"Process for recovering rare earth metals"

DESCRIPTION

[0001] Some exhausted catalysts of the petroleum industry that are used for catalytic cracking processes contain elements such as nitrogen, sulphur, and high amounts of alumina and silicon oxides. The oxides that are present in larger amounts are Si0 ₂ (-48%) and Al ₂ 0 ₃ (-43%); other oxides are La ₂ 0 ₃ (-2.5%), P ₂ 0 ₅ (~ 1.5%), Fe ₂ 0 ₃ (-1%), V ₂ 0 ₅ (-1%), NiO (<1%) and Ce ₂ 0 ₃ (~ 0.5%). Such catalysts have, by virtue of the high content of Al and Si oxides, a good pozzolanic activity, which advances their use as additives in concretes. In fact, it has been shown that the natural zeolite or pozzolanic materials can improve the physical and mechanical properties of concrete by virtue of the silicon oxides that react with calcium hydroxide. Furthermore, cerium oxide is a chromophore compound, and as such, it finds wide use in the ceramic industry.

Moreover, in these catalysts there are rare earth oxides (such as lanthanum and cerium oxides), of a great strategic importance for both the development and production of the new electronic technologies, and for their economic value. The extraction and refining of rare earth elements is concentrated in a few Countries, including China, which holds the record for production. However, the demand for such elements has already exceeded the offer, and it is expected that the recovery of rare earth elements from secondary raw materials, such as exhausted catalysts, will be more and more necessary in the future.

The European Union also classifies the rare earth elements as a critical, strategically important material, since they are essential raw materials both for high-tech products and routinely used materials, such as, for example, cell phones, thin-layer photovoltaic elements, lithium ion accumulators, optic fiber cables, synthetic fuels, etc. It is believed that within 2030, the request of a series of fundamental raw materials could even triplicate compared to that of 2006. The main risk related to their provision is further related to the fact that they have a low degree of replaceability and a reduced degree of recycling.

[0002] Currently, processes for recovering rare earth elements, and particularly Lanthanum and Cerium, from catalysts, are known, but these have some drawbacks that limit their application.

OBJECT OF THE INVENTION

[0003] Therefore, the object of the present invention is to provide a process for recovering rare earth elements from exhausted catalysts containing rare earth elements, and particularly Lanthanum and Cerium, which at least partially overcome the limitations of the known processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Fig. 1 is a schematic representation of the process according to an embodiment of the present invention, while Fig. 2 is a schematic representation of the process according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0005] For the purposes of the present invention, a catalyst used for recovering rare earth elements comprises a gel of an inorganic oxide, such as a silica-aluminium gel, and/or a crystalline zeolite dispersed (10-50% by weight) in an inorganic matrix (50-90% by weight) having excellent mechanical properties and some catalytic properties. The remainder (0-10% by weight) is composed of additives, such as platinum, rare earth elements, antimony, and other elements. A typical composition of a catalyst object of the present invention is as follows:

Compounds % by

weight:

A1 ₂ 0 ₃ >20

Si0 ₂ >20

S 0-5

Ti 0-5

Fe 0-5 Zr 0-5

La 0-15

Ce 0-15

Other rare earth 0-15

elements

from which it is inferred that such catalysts comprise a substantial portion of Aluminium.

Similarly, binders, filler agents, and other functional additives may be present.

[0006] In particular, the process of the present invention is carried out on a raw material represented by a powder catalyst.

[0007] The process for recovering rare earth metals, preferably Lanthanum and Cerium, and still more preferably Cerium, according to the present invention, comprises a first step a) of subjecting the powder of such catalysts to leaching with a strong mineral acid.

Particularly, such acid may be sulfuric acid, hydrochloric acid, or nitric acid.

In a preferred aspect, such acid is selected from hydrochloric acid and nitric acid.

The concentration of the acid may range between 1-4 M, so as to bring the pH to values lower than 1.

Particularly, sulfuric acid will preferably be in a concentration of 1-2 M, while hydrochloric acid and nitric acid will have a concentration of about 1-4 M, and preferably 2-3 M.

Particularly, such step a) is carried out for a time ranging between 1-4 hours, and preferably about 2-3 hours at a temperature of 10-100° C and preferably 25- 80° C.

It has been noticed that the temperature increase promotes the dissolution of the elements present in the starting powder.

[0008] Particularly, the ratio between the leaching solution and the solid (represented by the catalyst powder) is about 20-30%, and preferably about 10-20% (weight/volume) .

