GB2054545A - Recovery of uranium from an impure phosphoric acid - Google Patents

Abstract

Uranium (VI) contained in an impure phosphoric acid, particularly a wet-process phosphoric acid, is recovered in a process that comprises a first cycle of extracting the impure acid by an organic phase comprising a dialkyl phosphoric acid, a trialkylphosphine oxide, and an inert diluent, re-extraction of the uranium (IV) by means of an aqueous solution of phosphoric acid containing ferrous ions and a second extraction step after oxidation by a second organic phase comprising a dialkyl phosphoric acid, an inert diluent, and if appropriate a synergistic extraction agent which is a trialkyl phosphine oxide dibutyl butyl phosphonate or trialkylphosphate which, before regeneration, is washed by means of a sulphuric solution containing less than 40 ppm of iron. The process can be used for the production of uranium with a Fe/U weight ratio of less than 0.2%.

Description

SPECIFICATION Recovery of uranium from an impure phosphoric acid The present invention concerns the recovery by liquid-liquid extraction of the uranium contained in a phosphoric acid, more particularly but not exclusively a phosphoric acid produced by a wet-process method.

It is known that uranium can be recovered from aqueous solutions containing the uranium at low levels of concentration by separating it from the other constituent substances (which if appropriate can be put to good use) of the treated ores by means of a combination of liquid-liquid extraction operations and chemical treatment operations, the aim of which is to isolate the uranium and recover it in the form of oxide U308 in a high state of purity so that it can be used as a nuclear fuel source. These processes are applied for recovery purposes to ores such as phosphate-bearing rocks, which also provide phosphoric acid, or ores of various origins with a greater or lesser content of uranium, which in most cases occurs in the form of oxides.The process generally comprises treating the ore by means of a strong concentrated acid such as sulphuric, phosphoric, hydrochloric or nitric acid to provide an aqueous solution that contains uranyl ions in a highly dilute state together with other contaminating ions from which the uranium is recovered.

A typical example of a process for the recovery of uranium from wet-process phosphoric acids is described in U.S. Patent No.3,711,591. This process breaks down into two successive extraction cycles.

In the first cycle of this process, the uranium of valency 6 in the impure phosphoric acid is extracted by means of a first extractive organic phase comprising an inert diluent, a primary extracting agent, viz.

di-(2 ethylhexyl) phosphoric acid, and a synergistic extracting agent, viz. tri-octylphosphine oxide, at selected concentrations in the diluent. After separation of the phases, the impure acid, exhausted in respect of uranium, is returned to the phosphoric concentration unit while the organic phase, which is charged with uranium (VI), is treated at a moderate flow rate with an aqueous solution of phosphoric acid containing a reducing agent based on metallic iron or a ferrous salt, thereby selectively reextracting uranium (IV) in the aqueous phase.

In the second cycle of this process, the resulting aqueous solution of phosphoric acid, which has a high content of uranium and iron, is, after reoxidation of the uranium to a valency of 6, continuously treated with a second extractive organic phase of the same nature as the first phase to produce an organic phase charged with uranium (VI) and an exhausted aqueous phase, which is recycled to the re-extraction operation of the first cycle or is passed to the phosphoric concentration unit. The preceding organic phase is washed with water and then treated to recover the uranium by precipitation, in the form of a mixed carbonate of uranium and ammonium, which is generally known as AUT. The resulting uranium-depleted organic phase is recycled.

Industrial use of this process requires that, in order to achieve effective re-extraction ofthe uranium of the first extractive phase, the concentration of the iron in the aqueous solution of phosphoric acid must be relatively high, say 15 to 30 g/litre of aqueous solution. Under these conditions, and in spite of a very low degree of extraction in respect of the iron by the organic phase in the second cycle, the uranium produced contains substantial amounts of iron, which can be up to 4 or 5% by weight. There is therefore a need for an improved process that makes it possible to produce uranium substantially free from iron under economically viable conditions.

