AU768814B2 - Method of producing highly pure aluminium primary base metal - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Method of producing highly pure aluminium primary base metal e The following statement is a full description of this invention, including the best method of performing it known to us: Freehills Carter Smith Beadle Melbourne\003872509 Printed 7 August 2001 (12:16) page 2 Freehills Carter Smith Beadle Melbourne\003872509 Printed 7 August 2001 (12:16) page 2 2 METHOD OF PRODUCING HIGHLY PURE ALUMINIUM PRIMARY BASE METAL Technical Field The present invention relates to a method of producing an aluminium primary base metal by electrolysis.

Background Art An aluminium base metal has been principally produced by Hall Heroult electrolysis. In Hall Heroult electrolysis, alumina, that is aluminium oxide, and a carbon material for an anode are used as a main raw material and an additive raw material, respectively.

Alumina is usually prepared from alumina containing ore such as bauxite by alkali extracting and calcining, and is supplied as powder to an electrolysis cell.

Alumina prepared in such a manner as explained above usually has a purity of about 98.5 wt%. The alumina has a moisture content and contains from several tens to several hundred of ppm each of metal oxides such as Fe, 20 Si, Ga, V and Ti as shown in Table i.

The carbon material for anode used as an additive raw material is prepared by mixing calcined coke and a binder in a predetermined proportion and compacting the mixture into briquettes, and is supplied to the top of the anode of an electrolysis cell. Moreover, these materials are sometimes compared and fired in advance, and set in the electrolysis cell. The carbon material for anode is consumed as the electrolytic reduction of alumina (aluminium oxide) proceeds. The carbon material used in the anode is a mixture of coke and pitch, and contains about several hundred ppm each of oxides such as Fe, Si, V and Ti. This is because the ordinary purity of coke and pitch is as shown in Table. 2.

Melboume\003872190 Printed 7 August 2001 (9:41) Table 1: Contents of Impurity Elements Usually Contained in Alumina.

Impurity Elements Content (ppm) Si 60 350 Fe 30 200 Cu 0 Ni 0 Ti 0 Mn 0 V 0 Sn 0 Zn 0 Cr 0 Pb 0 Zr 0 Bi 0 Ga 30 200 r c Melboume\003872190 Printed 7 August 2001 (9:41) a Table 2: Contents (ppm) of Impurity Elements Usually Contained in Coke and Pitch Impurity Elements Content in Coke Content in Pitch Si 100 300 70 210 Fe 20 150 14 110 Cu 0 20 0 14 Ni 0 100 0 Ti 1 100 1 Mn 1 100 1 V 1 300 1 210 Sn 1 200 1 140 Zn 0 -60 0 -42 Cr 0 100 0 Pb 0 50 0 Zr 0 -50 0 Bi 0 10 0 7 Ga 0 20 0 14 Although the impurities contained in the alumina (main raw material)and the carbon material for anode (additive raw material) are partly removed during electrolysis, a significant amount is transferred to the product. As a result, the maximum purity of primary aluminium obtained by electrolysis is 99.9 wt% (hereinafter referred to as 3N).

In the present specification, the purity of an aluminium base metal is defined as a value obtained by subtracting the total content of the main impurity elements of Si, Fe, Cu, Ni, Ti, Mn, V, Sn, Zn, Cr, Pb, Zr, Bi and Ga (14 elements) from 100 wt%.

On the other hand, in the field of electrolytic capacitors, magnetic discs and the like where demand for Melboume\003872190 Printed 7 August 2001 (9:41) highly pure aluminium has been growing in recent years, aluminium having a purity of about 3N cannot meet the requirements for the properties of the capacitors, discs and the like; demand for highly pure aluminium having a purity of at least 99.95% (hereinafter referred to as 3N5) has been growing.

In order to surely meeting the quality requirements explained above, the purity of the aluminium base metal has heretofore been improved by a secondary refining step, by the three layer electrolysis and by the segregation process. However, since the improvement requires a secondary refining step, the production cost rises, and the production efficiency declines.

