Ammonium hydroxide stripping of tungsten from organic solvents

A process for stripping tungsten values from a tungsten-bearing acidic liquid organic phase into a basic aqueous ammoniacal stripping solution comprises mixing the organic phase and the stripping solution with a high-shear mixing device to maximize the pH gradient between the organic phase and the aqueous solution whereby growth of any precipitated ammonium paratungstate crystals is minimized and the dissolution thereof is maximized and to strip the tungsten values from the organic phase into the stripping solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the recovery of tungsten from its ores, 
particularly as a compound such as ammonium paratungstate (APT). In 
particular, the present invention relates to the treatment of an aqueous 
solution containing sodium tungstate (Na.sub.2 WO.sub.4) and dissolved 
impurities to recover an aqueous solution of ammonium tungstate via 
extraction of tungstate values from the aqueous sodium tungstate solution 
into an acidic organic phase, followed by stripping of the tungsten values 
from the organic phase into a basic aqueous ammoniacal solution. More 
particularly, the present invention relates to improvements in the process 
of stripping the tungsten-rich organic phase with the ammoniacal aqueous 
solution. 
Tungsten is frequently recovered from its ore by a series of steps 
including alkali digestion of the ore to recover an aqueous solution of 
sodium tungstate. Following removal of impurities such as silica and 
molybdenum from this aqueous solution, it is passed through solvent 
extraction and stripping steps to produce an aqueous solution of ammonium 
tungstate that should be essentially free from sodium ions and should 
contain only minor amounts of sulphate ion. In the extraction step, the 
sodium tungstate solution is mixed under acidic conditions in several 
stages with a water-immiscible organic phase comprising, for instance, an 
alkylamine diluted in kerosene, and substantial amounts of tungsten values 
pass into the organic phase. The aqueous-organic mixture is then allowed 
to separate into two discrete phases. The mixing and separating operations 
of the extraction step can be performed in mixer-settler units of 
conventional design. The tungsten-enriched organic phase is typically 
washed, and then conveyed to the stripping step. 
The stripping step comprises one or more stages each comprising a stripping 
unit and a phase separation unit such as a settler. In the stripping unit, 
the tungsten-bearing organic phase is mixed with an aqueous stripping 
solution of ammonia which also contains some ammonium tungstate in 
solution. Tungsten values are stripped from the organic phase into the 
aqueous ammoniacal stripping solution, forming a mixture of the 
tungsten-depleted organic phase and a tungstate-enriched aqueous solution. 
The mixture is then separated, for instance by allowing the aqueous 
solution to settle from the organic phase in the settler. The stripped 
organic phase can be washed and recycled to the extraction step, and the 
tungstate-rich aqueous solution is processed for the recovery of, e.g., 
solid ammonium paratungstate ((NH.sub.4).sub.10 W.sub.12 O.sub.41) (APT) 
crystals. 
In practice, however, the conditions under which conventional stripping 
techniques have been carried out previously have favored the formation in 
the stripping unit of solid reaction products, particularly APT, which 
interfere with the normal operation of the settler. It is desirable that 
the aqueous and organic phases form two sharply divided layers in the 
settling stage as quickly as possible, to maximize efficient recovery of 
tungsten while minimizing contamination of the product, but solid reaction 
products that are formed in the stripping unit and are carried into the 
settler interfere with the separation that must be attained between the 
organic and aqueous phases. Since solids that are carried into the settler 
generally do not redissolve there, the solids must be dealt with before 
reaching the settler. Physically removing the solids necessitates 
additional process time and equipment, and to the extent that the removed 
material contains tungsten its removal represents a decrease in the amount 
of tungsten which would otherwise report to subsequent recovery stages. 
Thus, the formation of APT or other solid reaction products in the 
stripping unit should be minimized. 
Past efforts directed toward minimizing the formation of APT in the 
stripping unit have met with less than complete success, while imposing 
restrictive and expensive requirements as to equipment size and as to 
operating conditions and controls. Thus, there is a need for a process for 
stripping a high percentage of tungsten values from the organic phase into 
the aqueous ammoniacal solution quickly while minimizing the formation of 
solid APT. 