The present invention provides a process for the recovery of uranium in an impure phosphoric acid comprising, in a first cycle, treating the impure acid by means of a first extractive organic phase comprising a dialkyl phosphoric acid, a trialkylphosphine oxide and an inert diluent, and then separating the phases; treating the above-mentioned uraniumcharged organic phase by means of an aqueous solution of phosphoric acid containing ferrous ions, to extract the uranium (IV) in the said aqueous phase, then separating the phases and recycling the exhausted organic phase to the impure acid extraction operation; in a second cycle, treating the above-mentioned aqueous phase after oxidation by means of a second extractive organic phase comprising an inert diluent, a dialkyl phosphoric acid and, if appropriate, a synergistic extraction agent that is a trialkylphosphine oxide, dibutyl butyl phosphonate or a trialkylphosphate to recover, after separation of the phases, an aqueous phase that is exhausted in respect of uranium and a uraniumcharged organic phase, treating the uraniumcharged extractive organic phase of the second cycle by means of an aqueous solution containing from 25 to 60% and preferably about 45% of sulphuric acid substantially free of and preferably containing less than 40 ppm of iron to reduce the weight ratio Fe/u to less than 0.5% and preferably less than 0.2%, washing the thus-treated extractive phase with water, recovering the uranium from the extractive phase and recycling that phase. The solution of sulphuric acid may have been produced by dissolution of substantially iron-free concentrated sulphuric acid in the aqueous washing solution issuing from the stage of washing the uranium-charged extractive organic phase of the second cycle with water.

The present invention will be better appreciated by referring to the accompanying drawing, the single figure of which shows an illustrative flow diagram.

The impure phosphoric acid is represented typically by a crude wet-process phosphoric acid produced by attacking phosphate-bearing rocks by means of sulphuric acid. After filtration of the gypsum, the crude phosphoric liquor, which is introduced by way of conduit 1, is usually subjected at 2 to known pre-treatment operations such as those for stabilisation and concentration, issuing from unit 2 at 3, in a concentration by weight in respect of P2O5 which is usually from 25 to 40%, the uranium content usually being from 80 to 250 mg/litre and the iron content being from 3 to 10 g/litre. The above-mentioned acid is used to feed an extraction region 4 which generally comprises a battery of mixer-settlers, a filled column or a pulsed column, in counter-flow or in co-flow with respect to a first organic extraction phase which enters at 5 and which circulates in a closed circuit.The ratio between the flow rate of the impure acid and the flow rate of the organic phase is generally from 0.5:1 to 5:1 and generally close to 2:1. The first organic extraction phase comprises a well known inert organic diluent such as kerosene, a dialkylphosphoric acid as principal extraction agent and trialkylphosphine oxide as synergistic extraction agent. Usually, the preferred constituents are di-(2ethylhexyl)-phosphoric acid (HDEHP) and trioctylphosphine oxide (TOPO). The concentration of HDEHP in the extractive phase is generally from 0.1 Mto 1.5 M, preferably close to 0.5 M. The concentration of TOPO in the extraction phase is generally from 0.05 Mto 0.5 M, preferably close to 0.125 M.

At the outlet from the extraction region 4, a solution of phosphoric acid exhausted in respect of uranium is recovered at 6, while an organic phase, which is charged with uranium (VI), is recovered at 7.

The organic flow 7 is then used to feed a uranium re-extraction region 8 usually comprising a battery of mixer-settlers, a pulsed column or a filled column, where it is treated in counter-flow or co-flow by means of an aqueous solution of phosphoric acid 9 containing ferrous ions, the ratio between the flow rate of the flow 9 and that of the flow 7 generally being from 0.015:1 to 0.10:1 and preferably close to 0.025:1. Its content by weight of P205 is generally from 28 to 45%, preferably close to 35%, and the amount of ferrous ions is generally from 10 to 40 g/litre, preferably close to 25 g/litre. In a preferred alternative form, the flow 9 is produced by taking off a part of the flow of the uranium-depleted impure acid 6 that issues from the region 4, while the ferrous ions can be produced by dissolution of metallic iron, which is introduced at 10.

An organic phase, which is deprived of uranium and which forms the recycled flow 5, issues from the region 8 at 11, while an aqueous flow, which is enriched in respect of uranium (IV) and which contains iron ions, issues from the region 8 at 12.

The flow 12 is used to supply a region 13 where it is oxidised by means of an oxidising agent such as hydrogen peroxide, air or a chlorate, or by passing into the anodic compartment of a d-c separator-type electrolytic cell from which it issues at 14 to pass into the second cycle.

In the second cycle, the flow 14 is used to supply an extraction region 15 formed by a battery of mixer-settlers in conjunction with the flow 16 from the second organic extraction phase. The second organic phase usually comprises a well known inert diluent such as kerosene, a dialkylphosphoric acid such as HDEHP and, if appropriate, a synergistic extraction agent, preferably TOPO. However, the molar concentrations of the principal extracting agent and the synergistic extracting agent in the inert diluent may be substantially different from those in the organic phase of the first cycle. Thus, the concentration in respect of HDEHP is usually from 0.1M to 1M, preferably close to 0.3M, and the concentration in respect of TOPO is usually from 0.01Mto 0.5M, preferably close to 0.075M.The ratio between the flow rate of flow 14 and the flow rate of flow 16 usually varies from 0.1:1 to 5:1, and is preferably close to 0.5:1.

