WO2022140804A1

WO2022140804A1 - Method for filtering impurities in concentrated extracted phosphoric acid for enhancement of diammonium phosphate fertilizer content and for treatment of post-filtration solid waste into phosphate fertilizer, np fertilizer or npk fertilizer.

Info

Publication number: WO2022140804A1
Application number: PCT/VN2021/000024
Authority: WO
Inventors: Trinh Hong TU; Trinh Duc ANH; Le Ngoc DIEP; Nguyen Xuan LONG; Le Thi Ngoc ANH; Tran Kim TUYEN; Trinh Tuan LINH
Original assignee: Tu Trinh Hong
Priority date: 2020-12-21
Filing date: 2021-12-20
Publication date: 2022-06-30

Abstract

The invention relates to a method for filtering impurities in concentrated extracted H₃PO₄ for enhancement of diammonium phosphate (DAP) fertilizer content, and for treatment of post-filtration solid waste into phosphate fertilizer, NP fertilizer or NPK fertilizer comprising: (i) preparing concentrated extracted H₃PO₄ having a content, with respect to P₂O₅, in the range of 45-49% by mass and having a temperature in the range of 45 - 100°C; (ii) preparing a solution of coagulant mixture by adding polyacrylamide (PAM), in the form of cation alone or in the form of cation-anion combination, and Na₂S to water, wherein the proportion of water : PAM : Na₂S per ton of concentrated H₃PO₄ is (12.5-20 L) : (60-70 g) : (20-50 g), respectively; (iii) adding the solution obtained in step (ii), as a sedimentation aid, to the concentrated H₃PO₄ solution in step (i) with the proportion of 12.5 -20.0 L per ton of concentrated H₃PO₄ and regularly stirring, to coagulate and flocculate suspended sediment (SS); and (iv) filtering for solid-liquid separation of the solution obtained from step (iii), clean H₃PO₄ solution obtained after separation is used for manufacturing DAP, wherein the recovery rate of H₃PO₄solution qualified for the DAP manufacture is 80-85% and the SS content in the post-filtration solution is less than 1% by mass.

Description

Method for filtering impurities in concentrated extracted phosphoric acid for enhancement of diammonium phosphate fertilizer content and for treatment of post-filtration solid waste into phosphate fertilizer, NP fertilizer or NPK fertilizer

Technical field

The present invention relates to the field of fertilizer manufacture, particularly a method for filtering impurities in concentrated extracted phosphoric acid (H₃PO₄) which is used in the manufacture of diammonium phosphate (DAP) fertilizer ((NH₄)₂HPO₄) for enhancement of the fertilizer content, and for treatment of post-filtration solid waste (solid residue) into phosphate fertilizer ( P₂O₅), nitrogen-phosphorus (NP) fertilizer or nitrogen-phosphorus-potassium (NPK) fertilizer having a maximum amount of effective P₂O₅ component.

Prior art

DAP fertilizer production technology comprises two stages: manufacturing H₃PO₄ and manufacturing DAP from the obtained H₃PO₄. H₃PO₄ has been manufactured by two methods, namely the thermal method from elemental phosphorus and the wet method by means of processing apatite ore ( Ca₃F(PO₄)₃) with sulfuric acid ( H₂SO₄) (the dehydrate method), therefore this product is referred to as extracted H₃PO₄, which is less costly than H₃PO₄ obtained by the thermal method.

Technical processes for manufacturing extracted H₃PO₄ are divided into two stages, comprising manufacturing dilute H₃PO₄ and manufacturing concentrated H₃PO₄.

The process for manufacturing (extracting) dilute H₃PO₄ is carried out according to the following reaction:

Ca₃F(PO₄)₃ + 5H₂SO₄ -> 3H₃PO₄ + 5CaSO₄ + HF

Depending on the ore quality, the ore humidity and the H₂SO₄ concentration, the solution of extracted H₃PO₄ generally has a concentration in the range of 25 - 28% by mass with respect to P₂O₅. In practice, the apatite ore also contains other metal impurities partly dissolved in newly formed H₃PO₄, and residual H₂SO₄, for example:

The remaining metal impurities together with generated metal salts, H₂SiF₆ and salts CaSiF₆, MgSiF₆ thereof are considered the impurities existing in solution. Such impurities existing in solution will significantly affect the concentration process as well as the DAP manufacturing process. Particular impurity contents are presented in Table 1 below.