2. Description of the Prior Art 
Previous techniques for stripping tungsten have dealt with the formation of 
solids in the stripping unit by increasing the size and/or number of 
mixing compartments so as to provide enough residence time for the solids 
to redissolve before the liquid passes to the settler. This approach 
increases overall process time, and raises costs, for a given amount of 
production. 
Also, the prior art has favored operating the stripping unit with 
relatively dilute concentrations of ammonium tungstate so as to lessen the 
proximity to saturation of the aqueous phase with respect to tungsten, and 
thus minimize formation of APT solids in the stripping unit. These 
techniques require the operator to sacrifice rate of production as well as 
flexibility of operating conditions, and they require wasteful commitment 
of equipment capacity. In particular, the load on the APT crystallizer is 
increased due to the relatively low WO.sub.3 concentration of the 
ammoniacal liquor fed to the crystallizer and the larger quantity of water 
that must be evaporated. 
South African published patent application Ser. No. 68-492 shows the 
inevitability with which the prior art has viewed both the formation of 
undesired solids in the stripping column and the need for sizing the 
equipment so as to allow the undesired solids to redissolve in the 
stripping column. The applicant teaches stripping tungstate from an 
organic-amine phase by feeding the organic phase into the side of a column 
agitated by a marine-type propeller lying in a horizontal plane just above 
the top of the side inlet. The applicant states that upon initial contact 
of the loaded organic phase with the stripping solution in accordance with 
the disclosed method there occurs some precipitation of a "white, 
tungsten-containing compound" which redissolves before reaching the 
settler stage "providing the column is of sufficient length". It is 
submitted that this teaching suggests increasing the size and, therefore, 
the residence time of the stripping unit in order to keep APT solids from 
passing into the settler. 
In the South African application, an aqueous ammonium tungstate strip 
liquor containing 350 to 370 gpl of WO.sub.3 is formed by contacting the 
organic phase with a stripping solution having a pH of 10 to 11 and 
containing 20 to 125 gpl WO.sub.3. The preferred WO.sub.3 concentration in 
the stripping solution is 100 gpl or less, such as 20 to 40 or 50 gpl; it 
is believed that the lower WO.sub.3 concentrations are preferred in order 
to reduce the formation of solid APT in the stripping unit. 
U.S. Pat. No. 4,092,400 describes stripping an organic phase containing 100 
to 150 gpl of WO.sub.3 with an aqueous solution containing about 1.3 wt.-% 
ammonia. Solid APT forms on contact between the organic and aqueous 
phases, and a retention time of at least 10 minutes is required to assure 
that the APT solids that form are redissolved. 
The prior art thus has not recognized the particular combination of 
conditions under which as tungsten-laden acidic organic stream can be 
stripped of tungsten values in a short residence time without passage of 
undesired solids to the settling unit. 
SUMMARY OF THE INVENTION 
Generally speaking, the present invention comprises a process for stripping 
tungsten values from a tungsten-bearing acidic liquid organic phase into a 
basic aqueous ammoniacal stripping solution comprising mixing the organic 
phase and the stripping solution with a high-shear mixing device to 
maximize the pH gradient between the organic phase and the aqueous 
solution whereby growth of any precipitated ammonium paratungstate 
crystals is minimized and dissolution thereof is maximized and to strip 
the tungsten values from the organic phase into the stripping solution. 
The invention can be carried out in a stripping unit by feeding an aqueous 
ammoniacal stripping solution to the stripping unit, establishing a zone 
of high shear comprising an intimate mixture of the organic and aqueous 
phases with the high-shear mixing device, and, feeding the acidic liquid 
organic phase to the zone of high shear, wherein high-shear mixing is 
imparted to the organic phase which is effective to strip tungsten values 
from the organic phase into the aqueous ammoniacal stripping solution.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates in particular to the conditions under which 
the organic and aqueous phases are contacted with each other in a 
stripping unit such that tungsten values are stripped from the organic 
phase into the aqueous phase in a short residence time without the 
carryover of unwanted solids reaction products into the settling unit. By 
"solid reaction products" is meant solid APT and solid silica-based 
compounds that can form in the stripping unit during mixing of the acidic 
organic tungsten-laden stream with the basic aqueous ammoniacal stripping 
solution. 
