METHOD AND DEVICE FOR EXTRACTING PHOSPHORUS FROM SEWAGE SLUDGE

Description

[0001] The invention relates to a method for obtaining phosphorus from sewage sludge according to the preamble of claim 1.

[0002] EP 2 160 438 B1 discloses a process for the production of phosphorus pentoxide (P₂O₅) based on a process originally invented by Robert A. Hard, and therefore also known as the Hard process after its inventor. The process involves the formation of a furnace bed using charge agglomerates in a counter-current rotary kiln. The agglomerates contain phosphate ore particles, carbonaceous material particles, and sufficient silicon dioxide particles. The agglomerates are intended to have a calcium-to-silicon dioxide molar ratio of less than 1.0, with individual agglomerates having essentially the same elemental composition, the same calcium-to-silicon dioxide molar ratio, and the same proportion of excess solid carbon compared to the theoretical carbon requirement for the reduction of all phosphate in the ore.During the process, a bed temperature of or above 1180 °C is maintained along a portion of the bed length. Furnace exhaust gas is generated, and phosphorus pentoxide is simultaneously recovered from this exhaust gas. The furnace discharges a residue containing processed agglomerates, with less than 10% of the phosphate input from the agglomerates to the furnace remaining as phosphate in the residue.

[0003] Processes for the production of phosphorus pentoxide are known from US 2013 / 0136682 A1. EP 0 972 577 A2 discloses a process for the extraction of phosphorus from organic sludge. DE 20 2018 004 168 U1 relates to a phosphorus plant in an air-conditioning design. A process for the production of phosphorus pentoxide is known from WO 2008 / 153521 A1. WO 93 / 00555 A1 discloses a process for the combined incineration of sewage sludge and waste.

[0004] From MATTENBERGER H ET AL: "Sewage sludge ash to phosphorus fertilizer (II): Influences of ash and granulate type on heavy metal removal", WASTE MANAGEMENT, ELSEVIER, NEW YORK, NY, US, Vol. 30, No. 8 - 9, August 1, 2010 (2010-08-01), pages 1622-1633, XP027080243, ISSN: 0956-053X, DOI: 10.1016 / J. WASMAN.2010.03.037, a device for the production of phosphorus from sewage sludge using a rotary kiln is known.

[0005] According to the Ordinance on the Utilization of Sewage Sludge, Sewage Sludge Mixture and Sewage Sludge Compost (Sewage Sludge Ordinance - AbfKlärV), a reorganization of sewage sludge treatment and disposal in Germany is being pursued. In particular, the ordinance aims to return phosphorus to the economic cycle (Federal Law Gazette 27.09.2017).

[0006] According to this amended version of the Sewage Sludge Ordinance, phosphorus recovery is mandatory for sewage sludge with a phosphorus content exceeding 20 g / kg dry matter. This limit is mandatory for wastewater treatment plants with a capacity of over 100,000 population equivalents down to 50,000 population equivalents after a transition period of 12 or 15 years, respectively. For treatment plants with a capacity of less than 50,000 population equivalents and phosphorus concentrations of less than 20 g / kg dry matter, land application is permitted indefinitely (Federal Law Gazette 27.09.2017; Sewage Sludge Ordinance).

[0007] The starting point for phosphorus recycling is the annual phosphorus load of 61,000 tons per year generated by municipal wastewater treatment plants. This involves considering different material flows generated during wastewater treatment. A distinction is made between wastewater (treatment plant effluent), process water (sludge water), sewage sludge, and sewage sludge ash. All of these sources are available for potential phosphorus recovery; however, the highest phosphorus concentrations are found in dewatered sewage sludge and sewage sludge ash.

[0008] The current disposal situation for sewage sludge in Germany is primarily characterized by thermal treatment. Agricultural use represents the second largest disposal method.

[0009] The new regulations significantly restrict the proportion of sewage sludge that can be used in agriculture. Therefore, phosphorus recovery processes that utilize raw sludge, digested sewage sludge, and sewage sludge ash are becoming increasingly important.

[0010] In wastewater from sewage treatment plants, phosphorus most commonly occurs as orthophosphate (PO₄³⁻) in anionic form. Organically bound phosphorus and polyphosphates also exist. Both organically bound phosphorus and polyphosphates can be mineralized or hydrolyzed to orthophosphate by microorganisms.

[0011] Organically bound phosphorus is primarily biologically bound phosphorus. This form is found in biological wastewater treatment. Under anaerobic conditions, bacteria utilize the phosphorus stored in their cell mass (polyphosphates) as an energy source. When the bacteria return to an aerobic environment, they reabsorb dissolved phosphates.

