settling characteristics of qal red mud · 2015-07-28 · titanium, soda-alumina-silicate...

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(ii)

SUMMARY

Red mud is a waste product of Bayer process. The composition of the red mud is

mostly oxides of iron, titanium, soda-alumina-silicate compounds, unreacted quartz,

and other insolubles in the bauxite. It must be removed from the pregnant caustic

liquor prior to precipitation or impurities will be present in the final product. The

removal is achieved in gravity settling tanks under flocculated conditions. The size of

these settling tanks is determined by the rate at which the red mud settles.

The initial aim of this thesis was to develop a simple, reproducible test that could be

used to assess the performance of flocculents in aiding settling. However, due to

equipment constraints, the aim was changed to developing an understanding of the

settling characteristics of the mud. This was investigated using a settling vessel

equipped with light sensors. Some difficulties were experienced owing to the fact that

the intensity of the light was not sufficient to obtain readings at all but very low

concentrations. Hence, the settling characteristics of the red mud slurry could not be

obtained. Other deficiencies in the experimental process were identified as the

method of flocculent preparation, the dispersion of flocculent in the slurry and

influence of wall effects on the settling of flocculated particles.

This thesis did not achieve the aim that it set out to, but it is the opinion of the author

that this is an important topic worthy of further study. Suggestions have been made

with regards to how more indicative results could be obtained and could these be

made it is likely that the settling characteristics of red mud could be obtained.

(iii)

TABLE OF CONTENTS

1 INTRODUCTION ................................ ................................ ........................... 2

2 LITERATURE REVIEW................................ ................................ ................ 3

2.1 FLOCCULEN TESTING TECHNIQUE................................ ................................ . 3

2.11 Flocculation Theory................................ ................................ ................... 3

2.12 Factors Influencing Flocculation ................................ ............................... 5

2.121 Conformation of Adsorbed Polymer................................ ................................ ...................... 6

2.122 Agitation Intensit ................................ ................................ ................................ ................ 6

2.213 Agitation Duration ................................ ................................ ................................ ............... 7

2.123 Temperature................................ ................................ ................................ ......................... 7

2.13 Procedures For Characterising Settling ................................ ..................... 8

2.131 Batch Procedures................................ ................................ ................................ .................. 8

2.132 Continuous Procedures ................................ ................................ ................................ ......... 9

3 THEORY................................ ................................ ................................ ........ 11

3.1 COARSE PARTICL SEDIMENTATION ................................ .............................. 11

3.11 Suspensions with Uniform Size Distributions................................ ............ 12

3.2 FLOCCULATION ................................ ................................ ............................ 16

3.21 Flocculent Effects on Stability................................ ................................ .. 17

3.22 Effect of Flocculation on Sedimentation ................................ ................... 17

4 PLAN OF STUDY ................................ ................................ ......................... 18

5 EXPERIMENTAL................................ ................................ ......................... 19

5.1 SETTLING VESSE DESCRIPTION................................ ................................ .... 19

5.2 MATERIAL ................................ ................................ ................................ .. 20

6 RESULTS................................ ................................ ................................ ....... 21

7 DISCUSSION................................ ................................ ................................ . 23

8 SUGGESTIONS FOR FURTHER WORK ................................ .................. 27

9 CONCLUSION ................................ ................................ .............................. 28

10 REFERENCES ................................ ................................ .............................. 29

(iv)

LIST OF FIGURES

Figure 2.1 Aggregation by polymer bridging 5

Figure 2.2 Agitation speed effects on flocculation 6

Figure 2.3 Flocculent testing 8

Figure 2.4 Continuous flocculent testing 9

Figure 5.1 Layout of experimental settling vesse 19

Figure 6.1 (a) Settling Rate vs Concentration (Flocculent Dosage = 0 g/t 21

Figure 6.1 (b) Settling Rate vs Concentration (Flocculent Dosage = 100 g/t 21

Figure 6.1 (c) Settling Rate vs Concentration (Flocculent Dosage = 200 g/t 21

Figure 6.2 (a) Sedimentation velocity as a function of voidage (0 g/t) 22

Figure 6.2 (b) Sedimentation velocity as a function of voidage (100 g/t) 22

Figure 6.2 (c) Sedimentation velocity as a function of voidage (200 g/t) 22

LIST OF TABLES

Table 6.1 Experimental and theoretical values of n

E1445 Thesis Settling Characteristics of QAL Red Mud Toby Marsh

Page 2

1 INTRODUCTION

The Bayer Process was developed in 1887 and is the most widely used method o

extracting aluminium oxide (A2O3) from bauxite ore before it is smelted to aluminium

metal. It involves the dissolving of the ore in a caustic (NaOH) solution at high

temperature and pressure in the digestion stage. The chemistry of the main reaction is:

Al2O3 + 2NaOH 2NaAlO2 + H2O

The digestors discharge to the clarification section in which the red mud is separated

from the pregnant liquor. The composition of the red mud is mostly oxides of iron,

titanium, soda-alumina-silicate compounds, unreacted quartz, and other insolubles in the

bauxite. The pregnant liquor is then seeded and the alumina precipitates as an alumina

trihydrate according to the reaction:

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

The hydrated alumina is then calcined to dry off the water and alumina is produced.

The area of interest in this thesis is the clarification of the pregnant liquor to remove the

red mud. At QAL (Queensland Alumina Limited), the red mud is removed via

flocculated gravitational separation. Although the digested slurry is naturally slightly

flocculated, flocculent must be added to achieve a practical settling rate and compaction.