Advantageously, the leaching step a) according to the present invention allows the passage of Lanthanum and Cerium into the solution with very high yields, above 50% even at room temperature, and even above 80% at high temperatures.

[0009] In a particular aspect, the leaching step may be carried out in countercurrent with 2-6 stages, in order to concentrate the solution.

[0010] In a first embodiment of the process of the present invention, after the leaching step a) , a precipitation step (step b) is carried out.

Such step b) is carried out by increasing the pH of the solution previously obtained by adding a base.

In a preferred aspect, sodium hydroxide is added in concentrations ranging between 4.5-5.5 M and until reaching a pH of about 0.7-2.

Therefore, the precipitation step is carried out for a period of 1-2 hours.

[0011] From the precipitation step:

• an exhausted solution, intended to undergo treatments provided for the waste waters;

• a concentrated solid comprising Lanthanum and Cerium plus aluminium and other impurities are obtained.

[0012] According to a preferred aspect of the present invention, the process providing the precipitation step b) comprises a leaching _. step a) carried out by sulfuric acid.

[0013] Advantageously, the yields of the precipitation step according to the present invention are very high, and allow an almost quantitative recovery of Lanthanum and Cerium.

Therefore, on the whole, the overall yield of the process of the present invention is above 80% and ranges between 85-95%.

[0014] According to a particularly preferred aspect of the invention, in an alternative form illustrated for example in Fig. 2, the process for recovering rare earth metals from catalysts according to the present invention comprises, after the leaching step a), a solvent extraction step. To this aim, step a) is preferably carried out using hydrochloric acid or nitric acid.

In this case also, the ratio between leaching solution and the solid (represented by the catalyst powder) is about 20-30%, and preferably about 10-20% (weight/volume) .

Preferably, such step a) is carried out for 1-3 hours, preferably for about 2 hours and at a temperature of 50-80° C.

[0015] In a particular aspect, the leaching step may take place in countercurrent with 2-6 stages, in order to concentrate the solution.

[0016] Next, in a step b' ) , the pH of the leaching solution is preferably increased up to 4 by adding NaOH.

[0017] At this point, an optional filtration step b'l) may be provided, so as to separate the solid from the solution, which is subsequently processed.

[0018] Subsequently, it is then proceeded with the solvent extraction step c'), in which the leaching solution is contacted under vigorous stirring with an organic extracting agent.

Preferably, such extracting agent is selected from (2- ethylhexyl) phosphoric acid (for example, available as D2EHPA) , or the di (2, 4 , 4-trimethylpentyl) phosphine acid (available as CYANEX 72) .

According to a preferred aspect, such extracting agent is in an organic solvent, for example, represented by n-heptane or kerosene, at about 20% (vol/vol) .

Furthermore, the aqueous phase/organic phase ratio is preferably about 1:1.

[0019] After the extraction step, the stripping (removal) step d' ) is performed, wherein Cerium and Lanthanum are extracted from the solution previously obtained by adding a solution of the same acid used for the leaching step, i.e., hydrochloric acid or nitric acid.

The organic solution/added acid (hydrochloric acid or nitric acid) ratio preferably ranges between 1:1 and 4:1.

In an aspect of the present invention, the stripping step is preferably repeated on the obtained aqueous solution up to 4 times, in cascade, before going through the step e' ) .

[0020] A precipitation step e' ) follows the step d' ) .

Particularly, Cerium and Lanthanum are precipitated from the aqueous solution previously obtained with a concentrated oxalic acid solution (for example, 100 g/1) .

In a preferred aspect, concentrated NaOH is added to the precipitation solution in order to maintain the pH between about 0.5-3.

[0021] Cerium and Lanthanum (Ce2(C20 ₄ ) ₃ ) and (La ₂ (C ₂ 0 ) 3) oxalate thus precipitated comprise about 45-50% Lanthanum and 3% Cerium.

[0022] With the calcination step f' ) , the oxalate is subjected to a temperature of about 600° C for at least 1 hour, so as to obtain a mixed Lanthanum and Cerium trioxide, the purity of which is about 98%. Experimental section

[0023] In the following experimental section, some examples of processes implemented according to the present invention are described.

EXAMPLE 1

a) Characterization of a catalyst powder containing rare earth elements

A sample of a powder obtained from exhausted catalysts was characterized by XRF analysis as having the following composition:

Element % by

weight

Al 17.35

Si 12.75

S 0.013

Ti 0.43

Fe 0.33

Ni 0.02

Zr 0.16

La 3.02

Ce 0.23 b) Leaching

The powder sample of the point a) is subjected to a leaching step during 3 hours with a 2 M sulfuric acid solution.