[0012] In wastewater treatment plants, during the phosphorus removal process, phosphate is bound using precipitating agents such as aluminum or iron salts, as well as lime. This chemically binds the phosphates, which are initially dissolved in the water, in the form of insoluble salts.

[0013] Depending on the location of the sampling points, the recovery potential and the type of phosphorus compound differ. In wastewater treatment plant effluent, phosphorus is present in dissolved form as orthophosphate. When recovering phosphorus from the sludge water, the recovery rate depends heavily on the operating mode of the wastewater treatment plant. The points of application after dewatering and thermal treatment offer the greatest recovery potential. There, the phosphorus is biologically and chemically bound in the sludge matrix. However, for recovery, it must be released. After thermal treatment, usually in a dedicated incineration plant, the phosphorus is chemically bound in the sewage sludge ash.

[0014] Due to the significant mass loss during the combustion of sewage sludge, a concentration of phosphorus is achieved in the sewage sludge ash. Therefore, sewage sludge ash has the highest phosphorus content compared to other forms of residue.

[0015] In the process water of wastewater treatment plants, dissolved phosphorus accumulates through precipitation and separation in the sewage sludge. During subsequent dewatering, 50 to 80% of the water is removed.

[0016] Depending on the type of dosing points, three types of treatment procedures can be distinguished. In pre-precipitation, the precipitating agent is added before the settling tank. Generally, almost all precipitating agents can be used.

[0017] In simultaneous precipitation, the precipitating agent is added before, after, or directly into the aeration tank. Iron(III) salts are preferably used in this treatment process, but aluminum(III) salts and iron(II) salts are also possible. Simultaneous precipitation represents an immediate measure for phosphorus removal; subsequently, the precipitated phosphorus is removed along with the excess sludge.

[0018] Post-precipitation is an independent precipitation stage installed downstream of the secondary clarifiers. In addition to dosing and mixing systems, this requires additional reaction tanks such as flocculation tanks. All precipitating agents can be used in this process, however, consumption is significantly higher than in other treatment methods.

[0019] Iron (Fe³⁺) in its trivalent form is used as iron chloride (FeCl₃) or iron(III) sulfate (Fe₂SO₄)₃. Iron (Fe₂SO₄) in its divalent form, iron sulfate (FeSO₄), which is oxidized to Fe³⁺ by reaction with oxygen, can also be used. 4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

[0020] These precipitating agents form sparingly soluble iron phosphate (FePO₄), which improves floc formation and settling properties. Simultaneously, the precipitating agent also leads to the elimination of polyphosphates and organic phosphorus. FeCl₃ + PO₄³⁻ → FePO₄ + 3 Cl⁻ Fe₂(SO₄)₃ + 2 PO₄³⁻ → 2 FePO₄ + 3 SO₄²⁻

[0021] In the form of aluminum sulfate, Al₂(SO₄)₃ · 18 H₂O, the phosphorus present is precipitated by means of Al³⁺. The trivalent aluminum ion forms readily settleable flakes and is therefore often used in the corresponding treatment process. Al₂(SO₄)₃ + 18 H₂O + 2 PO₄³⁻ → 2 AlPO₄ + 3 SO₄²⁻ + 18 H₂O

[0022] In calcium phosphate precipitation, a softening process of the wastewater is initiated using calcium hydroxide, resulting in a precipitate of calcium carbonate. Calcium phosphate precipitation only begins when 60 to 80% of the calcium carbonate has been formed. 3 Ca(OH)₂ + 2 PO₄³⁻ → Ca₃(PO₄)₂ + 6 OH⁻

[0023] Precipitation with lime milk is becoming increasingly difficult due to the problem of high sludge production and lime precipitation in the sludge treatment plant's pipelines. Phosphorus is found in various fractions of the wastewater treatment plant. Consequently, the phosphorus load also differs in the material flows under consideration.

[0024] There are various phosphorus recycling processes that are operated on a large industrial scale. These are primarily thermochemical and metallurgical processes.

[0025] From DE 102 43 840 B4, a process for separating heavy metals from phosphate-containing sewage sludge ash is known. In this process, alkali and / or alkaline earth chlorides are mixed into the ash. The mixture is then heated above the boiling point of the heavy metal chlorides that form in a closed system, for example, in a rotary kiln. The heavy metal chlorides such as cadmium, copper, mercury, lead, molybdenum, tin, and zinc that escape from the mixture form volatile metal chlorides and oxide chlorides, which volatilize from the ash into the exhaust gas and are then collected separately.

