RU2220228C2 - Gear for electrolyte circulation in bath of electrolyzer with salt melt - Google Patents

Abstract

FIELD: electrolytic reduction of metal oxides in electrolyzer. SUBSTANCE: invention refers to electrolytic reduction of metal oxide in electrolyzer to metal and oxygen, specifically, winning of aluminum and oxygen from aluminum oxide in salt melt bath containing fluorides of metals. Invention is aimed at deviation of oxygen bubbles released near anode into lifting conduit, at mixing salt melt bath in lifting conduit and at improved solution of metal oxide in salt melt bath. Electrolyzer has chamber with bottom carrying salt melt bath containing molten salts of metal oxide soluble in molten salt, at least one cathode, inert anode, at least one side wall extending upwards from bottom. Inert anode has two end parts, is mounted in chamber for formation of lowered conduit close to first end part and lifting conduit near second end part distant in lateral direction from lowered conduit and is equipped with arch located above anode. Part of lower surface of arch is inclined upwards from first end part towards second end part. EFFECT: improved solution of metal oxide in salt melt bath. 11 cl, 4 dwg

Description

 The present invention relates to the electrolytic production of metal in an electrolyzer containing a cathode, an inert anode and a bath of molten salts containing metal oxide. Preferably, the electrolyzer is used to produce aluminum from a molten bath of salts containing metal fluorides and alumina. In particular, the invention relates to a device for circulating a bath of molten salts in an electrolytic cell.

 The cost of producing aluminum can be reduced by replacing the inert anodes with the carbon anodes currently used in most industrial electrolyzers. Inert anodes have stable dimensions, since they are not consumed in the process of aluminum production. The use of an inert anode of stable dimensions in conjunction with a wettable cathode allows one to obtain more efficient designs of the electrolyzer, to reduce the current density and the gap between the anode and cathode, which saves energy.

The disadvantage of inert anodes is that they can contain metal oxides having some solubility in molten baths of fluoride salts. To reduce corrosion of inert anodes, the electrolysers containing them must work at temperatures below the normal operating temperature range of the Hall cell (approximately 948-972 ^o C). However, operating at lower temperatures also leads to some problems, such as the difficulty of maintaining the electrolyte in a state of saturation with alumina, solidification of the electrolyte in the electrolyzer (precipitate formation), and flooding of aluminum. In addition, some types of inert anodes tend to form resistive layers at low operating temperatures.

 To reduce the degree of corrosion of inert anodes, it is necessary to maintain the concentration of alumina close to saturation, but without high speed in the bath near the anodes and without the formation of a precipitate in the cell. Some electrolyte circulation is required to dissolve the alumina, however, circulation can also accelerate the wear of the anode by circulating droplets of aluminum. The authors found that these problems can be solved by providing strong mixing in the supply area of alumina insulated from the electrodes in order to improve the dissolution of alumina without increasing corrosion of inert anodes.

 Known electrolyzer for producing metal by electrolytic reduction of metal oxide to metal and oxygen, containing a chamber with a bottom, at least one side wall extending upward from the bottom, for placement in the bath chamber of a molten salt containing molten salts and metal oxide soluble in molten salts, at least one cathode and at least one inert anode placed in the chamber (WO 93/10281, C 25 C 3/08, 05/27/1993).

 The basis of the present invention is the creation of an electrolyzer containing an inert anode and an inclined arch that deflects the oxygen bubbles formed on the anode in the direction of the lifting channel, in which the metal oxide is dissolved.

 These problems are solved in an electrolytic cell for producing metal by electrolytic reduction of metal oxide to metal and oxygen, containing a chamber with a bottom, at least one side wall extending upward from the bottom to place soluble salts containing molten salts and metal oxide in the bath chamber in molten salts, at least one cathode and at least one inert anode located in the chamber, due to the fact that the inert anode having two end parts is installed in the chamber with the possibility of I lowering channel near the first end part and the lifting channel near the second end part, remote in the transverse direction from the lower channel, and is equipped with a arch located above the anode, and part of the lower surface of the arch is inclined upward from the first end part to the second end part to reject oxygen bubbles released near the anode into the lifting channel, mixing the molten salt bath in the lifting channel and improving dissolution of metal oxide in the molten salt bath.