The flocculents used are synthetic polymers and they are extremely costly. The mos

common way of determining the correct flocculent dosage is to perform batch settling

tests in a laboratory, though these can only be used as a rough guide to actual plant

performance as they are subject to operator bias and are seldom reproducible. This leads

to overdosing to ensure complete flocculation and due to the high cost of the flocculent

incorrect dosing can be an expensive procedure. The results from batch settling tests ar


Page 3

also used to size the clarifiers. Consequently, it is often the case that these are oversized

as well to ensure adequate performance, also an expensive exercise.

The original aim of this thesis was to develop a procedure that could be performed

reproducibly in the laboratory and removed the emphasis away from the operator,

focussing it on the equipment. Due to equipment and time constraints however, the ai

had to be re-evaluated. Consequently the main aim was shifted to investigating the

settling characteristics of red mud from the QAL alumina refinery. The second aim was

to observe the effect of flocculation on settling characteristics. Both were investigated

concurrently using the same apparatus and slurry.

2 LITERATURE REVIEW

2.1 Flocculent Testing Technique

This section covers the design a standard, simple, reproducible laboratory technique for

the characterisation of red mud settling. It will deal with the underlying molecular and

chemical theory behind flocculation, the factors affecting the performance of a flocculent,

and previous methods developed characterising settling. All literature quoted can be

found in the reference section of this report.

2.11 Flocculation Theor

Before bauxite ore can be slurried with caustic solution in the digestion stage, it must be

crushed to an appropriate size in a ball mill. When fines are produced, the particles gain

energy in the form of surface energy and deformation energy. This energy is stored in

the sub-surface region of the particle (Somasundaran, 1980). This increase in system

energy has to be eliminated and it is done by interaction with the solvent and solute

particles, as well as aggregation of the particles themselves. The rate of aggregation can

be extremely slow depending on the nature of interactions between the particles as well


Page 4

as the energy changes involved. It is dependent essentially on the probability of particle

collisions (governed by Brownian motion dependent on solution temperature), probability

of attachment during such collisions and probability of detachment of particles from

aggregates. The fraction of effective collisions (α) between two particles of radius ri and

rj separated by a distance d is given by the equation derived by Fuchs (Fuchs, 1934) as:

22exp2

1

S

dS

kT

V∫

∞

=

α [2-1]

,where k = Boltzmann constant

S = 2d/( ri + rj)

V = Total energy of interaction

T = Temperature

The total energy of interaction (V) is given by:

V = Va + Ve + Vb + Vs [2-2]

Va are the London-van der Waals attractive forces, Ve are the electrical double layer

forces, Vb are the bridging forces and Vs are the steric forces. In the discussion o

flocculation, the forces of interest are the bridging forces (Vb).

Aggregation can be induced by long chain adsorbates with several active sites attaching

to two or more particles. Polymers (flocculents) can provide such bridging mos

effectively if the surface of the particle is not completely covered by the polymeric

species (Figure 2.1). If the particle is completely covered (termed ‘overflocculated’),

bridging can only occur if some of the polymer detaches thus freeing up active sites on

the particle surface, or if polymer-polymer bonds are formed. If there are too few

polymers attached to the particle surface (termed ‘underflocculated’), the chances o

aggregation are also reduced due to the reduced probability of interaction of polymer and

particle. It has been suggested that the optimum surface fraction covered by polymers to


Page 5

induce aggregation is 0.5 (Healy and La Mer, 1964), however this was not supported by

experimental evidence and therefore has not been proven.

Whether or not a polymer will adsorb to the surface of a mineral particle, and how

quickly it will do so is largely dependent on the properties of the polymer and the

solution. Polymer properties such as molecular weight, and the nature and concentration

of the functional groups play a major role, as well as solution properties such as agitation,

ionic strength, solvent power, pH and temperature (Somasundaran, 1980). Forces

responsible for adsorption result mainly from three types of bonding, electrostatic,

hydrogen and covalent

2.12 Factors Influencing Flocculation

As mentioned previously, there are a number of factors that influence flocculation. These

relate to both the flocculent itself and the conditions of the solution to which it is added.

This section will provide a detailed account of how these factors effect flocculation, in an

attempt to identify the parameters that must be calculated when optimising flocculen

performance.

Polymer (flocculent)

Particle Aggregated Particles

Figure 2.1 Aggregation by polymer bridging


Page 6

2.121 Conformation of Adsorbed Polymer

The effect of the conformation of adsorbed polymeric flocculents on the flocculation

process, although considered to be substantial, has not been investigated yet to the point

of being able to ascertain the optimum conformation or conformations required. Studies

conducted on alumina flocculation with adsorbed polyacrylic acid have confirmed the

conformation is a controlling factor in flocculation (Huang, Tjipangandjara and

Somasundaran, 1988). Under fixed pH conditions, stretched polymers perform better

than coiled polymers. Under varying pH conditions, coiled polymers that undergo a

partial transformation to stretched polymers induce better flocculation. As noted, these

tests were performed with alumina and adsorbed polyacrylic acids. Whether these results

are valid for red mud flocculated with a different polymer could only be determined

empirically. However, it is interesting to note the effect each conformation can have and

the reasons for this.

