Pilot Scale Treatment of Wastes from the Iron and Steel
Industry
Antonio E. C. Peresa*, Helio C. Pereirab, Dinalva C. Fonsecac and Fernando L. Krügerd
a
UFMG Federal University of Minas Gerais
Rua Espirito Santo, 35/206, 30160-030 Belo Horizonte, MG, Brazil
b
Samarco Mineração S.A.
Caixa Postal 720004, 29230-000 Anchieta, ES, Brazil
c
Companhia Siderúrgica Nacional
Mina de Casa de Pedra 36415-000 Congonhas, MG
d
UFOP Federal University of Ouro Preto
Campus Morro do Cruzeiro, 35400-000 Ouro Preto, MG, Brazil
Abstract
The topic of treating wastes produced in the iron and steel industry, at pilot plant scale,
is addressed. Two case studies are presented and discussed. Both investigations yielded
products adequate for further utilization in steelworks, the wastes being processed by
means of low cost techniques. The innovative character of the investigation was the
development of routes for the full economical re-use of the wastes, produced at large
scale.
The first case study refers to the testwork performed with two kinds of residues
generated by a wire drawing company: cake from the acidulated effluents plant, and
wire drawing scale. The technological solution proposed was pelletising the two
residues mixed with iron ore. The recommended ratio of materials was defined at 8:1:1
(ore:cake:scale). The mechanical characteristics of the product met the market
specifications for blast furnace pellets.
Pellet screenings represents the undersize generated in the operation of screening the
final product of pelletising plants. Compared with natural sinter feed, pellet screenings
presents the advantages of extremely low loss on ignition and high content of fluxing
elements. Pellet screenings can not be re-utilised in the pelletising process. The
objective of the present investigation was evaluating, at pilot plant scale, the
substitution (50% and 100%) of natural sinter feed by pellet screenings in the sintering
machine feed cake. No major differences were observed in the chemical, physical, and
metallurgical characteristics of sinters.
Keywords: acidulated effluents cake, wire drawing scale, pellet screenings, sinter feed,
ferrous wastes
1. Introduction
The purpose of this paper is to communicate results of two case studies concerning the
recycling of wastes from the Brazilian iron and steel industry. Iron and steelmaking
represent a relevant economical activity in Brazil. The industries generate solid waste
material, atmospheric emissions and liquid effluents that require treatment under
different conditions to mitigate the environmental impact of the activities. A statistical
analysis of the distribution of waste material from this industrial segment indicated:
blast furnace slags 38.7%; steelmaking slags 21.7%; dust and sludges 13.7%; scarfing
sludges 4.7%; scale 4.4%; oil and grease 3.6% and other residues 13.2% (Chehebe, J.R.
et al., 1987).
Priority was first given to blast furnace slags, due to the large amounts produced. The
treatment includes magnetic concentration and size separation. Some uses of the
processed slags are (Assis, R.M, 1997): raw material in the cement industry, aggregates
for soil and land stabilisation, aggregates used to pave the way for asphalt surfaces and
surface drainage. aggregates for the construction of damns and banks, source of metals,
soil corrective especially for eucalyptus cultures.
The disposal of sludges and powders represents a serious challenge and deserves
especial attention.
According to the International Iron and Steel Institute (1987), the destination of
powders and sludges produced by the Japanese metallurgical and iron and steelmaking
industries is: 42% sintering plants, 13% blast furnaces, 25% cement industry, 12%
briquetting and pelletising plants, 8% final disposal. Residues from the raw materials
yard are recycled in sintering and pelletising, after mixture with other powders and, if
necessary, moisture content adjustment. Residues from cupola furnaces represent a
source of carbon and are fully re-utilised in iron and steel plants. Tar residues may be
recycled in cupola foundry, mixed with charcoal, or burned in incinerators, together
with oil residues.
Sinters represent a blast furnace charge with physical, chemical, and metallurgical
characteristics adequate for enhanced performance and also contribute to the mitigation
of the environmental impact of steelworks activities by incorporating wastes such as
fines, powders, dusts, and sludges.
Ametrano et al. (1999) described the process for utilisation of residues at Siderar, that
utilises a sintering machine with area of 72.5 m2, and production capacity of
3,000 t/day. The production of residues reaches 280,000 t/year, consisting of: fines of
pellets, sinter fines, steelmaking slag and sludge, scrap, dust from blast furnace gas,
blast furnace dust, and rolling scale. Minor benefits in productivity, (CaO/SiO2) ratio,
shatter test index were achieved. The major benefits came from RDI (reduction
degradation index) increasing from 20% to 25%, and the coke consumption being
reduced from 49 kg/t to 32 g/t. The production cost reduction reached 7%.
