I
LOWELL E. PETERSON and JOSEPH W. OPlE Central Research Laboratories, General Mills, Inc., Minneapolis, 1 3, Minn.
Flocculation of Slimes
by Guar
This first systematic study of flocculation by nonionic hydrocolloids reveals principles that should be generally applicable in purifying industrial waste waters
which has the structure ( 5 ) shown at WATER-DISPERSIBLE polysaccharides lower left. have been used in purification of industrial waste waters for several decades. They clarify slimes by flocculating the fine suspended inorganic particles into aggregates large enough to settle rapidly under the influence of gravity. As early as 1938 Overbeek (7) explained how addition of a hydrophilic colloid to a hydrophobic colloid could either sensitize or inhibit its flocculation by an electrolyte. More recent explanations (6) of the flocculation of slimes by hydrophilic polymers alone are based on a model similar to Overbeek‘s: A single molecule of flocculant adsorbs on the surfaces of two different slime particles, binding them together. The process continues until aggregates are large enough to settle under the influence of gravity. Although this simple picture seems to offer anadequateexplanation for flocculation phenomena, it does not make it simple to predict the action of a given flocculant on a novel slime. The distinctive behavior of each . individual slime discloses a need for understanding the fine details of flocculation. The flocculation of four different slimes by guar was studied in an attempt to learn principles of value in predicting the behavior of other slimes. Guar is native to India, recently was introduced into southwest United States as a crop to be grown in rotation with cotton fdr soil improvement. The guar plant yields a bean, the endosperm of which is composed of a galactomannan gum of molecular weight 220,000 (7),
When the endosperm is reduced to a fine granulation, it can be readily dispersed in cold water to form a viscous sol. As guar is a nonionic material, its sols are relatively stable toward the addition of most electrolytes. A notable exception is the borate ion, which causes gelation by forming a complex at the sites of the &-hydroxyl groups. Experimental Methods
Settling Rates. Settling rates were observed visually in 100-ml. graduated cylinders. A small measured volume (usually 2 ml.) of 0.1% solution of General Mills’ Guartec was added to 100 ml. of slime in a graduated cylinder. The container was inverted four times to ensure thorough mixing, and a stopwatch started. The position of the interface between settling solids and clear supernatant was noted a t intervals. If no distinct boundary was visible the settling characteristics were described qualitatively. Silica Slime. Pure silica slimes were prepared from ground Ottawa sand, (5 pounds), leached successively in concentrated hydrochloric acid and acid dichromate solution. The acid was rinsed away by washing the sand 10 times with 500-ml. portions of distilled water and then pouring the sand in a fine stream into ten successive 1-liter portions of distilled water. The leached sand was oven-dried at 110’ C. and ground in an Abbe pebble mill overnight. The -325-mesh fraction was set aside as a dry powder to be used in making standard slimes. Microscopic examination showed that the powder contained an abundance of particles smaller than 10 microns. Slimes were prepared as needed simply by adding a weighed portion of the dried powder to distilled water or a n aqueous solution of a known electrolyte. Bentonite. Bentonite slimes were prepared by dispersing dry Wyoming ben: tonite in an aqueous medium, using a
Waring Blendor. Clay slimes are not easily prepared in reproducible fashion, for their stability depends on age, speed of stirring, and apparently other factors not clearly understood. For this reason, in comparative tests on bentonite slime, a single batch was divided into portions for the individual tests, The slime was aged several hours, and then the tests were carried out within a few minutes to minimize the effect of additional aging. Iron Ore. As slimes washed from iron ore changed on aging, fresh samples were prepared as needed from dry crushed ore. Two ores from nearby areas of the Mesabi Range were chosen. One was known to yield a very refractory slime ; the other produced a slime which (at 10% solids) could be settled rapidly with 20 p.p.m. of Guartec alone. Slimes were prepared by washing 1 part of ore with 1.5 parts of water. Coarse particles were removed by passing the slime successively through a 20mesh