US3431069A - Production of sodium tripolyphosphate - Google Patents

Description

United States Patent PRODUCTION OF SODIUM TRIPOLYPHOSPHATE Robert B. Hudson, St. Louis, Mo., and Robert E. Mesmer,

Oak Ridge, Tenn, assignors to Monsanto Company, St.

Louis, Mo., a corporation of Delaware No Drawing. Filed June 12, 1967, Ser. No. 645,498 US. Cl. 231.06 8 Claims Int. Cl. C01b 25/30 ABSTRACT OF THE DISCLOSURE Sodium tripolyphosphate products are produced having an assay of from about 91% to essentially 100% by weight of sodium tripolyphosphate, a Form I sodium tripolyphosphate to Form II sodium tripolyphosphate weight ratio of from about 1:5 to about 3:1 and a water insoluble content of less than 0.20% by weight by calcining sodium orthophosphate salts at elevated temperatures in the presence of from about 0.03% to about 5% of a potassium source. Calcination temperatures are significantly lowered by the practice of the invention.

which are being calcined, in general, determines the particular sodium phosphates that are produced. As used herein the Na/ P ratio is on an atomic basis unless otherwise specified. While it is theoretically possible to produce pure sodium tripolyphosphate at a Na/ P ratio of 5:3, competing reactions cause the product to contain from about 90% to about 97% sodium tripolyphosphate and the remaining 3% to is a mixture of pyrophosphates, metaphosphates and some uncalcined orthophosphates. Among the metaphosphates which can be formed are the water insoluble salts, NaPO II, NaPO III and NaPO IV. A slight excess of sodium is generally used to suppress the formation of water insoluble metaphosphates which have a Na/P ratio of 1.0. By employing these higher Na/P ratios the amount of sodium tripolyphosphate in the product is lower and the amount of the tetrasodium pyrophosphate is higher than when Na/P ratios closer to the theoretical 5:3 ratio are used. However, in most instances a product having a lower sodium tripolyphosphate assay is preferred to one having a high water insolubles content.

Sodium tripolyphosphate is generally preferred to tetrasodium pyrophosphate as a detergent builder, therefore, a product having a high sodium tripolyphosphate assay provided that the insolubles are low, that is below about 0.2%, would be preferred to one having a lower sodium tripolyphosphate assay.

Anhydrous sodium tripolyphosphate is known to exist in two crystalline forms, Form I normally referred to as the high temperature form and Form 11 generally referred to as the low temperature form. The sodium tripolyphosphate assay and the ratio of Form I to Form II in the so dium tripolyphosphate product affects the hydration characteristics of the product. Hydration characteristics generally relate to detergent crutching performance. The Temperature Rise Test, commonly referred to as TR of the product, is generally used to determine the relative amounts of Form I and Form 11 in the product. The procedure for the TR determination is described in A.S.T.M.

Bulletin No. 191 by McGilvery, July 1953. It has been found that the hydration characteristics of a sodium tripolyphosphate product having a Form I to Form II weight ratio of from about 1:5 to about 3:1 (TR values of from about 10 to 24.8 C.) and a sodium tripolyphosphate assay of from about 91% to essentially is highly desirable for some detergent uses. To produce those products having such highly desirable hydration characteristics a Na/P ratio close to the theoretical 5/3 ratio must be used and the calcining temperatures to obtain the desired Form I to Form II ratio are most favorable to the production of insoluble metaphosphate, therefore, a significant problem with the formation of a relatively high level of water insolubles is created.

It is believed, therefore, a process which enables the production of a sodium tripolyphosphate product relatively free of water insoluble material and having a Form I to Form II weight ratio of from about 1:5 to about 3:1 and a sodium tripolyphosphate content of from about 91% to essentially 100% by weight would be an advancement in the art.

It has been discovered that by adding a relatively small amount of potassium prior to the time when the orthophosphates are converted to sodium tripolyphosphate that a product having a Form I to Form 11 weight ratio of from about 1:5 to about 3:1 and a sodium tripolyphosphate assay of from about 91% to essentially 100% by weight and a water insoluble content of less than about 0.20% by weight can be produced even when the Na/P ratio in the raw materials approaches the theoretical 5/3. Additionally, when relatively small amounts of potassium are present, the calcining temperature can be lowered by a significant amount and achieve a product having the desirable sodium tripolyphosphate assay from Form I to Form II ratio. Significant savings in fuel or an increase in calcination capacity or both can be achieved by the prac-, tice of this invention.