Brazilian CVRD and BHP-Billiton hold each 50% of the shares of Samarco Mineração
S/A, a company that produces pellets, pellet feed (iron ore concentrate) and pellet
screenings for the international iron ores market, being the second transoceanic exporter
of pellets for steel production, representing 17% of this market. The products are sold to
China (37%), other countries in Asia (19%), Middle East and Africa (19%), Europe
(16%) e Americas (9%).
Pellet screenings is the undersize resulting from screening the product of the pelletising
plant after full processing. Compared with natural sinter feed, the pellet screenings
presents the advantages of very low loss on ignition and of containing fluxes. Due to
the unfeasibility of re-utilisation of the pellet screenings in the pelletising plant, it is
sold to sinter makers. Samarco’s challenges are reducing the amount of pellet
screenings produced in the pelletising plant and searching for applications for this
material.
The current evolution stages of sintering operations and blast furnaces all over the world
stress the need of utilising raw materials meeting strict standards of quality, costs,
supply, and environment preservation. This investigation contributes to bring more
knowledge to the field of iron ores sintering
2. Recycling of cake from acidulated effluents plant and wire
drawing scale
This case study addresses the processing of wastes produced by a wire drawing
company in Brazil.
The cake is the residue produced in the processing station of acidulated effluents from
the surface treatment of wire rods and wires. The effluents processing consists of
neutralisation with lime followed by aeration/oxigenation with compressed air and the
flocculation, decantation and pressing. The liquid effluent from the pressing stage is
recycled to the tank of effluents to be treated and the pressed residue constitutes the
cake. The production of cake reaches 245 t/month. This residue was classified according
to the Brazilian standard as: class II – non inert. The chemical analysis of the cake
indicated the presence of Fe 37.30%, SiO2 4.92%, CaO 15.95%, MgO 1.03%, loss on
ignition 1.91% as major species. The only phase detected by X-Ray diffraction was
Fe2O3. Size analysis showed 97.5% passing 44 m.
The production of wire drawing scale reaches 725 t/month. This residue was classified
according to the Brazilian standard as: class II – non inert. The results of chemical
analysis are: Fe 75%, SiO2 2.5%, Al2O3 0.25%. X-Ray diffraction detected the presence
of wustite (~80%) and magnetite (~20%), hematite being a minor component
2.1 Experimental procedure and results
The samples of cake and scale were homogenised and mixed with iron ore fines in
proportions adequate to meet the desired moisture content for the pelletising process.
Details on the operation of the pelletising disk are presented in Table I. Exploratory
experiments aimed at establishing the process control parameters and the proportion of
components in the mixture. The proportion of components in the mixture was defined as
a function of the initial moisture and the final desired moisture for the pellet or micro
pellet. Ball et al. (1973) suggest an optimum range of moisture content in the pelletising
process between 8% and 15%.
The pellets and micro pellets produced with 11% moisture content were stored in trays
under dry room conditions for 24 hours, before being fired in electrical kilns. The firing
cycle (temperature x time curve) was defined as a function of the destination of the
product. Pellets are fed to a blast furnace and micropellets to a sintering plant. Blast
furnace pellets must present a larger diameter (10 to 12.5 mm) and higher compression
and abrasion resistance (tumbling test). The micro pellets, employed in the sintering
process, are smaller and require only a minimum resistance to be handled and fed to the
sintering machine. The maximum firing temperature was 1300 °C for pellets and 800 °C
for micro pellets.
Table I. Operation of the pelletising disk.
Parameters
disk slope
disk velocity
disk diameter
water flow rate
pelletising time
moisture (mixture)
moisture (final product)
mixture weight
weight % iron in the mixture
weight % scale in the mixture
weight % cake in the mixture
weight % bentonite
weight % sodium silicate
Pellets 10 to 12.5 mm
45°
21 rpm
600 mm
50 mL/min
10 min
7%
11%
3,100 g
79.5%
9.75%
9.75%
1%
-
Micro pellets 5 to 6 mm
45°
18 rpm
600 mm
50 mL/min
5 min
7%
11%
3,100 g
77.0%
9.70%
9.70%
1%
2.5%
The quality of the pellets was checked with the use blast furnace pellet standard tests:
tumbling and abrasion (ISO 3271/1985), compression resistance (ISO 4700/1983), and
reducibility (ISO 4695/1984). Results of tumbling and abrasion tests indicated figures
lower than the desirable values for blast furnace pellets, but still acceptable considering
the composition of the feed of the pelletising disk. Compression resistance and
reducibility values for the pellets were compatible with levels for ordinary pellets made
of pellet feed fines.