Table 1 : Impurity contents in the solution of extracted H₃PO₄ before concentration, equivalent to the P₂O₅ content in the range of 25-28% by mass

Next, dilute H₃PO₄ solution with a concentration in the range of 25 - 28% by mass is concentrated to a concentration in the range of 45 - 50% by mass, with respect to P₂O₅, so as to be used in the DAP production technology. However, during the concentration process, such salt systems as CaSO₄ - H₃PO₄ - H₂O, MgSiF₆ - H₃PO₄ - H₂O, and CaSiF₆ - H₃PO₄ - H₂O shall precipitate. In addition, when the P₂O₅ concentration in the systems increases due to the concentration process, the solubilities of CaSO₄, MgSiF₆ CaSiF₆ as well as other soluble salts such as Fe(H₂PO₄)₃, A1(H₂PO₄)₃, Mn(H₂PO₄)₃, etc., will remarkably be decreased. This means that the more the solution is concentrated, the more precipitates occur (as indicated by an increase in impurity contents in Table 2), causing the acid to thicken.

Table 2: Components in extracted H₃PO₄ after concentration, equivalent to the P₂O₅ content in the range of 45%-49%

In the DAP manufacturing stage, when H₃PO₄ is neutralized to obtain a pH of 3.5 - 5, phosphate salts of iron, aluminium and other heavy metals in one hand form hydroxides, and in the other hand form FePO₄, A1PO₄, MnPO₄, Ca₃(PO₄)₂, etc., which are stable phosphate salts. Being insoluble and losing effective P₂O₅ amount, and thus not useful for crops, the said salts together with impurities such as CaSO₄ MnF₂, CaSiF₆, will degrade the quality of DAP product, make it unqualified in terms of the N and P₂O₅ contents.

Exemplified reactions are as follows:

. Phosphate salts of heavy metals, earth alkaline metals, such as A1PO₄, FePO₄, Ca3(PO₄)₂, etc., with pH = 3.5 - 5 are insoluble, thus being unable to be absorbed by crops. Only salt NH₄MgPO₄ is absorbable to crops.

Such sediments degrade the quality of DAP fertilizer, in other words they cause a degradation in quality of the DAP fertilizer, a slow dissolution and unqualified contents of N and P₂O₅.

In order to decrease the suspended sediment content, known methods merely use the sedimentation process, however these methods have disadvantages in that the volume of the tank for to-be-concentrated solution is large, the time for the suspended sediment to settle is long and H₃PO₄ qualified for the DAP manufacture can only be recovered with the rate of 25-40%. Because of the increased content of suspended sediment, in case where remaining H₃PO₄ is used for manufacturing a fertilizer, quality of the fertilizer will be quite low.

Summary of the invention

It is therefore an objective of the present invention to overcome disadvantages of the technologies of the prior art, so as to effectively reduce suspended sediment (SS) in concentrated extracted H₃PO₄ to be less than 1% by mass which is impossible to the known methods, thus recovering a maximum amount of clean H₃PO₄.

Another objective of the present invention is to treat solid waste from SS after being separated from concentrated H₃PO₄ solution into phosphate fertilizer (P₂O₅), nitrogen-phosphorus (NP) fertilizer or nitrogen-phosphorus-potassium (NPK) fertilizer having a maximum amount of effective P₂O₅ component.

To obtain the said objectives, the present invention relates to:

[1] A method for filtering impurities in concentrated extracted H₃PO₄ for enhancement of diammonium phosphate (DAP) fertilizer content, comprising:

(1) preparing concentrated extracted H₃PO₄ having a content, with respect to P₂O₅, in the range of 45-49% by mass and having a temperature in the range of 45 - 100°C;

(ii) preparing a solution of coagulant mixture by adding polyacrylamide (PAM), in the form of cation alone or in the form of cation-anion combination, and Na2S to water, wherein the proportion of water : PAM : NaiS per ton of concentrated H₃PO₄ is (12.5-20 L) : (60-70 g) : (20-50 g), respectively;

(iii) adding the solution obtained in step (ii), as a sedimentation aid, to the concentrated H₃PO₄ solution in step (i) with the proportion of 12.5 - 20.0 L per ton of concentrated H₃PO₄ and regularly stirring, to coagulate and flocculate suspended sediment (SS); and

(iv) filtering for solid-liquid separation of the solution obtained from step (iii), clean H₃PO₄ solution obtained after separation is used for manufacturing DAP, wherein the recovery rate of H₃PO₄ solution qualified for the DAP manufacture is 80-85% and the SS content in the post-filtration solution is less than 1% by mass.

