The project "Concentrating phosphorus" developed a production process for a highly effective fertilizer made from steel slag and residues containing phosphorous.
Phosphorus is an important plant nutrient: In Germany 85 percent of all phosphate imports are used for agriculture. However, natural resources are becoming increasingly scarce and phosphates from domestic secondary sources such as sludge and meat and bone meal are not readily available to plants. It is necessary to improve phosphorous recycling and tap new sources to meet the growing demand for agriculture world-wide and to limit the cost increase for phosphorous fertilizers.
The project partners developed a process which produces a new, highly effective converter lime made of residues from burning sludge and meat and bone meal. Secondary phosphate carriers such as sludge ash are added to the liquid steel slag. Thermal digestion enriches the steel slag with phosphorous and removes heavy metals from the sewage sludge ash.
Phosphorous occurs as a bio available calcium silicate phosphate in the solidified slag. The steel slag has been enriched with phosphorous and complies with the heavy metal content of the German Fertilizer Ordinance. As this new fertilizer contains phosphorous and lime, it is possible to combine lime and phosphorous fertilization in a single cycle. The produced fertilizer is very similar to the Thomas phosphate or Thomas lime® which was previously produced as a result of smelting high-phosphorous ores.
Converting so far unused ashes containing phosphorous into an effective fertilizer saves natural resources and reduces phosphorous imports. It is possible to save a total of 4,000 tons of P2O5 in phosphorous imports per year.
As the thermal phosphate digestion uses the latent heat of the molten slag, this process does not require additional energy. The production of this new fertilizer is practically energy self-sufficient. Compared to other production processes, which do not use these energy-related advantages, energy expenditure of roughly 200 giga watt hours and emissions of around 33,500 tons of CO2 can be avoided when producing 80,000 tons of steel slag enriched with phosphates. If the infra structure is adapted accordingly, the new process can be incorporated into an integrated steel mill.