Recovery and Characterization Studies of Post-Production Alloy Waste from the Automotive Industry

materials

Article Recovery and Characterization Studies of Post-Production Alloy Waste from the Automotive Industry

Sylwester Zelazny˙ 1 , Witold Zukowski˙ 1 , Dariusz Bogdał 2,*, Szczepan Bednarz 2, Wiktor Kasprzyk 2 and Tomasz Swiergosz´ 3,* 1 Department of Inorganic Chemistry, Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155 Kraków, Poland; [email protected] (S.Z.);˙ [email protected] (W.Z.)˙ 2 Department of Biotechnology and Physical Chemistry, Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155 Kraków, Poland; [email protected] (S.B.); [email protected] (W.K.) 3 Department of Analytical Chemistry, Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155 Kraków, Poland * Correspondence: [email protected] (D.B.); [email protected] (T.S.)´

Received: 12 November 2020; Accepted: 6 December 2020; Published: 8 December 2020

Abstract: Superalloys provide high corrosion resistance and are widely used as high-performance materials in aerospace, automotive, chemical, and other industries. Herein, the investigation into the characteristics and properties of alloy waste; Inconel 625, Inconel 718, and Titanium Grade 5, from the automotive industry, was introduced as a result of a recovery in various processes. For this reason, the following procedures were carried as follows; the washing process to remove oil from the swarf was evaluated using several commercial agents and for the process of thermal disposal of processing fluids, a temperature of 900 ◦C was used in a muffle furnace without air access. The presented studies show that the commercially available series of washing agents did not modify the composition of the surface. However, the high temperatures during the calcination of oil residues are affecting the elemental composition of the alloys. According to the results of the analyses, it is not possible to remove 100% of the oil residues from alloy waste using washing agents based on light organic fractions; however, the effectiveness of this method reaches 99%. In this report, accurate SEM-EDS analyses show changes that occur on the surface after machining and removal of processing fluids. The NMR and GC/MS investigations indicate contaminants as a mixture of aliphatic and cycloaliphatic hydrocarbons with carbon numbers from C8–C30.

Keywords: alloy waste; Inconel 625; Inconel 718; Titanium Grade 5; thermal process; washing process; recovery; characterization; surface degradation; NMR; GC/MS; SEM-EDS

1. Introduction The development of the automotive industry imposes the need to improve technologies and methods of producing highly demanding components [1–3]. The high-quality requirements placed on machine elements promote the design of resistant and durable materials, such as titanium and nickel alloys [4,5]. This type of materials maintains high pressures on processing. Currently, the most basic techniques of machining in the industry are cavity techniques such as turning, milling, drilling and grinding [6]. All the processes applied, produce signiﬁcant amounts of swarf waste [7]. However, since no statistical information is available, the exact amount of swarf production worldwide remains unknown. The technical document states that only one of the three major car manufacturers

Materials 2020, 13, 5600; doi:10.3390/ma13245600 www.mdpi.com/journal/materials Materials 2020, 13, 5600 2 of 13 has an annual production of over 40,000 tons of swarf [8]. Furthermore, it is claimed that using current techniques in metal processing, 10% of all the metals production is converted into swarf waste [9]. Moreover, waste alloy swarfs are estimated to be between 2.3 and 5.8 million tones based on the global use of metal removal fluids. Filtration and oil recycling systems are implemented to reduce the use of metal removal fluids and solid waste [10]. It is common procedure in the automobile industry to use different alloys with the high market value as the material for specific elements, such as Titanium Grade 5, Inconel 625, Inconel 718 [11]. The main waste stream is the swarf generated during the processing and coolants [8]. These swarfs need to be stored and recycled due to the extremely high costs of steel production. Swarf is currently classified as solid or hazardous waste, depending on its composition and legislative definitions in different countries. Therefore, they should be processed following the rules so that they can be reused as raw materials [12]. They should be thoroughly cleaned from fluids used in the treatment process. The type of machining (milling, turning, grinding) has a fundamental influence on the amount, form and type of contaminants generated during operating, mainly particulates [13–15]. No influence of machining type on values of the machining fluids parameters was found [16]. On the other hand, microbiological contamination of emulsion machining fluids is a problem concerning each stage of fluid exploitation [17]. No significant influence of machining type on the content of bacteria and fungi in liquids was determined [18]. The process of microbiological contamination is favored by the complex chemical composition of emulsion processing fluids [19]. Not without significance is determination of the pH value of working solutions, favorable for the processes of decomposition of liquid components by microorganisms [20]. Environmentally friendly technologies to reduce pollution and recover resources are being explored more recently to strengthen environmental legislation related to pollution [21]. Especially in the engineering and machining industry, interest in addressing recycling of metal swarf and cutting fluids [7,21]. Herein, the swarf waste from three alloys Inconel 625, Inconel 718 and Titanium Grade 5 as an inherent by-product of the automotive industry within the technique of metal recovery through washing and thermal processing of oil maximizes the value of the recovered alloys swarf and reduces the costs of the process. In this study, for the first time, waste from alloys after processing in the automobile industry was characterized. The waste was recovered in a washing treatment process using available solutions to become a reusable material. Data from the analysis of the material surface and elemental analysis were compared to the reference data of the originating materials to verify that the materials do not lose their initial value during the machining and treatment processes. The contamination generated by the cooling liquids during machining was also investigated and the most effective washing liquids were selected. Moreover, for the qualities of metals, a process of thermal oil disposal from the surface of the alloy’s swarf was carried out. Alloys have been characterized using available research techniques.

2. Materials and Methods

2.1. Recycling Materials The swarfs from machining, i.e., alloys Inconel 625 (I625), Inconel 718 (I718), and Titanium Grade 5 (TG5), were received from the automobile company (Poznań,Great Poland, Poland). During the investigations, materials were used in the form received from the vehicle component manufacturer. In addition, the obtained materials had different sizes and forms including thickness and width (from less than 1.6 mm to more than 6.3 mm). Furthermore, the materials were covered with unknown oily substances derived from metalworking fluids.

