RU2414426C2 - Calcium nitrate - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: present invention relates to a method of cleaning calcium nitrate solutions/melt and a calcium nitrate solution/melt obtained using the said method, and can be used in chemical industry. The method of cleaning solutions or melt of calcium nitrate involves the following steps: a) dissolving or diluting calcium nitrate solution/melt to concentration 1.3-1.75 kg/l at temperature 20-90°C, b) neutralising the obtained solution, c) adding a flocculant and correcting molar ratio of phosphorus to fluorine, d) precipitation, e) centrifuging the precipitated phase. At step (a) the melt is diluted to approximately 1.6 kg/l at 70°C. At step (c) molar ratio of phosphorus to fluorine is corrected to a value higher than 0.30, preferably higher than 0.45. Precipitation at step (d) is carried out in a separate tank which is fitted with think plates and lamellae. The calcium nitrate solution/melt which is cleaned using the said method contains less than 3% insoluble substances.

EFFECT: method enables to lower content of insoluble impurities in calcium nitrate to a value of 50-100 mln^-1 and lower power consumption at the step for cleaning calcium nitrate.

6 cl, 2 tbl, 7 dwg, 4 ex

Description

Field of Invention

The present invention relates to a method for cleaning solutions or floats of calcium nitrate (NK).

BACKGROUND OF THE INVENTION

In the nitrophosphate method, phosphorite is dissolved in an excess of nitric acid, the liquid after the reaction is cooled to approximately 0 ° C to precipitate calcium nitrate tetrahydrate.

Acidic precipitate is removed by centrifugation and neutralization with ammonia. The water content is adjusted by evaporation before granulation or prilling.

Natural phosphate minerals, such as apatite, contain high concentrations of ions such as Si ²⁺ , Fe ²⁺ , Fe ³⁺ , Al ³⁺ , F ^- etc. in addition to calcium and phosphorus. Therefore, calcium nitrate crystals obtained by processing phosphorites will contain different amounts of impurities of fluorides, phosphates and silicates. Particles of sand, silicates and sludge from the processing stage may also be present. Most of the impurities must be removed to obtain calcium nitrate, suitable for technical applications or drip irrigation systems in greenhouses.

Modern methods of purification, the stage of obtaining NK in the nitrophosphate method are shown schematically in FIG. 1. NK purification is carried out at stages 5–7 and involves diluting the NK melt with water to a density of 1.45–1.48 kg / l at 60 ° C before neutralizing with ammonia to a pH of 5–8.

Neutralization leads to the deposition of inorganic components such as apatite, calcium fluoride, silicates and silicon dioxide, etc.

To reduce the amount of insoluble components, the solution from step 5 is mixed with flocculants. Flakes form and a significant portion of the inorganic matter is removed by using a decanter centrifuge.

The supernatant is transferred to stage 8, where the water content is adjusted and a melt is formed containing 77% calcium nitrate, CaN, 7% ammonium nitrate, AN, and 15% water.

Granulation or prilling results in a solid product (solid NR) having insolubles in the range 2400-900 mn ^-1. Insoluble impurities are a mixture of insoluble compounds, such as, for example, silicon dioxide, apatite, calcium fluoride, Al-Fe silicates, etc.

This cleaning procedure is common and this method is easy to implement. However, the above cleaning method has some disadvantages. This cleaning method requires the addition of a large amount of water to reduce the density from about 1.6 (70 ° C) to about 1.45 (70 ° C). This water should then be evaporated before granulation / prilling and a significant additional energy consumption is required. Furthermore, the level of purification from 2400 to 900 million ^-1 is insufficient for certain technical applications. In greenhouses, this level of insoluble impurities leads to such significant deposition in storage tanks over time that procedures are required to clean the equipment. And finally, the described method requires the operation of several decanting centrifuges, the maintenance and maintenance of which is expensive.

Other cleaning methods: Cleaning with a filter press (frame filter press) or other types of filtering equipment (such as a candle filter) are well known methods for cleaning salt solutions to high purity levels. However, for use in the purification of calcium nitrate from apatite, these filter systems will require the use of an auxiliary filter material (i.e. diatomite) to maintain an acceptable flow through the filter cake. The amount of auxiliary filtering material limits the use of this equipment for smaller production volumes, since the filter cake must be removed in an environmentally sound manner, which leads to a high cost. Most filters are batch filters, which is a drawback when operating in continuous mode. Another disadvantage is the need to dilute the NK solution to be purified to a value of less than 1.5 kg / l to achieve an acceptable viscosity and filtration rate. This again increases the cost, since all added water must be removed in order to obtain a solid NK.

