RECOVERY OF URANIUM FROM DENSE SLURRIES VIA · PDF fileRECOVERY OF URANIUM FROM DENSE SLURRIES...

The Southern African Institute of Mining and Metallurgy

6th Southern African Base Metals Conference 2011

T Udayar, M H Kotze and V Yahorava

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RECOVERY OF URANIUM FROM DENSE SLURRIES VIA

RESIN-IN-PULP


ABSTRACT

The revival of uranium as a valued commodity has prompted a drive in the mining

industry to identify the most cost-effective processing routes for the recovery of

uranium from low-grade uranium ores or wastes. Various published studies indicated

that direct recovery of uranium from the leached slurry via resin-in-pulp offers major

capital and operating cost advantages. Mintek, in partnership with Bateman

Engineering, developed the MetRIXTM

RIP technology that allows continuous counter-

current transfer of resin and pulp. Laboratory and pilot plant test work was done on

uranium recovery from a South African gold ore. Based on these results, operating

parameters were developed for a demonstration plant on site at the Harmony’s One

Plant near Welkom in the Free State for the recovery of uranium from their current gold

plant feed prior to cyanidation. This paper summarises the metallurgical results

obtained during various laboratory studies, a pilot plant campaign, and the on-site

demonstration plant operation.

1. INTRODUCTION

During the 1970s the demand for uranium had catapulted, resulting in renewed interest

for processing routes that were cost-effective for the recovery of uranium from low

grade deposits. This resulted in the commercialisation (during the 1970s) of the

NIMCIX technology, a continuous counter-current ion-exchange (CCIX) adsorption

contactor that was developed by Mintek for the recovery of uranium from relatively low

grade, un-clarified solutions. A number of pilot and commercial scale plants were built

and operated successfully, including pilot plants at Rössing and Durban Deep, and full

scale plants at Chemwes and Vaal River South. Currently, NIMCIX columns are still

in use at Anglo Gold Ashanti’s Vaal River South operation and Ezulwini, and they are

being tested at Trekkopje on a relatively large pilot scale (referred to as Midi

Commercial Demonstration plant with a heap stacking rate of a 9 000 t/day). The

NIMCIX columns could be used for both adsorption and elution processes, but it has to

be designed for the specific duty.

The use of resin-in-pulp (RIP) for the recovery of gold, uranium or base metals is still

very limited in the Western World. One of the major uncertainties with regards to the

commercial implementation of RIP has been the potential resin loss or consumption on

a full scale operation. In recent times, all the major resin manufacturers have made

significant technological advances resulting in a number of durable, commercially

available, RIP-grade strong-base resins for the recovery of uranium. This has renewed

interest in RIP as a potential economically attractive processing route for the recovery

of valuable metals from low grade slurries.




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Mintek has been conducting intensive research and development over the past 5 years

on the recovery of uranium from low grade uranium slurries. The MetRIXTM

technology, originally designed for the recovery of base metals from waste slurries, was

evaluated for counter-current transport of resin and pulp. This technology is a joint

development between Mintek and Bateman Engineering. The research included resin

durability and metallurgical performance test work. Based on the results developed

over this period, Harmony Gold Mining Company Limited commissioned Mintek and

Bateman Engineering to demonstrate the MetRIXTM

technology at their Harmony One

Plant near Welkom in the Free State.

This paper summarises metallurgical results obtained during various laboratory studies,

a pilot plant campaign on a uranium-bearing gold ore, and the demonstration plant

operation.

2. EXPERIMENTAL

2.1 Laboratory Scale Test work

2.1.1 Equilibrium adsorption and elution isotherms

Eight point adsorption equilibrium isotherms were generated for the adsorption of

uranium onto the strong-base anion exchange resin, in the sulphate form, by batch

contacting the resin and uranium-rich pregnant leach solution (PLS) at different resin:

solution ratios. Contact was made over a period of 24 hours under ambient conditions.

After adsorption, the uranium-loaded resin was separated from the solution, washed to

remove entrained solution, and then eluted in a column with excess eluate.

For the generation of elution equilibrium isotherms, a batch of resin in the sulphate form

was pre-loaded from uranium PLS. The loaded resin was then contacted with eluant at

8 different resins: eluant ratios over a period of 24 hours. Upon completion of the

equilibrium elution test work, the resin underwent an additional elution step using 2M

HNO3 to remove any residual uranium that might still have been present on the resin.

