Acid-leaching and consequent pore structure and bleaching capacity modifications of egyptian clays

Colloids

and Surfaces,

17 (1986)

241

241-249

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ACID-LEACHING AND CONSEQUENT PORE STRUCTURE BLEACHING CAPACITY MODIFICATIONS OF EGYPTIAN

M.I. ZAKI’>*, M. ABDEL-KHALIKz

AND CLAYS

and G.M. HABASHY’

‘Chemistry Department, Faculty of Science, ‘Chemistry Department, Faculty of Science, ‘The American University, Cairo (Egypt)

Minia University, El-Mink (Egypt) Ain Shams University, Cairo (Egypt)

(Received 15 January 1985; accepted in final form 5 September 1985)

ABSTRACT Three Egyptian clays were characterized by differential thermal (DT) and thermogravimetric (TG) analyses, infrared absorption (IR) and pore structure. The influence of HCl leaching on the cotton seed oil bleaching capacity of these clays (measured relative to the standard Tonsil AC) was correlated with consequent pore structure modifications. The results obtained show that the bleaching capacity is maximized on clay specimens having an average pore radius of 50-60 k

INTRODUCTION

Acid-leaching is widely adopted to upgrade the bleaching capacity of clays for both edible and mineral oils [ 1,2]. It has been established [3] that acidleaching leads to partial removal of octahedrally situated A13+ ions, leaving behind a framework possessing a large surface area and residual negative charge. Such modifications are believed [4] to cause not only colour-body removal, but also the removal of traces of metal, adsorption of phospholipids and soaps, and decomposition of oxidation products, such as peroxides. Acid-leaching is also expected to assist in the commercial utilization of alumina-rich clays (e.g. kaolinites) as a bauxite replacement for alumina ore r51. In the past decade, a number of papers [6-91 and industrial reports [lo, 111, dealing primarily with mineralogical and structural characterization of raw and acid-leached Egyptian clays, have been published, but with insufficient emphasis on the interrelation between their surface properties and bleaching capacities. The present paper, however, deals with the characterization, pore structure and cotton seed oil bleaching capacity of some acid-leached local clays, using Tonsil AC as a standard. *To whom all correspondence should be addressed.

0166-6622/86/$03.50

o 1986 Elsevier Science Publishers B.V.

242 EXPERIMENTAL

Materials

The standard Tonsil AC (Siidchemie, F.R.G.) was provided by El-Nile Co. (El-Minia, Egypt) in powdered form with a grain size of micrometer dimensions. Local specimens of Aswan, Alexandria--Cairo road and Quena clay (denoted respectively by C,, C, and C,) were crushed and ground to pass through a 74-pm screen. The fine powders obtained were vigorously stirred in hot (90°C) 6 vol.% HzOz solution (5 ml g-’ sample), and then in 2 N CH3COOH solution (5 ml g-’ sample), to eliminate respectively organic and carbonaceous materials [ 121. The samples were then filtered, washed thoroughly with distilled water and dried at room temperature to constant weight (5-7 days). The crude cotton seed oil used was supplied by El-Nile Co. (ElMinia, Egypt). Its deep red colour has been ascribed [lo, 111 principally to the coexistence of gossypol, carotenes, xanthophyll, chlorophyll and anthocyanins. It has been emphasized [lo, 111 that degradation products of these natural pigments may also occur in the oil due to field and storage damage, as well as improper processing. The hydrochloric acid and other chemicals used were of analytical grade (BDH). Apparatus

and techniques

Characterization

The H201 and CH,COOH pretreated clays and the Tonsil were characterized by differential thermal (DT) and thermogravimetric (TG) analyses, infrared absorption (IR) and X-ray diffraction (XRD) analyses. The apparatus and techniques used have been described previously [ 131. Acid treatment

and determination of the bleaching capacity The pretreated local clay specimens (166 g) were added to 1 1 of 1 N HCl solution in a two-necked flask, equipped with a thermometer and a condenser. The flask was placed in a water bath, thermostatically controlled at 95 + 2”C, and the slurry subjected to leaching with continuous magnetic stirring for varying periods, namely 60, 90 or 120 min. The slurry was then filtered, thoroughly washed until acid-free and dried at 110°C. The dried material was ground to pass through a 74-pm screen and kept in a desiccator. The A13+ content of the filtrate was determined by atomic absorption (Shimadzu Unit, Japan). The leached samples are denoted by the leaching period parenthesized next to the raw clay designation. Thus, Cx(60) indicates Aswan clay leached for 60 min, whereas C&(120) signifies Quena clay leached for 120 min. It is worth mentioning that the HCl leaching conditions were determined after careful consideration of the results of a comprehensive kinetic study by Huff and Hulbert [ 51. The bleaching capacity of the raw and acid-leached clays towards crude