2.2. Content of the Grain Fraction To remove the organic parts from the alloys, samples of 100 g each were transferred into 2-L beakers, then poured 500 mL of acetone until complete immersion. The degreasing time was 3 h. After this time, the samples were successively seeped on a funnel with a quality ﬁlter Type 289. Materials 2020, 13, 5600 3 of 13

After the seepage, the samples together with the filter were placed in an oven and dried at 35 ◦C for 4 h. Afterwards, particle size distribution was carried out to illustrate the amount of different sized swarf generated in the machining process. The analysis was carried out on an apparatus LAB-06-300 produced by the Scientific and Technical Company.

2.3. Moisture Content For determination of the swarfs physical properties, the analysis of moisture and oil content is the most crucial issue. For this purpose, a drying technique in a convection oven to constant moisture is required in accordance with existing speciﬁcations. According to the ASTM D2216-10 to determine moisture content, a portion of 20 g of each swarfs are placed in the oven and are performed at 110 5 C[22]. ± ◦ 2.4. Oil Content The method for the oil content of the material was performed in three repetitions. This was achieved by mixing 100 g of the sample dried in an oven with 500 mL of acetone and then shaking for 60 min. The liquid fraction was then decanted, and the precipitated swarfs were rinsed with 250 mL of water for 30 min by ultrasonic treatment. The above operations were repeated twice, and the solution was decanted. After the water treatment, the sample was dried in an oven at 110 ◦C for 3 h.

2.5. Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM–EDS) The Scanning Electron Microscope (SEM) model TM3000 from Hitachi (Fukuoka, Japan) was used to observe the changes on the surface.

2.6. Gas Chromatography–Mass Spectrometry (GC-MS) The gas chromatography (model: Agilent 7890B GC, Waltham, MA, USA) with mass spectrometry (model: Agilent 5977A GC/MSD) was used to verify the purity of the waste material in processes of washing and thermal treatment from oily substances before and after treatments to show what organic contaminants have to be dealt with. The GC/MS technique is overly sensitive and allows determining the degree of purification from organic compounds. Chromatographic analysis was performed to separate individual pollutants in extracted solutions allowing to define their mass spectra and the size of signals coming from individual compounds from the first and second stage of purification. The chromatographic separation program used the temperature increase of the chromatographic column (DB-5 Agilent) as follows: Initial temperature: 50 ◦C for 2 min and increase 10 ◦C/min to 250 ◦C. Inlet temperature 200 ◦C and injection volume 1 µL have been applied to all analyses in the investigation. The mass spectrometer worked in a full-scan mode in the m/z range from 10 to 600 u. Chromatograms were obtained from all the analyses.

2.7. Nuclear Magnetic Resonance (1H NMR and 13C NMR) The precision of NMR spectroscopy allows measuring the chemical shifts, thus obtaining information about chemical bonds and the structure of molecules occurring in oily liquid extracts. The analyses with the NMR technique were performed on a JEOL FT-NMR 500 MHz apparatus (JNM-ECZR500 RS1 version ECZR, Peabody, MA, USA

2.8. Oil Removal from Alloy Swarfs in Washing Process Oil residues from alloy processing aﬀect the possibility of metal recovery; therefore, a washing process was used to remove the oil from the swarfs. The choice of washing agents to remove contaminants from the swarfs was restricted according to the approval of the European market. The veriﬁcation of the selected agents was determined regarding environmental safety. Oil removal from alloy swarf was carried out using the solvent-based solutions: Spirdane D25; Spirdane D40 Materials 2020, 13, 5600 4 of 13 and Spirdane D60. These organic solutions were selected after determining precisely which organic compounds must be treated. For washing process 100 g of the washing agent and 20 g of swarfs in a beaker was used. The complete mixture was performed with a mechanical stirrer with a speed of 120 rpm. After 20, 40, and 60 min, the swarf samples were taken, rinsed twice with distilled water, and dried in a laboratory dryer at 110 ◦C for 2 h. The purity of the material was analyzed on a scanning microscope with an EDS attachment.

2.9. Oil Removal from Alloy Swarfs in a Thermal Process It is hazardous to utilize oil residues as part of hydrocarbon fuels on metal alloy swarfs since Materials 2020, 13, 5600 4 of 13 oil is explosive at high temperatures [23]. Nevertheless, the swarf samples of individual alloys were compoundscalcined at 900 must◦C be in atreated. muﬄe For furnace washing to examine process the 100 level g ofof the contamination washing agent and and structure 20 g of ofswarfs the alloy in a beakerafter high-temperature was used. The complete application mixture treatment was performe due tod the with low a mechanical content of oilstirrer residue with ona speed the surface. of 120 rpm.The process After 20, of thermal40, and 60 removal min, the each swarf time samples involved we 10re g taken, of swarfs rinsed in an twice alumina with ceramicdistilled crucible water, and driedlasted in for a 2laboratory h at a given dryer temperature. at 110 °C Afterwards,for 2 h. The purity the swarfs of the were material cooled was down analyzed and analyzed on a scanning using a microscopescanning microscope with an EDS with attachment. an EDS attachment.

