Process and plant for refining a metallic material including at least one impure precious metal

Abstract

A process for refining a metal material comprising at least one impure precious metal is described; the process comprises the steps of providing a metal material comprising at least one impure precious metal in a solid state; reducing said metal material into shavings; chemically or electrochemically dissolving the shavings of the metal material to obtain ions of the precious me tai; precipitating or depositing the precious metal thus refined. A plant for carrying out such a process is also described.

Description

[0001] PROCESS AND PLANT FOR REFINING A METALLIC MATERIAL INCLUDING AT LEAST ONE IMPURE PRECIOUS METAL

[0002] The present invention relates to a process for refining a metal material comprising at least one impure precious metal . The present invention further relates to a plant carrying out such a process .

[0003] The present invention finds useful use in the purification of at least one impure precious metal present in a metal material, whether scrap or new, in the form of bars or the like but also in other forms . For example, the present invention is applicable for the purification of gold or other precious metals in dore bars, where the latter commonly denote raw bars for example of a mining origin or from recycling, for example from "urban mines" that recycle Waste from Electrical and Electronic Equipment (WEEE) , mainly comprising precious metals but also other metals in various compositions, which are processed to obtain precious metals in high degrees of purity, higher than 99.5% .

[0004] It is known to refine materials containing precious metals with impurities to increase their degree of purity. It is in particular known to refine gold and silver alloys to obtain gold and silver with greater purity, as well as to refine platinum and palladium alloys .

[0005] To separate the precious metal from the impurity elements, the known processes in particular provide for dissolving with chemical reagents, for example acids, or electrochemically (electrolysis) the starting metal material so as to obtain ions of the precious metal, subsequently precipitating or depositing the precious metal in a solid form, with a greater degree of purity. For example, it is known to chemically refine gold alloys, for example from dore bars, with "aqua regia" , i . e . a mixture of hydrochloric acid and nitric acid. To carry out this process, the impure metal material to be refined is initially melted and poured into a vessel of water which is kept in motion by a pump to obtain flakes, so as to increase the surface available for the reaction, thus increasing the dissolution rate .

[0006] Disadvantageously, the process employing aqua regia allows the refining of gold alloys in which a maximum silver content of 10% by mass is present .

[0007] In fact, when the concentration of silver is greater than 10% by mass, it tends to react with the hydrochloric acid present in the aqua regia, creating silver chlorides that are solid, insoluble and that deposit on the flakes of metal material to be refined, stopping the reaction.

[0008] Other known processes provide for transforming the impure metal material to be refined, once melted, into the form of powder or microgranules instead of flakes, for example by atomization. In this case it is possible to refine gold alloys with a maximum silver content of about 12.5% by mass .

[0009] Disadvantageously, the presence of powders or microgranules introduces complications in the process, as the powders or microgranules tend to pack and stick to the surfaces of the atomization plant, causing metal losses and difficulties in washing and recovering.

[0010] The task underlying the present invention consists in making available a process for refining a metal material comprising at least one impure precious metal, and also a plant, which allow to optimize the dissolution in terms of increased reaction rate, lower concentration of chemicals and lower energy consumption.

[0011] A further obj ect of the present invention is to provide a process and a plant that allow a metal material to be made available in a form that is easy to handle, requiring less energy for movement, transfers and logistics compared to the known processes .

[0012] The obj ect of the present invention is also to provide a process and a plant that allow to increase the acceptability of the silver content in the alloy with respect to the known processes .

[0013] The task outlined above, as well as the purposes mentioned and others that will better appear later, are achieved by a process as recited in claim 1 and a plant as recited in claim 8 .

[0014] Other features are provided in the dependent claims .

[0015] A process for refining a metal material comprising at least one impure precious metal is described below.

[0016] In accordance with the present invention, the process comprises a first step of providing a metal material comprising at least one impure precious metal in a solid state .

[0017] The metal material can be scrap or new.

