GB1570368A - Manufacture of soil conditioners/fertilizers containing colloidal silicic acid and phosphate - Google Patents

Description

(54) MANUFACTURE OF SOIL CONDITIONERS/FERTILIZERS CONTAINING COLLOIDAL SILICIC ACID AND PHOSPHATE (71) We, GUANO-WERKE AG, a Joint Stock Company of Spitaler Strasse 10, 2000 Hamburg 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to a process for the manufacture of granular soil conditioners/fertilizers containing colloidal silicic acid.

It is known to apply sodium silicates to the soil in order to improve its capacity for retaining water and nutrients. To this end, sodium silicate is suspended in water, stabilized, if necessary, by organic substances such as urea, and applied to the soil.

Combined soil conditioners/fertilizers are known which in addition to colloidal silicic acid contain fertilizers, in particular mineral fertilizers. The manufacture of such soil conditioners/fertilizers, which is effected by mixing alkali metal silicate solutions with the fertilizer salts, frequently presents difficulties, since nitrogenous fertilizers are not stable at high pH's, whilst phosphate-containing fertilizers are converted to insoluble forms at alkaline pH's, with the result that they can no longer be taken up by plants.

German Published Application DAS 1,592,669 describes a method of overcoming this drawback which consists in increasing the viscosity of the alkali metal silicate solution to at least 7,000 cp by the addition of inert salts followed by reaction of the alkali metal silicate solution with solid mineral fertilizers in a mixing apparatus and simultaneous agglomeration of the product without further drying. This method permits the manufacture of small batches only, which must be used quickly on account of their poor stability.

According to another conventional process described in German Laid-Open Application DOS 2,208,917, scatterable soil conditioners containing silica gel and urea are produced by spraying mixtures of finely divided alkali metal silicates, containing from 15 to 22 wt. % water, and from 2 to 10 wt. % urea with acids or solutions or dispersions of acid salts while mixing, the pH in the final product being adjusted to 2 to 6 and the water content from 6 to 14 wt. %. Soil conditioners thus prepared again have poor storage stability.

The present invention seeks to provide an improved process for the manufacture of granular soil conditioners/fertilizers containing colloidal silicic acid and phosphates, which gives a product that is distinguished by good storage stability over long periods.

According to the present invention there is provided a process for the manufacture of a granular soil conditioner/fertilizer containing colloidal silicic acid and phosphate, wherein sodium silicate is reacted with an aqueous solution of monosodium dihydrogen phosphate and/or monopotassium dihydrogen phosphate and/or phosphoric acid in a granulating stage in the presence of agitated granules of recycled dried soil conditioner/fertilizer, the reactants being used in such proportions that the granules obtained have a pH of from 3.5 to 11, as measured on a 10% w/w aqueous slurry, and the product is passed from the granulating stage and dried in a drying stage.

In general, the recycled granules consist of fines and oversize granules separated after drying the product, the fines having a particle size, e.g., of below 0.5mm and the oversize granules a particle size, e.g. of above 2.0mm.

The process of the invention may be carried out in conventional granulating apparatus, for example drums, discs and fluidized-bed units. However, drums having longitudinal shovel-type vanes on their inner wall are particularly advantageous. Such drums are rotated at a speed which enables the solid material introduced into the drum to be transported to the top of the drum by the vanes, from which point it falls in the form of a curtain spread over the length and width of the drum. The phosphate and/or phosphoric acid and, if desired, other reactants in solution or suspension are sprayed onto this curtain. The heat of neutralization produced by the reaction of the phosphate and/or phosphoric acid with the alkaline reactants causes evaporation of the water introduced and there is immediately produced a moist granular product.

Phosphoric acid is the preferred reagent. The phosphoric acid is conveniently used in the form of a commercial 50 to 75% w/w acid. The sodium silicate required for the reaction may be introduced at the granulating stage either in the solid form together with recycled material or in the form of an aqueous solution or suspension. Other alkaline-reacting components which may be introduced are aqueous solutions of sodium or potassium hydroxide.

It is also convenient to add organic nitrogen compounds which act as aging retardants for the colloidal silicic acid formed during the reaction and which also ave fertilizing properties. Particularly suitable is urea which is usually used in amounts of from 1 to 5% by weight, based on the finished product. The urea may be used in the form of an aqueous solution or in solid or molten form.

Sodium silicate together with phosphoric acid and/or potassium or sodium dihydrogen phosphates and optionally potassium or sodium hydroxide are used in amounts such that the pH of the granules, as measured on a 10% w/w aqueous slurry, is from 3.5 to 11 and preferably from 6 to 9.

