MECHANOCHEMICAL SYNTHESIS OF RARE EARTH SULFIDES

Abstract

The present invention pertains to a process for preparing particles of rare earth sulfide comprising the steps of:preparing a reaction mixture comprising at least one compound comprising at least one rare earth element (A) and at least one alkali metal sulfide (B),submitting said reaction mixture to a mechanical stress so as to cause a chemical reaction that produces the particles of rare earth sulfide.

Claims

1. A process for preparing particles of rare earth sulfide comprising the steps of: preparing a reaction mixture comprising at least one compound comprising at least one rare earth element (A) and at least one alkali metal sulfide (B), submitting said reaction mixture to a mechanical stress so as to cause a chemical reaction that produces the particles of rare earth sulfide.

2. The process according to claim 1, wherein (A) is a rare earth halide.

3. The process according to claim 2, wherein the rare earth halide is a rare earth chloride.

4. The process according to claim 3, wherein the rare earth chloride is CeCl.sub.3, GdCl.sub.3, LaCl.sub.3 or a mixture thereof.

5. The process according to claim 4, wherein the rare earth chloride is LaCl.sub.3.

6. The process according to claim 1, wherein mechanical stress is provided by grinding together (A) and (B), optionally in the presence of at least one alkali metal halide (C).

7. The process according to claim 1, wherein the alkali metal sulfide (B) is Na.sub.2S.

8. The process according to claim 1, wherein the reaction mixture further comprises at least one solvent.

9. The process according to claim 1, wherein the reaction mixture further comprises grinding media which are objects consisting of a rigid material.

10. The process according to claim 9, wherein the objects are balls, beads, banded satellite spheres, rings or rods.

11. The process according to claim 9, wherein the objects are substantially spherical and have a mean diameter ranging from 0.5 mm to 150.0 mm.

12. The process according to claim 9, wherein the rigid material is selected from the list consisting of agate, corundum, zirconia, stainless steel, tempered steel, silicon nitride, tungsten carbide and mixtures thereof.

13. The process according to claim 12, wherein the rigid material is zirconia.

14. The process according to claim 9, wherein the weight ratio of the grinding media with regard to (A) and (B) is ranging from 1 to 50.

Description

EXAMPLES

[0088] In each example and comparative example, the reaction was conducted under atmosphere of Argon, the molar ratio between LaCl.sub.3 and the respective sulfur source was kept constant () and the weight ratio between powder reactants and zirconia grinding balls was also kept constant.

Example 1: Mechanochemical Synthesis of La.SUB.2.S.SUB.3 .Using Na.SUB.2.S

[0089] In a glove box under Argon atmosphere, 1.962 g (8.0.10.sup.3 mol) of anhydrous LaCl.sub.3 and 0.936 g (12.0.10.sup.3 mol) of Na.sub.2S were introduced in a zirconia grinding bowl. 18 zirconia grinding balls of 10 mm diameter were then added and the bowl closed and inserted in the planetary ball mill Pulverisette 7 available from Fritsh. The rotation speed of the bowl was set at 500 rpm for 20 min. followed by 10 min. pause. The cycle was repeated 4 times so that the overall effective grinding time was 80 min. The crude product was recovered as a powder which was then dispersed in absolute methanol in order to solubilize NaCl formed. Typically 1 g of resulting powder was dispersed in 100 ml of methanol under stirring for 2 hours using an ultrasonic water bath Bransonic 221 provided by Branson.

[0090] After what the dispersion was filtered over a Buchner funnel, the recovered powder was washed with methanol and dried under vacuum at 40 C. until constant weight was obtained. SEM images showed that the resulting La.sub.2S.sub.3 powder after washing with MeOH was composed of agglomerated nanoparticles.

Example 2: Mechanochemical Synthesis of La.SUB.2.S.SUB.3 .Using Li.SUB.2.S

[0091] The same procedure was carried out using 2.264 g (9.2.10.sup.3 mol) of LaCl.sub.3 and 0.635 g (13.8.10.sup.3 mol) of Li.sub.2S. SEM images showed that the resulting La.sub.2S.sub.3 powder after washing with MeOH was composed of agglomerated nanoparticles.

