Alumina Technology

R.N.Goyal JNARDDC, Nagpur

1. Introduction

Commercially alumina is produced from bauxite by age old Bayer process. It consists of following main processes: ! Desilication ! Digestion ! Precipitation

2. Desilication In the Bayer process the desilication of bauxite is an important step to maintain the alumina quality. It requires conversion of reactive silica of the bauxite into a non-soluble sodalite complex (Sodium aluminium silicate or sodalite). The desilication can be carried out before digestion or after the digestion process. If the desilication is carried out before digestion it is termed as pre-desilication whereas if the desilication follows after digestion, it is termed as post desilication. The advantage of pre-desilication is that there is no risk of auto-precipitation occurring during the post desilication process and overall energy requirement in the pre-desilication is lower as compared to post-desilication. 2.1 Pre-desilication During desilication the caustic soda reacts with the kaolinitic portion of the bauxite forming sodium silicate. Apart from this the caustic soda also reacts with the gibbsite to form sodium aluminate which further reacts with sodium silicate to form sodium aluminosilicate complex having the structure as x Na2O. y Al2O3. z SiO2. n H2O. This complex is solid in nature and goes along with the red mud thus desilicating the bauxite with respect to silica content. The values of x, y, z and n of the sodalite complex varies from plant to plant depending on the bauxite characteristic and the digestion parameters. The major reactions occurring during desilication are as follows: Na2O + SiO2 = Na2SiO3 ------ (1)

Na2O + Al2O3 = 2NaAlO2 ------ (2)

Na2SiO3 + NaAlO2 = x Na2O y Al2O3 z SiO2 n H2O ------ (3)

The desilication reaction is thus a consecutive reaction in which the reaction can be represented as : A (Kaolinitic silica) → B (sodium silicate) → C (Sodalite complex)

In this reaction the rate constant k1 for A → B is higher than the second reaction rate constant k2 for B → C. The dissolution of kaolinitic silica is dependent on

temperature and caustic concentration. Higher the temperature and caustic concentration, higher is the silica dissolution resulting in increase in silica level in the liquor. The second reaction rate constant is lower as compared to the first reaction requiring in an increased retention time for the second step. The reaction proceeds faster in presence of seed of sodalite. Desilication reaction is proportional to the reactive silica content of the bauxite. Higher the silica content, higher is the driving force of this reaction. Due to this reason, it is difficult to get satisfactory desilication when the silica content in the bauxite is lower than 1-1.5 %. Desilication rate constant varies depending on temperature, holding time, bauxite solids concentration, and caustic and alumina concentration of digestion liquor. 2.2 Factors Affecting Desilication 2.2.1 Temperature Higher the temperature faster is the rate of desilication. If the temperature is further raised beyond the critical temperature, the sodalite complex undergoes dissolution and proceeds faster than the desilication, with the result that the silica concentration of the final liquor becomes high. So as to bring down the silica concentration in the liquor to a lower level, temperature lower than the critical

temperatures need to be maintained. A temperature above critical temperature results in poor desilication output. Also at higher temperature the equilibrium concentration of the silica in the liquor is high. Normally plant prefers to carry out desilication at temperature ranging from 90 to 100oC. 2.2.2 Effect of Bauxite Solid Concentration The higher solids concentration of bauxite promotes higher desilication. A change of solids concentration from 200 gpl to 800 gpl can sensibly reduce the silica level in the liquor at a lesser residence time. The reason for the same is that the higher bauxite solids concentration in the slurry would yield more desilication product seed, which could in turn accelerate the desilication process. Nowadays with the current technological advances, it is possible to pump and agitate slurry with solids concentration up to 800 - 1000 gpl. The effect of bauxite solids

Variation of S ilica C oncentration w ith Temperature at 800 gpl bauxite

0.51

1.52

2.53

0 1 2 3 4 5 6 7 8 9 10Tim e , Hrs.

Silic

a C

onc.

, gpl

90°C95°C100°C

concentration on silica concentration in the liquor at various residence time is shown below

2.2.3 Caustic Concentration of Liquor Higher the caustic concentration faster will be the dissolution of reactive silica from bauxite in the liquor. But at the same time high concentration of caustic liquor will have a higher equilibrium concentration of the silica lowering the desilication rate.

2.2.4 Alumina Concentration of the Liquor It is quite clear that for the second step, the dissolved silica reacts with alumina to form sodalite complex. During desilication step some dissolution of alumina also occurs. It is observed that desilication rate is better when the starting concentration of the alumina is low. The higher alumina concentration in the solution reduces the degree of silica supersaturation, with the result reducing driving force for the desilication reaction. 2.2.5 Residence Time The residence time required for desilication varies with the silica content of the bauxite and solids concentration. If the silica content of the bauxite is low then appreciable desilication output is not achieved in 8-10 hours and hence it becomes necessary to continue desilication up to 16 hours. The retention time for desilication can be reduced for achieving the same silica concentration in the liquor by increasing the bauxite solids concentration.

Variation of Silica concentration in liquor at different bauxite solids concentration at 90°C

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8 9 10

Time, Hrs

Silic

a C

onc.

, gpl 200 gpl

300 gpl400 gpl500 gpl600 gpl800 gpl1000 gpl

2.3 Post-desilication In some plant post-desilication is practiced after digestion and after adding dilution liquor to reduce the silica concentration in the feed to settler. Post-desilication step has its own disadvantages. In post-desilication slurry volume increases due to addition of dilution liquor, which is necessary to avoid auto-precipitation during the post-desilication step. Because of this the energy required increases and at the same time during post-desilication step, the auto-precipitation losses are higher resulting in net reduction in extraction efficiency.

3. Digestion Process

Presently three digestion technologies are being employed through out the world. These are # Atmospheric Digestion # Low Pressure Digestion # High Temperature Digestion

Bauxite is digested at temperatures ranging between 105°- 260 °C with caustic soda concentration varying from 165 to 220 gpl as Na2O depending upon quality/mineralogy of the bauxite. Super saturation attainable for solubility of gibbsite at various caustic concentrations and temperatures is shown in the figure below.

Gibbsite solubility data in caustic liquors

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40 60 80 100 120 140 160

Temperature ( oC )

Alu

min

a co

nc. (

gpl)

250 gpl

200 gpl

150 gpl

100 gpl

From the solubility data of the gibbsite, it is possible to attain a RP of 1.40 at 140oC with 200 gpl Na2O caustic concentration whereas at atmospheric pressure digestion (107oC) with Na2O concentration of 200 gpl caustic concentration the achievable RP is 1.12. The monohydrate bauxite contains mostly boehmite and diaspore. The boehmite requires relatively higher caustic concentration and temperature for its extraction. For bauxite containing higher content of boehmite, it requires temperature in the vicinity of 200 – 240oC for economical extraction of alumina. Most of central Indian bauxite deposits contain higher content of

boehmite and hence plant operating with these deposits like BALCO and HINDALCO operate at a temperature of 240oC. The equilibrium solubility curve of boehmite dissolution at various caustic concentration and temperatures is shown in the figure given below: Usually a higher residence time is given in the plant to obtain alumina concentration near equilibrium.

Boehmite solubility data in caustic liquors

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60 80 100 120 140 160 180 200 220 240 260 280

Temperature, oC

Alu

min

a co

nc. (

gpl)

250 gpl

200 gpl

150 gpl

100 gpl

From the boehmite solubility data, it is possible to attain an RP of 1.1 – 1.28 at a Na2O concentration of 150 gpl in the temperature range of 220 – 260oC. Similarly with a caustic concentration of 200 gpl as Na2O and above temperature range it is possible to attain a RP of 1.13 – 1.30. 3.1 Atmospheric Digestion The atmospheric digestion process, developed by Aluminium Pechiney (France) ensures lower reactivity of silica, uses low digestion temperature (105°-108°C). Atmospheric pressure digestion because of its extreme simplicity presents many operational advantages: • Higher plant operating factor • Less digestion maintenance and cleaning costs • No pressure safety specific problems • No wear problems during digestion and flashing etc.

This technology is in operation in two plants namely NALCO and Guinea. However it has the following disadvantages: - • During atmospheric digestion, part of the well crystalline gibbsite remains

undigested causing lower digestion efficiency and higher bauxite consumption.

