Hydrometallurgy, 5 (1980) 9 7 - 1 0 7
97
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CRUD IN SOLVENT EXTRACTION PROCESSING
CAUSES AND TREATMENT
-
A REVIEW OF
G.M. RITCEY
Extractive, Metallurgy Section, Ore Processing Laboratory, CANMET, Energy, Mines and
Resources Canada, Ottawa (Canada)
(Received May 3rd, 1979; accepted May 25th, 1979)
ABSTRACT
Ritcey, G.M., 1980. Crud in solvent extraction processing -- A review of causes and treatment. Hydrometallurgy, 5: 97--107.
An overview is presented on the various aspects affecting solvent loss through crud
formation, encountered in many hydrometallurgical plants that employ solvent extraction
processing. This crud, or stable emulsion mixed with solids, can be caused by properties
of the feed solution, the solvent and its composition, the type of contactor selected and
the method of operation. Cruds differ between plants and therefore treatment schemes
and preventative measures are impossible to define without actual testing and evaluation.
A summary of items for consideration in solving crud problems is given.
INTRODUCTION
Common to all or most solvent extraction operations in the mining industry is the problem of stable emulsions and the eventual formation of cruds.
The crud can constitute a major solvent loss to a circuit and therefore adversely affect the operating costs. Because there can be many causes of crud
formation, each plant may have a crud problem unique to that operation.
Factors such as ore type, solution composition, solvent composition, presence of other organic constituents, design and type of agitation, etc. can
adversely affect the chemical and physical operation of the solvent extraction
circuit, and result in crud formation.
Crud is defined as the material resulting from the agitation of an organic
phase, an aqueous phase, and fine solid particles, that form a stable mixture.
Crud usually collects at the interface between the organic and aqueous phases
Other names that have been used for the phenomena are grungies, mung,
gunk, sludge, etc.
This paper will cover, in an overview, the general aspects of solvent losses
by crud; its formation and characteristics, and its treatment and prevention.
Solvent losses in processing can be attributed to: (1) entrainment, (2)
solubility, (3) evaporation, (4) degradation, (5) adsorption on solids, (6)
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formation of stable emulsions and cruds, (7) spillage and (8) sampling. These
solvent losses, and particularly those losses attributed to emulsion and crud
formation, can in part be related to: (1) nature of feed, (2) reagent choice,
(3) equipment selection and (4) m e t h o d of operation, such as the droplet
size, continuous phase, excessive turbulence, etc. Additional details on solvent
losses are noted in a text co-authored by the writer [1].
POSSIBLE CAUSES OF CRUD F O R M A T I O N
Nature of feed
The nature of the feed composition can be a major determining factor as
to whether crud will be formed in the subsequent extractive operations.
Solids must be absent from most solvent extraction circuits, and clarification
is usually aimed at achieving about 10 ppm solids. One of the major causes
of crud is the lack of good clarification, with the result that solids get through
to the solvent extraction circuit. The presence of colloids, such as silica [2],
can produce stable emulsions and crud during the mixing of the phases to
achieve mass transfer. Aged feeds can constitute a greater potential crud
problem than fresh leach solutions [3]. In plants where bacteria have been
prevalent, due to favourable environmental conditions, crud has resulted and
expensive circuit modifications were subsequently required. The elimination
of air to such circuits is often necessary to minimize bacteria and fugal growth
[4]. Certain systems may have hydrolysed compounds precipitating out of
solution, and thus a crud results. In certain extraction systems the anionic
strength of the aqueous feed solution may be insufficient, so that stable
emulsions occur when the two phases are mixed. If sufficient agitation is applied over a period of time, then crud can result. One other important cause
of cruds in solvent extraction plants is the dust from the air, if permitted to
be drawn into the agitation in a mixer-settler circuit. Thus, vessels should be
covered to prevent dust accumulation. Organic matter in the feed, such as
lignin or humic acids, may also promote crud formation.
Nature of solvent
The choice of the extractant and solvent composition is an important
aspect in the successful solvent extraction operation, but the possibility of
crud due to the solvent composition must not be overlooked. Many systems
require a modifier, to improve phase disengagement, to assist in solubilizing
the metal-organic species and to reduce third phase and emulsion tendency.
