IMBSiR 83-2773

Characterization of Miiiscaie

Steel Wastes

October 1 933

Prepared for: Orfice of Recycled Materials

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

NBSIR 83-2773

CHARACTERIZATION OF MILLSCALE

STEEL WASTES

E. S. Domalski^, W. A. MacCrehan^, J. R. Moody

Y. B. Tewari \ and J. A. Walker'*

October 1983

* Chemical Thermodynamics Division

2 Organic Analytical Research Division

^ Inorganic Analytical Research Division ^ Chemical Kinetics Division

Prepared for; Office of Recycled Materials

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

Preface

The American Iron and Steel Institute (AISI) requested the assistance of

expertise at the National Bureau of Standards in cooperating on the problem of

reprocessing of millscale and millscale sludge. Hence, the purpose of this

study is to investigate the appropriate chemistry to remove the oils and

greases (hydrocarbons) from the millscale waste so that an iron-rich material

can be recycled into the steelmaking process. The emphasis will be on the

determination of the physical properties of the millscale and hydrocarbon oil,

which will affect the extraction chemistry, and not on the identification and

precise determination of a few compounds. Comparison of unused grease and the

oil extracted from the millscale will be sought with a view toward interpreting

the changes in chemistry as they relate to steel processing.

Di scl aimer

Certain commercial equipment, instruments, or materials are identified in

this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the materials or equipment identified are

necessarily the best available for the purpose.
1 1

Table of Contents

Page

Abstract 1

I Introduction 2

II Particle Size Distribution Study 4

III Extraction of Hydrocarbon Oil from Mi 11 scale 6

IV Oil Recovery 10

V Gel Permeation Liquid Chromatography 12

VI DSC Experiments 19

VII Summary and Conclusions of Experimental Results. ... 20

Appendix - Description of Deoiling Plant 24
IV

List of Figures

Figure No. Page

1. GPL Chromatogram, Millscale Extract

Methyl isobutyl ketone solvent 33

2. GPC Chromatogram, Millscale Extract

Freon 114 solvent 34

3. GPC Chromatogram, Millscale Extract

Methyl ethyl ketone solvent 35

4. GPC Chromatogram, Millscale Extract

Hexane solvent 36

5. GPC Chromatogram, Millscale Extract

XAD-2 solvent 37

6. GPC Chromatogram, Veedol #57 Lubricant

(Prior to use) 38

7. - GPC Chromatogram, Brooks Lubricant

(Prior to use) 39

8. GPC Chromatogram, Veedol #56 Lubricant

(Prior to use) 40

9* DSC Thermogram, Brooks Mill Lubricant 41

10. DSC Thermogram, Veedol #56 Mill Lubricant .... 42

11. DSC Thermogram, Veedol #57 Mill Lubricant .... 43

12. DSC Thermogram, Millscale Extract

Bethlehem #2 44

13. DSC Thermogram, Millscale Extract

Republic #1 45

14. DSC Thermogram, Millscale Extract

Inland #2 46
V

List of Tables

Table No.

I Particle Size Distribution of Millscale (as received)

II Particle Size Distribution of Deoiled Millscale. . .

III Apparent Extraction Efficiencies (wt. percent) . . .

IV Apparent Mass of Millscale Extracts

V Retention of Molecular Weight Standards

VI Chromatographic Conditions

Comparison of Extraction Efficiency of Different

Solvents

5

5

9

11

14

15

17

VII
1

Characterization of Mi 11 scale Steel Wastes

E. S. Domalski, W. A. MacCrehan, J. R. Moody, Y. B. Tewari , and

J. A. Walker

National Bureau of Standards

Washington, DC 20234

ABSTRACT

This study was carried out to investigate the appropriate chemistry

required to remove the oils and greases (hydrocarbons) from the mi 11 scale

waste so that an iron-material could be recycled into the steel making process.

Characterization of the mi 11 scale was made as a result of examining the

following parameters: particle size distribution of the millscale, extraction

of the hydrocarbon grease by means of ignition and solvent reflux, gross

molecular weight distribution of the grease, and thermal decomposition of the

grease. The data obtained in this study may prove useful in evaluating and

optimizing operational millscale deoiling operations.
2

I. Introduction

The manufacture of steel products into slabs, sheets, blooms, billets, or

bars requires that they be worked or formed in a hot strip-rolling process.

During the working or forming, these products undergo oxidation, cooling, and

washing with a high pressure water spray. The hot steel surface is exposed to

both oxygen from the air and wash water. As a result of the exposure, layers

of iron oxides formed on the surface are flaked off. As the steel product is

rolled into the desired form, one layer of scale is broken off and is replaced

by a new layer. A new scale is generated everytime the steel product is

shaped or rolled. During the rolling process, the scale falls through the

roll tables into a trough or sewer through which rapidly-flowing water is

passing. A large amount of oils and greases enter and are used to lubricate

the roll tables and shaping equipment. The scale, water, oil, and grease

combine to form a steel waste of heterogeneous particle size. The easily

filtered, large particle material (about 1 mm to 1 cm) is referred to as

mill-scale. A mill-scale sludge is also generated by sand filtration of the

waste water and has a smaller particle size (about 1 pm to 1 mm). Past

practice has been to dispose of the mill-scale and sludge in land fill

operations at the plant site. The sludge required special treatment such as

dumping into lagoons or ponds. Filtering of some of the solids was often

necessary as well as special treatment of the leachate. In certain cases,

blending of the sludge with inert slag materials was required prior to the

landfill operation.