According to preferred embodiments of the cell, the latter contains several alternating cathodes and inert anodes; the molten salt bath placed in the chamber contains at least one metal fluoride selected from sodium fluoride, aluminum fluoride and cryolite, and alumina is used as the metal oxide; the cell is equipped with a partition that goes down from the arch next to the lower channel; a part of the lower surface of the arch between the first end part of the inert anode and its second end part forms at least one groove; the angle of inclination of part of the lower surface of the arch is about 2-50 ^o up from the horizontal; the angle of inclination of part of the lower surface of the arch is about 3-25 ^o up from the horizontal; the angle of inclination of part of the lower surface of the arch is about 10 ^o up from the horizontal; the electrolyzer comprises a cover located above the chamber, a metal support cylinder extending downward through the cover into the chamber, and at least one support wall attached to the metal support cylinder to support the arch; the electrolyzer contains at least one bolt resting on the arch and passing through a hole made in the inert anode; the bath of molten salts, placed in the chamber, has a temperature of about 700-940 ^o or a temperature of about 900-930 ^o .

 Other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description.

 The present invention relates to the production of metal by electrolytic reduction of metal oxide to metal and oxygen. A preferred embodiment of the invention relates to the production of aluminum by electrolytic reduction of alumina dissolved in a molten salt bath. An electric current is passed between the inert anode and the cathode through a salt bath to obtain aluminum at the cathode and oxygen at the anode. The inert anode preferably contains at least one metal oxide and copper, more preferably at least two different metal oxides and a mixture or alloy of copper and silver.

The proposed cell operates at a temperature in the range of about 700-940 ^° C, preferably about 900-940 ^° C, more preferably about 900-930 ^° C, and most preferably about 900-920 ^° C. An electric current is passed between the inert anode and cathode through the bath molten salt containing electrolyte and alumina. In a preferred embodiment of the electrolyzer, the electrolyte contains aluminum fluoride and sodium fluoride, and it may also contain calcium fluoride, magnesium fluoride and / or lithium fluoride. The weight ratio of sodium fluoride to aluminum fluoride is preferably about 0.7-1.1. At an operating temperature of 920 ^{° C.,} this ratio in the bath is preferably about 0.8-1.0, and more preferably about 0.96. A preferred molten salt bath suitable for use at 920 ^{° C.} contains about 45.9 wt.% NaF, 47.85 wt.% AlF ₃ , 6.0 wt.% CaF ₂ and 0.25 wt.% MgF ₂ .

 The most preferred embodiment of the electrolyzer contains several mainly vertical inert anodes, which alternate with predominantly vertical cathodes. Inert anodes preferably have an active surface area of about 0.5-1.3 cathode surface areas.

Lowering the temperature of the cell bath to the interval 900-920 ^o With reduces corrosion of the inert anode. Lower temperatures reduce the solubility of the ceramic components of the inert anode in the bath. In addition, lower temperatures reduce the solubility of aluminum and other types of metals produced at the cathode, such as sodium and lithium, which have a corrosive effect on both the metal phase of the anode and its ceramic components.

 The inert anodes used in this invention are made by reacting the reaction mixture with a gas atmosphere at elevated temperatures. The reaction mixture contains particles of copper and oxides of at least two different metals. Copper can be mixed or fused with silver. The oxides are preferably iron oxide and at least one oxide of another metal, which may be nickel, tin, zinc, yttrium or zirconium oxide. Nickel oxide is preferred. Preferred mixtures and alloys of copper and silver contain up to about 30 wt.% Silver. The silver content is preferably about 2-30 wt.%, More preferably about 4-20 wt.% And optimally about 5-10 wt.%, The rest is copper. The reaction mixture preferably contains about 50-90 parts by weight. metal oxides and about 10-50 parts by weight copper and silver.