3. Results and Discussion 2.9. Oil Removal from Alloy Swarfs in a Thermal Process ItFor is allhazardous alloy waste, to utilize a clean oil surfaceresidues is as essential. part of hydrocarbon The cleaning fuels process on metal must alloy remove swarfs oils, since greases, oil isand explosive traces of at chemicals high temperatures on the surface. [23]. Therefore,Nevertheless, before the theswarf treatment, samples a of number individual of analyses alloys were calcinedperformed at to900 check °C in parameters a muffle furnace of waste to andexamine oily substances. the level of Initially,contamination the sieve and analysis structure was of performed the alloy afteron sieves high-temperature with 1.6 mm; 3.15application mm; 6.3 treatment mm diameters. due to Totalthe low sieving content time of wasoil residue 5 min and on the 100 surface. g of each The of processthe swarfs of thermal were taken removal for analysis. each time The involved results of 10 the g analysisof swarfs are in presentedan alumina in ceramic Table1 and crucible Figure and1. lastedSincethe for swarfs2 h at a constitute given temperature. irregular shapes, Afterwards, a given the grain swarfs fraction were corresponds cooled down to and the lowestanalyzed diameter using aof scanning a single swarf.microscope Moreover, with an EDS exemplary attachment. graph of the granulometric analysis using the dimensions of 6.3–3.15 mm of TG5, I625, and I718 swarfs is shown in Figure2. 3. Results and Discussion Table 1. Granulometric composition of alloy swarfs: Titanium Grade 5, Inconel 625, Inconel 718 in Forpercentage all alloy after waste, previous a clean removal surfac ofe organic is essential. parts from The the cleaning surface. process must remove oils, greases, and traces of chemicals on the surface. Therefore, before the treatment, a number of analyses were Content of the Grain Fraction [%] performed to checkSieves parameters (mm) of waste and oily substances. Initially, the sieve analysis was performed on sieves with 1.6 mm; 3.15Titanium mm; Grade6.3 mm 5 diameters. Inconel 625Total sieving Inconel time 718 was 5 min and 100 g of each of the swarfs> 6.3were taken for analysis. 58.53 The results 4.73 of the analysis 24.93 are presented in Table 1 and Figure 1. Since the6.3–3.15 swarfs constitute 26.82irregular shapes, 10.41a given grain fraction 54.05 corresponds to the lowest diameter of a 3.15–1.6single swarf. Moreover, 9.76 an exemplary 54.57graph of the granulometric 13.81 analysis using <1.6 4.89 30.29 7.21 the dimensions of 6.3–3.15 mm of TG5, I625, and I718 swarfs is shown in Figure 2.

Figure 1. 1. IllustrationIllustration of ofa granulometric a granulometric analysis analysis usin usingg the dimensions the dimensions of 6.3–3.15 of 6.3–3.15 mm Titanium mm Titanium Grade 5Grade (A), Inconel 5 (A), Inconel 625 (B), 625 and (B Inconel), and Inconel 718 (C). 718 (C).

Table 1. Granulometric composition of alloy swarfs: Titanium Grade 5, Inconel 625, Inconel 718 in percentage after previous removal of organic parts from the surface.

Content of the Grain Fraction [%] Sieves (mm) Titanium Grade 5 Inconel 625 Inconel 718 >6.3 58.53 4.73 24.93 6.3–3.15 26.82 10.41 54.05 3.15–1.6 9.76 54.57 13.81 <1.6 4.89 30.29 7.21

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Figure 2. Granulometric composition of alloys after sieve analysis. Figure 2. Granulometric composition of alloys after sieve analysis. Furthermore, based on the standard, the moisture content of the samples for TG5, I625 and I718 was 12Furthermore, wt.%, 3.1 wt.%, based and on the 3.4 standard, wt.%, respectively, the moisture not content changing of the in timesamples after for 2 TG5, and 3I625 h of and drying. I718 Inwas addition, 12 wt.%, after 3.1 onewt.%, month and of3.4 storage, wt.%, respectively, the materials not show changing the same in moisturetime after content 2 and as3 h after of drying. receiving In fromaddition, automobile after one company month of (Tablestorage, S1). the Duematerials to the show type the of same method mo usedisture tocontent remove as after residues receiving from thefrom alloy automobile surface, company the amount (Table of oil S1). deposit Due to in the swarfs type is of important. method used Therefore, to remove for thisresidues investigation, from the thealloy oil surface, content the was amount determined of oil by deposit the gravimetric in swarfs method is important. during Therefore, the solution-based for this investigation, washing removal. the Followingoil content thewas findings determined of Ru byffi theno gravimetric and Zanetti meth [24].od As during a result, the thesolution-based final mass ofwashing oil content removal. was calculatedFollowing throughthe findings a comparison of Ruffino to theand initial Zanetti mass [24]. of each As sample.a result, Thethe oilfinal content mass of of the oil swarf content samples was ofcalculated TG5, I625, through and I718 a comparison was 22 wt.%, to 4.2the wt.%, initial and mass 4.6 of wt.%, each respectively. sample. The The oil respectivecontent of valuesthe swarf for moisturesamples of and TG5, oil contentI625, and are I718 shown was in 22 Table wt.%,2. In4.2 addition, wt.%, and after 4.6 onewt.%, month respectively. of storage, The the respective material swarfsvalues remainedfor moisture the and same oil oil content content are (Table shown S1). in Table 2. In addition, after one month of storage, the material swarfs remained the same oil content (Table S1). Table 2. Respective values for physical analysis of swarf materials. Table 2. Respective values for physical analysis of swarf materials. Alloy Type Titanium Grade 5 Inconel 625 Inconel 718 Alloy TypeOil Content Titanium 22 wt.% Grade 5 4.2 wt.% Inconel 625 4.6 wt.% Inconel 718 Oil ContentMoisture Content 12 22 wt.% wt.% 3.1 wt.% 4.2 wt.% 3.4 wt.% 4.6 wt.% Moisture Content 12 wt.% 3.1 wt.% 3.4 wt.% Based on the SEM photographs (Figure2) at various magnifications, it was found that the research materialBased comes on the from SEM machining photographs process. (Figure In this 2) type at ofva treatment,rious magnifications, the swarfs areit was wrinkled found as that a result the ofresearch the blade material moving comes along from the metalmachining surface. process. Moreover, In this the type distribution of treatment, of machining the swarfs fluid are impurities wrinkled areas a characterized result of the blade by the moving dark stains along on the the metal SEM surf images.ace. Moreover, the distribution of machining fluid impuritiesThe alloys are characterized surface distinguishes by the dark between stains theon the two SEM sides images. due to the amount of oil accumulating on eachThe side alloys of thesurface material distinguishes (Figure S1). between The bright the two side sides of the due swarfs to the is amount characterized of oil accumulating by a smaller surfaceon each wrinkle side of andthe material impurities (Figure are much S1). reducedThe bright and side distributed of the swarfs relatively is characterized regularly over by thea smaller entire metalsurface surface. wrinkle However, and impurities the dark are sidemuch of reduced the swarfs and is distributed wrinkled, andrelatively oil residues regularly are over mainly the foundentire inmetal metal surface. cavities However, and microcracks the dark side (Figure of the3). Therefore,swarfs is wrinkled, considering and that, oil residues in the process are mainly of removing found in oilymetal substances, cavities and the microcracks selection of (Figure proper 3). cleaners Therefore, plays considering a crucial role. that, Also, in the the process EDS analysis of removing confirms oily thesubstances, existence the of selection substantial of changesproper cleaners between plays the two a crucial sides ofrole. the Also, swarf the surface. EDS analysis The EDS confirms analysis the of theexistence dark area of substantial shows large changes coal accumulations. between the two The side analyzeds of the area swarf contains surface. carbon The EDS and oxygen,analysis whichof the indicatesdark area theshows presence large ofcoal large accumulations. quantities of The organic analyzed compounds area contains in this carbon area. The and EDS oxygen, analysis which of theindicates bright the area presence shows of that large the quantities carbon stock of organi is muchc compounds smaller; this in indicatesthis area. theThe presenceEDS analysis of organic of the compoundsbright area inshows the area; that however, the carbon organic stock contaminants is much smaller; cover thethis surface indicates in smaller the presence quantities. of organic Due to thecompounds above, significant in the area; changes however, in theorganic elemental contaminan analysists cover of a surface the surface and in oily smaller impurities quantities. are noted. Due Ato detailedthe above, elemental significant analysis changes of the in the contaminants elemental analysis occur on of the a surface surface, and as well oily as impurities elemental are analyses, noted. areA detailed shown inelemental Figures analysis S2–S7. of the contaminants occur on the surface, as well as elemental analyses, are shown in Figures S2–S7.