If a solvent has the tendency to produce emulsions on mixing with the aqueous feed solution, which could cause cruds if colloids or suspended solids
are present in the aqueous feed, then the cause may be due to several factors.
Perhaps the system requires the addition of a modifier, a change to a different
modifier, or a higher modifier concentration is demanded. Also, possibly the
diluent type and composition may not be compatible with the system. An
aromatic diluent or an aliphatic diluent with some aromatic content may be
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more desirable than a completely aliphatic diluent for that particular process.
Frequently, in the solvent makeup, there are unreacted chemicals from the
manufacturing process or, possibly, impurities from the containers used to
transport the solvent components. The problems associated with such impurities become particularly evident if their effects are not considered during
bench scale investigations [1]. Their presence in a solvent system can produce
cruds, good or poor phase separation and enhanced or poor loading characteristics. Such effects could result in abandoning a particular solvent because of
its poor chemical and physical characteristics.
Degradation of the solvent, due to the presence of certain metals in the
feed solution, use of oxidizing agents during stripping, high temperature
processing, biodegradability, etc. may result in decomposition products
forming stable emulsions and cruds. Several uranium plants have reported the
degradation of the isodecanol modifier to isodecanoic acid in the amineisodecanol system. Naphthenic acid extraction of copper from leach liquors
also showed degradation to an insoluble crud composed of 23% of the extractant [4]. In the refining of uranium using tributyl phosphate in contact
with nitric acid, degradation products of mono- and dibutyl phosphates are
produced. Amines are susceptible to degradation in the presence of oxidizing
agents. Carboxylic acids and D2EHPA have been reported to withstand lowpressure reductions in a process to produce metal powders [ 5]. The LIX
reagents containing LIX 63 (e.g., LIX 64, LIX 64N, LIX 70 and LIX 73) are
stable up to temperatures of 40°C, while those not containing LIX 63 {e.g.,
LIX 65N, and LIX 71) can safely withstand higher temperatures.
Equipment selection
There is no universal contacting equipment suitable for all solvent extraction operations. Even within a plant, it may be completely wrong to
select the same type of contactor for all stages of the extraction process. Each
plant, therefore, has to make a choice; the final selection is governed by the
t y p e of aqueous feed and composition, the solvent type and composition,
and how the respective physical characteristics affect the mixing process, flow
patterns and coalescence [6]. Naturally the mass transfer efficiency must also
be considered. With an adequate understanding of all physical--chemical
variables present in the process that will thus have an effect on the extraction
as well as the minimizing of emulsion and crud, then the right equipment for
that plant can be selected [1, 6 ] . That is, the type of equipment and the
method of agitation used to achieve mass transfer is of concern, if emulsion
tendency is to be minimized. Degradation of the solvent may have to be
considered in the equipment choice, due to the chemical system. In one
plant, centrifugal contactors were chosen over mixer-settlers because of lower
solvent degradation [ 7 ].
Method of operating
In the solvent extraction process, one of the major concerns should be the
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technique by which mass transfer is achieved. That is, the physical design and
operation can contribute n o t only to high solvent losses, such as by entrainment, b u t also to the formation of stable emulsions and cruds. The physical
aspects of the process are concerned with the dispersion of the two phases
on mixing, the t y p e of droplet formation and the rate and completeness of
coalescence. These are important aspects in the choice of suitable contacting
equipment and in the design and operation of the plant. Depending upon the
t y p e of contacting equipment selected and the energy input to the system,
different types of dispersion will be created for a particular system. Differences will be evidenced b y rates of mass transfer, drop size distribution,
wetting of surfaces, sedimentation and coalescence rates, and entrainment.
Depending upon the physical--chemical properties of the two phases being
dispersed, such as viscosity, surface tension, presence of solids, colloids, etc.,
then with increasing agitation and decrease in drop sizes, a region of instability will be reached followed by a stable emulsion. If solids or colloids are also
present, then a crud will result. This is demonstrated in Fig. 1 showing the
operating regions of pulsed columns [ 8]. The information is readily related
to excessive turbulence in mixer-settlers, particularly of the pump--mix
design, and in certain agitated columns where backmixing is severe. Flow
patterns during mixing can influence emulsion tendency, which can be further
influenced b y the continuous phase.Thus, if solids are present in mixer-settler
operations, and excessive turbulence exists, it would be advisable to use a
t y p e of contactor more suited to the physical--chemical characteristics of
the system. Centrifugal contactors would also be an unwise choice if solids
are present or crud formation is likely. Equipment such as the Graesser
Contactor, pulse sieve-plate column, ARD contactor and possibly in-line
mixers could be considered. Pulse columns have been described in the extraction of uranium from ore leach slurries and in the presence of crud [9, 10]
oJ
~0//
Flooding
(insufficient
pulse)
~%~/
Flooding
Region
~/
~/
Pulse Frequency x Amplitude
Fig. 1. Operating regions of a pulsed column.