3. Pellet screenings substituting for sinter feed
This case study addresses the evaluation, at pilot plant scale, of the influence of
replacing sinter feed by pellet screenings, at proportions of 50% and 100%, in the
sintering machine feed cake, without impairing the chemical, physical and metallurgical
characteristics of the product.
The following parameters were determined and compared: granulation index, sintering
time, < 5mm return, total return, coke rate, shatter and tumble tests results, RDI
(reduction degradation index), and reducibility.
3.1 Experimental procedure and results
The raw materials utilised in the investigation were homogenised and then sampled for
chemical and size distribution characterisations. Results of the chemical characterisation
are presented in Table II.
TableII. Chemical composition of the raw materials.
pellet screenings
ore A
ore B
ore C
ore D
limestone
quartz
dunite
manganese
hydrated lime
coke
Fe(t)
66.30
65.40
65.90
67.54
67.45
0.30
0.46
4.95
34.40
0.08
0.66
FeO
0.52
0.27
0.24
0.41
0.18
2.80
0.31
0.14
0.22
CaO
1.52
0.03
0.02
0.02
0.02
54.55
0.03
2.17
0.14
75.40
0.37
SiO2
2.13
3.95
4.55
0.69
1.32
0.92
99.08
38.50
6.51
1.22
6.35
Al2O3
0.46
1.50
0.59
.84
0.57
0.34
0.07
3.83
1.42
0.35
3.08
MgO
0.21
0.02
0.02
0.02
0.06
0.49
0.02
34.20
0.34
0.42
0.10
P2O5
0.105
0.076
0.050
0.085
0.062
0.114
0.014
0.240
0.200
0.110
The evaluation of the performance of the products regarding sinterability was
performed. A feed blend consisting of four different iron ore types was taken as
reference. The choice of this blend was based on preliminary comparative tests,
performed at the sintering pilot plant. The quality of the sinter and productivity of the
tests with this blend were equivalent to the levels achieved with the standard ores blend
industrially utilised at Usiminas.
The product pellet screenings was tested replacing partially (50%) and totally (100%)
one of the iron ore types of the blend that presents low SiO 2 content. This option was
selected due to the low SiO2 of the pellet screenings.
The test conditions are described next: (i) condition 1 – standard blend of iron ores
(reference); (ii) condition 2 – pellet screenings replacing 50% ore D; (iii) condition 3 –
pellet screenings replacing 100% ore D.
The target chemical composition of the sinter was: SiO2 = 4.70%, MgO = 1.40%, Al2O3
= 100%, (CaO/SiO2) ratio = 1.65
Table III shows the compositions of the blends utilised under different test conditions.
The methodology for simulating, in pilot scale, the industrial sintering process was
defined aiming at approaching the industrial conditions concerning the aspects:
chemical and metallurgical characteristics, productivity indexes, and overall process
trends.
In the sinterability tests two burning stages (for obtaining return fines) and three regular
burning stages are performed for each condition investigated. The control parameters
are: (i) all sinter cakes of a certain test condition should produce, after degradation, the
same percentage of return fines, (ii) the differences between the sintering times of each
individual burning stage should be lower than 1.5 min, (iii) the differences between the
weights of each individual feed blend, for the same test condition should be lower than
1.5 kg.
Table III. Composition of blends utilised under different test conditions.
Component
pellet
screenings
ore A
ore B
ore C
ore D
limestone
quartz
hydrated
lime
dunite
manganese
coke
reference
blend (%)
-
pellet screenings
pellet screenings replacing
replacing 50% D ore (%)
100% D ore (%)
12.12
24.28
26.28
17.53
13.02
24.22
12.87
0.61
1.00
26.32
17.55
13.03
12.12
12.56
0.51
1.00
26.36
17.58
13.05
12.24
0.41
1.00
3.64
0.83
3.40
3.60
1.17
3.40
3.55
1.52
3.40
The return fines percentage corresponds to 25% of the blend, the size distribution being
25% in the size range 5 to 10 mm and 75% in the size range 0 to 5 mm, same figures as
those in Usiminas plant practice.
The amount of added fuel is that required to balance the return fines (“return fines
produced” = “return fines introduced” in the corresponding blend), the tolerance value
being 10% with respect to the < 5.0 mm fraction.
The optimum moisture content of each blend is previously determined by means of a
specific methodology.
The basic parameters of the pilot sintering machine tests are:
i. pot average diameter = 300 mm;
ii. layer height = 530 mm (blend + bedding), bedding = 20 mm;
iii. suction pressure during burning = 1600 mm H 2O; during ignition 600 mm H2O;
iv. ignition time = 1 min;
v. ignition intensity = 25,000 kcal/m2/min (COG);
vi. target moisture in blends = optimum moisture previously determined;
vii. air/gas ratio in ignition = 5/1.