[0026] Current industrial applications involve the use of sodium sulfate as an additive to ash. Rhenanite (CaNaPO₄) is intended to be the main component of the mineral phosphorus stage, which is also available as a phosphorus fertilizer. The process is based on the calcination of the phosphorus. Sodium sulfate, sewage sludge (as a dry substance), and ash from the hot gas cyclone are added and treated at 900 to 1,000°C in a ventilated rotary kiln. After thermochemical treatment, the product is granulated and dried. The resulting exhaust gas, which contains heavy metals among other substances, is treated in several stages using an exhaust gas purification system (see Schaaf, T., Hermann, L. (2016): Process for fertilizer production from sewage sludge ash - ASH DEC process. Published by Outotec GmbH & Co. KG. Hesse. Available online at https: / / umwelt.hessen.de / sites / default / files / media / hmuelv / 10 impulsvortrag ash dec verfahr en.pdf; Adam, C.; Peplinski, B.; Michaelis, M.; Kley, G.; Simon, F-G (2009): Thermochemical treatment of sewage sludge ashes for phosphorus recovery. In: Waste management (New York, N.Y.) 29 (3), S. 1122 bis 1128. DOI: 10.1016 / j.wasman.2008.09.011; Stemann, Jan; Peplinski, Burkhard; Adam, Christian (2015): Thermochemical treatment of sewage sludge ash with sodium salt additives for phosphorus fertilizer production - Analysis of underlying chemical reactions. In: Waste management (New York, N.Y.) 45, S. 385-390. DOI: 10.1016 / j.wasman.2015.07.029).

[0027] A metallurgical phosphorus recycling process, combining the material and energy recovery of phosphate-containing waste, is also known from EP 2 874 763B1. For this purpose, sewage sludge and sewage sludge ash are pressed into briquettes and mixed with limestone and foundry coke. The coke is intended to provide the necessary thermal energy and contribute to the reducing atmosphere in the furnace shaft. The mineral components of the sewage sludge are melted into slag, for example, in a cupola furnace at 1,450 to 2,000 °C. In the furnace shaft, volatile heavy metals evaporate and are separated in the gas cleaning system. The synthesis gas formed at these temperatures can be used for energy recovery together with the waste heat. At higher temperatures, the remaining metals melt and form an iron-rich slag, which, due to its higher density, collects in the hearth of the furnace.Phosphorus-rich liquid slag, located above the molten metal in the furnace hearth, is separated from the iron-rich melt by tapping at varying heights. This process yields phosphate-containing slag, an iron-rich metal alloy, and synthesis gas as a byproduct. It is known that the phosphorus content varies among the recovered products. Phosphorus is found in both the iron tapping and the filter dust. In the granulated slag, a phosphorus content of only 2.2 to 2.5 percent by mass is achieved.

[0028] In another known thermal process for phosphorus recycling, elemental phosphorus is obtained from sewage sludge ash at temperatures of at least 1,500 °C under reducing conditions and subsequently converted to phosphoric acid.

[0029] In another known thermal process, phosphorus pentoxide can be produced via two zones using a rotary kiln. Sufficient addition of silica prevents the formation of melts. In the first reducing zone, carbon monoxide is formed: 2 Ca3(PO4)2 + 6 SiO2 + 10 C → 6 CaSiOs + 10 CO + P4

[0030] In a second oxidizing zone, afterburning takes place in the gas phase: P₄ + 5 O₂ → 2 P₂O 5 CO + ½ O₂ → CO₂

[0031] In a downstream exhaust gas cleaning system, the gas is dedusted in a cyclone. Subsequently, the phosphorus pentoxide is absorbed in a scrubber, forming phosphoric acid.

[0032] This procedure was improved according to WO 2005 / 118468 A2, US 7 378 070 B2, US 7 910 080 B2, US 2013 / 0136682 A1 and US 2016 / 0090305 A1.

[0033] A high-temperature reaction is revealed that takes place within the furnace bed: Ca 10 (PO 4 ) 6 F 2 + 9 SiO 2 + 15 C → 3 P 2 ↑ +15 CO ↑ +9 CaSiO 3 + CaF 2

[0034] For the described reaction, it is advantageous to maintain a uniform temperature profile with sufficient residence times. A minimum temperature of 1180 °C is mentioned, but a temperature of 1225 to 1250 °C is recommended. As shown in the reaction equation, the carbon should be in reactive form with a molar ratio of C : P of at least 2.5. To shift the chemical equilibrium towards the products, the carbon should be present in a superstoichiometric ratio relative to phosphorus. Furthermore, the formation of iron phosphides is described in connection with the complete removal of phosphorus from the furnace load. When fluorapatite is reduced, phosphorus metal vapor and carbon monoxide are formed as vaporous or gaseous reaction products. When atmospheric pressure is exceeded, the gas mixture escapes from the pellets into the surrounding furnace atmosphere (the furnace is generally operated at atmospheric pressure).The remaining phosphorus in the pellet residue is completely bound to iron in the form of FeP and Fe₂P. The object of the invention is to provide a process for recovering phosphorus from sewage sludge.

