Don't Blow It!
As already mentioned on the nitric acid page, the main use for nitric acid is to manufacture fertilizer. The other reagent needed is (pure) ammonia. If these two components are mixed, they will react and form ammonium nitrate. Ammonium nitrate is a substance that should be treated with care. At high concentration it can be detonated if the right conditions are met. In the fertilizer industry seven factors have been identified which can enhance the detonation of ammonium nitrate. The most well known factor is contamination with organics (i.e. gasoline), but other aspects can play an equally important role. Although the industry is well aware of these factors, history regrettably proves that every now and then ammonium nitrate handling leads to an (often vigorous) explosion. It goes without say that currently the industry mainly focuses on the improvement of safety in processes where ammonium nitrate is involved.
Ammonium nitrate is a basis for a wide range of fertilizers. It can be used in pure form, as calcium ammonium nitrate (CAN) or as NPK-fertilizer. N, P and K are the chemical symbols for nitrogen, phosphorous and potassium and NPK fertilizers are "custom made" fertilizers with varying amounts of nitrogen, phosphorous and potassium for specific fertilizing purposes. An NPK-fertilizers is in general identified by three numbers, i.e. 6-3-6 stands for a fertilizer grade (in this example a liquid fertilizer), containing 6% of nitrogen, 3% of phosphorous and 6% of potassium. Since the concentration of ammonium nitrate is lower in CAN and NPK, the risk of explosion is minimal. It may however decompose if exposed to high temperature. This page will focus on calcium ammonium nitrate production (CAN).
In a typical CAN-plant, the first stage is the production of a solution of ammonium nitrate. This is done by contacting gaseous ammonia with nitric acid (with a concentration of 50% - 60%). The reaction generates a lot of heat, evaporating a part of the water present in the nitric acid. The resulting ammonium nitrate solution is concentrated in a couple of evaporators to a concentration of around 97%.
The second component needed to produce CAN is calcium. Calcium is added in the form of finely grinded limestone, often referred to as lime flour. The lime flour can be obtained directly as a raw material or can be grinded on site from lumps of limestone. Often you will find a ball mill as a grinding tool. A ball mill can most accurately be described as a piece of equipment that makes a lot of noise, consumes a lot of power and creates vibrations all over the place. It consists of a (large) rotating cylinder equipped with spikes in which steel balls are placed. The lumps of limestone are fed to the ball mill and are crushed by the steel balls., producing a fine powder of limestone.
The next step is putting the two components together. This is done in a granulator. A granulator not only mixes the components but also, surprise(!), creates granules of the mixture. Since one granulation step is not enough to create granules that are large enough, the granules are put into the granulator several times. Every time a small layer is added to the granule, increasing the diameter of the granule. As a result, the granulator contains several "generations" of granules. After exiting the granulator, the product is screened. The granules with the right diameter are separated from the fines and the coarse particles and are further continue in the process. The fines and the coarse product (the latter after crushing) are returned to the granulator.
In the granulator, a limited amount of lime flour and ammonium nitrate reacts to calcium nitrate. This reaction is more or less undesired, since calcium nitrate is very hygroscopic (that means it attracts water out of the atmosphere). If the granules absorb too much water, they will get soft and disintegrate easily. To prevent this from happening the granules are sprayed with a thin layer of coating that protects it from attracting water.
Finally, the finished product is stored in large bins or silo's. The storage area has to be kept absolutely dry and no open fire is allowed in a CAN storage area, mainly to prevent decomposition. The risk in nitrate decomposition lies in the fact that nitrate contains oxygen. If a decomposition reaction starts, it will keep itself alive because it supplies the oxygen needed to continue the reaction. The only way to stop the reaction is by "drowning" the product with huge amounts of water.