Other cleaning methods are, for example, microfiltration using ceramic filters and disk centrifuges. The microfiltration method requires a large investment.

Additional disc centrifuges showed excessive sedimentation in the tests, leading to equipment clogging.

RU 2228906 (abstract) discloses a method for purifying a melt or a solution of calcium nitrate by isolating crystals of calcium nitrate tetrahydrate by cooling the nitric acid extract of phosphate ore, melting the crystals, diluting the melt / solution of calcium nitrate with a 0.5-60% solution of ammonium nitrate and neutralizing the diluted ammonia solution to a pH of 6.1-7.6, followed by separation of solid impurities by sedimentation of the suspension in a precipitation tank in one stage.

RU 2154045 discloses a method for producing complex mineral fertilizer, where phosphorite calcined at 850-1050 ° C is decomposed with non-concentrated nitric acid and the suspension obtained by decomposition is added to water in an amount of 0.5-2.5 volumes per volume of suspension, the insoluble residue is removed, allowing it to precipitate, part of the calcium nitrate is removed by cooling, the solution is treated with ammonia and further treated with the formation of AF (nitrogen-phosphate) or ROS (nitrogen-phosphate-potassium) fertilizer by evaporation, granules Hovhan and drying, the added water is removed by evaporation prior to the crystallization of calcium nitrate. The process of precipitation of insoluble sediment is accelerated many times and a standardized fertilizer is obtained from phosphite.

JP 2006225175 relates to a method for producing a transparent liquid fertilizer containing magnesium nitrate and calcium nitrate as main components, where nitric acid is added to water, in which dolomite is distributed with stirring and mixing with water in order to neutralize to produce magnesium nitrate and calcium nitrate and then at least one type of component selected from the group consisting of a component that is a potassium-containing component and a compound that is a nitrogen-containing Ponent, and a compound which is a component shown in trace amounts, and then added a high molecular flocculating agent and mixed at a temperature of 40-80 ° C and the mixture allowed to stand for sedimentation and separation of the insoluble part.

A brief description of the graphic materials

Figure 1 shows a block diagram of the stages of obtaining NC in the nitrophosphate method.

Figure 2 shows a pilot plant for continuous purification of the melt / solution of NK by deposition.

Figure 3 shows an example of a lamella type equipment.

Figure 4 shows a sectional view of a pilot plant, i.e. sectional view of a lamella installation with connections.

Figure 5 shows the water-insoluble content of the purified melt leaving the lamella separator, depending on the density of the melt (at 60-70 ° C, including the flocculant) and the supply of melt.

Figure 6 shows the amount of water-insoluble substance in the NK solution exiting the lamellar installation, and the molar ratio of phosphorus to fluorine (mol. P / mol. F) in the crude NK for 3 days.

Figure 7 shows the purification scheme of NK obtained after the implementation of the nitrophosphate method.

Summary of the invention

One object of the present invention is to provide an improved method for purifying NK solutions / floats, i.e. the method as a result of which a solution / melt of NK is obtained, where the amount of water-insoluble impurities is reduced.

Another objective of the invention is to reduce energy consumption at the stage of cleaning NK.

The present invention provides a method for purifying a NK solution obtained from the nitrophosphate method, comprising the step of precipitating an insoluble matter. The insoluble substance partially comes from decomposed natural phosphate, and partially obtained after neutralizing the acid crude solution of NK.

Precipitation can be carried out in any equipment suitable for this purpose, in particular in a tank containing plates or lamellas.

In the invention, in particular, a method for purification of a solution of NK by precipitation after adjusting the ratio of mol. R / mol. F to a value above 0.4 using phosphoric acid or another source of phosphorus soluble in the acidic solution of NK.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for purifying calcium nitrate solutions or floats, comprising a precipitation step. The precipitation step is followed by centrifugation of the precipitated phase. In addition, the method includes reusing the precipitated phase by returning it to the phosphorite decomposition step.

