Iron minerals and their influence on the optical properties of two Indian kaolins

Applied Clay Science 33 (2006) 269 – 277 www.elsevier.com/locate/clay

Iron minerals and their influence on the optical properties of two Indian kaolins Sathy Chandrasekhar ⁎, S. Ramaswamy Regional Research Laboratory (CSIR), Thiruvananthapuram, 695 019, India Received 25 November 2005; accepted 10 June 2006

Abstract Kaolin is mostly associated with minor quantities of ancillary minerals containing transition elements such as iron and titanium. These ions impart color to the white kaolin which adversely affects its application in paper and paint industries. Hence their removal is of prime importance in the optimum utilization of kaolin. The coloring effect as well as the mode of removal of these impurities depends on the “species” of the ion and/or the type of mineral. The present paper deals with the investigation on two Indian kaolins of different geological origin, one from Gujarat state at the western part of India and the other from Kerala State at the southern most part. Detailed physical, chemical and mineralogical characterization of the samples was carried out. The product clays after beneficiation by size classification, high gradient magnetic separation and chemical leaching were found to be of acceptable grade for paper industry with respect to optical properties and particle size. The impurity minerals were concentrated by different methods so that their identification was easier. Attempts were made to study the Fe species by correlating the XRD, chemical assay, DCB treatment and EPR spectral information of the clay samples before and after beneficiation. Iron stained anatase was found to be the major impurity in the Gujarat clay whereas iron was present as oxide/hydroxide in the Kerala sample. The beneficiated products from the Kerala clay were found to have better optical properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Kaolin; Beneficiation; SC-HGMS; DCB; Iron minerals; EPR spectra; Optical properties

1. Introduction Kaolin or china clay is a versatile industrial mineral and one of the highest value additions is achieved when it is beneficiated to pigment grade suitable for paper and Abbreviations: GC, Gujarat clay; KC, Kerala clay; ROM, Run-ofmines; SCP, Size classified product; SC-HGMS, Super conducting high gradient magnetic separation; IM, Impurity; NM, Non magnetics; DCBTP, Dithionite–citrate–bicarbonate treatment product; BDL, Below detectable limit; ND, Not determined. ⁎ Corresponding author. Tel.: +91 471 2515223; fax: +91 471 2491712. E-mail address: [email protected] (S. Chandrasekhar). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2006.06.008

paint industries. In kaolin, minor quantities of transition elements such as iron, titanium and manganese are generally present as ancillary minerals which adversely affect its optical properties. The valance state of the ion and atomic position in the structure depend on the conditions of formation of the mineral (Muller and Calas, 1993; Muller et al., 1995). Extensive research has been carried out on the nature of iron impurities in kaolin, which leads to the conclusion that iron is present as a part of the kaolinite or ancillary mineral (mica or titania) structure ie., “structural iron” or as separate iron minerals such as oxides, hydroxides, oxy-hydroxides, sulphides and carbonates ie., “free iron” (Jepson, 1988).

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The common Fe-oxide minerals in clays are reddish hematite and yellow goethite (Malengreau et al., 1996). Free and crystalline iron oxides are frequently separated from kaolin for identification and quantification. Most of the methods used to remove these oxides involve reduction and mobilization of iron. The Dithionite–Citrate–Bicarbonate (DCB) method developed by Mehra and Jackson (1960) removes the free iron oxides with a minimal effect on the crystalline clay minerals and this extractable iron is a fair estimate of pedogenic iron oxides in soils and kaolins (Borggaard, 1988). Identification and quantification of iron oxides in kaolin is very difficult due to their poor crystallinity and low concentrations. In order to overcome this difficulty Norrish and Taylor developed a selective dissolution method for concentrating Fe-oxides in kaolin by digestion with 5 M NaOH solution (Norrish and Taylor, 1961). A high ratio of dissolution reagent to sample weight ensures much better dissolution and avoids saturation of the solution with Si and Al (Smith, 1994). Size classification of clays removes part of the Fe and Ti minerals of coarser size. The higher specific gravity of these minerals also plays a role in this separation. Super Conducting High Gradient Magnetic Separation (SC-HGMS) has been established as an excellent technique in kaolin beneficiation for the removal of magnetic and feebly magnetic impurities even in small size range (Jepson, 1988). The important optical properties of white pigments are brightness, whiteness, yellowness and Lab color values (expressed in ISO units). Brightness represents the % of reflectance of light at a wavelength of 457 nm and the difference in reflectance values at 457 and 570 nm gives the yellowness. Hunter whiteness and yellowness indices have the advantage of single number quantities that are based on the entire visible spectrum. The whiteness formula relates better to people's visual assessment than brightness whereas yellowness indices normally do not correlate well with the visual judgment of yellowness. The Lab system based on the color opposites gives a better representation of the colors. The term L is a measure of lightness/darkness and varies from 100 for perfect white to 0 for absolute black. The red/green color is indicated by a. The more positive is its value, the greater is the reddishness and negative value indicating greenishness. Similarly, the yellow/blue shade is represented by b, positive value for yellow and negative for blueishness. Brightness and Lab color components are influenced by iron minerals and iron bearing anatase. Iron decreases brightness and L value whereas anatase increases b to cause yellowness. In the present investigation, two kaolin samples of different geological origin from India have been studied in detail. The physical, chemical and mineralogical charac-