The problem with water insolubles is not completely understood. Most of the water insoluble material can be attributed to the formation of water insoluble metaphosphates that are formed atthe calcining temperatures and Na/ P ratios which are used to produce a sodium tripolyphosphate product having a Form I to Form II weight ratio of from about 1:5 to about 3:1 and a sodium tripolyphosphate assay of above about 91% by weight. Formation of insoluble metaphosphates is also affected by rates of heating, particle size and density of the salts which are calcined and final calcination temperature. Control of these variables to insure low insoluble formation is not practical under many processing conditions used to produce the more desirable products. Also, it is known that the presence of certain other metal ions cause the water insolubles to be increased. The amount contributed from each source is believed to be dependent upon the particular system and the operating conditions. It is not known whether the potassium acts as a catalyst to promote the formation of a soluble material such as sodium tripolyphosphate, whether the potassium forms a water soluble complex with the material which would normally form the water insoluble material or whether the potassium inhibits the formation of the insoluble materials.

As used herein a water insoluble material means that percentage of material which is left undissolved after a sample has been dissolved in boiling water for 10 minutes. A typical method for determining the amount is given in Example 1. For sodium tripolyphosphate products the insolubles can also be determined by a turbidity test, details of which are presented in Example 2. In most instances the amount of insolubles which is normally considered as excessive is above about 0.2% by weight and products are preferred which contain less than 0.15% by weight of insolubles. Although the problem with water insoluble material exists when producing a product having a sodium tripolyphosphate assay of from about 91% to essentially about 100% by weight and a Form I to Form 11 ratio of from about 1:5 to about 3:1, the problem is particularly acute when products having a sodium tripolyphosphate assay of above from 92% by weight and a Form I to Form II ratio of from about 1:4 to about 2:3 are produced. This invention is, therefore, particularly well suited for producing products having these above narrower ranges of assay and Form I to Form II ratios.

A wide variety in sources of potassium can be used although in most instances from an economic standpoint it will be preferred to use a water soluble inorganic potassium salt or a potassium base. Use of a water soluble potassium source enables the potassium to be added to the precursor slurry or solution of the sodium orthophosphates which are generally dried to form a solid intimate mixture of the salts which is thereafter calcined to form the sodium tripolyphosphate product. A water soluble potassium source as used herein means that at least 0.1 gram of the potassium source will dissolve in 100 grams of water at 25 C. The addition of the potassium source to an aqueous precursor slurry having the desired Na/P ratio will generally be the preferred method of utilizing this invention; however, any method of adding a potassium source can be used as long as the potassium source is relatively uniformly distributed throughout the raw materials prior to the formation of a substantial amount of sodium tripolyphosphate that is before about 20% of the orthophosphate are converted to sodium tripolyphosphate. An aqueous solution containing the potassium source can be sprayed onto an agitated bed in a rotating calciner or onto a fluidized bed if a fluidized calciner is used before a substantial amount of the othrophosphates have been converted to sodium tripolyphosphate. If desired, the potassium source can be added as a solid to a bed in a rotating calciner prior to a substantial conversion to sodium tripolyphosphate. Additionally, if desired, a solid potassium source can be added to the solid mixture of orthophosphate salts. Additionally, if a conventional rotary drier is used to produce the mixture of sodium orthophosphate salts, an aqueous solution of a potassium source can be sprayed onto the drier bed. Therefore, this invention is independent of the method of adding the potassium source as long as the potassium is added before about 20% and preferably before about by weight of the orthophosphate salts have been converted to sodium tripolyphosphate and the potassium source is relatively uniformly distributed throughout the mixture of phosphate salts.

The source of the potassium can be selected from a relatively wide variety of materials. Potassium salts of inorganic acids can be used such as the potassium salts of the lower saturated aliphatic acids such as potassium formate, potassium acetate, potassium propionate, potassium butyrate and the like. Potassium salts of inorganic acids such as the mineral acids are preferred such as potassium carbonates, potassium phosphates, potassium chloride, potassium nitrate, potassium sulfate and the like. Of these, the potassium carbonates and phosphates such as potassium pyrophophate, potassium tripolyphosphate and mixtures thereof are especially preferred. Elemental potassium can also be used, however, the extensive safety precautions which have to be taken in handling elemental potassium generally will make elemental potassium a less desirable potassium source than some of the less dangerous potassium salts and bases because of its availability and its relative inexpensiveness. As previously mentioned, water soluble inorganic potassium salts, bases and mixtures thereof are preferred because it is easier to obtain a relatively uniform distribution of potassium by either dissolving the desired amount of potassium source in the sodium orthophosphate precursor slurry or by spraying an aqueous solution of the potassium S UICe Onto a rotating drier bed of orthophosphate salts or onto a bed of orthophosphate salts in a rotating drier bed of orthophosphate salts or onto a bed of orthophosphate salts in a rotating calciner before the salts are molecularly dehydrated to sodium tripolyphosphate.

