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Faculty of Science

ELUCIDATING THE DECOMPOSITION KINETICS OF XANTHATE COMPOUNDS IN

MINING WATERS BY HEADSPACE GAS CHROMATROGRAPHY-MASS
SPECTROMETRY

2018 | ADRIAN MCLEOD BATISTA

B.Sc. Honours thesis – Chemical Biology

ELUCIDATING THE DECOMPOSITION KINETICS OF XANTHATE COMPOUNDS
IN MINING WATERS BY HEADSPACE GAS CHROMATROGRAPHY-MASS
SPECTROMETRY
By

ADRIAN MCLEOD BATISTA

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF BACHELOR OF SCIENCE (HONS.)

In the

DEPARTMENTS OF BIOLOGICAL AND PHYSICAL SCIENCES

(CHEMICAL BIOLOGY)

This thesis has been accepted as conforming to the required standards by:

Kingsley Donkor (Ph.D.), Thesis Supervisor, Dept. Physical Sciences

Jonathan Van Hamme (Ph.D.), Co-Supervisor, Dept. Biological Sciences

Heidi Huttunen-Hennelly (Ph.D.), Examining Committee member, Dept. Physical Sciences

Dated this 1st day of May, 2018, in Kamloops, British Columbia, Canada

© Adrian Mcleod Batista, 2018

ABSTRACT

Xanthates are a widely used class of floatation reagents used for the recovery of valuable

sulphide minerals. Floatation utilizes large quantities of water that lead to rapid hydrolysis of

xanthates in circuit leading to a reduction in mineral recovery and an overall decrease in mill

efficiency. Currently the knowledge on xanthate degradation is sparse with only a few studies

reporting kinetic parameters for xanthate degradation into carbon disulphide (CS2). A headspace

GC-MS method was developed to directly measure the presence of CS2 in the gas phase. Kinetic

parameters were then established for potassium amyl xanthate (PAX), sodium isobutyl xanthate

(SIBX), potassium isopropyl xanthate (PIPX), and sodium ethyl xanthate (SEX) based on the

generation of CS2. The decomposition of all xanthate studied followed first order kinetics and the

rate constants were determined using a 7650A auto sampler and a PAL3 auto sampler. The rate

constants were found to be 7.05 x 10-4, 4.07 x 10-4, 5.11 x 10-4, and 1.48 x 10-4 h-1 for PAX,

SIBX, PIPX, and SEX respectively at 25 oC using the 7650A auto sampler. For the PAL3 auto

sample the rate constants were found to be 3.71 x 10-6, 4.39 x 10-6, 1.86 x 10-6, and 4.03 x 10-6 h-1

for PAX, SIBX, PIPX, and SEX respectively at 30 oC. The rate constants were found to increase

as the temperature was increased. These data were used to calculate the activation energies for

each decomposition reaction. The activation energies were found to be 19.83, 10.80, 34.44, and

22.64 kJ/mole for PAX, SIBX, PIPX, and SEX respectively. These data show that the methods

presented are viable options for further analytical studies on xanthate decomposition kinetics and

for higher throughput applications in industrial settings.

ii
ACKNOWLEDGEMENTS

I would like to thanks my supervisor Dr. Kingsley Donkor for all the support and

guidance provided throughout the duration of the project. I also would like to thank Trent

Hammer for training me to use the GC-MS and for providing expertise on instrumentation. John

Andre from New Afton Gold Mine in Kamloops also deserves a thank you for providing samples

and approaching myself and Dr. Donkor about this research opportunity. A special thanks you to

the Thompson Rivers University Chemistry department, honours coordinators, and research

office for making this research possible. Finally, I would like to extend thanks to Dr. Jon Van

Hamme and Dr. Heidi Huttunen-Hennelly for agreeing to be a part of my honours committee.

iii
TABLE OF CONTENTS

ABSTRACT…………………………………………………………………………………… II

ACKNOWLEDGEMENTS…………………………………………………………………. III

LIST OF FIGURES………………………………………………….……………………… V

LIST OF TABLES…………………………………………………………………………… VI

1. INTRODUCTION…………………………………………………………………………. 1

2. MATERIALS AND MEHTODS………………………………………………………….. 7

2.1 REAGENTS…………………………………………………………………………… 7
2.2 INSTRUMENTATION……………………………………………………………...... 7
2.3 PREPARATION OF CS2 STANDARDS…………………………………………….. 8
2.4 PREPARATION OF XANTHATE STANDARDS………………………………….. 8

3. RESULTS AND DISCUSSION…………………………………………………………… 9

3.1 DETECTION AND CALIBRATION OF CS2………………………………………. 9

3.2 DERIVING THE RATE LAW……………………………………………………… 13
3.3 DECOMPOSITION KINETICS……………………………………………….……. 14
3.4 THERMODYNAMIC STUDIES…………………………………………………… 18

4. CONCLUSIONS……………………………………………………………………….... 22

5. FUTURE WORK……………………………………………………………………..…. 23

6. LITERATURE CITED…………………………………………………………………. 24

7. APPENDIX………………………………………………………………………………. 25

7.1 CHROMATOGRAMS OF XANTHATE STANDARDS………………………… 25

7.2 MASS SPECTRA OF XANTHATE DEGRADATION PROCCESS……………. 27

iv
LIST OF FIGURES

Figure 1. General structure of a xanthate anion. The R-group will always be one of four 1
alkyl groups: ethyl, isopropyl, isobutyl, or an amyl group.

Decomposition reactions of potassium ethyl xanthate at acidic (A), and basic

(B) pH.
Figure 2. 3

Figure 3. Schematic diagram of a gas chromatograph single quadrupole mass 6

spectrometer (GC-MS) (Li et al., 2015).

Figure 4. A chromatogram of a 1 ppm CS2 standard solution. Sample volume injected 10

was 5 µm with a split of 20:1.

Figure 5. Calibration curve for CS2 using the 7650A auto sampler. An injection volume 11
of 5 µL was used with a 20:1 split. The headspace volume was 1 ml and the
temperature was 25 °C. The equation of the line and correlation coefficient are
shown within the graph area.

Figure 6. Calibration curve for CS2 using the PAL3 auto sampler. An injection volume 12
of 75 µL was used with a 100:1 split. The headspace volume was 10 ml and
the temperature was 30 °C. The equation of the line and correlation coefficient
are shown within the graph area.

Figure 7. Plot of CS2 evolved as a function of xanthate concentration. Potassium amyl 14

xanthate (PAX), sodium isobutyl xanthate (SIBX), potassium isopropyl
xanthate (PIPX), and sodium ethyl xanthate (SEX) all showed linear
relationships indicating first order kinetics for the decomposition.

Figure 8. Plot of CS2 evolved from a specific xanthate: PAX, SIBX, PIPX, or SEX, 15
against time using the 7650A auto sampler. Samples were run at 25 °C and 5
µL was sampled from 2 mL of headspace.

Figure 9. Arrhenius plot of ln(k) against the reciprocal of temperature. Temperature 20

used were 30 °C, 50 °C, and 80 °C. All k values were calculated from data
generated using the PAL3 auto sampler.

v
LIST OF TABLES

Table 1. Rate constants for PAX, SIBX, PIPX, and SEX determined using the 7650A 15-16
auto sampler. The constant was calculated using curve of CS2 generated over
time.
Table 2. Rate constants for PAX, SIBX, PIPX, and SEX determined using the PAL3 17
auto sampler. The constant was calculated using curve of CS2 generated over
time.
Table 3. Activation energy of PAX, SIBX, PIPX, and SEX determined using the 20
Arrhenius equation and the rate constants calculates using the PAL3 auto
sampler.

vi
1. INTRODUCTION

Alkyl dithiocarbonates, more commonly known as xanthates, are a class of widely used

mineral collectors in mine ore slurry. When combined with a technique know as froth

floatation, xanthates are able to selectively and efficiently concentrate sulphide minerals such as

chalcopyrite (CuFeS2) (Li, et al., 2015). This has propelled xanthates to the forefront of choice

reagents for the floatation of copper and gold sulphide minerals (Rezaei, Massinaei, and

Zeraatkar Moghaddam, 2018). Xanthate molecules, shown in Figure 1, all share a common

dithiocarbonate group (-OCS2) and have a variable alkyl chain attached through an ester

Figure 1 General structure of a xanthate anion. The R-group will always

be one of four alkyl groups: ethyl, isopropyl, isobutyl, or amyl
group.
linkage. Froth floatation begins with the grinding of raw ore into coarse particles to liberate the

valuable mineral particles from the matrix. The coarse mill product is placed into a large

floatation vat containing water and xanthates are added to begin the floatation process (911

Metallurgist, 2017). The negative sulphur atom in the xanthate will bind and reduce metal

containing sulphides. The xanthate will selectively impart hydrophobicity on the mineral of

interest so that it can be separated from the aqueous matrix and the other hydrophilic gangue

(Kemppinen, Aaltonen, Sihvonen, Leppinen, and Siren, 2015) The result is a micelle like layer

of xanthates with the alkyl tails facing outwards surrounding the mineral particle. An agitator

then aerates the tank to create bubbles. The hydrophobic tail of the xanthate molecule will

1
interact with these bubbles as they are produced and float to the surface carrying the mineral

with it. This process may be repeated many times with grinding between each collection to

further concentrate the mineral of interest. Previous research has suggested that floatation

efficiency and selectivity increase as the alkyl chain length is increased (Taguta, O'Connor, and

McFadzean, 2017); however, aside from the additional hydrophobicity that is imparted by the

larger hydrophobic group, a consensus has not been reached on an exact mechanism.

The drawback of concentrating minerals repeatedly in large volumes of water is the

catalytic nature of water on xanthates in solution (Shen, Nagaraj, Farinato, and Somasundaran,

2016). Xanthates are typically supplied to mills in a powder or pellet form (Mining World,

2017), which can then be prepared to a 5-10% solution to be used for floatation. The standard

practice is to consume these solutions within three weeks due to degradation in the aqueous

media. As the xanthates degrade in solution the concentration is decreased at an accelerated rate

and produces multiple by-products, which can severely impact the recovery of sulphide

minerals and floatation productivity on a massive scale (Kemppinen, Aaltonen, Sihvonen,

Leppinen, and Siren, 2015). The most common by-product of the degradation process is carbon

disulphide (CS2), a toxic volatile species known to accumulate in floatation mills (Shen,

Nagaraj, Farinato, and Somasundaran, 2016). Since the degradation process is continually

occurring within the floatation reactions, the generation of toxic CS2 presents not only an issue

with decreasing flotation efficiency, but on the health and safety of workers and the surrounding

environment. It is estimated that about half of the total xanthates are consumed during the

floatation reaction while the remainder is discharged into tailings waste (Li, et al., 2015). This

tailings waste will continue to undergo degradation and produce toxic species, which can

contaminate surrounding areas. Often the tailings water is discharged into a natural body of

2
water without any treatment. This makes it important to understand the decomposition process

of xanthates not only for industrial applications but also for waste management.

Xanthate literature has mostly focused on determination of xanthates using

ultraviolet/visible (UV/Vis) spectrophotometry (Hao, Silvester, and David, 2000); however this

method cannot distinguish between metal species and degradation products. The goal of these

studies was to track the decrease in xanthate concentration over time in order to elucidate its

decomposition behaviour. Mustafa and colleagues (2004) investigated the effects of

temperature, time and pH on the stability of potassium ethyl xanthate (PEX) using a UV/Vis

method. It was found that after thirteen days at a pH of 9, PEX absorbance did not decrease

significantly (0.451%/day). When the pH was decreased to 7 the rate of degradation increased

to 0.9024% per day, and increased even further to 2.099% as the pH was decreased to 5.

Similarly, when the temperature was increased from 10 oC to 25 oC the rates of decomposition

were at least tripled at each pH. By observing the appearance and disappearance of peaks when

the pH was changed, two theoretical decomposition reacts were proposed (Figure 2). In acidic

solution the formation of a xanthic acid intermediate (C2H5OCS2H) dominated; however they

did not describe how this reaction leads to an increase in the rate of decomposition. In basic

solution a similar reaction occurs but instead of a xanthic acid intermediate, the dominant

degradation products are ethanol and CS2. Again, the link between the different reactions and

how they potentiate the differences in the rate of decomposition has not been clarified.

Figure 2 Decomposition reactions of potassium ethyl xanthate at acidic (A), and

basic (B) pH.

3
 A similar study presented the decomposition rates by calculating the rate constant (k) for

each condition using SEX (Zhongxi and Forsling, 1997). At 40 oC the rate constants all had

orders of magnitude of 10-6. The order of magnitude decreased to 10-7 and then 10-8 as the

temperature was reduced to 20 oC and 5 oC respectively. It was also found that the difference

between the rate constants at pH 8.0 and 6.6 increased as the temperature was increased. This

suggests that temperature may be the dominant factor driving decomposition, and at high

enough temperatures pH may not significantly affect the rate compared to temperature.

The inability to distinguish between the different xanthates in solution has lead to the

development of alternative methods such as ion-interaction high performance liquid

chromatograph (HPLC) (Trudgett, 2005), and capillary electrophoresis (CE) (Kemppinen,

Aaltonen, Sihvonen, Leppinen, and Siren, 2015). These methods have allowed for the separation

and monitoring of the individual xanthates. A novel gas chromatography method was developed

by Li and colleagues (2015) for the detection of isobutyl xanthate (IBX) in surface and drinking

water. The research was based on the decomposition of IBX into CS2 under acidic conditions.

Li argued that previous methods that aimed to prevent the decomposition of the xanthate during

analysis have used pH levels that are too acidic for use with a C18 column. Additionally, CE

methods have suffered from high detection limits (10-6 to 10-7 g/ml), which are not compatible

with residual xanthate concentrations typically seen in floatation and wastewater circuits (7 x

10-7 g/ml). By detecting the evolution of volatile CS2 using a headspace GC method with an

electron capture detector a detection limit of 3 x 10-10 g/ml was obtained.

The use of a stable degradation product to quantify xanthates has allowed for the

development of methods where decomposition is examined by monitoring the appearance of

CS2 directly. A study by Shen and colleagues (2016) used a headspace GC - mass spectrometry

(GC-MS) for the detection of volatile CS2 evolved from the decomposition of SIBX. The
4
derived rate law from these experiments showed SIBX followed first order kinetics with respect

to the generation of CS2. The work also supported previous research by noting the increase in

degradation associated with increased temperatures and acidic pH. These changes were

reflected by the rate constants determined under various conditions. At 25 oC, k was found to be

9.3 x 10-4 h-1. This value increased to 1.7 x 10-2 h-1 when the temperature was increased to 50 oC,

and to 1.3 x 10-1 h-1 at 70 oC. A maximum rate of decomposition was measured at pH 2.2, and it

was noted that above pH 8, no significant change in the rate of decomposition was observed.

GC-MS is a technique that combines the separative technique of gas chromatography with

the detection ability of a mass spectrometer. A general schematic for the instrument is shown in

Figure 3. In GC, an inert gas such as helium is used as a mobile phase to carry the sample

through the column (Bacher, 2016). The column contains a solid support coated with a high

boiling point liquid polymer. Columns may be capillary based, containing a coating directly

around the inner diameter of the capillary. Separation is achieved based on interactions between

the stationary phase and the components of the sample. If a non-polar polymer is used, the

column will retain non-polar components of the sample while the polar components will be left

to flow through freely. Components are also separated based on their boiling points.

Compounds with low boiling points have high vapour pressure and will spend more time in the

gaseous phase than interacting with the liquid stationary phase, yielding shorter retention times

than compounds with higher boiling points.

5
Figure 3 Schematic diagram of a gas chromatograph single quadrupole mass spectrometer
(GC-MS) (Li et al., 2015).
As the components of the sample are separated and eluted at different times they are

detected by the mass spectrometer. MS ionizes molecules in a vacuum as they emerge from the

GC using a beam of high-energy electrons (Clinical Mass Spectrometry, 1995). This removes

an electron from the highest occupied molecular orbital of the molecule to produce a radical

with a charge of +1. The molecule may also be fragmented extensively by excess energy to

produce fragments that can help elucidate structure. The charged molecules are then subject to a

quadrupole mass analyzer. Four cylindrical rods are arranged parallel to one another and a

radiofrequency voltage is applied to one pair of rods while an opposing DC voltage is applied to

the other pair. Ions travel down the quadrupole and the voltages are adjusted so only ions of a

particular mass to charge (m/z) ratio will have stable trajectories and reach the detector. The

voltages are continually adjusted to detect ions with a mass range of up to 5000 m/z. The

detector collects and counts the ions to produce a signal proportional to the number of ions that

have impacted its surface. This yields not only a mass spectrum showing the molecular mass of

6
 the compounds in the sample, but a chromatogram that can yield quantitative information base

on parameters such as peak height or peak area, both of which are proportional to concentration.

Although literature does exist examining the decomposition of xanthates through the

evolution of CS2, no studies have been found comparing the kinetics of each of the xanthates

under a common method. This study was set out with two goals in mind. First, was to develop a

suitable headspace GC-MS method for the detection of CS2 in aqueous solution. Once the

instrument was calibrated, the decomposition kinetics of PAX, SIBX, PIPX, and SEX were

examined through the evolution of the prominent degradation product, CS2.

2. MATERIALS AND METHODS

2.1 Reagents
Pure CS2 solution was purchased from Sigma-Aldrich Canada Ltd., Oakville, Ontario, Canada

and was of analytical grade. Xanthate standards were supplied by New Afton Gold Mine,

Kamloops, British Columbia, Canada, as both dissolved solid and pure solid. All solutions were

prepared using filtered 18 MΩ water. Any solutions containing dissolved xanthate was filtered

through 0.45 µm nylon filter prior to use.

2.2 Instrumentation
All experiments were performed using an Agilent 7890B-GC coupled 5977A-MS (Agilent

Technologies, Santa Clara, CA). Half of the experiments used a 7650A (Agilent) automatic liquid

sampler with a 10 –µL headspace syringe, and the second half utilized a PAL3 (Agilent) auto

sampler system equipped with a headspace tool. In both cases an HP-5MS 5% phenyl methyl

silox capillary column was used (29.94 M x 250 µM x 0.25 µM) (Agilent Technologies, Santa

Clara, CA). The temperature program used was developed from a previous method (Shen,

7
Nagaraj, Farinato, & Somasundaran, 2016). The temperature was held at 35 oC for 2 min before

increasing the temperature by 50 oC per minute to 220 oC where it was held for an additional 1.3

min to yield a final run time of 7 min.

The optimized conditions for the 7650A auto sampler were a flow rate of 1 mL/min and a

split of 20:1. The injection volume was 5 µL a single ion monitor program was set up to observe

the presence of CS2 at 76.00 m/z with a gain factor of 10.00. For the PAL3 auto sampler the split

was increased to 100:1 and the gain was reduced to 1 to account for the increased sensitivity of

the headspace dedicated instrumentation. An injection volume of 75 µL was used to

accommodate the larger headspace needle used in the PAL3 system. The single ion monitoring

was also removed in favour of an MS scan from 40 to 350 m/z.

2.3 Preparation of CS2 Standards

For the 7650A auto sampler 10 ppm (mg/L) samples of CS2 were prepared by diluting 0.80

µL of pure CS2 to 100.0 mL with 18 MΩ water. Samples were then diluted further to 1 ppm

before being loaded into clean and dry 2 mL glass sample vials that were capped immediately.

Sample vials were shaken rapidly for 15 min in a flask shaker as suggested by Li and colleagues

(2015). These samples were used to confirm detection of CS2 and to optimize detection

conditions. Calibration standards were prepared using the same methodology to various

concentrations. For the PAL3 auto sampler the use of 20 mL glass headspace vials allowed

increased the loading volume to 10 mL. Samples were agitated at 30 oC for 15 min using the built

in agitator tool prior to injection.

2.4 Preparation of Xanthate Standards

All xanthate standard solutions were prepared from high purity solids. Approximately

0.0500g of solid PAX (97%), SIBX (95%), PIPX (98%), and SEX (95%) was dissolved in 18

8
MΩ water to a volume of 500 mL to prepare 1000 ppm stock solutions of each. Stock solutions

were stored at 4 oC in the dark to minimize aqueous degradation. Fresh stock solutions were

prepared weekly. Standards were prepared by first filtering the stock solution through a 0.45 µM

nylon filter prior to dilution. 1 mL of sample was loaded into a clean glass vial for the 7650A

sampler, and 10 mL was loaded for the PAL3 sampler. Samples were then shook or agitated for

at least 15 min prior to injection. All 7650A samples were equilibrated under ambient conditions

and a pH of about 8.50. PAL3 sample were equilibrated at varying temperatures and a pH of

about 8.50.

3. RESULTS AND DISCUSSION

3.1 Detection and Calibration of CS2

The development of a method to detect volatile CS2 was first investigated using a 1 ppm

standard solution prepared in water. This standard was chosen as it falls within the concentration

range of CS2 typically observed in mining circuits. A reproducible peak was observed around

1.88 min and had an m/z of 76 (Figure 4), using both auto sampler systems. This peak was

confirmed as the CS2 peak and a single ion-monitoring program was set to detect ions of 76 m/z

for the 7650A auto sampler. No interfering peaks were noted in the chromatogram as the solvent

peak comes out before 1 min, which is not detected by the mass spectrometer due to a minute

long solvent delay where no ions are detected for the first minute of a run.

9
Figure 4 A chromatogram of a 1 ppm CS2 standard solution. Sample volume injected was 5 µL
with a split of 20:1.
Once a suitable method had been developed for the detection of CS2, a linear range was

established and calibrated for. The solubility of CS2 at 20 oC has been reported to be as low as 2

ppm (World Health Organization, 2000), and as high as 2000 ppm (Chemical Book, 2017). To be

safe the highest calibration standard was chosen to be 3 ppm to see if linear range of 0.3 – 3 ppm

could be established. A calibration curve was generated from these standards by plotting the area

under the peak in the chromatogram against the concentration of CS2 (Figure 5). A strong

correlation between the concentration of CS2 and peak area was observed (R2 = 0.977). The limit

of detection (LOD), or the minimum amount of analyte that can be distinguished from the

absence of that analyte, and the limit of quantification (LOQ), or the minimum amount of analyte

that can be detected with analytical certainty, was found using the standard deviation of the

response and the slope. LOD was determined by multiplying the ratio of the standard deviation of

the lowest concentration standard over the slope of the line by 3.3, and LOQ multiplied the same

ratio by 10. These are the standard ratio multiples used for LOD and LOQ calculation in

analytical studies. The LOD was found to be 0.03 ppm, and the LOQ was found to be 0.09 ppm.

10
These values are higher than previously reported LODs using headspace GC-MS (0.002 ppm)

(Li, et al., 2015); however, by increasing the number of calibration standards and replicates this

LOD and LOQ can be reduced.

Figure 5 Calibration curve for CS2 using the 7650A auto sampler. An injection volume of 5 µL
was used with a 20:1 split. The headspace volume was 1 ml and the temperature was 25 °C.
The equation of the line and correlation coefficient are shown within the graph area.
A similar calibration protocol was carried out using the PAL3 auto sampler. Since the

same instrument, column, and temperature program was used for both auto samplers the retention

time of CS2 was unchanged using the PAL3 system. Since the PAL3 system is equipped with a

temperature-controlled agitator module and uses dedicated headspace equipment, the sensitivity

of the instrument increased, and the signals were too strong to continue using the same method as

before. The split ratio was increased 100:1 and the gain factor on the MS was reduced to 1. An

11
injection volume of 75 µL was also used to accommodate the larger headspace needle used by the

PAL3 system. Additionally, the single ion monitoring was removed from the MS, as the

increased sensitivity it provided was no longer needed. A new range was established from 0.1 to

2 ppm (R2 = 0.977) (Figure 6). The LOQ and LOD were also established for this curve using the

same method as described above. The LOD was found to be 0.04 ppm and the LOQ was found to

be 0.12 ppm. The lowest standard used is slightly below the LOQ, so the implications of

increasing the number of repeats to reduce the LOD and LOQ will be substantial for its

application in industry.

Figure 6 Calibration curve for CS2 using the PAL3 auto sampler. An injection volume of 75
µL was used with a 100:1 split. The headspace volume was 10 ml and the temperature was 30
°C. The equation of the line and correlation coefficient are shown within the graph area.
The direct use of CS2 for calibration allows for the use of ambient conditions and native

pH values. Previous methods that indirectly calibrate for CS2 by degrading xanthates must rely
12
on acidic pH and high temperatures to produce a detectable amount of CS2 in an analytical

timeframe (Li, et al., 2015). This extra sample preparation time limits industrial applicability. The

current method proposed is limited to a narrower range due to the low solubility of CS2 in water;

however, the window for industrial applicability still falls within this range making it more

suitable to be coupled to main industrial processes.

3.2 Deriving the Rate Law

To establish the rate law for the decomposition of xanthates into CS2 a plot of CS2

generated against initial xanthate concentration was prepared. The curve was prepared for each of

the four xanthates to be studied: PAX, SIBX, PIPX, and SEX. All the curves showed linear

relationships between the initial xanthate concentration and the amount of CS2 generated (Figure

7). All the correlations showed R2 values greater than 0.99, except for SIBX at 0.88. The linear

relationship seen confirms that the decomposition follows first order kinetics. The rate of change

in the production of CS2 equals that of the change in initial xanthate concentration. The rate law

can then be expressed as shown in Eq. (1):

𝑹𝒂𝒕𝒆 = 𝒌[𝑿𝒂𝒏𝒕𝒉𝒂𝒕𝒆] (1)

wherein rate equals the slope of the line of the curve of CS2 concentration generated during

decomposition against time, k is the rate constant, and [xanthate] is the initial concentration of

xanthate loaded into the sample vial.

13
 It should be noted that this experiment was carried out only on the 7650A sampling system,

as reaction orders are a property of the reaction, not the instrument.

Figure 7 Plot of CS2 evolved as a function of xanthate concentration at ambient temperature.

Potassium amyl xanthate (PAX), sodium isobutyl xanthate (SIBX), potassium isopropyl
xanthate (PIPX), and sodium ethyl xanthate (SEX) all showed linear relationships indicating
first order kinetics for the decomposition.
3.3 Decomposition Kinetics
A graph of CS2 generated by each xanthate against reaction time was developed to study the

rate constants of each xanthate. For the 7650A sampler the four xanthates all showed linear

relationships with R2 values greater than 0.92 (Figure 8). The rate constant for the degradation

reaction of each xanthate was calculated by dividing the slope of the line by the initial xanthate

concentration in the sample. In all cases the concentration of xanthate used was close to 100 ppm.

The rate constants at 25 oC were found to be 7.05 x 10-4, 4.07 x 10-4, 5.11 x 10-4, and 1.48 x 10-4

14
h-1 for PAX, SIBX, PIPX, and SEX respectively (Table 1). These values are comparable to those

found in the literature using similar methodology (Shen, Nagaraj, Farinato, & Somasundaran,

2016); however literature reported by Zhongxi and Forsling (1997) reported rate constants at 25
o
C to be on the magnitude of 10-6 and 10-7.

Figure 8 Plot of CS2 evolved from a specific xanthate: PAX, SIBX, PIPX, or SEX, against time
using the 7650A auto sampler. Samples were run at 25 °C and 5 µL was sampled from 2 mL of
headspace.
Table 1 Rate constants for PAX, SIBX, PIPX, and SEX at 25 °C determined using the 7650A
auto sampler. The constant was calculated using curve of CS2 generated over time.
Xanthate k (h-1) ± uncertainty

PAX 7.05 x 10-4 ± 0.89 x 10-4

SIBX 4.07 x 10-4 ± 0.28 x 10-4

15
 PIPX 5.11 x 10-4 ± 0.62 x 10-4

SEX 1.48 x 10-4 ± 0.09 x 10-4

A similar methodology was repeated using the PAL3 sampler; however, the addition of

the incubated agitator allows for the determination of k at varying temperatures. Another

adaption involved sampling from a single vial over time rather than from multiple vials due to the

capacity of the incubator. The rate constants for each xanthate at varying temperatures are shown

in Table 2. An important feature to note is that all the values are smaller than those calculated

using the 7650A auto sampler (Table 1). At 30 oC the rate constants were 3.71 x 10-6, 4.39 x 10-6,

1.86 x 10-6, and 4.03 x 10-6 h-1 for PAX, SIBX, PIPX, and SEX respectively at 30 oC. These

values did not increase significantly when the temperature was increased to 50 oC for PAX but

did increase slightly for SIBX, PIPIX, and SEX. At 80 oC all the rate constants except for SIBX

increased by an order of magnitude. These results are comparable to those published by Zhongxi

and Forsling (1997) which reported rate constants to the order of 10-6 for ethyl xanthates at 40 oC

using UV/Vis spectrophotometry.

16
Table 2 Rate constants for PAX, SIBX, PIPX, and SEX determined using the PAL3 auto
sampler. The constant was calculated using curve of CS2 generated over time.
Xanthate Temperature (oC) k (h-1) ± uncertainty

30 3.71 x 10-6 ± 0.56 x 10-6

PAX 50 4.26 x 10-6 ± 0.23 x 10-6

80 1.10 x 10-5 ± 0.19 x10-5

30 4.39 x 10-6 ± 0.60 x 10-6

SIBX 50 7.09 x 10-6 ± 0.59 x 10-6

80 8.19 x 10-6 ± 1.13 x 10-6

30 1.86 x 10-6 ± 0.27 x 10-6

PIPX 50 5.59 x 10-6 ± 0.59 x 10-6

80 1.31 x 10-5 ± 0.34 x 10-5

30 4.03 x 10-6 ± 0.19 x 10-6

SEX 50 5.46 x 10-6 ± 0.56 x 10-6

80 1.41 x 10-5 ± 0.08 x 10-5

Comparing the two methods, it appears that the 7650A auto sampler system appears

superior for the determination of the decomposition kinetics of xanthates; however, a few

considerations must be taken into account. Firstly, the 7650A method utilizes a different analysis

method where no vial is sampled from more than once. 8 samples are prepared into 8 separate

vials and each is analyzed once to generate the curve. The PAL3 system uses an incubator that

has the capacity for 6 vials. The method was modified to not only accompany the restrictions of

the incubator, but also to automate the method further for studies at higher temperatures. Rather

than separating each xanthate into a number of different vials, one vial was utilized for each

17
xanthate and was repeatedly sampled over time. This allowed for a much higher throughput of

samples and a level of automation that would be applicable to an industrial setting. The

consequence of this is that the septum of the sample vial is punctured multiple times which can

create leaks where volatile CS2 can escape. This might explain why the slopes generated from the

PAL3 curves are reduced, over the time course of the study some of the CS2 was likely escaping,

which would artificially reduce values for the slope of the line.

There may be a few reasons the results presented here differentiate from those in the

literature. In terms of developing a higher throughput method for use in industry all solutions

were prepared without pH adjustment. It was noted that the innate pH of xanthate solutions are all

around pH 8.5 and degradation is not severely impacted until the pH drops below 6 (Kemppinen,

Aaltonen, Sihvonen, Leppinen, & Siren, 2015). Values reported by Shen and colleagues (2016)

used a pH of 6.8 for analysis while Zhongxi and Forsling (1997) used a pH of 8 and 6.6 and

reported similar values for both. The methods used here all differ from Zhongxi and Forsling’s

(1997) study as the decrease in xanthate concentration was monitored rather than the evolution of

CS2. The method also utilized UV/Vis spectrophotometry and bands could not be assigned

unambiguously to xanthates and their related compounds giving larger uncertainties in

quantification compared to headspace GC-MS methods that measure CS2 directly. Both of the

comparative studies also only analyzed a single xanthate (SIBX or ethyl xanthate) so no

comparison can be made for both PAX and PIPX.

3.4 Thermodynamic Studies

Using the data generated from the PAL3 auto sampler the thermodynamic properties of

each xanthate were also investigated. A plot of Ln (k) against the reciprocal of temperature was

generated according to the Arrhenius equation. The activation energy (Ea) for the degradation of

18
each xanthate into CS2 was found by multiplying the slope of the graph (Ea/R) by the ideal gas

constant R (8.314 j mol-1). The activation energies were found to be 19.83, 10.80, 34.44, and

22.64 kj/mole for PAX, SIBX, PIPX, and SEX respectively. Only literature values for the

activation energy of O-ethyl S-methyl xanthate could be found in the literature (Adejoro, Esan,

Adeboye, & Adeleke, 2017). The researchers reported an Ea of 166.200 kj/mol for the S-

methylated ethyl xanthate. The values reported in this research are significantly lower which is

likely due to the chemical difference between the species. The methylated sulphur reduces the

overall polarity of the xanthate making it less susceptible to degradation reactions in water. This

would increase the activation energy so that more energy is needed to initiate the degradation of

the methylated compound. Additionally, Adejoro and colleagues (2017) utilized an in silico

quantum mechanics methodology to investigate thermodynamics rather than a direct method.

This work has shown that the PAL3 method may be suitable for investigating the thermodynamic

properties of the xanthates in tandem with their kinetic properties. This was a pilot study that

focused on simple data manipulation. Further work needs to be done to expand the Arrhenius

curve to produce results with analytical applicability.

19
Figure 9. Arrhenius plot of ln(k) against the reciprocal of temperature. Temperature used were
30 °C, 50 °C, and 80 °C. All k values were calculated from data generated using the PAL3 auto
sampler.

Table 3 Activation energy of PAX, SIBX, PIPX, and SEX determined using the Arrhenius
equation and the rate constants calculates using the PAL3 auto sampler.

Xanthate Ea (kj/mole)

PAX 19.83

SIBX 10.80

PIPX 34.44

SEX 22.64

20
4. CONCLUSIONS

Headspace GC-MS was shown to be a suitable technique for studying the decomposition

kinetics of xanthates in aqueous matrices at ambient and elevated temperatures. Conditions were

optimized for the GC-MS using both a 7650A and PAL3 auto sampler system. Both systems

were successfully calibrated for the detection of volatile CS2 and xanthates were confirmed to

follow first order decomposition into CS2. Rate constants were successfully calculated at 25, 30,

50, and 80 oC; however, the two sampling systems did not produce comparable results to one

another but did reflect values presented in the literature for both headspace GC-MS methods, and

UV/Vis spectrophotometric methods. This work has developed methods for both non-headspace

specific (7650A) and headspace dedicated (PAL3) instrumentation. The methods automation and

potential for high throughput make it ideal for use in an industrial setting where sample turnover

is fast and sample prep should be minimized. This work also provides foundations for analytical

studies into the degradation kinetics of xanthates in solution and decomposition thermodynamics.

