METHOD FOR SEPARING ELEMENTARY PHOSPHORUS FROM IRON OXIDE-CONTAINING AND PHOSPHATE-CONTAINING SUBSTANCES

Description

[0001] The present invention relates to a process for separating elemental phosphorus from iron oxide-containing and phosphate-containing substances, in particular from waste materials such as sewage sludge (ash), animal meal, bone meal and / or steelworks slag, preferably together with substances such as apatite, phosphate ores and / or phosphorite, as well as to an apparatus for carrying out the process according to the invention.

[0002] The publications WO 2020 / 225760 A1 and WO 2021 / 152386 A1 form the technical background to the subject matter of the present invention.

[0003] Prices for rock phosphate have recently experienced a steady increase due to the scarcity of suitable raw material sources. Furthermore, the mined phosphate ores increasingly contain significant levels of uranium, cadmium, selenium, and other harmful impurities. Such impurities can only be removed with great effort, making the use of phosphates from such contaminated ores increasingly uneconomical.

[0004] On the other hand, relatively high levels of phosphorus compounds can be found in some waste materials. This applies, for example, to sewage sludge, animal meal, bone meal, and / or steel mill slag, among others.

[0005] Particularly for food production and the pharmaceutical industry, high-purity phosphorus starting materials are required, which can be prepared from elemental phosphorus (P4, white phosphorus). However, production is becoming increasingly difficult and is currently no longer carried out, at least in Europe.

[0006] A major problem in the extraction of elemental phosphorus from the starting materials mentioned above, particularly from waste materials such as sewage sludge, animal meal, bone meal, and / or steel mill slag and / or from materials such as apatite, phosphate ores, and / or phosphorite, is the formation of phosphorus sludge, which, for example, forms in the Wöhler process from silicate- and carbon-containing impurities of the iron oxide- and phosphate-containing materials, incorporating various phosphorus species. Phosphorus sludge is highly toxic and poses a huge problem, as its disposal makes the extraction of elemental phosphorus using currently known processes more or less economically impossible.Furthermore, in conventional processes for the recovery of elemental phosphorus, the formation of phosphines (in particular PH 3 , in the context of the present invention also generally PH x ), which are volatile as an odorless gas and highly toxic, represents a major problem.

[0007] The present invention is therefore based on the object of creating a process and a corresponding device that enable the extraction of elemental phosphorus from the aforementioned starting materials without the formation of phosphorus sludge or phosphines. At the same time, the formation of undesirable iron-phosphorus alloys such as iron phosphides, ferrophosphate, and Fe 2 P, as well as CO 2 emissions, should be avoided compared to prior art processes.

[0008] To achieve this object, the method according to the invention comprises at least the steps according to claim 1.

[0009] In the process according to the invention, the starting materials are melted together with at least one aluminum carrier. In the resulting melt, which is an aluminum-containing phosphate slag melt and may initially contain aluminum oxide, an exothermic release of elemental phosphorus (P 2 , gaseous) and metallic iron occurs. The process according to the invention thus utilizes an autothermal redox reaction between the redox partners iron oxide, phosphorus oxide (phosphate), and aluminum, which are present in various reduction stages. The redox reaction is highly exothermic and thus self-sustaining.

[0010] In the melting phase, the phosphate slag forms easily mobile, electrically conductive cation and anion clusters (ion complexes). For example, cation clusters are formed from Fe 2+< , Ca 2+< , Mg 2+< , Na +< , K +< and the like, as well as anion clusters, for example, from PO 4 3-< , SiO 3-< , SiO 4 4-< , SiO 3 2-< and AlO 3 3-<. A deliberate electrothermal (molten salt electrolysis) and / or aluthermal partial reduction of the phosphorus oxides in the phosphate slag melt leads to an extremely rapid reduction of the phosphates to gaseous P 2 and the iron oxides to metallic iron. Due to the exothermic nature of the process according to the invention, large amounts of heat are released, so that the reaction is self-sustaining and may need to be cooled.

