Process Mineralogy of a Low Grade Ag-Pb-Zn-CaF2 Sulphide Ore and Its Implications for Mineral Processing

Yalong Guo

Abstract: Based on process mineralogical study of a low-grade Ag-Pb-Zn-CaF2 sulphide ore in SW China, the occurrence of Pb and Zn, the distribution of silver minerals and their relationships with other minerals are determined in details, which provides scientific reference for the forthcoming flotation process. The mineralogical results show that four silver minerals are determined, which aftonite occupies the most. Unlike other siliver minerals which are included in lead minerals and could be enriched in lead concentrates, aftonite could be liberated as an independent mineral after grinding and may be lost in the tailings, so key factors influencing the flotation of aftonite, such as pH value of pulp, effective collectors, etc., should be handled reasonably to get it better recovered. Besides, note that 2.1% calcite, 6.85% dolomite, which have close flotabilities to fluorite, will also be enriched affecting the quality of the fluorite concentrates. Thus, how to depress the quartz and the calcic gangue minerals during fluorite flotation is also the key factor of the whole process.

Keywords: Process mineralogy; lead-zinc sulphide ore; silver minerals; fluorite;

中圖分類號：O741文獻標識碼： A

Introduction

Historically, flotation is an important and versatile mineral processing step used to achieve selective separation of minerals and gangue, and process mineralogy, in which characteristics such as chemical composition, relative proportions, distribution, texture, types of intergrowths, size distribution, liberation degree and habits of the objective minerals are discussed, is always the first step to provide guidance in process design(C. Bazin, 1994; C.L. Evans, 2011; Vizcarra T.G., 2010; Liu, S.Q., 2012).

During the process mineralogy study, there are two important breakthroughs in the early eighties when two developments occurred: One was the realisation that the integrated approach using mineralogy and mineral processing would produce a synergy; the other, the development of a quantitative automated mineralogical measurement platform with quantitative evaluation of minerals by scanning electron microscope QEM×SEM (Miller et al., 1982; P. Gottlieb, 2000) and the second generation QEMSCAN, and later, the Mineral Liberation Analyser (MLA)( Jones, M.P., 1987; Petruk,W., 2000).

Mineral processing on multi-metallic ores is commonly complicated. Investigating where and how the target elements lie and the target minerals occur is necessary and important to choose reasonable grinding fineness and separating or enriching flowsheet during mineral processing. Fortunately, recent developments in techniques, such as QEMSCAN and MLA, have made it more applicable to the analysis of traditionally difficult multi-metal ores and more accessible to the industry. For the flotation concentrator, the question had always arisen as to what further gains could be made by way of recovery, selectivity or concentrate grade whilst sustaining or increasing throughput, and the process mineralogy of the ores is recognized as a key component in predicting their expected behaviour of any process design or optimisation study(Bradshaw, D.J., 2005).

China occupies abundant lead-zinc resources, but most of the lead-zinc deposits are polymetallic and associated with many useful composition such as Cu, Ag, Au, Sn, etc. Among all the associated compositions, silver resource is of greatest value in comprehensive utilization. Most researches has proved that silver grains are mainly included in the carrier minerals, andthe key mineralogical factors affecting the silver recovery are the variety, particle size and espically the distribution of silver in the main minerals which directly instructed the improvement of mineral processing technology.

There are a great variety of silver minerals, but only one or several of them are the main objects. Each silver mineral has its own floatability, which varies greatly during flotation process: Native silver, silver sulphides and halides always show good floatability; silver-antimony sulphides only show good floatability in demanding flotation condition; Silver-bearing tetrahedrite series minerals show different floatabilities with the changing content of Ag, Sb and As. Besides, lime, the common depressor for pyrite, always has a negative effect on silver flotation at a hgher pH(Xu Z.H.,1994).

