US3054656A - Process for the production of improved sodium tripolyphosphate - Google Patents

Description

ite

This invention relates to a process for the production of a sodium tripolyphosphate suitable for incorporation into detergent composition slurries or pastes which may be dried to produce granular detergent powders. More particularly it relates to a process which provides an improved sodium tripolyphosphate which forms a detergent composition slurry or paste with improved consistency characteristics for spray drying containing a minimum of small, hard lumps or gritty particles which do not disintegrate in the subsequent processing and carry through as such to the finished product.

A typical dry, granular detergent composition for household laundry and dishwashing use contains an organic surfactant such as a sodium alkylaryl sulfonate, an ethylene oxidehigher aliphatic alcohol condensation product or a higher aliphatic alcohol sulfate, sodium tripolyphosphate or tetrasodium pyrophosphate as an inorganic builder, and a small amount of an inorganic silicate such as sodium silicate as a corrosion inhibitor. One of the most effective ways of incorporating these ingredients into a dry granular product in which the ingredients are uniformly distributed throughout is to prepare a homogeneous slurry or solution of the ingredients and then spray dry this mixture to produce particles of the appropriate size each of which contains all the ingredients of the composition in the desired proportions. Because the evaporation of large amounts of water is costly and reduces the capacity of the spray drying equipment, it is the general practice to prepare the feed to the spray dryer with as high a solids content as possible. Usually the feed to the spray dryer is a concentrated slurry or paste having sufiicient fluidity so that it can be satisfactorily pumped and dispersed into small droplets in the spray drying equipment. A smooth, creamy, stable consistency is generally desired in order to allow the slurry to be pumped freely and which will not clog the nozzles of the dryer.

The consistency of the detergent slurry depends on a large number of factors including the type and concentrations of the various ingredients and the order and manner in which they are incorporated into the mixture. However, it is known that when all of these factors are held constant, the properties of the slurry may be markedly affected by variations in the behavior of the sodium tripolyphosphate builder. Some lots of commercial sodium tripolyphosphate have a much greater tendency than others to form small hard agglomerates or gritty particles in the detergent slurry mix. When this occurs to any substantial extent, enough of the gritty particles may pass unchanged through the mixing and spray drying operation and be present in the finished dry detergent product so that when the detergent is added to water for use by the housewife, the objectionable gritty particles may be noted. These particles do not dissolve as rapidly as the bulk of the detergent composition and are abrasive to the hands and to the equipment used.

Sodium tripolyphosphate is made commercially by heating a mixture of sodium phosphates having an Na O to P ratio of approximately 5 to 3 to temperatures between 300 and 550 C. There is a high temperature form known as Form I and a low temperature form known as Form II, the crystal structures of which may be distinguished by X-ray difiraction. Commercial sodium tripolyphosphate contains varying amounts of Form I material depending on the temperature conditions under which it is produced and is usually sold either as essentially Form 11 sodium tripolyphosphate or as a mixture of the two forms containing up to about 40% of Form I. The sodium tripolyphosphate containing Form I is used extensively in the household detergent compositions described above and it is for this type of sodium tripolyphosphate that the present invention is especially useful.

Sodium tripolyphosphate as it is discharged from the final heating zone is essentially anhydrous and generally picks up very little moisture during subsequent grinding operations. The loss of weight on ignition at 500 C. is usually about 0.10%. This ignition loss is partly adsorbed water or water of hydration which can be driven off at a much lower temperature (200 C.), and partly molecularly combined Water from incompletely converted orthophosphates, CO and other volatile matter resulting from the combustion of organic matter. The moisture content determined by heating at 200 C. for 1 hour is usually less than 0.1%. This essentially anhydrous sodium tripolyphosphate tends to form the gritty particles described previously when it is mixed with water and other ingredients of a detergent composition in the preparation of a relatively viscous slurry for feeding to a spray dryer in which it is converted into a dry, granular finished product.

