- Email: [email protected]
Evaluation of tailings stabilization methods as applied to uranium tailings
HydrometaUurgy, 12 (1984) 31--48
31
Elsevier Science Publishers B.V., Amsterdlm -- Printed in The Netherlands
E V A L U A T I O N OF TAILINGS STABILIZATION METHODS AS APPLIED TO U R A N I U M TAILINGS
V.I. LAKSHMANAN, L.M. LUCKEVICH,
Ontario Research Foundation, Mississauga, Ontario (Canada) G.M. RITCEY and J.M. SKEAFF
CANMET, Ottawa, Ontario (Canada) (Received May 9, 1983; accepted June 22, 1983)
ABSTRACT Lakshmanan, V.I., Luckevich, L.M., Ritcey, G.M. and Skeaff, J.M., 1984. Evaluation of railings stabilization methods as applied to uranium railings. Hydrometallurgy, 12: 31--48. In an attempt to obtain an environmentally acceptable uranium tailings disposal method, four commercial waste solidification processes were tested. These methods used inorganic and asphaltic binders. The solidified tailings were evaluated by measuring their physical and chemical properties immediately after solidification and as a function of time. Effluent from none of the solidified tailings during the tested period (30 days) contained less than 10 pCi/L. However, the test results indicated asphalt binder as superior but more expensive by an order of magnitude than a lime based method. Hence the lime based method was considered to be the best compromise, yielding relatively good properties at reasonable costs.
INTRODUCTION
Many types of metal extraction processes, including uranium extraction, produce tailings that require disposal. In general, mine-mill tailings are disposed of by: • i m p o u n d m e n t on land, • dumping into marine or inland water, • use as a construction material, or • use as a mine backfill. However, uranium milling is unique in that it requires the handling and disposal of large quantities of low-level radioactive materials. The special hazards presented by uranium taflings such as radioactivity, acidity and toxicity may prevent direct disposal by these usual methods. Proper management of uranium tailings involves environmental control with respect to: • ground water seepage, • surface water run-off, • 222 Rn diffusion,
0304-386X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
32 • air-borne dust (dry}, and • dam breakage. The Ontario Research Foundation, in a CANMET sponsored project (DSS contract serial No. OSQ80-00131} has assessed alternative methods of tailings management as applied to uranium milling. More specifically, the focus of the program was to investigate the stabilization (via solidification} of uranium mine-mill tailings so that impoundment on land is environmentally acceptable. The objectives of this program were to: (1) Acquire information and review waste stabilization technology as applied to uranium mine tailings. (2) Acquire and characterize uranium mine tailings. (3) Solidify uranium mine tailings b y applying existing solidification technology. (4) Evaluate the solidified tailings with an assessment emphasis on radionuclide release. 2. BACKGROUND INFORMATION
2.1 Origins and nature o f uranium railings Uranium is extracted from ore by either acid (H2SO4) or alkaline (CO~-) leaching. After extraction, residual uranium, pyrite, metals (Se, Pb, As, Cd, Mo), inorganic salts (F-, NO3-, C1-), and several decay products are found in the leach residues. The radioisotopes of most concern are 226Ra, 23°Th and 21°Pb. Systematic studies by various workers have shown that the majority (80--90%) of the 226Ra is in the fine fraction ( - 2 0 0 mesh) of the tailings solids [ 1,2]. Pyrite and pyrrhotite cause problems in the long term because their oxidation products (H2SO4 and Fe 3÷} solubilize radionuclides and heavy metals, hence increasing their migration to the surrounding environment~ [ 3]. Short-term management of uranium tailings usually consists of impoundment behind dams or embankments. Alternatives to i m p o u n d m e n t (marine/fresh water disposal, use as construction material or as mine backfill) are not y e t available because of the radioactive nature of the tailings.
2.2 Waste stabilization technology Waste stabilization denotes the conversion of toxic components in a waste to a chemical form that is resistant to leaching. Waste solidification denotes rendering o f a liquid or semi~solid waste into a cohesive solid. Often waste solidification results in waste stabilization and vice versa. In the case of uranium mine tailings, both stabilization and solidification are necessary. Waste solidification methods applicable to low or medium toxicity wastes can be grouped into the following categories: • silicate and cement based {inorganic binder), • lime based (inorganic binder),
33 TABLE 1 Advantages and disadvantages of solidification processes [4] Process
Advantages
Disadvantages
Cement-based
1. A d d i t i v e s are available at a r e a s o n a b l e price. 2. C e m e n t m i x i n g a n d h a n d l i n g t e c h n i ques are well d e v e l o p e d . 3. Processing e q u i p m e n t is r e a d i l y available. 4. Processing is r e a s o n a b l y t o l e r a n t o f c h e m i c a l v a r i a t i o n s in sludges. 5. T h e s t r e n g t h a n d p e r m e a b i l i t y of t h e e n d - p r o d u c t c a n be v a r i e d b y c o n t r o l ling t h e a m o u n t o f c e m e n t a d d e d .
1. L o w - s t r e n g t h c e m e n t - - w a s t e m i x t u r e s are o f t e n v u l n e r a b l e t o acidic leaching solutions. E x t r e m e condit i o n s c a n r e s u l t in d e c o m p o s i t i o n o f the fixed material and accelerated l e a c h i n g of t h e c o n t a m i n a n t s . 2. P r e t r e a t m e n t , m o r e e x p e n s i v e c e m e n t t y p e s , o r c o s t l y a d d i t i v e s m a y be necessary for stabilization of wastes containing impurities that affect the setting and curing of cement. 3. C e m e n t a n d o t h e r a d d i t i v e s a d d c o n siderably to weight and bulk of waste.
Lime-based
1. T h e a d d i t i v e s are g e n e r a l l y v e r y inex- 1. L i m e a n d o t h e r a d d i t i v e s a d d to weight and bulk of waste. p e n s i v e a n d w i d e l y available. 2. E q u i p m e n t r e q u i r e d f o r p r o c e s s i n g is 2. S t a b i l i z e d sludges are v u l n e r a b l e to s i m p l e t o o p e r a t e a n d w i d e l y available. acidic s o l u t i o n s a n d t o c u r i n g a n d 3. C h e m i s t r y o f p o z z o l a n i c r e a c t i o n s is s e t t i n g p r o b l e m s a s s o c i a t e d w i t h inwell k n o w n . o r g a n i c c o n t a m i n a n t s in t h e w a s t e .
Thermoplastic
1. C o n t a m i n a n t m i g r a t i o n r a t e s are g e n e - 1. E x p e n s i v e e q u i p m e n t a n d skilled rally l o w e r t h a n f o r m o s t o t h e r t e c h l a b o r are g e n e r a l l y r e q u i r e d . niques. 2. Sludges c o n t a i n i n g c o n t a m i n a n t s 2. E n d - p r o d u c t is fairly r e s i s t a n t t o m o s t t h a t volatilize a t l o w t e m p e r a t u r e s aqueous solutions. must be processed carefully. 3. T h e r m o p l a s t i c m a t e r i a l s a d h e r e well t o 3. T h e r m o p l a s t i c m a t e r i a l s are f l a m incorporated materials. mable. 4. Wet sludges m u s t b e d r i e d b e f o r e t h e y c a n be m i x e d w i t h t h e t h e r m o plastic m a t e r i a l .
Organic p o l y m e r
1. Only s m a l l q u a n t i t i e s o f a d d i t i v e s are u s u a l l y r e q u i r e d to c a u s e t h e m i x t u r e t o set. 2. T e c h n i q u e s c a n b e a p p l i e d t o e i t h e r w e t o r d r y sludges, 3. E n d - p r o d u c t h a s a l o w d e n s i t y as c o m p a r e d t o o t h e r f i x a t i o n techniques.
1o C o n t a m i n a n t s are t r a p p e d in o n l y a loose resin-matrix end-product. 2. C a t a l y s t s u s e d in t h e u r e a - - f o r m a l d e h y d e p r o c e s s are s t r o n g l y acidic. Most m e t a l s are e x t r e m e l y soluble at l o w p H a n d c a n e s c a p e in w a t e r n o t t r a p p e d in t h e m a s s d u r i n g t h e p o l y m e r i z a t i o n process. 3. S o m e o r g a n i c p o l y m e r s are b i o d e gradable. 4. E n d - p r o d u c t is g e n e r a l l y p l a c e d in a c o n t a i n e r b e f o r e disposal.
Encapsulation
I.
1. Materials u s e d are o f t e n e x p e n s i v e . 2. T e c h n i q u e s g e n e r a l l y r e q u i r e specialized e q u i p m e n t and heat t r e a t m e n t to f o r m the jackets. 3. T h e sludge has t o b e d r i e d b e f o r e t h e Process c a n b e a p p l i e d . 4. C e r t a i n j a c k e t m a t e r i a l s are f l a m mable.
V e r y s o l u b l e c o n t a m i n a n t s are t o t a l ly i s o l a t e d f r o m t h e e n v i r o n m e n t . 2. Usually n o s e c o n d a r y c o n t a i n e r is reqnired because the coating materials are strong and chemically inert.
• thermoplastic based (asphalt) (organic binder), • thermoset polymer based (organic binder), • encapsulation techniques {organic binder).
34 The advantages and disadvantages o f each of these processes are outlined in a table published by Pojasek [ 4], presented here as Table 1. Because of the large volumes of tailings requiring disposal and because they are of low radioactivity only the techniques based on inorganic and asphaltic binders were originally considered as economically feasible for the test program. Information regarding available waste solidification processes based on inorganic and asphaltic binders was obtained. Four of these methods were selected to be applied to uranium mine tailings; two were lime based, one was silicate based and one was thermoplastic based. The methods used were: • Atomic Energy of Canada Ltd. (AECL) bituminization (thermoplastic based), Canadian; • Canadian Waste Technology (lime based), Canadian; • Chemfix (silicate based), U.S.A.; • IUCS Poz-o-Tec (lime based), U.S.A. A brief discussion of each of the above is outlined below. 2.2.1 Chemfix The Chemfix process is a patented [ 5,6] system based on reactions between soluble silicates and silicate setting agents. The mobile reaction unit employed is capable of treating 450,000 L (100,000 gallons) or more of sludge per day at ambient temperature and pressure. The process consists of three stages: (i) waste homogenization; (ii) pumping of the waste to the reactor, mixing of waste and reagents, and discharge to disposal area; and {iii) solidification within 72 hours. The solidified product has soft-like properties and can be handled by standard earth moving equipment. 2.2.2 Canadian Waste Technology (CWT) The Canadian Waste Technology (CWT) solidification process is a patented system which "activates" silica in waste sludges so that a complex metal silicate matrix is formed [7]. The silica is present as clay minerals and sand. Treatment of such sludges with at least one of H2SO4, HC1, HNO3, CaO, MgO, CaC12, Ca(OH)2 and Mg(OH)2 (as determined by Canadian Waste Technology) yields a pourable slurry which solidifies over an unspecified time period. 2. 2. 3 IUCS Poz-o-Tec process The I.U. Conversion Systems (IUCS)Poz-o-Tec process is a patented [ 8] micro~ncapsulation process that involves the precise mixing of waste products, fly ash and lime-rich materials. The treated waste has the consistency of dry soil-cement and is compacted using vibratory rollers. After curing, compressive strengths of 7 MPa (1000 psi) and permeabilities of 0.13 nm s-1 (5 × 10 -9 in s-1 ) have been recorded. It should be noted that the treated waste is not pumpable. 2.2.4 AECL bituminization Bituminization consists of mixing the waste with asphalt (a petroleum byproduct composed of high molecular weight hydrocarbons), at 165--167 ° C.
