Analytica Chimica Acta 689 (2011) 8–21
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Analytica Chimica
Acta
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a c
小月子餐a
费仲
A review of the determination of organic compounds in Bayer process liquors
Greg Power a ,∗,Joanne S.C.Loh a ,Johannes E.Wajon b ,Francesco Butti c ,Cynthia Joll c
a
CSIRO Light Metals Flagship (CSIRO Process Science and Engineering)/Parker Centre,PO Box 7229,Karawara,WA 6152,Australia b
Wajon and Associates,16Eckersley Heights,Winthrop,WA 6150,Australia c
Curtin Water Quality Rearch Centre,Department of Chemistry,Curtin University G.P.O.Box U1987,Perth,WA 6845,Australia
a r t i c l e i n f o Article history:
Received 21December 2010
Received in revid form 17January 2011Accepted 18January 2011
Available online 25 January 2011Keywords:
Organics analysis Bayer process Review
a b s t r a c t
Bayer process liquors prent a difficult and complex matrix to the analytical chemist,and the history of the application of modern analytical techniques to this problem is a ca study in innovation.All Bayer process liquors contain organic compounds,in amounts varying from traces to veral grams per litre.The total organic carbon content of Bayer liquors may be less than 5g/L up to as much as 40g/L.The prence of the organic impurities is of concern to Bayer technologists becau they can have significant impacts on the economics of the process and the quality of the product.This review examines the history and current state-of-the-art of the analysis of organics in Bayer process liquors,and provides guidance on the applicable techniques matched to a comprehensive list of the compounds most likely to be prent.
属猴和属猪
Crown Copyright © 2011 Published by Elvier B.V. All rights rerved.
1.Introduction
The Bayer process,by which bauxite is treated with strong sodium hydroxide to refine alumina,is applied to about 97%of the over 200million annual tonnes of bauxite mined globally.The organic compounds prent in bauxite are primarily complex,water-insoluble materials derived from plant and animal matter,and include humic and fulvic matter,lignins and cellulo.Rela-tively minor amounts of organic compounds enter the liquor from other sources which include various chemical additives such as flocculants (in some cas starch but predominantly synthetic floc-culants),dewatering aids,crystal modifiers and water treatment chemicals [1].
The organic carbon content of bauxite is generally in the range 0.02–0.50%(w/w,carbon basis)[1].Bauxite digestion is usually carried out at temperatures in the range 135–245◦C at sodium hydroxide concentrations in the region of 3.5–5molar [2].Under the conditions a significant proportion of the organic matter prent in the bauxite is extracted into the liquor [1]or relead through the formation of volatile organic compounds [3].The compounds extracted into the liquor undergo alkaline degrada-tion reactions which lead to a predominance of low to medium molecular
weight compounds at steady-state,with typically 90%of compounds in the molecular weight range 90–500Da [1].
The prence of organic impurities in the liquor has significant implications for all aspects of the Bayer process,including process yield,product quality [4,5],scale formation [6]and environmental
∗Corresponding author.Tel.:+61410496400.E-mail address:greg.power@csiro.au (G.Power).
emissions [7],all of which affect the overall viability of the process as well as being a key factor in the design of each specific plant.The determination of organic impurities has been the subject of significant developmental effort and continues to be an active area of rearch.
2.Historical perspective
The prence of organic compounds in bauxite was recognid very early in the history of the extraction of alumina from bauxite.Only 13years after Bayer patented his process [8],improvements patented by C.M.Hall included heating bauxite to burn off organic matter prior to digestion [9].A study of the processing of Urals bauxite indicated the prence of soluble and insoluble organic matter [10],and that the soluble organic matter was 58%carbon.Utley reported that Arkansas bauxite contai
ned 0.3–0.4%organic matter which was about 50%carbon [11].Most of the bauxite now being mined originates from lateritic deposits which are or have been overlain by forests.Bauxite genesis relies on lective leach-ing of minerals by water percolation,resulting in a layered profile as depicted schematically in Fig.1.In such a profile the organic carbon content varies from a maximum in the order of 1%in the overburden to a minimum of 0.1%or less in the clay floor of the deposit [1].
