Effect of Cr, Ni, Cu and Zn in clinker on physical properties of cement via co-processing in a pilot-scale rotary cement kiln

Abstract

Co-processing of industrial wastes in cement kiln can be used for both destruction of hazardous organics and stabilization of heavy metals. This method provides a management option to use the wastes as raw materials or fuel in cement production. However, heavy metals in the wastes may adversely affect cement properties. This study investigated chemical and physical characteristics of various mixtures of heavy metals in synthetic raw materials to produce cement clinkers. Raw materials were mixed with different ratios of single oxide of heavy metals: namely, chromium oxides (Cr₂O₃), copper oxides (CuO), nickel oxides (NiO₂), zinc oxide (ZnO), and the two combinations of four kinds of heavy metals, i.e. 0.05%, 0.3%, 0.5%, 1.0% and 2.0%. The mixtures of Ordinary Portland Cement Type I were burned in a pilot-scale rotary cement kiln which the liquefied petroleum gas was used as fuel. Results showed that concentrations of heavy metals in the clinkers were higher than those in the mixtures before combustion. Moreover, the heavy metals were not destroyed in the burning process and they were found in the clinker associated with main phases of clinker, C₂S, C₃S, C₃A and C₄AF. High concentrations of heavy metals caused increase in free lime content, but decrease in C₃S. In particular, Cr and Cu concentrations greater than 1.0% by weight were produced more than 1.5% of free lime and caused unacceptable compressive strengths. Increase in Ni and Zn concentrations caused free lime contents to increase slightly although they were still within the SCCC’s quality standard of 1.5%. However, the mortars with higher Ni and Zn contents produced better compressive strength values than the standard values. Improvement of early strength at 1 day for 0.05%, 0.1%, 0.1% and 0.5% by weight of combination of heavy metals, Cu, Zn and Ni, respectively, were more than 130 kg cm⁻². The 28-day compressive strength values of clinkers containing 0.05-0.3%, 0.1%, 0.5% and 1.0% by weight of combination of heavy metals, Cu, Ni and Zn respectively, were more than 400 kg cm⁻². As for environmental impact evaluations, Waste Extraction Tests showed that heavy metal concentrations in leachates of all of the different ratios of heavy metals in clinker and cement mortar did not exceed the Thai regulatory standards (Cr, Cu, Ni and Zn less than 5, 25, 20 and 250 ppm respectively)