terization was carried out. Attempts were made to identify the iron and titanium minerals after separation from the clay by different methods. Size classification and high gradient magnetic separation of the Run-of-mine (ROM) clays were found to give product clays of particle size distribution and optical properties acceptable for pigment grades. The coarser fraction of size classification and magnetic portion of the SC-HGMS were found to contain more iron and titanium minerals. The impurity minerals were also concentrated by 5 M NaOH treatment and panning. The clay samples were subjected to DCB treatment to find out the soluble iron content. With respect to chemical leaching, the term “structural iron” has been used not only for the iron substituted in the kaolinite structure, but also to that in the structure of ancillary minerals like anatase, mica etc. which are not easily leachable. However, in EPR spectral analysis, the “structural iron” stands for the iron substituted in the kaolinite structure for aluminium and/or silicon. EPR spectra of ROM clays, impurity minerals, products of size classification, magnetic separation and chemical leaching etc. were studied to understand the nature of iron. Wet chemical and XRD analyses of selected samples were carried out and optical properties were measured to supplement the findings. 2. Experimental 2.1. ROM clays Kaolin samples of two different geological origins were chosen for investigation viz., Gujarat Clay (GC) from the Kutch district of Gujarat, North West part of India and Kerala Clay (KC) from Thiruvananthapuram district of Kerala, the southern most part of the country (Fig. 1). Kutch is well known in Indian geology because of typical Jurassic exposure. The Jurassic formations are predominantly composed of alternating sequences of limestone, shale and sandstone of marine origin. The feldspathic sandstone of Bhuj series of Jurassic have undergone weathering resulting in the formation of kaolinitic clays, which are intermittently exposed in various places of the Kutch district. At some places, these clays contain impurities such as Fe, Ti and Al minerals (Krishnan, 1997). Kaolin belonging to the upper sedimentary sequence (Warkalli formation) is encountered in the coastal tract of Kerala. It is a weathering crust developed over the Khondalite–migmatitic complex. Sedimentological data suggest that the kaolinite was transported and deposited in shallow waters where highly turbid waters suddenly lost transportation capacity (Soman, 1997). 2.2. Beneficiation of clays Nearly 10 tons each of the ROM samples from the two deposits were collected, thoroughly blended, blunged with water and screened through a 300 μm sieve in the Edwards and

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Grimshaw, 1960; Bennet and Reed, 1971). Wet chemical and instrumental analyses were adopted to determine the chemical assay of samples. Particle size distribution was found out by Micromeritics Sedigraph 5100. The XRD patterns were taken using X-ray diffractometer (Philips PW 1710) with Ni filtered Cu Kα radiation at 40 kV and 30 mA. Thermal analysis was carried out using computer controlled Seiko 320 DTA/TGA using α-Al2O3 as reference material and in air atmosphere. The heating rate was 10 °C per minute and the sample size was approximately 10 mg. EPR spectra were recorded at 9.4 GHz (X-band) at room temperature (303 K) using a Varian model E-112 spectrometer with 10 mW microwave power. The optical properties were measured by Technidyne Corporation Color Touch ISO model spectrophotometer.

3. Results and discussion

Fig. 1. Location map of the kaolin mines in India.

