GB2115800A - Process for producing granular compound fertilizer - Google Patents

Abstract

A granular compound fertilizer containing at least two components selected from nitrogen, a phosphate radical, potassium and other effective fertilizer ingredients is produced by steps such that at least one component is selected from the starting compounds urea, ammonium chloride, ammonium nitrate, sodium nitrate, calcium nitrate, ammonium phosphate and ammonium sulfate; a high-temperature aqueous solution having a concentration of at least 80 percent of saturation concentration is formed from all or a major quantity of said selected compound and water in a specified quantity of up to 40 weight percent on the basis of all or the major quantity of said selected compound; a high-temperature slurry containing at least 40 volume percent of liquid is formed by adding to said high- temperature aqueous solution the balance of said major quantity and/or a powdery substance which is selected from compounds other than said starting compounds and can be mixed with it as at least one fertilizer component; and said high- temperature slurry is attached to seed particles by spraying the slurry onto the seed particles which are moving in a granulating zone, and the resultant product is then dried and/or cooled.

Description

SPECIFICATION Process for producing granular compound fertilizer This invention relates to a process for producing a granular compound fertiliser including the steps of preparing a slurry by mixing solid particles with a concentrated solution formed by a dissolution and/or melting process, spraying the slurry onto seed particles which are moving, and solidifying the seed particles to which the sprayed droplets containing the solid particles are attached.

It is known in the art to produce a granular product such as a granular fertilizer by spraying a solution or molten liquid not containing solid particles onto seed particles. On the other hand, Japanese Patent Laid-Open No. 77252/1981 discloses a method of granulating urea by spraying a slurry. To restrict the increase in the biuret content of molten urea as the granulation starting material, this prior art sprays a molten urea slurry containing 0.01 to 5 wt% of fine solid particles of urea. In producing a granular product from a wide variety of starting materials such as a compound fertilizer, it is generally necessary to carry out mixing of various starting materials having different water solubility, different melting points and different thermal deterioration temperatures with each other.For example, potassium chloride which has low water solubility but a high melting point is used as one of the starting materials. For this reason, in the conventional process for producing a granular compound fertilizer by spraying a solution or molten liquid not containing solid particles, the temperature of the spray solution is set below the lowest thermal deterioration temperature and all the starting materials are dissolved in a relatively large quantity of solvent, in general water, to form the spray solution at this temperature as the solution or molten liquid not containing the solid particles. The spray solution is then sprayed.

Consequently, this prior art process consumes a great deal of energy twice, i.e., first in heatdissolving the starting materials in large quantities of water and then in evaporating and removing the water after spraying or coolsolidifying the molten matters. From the aspect of energy saving, therefore, none of the prior art processes inclusive of the aforementioned Japanese Patent Laid-Open No. 77252/1981 were entirely satisfactory.

The present invention provides a process for producing a granular compound fertilizer which can drastically reduce tha water content necessary for preparing a spray solution by spraying a slurry containing large quantities of solid particles onto seed particles and which can also save the energy necessary for dissolving or melting powdery starting materials as well as the energy necessary for removing the water from spray droplets attached to the seed particles after spraying by such means as drying or cooling or for cooling and solidifying the droplets. The present invention also provides optimum conditions for spraying a high-temperature slurry.

A very large number of granular compound fertilizers are now available. First, the ratio of the three fertilizer components, i.e., nitrogen, phosphorus and potassium, varies with the soil to which the fertilizer is applied and with the kinds of plants which grow on the soil. Moreover, some fertilizers contain components other than these three components while others use urea or ammonium sulfate as the nitrogen source.

Starting materials for these fertilizers are also numerous. The nitrogen sources includes ammonium sulfate, urea, ammonium chloride, ammonium nitrate, calcium nitrate, sodium nitrate, and so on. Examples of compounds containing both nitrogen and phosphorus include di-ammonium phosphate and mono-ammonium phosphate, while examples of compounds containing phosphorus include superphosphate of lime, double superphosphate of lime, fused phosphate, calcined phosphate and so on.

Examples of potassium-containing compounds include potassium sulfate, potassium chloride, potassium bicarbonate and so on. Other starting materials include those compounds which contain a specific element, for example, magnesium superphosphate, borate compounds and so on.

