Features of Draft Tube and Open Impeller Agitators for Precipitators in Alumina Refinery

Hi Friends,

Precipitators are most critical equipment considered in Alumina refinery because of its effect on process efficiency and product quality as well. Relevant process efficiency refers to liquor productivity during crystallization process in precipitators which has direct bearing on total thermal and electrical energy consumption for alumina production in Bayer process. On product quality front, product coarseness along with granulometry of hydrated alumina is controlled to the desired level in precipitators.

As discussed in earlier posts, in precipitators, freshly prepared seed hydrate is added in the saturated and filtered aluminate liquor at controlled temperature and the whole content is kept under agitation / circulation resulting in crystallization of hydrated alumina. The crystallization process requires about 40 hours of slurry holding time. There are three types of agitation / slurry circulation systems prevalent in Alumina refineries-

• Circulation  by air lift,
• Draft tube agitators and
• Open impeller agitators.

Slurry circulation by air lift requires huge flow of compressed air, thus it has lower slurry agitation efficiency. Till 1960s, many Alumina plants had compressed air based slurry agitation system in precipitators but now most of them have modified the system adopting either draft tube agitators or open impeller agitators. In addition to inefficiency, air lift system has the capacity limitation for bigger volume precipitators. Earlier days, the crystallization circuit had smaller capacity precipitation tanks with volume of each precipitation tank ranging from 1000 m3 to 3000 m3. Now a days, all modern Alumina refineries installed in 1980s and afterwards has higher volumetric capacity varying from 4000 m3 to 4500 m3 per precipitation tank. Installation of higher capacity precipitation tanks has resulted in lower space requirement for installation, lower man power requirement for operation and substantial reduction in operating and maintenance expenses. Agitators of precipitation tanks requires following key features-

1. Higher pumping capacity for keeping solids in suspension and homogeneity of solids,

2. Low shearing rate of impeller to minimize particle attrition and

3. Lower electrical energy requirement.

As per my personal experience and interactions with professionals of Alumina refineries in India and abroad, both draft tube agitators as well as open impeller agitators are equally efficient for solid consistency in precipitation tanks ranging from 400 to 500 gpl hydrate as alumina. This level of solids in precipitation tanks is adequate for achieving the liquor productivity upto 75 gpl Al2O3 and most of the plant in the World has liquor productivity in the same level. But, all new Alumina refineries under execution and planning stage have precipitation tanks of further higher capacity of 5500 m3 to 6000 m3 with solids in slurry over 600 gpl Al₂O₃ with the prime objective to have liquor productivity to the level of plus 92 gpl Al₂O₃.

For higher solid consistency, open impeller agitators will not have any problem, however, certain design modifications may be required in draft tube agitators for meeting the process requirement for its successful uninterrupted operation with high solids in precipitation tanks of Alumina refinery.

The views expressed here are purely my personal experience without any bias / prejudice for any particular individual / organization involved in designing of such a complex equipment. I have freely shared my views and request you to put your views / remarks / comments, if any, if you have different experience on the subject.

Rajendra Kunwar

www.ceti.co.in