Clay Minerals (1973) 10, 41.

ACTIVAT ION OF NON-SWELL ING BENTONITE

R. L . BLE IFUSS

Mineral Resources Research Centre, University of Minneapolis, Minnesota 554 55, U.S.A.

(Received 4 July 1972; revised 12 December 1972)

ABSTRACT: Examination of field samples shows that the alteration of the ben- tonite beds of Wyoming and Montana near the surface to produce high-quality, high- swelling, commercial bentonites is analogous to pedocal soil development. The near surface bentonite has undergone extensive calcification, and in some zones it is so heavily altered as to diminish its swelling properties. These low-swelling bentonites can be activated by conventional soda-ash additions. The naturally high-swelling materials have undergone only partial calcification and do not require soda-ash modi- fication. The relatively unaltered 'grey' or 'blue' bentonites have undergone little or no calcification. The latter have the highest Na to Ca ratios, are non-swelling, and do not respond markedly to soda-ash additions. Such low-swelling materials will respond to calcium-activation with markedly improved physical quality parameters of flocculation as measured by Marsh Funnel and Fann viscosimeter. The study indicates that vast reserves of low-swelling bentonites in Montana-Wyoming may be susceptible to con- trolled calcium activation and expanded commercial utilization.

INTRODUCTION

This paper is directed pr imari ly toward the modif ication of bentonites to meet the needs of the iron ore industry. Bentonite is an addit ive which is necessary for the successful bal l ing and pelletizing of finely ground iron-ore concentrates. Eighty mil l ion tons of pellets are produced annual ly on the North Amer ican continent requiring some 800,000 tons of bentonite, of which about 400,000 tons per year are shipped to Minnesota operators.

The feed mater ia l in the pelletizing process is finely ground iron-ore concentrate which is about 90% ~325 mesh and contains about 10% moisture. About 1% or some 20 lb / ton of bentonite is mixed into the fine concentrate. The mixed mater ia l is transferred into a bal l ing drum where the concentrate and admixed bentonite is rolled into 'green' balls about 0"5 in. in diameter. These green balls then move through a heat-hardening step where they are heated rapidly from room temperature to 1300~ in less than 20 rain.

Bentonite is considered to have a major role at three crit ical points in the pelletizing process. The first is in the green bal l stage where the bentonite contri- butes to the strength of the balls and permits them to stand up under mult iple screening and transfer steps on their way to the indurating machines. Green balls

42 R. L. Bleifuss without bentonite cannot survive these handling steps without excessive breakage. The second point is in the drying stage where the green balls are heated rapidly from room temperature to 250~ The bentonite seems to act as a buffer during this stage, moderating the release of moisture from the ball. Green balls of taconite concentrates with no binder often explode because of the internal steam pressure. The third point is in the preheating stage as the dry halls are brought from 250~ to about 900~ The dry ball is weakest in this range and the bentonite provides sufficient cementation to prevent breakage. Without the admixture of bentonite the balls are subject to both breakage in handling and to severe wind erosion and dusting commonly resulting in major operating problems. Bentonites, particularly the sodium-type bentonites, have been found to be uniquely suited to fulfilling the three critical roles mentioned.

Taconite companies in Minnesota have been using western bentonites for some 20 years, yet despite this no common bentonite specifications have been established. The companies specify a Wyoming-type or Na-type bentonite because there is clear evidence that they are superior to a Ca-type bentonite. Some taconite companies place minimum Na20 specifications, or set maximum CaO contents, or specify Na20:CaO ratios, but specifications vary from company to company. Companies usually specify a minimum Marsh Funnel value, which is a relative measure of the viscosity of a bentonite slurry at 5% solids, or sometimes a range of gel strengths. In addition, moisture, particle size, and the percentage of ~325 mesh material (grit) are controlled.

