FR2676730A1 - Process for converting gypsum to a fertilizing agent - Google Patents

Abstract

Process for converting natural or waste gypsum to a fertilizing agent by reaction of gypsum with urea in a sufficient proportion for the gypsum to be bound up in the form of a calcium sulphate tetraureate which is more or less mixed with the excess urea. New nitrogenous fertilizer containing urea with a delayed fertilizing action.

Description

PROCESS FOR THE TRANSFORMATION OF THE GYPSUM
IN A FERTILIZER
This memo describes a process for transforming gypsum into a nitrogenous compound essentially consisting of a combination between calcium sulfate and urea. This compound is a fertilizer with high nitrogen content and progressive action.

The process according to the invention makes it possible in particular to develop, in the form of this compound, the residual gypsum from industrial phosphoric acid manufacturing.

 It is known to transform gypsum into ammonia sulfate by aqueous digestion of the gypsum with ammonium carbonate. This process neither makes it possible to obtain a new product, nor to solve the problem of the elimination of by-products, the residual calcium carbonate also posing hardly soluble problems.

The Applicant has found a way to transform gypsum into a valuable added nitrogen compound.

 It is known to prepare the tetra-urea complex of calcium sulfate in the laboratory by digesting the gypsum in an aqueous urea solution for several days, even several weeks.

The applicant has studied this reaction and established that it is in fact the result of a balance represented by the following equation:

The study of this balance has shown the applicant that reaction A is rapid and complete when an excess of urea is added to the reaction compounds, while the reverse reaction B is facilitated by an excess of water.

The process exploiting this result consists in digestion of the gypsum in a supersaturated urea solution until complete transformation according to reaction A. Depending on the temperature, the reaction time required is between 1/2 hour and 3 h which makes then possible industrial exploitation.

To facilitate the latter, the applicant has established a ternary diagram represented by FIG. 1, in which 1 represents pure urea, 2 anhydrous calcium sulphate, 3 water. Point 4 corresponds to gypsum, point 5 to the new tretra-urea compound of calcium sulphate and point 6, the saturation point of a solution of urea in water at a given temperature.

The lines drawn between points 1 and 4 on the one hand, 5 and 6 on the other hand, delimit 4 zones in the diagram space.

Zone A corresponds to the existence of the tetra-urea compound of anhydrous calcium sulphate.

This is the zone in which it is necessary to place oneself in order to quantitatively obtain the desired compound. Area C represents the domain of pure gypsum.

Areas B and D correspond to proportions of reaction constituents showing mixtures, in particular of gypsum and of compound 5 for zone B.

The line x represents an example of the use of this diagram. For a constant calcium sulphate content, the zone of appearance of the desired compound is clearly seen. It is also easy to appreciate the conditions of the hydrolysis of the compound, which we have intentionally shown in dotted lines on the right x.

Those skilled in the art will be able, thanks to this diagram established for a chosen reaction temperature, to define the optimum conditions for production of the tetra-urea compound of calcium sulfate.

This product, very slightly soluble in water, can be filtered and dried by any known means. It has none of the hygroscopicity characteristics of pure urea, which makes it advantageous for storage and use.

Its hydrolysis with reappearance of urea is slow, which makes this product advantageous as a nitrogen fertilizer as opposed to urea itself. The hydrolysis can be delayed by the presence of an excess of urea as it appears clearly on examination of the diagram in FIG. 1.

Also, when this product is intended for agriculture, it may be advantageous to combine it with an excess of urea. The following examples give non-limiting examples of some embodiments according to the invention.

Example I 1 mole of gypsum is suspended at 300 in a saturated urea solution at this temperature. Four moles of urea are added and the whole is left to digest for 2 hours. The precipitate obtained is filtered and dried in an oven. The composition of the product obtained is as follows: N = 30% - CaO = 12% S = 7.5%
This composition is very similar to the tetra-urea compound of anhydrous calcium sulphate.

The mother liquors are recycled for a new operation.

Example 2
The procedure is as in Example 1, but 6 moles of urea are used in order to maintain the solution supersaturated despite the appearance of the two moles of reaction water.

The product obtained title: N = 33% CaO = 11.5% S = 7.1%
Example 3
The procedure is as in Example 2, but replacing the pure gypsum with residual precipitated gypsum from a manufacture of phosphoric acid.

This phospho-gypsum contains approximately 90% gypsum and 10% impurities inert towards
Urea. Since it contains 20 to 50% of soaking water, this is taken into account when calculating the saturation of urea.

In practice, it is no longer necessary to operate in the presence of mother liquors saturated with urea, the water contained in the precipitate sufficient to obtain the desired result.

Thus, for a precipitate containing 80% water, an addition of 6 moles of pure urea 1 mole of gypsum is perfectly suitable for obtaining the quantitative transformation of gypsum into a tetraureic compound of calcium sulfate. The drying of the mixture then leads directly to an industrial product containing, in addition to the impurities of the initial precipitate, 6 moles of urea per 1 of gypsum, including 2 moles of free urea facilitating the use of the product. This product can be easily granulated by any usual means. Its average composition is as follows: N = 31% CaO = 12% 5 = 7.5%
Example 4
1 mole of gypsum is treated with 4 moles of molten urea. The appearance of reaction water leads to digestion in accordance with the basic reaction described in the text. The product obtained is dried.

It can be granulated as long as an excess of urea is used. This excess urea must be of the order of 2 additional moles for 1 mole of gypsum.

Its average composition is as follows: N = 30% CaO = 12% S = 7.5%
Example 5
The procedure is as in Example 4, but using phosphogypsum and respecting the proportions of 6 moles of urea per 1 mole of gypsum. The product obtained has the following composition: N = 33% CaO = 12.5% S = 7%

Claims (5)

Claims 1- Method for transfarming gypsum into a fertilizer, characterized in that an aqueous suspension of gypsum is treated with an amount of urea calculated so that at least 4 moles of urea for one mole of gypsum, which is filtered and dried the product obtained, which essentially consists of calcium sulphate tetra-ureate. 2- A method according to claim 1, but using residual gypsum containing imbibition water and solid impurities, the amount of urea required is then determined so that after saturation with urea of the water has an additional quantity of at least 4 moles of urea for one mole of gypsum remains available. 3- A method according to claims 1 or 2, but in which a new excess of urea between 0 and 2 moles of urea is added for one mole of gypsum, which brings to 4 to 6 moles of urea available beyond saturation, the product obtained then containing urea in the form of a mixture of calcium sulphate tetra-ureate and pure urea. 4- A method according to one of claims 1,2 or 3 but wherein the urea is used in the molten state and that the mixture is granulated by cooling and / or drying. 5- New fertilizer obtained by any of the methods described in 1, 2, 3, and 4.