Disclosure of the Invention An object of the present invention is to solve the problems related to the conventional technologies described above, and to provide a method of stably producing an aluminium primary base metal having a purity of at least 99.95 wt% (3N5) by electrolysis.

According to the present invention, a method is provided of producing a highly pure aluminium primary base metal, comprising preparing an electrolysis anode using deashed coke and/or pitch as a carbon material for anode, and charging the electrolysis anode into a Hall Heroult electrolysis cell as an additive raw material, •wherein the deashed coke is prepared by treating raw material coal tar with an organic solvent, deashing the thus treated coal tar by redistillation, and calcining the deashed coal tar and the deashed pitch is prepared by treating raw material coal tar pitch with Melbourne\004389897 Printed 6 November 2003 (9:01) an organic solvent and deashing the thus treated coal tar by redistillation.

Best method for Carrying Out the Invention Alumina produced by a conventional apparatus is basically used as the main raw material alumina. The alumina is produced by a process designed to decrease the inclusion amounts of Fe and Si components derived from the production apparatus.

A mixture of aggregate coke and binder pitch is used as a carbon material for anode of an additive raw material. Although examples of a raw material for e *o o *oo o Melbourne\004389897 Printed 6 November 2003 (9:01) aggregate coke include pitch coke obtained by calcining coal tar pitch, and oil coke obtained by calcining crude oil, pitch coke prepared from relatively highly pure coal tar is preferred.

The aggregate coke is prepared by deashing raw material coal tar and calcining the deashed tar.

Although elements included in the raw material coal tar differ depending on the place of production of the coal, the coal tar usually contains 0.01 to 1% of an ash component mainly composed of SiO 2 and FeO. Since these elements show the same behaviour in the metal as in the alumina, the contents of these elements are desirably low when the aluminium is used for electrolytic capacitors, magnetic discs or the like. Accordingly, raw material coal tar is treated with an organic solvent, and the ash component of the coal tar is *.separated by redistillation to give highly pure coal tar. The resultant coal tar is then calcined, and the calcined coal tar is used as carbon aggregates for anode.

Prior to calcination of the coal tar, it is preferred to make the crystalline state during calcination equiaxed gains (granular crystals) by adding seeds for crystal formation and crystal growth of the raw material tar. The seeds are added for the following reasons. The crystallisation direction of the coke obtained by calcining the raw material tar without adding the seeds become nonuniform, and needle-like crystals grow. The needle-like crystals show poor chemical reactivity when electrolysis is conducted using a carbon anode prepared by mixing aggregate coke and binder pitch. As a result, the proportion of the coke that does not contribute to an effective electrochemical reaction, and that is consumed mechanically or by mere Melbourne\003872190 Printed 7 August 2001 (9:41) combustion increases.

Deashed highly pure tar pitch is used a binder for the anode of the additive raw material. The tar pitch can also be used without further processing; however, carbon black, mesophase carbon or crystallised carbon that is once pulverised is preferably added to the tar pitch to improve the binder properties, and the resultant tar pitch is preferably used.

The alumina (main raw material) and the carbon material for anode (additive raw material) thus prepared are charged into an electrolysis cell with fluorine compound containing cryolite used as an electrolytic bath, and subjected to an electrolytic reaction. The charged alumina is dissolved in molten cryolite, and an electrolytic reduction reaction proceeds with the carbon electrode material being contacted with the molten cryolite bath. Metallic impurities such as Fe and Si contained in the main and additive raw materials are also dissolved into the molten cryolite to cause a reduction reaction, and part of them are vapourised as fluorides and discharged together with an exhaust gas.

The discharge proportion of the impurities increases in accordance with a reduction potential "during the electrolysis. The discharge proportion of Fe is 30 wt%, and that of Ga is as much as 50 to 60 wt%.