To silver minerals, liberation or exposure is the precondition to show their flotation characteristics, and the most important determinant is the particle size of the silver minerals. Normally, only when the silver grains over 10μm occupy 60% above, the silver minerals could be liberated to show their own flotation characteristics after grinding, and key factors influencing the flotation of silver minerals, such as pH value of pulp, effective collectors, etc., should be handled reasonably to get them better recovered.

The deposit in Aba, Yunnan province, is a typical lead-zinc sulphides deposit with rich associated silver and fluorite resources, but few has been done to recover the associated minerals. Thus, before the process design and optimization, process minralogy has been studied to show the chemical properties, mineral compositions and main mineral dissemination characteristics of the ores.

Materials and Methods

Representative sample was taken from various locations within the ore body in Aba, Yunnan province. After the sample preparation, approximately 5kg bulk samples were chosen for phase analysis and mineralogical investigation. The samples had been crushed to -2mm.

To investigate the distribution of the main minerals in the mineralization and the role played by lead-zinc sulphides as carriers, process mineralogy studies were conducted, and a wide variety of instruments were used as below: The ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) was used to determine the elemental analysis of the sample; The optical microscope is used to observe mineral textures, and to determine mineral quantities by point counting; X-ray diffractometer (XRD) is used to identify minerals with a high degree of certainty, and to qualitatively determine mineral contents in powdered materials; The scanning electron microscope (SEM) is one of the most versatile and widely used tools of modern science as it allows the study of both the morphology and composition of materials. The mineral liberation analyzer (MLA) was used to reveal the mineral liberation characteristics and determine the mineral composition of the ores.

The assay data are given in Table 1, and the Pb and Zn phase analysis result is shown in Tables 2 and 3. The result of mineral content distribution is shown in Table 4.

Table 1 Result of chemical analysis of sample.

Table 2 Result of lead phase analysis.

Table 3 Result of zinc phase analysis.

Table 4 Result of mineral content distribution

Results and Discussions

According to the result in Table 1, the sample assaying 1.08% Zn, 2.56% Pb, 42.70% CaF2 and 130g/t Ag indicates that the main valuable elements are Ag, Pb, Zn and CaF2, and the silver minerals can be enriched in the flotation concentrates.

Lead sulphide

The lead sulphide refers primarily to the galena, whose cleavages are perfect. The cracked grains are cube shaped, and are easily to be over milled during grinding. The SEM images of mineralization and associations are shown in Fig. 1, and details are as follows.

Most of the galena grains are filling the holes or fractures of the quartz vein, and mainly distributed in irregular shapes (Fig 1-a, b); some of the galena grains are in irregular conjunction with sphalerite and calcite to form coarse compounds in the quartz vein (Fig 1-c, d); some grains are also disseminated along the fracture zone by filling metasomasis (Fig 1-e, f). It's worth mentioning that the ultrafine silver grains, such as boulangerite, pyrargyrite, freieslebenite, etc. mainly occur as inclusions in galena (Fig 1-g, h), and could be better recovered in lead concentrates by flotation.

Fig. 1. Selected SEM images of the lead sulphides

(abbreviations used: Ga=galena, Qu=quartz, SP=sphalerite, Ca=calcite, Ce=cerussite, Ag=silver minerals)

Zinc sulphide

The zinc sulphide refers primarily to the sphalerite, and ferrum is occasionally found as isomorphism hosting in the lattice of sphalerite. The SEM images of mineralization and associations are shown in Fig. 2, and details are as follows.

Fig. 2. Selected SEM images of the zinc sulphides

(abbreviations used: Ch=chalcopyrite, Py=pyrite)

Some of the former sphalerite grains are irregularly distributed in the gangue minerals, and micro-inclusions of chalcopyrite and pyrite can also be found in sphalerite (Fig 2-a); some of the former sphalerite grains exhibit in the form of metasomatism substituted by galena, and the conjunctures are mainly in harbor- or isolated island- shapes(Fig 2-b); the later sphalerite grains mainly associate with galena or calcite, and the said compounds are included in the quartz vein (Fig 2-c, d).

