Research On Condensation And Absorption Kinetics Of Heavy Metals At Low-temperatures During Co-processing In Industry Kilns

Disposal of hazardous wastes (HW) with industry kiln has been used in developed countries for more than thirty years, and now its development has aroused great interests at home and abroad. The environmental security problem, which is caused by toxic heavy metal elements introduced by wastes and raw materials, becomes a concern focus day by day. This subject is based on the environmental public welfare projects:"Environmental risk control technology study on using cement kiln to co-processing hazardous wastes". This paper has mainly studied the condensation and absorption kinetics of heavy metals at low-temperatures during co-processing hazardous wastes in cement kilns and ironmaking blast furnaces. The basic physical characteristics of cement raw meal (cement kilns) and the main material of ironmaking (ironmaking blast furnaces), such as surface area, pore volume and micro-structure were investigated by SEM, BET and particle size analysis. Under laboratory conditions, the coupling device of a rotary tube furnace and a vertical furnace with automatic temperature control was designed to simulate the real cement kiln and ironmaking blast furnace, and the heavy metals in hazardous wastes were replaced with heavy metal chemical reagents. The condensation and absorption characteristics of heavy metals (As, Pb, Cd) during co-processing in cement kilns and ironmaking blast furnaces were deeply studied in thethis paper. Besides, due to Zn could cause great harm to the ironmaking blast furnace, it is necessary to study the condensation and absorption characteristics of Zn during co-processing in ironmaking blast furnaces.Results showed that the cement raw meal has quite small surface area and compact structure, and the surface area is2.49m/g. Consequently, condensation of heavy metal vapor in the flue gas on the surface of cement raw meal was happened during co-processing in the principal part of cement kilns. Results of the experiment indicated that heavy metals (Pb, Cd, As) into the vertical furnace could be divided into three parts:condensing on the wall of the vertical furnace, which occupies approximately60%-70%for Pb, Cd and As; condensation and absorption on the cement raw meal, which occupies10%-20%; releasing into the atmosphere, which occupies10%-20%. The condensation and absorption of the cement raw meal for heavy metals (Pb, Cd, As) all increased with time, but decreased with temperature. The condensation and absorption kinetics could be well described by the double constant rate equation (lnqt=a+klnt). According to the data from double constant rate equation, combining the dynamic equation, the apparent activation energy could be obtained. The condensation and absorption activation energy of the different chemical forms of lead is PbO>PbS>PbCl2, and the activation energy of cadmium is CdO>CdS>CdCl2.Meanwhile, the same research was carryied out during co-processing in ironmaking blast furnaces. Results showed that the main materials of ironmaking (sinter, pellet, coke) have quite small surface area and compact structure. Consequently, condensation of heavy metal vapor in the flue gas on the surface of main material of ironmaking was happened during co-processing in the principal part of ironmaking blast furnaces. Results of the experiment indicated that heavy metals (Pb, Cd, As, Zn) into the vertical furnace could be divided into three parts: condensing on the wall of the vertical furnace, which occupies approximately50%for Pb, Cd, As and Zn; condensation and absorption on the main material of ironmaking, which occupies20%-30%; releasing into the atmosphere, which occupies about20%. The condensation and absorption of the main material of ironmaking for heavy metals (Pb, Cd, As, Zn) all increased with time, but decreased with temperature. The condensation and absorption kinetics could be well described by the double constant rate equation. According to the data from double constant rate equation, combining the dynamic equation, the apparent activation energy could be obtained. The condensation and absorption activation energy of the oxides of Pb, Cd, Zn were greater than their chlorines and sulfurs.The condensation and absorption kinetic equations of the three elements with different chemical forms during co-processing hazardous wastes in industry kilns (cement kilns and ironmaking blast furnaces) have been concluded and they could be used to predict the condensation and absorption quantity of heavy metals during co-processing in industry kilns. According to these equations, the condensation and absorption quantity at any temperature and any time during co-processing of hazardous wastes in industry kilns would be well calculated. The results could provide some reference on whether the hazardous waste was suitable for cement kiln disposal or not, where the hazardous wastes could be added and how much was the emission of heavy metals in the industry kilns. And it would also provide a data basis to evaluate the environmental risk causing by heavy metals during co-processing in industry kilns.