AU761819B2 - Method of dehydration of chlorides - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

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Name of Applicant/s: Actual Inventor/s: Address for Service: Invention Title: Russian National Aluminum and Magnesium Institute (VAMI) Atvminio AII(oe a9Mei aclu 'cal 0ocesses L'te4 Alexander N. Tatakin and Alexey B. Bezukladnikov and Vladimir I. Tschegolev and Galina J. Sandier BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "METHOD OF DEHYDRATION OF CHLORIDES" The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 22338.00 Method of Dehydration of Chlorides Field of the Invention This invention relates to nonferrous metallurgy, and more particularly, to preparation of camallite for magnesium production by electrolysis.

Background of the Invention A known prior art method of preparation of carallite or KC1 MgCl 2 6H 2 0 for electrolysis includes a step of thermal treatment of a solid state camallite consisting of two following phases: initial thermal treatment of carallite in a fuel-heated environment; and exposing the carnallite to gaseous hydrogen chloride in the heated environment.

In the first phase, carnallite is dehydrated by being heated to a temperature of about 300-330° C in a single fluidized bed dryer Then, the dehydrated product is loaded to another fluidized bed dryer and is chlorinated at a temperature reaching about 3700 C.

This method is disclosed in the USSR Inventor's Certificate 1,591,530.

An important drawback of this prior art method is that in the first phase, which occurs at the temperature of 300-3300 C, the dehydration of carnallite is accompanied by a significant hydrolysis of magnesium chloride. In the second phase which occurs at the 20 elevated temperature of about 3700 C, the products of hydrolysis undergo chlorination.

The resultant product contains not less than 0.45 percent by mass of MgO. In this prior art method, since electrolytic cells are supplied with the produced solid carnallite, the life span of the electrolytic cells is about three years or less.

Furthermore, the process of dehydration entailing the steps of heating of carnallite at the first phase to 3300 C, reloading of camallite to another fluidized bed dryer, and secondary heating thereof in such fluidized bed dryer up to 3700 C, resulted in the consumption of a substantial amount of energy.

Although in the prior-art method of production of dehydrated carallite, the specific consumption of chlorine is lower, compared to the method of the invention, the carnallite quality of the prior art method does not meet the standards related to the anode consumption during the electrolysis. Furthermore, in the prior art method the service life of electrolytic cells is unsatisfactorily short. For example, in the prior art method of processing camrnallite containing 0.45 percent by mass of MgO the service life of the s electrolytic cells is only twenty five month or 2.08 years.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Summary of the Invention The present invention relates to a method of dehydration of chlorides comprising the step of treating the chlorides within a fluidized bed of a multi-chamber continuous fluidized bed apparatus by a thermal medium containing hydrogen chloride. The hydrogen chloride is delivered to all production chambers of the fluidized bed apparatus.

15 The content of the hydrogen chloride in the thermal medium delivered to the fluidized bed of each successive production chamber is 1.02 to 4.00 times greater by volume than the content of the hydrogen chloride delivered to the respective preceding production chambers.

A temperature in the first production chamber is maintained at a level sufficient for S 20 removal of four molecules of water from the initial material. The temperature in the second production chamber is maintained at a level sufficient for removal of 1.0 to molecules of water.

The temperature in the third and fourth production chambers of the fluidized bed apparatus is maintained sufficient for removal of 0.1 to 1.0 molecules of water.

The utilized thermal medium is in the form of combustion gases in general and particularly the combustion gases generated within the combustion chambers of the fluidized bed apparatus.

An aspect of the present invention provides a method of dehydration of chlorides, comprising the step of: thermal treating of chlorides within a fluidized bed of a multi-chamber continuous fluidized bed apparatus having a first production chamber and at least one successive production chamber by a thermal medium containing hydrogen chloride; and delivering the hydrogen chloride to all production chambers of said fluidized bed apparatus, wherein the content of hydrogen chloride in the thermal medium delivered to the fluidized bed of each successive production chamber is 1.02 4.00 times greater by volume than the content of the hydrogen chloride delivered to the chamber preceding said successive production chamber.

Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

1is Description of the Preferred Embodiment In the invention, dehydration of chlorides occurs during the thermal treatment of camrnallite in a multi-chamber apparatus or a four-chamber continuous fluidized-bed dryer with a hydrogen-chloride ladened thermal medium. While a preferred thermal medium is in the form of combustion chamber gases which are deliverable to all production chambers of the dryer, other types of the thermal medium or heat carrier are within the scope of the invention. The hydrogen chloride content in the thermal medium supplied to :each successive production chamber is 1.02 4.0 times greater by volume than the respective content in the corresponding preceding chamber.

In the present invention, the temperature in the first production chamber is maintained at a level sufficient to remove four molecules of water out of six molecules of water present in the hydrated molecules of the initial material. The temperature in the second production chamber is maintained sufficient for removal of 1.0 1.5 molecules of water, whereas in the third and fourth production chambers the temperature is maintained sufficient for removal of the remaining 0.1-1.0 molecules of water.

The method of the invention enables the user by means of a single-stage dehydration in the four-chamber fluidized bed dryer to produce substantially dehydrated chlorides. One of the examples of such dehydrated chlorides is carnallite, in which the content of MgO does not exceed 0.3 percent by mass and the content of H 2 0 does not exceed 0.3 percent by mass. When feeding electrolytic cells with such solid product, the wear and tear of anodes during the three year term does not exceed 60 percent. This condition satisfies the requirements of electrolysis and extends the life span of the electrolytic cells in excess of three years.

The thermal treatment of chlorides is carried out by the thermal medium containing hydrogen chloride, which, in turn, is generated by burning of anodic chlorine in the combustion chambers of the fluidized bed dryer.

In the first production chamber of the fluidized bed dryer the temperature of the fluidized bed is maintained between 1300 and 150' C. At this temperature, the six-water molecules of camrnallite are transformed into two-water molecules of camrnallite and this 1s process is accompanied by minimal hydrolysis. To prevent the hydrolysis completely, the concentration of hydrogen chloride in the thermal medium should be maintained between 0.7 and 1.5 percent by volume.

In the second production chamber of the fluidized bed dryer, the temperature of the fluidized bed is maintained between 180 and 1950 C. In this condition, two-water molecules of carnallite are dehydrated into KC1 MgC1 2 nl 2 0, where n 1. Typically, this process is accompanied by the hydrolysis of carnallite accompanied by the formation of S: magnesium hydroxochloride. However, in the second production chamber of the fluidized bed dryer. when the HC1 content of the thermal medium is maintained in the range of between 1.4 and 2.0 percent by volume, the hydrolysis is inhibited.

In the third production chamber of the fluidized bed dryer the temperature of the fluidized bed is maintained between 2400 and 2600 C. In this chamber the hydrolysis of MgCL 2 is being continued and the decomposition of magnesium hydroxochloride occurs forming MgO. In this condition, the degree of dehydration reaches 99.5 percent. To prevent the hydrolysis and to chlorinate magnesium oxide, the content of HC1 in the thermal medium delivered to the fluidized bed is within the range of between 4.5 and percent by volume.

In the fourth production chamber of the fluidized bed dryer, the temperature of the fluidized bed is maintained about 3200 C. This temperature is sufficient to prevent rehydration and to assure maintaining the dehydration of carnallite at higher than 99.6 percent level. This temperature is also sufficient to provide the chlorination of the remaining magnesium oxide. Here, the HC1 concentration in the thermal medium is within the range of between 5 and 6 percent by volume.

The comparison between the efficiency of the method of the invention and that of the prior art methods is demonstrated by the following examples: Example 1 (Prior Art) The crystalline hydrate of carnallite (with six water molecules) is dehydrated in a fluidized bed dryer in two stages. At the first stage, camallite is heated by a chlorine-free 15 thermal medium to a temperature reaching up to 3300 C. This removes up to 99.5 percent of water.

