ABSTRACT
In the European Union fly ashes from municipal solid waste incinerators (MSWI) are classified as hazardous and must be then submitted to a specific leaching test before making decision regarding the proper landfill category [1]. From the point of view of contaminants release, chlorides pose the most relevant problems [2]. In fact, these waste components are present in such high concentration and mobility that a release as high as 25000 ppm can be reached from untreated MSWI ashes trough UNI 10802 standard test which is the one in force in Italy. This release value is ten times higher than the required limit from disposal to landfill for hazardous wastes and 16.7 times higher than the corresponding limits for non hazardous wastes [3]. A proper treatment must be then realized to reduce pollutant release. It is well known that the treatment of wastes (often hazardous) largely relies on cement-based stabilization/solidification processes which allow safer disposal and/or matter recovery for the manufacture of building materials [4-7]. Furthermore, the application of such processes has economical and environmental advantages due to the reduced energy requirement (process carried out at ordinary temperature) and the lack of secondary pollution (no gaseous emission is involved). In the case of MSWI fly ashes, cementitious stabilization/solidification processes are often not very effective. The reason for this is the scarce immobilizing capability of a cementitious matrix toward chlorides and other soluble salts. 1 Corresponding author: raffaele.cioffi@uniparthenope.it
As a consequence, if a cementitious process is to be economically applied to MSWI ashes (that is with a reasonable low binder : waste ratio), a preliminary washing step must be ensured. Consequently, it is not surprising that many cementitious treatment proposals can be found in literature either as scientific papers or as patents, in which soluble salts are previously removed in a washing step where water consumption is invariably high, ranging from 4 to 20 when referred to the unit mass of ashes [8-12]. Then, to make more economically attractive cementitious stabilization processes, the optimisation of the washing pre treatment step would be worthy of consideration. In fact, if MSWI ashes are stabilized without any pre-treatment, it is difficult to lower chlorides release below 5000 ppm, if the stabilization process is to be economically sound. For this reason, a water washing pre-treatment can be applied for chlorides (and other soluble salts such as sulphates) removal. However, the drawbacks are the related water consumption in the order of 4 m3/ton for complete chlorides removal and the secondary pollution arising from the transfer of chlorides and other soluble salts from ashes to the washing water. Hence, the washing pre treatment must be optimised in relation to the minimum washing water requirement and maximum allowed residual amount of chlorides (and other soluble salts) when a 1:4 binder to waste ratio is to be used in the stabilization process. This ratio is considered adequate for the economically sound management of MSWI ashes for landfilling disposal or for reusing as cement stabilized substrate in construction roads [12-16].
