Method for the preparation of granular ammonium sulphate

Method for the preparation of granular ammonium sulphate

• The invention relates to a method for the production of ammonium sulphate and to the resulting product, which is granular, free-flowing and non-agglomerating. The product produced by this method has a minimum title of 20.5% by weight, and a granular composition and hardness similar to that of ordinary fertilisers.

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• Ammonium sulphate has many applications, the most important being in the manufacture of fertiliser, for which it is a source of nitrogen (ammonium) and sulphur.
  Crystalline ammonium sulphate can be prepared in many ways, both as a by-product and as the primary product of the reaction between sulphuric acid and ammonia. It is very frequently used in blends with other types of fertilisers. Crystalline ammonium sulphate, however, has a number of major drawbacks, including the fact that its grains are small in size and irregular in shape, do not flow easily, have a tendency to agglomerate and are lacking in hardness. This makes it difficult to use as a fertiliser, given that fertilisers require uniformity of dispersion and specific grain properties (shape - dimensions).
  Numerous attempts have been made to reduce these defects by improving the crystallisation process. One such way is the use of a reverse-flow crystallisation process, in which the small crystals that are formed at low temperatures are mixed with a hot solution of ammonium sulphate or acidic ammonium sulphate. While there is an improvement in the crystalline ammonium sulphate thus produced, the production cost is high and, in addition, the grains are irregular in shape and dimensions, making them difficult to use as fertiliser.
  Further improvements in the product were achieved with the use of granulation techniques, yielding a granular product, enhanced in certain cases with the use of additives, such as a variety of metallic ions, phosphoric acid, phosphorites, urea, ammonium nitrate and other compounds. While these techniques have brought about further improvements, existing methods nonetheless still produce a granular product of little hardness and with a substantial percentage of grains outside the desired size range (2-4.5 mm).
  From the technical standpoint, all granular ammonium sulphate production methods present problems of corrosion of the material of the installations used, because of the low pH of the liquor produced during the reaction. Furthermore, none of the occasional improvements that have been achieved have brought about any substantial increase in production rates.
  This invention eliminates the above drawbacks and creates a series of advantages, which are analysed below. The main feature of this invention is the combination in series of a agitated cylindrical reaction tank and a tubular reactor, and the primary gain is the increase in production rate in comparison with the use of just one of the above machines. In addition, the extensive use of recycled water, either from the stripping of tail gases or from other factory units, allows the liquor produced in the reactor to be kept at a fairly low density, moderating the quantities of ammonia channelled into the tail gas stripping system, while finally, given the relatively high (compared to other methods) pH of the liquor, there is no corrosion of the reaction tank.

• As we have said, the production of granular ammonium sulphate is carried out in two successive stages, which are shown in diagrams N°1 and N°2, attached.
  In the first stage, ammonia (liquid or gas), sulphuric acid of at least 70% concentration by weight and recycled water from the stripping of tail gases are fed into a reaction tank fitted with co-axial agitators. Alternatively, two agitation tanks in series may be used. The reaction temperature is 100-120°C, with a usual value of 108-115°C, and the pressure is normal atmospheric pressure. The water added is acidic because of the presence of acidic ammonium sulphate. The reaction tank is made of steel with an acid-resistant lining. The liquor produced has a moisture level of 34-42%, a density of - g/l with a usual value of - g/l, and a pH of 2.5 - 6 with a usual value of 3- 5.
  From the overflow of the tank, the liquor produced flows into an agitated intermediate vessel, to which may be added liquid effluents from other parts of the plant. From the intermediate vessel, the liquor produced in the first stage is first mixed with the rest of the sulphuric acid of the reaction and then pumped into the tubular reactor. To the tubular reactor are also added water and ammonia, in either liquid or gaseous form. The pressure is 3-6 bar, usually 3.5-5 bar, and the temperature is over 130°C. The liquor that is projected from the tubular reactor into the rotating granulator has a moisture level of 5-12% after expansion. To the granulator is also added a small quantity of ammonia (liquid or gas) to complete the reaction. The pH of the product in the granulator, measured in a 1% solution by weight, is 3-5, with a usual value of 3.2-4.5, and the moisture level is 2-4.5%.
  The tail gases from the reaction tank and the granulator are conducted into 2 stripping columns, where the ammonia is neutralised and absorbed by recycled water and sulphuric acid. The acidic ammonium sulphate solution produced is fed into the reaction tank and the tubular reactor instead of plain recycled water.
  In both stages of preparation the proper feeding of raw materials and water is very important. This must be done as follows:

• Sulphuric acid : Discounting the acid from the stripping process, the remainder is fed into the reaction tank and the tubular reactor at a desired proportion of 45-55% respectively. In total 31-45% may be fed into the tank, 42-52% into the tubular reactor and 10-20% into the vessels where, mixed with recycled water, it is fed into the stripping columns to capture the ammonia.
• Ammonia : The ammonia is fed in equal quantities into the reaction tank and the tubular reactor. Specifically, 37-55% of the total ammonia is consumed in the reaction tank, 42-52% in the tubular reactor and 3-11% in the granulator.
• Acidic solution from stripping columns : As we have already mentioned, recycled water is fed into the vessels, where it is mixed with part of the sulphuric acid and then used to strip the tail gases. Of the resulting solution 5-11% is fed into the tubular reactor and the rest into the reaction tank.
  After granulation the product undergoes a series of processes to give it its final form; these are as follows :
• Drying in a rotary oven with a concurrent flow of exhaust gases from the combustion of natural gas (or diesel oil). The temperature of the exhaust gas at the entrance to the drying oven is 440-480°C, and may rise to 500°C, while the fertiliser exits at 95-110°C, with a usual value of 98-105°C.
• Sifting, in two stages, when the fine grains are returned to the granulator and the coarse ones sent for crushing, while the marketable grains (2-4.5 mm) move on to the cooling stage.
• Crushing of coarse grains, which are then returned to the granulator
• Cooling, with (cold) atmospheric air in a solid-fluid bed to a final temperature of 10-45°C (depending on climate conditions), and simultaneous dust removal.
• Coating with some suitable material, to keep the grains from agglomerating; this is done in a rotating drum called a coater.

