Friday, June 15, 2018

Tube Digestion Technology for Processing of Boehmitic Bauxite

Hi Friends,

In previous papers, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. It’s true that there is no end to advancements and thus tremendous scope for enriching our knowledge base. In continuation to our approach and efforts, we will present here in brief about Tube digestion technology as an alternative to Double digestion process for extraction of alumina. 

The digestion of predominantly Boehmitic Bauxite is carried out at an elevated temperature ranging from 220 deg C to 280 deg C, thus total thermal energy requirement for processing Boehmitic Bauxite is very high. As such Double digestion technology coupled with Pressure decantation system is a proven and established technology and many Alumina refineries are operative with this technology. The estimated overall energy consumption in efficient Alumina refinery with high temperature digestion technology is about 15 GJ per tonne of calcined alumina.

Another efficient digestion technology which was developed sometime in 1960s is popularly known as Tube Digestion Technology. In this process, wet ground Bauxite in caustic liquor followed by desilication is passed through long jacketed tubes / pipes at high velocity. The particle size of the Bauxite is controlled at around 100% minus 1000 microns. The tube jackets contain high pressure saturated steam for raising the temperature of reaction mixture indirectly to ensure dissolution of alumina. The higher velocity of slurry in the pipelines (tubes) enhances the heat transfer coefficient and thereby heat transfer rate resulting in lower digestion time requirement. The retention time during digestion is generally not dictated by high dissolution rate of alumina rather dependent on chemical kinetics of quartz with caustic liquor. It has been practically observed in laboratory as well as plants that higher retention time across tubes during digestion at higher temperature above 250 deg C causes Quartz attack due to dissolution of Quartz with caustic soda which increases the formation of DSP and Cancrinite and thus additional consumption of caustic soda for alumina production. As per my personal experience with Boehmitic Bauxite, 15 minutes of digestion time in tube digesters will be adequate. The outcome of thermal energy calculations reveal that energy requirement with Tube digestion technology is comparatively less than that for other high temperature digestion technology. 

Advantages of Tube digestion technology: 
There is a growing trend in adoption of Tube digestion technology in a few Alumina refineries in planning and execution stages in the World. An Alumina refinery of 1.8 Mtpa production capacity has been newly constructed and commissioned a few years back in Saudi Arabia adopting tube digestion technology. The plant has adopted this technology with Basic engineering developed jointly by two leading engineering consultancy companies. This will be the biggest Alumina refinery in the World adopting Tube digestion technology.

Tube digestion technology has following major advantages over other high temperature digestion technologies available in the World-
·         Lower thermal energy consumption for digestion,
·         Easy maintenance or replacement of equipment,
·         Lower retention time for digestion,
·         Flexibility in processing wide range of Bauxite,
·         Lower evaporation requirement,
·         Lower capital investment and
·         Marginally lower operating cost. 

Total thermal energy consumption for alumina production adopting Tube Digestion Technology works out to about 12 GJ/t alumina which accounts for 9.0 GJ/t alumina for Bayer process (Hydrate) area and 3.0 GJ/t for Calcination area of Alumina refinery.

Disadvantages of Tube digestion technology: 

The scaling rate in inner surface of tubes will be high because of Boehmite reversion in tube digestion stage because of high alumina loading in liquor in presence of solid Boehmite. As such, the hardness of scale will be very high due to presence of DSP and Cancrinite. Also, caustic soda consumption will be higher due to dissolution of quartz and formation of Cancrinite as well. Thus following are the disadvantages of Tube digestion technology-
·         High scaling rate in tube digesters and
·         High caustic soda consumption.
At times, certain chemicals, like MgO, are added to bauxite slurry before entry to tube digesters for making porous scales for easy dislodging.

Since high temperature digestion beyond 250oC causes conversion of quartz to reactive silica, thus it is always recommended to maintain the digestion temperature at around 240-245oC and should avoid crossing 250oC to combat high caustic soda consumption for alumina production.

The statements presented here are purely based on personal experience and technical studies of the author on the subject. Hence, it is advised to carry out detailed evaluation of process by thorough scrutiny of process control data and related efficiency figures before implementation in any plant for commercial production.

We solicit your valued comments / remarks for further improvement from our end.
Rajendra Kunwar

No comments:

Post a Comment