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The Purex uranium reprocessing flowsheet was examined by investigators
to determine which of mixer settlers, centrifugal contactors or pulse columns
would be the most suitable where there existed the possibility of crud formation during the extraction stage [11]. Both the mixer settlers and centrifugal
contactors proved unsatisfactory in the presence of the crud, while the
pulsed columns were self-cleaning with respect to solids, and no accumulation
of solids or plugging occurred. From their evaluations, pulsed columns were
r e c o m m e n d e d as extractors in large reprocessing plants.
Choice of the continuous phase, coupled with the contactor and flow patterns produced during the operation, may reduce the tendency of emulsification and crud formation if solids ~¢e present.
Although it is usually desirable to operate at saturation loading of the
solvent, there are certain situations where it is necessary to maintain less-thansaturation. For example, in the extraction and separation of zirconium from
hafnium in a nitric acid system, using TBP, the system operates only if run
at a b o u t 10% less than saturation. As saturation of the solvent is approached,
a zirconium c o m p o u n d is precipitated. In the presence of the solvent and
agitation, the result is a stable emulsion and crud [12]. Similar crud problems
can occur in rare~arth circuits using D2EHPA, where below saturation loading has to be maintained to prevent crud formation [13, 14].
D E S C R I P T I O N O F SOME C R U D S
The chemical and physical aspects of crud can differ for each separate
operation, and will vary in inorganic composition, organic content, color,
density, etc. The composition of many cruds appear to have in c o m m o n
such constituents as Si, A1, Fe, P, SO4, together with solvent, particles of
gypsum, clay and other fine particles, Often there is a direct relationship
between the feed liquor and the crud compositions, indicating possible
aqueous carry-over as well as inefficient clarification prior to solvent extraction.
In some of the South African plants a tar-like substance is generated and
determined to be aliphatic carboxylic acids. There is evidence of the presence
of isodecanoic acid, resulting from the oxidation of the isodecanol modifier.
More than one plant in North America has experienced some crud problems
when greater than 3% isodecanol is present. Possibly the vortex created on
mixing is a contributing factor to the crud produced b y the break<lown of
the isodecanol. Ferric iron, as an oxidant, is also present. In one early plant,
the animal glue flocculent used for filtration of the leach pulp caused severe
fungal growth in the solvent extraction circuit. The addition of a solventsoluble fungicide (a derivative of benzo-thiozole) alleviated the crud due to
the fungus. Use of an aromatic diluent, instead of aliphatic, was also affective
in minimizing the crud problem due to bacteria.
In some scrub and strip circuits, the crud is mainly composed of silica, as
well as inorganic sulphates. Such crud may be treated with dilute sulphuric
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acid, and recirculating through a pump results in the crud breaking down.
There is evidence in at least a few uranium circuits that the presence of
humic acids may be a possible cause of the crud problem. Lignin appears to
be another cause of crud formation.
Plants extracting uranium from phosphoric acid can also have excessive
crud formation, which is often a waxy material. This appears to be due to
the excessive agitation of the two phases in the presence of humic acids conrained in the phosphoric acid.
The presence of hydrolyzable compounds, which precipitate due to excessive agitation and high shear, can cause severe crud problems. Zirconium
presents a particular problem and is a major constituent of at least one crud
in a uranium processing circuit. Also, as stated earlier, zirconium will tend to
hydrolyze in a H N O 3 - T B P circuit given the right conditions of shear or energy
input (proximity to saturation of the solvent and the type and construction
of contactor, particularly in the coalescing zone). The use of Teflon plates
in columns for such an extraction process definitely increased coalescence and
decreased the tendency for hydrolysis and crud formation. [15].
