Recovery of Valuable Materials from Fertilizer Production Wastewater
Ammonium nitrate as a source for nitrogen is a well-known and accepted fertilizer.The first step of production is the neutralization reaction of gaseous ammonia (100% NH3) and nitric acid (54-60% HNO3) according to the following equation:
NH3 + HNO3 → NH4NO3
From the resulting 76% ammonium nitrate solution a solid product with a purity level of 99.6% is recovered by evaporation of water in multiple step vacuum-evaporizers, followed by prilling. The evaporated water is condensed and discharged with a temperature of 80°C to 90°C. Valuable materials are found in this condensed process stream as there are concentrations of 0.3 to 5g/L ammonia (NH3) and 3 to 10g/L ammonium nitrate (NH4NO3).
The loss of ammonia and ammonium nitrate along with this wastewater means a loss of 1.5% to 4% of the total production yield. Depending on the plant design and efficiency of the operation, the loss in yield can even be higher.
The loss of thermal energy along with the condensate is another deficit. Both these effects reduce the profitability of the operation. Depending on the plant size this can amount several million Euro per year that “go down the drain” in a proverbial sense of meaning.
The discharge of N-NH4 and N-NO3 into the environment is harmful since the fertilizers enhance growth of algae and bacteria in natural water bodies. Input of nutrients into the water above a natural balanced level causes so called “eutrophication“ of the water body and can result in fish kill and a loss of biodiversity.
There have been several attempts already to clean this type of wastewater e.g. by means of biological treatment methods. But it was found that this method is very costly since with conventional nitrification/denitrification huge amounts of organic material have to be dosed to the wastewater to feed the microorganisms with a carbon source. In addition to that nitrogen feed concentrations are relatively high. As a result it is very difficult to accomplish emission limits of 1.5mg/L N-NH3 to 10mg/L N-NO3: This would equal a removal rate of 95% and more, which would mean at least a multi-stage plant and most likely a polishing step.
Solution
A solution for this situation is offered by the so called Fertarex® water treatment p
rocess which has been developed by the Swiss company ArionexWasseraufbereitung. Six plants of this type have been built in Europe and Asia. The Fertarex® process delivers the following benefits:
- Almost all the ammonium nitrate and ammonia is recovered from the wastewater, resulting in a much improved production yield
- Reliably staying within discharge limits it protects the environment and prevents penalties
- The wastewater is transformed into a demineralized water with rest-conductivities of 0.06-0.1µS/cm, which can be beneficially reused on site and partially offset the requirement for pure water production
- The produced demineralized water can be used for the on site production of nitric acid or as boiler feed water
- Elegant safety philosophy
- Zero or significantly less discharge of water into the environment
Basics of the Fertarex®-Process
At a first glance a Fertarex® plant very much looks like a conventional demineralization plant with cation and anion exchanger followed by a mixed bed polisher. But there are several details that make it special. In the following the principle of operation is explained.
As shown in figure 1 in a very simplified way the strong acidic cation exchanger (RCat-H) removes ammonia (NH3) along with ammonium (NH4+) ions from the wastewater. The reaction scheme is as follows:
Exchange reaction taking place at the strong acidic cation exchanger (RCat-H):
NH3+NH4NO3 + 2 RCat-H ↔ 2 RCat -NH4+HNO3 (I)
In the following step a middle basic anion exchanger (RAn-OH) removes the nitric acid (HNO3), which is leaving the cation exchanger:
Exchange reaction taking place at the medium base anion exchanger (RAn-OH):
HNO3 + RAn -OH ↔ RAn-NO3 +H 2O (II)
As can be seen from reaction schemes (I) and (II) entirely all nitrogen compound entering into the system are bound by the ion exchangers and at the same time water is formed.
figure 1: Principle of removing ammonium nitrate and ammonia from wastewater by means of two in line ion exchange filters. By re-combining the regenerant solutions one obtains a concentrated ammonium-nitrate solution that can be passed back into the production. At the same time demineralised water is produced that can be further converted into high quality demineralised water.
After passing through the two filtration steps the water is fairly desalinated and exhibits a rest conductivity of 12 – 25µS/cm. Only traces of salt are left (5 to 15 mg/l NH4NO3). To remove the remaining traces the water is pumped through a mixed bed absorber, which is filled with a mixture of strong acidic cation- and strongly basic anion-exchangers. This mixed bed adsorber will remove rest traces of salt down to a rest conductivity of 0.06 – 0.1µS/cm. This demineralised water is suitable for high purity applications, such as for boiler feed.
Once exhausted, the ion exchange filters need to be regenerated and returned to their active form. They are then ready to be returned to service for the next loading cycle. The relevant exchange reactions are described below:
