applied sciences Article
Comparison of Grinding Characteristics of Converter Steel Slag with and without Pretreatment and Grinding Aids Jihui Zhao 1, *, Dongmin Wang 2 , Peiyu Yan 1 and Wenping Li 2 1 2
*
Institute of Building Materials, Department of Civil Engineering, Tsinghua University, Beijing 100084, China;
[email protected] School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing 100083, China;
[email protected] (D.W.);
[email protected] (W.L.) Correspondence:
[email protected]; Tel.: +86-159-0124-5174
Academic Editor: Stefano Invernizzi Received: 10 June 2016; Accepted: 8 August 2016; Published: 28 October 2016
Abstract: The converter steel slag cannot be widely used in building materials for its poor grindability. In this paper, the grinding characteristics of untreated and pretreated (i.e., magnetic separation) steel slag were compared. Additionally, the grinding property of pretreated steel slag was also studied after adding grinding aids. The results show that the residues (i.e., oversize substance) that passed a 0.9 mm square-hole screen can be considered as the hardly grinding phases (HGP) and its proportion is about 1.5%. After the initial 20 min grinding, the RO phase (RO phase is a continuous solid solution which is composed of some divalent metal oxides, such as FeO, MgO, MnO, CaO, etc.), calcium ferrite, and metallic iron phase made up most of the proportion of the HGP, while the metallic iron made up the most component after 70 min grinding. The D50 of untreated steel slag could only reach 32.89 µm after 50 min grinding, but that of pretreated steel slag could reach 18.16 µm after the same grinding time. The grinding efficiency of steel slag was obviously increased and the particle characteristics were improved after using grinding aids (GA), especially the particle proportions of 3–32 µm were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min, and 60 min grinding, respectively. This is mainly because of the reduction of agglomeration and this effect of GA was evidenced by SEM (scanning electron microscope) images. Keywords: steel slag; grinding property; particle characteristics; mineral phase; hardly grinding phases; magnetic separation; grinding aids; agglomeration
1. Introduction Steel slag is an industrial solid waste generated in the steel-making process, but it is rich in dicalcium silicate (C2 S) and tricalcium silicate (C3 S) in mineral compositions, which is similar to cement clinker [1–4]. Based on its mineral compositions, steel slag has great potential for application as supplementary cementitious materials (also known as mineral admixture) of cement and concrete [5–8]. Generally, only after being ground into powder, could steel slag have hydration activity and be used as supplementary cementitious materials [9,10]. The smaller the particle size of steel slag powder, the higher its hydration activity [11,12]. However, the grindability of steel slag is much worse than that of other mineral admixtures such as fly ash and granulated blast-furnace slag, seriously impeding the preparation efficiency of steel slag powder and its application performance in building materials. Therefore, its grinding property is an important consideration in the application of steel slag. In the grinding process of a solid, particles characteristics such as size, distribution, bulk density, and dispersion are in constant change on a macroscopic scale, and the internal chemical bonds of
Appl. Sci. 2016, 6, 237; doi:10.3390/app6110237
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particles are constantly broken on a microscopic scale [13–15]. Steel slag has poor grindability because of a large amount of iron oxides and continuous solid solution composed of some divalent metal oxides, which form a dense structure in the steel slag [16,17]. Currently, the researches on improvement of grinding efficiency for steel slag mainly focus on the optimization of grinding equipment—such as vertical mill and roller mill, while there are few researches on other measures, such as removal of hardly grinding phases (HGP) or the use of chemical admixtures. Although vertical mills have been applied in grinding of steel slag, many application researches indicate that steel slag powder prepared by vertical mills have poor particle morphology—such as flat, needle, and irregular shapes, resulting in the ball mill still being chosen by many manufacturers to prepare steel slag powder in China. Meanwhile, some studies indicate that metallic iron in steel slag is one main reason causing its poor grinding property. However, in different grinding periods, the factors influencing grinding efficiency usually change due to different grinding characteristics [18–20], but to date these factors have not been yet been fully studied. Some researches also show that the split-phase phenomenon occurs in the grinding process of steel slag, resulting in ground steel slag powder having some differences in chemical and mineral compositions during different grinding periods [21,22]. So, it is necessary to reveal the factors influencing the grinding efficiency of steel slag from studying grinding characteristics, hardly grinding phases, and split-phase phenomenon during different grinding periods. Moreover, in the grinding process of a solid, other phenomena such as agglomeration of fine particles and re-healing of particle fracture surfaces result in a lower grinding efficiency and higher energy consumption [23–26]. In recent years, grinding aids, most of which are polar organic compounds, are extensively applied to weaken agglomeration of fine particles and improve grinding efficiency of cement [27]. Therefore, the use of grinding aids is also an important way to improve preparation efficiency of steel slag powder. Based on the abovementioned discussion, the hardly grinding phases of converter steel slag were revealed from the proportion, morphology, mineral, and chemical compositions. Then the grinding characteristics of untreated and pretreated (i.e., magnetic separation) steel slag were compared from the proportion of iron phases, grinding efficiency, particle size, and particle distribution and morphology. Further, the grinding property of pretreated steel slag was also studied after adding organic grinding aids and its positive role and mechanism were discussed here as well. 2. Materials and Methods 2.1. Materials The converter steel slag used was provided from Laiwu Steel Corporation (Laiwu, China). The chemical compositions of steel slag, which were determined by X-ray fluorescence (XRF) analysis, (Rh, 40 kV, 70 mA), are given in Table 1. The mineral phases of steel slag, which were analyzed by X-ray diffraction (XRD), using a D6000 diffractometer with nickel-filtered Cu Kαl radiation (= 1.5405 Å, 40 kV, and 40 mA) from Shimadzu company (Kyoto, Japan), are given in Figure 1. The organic grinding aid used, produced by Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China), was chemical grade glycerol. Table 1. Chemical compositions of converter steel slag (%). CaO
SiO2
Al2 O3
Fe2 O3
MgO
K2 O
SO3
P2 O5
LOI
39.67
21.71
1.58
25.52
4.92
0.02
0.18
1.70
0.32
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1- C2S
2- C3S
3- C3A
4- C4AF
5-C2F
6- RO
7- 3CaO Fe2O3 3SiO2 8- 2CaO MgO 2SiO2
9 7 6 54 3 2 9 7 6 2 1
8 1
1 1
6
9- 3CaO 2TiO2
10-Fe
11- f-CaO
13- CaCO3
12- f-MgO 6
6 1
2 1 7 6
10
4 10
1 11
13
2
7 2
3
12
10
20
30
40
50
60
70
80
o
2( )
Figure 1. X‐ray diffraction (XRD) pattern of converter steel slag. Figure 1. X-ray diffraction (XRD) pattern of converter steel slag.
