Introduction
Molybdenum-polymetallic ores containing both molybdenum and lead are often characterized by complex mineral compositions, variable mineral dissemination sizes, and significant slime generation during grinding. These characteristics make the selection of flotation equipment critical to achieving high recovery and concentrate quality.
In the ore investigated in this study, molybdenite and galena are the principal valuable minerals. Molybdenite exhibits uneven medium-to-fine dissemination, with 53.19% of its particles occurring in the +0.074 mm fraction, whereas galena is relatively coarser, with 78.27% occurring in the +0.074 mm fraction. Some molybdenite and galena particles are closely intergrown. Meanwhile, calcite, mica, and chlorite account for 37.05% of the ore and are prone to sliming during grinding, which can adversely affect flotation selectivity.
The different operating principles of flotation machines and flotation columns lead to different responses to mineral particle size, pulp flow conditions, reagent dispersion, and bubble–particle attachment. Flotation machines rely primarily on mechanical agitation and turbulent mixing, which favors the suspension and recovery of relatively coarse particles. Flotation columns use counter-current contact between rising bubbles and descending pulp, together with fine bubbles and a relatively stable separation environment, making them particularly attractive for fine-grained mineral recovery.
Therefore, the key question is not simply which equipment provides higher recovery, but which flotation equipment is better matched to the mineral dissemination characteristics and the separation objective at each processing stage.
Comparative flotation tests were consequently conducted for both the bulk molybdenum-lead flotation stage and the subsequent molybdenum-lead separation stage. The results show that the flotation column increased molybdenum rougher recovery by 6.41 percentage points, but reduced lead recovery by 21.06 percentage points compared with the flotation machine. During molybdenum-lead separation, the flotation machine provided substantially better galena depression and concentrate quality, producing a final molybdenum concentrate containing only 1.73% Pb. Overall, the flotation machine demonstrated better comprehensive suitability for this ore.
Experimental Materials and Methods
1. Experimental Materials
The test samples were collected from a molybdenum-polymetallic ore deposit and consisted of 600 kg of run-of-mine (ROM) ore and 400 kg of molybdenum rougher concentrate.
The ROM ore was crushed to the required particle size, screened to remove oversized particles, homogenized, and split into representative samples. The molybdenum rougher concentrate was obtained from a beneficiation circuit comprising one roughing stage, three scavenging stages, and one pre-cleaning stage. It was similarly homogenized and split before testing.
Mineralogical characterization by X-ray diffraction (XRD) showed that the principal valuable minerals in the ROM ore were molybdenite and galena, while the major gangue minerals were quartz, feldspar, and calcite. In the molybdenum rougher concentrate, molybdenite and galena remained the principal valuable minerals, while pyrite and chalcopyrite constituted important gangue or associated sulfide minerals.
The chemical composition of the samples is summarized below.
| Sample | Mo (%) | Pb (%) |
| ROM Ore | 0.058 | 043 |
| Molybdenum rougher concentrate | 11.09 | 31.68 |
Molybdenum occurred predominantly as molybdenite with a particle-size range of 0.01–0.25 mm. Its oxidation rate was 11.11%, corresponding to a theoretical recovery of 88.89%.
Lead occurred mainly as galena, with a particle-size range of 0.037–0.38 mm. The oxidation rate was 13.33%, giving a theoretical recovery of 86.67%.
These characteristics indicate that the ore contains both relatively coarse galena and finer molybdenite, providing a suitable basis for evaluating the particle-size adaptability of flotation machines and flotation columns.
2. Test Equipment
A BM-Φ380×260 conical ball mill was used for grinding the ROM ore, while an LM-1300M vertical mill was employed for regrinding the molybdenum rougher concentrate.
Both flotation machines and flotation columns were used for comparative flotation tests. The flotation columns were equipped with spargers capable of producing bubbles approximately 1 μm in diameter, thereby increasing the available bubble surface area and improving the probability of fine-particle capture.
Before flotation, the pulp was conditioned to establish the required pulp concentration and pH conditions. The conditioned pulp was then transferred to the flotation equipment using a peristaltic pump.
Sampling was performed only after the equipment had reached a stable operating state. Samples were collected at predetermined intervals to minimize errors associated with start-up fluctuations, unstable pulp flow, or irregular sampling.
3. Test Procedure and Operating Parameters
The test consisted of two main stages: bulk molybdenum-lead flotation and molybdenum-lead separation. The specific procedures and parameters are detailed below.
The bulk molybdenum-lead flotation stage employed an open-circuit flowsheet comprising one roughing stage and one scavenging stage, with the grinding concentration of the raw ore maintained at 50%. To precisely determine the optimal grinding duration, grinding fineness tests were conducted. Regarding reagents, diesel was selected as the collector (dosage: 120 g/t), and No. 2 oil as the frother (dosage: 35 g/t). The flotation machine’s impeller speed was set to 1,800 r/min, with roughing and scavenging times of 10 min and 6 min, respectively. Additionally, the flotation column operated with a peristaltic pump speed of 160 r/min, a flotation time of 10 min, an aeration rate of 0.4 m³/h, an aeration pressure of 0.35 MPa, and a froth layer thickness maintained at 60 mm.
