EA010973B1 - Method for production of phosphoric acid - Google Patents

Abstract

The invention relates to a method for producing phosphoric acid involving: a formation of a first aqueous slurry containing calcium sulfate dihydrate and phosphoric acid; a formation of a second aqueous slurry containing calcium sulfate hemihydrate and phosphoric acid; a filtering of a mixture comprised of the two slurries through a filter while forming a filter cake simultaneously containing calcium sulfate dihydrate and hemihydrate, and; separating an aqueous phase containing the phosphoric acid.

Description

The present invention relates to a method for producing phosphoric acid, comprising preparing a first aqueous pulp containing calcium dihydrate sulfate and phosphoric acid, preparing a second aqueous pulp containing calcium sulfate hemihydrate and phosphoric acid, filtering through a filter accompanied by cake formation, and separating the aqueous phase with phosphoric acid content.

Known methods of filtering cake, consisting mainly of calcium dihydrate sulfate (Ca8O ₄ -2H ₂ O) (see, for example, BE-660216 and BE-683739). These patents provide for the subsequent conversion of calcium sulfate obtained at a higher temperature and acidity, with the aim of purifying and recrystallizing it to produce calcium sulfate hemihydrate calcium (Ca8O ₄ -1/2 H ₂ O).

British Patents No. 313.036 and US No. 1.902.648 describe the production of phosphoric acid, accompanied by the production of calcium sulfate hemihydrate, the latter being then converted to dihydrate to separate it by filtration and washing on a second filter.

US Pat. No. 1,836,672 discloses a process comprising two steps, the first of which is reacted with phosphate and sulfuric acid to simultaneously produce the hemihydrate and concentrated phosphoric acid, which is then separated by filtration, and the hemihydrate impregnated with phosphoric acid in the second step they are suspended in a sulfophosphoric medium at acidity and temperature at which calcium sulfate recrystallizes to form dihydrate.

Also known is a method for the continuous production of phosphoric acid and calcium sulfate, including crystallization after filtration of the sulfate in the form of dihydrate or hemihydrate in the same form or recrystallization in the form of anhydrite II or hemihydrate or dihydrate, respectively, containing a certain amount of anhydrite II (see EP-B- 0181029).

Finally, there is known a method for producing phosphoric acid and calcium sulfate by continuous filtration of streams coming from several reaction zones, each of which is supplied with water pulp formed during the decomposition of ore by phosphoric acid (ϋδ-Α-3911079). These types of pulp may contain calcium sulfate dihydrate or calcium sulfate hemihydrate. The obtained cake consists of layers of calcium dihydrate sulfate and calcium sulfate hemihydrate of calcium located on each other without penetration of one layer into another. The aim of this method is to increase the concentration of P ₂ O ₅ in phosphoric acid produced in the last reaction zone. This method is complex and difficult to implement, has the disadvantage, which is manifested in the low efficiency of the installation.

In all previous methods, the calcium sulfate hemihydrate or calcium sulfate dihydrate is always filtered separately.

In traditional dihydrate filtration methods, gypsum is a by-product to be dumped. However, its storage can be carried out only under strictly defined conditions in places sometimes remote from the place of production. The resulting gypsum contains a large amount of water, which makes it difficult to handle and increases transportation costs during land transportation. In addition, there is a danger of groundwater pollution at landfill sites.

The wet methods used in industry, which make it possible to obtain concentrated phosphoric acid (P ₂ O ₅ is more than 40%), have long been associated with difficulties caused by the simultaneous production of calcium sulfate hemihydrate and high temperature conditions: operating conditions cause corrosion and clogging of equipment, which causes additional production costs.

The aim of the present invention is to provide a method for producing phosphoric acid, which does not have these drawbacks and which allows the use of technically simple equipment or even use the equipment currently available.

The specified goal is achieved according to the invention using the above method, including:

mixing said first pulp and second pulp before and / or during filtration, filtering the resulting mixture to form a cake consisting of a mixture of calcium sulfate dihydrate and calcium sulfate hemihydrate.

By using this very simple method, significant advantages are achieved. Compared to the filtration of a cake consisting of calcium dihydrate sulfate, the filterability of the cake has increased since its porosity has increased due to the presence of hemihydrate crystals, whose structure (twins) differs from (needle) gypsum crystals. This helps to increase the productivity of the filter, which is already installed at the factory and allows for the filtration of gypsum. If a new plant for the production of phosphoric acid is to be installed, the size of the filter can be reduced, which will reduce capital costs.

