process plant evaluation

8/3/2019 Process Plant Evaluation

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SYNOPSIS

Energy is expensive. Process industries have always recognized that wasting energy

leads to reduced profits, but throughout most of this century the cost of energy was

often an insignificant part of the overall process cost, and gross operational

inefficiencies were tolerated. In 1970s, a sharp increase in the price of natural gas

and petroleum raised the cost of energy and intensified need to eliminate

unnecessary energy consumption. As an engineer designing a process, one of the

principal jobs would be to account carefully for the energy that flows into and out of

each process unit and to determine the overall energy requirement for the process.

This is done by writing energy balances and much the same principle applies to the

material balances to account for the mass flows to and from the process and its unit.

Heat transferis the transition of thermal energy from a heated item to a cooler item.

When an object or fluid is at a different temperature than its surroundings or another

object, transfer of thermal energy, also known as heat transfer, or heat exchange,

occurs in such a way that the body and the surroundings reach thermal equilibrium.

Mass transfer is the phrase commonly used in engineering for physical processes

that involve molecular and convective transport of atoms and molecules within

physical systems. Mass transfer includes both fluid flow and separation unit

operations.

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TABLE OF CONTENTS PAGE

SECTION 1: MATERIAL BALANCES

1. INTRODUCTION

Material balances (mass balances) are based on the fundamental law of

conservation of mass. In particular, chemical engineers are concerned with doing

mass balances around chemical process. They do a mass balance to account for

what happens to each of the chemicals that is used in a chemical process.

1.1 Conservation of Mass

The law of conservation of mass states that mass cannot be created or destroyed.

This law used in the form of a general mass balance equation to account for the total

mass all of the chemicals that are involved in the process. According to Richardson

and Coulson Volume 6, pg 34; the general conservation equation for any process

system can be written as:

Material in + generation consumption accumulation = Material out

For a steady state process the accumulation term will be zero. But if a chemical

reaction takes place a chemical species may be consumes or formed in the process.

If there is no chemical reaction the steady state balance reduces to:

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Material in = Material out

The balanced equation is also written for each separately identifiable species present

component and the total material as well.

2. MATERIAL BALANCE AROUND THE SETTLERS

Solvent degradation problems were encountered during 1985/6, 2000 and 2005 in

the solvent extraction plant.

The historical phenomena of solvent degradation occur from a combination of high

nitrates in the SX streams, high redox potential and a low strip pH. During solvent

degradation, the tertiary amine component of the solvent breaks down into

nitrosamine. As a result, the solvent loses the effectiveness of loading the uranium

hence lowering the efficiency of the stripping section. Thus, efforts have been put in

place to prevent the re-occurrence of solvent degradation at the SX plant.

What is being done: a target of less than 2% area of nitrosamines in the solvent has

been identified as a safe operating target. At the present moment, we are well below

the target and it was achieved via the following actions: An addition of soluble wire

for ferrous ion generation is currently in place. The addition of ferrous ions lowers the

ferric to ferrous ratio which as a result lowers the redox potential. There is a

consistent water addition to the ammonium sulphate to lower the nitrates

concentration. Solvent regeneration is also done to remove the nitrates from the

solvent. The nitrate mass balance will be one of the major controlling systems to be

carried out inorder to closely monitor the nitrates movement within the process. The

aim of the project (first part) was to provide a full mass balance on Nitrates and

Uranium.

2.1 Flow Diagram: Nitrate

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Figure 1: Nitrate Mass Balance Flowchart

2.2 Mass Balance: Nitrate

Nitrates Mass balance around SX plant- from 1st Aug 2008-to 15 September 2008

Streams NO-3 g/l Stream flow rate (m3/day) NO-3 In (Kg/day) NO-3 out (Kg/day)

Conc Eluate361.0

0 2,720.00981,920.0

0 -

Fresh Solvent363.0

0 2,176.00789,888.0

0 -

Raffinate50.0

0 2,720.00 -136,000.0

0

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Loaded solvent - 2,176.00 - -

Fresh Water - 670.00 - -

Scrub aq - 670.00 - -

Ok liqour4,004.0

0 720.00 -2,882,880.0

0

Amm. Sulphate2,420.00 720.00

1,742,400.00

Strip solvent363.0

0 2,176.00 -789,888.0

0

Strip solvent notregen

363.00 1,088.00

394,944.00 -

Regen aq9,680.00 45.00 -

435,600.00

Regen Solvent - 1,088.00 - -

3,909,152.00

4,244,368.00

Overall nitrates balance

Nitrate In (kg/day)3,909,152.0

0

Nitrate out (kg/day)4,244,368.0

0

Accumulation (kg/day) -335,216.00

Table 1: Nitrate Mass Balance Sheet

2.3 Flow Diagram: Uranium

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Figure 2: Uranium Mass Balance2.4 Mass Balance: Uranium

Uranium Mass balance around SX plant- from 1st Aug 2008-to 15 September 2008

Streams U3O8 g/l Stream flow rate (m3/day) U3O8 In (Kg/day) U3O8 out (Kg/day)

Conc Eluate 5.70 2,720.00 15,504.00 -

Fresh Solvent - 2,176.00 - -

Raffinate 0.01 2,720.0016.3

2

Loaded solvent 7.12 2,176.00 15,493.12

- -

Fresh Water - 670.00 - -

Scrub aq - 670.00 - -

- -

Ok liqour 17.80 720.0012,816.0

0

Amm. Sulphate - 720.00 - -

Strip solvent 0.05 2,176.00 108.80108.8

0

Regen aq - 45.00 - -

Regen Solvent - 1,088.00 - -

31,105.9212,941.1

2

Overall Uranium balance

Uranium In (kg/day) 31,105.92

Uranium out (kg/day) 12,941.12

Accumulation (kg/day) 18,164.80

Table 2: Uranium Mass Balance Sheet

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SECTION 2: PROCESS & INSTRUMENTATION DIAGRAM (PID) FOR SX

1. FLOW SUMMARY TABLE: SOLVENT EXTRACTION SECTION

Stream Number 1 2 3 4 5 6 7 8 9 10 11 12

Temperature C 28.2 27 25.7 26.9 40 28 25 26 25 28 28 25

Pressure (bar) 1 1 1 1 1 1 1 1 1 1 1 1

Flow rate (l/min) 19300 1025 648.5 1078 450 691 700 648 679 400 430 651

Uranium tenor g/l 6.7 6.7 0.010 0.017 0 18.7 18.7 0.008 0 18.4 0 0.011

Table 3: Flow Summary Table: Solvent Extraction section (12 August 2008 Datas)

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Raffinate Extr 5 Extr4 Extr 3 Extr 2 Extr 1 Scrub 1 Scrub 2 Strip 1 Strip 2 Strip 3 Strip 4 Regen

RAFFINATE

To CIX Fresh Eluate

OK LIQUOR TO

P/RECOVERYREGEN

SOLVENT TO

F/S TANK

FRESH SOLVENT CONC ELUATEFRESH

WATER

SCRUB AQ TO

CIX

AMMONIUM

SULPHATE

AMM HYDROXIDE

ADDITION FOR PH

CONTROL STRIP 2

Fresh Solvent

Tank

Conc.

Eluate tankAmm.Sulphate

1

9

10

4

1

1

1

2

Conc. Eluate from CIX

2

6

3

7

8

Conc. Heater

5

Loaded

SolventsTank

Amm.

HydroxideTank

AMM HYDROXIDE

ADDITION FOR PH

CONTROL STRIP 1

Sodium

Carbonate

Figure 3: Process and Instrumentation Diagram8


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SECTION 3: MASS TRANSFER OPERATIONS

1. INTRODUCTION

Mass transfer is the phrase commonly used in engineering for physical processes

that involve molecular and convective transport of atoms and molecules within

physical systems. Mass transfer includes both fluid flow and separation unit

operations. One of the separation unit operations are settlers. Settlers are commonly

used at Rossing in the Solvent Extraction Section, to separate liquids where there is

a sufficient difference in density between the liquids for the droplets to settle readily.

Settlers are essentially tanks which give sufficient residence time for the droplets of

the dispersed phase to rise or settle to the interface between the phases and

coalesce. In an operating settler there are three distinct zones or bands: clear heavy

liquid; separating dispersed liquid and clear light liquid.

Solvent Extraction is one of the crucial sections within the operation of Uranium

Oxide at Rossing. Solvent Extraction uses the principle of mass transfer to extract

uranium from the aqueous solution using the solvent as a medium of extraction in

order to concentrate the solution to the final product. Problems encountered included

the solvent entrainment in the exit streams. Streams also indicated clearly shown on

the diagram in section 2.

Aim of the project is to recover the entrained solvent from the aqueous stream as

much as possible.

2. ORGANIC ENTRAINMENT

A weekly experiment is carried out in the lab to determine the amount ofsolvent entrained in all the exit streams of the Solvent Extraction section. This

analytical method is suitable for the analysis of organic entrainment in

aqueous samples (e.g. Raffinate, Scrub water and OK Liquor). (See the

appendix for the method).

2.1 Results

The results obtained from the weekly organic entrainment are shown in the

graph below; for the data see the appendix.

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Graph 1: solvent losses in SX aqueous streams.

3. WAY FORWARD

Short term solution - Is to keep the organic: aqueous ratios stable. The flow

plays a very big role in the solvent entrainment and keeping the ratio constant

reduce the solvent losses though the aqueous streams.

Long term solution - An extra settler is required to recover the solvent mainly

in the stream that is loosing too much solvent. The settler should be fitted in

to account for the solvent lost in the aqueous stream as it is the highest. A

small settler will be able to recover at least up to 15m3 per month.

4. DESIGN OF THE SETTLER

Settlers are used to separate liquids where there is a difference in density

between the liquids for the liquids for the droplets to settle readily. Settlers are

essential tanks which gives sufficient residence time for the droplets of the

dispersed phase to settle to the interface between the phases and coalesce.

In an operating settler there will be three distinct zones: clear heavy liquid;

separating dispersed liquid; and clear light liquid. The settler vessel is sized

on the basis that the velocity of the continuous phase must be less than

settling velocity of the droplets of the dispersed phase. Plug flow is assumed,

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and the velocity of the continuous phase calculated using the area of the

interface: di

c

c uA

Lu < eg: 1

Where ud = Settling velocity of the dispersed phase droplets, m/s

uc = velocity of the continuous phase, m/s

Lc = continuous phase volumetric flow rate m3/s

AI = area if the interface, m2.

Stokes law is used to determine the settling velocity of the droplets:

c

cddd

gdu

18

)(2 =

eq: 2

Where dd = droplet diameter, m,

ud =settling (terminal) velocity of the dispersed phase droplets with

diameter d, m/s

c = density for the continuous phase, kg/m3

d = density of the dispersed phase, kg/m3

c = viscosity of the continuous phase, N s/m2

g = gravitational acceleration, 9.81 m/s

For a vertical small settler Ai =2

r

Data:

Strip Solvent Aqueous

Flow rate, kg/min 1000 6000

Density kg/m3 900 1030

Viscosity Nm s/m2 3 1.6

Calculation:

Assume - dd = 150 m

)sin(/0016.0

10118

)1030900(81.9)10150(3

26

grism

xx

xud

=

=

Vertically cylindrical vessel since the flow rate is small.

Lc = smxx /1067.13600

1

1000

6000 33=

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cu > andud ,

i

c

dA

Lu =

hence 23

044.10016.0

1067.1m

xAi ==

r = m58.0044.1 =

mdiameter 16.1=

The height is taken as twice as the diameter, a reasonable value for a cylinder:

Height = 2.32m

The dispersion bans is 10% if the height = 0.23 m

Residence time of the droplets: min4.276.1430016.0

23.0

== s

Very reasonable as it is between 2 to 5 min which is recommended.(Richardson and

Coulson, Volume 6, pg 444).

Velocity of the strip solvent =

smx

xx

/103

044.1

1

3600

1

900

1000

4=

The entrained droplets size is calculated from eq 2:

2/1

)(

18

=

cd

cd

dg

uud

=

mmx

xxxx

1281028.1

)9001000(81.9

10318103

4

2/134

=

The entrained droplets are 128 m which is satisfactory; below 150m.

Piping arrangements:

Flow rate = sm /9.13600

1

1030

6000

900

1000 33

=

+

Area of the Pipe = mmmxx

50049.04109.1 3

=

The position of the interface is half-way up the vessel and the aqueous liquid off-take

is taken at 90% of the vessel height, then

z1 = 0.9 x 2.3 = 2.07 mz3 = 0.5 x 2.3 = 1.15 m

z2 =mx 216.1900

1000

15.107.2=+

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Therefore the proposed design is:

Figure 4: vertical settler designs.

Note: Drain valves are fitted at the interface so that any tendency for the emulsion to

form can be checked, and the emulsion accumulated at the interface drained off

periodically as necessary.

SECTION 4: ENERGY BALANCE

2.0 m

1.15 m

1.15 m

2.07 m

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1. INTRODUCTION

In process design, energy balances are made to determine the requirements of the

process; heating, cooling and power required. In plant operation an energy balance

will tell us the pattern of energy usage, also it will tell us the areas conservation and

savings. The conservation of energy differs from that of mass in that energy can be

generated in a chemical process. The total enthalpy of the outlet streams will not

equal that of the inlet streams if energy is generated in the process, it depend in heat

of reaction. Energy can exist in several forms: heat, mechanical energy, electrical

energy and it is the total energy that is conserved.

One of the many statements of first law of thermodynamics is: although energy

assumes many forms, the total quantity of energy in the universe is constant and

when energy disappears in one form it appears simultaneously in other forms.

(energy of the system) + (energy of the surroundings) = 0

For a closed system, mass is not transferred across the boundary. Energy in the form

of heat or work may be transferred across boundary of a system:

(energy of surrounding) = +/-Q +/-W

The sign of Q and W depend on which direction of transfer is regarded as positive or

negative. By convention, Q is positive when heat is transferred to the system and

also W is positive when heat is transferred to the system.

(energy of the system) = EK + EP + U

U + EK + EP = Q + W

For closed system: EK = EP = 0

Conservation of energy:

Energy in + generation consumption accumulation = Energy out

2. HEAT TRANSFER OPERATIONS

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2.1 Conc. Heater

Maintaining a target temperature of 40C in Solvent Extraction plant during cold days

for the year has been a challenge since the Acid plant was decommissioned in 1998.

The current electrical line heater at SX is inadequate to temperature requirements at

high uranium throughput during the cold months. Previously, it was possible to

reduce throughput through SX because uranium produced target was not stretched

above 40,000t/annum. In 1985, the entire uranium plant was stopped for 3 days

during a cold spell and also in both May and August 1989.

Temperature is one of the most significant features which affects separation of

organic (solvent) and aqueous (conc. eluate) which is central to the SX process.

Usually, when the ambient temperature drop below 25C, separation times begin to

exceed 1 minutes making operation of SX plant difficult.

Temperature below 40C in the extraction settlers had led to phase disengagement

problems, stable emulsion and resulted in solvent losses through exit aqueous

streams.

2.1.1 Problem statement

The current shell and tube heat exchanger can only take a maximum flow of 700l/min

for both lines; the volume capacity is limited. In other word, each line is getting

350l/min heated and the rest amount directly from the tank at low temperature to

make up 1000l/min. Thus the two solution mix and the temperature drops by the time

they mix with organic in the mixer box to undesirable temperature of less than 35C.

With the high ore grade (5-6g/l Uranium tenor in the conc. eluate) SX circuit isrequired to run high conc. flows and this way, the ratio between heated and cold

conc. will decrease, resulted in emulsion formations and poor phase separation.

Hence, the current heat exchanger is not adequate, thus hampering the potential

economic benefit to Rossing.

2.2 Recommended - A Shell and Two Tube Heat Exchanger

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An upgrade of the conc. eluate heating will involve purchasing of higher volume

capacity a shell and two tube heat exchanger. This option will significantly improve

the T (30 C) to current T of 16 C. It will heat more volume/time than the current

one. It will use hot water circulating pump instead of steam generation. It has larger

heating surface area, environmental friendly compared to the fired boiler in terms of

gas emissions and easy to maintain.

The new setup in SX plant will be such that, each line will have its own heat

exchanger and run in parallel to achieve good required temperature range in the

settlers.

2.2.1 Design Area of the current heat exchanger

AREA = 430 ft2 x (0, 3048)2 = 39.95m2

2.2.2 Heat exchanger calculations

The determination of the heat exchanger area using the following basic of Kerns

method:

Data: Conc. eluate solution heated from 25 C to 35 C.

Flow - rate of the Conc. eluate 700 kg/min

Deionised water temperature fall from 60 C to 35 C

Heat capacity Conc. eluate = 3.1 KJ/kg C

Heat capacity of water = 4.2 KJ/kgC

Note: The conc. eluate solution is assigned to the shell side and water in the tubes.

Calculation:

Heat load = kWx

kg

7.361)2535(1.360

min/700

=

Cooling water flow = skgkW

/44.3)3560(2.4

7.361=

CTlm===

= 37.16

5.2ln

15

10

25ln

15

)2535(

)3560(ln

)2535()3560(

Since its a one shell pass and two tube passes:

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4.03560

2535

5.22535

3560

=

=

=

=

S

R

From figure 1 in the appendix: Ft =0.9

mT = 0.9 X16 = 14.4 C

From figure 2 in the appendix: U = 850 W/m2 C

Therefore the Area = 23

5.298504.14

107.361m

x

x=

2.3 Energy Required

It is proposed that a new heat exchanger be recommended that will take up the

volume of1000 kg/min at the same time. The current temperature difference is 16

degrees Celsius and the recommended temperature difference is 30 degrees

Celsius.

The energy needed to heat the 1000 kg/min will be:

Q=m Cp T

=skJ

x

/1550

)30(1.360

1000

Note: assuming steady temperature difference during operation

2.4 Benefits Expected

Reduced Solvent lossesStable emulsion formation (as a result of low

temperature in the settler) in extreme conditions further worsen the

problem to the extent that globules of emulsion (containing largely

organics) are carried over into the aqueous stream, further increasing

solvent losses. With a spare heater/bigger capacity 100% of the conc.

eluate will be heated and for that reason, it is expected that the solvent

entrainment in aqueous (raffinate) will be reduced.

Plant condition Phase Disengagement times - July 2008

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Settler

Temperature C

Separation time (s)

SX Line 1 35 57

SX Line 2 36 55

Comment:Poor separation between two phases, both lines has an approximate 7cm

layer ofemulsion, equivalent to 693L of solvent.

Laboratory condition phase disengagement times

SettlerTemperature C

Separation time (s)

Run 1 40 42

Run 2 50 38

Comment: Preferably, to run the plant at 40 C, as no emulsion was recorded hence

no solvent trapped between two phases.

Improved Uranium Recovery Efficiency Higher SX temperatures

enhance CIX resin stripping efficiencies resulting in better strip tenors

hence lower barrens.

Reduced risk of plant stoppage during cold weather

The weather forecast is Unpredictable, in 1985, the entire uranium plant stopped

for 3 days during a cold spell and also in both May and August 1989. Therefore

with a spare heater/bigger capacity this will not be the case anymore and the

plant will sustain high uranium transfer through the plant even during cold days.

3. DISCUSSION

The difference in the balances above are due to the possible reasons such as;

accumulation with the system; insufficient solvent to extract the uranium from the

aqueous stream; difference in flow ratios; impurities in the feed as the impurities

present in the feed could react with the components contained in the feed; Incorrect

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assumption of steady states; incorrect assumption that uranium is not reactive; and

errors due to approximations in the experimental data analysis.

Efficiency - The heat exchangers area has reduced dramatically from the area of

39.95m2 to the area of 29.9m2. The factor that is contributing to the decrease of the

area within the heat exchanger is fouling of dirt entrained. As it is well known that the

ferrous in the conc. eluate solution is reduced by adding wires in the conc. eluate

sump at Ion Continuous Exchange (CIX) before been pumped to SX. The conc.

solution is drawn from the bottom of the tank, where the dirt has settled and therefore

causing high fouling. The heat exchanger need to be cleaned on the regular basis

like on the module day to avoid the minimizing of the area.

4. CONCLUSION

It is very crucial to always consider the efficiency of the process as a priority. Mass

balances and energy balances play a big role in process optimization especially at

Rossing. Regular checks should always be carried out to maintain the efficiency of

the plant.

