Composition for making wettable cathode in aluminum smelting

Abstract

Compositions for making wettable cathodes to be used in aluminum electrolysis cells are disclosed. The compositions generally include titanium diboride (TiB.sub.2) and metal additives. The amount of selected metal additives may result in production of electrodes having a tailored density and/or porosity. The electrodes may be durable and used in aluminum electrolysis cells.

Claims

1. A component, comprising: 0.01 to not greater than 0.45 wt. % metal additives, the balance being TiB.sub.2 and unavoidable impurities, wherein the unavoidable impurities make up less than 2 wt. % of the component; wherein the metal additives at least include chromium (Cr), wherein the chromium content of the component is not greater than 0.35 wt. %; wherein the component is crack-free and has a density of at least 90% to not greater than 98% of its theoretical density, and wherein the component has an apparent porosity of 0.05% to 4%.

2. The component of claim 1, further wherein the component comprises a geometry selected from the group consisting of: a tube, a plate, a rod.

3. The component of claim 1, wherein the component is configured as an electrode for use in an aluminum electrolysis cell.

4. The component of claim 1, wherein the metal additives further include one or more of Fe, Ni, Co, and W.

5. The component of claim 1, wherein the metal additives further include one or more of Mn, Mo, Pt, and Pd.

6. An electrode for use in an aluminum electrolysis cell, the electrode comprising: 0.01 to less than 0.5 wt. % metal additives, the balance being TiB.sub.2 and unavoidable impurities, wherein the unavoidable impurities make up less than 2 wt. % of the electrode; wherein the metal additives at least include chromium (Cr), wherein the chromium content of electrode is not greater than 0.35 wt. %; wherein the electrode is crack-free and has a density of at least 90% to not greater than 98% of its theoretical density, and wherein the component has an apparent porosity of 0.05% to 4%.

7. The electrode of claim 6, wherein the electrode is configured as a cathode in an aluminum electrolysis cell.

8. A method comprising: (a) blending a first powder comprising TiB.sub.2 with a second powder comprising a selected amount of metal additives to make a TiB.sub.2 composition, the TiB.sub.2 composition comprising: from 0.01 to not greater than 0.45 wt. % of the metal additives, the balance being TiB.sub.2 and unavoidable impurities, wherein the unavoidable impurities make up less than 2 wt. % of the electrode, wherein the metal additives at least include chromium (Cr), wherein the chromium content of electrode is not greater than 0.35 wt. %; and (b) producing a TiB.sub.2 component from the TiB.sub.2 composition; wherein the TiB.sub.2 component is crack-free and has a density of at least 90% to not greater than 98% of its theoretical density, and wherein the component has an apparent porosity of 0.05% to 4%.

9. The method of claim 8, wherein the producing step further comprises: pressing the TiB.sub.2 composition; and sintering the pressed TiB.sub.2 composition to yield the TiB.sub.2 component.

10. The method of claim 8, wherein the method comprises forming the TiB.sub.2 component, the TiB.sub.2 component comprising a geometry selected from the group consisting of: a plate, a rod, and a tube.

11. The method of claim 8, wherein the producing step further comprises: pressureless sintering the TiB.sub.2 composition to yield the TiB.sub.2 component.

12. The method of claim 8, wherein the producing step further comprises: sintering the TiB.sub.2 composition at a temperature of between 1400° C. to 2100° C.

13. The method of claim 12, wherein the producing step further comprises: pressing the TiB.sub.2 composition at a pressure from 70 kg/cm.sup.3 to at least 350 kg/cm.sup.3.

14. The method of claim 8, wherein the metal additives further include one or more of Mn, Mo, Pt, and Pd.

15. The method of claim 8, wherein the metal additives further include one or more of Fe, Ni, Co, and W.

16. An electrode, comprising: 0.025 to 0.10 wt. % metal additives, wherein the metal additives at least include Cr; the balance being TiB.sub.2 and unavoidable impurities, wherein the unavoidable impurities make up less than 2 wt. % of the component; wherein the component has a density of 88.9% to 98.5% of its theoretical density, and wherein the component has an apparent porosity of 0.05% to 4%.

17. The electrode of claim 16, wherein the metal additives further include one or more of Mn, Mo, Pt, and Pd.

18. The electrode of claim 16, wherein the metal additives further include one or more of Fe, Ni, Co, and W.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is flow chart illustrating one embodiment of a method for producing electrodes having a selected density.

DETAILED DESCRIPTION

Example 1

(2) Three different TiB.sub.2 powders having the chemical make-up identified in Table 1, below, are produced by blending TiB.sub.2 powders (e.g., via a V-blender) with various other powders (all values are approximate. Composition D is pure TiB.sub.2 powder containing no metal additives. Various plates are made from Compositions A-D by pressing the compositions into plate form using a commercial-scale hot-press.

(3) TABLE-US-00001 TABLE 1 Chemical Makeup of Plates A-D Compo- Compo- Compo- Compo- Material (wt. %) sition A sition B sition C sition D Fe 0.14 0.08 0.05 Negligible Ni 0.16 0.08 0.04 Negligible Co 0.16 0.08 0.04 Negligible W 0.49 0.31 0.16 Negligible TiB.sub.2 and Balance Balance Balance Balance Unavoidable Impurities Ave. density 98.9% 98.2% 94.9% 68.8% (% of theoretical) Bulk density (g/cc) 4.47 4.44 4.29 3.11 Apparent Porosity, % 0.07 0.09 0.13 28.6  Total Metal Additives 0.95% 0.55% 0.29%   0% (wt. %)

(4) Plates made from compositions A-C are exposed to a molten salt bath of a 10,000 ampere pilot-scale aluminum electrolysis cell. The plates made from Composition A fail the testing, showing splitting/delamination. There is a mixed failure rate among plates made from Composition B. The plates made from composition C all pass the test, in that they survive about 120 days of testing without significant loss in thickness and without splitting/delamination.