Materials 2020, 13, 5600 6 of 13 Materials 2020, 13, 5600 6 of 13

Figure 3. The surface of materials from SEM pictures: Titanium Grade 5; dark side, magnification 40× Figure 3. The surface of materials from SEM pictures: Titanium Grade 5; dark side, magnification 40 (A), (A), Titanium Grade 5; dark side, magnification up to 1 mm (D), Inconel 625; dark side, magnification× Titanium Grade 5; dark side, magnification up to 1 mm (D), Inconel 625; dark side, magnification 40 (B), 40× (B), Inconel 625; dark side, magnification up to 1 mm (E), Inconel 718; dark side, magnification× Inconel 625; dark side, magnification up to 1 mm (E), Inconel 718; dark side, magnification 40 (C), 40× (C), Inconel 625; dark side, magnification up to 1 mm (F). × Inconel 625; dark side, magnification up to 1 mm (F).

ForFor quantitative analysis analysis of of the the chemical chemical composition composition of alloy of alloy samples, samples, the Optima the Optima 7300DV 7300DV ICP- ICP-OESOES System System from from Perkin Perkin Elmer Elmer was was used. used. Organi Organicc impurities impurities were were removed removed from from the the swarf swarf by by washingwashing thethe samples samples in in acetone acetone several several times. times. Analytical Analytical weights weights (1 g) (1 were g) were prepared prepared from thefrom cleaned the andcleaned dried and samples dried andsamples solved and in solved a mixture in a of mixture nitric acid of nitric (60%) acid and (60%) hydroﬂuoric and hydrofluoric acid (40%) acid in a (40%) Perkin in a Perkin Elmer microwave mineralizer at 230 °C under the pressure of 35 atmospheres [25,26]. Elmer microwave mineralizer at 230 ◦C under the pressure of 35 atmospheres [25,26]. After dissolution, theAfter sample dissolution, was analyzed the sample for the was content analyzed of certain for th elements.e content Theof certain results elements. of the analyses The results are presented of the inanalyses Table3 below.are presented in Table 3 below.

TableTable 3. 3. TheThe Inductively Coupled Plasma Plasma Optical Optical Emission Emission Spectrometry Spectrometry (ICP-OES) (ICP-OES) elemental elemental results results ofof alloyalloy swarfsswarfs analyses.analyses.

TitaniumTitanium Grade Grade 5 5 Inconel Inconel 625 625 Inconel Inconel 718 718 ElementsElements wt.% wt.% Elements wt.%wt.% ElementsElements wt.% wt.% Ti 82.79 Ni 59.21 Ni 53.92 Ti 82.79 Ni 59.21 Ni 53.92 Al Al 6.89 6.89 Cr Cr 21.1721.17 CrCr 18.17 18.17 V V4.48 4.48 MoMo 9.609.60 FeFe 17.78 17.78 Sb Sb1.90 1.90 FeFe 4.294.29 MoMo 4.50 4.50 Co Co0.23 0.23 AlAl 1.361.36 NbNb 2.87 2.87 Fe Fe0.22 0.22 SbSb 0.740.74 TiTi 1.05 1.05 Cr 0.04 Pb 0.48 Al 0.52 Cr Mn0.04 0.04 TiPb 0.380.48 CoAl 0.41 0.52 Mn Cd 0.04 0.01 Zr Ti 0.06 0.38 SbCo 0.37 0.41 Cd 0.01 Mn Zr 0.040.06 PbSb 0.27 0.37 Cu Mn 0.030.04 ZrPb 0.12 0.27 Cu 0.05 Cu 0.03 Zr 0.12 Cu 0.05 Identiﬁcation of the surface composition by the SEM-EDS method was performed to compare to ICP-OESIdentification analysis of the results. surface The composition research material by the SEM-EDS was prepared method through was performed several washingsto compare with to acetoneICP-OES and analysis then wereresults. placed The research in an oven material and driedwas prepared at 35 ◦C through for 4 h. several The elemental washings composition with acetone of theand surface then were is shown placed inin Tablean oven4. Toand provide dried at high 35 °C results for 4 h. with The the elemental SEM-EDS composition method, aof quantitative the surface is shown in Table 4. To provide high results with the SEM-EDS method, a quantitative analysis