• The final product produced by the above method has the following chemical properties:

• Nitrogen content : minimum 20.5% by weight, entirely in ammoniac form, using the method of analysis described in directive 77/535/EEC, Method 2.1.
• Sulphur content : minimum 23% by weight, using the method of analysis described in directive 89/519/EEC, Method 8.4, 8.9.
• Free moisture : maximum 0.3% by weight, measured by the ISO method.
• pH : between 2.8 and 4.5 , measured in an aqueous solution 1% by weight.

• Similarly, the physical properties of the final product produced by the above method are as follows :

Form

: granular, free-flowing, non-agglomerating

Granulometry

: 90% by weight minimum between 2 and 4.5 mm, and specifically
: > 4.5 mm&#;&#;&#;0.3% by weight maximum
4.5 mm - 4 mm&#;&#;&#;1-3% by weight
4 mm - 2 mm&#;&#;&#;87-95% by weight
< 2 mm&#;&#;&#;5-9% by weight
according to the Pr EN method (ISO modified)

Hardness

: Average value : 3 kg minimum, and usually more than 4 kg
Maximum value : 5 kg at least, and usually more than 5.5 kg
Minimum value : 2 kg at least, and usually more than 3 kg

determined on a sample of at least 10 grains with a diameter of between 3.15 and 4 mm by measurement of the necessary applied weight to crush each grain.
• SGN (size guide number)
  as determined by the formula: SGN = d50 x 100, where d50 is the diameter of 50% of the grains according to the cumulative distribution of the granulometric analysis.
• UI (uniformity index) 50-55
  as determined by the formula: UI = (d5 / d90) x 100, where d5, d90 are the diameters of 5% and 90% of the grains respectively, according to the cumulative distribution of the granulometric analysis.
• Tendency to agglomerate : 0.3 kg at most (usually 0)
  determined by an original method which measures the applied weight necessary to crush a sample of about 30 grams of product that has been centrifuged at rpm for 4 hours and then left to rest for another 4 hours. The method is repeated once, and the final value is the average of the two results.

• The advantages of this invention are many and varied. The direct and obvious advantage lies in the properties of the product itself, which has grains of uniform size (as demonstrated by the granulometric analysis, the SGN and the UI) and appropriate hardness (as demonstrated by the hardness index) that do not agglomerate.

• There are in addition, however, a number of secondary advantages, relating to the production process. These are :

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• 1. Full use of recycled water. Since the method requires large quantities of water to produce a reaction liquor of the desired low density, this permits the recycling of various liquid effluents not only from the production unit itself but from other parts of the plant and even from other factories. This has a twofold significance, since on the one hand it reduces the quantity of liquid effluents released into the final liquid receptor and on the other it reduces ammonia losses into the atmosphere.
• 2. Minimalisation of the corrosion of the reaction tank. The consequence of the low density of the liquor in the reaction tank and the capturing of gaseous ammonia in the stripping process, which is then returned to the reaction tank, is a higher pH than in other methods. This not only results in less chemical corrosion but at the same time prevents mechanical erosion, on the one hand because the higher pH prevents crystallisation and on the other because of the fact that the liquor is thinner than in traditional methods.
• 3. Use of smaller granulator. Since most of the gases created are captured by the reaction tank, the volume of gas in the granulator is less than in traditional methods. Thus in a new unit applying this process the granulator could be significantly smaller than in a traditional unit of the same capacity.
• 4. Increased production in existing units, as a consequence of advantage number 3. In the unit at SICNG the production rate using this method rose from 35 t/h to 65 t/h, for a period of 72 hours of continuous operation. This rate of production may be expected for a single production line and for a total consumption of electricity of 30-40 kWh/t of product or - kWh/hour of operation.

• The two figures below show in sequence the simplified flow chart of a compound fertiliser production unit that applies the method described above for the production of granular ammonium sulphate. Figure N°1 shows the standard reaction tank (1), the intermediate vessel (2), the granulator (3), the tubular reactor (4), the gas stripping system (5) and the water reservoirs. Figure N°2 shows the drying oven (7), the sieves (8), the crushers (9), the final sieve (10), the gas-stripping, drying and cooling system (11), the cooler (12) and the coater (13). The diagrams do not show various machines for the propulsion of solids, liquids and gases, which are not characteristic of the production installation for this product, while the arrangement of the machines as given on the diagrams is not binding and may differ in a real installation. Finally, these diagrams may in no sense be considered working drawings for the forms of the machines in question.

• Presented below are three examples of the application of this invention, which however must be taken as merely indicative and not be considered as defining the precise limits of the present invention.

Why Ammonium Sulfate