The ingredients are advantageously used in proportions such that the granules have a content of SiO2 of from 30 to 50% w/w, of P205 of from 8 to 20% w/w and of Na20 of from 10 to 25% w/w.

The concentration of the solutions introduced at the granulating stage is preferably such that the evaporation of the water introduced by the heat of neutralization gives a product having a water content of not more than 30% by weight.

The temperature at the granulating stage is usually from 40 to 700 C.

The granular product may then be dried in a drying drum at product temperatures of from 90 to 110C.

A process within the invention is described below with reference to the accompanying diagrammatic drawing: Charging containers 1,2,3 for fresh solid ingredients, sodium silicate, phosphates and if desired urea supply a mill 4.

Returns consisting of tails from a coarse-mesh sieve 9 and fines from a fine-mesh sieve 10, are ground in the mill 4 together with the ingredients freshly introduced through line 12, and the resulting mixture is passed to a granulating drum 7 provided with lifters. As the drum rotates, these lifters produce a dense curtain of solids having a particle size of from 0.1 to 2 mm, which curtain is evenly distributed over the cross-section of the drum.

Into this curtain there are then introduced, via lines 5 and 6, aqueous phosphoric acid, eg. 50 to 75% w/w H3PO4, and aqueous caustic soda, eg. 40 to 50% w/w aqueous NaOH. The resulting granules are dried in a drying drum 8 at temperatures of from 90 to 1 100C, the oversize and fines fractions being separated in sieves 9 and 10, respectively, and returned via line 12 and mill 4 to the granulating drum 7. The product is cooled in cooling drum 11, from which it passes to the hopper 13.

The special conditions maintained during the process produce a porous product which, as regards soil efficiency, is better than the products prepared by prior art processes in the slurry phase or in suspension.

EXAMPLE 1 4,000 kg/hr of returns consisting of fines and tails (below 0.5 mm and above 2.0 mm) and 1,352 kg/hr of sodium silicate spray concentrate a solid product prepared by spraying and containing (64% w/w of SiO2, 19% w/w of Na20, remainder H20) are milled together in a continuous stream. The particle size is 2 mm.

Appropriate conveyers pass the finely ground mixture of returns and sodium silicate to a granulating drum which is 5 m in length and has a diameter of 2 m. Lifters are provided along the inside wall of the drum, which lifters convey the material to the top of the drum as the latter rotates, from which point the material falls cascade-fashion to form a curtain distributed over the cross-section of the drum. The drum is rotated at a rate of 5 rpm. Into this curtain there are then sprayed, through nozzles, 1,208 kg/hr of aqueous sodium silicate solution (27% w/w of Six2, 8.1% w/w of Na20, 64.9% w/w of H20), 536 kg/hr of aqueous 50% w/w caustic soda solution, 160 kg/hr of 50% w/w urea solution and 1,136 kg/hr of 73%w/w commercial phosphoric acid.

The reaction between the phosphoric acid and the caustic soda and the solid and dissolved sodium silicate causes water to evaporate. The vapors are sucked off. The reac tion mixture is passed through a cooling drum having a length of 8 m and a diameter of 2 m, which drum is provided with lifters and buffles, cooling being effected by means of air sucked in countercurrently. A total of 530 kg/hr of water vaporizes in the two drums.

The material, now cooled and freed from adhering moisture, is screened on vibratory sieves. The fines below 0.5 mm and the tails above 2 mm are returned to the process and reground.

There are obtained 400 kg/hr of finished product. The water content is from 25 to 28% by weight, depending on the quality of the phosphoric acid used. It is mainly in the form of water of crystallization combined with sodium phosphate.

Analysis: 33% w/w of SiO2, 15% w/w of Na20, 15% w/w of P205, 0.9% w/w of N, 25 to 28% w/w of H20.

pH of a 10% w/w slurry is 9.5 and the ratio of SiO2: NaO is 2.2:1, by weight.

Sacks filled with the test product are stacked to a height of 12 sacks. The product is still free-flowing after a period of 6 months.

EXAMPLE 2 Using the same arrangement as described in Example 1, and in a similar manner, 4,000 kg of returns, 1,640 kg of sodium silicate spray concentrate (64% w/w of SiO2, 19% w/w of Na20, remainder H20), 180 kg of urea, 1,465 kg of sodium silicate solution (27% w/w of SiO2, 8.1% w/w of Na20, remainder H20), 650 kg of 50% w/w aqueous caustic soda and 1,375 kg of commercial phosphoric acid having a content of 53% w/w of P205 are reacted per hour. Vaporized water is sucked off. The reaction mixture is then partially dehydrated in a drying drum by means of a countercurrent of hot combustion gases having a temperature of 250"C. A total of 1,480 kg of water is evaporated per hour. After screening (0.5 to 2.0 mm particle size), 4,000 kg/hr of product are obtained, this then being cooled.

