Background of the Invention Urea superphosphate is made from urea, which is a cheap source of nitrogen, and from phosphate rock as a cheap source of P205. The powder or granules of urea superphosphate can be used as fertilizer. The urea superphosphate can also be granulated with other fertilizers, such as KC1, K2S04, MAP or DAP (monoammonium or diammonium phosphate), to obtain the desired NPK (nitrogen-phosphorus-potassium) compounds.

One of the advantages of the present invention is the production of urea superphosphate that in bulk blending is compatible with fertilizers such as urea and triple superphosphate (TSP).

USP 5,409,516 discloses a method of making phospho-nitrogenous products in two stages: first, preparing a sulfo-ureic reagent by reaction of concentrated sulfuric acid with urea, and secondly, reacting the sulfo-ureic reagent with tricalcium phosphate. The combination of urea with sulfuric acid, giving mono-ureic and di-ureic sulfate, is exothermic, so that the preparation of the sulfo-ureic reagent needs temperature control and heat exchanger means. The reaction between said reagent and the tricalcium phosphate forms tetra-urea calcium sulfate, which makes the product free of excess urea. First stage of this process-preparation of the sulfo-ureic reagent-besides requiring controlling conditions, is prohibitively expensive.

It is a purpose of this invention to provide a simpler and more economical process for the preparation of urea superphosphate.

It is another purpose of this invention to provide a single-stage process for the preparation of urea superphosphate.

It is a further purpose of this invention to provide such a process that does not require heat exchange and therefore can be carried out with simpler apparatus.

It is a further purpose of this invention to produce urea superphosphate that is compatible with fertilizers, such as urea and triple superphosphate (TSP).

It is a still further purpose of this invention to produce urea superphosphate as a powder or granules.

It is a still further purpose of this invention to use urea superphosphate as a raw material in the production of powdered and granulated NPK fertilizers.

Other purposes and advantages of the invention will appear as the description proceeds.

Summary of the Invention The process according to the present invention comprises reacting ground urea and ground phosphate rock with sulfuric acid in a single stage, in the presence of gypsum or phosphogypsum. Optionally, phosphoric acid can be added to the reaction mixture. The reagents are preferably added simultaneously to a reactor.

The urea is ground to an average granule size of not more than 3 mm and preferably not more than 1 mm. Preferably, the main fraction, viz. 80-90% by weight, of the ground urea has a granule size of less than 1 mm. The phosphate rock is also ground, preferably to such an extent that about 90% by weight thereof is reduced to a size of less than 0.15 mm.

The weight ratio (all ratios in this description and claims are by weight, unless otherwise indicated) of P205 to urea moieties depends on the composition of the final product desired.

Gypsum or phosphogypsum is added in an amount from 2% to 10%, and preferably about 6%, of the total weight of the raw materials, calculated as dry calcium sulfate dihydrate. It is added in the reaction stage as a binder to obtain the desired physical properties of the powdered urea superphosphate and the granulated urea superphosphate and to bind unreacted urea molecules, if such are present.

The ratio of sulfuric acid to urea depends on the desired final product composition. E. g., if the product is urea superphosphate 20-10-0, said ratio is about 3.

Water may be added to the reagents in order to obtain the desired humidity in the urea superphosphate powder finally obtained, but has no influence on the reaction. In general, it is from 4% to 14%.

The reaction is carried out at temperatures from 50 to about 110°C and preferably from 70 to 90°C. The duration of the reaction is typically from 0.5 to 6 minutes.

Detailed Description of Preferred Embodiments According to the invention, urea superphosphate (hereinafter, USP), is produced from rock phosphate, urea, sulphuric acid, and gypsum or phosphogypsum, with or without the addition of phosphoric acid. The following examples relate to the production of USP powder and granules with the composition 20-10-0 (20% N, 10% P205 and 0% K20), but this should not be construed as a limitation, since USP can be produced, according to the invention, with different compositions. Urea superphosphate (hereinafter, USP), produced according to the invention, can be granulated or bulk-blended or granulated with other fertilizers.

In the following examples, it is assumed that the phosphoric acid used was produced by a method called Wet Process Phosphoric Acid (WPA), which generates phosphogypsum as a waste. This method is described e. g. in Pierre Becker-Phosphates and phosphoric Acid, Marcel Dekker, Inc., 1993. Phosphogypsum contains about 98% of calcium sulfate dihydrate. As stated hereinbefore, calcium sulfate dihydrate (gypsum) or monohydrate or anhydrous calcium sulfate could be used in place of phosphogypsum.

Phosphogypsum is a convenient raw material and therefore the invention will be described with reference to it. It is useful as a binder for the excess urea molecules and as a binder between particles of the urea superphosphate in the production of the powder both in the reaction and in the granulation stages. Gypsum could also be used and would have the same functions. It was also found that phosphoric acid could be added to increase the P205 concentration in the product, and to compensate the effect of dilution caused by the addition of the gypsum or phosphogypsum.

The phosphogypsum used in the examples contained about 98% CaS04. 2H20 with 0.7% ground rock phosphate and 0.4% phosphoric acid, but in other cases the concentration of ground rock phosphate can vary from 0.2 to 1.0% and that of phosphoric acid can vary from 0.2 to 1%.

