AU3224199A - Method of dehydration of chlorides - Google Patents

Description

-1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant/s: Russian National Aluminum and Magnesium Institute (VAMI) Actual Inventor/s: Alexander N. Tatakin and Alexey B. Bezukladnikov and Vladimir I. Tschegolev and Galina J. Sandler Address for Service: BALDWIN SHELSTON WATERS 60 MARGARET STREET SYDNEY NSW 2000 *o Invention Title: "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 la Method of Dehydration of Chlorides Field of the Invention This invention relates to nonferrous metallurgy, and more particularly, to preparation of carnallite for magnesium production by electrolysis.

Background of the Invention A known prior art method of preparation of carnallite or KCl1 MgCl 2 6H20 for electrolysis includes a step of thermal treatment of a solid state carnallite consisting of 10 two following phases: an initial thermal treatment of carnallite in a fuelfired environment and 9 a thermal treatment of carnallite in an environment of gaseous hydrogen chloride.

15 In the first phase, carnallite is heated to 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 370° C.

This method is disclosed in the USSR Inventor's Certificate 20 1,591,530.

An important drawback of this prior art method is that in the first phase which occurs at the temperature of 300- 330° C, the dehydration of carnallite is accompanied by a significant hydrolysis of magnesium chloride. In the second phase which occurs only at a temperature increased up to 370° C the products of hydrolysis now undergo chlorination. The resultant product contains not less than 0.45 (wt) of MgO. In these circumstances, when electrolytic cells are supplied by solid carnallite 2 produced by the prior art method, the life span of these electrolytic cells does not exceed three years.

Furthermore, the process of dehydration entailing the steps of heating of carnallite at the first phase to 330* C, the reloading of the carnallite to another fluidized bed dryer, and the secondary heating thereof to 370 C, consumes substantial energy.

Although in the prior-art method of production of dehydrated carnallite, the specific chlorine consumption is lower compared with the method of the invention, the carnallite quality of the prior art methods does not meet the requirements of electrolysis for anode consumption.

Furthermore, in the prior art methods the service life of o• electrolytic cells is unsatisfactory short. For example, in 15 the prior art method of processing carnallite containing 0.45 of MgO the service life of the electrolytic cells is twenty five month or 2.08 years.

Summary of the Invention One aspect of the invention provides a method of dehydration of chlorides comprising the step of treating the chlorides within a fluidized bed of a multi-chamber Scontinuous fluidized bed apparatus by a thermal medium containing hydrogen chloride. The hydrogen chloride is delivered to all production chambers of the fluidized bed apparatus and 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 than the content of the hydrogen chloride delivered to the preceding production chambers.

As to another aspect of the invention, a temperature in the first production chamber is maintained at a level sufficient for removal of four molecules of water from the 3 initial material. The temperature in the second production chamber is maintained at a level sufficient for removal of to 1.5 molecules of water.

As to a further aspect of the invention, 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.

As to still another aspect of the invention, the utilized thermal medium employed is in the form of combustion gases in general and particularly the combustion ooo gases generated within the combustion chambers of the fluidized bed apparatus.

Thus, an objects of the invention is to upgrade the .quality of dehydrated carnallite by reducing the magnesium 15 oxide content in the carnallite up to 0.0 to 0.25 (wt), and reducing the H 2 0 content up to 0.2 to 0.3 (wt).

Description of the Preferred Embodiment *In the invention, dehydration of chlorides occurs during the thermal treatment of carnallite in a multichamber apparatus or a four-chamber continuous fluidized-bed dryer with a hydrogen-chloride ladened thermal medium. While a suitable thermal medium is formed by 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 higher 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 4 water present in the hydrated molecule 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 (wt) and the content of H 2 0 does not exceed 0.3% When feeding electrolytic cells with such solid product, the wear and tear of anodes during the three year term does not exceed 15 60%. 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 20 turn, is generated by burning of anodic chlorine in the combustion chambers of the fluidized bed dryer.