[0011] Advantageously, aluminum dross can be used as the aluminum carrier. Dross arises during the electrolysis of alumina to produce metallic aluminum or during the melting of scrap aluminum. The dross consists of metallic aluminum, but is surrounded by an aluminum oxide skin. The dross forms on the surface of the electrolysis or melting tanks due to the oxidation of aluminum to aluminum oxide and is difficult to dispose of. For example, attempts are being made to break down the oxide skin using fluorides, but this obviously leads to serious additional waste and exhaust gas problems associated with these processes.The use of dross in the process according to the invention has the advantage that, in addition to the aluminum required for the reduction, aluminum oxide, the reaction product of the process according to the invention, is also added, thus moderating the highly exothermic reaction and thus reducing the exothermicity. At the same time, a slag with a particularly high aluminum oxide content is obtained, which ensures outstanding early strength values ​​in cements containing this slag.

[0012] The conversion occurs so suddenly that any intermediates of phosphorus and compounds of phosphate and phosphorus with impurities in the starting materials cannot be formed, thus delaying and effectively preventing the formation of phosphorus sludge and phosphines. Aluminates and non-metallic impurities accumulate in the slag, while any heavy metals contained in sewage sludge, steel mill slag, or mill scale, for example, alloy with the resulting iron. In this way, pure phosphorus is obtained directly in the gas phase, with the initially formed P 2 being converted into further phosphorus modifications such as P 4 by known cooling processes such as cooling and quenching with water or by indirect cooling with preferably regenerative heat exchangers.

[0013] After shock cooling, the resulting slag can be used in the cement industry as a high-quality hydraulic binder. Depending on the requirements of the target slag analysis, additional additives can be added to the slag. For example, by increasing the addition of metallic aluminum, the SiO2 content of the target slag can be reduced while simultaneously increasing the aluminum content. In this case, an FeSi-containing iron melt with a slightly higher aluminum content is produced. This iron alloy can be advantageously used in steel metallurgy as a deox alloy additive.

[0014] With regard to the mixing of the starting materials and the aluminum carrier, according to a preferred embodiment of the present invention, the at least one iron oxide-containing and phosphate-containing substance is provided as a phosphate slag melt in the melting vessel, and the at least one aluminum carrier is added to the phosphate slag melt. The phosphate slag melt can be provided largely free of pollutants from an oxidative process, thus preventing the formation of undesirable, toxic substances from the outset.

[0015] According to a preferred alternative embodiment of the present invention with regard to the mixing of the starting materials and the aluminum carrier, it can also be provided that a solid, in particular sewage sludge ash with a lime carrier and / or apatite and / or phosphate ore, is provided as the iron oxide-containing and phosphate-containing substance, and that solid, metallic aluminum is added as the at least one aluminum carrier. In this case, the aluminum carrier is not added to an existing phosphate slag melt, but rather the starting material is brought into contact in solid form with a solid aluminum carrier, the metallic aluminum, during the addition. The substances are preferably ground into fine powders in order to provide a large reaction surface. The mixture obtained by the addition step must initially be ignited in some way.This can be accomplished with a spark plug or a flame, or even with phosphate slag melt already present in the melting vessel. The reaction subsequently sustains itself due to the exothermic nature of the redox reaction.

[0016] As already mentioned several times, the redox reaction between iron oxide, phosphorus oxide and aluminum is highly exothermic and therefore large amounts of heat are generated in the process according to the invention, which usually requires cooling the reaction. In addition to obvious possibilities for cooling the melting vessel or the phosphate slag melt using various cooling coils, heat exchangers and the like, according to a preferred embodiment of the present invention, this can also be done by adding an Al 2 O 3 carrier to the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt. Al 2 O 3 (aluminum oxide) is also a product of the redox reaction in the process according to the invention. The addition of the preferably cold aluminum oxide carrier shifts the equilibrium of the redox reaction towards the starting materials and thus inhibits the reaction rate and hence also the release of heat.In this way, overheating of the phosphate slag melt or the melting vessel at the point of heat release can be effectively avoided without introducing non-reactive chemical species into the process. A particularly advantageous aspect in this context is that the increased Al 2 O 3 content of the target slag due to the addition of Al 2 O 3 makes it suitable for the production of fast-setting cement.

