Determination of total caustic, total soda and alumina in Bayer process liquors with 859 Titrotherm

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1 Page 1/6 Determination of total caustic, total soda and alumina in Bayer process liquors with 859 Titrotherm Of interest to: Alumina refineries, manufacturers of sodium and potassium aluminate solutions Summary The determination of the total caustic, total soda and alumina contents in aluminate ( Bayer ) process liquors can be accomplished with high precision and speed using 859 Titrotherm in a thermometric acid-base titration. A complete titration can be accomplished in approximately 5 minutes. The procedure is an automated adaptation of the traditional Watts-Utley method 1, and is similar to the VanDalen-Ward thermometric titration method 2, but with the added advantage of being able to analyze the carbonate content of the liquor. Introduction In a titration, the titrant reacts with the analyte in the sample either exothermically (gives out heat) or endothermically (takes in heat). The Thermoprobe measures the temperature of the titrating solution. When all of the analyte in the sample has reacted with the titrant, the temperature of the solution will change, and the endpoint of the titration is revealed by an inflection in the temperature curve. The amount of analyte determined is not related to the change in temperature of the solution. Therefore, it is not necessary to use insulated titration vessels. Theory Thermometric titrations are conducted under conditions of constant titrant addition rate. In this respect they differ from potentiometric titrations, where the titrant addition rate may be varied during the titration according to the electrode response. In thermometric titrations, a constant addition rate of titrant equates to a constant amount of heat being given out or consumed, and hence a more or less constant temperature change up to the endpoint. Method outline present in the liquor are titrated with standard hydrochloric acid. H + + OH - H 2 O The titration is permitted to continue so that carbonate ions present in the liquor are also titrated CO H HCO 3 The titration is then automatically stopped, and potassium fluoride solution added to break the alumino-tartrate complex and release 3 mol hydroxyl ion per mol aluminium present. 2 2 Al (OH) 3(C4H4O6 ) n + 6F 3OH + n(c 4H4O6 ) + AlF 3 6 These liberated hydroxyl ions are then titrated with the standard hydrochloric acid, and determined as the alumina content of the liquor. Nomenclatures and definitions A number of nomenclatures are in use throughout the world to express the hydroxyl, carbonate and aluminate contents of Bayer Process liquors, as well as critical ratios for monitoring aluminate supersaturation and carbonate levels. Total caustic, C : the total hydroxyl ion content of the liquor comprising free hydroxyl plus one mol hydroxyl ion per mol aluminate ion. Expressed as Na 2 CO 3 g/l, Na 2 O g/l or NaOH g/l. Total soda, S : the total caustic content plus the carbonate content, expressed as Na 2CO 3 g/l, Na 2O g/l or NaOH g/l. Carbonate : Total soda minus total caustic. It may be expressed as Na 2CO 3 g/l, Na 2O g/l or NaOH g/l. Alumina, A : The aluminate content of the liquor, expressed as Al 2 O 3 g/l. An aliquot of Bayer liquor is dispensed into a titration vessel. Potassium sodium tartrate solution is added to complex aluminate ion, and release one mol OH - per mol aluminate ion present. Al(OH) n(c 4H4O6 ) Al(OH) 3(C4H4O6 ) n + OH These hydroxyl ions, together with hydroxyl ions already