The soluble organics which enter Bayer process liquors impart a red-brown colour to the liquor,the exact shade and intensity of which depends on both the bauxite source and the process-ing conditions [12].The significance of the prence of organics in the liquor appears to have been first highlighted in the English-language literature in Pearson’s 1955monograph on the aluminium industry [12],in which it was noted that the organic matter in
0003-2670/$–e front matter.Crown Copyright © 2011 Published by Elvier B.V. All rights rerved.doi:10.1016/j.aca.2011.01.040
G.Power et al./Analytica Chimica Acta689 (2011) 8–21
9
Fig.1.Schematic reprentation of a typical lateritic bauxite profile from Western Australia(diagram reproduced with the permission of BHP Billiton Worsley Alumina)[1].
liquor originated mainly from the bauxite,and that it had a number of negative influences on the oper
ation and economics of refin-ing operation,and on the quality of the product.A more detailed account of the origins and effects of organics in the Bayer process is given in the review by Solymár and Zsindely[13]of bauxites then being procesd in Hungary.The techniques available for the investigation of organic compounds in highly alkaline liquors at that time involved lengthy and complex wet chemical techniques [14],so the investigations in the industry were probably limited to determination of total organic carbon(TOC)by classical techniques such as permanganate titration[15,16].
It has long been known that oxalate is formed in the Bayer pro-cess,becau sodium oxalate has limited solubility at high pH, and so can crystallize out if the organic input to the refinery is sufficient[12,17].However,a deeper understanding of the nature and reactions of organic compounds in Bayer liquors awaited the development of instrumental techniques for organic determina-tion.Specialid sample preparation techniques also had to be developed for application to the highly concentrated and complex matrix of Bayer liquors.
Table1summaris as a timeline the main innovations in analyt-ical techniques and their application to the analysis of Bayer liquors in the past30years.Details of the application of the techniques to individual analytes are given in Appendix A.
3.Sample preparation and fractionation
The methods of sample preparation for the analysis of Bayer liquors range from a simple dilution in water to complex prepa-ration procedures,depending on the information required and the analytical methods to be ud.The following ctions summari the most important methods,in order of complexity.
3.1.Dilution,neutralization and acidification
Dilution in water has been found to be satisfactory for the deter-mination of the most prevalent organic anions prent in Bayer liquor by a number of analytical techniques.This has the advantages of simplicity and speed,which are particularly important for rou-tine applications.In addition,the risk of artefacts due to incomplete extraction or adsorption,loss encountered during fractionation or evaporation,and so on,are avoided.However,the dilutions required are often quite high(at least1:200,and often1:1000or more[18,19]),which limits both the nsitivity and the variety of compounds that can be analyd by the analytical technique that follows,such as ion chromatography(IC)or capillary zone elec-trophoresis(CZE).Nevertheless,simplicity of sample preparation was one of the key reasons for the early adoption of IC as a rou-tine method for the dete
张九州rmination of oxalate and other important anions in Bayer liquors[20],and remains the method of choice for analysis in that application.
Lever[21]ud CO2to reduce the alkalinity of the liquor and remove the aluminate content prior to determination under con-ditions favourable to the formation of a mixture of dawsonite (sodium aluminium carbonate)and aluminium hydroxide,which were then removed byfiltration.This procedure has the advantage of removing most of the aluminate from solution without adding mineral acid anions.The solution was then pasd through a cation exchange resin to ensure that all of the organic compounds were in their acid forms to facilitate derivatization,neutralized with NaOH, and evaporated to dryness prior to being butylated for determina-tion by gas chromatography(GC)with aflame ionization detector.
Guthrie et al.[22]ud a simpler procedure in which concen-trated HCl was added directly to a liquor sample in an ice bath until the precipitated aluminium hydroxide was redissolved.The resulting solution was butylated and analyd for low molecu-lar weight compounds by GC.To analy intermediate molecular weight compounds,the authors evaporated the butanol extracts to dryness and reacted the residues with“Tri-Sil”(dimethyl-(trimethylsilylamino)Si)to produce the silyl derivatives for GC analysis.Others[21,23]have ud methylation for this purpo. 3.2.Precipitation and liquid or
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solid pha extraction
The preparation for determination of the high molecular weight (“humic”)fractions of Bayer liquor generally involvesfirst precip-itating the“humic”material by acidification to a pH of2or less,as is the practice in the analysis of soils[24].Lever’s approach was to extract the precipitated organics with n-butanol and neutralize the extract with NaOH,followed by water-washing and ultrafiltration to produce a salt-free aqueous extract which could be parated into nominal molecular weight fractions by membranefiltration [21].Alternatively,Guthrie et al.[22]kept the initial steps of the sample preparation the same as described in Section3.1for GC analysis,and ud tetrahydrofuran(THF)as the solvent for the butyl esters.
