OA12717A

OA12717A - Method and apparatus for production of magnesium and chlorine.

Info

Publication number: OA12717A
Application number: OA1200400125A
Authority: OA
Inventors: Alexander S Chesnokov; Alexander A Larionov; Vladimir I Schyogolev; Bagdat M Shayahmetov; Alexander N Tatakin; Igor V Zabyelin
Original assignee: Aluminium Alloies & Metallurg; Russion Nat Aluminium And Magn
Priority date: 2003-05-05
Filing date: 2004-05-03
Publication date: 2006-06-27
Also published as: IL161678A0; RU2003113257A; IL161678A; RU2243295C1

Abstract

FIELD: electrolytic magnesium and chlorine manufacturing from MgCl2 containing fused salt. ^ SUBSTANCE: method includes maintaining of chlorine bubble-filled electrolyte in interelectrode gap to provide closed electrolyte circulation between electrolytic cell and cell for magnesium separation and to prevent electrolyte downstream in interelectrode gap; as well as providing of electrolyte and magnesium stream over cathodes, directed to top circulating channels. Velocity of electrolyte and magnesium stream over cathodes is controlled by changing of top circulating channel high in dependence of mean interelectrode distance. Gas-filling of electrolyte is maintained at 6-25 arbitrary units determined according to specific equation. Electrolysis is carried out providing increased interelectrode gap cross-section in direction of electrolyte upstream. Electrolytic cell contains electrolytic chamber with alternate anodes and cathodes and cell for magnesium separation, isolated from electrolytic chamber by divider with top and bottom circulation channels. Ratio of cathode high/interelectrode gap width is 25-60; and inclination of cathode ore anode working surface to vertical is 38'-1026'. ^ EFFECT: reduced energy consumption, increased current strength and capability of electrolytic cell. ^ 3 cl, 3 dwg, 1 ex

Description

012717 F0324

METHOP AND APPARATUS FOR

PRODUCTION OF MAGNESIUM AND CHLORINE

Field of the Invention

The invention relates to the magnésium and chlorine production by 5 electrolysis from the melt of salts containing MgCl2.

Description of the Prior Art

The method of production of magnésium and chlorine, utilizing thediaphragm free electrolytic cells, is known in the art. A typical electrolytic cellcomprises a production chamber containing multiple altemated anodes and 10 cathodes; a chamber for magnésium séparation, which is separated îrom theproduction chamber by a partition or separating wall having upper and lowercirculation channels. The electrolytic cell is formed with a closed bath circulationSystem between the production chamber and the chamber for magnésiumséparation.

012717

The velocity and direction of the streams of electrolyte or molten sait bathflows in the closed circulation loop are determined by the amount of chlorine inthe predetermined volume of bath, i.e. it is determined by filling of the bath withthe bubbles of chlorine gas. It is known that the quantity of such bubbles dépends 5 substantially upon the foliowing factors: electrical current density, operatingheight of the électrodes and the distance between the adjacent électrodes or inter-electrode gaps (see, for example: Journal ofNon-Ferrous Metals, 1976, No. 2, p.53; and Journal ofNon-Ferrous Metals-, 1975, No. 11, p. 43).

The operating height of électrodes and inter-electrodes distance of the 10 diaphragm ffee electrolytic cells used in the industiy hâve been adopted to besimilar to those of the diaphragm cells. For example, a typical cathode height isbetween 850 and 1000 mm and an average inter-electrode gap is between 50 and70mm. While the electrical current density at the cathodes is about 0.24 -0.3A/cm , the gas filling of the bath with chlorine bubbles in inter-electrode gaps 15 is between 3 and 5.

