Nitroguanidine is crystallized by maintaining at a temperature not exceeding about 100 DEG C. a supply of a slurry comprising nitroguanidine and sufficient water to dissolve it at a temperature below about 120 DEG C., causing a progressively renewed fraction thereof to flow under pressure through a space in which it is progressively heated to cause all the nitroguanidine to dissolve, filtering the resulting solution and discharging it through a narrow orifice into a region at a pressure not substantially exceeding atmospheric at a throughput rate which precludes any appreciable decomposition of the nitroguanidine. The pH of the slurry is kept below 9 by adding suitable substances (see Group IV) to reduce the acidity of the product. The space in which the slurry is heated under pressure should have a <PICT:0584907/III/1> large ratio of heating surface to volume and may be a narrow extended tube heated externally by a current of heating fluid. An electrical resistance wire wound round the tube or an external source of radiant heat may also be used or the slurry may be heated in a narrow annular space whose inner and outer walls are heated. In the figure, nitroguanidine, water and weakly alkaline material are introduced into the steam jacketed pot 4 and stirred to maintain a slurry. The jacket is heated to between 60 DEG C. and 100 DEG C. and the slurry forced by compressed air through the pipe 10 and the regulating valve 11 into the pipe 20 and thence into the pipe 21 surrounded by the jacket 14 into which dry pressure steam is introduced to heat the slurry. The rate of flow of the slurry and the steam pressure are adjusted so that the nitroguanidine is all dissolved only as it approaches the filter candle 17 which retains any extraneous matter. The liquid emerging from the fine jet 19 is atomised into a cloud of droplets which are rapidly cooled by the sudden release of pressure so that crystallisation of the nitroguanidine in each droplet takes place. The slurry from the vessel 4 may be passed to more than one unit for beating and atomising and a portion of it may be circulated p externally from and back to the vessel 4. The process can be worked continuously by supplying a cold slurry of nitroguanidine in water to the vessel 4 at the same rate as it is transferred to the pipe 21. In examples: (1) Crude nitroguanidine containing occluded sulphuric acid is made into a slurry with water and sodium acetate trihydrate. The filtered solution enters the jet at about 135 DEG C. and the spray is collected in a cooled receptacle into which a separate spray of cold water is introduced; (2) Nitroguanidine as in (1) is made into a slurry with water and calcium carbonate and attains a temperature of 142 DEG C. at the filter.ALSO:<PICT:0584907/IV/1> Nitroguanidine is crystallised by maintaining at a temperature not exceeding about 100 DEG C. a supply of a slurry comprising nitroguanidine and an amount of water sufficient only to dissolve it at a temperature below about 120 DEG C., causing a progressively renewed fraction thereof to flow under pressure through a space in which it is progressively heated to a temperature which causes all the nitroguanidine to dissolve, filtering the resulting solution and discharging it through a narrow orifice into a region at a pressure not substantially exceeding atmospheric at a throughout rate which precludes any appreciable decomposition of the nitroguanidine. The slurry may contain about 2 to 5 parts water per part of dry nitroguanidine and to reduce the acidity of the product there is included a substance which does not cause its pH, as measured at atmospheric temperature, to exceed 9 but which is in sufficient amount to react with the whole of the sulphuric acid with which the nitroguanidine may be contaminated. Suitable substances for this purpose are, e.g. borax, sodium acetate, potassium sulphite, di-sodium hydrogen phosphate, magnesium acetate, calcium acetate, potassium acetate, zinc oxide, and the carbonates of calcium, lead, magnesium or barium. For a desirable throughput the time for which the mixture exceeds a temperature of 120 DEG C. will seldom exceed 30 seconds and may be less than 10 seconds before it is discharged. The space in which the slurry is heated under pressure should have a large ratio of heating surface to volume and is preferably a narrow extended tube which is heated externally by means of a current of heating fluid but heating may be effected by an electrical resistance wire wound round the tube or from an external source of radiant heat. The slurry may also be heated in a narrow annular space whose inner and outer walls are heated. In the Figure: nitroguanidine, water and weakly alkaline material are introduced into the steam jacketed pot 4 and stirred to maintain a slurry. The jacket is heated to between 60 DEG C. and 100 DEG C. and the resulting slurry is forced by compressed air through the pipe 10 and through the regulating valve 11 into the pipe 20 and thence into the pipe 11. Dry pressure steam is introduced into the jacket 14 to heat the slurry and the rate of flow of the slurry and the steam pressure are adjusted so that the nitroguanidine is all dissolved only as it approaches the filter candle 17 which retains any extraneous matter. As the liquid emerges from the fine jet 19 it is atomised into a cloud of droplets which are rapidly cooled by the sudden p release of pressure so that crystallisation of the nitroguanidine in each droplet takes place. The slurry from the vessel 4 may be passed to more than one unit for heating and atomising and a portion of the slurry may be circulated externally from and back to the vessel 4. This vessel may be made of cast iron, the jet 19 of non-ferrous metal or stainless steel and the pipe 21 of ferrous metal, e.g. steel or stainless steel or of non-ferrous metal, e.g. copper. The process can also be worked in a continuous manner by supplying a cold slurry of nitroguanidine in water to the vessel 4 at the same rate as it is transferred to the pipe 21. In examples: (1) Crude nitroguanidine containing occluded sulphuric acid, produced by treating guanidine nitrate with sulphuric acid, is made into a slurry with water and sodium acetate trihydrate. The slurry takes about two-thirds of a minute to travel the length of the tube and the filtered solution enters the jet at about 135 DEG C. The spray is collected in a cooled receptacle beneath the jet, into which a separate spray of cold water is introduced. The crystals obtained are filtered, washed, dried and milled. (2) Nitroguanidine as in (1) is made into a slurry with water and calcium carbonate. The slurry takes 12 seconds to travel along the vertical pipe and attains a temperature of 142 DEG C. at the filter.