An exhaust gas containing impurities such as Fe and Ga as fluorine compounds is treated by wet recovery in which the fluorine component is absorbed into aqueous alkali. In the wet recovery, sodium hydroxide is commonly used for aqueous alkali for absorbing the exhaust gas, and the fluorine component is fixed as sodium fluoride (NaF). The NaF is treated with sodium aluminate for aluminium sulfate to regenerate cryolite.

Although the regenerated cryolite can be recycled as an Melbourne\003872190 Printed 7 August 2001 (9:41) 8 electrolytic bath, the cryolite is unsuitable for the production of highly pure aluminium because it contains impurities. On the other hand, the dry scrubbing method in which a discharged fluoride is absorbed into the raw material alumina is not preferred because the method results inn recovering even discharged impurities.

The aluminium primary base metal obtained by electrolysis using the alumina (main raw material) and/or the carbon material (additive raw material) for anode thus prepared is a highly pure base metal having a quality comparable to or practically identical to the conventional secondary refined base metal.

Example 1 Carbon black fine powder was added to highly pure coal tar obtained by dissolving coal tar pitch in an organic solvent, by redistilling the solution to effect deashing. The mixture was calcined at an average calcination temperature of 1,000C to give aggregate coke for electrolysis anode. Moreover, electrode 20 impregnating pitch prepared by deashing and adding carbon black in the same manner as explained above was purchased as binder pitch for anode. The Fe content of the purchased highly pure coke and that of the highly pure pitch were 2 ppm and 5 ppm, respectively; the Si content thereof and that of the highly pure pitch were ppm and 5 ppm, respectively; the Cu content thereof and that of the highly pure pitch were less than 1 ppm and Sless than 1 ppm, respectively. The total content of the other impurity elements excluding Al of the purchased coke and that of the highly pure pitch were each less than 3 ppm.

Highly pure self-firing anode briquettes were produced using the aggregate coke and electrode impregnating pitch. The briquettes were charged into Melboume\003872190 Printed 7 August 2001 (9:41) the top of the anode of an electrolysis cell to be made in process. Alumina produced in the step designed to decrease the inclusion amounts of Fe and Si, was supplied when the anode reached the reaction surface, i.e. in about 3 months; moreover, when the molten aluminium in process stored within the cell was completely replaced, the contents of the impurities were measured. As a result, it was found that the content of impurity Fe was lowered from the level of 250 ppm to ppm or less, and that the content of the impurity Si was lowered from the level of 200 ppm to 120 ppm or less.

Table 3 shows the contents of the main impurities and the purity of the aluminium base metal described above. In addition, Table 3 also shows, for comparison, conventional field proven values obtained by a procedure wherein a conventional anode material that was not deashed was used, alumina was produced by a conventional alumina production apparatus, and the alumina was not o o.cleaned.

2 Melbourne\003872190 Printed 7 August 2001 (9:41) Table 3: Contents (ppm) of Impurities and Purities (wt%)of Aluminium Base Metals Example 1 Conventional Example Alumina A Company, commercial A Company, commercial Coke Deashed Conventional Pitch Deashed Conventional Si 1'00 120 180 200 Fe 65 90 200 250 Cu 1 5 10 Ni 1> 1 Ti 20 30 20 Impurity Mn 5 5 7 Elements V 1 2 3 Sn 5 Zn 5 15 25 Cr 2 1 Pb 1> 3 Zr 5 Bi 1 1 Ga 70 90 70 100 Purity of base metal 99.972 99.963 99.947 99.934 Note: Table 3 shows analytical results obtained by sampling once a day for a test period (3 months).

Each of the data shown in a range is a maximum and a minimum value during the test period (3 months). Each of the data shown as a single numerical value is a value that did not vary within the significant digit or digits.

A step in which the inclusion amounts of the Fe and the Si component were decreased was performed.

