Ion-exchange equipment

The major disavantage of downflow contact of leach liquor with a fixed bed of resin is one of scale. The standard column for this duty has a diameter of 2.13 m, although columns of up to 3 m diameter have been used. Beyond this size range problems are encountered with the even distribution of solution flow

through the resin bed. The height of a settled resin bed and specific flow rate that can be applied bear a direct relation to the pressure drop resulting from flow.

High pressure drops are not only uneconomic but lead to external stresses on resin beads which result in bead distortion and breakdown and to channelling of solution flows within the bed.

The early ion-exchange plants had a capacity of several hundred tonnes of U³0⁸ per year. Productions an order of magnitude greater than this are now common. The multiplicity of columns required for these large productions would cancel many of the favourable economies that are available to large-scale

operation.

Continuous upflow contactors (CIX), of which a number of designs have achieved commercial acceptance, have been developed to provide advantages not realized with downflow columns. These advantages have been described else-where [1, 2]. Perhaps it should be emphasized here that, with respect to the two performance criteria that are governed by the chemistry of the solutions treated — uranium recovery efficiency and product purity — CIX contactors are capable of exceeding the performance of downflow units for an identical duty, if correctly designed.

Upflow resin contactors are capable of treating unclarified solutions con-taining appreciable quantities of suspended solids. The limits to the solids content of the slime thus treated are set by the difference in relative density between the resin beds and the fluid medium, and by the effective viscosity of the slime.

Operation with conventional strong-base resins is limited to around 8% solids for pulps which have favourable viscosity characteristics. A recently announced heavy resin [3] having a relative density in the sulphate form of 1.26 (compared to the more usual 1.10 to 1.15) will extend the operating range of upflow columns to around 15% solids. However, the full advantages of operating a resin system in the presence of loaded solids are realized when the major solid/liquid separation steps between leach and purification are eliminated. In this way, resin contactors are operated at a solids content similar to that under which leaching is conducted.

Russian technology in this field has been described in an earlier IAEA publication [4].

The kinetics of uranium absorption from a relatively thick leach pulp are slower than for a clear solution, owing to increased resistances to uranium transport external to the resin beads in the fluid phase. Resin-in-pulp (RIP) systems usually require a large-size resin bead to facilitate resin/pulp separation which introduces additional diffusional resistances within the resin beads. The purification available from RIP systems is essentially the same as that offered by conventional means.

2.2. Absorption

The selectivity with which strong-base anion exchange resins absorb uranyl sulphate complexes from solution is well documented [5, 6]. The predominant

H S 04

к so4

uo2(so4)3

so4 U 02( S 04)2

к so4 NO3

к so4

к CI

so4

5.10

= 7381.80

= 41.41

= 72.93

5.10

TABLE III. SUMMARY OF THE ION-EXCHANGE REACTIONS AND EQUILIBRIUM CONSTANTS AT 25°C IN THE SIX-COMPONENT SYSTEM AMBERLITE IRA 400, Na²S0⁴, NaCl, NaN0³, H²S0⁴ AND U 0²S 0⁴ Ion exchange reaction Equilibrium constant

R2 S04 + 2HS04" ^ 2RHS04 + S042 _

2R2S04 + U02S04 - R4U02(S04)3

R2S04 + U02(S04)2 2 - - R2U02(S04)2 + S042_

R2S04 + 2N03" ^ 2RN03 + S032_

R2 S04 + 2СГ =^ 2RC1 + S042"

reactions may be represented as

2R²S0⁴ + U 0²S 0⁴ =^ R⁴U0²(S0⁴)³ and

U 02( S 04)2 2- + R2S04 - R2U02(S04)2 + S04 2~

Substantial competition to uranium absorption is afforded by a number of reactions of the form

R2S 04+ 2 X = ^ 2 R X + S 04 2

-where X may be bisulphate, nitrate, chloride or other monovalent ions. The effect of these ions in equilibrium absorption is shown in Table III.

Bisulphate ion is alway present in acid leach liquors. The quantity is governed by both the total sulphate concentration in solution and the pH, as indicated by the equation

H S 04~ - H+ + S 04 2

-The equilibrium ratio for this reaction is often quoted as being of the order of 0.01. In fact this value is valid only for very dilute solutions; a more correct value

I 1 1 1

0.1 1.0 10 100 Total sulphate in solution

FIG.2. Distribution diagram: system Fe(III) — OH~— SOl' Fe(III) = 0.05M, 1 = 3.0, pH = 2.0.

for leach solutions having an ionic strength of 1.0 to 1.5 at 25°C is closer to 0.1.

