Sewage Sludge Ash To Phosphate Fertilizer By Thermochemical Treatment

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1 Sewage Sludge Ash To Phosphate Fertilizer By Thermochemical Treatment Benedikt Nowak, Institute of Chemical Engineering (TU Vienna) Libor Perutka, Institute of Chemical Engineering (TU Vienna) Philipp Aschenbrenner, Institute for Water Quality, Resources and Waste Management (TU Vienna) Petra Kraus, ASH DEC Umwelt AG Franz Winter, Institute of Chemical Engineering (TU Vienna) CONTACT DETAILS Benedikt Nowak Institute of Chemical Engineering, Vienna University of Technology Getreidemarkt 9/166-3, 1060 Wien, Austria Telephone: Fax: benedikt.nowak@tuwien.ac.at EXECUTIVE SUMMARY Phosphate recycling from sewage sludge can be performed by heavy metal removal from sewage sludge ash (SSA): Mixing SSA with an environmentally sound chloride (e.g. CaCl 2 ) and treating this mixture (possible after granulation) in a rotary kiln at 1000±100 C leads to the formation of volatile heavy metal compounds which evaporate and to P- phases with a high bio-availability. Due to economical and ecological reasons, it is necessary to improve this technology to reduce the energy consumption. As fluidized bed reactors are characterized by a high heat and mass transfer, the usage instead a rotary reactor promises the saving of thermal energy, the acceleration of removal processes and the material throughput. A rotary reactor and a fluidized bed reactor (both laboratory-scale and operated in batch mode) are used for the treatment of granulates containing SSA and CaCl2 (150 g Cl / kg ash). Residence time and in case of the fluidized bed reactor superficial velocity are varied between 10 and 30 min and 3.4 and 4.6 m s -1 at 850 C. Cd and Pb can be removed well (> 90%) in all experiments. The Cu removal ranged from 28-78%, the one of Zn from 75-84%. The amount of heavy metals removed increases with increasing in residence time which is most pronounced for Cu. In the pellet, three governing reactions occur: Formation of HCl and Cl 2 from CaCl 2 ; diffusion and reaction of these gases with heavy metal compounds; side reactions from heavy metal compounds with matrix material. Although heat and mass transfer are higher in the fluidized bed reactor, Pb and Zn removal is slightly better in the rotary reactor. This is due the accelerated migration of formed HCl and Cl 2 out of the pellets into the reactor atmosphere. Cu is apparently limited by the diffusion of its chloride thus the removal is higher in the fluidized bed unit.

