Ammonium sulfate is produced is produced as a byproduct in a number of processes both in and outside the chemical industry. In addition it is produced directly in small quantities by the neutralization of ic acid with ammonia, concentrating the solution and crystallization:

To obtain the coarse crystals required in agriculture, the reaction is carried out in special plants ("saturators") in which the small crystals have a sufficiently long residence time in which to grow.

70% is utilized with gaseous ammonia, the heat of neutralization then being sufficient to evaporate all the water. In a process operated in Japan, sulfuric acid is fed in at the top of a tower and gaseous ammonia at the bottom. The ammonium sulfate can be drawn off from the bottom, which requires no further drying.

In the USA in 1993, 14% of the total production of ammonium sulfate was manufactured from ammonia and sulfuric acid.

Ammonium sulfate is produced as a byproduct in:
(1) the working up of sulfur dioxide produced by oxidation of sulfide ores;
(2)the working up of sulfur dioxide from the desulfurization of power station flue gases. The sulfur dioxide is scrubbed from the cooled flue gases with ammonia as ammonium sulfite. The solution is then intensively treated with air to convert most of the sulfite to sulfate and is then sprayed into a 390°C gas stream, whereupon solid ammonium sulfate is produced and any ammonium sulfite is cracked to ammonia and sulfur dioxide, which is fed back into the process. The ammonium sulfate is finally granulated;
(3)the working up of ammonia produced as a byproduct in coke production to ammonium sulfate. 11% of the ammonium sulfate produced in the USA in 1993 came from this source;
(4)the working up of sulfuric acid produced in many organic processes e.g. nitration, alkylation etc. to ammonium sulfate. However, the organic impurities can interfere very strongly in the synthesis process e.g. by foaming or influencing the crystal morphology;
(5)the manufacture of caprolactam (starting material for polyamide 6) by the Beckmann rearrangement of cyclohexanone oxime in fuming sulfuric acid:

This is the most important source of ammonium sulfate and accounted for 75% of the USA production in 1993. In the "classical" caprolactam process, in which hydroxylamine sulfate produced by the Raschig process and the oxime is rearranged in sulfuric acid, 4.4 t of ammonium sulfate are produced per t caprolactam. In recently developed processes the incidence of byproducts has been significantly reduced or even eliminated.

Ammonium is manufactured by the neutralization of nitric acid with gaseous ammonia in a strongly exothermic reaction:



and is carried out in e.g. in circulation reactors, which ensure a rapid thorough mixing of the reaction components. In some cases these reactions are carried out under pressure to enable the steam formed to be used for preheating the ammonia and the acid. In this way reactor temperatures of up to 180°C can be obtained.

The thermal instability of ammonium nitrate means that the volume of the reactors must be kept as small as possible, the acid used must be as free from impurities as possible and the reaction must be controlled so as to avoid the presence of excess acid.

If nitric acid concentrations above 50% are used, it is possible, with appropriate process design, to evaporate all the water introduced without supplying additional energy.

The ammonium nitrate melt leaving the reactor generally has a water content of 3 to 5%, in some plants even as low as 0.5%. The melt is fed into the top of so-called "prilling towers" (up to 60 m high), in which the melt is so dispersed that droplets are formed which, upon cooling with air fed in at the bottom, solidify as they descend. If the melt only contains 0.5% water the resulting prills can be used directly, otherwise they have to be dried further. Granulation can be used instead of prilling.

Posttreatment is necessary, because ammonium nitrate is highly hygroscopic. In view of its strong oxidizing power, only inorganic substances such as attapulgite, kieselguhr or clay can be used.

In the Federal Republic of Germany, the use of pure ammonium nitrate as a fertilizer is forbidden on safety grounds. Mixtures, particularly with calcium carbonate, are used instead. Currently mixtures with N-contents of up to 28% are allowed.

