How to enhance liquor productivity in Alumina refinery?

Hi Friends,

In earlier posts, we have already discussed the principles of crystallization in Bayer circuit. In present post, we will discuss our views to enhance the liquor productivity in existing Alumina refineries operative at lower productivity level.

Production of sandy alumina with highest possible liquor productivity across crystallization circuit is the key issue in alumina production units so as to have minimum energy consumption resulting in lower production cost of calcined alumina. In earlier posts, we have also listed in detail the  factors affecting the liquor productivity in Alumina refinery. Thus by judicious changes in associated parameters will result in liquor productivity. It is prudent to mention here that changes in any of these parameters may have adverse effect on other parameters. For example, higher concentration profile of aluminate liquor in Bayer circuit may require changes in total water management for the plant otherwise there is a fair chance losing more caustic soda in residue washing circuit.

Most of the Alumina refineries in the World are operative with liquor productivity figures at around 60 gpl Al₂O₃, few at about 75 gpl Al₂O₃ and very few refineries are continuously achieving the liquor productivity figure at plus 95 gpl Al₂O₃. Such wide gap in productivity figure gives the tremendous scope for low productivity alumina producers to critically analyze the process conditions and make suitable changes in process circuit and process parameters for improvement in liquor productivity figure. Changes in process circuit may require some additional capital expenditure but the return on investment will be very high and thus the payback period for such investments will be around two to three years with certain realistic assumptions.

Changes required in the Alumina refineries will be purely plant specific but to start with following major aspects may be scrutinized in the existing plant and corrective actions may be taken suitably –

1. Check the bauxite charging ratio with respect to digestion liquor concentration and change these parameters to reasonably higher level considering the alumina saturation at particular concentration. The indicative range of A/C charging ratio (~ Blow-off ratio) may vary from 0.630 to 0.710 with digestion liquor concentration at 260 gpl to 290 gpl as Na₂CO₃.
2. Water draw for residue washing circuit to be controlled within 2.1 m³ per tonne of residue which will attribute in dilution control within concentration drop of about 30 gpl up to feed point of precipitation circuit.
3. Minimize process liquor dilution from miscellaneous sources to the extent possible.
4. Covert single stage precipitation circuit to two stage continuous circuit (agglomeration and growth circuit) immediately by planning necessary changes in piping and pumping system.
5. Operate the agglomeration circuit at high temperature of saturated aluminate liquor at around 80 to 85^oC and crystal growth circuit at around 56 to 60^oC.
6. Fine seed hydrate to be added to agglomeration stage at around 100 to 150 gpl hydrate as Al₂O₃, however seed charge to crystal growth circuit should be around 400 to 600 gpl Al₂O₃ depending on slurry handling capacity of installed agitation system in precipitators.
7. Liquor circulation time (retention time) in agglomeration stage should be kept at around 16 to 24 hours for generation of agglomerated seed hydrate to be used in growth circuit. However, the same is maintained at around 40 hours in crystal growth circuit. Required number of precipitators may be worked out based on the plant capacity and circulation time.
8. About 30% of total aluminate liquor should be passed through agglomeration circuit and balance 70% from the crystal growth circuit.
9. Adopt efficient hydrate classification techniques to ensure product quality with required coarseness as well as seed generation rate to meet the process requirement.

The above mentioned changes / modifications will result in improving the liquor productivity of the plant to plus 80 gpl Al₂O₃ level. Request to put your views / suggestions / remarks / comments, if any. We will welcome your suggestions. 

Regards.

Kunwar Rajendra