Patent Application
20140056786
The present invention relates to a device and a method for thermal treatment of noble metal-containing products which also contain fluorine aside from noble metals.
Various methods have become established for recovering noble metals from noble metal-containing products, such as, for example, catalysts or fuel cells. In the hydrometallurgical method, the noble metal-containing layer of catalysts is dissolved off the ceramic support by means of strong acids or bases. Subsequently, the noble metals are separated from the solution, for example through precipitation reaction.
In the pyrometallurgical method, the separation of noble metals proceeds through melting the noble metal-containing products in a metallurgical process. The ceramic fraction is transferred in a slag phase and tapped, the noble metals are alloyed into a collector metal, which is then also tapped and processed further.
The direct incineration of noble metal-containing sludges and multi-element waste materials, as described in DE 31 34 733 C2 and WO 99/037823, is also known. The noble metal-containing ash thus obtained is then leached in order to recover the noble metals.
In thermal reprocessing, noble metal-containing products that also contain fluorine aside from the noble metals have proven to be a problem. Thermal treatment of these products produces hydrogen fluoride gas, HF. Said gas reacts with water in the ambient air to produce hydrofluoric acid.
Conventional thermal reprocessing plants, in particular ashing plants, comprise a thermal treatment chamber and an exhaust gas purification system. The thermal treatment chamber is a component of a furnace and is provided with an insulating lining made, for example, of fireclay bricks or a ramming mass. These differ in composition. However, all insulating linings comprise, inter alia, silicon dioxide SiO2 (glass) and calcium oxide CaO. These components are attacked even by small amounts of the hydrofluoric acid and hydrogen fluoride that are generated and thus are dissolved out of the insulating lining which reduces the service life of the furnace and thus of the plant.
Another problem related to hydrogen fluoride gas is the condensation of hydrofluoric acid at the supporting external steel shells. These are the shaping and mechanical load-bearing framework of the components of an ashing plant. Hydrogen fluoride gas can diffuse through pores in the insulating lining and, in the region of the steel shells, react with water from the ambient air to produce hydrofluoric acid. Condensation of hydrofluoric acid on the steel shells causes them to corrode which can render the entire plant unstable.
A method, in which the generation of HF is to be prevented, is disclosed in EP 1 478 042 A1. In this method, components of fuel elements and catalysts are mixed with inorganic additives. In the subsequent thermal treatment process, the hydrogen fluorides and other fluorine compounds are absorbed and chemically bound by the additive. For this purpose, an up to 100-fold excess of the additive is added to the hydrogen fluoride gas that is being generated. However, it has been evident that the absorption at the additive is insufficient or too slow in the case of materials releasing hydrogen fluoride already at low temperatures allowing some hydrogen fluoride gas to escape. Moreover, the additive occupies a fraction of the volume of the incineration space such that the quantity of material that can be processed is reduced.
It is therefore the object of the present invention to provide a plant for thermal reprocessing of noble metal-containing products that contain fluorine in addition to noble metals. The plant according to the invention shall allow all fluorine-containing products to be reprocessed, regardless of the volatility of the materials contained therein.
Another object of the present invention is to provide a method for enriching noble metals from fluorine-containing materials.
A first embodiment meets the object on which the present invention is based through an ashing plant for enriching noble metals from fluorine-containing materials, comprising a thermal treatment chamber (1) having a refractory insulating lining on the inside of the thermal treatment chamber (1) and an exhaust gas cleaning system,
whereby the refractory insulating lining is resistant to hydrofluoric acid and the exhaust gas cleaning system comprises at least one or more acid scrubber(s) (3, 4) and at least one alkaline scrubber (5).
Another subject matter of the present invention is an ashing plant for enriching noble metals from fluorine-containing materials, comprising a thermal treatment chamber (1) having a refractory insulating lining on the inside of the thermal treatment chamber (1) and an exhaust gas cleaning system,
whereby the refractory insulating lining has an aluminium oxide content of 85% by weight or more and the exhaust gas cleaning system comprises at least one or more acid scrubber(s) (3, 4) and at least one alkaline scrubber (5).