Preliminary mineralogical analyses of commercial bentonites supplied to Min- nesota pelletizing plants show that although they are dominantly Na-montmoril- lonites they also contain a substantial percentage of mixed-layer clay components. This is somewhat surprising in view of the high sodium:calcium ratios specified in most of the contracts. Therefore, the Mineral Resources Research Center undertook a mineralogy-oriented study of both the commercial bentonites supplied to the taconite companies and field samples collected from bentonite producers in Montana and Wyoming in an effort to determine some of the characteristics which make a particular bentonite satisfactory as a pelletizing clay. During the summer of 1970 some 100 field samples of bentonite were collected with the cooperation of most of the bentonite suppliers.

Samples were collected from all of the bentonite layers being mined at each company's pit. Most samples collected represent material from active mine areas and are considered typical of each company's normal production.

M INERALOGICAL STUDY

Field samples were run through conventional physical and mineralogical tests in- cluding Marsh Funnel, gel strength, X-ray diffraction, differential thermal analyses, partial chemical analyses, grit determinations, and moisture content. A five-fold classification of the samples was made on the basis of the field occurrence and the XRD patterns of the clay-size fraction.

Activation of non-swelling bentonite 43 The five divisions are: (1) calcium bentonite--oxidized, near surface, and calcium

saturated; (2) grey bentonite--sodium saturated, some random mixed-layer com- ponents; (3) sodium bentonite--no random mixed-layer components, oxidized yellow- green material; (4) sodium bentonite--some random mixed-layer components, oxidized yellow-green material; (5) sodium bentonite--moderate random mixed-layer components, oxidized yellow-green material.

The relationship between the viscosity as determined by Marsh Funnel tests and the various sample groups is shown in Fig. 1. The first group, Ca-bentonites, in this case consisting only of oxidized yellow-green bentonites, shows very low viscosity with Marsh Funnel values distributed over a narrow range from 17 to 22 sec. The second group, the grey bentonite samples that show little evidence of surficial altera- tion, are also low viscosity with an average value of 25 sec. The third group, Na- bentonites with no mixed-layer components, has Marsh Funnel values that are also low and in the same range as those of the grey bentonites with the exception of three samples which gave values of + 50. The diffraction patterns for this group show sharp symmetrical peaks at 12.6 .K at 50~ humidity. The fourth group, Na- bentonites with some mixed-layer components, shows a wide range of viscosities and a substantial number of samples with very high viscosities. The fifth group, con- taining moderate to substantial mixed-layer components, has a range of viscosities essentially equivalent to group four.

The mineralogical studies show that bentonite groups four and five, which for the most part have relatively high viscosities, are characterized by mixed-layer com- ponents in these samples. The first order Na-montmorillonite X-ray peak is clearly asymmetrical toward the low-angle range at 50~ humidity. Glycolation and heating studies show that the asymmetry is not due to illite or chlorite mixed-layer components.

CHEMICAL ANALYSIS

Partial chemical analyses for Na20 and CaO were run on forty-five representative samples of the five groups. These data, Table 1, show that each group has a characteristic chemistry and Na~O:CaO ratio. The highest NazO:CaO ratios are associated with the grey bentonites which have relatively low Marsh Funnel values, average 25 sec, whereas a moderate increase in CaO is correlative with improved physical properties of flocculation as indicated by higher Marsh Funnel readings. The Ca-bentonites (Group 1) contain excess Ca and typically give low Marsh Funnel values.

These data show that both high-Na-bentonites and high-Ca-bentonites are characterized by low Marsh Funnel values. This trend is in keeping with the prior observations of Williams, Elsley & Weintritt (1954) reported in Grim (1962) where they showed that the viscosity of the clay slurries reached a maximum with 40~/o of the Na replaced by Ca. Their earlier work (Williams, Neznayko & Weintritt, 1953) on field samples showed that the exchangeable cations present as Ca + Mg was 35-67~'o of the total exchange capacity. Chemical analyses presented in this paper

44

I '~ 0 L

R. L. Bleifuss Group I

Ca-bentonites

1 I I

Group 2 Grey bentonites

I0

I I

Group 5 -- No-bentonites ~- I01-- No mixed layering

"5

E 0 u l l 9 I I Group 4

No-bentonites I I~ I Slight mixed layering 5~0 ~ l 1F i l l t l l i t

Group 5 Na-bentonites

010 20 50 40 50 +50 Marsh Funnel value (sec)

FIG. |. Distribution of Marsh Funnel values for each group of bentonite samples.

show substantially higher Na20:CaO ratios (total Na20, CaO) present in the crude blue bentonites which gave low Marsh Funnel values. These would seem to be susceptible to activation by addition of calcium.

