CA1154268A - Separation process for the recovery of uranium from wet-process phosphoric acid - Google Patents

Abstract

ABSTRACT

A synergistic extractant combination consisting of di(2-ethylhexyl) phosphoric acid (D2EHPA) and dibutyl butyl phosphonate (DBBP) in kerosene is employed in a two-cycle separation process for the recovery of uranium from wet-process phosphoric acid. The addition of the sulfuric acid and water scrubbing steps for the recycled extractant produces no problems to the contamination and dilution by the ammonium ion and water to the phos-phoric acid and also no precipitation occurs in the second cycle extraction step. The advantages of this process are lower chemical cost, higher product purity and better phase separation in comparison with the previous process.

Description

~5~268 The present invention xelates to a separation process for the recovery of uranium from wet-process phosphoric acid.
Wet-process phosphoric acid contains a significant amount of uranium, typically about 0.1 to 0.2 g/Q. Apart from being economically favourable, recovery of uranium as a by-product of phosphate fertilizer production supplies part of the world's nuclear fuel resource and reduces the environmental contamination. In order to recover the valuable source of uranium, a so-called "D2T-D2T process" has been developed by a research team at the Oak Ridge National Laboratory in U.S.
to separate it from phosphoric acid solution prior to further treatment to produce a fertilizer product. (See U.S. Patent No. 3,711,591 by F. J. ~lurst et al. titled "Reductive Stripping Process for the Recovery of Uranium from Wet-process Phosphoric Acid". Also, see the article in Ind. Eng. Chem. Process Des.
Develop., 11, 122 (1972) by F. J. ~urst et al. titled "Recovery of Uranium from Wet-process Phosphoric Acid").
The D2T-D2T process comprises two parts. A synergis-tic extractant combination consisting of di(2-ethylhexyl) phos-phoric acid ~D2EHPA) and trioctylphosphine oxide (TOPO) inkerosene is used in this process to extract uranium in the +6 oxidation state in the ~irst part. In the second part of the process, the concentrated acidic aqueous stream is ~54216~

X ;L.I j~
again~3*~ e~ and extracted with the same synergis~ic organic extractant, and finally s~ripped with ammonium carbonate solution to precipitate an~onium uranyl tricarbonate9 ~NH4)4UO~(C0~3~ (AUT~.

This process typically suffers ~rom several disadvanta~es: (1) The TOPO synergis~ic ayent is expensive and hence total chemical reagent cost is higher. (2) In the second purif~cation part of D~T-D~T process, the use of ammonium carbonate as the stripp;ng reagent results in the conversion of the D2E~PA to a highly hydrated ammonium salt, ~C~H9CH(C3H~)CHz0]2e(0NH4)-xHzO.
When the stripped solvent is recycled to the extraction step, the extracted water and NH~ transfer to the aqueous phosphoric phase9 d;lute and contami-nate the wet-process phosphoric acid~ which is unacceptable to the phosphoric acid plant to produce ~eed stocks and Yarious phosphate chemicals. These in turn have some other d;sadvantageous e~ec~s such as unclear phase separation~
ammonium salt precipitation ~Fe~NH4Hd(PO4)~-6H~O etc.~ and cause problems during operation.

SUMMARY OF TH~ INY~NTION

The present invention relates to a two-cycle separation process for the recovery of uranium from wet process phosphoric acid. A synergistic combination of di(2-ethylhexyl) phosphoric acid ~D2EHPA) and dibutyl butyl phosphonate ~DBBP, ~C4M90)z~(C4H9j3 in kerosene is used as extrac~ant.
Laboratory experimental results indicate that the maximum uranium extraction ocçurs when the DBBP : D2EHPA mole ratio is 1 :5 and this ratio has been used ~L~5426~

in the present invention. The concentration of D2EHPA is established based on the optimum condltions for uranium recovery and purification. The uranium in wet-process phosphoric acid is concentrated in ~he first cycle through extraction and reductive stripping operationsS then t~e uranium-concentrated ac;d passes to ~he second purification cycle, where i~s uranium is extracted, scrubbed with water and stripped with ammonium carbonake solution as ammonium uranyl tricarbonate. Before recycl;ng the solvent to the extraction step~
the stripped solven~ is scrubbed with sulfuric acid solution and water to remove the NH + and water of the hydrated ammonium salt of D2EHPA. More than 99% of NH + and the hydrated water can be re~oved when the concentration of H~ in the scrubbing sulfuric acid solution is above 0.6 N. This process offers the advantages of good phase separation, no problems to the contamina-tion of NH4t and dilution with water to the phosphoric acid, and no precipi-tation of ammonium salt ;s observed. Also, this D2EHPA-DBBP process is more economically favourable because of the lower reagent cost for DBB~ and higher purity of the product.
.'-, ' .
D ~RIPTION OF lHE INVENTION

The invention will be described in detail with reFerence to Figure 1 which is a schematic diagram illustrating the process oF the present invention.

FIRST CONCENT~TION CYCLE - After pretreatment for removing suspended solid and or~anic matter from wet-process phosphoric acid, the uranium in the feed acid is oxidized to the -~6 oxidation state by addin9 proper amount o~ H202 ~ S~Z6~

which is dependent on the EMF or Fe2~ concentration in the feed acid, some times this step is even unnecessary. Referring to Figure l, feed acid from line 1 enters the extrac~ion sectior, ~ (5 to ~ stages). In the extract10n section, the feed acid is countercurrently mixed and settled with the organic extractant from line 3 which reacts w~th the uranyl ions to form a complex soluble in the solvent. The composition of the feed acid varies with difFer ent batches9 it generally ranges from 4 to ~ M in HgP04 and contains 60 to 120 parts per million (ppm) by we;gh~ of uranium. The organic extractant used in this invent;on consists essentially of di(~-ethylhexyl) phosphoric acid (D2FHPA) and dibutyl butyl phosphona~e ~D8BP) dissolved in kerosene.
.In the firs~ c.ycle said organio extrac~ant is 0.5 - 0.7 M D2~HPA - 0.1 ~ 0.14 M
DBBP-kerosene (The optimum D2EHPA : DBBP mole ratio is 5:1~. The ratio of the feed aci.d to organic extractant can be adjus~ed from 2 :l t.o 4 :l which is dependent on the phosphoric acid concentration and uranium concentration of the aqueous phase. The uranium extraction coef~icien~s decrease as the temperature.of the acid is increased. The opera~ion temperature in the extraction section is better kept under room temperature. The organic extractant, containing complexed uraniu~, passes through line 4 to the reductive tripping section 5. A portion of the rafFinate from the extrac-i i ori ~, tion section 2 is added scrap-~ (Fe~) to reduce ferric ions and to bring ~ the ferrous ion concentration up to 25 to 50 grams per liter and passes through lines 6 and 7 to mix with the organic extractant. The uranyl ion is reduced to the quadravalent U"~ ion and therefore stripped to the aqueous stream due to the fact thdt the U4~ ion is not complexed by D2EHPA-D~P.
The volumetric flow ratios of the uranium-'oaded organic extractant stream 4 , ~lS~Z6i8 to strip liquor stream 7 is 35/1 to 5C/l. Intrastage recycle of the aqueous phase from the settlers to the mixers is provided to give aqueous/organic phase ratio of about 2 in the mixers-set~lers and operation at 45~ 55C to ensure efficient Inixing and stripping of the uraniurn. The organic extractant leaving the reductiYe stripping section is then recycled through line 3 to extraction section 2. The product solution fram ~he first concentration cycle contains about 6 to 15 9/~ uranium and about 25 to 50 g/~ iron.
.

SECOND PURIFIC~I9N CYCLE - The quadravalent U4+ ion in the first cycle product solution is oxidized by H~O~ to the uranyl ion before entering extraction section 9 (3 to 6 stages) ~hrough line 8. The amount of oxidant added is dependent upon the Fe2+ concentration in the ac;d. In the extrac-tion section, the oxidized acid is contacted with the organic extractant from line 10 containing 0.3~ 0.5 M D2EHPA - 0.06~ 0.1 M DBBP-kerosene. The volumetric flow ratio of aqueous phase to organ;c phase is about l and the concentration of the organic extractant is lower than that used in the first cycle. The main purpose is to obtain a high purity product by increasing the saturation,capacity of the extractant for uranium and minimizing the extraction of impurities. The operation temperature of the extraction sec-tion is preferably kept at room tem?erature. More than 99% of the uranium is extracted into the organic phase. In addition to uranium~ ~he extrac~
contains 0.5-1.5 g/R phosphate and 0.10- 0.20 9/~ iron. Organic phase from line 11 and water from line 13 enter three-stage water scriubbing section.
More than'98% of the phosl)hate in the organic phase is removed under opera-tion at an organic/aqueous ratio of 2/l to 5/l. The uran;um-loaded organic , ~59LZ~8 extractant stream 14 then enters -the ammonium carbonate striQping section 15.
Almost all of the uranium is removed from the organic phase with ammonium carbonate solution in two stages. Make-up ammonia and carbon dioxide are bubbled into the aqueous phase of the settler in the first stage to maintain the concentration o~ ammonium carbonate at about 2 to 2.5 M. Dilute ammonium carbonate solution of about 0.5 M is fed at a slow flow rate to the second.
stripping stage to scrub the very small amount of entra;ned uranium from the organic phase and to compensate the loss of water from the stripping system, which occurs on conversion of the D2EHPA to the hydrated ammonium salt.
Calcination of the AUT precipitate for two hours at 500~ SOO~C yields a product containing more khan 99% of U30~.