The uranium stripped with HNO3 was used to calculate the residual uranium

concentrations on the resin.

2.1.2 Kinetics of adsorption and elution

The rate of uranium adsorption onto the ion exchange resin was investigated by

contacting resin with a batch of uranium PLS, under agitation over a period of 24 hours.

Regular sampling of the barren solution was done. The resin loading was determined by

stripping the resin with excess 2 M HNO3.

Uranium loaded resin was contacted with 110 g/L H2SO4 in batch under agitation.

Regular samples were taken and analysed for uranium. At completion of the test, the

resin was separated from the eluate and stripped further with 2M HNO3 to determine the

residual uranium concentration on the resin.




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2.1.3 Pilot plant

The RIP pilot plant campaign for the recovery of uranium was run at Mintek during a

three week period. During this time, the main focus was metallurgical performance of

the resin and not durability. The pilot plant comprised of a leaching section in which

uranium-rich ore was slurried with municipal water, and then leached under optimised

conditions as determined during laboratory scale test work. The ore type used for the

pilot plant was a siliceous gold ore with a U3O8 content of 150 – 350 g/ton.

The leach section was run in a sequential manner to ensure that there was a constant

supply of freshly-leached pulp to the RIP adsorption circuit. This approach was taken

to ensure that the leached pulp did not age, which might have caused speciation changes

in the PLS over time (e.g. silica).

3. RESULTS AND DISCUSSION

Extensive laboratory investigations have been conducted at Mintek to evaluate the

metallurgical performance of various strong-base anion exchange resins for the recovery

of uranium. Various RIP-grade resins, which included both gel and macro porous types,

were tested. These results, and those on new resins, are compiled into a data base which

enables Mintek to recommend resins for specific applications in terms of metallurgical

performance and resin durability.

The ion exchange resins are submitted to a series of basic batch tests, including the

following:

. equilibrium adsorption isotherm in the specific leach liquor/pulp,

. equilibrium stripping isotherm, using different eluants,

. rate of adsorption,

. rate of elution and

. laboratory and pilot-scale durability tests.

The test work described in this paper was done on a RIP-grade strong-base, macro-

porous anion exchange resin. This resin was also used for the demonstration plant

campaign. Only the metallurgical performance of the resin is discussed.

3.1 Laboratory test work results

3.1.1 Uranium adsorption equilibrium isotherms and rate of adsorption

3.1.1.1 Equilibrium and kinetics for Harmony leached pulp

Uranium equilibrium adsorption isotherm and kinetics of uranium uptake, obtained for

Harmony-leached ore, are presented in Figure 1 and Figure 2. The Langmuir

equilibrium isotherm provided the best fit for the equilibrium adsorption data with the

determined constants as follows:




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a = 55.929

b = 0.032

A McCabe-Thiele construction was done using the equilibrium isotherm generated.

Resin to pulp ratio on the pilot plant was chosen to achieve 43 g/L U3O8 loading on the

resin from a leached pulp containing 180 mg/L U3O8 with a target of <2 mg/L of U3O8

in the barren pulp after RIP.

Figure 1: Uranium adsorption equilibrium isotherm (Harmony pulp)

Figure 2: Rate of uranium adsorption (Harmony pulp)




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3.1.1.2 Factors affecting uranium loading

Based on previous experience and information achieved during evaluation of RIP

technology for uranium recovery from the Harmony ore type, the following factors were

expected to have considerable impact on the resin performance:

• silica and

• chlorides

Silica present in uranium leach liquors is known to have an adverse impact on the rate

of adsorption of the metal, especially when the silica levels on the resin become

significant. Evaluation of silica impact on maximum uranium loading and rate of its

uptake was reported previously in some of the published Mintek papers [1, 3]. For resin

used in the current work, it was expected that the uranium adsorption capacity over a 24

hour period would decrease by >35% at silica fouling levels >20% (m/m).

Due to the negative impact of silica on the equilibrium and kinetics of adsorption, it was

therefore targeted to limit silica fouling (e.g. limiting overall resin residence time in the

adsorption circuit).

Another factor found to dramatically impact resin performance, was chlorides [6]. The

source of chlorides in the current project was process water and the levels expected to

be dealt with were 0 to 3 g/L. For prediction of the resin performance depending on the

process water quality that might vary on a site, depending as well on the season,

uranium adsorption equilibrium isotherms were generated using synthetic solution with

different chloride contents. The isotherms were fitted with the Freundlich model and

the constants are shown in

Table 1. The results obtained from the test work are given in Figure 3.

