JP5766492B2 - Molten salt electrolysis method - Google Patents

Description

  The present invention relates to a technique for efficiently producing a reducing agent metal zinc for obtaining metallic silicon.

  Metallic silicon is particularly attracting attention as a raw material for solar cells from the viewpoint of utilizing solar energy. As a process for producing high-purity silicon for use in silicon cells for solar cells, a Siemens method for obtaining silicon by reducing trichlorosilane is known. According to the Siemens method, high-purity silicon of 9N (99.9999999%) or more can be produced.

  However, in the above method, enormous energy is required for producing metal silicon by carbon reduction of silica, and trichlorosilane is highly reactive, so handling with great care is required. Furthermore, it is necessary to energize and heat the melted portion of the metal silicon to keep it at a high temperature, so that a large amount of electric energy is consumed, and thus there are problems such as requiring energy saving measures. Therefore, in recent years, a zinc reduction method for producing high-purity polycrystalline silicon by reducing silicon tetrachloride with metallic zinc has attracted attention as a polycrystalline silicon production method that can be produced at a lower cost than the Siemens method.

  For example, in Patent Document 1, a silicon core or a tantalum core that can be energized is installed in the reactor when vaporizing high-purity silicon tetrachloride and high-purity zinc and performing a reaction in a gas atmosphere at 900 to 1100 ° C. A method is disclosed in which silicon deposition is promoted on the wick, the reactor is opened after completion of the reaction, and the generated needle-like and flaky silicon are taken out.

  Further, in Patent Document 2, a vertical reactor having a silicon chloride gas supply nozzle, a reducing agent gas supply nozzle, and an exhaust gas extraction pipe installed at the top is used, and silicon chloride is contained in the reactor. A polycrystalline silicon manufacturing apparatus that supplies gas and reducing agent gas, generates polycrystalline silicon at the tip of the silicon chloride gas supply nozzle by the reaction of silicon chloride gas and reducing agent gas, and further grows downward as it is. It is disclosed.

  Since the zinc reduction method has a high reaction rate of about 70% and a reaction temperature lower than that of the conventional Siemens method, there is an advantage that high-purity silicon can be produced at a lower cost than the Siemens method.

  In the zinc reduction method, silicon tetrachloride is reduced with zinc to obtain high-purity silicon, and at the same time, zinc chloride is by-produced. By-product zinc chloride can be electrolyzed to obtain zinc and chlorine. The obtained zinc can be recycled to be used as a raw material for the reduction reaction, while chlorine can be used as a raw material for producing silicon tetrachloride by chlorinating metallic silicon and the like, so that the entire system is closed. be able to.

  In the closed cycle, by-product zinc chloride can be recovered in a molten state and electrolyzed as it is, so that energy loss can be minimized. Therefore, the molten salt electrolysis method is applied to the electrolysis of zinc chloride. desirable. In the molten salt, zinc generated by electrolysis has a higher density than the electrolytic solution, and therefore settles and deposits in the form of droplets from the electrode to the bottom of the molten salt electrolytic cell. Therefore, efficiently recovering molten zinc from the bottom of the molten salt electrolysis tank is a key technology for zinc chloride electrolysis.

Conventionally, the following techniques are known as a method for recovering zinc by molten salt electrolysis.
FIG. 2 and FIG. 11 of Non-Patent Document 1 show a structure in which a Pyrex (registered trademark) glass siphon tube is inserted into the electrolytic solution from the upper part of the electrolytic chamber, and the zinc settling in the electrolytic chamber bottom is extracted. Further, Patent Document 3 shows a structure in which a molten zinc take-out port is provided at the lower part of the side surface of the electrolytic cell to extract the molten zinc, and Patent Document 4 shows a structure in which a molten zinc take-out port is attached to the bottom of the electrolytic cell.

  The Pyrex (registered trademark) glass siphon tube insertion method of Non-Patent Document 1 has a problem that the structure is complicated and the siphon tube is difficult to maintain and has a short life, such as clogging of the siphon tube due to solidification and fixation of zinc. . In addition, since it is a vacuum suction extraction, there is a limit to the suction height (about 159 cm), and there is a limit in the depth direction of the electrolytic cell. Therefore, this technique can be applied only to a shallow electrolytic cell with low productivity. .