[2]. The method according to [1], further comprising:

(v) reducing residual H₃PO₄ in the solid waste obtained in step (iv) by adding 8-25% by mass of fine grained apatite ore with particle size of 65-70 μm and the P₂O₅ content in the range of 30-42% by mass to total amount of the solid waste, uniformly mixing, and retaining for 24-72 hours, to obtain a semi-product for manufacturing phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer.

[3]. The method according to [2], further comprising:

(vi) manufacturing phosphate fertilizer (P₂O₅) by uniformly mixing the semi-product obtained from step (v) with calcium sulfate hydrate, in particular calcium sulfate dihydrate (CaSO₄ ZHaO) and/or calcium sulfate hemihydrate (CaSO₄.1/2H₂O), preferably calcium sulfate hemihydrate, wherein the mass ratio of semi-product : calcium sulfate hydrate is in the range of 1 : 0.5 - 1 : 1.5.

[4]. The method according to [3], wherein the obtained phosphate fertilizer is dried to a desired humidity and fine ground to obtain the finished phosphate fertilizer (P₂O₅).

[5]. The method according to [2], further comprising:

(vii) manufacturing NP fertilizer by uniformly mixing the semi-product obtained from step (v) with ammonium sulfate - (NBU^SO*, wherein the mass ratio of the semi-product : ammonium sulfate is in the range of 1 : 0.5 - 1 : 2.

[6]. The method according to [5], wherein theNP fertilizer obtained is dried to a desired humidity and fine ground to obtain the finished NP fertilizer.

[7]. The method according to [5] or [6], further comprising:

(viii) manufacturing NPK fertilizer by mixing the NP fertilizer obtained from step (vii) with a proper amount of potassium chloride (KC1), depending on the KaO content requirement for the fertilizer.

[8]. The method according to any one of preceding items, wherein the proportion of water : PAM : Na₂S per ton of concentrated H₃PO₄ is 15 L : 65 g : 30 g.

[9]. The method according to any one of preceding items, wherein the mass ratio of PAM in the anion form : PAM in the cation form is in the range o

[10]. The method according to any one of preceding items, wherein the mass ratio of PAM in the anion form : PAM in the cation form is

Brief description of drawings

Fig.l is a schematic diagram showing a screw extrader used in examples.

Fig.2 shows a blade provided at an extrusion die head.

Fig.3 shows an extrusion die having multiple forming holes.

Detailed description of the invention

Hereinafter, the present invention will be described in detail as to each step and implementation conditions, as well as preferred embodiments thereof.

In an embodiment, the present invention provides a method for filtering impurities in concentrated extracted H₃PO₄ for enhancement of diammonium phosphate (DAP) fertilizer content comprising:

(i) Preparing concentrated extracted H₃PO₄ having a content, with respect to P₂O₅, in the range of 45-49% by mass and having a temperature in the range of 45 - 100°C.

If concentrated H₃PO₄ has the content with respect to P₂O₅ of less than 45% by mass, then such metal salts as Fe(H₂PO₄)₃, Al(H₂PO₄)₃, Mn(H₂PO₄)₃, etc., H₂S1F6 and its salts CaSiF₆, MgSiF₆ as generated will be considered impurities which exist in solution and incompletely precipitate, leading to the fact that the DAP fertilizer quality cannot meet the requirements.

If concentrated H₃PO₄ has the content with respect to P₂O₅ of more than 49% by mass, said acid shall become thickened, resulting in a low rate of clean acid recovery, and a decrease in the DAP fertilizer productivity.

If the temperature of concentrated H₃PO₄ is less than 45°C, an increase in the viscosity of concentrated H₃PO₄ will affect the acid recovery.

If the temperature of concentrated H₃PO₄ is higher than 100°C, the coagulation and flocculation of suspended sediment in next step (iii) will be affected, and the equipments will be damaged faster.

Preferably, concentrated H₃PO₄ has a temperature in the range of 45 - 90°C.

More preferably, the method according to the present invention is implemented straight after the concentration process so as to utilize the thermal from the process, lowering energy costs to heat the acid. (ii) Preparing a solution of coagulant mixture by adding polyacrylamide (PAM), in the form of cation alone or in the form of cation-anion combination, and Na₂S to water, wherein the proportion of water : PAM : Na₂S per ton of concentrated H₃PO₄ is (12.5-20 L) : (60-70 g) : (20-50 g), respectively;

If the amount of water is less than 12.5 L per ton of concentrated H₃PO₄, it will be difficult for PAM to be dissolved, the solution of coagulant mixture will be thick, viscous, difficult for pumping, as a result the quantitative pumping will be inaccurate.