In addition, it was found that, compared to pulps containing only dihydrate, feeding the filter with a mixture of hydrate and hemihydrate allows to obtain cakes, the content of free water and water-soluble P2O5 in which are the smallest with a minimum filter duration

- 1 010973, which is a surprise and significantly increases the profitability of the installation.

As noted above, cakes after processing on the filter contain significantly less free water than cakes obtained by filtering pulps containing only dihydrate. After unloading the mass, unstable hemihydrate crystals are rehydrated by the free water molecules present in the cake. This means that the amount of acidified water in the cake discharged into the dump decreases, which reduces the risk of contamination of the groundwater horizons. Over time, the cake dries, becomes easily transportable, and can be stored in piles, which require less floor space.

In addition, the overall efficiency of the plant for the production of phosphoric acid, on which calcium dihydrate sulfate is produced, increases, since the recrystallization of part of gypsum with the formation of calcium sulfate hemihydrate releases phosphate ions in the form of phosphoric acid.

According to an embodiment of the invention, the method includes mixing the first and second aqueous pulps before starting the filtration and supplying this mixture to the filter. The mixture can be prepared in a simple way, for example, by connecting the filter feed pipe to the outlet pipes of the reactors in which both pulps are prepared by the method according to the invention. An intermediate mixer may also be provided for mixing both pulps. The method according to the invention also provides for the simultaneous supply of the first and second pulps and their mixing on the filter itself.

Both types of water pulp can be prepared in any suitable way. So, for example, it is possible to provide for the production of one or the other pulp or both of them by the sulfuric acid decomposition of phosphate ores. It can also be envisaged to produce hemihydrate pulp in a known manner from a certain part of the dihydrate pulp, for example, by changing the temperature and / or acid conditions for a given part of the dihydrate pulp. Such a modification involves only the installation of one additional tank at the outlet of the decomposition reactor of the existing installation. Thus, it is preferable that the method includes:

sulfuric acid decomposition of phosphate ore and preparation of the first aqueous pulp, selection of a portion of the first aqueous pulp mentioned, treatment of this pulp with its conversion into a second aqueous pulp.

Such processing may include, for example, the addition of sulfuric acid to the drained portion of the aqueous pulp or its heating, for example, by heating the reactor or supplying water vapor to the pulp, or both of these processes simultaneously.

And vice versa, it can be envisaged that in a known manner a dihydrate pulp is obtained from part of the hemihydrate pulp.

In some cases, pulp prepared from hemihydrate is a more acidic medium than dihydrate pulp, i.e. it contains more sulfuric acid.

When a mixture of these two types of pulp is fed to the filter, the sulfuric acid content in the obtained phosphoric acid sometimes exceeds the technical conditions, and therefore it is necessary to desulfate the filtrate, for example, by adding phosphate ore and settling the calcium sulfate formed, which is advisable to reuse for sulfuric acid decomposition of phosphate ore.

Other embodiments of the method are given in the attached claims.

Below is described in more detail an example embodiment of the invention using the method according to the invention with reference to a single figure, which depicts a flow chart.

The installation comprises a decomposition reactor 1, into which crushed sulfate ore is loaded through inlet 2, and sulfuric acid is fed through inlet 3 and an aqueous solution of phosphoric acid through inlet 4, which is reused in the method according to the invention. A known decomposition mode is used in the reactor to produce aqueous pulp containing calcium sulfate dihydrate and phosphoric acid, which is discharged from the reactor 1 through the exhaust pipe 5.

A part of the pulp drained from the reactor 1 is withdrawn and fed through a conduit 6 to the conversion reactor 7. In this reactor, the pulp is processed in such a way that it can be used to prepare an aqueous pulp containing calcium sulfate hemihydrate and phosphoric acid. The processing used in this exemplary embodiment involves the addition of sulfuric acid through inlet 8 and / or heating the reactor and / or supplying steam to the pulp through inlet 9. It goes without saying that any other heat source can be used for this purpose. As a result of increasing acidity and / or temperature, the dihydrate is converted to hemihydrate.

The remaining part drained from the pulp reactor 1 is fed through the pipeline 10 to the mixing section, shown by arrow 11. The pulp processed in the reactor 7 is fed to this mixing section through the outlet pipe 12.

The mixing step can be carried out, as shown, using a common supply pipe with which pipelines 10 and 12 are connected, as well as, for example, using a mixing tank with which pipelines 10, 12 are connected, from which the mixture is fed to filter 13. You can also provide for the simultaneous supply of both types of pulp through pipelines 10, 12 directly to

- 2 010973 filter 13, which is then mixed.

The filter 13 can be any filter device, for example, a belt filter, a carousel-type device with filter cells, etc.