 The alloy or mixture of copper and silver preferably contains particles having an inner part that contains more copper than silver, and an outer part that contains more silver than copper. More preferably, the inner part contains at least about 70 wt.% Copper and less than about 30 wt.% Silver, and the outer part contains at least about 50 wt. % silver and less than about 30 wt.% copper. Optimally, the inner part contains at least about 90 wt.% Copper and less than about 10 wt.% Silver, and the outer part contains less than about 10 wt.% Copper and at least about 50 wt.% Silver. This alloy or mixture may be in the form of silver coated copper particles. Such a silver coating can be applied, for example, by electrolytic or chemical deposition.

The reaction mixture is carried out at an elevated temperature in the range of about 750-1500 ^{° C.} , preferably about 1000-1400 ^{° C.} and more preferably about 1300-1400 ^° C. In a particularly preferred embodiment, the reaction temperature is about 1350 ^° C.

 The gas atmosphere contains about 5-3000 ppm oxygen, preferably about 5-700 ppm, and more preferably about 10-350 ppm. Smaller oxygen concentrations result in a product with a larger metal phase than required, and excess oxygen results in a product having too much phase containing metal oxides (ferrite phase). The rest of the gaseous atmosphere preferably contains a gas such as argon inert to the metal at the reaction temperature.

 In a preferred embodiment, per 100 parts by weight about 1-10 parts by weight of metal oxide and metal particles are added. organic polymer binder. Some suitable binders include polyvinyl alcohol, acrylic polymers, polyglycols, polyvinyl acetate, polyisobutylene, polycarbonates, polystyrene, polyacrylates, and mixtures thereof and copolymers thereof. Preferably, 100 parts by weight about 3-6 parts by weight of metal, copper and silver oxides are added. binder.

 The inert anodes according to the invention comprise parts of a ceramic phase and parts of an alloy phase or part of a metal phase. Parts of the ceramic phase may contain both ferrite, for example nickel ferrite or zinc ferrite, and metal oxide, for example nickel oxide or zinc oxide. Parts of the alloy phase are scattered among parts of the ceramic phase. At least some parts of the alloy phase include an inner part containing more copper than silver, and an outer part containing more silver than copper.

 A particularly preferred embodiment of the cell comprises a chamber, at least one cathode and at least one inert anode in the chamber, and a vault above the inert anode. The camera has a bottom and at least one side wall extending upward from the bottom. The chamber contains a bath of molten salts. A preferred salt bath contains at least one metal fluoride selected from sodium fluoride, aluminum fluoride and cryolite.

 The cell preferably contains several cathodes alternating with inert anodes. Each of the cathodes and anodes has a first end portion adjacent to the lowering channel and a second end portion next to the lifting channel, which is transverse from the lowering channel. The vault, inclined upward from the first end portion to the second end portion, extends over alternating cathodes and inert anodes. In a preferred embodiment, the cell has a partition that extends downward from the arch next to the lower channel.

The arch rises upward at an angle of about 2-50 ^o from the horizontal, preferably about 3-25 ^o . It is particularly preferred that the arch rises upward at an angle of about 10 ^° . The inclined vault and the septum deflect the released oxygen bubbles from the anodes to the lift channel. An upward flow of oxygen bubbles in the lift channel mixes the molten salt bath and improves dissolution of the metal oxide. The molten salt bath has a higher velocity in the lifting channel than next to inert anodes in order to reduce the corrosion of inert anodes with dissolved aluminum or other substances in the bath.

 The arch has a lower surface or part of a lower surface. Alternatively, a portion of the bottom surface may form at least one groove extending between the first and second end portions. The groove increases the ability to transfer oxygen bubbles into the lifting channel, thereby eliminating excessive accumulation of bubbles near inert anodes.

Figure 1 shows the electrolyzer according to the invention in cross section;
FIG. 2 is a partial view of one assembly of the electrolyzer of FIG. 1;
figure 3 is a cross section along line 3-3 in figure 2;
FIG. 4 is a partial cross-sectional view of a roof for an alternative embodiment of the cell according to the invention along line 4-4 of FIG. 3.

