DE19727144A1 - Soluble, acidic polyphosphates and process and device for their production - Google Patents

Abstract

The invention relates to a method for producing soluble acid polyphosphates with an Na/P ratio of 0.3 to 0.6, a P2O5 content of over 77 %wt., a Na2O content of under 20 % and a residual water content of 3-10 %. The invention is characterized in that an aqueous phosphate solution is dried in order to form chain-like polyphosphates with an average chain length of 10 to 30, and then fused for 30-180 minutes at temperatures of 400-600 DEG C. The fusion is cooled, forming a vitreous product. During fusion, steam pressure of 0.1 to 0.5 bar is maintained in the atmosphere in contact with the fusion. The invention also relates to the polyphosphates produced by this method and to their use.

Description

The present invention relates to readily soluble, acidic polyphosphates with a high P ₂ O ₅ content and process for their preparation and furthermore describes a suitable device which permits continuous operation.

Acidic polyphosphates are understood by those skilled in the art to be those which have a sodium phosphorus molar ratio of <1, in particular <0.9, corresponding to an Na ₂ O content of <30% by weight, and in the solid state usually still 2-10% by weight .-% What water contained. The water content is essentially based on the presence of free phosphoric acid hydroxyl groups. The degree of polymerization also serves to characterize these phosphates, which are characterized by chain and ring-shaped structures. Due to the high content of free hydroxyl groups, the solutions of these phosphates are very acidic. These acidic groups are fundamentally polymerizable under water leakage, in which case crosslinking also takes place via PO ₄ tetrahedra and spatially crosslinked, substantially less water structures are formed. These structures with a low Na ₂ O and water content are then called ultraphosphates. An essential feature is the greatly reduced rate of dissolution in water, since the solution is only hydrolysed into smaller units.

The conditions under which sodium polyphosphates with a Na / P ratio <0.9 cross-linking and thus formation of ultraphosphate occurs a) high melting temperature and b) low water vapor pressure (see A. Winkler and E. Thilo, Z. anorg. General Chemistry 298 pp. 302-315 (1959)).

The relationships between the constitution of acidic sodium phosphate glasses in Dependence on their chemical analysis and the manufacturing conditions remains  Westman et al., Canadian Journal of Chemistry, Vol. 27, 1959, pages 1764 to 1775.

Furthermore, it has been found that special compositions of acidic polyphosphates are converted into crystalline products upon prolonged annealing of their melt. So an aqueous solution of NaH (PO ₃ ) ₂ after drying and melting at temperatures of 400 ° C and longer annealing at about 300 ° C in a crystalline NEN solid, which is believed to be a cyclic trimetaphosphate. The product is very slowly soluble in water. A mixture which corresponds to the composition Na ₂ H (PO ₃ ) ₃ can be converted at 300 ° C. into fibrous crystals which are sparingly soluble in water and which, according to the X-ray spectrum, contain long-chain polyphosphates (cf. Griffith, ACS 1954, p 5892).

Another crystalline form corresponds to the formula Na ₃ H (PO ₃ ) ₄ , which can be obtained in 12 hours by heating to 600 ° C. and tempering at 350 ° C. This product is also insoluble in water (Griffith, ACS 1956, pp. 3867-3870 and US-P 2,774,672).

DE 41 28 124 C2 describes acidic polyphosphates as additives or as melting salts for the preparation of cheese. These polyphosphates are produced from monosodium phosphate and phosphoric acid or sodium hydroxide solution and phosphoric acid in suitable mixing ratios directly by melting at 400 ° C to 500 ° C, with residence times of 20 minutes to 2 hours, the composition of the end product being determined by melting temperature, residence time and Na / P- Ratio is determined. For the general manufacturing conditions, reference is made to the above US Pat. No. 2,774,672. These polyphosphates are said to have stabilizing and preserving properties. The P ₂ O ₅ content is between 73 and 77% by weight, the Na ₂ O content is between 20 and 25% and the residual water content is 2 to 3% by weight. The Na / P ratio is therefore between 0.6 and 0.8. The solubility of such products is about 90 minutes, which is too slow for the intended use as a molten salt and stabilizing agent, inter alia, in the cheese industry. Because of the necessary processing times, dissolving times of less than 30 minutes, preferably less than 20 minutes, would be desirable.