[0035] This problem is solved as specified in claim 1.

[0036] According to the invention, it has been found that it is possible to transfer the Hard method, as shown in the patent documents referred to above, to use in sewage sludge.

[0037] The sewage sludge is mechanically dewatered beforehand. Depending on the chemical composition of the sewage sludge, it may be necessary to add and mix in finely ground quartz sand with a grain size of less than 100 µm.

[0038] The stoichiometric ratio of the reactants phosphorus (P), carbon (C), and silicon dioxide (SiO₂) is: P : C : SiO₂ = 2:5:3 (molar) or ≈ 1:1:3 (by mass fractions). For a technical process, this ratio must be modified so that all reactants except the target element are added in superstoichiometric amounts to maximize the phosphorus yield. Experience in phosphorus recovery has shown that carbon should be added 3 times and SiO₂ 1.7 times superstoichiometrically. Therefore, the actual ratio of the input materials P : C : SiO₂ ≈ 1:3:5 by mass fractions can be considered the standard mixing ratio. In practice, sewage sludge naturally contains a superstoichiometric proportion of carbon. However, the ratio of solid carbon to volatile components must be taken into account.Many sewage sludges contain volatile components of more than 50 percent by mass, which are no longer available for reaction at 900 °C on the bottom of the rotary kiln.

[0039] Therefore, the addition of solid carbon cannot always be avoided.

[0040] The quartz sand alone must be added in a superstoichiometric quantity. This pellet-like product mixture is then fed into a rotary kiln.

[0041] The highest phosphorus yields are achieved with increasing temperature, for example, at 1,250 °C or higher. At this temperature, the residence time should not be less than twenty minutes. Phosphorus recovery depends significantly on the precipitating agent and the Fe:P mass ratio or molar ratio of iron to phosphorus. In the high-temperature process, it has been shown that the iron-based precipitating agents (mostly FeCl₃) and the resulting high iron content severely limit phosphorus recovery from the sewage sludge. Phosphorus recovery decreases linearly with respect to the Fe:P mass ratio of 2. The higher the iron content, the lower the phosphorus recovery, so the Fe:P molar ratio in the sewage sludge must be lower than 0.95 to achieve a phosphorus recovery of more than 50% of the phosphorus present in the sewage sludge.

[0042] According to the invention, a phosphorus recovery rate of over 80% by mass is achieved in sewage sludge using the Hard process. The iron-to-phosphorus ratio is the decisive criterion for phosphorus recovery from sewage sludge. The less iron the sewage sludge contains, the better the phosphorus recovery rate.

[0043] According to one embodiment of the method, the heating process takes place within ten minutes, in particular within less than five minutes.

[0044] The invention also relates to a device for carrying out this process. According to the invention, a rotary kiln is used which is equipped with a feeding device for supplying sewage sludge.

[0045] Means for the removal of the slag are also provided.

[0046] The feeding device is connected to a transport means, in particular at least one conveyor belt, for transporting pelletized or coke-shaped pre-dried sewage sludge to a rotary kiln and includes heating means for heating the sewage sludge in the rotary kiln, as well as means for collecting phosphorus pentoxide and means for removing slag.

[0047] The invention is explained in more detail below using exemplary embodiments. The figures shown are: Fig. 1 shows the gaseous phosphorus discharge from sewage sludge in mass percent into the gas phase as a function of the molar ratio of iron to phosphorus in various wastewater treatment plants; Fig. 2 shows columnar representations of the percentage of phosphorus discharge depending on the precipitating agent; Fig. 3 shows a longitudinal section through a rotary kiln filled with sewage sludge particles in a first embodiment; Fig. 3 shows a cross-section through the rotary kiln according to Fig. 3a along a section line A - A and Fig. 4 a plant for feeding sewage sludge, carbon and silicon dioxide to a rotary kiln as well as for producing phosphorus pentoxide and for removing a sewage sludge residue from which the phosphorus has been removed.

