URANIUM SOLVENT EXTRACTION USING TERTIARY AMINES

URANIUM SOLVENT EXTRACTION USING

TERTIARY AMINES

by:

Dr J.M.W. Mackenzie

Manager Mining Chemicals

Henkel Australia Pty Ltd

PRESENTED AT

Uranium Ore Yellow Cake Seminar

February 1997

Melbourne, Australia

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Table of Contents

1.0 INTRODUCTION AND HISTORY .............................................................................................. 3

1.1 INTRODUCTION .................................................................................................................... 3

1.2 HISTORY ................................................................................................................................ 3

2.0 ACID LEACH CHEMISTRY FOR URANIUM ......................................................................... 4

3.0 TERTIARY AMINE EXTRACTION OF URANIUM ............................................................... 5

3.1 TYPE OF AMINE USED AND MAXIMUM LOADING CAPACITY .................................. 5

3.2 THE EFFECT OF COMPETING ANIONS ............................................................................. 6

3.3 DILUENTS AND THIRD PHASE INHIBITORS OR MODIFIERS....................................... 7

3.4 SECONDARY AMINES .......................................................................................................... 8

4.0 GENERAL CONCEPTS OF TERTIARY AMINE URANIUM ....EXTRACTION CIRCUITS

9

4.1 GENERAL CIRCUIT LAYOUT.............................................................................................. 9

4.2 LOADED ORGANIC SCRUBBING...................................................................................... 10

4.3 STRIPPING ............................................................................................................................ 10

4.4 REGENERATION OR SCRUBBING OF THE STRIPPED ORGANIC............................... 11

5.0 ASPECTS OF EQUIPMENT SELECTION AND PLANT DESIGN FOR TERTIARY

AMINE URANIUM SX CIRCUITS .................................................................................................. 11

6.0 SPECIAL PROBLEMS DUE TO ORGANIC CONTAMINANTS AND CRUD ................... 12

REFERENCES ................................................................................................................................... 13

TABLES AND FIGURES................................................................................................................... 15

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1.0 INTRODUCTION AND HISTORY

1.1 INTRODUCTION

Uranium was the first metal to be recovered in significant quantities

using solvent extraction. Much of the copper SX equipment technology

is a development from existing uranium technology. Because the

kinetics of extraction via an ion exchange mechanism are much faster

than for a chelation mechanism the mixer residence times are much

longer for copper SX than for uranium.

1.2 HISTORY

Following the development of the nuclear industry during and

immediately after World War II, attention was focussed on developing

technologies which could be used to upgrade and purify uranium from

low grade sources. Initially the nuclear industry had relied on high grade

uranium ores from the Belgium Congo and Canada. One of the main

participants in this development was the Union of South Africa and the

then Prime Minister, Jan Smuts, visited MIT in the USA in this regard.

Research in the USA lead to the first commercial use of amines in

uranium concentration at West Rand Consolidated Mines in South

Africa in 1952. The amines used in this plant were in the form of a

strong base resin and it was not until 1957 that the first commercial

solvent extraction plant using amines was opened in the USA.

Since those early days, the development of uranium hydrometallurgical

processing has developed along three main paths:

1. Ion exchange as the sole on site purification and

concentration route

This used a strong base (quaternary amine) functionality resin to

concentrate and purify the uranium.

2. Ion exchange followed by solvent extraction purification of

the IX concentration eluate

Acid leaching of uranium is a non-selective leach and ion

exchange using strong base resins is a non-selective extraction

process so it was not surprising that metallurgists looked to ways

of purifying the concentrated eluate produced by strong IX.

Attempts to improve the selectivity of the uranium IX process

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using weak base (tertiary amine) resins were not particularly

successful and a circuit based on strong base IX of the leach

solution followed by solvent extraction (SX) of the conc eluate

using tertiary amine extractants was developed. One of the first

plants of this type was installed at Buffelsfontein Gold Mine in

South Africa and this circuit is sometimes referred to as a

"Bufflex" circuit.

