Natural phosphate sources low in heavy metals are getting scarce. Containing about 15 mass-% of P2O5, sewage sludge ash can be considered a secondary phosphorus (P-) source. The P-content in the European sewage sludge could currently replace roughly 15% of the phosphate imports into the EU.
Hence already for many years, almost decades, it has been tried to recover phosphorus from sewage, sludge and ashes in various ways of which none has yet been realised at industrial scale. The reason for this failure lies firstly in the wet chemical approach, meaning complex and little efficient processes with liquids hard to handle; and secondly in the use of liquid or dewatered sludge as well as waste water, which results in a further decrease in efficiency mostly because of high mass flow and matrix effects.
The RecoPhos process is a thermal process using ash from sludge mono-incineration. The principle of the used so-called InduCarb process is similar to the one of the known Woehler process; dried sludge can be added as heat source or reducing agent as an option. The phosphate (amongst other constituents) is reduced on an inductively heated coke bed to white phosphorus, which is later condensed and thus separated from other gaseous reaction products; white phosphorous is the most valuable form of phosphorous and highly asked for by the industry. Further products are an iron alloy as well as a heavy metal mixture, both usable in steel industry; a silicate slag for the use in cement ovens as well as a high calorific gas.
The RecoPhos process uses an innovative reactor (InduCarb) designed for the reductive recovery of steel work dusts. By the use of ashes the material flow is minimal; if only sludge is available, it can be also used as input, adding flexibility to the concept. If additives are needed, suitable industrial wastes can be used.
The innovative RecoPhos process has never been realised before. It is planned to apply it for a patent .
Fields of science
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Funding SchemeCP - Collaborative project (generic)