2.2. Experimental Methods 2.2. Experimental Methods 2.2.1. Observation and Identification of Mineral Phases for Converter Steel Slag 2.2.1. Observation and Identification of Mineral Phases for Converter Steel Slag The steel slag sample needed to be prepared for the morphology observation of mineral phase. The steel slag sample needed to be prepared for the morphology observation of mineral phase. Firstly, the surface of the 3–8 mm blocky steel slag was burnished using P200#, P400#, P600#, P800#, Firstly, the surface of the 3–8 mm blocky steel slag was burnished using P200#, P400#, P600#, P800#, P1000#, and and P1200# P1200# sandpapers in turn, and then was polished then polished by a (UNIPOL-830, polisher (UNIPOL‐830, P1000#, sandpapers in turn, and was by a polisher Shenyang Shenyang Kejing Instrument Co., Ltd., Shenyang, China). The mineral phases were Kejing Instrument Co., Ltd., Shenyang, China). The mineral phases were observed andobserved identifiedand by identified by back‐scattered electron (BSE) imaging and energy dispersive X‐ray spectroscopy back-scattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDX), respectively, (EDX), respectively, using a scanning electron microscope (Quanta 200 OR, FEG, FEI In Company, using a scanning electron microscope (Quanta 200 FEG, FEI Company, Hillsboro, USA). addition, Hillsboro, OR, USA). In addition, the hardly grinding phases were observed using a metallographic the hardly grinding phases were observed using a metallographic microscope (BA210Met, Motic China microscope (BA210Met, Motic China Group Co., Ltd., Xiamen, China). Group Co., Ltd., Xiamen, China). 2.2.2. Measurement of Vickers Hardness of Mineral Phases for Converter Steel Slag 2.2.2. Measurement of Vickers Hardness of Mineral Phases for Converter Steel Slag The Vickers hardness of different mineral phases for steel slag was measured using a Vickers The Vickers hardness of different mineral phases for steel slag was measured using a Vickers hardness tester tester (HV-1000, (HV‐1000, Shanghai Materials Tester Factory, Shanghai, China), and the hardness hardness Shanghai Materials Tester Factory, Shanghai, China), and the hardness value value of each mineral phase was determined by taking the average of multiple points. of each mineral phase was determined by taking the average of multiple points. 2.2.3. Procedure of Grinding Experiment for Steel Slag 2.2.3. Procedure of Grinding Experiment for Steel Slag Grinding slag were carried outout by laboratory ball mill Wuxi Jianyi Grinding experiments experiments ofof steel steel slag were carried by laboratory ball (SM-500, mill (SM‐500, Wuxi Experiment Instrument Co., Ltd., Wuxi, China). The type of the ball mill was Φ500 mm × 500 mm, Jianyi Experiment Instrument Co., Ltd., Wuxi, China). The type of the ball mill was Ф500 mm × 500 48 r/min, and the grinding media was composed of 60 kg steel balls (Φ40 mm, Φ50 mm, Φ60 mm, and mm, 48 r/min, and the grinding media was composed of 60 kg steel balls (Φ40 mm, Φ50 mm, Φ60 Φ70 mm) and 40 kg small steel forgings (Φ25 mm × 35 mm). mm, and Φ70 mm) and 40 kg small steel forgings (Φ25 mm × 35 mm). The steel slag was firstly crushed to less than 5 mm by a jaw crusher (PE 60 × 100, Shanghai The steel slag was firstly crushed to less than 5 mm by a jaw crusher (PE 60 × 100, Shanghai Longshi Machinery Co., Ltd., Shanghai, China) before the grinding experiment. The weight of steel Longshi Machinery Co., Ltd., Shanghai, China) before the grinding experiment. The weight of steel slag for each grinding experiment was 3 kg and the grinding times were 10 min, 20 min, 30 min, 40 min, slag for each grinding experiment was 3 kg and the grinding times were 10 min, 20 min, 30 min, 40 50 min, 60 min, and 70 min. In addition, for the grinding aids experiment, 0.05% grinding aid was min, 50 min, 60 min, and 70 min. In addition, for the grinding aids experiment, 0.05% grinding aid added into the test group to compare with the blank group (i.e., without any grinding aids). was added into the test group to compare with the blank group (i.e., without any grinding aids). 2.2.4. Test Methods of Particle Size and Distribution of Ground Steel Slag Powder 2.2.4. Test Methods of Particle Size and Distribution of Ground Steel Slag Powder The surface area of ground steel slag powder was measured by Blaine method, conforming The specific specific surface area of ground steel slag powder was measured by Blaine method, to Chinese National Standard GB/T8074-2008. The sieving residue of steel slag powder was measured conforming to Chinese National Standard GB/T8074‐2008. The sieving residue of steel slag powder by sieving analysis methodanalysis with a 45 µm square-hole conforming to theconforming Chinese National was measured by sieving method with a 45 screen, μm square‐hole screen, to the Standard GB/T1345-2005. The particle size distribution was measured by laser-scattering method, Chinese National Standard GB/T1345‐2005. The particle size distribution was measured by conforming to the Chinese Industry Standard JC/T721-2006. laser‐scattering method, conforming to the Chinese Industry Standard JC/T721‐2006.
2.2.5. Observation of Particle Morphology of Ground Steel Slag Powder
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2.2.5. Observation of Particle Morphology of Ground Steel Slag Powder The particle morphology of ground steel slag powder was observed with a scanning electron The particle morphology of ground steel slag powder was observed with a scanning electron microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA) under high vacuum condition. microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA) under high vacuum condition.