Jones Clay washing plant. The −300 μm fraction slurry was subjected to successive hydrocycloning using a set of Mozley hydrocyclones (2″ stub, 2″ standard, 1″ and 10 mm) to get the size classified products of 2″ stub and 10 mm hydrocyclone overflow solids (SCP1 and SCP2 respectively). All the above operations were carried out at conditions optimized in the laboratory. The product clay was then subjected to SC-HGMS using M/s Carpco's 5 Tesla state-of-the-art separator. The SCP2 and SC-HGMS non-magnetics (NM) of the two clays were subjected to DCB treatment by the method developed by Mehra and Jackson (1960).

Both the clays are highly kaolinitic. GC is more colored which contains more impurity minerals than KC as indicated by XRD and chemical assay. GC appears to have originated from biotite rich granite/granite gneiss and schist, which are essentially colored minerals. Our earlier studies on this clay have shown that the size classification removed titanoferrous impurities only to a small extent and the impurity is uniformly distributed in all the size fractions (Chandrasekhar and Raghavan, 1999). The second sample KC has originated from a secondary clay deposit formed by the weathering of feldspar in granite rocks. The major impurity present is quartz and most of it is removed during size classification. 3.1. Mineralogical analysis

2.3. Concentration of impurity minerals Attempt was made to concentrate the impurity minerals in these two kaolins by the following methods. (i) Some of the impurity minerals were isolated during the size classification and magnetic separation of the clay samples (−300 μm, +45 μm size fraction and the magnetics). (ii) The denser impurity minerals were also separated from the clays by panning. A dilute suspension (10% w/w) of the clay was prepared in a distilled water and stirred well. The clayey portion was removed by decantation and repeated washing to get the impurity concentrate. (iii) The iron oxides were concentrated by digestion of the clay with 5 M NaOH solution (Norrish and Taylor, 1961). 2.4. Physical, chemical and mineralogical characterization All samples were characterized for their physical, chemical and mineralogical properties by standard methods (Searle and

The ROM clays, their beneficiated products and the impurity minerals were analyzed using X-ray diffraction. The major impurity mineral in GC is anatase along with small amounts of rutile and zircon. In KC, the rutile content is higher compared to anatase. Fig. 2 shows the XRD patterns of GC ROM and samples derived from it such as 2″ stub U/F (IM1), magnetic fraction from SCHGMS (IM2), panned minerals (IM3) and the NaOH treated sample (IM4). XRD patterns of the corresponding samples from KC are shown in Fig. 3 and all the peaks except those of kaolinite are marked in the figures. The impurity concentrates from GC and KC contain zircon, quartz, anatase and rutile. Kaolinite remains as the major mineral in IM1 and IM2 whereas ancillary minerals dominate in IM3. The IM4 samples are found to contain only the ancillary minerals. Even though the iron content is high in these samples especially in the panned fractions, no prominent peaks of any iron mineral are observed. This

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GC and probably this iron based titania is attributing to the higher b value (yellowness). Iron compounds appear to play a major role in the optical properties of KC. In the SCP1 of the two kaolins, TiO2 values differ but Fe2O3 values are almost the same and the optical properties are significantly different. The higher brightness of KC is probably due to the lower TiO2 content. SCHGMS removes more of the iron and titanium minerals and considerably improves the optical properties of both clays. The ROM and SCP2 samples of GC and KC were analyzed for trace elements and Table 3 gives the results. The vanadium and zirconium contents are higher in GC whereas KC contains significant quantities of chromium and manganese. Size classification reduces the quantities of only certain elements. Chemical assay of the impurity mineral concentrates (IM1–IM4) separated from the clays and their mineralogy is given in Table 4. The IM1 samples (2″ stub cyclone underflow solids) of both clays have more

Fig. 2. XRD patterns of GC samples (a) ROM (b) IM1 (c) IM2 (d) IM3 (e) IM4.

is probably due to the low crystallinity of the iron impurity. Thermal analysis of ROM and SCP2 samples of the two clays confirm the high kaolinite content with characteristic endotherm at ∼ 520 °C and exotherm at ∼ 999 °C and weight loss of ∼ 13%. The data for the impurity concentrates indicate that the IM1 and IM2 samples are more or less kaolinitic whereas IM3 samples do not give any prominent exotherm or endotherm in DTA and the weight loss (in TG) is relatively small. Table 1 gives the thermal analysis data of selected samples. 3.2. Chemical assay and optical properties The chemical assay, particle size distribution and optical properties of the ROM and beneficiated products of GC and KC are given in Table 2. Size classification increases the percentage of b 2 μm fraction to ∼ 82 and ∼ 92 for GC and KC respectively. Part of the iron and titanium is also removed with some improvement in optical properties. The TiO2 content is much higher in

Fig. 3. XRD patterns of KC samples (a) ROM (b) IM1 (c) IM2 (d) IM3 (e) IM4.