Wilson et al.[25]cautioned against the precipitation of alu-minium hydroxide in the preparation of liquors for determination of high molecular weight compounds on the basis that this may result in loss of some organic compounds by adsorption to the aluminium hydroxide surface.They recommend a1:10dilution followed by rapid acidification to pH1.5with1:1HCl to precip-
10G.Power et al./Analytica Chimica Acta689 (2011) 8–21
Table1
30-year timeline for the development and application of modern analytical methods to Bayer liquors.
Year Method Species Determined First Citation
1978Gas chromatography withflame ionization
detection(GC)
Oxalate and other small organic anions[21]
1982Gel permeation chromatography with UV
detection(GPC)
High molecular weight organic compounds[29]
1982Liquid chromatography with UV detection(LC)Products of degradation of high molecular
weight compounds
[29]
1983Ion chromatography with UV detection(IC)Oxalate and other small organic anions,as well
as chloride,sulphate andfluoride
[48]
1984Gas chromatography-mass spectrometry
(GC–MS)
High molecular weight organic compounds[22]
1986UV absorbance“Humates”[82]
1990Thermal decomposition Total organic carbon(TOC)[20]
1992Capillary zone electrophoresis with
conductivity detection(CZE)Oxalate and other small organic anions,as well
as chloride,sulphate andfluoride
[19]
1996UV-catalyd persulphate oxidation Total organic carbon(TOC)[99]
1997Infra-red spectroscopy(IR)including Fourier
transform IR(FTIR)Structure and composition of the solid/liquid
interface
[88]
199813C NMR Functional groups of organic compounds[38] 1998Differential thermal analysis(DTA)and
differential scanning calorimetry(DSC)
General organic substances[38]
1999Pyrolysis gas chromatography mass
spectrometry(py-GC/MS)Type of high molecular weight organic
compounds
[25]
2002Liquid chromatography-tandem mass
spectrometry(LC–MS/MS)
Variety of organic compounds[27] 20031H NMR Functional groups of organic compounds and
quantitative determination of small organic
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anions
[92]
2005Multi-dimensional high performance liquid
chromatography with UV detection(HPLC)Potentially variety of high molecular weight
organic compounds
[61]
2006Fourier transform infra-red spectroscopy
(FTIR)Total organic carbon(TOC)and other solution
parameters
[87]
itate the“humic”materials,which are then parated byfiltration, redissolved in NaOH and extracted onto a polar adsorption resin (Amberlite XAD-7).The organics are subquently washed from the resin with deionized water to produce a neutral,salt-free solution containing the acid forms of the“humic”materials.The collected dried solid humic material is then extracted quentially with diethyl ether,ethyl acetate,isopropyl alcohol and water[26].
Picard et al.[27]tested two parate extraction methods which were followed directly by multidimensional chromatography and mass spectrometric analysis(e Section5.1).
Thefirst extraction method trialled was a liquid/liquid para-tion which ud three different solvents
of increasing polarity in succession(ether,ether/n-butanol,and n-butanol)to parate the organics on the basis of their polarity.
The cond method was solid-pha extraction using a hydrophobic C18stationary pha.This was ud to parate the organics into high,medium and low molecular weight(HMW, MMW and LMW)fractions.The HMW fraction was defined as the material which precipitated at low pH,the MMW as that which was soluble at low pH and was retained on the C18stationary pha,and LMW as the soluble fraction which was not retained.On this basis, for samples taken from10different Bayer process plants,it was found that the LMW fractions reprented between30and50%of the TOC of the spent liquors analyd[27].