T R E A T M E N T AND SOLVENT RECOVERY
Because crud originating in one plant is usually different from that obtained in another, or in another circuit in the same plant, the treatment
scheme adapted for solvent recovery will, of necessity, be different. That is,
there is no universal treatment scheme that would be amenable to all cruds
because of the difference in the causes of crud formation. Some cruds are
broken down by settling and skimming [16], others by filtering [17], some
by acidification [16] or neutralization to an alkaline condition [9, 16], some
are broken by extreme agitation such as cycling through a centrifugal pump
[ 16], etc. After such treatment procedures, if successful, the solvent that is
released can be readily decanted for recovery. In some plants, a separate
solvent treatment stage is necessary, with a wash such as Na2COa being used
[16].
As an example of how different plant cruds react to treatment, CANMET
treated three by acid and alkali adjustment of a slurry-water mixture. None
of the three, one of which was from a stripping circuit, responded to alkali
treatment. In fact the stable emulsion and crud problem became greater.
With the first extraction crud containing a large a m o u n t of slimes from the
CCD circuit, 50% of the crud was dissolved at pH 1, and about 70% at pH
1. The second extraction crud, containing no slime solids from clarification, when mixed with water, 50% of the organic was released. Acidification
to pH 0 released no further organic, and in strong H2SO4, 95% of the organic
was released but as a fine emulsion. A sample of crud from a stripping circuit
was determined to be composed of 60% organic, 30% solids (as a precipitate)
and 10% aqueous. Acidification to pH 1.2 dissolved 20%, while at pH 0.5,
85% of the solvent was released. Further acidification resulted in reforming
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the crud. However, if the crud was slurried in the strip raffinate from that
circuit (initial pH 4.17), instead of water, only 50% of the crud dissolved at
pH 0.5, and only in strong acid was 90% of the solvent released.
Thus, with cruds differing in their chemical and physical-chemical history
of formation, treatment to break down the crud and recover the solvent is
only accomplished by the testing and evaluation of many possibilities. At
this time, only "trial and error" tests can be suggested to determine the best
treatment techniques.
CRUDPREVENTION
Because crud is a difficult phenomenon to completely characterize, is
often site specific, and indeed can vary within an extraction, scrub and stripcircuit operation, preventative measures are therefore difficult to cite. The
following are some of the methods that can be suggested as to the prevention
of crud. Again, it must be emphasized that because cruds have a different
history of formation one or more preventive measures may be necessary.
(a) Solids in the feed were mentioned as one of the major causes of subsequent crud formation in the solvent extraction circuit. Good clarification
is therefore necessary to minimize crud and therefore operating costs. Table
1 itemizes some benefits of good clarification [18].
TABLE 1
Benefits of good clarification
Better mixing efficiency
Less crud and therefore less solvent loss
Lower organic entrainment
Less difficulty with maintaining continuous phase
Decreased iron transfer to electrolyte (in copper processing) via suspended solids
Increased efficiency in the tank house
Decreased maintenance
In North America and South Africa, the objective is to obtain approximately 10 ppm suspended solids in the feed to extraction. This is usually obtained
by the use of sand filters after a CCD circuit. At least one plant has reported
that crud quickly developed when the sand filters were not in operation [19].
New plants are attempting to achieve this objective by the use of Enviroclear
thickners, leaf clarifiers and sand filters. Although sand filters are used successfully in some plants, other plants have not been able to achieve their
objective. Insufficient frequency of back-washing the sand filters is a probable cause of poor clarification.
Because dust can cause crud if permitted in a mixer-settler circuit, particularly if the settlers are located in the open, adequate covers over the settlers should be provided.
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(b) Colloidal silica in some circuits such as TBP--HNO3 in reprocessing of
uranium, can be reduced b y the addition of gelatine and heating to 80°C to
coagulate, followed b y centrifuge separation [2]. The addition of certain
surfactants, to lower the surface tension, may also reduce cruds due to colloidal silica [ 1 7 ] , as well as break emulsions [16]. Addition of sequestering
agents was successful in eliminating the deposition of calcium and the subsequent plugging of a centrifugal contactor with a calcium--rare earth precipitate, in a rare e a r t h - D 2 E H P A circuit [ 1 3 ] . However, the addition of
such reagents could also cause solvent degradation with continual cycling, so
the approach must be carefully investigated prior to adaption. Surfactants,
and other c o m p o u n d s used to enhance the liquid-solid separation after
leaching, could also enhance emulsion and crud tendency.
