DE1911134C3 - Process for the production of complex fertilizers - Google Patents

Description

19 Π 134

received umpnosphat.

A similar way of working is found in »Chem. Engineering ", 75 (1986), 5, pages 124 to 126, mentioned. However, it is expressly emphasized that this method of operation is still under development and still gives rise to considerable technical problems. These problems are so great that the aforementioned publication in Chem. Engineering describes a further method with which these difficulties are avoided. According to this process, the ammonium sulfate solution is introduced directly into the rock phosphate digestion and so all the calcium is precipitated as calcium sulfate. It is consciously accepted that this process can only produce a fertilizer which contains considerably more nitrogen than P ₂ O ₅

As already mentioned with the individual processes, the conversion of the calcium into the nitric acid Digestion solutions or the mother liquor of the calcium nitrate separation carried out with ammonium sulfate at temperatures at which the calcium sulfate is deposited in the form of its dihydrate (gypsum). However, this calcium sulfate dihydrate falls in one Crystal form, which only has a relatively low filter speed in the separation of the calcium sulfate dihydrate allowed by the remaining mother liquor

DE-AS 1134 094 winf a method described, after the total content of a digestion mixture of rock phosphate and nitric acid Calcium is precipitated by adding sulfuric acid. The precipitation and separation of the calcium sulfate takes place, depending on the precipitation temperature, as a dihydrate or as anhydrite

If after this procedure the calcium sulphate is also obtained in relatively easily filterable Fc .ti so is on the other hand, the large amount of sulfuric acid required for this is a considerable disadvantage. It must namely, despite the use of nitric acid for the raw phosphate digestion, still for calcium separation once the equivalent amount of sulfuric acid is used, which is caused by the nitric acid digestion should just be saved.

The task was therefore to find an economically justifiable process for extensive separation of calcium from rock phosphate digestion solutions, in which the resulting calcium sulfate in easily filterable form is obtained

There was a method of making complex fertilizers from a nitric acid. Rock phosphate digestion solution containing calcium * and phosphate clays with separation of the main amount of calcium as calcium nitrate tetrahydrate and the remaining amount of calcium in the form of calcium sulfate using ammonium sulfate solutions at elevated temperatures, Separation of the deposited calcium sulfate by filtration and work-up of the filtrate Compound fertilizers found. The process is characterized in that the implementation of the Rock phosphate digestion solution with the ammonium sulfate solution at a temperature of 110 to 120 ° C is carried out and the reaction mixture is either filtered at the precipitation temperature or on a Cooled temperature of about 70 to 60 ° C and filtered at this temperature.

To carry out the process, a rock phosphate is digested in a manner known per se in a digestion vessel with about 50 to 70 percent by weight nitric acid. The amount of nitric acid is to be measured so that for 1 mol of the PjO ₅ present in the rock phosphate, about 6.0 to 12 Mo! ENT ₃ come; the decomposition temperature is about 50 to 90 "C contains the digestion solution thus obtained, the nitric acid, calcium and phosphate ions, is first cooled and the precipitating calcium nitrate tetrahydrate by Ritration or centrifugation separated The filtrate is then heated to a temperature of 110 to 120 ⁰ C heated and reacted with an aqueous up to 45 weight percent ammonium sulfate solution, the amount of which is such that at least 1 mol of ammonium sulfate is added per equivalent of the calcium ions still present in the digestion mixture done 110 to 120 ⁰ C. the filtration of the thus deposited calcium sulphate hemihydrate proceeds with substantially höherr filter performance than the filtration of a directly precipitated calcium sulphate dihydrate. according to the invention but can also be the reaction mixture after the EI ner temperature of 110 to 120 ⁰ C were carried out precipitation are the Olciumsulfat hemihydrate, about cooled to 70 to 60 ° C. The originally precipitated calcium sulfate hemihydrate forms a dihydrate in very well-developed crystals. After a residence time of about 10 minutes to 2 hours, the filtration takes place at this temperature, which can be carried out just as easily as the filtration at the higher temperature. Ammonia can then be introduced into the filtrate in a manner known per se up to a pH of approx / or returned material is optionally granulated and dried.

