Use in soil improvement of solid particles containing a metal peroxide

The invention relates to the use of solid particles containing metal peroxides and more particularly alkaline earth metal peroxides, for soil improvement, in particular in agriculture, horticulture and forestry.

Metallic peroxides, such as alkaline earth metal peroxides, constitute very interesting sources of oxygen for various uses. Thus, it is known to amend the soils by incorporating solid particles of calcium peroxide therein with the aim of obtaining a slow release of oxygen. Oxygen is absorbed by the roots of plants and promotes their growth (Japanese patent application 51/141 265 filed May 20, 1975 in the name of Nippon Peroxide KK)

This slow and continuous supply of oxygen is particularly desirable for cultures carried out under anaerobic conditions. This is so when the soil is insufficiently aerated because it is impregnated or waterlogged, or even completely flooded or because the outer layer has a reduced porosity, for example due to the compaction of the earth. This is also the case for soils which contain a large proportion of constituents with high biochemical oxygen demand (BOD). These constituents can result from the decomposition of organic matter following, for example, the use of herbicides, or even from the decomposition of waste from previous harvests on plowed land.

When they are in the form of anhydrous solid particles, the alkaline earth metal peroxides, and more particularly calcium peroxide, decompose very little. On the other hand, in the presence of moisture, the calcium peroxide breaks down quickly by releasing its active oxygen. This decomposition is accelerated when the environment is acidic, as can happen naturally in soils or as it can result from the presence of certain additives usually used in agriculture, horticulture or forestry, such as certain fertilizers. Under these conditions, the peroxides in the form of particles do not exhibit a decomposition slow enough to ensure a continuous supply of oxygen, from the period of seed germination to the period of plant growth, or sufficiently complete to allow the use of all the oxygen available in principle.

To reduce the rate of decomposition of calcium peroxide during its use for water treatment, it has been proposed to coat it with water-insoluble coating agents (European patent application EP-A-000 25 43 filed November 15, 1978 on behalf of Interox). However, some of these insoluble coating agents form a protective layer so effective that it is difficult to recover all the oxygen available in principle.

The present invention aims to solve the problem of providing, for soil improvement, solid particles containing a metallic peroxide such as calcium peroxide which are well suited as a source of oxygen with slow and continuous release, in particular in environments such as soils. humid and possibly acidic. The particles intended for use according to the invention decompose slowly, releasing their active oxygen regularly, until almost complete emission of the oxygen available in principle. In addition, they do not contain non-biodegradable products. In addition, they have a speed of dissolution in water and a physical stability such that when in the presence of water or moisture, the accessibility of water to the peroxidized compound itself is controlled, without be suppressed or too strongly inhibited.

Therefore, when the particles are used to amend the cultivated soils, they release oxygen in sufficient quantity and for a period sufficient to cover the oxygen requirements from the period of seed germination until the development period. of the roots, without this release of oxygen being appreciably prolonged beyond the period during which it is useful.

The present invention relates to the use of solid particles containing a metal peroxide coated by means of a coating agent containing at a rate of 30% to 100% by weight, a condensed phosphate soluble in water.

In the particles intended for use according to the invention, the coating agent can constitute a layer, preferably continuous, arranged around the core of the particle which consists essentially of metallic peroxide. It can also serve as an agglomeration binder for elementary fine particles. In this case, each of the elementary fine particles of metal peroxide is surrounded by a layer, continuous or not, of coating agent.

The term “condensed phosphates” is intended to denote all the phosphates having at least one sequence of phosphorus-oxygen-phosphorus bonds in their molecule. Examples of such phosphates include pyrophosphates, polyphosphates such as tripolyphosphates, tetrapolyphosphates and pentapolyphosphates and metaphosphates such as trimetaphosphate, tetrametaphosphate and hexametaphosphate.

By condensed phosphates soluble in water is meant the condensed phosphates whose solubility in water is greater than 1% by weight under normal conditions of temperature and pressure.

Pyrophosphates correspond to the general formula:<ul id="ul0001" list-style="none"><li>(Me)_not(H)_mP₂0₇</li></ul>where n is an integer ranging from 1 to 4 and m is 4-n and where Me represents a metal atom of valence 1 or a group -NH₄.

Polyphosphates meet the general formula<ul id="ul0002" list-style="none"><li>Me₂0 (P0₃Me p</li></ul>where p is at least 3 and where Me has the same meaning as above.

Metaphosphates meet the general formula<ul id="ul0003" list-style="none"><li>(MeP0₃) q</li></ul>where q is at least 3 and where Me has the same meaning as above.

The condensed phosphates present in the coating agent are generally alkali metal or ammonium phosphates and preferably sodium phosphates. The coating agent may also contain mixtures of phosphates.

