Amorphous Silicate Decomposition from Non-Magnetic Coal Fly Ash using Sodium Hydroxide

Abstract

In five years, from 2015 to 2020, the need of coal in Indonesia is doubled to 166.2 million tonnes and increasing up to 2050 as stated in the Indonesian Energy Mix policy. Generally, coal is used in the coal-fired power plan. After the combustion, there are some residual materials such as mineral matter, fly ash and bottom ash (FABA) listed as hazardous materials due to the toxicity and metal contents. Referring to the amount of coal consumption in 2020, the fly ash and bottom ash will be of about 19.92 million tonnes. The utilization of FABA is then become a must thing to do. One of the alternatives is rare earth element (REE) extraction. Indonesian coal contains sufficient amount of REE to extract that is concentrated in the FABA to the value of more than 400 ppm. Regarding to the REE mode of occurrence that is mostly concentrated in the glass form or amorphous silicate, this study was conducted in order to observe the effect of sodium hydroxide to breakage the amorphous silicate structure. In order to obtain the amorphous silicate, magnetic separator was applied. Based on the XRF analysis, the non-magnetic fly ash (amorphous silicate of glass form) contained major elements of SiO2, Al2O3, Fe2O3, CaO, MgO and the rest were minor and trace elements such as Na2O, K2O, Cr2O3, MnO, SrO and P2O5. The particle size of the amorphous silicate is less than < 38 μm (- 400 mesh). To run the experiments, sodium hydroxide was used as the reagent with varied concentrations of 6 M, 8 M and 10 M. Other variables were temperature of 28 0C (room temperature), 60 0C and 90 0C and residence time of 2, 4, and 6 hour. Based on the results analysis, the amorphous silicate was decomposed to the most of about 22 % at 90 0C of temperature, 10 M of sodium hydroxide concentration, and 6 hour of residence time.