CHEMICAL ASPECTS OF SYNROC POWDER PREPARATION USING THE SANDIA PROCESS
A Thesis submitted in fulfilment of the requirement for admittance to the degree of
MASTER OF SCIENCE at
The University of New South Wales
by
ANTHONY CHETCUTI
School of Chemistry Department of Inorganic & Nuclear Chemistry SR.P.TlO
CERTIFICATE OF ORIGINALITY
I hereby declare that this thesis is my own work and that, to the best of my knowledge and belief, it contains no ~aterial previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgement is made in the text of the thesis.
(Signed) DECLARATION
The candidate, Anthony Michael Chetcuti, hereby declares that none of the work incorporated in this Thesis has been submitted by him to any other University or Institute for a higher degree. His work was carried out in the Ceramics Section, Materials Division, Australian Atomic Energy Commission, Lucas Heights Research Laboratories, Lucas Heights, New South Wales.
AM Chetcuti
...... ·jJ /ft( ABSTRACT
SYNROC is a synthetic three-phase titanate-based ceramic designed specifically for the disposal of radioactive HLW. Collectively, the three SYNROC phases - barium hollandite
(BaA1 2Ti 6016 ), perovskite (CaTi0 3 ) and zirconolite
(CaZrTi 207 ) can accept into solid solution almost all the radionuclides present in HLW.
The Sandia process was used to prepare SYNROC powder to study its morphology and sorptive characteristics. The hydrolysis and sorption/ion-exchange reaction steps were investigated by preparing SYNROC under controlled conditions. Electrokinetic techniques were utilized to study the behaviour of SYNROC, ST-SZ and Ti0 2-Zr0 2 powders.
Ca and some Ba losses occurred during SYNROC B preparation, which ultimately affected the final SYNROC chemical composition. The addition of acidified simulated HLW to SYNROC resulted in loss of SYNROC matrix elements at low pH. However, this had no apparent effect on SYNROC microstructure.
SYNROC prepared by the Sandia process is readily sintered and hot-pressed at lower temperatures and exhibits a much lower Cs leach rate in comparison with SYNROC prepared by the oxide route. ACKNOWLEDGEMENTS
The author wishes to express his gratitude for the encouragement and guidance afforded by Associate Professor D.J. Carswell (Supervisor), Head, Department of Nuclear Chemistry, University of New South Wales and Dr.J.L. Woolfrey, Dr. I.H. Harding and Dr K.D. Reeve of the Australian Atomic Energy Commission.
Sincere thanks are also due to Mr. P.A. James, Associate Professor H.A. Goodwin and Dr. B.R. Craven for valuable discussions and constructive criticism.
The candidate wishes to thank the staff of the AAEC/CSIRO for spectrographic analysis, X-ray diffraction and scanning electron microscopy and to Dr. J.C. Taylor and Dr. J.C. Hoare, CSIRO, Division of Energy Chemistry, Lucas Heights for helpful convnents. Also thanks to Mr. J.E. Croucher for fabricating SYNROC samples at the AAEC Lucas Heights.
I wish to thank my wife Gloria for her support through the MSc course and especially for typing this Thesis. ABBREVIATIONS
AA Atomic absorption AAEC Australian Atomic Energy Commission BAT Barium aluminium titanate Bq Becquerel BET Bruaner-Emmett-Teller CT Calcium titanate CZT Calcium zirconium titanate CIG Consolidated Industrial Gases Ci Curie OTA Differential thermal analysis GWe Gigawatt electrical g/L Grams per litre g/MT Grams per million tonne HLW High-level waste iep Isoelectric point Laser Light amplification by stimulated emission of radiation MBq Megabecquerel MPa Megapascals MWd Megawatt day MWe Megawatt electrical NOx Nitrogen oxides ppm Parts per million RE Rare earth SEM Scanning electron microscopy ST Sodium titanate ST-SZ Sodium titanate-sodium zirconate SYNROC Synthetic rock TBq Terabecquerel TTIP Titanium tetraisopropoxide TBP Tributyl phosphate XRD X-ray diffraction ZTB Zirconium tetra-n-butoxide TABLE OF CONTENTS
1. INTRODUCTION - Chapter I 1
1.1 Sources of Radioactive Wastes 2 1.2 Nature of Radioactive Liquid HLW 3
1.3 Chemical Composition of HLW 3 1.4 Radioactivity Associated with Liquid HLW 7 1.5 Current and Proposed Techniques Available for Disposal of Radioactive HLW 7
1.5.l Direct Burial of Encapsulated Unprocessed Nuclear Fuel Elements 7
1.5.2 Tank Storage of Liquid HLW 8
1.5.3 Solidification Processes 9
1.5.4 Borosilicate Glass 10 1.5.5 Ceramic 'Synthetic Mineral' Approach 12
1.6 Ceramic Wasteforms 12
1.6.l Alumina-Based Ceramics 12 1.6.2 Silicate-Based Ceramics 13 1.6.3 Titanate Based-Ceramics 13
1.7 SYNROC 14
1. 7. 1 SYNROC B 14 1.7.2 Preparation of SYNROC B by the Sandia Process 17 1.7.3 SYNROCC 19 2. EXPERIMENTAL - Chapter II 25
2.1.1 Starting Materials 26
2.1.2 Preparation of Hydrated Ti0 2 -Zr0 2 26 2.1.3 Preparation of ST-SZ 26 2.1.4 Preparation of SYNROC B by the Sandia Process 26
2.1.5 Characterization of Hydrated Ti0 2 -Zr0 2 , ST-SZ and SYNROC Powders 27 2.1.6 Surface Area Determination 27 2.1.7 Principle of Operation - Perkin-Elmer Sorptometer Model 212D 29 2. 1.8 Particle Size Distribution 29 2.1.9 Electrophoresis Measurements 30
2.2 Hydrolysis Reaction 30
2.3 Sorption/Ion-Exchange Reactions 33
2.4 Simulated HLW Addition to SYNROC B 33
2.5 Electrophoretic Mobility Measurements 34
3. RESULTS AND DISCUSSION - Chapter II I 36
3. l Metal Alkoxide Hydrolysis 37
3. 1.1 Effect of Acetone : Water Ratio on the Surface Area of ST-SZ 38 3. 1 . 2 Removal of Entrained Alcohols After Metal Alkoxide Hydrolysis 40 3. 1.3 Effect of Carbonate Concentration on the Surface Area of ST-SZ and SYNROC B Powders 43 Page
3.2 Sorption Experiments 46
3 .2. l Kinetics 46 3.2.2 Residual Sodium Removal of SYNROC B 49 3.2.3 Residual Nitrate Removal from SYNROC B 49
3.3 Electrokinetic Measurements 58
3.4 Sorption Capacity of SYNROC B for Cs and Sr 61
3.5 Effect of Excess Nitric Acid on SYNROC C Properties 61
3. 5. l Powder Properties 61 3.5.2 SYNROC Fabrication 63
4. CONCLUSION 65
5. REFERENCES 67 Chapter I
INTRODUCTION 1
1. INTRODUCTION
As at 30 June 1985 there were 331 nuclear power reactors operating worldwide and 191 under construction (Table 1). The fission of nuclear fuel, which provides the heat energy for the generation of electricity, also builds up fission products which absorb neutrons. Spent fuel is discharged from a reactor and replaced when the combination of neutron absorption by fission products and the reduction in fissile content no longer allows the nuclear chain reaction to continue efficiently.
At the time of discharge from the reactor, the spent fuel is intensely radioactive and generates significant heat, One tonne of spent light water reactor fuel typically generates - 2000 kW of thermal power from radioactive decay at the time of its removal from the reactor. This decreases to about 10 kW after one year, one kW after 10 years and 300W after - 100 years. The spent fuel elements are stored under water for periods ranging from several months to several years and then transported to a reprocessing plant where HLW is generated from the separation of re-usable uranium and plutonium.
The need for safe disposal of wastes containing radioactivity is an inevitable consequence of the harnessing of nuclear energy. The wastes can be solid, liquid or gaseous with concentration of radioactivity varying from hundreds of TBq+ to tens of MBq per cubic metre of waste. The radioactivity declines as the radioactive nuclei decay into stable
+ Becquerel (Bq) : the amount of radioisotope which decays at the rate of 1 disintegration per second, 3.7 x 10 10 Bq is 1 curie (Ci). 2
TABLE 1
Summary as at 30 June 1985 of all nuclear power reactors with a net electrical capacity over 30 MWe (megawatts electrical)
Country Building Operating
USA 38 85 USSR 41 40 France 22 41
UK 7 35 Japan 11 29
West Germany 7 16 Canada 7 15 Sweden 2 10 East Germany 6 5 Belgium 2 5 Spain 3 7 India 5 5
Taiwan 0 6 Switzerland l 5 Czechoslovakia 9 4 Bulgaria 3 4
Finland 0 4
South Korea 6 3 Italy 3 3 13 Others 18 9
191 331
Source: 1\1'[ C, Ju 1y l 9fl 5 3 isotopes, the rate being governed by the half-livest of the particular isotopes. The quantity of radioactive HLW is increasing annually and its ultimate disposal is a difficult but very important technological problem.