  In the structure in which the zinc outlet is provided in the lower part of the side surface of the electrolytic cell in Patent Document 3 and the bottom of the electrolytic cell in Patent Document 4, it is necessary to attach a stopper for liquid stop at the front end of the outlet, and repeated extraction and liquid stop of molten zinc are repeated. There is a possibility that molten zinc will overflow due to the tightness of the stopper due to zinc sticking in between. There is also a problem that the extraction of molten zinc by the use of a plug must be intermittent and it is difficult to continue the operation. In addition, the use of a metal material for the material of the plug is restricted because of the risk of alloying due to reaction with zinc. Furthermore, it is difficult to grasp the situation such as the level of molten zinc sinking to the bottom of the electrolytic cell, and there is a problem that it is difficult to measure the timing of extracting molten zinc.

JP 2004-18370 A JP 2007-223822 A Japanese Patent Laid-Open No. 2004-256907 JP 2003-328173 A

“Electrowinning zinc from zinc chloride in monopolar and bipolar fused salt cells”, Report of Investigations. United States Department of the Interior. Bureau of Mines, 8524 (1981)

  The present invention has been made in order to solve the problems of the prior art in extracting and recovering the generated molten zinc in molten salt electrolysis of zinc chloride. The object of the present invention is simple structure, molten salt electrolysis An object of the present invention is to provide a molten salt electrolysis method and a molten salt electrolysis tank capable of safely extracting and recovering molten zinc while keeping the liquid level of molten zinc deposited on the bottom of the tank constant.

  The molten salt electrolysis method of the present invention contains a molten salt containing zinc chloride, electrolyzes the molten salt to generate molten metal zinc and chlorine, contains the generated molten metal zinc, and contains the molten metal zinc. Using a molten salt electrolyzer having an extraction chamber having an opening to be recovered and a communicating portion filled with molten metal zinc with the electrolytic chamber and the extraction chamber communicating with each other at the bottom, the molten salt containing zinc chloride is electrolyzed and metal A molten salt electrolysis method for producing zinc, wherein the height of the opening is higher than the liquid level at the interface between the molten salt containing zinc chloride and the molten metal zinc in the electrolytic chamber, and the zinc chloride contained in the electrolytic chamber is The molten salt containing zinc chloride is set to be lower than the liquid level of the molten salt containing, and the molten salt containing zinc chloride is added to the electrolytic chamber so that the height of the molten salt containing zinc chloride contained in the electrolytic chamber is constant. It is characterized by electrolyzing zinc chloride.

In the present invention, the height h _2Zn of the opening is
_{h 2Zn = (ρ Zn × h} 1Zn + ρ s × h s) / ρ Zn
h _s + h _1Zn : More than the distance from the bottom of the electrolysis chamber to the lower end of the electrode, less than the height of the molten salt electrolyzer
h _1Zn : More than the height of the communication part, less than the distance from the bottom of the electrolysis chamber to the bottom of the electrode
(H _s : height of molten salt containing zinc chloride in electrolytic chamber, h _1Zn : height of molten metal zinc in electrolytic chamber, ρ _Zn : density of zinc, ρ _s : density of molten salt containing zinc chloride ) Can be set.

  According to the structure of the molten salt electrolysis method and the molten salt electrolysis tank of the present invention, the molten metal zinc level can be easily managed, and the effect that the molten metal zinc can be safely extracted and recovered is achieved. . Due to the complexity of the structure of the molten salt electrolytic cell and the risk of overflow when the molten metal zinc is extracted, the risk of an electrode short circuit due to the precipitated molten metal zinc, and the zinc head by the vacuum suction method This eliminates the depth limitation of the molten salt electrolyzer, and enables the provision of a molten salt electrolyzer that can easily extract molten metal zinc with a simple structure and can easily control the molten metal zinc level.

FIG. 1 is a schematic view of a molten salt electrolytic cell showing one embodiment of the present invention. FIG. 2 is a schematic view of a molten salt electrolytic cell showing another embodiment of the present invention.