Although the addition of small amounts such as less than 0.5% or even 0.03 by weight of potassium based upon the total amount of sodium orthophosphate salts will achieve at least some improvement, it is preferred to use greater amounts such as 0.1% by weight based upon the total weight of the sodium orthophosphate salts. Use of above about 5% by weight of potassium lowers the sodium tripolyphosphate assay below that which is desired for many uses. It is generally preferred to use from about 0.1% to about 2.5% by Weight with from about 0.2% to about 2.0 by weight being especially preferred.

The amount the temperature of the calcination step can be lowered is dependent upon the amount of potassium used, the total assay of sodium tripolyphosphate and the ratio of Form I and Form 11 in the finished product. For example, when producing a product having about a 92% sodium tripolyphosphate with about a Form I to Form II weight ratio of about 1:32 (TR of from about 12 to about 13) without any potassium a calcination temperature of about 480 C. is used; when about 0.3% potassium is added, a product having an equivalent sodium tripolyphosphate assay and a Form I to Form 11 ratio is produced at a calcination temperature of about 450 C. When the potassium level is increased to about 0.85% the calcination temperature is lowered from about 50 C. to about C. to obtain a product having similar quality characteristics to one which is produced at a temperature of about 510 C. when the same Na/P ratio is used in the raw materials. The particular calcination temperature will also depend to a degree upon the process used, for example, whether a rotary calciner or a fluidized bed calciner is used. One skilled in the art can determine the calcination temperature which is suitable at the level of potassium used by the Temperature Rise Test, which gives a quick and relatively accurate test for the determination of the relative amounts of Form I and Form II sodium tripoly phosphates which are present in the product. The formula for estimating the Form I content from the TR is as follows:

Form I=4 (TR 6) It can readily be appreciated that the preferred method of using a water soluble potassium source enables a person skilled in the art to control the level of potassium addition at a relatively constant level based upon the raw materials and thereby lower the calcination temperature by the desired amount to obtain the product having the desired Form -1 to Form II ratio. The calcination temperature, when utilizing this invention although dependent upon the above factors, will generally be from about 350 C. to about 480 C. depending upon the ratio of Form I to Form 11 desired and the level of potassium used. In most instances the calcination temperature will depend upon the level of potassium used in producing a particular product. The level of potassium will in many instances be governed by an economic evaluation based upon the cost savings of fuel in relation to the cost of adding additional potassium.

The following detailed examples also illustrate the changes from normal operating conditions (conditions which would be used in absence of the potassium) [which will be used in particular processes. All parts, proportions and percentages are by Weight unless otherwise specified. All ratios given are weight ratios except the Na/P ratio which is an atomic ratio.

EXAMPLE 1 A sodium orthophosphate precursor slurry is continuously prepared by metering into a mixing vessel the proper amounts of phosphoric acid and soda ash to obtain a Na/P ratio of from about 5.0/3 to about 5.1/3. The slurry is drum dried to form a solid sodium orthophosphate feed material. Analysis of the material from the drum drier determines if the desired Na/ P ratio is being attained in the precursor slurry which material fed 6 (3) The solution is filtered through an asbestos pad on a tared Gooch crucible.

(4) The material in the crucible is washed 5 times with distilled water.

(5) The crucible is placed in a 110 C. drying oven to a conventional concurrently heated, gas fired, stain- 5 for 1 hour and thereafter removed to a desiccator and less steel rotary calciner. The gas firing rate is adjusted to let stand for at least one hour. obtain a product temperature of about 450 C. The ma- (6) The crucible is reweighed and the percent insoluterial is cooled to below about 100 C. and milled and ble is calculated as follows: screened. Samples of the product are analyzed on an 10 hourly basis. Generally from about 1 to 2 hours after a Wei ht am of cruciblexloo change has been made in the process, such as a change insoluble matter in the Na/ P ratio in the precursor slurry, the effects of samp 1e Welght such changes are noticeable in the analysis of the product discharging from the calciner. Table 1 below gives the analysis of the product for the first four hours of the EXAMPLE 2 foregoing run.

