Method of treating phosphate-containing ash from waste-incineration plants by wet-chemical digestion in order to obtain compounds of aluminium, calcium, phosphorus and nitrogen

Abstract

The invention concerns a method of treating phosphate-containing waste, in particular phosphate-containing ash from waste-incineration plants, by wet-chemical digestion in order to obtain compounds of aluminum, calcium, phosphorus and nitrogen.

Claims

1. A method for obtaining precipitates of calcium sulphate (CaSO.sub.4) and for production of phosphoric acid from phosphate-containing ashes from waste incineration plants, characterized in that a) combining the phosphate-containing ashes with phosphoric acid to cause a reaction, b) separating an acid-insoluble portion of the phosphate-containing ashes from the reaction of step a), c) adding sulfuric acid to a filtrate or supernatant of step b) and thereby adjusting the pH to <1, to obtain and separate calcium sulphate precipitate and to thereby also obtain purified phosphoric acid as the filtrate or the supernatant of step c), d) recycling at least partially the filtrate or supernatant of step c) as the phosphoric acid for use in step a).

2. The method according to claim 1, additionally comprising: e) adding calcium oxide or calcium carbonate to the remainder of the filtrate or supernatant of step d) to obtain and separate calcium phosphate precipitate and calcium nitrate precipitate.

3. The method according to claim 1 additionally comprising: e) raising the pH-value of the filtrate or the supernatant of step d) to obtain and separate the dissolved aluminium as aluminium hydroxophosphate precipitate, and f) adding calcium oxide or calcium carbonate to the remainder of the filtrate or supernatant of step d) to obtain and separate calcium phosphate precipitate and calcium nitrate precipitate.

4. The method according to claim 1 further comprising the obtaining of precipitates selected from the group of calcium nitrate (CaNO.sub.3), calcium phosphate (Ca.sub.3(PO.sub.4).sub.2), calcium sulphate (CaSO.sub.4), and aluminium hydroxophosphate (Al(OH).sub.3AlPO.sub.4) from phosphate-containing ashes from waste incineration plants, by a) combining the phosphate-containing ashes with nitric acid or phosphoric acid or a mineral acid mixture of these two acids to cause a reaction, b) separating an acid-insoluble portion of the phosphate-containing ashes from the reaction of step a), c) addition of sulfuric acid to the filtrate or supernatant of step b) to obtain and separate calcium sulphate precipitate with a mechanical filtration and/or dewatering process, d) raising the pH-value of the filtrate or supernatant of step c) to obtain and separate the dissolved aluminium as aluminium hydroxophosphate precipitate with a mechanical filtration and/or dewatering process, e) recycling partially the filtrate or supernatant of step d) for use in step a), f) concentrating the remainder of the filtrate or the supernatant of step d) by evaporation, or g) adding of calcium oxide or calcium carbonate to the filtrate or supernatant of step f) to obtain and separate calcium phosphate precipitate and calcium nitrate precipitate with a mechanical filtration and/or dewatering process.

5. The method according to claim 2 characterized in that the sulfuric acid is added in a dilution from 10 to 98 wt %, in a stirred reactor, whereby the sulfuric acid is added in a molar ratio, corresponding to the dissolved calcium concentration of 0.5 Ca to 1.5 SO.sub.4 (sulphate).

6. The method according to claim 3 characterized in that the pH-value is adjusted to 2.0 to 3.0, with alkali or alkaline earth hydroxides or oxides, wherein the alkali or alkaline earth hydroxides or oxides is selected from the caustic soda, caustic potash, sodium silicate solution, calcium hydroxide or calcium carbonate.

7. Method according to claim 6, characterized in that the phosphate-containing ash is obtained by incineration Of phosphate-containing sewage sludges, biodegradable wastes, bio-wastes and/or animal wastes in a waste incineration plant.

8. The method according to claim 1 characterized in that the reaction of the phosphate-containing ashes is carried out with a mineral acid mixture, wherein said mineral acid mixture is present at a concentration of 5 wt % to 50 wt %, in aqueous dilution, and, optionally, the phosphate-containing ash is mixed in a reactor with the mineral acid mixture, wherein the proportion of ash is 5 wt % to 50 wt %, based on the diluted mineral acid mixture.

9. The method of claim 8, wherein the mineral acid mixture contains at least phosphoric acid (H.sub.3PO.sub.4) and nitric acid (HNO.sub.3), wherein the diluted mineral acid mixture contains 10 wt % to 50 wt % HNO.sub.3, and 90 to 50 parts of H.sub.3PO.sub.4, based on the complete mineral acid content of the aqueous dilution.