The present invention will be described below with reference to the drawings.
FIG. 1 is a schematic view of a molten salt electrolytic cell showing one embodiment of the present invention.
The molten salt electrolysis tank 1 is divided into two parts: an electrolytic chamber 2 for producing molten metal zinc 6 and chlorine by electrolyzing a molten salt 5 containing zinc chloride, and an extraction chamber 3 for accommodating the generated molten metal zinc 6. ing. The electrolysis chamber 2 and the extraction chamber 3 communicate with each other via the communication portion 4 at the bottom, and the lower portion of the electrolysis chamber 2 and the extraction chamber 3 are filled with molten metal zinc 6 having a higher density than the molten salt 5 containing zinc chloride. Yes. Therefore, the extraction chamber 3 is liquid-sealed so that the molten salt 5 containing zinc chloride does not enter.

The electrolysis chamber 2 is provided with a single or a plurality of sets of monopolar electrodes (monopolar) in which the anode 11 and the cathode 12 are arranged one-on-one. Moreover, it is good also as a structure which arrange | positions the bipolar electrode (bipolar) which has arrange | positioned the several intermediate electrode 13 (bipolar) between the anode and the cathode single or multiple sets. In order to increase the electrolytic reaction rate, it is preferable to use a bipolar electrode. The material and shape of the electrode are not particularly limited. The material of the anode 11 is preferably a carbon material, and graphite is particularly preferable because of its relatively low electric resistance. The material of cathode 12 can be applied or carbon materials, conductive ceramics such as TiB _2, Mo, inert metals such as W and Nb. As the shape of the electrode, a flat plate shape, a flat plate shape with a groove, or a cylindrical shape can be used.

  In the upper part of the electrolysis chamber 2, there is provided an inlet 14 for molten salt containing zinc chloride as a raw material and a chlorine suction pipe 15 which is an outlet for the generated chlorine.

  The bottom of the electrolysis chamber 2 may be horizontal, but if the bottom of the electrolysis chamber 2 is inclined toward the communication portion 4, the flow of molten metal zinc staying at the bottom of the electrolysis chamber toward the communication portion 4 is further promoted. Is done.

  The extraction chamber 3 has an opening 7 through which the molten metal zinc 6 is exposed. The height of the opening 7 is higher than the liquid level at the interface between the molten salt 5 containing zinc chloride and the molten metal salt accommodated in the electrolytic chamber 2, and the liquid of the molten salt 5 containing zinc chloride accommodated in the electrolytic chamber 2. It is set to be lower than

Liquid molten salt 5 containing zinc chloride molten liquid level of the metal zinc 6 (h _1Zn), the height of the opening 7 of the extraction chamber 3 and (h _2Zn) in the electrolytic chamber 2 to deposit into the electrolysis chamber 2 For the position (h _s ), the relational expression (1) is established. ρ _Zn is the density of zinc, and ρ _s is the density of the molten salt containing zinc chloride.
_{ρ Zn × h 2Zn = ρ Zn} × h 1Zn + ρ s × h s ···· (1)
_{h 2Zn = (ρ Zn × h} 1Zn + ρ s × h s) / ρ Zn ····· (2)
_{h 1Zn = (ρ Zn × h} 2Zn -ρ s × h s) / ρ Zn ···· (3)

h _1Zn is arbitrarily set within the range of the height of the communication portion 4 and less than the distance from the bottom of the electrolytic chamber tank to the lower end of the electrode. In the present invention, the level (h _s + h _1Zn ) of the molten salt 5 in the electrolysis chamber is arbitrarily selected within the range from the bottom of the electrolysis chamber tank to the lower end of the electrode and less than the height of the molten salt electrolysis tank, It is controlled so as to be constant (within ± 5% with respect to the set level of molten salt 5 in the electrolytic chamber (h _s + h _1Zn )). h _2Zn can be determined mechanically from the above equation (2).

