Hi Friends,
In earlier posts, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. it's a truth 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 discuss in brief about Tube digestion 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 reasonably 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.
A photograph of Tube digestion system of an operative Alumina refinery is given below-
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. Ma'aden Alumina refinery of 1.8 Mtpa production capacity at Ras az Zawr in Saudi Arabia is in advance stage of engineering and execution adopting this technology with the Basic engineering developed jointly by two leading engineering consultancy companies. This will be the biggest Alumina refinery in the World adopting Tube digestion technology. The plant is expected to be commissioned sometime in 2014. Tube digestion technology has following major advantages over other high temperature digestion technologies available in the World-
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 reasonably 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.
A photograph of Tube digestion system of an operative Alumina refinery is given below-
Facilities Installed for Tube Digestion System in an Alumina Refinery
- 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-
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- 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 250 deg C causes conversion of quartz to reactive silica, thus it is always recommended to maintain the digestion temperature at around 240-245 deg C and should avoid crossing 250 degree C 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.
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.
Please put your views / remarks / suggestions / comments, if any.
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Thanks and regards.
Kunwar Rajendra
Sir Greetings of the day
ReplyDeleteI m regular reader of your blogs on linkedin.
Sir i have one query hopefully you will help me out
Can you explain me the calculation of silica loss during desilication after formation of bayer solalite,i.e. how much caustic is lossed with the formation of sodalite which goes to mud separation unit with incomplete desilication.
Hoping for your positive reply at the earliest
Regards
Prashant Singh
Dear Prashant,
ReplyDeletePlease have a look at the following deliberations which will clarify your doubts about Desilication, Sodalite factor and Soda loss-
The silica in bauxite is in the form of Kaolinite (Al2O3.2SiO2.2H2O) and Halloysite (Al2O3.2SiO2.3H2O) are known as reactive silica as the react with Caustic soda.
The reactive silica present in Bauxite reacts with Caustic soda and forms soluble Sodium silicate as per the following chemical reaction-
SiO2 + 2NaOH = Na2SiO3 + H2O.
In subsequent step, Sodium silicate on reaction with Sodium aluminate forms Sodium aluminium silicate as per the following reaction,
Na2SiO3 + NaAlO2 = 3(Na2O.Al2O3.2SiO2.2H2O).Na2X
Where, X can be carbonate, chloride, sulphate or oxide ions.
Morphological studies of the complex indicate that one or more forms of DSP in Bayer's process. Hence, in addition to Sodalite complex, the presence of DSP as Cancrinite and Noselite also may be there but in minor quantity. The Sodium aluminium silicate thus formed is the Desilication product (DSP) and the complex is popularly known as Sodalite complex. The precipitation of Sodium aluminium silicate occurs by nucleation and crystal growth mechanism.
Since Sodium aluminium silicate is the impurity in Bayer liquor thus it is essential to remove it along with other solid impurities along with Bauxite residue (Red mud) in order to minimize the silica impurity in the product alumina and to reduce the potential of scale formation of the heat transfer surface of heat exchangers. For meeting the international quality of Calcined alumina, silica level in Bayer liquor should be controlled within 0.50 gpl SiO2. Initially, the silica concentration in liquor is low but slowly it increases and reaches to its saturation level before its precipitation. The Desilication rate is dependent on Reactive silica content in Bauxite, desilication temperature, % Solids in slurry and retention time. The optimum time for desilication is arrived by carrying out test work at plant condition with finely ground (~100 mesh) Bauxite in Teflon beaker with varying temperature, solid consistency and residence time under constant agitation. Only Teflon beaker is used for the test work because NaOH solution dissolves silica from glassware at temperature above 70 degree centigrade resulting in misleading results of the conducted tests.
3.6.3 Significance of Sodalite Factor
The chemistry involved in desilication clearly indicates the impact of Reactive silica content in Bauxite on alumina production process. In nut shell, major portion of Caustic soda, used for dissolution of alumina present in bauxite, is consumed by reactive silica present in Bauxite. This happens mainly because of the formation of Sodalite complex, 3(Na2O.Al2O3.2SiO2.2H2O).Na2X, by reaction of reactive silica with NaOH and Sodium aluminate liquor. The ratio of Na2O/R. SiO2 is termed as Sodalite factor which ultimately reflects the indicative figure of Chemical soda loss in production of alumina.
With above derivation, it is evident that theoretical Sodalite factor (Na2O/R.SiO2) works out to 0.67. Based on this factor, Chemical soda loss is equal to (% SiO2 x Dry Bauxite factor X Sodalite factor x 1.29)/100 toones of NaOH per tonne of alumina.
For example, if % R.SiO2 in Bauxite = 2.00%, Dry Bayxite factor = 2.60 tonne /tonne alumina and Sodalite factor = 0.67; the Chemical soda loss = 2.00 x 2.60 x 0.67 x 1.29/100 tonne of NaOH per tonne of alumina = 0.045 tonne of NaOH per tonne of alumina i.e. 45 kg of NaOH will be lost only because of the presence of Reactive silica in Bauxite.
Therefore, Chemical soda loss is a function of Reactive silica in Bauxite, Dry Bauxite consumption factor and Sodalite factor.
Regards.
Kunwar Rajendra