Patent Application
20180142327
The invention relates to a pyrometallurgical converting process for the production of a PGM-enriched alloy (PGM=platinum group metal) and to a gas lance which can be used in such process.
The abbreviation “PGM” used herein means platinum group metal.
In general, the enrichment of PGMs by means of pyrometallurgical converting is well-known, see, for example, S. D. MCCULLOUGH, Pyrometallurgical iron removal from a PGM-containing alloy, Third International Platinum Conference ‘Platinum in Transformation’, The Southern African Institute of Mining and Metallurgy, 2008, pages 1-8.
The process of the invention is a pyrometallurgical converting process which employs a gas-cooled gas lance for the supply of oxidizing gas.
The process of the invention is a process for the production of a PGM-enriched alloy comprising at least one PGM selected from the group consisting of platinum, palladium and rhodium. The process comprises the steps:
The term “tube” is used herein in connection with the gas-cooled gas lance. To avoid misunderstandings, any tube forming part of the gas-cooled gas lance may have a cross-section other than a circle. It may, for example, have a triangular, square or hexagonal cross-section. However, a circular cross-section is typical and preferred.
“0 wt.-%” or “0 vol.-%” appears several times in the description and the claims; it means that the respective component is not present or, if present, it is at best present in a proportion of no more than at a technically inevitable impurity level.
The process of the invention is a process for the production of a PGM-enriched alloy comprising one or more PGMs selected from the group consisting of platinum, palladium and rhodium. It is preferred that the PGM-enriched alloy produced by the process of the invention comprises iron, for example >0 to 60 wt.-% of iron, and one or more of said PGMs, for example 20 to <100 wt.-% of one or more of said PGMs. The PGM-enriched alloy made by the process of the invention may comprise nickel and copper. Examples of other elements (elements other than iron, nickel, copper, platinum, palladium and rhodium) which may be comprised by the PGM-enriched alloy made by the process of the invention include, in particular, silver, gold, aluminum, calcium, phosphorus and silicon.
In step (1) of the process of the invention a PGM collector alloy is provided.
PGM collector alloys are well-known to the person skilled in the art; they may typically be formed during pyrometallurgic recycling of appropriate PGM containing waste material like, for example, PGM containing waste catalysts, for example, used automotive exhaust catalysts. In the course of such pyrometallurgic recycling the PGMs are separated by smelting the PGM containing waste material, for example, ceramic supports having a PGM containing washcoat (like used automotive exhaust catalysts) together with a collector metal like, for example, iron in an oven, a so-called smelter. The PGMs form a PGM collector alloy with the collector metal, which is separated from slag formed as by-product during smelting.
The PGM collector alloy provided in step (1) comprises collector metal and one or more PGMs selected from the group consisting of platinum, palladium and rhodium, for example, 2 to 15 wt.-% of said one or more PGMs. The collector metal may comprise one, two or more collector metals, for example, iron alone or iron and nickel. The PGM collector alloy may comprise, for example, 30 to 95 wt.-% of iron and 2 to 15 wt.-% of one or more PGMs selected from the group consisting of platinum, palladium and rhodium. In an embodiment, the PGM collector alloy may comprise 40 to 70 wt.-% of iron, 0 to 20 wt.-% of nickel and 5 to 15 wt.-% of one or more of said PGMs. Examples of one or more other elements (elements other than iron, nickel, platinum, palladium and rhodium) which may be comprised by the PGM collector alloy include silver, gold, copper, aluminum, calcium, silicon, sulfur, phosphorus, titanium, chromium, manganese, molybdenum and vanadium.
If the PGM collector alloy comprises silicon, there may be two variants. In a first variant the silicon content of the PGM collector alloy may be in the range of 0 to 4 wt.-%, in a second variant it may be in the range of >4 to 15 wt.-%.
In step (2) of the process of the invention a material capable of forming a slag-like composition when molten is provided.
The term “material capable of forming a slag-like composition when molten” used herein shall illustrate that the molten material looks and behaves like a slag. It shall at the same time express that it is not to be confused with the slag formed as by-product of the process of the invention, i.e. the slag obtained after conclusion of step (4). Moreover, the material capable of forming a slag-like composition when molten is not necessarily identical in composition with the one or more upper low-density molten masses formed in step (3), although it forms at least a predominant part of the latter.