  The material of the electrolysis chamber 2, the extraction chamber 3, and the communication portion 4 may be any material that does not corrode or corrode the molten salt electrolyte containing zinc chloride and the generated chlorine and molten zinc, such as alumina, silica, nitrogen silicon, carbonized It is preferable to produce the refractory mainly composed of silicon or the like.

First, a molten salt containing an amount of metal zinc capable of filling the communicating portion 4 with molten metal zinc and an amount of zinc chloride that achieves a predetermined liquid level (h _s ) is charged into the molten salt electrolytic cell. The temperature of the molten salt electrolyzer is raised to dissolve the molten salt containing metallic zinc and zinc chloride, and then energization is started.

The molten salt 5 containing zinc chloride may be zinc chloride alone, or an alkali metal salt such as Li, Na, K, Ca, Ba (eg, LiCl, NaCl, KCl, CaCl ₂ , BaCl ₂ , LiF, etc.) as a supporting electrolyte. Zinc chloride containing NaF, KF, CaF ₂ , BaF ₂ ) alone or a plurality thereof may be used.

  As the electrolytic reaction proceeds, zinc is deposited near the cathode 12 and chlorine is produced near the anode. Since the molten metal zinc 6 has a higher density than the molten salt 5 containing zinc chloride, the molten metal zinc settles in the molten salt 5 and stays at the bottom of the electrolysis chamber 2. Chlorine rises in the molten salt 5 as bubbles, is collected above the surface of the molten salt 5, and is collected through the chlorine suction pipe 15.

  As the electrolytic reaction proceeds, the level of the molten salt 5 in the electrolysis chamber decreases, but zinc chloride or a molten salt containing zinc chloride is introduced from the inlet 14 so that the level of the molten salt 5 in the electrolysis chamber is kept constant. . The level of the molten salt 5 can be confirmed by visual observation, a camera, a level sensor, or the like. The molten metal zinc 6 moves to the extraction chamber 3 through the communication portion 4 as the electrolytic reaction proceeds, and eventually overflows from the opening 7 of the extraction chamber 3. The overflowing molten metal zinc is temporarily received in a storage tank or the like and appropriately transferred to the next process using a transfer means such as a pump or a transfer rod.

  In the case of the prior art in which the molten metal zinc 6 deposited on the bottom of the molten salt electrolysis tank 1 is appropriately extracted, the lower end of the electrode comes into contact with the molten metal zinc 6 gradually increasing in the bottom of the electrolytic chamber 2 and short-circuits. It is necessary to be careful not to. However, in the molten salt electrolyzer of the present invention, the bottom of the electrolysis chamber 2 and the bottom of the extraction chamber 3 communicate with each other via the communication portion 4, so that the molten salt 5 containing zinc chloride in the electrolysis chamber 2 and the bottom are retained. The total mass of the molten metal zinc and the mass of the molten metal zinc staying in the extraction chamber are balanced to maintain the liquid level of each other. Since the height of the molten metal zinc in the molten salt electrolytic cell 1 can be determined by setting the height of the opening 7 of the extraction chamber 3 and the level of the molten salt containing zinc chloride, the risk of short-circuiting must be reliably avoided. Can do.

Furthermore, the electrolytic chamber 2 has a horizontal average cross-sectional area S _a (average cross-sectional area in the molten metal zinc level fluctuation region) of the molten metal zinc 6 deposited on the bottom of the extraction chamber 3 from the electrolytic chamber 2 through the communication portion 4. The ratio (S _b / S _a ) of the horizontal average cross-sectional area S _b (the average cross-sectional area within the level fluctuation region of the molten salt containing zinc chloride) of the electrolyte solution (molten salt containing zinc chloride) of the molten metal zinc by reducing design than the density [rho _s ratio (ρ _{s /} ρ _Zn) of the electrolytic solution for the density [rho _Zn (molten salt containing zinc chloride), without replenishment of the molten salt containing zinc chloride or zinc chloride electrolyte Thus, it is possible to discharge molten metal zinc from the extraction chamber 3 only by generating zinc, and the zinc discharge rate can be further averaged.