[0017] For optimized process control, especially with regard to a desirable melt viscosity, it is advantageous to adjust the basicity, as the weight ratio of CaO / SiO 2 , of the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt to a value of 0.65 to 1.4, preferably 0.85 to 1.2, and particularly preferably 1, preferably by adding CaO carriers and / or SiO 2 carriers, as corresponds to a preferred embodiment of the present invention. The chemical analysis of the target slag shows, in particular, the following proportions of the main components (in wt.%): CaO 22 - 56 Al2O3 32 - 55 SiO2 1 - 22 Fe2O3 0,15 - 3 MgO 2 - 15 P 0,05 - 1,2

[0018] This corresponds to the standard analysis of a high-quality alumina cement.

[0019] According to the invention, a tin bath is provided as a cathode body in the melting vessel beneath the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt, and at least one anode body, preferably made of graphite, platinum, or magnetite spinel, is provided immersed in the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt. This enables the electrochemical influencing of the inventive process, as explained below. The material for the anode body can be selected according to various criteria. While graphite electrodes are undoubtedly inexpensive to purchase, they are degraded relatively quickly by nascent oxygen in the inventive process, forming carbon dioxide. However, the formation of carbon dioxide must be avoided for climate protection reasons, and frequent anode changes are undesirable from an economic perspective.It has been observed that anodes made of platinum or magnetite, and especially magnetite spinel, are expensive, especially in the case of platinum, but have an extremely long service life. Another advantage is that such electrodes only produce oxygen and no carbon dioxide.

[0020] To minimize the use of metallic aluminum as much as possible, the invention provides that a direct current, preferably a direct current of 3 V to 15 V, preferably of 6 V to 12 V, particularly preferably of 8 V to 10 V, is applied to the cathode body and the anode body, as is the case with a preferred embodiment of the present invention. This allows the reactions to be driven beyond the natural chemical equilibrium by electrolysis. The iron formed accumulates in the tin bath and forms a layer beneath the tin bath when the tin bath reaches saturation.

[0021] Alternatively, however, if the reaction proceeds more or less completely due to the quality of the starting materials and optimal adjustment of the basicity of the phosphate slag melt as well as the temperature, it can also be provided that a direct current is drawn from the cathode body and the anode body. In this case, the process according to the invention supplies electrical energy by developing an electrical voltage between the anode and cathode, which can trigger an external current flow that can be used in any conceivable way. At the same time, the reaction progress in the phosphate slag melt can also be measured via this galvanic chain. When iron oxide and phosphorus oxide in the phosphate slag melt are completely reduced and present in their elemental form, no more current flows. Accordingly, experiments have shown that the current flow decreases depending on the progress of the phosphate slag reduction.Further added aluminum (metallic aluminum) no longer reacts with the now reduced phosphate slag melt.

[0022] In order to be able to obtain the products of the cathode reaction and the anode reaction separately, according to a preferred embodiment of the present invention, the cathode body is provided in a first region of the melting vessel in a depression arranged in a partial region of the bottom of the melting vessel, and the at least one anode body is provided in a second region of the melting vessel different from the first region, and gaseous phosphorus is withdrawn from the first region and oxygen from the second region. By arranging the cathode body in a region different from the region in which the anode body is arranged, the products of the cathode and anode reactions are produced spatially separately in the melting vessel or in the gas space of the melting vessel and can therefore also be withdrawn separately.At the cathode, these are iron and gaseous phosphorus, while at the anode, with an inert anode, only oxygen gas is produced. The resulting iron accumulates in the tin bath, forms a layer beneath the tin bath when saturation is reached, and can be withdrawn from the sink.

[0023] The device according to the invention for carrying out the method according to the invention is defined in claim 7.

[0024] In this device according to the invention, the aluminum supports can be melted together with the iron oxide- and phosphate-containing materials, the starting materials. In the resulting melt, which is an aluminum-containing and optionally aluminum oxide-containing phosphate slag melt, an exothermic release of elemental phosphorus (P 2 , gaseous) and metallic iron occurs depending on the addition rate of the metallic aluminum. The original phosphate slag melt is practically phosphorus- and iron-free after reduction or conversion.If, as previously described in connection with a preferred embodiment of the method according to the invention, the at least one iron oxide-containing and phosphate-containing substance is provided as phosphate slag melt in the melting vessel and the at least one aluminum carrier is added to the phosphate slag melt, the device according to the invention is preferably characterized in that the feed device has a first pipe passing through the housing, in which a conveying device, preferably in the form of a conveyor screw, for at least one aluminum carrier is arranged. Through this first pipe, the aluminum carriers can be introduced into the housing of the device and ultimately into the phosphate slag melt, where they can be melted together with the phosphate slag melt. The reduction process of the phosphate slag melt corresponds to the addition rate of the metallic aluminum.