Moreover, the exhaust gas cleaning system according to the invention preferably comprises at least one or more thermal after-incineration chambers (2). Preferably, the exhaust gas cleaning system comprises one after-incineration chamber (2).
The thermal treatment chamber (1) is a component of a furnace, into which the materials to be processed are introduced. For this purpose, the thermal treatment chamber (1) comprises an opening for introduction of the corresponding materials. According to the invention, the thermal treatment chamber (1) can be operated with a sub-stoichiometric amount or with an excess of air. The inside of the thermal treatment chamber (1) can comprise devices for incineration of the fluorine-containing and noble metal-containing materials. This concerns, for example, grates for accommodation of troughs for incineration of solid materials. Liquid materials can be introduced into the thermal treatment chamber (1) and incinerated therein either batch-wise in troughs or continuously by means of corresponding dosing facilities.
The temperature on the inside of the thermal treatment chamber (1) usually is approx. 800° C. In this context, the refractory insulating lining is designed to be stable at this continuous temperature. Moreover, it is also resistant to temperature peaks of up to approx. 2,000° C. It is feasible according to the invention to heat the thermal treatment chamber (1) directly or indirectly. All means of heating known according to the prior art are feasible, for example gas and oil heating or electrical heating.
According to the invention, an acid scrubber (3, 4) is a scrubbing stage, in which exhaust gases from the thermal treatment chamber (1) are washed with water or with water acidified by the hydrogen fluoride gas to be washed out. According to the invention, an alkaline scrubber (5) is a scrubbing stage, in which the exhaust gases are washed with an alkaline agent.
Heating fluorine-containing materials in the thermal treatment chamber (1) in the ashing plant according to the invention produces exhaust gases that contain hydrogen fluoride gas. Since the thermal treatment chamber (1) is lined with the hydrofluoric acid-resistant insulating lining, the chamber is not attacked by the exhaust gases. In the exhaust gas cleaning system according to the invention, the exhaust gas is initially subjected to thermal reprocessing in a thermal after-treatment chamber (2) and then all hydrogen fluoride gas or hydrofluoric acid already formed is removed in the acidic and alkaline scrubbing stages (3, 4, 5) such that the exhaust gases are then harmless and can be guided to the outside, for example by means of a chimney.
The ashing plant according to the invention can provide further cleaning stages or cleaning agents for exhaust gas cleaning in order to remove, for example, soot, chlorine or nitrous gases from the exhaust gases. Pertinent cleaning agents or cleaning stages are described in the prior art.
According to the invention, the hydrofluoric acid-resistant insulating lining is resistant both to hydrogen fluoride gas and to hydrofluoric acid.
The noble metal-containing and fluorine-containing materials are placed in the thermal treatment chamber (1). The exhaust gases produced in the thermal treatment chamber (1) during thermal treatment can first be guided into a thermal after-incineration chamber (2). Preferably, said chamber is also provided with a hydrofluoric acid-resistant refractory insulating lining. Moreover, the ashing plant comprises an exhaust gas conduit (6) for guiding the exhaust gases out of the thermal treatment chamber (1). Preferably, the inside of said exhaust gas conduit (6) is also provided with a hydrofluoric acid-resistant refractory insulating lining.
Thermal treatment chamber (1), exhaust gas conduit (6), and the after-incineration chamber (2), which is preferably present, are the components of the ashing plant through which exhaust gas flows before hydrogen fluoride gas is removed from the exhaust gas in the acidic and alkaline scrubbers (3, 4, 5). Providing the thermal treatment chamber (1) as well as the after-incineration chamber (2), and the exhaust gas conduit (6) with a hydrofluoric acid-resistant refractory insulating lining increases the service life of the ashing plant according to the invention, since these components cannot be attacked by the hydrofluoric acid or hydrogen fluoride gas.
