minerals

Article Recovery of Iron from Mill Scale by Reduction with

Krzysztof Nowacki 1,* , Tomasz Maci ˛ag 2 and Teresa Lis 1

1 Department of Production Engineering, Faculty of Materials Engineering, Silesian University of Technology, Akademicka 2A Street, 44-100 Gliwice, Poland; [email protected] 2 Department of Metallurgy and Recycling, Faculty of Materials Engineering, Silesian University of Technology, Akademicka 2A Street, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-32-603-44-12

Abstract: The mill scale is a waste from the iron and steel industry. Due to the high content of iron in the form of oxides, it is an attractive material for the recovery of metallic iron by reduction. The product of mill scale reduction is an iron with a very extended surface and a high affinity for . The smaller iron particles are, the easier it is for spontaneous rapid oxidation, which can be linked to pyrophoricity. This article presents results of experiments using the TG/DTA thermal analysis method aimed at verifying the possibility of recovering iron from the mill scale by a reduction with carbon monoxide at 850 ◦C, 950 ◦C, and 1050 ◦C, taking into account the phenomenon of secondary oxidation in contact with oxygen from air at temperatures of 300 ◦C, 350 ◦C, and 400 ◦C. Two forms of mill scale were used for tests, in the original state and after grinding to develop the surface.

 Keywords: metallurgical waste; mill scale; iron reduction; pyrophoricity; TG/DTA thermal analysis 

Citation: Nowacki, K.; Maci ˛ag,T.; Lis, T. Recovery of Iron from Mill Scale by Reduction with Carbon 1. Introduction Monoxide. Minerals 2021, 11, 529. Protection of the natural environment is one of the biggest challenges for human https://doi.org/10.3390/min11050529 civilization. Constantly growing demand for construction materials causes an increase in the amount of industrial waste. In the European Union, the concept of waste is defined Academic Editors: George in Directive 2008/98/EC [1]. The term “waste” is defined as any substance or object Angelopoulos and which the owner disposes of, intends to dispose of, or is obliged to dispose of. The waste Luis Pérez Villarejo owner is obliged, in the first place, to prevent the generation of waste, and then prepare for the re-use of waste, production of which could not be prevented. Reducing waste Received: 22 April 2021 generation together with waste disposal is the basic principle of rational use of natural and Accepted: 15 May 2021 anthropogenic resources. Published: 17 May 2021 One industrial branch that generates large amounts of waste is metallurgy, especially iron and steel metallurgy, which is the largest producer of semi-finished and finished Publisher’s Note: MDPI stays neutral products [2,3]. As a result of enormous metallurgical production, significant amounts of with regard to jurisdictional claims in waste are generated, such as blast furnace and converter slag, various types of dust and published maps and institutional affil- sludge, scale, and gases [4–9]. Many metallurgical wastes contain a large amount of iron iations. and can be subjected to the processes of this element’s recovery. Wastes containing less iron or impurities go to other recycling processes or are landfilled. This paper presents research related to the reduction of mill scale. Oxidation and formation of scale is an inevitable phenomenon during the hot rolling process of steel [8]. Copyright: © 2021 by the authors. Mill scale harms the surface quality of steel products and mill rolls durability. Moreover, Licensee MDPI, Basel, Switzerland. its formation causes material loss, which, depending on the used rolling technique, ranges This article is an open access article from 1% to 3% [9,10]. Structurally, the rolling scale is a layered and brittle material distributed under the terms and consisting of iron oxides (wustite—FeO, magnetite—Fe O , hematite—Fe O ), with the conditions of the Creative Commons 3 4 2 3 Attribution (CC BY) license (https:// predominant phase in the form of wustite. It comes as a powder composed of small creativecommons.org/licenses/by/ particles, flakes, and chips. In addition to iron compounds, the mill scale may also contain 4.0/). C, Si, Ca, Ni, Al, Mn, S, P, and other metal oxides [10,11]. It is related to the type of

Minerals 2021, 11, 529. https://doi.org/10.3390/min11050529 https://www.mdpi.com/journal/minerals MineralsMinerals 20212021, 11, 11, 529, 529 2 of2 of14 13

productionproduction in inthe the steelworks. steelworks. Mill Mill scale scale is isgenerally generally free free from from hazardous, hazardous, flammable, flammable, radioactive,radioactive, or or explosive explosive substances. substances. The The permissible permissible limit limit for for oil oil content content in in the the mill mill scale scale is isless less than than 1%. 1%. As As a waste, a waste, the the mill mill scale scale is isused used inin several several ways ways (Figure (Figure 1).1). Some Some of of these these applicationsapplications are are widely widely used, used, others others are are still still being being developed developed [12–18]. [12–18 ].

FigureFigure 1. 1.DifferentDifferent uses uses of ofmill mill scale scale [19] [19. ].