(c) Certain organic constituents, such as lignins and humic acids, may be
solubilized in the aqueous feed and may cause problems in the solvent extraction circuit. At this time, very little is known about this particular area of
crud formation. In phosphoric acid circuits, it has been found useful to
remove humates b y coagulation with surfactants followed by filtration [20].
Lignin can be removed by passing the leach solution through a bed of activated carbon [ 1 6 ] . Pressure treatment, under oxygen, at 200--250°C will
destroy the lignin components [16].
(d) Crud due to bacterial and fungal growth may be minimized by elimination of as much air as possible to the system [21]. These growths, in association with any solids present in the feed liquor, lead to the formation of crud.
Fungus growth in some circuits, due to the isodecanol modifier in an amine-uranium system, was eliminated by using 35% Solvesso 150 aromatic diluent
[ 2 2 ] . The aromatic diluent acts as a bactericide and fungicide. Commercial
bactericides have also been used successfully.
(e) Proper selection of the extractant as well as the other solvent components can minimize emulsions and crud formation. Freshly prepared solvents
can often contain impurities which could subsequently cause operational
problems. Therefore, all solvents should be conditioned prior to their use.
Having selected a suitable solvent system, cyclic tests should be performed to
determine whether degradation of any of the solvents components is taking
place. Any degradation products could cause crud formation m the circuit.
(f) There are a number of items for consideration in the operation of the
circuit to minimize crud formation. In certain systems, solvent saturation
can result in the formation of gelatinous solids, as in the rare earth--D2EHPA
system [13, 14] and in the z i r c o n i u m - T B P circuit [ 1 2 ] . The p h e n o m e n o n
is partially due to the increase in viscosity as loading is reached. An increase
in the O / A ratio thus results in a decrease in crud formation. As the viscosity
increases, excessive agitation can produce stable emulsions.
Flow patterns can be altered b y change of the continuous phase, and therefore the tendency for the formation of emulsions or cruds is altered. At one
uranium refinery, the solvent is maintained in the continuous phase in order
to produce flow patterns to reduce emulsion tendency [23].
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Occasionally, the water used for solution makeup to scrub or strip circuits, because of impurities, can cause subsequent emulsions and cruds. At
one uranium plant in South Africa, deionized water was used to l~epare the
ammoniacal strip solution, rather than normal plant water which tended to
cause crud formation [ 2 2 ] .
Equipment selection is important, as is also the proper operation of the
contacting devices. It is generally recognized that high shear is the primary
cause of the droplet haze, and subsequent emulsion and crud formation.
TABLE 2
Possible items of information required for consideration in solving plant crud problems
(1) Ore
-- mineralogy and analysis
(2) Leach
Possible effects on degradation as well as
-- oxidant and quantity added
emulsion-production or crud-stabilization.
-- any other chemicals added
(3) Liquid--solids separation
-- type of separation (e.g., CCD)
-- type and quantity of surfactant added
use of sand filters or other types of clarifiers (e.g., anthracite and frequency of
regeneration.
(4) Feed solution to solvent extraction
suspended solids
dissolved solids
solution composition (e.g., silica, aluminum, molybdenum, zirconium, etc.)
presence of humic acids
presence of lignin
(5) Solvent ex trac rio n
(a) extraction
modifier and concentration
diluent
extractant
vortex in mixer
mixer design (e.g., baffling)
agitation (rpm and design, energy)
degradation of modifier and extractant (and surfactant from L/S separation)
settler design (entry to dispersion band, flow-rate design, baffling)
velocity across settler
continuous phase
any fungus or bacteria present
viscosity, surface tension or interracial tension
presence of precipitates
(b) stripping
mixer and settler operations as in " e x t r a c t i o n " above
degradation of modifiers and extractant
continuous phase
any fungus or bacteria present
stripping agent
viscosity, surface tension or interracial tension
presence of precipitates
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T h u s , t h e t y p e a n d a m o u n t o f agitation (shear) m u s t be o p t i m i z e d f o r mass
t r a n s f e r while m i n i m i z i n g e m u l s i o n and c r u d f o r m a t i o n . Table 2 indicates
items o f i n f o r m a t i o n w h i c h m a y be r e q u i r e d in analysis o f c r u d f o r m a t i o n
problems in a plant.
