KR910001581B1 - Metallothermic reduction of rare earth oxides with calcium metal - Google Patents

Abstract

Rare earth oxides can be reduced to rare earth metals by, a novel, high yield, metallothermic process. The oxides are dispersed in a suitable, molten, calcium chloride-based bath (44) along with calcium metal. The bath (44) is agitated and calcium metal reduces the rare earth oxides to rare earth metals. The metals collect in a discrete layer (43) in the reaction vessel (22).

Description

Non-electrolytic Reduction Process of Rare Earth Oxides Using Calcium Metals

1 is a side cross-sectional view of a device suitable for the process of reducing RE-oxides to RE metals in accordance with the present invention.

FIG. 2 is a process overview for producing neodymium-eutectic alloy by reducing neodymium oxide (Nd ₂ O ₃ ) together with calcium.

FIG. 3 is a graph showing the yield of neodymium (Nd) metal produced from neodymium oxide (Nd ₂ O ₃ ) as a CaCl ₂ concentration function in a salt bath.

* Explanation of symbols for main parts of the drawings

8, 10, 12: cram shell heater 15: thermocouple

20: furnace well 22: reactor

24: stirrer 28: variable speed motor

36: cooling water coil 38: stainless steel baffle

The present invention relates to an electroless reduction process for the direct reduction of rare earth oxides (RE-oxides), in particular neodymium oxide.

This process is particularly useful for the inexpensive production of neodymium metal for neodymium-iron-boronite.

In the past, the strongest commercial permanent magnet was produced from a sintered powder of samarium and cobalt alloy (SmCo ₅ ). More powerful magnets have recently been made from alloys of rare earth elements (especially neodymium and praseodymium), iron and boron. Such alloys and methods for producing magnets using them are described in European Patent Applications Nos. 0,108,474, 0,133,758 and 0,144,112.

Rare earth elements (RE) are elements of atomic number 57-71 and yttrium of atomic number 39 of the periodic table whose sources are monazide and bastnasite ores. Mixtures of rare earth elements can be extracted from these ores by various known techniques, and the extracted mixtures can be separated from one another by conventional known techniques such as elution, liquid-liquid extraction and the like.

Once the rare earth elements (metals) have been separated from each other, they must be reduced from oxide to pure (at least 95 atomic percent purity) to be useful for application to permanent magnets. In the prior art, this final reduction step is very complex and expensive, resulting in a very high price of rare earth metals. As the reduction method, both methods have been used in the electrolytic process and the electroless process.

The electrolytic reduction process includes (1) a process involving the decomposition of anhydrous rare earth metal chlorides dissolved in a molten alkali metal salt or an alkaline earth metal salt and (2) a process involving the decomposition of rare earth oxides dissolved in a molten rare earth metal fluoride. There is this.

However, both electrolytic processes require the use of expensive electrodes that will eventually be consumed, anhydrous chlorides or fluorides should be used to prevent the formation of undesirable RE-oxy salts (eg NaOCl) and high temperatures (1000 ° C). Defects that require cell operation, excessive power consumption due to low electrical efficiency, and low yield (less than 40%). In addition, corrosive chlorine gas is released in the RE-chloride reduction process, and in the RE-fluoride reduction process, careful control of the temperature grade in the electrolytic salt cell is required to cause solidification with the rare earth metal end.

Wetallothermic reduction processes include (1) reduction of RE-fluoride using calcium metal powder (called calciothermic process) and (2) calcium hydride (CaH ₂ ) or calcium metal There is a reduction-diffusion process of RE-oxide using (Ca). However, since both are batch processes, they must be carried out under a non-oxygen atmosphere and the energy consumption is very high.

In the case of reduction-diffusion, the product is in powder form and must be hydrated for purification before use. Moreover, both are multi-step processes. On the other hand, one advantage of the non-electrolytic reduction process is that the yield of metals from oxides or fluorides is generally higher than 90%.

Processes involving RE-fluorides or chlorides require pretreatment of RE-oxides for the production of such halides, and this additional step also increases the final cost of the rare earth metals.

Therefore, there is a natural need for high purity rare earth metals having low unit cost in light rare earth metal-iron permanent magnets.

However, none of the existing methods of reducing rare earth metal compounds reduce terminals or increase the usefulness of magnet-grade metals.