2 Page 2/6 Apparatus and accessories 1 x Titrotherm (1 Dosino and 1 x 10 ml Dosing unit included) 3 x Dosino 2 x Dosing unit 10 ml 1 x Dosing unit 50 ml 1 x way stopper with antidiffusion tip 1 x x SGJ stoppers 1 x Reagent organizer 1 x Stacking frame Reagents Solvent: Standard: Titrant: Complexant 1: Complexant 2: deionized water TRIS [Tris(hydroxymethyl)aminomethane, 99.9%] c(hcl) = 1.5 mol/l 614 g/l potassium sodium tartrate tetrahydrate 620 g/l potassium fluoride Sample Bayer Process spent liquor submitted by a customer. The sample was well characterized using Metrohm potentiometric titration equipment and tiamo TM 2.1. Calculations Titer HCl titrant Weigh five amounts of TRIS ranging from approximately 0.35 to 1.1 g directly into titration vessels. Add 30 ml, deionized water to each, and titrate according to the HCl standardization method. The method allows sufficient time for the dissolution of the TRIS before commencing the titration. The method automatically performs a regression analysis, computing the slope of the line of best fit, and from that, the molarity of the titrant. Effectively, the program plots ml of titrant consumed against g of standard. Calculation of titer in tiamo TM RS01 EP TET.EP(1).VOL RS02 Slope RS.EP.SLO RS03 Intercept RS.EP.ITC RS04 Correlation (R 2 ) RS. EP.COR* RS.EP.COR RS05 Molarity [mol/l] of titrant 1/ RS.EP.SLO *1000/ RS06 Titer of titrant RS.molarity / TET.CONC RS07 Filter factor MV.filter factor Method blank The method blank is determined by titrating different amounts of sample and plotting sample amount against the titrant consumption. The method blank is determined as the y-intercept from a linear regression of the titration data. Changes in titrant dose rate or filter factor will require a new determination of the method blank. This parameter is stored along with the other method parameters. For all determinations, the blank volume is subtracted from the volume of titrant. In the case of the analysis of Bayer Process liquors, three blanks are determined for total caustic, total soda and alumina respectively. Calculation of method blanks in tiamo TM Legend: EP C = endpoint #1, vol. for total caustic EP S = endpoint #2, vol. for total soda EP A = endpoint vol. for alumina titration plus excess HCl titrated after EP C before first titration stopped. Calculation of method blanks in tiamo TM RS08 EP C 'C and S Titration.EP{1}.VOL' RS09 EP C Slope 'RS.EP C.SLO' RS10 EP C Intercept (C Blank) 'RS.EP C.ITC' RS11 EP C Correlation 'RS.EP C.COR' * 'RS.EP C.COR' RS12 EP A 'Al titration.ep{1}.vol'+ ( 'C and S Titration.EVT' -'C and S Titration.EP{1}.VOL') RS13 EP A Slope 'RS.EP A.SLO' RS14 EP A Intercept (A Blank) 'RS.EP A.ITC' RS15 EP A Correlation 'RS.EP A.COR' * 'RS.EP A.COR' RS16 EP S 'C and S Titration.EP{2}.VOL' RS17 EP S Slope 'RS.EP S.SLO' RS18 EP S Intercept (S Blank) 'RS.EP S.ITC' RS19 EP S Correlation 'RS.EP S.COR' * 'RS.EP S.COR' RS07 Filter factor 'MV.filter factor' Legend formula: TET.EP(1).VOL = Thermometric titration end point volume RS.EP.SLO = Slope of linear regression RS.EP.ITC = Intercept for linear regression RS. EP.COR* RS.EP.COR = Correlation coefficient (R 2 ) 1000 = factor for conversion mg MW TRIS = g/mol TET.CONC = Concentration of titrant (1.5 mol/l) MV.filter factor = Titration parameter (smoothing factor) EP C = first end point for total caustic EP S = second end point for total soda EP A = endpoint vol. for alumina titration plus excess HCl titrated after EP C before first titration stopped.