Silver minerals

There are 4 main types of silver minerals: aftonite (CuAgSbS), freieslebenite (AgPbSbS3), pyrargyrite (Ag3SbS3) and acanthite (Ag2S), among which aftonite occupies the most. Most of the silver minerals could be treated as independent minerals, and the SEM images of mineralization and associations are shown in Fig. 3.

Fig. 3. Selected SEM images of the silver minerals

Aftonite(CuAgSbS)

The silver of aftonite is high in quantity, with a highest content of 26%. Most of the grains are in irregular conjunction with, or finely distributed along, or included in sphalerite (Fig 3-a); a few grains are in cataclastic shape, and exhibit in the form of metasomatism substituted by galena (Fig 3-b). Besides, few grains are finely included in quartz, and may be lost in the tailings. Most of the grains could be liberated in grinding products, and could be better recovered by finer grinding in a proper way. Besides, key factors influencing the flotation of silver minerals, such as pH value of pulp, effective collectors, etc., should be handled reasonably to get it better recovered in the lead concentrates.

Freieslebenite (AgPbSbS3)

Freieslebenite is one of the rare silver minerals, and the grains are mainly included in galena (Fig 3-c, d). Thus, the freieslebenite will surely be enriched in the lead concentrates by flotation.

Pyrargyrite (Ag3SbS3)

Pyrargyrite is one of the common silver minerals, and the grain mainly occurs as the form of short column. The pyrargyrite grains of the ores are mostly included in galena, and will also surely be enriched in the lead concentrates by flotation (Fig 3-e, f).

Acanthite (Ag2S)

Acanthite is the low-temperature isomorphic modification of argentite, and is also one of the common silver minerals. The content of acanthite in the ores is very low, and most grains are finely included in fluorite, which will be lost in the fluorite concentrates.

Fluorite

Fluorite is one of the major objective minerals, and the SEM images of mineralization and associations are shown in Fig. 4.

Most of the fluorite grains are filling in the crushed zone, and occur as veins or blocks. Note that some calcite veins are wind through the fluorite veins, and calcite will be the impurity in fluorite concentrates (Fig. 4-a, b).

Fig. 4. Selected SEM images of the fluorite

(abbreviations used: Fl=fluorite)

Conclusions

1)The deposit in Aba is a typical low grade lead-zinc sulphides deposit with large associated silver reserves, assaying 2.46% Pb, 0.96% Zn and 130g/t Ag. Furthermore, fluorite with a grade of 42.70% CaF2 is also an important associated resource that needs to be comprehensive recovered.

2)Studies show that four silver minerals, aftonite (CuAgSbS), freieslebenite (AgPbSbS3), pyrargyrite (Ag3SbS3) and acanthite (Ag2S), are determined, which aftonite occupies the most. Unlike other siliver minerals which are included in lead minerals and could be enriched in lead concentrates, aftonite could be liberated as an independent mineral after grinding and may be lost in the tailings, so key factors influencing the flotation of aftonite, such as pH value of pulp, effective collectors, etc., should be handled reasonably to get it better recovered.

3)The ore type is dominated by quartz-fluorite type. Besides, note that 2.1% calcite, 6.85% dolomite and other calcic minerals that have close flotabilities to fluorite will be also enriched affecting the quality of the fluorite concentrates. Thus, how to depress the quartz and the calcic gangue minerals during fluorite flotation is the key factor of the whole process.

Acknowledgment

Sincere thanks and appreciations go to the Yunnan Tin Group(Holding) Company Limited for financial support and kindly providing the ore samples. The authors are also thankful for the support from the Analyzing and Testing Centre of Guangzhou Research Institute .

References

Bazin, C., Grant, R., Cooper, M., Tessier, R., 1994. A method to predict metallurgical performances as a function of fineness of grind. Minerals Engineering 7 (10), 1243–1251;

Bradshaw, D.J., Oostendorp, B., Harris, P.J., 2005. Development of methodologies to improve the assessment of reagent behaviour in flotation with particular reference to collectors and depressants. Minerals Engineering 18 (2), 239–246;