Indian Journal of Engineering & Materials Sciences Vol. 8, June 2001, pp. 170-176

Selective flocculation separation of iron and titanium from Kutch kaolins (India)

G V S S Kumara, S K Menona

, Y K Agrawala* & S B Vorab

·Chemistry Department, School of Sciences, Gujarat University, Ahmedabad 380 009, India

bGMDC Science and Research Centre, Ahmedabad 380 009, India

Received 17 February 2000; accepted 14 FebruaJY 2001

A new selective flocculation separation (SFS) scheme has been described for upgrading Mamuara kaolins of Gujarat (Kutch district), India. The process embodies the implementation of polvinylpyrrolidone (PVP) as dispersant, carboxymethyl cellulose(CMC) as flocculant and new class of supramolecules such as hexasulfonato calix(6)arene [HSC(6)l as enriching agent.

The upgrading and utilisation of low quality kaolin ores has become one of the imperative problems to be solved by the mineral engineering professionals in the developing country like India. The iron oxide and anatase Ti02 contained as impurites in kaolin reduce the quality as well as price. In addition to anatase Ti02 there is also the presence of rutile Ti02 which is removed along with the coarse particle fraction, hence it is an insignificant discolouring impurity. Among the various physico-chemical methods such as calcination 1.2 gravity separation3

,4 chemical bleaching5-7 magnetic separation3.8.9 froth flotation3.8.10-12 selective flocculation 10.13-15 and bio­beneficiation 16. 17 the selective flocculation technique is found to be more efficient due to its cost effectiveness, environmental viability and its success in the processing of fine grained ores.

Selective flocculation technology, a recent development in mineral processing industry, is one of the most widely used separation processes for beneficiation of complex ore deposits. The processing of kaolin by adopting selective flocculation technique had been initiated by Maynard et al.18

•19

• The process was modified by Sennett and Olivier20. In 1972, Mercade developed a selective flocculation process for the separation of anatase from kaolin by applying weak anionic polyelectrolyte21 . Recent literature reveals the success of selective flocculation process in the beneficiation of various crude kaolin reserves using different polymers as flocculants22-27.

India is endowed with rich resources of kaolin with colouring impurities which are to be removed for improvement in quality. The various techniques

*For correspondence

which have been applied to date to process the kaolin reserves (located in Kutch district, Gujarat , India) are still rather conventional such as levigation, airfiotation, filtration and sun-drying but the quality of kaolin produced is not of commercial grade.

In the present investigation, a comprehensive study on the removal of iron and titanium metal impurities from Kutch kaolin reserves has been described. The selective flocculation scheme (SFS) for the trace removal of metal impurities has been reported. The SFS scheme comprises the use of polyvinylpyrrolidone (PVP) as dispersant, carboxymethyl cellulose (CMC) as flocculant and hexasulfonato calix(6)arene [HSC(6)] as enriching agent. The results have been confirmed by chemical analysis and brightness studies.

Experimental Procedure Minerals

The Mamuara kaolin (Kutch district, Gujarat state, India) is dull white in colour, fairly hard lumps (black particles are visible) in a hydrothermal product of granite and belongs to the kaolinite group. Major minerals in the kaolin include kaolinite, muscovite, feldspar, quartz etc. Other minor minerals are montmorillonite, hydrobiotite, chlorite, illite, hematite, anatase etc.

The kaolin used in this study was obtained from Gujarat Mineral Development Corporation (GMDC), Gujarat, India. CMC and PVP were obtained from BDH Chemicals. All the reagents were of analytical reagent quality.

The kaolin lumps were crushed and subjected to sieve analysis for particle size classification. After sieving, the material retained on each sieve was weighed.

KUMAR et af.: SEPARATION OF IRON AND TITANIUM FROM KUTCH KAOLINS (INDIA) 171

Apparatus pH-measurements were made with Systronics pH­

meter, Model 331. Viscosities of kaolin slurries were measured on Brookfield Synchro-Lectric Viscometer. Brightness studies were carried out with Elico, Model CL-28 reflectance meter using magnesium sulphate as standard.