[0018] The metal material may be a bar, for example a dore bar, but also an ingot or other three-dimensional body. Such metal material may comprise an alloy of gold, but also of silver, or in any case an alloy containing other precious metals to be refined, such as for example copper, platinum, palladium, rhodium. The precious metal may then be, for example, one or more of gold, silver, copper, platinum, palladium or rhodium. Preferably, the process in accordance with the present invention applies to gold and silver defined as "pure" , i . e . respectively with gold content of between 99.5% and 99.99% by mass and with silver content of between 90% and 99.99% by mass, as well as to gold and silver alloys, as defined below.

[0019] The process in accordance with the present invention is applied for example, but not only, for the refining of a gold or silver alloy coming from a dore bar .

[0020] The process comprises a subsequent step of reducing said metal material into shavings .

[0021] In particular, this step provides for directly reducing the metal material in a solid state into shavings, starting from a three-dimensional piece and obtaining smaller shavings therefrom.

[0022] The shavings can have, for example, the following dimensions : a width of 2 mm to 30 mm, a thickness of 0.01 mm to 0.5 mm and a length (considering the unrolled, stretched or flattened shaving) of 5 mm to 200 mm.

[0023] The shavings can have variable dimensions depending on the material to be treated. For example, for pure metals such as gold with purity greater than 99% and silver with purity greater than 99%, which are soft and tendentially can be worked, a shaving produced by the process has a width of between 10 mm and 30 mm, a thickness of between 0.02 mm and 0.1 mm, and a length of between 10 mm and 100 mm.

[0024] On the other hand, for alloyed metals such as a dore bar, in which gold, silver, copper, platinoids and other impurities are present, which are harder and hard to work, a shaving produced by the process has a width between 5 mm and 10 mm, a thickness between 0.01 mm and 0.05 mm, and a length between 5 mm and 20 mm.

[0025] In the present description, the term shaving is understood to mean any product of various shapes, such as for example in the form of a curl or helicoid or even a flake, provided that it is finely chopped with a thin thickness ( from 0.01 mm to 0.5 mm) , obtained by mechanical processing for the removal of material, for example by means of milling machines, boring machines, grouging machines or automatic planing machines .

[0026] The process comprises a step of chemically or electrochemically (electrolysis) dissolving the shavings of the metal material thus obtained to obtain ions of the precious metal . This step is carried out after the step of reducing into shavings .

[0027] The process comprises a step of precipitating or depositing the precious metal thus refined. This step is carried out after the step of dissolving.

[0028] Preferably, the step of reducing into shavings is carried out by mechanical processing of the metal material in a solid state, by removing the material .

[0029] In accordance with a first embodiment, the step of dissolving occurs chemically by means of a chemical compound or mixture of chemical compounds, depending on the composition of the alloy to be dissolved. For example, if the alloy to be dissolved contains more gold, the step of dissolving preferably occurs by means of aqua regia . The step of dissolving can also occur, for example, with nitric acid if the alloy to be dissolved contains more silver .

[0030] Preferably the first embodiment of the process in accordance with the present invention is applied to metal materials with precious metal such as gold, i . e . pure gold as mentioned above, gold alloys or even dore bars .

[0031] For example, the gold alloys can be 18-carat or 14-carat gold alloys, for example with a gold content of 75%, a silver content of 20% and a copper content of 5% by mass, or with a gold content of 58%, a copper content of 30% and a silver content of 12% by mass, respectively.

[0032] The first embodiment of the present invention is in other words also applicable to metal materials with a gold content greater or higher than 50% by mass .

[0033] With the process in accordance with the first embodiment, it is in particular possible to refine gold alloys with a higher silver content than those known. For example, in accordance with the present invention the gold alloy may have a maximum silver content of 20-25% by mass .

[0034] In accordance with a second embodiment, the step of dissolving occurs electrochemically (electrolysis) . In such a case, the process comprises a further step of pressing / compacting the shavings to form an anode .