F IELD RELAT IONSHIPS

The materials collected for this study represent a random sampling of the bentonites

Activation of non-swelling bentonite TABLE 1. Partial chemical analyses of Bentonites for Na20 and CaO

45

Sample No. Na20 CaO Sample No. Na20 CaO

Group 2. Grey bentonite Group 1. Calcium bentonite

66 1"00 1-20 49 1-33 1"18 08 1-19 0.88 92 1"12 1'50 34 0.69 1'25

034 0.49 1' 12 12 0"27 0.81 65 0'53 0.98

Average 0"82 1" 12 (R=0.73)

Group 3. Sodium bentonite, no mixed layer components

09 2"02 0'45 89 1-43 0-74 19 1-75 0.77 83 2"14 0.78 44 2"45 0.80 81 1.69 0"42 01 2"70 0.42 42 2"41 0.67 93 2-39 0.56 79 2"16 0.63

Average 2" 11 0.62 (R=3.40)

Group 5. Sodium bentonite, moderate mixed layer components

46 1 "46 2.92 18 1.44 1.90 62 1.75 1-16 03 1-43 0.77 53 1 "09 1.48 40 1"12 1"19 37 1"35 0'99 35 1"21 0"90 50 1"87 2"27 56 1"42 1"15 58 0"94 0'85

Average 1 "37 1.42 (R=0"96)

55 1"35 1"19 52 1.52 0-83 64 1-71 1-30 14 2.06 1-12 09 2-53 0"81 61 2"60 0"95 16 2"20 0' 90 51 2'01 0"70 25 1"75 0-83 00 2-49 0-56

Average 2-02 0-92 (R=2-20)

41" 2.31 0.39 80* 2.71 0.36

Average 2"51 0"37 (R = 6-78)

Group 4. Sodium bentonite, some mixed layer components

91 1-39 1"08 72 1-71 1-60 30 1-62 0"95 24 2"29 1-28 29 2"26 1"14 84 2.04 0"70 26 1-58 1"30 36 2"16 2"01 48 2-14 0-42 97 2"16 0"59 75 1"86 1"22

Average 1"93 1-12 (R = 1"72)

* No mixed layer components.

46 R. L. Bleifuss of Montana and Wyoming currently under commercial exploitation. The samples were taken with the purpose of obtaining all types of materials that might be included in the shipments of pelletizing plants, but not with the purpose of establishing a weathering or alteration profile. It was therefore encouraging to find that the sample groups established on the basis of clay mineralogy and field description do fit into a geological and geochemical pattern compatible with the climatic conditions of the region.

The five sample groups are related to a hypothetical weathering or alteration profile as shown in Fig. 2. The average Na~O and CaO analyses of each group and the range of analyses are indicated graphically. The surficial material commonly is composed of highly weathered yellow-green bentonite. It may be gypsum bearing and the montmorillonite is Ca saturated. The clay is difficult to disperse in water in its natural state, but will commonly respond to soda-ash modification. The yellow- green bentonites that contain moderate amounts of mixed-layer components represent somewhat less altered material that is characteristically high yield. The next layer beneath is very similar in physical qualities, but has a lower mixed-layer clay com- ponent. Materials from both layers commonly respond satisfactorily to soda-ash addition. The upper three layers show a significant increase from top to bottom in the total Na20 content from 0"82-1-93yo, more than a two-fold increase. At greater depth, layer four, the bentonites are oxidized, but do not contain mixed-layer com- ponents and the average NazO content becomes greater whereas the CaO content is substantially lower. The least altered material, represented by the grey bentonites, shown as layer five, has a slightly higher Na20 and CaO content. These last two layers characterized by low yields, seem to be sodium saturated and generally do not respond to the addition of soda ash.

