Post on 06-Apr-2018
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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