TheThe problem problem of ofthe the management management of ofmetallurgical metallurgical waste, waste, including including mill mill scale, scale, is isnow now anan important important issue issue in inPoland. Poland. For For this this reason, reason, a aproject project was was undertaken undertaken to todevelop develop a a methodmethod of ofmanaging managing the the oily oily rolling rolling scale scale wi withth the the simultaneous simultaneous use use of of gases gases generated generated during the pyrolysis of polymers, e.g., carbon monoxide. Due to the high iron content during the pyrolysis of polymers, e.g., carbon monoxide. Due to the high iron content (68– (68–72%) and the need to use thermal methods in the case of oily mill scale, it was decided 72%) and the need to use thermal methods in the case of oily mill scale, it was decided to to investigate the direct reduction of scale using CO. The main aim of the research was to investigate the direct reduction of scale using CO. The main aim of the research was to check the possibility of iron recovery and to investigate the susceptibility to re-oxidation check the possibility of iron recovery and to investigate the susceptibility to re-oxidation of reduced Fe and FeO in contact with air. Iron resulting from the reduction of rolling of reduced Fe and FeO in contact with air. Iron resulting from the reduction of rolling scale has a porous structure, as does the sponge iron directly reduced from the ores scale has a porous structure, as does the sponge iron directly reduced from the ores in the in the metallurgical process (DRI), with a specific surface area that is typically around metallurgical process (DRI), with a specific surface area that is typically around 1 m²/g 1 m2/g [20,21]. This is because reduction reaction with CO takes place in a solid-state, [20,21]. This is because reduction reaction with CO takes place in a solid-state, hence the hence the shape of mill scale pieces is maintained, and their mass is reduced due to the shape of mill scale pieces is maintained, and their mass is reduced due to the removal of removal of oxygen. Consequently, reduced iron has a large surface area in relation to its oxygen. Consequently, reduced iron has a large surface area in relation to its mass, which mass, which increases its reactivity. Works devoted to the direct reduction of rolling scale, increaseseither with its reactivity. carbon monoxide, Works devoted to the or dire solidctsubstances reduction of (coal, rollin coke),g scale, do either not describe with carbonthe problem monoxide, of low-temperaturehydrogen or solid oxidation substances [6 ,(coal,18,22 –coke),30]. We do findnot describe works describing the problem the ofso-called low-temperature low-temperature oxidation oxidation [6,18,22–30]. of iron We andfind steel,works but describing most often the from so-called the point low- of temperatureview of applications oxidation for of waste iron tanksand steel, [31–33 but]. The most phenomenon often from of rapidthe point oxidation of view of finely of applicationsdivided iron for in waste contact tanks with [31–33]. air should The bephen takenomenon into account of rapid when oxidat designingion of finely technological divided ironprocesses in contact related with to air the should reduction be oftaken mill into scale. account In this work,when thermaldesigning analysis technological methods processeswere used related to simulate to the thereduction process of of mill mill scal scalee. reductionIn this work, using thermal CO at 850analysis◦C, 950 methods◦C, and were1050 used◦C, to followed simulate by the synthetic process airof mill oxidation scale reduction at 300 ◦C, using 350 ◦ C,CO and at 850 400 °C,◦C. 950 The °C, process and 1050was °C, carried followed out by on synthetic material inair itsoxidation original at form, 300 °C, i.e., 350 with °C, the and fragmentation 400 °C. The process obtained was in carriedthe metallurgical out on material process in its and original mill scale form, subjected i.e., with tothe grinding fragmentation to facilitate obtained particle in the size metallurgicalreduction and process increase and the mill reaction scale surface. subjected The to chemical grinding composition to facilitate of particle mill scale size was reductionalso analyzed and increase beforeand the afterreaction the processsurface. inThe terms chemical of iron composition content. of mill scale was also analyzed before and after the process in terms of iron content.