With reference to FIG. 1, stream 1 comprises a tungsten-bearing acidic 
liquid organic phase such as is produced by solvent extraction of an 
aqueous tungstate leach liquor, and stream 2 comprises an aqueous 
ammoniacal stripping solution. Streams 1 and 2 are fed separately to 
stripping unit 3, which can comprise the mixer section of a mixer-settler 
unit 4. 
Stream 1 can comprise a tungten-bearing organic phase produced by the 
extraction process described in a patent application, filed on even date 
herewith and assigned to the assignee of the present application, entitled 
"Solvent Extraction of Tungsten from Aqueous Tungstate Solutions" (Ser. 
No. 06/225,906) which is incorporated herein by reference. 
The organic and aqueous phases are mixed in stripping unit 3, and the 
mixture of liquids is then conveyed to settler unit 5 from which stream 6, 
comprising the aqueous stripping solution now enriched in tungstate (the 
"strip liquor"), and stream 7, comprising the organic phase now depleted 
of tungsten values, are recovered. An aqueous recycle stream 8 is drawn 
off of stream 6 and recycled to stripping unit 3. The remaining portion of 
the strip liquor is conveyed to crystallizers in which water is evaporated 
from the strip liquor and APT is recovered. Organic stream 7 is washed at 
washing stage 9 with deionized water 10 to remove entrained ammonia and 
tungsten. The washed organic stream 11 is recycled to the solvent 
extraction step, and a stream 12 of wash water containing small amounts of 
NH.sub.3 and tungstate is recycled to stripping unit 3. 
Stream 1 can comprise a tungsten-loaded organic stream produced by solvent 
extraction of an aqueous solution of, for instance, sodium tungstate, 
advantageously followed by a water wash stage to remove entrained aqueous 
liquid from the organic stream. Stream 1 typically comprises an organic 
diluent, such as kerosene or a mixture of linear aliphatic hydrocarbons 10 
to 13 carbon atoms in length, in which is dissolved about 2 to about 20 
vol. % of an alkylamine extractant such as ditridecyl amine, and 
optionally about 2 to about 25 vol. % of an alkanol conditioning agent 
such as isodecyl alcohol. The alkylamine extractant is loaded with about 1 
to about 200 gpl of WO.sub.3, and can also contain traces of anionic silica 
impurity that has been loaded along with the tungsten. Advantageously, the 
WO.sub.3 concentration is about 10 to about 140 gpl, and more 
advantageously about 25 to about 100 gpl, to provide an increased rate of 
production of the final tungstate product (e.g. APT). 
Stream 2 comprises an aqueous ammoniacal stripping solution containing free 
ammonia, supplied as fresh NH.sub.4 OH in stream 13. While the free ammonia 
concentration can be between about 2 to about 80 gpl, it is advantageously 
about 10 to about 60 gpl and more advantageously about 15 to about 30 gpl 
to provide efficient stripping of tungsten from the organic phase. The pH 
of stream 2 is about 9.0 to about 11.5, and advantageously about 9.5 to 
about 11.0 to optimize stripping of tungsten values from the organic 
phase. 
It is advantageous that the aqueous stream 2 contain dissolved ammonium 
tungstate as well as free ammonia, to permit recovery of a tungstate-laden 
strip liquor containing a high concentration of tungstate. Again referring 
to FIG. 1, stream 6, containing the aqueous strip liquor, is conveyed from 
the settling unit to the crystallizers in which water and ammonia are 
volatilized, and APT is formed; higher concentrations of tungstate in the 
strip liquor advantageously require less time and energy to drive off 
water and ammonia in the APT crystallizers. Thus, to provide a high 
concentration of tungstate in the strip liquor, recycle stream 8 
comprising ammonium tungstate in solution is drawn off of stream 6 and fed 
to stripping unit 3 together with fresh ammoniacal feed (e.g. NH.sub.4 OH) 
and recycled water from washing stage 9. Thus, stream 2 also contains 
about 25 to about 200 gpl WO.sub.3, and advantageously about 100 to about 
175 gpl WO.sub.3 to optimize recovery of a satisfactory concentration of 
WO.sub.3 in the strip liquor. It is a surprising and advantageous feature 
of the present invention that a stripping unit can be operated with so 
high a WO.sub.3 concentration and a short residence time without passage 
of solid reaction products out of the stripping unit. 