Analysis: 40% w/w of SiO2, 18.0% w/w of Na20, 18% w/w of P205, 0.9% w/w of N, 14% w/w of H20 (combined in the form of water of crystallization).

The pH of a 10% w/w slurry is 9.5 and the ratio of SiO2 to Na20 is 2.2:1, by weight Sacks filled with the product are stored in stacks of 16 sacks. The product is still freeflowing after a period of 9 months. A similar product prepared by mixing alkali metal silicate with phosphate fertilizer goes completely hard when stored for 2 weeks under the same condition.

EXAMPLE 3 Using the arrangement described in Example 1, 3,000 kg of returns, 2,820 kg of sodium silicate spray concentrate (64% w/w of SiO2, 19% w/w of Na20, remainder H20) and 100 kg of urea are sprayed with 1,600 kg of commercial phosphoric acid having a P205 content of 53% w/w, per hour. The vapors are sucked off. The granules pass to the drying drum, where they are dried at about 110 C (solids temperature).

A total of 500 kg/hr of water is vaporized in the two drums. After screening and cooling, 4,000 kg/hr of product are obtained.

Analysis: 45% w/w of SiO2, 13.4% w/w of Na2O, 20% w/w of P205, 1.1% w/w of N, 7 to 10% w/w of H20 (combined). The pH of a 10% w/w slurry is 7 and the ratio of SiO2 to Na2O is 3.35:1 by weight.

The storability is the same as that of the product prepared according to Example 2.

EXAMPLE 4 Using an arrangement as described in Example 1, 6,000 kg of returns, consisting of fines, intermediate material and tails, after passing through the mill together with 3,340 kg of sodium silicate solution (27% w/w of SiO2, 8.1% w/w of Na2O, 64.9% w/w of H20), are sprayed with 756 kg of commercial phosphoric acid having a P205 content of 53% w/w and 150 kg of 35% w/w aqueous urea solution, per hour.

The granules are passed through the drying drum, where they are dried at temperatures of about 110 C (solids temperature). A total of 2,000 kg/hr of water is vaporized in the reaction drum and drying drum.

After screening and cooling, 2,000 kg/hr of finished product are obtained.

Analysis: 45% w/w of SiO2, 13.4% of Na20, 20% w/w of P205, 1.1% w/w of N. The pH of a 10% slurry is 7 and the particle size of the granules is from 0.8 to 2.5 mm.

The storability is the same as that of the product prepared according to Example 2.