2.122 Agitation Intensity

Excessive hydrodynamic forces can cause adsorbed flocculent molecules to either desorb

from the particle or snap completely. Both result in an irreversible deterioration in the

flocculation state. Studies

have been conducted with

red mud from Bayer primary

red mud thickeners to

observe the influence that

varying the agitation speed

had on flocculation (Farrow

and Swift, 1996). The

effects were monitored b

measuring the initial solids

settling flux (t/m2.h) and the residual turbidity (NTU) as the agitation speed was

increased. It was noted that as agitation speed increased, the settling flux fell sharpl

Initi

al S

olid

s S

ettli

ng F

lux

Res

idua

l Tur

bidi

t

Agitation speed

Flux

Turbidit

Figure 2.2 Agitation speed effects on flocculation


Page 7

while the residual turbidity passed through a shallow minimum before increasing rapidly,

probably due to the onset of aggregate rupture (Figure 2.2). It is apparent that agitation

intensity strongly affects flocculent performance. Further tests with increased flocculen

dosage showed that this only marginally compensated for the excessive agitation. It was

also noted that optimal agitation conditions varied for different flocculents, presumabl

due to the different adsorption characteristics of these flocculents. The equipment used to

conduct these tests was designed such that operator bias was removed. This makes the

study highly informative, especially considering the test material.

2.213 Agitation Duration

The study conducted by Farrow and Swift (Farrow and Swift, 1996) also investigated the

effect of solids residence time, or duration of agitation, on the settling performance o

Bayer red mud. It concludes that solids settling flux decreases as the residence time

increases. In contrast, the turbidity also decreased, rising only with excessively long

residence time. The drop in turbidity indicates the aggregation of fines into larger

particles, with the increase being explained by the eventual rupture of these aggregates.

The drop in solids settling flux is explained by the partial fragmentation of some of the

largest aggregates.

2.123 Temperature

Increasing the temperature of the solution increases the Brownian motion of the

suspended particles. This will have a beneficial effect on the formation of aggregates as

the probability of particles colliding with each other is increased (Somasundaran, 1980).

This is supported further by equation [2-1], in which the temperature of the solution is

directly proportional to the number of effective particle interactions.


Page 8

2.13 Procedures For Characterising Settling

Current sedimentation characterisation procedures can be broadly categorised into two

groups; batch procedures and continuous procedures. Some claim to be semi-continuous,

but these can usually be recognised as a combination of two other procedures. This

section outlines the most relevant methods of assessing settling characteristics.

2.131 Batch Procedures

The main aim of sedimentation characterisation

is to size settlers. One method proposed to do

this involved the setup illustrated in Figure 2.3

(Christian, 1994). To calculate the required data,

solutions of different concentration are added to

graduated cylinders being constant agitated by

wire stirrer at 1 rpm. The flocculent is added and

the solution is mixed thoroughly. The motor is

then switched off and the movement of the

interface between the solution and the settling solids is measured and timed. This allows

calculation of the solids settling velocity at given concentrations. However, it does not

allow for the effect of any other parameters such as temperature to be measured and as

such is not a completely satisfactory method of flocculent testing. A similar method was

proposed by Waters and Galvin (Waters and Galvin, 1991), but again, there was no

consideration of other parameters such as temperature.

The procedure proposed by Solym�r et al. (Solym�r et al., 1992) uses a graduated

cylinder as shown in Figure 2.3 with a soft γ-ray radiation source mounted on the side t

measure solids concentration and settling rate. It also has a temperature probe mounted

in the base to monitor temperature. However, it has no means of agitation or temperature

variation.

Motor

Figure 2.3 Flocculent testing


Page 9

A semi-continuous test was also proposed (Waters and Galvin, 1991), in which a constant

solids feed flux was maintained without underflow removal. This is not indicative of real

plant operation and hence can be disregarded.

2.132 Continuous Procedures

Several continuous procedures have been proposed, ranging from relatively simple

apparatus to very complex. The major difference between batch and continuous

procedures is the direction of fluid flow relative to particle flow. In batch procedures, the

thickening of the solids bed forces the fluid flow upward, opposing the direction of the

particles. In continuous procedures however, a fraction of the fluid flows down towards

the discharge stub. This is a more accurate representation of plant operation.

The most effective method of assessing the performance of flocculents that the author has

found is outlined in a paper by Farrow

and Swift (Farrow and Swift, 1996).

The experimental apparatus, detailed in

Figure 2.4, consists of an external fixed

cylinder and an internal rotating

cylinder driven by a variable speed

motor. The conical section at the base

of the cylinders is fixed at an angle of

45° and slurry is fed into the gap

between the two cylinders from an

agitated tank via a peristaltic pump.

The flocculent is introduced through the

side of the vessel below the level of the slurry, and the flocculated slurry is drawn out vi

an underflow peristaltic pump. A level sensor attached to this pump controls the slurry

level in the shear vessel. The settling rate of the slurry in the cylinder attached to the

bottom of the shear vessel is calculated using a Lasentec® instrument while the turbidity

of sampled slurry is calculated using a modified Analite nephelometer. This is the most

P

P

Computer

MotorSlurry

Figure 2.4 Continuous flocculent testing

Flocculent

Motor


Page 10

accurate and representative testing procedure that has been developed to date, but

exceeds the monetary and practical constraints of most plant laboratories.

Waters and Galvin have developed a simplified and possibly applicable continuous

procedure (Waters and Galvin, 1991). It involves a graduated cylinder with constant

solids feed flux and underflow removal of slurry via a peristaltic pump. The slurry is

mixed with flocculent in a separate agitated vessel before being introduced to the cylinder

about halfway down its height. There is an overflow to remove excess supernatant and a

rotating rake in the base of the cylinder. If this vessel could be modified to include

temperature variations, the procedure may be an inexpensive and representative method

of assessing flocculents.