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Minerals 2021, 11, 529 3 of 13 2. Materials and Methods The study’s research material was iron-containing metallurgical waste in the form of oily mill scale (oil content > 3% by mass), from one of the steelworks located in Poland. 2. Materials and Methods Several methods were used for an analysis of the scale chemical composition. Carbon determinationThe study’s was research performed material with wasa LECO iron-containing CS844 analyzer metallurgical (LECO, St. waste Joseph, in theMI, form USA), of oxygenoily mill with scale a LECO (oil content ONH836 > 3% analyzer by mass), (LECO, from St. one Joseph, of the MI, steelworks USA), and located the remaining in Poland. elementsSeveral methodsusing a wereHitachi used S-3400N for an analysis scanning of themicroscope scale chemical (HITACHI, composition. Tokyo, CarbonJapan) equippeddetermination with an was EDS performed and WDS with detector. a LECO CS844 analyzer (LECO, St. Joseph, MI, USA), oxygenSince with the airon LECO in mill ONH836 scale analyzeris in oxide (LECO, form, phase St. Joseph, analysis MI, was USA), performed and the remainingon the X- elements using a Hitachi S-3400N scanning microscope (HITACHI, Tokyo, Japan) equipped ray diffractometer X’Pert 3 (Philips, Amsterdam, The Netherlands). The phases were with an EDS and WDS detector. identified based on the ICCD PDF-2 and COD Database. Mass fractions of individual Since the iron in mill scale is in oxide form, phase analysis was performed on the X-ray phases were calculated using the external standard method (corundum). diffractometer X’Pert 3 (Philips, Amsterdam, The Netherlands). The phases were identified Two forms of scale were used for the thermal analysis method, in fraction, in which based on the ICCD PDF-2 and COD Database. Mass fractions of individual phases were it is collected in a metallurgical process (denoted as A) and, in ground (powdered) form, calculated using the external standard method (corundum). to obtain a larger reaction surface during the experiment (denoted as B). Bulk density and Two forms of scale were used for the thermal analysis method, in fraction, in which it grain composition were determined for both fractions. Bulk density was defined as the is collected in a metallurgical process (denoted as A) and, in ground (powdered) form, to ratio of the weight of loose powder to the volume of the vessel in which it was located. obtain a larger reaction surface during the experiment (denoted as B). Bulk density and grain Grain composition was determined using sieve analysis, consisting of sifting 100 g of composition were determined for both fractions. Bulk density was defined as the ratio of the powderweight of through loose powder a set toof thesieves, volume and of then the vessel determining in which the it was masses located. and Grain percentages composition of individualwas determined fractions. using sieve analysis, consisting of sifting 100 g of powder through a set of sieves,Thermal and then analysis determining of mill thescale masses in the and form percentages of fractions of A individual and B was fractions. performed on the STA Thermal449 Jupiter analysis F3 thermal of mill scale analyzer in the formfrom ofNETZSCH fractions A (NETZSCH and B was performed Holding, onSelb, the Germany).STA 449 Jupiter The device F3 thermal enables analyzer measurement from NETZSCH of the DTA (NETZSCH (Differential Holding, Thermal Selb, Germany).Analysis) signalThe device with enables simultaneous measurement continuous of the DTA (Differentialmeasurement Thermal of sample Analysis) mass signal withTG (Thermogravimetricsimultaneous continuous Analysis). measurement Each time, of samplesamples mass weighing TG (Thermogravimetric about 0.2 g were placed Analysis). in alundum crucibles (Al2O3 content 99.7%) with a capacity of 0.3 mL and tested according Each time, samples weighing about 0.2 g were placed in alundum crucibles (Al2O3 content to99.7%) the scheme with a presented capacity of in 0.3 Figure mL and 2. A tested heating according chamber to of the the scheme device presented after placement in Figure of crucible2. A heating was pumped chamber out of and the devicefilled with after high placement purity argon of crucible (N6.0). was Then, pumped samples out were and heatedfilled with to the high selected purity reduction argon (N6.0). temperature Then, samples (850 °C, were 950 heated °C, or to1050 the °C) selected with reductionan argon flowtemperature of 50 mL/min. (850 ◦C, The 950 reduction◦C, or 1050 was◦C) carried with an out argon by purging flow of 50the mL/min. heating chamber The reduction with purewas carriedCO (flow out 60 by mL/min). purging The the reduction heating chamber time was with selected pure experimentally CO (flow 60 mL/min). by observing The thereduction TG curve, time the was flat selectedcourse of experimentally which indicated by the observing end of the the process. TG curve, After the the flatreduction course wasof which completed, indicated samples the end were of thecooled process. under After argon the (flow reduction 50 mL/min) was completed, until the planned samples oxidationwere cooled temperature under argon was (flow reached 50 mL/min) (300 °C, 350 until °C the or planned400 °C). oxidationOxidation temperature was carried wasout usingreached synthetic (300 ◦C, air 350 with◦C ora flow 400 ◦ C).of 60 Oxidation mL/min. was Oxidation carried time out using was taken synthetic as similar air with to a reduction.flow of 60 Finally, mL/min. samples Oxidation were time cooled was takenfreely astosimilar room temperature to reduction. under Finally, an samples argon .were cooled freely to room temperature under an argon atmosphere.

FigureFigure 2. 2. DiagramDiagram of the of thereduction reduction and oxidation and oxidation process process of mill ofscale mill realized scale realizedduring thermal during analysis. thermal analysis.

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After thermal analysis, the samples were subjected to further chemical composition analyses. Total iron content was determined by Atomic Absorption Spectrometry (ASA).

Minerals 2021The, 11 samples, 529 were measured with the AAS 30 Carl Zeiss apparatus (Jena, Germany), with4 of 13 a lamp current for iron of 8 mA, aperture of 0.30 mm, measuring the wavelength of Fe- 248.3 nm, reading the absorbance, and calculating the iron content from the standard curve. DeterminationAfter of thermalFe(II) analysis,ions thewas samples carried were subjectedout by to furtherreaction chemical with composition the batophenanthrolineanalyses. solution. Total Measurement iron content was was determined made with by Atomic the HP Absorption 8452 A Spectrometryspectrometer (ASA). at a wavelength λ The= 534 samples nm. wereContents measured were with calculated the AAS 30 Carlfrom Zeiss the apparatus standard (Jena, curve Germany), of the with absorbance valuea lampversus current the for concentration iron of 8 mA, aperture of Fe of 0.30(II) mm, ions measuring in complex the wavelength with of Fe- 248.3 nm, reading the absorbance, and calculating the iron content from the standard curve. batophenanthroline. Determination of Fe(III) ions was carried out by reaction with Determination of Fe(II) ions was carried out by reaction with the batophenanthroline thiocyanate ions. Measurementsolution. Measurement was made was with made the with HP the 8452 HP A 8452 spectrometer A spectrometer (HP at Inc., a wavelength Palo Alto, CA, USA) at theλ = wavelength 534 nm. Contents λ = 476 were nm. calculated Contents from were the standard calculated curve from of the the absorbance standard value curve of the absorbanceversus thevalue concentration versus the of Fe conc (II) ionsentration in complex of withFe (III) batophenanthroline. ions in complex Determination with thiocyanates. of Fe(III) ions was carried out by reaction with thiocyanate ions. Measurement was made with the HP 8452 A spectrometer (HP Inc., Palo Alto, CA, USA) at the wavelength λ = 476 nm. Contents were calculated from the standard curve of the absorbance value 3. Results and Discussionversus the concentration of Fe (III) ions in complex with thiocyanates. Results of chemical analysis of the tested scale are presented in Figure 3. In Table 1, 3. Results and Discussion the value of averaged chemical composition with the standard deviation of elements Results of chemical analysis of the tested scale are presented in Figure3. In Table1 , concentration in millthe scale value is of shown. averaged Figure chemical 4 presents composition the withresult the of standard qualitative deviation XRD ofphase elements analysis, and in Tableconcentration 2, the inresult mill scaleof the is shown.quantitative Figure4 analysis.presents the Results result ofof qualitative chemical XRD composition analysisphase confirmed analysis, andhigh in iron Table content2, the result in tested of the quantitative mill scale (approx. analysis. Results 85%), ofwhich chemical is a strong argumentcomposition for attempts analysis to recover confirmed iron high from iron content this waste. in tested Almost mill scale all (approx. of the iron 85%), in which is a strong argument for attempts to recover iron from this waste. Almost all of the iron scale is in the oxidein scaleform, is inand the not oxide more form, than and not 1% more is metallic. than 1% is The metallic. studied The studiedscale is scale also is also characterized by a lowcharacterized content byof aimpurities low content (S of impuritiesand P) and (S andother P) andelements, other elements, the presence the presence of of which could not bewhich clearly could confir not bemed clearly during confirmed XRD duringphase XRD analysis. phase analysis.

