pelletizing steel mill desulfurization slag

Pelletizing steel mill desulfurization slag

S. Komar Kawatra *, S. Jayson Ripke

Department of Mining and Materials Processing Engineering, Michigan Technological University,

1400 Townsend Drive, Houghton, MI 49931, USA

Received 7 May 2001; received in revised form 23 July 2001; accepted 25 July 2001

Abstract

Hot metal desulfurization slag is a high-metallic iron content slag produced at a typical

steelmaking facility and is currently considered waste. Each year, 50,000 tons of this slag is

produced at one particular steelmaking plant. This material can be beneficiated so that it can be

utilized as blast furnace feed, but it is necessary to first develop a process for agglomerating it into

pellets. A beneficiated slag concentrate was pelletized with both conventional bentonite binder and a

new fly ash-based binder (FBB) that was formulated by the investigators. The FBB used a high-

carbon fly ash that is currently landfilled as waste. In addition to fly ash, FBB contained calcium

hydroxide as an activator that also acts as flux known to improve the pellet’s reducibility

characteristics. The desulfurization slag concentrate pellets bonded with the FBB had superior dry

compressive strengths compared to the bentonite-bonded pellets. D 2002 Published by Elsevier

Science B.V.

Keywords: metal recycling; pelletizing; agglomeration

1. Introduction

Steel mills produce two types of slags called ironmaking slag and steelmaking slag.

Slags operate as an efficient medium for metallurgical reactions and are then subjected to

a suitable treatment for end use, such as cooling or quenching. Initially, at a steel mill,

iron ore, flux, and fuel (coke) are added into an oxygen blast furnace. The materials

combine and react to form liquid hot metal, gasses, and blast furnace (ironmaking) slag.

Blast furnace slags typically contain 0.1–1.5% iron (NSA, 2000) and are commonly

used for applications such as industrial aggregates, cement admixtures, and fertilizers.

0301-7516/02/$ - see front matter D 2002 Published by Elsevier Science B.V.

PII: S0301 -7516 (01 )00073 -4

* Corresponding author. Fax: +1-906-487-2934.

E-mail address: [email protected] (S.K. Kawatra).

www.elsevier.com/locate/ijminpro

Int. J. Miner. Process. 65 (2002) 165–175

Recycling these products conserves resources and minimizes emissions. As the hot metal

is further processed to produce iron and steel products, various treatments produce

steelmaking slags. These steelmaking slags typically contain 20–25% iron. It is

surprising that only half of the iron from steelmaking slags is typically recovered and

returned to the process. The chemical concentration range of the slags from one steel

mill is shown in Table 1.

The iron in steelmaking slag has the advantage of either being partially reduced to

iron oxides such as magnetite (Fe3O4) or wustite (FeO) or being completely reduced to

metallic iron. The steelmaking slag is currently sold as aggregate, stockpiled, inven-

toried, or in the worst case, it is landfilled at costs of US$5–100 per ton. Technology

should be developed to obtain higher recoveries of the iron units in the steelmaking slag

before its final application, both to improve iron utilization efficiency and to reduce slag

volume. The technology that is developed should allow the remaining slag coproduct to

be used for current end uses; and perhaps even new end uses once identified. This

technology will have the advantage of expanding the versatility of steel mill products

and operations.

If the ferrous material can be recovered from slag and pelletized, it can be recycled

into blast furnace feed. Pelletization is the most common method used in North America

today for preparing finely ground iron ores to be used as blast furnace feed. Iron ore

pelletization usually requires a binder, most commonly bentonite clay. However, a fly

ash-based binder (FBB) has been used to pelletize magnetite concentrate, and this binder

has particular benefits (Ripke and Kawatra, 2000; Eisele et al., 1997; Kawatra et al.,

1998a,b). FBB uses fly ash that is considered poor quality due to its high carbon content

and would otherwise be landfilled, thus reducing pollution. There is an economic as well

as an environmental driving force behind this research, since fly ashes are also more

readily available and less expensive than bentonite. Pelletization of the slag concentrate

will allow the iron and steel industry to increase profits with higher iron recovery and

lower disposal costs, while preventing pollution through lower waste generation. This is

the first time that iron units, beneficiated from hot metal desulfurization slag, have been

pelletized for reuse as blast furnace feed.