The separated calcium sulfate is slurried with water and this slurry with gaseous Ammonia and carbon dioxide reacted. The calcium carbonate which has precipitated out is converted in the usual way separated and the ammonium sulfate solution formed at the same time again for calcium ion separation in the procedure was introduced

According to the method of the invention can thus be

Technically easy to implement, reduce the CaO: P ₂ Oj ratio, which is about 3 to 4: 1 in the rock phosphate, to about 0.1: 1 water-soluble P ₂ Os of about the composition 25/25/0 or 18/18/18. A few examples are intended to illustrate the method of the invention.

example 1

In a reaction vessel, raw phosphate is introduced into 58 percent strength by weight nitric acid (7.0 mol HNOj / mol P ₂ Os) and then the main amount of the formed by cooling eo to about 16 ° C

Ca (NOj) ₂ · _{4H 2} O

deposited. The mother liquor separated from the crystals is continuously fed into the first of 2 Vessels connected in series are fed in, so that 173 parts by weight of mother liquor flow in every hour.

19 H 134

The mother liquor has the following composition:

N = 5.5% P ₂ O ₅ = 23.4% CaO = 9.8% molar ratio; CaO; P ₂ O ₅

1.1: U

At the same time, a technical ammonium sulfate solution is added to the first vessel, every hour Flow towards the weight. The solution has the following composition:

N = 7.8%

P ₂ O ₅ = 0.06%

CaO = 03%

SO ₃ = 23.8%

The temperature in both »! The vessel is 110 ^° C., and the vessel size is dimensioned so that the solutions have a residence time of 2 hours. The solution flowing out of the second vessel after this time is filtered, resulting in a filter capacity of 6 m ³ / m ² · h. The filtrate has the following composition:

N = 7.14% P ₂ O ₅ = 16.6% CaO = 0.8% molar ratio: CaO: P ₂ O ₅ = 0.12: 1.

By adding nitric acid and ammonia gas or ammonium nitric acid, the filtrate can be

entered NP fertilizer | 24.5 / 24.5 / 0 and by adding more potassium chloride produce an NPK döngemHtel 17.5 / 17.5 / 17.5, the P ₂ O ₅ gel being 90% water-soluble.

Example 2

The same solutions as in Example 1 are fed into the same vessels at the same speed. While a temperature of 110 ^° C. is again maintained in vessel 1, the temperature in the 2nd vessel is ^{lowered to 60 °} C. The temperature which occurs after a dwell time of 2 hours The solution is filtered, resulting in a filter capacity of 5 m ³ / m ^J · h. The filtrate has practically the same composition as the corresponding filtrate in Example 1.

Example 3
(Comparison)

The same process sequence as in Examples 1 and 2; the temperature in both vessels is now 70 ⁰ C, wherein directly CaSO ₄ · 2 H ₂ O is formed. The filtration carried out after the residence time of 2 hours proceeds at a rate of 0.5 mVm * · h.

Claims (1)

