WO2007083169A1

WO2007083169A1 - Master alloy manufacturing

Info

Publication number: WO2007083169A1
Application number: PCT/GB2007/050034
Authority: WO
Inventors: Peter Gamon Johns
Original assignee: Middlesex Silver Co. Limited
Priority date: 2006-01-23
Filing date: 2007-01-23
Publication date: 2007-07-26
Also published as: GB0601317D0; GB2434376A

Abstract

A method is provided for method for making a gold alloy, including the steps of: (a) forming a master alloy consisting essentially of constituents of said gold alloy and inevitable impurities (e.g. an AgCu alloy, AgCuZn alloy, CuZn alloy or AgZn alloy optionally containing one or more incidental ingredients selected from Ni, Fe and Si.) ; and (b) melting together the master alloy and gold to form the desired gold alloy; wherein the step of forming the master alloy includes adding boron as a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides metal borohydrides metal borohalides, and mixtures thereof. The boron compound may be added to the master alloy in the gas, liquid or solid phase, and in the latter embodiment to minimize evaporation of the boron on contact with the precursor alloy melt, the boron compound may be wrapped in a metal foil formed from constituents of the precious metal alloy or master alloy. The cast precious metal allo y or master alloy has been found to have a reduced number of hard spots when compared to conventional casting methods.

Description

MASTER ALLOY MANUFACTURING

FIELD OF THE INVENTION

This invention relates to a method of making a master metal incorporating small quantities of boron therein e.g. as a deoxidant or grain refiner and useful in the manufacture of gold alloys.

BACKGROUND TO THE INVENTION

Goldl jewellery alloys are frequently worked into complex ornamental shapes. To withstand extensive working without fracture, the alloys require high ductility and high strength. High ductility and high strength are facilitated by an alloy having a low oxygen content and a fine grain structure.

Boron is known to both deoxidize and refine the grain of goldl alloys. When boron scavenges oxygen from a melt and other oxides in the melt, it cleanses surfaces of the metal. U.S. Patent No. 5,384,089 (Diamond; incorporated by reference in its entirety) discloses the use of boron as a deoxidizer for gold-base alloys. This patent discloses that boron causes hard spots. Throughout this patent application, all percentages are wt%, unless otherwise specified.

A conventional method of introducing boron into a precious metal alloy or master alloy is through the use of 98 wt% Cu, 2 wt% B master alloy. However, the use of such a master alloy frequently introduces hard spots into the products. These hard spots are believed to be non-equilibrium phase CuB₂₂ particles that form in copper saturated with boron when cooled from the liquid phase to the solid phase. Hard spots can also form with other metal-boride compounds such as iron borides (for example Fe₅B₂ and FeB₂). The hard spots are frequently not detected until after the precious metal jewellery alloy is polished and inspected resulting in needless expense for the processing of ultimately unsatisfactory product. A further disadvantage with the use of a 98 wt% Cu, 2 wt% B master alloy is that for addition of a given mass of boron, it is necessary to add a relatively high mass of copper, which may not be desirable. There remains, therefore, a need for a more effective way to introduce boron as a grain refiner and oxygen/oxide scavenger into a master alloy menlt and via the master alloy into a gold alloy melt.

Cu-2wt%B master alloys are frequently contaminated with silicon. This silicon contamination may lead to brittleness as a result of the formation of brittle intermetallic compounds, oxides and low melting eutectics.

SUMMARY OF THE INVENTION

The invention provides a method for making a gold alloy or a master alloy, including the steps of:

(a) forming a master alloy consisting essentially of constituents other than gold of said gold alloy and inevitable impurities; and (b) melting together the master alloy and gold to form the desired gold alloy; wherein the step of forming the master alloy includes adding boron as a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides metal borohydrides metal borohalides, and mixtures thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMINTS

The following definitions are used throughout this patent application: Master Alloy - Constituents of a precious metal alloy omitting the predominant precious metal. For example, a yellow 10, 14 or 18 karat alloy may contain both silver and gold, only the gold would be omitted in the master alloy. Master alloys are usually sent to an end user who adds the required amount of precious metal.

