GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION

Transcription

1 B.A.R.C D h A CONTINUOUS PROCESS FOR PRECIPITATION OF AMMONIUM DIURATE FROM URANYL SOLUTION PART I by I. A. Siddiqui, B. V Shah, S. H. Tadphale and S. V. Kumar Process Engineering and Systems Division 1968

2 B.A.R.C CM CN GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION U cri A CONTINUOUS PROCESS FOR PRECIPITATION OF AMMONIUM DIURATE FROM URANYL SOLUTION Part I by I.A. Siddiqui, B.V. Shah, S.H. Tadphale and S.V. Kumar Process Engineering and Sysstems Division BHABHA ATOMIC RESEARCH CENTRE BOMBAY, INDIA 1988

3 B.A.R.C. - -\kl'l INIS Subject Category : B16.20 Descriptors ADU PRECIPITATION AMMONIA URANYL NITRATES PH VALUE ON-LINE MEASUREMENT SYSTEMS AQUEOUS SOLUTIONS URANIUM SLURRIES EXPERIMENTAL DATA PILOT PLANTS ON-LINE CONTROL SYSTEMS FLOW RATE PUREX PROCESS PRODUCTION

4 A B S T R A C T The development of a continuous reactor system for conversion of uranium from the Purex tail-end to yellow cake using commercial ammonia gas is discussed. Precipitation of uranium is carried out as a continuous process in a compactly designed reactor. The reaccants are agitated by air passed into the reactor. The ph of the product overflow is measured in-line and is maintained at the desired value by automatic control of the flowrate of ammonia The design parameters are enumerated and selection of some parameters is discussed. The reactor system and the associated instrumentation are described in detail. The reactor is capable of handling uranyl nitrate feed over a wide range of acidity, uranium content and feed flowrates without difficulty and loss of material. The pilot plant reactor of 22 lit active volume is capable of processing to 12 kg uranium per hour. The system has been designed to operate reliably and efficiently with minimum necessity of operator-intervention. Suitable conditions to avoid troubles due to heavy slurries resulting from concentrated feed and low flow velocity have been identified.

5 A CONTINUOUS PROCESS FOR PRECIPITATION OF AMMONIUM DIURATE FROM URANYL SOLUTION Part I I.A.Siddiqui, B.V.Shah, S.H.Tadphale and S.V.Kumar Process Engineering and Systems Division Bhabha Atomic Research Centre 1^ Introduction The reprocessing of radioactive fuel is an important stage of the nuclear fuel cycle. Reprocessing in India is carried out almost entirely by the Purex process. The tail end of the Purex process requires converting the uranium as uranyl nitrate solution into uranium oxide through the ammonium diuranate route. It is, therefore, necessary to have an efficient and smooth process for the production of ammonium diuranate. Present processes are generally based on batch-wise contact of ammonia with uranyl nitrate. Considering the advantages offered by a continuous setup, work was taken up to develop a reliable continuous ammonium diuranate production process. 2. The Reaction Ammonium diuranate is precipitated by reacting ammonia gas or ammonia liquor with uranyl nitrate solution. The overall equation of the exothermic reaction is of the form 2UO 2 (NO 3 ) 2 + 6NH3 + 3H2O > (NH. )2U Z O7 + 4NH4 NO3 However, in aqueous solution the reactants would undergo the following reactions at different ph conditions. NH 3 + H2O > NHiOH 2UO2(NO 3 >2 + 2NH4OH > 2UO 2 (OH) (N0 3 ) + 2NH< NO3 (ph=4) 2UO 2 (OH) (NO 3 ) + 2NH4OH > 2UO2 (0H) 2 + 2NH4NO3 (ph=4 to 6) 2UO 2 (0H) 2 + 2NH4OH > (NH4)2U 2 O7 + 3H 2 O (ph=>7)

