CZ2009222A3 - Method of purifying calcium nitrate solutions or melts - Google Patents

Abstract

Sedimentation is applied to purify the calcium nitrate solution or melts.

Description

Process for the purification of calcium nitrate melt solutions and calcium difluorate melt solutions

Field of the invention

The invention relates to a method for purifying calcium nitrate (CN) solutions / melts and further to calcium nitrate solutions / melts prepared by this method.

BACKGROUND OF THE INVENTION

In the nitrophosphate process, the phosphate is decomposed in an excess of nitric acid, then the so-called decomposed slurry is cooled to about 0 ° C to precipitate calcium nitrate tetrahydrate.

The acid precipitate is removed by centrifugation and neutralized with ammonia. The water content is adjusted by evaporation prior to granulation or spray crystallization.

Mineral phosphates, such as apatite, contain, in addition to calcium and phosphorus, high concentrations of ions such as Si ²⁺ , Fe ²⁺ , Fe ³⁺ , Al ³⁺ , F ', etc. Calcium nitrate crystals formed from decomposed phosphate, therefore contain different amounts of impurities - fluorides, phosphates and silicates. Also sand, silicate and decomposition sludge particles may still be present. Most impurities must be removed to obtain calcium nitrate suitable for technical applications or drop irrigation systems in greenhouses.

Existing purification processes: The production phases of the CN in the nitrophosphate process are shown schematically in Figure 1. CN purification is carried out in cycles 5-7 and before the neutralization with ammonia to pH 5-8 the CN melt is diluted with water to a density of 1.45-1, 48 kg / l at 60 ° C,

Neutralization precipitates inorganic components such as apatite, calcium fluoride, silicates and silica, etc.

To reduce the amount of insoluble matter in the melt, the solution from Cycle 5 is mixed with a flocculating agent. Flakes are formed and a large amount of inorganic substances is removed by decanting centrifuges.

The supernatant is transferred to cycle 8, in which the water content is adjusted to form a melt containing 77% calcium nitrate (CaN), 7% ammonium nitrate (AN) and 15% water.

• · · ·

Granulation or spray crystallization gives a solid product (solid CN) in which the content of insoluble matter ranges from 900 to 2400 ppm. Insoluble substances are a mixture of eg silica, calcium fluoride, Al-Fe silicates, etc.

This cleaning procedure is well established and easy to perform. However, the above purification procedure also has some drawbacks and drawbacks. The cleaning method requires the addition of large amounts of water to reduce the density from approx

1.6 kg / l (70 ° C) to about 1.45 kg / l (70 ° C). This water must then be evaporated before granulation and spray crystallization, leading to a significant increase in energy consumption. In addition, a purity level of 900 to 2400 ppm is not sufficient for some technical applications. In the cultivation market, this level of insoluble substances causes excessive sedimentation in storage tanks, leading to the need to clean these tanks. Finally, the described process requires the operation of several decanting centrifuges, which are expensive to maintain.

Other Cleaning Techniques: Cleaning with a filter press (plate and frame) or other types of filter devices (such as a candle filter) are well known methods for purifying salt solutions to a high degree of purity. However, for use in apatite calcium nitrate purification, these filtration systems require the use of a filter aid (eg, diatomaceous earth) to maintain sufficient flow through the filter cake. The amount of filter aid limits the use of this device to small production volumes, since the filter cake must be disposed of in an environmentally friendly manner, which increases the cost. Most filters also work batchwise, which is a considerable disadvantage for an otherwise continuous process. In order to achieve an acceptable viscosity and filtration rate, the CN solution must be diluted to a density of at least 1.5 kg / l prior to cleaning, which is another disadvantage. In order to obtain a solid CN, the added water must subsequently be removed, which increases the cost.

Other cleaning methods include, for example, microfiltration using ceramic filters and disc centrifuges. However, this microfiltration method requires large financial investments.

In addition, during the experiments with disk centrifuges, excessive fouling of the device occurred.