9coo In the first production chamber of the fluidized bed dryer the temperature of the fluidized bed is maintained between 130* and 150 C. At this temperature, the six-water molecules of carnallite are transformed into two-water molecules of carnallite and this 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 by volume.

In the second production chamber of the fluidized bed dryer, the temperature of the fluidized bed is maintained at between 180' and 195' C. In this condition, two-water molecules of carnallite are dehydrated into KCI-MgCl 2 where n I. Typically, this process is accompanied by the 5 hydrolysis of carnallite with formation of 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 2.0 by volume, the hydrolysis is inhibited. In the third production chamber of the fluidized bed dryer the temperature of the fluidized bed is maintained at between 240° and 260° C. In this chamber the hydrolysis of MgC12 is being continued and the decomposition of magnesium hydroxochloride occurs forming MgO. In this condition, the degree of dehydration reaches 99.5%. 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 5.5 by volume.

15 In the fourth production chamber of the fluidized bed dryer, the temperature of the fluidized bed is maintained about 320* C. This temperature is sufficient to prevent rehydration and to assure maintaining the dehydration of carnallite at higher than 99.6 level. This temperature 20 is also sufficient to provide the chlorination of the remaining magnesium chloride. Here, the HC1 concentration in the thermal medium is within the range of between 5 and 6 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, carnallite is heated by a chlorine-free thermal medium to a temperature of up to 330° C. This drives off 99.5 of water.

6 At the second stage, the produced carnallite containing of H20 and 1.8 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 370* C.

Thus, the dehydrated carnallite is produced containing 0.2 of H20 and 0.45 of MgO, with a specific chlorine consumption of 115 kg/ton.

In this Example, two autonomous fluidized bed apparatuses are utilized to produce deeply dehydrated carnallite. The main purpose of the first apparatus is the removal of water. For this purpose, the carnallite is heated to 330' C. In the second apparatus, after the step of reloading of carnallite, the magnesium oxide formed during the dehydration process is chlorinated by the 15 chlorine-containing gases. In this step, the carnallite is additionally heated to 370* C. This method of dehydration of carnallite is associated with the significant overheating of carnallite which leads to the increase in power consumption. Furthermore, this method does not provide the 20 carnallite 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 a four-chamber fluidized bed dryer in the suspended state. The temperatures of the fluidized bed in the production chambers of the fluidized bed dryer is as follows: the first chamber 140 C; the second chamber 190° C; the third chamber 250* C; and the fourth chamber 320° C. These temperature conditions provide the following results. In the first chamber, an 7 output of partially dehydrated carnallite, retaining two water molecules, is generated. In the second chamber, practically all of the water molecules are driven from the output of the first production chamber to form a substantially dehydrated carnallite. 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 MgCl 2 •the dehydration process is carried out by the thermal medium containing hydrogen chloride.

In the first production chamber, the concentration of hydrogen chloride in the thermal medium is 0.7 (vol), 15 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 (wt).

In the second chamber, the concentration of hydrogen o 20 chloride is 2 (vol). Such a concentration inhibits the 0. hydrolysis and provides for the partial chlorination of the produced magnesium oxide.

In the third chamber, the concentration of hydrogen chloride is 4.5 (vol). Here, intensive decomposition of hydroxochloride occurs, such that the resultant HC1 concentration assures the chlorination of the formed magnesium oxide up to 0.29 (wt).

In the fourth production chamber, the concentration of hydrogen chloride in the thermal medium is 4.6 (vol).

Here, the final dehydration of carnallite occurs. This level of HC1 concentration completes the chlorination of the remaining magnesium oxide. The concentration of magnesium oxide in the finished product is now reduced to 0.25 (wt).

This satisfies the requirements of electrolysis when the 8 solid carnallite, according to the method of the invention, is directly loaded into the electrolytic cells. This 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 concentration between the second and third chambers increases in the ratio of 1:2.25 whereas the concentration obetween the third and fourth chambers is increased in the 15 ratio of 1:1.02 The specific consumption of chlorine is 280 kg per ton of deeply dehydrated carnallite, which is less than the chlorine quantity produced in electrolysis.