FigureFigure 3. Result 3. Result of measurement of measurement on a scanning on a microscopescanning mi Hitachicroscope S-3400N Hitachi (HITACHI, S-3400N Tokyo, (HITACHI, Japan) equipped Tokyo, with an EDSJapan) and WDS equipped detector. with On the an left, EDS an imageand WDS of mill detector. scale, on the On right, the theleft, spectrum an image of the ofchemical mill scale, composition on the right, obtained afterthe analysis spectrum of the of entire the area.chemical composition obtained after analysis of the entire area.

Table 1. Average chemical composition of tested mill scale. Table 1. Average chemical composition of tested mill scale. Element C O Al Si Ca Mn Fe Cr Cu Ti S P Mass/%Element 0.03 C 12.23 O 0.13 Al 0.47 Si 0.40 Ca 0.53 Mn 85.38 Fe Cr0.10 Cu 0.43 Ti 0.30 S 0.07 P 0.03 σ Mass/%±0.01 0.03±2.66 12.23± 0.060.13 ±0.120.47 ±0.400.10 ±0.530.06 ±85.380.20 ±0.100.02 0.43±0.06 0.30±0.08 0.07± 0.01 0.03± 0.01 σ ±0.01 ±2.66 ±0.06 ±0.12 ±0.10 ±0.06 ±0.20 ±0.02 ±0.06 ±0.08 ±0.01 ±0.01 Table 2. Mass fraction of phases identified in examined mill scale.

Phase Fe FeO Fe3O4 Average mass fraction/% 0.85 47.81 51.34

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Figure 4. Result of qualitative XRD analysis of examined mill scale.

Table 2. Mass fraction of phases identified in examined mill scale.

Phase Fe FeO Fe3O4 Average mass 0.85 47.81 51.34 fraction/%

Figure 4. Result of qualitative XRD analysis of examined mill scale. FigurePart 4. of Result the scale of qualitative was subjected XRD analys to additionalis of examined grinding mill scale. in a porcelain mortar (fraction B). ThePart size of reduction the scale was to subjected expand tothe additional surface of grinding the material in a porcelainand examine mortar the effect (fraction of Table 2. Mass fraction of phases identified in examined mill scale. suchB). The treatment size reduction on reduction was to and expand oxidation the surface reactions of the taking material place and during examine the thethermal effect analysisof such experiment. treatmentPhase on The reduction bulk density andFe oxidation of the primary reactions fraction FeO taking (A) place and duringa groundFe the3O fraction thermal4 (B)analysis wasAverage determined, experiment. mass with The the bulk results density per ofton the of primary 1.67 Mg/m fraction3 and 2.80 (A) andMg/m a ground3, respectively. fraction 0.85 47.813 3 51.34 Bulk(B) was densityfraction/% determined, is one of with the core the results technological per ton ofproperties 1.67 Mg/m and anddepends 2.80 Mg/mon the condition, respectively. in whichBulk densitymaterial is is one stored. of the Therefore, core technological an increase properties in the value and of depends bulk density on the along condition with an in increasewhichPart material in ofmass the is scaleof stored. stored was Therefore,subjectedmill scale to anshou additional increaseld be expected, in grinding the value inin ofwhicha porcelain bulk the density space mortar along between (fraction with particlesanB). increase The willsize inbe reduction massreduced of storedwas due toto mill expandthe effect scale the shouldof surface its own be of weight. expected, the material Bulk in whichdensity and examine the of thickened space the between effect mill of 3 scaleparticlessuch reaches treatment will 5.7 be Mg/m reducedon reduction [34]. due The toand the differences oxidation effect of itsinreactions ownthe structure weight. taking Bulkof place fractions density during A of andthe thickened thermalB are 3 shownmillanalysis scale even experiment. reaches more 5.7clearly Mg/m The by bulk the[34 density].grain The size differences of theresult primary (Figure in the fraction structure 5). Sieve (A) of analysis and fractions a ground showed A and fraction Bthat are approximatelyshown(B) was even determined, more 90% clearlyof thewith A by thefraction the results grain grains per size tonbelong result of 1.67 to (Figure a Mg/m size5 class).3 and Sieve above 2.80 analysis Mg/m 0.5 mm,3 showed, respectively. including that overapproximatelyBulk 20% density to the is 90%2.0–5.0 one of of the mmthe A coreclass. fraction technological In the grains case belong of properties the toB afraction, size and class depends almost above 80% on 0.5 the mm,belonged condition including to a in over 20% to the 2.0–5.0 mm class. In the case of the B fraction, almost 80% belonged to a sizewhich class material below 0.5 is stored.mm, and Therefore, for the classes an increase higher in than the value2 mm, of no bulk shares density were along found. with an size class below 0.5 mm, and for the classes higher than 2 mm, no shares were found. increase in mass of stored mill scale should be expected, in which the space between particles will be reduced due to the effect of its own weight. Bulk density of thickened mill scale reaches 5.7 Mg/m3 [34]. The differences in the structure of fractions A and B are shown even more clearly by the grain size result (Figure 5). Sieve analysis showed that approximately 90% of the A fraction grains belong to a size class above 0.5 mm, including over 20% to the 2.0–5.0 mm class. In the case of the B fraction, almost 80% belonged to a size class below 0.5 mm, and for the classes higher than 2 mm, no shares were found.