The current options that are available to solve this problem include: a. Direct burial of unprocessed (possibly encapsulated) nuclear reactor fuel elements. b. Reprocessing of spent nuclear fuel elements and storing the acidic radioactive HLW in stainless steel storage tanks. c. Solidification of acidic HLW into an inert solid wasteform either as borosilicate glass or a special ceramic and storage and final disposal in a deep geological repository.
This Thesis outlines the preparation of SYNROC using the Sandia process and discusses the associated chemical considerations in its manufacture as a titanate-based ceramic wasteform.
1.1 Sources of radioactive wastes
Sources of nuclear waste include materials that become activated in the course of neutron bombardment. The wastes themselves can be either liquid, solid or gaseous. The major source of acidic HLW comes from the reprocessing of spent nuclear fuel elements. Other wastes include the zircaloyH cladding which encapsulates nuclear fuel rods. The cooling air recirculating over an experiment inside a nuclear reactor t Half-life: the time needed for a number of atoms of a nuclide to be reduced by half through radioactive decay. -t-t Zircaloy is a zirconium alloy and in addition to zirconium, it contains l .5o/., tin, 0.15o/., iron, 0. H chromium and 0.05% nickel by weight. 4 will most certainly become radioactive because of the formation of 41 Ar (from the 40Ar already present in air). 1
1.2 Nature of radioactive liquid HLW
A high proportion of the activity(~ 99%) of the irradiated fuel enters the HLW liquid waste stream during reprocessing, when the fuel element is cut into small pieces and dissolved in nitric acid. An organic solvent, usually tributyl phosphate (TBP), extracts 99.5% of the uranium and plutonium from the acidic solution and less than 0.1% of the fission products. Tables 2 and 3 list the radioactivity and half-lives of the more important fission products and transuranium elements present in nuclear HLW. There are in excess of 200 different species of fission products; (eg 137 Cs and 90sr have long decay half-lives and significant radiotoxicity). 1
1.3 Chemical composition of HLW
Typical HLW liquor generated from the reprocessing of nuclear fuel elements will contain at least 50 elements including some anions (eg Pai-, Moot-). Table 4 illustrates the characteristic composition of nuclear HLW generated from the reprocessing of nuclear fuel elements.
The bulk of dissolved solids that make up typical HLW liquor include fission products (eg 90sr, 137 Cs), actinides (eg 2 38U, 2 38pu, 2 37Np} and process chemicals (eg Na 2C0 3, Mn0 2 , NaN0 2 etc). Corrosion products (eg Ni, Cr, Fe) make a minor contribution (approx. 1 percent) of the total solid content of the HLW liquor. The majority of fuel elements are mechanically decanned, and inevitably some traces of the canning material (zircaloy) enter the HLW stream. The HLW liquor may 5 TABLE 2 - RADIOACTIVITY OF IMPORTANT LONG-LIVED FISSION PRODUCTS IN HIGH ACTIVITY LIQUID WASTE (Basis: one year of operation of a typical 1 000 MWe light water reactor)
After one After ten After After year years 100 l 000 (curies) (curies) years years Isotope Half-life (curies) (curies)
Strontium 90 29 years 2 010 000 1 610 000 174 000 + Yttrium 90* 2.6 days 2 010 000 1 610 000 174 000 Zirconium 93 950 000 years 50 50 50 50 Niobium 93m 12 years 6 23 50 50 Technetium 99 213 000 years 380 380 380 380 Ruthenium 106 369 days 7 250 000 14 600 Rhodium 106* 2 .2 hours 7 250 000 14 600 Caesium 134 2 years 4 680 000 223 000 Caesium 137 30 years 2 790 000 2 270 000 281 000 Bari um 137m* 2.6 min. 2 620 000 2 130 000 263 000 Cerium 144 284 days 12 100 000 4 000 Praseodymium 144* 17 min. 12 100 000 4 000 Samarium 151 93 years 32 900 30 800 15 000 12 * Indicates short-lived 'daughter' of the isotope directly above it in the table. + Blanks indicate that radioactivity has decayed to less than one-thousandth of a curie. TABLE 3 - RADIOACTIVITY OF TRANSURANIUM ELEMENTS IN HIGH ACTIVITY LIQUID WASTE (Basis: one year of operation of a typical l 000 MWe light water reactor)
After 10 After After After After one years l 00 l 000 l O 000 mi 11 ion (curies) years years years years Isotope Half-life (curies)(curies)(curies)(curies)
Neptunium 237 2 140 000 years 9. l 9.2 9.8 10 7.3 Neptunium 239 2.35 days 480 480 440 200 Plutonium 238 87.8 years 2 500 1 300 5.9 Plutonium 239 24 390 years 43 44 55 110 Plutonium 240 6 540 years 120 230 210 84 Plutonium 241 15 years 8 500 130 8.4 3.9 Plutonium 242 387 000 years 0.2 0.2 0.2 0.2 0.03 Americium 241 433 years 4 300 4 000 950 3.9 Americium 242m 152 years 230 150 2.6 Americium 243 7 370 years 480 480 440 200 Curium 242 163 days 190 130 2. 1 Curium 243 28 years 80 11 Curium 244 17.9 years 44 000 1 400 Curium 245 8 500 years 9. 1 9.0 8.4 3.9 Curium 246 4 760 years 1.8 1.8 1.6 0.4 NOTE: This table was calculated on the assumption that, because of incomplete recovery, 0.5% of plutonium in the spent fuel enters the high activity wastes. Blanks indicate that radioactivity has decayed to less than one-thousandth of a curie.
Source: Alfredson, P.G., Levins, D.M., Atomic Energy in Australia 19(3)Julv 1976. 6
TABLE 4 TYPICAL HLW COMPOSITION (PW-4b)
Fission Products Process Chemicals Actinides wt% wt% wt% wt% in HLW in HLW in HLW in HLW
Rb 20 0.87 Te02 1. 78 Fe 203 3.70 U30a 2.86 SrO 2.59 Cs20 7.05 Cr 203 0.84 Np0 2 2 .12 Y203 1.46 Bao 3.84 NiO 0.35 Pu02 0.02 Zr0 2 12. 11 La 203 3.62 PzOs 1.65 Am203 0.44 Mo0 3 12.67 Ce02 8.14 Cm203 0.10 Tc207 3. 16 Pr6011 3.63 Ru02 7.28 Nd 203 11.07 Rh 203 1.18 Pm 203 0.30
PdO 3.63 Sm 203 2.26 Fission Products 88.00 Ag 20 0.22 Eu 203 0.49 Process Chemicals 6.50 Actinides 5.50 CdO 0.24 Gd 203 0.34
100.00
Source: Reeve, K.D., Levins, D.M., Ramm, E.J., Woolfrey, J.L., Metals Forum 6 No 2, 1983 7 also contain degradation products such as dibutyl phosphoric acid l as well as kerosene and tributyl phosphate emulsions.
1.4 Radioactivity associated with liquid HLW
Most of the fission products in radioactive liquid HLW are beta-gamma emitters, with typical half-lives ranging from days through years to
(in a very few cases) > 10 years. The important alpha-emitting actinides have half-lives of 163 days to 2 x 10 6 years.
In the first few years, the short half-life fission products dominate the very high total activity. From 10 to about 1000 years, 30 year half-life 90sr and 137Cs dominate the activity, which falls by a 5 factor of 10 during this period. After approximately 600 years, the actinides assume importance over the fission products and they decay very slowly up to 106 years.1
1.5 Current and proposed techniques available for disposal of radio active HLW
1.5.1 Direct burial of encapsulated unprocessed nuclear fuel elements
Spent fuel rods are highly radioactive and generate a large amount of heat. On removal from a reactor, rods are initially stored in water filled cooling pools located in the reactor complex.. Thick concrete walls lined with stainless steel and the depth of water (6 to 8 metres) provide the radiation shielding necessary to protect workers. The water also serves to absorb the decay heat generated by the spent fuel. 2
Corrosion of the cladding of the spent fuel may occur in the water depending on the material used for cladding, the period of cooling and 8
the water purity. To minimize corrosion the pool water is circulated through filtering equipment to remove traces of corrosion products and any radioactive contaminants.