S. Chandrasekhar, S. Ramaswamy / Applied Clay Science 33 (2006) 269–277 Table 1 Thermal analysis data of selected samples Sl. no.

Samples

TG (wt. loss %)

DTA (°C) Endotherm

Exotherm

1 2 3 4 5 6 7 8 9 10

GC ROM GC SCP2 GC IM1 GC IM2 GC IM3 KC ROM KS SCP2 KC IM1 KC IM2 KC IM3

13.20 14.29 8.76 14.47 2.28 12.78 14.24 3.12 14.40 2.15

539.7 524.7 528.6 513.7 197.9 521.9 525.4 527.8 513.1 196.4

994.4 985.4 994.7 977.1 869.4 999.9 999.9 994.0 976.5 867.8

quartz, iron and titanium minerals. GC is found to contain mica also. The low values for Al and ignition loss clearly indicate the low kaolinite content in these samples. The magnetic fraction (IM2) from SC-HGMS contains essentially kaolinite with higher amounts of Fe and Ti especially in GC. Panning of the clay removes most of the kaolinite and the impurity (IM3) has low LOI and high percentage of Fe and Ti. Alkali treatment removes most of the Al and Si and the IM4 sample contains very high values of Fe and Ti. The kaolinite mineral dissolved in alkali and the Fe and Ti minerals

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are retained. As discussed earlier, the iron minerals appear to have a very low crystallinity. The product clays from GC and KC beneficiated by sizing and magnetic separation were found to respond to DCB treatment. Tables 5 and 6 give the % of Fe2O3 and TiO2 and optical properties of the GC and KC samples respectively before and after DCB treatment. The improvement is almost the same for the second and third DCB treatments. The GC contains more Fe2O3 and TiO2 and has inferior optical properties. DCB treatment of the GC SCP2 improves the brightness by ∼ 4 units with a decrease in Fe2O3 and no change in TiO2 percentage. But SC-HGMS results in an increase of brightness by ∼ 9 units with a similar decrease in Fe2O3 content but with a sharp decrease in TiO2. DCB treatment of the SCHGMS product improves the brightness only by ∼ 2 units with a slight decrease in Fe content. The SCP2 of KC when subjected to DCB treatment gives a sharp increase in brightness (∼ 9 units) and the % of Fe2O3 reduces from 0.35 to 0.19 with no change in the TiO2 content. SC-HGMS also gives an improvement of ∼ 7.5 units and both Fe2O3 and TiO2 content were found to decrease. This indicates that most of the iron in KC is “free” and leachable. The TiO2 contains some iron in its structure which may be facilitating its removal from the clay by SC-HGMS. It is observed that DCB treatment removes some more iron resulting in higher brightness.

Table 2 Chemical assay, particle size and optical properties of the raw and beneficiated kaolins GC ROM

GC SCP1

GC SCP2

GC SCP2NM

KC ROM

KC SCP1

KC SCP2

KC SCP2NM

Chem. assay (wt.%) 45.81 SiO2 37.72 Al2O3 Fe2O3 0.86 TiO2 1.63 CaO 0.06 MgO .032 Na2O 0.29 K2O 0.03 LOI 13.56

44.21 36.30 0.61 1.53 ND ND 0.33 0.01 14.22

44.39 37.98 0.43 1.60 0.06 0.032 0.13 0.02 14.31

44.27 37.91 0.28 0.99 0.06 0.031 0.11 .015 14.40

45.86 36.02 0.39 0.66 0.19 0.041 0.36 0.39 13.60

46.18 36.42 0.62 0.43 0.80 ND 0.12 0.04 14.31

46.35 37.88 0.35 0.52 0.19 0.035 0.27 0.15 14.10

46.28 37.90 0.25 0.31 0.18 0.033 0.24 0.13 14.18

Particle size distribution, wt.% N45 μm 3.00 b45, N 2 μm 28.70 b2 μm 58.30

0.80 28.60 70.60

0.00 17.70 82.30

0.00 17.70 82.30

6.20 24.51 69.29

0.00 16.42 83.58

0.00 8.08 91.92

0.00 8.08 91.92

Optical properties (% ISO) Brightness 70.34 L 89.18 a 0.61 b 6.01 HUN W 45.14 HUN Y 10.58

