Abstract:
The present application relates to a process for recovering precious metals from anode slime or granulated material through the use of an inductive system for the melting that uses an induction source and a furnace with a tilting system where the load or material to be melted is placed. The present invention further relates to the product recovered via such process.

Description:
RELATED APPLICATION 
       [0001]    The present application claims Paris Convention priority from Chilean Patent Application No. 3238-2007, filed 9 Nov. 2007, the content of which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present application relates to a process for recovering precious metals from anode slime or granulated material through the use of an inductive system for the melting that uses an induction source and a furnace with a tilting system where the load or material to be melted is placed. Although the present invention will be described hereinafter with reference to such an application, it will be appreciated that the invention is not restricted to this field of use. 
       BACKGROUND OF THE INVENTION 
       [0003]    Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. 
         [0004]    Normally, induction furnaces are used to melt metal materials—mainly “scrap” or waste coming from different processes that use these metals as their main raw materials. Every induction furnace is characterised by a power supply and a furnace. When it comes to a non-metallic and/or non-ferrous material, a special crucible must be added to the furnace to allow the transfer of power from the magnetic field to the load or to the material to be melted, in such a way that heat is exchanged. In this way, an indirect melting process is achieved, the efficiency of which depends, amongst other factors, on the material of the crucible and the material to be melted. The present invention relates to the recuperation of precious metals through the melting of the anode slimes, a by-product of the copper electrolysis process, and/or the melting of the granulated material from processes such as gold and silver metallurgy. Nowadays, the melting process of these kinds of materials is performed in furnaces that use fossil fuels, such as reverberatory and tilting rotary furnaces. 
         [0005]    The invention further relates to an efficient melting process, through the use of a self-resonant magnetic induction source, such as that described in U.S. Pat. No. 6,466,467 B2, and a furnace with a silicon carbide crucible (or its equivalent). 
         [0006]    In addition to heat exchange processes utilising conduction and convection present in the currently developed processes, the present invention provides an extra component from a magnetic field to the load or material to be melted. In addition, the self-resonance capacity of the induction source allows eddy currents in an already liquefied load to be generated. 
         [0007]    In addition, a relatively efficient process is obtained for melting carried out at a high altitude, which represents a perceptible advantage over melting processes that use fossil fuel. The use of a self-resonant magnetic induction source allows the generation of a relatively optimised process, independent of the load about to be melted. 
         [0008]    To obtain refined copper, there are two types of processes, namely, hydrometallurgical and pyrometallurgical processes. 
         [0009]    In the copper deposits of oxidised minerals, the copper recuperation process that follows crushing is divided into three main stages: heap leaching, solvent extraction and electrolysis. In the case of sulfurous copper minerals, after crushing and milling, the mineral is then subjected to the melting stage and then on to electric refining. 
         [0010]    In the case of copper electric refining, blister copper anodes are connected to a negative line, and high-purity electrolytic copper cathodes are connected to a positive line. The electrolytic solution is often a sulfurous copper II solution. The impure copper that represents the anodes is dissolved and then deposited on the pure copper cathode, with impurities settling to the bottom of the cell. With this, a waste referred to as “anode slime” is obtained, which comprises waste from noble metals, such as gold, silver, tellurium, selenium, etc. 
         [0011]    The metal waste in the “anode slime” can be recovered in a first stage through hydrometallurgical processes or through melting processes. The product of each of these processes can later be refined or it can go through other processes to obtain the constituent noble metals. 
         [0012]    Therefore, elements like gold, silver, platinum, palladium and others are obtained through selective leaching as a secondary product of the hydrometallurgy of large amounts of minerals that contain them in very low concentrations. After purifying the resulting solutions that contain them to obtain higher concentrations, the elements are reduced to the solid metallic state in the form of a very fine granulated material. However, as a result from the melting of the granulated material, and for a subsequent refining and partition, it is necessary to have the product as an alloy with different geometrical forms. The electrochemical partition of the melting product for each separate element such as copper, silver and gold must be carried out in order to use them in a direct way. 
         [0013]    The process of the present invention is part of the recovery through the melting of precious metals contained in the anode slime, a secondary product of the electrorefining process of copper and other metals, and through the melting of the granulated material resultant of the gold and silver metallurgy. 
         [0014]    The recovery through the melting of metal elements in the “anode slime”, and the melting of the granulated material that result from gold and silver metallurgy, is carried out normally in furnaces that use fossil fuels such as coal, oil and/or gas. These types of furnaces are characterised by reverberatory furnaces and “Kaldo” or tilting rotary furnaces (for example, the TROF conversion process of Outokumpu). 
         [0015]    The reverberatory furnace is a generally rectangular type of furnace, covered by a firestone vault and with a chimney that reflects the heat produced in the combustion chamber to the location that contains the material about to be melted. 
         [0016]    A tilting rotary furnace contains a cylindrical container that can rotate and tilt vertically. Fuel, air and oxygen are injected through a primary lance to accomplish the melting of the anode slime inside the container, and a secondary lance injects air to the surface of the melted material for the refining. 
         [0017]    The use of these types of furnaces has many disadvantages, among them:
       They produce a large amount of flumes, due to the combustion of the fossil fuels.   A relatively low process efficiency is obtained, particularly for the application at a high altitude.   Due to the radiant warming method and the convection of up to 1200° C. (2192° F.), the time required to be invested in each load is extreme.       
 