Such polyphosphates must have a number of properties and functions for use as food phosphates, for example for the production of processed cheese:

• a) it must be solid, easy-to-handle powder that
• b) high solubility, especially a time to dissolve in water of below 30, preferably less than 20 minutes,
• c) a good complexing ability for alkaline earth metals, in particular calcium and Magnesium,
• d) a good buffering action in the acidic range, in particular for an application for example in salad dressings and mayonnaises,
• e) a preservative effect (expressed in the reduction in the number of bacteria per volume and quantity) also during the storage time of the finished cheese,
• f) a stabilizing effect compared to other additives, especially Vit show amine C.

According to DE 41 28 124 C2, weakly acidic polyphos have so far been used for this purpose phate with a Na / P ratio of 0.8 to 0.6, which is not sufficient Show appropriate dissolving behavior. Based on what has been said so far, it can be concluded that the proportion of ultraphosphate is too high for rapid solubility.

It was therefore the task of containing solid, soluble and acidic polyphosphates according to the share of cross-linked structures (ultraphosphate) and a process for Making the same.  

Surprisingly, it has now been found that strongly acidic polyphosphates (1% solution pH <2!) With a Na / P ratio of 0.3-0.6 under suitable conditions into a medium-long-chain form with about 10 to 30, preferably 20 up to 30 PO ₃ units can be transferred, which have only a small extent the crosslinking known from the literature and also do not have the ring or methaphosphate structure of (Na ₂ H ₂ PO ₃ ) ₄ described in the literature. (see above)

These medium-length chains surprisingly have a very large solution speed, with solution times for 10% by weight of about 10 in favorable cases Minutes can be reached. Due to the chain-like structure, a large part of the acid Groups blocked, so that these polymers are much less acidic than that analytical content of phosphoric acid corresponds. The connections are however able to hydrolyze slowly and to this extent exert a strong buffering effect. On the other hand, the polymeric structure is capable of divalent ions, in particular To complex magnesium and calcium ions and thus their precipitation as difficult to prevent soluble phosphates. Furthermore, these polyphosphates prove to be surprisingly good stabilizers. They also show against bacteria and mushrooms in particular have a slightly microbiocidal effect.

In contrast to previous methods, in which the chain length of polyphosphates had to be laboriously determined by end group titration, chain lengths and degree of crosslinking of polyphosphates can be determined very easily with modern ³¹ P-NMR methods by dissolving the polyphosphate in deuterium oxide and during or briefly after the dissolution process, ie before a noticeable hydrolysis begins and the result is falsified, picks up the resonance signals of the different phosphate groups. Terminal phosphate groups have a resonance at -6 to -12 ppm, medium-sized phosphate groups in the chain have a resonance frequency of -18 to -24 ppm, cyclic phosphates have a resonance at -23 (trimetaphosphate) or -21 ppm (tetrametaphosphate). The signal of the free orthophosphates is found under these circumstances at 0 ± 2 ppm, depending on the acidity.

The water content of the products, which in this case predominantly determines bound water, is usually determined by determining the loss on ignition at 600 to 800 ° C., zinc oxide being added to the determinations in order to avoid P ₂ O ₅ losses in the case of acidic polyphosphates .

A turbidity photometer was used to determine the dissolution rate Dr. Lange, model LTP5 and a stirrer from Heidolph, model RZR - 2000 (with tachometer) with a KPG blade stirrer for 50 ml flasks from the glassworks Wertheim No. 3,855 used.