21
5. FUTURE WORK

Future work for this research should focus on increasing the analytical certainty of the values.

Increasing the repeats of each sample and using fresh headspace vials and lids for each analysis

will likely accomplish this. Since xanthate degradation is heavily pH dependant future studies

should also explore how k changes in response to changes in pH. Additional thermodynamic

studies can also be carried out to further elucidate the thermodynamics of the degradation

reaction. Finally, since the floatation tank is a concoction of chemical activity, it may be

beneficial to monitor for any potential destabilizing or stabilizing cosolutes in solution that may

affect xanthate degradation. As more research is conducted a catalogue of the decomposition

behaviour of each xanthate can be developed to allow for mining operations to adjust current

floatation protocol to correct for any xanthate degradation that may be affecting mineral recovery

in circuit.

22
6. LITERATURE CITED

911 Metallurgist. (2017). 911 Metallurgist. Retrieved April 02, 2018 from 911 Metallurgist:
www.911metallurgist.com/blog/flotation_collectors

Adejoro, I., Esan, T., Adeboye, O., & Adeleke, B. (2017). Quantum mechanical studies of the
kinetics, mechanisms, and thermodynamics of gas-phase thermal decomposition of ethyl
dithiocarbonate (xanthate). Journal of Taibah University for Science , 11 (5), 700-709.

Bacher. (2016, April 01). Gas Chromatography Theory. Retrieved April 02, 2018 from UCLA
Chemistry: www.chem.ucla.edu/~bacher/General/30BL/gc/theory.html

Chemical Book. (2017). Carbon Disulphide . Retrieved April 14, 2018 from Chemical Book:
www.chemicalbook.com/ChemicalProductProperty_EN_CB6279761

Clinical Mass Spectrometry. (1995). Basic theory of mass spectrometry. Clinica Chimica Acta ,
241, 15-71.

Hao, F., Silvester, E., & David, G. (2000). Spectroscopic characterization of ethyl xanthate
oxidation products and analysis by ion interaction chromatography. Analytical Chemistry
, 72 (20), 4836-4845.

Kemppinen, J., Aaltonen, A., Sihvonen, T., Leppinen, J., & Siren, H. (2015). Xanthate
degradation occuring in floatation process waters of a gold concentrator plant. Minerals
Engineering , 80, 1-7.

Kim, I., Suh, S., Lee, I., & Wolfe, R. (2016). Applications of Stable, Nonradioactive Isotope
Tracers in In Vivo Human Metabolic Research. Experimental and Molecular Medicine ,
48 (1), 1-10.

Li, N., Chen, Y., Zhang, C., Chen, W., Fu, M. Y., Fu, M., et al. (2015). Highly Sensistive
Determination of Butyl Xanthate in Surface and Drinking Water by Headspace Gas
Chromatography. Chromatographia , 78, 1305-1310.

Mining World. (2017, June). Alternatives to xanthate collectors for sulphide flotation
applications. Mining World Processing , 14 (3), pp. 24-28.

Mustafa, S., Hamid, A., Naeem, A., & Sultana, Q. (2004). Effect of pH, Temperature and Time
on the Stability of Potassium Ethyl Xanthate. Journal of the Chemical Society of Pakistan
, 26 (4), 363-366.

Rezaei, R., Massinaei, M., & Zeraatkar Moghaddam, A. (2018). Removal of residual xanthate
from flotation plant tailings using modified bentonite. Minerals Engineering , 119, 1-10.

23
Shen, Y., Nagaraj, D., Farinato, R., & Somasundaran, P. (2016). Study of xanthate
decomposition in aqueous solutions. Minerals Engineering , 93, 10-15.

Taguta, J., O'Connor, C., & McFadzean, V. (2017). The effect of alkyl chain length and ligand
type of thiol collectors on the heat of adsorption and floatability of sulphide minerals.
Minerals Engineering , 110, 145-152.

Trudgett, M. (2005). The Ultra-Trace Level Analysis of Xanthates by HIgh Performance Liquid
Chromatography. University of Western Sydney, Physcial Sciences . Sydney: University
of Western Sydney.

World Health Organization. (2000, April). Carbon Disulfide. Retrieved April 14, 2018 from
www.inchem.org: www.inchem.org/documents/icsc/icsc/eics0022.htm

Zhongxi, S., & Forsling, W. (1997). The degradation kinetics of ethyl-xanthate as a function of
pH in aqueous solution. MInerals Engineering , 10 (4), 389-400.

24
 7. APPENDIX

7.1 Chromatograms of Xanthate Standards

750 ppm PAX

750 ppm SIBX

25
750 ppm PIPX

750 ppm SEX

26
7.2 Mass Spectra of Xanthate Degradation Process

PAX

27
SIBX

28
PIPX

SEX

29