The stripped organic extractant leaving the ammonium carbonate stripping section 15 contains 7.2~ 8.3 9/~ NH~ and 65- 80 9/~ H20. The NH+ and H~O
in the organic phase are scrubbed with the sulfur;c solution from line 18 at an organic/aqueous ratio of 2/1 to 5/1. The concentration of the Feed sulfuric acid solution in the two-stage scrubbing section is preferably 6N
to 16 N. The sulfuric acid solution leaYing the second scrubbing stage ~5 recycled to the f;rst stage. The scrubbing efficiency is less than 99%
when the H~ concentration of the recycled sulfuric acid solution gradually decreases to below 0.6 N. The sulfuric acid scrubbing section should be des;gned so that it can be by-passed and replaced with fresh solution when necessary. Operation temperature of 50 ~ 60C is preferred for efficient phase separation. After sulfuric acid scrubbing, organic extractant con-tains about 100 ppm of so42~. The concentration oF so,2~ ;s reduced to below 10 ppm when scrubbed ~lith water from line 21 No precipitate is ~S~;~63~

found in the extraction section when the recycled organic extractant is scrubbed with sul-furic acid solution and water. Also, phase separation is improved in the second cycle extraction step~ dilution and contamination of the wet~process phosphoric acid are preventedO

As described above, the present process for the recovery of uranlum from wet-process phosphoric acid has the following advantages :

1. The organic extractant used in this process consists of di~2-ethyl-hexyl) phosphoric acid and dibutyl butyl phosphonate in kerosene. The chemical reagent cost is lower for ~butyl butyl phosphonate than trioctyl phosphine oxide that is usually employed in the uranium recovery process.

2. ~he uranium in the firs~ cycle can be suitably concentrated to the desired concentration by altering the concentration of the extractant and volumetric flow ratio o-P the two phases according to the uranium content of the feed acid.

3. In the second purification cycle of this invention, two stages of sulfuric acid scrubbing and three stages of water scrubbing are employed to remove the NH~+ and H20 in the organic extractant. No precipitate such às FegNH4H~(P0~)6 6H~0 is found, problems of phase separation, contamination and dilution of wet-process phosphoric acid are eliminated. The problem oP NH~ contamination of phosphoric acid is especially important for the phosphoric acid plant to produce feed stocks and various phosphate cilemicals.

~5~26~

4. In the su1furic acid scrubbing unit, the concerltration of the feed acid solution is 6 to 16 N. Because sulfuric acid solutiorl is recycted between the two stages~ it is easier to deal with the less amount of the resulting liquid waste.

5. Synergistic extraction combination o~ D2EHPA and DBBP g;ves a poor iron extraction coefficient ~han D2~HPA-TOPO does, the purity of the final product in the presen~ invention increases to about 99% and the ;ron content reduces to 0.05- Q.10%.

The following exan)ple further illustrates this invention.

EX~MPLE

FIRST CONC~NTRATION CYCLE A test run was perforrned with wet-process phosphoric acid sampte obtained from a local commercial phosphate plant.
This acid sample was 4.8 M in HgPO~ and contained 0,067 grams of uranium per liter, its composition after pretreatment is shown in Figure 2 (lAF).
Uranium was converted to the hexavalent form (U~) by adding suitable amount of 0.3% of H20~ on a batch basis ~efore ex-~raction with 0.6M
D2EHPA-0.12M D8BP-kerosene. ~8~ of the uraniu~ was extracted in seven extraction stayes operated under room temperature at an organic/aqueous ratio of 1/3. The equilibrium composition of the organic phase is shown in Figure 2 (lAP). The composition of the uranium-barren acid after F,Y..~ ~
oost-treatment ;s shown ;n ~h~P~ (lAG), total organic contents in the raffina-te is less than 50 ppm (DBBP ~ 7 ppm). ~ranium was stripped by ~S9~;~6~

contacting the uranium-loaded organic stream with a small volume oF extracted raffinate in which scrap iron was added ~o produce about 35 grams of ferrous iron per liter. In the three-stage stripping sec~ion volumetric flow ratio of organic phase to aqueous phase was 5011.3 and intrastage recycle o~ the aqueous phase from the settlers to the mixers was provided to give an aqueous/
organic phase ratio o~ about 2 in the mixers-sectlers. The composition of the strip product solution is shown in Figure 2 (lBP). Strip solution con-r taining about ~ grams of uranium per liter is about a factor of 120 times of concentration in uranium than the original feed acid.