 Figure 1 shows the electrolyzer 10 according to the invention. The cell 10 contains a bottom 11 and side walls 12, 13 forming a chamber 15. The bottom 11 is made of carbon and electrically conductive material. Layer 17 of molten aluminum covers the bottom 11. The bath of molten salts partially fills the chamber 15 above the layer 17. Refractory material 20 is located around the side walls 12, 13 and under the bottom 11. An insulating cover 22 is located above the chamber 15. Gases exit the chamber 15 through the vent channel 23. The alumina feeder 24 passes through the cover 22.

 The cell 10 contains two electrolysis modules 25, 26, each of which has several alternating cathodes and inert anodes. Cathodes rest on the bottom 11.

 One of the electrolysis units 25 is shown in more detail in FIGS. 2 and 3. The assembly 25 contains four cathodes or cathode plates 28a, 28b, 28c, 28d of titanium boride mounted in the bottom 11 and extending upward into the molten salt bath 18. Three inert anodes 29a, 29b, 29c extend downward from the anode assembly plate 30 connected to the nickel alloy rod 32 inside the metal support cylinder 33. The support cylinder 33 is preferably made of a nickel alloy. Electric current is supplied to the inert anodes through the rod 32 and the assembly plate 30. It goes without saying that the industrial electrolyzer contains much more anodes and cathodes in each module than the experimental electrolyzer shown and described. The anodes and cathodes in an industrial electrolyzer are larger than those described and shown in this application.

 In the electrolyzer 10, aluminum is obtained by passing an electric current passing between the anodes and cathodes, as a result of which the aluminum oxide dissolved in the bath 18 is reduced to aluminum and oxygen. The aluminum obtained at the cathodes flows dropwise through the cathodes into the molten metal layer 17. Bubbles of oxygen formed on the anodes rise up into the space 37 in the chamber 15 above the bath 18. Then the oxygen is removed to the outside.

In known electrolyzers containing carbon anodes and operating at temperatures of about 948-972 ^° C, alumina readily dissolves in the molten bath of salts, so there is no need to accelerate dissolution by mechanically stirring the bath. However, in electrolytic cells with anodes made of cermet material, anodes tend to corrode at such temperatures. Corrosion of anodes made of cermet material can be controlled by cooling the bath to a temperature in the range of about 700-940 ^{° C.} , preferably about 900-940 ^° C. At such low temperatures, alumina dissolves more slowly, so there is a higher need for mixing the bath.

 As can be seen in figure 1, the above tasks are solved by creating a lifting channel 34, through which the oxygen bubbles formed on the anodes float up in the direction of arrows 35, 36. The rising bubbles mix the bath of molten salts in the channel 34, thereby improving dissolution of aluminum oxide supplied through feeder 24. The direction of circulation is established due to the presence of lowering channels 38, 39 between the side walls 12, 13 and electrolysis units 25, 26. A bath of molten salts containing dissolved aluminum oxide, opu reprobes down the channels 38, 39, eventually reaching the electrodes at nodes 25, 26.

The circulation of the molten salt bath 18 improves the presence of a vault 40 above the anodes 29a, 29b, 29c, as shown in FIGS. 2 and 3. The vault 40 has a first end portion 42 next to the lower channel 38 and a second end part 43 next to the lift channel 34. 40 has a bottom surface or a portion of a bottom surface 45 that rises at an angle upward from the first end portion 42 to the second end portion 43. In a particularly preferred embodiment of FIG. 3, the bottom surface 45 is at an angle of about 10 ^degrees to the horizontal.

 The vault 40 also includes a baffle 50 extending downward from the horizontal top surface 46 adjacent to the first end portion 42. The baffle 50 improves the circulation of the bath by preventing oxygen bubbles from rising upstream of the downcomer 38.

