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Sun, Zhiming, Yang, Xiaoping, Zhang, Guangxin, Zheng, Shuilin, & Frost,Ray(2013)A novel method for purification of low grade diatomite powders in centrifu-gal fields.International Journal of Mineral Processing, 125, pp. 18-26.

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https://doi.org/10.1016/j.minpro.2013.09.005

1

A novel method for purification of low grade diatomite powders in 1

centrifugal fields 2

3

Zhiming Suna,b, Xiaoping Yanga, Guangxin Zhanga, Shuilin Zhenga∗, Ray L. Frost b∗ 4 5

a School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 6 100083, P.R. China 7

8 b Nanotechnology and Molecular Science, Faculty of Science and Technology, Queensland University of 9

Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia 10

2

Abstract. 11

This study presented a novel method for purification of three different grades diatomite from China by 12

scrubbing technique using sodium hexametaphosphate (SHMP) as dispersant combined with 13

centrifugation. Effects of pH value and dispersant amount on the grade of purified diatomite were 14

studied and the optimum experimental conditions were obtained. The characterizations of original 15

diatomite and derived products after purification were determined by scanning electron microscopy 16

(SEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and specific surface area analyzer (BET). 17

The results indicated that the pore size distribution, impurity content and bulk density of purified 18

diatomite were improved significantly. The dispersive effect of pH and SHMP on the separation of 19

diatomite from clay minerals was discussed systematically through zeta potential test. Additionally, a 20

possible purification mechanism was proposed in the light of the obtained experimental results. 21

22

Keywords: Diatomite; Centrifugal separation; Purification; Sodium hexametaphosphate 23

• Corresponding authors. Fax: +86 10 62330972 (S. Zheng), +61 7 3138 2407(R. L. Frost) 24

E-mail addresses: shuilinzh@sina.com (S. Zheng), r.frost@qut.edu.au (R. L. Frost) 25

3

1. Introduction 26

Diatomite or diatomaceous earth is a non-metallic material composed mainly of a wide variety of shape 27

and sized skeletal remains of diatoms [1, 2]. Because of highly porous structure, low density, high 28

absorption capacity, low thermal conductivity, chemical inertness, relatively low price and high 29

abundance, diatomite can be used extensively as filtration media, adsorbents, conventional catalyst 30

supports [3-8]. It can also be applied in manufacturing of certain membranes and in the 31

separation of biocatalytic proteins and cells [9]. China has substantial diatomite resources, most of 32

which are of moderate quality, having a SiO2 content of about 60 %~80 %. There are a lot of 33

impurities such as alkaline and alkaline earth metals and some organic substance, which decrease the 34

microporosity and limit the use of diatomite. Hence, raw diatomite needs to be industrially purified 35

before its commercial utilization. Several approaches have been reported, such as acid leaching, 36

classification, crushing, gravitational sedimentation, calcinations and ultrasonic treatment [9-13]. 37

Chemical treatments, such as acid treatment, can significantly improve the quality of raw diatomite. 38

However, waste water generated after acid leaching is very difficult to deal with, which causes severe 39

environmental problems and great operative expenses. Other beneficiation processes have some other 40

weaknesses, such as inefficiency, high processing cost, inadequate effect, fusion problem and 41

destruction of diatom structure. The advantages and disadvantages of different diatomite purification 42

techniques are concluded in Table 1. Among these methods, scrubbing technique has been preferred 43

due to its cheapness and the high-efficiency of the low grade diatomite purification especially for 44

pretreatment. However, there are few industrial achievements on diatomite purification now. 45

High speed scrubbing technique is applied to make the impurities coated on the surface of raw 46

diatomite separated from the diatom units initially. Agitator scrubbing machine is a kind of 47

conventional dressing equipment, which has an extensive application in mineral processing. 48

Centrifugal separation is one of the technically and economically acceptable solutions for the treatment 49

of waste water, flotation slurries and chemical industry [14-16]. The advantage of centrifugal 50

separators lines in the fact that the settling velocity is much higher than that of gravitational separators, 51

which results in smaller separator dimensions, higher separation efficiency and precision. Because of 52

small particle size of impurities and diatomite, the centrifugal separation can shorten the particles 53

settling time, which can ensure the separation process efficiently and continuously. 54

