). While the original waste Fe sludge WFS/0 contained ~41 wt.% of Fe 2 O 3 , usable waste sludge containing up to 90 wt.% of α-Fe 2 O 3 was obtained by the calcination. Figure 5. Form and color of the untreated waste Fe sludge (WFS), waste Fe sludge washed in water (WFS/0), waste Fe sludge calcined at 180 ◦ C (WFS/180), and waste Fe sludge calcined at 900 ◦ C (WFS/900). Figure 5show untreated (labeled as WFS) and treated pigments (labeled as WFS/0, WFS/180, and WFS/900). Washing the waste with water removes soluble impurities. Washing and calcination lead to a partial reduction of the Zn and Cl amount contained in the sludge. After calcination, the amount of H 2 O decreased. WFS/180 and WFS/900 contained 2.2 wt.% and 0.2 wt.% of H 2 O, respectively. Density of WFS/0, WFS/180, and WFS/900 was 3.85 g/cm 3 , 4.95 g/cm 3 , and 4.46 g/cm 3 , respectively. During calcination of WFS/180, iron oxyhydroxides dehydrated to Fe 2 O 3 . This sludge must be well washed to remove most of the ZnCl2contained in the sludge. As mentioned before, the higher the calcination temperature, the more significant dehydration occurred. By calcination at 900 ◦ C, the washed sludge was stabilized. The prepared pigments WFS/180 and WFS/900 are very fine powders. This is proven by their specific surface area (SSA) and better absorption of loose moisture compared to the WFS/0 sample, which was slightly coarser. Cl − bound in the structure and α -FeOOH were released during heat treatments as HCl. At the same time, the amount of Zn is reduced by up to 97%. During calcination of the WFS/900 sample, the Cl−in the mixture was significantly eliminated (by almost 99%). While the original waste Fe sludge WFS/0 contained ~41 wt.% of Fe 2 O 3 , usable waste sludge containing up to 90 wt.% of α -Fe 2 O 3 was obtained by the calcination. 3.6. Granulometry of Waste Fe Sludge Inorganic pigments are largely present in the powder form. The ideal powder material contains only particles of the same size; however, the actual system consists of particles of different sizes. Particle size distribution is a very important criterion of powder pigments, as the grain size has a fundamental effect on the optical properties of pigments. It is associated with the scattering of light on the particles, so it highly affects the resulting opacity and color. The sample was measured in an aqueous medium, and with regard to the presence of clumps in the investigated pigments, ultrasound was used for better dispersion. The results are presented by distribution curves in Figure 6. Curves 2 and 3 have variable grain size. Pigment WFS/0 has grain sizes Dv (10) = 6.14 µ m, Dv (50) = 13.8 µ m, and Dv (90) = 30.3 µ m. The parameters of WFS/180 are Dv (10) = 0.898 µ m, Dv(50) = 2.61 µm , and Dv (90) = 7.7 µ m, while the parameters of the distribution curve for pigment WFS/900 are Dv (10) = 1.98 µ m, Dv (50) = 44.4 µ m, and Dv (90) = 1950 µ m. The particles can be divided into coarse—over 10 µ m; large—10 to 3 µ m; medium—3 to 1 µ m; fine—1 to 0.3 µm ; and very fine—below 0.3 µm. 3.7. SEM Analysis of Waste Fe Sludge The particle size was changed by calcination. The most common morphology of hematite particles obtained by peptization method and calcined at 500 ◦ C was truncated rhombohedrons, cubes, and octahedrons [29]. Minerals 2021,11, 313 9 of 16 Minerals 2021, 11, 313 9 of 16 3.6. Granulometry of Waste Fe Sludge Inorganic pigments are largely present in the powder form. The ideal powder material contains only particles of the same size; however, the actual system consists of particles of different sizes. Particle size distribution is a very important criterion of powder pigments, as the grain size has a fundamental effect on the optical properties of pigments. It is associated with the scattering of light on the particles, so it highly affects the resulting opacity and color. The sample was measured in an aqueous medium, and with regard to the presence of clumps in the investigated pigments, ultrasound was used for better dispersion. The results are presented by distribution curves in Figure 6. Curves 2 and 3 have variable grain size. Pigment WFS/0 has grain sizes Dv(10) = 6.14 µm, Dv(50) = 13.8 µm, and Dv(90) = 30.3 µm. The parameters of WFS/180 are Dv(10) = 0.898 µm, Dv(50) = 2.61 µm, and Dv(90) = 7.7 µm, while the parameters of the distribution curve for pigment WFS/900 are Dv(10) = 1.98 µm, Dv(50) = 44.4 µm, and Dv(90) = 1950 µm. The particles can be divided into coarse—over 10 µm; large—10 to 3 µm; medium—3 to 1 µm; fine—1 to 0.3 µm; and very fine—below 0.3 µm. Figure 6. Grain size distribution of pigment waste Fe sludge: 1) original (WFS/0); 2) calcined at 180 °C (WFS/180); 3) calcined at 900 °C (WFS/900). 