Method for recovering phosphorus

Abstract

The present invention concerns a method for recovering phosphorus by thermochemical reaction of a phosphorus-containing material such as an alternative fuel, for example, in the presence of calcium-containing particles in a moving bed reactor and subsequent separation of fines enriched with phosphorus from the moving bed reactor. Furthermore, the present invention concerns the use of a recyclable material obtained by the method as a fertilizer or fertilizer additive.

Claims

1. A method for recovering phosphorus, comprising the following steps: i) thermochemical reaction of a phosphorus-containing material in the presence of calcium-containing particles in a moving bed reactor; ii) separation of fines enriched with phosphorus from the moving bed reactor, wherein the separation of the fines enriched with phosphorus is carried out from the reactor discharge in one or more separation devices which are disposed inside or outside the moving bed reactor, wherein a first separation device of the one or more separation devices has an operating temperature above 400° C., in order to reduce a quantity of heavy metals and/or polyaromatic hydrocarbons; and iii) reaction of the fines enriched with phosphorus obtained from step ii) with an acid in order to obtain a phytoavailable phosphorus-enriched recyclable material comprising water-soluble phosphates including calcium dihydrogen phosphate (Ca(H.sub.2PO.sub.4).sub.2.H.sub.2O) and calcium hydrogen phosphate (Ca(HPO.sub.4).2H.sub.2O) for use as a fertilizer or a fertilizer additive.

2. The method as claimed in claim 1, wherein the thermochemical reaction is carried out at a temperature of up to approximately 1100° C.

3. The method as claimed in claim 1, wherein the thermochemical reaction is carried out in a fluidized bed reactor.

4. The method as claimed in claim 3, wherein the calcium-containing particles have a mean particle size in the range 0.3-3 mm.

5. The method as claimed in claim 1, wherein the phosphorus-containing material is a fuel.

6. The method as claimed in claim 5, wherein the fuel is selected from alternative fuel, biomass, sewage sludge, household waste, industrial waste, fermentation residue, slaughterhouse waste or combinations thereof.

7. The method as claimed in claim 1, wherein the calcium-containing particles essentially comprise CaO and/or CaCO.sub.3.

8. The method as claimed in claim 1, wherein the acid is selected from carbon dioxide, phosphoric acid, sulphuric acid, nitric acid or combinations thereof.

9. The method as claimed in claim 1, wherein the phosphorus content of the fines is from 1% to 10% by weight.

10. The method as claimed in claim 2, wherein the thermochemical reaction is carried out at a temperature of from 600° C.-1100° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an exemplary embodiment of a typical reactor apparatus for a gasification method; and

(2) FIG. 2 shows an exemplary embodiment of a typical reactor apparatus for a combustion method.

(3) Identical reference numerals in FIGS. 1 and 2 have identical meanings.

(4) Gasification processes (FIG. 1) are intended to transform the preferably solid fuel into a gaseous fuel, and thus to obtain chemically stored energy in the form of the calorific value in the product gas. The product gas is then upgraded materially and/or energetically.

(5) Combustion processes (FIG. 2) are intended to transform the chemical energy stored in the fuel into thermal energy as completely as possible. At the end of the process, a flue gas is left which preferably no longer contains chemical energy, but rather only latent and sensible heat.

(6) A typical reactor apparatus for a gasification method is shown in FIG. 1, wherein the reactor components are shown as follows: 1 educt inlet 2 reactor 3 cyclone/separator 4 heat exchanger 5 filter 6 product gas 7 bed material inlet 10 discharge for solid fractions a and b 11 discharge for solid fraction c 12 discharge for solid fraction d 13 discharge for solid fraction e 14 heat exchanger 15 discharge for solid fraction f as well as liquid fraction.

(7) Description of the gasification method shown in FIG. 1 by way of example:

(8) The fuel to be processed is introduced via the educt inlet system 1, in the example here into a fluidized bed gasifier as reactor 2, and reacted under autothermal and/or allothermal conditions. This transformation may be carried out in one or more coupled reactors and takes place at temperatures of >600° C.

(9) The bed material required for the reactor 2 is introduced into the reactor 2 via the bed material inlet system 7.

(10) The solid products in the reactor 2, which primarily consist of heavy inert components, are withdrawn as solid fractions a (bed material/ash enriched with impurities) and b (bed material/ash) via the discharge 10.