Method Into a 500-mL beaker containing 50 mL of distilled

water, 5 g of -325 mesh kaolin and dispersant were stirred till the wetting and dispersion of kaolin was completed. Then 50 mL of the water was added and the pH was adjusted with dilute HCl or NaOH. The CMC was added in small lots with stirring and the supernatant portion was separated by decantation. The collected portions were dried, weighed and analysed for kaolin, Fe203 and Ti02 contents. The experiments were repeated by adding various amounts of HSC(6) and recovered portions of the supernatant were subjected to brightness study using reflectance meter.

Results and Discussion The present study was envisaged to study the effect

of introduction of low molecular weight polymer «10,000) PVP as dispersant, high molecular weight CMC as a flocculant and HSC(6) as an enriching agent in the Selective Flocculation Separation (SFS) of iron and titanium contents from the Mamuara kaolins.

Table I-Particle size classification of kaolin

Product

+35 mesh -35 mesh + 100 mesh -100 mesh + 200 mesh -200 mesh + 325 mesh -325 mesh

Assay (%) kaolin

4.5 38.2 14.5 3.5

39.3

Table 2-Chemical composition of kaolin

Constituent

AI20 3

Si02

Fe203 TiO~ CaO MgO K20 Others

Assay (%)

38.87 44.23 0.66 1.70 0.13 0.24 0.Q7 13.69

* The assay was carried out by ICP - AES. * The values given are the average of six determinations.

The percentages of the various fractions of the Mamuara kaolins were determined through sieve analysis (Table 1). The -325 mesh sieve product was taken as the feed for the SFS of Fe203 and Ti02 from clay matrix. The results of the chemical analysis of -325 mesh fraction are mentioned in Table 2. The dispersant dosage, stirring time and settling time were optimised prior to SFS study (Tables 1 and 2).

Effect of pH

It has been observed that with the increase of the pH the viscosity decreases and remain almost constant between the pH 7.5 and 9.0 while beyond pH 9 again it increases (Fig. 1). The lower viscosity favours the separation of kaolin as higher viscocities takes more time for settling the impurities which effect the flocculation.

Effect of Settling Time SFS studies were carried out at the constant

dispersant and flocculant dosages (4 ppm). Decantation of the flocculated slurries was performed at different settling periods and much cleaner products were obtained within 2-3 min of settling period (Table 3). At lower settling periods, the obtained products were having high amounts of Fe203 and Ti02 which is not desirable.

Effect of Kaolin Slurry (%)

SFS studies of the supernatant portion were performed at various percentages of kaolin slurries. High grade kaolin product was obtained with 5%

~ r-------~-------------------.

b ·iii 200 0 180 Ii! ;; "tl 150 135 iii • ~ 0 0 ... 100 at 00

70 "10 70 "fO

o L-____________________________ ~

2 3 4 5 6 7 8 9 10

pH

Fig. l-Effect of pH on the viscosity of kaolin slurry

172 INDIAN J. ENG. MATER. SCI., JUNE 2001

(w/v) kaolin slurry while the selectivity of separation was found decreasing with higher kaolin slurry percentages (Table 4) which resulted in the low kaolin recoveries with high amounts of Fe203 and Ti02.

Flocculant Dosage The SFS studies were performed at different

f10cculant dosages as a function of pH. The amounts of kaolin, Fe20 3 and Ti02 recovered from the supernatant as well as residual portions were

Table 3--Effect of settling time on kaolin, Fe203 and Ti02 recoveries

Kaolin : 5% w/v Stirring rate: 100 rpm PVP dosage : 4ppm Initial Fe203 : 0.66%

CMC dosage : 4ppm Ti02 : 1.70%

Settling Recoveries time (min) Kaolin (g) Fe203 (g) Ti02 (g)

1.0 79.2 0.28 0.62 (79.2) (42.4) (36.4)

2.0 65 .5 0.17 0.41 (65 .5) (25.8) (24.0)

3.0 65.5 0.17 0.41 (65.5) (25.8) (24.0)

4.0 64.9 0.17 0.41 (64.9) (25 .8) (24.0)

All mentioned values are calculated for 100 g of kaolin . Values in parentheses represent the percentage recoveries.