Since the mill-scale is approximately 70 percent iron, and the sludge is

approximately 30 percent iron, economic considerations suggest recovery of
•3

these metal units. However, difficulties arise when these wastes are used

directly as feed stock in the sintering process because of the hydrocarbon

impurities acquired from the lubricants in the hot rolling process. The

hydrocarbon on the mi 11 scale is between 0.3 and 10 percent oil by weight and

as much as 50 percent oil by weight of the sludge. The hydrocarbon on the

scale interferes with the sintering process of preparing ore for the blast

furnace. Sintering involves a careful mixing of iron ore with limestone and

coal, followed by firing at red heat in a reducing gas atmosphere. The

resulting coarse mix is bound together by calcium ferrite and can be fed

directly into the blast furnace. Difficulties arise when the oily mi 11 scale

is used as a feed because the hydrocarbon residue of the heavy grease is not

burned in the sintering process, but is volatilized. When the waste air and

gas is filtered with an electrostatic precipitator, the filter bags get

clogged with hydrocarbon residues and can catch fire. In some instances,

explosions have occurred when volatilized hydrocarbons and air reach certain

mixed proportions in the electrostatic precipitator.

Disposal costs for both mill -scale and sludge are likely to increase as

the regulations for waste disposal established through the Resource Conservation and Recovery Act (RCRA) are implemented. Some estimates suggest that

these costs could possibly approach $50 per ton of waste. Recent figures

place the annual levels of mill-scale and sludge generation by the U. S. steel

industry at about three million tons per year (or 8000 tons per day).

A field trip to the Bethlehem Steel Corporation facility at Bethlehem,

Pennsylvania was made by E. S. Domalski, J. A. Norris, W. A. MacCrehan, and Y.

B. Tewari on Friday, February 5, 1982 to obtain a clearer understanding of the
4

steelwaste problem from actual observation of mill-scale generation by a hot

rolling steel process. The rolling or working of an I-beam (about 150 ft. in

length) was observed in which layers of iron oxides were flaked off during the

rolling process by the mechanical action of the mill and cooling water. About

2.5 percent of the metal units are lost in this hot-rolling process.

Millscale samples were received from the Bethlehem Steel Corp. , Inland

Steel Corp., and Republic Steel Corp. for study in this project. The Bethlehem

Steel Corp. also provided three samples of hydrocarbon lubricants so that

specimens would be available for testing as they are (i.e., unused) prior to

their application to roll tables or shaping equipment.

After having examined the available information on the recovery of metal

units from millscale, the following project goals were selected: (1) to

establish the particle size distribution of the millscale samples, (2) to

determine their hydrocarbon content by at least two methods, (3) to identify

the general nature of the hydrocarbon oil (i.e., aliphatic vs. aromatic) and

to indicate, when possible, any changes in the hydrocarbon lubricants by

comparison to lubricants prior to their use, (4) to study the efficiency of

different solvents with respect to the solubilization and extraction of

hydrocarbon oil from the millscale, and (5) to determine the temperature range

over which volatilization of the hydrocarbon lubricants occurs.

II . Particle Size Distrubution Study

The particle size distribution of millscale obtained from three steel

industry sources was determined by gravimetrical ly analyzing the fractions
5

initially obtained by sieving. Table I contains a listing of the particle

size distribution of oily millscale (as received), whereas in Table II the

particle size distribution of deoiled millscale (using a solvent extraction

method) is listed. The distribution listed in Table II shows that nearly 30

percent of millscale consists of very fine particle sizes (-50 mesh size).

Table I. Particle Size Distribution of Millscale (as received).

Screen Size

Bethlehem

weight fraction

Inland^

weight fraction

Republ ic

weight fraction

+10 mesh*^ 0.30^ 0 . 20

y

0.17g

-10 + 20 mesh 0.185 0.343 0.203

-20 + 30 mesh 0-085 0.33g 0.093

-30 + 40 mesh 0.07g 0-.05g 0.103

-40 + 50 mesh 0.13^ 0.04^ 0 . 12

g

-50 mesh 0-205 O.OOg 0.285

Millscale was ’very oily (wet) and difficult to sieve .

Table II. Particle Size Distribution of Deoiled Millscale.

Screen Size

Bethlehem

weight fraction

Inland

weight fraction

Republ ic

weight fraction

+10 mesh^ 0.26^ 0.13^ 0.15^

-10 + 20 mesh 0.195 0.29q 0.192

-20 + 30 mesh 0.09^ 0-115 0 . 12

g

-30 + 40 mesh 0.06g O.O84 O.IOq

-40 + 50 mesh 0.08^ 0 . 10

^ 0.132

-50 mesh 0.29^ 0.274 O. 3 O

4

^10 mesh = 1.7 x 10'^ meter

50 mesh = 0.29 x 10”^ meter
6

The Bethlehem Steel and Republic Steel samples were qualitatively similar

in particle size distribution when examined as-received. The Inland Steel

sample was oily (wet), and this condition apparently caused sufficient small

particle agglomeration to skew the particle size distribution as compared to

the other samples. However, after deoiling of this sample by solvent extraction, results for all three samples yield a similar particle size distribution, especially at smaller particle sizes. The figures listed in Table II

are indicative of the true solids distribution once the binding effects of the

oils have been removed.

III. Extraction of Hydrocarbon Oil from Mi 11 scale

Sampl i ng

A single five gallon^ can of millscale from Inland Steel was used for all

of the following tests. Preliminary observations indicated extreme problems

with sample heterogeneity. Because of this heterogeneity, the entire contents

of the can were subjected to a repetitive quartering process (cone and quarter)

until two dozen replicate samples of approximately three hundred grams each

were produced. The quartered samples appeared to be homogeneous, but, within

24 hours, signs of segregation were visible within the 24 sample jars.

Therefore, either the entire bottle was used for testing or the bottle was

thoroughly mixed before taking subsamples.

