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Sodium diformate production and use

Sodium diformate production and use description/claims

The present invention relates to a method for producing sodium diformate in solid form having a high formic acid content, use thereof in animal feeds, as acidifier, preservative, silage aid, fertilizer and growth and performance promoter, and also to animal feed additives comprising the inventive sodium diformate.

Formic acid formates have antimicrobial activity and are used, for example, for preserving and acidifying plant and animal materials, for instance grasses, agricultural products or meat, for treating biowastes, or as an additive for animal nutrition.

In the animal nutrition sector, as sodium compounds, use is generally made of either mixtures of sodium diformate with trisodium hydrogenformate, or the latter alone, see, e.g., WO 96/35337 and WO 04/57977. WO 96/35337, furthermore, reports on the use of sodium diformate, but no specific instructions on production of this compound are given.

Generally, a content of formate anions as high as possible as one of the active constituents is desirable for the use of hydrogenformates. From the economic aspect, it is advantageous in particular if this increased content of formate anions is accompanied by a formic acid fraction as high as possible, since this simultaneously offers the acidifying activity. Under these aspects, the use of formic acid sodium formate is particularly expedient, since, in this case, compared with trisodium hydrogentetraformate, and also compared with formic acid potassium formate, in each case a higher theoretical content, both in formate ions and in formic acid is present. Although both values are somewhat more expedient in the case of ammonium diformate, this is, however, a very unstable compound.

Formic acid formates in solid form and their production have long been known as such, e.g. from Gmelins Handbuch der anorganischen Chemie [Gmelin's handbook of inorganic chemistry], 8th edition, number 21, pages 816 to 819, Verlag Chemie GmbH, Berlin 1928 and also number 22, pages 919 to 921, Verlag Chemie GmbH, Berlin 1937. According to these citations, the formic acid formates potassium diformate and sodium diformate are said to be obtainable in principle by dissolving potassium formate or sodium formate in formic acid and subsequently cooling. In addition to sodium diformate, the more stable crystal form trisodium hydrogenformate exits. However, reference is made to the fact that especially sodium diformate is accessible only with difficulty in crystalline dry form and, furthermore, is relatively unstable. The statements in Gmelin's handbook only permit the conclusion that the products described there were not pure sodium diformate.

EP 0 824 511 B1 describes a method for producing products which comprise disalts of formic acid. In this process, certain alkali metal or ammonium formates, hydroxides, (bi)carbonates or ammonia are mixed at 40° C. to 100° C. with formic acid which has a content of at least 50%. The mixture is subsequently cooled and the disalts are obtained by filtration. Although the production of formic acid potassium formate and also of mixtures of formic acid sodium formate with trisodium hydrogentetraformate are presented by way of example, the production of solid pure sodium diformate, in contrast, is not taught. Thus, the temperatures and concentration limits specified for the method for the (aqueous) potassium and sodium formate solutions to be used in the method only permit the production of potassium diformate, since (aqueous) solutions of sodium formate cannot be produced in the concentrations specified, owing to the lower solubility limit compared with potassium formate. Therefore, although potassium diformate is obtained, sodium diformate is present solely in a mixture with trisodium hydrogentetraformate.

German patent DE 42 40 17 (of Jan. 14, 1926) teaches the production of formic acid sodium formates having various acid contents by introducing sodium formate into aqueous formic acid. The resultant crystals are obtained by cooling the solution to ambient temperature. Depending on the water content of the formic acid, in addition to trisodium hydrogenformate and mixtures of trisodium hydrogenformate with sodium diformate, sodium diformate is also said to be accessible. The latter is said to be obtained by the method of DE 424017 when the formic acid used has a content of more than 50%, e.g. 80%, as in example 2. The inventors' own experiments, however, found that, under the conditions specified in DE 424017, sodium diformate cannot be obtained in pure, crystalline form. Rather, in this procedure a mixture with trisodium hydrogenformate is obtained, the formic acid content of which is markedly below the expected theoretical value for pure sodium diformate of 40.36% by weight, based on the total dry weight.

Consequently, it has not been possible to date to produce solid sodium diformate having a formic acid content of at least 35% by weight in pure, stable and dry form.

Adequate stability of formic acid sodium formate in solid form is of particular importance not only with respect to handling and storage life, but also with respect to production. In particular, release to any great extent of the formic acid present in the formic acid sodium formate is undesirable owing to its corrosive activity.

In the animal nutrition sector, sodium diformate offers the advantage that the trace element sodium need not be added separately in the form of NaCl, as is otherwise customary, but already represents a sodium source as such. Owing to the high formic acid content in sodium diformate, e.g. compared with trisodium hydrogentetraformate, the content of sodium ions is limited. A low or limited content of cations, e.g. including potassium ions, is desirable inasmuch as the latter, in particular in the case of monogastric animals and especially in the case of poultry can lead to an increased liquid intake (increased drinking) and thus to dilution of the excreta of the animals, that is to say can develop diuretic activity.

The object underlying the present invention was to provide a method for producing sodium diformate in solid form and of high purity, i.e. especially having a high formic acid content. The inventive method should, in particular, enable the production of a sodium diformate in comparatively stable and dry form and thus open up a route to its industrial production.

This object has surprisingly been achieved by crystallizing out the target compound from a mixture of sodium formate with a more than one and a half times molar excess of concentrated or aqueous formic acid while maintaining a molar ratio of formic acid to water of at least 1.1:1.

The present invention therefore firstly relates to a method for producing sodium diformate in solid form having a formic acid content of at least 35% by weight, based on the total weight of sodium diformate, which comprises producing, at elevated temperature, from sodium formate and aqueous or concentrated formic acid having a formic acid content of at least 74% by weight, a homogeneous mixture which has a molar ratio of HCOOH to Na[HCOO] of greater than 1.5:1 and a molar ratio of formic acid to water of at least 1.1:1, cooling the mixture and separating off the solid phase from the mother liquor.