Figure 3: Uranium equilibrium loading: effect of Cl-

0 100 200 300 400 5000

10

20

30

40

50

60

U3O

8 loadin

g,

g/L

U3O

8 in the barren, mg/L 0g/L Cl 1g/L Cl 2g/L Cl 3g/L Cl

Freundlich isotherms




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Table 1: Freundlich equilibrium constants for uranium loading isotherms

Chloride concentrations in synthetic liquor 0 g/L 1 g/L 2 g/L 3 g/L

a 13.62 9.65 7.82 5.62

b 0.22 0.26 0.28 0.32

Quite a significant drop in uranium maximum loading was observed with an increase in

the chloride concentration. The uranium loading was depressed by as much as 23%

when the chloride content was increased from 0 – 3 g/L.

3.1.2 Elution equilibrium isotherm and rate of elution

Results of stripping equilibrium and rate of elution tests are presented in Figure 4.

Equilibrium elution isotherms and rate of stripping indicated that uranium elution by

using 110 g/L H2SO4 has an equilibrium constraint. For effective uranium stripping a

specific volume of eluant was required. Results indicated that the rate of stripping

would be less of a determining factor on the elution efficiency (with relatively long

stripping resin residence times) achieving almost complete uranium elution after 3

hours.

The assumptions made based on equilibrium and kinetics of elution were confirmed by

column elution tests, when stripping was performed at different flow rates (0.5 BV/h to

4 BV/h) as shown in Figure 5. There was minimal difference between the elution

profiles observed within the flow rates evaluated, with almost all flow rates achieving

100% uranium elution after 17 BVs (bed volumes) of eluant were passed through the

resin bed.

Figure 4: Equilibrium stripping isotherm and kinetics of uranium stripping with 110 g/L

H2SO4 as the eluant

0 5 10 15 20

0

1

2

3

4

5

0 2 4 6 8 20 24

0

20

40

60

80

100

U3O

8 i

n t

he

elu

ate

, g

/L

Residual U3O

8 in the resin, g/L

Fra

ctio

na

l st

rip

pin

go

f U

3O

8,

%

Time, hrs




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Figure 5: Effect of different flow rates on elution [2]

3.2 Pilot plant performance

Results obtained from the laboratory scale test were used to predict operating conditions

for a pilot plant campaign using a relatively low grade uranium feed. The pilot plant

was done essentially to confirm and optimise the operating parameters, including:

• number of adsorption stages,

• resin residence time,

• pulp residence time,

• resin loading and

• rate of silica fouling of the resin

The RIP section of the plant consisted of six 5 L, agitated RIP reactors. The reactors

were set-up in series, in a manner that enabled the flow of pulp by gravity through the

six reactors. Figure 6 shows a schematic diagram of the RIP section of the plant

operation. The plant was operated in a carousel mode with continuous flow of uranium-

leached pulp and counter-current batch movement of resin.

The fully loaded resin which exited the RIP adsorption stage from Position 1 went

through an elution step before its re-entry back into the adsorption stage at Position 6.




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Figure 6: Flow diagram of the RIP pilot plan

Table 2: RIP pilot plant test conditions

Condition

tested 1 2 3 4 5 6 7 8 9

Resin inventory

per stage 1% 1.2% 1.2% 1.2% 1.2% 1.2% 1.2% 1.2% 1.6%

Resin residence

time, hrs 2 2 2 2 2 2 2 3 3

Pulp residence

time, min 30 30 24 24 30 37.5 37.5 37.5 37.5

Transfer

numbers 1-16 17-26 27–36 37–43 44–51 52–72 73–80 81–93 94-100

The primary objectives of the RIP pilot plant was to investigate the impact of resin and

pulp residence times on the efficiency of uranium adsorption, and the rate of silica build

up on the resin. The conditions tested are shown in Table 2. The resin: pulp ratio was

chosen based on the equilibrium isotherm presented in Figure 1. The chloride

concentration in the pilot plant PLS was 0 g/L.