2.2.6. Test Methods of the Angle of Repose of Ground Steel Slag Powder 2.2.6. Test Methods of the Angle of Repose of Ground Steel Slag Powder The angle of repose of ground steel slag powder was tested conforming to the Chinese The angle of repose of ground steel slag powder was tested conforming to the Chinese National National Standard GB/T11986‐1989, as follows: steel slag powder was poured into a funnel, and Standard GB/T11986-1989, as follows: steel slag powder was poured into a funnel, and then powder then powder from the funnel fell onto and coated the disc below the funnel. Then the height, h, of from the funnel fell onto and coated the disc below the funnel. Then the height, h, of the powder layer the powder layer and the radius, R, of the disc were measured, thus the angle of repose, θ, of steel and the radius, R, of the disc were measured, thus the angle of repose, θ, of steel slag powder was slag powder was obtained according to the formula (tan θ = h/R). obtained according to the formula (tan θ = h/R). 3. Results and Discussion 3. Results and Discussion 3.1. Mineral Phases’Characteristics of Converter Steel Slag 3.1. Mineral Phases’Characteristics of Converter Steel Slag As shown in Figure 2(a), the morphologies of mineral phases in steel slag show different grey As shown in Figure 2a, the morphologies of mineral phases in steel slag show different grey levels levels under BSE images, such as black, grey‐black, grey, light‐grey, and white‐bright in grey level, under BSE images, such as black, grey-black, grey, light-grey, and white-bright in grey level, which which exhibit different shapes as well, such as round shape, leaf‐like shape, hexagonal‐plate shape, exhibit different shapes as well, such as round shape, leaf-like shape, hexagonal-plate shape, irregular irregular so on. The compositions of these minerals grey with different grey levels using were shape, andshape, so on. and The compositions of these minerals with different levels were determined determined using EDX analysis, and the results are shown in Figure EDX analysis, and the results are shown in Figure 2b. It can be seen from2(b). It can be seen from EDX EDX analysis results that the analysis results that the minerals with grey-black grey levels black and grey‐black are mainly silicate minerals with grey levels of black and areof mainly the silicate minerals phasesthe which are minerals phases which are composed of oxygen, silicon, and calcium elements; the irregular mineral composed of oxygen, silicon, and calcium elements; the irregular mineral phases with grey level of phases with grey level of light‐grey are mainly RO phase which is composed of oxygen, magnesium, light-grey are mainly RO phase which is composed of oxygen, magnesium, calcium, manganese, and calcium, manganese, and iron elements; irregular‐shaped minerals with grey level of grey, iron elements; the irregular-shaped mineralsthe with grey level of grey, which are filled in light-grey and which are filled in light‐grey and black minerals, are the calcium ferrite phases mainly composed of black minerals, are the calcium ferrite phases mainly composed of oxygen, calcium, iron, aluminum, oxygen, calcium, iron, the aluminum, and silicon elements; the with round granular‐shaped minerals and silicon elements; round granular-shaped minerals grey level of white-bright arewith the grey level white‐bright are the metallic iron above results that the mineral metallic ironof phase. The above results show that thephase. mineralThe phases of steel slagshow mainly contain silicate phases of steel RO slag mainly contain silicate mineral phase, phase, and mineral phase, phase, calcium ferrite phase, and phase, a smallRO amount of calcium metallic ferrite iron phase, etc. It isa small amount of metallic iron phase, etc. It is also evident from XRD analysis presented in Figure 1 also evident from XRD analysis presented in Figure 1 that the mineral phases converter steel slag that the contains mineral Cphases slag mainly 2S, C3phases) S, RO, 3CaO∙Fe 2O3(Comment: ∙3SiO2 (i.e., mainly RO, 3CaOsteel ·Fe2 O calcium C ferrite and so on. 2 S, C3 S,converter 3 ·3SiO 2 (i.e.,contains calcium ferrite phases) and so on. (Comment: RO phase is a continuous solid solution is RO phase is a continuous solid solution which is composed of some divalent metal oxides,which such as composed of some divalent metal oxides, such as FeO, MgO, MnO, CaO, etc.) FeO, MgO, MnO, CaO, etc.)
6
1 1 3
2
6
4
1
5 5
4
6
3
4
2
3 2
100μm
5 50μm
(a) Figure 2. Cont.
100μm
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Average result
1
Average result
2
Element
Wt %
At %
51.55
OK
29.34
48.97
01.10
01.20
MgK
00.22
00.24
AlK
01.95
01.91
AlK
00.82
00.81
SiK
11.33
10.69
SiK
14.59
13.87
PK
02.12
01.81
PK
CaK
43.91
29.03
CaK
45.35
30.21
FeK
05.32
02.52
FeK
03.06
01.47
ZnK
03.16
01.28
ZnK
02.84
01.16
Element
Wt %
At %
OK
31.12
MgK
03.78
Average result
3
03.26
Element
Wt %
At %
OK
17.89
35.77
MgK
22.02
28.97
CaK
03.49
02.79
MnK
05.29
03.08
FeK
51.31
29.39
4 Average result Element
Wt %
At %
OK
18.40
37.52
MgK
18.20
24.43
CaK
03.88
03.16
CrK
01.15
00.72
MnK
06.81
04.05
FeK
51.55
Average result
5
30.11
Element
Wt %
At %
OK
22.40
43.45
MgK
01.24
01.58
AlK
06.09
07.00
SiK
00.99
6 Average result Element
Wt %
At %
01.09
OK
01.42
04.77
01.36
01.82
97.22
93.41
CaK
35.34
27.36
CaK
TiK
06.42
04.16
FeK
CrK
01.33
00.79
FeK
26.19
14.56
(b) Figure 2.2. Morphologies identification of mineral in steel converter slag. (a): Figure Morphologies and and identification of mineral phases inphases converter slag. (a):steel Back-scattered Back‐scattered electron (BSE) images of various mineral phases; (b) Energy dispersive X‐ray electron (BSE) images of various mineral phases; (b) Energy dispersive X-ray spectroscopy (EDX) spectroscopy (EDX) analysis results of various mineral phases. analysis results of various mineral phases.
