IL33943A - Process for preparing granular fertilizers - Google Patents

Description

D"T» i Ο «Π naart^ » ηπ Process for preparing granular fertilizers PI30NS LIMITED 0: 52252 The present invention relates to the preparation of granular fertilizers using ammonium phosphate having an N:P atomic ratio in the range 0.8:1 to 0.95:1 as one of the raw materials.

A common method of forming particulate monoammonium phosphate into granules is to tumble or agitate the particulate monoammonium phosphate and such other materials which are also to be incorporated into the granules in the presence of water at an elevated temperature. The term granulation is used herein and in the claims to denote in general processes wherein particulate materials are tumbled or agitated in a vessel in the presence of a liquid phase so as to form larger particles containing the materials. The monoammonium phosphates used hitherto have had N:P atomic ratios of as near 1:1 as possible.

We have now found that granulation is facilitated if particulate ammonium phosphate having an N:P atomic ratio in the range 0.8:1 to 0.95:1 is used and if the mixture being granulated is treated with ammonia during granulation,since the reaction heat given off during reaction of the ammonia with the residual free acid in the ammonium phosphate feed leads to an increase in bed temperature and thus aids granulation.

Accordingly the present invention provides a process for preparing granular fertilizers containing an ammonium phosphate which comprises granulating a particulate material containing ammonium phosphate having an initial N:P atomic ratio in the range of 0.8:1 to 0.95:1, in the presence of water at an elevated temperature, and treating the particulate material with ammonia during granulation. Preferably the granular product is then dried. The particulate material may consist solely of the ammonium phosphate, although it is usually preferred that it contains other ingredients. Suitable other ingredients include nitrogen compounds, e.g. ammonium nitrate, potassium nitrate, ammonium sulphate and urea; phosphorus compounds, for example potassium —metaphosphate,-phosphate rock and -superphosphates ; potassium compounds, e.g. potassium chloride and potassium sulphate; trace elements; fillers and the like. Desirably the particulate material is a mixture of ammonium phosphate with potassium chloride and ammonium nitrate or urea. As indicated below, part of the particulate material may be provided by recycled product.

As indicated above, the ammonium phosphate in the particulate material has an :P atomic ratio of less than 0.95:1, eg. less than 0.94:1, referably in the range 0.85:1 to 0.94:1.

Where a mixture of materials is to be used, this mixture may be formed in situ within the vessel where granulation is to take place. Alternatively, some or all of the ingredients may be mixed together before they are fed to the vessel. As indicated " a1x>vev~it'may bt desired to recycle part of the~granuiar product so as to produce a desired mixed fertilizer product.- = The presence of water during the granulation process of the invention is essential since it controls and promotes granulation. Where dry feed materials are used, the water may be added as a separate ingredient either as a spray onto the granulating material or by passing steam into the material from underneath.

Alternatively, part or all of the water may be provided by using damp feed materials or aqueous suspensions or concentrated solutions of some or all of the feed materials. The overall initial water content of the material to be granulated is desirably f om 0.3 to 5% by weight of the solid material present. This may be achieved by, for example, using monoammonium phosphate containing from 3 to 10% by weight of water mixed with a drier feed material, e.g. recycled over-fine product.

The particulate ammonium phosphate feed material may be obtained by a number of ways involving the treatment of phosphoric acid with ammonia. For instance phosphoric acid may be treated with the appropriate amount of ammonia in a series of tanks and the product from the last tank dried >for example in a rotary drier. Another method involves the treatment of phosphoric acid with ammonia to give an ammonium phosphate having an N:P atomic ratio of about 1.4:1which is then treated with acid with concurrent evaporation of water to give a particulate ammonium phosphate having an N:P ratio suitable for use in the present invention. The preferred method of operation is to treat phosphoric acid, containing less than 60% by weight of with ammonia gas under superatmospheric pressure, preferably at a pressure of 15 to 50 pounds per square inch gauge,to give a fluid ammonium phosphate solution at its boiling point containing 4% to 15% by weight of water and expelling the fluid ammonium phosphate solution into a zone at ambient pressures, for example into the mixture being granulated, thereby reducing its water content and giving a solid ammonium phosphate product.

The temperature during granulation is preferably between 30° and 100°C. The water content of the mixed material will vary with temperature during the granulation, being for example 3.0% to 3.5% at the lower temperature and from 0.3 to 1.0% at the higher temperatures.

The material is treated with ammonia during granulation and this is desirably carried out so that the ammonium phosphate in the product has an N:P atomic ratio in the range 0.95:1 to 1.9:1 for example in the range 0.97:1 to 1.05:1.