The first strategy for long-term waste management treats spent fuel as waste, and disposes of it without reprocessing. Encapsulation of spent fuel involves sealing it directly into containers in a form suitable for burial in a deep geological repository~ Encapsulation of spent fuel releases only minor amounts of gaseous emissions and does not produce significant additional waste streams such as result from a reprocessing plant. 2
1.5.2 Tank storage of liquid HLW
Vitrification of radioactive liquid HLW into borosolicate glass monoliths is presently proceeding on a commercial scale at Marcoule in France. However, elsewhere (eg USA, UK) storage of acidic radioactive liquid HLW in stainless steel tanks is the only viable alternative technique available on a large scale. Although it is not the final option for storage, it will remain a necessary intermediate stage prior to solidification. The radioactive liquid HLW may require ageing (up to 6 years) in order to reduce the heat generation of waste liquor from reprocessing of uranium fuel elements by one order of magnitude~ However, the desired heat reduction can readily be achieved by prolonged ageing of the fuel elements prior to reprocessing. 3 9
The radioactive HLW liquids are usually concentrated by evaporation and stored as aqueous nitric acid solutions with up to
250 g/L salt content and 7M HN0 3 • However, the optimum nitric acid concentration for prolonged tank storage of HLW liquor falls within the range 2 to 4M and is thus determined by the corrosion behaviour of stainless steel used in the construction of liquid HLW storage tanks. The tanks are equipped with cooling coils to remove fission product decay heat and in some cases tanks may require continuous agitation to prevent any precipitated solids from settling and thus minimize hot spots at the walls.
The continued storage of liquid wastes in tanks involves constant supervision to ensure that the necessary services such as cooling, agitation and off-gas treatment are always available~ While liquid storage in stainless steel tanks may be considered safe and acceptable for some decades, with the expanding use of nuclear power it is necessary in the longer term to replace liquid storage by an alternative system that will not require constant surveillance, be potentially more safer and be in such a form that will make it possible for the waste to be transported away from the reprocessing plant for long-term storage or disposal. 3
1.5~3 Solidification processes
The primary objective of solidification techniques is to transform the HLW liquor into a mixture of oxides which can then be further processed by the addition of glass-forming materials, or ceramic components and reacting at high temperature(~ 1200-1500°C) to form a glass or ceramic. The major steps are denitration (chemical or thermal), 10
calcination, addition of glass formers or ceramic components and preparation of the glass or ceramic product (Fig. 1).
All current studied fixation technologies for radioactive wastes use the conversion of materials contained in the initial solution into a relatively insoluble compact that meets a number of requirements based on safety and economic criteria. In these waste treatment methods chemical conversion of the initial compounds gives crystalline melts, vitreous materials or combinations thereof. 4
There are currently three principal classes of oxide ceramics for the fixation of radioactive HLW; these include alumina-based, silicate based and titanate-based wasteforms. To best achieve the desired ceramic wasteform it is vital to establish which properties of the ceramic powder are critical during solidification. During the preparation of the powder the factors that influence these properties should be determined in order to produce the required precursor.
1.5.4 Borosilicate glass
Borosilicate glasses are the leading 'first generation' wasteforms being considered, having been developed notably by France and produced on an industrial scale. Glass can dissolve the majority of the HLW elements at an overall loading of up to 30 wt%. The wasteforms appear to be resistant to gamma radiation and actinide decay damage~ At temperatures up to 100°C their aqueous leach rates are acceptably low. 5 However, excessive temperature variations during long term burial are likely to cause devitrification of the glass and leads to accelerated leaching of radionuclides. 11
HLW LIQUOR
ADDITIVES SORPTION
EVAPORATION and/or PRESSURE CALCINATION EVAPORATOR SINTER ING and MELTER
MELTER
GLASS or CALCINE CERAMIC
FURTHER TREATMENT
METAL COATED or MATRIX PARTICLE
FIGURE 1. Basic solidification processes 12
The basic steps in the vitrification process are evaporation of the HLW, heating of the product to convert nitrates to oxides (calcination), addition of glass forming materials, melting at a temperature of about l000°C, and finally pouring or casting the resulting product into a container. Variations to this basic process include the production of glass marbles for encapsulation in a metal matrix, or addition of waste directly to a glass-forming mixture, with evaporation and melting both taking place in one piece of equipment. Typically, from 10 to 20 wt% of the calcined waste is incorporated in the glass although the trend is towards the lower concentration.
1.5.5 Ceramic 'Synthetic Mineral' approach
The technique of utilizing ceramics for immobilizing radioactive HLW involves the incorporation of the nuclides into solid solution in an assemblage of mineralogic phases. The concept was originally promulgated by Hatch 6 in 1953, but was unappreciated until demonstrated by workers at Pennsylvania State University. 7 ' 8 The original philosophy for using crystalline host phases remains the same today; namely, that particular mineral phases containing radioactive elements have been known to be geologically stable for 10 10 years. Thus, it is argued that analogous, synthetic mineral phases, eg ceramics, will also be stable over the required immobilization period. l.6 Ceramic wasteforms
1.6.1 Alumina-based ceramics
A class of high alumina-based ceramics for the fixation of military HLW at the Savannah River Plant was developed at the Science Center 13
of Rockwell International:' 10 Recently, the range of ceramics has been extended to immobi 1 ize commerc,a. 1 wastes. 11 The high alumina ceramic wasteform consists of the compatible phases of alumina, spinel (nominally MgA1 2 04 ), magnetoplumbite [nominally
X(Al,Fe) 23019 where X = Sr, Ba, or charge substitutions such as
Cs 0 • 5 + La 0 _5 1, and a fluorite-related uraninite, (U, Th)0 2 • For HLW containing a very high concentration of sodium, an additional crystalline phase, nepheline, NaA1Si0 4 , is produced to accommodate the monovalent ion.
1~6.2 Silicate-based ceramics
The 'supercalcine' ceramics developed by McCarthy et al 8 ' 12 at Pennsylvania State University represent the earliest demonstration that a ceramic wasteform could be used to immobilize HLW. The approach taken by McCarthy was to add Si, Al, Ca and Sr oxides to HLW before calcination, so that each radionuclide was optimally fixed by one or more phases. The key phases in the assemblage are pollucite
(CsA1Si 206 ), scheelite (CaMo0 4 ), fluorite [(U, Zr, Ce)0 2 ] apatite
(Ca 5 (P0 4 ) 3 0H) and monazite (REP04 ) for fixation of Cs, Sr, Mo, U and the rare earth elements respectively.
1~6.3 Titanate based-ceramics
Another range of ceramic materials, developed by Ringwood. et al 13 ' 14 at the Australian National University, is based on the incorporation of radioactive waste elements as dilute solid solutions in titania rich phases. The material known as SYNROC (synthetic rock) is a titanate based, fine-grained multiple component ceramic system, designed for the fixation of HLW arising from the reprocessing of nuclear power reactor fuel. 15 14
1.7 SYNROC
SYNROC consists of three major phases namely zirconolite (nominally
CaZrTi 207 ), perovskite (CaTi0 3 ), and hollandite (BaA1 2TiG0 16 ). The principal form SYNROC C was formulated to accommodate radioactive HLW commercial waste .. In addition to the three prescribed phases, minor crystalline phases such as Ti0 2 (rutile) can be present in SYNROC C including minor metallic phases and some amorphous material. SYNROC without added HLW is termed SYNROC B whereas SYNROC with added 10-20 wt% simulated or actual HLW is called SYNROC C; typical 14 SYNROC chemical compositions are shown in Table 5.
1..7 .1 SYNROC B
SYNROC B powder can be prepared by an oxide route or the Sandia process. 16 The oxide route consists of ball-milling metal oxide powders of Ti02 (anatase), Al 203 , Zr0 2 , BaC0 3 and CaC0 3 , adding nitric acid to convert the carbonates to nitrates, and spray-drying the oxide/ nitrate slurry and calcining in air to decompose the residual nitrate .d 14 t 0 OXl e .. The Sandia process involves the hydrolysis of alkaline methanolic solutions of titanium tetraisopropoxide and zirconium tetra n-butoxide in acetone-water media to yield a 'sodium titanate-sodium zirconate' powder mixture, which then acts as an ion-exchanger for an aluminium, barium and calcium nitrate solution to produce SYNROC B. 16
The metal alkoxides have the general formula M(OR) and are analogous X to alcohols (ROH) in which the hydroxylic hydrogen has been replaced by a metal (M) .. Metal halides {eg TiC1 4 , ZrC1 4 ) have been used to prepare such compounds, for example:
TiC1 4 + 4ROH + ( l) 15
TABLE 5
TYPICAL SYNROC COMPOSITIONS
SYNROC B SYNROC C wt% wt%
Ti02 71.4 57. l
Zr0 2 6.6 5.3
Al 2 03 5.4 4.3 BaOa 5.6 4.5 caoa 11.0 8.8 HLW 20.0
Total 100.0 100.0
(a) added as carbonates via the Oxide route
Source: Reeve, K.D., Levins, D.M., Ramm, E.J., and Woolfrey, J.L., Metals Forum 6 No. 2, 1983. 16
Hydrolysis of metal alkoxides results in metal hydroxide or hydrated oxide products 17 :
+ 4H 2 0 + M(OH) 4 + 4ROH (2)
The ion-exchange materials which have been developed have the general formula M[M~ OY H2 Jn' where Mis an exchangeable cation of charge +n and M' may be Ti or Zt. Because of economic considerations, a sodium form of the 1 titanate 1 NaTi 205H, has received the most attention. 18
The exchangers are prepared by reactions of a metal alkoxide with a base in non-aqueous solutions followed by hydrolysis. In the early stages of development, quaternary ammonium bases (e.g. tetramethyl ammonium hydroxide) were used to produce a water soluble intermediate which could be used directly as a precipitating agent or converted to the desired ion-exchanger 18 • However, all recent work has been performed with materials prepared simply by reacting the metal alkoxide
Ti(OC 3H7 ) 4 or Zr(OC 4H9 ) 4 with a solution of 10-20 wt% NaOH in methanol, (Eq. 3) followed by hydrolysis in excess water (Eq. 4) to form an insoluble product which is filtered and dried under vacuum at ambient temperature.
NaOH Intermediate (3) Methanol
(4) 17
1.7.2 Preparation of SYNROC B by the Sandia process
The preparation of Sandia SYNROC B involves a two-stage process (Fig.2). The first stage entails the preparation of hydrated sodium titanate-sodium zirconate (ST-SZ) ion-exchange material. This is produced by reacting methanolic sodium hydroxide solution with a mixture of titanium tetraisopropoxide and zirconium tetra n-butoxide. This alkaline metal alkoxide mixture is then poured into a acetone: water (10:1) mixture resulting in hydrolysis and a coarse filterable product. The filtered material is washed with acetone to remove residual alcohol and dried under vacuum at room temperature for 16 hours. The second stage of the reaction involves equilibrating a calculated amount of hydrated ST-SZ powder with a specified concentration of aqueous Al, Ba, Ca nitrate solution. Aqueous sodium hydroxide solution is added such that the concentration of sodium in the reaction mixture exactly balances the concentration of free nitrate in solution. The reaction mixture is agitated for approximately 1 hour to allow the ion-exchange/sorption process to proceed. During the reaction, sodium ions are displaced by aluminium, barium and calcium ions.* The SYNROC B material is filtered and the filtercake washed with demineralized water to remove residual sodium. The product is washed with acetone and dried under vacuum at room temperature for 16 hours.
* c.f. Section 3.2.3 page 53. 18 STAGE 1 STAGE 2
MIX ADD TTIP ST-SZ STEP 6 STEP l + POWDER ZTB
ION-EXCHANGE REACTION ADD STEPS 7,8 METHANOLIC Al,Ba,Ca ADD NaOH STEP 2 NaOH NITRATE SOLUTION (50 wt%) SOLUTION SOLUTION. STIR l HR.
HYDROLY SIS REACTIO N ACETONE STEP 3 + STEPS 9,10 STIR WATER FILTER RAPIDLY MIXTURE WASH WITH WATER AND FINALLY WAS; WITH ACETONE
VACUUM DRY STEP 11 STEP 4 FILTER (25°c) WASH 16h WITH I ACETONE TO SANDIA SYNROC B POWDER REMOVE ORGANICS
Vacuum STEP 5 Dry ADD STEP 12 (25°C) HLW 16h SOLUTION ST-SZ SANDIA SYNROC C SLURRY POWDER
FIGURE 2. Sandia SYNROC powder preparation 19
1.7.3 SYNROC C
The complete preparation and final fabrication of SYNROC C into ceramic monoliths basically involves three main process steps, these include:
a) HLW addition to SYNROC B b) Flash-drying/calcination c) Reaction hot-pressing
To prepare SYNROC C containing 10 or 20 wt% HLW, a simulated HLW solution type PW-4b-B (Table 6) is thoroughly mixed with SYNROC B and the resulting slurry is flash-dried and then calcined in a stream of Ar/3.5% H2 at 750°C for l hour to decompose residual nitrates (Fig. 3). Removal of nitrogen oxides (NO) is highly X desirable prior to hot-pressing as this reduces the probability of
Ru0 2 -+ Ru0 4 (volatile species) oxidation by NOx.
Titanium metal powder (2 wt%) is added as an oxygen scavenger to the calcined SYNROC C powder. The powder is placed inside a graphite die and hot-pressed at 1150 o C for 2 hours under 13.8 MPa pressure. 19
SYNROC prepared by the Sandia process offers the following advantages:
(i) The surface area (BET) of SYNROC B powder is six times greater than oxide-derived material after calcination
at 750°C under an atmosphere of Ar/3.5% H2 (Fig. 4). 20
40.----r--~---,----r---,--~---,.------.--~--r-,
SYNROC -C
SYNROC-B 30
"' "'0 20 ~
10 NITRATE
0 '---::....a...---L----L-----'------'----'------'----'------'---'-----11.--' 0 200 400 600 800 1000 TEMPERATURE ( 0 c)
FIGURE 3. Thermal decomposition of residual nitrates during calcination of SYNROC C. 21
TABLE 6
PW-4b-B SIMULATED HLW SOLUTIONt
Fission Products Process Chemicals Actinides
wt% wt% wt% in HLW in HLW in HLW SrO 2.83 FeO 3.61 U0 2 5.58 Y203 1. 58 Cr203 0.90 Zr0 2 13.00 NiO 0.33 Mo0 2 12. 18 P20s 0.16 Te0 2 3.53
Cs 20 9.05 Bao 4. 19 Ce0 2 8.77 Fission Products 89.42 Nd 203 18.63 Process Chemicals 5.00 Gd 203 1.20 Actinides 5.58 Ag 20 14.46 100.00
t Simulated PW-4b-B HLW solution was prepared at the AAEC Laboratories Lucas Heights by making the following substitutions:
Rb+ Cs= Cs; La+ Ce = Ce Pr+ Nd+ Pm+ Sm= Nd
Eu +Am+ Cm+ Gd = Gd Tc+ Ru+ Rh+ Pd+ Cd +Ag= Ag
U +Pu+ Np= U 22
10
60
-'Cl
..,C IIC C .... u ...C 2 0 IIC ::, Ill
C
60 0 700 100 900 1000 110 0 1200 C ALCINATION TEYP~RATURE (°C)
FIGURE 4. Comparison of Sandia and oxide derived SYNROC powder reactivity during calcination
A Sandia-Derived SYNROC B B Sandia-Derived SYNROC C C Oxide-Derived SYNROC B 23
(ii) Sandia SYNROC can be hot-pressed to near theoretical density (- 99.9%) at a lower temperature (1150-1200°C) than that prepared from ball-milling powdered oxides (Figs. 5 & 6). 20
(iii) Sandia SYNROC exhibits the lowest leach rate for the important HLW element, Cs. 20
The major disadvantages of the Sandia process for producing SYNROC include:
(i) High cost of metal alkoxide precursors.
(ii) Generation and ultimate disposal of large volumes of organic liquids.
(iii) Difficulty in controlling the final chemical composition of SYNROC.
However, because of the lower hot-pressing temperature and improved leach resistance of the Sandia SYNROC material an investigation was initiated to prepare SYNROC by the Sandia process on a laboratory scale and study the morphology and sorptive characteristics of the ceramic powder. The procedures and chemical aspects of the preparation are described in detail in the foregoing chapters of this Thesis. 24
FIGURE 5 • SEM photomicrograph of hot-pressed SYNROC C (oxide route).
'.... ·. .
FIGURE 6 . SEM photomic ro graph of hot-pressed SYNROC C (Sandi r1 route) .
The di s tribution of phases is different in both cases and is probably rel ated t o t he mi xing operation during samp l e preparation. 24
FIGURE 5 • SEM photomicrograph of hot-pressed SYNROC C (oxide route).
FIGURE 6 • SEM photomicrograph of hot-pressed SYNROC C (Sandi a route).
The distribution of phases is different in both cases and is probably related to the mixing operation during sample preparation. 25
Chapter I I
EXPERIMENTAL 26
2. EXPERIMENTAL
2.1.1 Starting materials
The following chemicals were used without any further purification: commercial grade titanium tetraisopropoxide, zirconium tetra-n butoxide, acetone and methanol, and A.R. aluminium nitrate, barium nitrate, calcium nitrate and sodium hydroxide.