These numerous starting compounds are generally used as a powdery solid, solution or slurry. In preparing the slurry to be sprayed from this large number of starting materials, the present invention first prepares a high-temperature concentrated solution having a concentration of at least 80 percent of saturated concentration and consisting of all or a major quantity of at least one compound selected from urea, ammonium chloride, ammonium nitrate, sodium nitrate, calcium nitrate, ammonium phosphate and ammonium sulfate among a large number of starting materials described above and up to 40 weight percent of water on the basis of all or the major quantity of the compound or compounds thus selected.Next, the present invention adds to the resulting high-temperature concentrated solution, a granular material corresponding to the balance of the selected compound or compounds and/or other powdery starting materials to be blended with the intended compound fertilizer, thereby forming a slurry. A slurry which contains at least 40 volume percent of liquid is used as the slurry. The present invention preferably uses also a nozzle for spraying the slurry, in which the minimum diameter of a slurry passage inside the nozzle is at least 4.5 times the maximum diameter of the solid particles contained in the slurry.It is a preferred condition for practising the present invention that the surface of the spray nozzle which comes into contact with the slurry be made of a hard material selected from i) metallic chromium or in alloy containing at least 17 wt% of chromium, ii) metallic titanium or a titanium alloy, iii) metallic zirconium or a zirconium alloy and iv) tungsten carbide.

Among the numerous starting materials for the compound fertilizer listed above, those belonging to the group consisting of urea, ammonium chloride, ammonium nitrate, sodium nitrate, calcium nitrate, ammonium phosphate and ammonium sulfate (hereinafter merely referred to as the "group") have high water solubility at high temperatures. Accordingly, if at least one compound is selected from the group the hightemperature concentrated solution for use as the mother liquor of the slurry can be prepared using a relatively limited quantity of water. A slurry having high fluidity and containing large quantities of solid particles and solutes can be prepared by mixing up to 50 volume percent of other powdery starting materials having low solubility with this solution. The aforementioned object of energy saving can be accomplished by spraying the resulting slurry.

On the contrary, in preparing a slurry having the same mixing ratio of the same compounds using the same quantity of water, if a solution consisting of a low-solubility compound other than that of the group and water, or a saturated solution or a saturated solution containing undissolved solid matter is first prepared and a high-solubility compound or compounds of the group are then added to the solution to prepare the slurry, the compound or compounds of the group preferentially dissolve in the solution so that the low-solubility compound other than that of the group, that has once been dissolved, is again precipitated as solid particles and exhibits a function of aggregating the solid particles during precipitation, resulting in choking of the nozzle in spraying the slurry and other problems. A specific example will be described later in the examples.

In the foregoing description, the term "hightemperature" means a temperature considerably lower than the thermal deterioration temperature of a material having the lowest thermal deterioration temperature among the compounds selected from the group and compounds other than those of the group for producing a desired compound fertilizer containing the desired components, the lowest thermal deterioration temperature being that at which the material deteriorates and prohibits the use of the fertilizer or at which the effective ingredients are lost.In the case of the production of a three-component compound fertilizer consisting of urea, monoammonium phosphate and potassium chloride, for example, the term "high temperature" refers to a temperature which is considerably lower than the deterioration temperatures of urea and monoammonium phosphate that are from 1000 to 1 37 C. Generally speaking, however, such deterioration becomes more rapid and vigorous with an increasing temperature and becomes slower with a decreasing temperature.

Accordingly, the residence time of the slurry inside a granulating machine must also be taken into consideration in deciding this temperature definitely. In other words, if the residence time is reduced, the temperature can be set to a point as high as possible while keeping the degree of deterioration on the same level. In this manner, a high-temperature slurry having a smaller water content can be prepared by making the most of the property that the solubility of each compound increases with an increasing temperature. This is important in enhancing the effects of the present invention.

The contents of the components in the compound fertilizer generally decrease in order of nitrogen, phosphorus and potassium. In other words, the nitrogen content is generally the largest. The present invention makes the most of the fact that among the compounds of the group that are to be blended in greater quantities, the nitrogen-containing compound has high water solubility at high temperature. Since the quantities of the phosphorus, potassium and other components are relatively small, a slurry having an extremely small water content can be prepared.

The slurry in the present invention is used for spraying as described already. Accordingly, the slurry must have fluidity to such an extent that it permits transportation and spraying by a pump.

To attain this fluidity, the slurry must contain at least 40 volume percent of liquid at the abovementioned high temperature at which it is handled, as will be described in detail elsewhere.