The molybdenum-lead separation stage utilized an open-circuit flowsheet consisting of one roughing stage and three cleaning stages. The regrinding concentration for the molybdenum rough concentrate was set at 55%, with strict control over regrinding fineness to ensure that the fraction finer than 0.038 mm accounted for 95% of the material. Reagent selection included CY-1 as the depressant (50 g/t), sodium silicate as the auxiliary depressant (50 g/t), diesel as the collector (5 g/t), and No. 2 oil as the frother (1.2 g/t). The flotation machine operated at a pulp concentration of 20%, with flotation times of 10 min for roughing and 6 min for cleaning. The flotation column operated with a pulp concentration of 10%, an aeration rate of 0.25 m³/h, and a froth layer thickness of 250 mm.
4. Testing and Analysis Methods
Following flotation, the products were filtered, dried to constant weight, and weighed. Chemical assays were conducted to determine molybdenum and lead grades.
Based on the assay results and product weights, yield, grade, and recovery were calculated.
XRD analysis was also performed to identify changes in mineral composition. In addition, laser particle-size analysis was conducted on selected products to investigate the relationship between particle-size distribution and flotation performance.
Results and Discussion
1. Grinding Fineness Optimization
To ensure the fairness of the comparative tests, grinding fineness optimization experiments were first conducted using a flotation machine; the relationship between the grinding time of the raw ore and the content of the -0.074 mm size fraction is shown in Figure 1.
As shown in Figure 1, the content of the -0.074 mm size fraction in the raw ore increased with prolonged grinding time. At a grinding time of 18 min, the -0.074 mm fraction content was 63.85%; at 22 min, it reached 66.52%; and at 25 min, it further increased to 71.50%. However, when the grinding time exceeded 22 min, the rate of increase in the -0.074 mm fraction content slowed down, while the phenomenon of over-grinding intensified, leading to an increase in fine slime content. Flotation tests were conducted to evaluate separation performance at various grinding fineness levels. The results showed that at a grinding fineness of 65% passing -0.074 mm, the rougher concentrate recovery rates were 79.06% for molybdenum and 77.94% for lead—increases of 3.98 and 3.64 percentage points, respectively, compared to a fineness of 55% passing -0.074 mm. When the grinding fineness was increased to 71.5% passing -0.074 mm, the molybdenum recovery rate rose by only 1.20 percentage points, while the lead recovery rate dropped by 2.80 percentage points; this decline is attributed to the brittle nature of galena, as over-grinding makes it difficult to effectively recover fine-grained lead minerals. Based on a comprehensive assessment of mineral liberation and flotation performance, the optimal grinding fineness for the bulk molybdenum-lead flotation stage was determined to be 65% passing -0.074 mm.
2. Flotation Column vs. Flotation Machine in Bulk Roughing
Using the established grinding fineness and reagent regime, open-circuit tests (one roughing stage and one scavenging stage) were conducted comparing a flotation column with a flotation machine; the test flowsheet is shown in Figure 2, and the results are presented in Table 1.
| Table 1. Comparative Test Results of Different Equipment for the Flotation of a Molybdenum–Lead Mixture (%) | ||||||
| Test Equipment | Product | Yield (%) | Grade Mo (%) | Grade Pb (%) | Recovery Mo (%) | Recovery Pb (%) |
| Flotation Machine | Rough Concentrate | 2.56 | 2.044 | 13.210 | 79.06 | 77.94 |
| Flotation Machine | Middlings | 1.46 | 0.161 | 0.950 | 3.54 | 3.20 |
| Flotation Machine | Tailings | 95.98 | 0.012 | 0.085 | 17.40 | 18.86 |
| Flotation Machine | Run-of-Mine Ore | 100.00 | 0.066 | 0.434 | 100.00 | 100.00 |
| Flotation Column | Rough Concentrate | 4.00 | 1.320 | 6.880 | 85.47 | 56.88 |
| Flotation Column | Middlings | 6.78 | 0.027 | 0.310 | 2.97 | 4.35 |
| Flotation Column | Tailings | 89.22 | 0.008 | 0.210 | 11.56 | 38.77 |
| Flotation Column | Run-of-Mine Ore | 100.00 | 0.062 | 0.483 | 100.00 | 100.00 |
As shown in Table 1, at the same grinding fineness (-0.074 mm, 65%), the flotation column improved molybdenum recovery in the rougher concentrate to 85.47%, which is 6.41 percentage points higher than that achieved by the flotation machine. However, the lead recovery in the rougher concentrate obtained using the flotation column was 56.88%, a decrease of 21.06 percentage points compared to the 77.94% recovery achieved by the flotation machine. This indicates that while the flotation column yields higher molybdenum recovery during the roughing stage, its lead recovery is significantly lower than that of the flotation machine; therefore, the flotation column is not recommended for the molybdenum-lead bulk flotation stage.
