Journal of Materials Research and Technology Journal of Materials Research and Technology
Short Communication
Flotation of fine pyrite by using N-dodecyl mercaptan as collector in natural pH pulp
Kaile Zhaoa,b, Guohua Gub, Wu Yana,, , Xiaohui Wanga,, , Chongqing Wangc, Longhua Xub
a Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Chengdu 610041, Sichuan, China
b School of Minerals Processing and Bio-Engineering, Central South University, Changsha 410083, Hunan, China
c School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, Henan, China
Received 18 December 2017, Accepted 04 May 2018
Abstract

Laboratory and pilot-scale flotation tests were conducted to investigate the effect of N-dodecyl mercaptan (NDM) on the flotation of fine pyrite. At bench scale, NDM resulted in higher recovery, and the mixed collector of NDM and potassium isoamyl xanthate (PIAX) (mass ratio of 2:1) caused the highest recovery of 96.5%. High grade and recovery of pyrite was obtained without adding any activator (CuSO4) and modifier (H2SO4) in a natural pH of the pulp. Pilot-scale tests indicated that a concentrate with the grade of 50.05% S, 42.23% Fe and the recovery of 98.51% S, 96.69% Fe was achieved by using the mixed collector of NDM+PIAX at pH 6.5.

Keywords
Fine pyrite, N-dodecyl mercaptan, Flotation, Pilot-scale tests
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1Introduction

Dongbafeng has about 250 million tones of pyrite reserves with S grades as low as 15.04% in Guangxi province, China, and the pyrite mainly coexists with pyrophyllite, kaolinite and carbonates [1]. In the practice of pyrite flotation, the general method is to add modifier H2SO4/H2C2O4 and activator CuSO4 in association with collector xanthates [2–5]. Due to the fine dissemination of pyrite [6,7] (−37μm 56.8%, −10μm 20.3%) and the high clay contents in this low-grade sulphide ore [8], H2SO4 and CuSO4 are used as modifier and activator respectively, and potassium isoamyl xanthate (PIAX) is used as collector [9] in the pyrite flotation. Zhao et al. reported that the recovery of pyrite might decrease by 10–20% in absence of H2SO4[8]. However, using large quantity of H2SO4 (8kg/t) may contaminate the environment, accelerate equipment corrosion and increase production costs.

Dodecyl mercaptan includes N-dodecyl mercaptan (NDM, C12H26S) and tert dodecyl mercaptan (TDM, C12H25SH). TDM has been used as the collector for sulphide ores (sphalerite, arsenopyrite, molybdenite) flotation [10,11], but its application is limited due to high price, complex procedure and bad odor. TDM is the isomer of NDM, and therefore they have the same molecular formula and functional group [12,13]. NDM is odorless and low price in China and can be used in sulphide ore flotation.

In this work, the introduction of N-dodecyl mercaptan (NDM) into the fine pyrite (clay-type, <37μm) flotation industry without adding any modifier (H2SO4) and activator (CuSO4) has been successful. The objective of this work is to float fine pyrite in a natural pH pulp at bench scale and pilot scale.

2Experimental

The raw ore used in this study was obtained from Dongbafeng Sulphur Mine of Guangxi Province, China. The sample was crushed, ground and screened into −3mm. It contains 15.04% S, 12.95% Fe, 31.29% Al2O3, 24.06% SiO2, 3.26% TiO2, 1.84% C and 1.12% CaO. The sample is composed mainly of pyrite, pyrophyllite, bauxite, kaolinite, anatase, graphite, chlorite and montmorillonite. Pyrite is the only mineral of economic value in the ore. The grain size of pyrite particles is between 1 and 1000μm. The degree of liberation of pyrite is below 74μm. In addition, the −37μm and −10μm size fractions account for 56.8% and 20.3% of the entire pyrite particles, respectively.

In laboratory and pilot-scale experiments, N-dodecyl mercaptan (NDM, CH3(CH2)10CH2SH, MW 202) and Potassium isoamyl xanthate (PIAX) were used as collectors, and sodium hexametaphosphate (SHMP) was used as the clay dispersant. Due to good natural floatability of pyrophyllite and other clay minerals [6,14], guar gum was used as the clay depressant. Sulphuric acid (H2SO4) and copper sulphate (CuSO4) were used as modifier and activator respectively, and methyl isobutyl carbinol (MIBC) was used as frother. The reagents used in this study were of technical purity. Pulp pH was recorded by the PHS-3 meter. Tap water was used in both grinding and flotation experiments.