  The obtained molten metal zinc 6 can be reused, for example, for zinc charged into a reduction reactor of a zinc reduction method. Chlorine generated during electrolysis can be used as a raw material for producing silicon tetrachloride by chlorinating metal silicon or the like.

FIG. 2 is a schematic view of a molten salt electrolyzer which is another embodiment of the present invention.
The extraction chamber 3 is not partitioned by the wall surface of the electrolysis chamber 2 as shown in FIG. 1, but the electrolysis chamber 2 and the extraction chamber 3 are separated from each other and communicated with each other at the bottom thereof. The cross-sectional area of the deposited zinc layer in the chamber 2 is made as small as necessary, and the cross-sectional area ratio between the electrolytic solution and molten metal zinc is set to ρ _s / ρ _Zn or less. According to the embodiment of FIG. 2, the generated zinc has a structure capable of making the discharge rate uniform and reducing the amount of zinc stored on the electrolysis chamber side from the molten salt electrolytic cell of the embodiment of FIG. 1. In the electrolysis chamber 2, a partition wall 23 provided with molten salt circulation ports 21 and 22 including two stages of zinc chloride in the upper and lower stages is disposed, and the molten salt can be circulated.

[Example 1]
Zinc chloride was electrolyzed using the bipolar molten salt electrolytic cell shown in FIG.
The height of the electrolysis chamber was 160 cm, the width was 100 cm, the depth was 100 cm, the height of the extraction chamber (h _2Zn ) was 70 cm, the width was 30 cm, the depth was 100 cm, and the height of the communicating portion was 10 cm.
As the electrode material, graphite was used for all of the cathode, the anode, and the bipolar electrode, and anhydrous zinc chloride molten salt was prepared as the electrolyte. The density (ρ _s ) of the molten salt is 2.4 g / cm ³ . The inside of the molten salt electrolyzer was heated and purged with argon gas to prevent moisture and oxygen from being mixed into the atmosphere. The molten metal zinc 3200kg (density ((rho) _Zn ) = 6.5g / cm < ³ >)) was injected | thrown-in there, and then the molten zinc chloride 3000kg was injected | thrown-in. At this time, the height (h _s ) of molten zinc chloride in the electrolytic chamber was 100 cm, the height of molten zinc (h _1Zn ) was 30 cm, and the height of molten zinc in the extraction chamber was 67 cm. The immersion depth of the electrode in the electrolytic solution was 55 cm. The distance from the bottom of the electrolysis chamber to the lower end of the electrode is 75 cm. After maintaining the melt temperature in the molten salt electrolytic cell at 550 ° C., electrolysis was performed at a current density of 0.3 A / cm ² .

As the electrolysis progresses, the liquid level of the electrolytic solution decreases. In order to keep the liquid level in the electrolysis chamber the same as when electrolysis was started, zinc chloride consumed by electrolysis was added every 60 minutes. The drop in liquid level in the electrolysis chamber before charging was about 2.5 cm (about 2% with respect to the level of molten salt 5 in the electrolysis chamber (h _s + h _1Zn )). After 60 minutes from the start of electrolysis, the molten metal zinc overflowed from the upper opening of the extraction chamber, and this molten metal zinc was recovered.

H _s and h _1Zn during electrolysis are the above equation (3) and the liquid level height of the electrolytic solution = h _1Zn + h _s = 130 cm,
_{h 1Zn = (ρ Zn × h} 2Zn -ρ s × h s) / ρ Zn ···· (3)
_h1Zn = ([rho] _Zn * _{h2Zn- [} rho] _s * (130- _h1Zn )) / [rho] _Zn
h1Zn = ([rho] _Zn * _{h2Zn- [} rho] _s * 130) / ([rho] _{Zn- [} rho] _s ) = 35 cm
h _s = 95cm
It changes in. Therefore, the electrode and the molten zinc at the bottom of the electrolysis chamber do not contact during electrolysis.

  Electrolysis was performed for 24 hours, and 123 kg of molten metal zinc was recovered in total. The current efficiency was 85%.