It is the principal object of the present invention to provide an improved process for producing a sodium tripolyphosphate builder for the formulation of a detergent composition which does not form an objectionable amount of hard gritty particles when it is mixed with the other ingredients of a detergent composition in the preparation of a slurry for feeding to a spray dryer. It is a further object of the invention to provide a simple and economical method for making the improved sodium tripolyphosphate.

We have discovered that if extremely small amounts of water are added to the anhydrous sodium tripolyphosphate its tendency to form hard gritty particles in the detergent slurry is substantially eliminated or reduced to the point where it no longer interferes with the production of satisfactory dry detergent compositions. The amount of water required varies somewhat with the particular type sodium tripolyphosphate being treated but is between 0.1 and 0.4% In general, the sodium tripolyphosphate is not further improved by the addition of more than 0.4% moisture and in most instances the optimum efiiect is obtained with the addition of .2 to .3% water.

The water may be added by any suitable means which results in good distribution throughout the material as by dropping or spraying water or introducing steam or humid airv into the sodium tripolyphosphate while it is being vigorously agitated. One of the most eflective ways wev have found is to add or spray water to the hot sodium tripolyphosphate soon after it is discharged from the final heating zone. The material is cooled by the evaporation of part of the added water and the amount of Water added is controlled so that only the desired small amount remains in the cooled product. The treated granular product is then ground to a powder. When the water is added to the hot sodium tripolyphosphate in this way, the very small amount of moisture remaining in the product is effective in eliminating the objectionable gritty particle forming tendency. When the water is added to sodium tripolyphosphate which has been cooled to or nearambient temperature, very efficient mechanical mixing is necessary to get uniform distribution. Repeated passes through the grinding mill will sometimes improve the distribution of the water when the mixing has been inadequate. Unless the added water is uniformly distributed throughout the product, larger amounts of water are required. Larger amounts of added water are undesirable because the ground product may tend to ball up on handling, clogging screening equipment, etc. or cake on storage causing difficulties in subsequent handling and use. The sodium tripolyphosphate assay of the product is obviously reduced in proportion to the amount of water added so that the use of the minimum amount of Water required to eliminate its gritty particle-forming property is advantageous. The few tenths of a percent of water added in accordance with this invention does not adversely affect the handling and storage properties of the sodium polyphosphate or change appreciably its assay, but does eliminate or very substantially reduce its tendency to form hard gritty particles when mixed into an aqueous solution or slurry.

. The tendency for a given sample of sodium tripolyphosphate to form gritty particles in detergent manufacture is determined by a laboratory procedure which approximates the commercial crutcher process for preparing detergent slurries for spray drying. A typical detergent slurry is prepared under controlled conditions in a laboratory mixer while thetemperature is maintained at 17018() F. by circulating water through a jacket or by placing the mixer in a constant temperature bath. The water-jacketed mixing chamber of a Brabender Plastograph is a convenient apparatus for the purpose and the test results reported in this application were obtained with it. A 7.5 x 5.0" diameter stainless steel beaker fitted with baffles and stirrer has also been found to give similar results.

V In testing the grit-forming tendency of a sample of sodium tripolyphosphate, 462 grams of an organic surfactant paste containing about 23% active surfactant, about 21% sodium sulfate and about 56% water is preheated to 170-180 F. and added to the mixer over a period of 30 seconds. The Brabender Plastograph mixer speed is set at 60 r.p.m. One minute after the surfactant paste has all been added, 96 grams of a sodium silicate solution containing 43% sodium silicate in which the weight ratio of Na O to SiO is l to 1.6 is added over a period of 30 seconds. Finally, one minute after the silicate has been added, 350 grams of the sodium tripolyphosphate to be tested is added over a period of 3 to 6 minutes depending on how easily the powder mixes into the liquid. One and a half minutes after all the sodium tripolyphosphate has been added, a level teaspoon sample of the mix is taken and examined for gritty particles by rubbing it with one finger on a sheet of paper. The number of gritty particles which do not break down when rolled under the finger with moderate pressure are counted. Teaspoon samples are taken again after and 10 minutes of mixing and the number of gritty particles again determined. After 10 minutes, the mixing is stopped and about 150 ml. of the mix is placed in a beaker and held at 170480 F. in a water bath for minutes. A final teaspoon sample is then taken from the beaker and examined.

The surfactant paste used in this test is made by mixing commercial surfactants with the calculated amounts of anhydrous sodium sulfate and water. For example, a surfactant paste was prepared by mixing 121 grams of Ultrawet K, an alkyl benzene sodium sulfonate containing about 88% active surfactant and 12% sodiumsulfate with 82 grams of sodium sulfate and 260 grams of water. Another surfactant paste for use in the laboratory mixing test was made by mixing 106 grams of Duponol ME Dry which is essentially 100% active lauryl alcohol sulfate, with 96 grams of sodium sulfate and 2.60 grams of water. A surfactant paste containing a mixture of alkylaryl sodium sufonate and higher aliphatic alcohol sulfate as the active surfactant ingredient has been found to give the same results.

The sodium tripolyphosphate can be rated for its tendency to form gritty particles in commercial detergent manufacture as the basis of the numberof gritty particles found in the samples taken during the laboratory mixing test. The fewer gritty particles found in this test, the less tendency there will be for the sodium tripolyphosphate to form similar gritty particles which in commercial detergent manufacturing may carry through the mixing and drying processes to the finished powdered detergent. In general, it has been found that if the sample taken from the test mix at the end of 10 minutes contains less than 5 grit particles, the sodium tripolyphosphate will perform satisfactorily in commercial use.

Table I shows the results obtained by the above laboratory test on a series of sodium tripolyphosphate samples taken from the product stream during the commercial production of sodium tripoly-phosphate containing about 25% Form I and 75% Form II Without the addition of small amounts of Water in accordance with the present invention.

TABLE I Laboratory Detergent Mixing Test Percent Number of Gritty Particles Found Sample No. H20 in iterproduct 1.5 Min. 5Min. 10 Min. 30 Min.

The sodium tn'polyphosphate represented by the samples in Table I was found to be unsatisfactory for the manufacture of a household detergent of the general composition described above because of the large number of gritty particles formed in the crutcher operation.

Table 11 below shows the results of similar tests run on samples of sodium tripolyphosphate taken from the same product stream when a controlled amount of water was added to the sodium tripolyphosphate after it was discharged from the final heating zone of the process.

TABLE II Laboratory Detergent Mixing Test Retained Number of Gritty Particles Found Sample No. Percent After- HsOiu Product 1.5 Min. 5Min. 10 Min. 30 Min.

The addition of from about .1 to .3 water changed the sodium tripolyphosphate in the product stream from material with a pronounced gritty particle forming tendency, and thereforeunsuitable for use in the manufacture of commercial household detergents, into a product with little or no tendency to form gritty particles in the detergent slurry mix.

Just how the addition of such small amounts of water changes so markedly the behavior of the sodium tripolyphosphate is not known. However, it is believed that the gritty particles are formed when finely ground sodium tripolyphosphate does not disperse easily in the aqueous detergent slurry and part of it becomes cemented by hydration into hard aggregates. Anhydrous sodium tripolyphosphate which has been treated by the addition of small quantities of water in accordance with the instant invention apparently disperses so rapidly in the slurry that such hydrated aggregates do not have a chance to develop. 7 i

' Example 1 Sodium tripolyphosphate prepared by heating a mixtially anhydrous having an average moisture content of less than 0.1%. It contains about 25% Form I and 75% Form H sodium tripolyphosphate. The hot discharge is fed into the upper end of a 34 x 30 diameter rotary tube mixer fitted with angle flights to insure good mixing and operated at 19 rpm. A 1 inch water line equipped with three spray nozzles extends into the rotary tube mixer to a distance of about 11 'feet from the feed end. Water is sprayed continuously onto the mixing bed of sodium tripolyphosphate at the rate of about 1 gallon per 100 pounds of sodium tripolyphosphate. The sodium tripolyphosphate is cooled by the evaporation of the water and discharges from the rotary mixer at a temperature of about 130 C. The rate at which water is sprayed into the rotary mixer is controlled so that a residual moisture content of 0.2 to 0.3% is maintained in the cold product. After grinding, representative samples of the sodium tripolyphosphate are found to have very little tendency to form gritty particles by the laboratory detergent slurry mixing test, the gritty particle count being consistently less than 5 after ten minutes, and the sodium triopolyphosphate performs satisfactorily in commercial detergent manufacture.

Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention. However, it should be clearly understood that, within the scope of the appended claims, the invention may be practiced by those skilled in the art, and having the benefit of this disclosure, otherewise than as specifically described and exemplified herein.

That which is claimed as patentably novel is:

1. Method of making an improved free-flowing sodium tripolyphosphate which does not produce gritty particles when mixed into detergent slurry compositions, which comprises heating sodium orthophosphate at a temperature of about 300 C. to 550 C. to produce an essentially anhydrous sodium tripolyphosphate containing a maximum of 0.1% moisture, mixing said substantially anhydrous sodium tripolyphosphate with sufiicient water to increase the moisture content by 0.1 to 0.4%, and recovering the treated sodium tripolyphosphate having an increased moisture content.

2. Method of claim 1 in which said moisture content is increased by 0.2 to 0.3%.

3. Method of making an improved tree-flowing sodium tripolyphosphate which does not produce gritty particles when mixed into detergent slurry compositions, which comprises heating sodium orthophosphate at a temperature of about 300 C. to 550 C. to produce an essentially anhydrous sodium tripolyphosphate containing a maximum of 0.1% moisture, mixing said substantially anhydrous sodium tripolyphosphate at a temperature above 200 C. with sufficient water to cool said tripolyphosphate by evaporation of part of said water, the remainder of said water being evenly distributed and increasing the moisture content of said sodium tripolyphosphate by 0.1 to 0.4%, and recovering the treated sodium tripolyphosphate having an increased moisture content.

4. Method of making an improved free-flowing sodium tripolyphosphate containing about 25% of the high temperature crystalline form and of the low temperature crystalline form and which does not produce gritty particles when mixed into detergent slurry compositions, which comprises heating a mixture of sodium orthophosphates having an Na O/P O ratio of 5/3, to temperatures between 300 C. and 550 C. to produce essentially anhydrous sodium tripolyphosphate containing a maximum of 0.1% moisture, mixing said substantially anhydous sodium tripolyphosphate With sufiicient Water to cool said tripolyphosphate by evaporation of a part of said water, the remainder of said Water being evenly distributed and increasing the moisture content of said sodium tripolyphosphate by 0.1 to 0.4%, and recovering the treated sodium tripolyphosphate having an increased moisture content.

References Cited in the file of this patent UNITED STATES PATENTS (2,244,158 Hubbard et a1. June 3, 1941 2,712,529 Mills et a1. July 5, 1955 2,737,443 Wright Mar. 6, 1956 2,796,324 Russell June 18, 1957 2,874,123 Schaafsma Feb. 17, 1959 2,895,916 Milenkevich et a1. July 21, 1959

Claims (1)

1. METHOD OF MAKING AN IMPROVED FREE-FLOWING SODIUM TRIPOLYPHOSPHATE WHICH DOES NOT PRODUCE GRITTY PARTICLES WHEN MIXED INTO DETERGENT SLURRY COMPOSITIONS, WHICH COMPRISES HEATING SODIUM ORTHOPHOSPHATE AT A TEMPERATURE OF ABOUT 300*C. TO 550*C. TO PRODUCE AN ESSENTILALLY ANHYDROUS SODIUM TRIPOLYPHOSPHATE CONTAINING A MAXIMUM OF 0.1% MOISTURE, MIXING SAID SUBSTANTIALLY ANHYDROUS SODIUM TRIPOLYPHOSPHATE WITH SUFFICIENT WATER TO INCREASE THE MOISTURE CONTENT BY 0.1 TO 0.4%, AND RECOVERING THE TREATED SODIUM TRIPOLYPHOSPHATE HAVING AN INCREASED MOISTURE CONTENT.