EXTRACTION OF RARE EARTH ELEMENTS FROM PHOSPHOGYPSUM (BY-PRODUCT OF PHOSPHATE FERTILIZER PRODUCTION)

Mugdha Walawalkar,a Connie K. Nichol,b and Gisele Azimi a,c,*

a Laboratory for Strategic Materials, Chemical Engineering and Applied Chemistry Department, University of Toronto, Toronto, ON, M5S3E5, Canadab Agrium Inc., 11751 River Road, Fort Saskatchewan, AB, T8L4J1, Canadac Materials Science and Engineering Department, University of Toronto, Toronto, ON, M5S3E5, Canada* corresponding author, Tel. +14169467568Email address: g.azimi@utoronto.ca

ABSTRACT

Rare earth elements (REEs) are essential to the production of technologically advanced products, including wind turbines, electric and hybrid cars, automotive catalytic convertors, hand held electronic devices, and military defense systems. Their application in environmentally beneficial technologies puts them at a high priority for ameliorating environmental concerns. Because of their increasingly high demand and limiting resources, it is highly beneficial to find new sources for them. The present study is focused on extracting REEs from a phosphate based fertilizer by-product, known as phosphogypsum (PG). Global estimated world production of PG is about 170 million tonnes that is stored in the form of wet stacks adjacent to fertilizer production facilities, occupying vast amounts of land. PG also contains trace metals, which are detrimental to the environment, and hence it is crucial to monitor these stacks. We obtained our PG sample from Agrium Inc. (a fertilizer manufacturing company in Canada). We performed a thorough characterization of the PG sample and found that it contains REEs, predominantly Y, La, Ce, Nd, and Sm. Figure 1 presents SEM, TEM, and STEM characterization results. For extraction purposes, we used hydrometallurgical acid leaching techniques. We investigated three acids over a range of operating conditions such as acid concentration, temperature, solid-to-liquid ratio, and residence time. We achieved an average extraction of 81% using 1.5 M hydrochloric acid using operating conditions that can be easily replicated in industrial settings. A systematic diagram of our process is presented in Figure 2. Our research was also focused on mechanistic studies of the extraction process using SEM/EDS and XRD. If our process is implemented, not only will it reduce the size of the existing PG stacks (mitigating their environmental risks), but also enable the development of new innovative routes for extracting REEs, making countries like Canada less vulnerable to supply disruptions.

Figure 1 – Characterization of the PG powder using SEM, TEM and STEM techniques identifying Ce, La, Nd, Y and Sm and the main REEs present

Figure 2 – Schematic diagram of our process for the extraction of REEs, from phosphogypsum (obtained from Agrium Inc., Alberta, Canada), using acid leaching

KEYWORDS

Rare earth elements, Fertilizer by-product, Rare earths recovery, Phosphogypsum, Acid leaching process