35
Water originally present in the waste is volatilized at this temperature. In this particular case, the tailings were dried and ground prior to bituminization. 3. DESCRIPTION OF TAILINGS AND SOLIDIFICATION METHODS TESTED
3.1. Tailings characterization Two barrels of mine tailings were received from Denison Mines located at Elliot Lake, Ontario. In the Denison uranium recovery process, the acidleached slurry is h y d r o c y c l o n e d and overflow is discharged to thickeners. The thickener overflow is discharged to an unclarified pregnant storage tank; the TABLE 2 Sieve analysis of uranium mine tailings Size fraction (mesh)
Wt. percent
+48 -48 -65 -100 -150 -200 -325
4.16 7.84 9.23 2.50 17.28 10.34 48.66
+65 +100 +150 +200 +325
TABLE 3 Semi-quantitative chemical analysis (XRF) of uranium mine tailings Sample 1
Sample 2
Element
Approximate weight percent
Element
Approximate weight percent
Si AI S K Ca Fe Ti Na Mg Pb Zn Rb
high 3 2 2 2 1.5 (3.2)a 0.3 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1
Si Al Fe K Ca Ti P Mg Zn Se Cu Pb Ni
35.1 3.3 2.4 (5.2) a 1.69 1.41 0.22 0.05 0.07 trace trace trace trace trace
a% Pyrite if all Fe is present as FeS 2.
36
thickener underflow is combined with the hydrocyclone underflow and filtered. The filter cake is washed in two stages of drum filters. The second stage cake is washed with water, reslurried with neutralized barren solution and pumped to the tailings disposal area. The tailings are approximately 80% solids. A pipe sample of the tailings was submitted for sieve, X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis. Based on the 226Ra analysis (352 pCi/g) and the sieve analysis (Table 2), the sample of Denison tailings received can be considered typical. Hence, it is presumed that elutriation of fines containing major portions of the 226Ra did not occur. Analyses by XRF and XRD are given in Tables 3 and 4, respectively. Since the pyrite analysis by XRD and XRF was relatively low, the tailings were analysed for iron using atomic absorption spectroscopy. The results indicated 2.06 percent iron, thus giving an upper limit on the pyrite of 4.4 percent. These values correspond to those expected for Denison tailings. TABLE 4 X R D analysis of uranium mine tailings Mineral
Wt. percent
Quartz Muscovite (or similar mineral) Gypsum Pyrite Others
70-80 10--20 5--10 1--2 a < 1--2
aWet chemical analysis yielded a value of 4.4% pyrite. 4. E X P E R I M E N T A L MIXES
In all of the solidification experiments the actual tailings solidification was performed by process vendor personnel. Three of the experiments were performed at O.R.F. with O.R.F. personnel observing. The AECL bituminization was performed at the AECL Chalk River Laboratories. No O.R.F. personnel were present but a detailed description of the solidification was provided by AECL.
4.1 Chemfix Two samples of uranium mine tailings were solidified using the Chemfix process. The first, designated C1, mixed 18.0 kg of tailings (84.5% solids) 1.7 L H20, 0.74 kg inorganic setting agent, and 0.18 L type N sodium silicate solution. The second, designated C2, contained 12.0 kg of tailings (84.5% solids), 0.74 kg inorganic setting agent, and 0.60 L H20. After mixing 6.0 kg more of tailings were added. C1 was a thixotropic slurry, easily placed by hand; C2 was a stiff slurry, readily placed using a small concrete vibrator.
37
4.2 Canadian Waste Technology (CWT) Two samples of uranium mine railings were solidified using the CWT process. The first, designated KC (Krofchak Cast), mixed 18.0 kg railings (80% solids), 2.6 kg water and 0.9 kg reagent for 5 minutes in a standard concrete mixer. The second, designated KF (Krofchak Filtered), had an increased water content (to give a 30% solids slurry) and was filtered through a Perin plate and frame filter press. Mix KC was a thixotropic, pourable slurry easily placed by hand; mix KF was a thixotropic, stiff slurry readily placed with mild vibration.
4.3 L U. Conversion System (IUCS) process Two samples of uranium mine tailings were solidified using the IUCS process. The first, designated IU1, mixed 1 L H20, 4.8 kg of IUCS blend No. 1 and 4.267 kg tailings (84.5% solids). The second, designated IU2, mixed 0.6 L H20, 3.524 kg IUCS Blend No. 2 and 6.101 kg tailings (84.5% solids). Both IU1 and IU2 were extremely dry and required vigorous hand compaction to form a continuous body; the moisture content of the solidified tailings was estimated to be 15--16%.
4.4 AECL bituminization The uranium mine tailings were dried (110 ° C) overnight and ground. The dry powder was screw fed (10--11 kg/h) to a bituminizer (ASTM type No. II Roofing Asphalt) with a bitumen flow rate of 3--3.5 kg/h. Production rate ranged from 13--14 kg/h with the final product being 75--76 weight percent tailings solids. The product was discharged to 11 L wooden moulds; each sample weighed 20--21 kg. 5. E V A L U A T I O N OF SOLIDIFIED TAILINGS
5.1 Physical and mechanical evaluation 5.1.1 Flow properties o f material for placement The consistency of the materials will determine the type of placement method and, to a large extent, the product strength and durability. The consistency of cement mortars is most commonly measured by Flow Value (ASTM C109-77). This method was used for the Chemfix and Krofchak processes. The IUCS material was too dry for the test to be applicable. The flow of the AECL material would, of course, be temperature-dependent and no tests were run on this material. The water/solids ratio, percentage dry tailings and flow properties of all mixes are presented in Table 5.
38 TABLE 5 Water/solids ratio, percentage dry tailings, and flow properties of mixes Sample
Percent solids
Dry tailings Total dry weight
Flow valuea
Comments
KC KF AECL C1 C2 IU1
72.1 75.7 78.3 82.4 83.5
94.1 94.1 75.7 94.5 94.5 42.9
150 128 -136 68 -
IU2
84.7
60.0
--
Flows readily. Placeable by hand. Flows readily at 165 ° C. Easily placeable; almost flows. Placeable with vibration. Requires hand or vibratory compaction. Requires hand or vibratory compaction.
aAs per ASTM C109-77.
5.1.2 Shrinkage, compressive strength and flexural strength T h e s h r i n k a g e o f s t a b i l i z e d t a i l i n g s w i l l b e i m p o r t a n t in d e t e r m i n i n g t h e e x t e n t o f b r e a k - u p o f a l a r g e s t a b i l i z e d m a s s o f t a i l i n g s . T h e i n i t i a l m o v e m e n t as the samples dry or the stabilizing reactions take place, will be the most imp o r t a n t in t h i s r e g a r d . P r i o r t o d e m o u l d i n g t h e s a m p l e s , m e a s u r e m e n t s w e r e m a d e o f t h e s h r i n k a g e w h i c h t h e s a m p l e s h a d u n d e r g o n e a n d t h i s is e x p r e s s e d as a p e r c e n t a g e o f t h e o r i g i n a l l e n g t h in T a b l e 6. TABLE 6 Shrinkage, compressive strength, and flexural strength of samples Sample
KC KF AECL C1 C2 IU1 IU2
Process used
Shrinkage at demoulding (%)
Krofchak Cast a 1.6 Krofchak filtered a 1.9 Bituminization b <0.1 Chemfix I b 0.4 Chemfix 2 b 0.3 IUCS 1 a 0.1 IUCS 2 a 0.1
Compressive strength
Flexural strength
kPa
psi
kPa
psi
435 580 1080 1230 2590 9215 8030
65 85 155 180 375 1335 1165
190 135 1480 425 415 1370 1260
30 20 215 60 60 200 180
a All tests 28 days after casting. bAll tests 14 days after casting. T h e m e a s u r e m e n t a c c u r a c y w a s 0.1 p e r c e n t ; t h a t is, t h e A E C L s a m p l e showed no measurable shrinkage at the time of demoulding. The IUCS samples had been cured, covered with a plastic sheet until demoulding while the
39
CWT and Chemfix samples had been covered for the first week of curing, and thereafter were left exposed in normal laboratory air. All o f these curing conditions were arrived at in consultation with the representatives of the companies involved in the stabilization. Prior to demoulding, all Krofchak Cast (KC) samples developed large open shrinkage cracks. This was not observed in the Krofchak Filtered (KF) samples, although they also shrank considerably. Compressive and flexural strength were measured at 14 or 28 days and are presented in Table 6 as well. Flexural testing was performed on 7.6 cm (3 in) cubes as described in ASTM C293-77, "Flexural Strength of Concrete (Using Simple Beam with Center-Point Loading}". Samples were cut from the cast specimens with a nominal 7.6 cm × 7.6 cm (3 in X 3 in) cross-section and were tested over a 30 cm (12 in) span. The compressive tests were performed as described in ASTM Cl16-68, "Compressive Strength of Concrete Using Portion of Beams Broken in Flexure". Samples were nominally 7.6 cm (3 in) cubes. The AECL samples did n o t have a well-defined failure point, b u t were loaded to 10 percent deformation. 5.1.3 Freeze-'thaw resistance All samples were subjected to freeze--thaw cycling b y ASTM C666-77, "Standard Method o f Test for Resistance of Concrete to Rapid Freezing and Thawing". Procedure A, which involves freezing and thawing the samples imTABLE7 Summary of freeze--thaw tests (2 samples of each material tested) Sample
KC
Cycles at end of testing
Comments
A
0
B
6
KF
A
6
AECL
A B
128 128
C1
A B
11 11
C2
A B
5 5
IU1
A B
40 80
Severe spalling and disintegration. Severe spalling and disintegration.