Page 11

3 THEORY

An isolated particle settling relative in a fluid in a gravitational field rapidly reaches a

terminal velocity. Terminal velocity (uo) occurs when the frictional forces acting on the

particle equal the net gravitational force. In industrial applications, concentrations o

suspensions are generally high enough for the interaction between settling particles t

significantly affect the rate of settling. Due to the modified flow pattern, the frictiona

forces may also be much greater than in the single particle case, and this condition is

known as hindered settling. The hindered settling velocity (up) may be considerably

lower than the terminal settling velocity of the particle.

3.1 Coarse Particle Sedimentation

The specific surface of coarse particles is much lower than that of fine particles, and thus

the surface forces and electrical interactions between particles are of little significance.

In general, the particles will not affect the rheology of the fluid. In the case of a coarse

particle settling in a suspension of much finer particles, the suspension can be considered

a continuum. The settling large particle will an displace equal volume of suspension in

which the movement between the fine particles and the liquid will be small. The

effective buoyancy in this case can be assumed equal to that of a liquid with the same

density as the suspension. However, the pressure distribution around small particles

settling in a suspension of coarse particles will hardly be affected by the larger particles,

and will therefore not contribute much to the buoyancy force. If the particles are all of a

comparable size, an intermediate situation will exist

The gravitational force on an individual particle in a sedimenting suspension is balanced

by buoyancy and fluid friction forces. These two forces are determined by the flow field

and the pressure distribution in the area surrounding the settling particle, but it is

considered academic to separate the total force into its two individual constituents.


Page 12

3.11 Suspensions with Uniform Size Distributions

To aid the interpretation of results from this thesis, the assumption was made that the

suspension had a uniform size distribution. This made it possible to correlate the results

to the Richardson-Zaki equation1, which was derived as follows:

The force balance for a single spherical particle settling at terminal velocity (u0):

R d d gs' ( )02 3

4 6

π π ρ ρ= − [4-1]

Rearranging:

R d gs' ( )0

2

3= −ρ ρ [4-2]

where R’0 = Drag force per unit projected area of particle (N/m2)

d = Diameter of particle (m)

ρs = Particle densit (kg/m3)

ρ = Liquid density (kg/m3)

g = Gravitational acceleration (m/s2)

The drag force is a function of the liquid density, liquid viscosity, terminal particle

velocity, and particle diameter. Due to the small diameter of the vessel used in this thesis

(symbolised as dt), wall effects are also significant, and as such;

R f u d dt' ( , , , , )0 0= ρ µ [4-3]

1 Richardson, J.F., Zaki, W.N., 1954, Sedimentation and Fluidisation: Part I, Trans. Inst. Chem. Eng.,Volume 32, Number 35


Page 13

As suspensions used in industry, and tested in this thesis are concentrated, the hindered

settling velocity (up) will be influenced by the particle concentrations. Hence, the drag

force is also a function of voidage (e), defined as the volume fraction of the liquid in

suspension. Therefore;

R f u d d ep t' ( , , , , , )0 2= ρ µ [4-4]

This argument will hold if the buoyancy force is attributed to the suspension density (ρc)

as opposed to the liquid density, and in this case the force balance becomes;

R d d gs s c' ( )02 3

4 6

π π ρ ρ= − [4-5]

As,

ρ ρ ρ ρs c se− = −( ) [4-6]

It follows that equation 4-5 becomes;

R d d egs s' ( )02 3

4 6

π π ρ ρ= − [4-7]

It is therefore possible to compare equations 4-2 and 4-7 and conclude;

R eRs' '0 0= [4-8]

Thus this condition introduces no additional variables to equation 4-4. By re-arranging

equations 4-3 and 4-4, the terminal settling velocity and hindered settling velocity can be

expressed explicitly;


Page 14

u f R d dt0 3 0= ( ' , , , , )ρ µ [4-9]

u f R d d ep t= 4 0( ' , , , , , )ρ µ [4-10]

Combining the two terms;

u

uf R d d e

p

t

05 0= ( ' , , , , , )ρ µ [4-11]

As up/u0 is dimensionless, the function, f5, must also be dimensionless;

= e

d

ddRf

u

u

t

p,,

'2

20

50 µ

ρ[4-12]

as R’0d2ρ/µ2 is the only dimensionless function of 0. As;

pc euu = [4-13]

Therefore;

= e

d

ddRf

u

u

t

c ,,'

2

20

60 µ

ρ[4-14]

From equation 4-2;

Ga

gddR s

3

2

)(

3

2'2

3

2

20

=

−=

µ

ρρρµ

ρ

[4-15]


Page 15

Where Ga is the Galileo number. The Galileo number can be expressed as a function o

terminal settling velocity, particle diameter, liquid density, and liquid viscosity. These

terms can be combined to give the Reynolds number for a particle falling under termina

velocity (Re’0). Therefore, equation [4-14] can be written thus;

= e

d

df

u

u

t

c ,,Re'070

[4-16]

Experimental results have shown that for most suspensions, the plot of log uc versus log e

exhibits linear characteristics;

)log()log()log( ic uenu += [4-17]

where n = gradient

ui = free falling velocity at infinite dilution

uc = sedimentation velocity

e = voidage (volume fraction of liquid)

The gradient of the curve (n) has been experimentally determined to be independent o

the voidage (e), and purely a function of the ratio of particle diameter to the diameter of

the settling vessel, in conjunction with either the Reynolds number of the Galileo

number;

=

td

dGafn ,Re'/ 07 [4-18]

The relationship between n and d/dt was investigated experimentally by Richardson and

Zaki..