Table 4-Effect of kaolin slurry (%) on the Recoveries of Kaolin, Fe203 and Ti02

Stirring rate: 100 rpm PVP dosage : 4 ppm Settling time: 2 min

CMC dosage : 4 ppm Initial Fe203 : 0.66% Ti02 : 1.70%

Kaolin slurry Recoveries (%) (w/v)

Kaolin (g) Fe203 (g) Ti02 (g)

5 65.5 0.17 0.41 (65.5) (25.8) (24.0)

10 53.2 0.20 0.47 (53 .2) (30.2) (27.5)

15 44.5 0.23 0.51 (44.5) (35 .1) (30.0)

All mentioned values are calculated for 100 g of kaolin . Values in parentheses represent the percentage recoveries.

determined at each pH. The iron and titanium contents were analysed by inductively coupled plasma-atomic emrnission spectrometry (lCP-AES) . Results revealed that around 65% kaolin could be recovered at pH ::::: 8, beyond this pH the kaolin recoveries were of lower percentage. The results showed that f10cculant dosage of 4 ppm was enough to obtain high grade kaolin. It was observed that at lower concentration of the f10cculant the recovered kaolin contained higher content of Fe203 and Ti02 (Table 5), while at higher f10cculant dosages, kaolin also flocculates and hence affects the selectivity. The optimum f10cculant dosage for the efficient SFS was found to be 4 ppm which resulted in 65.5% recovery of kaol in with the removal of 74.2% Fe203 and 76.0% Ti02 at pH ::::: 8 (Fig. 2). The brightness values of the products obtained with different f10cculant dosages are shown in Fig. 3. High brightness value of the product (82.0 units) was obtained with 4 ppm f10cculant dosage.

Preferential Adsorption The surface of Fe203 and Ti02 consists of

hydroxylated iron and titanium atoms joined by oxygen atoms, which themselves may be bonded to hydrogen atoms. Each surface iron and titanium atoms are bonded to, on average, two hydroxyl groups. The surface of kaolin contains silicon and aluminium atoms, both of which are hydroxylated. The hydroxylated aluminium atoms appear only at the edges of kaolin platelets28. In addition , some exchange cations such as iron, titanium, calcium, magnesium and sodium may also be present at the surface. Liu and Laskowski29 have shown that the

: ..--60-.9------------- ·.···· ,

70

20

10

54.6

44.4

El~V9. 1 4~ ~~ 33~ 33.5 26.~8

24 24

o ~----------------------------~ 2 3 4 5

Flocculant (Cr.t:) dosage, ppm

Fig. 2-Effect of flocculant dosage on kaolin and Ti02

recoveries

Table 5-Optirnization of flocculant concentration

Kaolin : 5% w/v Stirring rate: 100 rpm Dispersant (PVP) dosage :4ppm Settling time: 2 min

Initial Fe203 : 0.66% Ti02: 1.70%

Flocculant (CMC) Flocculant (CMC) Flocculant (CMC) Flocculant (CMC) dosage: 1 ppm dosage: 2 ppm dosage: 3 ppm dosage: 4 ppm

pH Weight Settled(g) Weight Settled(g) Weight Settled(g) Weight Settled(g) Kaolin F~03 Ti02 Kaolin Fe203 Ti02 Kaolin F~03 Ti02 Kaolin Fe203 Ti02

3 68.2 0.12 0.32 69.1 0.23 0.58 7204 0.36 0.91 65.2 0043 1.16 (68.2) (18.9) (19.1) (69.1) (35.1) (34.1) (72.4) (54.2) (53.5) (65.2) (65.3) (68.2)

4 50.4 0.15 0.39 51.1 0.21 0.61 63.3 0.39 1.00 56.4 0.44 1.21 (50.4) (23.2) (23.2) (51.1) (32.3) (36.2) (63.3) (58.9) (59.2) (56.4) (67.3) (71.4)