(11) Because of the intensive, thorough mixing of the Ca-based bed material, at the surface of the individual grains of bed material, abrasion of the calcium phosphate and of the bed material occurs which is dictated by the hardness of the bed material and by the intensity of the fluidization, and the dust-like fines which are formed are withdrawn overhead from the reactor with the fly ash. The product gas which is produced leaves the reactor 2 in the direction of the subsequent cyclone 3 together with the fly ash and smaller bed material particles (fines) which are entrained because of the speed of the flow.

(12) In cyclone 3, the fraction of the fly ash/bed material mixture which can be deposited is separated and trickles into the solid discharge 11 of the cyclone 3 as the solid fraction c.

(13) This ash has a higher phosphorus content than the bed ash in reactor 2.

(14) The fine fly ash/bed material mixture which cannot be separated in the cyclone 3 passes through the cyclone 3 and enters the downstream heat exchanger 4 where again, deposited fly ash/bed material mixture can be withdrawn from the discharge 12 as the solid fraction d.

(15) Next, the product gas together with the fly ash/bed material mixture still contained in the product gas enters the filter 5. The fly ash/bed material mixture is removed up to a specific particle size and extracted from the method via the discharge 13 for further use as the solid fraction e.

(16) The fly ash/bed material mixtures c, d and e have a higher phosphorus content compared with the solid fractions a and b in reactor 2.

(17) In a downstream heat exchanger 14, the product gas can be cooled further and further settling components of the fly ash/bed material mixture are withdrawn as the solid fraction f via the discharge 15. Upon cooling below the dew point, condensable gas components (vapours) can also be condensed and withdrawn as a liquid.

(18) The cleaned product gas 6 leaves the process and can then be used in subsequent process steps either energetically or materially.

(19) A typical reactor apparatus for a gasification method is shown in FIG. 2, wherein the reactor components are as follows: 1′ educt inlet 2′ reactor 7′ bed material inlet 10′ discharge for solid fractions a′ and b′ 16 vaporizer (heat exchanger) 17 cyclone/separator (can also be dispensed with) 18 filter and/or adsorber 19 economiser or flue gas condenser (heat exchanger) 20 discharge for solid fractions g and h 21 discharge for solid fraction i 22 discharge for solid fraction j 23 discharge for solid fraction k as well as liquid fraction 24 flue gas.

(20) Description of the combustion method shown in FIG. 2 by way of example:

(21) The fuel to be processed is introduced via the educt inlet system 1′, in the example here into a fluidized bed gasifier as reactor 2′, and transformed under autothermal conditions. This transformation may be carried out in one or more coupled reactors and takes place at temperatures of >600° C.

(22) The bed material required for the reactor 2′ is introduced into the reactor 2′ via the bed material inlet system 7′.

(23) The solid products in the reactor 2′, which primarily consist of heavy inert components, are withdrawn as solid fractions a′ (bed material/ash enriched with impurities) and b′ (bed material/ash) via the discharge 10′.

(24) Because of the intensive, thorough mixing of the Ca-based bed material, at the surface of the individual grains of bed material, abrasion of the calcium phosphate and of the bed material occurs which is dictated by the hardness of the bed material and by the intensity of the fluidization, and the dust-like fines which are formed are withdrawn overhead from the reactor with the fly ash.

(25) The flue gas from the combustion leaves the reactor 2′ and passes through the downstream vaporizer 16 together with the fly ash entrained by the speed of the flow and which could possibly be further softened, and bed material particles (fines). The flue gas cools down by exchange of heat; any softened ash solidifies into solid particles. During the turns of the flue gas, in which the direction of the flow is changed from vertically downwards to vertically upwards, the fly ash and bed material particles are separated out gravimetrically and extracted via a plurality of extraction points 20 as solid fractions g and h.

(26) The flue gas leaves the vaporizer 16 in the direction of the optional downstream cyclone/separator 17. In the cyclone 17, the separable fraction of the fly ash/bed material mixture is separated and trickles into the solid discharge 21 of the cyclone 17 as the solid fraction i.

(27) The fine fly ash/bed material mixture which cannot be separated in the cyclone 17 then reaches the filter 18 with the flue gas. The fly ash/bed material mixture is separated in filter 18 up to a specific particle size and is withdrawn from the method at the discharge 22 for a further use as solid fraction j.

(28) The fly ash/bed material mixtures g, h, i and j have a higher phosphorus content compared with the solid fractions a′ and b′ from reactor 2′.

(29) In a downstream economiser or flue gas condenser 19 (heat exchanger), the flue gas can be cooled further and further portions of the fly ash/bed material mixture which can be deposited are withdrawn via the discharge 23 as solid fraction k. Upon cooling below the respective dew points, condensable gas components (vapours) can also be condensed and withdrawn as a liquid.

