AU617018B2 - Production of anhydrous magnesium chloride - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1952 COMPLETE SPECIFICATION Form

(ORIGINAL)

FOR OFFICE USE 61 018 Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: SRelated Art:

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TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: NORANDA INC.

SUITE 4500 COMMERCE COURT WEST

TORONTO

ONTARIO M5L 1B6

CANADA

4i *i .t Actual Inventor: Address for Service: GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Complete Specification for the invention entitled: PRODUCTION OF ANHYDROUS MAGNESIUM CHLORIDE.

The following statement is a full description of this invention including the best method of performing it known to me:-

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I Il,_1 _l .4~ 1A o 0 a o oa oo a 0 0 0 000 000 0 a a a So a 0 0 0 0

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0 0 o 0 PRODUCTION OF ANHYDROUS MAGNESIUM CHLORIDE This invention relates to the production of anhydrous magnesium chloride from spray-dried magnesium chloride powder, or magiiesite or magnesia powders.

A key step in the production of magnesium metal by electrolysis of magnesium chloride is the preparation of the anhydrous magnesium chloride feed. The anhydrous magnesium chloride feed to the electrolysis cell must contain very low amounts of MgO, otherwise there will be excessive consumption of the graphite anodes and forma- 10 tion of sludge in the bottom of the cell that will eventually impair cell operation if it is not removed periodically. In addition, the most modern magnesium electrolysis cells, such as the Alcan multipolar cell, are very tightly sealed and are designed such that anodes cannot be changed or the cell desludged without closing down the cell. Thus, it is even more critical that the anhydrous magnesium chloride feed to such cells contains very little MgO, in this case preferably less than 0.2% MgO by weight.

1j: I Several processes are used commercially to produce anhydrous MgCl 2 The oldest is the IG Farben process, in which briquettes of MgO and coke are reacted with chlorine in an electrically heated vertical shaft furnace to produce molten magnesium chloride at about 800"C. The main drawbacks to this process are: its low productivity (less than 30 tpd of molten MgCl 2 per furnace), the need for periodic shutdowns to remove unreacted residue from T. the bottom, high chlorine requirements, and the presence o 0 10 of chlorinated hydrocarbons in the exhaust gases. More 0 0 o recently, a new shaft furnace chlorination process has 0 00 oo 0..0 been developed by Mineral Process Licensing Corp. (Can.

o000 Pat. No. 1,128,288) that produces molten anhydrous magnesium chloride directly from lump magnesite ore using 15 carbon monoxide as the reductant. This process has the 0oo 0 00 o0 advantage of eliminating the magnesite to MgO calcination 00 and MgO/coke briquetting steps but it requires a very pure magnesite feed to make acceptable quality Mg metal, 4a: and it still has not resolved the remaining drawbacks of S 20 the IG Farben chlorinator; namely, low productivity and chlorinated hydrocarbon emissions.

Norsk Hydro has developed a new process for producing anhydrous magnesium chloride prills from concentrated magnesium chloride brines. This process is described in U.S. Patent 3,742,100 and consists of the following unit operations: tt -3- 1) Evaporation of MgCl2 brine to a concentration of up to 55% MgCl 2 2) Prilling of the concentrated MgCl 2 brine to form prills of MgC1 2 .4-6 H 2 0 of suitable size for fluidized bed processing.

3) Fluidized bed dehydration with air at 200*C to produce MgC1 2 .2H 2 0 powder.

4) Three-stage fluidized bed dehydration with o anhydrous HC1 gas at about 300"C to give anhydrous o 10. magnesium chloride powder containing less than 0.2% 0 o" each by weight of MgO and H 2 0.

0 This process is operating commercially in Norway 0000 but it is very complex and capital intensive.

Amax Magnesium in Rowley, Utah produce molten magnesium chloride by reacting spray-dried magnesium chloride powder, containing about 5% by weight each of MgO and H 2 0, with a solid carbonaceous reductant and 00 chlorine gas at a temperature of about 800*C. This °o process is described in U.S. Patent No. 3,953,574. The A" 20 process is carried out in two in-series rectangular furoC naces, heated electrically via A.C. graphite electrodes installed in the furnace walls. Spray-dried MgCl 2 is fed A together with a solid carbon reductant into the first furnace and chlorine gas is bubbled through both furnaces using graphite lances to react the MgO and H 2 0 in the feed to MgCl2 and HC1, respectively. The final MgCl 2 ii ii

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"i melt contains less than 0.5% MgO. However, Amax has found that in order to obtain sufficiently high chlorine utilization efficiencies it was necessary to provide ferrous chloride to the melt, either by adding an iron metal or oxide to the chlorination furnace or, preferably, by adding ferrous chloride solution to the MgCl 2 brine before spray drying. Without such iron additions, chlorine efficiencies of less than 40% were achieved, which would be too low for a commercial process.

However, the use of iron results in several process drawbacks, in addition to its added cost, namely: a) the residual iron level of 0.5% in the product MgCl 2 melt decomposes in the electrolytic cell to iron metal that accumulates as a sludge and causes 15 losses in cell operating efficiency, b) part of the iron added volatilizes causing stack emission problems.

o The residual iron level in the MgCl 2 product from the Amax process is too high for use in modern, "sealed" 20 electrolysis cells and therefore Amax has had to develop 4 '0 a novel bipolar pre-electrolysis operation to remove the iron level down to less than 0.1% Fe in order to use its MgCl 2 as a feed to such cells. This is described in U.S.

Patent No. 4,510,029.

Applicant has found that the low chlorine utilization efficiency obtained with graphite lances as used by f

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d: it Amax for chlorine injection is because such lances produce coarse bubbles cm in diameter) and little gas hold-up within the melt of the melt volume). Therefore, lances do not create sufficient interfacial contacting area between the MgO and carbon particles and the chlorine gas bubbles to produce a rapid, efficient reaction.

It is therefore the object of the present invention to provide a new process for producing anhydrous magnesium chloride from spray-dried MgCl 2 powder that eliminates the need to use iron or FeCl 2 to achieve high chlorination efficiencies.

It is also an object of the present invention to provide a process which is applicable to: i upgrading anhydrous magnesium chloride from other processes to produce a product containing acceptable amounts of MgO.

producing anhydrous magnesium chloride directly ii .from magnesite or magnesia powders.

The process in accordance with the present invention for the preparation of anhydrous magnesium chloride containing less than 0,2% MgO from a magnesium chloride Ii. material containing more than 1% MgO comprises the steps of feeding spray-dried magnesium chloride powder, or a magnesite or magensia powder to a furnace containing a j molten magnesium chloride melt at a temperature of 750-850° C, and reacting the melt with a gaseous carbonaceous reductant and chlorine. The gases are introduced through a gas disperser consisting of a rotating vaned impeller with a tube in the center of the impeller to create gas bubbles having a diameter smaller than 5 mm and a gas hold-up as high as 30% of the original melt volume, and also ensure that the particles present in the melt are maintained in suspension.

The preferred carbonaceous reductant is carbon monoxide as this introduces no deleterious impurities into the melt. The carbon monoxide would be introduced together with the chlorine through the gas disperser. Solid 6 finely-divided carbonaceous reductants, such as calcined coke and graphite, can also be used to partially replace carbon monoxide. In this case, the levels of deleterious impurities, such as nickel, chromium must be carefully controlled if high-purity magnesium metal is to be produced from the resulting magnesium chloride melt. Normally a solid carbonaceous reductant would be added in a similar manner to the spray-dried MgC12 powder.

According to the present invention there is also provided an apparatus for the preparation of anhydrous magnesium chloride containing less than 0.2% MgO suitable for use in the electrolytic production of magnesium metal from a magnesium chloride material containing more than 1% MgO comprising: o° a furnace containing a molten magnesium chloride S° melt at a temperature of 750-850oC; 0 00 means for feeding spray-dried magnesium chloride 0oo powder, or magnesite or magnesia powder into said furnace; a gas disperser located in said furnace and consisting of a rotating vaned impeller with a tube in the center of the impeller; and means for reacting the melt with a gaseous carbonaceous reductant and chlorine, the gaseous reactants being introduced through said tube to create gas bubbles with a diameter smaller than 5 mm and a gas hold-up as high as 30% of the original melt volume, and also ensure that the MgO particles present in the melt are evenly suspended.

The furnace is preferably a multistage furnace having i a melt section consisting of interconnected stages followed by a multistage reduction section. The gas flow in the furnace is preferably countercurrent to the melt flow to achieve maximum utilization of the injected gases. Each section is provided with at least one gas disperser. ii -7- C' 0 0 C'1 0 C 0 0 0 0 0 o a o o o o o o o o o o C U The invention will now be disclosed by way of example, with reference to the following examples and to the accompanying drawings in which: Figure 1 shows the equipment used in laboratory chlorination tests; and Figures 2a and 2b show sectional side and top views of a chlorinator used to carry out the process in accordance with the invention.