Accordingly, it is an object of the invention to provide a new process that can produce rare earth metals at a more effective and lower cost.

Hereinafter, the process of the present invention will be described.

The reaction vessel (reactor) used in the process is equipped with an electric resistance heater or other heater for heating to the desired reaction temperature, and the vessel body is made of metal or refractory material which is inert to the reactants and harmless. A predetermined amount of RE-oxide is added to a reaction vessel filled with a salt mixture of about 70 wt% or more of calcium chloride and 5-30 wt% of sodium chloride.

Salt acts as a medium in the reduction reaction. Based on the amount of rare earth oxides (RE-oxides), stoichiometric excess calcium metal is added.

On the other hand, it may be advantageous to add an appropriate amount of another metal such as iron or zinc to form a co-alloy with the reduced rare earth metal so that the reaction is carried out at low temperature and the Re-metal product is obtained in the liquid state.

In order to proceed with the reaction, the vessel is heated to a temperature above the melting point of the reactive components (about 675 ℃). The molten reaction components are quickly stirred in the reactor to maintain mutual contact of each component during the reaction.

The reaction tank is replenished with an amount of CaCl ₂ sufficient to maintain the weight ratio of CaCl ₂ in the CaCl ₂ and NaCI mixture to 70% or more. If the reaction proceeds with the calcium chloride concentration of 70% or less, the yield is significantly reduced.

During the course of the reaction, various reactions take place in the reactor.

However, RE-oxides are believed to be reduced by the following empirical formula.

RE _n O _m + mCa → mCaO + mRE

Wherein m and n represent the number of moles of the components, and m and n are determined according to the oxidation state of the rare earth element. The reducing metal has a density of about 7 g / cc, and the salt bath has a density of about 1.9 g / cc. When stirring is stopped, the reducing metal forms a clear layer at the bottom of the vessel. This clear layer can be decanted and separated while in the molten state or separated from the salt layer after it has solidified.

The process of the present invention has several advantages over known processes.

The process is carried out at a relatively low temperature of about 700 ° C., in which rare earth metals can be recovered as eutectic alloys with zinc or iron, and relatively rare earth oxides (RE-oxides), calcium chloride (CaCl ₂ ) and sodium chloride ( NaCl ₁ ) and calcium metal (Ca) are used as reactants.

This process does not need to convert the RE-oxide into its chloride or fluoride in advance, nor does it require the use of expensive pure calcium metal powder or reducing agents such as calcium hydride (CaH ₂ ). Since the method itself is a non-electrolytic method and can be preferably performed at a temperature of about 700 ° C. and atmospheric pressure, energy consumption is low. The process can be carried out as a batch process or as a continuous process, and calcium chloride (CaCl ₂ ), sodium chloride (NcCl) by-product and calcium oxide (CaO) are easily processed. Moreover, rare earth metals, together with additional salts in the reactor, can be alloyed into RE-FE eutectic alloys and used in RE-FE magnets without expensive waste treatment.

As already mentioned, rare earth metals are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, erbium, dystrosium, holmium, erbium, torbium, ytterbium, tutetium and elements in the periodic table. Yttrium number 39.

Rare earth metal oxides are typically colored powders prepared in a metal separation process.

Here, light rare earth refers to lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd).

In this process, rare earth oxides (RE-oxides) may be used as sieves received from the separator, or calcined for removal of excess moisture and carbon dioxide.

In the following examples, the RE-oxide is oven dried for 2 hours at 1000 ° C. prior to use. CaCl ₂ , and NaCl, used in salt baths, were oven dried at 500 ° C. for 2 hours before use.

In the early stages, the most important thing is to prevent the introduction of moisture into the reactor to cause a dangerous reaction with Na or Ca.

When calcium is added to the NaCl-containing salt bath, some sodium metal is formed by the following reaction.

2NaCl + Ca → CaCl ₂ + 2Na

When Nd ₂ O ₃ is mixed with CaCl ₂ in the molten salted meat, oxychloride is formed by the following reaction.

Nd ₂ O ₃ + CaCl _2-> 2NdOCl + CaO The presence of such RE-oxychloride is known to lower the yield in conventional electrolytic processes. However, in the present invention, both RE-oxide and RE-oxychloride are easily reduced by calcium metal. In fact, the formation of such RE-oxychloride is not a problem because it floats on the molten layer of the reduced RE metal. RE-oxides, on the other hand, can be retained as contaminants in the molten layer of reduced RE metals because they have a density substantially similar to that of the reduced RE metals, which can render them unsuitable for magnet manufacture. However, as already mentioned, in this process, RE-oxide is reduced by calcium metal, so the resulting RE metal is essentially free of oxides.

Pure (non-alloy) neodymium (Nd) metals have a high melting point of about 1025 ° C., and other rare earth metals also have high melting points.

If desired, it is possible to obtain a pure rare earth metal in high yield by performing a reduction reaction at such a high temperature.

However, it is preferable to lower the reaction temperature by adding non-rare earth metals such as iron and zinc to the reaction tank to induce alloy formation with rare earth metals which are rarely dilute. For example, iron (Fe) together with neodymium forms a low melting eutectic alloy (Fe content -11.5wt%, melting point -640 ° C), and zinc (Zn) has a melting point of about 630 ° C. -11.9 wt%).

Thus, if a sufficient amount of iron is introduced into the Nd ₂ O ₃ reduction system, the reduced metal will form a liquid phase at about 640 ° C.

The Nd-Fe common alloy thus formed may be made of a magnet having an optimal Nd ₂ Fe ₁₄ B magnetic phase alloyed directly with additional iron and boron. (Reference: European Patent Application Nos. 0,108,474, 0,125,752, 0,133,758 and 110,144,122)

If a non-rare earth metal is added to lower the melting point of the reduced-recovered rare earth metal, but it is not desired to float the added rare earth metal together with the reduced rare earth metal, it has a boiling point much lower than the boiling point of the recovered rare earth metal. A metal can be added to the reactor and used.

That is, when the rare earth metal is neodymium, when the zinc (Zn) is used, the boiling point of neodymium (Nd) is 3150 ° C., whereas zinc (Zn) is 907 ° C., which can be easily separated by simple distillation.

The material to be used as the reactor or the reactor lining material should be a material which is not affected by the corrosiveness of the molten rare earth metal. Generally inert refractory materials such as alumina and boron nitride lined with yttria are preferred. It is also possible to use refractory linings composed of inert metals, such as tantalum, or harmless metals or harmless linings, such as iron. In particular, an iron metal lining body may be used to accommodate the reduced RE metal and then form a eutectic alloy therewith.

According to the present invention, a novel method of reducing rare earth metal oxides using calcium metal is provided. In the present invention, the molten calcium metal and RE-oxide are treated together to induce the following reaction.

RE _n O _m + mCa → nRE + mCaO

If no pressure is applied to the reactor, it is preferable to keep the reaction temperature below 910 ° C. to avoid excessive loss of Na formed by the reaction between Ca and NaCl. It is preferable to perform reaction under atmospheric pressure.

The most preferred operating temperature range is about 650-750 ° C.

Since the melting point of the Nd-Fe eutectic mixture and the Nd-Zn eutectic mixture is 700 ° C. or lower, the temperature range is suitable for reducing Nd ₂ O ₃ to Nd metal. Moreover, the solubility of Ca metal in the salt bath at about 700 ° C. is about 1.3 molecular percent, which is sufficient to rapidly reduce RE-oxides to RE metals.

If good separation of the reduced RE metal is desired, the reaction temperature should be higher than the melting point of the reduced RE metal (or the melting point of the alloy if the RE metal is alloyed or co-reduced with another metal). Relatively heavy RE metal or alloy melt is collected at the bottom of the reactor, which can be easily separated and solidified.

Table 1 shows the molecular weight (mw) and density (μ) of the compounds and elements used in the present invention (there is no special indication means the density at 25 ℃ (g / cc), * indicates that at 1000 ° K) Mean density.) Melting point (MP) and boiling point (BP) are indicated.

TABLE 1

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings and embodiments.

1 shows, in cross section, a device suitable for carrying out the process of the invention, used in the following examples.

All experiments were performed in furnace wells 20 with an inner diameter of 12.7 cm, a depth of 54.6 cm and a box flange 4 fastened to the bolt 6. During the experiment, the air bubbles in the box were kept in a helium air field containing 1 ppm or less of oxygen, nitrogen and water.