• The undigested gibbsite grains further acts as seed to enhance the hydrolysis loss of alumina in the post desilication and settling step.

• Due to atmospheric digestion, the supersaturation of the digested liquor cannot be increased to a desired level causing lower liquor productivity.

• Raw material consumption including energy is higher resulting in higher cost of production.

3.2 Variation of Digestion Efficiency With Holding Time at Atmospheric

Pressure Digestion The digestion studies carried out at JNARDDC for east coast bauxite at 106oC temperature and 210 gpl Na2O caustic concentration at various residence time indicates that the digestion efficiency increases upto 60 minutes holding time and beyond that there is no appreciable change.

3.3 Low Pressure Digestion Low pressure digestion is more suitable for gibbsitic bauxite and is being employed for alumina production in most of the plants in the world. Generally bauxite slurry after desilication is mixed with digestion liquor and this thin slurry is heated in single tube/multi-tube single pass or multi-tube multi-pass tubular heaters and finally pumped to the autoclave where live steam is fed to maintain the desired temperature. This system not only facilitate better heat recovery but also avoids caustic embitterment of carbon steel material of tanks and pipe lines by complexing of hydroxyl ions with dissolved alumina to form aluminate ions. The liquor to bauxite charge is a tricky situation in Bayer process. If the liquor quantity is less as compared to bauxite charge then higher super saturation is achieved but at the expense of lower digestion efficiency. On the contrary if the liquor quantity is higher than the desired, the super saturation achieved is less but the digestion efficiency is higher. Therefore it is essential to determine the optimum conditions at which both digestion efficiency and super saturation will be high.

Digestion Efficiency v/s Holding Time at Atmospheric Pressure (106°C) and at 210 gpl Na2O & 1.1 RP

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95

0 30 60 90 120 150

Holding Time, minutes

Dig

estio

n ef

ficie

ncy,

%

The required concentration of digestion liquor and temperature for digestion is usually decided on the basis of chemical and mineralogical analysis of bauxite. Low pressure digestion technology is applicable for those bauxite in which there is appreciable quantity of gibbsite and low quantity of boehmite. Low temperature digestion is carried out at elevated temperature ranging from 120 -180oC with caustic concentration ranging from 150 –220 gpl as Na2O. The only problem associated with extraction at higher temperature is boehmite reversion, which usually reduces the extraction. However it can be reduced by addition of lime during digestion. The experiments conducted at JNARDDC for low pressure digestion of east coast bauxite at different temperature indicates that as the temperature increases the digestion efficiency increases but after 160oC temperature there is a drop in digestion efficiency.

Low Pressure DigestionDigestion Efficiency v/s Temperature

at 180gpl Na2O, 1.1 RP, 30min. Res. time

70

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90

100 120 140 160 180 200

Temperature° C

Dig

estio

n ef

ficie

ncy,

%

3.4 Benefits of Low Pressure Digestion Over Atmospheric Pressure

Digestion The benefits in low pressure digestion are as follow as: - # Higher extractability as compared to atmospheric pressure digestion

leading to low bauxite consumption and less generation of red mud. # Higher super saturation of aluminate liquor in comparison to atmospheric

digestion leading to higher liquor productivity # Flashing of digested slurry will result in recovery of valuable flash steam,

which can be used in desilication or evaporation. # Flashing of steam will reduce the evaporation load. # The alkaline condensate obtained from flashed steam can be used for red

mud washing or for product hydrate washing.

Comparison of Low Pressure and Atmospheric Pressure Digestion of Gibbsite in Plant liquor

0.70.91.11.31.51.7

50 100 150 200 250 300 350

Caustic conc. (Na2O) gpl

RP

(Alu

min

a/C

aust

ic)

107°C

140°C

3.5 High Temperature Digestion High temperature digestion technology is useful for mostly gibbsitic and boehmitic mixed bauxite in which the percentage of boehmite is higher i.e. 8-10 %. The alumogoethite present in bauxite, which creates problem during settling, can also be extracted at high temperature with addition of lime as additive. For dissolving diasporic bauxite, high-pressure technology is adopted in which a temperature upto 260-280oC is required with lime as an additive upto 3 % by weight of bauxite. In Indian aluminium industry only BALCO and HINDALCO are operating at high temperature digestion i.e. at 240oC.