2 INTRODUCTION The current phosphate reserves are 16 billion tons; in million tons were mined [Jasinski2010]. So, the reserves-to-production-ratio is about 100 years. Phosphor is an essential element and not replaceable. 90% of the mined phosphates are used after digestion as fertilizer [Wagner1999]. When mined as a sedimentary mineral, phosphate rock is often contaminated by Cd and U, i.e. these elements end up in agricultural soils [Valsami-Jones2004]. The quality of mined ore is continuously decreasing resulting in higher concentrations of these elements. E.g. fertilizing with sedimentary rock phosphate fertilizer (22 kg P / ha) leads to a contamination of g U / ha [Kratz and Schnug2006]. Important phosphate reserves are in unstable world regions. Thus, phosphor recycling is necessary [Lederer and Rechberger2010]. The biggest secondary resource of phosphate is sewage sludge [Franz2008], which is polluted by organic [Harrison et al.2006] and inorganic compounds. Sludge incineration destroys organic pollutants and reduces mass and volume of the sludge. The ash cannot be used as fertilizer because it contains heavy metals: E.g., in a Japanese sewage sludge ash (SSA) the concentrations of Cd, Cu, Ni, Sn, Pb and Zn were about times higher than the concentrations in soil [Zhang et al.2002]. So, sewage sludge ash is usually deposited in landfills or used for mine filling. A possibility for heavy metal removal from SSA is mixing sewage sludge ash with environmentally sound chlorides (e.g. calcium chloride) and water and treating this mixture (possibly after pelletizing to avoid dust) at 1000±100 C in a rotary kiln. This leads to the formation and evaporation of volatile heavy metal compounds (mainly chlorides). As 90-99% of heavy metals like Cd, Cu, Pb and Zn can be removed, the solid product from the thermal treatment process can be directly used for fertilizer production [Mattenberger et al.2008] [Mattenberger et al.2010]. The P-availability of the product is higher than of ash because of the formation of new mineral phases [Adam et al.2009]. Compared to the direct use of sludge on agricultural land, co-incineration in cement kilns or in coal-fired power plants or mono-incineration and the disposal of the ashes in landfills or on soil, the described process (including mono-incineration) performs well with regard to P recycling from sewage sludge and environmental protection. However, this technology has to be improved to reduce the energy consumption [Lederer and Rechberger2010]. In the last few years, researches mainly focused on rotary reactor technology although gaseous reactants have to diffuse into the moving bed reducing the over-all reaction rates [Kunii and Chisaki2008]. Fluidized bed reactors (FBR) generally can be characterized by a high heat and mass transfer due to the formation of bubbles and the circulation of bed material [Kunii and Levenspiel1991]. Thus, using a FBR promises the saving of thermal energy, the acceleration of removal processes and therefore the increase of material throughput. A higher heat and mass transfer will accelerate the steps 1 and 2 from the listing below thus leading to an increase in heavy metal removal. On molecular scale, the process can be characterized by three governing reaction steps: 1. Formation of HCl or Cl 2 in dependence of the moisture of the reaction atmosphere according to CaCl 2 + H 2 O CaO + 2 HCl or CaCl 2 + ½ O 2 CaO + Cl 2 [Fraissler et al.2009]. HCl and Cl 2 diffuse inside the particles to heavy metal compounds (e.g. oxides) and to the reactor atmosphere as well.

3 2. Reaction of the intermediates HCl and Cl 2 with heavy metal compounds from the ash, e.g. 2 HCl + ZnO H 2 O + ZnCl 2 and Cl 2 + ZnO ½ O 2 + ZnCl 2. Volatile heavy metal compounds, mainly chlorides, are formed (e.g. boiling point of ZnCl 2 : 732 C [Lide2008]). 3. Heavy metal chlorides can be adsorbed on matrix material and react with matrix material (for Zn) according to ZnCl2 + SiO 2 + H 2 O ZnSiO HCl [Abanades et al.2001]. Heavy metal compounds from the ash may also react directly in solid state with matrix material forming stable silicates and aluminates, e.g. ZnO + SiO 2 ZnSiO 3 and ZnO + Al 2 O 3 ZnAl 2 O 4 [Stucki and Jakob1998]. These reactions reduce the maximum amount removable. The aim of this study is to test the described process in a laboratory-scale fluidized bed reactor at different superficial velocities and residence times and to compare the achieved results with those obtained in a laboratory-scale rotary kiln at similar conditions. Figure 1: Rotary reactor (left hand side) and fluidized bed reactor (right hand side) used MATERIAL AND METHODS A mixture of sewage sludge ash (from Fernwärme Wien, Vienna, Austria), CaCl 2 and water is granulated. The chloride content of the pellets is 150 g per kg ash. The pellets are sieved; the fraction between 1.4 and 4 mm is used for experiments. The pellets are treated in a laboratory-scale rotary kiln and in a laboratory-scale fluidized bed reactor (designed by using the Concept of Chemical Similarity [Winter2009]), see below. After the treatment the sample is milled, digested and analyzed with ICP-OES for the elements Cd, Cu, Pb and Zn. Pure ash contains 4 ppm Cd, 550 ppm Cu, 260 ppm Pb and 2000 ppm Zn. Rotary reactor RR The rotary reactor (see figure 1) consists of an indirectly heated tube (material: SiC-Si, heat conductivity approx. 100 W m -1 K -1 ) with a rotation speed of 4.2 min -1. At the reactor inlet, ambient air is taken in; the off-gas is sucked off by the chimney fan. The sample is filled into a batch container (quartz glass) which is pushed into the preheated reactor at the start of the treatment. At the end of the treatment, the container is pulled out again. A more detailed description of the rotary kiln used can be found in a previous publication [Nowak et al.2010]. Fluidized bed reactor FBR The stationary (bubbling) laboratory-scale FBR unit (see figure 1) consists of an indirectly heated quartz-glass tube (inner diameter approx. 35 mm). For fluidization preheated pressurized air is used. The superficial velocity is calculated from volume flow of the air, pressure and reactor temperature. Pellets are fed to the preheated reactor. After the