The furnace was heated using three tubular cell clamshell heaters 8, 10 and 12, each with an inner diameter of 13.3 cm and a total length of 45.7 cm.

The sides and bottom of furnace 20 were protected by a fireproof insulator 14.

Thermocouples 15 were installed on the outer wall 16 of furnace 20 at various locations along its length. One of the central thermocouples was connected to a thermostat (not shown) to enable automatic regulation of the clamshell heater 10 in the center. The other three thermocouples were connected to a digital temperature reading meter, and the top and bottom heaters 8 and 12 were connected to manual thermostats to maintain a nearly uniform temperature throughout the furnace.

The reduction reaction was carried out in a reaction tank 22 in a stainless steel crucible 18 (outer diameter -10.2 cm, depth -12.7 cm, thickness -0.15 cm) contained in a stainless steel furnace 20. Reactor 22 is made of tantalum metal, unless specifically mentioned in each example.

Tantalum agitator 24 was used for stirring the melt during the reaction. This stirrer has a shaft length of 48.32 cm and is attached with a stirring blade 26 at the end. The stirrer 24 is operated by a shifting mode 28 (100W) having a maximum rotational speed of 700 revolutions for 1 minute. The moder 28 is installed on the bracket 30 so that the depth of the stirring blade in the reactor 22 can be adjusted. The drive side is journaled in a bushing 32 supported by an annular support bracket 34. The bracket 34 is held by a collar 35 which is secured to the row 22 by bolts 37. The cooling water coil 36 was installed at the resident position of the furnace 20 to promote the condensation to prevent the volatile reaction components from escaping out of the reaction system. Stainless steel baffles 38 were installed in the furnace 20 and used to reflux Na vapor. The reflux product falls into the reactor through tube 40 installed in the bottom baffle.

When the reaction is complete and stirring is stopped, the components in the furnace are separated as bottom RE metal melt layer 43, middle RE-oxychloride and calcium / sodium chloride salt bath layer 44, and top unreacted calcium layer 45.

FIG. 2 is an ideal process overview for reducing Nd ₂ O ₃ to yield Nd metal in accordance with the present invention. This is described as follows.

Neodymium oxide (Nd ₂ O ₃ ) is added to the reactor in an appropriate ratio depending on the amount of calcium chloride and sodium chloride. Then, an amount of eutectic alloy forming metal (iron or zinc, etc.) sufficient to form calcium metal and Nd-common alloy is added.

The reaction mixture is rapidly stirred at a speed of about 300 to 700 RPM at about 700 ° C. for at least one hour. Preferably, the air space in the reactor is maintained by air of an inert gas such as helium. After the reaction was terminated and Nd ₂ O ₃ was reduced, stirring was continued at a rate of about 100 RPM for 1 hour, and then the reactor was left to stand to allow the liquid phase mixture to stratify.

The reduced Nd-eutectic alloy has the highest density and is collected at the bottom of the vessel. Residual salts and unreacted calcium are collected on the top of the co-alloy layer, which is easily separated and removed when the reactor is cooled and solidified.

The resulting Nd- eutectic metal alloy can be alloyed with other component elements to produce a permanent magnet composition. Such magnetic alloys may be processed by melt-spinning or may be finely ground to be powder metallurgically processed to produce magnets.

Example 1

265 g of neodymium (Nd) metal mass of 99% purity and 50 g of zinc (Zn) metal mass of 99.9% purity were placed in a reaction bath to prepare 315 g of an approximate eutectic alloy. At this time, the reactor was heated at 800 ° C. in the furnace to alloy Nd and Zn.

Furnace temperature (furnace tempernature) was lowered to 720 ℃ and 150g of sodium chloride (NaCl) and 350g of calcium chloride (CaCl ₂ ) was added to the reactor to form a salt bath containing 70wt% CaCl ₂ .

234 g (0.7 mole) of neodymium oxide (Nd ₂ O ₃ ) and 104 g (2.6 mole) of calcium metal were added, and the mixture was stirred at a rate other than 300 RPM for 2 hours, and then the stirring rate was lowered to 60 RPM and further stirred for 1 hour. The reactor was removed from the furnace and cooled.

