Improved Flotation of High-Clay-Content Gold Ore Using Fenugreek Polysaccharide Gum as the Depressant

Abstract

Due to the influence of the clay minerals, the grade of the concentrate from a gold concentrator in Northwest China is low, which significantly compromises product valuation and market competitivenes. This study evaluated the industrial potential of fenugreek polysaccharide gum (FGM) as a depressant to enhance the flotation through pilot-scale testing. Results showed that the FGM system effectively upgraded the concentrate grade by 4.7 g/t, while slightly increasing recovery by 0.37%. The application of FGM has generated an additional $1.715 million profit over the past two years. These findings suggest that FGM holds significant potential for large-scale industrialization in the flotation of high-clay-content sulfide ores.

1. Introduction

A gold mine located in Qaidam Basin of China is low-grade (with an average Au grade of 2.3 g/t) and high-clay-content (with clay accounting for 28.5%), and its dominant gold-bearing mineral is arsenopyrite [1,2]. It pertains to typical refractory gold ore and frequently coexists with clay minerals like pyrophyllite and mica [3]. Pyrophyllite and mica, with low hardness (moss hardness of 1–2.5) and good natural floatability, usually generate a significant amount of slime during grinding. This slime may seriously interfere with the flotation of sulfide minerals and is easily transported to flotation concentrate, leading to a reduction in concentrate grade [4,5]. In the flotation process of this ore, it is challenging to effectively separate the main Au-carrier mineral arsenopyrite and the primary gangue mineral pyrophyllite and mica without adding any depressant.

The depression of clay minerals in flotation can be achieved using inorganic or organic reagents designed to suppress silicate minerals. Commonly used depressants include water glass, hexametaphosphate, fluorocompounds, carboxymethyl cellulose, guar gum and lignosulphonate [6,7,8,9,10]. Among these, guar gum and other polysaccharide polymers have been widely used for sulfide mineral flotation, but this tends to pose severe disadvantages such as price sensitivity to market conditions, limited availability and high cost [5,7,10]. Therefore, there is a pressing need to develop more selective, cost-effective and high-performance depressants for the separation of sulfide minerals from hydrophobic clay minerals.

Fenugreek polysaccharide gum (FGM) is extensively present in beans, bitter beans and other leguminous plants. It possesses excellent water solubility and emulsifying characteristics and can react with a variety of metals [11,12]. It is widely used in the fields of flotation, food industry and pharmaceutical industry [13,14,15]. In this study, the pilot plant tests were conducted to examine the industrial potential of FGM as a depressant for this specific gold ore in Qaidam Basin. Additionally, the economic benefits of industrial operation over the past two years were evaluated.

2. Materials and Methods

FGM used in this experiment came from Shandong Gukang Biotechnology Co., Ltd. (Jinan, China), which was a white powder with excellent water solubility. In both the pilot plant test and industrial production, sodium carbonate (Na₂CO₃) was used as the pH regulator, cupric sulfate (CuSO₄) as the activator, sodium isobutyl xanthate (SIBX) and ammonium dibutyl dithiophosphate (ADD) as mixed collectors and pine oil as the frother. All other reagents and dosages remained constant during the trials while only the FGM depressant was added compared with the old flowchart.

Based on the lab closed-circuit flotation tests, a pilot-scale test (as Figure 1) was carried out at a gold concentrator in Northwest China. The raw ore, concentrate and tailings were sampled, filtered, dried and weighed. Subsequently, the gold grade of the sample was assayed, and the recovery was calculated.

The zeta potential measurements experimental procedure was conducted as follows: 30 mg of mineral powder was homogeneously dispersed in 50 mL of 1 mM potassium chloride (KCl) aqueous solution to prepare a stable suspension. The mixture was subjected to continuous magnetic stirring for initial conditioning. During this stirring phase, precise pH adjustments were first implemented using an appropriate buffer solution, followed by systematic addition of depressant agents. After complete reagent incorporation, the suspension underwent additional homogenization through 10 min of sustained mechanical agitation. Subsequently, the stirring apparatus was deactivated to allow undisturbed particle sedimentation for a minimum period of 30 min. Zeta potential measurements were ultimately performed using a Delsa 440sx Zeta Potential Analyzer manufactured by Malvern Instruments Ltd.(Worcestershire, UK), with triplicate readings taken to ensure measurement reproducibility.

3. Results and Discussion

Through comparative experiments, the technical superiority of FGM over traditional depressants such as sodium silicate and guar gum was demonstrated. In laboratory closed-circuit experiments, the new green organic depressant, FGM, has shown decent performance in increasing the enrichment ratio without loss of recovery.

