| Resources and environment have been the focuses at the present. They are also problems which were caused by the industry development. With the rapid development of economy, the consumption of natural resources accelerates, so does emissions of industrial waste. Lots of wastes are piling up due to the lack of appropriate treatment, which causes great damages to the ecological environment. In recent years, the resource recovery of waste draws more and more attentions for resource shortage, production cost increasing and environmental protection strengthening.The process of wastewater treatment in toothpaste factories produces a great deal of white precipitates which were not treated effectively because of high yields, mineral content and alkaline. The traditional ways cannot meet the economic and environmental requirements. The precipitates can be used as the raw material for calcium for containing calcium-basic substances. The traditional preparation of light calcium carbonate with limestone needs much energy, while the raw material extraction process can cause damage to the environment. The preparation of light calcium carbonate with the treatment sludge can improves comprehensive utilization of white precipitate from toothpaste factory, at the same time, eliminating the bad impact on the environment and reducing the calcium carbonate production costs, which has both practical and economic benefits.In this paper, double decomposition method was introduced to prepare the lightweight calcium carbonate with wastewater treatment sludge from the toothpaste factory as raw material, hydrochloric acid as the calcium extraction agent, ammonium carbonate as carburizing agent.Leaching agent concentration, leaching time, leaching temperature, liquid-solid ratio, mixing intensity and other factors on the effect of calcium leaching rate were studied in the leaching process. At last, optimum conditions of leaching were determined:leaching agent concentration was 0.4mol/L, extraction time was 30mins, extraction temperature 25?, liquid to solid ratio was 3.5:1, the stirring rate was 300r/min, the calcium leaching rate was 94.8% in such condition. And kinetics analysis shows that acid leaching process is in accordance with the shrinking core mode, and is controlled by stagnation of fluid membrane diffusion.The optimum reaction conditions of carbonation process were studied by orthogonal test method with the leaching solution as raw material. The results show that:the ratio of raw materials was 1:1.1, carbonization time was 45min, the carbonization temperature was 20?, the calcium carbonate yield rate was 94.5% under this condition.The synthesis process of calcium carbonate was studied, the effects of concentration of reactants, reaction temperature, reaction time, additive dosage, stirring intensity on the calcium carbonate crystal form and particle size were investigated with citric acid as the crystal control agent, six partial sodium phosphate as dispersing agent. The synthetic product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and laser particle size analyzer.The result shows that the light calcium carbonate under optimum preparation conditions can meet light calcium carbonate industry standard (HG/T2226-2000). |
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