3.5.1 Advantages of High Temperature Digestion # High temperature digestion is able to digest most of the boehmite content

of the bauxite. # High temperature digestion helps in extraction of large quantity of flashed

steam, which helps in reducing steam consumption as the flashed steam is reutilized in the process for heating the slurry.

# Removal of flashed steam helps in reducing overall evaporation load in the Bayer process.

# High temperature digested mud is more crystalline and hence it has better settling characteristic, requiring less quantity of starch or synthetic flocculant.

# It is possible to achieve higher super saturation in the aluminate liquor, resulting in higher liquor productivity.

4. Red Mud Settling The solid waste from the digestion process known as bauxite residue or red mud is required to be separated from the pregnant liquor containing sodium aluminate as quick as possible in order to reduce the hydrolysis loss of alumina (back reaction) with the red mud and also reduce the impurities entering the precipitation circuit. This is generally accomplished in large diameter conical or flat-bottomed tanks fixed with slow speed rakes. The digested slurry is usually diluted to appropriate concentrations with the first washer overflow or appropriate liquor to achieve reasonable settling rates. Red mud separation is tedious and time-consuming process and hence to enhance the rate either natural flocculants such as starch, rice bran etc. or synthetic flocculants which are either water soluble high molecular weight polyacrylates or the hydroxamated polyacrylamides are used. In the initial years only the natural flocculants were used. However because of the increased plant capacities, more stringent product granulometry and high productivity it has become essential to replace the natural flocculants which add up organic contaminants to the system and makes it almost impossible to control with the synthetic flocculants. However almost all the plants which use the synthetic flocculant use it in combination with the natural flocculant so as to get faster settling rate and also better overflow clarity. Settling tanks are generally of 3000 – 6000 m3 capacity depending on the settling rate of the mud and plant capacity with huge inbuilt rakes. Generally overflow clarities of 0.1 – 0.5 gpl is achieved. The underflow mud is removed at consistencies ranging between 400 – 600 gpl for further processing so as to recover the entrained caustic and reduce the soluble soda of the mud to enable safe disposal. The settling is thus an important step in the Bayer process. To check the suitability of flocculant and determine the exact dosages of the flocculant it is necessary to conduct jar settling test in laboratory. The jar settling test is carried out in a graduated cylinder of one litre capacity. The slurry after desired dilution is flocculated by addition of flocculant and the rising of the clear interface with time is noted and based on the sinking of the particles, the rate of settling is determined. The overflow clarity is measured by carefully pipetting the liquor from the clear zone and determining its mgpl solids. Further qualitative information on compaction may be noted after a fixed interval of time. The height vs time plot gives the settling rate. Normally the settling velocity is calculated based on the time required for interface to travel from 800 ml to 600 ml i.e. in the clear zone.

4.1 Interpretation of the Settling Test Data for Estimation of Area Requirement for Settling and Consolidation of Mud

Initially the height Vs time is plotted for the flocculated red mud slurry

Jar SettlingTest (Height vs Time)

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0 5 10 15 20 25 30 35 40

Time, minutes

Hei

ght,

mm

From the height vs time plot tangents are drawn to the curve. The intercept of the tangent on the Y axis indicate the solids concentration of the mud at the interface in the initial settling to these points.

Deriving values from Settling Curve

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0 5 10 15 20 25 30 35 40Time, minutes

Hei

ght,

mm

Thus a plot of solids concentration vs time is obtained. Applying the Kynch mathematical analysis the area required for settling can be calculated.

