FR2461679A1 - Food-pharmaceutical quality phosphoric acid preparation - by treating urea phosphate with nitric acid, then with nitrous acid, passing air and reducing nitrate with saccharide(s) or formic acid - Google Patents

Abstract

Process for preparing "human food" quality phosphoric acid starting from "purified" technical quality phosphoric acid, obtd. by treatment of urea phosphate with nitric acid and which contains urea nitrates, comprises the following successive stages effected in continuous or batch mode: -a) Treatment of the starting phosphoric acid, which contains 30-50%P205, 2-7%nitrate ions and 0.5-3% of urea, with nitrous acid formed in situ and a molar ratio of nitrous acid/urea of 2-4/1 until all the urea has been converted to nitrogen, carbon dioxide and water; b) entraining the gaseous nitrogen-containing cpds. forming from the reaction with urea, and other gas product of a current of air through the phosphoric acid, until the emerging gas is colourless; c) treating the phosphoric acid, in which the nitrate content has risen to 3-8% due to the stage a) treatment, with a reducing agent chosen from di- or monosaccharides and formic acid; d) concentrating the phosphoric acid to a 55-62% P205 content and treating it with 30% hydrogen peroxide or another substance producing uncontaminated nascent oxygen, until the acid is decolourised. This final prod. comprisises 75-85% of H3PO4 containing 0-20ppm nitrate ions, free of urea and at most 2ppm of arsenic, less than 10ppm fluorine and less than 5ppm of heavy metals (expressed as lead). Food quality phosphoric acid used to prepare soft drinks and refreshments and as a requesting agent in foods. It may be used to prepare othophosphates and polyphosphates used as buffering agents and preservatives in the food and pharmaceutical industry and may also be used in fermentation broths used to produce synthetic proteins etc. 'Food quality' phosphoric acid is obtd. from wet process phosphoric acid at reasonable cost avoiding costly purifs. or use of high cost thermic process phosphoric acid.

Description

 We know the vast field of applications of phosphoric acid which, by its purity, deserves to be approved as a substance suitable for human consumption. These applications can be classified into two main groups, related to two of its important properties: acidity and sequestration capacity. Thus, as a component providing a determined acidity, it finds a wide market as an additive, inter alia in the manufacture of frothy and refreshing drinks, while as a sequestering agent, it is used in foods to which a component should be added. of this type.

 Industrially, it constitutes the ideal raw material for obtaining orthophosphates and polyphosphates commonly used as buffering and preserving agents in the dedicated food industry.

Some of these products are also used in the pharmaceutical industry. In these cases, the rigorous requirements which must be met by the salts derived from phosphoric acid are guaranteed by the use, as a raw material for their preparation, of an acid suitable for human consumption such as the one in question. question.

 The field of application of this acid is considerably widened by its use for nutritive broths determined in the fermentation industry, among the branches of which that of the preparation of synthetic proteins is currently in evidence.

 The subject of the invention is therefore a process for the preparation of a phosphoric acid suitable for the abovementioned applications and which, therefore, complies with the conditions stipulated for its approval as a substance suitable for human consumption.

 The properties required, as they appear in the Food Chemicals Codex of 1974, boil down to the fact that it must be colorless and odorless, completely soluble in water and in balconies; and must not contain more than 10 ppm of F, nor of heavy metals (expressed as Pb), and not more than 3 ppm of As, for an H3PO4 content of between 75 and 85%.

The production of a phosphoric acid having the characteristics mentioned above is basically carried out along two very distinct lines:
- From a phosphoric acid obtained thermally, a process which makes it possible to obtain an acid suitable for human consumption of a high quality, but which is not easily accessible due to the high cost of the installation and treatment. Production does not cover the needs of the market.

- From a phosphoric acid obtained by the wet method, by subjecting it to an extraction operation with solvents. The working systems associated with this line are complex and sophisticated and are protected by a multitude of patents. It can generally be said that the removal of impurities is an expensive process, viable only, in this case, for very high productions, with the drawbacks which arise from the use of considerable quantities of organic solvents.

 In the process according to the invention, one starts, as above, from a phosphoric acid obtained by the wet process, one of the most widespread and economical raw materials on the market, but the use of organic solvents is entirely excluded. 'purification is carried out by a system of chemical treatments, with very traditional installations, and requiring only the consumption of materials having little impact on the cost of the product.

Overall, the operation is carried out in two phases, all of which form the total manufacturing scheme, in which a varied range of qualities of acids of distinct economic level can be obtained, depending on the applications for which they are intended. The quality of acid obtained at the end of the first phase of the total scheme constitutes the raw material which is subjected in the second phase to the purification process which is the subject of the present invention.

In the first phase of the process, an acid of high technical purity is obtained, with a concentration of P205 of between 30 and 55%, in which the im
25 typical purities of the starting phosphoric acid were reduced by 99%, leaving only a maximum of 100 ppm of SO4, 1 to 3 ppm of As, 10 ppm of F, 10 ppm of heavy metals (expressed as Pb) and 3 ppm Cr, the acid obtained being free of suspended matter.

 The process which is described in this specification collects the abovementioned raw material when it contains in its composition, in addition to the impurities which have just been listed, proportions of between 2 and 7% of N03 and between 0.5 and 3 d urea, inherent in the treatment to which the crude acid obtained by the wet process has been subjected.

2 N2 P C02 + H20
L'acide nitreux peut être produit "in situ", en introduisant un nitrite sous forme solide ou en solution concentrée, tout en agitant la masse en réaction, mais ceci présente l'inconvénient de contaminer l'acide phosphorique traité avec une nouvelle impureté: le cation du nitrite.In its implementation, the method according to the invention comprises the following successive steps: a) Treatment of the above-mentioned acid, containing from 30 to 50%
P205, with nitrous acid formed in its own mass, to eliminate the urea present, which turns into
N2, C02 and H20 according to the equation: 2HN02 + CO (NR
22

2 N2 P C02 + H20
Nitrous acid can be produced "in situ", by introducing a nitrite in solid form or in concentrated solution, while stirring the mass in reaction, but this has the drawback of contaminating the phosphoric acid treated with a new impurity: the nitrite cation.

HNO2 + HNO
Ceci revient à augmenter la teneur en impureté N03, qui atteint un niveau compris entre 3 et 8% à la fin du traitement, mais améliore l'aspect économique du processus sensiblement de la différence que représente la consommation d'un nitrite ou d'un gaz produit par synthèse ou provenant d'un brûleur d'ammoniac.For this reason, the most advantageous form consists in passing a gaseous current constituted by nitrous vapors and air, and the N02 which it comprises gives rise to the formation within the liquid of the necessary nitrous acid according to the known equation: 2N02 + H20

HNO2 + HNO
This amounts to increasing the N03 impurity content, which reaches a level of between 3 and 8% at the end of the treatment, but improves the economic aspect of the process appreciably by the difference represented by the consumption of a nitrite or a gas produced by synthesis or coming from an ammonia burner.

 This step, which can be carried out continuously or discontinuously, is, in the aforementioned preferred form, carried out in a column-shaped reactor provided with good dispersing means for the gas mixture which arrives via its base. When working discontinuously, the reaction can be considered practically complete when the outlet gases have the same coloring as those of inlet, an unequivocal sign of the fact that there is now no absorption of N02 in the liquid mass as a consequence of the reaction When working continuously, it is necessary to equalize 1R8 flow rates of the supply of the reactor in phosphoric acid subjected to the treatment and in gaseous mixture containing NO2, so that this identity of coloring between the gases d Entry and exit is always kept constant.

b) In a second step, the phosphoric acid leaving the treatment is degassed in an air stream and brought to the next step to reduce the content of NO 3 that it has at least as much as necessary.