CB

A

ED

F

Position 2

Position 3

Position 6

Position 5

Position 4

Position 1

G

DC

B

FE

G

Position 2

Position 3

Position 6

Position 5

Position 4

Position 1

A

Leached uranium pulp

PULP FLOW

RESIN FLOW

barren uranium pulp

Leached uranium pulp

barren uranium pulp

A

resin to elution

Pulp would have

generally been

recycled to feed

but was not in

this case due to

the plant set-up

Reactor A to the plant




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Uranium loading on the resin removed from Stage 1 varied significantly (Figure 7),

primarily due to a variation in the uranium grades of the ore that was fed to the leach

section of the plant. Uranium loadings of 30 – 48 g/L U3O8 were experienced for the

most part of the campaign. These loadings were achieved at equilibrium uranium

concentrations ranging from 64 to 276 mg/L in solution.

Figure 7: Uranium loading during the pilot plant campaign

The impact of resin residence time and pulp residence time on the maximum loading of

uranium achieved was seen to be minimal within the parameters evaluated.

The uranium-loaded resin from Stage 1 was eluted with 110 g/L H2SO4 at a rate of

5 BV/hr until a total of 30 BVs was passed through the resin bed. These conditions

were found to be optimal for consistent stripping of U3O8 from the resin to below 1g/L.

Such residual uranium content on the eluted resin did not have an impact on the

adsorption performance in RIP.

Build-up of silica content on the resin was closely monitored during the pilot plant in

order to determine a rate of silica fouling. As silica loads on the resin a portion of it was

eluted with acid together with uranium (“strippable” silica) and the other portion

remained on the resin (“un-strippable”) silica. Remained silica could be stripped only

with caustic and was allowed to build up for duration of the plant. Thus, rate of “un-

strippable” silica was determined.

The SiO2 content in the PLS over the duration of the pilot plant was <500 mg/L.

Profiles of “strippable” and “un-strippable” silica observed during the pilot plant

campaign are shown in Figure 8. The level of “strippable” silica was seen to be

constantly below 5 g/L (1% m/m); while a constant build-up in “un-strippable” silica

levels was observed. A steep gradient in “strippable” and “un-strippable” silica

observed between 45 and 55 hrs of plant operation was as a result of an excessive acid

addition in the leach circuit. This caused excessive silica to be brought forward to RIP

circuit. As the campaign proceeded, it was clear the amount of ‘un-strippable’ silica

increased progressively.

0 10 20 30 40 50 60 70 80 90 100

0

5

10

15

20

25

30

35

40

45

50

55U3O

8 loadin

g o

n resin

, g/L

Transfer number




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Figure 8: Silica fouling of the resin during the pilot plant campaign

3.3 MetRIX™ demonstration plant

Results obtained during the extensive laboratory and pilot test work conducted at

Mintek were used for evaluation of the MetRIXTM

technology as an option for recovery

of uranium from Harmony’s gold ore. Based on the potential capital and operating cost

savings when compared to the other processing flow sheets considered [7], Harmony

commissioned Mintek and Bateman to install and operate a MetRIXTM

demonstration

plant on their Harmony One Plant near Welkom in the Free State. A photograph of the

plant is shown Figure 9. The demonstration plant was operated over a 47 day period.

Figure 9: Demonstration plant installed on site at Harmony

0 20 40 60 80 100 120

0

5

10

15

20

SiO

2 loadin

g,

g/L

Time, hours

strippable SiO2 un-strippable SiO

2un-strippable SiO2 strippable SiO2

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_____________________________________________________________________________

Page 61

the PLS at 80 mg/L U3O8 and a barren pulp containing <5 mg/L U3O8 in solution.

Much higher uranium loadings were achieved during the pilot scale test work, however

the PLS did not contain Cl- and had significantly higher U3O8 concentrations in the

PLS.

Figure 12: Demonstration plant adsorption equilibrium isotherms

The silica fouling rate on the demonstration plant was within the limits predicted based

on the laboratory and pilot plant test work.

3.4 Elution circuit

The loaded resin that exited the RIP 1 tank was collected in a designated buffer tank and

once a sufficient volume of resin had been accumulated, an elution cycle was done.

Sulphuric acid at a concentration of 110g/L was used to strip the uranium loaded resin.

The elution section consisted of the following tanks:

• fresh eluant containing 110 g/L H2SO4,

• 2 intermediate eluates,

• a strong eluate tank containing uranium-rich fraction and

• a wash water tank

The elution circuit was run in a closed loop, with only the uranium-rich fraction exiting

the circuit after each elution cycle.

