I
EDGAR L. PIRET,' R. G. WHITE, H. C. WALTHER, Jr.:
and A. J. MADDEN, Jr.
University of Minnesota, Minneapolis, Minn.
Pelletizing Magnetic Taconite Concentrate The binder action of peat suggests a new use for this natural product M r x T m E s of coarsely ground airdried peat and a n alkali function effectively as binders for making pellets of magnetic taconite concentrate. This is of interest because extensive peat reserves are close to the pelletizing plants of northern Minnesota. This arricle deals primarily with factors influencing the crushing strength of such dried pellets prepared with a dry peat-aqueous sodium hydroxide binder system.
60 inches belo\v the bog surface, included material from two horizons : 36 to 48 inches: .4 moderately coarse and fibrous (herbaceous) peat. 48 to 60 inches: A well disintegrated coarse and fibrous peat with the plant remains partially preserved. After mining, the material was air dried a t room temperature to about 10% moisture, coarsely ground in a feed mill to 19% minus 100-mesh, and mixed thoroughly in a feed mixer. Subsamples of the 500-pound masterbatch were ground further, where necessary, in a hlikro-Sampl mill.
Organic Group Analyses of Peatsa Type I,
Pelletizing Process
I n the beneficiation of Mesabi magnetic taconite to a high-iron concentrate, finely divided magnetite is separated magnetically from the crushed ore slurry a n d rolled into marble-sized balls. These are then burned in the pelletizing furnace to produce finished pellets strong enough for shipping and charging to the blast furnace. T h e balling step, made in large rotary drums, requires the addition to the fine concentrate of a binder so that the d a m p green pellets will stand u p on the way to and through the pelletizing furnace (2, 3, 4 ) . A variety of additives have been tested as binders for the balling step. T h e most satisfactory of these are bentonite and gelatinized starch. T h e cost of any new binder must? of course, be competitive \vith present agents. A good binder must pass three major laboratory tests (1) : 0
0
T h e green Ia-inch pellets should have a wet crushing strength of about 4 to 6 pounds. After being dried a t 100' C. the balls should have a dry strength of about 10 pounds. There should be no decrepitation with rapid heating of the moist pellets to 500' C.
Peats and Taconites Used
T h e peat (Type I), taken from the Rice Lake bog near Duluth (5) was selected for most of the tests because of its relatively high content of humic acids. T h e master sample, mined 36 to 1 Present address, U. S. Embassy, Paris, France. * Present address, Continental Oil Co., Ponca City, Okla.
Constituent Group Bitumens Humic acids Humins Holocellulose a-Cellulose Hemicelluloses Ash ~
T>-pe11,
7%
w
11.2 26.9 21.8
9.0 12.4 42.8
8.2 23.4 9.4
3.5 32.5 8.3 __
Total 100.9 108.5 a Basis of anhydrous peat. Organic fractions corrected t o anhydrous, ash-free basis. Method developed at the University of hlinnesota.
.4 mixed herbaceous and sphagnum moss peat, Type 11, having a substantially lower content of humic acids, was obtained from the FloodwoodState bog 3 miles west of Floodwood, Minnesota, a t a depth of 24 to 48 inches. T h e wet peat (40 pounds) was dried to about 8y0 moisture. mixed, and ground to 55% or 78% minus 100-mesh in the mill. T h e two batches of magnetic taconite concentrate were prepared directly from crushed rock by grinding and concentration, and were equivalent in balling characteristics. Moisture contents were about 0.5%; screen analyses were 69 and 76% minus 325-mesh. Test Procedures
T h e laboratory balling operations and crushing strength tests ( 2 , 3 )were used. For each test run, a weighed amount of ground air-dried peat was blended by hand with 36 pounds of dry taconite concentrate. The desired weight of sodium hydroxide (dissolved in about a liter of water) or of dry sodium carbonate was then similarly blended in, together with sufficient water to raise the total moisture to about 8 to 10%. The damp mixture was then fluffed by rubbing through 6- or 10-mesh screens. Balling was carried out in a drum, 20
inches long by 24 inches in diameter, revolving at 48YG of critical speed. Test pellets, 3 / 4 inch in diameter, were formed by alternately adding the concentratebinder mix and spraying in water. Periodic selection by screening during formation resulted in the final pellets being smooth, round, and uniform. .4bout 50 of the final balls were weighed and dried on a hot plate at 103 C. to determine moisture and provide pellets for the dry-strength tests. Crushing strengths were measured by means of a platform spring scale. A compressive force was applied by hand with a light steel bar. The reading at failure was taken as the strength. Wet and dry strengths are the average measure on 25 pellets taken from the drum. T h e prat aqd taconite concentrate represent Lveights on a moisture-free basis. Results and Discussion
Amount of Peat Required. Various amounts of peat added to the fine concentraie improve the crushing strength ofdried taconite balls (Figure 1). Crushing strength rises bvith increasing peat levels to a maximum of about 12 to 16 pounds, depending upon the type and sieve size of the peat. About 12 pounds of peat per ton of concentrate (0.6 are required for the maximum weight 70) strength. Further addition of peat seems to increase the porosity of the pellets, with a slow decline of strength. Fine grinding of the peat does appear necessary for complete utilization of its binder action. M'ith 7.5 pounds of peat and 1.0 pound of sodium hydroxide per ton of concentrate, pellet dry strength increases by about 30% as the sieve size is reduced from 19% (feed mill grind) to 76% minus 100-mesh; the corresponding crushing strengths of dry taconite pellets were 10.8 and 14.4 pounds. Probably this modest improvement stems primarily from increased dissolution of the humic acids from the finer peat particles by the alkali. But the improvement seems to level off Lvhile the peat is still much coarser than the powdery taconite concentrate? showing a highly intimate blend is not needed. T h e Sodium Hydroxide Requirement. T h e amount of sodium hydroxide added with a given quantity of peat exerts a pronounced effect on the resultant pellet dry strength (Figure 2). Both types of peat give satisfactory crushing strengths by the admixture of 1.0 pound of sodium hydroxide and 7 . 5 pounds of pea: per ton of concentrate. Humic Acids Play Dominant Role. As seen in Figures 1 and 2, the binding effect achieved with Type I1 peat paralVOL. 53, NO. 3