RU 2228906 (Annotation) describes a method for purifying a melt or calcium nitrate solution by isolating calcium nitrate tetrahydrate crystals

freeze-drying from phosphate ore extract in nitric acid, melting crystals, diluting the melt / calcium nitrate solution 0.5 to 60% ammonium nitrate solution, and neutralizing the dilute solution with ammonia to a pH value of 6.1 to 7.6 and then separating the solid impurities by sedimentation in a decanter tank to form a slurry in one step.

RU 2154045 describes a method for preparing a complex mineral fertilizer in which phosphate, calcined at 850-1050 ° C, is decomposed with unconcentrated nitric acid. The suspension formed by decomposition is added to the water in a volume of 0.5 to 2.5 per volume of suspension, insoluble residues are removed so as to allow them to sediment, and a portion of the calcium nitrate is removed by freezing. The solution is then neutralized with ammonia and treated to the NP or NPK fertilizer type by evaporation, granulation and drying, the added water is removed by evaporation prior to the calcium nitrate crystallization. The process of sedimentation of the insoluble precipitate is accelerated several times, the phosphate is produced as a normalized fertilizer.

JP 2006225175 is directed to a method of producing a transparent liquid fertilizer containing magnesium nitrate and calcium nitrate as the main components. Nitric acid is added to the water in which calcium magnesium carbonate is mixed, and after neutralization magnesium nitrate and calcium nitrate are formed. Further, at least one substance selected from the group of potassium components, one substance selected from the group of nitrogen compounds and one substance from the group of trace element compounds are added and dissolved. Further, a high molecular weight flocculating agent is added and the mixture is stirred at a temperature in the range of 40 to 80 ° C. The mixture is then allowed to stand to allow the undissolved portion to settle, then separated.

BRIEF DESCRIPTION OF THE DRAWINGS

Giant. 1 schematically describes CN production cycles in a nitrophosphate process.

Giant. 2 depicts a test equipment for continuously purifying CN melts / solutions by sedimentation.

Giant. 3 shows an example of a vane unit.

Giant. 4 shows a cross section of a test unit, i.e. a cross section of a lamella unit with corresponding connections.

* φ

Giant. 5 shows a graph of the content of water-insoluble substances in the pure melt leaving the lamella separator, as a function of the melt density (at 60-70 ° C including flocculant) and the melt flow rate.

Giant. 6 shows a graph of the content of water-insoluble matter in the CN solution exiting the lamella separator and the ratio of moles P / moles F in crude CN over 3 days.

Giant. 7 schematically illustrates the purification of CN originating from the nitrophosphate process.

SUMMARY OF THE INVENTION

One object of the invention is to provide an improved method for purifying CN solutions / melt, that is, a method for providing CN solution / melt in which the water insoluble matter is reduced.

Another object of the invention is to reduce the energy consumption of the calcium nitrate purification process.

The present invention provides a method of purifying a CN solution originating from a nitrophosphate process, comprising the step of sedimenting the insoluble material. The insoluble material comes partly from phosphate and partly from neutralization of the acidic crude CN solution.

Sedimentation can be carried out in any apparatus suitable for this purpose, in particular in tanks containing thin plates or lamellae.

In particular, the invention provides a method of purifying a CN solution by sedimentation after adjusting the mol P / mol F ratio to a value greater than 0.4 with phosphoric acid or another phosphorus compound soluble in the acidic CN solution.

Exemplary embodiments of the invention

The present invention provides a method for purifying calcium nitrate solutions or melts comprising a sedimentation step. Sedimentation is followed by centrifugation of the sludge phase. Further, the method comprises reuse of the sludge phase in a phosphate decomposition cycle.

The method according to the invention comprises the following steps:

a) phosphate leaching,

b) freezing / crystallization,

c) filtration,

d) washing of crystals,

(e) dissolution and dilution to a density within the range of 1,3 to 1,75 kg / l at «φ

up to 90 ° C,

(f) neutralization;

g) addition of flocculant and adjustment of mol P / mol F ratio,

(h) sedimentation;

i) centrifuging the sludge phase and optionally reusing the sludge in a) _T

(j) evaporation;

(k) comminution.

A further embodiment of the process comprises, at the above point e), diluting the melt to a density of 1.6 kg / l at a temperature of 70 ° C.