4 treatment, they are removed pneumatically. Particles in the off-gas of the reactor are separated by a cyclone and returned to the reactor. Figure 2: Fraction of Cd, Cu, Pb and Zn removed at 850 C in dependency of residence time and superficial velocity (in case of fluidized bed reactor); RR rotary reactor. RESULTS AND DISCUSSION Experiments are carried out in the fluidized bed reactor at different gas velocities (3.4, 3.9 and 4.6 m s -1 ) and in the rotary reactor. The treatment temperature is 850 C; the residence time is varied between 10 and 30 min. The removal is calculated according to fraction removed = 1 c/c 0, (concentration of the investigated element in the treated sample c and in the untreated sample c0). Due to entrainment of particles during sampling in the fluidized bed reactor, the total mass of the sample cannot be factored into the equation above. The removal as defined above is thus smaller than the removal regarding element masses. Generally, Cd and Pb can be removed well (up to 99% Cd and 97% Pb, depending on reactor and residence time). The removal of Cu and Zn is in all experiments incomplete; a maximum of 78% of Cu and 84% of Zn can be achieved. Influence of the residence time on heavy metal removal Figure 7 illustrates the influence of the residence time at 850 C.

5 Cd can be removed well in all experiments carried out, independent of the residence time for the concerned temperature. The removal of Cu is incomplete in all experiments shown. The amounts removed range from 28% (RR, 850 C, 10 min) to 78% (FBR at 3.4 m s -1, 30 min). From all elements concerned, the Cu removal shows the biggest influence of the residence time: In RR experiments, 28% can be removed after 10 min at 850 C, after 30 min 69% are separated. In FBR experiments, tripling the residence time from 10 to 30 min leads to an augmentation from 28 to 78% (at 3.4 m s -1 ) and 60 to 73% (at 4.6 m s -1 ), respectively. Pb removal exceeds always 90% in the experiments shown in figure 2. The maximum amount removed is 97% (RR). Apparently, removal processes of Pb are finished after min as a longer residence time does not lead to higher removals at 850 C. Zn shows a small dependency on the residence time, the removal at 850 C is always ca. 80±5%. Longer residence times lead to a better heavy metal removal. Especially the removal of Cu proceeds slowly; long residence times are necessary to achieve maximum evaporation. Cd is not, Pb and Zn are hardly affected by the residence time. Regarding the boiling points of the heavy metal chlorides (CdCl 2 : 960 C, CuCl: 1400 C, PbCl 2 : 951 C, ZnCl 2 : 732 C [Lide2008]), the behavior of Cu can be explained by the relatively high boiling point of its chloride. Influence of the mass transfer on heavy metal removal It can generally be assumed that heat and mass transfer increase in the fluidized bed reactor with the superficial velocity; in the rotary reactor they are less. Reaction rates and diffusion coefficients (in solid and gas) depend on the local temperature in the pellets, thus on the reactor temperature. Cd does not show an influence of the mass transfer due to its good removal. For Cu, the maximum amount can be evaporated in the fluidized bed reactor at 3.4 m s -1 ; only at a residence time of 10 min the best result is 4.6 m s -1 ( figure 2). The difference of the removal between RR and FBR is significant; it is up to 30%. After long residence times (30 min) the influence of the mass transfer gets less pronounced; the difference is ca. 8% in the experiments shown. An increasing superficial velocity leads in the investigated range to a slightly lower removal (except the experiment at 10 min). The removal of Pb is affected negatively by a higher mass transfer as the removal is always ca. 6% higher in the rotary reactor than in the fluidized bed at the investigated temperature. Zn behaves similarly as Pb; the removal in the rotary reactor is at residence times shown ca. 1-4% higher than in the fluidized bed reactor. In the experiments shown, a higher mass transfer does not necessarily lead to a higher removal. Only Cu can be removed better in the fluidized bed reactor. This can be explained by the three governing reaction steps described in the introduction (assumption that heavy metals are present as oxide at the concerned temperature). Possible limiting steps in this reaction scheme could be 1. the heating rate of the particles in the reactor; 2. reaction kinetics of the decomposition of CaCl 2 (temperature dependent); 3. diffusion rates of HCl and Cl2 in the pellet (temperature dependent);