[Example 2]
Zinc chloride was electrolyzed using the bipolar molten salt electrolytic cell shown in FIG.
The height of the electrolytic chamber was 220 cm, the width was 100 cm, and the depth was 100 cm, and the height (h _2Zn ) of the extraction chamber was 100 cm, the width was 30 cm, and the depth was 30 cm. The height of the communication part was 10 cm.

As the electrode material, graphite was used for all of the cathode, the anode, and the bipolar electrode, and a mixed molten salt of anhydrous zinc chloride molten salt as an electrolytic solution and 20 mol% of sodium chloride as a supporting electrolyte was prepared. The density (ρ _s ) of the molten salt is 2.3 g / cm ³ . The inside of the molten salt electrolyzer was heated and purged with argon gas to prevent moisture and oxygen from being mixed into the atmosphere. 1400 kg of molten metal zinc (density (ρ _Zn ) = 6.5 g / cm ³ ) was added thereto, and then 4000 kg of mixed molten salt was added. At this time, the height of molten zinc chloride in the electrolytic chamber was 160 cm, the height of molten zinc was 40 cm, and the height of molten zinc in the extraction chamber was 97 cm. The immersion depth of the electrode in the electrolytic solution was 100 cm. The distance from the bottom of the electrolysis chamber to the lower end of the electrode is 100 cm. After maintaining the melt temperature in the molten salt electrolytic cell at 450 ° C., electrolysis was performed at a current density of 1.0 A / cm ² .

As the electrolysis progresses, the liquid level of the electrolytic solution decreases. In order to keep the liquid level in the electrolysis chamber the same as at the start of electrolysis, additional zinc chloride consumed by electrolysis was added every 20 minutes. The drop in liquid level in the electrolysis chamber before charging was about 2 cm (about 1% with respect to the level of molten salt 5 in the electrolysis chamber (h _s + h _1Zn )). After 20 minutes from the start of electrolysis, the molten zinc overflowed from the upper opening of the extraction chamber, and this molten zinc was recovered. H _s and h _1Zn during electrolysis are the above formula (3) and the liquid level height of the electrolytic solution = h _1Zn + h _s = 200 cm,
_h1Zn = ([rho] _Zn * _{h2Zn- [} rho] _s * 200) / ([rho] _{Zn- [} rho] _s ) = 45 cm
h _s = 155cm
It changes in. Therefore, the electrode and the molten zinc at the bottom of the electrolysis chamber do not contact during electrolysis.

  Electrolysis was carried out for 50 hours, and a total of 1646 kg of molten zinc was recovered. The current efficiency was 90%.

  The molten salt electrolysis method and molten salt electrolysis tank of the present invention can be used for recovery of zinc metal from zinc chloride, zinc reduction method which is attracting attention as a method for producing polycrystalline silicon, and the like.

DESCRIPTION OF SYMBOLS 1 ... Molten salt electrolytic cell 2 ... Electrolytic chamber 3 ... Extraction chamber 4 ... Communication part 5 ... Molten salt 6 containing zinc chloride ... Molten metal zinc 7 ... Opening part 11 ... Anode 12 ... Cathode 13 ... Intermediate electrode 14 ... Input port 15 ... Chlorine suction pipe 21 ... Distribution port 22 ... Distribution port 23 ... Partition

Claims (1)

An electrolytic chamber for containing a molten salt containing zinc chloride, electrolyzing the molten salt to generate molten metal zinc and chlorine, and an opening for receiving the generated molten metal zinc and collecting the molten metal zinc Using a molten salt electrolysis tank having an extraction chamber, and the electrolysis chamber and the extraction chamber communicating with each other at the bottom and filled with the molten metal zinc, the molten salt containing zinc chloride is electrolyzed to obtain the metal zinc A molten salt electrolysis method to be manufactured, comprising:
The height of the opening is higher than the liquid level at the interface between the molten salt containing zinc chloride and the molten metal zinc in the electrolytic chamber and lower than the liquid level of the molten salt contained in the electrolytic chamber. The zinc chloride is electrolyzed while adding the molten salt containing zinc chloride to the electrolytic chamber so that the liquid level of the molten salt containing zinc chloride contained in the electrolytic chamber is constant. A molten salt electrolysis method characterized by the above.

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