The concentration of tungstate in the strip liquor (stream 6) is typically 
about 50 to about 300 gpl WO.sub.3 ; advantageously it is about 100 to 
about 275 gpl WO.sub.3, and more advantageously about 150 to about 250 gpl 
WO.sub.3, to permit more economical operation in the crystallizer. 
With reference to FIG. 2, the invention will be described as it can be 
carried out in stripping unit 3. 
Organic stream 1 and aqueous stream 2 are separately fed to stripping unit 
3 where they are mixed. The mixture flows upward and through outlet 14 
into settler 5. The ratio of the volume feed rates of the organic and 
aqueous streams (O/A) is about 0.5 to about 3.0, and advantageously about 
0.8 to about 1.2 to optimize stripping of tungsten in the stripping unit. 
Advantageously, the flow rates of streams 1 and 2 are selected with 
respect to the volume of stripping unit 3 to provide a residence time of 
less than about 10 minutes, and more advantageously less than about 6 
minutes. 
The liquid in stripping unit 3 is under agitation by an impeller 15 which 
is being rotated via shaft 16 by externally mounted motor 17. Any of 
several types of impeller can be used. Very satisfactory results have been 
obtained with a "turbine-type" impeller, having a plurality of blades 
mounted around the axis of rotation. The impeller can be provided with a 
shroud to increase pumping action. 
Impeller 15 is rotated so as to impart high-shear to a zone 18 around the 
blades of the impeller. Organic stream 1 is fed into zone 18, and 
high-shear mixing is imparted to the liquid in zone 18 which is effective 
to strip tungsten from the organic phase into the aqueous phase and 
effective to permit recovery from the stripping unit 3 of the 
tungsten-depleted organic phase and the tungsten-enriched aqueous solution 
as a liquid mixture which is free of solid reaction products. Such 
high-shear mixing maximizes the pH gradient between the organic phase and 
the aqueous solution whereby growth of any precipitated ammonium 
paratungstate crystals is minimized and the dissolution thereof is 
maximized. Upon initial contact between the organic phase and the aqueous 
solution, the aqueous solution is momentarily depleted of ammonia by the 
stripping reaction and any precipitated ammonium paratungstate crystals 
can grow to sizes which are not readily redissolved. Effective high-shear 
mixing instantaneously re-establishes the initial high pH gradient between 
the organic phase and the aqueous solution thereby minimizing the growth of 
any precipitated ammonium paratungstate crystals. The effectiveness of the 
high-shear mixing can be ascertained by observing the initial contact 
between the phases. The region of initial contact should either remain 
clear (i.e., no precipitation of ammonium paratungstate) or become 
temporarily clouded followed by rapid clearing (i.e., precipitation of 
finely divided ammonium paratungstate followed by rapid redissolution). 
The high-shear mixing should not be so high, however, as to form an 
emulsion of the aqueous and organic phases; in such an event, phase 
separation would be very slow. The mixture is passed to settler 5, in 
which the organic and aqueous phases readily separate from each other with 
a desirable clear, well-defined interface. Tungsten is substantially 
completely stripped from the organic phase into the aqueous phase in the 
stripping unit; typically, at least about 90% and advantageously at least 
about 99% of the tungsten is stripped. 
The organic stream is fed to the stripping unit at a point near the blades 
of the impeller 15, where a zone of high shear can readily be established. 
Advantageously, to maximize contact between the organic phase and the 
aqueous ammoniacal stripping solution for more efficient stripping, the 
stripping solution is separately fed to the zone of high shear. One 
skilled in this art will recognize that the location of the zone of high 
shear can be readily ascertained, and that the location of this zone is 
relatively static in a unit operating at steady state. Thus, a stripping 
unit can be designed by reference to the foregoing description and the 
following Examples. Operating conditions should be established so that the 
aqueous phase is the continuous phase in the stripping unit. 