2.1.2 Preparation of hydrated Ti02-Zr02
Hydrated Ti0 2-Zr02 was prepared by mixing together stoichiometric quantities of Ti(OC 3H7 ) and Zr(OC 4H9 ) 4 • The metal alkoxide solution was hydrolyzed by pouring it into an acetone/water (10:l) solution. The hydrated oxide product was filtered, washed with acetone and dried under vacuum at room temperature for 16 hours.
2.1.3 Preparation of ST-SZ
Hydrated sodium titanate-sodium zirconate (ST-SZ) was prepared by hydrolyzing a methanolic alkaline solution comprising a mixture of titanium tetraisopropoxide and zirconium tetra-n-butoxide in an acetone/water (10:1) solution. The coarse product was filtered, washed with acetone and dried under vacuum at room temperature for 16 hours.
2.1.4 Preparation of SYNROC B by the Sandia process
SYNROC B was prepared by the Sandia process as previously described in Section 1.7.2 of the Introduction. 27
2.1.5 Characterization of hydrated Ti02-Zr02, ST-SZ and SYNROC powders
Sodium titanate-sodium zirconate, hydrated Ti0 2-Zr02 and SYNROC powders were characterized by determining the surface area, particle size distribution and electrophoretic mobility of these materials.
2.1.6 Surface area determination
The surface area of an irregular solid or a powder can be determined from the volume of nitrogen physically adsorbed at constant temperature at a series of pressures. The relationship used is the Brunauer Emmett-Teller, or BET, equation: 21
p l = [ l + (C - 1) p/p ) (5) v(po-p) 0 V C m
where
V = volume at STP of adsorbed gas p = gas pressure (N 2)
Po = saturation gas pressure Vm = volume of gas at STP required to form an adsorbed monolayer
C = a constant, characteristic of the adsorbent pair.
The surface area of all powders was determined by the BET continuous flow method on a Perkin-Elmer Sorptometer Model 212D (Fig. 7); the gas used for adsorption was CIG high purity nitrogen.
N N
CX> CX>
samples. samples.
NITROGEN NITROGEN
HELIUM HELIUM
powder powder
Sorptometer Sorptometer
3 3
-
-
and and
L L
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Perkin-Elmer Perkin-Elmer
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of of
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212D. 212D.
SORPTOMETER SORPTOMETER
Schematic Schematic
Model Model
7. 7.
FIGURE FIGURE
CHART CHART
RECORDER RECORDER INTEGRATOR INTEGRATOR 29
2.1.7 Principle of operation - Perkin-Elmer Sorptometer Model 212D
The Perkin-Elmer Sorptometer Model 212D operates on the principle whereby a known mixture of nitrogen and helium gas is passed over the solid in a sample U-tube, and the effluent is monitored by a thermal conductivity detector. With the gas flowing, the sample tube is cooled with liquid nitrogen. The cooled sample adsorbs a certain quantity of N2 from the gas stream, and the resultant dilution of the effluent is indicated on the recorder chart as a peak, the area of which is proportional to the volume of nitrogen adsorbed. After adsorption, equilibrium is attained and the recorder returns to the zero position.
After removing the liquid nitrogen bath,desorption of N2 occurs, giving a desorption peak on the chart recorder. At the completion of desorption a known volume of N2 is injected into the gas stream and the resulting calibration peak recorded. The actual volume of nitrogen adsorbed by the sample can be calculated by comparing the desorption/calibration peak areas. This information together with temperature and pressure data is then fed into an AAEC computer program for surface area computations.
2.1.8 Particle size distribution
The diameters of the individual particlest in a sample of powder are not all the same. There is a size distribution; some particles will be fine(~ O.lµm), some coarse(~ lOOOµm) and a large number intermediate in diameter. Most fine particle systems, whether formed by comminution of a bulk material or grown by accretion, have a size t A 'particle' may be a single primary particle or a solid agglomerate. As such, it is a small mass that is free to move as an entity when the powder is dispersed by the breaking of the surface bonds. 30
distribution that obeys the log-normal distribution function. 22 Particle size distributions of the powder samples were determined using the Malvern Laser Particle Size Analyzer Model 3600D (Fig. 8). All samples were dispersed inn-hexane to minimize hydrolysis prior to measurement.
2.1.9 Electrophoresis measurements
The electrophoretic mobility of SYNROC powder and precursor materials was measured on a Rank Brothers Mark II Micro-Electrophoresis apparatus (Fig. 9). All samples were outgassed under a stream of special grade CIG argon.
2.2 Hydrolysis reaction
Alkaline methanolic titanium-zirconium alkoxide solutions were hydrolyzed with various acetone/water mixtures to establish the
effect if any of the acetone : water ratio on the surface area of precipitated sodium titanate-sodium zirconate (ST-SZ). Samples of ST-SZ were prepared in batches by hydrolyzing alkaline mixtures of zirconium tetra-n-butoxide (3.25g), titanium tetraisopropoxide (20.96g) and 13.3 wt% methanolic NaOH solution (12.38g) in an 16 acetone:water mixture. The acetone : water ratio varied from l to 10.
The ST-SZ product was filtered on a Buchner funnel, washed with two bed volumes of acetone and dried under vacuum at room temperature for 16 hours. The surface area of individual powders was determined by the BET technique on a Perkin-Elmer Sorptometer Model 212D.
Precipitated ST-SZ from the hydrolysis of the metal alkoxides was
exposed to atmospheric carbon dioxide to establish the extent of Co 2 sorption and ultimately to examine the effect on the surface area of
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the material. Exposure to static air ranged from between 10 min and 400 min. An identical experiment was conducted in which SYNROC B powder was exposed to air for the same time interval. The carbonate concentration was determined as was the surface area of each sample using the BET method.
2.3 Sorption/Ion-Exchange Reactions
To investigate the sorption behaviour of ST-SZ powder a series of reactions between this material and aqueous Al, Ba, Ca nitrate solution was studied. The suspension was maintained at pH= 9 with reaction times between 2 min and 100 min.
A similar series of reactions between ST-SZ and the same metal nitrate solution was conducted over a pH range of l to 10 and a 2 hour reaction time. Another series of reactions was performed using hydrated Ti0 2 -Zr0 2 over the same pH range and reaction time.
2.4 Simulated HLW addition to SYNROC B
Acidified PW-4b-B simulated HLW (Table 6) which is a modified version of PW-4bt (Table 4) was thoroughly mixed with SYNROC Bin the proportions needed to yield 20 wt% waste oxides (anhydrous basis). The slurry samples containing 1, 2, 3 and 6M 'free' nitric acid were kept agitated for 2 hours. Small volumes of concentrated HN0 3 or NaOH were used for pH adjustment. The samples were centrifuged to separate the solid-liquid phases and the supernatant liquors were analyzed by atomic absorption (AA techniques). All pH measurements were made using a Radiometer Autocal pH meter Model pHM83. t PW-4b HLW is derived from US reprocessed irradiated nuclear fuel elements by the PUREX solvent extraction process using TBP. 34
SYNR0C C (20 wt% waste oxide loading) was prepared from acidified PW-4b-B simulated HLW (1,2,3 and 6M) by thoroughly mixing the HLW and SYNR0C B until a thick (straw coloured) slurry was produced.
The slurry was flash-dried and calcined in a stream of Ar/3.5% H2 at - 75o 0 c for 1 hour. The calcined powder (light grey) was placed inside a steel can, inserted in a graphite die and hot-pressed at - 1150°c for 2 hours under 13.BMPa pressure.
The surface area of powders was determined by the BET technique, as described above. Particle size distributions were measured using the Malvern laser particle size analyzer Model 3600D with the powder sample dispersed inn-hexane.
2.5 Electrophoretic mobility measurements
The samples ST-SZ, Ti0 2-Zr0 2 (hydrate) and SYNR0C B {Sandia) were characterized in terms of the isoelectric point (iep)t using the Rank Brothers Particle Micro-Electrophoresis apparatus.
To study the electrokinetic behaviour of the above materials separate mobility measurements were made at pH values range from 3 to 10. The pH of the suspension was controlled with sodium hydroxide and nitric acid, and all inorganic chemicals were A.R. grade. To ensure good dispersion of the solids the suspensions (solid/liquid ratio= 0.005/100) were conditioned by degassing under special grade argon for 24 hours whilst agitating the samples using magnetic stirrers before conducting electrophoresis measurements as above. The pH of t The isoelectric point (iep) of inorganic surfaces is defined as the solution pH of the net zero charge of the interface. 35
aqueous suspension was monitored using a Radiometer Autocal pH meter Model pH M 83.
The first series of experiments was done with suspensions of ST-SZ,
Ti0 2 -Zr0 2 (hydrate) and SYNROC B (Sandia) in demineralized water.