Page 16

Work conducted by Garside and Al-Dibouni2 suggests the effects of interaction between

particles is underestimated in equation 4-13, especially at high values of e. They

proposed the equation;

068.2

14.5

Re'06.0/

/=

−−

oc

oc

uue

euu[4-19]

The complexity of equation 4-19, coupled with the doubt that it actually more accurate is

justification enough to use equation 4-13 for the purposes of this thesis.

3.2 Flocculation

Flocculation affects the behavior of fine particles in suspension by causing the particles

to aggregate with occluded liquid. The density of the conglomerated particles is between

that of the particles and the liquid.

Aggregates form when particles collide due to relative motion, and their stability (agains

aggregation) is determined by the interaction that occurs upon collision. Attractive and

repulsive forces are dependent on the conditions of the suspension, including

concentration, pH and temperature. Flocculents are macromolecules (generally long

chain organic molecules) which adsorb to the surface of molecules. When collisions

occur, these adsorbed molecules may adsorb to the surface of the colliding particle and

cause the two to form an agglomerate. Other attractive/repulsive forces present are van

der Waals forces and electrostatic forces.

2 Garside, J., Al-Dibouni, M.R., 1977, Velocity-voidage relationships for fluidisation and sedimentation in

solid-liquid system, Ind. Eng. Chem. Proc. Des. Dev., Volume 16, Number 206


Page 17

3.21 Flocculent Effects on Stability

Adsorbed flocculents can strongly affect the stability of suspensions, depending on a

number of factors which are influenced by the mechanism of adsorption, the amount o

flocculent present and the method of mixing the polymer into the suspension. The total

potential energy of interaction (VT) is given by;

VT = VA + VR + VB + VS [4-20]

VA are the London-van der Waals attractive forces, VR are the electrical double layer

forces, VB are the bridging forces and VS are the steric forces. In the discussion o

flocculation, the forces of interest are the bridging forces (VB). Adsorbed flocculent can

decrease VA and increase VR, and can also introduce a steric force.

Flocculents can exhibit an optimum dosage, which depends on the flocculents molecular

weight and the solids concentration in the suspension. Overflocculation can result in the

restabilisation of the suspension.

3.22 Effect of Flocculation on Sedimentation

The effect of flocculation on the settling of particles in suspension is dependent on the

dose of flocculent and the chemical environment. There are two types of settling which

commonly occur. The first occurs at low initial solids concentrations, and the flocs settle

as discrete units of aggregated particles and entrained liquid. They settle at a constant

rate until they begin to accumulate at the bottom of the vessel, at which time the flocs

deform and the sediment layer compacts. At higher solids concentration, the maximum

settling rate may not be achieved immediately, and could increase with increasing initial

fluid height. The model suggested for this situation is one where flocs extend in a

continuous network to the walls of the vessel.


Page 18

4 PLAN OF STUDY

The aim of this thesis was to investigate settling characteristics of red mud from the QAL

alumina refinery, and to observe the effect of flocculation on settling characteristics. The

settling characteristics of the red mud were investigated by varying the concentration o

the mud and the flocculation dosage, and observing its sedimentation velocity. A

cylindrical settling vessel with light sensors was used to measure the change in the

concentration of the slurry as it settled. This data was interpreted to determine the

sedimentation velocity and correlated to the Richardson-Zaki equation.

Viscosity and density experiments were conducted to obtain data to be used in the

analysis of results.


Page 19

5 EXPERIMENTAL

5.1 Settling Vessel Description

The layout of the experimental settling vessel is shown in Figure 5.1. The settling vesse

consists of a vertical cylinder (I.D. 110 mm) with a volume of 0.009 3. The vessel is

constructed from transparent plastic painted with black paint to prevent light fro

penetrating the vessel. A plastic

lid with a motor built into it i

screwed onto the top of the vessel

prior to testing. The vessel was

equipped with a stirrer which was

designed to rotate slowly in order

to negate edge effects imparted

by the wall. Unfortunately, the

stirrer could not be attached to the

motor and as a result it was not

used.

Attached to the vessel is a

fluorescent light source running

vertically up one side. Directly

opposite the light source running

up the other side of the vessel is

16 ports for light sensors. The sensors begin 80 mm from the base of the settling vessel

and are spaced 55 mm apart. Only 12 light sensors and these inserted into the bottom 12

ports. The light sensors fed into a light sensor input and their readings were stored by the

data logger.

Light Sensor Input

Data

Logger

Light Sensor Output

Light

Source

Figure 5.1 Layout of experimental settling vessel


Page 20

Red mud slurry was premixed and the concentration was calculated before it was added

to the vessel. Based on the concentration, the flocculent dosage could be calculated.

Varying slurry concentrations were tested in conjunction with four different flocculent

dosages. The slurry was added to the vessel prior and mixed thoroughly prior to the

addition of flocculent. The flocculent was added in accordance with the procedur

outlined by Cytec. Prior to flocculent addition, the lid was placed on the vessel and data

acquisition began. Data was collected for at least three hours to ensure complete settling

had occurred. In the case of the highest flocculent dose however, the time required for

testing was much less. The rate of settling was determined by the degree of ligh

transmission through the slurry. Light intensity varied on a scale from 0 to 255, with 255

being the maximum light intensity recorded by the sensors.

5.2 Materials

The suspension used in this thesis was obtained from the overflow of washer number 4 at

the QAL alumina refinery in Gladstone, Qld. The solids were ‘red mud’, which consist

of the components of bauxite which are not dissolved in the caustic during digestion. The

liquor was essentially a dilute caustic (NaOH) solution.

The flocculent used in the thesis was the Cytec product HX-400. This is a Superfloc

HX emulsion flocculent and is used on-site at QAL. The details of the composition o

the flocculent as well as the instructions for preparing and testing it are regarded as

confidential. Subsequently they cannot be included in the thesis.