Among all the phosphates which can be used, polyphosphates and metaphosphates are preferred.

Good results have been obtained with sodium tripolyphosphate and more particularly sodium hexametaphosphate.

The coating agent is generally present in an amount of at least 0.1% and preferably at least 0.5% of the weight of the particles. The amount of coating agent generally does not exceed 40% and preferably 20% of the weight of the particles.

The coating agent consists of 30 to 100% by weight of the soluble condensed phosphates, the possible balance (from 0 to 70%) being able to consist of various additives depending inter alia on the use for which the particles are intended. Thus, the coating agent may contain additives for reducing attrition or pH regulators. As pH regulators, buffers such as carbonates, bicarbonates, sodium tetraborate alone or as a mixture with compounds such as sodium hydroxide, mixtures of sodium or potassium acid phosphates, etc. are suitable. The pH regulating agents are generally present in an amount of at most 70% and preferably from 10 to 60% of the total weight of coating agent. The other additives of the coating agent are generally present at a rate of at most 10% of its weight.

Preferably the coating agent contains from 40 to 90% by weight of soluble condensed phosphates, from 10 to 60% of pH regulators and up to 10% of other additives.

The metal peroxides contained in the particles intended for use according to the invention can be of various natures. In general, these are group 1 and 2 metal peroxides of the Periodic Table of the Elements. They can also be mixtures of peroxides. Preferably, they are group 2 metal peroxides such as calcium, magnesium and zinc peroxides. Good results have been obtained in the case of calcium peroxide.

The particles used according to the invention may contain, in mixture with the metallic peroxide, various additives in small amounts, generally not exceeding 20% ​​of the total weight of the particles. Among these additives are in particular stabilizers of peroxides.

The particles may also contain variable amounts, generally not exceeding 40% of the total weight of the particles, of products resulting from the manufacture of metallic peroxide such as unprocessed reactants or reaction by-products (oxides, hydroxides, carbonates metal, etc.). The particles for the use according to the invention may finally also contain fillers such as talc in variable quantities, generally not exceeding 40% of their total weight. These charges allow the active oxygen content to be adjusted to the desired value.

Particles intended for use according to the invention thus contain, in weight proportions:<ul id="ul0004" list-style="none"><li>- from 10 to 95% of metallic peroxides;</li><li>- from 0.1 to 20% of a coating agent containing from 30 to 100% by weight of soluble condensed phosphates, up to 70% of pH regulators and up to 10% of other additives;</li><li>- up to 20% of additives such as stabilizers;</li><li>- up to 40% of products resulting from the manufacture of metal peroxides; and</li><li>- up to 40% of charges.</li></ul>

The particles can contain varying amounts of active oxygen. In general, they contain at least 1% of their weight of active oxygen. Most often this proportion does not exceed 40%. In the case of calcium peroxide, the particles generally contain from 8 to 20% by weight of active oxygen.

The particles for the use according to the invention can have very variable dimensions. Their diameter is generally greater than 0.01 mm. Most often, it is from 0.01 to 50 mm and preferably from 0.1 to 25 mm.

As has been said previously, the particles can be in the form of granules, the outer layer of which contains the coating agent, or in the form of agglomerates of elementary fine particles linked together by the coating agent. These agglomerates can have any shape such as those of lozenges, tablets and tablets.

When the particles are in the form of agglomerates, they can be produced by dry mixing of fine particles of metal peroxide and of coating agent in the desired proportions. Dry mixing can be carried out in any known mixer and is followed by compaction intended to agglomerate the fine particles together to form the particles according to the invention. For this purpose, any known compactor capable of exerting sufficient pressure can be used. Preferably, the pressure exerted during compaction is greater than 50 kg / cm^{2 .} Well suited devices are roll presses and roll compactors. After compaction, grinding can still be carried out to bring the particles to the dimensions desired for their processing.

Another technique consists in agglomerating the fine particles of metal peroxide by means of a solution or a suspension of the coating agent in a suitable solvent. This treatment can be carried out in various types of apparatus known in themselves such as fluid bed dryers or drum dryers.

The fine particles used in the agglomeration processes generally have an average diameter of 0.1 to 300 micrometers and most often from 1 to 100 micrometers.

According to a preferred embodiment of the invention, the particles are in the form of granules, the outer layer of which contains the coating agent.

The method for manufacturing the particles consists in spraying on moving metal peroxide particles a solution of the coating agent in a liquid medium and evaporating the solvent from the solution.

The coating agent is generally used in the form of an aqueous solution.

The concentration of the coating agent in the solution can vary within fairly wide limits and is generally from 1 to 70% by weight of the total weight of solution. A concentration is preferably used as close as possible to the concentration at which a separation of a solid phase is observed under the temperature conditions used.