and a 100-mesh screen. Adsorption Measurements. The amount of guar adsorbing on the slime particles was determined by an analytical technique developed by Smith and others (2). Equal amounts of a centrifuged Guartec sol were added to 100 ml. of slime and to 100 ml. of distilled water. Both containers were immediately inverted four times to ensure thorough mixing. Mixing was repeated every 15 minutes for 2 hours to permit the adsorption to proceed to equilibrium. If the supernatant above the settled slime did not become perfectly clear, both the slime and control sample were centrifuged until the supernatant was clarified. Then 2-ml. portions of both samples were, withdrawn for guar analysis. T h e difference between these two analyses indicates the amount of guar adsorbed by slime. A colorimetric procedure is used to determine guar concentrations. A 2-ml. aliquot of the guar solution (containing from 0 to 150 y of guar) is placed in an optically clear test tube (Coleman cuvette) and treated with 0.1 ml. of 80% aqueous phenol and 5.0 ml. of concenVOL. 50, NO. 7
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JULY 1958
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trated sulfuric acid. X yellow color develops in proportion to the amount of carbohydrate present. The absorbance of the resulting solution is read on a Coleman Jr. spectrophotometer at 405 mp. This can be converted into a concentration measurement by using a standard curve. The standard curve is prepared by plotting absorbance us. concentration for series of solutions. each containing a known weight of mannose and galactose in the ratio of 2 to 1. Guar, upon hydrolysis, yields mannose and galactose in the same ratio. When this curve is used to determine guar concentrations from absorbance measurements, a factor of 1621180 is used to compensate for the fact that a molecule of \vater has been gained in hydrolyzing each link in the polymer molecule. Experimental Results and Discussion
Characterization of Iron Slimes. Slimes from the two samples of iron ore were compared. The easily settled (sandy) slime contained 10% solids. After settling unassisted for 1 hour in a 100-cc. graduated cylinder, the top 30 ml. of supernatant contained 0.5% solids. The supernatant from the refractory (painty) slime. which initially contained only 6.2% solids, still contained 0.9% solids after settling for the same period. The unsettled material contained 52.7y0 iron. Chemical analyses and p H measurements for these two slimes are: pH at 10%
70HCISlime Sandy Painty
%
in comparative experiments, for even very low concentrations of specific electrolytes may alter slime stability. Sorption Studies. The painty iron slime is not settled rapidly by the addition of Guartec alone. However, a few parts per million of lime condition the slime so that giant flocs form immediately and settle rapidly when Guartec is added. The question naturally arises, whether the lime increases the amount of guar adsorbed by the slime or acts in some other manner to sensitize the slime to flocculation by Guartec. Adsorption measurements were made to determine how much guar was adsorbed under normal flocculating conditions with and without 200 p,p.m. of lime. Figure 1 sliows that less sorption occurred when lime was present. This is probably because the lime causes minute flocs to form: and less surrace is available for adsorption of guar. It is therefore presumed that for this system the function of the lime is to reduce electrostatic repulsion between slime particles and permit them to come together close enough to be bridged by guar molecules. The foregoing experimenr has shown the adsorption of guar on iron slime to be significantly large. This iron slime contains about joy0 silica, Saturally the action of guar upon a pure silica slime is of interest. A 1% silica slime was prepared from the dry pulverized Ottawa sand described earlier. Ten 100-m!. graduates were filled to the 50-ml. mark with this slime. Five graduates were brought to the 80-ml. mark with a O.O1.M lime solation; the other five were filied to the
These electrolytes were added to the slimes in varying amounts until the minimum concentration was found which would settle the slime leaving a clear supernatant. The following conclusions were drawn:
same volume with distilled water. Minute flocs could be seen in the slime treated Lvith lime, but no settling could be detected within 2 minutes. Each graduate was then filled to the 100-m1. mark with a solution containing a known amount (betlveen 1 and 5 mg.) of guar.