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(Figures S8–S10) was carried out on selected swarfs, which were smooth and their surface was analysispolished ([27].Figures In addition, S8–S10) was in order carried to outdetermine on selected the extent swarfs, to which which were the alloy smooth has and oxidized their surfaceduring wasmachining, polished Figure [27]. 4 In shows addition, a surface in order analysis to determine for distribution the extent of oxygen. to which The the oxygen alloy content has oxidized show duringhigh oxygen machining, distribution Figure 4on shows the swarf a surface surface analysis due for to distributionthe oxidation of oxygen.of the alloy The oxygenpart during content the showmachining. high oxygenFollowing distribution the values on thefrom swarf the surfacetwo selected due to methods the oxidation (ICP-OES, of the SEM-EDS) alloy part duringof the theelemental machining. analysis Following they are theconsidered values fromas convergent. the two selected methods (ICP-OES, SEM-EDS) of the elemental analysis they are considered as convergent. Table 4. The SEM-EDS elemental results of alloy swarfs surface analyses. Table 4. The SEM-EDS elemental results of alloy swarfs surface analyses. Titanium Grade 5 Inconel 625 Inconel 718 Elements Titanium wt.% Grade 5 Elements Inconel 625wt.% Inconel 718Elements wt.% Ti Elements 80.40 wt.% Elements Ni wt.% 53.80 Elements wt.% Ni 54.50 O Ti12.20 80.40 NiCr 53.8028.00 Ni 54.50Fe 19.50 Al O5.20 12.20Mo Cr 28.009.20 Fe 19.50Cr 18.90 C Al2.00 5.20 MoFe 9.204.20 Cr 18.90Mo 6.10 C 2.00 Fe 4.20 Mo 6.10 Nb 3.40 Al 0.70 Nb 3.40 Al 0.70 OO 0.800.80 O 0.20 O 0.20 TiTi 0.60 0.60 Al Al 0.10 0.10

Figure 4. Analysis of the swarf surface of Inconel 718 (A); surface oxygen distribution ofof InconelInconel ((B).).

For GC-MS analyses, the extraction of oily substances was carried out using 500 mL of solventsolvent each time,time, in in open open beakers, beakers, since since during during the purificationthe purification process, process, gases maygases lead may to uncontrolledlead to uncontrolled pressure escalation.pressure escalation. However, However, during the during experiments, the experiments, no solvent evaporationno solvent evaporation was observed. was Purification observed. wasPurification intensified was by intensified immersion by of immersion beakers with of solutionbeakers andwith swarfsolution material and swarf in an ultrasonicmaterial in bath. an Acetoneultrasonic 500 bath. mL (AC)Acetone was 500 selected mL (AC) for the was extraction selected offo oilyr the substances extraction fromof oily 100 substances g of the waste from materials. 100 g of Thethe waste purification materials. was The carried purification out in two was separate carried stages. out in Thetwo second separate stage stages. of purification The second consisted stage of ofpurification repeating consisted the process of withrepeat aing new the portion process of with thesolvent. a new portion The second of the purification solvent. The stage second was introducedpurification sostage that thewas purification introduced effisociency that the of thepuri firstfication stage efficiency could be quantifiedof the first by stage comparing could thebe residualquantified impurities by comparing in the solutionthe residual after impurities the second in purification the solution stage after with the se thecond number purification of impurities stage (chromatographicwith the number of peaks) impurities in the solution(chromatographic from the firstpeaks) purification in the solution stage. Thefrom duration the first of purification each stage wasstage. 60 The min. duration The extracted of each solutionsstage was were 60 min. then Th analyzede extracted for solutions the content were of extractedthen analyzed compounds. for the Thecontent peak of areas extracted obtained compounds. from the chromatographicThe peak areas ob analysistained from after thethe firstchromatographic purification are analysis manytimes after largerthe first than purification the peak areasare many for the times solutions larger afterthan thethe secondpeak areas purification. for the solutions Quantitative after comparison the second ofpurification. these surfaces Quantitative allowed determiningcomparison of the these degree surf ofaces material allowed purification. determining Metal the degree surface of treatment material withpurification. solvents Metal and ultrasounds surface treatmen from residuest with solvents of oily and substances ultrasounds allows from removing residues organic of oily compounds substances fromallows the removing surfaces organic of the analyzed compounds metals from with the esurfacesfficiency of from the 98–99%analyzed (compared metals with with efficiency the baseline from of98-99% a pure (compared solvent) (Figures with the S11–S13). baseline of The a pure effectiveness solvent) (Figures of acetone S11–S13). removal The is significant,effectiveness as of observed acetone inremoval the chromatograms is significant, as of observed metal purification in the chroma stages.tograms The resultsof metal obtained purification from stages. GC/MS The analyses results obtained from GC/MS analyses of the oily extracts from I625, I718, and TG5 alloys show that they