IU2
A B
98 98
Severe spalling and disintegration. Severe spalling and disintegration.
Turned to mush during saturation period prior to freeze-thaw cycling. Broke prior to freeze--thaw test; turned to mush after 6 cycles. Broke after I cycle; turned to mush after 6 cycles. No visual change of surface; some warpage. No visual change of surface; some warpage. Ends disintegrating; severe spalling on one side. Ends disintegrating; one side turning to mush. Numerous cracks; shattered. Numerous cracks; shattered.
40 mersed in water, was used for all tests. Samples were cured for 14 days (AECL and Chemfix) or 28 days (IUCS or CWT) prior to testing. All samples were immersed for a minimum of 24 hours prior to beginning freeze--thaw cycling. An a t t e m p t was made to determine the length change o f the specimens on freeze--thaw cycling, but this was n o t possible due to surface scaling, or sample disintegration or warping. The results are summarized in Table 7. No quantitative comparison of the freeze--thaw samples was made. A qualitative classification of the freeze--thaw resistance (from observations) is that AECL is better than IUCS which is better than Chemfix which in turn is better than CWT. 5.1.4 Porosity and pore size distribution Measurements of porosity and pore size distribution were performed on the seven solidified tailings samples using an Amino 400 MPa (60,000 psi) mercury intrusion porosimeter. Intrusion volumes were measured as a function of both increasing and decreasing pressure. In all cases some hysteresis between intrusion and extrusion curves was observed. The shapes of the hysteresis curve can be related to the shape of the pores found in the solidified matrix. The total pore surface area of each sample was calculated using the acquired data. Porosity data are given in Table 8. The data indicated that the porosity of the samples varied over the range 4.9 to 39.7 volume percent. Reduction of the water used in preparation of the Krofchak and Chemfix samples gave the expected decrease in total porosity. The AECL bituminization process resulted in a very low-porosity material. The IUCS material was found to have pores in two ranges, the smaller-sized pores presumably associated with the added pozzolan. When this is taken into account, the pore size distributions of the six materials solidified with inorganic agents were quite similar, except for the expected shifts to smaller pore sizes observed with decreasing porosity. The materials solidified by IUCS and Chemfix both appeared to have a higher fraction of partially blocked pores in the gel structure than the Krofchak sample, as illustrated by the more pronounced hysteresis effect observed in their pore size distribution curves. 5.2 Radon emanation One of the important parameters in the evaluation of the effectiveness of fixation processes for uranium tailings should be the rate of radon gas emanation from the fixed materials. For a block of fixed materials which contains a percentage of uranium mine-mill railings, the radon will emanate from the surface layer and from the interior of the block. The depth within the block from which the gas originates is dependent upon the diffusion constant of the gas through the material. A procedure was designed to estimate the rates of radon emanation from the seven fixed materials and also from untreated tailings. The fixed materials were prepared in the shape of cubes having a volume of 0.44 L. Each of the samples was placed in a 4-1itre air-tight steel box fitted with a sealing connec-
41 Q~
C',1
T-q
N
n~
OD'~ C,1~1
OCt1 C',10
~1 Cq
~ ~
C~1 Cq
O C~!
(~JoO
C~lCr3
r~D
C,,1
¢b'3
~
C~3F,..~
ce3oD
c~r3
'~
"~
Li~
,..T,-T
,-T,-T
,~
~
,-T
v~
~ .,=
.=
.,= .rc~ n ~
r~ c~ n~J
r~
o
o
Q~
r~
=l ..o
42 t o r a n d a o n e - w a y valve. T o o b t a i n each r a d o n gas s a m p l e an e v a c u a t e d r a d o n cell having a v o l u m e o f 1 6 0 m L was c o n n e c t e d t o t h e s a m p l e c h a m b e r , t h u s t r a n s f e r r i n g a s a m p l e o f gas t o t h e cell t h r o u g h a 0.8 ~ m p o r e size Millipore filter. This p r o c e d u r e filtered o u t t h e a i r b o r n e r a d o n d a u g h t e r p r o d u c t s f r o m t h e gas s a m p l e . T h e gas cell was d i s c o n n e c t e d , a n d a f t e r a 2--3 h o u r p e r i o d , a t e n - m i n u t e c o u n t was m a d e o f t h e a c t i v i t y in t h e cell. S a m p l e s o f gas were t a k e n a n d c o u n t e d at regular daily intervals o v e r a p e r i o d o f a w e e k , in o r d e r t o d e t e r m i n e t h e t r e n d o f t h e r a d o n gas c o n c e n t r a t i o n as a f u n c t i o n o f t i m e . T h e d a t a w e r e c o r r e c t e d f o r b a c k g r o u n d c o u n t s and f o r t h e dilutions during t h e s a m p l i n g . A graphical r e p r e s e n t a t i o n o f t h e s e d a t a s h o w e d t h a t parallel rel a t i o n s h i p s o f r a d o n c o n c e n t r a t i o n versus t i m e w e r e o b t a i n e d . T h e c o u n t s t h a t w e r e a c q u i r e d o n t h e last d a y o f e a c h r u n c o u l d t h e r e f o r e be used to g e n e r a t e a c o m p a r i s o n b e t w e e n t h e f i x e d materials. T a b l e 9 s h o w s the results o f t h e r a d o n e m a n a t i o n m e a s u r e m e n t s . T h e t w o r i g h t - h a n d c o l u m n s r e p r e s e n t the r a d o n c o u n t s p e r u n i t w e i g h t o f f i x e d s a m p l e a n d p e r u n i t weight o f tails, res p e c t i v e l y , w i t h b o t h sets o f d a t a n o r m a l i z e d t o a r e a d i n g o f 1 0 0 f o r t h e unt r e a t e d tailings. TABLE 9 Radon emanation data Sample
KC KF AECL C1 C2 IU1 IU2 Tailings
Mass of
Weight dry tails Radon count a Radon count Radon c o u n t sample 100 × Total dry weight per unit per unit (g)
(%)
719 679 776 680 740 704 762 243
94.1 94.1 75.7 94.5 94.5 42.9 60.0 100
sample mass b tails mass b
43346 27121 884 26230 33824 8299 13618 5420
270 179 5 173 205 53 80 100
287 190 7 183 217 123 134 100
al0-minute count that has been corrected for the background and the dilutions when sampling. bData have been normalized to 100 for untreated tails. F r o m this simple t e s t p r o c e d u r e , t h e A E C L b i t u m i n i z a t i o n was m o s t effective in r e d u c t i o n o f t h e rate o f r a d o n gas release, while t h e K r o f c h a k p o u r e d s a m p l e (KC) a l l o w e d t h e greatest release o f r a d o n gas. T h e t w o IUCS s a m p l e s w e r e n e x t best at h i n d e r i n g r a d o n release. T h e filtered s a m p l e ( K F ) a n d t h e t w o C h e m f i x p r o c e d u r e s o f fixing were less effective, a n d gave app r o x i m a t e l y t h e s a m e results. H o w e v e r , w h e n c o m p a r e d on a p e r u n i t weight o f tails basis, o n l y t h e A E C L b i t u m i n i z a t i o n a c t u a l l y r e d u c e d t h e r a d o n release relative t o t h a t o f t h e u n t r e a t e d tails. When c o m p a r i n g o n t h e basis o f a u n i t w e i g h t o f fixed m a t e r i a l , b o t h t h e A E C L a n d I U C S processes r e d u c e d
43 the radon gas emanation below that of the untreated tails. Because of the variability of the water c o n t e n t among the samples, the aging of the samples over the two week measuring period, uncertainties in the normalization, and statistical uncertainty, the total uncertainty, associated with each of the relative radon counts per unit weight is estimated to be about 20%. It seems anomalous t h a t the radon counts per unit weight of railings are higher than the count for the untreated tailings for all of the samples but one. The histories of the seven fixed materials prior to the radon counting are similar in t h a t a period of two to four weeks elapsed between formation and the time of which the measurements were taken. During this period, the fixed materials aged at their own specific rates. The aging of the untreated tailings occurred over a much longer time. The difference in radon release rates between the group o f seven fixed samples and the untreated tailings due to the different time periods involved is not known. Also, because radon dissolves readily in water, the percentage of water in the seven fixed samples and the untreated tailings could result in a significant variation in the radon diffusion constants of the samples. Since these factors are n o t known, a direct comparison of the radon data between the group of seven fixated materials and the untreated tailings may n o t be possible.
5.3 Chemical durability Tests carried out to study the chemical stability of the various fixated materials (after crushing through jaw crusher to produce 2 cm (3/4 in) materials) include agitation and percolation leach studies for the release of 226Ra.