An aqueous phase exhausted in respect of uranium issues from the region 15 at 17, and in general is used to supply the extraction region 4, while an organic flow 18 charged with uranium issues from the region 15. The flow 18 is then used to supply a region 19 for removal of iron. This usually comprises a battery of mixer-settlers where flow 18 is treated with flow 20, which comprises an aqueous solution of sulphuric acid containing less than 40 ppm of iron.

The sulphuric concentration of this solution is from 25% to 60% and preferably close to 45%. In a first alternative form, this solution is produced by diluting with water 95% sulphuric acid containing less than 40 ppm of iron. In another preferred embodiment as shown in the accompanying drawing, the 98% concentrated sulphuric acid, which enters at 21, is diluted to the required level of concentration by means of the aqueous washing solution 22 coming from the washing region 24, as will be described hereinafter. The ratio between the flow rates of the flow 20 and the flow 18 may vary between 0.2:1 and 10:1 and is preferably close to 2:1.

As extraction of the iron contained in the organic solution 18 by means of the flow of sulphuric acid at 20 is relatively slow, the contact time between the two phases mentioned above must be long enough.

An increase in contact temperature, for example from 20 to 500C, makes it possible to improve this operation. However, bearing in mind that the amount of uranium co-extracted by the sulphuric acid increases with temperature, it is preferable for the purification operation to be carried out at a temperature close to the ambient and with suitably chosen contact time in respect of the phases 18 and 2Q. The contact time depends on the mixing-settling technology used. Likewise, the number of theoretical stages to be used in this washing step may vary within wide limits, depending on the volume ratio of the phases. For example, with a volume ratio of aqueous phase to organic phase of 2:1 at 30"C, about six stages are used, with a mixing time often minutes in each stage.

An iron-depleted organic flow 23 issues from the iron purification region 19, while an iron-enriched sulphuric and phosphoric aqueous solution 25 issues from the region 19. If appropriate, and after concentration in an evaporator, the solution 25 provides an acid flow which, for reasons concerned with the economics of the process, is re-introduced into the stage of attacking the phosphate-bearing rocks. The purified organic flow 23 is then used to feed a washing region 24 which usually comprises a battery of mixer-settlers in which it is washed by means of water introduced at 28 to recover P205 dissolved in the organic phase.

The aqueous washing solution 22 issues from the region 24. it can easily be used to dilute the sulphuric acid 22 used in the purification region 19, as just described above, or it can be added to the flows 3 and/or 14, Also issuing from the region 24 is a flow 29 of purified washed uranium-charged organic phase from which the uranium is recovered using known means. Preferably the flow 29 is treated in a two-stage region comprising in the first stage 30 treatment by means of a recycled flow 31 of an aqueous solution containing carbonate and ammonium ions and, in the second stage 32, treatment by means of an aqueous solution of ammonium carbonate whose concentration is preferably lower than that of the flow 31.At 33 there issues an uraniumexhausted organic flow, which is recycled to the region 15, and an aqueous suspension of AUT, as indicated at 34, which is filtered and washed at 35, the mother liquor being recycled at31 after its strength in respect of carbonate and ammonium ions has been adjusted. The filtration cake is then calcined to provide U308 containing iron in an Fe:U weight ratio of less than 0.5% and in most cases less than 0.2%.

The temperature throughout the process is not critical and is usually from 10 to SOOC, preferably from 20 to 50"C.

The invention is illustrated hereinafter by means of a non-limiting example, in which further reference will be made to the accompanying drawing: Example A pre-treated impure phosphoric acid containing 30% of P205, 110 mg/litre of uranium and 8 g/litre of iron is introduced at 3 into a battery comprising five mixer-settlers at a flow rate of 100 litres/hour. A flow 5 of 50 litres/hour of a recycled organic phase comprising a 0.5M concentration of HDEHP and a 0.125M concentration of TOPO is also introduced into the battery 4. The aqueous flow 6 issuing from the battery is passed to the concentration stage. The organic flow 7 contains 160 mg/litre of iron.This flow is passed into the uranium regeneration region 8 comprising four mixer-settlers where it is treated in counter-flow by a flow 9 of phoshoric acid containing 30% P205 and 25 g/litre of ferrous ions at a flow rate of 1.35 litres/hour.

The exhausted organic phase is recycled to the extraction stage and the aqueous phase 12 issuing contains 8 g/litre of uranium. After oxidation by means of hydrgen peroxide, the aqueous phase 14 is treated in a battery of four mixer-settlers by means of a kerosene phase with a 0.3M concentration of HDEHP and a 0.075M concentration of TOPO, at a flow rate of 2.02 litres/hour. The resulting organic phase 18 contains 170 mg/litre of iron and 5.3 g/litre of uranium. The aqueous phase 15 issuing is passed to the extractor 4 in the first cycle.

The organic phase 18 then feeds a battery comprising six mixer-settlers where it is treated in counter-flow by means of an aqueous solution 20 which is the combination of the flow 21 of 98% sulphuric acid (iron S 40 ppm) at a flow rate of 1.35 litres/hour and the aqueous flow 22 issuing from the washing battery 24 and corresponding to the flow of pure water 28 whose flow rate is 2.92 litres/hour. The aqueous flow 25 from the battery 19, if necessary after concentration, is returned to the stage of attacking the phosphate-bearing rocks. The organic flow 23 issuing from the battery 19 contains 15 mg/1 of iron and 4.7 g/litre of uranium. This flow is used to feed the battery 24 of two mixer-settlers where it is washed in counter-flow by means of water as just described above. The purified and washed organic flow 29 is then treated to recover the uranium from it in a two-stage apparatus 30 and 32, in which the first stage is fed with a recycling 2M aqueous solution of (N H4)2 003, which is saturated in respect of AUT, the second stage being fed by means of a 0.5Msolution of (NH4)2 C03. There issues at 33 an organic flow which is recycled at 15 and an aqueous suspension of AUT which is filtered, the mother liquor being adjusted and recycled. Calcination of the cake results in U308 in an Fe: U weight ratio of 0.2%.

Claims (14)

1. A process for the recovery of uranium (VI) contained in an impure phosphoric acid, comprising, in a first cycle, treating the impure acid by means of a first extractive organic phase comprising a dialkyl phosphoric acid, a trialkylphosphine oxide and an inert diluent, then separating the phases, treating the preceding uranium-charged organic phase by means of an aqueous solution of phosphoric acid containing ferrous ions, to extract the uranium (IV) in the said aqueous phase, then separating the phases and recycling the exhausted organic phase to the step of extracting the impure acid; in a second cycle, treating the preceding aqueous phase, after oxidation, by means of a second extractive phase comprising an inert diluent, a dialkyl phosphoric acid and, if appropriate, a synergistic extraction agent that is a trialkylphosphine oxide, dibutyl butyl phosphonate or a trialkyl phosphate to produce, after separation of the phases, an aqueous phase that is exhausted in respect of uranium and an organic phase that is charged with uranium, treating the extractive phase by means of an aqueous solution containing from 25% to 60% of sulphuric acid that is substantially free from iron to reduce the Fe/U weight ratio to less than 0.5%, washing the thustreated extractive phase with water, recovering the uranium from the extractive phase and recycling that phase.

2. A process as claimed in Claim 1, in which the aqueous solution of sulphuric acid used in the second cycle contains less than 40 ppm of iron.

3. A process as claimed in Claim 1, in which the Fe/U weight ratio is less than 0.2%.

4. A process as claimed in any preceding claim, in which the aqueous solution of sulphuric acid has been produced by dissolving substantially iron-free concentrated sulphuric acid in the aqueous washing solution issuing from the step of washing the uranium-charged extractive organic phase with water.

5. A process as claimed in any preceding claims, in which the extractive organic phase of the second cycle comprises an inert diluent, di-(2-ethylhexyl) phosphoric acid and trioctylphosphine oxide.

6. A process as claimed in any preceding claim, in which the aqueous solution of sulphuric acid issuing from the step of treating the organic phase is recycled, if necessary after concentration, to the stage of attacking the phosphate-bearing rocks.

7. A process as claimed in any preceding claim, in which the treatment is carried out in a liquid-liquid contact region comprising from 1 to 10 stages.

8. A process as claimed in Claim 7, in which the contact region comprises 6 stages.

9. A process as claimed in any preceding claim, in which the flow ratio between the flow of the aqueous solution of iron-free sulphuric acid and the flow of the organic phase is from 0.2:1 to 10:1.

10. A process as claimed in Claim 9, in which the said ratio is close to 2:1.

11. A process as claimed in any preceding claim, in which the uranium is recovered from the purified organic phase of the second cycle by precipitation of AUT by means of an aqueous solution containing ammonium and carbonate ions.

12. A process as claimed in Claim 1 carried out substantially as hereinbefore described in the foregoing Example.

13. A process as claimed in Claim 1 carried out in apparatus substantially as hereinbefore described with reference to the accompanying drawings.

14. A process as claimed in any preceding claim in which the impure phosphoric acid has been produced by the wet process.