3.2. Determination of Hardly GrindingPhases (HGP) in Converter Steel Slag 3.2. Determination of Hardly Grinding Phases (HGP) in Converter Steel Slag As the grindability of various mineral phases in steel slag have great differences, the easily As the grindability of various mineral phases in steel slag have great differences, the easily grinding phases (denoted as EGP) are firstly being ground to fine powder, while the poor grinding phases (denoted as EGP) are firstly being ground to fine powder, while the poor grindability grindability mineral phases are difficult to grind down, which seriously affects the grinding mineral phases are difficult to grind down, which seriously affects the grinding efficiency. However, efficiency. However, different mineral phases in steel slag are very difficult to be separated out, so it different mineral phases in steel slag are very difficult to be separated out, so it is impossible to is impossible to determine what is an easily grinding phase and hardly grinding phase from the determine what is an easily grinding phase and hardly grinding phase from the grindability index grindability index of various mineral phases. In this paper, a simple method for determination of of various mineral phases. In this paper, a simple method for determination of the hardly grinding the hardly grinding phase in steel slag by oversize substance (i.e., residue) is provided. After phase in steel slag by oversize substance (i.e., residue) is provided. After grinding and then screening grinding and then screening of steel slag powder, the oversize substance obtained can be considered of steel slag powder, the oversize substance obtained can be considered as the hardly grinding phases as the hardly grinding phases (denoted as HGP) in steel slag. In order to determine the HGP in the (denoted as HGP) in steel slag. In order to determine the HGP in the grinding process, the steel slag grinding process, the steel slag powder after 10, 20, 30, 40, 50, 60, and 70 min grinding were screened powder after 10, 20, 30, 40, 50, 60, and 70 min grinding were screened with a 0.9 mm square-hole with a 0.9 mm square‐hole screen, and then the proportion, morphology, and compositions of screen, and then the proportion, morphology, and compositions of oversized substances (i.e., HGP) oversized substances (i.e., HGP) were analyzed. were analyzed. 3.2.1. Proportion of HGP in Converter Steel Slag 3.2.1. Proportion of HGP in Converter Steel Slag The proportions of oversized The proportions of oversized substances substances after after different different grinding grinding times are shown in Figure times are shown in Figure 3. 3. Form the figure, with the increasing of grinding time of steel slag, the proportion proportion of of oversized oversized Form the figure, with the increasing of grinding time of steel slag, the substances that that passed passed the the 0.9 mm square‐hole screen rapidly declined in the range range of of 10–20 min substances 0.9 mm square-hole screen rapidly declined in the 10–20 min grinding time, and then starts to slowly decline (starting from 30 min grinding time), until the the grinding time, and then starts to slowly decline (starting from 30 min grinding time), until proportion finally reaches about 1.5%, which indicates that this part of oversize substances is the proportion finally reaches about 1.5%, which indicates that this part of oversize substances is the HGP in converter steel slag. In other words, the proportion of HGP determined by residue method HGP in converter steel slag. In other words, the proportion of HGP determined by residue method of of 0.9 mm square‐hole screen is about 1.5%. 0.9 mm square-hole screen is about 1.5%.
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Content Content of of oversize oversize products products for for 0.9mm 0.9mm screen screen (%) (%)
4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 10 10
20 20
30 30
40 40
50 50
60 60
70 70
Grindingtime time(min) (min) Grinding
Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time. Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time. Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time.
3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag 3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag 3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting of grey grey levels during the initial levels during the initial grinding period (20 min grinding time), while basically exhibiting of grinding period (20 min grinding time), while basically exhibiting one grey the middle–later grinding periods (i.e., 50 min and 70 min grinding times). 50 min and 70 min grinding times). one grey grey level level during during the middle–later grinding periods (i.e., 50 min and 70 min grinding times). one level during the middle–later grinding periods (i.e., Based on the identification and analysis results of mineral phases in Section 3.1, it can be known Based on the identification and analysis results of mineral phases in section 3.1, it can be known that Based on the identification and analysis results of mineral phases in section 3.1, it can be known that that the mineral phases with three kinds grey levels are mainly theRO phase, calcium RO phase, calcium ferrite, and ferrite, and the mineral phases with three kinds of of grey levels are mainly the RO phase, calcium ferrite, and the mineral phases with three kinds of grey levels are mainly the metallic phase. So,So, thethe HGP of converter steel slag during the initial grinding period is composed metallic iron iron phase. So, the HGP of converter converter steel slag during during the initial grinding grinding period is is metallic iron phase. HGP of steel slag the initial period of RO phase, calcium ferrite, and metallic iron phase, while the HGP is mainly the metallic iron phase composed of of RO RO phase, phase, calcium calcium ferrite, ferrite, and and metallic metallic iron iron phase, phase, while while the the HGP HGP is is mainly mainly the the composed during the later grinding period, and the longer grinding time, the higher the proportion of metallic metallic iron phase during the later grinding period, and the longer grinding time, the higher the metallic iron phase during the later grinding period, and the longer grinding time, the higher the iron phase. proportion of metallic iron phase. proportion of metallic iron phase.
33
11 2222
3333 2222
11 11
2222
3333
11
2222
11
11
2222
3333
(a) (a)
2222
(b) (b)
44
44
44 44
44
44
44
(c) (c)
(d) (d)
Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; (b) Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; (b) Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; 20 min; (c) 50 min; (d) 70 min. 20 min; (c) 50 min; (d) 70 min. (b) 20 min; (c) 50 min; (d) 70 min.
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To reflect HGP characteristics from another point of view, the Vickers hardness of selected regions in Figure 4 were tested and results are shown in Table 2. To some extent, the hardness of the mineral phase can reflect its grindability. Results of Vicker hardness show that the RO phase and calcium ferrite phase have high hardness compared with the silicate phase, which may be one reason why these two mineral phases are HGP. Meanwhile, metallic iron phase with very low hardness is also HGP because of its high flexibility. Thus, the HGP is explained from Vickers hardness of mineral phases. Table 2. Vickers hardness of selected region in Figure 4 (HV). Region number
1
2
3
4
Mineral phases Vickers hardness
Silicate phase 187.3
Calcium ferrite phase 335.0
RO phase 298.1
Metallic iron phase 29.4
3.2.3. Chemical Compositions of HGP in Converter Steel Slag The chemical compositions of oversized substances passed a 0.9 mm square-hole screen under different grinding times, which were determined by X-ray fluorescence analysis (XRF) and chemical titration, are shown in Table 3. The results show that the contents of CaO and SiO2 in HGP gradually decrease with the increase in grinding time, while the contents of metallic iron show a reverse trend, and the contents of Fe2 O3 and MgO are firstly increased and then decreased with the increasing of grinding time. In the initial grinding period (10–20 min grinding), the chemical compositions of HGP mainly consist of CaO, SiO2 , Fe2 O3 , and MgO; in the middle grinding period (40–50 min grinding), it mainly consists of Fe2 O3 and metallic iron; in the later grinding period (60–70 min grinding), metallic iron takes up the most component. So the HGP have different chemical compositions during different grinding period. Table 3. Chemical compositions of hardly grinding phases (HGP) in steel slag vs. grinding time (%). Grinding time (min)
CaO
SiO2
Al2 O3
Fe2 O3
Fe
MgO
10 20 30 40 50 60 70
32.45 26.12 20.30 14.70 6.14 5.19 4.04
12.05 8.26 5.81 4.16 3.05 2.03 1.14
1.15 1.06 0.92 0.75 0.62 0.51 0.44
34.80 37.07 38.13 39.08 32.12 18.09 11.25
3.53 7.60 13.36 23.10 45.53 65.33 76.20
8.43 12.29 13.07 10.13 7.20 4.12 3.21
3.3. Comparison of Grinding Characteristic between Untreated and Pre-treated Converter Steel Slag From the research result presented in the previous section, the iron-rich phases, especially metallic iron, are the main hardly grinding phases, so it is necessary to remove or recycle the iron-rich phases before the grinding process of steel slag. In this study, the grinding characteristics of untreated and pretreated converter steel slag were compared with respect to iron mineral phases, grinding efficiency, particle size distribution, and particle morphology (Comment: the pretreatment of steel slag is the preliminary magnetic separation and multistage screening and magnetic separation, as shown in Figure 5).