In another embodiment of the process, at the above point g), the ratio of mol P / mol F is set to a value higher than 0.30, preferably a value higher than 0.45.

In another embodiment of the invention, the sedimentation at point (h) above is carried out in a sedimentation tank equipped with thin plates and slats.

In another embodiment of the invention, a calcium nitrate solution / melt is obtained which contains less than 3% water-insoluble matter.

The calcium nitrate obtained by the process of the invention is comminuted by granulation, spray crystallization or another well known comminution method.

X-ray diffraction analysis revealed that the insoluble material that precipitates upon neutralization of the acidic CN solution contains mainly fluoroapatite (Ca ₅ (PO ₄ ) 3 F (70-95%), and small amounts of SiO ₂ and CaF ₂ . in range

2.2 to 3.2 g / cm ³ and can therefore sediment in aqueous CN. It was found that once the neutralized solution was left to stand, the insoluble substances slowly settled and formed a sludge at the bottom of the tank.

The sedimentation rate depends, among other factors, on the density of the solution and the mole ratio of P / mol F. Generally, the more dilute the solution, the higher the sedimentation rate, and the lower the mole ratio of P / mol F, the lower the sedimentation rate.

The fact that the more dilute the solution, the higher the sedimentation rate, stems directly from Stokes law:

Vs = K (pins-psol) d _p ² / η where

Vs is the sedimentation rate

K is constant pins is density of insoluble substances psol is density of solution η is solution viscosity d _p ² is particle diameter

The low mole ratio P / mole F results in a low sedimentation rate due to the fact that the main impurity that precipitates during neutralization is fluoroapatite (CasfPO4F). The ideal value of the mol P / mol F ratio for the precipitation of said compound is 3. If the ratio is too low, small amounts of apatite crystals precipitate and are small in size, i.e. low apatite crystal growth rate, the excess F precipitates as CaF ₂ and the whole insoluble mixture Material sedimentation significantly slower. The resulting CaF ₂ and SiO ₂ crystals are small, with a large surface area and more pronounced "flake" behavior.

Research has shown that a low F content and a high mol ratio P / mol F (> 0.7) in the CN solution results in precipitation of larger apatite crystals, which sediment more rapidly. On the other hand, a high F concentration and a low P / mol F ratio (<0.4) often cause small crystal formation and a low sedimentation rate.

As mentioned above, phosphates contain different amounts of F, Si, Al, Fe and other substances in addition to Ca and P. The amount of these substances varies according to the type of phosphate and the washing procedures applied, see Figure 1, Step 4.

Therefore, the ratio of mol P / mol F CN crystals that advance to the dilution and neutralization step varies depending on the crude phosphate used.

Commonly used phosphates include, for example, Coke phosphates, Boucraa, (Morocco) and Youssoufia (Morocco) or mixtures thereof. The content of Si, F, Fe, Al, Ca, P and the like varies from one type of phosphate to another and also within one type of phosphate. Therefore, the ratio of moles P / moles F in the CN solution obtained by the nitrophosphate process also varies greatly. The consistency of the insoluble matter and the amount of the individual compounds during the neutralization of the CN melt are different, which may affect the sedimentation process.

In a continuous sedimentation process, a CN flocculating agent is added to the solution / melt prior to entering the sedimentation tank. The addition of a flocculating agent improves the purity by reducing the amount of water insoluble substances and increasing the solid sludge content so that a more compact sludge is obtained. For better purity, the density of the solution / melt CN is reduced and the mole ratio P / mole F increased.

Surprisingly, the amount of insoluble matter in the purified solutions is much lower than in the solutions obtained by the purification method used in the CN production plant so far (Fig. 1), where a centrifuge having a centrifugal force of 500-2000 g is used to remove the insoluble matter.

A purification method, including sedimentation of insoluble matter, can therefore be used for purification of CN in the nitrophosphate process.

To further improve solution / CN melt cleaning, the sedimentation tank is equipped with metal plates or lamellas. The sedimentation tank with lamellae is connected to the solution / melt tank CN, for example to the neutralization tank and the CN production plant (cycle 6, Fig. 7). Before entering the sedimentation tank, a flocculating agent is optionally added to the CN solution / melt and / or the P / mol F ratio is adjusted. The sludge obtained by sedimentation is discharged into a decanting centrifuge where the sludge phase is drained.