A batch-wise laboratory process using a precursor slurry TABLE 1 prepared as described in Example 1 and dried on a laboratory scale drier followed by laboratory scale calcina- Hour 55.35g? TR (0 15: 33? tion gives results which can be reproduced in full plant scale operation, thereby enabling comparative test regig 33 8 g sults without excessively long periods of time before the gig 18-3 g-ig effect of a change can be observed. The following procedures are conducted in laboratory scale equipment A control precursor slurry is prepared as in Example After four hours the Na/ P ratio in the precursor slurry 1 eixcgpt the Na/P ratio is 5/3 and mm dned to form was increased to from about 5.24/3 to about 5.34/3. The solid A precursor slurry haYmg h Same Na/P following table gives the analytical results of samples 15 prepared and enough potassnlm mpolyphosphate taken at hourly intervals for the succeeding 8 hours. is added.to yleld abqut 043% potassmm based i the total weight of sodium phosphate salts present in the drum dried orthophosphate flakes. TABLE 2 The two solid orthophosphate materials are calcined O at various calcination temperatures ranging from about Hour 33 3 TR( iigr ii 350 C. to about 520 C. Ion exchange and X-ray anal- 9 9 0 198 ysis of samples of the calcined product show the amounts 3 01276 of the various sodium phosphates present and the amount ig g 8:22; of Form I and Form 11 sodium tripolyphosphate present 11.2 0.227 40 in the calcined product. Sample 1 is the designation given 1 8: to the control material which does not contain potassium. 9.3 0.200 Sample 2 is the material containing potassium. Table 3 gives the results of these analyses.

TABLE 3 Pyrophos- Trimeta- Orthophos- Turbidity 1 phate phosphate phate STP II STP I (p.p.m. SiOz) 9.65 13.06 0.72 0.70 84 4s 4 37 160 10 a 2.22 a; 2-2? 2; a 1: 2s as 5; a a 5:25 4:89 014s 0:38 37 0 57 94 20 "id 5. 35 4. 70 0. 0. 49 0 6 94 89 The foregoing run indicates the difficulties in obtaining a product having a desired sodium tripolyphosphate assay, the desired Form I to Form 1 1 ratio and relatively low insolubles. As the TR rose to the desired level, the insolubles increased and the assay steadily lowered. Operating with high Na/P ratios apparently lowered the insolubles slightly, enables the TR to rise to the proper level, however, the sodium tripolyphosphate assay was lower than is desirable for many products.

The insolubles were determined according to the following analytical procedure.

(1) A 10 gram sample is weighed on a standard triple beam balance.

(2) While stirring, the sample is added to 200 ml. of distilled water in a 400 ml. beaker. The material is boiled for 10 minutes.

4. All turbidity standards are shaken well before using. Sample solutions are not compared with suspended air bubbles. Turbidity is measured by looking through the solution and the measure of turbidity is the intensity of the black lines on a white background, illuminated by a Daylite fluorescent tube, the light source coming through the slit. Containers, beaker and bottle standards are placed in exactly the same position relative to the light source. The sample solution is always bracketed by a standard that is slightly higher in turbidity on one side and slightly lower on the other side.

A turbidity level equal to a standard containing about 60 ppm. SiO is equivalent to an insoluble level of about 0.1% by weight.

The results of the above tests indicate that the addition of low levels of potassium dramatically lower the insoluble level of the sodium tripolyphosphate product and indicate that the calcination temperature can be lowered from about 25 C. to about 50 C. and obtain the ratio of Form I to Form II which is desired. For example, the ratio of Form I to Form II in Sample 1 calcined at 475 C. and 500 C. was approximately the same as that of Sample 2 calcined at 425 C. and 450 C.

EXAMPLE 3 Using similar equipment and processing conditions as in Example 1 a test is conducted to evaluate the effect of potassium addition in plant scale sodium tripolyphosphate production. As is noted in Example 1 the effects of a change in the precursor slurry is not seen in the product until about 8 hours after the change due to the retention time in the system.

A Na/P ratio in the precursor slurry of from about 5/3 to about 5.2/3 is used throughout the test. After about 16 hours of running normal production potassium hydroxide is continuously added to the precursor slurry to obtain a level of about 0.4% by weight of potassium based upon the total amount of sodium phosphate present in the slurry. The potassium hydroxide is added for 18 hours and samples are taken and analyzed every two hours. Table 4 gives the results of the tests.