72.50 90.12 0.61 6.05 46.44 10.47

73.57 90.52 0.63 6.09 49.69 9.67

83.69 94.67 −0.28 4.59 64.78 6.93

77.71 90.30 2.75 3.02 65.96 4.78

78.17 90.95 1.76 3.60 64.01 5.65

80.86 92.20 2.39 3.27 67.83 5.07

88.22 95.80 0.30 2.60 77.04 4.00

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Table 3 Trace elements in Gujarat and Kerala clays (in ppm) Sl. no.

Elements

GC

KC

ROM

SCP2

ROM

SCP2

1 2 3 4 5 6 7 8 9 10 11

Vanadium Chromium Manganese Cobalt Nickel Copper Zinc Lead Zirconium Cadmium Bismuth

13.9 15.4 4.6 2.3 3.2 1.1 1.5 6.9 63.0 1.1 2.6

13.4 13.4 2.2 1.9 3.2 1.3 2.4 6.2 37.9 1.1 2.6

5.6 77.3 25.8 2.4 5.1 2.1 1.4 2.4 BDL BDL 1.1

5.8 12.2 10.5 2.6 4.9 1.8 1.2 3.1 BDL BDL 1.2

It is important to note that the SC-HGMS products of both clays have almost the same iron content but have significantly different optical properties, the GC sample being inferior. The reason can be again attributed to the higher TiO2 content and presence of some iron in TiO2 structure. The brightness/whiteness of kaolin are dependent on the overall effect of the Lab color values. It has been well established that the “free” iron is more soluble than the “structural” iron and hence the former is more easily removed by chemical leaching. However, the iron removed by SC-HGMS accounts to both “free” and “structural” type. The L value of GC increases after SCHGMS to a greater extent than that after DCB treatment. But in the case of KC, the increase is almost the same by both treatments. The values of a and b can be directly correlated to the “free” and “structural” iron respectively. The a value (reddishness) of GC is ∼ 0.6 and comes down to ∼ 0.4 on DCB treatment which indicates that the “free” iron is less in this clay. It is interesting to note

that there is a drastic decrease in the a value of KC (from 2.41 to 0.4) on DCB treatment which establishes that most of the iron in KC is “free” in nature. DCB treatment does not bring much change to the b value and it decreases to some extent by SC-HGMS. These results confirm that the “structural” iron in the form of Festained titania influences the optical properties of GC and SC-HGMS removes part of it thus enhancing the same. DCB treatment does not have appreciable effect because “free” iron is relatively less in this clay. Most of the iron in KC is “free” in nature and its removal is almost similar by both SC-HGMS and DCB treatment. The changes in the % of Fe2O3 and TiO2 during these treatments also support the same. Thus deferration by DCB treatment confirms the presence of minerals containing structural iron in GC and free iron in KC. 3.3. Electron paramagnetic resonance spectral studies The clay products SCP2, SCP2NM and SCP2DCBTP and the impurity minerals IM2 and IM3 of the GC and KC samples were studied by EPR spectroscopy. Figs. 4 and 5 give the EPR spectra of the GC and KC derived samples respectively. The SCP2 samples of GC (Fig. 4a) and KC (Fig. 5a) have lines at two resonance regions F1 (low magnetic field) and F2 (high magnetic field) centered at g ∼ 4.0 and at g ∼ 2.0 respectively. F1 is attributable to structural Fe+ 3 occupying positions in the octahedral sheet of kaolinite and F2 attributed to the crystal imperfections, holes, free radicals and the poorly understood domains (antiferromagnetic phases) in which Fe + 3 reside in close proximity to one another (Komusinski et al., 1981; Balan et al., 2000). GC has produced EPR lines in F1 region with g values 4.91 (1375 G), 4.10 (1650 G) and 3.57 (1900 G) and KC with g values 4.66 (1350 G) and

