CA2295687C - Soluble acid polyphosphates 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 ~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

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a Soluble acid polyphosphates and process and apparatus for their preparation The present invention relates to readily soluble acid polyphosphates having a high P205 content and a process for their preparation and in addition a suitable apparatus is described which permits a continuous mode of operation.
Acid 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 , equivalent to an Na20 content of < 30$ by weight, and in the solid state usually also comprise 2-10~ by weight of water.
The water content is essentially due to the presence of free phosphoric acid hydroxyl groups. The degree of polymerization further serves to characterize these phosphates which feature chain and ring structures. Due to their high content of free hydroxyl groups, the solutions of these phosphates are very strongly acidic.
These acid groups are in principle capable of further polymerization with elimination of water, cross-linking via P04 tetrahedra then also taking place and spatially crosslinked essentially lower-water structures being formed. These structures having a lower Na20 and water content are then termed ultraphosphates. An important feature is the greatly decreased rate of dissolution in water, since dissolution only takes place by hydrolysis into smaller units.
The conditions under which, in the case of sodium polyphosphates having an Na/P ratio < 0.9, crosslinking and thus formation of ultraphosphate takes place are a) high melt temperature and b) low water vapor pressure (cf. A. Winkler and E. Thilo, Z. anorg. allg. Chemie 298 pp. 302-315 (1959) ) .
The relationship between the constitution of acid sodium phosphate glasses as a function of their chemical analysis and the conditions of their preparation is further described by Westman et al., Canadian Journal of Chemistry, Vol. 27, 1959, pages 1764 to 1775.
In addition it has been found that specific compositions of acid polyphosphates are converted into crystalline products during relatively long term heating of their melts. An aqueous solution of NaH(P03)2, after drying and melting at temperatures of 400°C and relatively long heating at about 300°C, converts into a crystalline solid which is assumed to be a cyclic trimetaphosphate. The product dissolves very slowly in mater. A mixture which corresponds to the composition Na2H(P03)3 may be converted at 300°C
into fibrous crystals which are poorly soluble in mater and which, according to the X-ray spectrum, comprise long-chain polyphosphates (cf. Griffith, ACS 1954, p.
5892) .
A further crystalline form corresponds to the formula Na3H(P03)4, which can be obtained in 12 hours by heating to 600°C and holding at 350°C. This product is also insoluble in mater (Griffith, ACS 1956, p. 3867-3870 and US Patent 2,774,672).
DE 4128124 C2 describes acid polyphosphates as additive and as emulsifying salt for preparing cheese. These polyphosphates are prepared directly from monosodium phosphate and phosphoric acid or sodium hydroxide solution and phosphoric acid in suitable mixing ratios by melting at 400°C to 500°C at residence times of from 20 min to 2 hours, the composition of the end product being determined by melting temperature, residence time and Na/P ratio. For the general preparation conditions reference is made to the above US Patent 2,774,672.
These polyphosphates are said to have stabilizing and preserving properties. The P205 content is between 73 and 77$ by weight, the Na20 content is between 20 and ' CA 02295687 2000-03-27 W(a 99/00324 PCT/EP98/03823 25~ and the residual water content is from 2 to 3~ by weight. The Na/P ratio is thus between 0.6 and 0.8. The rate of dissolution of such products is about 90 minutes, which is too slow for the intended use as emulsifying salt and stabilizer, inter alia in the cheese industry. Because of the necessary processing times, dissolution times of less than 30 minutes, preferably less than 20 minutes, would be desirable.
For use as food phosphates, inter alia for preparing process cheese, such polyphosphates must have a number of properties or functions:
a) they must be solid easily handlesble powders which b) have a high solubility, in particular a time for dissolution in water of less than 30 minutes, preferably less than 20 minutes, c) have a good complexing capacity for alkaline earth metals, in particular calcium and magnesium, d) show a good buffering action in the acidic range, in particular for a use in salad dressings and mayonnaises, for example, e) have a preservative action (expressed in reduction in microorganisms per unit volume and amount) even during the storage time of the finished cheese, f) exhibit a stabilizing action toward other additives, in particular vitamin C.
According to DE 4128124 C2, for this purpose weakly acidic polyphosphates having an Na/P ratio of from 0.8 to 0.6 have been used to date, but these do not have a satisfactory dissolution behavior. On the basis of the facts discussed above, it may be concluded that the content of ultraphosphate is too high for rapid dissolution.
The object was therefore to find solid, soluble and acid polyphosphates having a low content of crosslinked .
Hlfl 99/00324 PCT/EP98/03823 structures (ultraphosphate) and a process for preparing the same.
Surprisingly it has now been found that strongly acidic polyphosphates (1~ strength solution pH<2!) having an Na/P ratio of 0.3-0.6 can be converted under suitable conditions into a medium-length chain form having from about 10 to 30, preferably from 20 to 30, P03 units, which have the literature-known crosslinking only to a small extent and also do not have the ring- or metaphosphate structure of (Na2H2P03)4 previously described in the literature (see above).
These medium-length chains surprisingly have a very high rate of dissolution, in favorable cases solution times for 10~ of about 10 minutes being achieved. Owing to the chain-like structure, a majority of the acid groups are blocked, so that these polymers are considerably less acidic than corresponds to the analytical phosphoric acid content. However, the compounds have the ability to be hydrolyzed slowly and to that extent exhibit a strong buffering action.
Furthermore, the polymeric structure has the capacity of complexing divalent ions, in particular magnesium ions and calcium ions, and thus to prevent their precipitation as poorly soluble phosphates. In addition, these polyphosphates prove to be surprisingly good stabilizers. They also show a slightly microbicidal action to bacteria and especially fungi.
In contrast to previous processes in which the chain length of polyphosphates had to be determined laboriously by end-group titration, using modern 31P-N1~
methods, chain lengths and degree of crosslinking of polyphosphates can be very simply determined by dissolving the polyphosphate in deuterium oxide and recording the resonance signals of the various phosphate groups during or shortly after the dissolution process, i.e. before a marked hydrolysis occurs and falsifies the result. Terminal phosphate groups have a resonance at from -6 to -12 ppm, central phosphate groups in the chain have a resonance frequency of from -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 conditions at 0 t 2 ppm, depending on acidity.
The water content of the products, which in this case predominantly defines bound water, is usually determined by determining the loss on ignition at from 600 to 800°C, zinc oxide being added in each case during the determinations in order to avoid P205 losses with acid polyphosphates.
To determine the rate of dissolution, a turbidity photometer from Dr. Lange, model LTP5 and a stirrer from Seidolph, model RZR-2000 (with rotary speed indicator) equipped with a KPG leaf stirrer for 50 ml flasks from Glaswerke Wertheim No. 3.855 were used.
To carry out the investigation, 45 ml of water are added in each case to 5 g of the polyphosphate in an analytical cuvette of the photometer and stirred at a rotary speed of 500 rpm. After the specified measuring times (5, 10, 15, 20, 30, 40, 50 and 60 minutes), the stirrer is removed and the turbidity is measured, in which case this operation should proceed as rapidly as possible in order to prevent substantially deposition of the phosphate.
The measured turbity is evaluated visually in TU/F
according to the following system:
1 - 2 TU/F - clear 2 - 10 TU/F - opalescent 10 - 15 TU/F - slightly turbid 15 - 20 TU/F - turbid > 20 TU/F - highly turbid The novel products are prepared by predrying an aqueous solution of phosphoric acid and sodium phosphate or sodium hydroxide solution in a sodium/phosphorus ratio of from 0.3 to 0.6 down to a water content of approximately 20~ and melting this mixture by slow heating in a suitable furnace at temperatures of from 400 to 600 ° C for a time of from 60 to 120 minutes . By passing appropriately moistened air over in the case of continuous processes or by matching the amount of water to be evaporated to the furnace volume, for example in the case of a muffle furnace, as a further parameter, the water vapor pressure over the melt is set to 0.1-0.5 bar.
After a short cooling phase, the melt is cooled in an anhydrous atmosphere to room temperature and ground to powder fineness. Products having an Na/P ratio of less than 0.3 can no longer be ground or no longer solidify at room temperature.
From the following experiments it may be concluded that the desired chain-type products are achieved having average chain lengths of from 10 to 30, which have a rate of dissolution of up to 20 minutes, at melt temperatures of from 400 to 550°C and residence times of from 60 to 120 minutes. At temperatures above 550°
and reaction times greater than 180 minutes, the chain length increases to over 30, as a result of which the rate of dissolution and also the cyclic phosphate content greatly increase, so that such products are not very suitable for the purpose of the invention. An optimal setting of chain length in the range from about 20 to 25 may be achieved by setting the water vapor pressure over the melt to from 0.2 to 0.3 bar.
Example 1 Determination of the reaction temperature WD 99r00324 PCTrEP98r03823 In a muffle furnace (Heraeus type lit 70 E) , which can be heated to temperatures of from 400 to 800 ° , in each case in platinum dishes, a solution of 131.6 g of NaH2P04 (1.l mol) 114.4 g of H3P04 (technical grade 82.2$ strength 0.96 mol) and 39.5 g of water (2.2 mol) are used as solution and heated slowly so as to avoid sputtering of the solution. The results of the experiments are reproduced in the tables below.

