(70bq) Mechanochemical Waste Processing Based on Grinding

Mechanochemistry of solids covers a whole range of interconnected phenomena taking place in the case of mechanical action on solids or their separate parts participating in chemical reactions with other substances or with each other. Although separate manifestations of heterogeneous mechanical actions on chemical reactions have been studies for many years, it was only in recent decades that the mechanochemistry of solids began to develop as an independent scientific trend. This was due to the growing demand of the industry for solving problems united by the need to use or to prevent chemical reactions caused or accelerated by mechanical activation.

Mechanical activation of a solid is normally caused by its grinding, which enables us to increase surface area with rupturing the bonds. There have been a lot of investigations on its relevant phenomena on various materials since last few ten years, and most of them are related to physicochemical changes of solids with structural change, syntheses with chemical reactions connected with surface phenomena. Mechanochemistry is not always to be able to achieve chemical reaction of solids, but, it has a potential to control or assist the chemical reaction of substances.

We would like to introduce three mechanochemical processes for different types of wastes such as an electric precipitation (EP) dust, the indium tin oxide (ITO) scrap, and the polyvinyl chloride (PVC) waste.

The first example, EP dust contains heavy metals and ammonium sulfate in a power station, in which heavy oil is used as fuel in their boilers. The dust includes small amount (1.5%) of vanadium (V) that is a target element to be separated. The authors have attempted to extract V by water leaching followed by grinding the dust using a planetary mill in air. The grinding leads to the formation of ammonium vanadium sulfate, which is a soluble in water. The yield of V in the solution increases from about 10 to 95% by only 45 minutes in grinding period. The second example is to extract In from the ITO scrap which contains Al2O3 powder. The process consists of grinding the scrap by a planetary mill in air and room temperature acid leaching of the ground scrap. The grinding causes structural change of In2O3 in the ITO scrap into amorphous state. The degradation of crystallinity resulted in easy extraction of In at high yield in the leaching process. The Al2O3 powder in the scrap plays a significant role to make In2O3 amorphous. The third example is to dechlorinate PVC ([CH2CHCl]n). The PVC powder is ground with -CaO/Ca(OH)2 by a planetary ball mill in air to investigate their mechanochemical (MC) reactions. The ground mixture was washed with distilled water to evaluate the reaction yield. The grinding causes dehydrochlorinating reaction, forming CaOHCl and partially dechlorinated PVC. The reactivity against PVC of CaO is superior to that of Ca(OH)2, but all the same, the reaction yield is improved as the grinding progresses. The yield and rate of the reaction are improved with an increase in the molar ratio of (CaO/PVC) as well as the rotational speed of the mill. The dechlorination yield reaches almost 100%, depending on the grinding time and CaO addition.

The mechanochemical treatment to these wastes before leaching would be an effective operation to be able to extract the target elements at high yield in the leaching stage.