Example 3: Mechanochemical Synthesis of Gd.SUB.2.S.SUB.3 .Using Na.SUB.2.S

[0092] The same procedure was carried out using 2.003 g (7.6.10.sup.3 mol) of GdCl.sub.3 and 0.889 g (11.4.10.sup.3 mol) of Na.sub.2S. SEM images showed that the resulting Gd.sub.2S.sub.3 powder after washing with MeOH was composed of agglomerated nanoparticles.

COMPARATIVE EXAMPLE

[0093] The same procedure was carried out using 2.012 g (8.2.10.sup.3 mol) of LaCl.sub.3 and 0.887 g (12.3.10.sup.3 mol) of CaS.

[0094] When comparing results from examples 1 and 2 with results from comparative example, XRD experiments conducted onto the crude products revealed, on one hand, that the reaction gave the expected product namely La.sub.2S.sub.3 when Na.sub.2S or Li.sub.2S were employed as sulfur source and, on the other hand, that the reaction did not or almost did not occur when CaS was used since no La.sub.2S.sub.3 was detected in that case.

[0095] Moreover, XRD experiments conducted onto the crude products showed that Na.sub.2S was consumed since no remaining Na.sub.2S was detected, that Li.sub.2S was almost totally consumed since remaining Li.sub.2S was detected in low quantity and that CaS was almost unreactive since it was the only compound detected with LaCl.sub.3. Thus unexpectedly, the reaction rate was improved when replacing CaS by Li.sub.2S or Na.sub.2S.

[0096] X-Ray diffraction measurements were performed with XPert PRO diffractometer available from PANalytical, using copper's K1 and K2 radiations (=1.54056 and =1.54439 respectively) in the Bragg-Brentano geometry.

[0097] Scanning Electron Microscopy (SEM) was carried out thanks to a Zeiss Leo 1525 apparatus equipped with an in-lens detector operating at 3 kV.

[0098] SEM images revealed that the resulting La.sub.2S.sub.3 and Gd.sub.2S.sub.3 powders after washing with MeOH were composed of agglomerated nanoparticles.

[0099] Measurements of sulfur content were carried out onto samples of product obtained after washing with methanol. The titration was performed by induced coupled plasma with an optical emission spectroscopy detector (ICP-OES) performed on PlasmaQuant PQ 9000 from Analytik Jena. The samples were solubilized in concentrated aqueous HNO.sub.3 solution by heating in a microwave oven. The limpid solution was diluted in a nitric acid 5% aqueous solution. The intensity measured on the Sulfur specific wavelength (eg. 180.669 nm and 181.975 nm) was compared to a calibration curve in the range of 0.05 to 22.0 mg/L of sulfur standards obtained in similar analytical conditions in order to determine the amount in the diluted solution. The amount in the solution was obtained by calculation using the dilution factor.

[0100] As La.sub.2S.sub.3 was the only sulfur containing compound detected by XRD in the product after washing, the sulfur content was attributed to La.sub.2S.sub.3 only. The yield of La.sub.2S.sub.3 is reported in table 1.

TABLE-US-00001 TABLE 1 sulfur content in the resulting product and yield of La.sub.2S.sub.3 Sulfur Recovered content in La.sub.2S.sub.3 product the washed La.sub.2S.sub.3 theoretical Recovered after washing product content in content in La.sub.2S.sub.3 S crude product in methanol ICP/OES the product the product yield Example source (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (%) 1 Na.sub.2S 100 42 11.4 44.4 51.6 36.1 2 Li.sub.2S 100 58 9.4 36.6 59.5 35.7 comparative CaS 100 10 n.d.* n.d.* 52.9 .sup.0** *n.d. = not determined **yield assumed to be equal to 0 because, as previously mentioned, no La.sub.2S.sub.3 was detected by XRD

[0101] The results summarized in table 1 clearly reveal the advantage of using alkali metal sulfide such as Li.sub.2S or Na.sub.2S for the mechanochemical synthesis of La.sub.2S.sub.3 starting from LaCl.sub.3.

[0102] Moreover, XRD experiments performed onto products resulting from examples 1 and 3 revealed that the obtained La.sub.2S.sub.3 and Gd.sub.2S.sub.3 powders were composed of particles comprising cristallites having a mean diameter of 15 nm. This diameter was calculated using the Scherrer model as previously described.