The pilot test of FGM followed the test flow consisting of one roughing, two cleaning and two scavenging. The average results with three repeated experiments, presented in Table 1, show that the addition of FGM increased the concentrate grade by 4.7 g/t, from 26.11 g/t to 30.81 g/t, and the enrichment ratio was improved by 1.96, while the gold recovery increased by 0.37 percentage points. The pilot test findings confirmed that FGM is suitable for industrial production. The effectiveness of FGM can be attributed to its strong depression of the main gangue minerals like pyrophyllite and mica. Furthermore, FGM can disperse the gangue minerals from the surface of Au-bearing arsenopyrite without negatively affecting arsenopyrite recovery.

The research findings were successfully implemented at a gold concentrator in Northwest China with a raw ore processing capacity of 4000 t/d. Despite dealing with similar raw ore grades but increased ore selectivity challenges, various flotation indexes were significantly improved. Additionally, the installed capacity of equipment was reduced by 24%, effectively lowering production energy consumption and generating substantial economic benefits for the enterprise. From 2022 to 2023, as the research results were gradually industrialized, notable economic improvements were achieved compared to 2021 (without FGM). The output value in 2022 and 2023 increased by $0.89 million and $1.24 million, while the new tax contributions rose by $0.175 million and $0.24 million, respectively. Over two years, the total additional profit generated by the implementation of the FGM system reached $1.715 million.

Preliminary investigations have been systematically performed to elucidate the inhibitory mechanism of the novel reagent in industrial ore flotation processes. Here, we take pyrite and muscovite minerals as representatives of valuable minerals and gangue minerals, respectively. The results are shown in Figure 2. The experimental results demonstrated that the introduction of FGM significantly altered the surface potential characteristics of both muscovite and pyrite across various pH conditions. FGM treatment induced a notable elevation in zeta potential for both minerals, which can be attributed to the competitive adsorption mechanism between FGM molecules and negatively charged hydroxyl ions (OH⁻) on mineral surfaces.

Notably, comparative analysis revealed a more pronounced enhancement in surface zeta potential for muscovite compared to pyrite, indicating preferential FGM adsorption on muscovite surfaces. This differential adsorption behavior suggests that FGM exhibits stronger affinity interactions with muscovite compared to pyrite. The selective surface modification mechanism subsequently reduces muscovite’s opportunity to enter concentrate, i.e., muscovite was depressed selectively.

4. Conclusions

The novel green depressant FGM (featured by inherent biocompatibility and biodegradability) demonstrated exceptional performance in enhancing flotation efficiency for refractory gold ores characterized by low grade and elevated clay content. Key findings reveal its significant potential for industrial-scale applications:

(1). Process optimization: FGM achieved a notable 4.7 g/t increase in concentrate grade while maintaining recovery rates (+0.37%) compared to conventional reagents, particularly effective in managing clay interference during separation.

(2). Economic validation: industrial implementation yielded an additional $1.715 million in profits over a two-year operational period, substantiating both technical efficacy and economic viability for mineral processing operations.

(3). The selective adsorption of FGM on gangue mineral (muscovite) and valuable mineral (pyrite) causes the selective depression of muscovite.

(4). Implementation considerations:

Operational stability: while promising, the reagent’s long-term performance consistency requires extended validation under continuous production conditions.

Mineralogical sensitivity: effectiveness appears ore-specific, showing variable responses to different clay types and content levels, necessitating tailored optimization strategies based on ore composition.

Author Contributions

Conceptualization, Z.W. and Z.G.; methodology, X.W. and Z.G.; software, M.Z. and W.Z.; investigation, X.W. and M.Z.; resources, Q.D. and K.Z.; writing—original draft preparation, X.W. and W.Z.; writing—review and editing, Z.W. and M.Z.; visualization, Z.W. and M.Z.; supervision, Z.G.; project administration, Z.W.; funding acquisition, Z.W., Z.G., Q.D. and K.Z. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

We would like to thank Hui Yang (Y.H.), a master’s student from Kunming University of Science and Technology, for her help with the zeta potential measurements.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1 Flotation flowchart of the industrial test (old flowchart without FGM).

Figure 2 Zeta potential values of pyrite and muscovite shown as a function of pH.

Table 1

Industrial flotation test results (average values ± standard deviation).

Reagent System	Au Grade (g/t)			Recovery (%)	Enrichment Ratio
	Raw Ore	Concentrate	Tailings
Old reagents system (without FGM)	2.26 ± 0.01	26.11 ± 0.12	0.36 ± 0.02	85.25 ± 0.24	11.55 ± 0.10
New reagents system	2.28 ± 0.01	30.81 ± 0.11	0.35 ± 0.01	85.62 ± 0.36	13.51 ± 0.11

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