[0025] According to a previously described alternative variant of the present invention, a solid, in particular sewage sludge ash with a lime carrier and / or apatite and / or phosphate ore, is provided as the iron oxide- and phosphate-containing substance, and solid, metallic aluminum is added as the aluminum carrier. At this point, the addition of aluminum scrap, preferably magnesium-alloyed, phosphate-containing steel slag, phosphate-containing rolling mill scale, and the like as solids would also be conceivable.In this case, the device according to the invention is preferably characterized in that the feeding device has a first tube passing through the housing, in which first tube a second tube is arranged to form an annular gap, preferably concentrically to the first tube, wherein one of the first and the second tube is designed to feed iron oxide-containing and phosphate-containing substances and the other of the first and second tube is designed to feed aluminum carriers and wherein the first tube projects beyond the second tube at its end passing through the housing.This feed device allows two solid components, namely the iron oxide- and phosphate-containing material or the starting material, and the aluminum carrier to be fed together into the device. Due to the fact that the first tube projects beyond the second tube at its end passing through the housing, contact is established between the two components in the feed device. This contact leads to the formation of an autothermal reaction front at which the aluminum carriers are melted together with the iron oxide- and phosphate-containing materials, and at which the exothermic reduction reaction begins. The resulting aluminum-containing and optionally aluminum oxide-containing phosphate slag melt then enters the interior of the melting vessel and into the existing phosphate slag melt, where complete conversion to iron and elemental phosphorus takes place.Both the first and second tubes can be provided with suitable conveying means for conveying the components. An ignition device for the aluminum carrier and the iron oxide- and phosphate-containing material can be provided near the end of the second tube; however, the mixture of the two components can also be ignited by the heat of the phosphate slag melt.

[0026] For the electrochemical influence of the process according to the invention, the device is designed according to the invention in such a way that the device has a cathode body arranged beneath the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt and at least one anode body immersed in the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt. The cathode body and the anode body are provided with electrical lines in a known manner to form an electrical circuit.

[0027] In order to be able to separately obtain the products of the cathode reaction and the anode reaction, according to a preferred embodiment of the present invention, the cathode body is arranged in a first region of the melting vessel in a depression arranged in the bottom of the melting vessel, and the at least one anode body is arranged in a second region of the melting vessel laterally spaced from the first region, and a further extraction device is arranged in the second region. By arranging the cathode body in a region that is different from the region in which the anode body is arranged, the products of the cathode and anode reactions arise spatially separately in the melting vessel or in the gas space of the melting vessel and can therefore also be extracted separately.At the cathode, these are iron and gaseous phosphorus, while at the anode, with an inert anode, only oxygen gas is produced. The resulting iron accumulates in the tin bath, forms a layer beneath the tin bath when saturation is reached, and can be withdrawn from the sink.

[0028] In order to achieve the most complete separation possible between the products of the cathode reaction and the anode reaction in the gas space of the device according to the invention, the invention is preferably further developed in such a way that a partition wall dividing the gas space into two separate sections is immersed in the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt between the first and second regions. Thus, products of the cathode reaction cannot enter the first region with the anode, and vice versa, and can thus be removed separately.

[0029] On the other hand, when the gaseous products of the cathode and anode reactions combine in an exothermic reaction, extremely pure P 2 O 5 is formed. The P 2 O 5 formed in this reaction is so pure that it can be directly converted into food-grade phosphoric acid by hydrolysis and used accordingly.

[0030] According to a preferred embodiment of the present invention, the cathode body is formed from a tin bath. A tin bath as a cathode has the advantage that, in addition to the iron formed in the inventive process in the inventive device, heavy metals, which may be contained in sewage sludge, steel mill slag, or mill scale, alloy with the iron formed and are thus bound. Once the tin bath is saturated with iron, the iron formed in the inventive process forms a layer beneath the tin bath and can be removed.

