Simulation Study On Existence Species Of Heavy Metals During The Co-processing Of Hazardous Wastes In The Cement Kiln

The environmental safety of co-processed cement products has been attached to more and more importance both at home and abroad. The releasing mechanism research of the heavy metals in the cement products is the prerequisite and basis of the environmental safety evaluation of co-processed cement products. But the existing species of heavy metals in the cement products is the main factor determining the release. Accordingly, it is necessary and indispensible to conduct the researches deeply about the micro species for the investigation of releasing mechanism of heavy metals in the co-processed cement products.By simulating the calcination of cement raw material adulterated with heavy metal chemicals, cement clinker and cement paste are produced. Modified Tessier sequential extraction procedure was firstly congducted to analyze the apparent chemical species of heavy metals both in the clinker and cement paste, then synchronized radiation X-ray absorption spectra (SR-XAS) and electron probe micro analysis (EPMA) are united to investigate the existing species of heavy metals both in the clinker and cement paste. The results are as follows.(1) The main chemical valences of heavy metals in the clinker are listed: Cr, +6; As,+5; Pb,+4,+2; Cd:+2; Ni:+2,+3. No new chemical valences appear in the hydration process; (2) Cr mainly exists in the acidified sodium acetate solution extractable form in the clinker (71%). Cr probably mixes in the clinker in the form of CaCrO4, which can accumulate with C3S and C2S during the calcinations process because of the comparability in structure. In addition, a little portion of Cr probably substitutes the Si in the C3S and C2S and enters the lattice. During the hydration process, CaCrO4 is mainly combined to the C-S-H gel, and the Cr substituting the Si in the C3S and C2S enters the C-S-H gel with the hydration. (3) As in the clinker predominantly distributes in the acidified hydrogen peroxide extractable form and acidified sodium acetate solution extractable form (51.8%,35.1%). During the calcinations process, As probably mainly substitutes the Si in the C3S and C2S and enters the lattice, and a little portion of As may blend in the clinker in the form of Ca3(AsO4)2. During the hydration process, As substituting the Si may mainly enter the C-S-H with the hydration, and Ca3(AsO4)2 may be absorbed to the surface of C-S-H gel grains and exist in the holes between grains. (4) Pb in the clinker predominantly distributes in the acidified hydrogen peroxide extractable form (72.7%). Pb may mainly blend in the clinker in the form of PbSO4 and PbO. In the hydration products, the main chemical form of Pb may be the same, yet the distribution may be more decentralization. (5) The main apparent chemical species of Cd in the clinker is the acidified sodium acetate solution extractable form (95.1% ). Cd may substitute the Ca in the mineral phases with high content of Ca like CaO, or blend with CaO in the form of CdO. In the hydration products Cd is predominantly enriched in the CH crystal. During the hydration process, Cd which enters the lattice of CaO may enter the CH phase with hydration of CaO, and CdO blending with CaO may be absorbed to the surfaces of CH crystal grains of exist in the holes between grains, and a little portion of Cd may substitute the Ca in the CH crystal and enter the lattice. (6) Ni in the clinker mainly exists as the acidified hydroxylamine hydrochloride extractable and residual form (65.6%,20.3%). Ni predominately exists as the Ni-Mg compound with the possible chemical formula of MgNiO2 and distributes in the clinker at random. This compound may not participate in the hydration because of low solubility or low chemical activity, so it is still the main existing compound of Ni in the hydration products.