The assumption made in this composite profile is that the chemical and mineralogical changes we observe going from top to bottom are the effects of decreasing alteration, i.e. weathering. The upper oxidized sections show calcification and replacement of sodium.

The bentonite beds occur within the Cretaceous Mowry shale as described by Slaughter & Early (1965) and it may be that part of the calcium is being introduced into the bentonite beds in solution from the overlying shale units. Calcium pro- gressively replaces sodium from the clay, putting sodium into solution. It is assumed that sodium then migrates out of the formation. However, some of the sodium will migrate ahead of the calcification front which might account for the somewhat higher sodium values in the oxidized bentonites of the fourth layer where no mixed- layer components are found. The low CaO values in this group also support this hypothesis. The grey bentonites beneath the latter oxidized layer contain mixed-layer components and are believed to represent the least altered material in the section. The bottom group is represented by only two samples of grey bentonite that con- tain no mixed-layer components. Only two such samples were collected and con- sequently they were not included as part of the hypothetical weathering sequence; however, they might represent an intermediate phase between the oxidized normal Na-montmorillonites and the mixed-layer grey bentonites. Under this scheme sodium

Hypothetical relationships

i Surficial material 2 Gypsum common 3 Ca 2+ saturated 4 Low yield (Group I)

I Yellow bentonite 2 2 Mixed layer compo-

nents moderately developed

5 High yield (Group 5) ! Yellow bentonite

3 2 Mixed layer compo- nents slightly deve{oped

3 High yield (Group 4)

4 7O

2

', Yellow oxidized bentonite

2 No mixed layer components

3 Low yield (Group 5)

I Grey bentonite 2 Layer components

moderately developed 3 Low yield (Group 2)

I Grey bentonite 2 No mixed layer

components 3 Low yield (Group 2)

Activation of non-swelling bentonite

1 I

j \ . / {

0

--am t I I ; '0 2 0 3"0 0 {'0 2"0 3'0

NO20 % CoO %

FIG. 2. Hypothetical relationships showing each bentonite group in its relative position in the postulated weathering profile.

47

or calcium additions and alteration of the bentonites is not necessarily coincident with oxidation. Though the alteration effects described for the composite section are considered to have been achieved over a long period of semi-arid climatic conditions in the area, the process is thought to parallel some of the transformations in pedocal soil formation under present day semi-arid conditions.

BENTONITE ACT IVAT ION

Measurements show that the maximum viscosity values are observed in montmoril- lonites in which calcium occupies about 40~/o of the exchangeable cation sites (Williams et al., 1954). The foregoing discussion suggests that many of the grey or blue bentonites and the oxidized yellow bentonites that normally do not respond to sodium activation are in fact calcium deficient. These bentonites are on the sodium- rich side and therefore show no better dispersion characteristics than the Ca-saturated bentonites and should respond to Ca additions. The effect of adding Ca to the bentonites was first investigated by running conventional Marsh Funnel tests. The test data obtained from seven representative samples are shown in Fig. 3. The tests show that grey bentonites respond markedly to Ca additions with increased Marsh Funnel values. Oxidized bentonites that do not respond to soda-ash treatment also

48 R. L. Bleifuss respond to Ca additions. One of the high-sodium grey bentonites that contains no mixed-layer component reacted to a limited extent. Calcium-saturated bentonites, as illustrated by one sample, do not respond to further calcium activation. The effect of adding 1-5~/o soda-ash to these samples is also shown in Fig. 3; with the exception of one sample sodium did not affect Marsh Funnel data.

+6 x 9

I i , , IC 0 0-5 0 -6 0-9 J .2 ~'-5 I -8 1.5

Ca (0H)2% No2C03% I I , ] ..... [ ,,. 0 I0 20 3'0 40

( Ib/ long ton)

FIG. 3. Marsh Funnel values obtained on bentonites with progressively larger Ca(OH)z additions compared with data obtained with Na2CO3. A, Grey bentonite; A , grey bentonite; O, grey bentonite; O, grey bentonite; x, grey bentonite; [], oxidized

bentonite Group 3 ; II, Ca bentonite.

