Note: Descriptions are shown in the official language in which they were submitted.
W CA 02295687 2000-03-27
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WO 99/00324 PCT/EP98/03823
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
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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
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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
CA 02295687 2000-03-27
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Hlfl 99/00324 PCT/EP98/03823
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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
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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
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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
CA 02295687 2000-03-27
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.
CA 02295687 2000-03-27
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CA 02295687 2000-03-27
WO 99/00324 PCT/EP98/03823
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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
CA 02295687 2000-03-27
W~ 99/00324 PCT/EP98/03823
- 13
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
CA 02295687 2000-03-27
WD 99/00324 PCT/EP98/03823
- 14
lengths of approximately 20 to 30 can be achieved at
melt temperatures up to 530°.
CA 02295687 2000-03-27
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