NO145262B - PROCEDURE FOR PREPARING A STABLE SOLUTION OF ERGOTAL KALOIDS OR THEIR SALTS - Google Patents

Description

Process for the production of anhydrous magnesium phosphates or magnesium calcium phosphates.

The present invention relates to the production of anhydrous magnesium phosphates from tertiary calcium phosphates, especially from naturally occurring raw phosphate. In particular, the invention relates to the production of tertiary magnesium orthophosphate, Mg,,(P04)2, by completely replacing the calcium with magnesium in the tertiary calcium phosphate molecule. But the invention also applies to working methods where calcium is only partially replaced by magnesium, so that mixed tertiary magnesium-calcium orthophosphates are formed. All these phosphates are hereinafter called "magnesium phosphate" for brevity, except in the case where the tertiary magnesium phosphate, Mgn(P04)2, is expressly mentioned, to distinguish it from the mixed magnesium-calcium phosphates.

Magnesium phosphate, especially tetriary

magnesium orthophosphate, due to its high P20B content, could be used as a valuable fertiliser, if it were possible to produce it technically at reasonable costs. However, this has so far not been possible.

From a chemical point of view, the simplest method for producing magnesium phosphate consists of letting a water-soluble phosphate react with a magnesium salt, or letting phosphoric acid react with magnesium oxide or

-hydroxide. Such processes are easy to carry out, but the water-soluble phosphates or phosphoric acid are expensive starting materials. It has therefore been proposed to produce magnesium phosphate with a reaction between

magnesium chloride and crude phosphate, especially by reaction between hydrated magnesium chloride (bischofite) and crude phosphate at a higher temperature. This method has the disadvantage that the hydrated magnesium chloride is split into magnesium oxide and hydrochloric acid when heated, and the latter escapes for the most part, without reacting with the calcium phosphate. It has therefore been proposed to carry out the reaction in a stream of HC1 or chlorine at temperatures of 700° C or higher. This is difficult to do on a technical scale, partly because it requires a high temperature and the use of expensive corrosion-resistant equipment. It has been proposed to react hydroxyapatite

(calcium phosphate in some types of raw phosphate)

with anhydrous magnesium chloride, but the reports show that this experiment has not given satisfactory results.

There are commercially available fertilizers which can be considered to contain magnesium phosphate. These fertilisers, which are usually referred to as "thermo-phosphates", or with a similar name, are obtained by reaction between clay phosphates (Tonerde Phosphat) and a magnesium salt, e.g. Mg sulphate or chloride, at a high temperature of the order of magnitude 850-1000° C. Typical preparations obtained in this way contain 15-25 per cent P205, i.e. far less than the P2Os content of pure magnesium phosphate. The large amount of calcium, iron and aluminum contained in these products means an unnecessary ballast. Furthermore, the high temperature required for the production of these fertilizers means that they become rather expensive.

The invention relates to a method for the production of anhydrous magnesium phosphates or magnesium-calcium phosphates by thermally splitting raw phosphate in a mixture with magnesium chloride, and the characteristic feature of the method is that the raw phosphate is heated to 300-600° C together with a mixture of magnesium chloride or ammonium chloride whose magnesium content essentially stoichiometric corresponds to the amount of calcium that must be replaced by magnesium in order for the salt mixture to sinter or melt, and the chlorides are removed by leaching with water from the reaction product.

In this reaction, a stoichiometric exchange of calcium with magnesium takes place.

If the desired product is tertiary magnesium orthophosphate, the amount of magnesium introduced into the reaction mixture is chosen to be stoichiometrically equivalent to, or preferably greater than, the total amount of calcium present in the raw phosphate. The amount of magnesium chloride used in excess, above the amount required for complete exchange of calcium with magnesium, is found in the product, mainly as free MgO.

If less than the stoichiometrically calculated amount of magnesium is used, a mixture of calcium and magnesium phosphates is obtained, but the original apatite structure is strongly modified, even with a partial replacement of calcium with magnesium, which is evident from the fact that the plants can easily absorb the product's P205 content. Such mixed Ca-Mg phosphates are of particular importance in soils that lack Ca but are rich in Mg.

