US3379497A - Tripolyphosphate processes - Google Patents

Description

3,379,497 TREPOLYPHOSPHATE lROCESSES Edward L. Moore, St. Louis, Mo., assigner to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Dee. 22, 1964, Ser. No. 420,441 Claims. (Cl. 23106) ABSTRACT 0F THE DESCLGSURE A process for manufacturing an alkali metal tripolyphosphate comprising calcining a mixture of tetraalkali metal pyrophosphate, trialkali metal trimetaphosphate and ammonium nitrate at a conversion temperature below the melting point of the alkali metal tripolyphosphate, the mixture beinfy substantially free of molecular hydrated phosphates and a similar Calcining process wherein the assay of a sodium tripolyphosphate product contaminated with sodium pyrophosphate and sodium trimetaphosphates is improved by heating the product in the presence of ammonium nitrate.

The present invention relates to the manufacture 0f alkali metal tripolyphosphatcs. More particularly the present invention relates to processes for manufacturing sodi um tripolyphosphate from mixtures of tetraalkali metal pyrophosphates and tiialkali metal trimetaphosphates.

Although it has been stated heretofore that alkali metal tripolyphosphates can be manufactured in calcining processes by simply heating mixtures of orthophosphates and/or polyphosphates (containing rappropriate ratios of alkali metal to phosphorus) at some temperature below the melting point of the desired alkali metal t-ripolyphosphate, experience has shown that when only phosphates which are completely molecularly dehydrated are present in the calciner feed streams the desired conversion to tripolyphosphates does not take place. For example, when intimate mixtures of tetrasodium pyrophosphate and trisodium trimetaphosphate are heated even for extended periods of time at temperatures as high as about 620 C., in a conventional tripolyphosphate calciner, substantially no interreaction of these salts occurs. The presence of other completely molecularly dehydrated phosphate salts in such mixture apparently has no beneficial effect on the desired reaction.

lt is an object of the present invention to provide processes for manufacturing alkali metal tripolyphosphates from mixtures or blends of completely molecularly dehydrated alkali metal pyrophosphate and trimetaphosphate salts, which mixtures or blends are substantially free of molecularly hydrated phosphate salts.

It is another object of the present invention to provide processes for improving the assay of completely molecularly dehydrated alkali metal tripolyphosphate products that are contaminated with alkali metal pyrophosphate and trirnetaphosphate salts.

These objects, as well as others which will become apparent from the following discussion and claims, can be attained by heating mixtures of completely molecularly dehydrated phosphate salts containing alkali metal pyrophosphates and alkali metal trimetaphosphates at a conversion temperature (of below the melting point of the alkali metal tripolyphosphate that results from reacting together the alkali metal pyrophosphate and trimetaphosphate salts in the mixture) in the presence of ammonium nitrate. The meaning of the terms molecularly dehydrated and molecularly hydrated are Well known in the art and need not be detailed here.

While the value of the processes of the present invention will perhaps be greatest where the raw material feed streams that are calcined in accordance with these proc- States Patent O esses contain only the py-rophosphate and trimetaphosphate reactants, or consist practically solely of mixtures of the pyrophosphate and trimetaphosphate reactants and alkali metal tripolyphosphate; completely molecularly dehydrated phosphate or polyphosphate can be present in the mixtures that are calcined in accordance with these processes without detracting substantially from all of the benefits that can be obtained by practicing the present invention. Generally, however, it is preferred that the mixtures that are useful in the practice of the present invention contain at least about 0.5 weight percent of tetraalkali metal pyrophosphate and at least about 0.25 weight per-cent of trialkali metal trimetap-hosphate. The term alkali metal is herein intended to encompass sodium, lithium, potassium, rubitlium, ccsium and francium. Mainly because of cost considerations, it is preferred that sodium and potassium phosphate salts be utilized and manufactured in the processes of the present invention. It is still further preferred that sodium phosphate salts be used and manufactured via these processes. Typical examples of completely molecularly dehydrated alkali metal phosphate salts include trisodium orthophosphate, tetrasodium pyrophosphate, pentasodium tripolyphosphate, trisodium trimetaphosphate, the sodium phosphate glasses (containing mixtures of sodium phosphates having from 3 to about 50 or more phosphorus atoms in their molecules) as well as the sodium ultraphosphates Other examples include the potassium, lithium, rubidium, ccsium and francium salts of tre aforementioned completely molecularly dehydrated phosphates.

Any means for heating the above-described completely molecularly dehydrated raw materials to a temperature within the aforesaid (calcining) conversion temperature range and held within this range for a period of time (if desired) can be utilized advantageously in the practice of this invention. Depending upon such factors as the purity of the tripolyphosphate product desired, the iniial assay of any tripolyphosphate material used as a raw material in the present processes, the rate of heating within the conversion temperature range, etc., it is often desirable to hold or maintain the temperature of the reaction products within the conversion temperature range for a significant period of time, although even extremely short exposures of the reaction mixtures of the present invention to conversion temperatures is beneficial. For optimum results in the particular aspect of this invention involving calcination of mixtures consisting practically solely of tetrasodium pyrophosphate and trisodium trimetaphosphate (plus ammonium nitrate), generally holding such mixtures at a temperature Within the preferred temperature range of from about 325 C. to about 500 C. for at least about 5 minutes is desired. Prolonged exposure to any conversion temperatures apparently has no deleterious effect on the quality of the resulting tripolyphosphate product.