[0048] In the plot of thermal phosphorus discharge 1 ( Fig. 1 From various sewage sludges, the ratio of Fe to P by mass is given by a linearly decreasing curve. Therefore, iron concentration has a large, if not the largest, influence on the degree of phosphate degradation. Fig.1 The results are presented using examples from various wastewater treatment plants 3, 4, and 5. The higher the iron content, the lower the phosphorus discharge, as is particularly evident in a rotary kiln.

[0049] A plausible explanation for the observed mechanism, involving the formation of iron phosphides, has been found: According to investigations by the Federal Institute for Materials Research and Testing (BAM), sewage sludge ash generally contains phosphorus contents of 1.5 to 13.1 wt%. Furthermore, it is assumed that sewage sludge ash contains aluminum contents of 0.7 to 20.2 wt%, iron contents of 1.8 to 20.3 wt%, and calcium contents of 6.1 to 37.8 wt%. Since these elements are used for phosphorus precipitation, they significantly influence the composition of the sewage sludge, while the apatite treated according to the Hard process consists mainly of calcium phosphate and the accompanying element fluorine. Structural investigations provided evidence that iron phosphides (Fe₂P and FeP), which have a low vapor pressure, are formed at high temperatures under reducing conditions.Therefore, according to the invention, it is assumed that the low phosphorus yield is caused by the formation of iron phosphides. The invention is explained in more detail below using an exemplary embodiment with reference to the drawings. Fig. 2 The results of the thermochemical phosphorus recovery from calcium-, aluminium- and iron-precipitated sewage sludge are presented.

[0050] In Fig. 2 The thermochemical phosphorus removal 1 is shown as a percentage of the total phosphorus contained in the sewage sludge; column 2 indicates the removal of phosphorus as calcium phosphate (Ca3(PO4)4) when calcium is used as a precipitating agent, column 3 the removal of phosphorus as aluminum phosphate (AlPO4) when aluminum is used as a precipitating agent, and column 4 the removal of phosphorus as iron phosphate (FePO4) when iron is used as a precipitating agent. It is evident that iron significantly reduces the efficiency of thermochemical phosphorus recovery.

[0051] When an aluminum-based precipitating agent is used, such as aluminum sulfate (Al₂(SO₄)₃·18H₂O), up to 87.5% of the phosphorus can be recovered via thermochemical high-temperature conversion. The residue then has a phosphorus content of less than 20 g / kg. In accordance with the sewage sludge ordinance, the requirement of a limit value of less than 20 g / kg dry matter sewage sludge and a recovery rate of at least 80% is met.

[0052] The revised Sewage Sludge Ordinance, cited above, has significantly tightened the requirements for the use of thermal processes. For example, phosphorus recovery from sewage sludge must be carried out using a process that guarantees a reduction in phosphorus content of at least 50% or to less than 20 grams per kilogram of dry matter. At least 80% of the phosphorus must be recovered from ash or carbonaceous residues remaining after pretreatment of the sewage sludge (see Sections 3a-3c of the Sewage Sludge Ordinance).

[0053] Since sewage sludge, unlike phosphate ore, has a higher diversity in its elemental composition, the extraction of a pure phosphorus-containing product is limited, for example, by heavy metals.

[0054] According to the invention, the process is carried out in a rotary kiln 10 ( Fig. 3a, 3b ) carried out according to the type of a rolling process, as is known from EP 3 243 915 A1 for the use of rolling oxide from zinc-containing raw materials, for example.

[0055] The rolling mill process belongs to a series of processes in which the enrichment of the oxide components to be recovered occurs via the formation of an intermediate metal phase followed by volatilization and reoxidation in the gas stream. The unwanted residues remain predominantly in a highly viscous residue.

[0056] Due to rotation and an inclination relative to the horizontal, the solid feed is gradually moved towards the discharge end against the gas flow. The system thus operates on the so-called countercurrent principle. The residence time of the feed material depends on the lining, length, inclination, and rotational speed of the rotary tube furnace. The sewage sludge passes through three zones: a drying zone, a heating zone with combustion of carbon-containing substances, a main reaction zone, and a reoxidation zone. The sewage sludge is heated for a period of ten minutes or at least five minutes.