Page 21

6 RESULTS

Experiments were performed using different concentrations of red mud slurry and

different flocculent dosages. The solutions were mixed before and after flocculen

addition. Each experiment was run for at least three hours to ensure that complete

settling occurred. However, with flocculent addition, it was noticed that settling occurred

much faster than this. Figures 6.1(a), (b) and (c) illustrate the affects of concentration

and flocculent dosage on settling rate.

0.976

0.98

0.984

0.988

0.992

0.996

1

8.2 8.7 9.0 17.8 18.5 19.7 34.3 35.1 37.1

Conc (g/L)

Uc

(mm

/s)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

8.1 8.4 8.9 16.8 17.1 18.2 33.1 35.5 41.2Conc (g/L)

Uc

(mm

/s)

Figure 6.1 (a). Settling Rate vs Concentration Figure 6.1 (b). Settling Rate vs Concentration

Flocculent Dosage = 0 g/t Flocculent Dosage = 100 g/t

0.00

2.00

4.00

6.00

8.00

10.00

24.2 25.1 27.2 41.3 42.4 44.3Conc (g/L)

Uc

(mm

/s)

Figure 6.1 (c) Settling Rate vs Concentration

Flocculent Dosage = 200 g/t

The log of the settling rate was plotted against the log of voidage (ie. volume fraction o

liquid in suspension) to assess how well the data correlated to the Richardson-Zaki


Page 22

equation for hindered settling. The results of this are shown in Figures 6.2 (a), (b) and

(c).

y = 0.0126x + 0.1082R2 = 0.9225

-0.016

-0.012

-0.008

-0.004

0.000-9.8 -9.6 -9.4 -9.2 -9.0 -8.8 -8.6

ln (e)

ln (

Uc)

y = 0.0062x + 0.048R2 = 0.9843

-0.016

-0.012

-0.008

-0.004

0.000-11.0 -10.0 -9.0 -8.0 -7.0

ln (e)

ln (

Uc)

Figure 6.2 (a). Sedimentation velocity as a Figure 6.2 (b). Sedimentation velocity as a

function of voidage (0 g/t) function of voidage (100 g/t)

y = 0.0024x + 0.0034R2 = 0.9527

-0.020

-0.016

-0.012

-0.008

-0.004

0.000-10 -8 -6 -4

ln (e)

ln (

Uc)

Figure 6.2 (c). Sedimentation velocity as a

functio of voidage (200 g/t)

From the results shown in Figure 6.2, an experimental value for the index n in the

Richardson-Zaki equation was calculated. This was compared with literature values for

the index, however the method of calculating this was questionable. The results are listed

in Table 6.1. Complete results and calculations can be found in Appendix B of this

thesis. Data from each run was recorded electronically and is stored on computer file.

The data can be obtained by contacting the author.


Page 23

Floc Dosage Experimental Theoreticallyn Ga Predicted n

0 0.0126 6.40E+04 2.40100 0.0062 1.06E+04 2.74200 0.0024 1.38E+04 2.69

Table 6.1 Experimental and theoretical values of n

7 DISCUSSION

The solutions with no flocculent addition and those dosed with 100 g/t, illustrated in

Figures 6.1 (a) and 6.1 (b) followed the expected trend with respect to sedimentation

velocity as a function of concentration. The reason for this is that as the concentration

increases, the probability of interaction between settling particles increases and the

sedimentation velocity decreases. This is illustrated in the Richardson-Zaki equation:

U Ucn= 0ε [7-1]

As the voidage of the solution (ε) increases, the hindered sedimentation velocity (Uc) wil

also increase as n remains constant. The solutions dosed with 200 g/t flocculent, shown

in Figure 6.1 (c) however do not follow the trend as closely as the first two. The

sedimentation velocity for the 25 g/L solution is lower than expected, and the velocities

for the higher concentrations are significantly lower than they should be. The fact that

the hindered settling velocities exhibited by the solutions that were flocculated to 100g/t

were less than corresponding settling velocities exhibited by unflocculated solutions was

unexpected, as was the settling velocities of the higher concentrations dosed with 200 g/t

flocculent. There are a number of possible explanations for this.

There are a number of possible reasons associated with the addition of flocculent and

mechanism of flocculation. These are as follows:

1. Preparation of flocculents: The specifications pertaining to the breaking and dosing

of HX-400 supplied by Cytec states that the flocculent should be prepared by stirring

with a magnetic stirring device. Previous experience has highlighted a deficiency in


Page 24

this technique occurring when clumps of flocculent form in the final test batch. It

was postulated by sources at QAL that this was due to an insufficient shear rate

generated by the magnetic stirrer, and in order to get the flocculent to disperse

correctly, overhead agitation should be used. This technique was implemented and

the formation of lumps was greatly reduced. However, they were noticed in a few of

the batches of flocculents that were prepared. Clumps of flocculent indicate that the

long chain macromolecules have failed to extend and disperse, and the concentration

of the flocculent mixture is of variable concentration. This results with inaccurate

dosing of flocculent, and would affect the settling rate. Alternately, the rate o

mixing could have caused the polymer flocculent molecules to shear, which would

render them ineffective in aiding the settling of the suspension.

2. Effect of concentration on flocculation: As mentioned in section 3.22 of this thesis,

at low initial solids concentration, the settling rate is fairly constant. However at

higher initial solids concentrations, the maximum rate of settling is not reached

immediately due to the formation of a network of flocs extending to the walls of the

vessel. As the vessel diameter is quite small, and the stirring apparatus could not be

used, this is at least plausible.

