Leaching, filtration and displacement washing in a treatment process for MSW combustion fly ash - laboratory scale and plant studies

Licentiate thesis, 2004

Municipal Solid Waste combustion, as one of the waste management tools, starts to play an important role nowadays in heat and energy generation. However, MSW combustion leads to generation of other wastes - combustion residues, including ashes, which require a proper management. Thus, a lot of efforts have been directed towards minimization or elimination of negative impact on the environment when ash is land filled or utilized and thus different treatment options for fly ashes are presently being tested. This study is focused on testing a treatment method for cyclone ash from MSW combustion in Bubbling Fluidised Bed Boiler. The proposed method is based on effective leaching of the ash with water to release easily soluble salts, mainly NaCl and KCl, from the ash. The leaching step is followed by filtration and washing. The study was carried out in two stages: laboratory scale experiments and pilot plant scale experiments. The washing process tested under laboratory conditions consisted of the following steps: the cyclone ash and de-ionized water was mixed in a leaching reactor and after a specified residence time, the slurry was filtered and finally washed with de-ionized water in a filtration unit. The laboratory tests provided necessary data for continuation of the research in the pilot plant scale.The pilot plant consisted of a leaching unit, a filtration/washing unit, a storage tank and an ash bin. The whole washing process, except for the filtration step, was working in continuous mode. The findings from the research show that the proposed treatment process works very well for this type of ash. The leaching with water results in a relatively quick release of soluble salts from the ash. The ash/water slurry seemed to be a suitable material for mechanical separation in the filter unit, independently of the L/S slurry ratio applied. The mother liquor (leaching liquor) trapped in the filter cake can be washed out by displacement washing using a water volume twice is the volume of the mother liquor. Thus, an overall liquid to solid ratio at 1 seems to be reasonable low and still sufficient to reduce the leaching of soluble components from the washed ash. At this liquid to solid ratio the leaching liquor is still far from being saturated in respect to NaCl and KCl. The cyclone ash washed in the pilot plant passed two leaching tests (batch leaching test and the column test) that are proposed as European standard methods for the evaluation of particulate waste.