FR2510550A1 - Nitrogen phosphorus fertiliser granules - having a core of low hygroscopicity and a urea or ammonium nitrate outer layer - Google Patents

Abstract

Nitrogen, phosphorus, and opt. potassium-contg. fertiliser granules have a central core of less hygroscopic material, such as ammonium phosphate and potassium chloride, and an outer layer of ammonium nitrate or urea which constitutes 10-70% of the total wt.. Greater resistance to humidity. Since the core is less hygroscopic the granules retain their strength even under conditions of high humidity.

Description

NEW PELLET A COMPOUND FERTILIZERS
IMPROVED PROPERTIES, AND METHOD FOR OBTAINING THEM
The present invention relates to granular compound fertilizers with improved physical characteristics, and to a method for obtaining them. It also relates to a device for implementing the method.

 We know that when it comes to fertilizers, ammonium nitrate and urea are widely used salts. Unfortunately, they have the disadvantage of being hygroscopic.

 NP and NPK compound fertilizers containing either of these salts are generally even more hygroscopic. Thus, for example, at the temperature of 300C, the critical relative humidity of ammonium nitrate, that is to say the humidity of the atmosphere above which this salt absorbs significant quantities of water, is 59.4% for ammonium nitrate, it is 75% for urea, while it is only 45 to 50% for fertilizers of formula 17.17.17 or 18.22.12, the three figures representing the weight percentages of N, P205 and K20.

 The hygroscopicity of these compounds affects their bulk storage. In a humid atmosphere, the surface of the heaps absorbs water, and the fertilizer takes on a muddy consistency, while below, the partial humidification of the grains reduces their mechanical resistance until they fall to dust. Farther inside the heaps, the penetration of even a small amount of moisture causes the grains to resume mass.

 To avoid these drawbacks, in British patent 944 135, it has been proposed to first prepare ammonium nitrate in the form of small granules, then to impregnate these granules with phosphoric acid and to neutralize this acid by l 'ammonia, to obtain a binary NP fertilizer of formula 30.10.0 or 40.10.0.

 Bleak admitting that this way of enclosing each ammonium nitrate granule inside a low hygroscopic monoammonic phosphate capsule improves the physical characteristics of the fertilizer, the essential preliminary step of granulating nitrate would increase the cost of production, and the process could only be applied to binary fertilizers with a high N / P ratio, excluding NPK fertilizers.

 The same principle is found in American patent 3,348,938 (column 2, lines 40-44 and column 3, lines 48-64) which proposes to impregnate the granules with a layer of ammonium phosphate which is less likely to clot than ammonium nitrate, in order to improve the anti-clumping properties of the granules.

 In these processes, the guiding idea apparently consists in coating a nucleus of hygroscopic fertilizer with a less hygroscopic constituent, namely ammonium phosphate. Let us recall that the critical relative humidity of - phosphate K> sooioe0tique- is 91.6%, that of diammonium phosphate 82.8 X, that of mixture of these two salts of about 78%.

 Unfortunately, experience has not confirmed this hope; it has been found that the NPK fertilizers thus prepared were practically as hygroscopic as those obtained by introducing all the constituents into the granulator. The explanation is probably that ammonium phosphate crystallizes on the surface of the grains in a very porous form.

Now it has been observed that, unexpectedly, better characteristics and granular compound fertilizers containing nitrogen, phosphorus and possibly potassium are obtained, of better conservation, better resistant to humidity, silve grains include a central core containing the least hygroscopic constituents, for example ammonium phosphate and potassium chloride, said core being impregnated and coated with the more hygroscopic constituent 12, namely ammonium nitrate or urea, in particular in a proportion of 10 to 70% by mass
The product according to the invention can in particular be obtained by a two-step process which consists in continuously introducing, into a first zone 1 fertilizers in solid form, at least one fertilizers in liquid form, optionally an ammoniacal fluid , to mix them and to form aggregates of low water content, to transfer them continuously in a second zone 2 to make them fall in rain through a current of hot gases and to spray on them a second fertilizing substance in liquid form , said process being characterized in that the fertilizing material sprayed in zone 2 is a concentrated solution of ammonium nitrate or urea, so as to impregnate and coat the aggregates formed in zone 1 by said sprayed material, and at the same time drying the resulting product.

 The products obtained at the exit from zone (2) are sieved to obtain the desired size grains and the fraction of product of size smaller than the desired size is recycled at the entrance to zone 1, as well as the fraction that is too large. after grinding.

 Zone 1, the essential function of which is to mix and agglomerate the materials in the form of more or less regular aggregates, consists of an apparatus commonly called a "granulator": it is preferably a rotary drum of slightly inclined axis, it can also be a mixer with two shafts, an inclined rotary table, etc. When the granulator is equipped with a device for injecting ammonia under the bed of the solid product in motion, it is commonly called ammonia-granulator.

 Zone 2, the essential function of which is to impregnate and coat the aggregates coming from zone 1 with ammonium nitrate or urea, and to dry the regular grains thus obtained, is a rotary dryer drum common type, internally equipped with fins to lift the product and make it fall in rain.

 The ammonium nitrate sprayer or sprayers are placed inside the dryer, for example near one of the ends, the axis of the jet (s) is preferably substantially parallel to the axis of the dryer, although it is also possible to spray the concentrated solution on the product bed at the bottom of the dryer.

 The solid raw materials introduced into the granulator depend on the formula produced. Simple or triple superphosphates, potassium chloride or sulphate are commonly used, possibly a diluting filler, such as sand, gypsum, dolomite. It is also possible to introduce ammonium sulphate and mono- or di-ammonium phosphate in solid form.

 The liquid fertilizer material introduced into the granulator is generally a concentrated phosphate and / or ammonium sulfate slurry, obtained by neutralizing phosphoric acid and / or sulfuric acid with ammonia. It is particularly advantageous for this purpose to use a tubular reactor supplied under pressure with acid and ammonia, and to discharge directly into the granulator the jet of reaction mixture, consisting of water vapor and pulverized spray. based on diammonium phosphate, the flow rates of phosphoric acid and ammonia in the tubular reactor are adjusted so as to obtain an NH3 / H3PO4 molecular ratio equal to 1.3-1.4 at the outlet of this device, and injected additional ammonia in the granulator under the bed of rotating solid particles, so as to obtain the desired molecular ratio in the finished product, for example 1.6-1.8.

 A granulation installation is generally equipped with a gas washing system, to stop dust escaping from the cyclones, and to recover the ammonia released from the granulator and the dryer, using part of the acids used. The aqueous solution or suspension produced in the gas washing system, at a concentration as high as possible, is recycled either directly to the granulator or to the tubular reactor.

 The ammonium nitrate or urea solution sprayed in the dryer is at a concentration preferably greater than 90%, for example 94-96%. These salts can also be used in the almost anhydrous molten state.

 The attached figure schematically represents a granulation circuit making it possible to carry out the method according to the invention.

The solid raw materials and (9) the recycled solid product are introduced into the rotary ammoniator-granulator drum (1).

 In (10) the anhydrous ammonia, in the liquid or gaseous state, in - (11) phosphoric acid and / or sulfuric acid and in ( 12) the liquid effluent from the gas washing system.

 In the dryer (2), the concentrated solution of ammonium nitrate or urea (13) is sprayed using a sprayer (5) placed near the inlet. Hot air is blown into the dryer at (14).

 The product leaving at the other end of the dryer is introduced at the top of a double bridge sieve (6). The too large product (15) is ground in a grinder (7) and recycled in (9) in the granulator at the same time as the too fine product.

 The commercial size product, generally between 2 and 3.75 mm, is discharged in (16). It is cooled and coated before being sent to stock.

 Instead of introducing the liquid effluent from the washing system into the tubular reactor (4), part or all of it can be sent directly to the granulator, by the line (17) shown in dotted lines.

 If the quantity of liquid matter introduced into the granulator is insufficient to obtain the desired preliminary granulation, part of the concentrated solution of ammonium nitrate or urea is diverted towards the granulator, by the line (18) shown in dotted lines. .

 The distribution of the concentrated solution of ammonium nitrate or urea between the granulator and the dryer constitutes a very simple means of regulating the granulation, while maintaining a low recycling rate, close to 2 times the production for example. Depending on whether the aggregates leaving the granulator tend to become too fine, or too large, the flow of concentrated solution introduced into the granulator is increased or decreased, the rest is sprayed into the dryer.

 The fertilizer grains obtained by this process have exceptional physical qualities. They are regular in shape, almost spherical, dense and very hard. Their smooth surface makes it possible to reduce by half the quantity of coating powder and surfactant necessary to avoid solidification. In a humid atmosphere, they absorb water, but this does not penetrate to the heart of the grain, which retains a hard core. When the air becomes drier, grains of acceptable quality are found instead of the usual crumbling product.

 These interesting characteristics of the grains are attributed to the fact that the concentrated and hot solutions of ammonium nitrate or urea, thanks to their fluidity, completely fill the cavities of the porous aggregates coming from the granulator, and that the excess coats the core thus obtained of a dense and compact layer which resists the penetration of moisture. Examination with a magnifying glass of a grain section shows a stratified structure, with large KC1 crystals in the center and on the outside a regular and homogeneous layer highly enriched in nitrate or urea. The absence of large potassium chloride crystals at the periphery explains the sphericity of the grains, and prevents the creation of cracks promoting the penetration of moisture into the heart of the grain.

