Recovering phosphorus in the form of its chlorides from sewage sludge ash

Abstract

The invention relates to a process for recovering phosphorus in the form of its chlorides, in which process sewage sludge ash, which has a phosphorus content expressed in wt.% of elemental phosphorus in the ash of at least 2 wt.%, is reacted in the presence of a carbon source with elemental chlorine at a temperature of 350 to 900°C and the phosphorus chlorides formed are removed from the waste gas stream.

Description

[0001] in the form of its chlorides from sewage sludge ash

[0002] The present invention relates to the recovery of phosphorus in the form of its chlorides from sewage sludge ash.

[0003] The desire to recover raw materials from sewage sludge has grown in recent years, primarily for ecological and political reasons.

[0004] It is common practice to burn partially dewatered sludge from large wastewater treatment plants in incineration plants.

[0005] The resulting ash is a primarily inorganic residue containing various elements such as phosphorus (P), iron (Fe), aluminum (Al), calcium (Ca), and silicon (Si). Disposing of this ash by spreading it into the environment without further treatment can lead to serious environmental pollution, primarily due to the presence of heavy metals and other toxic substances.

[0006] Furthermore, untreated ash is lightweight, finely dispersed, and dusty, making handling, transportation, storage, and disposal difficult. Therefore, the disposal of this ash is a growing problem in many parts of the world.

[0007] In view of these problems, it has become necessary to develop processes for the treatment of these residues with the aim of recovering the elements they contain and reusing them for various applications.

[0008] In fact, there are various attempts and methods in the state of the art to use sewage sludge ash as a source of valuable raw materials.

[0009] The recovery of valuable materials from sewage sludge ash is becoming increasingly important, particularly with regard to the utilization of phosphorus, for which there is a high global demand and, at the same time, limited availability.

[0010] Depending on the operation of the sewage treatment plant, the concentration of phosphorus in the sewage sludge ash, expressed in elemental form, is usually between 2 and 13 wt%.

[0011] Other main components of sewage sludge ash are, besides phosphorus, iron (Fe), aluminum (AI), calcium (Ca) and silicon (Si).

[0012] Various approaches are known in the prior art for recovering phosphorus from sewage sludge ash generated during incineration processes. Reported work on phosphorus recovery from sewage sludge ash is often associated with thermochemical treatment or acid leaching processes. In the thermochemical treatment of sewage sludge ash, as described, for example, in EP1669467A1, the sewage sludge ash is reacted with chlorine gas without the presence of a carbon carrier to remove heavy metal chlorides. However, no phosphorus chlorides are formed, and the phosphorus compounds remain in the residue.

[0013] In an acid leaching process, such as that described in EP3770113B1, sewage sludge ash is dissolved in an acid and later neutralized. This process has the disadvantage that large volumes of acids and alkalis must be used at comparatively low concentrations, resulting in large amounts of waste. Furthermore, a liquid-liquid extraction process is used, which involves the use of additional, cost-intensive chemicals such as complexing agents.

[0014] The objective of the present invention was therefore to recover phosphorus in the form of its chlorides from sewage sludge ash using a simple and cost-effective process. The process should also lead to technically usable and marketable products and produce as little waste as possible. By directly using the sewage sludge ash without prior dissolution in water or acid, the amount of waste generated for the separation of the phosphorus component is minimal.

[0015] Solution to the task

[0016] Surprisingly, a process for the recovery of phosphorus in the form of its chlorides was found, which is characterized in that sewage sludge ash having a phosphorus content, expressed as % by weight of elemental phosphorus in the ash, of at least 2 % by weight, is reacted with elemental chlorine in the presence of a carbon source at a temperature of 350 to 900°C and the phosphorus chlorides formed are separated from the exhaust gas stream.

[0017] Sewage sludge ash

[0018] For the purposes of this invention, sewage sludge ash refers to the ash resulting from the incineration of sewage sludge. This can be obtained both from sewage sludge incineration and from sewage sludge gasification to produce a fuel gas. In addition to phosphorus compounds, sewage sludge ash preferably contains compounds of iron (Fe), aluminum (Al), calcium (Ca), and silicon (Si).