Solids
(Giass
Insoluble Material
Si02
Fe
Electrode)
77.3 50.4
75.5 49.3
14.4 32.9
7.2 6.8
The stable portion of the painty slime was diluted with distilled water and placed in a crude electrophoretic cell. The red slime particles concentrated in the positive chamber and formed a coating on the positive electrode. \Yhen the polarity was reversed, the coating !vas rransferred to the other electrode. Thus, the natural iron oxide particles assume a negative charge in distilled water. The petrographic microscope reveals significant differences between the two slimes. The painty slime contains extremely fine particles and no visible quartz. (A considerable amount of quartz is present, buL is obscured by a coating of iron oxide.) Under darkfield illumination other differences appear. The refractory slime contains much hematite of a very fine granulation and practically no goethite. The sandy slime by contrast contains considerable coarse, crystalline goethite (including limonite) and very little hematite, 4 sample of slime solids from the refractory ore \vas leached with con-
10 14
centrated hydrochloric acid, washed repeatedly, and dialyzed until the wash water gave a negative chIoride test with silver nitrate. The solid material remaining appeared to be almost entirely quartz of a particle size from 2 to 40 microns, mostly about 5 microns. Perhaps the larger particles were merely clusters. A slime prepared from this quartz in distilled water was not flocculated by Guartec, even though Guartec possessed some activity toward the slime from which it was isolated. The presence of soluble salts in the ores could be important for two reasons. Even traces of polyvalent ions can exert a profound influence on the stability of colloidal dispersions. The ionic environment of a finely divided hydrous iron oxide affects its crystalline development ( 3 ) . T o detect possible reasons for the difference in the two slimes, the soluble salts were washed from the ores by dialysis. The residues left after evaporation of the wash water were examined spectrographically. Both residues contained sodium ions as the major constituent. Only the easily settled slime, however, contained calcium, magnesium, lead, manganese, and iron ions in trace quantities-i.e., less than lyO of total soluble salts. Influence of Electrolytes on Stability of Iron Slimes. 4 much more stable slime resulted lvhen the iron ore was washed with distilled water than when tap water 'tvas used. This observation led to an investigation of the effects of a number of different electrolytes on slime stability:
The valence of the cation is the most important factor in determining the ability of an electrolyte to settle the iron slime. The approximate molar concentrations required are (mole per liter) : Univalent cations Divalent cations Trivalent cations
X
25
1
o
2
x x
10-3 10-3
The silver ion, although monovalent, was nearly as effective as a divalent cation. Calcium hydroxide was more active than calcium chloride. Similar valency rules apply when bentonite slime replaces iron slime. I t is clear, then, that electrolyte concentrations must be carefully controlled
INDUSTRIAL AND ENGINEERING CHEMISTRY
Flocculation
Characteristics Series Without lime
Guartec. hIg.
after Guartec
1, 2, 3,
No flocculation
dddition
4,5
With lime
1, 2
Small flocs form immediately
3
Large flocs form immediately
5
Very large flocs form immediately
It is clear that lime sensitizes a pure silica slime to flocculation by Guartec. Adsorption isotherms of guar on silica
SLIME FLOCCULATION
.
were measured as before in solutions containing different concentrations of lime. Figure 2 shows that no adsorption takes place in the absence of lime. Each addition of lime increases the amount of guar adsorbed until nearly quantitative adsorption takes place in the presence of 900 p.p.m. of lime. As adsorption measurements are obtained by difference, greater scattering is to be expected (and is found) in the points on the isotherms showing low adsorption. Thus, the lime has an effect on the silica slime which could not be observed on the iron slime-it conditions the quartz surfaces so that guar can be adsorbed. Naturally the question arises whether the calcium ion or hydroxide ion alone is responsible for this increased guar adsorption, or whether both ions are required. Sorption measurements were carried out a t a single guar concentration in systems containing equivalent amounts of calcium hydroxide, calcium chloride, sodium chloride, and sodium hydroxide. Table I shows that neither calcium chloride nor sodium hydroxide alone will bring about adsorption; both calcium and hydroxide ions are necessary. A reasonable explanation seems to be that the hydroxide ions are first required to establish a substantial negative potential on the quartz particles. This negative potential makes it possible for calcium ions to be bound firmly to the surface (4). The adsorbed calcium ions then presumably furnish sorption sites for the guar molecules.
Table I. Adsorption of Guar by Silica in Presence of Electrolytes Electrolyte
Electrolyte
I
None Ca(OH)2 CaCll NaOH NaCl
Concn., Mole/ Liter
.... 0.004 0.004 0.008 0.008
Guar, M g . Added t o 100 ml. slime Adsorbed 3 3 3 3 3
0 1
0 0 0
0
1.4
Adsorption of Guartec on iron slime with and without lime
Figure 1.
3.0
-
o NO LIME A -'I25 ppm. LIME
v)
Q
J
5: 2.5
.
0 ,-250 x
,
LL
5
K K
a -
ppm, pp.m. 500 pp.m. 900 pp.m.