An acid is thus obtained in this step containing from 30 to 50% of P205, from 3 to 8% of NO and free of
3 urea.

c) In this step, the acid obtained in the previous step is treated with an agent capable of converting the NO 3 into volatile nitrogen compounds with a lower degree of oxidation.

For this, the acid is brought to the boiling temperature (as a function of the variable concentration of
P205 resulting from the previous step) and is treated with a carbohydrate, preferably a sucrose because of its low cost or, optionally, a monosaccharide such as glucose, or with substances of the type of 1 formic acid.

The treatment with sucrose or glucose takes place in a reactor provided with stirring and heating means, under atmospheric pressure, by conducting the nitrous gases which form to an absorber.

This reaction, which is not well known, takes place in an uncontrolled form, with intermittences of high production of foam and exotherm, which obliges to reduce the heating or to increase it irregularly to maintain the expected temperature. , and also requires an indefinite time (which can vary between a few minutes and 2 or 3 hours) to start.
US NO 3,158,527 studies this phenomenon applied to the reduction of NO in radioactive waste water, but
3 the residual N03 concentration after treatment is
3 very high, compared to the objective pursued by the invention (less than 50 ppm), with, as an additional drawback, a high carbonization of the sucrose, not to mention the aforementioned uncontrolled nature of the reaction, which, in certain cases, can extend for 24 hours.

 The Applicant has noted that the above-mentioned drawbacks occur when it treats any of the aforementioned reagents (sucrose, glucose or formic acid) phosphoric acid containing 3 to 6% of N03 which has not been previously submitted the action of nitrous acid to remove urea.

 The advantage of the present invention with respect to this step is that, perhaps as a result of the influence of a type of reaction promoter, resulting from the treatment described in the previous step, the reaction occurs. takes place in an easily controllable form, with little foaming and without carbonization, the removal of which is difficult and costly. The release of nitrous vapors within the reaction begins a few seconds after adding the sucrose, ending after a time which varies according to the amount of the mass treated, while stirring and heating are maintained. outside, without carbonization that is difficult to remove.

 When the release of nitrous gases ceases, the acid already reaches a concentration of H2PO4 of between 75 and 85%, due to the evaporation of the water which occurs simultaneously. The NO3 content varies between 0 and 20 ppm.

Optionally, the acid obtained is concentrated by evaporating, at atmospheric pressure or under vacuum, the amount of water necessary to arrive at 85% of H3PO4, if
3 04 this percentage is desired, or at the desired commercial concentration.

d) As an optional variant of the treatment for the elimination of NO3 indicated in the preceding stage, the phosphoric acid obtained in stage a) is subjected to the action of formic acid, either concentrated to 85%, or diluted to a lower bracing.

 This treatment is carried out by heating the mass under reflux to retain the acid within the reaction while it takes place, since this reaction is strongly exothermic and that, when the temperature rises, it would expel the formic acid in conjunction with nitrous vapors formed in the decomposition of nitrates.

The reaction is started by gentle heating, stirring until a temperature which is strongly linked to the amount of formic acid added to the reaction is reached and which the Applicant has been able to fix between 300 and 600 C, which gradually decreases as the amount of formic acid used in treatment increases.

 When the reaction ceases, which is recognized by the lowering of temperature which takes place, the reflux is removed to distill the excess formic acid.

The reaction behaves in an uncontrolled manner similar to that observed in the case of disaccharides or monosaccharides when the treatment is carried out on phosphoric acid without having previously acted on nitrous gases, therefore requiring for its regulation a promoter which l 'primer.

This suggests that this is a type of reduction (sugars and formic acid are reducing agents) which takes place by chain reaction with the formation of unstable intermediates. The dose of formic acid which is necessary in this variant to reduce the NO content to a level below 50 ppm
3 is given by a formic acid / NO su 3 molar ratio greater than 1/1.

e) Finally, the excess organic matter resulting from the treatment to which the phosphoric acid is subjected in stages c) or d) is eliminated, as well as the dissolved organic substances specific to the raw material obtained by the wet process and which have passed through. all previous treatment systems.