The term "liquid in the slurry" refers to a remaining liquid other than the powdery solid particles present in the slurry and consists of water and solutes dissolved in water at that high temperature. However, the water content in the slurry can be reduced if the content of the powdery solid particles in the slurry is greater provided that it does not deteriorate the fluidity of the slurry.

On the other hand, even if a liquid having a given composition and temperature forms mother liquor of the slurry, the slurry fluidity varies remarkably depending upon the kinds of the components present in the mother liquor as the powdery solid particles, their mixing ratio, the average particle size, the particle size distribution, the size and shape of individual particles, and the like. Accordingly, it is generally not possible to describe an optimal value of the content of the powdery solid particles in the slurry and the optimal value must be experimentally decided after the starting materials for a desired granular compound fertilizer are selected. Depending upon the blend ratio of the granular compound fertilizer, it is at times possible to keep a good slurry fluidity even if the blend quantity of the materials having low solubility and selected from the compounds other than the aforementioned group is relatively small and the whole of this material is dispersed and mixed as granular solid particles in the liquid.

In such a case, the slurry is prepared in such a manner that part of the compounds selected from the group exists not in the form of a solution or in a molten state but in the form of powdery solid particles in the slurry, thereby reducing the water content of the slurry and enhancing the effect of the present invention.

For the reason described above, it is preferred in the present invention that the balance of the slurry having the necessary fluidity other than the powdery solid particles in the slurry is liquid and that this liquid is a high-temperature saturated solution of at least one compound selected from the group or one having a concentration of at least 80 percent of the saturated concentration. If the concentration of this liquid is below 80 percent of the saturated concentration, preparation of the slurry becomes easier but, since the water content in the slurry increases, the effect of the present invention decreases. In order to secure the liquid content in the slurry, up to 40 weight percent of the water content may be employed per one kind of solute selected from the group to be contained in the liquid.This solute need not necessarily account for the whole of at least one kind of the starting material selected from the group but part of the material may be present as solid particles, as described above, so as to enhance the effect of the present invention.

In spraying the slurry, the slurry must contain at least 40 vol% of the liquid therein even if it is prepared in the manner described already.

Generally, a slurry containing below 40 vol% of the liquid has low fluidity and can not attain a well sprayed state (refer to the examples below). As the liquid content exceeds 40 vol%, spraying becomes easier but the energy saving becomes poor due to the increase in the water content of the slurry. Accordingly, the preferred range of the liquid content of the slurry is from 50 to 90 volume percent. A slurry having a large solid particle content and sufficient fluidity can be selected from within this range. Maximum diameters of individual solid particles contained in the slurry are preferably from 50 ym to 1 mm. A slurry containing large quantities of the same solid consisting of particles of below 50 ,um is likely to show inferior fluidity.If solid particles of greater than the maximum diameter of 1 mm are present, on the other hand, the spray nozzle is likely to be choked. In other words, since the spray nozzle has such a property that the diameter of the sprayed droplets becomes greater with an increasing nozzle diameter, a disadvantage would occur in attaching uniformly the sprayed droplets onto a very large number of seed particles during granulation.

As a result of experiments, the inventors of the present invention have found that in order that the solid particles in the slurry can smoothly pass through at least one slurry passage provided inside the spray nozzle, the diameter of the slurry passage having the minimum diameter of the slurry passages inside the spray nozzle is preferably at least 4.5 times the maximum diameter of solid particles contained in the slurry.

In view of the contradictory conditions described already, the maximum diameter of the solid particles in the slurry is preferably up to 1 mm.

The shapes of the particles present as the solid particles in the slurry are various. The term "maximum diameter of particles" used herein therefore means the diameter of the group of particles having the greatest diameter if the particles are spherical and means the length of the group of particles having the greatest length if the particles have a thinly elongated rod-like shape.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagram showing altogether the results of a large number of experiments for spraying the slurry; Figure 2 is a process flowchart of the granulator used in the examples Of the present invention; and Figure 3 is a schematic view of the slurry spray nozzle used in the examples of the present invention.

