Urban Mining At Our Cities' Door Steps: Results Of 2 Years Of Renewable Phosphate Fertilizer Production Ash In A Pilot Plant

Abstract:

Phosphorus is a vital component of every cell in all living organisms. No other element can replace phosphorus as a primary nutrient that is essential for plant growth. Plants take up phosphorus from soil. Plant available phosphorus pool in soils needs to be replenished by spraying phosphorus fertilizers. Limited natural resources, phosphate ores, predominantly concentrated in Morocco, China and USA, are going gradually into depletion. 159 million tons of phosphates have been mined 2009. Drivers for phosphate consumption are available cropland, growth of population, changing diets and bio-fuels. Sales shall grow by 2-3% per year. Renewable phosphates from sewage sludge and slaughterhouse residues can replace up to 40% of the natural rock phosphates. After widespread and constantly increasing energy recovery in more than 400 sludge incinerators, these materials produce 3 million tons of ash with a phosphate concentration of up to 30%, being available as a waste at negative cost. Without treatment, ash is not a fertilizer because of its high heavy metal concentration and deficient phosphate plant availability. Because of an increasing concern with nutrition safety, food manufacturers and policy makers prefer energy recovery from sewage sludge opposed to its direct application as a fertilizer on cropland, the vanishing alternative phosphate recovery route. In Europe, landfill disposal of sludge is not a legal option any more.ASH DEC has developed a thermochemical process to eliminate heavy metals from ash and simultaneously make nutrients plant available. For this purpose, a mix of phosphorus rich ash and – predominantly - magnesium salts is fired at 1.000°C in a reactor. At this temperature, the toxic heavy metals form gaseous metal chlorides that leave the kiln with the process gas and are captured as filter dust in a dry air pollution control system. Simultaneously, within the solid phosphate compounds, magnesium ions partly replace calcium ions and significantly improve the plant availability of phosphates. The result of the thermochemical process is a phosphate fertilizer that compares to traditional superphosphates in terms of efficiency. The product’s heavy metal loads, however, are one to two orders of magnitude below those of most rock phosphate based fertilizers (cadmium, uranium) and below those of the sludge ash (cadmium, copper, lead, zinc).Before the process could be transferred to industrial scale, a process controlled 7 ton per day, pilot plant was set-up and operated for two years in Austria. Operations aimed at proof of the technical concept, process optimization and test of machinery and equipment. Operational experience and results are shown in the full paper and at the conference.

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