Figure 5. Size composition of A and B fractions of tested mill scale. Figure 5. Size composition of A and B fractions of tested mill scale. Table3 summarizes the denotations and mass change of samples that took part in the experiment of reduction and oxidation during thermal analysis measurement. The curves recorded during the TG/DTA analysis are shown in Figures6 and7.

Figure 5. Size composition of A and B fractions of tested mill scale.

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A850-300 A850-350 A850-400

A950-300 A950-350 A950-400

A1050-300 A1050-350 A1050-400

Figure 6. TG/DTA curves recorded during the thermal analysis of mill scale from metallurgical process (fraction A) Figure 6. TG/DTA curves recorded during the thermal analysis of mill scale from metallurgical process (fraction A) subjected to reduction (carbon monoxide) at 850 ◦C, 950 ◦C, and 1050 ◦C, followed by exposure to an oxidizing atmosphere subjected to reduction (carbon monoxide) at 850 °C, 950 °C, and 1050 °C, followed by exposure to an oxidizing atmosphere (synthetic air) at temperatures of 300 ◦C, 350 ◦C, and 400 ◦C. (synthetic air) at temperatures of 300 °C, 350 °C, and 400 °C.

Table 3. Denotations and mass change of mill scale fractions A and B during thermal analysis.

◦ Sample Sample Temperature / C Mass Change /% Denotation Weight /g Reduction Oxidation Reduction Oxidation Fraction A A850-300 0.2054 850 300 −23.15 1.35 A850-350 0.1911 850 350 −21.98 3.48 A850-400 0.2039 850 400 −22.10 8.17 A950-300 0.1912 950 300 −24.90 0.90 A950-350 0.2067 950 350 −25.44 2.22 A950-400 0.2247 950 400 −24.48 6.22 A1050-300 0.2024 1050 300 −26.43 0.55 A1050-350 0.1932 1050 350 −25.89 1.77 A1050-400 0.2207 1050 400 −26.90 4.92

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Table 3. Cont.

◦ Sample Sample Temperature / C Mass Change /% Denotation Weight /g Reduction Oxidation Reduction Oxidation Fraction B B850-300 0.1957 850 300 −21.77 1.45 B850-350 0.1940 850 350 −22.00 4.55 B850-400 0.2140 850 400 −24.28 11.44 B950-300 0.1950 950 300 −24.93 1.09 B950-350 0.2136 950 350 −24.91 2.80 B950-400 0.1927 950 400 −25.48 8.10 B1050-300 0.1945 1050 300 −25.89 0.75

Minerals 2021, 11, 529 B1050-350 0.2076 1050 350 −25.94 2.10 8 of 14

B1050-400 0.2200 1050 400 −26.11 5.23

B850-300 B850-350 B850-400

B950-300 B950-350 B950-400

B1050-300 B1050-350 B1050-400

Figure 7. TG/DTA curves recorded during the thermal analysis of ground mill scale (fraction B) subjected to reduction Figure(carbon 7. monoxide) TG/DTA curves at 850 recorded◦C, 950 ◦ C,duri andng 1050the thermal◦C, followed analysis by of exposure ground tomill an scale oxidizing (fraction atmosphere B) subjected (synthetic to reduction air) at (carbon monoxide) at 850 °C, 950 °C, and 1050 °C, followed by exposure to an oxidizing atmosphere (synthetic air) at temperatures of 300 ◦C, 350 ◦C, and 400 ◦C. temperatures of 300 °C, 350 °C, and 400 °C.

The reduction of iron oxides is carried out through the reaction of heterogeneous solid, liquid, and gas phases. Reduction mechanisms consist of surface processes that accompany the removal of oxygen from reduced oxide and transport processes related to the flow of gaseous reagents and mass transfer inside the solid [35–37]. Due to the influence of several factors, such as temperature, vapor pressure, by reducing the agent composition or the degree of phase contact, the mechanism of reducing iron oxides is complicated and not fully understood [37–39]. It is commonly assumed that a reduction of iron oxides at temperatures below 560 °C occurs in several stages Fe2O3 → Fe3O4 → Fe, and at temperatures above 560 °C according to Fe2O3 → Fe3O4 → FeO → Fe [40,41]. The process of reducing iron oxides with carbon monoxide is illustrated by the following reactions:

3Fe2O3 + CO → 2Fe3O4 + CO2 (1)

Fe3O4 + CO → 3FeO + CO2 (2)

FeO + CO → Fe + CO2 (3) Results of the TG analysis showed that with an increase of temperature at which the reduction of mill scale with CO was carried out, loss of sample mass increased, and thus,

