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

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.

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)

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

Schematic of a Gas Chromatograph

Overview

1. 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

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

Basic Principles of GC Separation

•If KA > 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

Elution from a GC Column

1. 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

Example:Liquor samples are converted to methyl esters in order to make them volatile

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 > 1.5.

GC Components

The major components of a GC system are:

• Gas supply

• Injection System

• Column

• Detector

• Oven

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)

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 Injector2. 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

Injectors

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

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

GC Columns

Picture of a packed GC column Picture of a capillary GC column

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.

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

Specific GC Detectors

• Flame Ionization Detector (FID)

The FID consists of a hydrogen / air flameand a collector plate.Effluent from the GCcolumn passes through the flame whichbreaks 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 coversmany applications,and it’s only disadvantageis that it destroys the sample.

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.

Specific GC Detectors

• Mass Spectrometer (MS)-Uses the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate themfrom 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. TheTCD 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 andphosphorus.

GC Oven

• 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

•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.

GC Oven

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.

•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.

Quantification in GC

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.

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.

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

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.

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.

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 impurities:measurement 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.

AppendixIon Chromatogram of an Anion Standard Solution (Dionex, 1998)

(Oxalate elutes after 8 Minutes)

Acknowledgment

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