10. The method according to claim 2 additionally comprising: e) raising the pH-value of the filtrate or supernatant of step d) to obtain and separate the dissolved aluminium as aluminium hydroxophosphate precipitate, and f) adding calcium oxide or calcium carbonate to the remainder of filtrate or supernatant of step d) to obtain and separate calcium phosphate precipitate and calcium nitrate precipitate.

11. The method according to claim 2 further comprising the obtaining of precipitates selected from the group of calcium nitrate (CaNO.sub.3), calcium phosphate (Ca.sub.3(PO.sub.4).sub.2), calcium sulphate (CaSO.sub.4), and aluminium hydroxophosphate (Al(OH).sub.3AlPO.sub.4) from phosphate-containing ashes from waste incineration plants, by a) combining the phosphate-containing ashes with nitric acid or phosphoric acid or a mineral acid mixture of these two acids to cause a reaction, b) separating an acid-insoluble portion of the phosphate-containing ashes from the reaction of step a) with a mechanical filtration and/or dewatering process, c) addition of sulfuric acid to the filtrate or supernatant to obtain and separate calcium sulphate precipitate with a mechanical filtration and/or dewatering process, d) raising the pH-value, of the filtrate or the supernatant of step c) to obtain and separate the dissolved aluminium as aluminium hydroxophosphate precipitate with a mechanical filtration and/or dewatering process, e) recycling partially the filtrate or supernatant of step d) for use in step a), f) concentrating the remainder of the filtrate or the supernatant of step d) by evaporation, or g) adding of calcium oxide or calcium carbonate to the filtrate or supernatant of step f) to obtain and separate calcium phosphate precipitate and calcium nitrate precipitate with a mechanical filtration and/or dewatering process.

12. The method according to claim 3 comprising the obtaining of precipitates selected from the group of calcium nitrate (CaNO.sub.3), calcium phosphate (Ca.sub.3(PO.sub.4).sub.2), calcium sulphate (CaSO.sub.4), and aluminium hydroxophosphate (Al(OH).sub.3AlPO.sub.4) from phosphate-containing ashes from waste incineration plants, characterized in that a) combining the phosphate-containing ashes with nitric acid or phosphoric acid of a mineral acid mixture of these two acids to cause a reaction, b) separating an acid-insoluble portion of the phosphate-containing ashes from the reaction of step a) with a mechanical filtration and/or dewatering process, c) addition of sulfuric acid to the filtrate or supernatant to obtain and separate calcium sulphate precipitate with a mechanical filtration and/or dewatering process, d) raising the pH-value of the filtrate or the supernatant of step c) to obtain and separate the dissolved aluminium as aluminium hydroxophosphate precipitate with a mechanical filtration and/or dewatering process, e) recycling partially the filtrate or supernatant of step d) for use in step a), f) concentrating the remainder of the filtrate or the supernatant of step d) by evaporation, or g) adding of calcium oxide or calcium carbonate to the filtrate or supernatant of step f) to obtain and separate calcium phosphate precipitate and calcium nitrate precipitate with a mechanical filtration and/or dewatering process.

13. The method according to claim 3 characterized in that the sulfuric acid is added in a dilution from 10 to 98 wt %, in a stirred reactor, whereby the sulfuric acid is added in a molar ratio, corresponding to the dissolved calcium concentration of 0.5 Ca to 1.5 SO.sub.4 (sulphate).

14. The method according to claim 4 characterized in that the sulfuric acid is added in a dilution from 10 to 98 wt %, in a stirred reactor, whereby the sulfuric acid is added in a molar ratio, corresponding to the dissolved calcium concentration of 0.5 Ca to 1.5 SO.sub.4 (sulphate).

15. The method according to claim 4 characterized in that the pH-value is adjusted to 2.0 to 3.0, with alkali or alkaline earth hydroxides or oxides, wherein the alkali or alkaline earth hydroxides or oxides is selected from caustic soda, caustic potash, sodium silicate solution, calcium hydroxide or calcium carbonate.

16. Method according to claim 15, characterized in that the phosphate-containing ash is obtained by incineration of phosphate-containing sewage sludges, biodegradable wastes, bio-wastes and/or animal wastes in a waste incineration plant.

17. The method according to claim 2 characterized in that the reaction of the phosphate-containing ashes is carried out with a mineral acid mixture, wherein said mineral acid mixture is present at a concentration of 5 wt % to 50 wt %, in aqueous dilution, and, optionally, the phosphate-containing ash is mixed in a reactor with the mineral acid mixture, wherein the proportion of ash is 5 wt % to 50 wt %, based on the diluted mineral acid mixture.