6 4 The reaction has to be carried out at ph greater than 7 in order to ensure precipitation of all the available uranium. On the other hand, the ph should be maintained as low as practicable in order to reduce the consumption of ammonia. 3. The Design Parameters After a preliminary survey of the problem, the following aspects were identified for detailed study and determination with the aim of developing a suitable continuous ADU reactor system. 1. Choice of reagent - ammonia gas or liquid 2. Geometry of the reactor vessel and orientation of nozzles 3. Type of mixing device 4. Air requirement for reaction / mixing 5". Residence time requirement 6. Degree of conversion of UN 7. Effect of feed concentration on the quality of the product 8. Selection of instruments and design of control loop 9. Controller tuning. Effect of step change in feed flowrate 11. Degree of control of product quality in terms of ph 12. Range of acceptable feed solution compositions 13. Effect of air flowrate on ammonia consumption 14. Effect of. air flowrate on particle size of the precipita-t-e 15. Effect of the reactor ph s^t-point on ammonia consumption 16. Effect of reactor ph set-point on particle size of the precipitate 4 The Reactor Equipment A pilot plant was set up to develop a process that would meet the above objectives (fig. 1). After each precipitation the supernatant was separated from the slurry and the precipitate was dissolved in nitric acid to obtain the next batch of feed.

7 Fig. 6 shows the process and instrumentation schematic of the final configuration of the pilot plant after several stages of development.this pilot plant consisted of the following : 1.reactor 2.feed tank 3.product tank 4.metering pump mm diam x 400 mm, conical bottom.. capacity 00 lit.. capacity 00 lit.. plunger or diaphragm type 5.pressure regulating valve.. for control of ammonia gas pressure 6.diaphragm control valve (DCV) 7.draft-tube mixer.. for control of ammonia gas flow.. for agitation and mixing of product slurry The reactor is shown in fig. 2. The product is withdrawn through a 25 mm overflow line near the top of the reactor. A 50 mm line is also provided at the bottom for total draining of the reactor contents. A submerged draft tube of the size 40 mm diam x 350 mm was provided at the centre of the feed tank and the product tank to function as a mixer. About 50 1pm air flow rate was found to be sufficient for thorough mixing with 0% submergence. 5_. The Instrumentation The ph measurement loop consisted of a Philips Industrial ph Amplifier Model-PP 9041 used with a combined ph-electrode. The combined electrode was selected for simple insertion and easy cleaning. The ph signal was processed through an I-P transducer to drive the ammonia DCV. An On-line Flowrate Meter of Type B (Ref. 1) was used in the slurry line to obtain a direct digital indication of the flowrate. Probes for measurement of level, pressure and temperature in the reactor were also provided.

8 5 _1 _The Level Control Loop In the early stages of development, product was drdwn from the reactor by an on-off control driven by the lev*-", of the slurry in the reactor. A 12 mm glass side-tapping was i--. i.^-l at the appropriate level in the reactor. Visible i-lc.;,, j.ro.i.i a source placed near the glass tube at the desired elevation -. ss sensed by a light sensing diode. A rise in the slurry level caused change in intensity of the light passing through the sight class thereby triggering a solenoid valve which opened the remote-operated product valve for a set time interval of the ord-r -f 1 to 3 sec. and drained some product. This set-up as shown in fig. 3 was satisfactory except for problems with occasional coating of the inside of the glass tube with the ADLJ precipitate which resulted in false signals leading to maloperation of the r> \ P te >.n. It was, therefore, found necessary to devise * better level control loop. Conventional pneumatic level measurement using a purged probe, located so as to avoid choking, was used and found to be satisfactory. Later, a differential pressure switch which could be set to obtain low level and high level electrical signals was introduced. These signals were used to drive a solenoid valve (SV) which controlled the air supply to an airoperated on-off type remote valve regulating the outflow of product. The set-up is shown in fig. 4. This fiystem operated satisfactorily with an acceptable response time. 5 2 The In-Line ph Loop The product slurry was made to flow through an electrode chamber which consisted of a pot holding a ph electrode tor in-line monitoring of the ph of the product stream. Due to continuous refreshing, this represented the conditions in the reactor itself. The ph measurement was transduced to c pneumatic signal to drive the ammonia gas DCV in the ph control loop. Fig. 5 shows a schematic diagram of the arrangement of the ph measurement loop.