5.3.1 Agitation leach studies Agitation leach tests with various fixated materials were carried out in 1-L leach vessels at 10% solids, a one-hour residence time, room temperature and pH levels of 1.5, 3.0, and 5.5. Sulphuric acid was used to adjust the pH of the leach solution. Leaching was conducted in a vessel of 13.5 cm diameter having a pitch blade t y p e impeller of 5.08 cm diameter. The agitation rate was 98 min -1" The slurry samples at the end of the test work were filtered to obtain samples for 226Ra analysis. The solution samples were stored in containers previously washed with distilled water and nitric acid. In order to prevent loss of 226Ra values from solution, the samples were stabilized by the addition of 10 ml concentrated HNO3 per litre of filtered solution. Leach residues were subjected to screen analysis. 226Ra in solution was obtained by an ~-spectroscopic m e t h o d . A large n u m b e r of agitation leach studies investigating the effects of particle size, pH, and fixation m e t h o d on 22~Ra release were performed. A summary of the data only will be presented and is given in Table 10. The lowest solution 226Ra levels were obtained for the IU2 solidified taftings (less than 100 pCi/L). This may be due to the alkaline pozzolan reaction
44
+ ~
~+
LO
r~J
CO ~
0
~6
+~ ~+
H
~...= + ~
~÷ o
~
7"
,.C
c~ 0
~
Q~
~S
v
45
binding 22~Ra within the crystallinelattice,i.e.,with 226Ra substitution for Ca occurring during precipitationof cementitous compounds. Increasing the surface area of the solidifiedt~ilings(i.e.,decreasing the average particlesize) does not increase 226Ra release.This ispossibly due to simultaneous sorption, desorption, and precipitationof aqueous 226Ra and depends on complex solution equilibria. Both decreasing the p H (to 1.5 from 3.0) and increasing the pH (to 5.5 from 3.0) increases ~26Ra release. Fluctuations in the p H affect the complex solution equilibriain various ways, either precipitatingor dissolvingradium sulphate. It is,however, emphasized that comprehensive testing was not performed and that the data can only be interpreted within the limited scope of the experiments. 5.3.2 Percolation leach tests It is extremely difficult to simulate the seepage of rain water through natural and various fixated tailings piles on a laboratory scale. Nevertheless, in order to obtain an estimate of the total radium that could be released through seepage, a number of 2.5~centimetre (one-inch) diameter containers were packed with various fixated materials and tailings. The rate o f flow o f seepage was simulated to correlate with total precipitation in the Elliot Lake area, 86 c m / y e a r (34 in/year), and the pH of the water was maintained at 4. Based upon the pH of rain water east of the Mississippi River i n North America the pH of the leachant was chosen at 4. Seepage samples were collected daily and the pH of these seepage samples was measured and the samples were then preserved by the addition of 10 m L concentrated HNO3 per litre. 226Ra levels in these solutions were measured by a-spectroscopic methods. The test results (Table 11) generally show a decrease in 226Ra with an increase in time. Based u p o n the seepage analysis, effluent samples obtained TABLE 11 Percolation leach test results a
Sample
Head
22~Ra (pCi/L)
(pCi/g)
KF KC C1 C2 IU1 IU2 AECL
334 355 305 298 144 212 277
Day 1
Day 8
Day 15
Day 22
823 668 271 256 70 47 62
474 641 284 238 31 17 80
462 488 220 150 53 27 74
488 383 210 69 9 100
aConditions: Leach solution pH 4.0; flow rate: 0.170 L/day, equivalent to 86 cm/year (34 in/year).
46 f r o m I U 2 f i x a t e d m a t e r i a l c o n t a i n e d t h e l o w e s t t o t a l 226Ra in solution. T h e p H o f t h e seepage s a m p l e s c o l l e c t e d f o r t h e K r o f c h a k s a m p l e s w e r e highest at 1 2 . 1 - - 1 2 . 4 a n d f o r t h e A E C L s a m p l e s w e r e l o w e s t at 7 . 0 - - 8 . 2 . This s h o u l d be c o m p a r e d against t h e p H o f seepage s a m p l e s (2.8--3.2} o b t a i n e d f r o m unt r e a t e d tailings {Table 12). P e r c o l a t i o n leach tests d u r i n g t h e t e s t p e r i o d w i t h b l a n k tailings were n o t successful. Daily seepage s a m p l e s c o u l d n o t be collected since s u f f i c i e n t l e a c h a n t s o l u t i o n did n o t seep t h r o u g h . TABLE 12 Effect of
time on the
pH of various percolation seepage samplesa
Day
KF
KC
C1
C2
IU1
IU2
AECL
Untreated tailings
4 7 10 14 17 20 24 27 30
12.1 12.1 12.3 12.4 12.3 12.4 12.4 12.4 12.2
12.2 12.2 12.3 12.4 12.3 12.5 12.4 12.4 12.2
9.0 10.6 10.6 10.2 10.4 10.2 10.4 10.2 10.3
10.7 11.2 11.3 11.0 11.0 10.9 10.9 10.8 10.6
11.0 11.0 11.0 11.0 10.9 10.8 10.8 10.6 10.4
9.3 10.0 10.0 9.9 9.7 9.7 9.7 9.8 9.6
7.0 7.9 7.8 7.6 7.0 8.2 7.0 7.5 7.7
2.9 3.2 2.8 3.0 2.9 2.9 2.8 3.0 --
aInitial pH of leachant solution, 4.0. 6. DISCUSSION As c a n be seen f r o m t h e results given a b o v e , c o n s i d e r a b l e v a r i a t i o n is observed in t h e p r o p e r t i e s o f m a t e r i a l s f o r m e d f r o m t h e f o u r solidification processes. T h e f o l l o w i n g is a s u m m a r y o f s o m e o f t h e a d v a n t a g e s a n d disadvantages o f these processes, including s o m e p r e l i m i n a r y c o s t i n f o r m a t i o n . T h e solidification process d e v e l o p e d b y C a n a d i a n Waste T e c h n o l o g y was t h e least e x p e n s i v e o f t h e processes listed. N o e s t i m a t e has b e e n m a d e o f capital costs, b u t m a t e r i a l s costs w o u l d b e in t h e o r d e r o f $ 1 . 5 0 " p e r t o n o f solidified tailings. T h e t e s t results indicate, h o w e v e r , t h a t tailings solidified using this process e x h i b i t p o o r strength, shrinkage a n d f r e e z e - - t h a w r e s i s t a n c e As w o u l d b e e x p e c t e d , this results in relatively high rates o f r a d o n e m a n a t i o n a n d acidic leach rates o f r a d i u m , w h e n c o m p a r e d t o o t h e r processes tested. I t w o u l d a p p e a r t h a t t h e process suggested b y C a n a d i a n Waste T e c h n o l o g y m a y n o t be suitable f o r t h e solidification o f t e s t e d u r a n i u m mine-mill tailings. T h e b i t u m i n i z a t i o n process p e r f o r m e d b y A E C L resulted in solidified tailings with v e r y g o o d physical p r o p e r t i e s a n d v e r y l o w r a d o n e m a n a t i o n rates. T h e p r o d u c t was f o u n d t o have relatively high r a d i u m leach rates, this b e i n g s o m e w h a t surprising in light o f t h e g o o d p h y s i c a l p r o p e r t i e s . T h e results o f t h e leach studies are q u i t e significant since c o n t r o l o f t h e r a d i u m a n d r a d o n e m a n a t i o n is t h e p r i m a r y p u r p o s e o f t h e f i x a t i o n p r o c e d u r e . Because t h e *All prices in 1981 Canadian dollars.
47 Canadian climate precipitation rates exceed those of evaporation, the constant leaching o f 226Ra is of special significance. As a result, the bituminization process would be more suitable in a location with a more arid climate since, in this case, release of radon and surface erosion would be o f major concern. The cost of the bituminization process is its most serious disadvantage. As tested, materials costs alone were approximately $420 per ton o f solidified tailings. If the bitumen content were dropped to 10% this value would drop to approximately $140 per ton. For cost reasons alone, this t y p e of process is therefore n o t practical for solidification o f the bulk of the tailings. Perhaps it can find some limited application as a solidification process to be used to place a cover over an existing tailings, this cover representing only 1--2% of the total volume o f the railings. The cost o f the Chemfix process has been estimated at approximately 300 to 500 thousand dollars in set-up and $7 per ton of tailings for materials. This process is the second least expensive o f those tested. Solidification of railings using this process resulted in some improvement in strength and porosity values over unsolidified tailings, but resulted in a product with poor freeze-thaw resistance. A distinct advantage over the other processes is its rapid setting rate. Radon and radium release rates were generally in agreement with the physical properties, the values being lower than those observed for the CWT samples b u t n o t as low as observed for the AECL samples (except for 22~Ra agitation leach rates) and IUCS samples. The samples solidified using the IUCS process exhibited good strength and porosity values and fairly good resistance to freeze--thaw action. The relatively good physical properties were followed by similar results for the radon emanation and radium leaching studies. The IUCS solidified materials showed the best resistance to acid leaching and second lowest radon release rates when compared to other processes tested. At a cost (materials and processing) of $10--17 per t o n of solidified tailings, this process is considerably less expensive than the bituminization process, b u t the most costly of the inorganic processes. The IUCS costs assume pozzolan and additive sources relatively close to the site and an existing plant. Another advantage of the process is that placement of the material is performed using conventional construction equipment. 7. CONCLUSIONS The solidification of uranium mine-miU tailings to minimize risks of environmental contamination can be performed by several different processes. Although the present study is n o t comprehensive, it does indicate what properties may be expected from tailings solidified using processes of various types. As can be seen by the summary given in Table 13, the process marketed by Canadian Waste Technology was the poorest performer of the group tested. The AECL and IUCS processes gave the most improved properties of the uranium tailings, b u t the bituminization process is much more expensive. The
48
silicate based process (Chemfix) showed some improvement but did not perform as well as the lime--pozzolan or bituminization processes. Thesel categories of solidification processes were suggested as being feasible in section 2 of this r e p o r t The specific solidification process, however, is n o t necessarily representative of all processes encompassed by each category. Total solidification of the tailings may n o t be possible in terms of the costs involved. The solidified materials, however, may be quite useful for containment of the tailings either by capping or erection of barriers. TABLE 13 Summary of results
Percent tailings (approx.) Ease of placement Shrinkage Compressive strength Flexural strength Freeze--thaw resistance Porosity Pore shape Batch leach Percolation leach Radon emanation Cost -- materials -- process
CWT
AECL
Chemfix
IUCS
94 1 4 4 4 4 2 3 4 4 4 1 1
75 4 1 3 1 1 1 -2 2 1 4 4
95 2 3 2 3 3 2 2 2 2 3 3 2
50 3 2 1 2 2 2 1 1 1 2 2 3
Processes are ranked 1--4, 1 being the best performer of the group and 4 being the worst performer. ACKNOWLEDGEMENTS
Authors wish to thank Dr. R.B. Bruce, Dr. D.J. Pinchin, Dr. E.E. Berry, Mrs. M.K. Witte, and Dr. W.R. S t o t t for their participation in the project. REFERENCES 1 Lakshmanan, V.I. and Ashbrook, A.W., Radium balance studies at the Beaverlodge Mill of Eldorado Nuclear Ltd, Seminar on Management, Stabilization and Environmental Impact of Uranium Mill Tailings, Albuquerque, NM, July 24--28, 1978. 2 Lakshmanan, V.I. and Ashbrook, A.W., Second radium balance studies at Beaverlodge Mill, Annual Meeting, Canadian Uranium Producers Metallurgical Committee, Elliot Lake, Ontario, May 12--18, 1979. 3 Haque, K.E., Lucas, B.H. and Ritcey, G.M., CIM Bull., (July 1980) 141--147. 4 Pojasek, R.B., Solid waste disposal: Solidification, Chem. Eng., (August 13) (1979) 141--145. 5 Conner, J.R., U.S. Patent 3,837,872, September 24, 1974. 6 Conner, J.R. and Polosky, R.J., U.S. Patent 3,841,102, October 15, 1974. 7 Krofchak, D., Canadian Patent 102 4277, January 10, 1978. 8 Smith, C.L. and Webster, W.C., U.S. Patent 3,720,609, March 13, 1973; RE 29783, September 17, 1978.