[0035] The step of pressing / compacting the shavings is, in accordance with an embodiment, in particular carried out after the step of reducing into shavings and before the step of dissolving. In accordance with this embodiment, this step of pressing / compacting the shavings occurs by means of a press and a mould, of size and shape appropriate to the process, in which the shavings are arranged .

[0036] Alternatively, the step of pressing / compacting the shavings is carried out after the step of reducing into shavings and simultaneously with the step of dissolving.

[0037] In accordance with this alternative, this step of pressing / compacting the shavings occurs by means of an anode-pressor suitably designed for the purpose, such as a container, for example cylindrical or parallelepipedshaped, in which the shavings are arranged, which is provided with electrical contacts and which is immersed in the electrolyte liquid.

[0038] Alternatively, it is possible to use a rotating tumbler in which the shavings are arranged, which is provided with electrical contacts and which is immersed in the electrolyte, or filled with electrolyte .

[0039] By means of such an anode-pressor or rotating tumbler, it is possible to simultaneously press / compact the shavings and dissolve them.

[0040] The step of pressing occurs at a compression force of 20 to 200 tons .

[0041] For example, in the case of shavings of metal materials with high purity, the step of pressing occurs at a compression force of 20 and 100 tons .

[0042] Instead, in the case of shavings of metal materials with alloyed metals, the step of pressing occurs at a compression force of 100 to 200 tons .

[0043] In accordance with the present invention, with the second embodiment, i . e . by electrochemical dissolution, it is preferably possible to refine gold alloys or even silver alloys .

[0044] In such a case, the gold alloy may have for example the following composition: up to 10% by mass of silver, up to 30% by mass of copper and the remainder of gold.

[0045] Instead, the silver alloy may preferably have a silver content higher than 90% by mass, preferably higher than 92% by mass .

[0046] In accordance with the second embodiment, the step of dissolving is carried out with the use of an electrolytic solution.

[0047] For gold alloys, the electrolyte solution is a solution of chloroauric acid and hydrochloric acid. Such a solution comprises for example HC1 in a content of between 6% and 14% by volume, preferably between 9% and 11% by volume, and the remainder of chloroauric acid.

[0048] Preferably the gold content in the electrolyte solution is of between 80 and 100 g / 1, preferably between 85 and 95 g / 1.

[0049] For silver alloys, the electrolyte solution is a solution of silver nitrate and nitric acid.

[0050] Such a solution comprises for example HNO3 in a content of between 2% and 10% by volume, preferably of between 5% and 7% by volume, and the remainder of silver nitrate .

[0051] Preferably the silver content in the electrolyte solution is of between 40 and 100 g / 1, preferably between 60 and 80 g / 1, more preferably between 65 and 75 g / 1.

[0052] In accordance with the first and second embodiments, the methods and conditions, not specified in the present description, of the steps of dissolving and precipitating or depositing are determinable for a person skilled in the art .

[0053] Advantageously, the shavings maximize the ratio between surface and volume, being thus easy to manipulate .

[0054] Said shavings have a surface per unit of mass higher than both flakes and powders . For example, the shavings of a gold alloy have a surface per unit mass of between 0.5 and 5 m2 / kg, depending on the production parameters of the shavings . Advantageously, the dissolution and therefore refining time is reduced, since the reaction rate is a linear function of the surface available for the reaction itself .

[0055] Advantageously, gold alloys with a maximum silver content of 20-25% by mass can also be chemically refined, and in particular with aqua regia .

[0056] Advantageously, the anodes consisting of compacted shavings have a porous structure, characterized by a high number of channels and passages . Therefore, the solid shape that is obtained (generally cylindrical or parallelepiped-shaped) after pressing or compacting the shavings, allows the passage of the electrolyte also through the anode itself, increasing the dissolution rate since the surface exposed to the electrolyte is greater than a conventional anode that does not have a porous structure .

[0057] In fact, the movement of the electrolyte in an electrolytic refining vessel, in the case of a conventional non-porous anode, is carried out by a recirculation pump, which moves the flow of the electrolyte inside the vessel and therefore around the anode, affecting only the outer surface of the anode .

[0058] Advantageously, replacing the conventional anode with an anode consisting of compacted shavings increases the dissolution rate in the process and also the acceptability percentage of silver, as the surface exposed to the electrolyte is increased compared to a conventional anode .

[0059] A further improvement of the performance can be obtained by producing a forced flow of electrolyte through the anode consisting of compacted shavings, for example by creating compulsory passages for the electrolyte; in this way the reaction rate is further increased, also improving all the other consequent advantages .

[0060] In addition, the step of making the shavings is a mechanical process, which does not require melting the material and therefore results in lower energy consumption and great ease of use .

[0061] That is, in the process according to the present invention, the metal material comprising at least one impure precious metal in the form of a solid, such as for example bars (dore bars) , ingots or the like, is subj ected to mechanical processing for the reduction of the same in the form of shavings without intermediate steps of melting the same .

[0062] A plant configured to carry out the process described above is also an obj ect of the present invention .

[0063] The plant comprises a mechanical processing unit configured to accommodate a metal material comprising at least one impure precious metal to be refined in a solid state . The mechanical processing unit is in particular configured to reduce the metal material in a solid state into shavings .

[0064] The plant comprises a reactor, located downstream of the mechanical processing unit .

[0065] The reactor is configured to chemically or electrochemically (electrolysis) dissolve the shavings of the metal material to obtain precious metal ions . The reactor is further configured to precipitate or deposit the precious metal thus refined.

[0066] In accordance with the first embodiment, the reactor is a chemical reactor .

[0067] In accordance with the second embodiment, the reactor is an electrochemical reactor .

[0068] In accordance with this second embodiment, the plant also comprises a presser configured to press / compact the shavings, in order to form an anode . The presser is placed upstream of the reactor or at the reactor, for example integrated into the reactor itself .

[0069] For example, the presser comprises a mould, for example in the shape of a cylinder or parallelepiped or other shapes, and a press configured to compact the shavings within the mould. To this end the press, for example, has a compression force of 20 to 200 tons .

[0070] For example, in the case of shavings of metal materials with high purity, the press has a compression force of 20 and 100 tons .

[0071] Instead, in the case of metal shavings with alloyed metals, the press has a compression force of 100 to 200 tons .

[0072] In accordance with the variant that provides for the presser integrated in the reactor itself, according to a first alternative, the presser comprises an anodepressor comprising a container, for example cylindrical or parallelepiped-shaped, configured to contain the shavings .

[0073] In accordance with a second alternative, the presser comprises a rotating tumbler configured to contain the shavings .

[0074] In accordance with these two alternatives, the reactor comprises a process vessel comprising an anode and at least one cathode and in which an electrolyte is recirculatable for example by means of a pump . In particular, in accordance with the first alternative (anode-pressor) the anode comprises the container configured to contain the shavings . The container is provided with electrical contacts connected to a galvanic power supply and adapted to carry an anodic current to the shavings . A plunger is slidable in the container . This container is configured to be immersed in the process vessel and then in the electrolyte .

[0075] The electrolyte is moved around the container by a recirculation pump in the process vessel (reactor) , or, preferably, is forced through the container . In this second case, the container is perforated to allow the electrolyte to pass .

[0076] In particular the bottom and / or one or more parts of the container is electrically conductive, for example the container comprises metal plates (electrical contacts mentioned above) . Preferably, the metal plates connected to the galvanic power supply are positioned on said plunger .

[0077] The electrical contacts, i . e . the metal plates, carry anodic current to the shavings when they come into contact with them, generating an electrical current that flows to one or more cathodes the process vessel (reactor) is provided with, generating the electrochemical process of electrolysis and electrodeposition .

