WO2008070887A2

WO2008070887A2 - Method and apparatus for producing crystal grade anhydrous rare earth halides

Info

Publication number: WO2008070887A2
Application number: PCT/AT2007/000557
Authority: WO
Inventors: Robert Melinz; Andreas Smeritschnig
Original assignee: Treibacher Industrie Ag
Priority date: 2006-12-13
Filing date: 2007-12-10
Publication date: 2008-06-19
Also published as: WO2008070887A3; AT504602B1; AT504602A1

Abstract

The invention relates to a method for producing crystal grade anhydrous rare earth halides, comprising the steps of melting an oxyhalide-free anhydrous NH4X/REX3 mixed salt, wherein X = C1 or Br, and RE is selected from the group consisting of Y, La and lanthanides, passing the melt through a carbon adsorbing filter, and allowing the melt to solidify in a mold, wherein all process steps are performed under inert gas flow. The molded rare earth halides obtained are carbon free and allow easy handling.

Description

Method and apparatus for producing crystal grade anhydrous rare earth halides

Crystal grade anhydrous rare earth chlorides and bromides are applied as functional materials for scintillator crystals in high energy ray detectors. The growing of crystals sets uncompromising purity requirements on the feedstock for crystallization. The feedstock must be a stoichiometric rare earth halide that is free of any impurities which would impair the crystal's quality. Impurities may either arise from the raw materials (other elements) or the production process (water, oxygen, carbon).

Anhydrous rare earth chlorides and bromides are made from rare earth raw materials (RE oxides, RE carbonates) by first converting them into rare earth halide hydrate crystals and then removing the crystal water nominally to zero by physical and chemical means. In a first step the major part of water is removed by physical means, usually vacuumdrying, in a second step the remaining minor part of water is removed by chemical agents. Chemical agents are necessary to avoid the formation of oxyhalides which RE halides tend to form due to hydrolysis. The most effective chemical agents being applied according to the prior art are ammonium halide (NH₄X) sublimation process (e.g., Gmelin Handbook of Inorganic Chemistry (8^th Edition) Sc, Y, La-Lu RARE EARTH ELEMENTS Part C 4a) and hydrogen halide (HX) gas process (e.g., Gmelin Handbook of Inorganic Chemistry (8^th Edition) Sc, Y, La-Lu RARE EARTH ELEMENTS Part C 4a and WO 97/07057). The product obtained is an anhydrous rare earth halide powder which is very hygroscopic, has a high surface area and contains traces of carbon.

As the presence of carbon renders the rare earth halides inappropriate for crystal growth, a decarburization treatment is required subsequent to the sublimation and gas process. A further drawback is, however, that the product is obtained in the form of powder or granules which is very inconvenient to handle.

It is the object of the invention to avoid and overcome the problems and drawbacks of the prior art and to provide carbon-free anhydrous rare earth halides having a form that allows easy handling.

This object is achieved by a method for producing crystal grade anhydrous rare earth halides, comprising the steps of melting an oxyhalide-free anhydrous NH₄X/REX₃ mixed salt, wherein X = Cl or Br₅ and RE is selected from the group consisting of Y, La and lanthanides (the so-called rare earth elements), preferably La and Ce, passing the melt through a carbon adsorbing filter, and allowing the melt to solidify in a mold, wherein all process steps are performed under inert gas flow.

The lanthanides comprise the elements Ce, Pr₅ Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

The method according to the present invention allows to convert an anhydrous rare earth halide powder, obtained, for instance, from an NH₄X sublimation based or HX gas based process, into a surface area-reduced and carbon-free solid piece which is less sensitive to moisture and can be handled more easily than powder or granules.

According to an embodiment of the present invention, the oxyhalide-free anhydrous NH₄XZREX₃ mixed salt is obtained from an NH₄X-sublimation process.

According to another embodiment, the oxyhalide-free anhydrous NH₄XyREX₃ mixed salt is a mixture of oxyhalide-free anhydrous REX₃ obtained from anHX-gas process and 8-12 mass%, preferably 10 mass%, OfNH₄X.

Preferably, argon is used as the inert gas.

A preferred embodiment of the present invention is characterized in that the carbon adsorbing filter consists of silica, preferably silica granules, wherein the average size of the silica granules preferably ranges from 0.3 to 3 mm, more preferably 0.5 to 1 mm.

An essential feature of the invention is that all parts coming into direct contact with REX₃ are made of silica glass.