At the second stage, the produced carnallite containing 0.5 percent by mass of H 2 0 and 1.8 percent by mass of MgO is loaded into a second fluidized bed dryer and heated in the fluidized bed by an HCl-laden thermal medium to a temperature of about 3700 C.

S 20 Thus, the dehydrated camallite is produced containing 0.2 percent by mass of H 2 0 and S0.45 percent by mass of MgO. The specific chlorine consumption is about 115 kg per ton of dehydrated camallite.

In this Example, two autonomous fluidized bed apparatuses are utilized to produce substantially dehydrated camallite. A primary purpose of the first apparatus is the removal of water. For this purpose, the camallite is heated up to the temperature of 330° C. In the second apparatus, after the step of reloading of camallite, the magnesium oxide formed during the dehydration process is chlorinated by the chlorine-containing gases. In this step, the camallite is additionally heated to 3700 C. This method of dehydration of camallite is associated with the significant overheating of camallite which leads to the increase in power consumption. Furthermore, this method does not provide the camrnallite of quality required by the subsequent electrolysis.

Therefore, the life span of the electrolytic cells utilizing such carnallite is only about 2.08 years.

Example 2 (The Method of the Invention) The crystalline hydrate of carnallite (with six water molecules) is dehydrated in the suspended state in the four-chamber fluidized bed dryer. The temperatures of the fluidized bed in the production chambers of the fluidized bed dryer are as follows: the first chamber 1400 C; the second chamber 1900 C; the third chamber 2500 C; and the fourth chamber 3200 C. These temperature conditions provide the following results. In the first chamber, an output of partially dehydrated carnallite, retaining two water molecules, is .generated. In the second chamber, substantially all of the water molecules are driven from the output of the first production chamber. so as to form a substantially dehydrated S is camallite. This occurs concurrently with the formation of hydroxochloride. In the third chamber, the main decomposition of hydroxochloride occurs accompanied by the formation of magnesium oxide. In the fourth chamber, a complete removal of the bound water is accomplished. To prevent undesirable hydrolysis of MgC1 2 the dehydration process is carried out by the thermal medium containing hydrogen chloride.

20 In the first production chamber, the concentration of hydrogen chloride in the S"thermal medium is about 0.7 percent (vol), which almost completely suppresses the hydrolysis of MgC12. The content of magnesium oxide in the product generated in the fluidized bed of the first chamber does not exceed 0.004 percent by mass.

In the second chamber, the concentration of hydrogen chloride is about 2.00 percent (vol). Such a concentration inhibits the hydrolysis and provides for the partial chlorination of the produced magnesium oxide.

In the third chamber, the concentration of hydrogen chloride in the thermal medium is about 4.5 percent (vol). Here, intensive decomposition of hydroxochloride occurs in such a manner that the resultant HC1 concentration assures the chlorination of the formed magnesium oxide up to 0.29 percent by mass.

In the fourth production chamber, the concentration of hydrogen chloride in the thermal medium is about 4.6 percent (vol). In this region, the final dehydration of carnallite occurs. The level of HC1 concentration assures completeness of the chlorination of the remaining magnesium oxide. The concentration of magnesium oxide in the finished product is now reduced to 0.25 percent by mass. This satisfies the requirements of electrolysis. when the solid camrnallite, according to the method of the invention, is directly loaded into the electrolytic cells. Such loading occurs without previously required melting of the dehydrated carnallite and the chlorinating thereof in dedicated chlorinators.

The service life of the electrolytic cells when utilized with such carnallite is 3.12 years.

The hydrogen chloride concentration in the thermal medium varies from chamber to chamber of the fluidized bed apparatus. In this respect, the concentration of hydrogen chloride in the thermal medium between the first and second production chambers, is increased in the ratio of 1:2.86. The respective concentration between the second and third chambers increases in the ratio of 1:2.25, whereas the respective concentration S is between the third and fourth chambers is increased in the ratio of 1:1.02 The specific consumption of chlorine is 280 kg per ton of deeply dehydrated carnallite, which is less than the quantity of chlorine produced in electrolysis.