WHAT WE CLAIM IS: 1. A process for the manufacture of a granular soil conditioner/fertilizer containing colloidal silicid acid and phosphate, wherein sodium silicate is reacted with an aqueous solution of monosodium dihydrogen phosphate and/or monopotassium dihydrogen phosphate and/or phosphoric acid in a granulating stage in the presence of agitated granules of recycled dried soil conditioner/fertilizer, the reactants being used in such proportions that the granules obtained have a pH of from 3.5 to 11, as measured in a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. tion mixture is passed through a cooling drum having a length of 8 m and a diameter of 2 m, which drum is provided with lifters and buffles, cooling being effected by means of air sucked in countercurrently. A total of 530 kg/hr of water vaporizes in the two drums. The material, now cooled and freed from adhering moisture, is screened on vibratory sieves. The fines below 0.5 mm and the tails above 2 mm are returned to the process and reground. There are obtained 400 kg/hr of finished product. The water content is from 25 to 28% by weight, depending on the quality of the phosphoric acid used. It is mainly in the form of water of crystallization combined with sodium phosphate. Analysis: 33% w/w of SiO2, 15% w/w of Na20, 15% w/w of P205, 0.9% w/w of N, 25 to 28% w/w of H20. pH of a 10% w/w slurry is 9.5 and the ratio of SiO2: NaO is 2.2:1, by weight. Sacks filled with the test product are stacked to a height of 12 sacks. The product is still free-flowing after a period of 6 months. EXAMPLE 2 Using the same arrangement as described in Example 1, and in a similar manner, 4,000 kg of returns, 1,640 kg of sodium silicate spray concentrate (64% w/w of SiO2, 19% w/w of Na20, remainder H20), 180 kg of urea, 1,465 kg of sodium silicate solution (27% w/w of SiO2, 8.1% w/w of Na20, remainder H20), 650 kg of 50% w/w aqueous caustic soda and 1,375 kg of commercial phosphoric acid having a content of 53% w/w of P205 are reacted per hour. Vaporized water is sucked off. The reaction mixture is then partially dehydrated in a drying drum by means of a countercurrent of hot combustion gases having a temperature of 250"C. A total of 1,480 kg of water is evaporated per hour. After screening (0.5 to 2.0 mm particle size), 4,000 kg/hr of product are obtained, this then being cooled. Analysis: 40% w/w of SiO2, 18.0% w/w of Na20, 18% w/w of P205, 0.9% w/w of N, 14% w/w of H20 (combined in the form of water of crystallization). The pH of a 10% w/w slurry is 9.5 and the ratio of SiO2 to Na20 is 2.2:1, by weight Sacks filled with the product are stored in stacks of 16 sacks. The product is still freeflowing after a period of 9 months. A similar product prepared by mixing alkali metal silicate with phosphate fertilizer goes completely hard when stored for 2 weeks under the same condition. EXAMPLE 3 Using the arrangement described in Example 1, 3,000 kg of returns, 2,820 kg of sodium silicate spray concentrate (64% w/w of SiO2, 19% w/w of Na20, remainder H20) and 100 kg of urea are sprayed with 1,600 kg of commercial phosphoric acid having a P205 content of 53% w/w, per hour. The vapors are sucked off. The granules pass to the drying drum, where they are dried at about 110 C (solids temperature). A total of 500 kg/hr of water is vaporized in the two drums. After screening and cooling, 4,000 kg/hr of product are obtained. Analysis: 45% w/w of SiO2, 13.4% w/w of Na2O, 20% w/w of P205, 1.1% w/w of N, 7 to 10% w/w of H20 (combined). The pH of a 10% w/w slurry is 7 and the ratio of SiO2 to Na2O is 3.35:1 by weight. The storability is the same as that of the product prepared according to Example 2. EXAMPLE 4 Using an arrangement as described in Example 1, 6,000 kg of returns, consisting of fines, intermediate material and tails, after passing through the mill together with 3,340 kg of sodium silicate solution (27% w/w of SiO2, 8.1% w/w of Na2O, 64.9% w/w of H20), are sprayed with 756 kg of commercial phosphoric acid having a P205 content of 53% w/w and 150 kg of 35% w/w aqueous urea solution, per hour. The granules are passed through the drying drum, where they are dried at temperatures of about 110 C (solids temperature). A total of 2,000 kg/hr of water is vaporized in the reaction drum and drying drum. After screening and cooling, 2,000 kg/hr of finished product are obtained. Analysis: 45% w/w of SiO2, 13.4% of Na20, 20% w/w of P205, 1.1% w/w of N. The pH of a 10% slurry is 7 and the particle size of the granules is from 0.8 to 2.5 mm. The storability is the same as that of the product prepared according to Example 2. WHAT WE CLAIM IS:

1. A process for the manufacture of a granular soil conditioner/fertilizer containing colloidal silicid acid and phosphate, wherein sodium silicate is reacted with an aqueous solution of monosodium dihydrogen phosphate and/or monopotassium dihydrogen phosphate and/or phosphoric acid in a granulating stage in the presence of agitated granules of recycled dried soil conditioner/fertilizer, the reactants being used in such proportions that the granules obtained have a pH of from 3.5 to 11, as measured in a

10% w/w aqueous slurry, and the product is passed from the granulating stage and dried in a drying stage.

2. A process as claimed in claim 1, wherein the sodium silicate is introduced in solid form and/or in the form of an aqueous solution.

3. A process as claimed in claim 1 or 2, wherein granulation is carried out in the presence of urea in solid form or in the form of an aqueous solution or in the molten form.

4. A process as claimed in claim 3, wherein the urea is used in an amount of from 1 to 5% by weight, based on the finished product.

5. A process as claimed in any of claims 1 to 4, wherein an aqueous solution of sodium and/or potassium hydroxide is introduced at the granulating stage.

6. A process as claimed in any of claims 1 to 5, wherein the recycled dried soil conditioner/fertiliser is made up of fines and coarse particles outside the particle size range desired for the granular product.

7. A process for the manufacture of a granular soil conditioner/fertilizer as claimed in claim 1 carried out substantially as described in any of the foregoing Examples or with reference to the accompanying drawing.

8. Granular soil conditioners/fertilizers when manufactured by a process as claimed in any of claims 1 to 7.

9. Granular soil conditions/fertilizers as claimed in claim 8, containing 30 to 50% w/w of SiO2, 8 to 20 % w/w of P205 and 10 to 25 % w/w of Na2O.