TABLE 4 Calcination Time Insolubles T R C C terripeature Remarks 0.23 490 Nail ratio= 0. 28 12. 6 483 Started K addition.

0. 08 12. 9 433 Nil/P Iati0= The calciner temperature is reduced from about 30 C. to 40 C. to obtain a product having a TR of from about 11 to about 15 when the potassium is added.

The above results indicate that by the addition of potassium the insolubles are controlled below about 0.10% and that a significant reduction in calciner temperature is achieved.

A similar test in which the potassium level is increased to about 0.8% shows the same low level of insolubles and a reduction of the calciner temperature of about 70 C. to about 380 C. products having a TR of from about 11 to about 15 C.

EXAMPLE 4 An orthophosphate solution containing about 0.5% potassium is prepared by dissolving about 500 parts of disodium orthophosphate, 250 parts of monosodium orthophosphate and about 6.7 parts of pentapotassium tripolyphosphate in about 2,500 parts of water. The solution is dried to form a solid mixture of orthophosphate salts having an Na/ P ratio of about 5 :3 and containing about 0.5% potassium. The mixture is milled to have a maximum particle size which will pass through a U.S. Standard 120 mesh screen and 50% which will pass through a U.S. Standard 200 mesh screen.

The material is charged into the top of a conventional fluidized bed reactor which has gases passing upwardly through the bed which is composed essentially of sodium tripolyphosphate. The gases are maintained at a temperature sufficient to keep the bed at about 450 C. The product is withdrawn close to the bottom of the reactor and is analyzed for Form I to Form II ratio using the TR test. The Form I to Form II ratio weight is found to be about 6:4 which is considerably higher than the 3 :7 ratio which is normally in the product at the 450 C. By lowering the gas temperature by about 45 C. the Form I to Form II weight ratio is lowered to the desired 3:7 weight ratio based upon the sodium tripolyphosphate present. The material produced analyzes over 94% sodium tripolyphosphate and has a water insoluble content of less than about 0.15% by weight based upon the total weight of the product. Reducing the feed rate to the calciner increases the assay so that a product having an assay of essentially sodium tripolyphosphate is produced.

What is claimed is:

1. In a process wherein sodium orthophosphate salts are calcined at elevated temperature to produce a sodium tripolyphosphate product having a sodium tripolyphosphate assay of from about 91% to about 100% by weight and having a weight ratio of Form I sodium tripolyphosphate to Form II sodium tripolyphosphate of from about 1:5 to about 3:1, the improvement comprising adding an amount of potassium or potassium containing compound to said orthophosphate salts prior to a substantial conversion of said salts to sodium tripolyphosphate sufiicient to provide a potassium level of from about 0.03% to about 2.5% by weight based upon the sodium orthophosphates present to thereby produce a product having less than about 0.20% by weight of water insoluble salts NaPOg-II, NHIPO3-III and NaPO IV.

2. An improvement according to claim 1, wherein said potassium containing compound is selected from the group consisting of inorganic potassium salts, and mixtures thereof.

3. An improvement according to claim 1, wherein said potassium containing compound is selected from the group consisting of potassium phosphates, potassium carbonates, potassium hydroxide and mixtures thereof.

4. An improvement according to claim 3, wherein said potassium level is from about 0.2% to about 2.0% by weight.

5. An improvement according to claim 3, wherein less than about 20% of said orthophosphate salts are converted to sodium tripolyphosphate prior to the addition of said potassium or potassium containing compound.

6. An improvement according to claim 4, wherein said calcination temperature is from about 350 C. to about 480 C.

7. An improvement according to claim 6, wherein said sodium tripolyphosphate product has a sodium tripolyphosphate assay from about 92% to about 97% by weight and the ratio of Form I sodium tripolyphosphate to Form 11 sodium tripolyphosphate is from about 1:4 to about 2:3.

8. An improvement according to claim 7, wherein said water insolubles are less than about 0.15 by weight.

References Cited UNITED STATES PATENTS 3/1946 Hubbard et a1 23106 7/1958 Kramer et a1. 23106 P0405) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3331,06?! Dated IIzirch n, 1969 Inventor) Robert B. Hudson 8 Robert E. Mesmer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

olumn u, line 7, the figures "0.5%" should be written 0.05% the figures "0.03" should be written 0.03%

SIGNED AND SEALED Iii-19D GEAL) Attest:

Edmd Emmi" mm! x. sum, .m. Aw i ng Offi r Gomissioner or Patents