MARCH 1961
215
STRENCTH CURVE
i l g
-0
EXTRACTION CURVE
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BASIS
FOR DRY STRENGTH T E S T S
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7.5 L 0 . O F TYPE IO
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I O 10. NaOH PER TON O F TACONITE CONCENTRATE
O
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I PEAT
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TON OF TACONITE
CONCENTRATE
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18
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24
PEAT CONTENT ( L B . PER TON O f CONCENTRATE)
Figure 1. Addition of peat raises dry strength of taconite to acceptable levels
TYPE
-
24
I
PEAT
01 0
Y 1
I
I
02
0.4
06
SODIUM HYDROXIDE
0-
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A
-
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16-
t
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9
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Figure 2. Sodium hydroxide exerts a pronounced effect on the binder actions of peat
BOTH PEATS USED AT 1 5 LB. PER TON OF TACONITE CONCENTRATE SIEVE
Minimum caustic requirement v a r i e s w i t h t y p e of p e a t
SIZES i
:7 6 %
-100 MESH
T Y P E XI P E A T : 7 8 %
-100 M E S H
TYPE
I
PEAT
1
00
8
10
SODIUM HYDROXIDE CONTENT (LE. PER TON OF CONCENTRATE)
els that of Type I but the ball strength is consistently lower. T h e Type I1 peat had a considerably lower content of humic acids (12.4 us. 26.9y0). These comparative results suggest that the humic acids constituent of peat plays a major role in its binding properties. Supporting evidence is given in Figure 3, where the pellet strength data of Figure 2 for Type I peat are replotted along with the corresponding humic acids extraction curve for that peat. T h e latter represents the percentage of peat extracted using increasing amounts of sodium hydroxide. T h e close resemblance of the pellet strength and exrraction curves lends experimental support to the supposition that the crude humic acids fraction of peat is largely responsible for its binder action. The extraction data of Figure 3 represents those substances in peat simply extractable by a warm alkali solution
2 16
and precipitated by acid. These are often called crude, or whole humic acids. T h e portion of crude humic acids hydrolyzable by mineral acid appears for most part under the holocellulose fraction. T h e peat having a relatively low humic acids content still imparts satisfactory crushing strengths to the pellets, meeting the minimum requirement of 10 pounds. The data point up the importance of careful selection and control of raw material in chemical applications of peat. The wet strength of green pellets prepared with peat and sodium hydroxide was in all cases acceptable, lying in the range of 4 to 6 pounds. Tests for decrepitation behavior, involving rapid heating to 500 O C. of selected moist pellets, showed satisfactory performance. Peat-Sodium Carbonate System. T o eliminate the need for handling the alkali as a separate liquid reagent, sodium hydroxide was replaced by a nonhy-
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
1.4
O
L6
E
S e u a r a t e e x t r a c t i o n data c l o s e l y p a r a l l e l p e l l e t s t r e n g t h c u r v e
z
-x
I
Figure 3. In situ solubilization of peat humic acids component plays chief role in binder action
r
v)
I
0.0 1.0 1.2 LEVEL l L 0 PER L 0 OF PEAT]
groscopic alkali, sodium carbonate, which could be mixed with peat to provide a dry, poLvdered binder. About 2.5 times as much sodium carbonate as sodium hydroxide (pound for pound) is required for comparable and acceptable dry strengths. This is in line with the carbonate’s weaker basicity and lower effectiveness in solubilizing the humic acids. When using 2.5 pounds of sodium carbonate per ton of taconite concentrate, the variation of pellet dry strength with peat content (Type I) follows closely the results of the upper curve of Figure 1 obtained \\ith 1.0 pound of sodium hydroxide. These results suggest that a peat-sodium carbonate dry mixture also constitutes an effective binder for taconite concentrate on the basis of agglomerating qualities and crushing strength imparted. Acknowledgment
The authors thank H . H. Wade, E. LV. Davis, and L. S. Taylor of the University of Minnesota Mines Experiment Station for facilities and assistance. literature Cited
(1) Davis, E. W., Reserve Mining Co., Silver Bay, Minn., private communication March 16, 1955. ( 2 ) Davis, E. W., Wade, H. H., “Agglomeration of Iron Ore by the Pelletizing Process,” Information Circ. No. 6, Mines Expt. Station, University of Minnesota, Minneapolis, Minn., 1951. ( 3 ) Firth, C. V., A m . Inst. M i n i n g Engrs., Iron CY Stee! Diu., Blast Furnace and R a w .Materials Committee, Proc. 4, 46-69 (1944:. (4) Haley, K. M . , Skilling’s ‘Mining Reu. 45, 1-3 (Feb. 9, 1957). ( 5 ) Soper, E. K., “The Peat Deposits of Minnesota,” Minnesota Geological Survey, Bull. No. 16, p. 223, 1919. RECEIVED for review June 21, 1960 ACCEPTEDDecember 28, 1960 Work supported by a grant of the Office of
Iron Range Resources and Rehabilitation, State of Minnesota, and by the Graduate School of the University of Minnesota.
