IL161678A - Method and apparatus for production of magnesium and chlorine - Google Patents

Description

Ttoi O WJO npari ¾ χ-ΐ no \y METHOD AND APPARATUS FOR PRODUCTION OF MAGNESIUM AND CHLORINE Russian National Aluminium and Magnesium Institute (VAMI) & Aluminium Alloies & Metallurgical Processes Limited C: 51438 METHOD AND APPARATUS FOR PRODUCTION OF MAGNESIUM AND CHLORINE Field of the Invention The invention relates to the magnesium and chlorine production by electrolysis from the melt of salts containing MgCl2.

Description of the Prior Art The method of production of magnesium and chlorine, utilizing the diaphragm free electrolytic cells, is known in the art. A typical electrolytic cell comprises a production chamber containing multiple alternated anodes and cathodes; a chamber for magnesium separation, which is separated from the production chamber by a partition or separating wall having upper and lower circulation channels. The electrolytic cell is formed with a closed bath circulation system between the production chamber and the chamber for magnesium separation.

The velocity and direction of the streams of electrolyte or molten salt bath flows in the closed circulation loop are determined by the amount of chlorine in the predetermined volume of bath, i.e. it is determined by filling of the bath with the bubbles of chlorine gas. It is known that the quantity of such bubbles depends substantially upon the following factors: electrical current density, operating height of the electrodes and the distance between the adjacent electrodes or inter-electrode gaps (see, for example: Journal of 'Non-Ferrous Metals; 1976, No. 2, p. 53; and Journal of Non-Ferrous Metals; 1975, No. 11, p. 43).

The operating height of electrodes and inter-electrodes distance of the diaphragm free electrolytic cells used in the industry have been adopted to be similar to those of the diaphragm cells. For example, a typical cathode height is between 850 and 1000 mm and an average inter-electrode gap is between 50 and 70mm. While the electrical current density at the cathodes is about 0.24 -0.3A/cm2, the gas filling of the bath with chlorine bubbles in inter-electrode gaps is between 3 and 5.

The limited application of this known method should be considered as an important drawback thereof.

This prior art method is typically used in the diaphragm free electrolytic cells with the working width of the electrodes up to 0.6 m operating at the amperage up to 100 - 120 kA. Carrying out the electrolytic process in such electrolytic cells accompanied by filling inter electrode gaps with bubbles of chlorine gas (with such gas filling substantially equal 3 - 5), while utilizing electrodes having working width exceeding 0.60(m), causes formation of undesirable descending currents of the molten bath within such inter-electrode gaps. It should be noted that the velocity of the molten bath currents is not high enough, so as to assure the removal of the produced magnesium metal from the production into the magnesium separation chamber. One of the major drawbacks of the prior art method is that a significant part of the produced magnesium metal remains in the production chamber, where it is repeatedly circulated. Such remaining magnesium intensively reacts in the production chamber with the developed chlorine gas, causing the noticeable reduction of the magnesium current efficiency.

Summary of the Invention One aspect of the invention provides a method of producing magnesium metal and chlorine gas from a molten salt bath containing MgCl2 in an electrolytic cell. The electrolytic cell comprises at least one production chamber containing a plurality of alternated anodes and cathodes, with respective inter-electrode gaps being formed between two adjacent electrodes and at least one chamber for magnesium separation. The separation chamber is separated from the production chamber by a partition wall which is formed with upper and lower circulation channels. A closed molten bath or electrolyte circulation system is provided between the production chamber and the separation chamber. In the preferred embodiment of the invention the method consists of the following steps: (a) filling the molten salt bath in the inter-electrode gaps with bubbles of chlorine gas, so as to prevent formation of descending bath currents therein; (b) formation of the molten bath and magnesium flow in an area above the cathodes oriented in the direction of the upper circulation channels; and (c) regulating the flow velocity of such bath flow in the area above the cathodes by changing the height of the upper circulation channels.