In contrast to this research, metallized pellets have been produced for direct

reduction, from a different type of slag (Wang and Xiao, 1989); additionally, the

Inmetco process has been used to pelletize low-metal content ferrous wastes using direct

reduction of carbon-containing green pellets in a rotary hearth furnace (Bauer et al.,

Table 1

Typical range of chemistry for steel mill slags (Farrand, 2000)

Slag origin Slag chemistry (wt.%)

CaO SiO2 FeO MgO MnO Al2O3

Blast furnace (BF) 30–40 32–40 < 1 12–18 < 1 7–11

Basic oxygen furnace (BOF) 40–50 10–20 10–35 5–15 4–9 1–7

Electric arc furnace (EAF) 35–60 15–20 10–30 5–15 < 10 3–10

Ladle 40–60 3–15 0–10 5–15 0–10 20–40

S.K. Kawatra, S.J. Ripke / Int. J. Miner. Process. 65 (2002) 165–175166

1990). Steelmaking slags can contain 20–30% iron oxides. Steelmaking slags have been

pelletized and used as aggregate and in concrete masonry (Cotsworth, 1978) and

unbeneficiated slags have been reused in a blast furnace for the production of pig iron

(Dolgorukov, 1991).

An excellent study of BOF and EAF steelmaking slags is available from the

American Iron and Steel Institute’s (AISI) ‘‘Steel Industry Technology Roadmap’’ (AISI,

2000) and the United States Geological Survey’s (USGS) ‘‘Mineral Commodities

Summaries’’ (Kalyoncu, 2000). There are some additional steelmaking slags that should

be considered for recovery of iron units; Table 2 summarizes the information shared

with us from just one steel mill. There is an excellent opportunity for development of

technology to improve iron recovery for each of these steelmaking slags. According to

the AISI’s ‘‘Steel Industry Technology Roadmap,’’ 3.9 million tons of iron units is

generated each year as slag by US steel mills, while only 1.8 million tons is recycled.

However, these data are low estimates because only BOF and EAF slags were accounted

for, while other slags such as hot metal pretreatment (desulfurization) slags and ladle

slags also produced at steel mills were excluded (Table 2). According to the USGS

(Kalyoncu, 2000), steel slag from open hearth (OH) steel production was also sold from

stockpiles, but there was no domestic production of OH steel slag during 1999.

1.1. Slag processing

Steel mill slags are currently processed by standard methods common to the sand and

gravel industry such as crushing and screening. Typically, the slag material that is larger

than 1 in. in diameter is very highly metallic and is recycled as scrap. Processing of the

remaining finer material results in marketable slag products. Additional processing could

be integrated that would allow the recovery of iron units. Gravity, size, and magnetic

separation techniques, could be designed into flow sheets for recovering the iron units. The

resulting fine scrap concentrate could then be pelletized and recycled into the blast

furnace.

Table 2

Annual slag production summary from one North American steel mill producing 3.7 million metric tons of

steel (Farrand, 2000)

Slag type Production

(tons)

Usage

Primary Blast furnace slag 450,000 Land reclamation and landfill

construction to high value

cement lightweight aggregates

Steelmaking slag BOF 160,000 Asphaltic aggregate, fill, and

road bases; only about 50%

of the iron is recovered

EAF 90,000

Secondary Desulfurization slag

(hot metal pretreatment)

50,000 Mineral aggregate

Ladle slag 70,000 Mineral aggregate

S.K. Kawatra, S.J. Ripke / Int. J. Miner. Process. 65 (2002) 165–175 167

1.2. Fly ash and accelerators

The binding mechanisms associated with fly ashes have been explained by Helmuth

(1987) and Taylor (1990). Fly ash is composed of amorphous and fine crystalline

pozzolanic aluminosilicate particles. Pozzolans react in hydroxide ion solutions to form

precipitates that have binding characteristics.

Accelerators such as calcium chloride have been studied in cement and concrete

reactions for over 100 years. Cement reactions are essentially the same as fly-ash

pozzolanic reactions, and so accelerators have similar effects on both conventional cement

and on fly-ash pozzolans. The normal hardening of cement is the result of continuous

calcium silicate dissolution, resulting in precipitation of the gluing agent dicalcium silicate

hydrate. As calcium chloride exothermically dissolves, hydration reactions are accelerated

both because of the increased temperature, and because of the presence of lime that has a

higher solubility in the lower pH calcium chloride solutions than in water alone

(Platzmann, 1926). The lower pH also results in accelerated dissolution of the calcium

silicates. The hydrate is then more rapidly precipitated from increased calcium ion

concentration available from the calcium chloride and dissolved calcium hydroxide

(Forsen, 1938). Finally, the calcium chloride retains water throughout the reaction, which

prevents drying and allows the reaction to continue longer. Calcium chloride addition

accelerates the formation of tricalcium silicates, dicalcium silicates, and tricalcium

aluminates and increases the calcium oxide/silicon dioxide (CaO/SiO2) ratio of the

calcium-silicate-hydrate (C-S-H) phase (Odler and Abdul-Maula, 1987).