Example
A fertilizer corresponding to the formula 17.17.17 is obtained at a rate of 50 T / h in a granulation installation, using, per tonne of finished product
94 Z solution of NH4N03 and I300C .- .. 355 kg
Anhydrous liquid ammonia ... 65 kg
Wet phosphoric acid at 50 Z P205 ... 322 kg
Superphosphate at 18 Z of P205 soluble in ... 54 kg
citrate
Potassium chloride at 60 Z KH20 ... 284 kg
Coating products ... 6 kg
In the granulator, potassium chloride, superphosphate, and approximately 100 T / h of recycled product (fine and coarsely ground) are introduced in the solid state, twice the production. In the tubular reactor which feeds the granulator, phosphoric acid and about 75% of ammonia, the remaining 25 x being injected into the granulator under the bed of solid product.

 A part of the phosphoric acid, around 10 Z, passes through the gas washers in which water is also added to maintain the density of the recirculating liquid between 1.35 and 1.4.

The solution coming from the washers is introduced into the tubular reactor at the same time as the phosphoric acid.

 The ammonium nitrate solution is sprayed in the dryer at a pressure of 2.5 kg / cm2 using a sprayer giving a narrow jet in full cone, at high impact force, the angle at the top of the cone projection being about 150.

 The finished product obtained by sieving the grains leaving the dryer has a size between 2.25 and 3.75 mm. The crushing resistance of the 3 mm diameter grains is between 8 and 10 kg. The bulk density of the product is 1.03 kg / dm3. Their grain surface is smooth and their shape is almost spherical.

Crush resistance and bulk density were measured under the conditions set out in bulletin Y 147 of October 1979 from the National Fertilizer Development Center, Tennessee
Valley Authority - MUSCLE SHOALS, ALABAMA 35660, Crushing Strength page 9 and Bulk Density, page 11.

 When exposed to a humid atmosphere, the product gradually wets, but each grain retains a hard core: when the quantity of water absorbed reaches 5 g, the grain still has a crush resistance of 1 kg.

 By way of comparison, if the ammonium nitrate solution is introduced into the granulator in the same installation, as is the practice in the prior art, for the same recycled flow rate of 100 T / h, the hourly production is only 25 T / h, because the large quantity of liquids introduced into the granulator requires recycling at least four times the production.

 The grains obtained after drying and sieving are more or less regular, with protuberances which promote solidification and the formation of dust. Their crush resistance is only 3 to 5 kg, their bulk bulk density is 0.95 kg / dm.

 After exposure in a humid atmosphere, the grains having absorbed 5% of moisture no longer offer any resistance to crushing.

Claims (5)

CLAIMS 1 ") Granular compound fertilizers containing nitrogen, phosphorus and possibly potassium, characterized by the fact that they comprise a central core containing the least hygroscopic constituents, said core being impregnated and coated with the most hygroscopic constituent from the group of ammonium nitrate and urea nota vent in a proportion of 10 to 70 Z by mass. 2 ") Process for obtaining fertilizers according to claim 1, in two stages which consists in continuously introducing , in a first zone 1 of fertilizers in solid form, at least one fertilizer in liquid form, optionally an ammoniacal fluid, to mix them and to form aggregates of low water content, to transfer them continuously in a second zone 2, to make them rain through a stream of hot gases and to spray on them a second fertilizing substance in liquid form, characterized in that the fertilizing material pulverizes risée in zone 2 is a concentrated solution of ammonium nitrate or urea, so as to impregnate and coat the aggregates formed in zone 1, and at the same time to dry the resulting product.  30) Method according to claim 2, characterized in that the products obtained at the outlet of the zone (2) are sieved to obtain the grains of desired size and that the fraction of product of size smaller than the desired size is recycled to The entry of zone 1, as well as the excessively large fraction, after grinding.  40) Method according to one of claims 2 and 3, characterized in that the liquid fertilizing material introduced into zone 1 is obtained by neutralizing sulfuric and / or phosphoric acid with ammonia, so as to obtain a ammonium sulphate and / or mono or diammonium phosphate slurry.  50) Method according to one of claims 2 to 4, characterized in that the solid raw materials introduced into zone 1 are from the group of simple or triple superphosphates, potassium chloride and sulfate, ammonium sulfate and mono and di-ammonium phosphate, and diluent fillers.  6) Method according to one of claims 2 to 5, characterized in that the liquid material sprayed in the zone (2) consists of a solution with a dry matter concentration greater than DO% by weight and preferably between 94 and 96%.  70) Device for implementing the method according to one of claims 2 to 6, characterized in that it comprises a granulator (1), a rotary dryer drum (2) and a sprayer (5) placed at the entry of the rotary dryer drum (2).  80) Device according to claim 7, characterized in that it has a tubular reactor (4) which opens into the granulator (1) so as to neutralize the acids in this reactor with ammonia, and possibly to introduce therein. liquid effluent from the gas washing system.