The BET surface areas of the ST-SZ, Ti0 2-Zr02 (hydrated) and SYNROC B powders (determined by the three-point method using the Perkin-Elmer Sorptometer at liquid-nitrogen temperature) were 150,
200, and 150 m2/g respectively. 36
Chapter III
RESULTS AND DISCUSSION 37
3. RESULTS AND DISCUSSION
3.1 Metal alkoxide hydrolysis
When metal alkoxides M(OR)n, are reacted with an excess of water two 23 21+ simultaneous reactions take place: '
(a) Hydrolysis
(ROh M- OR + HOH + (R0)3 M-OH + ROH ( 6)
(b) Polymerization
where Mis a four-valence metal and R is an alkyl radical, C H • X 2 X + 1 In most alkoxide systems, hydrolysis and condensation reactions rapidly proceed until one of the reacting groups, OR or OH, is completely consumed.
Under a given hydrolysis condition, the relative concentration of OH and OR depends on the availability of water,ageing, dilution and type of alkoxide, and the host liquor.
The exact mechanism for metal alkoxide hydrolysis is not known. However, the following reaction sequence is a reasonable theory. It supposes that the initial step involves the coordination of a water molecule through its oxygen atom to the metal in a facile nucleophilic 25 process. 38
H~ 0: + M(OR)x
H /
-+ M(OH)(OR)x-l + ROH (8)
One of the protons on the water molecule interacts with the oxygen of an alkoxide group through hydrogen bonding and, following an electronic rearrangement, a molecule of alcohol is liberated.
3.1.1 Effect of acetone: water ratio on the surface area of ST-SZ
Surface area results (Fig.10) showed a small decreasing trend after an initial acetone: water ratio of 0.1. The decline continued until a ratio of 2.6 was reached. Surface area then increased and finally attained a value of 130 m2g- 1 , i.e. 8 m2g- 1 higher than the value obtained at the 0.1 ratio. It appeared that powder reactivity was relatively insensitive to variation in acetone: water ratio. Dosch et a1 16 found that changes in water: acetone ratio can produce order-of-magnitude differences in surface area.
From the curve (Fig. 10) it appears that as the quantity of hydrolysis water is increased the surface area is lowered. This apparent decrease in surface reactivity is not dramatic for ST-SZ powder but according to Dosch et al 16 , ST powder prepared under similar conditions shows large variations in surface area.
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Effect Effect
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...... _ ...... _
FIGURE FIGURE
I I
I I
0 0
20 20
60 60
uo uo
0 0
e e I I
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C C
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-
...... 40
The differences in surface area between ST-SZ and ST are most probably due to the presence of Zr in the structure of ST-SZ. During hydrolysis Zr has a tendency to repel hydroxyl groups 26 and results in preferential formation of a polymeric network of oxo and aquo bridging groups. 27 The network which is formed between hydrated Ti and Zr species may be of such a form,that ligand water molecules or hydroxyl groups may occupy a significant number of available surface sites, thus restricting the sorption of nitrogen molecules during surface area measurements.
Due to the complexity of metal ion hydrolysis, a detailed study as to why these differences in surface reactivity occur would require sophisticated instrumental techniques such as SEM, XRD, OTA and Raman spectroscopy etc.
3.1.2 Removal of entrained alcohols after metal alkoxide hydrolysis
Residual carbonaceous material is undesirable during SYNROC hot pressing procedures as this causes increased open porosity in the sample.
The metal alkoxide hydrolysis step generates the corresponding alcohols (i.e. isopropanol, n-butanol); methanol is already present and comes from the methanolic sodium hydroxide solution used in ST-SZ preparation. Washing the product with two bed volumes of acetone removes> 90% of the alcohol.
Infrared spectrat of all samples show the efficiency of the washing step (Figs. 11 & 12). t Infrared spectra were measured on a Jasco infrared spectrometer Model A-302.
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1100 1100
1230 1230
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1
1360 1360
from from
(cm-
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1710 1710
the the
WAVENUMBER WAVENUMBER
of of
spectrum spectrum
Infrared Infrared
3000 3000
3500 3500
11. 11.
FIGURE FIGURE
<( <(
I-
~ ~
z z
a:: a::
(/) (/)
(/) (/)
- 0 0 (/) (/) z z ~ ~ - - t 1420 z 0
Cl) Cl)
:E Cl) z 1220 1710 _, WAVENUMBER (cm ) FIGURE 12. Infrared spectrum of the filtrate after washing ST-SZ with acetone 43 3.1.3 Effect of carbonate concentration on the surface area of ST-SZ and SYNR0C B powders The sorption of atmospheric carbon dioxide by both ST-SZ and SYNROC B powders increases with exposure time in air (Fig. 13) and reaches a maximum after approximately 300h (- 4.5 wt% for ST-SZ and - 3 wt% for SYNR0C B respectively). The measured BET surface area is significantly lowered, for ST-SZ, falling from 150 m2g- 1 to 85 m2g- 1 after l00h. In the case of SYNR0C B,surface area declines from 150 m2g- 1 to 90 m2g- 1 for the same exposure time (Fig. 14). The carbonate species that is likely to form on-the particle surface can possibly be due to (1) the 'free' ion, (2) monodentate form or (3) the bidentate form: 28 2- M M M ( 1) (2) (3) The 'free' C0 3 ion is a planar XY 3 species and, of the six possible vibrational modes, a maximum of four can be observed owing to degeneracy. Three bands are usually observed in the infrared spectrum for C-0 stretching frequencies, one at 1440 cm-1 and two below 900 cm- 1 • According to Hair28 the surface carbonates contain the monodentate species. 44 8 • ST-SZ • SYNROC B 6 -,: z 0 ~ a: 4 z1- ...u z 0 u ... ~ z 0 a:i 2 a: "'u OL-----....l.------..L.------.L...------'------'0 10 0 200 300 400 500 EXPOSURE TIME (hours) FIGURE 13. Sorption of carbon dioxide by ST-SZ and SYNROC B powders after exposure to air. .i::,, .i::,, CJ'1 CJ'1 ~ ~ ______ .__ .__ B B 5 5 SYNROC SYNROC for for ) ) ______ S.andia S.andia ST-SZ ST-SZ • • .._ .._ • • 0 0 wt'l wt'l profile profile ( ( 3 3 CONCENTRATION CONCENTRATION concentration concentration powders powders ______ 2 2 B B _., _., CARBONATE CARBONATE carbonate carbonate vs vs SYNROC SYNROC area area and and Surface Surface ST-SZ ST-SZ ______ ~ ~ a a 14. 14. a.__ a.__ 0 0 50 50 10 10 uot- E E .. .. Ill Ill C C a: a: ::I ::I C C ...... u u a: a: ...... ...... C C i i ...... - - FIGURE FIGURE 46 From the surface area vs carbonate concentration curve (Fig. 14), it is obvious that after the preparation of both ST-SZ and SYNROC B these materials should be sealed in airtight containers in order to avoid deterioration of powder surface reactivity. 3.2 Sorption experiments 3.2.1 Kinetics The sorption of cations (Mn+) by hydrated metal oxides is frequently found to be extremely rapid, most of the exchange occurring within a matter of minutes. 29 Morgan and Stumm 30 found that Mn 2+ sorption (and H+ release) by dispersed colloidal Mn0 2 was very rapid, but in the presence of 10- 2M NaC10 4 the reaction rate was much slower. It was suggested that in l0-2M NaC10 4 , the colloidal Mn0 2 was flocculated and sorption involved diffusion of Mn 2+ to less readily available sites. Kurbator and Wood 31 also found that the rate of uptake of Co 2+ by coprecipitated Fe gel decreased with increased inert electrolyte concentration, but these results were not consistent with previous findings 32 and may have reflected gradual pH changes. Other workers in the field have described the approach to equilibrium for these sorption reactions in terms of one or more first- ord er reac t 1ons.. 29 The sorption of Al 3+, Ba 2+ and ca 2+ by freshly precipitated ST-SZ proceeded rapidly within the initial 10 minutes of reaction. Ba 2+ sorption proceeded at a faster rate then either the Al 3+ or Ca 2 + (Fig. 15). After the first 5 minutes of reaction the residual amounts of Al 3+, Ca 2 + and Ba 2+ were 0.5, 0.6 and 0.05 wt% of the initial += " " by by these these stoichiometry. stoichiometry. 