U.S. gallon = 3.78533 liters.
7

Ignition

The initial tests on these blended samples were designed to determine the

total amount of volatile residue actually present. For these tests, a tube

furnace was used to heat the samples in a flowing nitrogen stream which kept

oxygen and water away from the sample. At low temperatures , the condensate at

the outlet of the tube furnace appeared to be water but upon attaining 200 °C

the odor of some volatile material was quite apparent; at 250 °C the sample

generated a white smoke. Arbitrarily, 500 °C was chosen for the sample

ignition temperature, because no volatiles were detected in the N

2 gas stream

and because the ignitions produced reproducible results.

A number of samples from different bottles were ignited to 500 °C for one

hour under N

2 gas. The average sample size was about 15 g and the average

loss on ignition was 2.07 wt. percent. The precision of these measurements

was 0.15 percent, relative standard deviation (RSD). However, in this ignition

experiment, all volatiles are measured, and therefore a separate determination

of water by the Karl -Fischer titration was performed by S. Margolis of the

Organic Analytical Research Division. One disadvantage of this method is that

water which is occluded or bound as water of crystallization is not likely to

be titrated. We believe that, in this case, all of the water present was

adsorbed and hence titratable. In addition, because of the sensitivity of the

measuring instrument, only small (‘^'100 mg) samples could be used for analysis.

This size restriction created an additional problem since the bulk sample was

originally very heterogeneous. The average value found for the water content

was 0.72 wt. percent which in turn indicates, by difference, that the average

amount of volatile oil present was -'-1 . 35 wt. percent.
8

Extraction

To test the relative efficiency of the extraction power of various

solvents, a Soxhlet extraction apparatus was used; a 300 g sample size was

chosen in an effort to minimize the effects of sample heterogeneity. The

purpose of these extractions was to measure the weight loss of the samples

after 2, 6, and 18 extractions for each of the four solvents tested. To do

this, the experiment was stopped after the requisite number of extractions,

and the sample was drained of excess solvent before removing the sample from

the apparatus. The remaining solvent was removed by vacuum drying at room

temperature. The sample was then re-equi 1 ibrated at room temperature and

humidity before weighing. We assumed that the most volatile components, which

could be lost upon vacuum drying, were actually extracted before the first

vacuum drying.

The concentration of volatiles was calculated directly from the weight

loss on extraction. As a second means of deducing the concentration of

volatiles extracted, a small (10 g) sample was removed after each extraction

experiment and subjected to the one hour tube furnace ignition. The sum of

the loss on extraction and the residual volatiles should equal 2.07 percent.

The solvent complicated matters since it was not possible to know when the

procedure for removing solvent (drying) from the sample did not also remove

water or other volatiles. The removal of excess solvent was necessary in

order to determine the amount of oil extracted from the sample.

The results obtained from the extractions and ignitions are tabulated in

Table III. Freon 114 (1 ,2-dichlorotetrafluoroethane) was definitely less
9

effective than the other solvents. To some degree this effect may be

attributed to its lower boi 1 ing poi nt. Extraction temperatures in the Soxhlet

were lower than the boiling points of the solvents, but the extreme difference

in temperatures may well account for the inefficient extractions with Freon

114. Interestingly enough, all solvents yield approximately the same ultimate

degree of extraction efficiency. The efficiency differences between hexane,

methyl isobutyl ketone, and methyl ethyl ketone are less than the estimated

experimental errors.

Table III. Apparent Extraction Efficiencies (wt. percent).

Solvent Hexane

Methyl Isobutyl

Ketone

Methyl Ethyl

Ketone Freon 114

2 Extractions 1.5, 1 . 5

o 1.43 0.9q

Ignition 0.43 0 . 5

g

TOTAL 2.02 1.99 2.07 2.07

6 Extractions 1.63 1.63 1 . 2

g

Ignition 0.43 0 . 5

^ 0 . 5 ^ 0.83

TOTAL 2.19 2.18 2.23 2.12

18 Extractions 1.73 1.6, 1.47

Ignition 0 . 3

g

0.62 0 , 5

g

0 . 6

^ TOTAL 2.11 2.19 2.22 2.08

Total estimated volatiles 2.07%, including 0.72% H

2

O.

Also, of potential interest is the behavior of the dry (ignited) steel

samples. All of these samples exhibited an affinity for water and had an

average gain of 0.04 percent on exposure to air. Vacuum drying removed this
10

water as well as solvent (and presumably volatile oils) so that exposure to

the atmosphere after drying was used to preserve a consistent basis for a

sequence of weight measurements. One sample was ignited to 1000 °C under N

2

,

and it exhibited a further loss of 0.48 percent compared to the ignition at

500 °C. This step caused visible alteration in the sample (such as partial

fusion); and thus, it is difficult to know in detail what processes took

place. Some inorganics are volatile at this temperature while few, if any are

volatile at 500 °C. In addition, since no odorous emissions were detected or

visible emissions observed during the temperature programming from 500 to

1000 °C, most of the volatile organics are probably lost in the 20 to 500 °C

temperature range.

The total weight loss of these samples, whether by ignition to 500 °C or

by separated extraction and then ignition is about 2.1 percent. It is likely

that at least part of the 0.7 percent water in the samples is extracted by the

solvents and equally likely that part of the oil fraction is not estimated.

It is significant that by whatever path used, figures of '^'2.1 percent total

volatiles and extractables are obtained. For this extraction procedure. Freon

114 was the least efficient solvent while the others chosen for testing were

quite similar in performance.

