Improved dewatering of nickel laterite ore slurries using superabsorbent polymers

APT 1341

No. of Pages 9, Model 5G

20 July 2016 Advanced Powder Technology xxx (2016) xxx–xxx 1

Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

2

Original Research Paper

6 4 7 5

Improved dewatering of nickel laterite ore slurries using superabsorbent polymers

8

Sophia Joseph-Soly a, Ataollah Nosrati b, Jonas Addai- Mensah a,⇑

9 10 12 11 13 1 2 5 8 16 17 18 19 20 21 22 23 24 25 26 27

a b

Future Industry Institute, University of South Australia, Mawson Lakes, SA 5095, Australia School of Engineering, Edith Cowan University, Joondalup, WA 6027, Australia

a r t i c l e

i n f o

Article history: Received 3 May 2016 Received in revised form 29 June 2016 Accepted 5 July 2016 Available online xxxx Keywords: Enhanced dewatering Compact consolidation Sediment water recovery Nickel laterite Superabsorbent

a b s t r a c t High sedimentation rates, good supernatant clarity and compact consolidation of valuable mineral slurries and waste tailings are the main requirements for effective dewatering. The current conventional flocculant-mediated and gravity-assisted thickening processes used in industry are far from being efficient in terms of maximising pulp water recovery. The present work investigates an unconventional approach using anionic, highly cross-linked polyacrylate, superabsorbent polymer to dewater slurries of three, unflocculated and flocculated, low-grade nickel (Ni) laterites ores (goethetic, siliceous goethite and saprolitic). The superabsorbent (SAB) sealed in a water permeable polyester bag was applied over 24 h contact time at 1–5 wt.% dosage to dilute (2–8 wt.% solid) and 20–25 wt.% solid, self-settled and polyacrylamide (PAM, 0–400 g/t solid) flocculated sediments generated at 10
3 – 2 M solution ionic strength, pH 2.5–10.5 and 25 °C. The results showed that SAB water absorption of 80–90 wt.% occurred within 8 h, reflecting sediment consolidation to 40–55 wt.% solid. SAB water recovery capacity was maximum at dosages of 3 and 2 wt.%, respectively, for dilute and concentrated slurries and at pH 6.0–7.0. The SAB-mediated pulp dewaterability, however, decreased markedly with increasing ionic strength and smectite clay mineral content. Slurries’ pre-flocculation at up to 400 g PAM/t solid had a noticeable impact on SAB’s dewatering efficacy which decreased appreciably with increasing flocculant dosage. The regeneration and recycle trials of the superabsorbent showed that there is unique opportunity for multiple use and greater pulp water recovery with a given sample. Both the water absorption and release capacities, however, decreased steadily with increasing number of superabsorbent recycles. Crown Copyright Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. All rights reserved.

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

51 52

1. Introduction

53

As high grade ore deposits are rapidly exhausted, the mineral industry is forced to increasingly exploit lower grade ores, the processing of which is achieved through extractive hydrometallurgical processes. The voluminous amounts of the ore and water processed result in equivalently large quantities of dilute waste tailings [5,30]. Lateritic ores containing clay minerals, such as kaolinite and smectite, in tailings pose significant technical, socio-economic and environmental problems associated with the treatment and disposal [1,16,19,20,29,31]. The presence of clays even as a minor component in mineral ores, can cause serious dewatering and handling problems in terms of gelation or swelling and space-filling ‘‘card house” structures. The resulting high yield stress accompanied by high flocculant demand, low settling rates

54 55 56 57 58 59 60 61 62 63 64 65

⇑ Corresponding author. E-mail address: [email protected] (J. Addai- Mensah).

and poor supernatant clarity are some of the typical challenges [1]. Polyacrylamide-chemistry based flocculants and gravity thickening and pressure filtration processes are currently used by the minerals industry for dewatering. Such processes are however, far from being efficient in terms of compact consolidation of the sediments despite the numerous studies performed and significant advances in thickener technology and flocculant structure and chemistry design [2]. Thus to date compact consolidation still remains a major persistent challenge in dewatering. The capital and operational costs associated with tailings disposal are significant in terms impoundment dam construction, management and remediation and the loss of several millions of tonnes of entrapped processed water and valuable reagents. Reagent loss and seepage also pose important environmental risk and concern. Therefore, the sustainability of the mining industry is becoming more dependent on improved dewatering methods for waste tailings treatment and some upstream value mineral slurries.

http://dx.doi.org/10.1016/j.apt.2016.07.010 0921-8831/Crown Copyright Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83

APT 1341

No. of Pages 9, Model 5G

20 July 2016 2 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149

S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

Usually, flocculation and primary dewatering processes are optimized for fast settling rates with limited attention of the subsequent consolidation. Various studies [12,17,19,21,31] reported that some novel dewatering methods that provide a strategy for improving pulp consolidation efficiency subsequent to fast settling using stimulus responsive polymeric flocculants. Several studies involving high molecular weight polyethylene oxide (PEO) flocculant have shown that sedimented pulps exhibit significant compaction upon the application of moderate shear rates (10–42 s
1) [17–19,31], in contrast to conventional PAM-based sediments. As thickener operations involve a range of shear rates (0.1–50 s
1) in the consolidation zone [18,28], shear-enhanced consolidation is clearly a desirable sediment characteristic. Other recent studies [8,12,21,26,32] reported consolidation improvement using temperature-responsive flocculant (pNIPAM) and its associated hydrophobic interaction mechanism. Temperature is used as stimulus to change the inter-particle forces from repulsion to attraction to form aggregates at above the LCST (lower critical solution temperature). Upon cooling the sediment below the LCST, the polymer molecules desorb and go back into solution. This results in reduced inter-particle and inter-floc attractions within the sediment, inducing further consolidation. Furthermore, pH sensitive polymers such as chitosan can also be used as flocculants [7,9,13,24]. Through pH modifications, alternating attractive and repulsive chitosan-mediated pulp particle interactions may be invoked judiciously, fostering both fast settling and compact consolidation of the sediment. SAB polymers can absorb and retain huge amounts of water relative to their own masses [33]. They are hygroscopic materials which entrap water via capillary forces in their macro-porous structure and through the hydration of functional groups [33]. One of the most important characteristics of SAB is their particles shape preservation upon swelling, which is a significant advantage over other hydrogels. This characteristic allows SAB to resist internal pressure that will force the release of water from the hydrogel. Unlike SAB, other materials such as tissue papers and polyurethane foams will lose most of their absorbed water when squeezed [33]. The swelling behaviour of SAB depends on its type and the solvent. The number of crosslinks in a polymer chain will affect the expansion of the polymer so that more crosslinks result in less swelling. The other factor is the amount of ions and organic components present in the solvent, which affect the electrical stability of the polymer chain, thus reducing the swelling capacity [27]. The ability of SAB to specifically absorb water and release it upon modification offers new possibilities for effective dewatering. Dzinomwa et al. [10] exploited this concept for fine coal dewatering using pH and temperature sensitive superabsorbent polymers. They showed that it was possible to dewater fine coal from a moisture content of 29.4% to 12–14% using pH-sensitive superabsorbent polymer (2 wt.%) within 4 h contact time. Peer and Venter [22] also reported that it is possible to decrease moisture content in fine coal by 70% using SAB at 2 wt.% dosage. Farkish and Fall [11] demonstrated the potential of a SAB polymer in the consolidation of mature fine tailings (MFT) to produce a spadeable slurry of 80 wt.% solid with close to 9 kPa of undrained shear strength gained by using a dosage of 3 wt.% SAB. The SAB recycling can be obtained by temperature- and pH-induced regeneration [10]. Along with the regeneration, hydrogel released water can also be recycled. As water is pervasive in mineral tailings, obviously opportunities exist in the development and application of high-capacity and regenerative water SAB as a feasible dewatering method. Efficient dewaterability of complex laterite ore slurries is not straight forward. Sedimentation rates are affected by the fineness of the grind, the clay fraction, non-clay fine materials and the chemistry of the pore water [4]. In a study with pre-reduced nickeliferous laterite suspension, Anastassakis [3] reported that the cationic

flocculants are more effective among anionic, cationic and nonionic flocculants probably due to the negative charge of the minerals particularly at high pH. The settling rate increased with increasing polymer dosage reflecting increase polymer bridging forces. However the resulting sediment consolidation was mediocre. For the recovery of valuable metals (Ni, Co) from fine gained laterite mineral slurries, it is crucial to dewater the dilute slurries from milling and beneficiation process prior to subsequent processing such as high and atmospheric pressure acid leaching and agglomeration for heap leaching [23]. The main aim of this study is to investigate the pivotal role of the key primary variables on the dewatering behaviour of three complex, low grade laterite mineral dispersions for significant water recovery and concomitant sediment consolidation enhancement using superabsorbent as a dewatering agent. Specifically, primary variables affecting the absorption capacity of superabsorbent comprising pH, solution ionic strength, superabsorbent dosage, solid loading, feed mineralogy and time, were investigated. Furthermore, pH-induced SAB regeneration was tested for SAB and released water recycling from the absorbed water.

150

2. Materials and methods

169

2.1. Materials

170

Three types of complex low grade Ni laterite ores (Siliceous Goethite (SG), Goethitic (G) and Saprolitic (SAP)) from the same Western Australia deposit were used in this study. The
38 lm fractions obtained from rod milling the
15 mm run of mine (ROM) ores were used. The 10th, 50th and 90th percentile particle sizes, D10, D50 and D90 respectively, determined by laser diffraction (Malvern Mastersizer X, Malvern UK), were 2, 10, and 52 lm, respectively (Fig. 1A). A summary of chemical and mineralogical characteristics for
38 lm ore samples are given in Tables 1 and 2, respectively [23]. The mineralogical characteristics revealed that goethite, hematite, smectite, kaolinite and quartz are the major minerals present in the samples. Feed slurries at solid loadings 2, 4 and 8 wt.% were prepared by mixing known amount of the laterite ore sample with known amount of 10
3 M KNO3 solution as a background electrolyte. An anionic SAB polymer (Superabsorbent Polyacrylate, BASF Ltd.) was used in this work. It is sodium polyacrylate, made from sodium salt cross-linked with polyacrylic acid. Its particle size distribution is shown in Fig. 1B and was determined by sieving using a mechanical shaker (AS/NZS3760 Shaker). The SAB particles are in 100–600 lm size range, making the markedly coarser than laterite particles. Dzinomwa et al. [10] reported that for granules size >8 mm, the rate of water absorption is too slow and for those smaller than 2 mm, the rate of absorption is fast. High molecular wt. (3–4  106 Da), anionic polyacrylamide copolymer (Rheomax 1010, BASF Australia) was used to produce flocculated sediment for secondary dewatering tests. The flocculant solution was prepared as stated by the supplier’s instructions. Precisely 0.25 vol.% of flocculant was mixed with 5 vol.% of acetone and 94.75 vol.% of Milli-Q water (surface tension 72.8 mN m
1, specific conductivity 0.5 lS cm
1 and pH 5.6 at 20 °C) using a suspension mixer (Ratek, Rowe Scientific Pty Ltd) for 90 min to ensure homogeneity. All slurries were prepared using 10
3 M KNO3 solution, unless stated otherwise. All test were replicated at least, 3 times and the arithmetic mean values of measurements and their cognate 95% CI standard errors reported.

171

2.2. Methods

207

2.2.1. Direct method In the studies, two types of SAB based dewatering methods were investigated. The first was a direct method, where a known

208

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168

172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206

209 210

APT 1341

No. of Pages 9, Model 5G

20 July 2016 3

S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

B

100 SG SAP G

80

60

40

20

0

0.1

100

Cumulative undersize (%)

Cumulative undersize (%)

A

1

10

80

60

40

20

0

100

0

Particle size ( µ m)

200

400

Particle size ( µm)

600

Fig. 1. Particle size distribution of (A) laterite particle measured by laser diffraction and (B) superabsorbent particles measured by sieving.

Table 1 Summary of chemical analysis results of
38 lm Ni laterite samples [23]. Element

Goethitic

Siliceous goethitic

Saprolitic

Ni (%) Co (%) Mg (%) Fe (%) Mn (ppm) Zn (ppm) Cu (ppm) Al (%) Cr (%) Ca (%) Si (%) Cl (ppm)

1.09 0.032 0.32 43.0 3100 500 475 5.34 1.47 0.07 4.6 400

1.4 0.066 4.22 23.1 2300 370 300 3.3 0.84 0.4 18.1 300

1.08 0.025 4.76 22.7 1170 400 380 3.93 1.09 0.5 17.3 300

Table 2 Summary of major mineral deportment in
38 lm Ni laterites samples according to QXRD analysis [23].

211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228

Mineral

Goethitic

Siliceous goethitic

Saprolitic

Goethite (%) Hematite (%) Chromite (%) Quartz (%) Kaolinite (%) Serpentine (%) Smectite (%) Other (%)

70 11 3 1 8 1 1 5

28 2 3 9 3 6 23 26

18 5 2 1 1 5 34 34

amount of superabsorbent was directly mixed with a known amount of laterite dispersion in a container. The mixing was performed by using an overhead stirrer at 200 rpm for 10 min to ensure suspension homogeneity. Various SAB masses were used to prepare the mixtures as 1, 2 and 3 g SAB/100 g slurry of 2, 4, and 8 wt.%, of laterite dispersions in 10
3 M KNO3 background electrolyte at pH 6.0. The containers of the SAB-laterite slurries were then sealed to prevent evaporation. After predetermined contact time of 4 h, there was an attempt to separate the dewatered laterite particles from the swollen SAB hydrogel using sieve screening process based upon particle size. This step was a challenge due to attachment of the laterite particles to the surface of the swollen SAB granules. So the direct method was not successful for the retrieval of water from the superabsorbent hydrogel. 2.2.2. Indirect method The second method, the indirect method [22] involved placing the SAB inside sachets made of water-permeable polyester cloth which served as a barrier between the slurry and the polymer.

Sachets (17  8 cm) containing various amounts of SAB were placed into 200 g of laterite slurries held in well-sealed containers to prevent evaporation. These sachets are made of 100% polyester textile with 37 lm pore size, which allows the SAB to absorb water from the slurry without direct contact with the mineral particles. The use of SAB in sachets facilitated its separation from the laterite slurry after dewatering, enabling its recycling after regeneration. In the case of dilute slurries dewatering, sachets with dosages of 1, 2, 3, 4 and 5 g SAB/100 g of slurry were used. Laterite slurries of 2, 4 and 8 wt.% solid loading were prepared at pH 6.0 as described above. In SAB application for secondary dewatering of concentrated (20–25 wt.% solid) slurries, self-settled and flocculated sediments formed from 8 wt.% solid slurries at pH 2.5, 6.0, 7.0 and 10.5 and 25 °C were used. Flocculation was performed on 8 wt.% solid goethetic laterite slurries at PAM dosages of 0–400 g/t solid. The resulting sediments were subsequently dewatered at SAB dosage of 1 g/100 g slurry. The mass of each SAB sachet was weighed before contact with the dilute and concentrated slurries over 24 h. After 30 min, 1 h, 4 h, 8 h and 24 h, the wet SAB sachets were removed and weighed. A spatula was used to carefully clear any particulate matter from the slurries which may adhere to the sachets. The mass percent water absorption (S) by SAB after various contact time was calculated using Eq. (1) below.

S¼

y
m1  100 w

229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251

252

ð1Þ

254

where y is mass of the wet sachet and SAB after absorption with respect to various contact times, m1 mass of dry sachet with SAB before absorption and w is total mass of water initially in the slurry.

255

2.2.3. Recovery of absorbed water and SAB regeneration Recovery of the absorbed water from SAB was achieved by pH controlled regeneration. The pH of the swollen SAB was reduced to 1.0 using 37% HCl. The mass of the acid used was approximately 0.01 times of the absorbed water. During regeneration, while the polymer was kept inside the sachets, it started to release the absorbed water by the addition of the acid. After 30 min, 1 h, 4 h, 8 h and 24 h the SAB sachets were removed and weighed. An equilibrium level between the released and residual water was generally reached after 6 h. For water recovery, the mass percent water desorption (D) was calculated using the following Eq. (2).

258

Smax
m2 D¼  100 w

256 257

259 260 261 262 263 264 265 266 267 268

269

ð2Þ

where Smax is maximum water absorbed by SAB over 24 h contact time, m2 is mass of sachet with residual water at the recovery time

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

271 272 273

APT 1341

No. of Pages 9, Model 5G

20 July 2016 4

S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

100 80

B G SAP SG

Water absorption (wt.%)

Water absorption (wt.%)

A

60 40 20 0

1

2

3

4

100 90 80 70 G SAP SG

60 50

5

0

10

20

30

40

Contact time (h)

Superabsorbent dosage (wt.%)

Water absorption (wt.%)

A

100

80

60 G SAP SG

40

20

B

50

Solid sediment content (wt.%)

Fig. 2. Water absorption from laterite slurries as a function of (A) SAB dosage (1, 2, 3, 4 and 5 g SAB/100 g slurry) and (B) contact time at SAB dosage of 3 g/100 g slurry.

40

30 G SAP SG

20

10

0

0 8 wt. %

4 wt. %

8 wt. %

2 wt. %

4 wt. %

2 wt. %

Fig. 3. (A) Water absorption and (B) final consolidation behaviour after 24 h as a function of solid loading for 2–8 wt.% laterite slurries with SAB dosage of 3 g/100 g slurry at pH 6.

100

279

3. Results and discussion

280

3.1. Water absorption capacity of SAB polymer in dilute laterite suspensions

275 276 277

281 282 283 284 285 286 287 288 289 290 291 292 293 294 295

3.1.1. Effect of SAB dosage Fig. 2A displays the effect of SAB dosage on dewatering behaviour of 4 wt.% G, SG and SAP laterite suspensions at pH 6.0. Water absorption with respect to time showed a sharp increase over the initial 6–8 h contact time, reaching a maximum (Fig. 2B). The maximum wt.% water absorbed increased with SAB dosage up to 3 g SAB/100 g slurry and plateaued thereafter as shown in Fig. 2A. At dosages of 3 and 4 g SAB/100 g slurry, 95 ± 1, 92 ± 1 and 94 ± 1 wt.% of water were removed from the G, SG and SAP slurries, respectively (Fig. 2A). The water absorbed at 5 g SAB/100 g slurry was marginally lower. These water recoveries are 10–20% higher when compared with the values for SAB dosages 1 and 2 g/100 g slurry, in good agreement with the literature [10,11,22]. The absorbency at 5 g SAB/100 g slurry dosage decreased slightly due to the

10-3 M 0.1 M 1M 2M

80

Water absorbed (wt.%)

278

and w is initial mass of water in the slurry. In order to neutralize the effect of the acid and, the SAB was washed with 1 M NaOH solution. Thereafter, it was rinsed with superfluous amount of Milli-Q water to pH 5.5 and then oven dried at 50 °C overnight to remove any water.

274

60

40

20

0 KNO3

NaNO3

NaCl

Fig. 4. Maximum water absorbed from 8 wt.% G-laterite slurry at SAB dosage of 3 g/ 100 g slurry in different concentration of electrolytes as a function of 3 different salts at pH 6.

higher self-crosslinking tendency which can cause a shorter spacing of the network [14]. From the water recoveries (Fig. 2A), it was decided that a SAB dosage 3 g/100 g slurry would be the optimum value for maximum water absorption from unflocculated, dilute laterite slurries.

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

296 297 298 299 300

APT 1341

No. of Pages 9, Model 5G

20 July 2016 5

S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

80

B

75 70 65 60

pH 2.5 pH 6.0 pH 7.0 pH 10.5

55 50

60

Initial Self settled SAB cons.

50

Solid loading (wt.%)

Water absorption (wt.%)

A

0

3

6

9

12

15

18

21

40 30 20 10 0

24

pH 2.5

pH 6.0

pH 7.0

pH 10.5

Contact time (h) Fig. 5. (A) Water removed from self-settled G laterite slurry as a function of time and (B) subsequent sediment consolidation behaviour by using SAB dosage of 1 g/100 g slurry at different pH.

1 wt.% 2 wt.% 3 wt.%

80

60

40

20

0

B

0

50

100

200

300

400

PAM concentration (g/t solid)

60

1 wt.% 2 wt.% 3 wt.%

50

Solid loading (wt.%)

Water absorption (wt.%)

A

40 30 20 10 0

0

50

100

200

300

400

PAM concentration (g/t solid)

Fig. 6. (A) Water absorption and (B) final sediment solid loading of G laterite slurries at different SAB dosages and pH 7.0 as a function of PAM concentration. 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325

326 327 328

3.1.2. Effect of slurry solid loading and mineralogy The effect of slurry solid loading and mineralogy on SAB water absorption and dewatering behaviour of dilute dispersions of the 3 types of laterite ores was analysed at SAB dosage of 3 g/100 g slurry and pH 6. Taking into account that the pristine pH of G, SG and SAP laterite slurries were 5.8, 7.8 and 8.3, respectively, the reasons for selection of pH 6 for these tests were (i) minimizing slurry pH alterations prior each test and (ii) producing comparable results (SAB water absorption efficiency) for three slurries. Fig. 3 shows the results for slurries at 2, 4 and 8 wt.% solid loading after 24 h contact time. They indicate that water absorption capacity was inversely proportional to slurry solid loading with a decrease of 8–10% for solid loading increase from 2 to 8 wt.% (Fig. 3A). The results of final sediment solid content (Fig. 3B) indicate that the SAB dewatering method is effective, producing higher solid loading slurries. The maximum final solid contents achieved with G, SAP and SG slurries at 8 wt.% solid loading were 47 ± 1, 40 ± 1 and 44 ± 1 wt.%, respectively and 48 ± 1, 35 ± 1 and 43 ± 1 wt.%, respectively at 4 wt.%. This shows the effect of ore mineralogy on water absorption where the presence of clay minerals (smectite and kaolinite) impacts deleteriously on the dewatering behaviour. Thus, the SAP slurry containing 34% smectite showed less dewaterability when compared with G and SG laterites containing 23 and 1% smectite, respectively. 3.1.3. Effect of ionic strength SAB Water absorption behaviour for G laterite slurries in various salt (KNO3, NaNO3 and NaCl) solutions with 0.1, 1.0, and

2.0 M concentrations, and SAB dosage of 3 g/100 g slurry was investigated at pH 6.0 (Fig. 4). Evidently, the SAB water absorbency decreased with increasing salt solution ionic strength. Furthermore, insignificant dependency of the salt type was shown in Fig. 4. The overall results are in good agreement with the literature; the greater the ionic strength of the external solution, the lower the absorption capacity of the SAB [25]. Reported studies suggest that the anionic SAB swelling loss is often attributed to a ‘‘charge screening effect” of the high ionic strength and additional cations preventing anion-anion electrostatic repulsion. Consequently, the osmotic pressure due to the mobile ion concentration difference between the SAB and aqueous phases is decreased, and hence the swelling capacity is reduced. Staples and Chatterjee [27] reported that the amount of ions and organic components present in the solvent may also affect the electrical stability of the polymer chain, thus reducing its swelling capacity.

329

3.2. Secondary dewatering using SAB

345

3.2.1. Effect of pH The absorbency of SAB from 20–25 wt.% solid, pre-sedimented (self-settled) G laterite slurries was studied at pH 2.5, 6.0, 7.0 and 10.5. The study showed that the SAB was less sensitive to pH and the maximum absorbency obtained was 77 ± 1, 75 ± 1, 74 ± 1 and 73 ± 1 wt.%, respectively, for pH 7.0, 6.0, 2.5 and 10.5 (Fig. 5). Fig. 5A shows the variation in absorbency of 1 g SAB/100 g slurry in the self-settled sediment. Apart from the noticeably higher value observed at pH 7 there was no significant pH-dependency of SAB water absorbency in the pH range investigated and this is in agree-

346

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

330 331 332 333 334 335 336 337 338 339 340 341 342 343 344

347 348 349 350 351 352 353 354 355

APT 1341

No. of Pages 9, Model 5G

20 July 2016 S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

75 70

B Solid loading (wt.%)

A Water absorption (wt.%)

6

65 60 pH 2.5 pH 6.0 pH 7.0 pH 10.5

55 50

0

3

6

9

12

15

18

21

60

Initial PAM flocc. SAB cons.

50 40 30 20 10 0

24

pH 2.5

pH 6.0

pH 7.0

pH 10.5

Contact time (h) Fig. 7. (A) Water removed from flocculated (200 g PAM/t solid) G laterite slurries (18–23 wt.% solid) as a function of time and (B) subsequent sediment consolidation behaviour following treatment with SAB dosage of 1 g/100 g slurry at different pH values.

80

B

pH 2.5 pH 6.0 pH 7.0 pH 10.5

70

Water absorption (wt.%)

Water absorption (wt.%)

A

60

50

40

1

2

3

4

80

70 65 60 55 50 45 40

5

pH 2.5 pH 6.0 pH 7.0 pH 10.5

75

1

2

Number of cycles 60 55

Solid loading (wt.%)

D

pH 2.5 pH 6.0 pH 7.0 pH 10.5

50 45 40 35 30 25

4

60

5

pH 2.5 pH 6.0 ph 7.0 pH 10.5

55

Solid Loading (wt.%)

C

3

Number of cycles

50 45 40 35 30

1

2

3

4

5

Number of cycles

25

1

2

3

4

5

Number of cycles

Fig. 8. Maximum water absorbed by SAB (A and B) and consolidation behaviour (C and D) with increasing number of absorb-desorb cycles for sedimented G laterite slurry with no PAM (A and C) and 200 g PAM/t solid (B and D). 356 357 358 359 360 361 362

363 364 365 366

ment with the literature [6,15]. The final consolidation behaviour of G laterite slurries is displayed in Fig. 5B. It is observed that the solid loading is grater at pH 6 and 7 than at pH 2.5 and 10.5. Interestingly, the G-laterite sample has an isoelectric point of pH 6.5. It appears that the lack of significant electrical double layer repulsion between the particles at pH 6 and 7 facilitated greater inter particle attraction and enhanced dewaterability. 3.2.2. Effect of flocculant and superabsorbent dosages Fig. 6 shows SAB water absorption and final consolidation behaviour of sedimented goethitic laterite slurries pre-flocculated at 0, 50, 100, 200, 300 and 400 g PAM/t solid and further dewatered

with 1, 2, and 3 g SAB/100 g slurry at pH 7.0. It is worth noting that using pH 7 for these experiments was due to maximum SAB water absorbency observed at this pH (Fig. 5). Both water absorption (Fig. 6A) and final sediment solid content (Fig. 6B) showed a negligibly small dependence on flocculant dosage at all SAB dosages. The slight decrease in both absorbed water and sediment solid loading with increasing flocculant dosage is due to water solvation and space filling nature of the PAM polymer which is hydrophilic. The results also indicate that regardless of the flocculant dosage, SAB water absorption capacity and the resulting sediment consolidation followed a dosage sequence: 2 g/100 g slurry >1 g/100 g slurry >3 g/100 g slurry. Based on the closeness of the good dewa-

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

367 368 369 370 371 372 373 374 375 376 377 378

APT 1341

No. of Pages 9, Model 5G

20 July 2016 7

S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

120

Water abs/des (wt.%)

100 80 G:Abs G Des SAP:Abs SAP:Des SG:Abs SG:Des

60 40 20 0

8

4

2

Slurry solid loading (wt.%) Fig. 9. Water absorption (Abs) and desorption (Des) behaviour of SAB from three laterite slurries as a function of initial solid loading of dilute (pristine) slurries.

381

382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405

3.2.3. Effect of pH and flocculation Fig. 7A shows water absorption capacity of SAB for flocculated slurries (18–23 wt.% solid) as a function of pH. The highest water absorption is observed at pH 6.0, followed by pH 7.0 and 10.5. This observation suggests that the anionic polymer bridging forces and intra-floccules’ polymer-particle-water network structures formed are more opened at pH 6.0, and hence offer less resistance to SAB absorption of trapped and free pore water. There was a significant decrease in the water absorption at pH 2.5. This is believed to be due to the fact that at pH 2.5 (
100

Abs Des

80 60 40 20 0 pH 2.5

406

3.3. Recycle and dewatering capacity of regenerated SAB

408

Successful SAB regeneration is essential to economic feasibility of this non-conventional dewatering process. It allows the cost of SAB per unit mass of water removed to be reduced dramatically and hence, rendering the process cost-effective when compared with conventional methods. Fig. 8 shows the effect of the number of SAB recycle on water absorption behaviour/capacity, with and without the presence of PAM flocculant (A, B) and final consolidation behaviour (C, D), as a function of pH. There was a steady decrease in the water absorption capacity and hence, sediment solid loading as the number of cycles increased. After the fifth recycle the SAB absorbed 48–60 wt.% and 42–62 wt.% of water, respectively, from the self-settled (Fig. 8A) and flocculated slurries (Fig. 8B), leading to final solid contents in range 30–45 wt.% (Fig. 8C and D). This is in good agreement with the studies by Peer and Venter [22], which indicated that the absorption capacity of the SAB (in bags) decreased with increasing in water uptakeelution cycles as dissolved ions and substrate surface contamination were affecting the absorption process. Furthermore, there was a noticeable effect of pH on the SAB recycling and efficiency. In self-settled sediments, the differences in water absorption between first and fifth cycles were 23 ± 1, 17 ± 1, 20 ± 1 and 25 ± 1 wt.% and 21 ± 1, 26 ± 1, 10 ± 1 and 28 ± 1 wt.% for the flocculated sediments, respectively, at pH 2.5, 6.0, 7.0 and 10.5. Relatively, the SAB dewatering performance over 5 cycles was better at pH 6.0 and 7.0 than at pH 2.5 and 10.5. Fig. 9 shows the absorption and desorption of water for three, dilute laterite slurries (2, 4 and 8 wt.% solid loading) whilst Fig. 10 depicts the result for concentrated, self-settled and PAM flocculated slurries as a function of pH. In all cases desorption is little less than absorption and is in agreement with the literature. The residual water amounting to 5 times of the dry mass of SAB remained regardless of process conditions. This residual water may be explained in terms of ‘bound water’, i.e. water tightly bound to the polymer, which is distinct from free water (less bound) which requires less energy for releasing from SAB [10]. Therefore, the residual water in the SAB was removed by drying at 55 °C in an incubator and thus regained its initial structure.

409

3.4. Conventional versus superabsorbent dewatering method

446

Since the SAB is reusable, after the first use and regeneration, the sachet with dosage 1 g SAB/100 g slurry was used for further four cycles. It is pertinent to note that the PAM flocculant cannot be regenerated after use, hence only the result (Conv) obtained

447

100

0 PAM

Water abs/des (wt.%)

380

tering performances of the 1 and 2 g SAB/100 g slurry, further testing was performed using 1 g SAB/100 g slurry dosage as shown below.

Water abs/des (wt.%)

379

absorbency and the concomitant pulp dewaterability correlated well as expected.

pH 6.0

pH 7.0

pH 10.5

200 PAM

Abs Des

80 60 40 20 0

pH 2.5

pH 6.0

pH 7.0

pH 10.5

Fig. 10. Maximum water absorption and desorption (recovery) behaviour of SAB from (left) self-settled and (right) PAM flocculated (200 g/t solid) slurries at different pH.

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

407

410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445

448 449 450

APT 1341

No. of Pages 9, Model 5G

20 July 2016 S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx

Water recovered (kg)

A

4000

3000

pH 2.5 pH 6.0 pH 7.0 pH 10.5

2000

1000

B

4000

Water recovered (kg)

8

3000

2000

1000

0

0

1 2 3 4 5 nv Co Cycle Cycle Cycle Cycle Cycle

pH 2.5 pH 6.0 pH 7.0 ph 10.5

1 2 3 4 5 nv Co Cycle Cycle Cycle Cycle Cycle

Fig. 11. Water recovered from 1 ton of (A) unflocculated and (B) PAM flocculated G laterite slurries (25 wt.% solid) using the SAB dosage of 1 g/100 g slurry as a function of pH. The data for conventional PAM flocculated (Conv) sediments are also shown.

474

from the dewatering test at 200 g PAM/t solid is available for comparison with the cumulative (5 cycles) SAB dewatering results. Fig. 11 shows the cumulative water recovery from 1 t of 25 wt.% solid G laterite slurries without (A) and with (B) the addition of PAM flocculant using 1 wt.% SAB. Substantially similar water recovery behaviour are shown for both unflocculated and flocculated sediments over 5 cycles. The recycled SAB continued to further absorb water from pre-sedimented slurries and hence its cumulative amount of water recovered increased monotonically with the number of recycle, albeit with decrease in the water absorption capacity (Fig. 8A and B). The extent water recovery was, again, noticeably greater at pH 6.0 and 7.0 than at pH 2.5 and 10.5. Notably, such a remarkable feat is not achievable when benchmarked with the performance of conventional PAM flocculant-assisted dewatering method. The SAB-based cumulative water recovered after the fifth cycle at pH 6.0 was 3425 kg as opposed to 645 kg with 200 g PAM/t solid. The results show that the SAB application is an effective secondary dewatering method when compared with the other conventional dewatering processes such as flocculant-assisted thickening, filtration and centrifugation. Further investigations for optimization and improvement, in tandem with techno-economic evaluation and analysis, are certainly needed for potential commercial application to hydrophilic mineral slurries.

475

4. Conclusions

476

In this study the feasibility and efficacy of using SAB for the dewatering and densification of fine grained Ni laterite slurries were investigated. The results showed that:

451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473

477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495

 Significant dewatering, reflecting dramatic compact consolidation, can be achieved within 8 h by indirect contact between SAB and pristine and PAM flocculated, hydrophilic mineral slurries. The most conducive conditions for maximum dewaterability of dilute (2–8 wt.% solid) and pre-flocculated (20–25 wt.% solid) slurries were observed at SAB dosages of 3 and 2 wt.%, respectively.  SAB was effective for dewatering high solid loadings (>20 wt.%) slurries, a behaviour which is contrary to that of conventional polymeric flocculant. Hence, it may be used in secondary dewatering applications to achieve compact consolidation and maximum water recovery.  The presence of certain clay minerals in slurry adversely affected the SAB water absorption capacity. Saprolitic ore slurries containing high (34%) smectite clay mineral content displayed lower dewaterability when compared with goethetic and siliceous goethite laterite pulps.

 SAB water absorbency and hydrogel swelling capacity decreased markedly with an increasing solution ionic strength. Solution pH, on the other hand, had a subtle effect with optimum SAB dewatering performance indicated at pH 6–7.  Single SAB led up to 30% more water recovery from the laterite slurries over the amount recovered by using the conventional PAM flocculant-assisted dewatering method. Furthermore, the water absorption efficacy and hence, sediment consolidation, of pre-flocculated slurries achieved with SAB decreased with the increasing flocculant dosage.  SAB regeneration and re-use investigations show that, there is unique opportunity for its multiple recycling and greater water recovery with a given sample. A gradual decrease in the water absorption capacity or efficacy and hence, sediment solid loading, was however observed as the number of SAB recycling increased. Further studies are warranted to overcome this limitation.

496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513

Acknowledgment

514

The authors gratefully acknowledge University of South Australia (APA scholarship), BASF (Thai Ltd) for Superabsorbent Polyacrylate sample and CSIRO (Dr. D. Robinson) for the laterite ores.

515

References

518

[1] J. Addai-Mensah, Enhanced flocculation and dewatering of clay mineral dispersions, Powder Technol. 179 (1) (2007) 73–78. [2] J. Addai-Mensah, J. Ralston, Interfacial chemistry and particle interactions and their impact upon the dewatering behaviour of iron oxide dispersions, Hydrometallurgy 74 (3) (2004) 221–231. [3] G.N. Anastassakis, Physicochemical factors affecting flocculation of prereduced nickeliferous laterite suspension, Sep. Purif. Technol. 45 (1) (2005) 16–24. [4] S. Azam, R.J. Chalaturnyk, J.D. Scott, Geotechnical characterization and sedimentation behavior of laterite slurries, Geotech. Test. J. 28 (6) (2005) 523. [5] C.O. Brawner, D.B. Campbell, The Tailings Structure and Its Characteristics: A Soil Engineers Viewpoint, Golder, Brawner & Associates, Limited, 1972. [6] Y. Chen, Y. Liu, H. Tan, J. Jiang, Synthesis and characterization of a novel superabsorbent polymer of N,O-carboxymethyl chitosan graft copolymerized with vinyl monomers, Carbohydr. Polym. 75 (2) (2009) 287–292. [7] Y.-C. Chung, L.-C. Wu, C.-Y. Chen, The removal of kaolinite suspensions by acidsoluble and water-soluble chitosans, Environ. Technol. 34 (3) (2013) 283–288. [8] Y. Deng, H. Xiao, R. Pelton, Temperature-sensitive flocculants based on poly (Nisopropylacrylamide-co-diallyldimethylammonium chloride), J. Colloid Interf. Sci. 179 (1) (1996) 188–193. [9] R. Divakaran, V.S. Pillai, Mechanism of kaolinite and titanium dioxide flocculation using chitosan—assistance by fulvic acids?, Water Res 38 (8) (2004) 2135–2143. [10] G.P.T. Dzinomwa, C.J. Wood, D.J.T. Hill, Fine coal dewatering using pH- and temperature-sensitive superabsorbent polymers, Polym. Adv. Technol. 8 (12) (1997) 767–772. [11] A. Farkish, M. Fall, Rapid dewatering of oil sand mature fine tailings using super absorbent polymer (SAP), Miner. Eng. 50–51 (2013) 38–47.

519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

516 517

APT 1341

No. of Pages 9, Model 5G

20 July 2016 S. Joseph-Soly et al. / Advanced Powder Technology xxx (2016) xxx–xxx 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577

[12] G.V. Franks, H. Li, J.-P. O’Shea, G.G. Qiao, Temperature responsive polymers as multiple function reagents in mineral processing, Adv. Powder Technol. 20 (3) (2009) 273–279. [13] G.V. Franks, C.V. Sepulveda, G.J. Jameson, PH-sensitive flocculation: settling rates and sediment densities, AIChE J. 52 (8) (2006) 2774–2782. [14] F. Hua, M. Qian, Synthesis of self-crosslinking sodium polyacrylate hydrogel and water-absorbing mechanism, J. Mater. Sci. 36 (3) (2001) 731–738. [15] J. Liu, A. Wang, Study on superabsorbent composites. XXI. Synthesis, characterization and swelling behaviors of chitosan-g-poly (acrylic acid)/ organo-rectorite nanocomposite superabsorbents, J. Appl. Polym. Sci. 110 (2) (2008) 678–686. [16] A. McFarlane, K. Bremmell, J. Addai-Mensah, Improved dewatering behavior of clay minerals dispersions via interfacial chemistry and particle interactions optimization, J. Colloid Interf. Sci. 293 (1) (2006) 116–127. [17] A.J. McFarlane, K. Bremmell, J. Addai-Mensah, Optimising the dewatering behaviour of clay tailings through interfacial chemistry, orthokinetic flocculation and controlled shear, Powder Technol. 160 (1) (2005) 27–34. [18] P. Mpofu, J. Addai-Mensah, J. Ralston, Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int. J. Miner. Process. 71 (1) (2003) 247–268. [19] P. Mpofu, J. Addai-Mensah, J. Ralston, Flocculation and dewatering behaviour of smectite dispersions: effect of polymer structure type, Miner. Eng. 17 (3) (2004) 411–423. [20] J.V. O’Gorman, J.A. Kitchener, The flocculation and de-watering of kimberlite clay slimes, Int. J. Miner. Process. 1 (1) (1974) 33–49. [21] G. Qiao, G.V. Franks, J.P. O’Shea, The influence of addition method on the flocculation of model suspensions by Poly(N-isopropyl Acrylamide) and derivatives, in: Proceedings of the CHEMECA 2008 Conference, Engineers Australia, 393–402, Newcastle, Australia, 2008. [22] F. Peer, T. Venter, Dewatering of coal fines using a super absorbent polymer, J. South Afr. Inst. Min. Metall. 103 (6) (2003) 403–409.

9

[23] K. Quast, J.N. Connor, W. Skinner, D.J. Robinson, J. Addai-Mensah, Preconcentration strategies in the processing of nickel laterite ores Part 4: preliminary dewatering studies, Miner. Eng. 79 (2015) 287–294. [24] J. Roussy, M. Van Vooren, E. Guibal, Chitosan for the coagulation and flocculation of mineral colloids, J. Disper. Sci. Technol. 25 (5) (2005) 663–677. [25] H. Sadeghi, H. Shasavari, S. Mirdarikvande, M. Alahyari, Investigation Swelling Behavior of a Novel alginate-based Composite Hydrogel in various salinity solutions, Bull. Env. Pharmacol. Life Sci. 3 (1) (2013) 135–139. [26] S. Sakohara, T. Kimura, K. Nishikawa, Flocculation mechanism of suspended particles using the hydrophilic/hydrophobic transition of a thermosensitive polymerx [translated]y, KONA Powder Part. J. 20 (2002) 246–250. [27] T. Staples, P. Chatterjee, Synthetic superabsorbents, in: Absorbent Technology, Elsevier Science BV, Amsterdam, 2002, p. 284. [28] A. Sworska, J. Laskowski, G. Cymerman, Flocculation of the Syncrude fine tailings: Part II. Effect of hydrodynamic conditions, Int. J. Miner. Process. 60 (2) (2000) 153–161. [29] A. Sworska, J.S. Laskowski, G. Cymerman, Flocculation of the Syncrude fine tailings: Part I. Effect of pH, polymer dosage and Mg2+ and Ca2+ cations, Int. J. Miner. Process. 60 (2) (2000) 143–152. [30] S.G. Vick, Planning, design, and analysis of tailings dams, BiTech (1990). [31] K.Y. Yeap, J. Addai-Mensah, A.J. McFarlane, The influential role of pulp chemistry, flocculant structure type and shear rate on dewaterability of kaolinite and smectite clay dispersions under couette Taylor flow conditions, Powder Technol. 179 (1) (2007) 79–83. [32] K.Y. Yeap, J. Addai-Mensah, D.A. Beattie, The pivotal role of polymer adsorption and flocculation conditions on dewaterability of talcaceous dispersions, Chem. Eng. J. 162 (2) (2010) 457–465. [33] M.J. Zohuriaan-Mehr, K. Kabiri, Superabsorbent polymer materials: a review, Iran. Polym. J. 17 (6) (2008) 451.

Please cite this article in press as: S. Joseph-Soly et al., Improved dewatering of nickel laterite ore slurries using superabsorbent polymers, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.010

578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607