         [0021]    It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. 
         [0022]    Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. 
         [0023]    Although the invention will be described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 
       SUMMARY OF THE INVENTION 
       [0024]    According to a first aspect of the present invention there is provided a process for recovering one or more precious metals from anode slime or granulated material through the use of an inductive system for the melting of said anode slime or granulated material, said process employing an induction source and a furnace with a tilting system where the load or material to be melted is placed, said process comprising the steps of:
       a) using an induction source wherein the frequency is adjusted cycle-to-cycle to the resonant frequency of the load;   b) placing the load in a crucible associated with said furnace; and   c) applying three heat exchange processes to the load: conduction, convection and magnetic induction.       
 
         [0028]    In an embodiment, said one or more precious metals are selected from the group consisting of: gold, silver, tellurium, selenium, platinum, palladium and mixtures thereof. 
         [0029]    In another embodiment, said anode slime or granulated material can be directly refined by adding oxygen lances or oxygen gas. 
         [0030]    In another embodiment, said crucible comprises silicon carbide or another material with similar properties. 
         [0031]    In another embodiment, said material to be melted comprises anode slime or another type of material with metal elements. 
         [0032]    In another embodiment, said material to be melted comprises a non-metallic and/or non-ferrous material. 
         [0033]    According to a second aspect of the present invention there is provided precious metals from anode slime or granulated material, when recovered by a process according to the first aspect of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  (prior art) shows a cell layout for a copper electrorefining process. 
           [0035]      FIG. 2  (prior art) shows a diagram of a reverberatory furnace. 
           [0036]      FIG. 3  (prior art) shows a diagram of a tilting rotary furnace. 
           [0037]      FIG. 4  (prior art) shows the circuit of U.S. Pat. No. 6,466,467 B2, for the control of a self-resonant converter through the current injection. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    A preferred embodiment of the invention is described, by way of example only, with reference to the accompanying drawings. 
         [0039]    The process provides the melting of dry anode slime or of granulated material in an induction furnace having the following characteristics:
       a self-resonant magnetic induction source; and   a furnace provided with a silicon carbide crucible.       
 
         [0042]    In the case of the recovery of precious metals starting with the anode slime, the first stage of the process consists of extracting the anode slime from the bottom of electrorefining cells, to be later filtered and transported to a furnace to be dried. After this stage, the resulting material is referred to as “calcina” or “calcine”. 
         [0043]    Subsequently, the calcina/calcine is loaded into the silicon carbide crucible and mixed with flux inside the furnace that feeds the self-resonant magnetic induction source. Once the material is loaded into the crucible, a magnetic induction source must be turned on to generate the variable electromagnetic field for a period that will allow the melting of the load inside the crucible. 
         [0044]    At the beginning of the melting process, the magnetic field generated by the induction source will generate eddy currents or stray currents in the crucible, so an indirect heat exchange process to the load will be generated, i.e., the crucible will be heated due to the losses produced by the eddy currents, and through the conduction the heat exchange process to the calcina/calcine will be achieved. 
         [0045]    As soon as a stage of melted metal inside the crucible is created, due to the beginning of the melting of the calcina/calcine, the magnetic field generated by the induction source will induce eddy currents in the crucible and in such stage of melted metal. Therefore, once a melted metal stage inside the crucible is obtained, the heat exchange between the induction source and the load will be carried out in three processes:
       conduction: between the walls of the crucible and the calcina/calcine, and between the stage of melted metal and the calcina/calcine.   convection: in the melted metal stage.   electromagnetic: between the variable magnetic field through the self-resonant magnetic induction source and the melted metal stage.       
 
         [0049]    Because the magnetic induction source operates to the resonant frequency of the load formed by the crucible and the calcina/calcine during any of its stages, the electromagnetic heat exchange process may be optimised at every moment and in any operation condition (e.g. temperature, pressure, and humidity). 
         [0050]    Because of the melting of the calcina/calcine, the density of the material inside the crucible increases with time, so the volume of the load decreases and it is necessary to load the crucible again with the calcina/calcine mixed with the flux in several stages. The number of times that it is necessary to load the crucible depends mainly on the density of the metal material present in the anode slime. Once the melted material covers the entire capacity of the crucible and the load reaches the ideal temperature, the slag present in the crucible that operates the rotation system of the furnace is extracted. The slag is loaded in deposits of the correct material (e.g., iron), generally of a conical shape, for its accumulation and later processing, destined to recover the metal waste dragged by the slag. 
         [0051]    Subsequently scorifier material is added to refine the melted metal stage contained in the crucible, which has the largest deposit of the precious metals present in the anode slime. Finally, through operation of the furnace rotation system, the metal is extracted from the crucible to form ingots or anodes for subsequent electrorefining. 
         [0052]    When it comes to the recovery of precious metals from granulated material, the aforementioned process is also followed. 
         [0053]    The use of the self-resonant magnetic induction source has the following advantages over the processes of the prior art:
       The only fumes generated are due to the melting of the anode slime or of the granulated material.   The efficiency of the process is independent of the height the team works at.   Relatively high process efficiency, because of the facility to adjust the resonant frequency from cycle-to-cycle.   There is no lower limit to the initial purity or yield of the starting material to be melted.   The time period for the melting is reduced remarkably.       
 
         [0059]    Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 
         [0060]    All references cited in this specification are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference.