Application and mechanism of calcium hypochlorite in replacing sodium sulfide for the clean separation of low-grade molybdenum-bismuth ore

Abstract

Stemming from the high costs and environmental pollution associated with the use of sodium sulfide in the separation and extraction processes of molybdenum bismuth ore, calcium hypochlorite was introduced as a substitute to facilitate the cleaner production of low-grade molybdenum bismuth ore in this study. The effects of calcium hypochlorite on molybdenite, bismuthinite, and pyrite were investigated through micro-flotation, flotation kinetics, batch flotation, Fourier transform infrared (FTIR) spectra, scanning electron microscopy energy dispersion spectra (SEM-EDS), and inductively coupled plasma-optical emission spectra (ICP-OES). The flotation tests results showed that calcium hypochlorite could selectively depress bismuthinite and pyrite. In comparison to sodium sulfide, calcium hypochlorite not only improved the flotation indicators for molybdenum and bismuth concentrates but also reduced the dosage of flotation reagents. Moreover, the chemical oxygen demand (COD) of tailings wastewater significantly decreased when using calcium hypochlorite as a depressant. Mechanism research revealed that the use of calcium hypochlorite as a depressant led to BiOCl precipitation on bismuthinite, which hindered the attachment of the collector. In summary, calcium hypochlorite serves as a more efficient and environmentally friendly depressant compared to sodium sulfide in the industrial production processes of low-grade molybdenum bismuth ore.

1. Introduction

Molybdenum, a strategically valuable metal, has found applications in medical, aerospace, and other high-tech sectors owing to its exceptional physicochemical properties [1]. Molybdenite is the primary natural form of molybdenum and is recognized as the most commercially significant mineral for molybdenum extraction [2, 3]. Molybdenite commonly coexists with copper ore, and due to continuous development and utilization, the reserves of high-grade copper-molybdenum ore are nearly exhausted [4]. Faced with the burgeoning demand for molybdenum resources in society, beneficiation industries are now compelled to process low-grade molybdenum ores [5].

In recent years, scholarly interest in molybdenum bismuth ore has grown significantly [6−8]. Similar to molybdenite, bismuthinite also exhibits favorable floatability [6]. Therefore, it is a common practice to initially float molybdenite and bismuthinite together, followed by their subsequent separation in the flotation process [9]. During the separation process, processing plants typically use sodium sulfide as the depressant for bismuthinite [10]. Sodium sulfide generates hydrophobic products on molybdenite but forms hydrophilic components on bismuthinite, hindering the adhesion of collectors and thus suppressing the up-float of bismuthinite [11]. Sodium sulfide has been used to depress bismuthinite and facilitate its separation from molybdenite in flotation [9]. However, the application of sodium sulfide has posed numerous challenges in practical production for the plants. Sodium sulfide exhibits poor stability and susceptibility to oxidation, thereby reducing its efficacy in the pulp [12]. As a result, a substantial quantity of sodium sulfide is required during flotation. Exceeding a certain sodium sulfide concentration threshold can induce a non-selective removal effect on collectors [13]. This consequently leads to diminished recovery of valuable minerals and erratic fluctuations in product indicators [9, 12, 13]. Furthermore, storing a substantial quantity of sodium sulfide can generate hydrogen sulfide, a highly toxic gas posing significant threats to operator safety and the ecological environment [10, 14, 15]. Moreover, sodium sulfide’s extensive application poses challenges in treating beneficiation wastewater at the end of the flotation process [9, 16]. Therefore, the use of sodium sulfide, whether direct or indirect, imposes a significant burden on plants, underscoring the need for alternative depressants.