In Figs. 2 and 3, the reference numerals have the following meaning: 1-main body of granulator 2-slurry feed pipe 3-air feed pipe 4-air exhaust pipe 5,6-slurry material feed pipe 7-slurry spray nozzle 8-seed particle feed pipe 9-outlet pipe of grown product 1 0-mixing tank 11-particle tank 1 2-slurry pump 1 zi separator 1 5-cooler 16-mill 1 7-classifier 19-final product outlet pipe 22,23-tank 24,25-feeder 31,32,33,34,35-slurry passage 36-main body of spray nozzle A liquid spray nozzle having a conventional construction which receives only the liquid pressure or the action of a pressurized gas stream ejected in the proximity of a spray port of a pressurized liquid thereby to eject the liquid can be used as the slurry spray nozzle in the present invention. Figure 3 is a schematic sectional view showing an example of the spray nozzle used in the embodiments of the present invention. The slurry is supplied in the direction indicated by an arrow 31 and is divided so as to flow through a passage 32 in the atomizing direction inside the nozzle main body 36 and through at least two passages 33 each of which contains a bend or is curved in an arcuate form inside the nozzle main body 36.When these fiows gather together inside a mixing chamber 34, vigorous whirling motion occurs in the slurry inside the mixing chamber 34 and the slurry is sprayed as droplets from the spray port 35 as represented by arrows of dotted line. In the spray nozzle of the type described above, the diameter of the spray port 35 mostly corresponds to the minimum diameter of the slurry passage inside the nozzle. Since a solid having high hardness such as crystals is ejected, the slurry spray nozzle is generally worn out and the diameter of the slurry passage becomes gradually greater until at last a desired diameter of droplets cannot be obtained easily. For this reason, the aforementioned hard material is preferably used for the surface of the nozzle in contact with the slurry as the material for producing the nozzle.

The energy quantity that can be saved in the production of a granular compound fertilizer according to the process of the present invention changes remarkably with the components contained in the compound fertilizer. In the production of a.granular compound fertilizer containing urea, ammonium phosphate and potassium chloride and 1 8 weight percent each of nitrogen, phosphorus, calculated as P205 and potassium, calculated as K20 for example, the process of the present invention can save about 800 kg of water which must be evaporated per ton of the granular product and 20,000 kcal of the heat of crystallization which requires cooling in comparison with the case in which all starting materials are to be sprayed as a high-temperature solution.Hence, the present invention provides a remarkable effect from the aspect of energy saving.

Example 1 This example was carried out in order to compare the minimum diameter of the slurry passage among those capable of spraying the slurry inside the spray nozzle with the maximum diameter of the solid particles and to determine its relation to the liquid content of the slurry.

Figure 1 shows altogether the results of experiments when spray nozzles having various diameters but similar to one another as shown in Figure 3 were used and each slurry was prepared by selecting at least one component from monoammonium phosphate, di-ammonium phosphate, ammonium sulfate, potassium chloride and potassium sulfate, then classifying the crystals of the select component using a 20 to 60 mesh sieve and adding the crystals to a saturated aqueous urea solution under heating, followed by spraying of the resulting slurry. The values on the abscissa of the diagram represent those obtained by dividing the minimum diameter of the slurry passage inside the spray nozzle by the maximum diameter of the solid particles in the slurry while those on the ordinate represent the volume percentage of the liquid content in the slurry.The maximum diameter and the volume percentage were measured in accordance with the method described in the example below. The mark 0 signifies that the spraying was possible at the positions on the abscissa and ordinate corresponding to the mark position, the mark X signifies that the spraying was impossible and the mark A signifies that the spraying was mostly possible but sometimes impossible.

As can be seen clearly from Figure 1, the spraying was possible extremely frequently within the zone whose value on the abscissa was at least 4.5 and on the ordinate was at least 40 percent, whereas it was very often impossible within other zones. Substantially the same result could be obtained in the slurries prepared by adding each of the aforementioned crystal components to a saturated aqueous solution of ammonium nitrate.

Example 2 This example was carried out to granulate a compound fertilizer in accordance with the diagram shown in Figure 2.