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

This prior art method is typically used in the diaphragm ffee electrolytic cells with the working width of the électrodes up to 0.6 m operating at the 012717 amperage up to 100 - 120 kA. Carrying out the electrolytic process in suchelectrolytic cells accompanied by filling inter electrode gaps with bubbles ofchlorine gas (with such gas filling substantially equal 3 - 5), while utilizingélectrodes having working width exceeding 0.60(m), causes formation of 5 undesirable descending currents of the molten bath within such inter-electrodegaps. It should be noted that the velocity of the molten bath currents is not highenough, so as to assure the removal of the produced magnésium métal from theproduction into the magnésium séparation chamber. One of the major drawbacksof the prior art method is that a significant part of the produced magnésium métal 10 remains in the production chamber, where it is repeatedly circulated. Suchremaining magnésium intensively reacts in the production chamber with thedeveloped chlorine gas, causing the noticeable réduction of the magnésium currentefficiency.

Brief Description of the Drawings: 15 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 ; andFig. 3 is a partial section view according to section line 3-3 of Fig. 1. 012717.

Description of the Preferred Embodiment

Referring now to FIGs. 1-3, wherein the preferred embodiment of theelectrolytic cell of the invention utilized in the production of magnésium andchlorine is illustrated. A housing 1 of the electrolytic cell incorporâtes a 5 reffactory wall structure formed with a production or electrolysis chamber 3 whichis separated from a magnésium collecting chamber 6 by a reffactory curtain orpartitioning wall 7. Although the electrolytic cell having one production andmétal collecting chambers is illustrated by the drawings, electrolytic cells with aplurality of production and of métal collecting chambers separated by the 10 respective curtain or partitioning walls are within the scope of the invention.

As best illustrated on at least FIG. 3, the partitioning wall 7 extendssubstantially upwardly within the reffactory housing of the electrolytic cell froman area at the bottom floor 14 to a top part thereof. The walls and the floor of theelectrolytic cell can be made as heavy reffactory construction utilizing refractory 15 blocks.

As illustrated in FIG. 3, the partitioning wall 7 is formed with uppercirculation channels 8 and lower circulation channels 9 separated by a solidportion of the wall. The circulation channels 8 are provided at an upper région ofthe wall 7, whereas the lower circulation channels 9 are situated in the vicinity of 20 the floor area 14. 012717

The production or electrolysis chamber 3 is formed with a gas space 21having a gas discharge outlet duct 11 situated at the upper portion thereof adaptedfor removal of the accumulated chlorine gas. The electrolysis or productionchamber 3 is enclosed at the top by a refractory lined closure 10, so as to form a 5 gas-tight seal therebetween. A multiplicity of altemated anodes 4 and cathodes 5 are situated in the productionchamber 3. A plurality of heavy, plate-like graphite anodes 4 can be mounted withinthe top closure 10, so as to project downward into the production chamber with theirlower edges 15 situated near the bottom floor 14. Position of each anode 4 is such 10 that longitudinally it substantially extends ffom the front to the rear of the productionchamber 3. As best illustrated in FIG. 1, longitudinally the anodes 4 extend betweenthe partition wall 7 and a rear wall 16 of the production chamber 3. A suitableelectrical connecting means is also provided. The multiplicity of cathodes 5 arearranged at localities between successive anodes, so that the électrodes altémate in 15 mutually parallel arrays along the production chamber 3. The cathodes that aredisposed 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 averageinter-electrode gap or average distance between the adjacent électrodes (Lav) is theaverage distance or gap calculated based on the distance (Lu) at the upper part of the 20 cathode and the distance (Ll) at the lower part of the cathode (see FIG. 2). As 012717 ülustrated in FIG. 1, the cathodes 5 are carried by a suitable mounting structure 17which extends through the wall 16 and has a suitable electrical connecting means. Ineach respective pair, the cathodes are disposed suitably close to the respectiveadjacent anodes. The cathodes 5 may be formed of Steel plates. 5 In carrying out the method of the invention the electrolytic cell opérâtes in the following manner. The molten sait 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 électrodes. At the température of the bath between 660 and670°C, the chlorine gas is generated at the anodes 4, whereas the magnésium métalis produced at the cathodes 5. Thus, in the invention metallic magnésium isproduced by passing direct electric current between the anodes 4 and cathodes 5suspended in facing spaced relation in the molten sait bath containing magnésiumchloride. The electrolysis of magnésium chloride in the bath causes the magnésiummétal to be released at working surfaces 18 of the cathodes 5, while chlorine gas is 15 generated at working surfaces 20 of the anodes 4. The magnésium métal, being lighter than the bath, rises along the cathode working surfaces 18, while the chlorinegas rises through the bath in a plume of bubbles from each anode surface 20 to becollected 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 20 opérable between the production chamber 3 and the magnésium séparation chamber 012717 6. Since the electrolysis is intensive within the inter-electrode gaps 12, the bathcirculation is enhanced by filling of the bath situated within the inner électrodesgaps 12 with the bubbles of the chlorine gas. In the invention the gas filling ismaintained in such a manner, so as to prevent formation of the descending bath 5 currents. The flow of molten bath and magnésium is formed in the area above thecathodes 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 theheight of the upper circulating channels 8.