31
Elsevier Science Publishers B.V., Amsterdlm -- Printed in The Netherlands
E V A L U A T I O N OF TAILINGS STABILIZATION METHODS AS APPLIED TO U R A N I U M TAILINGS
V.I. LAKSHMANAN, L.M. LUCKEVICH,
Ontario Research Foundation, Mississauga, Ontario (Canada) G.M. RITCEY and J.M. SKEAFF
CANMET, Ottawa, Ontario (Canada) (Received May 9, 1983; accepted June 22, 1983)
ABSTRACT Lakshmanan, V.I., Luckevich, L.M., Ritcey, G.M. and Skeaff, J.M., 1984. Evaluation of railings stabilization methods as applied to uranium railings. Hydrometallurgy, 12: 31--48. In an attempt to obtain an environmentally acceptable uranium tailings disposal method, four commercial waste solidification processes were tested. These methods used inorganic and asphaltic binders. The solidified tailings were evaluated by measuring their physical and chemical properties immediately after solidification and as a function of time. Effluent from none of the solidified tailings during the tested period (30 days) contained less than 10 pCi/L. However, the test results indicated asphalt binder as superior but more expensive by an order of magnitude than a lime based method. Hence the lime based method was considered to be the best compromise, yielding relatively good properties at reasonable costs.
INTRODUCTION
Many types of metal extraction processes, including uranium extraction, produce tailings that require disposal. In general, mine-mill tailings are disposed of by: • i m p o u n d m e n t on land, • dumping into marine or inland water, • use as a construction material, or • use as a mine backfill. However, uranium milling is unique in that it requires the handling and disposal of large quantities of low-level radioactive materials. The special hazards presented by uranium taflings such as radioactivity, acidity and toxicity may prevent direct disposal by these usual methods. Proper management of uranium tailings involves environmental control with respect to: • ground water seepage, • surface water run-off, • 222 Rn diffusion,
0304-386X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
32 • air-borne dust (dry}, and • dam breakage. The Ontario Research Foundation, in a CANMET sponsored project (DSS contract serial No. OSQ80-00131} has assessed alternative methods of tailings management as applied to uranium milling. More specifically, the focus of the program was to investigate the stabilization (via solidification} of uranium mine-mill tailings so that impoundment on land is environmentally acceptable. The objectives of this program were to: (1) Acquire information and review waste stabilization technology as applied to uranium mine tailings. (2) Acquire and characterize uranium mine tailings. (3) Solidify uranium mine tailings b y applying existing solidification technology. (4) Evaluate the solidified tailings with an assessment emphasis on radionuclide release. 2. BACKGROUND INFORMATION
2.1 Origins and nature o f uranium railings Uranium is extracted from ore by either acid (H2SO4) or alkaline (CO~-) leaching. After extraction, residual uranium, pyrite, metals (Se, Pb, As, Cd, Mo), inorganic salts (F-, NO3-, C1-), and several decay products are found in the leach residues. The radioisotopes of most concern are 226Ra, 23°Th and 21°Pb. Systematic studies by various workers have shown that the majority (80--90%) of the 226Ra is in the fine fraction ( - 2 0 0 mesh) of the tailings solids [ 1,2]. Pyrite and pyrrhotite cause problems in the long term because their oxidation products (H2SO4 and Fe 3÷} solubilize radionuclides and heavy metals, hence increasing their migration to the surrounding environment~ [ 3]. Short-term management of uranium tailings usually consists of impoundment behind dams or embankments. Alternatives to i m p o u n d m e n t (marine/fresh water disposal, use as construction material or as mine backfill) are not y e t available because of the radioactive nature of the tailings.
2.2 Waste stabilization technology Waste stabilization denotes the conversion of toxic components in a waste to a chemical form that is resistant to leaching. Waste solidification denotes rendering o f a liquid or semi~solid waste into a cohesive solid. Often waste solidification results in waste stabilization and vice versa. In the case of uranium mine tailings, both stabilization and solidification are necessary. Waste solidification methods applicable to low or medium toxicity wastes can be grouped into the following categories: • silicate and cement based {inorganic binder), • lime based (inorganic binder),
33 TABLE 1 Advantages and disadvantages of solidification processes [4] Process
Advantages
Disadvantages
Cement-based
1. A d d i t i v e s are available at a r e a s o n a b l e price. 2. C e m e n t m i x i n g a n d h a n d l i n g t e c h n i ques are well d e v e l o p e d . 3. Processing e q u i p m e n t is r e a d i l y available. 4. Processing is r e a s o n a b l y t o l e r a n t o f c h e m i c a l v a r i a t i o n s in sludges. 5. T h e s t r e n g t h a n d p e r m e a b i l i t y of t h e e n d - p r o d u c t c a n be v a r i e d b y c o n t r o l ling t h e a m o u n t o f c e m e n t a d d e d .
1. L o w - s t r e n g t h c e m e n t - - w a s t e m i x t u r e s are o f t e n v u l n e r a b l e t o acidic leaching solutions. E x t r e m e condit i o n s c a n r e s u l t in d e c o m p o s i t i o n o f the fixed material and accelerated l e a c h i n g of t h e c o n t a m i n a n t s . 2. P r e t r e a t m e n t , m o r e e x p e n s i v e c e m e n t t y p e s , o r c o s t l y a d d i t i v e s m a y be necessary for stabilization of wastes containing impurities that affect the setting and curing of cement. 3. C e m e n t a n d o t h e r a d d i t i v e s a d d c o n siderably to weight and bulk of waste.
Lime-based
1. T h e a d d i t i v e s are g e n e r a l l y v e r y inex- 1. L i m e a n d o t h e r a d d i t i v e s a d d to weight and bulk of waste. p e n s i v e a n d w i d e l y available. 2. E q u i p m e n t r e q u i r e d f o r p r o c e s s i n g is 2. S t a b i l i z e d sludges are v u l n e r a b l e to s i m p l e t o o p e r a t e a n d w i d e l y available. acidic s o l u t i o n s a n d t o c u r i n g a n d 3. C h e m i s t r y o f p o z z o l a n i c r e a c t i o n s is s e t t i n g p r o b l e m s a s s o c i a t e d w i t h inwell k n o w n . o r g a n i c c o n t a m i n a n t s in t h e w a s t e .
Thermoplastic
1. C o n t a m i n a n t m i g r a t i o n r a t e s are g e n e - 1. E x p e n s i v e e q u i p m e n t a n d skilled rally l o w e r t h a n f o r m o s t o t h e r t e c h l a b o r are g e n e r a l l y r e q u i r e d . niques. 2. Sludges c o n t a i n i n g c o n t a m i n a n t s 2. E n d - p r o d u c t is fairly r e s i s t a n t t o m o s t t h a t volatilize a t l o w t e m p e r a t u r e s aqueous solutions. must be processed carefully. 3. T h e r m o p l a s t i c m a t e r i a l s a d h e r e well t o 3. T h e r m o p l a s t i c m a t e r i a l s are f l a m incorporated materials. mable. 4. Wet sludges m u s t b e d r i e d b e f o r e t h e y c a n be m i x e d w i t h t h e t h e r m o plastic m a t e r i a l .
Organic p o l y m e r
1. Only s m a l l q u a n t i t i e s o f a d d i t i v e s are u s u a l l y r e q u i r e d to c a u s e t h e m i x t u r e t o set. 2. T e c h n i q u e s c a n b e a p p l i e d t o e i t h e r w e t o r d r y sludges, 3. E n d - p r o d u c t h a s a l o w d e n s i t y as c o m p a r e d t o o t h e r f i x a t i o n techniques.
1o C o n t a m i n a n t s are t r a p p e d in o n l y a loose resin-matrix end-product. 2. C a t a l y s t s u s e d in t h e u r e a - - f o r m a l d e h y d e p r o c e s s are s t r o n g l y acidic. Most m e t a l s are e x t r e m e l y soluble at l o w p H a n d c a n e s c a p e in w a t e r n o t t r a p p e d in t h e m a s s d u r i n g t h e p o l y m e r i z a t i o n process. 3. S o m e o r g a n i c p o l y m e r s are b i o d e gradable. 4. E n d - p r o d u c t is g e n e r a l l y p l a c e d in a c o n t a i n e r b e f o r e disposal.
Encapsulation
I.
1. Materials u s e d are o f t e n e x p e n s i v e . 2. T e c h n i q u e s g e n e r a l l y r e q u i r e specialized e q u i p m e n t and heat t r e a t m e n t to f o r m the jackets. 3. T h e sludge has t o b e d r i e d b e f o r e t h e Process c a n b e a p p l i e d . 4. C e r t a i n j a c k e t m a t e r i a l s are f l a m mable.
V e r y s o l u b l e c o n t a m i n a n t s are t o t a l ly i s o l a t e d f r o m t h e e n v i r o n m e n t . 2. Usually n o s e c o n d a r y c o n t a i n e r is reqnired because the coating materials are strong and chemically inert.
• thermoplastic based (asphalt) (organic binder), • thermoset polymer based (organic binder), • encapsulation techniques {organic binder).
34 The advantages and disadvantages o f each of these processes are outlined in a table published by Pojasek [ 4], presented here as Table 1. Because of the large volumes of tailings requiring disposal and because they are of low radioactivity only the techniques based on inorganic and asphaltic binders were originally considered as economically feasible for the test program. Information regarding available waste solidification processes based on inorganic and asphaltic binders was obtained. Four of these methods were selected to be applied to uranium mine tailings; two were lime based, one was silicate based and one was thermoplastic based. The methods used were: • Atomic Energy of Canada Ltd. (AECL) bituminization (thermoplastic based), Canadian; • Canadian Waste Technology (lime based), Canadian; • Chemfix (silicate based), U.S.A.; • IUCS Poz-o-Tec (lime based), U.S.A. A brief discussion of each of the above is outlined below. 2.2.1 Chemfix The Chemfix process is a patented [ 5,6] system based on reactions between soluble silicates and silicate setting agents. The mobile reaction unit employed is capable of treating 450,000 L (100,000 gallons) or more of sludge per day at ambient temperature and pressure. The process consists of three stages: (i) waste homogenization; (ii) pumping of the waste to the reactor, mixing of waste and reagents, and discharge to disposal area; and {iii) solidification within 72 hours. The solidified product has soft-like properties and can be handled by standard earth moving equipment. 2.2.2 Canadian Waste Technology (CWT) The Canadian Waste Technology (CWT) solidification process is a patented system which "activates" silica in waste sludges so that a complex metal silicate matrix is formed [7]. The silica is present as clay minerals and sand. Treatment of such sludges with at least one of H2SO4, HC1, HNO3, CaO, MgO, CaC12, Ca(OH)2 and Mg(OH)2 (as determined by Canadian Waste Technology) yields a pourable slurry which solidifies over an unspecified time period. 2. 2. 3 IUCS Poz-o-Tec process The I.U. Conversion Systems (IUCS)Poz-o-Tec process is a patented [ 8] micro~ncapsulation process that involves the precise mixing of waste products, fly ash and lime-rich materials. The treated waste has the consistency of dry soil-cement and is compacted using vibratory rollers. After curing, compressive strengths of 7 MPa (1000 psi) and permeabilities of 0.13 nm s-1 (5 × 10 -9 in s-1 ) have been recorded. It should be noted that the treated waste is not pumpable. 2.2.4 AECL bituminization Bituminization consists of mixing the waste with asphalt (a petroleum byproduct composed of high molecular weight hydrocarbons), at 165--167 ° C.