The following Table shows the leaching percentages for each element comprised in the initial sample and the percentage that is present in the solid residue.

From the data set forth above, it shall be apparent that the leaching step allows recovering a high percentage of Lanthanum and Cerium.

c) Precipitation

The Lanthanum and Cerium-rich solution obtained according to the point b) is subjected to precipitation, by adding 5 M sodium hydroxide and bringing the pH to below 2.

In the following Table, the % composition of the precipitate after XRF analysis, and the precipitation yields are set forth.

EXAMPLE 2

Solvent extraction process

A leaching of 20 g catalyst in 100 mL 2M HN0 ₃ at 30° C for 2 hours is set forth by way of example.

The extraction yield was equal to 55% for La and 47% for Ce.

50 mL leaching solution were treated with an organic solution at 20% vol. D2EHPA in n-heptane in a 1:1 volumetric ratio, under constant stirring for a few minutes .

The results of the extraction are reported in the following Table.

Lanthanum extraction 0.73 0 2.503 0.128154 0.019596 13.3

0.7 0.5 2.729 0.135631 0.012119 8.2

0.75 2.5 2.441 0.124979 0.022771 15.4

1 2.5 1.944 0.102449 0.045301 30.7

1.43 1 1.357 0.071514 0.076236 51.6

1.9 0.5 0.545 0.028449 0.119301 80.7

2.3 0.5 0.1028 0.005315 0.142435 96.4

3.7 0.5 0.02226 0.00114 0.14661 99.2

Cerium extraction

Once it has been separated, the organic step was contacted with 25 mL 4 M HN0 ₃ solution for 30 minutes under constant stirring.

The results of the stripping operation are set forth in the followin Table. (g) (FA/FO)

LANTHANUM

Init . 4.47 0.11175 0.03486 76.2

15' 4.653 0.116325 0.030285 79.4

30' 4.819 0.120475 0.026135 82.2

CERIUM

Init. 0.2955 0.007388 0.0020625 78.2

15' 0.2965 0.007413 0.0020375 78.4

30' 0.2979 0.007448 0.0020025 78.8

To the aqueous solution, 5 mL of a solution of 40 g/L oxalic acid were added, and the pH was brought to 40% by weight with 10 mL NaOH. At pH 0.66, the formation of a cosiderable white precipitate is observed, which is separated by filtration and dried at 105° C for 24 hours .

The precipitate was then analyzed.

The concentration of the major elements is set forth in the following Table.

The main contaminant turns out to be aluminium, since silicon, the other most concentrated element in the exhausted catalysts, is not leached by strong acids.

The salt is composed of Cerium and Lanthanum oxalates having a purity of about 97.5%, which may be baked at 600° C for at least 1 hour, thus obtaining La ₂ 0 ₃ and Ce ₂ 0 ₃ in a high purity (>98%) .

[0024] The advantages provided by the process of the present invention are clear, since, as it has been shown, it allows recovering rare earth metals in a highly pure form.

For example, it is sufficient to note how, from the leaching and precipitation steps, percentages of Lanthanum and Cerium of 90% and 82% are obtained, respectively.

Furthermore, it allows recovering material, which otherwise would be disposed of, however losing a valuable source of important elements, such as Lanthanum and Cerium, which are metals for which a strong request increase in the market is expected. On the other hand, the waste material, for example, non-leached material, may be subjected to decontamination procedures provided for by the law in order to be subsequently used again in other application fields, for example, as an additive in cement plants, due to its high pozzolanic activity, or in the ceramic industry.

A further advantage is represented by the possibility to separate Aluminium, which is notoriously difficult to separate from the other elements, which is in fact recovered in a percentage lesser than 1% in the final product, from the rare earth metals, and particularly Lanthanum and Cerium, and still more particularly Cerium.

[0025] From the description provided above of the process of the invention for recovering rare earth metals, and particularly Lanthanum and Cerium, and still more particularly Cerium, from catalysts, those skilled in the art, in order to meet contingent, specific needs, will be able to make a number of modifications, additions, or replacements of elements with other functionally equivalent ones, without however departing from the scope of the appended claims. Each of the characteristics described as belonging to a possible embodiment may be implemented independently from the other embodiments described.