In light of the filling of the molten sait bath in general with the bubbles of ÎO chlorine gas and specifically in view of the substantial level of filling of the bathwithin the inter-electrode gaps 12 with the bubbles of chlorine gas, the ascendingcurrents of the molten bath (electrolyte) are formed containing magnésium andchlorine. The chlorine gas is accumulated in the gas space 21 at the area above thelevel of bath in the production chamber 3. The produced chlorine gas is 15 çontinuously evacuated from the gas space 21 by means branch pipe 11 into themain chlorine gas duct. The molten bath together with the produced magnésiumare transferred from the area of the production chamber 3 above the cathodes 5through the upper circulation channels 8 into the chamber 6 for magnésiumséparation. There, the magnésium métal is accumulated on the surface of the bath 20 and periodically taken out by the vacuum ladle. In the séparation chamber 6 the molten bath separated from the magnésium métal is directed downwardly and 012717 through the lower circulation channels 9 back to the production chamber 3, so asto penetrate into the inter-electrodes gaps 12.

One of the main objects of the method of the invention is to providefavorable characteristics related to the filling of the bath with the inter-electrode 5 gaps 12 with the bubbles of chlorine gas. This assures formation of the ascendingcurrents of the bath within the entire length of the inter-electrode gaps 12. Theseascending currents are oriented in the upper part of the inter-electrode gaps 12above the cathodes 5 in the direction of the magnésium séparation chamber 6 andaccelerate magnésium removal from the production chamber 3 into the séparation 10 chamber 6.

The gas filling in the inter-electrode gaps is defined by the following formula:

DcathHcath G =- . · · - - Lav wherein the electrolysis is carried out with the gas filling beingwithin the range between 6 and 25 (conventional units). 15 G - characteristic specifying filling of the bath with chlorine gas bubbles in the inter-electrode gaps (conventional units); 012717

Dca* - cathode electrical current density (A/cm2);

Hcath ~ height of the cathode; (cm) and

Lav - average distance between the électrodes or inter-electrode gap, (cm).

The ascending currents of the molten bath in the inter-electrodes gaps are 5 formed with variable cross-section, which is increased along the direction ofmovement of the currents; whereas the velocity of bath currents in the area abovethe cathodes is selected from the following condition: hchn -= from 1.0 to 10.0, where

Lav hchn - height of upper circulation channels, (cm); and 1° Lav - average distance between the électrodes or inter-electrode gap, (cm).