A recent investigation [6] into equilibria between strong-base ion-exchange resin and uranyl sulphate solutions containing nitrate and chloride ions lias shown that an appreciable fraction of the uranium in the resin phase is present as the U02(S04)22~ complex, and the greater part as U02(S04)34~. The activities of ionic species in solution were calculated using the extended Debye-Huckel equation, whereas resin-phase activities were correlated by the use of the Wilson equation. Table III contains a summary of a number of these reactions and their equilibrium constants (calculated by using the activity of each species). Ferric ion species were not included in this study.

Groenewald [8] has calculated equilibrium distributions in the system Fe(III) — S0⁴²~— OH" using a number of published stability constants. These curves (Fig. 2) indicate that the only anionic ferric species present in sulphate leach liquors is Fe(S0⁴)²~. The existence of the complex (Fe(OH)(S0⁴) 22-has often been canvassed to explain the decrease in ferric absorption at pH's

TABLE IV. THE CONCENTRATION OF COMPETING IONS REQUIRED TO REDUCE EQUILIBRIUM URANIUM EXTRACTION BY 50% FROM ACID LIQUORS

below 2.0. This decrease is probably owing to increased competition from bisulphate ions.

Table IV shows the effect of competing ions on the absorption of uranium.

The concentration necessary to reduce the absorption of uranium into a number о f organic media is shown with reference to the behaviour of a standard pregnant solution. For example, between 3 and 3.5 g/ltr of chloride ion will reduce the loading of a strong-base resin from between 20 and 25 g/ltr U³0⁸ when in equilibrium with a solution containing 50 ppm U3Og to a loading of 10 to

12.5 g/ltr U³0⁸.

The equilibrium loading attainable in a resin-purification system has a bearing on elution costs, equipment size and product purity. Because all the ion-exchange sites in a resin are converted during elution to the ionic form represented by the eluting solution, the consumption of eluant per kilogram of uranium eluted is directly proportional to the uranium loading on the resin entering the elution circuit. This loading is' in turn limited by the equilibrium value achievable. The size of continuous ion-exchange contactors is directly related to the rate of

uranium transport into and out of the resin beads. The rate of uranium absorption from a solution is governed in part by the equilibrium absorption because the difference in uranium content between the initial state of the resin and the equilibrium state represents a driving force for mass transfer. Finally, the lower

TABLE V. THE CONCENTRATION OF COMPETING IONS REQUIRED TO REDUCE EQUILIBRIUM URANIUM EXTRACTION BY 50% FROM ALKALINE CARBONATE LIQUORS

Ionic species

CI S 04

C 03

PH

u3o8 U²O^s

Reference solution (g/ltr) nil nil 2 - 8 10.5 150 ppm nil

Strong-base IX resin (g/ltr) 2 - 4 2 - 6 3 0 - 8 0

< 8 1 2 0 - 1 3 0

< 1 . 0

the equilibrium loading of uranium the greater will be the amount of extraneous ions that are carried forward into the elution circuit.

The absorption of uranium from alkaline leach solutions is marked by very favourable equilibrium loadings. The reaction is usually represented by

4RX + U 02( C 03)3 4- ^ R4U02(C03) + 4Х"

Equilibrium loadings of over 100 g/ltr U³0⁸ in the absence of competing ions suggest that an ionic species of lower valence may be involved. Chloride and sulphate exert a strong depressing effect on equilibrium loadings, which are more sensitive to pH and to the carbonate concentration than are the analogous values in the acid-sulphate system. The behaviour of vanadium ions, which are often encountered in alkaline leach liquors, is strongly pH dependent. Pentavalent vanadium is absorbed in preference to uranium in the pH range 8 to 10.5 whereas uranium absorption is favoured at pH's of 10.8 to 11.3 [11]. Molybdenum is absorbed from alkaline leach solutions but is held less strongly than the uranium carbonate complex and will be displaced from the resin as the uranium concentra­

tion in the resin phase increases. The effect of competing ions on the absorption of uranium from carbonate leach solutions is shown in Table V.