18. The method according to claim 3 characterized in that the reaction of the phosphate-containing ashes is carried out with a mineral acid mixture, wherein said mineral acid mixture is present at a concentration of 5 wt % to 50 wt %, in aqueous dilution, and, optionally, the phosphate-containing ash is mixed in a reactor with the mineral acid mixture, wherein the proportion of ash is 5 wt % to 50 wt %, based on the diluted mineral acid mixture.

19. The method according to claim 4 characterized in that the reaction of the phosphate-containing ashes is carried out with a mineral acid mixture, wherein said mineral acid mixture is present at a concentration of 5 wt % to 50 wt %, in aqueous dilution, and, optionally, the phosphate-containing ash is mixed in a reactor with the mineral acid mixture, wherein the proportion of ash is 5 wt % to 50 wt %, based on the diluted mineral acid mixture.

20. The method of claim 17, wherein the mineral acid mixture contains at least phosphoric acid (H.sub.3PO.sub.4) and nitric acid (HNO.sub.3), wherein the diluted mineral acid mixture contains 10 wt % to 50 wt % HNO.sub.3, and 90 to 50 parts of H.sub.3PO.sub.4, based on the complete mineral acid content of the aqueous dilution.

Description

EXAMPLES

(1) The entire process is described by the following experimental setup:

(2) Starting material is an ash from a sewage sludge incineration plant. The essential ingredients were analyzed as follows:

(3) TABLE-US-00002 Wt % P2O5 25.0 Wt % CaO 17.5 Wt % Fe2O3 26.9 Wt % Al2O3 6.7 Wt % SiO2 19.5

(4) 100 g ash is treated in a beaker glass with 300 g of diluted acid.

(5) The diluted acid is composed of: 70 wt % water; 15 wt % HNO.sub.3 and 15 wt % H.sub.3PO.sub.4. The suspension is stirred for 30 minutes at 40 C. and then filtered through a vacuum-nutsch (with filter) (Vacuum Bchner-Funnel with filter). The filter cake is then weighed and further dried at 100 C.

(6) Wet filter cake=122 g

(7) Dried filter cake=72 g

(8) It can be calculated that from 100 g ash 72 g have not been dissolved in acid, 28 g (=28%) are acid soluble. In total, 275 g filtrate was recovered and subsequently analysed. The analysis results in the following table were compared with theoretical values that arise, if the significant ash contents would have resolved to 100% (in the filtrate and the wet portion of the filter cake):

(9) TABLE-US-00003 (100%) (is) acid soluble Wt % P2O5 17.78* 13.68 88.7% Wt % CaO 5.37 4.80 89.4% Wt % Fe2O3 8.25 0.47 5.7% Wt % Al2O3 2.06 1.56 75.7% Wt % SiO2 5.98 0.02 0.3% *7.69% P.sub.2O.sub.5 result from the ash, 10.09% P.sub.2O.sub.5 from the added phosphoric acid.

(10) The results illustrate that high re-dissolving rates of phosphate, calcium and aluminium can be achieved, while iron is dissolved only slightly, and silicon as SiO.sub.2 is as expected almost insoluble. The concentration of H.sub.3PO.sub.4 has increased from the previous 15 wt % to 13.681.37=18.7 wt % (1.37=conversion factor from P.sub.2O.sub.5 to H.sub.3PO.sub.4). An acid with 37.4% H.sub.3PO.sub.4 and 30% HNO.sub.3 can be obtained by evaporation by the factor 2. This acid can be used for the production of fertilizers or is neutralized with CaO, and then evaporated, whereby a double salt of Ca(NO.sub.3).sub.2*Ca(H.sub.2PO.sub.4).sub.2 can be formed.

(11) This method has the disadvantage that on the one hand, considerable amounts of nitric acid and phosphoric acid are required, and on the other hand, contaminations of iron and aluminium salts impair the product quality. The process would be economically borderline and the value of the end products greatly limited.

(12) These problems, which are massive especially with higher Al concentrations in the sewage sludge ashes (that can account for more than 20% Al.sub.2O.sub.3), are solved, according to the invention, by the multistep process in step 2 and step 3 as follows:

(13) The filter cake (122 g wet weight) from the above-described 1.sup.st step (the ash dissolving process) is washed with 100 g of hot water (70-90 C.). The obtained wash filtrate is mixed together with 200 g filtrate from the 1.sup.st step (dilute acid) with 25 g of sulphuric acid (48 wt %) in a beaker glass (total weight=100+200+25=325 g). According to the above described reaction, white calcium sulphate precipitates after a few minutes. After 30 minutes, the precipitate was filtered through a vacuum-nutsch (with filter) (Vacuum Bchner-Funnel with filter). Obtained was 280 g filtrate and 43 g gypsum-wet-filter cake. The filtrate was analyzed.

