RU2459765C2 - Method of purifying calcium nitrate and obtained products - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used in inorganic chemistry. The method of producing purified calcium nitrate from molten calcium nitrate involves decomposition of apatite with acid (1), cooling and crystallisation of the resulting calcium nitrate solution (3), filtering calcium nitrate crystals (4), washing the filtrate with nitric acid and/or water (5), dissolving in water (6), neutralisation with ammonia (7) and adding a flocculant (8). Before crystallisation, the solution obtained from decomposition undergoes centrifuging (2) and floatation (9) is carried out after adding the flocculant (8).

EFFECT: invention increases purity of calcium nitrate, reduces content of water-insoluble solid calcium nitrate particles to less than 150 ppm.

12 cl, dwg, 3 ex

Description

FIELD OF TECHNOLOGY

The present invention relates to a method for purifying calcium nitrate obtained from phosphate, and to purified calcium nitrate obtained using the method according to the present invention.

BACKGROUND OF THE INVENTION

Calcium nitrate can be obtained by dissolving phosphorite in nitric acid, followed by precipitation of calcium nitrate tetrahydrate by cooling the resulting solution (extract). Precipitated calcium nitrate (NK) is separated from the solution in which the treatment took place and neutralized before grinding.

Phosphate raw minerals, such as apatite, contain high concentrations of OH ^- , F ^- or Cl ^- ions in the crystal. As a result, crystals of calcium nitrate obtained from apatite usually contain impurities in the form of fluorine and phosphorus compounds. In addition, other impurities may be present, such as iron, aluminum, and the like. Upon receipt of calcium nitrate, suitable for technical use, that is, as a coagulant for latex, cement setting accelerator, to prevent oil storage from acidification, and so on, these impurities should be removed.

BACKGROUND OF THE INVENTION

The separation of solid impurities in a solution / melt of calcium nitrate by sedimentation is described in patent document Russian Patent RU 2228906. Other purification methods include precipitation of fluorine and phosphorus-containing contaminants in the form of apatite and CaF ₂ at the stage of neutralization by adjusting the P / F ratio and neutralizing to pH 5-6, followed by separation of the precipitates obtained by decantation centrifuges (patent document US 4952379). However, these methods do not allow complete separation of insoluble particles without the use of excessive amounts of water in order to increase the density difference between solid particles / sediments and saline and / or without increasing the settling time to a practically inexpedient duration in the case of a continuous process.

US Pat. No. 5,009,792 describes a flotation method for the purification of aqueous salt solutions, however, it is based on the addition of organic blowing agents such as waxes, oils and surfactants. This may be effective, for example, in the case of purification of dissolved calcium nitrate containing oil / wax coatings. However, for technical products, organic components are undesirable and their use should be minimized to avoid contamination of the powdery final product.

A known method of cleaning a saline solution, providing a high degree of purity, is also cleaning using a filter press or other types of filter equipment. However, if filters are used in the purification of calcium nitrate from apatite, the use of an auxiliary filter substance (namely diatomite) will be required to maintain an acceptable flow through the filter cake. The amount of filter aid limits the use of such equipment to small production volumes due to the need to remove filter cake in an environmentally acceptable manner and thus leads to high cost. In addition, most filters operate in batch mode, which is a disadvantage in the case of a continuous process for the rest.

In addition, none of the cleaning methods of the prior art (with the exception of some filter systems) does not allow to obtain a very pure substance; as a rule, the content of water-insoluble particles is from 400 to 1000 ppm. (rt).

OBJECT OF THE INVENTION

The main objective of the present invention is to provide a continuous and effective method for purifying calcium nitrate melts obtained by the decomposition of phosphate raw materials.

Another objective is to create a method that allows to reduce the content of water-insoluble impurities in the melt of calcium nitrate to less than 100 ppm.

Another objective is to propose a production method in which the above-mentioned disadvantages would be eliminated.

The objectives of the invention can be solved using the features set forth in the following description of the invention and / or in the attached claims.

List of abbreviations:

NK calcium nitrate,

present insoluble in water.

SUMMARY OF THE INVENTION

According to the currently used method, the phosphate raw material is usually decomposed with acid in a decomposition reactor, after which the resulting solution is cooled to precipitate calcium nitrate tetrahydrate in the crystallizer. The resulting crystals of calcium nitrate are separated from the rest of the solution by filtration, washed with nitric acid and / or water, dissolved in water and neutralized with ammonia. During this neutralization process, many insoluble compounds are formed, such as calcium fluoride, apatite, (calcium phosphates, calcium hydroxides, iron phosphates and so on). In addition, the neutralized solution will contain insoluble substances originating from the phosphate raw material, such as silicon dioxide and silicates. To clean the neutralized NK solution, water is added until a certain density is reached, after which a flocculant is introduced into the NK solution. Impurities are mostly settled, but they can also be removed by centrifugation. The centrifugate is evaporated to adjust the water content and ground (granulated or prilled). With this method, solid NK is obtained, usually containing 1200 ppm. insoluble matter. When solid NK is dissolved in water, an opaque solution is formed due to these insoluble impurities.

It was assumed that the additional centrifugation introduced in order to increase the purity of calcium nitrate after the phosphate decomposition stage could lead to a decrease in the amount of impurities. The experiments showed that impurities decreased only partially.

The invention is based on the unexpected discovery that the placement of at least one centrifuge after the decomposition reactor and in front of the crystallizer makes it possible to obtain significantly better cleaning due to the use of a flotation unit instead of the previous centrifugation unit. The second purification step can be improved by adding a filtration step through the sand filter after flotation. By centrifuging immediately after decomposition, the sludge containing insoluble substances from the decomposition reactor is removed at the initial stage, while it was unexpectedly found out that such removal will contribute to the floatation of the remaining insoluble particles to the surface when a flocculant is added to the neutralized NC solution. Not wishing to be bound by any theory, it is believed that separation of the particles at the flotation stage unexpectedly improves when heavy particles are removed, since flotation is less disturbed.

Thus, one aspect of the present invention relates to a method for producing purified calcium nitrate, the method comprising:

- decomposition of phosphate raw material with nitric acid,

- centrifugation of the resulting solution with at least partial separation of solid residues;

- cooling and crystallization of the centrifuged liquid with the formation of crystals of calcium nitrate;

- filtering the crystallized solution to obtain a crystalline phase;

- washing with nitric acid and / or water;

- dissolution of filtered crystals of calcium nitrate in water;

- neutralization of a solution of calcium nitrate by adding ammonia to a predetermined pH;

- adding flocculant and flotation of a neutralized solution of calcium nitrate;

- concentration of the purified solution of calcium nitrate and

- dispersing a concentrated solution of calcium nitrate with the formation of solid particles of calcium nitrate.

According to another aspect of the invention, a filtration step, for example, filtration through a sand filter, can be added after the flotation step.

Another aspect of the present invention relates to purified calcium nitrate obtained by the method according to the present invention.

The method of the present invention is a continuous and efficient process. Due to the application of the claimed sequence of technological stages, it is possible to obtain calcium nitrate with a higher degree of purity compared to the currently used method. The content of water-insoluble particles will be significantly lower than 150 ppm.

Purified calcium nitrate, when dissolved in water, forms a clear aqueous solution.

The method and product are environmentally friendly, since the use of organic components is eliminated. The method is carried out at a high concentration of salts, which is a significant advantage, since water must be evaporated from the solution before the formation of solid particles.

BRIEF DESCRIPTION OF GRAPHIC MATERIALS

1 is a flowchart of an embodiment of a method according to the invention.

Figure 2 is a block diagram of a comparative example.

Figure 3 is a block diagram of a second comparative example.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The invention will now be described in more detail using examples of possible embodiments of the invention. These embodiments should not be construed as limiting the general idea of the invention using a centrifuge after decomposition and replacing a conventional centrifuge with a flotation unit. This general concept of the invention is valid with respect to all currently known and predictable nitrate melts.

DESCRIPTION OF EXAMPLES OF CARRYING OUT THE METHOD ACCORDING TO THE INVENTION

Example 1

At stage 1, apatite is decomposed with an excess of nitric acid. This step is a traditional process well known to those skilled in the art. At this stage, schematically represented by block 1 in FIG. 1, any of the known reactors and any method for the decomposition of phosphate raw material can be used.

After decomposition, the resulting solution (extract) is centrifuged in a decantation centrifuge. This is stage 2 in FIG. 1. Before the centrifuge, the hood is cooled to a temperature of 40 ° C. The amount of extract passing through the centrifuge is usually 20-45 m ³ , while the amount of sediment in front of the centrifuge is 3%, and after it - 5%. The centrifuge speed is 440 G.

Using a centrifuge, a significant amount of sediment is removed from the hood, usually the figures reach 300 kg of sand / gravel, silicates, fluorides per m ^{3 of} liquid. After centrifugation, the extract is cooled to a temperature in the range of 0-5 ° C to crystallize calcium nitrate crystals (stage 3 in FIG. 1). The crystals formed are collected and separated from the liquid by filtration (step 4). Next, the collected crystals of calcium nitrate are washed in nitric acid and water (stage 5) and then dissolved in water to form a solution of calcium nitrate (stage 6). After that, the NK solution is neutralized with ammonia to a pH value of 5.5-8 (stage 7). Steps 3 through 7 are standard processes and are well known to those skilled in the art.

After neutralization, one or more flocculants are added to the nitrate solution (step 8), usually 20-30 ppm. in the form of a flocculant solution (a flocculant dissolved in water).

Flocculant can be prepared in a flocculator. The flocculator is a reservoir with an effective volume of 0.87 m ³ , equipped with a mixing device. The speed of the mixing device can vary, and with it the mixing energy. In most experiments, 50 Hz was used, which corresponds to a stirring speed of 37 rpm. Also conducted experiments with 25 and 75 Hz, experiments showed that this does not have a positive effect on cleaning.

Suitable flocculants are anionic flocculants, especially high molecular weight flocculants. The flocculant can be mixed in a flocculation chamber before installing flotation. Mixing energy and residence time are important; mixing should be intense enough to evenly distribute the flocculant, but not too strong, as this will lead to the destruction of the flakes. The speed gradient (G-factor) in the chamber, corresponding to 100-150, was acceptable. The residence time should not be too long, since high temperatures can lead to the destruction of the polymer chains of the flocculant; it was found that an acceptable time is preferably 8-12 minutes. After that, the flocculated NK melt can be fed to the flotation unit of stage 9 by gravity, since the participation of the force should be minimized to preserve the “flakes” formed in the flocculation chamber.

To further enhance the effect of flocculants, a second dose of flocculant M can be added immediately before flotation. Dosing the entire amount of flocculating solution into the feed material only into the flocculator or into the feed material only into the flotation unit leads to poor cleaning.

Then the solution is floated in step 9. On the surface of the flotation unit, the sediment layer is continuously removed, for example, by means of rotating blades. The speed of the scraper to remove sludge should be adjusted in accordance with the number of N.V. particles in raw materials. Too high a speed can lead to the entrainment of flakes in the product stream, since the scraper blades create a turbulent movement under the sediment layer. Too low a speed led to a too thick sediment layer and, in addition, to entrainment of flakes.

At stage 9, at the inlet to the flotation unit, dispersion liquid N in an amount of 5 to 10 vol% can be added depending on the concentration of the NC melt. The dispersion liquid can be prepared in a high-pressure apparatus and can be supplied with cold water and air at a pressure of 6.0 bar, preferably either cold water saturated with air at 5-6.5 bar, or recycled purified NK melt saturated with air at the same pressure . As the dispersion liquid mixes with the NK melt, the pressure decreases towards atmospheric and air is released in the form of small dispersed bubbles in the NK melt. Air bubbles rise to the top of the flotation unit, capturing flakes containing n.v. particles. N.V. level particles at the exit of the flotation unit was 100-150 ppm The best results were obtained at a low concentration of NK melt (56-58%), about 25 ppm were added at two stages. flocculant and 8-10% dispersion liquid, so that 50-100 ppm were obtained in a pure NC solution. present particles.

A possible installation for filtering through a sand filter as step 10 has also been studied. In step 10, a clean NK melt can be fed through a sand filter, preferably through an internal decanter filter for post-treatment. A laboratory sand filter is connected to a laboratory flotation unit. As the starting material, a purified NK melt with a content of nv is obtained. in the range of 150-300 ppm At a surface load of 2-7 m / h, the refined NK melt after the sand filter contained less than 50 ppm. present particles. The sand filter worked continuously until the total load N.V. remained below 1500 g / m ² h. About 10% of the initial NC melt was used as a washing liquid for sand regeneration. The washing liquid in this case contained about 0.1% n.v. particles.

Preferably, the sand filters contain sand with a particle size of 1.2-2.0 mm. During the filtering process, most of the remaining n.o. particles. The sand filter is continuously regenerated by washing with a small amount of NK melt. This wash liquid should preferably be recycled to the flotation unit.

In those cases where the method according to the present invention allows to obtain an insoluble impurity content of 60-70 ppm in the final pure NK solution, the insoluble impurity content in solid NK will be less than 100 ppm.

"Flakes" are aggregated or coagulated particles that can be formed through the use of flocculants.

The product stream purified in stages 1 to 10 of the method according to the present invention is ready for further processing operations, such as evaporation and dispersion.

Reference Example 2

As for Figure 2, apatite was decomposed with an excess of nitric acid (stage 1). The resulting solution was cooled to a temperature of 0-5 ° C, while crystals of calcium nitrate tetrahydrate precipitated (stage 3).

NK crystals were separated by filtration (stage 4), washed (stage 5), dissolved and diluted with water to a density of 1.49 kg / m ³ at a temperature of 50 ° C (stage 6). The diluted NK solution was neutralized with ammonia to a pH of 6.5 (stage 7), after which a flocculant (stage 8) was added to form a solution of NK.

Part of solution A was transferred to a beaker and saturated with air (small air bubbles from a glass tube) (step 11).

Then the air-saturated solution was defended. Flocculated particles slowly settled to the bottom of the glass. There was practically no movement of particles to the surface.

Part of the solution was centrifuged in a decanter centrifuge to remove insoluble and / or flocculated particles (step 12). The centrifuge was analyzed for the content of water-insoluble impurities. It turned out that the centrifugate (filtrate) contains 770-850 ppm. insoluble particles. Visually, the centrifuge was cloudy.

Reference Example 3

As for Figure 3, apatite was decomposed with an excess of nitric acid (stage 1). The resulting solution was placed in a decantation centrifuge with the same rotation speed as in Example 2, where most of the insoluble particles were removed (stage 2). Then the centrifugate was cooled to a temperature of 0-5 ° C, while the formation of crystals of calcium nitrate tetrahydrate occurred (stage 3).

NK crystals were separated by filtration (stage 4), washed (stage 5) and diluted to a density of 1.53 kg / m ³ at a temperature of 50 ° C (stage 6).

The diluted NK solution was neutralized with ammonia to a pH of 6.5 (step 7) and a flocculant (step 8) was added to form a solution In the NK.

A portion of solution B was transferred to a beaker and saturated with air (step 11). After 10 minutes of sedimentation, almost all coagulated particles moved to the surface of the solution. This was the opposite of Example 2, where the bulk of the particles precipitated.

Part of solution B was transferred to a decanter centrifuge to remove insoluble impurities. The centrifugate was analyzed; it turned out that it contained 560 ppm. insoluble impurities. The solution was still cloudy, but to a lesser extent compared to Example 2.

Evaporation and dispersion of this liquid resulted in solid particles of nanocrystals containing approximately 800 ppm. insoluble material.

Example 3 shows that the mere addition of a centrifugation step to the prior art process does not provide the degree of purification claimed by the present invention.

Claims (12)

1. A method of obtaining purified calcium nitrate from a calcium nitrate melt, including decomposition of apatite with acid (1), cooling and crystallization of the resulting calcium nitrate solution (3), filtering calcium nitrate crystals (4), washing the filtrate with nitric acid and / or water (5) dissolving in water (6), neutralizing with ammonia (7) and adding a flocculant (8), characterized in that it further includes centrifuging (2) the resulting solution before decomposition and crystallization and flotation (9). 2. The method according to claim 1, characterized in that the method includes filtering (10) through a sand filter after adding a flocculant. 3. The method according to claim 1 or 2, characterized in that it is a continuous method. 4. The method according to claim 1, characterized in that the centrifugation (2) is performed using a decanter centrifuge. 5. The method according to claim 1 or 2, characterized in that the flocculant is an anionic flocculant. 6. The method according to claim 1 or 2, characterized in that the addition (8) of the flocculant is carried out in a flocculation chamber. 7. The method according to claim 6, characterized in that the speed gradient in the flocculation chamber is 100-150, and the residence time is in the range from 8 to 12 minutes. 8. The method according to claim 6, characterized in that the flocculant is added in two doses, before and after the flocculation chamber. 9. The method according to claim 1 or 2, characterized in that the flotation (9) includes the use of dispersion liquid. 10. The method according to claim 9, characterized in that the dispersion liquid is cold water saturated with air at a pressure of 5-6.5 bar. 11. The method according to claim 9, characterized in that the dispersion liquid is a recycled purified melt NK, saturated with air at a pressure of 5-6.5 bar. 12. The method according to claim 2, characterized in that the sand used for filtering through a sand filter has a particle size of from 1.2 to 2.0 mm

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