Table 4 Chemical assay and mineralogy of the impurity concentrates Properties

GC IM1

GC IM2

GC IM3

GC IM4

KC IM1

KC IM2

KC IM3

KC IM4

47.20 21.83 4.90 10.60 BDL BDL 0.30 BDL 8.57 K, Q, M

43.12 37.68 3.57 8.71 0.05 0.03 0.10 0.012 14.56 K, Q, A,

24.22 14.18 20.12 36.11 0.05 0.03 0.24 0.01 2.10 Z, R, K

8.17 ND 41.92 38.23 ND ND ND ND 7.47 Z, R

49.52 19.66 17.69 8.16 0.46 0.36 0.11 0.39 3.12 K, Q

45.48 37.15 1.12 1.13 0.18 0.03 0.45 0.08 14.56 K, Q, A

25.90 13.23 20.18 35.32 0.62 0.48 0.32 0.18 2.96 R, A, K, Z

5.64 ND 63.83 24.29 ND ND ND ND 5.21 A, R

Chemical (% by wt.) SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2 O LOI Minerals

A — Anatase; K — Kaolinite; Q — Quartz; R — Rutile; Z — Zircon; M — Mica.

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Table 5 Optical properties of DCB treated samples from Gujarat clay SCP2 Before DCBT Brightness, 74.08 % ISO Yellowness 8.71 L 90.33 a 0.61 b 6.06 HUN WI 50.32 HUN YI 9.58 % Fe2O3 0.43 % TiO2 1.60

SCP2NM After first DCBT

After second DCBT

After third DCBT

Before DCBT

After first DCBT

After second DCBT

After third DCBT

77.37

77.81

77.97

83.39

84.42

85.29

85.37

7.98 91.56 − 0.40 5.19 56.69 8.09 0.28 1.59

7.07 91.77 − 0.51 5.16 57.16 8.03 0.27 1.59

7.10 91.86 −0.53 5.15 57.34 8.01 0.27 1.58

7.00 94.68 − 0.29 4.85 64.40 6.92 0.28 1.00

7.13 94.72 − 0.64 4.15 67.24 6.26 0.24 1.00

5.44 95.05 − 0.74 3.96 68.86 5.95 0.24 1.00

5.39 95.09 − 0.70 3.89 69.29 5.84 0.24 0.99

4.10 (1650 G). In the F2 region, broad lines are produced at g = 2.60 (2600 G) by GC and at g = 2.81 (2400 G) by KC. The Fe+ 3 can occupy two different sites in the F1 region. GC and KC show a resonance line at g = 4.10 chemically referred to as Fe(I). The Fe(II) resonance lines are shown at g = 4.91 and 3.57 by GC and at 4.66 by KC. Though both Fe(I) and Fe(II) are due to Fe3+ substituting for Al+ 3 in the kaolinite lattice, their symmetry is different. There is a correlation between the degree of structural order of the kaolinite and the relative populations of Fe(I) and Fe(II) sites. Replacement of Al+ 3 by Fe3+ at Fe(I) probably at the periphery of the crystalline domains is accompanied by rhombic distortion (the deformation of the site of substitution), disturbing the regularity of the kaolinite lattice (Balan et al., 1999). Replacement of Al+ 3 with Fe3+ at Fe(II) makes no notable deformation in the crystal lattice showing that it is associated with high crystalline nature. Thus Fe(I) substitution is associated with poorly crystalline nature and even a small increase will result in appreciable

decrease in crystallinity of the sample. In other words Fe(I) signal corresponds to the changes of the site symmetry owing to the random distribution of vacant octahedral site in successive layers and Fe(II) corresponds to the sites within low defect kaolin. (Allard et al., 1994). Therefore an increase in the number of Fe(I) sites and decrease in the number of Fe(II) sites lead to an increase in structural disorder. 3.3.1. Effect of magnetic separation on the resonances F1 and F2 After magnetic separation, the intensity of resonance F2 has decreased considerably and it has become much broader in GC. Incidentally, the intensity of resonance F1 has remained almost the same. The magnetic portion (IM2) which contained more Fe-stained anatase, produced resonances F1 and F2 (with more intensity) matching very well with that of SCP2. It is therefore clear that the resonance F2 is associated with the Festained anatase. The impurity mineral (IM3) has also

Table 6 Optical properties of DCB treated samples from Kerala clay SCP2 Before DCBT Brightness, 80.72 % ISO Yellowness 3.86 L 92.18 a 2.41 b 3.30 HUN WI 67.58 HUN YI 5.12 % Fe2O3 0.35 % TiO2 0.52