The material capable of forming a slag-like composition when molten may have a composition such that the molten slag-like composition comprises or consists of magnesium oxide and/or calcium oxide, silicon dioxide, and the in each case optional components: iron oxide (in particular FeO), sodium oxide, boron oxide and aluminum oxide. The molten slag-like composition may comprise or consist of, for example, 40 to 90 wt.-% of magnesium oxide and/or calcium oxide, 10 to 60 wt.-% of silicon dioxide, 0 to 20 wt.-%, in particular 0 wt.-% of iron oxide, 0 to 10 wt.-% of sodium oxide, 0 to 10 wt.-% of boron oxide, and 0 to 2 wt.-%, in particular 0 wt.-% of aluminum oxide.
If the silicon content of the PGM collector alloy provided in step (1) is in the range of 0 to 4 wt.-%, it is expedient that the material capable of forming a slag-like composition when molten has a composition such that the molten slag-like composition comprises or consists of:
If the silicon content of the PGM collector alloy provided in step (1) is in the range of >4 to 15 wt.-%, it is expedient that the material capable of forming a slag-like composition when molten has a composition such that the molten slag-like composition comprises or consists of:
In an embodiment, and apart from said wt.-% proportions of silicon dioxide and magnesium oxide and/or calcium oxide, the material capable of forming a slag-like composition when molten has a composition such that the molten slag-like composition comprises no iron oxide, 0 to 10 wt.-% of sodium oxide, 0 to 10 wt.-% of boron oxide and no aluminum oxide.
The material capable of forming a slag-like composition when molten and, as a consequence thereof, also the molten slag-like composition itself does not comprise PGMs with the exception of technically inevitable impurities. However, if the latter is present its proportion should be low; preferably such proportion does not exceed, for example, 10 wt.-ppm in the material capable of forming a slag-like composition when molten.
The material capable of forming a slag-like composition when molten is a combination of substances and may comprise the afore mentioned oxides or only said oxides, however, this is not necessarily the case. It may instead or additionally comprise compounds capable of forming such oxides or oxide compositions when heated during formation of the one or more upper low-density molten masses. To name just a few examples of such type of compounds: carbonates are examples of compounds which may split off carbon dioxide and form the corresponding oxides when heated and melted during formation of the one or more upper low-density molten masses; silicates are examples of compounds which may form the corresponding oxides and silicon dioxide when heated and melted during formation of the one or more upper low-density molten masses; borates are examples of compounds which may form the corresponding oxides and boron oxide when heated and melted during formation of the one or more upper low-density molten masses.
In step (3) of the process of the invention the PGM collector alloy and the material capable of forming a slag-like composition when molten are melted within a converter until a multi- or two-phase system of a lower high-density molten mass comprising the molten PGM collector alloy and one or more upper low-density molten masses comprising the molten slag-like composition has formed or, in an embodiment, until a two-phase system of a lower high-density molten mass comprising the molten PGM collector alloy and an upper low-density molten mass comprising the molten slag-like composition has formed. The PGM collector alloy and the material capable of forming a slag-like composition when molten may be melted in a weight ratio of, for example, 1:0.2 to 1, preferably 1:0.2 to 0.6.
The converter is a conventional pyrometallurgical converter vessel or crucible furnace which allows for melting the PGM collector alloy and the material capable of forming a slag-like composition when molten. The converter has one or more openings at its top and it may have a cylinder- or pear-like shape, for example. Its construction may be such that it allows for a rotating and/or rocking movement to allow support of mixing of its contents. Preferably it is tiltable to allow for pouring out molten content thus enabling performing step (5) of the process of the invention. Its inner which has contact with the multi- or two-phase system of the lower high-density molten mass and the one or more upper low-density molten masses is of a heat-resistant material as is conventional for pyrometallurgical converter vessels, i.e. a material which withstands the high temperatures prevailing in process steps (3) and (4) and which is essentially inert towards the components of said multi- or two-phase system. Examples of useful heat-resistant materials include silica bricks, fireclay bricks, chrome-corundum bricks, zircon mullite bricks, zircon silicate bricks, magnesia bricks and calcium aluminate bricks.
In the course of step (3), first of all, the PGM collector alloy and the material capable of forming a slag-like composition when molten are introduced into the converter, either as premix or as separate components. The process of the invention is a batch process and it is preferred not to introduce the entire batch all at once and then to heat and melt the contents of the converter, but to introduce the materials to be melted portionwise and adapted to the melting speed. Once the entire batch has melted, said multi- or two-phase system of a lower high-density molten mass and the one or more upper low-density molten masses is obtained.