4.2 Factors Affecting Settling 4.2.1 Temperature Higher temperature normally enhances settling rate. If the slurry is flocculated, the higher temperature improves the flocculation and the frequency of interaction of particles is higher. 4.2.2 Liquor Concentration Too high liquor concentration reduces settling rate due to high viscosity of liquor. 4.2.3 Solids Concentration The settling rate of the suspended particles depend on the solids concentration and on the degree of agglomeration/flocculation as it determines the frequency of particle interaction. 4.2.4 Settling Aid Settling rate can be enhanced by promoting flocculation using settling aids. The dosage of it depends upon the properties of the solid and electrolyte. There is an optimum dosage for any flocculant depending upon the attainable floc size, maximum settling rate and maximum underflow solids concentration. 4.2.5 Surface Area and Mud Height Surface area of the mud particle is an important factor while settling. Similarly settled mud height in the settler determines the degree of compaction. If the mud height is kept more than there is a possibility that the mud in the unsettled form may decrease overflow clarity. 4.3 Measurement of Settling Rate Using Radiometric Equipment Use of radiometric methods for measurement of settling rate is based on the principle that during settling only the solids concentration varies. The radiation source mounted onto a common elevator will scan the whole height of the settling tube. The detector on the other side measures the intensity of gamma rays. The solids in the tube will absorb some of the gamma rays and the rate of absorption depends on the local solids concentration. The measurements are carried out by time programmed radiometric scanning of a substance (in suspension) filled in the inner settling tube. The inbuilt software based on the measurement of the intensity of radiation calculates and gives: -

• Height (in cm) vs Solids concentration (kg/m3) at different time interval • Velocity diagram i.e. velocity (in m/hr) vs solid concentration (kg/m3) • Solid flux diagram i.e. solid flux (in kg/m2.hr) vs Solid concentration

(kg/m3)

• Loading diagram i.e. Discharged solid concentration (kg/m3) vs Specific solid loading (kg/m2.hr) to thickener

4.3.1 Solids Concentration Profile Gives complete characteristics of the settling of the red mud with time as a function of height. This reveals the rate of settling of various red mud particles in the settler. It also gives information such as residue level height vs time. The three profiles given below indicates the level of flocculation attain during settling. These are under flocculated when the dosage of flocculant is inadequate, well flocculated when the flocculant dosage is sufficient and over flocculated when the dosage is higher.

4.3.2 Velocity Diagram It gives solid content vs settling velocity graph on a semi logarithmic scale. With this various parameters such as stokes velocity, average particle diameter etc. can be calculated. Velocity diagram simulates the zone of actual settler (i.e. two zone settling or three zone settling).

Velocity Diagram

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Solids Content, gpl

Velo

city

, m/h

r

4.3.3 Solid Flux Diagram The solid flux diagram is derived from the concentration profile. The product of the concentration and settling rate is termed as solid flux. The plot of solid flux vs solid concentration gives the behavior of the settling.

Solid Flux Diagram

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100

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Solid Content, gpl

Solid

Flu

x, k

g/m

² .hr

4.3.4 Loading Diagram It gives the plot of the discharge solids concentration vs specific loading. This data is useful in estimating allowable loading of solids in a continuous thickener. This data may also be used for designing a new thickener.

Loading Diagram

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0 10 20 30 40 50 60 70Specific Loading, kg/m² .hr

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char

ge S

olid

s C

once

ntra

tion,

K

g/m

³

4.3 Limitation of the Conventional Jar Test Method

• The temperature of the slurry cannot be maintained as that of the actual settler

• The complete compaction of the underflow requires many hours which cannot be studied by the conventional batch test

• To obtain rate of settling, specific loading solid concentration, underflow compaction, and specific area requirement a series of experiments have to be conducted along with tedious manual calculation

Though some of the data are calculated by kynch mathematical analysis, there are some limitation to this empirical relations such as it assumes that particles are of the same size and the velocity of fall depends only on the local particle density. 4.4 Merits of SAM Over Conventional Jar Test Method

• The relationship between settling rate and underflow solids concentration, specific loading of settlers, degree of compaction and specific area requirement all can be obtained by a single laboratory test.

• The settling rate at each of the solids concentration found with sam are for solids in the compaction zone, which are not possible by laboratory jar tests.

• Although, the simple jar tests are used to screen flocculant type and approximate dosage, they very often mislead as to the thickenability of the underflow, whereas, with sam this important aspect of the behavior operating with the particular slurry and flocculant can be predicted.

• The condition of the settler can be very well maintained in this unit, i.e. temperature.

4.6 Application of SAM The traversing soft gamma ray absorption model (sam) can be used for the following applications: 1. To select effective flocculant which may be starch, synthetic or

combination of both for each source of bauxite. 2. To optimise the dose of flocculant for proper thickening of mud. 3. To optimise the capacity of the thickener for full production rate of alumina. 4. To design thickener and deep thickener for future coming alumina plants

processing new source of bauxite.