Referring to Figure 1, blended, MgCl 2 powder, 10 produced from spray-dried MgCl 2 and of known MgO content, was loaded into a graphite crucible 10 located in a clay-graphite enclosure 11 and melted under a N 2 atmosphere. A charge weight of 6 kg was used to give a melt volume of about 3.6 L (6.5-in. diameter by 6.5-in. high).

15 When the test temperature measured by thermocouple 12 was reached, usually in range 750-820°C, a six-bladed graphite stirrer 13, 2,5-in. diameter, was inserted and started together with CO/Cl 2 injection through a 0.6 cm diameter tube drilled through the center of the impeller.

Melt samples were taken periodically during the tests. A constant CO/Cl 2 gas flowrate, in the range 0.5-2.0 L/min., was used throughout each test and stirrer speeds in the range 300-800 rpm. The exhaust gases were removed from the crucible via a ceramic tube 14 and then sparged through water and caustic solution to remove HC1 and C1 2 At the end of each test the gas flow was stopped, the stirrer removed and the melt allowed to cool and solidify.

000oo 0 C i1 4 -8- A typical test result is given in the following Table I.

Table I Test Co'nditions Melt Temperature CO, Cl 2 Flowrate 815'C 500 mL/min each, constant throughout test %MgO Calculated Cl 2 Utilization Efficiency 'C 'C LO 0 'CC, 'C C 'C 'C 'CC,

C,

o 'C 'C C- TIME, min 0 60 120 180 240 300 360 420 480 540 3.91 3 .05 2.24 2 .01 0 90 0.37 0.14 0.026 0.008 84 .6 89.8 84.6 70.0 70.0 55.4 24.0 11.9 Average Cl 2 utilization efficiency 46.6% over whole test.

ii ~il -9- \i iI Initially, when the MgO level in the molten MgCl2 bath is relatively high (above 1% MgO), the chlorine utilization efficiency is very high, up to 90%, especially considering the small size of the laboratory reactor. Chlorine utilization efficiency decreases steadily as the MgO level falls to values of about 20% at 0.1% MgO in the melt. The overall average Cl 2 utilization efficiency for this test was 46.6%. If the CO and Cl 2 gas 'o flowrates are reduced proportionately as the MgO level o S 10 falls below 1% MgO, then the overall Cl2 utilization is o oa o increased to over o o a 0 0 ooo 0 The stirrer design and speed used in this test was 0 not considered to be the optimum but it still showed that MgO levels less than 0.01% can be achieved at a high chlorine utilization efficiency.

4 Water model studies have shown that the rotating vaned impeller used in the above tests typically produced gas bubbles with a diameter in the range 2-3 mm in diameter. The impeller disperses the gas bubbles in the melt to create a gas hold-up as high as 30% of the original melt volume. This creates a very large interfacial contacting area betwee: gas bubbles and the melt and produces a rapid efficie, reaction. The relationship between interfacial arc. id gas bubble diameter and gas hold-up can be represented by the equation: Interfacial area 6 x fractional volumetric hold-up/mean bubble diameter.

i lO When the same test was carried out with chlorine and carbon monoxide gases sparged through a 0.6 cm diameter quartz tube, very little reaction occurred. The gas bubbles produced were estimated to be about 1 cm in diameter and the gas hold-up in the melt was very low thus giving a much smaller interfacial contacting area compared to the vaned impeller. Thus the MgO level in the melt was still above 4% MgO after several hours.

Further test results showing the effect of gas 10 flowrate and melt temperature on the reaction rate and Schlorine utilization efficiency are given in the following Table II.

S a ooo 0 000 C C aaoO o C'.o *000 .400 C4 0u 0 000 000 0 0 C &C 0 4, 00~ 0 C 44 0 0 3 0 0 Batch Test Results

TEST

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1 2 3 4 TEMP GAS FLOWRATES, ml/mn~± 0C CO C1 2 820 710 700 820 1100 1050 820 2020 2040 750 1020 1000 750 1800 1750

TIME

240 240 135 270 150 %MgO in Initial 4 .89 4.01 4.91 5.32 4.31 MgCl 2 Final 0. 10 0.06 0. 03 0. 08 0. 05 Average Cl 2 Efficiency, 84.6 54.3 59. 3 60.6 53.1 L -12i These tests were all carried out at a strirrer i speed of 650 rpm, which was found to be the optimum for this vessel size and stirrer design. Temperature in the Srange 750 to 820°C was found to have little effect.

