This material is a fly ash collected in the electrostatic filters of an incineration plant for city waste. The fly ash was sampled by Dr. F. Ariese of the Institute for Environmental Studies (IVM) of the Free University of Amsterdam at the municipal waste incineration plant (AVI) of Amsterdam. The municipal waste consists of about 60 % domestic waste and 40 % inflammable, non-chemical waste from small commercial companies. The waste was burnt without prior sorting treatment and no combustion additives were applied. The temperature in the furnace was about 900 – 1000 °C. The plant operates four furnaces with a total emission of 500 000 m3·h-1 exhaust gases with a solid content of 2 g·m-3. The batch of 1000 kg, packed in two large bags, was collected from the electrostatic filters of one furnace over a period of 4 h and dispatched to IRMM for further treatment [2]. The water content of the starting material corresponded to less than 1 % (mass fraction).
Fine grinding of the incineration ash was carried out using a jet mill and ultrafine classification system. In the jet mill, three nozzles are mounted at the bottom of the grinding chamber to blow air jets of 6 bar. The three-dimensional nozzle arrangement enables the feed material to be completely ground without residue. The air jets accelerate the feed particles to impact on each other, thus reducing in size through wear from identical material without any contamination from foreign grinder material. Furthermore, all surfaces of the milling and sorting system, which come into contact with the products, are made of aluminium oxide ceramics, polypropylene or polyurethane in order to minimize contamination risks. Fine particles are extracted through the classifying wheel. They can be extracted into a sharp size distribution within a range between 5 and 120 ìm depending on the feed material and operating parameters. Coarse particles flow back along the wall into the grinding chamber and are re-injected in the grinding process. The grinding process was performed with a speed of the classifying wheel of 7000 rotations per minute. A total amount of 195 kg ground incineration ash powder with a top particle size < 105 ìm was produced with a mean production rate of 2.2 kg·h-1. Homogenisation and bottling were carried out in a multi-purpose cone mixer of 250 L volume with semi-automatic filling equipment.
After jet milling the material was directly introduced into the cone mixer by a central filling nozzle on top of the mixer. All drives and gears are placed outside the mixing chamber in order to exclude contamination by oil. All parts of the mixer in contact with the powder are made of polished stainless steel. A potential contamination from the processing equipment would not have an impact on the quality of the final CRM as certification is done after processing and bottling. A feed screw at 100 mm from the bottom of the mixer allows filling of vials without stopping mixing with a given mass of material. Directly after filling of a bottle, the feeder turns into the opposite direction to push the powder back into the cone mixer. A mean production rate of 100 bottles per hour was achieved. A total number of 4345 bottles were filled with 40 g powder in 100 mL amber glass bottles, which were closed with an insert of polyethylene and a screw cap . keywords: environmental material , solid waste/ash , trace elements , heavy metals , fractionation analysis , aqua regia soluble fraction |