Heating of the converter contents in order to melt the latter and thus form the multi- or two-phase system means raising the temperature of the converter contents to, for example, 1200 to 1800° C., preferably 1500 to 1700° C. Such heating may be performed by various means either alone or in combination, i.e., for example, plasma heating, indirect electrical heating, arc heating, inductive heating, indirect heating with burners, direct heating with one or more gas burners from the above and any combination of said heating methods. Direct heating with gas burners capable of producing said high temperatures is a preferred method. Examples of useful gas burners include gas burners run with hydrogen or a hydrocarbon-based fuel gas and oxygen or nitrous oxide as oxidant.
After conclusion of step (3), i.e. once the multi- or two-phase system has formed, step (4) of the process of the invention is performed. In step (4) an oxidizing gas comprising or consisting of 0 to 80 vol.-% of inert gas and 20 to 100 vol.-% of oxygen, preferably 0 to 50 vol.-% of inert gas and 50 to 100 vol.-% of oxygen, in particular 0 vol.-% inert gas and 100 vol.-% of oxygen (i.e. oxygen gas) is contacted with the lower high-density molten mass obtained in step (3) until the latter has been converted into a lower high-density molten mass of the PGM-enriched alloy, i.e. until the PGM-enriched alloy, has formed. Any gas inert towards the lower high-density molten mass can be taken as the inert gas, in particular argon and/or nitrogen.
The contact with the oxidizing gas leads to an exothermic oxidation reaction in the course of which nonprecious elements or metals are converted into oxides and absorbed by the one or more upper low-density molten masses. This oxidation process of step (4) results in depletion of elements or metals other than the PGMs, in particular in depletion of iron and, if present, other nonprecious elements or metals within the lower high-density molten mass or, if taking the reverse view, in PGM enrichment within the lower high-density molten mass.
The contact between the oxygen or oxygen containing oxidizing gas and the lower high-density molten mass is made by passing or bubbling the oxidizing gas into the lower high-density molten mass by means of a gas lance the oxidizing gas exhaust of which being immersed into the lower high-density molten mass. The duration of the contact with the oxidizing gas or, in other words, the amount of oxidizing gas employed depends on when the PGM-enriched alloy of a desired composition has formed. In still other words, the contact with the oxidizing gas is maintained for such period of time, until a PGM-enriched alloy with a desired composition has formed; this will typically take 1 to 5 hours or 2 to 4 hours, for example. The development of the composition of the lower high-density molten mass during performance of step (4) until the PGM-enriched alloy of the desired composition has formed, can be tracked by standard analytical techniques, for example, XRF (X-ray fluorescence) analysis. As by-product an upper low-density molten slag is formed in the course of step (4).
The oxidizing gas exhaust of the gas lance is immersed into the lower high-density molten mass thus enabling passing or bubbling the oxidizing gas leaving the exhaust into the lower high-density molten mass. There is no need to immerse the exhaust deeply into the the lower high-density molten mass. Rather, it is preferred to choose an only low immersion depth in the range of from, for example, >0 to 10 cm.
The gas lance comprises an inner tube for the supply of oxidizing gas, i.e. for the supply thereof into the lower high-density molten mass. The inner tube is surrounded by an outer tube. Preferably it is equidistantly surrounded by the outer tube. Both tubes together, i.e. the inner tube and the outer tube surrounding the inner tube form a hollow space between themselves, i.e. between the inner wall of the outer tube and the outer wall of the inner tube. It is preferred that the arrangement of the inner tube and the outer tube is symmetrical along the gas lance's length axis. The distance or equidistance between the inner wall of the outer tube and the outer wall of the inner tube may be in the range of, for example, 2 to 10 cm.
It goes without saying that the gas lance takes a non-horizontal orientation during the oxidizing gas supply of step (4). The orientation will typically be vertical or deviating from a vertical orientation by no more than 30°, for example. In other words, the gas lance has a bottom end and a top end.
The inner tube has a bottom opening comprising the oxidizing gas exhaust. In a simple embodiment, the bottom opening is the oxidizing gas exhaust. In another embodiment, the bottom opening may comprise the oxidizing gas exhaust, for example, in the form of a nozzle.