The ammonia may be added in any suitable form, e.g. as gaseous ammonia or as liquid ammonia.

It has been found that the addition of phosphate rock in powder form or superphosphate, for example single superphosphate, to the material which is to be granulated considerably improves the granulation characteristics of the mixture and also promotes the formation of harder granules.

According to a preferred embodiment of the invention, therefore, rock phosphate or superphosphate in a powdered condition is present in the material being granulated. The amount of rock phosphate or superphosphate used is preferably in the range 1% to 10% by weight based on the total weight of the material.

The process of the invention may be carried out in any suitable form of apparatus. Preferably the material is granulated in a rotating drum.

As indicated above, the product of the granulation process of the invention may be dried if desired. Where drying is carried out, this is desirably done at a temperature of below 120°C, e.g. at about 110°C, and may be effected by, for example, passing hot gases through a vessel, : e.g. a rotating drum, in which the product is tumbled or agitated.

After the product has been dried to the desired extent, it is preferred to cool it to below kS°C , before storage.

It will usually be desirable to classify the granular product to obtain a final product of the desired particle size. The under-size material or ground over-size material is desirably recycled to the granulation stage in order to achieve a satisfactory water content, since the recycled product will usually contain a very small amount of water, e.g. less than 1%, and may be used to achieve the desired water balance where very wet feed materials are used. If need be, on-size granular product may be comminuted to provide further recycle material.

The following Example is given to illustrate the process of the present invention, all parts and percentages being by weight unless otherwise stated: Example A stirred tank reactor operated at a pressure of 33 pounds per square inch gauge was continuously fed with phosphoric acid (49% P2O5) and ammonia at a rate sufficient to maintain the pH of the reactor contents at 2.8, which was in this case equivalent to an N:P atomic ratio of 0.9:1. Under these conditions a solution of anmonium phosphate containing 12% by weight of water at its boiling point of 168°C was produced. The desired reaction temperature and water content were maintained by a small flow of water into the reactor. The product from the reactor was sprayed into a void tower where it solidified to a powder containing about 6% by weight of water. The powder was recovered from the bottom of the tower and conveyed to a rotating drum type of granulator. Also fed to the granulator were ammonium nitrate solution containing 7% by weight of water at 135°C. , potassium chloride, 3% by weight of triple superphosphate, recycle product material, and gaseous ammonia.

The quantities of these materials were such as to give an Ν:Ρ„0.-:Κ_0 ratio of 17:17:17 and the amount of ammonia was i o I such as to give a N:P atomic ratio of 1:1 in the ammonium phosphate of the product. The water content of the mixture during granulation was 2 to 3% and the temperature was 55 - 65°C. The granules so obtained were dried at 110°C to give a 17:17:17 product having a water content of 0.3%.

The pH as measured in this example was that of a % solution in water.

Claims (12)

VJhat we claim is:

1. ) A process for preparing granular fertilizers containing an ammonium phosphate which comprises granulating a particulate material containing ammonium phosphate having an initial N:P atomic ratio in the range of 0.8:1 to 0.95:1 in the presence of water at an elevated temperature and treating the particulate material with ammonia during granulation.

2. ) A process as claimed in claim 1 wherein the ammonium phosphate has an initial N:P atomic ratio of from 0.85:1 to 0.94:1.

3. ) A process as claimed in either of claims 1 or 2 wherein the particulate material also contains potassium chloride and ammonium nitrate or urea.

4. ) A process according to any of the preceding claims wherein powdered phosphate rock or superphosphate is present in the material being granulated.

5. ) A process according to claim 4 wherein the phosphate rock or superphosphate is present in frcml to 10% by weight of the total weight of the material being granulated.

6. ) A process according to any of the preceding claims wherein the overall initial water content of the materials being granulated is from 0.3 to 5% of the solid material present.

7. ) A process according to any of the preceding claims wherein the treatment with ammonia is carried out so that the ammonium phosphate in the product has an N:P atomic ratio in the range 0.95:1 toi.9;i^

8. ) A process according to any of the preceding claims wherein the particulate material being ^anuldteS contains recycled product.

9. ) A process according to any of the preceding claims wherein the final product is dried.

10. ) A process according to claim 1 substantially as hereinbefore described.

11. ) A process according to claim 1 substantially as hereinbefore described in the Example.

12. ) A granular material containing ammonium phosphate whenever prepared by a process as claimed in any of the preceding claims.