[0019] The sewage sludge ash used in the process according to the invention preferably has a phosphorus content, expressed as a percentage by weight of elemental phosphorus in the ash, of at least 3%, preferably from 3 to 29% by weight, in particular from 3 to 15% by weight. The sewage sludge ash preferably contains a proportion of iron compounds of 5 to 21% by weight (calculated as elemental iron), and preferably a proportion of calcium compounds of 4 to 38% by weight (calculated as elemental calcium). The preferred composition of a sewage sludge ash, calculated on the basis of the respective element, preferably also contains:

[0020] Aluminum from 0.7 to 20 wt.%,

[0021] Potassium from 0 to 2 wt.%,

[0022] Magnesium from 0.1 to 4 wt.%,

[0023] Sodium from 0.1 to 4% by weight

[0024] Sulphur from 0 to 7 wt.%,

[0025] Silicon from 0 to 24 wt% and

[0026] Titanium from 0 to 2 wt%.

[0027] The elements together, calculated as oxides, preferably make up more than 70 wt.% of the sewage sludge ash.

[0028] The exact structural composition of the sewage sludge ash cannot be determined using the usual analytical methods available to the expert, such as X-ray diffraction (XRD), since the diffractogram only shows reflections for the SiO2 present.

[0029] Preferably, the chloride content in the sewage sludge ash is less than 1 wt.%.

[0030] Carbon source

[0031] In principle, all carbon modifications such as graphite, soot, coal, coke, activated carbon, as well as carbon-containing gases such as carbon monoxide, methane, or phosgene, as well as liquid materials such as polyethylene glycol or various oils, or solid materials such as biowaste or sewage sludge can be used as carbon sources. Sewage sludge is particularly preferred. This preferably contains carbon in a proportion of at least 5 wt.%, preferably 20 to 50 wt.%, based on the dry mass.

[0032] In the context of the present invention, the term "sewage sludge" refers to any suspension of finely divided particles of a solid substrate in a liquid. The sewage sludge preferably contains a carbon content, expressed as a weight percent elemental carbon, of at least 5 weight percent carbon. In a preferred embodiment, the liquid in which the particles are suspended is wastewater as defined herein. The term "wastewater" refers to all liquids of an aqueous and / or organic nature or mixtures thereof that do not have drinking water quality within the meaning of drinking water standards. In a particular embodiment, the sewage sludge is in the form of primary sludge, raw sludge, excess sludge, or treated and / or stabilized sewage sludge (aerobic / anaerobic).

[0033] The term "biowaste" refers to all organic waste of animal or plant origin that is generated in a household or factory and can be broken down by microorganisms, soil organisms, or enzymes. Examples of such waste include straw, sawdust, waxes, fats, and bird droppings.

[0034] The carbon source can be solid, liquid, or gaseous. The use of a solid carbon source is preferred.

[0035] If the carbon content of the carbon source is less than 70 wt.%, pyrolysis is preferably carried out prior to the reaction with chlorine gas. This is preferably carried out under an inert gas such as nitrogen at temperatures of 250 to 800°C, preferably at 350 to 550°C, until the gas formation of volatile components is less than 1 / 1 kg of carbon source used per hour.

[0036] Sewage sludge is particularly preferred as a carbon source.

[0037] Preferably, the chloride content in the carbon source, in particular based on its dry weight, is less than 1 wt.%.

[0038] Mixing ratios

[0039] The ratio of the weight fractions of carbon from the carbon source to sewage sludge ash is preferably greater than 0.01, preferably from 0.04 to 0.5.

[0040] reactor

[0041] The sewage sludge ash to be used in the process according to the invention, preferably in a mixture with the carbon source, is preferably introduced into a reactor that is preferably provided with a layer resistant to the reaction conditions to be established. Preferred reactor materials are reactors coated with nickel or nickel-based alloys or graphite, or reactors consisting of a ceramic such as silicon carbide or aluminum oxide or aluminum oxide-silicon dioxide mixed ceramic. Tubular reactors such as rotary tube reactors or other reactors can be used as such. Particular preference is given to reactors that allow movement of the sewage sludge ash, preferably in a mixture with the carbon source, during the reaction in order to allow the most effective contact of the sewage sludge ash, preferably in a mixture with the carbon source, and chlorine gas.Preferred are fluidized-bed reactors, rotary tube reactors, or a reaction in an extruder with screw propulsion. A fixed-bed reactor or a shaft-furnace reactor can also be used.