[0057] Cool feed material, for example, sewage sludge 12 pelletized with quartz sand, is fed in via an inlet 11, for example, via a chute, a product chute, or a conveyor belt. The sewage sludge 12 has a moisture content of no more than 10% or less, for example, 7%, and forms a bed 13 on the bottom of the rotary kiln 10. Above this bed, a hot kiln atmosphere develops in a drying zone 14. This causes free and bound water to evaporate, and the batch or feed of pelletized sewage sludge 12 dries. Some carbon-containing volatile components from the sewage sludge 12 are driven off in the drying zone 14.Due to the temperature present in the rotary kiln 10 in the area above the drying zone 14, a combustion process takes place there exclusively in the kiln atmosphere above the packing 13 or at the contact surface between the kiln atmosphere and the packing 13. In a main zone 15, phosphorus oxide-rich raw gas escapes from the packing into the kiln atmosphere and leaves the rotary kiln 10 on the inlet side via a flue pipe 16 and is subjected to a multi-stage exhaust gas treatment for product recovery and purification.

[0058] The sewage sludge 12 introduced into the rotary kiln 10 can be in the form of sewage sludge coke, sewage sludge briquettes or sewage sludge pellets or as other granules.

[0059] In this main zone 15, the reduction of the phosphorus compounds present in the bed 13 begins. Since phosphorus reduction is an endothermic process, the amount of carbon required in the rotary process is not determined by the stoichiometric amount needed for phosphorus reduction, but rather by the heat demand of the process. Therefore, carbon must be present in significantly superstoichiometric amounts or added in the form of, for example, coke. The reducing agent carbon contained in or added to the sewage sludge 12 initially reacts with atmospheric oxygen to form carbon dioxide, which then reacts with solid carbon according to the Boudouard reaction to form carbon monoxide. The carbon monoxide can then reduce the phosphorus compounds present.The rolling motion generated by the rotation of the rotary kiln 10 on rotating rollers 17, 18 supports this effect by continuously renewing a contact zone 19 between the charge 13 and the kiln atmosphere in the main zone 15. This results in the discharge 20 of reoxidized phosphate into the gas phase in the form of phosphorus pentoxide, starting from the drying zone 13, producing slag. To maintain this process, the slag must not melt. Therefore, additives are introduced when the sewage sludge 12 is fed into the rotary kiln 10 to prevent melting. Preferably, an excess of silicon dioxide is added, which forces the formation of silicates. Due to the prevailing process conditions—high temperature and sufficiently high vapor pressure—phosphorus evaporates from the charge into the gas space. In the gas phase, the phosphorus vapors are exothermically reoxidized to phosphorus pentoxide.In addition to this reaction, the afterburning of the carbon monoxide contained in the furnace atmosphere also provides heat energy, which is why the process gas heats up further. At the same time, the furnace atmosphere is already becoming depleted of free oxygen.

[0060] In a reoxidation zone 21, adjoining the main zone 15 and also referred to as the ash formation zone, cold, oxygen-rich air is supplied from one end of the rotary kiln 10 via an inlet 22 in a counterflow principle. This cold, oxygen-rich air encounters the bed 13 at the end of the kiln, which is heated there by a burner 23, thus warming the air. Metal compounds present in reduced form in the product batch that have not evaporated are reoxidized here. If, for example, iron is present, it would be reoxidized exothermically to iron oxide. The remaining SiO₂ content prevents the ash from softening and clumping together.

[0061] The mixture of sewage sludge and necessary additives intended for the rotary kiln 10 has been previously micro-pelleted, for example. The rotary kiln 10 is inclined downwards towards the burner 23, so that the packing material 13 is gradually moved towards the burner 23 as the rotary kiln 10 slowly rotates. Below the burner 23, the packing material 13 is discharged from the rotary kiln 10 as ash via an outlet 24.

[0062] To cool the residue, it is passed through a cooler (not shown here). The heat extracted from the residue in the cooler is simultaneously used to heat the inlet air supplied to the rotary kiln 10 via inlet 22. The product gas or product vapor 4, in particular phosphorus pentoxide, escaping as discharge 20, is, after being removed from the rotary kiln 10 via the exhaust pipe 16, passed through a dust removal stage and a hydrator to form phosphoric acid, and then purified to produce product phosphoric acid.

[0063] The rotary kiln 10 is particularly suitable for the reduction of phosphate-containing sewage sludge, as it transfers heat directly to a bed of pelletized feed particles. The rotary kiln 10 used according to the invention is of conventional design; it has, for example, stationary end sections and a rotating central section or cylinder, which is lined with and connected to a suitable refractory material. If the burner 23 is arranged off-center at the end wall and the rotary kiln 10 also rotates in the area of ​​the reoxidation zone 21, shingle plates are arranged around the inlet of the burner 23 in the end wall, which prevent uncontrolled intake of air or escape of phosphorus pentoxide from the interior of the rotary kiln 10.