If the amount of water is more than 20 L per ton of concentrated H₃PO₄, concentrated H₃PO₄ is diluted, and more energy will be consumed to dry the fertilizer product.

Using PAM in the cation form with negative charge serves to flocculate suspended sediment Preferably, in addition to PAM in the cation form, PAM in the anion form is also used in combination to enhance the ability of binding suspended sediment in concentrated H₃PO₄. According to an embodiment, the mass ratio of PAM in the anion form: PAM in the cation form is in the range of

If this ratio is less than the ability of coagulating aluminium compounds in concentrated H₃PO₄ is not high. If this ratio is higher than both PAMs in the anion form and cation form are incompatible with each other, causing a decrease in coagulation effect

If the total amount of PAM per ton of concentrated H₃PO₄ is less than 60 g, there will be an incompetency in creating bonds with suspended sediment (SS) existing in concentrated H₃PO₄, resulting in the fact that SS cannot be exhaustively filtered by the solid waste separator.

If the total amount of PAM per ton of concentrated H₃PO₄ is more than 70 g, there will be a PAM redundancy in the concentrated H₃PO₄ solution, the acid solution becomes viscous, resulting in an ineffective operation of the solid waste separator.

The Na₂S amount of less than 20 g per ton of concentrated H₃PO₄ will not be sufficient to enhance the liquidity of the PAM solution for an easy pumping, and will not be sufficient to precipitate cadmium and heavy metals in concentrated H₃PO₄ into metal sulfides, for example under the following reactions:

Cd⁺, Fe⁺, Pb⁺, As⁺ ... + H₂S -> CdS, PbS, As₂S₃

... + 2H⁺

If the Na₂S amount is more than 50 g per ton of concentrated H₃PO₄, when mixing the solution of coagulant mixture with concentrated H₃PO₄, H₂S generated according to the following reaction will become redundant, causing air pollution.

Na₂S + 2H₃PO₄ 2NaH₂PO₄ + H₂S

Preferably, the proportion of water : PAM : Na₂S per ton of concentrated H₃PO₄is 15 L : 65 g : 30 g.

Preferably, the mass ratio of PAM in the anion form: PAM in the cation form is

(iii) Adding the solution obtained in step (ii), as a sedimentation aid, to the concentrated H₃PO₄ solution in step (i) with the proportion of 12.5 - 20.0 L per ton of concentrated H₃PO₄ and regularly stirring, to coagulate and flocculate suspended sediment.

(iv) Filtering for solid-liquid separation of the solution obtained from step (iii), clean H₃PO₄ solution obtained after separation is used for manufacturing DAP, wherein the recovery rate of H₃PO₄ solution qualified for the DAP manufacture is 80-85% and the SS content in the post-filtration solution is less than l% by mass.

Solid-liquid separation process can be implemented by a hydraulic press or a filter press. However, the invention is not limited thereto, any method and means for solid-liquid separation known in the art which can separate the solid phase in the form of solid waste from the liquid phase can be used.

According to an embodiment, the method according to the present invention further comprises:

(v) reducing residual H₃PO₄ in the solid waste obtained in step (iv) by adding 8-25% by mass of apatite ore with particle size of 65-70 μm and the P₂O₅ content in the range of 30-42% by mass to total amount of the solid waste, uniformly mixing, and retaining for 24-72 hours, to obtain a semi-product for manufacturing phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer.

Some reactions occurring in step (v) are as follows:

If the apatite ore amount is less than 8% by mass, the concentration of residual free H₃PO₄ in the finished fertilizer is still high, affecting the plant growth.

If the apatite ore amount is more than 25% by mass, the concentration of residual free H₃PO₄ significantly decreases, but the effective P₂O₅ content also decreases.

If retention time is shorter than 24 hours, residual H₃PO₄ cannot completely react with apatite ore.

If retention time is longer than 72 hours, the semi-product will be stiffened, affecting the use in following steps of the manufacturing process because the loosening of the semi-product will be required.

(vi) manufacturing phosphate fertilizer (P₂O₅) by uniformly mixing the semi-product obtained from step (v) with calcium sulfate hydrate, in particular calcium sulfate dihydrate (CaSO₄.2H₂O) and/or calcium sulfate hemihydrate (CaSO₄.1/2H₂O), preferably calcium sulfate hemihydrate having a mass ratio of the semi-product : calcium sulfate hydrate in the range of 1 : 0.5 - 1 : 1.5.