SECOND PURIFlCATXON CYCLE - The firs~-cycle product soiutions from several test runs were mixed for the continuous demonstrations of the second-cycle purification process. The compos1tion of the mixed solution is shown in Figure 3 (2AF~). The uranium in the solution was oxidized with 35% H202 to the hexavalent state ~Uo2+) and then extracted in five stages with 0.4 M
D2EHPA-0.08M D~P-kerosene at an organic/aqueous flow ratio of 1/1. The composition of the extract is shown in Figure 3 (2AP), more than 99% of the uranium was e~tracted. Almost all of the phosphate which was extracted into the organic phase was removed in three water ~crubbing stages at room temperature. Then the uranium-loaded organic phase was stripped with ammonium carbonate solution to precipitate ammonium uranyl tricarbonate (AUT). The oo~position of the AUT is shown in Figure 3~ calcination of the air-dried precipitate for two hours at 600C yields a hi~h-grade product ~hich contains about 99.3% of U30~. The composition of the organ;c _g_ 3L~ S~2~3 extractant leaving the ammonium carbonate stripping section is shown in Figure 3 (2CE1), organic phase contains about 7.8 9/~ ~IH4+ and 73 9/~ HaO.
The composition of the stripped organic extractant a~t~!r t~Jo-stage sulfuric acid scrubbing unit is shown in Figure 3 (2CE~), more l;han 99X Of the NH4 and H~O were removed. The entrained so42~ in the organic phase due to contact with sulfuric acid solution was reduced to less than 5 ppm ~hrough three stages o~ water scrubbing under roo~ tempera~ure. After sulfuric acid and water scrubbing, the organic ex~ractant was recycled to the extrac-tion step. Good phase separation was sbserved and no Fe9NH4H~(PO4)6 6H~O
precipitate occurred. The recycled extractant was also rree of a~nonium ion and water during the continuous operation.

~`

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLU-SIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A separation process of recovering uranium from 4-6 M wet-process phosphoric acid comprising: 1) extracting uranyl ions from the acid, pre-oxidized by hydrogen peroxide, with a synergistic extractant mixture which contains di(2-ethylhexyl) phosphoric acid and dibutyl butyl phosphonate in kerosene; 2) concentrating the uranium by reductive stripping of the uranium loaded organic phase with 4-7 M phosphoric acid containing 25-50 g/? of ferrous ions at 45-55°C, 3) re-oxidizing and in a second extracting step extracting the concentrated uranium solution with a synergistic extractant mixture which contains di(2-ethylhexyl) phosphoric acid and dibutyl butyl phosphonate in kerosene; 4) scrubbing the uranium-loaded organic phase with water and stripping said organic solvent with ammonium carbonate solution to precipitate the uranium as ammonium uranyl tricarbonate; and 5) scrubbing the stripped organic phase with sulfuric acid solution and water before recycling to the second extraction step.

2. The process according to claim 1, wherein said synergistic extractant contains 0.5-0.7 M of di(2-ethylhexyl)-phosphonic acid, 0.1-0.14 M of dibutyl butyl phosphonate in kerosene in the first extraction step and 0.3-0.5 M of di(2-ethylhexyl) phosphonic acid, 0.06-0.1 M of dibutyl butyl phos-phonate in kerosene in the second extraction step.

3. The process according to claim 1, wherein con-centrations of sulfuric acid feed solution in step (5) is 6-16 N.

4. The process according to claim 3, wherein the volume ratio of organic solvent to the sulfuric acid solution is about 2/1 to 5/1, and operation is at a temperature of about 50-60°C.

5. A process according to claim 1, in which the water under scrubbing step is effected in 2 to 3 stages.

6. The process according to claim 1, wherein the water scrubbing in step (5) is performed in 2 to 3 stages at room temperature with volume ratio of organic solvent to water about 1/1 to 1/5.