IL42828A - Process and apparatus for the manufacture of alkali metal polyphosphates - Google Patents

Abstract

1414369 Alkali metal polyphosphates RHONE-PROGIL 30 July 1973 [31 July 1972 14 June 1973] 36145/73 Heading C1A A process for preparing alkali metal polyphosphates having a predetermined crystalline form comprises, in a first stage, joining a mixture of alkali metal orthophosphates into spherical granules in the presence of water, the granules obtained having a molar ratio of alkali metal oxide/P 2 O 5 of from 1 to 2, drying the spherical granules at a temperature less than 120‹ C. and, in a second stage, heating the granules to form polyphosphates. It is preferred to prepare the granules from a mixture of mono and disodium orthophosphates, having a molar ratio of Na 2 O/P 2 O 5 close to 5/3, using a 50% by wt. aqueous solution of these salts to give granules of an average grain size of 0À1-3À15 mm. which are dried at 100‹ C. in, for example, a fluidized bed. The second stage of the process in which polyphosphates are formed is carried out either by (a) heating the dried granules in a fluidized bed or (b) introducing the dried granules into a fluidized bed by means of a pneumatic conveyer at a high temperature, in the presence in both cases of water vapour. In the case of processes (a) the average residence time of the granules in the bed is 5-30 mins. At a temperature of 300-400‹ C. in the bed granules of phase II tripolyphosphates are attained, whilst at temperatures of at least 480‹ C. phase I tripolyphosphates are obtained. The use of intermediate temperatures allows the formation of both phases in a predetermined manner. In the case of process (b) the dried granules are introduced into a pneumatic conveyer at a high temperature, e.g. 550‹ C. to give a residence time of a few seconds in the conveyer and 1-10 mins. in the following fluidized bed. The ranges of temperature in which phase I and phase II are formed are less high than in process (a). The water vapour content of the bed gases is preferably 14-100% by weight. [GB1414369A]

Claims (10)

1. A process for preparing alkali metal polyphosphates having a predetermined crystalline form* which comprises, in a first stage orming a mixture of alkali metal ortho- phosphates into spherical granules in the presence of water, the spherical granules obtained having a molar ratio of alkali metal oxide to phosphorous pentoxide from 1 to 2, drying the spherical granules and, in a second stage, heating the spherical granules of orthophosphates to a temperature sufficient to yield granules of polyphosphates having the desired crystalline form*

2. A process according to Claim 1, in which the polyphosphates obtained is sodium tripolyphosphate and the granules in the first stage comprise a mixture of mono- and disodlum ortho- phosphates having a molar ra ie Na20/P20j of about 5/3.

3. A process according to Claim 2, in vhich the granulation is achieved by treating the particles in the first stage with an aqueous solution of a mixture of mono- and diso&ium orthophosphates, having a molar ratio 3Ka20/P20j of about 5/3. ·

4. A process according to any one of Claims 1 to 3, in which , the granules have an average diameter from 0·1 to 3.15 mm, the granules are sieved after granulation and any oversize particles are crushed and returned to the granulator.

5. A process according to any preceding claim, in which the granules of orthophosphates are dried at a temperature not exceeding 120°C. 428!28/2

6. · A process according to any preceding claim, in which the granules of orthophosphates are heated by means of a fluidieed bed.

7. · A process according to Claim 6, in which the granules of orthophosphates are introduced into the fluidieed bed ^i means of a pneumatic conveyor at a high temperature.

8. A process according to Claim 7t appendent to in which the gases in the fluidieed bed are temperature between 360°C and 480°C and have a water vapour content from 14% to 100% by weight to obtain granulated tripolyphosphate having a form I content (as hereinbefore defined) which ie greater the higher the temperature of the fluidieed bed,

9. A process according to Claim 7, appendent to Claim 2, in which the gases in the fluidieed bed are at a temperature of at leaet 480°C and have a water vapour content from 14% to 100% to obtain granular tripolyphosphate substantially all of which is of form 1 (as hereinbefore de ined).

10.A process according to Claim 7, appendent to Claim 2, in which the gases in the fluidieed bed are at a temperature of not more than 360°C and have a water vapour content from 14% to 100% to obtain granular tripolyphosphate substantially all of which is of form XI ( as hereinbefore defined). - 20 - 2828/2 ·A process according to Claim 8, in which said gases have a water vapour content of 20% by weight and the proportion of tripolyphosphate of form I obtained is in linear relation to the temperature of the fluidised bed. .A process according to any one of Claim 7 to 11, in which the gas temperature in the pneumatic conveyor is from 400 to 600°C» .A process according to any preceding claim, in which the polyphosphate particles obtained are crushed. .A process according to Claim 1, substantially as hereinbefore described with reference to any one of the Examples* ·Apparatus for preparing alkali metal polyphosphates by the process according to any of Claims 1 to 14 having a predetermined crystalline form,/ comprising in combination a granulator, a drier and a fluidised bed for heating granules of orthophosphates obtained from the granulator and drier. .Apparatus according to Claim 15» having a pneumatic conveyor operating at a high temperature for conveying the orthephosphate granules from the drier to the fluidised bed. .Alkali metal polyphosphates, when prepared by a process according to any one of Claim 1 to 14. For the Applicants Dr.Yitzhak Hess