[0078] The plunger is configured to be held pressed against the mass of shavings by pneumatic, electrical or mechanical means controlled in retro-action to the anodic current, in order to keep the mass of shavings pressed onto the electrical contact (s) (i . e . the metal plates) as the metal mass decreases due to dissolution. In accordance with the second alternative, the anode comprises the rotating tumbler configured to contain the shavings . In this case the rotating tumbler can be configured to be immersed in the electrolyte or filled with electrolyte .

[0079] The rotating tumbler is provided with electrical contacts that are connected to a galvanic power supply and adapted to carry anodic current to the shavings . When the tumbler rotates, the shavings inside it come into contact with the electrical contacts that carry anodic current, generating an electrical current that flows towards the one or more cathodes the process vessel (reactor) or the tumbler itself is provided with, generating the electrochemical process of electrolysis and electrodeposition.

[0080] The processes and the plants thus conceived are susceptible to numerous modifications and variants, without departing from the scope of the inventive concept; moreover, all details can be replaced with technically equivalent elements . In practice, any materials can be used according to requirements, as long as they are compatible with the specific use, the dimensions and the contingent shapes .

Claims

CLAIMS1. Process for refining a metal material comprising at least one impure precious metal, comprising the following steps :- providing a metal material comprising at least one impure precious metal in a solid state;- reducing said metal material into shavings;chemically or electrochemically dissolving said shavings of said metal material to obtain ions of the precious metal;- precipitating or depositing the precious metal thus refined .

2. Process according to claim 1, wherein said step of reducing said metal material into shavings occurs by mechanical processing of said metal material in a solid state .

3. Process according to any one of the preceding claims, wherein the shavings have a width of between 2 mm and 30 mm, a thickness of between 0.01 mm and 0.5 mm and a length of between 5 mm and 200 mm.

4. Process according to any one of the preceding claims, wherein said metal material comprises an alloy of gold, silver or in any case an alloy containing other precious metals to be refined, such as copper, platinum, palladium, rhodium; preferably coming from a dore bar .

5. Process according to any one of the preceding claims, wherein said step of dissolving occurs chemically.

6. Process according to the preceding claim, wherein said metal material comprises a gold alloy having a maximum silver content of 20-25% by mass .

7. Process according to any one of claims 1 to 4, wherein said step of dissolving occurs electrochemically, saidprocess comprising a further step of pressing / compacting the shavings to form an anode, said step being carried out after the step of reducing into shavings and prior to the step of dissolving.

8. Process according to any one of claims 1 to 4, wherein said step of dissolving occurs electrochemically, said process comprising a further step of pressing / compacting the shavings to form an anode, said step being carried out after the step of reducing into shavings and simultaneously to the step of dissolving.

9. Plant configured to carry out the process according to any one of the preceding claims, comprising:- a machining unit, configured to accommodate a metal material comprising at least one impure precious metal to be refined in a solid state and reduce it into shavings ;a reactor, placed downstream of said mechanical processing unit, said reactor being configured to chemically or electrochemically dissolve the shavings of said metal material to obtain ions of the precious metal, said reactor being configured to precipitate or deposit the precious metal thus refined.

10. Plant according to the preceding claim, comprising a presser configured to press / compact the shavings to form an anode; said presser being placed upstream of the reactor or at the reactor .

11. Plant according to the preceding claim, wherein said reactor comprises a process vessel comprising an anode and at least one cathode, an electrolyte being recirculatable in said vessel, wherein said anode comprises a container configured to contain shavings and provided with electrical contacts connected to agalvanic power supply and adapted to carry an anodic current to the shavings, a plunger being slidable in said container; or said anode comprises a rotating tumbler configured to contain the shavings and provided with electrical contacts connected to a galvanic power supply and adapted to carry an anodic current to the shavings .