(5) Plates made from Composition D, i.e., pure titanium diboride, are machined into test coupons (e.g., 2″×2″×0.5″), and the test coupons are exposed a molten aluminum bath having a salt cover in an alumina crucible. The temperature of the molten aluminum was comparable to the conditions used in the aluminum electrolysis cell using inert anodes (e.g., in the range of 840-910° C.). The test coupons were exposed to the molten aluminum for about 480 hours. After the exposure period, the test coupons are removed hot from the crucible and air quenched. The test coupons are examined both by macroscopic inspection and by microstructure analysis (e.g., via SEM metallography). A test coupon “passes” if it is (a) intact as shown via macroscopic inspection, and (b) there is no visually apparent cracking due to aluminum filled cracks, as shown via the microstructure analysis. If either criteria is not met, the test coupon is considered a “fail”. The test coupons made from Composition D failed, show grain boundary attack and disintegration after anywhere from 7 to 20 days of testing, illustrating the inadequacy of pure TiB.sub.2 electrode plates.

(6) With respect to Plates A and B, it is theorized, but not being bound by this theory, that higher concentration of additives such as the likes of Ni, Co, Fe and/or W, may have led to stress corrosion cracking. The higher additive levels may have also led to potential volumetric expansion reactions between the commonly-used metals and aluminum during metal making. However, when the metal additive levels are low enough, stress corrosion cracking is not realized (e.g., due to insufficient materials to react with the aluminum metal of the bath).

(7) Plates having too high of a theoretical density, i.e., plates made from Composition A, and some made from Composition B, fail the test. This indicates that the theoretical density should be below about 98%. Indeed, plates made from composition C, which have a density of about 95% of theoretical, were successful in passing the pilot testing. Thus, it is anticipated that plates having a density in the range of 90-98% of theoretical may be effectively used as electrodes in an aluminum electrolysis cell. The noted metal additives may be useful in producing such plates and with the appropriate porosity.

(8) This data also suggests that the total amount of metal additives should be less than 0.55 wt. %. However, it is anticipated that higher amounts of metal additives (e.g., up to about 0.75 wt. % total) could be employed in some circumstances. The data also shows that at least some metal additives are required; plates made from pure TiB.sub.2 (Composition D) were the worst performing, indicating that at least some metal additive is required.

Example 2

(9) Similar to Example 1, various powder blends are produced by blending. The weight percent of the metal additives of the various blended samples are provided in Table 2, below, the balance being TiB.sub.2 and unavoidable impurities. TiB.sub.2 powder samples are pressed into plate form using a lab-scale, hot-press. After pressing, the plates are machined into test coupons (e.g., 2″×2″×0.5″).

(10) TABLE-US-00002 TABLE 2 Chemical Makeup of Samples 1-9 Total Ave. Metal Density Apparent Material Add. (% of Porosity Sample (weight %) (wt. %) theoret.) (%) Result 1 0.125 Ni 0.125 97.2 0.09 Pass 2 0.25 Ni 0.25 98.5 0.23 Pass 3 0.063 Fe 0.063 88.9 3.79 Pass 4 0.125 Fe 0.125 97.0 0.10 Pass 5 0.25 Fe 0.25 98.0 0.05 Pass 6 0.50 Fe 0.50 98.8 0.12 Fail 7 0.6 W 0.60 61.9 37.2 Fail 8 0.5 Fe + 0.6 W 1.1 99.6 0.07 Fail 9 0.05 each of Fe, 0.30 97.8 0.18 Pass Ni, Co + 0.15 W

(11) The test coupons are exposed to a molten aluminum bath having a salt cover in an alumina crucible. The temperature of the molten aluminum was comparable to the conditions used in aluminum electrolysis cells employing inert anodes (e.g., in the range of 840−910° C.). The test coupons were exposed to the molten aluminum for about 480 hours. After the exposure period, the test coupons are removed hot from the crucible and air quenched. The test coupons are examined both by macroscopic inspection and by microstructure analysis (e.g., via SEM metallography). A test coupon “passes” if it is (a) intact as shown via macroscopic inspection, and (b) there is no visually apparent cracking due to aluminum filled cracks, as shown via the microstructure analysis. If either criteria is not met, the test coupon is considered a “fail”.

(12) Plates having too high of a theoretical density, i.e., plates made from samples 6 and 8 failed the test. However, plates having a density below about 98.5%, but above about 88.9% (of theoretical) were able to pass the test. Similarly, plates having too low of a of density, i.e., plates made from sample 7, failed the test. This data suggests that any of the metal additives of Fe, Ni, and/or Co may be selected as the metal additive so long as the end products have a density of from about 85% to about 98.5% of the theoretical density. In some instances, W and/or other substitutes, described above, may be used in place of and/or in addition to the Fe, Ni, and Co metal additives. This data suggests that the total amount of metal additives should be less than 0.50 wt. %. However, it is anticipated that higher amounts of metal additives (e.g., up to about 0.75 wt. % total) could be employed in some circumstances.

(13) While various embodiments of the present disclosure have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.