IV. Oil Recovery

The objective of the solvent study was to compare the efficiency of

different solvents for the extraction of oil from mi 11 scale. Solvents of

different polarity were chosen ranging from methyl ethyl ketone (the most

polar), methyl isobutyl ketone. Freon 114, to hexane (least polar). Also a
11

solid extractant, Rohm and Haas Amberlite XAD-2 (a polystyrene/di vinyl benzene

copolymer), was used as a model for solid phase oil recovery.

The mi 11 scale solvent extracts from a Soxhlet apparatus were taken for

rotary evaporation and analysis. For the solid extraction sample, XAD-2 was

mixed with the mi 11 scale in a ratio of 1:10, and the mixture was heated at

100 °C for 1 hour while being shaken. The XAD-2 was then separated by

floatation on water, collected, dried, and then extracted with methyl ethyl

ketone. The resulting extract was then handled in the same manner as the

Soxhlet extracts. In each case, the solvents were removed by rotary evaporation under vacuum at 50 °C. The resulting "millscale extracts" from each

solvent and XAD-2 were then weighed. The results were as listed in Table IV.

Table IV. Apparent Mass of Millscale Extracts.

Solvent

Grams Oil Extracted

per 300 g Sample Percent Oil Extracted

Methyl isobutyl ketone 2.34 0.78

Freon 114 2.11 0.70

Methyl ethyl ketone 1.88 0.63

Hexane 1.65 0.55

XAD-2 0.19 0.063

Few inferences may be drawn concerning the recovery of oil (by mass) as

considered from the viewpoint of the effect of solvent polarity. The

moderately polar solvent methyl isobutyl ketone provided the best recovery of

oil by weight. However, Freon 114 (a relatively nonpolar solvent) ranked

second in extraction efficiency, followed by the most polar solvent (methyl
12

ethyl ketone), and finally the poorest efficiency was found using hexane.

Differences between these results and the previously reported gravimetric

results for the extractions may have been caused by loss of very volatile

hydrocarbons and water during rotary evaporation.

These results would lead us to conclude that a moderately polar solvent

is best for de-oiling mill scale. Thus, solvents with polarities between that

of methyl isobutyl ketone and Freon 114 should provide satisfactory oil

recovery. However, this conclusion must be tempered with the realization that

many other parameters, besides the polarity of the solvent, must be considered

in the extraction efficiency. The boiling point of the solvent comes into

play since it determines the temperature of the Soxhlet extraction. Other

properties such as solvent viscosity may determine the rate at which the

solvent can diffuse into the mill scale particles.

The oil recovery achieved by using XAD-2 (the solid polystyrene adsorbent)

was fairly low. However, this experiment demonstrated that at least some oil

could be recovered by a solid adsorbent and perhaps more oil could be recovered

if higher temperatures than 100 °C were used. XAD-2 is too expensive ($6/lb.)

to use for de-oiling millscale, but other solids such as dry wood chips or

charcoal might be more cost-effective.

V. Gel Permeation Chromatography

Gel permeation chromatography (GPC) was used to provide information about

the molecular weight fractions of the mill scale extracts. This approach

employs a high pressure pump which forces the solvent through a packed bed
13

consisting of a polymer gel with a controlled, uniform pore size. In this

°a

work, a polystyrene column with a 200 A pore size was used with toluene as

the flowing solvent. The solvent and polymer gel must possess similar

polarities because the separation is based on a purely physical separation -

diffusion. The sample molecules are forced along with the flowing solvent,

but are also diffusing in and out of the gel. Molecules that are smaller than

the pore size can diffuse into the stagnant solvent of the gel. Larger

molecules will not so diffuse and thus will be carried more rapidly by the

flowing solvent. Large molecules will elute from the column first followed by

progressively smaller molecules. Although the separation mechanism is based

on molecular size, this quantity is linearly related to the molecular weight

for compounds of a homologous series.

The GPC system can be calibrated with a series of standards of known

molecular weight, and the log of the retention volume or time can then be

plotted against the molecular weight. A straight line curve was obtained for

a series of n-alkanes. Several standards for aromatic compounds were also

tested in order to determine their respective retention characteristics;

results are reported in Table V. Aromatic compounds (benzene, perylene, and

the polystyrene standards) have much smaller size (and hence, longer retention

times) relative to the alkanes for an equivalent molecular weight.

3 o —TP

1 Angstrom (A) = 1 x 10 meter.
14

Table V. Retention of Molecular Weight Standards.

Compound Class

Retention Time

Molecular Weight (minutes)

Pentane (C^) n-al kane 72 3.325

Heptane (C^) n-al kane 100 3.175

Decane (C-jg) n-al kane 142 3.000

Tetradecane (C-j^) n-al kane 198 2.900

Hexadecane (C-jg) n-al kane 226 2.775

Benzene aromati

102 4.45

Peryl ene aromatic 252 4.20

Polystyrene

Standards

PS-1 aromatic 800 2.75

PS-3 aromatic 4,000 2.35

PS-6 aromatic 50,000 2.25

The retention times for PS-3 and PS-6 are almost identical (2.35 versus

2.25 minutes) even though the molecular weights are 4,000 and 50,000,

respectively. The structure of these two compounds is much larger than the

average pore size of the column. This property causes them to be totally

excluded from the pores of the gel, and thus elute at very short retention

times with very little resolution between the two standards.
15

The detection approaches used were differential refractive index (RI) and

ultraviolet absorbance (UV). The refractive index detector produces an output

signal that is the difference in refractive index between the sample and the

reference solvent. For this work, components with lov/er refractive indices

than toluene (the reference) are displayed above the baseline. Compounds that

have lower refractive indices include straight and branched chain alkanes.

Multi-ring aromatic compounds have a higher refractive index than toluene and

are displayed below the baseline. An absorbance detector was also used being

set at a wavelength of 315 nm (the cut-off of the toluene solvent). This

detector measures the absorbance of sample molecules so provides a selective

response for aromatic compounds of moderate and high molecular weight.