3. Direct solvent extraction of the leach solution

This circuit uses a single extraction process, solvent extraction

using a tertiary amine solvent and yields a product, which is as

pure as circuit (2). The direct SX circuit ("Purlex") become the

standard uranium circuit in Canada, Australia and the US. In

South Africa both direct SX and IX-SX circuits co-existed but SX

circuits predominated.

There are some hydrometallurgical circuits for uranium which use

none of the above options. These include the use of TBP to treat

nitric acid leach solutions produced in nuclear fuel reprocessing

(1) (2), and to treat nitric acid leach solutions produced at

Palaborwa, South Africa. D2EHPA/TOPO mixtures are used to

extract uranium from phosphoric acid solutions

2.0 ACID LEACH CHEMISTRY FOR URANIUM

Sulphuric acid leaching of uranium ores in the presence of an oxidising agent

(usually manganese or chlorate based) which provides a leach oxidationreduction

potential of 400 - 500 mv relative to a saturated calomel electrode,

results in virtually all the leached uranium being present in the U(6) valency.

The uranium dissolves as UO2

2+ and then forms two uranyl sulphate anion

complexes viz:

UO2

2+ + 2SO4

2- .> UO2(SO4)2

2-

UO2(SO4)2

2- + (SO4)2- .> UO2(SO4)3

4-

The uranyl sulphate anion complexes are the species, which are extracted by

amines.

Unfortunately, the oxidising sulphuric acid leach, which is often carried out at a

temperature of 40 - 80oC is aggressive and non-selective resulting in many

other species besides uranium being leached.

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The presence of these anionic species can present problems in uranium

solvent extraction. Some of the more important species involved are:

Soluble silica Si(OH)4 Si02 amorphous

Tungsten WO4

2-

Antimony SbO4

3-

Arsenic ASO3

3-

Molybdenum MnO4

2-

Vanadium VO3

-

Zircon ZrO3

2-

Titanium

Phosphate PO4

3-

In addition, sulphuric acid dissociates in water as follows:

H2SO4 .> HSO4

- + H+ K1 = 4 x 10-1

HSO4

- .> H+ + SO4

2- K2 = 1.27 x 10-2

Chloride (Cl-) and nitrate (NO3

-) anions may also be present in the leach liquor.

The problems caused by these species and possible solutions to these

problems are listed in Table 1.

3.0 TERTIARY AMINE EXTRACTION OF URANIUM

There are a number of extractants, which can be used to recover uranium, but

only the amines (tertiary and quaternary) and the organic phosphates have

found widespread commercial acceptance in the recovery of uranium from

ores. SX recovery of uranium is restricted to acid leach solutions. Carbonate

leach recovery systems do not use SX as a recovery or purification stage. By

far the most widely used extractants for uranium are the tertiary amines

specifically the C8-C10 symmetrical amines. Table 2 shows the loading and

stripping chemistry for these systems.

3.1 TYPE OF AMINE USED AND MAXIMUM LOADING CAPACITY

The tri-octyl, tridecyl amine mixtures are used almost exclusively

however amines of longer chain lengths have been used.

Trilaurylamine can be used when molybdenum is present as the amine

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molybdate complex formed with the C12 amine is much more organic

soluble than the C8 amine complex. Obviously higher molecular amines

require greater mass concentrations to give the same loading capacity

as the lower molecular weight amines.

The approximate molecular weight of a typical C8-C10 tertiary amine,

AlamineR 336, is 388 - 391 and the product is 96% tertiary amine.

Based on this molecular weight, and the specific gravity of AlamineR 336

of 0.81, it is possible to calculate the theoretical maximum loading of a

1% V/V mixture of AlamineR 336 in a diluent of specific gravity of 0.81.

The calculated maximum loading depends on the speciation of the

uranyl sulphates in the leach liquor. The appropriate values are:

Species Theoretical maximum loading

of

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