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Untreated converter steel slag
First step
Crushing
Steel slag after primary multi-step Steel slag after multi-stage magnetic separation screening and magnetic (PMS-steel slag) separation (MSMS-steel slag)
Magnetic separation
Pre-milling
Screening
Recycling
Magnetic separation
Recycling
Scrap steel in slag
Scrap steel and iron concentrate in slag
Figure 5. The technical flow process of recycling iron‐rich phases in steel slag. Figure 5. The technical flow process of recycling iron-rich phases in steel slag.
3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag After Pre‐treatment
3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag after Pre-treatment Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic
Totalseparation (denoted analysis results as PMS) of iron and mineral phases in converter steel slag as SMS) after preliminary multistage magnetic separation (denoted are shown in magnetic separationTable 4. (denoted as PMS) and multistage magnetic separation (denoted as SMS) are shown in Table 4. Appl. Sci.2016, 9, 237 8 of 16 From the results, it can be seen that the proportions of iron phases in steel slag are obviously
reduced after pretreatment. The proportion of metallic iron in steel slag is decreased from 2.38% to Table 4. Total analysis results of iron mineral phases in converter steel slag. 1.18% by preliminary magnetic separation, and both the metallic iron proportion and total content Untreated Steel slag after primary multi-step Steel slag after multi-stage First step converter magnetic separation screening andfrom 2.38% to 0.45% magnetic of iron phases in steel slag are decreased by multistage magnetic separation, Proportions of iron mineral (MSMS-steel phases (%) slag) steel slag (PMS-steel slag) separation Iron mineral phases and 18.26% to 9.20%, respectively, indicating that the effect of pretreatment on iron‐rich phases is Untreated steel slag PMS-steel slag SMS-steel slag obvious. The removal of iron‐rich phases in steel slag is a very favorable factor for preparation and Metallic iron & magnetite 2.38 1.18 0.45 application of steel slag powder. Hematite/limonite 13.11 13.05 6.67 Sulfide 0.04 0.04 0.04 Table 4. Total analysis results of iron mineral phases in converter steel slag. Siderite Pre-milling2.26 Magnetic separation 1.98 Crushing Magnetic separation 2.52 Screening Proportions of iron mineral phases (%) Iron silicate 0.21 0.10 0.06 Iron mineral phases Total 18.26 16.63 9.20 Untreated steel slag PMS‐steel slag SMS‐steel slag Recycling Recycling Metallic iron & magnetite 2.38 1.18 0.45 Scrap steel in slag concentrate in slag Hematite/limonite 13.11 Scrap steel and iron13.05 6.67 From the results, it can be seen that the proportions of iron phases in steel0.04 slag are obviously Sulfide 0.04 0.04 reduced after pretreatment. The proportion of iron in2.26 steel slag is slag. decreased Siderite 2.52 1.98 from 2.38% to Figure 5. The technical flow process of metallic recycling iron-rich phases in steel Iron silicate 0.21 both the metallic 0.10 1.18% by preliminary magnetic separation, and iron proportion0.06 and total content of Total 18.26 16.63 9.20 3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag After Pre-treatment iron phases in steel slag are decreased by multistage magnetic separation, from 2.38% to 0.45% and
18.26% toTotal 9.20%, respectively, that the effect of pretreatment iron-rich phases is obvious. analysis results ofindicating iron mineral phases in converter steel slag on after preliminary magnetic 3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag The removal of iron-rich phases in steel slag is a very favorable factor for preparation and application separation (denoted as PMS) and multistage magnetic separation (denoted as SMS) are shown in The specific surface areas of untreated and pretreated steel slag under different grinding times Table 4. powder. of steel slag are shown in Figure 6.
Grinding Efficiency Untreatedand and Pretreated Pretreated Steel 3.3.2.3.3.2. Grinding Efficiency ofof Untreated SteelSlag Slag 500
500
Untreated steel slag Pre-treated steel slag
specific surface areas untreatedand andpretreated pretreatedsteel steel slag slag under different grinding times The The specific surface areas of of untreated under Pre-treated steel slagdifferent grinding times are are shown in Figure 6. shown in Figure 6.
350
350
SSA=157.44x-207.28 2 R =0.9906
300
Untreated steel slag Pre-treated steel slag
250 200 150 10
300
350
500
300
Untreated steel slag Pre-treated steel slag
450 250
20
30
40
50
60
70
Grinding time (min) 250 200
SSA=124.7x-155.85 2 R =0.9605
400 200
SSA=157.44x-207.28 2 R =0.9906
2
400
2
Specific surface area (m /kg)
450
400
2
400
surface area (m /kg) Specific surface area (m Specific /kg)
500
Untreated steel slag
450
2
Specific surface area (m /kg)
450
150 350 100
300
2.0
2.5
3.0
3.5
x=lnt 250
4.0
4.5
SSA=124.7x-155.85 2 R =0.9605
200 150
150 100
10
20
30
40
50
60
70
2.0
2.5
3.0
3.5
4.0
4.5
x=lnt
Grinding time (min)
(b)
(a)
Figure 6. Specific surface areas(SSA) (SSA)of ofsteel steel slag grinding time: (a) SSA vs. grinding Figure 6. Specific surface areas slag under underdifferent different grinding time: (a) SSA vs. grinding time; (b) linear fitting of SSA. time; (b) linear fitting of SSA.