Example 3. (The Method of the Invention).

This Example is similar to Example 2, but the HC1 concentrations maintained in the thermal medium is as follows:

S*

-in the first chamber 0.86 (vol); -in the second chamber 1.5 (vol); -in the third chamber 5.12 (vol); -in the fourth chamber 5.24 (vol).

The increase in the HC1 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 9 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 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) *o This method is similar to the method of Example 2.

However, the HC1 concentration maintained in the thermal medium is as follows: 15 -in the first production chamber 1.14 (vol); -in the second production chamber 1.30 (vol); -in the third production chamber 5.2 (vol); -in the fourth production chamber 6.0 (vol).

The concentration of HC1 in the thermal medium between 20 the first and second chambers is increased in the ratio of 1:1.14 The concentration of HC1 between the second and third chambers is increased in the ratio of 1:4.0. times.

The concentration of HC1 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 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 10 during electrolysis. Therefore, additional chlorine is required in the method of Example 4.

In the present invention, the dehydration of chlorides occurs in a four-chamber 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 HCI content delivered to the preceding chambers. The temperature in the first chamber is high enough for the removal from the e initial material of four molecules of water. The ptemperature in the second chamber is high enough for the p 15 removal of from 1 to 1.5 molecules of water, whereas in the third and fourth chambers the level of temperature enables pp..

p.othe invention to remove the remaining 0.1 to 1.0 molecule 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 p. example, carnallite. The content of each component, such as MgO and H 2 0 in dehydrated carnallite does not exceed 0.3 When the electrolytic cells are used with such solid product, the wear and tear of the anodes used in the cells during the three year term does not exceed 60 This condition satisfies the requirements of electrolysis and extends the life span of the electrolytic cells in excess of three years.

The chlorides are thermally treated by the HCl 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 11 anodic chlorine in the burners of the fluidized bed apparatus.

The temperature of the fluidized bed in the first chamber of the fluidized bed apparatus is between 130' and 150' C. At this condition, the six-water molecules of carnallite are partially dehydrated and transformed into two-water molecules of carnallite. 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 (vol).

Thus in utilizing the method of carnallite dehydration in the four-chamber fluidized bed apparatus, supplying the chlorine-containing gases to all production chambers 15 enables the user to obtain dehydrated carnallite with the content of each component such as MgO and H 2 0 less than 0.3 By utilizing a solid carnallite, the method of the invention extends the service life of the electrolytic cells in the excess of three years.

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

*°o 12

TABLE

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

Parameters Stage 1, continuous FB dryer Stage 2, batch FB chamb.1 ch.2 ch.3 ch.4 Total dryer Termpera- Prior Art ture of Method 330 370 fluid Method of 140 190 250 320 bed, oC Invention HCl con- Prior Art concent. Method *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- 337con- 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 0.004 0.13 0.25 0.1 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 treatment of 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 said fluidized bed apparatus and the content of hydrogen chloride in the thermal medium delivered to the fluidized bed of each 0 successive production chamber is 1.02 4.00 times greater than the content of the hydrogen chloride delivered to the preceding chambers.

2. The method of Claim 1, wherein said multi-chamber fluidized bed apparatus is a four-chamber continuous 15 fluidized bed apparatus.

S3. 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.

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 production chambers of the fluidized bed apparatus is maintained sufficient enough for removal of 0.1 1.0 molecules of water. 14

6. The method of Claim 2, 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 continuos 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 Claim 1, wherein said chlorides are in the 10 form of carnallite and concentration of magnesium oxide in a final product is substantially reduced, so that the 6 dehydrated carnallite is loaded into an electrolytic cell without melting and chlorination thereof in dedicated chlorinators.

10. A method of dehydration of chlorides substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying examples. DATED this 26th Day of May, 1999 RUSSIAN NATIONAL ALUMINUM AND MAGNESIUM INSTITUTE (VAMI) Attorney: IVAN A. RAJKOVIC Fellow Institute of Patent and Trade Mark Attorneys of Australia of BALDWIN SHELSTON WATERS