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The reduction of iron oxides is carried out through the reaction of heterogeneous solid, liquid, and gas phases. Reduction mechanisms consist of surface processes that accompany the removal of oxygen from reduced oxide and transport processes related to the flow of gaseous reagents and mass transfer inside the solid [35–37]. Due to the influence of several factors, such as temperature, vapor pressure, by reducing the agent composition or the degree of phase contact, the mechanism of reducing iron oxides is complicated and not fully understood [37–39]. It is commonly assumed that a reduction of iron oxides at temperatures ◦ below 560 C occurs in several stages Fe2O3 → Fe3O4 → Fe, and at temperatures above 560 ◦ C according to Fe2O3 → Fe3O4 → FeO → Fe [40,41]. The process of reducing iron oxides with carbon monoxide is illustrated by the following reactions:

3Fe2O3 + CO → 2Fe3O4 + CO2 (1)

Fe3O4 + CO → 3FeO + CO2 (2)

Minerals 2021, 11, 529 FeO + CO → Fe + CO2 9 of 14(3)

Results of the TG analysis showed that with an increase of temperature at which the reduction of mill scale with CO was carried out, loss of sample mass increased, and thus, itsits reducibility. reducibility. Average Average weight weight loss loss during during reduction reduction for for fractions fractions A A and and B Bis is presented presented inin Figure Figure 8.8 .It It is is similar similar for for both both tested tested fractions fractions and and varies varies from from approximately approximately 22.5% 22.5% at at ◦ ◦ 850850 °CC to to approx. approx. 26.2% 26.2% at at 1050 1050 °C.C.

◦ FigureFigure 8. 8.AverageAverage weight weight loss loss of of studied studied mill mill scale scale during during reduction with carboncarbon monoxidemonoxide at at 850 850 C, ◦ ◦ °C,950 950°CC and and 1050 1050C—fractions °C—fractions A A and and B. B.

TheThe time time when when the reductionreduction processprocess was was completed completed is moreis more important important from from the pointthe of view of the application in the technological process. It revealed itself in the achievement point of view of the application in the technological process. It revealed itself in the◦ achievementof a horizontal of a course horizontal on the course TG curve on recordedthe TG curve during recorded reduction. during For millreduction. scale A For at 850 millC, the reduction lasted approximately 180 min, while at 950 ◦C and 1050 ◦C, stabilization took scale A at 850°C, the reduction lasted approximately 180 min, while at 950 °C and 1050 place after approximately 90 min. Grinding the scale to fraction B resulted in shortening of °C, stabilization took place after approximately 90 min. Grinding the scale to fraction B reduction time at 850 ◦C to approximately 120 min; however, it did not significantly change resulted in shortening of reduction time at 850 °C to approximately 120 min; however, it reduction time at temperatures of 950 ◦C and 1050 ◦C. did not significantly change reduction time at temperatures of 950 °C and 1050 °C. After heating in a reducing atmosphere, samples were cooled to a temperature of After heating in a reducing atmosphere, samples were cooled to a temperature of 300 300 ◦C, 350 ◦C, or 400 ◦C, where they were subjected to heating in an atmosphere of °C, 350 °C, or 400 °C, where they were subjected to heating in an atmosphere of synthetic synthetic air for 180 min. The product of the mill scale reduction process is iron in a air for 180 min. The product of the mill scale reduction process is iron in a spongy form, spongy form, similar to industrial iron obtained by direct reduction from ores, the so- similar to industrial iron obtained by direct reduction from ores, the so-called DRI (Direct called DRI (Direct Reduced Iron). As a result of oxygen removal, it has a highly porous Reduced Iron). As a result of oxygen removal, it has a highly porous structure and is very structure and is very reactive with moisture and oxygen. The re-oxidation process revealed reactiveitself in with THE moisture form of and an increaseoxygen. inThe mass re-oxi recordeddation process on the TGrevealed curve. itself Unlike in THE mill form scale, ofwhich an increase is non-flammable, in mass recorded finely divided on the reduced TG curve. iron canUnlike oxidize mill rapidlyscale, which when inis contactnon- flammable,with a sufficient finely amountdivided of reduced oxygen. iron This can phenomenon oxidize rapidly is called when pyrophoric, in contact from with “pyro” a sufficient(fire) + “phor” amount (carry) of oxygen. and literally This phenomenon means “fire is bearing” called pyrophoric, [42]. Pyrophoricity from “pyro” is one (fire) of the + “phor”chemical (carry) properties and literally of an element means “fire and determinesbearing” [42]. the Pyrophoricity powder’s tendency is one toof self-ignitionthe chemical or propertiesrapid oxidation of an element associated and with determines the release the of powder’s significant tendency amounts to of self-ignition heat. Atoms or forming rapid oxidation associated with the release of significant amounts of heat. Atoms forming the subsurface layer take part in the oxidation reaction, and due to the good thermal conductivity of metal and bad of gas atmosphere, practically all the heat generated during oxidation is used to heat particles, which may consequently lead to its ignition [43–45]. Most often, for such a phenomenon to occur, the substance must be finely divided, and the smaller the powder particles, the higher temperature they heat up. Apart from many metals, including iron, one of the materials showing pyrophoric properties is FeO [46,47]. To reduce the oxidation of DRI, one can turn to industrial practice, where we can find methods such as DRI passivation by blowing with a mixture with a small amount of oxygen, coating DRI with protective substances, or reducing the surface area of DRI by densifying it in form of briquettes [48]. With the increase of temperature at which oxidation was carried out, the weight gain also increased. The intensity of the process was revealed in the form of an increase in exothermic peak area accompanying oxidation reaction and recorded on the DTA curve. At the same time, it was found that the higher the reduction temperature was, the smaller the weight increase during oxidation follow. Again, this is related to both fractions A and B, with the weight increase of fraction B being higher. The explanation for this phenomenon is the sintering of finely divided iron obtained by reducing the mill scale.

