Phosphate recovery process

Outotec is the only company in the world that is mastering the entire value chain from ore to refined metals and delivers life cycle solutions. Now this knowledge is extended to renewable raw materials starting with the phosphorus rich by-products from wastewater treatment, biogas and meat processing plants.

The ASH DEC process ideally complements Outotec's existing biomass incineration solutions based on fluidized bed technology and facilitates the integration of the ash-to-fertilizer process into a single, sustainable facility to recover energy and nutrients from biomass and sludge.

The ASH DEC technology is a thermal decontamination process comparable to calcination. It is appropriate for phosphate recovery from ash that remains as a by-product from incineration of municipal sewage sludge, manure and chicken litter and residues from anaerobic digestion and slaughterhouses.

Outotec’s scope:

• Project studies
• Technology customizing
• Turnkey sludge-to-energy
• Sludge-to-energy & fertilizer
• Ash-to-fertilizer plants
• Manufacturing licenses 
• Service solutions for the plant’s entire lifetime.

Typical customers for turnkey plants are municipal operators of wastewater treatment plants and sludge incinerators or companies of the energy and the waste & sludge management sector. The process is on the point of transfer to the industrial scale – Outotec is currently working on the conceptual design of two industrial projects.

The ASH DEC technology is a thermal decontamination process comparable to calcination. It is appropriate for phosphate recovery from ash that remains as a by-product from incineration of municipal sewage sludge, manure and chicken litter and residues from anaerobic digestion and slaughterhouses.

Ash and natural earth alkali salts are fed to a thermal reactor and exposed for about 20 minutes to a temperature of 1.000-1.050°C. At this temperature, the target heavy metals – usually mercury, cadmium, lead, copper and zinc - react with the salts, become gaseous and evaporate. Simultaneously, the hardly soluble phosphate compounds are transformed into plant available species.

A phosphate powder, rich in secondary and trace nutrients and free of toxic contaminants is continuously discharged from the reactor and cooled. Heavy metals, representing about 1% of the input material flow, are retained in the air pollution control system in the form of filter dust. If and when organic substances survive the incineration, they are definitely destroyed during the thermal treatment.

To produce fertilizers that meet the requirements of crops, soils and markets, the semi-product may be enriched with additional primary, secondary and trace nutrients and compacted to fertilizer granules.

The process was tested and optimized for almost two years in a 7 ton per day pilot plant between June 2008 and April 2010.

The renewable calcined phosphate fertilizers, sold under the PhosKraft® brand, meet the most stringent existing and foreseeable requirements of fertilizer regulations in Europe.

They contain two orders of magnitude less cadmium and one order of magnitude less uranium than most phosphate rock based fertilizers and are equally effective in terms of yield and phosphate uptake by crops.

Calcined phosphates perform particularly well on soils with pH-values below pH=7. On soils with pH=6 and lower, they usually outperform water soluble, traditional phosphate fertilizers.

Their characteristics are close to the well proven Thomas phosphate that used to be a by-product from steel production and is no longer available.

The product matches the principles of organic farming and is at present under scrutiny for being admitted in Annex II A of Regulation (EC) 889/2008. 

Leading soil and plant nutrition research institutes in Germany, Switzerland, The Netherlands and Austria have tested and confirmed the product quality in numerous pot and field tests. As a consequence, Austrian and German governments have licensed PhosKraft® fertilizers for application on pasture and cropland.