Materials 2020, 13, 5600 8 of 13 of the oily extracts from I625, I718, and TG5 alloys show that they contain mainly a mixture of aliphatic and cycloaliphatic hydrocarbons with the number of carbon atoms ranging from C8–C30. The quality analyses were confirmed with 90% compatibility with the NIST database provided by the Agilent company. Additionally, for NMR analyses, an oily extract from the alloys surface has been 13 extracted to prepare each time a sample of 40 mg/mL in CDCl3 + TMS. For each extract, C NMR and 1H NMR analyses were performed to determine functional groups and identify compounds present in working fluids. The spectra from the NMR analyses are shown in Figures S14–S19. The analysis of the NMR spectra shows that no significant chemical compounds are containing the following groups: Carboxylic, ketone, aldehyde (no signals > 165 ppm in the 13C NMR spectrum), aromatic derivatives (benzene, naphthalene type), and compounds containing double C=C bonds. The lack of <0.5 ppm signals also indicate the absence of organosilicon compounds: Poly (dimethylsiloxane) type. However, the spectra indicate the presence of aliphatic and cycloaliphatic hydrocarbons and possibly small amounts of hydroxyl groups -OH. The sample obtained from the extraction of I625 alloy contains aliphatic and cycloaliphatic hydrocarbons containing methylene, methine and methyl groups (0.5–1.9 ppm signals in the 1H NMR spectrum and 10–30 ppm in the 13C spectrum). The samples obtained from the extraction of I718 and TG5 alloys have a similar composition (Figure S20). Process of washing alloy waste from the oil residues, to be reusable for industry, with solvent-based solutions was carried out using Spirdane D25, Spirdane D40, Spirdane D60. These solvents are called a white spirit consisting of complex hydrocarbon substances obtained by hydro-refining a crude oil cut (n-alkanes, isoalkanes). These solvents offer toxicological and ecotoxicological levels below values specified by legislation in force. Moreover, the solvents are especially designed for the degreasing industry or applications requiring a fast evaporation rate. Additionally, the degree of washing is related to oil content (Table2). The sieve analysis shows that the highest amount of oil on the surface contains TG5 and has the highest adhesion to the alloy surface (Table1). The oil content is also directly related to the degree of washing resulting in the lowest degree for TG5 washing (Table5). Following a series of investigations into the surface washing using the Spirdane cleaning solution, the contamination was practically completely removed from the surface of the alloy (Figure4 and Figures S21–S29). The longer the time it takes to wash the swarfs in the solvents, the more effective the surface treatment is. However, there are only residual amounts of contamination on the bright (smoother) side, which is probably since the blade of the cutting machine more permanently left the contamination on the surface. In addition, no clusters of washing liquids used during cleaning have been observed on the surface (Figures S21–S29). The percentage of oil residues was determined using GC/MS technique according to the protocol used to identify the type of contaminations. Analyses have shown that washing with solvents for 60 min almost completely removes surface contamination. The removal range for oily substances is between 93.7 and 98.6 percent, indicating high process efficiency (Table5). Solvent-based washing processes give the same results in three repetitions. Effective washing capacity depends primarily on its ability to disperse, transport, dissolve, and thus remove impurities from the solid matrix [10]. However, it is considered that the reason why the oil removal efficiency decreases with an increase in specific mass density is that the sample molecules are more agglomerated as the mass density increases, making contact difficult between the molecules and the washing agents. This means that the specific gravity has played a significant role in removing oil from the alloy swarfs [28].

Table 5. Removal percentages of oil residues from swarfs after 60 min using washing agents.

Alloy Type Titanium Grade 5 Inconel 625 Inconel 718 Washing agent Oil removal [%] Oil removal [%] Oil removal [%] SPIRDANE D25 96.2 98.6 97.8 SPIRDANE D40 94.6 96.4 96.7 SPIRDANE D60 93.7 95.8 95.9 Materials 2020, 13, 5600 9 of 13 Materials 2020, 13, 5600 9 of 13

InIn addition, the the microcracks microcracks that that form form during during the the alloying alloying process process create create areas areas where where the the oil oilenters enters deeper, deeper, resulting resulting in exceedingly in exceedingly difficult diﬃcult for forthe thewashing washing agent agent to penetrate to penetrate and andwash wash out outcontaminants. contaminants. Even Even a long a long washing washing process process does does not eliminate not eliminate 100% 100% of the of oi thel residues. oil residues. It has It been has beenobserved observed that washing that washing the surfaces the surfaces with with solvents solvents has hasa higher a higher or lower or lower oxygen oxygen distribution distribution than than in inthe the case case of ofuntreated untreated material. material. (Tables (Tables 44 andand 55).). However, thethe remainingremaining composition composition of of the the surface surface staysstays unchangedunchanged (Table(Table6 6 and and Figure Figure5 ).5). This This shows shows that that the the solvents solvents used used for for washing washing do do not not cause cause oxidationoxidation ofof thethe alloys’alloys’ surfacessurfaces and can be employedemployed as washing agent to recover alloys for re-use, whilewhile maintainingmaintaining theirtheir originaloriginal properties.properties.

TableTable 6.6. OxygenOxygen distributiondistribution on the surfacesurface ofof swarfsswarfs inin aqueousaqueous treatmenttreatment processprocess andand thermalthermal treatmenttreatment process.process.

AlloyAlloy Type Type Titanium Titanium Grade Grade 5 5 Inconel Inconel 625 625 Inconel Inconel718 718 TreatmentTreatment Process Process Type Type Oxygen Oxygen [%] [%] Oxygen Oxygen [%] [%] Oxygen Oxygen [%] [%] AqueousAqueous Treatment Treatment Process Process 10.1 10.1 0.8 0.8 1.2 1.2 ThermalThermal Treatment Treatment Proc Processess 47.947.9 32.1 32.1 29.1 29.1

Figure 5. SEM-EDS photos after oil removal from Inconel 625 (A,A1), Inconel 718 (B,B1), Titanium GradeFigure 55. (C SEM-EDS,C1) swarfs photos in aqueous after oil treatment removal process from Inconel (Spirdane 625 D40(A,A1 after), Inconel 60 min) 718 with (B, theB1), elemental Titanium compositionGrade 5 (C,C1 of) theswarfs surfaces. in aqueous treatment process (Spirdane D40 after 60 min) with the elemental composition of the surfaces. In the process of thermal removal of oil residues at temperature 900 ◦C in the muffle furnace, changesIn the in theprocess structure of thermal of the removal alloys and of significantoil residues oxygen at temperature content on 900 the °C surface in the muffle were observed furnace, (Figurechanges6 ).in Duethe structure to the lack of ofthe oxygen alloys and availability significant in theoxygen mu fflcontente furnace, on the the surface pyrolysis were caused observed the oxidation(Figure 6). on Due the surface.to the lack The mainof oxygen components availability of the in alloys the havemuffle been furnace, reduced the in pyrolysis significant caused quantities. the Onoxidation the other on hand,the surface. there is The a high main oxygen components distribution of the on thealloys surface, havereaching been reduced almost in 50% significant for TG5 (Tablequantities.6). In On the the process other of hand, thermal there removal is a high of oiloxygen residues, distribution the microcracks on the surface, formed reaching on both sidesalmost of 50% the materialsfor TG5 (Table have an 6). impact In the onprocess the high-temperature of thermal remova oil,l whoseof oil residues, main component the microcracks is carbon, formed which reduceson both thesides composition of the materials on the have surface an impact of the alloys.on the high The-temperature enormous changes oil, whose in the main composition component of theis carbon, alloys dowhich not allowreduces the the use composition of a thermal on treatment the surface process of the to alloys. dispose The of enormous oil residues. changes The materials in the composition treated in thisof the manner alloys cannot do not be allow fully recoveredthe use of fora thermal reapplication. treatment process to dispose of oil residues. The materials treated in this manner cannot be fully recovered for reapplication.

Materials 2020, 13, 5600 10 of 13 Materials 2020, 13, 5600 10 of 13

Figure 6. SEM-EDS photosphotos after after oil oil removal removal from from Inconel Inconel 625 (A625,A1 (A), Inconel,A1), Inconel 718 (B ,718B1), ( TitaniumB,B1), Titanium Grade 5 Grade(C,C1) 5 swarfs (C,C1 in) swarfs a thermal in a processthermal (900 process◦C) with(900 °C) the with elemental the elemental composition composition of the surfaces. of the surfaces.

Although it it is is not not possible possible to remove to remove 100% 100% of the of oil the residues oil residues employing employing washing washing with solvent- with basedsolvent-based detergents, detergents, it is advantageous it is advantageous to obtain a toma obtainterial with a material a degree with of purification a degree of close purification to 100% forclose use to to 100% other for applications. use to other applications.Mishra et al. Mishrafor the etrecovery al. for the of recoveryferrous grinding of ferrous swarf grinding used swarf also commercialused also commercial detergents detergents achieving achieving100% of cleaning 100% of ef cleaningfectiveness effectiveness [29]. However, [29]. However,the studies the available studies inavailable the literature in the literature are not arebased not basedon effectiveness on effectiveness verification verification based based on onthe the GCMS GCMS technique technique and therefore cannotcannot be be compared compared with with other other results. results. In this In case,this acase, combination a combination of heavy of organic heavy fractionsorganic fractionsis better tois better clean theto clean metal the surface metal withsurface light with organic light fractionsorganic fractions such as Spirdanesuch as Spirdane D25. The D25. results The resultsof the presentedof the presented analyses analyses show thatshow it that is possible it is possible to dispose to dispose of up of to up 99% to of99% the of contaminants the contaminants in a inshort a short time. time.Therefore, Therefore, the the above above studies studies,, due due to to the the surface surface coverage coverage with with oxygen oxygen inin smallsmall amount after washing, makemake thethe processprocess of of passivation passivation more more convenient convenient and and it canit can be be carried carried out out to maintainto maintain the thematerial’s material’s corrosion corrosion resistance. resistance. A standard A standard treatment treatment process process slightly reducesslightly thereduces corrosion the resistancecorrosion resistanceof the materials. of the Thematerials. passivation The passivation process first process removes first free removes iron, which free mayiron, bewhich on the may surface be on from the surfacecontact from with machinecontact with tools machine or oil residuals. tools or oil Cleaning residuals. procedures Cleaning ofprocedures I625, I728, of and I625, TG5, I728, reported and TG5, in reportedthese studies. in these are studies. sufficient are to sufficient start the passivationto start the passivation process in alkaline process solutions. in alkaline The solutions. lack of availableThe lack ofalloy available cleaning alloy techniques cleaning intechniques the literature in the shows literature that this shows issue that is worththis issue investigation. is worth investigation.

4. Conclusions Conclusions The feasibility of the washing process and thermal treatment for swarf recovery into reusable materials for the industry was inve investigated.stigated. Microscopic Microscopic analysis of I625, I718, and TG5 have shown that thethe swarf swarf surfaces surfaces di ﬀdifferer in texture. in texture. Besides, Beside thes, analysis the analysis showed showed that the impuritiesthat the impurities are irregularly are irregularlydistributed, distributed, and in the and case in of the the case wrinkled of the wrinkl side (theed side dark (the side) dark the side) impurities the impurities are mainly are locatedmainly locatedin its cavities. in its Thecavities. contaminants The contaminants contain mainly contain carbon mainly indicating carbon that indicating the covering that substancesthe covering are substancesmainly aliphatic are mainly and cycloaliphatic aliphatic and hydrocarbons, cycloaliphatic with hydrocarbons, no nitrogen. with The no gravimetric nitrogen. analysisThe gravimetric showed analysisthat the showed content that of impurities the content is of about impurities 4.2%, 4.6%,is about and 4.2%, 22% 4.6%, for I625, and I718,22% for and I625, TG5, I718, respectively, and TG5, respectively,and the amount and of the impurities amount of does impurities not change does during not change storage. during Additionally, storage. Additionally, during the particle during sizethe particleanalysis, size discrepancies analysis, discrepancies in the particle in size the were particle found, size with were the found, highest with percentage the highest fractions percentage in the fractionsrange 3.5–1.6 in the mm range (I625) 3.5–1.6, 6.3–3.15 mm (I625), mm (I718) 6.3–3.15 and abovemm (I718) 6.3 mm and (TG5). above Attempts 6.3 mm (TG5). to use Attempts washing agentsto use washingshow that agents organic show solvents that organic have a goodsolvents impact have on a thegood surface impact structure on the surface and washes structure oil residues and washes with oilalmost residues 99% ofwith eﬀ ectiveness.almost 99% Also, of effectiveness. it has been found Also, thatit has the been thermal found treatment that the ofthermal I625, I718, treatment and TG5 of I625, I718, and TG5 alloys at 900 °C removes contaminants; oxidizing significantly the surface of the alloys and leads to irreversible changes in structure.