3. Flotation Machine vs. Flotation Column in Molybdenum-Lead Separation
To compare the performance metrics of flotation machines and flotation columns during the molybdenum-lead separation stage, comparative tests were conducted using a flowsheet consisting of one roughing stage and three cleaning stages. The pulp densities for the flotation column and the flotation machine were set at 10% and 20%, respectively. The test flowsheet is shown in Figure 3, and the results are presented in Table 2.
| Table 2. Comparative Test Results of Lead–Molybdenum Separation Equipment (%) | ||||||
| Separation Equipment | Product | Yield (%) | Grade Mo (%) | Grade Pb (%) | Recovery Mo (%) | Recovery Pb (%) |
| Flotation Machine | Molybdenum Concentrate | 1.95 | 42.56 | 1.73 | 7.67 | 0.11 |
| Tailings | 79.56 | 5.00 | 37.40 | 36.82 | 95.28 | |
| Concentrate 1 Tailings | 10.17 | 26.42 | 11.73 | 24.88 | 3.82 | |
| Concentrate 2 Tailings | 4.94 | 36.97 | 3.51 | 16.91 | 0.56 | |
| Concentrate 3 Tailings | 3.38 | 43.82 | 2.18 | 13.72 | 0.23 | |
| Mixed Concentrate | 100.00 | 10.803 | 31.23 | 100.00 | 100.00 | |
| Flotation Column | Molybdenum Concentrate | 41.56 | 39.37 | 9.96 | 42.53 | 13.25 |
| Tailings | 45.00 | 18.00 | 10.50 | 49.47 | 85.00 | |
| Concentrate 1 Tailings | 8.00 | 22.00 | 11.00 | 5.00 | 1.15 | |
| Concentrate 2 Tailings | 3.50 | 28.00 | 12.00 | 2.00 | 0.36 | |
| Concentrate 3 Tailings | 1.94 | 35.00 | 13.00 | 1.00 | 0.24 | |
| Mixed Concentrate | 100.00 | 16.38 | 9.35 | 100.00 | 100.00 | |
As shown in Table 2, the lead grade of the molybdenum concentrate obtained using the flotation machine was only 1.73%—a 94.46% reduction compared to the feed. This is because the mechanical agitation in the flotation machine ensures the uniform dispersion of depressants and their rapid adsorption onto the galena surface, effectively reducing its floatability. In contrast, the pulp flow in the flotation column is gentle and reagent dispersion is slow; consequently, the surfaces of some galena particles are not fully covered by the depressant, allowing them to attach to bubbles and float, which results in a higher lead content in the molybdenum concentrate. The final molybdenum recovery rate for the flotation column was 42.53%—significantly higher than that of the flotation machine—aligning with its advantages in capturing fine-grained minerals; however, while recovering fine molybdenite, the column also entrained more fine-grained gangue minerals and incompletely depressed galena, resulting in a lower molybdenum grade compared to the flotation machine. A “one-roughing, one-cleaning” flowsheet using the flotation machine reduced the lead grade in the molybdenum concentrate to 11.73%, meeting the standard after three cleaning stages; conversely, the flotation column required two cleaning stages to reach a similar level but failed to meet the standard even after three stages.
Conclusions
The comparative tests establish that flotation machine selection is more suitable than flotation column selection for the overall beneficiation of the investigated molybdenum-polymetallic ore.
First, the flotation machine and flotation column exhibit fundamentally different hydrodynamic characteristics. Mechanical agitation in the flotation machine generates strong turbulence and promotes coarse-particle suspension, reagent dispersion, and bubble-particle collision. This makes the flotation machine particularly suitable for the relatively coarse galena in the ore. The flotation column relies on counter-current contact and fine bubbles, which provide advantages in the recovery of fine-grained molybdenite but offer less favorable conditions for coarse-particle recovery and rapid reagent dispersion.
Second, during bulk molybdenum-lead flotation, the flotation column increased molybdenum recovery by 6.41 percentage points, reaching 85.47%, but reduced lead recovery by 21.06 percentage points compared with the flotation machine. Therefore, the higher molybdenum recovery of the flotation column did not compensate for its significant loss of lead recovery, making it unsuitable as the primary equipment for bulk molybdenum-lead flotation.
Third, during molybdenum-lead separation, the flotation machine demonstrated superior galena depression and separation selectivity. The final molybdenum concentrate produced by the flotation machine contained only 1.73% Pb, while the flotation column exhibited greater entrainment of fine gangue and incompletely depressed galena. Although the flotation column achieved a higher molybdenum recovery of 42.53%, its concentrate quality and overall separation performance were inferior.
Finally, the principal factor governing equipment performance is the compatibility between flotation equipment characteristics and mineral dissemination characteristics. The fine-grained molybdenite is theoretically more compatible with the fine-bubble and counter-current environment of a flotation column, whereas the relatively coarse galena is better suited to the strong mixing and suspension characteristics of a flotation machine. When both molybdenum and lead recovery, concentrate quality, and overall separation efficiency are considered simultaneously, the flotation machine provides the better overall solution.
Therefore, flotation machines are recommended as the primary flotation equipment for this molybdenum-polymetallic ore. This equipment selection can provide a more balanced combination of molybdenum and lead recovery, stronger galena depression, and better concentrate quality, while reducing the adverse effects associated with fine gangue entrainment and over-reliance on fine-particle recovery.