Another aspect of the present invention refers to solid block crystal grade anhydrous rare earth halides which are obtained by the novel and inventive process.

A further aspect of the invention refers to an apparatus for producing crystal grade anhydrous rare earth halides, comprising

- an inner tube for receiving in its upper part an oxyhalide-free anhydrous NH₄X/REX₃ mixed salt, wherein X = Cl or Br, and RE is selected from the group consisting of Y, La and lanthanides, and further containing a bed of carbon adsorbing filter material in its lower part which is sealed by a bottom plate having a plurality of orifices, wherein the orifices are smaller than the average size of the filter material, and a tubular, preferably conical, mold adjoining the inner tube below the bottom plate,

- an outer tube for receiving the inner tube, having its lower end connected to an inert gas supply and its upper end to a gas line for carrying off the inert gas and gaseous byproducts, wherein the gas line is connected to a condenser means, and

- a tube furnace surrounding the outer tube, wherein the inner and outer tubes as well as the tube furnace are vertically arranged.

It is preferred that the apparatus parts directly contacting REX₃, preferably the inner and outer tubes, are made of silica glass and cleaned by etching with HNO₃/H₂O₂ prior to use.

According to a preferred embodiment of the apparatus, the gas line is heated, in order to avoid ISlH₄X depositions.

Preferably, the gaps between the tube furnace and the outer silica tube are isolated by silica wool.

The invention is illustrated and explained in more detail byway of the following examples

Examples

State of the art process steps

Dissolution + Filtration + Evaporation + Crystallization

Rare earth halide hydrates are produced from rare earth starting materials (RE oxides, RE carbonates) by dissolution in hydrochloric or hydrobromic acid, filtration, evaporation, saturation and crystallization, at least of RE halide hydrate crystals which stoichiometrically contain 7 water molecules per rare earth halide formula unit. These steps, especially filtration and crystallization, serve further purification.

Exemplary production of anhydrous lanthanum bromide LaBr₃

Dissolution (stoichiometric quantities)

La₂O₃ + 6 HBr (acid, 48%) → 2 LaBr₃ (solution)

Heating (>100°C) + adding active charcoal (for carbon removal)

LaBr₃ (solution containing carbon) -→ LaBr₃ (solution, adsorbed carbon) Filtration (glass fibre filter)

LaBr₃ (solution + adsorbent) → LaBr₃ (clear solution)

Evaporation (RE bromides 130°C [RE chlorides 120⁰C])

LaBr₃ (unsaturated solution) → LaBr₃ (saturated solution) + HBrZH₂O (vapour T)

Crystallization (stirring and cooling)

LaBr₃ (saturated solution) -→ LaBr₃.7 H₂O (crystals) + LaBr₃ (liquor)

Separation (filtration or centrifugation)

LaBr₃J H₂O + LaBr₃ → LaBr₃J H₂O (crystals)

Physical dehydration (vacuumdrying)

a) for the NH₄X sublimation process

LaBr₃J H₂O + 5 NH₄Br → LaBr₃.5 NH₄Br-H₂O + 6 H₂O

LaBr₃J H₂O crystals are placed into a flask, and NH₄Br crystals are admixed in a molar surplus of 3-5. Then water is added till a liquid mixture is formed. This mixture is heated up under rotation, at which a clear solution arises, and evaporated at approximately 140°C under atmospheric pressure till particles start to segregate from the boiling solution. The suspension formed is cooled down under rotation (by lifting out of the heating bath), at which a viscous mass is formed that covers the inner wall of the flask.

The flask is slowly evacuated while the viscous mass is bubbling. After applying full vacuum the flask is heated up again (by immersing into the heating bath) up to a maximum temperature of 180⁰C. At that temperature the viscous mass converts into a solid crust of NH₄BrZLaBr₃ mixed salt with a minimal amount of crystal water. This crust detaches from the glass easily, is broken up, removed from the flask and taken as precursor for the NH₄X sublimation process.

b) for the HX gas process

LaBr₃J H₂O (crystals) → LaBr₃. H₂O (granules) + 6 H₂O LaBr₃.7 H₂O crystals are placed into a flask and slightly heated up under rotation and Ml vacuum. It has to be taken care that the crystals do not melt or agglomerate. If done carefully, the crystals strat to crunch and convert into free-flowing granules. The temperature is raised up to 160°C at Ml vacuum. The product obtained is removed from the flask and can be immediately used for the HX gas process.

