Metal Elution and Content of Fused Slags Produced from Incinerated Ash

Jun Kobayashi, Ryoichi Kizu, Kazuyuki Torii, Hideo Sugiyama
2004 Chemical and pharmaceutical bulletin  
From the viewpoints of environmental risk mitigation and of the recycling of industrial products, many items conventionally destined for disposal became candidate objects for use as other products. Moreover, the technology utilized in the recovery/processing for reusing materials has been considerably developed, and recently the safety of reusing these materials has become a subject for investigation. Waste materials such as city garbage are thoughtlessly generated in great quantities. In
more » ... most such waste undergoes incineration processing and the residue (incinerated ash) is reclaimed. The residual capacities of disposal plants are decreasing every year, and the control of the generation of city garbage, by minimizing production and the recycling of incineration ash has been necessitated out of considerations for the environment. Recently incinerated ashes are being fused even further to create fused slags. This processing of the slags has been tried to create concrete building materials such as bricks for use in sidewalks. 1) However, as these products are now beginning to be processed for various practical uses in some cities, it has become necessary to better understand the safety (toxicity and durability) of such products. Since there are various types of melting furnaces and materials, especially such as sewer sludges, it is expected that the character of the slags produced will vary greatly with the place of processing and the season in which they are produced. To date, most investigations have been into the physical aspects of their use; but only from a viewpoint of the density and durability of concrete products prepared from slag for use as building materials. 2) Almost no research investigating the chemical components of slag aggregates has been reported. Among the possible chemical components, inorganic substances are considered to be very important. These do not decompose in the fusion process and the mostly remain intact in the end product. It is especially important because metals also have toxic actions. 3, 4) In this study, in order to check the safety when reusing fused slag as building materials, elution tests and the metal contents were examined using 13 fused slags having different preparation methods and sampling points. Experimental Reagents Standard metal solutions of atomic absorption spectrometry (AAS) grade were purchased from Wako and Kanto Chemicals, and diluted suitably with 0.1 M nitric acid. Hydrochloric acid (Kanto Chemicals, AAS grade), nitric acid (Wako, toxic metal analysis grade) and hydrofluoric acid (Kanto Chemicals, AAS grade) were used. All water used was refined by the Milli-Q system. Apparatus Decomposition of slags was performed using microwave decomposition equipment (Milestone MLS-1200MEGA). The metal concentrations in the decomposition liquids and eluates were measured by an AAS (Hitachi Z-8000) with graphite furnace mode. Nine metals were measured (aluminum, arsenic, cadmium, chromium, copper, manganese, lead, selenium and tin), and each measurement condition was optimized according to the manufacturers instruction manual. All containers were washed by acid soaking before use. Fused Slags In this study, 13 slags in which the fusing (gasification, coke bed, plasma, etc.), cooling (water, air and gradual) and materials (city garbage and sewer sludge) differed were used (Table 1) . Water cooling is defined as being dropped in water after fusion; a method that institutes sudden cooling and solidification. Air cooling is defined as being left in air for natural cooling (about 15°C/min). Gradual cooling is defined as being kept in air at a controlled cooling temperature (1°C/min), and thus promoting crystallization. Elution Testing of Metals Elution tests were performed based by an officially sanctioned method. 5) 3.5 g of slag was crushed with a pestle in a mortar and passed through two sieves of 8.6 and 60 meshes to be fractioned to three parts. The middle size granule (0.425-2 mmf) was put into a 50 ml conical tube, and 35 ml water added. The suspended solution was shaken in a mechanical shaker (200 min Ϫ1 , 4 cm of width) at room temperature for 6 h. After shaking, the supernatant was filtrated with a membrane filter (0.22 mmf pore size), and nitric acid was added to 0.1 M acidity. Metals in the eluate were measured by AAS. In addition to using water, elution was also performed using pH 4 nitric acid and pH 10 potassium hydroxide for eluents, simulating acid rain and snow, and soil exudation liquids. Each slag was measured in duplicate and that the examination value was below sensitivity was checked (Al Ͻ0.5 ppb; As Ͻ5 ppb; Cd Ͻ0.05 ppb; Cr Ͻ0.5 ppb; Cu Ͻ0.5 ppb; Mn Ͻ0.5 ppb; Pb Ͻ1 ppb; Se Ͻ10 ppb; Sn Ͻ5 ppb). Metal Contents 0.2 g of slag and 2 ml of hydrochloric, nitric, and hydrofluoric acid were each added into the Teflon decomposition container. This container was applied to the microwave decomposition equipment and sample decomposition performed. The conditions used in the decomposition were a frequency of 2450 MHz (12.25 cm wavelength), 120°C, 60 min, and 400 W. The device decompresses the container so that it undergoes above 110 atmospheric pressures. The sample after decomposition is transferred to another tube, and the supernatant fluid moves to another tube after centrifuging at 3000 rpm for 10 min. Centrifuging was repeated after adding of water to the remainder, and a decomposition liquid of 20 ml was prepared. Metals 1378 Notes Chem. In recent years, trials to reuse fused slags, obtained through the further treatment of incinerated ash produced in the process of the disposal of garbage or sewer sludge, such as for building materials including bricks, have drawn attention. To establish the safety of using such substances, elution tests for metals and metal contents were investigated in this study. As the result of examining 13 slags, which were made by different methods, little elution of toxic metals was found in the elution tests when water was used as a solvent. However when acid was used, there was a tendency for the elution of many kinds of metals to become greater. On other hand, the results of content measurements showed that arsenic and selenium, which were not seen to be eluted from any of the slags tested, were detected; and that aluminum and manganese existed in the slags in high concentrations. Key words fused slag; reuse; safety; metal in the liquid were measured by AAS.
doi:10.1248/cpb.52.1378 pmid:15516769 fatcat:x2z4artni5crvieyx6opl3vkre