(14) The analysis results in the following table are compared with values that result from a parallel experiment, whereby the filtrate from the 1.sup.st step plus the washing water have been analyzed in the ratios mentioned above:

(15) TABLE-US-00004 filtrate + WW after Ca-precipitation pH-value 1.9 0.9 Wt % P2O5 9.58 9.43 Wt % CaO 3.70 1.45 Wt % Fe2O3 0.32 0.33 Wt % Al2O3 0.98 1.01

(16) It is clear from the data that the Ca-content is reduced and at the same time also the pH-value, due to the fact that additional H-ions could form. The acidic filtrate of the 2.sup.nd step can thus be used for dissolving the ash, whereas the dosage of H.sub.3PO.sub.4 can be completely dispensed with, since new phosphoric acid is constantly formed from the phosphate-containing ash. This step is of particular note in the multistep method, due to the fact that by using the sulphuric acid overall costs are reduced (sulphuric acid is, according to its efficiency, the cheapest acid), and because gypsum is additionally recovered.

(17) With the recycled filtrate (low calcium dilute acid) is the recipe of the 1.sup.st step now as follows: 100 g ash are treated in a beaker glass with 300 g of dilute acid. The dilute acid is composed of: 270 g recyclate and 20 g HNO.sub.3. The suspension is stirred for 30 minutes at 40 C. and then filtrated through a vacuum-nutsch (with filter) (Vacuum Bchner-Funnel with filter). The filter cake is subsequently weighed and further dried at 100 C.

(18) Wet filter cake=130 g

(19) Dried filter cake=75 g

(20) It can be calculated that from 100 g ash 75 g have not been dissolved in acid, i.e. 25 g (=25%) are acid soluble. In total, 258 g filtrate (dilute acid) was recovered and then analyzed. The analysis results essentially correspond with the results of the 1.sup.st starting step, only the CaO-content was with 5.8% CaO slightly higher (because even extra calcium was added over the recyclate), while the Al.sub.2O.sub.3 content has increased further from 1.5 to 2.6%. In a 3.sup.rd step, the aluminium concentration in the dilute acid is reduced by adding small amounts of calcium oxide. For this purpose, 150 g dilute add is mixed at room temperature with 1 g CaO. A precipitate was formed, which was filtered with a vacuum-nutsch (with filter) (Vacuum Bchner-Funnel with filter) after 15 minutes. Obtained were 123 g filtrate and 25 g wet filter cake. The filtrate was analyzed.

(21) The analysis results in the following table are compared with values that result from a parallel experiment, whereby the dilute acid has been analyzed before the addition of CaO

(22) TABLE-US-00005 Dilute acid after Al-precipitation pH-value 0.9 2.2 Wt % P2O5 9.35 8.95 Wt % CaO 5.84 6.42 Wt % Fe2O3 0.54 0.48 Wt % Al2O3 2.61 0.84

(23) AlPO.sub.4-precipitate is formed due to a minor increase of the pH-value, what can be seen based on the decreased values for P.sub.2O.sub.5 and Al.sub.2O.sub.3. The values are deliberately only slightly reduced by the precipitation, since an excessive accumulation of aluminium ions shall be entirely prevented. The water-washed and dried precipitate was analyzed as follows:

(24) TABLE-US-00006 Wt % P2O5 31.3 Wt % CaO 1.1 Wt % Fe2O3 3.5 Wt % Al2O3 56.1

(25) The precipitate was further treated in an additional experiment, according to patent DE 10 2012 015 065 B3 Jul. 18, 2013, and has been converted to sodium aluminate solution and a calcium phosphate precipitate.

(26) The process steps described herein are applied variably, depending on the concentration of Ca and Al ions in the digestion acid (or the corresponding oxide concentrations in the ash). The essence of the invention is that an economic process management is possible through combination of process steps and particularly low-aluminium end products are obtained.

(27) The purified HNO.sub.3H.sub.3PO.sub.4 acid is preferably neutralized with CaO to a pH-value of 6 and concentrated by evaporation of water, e.g. by spray-drying. The result is a double salt of calcium nitrate and calcium phosphate. According to the invention, a salt with the following composition could be formed: 19.5 wt % P2O5; 28.5 wt % CaO; 24.3 wt % NO3; 0.8 wt % Al2O3, 0.6 wt % Fe2O3.

(28) FIG. 3 shows the scheme of the various processes.