Some kinds of dispersants, such as sodium hexametaphosphate and sodium pyrophosphate, are 55

4

employed during the scrubbing process. Uniform and relatively stable dispersion of diatomite and its 56

impurities in the scrubbing suspension is the premise of effective centrifugal separation. The sodium 57

hexametaphosphate (SHMP) is widely used in mineral processing as floatation regulator and dispersant 58

[17]. The influence and mechanism of SHMP on the floatation and dispersion of different minerals has 59

been studied by many researchers [18-19]. However, the relationship between the settling property and 60

the dispersion of diatomite and its impurities has not been systematically studied before. 61

In the light of these, three natural diatomite powders, selected from different regions of China, 62

were purified by scrubbing technique combined with centrifugal separation using sodium 63

hexametaphosphate as the dispersant. It is the aim of the investigation to show whether different grades 64

of raw diatomite can be effectively purified through this method. Effects of pH value and dispersant 65

amount on the grade of purified diatomite were discussed. The influence of pH and sodium 66

hexametaphosphate on the dispersion of the diatomite and clay minerals was systematically studied 67

based on the zeta potential analysis of pure materials, and the possible purification mechanism was 68

further proposed. 69

70

2. Materials and methods 71

2.1. Materials 72

Three typical natural original diatomite powders were obtained from Linjiang of Jilin province 73

(LJ-DE), Huade of Inner Mongolia (HD-DE), and Shengxian of Zhejiang province (SX-DE) in China 74

respectively, respectively. Sodium hexametaphosphate and sodium hydroxide were purchased from 75

Beijing Chemical Reagent Company (Beijing, China). The chemical compositions of the raw diatomite 76

are listed in Table 2. As shown in Table 2, the main component of raw diatomite is SiO2. Ferric oxide 77

and alumina are the main impurities in raw diatomite. Based on the SiO2 content, the grade of three 78

diatomites decreases in the following order: LJ-DE > HD-DE > SX-DE. The pure minerals for zeta 79

potential analysis were purchased from NCS Testing Technology Co., Ltd. (Beijing, China) without 80

any further purification. The chemical compositions of the pure minerals were summarized in Table 2. 81

The microstructure of the pure diatomite is displayed in Fig. 1. It is clear that the purity of the diatomite 82

sample was very high from the chemical compositions analysis and SEM pictures. 83

84 2.2. Purifying methods 85

5

The flowsheet for the processing of raw diatomite is given in Fig. 2. As showen in Fig. 2, the 86

purification was undertaken by the following procedure: 100g of raw diatomite samples was initially 87

dispersed in 400 mL of water with a laboratory-scale model XFD12 agitator scrubbing machine for 88

about 25 min with a stirring rate of 1500 rpm. A certain amount of sodium hydroxide was 89

pre-dissolved in 20 mL of deionised water and then it was slowly added to the scrubbing suspension at 90

room temperature in order to adjust the pH value of the suspension. The suspension was stirred for 91

further 10 min, and then the selected amount of sodium hexametaphosphate was added to the 92

suspension. After stirring for a further 5 min, the suspension was collected and sieved using a 325 mesh 93

standard screen (45 μm). After sieving, the oversized products were collected and dried in an oven at 94

105 °C for 12h. The obtained sample was labeled as oversized product. The undersized suspension was 95

then transferred to a 100 mL centrifuged tube. After shaking, the mixture was centrifuged at 1500 rpm 96

(RCF (relative centrifugal force) = 300) for 15 min. After the centrifugation, the centrifugal sediment 97

was obtained and then dried at 105 °C for 12 h. The centrifugal overflow was filtrated by vacuum filter 98

and washed with distilled water 2-3 times and then dried at 105 °C for 12h. 99

100

2.3. Characterization 101

The chemical compositions of raw diatomite and purified diatomite are determined by X-ray 102

fluorescence (SPECTRO X-LAB2000). The crystalline phase identification of samples was undertaken 103

by a PANalytical X’Pert PRO X-ray diffractometer employing CuKα radiation at a goniometer rate of 104