The lamella sedimentation tank is in fact a tank equipped with a plurality of thin metal plates (slats) spaced 0.05 to 0.1 m apart. The angle of inclination of the plates is 40 to 70 degrees. The lamellas increase the total sedimentation area in the tank and sedimentation is more efficient. The space between the plates allows the free upward movement of the liquid, but the movement of the insoluble particles is limited, they tend to settle on the plates and sink down into the conical sludge tank 20.

As mentioned above, a flocculating agent is optionally added before the solution / melt CN enters the sedimentation tank. The added flocculant is first transferred to the first mixing compartment where it is well mixed into the CN solution / melt. This mixing compartment is equipped with a rotary mixer or 25 other suitable mixer. The solution / melt CN, mixed with the flocculating agent, subsequently flows into a second mixing section, equipped with a slow-motion mixer, by means of which the flocculated particles are kept suspended.

The solution / melt CN flows from the second mixing compartment into the vane sedimentation tank and enters the bottom of the plates. The solution moves upward between the plates and exits the lamella separator through the outlet channels located above the plates. The drain channels have a large number of openings through which liquid flows out of the vane space. In this way, an even distribution of the liquid over the plates is ensured.

• ·· ·

The solid particles settle on the plates and sink down into a sludge pit equipped with a rotating wiper with a very low rotation speed. The sludge level in the sludge well is kept constant or fluctuates as the sludge phase is pumped into a decantation centrifuge for dewatering.

In one embodiment of the present invention, there is provided a method of purifying CN solution / melt in a nitrophosphate process, wherein the purification step comprises sedimenting insoluble materials and removing them from the bottom of the sedimentation tank.

In another embodiment of the present invention there is provided a method of purifying CN solution / melt in a nitrophosphate process, wherein the purification step comprises sedimentation of insoluble matter in a vane sedimentation tank.

In another embodiment of the present invention, a flocculating agent is added before the solution / melt CN enters the sedimentation tank. Various types of flocculating agents may be used, preferably the flocculating agent is selected from the following: Fennopol A3304, Nordfloc A172 and Superfloc AF126.

In another embodiment of the present invention, the sludge formed by the suspended insoluble matter is discharged from the bottom of the sedimentation tank and optionally used for other processes such as centrifugation. By further centrifugation, the sludge is divided into a precipitate, i. a concentrated mixture of insoluble matter, and a supernatant, a purified melt.

The solution / melt density of the CN to be cleaned is in the range of 1.3 to 1.5

1.75 kg / l, preferably 1.45 to 1.65 kg / l, and the solution / melt temperature of the CN is maintained at 25 to 90 ° C, preferably 40 to 80 ° C.

The mol P / mol F ratio is characteristic of the untreated CN melt and can be changed by using another phosphate in the NPK production plant or by adding concentrated phosphoric acid to the untreated CN slurry. The mole P / mole F ratio can also be changed by adding another phosphorus compound soluble in the acidic untreated CN solution. The amount of concentrated phosphoric acid added may vary according to the ratio of moles P / moles F in solution / slurry CN, but a dose of 0 to 9 kg / m ³ of the CN solution is normally sufficient at 50% CN in solution.

CN purification in the nitrophosphate process of the present invention is schematically shown in Figure 7.

Cleaning includes an additional cycle 8, sedimentation cleaning. Cycle 7 optionally comprises adding a flocculating agent and adjusting the mol P / mol F ratio by adding phosphoric acid or another phosphorus compound.

Modification of cycle 7, including the adjustment of the mol P / mol F ratio and including the sedimentation cycle, which includes decanting sludge centrifugation, brings several advantages over the process shown in Fig. 1.

These advantages include significant energy savings as the CN solution to be processed has a density of up to 1.67 kg / l. In the current cleaning process, the CN melt (cycle 4, Figure 1) needs to be diluted to a density of approximately 1.47 kg / l. In the process according to the invention, this step can normally be omitted completely. From the sedimentation cycle a solid CN containing insoluble matter at a concentration of 350 ppm is obtained. This is a significant improvement compared to the current process shown in FIG. 1 to obtain CN containing insoluble matter at 1000 ppm.