Marsh Funnnel data indicate that with no additives the grey bentonites are in a nondispersed state, i.e. they contain a relatively small percentage of clay-size particles. With the progressive addition of calcium, dispersion into clay-size particles is in- creased and we observe a gradual increase in viscosity. The calcium-dispersed clays are then reflocculated at higher lime levels as indicated by the very high Marsh Funnel values. The lime addition can be kept low enough to accomplish dispersion without causing massive flocculation; flocculation may be undesirable in some appli- cations, although not necessarily deleterious to a pelletizing operation.

CoIIoid* determinations were run on selected samples of bentonite at various levels of lime addition to demonstrate that activation with lime hydrate promotes

* The percent colloids was determined by dispersing I0 g of clay in 500 ml of distilled water and determining the percentage still in suspension after 24 hr.

Activation of non-swelling bentonite 49 complete dispersion of the clay. The graphs in Fig. 4 indicate the change in percentage of colloids as a function of percentage of calcium hydroxide addition and of equiva- lent soda-ash additions. These graphs illustrate the significant increase in the percent- age of colloids with increasing calcium activation. The influence of soda ash on these samples is negligible, as indicated--there is no appreciable increase in the measured colloid content. Samples No. 00 and No. 14 are grey bentonites, the third, No. 93, is an oxidized bentonite that shows no mixed layering, and No. 24 is a yellow-green bentonite that shows slight mixed-layer components, but did not respond to soda-ash treatment.

"0.. .... 00 .80 1 40 400 1-0 2-0 4-0 0 I-0

lOG

8O

6O

~ 4C

o

S lOOt- . . . . . .

60

40 0 I "0 2"0

I I I I "0 2 -0 4 -0

I 4-0

,0 7 -

f 60 40 I 0 I-0

J

I ,,I 2-0 4.0

I 9 2-0 4-0

'~176 F

60

40 I ,I 0 I "0 2"0 4-0

I001- . . . . . IOOF

8 0 ~ 0 80f 60 60

I 40 40 I "0 2"0 4"0 0

Ca (OH) 2 %

I I I I-0 2.0 4-0

NozC03%

Sample no. O0

Grey benton i te Group 2

Sample no. 14

Grey benton i te Group 2

Sample no. 93

Oxid ized ye l low benton i te Group 3

Sample no. 24

Oxidized ye l low benton i te Group 4

D

FIG. 4. Percentage of colloids as a function of the progressive addition of Ca(OH)2 or Na2CO3.

50 R. L. Bleifuss SUMMARY

The samples collected in the field seem to fit into a hypothetical weathering pattern that is compatible with their chemistry, mineralogy and geological environment. The data show that the grey or blue bentonites, and many of the oxidized bentonites that are low-yield materials in their natural state and of restricted commercial application at present are calcium deficient. Addition of calcium progressively increases their dispersion as measured by the percentage of colloids. The addition of excess calcium above that required for dispersion results in concomitant flocculation. The high dis- persion and sequential flocculation attainable by calcium activation of grey bentonites seem to be uniquely suited to the bailing and pelletizing of taconite concentrates. It seems logical that calcium activation might be equally suited to some of the other common commercial bentonite uses.

ACKNOWLEDGMENTS

The supporting funds for this study were provided primarily through a grant by the American Iron and Steel Institute. Further support was provided by funds made available by the Mineral Resources Research Center of the University of Minnesota.

REFERENCES

G~ R.W. (1962) Applied Clay Mineralogy, McGraw Hill, New York. SLAUGHTER M. & EARLY J.W. (1965) Spec. Pap. geol. Soe. Am. 83, pp. 95. WILLIAMS F.J., ELSLI~Y B.C. & WEISTPOTT, D.J. (1954) The variations of Wyoming Bentonite as a

function of the overburden. In Clays and Clay Minerals (A. Swineford and N. V. Plummet, editors). Natl. Research Council Publ. 327, 141.

WrLLIAMS F.J., NEZNAYKO M. & W~INrParr D.J. (1953) J. phys. Chem. 57, 6.