[0031] To increase the service life of the device according to the invention, the tin bath is contained in a carbon-containing body arranged in the depression, as is the case with a preferred embodiment of the present invention. The carbon-containing body can preferably be designed as a graphite body or consist of a carbon-containing mass.

[0032] According to a preferred embodiment of the present invention, the anode body can be made of graphite, platinum, or magnetite spinel. Graphite electrodes are widely used in the prior art and, when the inventive method is carried out in the inventive device, lead to the formation of CO and optionally CO2 at the anode. The CO (carbon monoxide) can subsequently be thermally utilized. The graphite anode is consumed in this process and must be replaced regularly. The advantages of the other described anode materials have already been explained above in connection with the inventive method.

[0033] To allow a CO2-free process, the invention can be further developed according to an alternative embodiment such that the at least one anode body is made of a high-alloy steel, wherein the steel is preferably coated with platinum. A platinum coating increases the already good service life of an anode made of high-alloy steel and also offers catalytic properties.

[0034] Furthermore, the use of a Söderberg electrode as an anode body is also conceivable.

[0035] As already mentioned, large amounts of heat are released during the implementation of the method according to the invention in the device according to the invention, which can sometimes adversely affect the equipment. To prevent this, the at least one anode body can be designed to be coolable, preferably by channels arranged inside the cathode body for a tin melt, a molten salt or sodium melt, or for gas cooling, preferably by nitrogen or argon, as is the case in a preferred embodiment of the present invention.

[0036] As already described above, the invention provides that a direct current, preferably a direct current of 3 V to 15 V, preferably of 6 V to 12 V, particularly preferably of 8 V to 10 V, is applied to the cathode body and the anode body in order to minimize the use of aluminum as much as possible. This allows the reactions to be driven beyond the natural chemical equilibrium by electrolysis. For this purpose, the device according to a preferred embodiment of the present invention comprises means for applying a direct current to the cathode body and the anode body.

[0037] Alternatively, however, in the case of a reaction that proceeds more or less completely due to the quality of the starting materials and optimal adjustment of the basicity of the phosphate slag melt as well as the temperature, it can also be provided according to the invention that a direct current is taken from the cathode body and the anode body. In this case, the process according to the invention supplies electrical energy that can be used in any conceivable way. For this purpose, the device has means for taking a direct current from the cathode body and the anode body, as corresponds to a preferred embodiment of the present invention. The anode reaction can be specified as follows: Al -> Al 3+< + 3 e -<

[0038] This represents a release of electrons and consequently an oxidation of metallic aluminum.

[0039] The cathode reactions can be given as follows: P 3+< + 3e -< -> P Fe 3+< + 3 e -< -> Fe

[0040] This represents an electron uptake and thus a reduction of the iron oxides and phosphorus oxides.

[0041] The invention will be explained in more detail below with reference to an embodiment shown in the drawing. Figure 1 a schematic representation of a first device according to the invention, Figure 2 a representation of a device according to the invention with a cathode body and several anode bodies, Figure 3 a feeding device according to a first embodiment of the device according to the invention, preferably for the device according to Figure 1 , and Fig. 4 an embodiment of the present invention in which a direct current can be taken from the electrodes.

[0042] In Figure 1The device according to the invention is provided with the reference numeral 1. The device 1 comprises a housing 3 made of structural steel sheet, lined with a refractory material 2 to form a melting vessel 11. A refractory material suitable for the present invention was found to be magnesium spinel in an Al 2 O 3 matrix, which material is known as sintered refractory concrete. Corundum bricks are also suitable as refractory material. Phosphate slag melt 4 is arranged in the housing as the iron oxide-containing and phosphate-containing substances. A feed device 5 of the device 1 passes through the housing 3 with a first pipe 6. Aluminum supports 7 or Al 2 O 3 supports containing metallic aluminum can be fed through the first pipe 6 of the feed device 5, which is accomplished by a conveying device 8 in the form of a screw conveyor.The feed device 5 is immersed in the phosphate slag melt 4, whereby the aluminum carriers 7 are conveyed directly into the depths of the phosphate slag melt. There, together with the iron oxide- and phosphate-containing substances, namely the phosphate slag melt 4, they are melted and rapidly converted into elemental phosphorus and iron. The phosphorus is removed via the extraction device 9, and the iron collects at the bottom 10 of the melting vessel 11. There, the iron can be tapped. The phosphate slag melt 4 is converted into phosphate-free cement slag without the addition of further starting materials.