6 4. reaction kinetics of the reaction of HCl and Cl 2 with heavy metal compounds (temperature dependent); 5. evaporation of formed volatile heavy metal compounds (e.g. chlorides) 6. diffusion rates of formed heavy metal chlorides (temperature dependent); 7. reaction kinetics of matrix reactions (temperature dependent). Obviously, the limiting step may differ for every single heavy metal: The removal of Cu is superior in the fluidized bed reactor. Regarding the experiments one limiting step is the heating rate of the particle: In the rotary reactor, the heat-up time takes rather long over all in experiments with a residence time of 10 min. This fits also to the relatively high boiling point of CuCl (ca C [Lide2008]). Apparently, HCl and Cl 2 are formed within a short time (as can be seen by the high removal of Pb and Zn after only short treatment times). Thus, steps 2 and 3 from the list above are not limiting the removal of Cu. Step 4 is also not limiting because it is competing with step 3: HCl and Cl 2 either diffuse to the particle surface and migrate to the reactor atmosphere (where they are more or less lost for further reactions) or they react in the particle. Step 6 and 7 are also competing: heavy metal chlorides either migrate to the reactor atmosphere (they are removed) or they react with the matrix. Thus, the removal of Cu is limited by the evaporation of CuCl and / or the diffusion of CuCl to the reactor atmosphere. I.e. a higher heat and mass transfer will enhance the removal of Cu. Probably a too high mass transfer will decrease the Cu removal because of changing the limiting step to e.g. step 4 (see FBR experiments). The removal of Pb, which is very high, is limited by the availability of Cl. I.e. the limiting step is step 3 (hindering step 4: once HCl and Cl 2 are migrated to the reactor atmosphere they are lost). Zn behaves similarly to Pb. Thus, its removal is also limited by the reaction rate of Zn compounds with HCl and Cl 2 and/or the fast diffusion of HCl and Cl 2. However, removals do not differ much between rotary reactor and fluidized bed reactor. Nevertheless, the limiting steps of the removal of a certain heavy metal can be shifted by different reactor temperatures and residence times exceeding the investigated range. Due to the removal mechanism and its kinetic constants which depend on several parameters (e.g. temperature), the removal may be de- or increased in a fluidized bed reactor compared to a rotary kiln at e.g. higher temperatures. Changing other parameters (as the reactor filling) will also shift the results. CONCLUSIONS Experiments with the aim of heavy metal removal of a granulated mixture of sewage sludge ash and CaCl 2 were performed in a rotary reactor and a fluidized bed reactor (both batch operated). The residence time was varied from 10 to 30 min at 850 C. Cd and Pb could be removed well in all experiments (99% Cd, 90-97% Pb). The removal of Cu and Zn was significantly lower and reached 28-78% and 74-84%, respectively, depending on the residence time. A higher residence time led to a higher removal, over all for Cu. Increasing the residence time from min leads to a 41% (from 28 to 69%) higher removal of Cu. In the fluidized bed reactor these value is ca. 17% (from 59 to 76%), depending on the superficial velocity. Although the mass transfer is higher in the fluidized bed reactor, the removal especially of Pb and Zn is slightly higher in the rotary reactor. This was found to be due to the accelerated migration of formed HCl and Cl 2 out of the particles into the reactor