During the mixing period, metal salts will intensively exchange with acid stronger than H₃PO₄, releasing less soluble salts such as CaHPO₄ and forming hydrated salts, therefore the obtained phosphate fertilizer has a low humidity, which advantageously reduces the energy consumption to dry the finished phosphate fertilizer. Some reactions occurring in step (v) are as follows: 4

If the content by mass of calcium sulfate hydrate is more than 1.5 parts per 1 part of the semi-product, the P₂O₅ amount in the product is low.

If the content by mass of calcium sulfate hydrate is less than 0.5 parts per 1 part of the semi-product, the CaSO₄ amount will be insufficient to exchange with phosphate metal salts in the solid waste.

With the adjustment of the mass ratio of the semi-product : calcium sulfate hydrate to be within the said range, it is possible to obtain the P₂O₅ content in the range of 16-30% by mass, which can meet various demands from clients. For example, if it is desired to increase the P₂O₅ content, the consumed calcium sulfate hydrate should be decreased.

Phosphate fertilizer obtained in this step may be further dried to a desired humidity and fine ground to obtain the finished phosphate fertilizer (P₂O₅).

(vii) manufacturing NP fertilizer by uniformly mixing the semi-product obtained from step (v) with ammonium sulfate - (NH₄)₂SO₄ having a mass ratio of the semi-product : ammonium sulfate in the range of 1 : 0.5 - 1 : 2.

During the mixing period, metal salts will intensively exchange with acid stronger than H₃PO₄, releasing salts such as (NH₄)₂HPO₄ and forming hydrated salts, therefore the obtained NP fertilizer has a low humidity, which advantageously reduces the energy consumption to dry the finished NP fertilizer. Some reactions occurring in step (v) are as follows:

If the content by mass of ammonium sulfate is more than 2 parts per 1 part of the semi-product, the large amount of ammonium sulfate in the fertilizer will acidify soil.

If the content by mass of ammonium sulfate is less than 0.5 parts per 1 part of the semi-product, the amount of ammonium sulfate will be insufficient to exchange with phosphate metal salt radicals in the solid waste.

The NP fertilizer obtained in this step can be dried to a desired humidity and fine ground to obtain the finished NP fertilizer.

(viii) manufacturing NPK fertilizer by mixing the NP fertilizer obtained from step (vii) with a proper amount of potassium chloride (KC1), depending on the KzO content requirement for the fertilizer, wherein the NP fertilizer may either be dried and fine ground or not.

Examples

Hereinafter, examples relating to steps of separation filtering of suspended sediment (SS) in concentrated extracted H₃PO₄, and treating the solid waste from SS into phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer in the method according to the present invention will be described.

Preparing:

- concentrated extracted H₃PO₄: 10 tons;

- sedimentation aid: polyacrylamide (PAM) having the chemical formula of CONH₂(CH₂-CH-)n, including polyacrylamide in the cation form and polyacrylamide in the anion form;

- Na₂S;

- a tank for storing concentrated extracted H₃PO₄ (acid tank) in the form of a circular tank equipped with stirring shaft having a speed in the range of 120 - 300 rpm;

- a tank for preparing coagulant mixture (coagulant mixture tank) in the form of a circular tank equipped with stirring shaft having a speed in the range of 400 - 500 rpm and having a volume equal to about 1/40 of the acid tank volume; - a solid-liquid separator in the form of a filter press; and

- a screw extruder, as shown in Figs. 1-3, including: motor 1 attached to screw 3 to rotate the same; feeding hopper 2 for feeding materials to screw 3; and extrusion die 5 for shaping extruded product. Preferably, the extruder further comprises cutter 4 adjacent to extrusion die 5.

Tanks and solid-liquid separator are common in the art, thus not necessary to be further described.

Example 1: Separation filtering of suspended sediment (SS) in concentrated extracted H₃PO₄ having components shown in Table 3 below.

Table 3: Components in concentrated extracted H₃PO₄

(i) 10 tons of concentrated extracted H₃PO₄ having a temperature of 50°C are pumped to the acid tank. The acid is taken straight after the concentration process so as to utilize the thermal from the process, lowering energy costs to heat the acid.

(ii) 150 L of water are pumped to the coagulant mixture tank. 600g of PAM in the cation form are weighted and gradually added to the tank while being stirred. After that further 50g of PAM in the anion form are added and stirred until completely dissolved. Finally, 300g of Na₂S are added and stirred until completely dissolved.

(iii) The coagulant mixture is pumped to the acid tank while being stirred at the speed of 150 rpm.

