Ammonium Phosphate Recovery from Incinerated Ash of Sewage Sludge
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Journal of Materials Science and Engineering A 5 (11-12) (2015) 415-420 doi: 10.17265/2161-6213/2015.11-12.005 D DAVID PUBLISHING Ammonium Phosphate Recovery from Incinerated Ash of Sewage Sludge Masaaki Takahashi1*,Yukimasa Takemoto1 and Ken Onishi2 1. Faculty of environmental and information science, Yokkaichi University, Yokkaichi, Mie 512-8512, Japan 2. Mie Chuo Kaihatsu Co., Ltd, Hachiya 4713, Iga, Mie, Japan Abstract: In order to establish a phosphorus recovery method, some phosphorus recovery methods from incinerated ash of sewage sludge were investigated. The phosphorus which is recovered from ash with acidic treatment, is mainly composed of aluminum phosphate. In order to remove aluminum component from aluminum phosphate, alum formation processes was investigated. The phosphorus extracted from ash was mixed with ammonium sulfate in existence of sulfuric acid, and formed alum (aluminum ammonium sulfate), and was separated by crystallization. The phosphorus which had aluminum removed, was recovered by addition of ammonium hydroxide. The recovered phosphorus was considered ammonium phosphate by X-ray analysis and XRD. Key words: Incinerated ash, sewage sludge, acid treatment, ammonium phosphate, alum. 1. Introduction extraction [9]. Aluminum ions react with monovalent cations like Sewage sludge contains significant amounts of + + + NH4 , K , Na under existence of H2SO4, and form phosphorus, and in order to recover the phosphorus alum, it is widely known. Some alum like from it, many investigations have been carried out AlNH4(SO4)2 or AlK(SO4)2 has low solubility in water [1-5]. at low temperature. As mentioned, phosphorus is The phosphorus in the sludge is regarded as thought to exist mainly as a form of aluminum existing mainly as a form of aluminum phosphate [6], phosphate, aluminum element is considered to react which has few uses, and the existence of aluminum with these cations, and formed alum which can be component is not good for fertilizer and is a bad separate by crystallization resulting in formation of influence on many plants, therefore, some recovery H3PO4 as shown in Eq.(1) [10-12]. methods which can recover phosphorus with low AlPO4 + H2SO4 + ( NH4)2SO4 → aluminum content are recommended. In acidic AlNH4 (SO4)2 + H3PO4 (1) treatments, almost of all phosphorus in the ash can be The formed phosphoric acid is expected to contain extracted from the ash, however, the extracted significant amounts of non-reacted H2SO4, and phosphorus consists mainly of aluminum phosphate separation of these contaminants is thought not easy. [7]. On the contrary, in alkali treatments, low In order to confirm the reaction, neutralization of the aluminum contents of phosphorus can be recovered phosphoric acid using NH4OH is considered easy, and [8], however, the recovery rate is low compared to we investigated the basic condition of the method. acidic treatments, and in order to attain higher extraction rates, hydrothermal processes are 2. Method considered useful compared to conventional alkali 2.1 Ash Corresponding author: Masaaki Takahashi, researcher of The incinerated ash which was discharged from Yokkaichi University, research field: environmental technology. Yokkaichi municipal waste water treatment facility 416 Ammonium Phosphate Recovery from Incinerated Ash of Sewage Sludge was used in the experiment. The ash is made from ratio 1:10), aluminum phosphate concentration in the sewage sludge which is incinerated in a fluidized bed extract is expected to be low, and further incinerator at 850 ºС. Chemical components of the ash concentration will be needed. In order to solve the is shown in Table 1. matter, we adopted a high solid- liquid ratio and counter current extraction method [15, 16], and 2.2 Phosphorus recovery Method possible aluminum phosphate concentration (about 80 The outline of the phosphorus recovery method is g AlPO4/L) can be attained by adoption of solid-liquid shown in Fig. 1. Phosphorus in the ash is extracted ratio 1:3. using an aqueous solution of H2SO4. In order to Raw ash and treated ash was analyzed using X-ray complete alum formation, further, small amounts of analyzer followed by dehydration. Phosphorus and H2SO4 was added to the extract. Later, powder of aluminum in the ash was degreased by acid treatment, ammonium sulfate was dissolved to the extract at room and SO3 component was increased depending on the temperature, and the mixture is cooled below 5 ºС. reaction of H2SO4 and calcium component in the ash After one day, crystal of alum is formed, and (Table 1). The phosphorus extraction rate was separated by filtration. Aq. solution of ammonium calculated from the elemental composition, and hydroxide is added to the extract to adjust the pH 4 ~ considered about 80%. 