5 g of the polyphosphate in a measuring cuvette of the photometer with 45 ml of water are each added and stirred at a speed of 500 min ^-1 Umdrehungsge for carrying out the examination. After the intended measuring times (5, 10, 15, 20, 30, 40, 50 and 60 minutes), the stirrer is removed and the turbidity is measured, this process should proceed as quickly as possible in order to largely avoid settling of the phosphate.

The measured turbidity in TE / F is assessed visually according to the following scheme:

1-2 TE / F = clear
2-10 TE / F = opal
10-15 TE / F = slightly cloudy
15-20 TE / F = cloudy
<20 TE1F = very cloudy

The products according to the invention are produced by predrying an aqueous solution solution of phosphoric acid and sodium phosphate or sodium hydroxide solution in one Sodium / phosphorus ratio from 0.3 to 0.6 up to a water content of approx. 20% and melting by slowly heating this mixture in a suitable oven to temperatures of 400 to 600 °, for a time of 60 to 120 minutes. By Transfer of appropriately humidified air in continuous processes or by adapting the amount of water to be evaporated to the furnace volume for example in a muffle furnace, water vapor pressure is another parameter adjusted to 0.1-0.5 bar above the melt.

After a short cooling phase, the melt is left in an anhydrous atmosphere cooled to room temperature and ground to powder fineness. Products with one Na / P ratio of less than 0.3 can no longer be ground or solidified no longer at room temperature.

From the following experiments it can be seen that the desired chain-shaped moderate products with average chain lengths of 10 to 30, which a Have a dissolution rate of up to 20 minutes at melting temperatures of 400 to 550 ° C and residence times of 60 to 120 minutes can be achieved. At Temperatures over 550 ° and reaction times over 180 minutes increase the chain length to over 30, with which the dissolution rate and also the cyclic phosphatane partly increase strongly, so that such products for the purpose of the invention little are suitable. An optimal setting of chain lengths in the range of approximately 20 to 25 can be adjusted by adjusting the water vapor pressure above the melt Reach 0.2 to 0.3 bar.

example 1 Determination of the reaction temperature

In a muffle furnace (Heraeus type MR 70 E), which can be heated to temperatures of 400 to 800 °, a solution of 131.6 g NaH ₂ PO ₄ (1.1 mol) is placed in platinum dishes.
114.4 g H ₃ PO ₄ (technically 82.2% ≈ 0.96 mol) and
39.5 g water (2.2 mol)
used as a solution and warmed up slowly so that splashing of the solution is avoided. The results of the tests are given in the following tables.

Table 1

Table 2

Table 3

Table 4

As Table 1 shows, the solubility is after 20 minutes at melting temperatures from 400 to 500 ° excellent. At the same time, as expected, the hygroscopicity increases act. With increasing temperature at the same water vapor pressure, the loss on ignition decreases that the average degree of condensation and the cyclic phosphate proportion accordingly Table 2 increases, with the result that the solubility decreases accordingly. The Specification P1 to P50 in Table 2 gives the analytically determined proportion of the ent speaking chain lengths again.

Table 3 shows that with a very long residence time at temperatures of 500 ° or at shorter dwell times at even higher temperatures the average chains length increases sharply, which is also shown by the decrease in ignition loss. The corresponding, more networked products are no longer sufficiently soluble to to be used according to the invention.  

Example 2

In a further experiment, the weight was changed so that an Na ₂ O / P ₂ O ₅ molar ratio of 0.5, corresponding to a gross formula NaHP ₂ O ₆ × H ₂ O, results. As Table 4 shows, such products do not differ significantly in their composition from the products according to Example 1. Only the hygroscopicity is increased by increasing the acidity and the solubility is obviously somewhat reduced by increasing crosslinking.

Example 3 Continuous production in the tube furnace

13.8 kg of an 85% technical H ₃ PO ₄ solution are placed in a quartz reactor with stirrer and cooling, and 5.9 kg of 49.2% sodium hydroxide solution are added so slowly with stirring that the temperature remains below the boiling point. 19 kg of phosphate solution with an Na ₂ O / P ₂ O ₅ ratio of 0.533 and a density of 1.61 are obtained.