[0042] Preferably, the reactor has an outlet for the exhaust gas stream. The exhaust gas stream contains the gaseous reaction products, volatile components of the sewage sludge ash, possibly mixed with the carbon source, and any excess chlorine gas, which can be discharged together from the reaction chamber.

[0043] In the case of a tubular reactor, the reactor length is preferably 0.2 to 40 m. The residence time in the reactor during the reaction generally depends on the temperature and the possibility of contact between the material and chlorine gas. The residence time in the reactor can range, for example, from one minute to five hours. The process according to the invention can be operated as a batch or continuously.

[0044] Proceedings

[0045] Preferably, the sewage sludge ash and the carbon source are mixed first. If the carbon content of the carbon source is less than 70 wt.%, pyrolysis preferably takes place before the reaction with chlorine gas. This is preferably carried out under an inert gas such as nitrogen at temperatures of 250 to 800°C, preferably at 350 to 550°C, until the gas formation of volatile components is less than 1 kg of carbon source used per hour. The off-gases are preferably discharged.

[0046] Pyrolysis preferably takes place in an inert gas stream, preferably nitrogen or argon, with an oxygen content of preferably less than 0.1 percent by volume at a temperature of 200 to 800°C, preferably 350 to 550°C, in particular until gas evolution ceases. The escaping gases are preferably combusted and can be used to generate heat or electricity, or be recycled.

[0047] After mixing the components, sludge ash and carbon source, and pyrolysis if necessary, the reaction with chlorine gas takes place. The chloride content of this mixture is preferably less than 1 wt.%.

[0048] The chlorine gas can be brought into contact with the sewage sludge ash-carbon source mixture in various ways. The reaction preferably takes place with an oxygen content of preferably less than 0.5, in particular less than 0.3 percent by volume. It is also preferred to carry out the reaction at a water content of preferably less than 0.5, in particular less than 0.3 percent by volume. Chlorine is preferably passed over or through the mixture, with the mixture preferably being agitated during the reaction for effective conversion. This can take place in a rotary kiln or in a paddle dryer in which the mixture is agitated. However, the chlorine gas can also be passed through the mixture, which can be achieved, for example, in a fluidized bed or fixed bed. If necessary, the sewage sludge ash can first be subjected to a shaping process, for example, compaction or pelletization.If the carbon source is a gaseous compound, the chlorine gas application described above is preferably carried out together or separately with the gaseous carbon source for sewage sludge ash.

[0049] The reaction with chlorine preferably takes place at a temperature of 450 to 850°C. The reaction is preferably complete when the proportion of phosphorus compounds in the exhaust gas stream, preferably measured using a gas-phase IR spectrometer calibrated accordingly in weight percent, is less than 0.1 wt.%, in particular less than 0.01 wt.%.

[0050] In addition to the phosphorus chlorides, which include in particular phosphorus oxychloride and phosphorus trichloride, the exhaust gas stream may also contain gaseous iron(III) chloride and possibly also AlCl, if iron and / or aluminum are contained in the sewage sludge ash.

[0051] At a reaction temperature of 300 to 320°C, the proportion of iron(I) chloride in the exhaust stream is generally comparatively small and only increases after a temperature increase to 350 to 400°C. Iron(III) chloride and also AlCl3, if aluminum is present in the sewage sludge ash, can be separated from the exhaust stream and separated from each other, preferably by resublimation on surfaces of different temperatures. If iron chloride is present together with AlCl3 in the exhaust stream, the respective chlorides can also be fractionally resublimated on different surfaces at different temperatures due to their sufficiently different boiling points, thus allowing them to be separated very cleanly.

[0052] Preferred deposition temperatures for FeCl3 are less than or equal to 307°C, in particular 150 to 300°C, and for AlCl3 less than or equal to 150°C, in particular 110 to 149°C. The phosphorus-chlorine compound can also form adduct compounds with FeCl3, which have lower melting points of approximately 100°C. The separation of the P- and Fe-containing compounds can, if necessary, be achieved by distillation.

[0053] The iron recovered according to the invention in the form of iron(III) chloride can, optionally after purification as described above, be converted into the desired raw material form of iron, for example, for the production of cathode active materials for lithium-ion batteries. Examples include iron sulfate, iron nitrate, iron phosphate, or various forms of iron oxide.