Materials 2020, 13, 5600 11 of 13

alloys at 900 ◦C removes contaminants; oxidizing signiﬁcantly the surface of the alloys and leads to irreversible changes in structure.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/24/5600/s1, Table S1. Relevant values for physical analysis of swarf materials after one month of storage. Figure S1. The surface of swarf materials from SEM pictures: Titanium Grade 5; dark side; magnification up to 500 µm (A), Titanium Grade 5; bright side; magnification up to 500 µm (D), Inconel 625; dark side; magnification up to 500 µm (B), Inconel 625 bright side; magnification up to 500 µm (E), Inconel 718; dark side; magnification up to 500 µm (C), Inconel 625; bright side; magnification up to 500 µm (F). Figure S2. SEM image of Inconel 718—the dark side—magnification up to 20 µm (A-B). EDS elemental analyses (C-D). Figure S3. SEM image of Inconel 718—the bright side—magnification up to 20 µm (A-B). EDS elemental analyses (C-D). Figure S4. SEM image of Inconel 625—the dark side—magnification up to 20 µm (A-B). EDS elemental analyses (C-D). Figure S5. SEM image of Inconel 625—the bright side—magnification up to 20 µm (A-B). EDS elemental analyses (C-D). Figure S6. SEM image of Titanium Grade 5—the dark side—magnification up to 10 µm (A-B). EDS elemental analyses (C-D). Figure S7. SEM image of Titanium Grade 5—the bright side—magnification up to 10 µm (A-B). EDS elemental analyses (C-D). Figure S8. SEM-EDS analysis on cleaned swarf surface of Titanium Grade 5 alloy at the selected point. Figure S9. SEM-EDS analysis on cleaned swarf surface of Inconel 625 alloy at the selected point. Figure S10. SEM-EDS analysis on cleaned swarf surface of Inconel 718 alloy at the selected point. Figure S11. Chromatogram of acetone oily extract by GC/MS technique for Inconel 625. First metal ultrasonic treatment. Figure S12. Chromatogram of acetone oily extract by GC/MS technique for Inconel 718. First metal ultrasonic treatment. Figure S13. Chromatogram of acetone oily extract by GC/MS technique for Inconel 625, 718 and Titanium Grade 5. Second metal ultrasonic treatment. Figure S14. The 13C NMR spectrum obtained by extraction of the organic fraction of the Inconel 625 alloy. Figure S15. The 1H NMR spectrum obtained by extraction of the organic fraction of the Inconel 625 alloy. Figure S16. The 13C NMR spectrum obtained by extraction of the organic fraction of the Inconel 718 alloy. Figure S17. The 1H NMR spectrum obtained by extraction of the organic fraction of the Inconel 718 alloy. Figure S18. The 13C NMR spectrum obtained by extraction of the organic fraction of the Titanium Grade 5 alloy. Figure S19. The 1H NMR spectrum obtained by extraction of the organic fraction of the Titanium Grade 5 alloy. Figure S20. The comparison of 1H NMR spectra of the analyzed organic fractions: (A) Inconel 625; (B) Inconel 718; (C) Titanium Grade 5, indicates their high chemical similarity. Figure S21. SEM-EDS photos after oil removal from Inconel 625 swarfs by washing with Spirdane D25 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S22. SEM-EDS photos after oil removal from Inconel 718 swarfs by washing with Spirdane D25 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S23. SEM-EDS photos after oil removal from Titanium Grade 5 swarfs by washing with Spirdane D25 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S24. SEM-EDS photos after oil removal from Inconel 625 swarfs by washing with Spirdane D40 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S25. SEM-EDS photos after oil removal from Inconel 718 swarfs by washing with Spirdane D40 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S26. SEM-EDS photos after oil removal from Titanium Grade 5 swarfs by washing with Spirdane D40 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S27. SEM-EDS photos after oil removal from Inconel 625 swarfs by washing with Spirdane D60 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S28. SEM-EDS photos after oil removal from Inconel 718 swarfs by washing with Spirdane D60 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Figure S29. SEM-EDS photos after oil removal from Titanium Grade 5 swarfs by washing with Spirdane D60 after 20 (A-dark side; A1-bright side), 40 (B-dark side; B1-bright side), and 60 (C-dark side; C1-bright side) minutes. Author Contributions: Conceptualization, D.B.; methodology, D.B. and S.Z.;˙ software, D.B., S.Z.,˙ and T.S.;´ validation, D.B., S.Z.,˙ W.Z.˙ and T.S.;´ formal analysis, D.B., S.Z.,˙ W.Z.,˙ and T.S.;´ investigation, D.B., S.Z.,˙ W.Z.,˙ S.B. and T.S´ resources, D.B. and S.Z.;˙ data curation, D.B., S.Z.,˙ W.Z.˙ and T.S.;´ writing—original draft preparation, T.S.;´ writing—review and editing, T.S.;´ visualization, W.K., S.B. and T.S.;´ supervision, D.B.; project administration, D.B., All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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