In Figure 2, reference numeral 1 represents the main body of a granulator in accordance with a spouted layer granulation system. Seed particles supplied from a seed particle supply pipe 8 were placed in a desired quantity in this main body and an air stream, continuously introduced at a high speed from a pipe 3 and having a temperature considerably higher than the normal (ambient) temperature, formed an upward spouted layer together with the seed particles near the center of the main body 1 as shown in the drawing. At the same time, a high-temperature slurry was upwardly ejected from a slurry spray nozzle 7 disposed at the lower part of the main body 1 into this layer so that the spray droplets were attached onto the moving seed particles and cool-dried by the air stream.Thus, the seed particles were grown into macro-particles and the product thus produced was withdrawn from a pipe 9. In this instance, the seed particles that were blown up to the upper part of the granulator 1 inside the spouted layer dropped again onto the upper surface of a particle layer 11 at the lower part of the granulator, entered again the spouted layer, were then blown up and circulated inside the granulator 1. In the interim, the seed particles received repeated operations of the attachment of the spray droplets, cooling and drying, gradually grew in size and were finally discharged from the pipe 9.

After each component to be incorporated in the complex fertilizer such as nitrogen, phosphorus, potassium and the like was supplied in a desired quantity in the form of an aqueous solution, a molten liquid, a slurry or solid particles from the pipe 5 or 6 to a mixing tank, the components were mixed with each other inside the mixing tank 10. The resulting slurry was pressurized by a slurry pump 12 and was supplied and sprayed to the nozzle 7 from the pipe 2.

The product having an increased particle size which was discharged from the pipe 9 was a mixture of particles having a variety of particle sizes. After the product was cooled by a cooler 15, it was classified by a classifier 1 7 into the product having particle sizes exceeding the desired particle size, the final product having the desired particle size and the product having a particle size smaller than the desired particle size.

The product having the greater particle size was stored in a tank 23, while the one having the smaller particle size was stored in a tank 22. The final product alone was fed out of the system through a pipe 1 9. The pipe 4 was a discharge pipe for the air stream blown into the granulator 1 from the pipe 3. After fine particles carried by the exhaust air were separated by a separator 14, the exhaust air was discharged into the atmosphere and the fine particles collected by the separator 14 were sent to the tank 23.

The product having the greater particle size that had temporarily been stored in the tank 23, was milled by a mill 16 and was then circulated again to the granulator 1 as the seed particles through the pipe 8 together with the product having the smaller particle size that had been stored in the tank 22. In this case, the ratio was changed between the recircuiating seed particle feed quantity from the tank 22 and that from the tank 23 by feeders 24 and 25 in order to adjust the particle size distribution of the recirculating seed particles.

Using the series of granulation equipments having the functions described above, the production test of a granular compound fertilizer containing 1 8 wt.% each of nitrogen, phosphorus (as P205) and potassium (as K2O) components and a particle size in the range of 2 to 5 mm as the final product was carried out in the following manner. The granulator used had a cylindrical shape having an inverted circular truncated cone at its lower part and the diameter of the cylindrical portion was 1 ,000 mm. Its inner capacity was 250 1. The spray nozzle 7 used for spraying the slurry had a construction as shown in Figure 3. Among the divided slurry passages 32, 33, 34, and 35 inside the nozzle, the passage 35 had the minimum diameter of 4 mm.The nozzle was of the type not using an impinging gas.

The slurry to be ejected from this nozzle was prepared in the following way. An aqueous urea solution was first prepared by dissolving 29.7 parts of urea in 7.9 parts of water at 1000C. Next, 7.2 parts of di-ammonium phosphate were dissolved in this aqueous solution at the same temperature to form a saturated aqueous solution. To this saturated aqueous solution were added 26.9 parts of di-ammonium phosphate passing through a 28-mesh sieve. The resulting slurry was sent from the pipe 5 to the mixing tank 10. Furthermore, 28.3 parts of the crystals of potassium chloride passing through a 28-mesh sieve was fed to the mixing tank 10 from the pipe 6 and the mixture was mixed to form the slurry for spraying. The slurry temperature was 1000C.

The slurry was pressurized by the slurry pump 1 2 while being heated to prevent its temperature dropping, and was ejected from the spray nozzle 7. Before the start of spraying, jet air at 580C was fed into the granulator 1 from the pipe 3 at a rate of 4,100 Nm3/hr and 250 1 of granular urea as the starting seed particles were fed so as to form in advance the spouted layer inside the granulator 1.

Upon observation with a magnifier, the slurry was found to contain crystals having the maximum length of 0.59 mm. Hot filtration was carried out to determine the volume and weight of the filtrate and the components were analyzed. As a result, the slurry was found to contain 52 vol % of liquid from the water contained in the cake. This slurry was ejected from the nozzle 7 at a rate of 1,900 kg/hr and the granulation apparatus was operated as a whole. The spray nozzle 7 could be smoothly operated without choking and, after the steady state was reached, the desired granular compound fertilizer having a particle size of 2 to 5 mm could be obtained as the final product at a rate of 1,710 kg/hr.