As shown in Figure 6, the specific surface areas of untreated and pretreated steel slag are gradually increased with increasing grinding time, and starting from 50 min grinding time, the tendency to increase is slowed down due to the agglomeration of fine particles. The specific surface area of pretreated steel slag is obviously higher than that of untreated steel slag after the same grinding time. For example, the specific surface area of untreated steel slag is only 360 m2/kg after 60 2
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As shown in Figure 6, the specific surface areas of untreated and pretreated steel slag are gradually increased with increasing grinding time, and starting from 50 min grinding time, the tendency to increase is slowed down due to the agglomeration of fine particles. The specific surface area of pretreated steel slag is obviously higher than that of untreated steel slag after the same grinding time. For example, the specific surface area of untreated steel slag is only 360 m2 /kg after 60 min grinding, while that of pretreated steel slag reaches 361 m2 /kg after 40 min grinding, thus 20 min grinding time is saved. The relationship of specific surface area vs. grinding time was fitted, and the result is as follows: SSA = 124.7ln t − 155.85 ( R2 = 0.9605) (Untreated steel slag) (1) SSA = 157.44ln t − 207.28 ( R2 = 0.9906) (Pretreated steel slag)
(2)
From the above fitted result, it can be seen that the relationship of specific surface area with the logarithm of grinding time shows a good linear relationship. After applying derivation calculus to the above equations, we can get the increasing speeds of specific surface area: dSSA 124.7 = (Untreated steel slag) dt t
(3)
dSSA 157.44 = (Pretreated steel slag) dt t
(4)
It is obvious that the grinding speed of pretreated steel slag is higher than that of untreated steel slag. 3.3.3. Particle Size Distributions of Ground Untreated and Pretreated Steel Slag The particle size distribution and median diameter of ground untreated and pretreated steel slag powder are shown in Table 5 and Figure 7, respectively. Table 5. Particle size distribution of ground steel slag powder under different grinding time (%). Untreated steel slag
Pretreated steel slag
Grinding time (min)
65 µm
65 µm
10 20 30 40 50 60 70
2.77 4.97 6.64 8.16 11.18 8.99 10.91
27.88 37.16 36.62 38.24 37.90 32.52 30.99
25.24 27.54 28.04 27.46 22.19 26.57 19.12
44.11 30.33 28.70 26.15 28.73 31.91 38.97
2.44 5.66 9.00 10.54 14.87 16.36 14.50
25.42 40.06 44.22 48.80 56.99 53.83 55.74
21.95 30.84 32.10 30.51 22.65 20.96 21.47
50.19 23.45 14.68 10.15 5.49 8.84 8.29
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70
Untreated steel slag Pre-treated steel slag
Median diameter (m)
60
50
40
30
20
10 10
20
30
40
50
Grinding time (min)
60
70
Figure 7. Median diameter of ground steel slag powder under different grinding time.
Figure 7. Median diameter of ground steel slag powder under different grinding time. As shown in Table 5, with increasing grinding time (≤ 50 min), the proportion of particles for more than 32 μm (especially particles for more than 65 μm) is gradually and obviously decreased, while the proportion of particles for less than 32 μm shows a reverse tendency, which indicates that the particle size distribution of steel slag is optimized by mechanical grinding. However, when grinding time is more than 50 min, it shows a complex or chaotic tendency due to the agglomeration of fine particles. Compared with untreated steel slag, the proportion of particles for less than 32 μm
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As shown in Table 5, with increasing grinding time (≤ 50 min), the proportion of particles for more than 32 µm (especially particles for more than 65 µm) is gradually and obviously decreased, while the proportion of particles for less than 32 µm shows a reverse tendency, which indicates that the particle size distribution of steel slag is optimized by mechanical grinding. However, when grinding time is more than 50 min, it shows a complex or chaotic tendency due to the agglomeration of fine particles. Compared with untreated steel slag, the proportion of particles for less than 32 µm in pretreated steel slag powder is much higher after the same grinding time, indicating that the increasing tendency of Appl. Sci.2016, 9, 237 10 of 16 fine particles for pretreated steel slag powder is faster. For median diameter (D50), with the increasing of grinding time, the median diameter of steel to the variation tendency of particle proportion of more than 32 μm. At 50 min grinding time, the slag is firstly decreased before 50min grinding time, and then is increased after that, which is similar to median diameter of untreated steel slag reaches a minimum, 32.89 μm, which is much larger than the variation tendency of particle proportion of more than 32 µm. At 50 min grinding time, the median that of pretreated steel slag, 18.16 μm. This shows that it is difficult for untreated steel slag to be diameter of untreated steel slag reaches a minimum, 32.89 µm, which is much larger than that of finely ground. pretreated steel slag, 18.16 µm. This shows that it is difficult for untreated steel slag to be finely ground.
3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag 3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical in shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical in shape, shape, and fine particles adhere to the larger particles. By comparison, the overall particle size of and fine particles adhere to the larger particles. By comparison, the overall particle size of pretreated pretreated steel slag powder is smaller than that of untreated steel slag, and particle uniformity in steel slag powder is smaller than that of untreated steel slag, and particle uniformity in size is also size is also superior to that of untreated steel slag. To some extent, both the ground untreated and superior to that of untreated steel slag. To some extent, both the ground untreated and pretreated steel pretreated steel slag powders show some agglomeration phenomenon. slag powders show some agglomeration phenomenon.