(iv) After 15 minutes of stirring, the solution in the acid tank is pumped to the solid-liquid separator, which is a filter press to filter for separating suspended sediment from the acid solution. The clean H₃PO₄ solution obtained is subjected to the DAP manufacturing process, while the solid waste is subjected to step (v) to treat the residual acid.

Table 4: Quality of H₃PO₄ solution after filtering suspended sediment

Total amount of the solid waste obtained is 1800 kg. Components in the solid waste are shown in Table 5 below. Table 5: Components in the solid waste obtained after filtering suspended sediment

It can be seen from Table 3 and Table 4 that the content of SS in the H₃PO₄ solution is decreased by ten times (from 8.9% to 0.77%), showing a significant separation of impurities which degrade the DAP fertilizer quality.

From Table 3 and Table 5, it is possible to calculate the P₂O₅ amount obtained in the acid solution for manufacturing DAP to take 85.2% by mass of the original P₂O₅ amount.

The solid waste is separated with an amount of up to 18% by mass with respect to the total mass of the original concentrated extracted acid solution, wherein the P₂O₅ amount remaining in the waste takes 14.8% by mass of the original P₂O₅ amount. Table 6: Quality criteria of the DAP fertilizer manufactured from the H₃PO₄ solution after separating suspended sediment according to Table 4

It can be seen from Table 6 that after filtering to remove impurities, the DAP obtained can provide an excellent total nutrient content, which is higher than DAP 64% product containing 18% by mass of total N and 46% by mass of P₂O₅, as requested by clients.

Example 2: Separation filtering of suspended sediment (SS) in concentrated extracted H₃PO₄ having components shown in Table 7 below.

Table 7: Components in concentrated extracted H₃PO₄

Preparation and filtering steps are implemented as in Example 1. Quality of H₃PO₄ solution after filtering suspended sediment and components in the postfiltration solid waste of Example 2 are shown in Table 8 and Table 9 below. Table 8: Quality criteria of H₃PO₄ solution after filtering suspended sediment

Table 9: Components in the post-filtration solid waste

It can be seen from Table 7 and Table 8 that the content of SS in the H₃PO₄ solution is decreased by 40 times (from 12.7% to 0.32%), showing a significant separation of impurities which degrade the DAP fertilizer quality.

From Table 7 and Table 9, with similar calculation as in Example 1, the amount of recovered effective P₂O₅ for manufacturing DAP takes 81.1 % by mass of the original P₂O₅ amount, lower than the case of using an acid with P₂O₅ content of 45% by mass. This shows that the higher concentration of acid is used, the lower DAP productivity is obtained. Therefore, as discussed, the acid content is preferably in the range of 45 - 49% by mass.

Total amount of the solid waste obtained is 2300 kg, taking 23% by mass of total amount of original concentrated extracted acid solution, wherein P₂O₅ remained in the waste takesl8.9% by mass of the original P₂O₅ amount.

Table 10: Quality criteria of the DAP fertilizer manufactured from the H₃PO₄ solution after separating suspended sediment according to Table 8

It can be seen from Table 10 that after filtering to remove impurities, the DAP obtained can provide an excellent total nutrient content, which is higher than DAP 64% product containing 18% by mass of total N and 46% by mass of P₂O₅, as requested by clients.

Example 3: Treating solid waste from SS obtained in Example 1 into phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer

(v) Reducing residual H₃PO₄ in the solid waste obtained in step (iv) of Example 1.

1800 kg of the solid waste in Example 1 are uniformly mixed with 144 kg of apatite ore having the P₂O₅ content of 32% by mass (equivalent to 8% by mass of the solid waste) in the screw extruder as shown in Fig.l. For the solid waste and the apatite ore to be uniformly mixed, so that acid can well penetrate into ore grains to completely decompose apatite, the mixture thereof can be allowed to run through the extruder for 2 - 5 times. After mixing, the obtained mixture is retained for 36 hours, providing 1944 kg of a semi-product for manufacturing phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer. After reducing residual free H₃PO₄, the P₂O₅ content in the semi-product decreases and can be calculated as follows:

1944

(vi) Manufacturing phosphate fertilizer (P₂O₅) from the semi-product obtained in step (v)

Phosphate fertilizer can be manufactured according to the requirement regarding P₂O₅ content. Maximum P₂O₅ content obtainable with the method according to the present invention corresponds to a mixing ratio of 0.5 parts of calcium sulfate hemihydrate per 1 part of the semi-product after the reduction of residual acid. As such, the maximum P₂O₅ content will be The

amount of calcium sulfate hemihydrate should not be less than 0.5 parts per 1 part of the semi-product because the amount of CaSO* will be insufficient to exchange with other metal salts present in the semi-product. Therefore, when being dissolved in water, metal salts shall be converted into stable phosphates, causing a loss in P₂O₅ and a difficulty in drying the product. If the amount of calcium sulfate hemihydrate is more than 1.5 parts per 1 part of the semi-product, the P₂O₅ content will be less than the minimum content of 16% according to the National Standard TCVN 4440:2018 on single super phosphate fertilizers.