While the presence of even extremely small amounts of ammonium nitrate in the calciner raw material feed streams of this invention is beneficial, generally, it is preferred that vat least about 0.5 weight percent, based upon the total weight of tetra-alkali metal pyrophosphate and trialkali trimetaphosphate in the completely molecularly dehydrated raw material `feed stream to be used. Although [generally no added benefits can be observed by using more than about 5 weight percent of ammonium nitrate, any amount can be used without detractintg substantially from all of the benefits that can result from practicing the processes of this invention.

ln the following examples, all parts and percentages given are by weight unless otherwise specified.

EXAMPLE I Into 5000 parts of a powdered mesh) conventional commercial sodium tripolyphosphate product containing about 94% of sodium tripolyphosphate, 2% of trisodium trimetaphosp'nate and 4% of tetrasodium pyrophosphate, and less than 0.5% of sodium orthophosphates are blended (in a conventional ribbon-type mixer) 100 parts of powdered (-l00 mesh) ammonium nitrate. One thousand parts of the resulting mixture is then heated in a mutlie furnace over a period of about 20 minutes to 460 C. and then immediately cooled. This is Sample I-A in Table I, below. Another thousand parts of the same mixture are treated similarly in the mutile furnace, except they are held at yabout 460 C. for an hour and then cooled. This is Sample I-B in Table I, below. Table I lists analytical data for these materials. A third sample, 1000 parts of the original commercial sodium tripolyphosphate, is treated just like Sample I-B, except that is contains no ammonium nitrate. This is Sample LC in Table I, below.

TABLE I An aqueous solution containing 2169 parts of tetrasodium pyrophosphate, 831 parts of trisodium trimetaphosphate and 7000 parts of water (a 30% solidsl slurry) is drum dried on a conventional stainless steel steam heated drum drier. Two thousand parts of the resulting dried product are first pulverized to pass through a G-mesh U.S. Standard screen and then heated in a slowly revolving conventional, indirectly tired rotary calciner (over a period of minutes) from about 25 C. to 450 C. The mixture is held yat this temperature for l5 minutes, and then cooled to room temperature. The resulting mixture contains the starting phosphates uncharged and essentially no sodium tripolyphosphate.

A similarly treated mixture containing the same ingredients, except that 1% of NH4NO3 is also blended thereinto, yields 89% sodium tripolyphosphate.

EXAMPLE III An aqueous slurry containing 5155 parts of tetrapotassium pyrophosphate, 1845 parts of tripotassium trimetaphosphate, 300 parts of ammonium nitrate, and 3000 parts of w'ater is spray-dried in a conventional spray tower. Two thousand parts of the resulting blend of salts are then calcined at a temperature of 500 C. for 15 minutes. The resulting cooled product contains 50% ot potassium tripolyphosphate. A similarly calcined blend of salts that do not contain any ammonium nitrate is found to contain essentially no tripolyphosphate.

4 EXAMPLE IV An intimate blend of iinely divided tetrasodium pyrophosphate (479 parts) tetrasodium pyrophosphate (396 parts) tripotassium trimetaphosphate (142 parts) land trisodium trimetaphosphate (184 parts) is calcined at a temperature of 550 C. for 10 minutes. Only 0.3% of tripolyphosphate is found in the resulting product. An identical blend containing, additionally, 3% of ammonium nitrate intimately intermixed therewith, which. iscalcined at the same time and in the same manner contains of trisodium dipotassium tripolyphosphate.

What is claimed is:

l. A process for manufacturing an alkali metal tripolyphosphate, which process comprises calcining a mixture of tetraalkali metal pyrophosphate, trialkali metal trimetaphosphate and ammonium nitrate at a conversion temperature below the melting `point of said alkali metal tripolyphosphate; said mixture being substantially free of molecularly hydrated phosphates.

2. A process as in claim 1, wherein said alkali metal is selected from the group consisting of sodium and potassiurn.

3. A process as in claim 1, wherein said alkali metal is sodium and said conversion temperature is wit-hin the range of from about 325 to about 620 C.

4. A process for improving the assay of a competely molecularly dehydrated sodium tripolyphosphate product contaminated with tetrasodium pyrophosphate and sodium trimetaphosphate, which process comprises subjecting an intimate mixture of said tripolyphosphate product with from labout 0.5 to about 5 weight percent b'asedton the weight of said tripolyphosphate product, of ammonium nitrate` to a temperature within the range of from about 350 to about 500 C. for at least about 5 minutes.

5. A process as in claim 4, wherein the sodium tripoly-` phosphate product which is contaminated with tetrasodium pyrophosphate and trisodium trimetaphosphate contains at least about 0.5 weight percent of said tctrasodium pyrophosphate and at least about 0.25 Weight percent of said trisodium trimetaphosphate.

References Cited UNITED STATES PATENTS 10/l957 Pfrengle 23-106 1/1957 Pfrengle 23-106