[0064] Fuel and air or oxygen are supplied to the burner 23, so that the burner 23 generates a flame for directly heating the bed 13. The term "flame" is understood to mean either the luminous components of an oxidation reaction, the associated hot gases, or both.

[0065] To initiate the process, a conventional fuel can be used to preheat the central part of the rotary kiln 10 and the bed 13. However, since the reaction in the bed 13 produces elemental phosphorus vapor and carbon monoxide, which are combusted in the main zone 15, also known as the oxidation zone, less fuel is required once the process is running. Sufficient air or oxygen must be provided to safely oxidize the phosphorus and carbon monoxide above the bed 15.

[0066] In summary, the following conditions can be defined for carrying out the process according to the invention: The process requires a strongly reducing atmosphere in the product batch of the furnace and immediately above it (freedom from oxygen, presence of carbon monoxide).

[0067] In the free furnace chamber, an oxidizing atmosphere is required to maintain the basic reactions and to ensure the safe afterburning of elemental phosphorus to phosphorus pentoxide and of carbon monoxide to carbon dioxide.

[0068] The reaction partners of the phosphate, namely carbon and silicon dioxide, must be present in excess and well mixed.

[0069] The sewage sludge must not contain too much iron – iron is frequently used as an efficient phosphorus precipitant, but the phosphorus is hardly recoverable from these compounds, as demonstrated by the invention. The presence of iron phosphate reduces the phosphorus evaporation rate to a low level.

[0070] Sewage sludge treated with aluminum- or calcium-based precipitants is more suitable. Tests with pure aluminum or calcium phosphate show a very high phosphorus evaporation rate.

[0071] The process temperature is above 1,200 °C, preferably above 1,250 °C.

[0072] The residence time of the feed material, i.e., the bulk material 13, in the rotary kiln 10 at the highest temperature used here, 1280 °C, is at least twenty minutes, preferably between twenty and forty minutes. This ensures rapid heating to the process temperature.

[0073] In a further embodiment of the invention ( Fig. 4 Sewage sludge 32, originating from a wastewater treatment plant and pre-dried by centrifuge to a moisture content of 75 to 80%, is applied to a belt dryer 31, where it is further dried at temperatures of 120 to 125 °C until it has a residual moisture content of approximately 10%. The sewage sludge particles 32 present on the conveyor belt 31 form, for example, granules.

[0074] Before the sewage sludge particles 32 are fed to the rotary kiln 10, carbon particles and silicon dioxide particles, in particular in the form of quartz sand, which are fed via a funnel 33, are added to a mixer 26, whereby the sewage sludge particles 32 themselves enter the mixer 26 via a funnel 34.

[0075] From the mixer 26, a sewage sludge mixture 28 produced in this way is fed via a rotary valve 35 to a screw conveyor 25, which introduces the sewage sludge particles 32 into the rotary kiln 10. The screw conveyor 25 extends into the interior of the rotary kiln 10 so that the sewage sludge particles 32 are preheated before they fall to the bottom of the rotary kiln 10. The screw conveyor 25 extends into the side wall of the rotary kiln 10 approximately in the middle or in the lower third of the side wall. A scale seal ensures sufficient sealing of the interior of the rotary kiln 10 from the exterior.

[0076] The mass ratio of phosphorus to silicon dioxide contained in the sewage sludge required for mixing in the mixer 26 is determined continuously or preferably at time intervals after being analyzed in the wastewater treatment plant by X-ray fluorescence analysis or by ICP emission spectrometry (ICP OES) (= Inductively-Coupled Plasma Optical Emission Spectrometry), i.e., in a process of optical emission spectrometry with inductively coupled plasma, it has been determined in what proportions of phosphorus and silicon dioxide are already present in the sewage sludge, whereby carbon is analyzed, for example, coulometrically, so that on the basis of this result, sufficient amounts of carbon and silicon dioxide are added to the mixer 26 until at least a stoichiometric ratio of the mass of phosphorus to the mass of carbon and to the mass of silicon dioxide of 1:1:3 is achieved, so that the silicon dioxide and carbon added to the sewage sludge, if necessary, together with the sewage sludge mixture 28, are fed into the rotary kiln 10, so that the sewage sludge orThe mixture of sewage sludge, added silicon dioxide and added carbon is subjected to a churning process and the expelled phosphorus is collected in the form of gaseous phosphorus pentoxide.

[0077] The addition of carbon and silicon dioxide is achieved in a particularly advantageous manner by adding carbon and silicon dioxide to the sewage sludge in superstoichiometric masses until the ratio of the mass of phosphorus to the mass of carbon and to the mass of silicon dioxide reaches 1:3:5. In this way, a very high proportion of phosphorus, for example 80%, can be extracted from the sewage sludge mixture 28 in the rotary kiln 10, which is constructed as shown in Fig. 3 is shown.