[0054] Alternatively, the iron(III) chloride or the adduct compounds, even without being separated by resublimation, can be introduced directly as a gas stream into an aqueous medium containing sulfuric acid, nitric acid, or phosphoric acid, thereby resulting in the formation of the corresponding iron(III) sulfates, nitrates, or phosphates. If necessary, a suitable reducing agent is used in the reaction to obtain iron(II) sulfate, iron(II) nitrate, or iron(II) phosphate, respectively.

[0055] Alternatively, the exhaust gas stream can be introduced into a non-aqueous medium such as liquid POCl3 and the iron compounds can be dissolved there.

[0056] However, the separation of iron chloride from the exhaust gas stream by resublimation is preferred.

[0057] In the process according to the invention, the chlorides of phosphorus, preferably the phosphorus oxychloride and any phosphorus trichloride also formed, can be removed from the exhaust gas stream, preferably by means of a condenser, and any excess chlorine gas can be recycled.

[0058] From the exhaust stream, the phosphorus oxychloride, which is gaseous at reaction temperature, and any gaseous phosphorus trichloride that may also be formed are preferably separated from each other using a condenser. Typically, a mixture of phosphorus oxychloride and phosphorus trichloride is formed, which can be further separated by distillation. This allows the phosphorus components to be recovered in a very pure form.

[0059] The process according to the invention is preferably characterized in that the exhaust gas stream containing phosphorus trichloride, or the phosphorus trichloride after separation therefrom, is reacted with chlorine gas at a temperature of 20 to 160°C to form PCl5. Phosphorus pentachloride is formed during this reaction. A molar ratio of phosphorus trichloride to phosphorus trichloride of 1:1 to 10:1 is preferred.

[0060] The process according to the invention is preferably used for the recovery of phosphorus compounds in the form of chlorophosphorus compounds, in particular in the form of a mixture of phosphorus oxychloride and phosphorus trichloride.

[0061] Phosphorus oxychloride and phosphorus trichloride usually fall in the exhaust gas stream in a

[0062] Weight ratio of 10 to 1 to 1 to 10. The phosphorus chlorides from the process according to the invention can be converted to phosphorus trichloride, and thus to the preferred starting material for phosphorus pentachloride formation, by conducting the reaction preferably above 500°C. It is also preferred to carry out the process at a carbon / sewage sludge ash weight ratio of greater than 0.01, in particular from 0.04 to 0.5.

[0063] If the process according to the invention is operated without a stoichiometric excess of chlorine and thus there is little to no chlorine in the exhaust gas stream, the exhaust gas stream containing the chlorides of phosphorus, in particular phosphorus oxychloride and optionally phosphorus trichloride, can also be introduced into water or a lye, preferably after it has been freed from iron chloride, in order to obtain the corresponding acids of phosphorus such as phosphoric acid and phosphonic acids, from which further phosphorus derivatives, such as their esters, can then be produced.

[0064] Examples

[0065] Analytics

[0066] The analysis for phosphorus compounds, especially POCh and PCl3, is preferably carried out during the reaction using online IR in the exhaust stream. For this purpose, the gas stream from the reactor is passed through a glass cuvette that allows the passage of IR radiation over the widest possible spectral range, for example, by using windows made of a thalium compound. The mass fraction of phosphorus compounds in the exhaust stream can be determined by prior calibration (total evaporation of known PCl or POCh mass flows into a nitrogen gas stream with a known volume flow and quantification of characteristic bands in the IR spectrum).

[0067] The stated weight percentages were determined by an ICP-OES measurement. For this purpose, a weighed amount of the solid is first dissolved in a known amount of an acid, the concentration of the indicated elements is determined using an ICP analysis against a calibration measurement, and from this, the percentage of the solid is calculated.

[0068] Carbon analyses were carried out using a special carbon analyzer type G4 ICARUS HF.

[0069] The analysis of phosphorus compounds, particularly POCl3 and PCl3, in the liquid phase is preferably performed using a GC method. An Agilent Technologies 7890A series instrument is used. A TCD and an FID serve as detectors. A Gerstel MPS is used as the injection system. An HP5 column with the following dimensions is installed: length = 30 m, inner diameter = 320 μm, and a film thickness of 0.25 μm.