It has been observed that in order to maintain the désirable characteristics of the filling of the bath with the chlorine gas bubbles, while keeping constant theheight of électrodes, it is necessary to considerably reduce the distance betweenthe adjacent électrodes or the inter-electrode gaps 12. Compared to the prior art 15 methods, it is necessary to reduce the gap distance from 1/2 to 1/3 of the prior artgaps 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 tosubstantially increase the amperage in the electrolytic cell and its production rate. 012717

The excessive fîlling of the bath with the chlorine gas bubbles isundesirable. In this respect, when characteristics of the gas fîlling of the bath areincreased more than 25 times, in view of the high velocity of the developed thrustof the gas bubble-bath mixture, the generated small drops of magnésium are 5 separated fforn the working surfaces 18 of the cathodes 5 and are easilychlorinated in the turbulent flow of bath and chlorine gas bubbles mixture. Thisleads to a substantial réduction of the electrical current efftciency and to aréduction of the production rate of the electrolytic cell. Furthermore, the chlorinegas losses attributable to the gas removal through the gas aspiration System are 10 directly proportional to the excessive gas fîlling of the molten bath with chlorinegas bubbles. With the increased saturation of the melt with the chlorine gasbubbles, the velocity of the molten bath flow also increases. In this condition, thenumber of chlorine gas bubbles that are carried away from the inter-electrodesgaps 12 to the magnésium séparation chamber 6 is also increased. 15 When the level of gas fîlling is reduced to less than 6 conventional units, the descending currents of the bath appear with the inter-electrodes gaps 12,causing noticeable réduction 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 10 012717 réduction of the différence in the velocity of the currents distributed along thevertical extension of the inter-electrode gap. This contributes to the improveddistribution and coverage of the working surfaces 18 of the cathodes 5 with theformed metallic magnésium. Specifically, such improvement relates to the ascent 5 of magnésium on the working surfaces 18 of cathodes 5 and séparation of themagnésium métal from the surface of the cathode at the upper part thereof. Thelatter factor contributes to the réduction of magnésium losses due to its undesirablechlorination and to the increase in the electrical current efficiency.

In the invention, the velocity of the bath and magnésium mixture flow over W the working surfaces 18 of cathodes is controlled by varying the height of theupper circulation channels 8. The height of the upper circulation channel (hChn)can be defined as a minimal distance between upper and lower régions thereof at aparticular location. Upon variation of the height of the upper circulation channel,the cross-sectional area of such channel varies accordingly. This also causes 15 respective variations in the velocity of the molten bath-magnesium flow betweenthe production (3) and séparation (6) chambers. The flow velocity is a fonction ofthe average distance between two adjacent électrodes or inter-electrode gap( Lav ).When the ratio of the height of the upper circulation channel (h^) to the averagedistance between the électrodes or inter-electrode gap (Lav) is less than 1.0, the 20 hydraulic résistance of the upper circulation channels 8 to the flow of the bath- magnesium mixture passing therethrough is increased. This causes détérioration 11 012717 in the conditions of magnésium transfer from the production chamber 3 to themagnésium séparation chamber 6. The increase in height of the upper circulationchannels (hchn) to the average inter-electrode distance (Lav) to more than 10.0causes a substantial increase in the losses of chlorine gas with the exhaust gasses, 5 which are evacuated from the electrolytic cell through the branch pipe 13.

Due to the above-discussed ratios of the height of the cathode (Hca1h) to the average distance between the électrodes or inter-electrode gap (Lav), the favorableconditions of the gas filling are assured. This results in the formation of the flowof the molten bath and magnésium oriented toward the upper circulation channels tO 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 takesplace. Such division results in the détérioration of the removal of magnésiummétal into magnésium séparation chamber 6 and in the reduced electrical current 15 efficiency of the electrolytic cell. When the above-discussed ratio exceeds 60,there is an increase in the bath-magnesium flow velocity within the uppercirculation channel 8. This causes détérioration of the magnésium séparation inthe séparation chamber 6. Specifically, this condition results in the séparation ofmagnésium drops and their entrainment with bath flow into the production 20 chamber 3, which ultimately causes the réduction in the electrical current efficiency. 12 012717