 This elimination is carried out with nascent oxygen, preferably produced by the "in decomposition of H 2 O 2 at 30%. The H 2 O 2 is incorporated into the concentrated acid, with stirring and preferably at ambient temperature, the mass being heated. once dispersed, to completely decompose the reagent and restore the P205 concentration of the final product.

 Other non-contaminating nascent oxygen generating systems can be adopted, such as the use of ozonizers, but the impact of the consumption of H 2 O 2 necessary to obtain the discoloration of the acid on the cost of the final product is sufficient low to consider it as a preferred method, this consumption can be estimated at a maximum of 45 to 50 1 of H 2 O 2 to 30 per tonne of acid to 62% of P 2 O 5.

The "food grade" quality acid resulting from the treatment adopted has the following fundamental characteristics:
It is colorless, odorless and soluble in water and in alcohol in all proportions; it has an H3PO4 richness of 85% and contains less than 2 ppm of As, less than 10 ppm of F and less than 5 ppm of heavy metals (expressed as Pb).

It therefore meets the required conditions stipulated for this type of acid, and one can add, given the impact that the treatment and the raw material used (crude phosphoric acid obtained by wet process) could have on other impurities , that it is free of urea, organic matter and Ca and has less than 20 ppm NO, and less than 50 ppm Fe and Mg.

The invention will be better understood with the aid of the following examples, which illustrate the process which it concerns and are in no way limiting.

EXAMPLE 1
Is introduced into a column reactor, provided with a gas diffusion plate in its zone of its base,? 2500 parts by weight of a phosphoric acid prepared by treatment of urea phosphate with nitric acid and which has the following composition:
40.0% P205; 4.0% NO3; 1.0% urea
41 ppm Fe; 1 ppm As; 2 ppm heavy metals (Pb)
9 ppm Mg; 10 ppm F; 0.05% S04 and 9 ppm Ca.

 The urea phosphate with which this raw material was produced comes in turn from the treatment to which a crude phosphoric acid obtained by the wet method was subjected with urea; the urea phosphate thus obtained in turn recrystallizes.

 Through the base of the diffusion plate, a gas stream is introduced, consisting of a mixture of NO + N02 and air, in a volume ratio of nitrous gas / air equal to 108/300. The supply takes place with a flow rate of 350 to 400 l / h of gaseous mixture for 40-minutes while maintaining a residence time of 12 seconds in the liquid mass, then the supply of nitrous gas is cut off to continue with the air alone.

 The absorption of nitrous gases in the reaction mass is noted by the different coloring; intense red at the entrance and orange-yellow at the exit of the column.

 The subsequent passage of air is terminated when the outlet gases are completely colorless.

The acid leaving the reactor presents the following analysis:
42% P205; 4.8% NO3 and below.

EXAMPLE 2
1000 parts by weight of an acid treated with nitrous gases as in Example 1, which contains 42.0% of P205, 4.8% of NO3 and is free of urea. The mixture is heated to boiling point and a dose of sucrose in a 2 M solution is added thereto, sufficient to establish a sucrose / NO molar ratio; in the treatment which is equiva
3 slow at 1/33.

 The reaction, which begins in a few seconds, continues with the formation of foam which can be controlled by the release of nitrous vapors, which ceases after approximately 25 minutes.

 The acid, which now has a slightly yellow tone, contains 63 P205 bones and does not precipitate with the "nitron" reagent, even when taking 25 g of sample. By colorimetry, there is the presence of 1 ppm of Nu,.

3
EXAMPLE 3
100 parts by weight of phosphoric acid treated with nitrous gases as in Example 1 are introduced into a reactor having characteristics similar to those of the reactor of Example 2 and, after boiling, treatment is carried out with 12 parts. by volume of a 2M sucrose solution. Samples are taken every 5 minutes and their NO3 content is determined. The results show that the decomposition of NO3- takes place gradually at the same time as the concentration of phosphoric acid increases in the mass, and it ends after 25 to 30 minutes, when a product containing 65.1% of P205 and which, with the "nitron" reagent, indicates the absence of
N03, even with 25g samples.
NO3, indicates the presence of 1 ppm.