The Nd-Zn eutectic alloy collected at the bottom of the reactor was distilled to recover 189 g of neodymium (Nd) metal having a purity of 99% or more (excluding the first added 265 g of Nd metal). The yield of Nd metal from the oxide is about 94%.

Example 2

350g of neodymium (Nd) metal mass of 99% purity and 64g of high purity electrolytic iron (Fe) were added to a steel reactor, and the reactor was put into a furnace and heated to 800 ° C to form 414g of a near-fusion alloy.

The furnace temperature was lowered to about 720 ° C. and 300 g of sodium chloride (NaCl) and 700 g of calcium chloride (CaCl ₂ ) were added to the reactor to form a salt bath having a calcium chloride content of 70 wt%. 117 g (0.35 mole) of neodymium oxide (Nd ₂ O ₃ ) and 46 g (1.15 mole) of calcium (Ca) metal and 10.8 g (0.47 mole) of sodium (Na) were added and the mixture was stirred for 135 minutes at a speed of 300 RPM. Then quickly 117 g of Nd ₂ O ₃ and 46 g of Ca and 10.8 g of Na were additionally added, and then the mixture was stirred at 300 RPM for 114 minutes, followed by further stirring at 60 RPM for 1 hour.

The reactor was removed from the furnace and cooled. An unreacted Na-Ca layer is formed on top of the salt layer.

594 g of an Nd-Fe alloy having a purity of 97% or higher was recovered.

This alloy may be alloyed with the additional additive iron and boron in the recovered state to form an ideal Nd-Fe-B alloy suitable for permanent magnet production.

As a result, 180 g of Nd metal having a purity of 99% or more was recovered.

It can be seen from this example that the calcium and sodium melts reduce the rare earth oxides in the solvent bath (salt bath).

Example 3-5

In Example 1 described above, the amount of sodium chloride (NaCl), calcium chloride (CaCl ₂ ) and calcium (Ca) added was changed and stirring was first performed at 300 RPM for 4 hours and then at 60 RPM for 1 hour. Reaction was performed similarly to a process.

The results are shown in Table 2 below.

TABLE 2

* Example 3 used 117 g of Nd ₂ O ₃ .

As can be seen from Table 2, when the composition of the salt bath is 65.5wt% CaCl ₂ and 34.5wt% NaCl, the yield is as low as 65.2%.

However, when the concentration of CaCl ₂ in the composition of the salt bath is higher than 70wt%, the Nd yield is more than 85%, usually 95% or more.

3 is a graph showing the yield of neodymium (Nd) metal reduced from neodymium oxide (Nd ₂ O ₃ ) as a function of CaCl ₂ concentration in NaCl-CaCl ₂ salt bath. As can be seen from the graph, in order to yield high yielding reduced Nd metal, CaCl ₂ concentration in NaCl-CaCl ₂ salt bath should be maintained above 70wt%.

On the other hand, in order to provide a suitable melt for the dispersion of RE-oxides, it is preferable that the volume ratio of the salt bath and the RE-oxide is at least 2: 1.

As the volume ratio of the salt bath and the RE-oxide increases, the stirring speed for obtaining a similar yield for the same time decreases.

The Nd metal reduced in Example 1 may be recovered by separating and removing Zn by vacuum distillation. Analysis of the resulting metals confirmed that it was a high purity Nd metal with a purity of 99% or more, containing only 0.4% aluminum, 0.1% silicon, 0.01% calcium and very small amounts of zinc, magnesium and iron contaminants.

The calculated Nd metal may be melted together with electrolytic iron and ferroboron in a vacuum furnace to prepare an alloy for permanent magnets having a composition of Nd _0.15 B _0.05 Fe _0.80 . The alloy can then be made of a microcrystalline ribbon having a coercive force of about 10 megaussulelet according to the method described in EP 0108474.

Although the examples have described the process of the present invention with respect to Nd ₂ O ₃ , the process can be equally applicable to other single rare earth oxides and mixture reductions of rare earth oxides. This can be easily seen from the fact that CaO is a more stable compound than the oxide of any rare earth element.

Although state-of-the-art free energy measurements of RE-oxides and CaO are already known in the art, it has not been found that RE-oxides can be reduced by Ca metal as an electroless, liquid phase process.

In some cases, oxides of transition metals such as Fe or Co may be co-reduced together with the RE-oxides by the process of the present invention.