As to another aspect of the invention, the gas filling in the inter-electrode gaps is defined by the following formula: DCath¾ath wherein the electrolysis process takes place with the gas filling being between 6 and 25 (conventional units), where: G - characteristic specifying filling of the bath with chlorine gas bubbles in the inter-electrode gaps (conventional units); Dcath - cathode electrical current density (A/cm2); Heath - height of the cathode (cm); and Lav - average distance between the electrodes or inter-electrode gap, (cm).

As to a further aspect of the invention, the velocity of the flow in the area above the cathodes is selected by the following ratio: = from 1.0 to 10.0, where hchn - height of upper circulation channels, (cm); and Lav - average distance between the electrodes or inter-electrode gap, (cm).

As to still another aspect of the invention, the ascending bath currents in the inter-electrode gaps are formed having a variable cross-section, wherein such variable cross-section bath currents is increased in the direction of the melt movement.

Still another aspect of the invention provides an electrolytic cell comprising at least one production chamber, a plurality of alternated anodes and cathodes with a respective inter-electrode gap being formed between two adjacent anodes and cathodes, and a chamber for magnesium separation separated by a partition from the production chamber, wherein the partition is formed with upper and lower circulation channels. The electrolytic cell includes a closed molten bath circulation system operable between the production chamber and the chamber for magnesium separation, with the height of cathodes exceeding the inter-electrode gaps in between about 25 and 60 times.

A still further aspect of the invention provides an electrolytic cell, wherein a working outside surface of each cathode is inclined to the vertical at an angle between 0°38' and 1°26\ The working outside surface of each anode is inclined to the vertical at an angle of between 0°38' and 1°26\ Brief Description of the Drawings: Fig. 1 is a top plan view of the electrolytic cell according to the invention; Fig. 2 is a partial section view according to section line 2-2 of Fig. 1; and Fig. 3 is a partial section view according to section line 3-3 of Fig. 1.

Description of the Preferred Embodiment Referring now to FIGs. 1-3, wherein the preferred embodiment of the electrolytic cell of the invention utilized in the production of magnesium and chlorine is illustrated. A housing 1 of the electrolytic cell incorporates a refractory wall structure formed with a production or electrolysis chamber 3 which is separated from a magnesium collecting chamber 6 by a refractory curtain or partitioning wall 7. Although the electrolytic cell having one production and metal collecting chambers is illustrated by the drawings, electrolytic cells with a plurality of production and of metal collecting chambers separated by the respective curtain or partitioning walls are within the scope of the invention.

As best illustrated on at least FIG. 3, the partitioning wall 7 extends substantially upwardly within the refractory housing of the electrolytic cell from an area at the bottom floor 14 to a top part thereof. The walls and the floor of the electrolytic cell can be made as heavy refractory construction utilizing refractory blocks.

As illustrated in FIG. 3, the partitioning wall 7 is formed with upper circulation channels 8 and lower circulation channels 9 separated by a solid portion of the wall. The circulation channels 8 are provided at an upper region of the wall 7, whereas the lower circulation channels 9 are situated in the vicinity of the floor area 14.

The production or electrolysis chamber 3 is formed with a gas space 21 having a gas discharge outlet duct 11 situated at the upper portion thereof adapted for removal of the accumulated chlorine gas. The electrolysis or production chamber 3 is enclosed at the top by a refractory lined closure 10, so as to form a gas-tight seal therebetween.

A multiplicity of alternated anodes 4 and cathodes 5 are situated in the production chamber 3. A plurality of heavy, plate-like graphite anodes 4 can be mounted within the top closure 10, so as to project downward into the production chamber with their lower edges 15 situated near the bottom floor 14. Position of each anode 4 is such that longitudinally it substantially extends from the front to the rear of the production chamber 3. As best illustrated in FIG. 1, longitudinally the anodes 4 extend between the partition wall 7 and a rear wall 16 of the production chamber 3. A suitable electrical connecting means is also provided. The multiplicity of cathodes 5 are arranged at localities between successive anodes, so that the electrodes alternate in mutually parallel arrays along the production chamber 3. The cathodes that are disposed between pairs of anodes so as to be arranged in spaced pairs forming inter-electrodes gaps 12 therebetween. For the purposes of this application the average inter-electrode gap or average distance between the adjacent electrodes (Lav) is the average distance or gap calculated based on the distance (Lu) at the upper part of the cathode and the distance (LL) at the lower part of the cathode (see FIG. 2). As illustrated in FIG. 1, the cathodes 5 are carried by a suitable mounting structure 17 which extends through the wall 16 and has a suitable electrical connecting means. In each respective pair, the cathodes are disposed suitably close to the respective adjacent anodes. The cathodes 5 may be formed of steel plates.