The conditions for the gel permeation analysis are detailed in Table VI.

Table VI. Chromatographic Conditions.

Parameter Value

Sample size 20 pL loop injection of 50 mg oil /1 000 mg toluene

Solvent Toluene flowing at 1.5 mL/minute

o Column Polymer Laboratories PL gel lOOA (200 A effective

pore diameter), 10 pm particles, 60 x 0.77 cm

Detectors Waters R401 differential refractometer sensitivity 4X

Kratos Spectroflow 773 UV absorbance detector,

wavelength 315 nm, sensitivity 0.75 AUFS
16

The actual chromatograms obtained from the mill scale extracts

(Figures 1-5) and roller lubrication greases (Figures 6-8) are included. The

molecular weight and class identifications are detailed in Figure 1, based on

the response obtained for the calibration standards.

There are several characteristic features of the lubrication greases that

should be noted: All have a fairly large amount of an aromatic fraction of

molecular weight greater than 3500 amu. They also have a large aliphatic

fraction of molecular weight around 500. There is a large quantity of material

having an aromatic fraction with a molecular weight between 3500 and 300, with

a maximum concentration of compounds of molecular weight 600 to 800. Finally,

there is a small aliphatic fraction with molecular weights below 100.

The mi 11 scale extracts show several distinct differences from the original

lubrication greases. The high molecular weight aromatic fraction (greater

than 3500) is substantially diminished. The moderate molecular weight

aromatics (3500 to 300) are also diminished, especially the fraction with

molecular weight below 1000. The high molecular weight aliphatic fraction (at

about 500 amu) is virtually the same in the extracts as in the lubrication

greases. There is actually a higher proportion of low molecular weight

aliphatics in the extracts than in the parent lubricants.

The respective individual solvents can be compared for their efficiency

of extracting the different fractions of the mi 11 scale oil by a qualitative

examination of the peak heights. Table VII summarizes the results of this

compan son:
17

Table VII. Comparison of Extraction Efficiency of Different Solvents.

- - Molecular Weight Fraction Recovery^ - -

- - - Aromatics - - - - Aliphatics -

Sol vent

High MW

(>3500)

Moderate MW

(3500-300)

High MW

(<500)

Low MW

(<100)

Hexane + +++ ++ +++

Freon 114 ++ +++ +++ ++

Methyl isobutyl ketone + ++ ++ +++

Methyl ethyl ketone ++ +++ +++ ++

XAD-2 + + + +

^here best recovery is symbolized by +++.

The GPC data are a direct indication of the concentration of detectable

components in different molecular weight categories. However, for the millscale extracts an additional parameter must be considered when evaluating the

results given in Table VII. The mass of oil extracted will also determine the

relative response for each molecular weight fraction. This effect is particularly

apparent for the results obtained for methyl isobutyl ketone. The experimentally determined peak heights indicate that this solvent is not very good

for extracting the first three molecular weight fractions. However, because

this solvent was so efficient at removing low molecular weight aliphatic

compounds (and perhaps undetected low molecular weight aromatic compounds),

the other fractions are effectively diluted by these components. The most oil

was recovered by methyl isobutyl ketone; and thus, some of the fractions are

diluted and give correspondi ngly low peak responses.
18

Several general statements can be made about the oil removed from

mi 11 scale by the various liquid solvents. The oil is depleted in the high

molecular weight aromatic compounds (>3500) compared to the mill lubricants.

This result could be caused by one of two distinct factors: Some of the large

aromatic compounds may have been "cracked" by the high temperature milling

process into smaller fragments and then lost by volatilization. The second,

and perhaps more likely possibility, is that this high molecular weight

fraction is not being completely removed from the mill scale by the solvents

tested under the conditions chosen.

Another characteristic of the extracts is the lower proportion of low

molecular weight aromatics, especially of molecular weight below about 1000.

These components are significantly depleted compared to the lubricants. Since

these components should be easily extracted by all the solvents, they may have

been lost by volatilization in the milling process or lost during sampling,

shipping, and storage.

The high molecular weight aliphatics recovered by the Soxhlet extraction

are very nearly equal to what is present in the lubricants for all solvents.

However, a larger amount of low molecular weight aliphatic components occurs

in the extracts, especially for hexane and methyl isobutyl ketone. This

observation is of particular note since some of this fraction will be

volatilized in the rotary evaporation step. This significant increase,

relative to the other components, is probably due to cracking of larger

aliphatic components in the heat of the milling process. This fraction may be

of the greatest significance to the explosion problem found when untreated
19

mill scale is used as sinter feed. These low molecular weight aliphatics, such

as butane, perhaps branched pentanes, and hexanes, are quite volatile and form

explosive mixtures with air even at low concentrations of vapor and may

contribute significantly to the explosions observed when untreated millscale

is used as sinter feed.

VI . DSC Experiments

Three lubricating greases which are commonly used in rolling mills were

examined using differential scanning calorimetry (DSC) to determine their

initial and peak decomposition temperatures and when possible to obtain an

estimate of their calorific values. Experiments were conducted with air as

the purge gas at 700 kPa (100 psig) and at a controlled heating rate of 20 °C

per minute. The resulting data are shown in Figures 9, 10, and 11. These

figures indicate the presence of two prominent classes of organic compounds

with peak temperatures in a low (257, 275, 276 °C) range and high (472, 480,

484 °C) range. A third less prominent peak (or shoulder) appears at about

300 °C.