The method according to the present invention includes the steps of:

a) decomposition of phosphorite,

b) cooling / crystallization,

c) filtration,

g) washing the crystals,

d) dissolution and dilution to 1.3-1.75 kg / l at temperatures from 20 to 90 ° C,

e) neutralization,

g) adding flocculant and adjusting the ratio of mol. R / mol. F

h) deposition,

i) centrifuging the precipitated phase and possibly recycling the precipitated phase to step (a),

k) evaporation

k) dispersion.

In one embodiment of the method in step (e) above, the melt is diluted to about 1.6 kg / l at 70 ° C.

In another embodiment of the method in step (g), the molar ratio is higher. R / mol. F is adjusted to a value above 0.30, preferably above 0.45.

In yet another embodiment of the method, the deposition in step (h) is carried out in a precipitation tank provided with thin plates, lamellae.

In another embodiment of the invention, there is provided a solution / melt of calcium nitrate that contains less than 3% insoluble impurities.

The calcium nitrate obtained by the method according to the invention is dispersed by granulation, prilling or any other method known in the art.

From X-ray diffraction analyzes, it is known that the insoluble substance that precipitates upon neutralization of the acidic solution of nanocrystals mainly consists of fluorapatite (Ca ₅ (PO ₄ ) ₃ F (70-95%)), and a small amount of SiO ₂ and CaF ₂ . The densities of these minerals are in the range of 2.2-3.2 g / cm ³ , and therefore they must be precipitated in an aqueous solution of NK. It is observed that insoluble impurities precipitate slowly and form a sludge phase at the bottom of the container when the neutralized solution is left alone.

The deposition rate depends, among other factors, on the density of the solution and the ratio of mol. R / mol. F. Typically, the more diluted the solution, the faster the deposition rate, and the low molar ratio. R / mol. F gives a low deposition rate.

The fact that a more dilute solution precipitates faster follows directly from the Stokes law;

Vs = K (ρins-ρsol) d _p ² / η,

Where

Vs means deposition rate,

K is a constant

ρins means the density of insoluble matter,

ρsol means the density of the solution,

η means the viscosity of the solution,

d _p ² means particle diameter.

The second factor, namely that the low ratio of mol. R / mol. F leads to a low deposition rate, stems from the fact that the main impurity that precipitates during neutralization is fluorapatite (Ca ₅ (PO ₄ ) ₃ F). The ideal ratio of mol. P to the pier. F for precipitation of said compound is 3. If the ratio is too low, then the precipitated apatite crystals become few and small, i.e. the low growth rate of apatite crystals, the remaining F precipitates in the form of CaF ₂ , and the entire mixture of insoluble matter precipitates much more slowly. The resulting crystals of CaF ₂ and SiO ₂ are small, with a large surface area and more loose.

Studies have shown that low F content and high molar ratio. P / mol. F (not less than 0.7) in the NC solution gives large apatite crystals, which rapidly precipitate. On the other hand, high concentrations of F and a low molar ratio. P / mol. F (below 0.4) often leads to the formation of small crystals with a low deposition rate.

As mentioned above, phosphorites contain different amounts of F, Si, Al, Fe and other elements in addition to Ca and P. These amounts will vary depending on the type of phosphorites and the washing methods used, FIG. 1, step 4.

Therefore, the ratio of mol. P / mol. F NC crystals entering the dilution and neutralization stage will vary depending on the phosphate feed used.

Some typical phosphorites used are, for example, phosphates from the Kola Peninsula, Bukra (Boucraa, Morocco) and Yusufia (Youssoufia, Morocco), or mixtures thereof. The content of Si, F, Fe, Al, Ca, P, etc. varies from type to type, and also within the same type. Therefore, they say. P / mol. F in the NK solution obtained from the nitrophosphate method will also vary to a large extent. During neutralization of the NK melt, the consistency of the formed insoluble impurities and the amount of each compound will vary and will be able to influence the deposition process.

In the process of continuous deposition, the flocculant is added to the solution / melt NK before introducing into the sedimentation tank. The addition of a flocculant improves the purification in such a way that the amount of water-insoluble impurities used is reduced and the solids content increases, i.e. get a more condensed precipitate. To obtain better cleaning, the density of the solution / melt NK is reduced, and the ratio of mol. P: mol. F increase.

The amounts of insoluble impurities in the purified solutions turned out to be unexpectedly much smaller than the amounts obtained by the purification method used in the conventional NK apparatus, Figure 1, where a centrifuge giving 500-2000 g (centrifugal force) was used to remove insoluble particles.

Therefore, the purification method, including the precipitation of insoluble impurities, could be used to purify NK in the nitrophosphate method.

To further improve the cleaning of the solution / melt of NK, a precipitation tank equipped with metal plates or lamellae is used. Such precipitation equipment is developed and used in water treatment. The unit for deposition with lamellae is connected to the loading tank for the solution / melt of the NK, for example, a tank for neutralizing from the NK installation (stage 6, Fig.7). Before introducing into the aggregate for precipitation into the solution / melt NK, it is possible to add a flocculant and / or adjust the ratio of mol. P / mol. F. Sludge obtained after sedimentation is discharged into a decanter centrifuge for dehydration.

The lamellar sedimentation unit is essentially a tank provided with a plurality of thin metal plates (lamellae) mounted at a distance of 0.05-0.1 m from each other. The inclination of the plates is 40-70 degrees. Lamellas increase the total deposition area in the tank, and deposition is more efficient. The space between the plates allows the liquid to move upward freely, and insoluble particles are delayed and tend to settle on the plates and slip down into the conical sludge tank.

As mentioned above, the flocculant can be added to the solution / melt of NK before transferring the solution to the sedimentation tank. The solution with added flocculant is first transferred to the first mixing chamber, where the flocculant is thoroughly mixed with the solution / melt. This chamber is equipped with a rotary mixer or other suitable mixing equipment. The solution / melt mixed with the flocculant flows into the second chamber, which has a slowly moving stirrer to keep the flocculated particles in suspension.

From the second chamber, the melt / solution is poured into the lamella sedimentation tank and enters the lower part of the plates. The solution moves up between the plates and leaves the lamella separator through the drain troughs located above the plates. The drain troughs have many openings through which fluid must flow out in order to leave the lamella. In this way, the gutters guarantee an even distribution of the solution over the plates.

Solids are deposited on the plates and slide down into the sludge hopper, which is equipped with a very slowly rotating scraper. The level of sludge in the hopper is kept constant or is varied by pumping the precipitated phase into a decanter centrifuge for dehydration.

In one embodiment of the present invention, there is provided a method for purifying NK solutions / floats in a nitrophosphate method, wherein the purification step involves precipitating insoluble impurities and removing said insoluble impurities from the bottom of the precipitation tank.

In another embodiment of the invention, there is provided a method for purifying NK solutions / floats in a nitrophosphate method, wherein the purification step involves the precipitation of insoluble impurities in a lamella sedimentation tank.

In another embodiment of the invention, the flocculant is added to the solution / melt NK before introducing into the sedimentation tank. Various types of flocculating agents may be used; preferably, the flocculating agent is selected from Fennopol A3304, Nordfloc A172 and Superfloc AF126.

In another embodiment of the invention, the sludge generated by precipitated insoluble substances is discharged from the bottom of the precipitation tank and optionally subjected to further processing, such as centrifugation. In an additional centrifugation step, the sludge is further separated into a precipitate, i.e. concentrated precipitate of insoluble impurities, and supernatant, i.e. refined melt.

The density of the solution / melt of the NC to be cleaned is in the range of 1.3-1.75 kg / l, preferably 1.45-1.65 kg / l, and the temperature of the solution / melt is maintained in the range of 25-90 ° C, preferably 40-80 ° C.

The ratio of mol. R / mol. F is a characteristic of crude NK melt and can vary by changing natural phosphates in an ROS unit (production of nitrogen-phosphate-potassium fertilizer) or by adding concentrated phosphoric acid to the crude NK melt. The ratio of mol. P / mol. F may also vary by the addition of another source of phosphate soluble in the acidic crude NA solution. The amount of added concentrated phosphoric acid will vary according to the mol. P / mol. F in the solution / melt of the NC, however, the amount of 0-9 kg / m ³ of the NC solution will normally be sufficient based on 50% CaN in the solution.

The NK purification in the nitrophosphate method according to the present invention is illustrated schematically in FIG. 7.

Purification involves an additional step 8, purification by precipitation. Stage 7 may include the addition of a flocculant and the adjustment of the ratio of mol. P / mol. F, with the addition of phosphoric acid or another source of phosphate.

When modifying stage 7, including adjusting the ratio of mol. P / mol. F, and a precipitation step including a decanter centrifuge for treating sludge, some advantages are achieved compared to the method shown in FIG. 1.

Among these advantages, a significant amount of energy is saved, since the NC solution to be processed has a density of up to 1.67. In the existing cleaning method, the NK melt (from stage 4, FIG. 1) should be diluted to a density of about 1.47 kg / l. In the method of the present invention, normal dilution can be omitted.

Step deposition allows to obtain solid NK containing insoluble substance in a concentration of usually 350 million ^-1. Compared with the known method illustrated in Figure 1, the solid preparation of NC containing 1000 million ^-1-insoluble substance is a significant improvement.

With the same power consumption as in the method illustrated in Figure 1, the application of the method of the invention can reduce the content of insoluble substances to values between 50 and 100 million ^-1.

The operating costs for decanting centrifuges will be significantly reduced, since only one small centrifuge will be used in the method according to the invention, compared with three large centrifuges in the known method.

The invention will now be illustrated by the following non-limiting examples.

Examples

If it is not indicated that solutions of other flocculants are used, then all examples were carried out using Nordfloc A172 (supplied by SNF). A flocculant solution was prepared in batches by dissolving 1 kg of flocculant in 1 m ^{3 of} water in a flocculant tank. The resulting 0.1% solution was supplied both to the crude melt and to the sludge phase, entering the decanter centrifuge.

Example 1

In laboratory tests, various neutralized NK solutions (approximately 1000 ml) from the apparatus were mixed with a flocculant and transferred to a graduated cylinder (1000 ml) and left alone for 2 minutes. The volume of the slurry phase, i.e. precipitated insoluble matter. The results are presented in Table 1 below:

S1 and S2 mean samples 1 and 2 from the installation.

Table 1 NK solution neutralized to pH 6-7 (1 + 10) with ammonia The density of the solution (g / cm ³ ) Superfloc flocculant (Cytec Ind) 1% (ml / L solution) was added. Slurry volume (ml) % P % F pier P / mol. F The number of slurries. deposited in the liquid phase over (mn ^-1) 1.57 (S1) 6 330 0.15 0.12 0.77 NA 1.57 (S1) 6 350 0.15 0.12 0.77 80 1.49 6 190 0.15 0.12 0.77 72 (S1 + water) 1.49 6 180 0.15 0.12 0.77 68 (S1 + water) 1.55 (S2) 6 800 0,065 0.14 0.28 60 1.48 6 750 0,065 0.14 0.28 55 (S2 + water) NA = no data

The results of Table 1 clearly show that insoluble matter precipitates much faster if certain preconditions are met. Nevertheless, the following conclusions can be made: 1) the more diluted the solution, the higher the deposition rate, 2) at low molar ratios. P / mol. F the deposition rate is low, and 3) the amount of insoluble matter in the solution of the above sludge is very low.

Example 2

Testing in a pilot installation

The test described in example 1 is a batch process. For additional testing of the use of deposition to obtain purified melt, i.e. for effective purification of NK melt from the nitrophosphate precipitation method, a pilot plant was constructed, as shown in FIG. 2. The pilot plant had a volume of approximately 1.5 m ³ and was made of Plexiglass for visual observation of deposition.

Description of equipment:

Tank A: Thorough mixing of the flocculant and the crude solution of NK.

Tank B: Gentle mixing of the flocculant and the crude solution of NK.

Volume D: The zone of deposition of water-insoluble matter.

Volume C: Purified NK solution.

The crude NK solution was pumped into tank A, where it was thoroughly mixed with a suitable Nordfloc A172 flocculant (high speed mixer, short residence time). Then, the solution was carefully mixed in tank B for 2-6 minutes in order to ensure aggregation of larger flakes. The solution flows into zone D under the action of gravity and is distributed throughout the tank, and moves slowly up to zone C.

Flakes (insoluble matter) migrated down to zone D, the conical part of the tank and drained to the bottom. From zone C, the solution was forced upward and poured through the wall into the bypass chamber before leaving the “clean melt” outlet pipe.

Used a neutralized solution of NK from the installation ("KS-melt" in Figure 2) and carried out several test runs under various conditions.

The results are presented in Table 2 below.

table 2 Purification results by sedimentation in a pilot plant Dirty density. solution of the Tax Code 1% of solutions. pH r-ra NK % P % F pier P / mol. F Duration of sedimentation tank (min) % insoluble. substances in the sludge (sludge compaction) mn ^-1 flocculation. agent Addition of H ₃ PO ₄ + Δ mol. P / F ppm ^-1 substances in the NK 1630 6.2 NA NA NA 86 5.7 7.4 NA 484 1500 0.10 0.15 0.41 36 8.0 8.0 NA 250 1500 6.1 0.50 36 10.6 8.0 +0.1 189 1515 6.1 0.10 0.14 0.44 89 NA 8.3 NA 204 1535 6.1 0.13 0.14 0.57 89 NA 8.2 NA 150 1510 6.1 NA NA 0.70 60 13.0 9.0 +0.2 120 NA = no data

Example 3

In another example, the pilot plant of FIG. 2 was replaced with more advanced precipitation equipment designed for water treatment. The lamella sedimentation unit (model LF, supplied by Nordic Water) was connected to a neutralization tank from the NK plant (stage 5 in FIG. 1) and a decanter centrifuge was attached to dewater the precipitated phase.

In principle, the lamella unit is very similar to the deposition equipment in Figure 2, but this unit is equipped with several plates (lamellae) installed at a distance of 0.05-0.1 m from each other. The inclination of the plates is 55 degrees. The total area of planned horizontal deposition is 5 m ² . The space between the plates allows the liquid to move upward freely, and insoluble particles are trapped and deposited on the plates and slide down into the conical zone for sludge.

A cross-sectional view of a pilot plant showing a lamella assembly with connections is shown in FIG. 4.

At the top left of the "Lamella unit" there is a flocculator consisting of two chambers with a total volume of 1 m ³ . The first chamber is small and equipped with a stirrer having a high speed in order to thoroughly mix flocculant and melt.

The melt flows into the second chamber, which has a slowly moving stirrer to maintain the resulting flocculated particles in suspension.

From the second chamber, the melt / solution flows into the lamella tank and enters the lower part of the plates. The solution moves up between the plates and leaves the lamella separator through the drain troughs located above the plates. The drain troughs have many openings through which fluid must flow out in order to leave the lamella. Thus, the gutters provide an even distribution of the solution over the plates.

Solids are deposited on the plates and slide down into the sludge hopper, which is equipped with a very slowly rotating scraper. The sludge level in the hopper is kept constant or is changed by pumping the precipitated phase into a decanter centrifuge for dehydration.

Figure 5 presents the results with different retention time and density of the crude solution of NK. Figure 5 shows the amount of water-insoluble substance in the purified NK melt leaving the lamella aggregate, depending on the melt density (at 60-70 ° C, including the flocculant) and the supplied melt flow.

These results show that the lower the density of the crude NA solution, the lower the content of insoluble matter in the purified solution exiting the lamellar aggregate. At densities between 1.58 and 1.60 kg / l (70 ° C) insoluble matter content will be generally below 300 million ^-1. At densities of about 1,46-1,47 kg / l is achieved insolubles content below 50 million ^-1.

Changing the retention time between 60 and 35 min slightly affects the cleaning of the NK solution.

Example 4

The effect of the ratio mol. R / mol. F untreated NK solution

The lamellar aggregate for precipitation was connected to the NK plant for the implementation of the continuous deposition process for three days. The ratio of mol. P / mol. F of crude NK was varied by replacing phosphorite in an ROS unit and by adding phosphoric acid to the crude mother liquor stock. The content of insoluble matter in the purified NK solution was measured.

Used the following conditions:

Flow: 2.5 m ³ NK / h solution.

Retention time: 34 minutes

Flocculant Dosage: 20 million ¹ (0.1% solution).

Density of fusion: an average of 1580 kg / m ³ at 80 ° C (1550-1620).