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As Table 1 shows, the solubility after 20 minutes at melt temperatures of from 400 to 500° is excellent. The hygroscopicity increases simultaneously, as expected.
With increasing temperature, for the same water vapor pressure, the loss on ignition decreases, so that the mean degree of condensation and the cyclic phosphate content increase in accordance with Table 2, with the result that the solubility decreases accordingly. The presentation of P1 to P50 in Table 2 gives here the analytically determined content of the corresponding chain lengths.
Table 3 shows that with a very long residence time at temperatures of 500°C or with shorter residence times at still higher temperatures, the mean chain length increases greatly, which is also indicated by the decrease in loss on ignition. The corresponding more greatly crosslinked products are no longer sufficiently soluble in order to be used according to the invention.
Example 2 In a further experiment the initial weight was changed so as to give an Na20/P205 molar ratio of 0 . 5 , 2 5 equivalent to an empirical formula of NaHP206 ~ H20 . As shown in Table 4, such products do not differ significantly in their composition from the products according to Example 1. Only the hygroscopicity is increased due to the increase of the degree of acidity and the solubility somewhat decreased apparently due to increasing crosslinking.
Example 3 Continuous preparation in a tubular furnace 13.8 kg of an 85~ strength technical-grade H3P04 solution are placed in a quartz reactor equipped with stirrer and cooling and 5.9 kg of 49.2 strength sodium W~ 99/00324 PCT/EP98/03823 hydroxide solution are added with stirring sufficiently slowly that the temperature remains below boiling point. 19 kg of phosphate solution having an Na20/P205 ratio of 0.533 and a density of 1.61 are obtained.
The abovementioned phosphate solution is charged continuously via a diaphragm metering pump, whose stroke volume and cycle frequency are adjustable, into a melting furnace according to Figure 1. As can be seen, in a tubular furnace 1 (in the present case a type F 500 furnace from Gero was used which had a total length of 750 cm and a heating zone of 500 cm) there is situated as reaction vessel a quartz tube 3 having a length of 880 cm and a diameter of 55 cm. Placed in the quartz tube is situated a slightly inclined melting trough 2, which is milled out of a graphite round rod and has an area of 256 cm2 and a volume of 1024 cm3, which can be decreased, if appropriate, to 256 cm3 by introducing an insert wedge (apart from graphite, silicon carbide can also be used as material, other ceramic materials are sometimes attacked by the phosphate melts). The phosphate solution is introduced at a metering rate of 300 g/h via line 4, the melt produced flows off continuously via line 5 owing to the inclination at the end of the melting trough 2 and vitrifies via a cooled roller 6. The resultant phosphate glass is broken with scraper 7 under exclusion of atmospheric humidity and after intermediate storage in vessel 8 is ground to form powder. The inlet zone of the quartz tube la is preheated to 100°, the actual reaction zone 1b is set to from 650 to 675°, giving melt temperatures of from 515 to 560°. To set the water vapor pressure, a 10 1/min nitrogen stream is continuously passed through the apparatus via line 9, which nitrogen stream is set to 150 mbar water vapor pressure by passing it through water at 60° in the scrubber 10. The results of these experiments are reproduced in Table 5 below. It is shown that optimum solubilities corresponding to chain lengths of approximately 20 to 30 can be achieved at melt temperatures up to 530°.