2θ= 4°/min. Morphology of samples was examined by a scanning electron microscopy (S-3500N, FEI 105

Company). Infrared emission spectroscopy was carried out on a Nicolet Nexus 870 FTIR spectrometer, 106

which was modified by replacing the IR source with an emission cell. Spectral manipulation such as 107

baseline adjustment, smoothing and normalization was performed using the Spectra calc software 108

package (Galactic Industries Corporation, NH, USA). Surface area analyses based upon N2 109

adsorption/desorption were carried out with a Micromeritics Tristar 3000 automated gas adsorption. 110

From the measurement, pore structure parameters were also characterized at liquid nitrogen 111

temperature. The samples were pre-heated at 105 °C under the flow of N2 on a Micrometrics Flowprep 112

060 degasser before the measurement. 113

114

2.4. Zeta potential measurement 115

6

A certain amount of pure diatomite, kaolin and montmorillite was added into distilled water firstly. 116

The suspension was stirred for 15 min using a magnetic stirring apparatus before the test. At different 117

pH values, zeta potential of different samples was measured in a Nano ZS90 type ξ-potential analyzer 118

(Malvern, Britain). The pH value of the solution was automatically adjusted by adding HC1 or NaOH 119

in range of 2–12. 120

3. Results and discussion 121

3.1 Characterizations of purified diatomite 122

3.1.1 Chemical analysis 123

The effect of pH values of the scrubbing suspension on SiO2 content of purified diatomite was 124

examined through a series of centrifugal separation experiments while other variable kept constant. The 125

dosage of SHMP keeps 3 wt. ‰. Changes of the SiO2 content of the purified diatomite at different pH 126

are given in Fig. 3. It is clear that the SiO2 content of purified diatomite was enhanced with the 127

increase in pH. The SiO2 content of purified diatomite reached the highest values when the pH was 128

up to 10, so the optimum pH value is determined as 10. It can be confirmed that a high-grade silica 129

material, more than 87 wt. % SiO2 was obtained when pH was 10 for the purification of LJ-DE and 130

HD-DE. On the other hand, the removal of the impurities for SX-DE was more difficult compared with 131

the other two diatomites, and the SiO2 content of the purified diatomite was just increased to 74.01 wt. % 132

when pH was 10. 133

The dosage of SHMP is another key factor on the grade of purified diatomite and its influence is 134

shown in Fig. 4. The pH values of the scrubbing suspension were kept 10. From the Fig. 4, the SiO2 135

content of purified diatomite was increased with the increase in the dosage of SHMP. As for LJ-DE, 136

when the dosage of SHMP was up to 2 wt. ‰, the grade of purified diatomite reached the highest 137

value. As for HD-DE and SX-DE, the optimum SHMP dosages are determined as 3.5 wt. ‰ and 3 138

wt. ‰, respectively. The difference of optimum SHMP dosage might be caused by the different 139

contents and forms of impurities in three raw diatomites. The yield of the purified diatomite for LJ-DE, 140

HD-DE and SX-DE are 81.37%, 64.21% and 77.28% respectively under the optimum experimental 141

conditions. 142

143

3.1.2 Mineralogical analysis 144

X-ray diffraction (XRD) is one of the most useful techniques to study the structural geometry and 145

7

texture of impurities in diatomites. The XRD patterns of three raw diatomites before purification and 146

different products (oversized product, centrifugal overflow and centrifugal sediment) obtained after 147

purification are presented in Fig. 5. An essentially amorphous phase was observed in three raw 148

diatomites, but a significant amount of crystalline phases were also found. Among three raw 149

diatomites, the XRD pattern of LJ-DE showed the highest content of amorphous SiO2, followed by 150

HD-DE and then SX-DE. The main impurities in raw LJ-DE samples can be divided into two types. 151

One is clay minerals such as montmorillite, kaolin and muscovite, and the other one is crystalline 152

siliceous sands, such as quartz and feldspar. The main mineral compositions of the oversized product 153

are coarse feldspar and quartz, which agrees with our expectations. The typical XRD patterns of clay 154

minerals such as montmorillite, kaolin and muscovite appear in the centrifugal overflow, determined as 155

the tailings. All the purified powder samples exhibit less crystalline structure after purification. From 156