[0043] In Figure 2 The same parts are provided with the same reference numerals. The device 1 according to Figure 2has a cathode body 15 arranged beneath the aluminum-containing phosphate slag melt 4 and several anode bodies 13 immersed in the aluminum-containing phosphate slag melt 4, wherein the cathode body 15 is formed by a tin bath. The tin bath is arranged in a depression 14 in the bottom 10 of the melting vessel 11 and, as the reaction progresses, is underlain by the iron formed. The tin bath is held in a graphite body 18 arranged in the depression. An iron layer forming beneath the cathode body 15 is designated by the reference numeral 12. The current is then introduced through the graphite body 18, the iron layer 12, and the tin bath 15. The anode bodies 13 can be made of graphite or high-alloy steels and, if appropriate, provided with a coating 16.

[0044] The cathode body 15 is arranged in a first region A of the melting vessel 11, wherein the first region A is separated from a second region B, which is laterally spaced from the region A, by a partition wall 16a immersed in the phosphate slag melt 4. Elemental phosphorus in the form of P 2 is withdrawn at the withdrawal device 9, and in the case of coated anode bodies, oxygen (O 2 ) escapes at a further withdrawal device 17 assigned to the second region B. A direct current is applied to the cathode and the anode in a known manner.

[0045] The feeding device 5 according to Figure 3 has a first tube 6 passing through the housing 3, in which first tube 6 a second tube 19 is arranged concentrically to the first tube 6, forming an annular gap 20. The first tube 6 projects beyond the second tube 19 at its end 6a passing through the housing 3. .Two solid components, namely the iron oxide-containing and phosphate-containing material 21 or the starting material 21 and an aluminum carrier 7, in this case metallic aluminum, which are fed together into the device, come into contact with each other, resulting in the formation of an autothermally maintained reaction front 22, at which the aluminum carriers 7 are melted together with the iron oxide-containing and phosphate-containing material 21. The resulting phosphate slag melt 4 containing aluminum and aluminum oxide is symbolized by drops and then enters the interior of the melting vessel and into the existing phosphate slag melt, where complete conversion to iron and elemental phosphorus takes place. In the region of the end 6a of the second tube 6, an ignition device 23 for the aluminum carrier 7 and the iron oxide-containing and phosphate-containing material 21 can be provided.

[0046] In the representation according to Figure 4 , the anode body 13 is arranged around the first tube 6 of the feed device 5 and means are provided for taking a direct current from the cathode body 15 and the anode body 13 .

Claims

1. Method for separating elemental phosphorus from iron oxide-containing and phosphate-containing materials, in particular from waste materials such as sewage sludge (ash), animal meal, bone meal and / or steelworks slag, preferably together with materials such as apatite, phosphate ores and / or phosphorite, comprising at least the following steps: providing at least one iron oxide- and phosphate-containing material, adding at least one aluminum carrier to the at least one iron oxide- and phosphate-containing material and melting the at least one aluminum carrier together with the at least one iron oxide- and phosphate-containing material to form an aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4), reacting the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt to elemental gaseous phosphorus, iron and an Al2O3-containing slag in a melting vessel (11), drawing off elemental, gaseous phosphorus and tapping off the iron and the Al2O3-containing slag, characterized in that a tin bath is arranged as cathode body (15) in the melting vessel (11) under the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4) and at least one anode body (13), preferably of graphite, platinum or magnetite spinel, is arranged to be immersed in the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4) and a direct current, preferably a direct current of 3 V to 15 V, preferably of 6 V to 12 V, in particular preferably of 8 V to 10 V, is applied to the cathode body and the anode body or a direct current is taken from the cathode body (15) and from the anode body (13).

2. Method according to claim 1, characterized in that the at least one iron oxide- and phosphate-containing material (21) is provided as a phosphate slag melt (4) in the melting vessel (11) and the at least one aluminum carrier (7) is added to the phosphate slag melt (4).