The inner tube has a top opening or an open top end. The oxidizing gas is fed into the inner tube at its top opening or at its top end opening and leaves the tube through the exhaust at the bottom into the lower high-density molten mass.
The hollow space has a closed bottom but it has a top opening. Examples of bottom closing means include a closed bottom as such, a lid, a cover and the like. Hence, the outer tube and the inner tube together form a double-walled tube with a closed bottom arranged around the inner tube's bottom opening. Said closed bottom may be located either at the level of the inner tube's bottom opening or closely recessed therefrom, for example, with a recess of ≤2 cm into top direction.
The hollow space comprises an arrangement of tubes for a supply of cooling gas to the bottom region of the hollow space. The cooling gas outlets at said tubes' bottom ends are located close to the bottom region of the hollow space, for example, no more than 1 to 10 cm from the inner wall of the hollow space's bottom. It is preferred that the arrangement of tubes for the cooling gas supply is symmetrical along the gas lance's length axis. The arrangement of tubes for the cooling gas supply may comprise 2 to 10 tubes, for example. The arrangement of tubes for the cooling gas supply to the bottom region of the hollow space makes the gas lance gas-coolable, i.e. it gives the gas lance the capability of being cooled by a cooling gas like air during step (4). It goes without saying that the arrangement of tubes does not fully consume the hollow space; rather, it leaves enough space for the cooling gas to escape the hollow space at its top opening.
Those parts of the gas lance coming into contact with the already mentioned lower high-density molten mass and the one or more upper low-density molten masses are made of stainless steel. This is at least true for the outer tube of the gas lance, the hollow space's bottom and the oxidizing gas exhaust. However, typically and preferably, also the inner tube and preferably also the tubes of said arrangement of tubes for the cooling gas supply are made of stainless steel.
The size of the gas lance or the size of its parts, of course, depends on the process' batch size and the shape and dimensions of the converter. To illustrate, in case of a converter with a total inner volume of, for example, 500 liters, the length of the gas lance from its top end to its bottom end may be in the range of from, for example, 1 to 3 meters.
It is an essential feature of the process of the invention that the gas lance is capable of being cooled by a cooling gas like air. Hence, the gas lance is cooled during step (4) by means of cooling gas supplied to the bottom region of the hollow space via said arrangement of tubes. In other words, the cooling gas is fed into the tubes of said arrangement of tubes and flows in downward direction to said hollow space's bottom region. The cooling gas is typically supplied in the form of compressed air of ambient temperature. The cooling gas is typically supplied with a flow rate allowing for the stainless steel parts of the gas lance to be cooled below the stainless steel's softening temperature, i.e. to a temperature of, for example, below 1200° C., or, in particular, to a temperature in the range of, for example, 800 to <1200° C.
In an embodiment of the process of the invention, the oxidizing gas and the cooling gas have the same composition. In such embodiment air may be used as oxidizing gas and as cooling gas.
Once the cooling gas has left the cooling gas outlets of the tubes of said arrangement of tubes and has performed its cooling task, it escapes hot at the top opening of the hollow space, typically into the atmosphere, for example. It goes without saying that the main cooling effect of the cooling gas is developed in the bottom region of the hollow space, i.e. in that region where cooling is most required. It is an advantage that the strongest cooling effect is achieved at the bottom end of the gas lance, where the highest temperatures due to the oxidation develop.
It has turned out that the afore disclosed gas-cooling measure enables the use of a mechanical stable gas lance which has a satisfactory resistibility against the challenging conditions prevailing during step (4) in terms of preventing a weakening or an unacceptably fast destruction of the gas lance under said conditions. In other words, the service-life of the expensive gas lance can be extended compared to running the pyrometallurgical process with a similar gas lance without gas-cooling. It is believed, although not confirmed by tests or detailed investigation, that the gas-cooling does not only simply cool the gas lance. More precisely, it is believed, that the gas-cooling does not only cool the gas lance's outer surface facing the conditions prevailing in step (4). It appears that the gas-cooling allows for the formation of a layer of refractory material on said outer surface, for example, in the form of a crust of oxidic material comparable with a thin crust of solidified slag. It is believed that such crust behaves as a thermal insulator and/or as a protective shield against the chemical environment the outer surface of the gas lance is exposed to during step (4).
