EFFECT OF CALCIUM IONS ON THE ADSORPTION OF CMC ONTO THE BASAL PLANES OF NEW YORK TALC – A TOF-SIMS STUDY

E. Burdukova1, G.C. Van Leerdam2, F.E. Prins3, R.G. Smeink3, D.J. Bradshaw1 & J.S. Laskowski4 1

Department of Chemical Engineering

University of Cape Town Cape Town, South Africa 2

Akzo Nobel Chemicals Research

Arnhem, Netherlands 3

Akzo Nobel Functional Chemicals

Arnhem, Netherlands 4

Department of Mining Engineering

The University of British Columbia Vancouver, B.C., Canada

Correspondence to be addressed to: E. Burdukova e-mail:

[email protected]

Tel:

– +27 21 650 5524

Fax:

– +27 21 650 5501

ABSTRACT Talc is a magnesium rich phylosilicate mineral that occurs as a gangue component of many base metal sulfide ore deposits around the world. The basal planes of talc are naturally hydrophobic and therefore highly floatable. In order to reduce their floatability, polymeric depressants such carboxymethyl cellulose (CMC) are commonly utilised. Typical depressants are natural cellulose based polymers such Depramin®, produced by Akzo Nobel. This paper examines the adsorption characteristics of a charged polymeric depressant (CMC) on the basal planes of New York talc using surface specific ToF-SIMS measurements. The effect of the presence of calcium ions in solution on the adsorption density of charged polymers is also studied. The adsorption density of CMC was measured both in the presence and absence of calcium ions in solution. It was found the presence of calcium greatly enhanced the adsorption density of the polymer. This was attributed to the increased concentration of CaOH+ ions on the mineral surface, which facilitated polymer adsorption though acid/base interactions. The adsorption density was also found to be enhanced by the increased amount of coiling of the CMC macromolecules due to their reaction with Ca2+ ions.

Keywords: Froth flotation, Process instrumentation, Particle morphology, Flotation depressants

1

INTRODUCTION & BACKGROUND

Talc is a magnesium rich phylosilicate mineral that occurs as a gangue component of many base metal sulfide ore deposits around the world. These ores are generally beneficiated by flotation. Due to its natural floatability, talc readily enters the flotation concentrate, reducing its grade. In order to render talc and other gangue minerals non-floatable, polymeric depressants such as guar gum and carboxymethyl cellulose (CMC) are commonly utilised. Typical depressants are natural cellulose based polymers such Depramin®, produced by Akzo Nobel. Talc is an anisotropic mineral and the particles exhibit distinct edges and basal planes. Figure 1 shows the layered structure of talc, pointing out the chemical structure of the basal planes and edges (Flegmann and George 1975).

OH

-

-2

EDGE

O

Si+4 Mg

+2

TETRAHEDRAL BASAL PLANE

Figure 1 – Crystallographic structure of talc Small amounts of Ti3+ or Al3+ ions can substitute for Si4+ ions in the tetrahedral layer (Deer et al 1992). Such substituions cause a proton defficiency on the basal planes of talc, resulting in a presnece of a negative charge on talc basal planes (Okuda et al 1969; Burdukova et al 2007). While the edges of talc are hydrophilic, the basal planes are mildly hydrophobic with a contact angle of ≈ 60º (Fuerstenau and Huang 2003). This makes the basal planes chiefly responsible for the high floatability of talc particles. This makes the basal planes chiefly responsible for the high floatability of talc particles. Therefore, in order to effectively retard talc flotation, polymeric depressants such as CMCs need to adsorb specifically/preferentially onto the talc basal planes as opposed to talc edges (Smeink et al 2005).

Carboxymethyl cellulose depressants are anionic polysaccharides, with molecular weights ranging from 103 and 106 Dalton. The structure of Depramin® (a carboxymethyl cellulose macromolecule used in this study) is show in Figure 2.

Figure 2 – Structure of Depramin® (Smeink et al 2005) The mechanisms of adsorption of CMC’s onto talc as well as other minerals has been widely studied, with hydrophobic and hydrogen bonding considered to be the primary adsorption mechanisms (Steenberg and Harris 1984; Morris et al 2002; Fuerstenau and Huang 2003; Wang and Somasundaran 2005). A different mechanism was proposed by Liu and Laskowski, in which adsorption of polysaccharide macromolecules results from acid/base interactions between polysaccharide and metal hydroxyl complexes present on the mineral surface (Liu and Laskowski 1989; Liu et al 2000; Liu and Laskowski 2006a; Liu and Laskowski 2006b). It has also been shown that in some cases the interaction of anionic polymer with metal hydroxide sites acts in synergy with the hydrophobicity of the mineral surface (Rath et al 1997). Researchers such as Fuerstenau & Huang and Wang et al. considered hydrogen bonding to be a dominant mechanism in polymer adsorption (Fuerstenau and Huang 2003; Wang and Somasundaran 2005). However, hydrogen bonding itself can be treated as an acid base interaction (Fowkes 1989). Most polymer macromolecules placed in solution possess acidic properties. Mineral surfaces in solution show either prevailing acidic or basic properties (for instance, while quartz is acidic, alumina is basic) Therefore, an interaction between an acidic polymer and a basic mineral would be strong enough to be considered a chemical bond. An interaction between a weakly acidic polymer and a weakly basic mineral would take form of

a weak interaction, commonly labelled as hydrogen bonding (Liu et al 2000; Liu and Laskowski 2006a). Many studies have demonstrated the CMC adsorption density onto talc to be strongly dependent on the ionic strength of the solution (Morris et al 2002; Pawlik et al 2003; Khraisheh et al 2005; Parolis et al 2006; Pawlik and Laskowski 2006). This phenomenon is chiefly attributed to two factors: the thinning of electrical double layers around both the mineral surface and the CMC macromolecules, thus reducing the electrostatic repulsion, as well as the increased degree of coiling of a CMC macromolecule in the presence of a high concentration of electrolyte. The latter was confirmed by intrinsic viscosity measurements of polymer solutions at varying levels of supporting electrolyte (Pawlik et al 2003; Parolis et al 2006). The effect of cation type as well as ionic strength was measured by Parolis et al (2006), who found that the presence of calcium caused a greater degree of coiling of polymer molecules than that of potassium for the same ionic strength. The results also suggested that calcium ions interact chemically with the polymer macromolecules, while the potassium ions do not. The dependence of CMC adsorption on the type of ions present in solution was tested by Shortridge et al. It was shown that the CMCs adsorbs more densely onto the surface of talc in the presence of hydrolysable Ca2+ ions than K+ ions, for the same solution ionic strength at a constant pH of 9 (Shortridge et al 2000). These findings were later confirmed by Parolis et al (2006), who found similar trends. The effect of metal cations on the surface of the minerals is fairly well known (James and Healy 1972a; James and Healy 1972b; James 1981; Fuerstenau et al 1988; Bremmell and Addai-Mensah 2005; Ma and Pawlik 2005; Ma and Pawlik 2006). The effect is generally viewed in terms of the structure making/water breaking properties of these ions. Structure breaking, monovalent K+ ions readily adsorb onto a negatively charged mineral surface, reducing its surface potential. However, structure making, divalent Ca2+ ions tend not to adsorb onto the mineral surfaces, as such a process is thermodynamically unfavourable (James and Healy 1972a), with Ca2+ hydrolysis products adsorbing strongly.

The adsorption of hydrolysable polyvalent cations is highly dependent on pH. The adsorption of ions increases with the increasing amount of hydroxy complexes (e.g. CaOH+), (Ahmed and Van Cleave 1965; James and Healy 1972b; James 1981). The vast majority of the studies that focused on the adsorption mechanisms of polymers onto talc have made use of bulk measurement techniques such as ToC adsorption and microflotation. While these techniques have unquestionable merit, they cannot decouple the adsorption of polymers on the talc edges from that on talc basal planes. For that purpose, advanced micro-analytical techniques are required with can perform chemical identification at a very local scale with high sensitivity. Recent developments in ToF-SIMS technology have allowed detection of the characteristic fragments of CMC macromolecules on the surfaces of randomly orientated mineral particles (Smeink et al 2005; Parolis et al 2007). In this paper, the adsorption characteristics of charged polymeric depressants on specific particle planes are studied. The paper aims to examine the role of calcium ions in the adsorption of CMC macromolecules onto New York, focusing on the basal plane surface using surface specific ToF-SIMS measurements.

2 2.1

EXPERIMENTAL DETAILS Materials

New York talc, obtained from Wards Minerals, was used in the test work. The talc was dry ground in a titanium ring mill and screened to obtain a particle size fraction of P100 +150 microns. Electron microprobe analysis showed that the talc sample was ≈ 98.5 % pure, with approximately 0.3 % of the Si4+ ions being substituted with Al3+ ions in the tetrahedral layers. The CMC used was Depramin® 267 (supplied by Akzo Nobel), with a molecular weight of approximately 350 000 Daltons, with the degree of substitution of 0.65. Calcium nitrate and potassium nitrate were used as the source of Ca2+ and K+ ions respectively. All the tests were conducted at pH 9, with nitric acid, potassium hydroxide and calcium hydroxide used to adjust the pH of the solutions (depending on which cation was present).

2.2

Sample Preparation

Large talc flakes were manually picked out of a +150 µm talc sample. Solutions of varying ionic strengths, pH values and concentrations of depressant were made up in 10 ml sealable plastic test tubes. 0.5 g aliquots of talc particles were placed inside each test tube and allowed to equilibrate for 1 hour. The dosage of Depramin® used in the test work was 400g/ton of NY talc, which was equivalent to 2.6×10-3 M concentration of COO- functional groups. In all cases, the dosage of supporting electrolyte was 10-2 IS. After an hour the particles were filtered out using a vacuum funnel and gently rinsed with distilled water to prevent any bulk solution from adhering to the mineral surface. The particles were dried in a convection oven at 50° C. Out of the 0.5 g sample, three particles were randomly selected and manually placed on an adhesive surface (post note) such that the orientation of three of the particles was face-up.

2.3

ToF – SIMS Measurement and Analysis

The measurements were performed on the ION-TOF V Secondary Ion Mass Spectrometer, provided by TASCON GMBH, in Munster, Germany. The primary excitation ion was Bi3+. The spectra and images were taken in bunched mode (focus: 3-5 µm), with a mass resolution of 5000-10000. Cycling time of the instrument was set to 200 µs, allowing the acquisition of spectra up to a mass to charge ratio of 1800. The ToF-SIMS instrument also contains a Secondary Electron imaging detector, which was utilised during the course of this work. Since acquisition of SE pictures is done at relatively high ion current, they SE images were gathered after recording of the ToF-SIMS images. In order to make the data obtained for the different samples comparable, the intensities of the CMC specific mass fragments were normalised to the Si intensity of the talc substrate. Characteristic CMC fragments with relatively high sensitivity were CH3−COO- (mass 60.02). The ToF-SIMS data of the CMC exposed talc were always compared to the data of the blank mineral surface, which was handled according to exactly the same procedure but without

exposure to ions or polymer species. The results of such a blank surface act as a base line for evaluation of the rest of the measurements.

3

RESULTS AND DISCUSSION

3.1

Isolation of a Basal Plane Surface

In order to ensure that the measured surfaces correspond to the basal planes of the talc particles, all the ToF-SIMS measurements were performed in conjunction with low resolution SE imaging on the same area of the talc particles. Figure 3 (A) shows an example of a total ion ToF-SIMS spectral image of a basal plane of a particle of New York talc. It is clear that the spectral image alone is insufficient to determine whether it originates from a basal plane of a talc particle or its edge or both. In addition, Figure 3 (B) shows a low resolution SE image of the same particle surface. It can be seen from the image that the surface contains a number of imperfections due to the sample preparation process (e.g. handling the particles with tweezers) but it is still sufficiently intact to be considered representative of a talc basal plane.

(A)

(B)

Figure 2 – (A) ToF-SIMS total ion spectral image of a basal plane of a talc particle, (B) corresponding low resolution SE image of the same surface

Figure 3 – Low resolution SE image of a basal plane of a talc particle with a superimposed ToF-SIMS spectral image In order to further combine morphological (SE) data with chemical data (ToF-SIMS), the two images can be manually superimposed by matching the texture of the ToF-SIMS image to the features of the analysed particle. It is important to note that the superimposition is not exact and should only be used as a general guideline in determining whether or not a spectral image corresponds to a desired particle plane. An example of such a superimposed image is presented in Figure 3. This particular image represents the distribution of all the ionic species on the particle surface. However, such superimposition can be performed for any desired ion or fragment from the ToF-SIMS spectrum.

3.2

Adsorption of CMC on the Basal Planes of Talc

In order to determine the effect that calcium ions have on the adsorption characteristics of CMC macromolecules onto talc, ToF-SIMS measurements were performed on basal planes of talc particles. To decouple the effect of ion type from the effect of ionic strength, the tests were performed in the presence of divalent calcium ions as well as monovalent potassium ions at the same ionic strength. Potassium ions serve as indifferent electrolyte and provide a point of reference for the calcium results. The measurements were performed at a constant pH of 9 and at an ionic strength of 10-2 for both calcium and potassium cations. The SE images of talc basal planes with corresponding ToF-SIMS images of CMC fragments (CH3−COO-) in the presence of both potassium and calcium ions are shown in Figure 4. The

images demonstrate that the talc particles do not always consist of an ideal basal plane surface, but rather depict a surface that shows a degree of damage caused by sample preparation.

(A)

(B)

Figure 4- ToF- SIMMS image of CMC in the presence of (A) 0.01 IS potassium nitrate, (B) 0.01 IS calcium nitrate, superimposed onto the SE image of the talc particles, with selected analysis area For this reason, in the cases were the particle surfaces exhibited such apparent damage, the damaged areas were excluded from analysis. The surface areas selected for evaluation therefore consisted of undamaged basal planes. The images confirm that the ToF-SIMS intensities of CMC molecules in the presence of both calcium and potassium electrolytes, originate from the basal plane surfaces of talc. The Si - normalised intensities of CMC adsorbed in the presence of both potassium and calcium ions at pH 9 are shown in Figure 6. The readings are shown alongside those obtained from a blank reference sample. The results show that in the presence of both potassium nitrate and calcium nitrate solutions, the normalised intensities of CMC are greater than those of the reference baseline sample. This indicates that the adsorption of CMC molecules takes place on the basal planes of talc at pH 9 in the presence of both ions.

Normalised CMC intensity

0.45 0.40

Error bars - 95% Confidence interval of the mean

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Blank

0.01 IS Potassium Nitrate

0.01 IS Calcium Nitrate

Test Type

Figure 5 – Normalised intensities of CMC adsorbed in the presence of both calcium and potassium ions on the basal planes of talc, at the ionic strength of 0.01, at pH 9 Figure 6 also shows that the normalised intensities of CMC adsorbed in the presence of calcium nitrate are significantly higher than those in the presence of potassium solutions at the same ionic strength. This indicates that CMC adsorbs more strongly onto talc basal planes in the presence of calcium ions than it does in the presence of potassium ions at the same ionic strength. These findings are in agreement Parolis et al (2004, 2006) who produced similar results using bulk adsorption techniques.

3.3

Effect of Calcium on CMC Adsorption

In order to investigate the reason for the elevated CMC adsorption in the presence of calcium nitrate electrolyte, one needs to evaluate how the calcium ions interact with the talc surface. To ascertain this, talc particles were analysed in the absence of CMC, with only a 0.01 IS solution of calcium nitrate present. The SE images of the basal planes of talc particles with superimposed ToF-SIMS images of CaOH+ and Ca2+ ions at pH 9 are presented in Figure 7.

(A)

(B)

Figure 6 - (A) ToF- SIMMS image of calcium hydroxide ions, (B) ToF - SIMMS image of calcium ions superimposed onto the SE image of the talc particle The normalised intensities of CaOH+ and Ca2+ ions adsorbed at pH 9 are presented in Figure 8. The readings are shown alongside those obtained from a blank reference sample. Figure 8 demonstrates that the normalised intensity of Ca2+ ions on the talc surface in the presence of calcium nitrate solution is of a similar order of magnitude as the reference sample. However the error analysis indicates that the difference between the blank and non-blank sample is small but statistically significant. This is an unexpected result, since the adsorption of divalent ions onto mineral surfaces is thermodynamically unfavourable (James and Healy 1972a). The slight indication of the presence of Ca2+ on the basal plane surface can be attributed to the measurement artefact associated with the presence of CaOH+ ions on the surface. Based on this it was assumed that the adsorption of Ca2+ ions onto talc basal planes at pH 9 is negligible.

Normalised Ion Intensity

3.50 3.00 2.50 2.00

Error bars - 95% Confidence interval of the mean +2 Calcium Ca + Calcium CaOH Hydroxide

1.50 1.00 0.50

-3

0.014 ± 1.5×10

-3

0.029 ± 4.3×10

0.00 Blank

0.01 IS Calcium Nitrate

Blank

0.01 IS Calcium Nitrate

Test Type

Figure 7– Normalised Intensities of both Ca2+ and CaOH+ ions on the basal planes of talc at

Percentage of ions present in solution -2 of 10 M Ca(NO3)2

pH 9 and 0.01 IS solution of Ca(NO3)2

100% 80% 60% 40%

+2 Ca+2 Ca + CaOH CaOH+

20% 0% 5

6

7

8

9

10

11

12

13

14

15

pH

Figure 8– Solubility diagram of CaOH+ ions in solution as a function of pH On the other hand, CaOH+ ions are known for their high affinity or specific adsorption on mineral surfaces (Ahmed and Van Cleave 1965; James and Healy 1972a). This is confirmed by the ToF-SIMS spectra which demonstrate that the adsorption of CaOH+ in the presence of calcium nitrate is significantly larger than that on a blank reference sample. This

demonstrates that a significant amount of CaOH+ ions specifically adsorb onto the basal plane surface at pH 9. It is important to note that at pH 9, only a tiny fraction (0.015 %) of calcium ions exist in the form of a hydroxyl complex, as shown in Figure 8. The results provide a partial explanation for the increased degree of adsorption of CMC macromolecules in the presence of calcium ions. The adsorption of CaOH+ species on the surface of talc increases the overall number of metal hydroxide sites on the mineral surface. This precipitates an increased amount of CMC adsorption though acid/base interactions.

3.4

Effect of Calcium on CMC Conformation

In order to see whether or not divalent Ca2+ ions, abundant in the calcium nitrate solution at pH 9, have any effect on CMC adsorption, ToF-SIMS measurements were performed to determine the amount of Ca2+ ions present on the surface of talc in the presence and absence of CMC. The results are presented in Figure 9. 5.0

+2

Normalised Ca Intensity

4.5 4.0

Error values - 95% Confidence interval of the mean

4.12 ± 0.59

3.5 3.0 2.5 2.0 1.5 1.0 0.5

-3

0.01 ± 1.24×10

-3

0.03 ± 1.74×10

0.0 Blank

0.01 IS Ca, Absence of CMC

0.01 IS Ca, Presence of CMC

Test Type

Figure 9 – Normalised Intensities of Ca2+ ions on the basal planes of talc in the presence and absence of CMC, at pH 9 and 0.01 IS solution of Ca(NO3)2

The results demonstrate that in the absence of CMC in solution, the adsorption of Ca2+ ions onto the basal planes of talc is practically negligible and is similar in magnitude to that found on a blank sample. However when CMC macromolecules are present in solution, the adsorption of Ca2+ ions increases dramatically, by orders of magnitude. Since the adsorption of these ions directly onto the surface is unfavourable, the ions must therefore be brought in touch with the talc basal planes by an indirect method – by catching a ride on the CMC macromolecules. The carboxylic acid groups in the CMC macromolecules are known to react with calcium ions (Fixman 1964) and an increase in adsorption density of calcium ions in the presence of CMC molecules in solution is caused by the chemical reaction which takes place in the solution between Ca2+ and CMC molecules. Such a reaction t should also increase the degree of coiling of CMC macromolecules; this degree should be greater in the presence of divalent calcium ions than in the presence of monovalent ions such as potassium, at the same ionic strength. Such results were demonstrated by Mohammed et al who studied the effect of calcium ions on xanthan (a polysaccharide macromolecule similar to CMC). They found that the presence of calcium caused a significant increase in the storage modulus of the polymer, which is consistent with gel formation. This was attributed to the binding of calcium ions to pairs of COOH- groups, promoting intermolecular coiling (Mohammed et al 2007). This result demonstrates that the increased adsorption of CMC onto the basal planes of talc can also be attributed to the creased degree of coiling of the CMC macromolecules. As the molecules coil, they are able to adsorb more densely onto the mineral surface due to a decrease in a spatial constraints.

4

SUMMARY & CONCLUSIONS

Surface specific analysis techniques such as ToF-SIMS allow for a direct ex situ determination of the degree of adsorption of charged polymeric depressants such as carboxymethyl cellulose on mineral particles. Due to the ability to probe on a very local scale (sub micron), these measurements can be made with reference to a specific plane of a mineral, in this case basal plane of the particles of New York talc. The morphology of the measurement position on the talc particles can be confirmed by using the combination of ToF-SIMS spectral images with low resolution SE images. The surface specific study confirmed that CMC macromolecules adsorb more densely onto the basal planes of talc in the presence of calcium ions than in the presence of potassium ions at the same ionic strength. The ToF-SIMS measurements also showed that calcium ions adsorb specifically onto the talc basal plane surface in the form of CaOH+ ions, despite their low concentration in the electrolyte solution at pH 9. This confirmed that a major mechanism of adsorption of CMC macromolecules onto talc was acid/base interactions with metal hydroxy species on the mineral surface.

The presence of CaOH+ ions on the surface

increases the number of the available metallic sites, thus promoting CMC adsorption. It was also found that the adsorption of divalent Ca2+ ions on the surface of talc in the absence of CMC was practically negligible. However, the magnitude of Ca2+ ion adsorption increased dramatically in the presence of CMC in solution. This was attributed to the chemical interaction between CMC macromolecules and divalent calcium ions, whereby these ions react with carboxylic acid groups of the polymers, causing them to coil. The ions are then indirectly transferred to the mineral surface by being part of the polymer macromolecule. Therefore, the presence of Ca2+ ions in solution further increased CMC adsorption onto talc basal plane by decreasing the effective molecule surface area and promoting denser spatial adsorption.

5

ACKNOWLEDGEMENTS

The authors would like to acknowledge and thank Akzo Nobel Functional Chemicals and Akzo Nobel Chemicals Research for their financial support of this work, for providing technical expertise as well as access to ToF-SIMS equipment.

6

REFERENCES

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a ToF-SIMS Study -

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Nearly 90% of the Internet traffic volume is carried by TCP, the transmission control ... LAS (Least Attained Service) and PS+PS scheduling algorithms improve the ... improves the response time of small flows by avoiding slow-start penalty.

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Dec 19, 2008 - Materials and methods Twenty-four chronic low back pain. (LBP) patients with M1 ... changes has been found in a follow-up study [4]. Patients.

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Jul 15, 2016 - The Connecticut Department of Transportation (ConnDOT) provides a variety of public transportation services in local areas and regions in Connecticut and coordinates services with other public transportation agencies both in Connecticu

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Asia Pacific Journal of Language in Education. (2000) Vol. 3, No. .... foreign language to satisfy part of a graduate degree requirement. In their I diary entries ...

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