3. Method according to claim 1, characterized in that a solid material, in particular sewage sludge ash with a lime carrier and / or apatite and / or phosphate ore, is provided as the iron oxide- and phosphate-containing material (21) and that solid, metallic aluminum is added as the at least one aluminum carrier (7).

4. Method according to claim 1, 2 or 3, characterized in that an Al2O3 carrier is added to the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4).

5. Method according to any one of claims 1 to 4, characterized in that the basicity, being the weight ratio of CaO / SiO2, of the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4) is adjusted to a value of 0.65 to 1.4, preferably 0.85 to 1.2 and more preferably 1, preferably by addition of CaO carriers and / or SiO2 carriers.

6. Method according to any one of claims 1 to 5, characterized in that the cathode body (15) is arranged in a first region (A) of the melting vessel (11) in a depression (14) arranged in a partial region of the bottom (10) in the melting vessel (11) and that the at least one anode body (13) is arranged in a second region (B) of the melting vessel (11) different from the first region (A), and in that gaseous phosphorus is withdrawn from the first region (A) and oxygen is withdrawn from the second region (B).

7. Device for carrying out the method according to any one of claims 1 to 6, comprising a melting vessel (11) formed by a refractory-lined housing (3), a phosphate slag melt (4) containing aluminum and optionally aluminum oxide arranged in the melting vessel (11), a gas chamber closed off by the housing (3) above the phosphate slag melt (4), and a feed device (5) for iron oxide- and phosphate-containing substances (21) and at least one extraction device (9) for gaseous, elemental phosphorus, characterized in that the device (1) comprises a cathode body (15) arranged below the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4) and at least one anode body (13) immersed in the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4).

8. Device according to claim 7, characterized in that the feed device (5) comprises a first tube (6) passing through the housing (3), in which a conveying device (8), preferably in the form of a screw conveyor, for at least one aluminum carrier (7) is arranged.

9. Device according to claim 7, characterized in that the feed device (5) comprises a first tube (6) passing through the housing (3), in which first tube (6) a second tube (19) is arranged forming an annular gap (20), preferably concentric to the first tube (6), wherein one of the first tube (6) and the second tube (19) is designed for feeding iron oxide and phosphate-containing substances (21) and the other of the first tube (6) and the second tube (19) is designed for feeding aluminum carriers (7), and wherein the first tube (6) projects beyond the second tube (19) at its end (6a) penetrating the housing (3).

10. Device according to any one of claims 7 to 9, characterized in that the cathode body (15) is arranged in a first region (A) of the melting vessel (11) in a depression (14) arranged in the bottom (10) of the melting vessel (11), and in that the at least one anode body (13) is arranged in a second region (B) of the melting vessel (11) laterally spaced from the first region (A), and a further extraction device (17) is arranged in the second region (B).

11. Device according to claim 10, characterized in that between the first region (A) and the second region (B) a separating wall (16a) dividing the gas space into two sections separated from each other is immersed in the aluminum-containing and optionally aluminum oxide-containing phosphate slag melt (4).

12. Device according to any one of claims 7 to 11, characterized in that the cathode body (15) is formed by a tin bath.

13. Device according to claim 12, characterized in that the tin bath is received in a carbon-containing body (18) disposed in the depression (14).

14. Device according to any one of claims 7 to 11, characterized in that the at least one anode body (13) is formed of graphite, platinum or magnetite spinel.

15. Device according to any one of claims 7 to 11, characterized in that the at least one anode body (13) is formed of a high-alloy steel, the steel preferably being coated with platinum (16).

16. Device according to one of claims 7 to 13, characterized in that the at least one anode body (13) is designed to be coolable, preferably by channels arranged inside the anode body (13) for a tin melt, a salt melt or sodium melt or for gas cooling, preferably by nitrogen or argon.

17. Device according to any one of claims 7 to 14, characterized in that the device (1) comprises means for applying a direct current to the cathode body (15) and the anode body (13).

18. Device according to any one of claims 7 to 15, characterized in that the device (1) comprises means for taking a direct current from the cathode body (15) and from the anode body (13).