Hydrochloric Acid Leaching of Philippine Coal Fly Ash: Investigation and Optimisation of Leaching Parameters by Response Surface Methodology (RSM)

Al Mon E Dahan,Mona Lisa C Pacaña,Vannie Joy T Resabal,Leaniel C Silva,Ronben M Baute,Carlito B Tabelin,Richard D Alorro

doi:10.3390/suschem3010006

Abstract

Coal Fly Ash (CFA) is a hazardous waste from coal-fired power plants, but has increasingly become a popular supplementary cementitious material for cement in the construction industry. As a secondary resource of REE, its main advantage lies in its fine particle size that eliminates the need for costly and energy-intensive comminution. In this study, the potential of CFA from the Philippines as a secondary REE resource was investigated by direct leaching of REEs with hydrochloric acid (HCl). The CFA sample came from a coal-fired power plant with a circulating fluidized bed combustion (CFBC) technology. For the leaching tests, the effects of HCl concentration, leaching time and leaching temperature on REE extractions were elucidate optimized via Response Surface Methodology (RSM). The RSM results showed that the optimum leaching occurred at 3M HCl, 65 °C and 270 min with extractions of Nd, Er, Eu, Tb and Dy at 70.8%, 76.34%, 88.02%, 90.01% and 73.38%, respectively. According to these results, the CFA from the Philippines is a promising secondary resource of REE and the extraction method employed was effective in achieving a relatively high REE dissolution. Moreover, the empirical model that was established accurately predicted the dissolution of REE with an accuracy of 98.20%, 96.66%, 97.09%, 98.17% and 97.78% for Nd, Er, Eu, Tb and Dy, respectively.

Highlights

This research has proven that the Philippine Coal Fly Ash (CFA) is a potential secondary resource of Rare earth elements (REE) based on the criteria of Seredin and Dai (2012) with an outlook coefficient of 1.03 and a relative percentage of critical elements of 33.3%
The REEs were found to be finely disseminated throught the matrix of the CFA sample; that is, they are associated with the other minerals present that are relatively abundant such as the aluminosilicates, oxides of iron and calcium
The statistical analyses conducted have shown that the three main factors (HCl concentration, leaching temperature and leaching time) investigated have significant effects on the extraction of

Summary

Introduction

A three-megawatt wind turbine, for example, requires three tons of REEs [1]. With the global push to decarbonize society to combat climate change, the demand for REEs is projected to increase by 5–9% for the 25 years [1]. The demand for REEs is outstripping its supply by about 3000 tons per year [2,3]. This supply–demand imbalance in the market is driving up the REE prices, which in turn is fueling the need to explore new REE resources and cost-effective extraction technologies [4,5,6]

Methods

Results

Conclusion