5 which is considered to be best for the separation of 3.2 Addition Rate of the Ammonium Sulfate non-reacted phosphorus (AlPO4 or FePO4) [13]. Non-reacted aluminum phosphate is precipitated, and In order to find the optimal addition rate of removed by filtration as residue, and the aluminum ammonium sulfate, 100 mL of aluminum phosphate removed phosphorus can be recovered by vaporization solution (P: 1.6%, H2SO4: 7%) which was extracted of the filtrate. from ash was mixed with sulfuric acid (9 g), and 3 g to 3. Result and Discussion Table 1 Elemental composition of the ash. 3.1 Aluminum Phosphate Preparation Element Raw ash Treated ash Al2O3 18.1 7.6 Aluminum ammonium sulfate SiO2 31.2 44.3 (AlNH4(SO4)2·12·H2O) has low solubility in water at P2O5 18.1 3.4 low temperature (4 g in 100 mL water at 0 ºС) [14]. In SO3 1.6 14.6 K O 1.7 1.8 order to make crystallization of the alum, 2 CaO 9.5 10.5 concentration, about 100 g/L of the aluminum Fe2O3 12.3 13.4 phosphate is considered to be needed. However, in Others 7.5 4.6 consideration of the usual extraction rate (solid-liquid Unit: wt %. H2SO4 ＋ Water (NH4)2SO4 aq. NH4OH Filtrate Extract Ash Crystallization pH 4 ~ 5 Vaporization Extraction Mixing º Temp. < 5 С Filtration H2SO4 Treated ash Recovered alum Residue Phosphorus (AlNH4(SO4)2) (Non-reacted AlPO4) (NH4H2PO4) Fig. 1 Recovery method. Ammonium Phosphate Recovery from Incinerated Ash of Sewage Sludge 417 15 g of ammonium sulfate powder was added, and alum, and dehydrated at 180 ºС for analysis, and dissolved at room temperature. Later, the mixture was analyzed using chemical mass balance, and the result cooled to 3 ºС, 24 h later, formed crystal (which (shown in Fig. 4) indicates the formation of alum. expected to be alum) was recovered. The amount of Small amount of P2O5 is found, which is considered to the formed crystal has increased by the addition of be derived from formed phosphoric acid, and ammonium sulfate toward the equivalent ratio, when remained in the crystal. the addition rate reaches 7 g larger than the chemical In order to recover phosphorus, 58mL of equivalent amount, increasing rate of the formed ammonium hydroxide solution (NH3: 28%) was added crystal tend to be plateau (Fig. 2). The most suitable to the filtrate, and adjusted at the pH 4 ~ 5. Light addition rate is considered to be 7 g more than charcoal precipitate was formed which was expected equivalent (surplus). as non-reacted phosphate, and removed by filtration using filter paper as a residue, and dried at 105 ºС 3.3 Addition rate of Sulfuric Acid (amount: 48 g). The phosphorus which was expected In order to find the optimal addition rate of sulfuric to be ammonium phosphate (64 g), was recovered by acid, 100 mL of aluminum phosphate solution (P: evaporation of the filtrate, and analyzed using XRD º 2.3%, H2SO4: 8.6%) which was extracted from ash and X-ray analyzer followed by drying (105 С). was mixed with ammonium sulfate (12 g), and 7 g to The residue was mainly composed of P2O5, SO3, 24 g of sulfuric acid was added at room temperature. Fe2O3 and NH3 (estimated by chemical composition), Later, the mixture was cooled at 3 ºС, 24 h later, and most of the aluminum component was removed. formed crystal (which expected to be alum) was Crystal (g) recovered. The amount of the formed crystal has also 25 increased by the addition of sulfuric acid toward the 20 equivalent ratio, when the addition rate reaches 10g 15 larger than the chemical equivalent amount, increasing 10 rate of the crystal tend to be plateau (Fig. 3). The most 5 Equivalent ratio suitable addition rate is considered to be 10g more 0 than equivalent (surplus). 0 5 10 15 20 (NH ) SO addtion rate (g) 3.4 Composition of the Recovered Phosphorus 4 2 4 Fig. 2 Relation with formed crystal and ammonium In order to confirm the formation of the ammonium sulfate. phosphate, phosphorus containing water (300 mL) which was extracted from ash using H SO was Crystal (g) 2 4 50 prepared. The phosphorus and H2SO4 concentration of 40 the phosphorus containing water is estimated to be P: 30 3%, H2SO4: 7%. Later, 43 g of H2SO4 and 34 g of 20 (NH4)2SO4 was added to the phosphorus containing water, and dissolved at room temperature. The 10 Equivalent ratio mixture was cooled for 24 h at 3 ºС, and formed 0 crystal (108 g) was separated using filter paper (Toyo 0102030 H2SO4 addition rate (g) Roshi Kaisha, Ltd. ADVANTEC). The crystal formed by cooling was expected to be Fig. 3 Relation with formed crystal and sulfuric acid 418 Ammonium Phosphate Recovery from Incinerated Ash of Sewage Sludge The residue is considered to consist of ferric The recovered phosphorus was also analyzed using phosphate and ammonium sulfate which remained in an X-ray analyzer, and is mainly composed of P2O5 the residue.