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the subsurface layer take part in the oxidation reaction, and due to the good thermal conductivity of metal and bad of gas atmosphere, practically all the heat generated during oxidation is used to heat particles, which may consequently lead to its ignition [43–45]. Most often, for such a phenomenon to occur, the substance must be finely divided, and the smaller the powder particles, the higher temperature they heat up. Apart from many metals, including iron, one of the materials showing pyrophoric properties is FeO [46,47]. To reduce the oxidation of DRI, one can turn to industrial practice, where we can find methods such as DRI passivation by blowing with a nitrogen mixture with a small amount of oxygen, coating DRI with protective substances, or reducing the surface area of DRI by densifying it in form of briquettes [48]. With the increase of temperature at which oxidation was carried out, the weight gain also increased. The intensity of the process was revealed in the form of an increase in exothermic peak area accompanying oxidation reaction and recorded on the DTA curve. At Minerals 2021, 11, 529 the same time, it was found that the higher the reduction temperature was, the smaller10 of 14 the weight increase during oxidation follow. Again, this is related to both fractions A and B, with the weight increase of fraction B being higher. The explanation for this phenomenon is the sintering of finely divided iron obtained by reducing the mill scale. Sintered particles Sinteredhave a particles smaller reaction have a surface.smaller reaction Tendency surface. to sintering Tendency increases to sintering with the temperatureincreases with and thetime temperature of reduction and as time stated of reduction in Reference as stated [49]. in Reference [49]. WeightWeight gain gain during during oxidation oxidation of ofsamples samples from from fractions fractions A Aand and B at B at300 300 °C,◦ C,350 350 °C,◦ C, andand 400 400 °C◦ Cduring during 180 180 min min is is shown shown in Figure9 9.. The The smallest smallest weight weight increase increase of of 0.55% 0.55% was wasrecorded recorded for samplefor sample from fractionfrom fraction A oxidized A oxidized at 300 ◦C at after 300 reduction °C after atreduction a temperature at a of temperature1050 ◦C, and of highest1050 °C, (11.44%) and highest for sample (11.44%) from for B sample fraction from oxidized B fraction at 400 ◦oxidizedC after reduction at 400 °Cat after 850 reduction◦C. at 850 °C.

◦ ◦ ◦ FigureFigure 9. Mass 9. Mass increase increase during during oxidation oxidation of sample of sampless at 300 at 300°C, 350C, 350°C, andC, and400 °C 400 overC over 180 min— 180 min— fractionsfractions A and A and B. B.

In Inorder order to to check check the the influenceinfluence ofof thethe accomplished accomplished thermal thermal analysis analysis research research pro- programgram on on the the state state of of the the tested tested mill mill scale, scale, samples samplesafter after thethe experimentexperiment werewere subjectedsubjected to to chemical analysis for for iron iron determination. determination. Total Total iron iron content content was was determined determined by by Atomic Atomic AbsorptionAbsorption Spectrometry Spectrometry (ASA), (ASA), while while iron iron ions ions were were determined determined by by spectrophotometric spectrophotometric methods (iron (II) ions in reaction with batophenanthroline solution, and iron (III) ions in methods (iron (II) ions in reaction with batophenanthroline solution, and iron (III) ions in reaction with thiocyanate ions). The results of the chemical analysis of both fractions are reaction with thiocyanate ions). The results of the chemical analysis of both fractions are shown in Figure 10. The mass fraction of individual components of the mill scale changed shown in Figure 10. The mass fraction of individual components of the mill scale changed significantly. The average iron (II) content for both fractions was about 82%. The difference significantly. The average iron (II) content for both fractions was about 82%. The between A and B fractions was shown by a lower total iron content and a higher iron (III) difference between A and B fractions was shown by a lower total iron content and a higher content in the case of scale B. This is reflected in the results of thermal analysis, which iron (III) content in the case of scale B. This is reflected in the results of thermal analysis, which showed that the development of scale B surface by grinding increased its susceptibility to oxidation.

Minerals 2021, 11, 529 10 of 13 Minerals 2021, 11, 529 11 of 14

Minerals 2021, 11, 529 11 of 14 showed that the development of scale B surface by grinding increased its susceptibility to oxidation.

Figure 10. Summary of chemical composition analysis results of mill scale (fractions A and B) subjected to reduction and secondary oxidation process. Figure 10. Summary of chemical composition analysis results of mill scale (fractions A and B) subjected to reduction and Figure 10. Summary of chemical composition analysis results of mill scale (fractions A and B) subjected to reduction and secondary oxidation process. secondary oxidation process. The content of metallic iron estimated based on the above results in samples subjectedThe to content thermal of metallicanalysis iron is shown estimated in Figu basedre on11. theIt clearly above resultsshows inthat samples the content subjected of metallicto thermalThe iron content analysis is each of is timemetallic shown lower iniron Figurein millestimated 11scale. It clearlyofbased fraction showson theB. thatThe above thehighest contentresults share, in of metallicsampleswhich amountedironsubjected is each toto time approximatelythermal lower analysis in mill 15%, is scale shown was of found fraction in Figu for B.re the The11. sample,It highest clearly which share, shows whichwas that reduced the amounted content with toof carbonapproximatelymetallic monoxide iron is 15%, each at was atime temperature found lower for thein ofmill sample, 1050 scale which°C of andfraction was oxidized reduced B. The in with highestsynthetic carbon share, monoxideair whichat a temperatureatamounted a temperature toof 300approximately of °C 1050 (A1050-300).◦C and 15%, oxidized The was lowest found in synthetic quantity for the air ofsample, at metallic a temperature which iron (0.9%)was of 300reduced was◦C (A1050-found with for300).carbon scale The samplesmonoxide lowest subjected quantity at a temperature ofto metallicreduction iron atof 850 (0.9%)1050 °C and was°C andoxidation found oxidized for at scale 400 insamples °C synthetic (B850-400). subjected air This at to a valuereductiontemperature is almost at 850 of identical 300◦C and°C (A1050-300). oxidationto the metallic at The 400 ir ◦lowestonC (B850-400). content quantity determined This of metallic value by is almostironXRD (0.9%) phase identical was analysis tofound the formetallicfor the scale primary iron samples content mill subjected scale determined (Table to reduction 2). by XRD at phase 850 °C analysis and oxidation for the primary at 400 °C mill (B850-400). scale (Table This2). value is almost identical to the metallic iron content determined by XRD phase analysis for the primary mill scale (Table 2).

Figure 11. Estimated content of metallic iron in scale samples A and B subjected to thermal analysis. Figure 11. Estimated content of metallic iron in scale samples A and B subjected to thermal analysis. 4. Conclusions 4.Figure ConclusionsThis 11. Estimated work presents content theof metallic results iron of in experiments scale samplesaimed A and B at subjecte checkingd to thermal the possibility analysis. of recoveringThis work iron presents from oily the mill results scale of by experiments reducing with aimed carbon at checking monoxide the and possibility taking intoof recoveringaccount4. Conclusions the iron phenomenon from oily mill of secondary scale by oxidationreducing inwith contact carbon with monoxide oxygenfrom and air.taking Analysis into accountof chemicalThis the work andphenomenon presents phase composition theof resultssecondary showed of experiments oxid thatation the in examinedaimed contact at checking millwith scale oxygen the contained possibility from aboutair. of Analysis85%recovering of theof ironironchemical in from the formandoily millphase of oxides scale composit (FeO,by reducing Feion3O 4showed). with carbon that themonoxide examined and milltaking scale into containedaccountThe about mainthe phenomenon research85% of the was iron of carried insecondary the out form with ofoxid theoxidesation use (FeO, ofin thecontact Fe thermal3O4). with analysis oxygen methods from air. on theAnalysisThe mill main scale of chemicalresearch in primary wasand form carried phase (fraction outcomposit with A) andthione use groundshowed of the to thatthermal extend the their analysisexamined surface methods mill (fraction scale on thecontained mill scale about in primary 85% of form the iron (fraction in the A) form and of ground oxides to (FeO, extend Fe3 Otheir4). surface (fraction B). The main research was carried out with the use of the thermal analysis methods on the mill scale in primary form (fraction A) and ground to extend their surface (fraction B).

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B). Bulk density of fraction A and fraction B was determined, with the results per ton of 1.67 Mg/m3 and 2.80 Mg/m3, respectively. Sieve analysis was also performed. The reduction was carried out for approximately 180 min at temperatures of 850 ◦C, 950 ◦C, and 1050 ◦C. The course of the TG curves showed that the degree of mill scale reduction increases with temperature, while the kinetics of the process changes as well. The smallest weight loss and the lowest reaction rate were found for the reduction of mill scale with carbon monoxide at 850 ◦C while the highest weight loss and the highest reaction rate were found during the reduction of scale at 1050 ◦C. This applies to the mill scale in both fractions (A and B). Re-oxidation effect was observed during exposition of reduced mill scale to synthetic air at a temperature of 300 ◦C, 350 ◦C, and 400 ◦C for 180 min. Each time fraction B, due to a more unfolded surface, was characterized by a higher weight gain due to oxidation than fraction A. The smallest weight increase of 0.55% was recorded for the sample from fraction A oxidized at 300 ◦C after reduction at temperature 1050 ◦C, and highest (11.44%) for sample from fraction B oxidized at 400◦C after reduction at 850 ◦C. At the same time, the tendency for sintering of reaction products with increasing temperature was confirmed. Thermal analysis results was also reflected in iron content analysis after the process, where each time samples of fraction B had a higher content of iron in II and III degrees of oxidation and a lower metallic iron content compared to samples of fraction A. The occurrence of secondary oxidation of reduced iron should be taken into account when designing technological processes related to the recovery of iron from metallurgical waste by reduction. Most favorable results in this work were obtained for the reduction of mill scale in the primary form (fraction A) at a temperature of 1050 ◦C. The authors emphasize the need for further research, going beyond the limitations of the used research methods and supplementing the results obtained here with a more complex analysis of mill scale reduction products.

Author Contributions: Conceptualization, K.N., T.M., T.L.; methodology, T.M.; validation, T.M., K.N.; formal analysis, T.L.; resources, K.N., T.M.; writing—original draft preparation, K.N., T.M.; writing— review and editing, K.N., T.M., T.L.; visualization, T.M.; supervision, T.M.; funding acquisition, K.N. All authors have read and agreed to the published version of the manuscript. Funding: The Silesian University of Technology (Faculty of Materials Engineering, Department of Production Engineering) supported this work as a part of Statutory Research BK-229/RM1/2021 (11/010/BK_21/0032). Conflicts of Interest: The authors declare no conflict of interest.

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