Factors Affecting Extraction Efficiency of Alumina

Hi Friends,

Extraction efficiency is the measure of percentage dissolution of total alumina present in Bauxite across digesters and thus calculated by using analytical results of alumina in Bauxite processed and alumina present in blow-off liquor coming out from digesters.  It is always desirable to achieve alumina extraction efficiency in digesters close to 100%. Main factors affecting the alumina extraction efficiency are briefly described in subsequent paragraphs-

1. Particle size of Bauxite: We are aware that grinding of Bauxite is done to increase the surface area of Bauxite for improving its reactivity with caustic soda. Finer Bauxite particles get dissolved very easily but may cause settling problem of finer residue particles in decanters and residue washers. The deterioration in residue settling rate poses problems in polishing filtration of decanter's overflow liquor and may limit the production rate and quality of product as well. On the other hand, coarser Bauxite particles may not dissolve completely in caustic soda and part of Bauxite may remain unreacted. Hence grinding of Bauxite is done to achieve optimum size of Bauxite particles feeding to digesters. It has been experienced in many operative Alumina plants in the World that Bauxite particle size  of over 80% of minus 65 mesh fractions is the optimum particle size, however, the exact particle size of Bauxite of particular deposit may be  worked out based on laboratory test work under similar digestion conditions. 
2. Steady control of digestion liquor concentration: Digestion liquor prepared with uniform dosing of fresh caustic to spent liquor feeding to grinding mill and digestion circuit results in steady control of digestion liquor concentration and ratio as well. The steady control of digestion liquor concentration results in improved digestion efficiency. 
3. Stability of Digestion liquor: The addition of lime slurry to the process circuit maintains the stability of NaOH in liquor to the maximum level and hence better extraction efficiency of alumina across digesters.
4. Optimum Retention time: Adequate retention time in digesters must be provided to attain complete dissolution of alumina present in Bauxite. Excessive retention time in digesters to be avoided.
5. Optimum digestion temperature: Control of digestion temperature at optimum level is the key parameter for higher extraction efficiency as lower digestion temperature may not dissolve the alumina in caustic liquor completely. At times, the higher digestion temperature causes reversion of gibbsitic alumina due to presence of boehmitic alumina which acts as seed in digesters. Thus higher temperature as well as higher residence time may affect the extraction efficiency.
6. Continuous agitation of slurry in digesters: It has been established from laboratory and pilot plant tests that continuous digestion of reacting mixture in digesters improves the dissolution rate and thereby extraction efficiency of alumina in digesters.

The above stated parameters are the key factors for maximizing the extraction efficiency of alumina in digesters.

Please process your valued remarks highlighting the missing parameters.

Regards.

Rajendra Kunwar