The refractory insulating lining of the present invention can be a ramming mass. Said ramming mass preferably has an aluminium oxide (Al2O3) content of 85% by weight or more, in particular of 88% by weight or more. Said insulating lining is stable at a working temperature and at a continuous temperature of approx. 800° C. However, it also withstands peak temperatures of up to approx. 2,000° C.
Ramming masses usually contain silicon dioxide (SiO2) and/or calcium oxide (CaO) in addition to aluminium oxide. These components are also present in conventional refractory insulating linings. These are dissolved by hydrofluoric acid, which destroys the insulating lining.
Surprisingly, it has been evident that an aluminium oxide fraction of 85% by weight or more, in particular of 88% by weight or more, being present in a ramming mass is sufficient to provide hydrofluoric acid resistance. The ramming masses according to the invention can also contain different fractions of calcium oxide and silicon dioxide as further components in addition to aluminium oxide. Despite the calcium oxide and/or silicon dioxide fraction of the ramming mass being up to 15% by weight, in particular up to 12% by weight, the ramming mass is not attacked by hydrofluoric acid. In this context, the relative content of calcium oxide and/or silicon oxide or their ratio with respect to each other is irrelevant. Moreover, the corresponding ramming mass is easy to process and easily adapts to the internal wall of the thermal treatment chamber (1), exhaust gas conduit (6), and after-incineration chamber (2).
In this context, it is feasible according to the invention that the thermal treatment chamber (1), the after-incineration chamber (2), and the exhaust gas conduit (6) comprise the same or different insulating lining(s).
Preferably, the thermal treatment chamber (1), the after-incineration chamber (2) and/or the exhaust gas conduit (6) further comprise an external lining. The external lining can be provided using materials that are known according to the prior art. Preferably, the external lining is a mineral fibre. The external insulation is surrounded by a steel plate. The steel plate fixes the external insulation in place and serves for stabilisation and shaping of the components of the ashing plant.
One problem during the incineration of fluorine-containing products is the generation of hydrofluoric acid that causes corrosion in the region of the supporting external steel shells. If hydrogen fluoride gas HF diffuses through pores to reach the space behind the temperature-resistant insulating lining in the reaction space, it reaches the external steel constructs part of which are load-bearing. Hydrogen fluoride gas can react with water from the ambient air to form hydrofluoric acid in this location. Hydrofluoric acid forms an azeotropic mixture with water at a concentration of 38.2% HF, whereby the boiling temperature of the azeotropic mixture is 112° C. If hydrofluoric acid condenses at the steel walls, it causes these to corrode.
The effect of having the external insulation is that the temperature at the steel shell, i.e. at the external steel wall of the plant components, does not drop below 120° C. In this context, the thickness of the external insulation is a function of the temperature profile on the inside of the corresponding component of the ashing plant. Condensation of hydrofluoric acid does not take place at a temperature of 120° C. Accordingly, if hydrogen fluoride gas were to diffuse through the refractory insulating lining towards the outside, no corrosion damage to load-bearing steel constructs is to be expected.
According to the invention, the thickness of the insulation on the thermal treatment chamber (1), exhaust gas conduit (6), and after-incineration chamber (2) can differ. However, it is feasible just as well that the insulation of all components is equal in thickness. Accordingly, thermal treatment chamber (1), exhaust gas conduit (6), and after-incineration chamber (2) can, for example, comprise an external insulation made of a mineral fibre at a thickness of approx. 10 cm.
In a preferred embodiment, the thermal treatment chamber (1) comprises a refractory insulating lining on the inside, whereby the thickness of the wall of the chamber plus the insulating lining is approx. 30 cm, and an external insulation with a thickness of approx. 10 cm.
