Bergaya F. Handbook of Clay Science

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magnetic separation; (v) ﬂotation and selective ﬂocculation; (vi) drying and calci-

nation; and (vii) chemi cal modiﬁcation and activation. A typical dry processing plant

for kaolin or bentonite is shown in Fig. 10.1.10. Once the basic composition of a raw

material was understood, a series of processing steps may be introduced. For high-

quality clays the processing may be relative ly complex.

B. Processing of Kaolins

Common clays for production of tiles, bricks and ceramics are generally used with-

out any processing. In some cases the largest particles are removed by sim ple sed-

imentation techniques without pre-puriﬁcation to obtain homogeneous ceramic

bodies. Antique ceramists used advanced processing of common clays to obtain the

reﬁned clays for shaping and the slips for decorat ion. The red/black decoration

(‘Glanzton’ technique) of the famous Antique vases with the appealing gloss was

produced by using clay masses and slurries with different contents of kaolinite and

illite (Hofmann, 1962b; Noll, 1982, 1991).

Kaolins for the paper industry now require sophisticated and extensive processing

technologies (Table 10.1.4). Fig. 10.1.11 summarizes the applied technologies that

may be used to reﬁne and improve the industrial properties of kaolins. Fig. 10.1.12

shows a US kaolin mining and degritting operation from which the degritted kaolin

is piped up to 40 km to a processing plant (Fig. 10.1.13) that may typically produce

over 1 million tons of processed kaolin.

C. Raw and Activated Bentonites

Bentonites are ﬁne-sized materials, and hence are generally used without significant

size fractionation (Figs. 10.1.14 and 10.1.15). As certain pr operties of bentonites can

change between neighbouring pits or between individual layers within a pit, blending

is common ly used to maintain a consistent product with standard properties.

Bentonites may be used in the raw form (as mined) or they may be activated (Tables

10.1.2b and 10.1.3). Alkali or soda-activation, using sodium carbonate (Na

transforms the calcium (magnesium) bentonites into sodium-calcium forms. The

bentonite with water contents of 35–40% (w/w) is usually kneaded or milled with

1–5% (w/w) sodium carbonate and homogenized (Fig. 10.1.16). This process of soda-

activation was ﬁrst proposed by Hofmann and Endell (1935). Fluid dispersions are

usually avoided because sodium bentonite dispersions are extremely difﬁcult to ﬁlter

due to their high viscosities (see Chapter 5). Nevertheless, smaller amounts of high-

quality sodium bentonites, or especially hectorites for certain applications, are pro-

duced in dispersions after careful selection of the raw bentonite. The quality of the

products can be improved by special processing of the raw bentonite (fractionation or

hydrocycloning, mechanical treatments, reaction with heated water vapour, etc.).

During soda-activation a large part of the interlayer calcium ions is precipitated

as calcium carbonate. As the calcium ions are still present in the system when this

Chapter 10.1: Conventional Applications518

dust collector

product collector

product

classifier

feed

ersize

mill

hot air

furnace

exhaust fan

vent

(a)

(b)

Fig. 10.1.10. Dry processing for kaolin or bentonite. (a) Mill, cyclone, and dust collection

circuit, (b) hot air circuit for drying in grinding mill.

10.1.2. Processing Industrial Clays 519

Fig. 10.1.11. Flow sheet showing the various stages and complexities in the modern wet

processing of kaolin.

Table 10.1.4. Applied technologies used in kaolin processing

Delamination

High gradient magnetic separation

Selective ﬂocculation

Column ﬂotation

Ozone bleaching

Ultraﬁne particle size separation

Surface modiﬁcation

Calcination

Chapter 10.1: Conventional Applications520

bentonite is dispersed in water, the degree of delamination is not optimal, and the

colloidal properties depend on the amount of soda added (Fig. 10.1.17) (see Chapters

4 and 5). The degree of thixotropy (hysteresis of the ﬂow curves) reaches a maximum

as a function of the amount of soda added. The yield value also increases to a

Fig. 10.1.13. A US kaolin processing plant, Georgia, USA.

Fig. 10.1.12. Mining and degritting operation in a kaolin mine, Georgia USA.

10.1.2. Processing Industrial Clays 521

Fig. 10.1.14. Bentonite pit in Bavaria, Germany (Courtesy N. Schall, Su

d-Chemie AG,

Germany).

Fig. 10.1.15. Storage of raw bentonite (Courtesy N. Schall, Su

d-Chemie AG, Germany).

Chapter 10.1: Conventional Applications522

maximum at an optimum addition of soda (Fig. 10.1.18). The properties of bent-

onites respond differently to soda-activation. This is not only due to differences in

particle size distribution betw een the raw bentonites but also because of differences

in layer arrangement within individual mo ntmorillonite particles (see Chapter 5).

Domains of silicate layers with a certain degree of ordering are separated by areas of

loose contact s (‘breaking points’), deno ted by arrows in Fig. 10.1.19. Depending on

the distribution and the extent of these defects in the parent montmorillonite, soda-

activation yields different size distributions of the delaminated particles .

The strong dependence of rheological properties on the Na

/Ca

ratio is illus-

trated in Fig. 10.1.20 (see also Chapter 5). Varying the Na

/Ca

ratio is, therefore,

an important way to optimize and tailor the propert ies of soda-activated bentonites

for the intended applications.