The content of insoluble matter in the crude NK: 1-1.6%.

The pH of the crude NA solution was from 5.5 to 6.5.

Precipitated stream: 250 liters / hour.

The results are presented in Fig.6. The amount of water-insoluble substance in a solution of NK exiting the lamellar aggregate and the ratio of mol. P / mol. F in crude NK for 3 days.

Figure 6 shows that the amount of water-insoluble impurities decreased to 150-350 ppm ^-1 until May 23, 1800 hours. During this period, the ratio of mol. R / mol. F ranged from 0.4 to 0.5.

From May 23, 1800 hours, the amount of water-insoluble impurities began to increase and the ratio of mol. P / mol. F in the crude NK decreased to 0.3. From May 24, 0.4, etc. number of insoluble impurities was reduced from about 1000 million to about 200 ^{-1 -1} million as the ratio mol. R / mol. F gradually shifted to 0.7.

The last change in the amount of water-insoluble impurities was interrupted from 1200 to 1600, on May 24, since pH values appeared outside the acceptable range (above 7 due to HNO ₃ saturation in the installation).

When they say. P / mol. F in the crude solution of NK (and the acid solution of NK from the ROS unit) decreased, then the water-insoluble crystals formed at the stage of neutralization became smaller and looser. Therefore, a more bulky sludge was formed in the sedimentation unit. Such a sludge precipitated much more slowly, and the amount of water-insoluble impurities from the nanocrystal leaving the lamella increased.

Like P / mol. F is a characteristic of crude NK and can vary by changing phosphorites in an ROS unit or by adding concentrated phosphoric acid.

Precipitated phase treatment:

The precipitated phase exiting the lamellar aggregate was connected to a decanter centrifuge to remove dry matter. The decanter centrifuge has a centrifugal force of about 550.

Before introduction into the centrifuge in the precipitated phase is added 25-35 million ^-1 flocculant.

Concentrated sludge (approximately 15% lamella mixture) exiting the centrifuge was reused at the decomposition stage, i.e. stage 1 in figure 1.

NK solution discharged from the centrifuge (about 85% of a centrifuge mixture) was below 450 million ^-1 insolubles and purified NK mixed stream exiting the lamellar aggregate or reused in a stream of crude NK entering the lamellar aggregate.

The results show that crude solution NK derived from nitrophosphate process and having a density up to 1620 kg / m ³ at 70 ° C, can be purified to 200-300 mln ^-1 insoluble substance by precipitation tank, the precipitation tank preferably having a lamella plate, provided that the retention time is up to 35 min and the ratio of mol. P / mol. F in the NC solution is above 0.4, preferably above 0.5.

If the ratio of mol. P / mol. F below 0.4, then the sedimentation rate of the sludge decreases and makes cleaning less efficient. Cleaning can still be carried out, but the required stream of sludge leaving the sedimentation tank will be much higher.

Like P / mol. F can be easily varied by adding concentrated phosphoric acid or another source of phosphate soluble in an acidic crude NK solution.

Claims (6)

1. The method of purification of solutions or floats of calcium nitrate, characterized in that it includes the following stages:
a) dissolving and diluting the solution / melt of calcium nitrate to a concentration of 1.3-1.75 kg / l at temperatures from 20 to 90 ° C,
b) neutralizing the resulting solution,
C) adding flocculant and adjusting the molar ratio of phosphorus to fluorine,
g) deposition,
d) centrifuging the precipitated phase. 2. The method according to claim 1, characterized in that in stage (a) the melt is diluted to about 1.6 kg / l at 70 ° C. 3. The method according to claim 1, characterized in that at stage (c) the molar ratio of phosphorus to fluorine is adjusted to a value above 0.30, preferably above 0.45. 4. The method according to claim 1, characterized in that at stage (a) the melt is diluted to about 1.6 kg / l at 70 ° C and at stage (c) the molar ratio of phosphorus to fluorine is adjusted to a value above 0.30, preferably above 0.45. 5. The method according to claim 1, characterized in that the deposition in stage (g) is carried out in a precipitation tank equipped with thin plates, lamellae. 6. The solution / melt of calcium nitrate, purified by the method according to any one of claims 1 to 5, characterized in that it contains less than 3% insoluble substances.

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