In carrying out the method of the invention the electrolytic cell operates in the following manner. The molten salt bath containing 10-18% MgCl2, 35-40% NaCl and 45-50% KC1 is introduced into the cell and the direct electrical current (DC) is supplied to the electrodes. At the temperature of the bath between 660 and 670°C, the chlorine gas is generated at the anodes 4, whereas the magnesium metal is produced at the cathodes 5. Thus, in the invention metallic magnesium is produced by passing direct electric current between the anodes 4 and cathodes 5 suspended in facing spaced relation in the molten salt bath containing magnesium chloride. The electrolysis of magnesium chloride in the bath causes the magnesium metal to be released at working surfaces 18 of the cathodes 5, while chlorine gas is generated at working surfaces 20 of the anodes 4. The magnesium metal, being lighter than the bath, rises along the cathode working surfaces 18, while the chlorine gas rises through the bath in a plume of bubbles from each anode surface 20 to be collected in a gas space 21 of the production chamber 3 above the level of the bath.

The electrolytic cell includes the closed bath circulation system which is operable between the production chamber 3 and the magnesium separation chamber 6. Since the electrolysis is intensive within the inter-electrode gaps 12, the bath circulation is enhanced by filling of the bath situated within the inner electrodes gaps 12 with the bubbles of the chlorine gas. In the invention the gas filling is maintained in such a manner, so as to prevent formation of the descending bath currents. The flow of molten bath and magnesium is formed in the area above the cathodes 5, so that such flow is oriented toward the upper circulating channels 8.

The velocity of such flow passing above the cathodes 5 is regulated by changing the height of the upper circulating channels 8.

In light of the filling of the molten salt bath in general with the bubbles of chlorine gas and specifically in view of the substantial level of filling of the bath within the inter-electrode gaps 12 with the bubbles of chlorine gas, the ascending currents of the molten bath (electrolyte) are formed containing magnesium and chlorine. The chlorine gas is accumulated in the gas space 21 at the area above the level of bath in the production chamber 3. The produced chlorine gas is continuously evacuated from the gas space 21 by means branch pipe 11 into the main chlorine gas duct. The molten bath together with the produced magnesium are transferred from the area of the production chamber 3 above the cathodes 5 through the upper circulation channels 8 into the chamber 6 for magnesium separation. There, the magnesium metal is accumulated on the surface of the bath and periodically taken out by the vacuum ladle. In the separation chamber 6 the molten bath separated from the magnesium metal is directed downwardly and through the lower circulation channels 9 back to the production chamber 3, so as to penetrate into the inter-electrodes gaps 12.

One of the main objects of the method of the invention is to provide favorable characteristics related to the filling of the bath with the inter-electrode gaps 12 with the bubbles of chlorine gas. This assures formation of the ascending currents of the bath within the entire length of the inter-electrode gaps 12. These ascending currents are oriented in the upper part of the inter-electrode gaps 12 above the cathodes 5 in the direction of the magnesium separation chamber 6 and accelerate magnesium removal from the production chamber 3 into the separation chamber 6.

The gas filling in the inter-electrode gaps is defined by the following formula: DCathHcath G = wherein the electrolysis is carried out with the gas filling being within the range between 6 and 25 (conventional units).

G - characteristic specifying filling of the bath with chlorine gas bubbles in the inter-electrode gaps (conventional units); DCath - cathode electrical current density (A/cm ); Heath - height of the cathode; (cm) and Lav - average distance between the electrodes or inter-electrode gap, (cm).