 The vault 40 is supported by vertical support walls 55, 56 attached to the horizontal support arm 58. The arm 58 is attached to the lower end of the support cylinder 33. The vault 40 holds the anodes 29a, 29b, 29c with bolts 60a, 60b, 60c passing through the holes 61 next to the upper surface 46 of the arch. When lifting the support cylinder 33 and the bracket 38, the support walls 55, 56 raise the arch 40 upward, so that the bolts 60a, 60b, 60c also raise the anodes 29a, 29b, 29c. Anodes 29a, 29b, 29c rise up to reduce the effective surface area between the anodes 29a, 29b, 29c and the cathodes 28a, 28b, 28c, 28d. Similarly, when lowering the anodes 29a, 29b, 29c, 29d, the interelectrode surface area increases. If the current in the cell is constant, then an increase in the effective interelectrode area will cause a decrease in voltage and temperature in the cell, and a decrease in the effective interelectrode area will cause an increase in voltage and temperature in the cell.

 The arch 40, the partition 50, the supporting walls 55, 56, the bracket 58 and the bolts 60a, 60b, 60c can be made of cermet materials of the anode or similar materials.

 In an alternative embodiment of the invention shown in FIG. 4, the arch 40 has a lower surface portion 45 forming two grooves 70, 71. The grooves 70, 71 extend between the partition 50 and the second end portion 43. The grooves 70, 71 increase the ability to transport oxygen bubbles from inert anodes to the lift channel, thereby eliminating excessive accumulation of such bubbles under vault 40.

 Although preferred embodiments of the invention are described in the application, other modifications are also possible without departing from the scope of the attached claims.

Claims (12)

1. Electrolyzer for producing metal by electrolytic reduction of metal oxide to metal and oxygen, containing a chamber with a bottom, at least one side wall extending upward from the bottom, for placement in the bath chamber of a molten salt containing molten salts and metal oxide soluble in molten salts, at least one cathode and at least one inert anode placed in the chamber, characterized in that the inert anode having two end parts is installed in the chamber with the possibility of the formation of the lowering chamber Ala near the first end part and the lifting channel near the second end part, remote in the transverse direction from the lower channel, and is equipped with a arch located above the anode, and part of the lower surface of the arch is inclined upward from the first end part to the second end part to deflect standing out nearby with an anode of oxygen bubbles in the lifting channel, mixing the bath of molten salts in the lifting channel and improving the dissolution of metal oxide in the bath of molten salts. 2. The electrolyzer according to claim 1, characterized in that it contains several alternating cathodes and inert anodes. 3. The electrolyzer according to claim 1, characterized in that the molten salt bath placed in the chamber contains at least one metal fluoride selected from sodium fluoride, aluminum fluoride and cryolite, and aluminum oxide is used as the metal oxide. 4. The electrolyzer according to claim 1, characterized in that it is equipped with a partition that goes down from the arch next to the lower channel. 5. The cell according to claim 1, characterized in that a part of the lower surface of the arch between the first end part of the inert anode and its second end part forms at least one groove. 6. The cell according to claim 1, characterized in that the angle of inclination of a portion of the lower surface of the arch is about 2-50 ° up from the horizontal. 7. The cell according to claim 1, characterized in that the angle of inclination of a portion of the lower surface of the arch is about 3-25 ° up from the horizontal. 8. The cell according to claim 1, characterized in that the angle of inclination of the lower surface of the arch is about 10 ° up from the horizontal. 9. The electrolyzer according to claim 1, characterized in that it comprises a cover located above the chamber, a metal support cylinder extending downward through the cover into the chamber, and at least one support wall connected to the metal support cylinder to support the arch. 10. The electrolyzer according to claim 9, characterized in that it contains at least one bolt resting on the arch and passing through a hole made in an inert anode. 11. The electrolyzer according to claim 1, characterized in that the bath of molten salts, placed in the chamber, has a temperature of about 700-940 ° C. 12. The electrolyzer according to claim 1, characterized in that the bath of molten salts, placed in the chamber, has a temperature of about 900-930 ° C.

Images