Chemical dehydration

a) NH₄X sublimation process

LaBr₃.5 NH₄Br.H₂O → LaBr₃ (anhydrous) + 5 NH₃ + 5 HBr + H₂O

The reaction is carried out in a chemically resistent and thermoresistent silica apparatus within a furnace. The vacuumdried precursor (NH₄Br/LaBr₃ mixed salt) is placed in silica vessels which are passed by an inert gas flow, for example argon, from bottom to top, assuring inert conditions and transporting the sublimation products off.

As the temperature is rising the ammonium bromide decomposes into NH₃ and HBr gas, which both serve as water carriers upon release from the precursor. The temperature is raised at a rate of 0.5-1 °C/minute beyond 600°C (complete removal of NH₄Br). The sublimation products (NH₄Br + H₂O) are collected in a cooling trap.

The product obtained by the sublimation reaction is a crude anhydrous RE halide powder which has a high surface area, is very hygroscopic and contains small amounts of carbon originating from the starting materials.

The formation of oxyhalides can be completely avoided in a well-run process, but the quantity of remaining carbon is varying. Traces of contaminating carbon are regularly present, rendering the material inappropriate for crystal growth and making a refinement step necessary. Remelting can be accomplished immediately following chemical dehydration.

The ammonium halide sublimation process is suitable for chlorides and bromides of basic RE elements (e.g., La, Ce). When applied to more acidic RE elements (e.g., Y, Yb) the formation of oxyhalides becomes likely.

b) HX gas process LaBr₃-H₂O (granules) + HBr (gas) -> LaBr₃ (granules) + HBr (acid)

Vacuumdried LaBr₃-H₂O granules are placed into a rotating silica tube which is mounted within a tube furnace as a heating source. HBr process gas is circulated through the silica tube reactor over the agitated granules and units for gas treatment. The gas transport is accomplished by a chemically resistant membrane pump. AU parts are connected with teflon tubes.

Dry HBr process gas gets loaded with H₂O from the vacuumdried precursor and unloads the vapor while passing the condenser. The vacuumdried LaBr₃-H₂O precursor, initially containing remnants of water, is thus converted into anhydrous LaBr₃. By the presence of dry HBr the formation of oxybromides is avoided, due to a steady unloading OfH₂O vapor. The temperature is increased stepwise from 150⁰C to 500⁰C (50⁰C steps from 15O⁰C to 300⁰C, 100⁰C steps from 300⁰C to 500⁰C). Despite circulation of the HBr gas, new HBr gas has to be continuously added to the drying process, due to HBr absorption in the condensate forming HBr acid. The occuring pressure loss serves as an indicator for the progress of the drying process. The dehydration lasts up to 4h.

The dehydration according the HX gas process is very suitable for acidic and heavy RE element halides, e. g. YX₃, YbX₃. Anhydrous RE halide granules obtained from the HX gas process can additionally be transformed into a solid block by admixing NH₄X and remelting.

After complete conversion into the anhydrous state traces of carbon are still inside the product, producing a grey tinge. The carbon is formed by pyrolysis of hydrocarbons present in the HBr gas, the initial La₂O₃ or the HBr acid. When the RE halide product is dissolved in deionized water or crystallized the carbon contamination remains as insoluble black particles or black spots inside the crystal, respectively. Carbon must be removed by an additional treatment (decarburization).

Decarburization

Decarburization is carried out subsequent to dehydration. For that the temperature is raised near the LaBr₃ melting point (i.e., up to about 700⁰C). Below the melting point the powdery state and the high surface area of the halide are providing good access for the process gas. The HBr gas flow is maintained or clocked, respectively, so as to avoid temperature losses. HBr gas also passes a bubble column filled with sulfuric acid. At the contact with sulfuric acid HBr is partially oxidized to Br₂, imparting a red-brown tinge to the originally colourless HBr gas. Br₂ is necessary to effectively remove the carbon. The decarburization step lasts up to 4 hours.

Afterwards heating is stopped and the reactor is cooled down. The reaction vessel is flushed with inert gas (argon) and unloaded. The anhydrous and spotless white RE halide granules are packed into air and moisture tight containers.

Production of crystal grade anhydrous rare earth halides according to the invention

Principally, the invention comprises the transformation of powdery anhydrous RE halide into a solid piece of molten anhydrous RE halide, wherein the liquid melt passes a filter bed for the adsorption of insoluble impurities, especially carbon. The feedstock for this refinement is preferably either granules from the HX gas process or powder from the NH₄X sublimation process.