As shown in Table 3, the field proven values did not attain the purity 3N5 in Conventional Example. In contrast to the results explained above, in Example 1 in which coke and pitch (anode materials) were deashed according to the present invention, all the field proven values including the lower limit attained the purity *5 As explained above, Melbourne\003872190 Printed 7 August 2001 (9:41) according to the present 11 invention, a purity of at least 99.95 wt% (3N5) can be stably ensured for the aluminium base metal.

Concerning the effect of decreasing the amount of impurities by deashing the anode materials in the present example, it should be particularly noted that the Pb content was lowered from the conventional value of 3 to 5 ppm to less than 1 ppm.

For example, when the aluminium base metal is worked to form a foil for an electrolytic capacitor, the foil must be heat treated, whereby Pb is concentrated on the foil surface. As a result, the foil surface portion subsequent to the heat treatment has a Pb content that is from 10 to 100 times as great as the average Pb content. The concentration of Pb therefore exerts adverse effects on the capacitor characteristics. No such adverse effects are produced after decreasing the Pb content in the present invention.

Example 2 The following aggregate coke for electrolysis anode was purchased and prepared. In the same manner as in :.:-.Example i, carbon black fine powder was added to highly pure coal tar obtained by dissolving coal tar pitch in an organic solvent, and redistilling the solution to effect deashing. The mixture was calcined at an average calcination temperature of 1,000 0 C to give aggregate coke for electrolysis anode. Conventional electrode pitch was purchased and prepared as binder pitch for anode. The purity of the purchased highly pure coke was the same as in Example 1. The purchased conventional electrode pitch had an Fe content of 37 ppm, a Si content of 171 ppm and a Cu content of less than 1 ppm.

Self firing anode briquettes were produced using the aggregate coke and electrode pitch. The briquettes were charged into the top of the anode of an Melboume\003872190 Printed 7 August 2001 (9:41) 12 electrolysis cell to be made in process. Alumina produced in the step that was designed to decrease the inclusion amount of the Fe and the Si component was supplied at the stage where the anode reached the reaction surface in about 3 months; moreover, when the molten aluminium in process stored within the cell was completely replaced, the contents of the impurities were measured. As a result, it was found that the content of impurity Fe was lowered from the level of 250 ppm to 150 ppm, and that the content of impurity Si was lowered from the level of 200 ppm to 170 ppm.

Table 4 shows the contents of the main impurities and the purity of the aluminium base metal described above. In addition, Table 4 also shows for comparison conventional field proven values obtained by a procedure wherein a conventional anode material that was not deashed was used, and alumina was not cleaned.

Melbourne\003872190 Printed 7 August 2001 (9:41) 13 Table 4: Contents (ppm) of Impurities and Purities (wt%) of Aluminium Base Metals Example 2 Conventional Example Alumina A Company, commercial A Company, commercial Coke Deashed Conventional Pitch Conventional Conventional Si 120 145 180 200 Fe 70 95 200 250 Cu 1 5 10 Ni 1 1 Ti 20 30 20 Impurity Mn 5 6 5 7 Elements V 1 2 3 Sn 5 Zn 20 30 25 Cr 1 1 Pb 2 3 Zr 5 Bi 1 1 Ga 70 90 70 100 Purity of base metal 99.967 99.958 99.947 99.934 Note: Table 4 shows analytical results obtained by sampling once a day for a test period (3 months).

Each of the data shown in a range is a maximum and a minimum value during the test period (3 months) Each of the data shown as a single numerical value is a value that did not vary within the significant digit or digits.

A step in which the inclusion amounts of the Fe and the Si component were decreased was performed.

As shown in Table 4, even the upper limit of the field proven values in Conventional Example did not attain the purity 3N5. In contrast to the results explained above, in Example 2 in which coke (anode material) alone was deashed, all the field proven values including the lower limit attained the purity The effect of decreasing impurities by deashing coke alone in the present example is small in comparison with the example in which both the coke and the pitch a Melbourne\003872190 Printed 7 August 2001 (9:41) 14 were deashed. That is, it is more desirable to deash both the coke and pitch than to deash the coke alone.