3. Method of flocculent addition: Correct flocculent addition calls for the plunging of

the mixture with a horizontal perforated disc. As this piece of equipment was

unavailable, the mixture was agitated with a large stirrer. This was an unfortunate,

but unavoidable consequence of the lack of equipment, but one which probably led to

poor distribution of the flocculent through the suspension.

4. Dosage of flocculent: It is highly likely that the dosage of flocculent added to the

suspension was incorrect. This is due to the extreme difficulty experienced in

obtaining an accurate measure of the concentration of the suspension. Concentration

was calculated by syringing a known volume of suspension into a pre-weighed

container, evaporating the liquid in an oven, and measuring the change in mass.

Preliminary visual settling experiments conducted with samples from the top and

bottom of the sample container indicated that the suspension, if thoroughly mixed had

a uniform concentration. Subsequent more accurate measurements of solids conten

of samples taken from the top, middle and bottom of the suspension showed a marked


Page 25

difference. This was not discovered until towards the end of the experimental runs,

when they were conducted in an effort to account for erroneous results. At this stage,

it was not feasible to retest all the samples, so as the middle result was approximatel

the mean of those from the top and the bottom, it was taken as valid. However, this

does imply that the correct solids concentration was obtained, and indeed it probably

was not. This would have resulted in an error in the calculated amount of flocculen

used to obtain the required dosage, and would affect the sedimentation rate.

The method for obtaining a settling rate of the suspensions is another possible area where

error may have been introduced into the experiments. As outlined in section 4 of this

thesis, the settling vessel used light sensors to measure the concentration of the slurry.

number of deficiencies can be recognised in this method:

1. Method of identifying interface: The interface was assumed taken as the poin

where the light intensity measures 127.5, the halfway point. However, to be

completely accurate, the light sensors would have to be calibrated to transform

readings of light intensity to concentration, giving a change in concentration with

time (∂C/dt) and height ∂C/∂H). This would allow calculation of the sedimentati

velocity in accordance with the Kynch theory of sedimentation. However, it was

noted in work by Drake3 that the light sensors were only sensitive enough to measure

extremely low concentrations, in the order of 10-3 g/L. As the lowest initial solids

concentration tested in this thesis was 8.1 g/L, it was assumed that the reading would

be too low to gain any meaningful concentration data from the light intensity.

Instead, it was assumed that below concentrations where the light sensor would

actually register a reading, the liquor above the sensor was close enough to clear.

This allowed the progression of the interface to be observed. This was a necessary

simplification, but one that would have imparted considerable error into the

experiment.

2. Dispersion of Light: As the light source consisted of a single fluorescent tube

running the length of the settling vessel, the amount of light registered by a sensor at

height H above the base would not be only the light transmitted at height H. Instead,


Page 26

light transmitted from above and possibly below the sensor would also be detected

and as a result an inaccurate measurement would occur. This could possibly have

lead to an overestimation of the sedimentation velocity. This problem may be able to

be remedied by the use of LASER light sources positioned directly opposite the light

sensor. These would not deviate from the its path and would not influence the

readings of other light sensors. Additionally, the intensity of the LASER could be

increased beyond that of fluorescent light to make it possible to obtain readings in

more concentrated slurries.

1. Wall effects: Wall effects are considered negligible if the ratio of the diameter of the

vessel to the particle diameter is more than 100. In this case, it is possible that the

ratio for unflocculated particles was greater than 100, and the sedimentation velocity

was not affected. However, when the slurry was flocculated, the particle diameter

increased and the resulting ratio may have been small enough for the sedimentati

velocity to be reduced by the retarding effect of the walls. The vessel was equipped

with a stirrer aimed at negating wall effects. However, this could not be used and as a

result wall effects may have been influential.

As can be seen from Table 6.1, the results did not correlate well with the Richardson-

Zaki equation. Values for n must be greater than 1, or the hindered settling velocity will

be greater than the terminal settling velocity; a situation which is physically impossible

under gravitational settling conditions. This result can be attributed to the poor settling

data obtained from the experiments, the possible causes of which have been outlined

previously.

3 Drake, A., 1998, SMACS, The University of Queensland


Page 27

8 SUGGESTIONS FOR FURTHER WORK

There are a number of ways that this thesis could be improved in attempt to obtain more

accurate results:

1. Flocculent preparation and addition: A method for preparing flocculent solutions

without the formation of clumps must be developed. Whereas solutions with visible

clumps were not used in this thesis, their presence in some of the solutions prepared

makes it difficult to be confident that those in which clumps were not visible were

indeed of a uniform concentration. It is therefore recommended that the preparation

method be reviewed and modified if possible to ensure complete mixing of the

flocculent in the liquor. The method of addition of flocculent was not ideal. Tests

should be repeated using a perforated horizontal disk to disperse the flocculent in the

liquor prior to settling. This disk should have a diameter slightly less than that of the

settling vessel.

2. Concentration of slurry: A more accurate method of determining slurry

concentration must be developed to aid in the analysis of results, and in the

calculation of flocculent dosages.

3. Equipment modifications: To ensure that the readings of the light sensors represents

the slurry concentration at the level of the sensor, a more appropriate light source

could be found. One suggestion is that a LASER be used. This would also allow the

intensity to be increased in order for the concentration to be more accuratel

measured. Additionally, the stirrer must be attached to ensure that wall effects

(significant when particles flocculate) are negated.