Example 3. (The Method of the Invention).

This Example is similar to Example 2, but the following HC1 concentrations is maintained in the thermal medium: -in the first chamber 0.86 percent (vol); -in the second chamber 1.5 percent (vol); -in the third chamber 5.12 percent (vol); -in the fourth chamber 5.24 percent (vol).

The increase in the HCI concentration between the adjacent chambers of the fluidized bed apparatus is as follows. Between the first and second chambers the HC1 concentration is increased in the ratio of 1:1.86 Between the second and third chambers the concentration is increased in the ratio of 1:3.2. Between the third and fourth chambers the concentration is increased in the ratio of 1:1.02. Because of the HC1 concentration increase in the last chambers, the MgO content in the finished product is decreased to 0.15 percent by mass.

The specific consumption of chlorine in this Example is 310 kg per ton of deeply dehydrated carnallite. This is almost equal to the quantity of chlorine generated during the electrolysis of the carnallite. The life span of the electrolytic cells when utilized with such carnallite is 3.27 years.

Example 4 (The Method of the Invention) This method is similar to the method of Example 2. However, the level of HC1 concentration maintained in the thermal medium is as follows: -in the first production chamber 1.14 percent (vol); -in the second production chamber 1.30 percent (vol); -in the third production chamber 5.2 percent (vol); 15 -in the fourth production chamber 6.0 percent (vol).

The concentration of HC1 in the thermal medium between the first and second chambers is increased in the ratio of 1:1.14 The concentration of HCl between the second and third chambers is increased in the ratio of 1:4.0. The concentration of HC1 20 between the third and fourth chambers is increased in the ratio of 1:1.15. In this condition, the MgO content in the finished product is now reduced to 0.10 percent by S mass. The service life of electrolytic cells, when utilized with such carnallite, is 3.4 years.

However, in this Example, the specific chlorine consumption is 337 kg per ton of deeply dehydrated carnallite. Such consumption exceeds the quantity of chlorine generated during electrolysis. Therefore, additional chlorine is required in the method of the Example 4.

In the present invention, the dehydration of chlorides occurs in the fourchamber continuous fluidized bed dryer by thermally treating the initial material with the combustion chamber gases or a thermal medium containing hydrogen chloride, which admixture is delivered to all production chambers of the apparatus. The hydrogen chloride content in the thermal medium provided to each successive chamber is from 1.02 to 4 times higher than the respective HC1 content delivered to the respective preceding chamber. The temperature in the first chamber is high enough for the removal from the initial material of four molecules of water. The temperature in the second chamber is substantially high for the removal between 1 to and 1.5 molecules of water, whereas in the third and fourth chambers the level of temperature enables the invention to remove the remaining 0.1 to 1.0 molecules of water from the hydrated molecule of the material.

According to the method of the invention, one-stage dehydration of material in the four-chamber fluidized bed apparatus produces dehydrated chlorides, such as for example, carnallite. The content of each component, such as MgO and H 2 0 in dehydrated camallite does not exceed 0.3 percent by mass. When the electrolytic cells utilize such ooooo solid product, the wear and tear of the anodes used in the cells during the three year term does not exceed 60 percent. This condition satisfies the requirements of electrolysis and extends the life span of the electrolytic cells in the excess of three years.

15 The chlorides are thermally treated by the HC1 containing thermal medium. For example, in general, the thermal medium is a gaseous mixture with hydrogen chloride added thereto; and, in particular, the thermal medium is HCl-laden combustion chamber gases generated by burning of anodic chlorine in the burners of the fluidized bed apparatus.

The temperature of the fluidized bed in the first chamber of the fluidized bed oapparatus is between 1300 and 150' C. At this condition, the six-water molecules oo:associated with camallite are partially dehydrated and transformed into two-water :o molecules ofcarnallite. This step is accompanied by the minimal hydrolysis. It has been found that to avoid extensive hydrolysis, the hydrogen chloride content in the combustion chamber gases should be maintained within the range of from 0.7 to 1.5 percent (vol).