In laboratory experiments, the 300g ore was ground to −74μm (97wt%) in a XMB-70 rod mill at a pulp density of 50wt%. Flotation tests were performed in a XFD-63 flotation cell (self-aeration), 1.5dm3 for rougher-scavenger flotation and 1.0dm3 for cleaner flotation, respectively. The impeller speed was 1590rpm. The froth product was collected with an automatic froth scraper device. In the rougher flotation stage, the desired amount of reagents (SHMP 750g/t, guar gum 300g/t, NDM 100g/t, PIAX 50g/t, and MIBC 50g/t) were added in the sequence to the rougher flotation cell with 3min interval, and flotation time was 6min. In pilot-scale trials, the processed capacity of the pilot-scale plant was 1.25t/d (52kg/h) feed. The pyrite flotation was carried out in a FX2-39 flotation cell (self-aeration) at a feed particle size of −74μm (97wt%, using a XMBL-420×600mm rod mill). The volume of flotation cells for rougher-scavenger and cleaner flotation were 39dm3 and 24dm3, respectively. The products were collected, dried, and weighed. Determination of S was conducted by infrared carbon and sulphur analyzer (Multi EA 2000, Analytikjena Germany), and Fe content was analyzed by volumetric method (K2Cr2O7). To assure the accuracy of laboratory experiments, the calculated S grade of feed should be in the range of 15.04±0.04%, otherwise the experiments were repeated. The flotation flowsheet and corresponding reagent concentrations of closed flotation circuit and pilot-scale tests are given in Fig. 1.

Fig. 1.
(0.19MB).

The flotation flowsheet and corresponding reagent concentrations of closed flotation circuit and pilot-scale tests.

3Results

Fig. 2a shows the S recovery as a function of PIAX, NDM, or NDM+PIAX dosage in the rougher operation. The recovery of pyrite increased markedly with the increasing of collector dosage. Compared to PIAX, NDM resulted in higher recovery with the same collector addition. Moreover, the mixed collector greatly improved the pyrite recovery, and the highest recovery was 96.5% at 150g/t NDM+PIAX (preferred mass ratio of 2:1). The effect of PIAX and NDM addition in the rougher operation is shown in Fig. 2b and c, respectively. Compared with PIAX, NDM showed larger effect on the recovery of pyrite. The optimum dosage was 100g/t NDM and 50g/t PIAX, achieving high grade pyrite with grade of 41.2% S and recovery of 96.5% S.

Fig. 2.
(0.23MB).

The effects of collector (PIAX, NDM, or NDM+PIAX as collector) dosage on the S recovery (a) and grade (b, c) in the rougher concentrates at pH 6.5 without H2SO4 and CuSO4 addition.

Based on the optimum conditions of open circuit tests, locked cycle tests were carried out at batch scale using NDM+PIAX as a mixed collector in natural pH pulp. The comparative tests (S2) were conducted using H2SO4 and CuSO4 without NDM. As shown in Fig. 1, the closed flotation circuit includes one roughing, three scavenging, and four cleaning steps. Flotation results of the average values are displayed in Table 1 (S1, S2). The dispersive values of the 4th, 5th and 6th cycles and their standard deviations are shown in Table 2 (S1, S2).

Table 1.

The average values of closed flotation circuit (S1, S2) and pilot-scale trials (S3).

System  Products  Ratio (w/%)  Grade (%)Recovery (%)
      Fe  Fe 
Locked cycle flotation tests (S1) NDM+PIAX (pH 6.5)Pyrite concentrate  30.06  49.23  41.71  98.33  96.81 
Tailing  69.94  0.36  0.59  1.67  3.19 
Feed  100.00  15.05  12.95  100.00  100.00 
Locked cycle flotation tests (S2)a PIAX (pH 4)Pyrite concentrate  29.02  45.53  38.23  87.95  85.64 
Tailing  70.98  2.55  2.62  12.05  14.36 
Feed  100.00  15.03  12.96  100.00  100.00 
Pilot-scale tests (S3) NDM+PIAX (pH 6.5)Pyrite concentrate  29.65  50.05  42.23  98.51  96.69 
Tailing  70.35  0.35  0.61  1.49  3.31 
Feed  100.00  15.06  12.95  100.00  100.00 
a

The comparative system S2, acidic without NDM (pH 4).

Table 2.

The dispersive values of closed flotation circuit (S1, S2) and pilot-scale trials (S3).

