Investigations on flotation separation of scheelite from calcite by using a novel depressant: Sodium phytate
Graphical abstract
Introduction
Tungsten is an important rare metal and has wide application in modern industry. Scheelite, a typical tungsten-containing mineral (CaWO4), is the main resource for producing tungsten from ore deposits in the field of mineral processing. Scheelite is usually associated with other calcium-containing minerals in deposits, such as calcite (CaCO3), fluorite (CaF2) and fluorapatite [Ca10(PO4)6F2]. Among these gangue minerals, calcite is often regarded as the least valuable one and must be removed (Castro and de Borrego, 1996). Flotation, a physico-chemical separation process that utilizes the difference in surface properties of the valuable minerals and the unwanted gangue minerals (Wills and Napier-Munn, 2005), is the main technique to recover scheelite from calcite and other gangue minerals at the present time (Hu et al., 2012).
In scheelite flotation, fatty acids and their derivatives are the most commonly used collectors. However, due to the same active Ca2+ species on the cleavage surfaces of scheelite and calcite, the two minerals often exhibit similar reactivity to these fatty acids collectors, which causes poor selectivity in flotation (Pradip et al., 2002). It has been widely recognized that the fatty acids collectors can collect both scheelite and calcite through chemisorption by forming calcium dicarboxylates on the mineral surfaces (Atademir et al., 1981, Ozcan and Bulutcu, 1993, Rao et al., 1990, Rao and Forssberg, 1991). Therefore, it is almost impossible to separate scheelite and calcite without any depressant, when the fatty acids are used as collectors in flotation of calcitic scheelite ore.
Depressants, aiming at reducing the flotability of calcite under scheelite flotation conditions, are widely used in scheelite flotation plants to improve the flotation selectivity. The depressant effect is mostly realized through the selective interactions between the Ca2+ on calcite surface and the functional groups in the inhibitors. The most common functional groups are silicate, hydroxy, carboxyl and phosphate, corresponding to the reported inhibitors shown in Table 1. The depressants with functional groups including silicate, hydroxy and carboxyl, are reported to make calcite surface hydrophilic by adsorbing and forming new hydrophilic species on particle surface. However, these reagents often show limited selectivity in scheelite flotation practice because they also depress scheelite to a large extent.
Of all the reported depressants in Table 1, phosphates (see Fig. 1) are special kinds because their depressant effect on calcium minerals is not related to their adsorption and formation of new species on particle surface. It has been demonstrated that the phosphates depress the calcium minerals by dissolving and complexing the Ca2+ on mineral surfaces, which is intimately influenced by the pulp pH and phosphate structure in solutions (Changgen and Yongxin, 1983). It has been found out that the preferential flotation separation of scheelite from calcite could be realized by using phosphates as flotation depressants, by control of pulp pH range of 9.0 for Na3PO4, 8.0 for Na4P2O7 and 9.5–10.5 for (NaPO3)6, respectively (Changgen and Yongxin, 1983, Gao et al., 2018). However, the best pH ranges that the phosphates work are too narrow to be precisely controlled in flotation plants. As a result, the depressant effect of phosphates in scheelite flotation is often discounted and the efficient separation of scheelite and calcite remains challenging.
Phytic acid (C6H18O24P6) is a naturally occurring phosphate ester in legume seed, cereal grains and beans (Sandberg and Scheers, 2016). It is easily available from agricultural products and can exist in aqueous solution stably. As an organic macromolecule compound, phytic acid has 12 free hydroxyl groups and 6 phosphate groups on its carbon ring in one molecule, hence exhibiting powerful chelating capability with many metal ions (Torres et al., 2005), as shown in Fig. 3. Phytic acid is generally used as the corrosion resistant inhibitor for the paint or pigment (Nan et al., 2015, Notoya et al., 1995). It is also reported to act as the reactant to form conversion coating in alloy materials (Cui et al., 2008, Pan et al., 2009). However, there are few published papers referring to utilizing its chelating capability in flotation field, especially in the separation of scheelite and calcite.
In this paper, a sodium salt of phytic acid named sodium phytate (SP) was evaluated as a novel phosphates depressant for calcite in scheelite flotation using sodium oleate (NaOl) as collector. The effects of SP on the flotation performance of scheelite and calcite were investigated. The adsorption mechanism of SP and NaOl on mineral surfaces was evaluated by zeta potential and FTIR spectra. Based on the micro flotation results and surface analysis, the depressant-collector scheme (sequencing and dosage) and a chelating adsorption model of SP at the particle/water interface were proposed.
Section snippets
Mineral samples and reagents
The samples of scheelite and calcite were all obtained from Xintianling mine, Hunan, China. Hand-selected crystals were crushed to −1 mm with a laboratory roll crusher and were ground in a ceramic ball mill. The ground samples were wet- sieved and the −74 + 38 μm-size fraction was collected for micro-flotation test. A portion of 38 μm-size fraction was further ground in an agate mortar to obtain −2 μm-size particles for zeta potential measurements and FTIR spectra studies. The purities of the
Micro-flotation of single minerals
Flotation studies of scheelite and calcite as a function of pulp pH, NaOl concentration, SP concentration and addition order of reagents were investigated, and the results are shown in Fig. 4, Fig. 5, Fig. 6.
Fig. 4 shows the effects of NaOl concentration and pulp pH on flotation behaviors of scheelite and calcite. When pulp pH was adjusted to 7.0, the flotation recovery of scheelite and calcite all increased with the increase of NaOl concentration. When NaOl concentration increased to 2 × 10−4
Conclusions
Sodium phytate was found to be a depressant for selectively flotation of scheelite from calcite. The flotation results of both single and mixed binary minerals indicated that scheelite presented good flotability while calcite exhibited poor flotability by using sodium phytate as depressant and sodium oleate as collector. A reagent scheme, i.e., 2 × 10−5 mol/L of sodium phytate and 2 × 10−4 mol/L of sodium oleate improved the separation of scheelite from calcite in the pH range of 7–12.
The
Acknowledgements
This work was supported by the Major State Basic Research Development Program of China (973program) (2014CB643402), the Collaborative Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources-Innovation Driven Plan of Central South University (Grant No. 2015CX005).
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