Proven Process Technologies for Urea Fertilisers Enriched with Elemental Sulphur or Ammonium Sulphate

Sulphur (S) deficiency occurs widely in many parts of the world, due to a combination of factors: reduced use of Single super phosphate, which contains gypsum (CaSO4), more S being removed from fields due to increasing yields, and declining soil reserves due to erosion and leaching. In addition, emissions of sulphur dioxide from burning fossil fuels have historically provided a large S input to soil, as both rain and deposition of dust. As these emissions reduce, so soil deficiencies are increasing. Fertilisers enriched with S are now increasingly used to correct sulphur deficiencies.

A better source of S may be ammonium sulphate (AS), which makes the S available in a form that can be directly taken up by plants. By itself, AS is less attractive as a fertiliser for fertilising purposes, due to its relatively low nitrogen (N) content (21%) and its high S content. But, by combining AS with urea, it is possible to produce fertilisers with N and S concentrations suited to specific requirements of crops. Thus, for example, by combining the components in a urea / AS weight ratio of about 4 to 1, a fertiliser can be obtained which contains about 40% N and about 5% S, which is suitable for many fertilising purposes.

Urea fertiliser is characterised by significant loss of N (up to 70%) due to its high mobility in the environment. The main problems associated with low Nutrient Use Efficiency are a significant impact on climate change and possible environmental toxicity, eutrophication and nitrate pollution, ozone and air quality degradation, and emissions of greenhouse gases.

A N plus S fertiliser can enhance the utilisation rate of N due to their well-known synergy; plants need sufficient levels of S to be able to utilise N efficiently. Together, N and S are vital building blocks for protein, so N and S should be applied at the same time.

The challenge when adding AS to urea melt is its limited solubility, and thus for high N/S ratios a slurry needs to be handled. With S the challenge is different but no less difficult. This paper will describe which production technologies are available to produce these high value urea with elemental sulphur and urea with ammonium sulphate fertilisers and which reference installations are already available for each.