5 44.3 0.20 0.52 44.3 0.31 0.89 49.4 0.41 1.07 50.3 0.45 1.22 (44.3) (31.1) (30.4) (44.3) (47.4) (52.7) (49.4) (62.3) (63.3) (50.3) (68.4) (71.7)

6 35.0 0.20 0.60 37.5 0.33 0.92 41.5 0.42 1.13 40.5 0.47 1.24 (35.0) (32.3) (35.1) (37.5) (50.7) (54.4) (41.5) (63.4) (66.7) (40.5) (71.3) (73.0)

7 22.0 0.31 0.87 25.4 0.38 1.00 32.4 0.45 0.46 37.0 0.48 1.28 (22.0) (47.5) (51.2) (2504) (57.8) (59.3) (3204) (67.8) (69.1) (37.0) (73.2) (75.2)

8 19.1 0.37 1.01 23.2 0040 1.13 29.0 0.46 1.24 34.5 0049 1.29 (19.1) (55.6) (59.5) (23.2) (60.9) (66.5) (29.0) (70.3) (73.2) (34.5) (74.2) (76.0)

9 22.0 0.35 0.96 24.0 0.40 1.11 30.1 0.46 1.23 36.2 0.48 1.28 (20.2) (52.7) (65.6) (24.0) (60.7) (65.2) (30.1) (70.2) (72.4) (36.2) (74.0) (75.2)

10 80.0 0.15 0.43 75.0 0.25 0.70 80.0 0.34 0.90 74.0 0.43 1.14 (80.0) (22.9) (2504) (75.0) (37.4) (41.3) (80.0) (52.4) (53.2) (74.0) (65.0) (67.4)

All the mentioned values are calculated for 100 g of kaolin. Values in parentheses represent the percentage recoveries

FIocculant (CMC) dosage: 5 ppm

Weight Settled(g) Kaolin Fe203 Ti02

68.1 0.43 1.27 (68.1) (66.0) (73.9)

59.2 0.44 1.26 (59.2) (66.5) (74.0)

53.4 0.44 1.26 (53.4) (67.0) (74.3)

48.0 0.47 1.28 (48.0) (71.4) (75.2)

46.2 0.49 1.29 (46.2) (75.0) (76.0)

45.4 0.49 1.30 (45.4) (75.1) (76.5)

47.9 0.49 1.29 (47.9) (74.0) (76.1)

66.1 0.42 1.14 (66.1) (63.2) (67.4)

;;>::: C s:: >-::0 ~

~ en tTl '"t:I >-::0 >--l (3 Z 0 '"!1

sa 0 Z >-Z 0 -l ::j >-Z C s:: '"!1 ::0 0 s:: ;;>::: c -l n ::r: ;;>::: >-0 r< Z en ~

Z 0

~

-..J w

174 INDIAN J. ENG. MATER. SCI., JUNE 200 1

84

82

00 <II III

.2 78 til > lil76 III l: ...

.s::. 74

.!:!' cD

72

70

E8

r--------.-.-- .- .--.- .... .

82 82.1

77.4

74

I

I I I

L-______________J 2 34

ROCCt~art (HSC(6)) 00sng; PfIT1

5

Fig. 3--Effect of flocculant dosage on the brightness of kaolin

adsorption of dextrin on the mineral surfaces depends critically on the hydroxylated silicon surface sites of quartz which is relatively poor. This suggests that CMC may adsorb predominantly onto hydroxylated aluminium surface sites of kaolin and thus adsorbs on only a small proportion (platelet edges) of the surface. In comparison, the surface of Fe203 and Ti02 particles have high concentration of hydroxylated iron and titanium surface sites . Hence, the preferential adsorption of CMC on the Fe203 and Ti02 may be associated with its relatively high number of hydroxylated metal adsorption si tes compared to kaolinite.

Results given in Table 6 show that the maximum recovery of Fe203 (74.2%) and Ti02 (76%) was obtained at low floccu lant dosages (4 ppm). This suggests that bridging floccu lation mechanism is the most probable adsorption mechanism. By measuring the settling rate of many individual aggregates, it is observed that flocculation of Fe203 and Ti02 by CMC produced larger and faster settling aggregates and that floc 30 growth most probably occurs by a cluster addition mechanism.