Precursor Alloy - A composition slightly off specification for a desired master alloy or precious metal alloy. Gold alloys

The present method may be used for the treatment of fine gold. In addition, jewellery gold alloys, to which boron may be added by the methods described herein e.g. to improve fluidity when investment casting are disclosed, for example in US-A-5340529 (DeWitt et al; 10-18K alloys), 5180551 (Agarwal, 14K alloys); 5045411 (Taylor et al), 5749979 (Carrano et al, 14K), 6045635 (Ogasa, high-purity gold); 6406568 (Agarwal et al, 18K), 6676776 (Agarwal et al, 14K). US-A- 5384089 (Diamond) discloses yellow karat gold metal alloys particularly suited for the casting of jewellery articles and which include varying amounts of up to about 1 wt% Ge which serves as an oxygen scavenger. By varying the amounts of grain refiner, deoxidizing agents such as Si and B can be substantially eliminated. However, where melting is performed in an open non-graphitic crucible using a gas fired iurnace, there will be normally be significant free oxygen present and trace amounts of B or Si or both can be introduced with a limit of no more than 30 parts per million of B and up to 0.058% silicon in 14 karat yellow gold. US-A- 2002/0098108 (Taylor) discloses 22K gold alloys including silver, boron, cobalt and copper which are stated to meet modem jewellery fabrication techniques requirements and to exhibit pleasing yellow colour. These formulations possess work hardening properties consistent with gold alloys of lower karat while maintaining colour properties consistent with traditional 22K gold jewellery. In these alloys, boron is said to sharply decrease the rate of copper oxidation during melting by preferentially combining with oxygen over copper thus increasing the number of times the alloy may be reused prior to chemical refining. In comparison to the same alloy without boron, the boron containing formulation is also stated to have increased fluidity at temperatures above the melting point. Thus, casting operations can be conducted a lower temperatures than comparable 22 karat alloys. US-A-4804517 (Schaffer) discloses the use of boron as an oxygen scavenger for use in yellow dental alloys containing palladium, indium, silver and gold. US-A- 4557895 discloses a yellow gold alloy which resembles 10 karat gold in appearance and physical properties but has a gold content substantially less than 10 karat gold. The alloy consists essentially of 11% to 16% by weight gold, 28% to 34% by weight silver, 30% to 38% by weight copper, 8% to 12% by weight palladium, 7% to 10% by weight zinc, 0.2% to 0.4% silicon, and about 0.02% boron, and is said to be particularly well suited for use in the manufacture of rings and other jewellery articles. In the manufacture of the alloy, it is recommended to use a copper-boron alloy, e.g. an alloy containing about 2% boron. The disclosure of all the above specifications is incorporated herein for all purposes by reference.

The present process is typically used for making via master alloys gold jewellery alloys having at least 33% (8 karat) of gold with the balance being alloying elements including, but not limited to silver, nickel, copper and zinc as well as inevitable impurities. Most benefited by the process of the invention are those gold alloys having between 37.5 wt% gold (9K) and 77% gold (>18K) or even up to 22K and fine gold. Particular materials that might be made according to the invention using master alloys are:

24K gold

Au > 99.7 wt%, balance grain refining and/or hardening additives and impurities).

22 K gold , for example.

Au, 91.67 wt%, Ag 5 wt%, Cu 2 wt%, Zn 1.33 wt%)

18K gold, for example:

Au 75 wt%, Ag 20 wt%, Cu 5 wt%, Au 75 wt%, Ag 15 wt%, Cu 10 wt%, or

Au 75 wt%, Ag 13 wt%, Cu 12 wt%; or

Au 75 wt%, Ag 5 wt, Cu 20 wt%;

Au 75 wt%, Ag 2.75 wt%, Cu 22.25 wt%; or

Au 75 wt%, Cu 25 wt%; or Au 80%, Al 20%. or

Au 75%, Pt, Pd or Ag 25%;

Au75 wt%, Pd 10 wt%, Ni 10 wt%, Zn 5wt%; or Au 75 wt%, Fe 17 wt%, Cu 8 wt%; or Au 75 wt%, Cu 23 wt%, Cd 2 wt%.

14K gold, for example Au 58.33 wt$, Ag 24.78 wt%, Cu 26.75wt%, Zn 0.14 wt%; or

Au 58.33 wt%, Ag 4.00 wt%, Cu 31.24 wt%, Zn 6.43 wt%, Ni 0.10 wt%, Fe

0.05 wt%, Si 0.01 wt%; or

Au 58.33 wt%, Ag 2.08 wt%, Cu 39.59 wt%.

1OK gold, for example

Au 41.70 wt%, Ag 11.66 wt%, Cu 40.81 wt%, Zn 5.83 wt%, Si 0.03 wt%,

B 0.02 wt%; or

Au 41.70 wt%, Ag 5.50 wt%, Cu 43.80 wt%, Zn 9.00 wt% ; or

Au 41.70 wt%, Ag 2.82 wt% Cu 55.48 wt%.