Lubricating greases obtained by means of de-oiling millscale samples from

Bethlehem, Republic, and Inland Steel Corps, using solvent extraction procedures were examined. DSC thermograms showed peak temperatures for the

initial class of organic compounds undergoing decomposition at 310, 324,

325 °C; see Figures 12, 13 and 14. Calculation of calorific values for the

extracted lubricating greases from Republic and Bethlehem millscale samples

yielded 5162 J/g and 4817 J/g.
The thermal stability of each of the three extracted greases measured by

DSC was limited to 400 °C. Above this temperature, a rapid transition was

observed which was manifested by several sharp temperature spikes in the

thermogram. This phenomenon is usually associated with sample ignition or

explosive decomposition. This result was not observed in the unused grease.

The cause of this difference may be related to the thermal history of the

greases during the rolling process which resulted in a higher molecular weight

material that may be oxidized very rapidly above 400 °C.

VII, Summary and Conclusions of Experimental Results

Particle Size Studies

Considering the source and nature of millscale samples, it is not too

surprising that the millscale shows evidence of considerable heterogeneity

(Tables I, II). As with the Inland sample, the presence of a slight excess of

oil can cause a large decrease in the apparent number of fine particles. Yet,

when this sample was de-oiled by solvent extraction, the number of -50 mesh

particles found was very similar to the other millscale samples.

These considerations .suggest difficulty in any attempt to prepare a

"representative" sample or in preparing a single large homogeneous lot of

material for use as a reference standard. Indeed, later experience in

handling these samples demonstrated that after the process of laboriously

coning and quartering a very large sample to obtain 300 g samples, the sub

samples themselves later experienced some re-segregation.
21

Extraction Studies

Two different procedures were developed in order to determine the amount

of oil in a specially prepared set of replicate samples. In the first procedure, a simple ignition to 500 °C under nitrogen was used to determine the

weight loss from the samples. Although the temperature chosen was somewhat

arbitrary, we believe it to be high enough to remove most volatiles and oils.

Above 500 °C, no condensate could be observed on the cold parts of the combustion apparatus. In addition, most of the DSC thermogram results (Figures

9-14) suggest that the oils are volatilized below 500 °C.

The water content of the samples was unexpectedly high (0.72 percent) and

this in turn caused increased difficulty in determining the amount of oil

present. The ignition yielded an average of 2.1 percent volatile with a RSD

of 0.003 percent (absolute). By difference, this result suggests an oil

content of -'-1.35 percent with a substantial uncertainty.

The second method used four different solvents in a Soxhlet apparatus in

an attempt to characterize the relative extraction efficiency of the solvents.

As shown in Table III, this effort was largely unsuccessful because all of the

solvents, except for Freon 114, proved to be very good solvents for these

extraction studies. If the average value of 2.15 percent oil found after 18

separate extractions by the four solvents is used, and the same correction for

a water content of 0.72 percent is used, an apparent oil content of 1.43

percent is found. Considering the substantial uncertainties involved, this

finding is in very good agreement with the 1.35 percent figure obtained by the

combustion tube method.
22

Oil Recovery

After the removal of solvent by rotary evaporation, the four sample

extracts were weighed to yield a figure for the percentage of oil recovered

from the samples (Table 10). Qualitatively, the amount of oil recovered from

each solvent is similar — yet all samples yield an assay for oil which is

about one half that obtained by solvent extraction and gravimetry (-v-O-OO

percent average vs 1.43 percent). Although there may be a procedural bias

built into one or both methods, the rotary evaporation probably caused a loss

of a substantial amount of oil through volatilization.

Gel Permeation Chromatography

All of the extracted oil residues (as described above) were analyzed by

gel permeation liquid chromatography together with samples of typical lubricants

(e.g. , Veedol). A comparison of the results, as illustrated in Figures 1-8,

demonstrates that all of the solvents used gave good recovery of moderate

molecular weight aromatics as well as all aliphatics. High molecular weight

aromatics were not well extracted, presumably due to the relatively low

temperature of the Soxhlet extraction.

The relative proportions of low molecular weight aromatics are depleted

in the extracts relative to the lubricants whereas the relative proportions of

the low molecular weight aliphatics are increased. No clear explanation for

this difference was found.
23

DSC Experiments

The results of these experiments are illustrated as DSC thermograms in

Figures 9-14 and may be summarized as follows: Two prominent peaks were found

for the unused lubricating greases, and these peaks correlate well with the

gel permeation-liquid chromatography studies of the same materials. Peak

decomposition temperatures between 470-485 °C for unused lubricating oil may

correspond to the high molecular weight formation observed by gel permeationliquid chromatography. The 500 °C temperature used for the combustion tubenitrogen sweep of the samples is well above the highest peak observed. The

peak decomposition temperatures between 310-325 °C for the mi 11 scale extracts

could correspond to the high molecular weight aliphatic fraction observed in

the gel permeation liquid chromatography study.

Concl usions

Fairly large samples (300 g) were required to give reasonably significant

data for the comparison of the relative extraction efficiencies of four

solvents. Since all of the solvents except Freon 114 exhibited high efficiency,

the chances are good that a less costly and more environmentally acceptable

solvent than Freon 114 can be found. Experiments such as these may prove

useful in studying and optimizing operational mi 11 scale deoiling operations.

The very simple measurement of loss on heating to 500 °C (under N

2

) may prove

to be a useful method for measuring the efficiency of deoiling plants. The

method is quick (1 hour), precise (>>1 percent relative), consumes little

sample (15 g), but does require a correction for the water content of the

sample.
24

APPENDIX

Description of a Deoiling Plant

I. Selection of a Process and Solvent

Several methods of deoiling of steelwaste have been investigated and are

described in the literature [1]. These may be classified into three groups:

1) Thermal incineration

2) Saponification and solution in large quantities of

water

3) Washing with organic solvent

The incineration method is high in energy consumption. Also, it creates

a secondary exhaust control problem with additional energy requirements, and a

tertiary waste water problem. (Such an installation can scarcely be constructed

without a gas scrubber.)