[0070] Example 1 :

[0071] 160g of sewage sludge ash from a sewage sludge incineration plant (containing calcium 9.1 wt.%, iron 6.5 wt.%, 7.4 wt.% phosphorus, 7.0 wt.% aluminum, 793 ppm copper, 0.36% potassium, 1.2% magnesium, 879 ppm manganese, each calculated as elements; color: red) are mixed with 40g of activated carbon and heated to 780°C in a quartz glass tube placed in an electric furnace under exclusion of air in a nitrogen gas flow of 11 / min.

[0072] The nitrogen stream is then replaced with a stream of 200 ml / min of chlorine gas (99.8% purity) and passed over the mixture for 14 hours at 780°C. The resulting FeCh, partially together with p-chlorine compounds, is resublimated ("sublimate") in a cooler at a temperature of 200°C. The product gas stream, which is gaseous at this temperature, is disposed of via a cold trap and scrubber.

[0073] The residue remaining in the reactor tube (86 g) is still blackened by carbon residues. The HCl-acid extract shows, by ICP-OES, a content (each calculated as elements): calcium 19.3 wt.%, iron 610 ppm, phosphorus 0.83 wt.%, aluminum 0.86 wt.%, copper 785 ppm, potassium 0.4 wt.%, magnesium 2.7 wt.%, and manganese 0.2 wt.%.

[0074] 74 g of the resulting residue is suspended in 350 g of water with stirring and then filtered. The residue (83 g) is dried to constant weight at 110°C. 38 g remain.

[0075] The filtrate, 336g, contains, according to ICP-OES, calcium 3.9 wt%, iron 43 ppm, phosphorus 53 ppm, aluminum 94 ppm, copper 114 ppm, potassium 704 ppm, magnesium 0.49 wt%, and manganese 407 ppm.

[0076] The filtrate is passed through a moist Lewatit® TP207 sodium ion exchanger. According to ICP-OES, the resulting solution contains 2.4 wt.% calcium, <4 ppm iron, <4 ppm phosphorus, <4 ppm aluminum, <4 ppm copper, <4 ppm potassium, 596 ppm magnesium, and <4 ppm manganese. The filtrate used was slightly diluted with water, which was used to rinse the ion exchanger. By rinsing the loaded ion exchanger with NaCl solution, the two heavy metals copper and manganese can be recovered as a solution. The manganese can be oxidized from the copper by introducing chlorine gas into the ion exchanger to form manganese dioxide, which is then filtered off, leaving a copper chloride solution.

[0077] This solution, obtained after treatment with Lewatit® TP207, is passed through another ion exchanger, Lewatit® MonoPlus SP112 in the sodium form, to recover the potassium contained. After loading, the column is discharged with NaCl. The resulting potassium chloride can be used as a fertilizer raw material.

[0078] The resulting FeCh / POCh-containing sublimate (88 g, melting point 110 °C) was heated to 170 °C under reduced pressure to release adhering POOL, which was condensed in a cold trap. 7 g of POOL was released and condensed. The identity was confirmed by GC-TCD.

[0079] Alternatively, the sublimate can be dissolved in concentrated sulfuric acid to release adhering POOL along with HCl and convert the ferric chloride into ferrous sulfate. Example 2:

[0080] 60g of sewage sludge ash from a sewage sludge incineration plant containing (calcium 8.1 wt.%, iron 6.4 wt.%, phosphorus 5.0 wt.%, aluminum 6.0 wt.%, each calculated as elements; color: red) are mixed with 2.5g of activated carbon and heated to 600°C in a quartz glass tube placed in an electric furnace under exclusion of air in a nitrogen gas flow of 11 / min.

[0081] The nitrogen stream is then replaced with a stream of 100 ml / min of chlorine gas (99.8% purity) and passed over the mixture for 6.5 hours at 600°C. The resulting FeCl is resublimated in a cooler at a temperature of 200°C. The product gas stream, which is gaseous at this temperature, is analyzed at irregular intervals in a gas-phase infrared spectrometer and then disposed of via a cold trap and scrubber.

[0082] After about 10 minutes of reaction time, two adsorption bands appear in the IR spectrum of the gas phase at 593 cm -1 and at 1322 cm-1 that can be assigned to POCh.