EXAMPLE 4
Fractions of 250 parts by weight of a phosphoric acid previously treated with nitrous gases are introduced successively into a reactor fitted with a stirring, heating and reflux system, which contains 40.4% P205, 6.096 N03 and is free of urea.

 A proportion of formic acid 10 M is added to each of these fractions such that it allows a formic acid / NO 3 molar ratio of between 1 / 0.44 and 1/10 to be obtained and the temperatures at which starting is determined the reaction.

At the end of the reaction (the evolution of nitrous gases ceases), the remaining formic acid is distilled in each case by removing the reflux and putting back water to facilitate its expulsion.

 The samples obtained are discolored and analyzed.

The data for this example is shown in the table below: rap. molar temp. start end product ac.form./N03 of the reaction% P205 ppm N03 1 / 0.44 500C 61.7 20 (calorimated1 / 1600c 64.2 17 trie) 1/5 800c 65.2 63 -
EXAMPLE 5
The preceding test is repeated on two fractions of an acid previously treated with nitrous gases by proceeding as in Example 1 and which contains 40.4% of P205, 5.85% of N03 and is free of urea.

85% formic acid is used for the treatment while maintaining formic acid / NO 3 molar ratios respectively equal to 1 / i and 1 / 0.5. The temperature of the reaction mass spontaneously rises to 950C.

 Once the reaction has started, when a rapid release of nitrous gases takes place, the reflux is removed, facilitating the exit of these gases from the reactor in order to avoid as much as possible their return to the reaction mass.

It can be seen that the temperatures at which the reaction begins coincide substantially with those indicated in Example 4: 650C for the 1/1 NO formic acid molar ratio and 450C for that 1 / 0.5.

3
Once the formic acid has been expelled by proceeding as in Example 4, acids of variable concentration between 61 and 62% of
P205 and which, on analysis with "nitron" with 25g samples, do not produce a precipitate, while the colorimetry indicates 1 ppm.

EXAMPLE 6
1000 parts by weight of each of the acids which result respectively from the treatments according to Examples 2 and 5 are introduced successively into an open reactor.

At room temperature and with stirring, 30 parts by volume of 30% H 2 O 2 (110 volumes) are added in each case. The mass is then heated and the treatment is continued until complete decomposition of the H 2 O 2 is obtained, the initial weight of acid introduced into the reactor being recovered.

 The products thus obtained have similar characteristics and similar analyzes.

They are colorless, completely soluble in water and in alcohol, have a H3PO4 richness of 85% and their micro-impurity content is established as follows:
Urea: free; N03 t 1 ppm
As: 2 ppm F: 10 ppm
Heavy metals (expressed as Pb): 3 ppm.

Claims (10)

