Thermometric titrimetry has had a long association with the production of alumina from bauxite; where it has traditionally been applied to the determination of the caustic and aluminate contents of recirculating “Bayer Process” liquors. However, the versatile nature of this technique means that it can be applied to other important areas of process and quality control in alumina refining.

1. INTRODUCTION.

In the thermometric titrimetry, titrant is delivered at a constant rate, and the rate of heat evolution or absorption is substantially constant while unreacted analyte remains in the sample solution. When all analyte is consumed, the temperature rate change signals the titration endpoint. Because only the temperature rate increase or decrease is important, it is unnecessary to calibrate the sensing thermistor (although this can be done if desired). It is also unnecessary to use hermetically sealed calorimeter vessels; styrofoam coffee cups make ideal titration vessels in most aqueous titrations. Otherwise, polypropylene beakers or small “Thermos” flasks may be used.

Thermotitration is an ideal technique for process and quality control in industry. As previously stated, the sensing thermistor does not require calibration, and will last indefinitely (provided that the protecting glass envelope is not broken or chemically attacked). The same sensor may be used for acid-base, redox, and complexometric titrations, as well as those where precipitates are formed. In many cases, it is unnecessary to dilute the sample solution prior to analysis. Non-aqueous titrations may be easily performed. Applications so far developed span many industries; including mining, hydrometallurgical, metal finishing, catalysts, pigments and fillers, pharmaceutical, fertilisers, petrochemicals and food.

Although thermotitrimetry can trace its origins back to the early years of the century¹, it is only since the advent of fast-response thermistors in the 1950’s that the technique has become of practical use to analysts. The first really practical exponent of this technology was the aluminium major Alcan Limited, who had developed it for the analysis of Bayer Process liquors².

The electronic basis of the thermotitratorhas been traditionally a  Wheatstone bridge, of which the thermistor forms one arm, although other variants are possible. The thermotitrator consists of three modules:

• A stepper-motor driven precision burette pump

• A control module which controls input and output signals

• A personal computer

2.1. Analysis of Sodium Aluminate (Bayer Process) Liquors.

An aliquot of Bayer Process of sodium aluminate liquor is treated with a solution of an aluminium complexing ion, preferably tartrate. In complexing aluminate species in solution, one gram ion of hydroxyl is released for each gram atom of aluminium in solution:

Together with “free” hydroxyl ions already present in solution, these hydroxyls are titrated with protons (acid), to a thermometrically determined endpoint.

The reaction:

does not interfere, due to the  large differences in reaction enthalpies.

After the caustic titration is complete, fluoride ion is added to break the aluminate complex, releasing 3 gram ion hydroxyl for every gram atom aluminium present:

These hydroxyls are then titrated with hydrogen ions to the same thermometrically-determined endpoint.

Reference may also be made to VanDalen and Ward’s original paper on this method².

It is convenient to determine carbonate sequentially after the caustic and alumina determinations. The thermotitrator is sufficiently sensitive to determine the bicarbonate endpoint, in spite of the relatively low reaction enthalpies. The protonation of bicarbonate may also be observed:

However, the reproducibility afforded by this reaction is not as good as for reaction (3), and is not used for the determination of the carbonate content of the liquor.

Figure 1 illustrates a typical titration thermogram for Bayer Process liquor. The "y" axis represents solution temperature, and refers to the red curve (direct heat output). The "x" axis represents mL of HCl titrant. The white curve represents the first derivative of the direct heat curve. The titration endpoints are located by the second derivative (green) curve. From left to right, the endpoints (white points) identify the caustic, alumina, pK_{CARBONATE 2} and pK_{CARBONATE 1} endpoints respectively.

2.2. Determination of Hygroscopic Moisture.

The traditional titrimetric procedure for analysis of water has been the Karl Fischer method. While improvements have been made to improve the stability and reduce the toxicity of the Karl Fischer reagent, it is still a noxious reagent. Further, the method requires a dedicated instrument and is restricted in the range of samples to which it may be applied. A thermometric method ³ relies on a  strong endothermic response between acid-catalysed 2,2-dimethoxypropane (DMP) and water⁴.

The titrant is indefinitely stable, and is of low toxicity. Samples may be dissolved or suspended in a range of polar solvents (but not acetone or methanol, since they are reaction products).

Because the reaction requires an acidic environment, only samples which are either acidic, neutral, or slightly alkaline can be analysed. Neutral or slightly alkaline samples can be rendered acidic with the addition of a suitable organic acid such as methyl sulphonic acid. Suitable samples for analysis are:

· Calcined alumina
· Washed aluminium hydroxide (“hydrate”) filter cakes
· Acid wash solutions (for determination of dissolved solids content by difference).

