Electrolytic method, apparatus and product

Abstract

In a method for removing a substance from a feedstock comprising a solid metal or a solid metal compound, the feedstock is contacted with a fused-salt melt. The fused-salt melt contains a fused salt, a reactive-metal compound, and a reactive metal. The fused salt comprises an anion species which is different from the substance, the reactive-metal compound comprises the reactive metal and the substance, and the reactive metal is capable of reaction to remove at least some of the substance from the feedstock. A cathode and an anode contact the melt, and the feedstock contacts the cathode. An electrical current is applied between the cathode and the anode such that at least a portion of the substance is removed from the feedstock. During the application of the current, a quantity of the reactive metal in the melt is maintained sufficient to prevent oxidation of the anion species of the fused salt at the anode. The method may advantageously be usable for removing the substance from successive batches of the feedstock, where the applied current is controlled such that the fused-salt melt after processing a batch contains the quantity of the reactive metal sufficient to prevent oxidation of the anion species at the anode.

Claims

1. A method for removing a substance from batches of a feedstock comprising a solid metal, containing the substance in solid solution, or a metal compound comprising the substance and a metal, to produce batches of a product comprising the metal, comprising the steps of: (A) producing a batch of the product by; providing a fused-salt melt comprising a fused salt, a reactive-metal compound and a reactive metal, the fused salt comprising an anion species which is different from the substance, the reactive-metal compound comprising the reactive metal and the substance, and the reactive metal being capable of reaction to remove at least a portion of the substance from the feedstock; contacting the melt with a cathode; contacting the cathode and the melt with a batch of the feedstock such that the batch feedstock is cathodically connected; contacting the melt with an anode; and applying a current between the cathode and the anode to remove at least a portion of the substance from the cathodically-connected batch of feedstock so as to produce the product; in which a portion of the applied current during step (A) is carried by a reaction in which the reactive metal in the melt is oxidized at the anode; and in which a quantity of the reactive metal in the melt is sufficient to prevent oxidation of the anion species at the anode when the current is initially applied and at all times during step (A); and then (B) applying the current between the cathode and the anode for a further period of time, during which time the product remains cathodically connected in the melt, to decompose a portion of the reactive-metal compound in the melt and so increase the quantity of the reactive metal in the melt; in which steps (A) and (B) are carried out under current control; (C) removing the batch of product from the melt; and (D) re-using the melt to process a further batch of feedstock as defined in steps (A) to (C).

2. The method according to claim 1, in which the applied current is a predetermined variable current or is applied according to a predetermined current profile or is a constant current.

3. The method according to claim 1, in which a reaction between the feedstock and the reactive-metal compound changes a concentration of the reactive-metal compound in the melt during step (A).

4. The method according to claim 3, in which the reaction between the feedstock and the reactive-metal compound forms an intermediate compound, which reduces the concentration of the reactive-metal compound in the melt during an intermediate phase of step (A), and comprising carrying out step (B) such that said quantity of the reactive metal in the melt at an end of step (B) is above a threshold quantity, below which, application of the applied current would cause oxidation of the anion species at the anode.

5. The method according to claim 1, in which the melt is re-used to process 10 or more batches.

6. The method according to claim 1, in which cations of the reactive metal are correspondingly reduced at the cathode.

7. The method according to claim 1, in which the feedstock comprises a metal selected from beryllium, boron, magnesium, aluminium, silicon, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, germanium, yttrium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, the lanthanides.

8. The method according to claim 1, in which the substance comprises oxygen.

9. The method according to claim 1, in which the reactive metal comprises Ca, Li, Na or Mg.

10. The method according to claim 1, in which the anion species comprises chloride.

11. The method according to claim 1, in which the fused salt comprises calcium chloride.

12. The method according to claim 11, in which the quantity of the reactive metal in the melt before the melt is contacted with the feedstock at a start of step (A), and at an end of step (B), is between 0.1 wt % and 0.7 wt %.

13. The method according to claim 11, in which the quantity of the reactive-metal compound in the melt before the melt is contacted with the feedstock at a start of step (A), and at an end of step (B), is between 0.5 wt % and 2.0 wt %.

14. The method according to claim 11, in which the quantity of the reactive metal in the melt before the melt is contacted with the feedstock at a start of step (A), and at an end of step (B), is between 0.2 wt % and 0.5 wt %.

15. The method according to claim 11, in which the quantity of the reactive-metal compound in the melt before the melt is contacted with the feedstock at a start of step (A), and at an end of step (B), is between 0.8 wt % and 1.5 wt %.

16. The method according to claim 1, in which a current density at the anode when the current is applied at a start of step (A) is greater than 1000 Am.sup.?2.

17. The method according to claim 1, in which a predetermined current is applied during an intermediate phase of step (A), and lower predetermined currents are applied before and after the intermediate phase.

18. The method according to claim 1, in which the product comprising the metal is a metal product, an alloy product or an intermetallic product.

19. The method according to claim 1, in which a current density at the anode when the current is applied at a start of step (A) is greater than 1500 Am.sup.?2.

20. The method according to claim 1, in which a current density at the anode when the current is applied at a start of step (A) is greater than 2000 Am.sup.?2.

21. The method according to claim 1, in which the feedstock comprises a metal selected from lanthanum, cerium, praseodymium, neodymium, samarium, actinium, thorium, protactinium, uranium, neptunium or plutonium.

