Chemical reactions during the preparation of P and NPK fertilizers from thermochemically treated sewage sludge ashes

Figures & data

Table 1 Mass fractions of the main elements and heavy metals contained in raw sewage sludge ashes (SSA), SSA-Ca and SSA-Mg, and phosphorus solubility in water and 2% citric acid

	Raw SSA (mg kg^−1)	SSA-Ca (mg kg^−1)	SSA-Mg (mg kg^−1)
Al	55,920	55,410	56,140
As	34	30	29
Ca	134,200	196,800	123,500
Cd	4	<0.06	<0.06
Cr	146	139	137
Cu	1,113	114	50
Fe	100,100	84,990	92,070
Hg	<0.07	<0.07	<0.07
K	10,580	6,455	7,063
Mg	15,020	11,860	57,930
Mn	1,043	830	900
Na	5,586	4,762	5,060
Ni	83	77	74
P	90,040	80,610	84,890
Pb	237	<0.6	<0.6
S	12,970	8,345	6,898
Sn	48	11	20
Zn	2,543	16	55
Phoshorus solubility in water (%)	0	0	0
Phosphorus solubility in citric acid (%)	31	76	96

Table 2 Mass fractions of P2O5 and CaO contained in sewage sludge ashes (SSA) SSA-Ca-1 to SSA-Ca-5

	P2O5 (%)	CaO (%)	CaO/P2O5
SSA-Ca-1 (0% PA)	18	28	1.6
SSA-Ca-2 (10% PA)	21	25	1.2
SSA-Ca-3 (20% PA)	25	22	0.88
SSA-Ca-4 (50% PA)	29	19	0.66
SSA-Ca-5 (100% PA)	37	14	0.38
PA, phosphoric acid.

Table 3 Mass fractions of P2O5 and CaO contained in sewage sludge ashes (SSA) SSA-Mg-1 to SSA-Mg-5

	P2O5 (%)	CaO (%)	CaO/P2O5
SSA-Mg-1 (0% PA)	19	17	0.89
SSA-Mg-2 (10% PA)	22	15	0.68
SSA-Mg-3 (20% PA)	24	14	0.58
SSA-Mg-4 (50% PA)	30	12	0.40
SSA-Mg-5 (100% PA)	37	9	0.24
PA, phosphoric acid.

Table 4 Amounts of the educts used to prepare the NPK fertilizers

	NH4NO3 (g)	SSA-Ca (g)	MCPM (g)	K2SO4 (g)	CaSO4 (g)
NPK-1	28.5	13.5	–	8	–
NPK-2	28.5	7	2	8.5	4
NPK-3	28.5	–	4	8.5	9
MCPM, monocalcium phosphate monohydrate; SSA, sewage sludge ashes.

Table 5 Mineral phases detected by the X-ray diffraction in sewage sludge ashes (SSA) SSA-Ca-1 to SSA-Ca-5

Figure 1 Sections of the X-ray diffraction patterns of sewage sludge ashes (SSA) (a) SSA-Ca-1 (0% PA), (b) SSA-Ca-4 (50% PA) and (c) SSA-Mg-1 (0% phosphoric acid [PA]). B, brushite; C, chlorapatite; F, farringtonite; M, monocalcium phosphate monohydrate; W, whitlockite; 1, anhydrite; 2, anorthite; 3, hematite; 4, quartz.

Figure 2 Close up of the X-ray diffraction patterns of sewage sludge ashes (SSA) SSA-Ca-1 to SSA-Ca-5 showing diffraction lines characteristic of monocalcium phosphate monohydrate (left) and brushite (right). PA, phosphoric acid.

Figure 3 Close up of the X-ray diffraction patterns of sewage sludge ashes (SSA) SSA-Mg-1 to SSA-Mg-5 showing diffraction lines characteristic of monocalcium phosphate monohydrate (left) and brushite (right). PA, phosphoric acid.

Table 6 Phosphorus solubility in water and citric acid (2%) for sewage sludge ashes (SSA) SSA-Ca-1 to SSA-Ca-5

	SSA-Ca-1 (0% PA) (%)	SSA-Ca-2 (10% PA) (%)	SSA-Ca-3 (20% PA) (%)	SSA-Ca-4 (50% PA) (%)	SSA-Ca-5 (100% PA) (%)
Phosphorus solubility in water	0	6	13	67	100
Phosphorus solubility in 2% citric acid	76	78	100	100	100
PA, phosphoric acid.

Table 7 Mineral phases detected by the X-ray diffraction in sewage sludge ashes (SSA) SSA-Mg-1 to SSA-Mg-5

Table 8 Phosphorus solubility in water and citric acid (2%) for sewage sludge ashes (SSA) SSA-Mg-1 to SSA-Mg-5

	SSA-Mg-1 (0% PA) (%)	SSA-Mg-2 (10% PA) (%)	SSA-Mg-3 (20% PA) (%)	SSA-Mg-4 (50% PA) (%)	SSA-Mg-5 (100% PA) (%)
Phosphorus solubility in water	0	14	39	87	100
Phosphorus solubility in 2% citric acid	93	99	100	100	100
PA, phosphoric acid.

Figure 4 Sections of the X-ray diffraction patterns of (a) NPK-1 and (b) NPK-2. A, arcanite; C, chlorapatite; G, gwihabaite; N, nitramite; P, biphosphamite; S, (NH4)5(NO3)3SO4; 1, anhydrite; 3, hematite; 4, quartz; 5, bassanite.

Figure 5 Close up of the X-ray diffraction patterns of three NPK fertilizers (♦, NPK-1, NPK-2, ○, NPK-3) showing diffraction lines characteristic of (a) biphosphammite and (b,c) chlorapatite.

Table 9 Mineral phases detected by the X-ray diffraction in the fertilizers NPK-1 to NPK-3

Table 10 Phosphorus solubility in water and citric acid (2%) for the fertilizers NPK-1 to NPK-3

	NPK-1 (%)	NPK-2 (%)	NPK-3 (%)
Phosphorus solubility in water	0	30	93
Phosphorus solubility in 2% citric acid	98	85	100
Phosphorus solubility in alkaline ammonium citrate (Joulie)	3	30	65