Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2001-04-18
2003-11-25
Kastler, Scott (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S741000
Reexamination Certificate
active
06652623
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention mainly relates to a method and to a kit for working up a gold bath or a gold alloy bath containing gold in the form of a gold sulphite complex.
2. Description of the Prior Art
For a number of purposes gold or gold alloys are separated from in particular aqueous solutions, which contain the gold and optionally further alloy metals, usually in the form of complexes (so-called baths) and this usually takes place galvanically, i.e. using current. In particular due to its low toxicity compared with other gold baths, e.g. gold cyanide baths, particularly in the field of dental galvanic technology gold or gold alloy baths have become widely used and contain the gold in the form of a gold sulphite complex.
It is desirable, e.g. in dental technology, to be able to work up the residual gold content remaining after separation in the bath and to reuse the gold obtained.
In this connection it has long been known to galvanically work up gold sulphite baths. However, for this it is necessary to have a galvanizing apparatus comprising electronics, heating system, galvanizing vessel, electrode system, stirrer, etc., as well as a process control suitable for residual gold separation. The gold still contained in the complex is separated at the cathode. It is possible to use the most varied cathode materials, such as e.g. a metal and in particular stainless steel rod, a metallized and in particular nickel-plated plastic sponge electrode, etc. The separated gold is usually very firmly connected to the electrode (metal rod) and must be removed therefrom mechanically, e.g. by scratching off or only following separation is it available in fine gold form (plastic sponge electrode). Particularly in the case of highly concentrated gold sulphite baths gold separation is very complicated or time-consuming and may even be completely impossible as a result of the apparatus. For example, it is then often difficult and costly to introduce and appropriately arrange the electrodes in the bath. Thus, frequently galvanic gold separation is not a suitable processing process. Another disadvantage of galvanic residual gold separation is that any alloying elements contained in the bath are also separated.
The problem of the invention is to permit a current-free working up of gold baths or gold alloy baths. It is in particular to be achieved that the reprocessing is easily performed, selective and quantitative. It must lead to pure gold, because no alloying metals contained are to be precipitated. The process must not represent a health hazard for the user and as a result of the ease of disposal of the reaction products must be ecologically unobjectionable and in particular inexpensive.
SUMMARY OF THE INVENTION
This problem is solved by a method having the features of claim 1 and a kit having the features of claim 14, together with the use of an oxidant according to the features of claim 18. Preferred developments are described in the dependent claims 2 to 13, 15 to 17 and 19. By reference the wording of all the claims is made into part of the content of the present description.
The aforementioned method is inventively characterized in that at least one oxidant is added, which oxidizes the sulphite contained in the complex and reduces the gold contained in the complex to its metallic form. Thus, the oxidant added firstly acts as an oxidizing agent for the sulphite and secondly as a reducing agent for the gold contained in the complex with a positive oxidation number. The sulphite is preferably oxidized to sulphate and the gold contained in the complex, preferably gold-(I), is reduced to its metallic form.
Within the sense of the present invention gold sulphite baths are understood to mean all preferably aqueous gold or gold alloy baths based on sulphite with or without additives or impurities.
As a result of the sulphite oxidation according to the invention the water-soluble gold sulphite complex is destroyed and the gold is essentially precipitated in pure form as a metallic gold precipitate. However, in the case of reductive processes in which a reducing agent is added for reducing the sulphite present in the complex and consequently the gold sulphite complex is destroyed, it is firstly possible for toxic, malodorous hydrogen sulphide (H2S) to form and secondly any alloying metals contained in the bath such as manganese can also be precipitated and contaminate the gold precipitate. The gold precipitate obtained can also be contaminated with gold sulphide. this is not the case in the method according to the invention, i.e. there are no unpleasant smells and the precipitated gold is in pure form.
It is possible with the method according to the invention to work up gold baths having an alkaline, neutral or acid pH-value. However, preferably working up takes place in an acid medium, particularly a weak acid medium (pH under 7 to 5). Preferred gold sulphite baths which can be used frequently have a slightly alkaline pH-value. Optionally it is possible to adjust the pH-value weakly acid by adding corresponding substances and optionally by the choice of the oxidant.
Fundamentally the method according to the invention makes it possible to oxidatively work up all known galvanic gold or gold alloy baths based on sulphite and having a gold content of >0 to 80 g/l, particularly 2 to 40 g/l, i.e. also highly concentrated gold sulphite baths.
The oxidant can be any compound able to oxidize the sulphite contained in the complex and also reduce the gold contained in the complex. Thus, the oxidant is classified in the electrochemical series between sulphite and gold. Thus, the normal potential of the oxidant must be more positive than that of sulphite and more negative than that of gold. The preferred oxidant is hydrogen peroxide (H
2
O
2
), which can be alone or combined with other oxidants. Hydrogen peroxide is miscible in any ratio with an aqueous gold sulphite bath. It can be used in any commercial concentration. In particular use is made of a 3 to 15 wt. %, preferably 5 to 10 wt. % aqueous hydrogen peroxide solution. Hydrogen peroxide addition offers the advantage that hydrogen peroxide is an oxidant containing no additional constituents, such as e.g. metallic and/or miscellaneous constituents, which lead to gold impurities and/or which are ecologically objectionable. Thus, no further metal constituents are precipitated during its decomposition reaction with the gold sulphite complex.
It is also possible to use a compound as the oxidant which, during the reaction, “liberates” hydrogen peroxide. This compound can be added alone or combined with other oxidants, preferably hydrogen peroxide itself. Corresponding compounds which can be added are so-called per or peroxo compounds.
It is e.g. possible to use per(oxo)borates, such as sodium perborate or calcium perborate or per(oxo)carbonates, such as sodium per(oxo)carbonate. In the method according to the invention use is more particularly made of e.g. peroxo disulphate, peroxo urea (urea-H
2
O
2
) or in particular potassium-peroxo-monosulphate (KPMS). Through the addition of potassium-peroxo-monosulphate it is possible to detect the end of the reaction, because this can easily be established in the gold bath, e.g. with potassium iodide starch paper. It is consequently possible to detect in the gold bath any undecomposed potassium-peroxo-monosulphate in excess, in order to establish whether the gold contained in the complex has been completely precipitated. It is consequently possible to work in substance-economizing manner. The use of KPMS also has the advantage that of its own accord a KPMS excess can be completely reduced after a short period of time, e.g. 1 to 2 hours at a pH of 6.
According to the invention the oxidants can be added either all at once or portionwise, i.e. in partial quantities, to the gold bath.
In the method according to the invention preferably several and in particular two oxidants are added. It is possible to add the oxidants together, i.e. in a single-stage process, but preference is g
Mzyk Waldemar
Ruebel Susanne
Kastler Scott
Meyer Jerald L.
Nath Gary M.
Nath & Associates PLLC
Wieland Dental + Technik GmbH & Co. KG
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