CONCLUSIONS
The f o r e g o i n g has been an a t t e m p t t o provide an overview o f the various
aspects o f c r u d , its f o r m a t i o n , r e c o v e r y o f solvent and possible m e t h o d s o f
p r e v e n t i o n . O b v i o u s l y there are m a n y items for c o n s i d e r a t i o n with respect t o
the c h e m i c a l and physical parameters, the engineering, design and o p e r a t i o n
o f solvent e x t r a c t i o n circuits if we are to fully u n d e r s t a n d and solve the
p r o b l e m o f c r u d f o r m a t i o n . Thus, m u c h research and basic studies are necessary in the f u t u r e if the p r o b l e m s , and expense, caused b y crud f o r m a t i o n
are t o be eliminated.
REFERENCES
1 Ritcey, C.M. and Ashbrook, A.W. Solvent Extraction--Principles and Applications to
Process Metallurgy. 2 volumes, Elsevier, Amsterdam. 1979.
2 Cao, S., Dworschak, H. and Hall, A. In: Proceedings of the International Solvent Extraction Conference, ISEC '74, Lyon. Soc. Chem. Industry, London, pp. 1453--1480.
3 Ritcey, G.M., Slater, M.J. and Lucas, B.H. Proceedings of International Hydrometallurgy Symposium, AIME, Chicago, Feb., 1973. AIME, New York, 1973, pp. 419--474.
4 Fletcher, A.W. and Hester, K.W. A new approach to copper-nickel ore processing,
paper presented at the Annual AIME Meeting, New York, Feb., 1964.
5 Burkin, A.R. In: Proceedings of First Hydrometallurgy Meeting, CIM, Ottawa, Oct.,
1971.
6 Ritcey, G.M. Solvent extraction contactors. In: Proceedings of AIChE Symposium
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7 Orth, D.A., McKibben, J.M. and Scotten, W.C. Proceedings of International Solvent
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514--433.
8 Sege, G. and Woodfield, F.W. Chem. Eng. Prog., Aug., 1954, p. 396.
9 Ritcey, G.M., Joe, E.G. and Ashbrook, A.W. Trans. A.I.M.E., 238 (1967), 330--334.
10 Ritcey, G.M., Slater, M.J. and Lucas, B.H. Proceedings of International Hydrometallurgy Symposium, AIME, Chicago, Feb., 1973. AIME, New York, 1973, pp. 419--474.
11 Huppert, K.L., Issel, W. and Knoch, W. Proceedings of International Solvent Extraction Conference, ISEC '71, The Hague. Soc. Chem. Industry, London, 1971, pp.
2063--2074.
12 Ritcey, G.M. and Conn, K. Liquid--liquid separation of zirconium and hafnium; Eldorado Nuclear, R&D Division, Ottawa, Report T67-7; 1967.
13 Lucas, B.H. and Ritcey, G.M. CIM Bulletin, January, 1975.
14 Gaudernack, B. and Braaten, O. Occurrence and extraction of rare earths in Norway.
Presented at the 9th Rare Earth Research Conference, Oct., 10--14, 1971, Virginia.
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16 Personal communication.
17 Lucas, B.H. and Ritcey, G.M. CIM Bulletin, June 1975; Canadian Patent No. 101759,
Sept. 1977, U.S. Patent 3,969,476, 1976.
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18
19
20
21
22
23
Rossiter, G. Anamax Twin Buttes Oxide Plant Operating Experience -- First Year.
Presented at the Arizona Section, AIME, Hydrometallurgical Division, spring, 1976.
Abramo, J.A. and Lowings, S.W.H. Uranium processing at Exxon's Highland Operation. Presented at Symposium on Solvent Ion Exchange, AIChE, Tucson, Arizona, May,
1973.
Hurst, F.J. Recovery of uranium from wet-process phosphoric acid by solvent extraction. Presented at Annual AIME meeting, Las Vegas, February, 1976.
Reference 15, Davy-Power Gas, p. 59.
Meyburgh, R.G.J. South African Inst. Min. and Metall., Oct., 1970, pp. 54--66, and
April 1971, pp. 190--197.
Ryle, B.G. USAEC Report TID 5295, 1956.