According to the invention, the ashing plant preferably comprises at least one scrubber made of graphite (3) with a double-walled design. Said double-walled scrubber (3) preferably comprises a cooling system, in particular a water cooling system. The scrubber (3) can comprise a steel plate as external shell. The double-walled scrubber (3) comprises valves for feeding and discharging the coolant.
The coolant, for example water, flows between the external graphite wall and the external shell. This leads to indirect dissipation of heat from the exhaust gas via the scrubbing medium and the graphite walls.
Water has proven to be particularly well-suited as coolant. Water is inexpensive and easy to handle. Moreover, if there was any exposure to the exhaust gases, there would be no hazard of undesired chemical reactions occurring.
The thickness of the graphite walls preferably is in the range of 3 cm to 4 cm. It has been evident that this thickness is sufficient to maintain sufficient temperature and acid stability.
Water flows into the double-walled scrubber (3) as scrubbing agent for the exhaust gas. The water reacts with hydrogen fluoride gas HF in the exhaust gas and removes it by scrubbing. The scrubbing water containing the hydrogen fluoride gas in the form of a salt is then collected and subjected to disposal.
Graphite is characterised not only by acid resistance, but by high temperature resistance as well. The exhaust gases flow from the thermal after-incineration chamber (2) or the thermal treatment chamber (1) into an acid scrubber. The temperature of said exhaust gases is up to 1,000° C. It has been evident that the temperature resistance of graphite is sufficient in this context.
Preferably, the exhaust gas cleaning system according to the invention further comprises at least one single-walled scrubber (4) made of graphite. This scrubber also comprises a steel plate for its external shell. Preferably, the thickness of the graphite wall is in the range of 3 cm to 4 cm.
In a preferred embodiment, the exhaust gas cleaning system comprises a double-walled scrubber made of graphite (3) and a single-walled scrubber made of graphite (4). This embodiment is shown in
In the first double-walled scrubber (3), at least a majority or all of the hydrogen fluoride gas is washed out of the exhaust gas with water. Moreover, the exhaust gas is also being cooled. In the second single-walled scrubber (4), hydrogen fluoride gas that may still be present is bound and thus removed from the exhaust gas. No further cooling of the exhaust gases is needed.
The ashing plant of the present invention further comprises an alkaline scrubber (5). The exhaust gases are guided from the at least one acid scrubber (3, 4) into said alkaline scrubber after most or all of the hydrogen fluoride has been removed. The alkaline scrubber (5) can comprise a coating on its inside that is resistant to alkaline scrubbing water and any traces of hydrogen fluoride gas that may still be present in the exhaust gas. Specifically, the coating is stable when exposed to bases having a pH of at least 10 or more, in particular of at least 11 or more. Preferably, the alkaline scrubber (5) comprises on its inside a coating made of a plastic material, in particular made of polypropylene. The external shell of the alkaline scrubber can consist of steel.
A plastic coating has proven to be easy to handle. The lining is made to be homogeneous and is not associated with a risk of cracks. Moreover, plastic materials, in particular polypropylene, are resistant to alkaline scrubbing water as is used in this scrubbing stage. If any residual HF were still to be present at this stage, the coating would not be attacked by it.
The exhaust gas guided into the alkaline scrubber (5) is largely free of hydrogen fluoride gas. However, it cannot be excluded that some traces of hydrogen fluoride gas may still be present. If the alkaline scrubber (5) were lined with the otherwise common glass fibre-reinforced plastic materials (GFR), these residual amounts of hydrogen fluoride gas would be sufficient to attack and quickly etch away the glass fibres in the plastic material. The internal lining would have to be replaced after just a short time under these conditions.
The ashing plant according to the invention can further comprise a control unit. The control unit controls the temperature profiles needed during the thermal treatment depending on the specific material and also controls the exhaust air line as a function of negative pressure, temperature, and oxygen content of the exhaust gas. As a matter of principle, both continuous and discontinuous operation are feasible. A discontinuous operation is preferred.