SCP2NM After first DCBT

After second DCBT

After third DCBT

Before DCBT

After first DCBT

After second DCBT

After third DCBT

88.71

89.85

89.93

88.22

90.82

91.70

91.80

3.40 95.95 − 0.39 2.57 77.96 3.83 0.19 0.51

2.99 96.23 − 0.41 2.12 80.97 3.14 0.18 0.50

3.09 96.27 −0.40 2.11 81.05 3.13 0.18 0.50

2.60 95.77 0.30 2.68 77.04 4.00 0.25 0.31

3.01 96.62 − 0.32 1.95 82.57 2.88 0.15 0.31

1.93 96.83 − 0.33 1.57 85.07 2.32 0.14 0.30

1.95 96.83 − 0.30 1.57 85.08 2.31 0.14 0.30

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3.3.2. Effect of deferration by DCB treatment on resonances F1 and F2 It is expected that resonance F2 is associated with surface contaminants and probably it would be affected by chemical treatment of the kaolin, which removes the iron species. In the case of SCP2 of KC, the DCB treatment has no effect on the resonance F1 but almost eliminated the resonance F2 showing the presence of appreciable amount of free iron in the sample. The brightness improvement obtained for KC after the DCB treatment also substantiates the point that almost all the iron in KC is in the non-structural form. But in the case of SCP2NM products, DCB treatment does not have much effect on the intensities of the resonance lines. The spectra of both SCP2 and SCP2NM products of GC show that DCB treatment has no effect on the resonance F1, but reduced the intensity of the resonance F2 indicating the removal of some free iron present in it.

Fig. 4. EPR spectra of GC samples (a) SCP2 (b) SCP2NM (c) SCP2DCBTP (d) SCP2NM-DCBTP (e) IM2 (f) IM3.

given a much more intense and broad resonance at F1 (g = 4.10 with a weak shoulder at 2.40) showing that most of the iron in GC is in the structural form. In sample KC it is found that after the magnetic separation there was a sharp decrease in the intensity of resonance F1 and F2. The resonance F1 has almost disappeared and the resonance F2 has become broader and less intense indicating the removal of both “structural” and “free” iron in clay. The spectra of magnetic residue (IM2) which contained mainly the “non-structural iron” produced a resonance F2 similar to that of SCP2 with much higher intensity. Also the impurity mineral separated by panning (IM3) has produced only resonance F2 (with more intensity), showing clearly that this resonance is associated with a surface contaminant which is expected in the sample KC.

Fig. 5. EPR spectra of KC samples (a) SCP2 (b) SCP2NM (c) SCP2DCBTP (d) SCP2NM-DCBTP (e) IM2 (f) IM3.

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4. Conclusions The two kaolin samples from Gujarat and Kerala states of India were found to be highly kaolinitic with different types of Fe impurities. Size classification followed by SC-HGMS and DCB treatment resulted in product clays of high brightness and fine particle size matching with the specifications for pigment grade kaolin. The crystallo chemical studies on the clays and the mineral impurities showed that GC contains titanoferrous minerals whereas KC has ferruginous minerals. Iron in the “titanoferrous” form was found to have a more deleterious effect on the optical properties of kaolin even though the total analytical Fe2O3 content is small. SC-HGMS improved the brightness of both clays especially that of GC. However, KC responded to the DCB treatment more effectively indicating that it contains higher amount of “free” iron. EPR spectral study of the beneficiated kaolins and impurity concentrates confirmed that (i) most of the iron in KC is “free” and GC contains both “free” and “structural” type (ii) “free” iron is removed by DCB treatment whereas both “free” and “structural” iron are removed by SC-HGMS. Acknowledgements The authors are grateful to the Director, Regional Research Laboratory for giving permission to publish this work. Thanks are also due to Mr. P. Raghavan for supplying the samples during size classification and SCHGMS processes. The authors are thankful to Mr. P. Guruswamy for providing the XRD patterns and to the RSIC, IIT Chennai for the EPR spectra. Discussions with Dr. Rao of Pondicherry University on EPR spectral studies have been helpful. References Allard, T., Muller, J.P., Dran, J.C., Menager, M.T., 1994. Radiation induced paramagnetic defects in natural kaolinites: alpha dosimetry with ion beam irradiation. Physics and Chemistry of Minerals 21, 85–96. Balan, A., Allard, T., Biozot, B., Morin, G., Muller, J.P., 1999. Structural Fe+ 3 in natural kaolinites: new insights from electron

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