In the above example, the mixture of both types of pulp is filtered in the first section of the filter with the formation of the filtrate 14, which is the target product of the process, i.e. an aqueous solution of phosphoric acid.

Presented on the figure, the method contains two stages of washing the cake after filtering. The second washing is carried out using a washing liquid, for example, water introduced in section 15.

The product 16 obtained after washing is returned to the process as a washing liquid through the inlet 17 for use in the first washing stage. The product 18 obtained in the first washing stage is an aqueous solution of phosphoric acid, which can be returned to the process through a recirculation pipe 19 and introduced into decomposition reactor 1 through inlet 4.

If the filtered phosphoric acid entering through the outlet pipe 14 has a too high residual sulfuric acid content, then the pipe 14 can be communicated with a desulfation tank 20 with a stirrer, into which, for example, an additional amount of phosphate ore is fed through the inlet 21. In this tank, calcium sulfate is formed, which is supplied via pipeline 22, for example, to clarifier 27 for separation by settling of calcium sulfate, which is discharged from clarifier 25 through bypass pipe 23, while purified phosphoric acid is collected in section 28. Calcium sulfate can be returned to decomposition reactor 1 at site 24.

Finally, at the outlet end of the filter 13, the cake is unloaded, as shown by arrow 25, which, if necessary, can be moved somewhere. Then it can be dumped into the stack 26. During unloading, moving and / or dumping into the stack, the hemihydrate gradually turns into dihydrate under the influence of residual water in the cake. Consequently, the cake gradually loses water, which simplifies its transportation and dumping into the stack.

Preferably, in the mixing step 11, both types of pulp contain calcium sulfate dihydrate and calcium sulfate hemihydrate in a weight ratio of 90: 10-10: 90, preferably 70: 3030: 70.

It can also be envisaged that at least an additional aqueous pulp containing, for example, only dihydrate, only hemihydrate or a mixture of both of them, in particular in different proportions obtained in step mix 11.

Below the method according to the invention is explained in more detail using examples of execution, given for explanation and not limiting.

Example 1. Two series of four experiments in each were carried out. In each series of experiments, different types of pulp were fed to the filter in four different ways:

1) supply first hemihydrate pulp (20 wt.%) And then pulp from gypsum (80 wt.%);

2) first supply of pulp from gypsum (80 wt.%) And then hemihydrate pulp (20 wt.%);

3) supply of a mixture of hemihydrate pulp and pulp from gypsum with a weight ratio between dihydrate and hemihydrate of 80:20;

4) the supply of pulp with a content of 100 wt.% Gypsum.

Filtration was carried out at a reduced to 400 mm Hg. pressure in the Buchner funnel with a useful surface area of 1 dm ² . It consisted of four successive stages: the formation of cake (s) by the separation of phosphoric acid and crystals from both types of pulp (the uterine solutions section in Tables 1, 2 below), the first washing of the cake with the filtrate of the second washing (section "1st washing ”), The second washing of the cake with industrial water (section“ 2nd washing ”) and air drying of the cake (section“ Dehydration ”).

The first series of experiments

Table 1

No. An experience Time, sec Cake Composition Thick- on Mother solutions 1st flushing 2nd flushing Dehydration Amount Free water,% P ₃ O _} 8E mm 1a 20% H 80% ϋ 33 38 nineteen thirty 120 47.6 0.43 53 2a 80% O- 20% N 40 43 21 thirty 134 47.4 0.35 48 Per Mixture 35 33 22 thirty 120 44,2 0.36 48 4a 100% O 36 41 29th thirty 136 46.8 0.41 55

- 3 010973

The second series of experiments

table 2

No. An experience Time, sec Cake Composition Thickness Mother solutions 1st flushing 2nd flushing Dehydration Amount Free water,% P ₂ O ₅ 5E mm 1b 20% H 80% ϋ 22 23 sixteen thirty 91 49.1 0.50 fifty 2b 80% ϋ 20% Η 37 31 27 thirty 115 46.5 0.43 53 B Mixture sixteen 21 fifteen thirty 82 45.4 0.37 55 4b 100% ϋ 21 32 twenty thirty 103 53.8 0.50 58

H is hemihydrate; Ό - dihydrate; P ₂ O ₅ 8E - water soluble P ₂ O ₅ = phosphoric acid in cake.

Both series of experiments unequivocally indicate that the experiments in which the hemihydrate pulp was supplied first (experiments 1a, 1b), and the experiments in which different types of pulp were mixed beforehand (experiments 3a, 3b), are characterized by the minimum filtration time, while more optimal results for the mixture in experiment 3b.

On the other hand, the use of a mixture of hemihydrate pulp and gypsum pulp systematically ensures a minimum content of free water and water-soluble P ₂ O ₅ in the cake.

Example 2. Five series of experiments were conducted using mixtures of dihydrate and hemihydrate in different weight ratios for comparison using only one dihydrate.

The total duration of the filtration, the content of P ₂ O ₅ in the cake, the yield of free water from the cake after rehydration of the hemihydrate are shown in the table below.

An experience Duration of filtration, sec. Cake Composition The output of P ₂ About ₅ ,% P ₂ O ₅ 5E,% ρ ₂ 0 _5ϊ %, total Free water after rehydration,% 100% O 184.3 4.8 12.3 32,53 93.8 Mixture: 80% ϋ 20% N 202 4,5 11.6 31.08 94.2 Mixture: 70% ϋ 30% N 159.9 4.2 10.5 30,43 94.8 Mixture: 60% ϋ 40% N 162.6 4.4 10.1 27.13 95.0 Mixture: 50% ϋ 50% N 179 4.8 9.5 24.74 95.3

Ό - dihydrate; H is hemihydrate; P ₂ O ₅ 8E - water soluble P ₂ O ₅ = phosphoric acid per cake; P ₂ O ₅ TOT is the total amount of phosphoric acid.

These results indicate the advantages of filtering the mixture compared to filtering the dihydrate alone, these advantages are: a decrease in the duration of the filtration, an increased yield of P ₂ O ₅ , a decrease in the content of free water in the cake after repeated hydration of the hemihydrate phase.

It should be noted that the present invention is not limited to the above options for its implementation and that it can be modified without departing from the scope of the claims.

Claims (15)

CLAIM 1. The method of obtaining phosphoric acid, which includes the preparation of the first aqueous pulp containing sulphate of calcium dihydrate and phosphoric acid, the preparation of the second aqueous pulp containing sulphate of calcium hemihydrate and phosphoric acid, filtration through a filter to form a cake and separating the aqueous phase containing phosphoric acid , characterized in that it also includes mixing between the first and second slurries before the start of filtration and / or during it, filtering the resulting mixture with the formation of a cake, consisting from a mixture of calcium dihydrate sulfate with calcium hemihydrate sulfate. 2. The method according to claim 1, characterized in that it includes mixing the first and second aqueous slurries prior to filtration and feeding the resulting mixture to the filter. - 4 010973 3. The method according to claim 1, characterized in that it includes the simultaneous feeding of the first and second pulps to the filter and mixing them on it. 4. The method according to any one of claims 1 to 3, characterized in that the first and / or second pulp is obtained by the sulfuric acid decomposition of phosphate ore. 5. The method according to any one of claims 1 to 4, characterized in that it includes the sulfuric acid decomposition of phosphate ore with the formation of the first water pulp, the selection of part of the first water pulp, its processing with the transformation of the first water pulp into the second water pulp. 6. The method according to claim 5, wherein the treatment includes the addition of sulfuric acid to the selected part of the first water pulp and / or its heating. 7. The method according to any one of claims 1 to 6, characterized in that it includes unloading the cake containing both calcium dihydrate sulfate and calcium hemihydrate sulfate, as well as residual water, and subsequent hydration of calcium hemihydrate sulfate to form calcium dihydrate in the presence of residual water. 8. The method according to p. 7, characterized in that the unloading of the cake is made with dumping into the stack by the dry method. 9. The method according to any one of claims 1 to 8, characterized in that the aqueous phase containing phosphoric acid is separated during filtration and is subjected to subsequent desulfation. 10. The method according to claim 9, characterized in that the subsequent desulfation includes the addition of phosphate ore to the aqueous phase and the separation of the resulting calcium sulfate. 11. The method according to p. 10, characterized in that the resulting calcium sulfate is reused to form the first aqueous pulp. 12. The method according to any one of claims 1 to 11, characterized in that after separation of the aqueous phase with the content of phosphoric acid, it contains at least one step of washing the cake with a washing liquid. 13. The method according to p. 12, characterized in that it includes the reuse of the washing liquid obtained at least after one washing step, for the formation of at least one first or second pulp 14. The method according to any one of claims 1 to 13, characterized in that the resulting mixture contains calcium dihydrate sulfate and calcium hemihydrate sulfate with a weight ratio of 90: 10-10: 90. 15. The method according to any one of claims 1 to 14, characterized in that it additionally includes at least one feed to the filter of the first or second aqueous pulp before or after filtering said mixture.