In a further embodiment, the present invention comprises a method for enriching noble metals from fluorine-containing materials, comprising a thermal treatment of the materials in a thermal treatment chamber (1) having a hydrofluoric acid-resistant refractory insulating lining and cleaning of the exhaust gases generated during the thermal treatment, whereby the cleaning comprises the following steps in the following order:
A noble metal-containing ash is generated during the thermal treatment of the noble metal-containing and fluorine-containing materials. Said ash is then reprocessed according to wet chemical methods known according to the prior art in order to recover the noble metals it contains. In the scope of the present invention, noble metals are gold, silver, and the metals of the platinum group.
The thermal treatment usually proceeds at a temperature of up to 800° C. Peak temperatures of up to approx. 2,000° C. may occur briefly. Following the introduction of the materials into the thermal treatment chamber (1), the temperature is increased slowly up to a temperature of approx. 600° C. to 800° C. The specific temperature depends on the materials to be processed.
The exhaust gases generated during the thermal treatment are first subjected to thermal after-incineration in an after-incineration chamber (2), if applicable. Subsequently, the exhaust gases are washed with water or an acid in an acid scrubber. According to the invention, water is used as the scrubbing agent. Water washes hydrogen fluoride gas out of the exhaust gas. This produces an acid, which washes out more hydrogen fluoride gas such that, in the course of the entire scrubbing process, both water and an acid wash the exhaust gases.
The scrubbing water can have room temperature in this context. However, it is feasible just as well that the scrubbing water has a temperature that is slightly higher than room temperature. In this context, the temperature should not be more than 10 to 20° C. above ambient temperature.
The salt-enriched scrubbing water obtained in step b) of the scrubbing process can be fed to a waste water treatment plant periodically or continuously as side stream.
Preferably, scrubbing of the exhaust gases with water and/or an acid comprises two steps, namely
It is preferred to cool the exhaust gas in the double-walled graphite scrubber (3) by means of a cooling system, in particular a water cooling system. Provided water is used as coolant, the temperature of the water preferably is approx. 60° C.
Hydrogen fluoride gas is removed from the exhaust gas by introducing water into the inside of the double-walled scrubber (3).
In order to ensure that the hydrogen fluoride gas is removed from the exhaust gas as close to completely as possible, the exhaust gas is washed twice with water and/or an acid in a preferred embodiment and, for this purpose, is guided from the first double-walled graphite scrubber (3) into a second single-walled graphite scrubber (4). The exhaust gas is washed with water and/or an acid in both scrubbers (3, 4). In both scrubbing stages, the initial scrubbing agent is water. The water becomes acidified by the hydrogen fluoride gas washed out of the exhaust gas such that, overall, both water and an acid wash the exhaust gas at the respective stages. If the hydrogen fluoride gas is already removed completely from the exhaust gas in the first stage, the second stage entails a cleaning with water only.
In scrubbing step c), the exhaust gas is neutralised and acidic components originating from step b) are removed. A base with a pH of at least 10 or more, in particular of at least 11 or more, can be used as base in this context. It is preferred to use sodium hydroxide solution for scrubbing the exhaust gases in step c). Sodium hydroxide solution does not undergo any undesired reaction with the exhaust gas components. Moreover, it is inexpensive and easy to handle.
The method according to the invention is suitable preferably for the processing of materials that have a fluorine content of up to 5% by weight. It is preferred to use fluoro-organic materials, PTFE films, fuel cells, catalysts and/or pastes as materials to be subjected to the thermal treatment. As a matter of principle, the method is suitable for all materials having a fluorine content of up to 5% by weight that decompose at a temperature of approx. 800° C., in particular of approx. 600° C.
It is preferred to implement the method according to the invention in an ashing plant of the type described above. The ashing plant according to the invention is suitable for the use and implementation of the method according to the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102012016420.3 | Aug 2012 | DE | national |