The purpose of the work described in this paper was to determine the conditions

necessary for pelletizing iron units recovered from desulfurization slag, and to evaluate

FBB as an alternative to bentonite for this application.

2. Experimental

2.1. Materials

The hot metal desulfurization slag was beneficiated using a conventional process. The

particle size was 80% passing 550 Am. The steelmaker produces 50,000 tons of the slag

annually at their facilities. Two concentrates were produced from the beneficiation

process, a high-grade product that could be used directly, and a relatively low-grade

product that the source requested to have pelletized. The low-grade product accounted for

27% of the head weight. Elemental analysis was performed with inductively coupled

plasma (ICP) spectrophotometry, as shown in Table 3. A separate FeO digestion method

was used to determine metallic iron concentration by ICP spectrophotometry that was

verified with quantitative X-ray diffraction (XRD). Particle size was determined both by

screening and laser diffraction (Microtrac).

Fly ash with a high carbon content (according to ASTM C618-98, 1998) was

collected from the Central Illinois Light’s (CILCo) E.D. Edwards power plant Unit 2

burner, designated as Unit 2 fly ash. Approximately 35 kg (75 lb) of the fly ash was

thoroughly mixed and divided into representative samples of approximately 750 g (1.7


lb) by coning, quartering, and incremental sampling. Then, the individual samples were

sealed into plastic bags. Reagent grade calcium hydroxide [Ca(OH)2] and calcium

chloride dihydrate [CaCl2�2H2O] were used to activate and accelerate the fly-ash

pozzolanic reactions. The fly ash had a particle size of 80% passing 53 Am (270 mesh).

Particle size was determined by laser diffraction (Microtrac).

The sample of bentonite clay was obtained from American Colloid. It was a western

bentonite, classified as SPV 200. This was a high-performance bentonite with a particle

size of 80% passing 75 Am (200 mesh). Quantitative elemental analysis was performed

with X-ray fluorescence (XRF) on the bentonite and fly ash by a commercial testing firm

specializing in fly-ash analysis. Chemical analyses of the Edwards plant fly ash and

bentonite are compared in Table 4.

Table 4

Chemical analyses of the high-carbon fly ash and the American Colloid SPV 200 bentonite

Analyte (%) High-C fly ash SPV 200 bentonite

SiO2 38.87 34.62

Al2O3 22.92 23.16

Fe2O3 6.09 5.49

Total (SiO2 +Al2O3 + Fe2O3) 67.88 63.26

CaO 15.11 9.63

MgO 3.56 2.11

Na2O 1.71 1.06

K2O 0.99 0.39

TiO2 1.18 1.25

MnO2 0.05 0.01

P2O5 0.91 2.20

SrO 0.20 0.39

BaO 0.48 0.53

SO3 1.20 5.93

LOI 6.78 13.24

Bentonite clay is the material currently used as an iron ore concentrate pellet binder.

Table 3

Chemical analysis of the hot metal desulfurization slag concentrate

Chemical Concentrate (wt.%)

Fe, total 61.7

Fe, metal 55.0

FeO 8.6

CaO 17.2

MnO 0.1

SiO2 9.8

MgO 2.3

Na2O 0.1

Al2O3 2.2

C 4.2

S 2.1

Total 101.6


2.2. Equipment

Binder components were mixed together with a Spex mixer/mill (Spex Industries, New

Jersey, USA). The binder was mixed into the concentrate with a Readco Type-A kneader–

mixer. Pellets were formed with a laboratory-scale balling drum, shown schematically in

Fig. 1. Pellets were dried in a Blue M (Illinois, USA) drying oven. Pellets were fired in a

Thermolyne Type 46100 high-temperature furnace. Ultimate compressive strengths of the

pellets were determined with an Instron 4206 using an 8896-N (2000 lbf) load cell at a

constant cross-head speed of 40 mm/min (1.57 in./min). Particle size was determined by

laser diffraction with a Microtrac SRA (Leeds and Northrup Instruments, Philadelphia,

PA). X-ray diffraction was performed with a Scintag, XDS 2000 Q/Q diffractometer and

data were analyzed with DMSNT version 1.33 software.

3. Pelletization procedure

To prepare the balling drum feed, a dry sample of the slag concentrate weighing 36 kg

(86 lb) was split into 16 samples with a riffle. Dosages of 1.0% binder by weight of slag

concentrate were added of either the bentonite or the fly ash (as FBB) and mixed with the

concentrate by rolling 100 times with a mixing cloth. Moist concentrate, created from dry

concentrate moistened by mixing with about 50% water and pressure filtered to 13%

moisture, was also used. In this case, the binder was mixed with moist concentrate with a

kneader–mixer for 5 min at 350 rpm with 150 rpm orbital motion.

A small amount of the balling drum feed was added to a 40.7-cm (16 in.)-diameter,

17.8-cm (7 in.)-wide pelletizing drum (Fig. 1) rotating at 25 rpm. Water mist was

sprayed onto the material to produce small granules commonly called ‘‘seeds.’’ Addi-

tional material and water mist were added to enlarge the pellets. Pellets were periodi-

cally removed and screened to control pellet sizes. This procedure continued until

Fig. 1. Dimensions of the laboratory-scale pelletizing drum used for pellet production. The 17.8-cm (7 in.)-

deep steel drum rotated at 25 rpm. Feed and water spray were added and pellets were removed through the

mouth.


approximately 1 kg of finished green pellets was produced with diameters ranging from

11.2 to 12.7 Am (7/16–1/2 in.). Green pellets were sampled and 20 pellets each were

selected for wet knock and wet crush strength testing. The remainder of the pellets were

oven dried at 105 jC for 20 h and the pellet moisture content was determined by the wet

and dry weight difference. Fifty dry pellets were sampled for strength testing, of which

20 were tested as dry pellets, and 30 were sintered at 1200 jC for 10–30 min. The

pellets were tested using the procedures given in Table 5.

For each individual experiment, the mean and standard deviation was reported for a

sample of 20 pellets. The 95% confidence interval (P95) is the range within which it is

95% certain that the true mean for the entire batch of pellets lies. P95 is based on the

mean, standard deviation, and sample size of 20 pellets tested. This value is calculated

using the standard t-distribution (Dixon and Massey, 1983). For 20 pellets (df = 19), the

95% confidence interval was calculated from the measured standard deviation using the

relationship: where s =measured standard deviation for the data set, N = number of data

points (equal to 20 for the procedures used here), t95(19) = the t-value for the 95%

confidence interval, at df = 19, and P95 = 95% confidence interval for the mean

calculated for the data set.

P95 ¼ Ft95ð19Þffiffiffiffi

Np

� �s ¼ Fð0:386Þs

4. Results

Two sets of experiments used as-received concentrate mixed with 0.66% bentonite

binder. The first used a dry concentrate and the second used a moist concentrate. As was

Table 5

Descriptions of the tests used for evaluating pellet quality (ASTM E382-97, 1998)

Test Procedure and use of data

Wet knock A single freshly made (undried) pellet was dropped repeatedly from a height of 18 in.

onto a steel plate. The number of drops required for fracture was recorded. This was

repeated for 20 pellets, and the results averaged. Measures the ability of the wet pellet

to remain intact during handling.

Wet crush A single (undried) pellet was compressed using an Instron compression test machine with

a cross-head speed of 40 mm/min. The peak load required to fracture the pellet was

recorded. This was repeated for 20 pellets, and the results averaged. Measures the ability

of the wet pellets to retain their shape during handling.

Dried crush Pellets were dried at 105 jC (221 jF) for at least 1 h; single pellets were then compressed

using an Instron compression test machine. The peak load required to fracture the pellet

was recorded. This was repeated for 20 pellets, and the results averaged. Measures the

ability of dried pellets to survive handling during the firing process. Should be at least

22.2 N (5 lbf) per pellet. This is the most critical measurement of binder performance.

Sintered crush Pellets were sintered in a crucible by preheating at 120 jC/min to 1200 jC, held for

another 10 min, and then air cooled. Individual pellets were then compression tested with

an Instron compression testing machine. The peak load required to fracture the pellet was

recorded. This was repeated for 20 pellets, and the results averaged. Measures the ability

of dried pellets to survive handling during shipment and reduction. Values should be at

least 1780 N (400 lbf) per pellet.


expected from prior experience, pellets could not be produced from either test because the

material was too coarse.

Additional experiments used concentrate that was rod milled for 5, 15, 30, and 45

min. A rod mill with dimensions of 20.3 cm (8 in.) diameter� 25.4 cm (10 in.) length,

that had four evenly spaced 0.19-cm (3/4 in.) lifters and contained stainless steel rods,

was used to comminute the slag concentrate. Microtrac size analyses were performed for

the resulting products, in distilled water. Particle size is represented by the cumulative

distribution curve shown in Fig. 2. Detailed descriptions of the cumulative distribution

curve are available in the literature (Wills, 1992). A typical magnetite pelletizing

concentrate is shown for comparison.

Since grinding the slag was necessary to produce pellets, a standard Bond (1952)

grindability work index was determined for a target size of 100 mesh (Deister, 1987).

The work index by this method was determined to be 162 kW h/st, which is

unreasonably high. The work index was unusually high because the hard-to-grind,

ductile metallic iron accumulated in the mill, while the Bond procedure assumes that

everything is being ground to pass the target size. Therefore, the Berry and Bruce (1966)

method for determining work index was employed instead, with quartz as the reference

ore. This resulted in a work index of 8.83 kW h/st to reach a pelletizable size

distribution.

The material became marginally pelletizable after grinding from 80% passing 500 Amto 80% passing 450 Am. Table 6 shows that acceptable pellets could be made from

Fig. 2. Cumulative size distributions of hot metal desulfurization slag concentrate before and after grinding with

a rod mill, determined by Microtrac laser diffraction. A typical feed size distribution for the magnetite

pelletizing concentrate is also shown.


concentrate milled to 80% passing 290 Am. Additional grinding produced superior-quality

pellets. The FBB, consisted of a mixture of fly ash, calcium hydroxide, and calcium

chloride added to the slag at rates of 1.0% fly ash, 1.0% calcium hydroxide, and 0.2%

calcium chloride by weight. This dosage rate produced excellent-quality dry pellets, as

shown in Fig. 3. Dry compressive strength is the first consideration to determine

acceptability of a pellet binder. Sintered strengths were lower for the FBB-bonded pellets

than for bentonite-bonded pellets. However, sintering profiles and procedures can easily be

optimized to produce acceptable strength-sintered pellets.

Fig. 3. Compressive strengths of hot metal desulfurization slag concentrate pellets bonded with either bentonite

clay or high carbon fly ash-based binder (FBB). The dashed line represents the minimum dry (presintered)

compressive strength of 22 N (5 lbf) for an industrially acceptable blast furnace feed pellet.

Table 6

Mechanical properties of hot metal desulfurization slag concentrate pellets

Binder P80

(Am)

Wet knock

(drops)

Wet crush [N (lbf)] Dry crush [N (lbf)] Sintered crush [N (lbf)]

Bentonite 450 not

determined

not determined 10.7F 0.9 (2.4F 0.2) 1041F 334 (234F 75)

Bentonite 350 4.6F 0.5 6.7F 1.3 (1.5F 0.3) 12.0F 0.9 (2.7F 0.2) 1072F 356 (241F 80)

Bentonite 290 13.9F 1.1 11.1F1.8 (2.5F 0.4) 22.7F 1.8 (5.1F 0.4) 1806F 444 (406F 100)

Bentonite 220 7.4F 0.3 13.8F 1.8 (3.1F 0.4) 27.6F 2.7 (6.2F 0.6) 1832F 374 (412F 33)

FBB 220 7.4F 0.8 22.2F 1.3 (5.0F 0.3) 53.4F 1.8 (12F 0.4) 1378F 222 (310F 50)


5. Conclusions

bThe desulfurization slag concentrate could be used to produce pellets having

acceptable strength after it was rod milled to 80% passing 290 Am, using 1.0% bentonite

as the binder. These pellets have acceptable mechanical properties for use as blast furnace

feed for producing iron and steel. Finer grinding of the hot metal desulfurization slag

concentrate gave higher quality pellets.

bFBB was used to produce pellets with dry compressive strengths superior to pellets

bonded with bentonite. The superiority of the FBB was due to the different binding

mechanism involved. Since fly ash is more readily available and costs less than the

bentonite, it is recommended as the preferred binder for pelletization of this material.

bFurther research is warranted to optimize the sintering profile and procedures and

to determine the reducibility characteristics of these hot metal desulfurization slag

pellets.

Acknowledgements

We thank the National Science Foundation (NSF) under Grant BES-9802198 and Dr.

Tim Eisele of Michigan Tech for his invaluable technical advice. We also thank the

following undergraduate students: Kari Buckmaster, Karla Andrade, Gabriella Ramirez,

Toby Lee, Frank Perras, Elise Anderson, and Jaime Krull.

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pelletizing steel mill desulfurization slag

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