Thus. in utilizing the method of carnallite dehydration in the four-chamber fluidized bed apparatus, and supplying the chlorine-containing gases to all production chambers. enables the user to obtain dehydrated carnallite with the content of each component such as MgO and H20 less than 0.3 percent by mass. By utilizing a solid carnallite, the method of the invention enables the user to extend the service life of the electrolytic cells in the excess of three years.

The Table presented hereinbelow illustrates the results of producing a solid dehydrated carnallite under various experimental conditions.

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TABLE

Examples of dehydration of chlorides in general, and carnallite in particular.

Parameters Stage 1, continuous FB dryer Stage 2, batch FB chamb.l ch.2 ch.3 ch.4 Total dryer Termpera- Prior Art ture of Method 330 370 fluid Method of 140 190 250 320 Sbed, oC Invention HC1 con- Prior Art concent. Method S. in ther- Method 0.7 2.0 4.5 4.6 mal med- of Invention 0.86 1.6 5.12 5.24 ium vol. 1.14 1.30 5.2 Specific Prior Art chlorine Method 115 consump. Method 280 kg/ton of Invention 310 337 MgO con- Prior Art tent in Method 1.8 0.45 finished Method 0.004 0.12 0.29 0.25 product, of Invention 0.004 0.126 0.25 0.15 wt. 0.004 0.13 0.25 0.1

H

2 0 con- Prior Art tent in Method 0.5 0.2 finished Method 15.0 3.80 0.90 0.21 product, of Invention 15.0 3.76 0.87 0.21 wt. 15.0 3.60 0.86 0.21 Cell Prior Art 2.08 service Method 3.12 life, Method of 3.27 years Invention 3.4

Claims (10)

1. A method of dehydration of chlorides, comprising the step of: thermal treating of chlorides within a fluidized bed of a multi-chamber continuous fluidized bed apparatus having a first production chamber and at least one successive production chamber by a thermal medium containing hydrogen chloride; and delivering the hydrogen chloride to all production chambers of said fluidized bed apparatus, wherein the content of hydrogen chloride in the thermal medium delivered to the fluidized bed of each successive production chamber is 1.02 4.00 times greater by volume than the content of the hydrogen chloride delivered to the chamber preceding said successive production chamber.

2. The method of Claim 1, wherein said multi-chamber fluidized bed apparatus is a four-chamber continuous fluidized bed apparatus.

3. The method of Claim 2, wherein a temperature in the first production chamber is maintained at a level sufficient for removal of four molecules of water from the initial material. 2

4. The method of Claim 3, wherein the temperature in the second production chamber is maintained sufficient enough for removal of 1.0 1.5 molecules of water.

The method of Claim 4, wherein the temperature in the third and fourth successive production chambers of the fluidized bed apparatus is maintained sufficient enough for removal of 0.1 1.0 molecules of water.

6. The method of any one of Claims 2 to 5, wherein said thermal medium is in the form of combustion gases.

7. The method of Claim 6, wherein said combustion gases are combustion gases generated within a combustion chamber of said four-chamber continuous fluidized bed apparatus.

8. The method of Claim 7, wherein said four-chamber fluidized bed apparatus is a four-chamber continuous fluidized bed dryer adapted for dehydration of carnallite.

9. The method of any one of Claims 1 to 8, wherein said chlorides are in the form of carnallite and concentration of magnesium oxide in a final product is substantially reduced, so that the dehydrated carnallite is loaded into an electrolytic cell without melting oand chlorination thereof in dedicated chlorinators.

10. A method of dehydration of chlorides, substantially as herein described with reference 15 to any one of the embodiments of the invention illustrated other than the comparative examples. DATED this 21st Day of January 2003 RUSSIAN NATIONAL ALUMINUM AND MAGNESIUM INSTITUTE (VAMI) Attorney: PAUL G. HARRISON Fellow Institute of Patent and Trade Mark Attorneys of Australia of BALDWIN SHELSTON WATERS *L 9o