Master alloys for gold

Master alloys containing the non-gold ingredients of any of the above mentioned alloys may be treated with a decomposable boron-containing compound to introduce boron in desired quantities. The master alloy into which boron is to be introduced is e.g. an AgCu alloy, AgCuZn alloy, CuZn alloy or AgZn alloy optionally containing one or more incidental ingredients selected from Ni, Fe and Si.

Incorporation of boron

The boron compound may be introduced into molten master alloy in the gas phase, advantageously in admixture with a carrier gas which assists in creating a stirring action in the molten alloy and dispersing the boron content of the gas mixture into said alloy. Suitable carrier gases include, for example, hydrogen, nitrogen and argon. The gaseous boron compound and the carrier gas may be introduced from above into a vessel containing molten master alloy e.g. a crucible in a melting furnace, a casting ladle or a tundish using a metallurgical lance which may be a elongated tubular body of refractory material e.g. graphite or may be a metal tube clad in refractory material and is immersed at its lower end in the molten metal. The lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep into the molten master alloy. Alternatively the boron-containing gas may be introduced into the molten master alloy from the side or from below e.g. using a gas-permeable bubbling plug or a submerged injection nozzle. For example, Rautomead International of Dundee, Scotland manufacture horizontal continuous casting machines in the RMK series for continuous casting. The alloy to be heated is placed in a solid graphite crucible, protected by an inert gas atmosphere which may for example be oxygen-free nitrogen containing <5 ppm oxygen and <2 ppm moisture and is heated by electrical resistance heating using graphite blocks. Such furnaces have a built-in facility for bubbling inert gas through the melt. Addition of small quantities of thermally decomposable boron-containing gas to the inert gas being bubbled through the melt readily provides a desired few ppm or few tens of ppm boron content The introduction of the boron compound into the master alloy as a dilute gas stream over an period of time, the carrier gas of the gas stream serving to stir the molten master alloy, rather than in one or more relatively large quantities is believed to be favourable from the standpoint of avoiding development in the resulting gold alloy of boron hard spots.

Compounds which may be introduced into molten master alloy in this way include boron trifluoride, diborane or trimethylboron which are available in pressurised cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being preferred because apart from the boron, the only other element is introduced into the alloy is hydrogen. A yet further possibility is to bubble carrier gas through the molten master to effect stirring thereof and to add a solid boron compound e.g. a higher borane into the fluidized gas stream as a finely divided powder which forms an aerosol. The boron compound may also be introduced into the molten master alloy in the liquid phase, either as such or in an inert organic solvent. Compounds which may be introduced in this way include alkylboranes or alkoxy-alkyl boranes such as triethylborane, tripropylborane, tri-«-butylborane and methoxydiethylborane which for safe handling may be dissolved in hexane or THF. The liquid boron compound may be filled and sealed into containers of silver or of copper foil resembling a capsule or sachet using known liquid/capsule or liquid/sachet filling machinery and using a protective atmosphere to give filled capsules sachets or other small containers typically of capacity 0.5-5 ml, more typically about 1-1.5 ml. As an alternative, especially for gold casting, the capsules or sachets may be of a polymer e.g. polyethylene or polypropylene. The filled capsules or sachets in appropriate number may then be plunged individually or as one or more groups into the molten master alloy. A yet further possibility is to atomize the liquid boron-containing compound into a stream of carrier gas which is used to stir the molten master alloy as described above. The droplets may take the form of an aerosol in the carrier gas stream, or they may become vaporised therein.

The boron compound may also be introduced into the molten master alloy in the solid phase, e.g. using a solid borane e.g. decaborane B₁₀H₁₄ (m.p. 100°C, b.p. 213°C) or by using a metal borohydride or borohalide, e.g. sodium borohydride.

A precursor melt of master alloy is formed by melting appropriate amounts of the alloying elements in a suitable crucible. For gold master alloys a suitable crucible is ceramic, clay-graphite, iused silica, silicon carbide, graphite and zirconia may also be used. The metals are heated to a temperature effective to fully liquefy and flow the mixture. The melting temperature influences the kinetics of boron evaporation which determines the final boron concentration in the cast master alloy. The selected temperature should be sufficiently above the liquidus temperature of the master alloy to prevent freezing in a grain box during grain making. While the alloys are readily cast at atmospheric pressures, higher or lower pressures should not affect the benefits of the invention, but may affect the kinetics of boron evaporation. Once the master alloy melt is at the desired molten temperature, a boron containing compound is added as described above. It has surprisingly been found that e.g. when adding a decomposable boron compound such as sodium borohydride, an alkyl boron, boron hydride, boron halide or mixture thereof that more than 20 ppm of boron can be incorporated without the development of boron hard spots. This is advantageous because boron is rapidly lost from molten metal. It is therefore desirable to incorporate more than 20 ppm boron into master alloy and amounts of e.g. up to 50 ppm, typically up to 80 ppm, and in some instances up to 800 or even 1000 ppm may be incorporated. Relatively high boron content is desirable for master alloys which will be melted with gold to make casting grain and then further melted to form rod, wire, or investment casting. In an embodiment, sufficient boron is added so that an effective amount remains in the cast master alloy for effective grain refinement and deoxidation ofgold alloy made using said master alloy. Between 1 ppm and 1600 ppm boron remaining is effective. Preferably, the boron content is between 100 ppm and 1600 ppm.