The reaction mixture may be a melt in which all or most of the chloride constituents have been completely melted, or a sintered mass in which most of the salt crystals have not yet lost their identity. By experiment, it can be found for each individual composition of the starting materials whether the sintered or the completely melted state is preferable.

Raw natural phosphate usually contains some calcium carbonate and calcium fluoride, which when reacted with magnesium chloride is converted into magnesium carbonate and magnesium fluoride and a corresponding amount of calcium chloride. Depending on the desired degree of purity of the magnesium phosphate to be produced, magnesium carbonate, and possibly magnesium oxide formed during the process, can remain connected to the magnesium phosphate, or it can be separated from the phosphate in a manner known per se. In each case, the amount of magnesium chloride used for the process must be calculated in such a way, taking into account the calcium carbonate and fluoride contained in the raw phosphate, that the total amount of magnesium chloride is stoichiometrically sufficient for all the calcium present to be converted into the corresponding magnesium compounds.

The reaction mixture can contain the magnesium chloride and the alkali metal or ammonium chloride either in the form of a double salt with the molar ratio 1:1, e.g. carnallite or sodium or ammonium analogues of carnallite, or in any other suitable molar ratio.

It is known that anhydrous magnesium chloride can be prepared by heating a mixture of magnesium chloride hexahydrate with ammonium chloride, e.g. in the form of ammonium camalite. In a similar way, anhydrous mixtures of magnesium chloride and an alkali metal chloride have been prepared by heating a mixture of magnesium chloride hexahydrate with sodium or potassium chloride, e.g. in the form of sodium carnallite or potassium carnallite. However, the conclusion could not be drawn from these known facts that magnesium phosphate or magnesium calcium phosphate could be produced by heating raw phosphate with a mixture of magnesium chloride and alkali chloride or ammonium chloride in the sintered or molten state at lower temperatures and under more favorable conditions than can be achieved by heating crude phosphate with magnesium chloride in the absence of alkali metal or ammonium chloride.

In contrast to what happens if crude phosphate is reacted in heat with only hydrated magnesium chloride, no thermal decomposition takes place in the method according to the invention, but a simple dehydration of the magnesium chloride. The reaction between the calcium phosphate and the magnesium chloride, by which the desired magnesium phosphate is formed, thus becomes a purely metathetic (ion exchange) process, without the participation or presence of significant amounts of hydrochloric acid. This straightforward and quantitative exchange reaction between tertiary calcium phosphate and magnesium chloride, which could not be achieved by the previously known methods, is presumably due to the presence of alkali metal or ammonium chloride, which seems to facilitate the dehydration of the magnesium chloride. In particular, the presence of ammonium chloride and potassium chloride counteracts thermal decomposition of the magnesium chloride while the dehydration takes place. The presence of sodium chloride acts in the same direction, albeit to a less pronounced degree. Depending on the splitting of the salt mixture, it is often possible to carry out the reaction while the mass is only sintered, without becoming truly fluid. When sodium chloride replaces ammonium or potassium chloride, this can result in an increase in the MgO content in the reaction product. For many soils, magnesium phosphate containing MgO is an excellent fertiliser. If the presence of MgO is not desired, this can be separated and extracted by work operations known per se.

The required reaction temperature is usually between 300-600° C and depends on the composition of the chloride mixture and the nature of the raw phosphate used. Depending on the nature of the starting materials and the composition of the reaction mixture, the products obtained have different P205 contents. A typical from raw phosphate with 30-34 weight-percent. P20-obtained product can contain 30-40 per cent P2Os and 46-53 per cent MgO.

The reaction according to the invention can be carried out in many different ways.

According to one method, the salt mixture is e.g. carnallite, melted, and the phosphate introduced into the smelter.

In another working method, starting material is mixed in solid form, and the mixture is dried and heated so that the magnesium chloride and the alkali metal or ammonium chloride are melted.

In a further method, a reaction mixture is produced in solid form, which consists of magnesium chloride hexahydrate, alkali metal chloride and crude phosphate. This reaction mixture is first dewatered at a moderate temperature and is then introduced into a bath consisting of molten calcium chloride and alkali metal chloride. In the bath, magnesium sulphate falls out, which is separated, while a new portion of dewatered reaction mixture can be introduced into the bath.