When the inclination of the working surfaces of the cathodes or anodes (18and 20 respectively) relative to the vertical exceeds 1°26', the total workingsurface of the électrodes is reduced. This causes the réduction of the cell amperage, as well as the réduction of its production rate. 3 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 theworking surfaces of the cathodes. This results in the prématuré séparation of thedrops of magnésium métal from the working surfaces 18 of the cathodes andincreases the losses of magnésium. 1° The invention enables the user to increase the working width of électrodes up to 2.0 m and greater, to decrease the distance between adjacent électrodes orthe inter-electrodes gap/distance, and as a resuit, to increase the electrolytic cellçapacity and to reduce the spécifie power consumption. In this respect, during dieproduction of the magnésium métal and chlorine gas by the method of the ^5 invention, a spécifie 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 ofthe cathode exceeds the inter-electrode gap by 25-60 times. The working surfaces 13 012717 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 anglebetween 0°38' and 1°26'. Such inclination causes an increase in the total workingsurface of the électrodes. As a resuit, the amperage of the cell and consequently itsproduction rate are increased as well.

The method of the invention causes réduction in the spécifie electricalpower consumption up to lOOOkW h/t Mg and also results in the increase of theelectrolytic cell production rate up to 40 percent. 14

Claims

012717 What is Claimed is:

1. A method of production of magnésium and chlorine from a molten sait bathcontaining MgCl2 in an electrolytic cell comprising: at least one production chamber containing a plurality of altemated anodes5 and cathodes with respective inter-electrode gaps formed therebetween, at leastone chamber for magnésium séparation being separated by a partition wall fromsaid at least one production chamber, said partition wall formed with upper andlower circulation channels, a closed bath circulation System associated with said atleast one production chamber and said at least one séparation chamber, said 10 method comprising the steps of: filling said molten sait bath within said inter-electrode gaps with bubbles ofthe chlorine gas and maintaining said bubbles within said gaps, so as to preventformation of descending currents therein; formation of a flow of said molten sait bath and magnésium oriented in the15 direction of the upper circulation channels in an area above said cathodes; and regulating velocity of said flow in said area above said cathodes by varying the height of said upper circulation channels. 15 012717
2. The method according to claim 1, wherein the gas filling in the inter-electrode gaps is defined by the following formula: DcathHcath wherein the electrolysis is carried out with the gas filling being within therange between 6 and 25 (conventional units), and wherein: 5 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); Hcath - height of the cathode (cm); and Lav - average distance between the électrodes or inter-electrode gap, (cm).
3. The method according to claim 1, wherein the velocity of said flow of the molten sait bath and magnésium in the area above said cathodes is selected by thefollowing ratio: hchn - = from 1.0 to 10.0, where Lav hchn - height of upper circulation channels, (cm); and 15 Lav - average distance between the électrodes or inter-electrode gap, (cm). 16 012717
4. The method according to claim 1, wherein ascending currents of saidmolten sait bath in said inter-electrode gaps are formed having a variable cross- section.
5. The method according to claim 4, wherein said variable cross-section of the 5 ascending currents within said inter-electrode gaps is increased in the direction of movement of said molten sait bath.
6. An electrolytic cell, comprising at least one production chamber and at least one magnésium séparationchamber separated fforn each other, said production chamber containing a 10 plurality of altemated anodes and cathodes with respective inter-electrode gaps formed therebetween, at least one chamber for magnésium séparation, said at leastone production chamber and said at least one chamber for magnésium séparationbeing separated by a partition, said partition being formed with upper and lowercirculation channels, a closed electrolyte circulation System associated with said at 15 least one production chamber and séparation chambers, wherein the height of each said cathode exceeds the length of each said inter-electrode gap by between 25 and 60 times.
7. The electrolytic cell of claim 6, wherein working surface of each saidcathode is positioned at an angle between 0°38' and 1 °26' to a vertical. 17 012717
8. The electrolytic cell of claim 6, wherein working surface of each saidanode is positioned at an angle between 0°38' and 1 °26' to a vertical.
9. The electrolytic cell of claim 6, wherein said at least one productionchamber comprises a multiplicity of production chambers.
10. The electrolytic cell of claim 6, wherein said at least one magnésium séparation chamber comprises a multiplicity of magnésium séparation chambers. 18