EP0109327B1 - Process for the complete recovery of uranium, yttrium, thorium and rare earth metals from a phosphate ore while preparing wet process phosphoric acid - Google Patents

Description

The present invention relates to a process for the overall recovery of uranium, yttrium, thorium and rare earths contained in a phosphate ore during the preparation of phosphoric acid by wet process.

It is known that the phosphate ores used for the manufacture of phosphoric acid contain significant amounts of uranium, yttrium, thorium and rare earths. For these minerals, in the thorium, rare earths and yttrium group, yttrium represents in quantity approximately half of the whole.

It is also known that during the attack on a phosphate ore with sulfuric acid, most of the uranium (around 95%) is dissolved in the phosphoric acid formed and well-known processes make it possible to recover this element in particular by liquid-liquid extraction or by precipitation from phosphoric acid from a secondary gypsum containing uranium.

However, most of the rare earths and yttrium present in the ore is not dissolved during the attack and co-precipitates with gypsum. The quantity of these elements which goes into solution depends on the nature of the ore and generally constitutes 5 to 20% approximately of the total quantity present in the ore. To then recover these elements, it is necessary to treat this gypsum, for example by washing with sulfuric acid.

The recovery of uranium on the one hand and of the other elements mentioned on the other hand therefore requires two separate treatments, one on phosphoric acid, the other on gypsum.

The problem therefore arose of a process allowing in a single operation the joint recovery of all the uranium and the other elements.

This problem has been partially resolved. A process is known in fact (British patent 793.801) in which the solubilization of the yttrium and the rare earths is increased at the time of the attack by the addition of silica. This gives a phosphoric acid solution containing uranium and a part of the yttrium and rare earths greater than that obtained under the usual attack conditions.

However, the addition of silica has several drawbacks.

First of all if the proportion of rare earths, yttrium and thorium solubilized at the attack increases with the quantity of added silica, one however quickly arrives at a stage. This is how it turns out to be difficult to dissolve on attack more than around 40% of the total quantity of the elements in question.

Furthermore, the addition of silica hinders the filtration of the attack slurry during the separation of the gypsum and the phosphoric acid. The more the quantity of silica increases, the more the filtration speed decreases. This is a very serious industrial disadvantage.

Finally, silica can prove to be a nuisance in the subsequent stages of a process for manufacturing phosphoric acid, in particular during liquid-liquid extractions.

The object of the invention is to further improve the solubilization of rare earths, of yttrium and of thorium on attack without harming the subsequent progress of the process for the manufacture of phosphoric acid.

For this purpose, the process according to the invention, for the overall recovery of uranium, yttrium, thorium and rare earths contained in a phosphate ore, during the preparation of phosphoric acid by wet process, is characterized in that during the acid attack of the ore, optionally carried out in the presence of silica, aluminum is added to the attack medium in trivalent form and / or a ferric compound in such a way that the amount of aluminum and / or iron present in this attack medium varies between 0.8% and 1.5% by weight expressed as A1 ₂ 0 ₃ and / or Fe ₂ O ₃ relative to the ore subjected to attack.

The process of the invention makes it possible to reach percentages of solubilization of the above-mentioned elements greater than those of silica while keeping a shorter filtration time.

Other characteristics of the invention will appear more clearly on reading the description which follows and concrete but non-limiting examples of implementation of the method.

The attack on the phosphate ore which can be carried out more particularly with sulfuric acid takes place under known and usual conditions of temperature and concentration of acids.

Aluminum or iron can be introduced either with the attack acid or in the attack slurry.

Aluminum is added in the form of a salt of this element, for example in the form of a sulfate, a phosphate, an alumina or any other precursor capable of releasing the aluminum ion under the conditions of 'attack. It is the same for iron which can in particular be added in the form of sulfate, of oxide, such as ferric oxide.

In a particular embodiment, aluminum and iron can be added in the form of aluminum-calcium phosphates containing iron such as Thies phosphates and Taïba fines, so that the proportions of aluminum and iron in the attack medium are within the limits indicated in claim 1.