The solution is generally used at a temperature ranging from room temperature to a temperature slightly higher than that of the granules on which it is sprayed. In general, the temperature of the solution ranges from room temperature to the temperature of the granules on which it is sprayed plus 25 ° C.

The temperatures at which the spraying and evaporation are carried out are lower than the decomposition temperature of the coating agent and in general do not exceed 120 ° C. For evaporation, temperatures of 25 to 110 ° C are generally used.

The spraying can be carried out according to different techniques, for example in a fluidized bed, on a rotating floor, in a rotary drum or in any other similar device known in itself.

Evaporation can take place at the same time and in the same enclosure as the spraying or in a separate device. We generally operate according to a continuous process. A device such as a fluid bed or any other device known per se may be suitable. In this case, the temperature of the fluid bed is 30 to 105 ° C in general.

The use of a fluid bed has proved to be particularly advantageous, on the one hand because the spraying and the evaporation can be carried out simultaneously in the same apparatus and on the other hand because this technique makes it possible to obtain a more compact and more homogeneous coating.

As the fluidizing gas, any inert gas and in particular air can be used. This gas can be heated to maintain the temperature of the fluid bed at the desired value. One can also use other heating means such as a tube bundle placed in the fluid bed.

One can also operate according to a similar process but discontinuously.

The particles can be used advantageously for the amendment of highly humid soils. They can also be used when the coated seeds containing an oxygen source in their coating are not suitable, either for physical reasons as is the case when planting tree seedlings, vine stocks, etc., or for chemical reasons as is the case when the seeds are particularly sensitive to mineral salts.

In the use for soil improvement according to the invention, the particles are incorporated in general by burying them at a depth of 1 to 5 cm. The particles are generally used at a rate of 1 to 1000 g / m² of treated surface.

The examples below serve to better understand the invention and to show the remarkable results obtained during the use of the coating agents according to the invention, without however limiting the scope of the invention to the embodiments described. .

Example 1 gives a method of preparing the particles containing coated calcium peroxide.

Examples 2 to 9 demonstrate the reduction in the rate of decomposition of the calcium peroxide contained in the particles used according to the invention.

Examples 5 to 7 and 9 are given for comparison.

Example 1

Manufacture of agglomerated calcium peroxide particles using sodium hexametaphosphate (HMP)

A calcium peroxide powder (80% Ca0) is used₂) obtained by reaction of hydrogen peroxide with lime. The average particle diameter is 8 microns.

The powder is agglomerated by a continuous process in a fluidized bed.

The device used consists of a cylinder 15 cm in diameter and 77 cm in height, fitted at its base with a gas distribution plate (2 mm holes).

Initially, 3.5 kg of calcium peroxide particles with a diameter of less than 2 mm are introduced.

Air is passed through the gas distribution plate and an aqueous solution containing 3% by weight of sodium hexametaphosphate, 2, is introduced by a pneumatic sprayer (nozzle) placed at the wall 11 cm from the bottom. 5% by weight of sodium carbonate (buffer), and 0.32% by weight of 100% hydrogen peroxide at the rate of 2.7 kg per hour. The temperature of the solution is 20 to 25 ° C.

Simultaneously, the calcium peroxide powder is introduced, via a vibrating passage, into the fluidized bed, by means of a venturi supplied with compressed air, at the rate of 2.7 kg per hour.

The height of the bed is 25 cm and its temperature is 32 ° C.

Is withdrawn from the device, by overflow, agglomerated particles of dimensions between 0.5 and 4 mm and containing about 65% by weight of calcium peroxide.

Examples 2 to 7

Rate of decomposition in the soil of calcium peroxide of particles

Two series of tests were carried out respectively with particles containing calcium peroxide obtained according to the method of example 1 (examples 2 to 4) and particles of calcium peroxide not coated (examples 5 to 7 for comparison).

10 parts by weight of calcium peroxide particles are mixed with 90 parts by weight of moist potting soil (with 15.6% water) for potting JOHN INNES n ° 1.

The mixtures are placed in sealed glass bottles and stored overnight.

The oxygen loss of the calcium peroxide particles is measured and is given in Table 1 below.<tables id="tabl0001" num="0001"><img file="EP0063844B1_D0001.tif" /></tables>

Examples 8 and 9

Hydrolysis rate of calcium peroxide of particles in an aqueous medium

Was placed in two containers stored under static conditions at room temperature and containing demineralized water, doses corresponding to 500 mg / I of calcium peroxide particles obtained according to Example 1 (Example 8) and particles of uncoated calcium peroxide (Example 9).

After 25 days, the active oxygen loss of the particles was measured. The loss of active oxygen per day is given in Table 2 below.<tables id="tabl0002" num="0002"><img file="EP0063844B1_D0002.tif" /></tables>