• Manufacturing phosphate fertilizer with the maximum P₂O₅ content of24.4%

Required amount of the calcium sulfate hemihydrate can be calculated as follows: 1944 kg of the semi-product x 0.5 = 972 kg of the calcium sulfate hemihydrate

1944 kg of the semi-product are uniformly mixed with 972 kg of the calcium sulfate hemihydrate in the screw extruder as shown in Fig.1 to provide 2916 kg of finished phosphate fertilizer. For the solid waste of the semi-product and the calcium sulfate hemihydrate to be uniformly mixed, the mixture thereof can be allowed to run through the extruder for 2 - 5 times. Table 11 : Quality analysis result of the phosphate fertilizer with the maximum P₂O₅ content of 24.4%

The result shows that the effective P₂O₅ takes 93% by mass of the total P₂O₅.

Manufacturing phosphate fertilizer with the minimum P₂O₅ content of 16%

For manufacturing phosphate fertilizer with the minimum P₂O₅ content of 16%, total parts by mass of the semi-product + the calcium sulfate hemihydrate after mixing will be — - =2.29 parts. That is to say, in this case, the mass ratio of the semi-product : the calcium sulfate hemihydrate will be 1 : 1.29.

Total amount of the phosphate fertilizer having the minimum P₂O₅ content of 16% can be calculated as follows:

1944 kg of the semi-product + (1.29 x 1944) kg the calcium sulfate hemihydrate = 4451 kg.

Implementation manner:

Step 1 : Total amount of the semi-product and a half of the calcium sulfate hemihydrate are uniformly mixed in the extruder for 3 - 5 times. With such mixing ratio, the phosphate fertilizer has sufficient humidity for the extruder to operate, and creates a competent medium for exchange reactions.

Step 2: The remaining calcium sulfate hemihydrate is added and uniformly mixed. In this step, the calcium sulfate hemihydrate serves as a filler as well as a desiccant. There is no need to dry the obtained phosphate fertilizer.

Table 12: Quality analysis result of the phosphate fertilizer with the minimum P₂O₅ content of 16%

The result shows that the effective P₂O₅ takes 92.5% by mass of the total P₂O₅.

(vii) Manufacturing NP fertilizer

The semi-product obtained from step (v) and ammonium sulfate are uniformly mixed with the mass ratio thereof being 1 : 1.

The amount of obtained NP fertilizer can be calculated as follows: 1944 kg of the semi-product + 1944 kg of ammonium sulfete = 3888 kg.

Table 13: Quality analysis result of NP fertilizer .

The result shows that fee effective P₂O₅ takes 94% by mass of fee total P₂O₅.

Example 4: Treating the solid waste from SS obtained in Example 2 into NPK fertilizer

2300 kg of the solid waste of Example 2 are uniformly mixed wife 345 kg of apatite ore having fee P₂O₅ content of 32% by mass (equivalent to 15% by mass of fee solid waste) in a screw extruder as shown in Fig. 1, to reduce residual free H₃PO₄ present in the solid waste.

For fee solid waste and fee apatite ore to be uniformly mixed, so feat acid can well penetrate into ore grains to completely decompose apatite, the mixture thereof can be allowed to run through fee extruder for 2 - 5 times. After mixing, fee obtained mixture is retained for 36 hours, providing 2645 kg of a semiproduct. After reducing residual free H₃PO₄, fee P₂O₅ content in the semi-product decreases and can be calculated as follows:

For manufacturing NP fertilizer wife fee minimum P₂O₅ content of 15%, total parts by mass of the semi-product + ammonium sulfate after mixing will be =2.4 parts. That is to say, in this case, fee mass ratio of the semi-product : ammonium sulfate will be 1 : 1.4.

Total amount of the NP fertilizer can be calculated as follows:

2645 kg of the semi-product + (1.4 x 2645) kg of ammonium sulfate = 6348 kg.

In 6348 kg of the NP fertilizer:

- N content (from ammonium sulfate) = 12.25%

- P₂O₅ content = 15%.