After conclusion of step (4), i.e. once the PGM-enriched alloy of the desired composition has formed, the gas-cooled gas lance is removed from the converter contents and step (5) of the process of the invention is performed. In said step (5) the upper low-density molten slag formed in the course of step (4) is separated from the lower high-density molten mass of the PGM-enriched alloy making use of the difference in density. To this end, the content of the converter is carefully poured out making use of the well-known decantation principle. Once the upper low-density molten slag is decanted the lower high-density molten mass of the PGM-enriched alloy is poured into suitable containers.
Steps (3) to (5) of the process of the invention constitute a sequence of steps, in particular in direct succession. This needs to be understood in such sense that no further steps or at least no further fundamental steps are required or performed between or during said steps (3) to (5). Examples of optional non-fundamental steps are (i) the removal of part of upper low-density molten mass in the course of step (4) or (ii) addition of PGM collector alloy and/or material capable of forming a slag-like composition when molten in the course of step (4).
After conclusion of step (5) subsequent step (6) is performed, in which the separated molten masses are allowed to cool down and solidify.
After solidification the solidified PGM-enriched alloy is collected in step (7). It may then be subject to further conventional refinement, for example, electrometallurgical and/or hydrometallurgical refinement in order to finally obtain the individual PGMs either as metal or as PGM compound or as a solution of the latter.
The PGM-enriched alloy collected in step (7) is distinguished by a relatively high PGM content. This relatively high PGM content means less effort and less consumption of chemicals with a view to said further refinement processes.
In view of the foregoing disclosure the skilled person will understand that the invention relates also to a gas lance according to any of its afore disclosed embodiments. Hence, the invention relates also to a gas-coolable gas lance comprising an inner tube for a supply of a gas A,
Such gas-coolable gas lance can be used in a pyrometallurgical converting process for the production of a PGM-enriched alloy from a PGM collector alloy, in particular, for a supply of oxidizing gas in such process, as disclosed in the foregoing. In said pyrometallurgical converting process the oxidizing gas corresponds to gas A and the cooling gas or cooling air corresponds to gas B.
The invention comprises the following embodiments:
2. The process of embodiment 1, wherein the PGM-enriched alloy comprises >0 to 60 wt.-% of iron and 20 to <100 wt.-% of the one or more PGMs.
A premix of 500 kg of a PGM collector alloy comprising 47 wt.-% of iron, 14.1 wt.-% of nickel, 8.1 wt.-% of silicon, 4.6 wt.-% of palladium, 3.2 wt.-% of chromium, 2.5 wt.-% of titanium, 2.2 wt.-% of platinum, 1.8 wt.-% of manganese, 0.6 wt.-% of rhodium and 0.9 wt.-% of copper, 123 kg of calcium oxide, 75 kg of silicon dioxide, 15 kg of sodium carbonate and 15 kg of borax was portionwise introduced into an already 1500° C. hot cylindrical natural gas-heated furnace and further heated to 1700° C. After a melting time of 10 hours a two-phase system of a lower high-density molten mass comprising the PGM collector alloy and an upper low-density molten mass comprising a slag-like composition was formed. Oxygen was introduced into the lower high-density molten mass via the exhaust of an air-cooled gas lance with an oxygen flow of 900 l/min.
The air-cooled gas lance had a 1.70 meters long inner stainless steel tube with an inner/outer diameter of 0.5 cm/2.5 cm for the oxygen gas supply. The inner tube was equidistantly surrounded by an outer stainless steel tube (1.70 meters long, inner/outer diameter of 7.5 cm/8.0 cm) forming a hollow space between the inner wall of the outer tube and the outer wall of the inner tube.
The inner tube's bottom opening served as oxygen gas exhaust. The hollow space had a closed bottom and an open top and it comprised a symmetrical arrangement of six stainless steel tubes (1.68 meters long, inner/outer diameter of 0.5 cm/2.5 cm) for cooling air supply to the bottom region of the hollow space. The six tubes were arranged such that their bottom openings had a distance of 2 cm from the inner bottom wall of the hollow space. Air-cooling was performed with a total air flow of 2500 l of air of 20° C. per minute. The cooling air was fed into the six tubes and left those at the tubes' bottom ends finally escaping the hollow space hot at its open top into the atmosphere.
After 2.5 hours the oxygen introduction was stopped and the gas lance was removed. The upper low-density molten mass was poured into cast iron slag pots in order to cool down and solidify. The lower high-density molten mass was then poured into graphite molds in order to cool down and solidify. After solidification and cooling down to ambient temperature both materials were analyzed by XRF.