35
Water originally present in the waste is volatilized at this temperature. In this particular case, the tailings were dried and ground prior to bituminization. 3. DESCRIPTION OF TAILINGS AND SOLIDIFICATION METHODS TESTED
3.1. Tailings characterization Two barrels of mine tailings were received from Denison Mines located at Elliot Lake, Ontario. In the Denison uranium recovery process, the acidleached slurry is h y d r o c y c l o n e d and overflow is discharged to thickeners. The thickener overflow is discharged to an unclarified pregnant storage tank; the TABLE 2 Sieve analysis of uranium mine tailings Size fraction (mesh)
Wt. percent
+48 -48 -65 -100 -150 -200 -325
4.16 7.84 9.23 2.50 17.28 10.34 48.66
+65 +100 +150 +200 +325
TABLE 3 Semi-quantitative chemical analysis (XRF) of uranium mine tailings Sample 1
Sample 2
Element
Approximate weight percent
Element
Approximate weight percent
Si AI S K Ca Fe Ti Na Mg Pb Zn Rb
high 3 2 2 2 1.5 (3.2)a 0.3 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1
Si Al Fe K Ca Ti P Mg Zn Se Cu Pb Ni
35.1 3.3 2.4 (5.2) a 1.69 1.41 0.22 0.05 0.07 trace trace trace trace trace
a% Pyrite if all Fe is present as FeS 2.
36
thickener underflow is combined with the hydrocyclone underflow and filtered. The filter cake is washed in two stages of drum filters. The second stage cake is washed with water, reslurried with neutralized barren solution and pumped to the tailings disposal area. The tailings are approximately 80% solids. A pipe sample of the tailings was submitted for sieve, X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis. Based on the 226Ra analysis (352 pCi/g) and the sieve analysis (Table 2), the sample of Denison tailings received can be considered typical. Hence, it is presumed that elutriation of fines containing major portions of the 226Ra did not occur. Analyses by XRF and XRD are given in Tables 3 and 4, respectively. Since the pyrite analysis by XRD and XRF was relatively low, the tailings were analysed for iron using atomic absorption spectroscopy. The results indicated 2.06 percent iron, thus giving an upper limit on the pyrite of 4.4 percent. These values correspond to those expected for Denison tailings. TABLE 4 X R D analysis of uranium mine tailings Mineral
Wt. percent
Quartz Muscovite (or similar mineral) Gypsum Pyrite Others
70-80 10--20 5--10 1--2 a < 1--2
aWet chemical analysis yielded a value of 4.4% pyrite. 4. E X P E R I M E N T A L MIXES
In all of the solidification experiments the actual tailings solidification was performed by process vendor personnel. Three of the experiments were performed at O.R.F. with O.R.F. personnel observing. The AECL bituminization was performed at the AECL Chalk River Laboratories. No O.R.F. personnel were present but a detailed description of the solidification was provided by AECL.
4.1 Chemfix Two samples of uranium mine tailings were solidified using the Chemfix process. The first, designated C1, mixed 18.0 kg of tailings (84.5% solids) 1.7 L H20, 0.74 kg inorganic setting agent, and 0.18 L type N sodium silicate solution. The second, designated C2, contained 12.0 kg of tailings (84.5% solids), 0.74 kg inorganic setting agent, and 0.60 L H20. After mixing 6.0 kg more of tailings were added. C1 was a thixotropic slurry, easily placed by hand; C2 was a stiff slurry, readily placed using a small concrete vibrator.
37
4.2 Canadian Waste Technology (CWT) Two samples of uranium mine railings were solidified using the CWT process. The first, designated KC (Krofchak Cast), mixed 18.0 kg railings (80% solids), 2.6 kg water and 0.9 kg reagent for 5 minutes in a standard concrete mixer. The second, designated KF (Krofchak Filtered), had an increased water content (to give a 30% solids slurry) and was filtered through a Perin plate and frame filter press. Mix KC was a thixotropic, pourable slurry easily placed by hand; mix KF was a thixotropic, stiff slurry readily placed with mild vibration.
4.3 L U. Conversion System (IUCS) process Two samples of uranium mine tailings were solidified using the IUCS process. The first, designated IU1, mixed 1 L H20, 4.8 kg of IUCS blend No. 1 and 4.267 kg tailings (84.5% solids). The second, designated IU2, mixed 0.6 L H20, 3.524 kg IUCS Blend No. 2 and 6.101 kg tailings (84.5% solids). Both IU1 and IU2 were extremely dry and required vigorous hand compaction to form a continuous body; the moisture content of the solidified tailings was estimated to be 15--16%.
4.4 AECL bituminization The uranium mine tailings were dried (110 ° C) overnight and ground. The dry powder was screw fed (10--11 kg/h) to a bituminizer (ASTM type No. II Roofing Asphalt) with a bitumen flow rate of 3--3.5 kg/h. Production rate ranged from 13--14 kg/h with the final product being 75--76 weight percent tailings solids. The product was discharged to 11 L wooden moulds; each sample weighed 20--21 kg. 5. E V A L U A T I O N OF SOLIDIFIED TAILINGS
5.1 Physical and mechanical evaluation 5.1.1 Flow properties o f material for placement The consistency of the materials will determine the type of placement method and, to a large extent, the product strength and durability. The consistency of cement mortars is most commonly measured by Flow Value (ASTM C109-77). This method was used for the Chemfix and Krofchak processes. The IUCS material was too dry for the test to be applicable. The flow of the AECL material would, of course, be temperature-dependent and no tests were run on this material. The water/solids ratio, percentage dry tailings and flow properties of all mixes are presented in Table 5.
38 TABLE 5 Water/solids ratio, percentage dry tailings, and flow properties of mixes Sample
Percent solids
Dry tailings Total dry weight
Flow valuea
Comments
KC KF AECL C1 C2 IU1
72.1 75.7 78.3 82.4 83.5
94.1 94.1 75.7 94.5 94.5 42.9
150 128 -136 68 -
IU2
84.7
60.0
--
Flows readily. Placeable by hand. Flows readily at 165 ° C. Easily placeable; almost flows. Placeable with vibration. Requires hand or vibratory compaction. Requires hand or vibratory compaction.
aAs per ASTM C109-77.
5.1.2 Shrinkage, compressive strength and flexural strength T h e s h r i n k a g e o f s t a b i l i z e d t a i l i n g s w i l l b e i m p o r t a n t in d e t e r m i n i n g t h e e x t e n t o f b r e a k - u p o f a l a r g e s t a b i l i z e d m a s s o f t a i l i n g s . T h e i n i t i a l m o v e m e n t as the samples dry or the stabilizing reactions take place, will be the most imp o r t a n t in t h i s r e g a r d . P r i o r t o d e m o u l d i n g t h e s a m p l e s , m e a s u r e m e n t s w e r e m a d e o f t h e s h r i n k a g e w h i c h t h e s a m p l e s h a d u n d e r g o n e a n d t h i s is e x p r e s s e d as a p e r c e n t a g e o f t h e o r i g i n a l l e n g t h in T a b l e 6. TABLE 6 Shrinkage, compressive strength, and flexural strength of samples Sample
KC KF AECL C1 C2 IU1 IU2
Process used
Shrinkage at demoulding (%)
Krofchak Cast a 1.6 Krofchak filtered a 1.9 Bituminization b <0.1 Chemfix I b 0.4 Chemfix 2 b 0.3 IUCS 1 a 0.1 IUCS 2 a 0.1
Compressive strength
Flexural strength
kPa
psi
kPa
psi
435 580 1080 1230 2590 9215 8030
65 85 155 180 375 1335 1165
190 135 1480 425 415 1370 1260
30 20 215 60 60 200 180
a All tests 28 days after casting. bAll tests 14 days after casting. T h e m e a s u r e m e n t a c c u r a c y w a s 0.1 p e r c e n t ; t h a t is, t h e A E C L s a m p l e showed no measurable shrinkage at the time of demoulding. The IUCS samples had been cured, covered with a plastic sheet until demoulding while the
39
CWT and Chemfix samples had been covered for the first week of curing, and thereafter were left exposed in normal laboratory air. All o f these curing conditions were arrived at in consultation with the representatives of the companies involved in the stabilization. Prior to demoulding, all Krofchak Cast (KC) samples developed large open shrinkage cracks. This was not observed in the Krofchak Filtered (KF) samples, although they also shrank considerably. Compressive and flexural strength were measured at 14 or 28 days and are presented in Table 6 as well. Flexural testing was performed on 7.6 cm (3 in) cubes as described in ASTM C293-77, "Flexural Strength of Concrete (Using Simple Beam with Center-Point Loading}". Samples were cut from the cast specimens with a nominal 7.6 cm × 7.6 cm (3 in X 3 in) cross-section and were tested over a 30 cm (12 in) span. The compressive tests were performed as described in ASTM Cl16-68, "Compressive Strength of Concrete Using Portion of Beams Broken in Flexure". Samples were nominally 7.6 cm (3 in) cubes. The AECL samples did n o t have a well-defined failure point, b u t were loaded to 10 percent deformation. 5.1.3 Freeze-'thaw resistance All samples were subjected to freeze--thaw cycling b y ASTM C666-77, "Standard Method o f Test for Resistance of Concrete to Rapid Freezing and Thawing". Procedure A, which involves freezing and thawing the samples imTABLE7 Summary of freeze--thaw tests (2 samples of each material tested) Sample
KC
Cycles at end of testing
Comments
A
0
B
6
KF
A
6
AECL
A B
128 128
C1
A B
11 11
C2
A B
5 5
IU1
A B
40 80
Severe spalling and disintegration. Severe spalling and disintegration.