Industrially urea is only produced from ammonia and carbon dioxide. Since carbon dioxide is a byproduct in the production of hydrogen for use in the synthesis of ammonia from natural gas or crude oil (in the case of natural gas only in 90% of the required amount), a urea plant is often coupled with an ammonia synthesis plant.

In the first step ammonia and carbon dioxide react forming ammonium carbamate:

At high pressures this reaction is quantitative. Ammonium carbamate is in equilibrium with urea and water:



Ca. 70% of the carbon dioxide is converted into urea at an ammonia to carbon dioxide molar ratio of 4:1, a temperature of 200°C and a pressure of 250 bar.

The unconverted ammonium carbamate and unreacted ammonia have to be removed from the reaction mixture which consists of an aqueous solution of urea, ammonium carbamate and ammonia. The numerous industrial processes particularly differ in the way in which this separation and the recycling of ammonia and carbon dioxide are carried out, the minimizing of the energy consumption of these large plants (up to 1700 t uredd) being crucial.

In the first plants, using the "once-through" process, the mixture of carbon dioxide and ammonia resulting from pressure release and thermal decomposition of ammonium carbamate was worked up to ammonium nitrate or sulfate, the carbon dioxide being vented. Modern plants operate with total recycling of carbon dioxide and ammonia achieving yields based on ammonia consumed of 98.6 to 99.5%. Since considerable product loss occurs in the subsequent prilling of the urea, the real yields are even higher.

Current processes comprise:
(1)solution recycling processes
(2)stripping processes

In the former, the pressure applied to the solution leaving the reactor is reduced stepwise, the carbon dioxide and ammonia released upon each pressure reduction being returned at their respective pressures to the absorber for absorbtion in water or in the urea mother liquor, in countercurrent.

The pressure on these absorption solutions is increased from step to step, until the synthesis pressure is regained. With this countercurrent absorption a concentrated ammonium carbamate solution is obtained containing little water, which produces equilibrium conditions favorable for urea formation.

Recycling processes in which the first decomposition stage is operated at 60 to 80 bar require even less heat (Mitsui-Toatsu, Montedison process). In the stripping process, the solution, upon leaving the reactor, is fed into the top of a film evaporator operated at the reaction pressure in which the solution flowing downwards is contacted countercurrently with all the carbon dioxide required in the process. The gases which leave via the head of the film evaporator are partly condensed in the pressurized solution coming from the low pressure decomposer, together with part of the freshly supplied ammonia. This mixture of liquid and gas is fed into the reactor. Since 85% of the carbamate is decomposed in the stripper, a single adjoining low pressure decomposer is sufficient to complete the dissociation. The heat produced in the high pressure condenser is converted into steam, which is used in the operation of the low pressure decomposer (Stamicarbon process).

Instead of stripping with carbon dioxide, ammonia can be used (Snam Progetti process). The stripping processes are energetically somewhat more favorable than the recycling processes with high pressure decomposers.

The 72 to 77% urea solutions obtained with these processes are, after prepurification e.g. over activated charcoal to remove oil, either vacuum evaporated until crystallization takes place or evaporated in a falling film evaporator to a urea melt (m.p. 132.7 °C). These urea melts or molten crystals are generally prilled (however granulated urea is aIso produced).

Urea cakes fairly easily upon storage. This tendency can be reduced by posttreatment e.g. with kielelguhr or formaldehyde. Sulfur- or polymer-coated urea can be used as a controlled release fertilizer, particularly in the USA (114^.10³ t/a and 42^.10³ t/a respectively).

Urea forms upon heating:



which is harmful to some plants. The biuret concentration must therefore be kept low (technical urea contains 0.3 to 1 % biuret).
 
Since urea is relatively quickly metabolized, so-called "controlled release fertilizers" have been developed. Examples are crotonylidene urea (Ⅰ) (from urea and ) and isobutylidene urea IBDH (Ⅱ) (from urea and isobutyraldehyde). The most important are, however, the urea-formaldehyde products of which 191.103 t were produced in the USA in 1993.