9 7 Use of feed containing 120 to 150 g/1 uranium resulted in thicker product slurry which necessitated further improvements in the method of product" drawal. The thicker product slurry caused frequent, choking of the electrode chamber and of the side-tapping carrying the slurry into the pot, resulting in interruption of the process. This led to a search for a better configuration of the in-line ph chamber. The glass pot used in earlier stages was in any case not desirable for plant scale operations. In order to avoid these problems, it was decided to dispense with the level control loop and to draw the product through an overflow loop. The new version of the electrode chamber was an inclined 20 mm tee-shaped stainless steel pot installed in the product overflow line itself instead of being placed in a tapping on the slurry product line. The complete process instrumentation and control system is shown schematically in fig. 6. The precipitation reaction is exothermic, but this was not found to be a consideration during design or operation. 6. The Experimental Procedure It had been established in the early exploratory runs that satisfactory recovery of uranium was possible if the reactor ph was maintained in the range 7.0 to. Initial runs were conducted using feeds containing 20 to 30 g/1 uranium and approximately 0.5 M nitric acid in aqueous feed. After prolonged experimentation with tuning of the pneumatic controller, it was possible to obtain control of ph within 0.1 ph points. Feed containing 60 to 80 g/1 uranium could be processed without difficulty. These data cire presented in Table I. It was observed that the performance of the level control loop could be affected by choking of the level probe, a drift in the d.p. switch setting, due to a maloperation of the SV or the RV as well as the on-off nature of level control and the inability to freeze and instantly restart process

10 8 In order to eliminate the above problems as well as those encountered in regulating the flow through the ph measurement line and of choking of the ph pot and the line, the product line was reconfigured to permit drawal of product by overflow and the ph electrode was relocated in the overflow line. These changes succeeded in eliminating the problems. This could be possible as the reactor contents were subjected to continuous mixing by sparging of air in order to avoid settling of solids, due to which the behaviour of the ADU reactor approximated a continuous stirred tank type reactor (CSTR). This behaviour was established during tests conducted to study the degree of mixing obtained in the reactor when it was found that there was no settling of solids at the bottom of the reactor over a period of three hours. After making the above changes it was possible to continuously maintain the product quality even when uranyl citrate was fed from the top and product was withdrawn through an overflow line located at a level below the inlet line, indicating that there was no short circuiting of feed to the product line. This factor permitted considerable flexibility in the location of the feed nozzles on the reactor. Operating the reactor with product withdrawal by overflow was found to offer the following advantages : 1. No requirement of level control 2. Really continuous (not intermittent) product withdrawal 3. Low chance of ammonia carry over with the product since ammonia gas inlet is located at a lower level in the side of the reactor

11 4. Continuous measurement of ph in the product line bymonitoring the entire outflow of the product and not a tapped stream, and reduced chances of choking thus avoiding the necessity of flow regulation 5. Truly continuous operation; free from the necessity of operator intervention, more reliable and economical. 7. Extended Tests Several series of experiments were designed and conducted in order to further probe the nature of the Continuous Precipitation System and to determine the scope for further improvement. The experimental data are shown in Table II. The details of these extended experiments are proposed to be dealt with in Part II of this report. The major observations are summarised here. 7.1 Mixing: It was found that air rate required for uniform mixing in the reactor is 1pm. High concentration feed: The process was tested for continuous operation with feed of concentration upto 258 g/1 uranium. This could be done without encountering any problem of choking or of product quality. 7.3 Dynamic response to fetid disturbance: The system was studied for response to a si:ep change in feed rate. A negative step as high as 50% of the normal flowrate was imposed upon the system. It was observed that system quickly responded without upsetting any parameter like quality and level. Similar results were obtained with the imposition of the reverse step.

12 7.4 Residence time and throughput: It was found that with a reactor of 22 lit active volume it is possible to achieve upto 12 kg/hr uranium throughput over a range of feed quality and acidity. It was found that uniform throughput was obtained for concentrations of feed ranging from 40 to 240 g/1. This was not affected adversely by the low and high feed concentrations which would be expected to alter the residence time significantly due to the change in flow conditions. This indicated the possibility of a short residence time requirement for a complete reaction. It was also felt that the possibility' existed -of the reaction time being a function of variables sucii as feed concentration, ammonia concentration,- s-lurry " percentage, viscosity, mass transfer etc. However, further studies of reaction time were constrained due to the limitations of the experimental set-up. These studies are proposed to be continued in future. 8 Conclusions It can be concluded from results obtained from the prolonged testing of the pilot plant reactor and the associated process that, a continuous process for the precipitation of uranium is practically feasible. The continuous reactor required is compactly sized and of simple design, with a small requirement of vessels. and. tankage... The system does not require elaborate or expensive instrumentation. -io- The process is highly flexible with respect to feed quality. The operation is uncomplicated with straight-forward start-up, shutdown and freeze procedures. The. system is very stable to disturbance due to changes in feed etc. The reactor system is capable of being operated in a truly automatic mode and the product quality does not depend upon the operator.