7 atmosphere decreasing the availability of these gases in the pellet. The Cu removal is higher in the fluidized bed reactor as it is apparently limited by the diffusion of its chloride. As the experiments were carried out in batch operation under conditions investigated, continuous operation is part of future work. ACKNOWLEDGEMENTS This research is part of the Vienna Spot of Excellence on Urban Mining which is funded by the Zentrum für innovative Technologien GmbH (ZIT) of the City of Vienna. Additionally we want to express our thanks for the financial support of project AKTION Austria-Czech Republic (No. 54p1). REFERENCES Abanades, S., Flamant, G., and Gauthier, D. (2001). Modelling of heavy metal vaporisation from a mineral matrix. Journal of Hazardous Materials, 88(1): Adam, C., Peplinski, B., Michaelis, M., Kley, G., and Simon, F.-G. (2009). Thermochemical treatment of sewage sludge ashes for phosphorus recovery. Waste Management, 29(3): Fraissler, G., Jöller, M., Brunner, T., and Obernberger, I. (2009). Influence of dry and humid gaseous atmosphere on the thermal decomposition of calcium chloride and its impact on the remove of heavy metals by chlorination. Chemical Engineering and Processing: Process Intensification, 48(1): Franz, M. (2008). Phosphate fertilizer from sewage sludge ash (ssa). Waste Management, 28(10): Harrison, E. Z., Oakes, S. R., Hysell, M., and Hay, A. (2006). Organic chemicals in sewage sludges. Science of The Total Environment, 367(2-3): Jasinski, S. M. (2010). U.S. Geological survey, Mineral Commodity Summaries, chapter Phosphate rock, pages United States Government Printing Office, Washington. ISBN Kratz, S. and Schnug, E. (2006). Uranium in the Environment: Mining impact and Consequences, chapter Rock phosphates and P fertilizer as sources of U contamination in agricultural soils. Springer Berlin Heidelberg. Kunii, D. and Chisaki, T. (2008). Rotary Reactor Engineering. Elsevier, 1 edition. ISBN Kunii, D. and Levenspiel, O. (1991). Fluidization engineering. Butterworth-Heinemann Boston London Singapore Sydney Toronto Wellington. ISBN Lederer, J. and Rechberger, H. (2010). Comparative goal-oriented assessment of conventional and alternative sewage sludge treatment options. Waste Management, 30(6): Lide, D. R. ( ). CRC Handbook of Chemistry and Physics First Edition for the 21st Century.. Mattenberger, H., Fraissler, G., Brunner, T., Herk, P., Hermann, L., and Obernberger, I. (2008). Sewage sludge ash to phosphorus fertiliser: Variables influencing heavy metal removal during thermochemical treatment. Waste Management, 28(12):

8 Mattenberger, H., Fraissler, G., Jöller, M., Brunner, T., Obernberger, I., Herk, P., and Hermann, L. (2010). Sewage sludge ash to phosphorus fertiliser (ii): Influences of ash and granulate type on heavy metal removal. Waste Management, 30(8-9): Nowak, B., Pessl, A., Aschenbrenner, P., Szentannai, P., Mattenberger, H., Rechberger, H., Hermann, L., and Winter, F. (2010). Heavy metal removal from municipal solid waste fly ash by chlorination and thermal treatment. Journal of Hazardous Materials, 179(1-3): Stucki, S. and Jakob, A. (1998). Thermal treatment of incinerator fly ash: Factors influencing the evaporation of ZnCl2. Waste Management, 17(4): Valsami-Jones, E. (2004). Phosphorus in Environmental Technologies - Principles and Application. IWA Publishing, London UK. Wagner, H. (1999). Stoffmengenflüsse und Energiebedarf bei der Gewinnung ausgewählter mineralischer Rohstoffe, Teilstudie Phosphat. Geologisches Jahrbuch/Sonderhefte, Reihe H(Heft SH 5). Winter, F. (2009). The concept of chemical similarity. In Scale-up in Combustion. Verlag ProcessEng Engineering GmbH, Vienna, Austria. ISBN Zhang, F.-S., Yamasaki, S., and Nanzyo, M. (2002). Waste ashes for use in agricultural production: I. liming effect, contents of plant nutrients and chemical characteristics of some metals. The Science of The Total Environment, 284(1-3):

COOPERATION IN THE FIELD OF EMISSIONS ABATEMENT TECHNOLOGY

Report on the Aktion project No. 58p21: COOPERATION IN THE FIELD OF EMISSIONS ABATEMENT TECHNOLOGY Antragsteller (Name, Titel, Funktion): Universität o. sonst. Institution: Institut: Adresse: Projektpartner