Effect of HSC(6)

The introduction of calixarenes in the SFS system helped in enhancing the selectivity of separation. Various SFS experiments are performed with different HSC(6) dosages at optimum flocculant and dispersant dosage (4 ppm). Results reveal that efficient recoveries are obtained at pH "" 8. Beyond

80

70 ~8 7

A

b " 68.8 65.8 66.1 66.6

60 ::;,' I/)

~ 50 C> :> 0 I)

~ 40 '2 s ~ 30

23.8 c. :l 21

19.5 <11

20 16.3 15.7 21.9

16.5 10 14 14

0

0.25 0.5 0.75 1.25

Calixarene (HSC(S)) Dosage. ppm

Fig. 4-Effect of calixarene dosage on the recoveries of kaolin, Fe20) and Ti02

891

88 88.5 88.5 .

87

",86 03.1 <V :l iii 85 > (;j

~84 c :E 83 OJ .;::; 824 Ql 82

84

I i

I 81

80

79 0.25 0.5 0.75 1.25

calixarene (HSC{G)), PPm

Fig. 5-Effect of calixarene dosage on the brightness of kaolin

th is pH the kaolin recoveries were found decreasing (Table 6). The data in Table show that the HSC(6) dosage of 1 ppm has a pronounced effect on the system. Higher concentration of HSC(6) has no added advantage on the system. The enhanced recoveries of kaolin (68.7%) with the efficient removal of Fe20 3 (83.2%) and Ti02 (86.0%) are due to the calixarenes in the SFS study of Mamuara kaolin (Fig. 4).

It is found that HSC(6) molecules were acting as enriching agents in the new SFS scheme. As HSC(6) molecules were reported as a new class of surfactants, they are expected to decrease the surface tension to promote much efficient selective flocculation of

Table 6-Oplimizalion of calixarene dosage

Kaolin : 5% w/v Stirring rate : Dispersant (PVP) dosage : 4 ppm Settling time : F1occulant(CMC) dosage : 4 ppm Initial Fe203 :

TiOz :

HSC(6) 0.25 ppm HSC(6) 0.50 ppm HSC(6) 0.75 ppm HSC(6) 1.0 ppm pH Weight Settled(g} Weight Settled(g) Weight Settled{g) Weight Settled(g)

Kaolin FezOJ TiOz Kaolin Fez0 3 Ti02 Kaolin Fe203 Ti02 Kaolin FezOJ TiOz

3 66.2 0.45 1.03 64.3 0.37 1.09 63.0 0.42 1.12 62.1 0.43 1.24 (66.2) (68.5) (60.4) (64.3) (56.3) (64.1) (63.0) (63.0) (66.1) (62.1) (65.4) (72.9)

4 54.5 0.46 1.08 57.1 0.38 1.11 58.0 0.44 1.15 59.3 0.47 1.26 (54.5) (69.2) (64.0) (57.1) (58.4) (65.2) (58.0) (67.0) (68.2) (59.3) (71.2) (74.1)

5 52.3 0.46 l.l2 51.0 0.42 1.12 53.0 0046 1.22 54.0 0048 1.29 (52.3) (70.3) (66.0) (51.0) (63.1) (66.0) (53.0) (70.4) (72.0) (54.0) (72.5) (76.2)

6 39.1 0.46 1.16 43.2 0.43 1.19 41.0 0048 1.27 44.3 0.50 1.36 (39.1) (70.1) (68.2) (43.2) (6504) (70.2) (41.0) (73.0) (75.3) (44.3) (76.3) (80.1)

7 37.6 0049 1.31 36.9 0.51 1.28 35.0 0.51 1.41 33.1 0.53 1.43 (37.6) (74.0) (77.0) (36.9) (77.1) (75.2) (35.0) (177.6) (83.0) (33.1) (80.3) (84.3)

