US2337491A - Process of producing sodium acid pyrophosphate - Google Patents

Description

Patented Dec. 21, 1943 PROCESS OF PRODUCING SODIUM ACID PYROPHOSPHATE Frank H. Perrln and Otis w. Allen, Pulaski, Tenn.,

assignorsto International Minerals & Chemical Corporation, Chicago, Ill., a corporation of New York No Drawing. Application September 14, 1940,

Serial No. 356,834

Claims.

This invention relates to the production of pyrophos'phates that are particularly suitable, for example, for leavening agents or baking powders.

In carrying out the invention, sodium acid pyrophosphate, NazHzPaOr, or a double salt of sodium acid pyrophosphate-monosodium phosphate, Na2H2P2O72NaH2PO4, or a physical mixture of the two salts. is produced. It is immaterial in carrying out the process whether the result is a double salt or a physical mixture of the two salts.

Heretofore, acid sodium pyrophosphate has been produced by heating mono-sodiumphosphate, NaH2PO4, at about 200 C. for about six or eight hours in accordance with the formula 2NaH2PO4=Na2H2PzO7+H2Q ,Many mechanical difficulties have been encountered in attempts to produce mono-sodiumphosphate, and corrosive-proof containers such as stainless steel are needed. It is difficult to dry the product to prevent it from melting or fusing. The mixture becomes very sticky at elevated temperatures and is inconvenient to stir or handle. Also, when stored it cakes into a very hard solid lump in a few days.

By the present invention prior difficulties are overcome and sodium acid pyrophosphate is produced in a rapid and satisfactory way at comparatively low cost without encountering many of the mechanical difficulties heretofore experienced.

The following examples are given to illustrate the invention, but it is to be understood that the invention is not limited to the details given.

Example I Tetrasodium pyrophosphate of any fineness, finely ground being preferred, is reacted with phosphoric acid, at room temperature, with the heat of reaction being all that is required to complete the reaction, using one mol of the former to two mols of the latter:

This reaction is reversible only when the compounds are in solution. The physical condition of the double salt (or mixed salts) will be dependent on the strength of phosphoric acid used, but in all cases some drying, at less than the temperature required to convert monosodium phosphate into sodium acid pyrophosphate, is required so that the final product will be in such physical condition as to permit grinding into a powder. The preferred drying temperature is near that of boiling water, 212 F. Any of the condition.

standard types of dryers are satisfactory for this phosphoric acid the product can be kept agitated .for about 20 minutes, to prevent setting as a mass. It will then be in such physical condition as to permit grinding in a swinghammer mill, for example, and subsequently screened to minus 6 mesh. This 6 mesh product can then be introduced into a rotary dryer and heated to a temperature of about 212 F. for 20 minutes, and discharged from the dryer in'the physical desired This product, the double salt (or mixed salts) of NazHaPaOmZNaI-IzPO-r, has a Slight tendency to be hygroscopic after final drying, but not nearly so much as monosodium phosphate alone. This hygroscopic tendency can be taken care of in the usual ways, for instance, by coating with starch. Or it can be marketed as such, notwithstanding its tendency to be slightly hygroscopic.

Example II The same reaction as Example I is used. The 7 resulting product of the double (or mixed salts) can be converted by heating at the required temperature into sodium acidpyrophosphate:

minutes the change to NazHzPzo-z is complete.

The final product. Nazl-lzPzo-z, is stable, and shows no signs of being hydroscopic.

Example III 200 lbs. of tetrasodium pyrophosphate analyzing 53.4% P205 were mixed with 194.19 lbs. of phosphoric acid analyzing 55% P205, and the mass was kept agitated for about 20 minutes, then passed through a swinghammer mill, screened to minus 6 mesh, then dried as in Example I. or Example II, to get, respectively. sodium acid pyrophosphate-monosod um phosphate (or a mixture of the two salts) and sodium acid pyrophosphate.

It has been found that crystalline tetrasodium pyrophosphate would work just as well, and that different strengths of phosphoric acid will also work just as well in carrying out the process. The reaction is dependent on the total pounds of P205 in the tetrasodium pyrophosphate being equal to the total pounds of P205 in the phosphoric acid. Either or both of the two products will serve as a source of the acid radical in leavening agents, or they can be used in any manner a user might desire, their use not being limited to leavening agents.

What is claimed is:

1. The process of producing a sodium acid phosphate product, which comprises mixing and reacting tetra sodium pyrophosphate with phosphoric acid using approximately one mol of the former to two mols of the latter, and heating the product under such conditions that the material is dried and not more than one molecule of combined water is driven off for each two'mols of monosodium phosphate present in the said product.

2. The process of claim 1, in which the drying I temperature is approximately 212 F.

8.The process of producing a sodium acid phosphate product which comprises mixing tetra sodium pyrophosphate with phosphoric acid,

15 phoric acid, using approximately one mol of the former to two mols of the latter to produce a I mixture of sodium acid pyrophosphate and sodium acid phosphate, and heating the resulting mixture at a temperature of approximately 400 20 to approximately 420 F. for approximately 60 minutes.

FRANK H. PERRIN. OTIS W. ALLEN.