0 0 10 10 species species SYNROC SYNROC + + + + 2 2 All+ All+ Ca Ba containinq containinq • • 0 0 • • • • 80 80 desired desired hydrated hydrated Ba Ba the the solution solution a a Ca, Ca, give give from from 60 60 (min.) (min.) Al, Al, to to of of - 9 TIME TIME • • pH pH 40 40 exchange exchange concentration concentration REACTION REACTION constant constant a a at at at at 20 20 ions ions ST-SZ ST-SZ Sorption/ion Sorption/ion 15. 15. 0 0 L L 0 0 FIGURE FIGURE 1.0 1.0 a: a: Cl. Cl. z z ...... u u ...... z z ...... 0 0 I- ...... .,j .,j 0 0 z z u u u u "" "" z z ...... ~0.5 ~0.5 a: a: z z 0 0 48 concentration respectively. The sorption /ion-exchange rate for aqueous metal ions is related to the hydration energy 6G of the particular species in solution by the expression for divalent ions NZ~e 2 1 0 I ( 10) 2r. £ ] 1 where the ions are assumed to be charged hard spheres, N is Avogardro's number, Zi the ionic charge, e0 the charge, r; the radius of ion i and £ the dielectric constant of water. Voet 33 has shown that the hydration energies of alkaline earth metal ions are consistent with the Born 34 model when r'. is given by equation 1 ( 11) I r. = r~ + 6 ( 11) 1 1 where r~ is the Goldschmidt 35 crystal radius of the ion and 6 = 0.7 ~ for alkaline-earth metal ions. Goldschmidt 35 and Pauling 36 radii for Ba 2+ and Ca 2+ are - 1317.1 and - 1591.7kJ mol- 1 as calculated by equation (11) respectively and since the 6G 2 is inversely proportional to the ionic radius, Ca 2+ has the highest (negative) hydration energy whereas the larger Ba 2 + ion has a smaller hydration energy. Consequently, removal of water molecules from solvated Ca 2+ ions requires more work. Hence Ba 2+ ions, which have the least negative hydration energy are sorbed at a much faster rate. 49 3.2.2 Residual sodium removal of SYNROC B The removal of sodium from SYNROC B filtercake was attempted by reslurrying the SYNROC B with demineralized water and filtering on a Buchner funnel. However, this method proved to be unsuitable (Fig. 16), the most plausable explanation being that channelling through the bed made it difficult to wash out the more strongly adsorbed sodium. A second technique for removing sodium proved successful and involved reslurrying the SYNROC B filtercake with demineralized water and then centrifuging the slurry rapidly. This gave a more efficient extraction of sodium, as can be seen from (Fig. 16). Centrifugation reduced the sodium content of SYNROC B down to levels of between 800 and lOOOppmNa (Table 7). The washing procedure depletes Ca and Ba by - 20-25% and 1-2% respectively from the SYNROC B product. This loss is not desirable, as it alters the chemical composition of SYNROC B. 3.2.3 Residual nitrate removal from SYNROC B The infrared spectra (Fig. 17) shows the reduction of nitrate concentration after washing SYNROC B with demineralized water. It is noted that after the seventh wash, nitrate ion is not detected by the IR spectrometer. The spectrum for sample No. 7 implies a residual nitrate concentration in SYNROC B - ~ 5 wt% (50,000 ppm ).t However, the usual washing regime for SYNROC B preparation requires 10 washings of the filtercake thereby reducing the nitrate level down to much less than the 5 wt% quoted above. t The detection limit of the infrared spectrometer is - 5%. 50 4 ' \ ' \ ' \ A VACUUM FILTRATION '\ 3 \ e CENTRIFUGATION \ ..... \ 'all \ \ -I \ \ --z Q ~ a:: 2 ' zI- ...u z 0 u .... ~ ::, 0 0 VI ~. ~ . ·--./~.- 01______L.______L.______L.______...______,J 0 2 4 6 8 1 0 WASH-WATER VOLUME (litres) FIGURE 16. Removal of residual sodium from Sandia SYNROC B filtercake. 51 TABLE 7 RESIDUAL SODIUM CONTENT IN SANDIA SYNROC B FILTERCAKE a b Na(wt %) Wash-Watert Na(wt %) Wash-Water t Volume Volume (L) ( L) 2.4 1 2.0 l 1.0 2 1.4 2 0.5 3 0.8 3 1.5 4 0.3 4 0.9 5 0.2 5 0.4 6 0. 1 6 0.3 7 0. l 7 0.7 8 0.08 8 0.3 9 0.08 9 t The wash-water was demineralized water in each case. (a) Buchner funnel method. (b) Centrifugation technique. 52 2 3 ...... ,.~ 4 z 0 (/) (/) :!: (/) z 5 < IX I- 6 7 4000 2000 1600 1200 8 0 0 WAVENUMBER ( cm-1 ) FIGURE 17. Infrared spectrum of SYNROC B showing nitrate ion depletion with washing. Nitrate vibrational frequency occurs at 1340 cm- 1 • 53 The sorption of Al 3+, Ba 2+ and Ca 2+ by ST-SZ (Fig. 18) is very similar to the sorption behaviour for these species by hydrated Ti0 2 -Zr0 2 (Fig. 19). Sorption of metal ion species from aqueous solution follows the typical sigmoidal form for all species with ST-SZ as the sorbent. However,for Ti0 2 -Zr0 2 sorption of Ca 2+ does not follow the same trend. For the system Al 3+ {aqueous)/ST-SZ at between the narrow pH range 3.4-4.0 there is an abrupt increase in sorption of hydrated A1 3+ and a similar pattern is observed for Ba 2+ {aqueous)/ST-SZ in the pH range 4.5 - 5.5. However, with ca 2 +, sorption occurs over a wider range of pH and is not as dramatic as with A1 3+ and Ba 2+. Similarly, the affinity for aqueous Al 3+ and Ba 2+ by Ti0 2-Zr02 sorbent (Fig. 19) shows increased sorption at between pH 3.5 - 5.0 and pH 5.0- 7.0 for Al 3+ and Ba 2+ respectively. Ca 2+ sorption by Ti0 2 -Zr02 again occurs over a wide range of pH and does not follow the same trend as the Ca 2+ {aqueous)/ST-SZ system. From the experimental data (Figs. 18 & 19) it is apparent that Al 3+ and Ba 2+ undergo hydrolysis at a much faster rate than Ca2 +. According to James and Healy 37 as the hydrolysis proceeds the ionic charge Z decreases, the coulombic 6G~oul and chemical energy 6G~hem contributions dominate the sorption energy change and hence the affinity for the metal ion species by the surface is greatly enhanced. Chemical analysis of the filtrate from the reaction between A1 3+, Ba 2+ and Ca 2+ nitrate solution and ST-SZ shows an increase in the concentration of sodium~ 100% thus indicating ion-exchange processes rather than a direct sorption mechanism. ~ ~ u, u, I I species species 10 10 i i I I I I hydrated hydrated Ca Ca I I I I iii iii Ba, Ba, Al, Al, :;::::s= :;::::s= I I i i with with I I 6 6 I I ST-SZ ST-SZ I I for for 0 0 PH PH i7 i7 I I 4 4 ~ ~ profile profile pH pH vs vs -t: -t: ST-SZ ST-SZ I I 2 2 120min. 120min. Sorption Sorption = = 1 1 18. 18. I I I I I sorp. sorp. 1 HYDRATED HYDRATED 0 0 t- FIGURE FIGURE I I 111 111 o o 20 20 40 40 60 60 80 80 o o , , 0 0 0 0 Ill Ill a: a: ~ ~ z z a. a. 0 0 - -. -. o'- U'1 U'1 U'1 U'1 I I 1 0 0 1 _x= _x= Ca Ca Ba, Ba, I I P P Al, Al, C C with with 2 2 -zro I I 6 6 2 Ti0 for for I I PH PH I I 4 4 profile profile pH pH 2 2 species. species. vs vs ZrO I I - 2 TiO I I hydrated hydrated Sorption Sorption 2 2 "'-====:::L..--....1.---1---L---.L---.J.---.i....-~ "'-====:::L..--....1.---1---L---.L---.J.---.i....-~ 120min. 120min. 19. 19. = = __ __ I I ._ ._ sorp. sorp. HYDRATED HYDRATED 1 __ __ FIGURE FIGURE 0 0 OL_ OL_ 20 20 80 80 40 40 60 60 1001 1001 z z 0 0 II) II) 0 0 - a: a: ~ ~ I- a< a< ...... - 56 Although, at high metal ion concentration it is difficult to determine whether the cation is sorbed or precipitated, the percent uptake vs pH curves usually show two features that suggest sorption is the dominant mechanism. 29 Firstly, the sorption curves shift to a lower pH as the initial cation concentration is decreased, which is contrary of what would be predicted for hydroxide precipitation. Secondly, the tailing of the curves on the low pH side of the inflection point (Figs. 18 & 19) is characteristic of sorption but not of precipitation as predicted from a strict solubility product approach. 29 From the potentiometric titration data for aqueous Al 3+, Ba 2+ and Ca 2+ nitrate at 25°c (Fig. 20) it is immediately apparent that both Ca 2+ and Al 3+ form complex hydrolysis species as the solution pH is increased. Messmer and Baes 38 have suggested species of the type 5 Al 2 (0H) 2 4 + and A1 3 (0H) 4 + in the case of A1 3+ in aqueous solution. Kubota 39 performed measurements using low concentations and showed Al(OH) 2+ to be the only hydrolysis product. Ca 2+ hydrolysis leads to complex formation which is most probably the species Ca(OH)+ and the Ca 2+ concentration falls by 31% of the initial concentration at pH 9. When ST-SZ powder is dispersed in demineralized water the suspension acquires a pH - 9 at room temperature. This means that formation of transient hydroxy complexes of both A1 3+ and Ca 2+ is likely, due to localized regions of high pH on the particle surface. Such complex species of Ba 2 + are not expected because of the high solubility of Ba 2 + salts. 57 Ca I - - - • 0 0 • 0 X ,-- 1__, . ea -e., z Ba • ...0 ------•·'------■-• I ' 4( • ...ac z ""u 4 z 0 u z - - - - - 0 -' 2 ...4( "". :E PH FIGURE 20. Potentiometric titration data for aqueous Al, Ba, Ca nitrate solution at 25°C 58 3.3 Electrokinetic measurements During electrophoresis experiments a particle suspended in water migrates when an electric field is applied by two immersed electrodes. The velocity of the particle is proportional to this electric field (E). The proportionality constant is called the electrophoretic mobility (JJ). The Smoluchowski theory in its modified form 40 relates the elctrophoretic mobility to the 'zeta' potential (~), which is the electric potential at a 'shear' planet near the surface of the particle. The iep's for ST-SZ, hydrated Ti0 2 -Zr0 2 and Sandia SYNROC B were determined from electrophoretic mobility measurements and found to occur at pH's 3.5, 4.2 and 5.7 respectively. It is apparent from these data that ST-SZ is capable of sorbing cations from solution at lower pH values (Fig. 21). Addition of simulated PW-4b-B HLW containing 'free' nitric acid caused loss of SYNROC B matrix elements Al 3+, Ba2+, Ca 2+ {Fig. 22) into solution, the effect increasing rapidly with lowering of HLW solution pH. At pH =l, - 85 to 90% of the Ba 2+ and Ca 2+ species went into solution. Loss of Ca 2+ (- 52%) still occurred at pH =5. However, Ca 2+ desorption is predicted to cease at pH - 7-8. t The shear plane is the furthermost distance from the surface where the liquid is stagnant. u, u, I.O I.O 8 8 B. B. SYNROC SYNROC 7 7 B B Sandia Sandia and and 2 2 2 2 SYNROC SYNROC • • -zro 6 6 2 -zro 2 Ti0 ----.....::::: ----.....::::: Sandia Sandia ST-.SZ ST-.SZ Ti0 ■ • • .,. .,. ST-SZ, ST-SZ, 5 5 for for (iep) (iep) pH pH points points 4 4 Isoelectric Isoelectric 3 3 :------ 21. 21. "- FIGURE' FIGURE' 2 2 1 1 JL JL 0 0 2 2 4~ 4~ -4 -4 -3 -3 _, _, ;J. ;J. E E Ill Ill > > ~ ~ 0:::: 0:::: u.J u.J u u 0:::: 0:::: :::r: :::r: ...... g,-z g,-z u.J u.J 0 0 ::::E ::::E a::i a::i ~ ~ ...... ....J ....J 0 0 ...... >- ...... ...... 0 0 ...... - - I I I I I I co co ...... ('J ('J ...... 60 100------..------. • AIJ+ D Ba2+ 80 ■ ca2+ z 0 .... 0.. 0::: 0 I.I) UJ O 60 l'CI u +- N l'CI DJ + _40 M .... z UJ u 0::: ~ 20 oL_ _l__ ----1. ___1__-=:t::=-.L....a.._J 0 1 2 3 4 s 6 PH FIGURE 22. Desorption of SYNROC B matrix elements vs pH 61 This may well explain why simple water washing (pH - 6.5) of SYNROC B leads to loss of Ca 2+ and relatively lower Ba 2+ desorption. Loss of Ca 2+ is expected from the SYNROC B surface since it has been shown by Fuerstenau et al 41 that the affinity for divalent ions (eg Ba, Sr, Ca and Mg) for the Ti0 2 (rutile) surface (pH.,ep =5.8) was found to follow the sequence Ba> Sr> Ca> Mg. 3.4 Sorption capacity of SYNROC B for Cs and Sr In the absence of other HLW species the sorption capacity of Sr and Cs at the iep for SYNROC B (pHiep =5.7) is - 0.1 meq.g- 1 (Fig. 23). 0 The crystal radii for Cs and Sr are 1.67 and 1.12A respectively. According to hydration theory33 Sr ions should undergo sorption far more readily than solvated Cs ions since the hydration energies are - 1445.9 and 255.4 kJmol-1 for Sr and Cs respectively. However, it appears that at low pH Cs undergoes sorption quite readily, but with increasing pH, Sr sorption increases. It is quite 4 probable to suspect that Sr which forms Sr(OH) 2 (K sp = 3 x 10- ) is merely hydrolyzing and forming the hydroxy metal complex. 3.5 Effect of excess nitric acid on SYNROC C properties 3.5.1 Powder properties The surface area (BET) of the calcined SYNROC powder decreased dramatically as the HLW pH was lowered (Table 8), from 13.7 to 5.1 but then increased to 5.7 m2 g- 1 with 6M excess acid. The particle 62 -14 ·12 t 12 0 mi n, .... s or p . -I O') . ·10 r::, II E -> I- ·O 8 u et Q. et Cs u ·O 6 z 0 I- Q. 0:: ·04 0 C/) Sr ·O 2 -□ _ _..__~_--:-". OL-----1---L--..L.--.IL----L--..J..--..__ 0 2 J 4 5 6 7 8 9 1 0 PH FIGURE 23. Sorption capacity of SYNROC B for Cs, Sr vs pH 63 size distribution showed the reverse trend (Table 8), the modal size initially increasing from 14.5 - l8.5µm to peak at 160.4 - 26l .4µm before decreasing to 50. 2 - 64. 6µm. The tap density of the powder increased from 0.8 to 1.2 g.cm -3 • Calcined SYNROC C powder derived from 0.5 molar PW-4b-B simulated HLW had a surface area 4 times greater than for the 6 molar-derived powder. However, the modal size was identical for both samples. 3.5.2 SYNROC fabrication The observed changes in powder properties did not appear to have any significant effect on hot-pressed density (Table 8), although any such effects may have been masked by the initial prepressing procedure. SYNROC microstructure was apparently not affected, although the desorption of matrix elements during initial HLW addition created the potential to enhance segregation during fabrication processes. The pellets hot-pressed from the 3 and 6 molar-derived powder appeared to be brittle and more susceptible to cracking. Contrary to this apparent brittleness, the 6 molar-derived product exhibited the most favourable leach resistance (Table 8). TABLE8 EFFECTOF EXCESSHNO3 IN HLWON SYNROC PROPERTIES Leach Ratet Excess pH pH Surface Modal Hot-Pressed (g.m-2.d-l) HNO3 Area Particle Tap (M) SYNROC (m2.g-l) Size Range Density Density Simulated Cs Sr HLW Slurry (µm) (g.cm- 3 ) (g.cm- 3 ) 0.5 2.5 4.8 25.0 50.2 - 64.6 0.7 4.40 0.15 0.05 1.3 3.4 13.5 14.5 - 18.5 0.8 4.46 0.24 0.07 2 0.9 1.2 9.1 64.6 - 84.3 l.3 4. 51 1.10 0.37 3 0.5 0.8 5.1 160.4 - 261.6 1.2 4.45 0.30 0.03 6 0. l 0.2 5.7 50.2 - 64.6 l • 2 4.48 0.07 0.03 t Leach data were supplied by Dr D Levins. °'.i:,. 65 CONCLUSION An investigation into the use of the Sandia process for the preparation of SYNROC has been completed. Comparison of surface morphology (eg surface area) between Sandia and oxide route SYNROC powder after calcination clearly indicates the former material to be superior. The Sandia process itself is simple and utilizes established chemical process technology and SYNROC produced by this method is free of contaminants (eg sulphur and silica). Ball-milling metal oxide powders to prepare SYNROC has the inherent potential of introducing undesirable impurities which are likely to produce a low grade ceramic product. The Sandia process however, has several disadvantages: (a) difficulty in controlling the bulk chemical composition of the final product because of some Ca and Ba losses during the preparation; (b) generation of large volumes of organic solvents especially if SYNROC is prepared on a large scale. This would require reclaiming and recycling of these liquids to minimize costs of production. Apart from these disadvantages, the Sandia process has the distinct advantage of producing highly reactive powder which sinters and 66 therefore can be hot-pressed at lower temperatures(~ 1150-1200°C) than oxide-derived material. 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