The method according to the invention makes possible for the first time the production of a sodium diformate in solid dry form having a formic acid content of at least 35% by weight, frequently at least 36% by weight, in particular at least 37% by weight, especially at least 38% by weight, very especially at least 39% by weight, and still more especially at least 40% by weight, in each case based on the total weight of the sodium diformate. A sodium diformate of this type is therefore a further subject matter of the invention.

A homogeneous mixture in the meaning of the present invention is taken to mean a clear aqueous solution of formic acid in which all of the sodium formate present in the solution is in dissolved form. If appropriate this inventively produced homogeneous mixture comprises sodium diformate crystals for the purpose of seeding the crystallization, as described hereinafter in detail. To this extent, the term “homogeneous mixture” used here comprises not only the aqueous solution comprising formic acid and dissolved sodium formate, but also the sodium diformate crystals which are added if appropriate.

The starting materials sodium formate and formic acid used in the present invention are commercially available and can be used as such without pretreatment.

The sodium formate present as solid at room temperature can be used, e.g., as technical-grade sodium formate. Sodium formate occurring in the production of polyols as waste product is also suitable for use in the present invention. Generally, use is made of a sodium formate whose Na[HCOO] content is at least 97% by weight, based on the total weight of the sodium formate source used. Preferably, use is made of a sodium formate which comprises less than 0.1% by weight, and in particular less than 0.05% by weight, potassium ions, in each case based on the total weight of the sodium formate source used.

It is likewise possible to produce in situ the sodium formate intended for the reaction with formic acid, e.g. by reacting sodium hydroxide, carbonate or hydrogencarbonate with formic acid in concentrated aqueous solution, by reacting carbon monoxide with liquid sodium hydroxide, or by reacting methyl formate with sodium hydroxide. In the case of this variant, a procedure can be followed, e.g., such that solid NaOH or a concentrated aqueous solution thereof, if appropriate with cooling and/or stirring, is dissolved in concentrated formic acid. The ratios of the starting materials can advantageously be directly chosen such that the components formic acid, sodium formate and water in the resultant mixture are already in the abovementioned molar ratios which are required. Otherwise, generally neutralization of excess formic acid and/or a reduction of the water content of the mixture by conventional methods known to those skilled in the art, e.g. evaporation, extraction, distillation and the like, are necessary. Moreover, in this variant the statements made for the general process procedure can be followed.

According to the invention use is made of an aqueous formic acid solution having a formic acid content of at least 74% by weight, or a concentrated formic acid. The concentrated formic acid is taken to mean by those skilled in the art a formic acid solution having a formic acid content of 94% by weight or more, i.e. having a residual water content of less than 6% by weight, in each case based on the total weight of formic acid solution. As aqueous formic acid, a solution of formic acid in water having a formic acid content of less than 94% by weight, based on the total weight of the aqueous formic acid solution. The aqueous formic acid solution used preferably has a concentration of at least 75% by weight, preferably at least 80% by weight, and particularly preferably at least 90% by weight. Very particularly preferably, use is made of concentrated formic acid having a formic acid content of at least 94% by weight. The concentration of the formic acid or solution will preferably not exceed 99% by weight, and is particularly preferably in the range from 80 to 99% by weight, and especially in the range from 94 to 98% by weight.

Preferably, use is made of concentrated or aqueous formic acid in an amount of at least 1.6 mol, in particular at least 1.8 mol, and especially at least 2.0 mol of HCOOH per mol of Na[HCOO]. Preferably, the molar ratio used of HCOOH:Na[HCOO] will lie in the range from 1.6:1 to 3:1, and in particular in the range from 1.8:1 to 2.5:1.

In addition, use will preferably be made of the respective starting materials in amounts such that the molar ratio of HCOOH:H2O in the homogeneous mixture is at least 1.5:1, and particularly preferably at least 1.8:1; very particularly preferably, it is in the range 1.5:1 to 10:1, and in particular in the range from 1.8:1 to 6.1:1.

The sequence in which the starting materials are used is of subsidiary importance. Advantageously, the mixture proceeds in such a manner that a homogeneous liquid mixture of the starting materials is obtained in the molar ratio which is to be kept. According to the invention, the homogeneous mixture is produced at elevated temperature. This is generally taken to mean temperatures of at least 30° C., in particular at least 40° C., and especially at least 50° C. Said homogeneous mixture can be produced using conventional procedures known to those skilled in the art, e.g. by mixing, stirring and/or dissolving, using elevated temperature, or by a combined use of these methods.

To carry out the inventive method, generally a procedure is followed such that an aqueous or concentrated, preferably concentrated, solution of formic acid is initially charged. To this formic acid solution is added the sodium formate in solid form, or in the form of an aqueous solution or suspension. The addition can proceed in portions, e.g. in 2, 3, 4 or more individual portions which are added to the mixture at a predetermined time interval from one another, or continuously, i.e. at constant, decreasing, or increasing, rate. During the addition, generally a temperature increase occurs, so that if appropriate additional heating is not necessary. Customarily, the temperature of the mixture is set, e.g. by matching the addition rate and/or cooling or heating of the mixture and/or of the solution or suspension added, in such a manner that in the mixture a temperature in the range from 30° C. to 80° C., and in particular from 40° C. to 70° C., is maintained. Preferably, the temperature of the mixture is not above 65° C. It is essential to the invention that the crystallization proceeds from an aqueous solution. This solution, as set forth below, can be admixed, or become admixed, with seed crystals as soon as before the start of crystallization.

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