Claims (12)

In a non-electrolytic reduction step of reducing rare earth oxides to rare earth metals, a molten salt bath whose main component is calcium chloride (CaCl ₂ ) is synthesized; Dispersing less volume of rare earth oxides in the salt bath than in the salt bath; Adding a stoichiometric excess of calcium metal to the salt bath and stirring the salt bath while maintaining a molten state; Electroless reduction process of rare earth oxides, characterized in that the oxides are reduced to metals according to the following scheme RE _n O _m + mCa → nRE + mCaO In the formula, RE means rare earth elements having valence of 2, 3 or 4, O means oxygen, Ca means calcium, CaO means calcium oxide, n and m are oxidation of rare earth elements. As an integer determined according to the state, the number of times the valence of the rare earth element multiplied by n is determined to be equal to the number of times the oxygen atom (2) times m. The process of claim 1, wherein the rare earth oxide is selected from the group consisting of lanthanum oxide, cerium oxide, praseodymium oxide, and coral neodymium. The process of claim 1 or 2, wherein the rare earth oxide is neodymium oxide and is reduced to neodymium metal according to the following formula. Nd ₂ O ₃ + 3Ca → 2Nd + 3CaO Forming a molten salt bath comprising at least 70% by weight of calcium chloride (CaCl ₂ ); Adding and dispersing a rare earth oxide having a smaller volume than the salt bath to the salt bath; Adding a stoichiometric excess of calcium metal to the salt bath; Stirring is continued while calcium metal reduces the rare earth metal oxide to the rare earth metal while maintaining the salt bath in a molten state; Stopping the stirring to form a separation layer containing rare earth metal in the salt bath; Non-electrolytic reduction process of rare earth oxide, characterized in that. The process of claim 4, wherein the rare earth oxide is selected from the group consisting of lanthanum oxide, cerium oxide, praseodymium oxide, and neodymium oxide. Forming a molten salt bath comprising at least 70% by weight of calcium chloride and the remaining weight of sodium chloride; Adding neodymium oxide (Nd ₂ O ₃ ) in a volume of 50% or less of the volume of the salt bath to the salt bath; Adding excess calcium metal stoichiometrically to said neodymium oxide; Stirring was continued while the salt bath was kept molten until an intrinsic amount of neodymium oxide was reduced to neodymium metal; Stirring was stopped while the components were kept molten to form a separation layer in the salt bath comprising a reduced neodymium metal, essentially free of neodymium oxide; Non-electrolytic reduction process of rare earth oxide, characterized in that. Forming a molten salt bath consisting of at least 70 wt% calcium chloride and up to 30 wt% sodium chloride; Less rare earth oxides are added to the salt bath than the salt bath; Adding a stoichiometric excess of calcium metal to the salt bath; Stirring the calcium metal to reduce the rare earth oxide to the rare earth metal while maintaining the salt bath in a molten state; Adding an amount of the non-rare earth metal to the salt bath in an amount sufficient to form a rare earth metal / non-rare earth metal alloy having a melting point substantially lower than that of the rare earth metal; Stopping the agitation so that the rare earth metal / non-rare earth metal alloy is collected into the separation layer in the salt bath; Non-electrolytic reduction process of rare earth oxides characterized in that. The process of claim 7, wherein the rare earth oxide is selected from the group consisting of lanthanum oxide, cerium oxide, praseodymium oxide, and neodymium oxide. 8. The process of claim 7, wherein the rare earth oxide is neodymium oxide. The electroless reduction process of rare earth oxides according to any of claims 7 to 9, wherein the non-rare earth metal is iron (Fe). 10. The process of any of claims 7 to 9, wherein the non-rare earth metal is zinc (Zn). Forming a salt bath consisting of at least 70% by weight calcium chloride and the remainder of sodium chloride; Adding a rare earth oxide having a volume of 50% or less of the volume of the salt bath to the salt bath; Adding a sufficient amount of calcium metal to the salt bath to reduce the rare earth oxide; Stirring the calcium metal until the rare earth metal oxide is reduced to the rare earth metal while maintaining the salt bath in a molten state; Non-electrolytic reduction process of rare earth oxides, characterized in that the stirring is stopped while maintaining the components in a molten state to form a separation layer containing the reduced rare earth metal in the salt bath.

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