5μm
5μm
50μm
(a)
50μm
(b)
Figure 8. Particle Particle morphology morphology of of ground ground steel steel slag slag powder powder after after 50min 50min grinding grinding time: time: (a) (a) untreated untreated Figure 8. steel slag; (b) (b) pretreated pretreated steel steel slag. slag. steel slag;
3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag 3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag From the above results of studies, it can be seen that the agglomeration phenomenon of fine From the above results of studies, it can be seen that the agglomeration phenomenon of fine particles will occur during deep grinding periods, which seriously affect or reduce the grinding particles will occur during deep grinding periods, which seriously affect or reduce the grinding efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to use GA in the grinding process of steel slag for many manufacturers to improve preparation use GA in the grinding process of steel slag for many manufacturers to improve preparation efficiency efficiency of steel slag powder. In additional, on the basis of pretreatment, the grinding property of of steel slag powder. In additional, on the basis of pretreatment, the grinding property of steel slag can steel slag can be further improved by GA. In this study, glycerol, which is a common cement GA, be further improved by GA. In this study, glycerol, which is a common cement GA, was selected as the was selected as the GA of steel slag. The effects of GA on the grinding property of steel slag were GA of steel slag. The effects of GA on the grinding property of steel slag were studied from sieving studied from sieving residue, particle size distribution, angle of repose, and particle morphology. residue, particle size distribution, angle of repose, and particle morphology. 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving
improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to use GA in the grinding process of steel slag for many manufacturers to improve preparation efficiency of steel slag powder. In additional, on the basis of pretreatment, the grinding property of steel slag can be further improved by GA. In this study, glycerol, which is a common cement GA, was selected as the GA of steel slag. The effects of GA on the grinding property of steel slag were Appl. Sci. 2016, 6, 237 11 of 15 studied from sieving residue, particle size distribution, angle of repose, and particle morphology. 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious after 20 min and reaches an optimum grinding role at 50 min grinding time. after 20 min and reaches an optimum grinding role at 50 min grinding time. 60
Appl. Sci.2016, 9, 237
45m sieve residue (%)
50
Blank With 0.05% GA
40
30
12 of 16
20
coarse and have a good dispersion at this stage, basically having no agglomeration of fine particles. 10 Reducing role of GAnot on sieve residue The grinding efficiency of steel slag is basically influenced by dispersion of particles, so the 10 20 30 40 50 60 70 optimization role of GA on the particle size distribution is not obvious. During 40 min, 50 min, and Grinding time (min) 60 min grinding time, the proportions of particles in the range of 3–32 μm are obviously increased by 7.24% (absolute value, same below), 7.22%, and 10.63%, respectively, after adding GA, while the Figure 9. Sieving residue of steel slag powder with grinding aid (GA) vs. grinding time. Figure 9. Sieving residue of steel slag powder with grinding aid (GA) vs. grinding time. proportions of particles in the range of more than 32 μm are also obviously decreased, indicating that GA can efficiently weaken agglomeration of fine particles, then improve the dispersion of steel 3.4.2. Effect of GA on the Particle Size Distribution of Steel Slag Powder 3.4.2. Effect of GA on the Particle Size Distribution of Steel Slag Powder slag powder. Meanwhile, many researches indicate that 3–32 μm particles have the greatest The uniformity coefficient of steel slag powder can reflect the extent of the width of particle The uniformity coefficient of steel slag powder can reflect the extent of the width of particle size contribution on the property of cement‐based materials, so the improvement of 3–32 μm particles size distribution, and the smaller uniformity coefficient, the wider the particle size distribution. distribution, and the smaller the uniformity coefficient, the wider the particle size distribution. From proportion after using GA shows the that GA can significantly optimize particle size distribution of From Figure 10, it can be seen that the uniformity coefficient of steel slag powder is firstly increased Figure 10, it can be seen that the uniformity coefficient of steel slag powder is firstly increased and steel slag powder. At 70 min grinding time, as the agglomeration of particles is very serious, GA can and completely then gradually decreased with the increasing grindingtime. time.By Bycomparison, comparison, the uniformity uniformity then gradually decreased with the increasing of of grinding the not resist it due to the fixed dosage (0.05%), resulting in the weakening of GA’s coefficient ofrole. steelIt slag powder GA is largerrole thanof that GA forGA the on same grinding coefficient of steel slag powder with GA is larger than that without GA for the same grinding time, optimizing shows that with the optimizing the without fixed dosage the particle time, size indicating that the particle size distribution of steel slag powder can be narrowed by GA. The effect is indicating that the particle size distribution of steel slag powder can be narrowed by GA. The effect distribution of steel slag powder is different under different grinding times, and the role of 0.05% the strongest at 50 min and 60 min grinding times. is the strongest at 50 min and 60 min grinding times. dosage of GA is best during 40–60 min grinding time. As shown in Table 6, it can be seen that the effect of GA on the particle size distribution of steel slag powder is relatively small before reaching 40 min grinding time. The reason is that particles are 1.25 Blank With 0.5% GA 1.20
Uniformity coefficient
1.15 1.10 1.05 1.00 0.95 0.90 0.85 10
20
30
40
50
60
Grinding time (min)
70
Figure 10. Uniformity coefficient of steel slag powder with GA vs. grinding time. Figure 10. Uniformity coefficient of steel slag powder with GA vs. grinding time.
Table 6. Particle size distribution of steel slag powder with GA under different grinding time (%).