(30) The flue gas 24 from which the dust has been removed leaves the process and can now be used energetically by further cooling, for example by means of an ORC process.

(31) Binding of phosphorus is carried out: in the fluidized bed process, in the calcium-containing fluidized bed material and/or in the calcium-containing fluidized bed material fines, and/or in the fluidized bed discharge, preferably in the fluidized bed discharge.

(32) The thermochemical reaction is usually carried out at a temperature below the ash softening temperature, in order to prevent vitrification of the ash and the formation of agglomerates and/or deposits. The preferred temperature range is between 400° C. and 1100° C., particularly preferably between 600° C. and 1100° C. The expression “thermochemical reaction” as used herein in particular encompasses pyrolysis and/or gasification and/or combustion of the phosphorus-containing material.

(33) The calcium-containing particles contained in the bed material in particular comprise calcium oxide (CaO) and/or calcium carbonate particles (CaCO.sub.3). Normally, the calcium-containing particles which are in the bed material prior to thermochemical reaction as the starting material essentially comprise calcium carbonate (CaCO.sub.3), preferably in a fraction of >90% by weight with respect to the total weight of the calcium-containing particles. In addition, further mineral components may be present, in particular carbonates of other metals, such as magnesium carbonate, for example. When using dolomite in particular, or fractions of dolomitic materials such as dolomitic limestone, as the bed material, the fraction of magnesium carbonate may also be >10% by weight, with respect to the total weight of the bed material. Dolomite or dolomitic fractions in the bed material have a catalytic property, in that tars produced upon pyrolysis or gasification can be transformed (cracked) into short-chain hydrocarbon compounds. Finally, quartz sand and/or silica sand may be used as the bed material.

(34) During the thermochemical reaction, the corresponding oxides are formed from the carbonates in the starting material for the reactor bed, i.e. more and more calcium oxide is formed from calcium carbonate, so that the fraction of oxides such as calcium oxide, for example, initially increases during the thermochemical reaction, but during the further course of the thermochemical reaction, the oxides particularly on the surface of the particles are increasingly transformed into the corresponding phosphates.

(35) In a preferred embodiment, the calcium-containing particles which are in the reactor prior to the thermochemical reaction as the starting material essentially consist of calcium oxide (CaO)/magnesium oxide (MgO) and/or calcium carbonate (CaCO.sub.3)/magnesium carbonate (MgCO.sub.3) particles, wherein the quantity of CaO/MgO and/or CaCO.sub.3/MgCO.sub.3 is preferably >90% by weight.

(36) The calcium-containing particles are preferably present in the form of bulk material, preferably in the bed material of the reactor, in particular as reactive bed material. A preferred bed material is the material described in WO 2009/100937, which preferably consists of micritic lime. The fraction of calcium and/or magnesium-containing particles in the bulk material is preferably 90% by weight, with respect to the total weight of the bulk material. In particular, the quantity of calcium- and/or magnesium-containing particles in the bulk material is selected in a manner such that the phosphorus-binding particles are in a stoichiometric excess with respect to the phosphorus content of the phosphorus-containing material so that as complete a reaction of the phosphorus into phosphates as possible (essentially calcium phosphates and/or magnesium phosphates) is made possible. In the present invention, it was surprisingly discovered that the use in particular of CaO/CaCO.sub.3-based fluidized bed materials not only results in positive fluidized bed material properties such as a high catalytic activity and mechanical stability, but also results in a significant increase in the vitrification temperature.

(37) In addition to the calcium-containing particles, as mentioned above, the reactor material may also comprise oxides and carbonates of other metals. Preferred metal oxides are of the M.sub.xO.sub.y type, wherein M designates a metal, for example Sr, Ba, La, Mn or Y, and x and y, as is customary, designate whole numbers.

(38) In further embodiments, the metal oxide may comprise a fraction of iron oxide and/or silicon dioxide. Optionally, it may also contain aluminium oxide.

(39) Preferred carbonates are of the type M.sub.x(CO.sub.3).sub.y, wherein M is a metal such as Sr, Ba, La, Mn or Y, and x and y, as is customary, designate whole numbers.

(40) The calcium-containing particles which are in particular used in the thermochemical reaction in a fluidized bed reactor usually have a mean particle size in the range 0.1 mm to 3 mm, particularly preferably in the range 0.5 mm to 1.5 mm. The particles are preferably essentially free from particles with a particle size of <100 μm, in particular <50 μm, which as a rule constitute a large proportion of the fines which are discharged at the normal gas velocities.