13 -11- Ac knowledqements The authors are grateful to Shri A.N.Prasad, Director, Reprocessing Group whose continuous encouragement made this work possible. The authors must thank Head, Uranium Metal Plant, Chemical Engineering Group and his staff for timely response to our demands for uranium solution. Shri B.K.Jain and his staff deserve our thanks for their constant and timely help with the system instrumentation. Shri S.U. Salunke, Shri C.K.R. Kaimal, Shri A.S. Chaudhary and Shri R. D'Silva contributed greatly with the prolonged experimental work. References 1. B.V.Shah, C.K.R.Kaimal, I.A.Siddiqui and S.V.Kumar: "A Direct Reading On-Line Flowrate Meter for Use in Radiochemical Plant" - Report BARC-1387 (1987)

14 Table I Experimental Data of Pilot Plant Operation of Continuous ADU Precipitation System Feed Uranium g/i v Feed Acid moles Feed Rate lph Hours of Operation

15 13 Table I - continued Experimental Data of Pilot Plant Operation of Continuous ADU Precipitation System Feed Uranium g/i Feed Acid moles Feed Rate lph " 64.0C Hours of Operation

16 H Table II Experimental Data of Pilot Plant Operation of Continuous ADU Precipitation System Feed Uranium 3/ ' Feed Acid Feed rate moles lph Hours of Opern P PH TO II m lpm ^ A kg DO B g/l 122 HA DCV Air psi J Reactor Capacity kg/hr > Batch Volume lit continued >> o

17 -45- Table II - continued Experimental Data of Pilot Plant Operation of Continuous ADU Precipitation System Feed Uranium g/i Feed Acid moles Feed rate lph Hours of Opern P PH S lpm A kg B g/i DCV Air psi Reactor Capacity kg/hr Batch Volume lit >> continued VAR Notes: P = ph of the product from reactor S = Flowrate of sparging air in the reactor, lpm A = Consumption of ammonia gas for the batch, kg B = Concentration of Uranium in the supernatant, ppm VAR = Constant value not maintained = Not available

18 AIR VENT DCV REACTOR ph ELECTRODE CHAMBER AIR \l PRODUCT AMMONIA GAS CYLINDER FEED TANK DRAFT TUBE MIXER TERING PIWP Flg.lt SCHEMATIC OF CONTINUOUS URANIUM PRECIPITATION PILDT PLANT

19 AIR VENT PRESSURE PROBE LEVEL PROBE URANTL NITRATE FEED SIDE GLASS LEVEL MEASUREMENT VIEWING VINDOW AMMDNJA GAS RV PINCH VALVE IN-LINE ~ph MEASUREMENT BALL VALVE X y DRAIN PRODUCT Flg.2i ADU REACTDR

20 : :! SLURRY LEVEL MEASUREMENT VENT LIGHT UN FEED MAXIMUM LEV L CLASS i.,- ) i i ADJUSTABLE.UNIMUM LEVEL LAMP I i SV NH3 GAS DRAIN Flg.3 OPTICAL SLURRY LEVE:. AND SLURRY FLDV CCi\

21 D.P SWITCH UN FEED AIR MAXIMUM LEVEL ADJUSTABLE MINIMUM LEVEL NH3 GAS DRAIN PRODUCT Fig.4i SLURRY LEVEL CDNTRDL USING D.P. SWITCH

22 . ' " ) ph CONTROLLER J-P CONVERTER ph IKHCATDR VENT 1 PRV o DCV ; i RV, -'. i AMMONIA GAS A AJ ph ELECTRtJDE DRAIN SLURRY Flg,5i LOOP FDR IN-LINE on MEASUREMENT AND CGNTRP

23 21 PH ph INDICATOR DIGITAL CONTROLLER RECORDER FLDVRATE DISPLAY I-P CONVERTER AMPLIFER r r I h AIR PRESSURE INDICATOR VENT I I I PH ELECTRUDE i 1 1 UN FEED CONSTANT LEVEL IN-LINE ph PDT REACTOR AIR DCV AMMONIA GAS PRODUCT DVERFLDV CYLINDER FIQ.6" CDNTINUDUS ADU PRECIPITATIDN PRDCESS AND INSTRUMENTATIDN SCHEMATIC

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