[0083] The residue remaining in the reactor tube (54.6 g) is 36 wt.% water-soluble. The resulting solution is an aqueous calcium chloride solution containing alkali, alkaline earth, and heavy metals and can be separated from any traces of heavy metals using methods known to those skilled in the art, such as ion exchange resins.

[0084] The resulting FeCl-containing product can be dissolved in concentrated sulfuric acid to release any adhering POCl3 and convert the ferric chloride into ferrous sulfate. Alternatively, it can be heated to release any adhering POCl3.

[0085] Example 3:

[0086] 62.3 g of sewage sludge ash from a sewage sludge incineration plant (containing calcium 8.1 wt.%, iron 6.4 wt.%, phosphorus 5.0 wt.%, aluminum 6.0 wt.%, each calculated as elements; color: red) are mixed with 2.5 g of activated carbon and heated to 500°C in a quartz glass tube placed in an electric furnace under exclusion of air in a nitrogen gas flow of 1 l / min.

[0087] The nitrogen stream is then replaced with a stream of 100 ml / min of chlorine gas (99.8% purity) and passed over the mixture for 6 hours at 500°C. The resulting FeCh, partially together with p-chlorine compounds, is resublimated in a cooler at a temperature of 200°C. The product gas stream, which is gaseous at this temperature, is analyzed at irregular intervals in a gas-phase infrared spectrometer and then disposed of via a cold trap and scrubber.

[0088] After about 10 minutes of reaction time, two adsorption bands appear in the IR spectrum of the gas phase at 593 cm -1 and at 1322 cm -1 that can be assigned to POCh.

[0089] The residue remaining in the reactor tube (60.2 g) is yellow-red, and grey in deeper layers.

[0090] The resulting FeCl-containing product can be dissolved in concentrated sulfuric acid to release any adhering POCl3 and convert the ferric chloride into ferrous sulfate. Alternatively, it can be heated to release any adhering POCl3.

Claims

Patent claims:

1. A process for the recovery of phosphorus in the form of its chlorides, in which sewage sludge ash having a phosphorus content, expressed as % by weight of elemental phosphorus in the ash, of at least 2 % by weight is reacted with elemental chlorine in the presence of a carbon source at a temperature of 350 to 900°C and the phosphorus chlorides formed are separated from the exhaust gas stream.

2. Process according to claim 1, characterized in that the sewage sludge ash used has a phosphorus content, expressed as percentage by weight of elemental phosphorus in the ash, of at least 3%, preferably from 3 to 29% by weight.

3. Process according to at least one of claims 1 or 2, characterized in that the sewage sludge ash used has an iron content of 5 to 21 wt.%, calculated as elemental iron.

4. Process according to at least one of claims 1 to 3, characterized in that the reaction with elemental chlorine takes place at a temperature of 450 to 850°C.

5. Process according to at least one of claims 1 to 4, characterized in that the sewage sludge ash used has an iron content and iron chloride formed is discharged together with phosphorus chlorides formed in the exhaust gas stream, and the iron chloride and the phosphorus chlorides are separated from the exhaust gas stream.

6. Process according to at least one of claims 1 to 5, characterized in that phosphorus chlorides formed from the exhaust gas stream, in particular phosphorus oxychloride and optionally phosphorus trichloride and / or phosphorus pentachloride, are separated by condensation.

7. Process according to at least one of claims 1 to 6, characterized in that the carbon source is selected from the group consisting of graphite, soot, coal, coke, activated carbon, carbon monoxide, oils, methane, polyethylene glycol, biowaste, and in particular sewage sludge.

8. A process according to at least one of claims 1 to 7, characterized in that the ratio of the weight fractions of the carbon in the carbon source to sewage sludge ash is greater than 0.01, preferably from 0.04 to 0.

5.

9. A process according to at least one of claims 1 to 8, characterized in that, if the carbon content in the carbon source is less than 70 wt. %, the mixture of sewage sludge ash and carbon source is treated with chlorine gas under inert gas at temperatures of 250 to 800°C, preferably at 350 to 550°C, before the reaction until the gas formation of volatile components is less than 1 l / 1 kg of carbon source used / hour.

10. Process according to at least one of claims 1 to 9, characterized in that the residue is separated from insoluble components with water.