Mineralogical and chemical characteristics of a powder and purified quartz from Yunnan Province

Ran-Fang Zuo; Gao-Xiang Du; Wei-Gang Yang; Li-Bing Liao; Zhaohui Li

doi:10.1515/geo-2016-0055

Article Open Access

Mineralogical and chemical characteristics of a powder and purified quartz from Yunnan Province

Ran-Fang Zuo , Gao-Xiang Du , Wei-Gang Yang , Li-Bing Liao and Zhaohui Li

Published/Copyright: December 3, 2016

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From the journal Open Geosciences Volume 8 Issue 1

Abstract

For comprehensive utilization of powder quartz, a detailed mineralogical analysis is needed. In this paper, the minerals collected from a powder quartz deposit in Yunnan Province and its purification products were characterized by X-ray diffraction (XRD), chemical composition, and scanning electron microscopy (SEM) analyses. The raw powder quartz deposit had a simple mineral composition made of 85% quartz and 15% clay minerals kaolinite and illite with a trace amount of feldspar. It had narrow particle size distribution with whiteness of 81.4. The impurity minerals kaolinite and illite existed in the forms of aggregates with quartz or inclusions in quartz particles and could be easily removed. The mineralogical characterization suggested that the powder quartz deposit was formed by long-term weathering of feldspathic or felsic rocks, possible of pegmatitic origin.

Keywords: Powder quartz; Mineralogical characterization; Purification technology

1 Introduction

Handling of fine, usually submicrometer-sized, powders in large quantities requires a high degree of process control to achieve the desired microstructural characteristics, e.g., small defect size, good dispersion in other phases, and homogeneous grain-boundary composition [1]. Powder quartz, a novel siliceous material of unique application, was formed mostly by weathering of microcrystalline quartzite [2]. In China, the first powder quartz deposit was discovered in Jiangxi Province in the 1980’s, which accounts for 99% of the total reserves in China, but with low quality [3]. It was characterized with small particle sizes, easy processing, and wide application compared with traditional siliceous materials such as quartzite, quartz sand, and vein quartz, respectively [4].

The particle size of the quartz added a significant influence on its potential development [5]. Thus, fine powder quartz had its distinct advantages in its utilization. Particularly, high purity quartz has become one of today’s key strategic minerals with applications in high-tech industries that include semiconductors, high temperature lamp tubing, telecommunications and optics, microelectronics, and solar silicon applications [6]. In the lamp tubing and optics industries aluminium should not exceed 20 ppm, while other metals should be less than 1 ppm, and total impurities less than 30 ppm [6]. For semiconductor base materials and crucibles aluminium content should be even lower, specified to less than 10 ppm, other metals less than 0.1 ppm, and total impurities not to exceed 15 ppm [6].

High-purity and ultra-fine micro powder quartz could be obtained by scrubbing, magnetic separation, flotation, acid-cleaning of conventional quartz deposits [7 8 9 10 11]. Comprehensive studies of the purification technology of quartz deposits were also undertaken extensively [12], so as to expand its application and improve its industrial value. As quartz sands have variable degrees of purity due to variable mineral composition, only a small fraction is suitable for glassmaking and other high-grade industrial applications including foundries [13].

The production of high purity materials requires precise control of impurity contents [14]. The forms of impurities existed in powder quartz would affect its purification technology and purification products directly. Thus, mineralogical analyses of powder quartz were conducted to help promoting its comprehensive utilization. In this paper, the powder quartz from Yunnan Province was evaluated on its compositional properties and analyzed for its possible genesis. The results would be used to provide a basis for improving its mechanical properties, comprehensive development, and utilization.

2 Experimental

2.1 Materials

The powder quartz samples were collected from a powder quartz deposit in Yunnan Province, China. The deposit occurred in the lower to mid Devonian with an average thickness of 10 m. The deposit is about 3.0 M ton with an average SiO₂ content of 92.47% [15]. It occurs on a shallow depth and can be mined in open pit mining.

They consisted of silica powder and silica sand with loose texture. The raw materials were analyzed first and then subject to further treatment such as dry ultrafine grinding with liquid additives [16]. The grinding consisted of two stages: (1) grinding the samples to <200 meshes; and (2) further grinding the samples to <800 meshes. In brief, the ultrafine grinding was performed in a vibration rod mill. A desired amount of quartz sample (50 g) and ethanol (5 mL) were added to the pot filled with the grinding media. After grinding the samples to desired particle size, the samples were collected for SEM observation and EDS analyses. The grinding will result in two distinct consequences: particle breakdown and mechanical activation [17]. Our goal was to utilize the fine grinding process while minimizing mechanical activation during the purification stage.

2.2 Methods of characterization

The chemical composition was analyzed using ICP-AES [18], while the mineral identification was conducted using XRD. The relationships between the host and the accessory minerals were determined by grinding the powder quartz first and then purifying the samples to different degrees as indicated by the SiO₂ content. The purified minerals were further analyzed for their particle morphology. Residual impurity minerals were further identified. The genesis of the powder quartz deposit was deduced from the analytic results following the principles of minerals weathering.

The ICP analyses were performed on an ICP-AES (Thermo Jerell Ash, USA). Accuracy was checked in each analytical batch by measuring certified reference material and was found to be within ±5% of the certified values. Precision was also evaluated by measuring standard deviation of three individual runs, which was typically better than 2%.

The micromorphology of raw powder quartz and powder quartz samples purified to 98.6% and 99.6% SiO₂ contents was further analyzed using SEM supplemented with energy dispersion spectrum (EDS) analysis. The SEM images were taken using a QUANATA 6000 scanning electron microscope from American FEI Company at an accelerating voltage of 25 kV.

XRD patterns were recorded by a Rigaku D/MAX-rA diffractometer (Rigaku, Japan) with a Ni-filtered CuKα radiation at 40 kV and 100 mA and a scanning speed of 2° 2θ/min at 0.02° per step. The whiteness was measured by SBDY-1 digital whiteness meter (Shanghai, China).

3 Results and Discussion

3.1 Chemical composition analysis

The species and content of impurity minerals existing in powder quartz were the main factors to affect purification of the powder quartz. The chemical composition analysis and microelement analysis of powder quartz were performed to obtain the accurate content of each component.

The major chemical analyses revealed that the powder quartz was mainly made of SiO₂, yielding 93.7% (Table 1). The main impurity was Al₂O₃, accounting for 3.62%, followed by K₂O, MgO, and Fe₂O₃, accounting for 0.64%, 0.26%, and 0.18% respectively. Microscopic observation showed that the impurities existing in powder quartz were in the form of K-feldspar and magnesium silicates. The contents of CaO, Na₂O, MnO₂, and SO₃, were very low. The trace element analyses showed that Co, Cu, Pb, and Zn were at the ppb level. However, the concentrations of B and Ti (Table 2) were much higher than typical for quartz deposits of different origins [19]. The major, minor, and trace element contents, respectively, suggested that the powder quartz mine was associated with geological environment and mineralization of felsic rocks of pegmatitic origin as indicated by the higher B and Ti contents [20].

The whiteness of powder quartz was 81.4, a value suggesting that it could be used as raw material for ceramics directly. It could also be used as raw refractory materials after removal of Al. Resource exploration in this area revealed a great prospect for a large mineral deposit.

Table 1

The chemical composition analysis of powder quartz (%).

Composition	SiO₂	Al₂O₃	Fe₂O₃	CaO	MgO	K₂O	Na₂O	MnO₂	SO3	L.O.I

Content	93.7	3.62	0.18	0.048	0.26	0.64	0.023	0.003	0.019	1.43

Table 2

The trace element analysis of powder quartz (mg/kg).

Element	B	Co	Cr	Cu	Mo	Ni	Pb	Sn	Ti	V	Zn

Content	360	<1	5	<1	5	2	<1	<1	310	11	2

3.2 XRD pattern analysis

The semi-quantitative analysis showed that the contents of quartz and clay minerals were 85% and 15%, respectively. The clay minerals mainly consist of illite and kaolinite (Fig. 1).

Figure 1

The XRD pattern of powder quartz from Yunnan province.

3.3 The particle morphology and energy spectrum analysis

After the powder quartz was ground to less than 200 meshes and then purified, the SiO₂ content of the sample increased to 98.6%. Further purification after grinding to 800 meshes resulted in a SiO₂ content of 99.6%. The impurity minerals were significantly removed by the grading and subsequent purification processes.

3.3.1 The micromorphology analysis of raw powder quartz

Several spots were analyzed using EDS attached to the SEM (Fig. 2). Each spot was about 2 μm in diameter and the given content of impurity elements thus determined was semi-quantitative. The powder quartz samples had a characteristic high degree of weathering and flake-like impurities distributed in between quartz particles (points A and C in Fig. 2a) or on the surface of quartz particles (point B in Fig. 2a). A part of it was wrapped in weathered quartz particles or mingled in quartz particle gaps. Point I in Fig. 2b was a clay mineral particle surrounded by agglomerated particles. The EDS analyses of each point suggested that the impurity minerals were mainly kaolinite or illite as indicated by the substantial amount of K (Table 3).

Figure 2

SEM observation of raw powder quartz.

Table 3

The energy spectrum analysis of raw powder quartz samples.

Position	Morphology	Form	Content (%)				Impurity minerals
Position	Morphology	Form	O	Al	Si	K	Impurity minerals
A	scale aggregate	free	45.50	6.25	47.04	1.21	kaolinite or illite
B	sheet	stick to quartz surface	50.55	1.29	47.38	0.78	kaolinite or illite
C	scale aggregate	free	55.59	6.59	36.31	1.50	kaolinite or illite
D	unclear	particle gap	42.46	4.98	51.23	1.33	kaolinite or illite
E	unclear	particle gap	42.68	2.55	54.05	0.72	kaolinite or illite
I	sheet	wrapped in particles	51.50	12.87	35.19	0.44	kaolinite or illite

3.3.2 The micromorphology of purified powder quartz samples

As the particle sizes of the impurity minerals were overall less than 2 μm, grinding the materials to less than 200 or 800 meshes was needed in order to remove most of the impurity minerals. The SEM analyses of powder quartz purified after grinding to 200 and 800 meshes were shown in Figs. 3 and 4. The results of EDS analyses of impurity particle were listed in Table 4.

Table 4

The energy spectrum analysis of powder quartz after first purification.

Position	Morphology	Form	Content (%)				Impurity minerals
Position	Morphology	Form	O	Al	Si	K	Impurity minerals
A	single grain	inclusion	42.07	3.55	51.57	1.89	kaolinite or illite
C	scale aggregate	stick to quartz surface	48.32	14.86	36.73	0.09	kaolinite or illite
E	scale aggregate	particle gap	49.26	8.53	41.08	1.13	kaolinite or illite
F	platy aggregate	particle gap	35.49	6.19	53.74	4.58	potash feldspar
G	platy	particle gap	30.80	6.07	58.49	4.64	potash feldspar

Most of the impurity minerals were removed upon purification after ground to 200 meshes. The surface of quartz particle was clean and had regular edges (Fig. 3). However, there are still some sheet-like impurity minerals sticking to the surface of quartz particles (point C in Fig. 3a). Some platy impurity minerals were mingled in the gaps of weathered quartz particles (point E in Fig. 3b). Again, the EDS analyses suggested that they were mainly kaolinite, illite, and K-feldspar. Further purification and separation of quartz from the impurity minerals could be achieved via the size differences between quartz and impurity particles (in the range of 2 to 4 μm).

Figure 3

SEM analyses of powder quartz after first purification.

Upon further purification (i.e. after ground to 800 mesh), the particle size of quartz became uniform, about 10 μm (Fig. 4a). The impurity minerals on the surface of quartz particles disappeared. The remaining impurity particles may be attributed mainly to inclusions in the quartz particles (point A in Fig. 4b). The EDS analysis at point A showed chemical composition similar to that of kaolinite or illite, owing to the sheet-like morphology and substantial amount of Al. After the second step of purification, the remaining impurity particles (about 2 μm) existed only in the gap of quartz particles in the forms of inclusion, making further purification very difficult.

Figure 4

SEM analyses of powder quartz after second purification.

Unlike kaolinite and illite, K-feldspar was present in the gaps of quartz particles or covered by layers of platy minerals with extremely low quality, making it difficult to be observed in comparison to platy minerals.

3.4 Genetic analysis

Under acidic conditions of the diagenetic process, K-feldspar would undergo incongruent dissolution and weathering to form kaolinite, illite, and quartz. Under specific conditions, kaolinite can react with K-feldspar to form illite and quartz. The reactions could be expressed as follows:

2KAlSi₃O₈ (K-feldspar) + 2H⁺+ H₂O → Al₂Si₂O₅(OH)4 (kaolinite) + 4SiO₂ (quartz) + 2K⁺
3KAlSi₃O₈ (K-feldspar) + 2H⁺+ H₂O → KAl₃Si₃O₁₀(OH)2 (illite) + 6SiO₂ (quartz) + 2K⁺+ H₂O
Al₂Si₂O₅(OH)₄ (kaolinite) + 2KAlSi₃O₈ (K-feldspar) → KAl₃Si₃O₁₀(OH)₂ (illite) + SiO₂ (quartz) + H₂0

The relationships between quartz and feldspar indicated that quartz was the dominant residual mineral or newly formed one, while K-feldspar was consumed in the process of incongruent dissolution. The absence or near absence of K-feldspar suggested that weathering process proceeded over a long period of time and was almost completed. The minor K-feldspar was only detected as inclusions or surrounded by clay minerals, which protect its further decomposition. The high B and Ti content suggested its pegmatitic origin.

4 Conclusion

1) The whiteness of powder quartz from Yunnan Province was 81.4 and the content of SiO₂ was 93.7%. The material was mainly composed of quartz under a higher degree of weathering, coexisted with some kaolinite, illite, and trace amount of feldspar. Its simple mineral composition and small particle size distribution made it a valuable mineral resource.

2) The impurity minerals stick to the surface of quartz particles and were wrapped in quartz particles or mingled in the gap of quartz particles. The size of impurity particles was less than 2 μm. These impurity minerals could be easily removed from the quartz after grinding and purification.

3) The powder quartz was formed from feldspar or feldspar and quartz over long-term weathering, while the minor remaining K-feldspar was present as inclusions or wrapped by newly formed clay minerals to protect a further decomposition.

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Acknowledgment

The project was partially supported by the Fundamental Research Funds for the Central Universities” (No. 2652013037 and 2652013043).

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Received: 2015-6-13

Accepted: 2016-4-12

Published Online: 2016-12-3

Published in Print: 2016-1-1

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