3.7. SEM Analysis of Waste Fe Sludge The particle size was changed by calcination. The most common morphology of hematite particles obtained by peptization method and calcined at 500 °C was truncated rhombohedrons, cubes, and octahedrons [29]. For inorganic pigments, it is suitable that particle sizes range from 0.1 to 10 µm [30], so the pigments have good covering and absorption qualities. Figure 7 shows WFS/0 sample morphology as determined by SEM by all samples and EDAX by (Figure 7A-1). SEM images show a not quite regular round particle system (Figure 7C-2). Moreover, particles larger than 1000 µm are also included in this sample, as determined by granulometry (Figure 6). Figure 7A-1 shows more clearly that there are round particles of different sizes, formed by clusters of smaller round particles of approximately 20 µm. Thus, if the particles are clustered into even smaller objects, the granulometric curves may be so variable on this basis. Figure 6. Grain size distribution of pigment waste Fe sludge: (1) original (WFS/0); (2) calcined at 180 ◦C (WFS/180); (3) calcined at 900 ◦C (WFS/900). For inorganic pigments, it is suitable that particle sizes range from 0.1 to 10 µ m [ 30 ], so the pigments have good covering and absorption qualities. Figure 7shows WFS/0 sample morphology as determined by SEM by all samples and EDAX by (Figure 7A-1). SEM images show a not quite regular round particle system (Figure 7C-2). Moreover, particles larger than 1000 µ m are also included in this sample, as determined by granulometry (Figure 6). Figure 7A-1 shows more clearly that there are round particles of different sizes, formed by clusters of smaller round particles of approximately 20 µ m. Thus, if the particles are clustered into even smaller objects, the granulometric curves may be so variable on this basis. The chlorine present in the WFS/0 (see Figure 7A-2) comes from the ZnCl 2 purification process. The minimum amount of Al probably comes from a very different compound contained in the sludge precipitate (Figure 7A-2). The highest density, 4.95 g/cm 3 , was found for the WFS/180 sample, while the original untreated WFS/0 sample had the lowest density of all. Thermal calcination led to an increase in density by 28%. With an increase of the temperature, the SSA changes from 6.89 m 2 /g (for WFS/0) to 23.6 m 2 /g (WFS/180) and 14.86 m 2 /g (WFS/900). The SSA of commercial samples was determined to be 23.67 m 2 /g. The SSA value for all iron oxides prepared in this study is comparable to the commercial pigments used in this experiment. 3.8. Phase Analysis of Waste Fe Sludge Results of the XRPD phase analysis are shown in Figure 8. The pattern of the sample WFS/0 (see Figure 8a) does not show any reflections. This sample contains Fe 2 O 3 in amorphous form. Wider reflections of hematite phase were found in the case of sample WFS/180; dominant reflection is in the position 38 ◦ 2 θ (PDF-2 card no. 00-033-064-64). From the point of view of the XRPD measurement, the sample WFS/180 (see Figure 8b) is comparable with the sample SP1. XRPD pattern of the sample WFS/900 (see Figure 8c ) contains crystalline phase of the α -Fe 2 O 3 (hematite) (PDF-2 card no.01-076-45-79) with the most intensive reflection in the position 39 ◦ 2 θ . With increasing temperature of calcination, the reflections are narrower. In some cases, the goethite α -FeOOH transforms into crystalline phase of hematite, present as narrowly shaped peaks [ 31 ]. But the goethite phase was not found in tested pigments (Figure 8). When mixing the pigments with the transparent glaze, certain phase changes occurred. The α -Fe 2 O 3 form is stable up to 800 ◦C , as declared by the manufacturer. When fired at 1060 ◦ C, the magnetite form changes to hematite, which is stable up to 1000 ◦C [24]. 3.9. TG and DTA of Waste Fe Sludge TG a DTA curves for WFS/0 sample are shown in Figure 9. Significant endothermic reactions are present on the temperature at 110 ◦ C, and 562 ◦ C can be connected with moisture decomposition. Hematite form under atmospheric conditions and often the end of oxidative transformation of other iron oxides. Then, with temperature rising, goethite can transform to α -Fe 2 O 3 or the peak is attributed to the oxidation of FeO to Fe 3 O 4 Minerals 2021,11, 313 16 of 16 40. Pekkan, K.; Karasu, B. Evaluation of borax solid wastes in production of frits suitable for fast single-fired wall tile opaque glass-ceramic glazes. Bull. Mater. Sci. 2010,33, 135–144. [CrossRef] 41. Bernardin, A.M. The influence of particle size distribution on the surface appearance of glazed tile. Dyes Pigment. 2009 ,80, 121–124. [CrossRef]