243

cotton seed oil was tested using the Lovibond Tintometer (Quality Control Laboratories, El-Nile Co., El-Minia, Egypt) adopting the procedure detailed elsewhere [ 31. A calibration curve was constructed using the standard Tonsil AC and different oil/Tonsil weight ratios. The bleaching capacity (BC) of a given clay was then determined by applying the following equation: BC = [(A,-A,)/(A,-A,)] X 100, where Al is absorbance of the crude oil, A2 and A3 are the absorbances of oil bleached with equal weights amounts of the clay and the standard Tonsil, respectively. Pore structure

The sorption isotherms of N, (at -195°C) were determined volumetrically using a microapparatus based on the design of Lippens et al. [14], and following strictly the experimental recommendations reported in a recent IUPAC/SCI study [15]. The isotherms were of type-IV and the hysteresis loops displayed were of type-B, thus suggesting a pore structure consisting of slit-shaped capillaries [ 161. The surface area (&ET) was calculated using the BET method [16]. A computer-oriented analysis, based on the Kelvin method [ 161, was devised [17 3 for calculating and plotting the pore size distribution curves (PSD). Since, as Everett [18] has emphasized, the PSD curve calculated from the desorption branch of the isotherm will often give a misleading picture of the pore structure, the input data were derived from the adsorption branch. RESULTS

AND DISCUSSION

Characterization Aswan clay (C,)

The TG and DT curves, given in Fig. lA, indicate that C!, suffers a total loss of 13.3% of its original weight in two endothermic steps: a minor loss of 1.5% at <2OO”C, most probably due to elimination of adsorbed water, and a major loss of 11.8%, maximized at 550°C and completed at 6OO”C, due to elimination of structural water [19], i.e. dehydroxylation. The amount of adsorbed water (200°C) may be used as a distinctive parameter to determine the nature of the clay mineral [9, 201, and to differentiate between kaolinic and non-kaolinic clays [9]. For kaolinites to be present in a significant proportion, the amount of structural water associated with unit weight of adsorbed water (i.e. (13.3-1.5)/1.5)], denoted by W, in Table 1, must exceed at least 5. In contrast, for montmorillonites or illites, for example, it is below 2. The value of W, calculated for C, (Table 1) is well above 5, showing it to be predominantly composed of kaolinites. Further strong evidence for the kaolinic nature of C, is given by: (i) the diagnostic sharp exothermic effect located at ca. 995°C in the DT curve (Fig. 1A); (ii) the two distinct O-H stretching absorptions displayed at 3700 and 3620 cm-’ in the IR spectrum (Fig. 2A) [21] ; and (iii) the characteristic

I,

0

200

1

400

I,

I,

600 800 Temperature

Fig. 1. TG and DT curves Tonsil (D).

I

1000

of the raw local

I

12oBc clays

C, (A),

C, (B) and C, (C), and standard

Wavenumber Fig. 2. IR spectra

from the raw local clays C, (A),

C, (B) and C, (C), and Tonsil

(D).

XRD lines given in Table 1 (viz. at 2.56, 3.56, 4.35 and 7.15 a (cf. ASTMcard No. 14-164 [22]). Moreover, the low adsorbed water content (1.5%) [20, 231, together with the O-H stretching IR absorption displayed at 3655 cm-’ (Fig. 2A) [21], suggest that the kaolinites contained in C, are arranged in a well-ordered structure.

245 TABLE 1 Bleaching capacity, BET surface area, pore structure and amount of Al,O, removed of raw and leached clays

Clay

specimen

BC

wt%

(W)

A’#‘, removedb

70

160

45

00

110 80 30

220 246 190

58 38 89

17 53 71

CY

40

186

35

00

C, (60) c, (90) c, (120)

59

85 65

197 220 180

65 45 78

10 27 55

33

220

27

00

53 65 53

240 265 230

40

45 70

6 16 30

100

259

53

00

cx C, (60)

c, (90) c, (120)

C, (60)

c, (90) G (120)

Tonsil

W,

d values (A) of

7.9

2.56, 3.56, 4.35, 7.15

3.2

2.56, 3.27, 3.37, 3.52, 4.4, 9.94, 10.5

1.8

3.37,4.49,9.94,10.5, 16.3

0.9

3.37, 4.47, 9.96, 10.41, 16.3

a Values accurate to within * 3 mz g-‘. bValues calculated with reference to the average of the moved out of the three local clay specimens in 36 h.