Claim: Process for the production of complex fertilizers from a raw phosphate digestion solution containing nitric acids, calcium and phosphate ions with separation of the main amount of calcium as calcium nitrate tetrahydrate and the remaining amount of calcium in the form of calcium sulphate by means of ammonium sulphate solutions at elevated temperatures, separation of the separated calcium sulphate by filtration and processing of the filtrate to complex fertilizers, characterized in that the reaction of the raw phosphate digestion solution with the ammonium sulfate solution is carried out at a temperature of 110 to 120 ⁰ C, and the reaction mixture is either filtered at the precipitation temperature or ^{cooled to a temperature of about 70 to 60 0} C and is filtered at this temperature. The invention relates to the production of nutrient-rich fertilizers from nitric acid rock phosphate digestion products, the use of sulfuric acid should be avoided as far as possible. In the In addition, the proportion of water-soluble P2O5 in the fertilizers to be produced should be as high as possible, preferably over 80%. When raw phosphate is digested with a sufficient or excess amount of nitric acid, preferably in a concentration of 50 to 70 percent by weight HNO ₃ , practically all of the calcium contained in the rock phosphate is dissolved under the digestion conditions. By cooling the digestion products - depending on the degree of cooling - the calcium can be partially or practically completely as Ca (NOj) ₂ · _{4H 2} O separate and separate. As from »Phosphorus and Potassium ", No. 26 (1966), page 25, and from T. P. Hignett" Proceedings of the 15th Annual Meeting Fertilizer Industry Round Table «, (November 10-12 1965), page 98, and from "Chemical Engineering", 74 (1967), 22, page 134, is the deposition of more than 70% of the amount of calcium contained in the rock phosphate is only due to the strong cooling of the To achieve nitric acid rock phosphate digestion products that have a high technical and in particular energetic effort required In addition, the separation of the calcium nitrate hydrate crystals by ss the greatly increased viscosity of the digestion products at such low temperatures is very difficult Another possibility of separating the calcium from nitric acid raw phosphate digestion products is to carry out the raw phosphate digestion in the presence of amorphous sulphate. According to US Pat. No. 2,689,175, the temperature of the Aufschilußgemisches must be kept between 75 and 100 ⁰ C in order to dissolve the rock phosphate as quantitatively as possible and to ensure good crystallization and filtration of the calcium sulfate dihydrate formed. The resulting solution containing calcium nitrate and phosphoric acid is then neutralized with ammonia and evaporated. P ₂ O ₅ weight ratio of about 2; 1 received. In order to produce a fertilizer with balanced N; To produce PiOs weight ratio, it is necessary to fractionally crystallize the ammonium product, whereby a product is obtained which consists mainly of ammonium phosphate and a little ammonium nitrate and a further product which consists mainly of ammonium nitrate. However, this work-up of the ammonation products is technically very complex and laborious. The calcium sulfate obtained as a by-product can be reacted with ammonia and carbon dioxide to form ammonium sulfate, which is reintroduced into the process. A variant of this method is described by R. C Cannon et al. described in the publication "An improved route to nitric phosphates" by the USS Agri-chemicals Ina, Decatur (Georgiaj, and consists in crystallizing part of the ammonium nitrate formed by the reaction of calcium nitrate with ammonium sulfate in the rock phosphate digestion products and This process is technically less expensive than fractional crystallization, as described in US Pat. No. 2,689,175. Both processes, however, have the disadvantage that as much ammonium sulfate has to be introduced into the raw phosphate digestion mixture as the calcium content The raw phosphate used corresponds to precipitate all of the calcium as sulphate. The ammonia nitrogen added with the ammonium sulphate must then be removed in the form of ammonium nitrate from the raw phosphate decomposition products about half as that by crystallization and separation, provided a fertilizer with balanced N: P ₂ O ₅ -Weight adj In addition, the ammonium sulphate is always used as an approximately 40% ammonium sulphate solution, so that in this way large amounts of water get into the digestion mixture. These amounts of water must then be evaporated again if ammonium nitrate and ammonium phosphate are to crystallize separately to produce a fertilizer with a balanced N: P2O5 ratio. However, evaporation and crystallization in relatively concentrated solutions always entails considerable technical difficulties, such as, for example, incrustations on the heat exchanger. Thus, these processes are always afflicted with considerable disadvantages when fertilizers with a balanced N: PjOs weight ratio are to be produced. As is apparent from DE-PS 5 79 629, to production of practically calcium-free fertilizers from nitric acid rock phosphate digestion solutions first a part of the Calciumniiraltrahydrate crystallized by cooling and then separated will. The mother liquor obtained is then reacted with ammonium sulfate solution to remove the remaining To precipitate and separate calcium as sulfate. This implementation is apparently intended to result from association the cold mother liquor with the ammonium sulphate solution which is at room temperature be performed. From the filtrate remaining after the calcium sulfate has been separated off the nitric acid contained therein is distilled off in vacuo at a possibly low temperature. These Gentle distillation conditions must be adhered to in order to avoid side reactions avoid. Ammonia is used as the distillation residue