(41) In a preferred embodiment, the calcium-containing particles essentially consist of particles which have a core formed by at least one carbonate and a sheath (or cover) surrounding the core formed from at least one metal oxide. In a further, particularly preferred embodiment, the particles have a core formed by at least one metal oxide and a sheath (or cover) surrounding the core formed by at least one carbonate. Preferably, the sheath (or cover) essentially completely encloses the core.

(42) By means of the thermochemical reaction of the calcium-containing particles in the presence of phosphorus-containing material below the ash softening temperature, phosphorus contained in the phosphorus-containing material is chemically bound in the form of a phosphate and thus rendered useful, for example in the form of calcium metaphosphate (Ca.sub.2P.sub.2O.sub.7) or tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2).

(43) Because binding of the phosphorus contained in the fuel occurs primarily on the surface of the calcium-containing particles, phosphorus enrichment occurs, dictated by the material abrasion during the thermochemical reaction, in the discharge of the reactor such as, for example, in the fluidized bed discharge, or dictated by the material fines entrained with the product gas in the discharge points disposed downstream on the product gas side. Thus, by using the mechanically stable particles described above, the fraction of phosphorus in the fines can be raised, and thus the efficiency of the method can be raised further. Furthermore, with the fines, other fertilizer-relevant substances such as potassium carbonate, potassium phosphate, magnesium carbonate and magnesium phosphate, can be discharged. The fraction of these further fertilizer-relevant substances can advantageously be controlled by means of the fraction of the corresponding metal oxides or metal carbonates in the bulk material.

(44) Since, as already described, the phosphorus enrichment occurs above all by abrasion of the calcium-containing particles, separation of the particles enriched with phosphorus is usually carried out from the reactor discharge, preferably in one or more separation devices such as, for example, in one or more separation devices of a fluidized bed reactor or in the discharge points disposed downstream on the product gas side. Separation of the fines thus occurs in at least one separation device or in a plurality of separation devices connected in series. By binding pollutants such as, for example, the long chain hydrocarbons described above, sulphur, chlorine, fluorine or heavy metals, into the bed material of the reactors, separating the particles enriched with phosphorus from the reactor discharge, means that a phosphorus-enriched product can be obtained which is free from disruptive side products or which only contains a small quantity thereof.

(45) In an alternative embodiment, the phosphorus enrichment may also occur in the bed material of the reactor. In this regard, the bed material binds the phosphorus by forming the phosphate compounds described above in the bed material of the reactor. In FIG. 1, the bed material enriched in this manner is discharged as material stream b via the discharge 10 which is withdrawn directly from the reactor 2.

(46) In a preferred embodiment, the reactor is provided with one or more separation devices which usually operate at different operating temperatures and which may be disposed inside or outside the moving bed reactor. As an example, the temperature of a first separation device may be in the range 500-150° C., preferably in the range 600-700° C., and the temperature of a second separation device may be in the range 100-400° C., preferably in the range 150-250° C.

(47) Separation of the typically particulate fines enriched with phosphorus is primarily carried out from the discharge for the flue gas and/or gasification gas, for example from the fluidized bed discharge, in a first or more separation devices disposed downstream. The operating temperature for the first separation device or plurality of separation devices should preferably be over 400° C., preferably between 600° C. and 1100° C., with respect to a fluidized bed method operated at atmospheric pressure, in order to reduce the quantity of heavy metals, i.e. metals with a density of >5 g/cm.sup.3 such as, for example, cadmium, lead, chromium, mercury, arsenic, cobalt and thallium, and/or other pollutants such as, for example polyaromatic hydrocarbons from pyrolysis and/or gasification and/or combustion.

(48) The separation of residual materials such as, for example, residual amounts of calcium-containing particles which have not reacted with phosphorus and/or heavy metals with a density of >5 g/cm.sup.3 such as, for example, cadmium, lead, chromium, mercury, arsenic, cobalt, thallium and/or other pollutants such as, for example polyaromatic hydrocarbons from pyrolysis and/or gasification, is usually carried out in a second or more downstream separation devices. The operating temperature of these separation device (s) should preferably be below 400° C., preferably below 200° C.

(49) Depending on the phosphorus content of the material used for the thermochemical reaction, the fines enriched with phosphorus obtained after separation usually have a phosphorus content of 1% to 5% by weight, preferably up to 10% by weight, with respect to the total weight of the fines. Particularly when a phosphorus-rich starting material is used such as sewage sludge, for example, phosphorus contents in the fines of 5-10% by weight may be obtained using the method in accordance with the invention.

