COPRECIPITATION AND ADSORPTION FOR REMOVAL OF CADMIUM, LEAD, AND ZINC BY THE LIME - SODA ASH WATER SOFTENING PROCESS (CRYSTALLIZATION, HEAVY METAL, CALCIUM-CARBONATE).

摘要

The crystallization kinetics for the precipitation of calcium carbonate and magnesium hydroxide in the presence of cadmium, lead, and zinc were studied. The kinetics for the precipitation have been studied in a continuous reactor operated under MSMPR (mixed suspension mixed product removal) conditions. Reactor residence times, effluent alkalinity conditions, and initial heavy metal concentrations (acting as impurity) were varied to determine these kinetics. The crystal size distribution was measured using the Coulter Counter Model TA II, allowing sizing of the crystals in the 6 to 75 micron size range. Steady state conditions in which the residual hardness, alkalinity distribution, and particle size distribution remains fairly constant after approximately 11 residence times, was observed. The MSMPR crystallizer analysis of the population balance provides an adequate description of the particle size distribution.; Residual calcium levels were varied in the presence of different types of heavy metals. Cadmium and zinc inhibit the precipitation of calcium carbonate resulting in a higher residual hardness level, about 50-85 mg/l as CaCO(,3), while lead promotes the crystallization of calcium carbonate resulting in lower residual hardness level, about 25-50 mg/l as CaCO(,3). The removal of magnesium hardness was not affected in the presence of cadmium, lead, or zinc. The predominant form of the calcium carbonate precipitated in this research was the calcite form, with a small amount of the aragonite formed. The percentage of aragonite crystals increased with the addition of magnesium hardness.; Heavy metals can be removed effectively in this operation. The removal efficiency exceeded 97.0% for cadmium and zinc for an initial concentration level of 5.0 mg/l and removals exceeded 99.0% for lead with an initial concentration level of 1.22 mg/l. The X-ray diffraction pattern data, the scanning electron microscope photographs, and the residual heavy metals concentration all suggest the major mechanisms involved for removal of heavy metals from solution in the lime-soda ash water softening process are: surface adsorption for cadmium and zinc removal, and isomorphic inclusion for lead removal.; A batch softening study was also performed which exhibited a different type of behavior as compared to those performed with the continuous MSMPR crystallization. (Abstract shortened with permission of author.)