Red mud thickener underflow slurries are not suitable, but washed red mud filter cakes could be considered as possible candidates.

Calcined Alumina. A strong argument may be mounted that the thermometric method is intrinsically superior to the ISO method for the determination of loss of mass by heating at 300°C. Because Al(OH)₃ decomposes rapidly around this temperature, any Al(OH)₃ present in the alumina due to recycling of kiln fines to the product will report erroneously as hygroscopic moisture in the product. Further, adsorbed moisture can readily hydroxylate the surfaces of some transition alumina phases⁵, and will dehydroxylate again around 300°C. Thus the true hygroscopic moisture content is unlikely to be determined by this method. The thermometric method only determines free water, and not “structural water” or “water of crystallisation”. The thermometric method is ideal for hour-to-hour process control, as it only takes a minute or so to perform. It is also ideal for studies on the changing nature of alumina along the route from the refinery kiln to the smelter pot, via storage facilities, shipping, and dry scrubbing units in the smelters.

Washed Hydrate. The moisture content of hydrate filter cakes is normally determined by loss of mass on drying at 105 or 110°C. It is regarded as unwise to exceed these temperatures, as Al(OH)₃ decomposition kinetics increase significantly, and the loss of mass from Al(OH)₃ decomposition may be wrongly reported as hygroscopic moisture. The low drying temperature means that relatively long drying times are required (2 - 4 hours, depending on the accuracy required). With such long analysis times, it is difficult to exercise tight control over filter performance, and kiln feed quality may vary as a result. Although adhering moisture on the Al(OH)₃  is slightly alkaline, it is likely that only a small quantity of methyl sulphonic acid would be required to acidify the sample for analysis.

A typical titration thermogram for the analysis of hygroscopic moisture with DMP is illustrated in Figure 2. The endothermic nature of the reaction may be observed from the direct heat curve.

2.3. Determination of Specific Surface Area.

The BET method for the determination of specific area of alumina is the industry standard for certification of product. While automated equipment has alleviated much of the arduous nature of this analysis, it still requires skill in performance, requires constant on-site supplies of liquid and gaseous nitrogen, and takes a relatively long time to obtain results. Such characteristics are not the hallmarks of a modern process control method. In researching the application of thermometric titration to the determination of catalyst activity, it has been established that a good correlation exists between surface acidic site density on alumina and the specific surface area as determined by BET.

The thermometric method determines acidic sites by suspending the alumina in a non-polar solvent such as cyclohexane or toluene. The titrant is dry n-butylamine. The titration time is less than a minute. There is evidence that the titration curves themselves can also give information concerning the catalytic activity, and "good" and "bad" catalysts may be differentiated. It is possible that this method may be useful in predicting the dry scrubbing performance of smelter grade aluminas.

2.4. Determination of True “Free Acid” Concentration of Plant Cleaning Acids.

Where mechanical descaling of alumina refinery processing units is not cost-effective or possible, or where caustic cleaning is ineffective, acid cleaning must be employed. Inhibited solutions of acid (mainly sulphuric) are circulated through scaled items of equipment until satisfactory cleaning is obtained. Cost and environmental considerations dictate that the cleaning solutions be “topped up” with fresh acid as the strength becomes depleted; both with neutralisation of residual caustic in residues, and with dissolution of iron, aluminium and other cations present in the scale matrices. It is important to monitor the strength of the acid cleaning solutions, both to maintain effectiveness, and to prevent excessive attack on the walls of the process vessels and pipes. Normally, a simple acid-base titration with an indicator is used for this purpose. Less frequently, potentiometric titration using a pH sensor may be employed. Both methods are inadequate, as they are unable to satisfactorily differentiate between the free protons present in solution and the aquo ions of metallic cations, particularly Fe(H₂O)₆³⁺ and Al(H₂O)₆³⁺. The hydrolysis pKa’s of these ions (approximately 2 and 3 respectively) are too low to be differentiated from “free acid” by conventional titrimetric methods. However, thermometric titrimetry has been successfully demonstrated in a number of similar s applications in the metal finishing industries.
 

REFERENCES.

1. J.M. Bell and C. F. J. Cowell, J. Am. Chem. Soc., 35, 49, 1913
2. E. VanDalen, and L. G. Ward, Anal. Chem., 45 (13), 2248, 1973
3. P. Sadtler and T. Sadtler, Am. Lab., 69, 85, 1982
4. K. B. Wiberg et al., J. Org. Chem., 50, 4717, 1985
5. K. Wefers and C. Misra, Alcoa Technical Paper No. 19 Revised, 1987