- CLAIMS  1 - Process for the preparation of phosphoric acid of "human food" quality, essentially from a phosphoric acid of "purified" technical quality, obtained by treatment of urea phosphate with nitric acid and which comprises nitrates and urea in its composition, characterized in that it comprises the following successive stages, carried out by working continuously or in discontinuous a) treating the starting phosphoric acid which has in its composition 30 to 50% of P205, from 2 to 7% of NO and from 0.5 to 3% of urea, with neither acid nor  3 treux formed "in situ", maintaining during the treatment a molar ratio nitrous acid / urea of between 2/1 and 4/1 to convert all of the urea into nitrogen, C 2 and water  b) entraining, by means of an air stream, the gaseous nitrogen compounds in excess of the reaction with urea and the rest of the gases produced in this until a completely colorless gas flow is obtained at the exit from the reaction mass  c) reacting the phosphoric acid thus treated in which the nitrate content has increased to a level of between 3 and 8% as a result of the treatment carried out in step a), with a reducing agent chosen from di- or monosaccharides and formic acid  d) concentrating the acid resulting from the preceding stage has a P205 richness of between 55 and 62% and treating it with H202 at 30% or with a substance producing nascent oxygen which is not contaminating, until discoloration of the acid is obtained, which constitutes the final product which has a H3PO4 content of between 75 and 85 N / A, contains from O to 20 ppm of NO, is free of urea and contains a maximum of 2 ppm of As, less than 10 µm of F and less than 5 ppm of heavy metals (expressed as Pb).  2 - Method according to claim 1, wherein the reducing agent is a di- or a monosaccharide, characterized in that in step c) of the treatment, this reagent is incorporated in aqueous solution and a molar ratio is maintained disaccharide / NO, phosphoric acid between 1/20 and 1/50, or double in the case of a monosaccharide, the reaction taking place at the boiling temperature of the acid and having the effect to reduce the nitrates to nitrogenous compounds of lower order of oxidation, which are evacuated from the reaction mass. 3 - Process according to claim 1, wherein the reducing agent is formic acid, characterized in that in step c) of the treatment the reaction takes place under reflux while maintaining a molar ratio of formic acid / NO 3 of l phosphoric acid equal to or greater than 1/1, the reaction being initiated by heating the mass to temperatures between 300 and 600, these temperatures being lowered as the aforementioned molar ratio is increased by stopping the reflux at the end of the reaction, and continuing the heat treatment to distill the excess formic acid and expel the nitrous gases formed. 4 - Process according to any one of claims 1 to 3, characterized in that in a preferred embodiment, step a) is carried out by treating the starting phosphoric acid, containing from 0.5 to 3% d urea, with a gas stream produced by synthesis or in an ammonia burner, and constituted by a mixture of nitrous gases (NO + N02) and air, which contains a volume ratio of 50 to 150 parts of the former for 150 to 500 parts of the second, maintaining in the reacted gas stream flow rates such that they allow residence times in the mass of the phosphoric acid which are preferably not less than 10 seconds, in view to obtain a maximum yield in the reaction with the urea of nitrous acid which is formed "in situ" by NO2.  5 - Method according to any one of claims 1,2 and 4, characterized in that step c) is preferably carried out with the aid of the phosphoric acid obtained at the end of step b) brought to its temperature of boiling, with good mechanical stirring or by diffusion with air, with a concentrated solution of sucrose making it possible to obtain between the latter and the NO3 that the treated acid contains a molar ratio preferably between 1/25 and 1/40, then continuing the heating until complete evacuation of the gases formed during the reaction.  6 - Method according to any one of claims 1, 3 and 4, characterized in that in step c), the treatment of phosphoric acid from step b) is carried out with formic acid concentrated to 85 5 'or less and preferably with a molar ratio of formic acid / NO equal to 1/1, by initiating the reaction  3 at a temperature between 300 and 40 ° C. and which rises to 950 at 100 ° C. as a result of the exothermic nature of the reaction, and by stopping the reflux in order to distill the excess formic acid.  7 - Process according to claims 1 and 2, characterized in that the concentration of the acid in step d) is carried out at atmospheric pressure. 8 - Process according to claims 1 and 3, characterized in that the concentration of the acid in step d) takes place during the distillation of excess formic acid. 9 - Process according to claims 1, 2 and 3, characterized in that the concentration of the acid in step d) is carried out under vacuum.  10 - Process according to claim 1, characterized in that the final bleaching treatment of the acid is preferably carried out with a proportion of H 2 O 2 at 30 5 'which is not more than 5% (weight / volume) by ratio to the acid resulting from the concentration carried out in step d), concentration which, optionally, is carried out to a degree slightly greater than that of the final product to take account of the dilution which occurs during the treatment of discoloration, which preferably begins at room temperature, to facilitate the dispersion of H 2 O 2, before causing its decomposition by adding heat to the mass subjected to stirring.