3 Page 3/6 Calculation of total caustic, total soda and alumina in tiamo TM RS20 C g/l (( 'C and S Titration.EP{1}.VOL' - 'CV.C Blank' )* 'C and S Titration.CONC' * * 'C and S Titration.TITER' )/ RS21 S g/l (( 'C and S Titration.EP{2}.VOL' - 'CV.S Blank' )* 'C and S Titration.CONC' * * 'C and S Titration.TITER' )/ RS22 A g/l ((( 'Al titration.ep{1}.vol' - 'CV.A Blank' )+( 'C and S Titration.EVT' - 'C and S Titration.EP{1}.VOL' ))* 'Al titration.conc' * * 'Al titration.titer' )/ ( 'MV.Sample size' *6) RS23 Na 2O/Al 2O 3 molar ratio ('RS.C g/l' * )/ ( 'RS.A g/l' * ) RS24 Na 2O carbonate 'RS.S g/l' - 'RS.C g/l' RS07 Filter factor 'MV.filter factor' Calculation of liquor components in tiamo TM C g/l Total Caustic content, either as Na 2 CO 3 (MW = ) or Na 2O (MW = ) S g/l Total Soda content, units as for C g/l Carb. Carbonate content of liquor, calculated as (S g/l C g/l) A g/l Alumina content, as Al 2 O 3 (MW = ) A/C Liquor supersaturation ratio (US style) C/S Liquor carbonation ratio (US style) MR Liquor supersaturation molar ratio [C]/[A] Item C g/l S g/l A g/l (US, EU) A/C C/S C g/l S g/l Carb g/l MR Calculation formula (( 'C and S Titration.EP{1}.VOL' - 'CV.C Blank' )* 'C and S Titration.CONC' * *'C and S Titration.TITER' )/ ('MV.Sample size' *2) (( 'C and S Titration.EP{2}.VOL' - 'CV.S Blank' )* 'C and S Titration.CONC' * * 'C and S Titration.TITER' )/ ((( 'Al titration.ep{1}.vol' - 'CV.A Blank' )+( 'C and S Titration.EVT' - 'C and S Titration.EP{1}.VOL' ))* 'Al titration.conc' * * 'Al titration.titer' )/ ( 'MV.Sample size' *6) = RS.A g/l / RS.C g/l = RS.C g/l / RS.S g/l (( 'C and S Titration.EP{1}.VOL' - 'CV.C Blank' )* 'C and S Titration.CONC' * * 'C and S Titration.TITER' )/ (( 'C and S Titration.EP{2}.VOL' - 'CV.S Blank' )* 'C and S Titration.CONC' * * 'C and S Titration.TITER' )/ = RS.S g/l RS.C g/l = ( RS.C g/l * )/( RS.A g/l * ) Methods Procedure for determination of HCl titrant molarity: Weigh accurately amounts of TRIS approximating 0.3, 0.5, 0.7, 0.9 and 1.1 g into clean, dry titration vessels. Add 30 ml deionized water, and titrate each with the method TET_titer_1-5mol_L_HCl determination. Procedure for determination of method blank: Prepare a diluted solution of a suitable spent liquor by pipetting 25 ml into a 500 ml volumetric flask, and making to volume with deionized water. Allow pipette to drain for 10 minutes due to the viscosity of process liquors. Pipette successive aliquots of 30, 25, 20 and 15 ml of this diluted solution into titration vessels, making to approximately 30 ml with deionized water. These aliquots are equivalent to 1.5, 1.25, 1.0 and 0.75 ml of undiluted liquor. Prepare each aliquot only a few minutes prior to titration, due to risk of absorption of atmospheric CO 2. Titrate with the method TET_blank_Bayer_liquor determination. Procedure for determination of C, S and A in liquors: For spent liquors, prepare an aliquot which contains an equivalent of 1.5 ml of undiluted liquor. This may be prepared by serial dilution or using an automatic pipette. Aliquots for pregnant liquors may range between 1 to 1.5 ml, depending on the alumina concentration of the liquor. Larger aliquots may be used for weaker liquors to the extent that very weak liquors may be used at full strength without any intermediate dilution. Liquors down to ~2 g/l C may be analyzed using a suitably large aliquot (up to 50 ml). Normally, the initial titration solution should approximate ml, making to this volume with deionized water. Titrate with the method TET_sample_Bayer_liquor_EU for example. Titrate with the method Bayer liquor EU for results according to European style nomenclature, Bayer liquor US for American style nomenclature, or Bayer liquor CN for Chinese style nomenclature.

4 Page 4/6 Determination Sequences Titration Parameters Titer determination Determination of method blanks Bayer liquor Titration Parameters for Bayer liquor sample preparation - see procedure see procedure Titer determination Blank/sample C and S Blank/sample A TRIS, [g] deionized water, Add 614 g/l KNa tartrate, Wait [s] Titration with 1.5 mol/l HCl yes yes yes Stirring rate Start volume Pause [s] Switch off autom. Dosing rate [ml/min] yes yes yes Filter factor Add 620 g/l KF, Damping until Wait, [s] Titration with 1.5 mol/l HCl - yes yes Stop volume Stop slope < 0.15 < < Add. volume after stop Evaluation start End points [Reaction type] [ERC] EP1 [ERC] EP2 [ERC] for Al ex* ex* ex* * exothermic

5 Page 5/6 Results Results titer determination Titer 1.5 mol/l HCl Thermometric Titration Plots Total Caustic and Total Soda titration slope Intercept Correlation (R 2 ) molarity [mol/l] Filter factor 35 titer Results blank determination Bayer liquor 2 EP1 C EP2 S EP A slope Intercept Correlation (R 2 ) Filter factor Results of Bayer spent liquor Alumina titration Customer results 859 Titrotherm, tiamo TM Mean Accepted range of values Mean, standard deviation and % rsd (10 determinations) C g/l (as Na 2 O) ±0.12, %rsd = 0.06 S g/l (as Na 2O) ±0.21 %rsd = 0.10 Carb. (as Na 2 O) A g/l (as Al 2O 3) ±0.16*, %rsd = 0.16 Molar ratio [C]/[A] C g/l (as Na 2CO 3) ±0.21 %rsd =0.06 S g/l (as Na 2 CO 3 ) ±0.36 %rsd = A/C C/S * Note: the alumina content of the liquor was checked by a completely independent determination based on thermometric titration of an acidified and buffered aliquot of liquor titrated with standard potassium fluoride solution. The titrant was calibrated against a primary standard solution prepared from 99.99% pure Al metal. The result of this analysis was A = 98.4±0.06 g/l Al 2O 3 (n = 5). Legend: 1 = solution temperature curve 2 = second derivative curve ( Endpoint Recognition Criterion ERC- curve for endpoints) References: 1 H. L. Watts and D. W. Utley, Anal. Chem. 28, 1731 (1956) 2 E. VanDalen and L. G. Ward, Anal. Chem. 45, 2248 (1973)