System  Products  Ratio (w/%)  Grade (%)Recovery (%)
      Fe  Fe 
S14th pyrite concentrate  30.11  49.18  41.42  98.42  96.30 
5th pyrite concentrate  29.84  49.47  42.39  98.11  97.52 
6th pyrite concentrate  30.23  49.04  41.31  98.45  96.61 
Average value  30.06  49.23  41.71  98.33  96.81 
Standard deviation  0.199  0.219  0.594  0.188  0.634 
S24th pyrite concentrate  29.24  45.05  37.73  87.74  85.13 
5th pyrite concentrate  29.12  45.23  37.92  87.62  85.35 
6th pyrite concentrate  28.71  46.32  39.03  88.48  86.43 
Average value  29.02  45.53  38.23  87.95  85.64 
Standard deviation  0.278  0.687  0.702  0.466  0.696 
S31–12h pyrite concentrate  29.74  49.87  41.69  98.48  95.87 
13–24h pyrite concentrate  29.72  49.75  41.73  98.31  96.09 
25–36h pyrite concentrate  29.62  50.02  42.26  98.45  96.73 
37–48h pyrite concentrate  29.54  50.21  43.13  98.56  97.71 
49–60h pyrite concentrate  29.76  49.69  41.53  98.29  95.74 
51–72h pyrite concentrate  29.82  49.61  41.36  98.36  95.32 
73–84h pyrite concentrate  29.56  50.35  42.75  98.78  97.64 
85–96h pyrite concentrate  29.61  50.11  42.33  98.59  96.86 
97–108h pyrite concentrate  29.45  50.83  43.37  98.53  98.01 
109–120h pyrite concentrate  29.69  50.01  42.13  98.72  96.89 
Average value  29.65  50.05  42.23  98.48  95.87 
Standard deviation  0.115  0.361  0.682  0.164  0.917 

In the comparative system S2, the reagents used in the flotation were H2SO4 of 8kg/t (pH 4), CuSO4 of 850g/t, guar gum of 700g/t, PIAX of 450g/t and MIBC of 80g/t. Under these conditions, a concentrate grade of 45.53% S at a recovery of 87.95% was achieved. To reduce reagent costs and improve the pyrite recovery, a mixed collector of NDM+PIAX (mass ratio of 2:1, the rougher concentration 150g/t, the scavenger concentration 75g/t, the cleaner concentration 30g/t) was adopted in the flotation circuit. A concentrate grade of 49.23% S at a recovery of 98.33% was achieved on a feed of 15.04% S using NDM+PIAX as a mixed collector at pH 6.5. Compared with S2, the S1 procedure increased the grade and recovery of S by 3.70% and 10.38%, respectively. Meanwhile the tailing grade decreased from 2.55% S to 0.36% S, implying that NDM is a powerful and selective collector for fine pyrite particles.

Because of the good collection performance for fine pyrite, collector complexes of NDM+PIAX were selected in the pilot-scale tests. The pilot-scale trials were performed at the Ermei pilot-scale plant located in Sichuan, China. The pilot-scale continuous running tests were carried out more than 120h (August, 2016). The flotation flowsheet is given in Fig. 1. The average values of flotation results are shown in Table 1 (S3). The dispersive values of 10 groups of pyrite concentrates (every 12h) and their standard deviations are shown in Table 2 (S3).

In S3 procedure, a concentrate grade of 50.05% S, 42.23% Fe at a recovery of 98.51% S, 96.69% Fe was achieved in the neutral circuit using NDM+PIAX as a mixed collector. Meanwhile, a tailing with 0.35% of S and 0.61% of Fe was produced. For this low-grade sulphide ore, NDM+PIAX showed an excellent collection for fine pyrite (−37μm 56.8%), confirmed by the high grade and recovery of pyrite achieved using NDM+PIAX as a mixed collector. The 2:1 collector mixture NDM+PIAX showed great potential for industrial application. This study solved the problem of flotation separation of fine pyrite and clay minerals, providing technical support for flotation recovery of clay-type pyrite flotation.

4Conclusions

In this study, a mixed collector of N-dodecyl mercaptan (NDM) and potassium isoamyl xanthate (PIAX) was developed to float fine pyrite from clay-type sulphide ore. The laboratory experiments indicated that NDM resulted in higher recovery, and the mixed collector of NDM+PIAX (mass ratio of 2:1) caused the highest recovery of 96.5% S. High grade and recovery of pyrite was obtained without adding any activator (CuSO4) and modifier (H2SO4) in a natural pH of the pulp. 120-h pilot-scale tests showed that a concentrate with the grade of 50.05% S, 42.23% Fe and the recovery of 98.51% S, 96.69% Fe was achieved by using the mixed collector of NDM+PIAX at pH 6.5. The 2:1 collector mixture NDM+PIAX showed great potential for industrial application. The NDM+PIAX flotation procedure will be adopted at the Dongbafeng sulphur mine.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 501404218, 51674207) and the Program of China Geological Survey (D1603.2.2) for the financial support of this research.

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Corresponding authors. (Xiaohui Wang luomodeyu@163.com)
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