1

H-026 Title: Determination of caustic, carbonate and

alumina in Bayer Process liquors

Scope: Determination of caustic, carbonate and alumina in Bayer Process liquors using a method based on procedures developed by Watts-Utley1 and VanDalen-Ward2

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)

Principle: An aliquot of alumina refinery sodium aluminate liquor is treated with sodium potassium tartrate solution to complex aluminate and release one mole hydroxyl for each mole aluminate present. The total hydroxyl content of the liquor (“total caustic”) and the carbonate content is determined by titration with standard hydrochloric acid.

−−−− +→+ OHOHCOHAlOHCnOHAl n2

64432

6444 )()()()(

−+− ↔+ 323 HCOHCO

A second titration is carried out contiguously with the first to determine the aluminate content (as “alumina”) immediately thereafter. This second titration is preceded by the addition of potassium fluoride solution, which destroys the alumino-tartrate complex, forming an insoluble potassium sodium aluminium fluoride and releasing three moles of hydroxyl for each mole of aluminate.

↓++↔+ −−−−− 36

2644

26443 )(36)()( AlFOHCnOHFOHCOHAl n

This hydroxyl content is determined again by the standard hydrochloric acid titrant.

The entire two-titration sequence is carried out automatically.

Reagents: Standard 1.5 mol/L HCl solution Potassium sodium tartrate solution, 450g/L Potassium fluoride solution, 600g/L (neutralized with

NaOH to faint pink with phenolphthalein)

Thermo. Titr. Application Note No.

2

Method: Basic Experimental Parameters: Data rate (per second) 10 Titrant delivery rate (mL/min.) 2.5 No. of exothermic endpoints 2, 1 Data smoothing factor 45 Procedure: An aliquot of liquor is pipetted into a titration

vessel, and treated with 10mL potassium sodium tartrate solution. The total liquid volume is made to approximately 25mL and the titration sequence commenced. The first titration (“caustic” and “carbonate”) stops automatically after the bicarbonate endpoint is detected. The second titration sequence (“alumina”) begins automatically with the addition of 10mL potassium fluoride solution. A brief delay is programmed; to allow full reaction. The liberated hydroxyl ions are then titrated.

Results: Replicate analysis (n=10) of alumina refinery digestion

feed liquor

“Total Caustic” (“C”) as g/L Na2CO3 343.1±0.37 “Total Soda” (“S”) as g/L Na2CO3 409.1±0.65 “Alumina” (“A”) as g/L Al2O3 125.3±0.13 A/C 0.365±0.0006

C/S 0.839±0.013

Calculations: (see Titration Plots below for an explanation of terms)

2,)99.105)((/"" 32 ×

××−=mLAliquot

HClMABLgCONaCCausticTotal

TCmLAliquot

HClMBCLgCONaSSodaTotal +××−=,

)99.105)((/"" 32

)6,()96.101))()(((/""32 ×

××−+−=mLAliquot

HClMBDEFLgAOAl

3

Thermometric Titration Plots:

BA

C

D

Fig. 1. First titration in sequence A = First titration blank volume, mL B = “Total caustic” titre volume, mL C = “Total caustic” + bicarbonate titre, mL D = Total volume dispensed, mL

E

F

Fig. 2. Second titration in sequence E = Second titration blank volume, mL F = “Alumina” titre, mL Legend: Red = solution temperature curve Green = first derivative curve Black = second derivative curve