The process of the invention is preferably carried out as follows. The raw materials-ground phosphate rock, urea, gypsum or phosphogypsum, sulfuric acid, with or without phosphoric acid and water-are added simultaneously to a reactor, e. g. a Khulman type reactor, or a pug mill.

The urea has been previously ground to grain size : 3 mm, preferably < 1 mm. In these examples, the granulometry of the urea is: 5% +2mm, 6% +1 mm, and 89% lmm or less. The temperature of the reactor is about 70°C.

The reaction product solidifies after about a minute or two. The resulting powder of urea superphosphate, has a humidity of 5.5-6.5% and generally comprises granules of sizes up to about 2 mm, is not hygroscopic, and has good friability and storage properties.

Granulation of the said powder, to granules of sizes generally about 2 to 5 mm, could take place immediately after the reaction product has solidified, or after curing for several days. It is carried out in several stages, wherein a recycle to each stage from the previous stage depends on the composition of the NP fertilizer. For NP 20-10, the granulation was made by using fresh urea superphosphate and recycle of the same from the previous granulation stage, in a weight ratio of about 1: 1. The humidity in the mixture of fresh material and recycle is about 5%. The granulation is sensitive to the addition of water. The granulation is carried out at a temperature from 40 to 80°C, preferably at about 50°C, this temperature being achieved by the addition of steam or, preferably, of hot water. The humidity of the granules from the granulator was kept between 5.5 and 6%.

In some of the NPK (nitrogen-phosphorus-potassium) formula, pH correction is needed in order to obtain the desired physical properties. This can be done by adding a base, preferably, ammonia.

The granules thus obtained are then dried to obtain a granular humidity of about from 1% to 2%. The drying of the granules is carried out with hot air at about 80°C to 90°C. The temperature of the granules does not exceed 70°C. The dried granules are screened to obtain a product size of 2-5 mm. The +5 mm fraction is crushed and used together with the-2 mm fraction as a recycle for the next granulation.

To examine the compatibility of the product of the process of the invention with urea, the dried product granules were put in a humidity chamber at 70% humidity and 25°C for several hours. Then they were mixed with urea at a weight ratio of 1: 1. The mixture was placed in a furnace and heated at 50°C for several days. The result showed that the product did not react with the urea. It was further found that the product, which did not contain an excess of urea, was compatible with TSP (triple superphosphate).

The powder urea superphosphate was granulated with KC1, TSP, SSP, MAP and ammonia to yield NPK fertilizers, such as 8-24-8 or 12-12-12.

By the same method, other compounds of NPK, such as 15-15-15, could be produced.

Example 1 A solid mixture of 344 g of ground phosphate rock (32.2% P20s), 445 g of ground (-1 mm) urea (44,2% N) and 63 g of phosphogypsum was added to the reactor. Simultaneously were added 196 g of 98% H2SO4 and 63 g of water. The temperature in the reactor reached about 80°C. The urea superphosphate product in the reactor solidified after one minute, and contained about 8% moisture.

After one week of curing, the powder was granulated in rotary drum. On granulation, the weight ratio of feed (powder of urea superphosphate) to recycled product (fine product from the previous granulation) was 1 to 1.

The reaction temperature was about 50°C, and was achieved by addition of steam. The wet granules contained moisture of about 11%. The granules were dried at 80-90°C to a moisture content of 1-2%. The dried product contained 20.1% N, 11.4% total P205, 8.4% citrate soluble P205, 9.1% water soluble P20s and 2% free acid as P205.

The granules were compatible with urea.

Example 2 Example 1 was repeated, but with the addition to the reagents of phosphoric acid, which in this case was obtained by the aforementioned WPA process, in order to increase the amount of citrate soluble P20s and of water soluble P205 in the product.

The quantities of the raw materials that were introduced to the reactor were: 588 g of ground phosphate rock (32.2% P20s), 890 g of ground urea, 120 g of phosphogypsum, 320 g of 98% H2SO4, 84 g of WPA (50% P20s) and 107 g of water. The reaction temperature reached about 75°C.

Solidification of the reaction product occurred after 1 minute. Moisture content was about 4%.

Again, the powder, after one week of curing, was granulated in a rotary drum. The weight ratio of the feed to the recycled product was 1 to 1. The temperature of about 50°C was achieved by the addition of steam. The wet granules contained about 8% moisture. The granules were dried at 80-90°C to a moisture content of about 1%.

The product contained 21.1% N, 12.3% total P20s, 9.2% citrate soluble P20s, 10% water soluble P205, and moisture content of 1.5%.

Again, the product was compatible with urea.

Example 3 Example 2 was repeated, except that the reaction was carried in a pug mill. The following raw materials were fed at the following rates: 400 g/minute of solid mixture containing ground phosphate rock, ground (-1 mm) urea and phosphogypsum (ratio as in Example 2);.

130 g/minute of liquid mixture containing H2SO4, WPA and water (ratio as in Example 2).

The reaction temperature in the pug mill was about 70°C, residence time was about 1 minute, and solidification occurred within two minutes.

Powdered urea superphosphate was collected for about two hours.

Analysis of the urea superphosphate: 20.2% N, 12.2% total P205, 9.11% water soluble P205, 1.7% free acid as P205, moisture content 3.1%.

Although examples of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, without departing from its spirit, or exceeding the scope of the claims.