The aforementioned phosphate solution is continuously introduced into a melting furnace according to FIG. 1 via a diaphragm metering pump, whose stroke volume and cycle frequency can be adjusted. As can be seen, a tube furnace 1 (in the current case a type F 500 from Gero, with a total length of 750 cm and a heating zone of 500 cm was used) as a reaction vessel, a quartz tube 3 with a length of 880 cm and a diameter of 55 cm. Inserted in the quartz tube is a slightly inclined melting trough 2 , which is milled out of a graphite rod, which has an area of 256 cm ² and a volume of 1024 cm ³ , which can be reduced to 256 cm ³ if necessary by inserting an insert wedge (except Graphite can also be used as a silicon carbide, other ceramic materials are partially attacked by the phosphate melts). The phosphate solution is fed in at a metering rate of 300 g / h via line 4 , the melt produced runs continuously via line 5 due to the inclination at the end of the melting tank 2 and is solidified in a glass-like manner via a cooled roller 6 . The phosphate glass obtained is broken with the exclusion of atmospheric humidity with a scraper 7 and, after intermediate storage in container 8, ground to powder ver. The input zone of the quartz tube 1 a is preheated to 100 °, the actual reaction zone 1 b is set to 650 to 675 °, resulting in melt temperatures of 515 to 560 °. To set the water vapor pressure, the apparatus is continuously flowed through with a 10 l / min nitrogen stream via line 9 , which is set to 150 mbar water vapor pressure by passing through 60 ° warm water in the scrubber 10 . The results of these tests are shown in Table 5 below. It shows that optimal solubilities can be achieved accordingly chain lengths of about 20 to 30 at melt temperatures up to 530 °.

Table 5

.

Claims (9)

1. Process for the production of soluble acidic polyphosphates with an Na / P ratio of 0.3 to 0.6, a P ₂ O ₅ content of over 77% by weight, an Na ₂ O content of less than 20% and a residual water content of 3 to 10%, characterized in that to form chain-like polyphosphates with average chain lengths of 10 to 30, an aqueous solution of the phosphates is dried and melted at temperatures of 400 to 600 ° for 30 to 180 minutes and the melt to form a cools glassy product, while a water vapor pressure of the atmosphere in contact with the melt of 0.1 to 0.5 bar is maintained during melting. 2. The method according to claim 1, characterized in that the Na / P ratio <0.5 and <0.6. 3. The method according to any one of claims 1 to 2, characterized in that the Phosphate solution by mixing aqueous phosphoric acid with sodium phosphate or caustic soda is generated. 4. The method according to any one of claims 1 to 3, characterized in that the Process is carried out continuously in a tube furnace, the Phos phate solution in a melting channel through the furnace and the water vapor pressure by passing an inert gas containing a corresponding amount of what contains steam, is set.   5. The method according to any one of claims 1 to 4, characterized in that the melting temperature is 450 to 550 ° and the phosphate chains have a length of 20 to 30 PO ₃ units. 6. polyphosphates produced by a process according to claims 1 to 5. 7. Use of polyphosphates according to claim 6 as melting salts and stabilizers sizing agent in processed cheese. 8. Device for the continuous production of acidic polyphosphates according to claims 1 to 6, consisting of

• a) with a tube furnace ( 1 )
• b) a lining made of a quartz tube ( 3 ) in which
• c) a melting tank ( 2 ) with a feed line ( 4 ) for the phosphates and discharge line ( 5 ) for the melt of the polyphosphates is slightly inclined to allow the flow of the melt, and
• d) a steam supply ( 9 ) is provided.

9. The device according to claim 8, characterized in that a cooling roller ( 6 ) is present for the running polyphosphate melt and the phosphate glass formed is broken by a scraper ( 7 ) and brought into a storage container ( 8 ).