The role of citric acid in the flotation separation of rare earth from the silicates Liuyin Xia a,b,⇑ , Brian Hart a , Kyle Douglas a a Surface Science Western, Research Park, University of Western Ontario, London, Ont. N6G0J3, Canada b Department of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China article info Article history: Received 20 August 2014 Accepted 9 February 2015 Available online 3 March 2015 Keywords: Citric acid Rare earth ToF-SIMS Froth flotation abstract The Nechalacho project is the most advanced large heavy rare earth elements (HREE) project outside of China. Open circuit and locked cycle flotation tests along with pilot plant testing of rare earth elements (REE) concentration from the host rocks are accomplished with collectors of alkyl phosphates and the modifier of citric acid. In this study, the function of citric acid in the separation of rare metals against sili- cates is investigated by a combination of micro-flotation tests and time of flight secondary ion mass spec- trometry (ToF-SIMS) surface chemical analysis. It was observed that there was little effect of citric acid on the REE recovery in the micro-flotation tests conditioned with de-ionized water (DIW). To evaluate the flotation response with excess secondary ions in the pulp, micro-flotation tests were performed to look at changes in recovery as a result of adding Al ions and the subsequent presence of citric acid. The results from three micro-flotation tests (DIW, DIW with the addition of 100 mg/L Al and DIW + 100 mg/L Al and 500 g/t citric acid) revealed that the addition of Al ions led to a decrease of REE grade, a lower REE min- erals recovery and/or an unexpected promotion of silicates to the concentrate. Citric acid reduced the negative effect generated by the Al ions in the flotation, which was shown by an improvement in REE grade. ToF-SIMS surface analysis of undifferentiated grains from the tests with and without citric acid revealed that grains reporting to the concentrate are doing so in response to collector attachment in com- bination with having more secondary Al on their surface. Citric acid may partially form aqueous soluble metal–ligand complexes resulting in less Al ions on the grains surface, which were rejected to the tailings. Citric acid also may form chelation competing for adsorption on gangue minerals, resulting in a dimin- ished effectiveness of the activation site. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Previous investigations on the Nechalacho project indicated that the separation of rare earth minerals from oxide and silicate gangue minerals has been successfully carried out by flotation. The following flotation concentration recoveries have been report- ed: 90% of the zirconium oxide, 69% of the niobium oxide and 63% of the tantalum oxide (Cox et al., 2010). As part of the project, micro-flotation tests were conducted on a feed sample in order to examine factors affecting stream partitioning. SEM–EDX was performed to evaluate variability in grain composition between streams (concentrate and tailings). The results showed that REE containing grains are tentatively identified as monazite, allenite and synchysite. Other minerals examined include zircon and pyr- ite, along with gangue aluminosilicate phases. The SEM/EDX eval- uation indicated that the micro-flotation separation was very successful, as almost no REE grains were identified in the tailings samples (Chehreh Chelgani et al., 2013). However, the reagent scheme used in that micro-flotation was relatively complex and at plant concentration levels. Although reagent signal intensity dis- crimination on test stream mineral surfaces was observed by the TOF-SIMS analysis, the details of the differences in surface species, particularly related to potential activation (or depression) of the examined mineral phases, is still not clear. One of the organic acids used in previous investigations was citric acid, which is a relatively common chelating agent employed in a variety of industries. It forms soluble, weakly bound complexes with metal ions, presumably by means of its carboxylate groups. Liu and Zhang (2000) found that citric acid could effectively remove Ca 2+ from chalcopyrite surfaces to increase the selectivity of flotation separation between chalcopyrite and galena using http://dx.doi.org/10.1016/j.mineng.2015.02.008 0892-6875/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Surface Science Western, Research Park, University of Western Ontario, 999 Collip Cir, London, Ont. N6G 0J3, Canada. Tel.: +1 (519) 661 2173; fax: +1 (519)661 3709. E-mail address: lxia22@uwo.ca (L. Xia). Minerals Engineering 74 (2015) 123–129 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng