Max Wellington ID UD3587SCH8551

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Max WellingtonID UD3587SCH8551

• Course
• INSTRUMENTAL METHODS CHROMATOGRAPHY
• Topic
• GAS CHROMATOGRAPHY AND ITS USE IN THE
  DETERMINATION OF OXALATES IN BAYER PROCESS
  LIQUORS
• ATLANTIC INTERNATIONAL UNIVERSITY

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INTRODUCTION

• Organic carbon enters the Bayer process liquors
  from bauxite in the form of humic substances (Rao
  and Goyal, 2006)
• 5-10 of organic carbon is converted to Sodium
  Oxalate (Lever, 1983 Grocott, 1988)
• The recycling of Bayer liquor will result in
  oxalate build up and ultimate preciptation as
  fine needles in the cooler parts of the circuit
  (Sipos, 2001).
• Sodium oxalate has been shown to be harmful to
  alumina productivity and size (Calalo and Tran,
  1993 Brown and Cole, 1980) and so its control
  and removal is critical to Bayer process
  productivity (The and Bush, 1987).
• Effective Oxalate control requires its accurate
  determination in process liquor streams.
  Chromatography can be used to facilitate the
  isolation and determination of oxalates from the
  milieu of structurally similar impurities as
  found in Bayer liquor.
• This Presentation will look at the use of Gas
  Chromatography and its use in the analysis of
  oxalate in Bayer liquor streams.

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INTRODUCTION TO GAS CHROMATOGRAPHY

• Gas chromatography is a process by which a
  mixture is
• separated into its constituents by a gas
  phase moving
• over a stationary phase
• Mobile Phase gas
• Stationary Phase liquid (or solid)

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Advantages of Gas Chromatography

• High Resolution Separations
• - Analysis of complex mixtures

• Greater sensitivity

• Better Peak Shape
• -Greater Efficiency
• -Greater Inertness

• More accurate Qualitative Analysis
• - Analysis of Closely Related Substances

• More accurate Quantitative Analysis

• Easy to maintain and durable

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Schematic of a Gas Chromatograph
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Overview

• The sample solution is injected into the heated
  (250 oC) injection port where
• it is rapidly volatilized

2. The volatilized sample is then swept via the
carrier gas into the heated column where
volatile compounds separate and are eluted
separately
3. The eluted compounds are then detected in a
heated detector to give an electrical signal
which is amplified and recorded.
4. The output is a plot of detector response vs.
time called a chromatogram
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Basic Principles of GC Separation

• Different compounds have different partition
• coefficients, K

• Example Compounds A and B

KA CmA / CsA
KB CmB / CsB

• CmA conc. of A in mobile phase

• CsA conc. of A in stationary phase

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Basic Principles of GC Separation

• If KA gt KB then compound A spends more time
  on
• average than compound B in the mobile phase

• Compound A migrates faster and separation occurs,
  and
• A is eluted first

• The Partition coefficient, K depends on the
  volatility
• (and bp) of the compounds being separated

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Elution from a GC Column

• Compounds are eluted in order of volatility, ie
  the most
• volatile (generally lowest boiling) off
  column first
• 2 . Compounds must be volatile to pass through
  the column
• If not, they must be chemically modified
  to do so
• ExampleLiquor samples are converted to methyl
  esters in
• order to make them volatile

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Separation Efficiency (Resolution)
The resolution (Rs) between two peaks in a
chromatogram is given by
where Z is the separation between peaks A and B
and Wa and Wb are the widths at the base of peaks
A and B, respectively.
Acceptable resolution is on the order of Rs
1.0, and baseline resolution between two peaks
(as shown in the figure) requires an Rs gt 1.5.
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GC Components
The major components of a GC system are

• Gas supply

• Injection System

• Column

• Detector

• Oven

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Gas Supply

• Inert gases are commonly used as the Mobile
  Phase for GC work

• The most commonly used gases are Helium, Argon,
  Hydrogen and
• Nitrogen

• Some GC units use a detector gas depending on
  the application and
• the type of detector.

• Example Hydrogen gas is commonly used with a
  Flame Ionization
• Detector (FID)

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Injection System

• The injection port consists of a rubber septum
  through which a syringe
• needle is inserted to inject the sample.

• The injection port is maintained at a higher
  temperature than the boiling
• point of the least volatile component in the
  sample mixture

• There are two types of injectors
• 1. Normal Packed Column Injector
• 2. Split/Splitless capillary Injector

• In the packed column injector, ALL the
  vapourized sample enters onto
• the column

• In the split/splitless injector the amount of
  vapourized sample that enters
• onto the capillary column can be controlled

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Injectors
Schematic of packed GC column injector
Schematic of split/splitless GC column injector
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GC Columns

• Gas chromatography columns are of two designs
• 1. Packed column
• 2. Capillary column

• Packed columns are typically a glass or
  stainless steel coil(typically 1-5m
• total length and 5mm inner diameter) that is
  filled with the stationary phase,
• or a packing coated with the stationary phase

• Capillary columns are a thin fused-silica
  (purified silicate glass) capillary
• (typically 10-100m in length and 250 micron
  inner diameter) that has the
• stationary phase coated on the inner surface.

• Capillary columns provide much higher separation
  efficiency than packed
• columns but are more easily overloaded by too
  much sample

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GC Columns
Picture of a packed GC column
Picture of a capillary GC column
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Stationary Phases

• The most common stationary phases in gas
  chromatography columns are
• polysiloxanes, which contain various
  substituent groups to change the
• polarity of the phase.

• The nonpolar end of the spectrum is polydimethyl
  siloxane, which can
• be made more polar by increasing the
  percentage of phenyl groups on
• the polymer.

• For very polar analytes, polyethylene glycol
  (also known as carbowax)
• is commonly used as the stationary phase

• After the polymer coats the column wall or
  packing material, it is often
• cross-linked to increase the thermal stability
  of the stationary phase and
• prevent it from gradually bleeding out of the
  column.

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Detectors

• After the components of a mixture are separated
  using gas chromatography,
• they must be separated as they exit the GC
  column.The requirements of a
• GC detector depends on the separation
  application.

• Example One analysis might require a detector
  that is selective for
• chlorine-containing molecules, another
  analysis might require a detector
• that is non-destructive so that the analyte
  can be recovered for further
• analysis

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Specific GC Detectors

• Flame Ionization Detector (FID)

The FID consists of a hydrogen / air flame and a
collector plate.Effluent from the GC column
passes through the flame which breaks down
organic molecules to produce ions.The ions are
collected on a biased electrode and produces an
electrical signal. The FID is extremely sensitive
and covers many applications,and its only
disadvantage is that it destroys the sample.
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Specific GC Detectors

• Atomic Emission Detector (AED)

- Simultaneously determines the atomic emission
of many elements in analyte that elutes from
GC capillary column.

• Chemiluminescence Detector (CD)

• Uses quantitative measurements of optical
  emission from excited chemical species to
• determine analyte concentration (energized
  molecules)

• Electron Capture Detector (ECD)

• Uses a radioactive beta emitter (electrons) to
  ionize some of the carrier gas and produces a
• current between a biased pair of electrodes.Has
  application for organic functional groups such
• as halogens,phosphorus and nitrogen compounds.

• Flame Photometric Detector (FPD)

• Used to detect phosphorus and nitrogen containing
  compounds.Uses the chemilumiescent
• reactions of these compounds in a hydrogen / air
  flame.

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Specific GC Detectors

• Mass Spectrometer (MS)

-Uses the difference in mass-to-charge ratio
(m/e) of ionized atoms or molecules to separate
them from each other.

• Photo Ionization Detector (PID)

-Uses ultraviolet light as a means of ionizing an
analyte exiting from a GC column

• Thermal Conductivity Detector (TCD)

• Consists of an electrically-heated wire or
  thermistor.Changes in thermal conductivity such
  as
• when organic molecules displace some of the
  carrier gas, cause a temperature rise which is
  sensed
• as a change in resistance.Change in resistance
  is proportional to the amount of analyte. The
• TCD is not as sensitive as the other detectors
  but it is non-specific and non-destructive.

• Nitrogen-Phosphorus Detector (NPD)

• Similar in design to a FID but with selectivity
  for compounds containing nitrogen and
• phosphorus.

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GC Oven
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GC Oven

• The oven consist of a wire coil that radiates
  into the inner volume of the
• oven.Heat from the resistive wire source is
  spread in an even manner,
• throughout the oven volume using a fan
  attached to an electric motor. A
• thermocouple inside the oven is part of
  regulating the oven temperature
• via the amount of heat released by the heating
  element.

• The GC oven is used to keep the column at
  temperatures between 40 to
• 350oC

• Most liquids must be converted to vapour state
  and maintained as a vapour
• throughout the GC separation

• GC ovens are temperature-programmed to allow
  separation of analytes
• spanning a range of vapour pressures in a
  single analysis

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Quantification in GC
Internal Standard Method

• A known amount of reference substance is added
  to the sample before
• injection onto the column.

Why use an internal standard?

• It eliminates any variations in those factors
  which influence the sensitivity
• and response of the detector.

Characteristics of an Effective Internal Standard

• The internal standard must be volatile
• The internal standard must produce completely
  resolved (separated)
• peaks in the chromatogram, and be eluted close
  to the analytes of interest.