Whelan and co-workers identified some anomalies from sol-vent extraction in which compounds with a range of polarities were found in a particular solvent[26].They suggested that the solubility of humic materials may be controlled by association,in which small molecules can aggregate by arranging their polar groups internally to produce relatively hydrophobic micellular structures which are more soluble in less polar organic solvents than might be expected. This phenomenon may be a function of concentration as suggested, or it could be an artefact of the extraction procedure which entails dissolving organic matter from concentrated Bayer humic material that has been dehydrated
and solidified.
Other approaches to the determination of molecular weight fractions are discusd in the next ction.
致运动员3.3.Fractionation by molecular weight and size
Separation of organic compounds into fractions bad on appar-ent molecular weight or molecular size by ultrafiltration(UF)or dialysis has been ud to characteri the organic matter prent in Bayer liquor.Lever[21]ud UF through membranes with nomi-nal molecular weight cutoff values(MW cutoff)of0.5,1,5and10kDa. The fractions collected were then evaporated and weighed to obtain a coar apparent molecular weight or size distribution.Dialysis into1.2,6,12,25,50,100,and300kDa MW cutoff fractions has been described by Wilson et al.[25,28].
Gel permeation chromatography(GPC),also known as size exclusion chromatography(SEC),has been ud to obtain a continuous apparent size distribution of the organic matter prent in Bayer liquor[29].Separation has been achieved on 500mm×7mm×140˚A and100A Spherosil100/200porous silica bead columns in ries[21]or on300×7.8mm×500˚A and100˚A -Styragel columns in ries[22]with UV detection to obtain a molecular weight distribution of Bayer liquor extracted with bu
tanol.Each of the chromatograms was a continuum with no discrete peaks.
It should be noted that SEC with UV detection underestimates saturated aliphatic carboxylic anions,which are known to consti-tute a significant proportion of the total organic carbon(TOC)in Bayer liquors but are not good absorbers of UV.A solution to this could be to incorporate continuous TOC(or,more strictly,dissolved organic carbon(DOC))detection as well[30,31],but application of this to Bayer liquors has not yet been reported.
All parations bad on molecular size should be treated with caution when applied to Bayer liquor extracts.Membranes and size exclusion gels have long been ud for the fractionation of pro-teins and peptides,for which purpo they are calibrated with
G.Power et al./Analytica Chimica Acta689 (2011) 8–2111
particles of known size which are uniform,spherical,non-polar and relatively chemically inert.The technique has been extended to the characterization of natural organic matter(NOM),but the interpretation of results becomes more complex becau para-tion is no longer purely on the basis of molecular size,and there are no universally applicable calibration standards[32,33].Inter-preting the information obtained from molecular size paration methods in such systems is therefore not str
aightforward,and can lead to gross errors when specific chemical and physical interac-tions between the analytes and the stationary pha are significant [34,35].Bayer liquor organics contain a high proportion of highly polar groups,in particular carboxylic acids.Electrostatic effects and hydrogen bonding can therefore be expected to play an important, even dominant,role in the retention behaviour,so correlations of retention times with molecular size alone are unlikely to be valid, and aggregation of small molecules into micellular structures noted in Section3.2can cau them to behave as if they have a much higher molecular weight than is in fact the ca[28].
Notwithstanding the complications,parations using mem-branes and columns of a variety of types are an important aspect of the sample preparation methodologies available for investigating the nature of Bayer liquor organics.本年
3.4.Consolidated sample preparation strategy for determination
of the high molecular weight fraction
Using the experience outlined in the previous ctions,a prepa-ration method suitable for the determination of the high molecular weight fractions of Bayer liquors is that developed by Wilson et al.
[25].To enable this quite complex scheme to be appreciated visu-ally,we have prepared aflow-sheet reprentation of it(Fig.2).It consists of the following ven main elements:
1.dilution
2.acidification to precipitate humics
5.washing and elution
6.acidification on Amberlite120resin
7.filtration and aliquot preparation/storage.
This scheme enables quantitative paration of the high molec-ular weight organics from the liquor,to produce a stock solution of the organics in acid form,free from aluminate and other salts. The stock solution may be sub-sampled for size paration and/or other determinations,or freeze-dried for storage.
