Optimisation of bioflotation of carbonaceous impurities from phosphate ore N. A. Abdel-Khalek 1 , A. M. Elmahdy 1 , A. A. El-Midany* 1 and S. Farrah 2 The amenability of carbonate separation from a sedimentary phosphate ore via the bioflotation process was investigated. The conventional flotation experiments using oleic acid as a collector could not give good results as in the presence of bacteria. Therefore, two types of bacteria were tested in beneficiation of carbonaceous phosphate ore. Flotation experiments were carried out using statistical designs for optimising the main operating parameters. The main parameters used in this study were micro-organism concentration, pH and collector dosage. The optimisation process was conducted using rotatable central composite design (RCCD) as a tool for optimisation. The design results showed that selective separation of carbonate from phosphate can be obtained using bacteria. An optimum concentrate containing 0?7%MgO and ,31%P 2 O 5 with a recovery of ,93% can be obtained at pH 5?5, a micro-organism concentration of 10610 7 cells and a collector dosage of 2 kg t 21 . Keywords: Dolomite, Phosphate, Statistical design, CCD, Bacteria, Micro-organism, Bioflotation Introduction The main problem that the current phosphate industry faces is the presence of carbonate impurities. Different flowsheets were suggested to separate carbonaceous gangue from sedimentary phosphate ores. However, only one process was applied on an industrial scale (heavy media separation) but it was stopped after a while owing to technical or/and economic reasons (El-Shall et al., 1996; El-Gillani and Abouzeid, 1993; Anazia and Hanna, 1987; Houot, 1982). The biggest difficulty comes from the similarity of the physicochemical properties of phosphate and carbonates minerals. The solution chemistry of phosphate and the presence of dissolved species from other salt type minerals lead to phosphate surface transformation (Moudgil and Somasundaran, 1986; Anazia and Hanna, 1987; Houot, 1982). Recently, biorelated processes have been used for processing of different ores such as coal, sulphides and gold (Somasundaran, 1998; Rao et al., 1992; Rao and Sumasundaran, 1995; Natarajan and Deo, 2001; Attia and Eizeky, 1985). However, the application of this process in phosphate beneficiation is very limited (Smith et al., 1991; Boice, 2000). In the present study, two types of bacteria were applied as surface modifiers in carbonate separation from sedimentary phosphate ores (Abdel-Khalek and S. Farrah, 2004). The amenability of carbonate separa- tion from a phosphate ore (Abu-Tartur area, Egypt) was tested using the bioflotation process. The separation process was investigated in terms of controlling factors. Moreover, statistical designs were used to screen and optimise the operating parameters affecting the flotation process. Experimental Phosphate samples A low grade sample of phosphate ore (Abu-Tartur area, New Valley, Egypt) was used. The representative sample was primarily crushed using a jaw crusher and then using a roller crusher to about –2 mm. The crushed ore was then divided to patches of ,3 kg using a Jone Riffle sampler. To prepare the feed for flotation experiments, samples were attrition scrubbed at 55% solid for 15 min and then, screened on a 0?25 mm screen. The oversize (22?0z0?25 mm) fraction was separated as a coarse concentrate (,30%P 2 O 5 ) and (0?6%MgO). The under- size fraction (20?25 mm) was further screened on a screen (0?044 mm) where the undersize fraction (20?044 mm) was considered as phosphatic clay slimes (,31% A.I.). The size fraction (20?25z0?044 mm) of high MgO content was taken as a flotation feed. Such intermediate size fraction (20?25z0?044 mm) has a low content of P 2 O 5 (27?07%) as a result of its highly contamination with MgO (,2?5%). Chemicals Sodium oleate of 99% purity was used as a collector (Aldrich Chemicals, Germany). A analytical grade of NaOH and H 2 SO 4 are used as pH modifiers. All reagents were used without further modification. Bacteria Two types of bacteria were used in the bioflotation process. These bacteria were isolated from the surface of 1 Central Metallurgical R & D Institute, PO Box 87 Helwan, Cairo, Egypt 2 Particle Engineering Research Center, University of Florida, Gainesville, FL, USA *Corresponding author, email aelmidany@gmail.com ß 2008 Institute of Materials, Minerals and Mining and The AusIMM Published by Maney on behalf of the Institute and The AusIMM Received 27 March 2006; accepted 11 December 2006 38 DOI 10.1179/174328507X198735 Mineral Processing and Extractive Metallurgy (Trans. Inst. Min. Metall. C) 2008 VOL 117 NO 1