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Quantification in GC

• The peak height or peak area for the internal
  standard peak
• in the chromatogram should be similar to those
  of
• the components to be measured.
• The internal standard should be chemically
  similar to the sample components
• of interest.
• The compound to be used as the internal standard
  must not be naturally
• present in the original sample.

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Oxalate Analysis by GC

• Oxalate standards are prepared from pure Sodium
  Oxalate,dried at 110oC
• for five hours.
• Both standard and samples are diluted with
  de-ionized water
• An internal standard and derivatizing reagent
  components are added to
• both standards and samples.
• Standard and samples are then digested at 65oC
  for 30 minutes in a water
• bath fitted with a rotating carousel.
• The organic layer of each standard and sample is
  extracted and placed in
• 2ml vial which are sealed with a rubber cap.
• Both standards and samples are analyzed using
  GC.
• Calibrations and results are processed using the
  instrument software.

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Reagents used and Reason(s)
Monochloroacetic acid

• Used as an internal standard.Very similar in
  structure and has a different
• retention time to the peaks of interest.Also,it
  is not present in the samples

Methanol

• The compounds are converted to the methyl esters
  using the methanol as
• these have relatively simple structure, low
  boiling points, and can be easily
• analyzed using FID detectors.

Sulphuric Acid
-Strong, pure acid is used to increase the ionic
strength of the solution and increase the
partition coefficient between the aqueous and
organic layers.
Chloroform

• The organic layer to which the methyl esters are
  extracted.Has the advantage of
• being a low boiling point solvent, which allows
  the sample to be vapourized and
• analyzed by GC FID.Also, it is not miscible with
  water.

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Bayer Liquor Chromatogram from GC unit (Oxalate
elutes at 3.25 minutes)
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Discussion and Conclusion

• Gas chromatography presents a simple and
  relatively rapid method for oxalate determination
  in Bayer liquor streams. The sample oxalate
  concentration is determined by comparing with
  peak area of internal standard.
• Gas chromatography is less costly in terms of
  maintenance when compared to ion chromatography
  as ion chromatogram requires the regular
  replacement of Guard Columns, Analytical Columns
  and Suppresors which are quite costly.
• In Bayer plants where Sulfate and Chloride levels
  are a concern then ion chromatography may be the
  method of choice as it can analyze each sample
  for a number of anions e.g Sulfate, Chloride,
  Oxalate, Fluoride, Nitrate and Phosphate (see
  Appendix 1) whereas the Gas Chromatogram can only
  be used for specified organic analyses of which
  only oxalate is of major concern to Bayer process
  operations.

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Reccomendations

• Gas Chromatography be used for the analysis of
  oxalates in Bayer process liquor streams in
  instances where anions such as Sulfate and
  Chlorides are not a concern.
• In cases where anions such as Sulfates and
  Chlorides are a concern then it is reccomended
  that ion chromatography be used.

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References

• Barnett N W, Bowser T A and Russel R A (1995)
  Determination of oxalate in alumina process
  liquors by ion chromatography. Analytical
  Proceedings and Communications, Vol 32, p57-59.
• Brown N and Cole T J (1980) The behaviour of
  sodium oxalate in a Bayer alumina plant. Light
  Metals, 105-117.
• Calalo R and Tran T (1993) Effects of sodium
  oxalate on the precipitation of alumina
  trihydrate from synthetic sodium aluminate
  liquors. Light Metals, 125-133.
• Grocott S C (1988) Bayer liquor
  impuritiesmeasurement of organic carbon, oxalate
  and carbonate extraction from bauxite digestion.
  Light Metals, 833-841.
• Harris Daniel (1996) Exploring chemical analysis.
  W.H. Freeman and Company, New York.
• Lever G (1978) Identification of organics in
  Bayer liquor (1978) Light Metals, 71-83.
• Rao K V and Goyal R N (2006) Organic carbon in
  indian bauxites and its control in alumina
  plants. Light Metals, 71-74.
• Sipos G (2001) The mechnism and action of sodium
  oxalate seed stabilizer molecules under Bayer
  conditions. PhD Thesis, School of Applied
  Chemistry, Curtin University of Technology,
  Australia.
• Skoog D A, Holler J F and Nieman T (1998)
  Principles of instrumental analysis. Harcourt and
  Saunders College Publishing, Chicago.
• The P J and Bush J F (1987) Solubility of sodium
  oxalate in Bayer liquor and a method of control.
  Light Metals, 5-10.

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AppendixIon Chromatogram of an Anion Standard
Solution (Dionex, 1998)(Oxalate elutes after 8
Minutes)
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Acknowledgment

• Acknowledgment is extended to Mr Glenroy
  Lawrence, Chemist of Jamalco for supplying some
  of the data used in this presentation.