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Claims (7)

1. A process for preparing soluble acid polyphosphates having an Na/P ratio of from 0.5 to 0.6, a P2O5 content of greater than 77% by weight, an Na2O
content of below 20% by weight and a residual water content of from 3 to 10% by weight, which in order to form chain-type polyphosphates having mean chain lengths of from 10 to 30, comprises drying an aqueous solution of the phosphates and fusing it at temperatures of from 400°C to 550°C for 60 to 120 minutes and cooling the melt forming a vitreous product, in which case during the fusion a water vapor pressure of from 0.1 to 0.5 bar is maintained in the atmosphere in contact with the melt.

2. The process as claimed in claim 1 wherein the phosphate solution is generated by mixing aqueous phosphoric acid with sodium phosphate or sodium hydroxide solution.

3. The process as claimed in any one of claims 1 or 2 wherein the process is carried out continuously in a tubular furnace, in which case the phosphate solution passes through the furnace in a melt channel and the water vapor pressure is set by passing over an inert gas which comprises an appropriate amount of water vapor.

4. A polyphosphate prepared by a process as claimed in any one of claims 1 in 3.

5. The use of polyphosphates as claimed in claim 4 as emulsifying salts and stabilizers in process cheese.

6. An apparatus for the continuous preparation of acid polyphosphates as claimed in claims 1 to 4, consisting of:
a) a tubular furnace equipped with b) a liming of a quartz tube in which c) a melting trough is mounted having a feed line for the phosphates and outlet line for the polyphosphate melt slightly inclined in order to allow the flow of the melt; and d) a wager vapor supply is provided.

7. The apparatus as claimed in claim 6 wherein a cooled roller is present for the outflowing polyphosphate melt and the resultant phosphate glass is broken via a scraper and brought into a storage vessel.