IU2
A B
98 98
Severe spalling and disintegration. Severe spalling and disintegration.
Turned to mush during saturation period prior to freeze-thaw cycling. Broke prior to freeze--thaw test; turned to mush after 6 cycles. Broke after I cycle; turned to mush after 6 cycles. No visual change of surface; some warpage. No visual change of surface; some warpage. Ends disintegrating; severe spalling on one side. Ends disintegrating; one side turning to mush. Numerous cracks; shattered. Numerous cracks; shattered.
40 mersed in water, was used for all tests. Samples were cured for 14 days (AECL and Chemfix) or 28 days (IUCS or CWT) prior to testing. All samples were immersed for a minimum of 24 hours prior to beginning freeze--thaw cycling. An a t t e m p t was made to determine the length change o f the specimens on freeze--thaw cycling, but this was n o t possible due to surface scaling, or sample disintegration or warping. The results are summarized in Table 7. No quantitative comparison of the freeze--thaw samples was made. A qualitative classification of the freeze--thaw resistance (from observations) is that AECL is better than IUCS which is better than Chemfix which in turn is better than CWT. 5.1.4 Porosity and pore size distribution Measurements of porosity and pore size distribution were performed on the seven solidified tailings samples using an Amino 400 MPa (60,000 psi) mercury intrusion porosimeter. Intrusion volumes were measured as a function of both increasing and decreasing pressure. In all cases some hysteresis between intrusion and extrusion curves was observed. The shapes of the hysteresis curve can be related to the shape of the pores found in the solidified matrix. The total pore surface area of each sample was calculated using the acquired data. Porosity data are given in Table 8. The data indicated that the porosity of the samples varied over the range 4.9 to 39.7 volume percent. Reduction of the water used in preparation of the Krofchak and Chemfix samples gave the expected decrease in total porosity. The AECL bituminization process resulted in a very low-porosity material. The IUCS material was found to have pores in two ranges, the smaller-sized pores presumably associated with the added pozzolan. When this is taken into account, the pore size distributions of the six materials solidified with inorganic agents were quite similar, except for the expected shifts to smaller pore sizes observed with decreasing porosity. The materials solidified by IUCS and Chemfix both appeared to have a higher fraction of partially blocked pores in the gel structure than the Krofchak sample, as illustrated by the more pronounced hysteresis effect observed in their pore size distribution curves. 5.2 Radon emanation One of the important parameters in the evaluation of the effectiveness of fixation processes for uranium tailings should be the rate of radon gas emanation from the fixed materials. For a block of fixed materials which contains a percentage of uranium mine-mill railings, the radon will emanate from the surface layer and from the interior of the block. The depth within the block from which the gas originates is dependent upon the diffusion constant of the gas through the material. A procedure was designed to estimate the rates of radon emanation from the seven fixed materials and also from untreated tailings. The fixed materials were prepared in the shape of cubes having a volume of 0.44 L. Each of the samples was placed in a 4-1itre air-tight steel box fitted with a sealing connec-
41 Q~
C',1
T-q
N
n~
OD'~ C,1~1
OCt1 C',10
~1 Cq
~ ~
C~1 Cq
O C~!
(~JoO
C~lCr3
r~D
C,,1
¢b'3
~
C~3F,..~
ce3oD
c~r3
'~
"~
Li~
,..T,-T
,-T,-T
,~
~
,-T
v~
~ .,=
.=
.,= .rc~ n ~
r~ c~ n~J
r~
o
o
Q~
r~
=l ..o
42 t o r a n d a o n e - w a y valve. T o o b t a i n each r a d o n gas s a m p l e an e v a c u a t e d r a d o n cell having a v o l u m e o f 1 6 0 m L was c o n n e c t e d t o t h e s a m p l e c h a m b e r , t h u s t r a n s f e r r i n g a s a m p l e o f gas t o t h e cell t h r o u g h a 0.8 ~ m p o r e size Millipore filter. This p r o c e d u r e filtered o u t t h e a i r b o r n e r a d o n d a u g h t e r p r o d u c t s f r o m t h e gas s a m p l e . T h e gas cell was d i s c o n n e c t e d , a n d a f t e r a 2--3 h o u r p e r i o d , a t e n - m i n u t e c o u n t was m a d e o f t h e a c t i v i t y in t h e cell. S a m p l e s o f gas were t a k e n a n d c o u n t e d at regular daily intervals o v e r a p e r i o d o f a w e e k , in o r d e r t o d e t e r m i n e t h e t r e n d o f t h e r a d o n gas c o n c e n t r a t i o n as a f u n c t i o n o f t i m e . T h e d a t a w e r e c o r r e c t e d f o r b a c k g r o u n d c o u n t s and f o r t h e dilutions during t h e s a m p l i n g . A graphical r e p r e s e n t a t i o n o f t h e s e d a t a s h o w e d t h a t parallel rel a t i o n s h i p s o f r a d o n c o n c e n t r a t i o n versus t i m e w e r e o b t a i n e d . T h e c o u n t s t h a t w e r e a c q u i r e d o n t h e last d a y o f e a c h r u n c o u l d t h e r e f o r e be used to g e n e r a t e a c o m p a r i s o n b e t w e e n t h e f i x e d materials. T a b l e 9 s h o w s the results o f t h e r a d o n e m a n a t i o n m e a s u r e m e n t s . T h e t w o r i g h t - h a n d c o l u m n s r e p r e s e n t the r a d o n c o u n t s p e r u n i t w e i g h t o f f i x e d s a m p l e a n d p e r u n i t weight o f tails, res p e c t i v e l y , w i t h b o t h sets o f d a t a n o r m a l i z e d t o a r e a d i n g o f 1 0 0 f o r t h e unt r e a t e d tailings. TABLE 9 Radon emanation data Sample
KC KF AECL C1 C2 IU1 IU2 Tailings
Mass of
Weight dry tails Radon count a Radon count Radon c o u n t sample 100 × Total dry weight per unit per unit (g)
(%)
719 679 776 680 740 704 762 243
94.1 94.1 75.7 94.5 94.5 42.9 60.0 100
sample mass b tails mass b
43346 27121 884 26230 33824 8299 13618 5420
270 179 5 173 205 53 80 100
287 190 7 183 217 123 134 100
al0-minute count that has been corrected for the background and the dilutions when sampling. bData have been normalized to 100 for untreated tails. F r o m this simple t e s t p r o c e d u r e , t h e A E C L b i t u m i n i z a t i o n was m o s t effective in r e d u c t i o n o f t h e rate o f r a d o n gas release, while t h e K r o f c h a k p o u r e d s a m p l e (KC) a l l o w e d t h e greatest release o f r a d o n gas. T h e t w o IUCS s a m p l e s w e r e n e x t best at h i n d e r i n g r a d o n release. T h e filtered s a m p l e ( K F ) a n d t h e t w o C h e m f i x p r o c e d u r e s o f fixing were less effective, a n d gave app r o x i m a t e l y t h e s a m e results. H o w e v e r , w h e n c o m p a r e d on a p e r u n i t weight o f tails basis, o n l y t h e A E C L b i t u m i n i z a t i o n a c t u a l l y r e d u c e d t h e r a d o n release relative t o t h a t o f t h e u n t r e a t e d tails. When c o m p a r i n g o n t h e basis o f a u n i t w e i g h t o f fixed m a t e r i a l , b o t h t h e A E C L a n d I U C S processes r e d u c e d
43 the radon gas emanation below that of the untreated tails. Because of the variability of the water c o n t e n t among the samples, the aging of the samples over the two week measuring period, uncertainties in the normalization, and statistical uncertainty, the total uncertainty, associated with each of the relative radon counts per unit weight is estimated to be about 20%. It seems anomalous t h a t the radon counts per unit weight of railings are higher than the count for the untreated tailings for all of the samples but one. The histories of the seven fixed materials prior to the radon counting are similar in t h a t a period of two to four weeks elapsed between formation and the time of which the measurements were taken. During this period, the fixed materials aged at their own specific rates. The aging of the untreated tailings occurred over a much longer time. The difference in radon release rates between the group o f seven fixed samples and the untreated tailings due to the different time periods involved is not known. Also, because radon dissolves readily in water, the percentage of water in the seven fixed samples and the untreated tailings could result in a significant variation in the radon diffusion constants of the samples. Since these factors are n o t known, a direct comparison of the radon data between the group of seven fixated materials and the untreated tailings may n o t be possible.
5.3 Chemical durability Tests carried out to study the chemical stability of the various fixated materials (after crushing through jaw crusher to produce 2 cm (3/4 in) materials) include agitation and percolation leach studies for the release of 226Ra.
5.3.1 Agitation leach studies Agitation leach tests with various fixated materials were carried out in 1-L leach vessels at 10% solids, a one-hour residence time, room temperature and pH levels of 1.5, 3.0, and 5.5. Sulphuric acid was used to adjust the pH of the leach solution. Leaching was conducted in a vessel of 13.5 cm diameter having a pitch blade t y p e impeller of 5.08 cm diameter. The agitation rate was 98 min -1" The slurry samples at the end of the test work were filtered to obtain samples for 226Ra analysis. The solution samples were stored in containers previously washed with distilled water and nitric acid. In order to prevent loss of 226Ra values from solution, the samples were stabilized by the addition of 10 ml concentrated HNO3 per litre of filtered solution. Leach residues were subjected to screen analysis. 226Ra in solution was obtained by an ~-spectroscopic m e t h o d . A large n u m b e r of agitation leach studies investigating the effects of particle size, pH, and fixation m e t h o d on 22~Ra release were performed. A summary of the data only will be presented and is given in Table 10. The lowest solution 226Ra levels were obtained for the IU2 solidified taftings (less than 100 pCi/L). This may be due to the alkaline pozzolan reaction
44
+ ~
~+
LO
r~J
CO ~
0
~6
+~ ~+
H
~...= + ~
~÷ o
~
7"
,.C
c~ 0
~
Q~
~S
v
45
binding 22~Ra within the crystallinelattice,i.e.,with 226Ra substitution for Ca occurring during precipitationof cementitous compounds. Increasing the surface area of the solidifiedt~ilings(i.e.,decreasing the average particlesize) does not increase 226Ra release.This ispossibly due to simultaneous sorption, desorption, and precipitationof aqueous 226Ra and depends on complex solution equilibria. Both decreasing the p H (to 1.5 from 3.0) and increasing the pH (to 5.5 from 3.0) increases ~26Ra release. Fluctuations in the p H affect the complex solution equilibriain various ways, either precipitatingor dissolvingradium sulphate. It is,however, emphasized that comprehensive testing was not performed and that the data can only be interpreted within the limited scope of the experiments. 5.3.2 Percolation leach tests It is extremely difficult to simulate the seepage of rain water through natural and various fixated tailings piles on a laboratory scale. Nevertheless, in order to obtain an estimate of the total radium that could be released through seepage, a number of 2.5~centimetre (one-inch) diameter containers were packed with various fixated materials and tailings. The rate o f flow o f seepage was simulated to correlate with total precipitation in the Elliot Lake area, 86 c m / y e a r (34 in/year), and the pH of the water was maintained at 4. Based upon the pH of rain water east of the Mississippi River i n North America the pH of the leachant was chosen at 4. Seepage samples were collected daily and the pH of these seepage samples was measured and the samples were then preserved by the addition of 10 m L concentrated HNO3 per litre. 226Ra levels in these solutions were measured by a-spectroscopic methods. The test results (Table 11) generally show a decrease in 226Ra with an increase in time. Based u p o n the seepage analysis, effluent samples obtained TABLE 11 Percolation leach test results a
Sample
Head
22~Ra (pCi/L)
(pCi/g)
KF KC C1 C2 IU1 IU2 AECL
334 355 305 298 144 212 277
Day 1
Day 8
Day 15
Day 22
823 668 271 256 70 47 62
474 641 284 238 31 17 80
462 488 220 150 53 27 74
488 383 210 69 9 100
aConditions: Leach solution pH 4.0; flow rate: 0.170 L/day, equivalent to 86 cm/year (34 in/year).