Many applications, listed in Tables 10.1.2b and 10.1.3, require hydrophobized

bentonites. To this end, the bentonite is usually reacted with quaternary alkylam-

monium salts such as dialky l dimethylammonium (I), dialky l benzylmethylammo-

nium (II), and alkyl benzyldimethylammonium salts (III),

I II III

Fig. 10.1.16. Activation of bentonite by kneading with soda (Courtesy N. Schall, Su

d-Chemie

AG, Germany).

10.1.2. Processing Industrial Clays 523

The technical products contain alkyl chains (R) of different lengths. For instance,

a technical dioctadecyl dimethylammonium chloride had the composition (% w/w):

1.5 C

, 0.8 C

, 27.5 C

,2C

,67C

, 1.2 C

(Favre and Lagaly, 1991). Ditalloyl

ammonium salts (derived from by-products of cellulose production from deal and

pine wood) are mixtures of dialkyl dimethylammonium surfactants with varied alkyl

chain length s, for instance 65% C

, 30% C

and 5% C

The amount of alkylammonium ions added corresponds to the cation exchange

capacity (CEC) of the clay, and the exchange is nearly quantitative. Products with

smaller amounts of alkylammonium salts can also be obtained.

rate of shear (s

-1

)

100

500

100

shear stress (mPa)

100

500 700

Fig. 10.1.17. Soda-activation of bentonites. Inﬂuence of soda addition on the ﬂow behaviour

(shear rate vs. shear stress) and thixotropy. 2% dispersions of Amory bentonite (Mississippi,

API 22b, Wards). A 0.5 mmol Na

/g bentonite, B 2.5 mmol Na

/g bentonite,

C 5 mmol Na

/g bentonite.

Chapter 10.1: Conventional Applications524

In most cases the products are not washed after cation exchange so that some free

(unreacted) alkylammonium salts may still be present. These free salt inﬂuences the

properties of the product such as ion-exchange and rheological behaviour (see Fig.

5.43, Chapter 5). The ratio alkylammonium salt/CEC and the treatment of the

organo-bentonite after the exchange reaction are parameters that can be used to

tailor these products.

Bleaching earths are obtained by treating bentonite with acids in such a way that

the layer structure is largely decomposed (see Chapter 7.1). Typically, the bentonite

is reacted with hydrochloric acid (up to 17 mmol/g bentonite) at 90–100 1C for a few

hours, then washed to reach pH 4, dried, milled and sieved (Fig. 10.1.21). Con-

centration of the acid, temperature and activation time determine the properties of

the prod uct (Fahn, 1973 ). The speciﬁc surface area and bleaching activity usually

0 0.05

0.25 0.5 1 2.5 5 10

soda addition (mmol/

bentonite)

maximum thixotropy

Dakota

Cameron

Bavaria

Otay

Amory

0.5

shear stress at a shear rate of 100 s

-1

Fig. 10.1.18. Rheological properties of 2% dispersions of different bentonites. The curves

show the effect of soda addition on shear stress measured at a rate of shear of 100 s

1

. Arrows:

denote maximum thixotropy.  Newtonian ﬂow, – d – d – (pseudoplastic), ——

thixotropic, – – – antithixotropic. Bentonite from Amory, Mississippi (API 22b, Wards);

Bavaria (Schwaiba, Su

d-Chemie, Germany); Cameron (Arizona, API 31); Dakota; Otay

(California, API 24). From Jasmund and Lagaly (1993).

10.1.2. Processing Industrial Clays 525

show maximum values at distinct activation conditions whereas the pore volume and

the silica content increase continuously (Fig. 10.1.22)(Fahn, 1973; Christidis et al.,

1997). The large volumes of acidic waste waters, containing aluminium salts, that are

produced can give rise to environmental problems. Clays of certain localities exhibit

similar adsorption properties without acid-activation. Before bleaching earths were

produced at an industrial scale, fuller’s earth was used for oil bleaching (Bene ke and

Lagaly, 2002 ).

10.1.3. LABORATORY EVALUATION OF CLAY SAMPLES

Any clay material may be recognized by some plasticity in its raw stat e. Some clay

materials may meet the required speciﬁcations for industrial uses with no processing

or blending with other raw materials. Other clays may be components in simple or

complex blends of various raw materials. Other clay raw materials require simple or

complex processing to remove contaminants or non-clay minerals.

The assessment of an undeveloped clay resource requires a series of screen-

ing techniques by which the clay technician can progressively move towards the

Fig. 10.1.19. Montmorillonite particles with inherent ‘‘breaking points’’ (arrows). From Jas-

mund and Lagaly (1993).

Chapter 10.1: Conventional Applications526

0.5 %

0 100 200 300 400

shear stress (mPa)

150

rate of shear (s

-1

)

1.5 % 2 %

2.5 %

1 %

1.5 % 2 % 0.5 2.5%

0 600 1200 1800 2400 3000 3600

shear stress (mPa)

150

rate of shear (s

-1

)

0.5 % 1 % 1.5 % 2 % 2.56 %

shear stress (mPa)

150

rate of shear (s

-1

)

0 120 240 360 480 600 720

(a)

(b)

(c)

Fig. 10.1.20. Flow curves of sodium bentonite (Bavaria, Germany), (a) in water, (b) in

0.01 M CaCl

, (c) in 0.025 M CaCl

. Bentonite content 0.5–2.5% (w/w), pH ¼ 6.5, 20 1C. From

Permien and Lagaly (1994).

10.1.3. Laboratory Evaluation of Clay Samples 527