In contrast, all the materials to be sprayed were prepared in solutions in accordance with the conventional process and then sprayed at the same temperature of the spray solution using the same apparatus as described above without using the slurry spray process of the present invention.

In this case, the water content to be fed to the granulator 1 increased by 1,266 kg/hr in comparison with the example described above and heat energy to be applied to the granulator 1 increased in response to the increase of this water content. It is generally necessary to keep the particle layer 11 inside the granulator 1 at a temperature considerably lower than that at which melting of the particles takes place. The air feed quantity could not be increased in this apparatus. When the solution was sprayed so that the solid feed quantity became the same as that of the slurry spraying, therefore, the input heat energy became excessive in comparison with the cool-drying capacity of the air stream.

Consequently the melting of the particles occurred inside the granulator and the operation could not be continued any longer. If the solution spray quantity alone was reduced to about 1 90 kg/hr, the operation became feasible but the energy required for the production of the same quantity of the granular compound fertilizer increased remarkably.

For the sake of comparison, the sequence of preparation of the starting materials in preparing the slurry was reversed. That is, 28.3 parts of the crystals of potassium chloride and 7.9 parts of water were mixed and dissolved at 1000C, and thereafter 29.7 parts of urea and 34.1 parts of the crystals of di-ammonium phosphate were added to and dissolved in the solution so as to prepare the slurry. In this case, potassium chloride that had been dissolved was precipitated and the potassium chloride particles grew to large particles. Consequently, the slurry spraying could not be smoothly carried out and the operation of the apparatus as a whole has to be frequently stopped so as to replace or clean the spray nozzle 7.

The advantages of the present invention can be obtained exactly in the same manner as in the aforementioned embodiments even when the granulation method is not the spouted layer type granulation method, such as a conventional drum granulation method or a fluidized bed granulation method disclosed in Japanese Patent Laid-Open No. 1 6427/1979, Japanese Patent Publication No. 7442/1972 or Japanese Patent Publication No.28106/1974.

Claims (7)

Claims

1. A process for producing a granular compound fertilizer containing at least two components selected from nitrogen, phosphorus, potassium and other agronomically effective components, wherein at least one component is selected from the starting compounds urea, ammonium chloride, ammonium nitrate, sodium nitrate, calcium nitrate, ammonium phosphate and ammonium sulfate; a high temperature aqueous solution having a concentration of at least 80 percent of saturated concentration is formed from all or a major quantity of said selected compound and water in a specified quantity of up to 40 weight percent on the basis of all or the major quantity of said selected compound; a high-temperature slurry containing at least 40 volume percent of liquid is formed by adding to said high temperature aqueous solution the balance of said major quantity and/or a powdery substance which is selected from compounds other than said starting compounds and can be mixed with it as at least one fertilizer component and said high-temperature slurry is attached to seed particles by spraying the slurry onto the seed particles which are moving in a granulating zone, and the resultant product is then dried and/or cooled.

2. A process as claimed in claim 1, wherein the maximum diameter of particles present as solid granules in said high-temperature slurry is from 50 to 1,000 um.

3. A process as claimed in claim 1 or 2, which uses a nozzle for spraying said high-temperature slurry, wherein the minimum diameter of a hightemperature slurry passage inside said nozzle is at least 4.5 times the maximum diameter of solid particles present in said high-temperature slurry.

4. A process as claimed in any of claims 1 to 3, wherein said high-temperature slurry contains 40 to 90 volume percent of liquid.

5. A process as claimed in any of claims 1 to 4 wherein the surface of said nozzle in contact with said high-temperature slurry is made of a material selected from i) metallic chromium or an alloy containing at least 17 weight percent of chromium, ii) metallic titanium or a titanium alloy, iii) metallic zirconium or a zirconium alloy and iv) tungsten carbide.

6. A process as claimed in any one of claims 1 to 5, in which said high temperature is a temperature higher than ambient temperature but lower than the thermal deterioration temperature of the selected starting compound.

7. A process as claimed in claim 1 and substantially as hereinbefore described with reference to Example 2.