Fig. 5 (a) and (b), it can be seen that a high-grade amphorous silica material with less crystalline 157

structure is obtained after purification. As shown in Fig. 5(c), the reduction of impurities is rather 158

insignificant for SX-DE, indicating that the purification effect of SX-DE is the worst. In addition, the 159

content of clay minerals is the highest for SX-DE among three diatomites. The results indicate the 160

effect of purification might be influenced to a large extent by the content of clay minerals in raw 161

diatomites and the relative higher content of clay minerals in SX-DE results in the undesirable 162

purification effect. 163

164

3.1.3 Microstructural analysis 165

Microstructural observations have been applied to identify the success of producing highly-porous 166

diatomite particles and maintaining porous structural integrity of diatom through the applied scrubbing 167

and centrifugation treatment. A selection of SEM images of raw diatomite and purified diatomite from 168

Linjiang are present in Fig. 6. The characteristic disc-shaped structure of diatom particles is clearly 169

shown in raw LJ-DE with the diameter of about 20 μm. Compared with purified diatomite, the raw 170

diatomite exhibits diatomite frustules with closed-pores dispersed in clay and some other fine particles. 171

In addition, integrated morphology of diatom in raw diatomite and purified diatomite is also displayed 172

(Fig. 6 LJ-a2 and LJ-b2). It was found that the pores on the surface of diatoms could be seen more 173

clearly after purification, which might be due to the fact that the impurities were removed from the 174

clogged pores. Moreover, the purified diatomite still kept integrality after purification. 175

8

Naturally it is also observed that some impurities have been deposited on the diatomite particles in 176

HD-DE (see Fig. 7 HD-a1 and HD-a2). The diatoms in raw HD-DE are mostly cracked inherently. The 177

diameter of integrated diatom in HD-DE sample is approximately 10 μm, which is smaller than that of 178

LJ-DE sample. There are fewer impurities in purified diatomite (Fig.7 HD-b1). Fig.7 HD-b2 indicates 179

undamaged forms of diatom in purified diatomite with extremely clean pores, which demonstrates the 180

approach has a significant improvement on the pore structure of diatomite. 181

Fig. 8 displays the microstructure of raw SX-DE sample and purified diatomite samples. The 182

presence of cylindrical diatom together with small particles of impurities could be noticed in 183

micrographs of raw SX-DE sample and purified diatomite. Considerable content of small impurities 184

such as quartz, feldspar and clay minerals was detected on the surface of diatomaceous earth and such 185

high content of impurities makes SX-DE greyer than the other two raw diatomite samples. Compared 186

with the undamaged forms of diatomaceous earth in raw SX-DE sample, the outside pore structure on 187

the surface of cylindrical diatom looks cleaner after purification. However, there are still a mass of 188

impurities embedded in the pore of cylindrical diatom. The shearing force under the current scrubbing 189

conditions can not remove such embedded impurities quickly and effectively. The embedded 190

distribution of impurities might be another key factor why the SX-DE sample is harder to be purified 191

than the other two samples. 192

193

3.1.4 IR analysis 194

A comparison of the IR spectra of three raw diatomite samples and purified diatomite are 195

presented in Fig. 9. As for all samples, the IR spectra show two major absorption bands which were 196

observed at about 3430 and 1630 cm-1 [21]. The band at 3430 cm-1 is due to the stretching vibrations of 197

adsorbed water, and the band at 1630 cm-1 represents H-O-H bending vibration of water. The bands at 198

about 1100 cm-1 reflects the asymmetric stretching modes of Si–O–Si bonds and the Si-O-Si symmetric 199

stretching vibration peaks appear at about 800 cm-1 [22, 23]. Compared with the raw LJ-DE and 200

HD-DE sample, the bending modes of Al-O-H at 912 and 914 cm-1 appear in the IR spectrum of 201

centrifugal overflow sample, which indicates that the presents of some clay minerals [24]. The bending 202

vibrations of Al-O-H are observed in all three samples of SX-DE, which means the content of clay 203

minerals are high and the removal effect after purification is not very well, which is in line with the 204

result of the previous SEM analysis. 205

9

206

3.1.5 Surface area measurement 207

As shown in Fig. 10, the nitrogen adsorption/desorption isotherms of raw diatomite, centrifugal 208

overflow and sediment exhibit a type II sorption behavior in the classification of Brunauer, Deming, 209

Deming and Teller (BDDT) [25]. The specific surface area, pore volume and average pore size of raw 210

diatomites and products after purification are summarized in Table 3. It is clear that the raw diatomite, 211

tailing and purified diatomite have the same form of isotherms and hysteresis loop. The hysteresis loop 212

is associated with the filling and emptying of the mesopores by capillary condensation [26]. The 213

adsorption of N2 of centrifugal overflow is much higher than that of purified diatomite, indicating that 214

the overflow possesses a higher specific surface area. The result is in line with the XRD result since the 215

overflow is mainly composed of clay minerals. Compared with the raw diatomite, purification did not 216

evidently change the shape of the isotherms. However, purification resulted in a dramatic increase in 217

nitrogen adsorption and an enhancement of the hysteresis, indicating the improvement of the 218

microporous structure of diatom. As shown in Table 3, the specific surface areas of purified diatomite 219

powder become smaller than that of raw diatomites. It is easy to understand that the decrease of 220

specific surface areas is caused by the removal of impurities with relative higher specific surface areas. 221

Similar specific surface areas changes were also obtained in previous studies [9, 10, 27, 28]. 222

223

3.2 Mechanism of purification 224

The relationship between zeta potential and pH value of pure diatomite, kaolin and 225

montmorillite is presented in Fig. 11. It can be seen that the pH value at the isoelectric point of 226

diatomite, kaolin and montmorillite is about 2.0. The surface of diatomite, kaolin and montmorillite 227

particles shows negative electricity in the pH range of 2-12. In the acidic environment, the absolute 228

value of zeta potential of diatomite, kaolin and montmorillite particles is remarkably lower and 229

aggregation occurs easily among the particles due to the relative lower electrostatic repulsive force 230

[18, 19]. Moreover, with the increase in the concentration of SHMP, it is obvious that sodium 231

hexametaphosphate could lower the zeta potential of diatomite, kaolin and montmorillite. As for 232

diatomite and kaolin, the absolute value of surface charge of diatomite and kaolin kept increasing 233

with increase of pH in the range of 2-12. On the other hand, the surface charge of montmorillite 234

decreased firstly and then increased when pH was up to 9. A similar variation trend for the three 235

10

minerals was observed in different concentrations of SHMP. The above results illustrate that the 236

optimum dispersion conditions for diatomite and clay minerals are in the weak alkaline condition at 237

a relative high concentration of SHMP, which is in agreement with the previous obtained optimum 238

purification conditions. 239

Based on the above results, the mechanism of purification can be comprehensively concluded as 240

follows: Under the high speed shear force, the impurities were taken out from the surface and the 241

porous structure of raw diatomite. Due to the high electrostatic repulsive force, uniform and 242

relatively stable dispersion of diatomite and impurities in the scrubbing suspension can be obtained 243

in the weak alkaline condition at a relative high concentration of SHMP. Then, the coarse fractions 244

(+45 μm) containing mainly quartz and feldspar were firstly removed to prevent these large and 245

heavy impurities from entering the centrifugal process. After suitable centrifugation, the diatom and 246

impurities were separated from each other. The grade of diatomite was further improved due to the 247

remarkable difference of particle density and size in centrifugal field. 248

4. Conclusions 249

Purification effects of scrubbing technique combined with centrifugal separation using sodium 250

hexametaphosphate as dispersant on three typical diatomite powders were studied. The simple 251

technique could effectively remove most of the impurities containing clay minerals and organic matters 252

from LJ-DE and HD-DE samples. From the SEM analysis, the porous structure of all three raw 253

diatomites can be improved after purification. It is deduced that the strong centrifugal force in 254

scrubbing process can effectively separate the diatomite and impurities. Uniform and relatively stable 255

dispersion of diatomite and clay minerals in the scrubbing suspension can be obtained in the weak 256

alkaline condition at a relative high concentration of SHMP. Based on the difference of partial density 257

and size between diatom and impurities, the diatomite was effectively purified in centrifugal field. The 258

embedded distribution and relatively high content of clay minerals result in the inadequate separation 259

before centrifugation, which might be the main reason for the undesirable purification effect of SX-DE. 260

It is concluded that the technique will be potentially used as good purification method of low grade 261

diatomite in the industry. 262

263

Acknowledgement 264

The work is supported by National Technology R&D Program in the 12th Five year Plan of China 265

11

(2011BAB03B07). The first author also thanks the China Scholarship Council (CSC) for financial 266

support. 267

268

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328

14

LIST OF FIGURES 329

Fig. 1. SEM pictures of pure diatomite sample, (a)×600 and (b)×1500. 330

Fig. 2. Proposed flowsheet for the purification of low grade diatomites. 331

Fig. 3. Relationship between the SiO2 content and pH value in purification of three diatomites. 332

Fig. 4. Relationship between the SiO2 content and the dosage of SHMP in purification of three 333

diatomites. 334

Fig. 5. XRD patterns of three raw diatomites and different products obtained after purification, 335

(a) LJ-DE, (b) HD-DE, (c) SX-DE. 336

Fig. 6. SEM pictures of raw diatomite sample (LJ-a1 ×500 and LJ-a2×1000) and purified 337

diatomite sample (LJ-b1 ×800 and LJ-b2×1500) from Linjiang. 338

Fig. 7. SEM pictures of raw diatomite sample (HD-a1 ×500 and HD-a2×1000) and purified 339

diatomite sample (HD-b1 ×1000 and HD-b2×1500) from Huade. 340

Fig. 8. SEM pictures of raw diatomite sample (SX-a1 ×500 and SX-a2×1000) and purified 341

diatomite sample (SX-b1 ×1000 and SX-b2×1500) from Shengxian. 342

Fig. 9. Infrared emission spectra of three raw diatomite samples (LJ-R, HD-R and SX-R), 343

centrifugal sediment samples (LJ-S, HD-S and SX-S) and centrifugal overflow samples (LJ-S, 344

HD-S and SX-S). 345

Fig. 10. Nitrogen adsorption-desorption isotherms of three raw diatomites and different products 346

obtained after purification, (a) LJ-DE, (b) HD-DE, (c) SX-DE. 347

Fig. 11. Relationship between zeta potential and pH value at different concentrations of SHMP, 348

(a) diatomite, (b) kaolin, (c) montmorillite 349

350

15

351

Fig. 1. SEM pictures of pure diatomite sample, (a)×600 and (b)×1500. 352

353

16

354

Fig. 2. Proposed flowsheet for the purification of low grade diatomites. 355

356

17

357

Fig. 3. Relationship between the SiO2 content and pH value in purification of three 358

diatomites. 359

360

Fig. 4. Relationship between the SiO2 content and the dosage of SHMP in purification of 361

three diatomites. 362

18

363

364

Fig. 5. XRD patterns of three raw diatomites and different products obtained after 365

purification, (a) LJ-DE, (b) HD-DE, (c) SX-DE. 366

19

367

Fig. 6. SEM pictures of raw diatomite sample (LJ-a1 ×500 and LJ-a2×1000) and purified 368

diatomite sample (LJ-b1 ×800 and LJ-b2×1500) from Linjiang. 369

370

20

371

Fig. 7. SEM pictures of raw diatomite sample (HD-a1 ×500 and HD-a2×1000) and purified 372

diatomite sample (HD-b1 ×1000 and HD-b2×1500) from Huade. 373

374

21

375

Fig. 8. SEM pictures of raw diatomite sample (SX-a1 ×500 and SX-a2×1000) and purified 376

diatomite sample (SX-b1 ×1000 and SX-b2×1500) from Shengxian. 377

22

378

Fig. 9. Infrared emission spectra of three raw diatomite samples (LJ-R, HD-R and SX-R), 379

centrifugal sediment samples (LJ-S, HD-S and SX-S) and centrifugal overflow samples (LJ-S, 380

HD-S and SX-S). 381

382

23

383

384

385

Fig. 10. Nitrogen adsorption-desorption isotherms of three raw diatomites and different 386

products obtained after purification, (a) LJ-DE, (b) HD-DE, (c) SX-DE. 387

24

388

389

25

390 Fig. 11. Relationship between zeta potential and pH value at different concentrations of 391

SHMP, (a) diatomite, (b) kaolin, (c) montmorillite. 392

26

LIST OF TABLES 393

Table 1. Main advantages and disadvantages of different diatomite purification methods [8-11]. 394

Table 2. Chemical compositions of three raw diatomite samples (wt. %). 395

Table 3. BET specific surface area (SBET), pore volume (Vp) and pore diameter for raw 396

diatomites and products after purification. 397

27

Table 1. Main advantages and disadvantages of different diatomite purification methods [8-11] 398

Techniques Main advantages Main disadvantage

Acid leaching treatment Good removal of impurities

High cost; Release of a large

amount of acid wastewater

causing environmental problems

Scrubbing method Simplicity of application Not effective for purification of all

raw diatomite

Classification Simplicity of application Low efficiency; Difficult for large

scale industrial application

Calcination Simplicity of application; good

removal of organic matters

High energy consumption;

Incapable of removal of some

impurities, such as clay minerals

Gravitational sedimentation Simplicity of application

Low efficiency; Large water

consumption; Rigorous weather

conditions

Ultrasound treatment

Effective purification at lab scale;

Good removal of impurities

embedded in the porous structure of

diatom

Difficult for large scale industrial

application

Magnetic separation Good removal of magnetic impurities Incapable of removal of

nonmagnetic substance

Flotation Good removal of impurities

High cost; Release of a large

amount of wastewater causing

environmental problems because

of the addition of the flotation

reagents

Grinding

Good removal of impurities

embedded in the porous structure of

diatom

Undesirable damage of original

porous structure of diatom

399

400

28

Table 2. Chemical compositions of three raw diatomite samples (wt. %). 401

Samples SiO2 Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O P2O5 L.O.I.a

LJ-DE 81.78 5.49 2.01 0.20 1.05 0.52 1.07 0.29 0.075 6.89

HD-DE 76.60 10.13 3.36 0.26 0.89 0.91 1.07 0.45 0.024 7.00

SX-DE 66.88 17.49 2.90 0.21 0.38 0.17 1.07 0.32 0.035 8.78

Pure DE 96.47 0.031 0.12 0.013 0.10 0.05 0.02 0.032 0.021 2.79

Pure kaolin 54.55 34.41 0.50 0.69 0.34 0.05 0.012 0.12 0.099 11.94

Pure montormolite 66.86 15.07 3.06 0.13 1.68 3.09 0.59 2.18 0.003 7.33

a Loss on ignition 402

29

Table 3. BET specific surface area (SBET), pore volume (Vp) and pore diameter for raw 403

diatomites and products after purification. 404

Sample ID SBET (m2 g-1) Vpa (cm3 g-1) BETb (nm)

Raw diatomite (LJ-DE) 22.182 0.061679 12.5903

Centrifugal overflow(LJ-DE) 49.6763 0.086132 10.1253

Centrifugal sediment(LJ-DE) 11.7082 0.039341 21.0529

Raw diatomite (HD-DE) 38.0799 0.055558 6.7341

Centrifugal overflow(HD-DE) 42.4162 0.087123 9.2536

Centrifugal sediment(HD-DE) 8.0296 0.030652 17.1252

Raw diatomite (SX-DE) 28.1562 0.062092 8.0125

Centrifugal overflow(SX-DE) 41.2365 0.082122 9.1253

Centrifugal sediment(SX-DE) 18.5692 0.048569 13.5624

a BJH desorption cumulative pore volume of pores between 1.7 and 300 nm in diameter. 405

b Adsorption average pore diameter (4V/A by BET) 406