Alexandria-Cairo

characteristic XRD lines

maximum amount of AJO, re-

road clay (C,)

The TG curve of this specimen (Fig. 1B) reflects thermal behaviour comparable to that of C,, but reveals a relatively higher total weight loss (16.6%) at 600°C due to the loss of more adsorbed water (4.0%) at
246

Quena clay (C,)

The TG curve given in Fig. 1C shows three weight loss processes at <15O”C, 250°C and <300”-900°C. The first at <15O”C (6.5%) is associated with two overlapping endothermic effects (DT curve, Fig. lC), suggesting the existence of two types of adsorbed water. The second at 250°C (1.8%), which is accompanied by a weak endothermic effect, can be associated with the decomposition of a carbonaceous contaminant [19]. The succeeding strong and gradual step at <300”-9OO”C, which brings the total loss up to 22.0%, is associated with a strong exothermic effect centred around 560°C. Since dehydroxylation should take place in this temperature range, the expected endothermic effect appears to be almost completely dominated by this strong exothermic effect. Exothermic processes encountered in this temperature range have been related to oxidation of pyrite contaminants [ 261. The weak endothermic effects at 750” and 85O”C, which intersect the termination of the third weight loss step, are considered to be due to decomposition of dolomite contaminants [26]. The W, value (Table 1) calculated after the substraction of the weight loss at 250°C (1.8%) from the weight loss at 600°C (20.0%), to avoid the losses due to the other contaminants, is sufficiently low to rule out the possibility of the existence of kaolinites. The IR spectrum (Fig. 2C) displays strong evidence for montmorillonites (band at 918 cm-’ [27]), micas (the weak O-H stretching absorptions at 3650 and 3630 cm-’ [ 211) and carbonate contaminants (the broad absorption centred around 1440 cm-‘) [17]. Again, strong absorptions are shown for the large amount of adsorbed water (at 3400 and 1650 cm-‘). The XRD results compiled in Table 1 lend further support for the presence of these constitutents and also for the absence of any kaolinites or kaolinitic structures. It is worth mentioning that the XRD peaks were fuzzy and of smeared maxima indicating very poor crystallinity. Tonsil

The thermal analysis results of the standard Tonsil AC, as shown by the TG and DT results exhibited in Fig. lD, together with the corresponding W, value (Table l), indicate clearly the presence of montmorillonite as the dominating mineral, and carbonaceous and mica contaminants. The IR (Fig. 2D) and XRD (Table 1) results support these findings. In conclusion, the montmorillonic content increases in the following order C,
and bleaching

capacity

The results of the textural analysis compiled in Table 1 show clearly that the increase of the montmorillonite content, in the order C,
247

LO

60

80

Fig. 3. Pore size distribution curves for Tonsil and the raw local and C, (C) before and after leaching for the periods indicated.

clays

C, (A),

C, (B)

both the Fr,,, values (Table 1) and the PSD curves (Fig. 3), develops consistently [16]. As far as the BC toward cotton seed oil is concerned, all of the raw local clays show (Table 1) weaker capacities than the Tonsil but to different degrees, e.g. the BC of C, is much higher than that of C,, which is in turn slightly higher than that of Cz. This trend in BC values parallels that of the mean pore radius but contravenes that of the &ET values. Upon leaching with HCl, the local clays show varying increases in the SBET values with an increase in the leaching period, but only up to 90 min; at the leaching period of 120 min, the SBET values suffer a marked drop. For all the clays, the amount of A1203 removed increases with the leaching period, but to different extents (Table 1). However, at a given period, the amount of A1103 removed varies in the order Cx>C,>C,. This trend is certainly a consequence of the increasing montmorillonite content, C,
248

creases markedly the surface area; however, further removal leads to a partial lattice collapse and a consequent drop of surface area. For non-kaolinitic clays (e.g. C,), the removal of an amount of AlaO as low as 30% seems to cause similar lattice damage (Table 1). The PSD curves, shown in Fig. 3, and theFpmax values (Table 1) show that the BC assumes a maximum value (for Tonsil and C, (60)) when the I”P,, has a value within the range 50-60 a. However, when FPPmaxassumes a value >60 A or <50 A, the BC decreases notably. This is the case with C, and C, (go), Fig. 3A; C, (60) and C, (120), Fig. 3B; and C, (90) and C, (120), Fig. 3C. The minimum BC value (Table 1) is observed for C, (120) and C,, for which Fp (Table 1) assumes respectively the highest (85 a) and the lowest (27 x7 v al ue. A detailed explanation of such a correlation between BC and rPmax requires fuller knowledge of the relationship between BC and the composition of the cotton seed oil, in particular the size and structure of the pigment molecules. Finally, one may conclude that the bleaching capacity towards cotton seed oil is maximized for clays having mesopores of 50-60 A. The present investigation does not lend support to the results reported previously [33 claiming a relation between bleaching capacity and the surface area of the clay. Ad hoc acid-leaching of clays, i.e. leaching under arbitrary conditions, may or may not lead to an increase in bleaching capacity. It can be also concluded that C, (60), Aswan clay, has a bleaching capacity superior to that of the Tonsil AC. ACKNOWLEDGEMENTS

We thank Professor S. Nashed (Ain Shams University, Egypt) for helpful discussion, and Mr N.H. Yacoub (El-Nile Co., El-Minia, Egypt) for providing the clay specimens and performing the Lavibond tintametric measurements.

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