Without GA
Grinding time (min)
＜3μm
10
2.44
3–32μm 32–65μm 25.42 21.95
With GA ＞65μm
＜3μm
50.19
2.83
3–32μm 23.83
32–65μm 22.80
＞65μm 50.54
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As shown in Table 6, it can be seen that the effect of GA on the particle size distribution of steel slag powder is relatively small before reaching 40 min grinding time. The reason is that particles are coarse and have a good dispersion at this stage, basically having no agglomeration of fine particles. The grinding efficiency of steel slag is basically not influenced by dispersion of particles, so the optimization role of GA on the particle size distribution is not obvious. During 40 min, 50 min, and 60 min grinding time, the proportions of particles in the range of 3–32 µm are obviously increased by 7.24% (absolute value, same below), 7.22%, and 10.63%, respectively, after adding GA, while the proportions of particles in the range of more than 32 µm are also obviously decreased, indicating that GA can efficiently weaken agglomeration of fine particles, then improve the dispersion of steel slag powder. Meanwhile, many researches indicate that 3–32 µm particles have the greatest contribution on the property of cement-based materials, so the improvement of 3–32 µm particles proportion after using GA shows that GA can significantly optimize particle size distribution of steel slag powder. At 70 min grinding time, as the agglomeration of particles is very serious, GA can not completely resist it due to the fixed dosage (0.05%), resulting in the weakening of GA’s optimizing role. It shows that the optimizing role of the fixed dosage GA on the particle size distribution of steel slag powder is different under different grinding times, and the role of 0.05% dosage of GA is best during 40–60 min grinding time. Table 6. Particle size distribution of steel slag powder with GA under different grinding time (%). Without GA
With GA
Grinding time (min)
65 µm
65 µm
10 20 30 40 50 60 70
2.44 5.66 9.00 10.54 14.87 16.36 14.50
25.42 40.06 44.22 48.80 56.99 53.83 55.74
21.95 30.84 32.10 30.51 22.65 20.96 21.47
50.19 23.45 14.68 10.15 5.49 8.84 8.29
2.83 4.81 7.63 10.44 14.32 15.73 15.05
23.83 37.44 47.08 56.04 64.21 64.46 57.33
22.80 34.14 32.74 28.47 21.10 18.61 19.49
50.54 23.62 12.55 5.04 0.38 1.20 8.13
3.4.3. Effect of GA on the Angle of Repose of Steel Slag Powder Powder fluidity is usually characterized by the angle of repose. The smaller the angle of repose, the better the powder fluidity. As shown in Figure 11, it can be seen that the angle of repose of steel slag powder is decreased after adding GA, indicating that GA can improve the fluidity of steel slag powder. The role of GA on the angle of repose is firstly enhanced and then weakened on the whole, which is similar to the role of GA on the sieving residue, and it is reaches maximum effectiveness at 50 min grinding time, i.e., the action effect of GA on powder fluidity is the greatest at 50 min grinding time. Appl. Sci.2016, 9, 237 13 of 16 Blank With 0.05wt% GA
52
o
Repose angle of powder ( )
50
48
46
44
42
40 10
20
Role of GA on angle of repose 30
40
50
60
Grinding time (min)
70
Figure 11. Repose angle of steel slag powder with GA vs. grinding time. Figure 11. Repose angle of steel slag powder with GA vs. grinding time.
3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder The particle morphologies of steel slag powder with and without GA are shown in Figure 12. It can be seen from the figure that the particles of steel slag powder without GA exhibit the agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding
40 10
20
30
40
50
60
70
Grinding time (min)
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Figure 11. Repose angle of steel slag powder with GA vs. grinding time.
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3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder 3.4.4.The particle morphologies of steel slag powder with and without GA are shown in Figure 12. It Effect of GA on the Particle Morphology of Steel Slag Powder can be from the figure that the slag particles of with steel and slag powder without GA inexhibit Theseen particle morphologies of steel powder without GA are shown Figurethe 12. agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding It can be seen from the figure that the particles of steel slag powder without GA exhibit the time, agglomeration of particles very serious and to many regenerated by agglomeration phenomenon whereis fine particles adhere each large other. particles Especiallyare at 70 min grinding adhering of fine particles. By is comparison, using large GA, particles the agglomeration phenomenon of time, agglomeration of particles very seriousafter and many are regenerated by adhering particles is not obvious at 50min grinding time, still keeping a relatively good dispersion. Although of fine particles. By comparison, after using GA, the agglomeration phenomenon of particles is not the particles with GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is obvious at 50min grinding time, still keeping a relatively good dispersion. Although the particles with still obviously those without GA. grinding The above results confirm that GA can efficiently GA also exhibit better a little than agglomeration at 70 min time, their dispersion is still obviously better weaken the agglomeration of fine particles when particle size becomes small and agglomeration of than those without GA. The above results confirm that GA can efficiently weaken the agglomeration fine particles occurs. of fine particles when particle size becomes small and agglomeration of fine particles occurs.
5μm
5μm
Agglomeration of particles Agglomeration of particles
50min
50μm
70min
20μm
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(a)
3μm 3μm Dispersion of particles
Dispersion of particles a little agglomeration
50min
20μm
70min
10μm
(b) Figure 12. slag powder with and without GA: (a) without GA; GA; (b) Figure 12. Particle Particlemorphologies morphologiesof ofsteel steel slag powder with and without GA: (a) without with GA. (b) with GA.
4. Conclusions 4. Conclusions From this study, we can conclude that: From this study, we can conclude that: (1) from analysis of BSE–EDX that that the mineral phases of converter steel slag mainly (1) ItIt was was evident evident from analysis of BSE–EDX the mineral phases of converter steel slag contain RO phase,RO calcium ferrite phase, and phase, a smalland amount of metallic iron mainly silicate contain mineral silicate phase, mineral phase, phase, calcium ferrite a small amount of phase, among others. metallic iron phase, among others. (2) The oversize screening cancan be considered as theas hardly grinding phases phases (HGP), (2)The oversize substance substance after after screening be considered the hardly grinding which provides a simple method of determining the HGP in steel slag. The HGP proportion which (HGP), which provides a simple method of determining the HGP in steel slag. The HGP proportion which was determined by a 0.9 mm square‐hole screen is about 1.5%. After the initial 20 min grinding, the RO phase, calcium ferrite, and metallic iron phase made up most of the proportion in the HGP, while the metallic iron made up the most component after 70 min grinding. (3) For steel slag powder with about 360 m2/kg specific surface area (SSA), 20 min of grinding time can be saved with pretreatment. The relationships of SSA with the logarithm of grinding time
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was determined by a 0.9 mm square-hole screen is about 1.5%. After the initial 20 min grinding, the RO phase, calcium ferrite, and metallic iron phase made up most of the proportion in the HGP, while the metallic iron made up the most component after 70 min grinding. (3) For steel slag powder with about 360 m2 /kg specific surface area (SSA), 20 min of grinding time can be saved with pretreatment. The relationships of SSA with the logarithm of grinding time show a good linear relationship, but pretreated steel slag has a higher grinding efficiency. In addition, the D50 of untreated steel slag can only reach 32.89 µm after 50 min grinding, but that of pretreated steel slag can reach 18.16 µm after the same grinding time. (4) Organic grinding aids (GA) can obviously improve the grinding property of steel slag, and the action effects of 0.05% dosage of GA on the grinding efficiency and particle characteristics were the best during 40–60 min grinding time. especially the proportions of particles in the range of 3–32 µm, which were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min and 60 min grinding, respectively. This is mainly because of the reduction of agglomeration after the use of GA, as evidenced by SEM images. Acknowledgments: This work was financially supported by the China’s Post-doctoral Science Fund (No. 2016M591170), the Open Fund of Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education (No. 201602). Author Contributions: J.Z. conceived of, designed and performed the experiments. D.W. and P.Y. analyzed the data and discussed the results. W.L. participated in writing this paper. Conflicts of Interest: The authors declare no conflict of interest.
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