Page 28

9 CONCLUSION

As previously mentioned, the aim of this thesis was changed due to equipmen

constraints. The amount of time available to investigate this topic meant that this was a

preliminary investigation. Although no conclusions can be drawn with regards to the

settling characteristics of red mud, this is indeed an important and relevant topic, and one

which should be investigated further in the future. The conclusions that can be drawn

from this thesis relate to the method in which the settling characteristics are determined.

Given a few modifications to experimental technique and equipment, it is my firm belie

that the aim of this thesis can be achieved. The following are the conclusions that have

been made:

1. The method of flocculent preparation must be improved to ensure homogenous

batches of flocculent are obtained.

2. A horizontal perforated disk must be used to agitate the mixture both before and after

flocculent addition to ensure complete dispersion of the flocculent in the slurry.

3. The light source used in the settling vessel must be modified to allow more accurate

results to be recorded at much higher concentrations than is currently possible. This

could be achieved by using a LASER.

If these suggestions were to be implemented, it is highly likely more meaningful results

can be obtained and the settling characteristics of red mud can be determined.


Page 29

10 REFERENCES

1. Christian, J.B., 1994, Improve Clarifier and Thickener Design and Operation,

Chemical Engineering Progress, Volume 90, Number 7, July 1994, pp 50-56

2. Farrow, J.B., Swift, J.D., 1996, A new procedure for assessing the performance of

flocculents, International Journal of Minerals Processing, Volume 46, 1996, pp 263-

275

3. Farrow, J.B., Swift, J.D., 1996, Agitation and Residence Time Effects during the

Flocculation of Mineral Suspensions, Proceedings of the Fourth International

Alumina Quality Workshop, June 1996, pp 355-363

4. Fitch, B., 1993, Thickening Theories – an Analysis, AIChE Journal, Volume 36,

Number 1, January 1993, pp 27-36

5. Fuchs, N., 1934, Uber Die Stabilitat und Aufladung der Aerosole, Z. Physik, Number

89, 1934, p 736

6. Healy, T.W., La Mer, V.K., 1964, The Energetics of Flocculation and Redispersion

by Polymers, Journal of Colloid Science, Volume 19, 1964, p 323

7. Leontaridis, N., Marinos-Kouris, D., 1992, Grecian Red Mud – Thickening and

Filtration Parameters and Process Design, Filtration and Separation, Volume 29,

Number 1, January-February 1992, pp 51-56

8. Moudgil, B.M., Scheiner, B.J., 1988, Flocculation and Dewatering, Proceeding of the

Engineering Foundation Conference, Palm Coast Florida, January 10-15, 1988


Page 30

9. Peiwang, L., Zhijian, L., Yucai, L., Hailong, C., Fengling, W., Hong, W., 1995, The

Influence of the Predesilication Temperature of Bauxite Slurry on th

Sedimentation of Red Mud and the Utilization of which in the Alumina Industry,

International Journal of Light Metals, 1995, pp 1417-1420

10. Plumpton, A.J., 1988, Production and Processing of Fine Particles, Proceedings o

the International Symposium on the Production and Proceedings of Fine Particles,

Montreal, August 28-31, 1988

11. Ryles, R.G., Avotins, P.V., 1996, Superfloc HX, A New Technology For The

Alumina Industry, Proceedings of the Fourth International Alumina Quality

Workshop, June 1996, pp 205-215

12. Solym�r, K., Saj�, I., Steiner, J., Z�ldi, J., 1992, Characteristics and Separability of

Red Mud, International Journal of Light Metals, 1992, pp 209-223

13. Coulson, J.M., Richardson, J.F., Brackhurst, J.F., Harker, J.H., 1991, Chemical

Engineering Volume 2, Fourth Edition, Particle Technology and Separation

Processes, Pergamon Press, Oxford

14. Richardson, J.F., Zaki, W.N., 1954, Sedimentation and Fluidisation: Part I, Trans.

Inst. Chem. Eng., Volume 32, Number 35

15. Garside, J., Al-Dibouni, M.R., 1977, Velocity-voidage relationships for fluidisation

and sedimentation in solid-liquid systems, Ind. Eng. Chem. Proc. Des. Dev., Volume

16, Number 206

16. Drake, A, 1998, SMACS (Fourth Year Engineering Thesis), Department of Chemica

Engineering, The University of Queensland

APPENDIX A

EXPERIMENTAL DATA

(Data and Charts available from author)

APPENDIX B

RESULTS AND

CALCULATIONS

B-1

APPENDIX C

EXPERIMENTAL

PROCEDURE

C-1

SLURRY PREPARATION

1. From a concentrated batch of known concentration, aliquots of 1000, 500 and 250 mL

were taken and diluted in 7L of water.

2. Following thorough mixing, 100 mL aliquots were drawn from the middle of the

suspension and placed in pre-weighed trays.

3. The trays were placed in an oven at 90 °C and allowed to evaporate.

4. Once all liquor had evaporated, the trays were weighed and the solids concentration

of the slurry calculated.

FLOCCULENT PREPARATION

Due to the confidentiality of the methods of preparing the flocculent, the details cannot

be included in this thesis.

SETTLING EXPERIMENTS

1. The slurry of known concentration was transferred to the settling vessel. All possible

precautionary measures were taken to ensure all solids were transferred to the vessel.

2. The light sensors were inserted into the bottom 12 ports and the light source was

turned on.

3. The slurry was mixed thoroughly and the flocculent was added by syringe if required.

The slurry was then mixed again to disperse the flocculent.

4. The lid was fixed to the settling vessel and the data logger was initiated.

5. After at least 3 hours (required to ensure complete settling), the data logger was

stopped.

6. Data was copied and interpreted in Microsoft Excel.

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