With the same energy consumption as the process illustrated in Figure 1, the insoluble matter content of the present invention can be reduced to values in the range of 50 to 100 ppm. The maintenance costs of the decanter centrifuge are significantly reduced because only one small centrifuge is used in the process of the present invention as opposed to the three large centrifuges used in the current process.

The invention will now be demonstrated in several non-limiting examples.

Examples

Unless indicated that other flocculating agents were used, all examples were performed using Nordfloc A172 (supplied by SNF). The flocculating agent solution was prepared in batches by dissolving 1 kg of flocculating agent in 1m ^{3 of} water in the flocculating agent tank. The resulting 0.1% solution was drained to both the untreated melt and the sludge entering the decanter centrifuge.

Example 1

In the laboratory tests, various neutralized solutions from the CN production apparatus (ca. 1000 ml) were mixed with the flocculating agent and pumped into a sorting cylinder (1000 ml) where they remained for 2 minutes. The volume of the sludge phase, the deposited insoluble matter, was observed. The results are shown in Table 1 below:

Samples 1 and 2 of the device are labeled S1 and S2.

Table 1 CN solution neutralized to pH 6-7 (1 + 10) with ammonia

Density of solution (g / cm ³ ) Flocculant added Superfloc (Cytec Ind) 1% (ml / l solution) Sludge volume (ml) % P %F The ratio of mol P / mol F Amount of insoluble matter in the liquid above the sludge phase (ppm) 1.57 6 330 0.15 0.12 0.77 ON 1.57 6 350 0.15 0.12 0.77 80 1.49 (S1 + water) 6 190 0.15 0.12 0.77 72 1.49 (S1 + water) 6 180 0.15 0.12 0.77 68 1.55 6 800 0.06 5 0.14 0.28 60 1.48 (S2 + water) 6 750 0.06 5 0.14 0.28 55 NA = not reached upný

The results from Table 1 clearly show that insoluble substances settle faster, provided that some necessary conditions are met. The following conclusions can be drawn: (i) the more dilute the solution, the higher the sedimentation rate; (ii) at low moles P / mol F the sedimentation rate is low;

Example 2

Testing in a test facility

The assay described in Example 1 is a dosing process. For a further test of the use of sedimentation to obtain a purified melt, i.e. to effectively purify the CN melt from the nitrophosphate sedimentation process, a test device was designed, as shown in Fig. 2. A test device of about 1.5 m ³ was made of plexiglass to sedimentation was observed.

Description of the device:

· Λ a

Tank A: Mix the flocculating agent and the crude CN solution carefully

Tank B: Gently mix flocculant and crude CN solution

Part D: Sedimentation surface for sludge, insoluble in water

Part C: Purified CN solution

The crude CN solution was pumped into tank A, where it was carefully mixed with a suitable flocculating agent, Nordfloc A172 (high speed mixer, short residence time). Then the solution was stirred in tank B for 2 to 6 minutes to allow flocculation of larger flakes. The solution flowed by gravity to Part D, spreading over the entire tank and moving slowly upward to Part C.

The flakes (insoluble matter) were moved to part D - the conical part of the tank, and were removed from the bottom of the tank. The solution from part C was propelled up, overflowed through the wall into the outlet chamber, and the tank left the clean melt through the outlet throat.

A neutralized CN solution from the production equipment (CN melt "in Fig. 2) was used for testing and several tests were performed under various conditions. The results are shown in Table 2.