22. The method according to claim 1, in which the feedstock comprises a metal compound containing a metal selected from beryllium, boron, magnesium, aluminium, silicon, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, germanium, yttrium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, the lanthanides or the actinides.

23. The method according to claim 1, in which the feedstock comprises a metal compound containing a metal selected from lanthanum, cerium, praseodymium, neodymium, samarium, actinium, thorium, protactinium, uranium, neptunium or plutonium.

24. The method according to claim 1, in which the feedstock comprises more than one metal such that the product of the method is an alloy or an intermetallic compound.

Description

EXAMPLE 1

(1) An electro-reduction process is used to reduce 100 g of Tantalum pentoxide to Tantalum metal. The electrolytic cell contains 1.5 kg of molten CaCl.sub.2 electrolyte and is fitted with a graphite anode of area 0.0128 m.sup.2. The level of CaO in the electrolyte is 1 wt %. The mass transfer coefficient at the anode has been determined as 0.00008 ms.sup.?1.

(2) When a current of 15 ? is applied to the cell chlorine gas is evolved at the anode. Using equation 9 above Da=1.37. When the current is reduced to 10 ? chlorine evolution stops (Da 0.97) but the electrolysis takes 33% longer to achieve full reduction.

(3) An identical experiment is carried out with the addition of 0.3 wt % Ca and no chlorine is evolved. Using equation 9 above Da=0.96. The electrolysis takes only 67% as long as when operating at 10 ?.

EXAMPLE 2

(4) An electro-reduction process is used to reduce 37 g of Titanium Oxide to Titanium metal. The electrolytic cell contains 1.5 kg of molten CaCl.sub.2 electrolyte and is fitted with a graphite anode of area 0.0128 m.sup.2. The level of CaO in the electrolyte is 1 wt %. The mass transfer coefficient at the anode has been determined as 0.00008 ms.sup.?1.

(5) When a current of 15 ? is applied to the cell chlorine gas is evolved at the anode. Using equation 9 above Da=1.55. When a similar experiment is carried out using only 30 g of TiO.sub.2 no chlorine is evolved (Da 0.77) but the cell loading (and hence productivity) has been reduced by 19%.

(6) An identical experiment is carried out using 37 g of Titanium Oxide and with the addition of 0.42 wt % Ca and no chlorine is evolved. Using equation 9 above Da=0.98.

(7) The above examples illustrate that the addition of Ca metal at the start of the electrolysis can avoid the production of chlorine at the anode and lead to higher rates of productivity. Similar outcomes may advantageously be achieved using other reactive metals in other melts, such Ba in BaCl.sub.2 or Na in NaCl.

(8) As illustrated in the Examples, preferred implementations of the invention, in which the electrolyte composition is modified by a deliberate increase in concentration of the reactive metal, may advantageously allow the current in an electro-reduction process for a predetermined batch of feedstock to be increased by more than 10% or 20% or 30%, and preferably more than 40%, above a maximum current that may be sustained without (for example) chlorine evolution in a similar process which does not involve the deliberate increase in concentration of the reactive metal. In the cell without the deliberately increased concentration of reactive metal, the (for example) chlorine evolution may not occur continuously as the feedstock is reduced (depending on the current or current profile applied) but the implementation of the invention may advantageously allow an increased current, as described above, at any point when (for example) chlorine would otherwise be evolved.

(9) As shown in Example 2, the invention may similarly be applied to increase the mass of a batch of feedstock that can be processed in a given electrolytic cell without (for example) chlorine evolution. The mass of feedstock may advantageously be increased by more than 10% or 15% or 20%.

EXAMPLE 3

(10) In one embodiment, a method of the invention concerns removing a substance from batches of a feedstock comprising a solid metal, containing the substance in solid solution, or a metal compound comprising the substance and a metal, to produce batches of a product comprising the metal, comprising the steps of:

(11) (A) producing a batch of the product by;

(12) providing a fused-salt melt comprising a fused salt, a reactive-metal compound and a reactive metal, the fused salt comprising an anion species which is different from the substance, the reactive-metal compound comprising the reactive metal and the substance, and the reactive metal being capable of reaction to remove at least a portion of the substance from the feedstock; contacting the melt with a cathode; contacting the cathode and the melt with a batch of the feedstock such that the batch feedstock is cathodically connected; contacting the melt with an anode; and applying a current between the cathode and the anode to remove at least a portion of the substance from the cathodically-connected batch of feedstock so as to produce the product; in which a portion of the applied current during step (A) is carried by a reaction in which the reactive metal in the melt is oxidized at the anode; and in which a quantity of the reactive metal in the melt is sufficient to prevent oxidation of the anion species at the anode when the current is initially applied and at all times during step (A); and then

(13) (B) applying the current between the cathode and the anode for a further period of time, during which time the product remains cathodically connected in the melt, to decompose a portion of the reactive-metal compound in the melt and so increase the quantity of the reactive metal in the melt; in which steps (A) and (B) are carried out under current control;

(14) (C) removing the batch of product from the melt; and

(15) (D) re-using the melt to process a further batch of feedstock as defined in steps (A) to (C);

(16) wherein the reaction between the feedstock and the reactive-metal compound forms an intermediate compound, which reduces the concentration of the reactive-metal compound in the melt during an intermediate phase of step (A), and comprising carrying out step (B) such that said quantity of the reactive metal in the melt at an end of step (B) is above a threshold quantity, below which, application of the applied current would cause oxidation of the anion species at the anode.