Uranium-rich eluate contained between 1.9 and 3.6 g/L of U3O8. Such uranium

concentrations were relatively low (when related to the stripping isotherm in Figure 4)

due to the limited number of intermediate eluate tanks and ineffective manual control of

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100

U3O

8lo

adin

gs,

g/L

U3O8 in the barren, mg/L

experimental data experimental data

experimantal data demoplant Freundlich Isotherm

Operating line_demoplant Operational stages_demoplant




_____________________________________________________________________________

Page 62

the relative resin and eluant volumes. About 30 BVs of eluant was passed through the

resin column over a 6 hour period, consistently resulting in a residual uranium

concentration of < 0.8 g/L on the resin.

4. CONCLUSIONS

Extensive laboratory and pilot scale test work at Mintek resulted in the selection of the

most cost-effective operating conditions for the recovery of uranium from acidic-

leached slurry via RIP. The pilot plant and demonstration plant adsorption and elution

results correlated reasonably well with the predictions that were made from the

laboratory test work. Moreover, these results confirmed the improved performance of

the MetRIXTM

technology under the optimised operating conditions employed.

The demonstration plant campaign was successful and achieved the following:

• The MetRIXTM

RIP process was successfully demonstrated and the process

targets were achieved.

• A relatively low rate of SiO2 fouling was measured under the final optimum

operating conditions.

• Sufficient elution data were generated for the elution strategy optimization that

would be employed on the full scale plant.

Demonstration of MetRIXTM

technology gave Harmony more confidence in

implementation of RIP technology for uranium recovery from their gold ore.

Commercialisation of MetRIXTM

technology in South Africa would give a boost for

RIP application in various hydrometallurgical areas (base metals, rare earths, etc.).

5. ACKNOWLEDGEMENTS

The authors wish to thank the management of Harmony Gold Mining Company Limited

for permission to publish this paper. A special thanks to the teams at both Mintek and

Bateman Engineering Projects for their support during the test work program, design,

construction and operational phase of the MetRIXTM

demonstration plant. The

assistance from the Harmony One Plant personnel was exceptional during the

commissioning and operational phase of the plant.

6. REFERENCES

1. Yahorava, V., Scheepers, J., Kotze M. H. and Auerswald, D. 2009. Impact of

silica on hydrometallurgical and mechanical properties of RIP grade resins for

uranium recovery. The Southern African Institute of Mining and Metallurgy,

Mintek75 Anniversary.

2. Yahorava, V., Scheepers, J., Kotze M. H. and Auerswald, D. 2009. Evaluation

of various durability tests to assess resins for in-pulp applications. The Southern

African Institute of Mining and Metallurgy. The Sixth Southern African

Conference on Base Metals.




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Page 63

3. Yahorava, V., Scheepers, J. and Kotze M. H. 2009 .Comparison of various anion

exchange resins for the recovery of uranium by means of RIP. Proceedings,

ALTA Uranium Conference, Perth, Australia, 2010

4. Jooste, M., Kotze, M. H., Auerswald, D. 2010. The advantages of true

continuous counter-current elution in uranium processing. Proceedings, ALTA

Uranium Conference, Perth, Australia, 2010.

5. Auerswald, D., Udayar, T., Kotze, M. H. and Scheepers, J. 2011. Operation of a

Resin-in-Pulp (RIP) plant for recovering uranium from a South African gold

pulp. Proceedings, ALTA Uranium Conference, Perth, Australia, 2011.

6. Wilson, A., Fainerman-Melnikova, M. and Soldenhoff, K. 2011. What are the

options for an integrated IX process to recover uranium from saline and hyper

saline liquors? Proceedings, ALTA Uranium Conference, Perth, Australia, 2011.

7. Van Tonder, D. and Van Hege, B. 2007. Uranium recovery from acid leach

liquors: The optimisation of RIP/SX flow sheets. The Southern African Institute

of Mining and Metallurgy. The Fourth Southern African Conference on Base

Metals.

The Author

Tresha Udayar, Senior Engineer, Mintek

I graduated with a BSc. Degree in Chemical Engineering from the University of

KwaZulu-Natal and thereafter joined Mintek in the Hydrometallurgical Division (2008

– present).

I initially worked in the extraction group on laboratory and pilot scale investigations for

base metal recovery. Thereafter moved to the processing group and worked on

laboratory, pilot and demonstration plant test work (at Harmony Gold Mining Company

Limited) for the recovery of uranium via RIP process routes.




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