· ♦ ♦ · * * * * * * * *

Table 2. Sedimentation cleaning results in a test facility

Density of crude CN solution 1% insoluble matter PH of CN solution % P % F The ratio of mol P / mol F Dwell time in sedimentation tank (min) % insoluble matter in sludge (sludge compaction) Ppm of flocculating agent Addition h ₃ after ₄ + ΙηηοΙ P / F Ppm of insoluble matter in CN solution 1630 6.2 ON ON ON 86 5.7 7.4 ON 484 1500 0.1 0 0.1 5 0.41 36 8.0 8.0 ON 250 1500 6.1 0.50 36 10.6 8.0 +0.1 189 1515 6.1 0.1 0 0.1 4 Γ 0.44 89 ON 8.3 ON 204 1535 6.1 0.1 3 0.1 4 0.57 89 ON 8.2 ON 150 1510 6.1 ON ON 0.70 60 13.0 9.0 +0.2 120

NA = not available

Example 3

In another example, the test apparatus of Figure 2 has been replaced by a much more advanced sedimentation unit developed for water purification. The Lamella sedimentation unit (model LF, supplied by Nordic Water) was connected to the neutralization tank of the CN production plant (Cycle 5 in Fig. 1) and a decanting centrifuge for sludge dewatering was connected.

Basically, the Lamella unit is very similar to the sedimentation device of FIG.

2, but this unit is additionally provided with plates (slats) 0.05 to 0.1 m apart. The angle of inclination of the plates is 55 degrees. The total planned 15 horizontal sedimentation area is 5 m ² . The space between the plates allows the liquid to move upward freely, but the movement of the insoluble particles is limited and these deposit on the plates and sink downwards into the conical part of the device.

«· * * * * * * * *

A cross-sectional view of a test apparatus showing a Lamella unit with connections is shown in Fig. 4.

In the upper left part of the Lamella unit there is a flocculator consisting of two compartments with a total volume of 1 m ³ . The first compartment is small and is equipped with a high speed mixer to properly mix the flocculating agent and the melt. The melt then flows into a second compartment whose agitator moving at low speed keeps the flocculated particles suspended.

From the second compartment, the melt / solution flows into the Lamella tank and enters the bottom of the plates. The solution moves upward between the plates and exits the lamella separator through the outlet channels located above the plates. The drain channels have plenty of holes through which liquid flows out of the Lamella unit. In this way, an even distribution of the liquid over the plates is ensured.

The solid particles settle on the plates and sink into a sludge pit equipped with a rotating wiper with a very low rotation speed. The sludge level in the sludge well is kept constant or fluctuates as the sludge phase is pumped into a decantation centrifuge for dewatering.

The results for which the residence time and density of the crude CN solution differ are shown in the graph in FIG. 5. The graph shows the water insoluble content of the pure CN melt flowing from the Lamella unit as a function of the melt density (at 60 ° C). up to 70 ° C, including the addition of a flocculating agent) and depending on the melt flow rate.

These results show that the lower the density of the crude CN solution, the lower the insoluble matter content in the pure solution that flows from the Lamella unit. At densities in the range of 1.58 to 1.60 kg / l (70 ° C), the insoluble content is normally less than 300 ppm. At densities of about 1.46 to 1.47 kg / l, the achievable insoluble matter content is less than 50 ppm.

Changes in the residence time between 35 and 60 min do not significantly affect the cleaning of the CN solution.

Example 4

Effect of mol P / mol F ratio of crude CN solution

The lamella sedimentation unit was connected to a CN production facility and a continuous sedimentation experiment was performed for three days. The ratio of mol P / mol F of the crude solution CN was varied by changing the crude phosphate in the NPK a

“TT

ea ·· • a • «A • · • · 4 and a • · • a • * and a • · · • · a ···· and ♦ and 4 a • and a  and aa 4

by adding phosphoric acid to the crude CN slurry. In the experiment, the content of insoluble substances in the purified CN solution was measured.

The following conditions were used in the experiment:

Flow rate: 2.5 m ³ of CN solution / hour

Delay time: 34 min

Flocculant dosing: 20 ppm (0.1% solution)

Melt density: average 1580 kg / m ³ at 80 ° C (1550 to 1620)

Insoluble matter content in crude CN: 1 to 1.6% pH of crude CN: between 5.5 and 6.5.

Sludge phase flow rate: 250 l / h

The results are shown in the graph of FIG. 6. The graph shows the amount of water insoluble matter in the CN solution that exited from the Lamella unit and the ratio of moles P / mol F in the crude CN melt over 3 days.