8 34.2 0.50 1.33 33.9 0.52 1.38 33.0 0.53 1.43 31.3 0.55 1.46 (34.2) (76.2) (78.1) (33.9) (79.0) (81.2) (33.0) (80.4) (84.1) 31.3) (83.2) (86.0)

9 37.1 0.50 1.30 35.2 0.52 1.36 36.0 0.52 1.41 32.3 0.54 1.45 (37.1) (75.2) (76.7) (35.2) (7704) (79.8) (36.0) (79.0) (83.2) (32.3) (81.1) (85.4)

10 75.3 0.30 0.95 76.4 0.32 0.01 76.0 0.36 1.07 79.3 0041 1.13 (75.3) (45.2) (55.9) (76.4) (49.1) (59.7) (76.0) (55.2) (63.0) (79.3) (62.1) (66.4)

All the mentioned values are calculated for 100 g of kaolin. Values in parentheses represent the percentage recoveries.

100 rpm 2 min 0.66% 1.70%

HSC(6) 1.25 ppm Weight Settled(g)

Kaolin FeZ0 3

62.2 0.43 (62.2) (65.4)

59.4 0.47 (5904) (71.3)

54.1 0.48 (54.1) (72.6)

44.5 0.50 (44.5) (76.4)

32.1 0.53 (32.1) (80.0)

31.2 0.55 (31.2) (83.3)

34.2 0.54 (32.4) (81.1)

78.4 0.30 (78.4) (45.3)

TiOz

1.23 (72.5)

1.26 (74.1)

1.29 (76.2)

1.36 (80.2)

1.43 (84.4)

1.46 (86.0)

1.45 (85.4)

0.82 (48.2)

A c::: 3: >-;;0

~

~ en en '"0 >-;;0

>--l 0 Z 0 'Tl

;:a 0 z >-Z Cj

-l ::J >-z 2 3: 'Tl ;;0 0 3: A c::: -l ('J :r::

'" >-0 r Z en ~

Z Cj

~

-.J Vl

176 INDIAN J. ENG. MATER. SCI., JUNE 2001

O.S r----- ----------.

0.7

~ 0.5 > o v

~ 0.4 c ~ 0.3

" 0.

Jl 0.2

0. 1

0.69

(1

0.6"

0.29

057

0.53 0.46

~.26 0.31

0.2 0.17 0.16

-->,--J>,

0.08 o L-____________________ __ ___ I

2 3 4 5

Floccu lant (CMC) I Ca!ixarene (HSC) d )5age, ppm

Fig. 6--Effect of fl occulant and calixarene kaolin, Fe20 3 and Ti02

Fe20 3 and Ti02. The obtained kaolin with the improved recoveries and quality were in good agreement with the above expectation. Further the brightness values of the recovered kaolins are shown in Fig. 5. High brightness value of the kaolin product (88.5 units) was obtained with 1 ppm calixarene dosage.

Conclusions The study show that the reagent combination of

PVP-CMC-HSC(6) gives increased kaolin recoveries (68.7%) with lower amounts of Fe20 3 (16.8 %) and Ti02 (14.0%). Whereas with the PVP-CMC combination the reasonably good quality of kaolin [(65.8%) with Fe203 (26%) and Ti02 (24%)]was obtained. The increased recovery of kaolin upto 3% with PVC-CMC-HSC(6) combination showed the role of calixarenes as enriching agents 111 mineral beneficiation field Fig. 6.

The brightness values obtained from SFS due to PVP-CMC-HSC(6) combination were higher as compared to that of PVP-CMC combination. The kaolin products obtained from former SFS scheme showed == 5 units increase in brightness values as compared to that of the latter (Fig. 3). The results showed pronounced effect of HSC (6) in the beneficiation study of Mamura kaolins . The resultant products find various industrial applications, viz., ceramic, rubber, paper, textile, paint etc.

Acknowledgement The fin ancial assistance given by GMDC Science

and Research Centre, Ahmedabad and CSIR, New Delhi is gratefully acknowledged.