To enable better mixing into the master alloy, the boron compound if in solid form may be wrapped in a thin metallic foil. The foil may be any constituent of the master melt or an inert material (i.e. a material which decomposes in the molten master alloy substantially without residue), such as paper or plastics sheet, and is preferably a ductile metal that may be formed into a relatively thin foil. Preferred metals for the foil include silver and copper. The foil preferably has a thickness of from about 0.01 mm to about 0.3 mm to enable the foil-wrapped boron compound to be well submerged in the master melt before the foil melts through releasing the boron compound. Once released, the constituents of the boron compound combine with oxygen in the master melt to effectively deoxidize the melt and the boron is believed to react (although the effectiveness of the invention does not depend on the accuracy of this theory) with some of the elements in the melt to form discrete insoluble particles dispersed throughout the master melt which act as nucleation sites promoting the formation of fine grains that are uniform in size and resist growth. When boron is added to molten master metal e.g. as diborane, the compound decomposes to boron and hydrogen e.g.

B₂H₆ -> 2B(s) + 3H₂ (g).

The hydrogen is effective to deoxidize the melt

To achieve a uniform casting, the boron may be dispersed throughout the master melt by stirring. Preferably, the boron is stirred for in excess of 1 minute and typically for from 1-5 minutes. Stirring may be by any means which does not contaminate the master melt such as with a graphite stirring rod or with a stream of carrier gas. The molten master alloy is then cast by a method suitable for forming a desired intermediate master metal product e.g. casting grain.

The resulting casting grain can then be re-melted with fine gold to make a range of jewellery and other products.

Claims

1. A method for making a gold alloy or a master alloy, including the steps of:

(a) forming a master alloy consisting essentially of constituents other than gold of said gold alloy and inevitable impurities; and

(b) melting together the master alloy and gold to form the desired gold alloy; wherein the step of forming the master alloy includes adding boron as a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides metal borohydrides metal borohalides, and mixtures thereof.

2. The method of claim 1, wherein the master alloy is an AgCu alloy, AgCuZn alloy, CuZn alloy or AgZn alloy optionally containing one or more incidental ingredients selected from Ni, Fe and Si.

3. The method of claim 1 or 2, including dispersing said boron compound into said master alloy by bubbling an inert carrier gas containing a gaseous hydride or halide of boron through said master alloy.

4. The method of claim 3, wherein said boron compound is one or more selected from boron trifluoride, diborane and trimethylboron.

5. The method of claim 1 or 2, wherein said boron compound is introduced into said master alloy melt in the liquid phase optionally in an inert organic solvent and optionally sealed into one or more containers of silver or copper foil.

6. The method of claim 5, wherein said boron compound is selected from the group consisting of triethylborane, tripropylborane, tri-«-butylborane, methoxydiethylborane and dispersions of any of them in hexane or THF.

7. The method of claim 1 or 2, wherein said boron compound is a higher borane that is solid at ambient temperatures or is a metal borohydride or borohalide.

8. The method of claim 7 wherein said boron compound is sodium borohydride.

9. The method of claim 7 or 8, further comprising the step of: (a) wrapping said boron compound in a metal foil selected to be one of said constituents of said precious gold alloy prior to dispersion in said master alloy melt, or (b) wrapping said boron compound an envelope or bag of paper or plastics prior to dispersion in said master alloy melt.

10. The method of claim 9, wherein said metal foil is selected to have a thickness of between 0.01 mm and 0.3 mm.

11. The method of claim 9 or 10, wherein said metal foil is selected to be copper or a copper-base alloy.

12. The method of any preceding claim, iurther comprising the step of transferring said precious metal alloy or master alloy to a grain box.

13. The method of any preceding claim, wherein sufficient boron is added to obtain a master alloy having, by weight, from 1 ppm to 1600 ppm of boron.