[0078] As in Fig. 3aAs shown, the phosphorus oxide-rich raw gas formed in the furnace atmosphere, i.e., the volatile degassed components of the sewage sludge, leaves the rotary kiln 10 via the exhaust pipe 16, which is located in the upper area of ​​the end wall on the side opposite the burner 23.

[0079] Simultaneously, heated air exits the rotary kiln 10 in the opposite direction to the conveying direction of the sewage sludge mixture in the screw conveyor 25, in a counter-current principle above the sewage sludge mixture 28 supplied via the screw conveyor 25, and removes the volatile hydrocarbons contained in the sewage sludge mixture 28, which has already been preheated by the hot atmosphere of the rotary kiln 10. These hydrocarbons, drawn in by a fan 36, are preferably combusted together with externally supplied combustion air 29 in a combustion chamber 30; the exhaust gases are preferably directed to the belt dryer 31 to assist in the heating of the sewage sludge particles 32.

[0080] Residual sewage sludge, from which the phosphorus has been largely removed, is discharged from the rotary kiln 10 via an outlet 36.

Claims

1. Process for obtaining phosphorus from dried sewage sludge (12), characterized in that the mass ratio of the phosphorus contained in the sewage sludge (12) is determined, that as much carbon and as much silicon dioxide are added to the sewage sludge (12) until an at least stoichiometric ratio of the mass of the phosphorus to the mass of the carbon and to the mass of the silicon dioxide of 1: 1: 3 is achieved, that the sewage sludge (12) together with the silicon dioxide and the carbon are fed to a rotary kiln (10), that the sewage sludge (12) or the mixture of the sewage sludge (12), of the added silicon dioxide and of the added carbon is subjected to a rolling process and the extracted phosphorus is collected in the form of gaseous phosphorus pentoxide.

2. Process according to claim 1, characterized in that carbon and silicon dioxide are added to the sewage sludge (12) in superstoichiometric masses until the ratio of the mass of the phosphorus to the mass of the carbon and to the mass of the silicon dioxide of 1: 3: 5 is achieved.

3. Process according to claim 1 or 2, characterized in that the sewage sludge is introduced into the rotary kiln (10) in the form of sewage sludge coke, sewage sludge briquettes, or sewage sludge pellets or as another granulate.

4. Process according to any of claims 1 to 3, characterized in that a bed (13) formed on the bottom of the rotary kiln (10) and formed from sewage sludge (12) is reduced in a reducing substance, or in an environment of carbon coke or in the presence of a reducing atmosphere, or in the absence of oxygen.

5. Process according to claim 4, characterized in that the process is carried out in the presence of carbon monoxide.

6. Process according to any of claims 1 to 5, characterized in that sewage sludge (12) is used which has previously been treated with an aluminium-based or a calcium-based precipitant.

7. Process according to any of claims 1 to 6, characterized in that the process temperature is above 1.200 °C, in particular above 1.250 °C.

8. Process according to any of claims 1 to 7, characterized in that the exposure time of the sewage sludge (12) in the rotary kiln (10) does not fall below a period of twenty minutes, in particular at a temperature of 1.280 °C.

9. Device for carrying out the method according to one of claims 1 to 8 using a rotary kiln (10), characterized in that it comprises transport means, in particular at least one conveyor belt, for transporting pelletized or coke-shaped pre-dried sewage sludge (12) to the rotary kiln (10) and heating means (21) for heating the sewage sludge (12) in the rotary kiln (10) and means for collecting phosphorus pentoxide and means for removing slag.

10. Device according to claim 9, characterized in that the sewage sludge (12) in the form of sewage sludge particles (32) is conveyed via a belt dryer (31) to a mixing plant comprising a mixer (26) in which carbon particles and quartz sand are admixed to the sewage sludge particles (32) as required to obtain a stoichiometric mass ratio of the mass of the phosphorus in the sewage sludge particles (32), or in that contain carbon and silicon dioxide are added.

11. Device according to claim 10, characterized in that a sewage sludge mixture (28) obtained from the mixer (26) is introduced into the rotary kiln (10) via a screw conveyor (25), in particular via a cellular wheel sluice (35).

12. Device according to any of claims 9 to 11, characterized in that phosphorus-containing raw gas (20), in particular phosphorus pentoxide, produced in the rotary kiln (10) is led out of the rotary kiln (10) via an outlet (16).