Residence times in the stripping unit of less than about 10 minutes, and 
more advantageously less than about 6 minutes, are preferred so as to 
increase the rate of production of tungsten-bearing strip liquor, and are 
feasible by stripping in accordance with this invention. As is well known, 
the residence time is a function of the stripping unit volume and the total 
flow rate of liquid (organic phase and aqueous stripping solution) into the 
stripping unit. 
The relatively short retention time in the stripping unit minimizes the 
opportunity for silica to precipitate within the stripping unit and the 
mixer-settlers. If silica concentration is relatively high in the feed to 
the solvent extraction step, significant quantities of silica can enter 
the stripping operation. In conventional practice, silica then 
precipitates not only after the stripping operation, but during stripping, 
necessitating periodic equipment cleaning and special holding and digestion 
tanks for precipitation and removal of precipitated silica. 
While the apparatus shown in FIG. 2 depicts one stripping unit feeding 
directly to the setttler, the present invention is also applicable to the 
treatment of a tungsten-bearing organic stream with a plurality of 
stripping units. For example, two stripping units may be operated in 
series; in such a case the aqueous-organic mixture formed by operation of 
the first stripping unit in accordance with the present invention 
constitutes the feed to the second stripping unit, which in turn further 
agitates the mixture sufficiently fast to avoid the formation of solid 
reaction products, and discharges the mixture to the settling unit. 
The invention will be further illustrated with reference to the following 
non-limiting Examples: 
EXAMPLE 1 
An organic stream composed of 7% ditridecyl amine, 12% isodecanol, and 81% 
kerosene (by volume), and containing 68.1 gpl of tungsten (as WO.sub.3), 
and an aqueous stream which contained 148 gpl WO.sub.3 and sufficient free 
NH.sub.3 to establish a pH of 10.5, were fed separately but simultaneously 
to a stripping unit at an average O/A ratio of 1 to 1.1. The residence 
time in the stripping unit was 6 minutes. The organic stream was fed into 
the stripping unit adjacent the outside edges of a 13/4 inch diameter 
shrouded radial turbine impeller which was rotating at about 1400 
rotations per minute. The aqueous stream was fed into the bottom of the 
unit. The unit was operating at a temperature of 40.degree. to 45.degree. 
C. The organic-aqueous mixture leaving the stripping unit contained no 
solid reaction products. Doubling the total flow rate of the aqueous and 
organic phases to the unit, e.g., decreasing the retention time to 3 
minutes, also generated a discharged organic-aqueous mixture containing no 
solids. Tungsten transfer from the organic to the aqueous phase was 
essentially complete. 
EXAMPLE 2 
An organic stream composed of 7% ditridecyl amine, 12% isodecanol, and 81% 
kerosene (vol.%), and containing 67.8 gpl of tungsten (as WO.sub.3), and 
an aqueous stream which contained 130 gpl WO.sub.3 and sufficient free 
NH.sub.3 to establish a pH of 10.5, were fed separately through the bottom 
of the stripping unit, at an average O/A ratio of 1 to 1.1. The residence 
time in the stripping unit was 12 minutes. The organic and aqueous streams 
were fed into the stripping unit underneath the outside edges of a 13/4 
inch diameter shrouded radial turbine impeller, which was rotating at 
about 1400 rotations per minute. The aqueous stream was also fed into the 
stripping unit underneath the outside edges of the impeller. Because of 
the physical arrangement of the inlet streams, back-mixing of the two 
phases occurred in a region of low shear. Copious quantities of white APT 
solids were precipitated, and were carried out into the settler 
compartment. These solids did not redissolve after standing overnight. The 
problem of solids formation was corrected by relocating the organic inlet 
such that the organic phase was injected directly into the high-shear zone 
of the impeller, such that back-mixing was avoided. The aqueous stream was 
also fed into the stripping unit underneath the impeller.