5. Precipitation Processes

5.1 Introduction The precipitation is basically the hydrolysis of sodium aluminate of the pregnant/precipitation liquor with recovery of caustic soda according to the following equation:

2NaAlO2 + 4H2O Al2O3.3H2O + 2NaOH

After bauxite digestion, the digested slurry is sent to the settlers where mud is removed while the clear aluminate liquor is pumped to precipitation. The seeded crystal growth technique is used for precipitation of the aluminate liquor, which is achieved by chemical and physical effects. This process helps in producing hydrated alumina along with its quality and granulometry. Liquor volume recirculated per kilogram of alumina hydrate production is called as liquor productivity. Improving precipitation process, optimizing liquor productivity and hydrate quality is a subject of interest for alumina technologists world wide.

5.2 Mechanism of Precipitation

• Growth: Transfer of alumina from the solution to the growing particles • Nucleation: Generation of new particles of very small size less than 1

micron. Rate of nucleation decrease with increase in temperature and may be zero above 75°C

• Agglomeration: Combining or cementing of many fine particles into one larger unit or agglomerate. It is favoured at high temperatures say above 70oC

• Breakage and Arition: It is reverse of agglomeration. This includes gross breakage of particles by direct impact with walls, impeller or other particles.

5.3 Factors Affecting Precipitation

• Caustic concentration • Molar ratio of the pregnant liquor • Temperature profile • Seed charge

a. Caustic Concentration

With the same molar ratio, a higher caustic concentration allows the precipitation of more aluminium hydroxide per unit volume of liquor. A higher caustic concentration at precipitation helps in achieving higher digestion liquor conc. through the existing evaporation stage without any additional energy consumption, which in turn offers the advantage of increasing the alumina content in the digestion liquor. Molar ratio as low as 1.25 (0.77 A/C) can be obtained with 150 gpl Na2O while a molar ratio of 1.45 (0.66 A/C) can be obtained at 110 gpl Na2O concentration thus enhancing the liquor productivity by nearly 35 gpl. Fig 1 shows the characteristics of caustic concentration, MR with alumina precipitated.

b. Molar Ratio of The Pregnant Liquor

The starting molar ratio (Na2O/Al2O3) determines thhydroxide, which can be precipitated per unit voalumina charge at digestion stage is to be maximratio resulting in higher supersaturation of precipitmore tendency to precipitate with the reduction of te

Fig. 1 Effect of Na2O/Al2O3-MR and caustic concentration on precipitated hydrate

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2.2 2.3 2.4 2.5 2.6 2.7 2.8 2Al 2O

3 pre

cipi

tate

d pe

r litr

e of

pr

egna

nt li

quor

Ending MR

M

MR=1.25,Na2O=150gpl

MR=1.45,Na2O=150gpl

MR=1.35,Na2O=110gpl

i

MR=1.35,Na2O=150gpl

e amount of aluminium lume of liquor. So the zed to get lower molar ation liquor, which has mperature.

.9 3

R=1.45,Na2O =110gpl

Fig.2 shows the value of molar ratio (A/C ratio) at which additional alumina dissolved in digestion will yield extra alumina during precipitation.

c. Temperatu The starting anand its granulconcentration following a higliquor productivproducing coaproduce sandystarting temperin precipitationalumina concenprofile is necesmolar ratio of thIn practice, thetemperature ancooling. The toperating alum d. Seed Charg

The rate ofcrystallisatiomore liquor

Typical Precipitation Profiles Showing the Influence of the Starting A/C (or MR) and the Productivity

0.35 0.4

0.45 0.5

0.55 0.6

0.65 0.7

0.75 0.8

0

A/C

60

80

100

1 ctiv

ity, g

pl

MR=1.38

MR=1.32

PRODUCTIVITY

Fig.2:

Liquor

re Pr

d enomeresuher ity.

rser alumature resutratisarye liq optd b

empeina p

e

crysn. Hprod

5

Liquo

ofile Across Precipitation

ding temperature of precipitation process dictates the yield try. Higher temperature has higher equilibrium alumina lting in decreased liquor productivity. Hence a plant starting and higher ending temperature will have lower The temperature profile is selected based on the aim of (sandy)/ floury alumina. To maximize the yield and to ina, it is essential to have a higher supersaturation, higher and a lower ending temperature. Lower end temperature lts in a higher precipitation rate due to lower equilibrium on in the liquor. Hence a search of optimum temperature for maximizing the yield for any given concentration and uor. imum profile can be approximated by optimisation of first y introducing one or the several stages of intermittent rature profile of 70– 50°C is considered ideal for the

lants.

tallisation is proportional to the seed surface available for igher seed charge leads to higher surface area and hence uctivity. The excess seed charging will lead to generation

10 15 20 25 30 35 Residence Time (hours)

0

20

40

r 2 Liq

uor

prod

u

A/C

of more fines in the circuit. In the plants not following seed filtration, the seed charge can be increased upto 300 – 400 gpl and the plants following seed filtration, the seed charge can be increased upto of 400 – 600 gpl .

Fig.3 shows the influence of seed charge vs liquor productivity. 5.4 Technologies Used The two basic technologies prevalent for precipitationand American technology. The European variant productivity operating at low temperature profile acrocharge and results in the generation of floury aluminauses low caustic conc., low seed charge and has lowthe advantage of coarsened alumina. The European process is operated closer to optimumlower energy consumption due to higher liquor technology is at present being followed by BALCO reAluterv – FKI, Hungary as shown in Fig.4.

Fig. 3 : Influence of the seed charge ,with and without seed filtration ,on the liquor productivity

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0 100 200 300 4

Solids in the end of precipita

Liqu

or p

rodu

ctiv

ity

WITH SEED FILTRATION

process are the European is characterized by higher ss precipitators, high seed

, while the American variant liquor productivity but has

conditions resulting in the productivity. This type of

finery with the technology of

00 500 600

tion slurry

WITHOUT SEED FILTRATION

One of the pecularities of the American process technology is the agglomeration of fine seed particles at higher temperature. The important process stage in this is the hydrate classification and the use of two different seed: fine and coarse seed. This type of variant is being followed by HINDALCO refinery precipitation process as shown in Fig.5.

Table-1 shows the comparison of the two processes

25 –30 hours

Pregnant liquor

Product hydrate for calcination

PT ST TT Precipitator

Fig. 5: American Process

Seed Tank

Pregnant liquor

Spent liquor

40 – 50 hours

)

n

Fine

Spent liquoroverflow

Precipitators

Fig. 4: European Process

(filtrate

Productio

seed

Coarse seed

Table-1: Comparison of the European and American process

Characteristics European American Pregnant liquor: Na2Oc,gpl MR

145-150 1.65-1.75

105-110 1.5-1.55

Seed solids conc.,gpl 200-400 50-120 Residence time, hours 50-70 30-40 Temp; °C – Starting Ending

60 50-55

75 55-60

Liquor productivity, kg/m3 65-70 50-55 Product size (-45 µm), % 35-45, Intermediate 8-12, Sandy Crystallisation mechanism adopted

Nucleation + growth

Agglomeration + growth

The trend of conversion of floury to sandy alumina suiting to modern smelter requirement is being practiced for material and energy saving measures. In this context the productivity is also to be maintained intact and on the higher side. To cope up with the recent requirement, the latest Alusuisse precipitation process and Aluminium Pechiney high solid technology were developed having advantage of higher liquor productivity and better granulometry of the product. Alusuisse precipitation process or Alusuisse Hybrid technology is the hybrid of the European and American variants achieving the high precipitation yield and the required product quality together with granulometry. The temperature can also be adjusted during the precipitation by means of appropriate selection of interstage cooling to maintain a sufficient but not too high supersaturation. It is also necessary to have control on agglomeration product by regulation of fine seed. This process is divided into two phases. The salient features of the process are detailed in Table-2.