Instead, a mixture of the starting materials in solid form can be heated so that the magnesium chloride is gradually dewatered, and the mixture begins. to sinter and granules are formed.

If a mixture of magnesium chloride and ammonium chloride, or an ammonium chloride-magnesium chloride double salt of the carnallite type, is used, the ammonium chloride is volatilized during the reaction. After the calcium chloride has been removed from the reaction product by leaching, the residue constitutes a technical magnesium phosphate, which has a high content of P205 and MgO.

If the starting material contains sodium or potassium chloride, this chloride enters the reaction product. From a reaction product containing potassium chloride, the calcium chloride can be preferentially washed out, while the remaining mixture of magnesium phosphate and potassium chloride forms a valuable fertiliser. In this case, almost all the potassium that was present in the starting material, e.g. carnallite, used as fertilizer. If for some reason it is not desired to retain potassium chloride in mixture with magnesium phosphate, it is easy to separate the potassium chloride by flotation or by hot extraction.

If the starting material contains sodium chloride, this is removed from the reaction product by leaching, together with the calcium chloride.

The technical magnesium phosphate obtained according to the present invention is not only a valuable fertilizer in and of itself, but it also has all the essential chemical properties of magnesium phosphate. For example, it reacts with ammonium chloride to form ammonium magnesium phosphate, or with sodium carbonate to form sodium phosphate, and in this respect can replace phosphoric acid.

The invention is illustrated by the following examples.

Example 1.

100 g of raw phosphate from Oron (Israel) which has been freed from sludge and which contains

was mixed with 300 g of dewatered natural technical carnallite, which contained

In this mixture, the amount of magnesium was approx. 30 percent excess over the stoichiometrically equivalent amount of calcium present. The mixture was heated to 490-520° C in a tubular container and held at this temperature for one minute. Thereby, the magnesium chloride was first dewatered and then melted. Condensation of evaporated water on the colder parts of the pipe walls during the heating period was prevented by applying a vacuum. After cooling, the solid mass was leached with water until it became chloride-free, whereby all calcium chloride and potassium chloride were removed, and then the magnesium phosphate residue was filtered off, and the filter cake was washed with water. After drying, it had the following composition:

The Mg balance was as follows:

In a modified embodiment of this method, the cooled solid mass was leached out with a small amount of water, whereby calcium chloride was removed from the mass in an amount corresponding to 99.5 percent of the calcium that had been added for the reaction and 7 .3 g of magnesium, corresponding to the amount of magnesium chloride which had been added in excess of the amount required to form magnesium phosphate, but practically all the potassium chloride remained in the mass.

Example 2.

100 g of chemically pure tricalcium phosphate was mixed with 400 g of carnallite of the same kind as that used in ex. 1. The mixture melted at 470-510° C, and was held at this temperature for 1 minute. Condensation of evaporated water was prevented by using vacuum in the reaction vessel.

The processing of the product took place in the same way as described in ex. 1. The aqueous extract contained 82.8 per cent of the added calcium, as well as 17.15 g of Mg, which corresponds to the amount of MgCl2 that was added in excess of the amount required for the formation of magnesium phosphate.

The Mg balance was as follows:

Example 3.

300 g ammonium chloride-magnesium chloride double salt, NH4C1. MgCl2. 6H20, was placed in a reaction tube, which was heated gradually, so that the salt was freed from water. Condensation of the evaporated water on the colder parts of the tube was prevented by applying a vacuum. Some of the starting NH4C1 sublimed at this point and was collected.

When the temperature reached 410° C, 100 g of the same crude phosphate freed from sludge, which is mentioned in example 1, was introduced into the tube, and the mixture was heated to 600° C with stirring. In this working step, the remaining NH4C1 sublimed, while the magnesium chloride reacted with crude phosphate. The reaction mass was then allowed to cool and was then leached with water and the solid cake washed until it was free of chloride. The aqueous extract and the wash water contained 90 per cent of the calcium.

The dry product had a similar composition to the product in Example 1.