The NP fertilizer is dried and mixed with 634 kg of KC1 (i.e., 10% by mass), providing 6348+634 = 6982kg of NPK fertilizer.

Nutrient content in the NPK fertilizer is as follows:

N = 12.25% x 0.9 = 11%

Effective P₂O₅= 15% x 0.9 = 13.5%

K₂O = 634/6982 x 0.62 x 0.9 = 5%

Therefore, according to this Example, the NPK fertilizer is obtained with a N:P:K ratio being 11: 13.5 : 5.

Effects of the present invention

By the method according to the present invention, the recovery rate of H₃PO₄ solution qualified for the DAP manufacture is within the range of 80- 85%, higher than those of 25-40% obtained with the sedimentation method of the prior art. As such, from the same material which is the post-concentration extracted H₃PO₄ solution, there is an increase in the DAP productivity because the method according to the present invention creates low-quality HaPO₄(in the waste) only in the range of 15- 20%, much lower than those of 60-75% in the sedimentation method.

By using the recovered H₃PO₄ solution according to the present invention, the quality of DAP fertilizer can meet the minimum standard N=18%, P₂O₅ - 46% as desired by clients.

By the method according to the present invention, the solid waste after separation filtering is treated into the semi-product which is used as the material for manufacturing phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer. In the common DAP manufacturing processes, sediment after the natural sedimentation can be used for manufacturing low-quality NP fertilizer only. However, the semi-product according to the present invention can provide not only NP fertilizer with better quality, but also other high quality products such as phosphate fertilizer and NPK fertilizer. This will bring about value added for fertilizer manufacturing plants.

Claims

1. A method for filtering impurities in concentrated extracted H₃PO₄ for enhancement of diammonium phosphate (DAP) fertilizer content, comprising:

(i) preparing concentrated extracted H₃PO₄ having a content, with respect to P₂O₅, in the range of 45-49% by mass and having a temperature in the range of 45 - 100°C;

(ii) preparing a solution of coagulant mixture by adding polyacrylamide (PAM), in the form of cation alone or in the form of cation-anion combination, and Na₂S to water, wherein the proportion of water : PAM : NaiS per ton of concentrated H₃PO₄ is (12.5-20 L) : (60-70 g) : (20-50 g), respectively;

2. The method according to claim 1, further comprising:

(v) reducing residual H₃PO₄ in the solid waste obtained in step (iv) by adding 8-25% by mass of apatite ore with particle size of 65-70 μm and the P₂O₅ content in the range of 30-42% by mass to total amount of the solid waste, uniformly mixing, and retaining for 24-72 hours, to treat the solid waste into a semi-product for manufacturing phosphate fertilizer (P₂O₅), NP fertilizer or NPK fertilizer.

3. The method according to claim 2, further comprising:

(vi) manufacturing phosphate fertilizer (P₂O₅) by uniformly mixing the semiproduct obtained from step (v) with calcium sulfate hydrate, in particular calcium sulfate dihydrate (CaSO₄.2H₂O) and/or calcium sulfate hemihydrate (CaSO₄.1/2H₂O), preferably calcium sulfate hemihydrate, wherein the mass ratio of semi-product : calcium sulfate hydrate is in the range of 1 : 0.5 - 1 : 1.5.

4. The method according to claim 3, wherein the obtained phosphate fertilizer is dried to a desired humidity and fine ground to obtain the finished phosphate fertilizer (P₂O₅).

5. The method according to claim 2, further comprising:

(vii) manufacturing NP fertilizer by uniformly mixing the semi-product obtained from step (v) with ammonium sulfate - (NH₄)₂SO₄ having a mass ratio of the semi-product : ammonium sulfate in the range of 1 : 0.5 to 1 : 2.

6. The method according to claim 5, wherein the NP fertilizer obtained is dried to a desired humidity and fine ground to obtain the finished NP fertilizer.

7. The method according to claim 5 or 6, further comprising:

(viii) manufacturing NPK fertilizer by mixing the NP fertilizer obtained from step (vii) with a proper amount of potassium chloride (KC1), depending on the K₂O content requirement for the fertilizer.

8. The method according to anyone of preceding claims, wherein the proportion of water: PAM : Na₂S per ton of concentrated H₃PO₄ is 15 L : 65 g : 30 g.

9. The method according to anyone of preceding claims, wherein the mass ratio of PAM in the anion form: PAM in the cation form is in the range of

10. The method according to anyone of preceding claims, wherein the mass ratio of PAM in tiie anion form: PAM in the cation form is