(50) In order to improve the phytoavailability of the phosphorus in the fines enriched with phosphorus, the particles may optionally be acidified. In particular, neither calcium metaphosphate (Ca.sub.2P.sub.2O.sub.7) nor tricalcium phosphate (Ca.sub.3(PO.sub.4).sub.2) is sufficiently soluble in water, and so these substances do not have direct phytoavailability. The formation of water-soluble HPO.sub.4.sup.2− and H.sub.2PO.sub.4.sup.− ions is important in obtaining good phytoavailability. The formation of these ions is obtained in the present invention by adding an acid to the fines. The acid is preferably selected from CO.sub.2 or carbonic acid, phosphoric acid, sulphuric acid, nitric acid or combinations thereof.

(51) By using an acid, water-soluble phosphates such as calcium dihydrogen phosphate (Ca(H.sub.2PO.sub.4).sub.2)*H.sub.2O and calcium hydrogen phosphate (Ca(HPO.sub.4)*.sub.2H.sub.2O) can be formed. In particular, calcium dihydrogen phosphate has a high solubility in water of 18 g/L under standard conditions of 25° C. and 1 bar, and thus is highly phytoavailable. As an example, upon reaction with an acid, in a first step tricalcium phosphate is transformed into calcium hydrogen phosphate, as shown in the following formula (1):
Ca.sub.3(PO.sub.4).sub.2+2H.sup.+⇄Ca(HPO.sub.4)+Ca.sup.2+  (1)

(52) In a second step, the reaction may be carried out in phosphoric acid, which is an important starting component in fertilizer production, as shown in the following formula (2):
Ca(HPO.sub.4)+2H.sup.+⇄=H.sub.3PO.sub.4+Ca.sup.2+  (2)

(53) In a preferred embodiment, the acidification is carried out with CO.sub.2 or carbonic acid, for example by adding water and CO.sub.2. In this regard, the CO.sub.2 may, for example, be obtained from biogas, gas from purification plants, flue gas or product gas, in particular from the combustion of the phosphorus-containing material in the method in accordance with the invention. In this regard, it is generally sufficient for the CO.sub.2 formed by the thermochemical reaction to be brought into contact with steam which is also formed during the combustion or with moisture from the air prior to acidification, so that an external addition of water is not necessary. By using the CO.sub.2 formed during the thermochemical reaction, the efficiency of the method can be further increased and the CO.sub.2 dump can be reduced. Particularly in the case when biogas is used, its CO.sub.2 content can advantageously be reduced.

(54) In this case, calcium hydrogen phosphate and calcium dihydrogen phosphate are formed in accordance with the following formulae (3) and (4):
Ca.sub.3(PO.sub.4).sub.2+3H.sub.2O+CO.sub.2⇄2Ca(HPO.sub.4).Math.2H.sub.2O+CaCO.sub.3  (3)
Ca.sub.3(PO.sub.4).sub.2+3H.sub.2O+2CO.sub.2⇄Ca(H.sub.2PO.sub.4).sub.2.Math.H.sub.2O+2CaCO.sub.3  (4)

(55) In the reaction represented in formula (3), the thermodynamic equilibrium at 25° C. is completely on the product side even at a system pressure of 1 bar. The graph of the thermodynamic equilibrium of the reaction represented in formula (4) at 25° C. with increasing pressure shows that even at a system pressure of 1 bar, calcium dihydrogen phosphate is formed more strongly. Thus, according to this preferred embodiment, by using the practically freely available substances H.sub.2O and CO.sub.2 which have already been formed, calcium hydrogen phosphate and calcium dihydrogen phosphate are formed, i.e. phosphorus in a phytoavailable form.

(56) In a further preferred embodiment, the acidification is carried out in two stages. In this regard, in a first step, acidification is carried out with CO.sub.2/carbonic acid, and in a second step, acidification is carried out with a further acid selected, for example, from phosphoric acid, sulphuric acid and/or nitric acid.

(57) The useful or recyclable material enriched with phosphorus obtained by the method in accordance with the invention is of particular application for use as a fertilizer or fertilizer additive such as, for example, as an additive for a multi-nutrient fertilizer or its production by processing the recyclable material enriched with phosphorus which is obtained. In a preferred embodiment, the recyclable material enriched with phosphorus may be used as a fertilizer or fertilizer additive without any further processing. By adding a nitrogen (N) and potassium (K) carrier, and optionally a sulphur (S) carrier, a NPK(S) multi-nutrient fertilizer can in particular be obtained which can be used in any agricultural areas as well as in the garden and ornamental plant areas.