Example 4.

A mixture of MgCl2. 6H2O and KC1 as contained

was dewatered and then finely ground. The product contained 0.92 percent H2O.

180 g of the dewatered salt mixture was heated to 540 C, at which temperature it melted. During the heating period, the reaction vessel was connected to a vacuum pump to prevent condensation of the evaporated water. 100 g of ground, dry Moroccan crude phosphate, which contained 32.3 percent P20 and 34.8 percent Ca, was introduced into the smelter, and after cooling the solid mass was worked up in the manner described in example 1. The final product contained 34.8 per cent.

Example 5.

70 g of the same sludge-freed Oron raw phosphate as in example 1 was mixed with 220 g of natural carnallite which contained

so that the excess of MgCl2 was approx. 22 percent. The mixture was heated to 520-550° C; condensation of water evaporated during the heating period was prevented by applying a vacuum. After cooling the mass, it was leached and washed with water until it was chloride-free. The aqueous extract contained 91.5 per cent of the added Ca and also 3.8 g of Mg, corresponding to the added excess of MgCl2.

The Mg balance was as follows:

Example 6.

A mixture of 11.5 kg of the same type of natural carnallite used in example 5 and 3 kg of the same sludge-free Oron raw phosphate as in example 1 was continuously charged over the course of 1 hour in a rotary kiln, which was heated directly to 550° C by means of combustion gases, which were led in countercurrent to the charge. The water present in the charge mixture escaped together with the exhaust gas, which contained 14% CO 2 and had a temperature of 300° C. The mixture melted completely. The product was allowed to cool and was washed and dried. It contained:

Example 7.

A mixture of 13.3 kg of carnallite (content 8.3% Mg, 13.3% K and 6% Na) and 3.8 kg of the same Moroccan crude phosphate as in example 4 was continuously charged in a rotary kiln, which was heated directly by means of combustion gases, which were led in countercurrent to the charge. The water in the material mixture went away with the gases. The mixture sintered at 460-480° C and was not heated above this temperature. The reaction product was allowed to cool and was washed free of chloride with water and was then dried.

It contained:

Example 8.

In a similar way as in example 7, a mixture of 21 kg of carnallite (content 7.8% Mg, 10.4% K and 8% Na) and 6 kg of sludge freed Oron raw phosphate (content 28% P2Og and 36 percent Ca) melted completely at 450-480° C. After leaching in the same way as in example 1, the product had the following composition:

As a variant of this example, the reaction product was not leached with water, but was quenched by being plunged directly from the oven into a water bath. The soluble salts contained in the reaction mixture were then dissolved in the water, while the insoluble magnesium phosphate was separated and dried.

Example 9.

A blinding of 13.3 kg of carnallite (content 8.3 per cent Mg, 13.5 per cent K and 6.9 per cent Na) and 3.8 kg malted Moroccan crude phosphate (content 32.2 per cent P205 and 34.8 percent Ca) was charged continuously and over the course of 1 hour in a rotary kiln, which was heated in the charge to 200-230° C by means of combustion gases in countercurrent. At this temperature the mixture was dewatered.

The dewatered mixture was charged again in the same furnace, which was now heated to 520-550° C, whereby the mass melted completely. The product was allowed to cool and was washed with water until it was free of chloride, and was then dried. It contained

Example 10.

A mixture of 13.3 kg carnallite (content 8.3% Mg, 13.5% K and 6.9% Na) and 3.8 kg ground Moroccan crude phosphate (content 32.2% P2Os and 34 .8 per cent Ca) was continuously charged over the course of 1 hour in a rotary kiln and heated to 200-230° C by means of counter-flowing combustion gases. The mixture was thereby dewatered.

The dewatered mixture was poured into a molten salt bath containing CaCl 2 , NaCl and KCl in the ratio in which they are present in the reaction product of the carnallite and the crude phosphate used as starting materials. The dry mixture coming from the rotary kiln then instantly reacted with the salts in the molten salt bath. An insoluble precipitate of anhydrous trimagnesium phosphate formed in the salt bath was recovered, while the CaCl2 formed by the reaction as well as the NaCl and KC1 introduced into the bath with the reaction product remained in the bath, without changing the composition of the latter. The same bath could therefore be used again and again for the treatment of additional quantities of dewatered carnallite triphosphate mixture.