+ 2 st's 0
-
LIME LIME LIME LIME
1.5
1.0
-I
d o
H U m 0.5 a 0
v)
NO LIME
Q
The sandy iron ore slime, which contains more than 75% silica, can be settled rapidly by Guartec, while silica alone is not flocculated in the absence of certain ions. The reason for the activity of guar on the iron slimes is of interest. The amount of soluble multivalent ions found in the slime does not account for more than a minor portion of the total adsorption as calcium-activated adsorption on silica. A sample of almost pure silica was obtained from the iron slime by acid leaching. An iron-rich portion of the same slime was obtained by decanting the top 500 cc. from a slime which had been permitted to settle 5
1.0
0.5
MILLIGRAMS OF GUARTEC ADDED PER GRAM O F SLIME SOLIDS
a . 0
Figure 2. of lime
n
I .o 15 2.0 2.5 30 MILLIGRAMS OF GUARTEC ADDED PER GRAM OF SLIME SOLIDS 0.5
Adsorption of Guartec on silica in presence of different concentrations
minutes in a 1-liter graduate. Slimes made from the silica, the ironrich fraction, and a mixture of the two were set aside for 80 days. Periodic microscopic examination revealed that in the mixture the silica gradually became enclosed in loose masses of iron
oxide. After aging, both the iron-rich fraction and the mixture were readily settled by 20 p.p.m. of Guartec; the pure silica slime was not settled. Quantitative adsorption took place on the two slimes which were settled, but no detectable adsorption occurred on the silica. VOL. 50, NO. 7
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p H = 8.0
IlOh-
= 7.Q
40 I
I
I
Q
I
2
I
3 TIME
I
I
I
4 5 6 (MINUTES)
I
I
7
8
Figure 3. Settling rates for bentonite slimes treated with Guartec at different pH values
Both 100-ml. graduates contain an iron ore slime of 10% solids and have settled for about 2 minutes. The one on the left has been treated with 2 mg. of Guartec
guar molecule completely saturates its bonding tendencies on the surface of a single particle. At low pH values, the negative potential on the clay particles is diminished, permitting the particles to come sufficiently close together to be bridged by guar molecules, as illustrated in B.
A
B Figure A.
B.
4.
Type of adsorption
Highly charged particles Neutral or weakly charged particles
Apparently, in the mixture the silica particles are dragged down in loose masses of flocculated iron oxide. In natural iron slimes also the iron oxide coating, so plainly visible on the quartz, probably makes it possible for guar to bind the silica particles into aggregates. Clays are common constituents of industrial slimes, and Wyoming bentonite was studied to gain some insight into their behavior. A 2y0 bentonite slime was prepared as described earlier. Five 50-cc. portions were diluted twofold with solutions of commercial buffer tablets of p H 4, 5, 6, 7, and 8. Each solution was treated with 10 ml. of a 0.1% guar solution. It appears from Figure 3 that low pH favors rapid settling, but it would be presumptuous to assume that only hydrogen ions affect settling rates. Potassium acid phthalate and sodium
10 16
phosphate were used to compound the tablets for solutions of p H 4, 5, and 6, while sodium tetraborate and potassium phosphate were used a t p H 7 and 8. The significant fact is that ionic environment markedly affects the interaction of guar with the slime. Adsorption measurements showed that the amount of guar adsorbed remained between 85 and 9570 for all p H values. Thus, the dissimilarity in settling characteristics cannot be ascribed to a difference in the sorption affinity of the clay for guar. A more plausible hypothesis is illustrated in Figure 4. At high p H values the surfaces of the clay particles have a strong negative charge and exert a strong repulsive force on like particles. The particles are thereby prevented from coming sufficiently close together to be bridged by guar molecules. Sorption proceeds as illustrated in A until each
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Acknowledgment The authors gratefully acknowledge the assistance of Strathmore R. B. Cooke in characterizing the slimes used and in providing helpful suggestions.
Literature Cited (1) Boggs, A. D.: master’s thesis, Purdue University, 1949. (2) Dubois, M., Gilles, K., Hamilton, J. D., Rebers, P. A., Smith, F., Nature 168, 167 (1951): Anal. Chem. 28, 350 (1956j. ’. (3) Gheith, Mohamed, Ph.D. thesis, University of Minnesota, 1951. (4) Guadin, A. M., Am. Inst. Mining Met. Engrs. 7 , 66 (1955). (5) Heyne, E., Whistler, R. L., J . Am. Cham. SOC. 7 0 , 2249 (1948). (6) Michaels, A. S., IND.ENC. CHEM.46,
1485 (1954).
(7) Overbeek, J. Th. G., Chem. Weekblad 35, 117 (1938).
RECEIVED for review April 19, 1957 ACCEPTED November 27, 1957 Division of Industrial and Engineering Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