The ascending currents of the molten bath in the inter-electrodes gaps are formed with variable cross-section, which is increased along the direction of movement of the currents; whereas the velocity of bath currents in the area above the cathodes is selected from the following condition: nchn = from 1.0 to 10.0, where hchn - height of upper circulation channels, (cm); and Lav - average distance between the electrodes or inter-electrode gap, (cm).

It has been observed that in order to maintain the desirable characteristics of the filling of the bath with the chlorine gas bubbles, while keeping constant the height of electrodes, it is necessary to considerably reduce the distance between the adjacent electrodes or the inter-electrode gaps 12. Compared to the prior art methods, it is necessary to reduce the gap distance from 1/2 to 1/3 of the prior art gaps and to increase the electrical current density by about 1.5 - 2.0 times, compared to the prior art methods. Such arrangement enables the invention to substantially increase the amperage in the electrolytic cell and its production rate.

The excessive filling of the bath with the chlorine gas bubbles is undesirable. In this respect, when characteristics of the gas filling of the bath are increased more than 25 times, in view of the high velocity of the developed thrust of the gas bubble-bath mixture, the generated small drops of magnesium are separated from the working surfaces 18 of the cathodes 5 and are easily chlorinated in the turbulent flow of bath and chlorine gas bubbles mixture. This leads to a substantial reduction of the electrical current efficiency and to a reduction of the production rate of the electrolytic cell. Furthermore, the chlorine gas losses attributable to the gas removal through the gas aspiration system are directly proportional to the excessive gas filling of the molten bath with chlorine gas bubbles. With the increased saturation of the melt with the chlorine gas bubbles, the velocity of the molten bath flow also increases. In this condition, the number of chlorine gas bubbles that are carried away from the inter-electrodes gaps 12 to the magnesium separation chamber 6 is also increased.

When the level of gas filling is reduced to less than 6 conventional units, the descending currents of the bath appear with the inter-electrodes gaps 12, causing noticeable reduction of the electrical current efficiency.

Increase in the cross-section of the molten bath ascending currents within the inter-electrode gaps 12 along the direction of the bath movement results in the reduction of the difference in the velocity of the currents distributed along the vertical extension of the inter-electrode gap. This contributes to the improved distribution and coverage of the working surfaces 18 of the cathodes 5 with the formed metallic magnesium. Specifically, such improvement relates to the ascent of magnesium on the working surfaces 18 of cathodes 5 and separation of the magnesium metal from the surface of the cathode at the upper part thereof. The latter factor contributes to the reduction of magnesium losses due to its undesirable chlorination and to the increase in the electrical current efficiency.

In the invention, the velocity of the bath and magnesium mixture flow over the working surfaces 18 of cathodes is controlled by varying the height of the upper circulation channels 8. The height of the upper circulation channel (η^) can be defined as a minimal distance between upper and lower regions thereof at a particular location. Upon variation of the height of the upper circulation channel, the cross-sectional area of such channel varies accordingly. This also causes respective variations in the velocity of the molten bath-magnesium flow between the production (3) and separation (6) chambers. The flow velocity is a function of the average distance between two adjacent electrodes or inter-electrode gap( Lav ). When the ratio of the height of the upper circulation channel (hchn) to the average distance between the electrodes or inter-electrode gap (Lav) is less than 1.0, the hydraulic resistance of the upper circulation channels 8 to the flow of the bath-magnesium mixture passing therethrough is increased. This causes deterioration in the conditions of magnesium transfer from the production chamber 3 to the magnesium separation chamber 6. The increase in height of the upper circulation channels (hchn) to the average inter-electrode distance (Lav) to more than 10.0 causes a substantial increase in the losses of chlorine gas with the exhaust gasses, which are evacuated from the electrolytic cell through the branch pipe 13.

Due to the above-discussed ratios of the height of the cathode (Hcath) to the average distance between the electrodes or inter-electrode gap (Lav), the favorable conditions of the gas filling are assured. This results in the formation of the flow of the molten bath and magnesium oriented toward the upper circulation channels in the space between the anodes and above the cathodes.