As explained above, according to the present invention, a purity of at least 99.95 wt% (3N5) can be stably ensured for the aluminium base metal.

Although the effect of decreasing Pb is more reduced in the present example than in Example 1, the content of the Pb was lowered from the conventional value of 3 to 5 ppm to 2 ppm.

Example 3 The highly pure self firing anode briquettes used in Example 1 were charged into the top of the anode of an electrolysis cell to be made in process. When the anode reached the reaction surface, i.e. in about 2 months, supply of highly pure alumina S was started.

When molten aluminium in process stored within the cell was completely replaced, the contents of the "'"impurities were measured. It has been seen from the measurement results in Table 3 that as a result of using 20 highly pure alumina S, deashed coke and deashed pitch, aluminium primary base metal each having an Si content of 60 ppm or less, and an Fe content of 80 ppm or less were obtained.

Table 5 shows the contents of the main impurities and the purities of the aluminium base metals described *."above. In addition, Table 5 also shows for comparison conventional field proven values obtained by a procedure wherein a conventional anode material that was not deashed was used, alumina was produced with a conventional alumina production apparatus, and the alumina was not cleaned.

Melbourne\003872190 Printed 7 August 2001 (9:41) Table 5: Contents (ppm) of Impurities and Purities (wt%) of aluminium Base Metals Example 3 Conventional Example Alumina A Company, B Company, alumina A Company, Alumina S S commercial Coke Deashed Deashed Conventional Pitch Deashed Deashed Conventional Si 40 60 40 60 180 200 Fe 50 80 45 70 200 250 Cu 1 5 1 5 10 Ni 1> 1> 1 Ti 20 30 10 20 20 Impurity Mn 5 2 5 7 Elements V 1 1 2 3 Sn 5 1 Zn 5 15 3 10 25 Cr 2 1 1 Pb 1 1> 3 Zr 5 2 Bi 1 1 1 Ga 70 70 30 40 70 100 Purity of base 99.980 99.970 99.986 99.979 99.947 99.934 metal Note: Table 4 shows analytical results obtained by sampling once a day for a test period (3 months).

Each of the data shown in a range is a maximum and a minimum value during the test period (3 months) Each of the data shown as a single numerical value is a value that did not vary within 10 the significant digit or digits.

As shown in Table 5, the field proven values in Conventional Example did not attain the purity In contrast to the results explained above, in Example 3 in which the anode materials were deashed and/or the alumina was cleaned, all the field proven values including the lower limit exceeded the purity and attained the purity of at least 99.97 wt% which is close to 4N.

0 0**0 0 Melboure\003872190 Printed 7 August 2001 (9:41) As explained above, the present invention can stably ensure the purity of at least 99.95 wt% Industrial Applicability Use of a deashed carbon material for anode in combination with the alumina gives an aluminium primary base metal meeting the requirements for the properties of electrolytic capacitors, discs and the like and having a purity close to the purity 4N of an aluminium secondary refined base metal.

eoo e* Melbourne\003872190 Printed 7 August 2001 (9:41)

Claims (1)

1. A method of producing a highly pure aluminium primary base metal, comprising preparing an electrolysis anode using deashed coke and/or pitch as a carbon material for anode, and charging the electrolysis anode into a Hall Heroult electrolysis cell as an additive raw material, wherein the deashed coke is prepared by treating raw material coal tar with an organic solvent, deashing the thus treated coal tar by redistillation, and calcining the deashed coal tar and the deashed pitch is prepared by treating raw material coal tar pitch with an organic solvent and deashing the thus treated coal tar by redistillation. 6 November 2003 FREEHILLS CARTER SMITH BEADLE Patent Attorneys for the Applicant *L C 20 NIPPON LIGHT METAL CO., LTD oo *ooo *o• *o* *eoee **oo Melboume\004389897 Printed 6 November 2003 (9:01)