Working with such huge machines and operating the plant is a challenge and it

remains a challenge. Running projects in plant is a very good learning tool as one

turn to learn faster and discover something new during the process. But the bad side

of it is that learning through projects in the company wont actually let you get into the

detail of an operation as time wont permit. The time in evaluating and researching for

the solution is very much limited for the sake of solving problems much faster

enabling high profit for the company. Therefore, it is advisable for the company to

keep students into consideration in projects allocation as per projects priority and not

per student capabilities. Sometimes projects are very long that will take the studenttime to be able to Finnish all the modules allocated to him/her by the university within

the limited time.

Theory is very important, it is called initial knowledge but it can be worthless at times,

especially when one cannot put what you learned in the classrooms into practice.

Theory and practice goes hand in hand. In order to practically design an equipment,

theory should be applied to maintain the correct standards of the equipments. Inorder

to calculate the heat required to heat up water ambient temperature to 80 degrees

Celsius you need to apply the theory learned in the classroom.

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5. REFERENCES

1. Applied thermodynamic Hand Book.

2. Coulson. JM & Richardson. JF, 2007, Chemical Engineering Design Volume6 (4th Ed), Elsevier, Oxford.

3. Coulson. JM & Richardson. JF, 2007, Chemical Engineering Volume 1 (6th

Ed), Elsevier, Oxford. Pg 381

4. Felder. Richard M & Rousseau. Ronald W, 2000, Elementary Principles ofChemical Processes (3rd Ed), John Wiley & Sons, New York.

5. Nakathingo. Elizabeth. Metallurgist. Supervisor

6. APPENDICES

1. Analytical method to determine the organic entrainment in the aqueous

streams

1.1 Reagents

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a) Diluent (Shellsol 2325 or Sasol SSX 210).b) Synthetic Conc. Eluate solution.

1.2 Analytical Method

a) 200 mls of fresh aqueous sample was added directly into a glass 250ml separation flask.

b) Then 20.0 mls of diluent was added using a burette into the same 250ml separating flask.

c) Shaken vigorously for 3 minutes and then allowed the liquid phases toseparate.

d) The aqueous phase was drained into a glass beaker. Keep theorganic phase in the separation flask.

e) 50 mls of the synthetic Conc. Eluate solution was added into theseparating flask and shake vigorously for 3 minutes.

f) The liquid phases were allowed to settle out and then the syntheticConc. Solution was drained out. (Do not drain the organic phase out.)

g) Step e) and f) was repeated for further two times giving a total of threecontacts between the synthetic Conc. Eluate solution and the sample

h) After the 3 contacts, the organic phase was filtered into a test tubethrough a 1 PS filter paper. (Not all of the organic phase need be

filtered.)

i) The organic was analyzed in the test tube for Uranium using ICP

j) The Uranium max. load of a sample of plant solvent was measured.

1.3 Calculation

The ppm organic entrainment = ppm Uranium from ICP in (i) x 100g/l Uranium in (j) max. load

e.g. 1.21 ppm Uranium in sample by ICP5.27 g/l Uranium in max. loading

then ppm entrainment = 1.21 x 100 = 22.96 ppm5.27

1.4 Organic Entrainment Results

Entrainment (ppm)

Date Raffinate 1 Raffinate 2 Scrub 1 Strip 1 Regen Aq Flow Ratio

2007/11/16 1658 942 1485 1631 900 1.00

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2007/11/22 467 413 400 27 1051 0.97

2007/12/29 151 174 279 58 857 0.88

2008/01/12 500 538 218 51 921 0.98

2008/02/06 467 167 250 183 876 0.98

2008/03/07 1404 296 62 1059 659 0.98

2008/03/18 64 0 0 0 1578 0.96

2008/03/29 251 214 138 63 1105 0.97

2008/04/13 258 760 1109 866 2057 0.91

2008/04/26 192 110 165 179 2734 0.98

2008/05/22 73 85 973 195 255 0.93

2008/09/13 274 442 518 686 2064 0.98

2008/09/24 190 391 159 63 1904 0.78

Table 1: Organic Entrainment Results over a period indicated in the table

process plant evaluation

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