Example 11.

A mixture of MgCl2. 6H 2 O and NaCl were dewatered in vacuum at 175° C. It then contained 12% Mg, 15.6% Na and 59.5% Cl. 100 g of enriched Oron raw phosphate (content 33.5% P2Os and 37% Ca) was mixed with 250 g of the above-mentioned dewatered salt mixture, and the whole mixture was heated in a reaction tube in the manner described in example 3. The heating was stopped at 555°C, at which temperature the mixture had sintered but was not completely melted. The mixture was allowed to cool and was leached with water. The residue was filtered and the filter cake was washed with water until it was free of chloride. The filtrate contained 81 percent of the calcium that had been added with the raw phosphate and 2.7 g of Mg that had been added in excess of the amount needed to form magnesium phosphate. The dried product had the following content:

Example 12.

100 g of Moroccan crude phosphate, which contained 32.2% P2O5 and 48.6% CaO, was mixed with 180 g of a dewatered natural carnallite, which contained

In this mixture, the amount of magnesium chloride was approx. 80 percent of the amount that is stoichiometrically equivalent to the total amount of calcium present. The mixture was heated in a reaction tube to 500-5550°C, where it began to sinter, and was held in this temperature range for 1 minute. Condensation of water was prevented by connecting the tube with vacuum.

After cooling, the product was washed with water until it was free of chloride.

The dry product contained:

Example 13.

100 g of the same sludge-free Oron raw phosphate as that used in example 1 was mixed with 155 g of natural carnallite which contained

In this mixture, the amount of MgCl2 was approx. 50 per cent of the amount that is stoichiometrically equivalent to the total amount of calcium in the raw phosphate.

The mixture was heated to 520-550° C; condensation of water in the first part of the heating period was prevented by means of a vacuum.

The cooled reaction mass was washed with water until it was free of chloride. The dry product contained

Claims (8)

1. Process for the production of anhydrous magnesium phosphates or magnesium-calcium phosphates by thermal decomposition of raw phosphate mixed with magnesium chloride, characterized in that the raw phosphate is heated to 300-600° C together with a mixture of magnesium chloride and an alkali metal chloride or ammonium chloride whose magnesium content essentially corresponds to stoichiometrically to the amount of calcium that must be replaced by magnesium for the salt mixture to sinter or melt, and the chlorides are removed by leaching with water from the reaction product. 2. Method according to claim 1, characterized in that the amount of magnesium introduced into the reaction mixture is stoichiometrically equivalent to, or greater than, the total amount of calcium contained in the crude phosphate, so that all tertiary calcium phosphate in the crude phosphate can be converted into tertiary magnesium orthophosphate. 3. Method according to claim 1 or 2, characterized in that the method is carried out with a reaction mixture containing magnesium chloride and potassium and/or sodium chloride. 4. Method according to claim 1, 2 or 3, characterized in that a molar ratio is used between the magnesium chloride and potassium, sodium or am the ammonium chloride in the reaction mixture at practically 1:1. 5. Method according to any one of claims 1-4, characterized in that the raw phosphate is introduced into a bath of molten magnesium chloride and alkali metal chloride. 6. Method according to any one of claims 1-4, characterized in that the raw phosphate is first mixed with magnesium chloride and alkali metal or ammonium chloride, and that the mixture is then heated to sintering or to complete melting. 7. Method according to any one of claims 1-4, characterized in that a solid reaction mixture of raw phosphate, magnesium chloride and alkali metal chloride is dewatered by heating to 100-180° C, and then introduced into a bath of molten calcium chloride and alkali metal chloride, and that the resulting precipitate of anhydrous tertiary magnesium phosphate is separated from the bath. 8. Modification of the method according to any one of claims 1-6, characterized in that the hot reaction mixture is quenched in water, instead of being leached after cooling. Publications cited: Gmelins Handbuch der anorganischen Chemie, 8. Aufl., Magnesium, Teil B, pp 105 and 535.