46 f r o m I U 2 f i x a t e d m a t e r i a l c o n t a i n e d t h e l o w e s t t o t a l 226Ra in solution. T h e p H o f t h e seepage s a m p l e s c o l l e c t e d f o r t h e K r o f c h a k s a m p l e s w e r e highest at 1 2 . 1 - - 1 2 . 4 a n d f o r t h e A E C L s a m p l e s w e r e l o w e s t at 7 . 0 - - 8 . 2 . This s h o u l d be c o m p a r e d against t h e p H o f seepage s a m p l e s (2.8--3.2} o b t a i n e d f r o m unt r e a t e d tailings {Table 12). P e r c o l a t i o n leach tests d u r i n g t h e t e s t p e r i o d w i t h b l a n k tailings were n o t successful. Daily seepage s a m p l e s c o u l d n o t be collected since s u f f i c i e n t l e a c h a n t s o l u t i o n did n o t seep t h r o u g h . TABLE 12 Effect of
time on the
pH of various percolation seepage samplesa
Day
KF
KC
C1
C2
IU1
IU2
AECL
Untreated tailings
4 7 10 14 17 20 24 27 30
12.1 12.1 12.3 12.4 12.3 12.4 12.4 12.4 12.2
12.2 12.2 12.3 12.4 12.3 12.5 12.4 12.4 12.2
9.0 10.6 10.6 10.2 10.4 10.2 10.4 10.2 10.3
10.7 11.2 11.3 11.0 11.0 10.9 10.9 10.8 10.6
11.0 11.0 11.0 11.0 10.9 10.8 10.8 10.6 10.4
9.3 10.0 10.0 9.9 9.7 9.7 9.7 9.8 9.6
7.0 7.9 7.8 7.6 7.0 8.2 7.0 7.5 7.7
2.9 3.2 2.8 3.0 2.9 2.9 2.8 3.0 --
aInitial pH of leachant solution, 4.0. 6. DISCUSSION As c a n be seen f r o m t h e results given a b o v e , c o n s i d e r a b l e v a r i a t i o n is observed in t h e p r o p e r t i e s o f m a t e r i a l s f o r m e d f r o m t h e f o u r solidification processes. T h e f o l l o w i n g is a s u m m a r y o f s o m e o f t h e a d v a n t a g e s a n d disadvantages o f these processes, including s o m e p r e l i m i n a r y c o s t i n f o r m a t i o n . T h e solidification process d e v e l o p e d b y C a n a d i a n Waste T e c h n o l o g y was t h e least e x p e n s i v e o f t h e processes listed. N o e s t i m a t e has b e e n m a d e o f capital costs, b u t m a t e r i a l s costs w o u l d b e in t h e o r d e r o f $ 1 . 5 0 " p e r t o n o f solidified tailings. T h e t e s t results indicate, h o w e v e r , t h a t tailings solidified using this process e x h i b i t p o o r strength, shrinkage a n d f r e e z e - - t h a w r e s i s t a n c e As w o u l d b e e x p e c t e d , this results in relatively high rates o f r a d o n e m a n a t i o n a n d acidic leach rates o f r a d i u m , w h e n c o m p a r e d t o o t h e r processes tested. I t w o u l d a p p e a r t h a t t h e process suggested b y C a n a d i a n Waste T e c h n o l o g y m a y n o t be suitable f o r t h e solidification o f t e s t e d u r a n i u m mine-mill tailings. T h e b i t u m i n i z a t i o n process p e r f o r m e d b y A E C L resulted in solidified tailings with v e r y g o o d physical p r o p e r t i e s a n d v e r y l o w r a d o n e m a n a t i o n rates. T h e p r o d u c t was f o u n d t o have relatively high r a d i u m leach rates, this b e i n g s o m e w h a t surprising in light o f t h e g o o d p h y s i c a l p r o p e r t i e s . T h e results o f t h e leach studies are q u i t e significant since c o n t r o l o f t h e r a d i u m a n d r a d o n e m a n a t i o n is t h e p r i m a r y p u r p o s e o f t h e f i x a t i o n p r o c e d u r e . Because t h e *All prices in 1981 Canadian dollars.
47 Canadian climate precipitation rates exceed those of evaporation, the constant leaching o f 226Ra is of special significance. As a result, the bituminization process would be more suitable in a location with a more arid climate since, in this case, release of radon and surface erosion would be o f major concern. The cost of the bituminization process is its most serious disadvantage. As tested, materials costs alone were approximately $420 per ton o f solidified tailings. If the bitumen content were dropped to 10% this value would drop to approximately $140 per ton. For cost reasons alone, this t y p e of process is therefore n o t practical for solidification o f the bulk of the tailings. Perhaps it can find some limited application as a solidification process to be used to place a cover over an existing tailings, this cover representing only 1--2% of the total volume o f the railings. The cost o f the Chemfix process has been estimated at approximately 300 to 500 thousand dollars in set-up and $7 per ton of tailings for materials. This process is the second least expensive o f those tested. Solidification of railings using this process resulted in some improvement in strength and porosity values over unsolidified tailings, but resulted in a product with poor freeze-thaw resistance. A distinct advantage over the other processes is its rapid setting rate. Radon and radium release rates were generally in agreement with the physical properties, the values being lower than those observed for the CWT samples b u t n o t as low as observed for the AECL samples (except for 22~Ra agitation leach rates) and IUCS samples. The samples solidified using the IUCS process exhibited good strength and porosity values and fairly good resistance to freeze--thaw action. The relatively good physical properties were followed by similar results for the radon emanation and radium leaching studies. The IUCS solidified materials showed the best resistance to acid leaching and second lowest radon release rates when compared to other processes tested. At a cost (materials and processing) of $10--17 per t o n of solidified tailings, this process is considerably less expensive than the bituminization process, b u t the most costly of the inorganic processes. The IUCS costs assume pozzolan and additive sources relatively close to the site and an existing plant. Another advantage of the process is that placement of the material is performed using conventional construction equipment. 7. CONCLUSIONS The solidification of uranium mine-miU tailings to minimize risks of environmental contamination can be performed by several different processes. Although the present study is n o t comprehensive, it does indicate what properties may be expected from tailings solidified using processes of various types. As can be seen by the summary given in Table 13, the process marketed by Canadian Waste Technology was the poorest performer of the group tested. The AECL and IUCS processes gave the most improved properties of the uranium tailings, b u t the bituminization process is much more expensive. The
48
silicate based process (Chemfix) showed some improvement but did not perform as well as the lime--pozzolan or bituminization processes. Thesel categories of solidification processes were suggested as being feasible in section 2 of this r e p o r t The specific solidification process, however, is n o t necessarily representative of all processes encompassed by each category. Total solidification of the tailings may n o t be possible in terms of the costs involved. The solidified materials, however, may be quite useful for containment of the tailings either by capping or erection of barriers. TABLE 13 Summary of results
Percent tailings (approx.) Ease of placement Shrinkage Compressive strength Flexural strength Freeze--thaw resistance Porosity Pore shape Batch leach Percolation leach Radon emanation Cost -- materials -- process
CWT
AECL
Chemfix
IUCS
94 1 4 4 4 4 2 3 4 4 4 1 1
75 4 1 3 1 1 1 -2 2 1 4 4
95 2 3 2 3 3 2 2 2 2 3 3 2
50 3 2 1 2 2 2 1 1 1 2 2 3
Processes are ranked 1--4, 1 being the best performer of the group and 4 being the worst performer. ACKNOWLEDGEMENTS
Authors wish to thank Dr. R.B. Bruce, Dr. D.J. Pinchin, Dr. E.E. Berry, Mrs. M.K. Witte, and Dr. W.R. S t o t t for their participation in the project. REFERENCES 1 Lakshmanan, V.I. and Ashbrook, A.W., Radium balance studies at the Beaverlodge Mill of Eldorado Nuclear Ltd, Seminar on Management, Stabilization and Environmental Impact of Uranium Mill Tailings, Albuquerque, NM, July 24--28, 1978. 2 Lakshmanan, V.I. and Ashbrook, A.W., Second radium balance studies at Beaverlodge Mill, Annual Meeting, Canadian Uranium Producers Metallurgical Committee, Elliot Lake, Ontario, May 12--18, 1979. 3 Haque, K.E., Lucas, B.H. and Ritcey, G.M., CIM Bull., (July 1980) 141--147. 4 Pojasek, R.B., Solid waste disposal: Solidification, Chem. Eng., (August 13) (1979) 141--145. 5 Conner, J.R., U.S. Patent 3,837,872, September 24, 1974. 6 Conner, J.R. and Polosky, R.J., U.S. Patent 3,841,102, October 15, 1974. 7 Krofchak, D., Canadian Patent 102 4277, January 10, 1978. 8 Smith, C.L. and Webster, W.C., U.S. Patent 3,720,609, March 13, 1973; RE 29783, September 17, 1978.