Figure 6 shows that the amount of water-insoluble substances was reduced to 150 to 350 ppm in the period up to May 23, 6:00 pm. In this period, the ratio of mol P / mol F is between 0.4 and 0.5.

From 23 May, 6:00 pm, the amount of water insoluble substances began to increase and the ratio of moles P / moles F of crude CN decreased to 0.3. As of May 24, 04:00 hrs and beyond, the mol P / mol F ratio has steadily shifted to 0.7 and the amount of water insoluble substances has started to decline from about 1000 ppm to about 200 ppm.

The latter development of the content of water-insoluble substances was interrupted on 24 May, between 12:00 and 16:00, as the pH values began to change outside the acceptable range (above pH 7, due to HNO ₃ blockage in the production facility) .

If the mole P / mol F ratio in the crude CN solution (and in the acidic CN solution from the NPK production apparatus) decreased, small and more fluffy, water-insoluble crystals formed in the neutralization cycle. This sludge settled significantly more slowly and the insoluble matter content of the CN flowing out of the Lamella unit increased.

The mole P / mole F ratio is characteristic of crude CN and can be influenced by phosphate change in the NPK production plant or by the addition of concentrated phosphoric acid.

• · *

Sludge phase treatment:

The sludge phase discharged from the vane unit was drained using a decanter centrifuge. The centrifugal force of the decanter centrifuge was approximately 550g.

Prior to entering the centrifuge, a 25 to 35 ppm flocculating agent was added to the sludge phase.

The concentrated sludge (approximately 15% of the vane unit delivery) that leaves the centrifuge is recycled by returning to the leaching cycle, that is, cycle 1 in Fig. 1.

The CN solution obtained by centrifugation (about 85% of the centrifuge supply) contained 10,450 ppm of insoluble matter and was mixed with the flow of purified CN solution exiting the vane unit, or recycled by adding to the raw CN solution flow to the vane unit.

The results show that the crude CN solution originating from the nitrophosphate process, with a density of up to 1620 kg / m 3 at 70 ° C, can be purified by the sedimentation tank 15 to a concentration of insoluble matter in the range 200-300 ppm, preferably using a sedimentation tank. equipped with lamellar plates, with a residence time of up to 35 minutes and a mole P / mol F ratio of the CN solution greater than 0.4, preferably greater than 0.5.

If the mole ratio P / mol F is less than 0.4, the sludge sedimentation rate 20 will decrease and at the same time the cleaning efficiency will be reduced, cleaning is still possible in this case, but the sludge flow from the sedimentation tank will be much higher.

The mole P / mole F value can be easily influenced by the addition of concentrated phosphoric acid or another phosphorus compound soluble in the acidic CN solution.

Claims (10)

CLAIMS 1. A method for purifying calcium nitrate solutions or melts, comprising the step of sedimentation. 2. The process according to claim 1, wherein the sedimentation is followed by centrifugation of the sludge phase. Method according to claim 1 or 2, characterized in that the sludge is recycled by returning to the phosphate leaching step. A method according to claim 1, 2 or 3, comprising the steps of a) phosphate leaching, b) cooling / crystallization, c) filtration, d) washing the crystals, (e) dissolution and dilution to a density of 1,3 to 1,75 kg / l at a temperature between 20 and 90 ° C; f) neutralization, g) addition of flocculant and adjustment of the mol P / mol F ratio, h) sedimentation, i) centrifuging the sludge phase and possibly recycling the sludge in step a) (j) evaporation; (k) pulverization. The process according to claim 4, wherein in step e) the melt is diluted to a density of about 1.6 kg / l at a temperature of 70 ° C. Method according to claim 4 or 5, characterized in that in step g) the mol P / mol F ratio is set to a value higher than 0.30, preferably higher than 0.45. Method according to claim 4, 5 or 6, characterized in that the sedimentation in step h) is carried out in a sedimentation tank equipped with thin plates, with slats. 5 8. A calcium nitrate solution / melt comprising less than 3% water insoluble matter. The calcium nitrate solution / melt according to claim 8, 10, characterized in that it is prepared by a process according to claims 1 to 7.