References I McKetta J J , Encyclo Chem Proc Design, (Marcel Dekker,

Inc., New York), 5 (1977) 440.

2 Chen X & Yang S, Kuagchan Liyong, 2 (1994) 38. 3 Pradip, Min Proc Extr Metall Rev, 10 (1 992) 121.

4 Dalmann W, Neese T, Helfri cht R & Buechner J, Aujbereit­Tech, 29 (1988) 186.

5 Gong W & Wu X, J Wuhan Univ Tec:mol, Mater Sci Ed, 12 (1997) 47 .

6 Forbus E S, Young R H & Willi s M J, Mill Eng, 45 (1993) 603.

7 McKetta J J, Encyclopedia of Chemical Processing and Design, (Marcel Dekker, Inc ., New York ), 4 ( 1977) 430.

8 Roe-Holln Y, Joseph S, Advances ill Mineral Processing Proceedings of a Symposium (honoring Nathanier Arbitor on his 75th birthday), edited by Soamsundran P, New Orlens, Loui siana, March 3-5,1986.

9 Chandra B M & Sharad G D, Bull Marer Sci , 10 (1988) 8. 10 Banks A F, Benejicianlion of Mineral Fines, ed ited by

Somasundaran P & Arbiter N, AlME, New York, (1979) 115.

11 Wang Y H C & Somasundaran P, Trans AlME, 272 (1970) 983.

12 McKetta J J, Encyclo Chelll Proc Design, (Marcel Dekker, Inc. , New York), 23 ( 1985) 454.

13 Banik N, Suresh N & Mandre N R, Min Proc Extr Metedl Rev, 14 ( 1995) 169.

14 Ravi shankar S A, Pradip, Dco M S, Kulkarni R A & Gundians S, Bull Mater Sci, 10 (1'988) 423.

15 Colombo A F, Fine Particles Processing , edited by Somasundaran P, (AlME, New York) , I (1 980) 1034.

16 Mohanty B K & Mishra A K, J Cell Appl Microbiol, 34 (1988) 233.

17 En rich H L, Min Metall Process, 5 (1988) 57. 18 Maynard R N, Millman N & lannicelli J, Calys Clay Mill, 17

(1969) 59.

19 Maynard R N, Sk ipper B R & Millman N, US Pat., 3, 37 1, 988, March 5, 1968, Chel1l Abstr 8109 1j , 68 (1968) no 18.

20 Sennett P & Olivier J P, US Pa:., 3, 446, 348, May 27, (1969), Chel11 Abstr 53144x , 71 C969) no 12.

2 1 Mercade V V, US Pat., 3, 701, 417; "Jctober 3\ ,1 972, Chelll Abstr472 194,78 (1973) no 8.

22 Yuan J, Everett B E & A'ld rews 'v R, US Dat., 5, 685, 900, 1997 , Chem Abstr 25980 'J , 128 ( 1998) no 3.

23 Bchl S, Willi s M J, Young R N, PCT lilt , Appl \\1"0 9628 , 516, Sep. 19, 1996 Chelll Abslr 303367r, 125 (1966) no 24.

24 Behl S, Willi s M J & Young R N, PC~ /111, Appl WO 9617, 688, June 13, 1996, Chem Abstr [4934 1 LI , 125 (1996) no 12.

25 Gong W, Yuan J & Cao M, FWl1il/ zhual/li Shengil/g COl/g/;ui Shuomil1gsllll, 1 (1995) 10 I.

26 Chen L, Yuan J & Gong W, Wuhan COl/gye Da.w e XUc!J{/(), 17 ( 1995) 36.

27 Gong W & Yalln J, J Wuhan Ull iv Tee/mol, Maler Sci Ed, 9 (1994) 39.

28 Stuillm W, Chemistry of the Solid- Water lnterferlace, (John Wiley & SOilS, Inc, New York), 1992.

29 Liu Q & Laskowski J S, lilt. J Mil/er. Process, (1976) 133.

30 Deason D M, Flocclilatiol/ ill Biotechl/ology al/d Separillioll Systellls, (El sev ier, Amsterdam), 1987, 21 .