Bauxite is a sedimentary rock with relatively high alumina content. It is the world’s main source of aluminium and consists mostly of aluminium minerals gibbsite (Al2O3,3H2O), boehmite (Al2O3,H2O), diaspore (α-AlO(OH)), mixed with iron oxides like goethite (FeO(OH)), haematite (Fe2O3,H2O), the aluminium clay minerals kaolinite (Al2O3,2SiO2,2H2O) and ilmenite (FeTiO3).
Approximately 85% of all the bauxite is converted to alumina for the production of aluminium metal, 10% is utilized for non-metal products and the remaining 5% is used for non-metallurgical applications such as production of refractory and abrasive materials.
The total amount of alumina that is extractable in solution with caustic alkali from bauxite is called available alumina.
Available alumina in bauxite has been analysed with varying quantity of sodium hydroxide to extract available alumina and this study clearly shows that extraction of available alumina definitely depends on the concentration of sodium hydroxide.
The estimation of available alumina in bauxite has been carried out using the certified reference material (CRM) CETEM BXMG-5. It has been observed during the analysis that when 0.5 gm of bauxite sample treated with 50 ml 10% sodium hydroxide solution at 140°C for 1 hour under normal atmospheric pressure, the values of available alumina obtained are 42.94%, 43.15% and 42.80%. On the other hand, when the digestion was carried out in a sealed container(Bomb digestion) with same amount of sample, same quantity of 10% sodium hydroxide solution at 140°C for 1 hour, the values obtained are 43.20%, 43.05% and 42.98% respectively. In both the cases, available alumina values were high as compared to the declared value (39.70%), most probably due to the fact that considerable amount of kaolinite goes into the solution at higher concentration of alkali increasing the amount of available alumina content. On the other hand when the experiment is carried out with 1 gm sample containing 25ml of sodium hydroxide (10%) solution and digested for 1 hour at 140°C, the values obtained are 39.95%, 40.12%, 39.62%, 39.80% and 39.50% respectively which are at par with the declared value of 39.70%.
The experimental procedure involves digestion of 1 gm sample with 25ml 10% NaOH solution at 140°C for 1 hour, followed by filtration through whatman 40, residue washed with hot water containing 1% NaOH soln. Aliquot taken from the filtrate and alumina estimated by complexometric back titration with std. zinc solution using xylenol orange as complexometric indicator.
|No. of tests||Results (obtained)||Mean||Declared value||Std. deviation||%RSD||MU|
±0.24 at 39.80%
Evaluation of available alumina greatly depends upon the quantity of NaOH solution used during the estimation. When testing was carried out with 50 ml 10% NaOH solution with 0.5 gm sample under normal atmospheric condition as well as by the bomb digestion process, 3-4% higher values of the declared CRM was obtained (given in the experimental). Desired value of the CRM was obtained only when the estimation was carried out taking 1 gm of sample and the digestion was done with 25 ml caustic soda solution (10%) for 1 hour at 140°C. Consequently this condition is to be maintained in order to achieve the desired value.
This method for the determination of available alumina is less time consuming, accurate, inexpensive with low standard deviation. Consequently this method can be adopted for the estimation of available alumina in bauxite.
CONTRIBUTED BY MR. KAJAL ROY UNDER THE GUIDANCE OF PROFESSOR BARUN KUMAR GUPTA