Möglichkeiten der Rücklösung und Rückgewinnung von Phosphor und anderen Wertstoffen aus Ruhlebener Klärschlammasche durch Säuren, Basen und/oder Additiven

In this master thesis the redissolution and recovery of phosphorus (P) and other valuable materials from Ruhleben sewage sludge ash (R-SSA) with HCl and H2SO4 was investigated using experimental laboratory tests. The parameters acid amount, solid liquid ratio (s/l ratio) and reaction time were varied and their influence on the redissolution of Ca, P, Al, Fe and SO4 was measured. Results showed that HCl(37%) resolved 91 ± 4 % P with an acid amount of 4 mL on 5 g R-SSA, a s/l ratio of 1:20 (acid concentration 0.46 mol/L) and a reaction time of 60 min. H2SO4(30%) on the other hand resolved 89 ± 3 % P at 6 mL on 5 g R-SSA, a s/l ratio of 1:10 (acid concentration 0.6 mol/L) and a reaction time of 60 min. Ca and SO4 showed very good redissolution of up to 100 % with HCl. The Ca redissolution with H2SO4 is clearly below that of HCl and is 63%. Al resolved to a lesser extent and reaches 65% with H2SO4 and 53% with HCl. Fe shows the lowest redissolution of 14% with HCl and 12% with H2SO4. The H2SO4 leaching experiments also showed that it is possible to separate the gypsum from the R-SSA if the leaching liquid is separated from the R-SSA after short contact times with H2SO4. The contact time, the acid amount and the s/l ratio have a big impact on the precipitation. It was possible to recover 75% of the total amount of gypsum that can be precipitated from R-SSA. In addition to this, the gypsum-free leaching liquid was then added to the R-SSA again which had already been leached, in order to dissolve P. There was no loss of P redissolution when the gypsum was extracted. This approach could not be observed in any other study. Based on the results, a recovery of 2,114 ± 130 t P/a with HCl and 2062 ± 130 t P/a with H2SO4 are possible if an amount of 30,000 t R-SSA/a is treated, which are forecasted for Berlin in the future. Since the redissolution of 91 ± 4% P requires an acid amount of 946 kg HCl(37%)/t R-SSA, this would result in an absolute annual acid requirement of 28,380 t HCl(37%) for 30,000 t R-SSA/a. H2SO4 experiments showed that 725 kg H2SO4(90%)/t R-SSA would be required for 89 ± 3% P redissolution, resulting in an absolute acid requirement of 21,750 t H2SO4(90%). Since H2SO4 has a lower consumption due to the higher concentration, is the cheaper acid of the two and has the possibility of recovering gypsum, H2SO4 would be preferred for leaching R-SSA from an economic point of view. In addition, about 75% of the gypsum can be precipitated with H2SO4, which corresponds to a quantity of 255 kg of gypsum(dry)/t R-SSA. At 30,000 t R-SSA/a, this results in an annual amount of 6630 ± 51 t gypsum that can be recovered.

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