It has also been found that it is possible to use a mixture of silica and aluminum. A percentage of solubilization of the yttrium and of the rare earths is then obtained which is higher than that obtained by adding silica alone with a filtration time which remains acceptable. In this case, it is possible to use a natural silica of the Kieselguhr type, a globular silica, or a precipitated silica. Aluminum can be used in the forms described above.

Finally, one can also use a mixture of silica and iron or silica, iron and aluminum.

The amounts of aluminum, iron and silica used depend on the type of ore processed, the attack conditions that one wishes to observe and the type of acid that one wishes to obtain.

After the attack, the resulting porridge is filtered. A residue or primary gypsum is obtained in the case of a sulfuric attack and a phosphoric acid solution. The whole solid obtained after this filtration is called gypsum here. The phosphoric acid solution notably includes uranium in almost the entire quantity present in the starting ore and a significant proportion of yttrium, thorium and rare earths.

The recovery of all of these elements can be done as described in European patent application 26132. In this case, the acid is brought into contact with an organic phase comprising a di (alkylphenyl) phosphoric acid, dissolved in an inert organic solvent and in the presence of a trialkylphosphine oxide. After separation of the phases, the organic phase is reextracted using a solution containing hydrofluoric acid and phosphoric acid.

Example 1

We start with a Kouribga phosphate ore of the following composition: 31.07% P ₂ 0 ₅ ; 344 ppm yttrium; 140 ppm uranium; Ce0 ₂ : 42 ppm; LaC0 ₃ : 132 ppm; Tb ₄ O ₇ : 9 ppm; Yb ₂ O ₃ : 21 ppm.

This mineral is attacked with sulfuric acid without any additives, then in another series of tests in the presence of precipitated silica in variable quantity and in a third series of tests in the presence of aluminum sulphate and d '' a mixture of aluminum sulphate and silica in variable quantity.

The percentages of solubilization for different elements are given in table 1 and the results obtained in the case of yttrium and in table 2.

The amount of aluminum is calculated as AI _z 0 ₃ .

It will be noted that for these examples and the following, the filtration times given were obtained by measuring the filtration time on buchner of the attack slurry and the filtration time of the cake after addition of a representative quantity of washing water. of the quantity of washing water used industrially. The sum of these two times for each test corresponds to the time indicated in Table 1.

It can be seen that, according to the process of the invention, the percentage recovered of rare earths and yttrium, and in particular yttric elements such as Tb and Yb, is markedly increased.

In addition, together with this increase, much better filtration times are obtained than when using silica. This is a particularly important advantage from an industrial point of view since the attack productivity is a function of the filtration time.

Example 2

We attack the same ore as in the previous example but this time in the presence of ferric sulfate,

For an amount of iron calculated as Fe ₂ 0 ₃ of 0.8% by weight relative to the ore, 40% of the amount of Y ₂ 0 ₃ present in the ore is solubilized and the filtration time is 109 s.

Claims (6)

1. A process for the overall recovery of uranium, yttrium, thorium and rare earths contained in a phosphate-bearing ore in the course of preparation of phosphoric acid by the wet process characterised in that, in the acid attack on the ore, which is possibly carried out in the presence of silica, there is added to the attack medium aluminium in trivalent form and/or a ferric compound in such a way that the amount of aluminium and/or iron present in said attack medium varies between 0.8 % and 1.5 % by weight expressed as AI₂0₃ and/or Fe₂0₃ with respect to the ore which is subjected to the attack operation.

2. A process according to claim 1 characterised in that the aluminium is introduced in the form of sulphate, phosphate or alumina.

3. A process according to claim 1 characterised in that the iron is introduced in the form of a ferric sulphate or a ferric oxide.

4. A process according to claim 1 or claim 2 characterised in that the addition of aluminium and/or iron to obtain said quantities of such elements in the attack medium is effected in the form of an aluminium-calcium phosphate containing iron.

5. A process according to claim 1 characterised in that the attack operation is carried out in the further presence of silica.

6. A process according to claim 7 characterised in that the attack operation is carried out in the presence of aluminium and silica.