The Behavior Research Of Heavy Metal In Cement Clinker Burning And Cement Hydration Process

Disposal of hazardous industrial wastes (HIW) with cement rotary kiln has been used in industrial developed countries for more than thirty years, and now this becomes the world wide concern for cement industry. In particularly the flow direction in the production and application of toxic heavy metal elements introduced by cement raw material, fuel and waste is drawing the focus of attention. This subject is based on the"technology and application study on using cement rotary kiln to incinerate hazardous wastes", which is a key project of Beijing Scientific Committee. Cr,Pb,Hg,Co,Ni,Cd,As and Zn as eight common heavy metals mainly existed in cement production, their behavior during cement clinker calcinations and cement hydration have been studied in this paper.According to the production practice and theoretical research demand, cement raw meal were made by industrial incineration waste and chemical reagent respectively, and then was calcined with simulated experimental study on using cement rotary kiln to incinerate hazardous industrial waste; leaching test of the heavy metal elements in cements prepared with varying addition of wastes was implemented under different conditions; the encapsulating ratio of heavy metal in clinker and cement hydration products was calculated by balance of material; the impact of the quantity of waste, the causticity solution and the pore structure on the leaching of heavy metals was analyzed; the existing form and distribution of heavy metals in cement clinker minerals and cement hydration products as well as the valence of heavy metals in clinker by means of XRD,XPS,DTA and EMPA were explored; therefore the influence of heavy metals on the formation of clinker minerals and hydrated process was discussed, and the solidification mechanism of clinker and hydration products were studied.Meanwhile, the technology of using cement rotary kiln to incinerate hazardous waste was studied, the characteristics of waste were analyzed, the effect of combustion with waste on cement burden, process system and quality of clinker were studied. The research progresses are as followings:The solidification ratio of heavy metals in lab are:Cr 83.8%;Co 86.1%;Ni 86.5%; Cu 74.3%;Zn 74.3%;Cd 88.1%;Pb 86.3%;As 89.3%. The high temperature furnace in lab is intermittent open, but the rotary kiln is continuous and closed circular system, so the solidification ratio in lab is far lower than in rotary kiln.Under the addition of this study, the heavy metals do not change substantially the clinker minerals. The incorporation of heavy metals to different clinker minerals is of selectivity: Zn is concentrated in the interstitial phase of clinker; As,Co,Cu and Ni is mainly incorporated by C3A and C4AF, while small part of them was found in C3S and C2S. Cd and Pb can not be separated clearly from interstitial phase and C3S and C2S, they are thus distributed homogeneously in clinker minerals.By means of XPS and thermodynamics analysis, the main valence of the 8 kinds of heavy metals appearing in the clinker are Cr,+3; Co,+3;Ni,+3;Cu,+2;Zn,+2; Cd,+2; Pb,+4; As,+3.Different heavy metals have different form and distribution in cement. The heavy metals of Pb, Zn and Ni were adsorbed on surface of the cement grains by means of compounds in the course of hydrating. Cr participated in hydration reaction and formed new compounds, which was similar with AFm in structure. The heavy metals of Co and Cd can exchange Ca in hydration product and be trapped in C-S-H, which formed new Co-containing calcium silicates and Cd-containing calcium silicates.For different moulding mode and application environment, the leaching rate of heavy metal in concrete is different. But for most of them the leaching rate is below 1%. Taking Sample No. AP3 as an example, under neutral condition, the leaching rate is: Cr 0.107%, Co 0.026%, Ni 0.203%, Cu 0.614%, Zn 0.112%, Cd 0.274%, Pb 0.126% and As 4.822%. The leaching content of heavy metal ions is dependant on the species, but no direct relation with absolute content is found. Under acidic condition, the leaching content of As and Hg increases, while the leaching content of Zn and Pb decreases, the leaching content of Cr,Cd,Ni and Co have no change under acidic media. Transport passage of ions can be more easily formed for the structure of model A, which makes heavy metal ions leach into external environment. Solidification ratio of most of the heavy metals increases with the increase of incorporation of waste in the raw meal. The pre-treatment technological process of waste was established, the technical regulation of using cement rotary kiln to incinerate hazardous waste was established, and experimental standard of the leaching of heavy metals in concrete was suggested.