Aiming at simulating handling and transportation effects on the size stabilisation degree
of the industrial sinter, the pilot sinter is submitted to mechanical treatments of tumbler
and shatter tests.
The following procedures were utilised for the physical and metallurgical
characterisation of the sinters:
i. shatter test: JIS M 8711 > 10 mm material;
ii. tumbler test: ASTM E-279-92 > 6mm material;
iii. RDI: based on procedures of Nippon Steel Corp.: temperature 550°C; < 2,83 mm
material;
iv. reducibility: JIS M 8713 temperature: 900°C, composition of the reducing gas: 30%
CO e 70% N2.
The chemical compositions of sinters produced with different blends are presented in
Table IV.
Table IV. Chemical compositions of sinters produced with different blends.
component
FeT
FeO
SiO2
Al2O3
CaO
MgO
P
CaO/SiO2
Zn
S
Cu
K2O
Na2O
Mn
TiO
reference
blend (%)
59.52
4.61
4.71
0.95
7.62
1.40
0.047
1.62
0.015
0.005
0.006
0.027
0.013
0.321
0.050
pellet screenings replacing pellet screenings replacing
50% D ore (%)
100% D ore (%)
59.90
59.21
4.87
4.64
4.71
4.68
0.97
1.00
7.66
7.61
1.42
1.40
0.048
0.056
1.63
1.63
0.017
0.030
0.005
0.005
0.007
0.006
0.031
0.030
0.019
0.020
0.321
0.330
0.053
0.051
Sinterability and sinter quality parameters for different blends are presented in Table V.
The granulation indexes of the blends, together with the sinterability parameters, the
chemical composition and the metallurgical characteristics of the sinter are the tools for
evaluating the sinter quality and predicting its performance in the blast furnace.
High granulation indexes are an indication of the ability of the blend components for
cold agglomeration.
The average size of the primary particles of the blend is a direct function of the size
distribution of the components. The addition of fine particles decreases the average size
of the primary particles. The quasi particles represent the micro pellets, formed after
mixing the blend, that will be fed to the sintering pot.
The average diameter of the quasi-particles (micro pellets) was approximately 3.5 mm
for the three blends.
4. Conclusions
4.1 Cake from acidulated effluents and wire drawing scale
The pelletising of the residues cake from acidulated liquid effluents treatment and wire
drawing scale is technically viable utilising a blend of these materials with iron ore, at a
ratio 8:1:1 (iron ore:cake:scale), yielding pellets presenting final moisture content of
11%.
Blast furnace pellets (diameter 10 to 12.5 mm), made with bentonite as binding agent,
reached mechanical resistance levels, determined in compression and tumbling tests,
within the market specifications for commercial pellets.
Despite the lack of specifications for sintering pellets, the micro pellets (diameter 4 to
6 mm), produced with bentonite and sodium silicate as binding agents, present visual
characteristics similar to other materials fed to the sintering machine.
4.2 Pellet screenings
The higher degree of porosity of the pellet screenings in comparison with iron ores
causes a slight reduction in the density of the sintering feed blend. On the other hand it
enhances the bed permeability, contributing to a decrease in the sintering time,
increasing the productivity.
Sinters produced with pellet screenings present slightly lower tumble and shatter
indexes in comparison with those produced with the reference blend.
No major differences were observed in the metallurgical characteristics, reducibility
and.RDI, of sinters produced with pellet screenings and with the reference blend.
Table V. Comparison among parameters of sinters produced with different blends.
parameter
reference
blend
(%)
dry weight (kg)
79.58
coke rate (kg/t)
51.04
time (min)
27.50
2
productivity (t/h/m )
1.585
shatter index (%)
81.23
reducibility (%)
74.47
tumbler index (%)
68.19
RDI (%)
23.64
mean size quasi-particle (mm)
3.61
mean size primary particle (mm)
2.08
granulation index
96.99
pellet screenings
replacing 50%
D ore (%)
75.96
50.66
26.30
1.592
77.55
73.60
66.55
24.54
3.43
2.09
98.17
pellet screenings
replacing 100%
D ore (%)
74.99
51.02
26.20
1.566
79.13
75.08
65.09
23.33
3.26
2.09
95.70
References
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Portuguese).
Ball, D.F. et al., 1973, Agglomeration of iron ores. Heineman. London.
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industry, Proceedings of the Seminar on Recycling of Residues, B3-B38 (in Portuguese).
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