4.Chromatographic paration
4.1.Gas chromatography(GC)
According to the published literature,thefirst significant advance in the application of modern chromatographic techniques to the determination of organic compounds in Bayer liquors was the work of Lever in the1970s[21],in which capillary GC was ud to analy species with low to medium molecular weights (40–350Da).The method relied on methylation and butylation of methanol extracts using diazomethane or diazobutane in ether to produce volatile compounds suitable for paration by gas chro-matography.Using the methods,Lever was able to confirm the identity and quantify the amounts offive key degradation products already believed to be prent:formate,acetate,lactate,oxalate and succinate.He was also able to identify a range of previously unidentified molecules,in particular a comprehensive range of mono-aromatic carboxylic acids from the degradation of humic molecules[21].The low molecular weight molecules were iden-tified and quantified by comparing their respons in theflame ionization detector(FID)to known standards.
GC quickly became the basis of a variety of methods for the investigation of low to medium molecula
r weight organic com-pounds in Bayer liquors.The differences in the methods ud by various workers were atfirst mainly in the sample preparation techniques ud,but later developments in column technology and detection methods have also had a significant influence.
The main variants in the derivatization methods are as follows:•Methylation of a methanol extract using diazomethane in ether [21,36],or by direct application to Bayer liquor using acidified tris(hydroxymethyl)aminomethane in chloroform and methanol [37];
•Methylation of an aqueous solid pha alkaline extract using tetrabutylammonium hydroxide added at pH8.5[38],or of a Bayer liquor butanol extract using acidified methanol[39];•Butylation:Lever[21]derivatized dried neutralid Bayer liquor using acidified butanol.Baker et al.[40]derivatized butanol extracts using acidified butanol in a microwave oven,Guthrie et al.[22]and Wellington and Valcin[41]derivatized Bayer liquor directly using acidified butanol,while Xiao[39]derivatized acidified,solvent extracted Bayer liquor using acidified butanol followed by hexane extraction.
Caution must be exercid in the u of derivatization tech-niques and in the interpretation of the results obtained.For example,Wilson et al.[38]found that the methyl ester did not form quantitatively f
or some compounds and that some methyl esters were non-volatile.Xiao[39]found that loss of low molecular weight acids could occur due to evaporation during concentration procedures.He also found that butylation could result in dibutyl ether artefacts,that it was difficult to identify unknowns from their butyl derivatives,and that butylation was not uful for high molec-ular weight acids.Xiao therefore recommended that methylation and butylation be ud in combination to optimi recoveries and improve the confidence in the identification of analytes.
Guthrie et al.[22]derivatized the butanol extracts with Tri-Sil for the determination of low and intermediate molecular weight aliphatic and aromatic acids.Silylation(using hexamethyldisi-lazane and trimethylchlorosilane)was also ud by Ellis et al.[42]to analy plant extracts and digested plant extracts.Using this proce-dure,it was possible to determine low and intermediate molecular weight mono-,di-and tri-carboxylic aliphatic and aromatic acids, aliphatic and aromatic hydroxy carboxylic acids,polyhydric alco-hols,alkanes,carbohydrates and furans.According to Eyer[43], Alcoa World Alumina has developed a GC method bad on methylation followed by chloroform extraction for the routine determination of oxalate,malonate and succinate.It was found that the method could be extended to include benzene as an analyte directly,but it was necessary to u butanol to derivatize acetate and formate for determination.Tardio[44]ud a similar method to determine formate,acetate,
butyrate,oxalate,malonate,succinate, glutarate,lactate,malate and fumarate as the methyl esters.
The advent of GC with mass spectrometry detection(GC–MS) in place of or in addition to non-specific detection by FID brought a major advance in analytical capability by enabling the identifica-tion of individual compounds,for example according to the scheme illustrated in Fig.3[22].
The complexity of the mixture of organic compounds in Bayer liquor is illustrated by the GC trace in Fig.4for compounds in the MW range90–300Da.The numbered peaks were identified by MS [22].The addition of modern multi-dimensional mass spectrometry has since demonstrated the potential for the identification of many hundreds of compounds[27].To date however,a total of only85 individual compounds,all of which have molecular weights below
12G.Power et al./Analytica Chimica Acta689 (2011) 8–21
Fig.2.Sample preparation scheme for the paration of high molecular weight organics from Bayer liquors,derived from the descriptions given by Wilson et al.[25].
Fig.3.Example of a determination scheme using GC–MS and GPC(SEC),adapted from Guthrie et al.[22].