Average overall chlorine utlization efficiencies ranged from 85% at 700 mL/min to 50-60% at up to 2000 mL/min.

Again these efficiencies would be much higher if the CO/C12 flowrates were reduced as the MgO level falls and are high enough for a commercial process.

i 10 A commercial chlorinator to produce 8.5 mtph of anhydrous magnesium chloride, containing less than 0.1% ,00 o MgO, is shown in Figures 2a and 2b. It is a multi-stage reactor, with each unit approximately 2.5 m L x 2.5 m W x 2 m high with about c. 1 metre melt depth and equipped with a special rotary gas disperser 20. The type of gas S dispersers used in the commercial chlorinator are 30-60 cm in diameter, larger than those currently used in the Aluminum Industry. The reactor has a melt/reaction section 22 consisting of 3 interconnected unit stages followed by a two-stage reduction section 24. A gas disperser is located in each stage and CO/C12 is blown through the center of the impellers. The gas flow is countercurrent to the MgC12 flow.

8.2 mtph of spray-dried MgC12 powder (approximately 5% MgO, 5% H 2 0) is fed continuously through feed ports 26 into the melting section 22 where most of the MgO and

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-13in the powder is reacted to MgCl, and HC1, respectively.

Heat is provided to melt the magnesium chloride by A.C.

powered graphite electrodes 28. CO and Cl 2 are injected into the melt section through several gas dispersion units at a total rate of about 2400 Nm 3 /h each of CO and Cl 2 The level of MgO in the MgCl 2 melt leaving the melt section is less than The remaining MgO is reacted in two additional stages to ensure the final level of MgO o" in the molten MgCl 2 product is less than 0.1% MgO. The o 10 amount of CO and Cl 2 injected in the last two stages is 0 0 2 c a very small, less than 50 Nm 3 /h each of CO and Cl 2 -I 2 0 0 00 0 0 0 0 00, CI IC

Claims (2)

1. A process for the preparation of anhydrous magnesium chloride containing smaller than 0.2% MgO suitable for use in the electrolytic production of magnesium metal from a magnesium chloride material containing more than 1% MgO comprising the steps of: feeding spray-dried magnesium chloride powder, or magnesite or magnesia powder to a furnace containing a molten magnesium chloride melt at a temperature of

750-850 C; providing a gas disperser in said furnace consisting of a rotating vaned impeller with a tube in the 0 center of the impeller; and 0 o: o(c) reacting the resulting melt with a gaseous carbonaceous reductant and chlorine, the gaseous reactants i°o being introduced through the tube in the impeller to create gas bubbles with a diameter smaller than 5 mm and a gas hold-up as high as 30% of the original melt volume, and ensure that the MgO particles present in the melt are evenly suspended. I 0 2. A process as defined in claim 1, wherein the anhydrous magnesium chloride contains less than 0.1% MgO. 3. A process as defined in claim 1, wherein the carbonaceous reductant is carbon monoxide. 4. A process as defined in claim 3, wherein a solid powdered carbonaceous material is introduced into said bath with the spray dried powder. An apparatus for the preparation of anhydrous magnesium chloride containing less than 0.2% MgO suitable for use in the electrolytic production of magnesium metal from a magnesium chloride material containing more than 1% MgO comprising: a furnace containing a molten magnesium chloride i i, 15 melt at a temperature of 750-850C; means for feeding spray-dried magnesium chloride powder, or magnesite or magnesia powder into said furnace; a gas disperser located in said furnace and consisting of a rotating vaned impeller with a tube in the center of the impeller; and means for reacting the melt with a gaseous carbonaceous reductant and chlorine, the gaseous reactants being introduced through said tube to create gas bubbles with a diameter smaller than 5 mm and a gas hold-up as high as 30% of the original melt volume, and also ensure that the MgO particles present in the melt are evenly suspended. 6. An apparatus as defined in claim 5, wherein the furnace is a multistage elongated furnace having a melt section consisting of interconnected stages followed by a multistage reduction section, at least one gas disperser being provided in each section. 7. An apparatus as defined in claim 6, wherein the gas flow is countercurrent to the melt flow. 8. A process for the preparation of anhydrous magnesium chloride substantially as herein described with reference to the accompanying drawings. 9. An apparatus for the preparation of anhydrous magnesium chloride substantially as herein described with reference to the accompanying drawings. DATED THIS 17TH DAY OF JULY 1991 NORANDA INC. By its Patent Attorneys: GRIFFITH HACK CO Fellows Institute of Patent Attorneys of Australia ,r