Table-2: Salient Features of Alusuisse Hybrid Technology

Characteristics Phase-I (Agglomeration) Phase –II (Growth) Na2Oc, gpl Upto 150 (fine seed) Upto 150 (coarse seed)Seed in preg. liquor, gpl 50-120 200-400 Starting temp.,°C 70 <60 Residence time, hours 4-6 40 Productivity, kg/m3 80 Grain size < 45 µm 5-10%

The most important aspect of the improved precipitation technology is the hydrate classification and the use of different seeds. i.e. fine and coarse respectively. The fine seed after filtration and washing can be subjected to agglomeration while the coarse seed can be sent for growth directly. Liquor productivity of 70- 80 gpl has been obtained in Gove alumina plant while in Interalumina Venezuela more than 80 kg/m3 liquor productivity could be achieved by using this technology. Fig.6 and Fig.7 detail the scheme of this type of precipitation process.

Pechineplants utechnolocharacte

Hydrate ClassificationGrowth phase

Cooling

F

PR

F

Fig. 6: The Alusuisse Precipitation Process

Agglomeration

y technology has beense European, Americagy. The modified Eurised by the high seed

Agglomeration

FIW

INE SEED

GP

CALCINATION

EGNANT LIQUOR

SPENT LIQUOR

ig. 7: Process Flow D

IS

Coarse seed

Product

Fine Seed

intron andropea char

LTRATIASHING

ROWTRECIP

iagra

Coarse Seed Filtration

du nge

O

HIT

m

Fine Seed

ced in new alumodified Europ or Pechiney using the hyd

N &

ATORS PUMP OF

Coarse

PRODUC

of Alusuisse

Growth Phase

- Low seed charge e - High temperature

mepr

F

S

T

Sc

P

- High seed charge

- Low temperatur

eed

HYDRATE

Primary classification

recipitation Process

DELIQUORING & WASHING

econdary

Thickener

AGGLOMERATION PRECIPITATIORS

ina plants. Indian alumina an (old version Pechiney) recipitation technology is

ate growth mechanism but

lassification

having the medium level liquor productivity (Fig.8). The HINDALCO plant employs the American technology, the BALCO plant employs the conventional European technology, and the NALCO plant adopts the modified European technology.

Salient features of Aluminium Pechiney ancompared in Table 3 Table-3: Comparison of The New Develo

Pechiney & Alusuisse processCharacteristics Aluminium

PechineyPregnant liquor: Na2Oc,gpl MR

140-145

1.65-1.70Seed solids conc.,gpl 800 Residence time, hours 60-65 Temp;°C – Starting Ending

60 55

Liquor productivity, kg/m3

60-65

Product size (-45 µm), % 8- 10 Crystallisation mechanism adopted

Growth andsecondary nucle

15% < 45 µ

Pregnant liquor

Seed tank

50 0C

58 0C

75 kg productivity

Fig. 8: Pechiney Process with One Stage Classification

14 % < 45 µ

d Alusisse technology ha

ped Processes: Alumi

Alusu

140-1

1.45-140

50-675-850-580-8

<5 ation

Agglomerationnucleation a

Tbx

20 % < 45 µ Production 7 % < 45 µ

v

ni

is

4.6

0 0055

, n

Liquor

e been

um

se

5 0

secondary d growth

The Pechiney process being followed by NALCO Damanjodi refinery is characterised by high caustic concentration as high as 145- 155 gpl and molar ratio of 1.50 –1.55, high seed ratio of 3.0 - 3.5, low to medium temperature upto initial 60oC with the limitation of low 2 -2.5 gpl organic content. The seed is coarse 1 –12 % minus 45 micron and residence time of 45 hours. The most significant mechanism is the crystal growth without agglomeration. The hybrid technology consisting of American and European variants with high utility of agglomeration has been successfully adapted in alumina refineries abroad. The need of the hour is to optimize the precipitation process in Indian alumina plants to get the maximum productivity and good alumina quality. 6. Future Alumina Quality

The experts of ALCAN and COMALCO have advised the long term alumina quality specification expected on the “open market” with consideration of both the metal market quality trends and reduction cell technology change in their Weipa Feasibility study for a green field alumina plant. The forecast is reproduced in Table 1. Table.1: Future Alumina Quality Projections

Parameter Typical Range Fe2O3, SiO2, % 0.012 max. 0.015 Na2O, % – short term - long term

0.30 0.25

max. 0.35

Specific surface area, m2/g 70 60-80 Particle size, -45 microns, %

-20 microns, % +150 microns, %

6 <1 1

max.10 max.2 max. 5