Quality control of chemical grade calcium carbonate from Lopburi limestone deposit in Thailand

Abstract

Calcium carbonate (CaCO3) is naturally produced from calcite, limestone, chalk, and marble. It is being used as raw material in various industries (e.g., cement, plastic, paint, paper, and pharmaceutical industries). The low-grade CaCO3 samples obtained from a calcite plant at Khoktum Sub-district, Muang Lopburi District, Lopburi Province, Thailand were used in this study. These samples contain more than 98% of CaCO3 while Fe2O3 contents are higher than 0.1% and increased after finished the process. Because of this problem, the products could not be sold since lower Fe2O3 contents are required (e.g., ≤0.05% for filler grade and ≤ 0.1% for animal feeding). This study aims to find a source that cause the increasing of Fe2O3 content in the processing plant, removing the Fe2O3 content using a dry-type magnetic separator, and suggest the selection of suitable installation of dry-type magnetic separators in term the economic evaluation. For sample characterization, the chemical composition of the samples were analyzed using X-ray fluorescence (XRF) spectrometer, and X-ray diffractometer (XRD). The results showed that the phase of iron found in the sample were magnetite, pyrrhotite, and iron scrap, indicating that the increasing of Fe2O3 content was from both raw material and ball mill. Induced roll magnetic separator was used to improve the quality of CaCO3 products by decreasing Fe2O3 content to achieve the standard requirements. The results showed that the Fe2O3 content was decreased from 0.67 to 0.08 wt% and CaCO3 content was increased from 98.06 to 98.73 wt% at the optimum condition. From these results, magnetic separators are suggested to be installed at the point of ball milling discharge and underflow of air classifier to reduce the iron content. The financial analysis indicated that the net present value (NPV), discount rate, internal rate of return, modified internal rate of return, and discounted payback period are equal to 17,992,947 baht, 15%, 53%, 24%, and 1.6 years, respectively.