The heating of one ton of scale to 600 °C (red heat) consumes about 110

kWh as thermal energy. The fuel consumption of a rotary kiln installation

with flue gas afterburner and wet scrubber, working in the USA, is reported to

be 261 kWh/t scale.

Washing with detergents to remove oil is also judged to be unfavorable

and expensive as the costs for detergents are intolerably high especially when
25

combined with the cost of treating the resultant waste water. The latter task

stretches prevailing legal tolerances. It seems absurd to convert a disposal

problem, with much effort, into a waste water problem.

Only the process of deoiling of mi 11 scale with solvents seems practical

without significant effort and expense to avoid secondary pollution of the

environment. The necessary process technology has been developed and utilized

successfully [2] to deoil steelwaste in Europe. Freon 11 (trichlorofluoromethane was the solvent used for the extraction process. However, this

solvent may not be acceptable to the U.S. Environmental Protection Agency due

to its adverse environmental effects. Therefore, one wishes to select a low

boiling hydrocarbon as an extracting solvent. The selection of an appropriate

solvent should be based on the following criteria:

1) Favorable threshold limit value for work place atmosphere and not

injurious to health.

2) Trace amount remaining in deoiled grease should be nonflammable or

burn cleanly, thus to avoid explosions during sintering.

3) Chemically stable and easy to recycle.

4) Effective removal of greases and oils from the mi 11 scale.

5) Economic practicabil ity, favorable price.
26

Several hydrocarbons have been examined as solvents for extraction of oil

from millscale. Based on deoiling efficiency, hexane may be used as an

extracting solvent. Hexane is relatively cheap and non-toxic, and trace

amounts of hexane remaining in deoiled grease will burn cleanly and will not

pollute the atmosphere. Unlike Freon 11 used in European Plant, hexanes are

highly flammable liquids and increase the risk of fire in work place. However,

U.S. food industries have been using hexane as a solvent to extract oil from

grains. So the flammability problem apparently can be handled by means of the

existing technology in food industries.

II. Major Features of the Plant

The plant consists of three processing units:

(A) the deoiling unit

(B) the solvent reclamation unit

(C) the exhaust air cleaning unit

III . Deoiling of Millscale

In the millscale doiling unit, adherent grease and oil are removed from

the scale by the solvent Freon 11*; the scale is then washed with hot water to

vaporize adherent solvent. This unit is covered by a gas and vapor-tight

hood, into which a large, refrigerant-cooled condensor is incorporated.

^Solvent Freon 11 can be replaced by hexane.
27

The screened greasy scale is taken from the hopper and fed by weight into

the material entrance sluice. The drawing off of scale from this entryway is

regulated by means of a load cell in such a manner that a full material column

will always be maintained sufficient to form a seal against the escape of

solvent vapors.

Inside the subsequent double screw mixer, the greasy material is mixed

with solvent in order to break up scale and sludge lumps held together by

grease. The mixture then falls into the solvent bath. While the scale is

sinking in free fall to to the bottom of the bath, the oils and greases are

dissolved. The scale is drawn off the bottom of the solvent bath by an

ascending screw conveyor, and above the bath level it is rewashed with clean

solvent. As the scale is conveyed further upwards the solvent is drained from

it. Still-adherent solvent remains are vaporized by means of hot water in a

scavenging rinse trough which brings the scale into a hot water bath. With a

dewatering screw conveyor, the scale is then moved out of the water system to

the draining area, outside of the building. At the same time, the rinse

trough and screw conveyor serve as a seal against the escape of solvent vapors

on the discharge side.

The hot process water moves in a closed circuit, losses to the scale

being made up with fresh water additions. The overflow from the rinse trough

is cleaned in a laminated settler and a non-clogging screen before it is

returned to the sump.

The heat losses from the water due to the evaporating of residual solvent,

and from the associated heating of the scale, are restored in the condenser of
28

the refrigerating engine. Water is reheated to about 53 °C* and is then

returned to the solvent scavenging rinse trough.

IV. Solvent Reclamation

The solvent utilized for the dissolving of oils and greases is completely

reclaimed in a three-stage distillation plant:

The grease and oil-bearing solvent, the "mi seel la", flows over the brim

of the solvent bath into a collecting channel leading to the miscella collecting

tank. On its way, the miscella passes a magnetic filter and a non-clogging

screen for filtration.

A partial flow is pumped back into the double screw mixer of the deoiling

unit; the main part is charged to the solvent recovery system.

In the first distillation stage, which is constructed as a forced circulation evaporator, about 90 percent of the solvent is recovered. The miscella

is preheated with hot water to 23 ®C*, then heated up to 24.5 °C* under

atmospheric pressure (1 atm). The required process heat is collected from the

connected solvent condenser by means of a heat pump. The heat is transferred

via a secondary refrigerant circuit. The pressure gradient necessary for heat

recovery is induced by a turbo compressor.

*Conditions will change due to the use of hexane as solvent.
29

The liquefied clean solvent returns to the ascending screw conveyor of

the deoi.ling unit, above the solvent bath level, for rewashing of the scale.

From the first-stage evaporator, the concentrated liquid oil/solvent

mixture is pumped into a second distillation stage, which is also constructed

as a forced circulation boiler.

This stage works at 50 °C* and 1.1 atmosphere pressure. The process heat

is delivered by hot water. The vaporized solvent is recondensed by refrigerant

and flows also into the screw conveyor of the deoiling unit.

This (further) concentrated mi seel la flows continuously into the batch

evaporators of the third distillation stage. In this stage, final solvent

vaporization is effected at about 100 °C* and under vacuum. During filling of

the evaporator and heating of the mi seel la to 100 the solvent vapors are

vented under normal pressure to the condenser inside the gas-tight hood

covering the deoiling unit and thereby liquefied.

When filling is finished, the apparatus is set for vacuum evaporation.

At first the pressure is lowered to 133 mbar^ (100 mm Hg pressure), thereafter

to about 27 mbar (20 mm Hg pressure)*. As soon as this pressure is reached,

(at about 100 °C*) the discharge valve opens.

The oil /grease mixture obtained in this manner contains only traces of

solvent, lying near the limit of analytical determination. The oil/grease

^Conditions will change due to the use of hexane as solvent.
30

mixture is delivered to the waste oil treatment plant of the steel works. The

oil has a calorific value which is in the range of that of heavy fuel oil.

VI . Cleaning of Exhaust Air

As already mentioned, the deoiling unit is sealed by the feed hopper on

one end and by the water bath on the other side. Gases in this range are

captured in a gas hood. The solvents are mostly liquefied by means of a

condenser in the hood, and are returned to the screw conveyor. The air which

is brought in with the scale, saturated with solvent vapor, is drawn out and

cleaned in an air cleaning installation.

The solvent-saturated air is treated in an ejector and a bubble tray

column with cold absorption oil. The solvent vapors are absorbed by the oil,

and the cleaned air is discharged. Regeneration of the absorption oil is

effected by heating it up to 90 and by expanding, first into a flash

cyclone at 100 mm Hg* and then into a falling-film evaporator at 20 mm Hg

pressure*.

The solvent vapors are compressed and recondensed by the vacuum pumps,

and the liquified Freon 11* is recycled into the deoiling process.

Summary

Fires occurring in precipitators of modern sinter plants can be

attributed to the introduction of hydrocarbons into the sinter mixture.

*Conditions will change due to the use of hexane as solvent.
31

Oil-containing scale must, therefore, be de-oiled before it is sintered if

conventional technology is used. For this purpose, a process has been

developed, by which oils and greases are removed from the scale by use of the

solvent Freon 11; hexane can be substituted, however. The use of this solvent

allows for a nearly total solvent recovery from the separated oil/grease

mixture, and also permits the application of large heat pumps to the circuit.

A plant constructed for 50 t/h scale would have a solvent consumption of

about 1 kg/t scale at an average inbound oil content of 0.4 percent. The

overall energy consumption, thermal and electrical, amounts to 14 kWh/t. The

residual oil content in the deoiled mi 11 scale is about 0.01 - 0.03 percent.

This deoiled mi 11 scale is usable in the sinter mix without any

difficulties.

This study was funded by the NBS Office of Recycled Materials in response

to the needs of the U.S. steel industry as represented by the American Iron

and Steel Institute (AISI), Committee on General Research, Unit 2 - Removal of

Hydrocarbons from Iron-Bearing Wastes.
32

References

[1] Balajee, S. R. , Deoiling and Utilization of Millscale. First ERA

Symposium on the Iron and Steel Pollution Abatement Technology. November

1, 1979.

[2] Barnes, T., AISI Committee Correspondence. December 29, 1981.
33

Methyl Isobutyl Ketone

Aliphatics

Aromatics MW 500

Figure 1
34

Freon Extract

Figure 2
35

Methylethylketone Extract

Figure 3
36

Hexane Extract

Figure 4
37

XAD-2 Extract

Figure 5
Brooks

38

Figure 6
39

Veedol # 56

Figure 7
40

Veedol #57

Figure 8
Samples

BROOKS

MILL

LUBRICANT

Dates

l-Apr‘-82

'

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41

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160

200

240

200

320

360

400

440

400

520

560

600

Temperature

C®C)

Figure

9

!
!

VEEDOL

MILL

LUBRICANT

#56

Date:

1

-Apr-82

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0

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42

CM*") MOTJ ^oaH

160

200

240

280

320

360

400

440

480

520

560

600
43

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Figure

11
Sample:

MILL

SCALE

EXT.

BETH.

lilS

Date:

31-Mar-82

sc

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13
le:

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Date:

31-Mar-82

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^MUJ^ MOT -I
U.S. DEPT. OF COMM. 1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

BIBLIOGRAPHIC DATA

SHEET (See instructions) NBSIR 83-2773 543 October 1983

4. TITLE AND SUBTITLE

Characterization of Mi 11 scale Steel Wastes

5. AUTHOR(S)

E. S. Domalski, W.A. MacCrehan, J.R. Moody, Y. B. Tewari , and J. A. Walker

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Interim FY 1982

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Office of Recycled Materials

National Bureau of Standards

Washington, DC 20234

10. SUPPLEMENTARY NOTES

rn Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This study was carried out to investigate the appropriate chemistry required to

remove the oils and greases (hydrocarbons) from the mi 11 scale waste so that an ironmaterial could be recycled into the steelmaking process. Characterization of the

mi 11 scale was made as a result of examining the following parameters: particle size

distribution of the mi 11 scale, extraction of the hydrocarbon grease by means of

ignition and solvent reflux, gross molecular weight distribution of the grease, and

thermal decomposition of the grease. The data obtained in this study may prove useful

in evaluating and optimizing operational mi 11 scale deoiling operations.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

characterization; hydrocarbon grease; mi 11 scale; particle size distribution;

solvent extraction; steel waste; tests

13. AVAILABILITY 14, NO. OF

PRINTED PAGES

IX i Unlimited

I I For Official Distribution, Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S, Government Printing Office, Washington, D.C. 20402.

^ 15. Price

IX I Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $ 10.00

USCOMM-OC 6043-P80
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