A prerequisite of the method according to the invention is that the crude anhydrous RE halide used as a starting material is free of oxyhalides, because the presence of oxyhalides cannot be reversed in the refinement process and will result in a material that is not suitable for crystal growth. Thus, the crude anhydrous RE halide used in the method according to the invention must be the product of a well-run dehydration process.

a) following the NH₄X sublimation process

When following the ammonium halide sublimation process the refinement can be carried out immediately after the chemical dehydration within the same apparatus in specially designed vessels. This remelting operation completely eliminates the carbon contamination regularly present after the dehydration step.

The apparatus of the present invention allows heating under an inert gas flow from bottom to top, the removal of the sublimate with the inert gas flow and the trickling of the melt through a carbon adsorbing filter bed into a mold where the purified product can solidify and cool down under inert conditions.

According to the present invention, a vertical assembly of the components is provided. The precursor is placed in the upper part of the inner silica tube and dehydrates as it is heated. The sublimate is carried off from the outer silica tube by the inert gas flow. The escaping NH₄X is collected in a cooling trap and can be recycled. An anhydrous but still crude RE halide powder is obtained. The powder is then melted by increasing the temperature beyond the melting point (regularly 950°C/2h holding) in the same vessel, at which lower part a filter bed is provided. At the bottom of the tube itself there are small openings, through which the melt trickles and drops into the mold placed just below.

The filter bed is decisive for refinement. It consists, for example, of silica granules in a size of 0.3-3 mm, preferably 0.5-1 mm, filled up to a height of 10 cm in the tube which is 40 cm in length. The surface of the granules is cleaned by etching with HNO₃ and H₂O₂. These granules do not fit through the orifices (where the melt trickles through) and adsorb segregated impurities, especially carbon, from the passing melt on their surface. Due to the resistance of silica against RE halide melts and the clean surface no contaminants are introduced into the melt.

The filtering material must be thoroughly cleaned before use. After the process run the remains of carbon are clearly visible as a black substance in the upper part of the filter bed. The halide product itself is free of black impurities. When the halide is dissolved in water or crystallized no black spots can be found.

The solidified anhydrous RE halide block is toppled out of the conical mold, packed in air and moisture tight bags which are evacuated and sealed.

b) following the HX gas process

The apparatus used is the same as for the ammonium halide sublimation, inclusive an inert gas flow. Crude anhydrous RE halide granules, either from an HX gas process or an interrupted NH₄X sublimation process, are thoroughly mixed with 10 mass% of ammonium halide and placed in the vessel.

The temperature is raised with a rate of 2°C/min to beyond the melting point of the anhydrous RE halide. During the heating the admixed ammonium halide decomposes and flushes the space between the granules in the silica tube with HX gas and NH₃. Atmospheric remains are displaced and traces of moisture are carried off thereby avoiding the formation ofoxyhalides. The anhydrous RE halide granules melt when the temperature passes the melting point and the melt trickles over the filter bed into the mold below. The following steps are the same as described under (a).

Apparatus according to the invention

For the reactor parts coming in direct contact with starting materials, intermediates and end products preferably only silica is used.

The apparatus can principally be divided in a part containing the reacting material, a part ensuring inert conditions, a part serving as a heating source and a part collecting side products.

The apparatus part which contains the reacting material comprises a silica tube 1 open at the top and sealed with a bottom plate 2 provided with small orifices 3. In its upper part the tube receives the RE halide 4, in its lower part the tube contains a bed 5 of carbon adsorbing filter material, e.g. is filled up to one third of the tube's height with silica granules that are slightly larger than the orifices 3 in the bottom plate 2 below. Adjoining the bottom plate a mold 6 of preferably conical shape is provided, which is supported by a carrier 7 that holds the tube in the heating zone.

The apparatus part which ensures inert conditions comprises an outer silica tube 8 receiving the inner silica tube 1 containing the RE halide 4 and the filter bed 5 and establishing an inert atmosphere therein, an inert gas supply 9 connected to the lower end of the outer silica tube 8, and a gas line 10 connected to the upper end of the outer silica tube 8. The gas linelO is connected to a collector 11, namely a condenser means, for gaseous byproducts. The inert gas flows from bottom to top through the silica tubes 1 and 8, thereby passing the RE halide 4 and carrying off gaseous byproducts. The inert gas used is preferably argon.

The apparatus part serving as a heating source comprises a vertically mounted tube furnace 12 enclosing the silica tube 8. The gaps between the tube furnace 12 and the outer silica tube 8 are preferably isolated by silica wool.

The apparatus part which collects byproducts comprises a collector 11, for example a water- cooled three-neck flask, connected to the outer silica tube 8 via the gas line 10 that is heated for avoiding NH₄X depositions therein. NH₄X carried off from the silica tube is condensed and collected in the flask and can be reused. The argon carrier gas is released over an outlet 13.

Production QfLaBr₃

101 g of a vacuumdried NH₄BrZLaBr₃ mixed salt were placed in the above described apparatus. The temperature was gradually increased to 950⁰C under argon gas. The obtained material was removed from the apparatus, packed in a glass eprouvette and sealed in air-tight bags. From the material crystals were grown (see Fig. 2a: crystal in transmitted light, and Fig. 2b: crystal in reflected light).

Production of CeBr₃

130 g of a vacuumdried NH₄BrZCeBr₃ mixed salt were placed in the described apparatus. The temperature was gradually increased to 950⁰C under argon gas. The obtained material was removed from the apparatus, packed in a glass eprouvette and sealed in air-tight bags. From the material crystals were grown (see Fig. 3a: crystal in transmitted light, and Fig. 3b: crystal in reflected light).

Production of CeCl₃

50 g OfCeCl₃ anhydrous granules were admixed with 5,8 g OfNH₄Cl and placed in the described apparatus. The temperature was gradually increased to 950°C under argon gas. The obtained material was removed from the apparatus, packed in a glass eprouvette and sealed in air-tight bags. From the material crystals were grown (see Fig. 4a: crystal in transmitted light, and Fig. 4b: crystal in reflected light).

Claims

Claims:

1. A method for producing crystal grade anhydrous rare earth halides, comprising the steps of melting an oxyhalide-free anhydrous NH₄XZREX₃ mixed salt, wherein X = Cl or Br, and RE is selected from the group consisting of Y, La and lanthanides, preferably La and Ce, passing the melt through a carbon adsorbing filter, and allowing the melt to solidify in a mold, wherein all process steps are performed under inert gas flow.

2. The method according to claim 1, wherein the oxyhalide-free anhydrous NH₄XZREX₃ mixed salt is obtained from an NH₄X-sublimation process.

3. The method according to claim 1, wherein the oxyhalide-free anhydrous NH₄XZREXa mixed salt is a mixture of oxyhalide-free anhydrous REX₃ obtained from an HX-gas process and 8-12 mass%, preferably 10 mass%, OfNH₄X.

4. The method according to any of claims 1 to 3, wherein the inert gas is argon.

5. The method according to any of claims 1 to 4, wherein the carbon adsorbing filter consists of silica, preferably silica granules.

6. The method according to claim 5, wherein the average size of the silica granules ranges from 0.3 to 3 mm.

7. The method according to claim 6, wherein the average size of the silica granules ranges from 0.5 to 1 mm.

8. Solid block crystal grade anhydrous rare earth halides, obtained by the process according to any of claims 1 to 7.

9. An apparatus for producing crystal grade anhydrous rare earth halides, comprising - an inner tube (1) for receiving in its upper part an oxyhalide-free anhydrous NH₄XZREX₃ mixed salt (4), wherein X = Cl or Br, and RE is selected from the group consisting of Y, La and lanthanides, and further containing a bed (5) of carbon adsorbing filter material in its lower part which is sealed by a bottom plate (2) having a plurality of orifices (3), wherein the orifices (3) are smaller than the average size of the filter material, and a tubular, preferably conical, mold (6) adjoining the inner tube (1) below the bottom plate (2),

- an outer tube (8) for receiving the inner tube (1), having its lower end connected to an inert gas supply (9) and its upper end to a gas line (10) for carrying off the inert gas and gaseous byproducts, wherein the gas line (10) is connected to a condenser means (11), and

- a tube furnace (12) surrounding the outer tube (8), wherein the inner and outer tubes (1, 8) as well as the tube furnace (12) are vertically arranged.

10. The apparatus according to claim 9, wherein the apparatus parts directly contacting REX₃, preferably the inner and outer tubes (1, 8), are made of silica glass and cleaned by etching with HNO₃ZH₂O₂ prior to use.

11. The apparatus according to claim 9 or 10, wherein the gas line (10) is heated.

12. The apparatus according to any of claims 9 to 11, wherein the gaps between the tube furnace (12) and the outer silica tube (8) are isolated by silica wool.