EP0991589A1

EP0991589A1 - Soluble acid polyphosphates and device for their production

Info

Publication number: EP0991589A1
Application number: EP98938627A
Authority: EP
Inventors: Thomas Klein; Thomas Staffel; Alexander Maurer; Friedrich Wahl
Original assignee: BK Giulini Chemie GmbH
Current assignee: BK Giulini Chemie GmbH
Priority date: 1997-06-26
Filing date: 1998-06-23
Publication date: 2000-04-12
Also published as: AU8728198A; DE19727144A1; CA2295687C; CZ300062B6; WO1999000324A1; CZ451099A3; PL192090B1; HUP0003050A2; SK176899A3; KR20010014220A; AU734894B2; SK286395B6; HUP0003050A3; PL337655A1; JP2002507957A; KR100522290B1; CN1261328A; BR9811271A; CN1161273C; CA2295687A1

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

Soluble, acidic polyphosphates and method and device for their

Manufacturing

The present invention relates to readily soluble, acidic polyphosphates with a high P ₂ O ₅ content and processes 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 2- Contain 10% by weight of water. 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 still polymerizable under water leakage, whereby cross-linking also takes place via PO ₄ tetrahedra and spatially cross-linked, significantly less water structures arise. These structures with a low Na ₂ 0 and water content are then called ultraphosphates. An essential feature is the greatly reduced rate of dissolution in water, since the solution is only separated into smaller units by hydrolysis.

The conditions under which sodium polyphosphates with a Na / P ratio <0.9 crosslink and thus form ultraphosphate are a) high melting temperature and b) low water vapor pressure (see A. Winkler and E. Thilo, Z. inorganic chemistry 298 pp. 302-315 (1959))

The relationship between the constitution of acidic sodium phosphate glasses depending on their chemical analysis and the manufacturing conditions is still by Westman et al., Canadian Journal of Chemistry, Vol. 27, 1959, pages 1764 to 1775.

It has also been found that special compositions of acidic polyphosphates are converted into crystalline products when their melt is heated for a prolonged period. Thus, an aqueous solution of NaH (PO ₃ ) _2, after drying and melting at temperatures of 400 ° C. and longer tempering at about 300 ° C., turns into a crystalline 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 (P0 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 4128124 C2 describes acidic polyphosphates as an additive or as a molten salt 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 min 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 necessary for the intended use as a molten salt and stabilizing agent. tel in the cheese industry is too slow. Because of the necessary processing times, dissolving times of less than 30 minutes, preferably less than 20 minutes, would be desirable.

For use as food phosphates, including for processed cheese production, such polyphosphates must have a number of properties and functions:

a) there must be solid, easy-to-handle powders which b) have a high solubility, in particular a time to dissolution in water of less than 30, preferably less than 20 minutes, c) a good complexing ability for alkaline earth metals, in particular calcium and magnesium, d ) a good buffering effect in the acidic range, especially for use in salad dressings and mayonnaises, for example, e) a preserving effect (expressed in the reduction in the number of germs per volume and amount) even during the storage time of the finished cheese, f) a stabilizing effect compared to other additives, in particular Show vitamin C.

According to DE 4128124 C2, weakly acidic polyphosphates with a Na / P ratio of 0.8 to 0.6 have been used for this purpose, but these do not show sufficient 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 finding solid, soluble and acidic polyphosphates with a small proportion of crosslinked structures (ultraphosphate) and a process for producing 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 in a medium-long-chain form with about 10 to 30, preferably 20 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. (so)

These medium-length chains surprisingly have a very high dissolution rate, with dissolution times for 10% by weight of about 10 minutes being achieved in favorable cases. A large part of the acidic groups is blocked by the chain-like structure, so that these polymers are significantly less acidic than the analytical content of phosphoric acid. However, the compounds are able to hydrolyze slowly and to this extent exert a strong buffering effect. On the other hand, the polymeric structure is capable of complexing divalent ions, especially magnesium and calcium ions, and thus preventing their precipitation as poorly soluble phosphates. Furthermore, these polyphosphates have proven to be surprisingly good stabilizers. They also show a slightly microbiocidal effect on bacteria and especially fungi

In contrast to previous methods, in which the chain length of polyphosphates had to be laboriously determined by end group titration, chain lengths and the 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 determines predominantly 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 from Dr. Lange, model LTP5 and a stirrer from Heidolph, model RZR - 2000 (with tachometer) with a KPG blade stirrer for 50 ml flasks from Glaswerke Wertheim No. 3.855.

To carry out the test, 5 g of the polyphosphate in a measuring cuvette of the photometer are mixed with 45 ml of water and stirred at a rotation speed of 500 min ^"1. 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 TE / F = very cloudy The products according to the invention are prepared by predrying an aqueous solution of phosphoric acid and sodium phosphate or sodium hydroxide solution in a sodium / phosphorus ratio of 0.3 to 0.6 to a water content of approximately 20% and melting by slowly heating this mixture in a suitable oven at temperatures of 400 to 600 °, for a period of 60 to 120 minutes. By passing 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, the water vapor pressure above the melt is set to 0.1-0.5 bar as a further parameter.

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

From the following experiments it can be seen that the desired chain-like products with average chain lengths of 10 to 30, which have a dissolution rate of up to 20 minutes, are achieved at melting temperatures of 400 to 550 ° C. and residence times of 60 to 120 minutes. At temperatures above 550 ° and reaction times over 180 minutes, the chain length increases to over 30, which increases the rate of dissolution and also the cyclic phosphate content, so that such products are not very suitable for the purpose according to the invention. An optimal setting of chain lengths in the range of approximately 20 to 25 can be achieved by setting the water vapor pressure above the melt to 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 of H ₃ PO ₄ (technically 82.2% * 0.96 mol) and 39.5 g of water (2.2 mol) are used as the solution and warmed up slowly, so that splashing of the solution is avoided. The results of the tests are shown in the following tables.

Table 1

Table 2

Table 3

Table 4

^; Water vapor pressure

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

Table 3 shows that with a very long residence time at temperatures of 500 ° or with shorter residence times at even higher temperatures, the average chain length increases significantly, which is also shown by the decrease in the loss on ignition. The corresponding, more crosslinked products are no longer sufficiently soluble 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 0 ₆ x 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 tank 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 by inserting an insert wedge if necessary ( In addition to graphite, silicon carbide can also be used as the material; 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 moisture with a scraper 7 and ground to powder after being stored temporarily in container 8. The inlet zone of the quartz tube 1a is preheated to 100 °, the actual reaction zone 1b 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 is shown that optimal solubilities corresponding to chain lengths of approximately 20 to 30 can be achieved at melt temperatures of up to 530 °.

Table 5

'' all opal to slightly cloudy soluble

Claims

claims

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 an aqueous solution of the phosphates is dried to form chain-shaped polyphosphates with average chain lengths of 10 to 30 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 is> 0.5 and <0.6.

3. The method according to any one of claims 1 to 2, characterized in that the phosphate solution is produced by mixing aqueous phosphoric acid with sodium phosphate or sodium hydroxide solution.

4. The method according to any one of claims 1 to 3, characterized in that the method is carried out continuously in a tube furnace, wherein the phosphate solution passes through the furnace in a melting channel and the water vapor pressure is adjusted by passing an inert gas which contains a corresponding amount of water vapor becomes.

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 stabilizing agents 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).