When the ratio of the cathode height (Hcath) to the inter-electrode gap (Lav) in the area above the cathodes is less than 25, the bifurcation of the bath flow takes place. Such division results in the deterioration of the removal of magnesium metal into magnesium separation chamber 6 and in the reduced electrical current efficiency of the electrolytic cell. When the above-discussed ratio exceeds 60, there is an increase in the bath-magnesium flow velocity within the upper circulation channel 8. This causes deterioration of the magnesium separation in the separation chamber 6. Specifically, this condition results in the separation of magnesium drops and their entrainment with bath flow into the production chamber 3, which ultimately causes the reduction in the electrical current efficiency.

When the inclination of the working surfaces of the cathodes or anodes (18 and 20 respectively) relative to the vertical exceeds 1°26', the total working surface of the electrodes is reduced. This causes the reduction of the cell amperage, as well as the reduction of its production rate.

When the inclination of the working surfaces of the cathodes or anodes (18 and 20) is less than 0°38', the flow of chlorine gas engages the upper part of the working surfaces of the cathodes. This results in the premature separation of the drops of magnesium metal from the working surfaces 18 of the cathodes and increases the losses of magnesium.

The invention enables the user to increase the working width of electrodes up to 2.0 m and greater, to decrease the distance between adjacent electrodes or the inter-electrodes gap/distance, and as a result, to increase the electrolytic cell capacity and to reduce the specific power consumption. In this respect, during the production of the magnesium metal and chlorine gas by the method of the invention, a specific power consumption is achieved with simultaneous increase of the electrolytic cell amperage and its production efficiency.

To achieve the objects of the invention in the electrolytic cell, the height of the cathode exceeds the inter-electrode gap by 25-60 times. The working surfaces of the cathodes are inclined to the vertical at the angle between 0°38' and 1°26'. The working surfaces of the anodes are inclined to the vertical at the angle between 0°38' and 1°26\ Such inclination causes an increase in the total working surface of the electrodes. As a result, the amperage of the cell and consequently its production rate are increased as well.

The method of the invention causes reduction in the specific electrical power consumption up to lOOOkW h/t Mg and also results in the increase of the electrolytic cell production rate up to 40 percent.

Claims (2)

1. The method of production of magnesium and chlorine by electrolysis of a melt of salts containing MgCl2, using an electrolysis cell with one or several electrolysis chambers having alternated anodes and cathodes, a magnesium separation cell separated from the electrolysis chambers by a partition formed with upper and lower circulation channels, said method comprising the steps of retaining filling of electrolyte with bubbles of chlorine in inter-electrode gaps providing closed electrolyte circulation between the electrolysis chambers and the magnesium separation cell to prevent formation of a descending stream of electrolyte within the inter-electrode gaps and formation of electrolyte and magnesium stream directed over the -cathodes and toward the upper circulation channels, characterized in that the velocity of electrolyte and magnesium stream over the cathodes is controlled by changing the height of upper circulation channels as a function of an average inter-electrode gap distance, the average inter-electrode gap distance being selected based on the following conditions: h channel = (between 1.0 and 10.0) 1 average where h channel is height of upper circulation channels (cm); 1 average is average inter-electrode gap distance (cm), the gas filling of the electrolyte with bubbles of chlorine in the inter-electrode gap is maintained between 6 and 25 of arbitrarily chosen units, the gas filling of the electrolyte is defined by the formula: d c . H c G = 1 average G is gas filling of the electrolyte with bubbles of chlorine in the inter-electrode gaps; d c is cathode current density (A/cm2); H c is cathode height (cm); wherein the electrolysis process is carried out by providing a variable cross-section area of the inter-electrode gaps in the direction of the ascending streams of electrolyte, the variable cross-section area is being increased in the direction the melt movement.

2. The electrolysis cell for production of magnesium and chlorine, comprising an electrolysis chamber having alternating anodes and cathodes, a magnesium separation cell separated from the electrolysis chamber by a paitition having upper and lower circulation channels, characterized on that the height of the cathode exceeds an inter-electrode gap between 25 and 60 times, and working surfaces of the cathode or anode are arranged with a slope to the vertical at an angle between 38' and 1°26'.