Elsevier

Carbon

Volume 173, March 2021, Pages 769-781
Carbon

Research Article
Thermal and gas purification of natural graphite for nuclear applications

https://doi.org/10.1016/j.carbon.2020.11.062Get rights and content

Abstract

Natural graphite such as flake graphite and microcrystalline graphite have important applications in nuclear engineering, especially in high-temperature gas-cooled reactors. Owing to requirements for low ash content and total equivalent boron content, thermal and gas purification is necessary for producing natural graphite powder with nuclear-grade purity. In this study, representative Chinese flake graphite and microcrystalline graphite were purified thermally or with halogens at graphitization temperatures. By thermal purification at 3000 °C, natural flake graphite and microcrystalline graphite containing at least 99.9% carbon was achieved. The metallic impurities remaining after thermal purification were mainly elements that formed carbides with extraordinarily high melting/boiling points, such as B, Ti, Ta, V, W, and Mo. Gas purification with halogens further increased the purity of natural flake graphite and microcrystalline graphite to nuclear grade, that is, the ash content and total equivalent boron content were less than 50 ppm and 0.01 ppm, respectively. Meanwhile, an investigation of the purification mechanism helped to understand the chemistry and thermodynamics of thermal purification of natural graphite.

Introduction

Natural graphite can generally be classified as flake graphite (FG), microcrystalline graphite (MG) (or amorphous graphite), and vein graphite. Flat, plate-like FG particles are usually found disseminated in metamorphic rocks [[1], [2], [3]]. Usually, FG ores are approximately 10% fixed carbon and easy to beneficiate. MG is a seam mineral formed by the metamorphism of previously existing anthracite coal seams [4,5]. MG is not amorphous; it consists of an enormous amount of tiny graphite crystallites that are apparent only under microscopic examination [1]. Commercial grades of MG ores have purity of 75–90%.

Both FG and MG have important applications in nuclear engineering. FG is used for manufacturing spherical fuel elements in a pebble-bed high-temperature gas-cooled reactor (pebble-bed HTR) [[6], [7], [8], [9], [10]], and MG is a potential filler material for the structural graphite of HTRs [11]. Because materials used in nuclear reactors must be highly pure, purification technology for natural graphite is critical for its nuclear applications.

The beneficiation of natural FG includes comminution [12,13], flotation [[14], [15], [16], [17], [18]], and refining. Two recent review papers [19,20] provide an excellent summary of this technology from the perspective of mineral processing. In general, FG can be upgraded to approximately 95% carbon by flotation [20]. Further upgrading of FG can be achieved by chemical refining (purification) such as acid leaching [21,22] or alkali roasting [[23], [24], [25], [26]]. After floatation, the chemical purification technique illustrated in Fig. 1(a) is the most common technique for producing high-purity graphite [20]. The reported purity of FG achieved by chemical purification is as high as 99.9% [[25], [26], [27]]. This material is widely used as a Li-ion battery anode material [28,29].

It should be noted that “natural graphite” used in purification studies in most of the literature refers to natural FG, and the purification of natural MG has been investigated in few studies.

Owing to the dense, fine texture of MG and the intense intergrowth of graphite with gangue minerals, flotation [30,31] as well as chemical purification [32] are less effective for MG. Under optimum conditions, an MG product with a fixed carbon content of 99.0% was obtained from ores with a carbon content of 90.2% [33].

However, the application of graphite in HTRs requires even higher purity than that achieved by chemical purification [11,34]. Nuclear-grade purity is mainly determined by the ash content and total equivalent boron content (EBC), which is used to provide a measure of the macroscopic neutron absorption cross-section of a nuclear material [35]. The idea of EBC is to convert the content of impurity elements other than boron to the equivalent boron content. The EBC factors for the impurity elements are determined from their atomic masses and atomic neutron absorption cross sections (σa), using the following equation:EBCfactor=(atomicmassboron)(σaimpurity)(atomicmassimpurity)(σaboron)EBC of impurity = (EBC factor) (μg of impurity/g base material). Therefore, the total EBC value is determined by the summation of individual EBC values.

In high-temperature gas-cooled reactor pebble-bed module (HTR-PM) [[8], [9], [10]], the ash content and total EBC of the structural graphite have to be less than 100 ppm and 0.9 ppm [36], respectively. For the matrix graphite powder, the ash content and total EBC must be less than 50 ppm and 1 ppm, respectively. The removal of isotopes that can become activated by slow neutrons leading to difficulties with fuel-route management, such as 6Li, is also an important issue.

Previously, thermal purification and gas purification techniques have been developed for producing artificial graphite to the high purity required for nuclear reactors [37]. Therefore, the thermal and gas purification of natural graphite (including FG and MG) are explored in this study.

Thermal purification depends on the fact that impurities are removed through distillation at temperatures higher than their boiling points, as shown in Fig. 1(b) and (c). Gas purification in this paper refers to the purification of graphite at graphitization temperature in the presence of halogens [37]. Thermal or gas purification may be achieved through two processes: batch and continuous. In the batch process, which has already been applied by some facilities in Inner Mongolia, purification takes place in an Acheson furnace. The integrated energy ranges from 15,000 to 18,000 kWh per ton of finished graphite. In the case of continuous purification, the graphite feedstock is fed continuously; therefore, it is more economical than the batch process. These furnaces can be of varied designs, but the basic concept is shown in Fig. 2; the arrangement can be either vertical or horizontal. For a horizontal arrangement, feedstock is fed by a rotating screw into the high-temperature zone, which is heated by resistive heaters [38]. For a typical vertical arrangement, a crucible containing raw graphite ore is fed into a tube still made of refractory materials, at room temperature. The temperature is increased to approximately 2800 °C in the middle part of the furnace, where the impurities are distilled. The residue at the bottom is natural graphite with high purity.

The objectives of this study were (1) to demonstrate the effect of thermal and gas purification on natural FG and MG; (2) to provide basic experimental data on the purification of natural FG and MG from different ore sources within China; and (3) to investigate the chemistry and thermodynamics involved in the thermal purification of natural graphite.

Section snippets

Natural graphite ores

Six FG ores (Jixi, Bayan Nur, Xinghe, LuoBei, Nanshu, and Pingdu) and four MG ores (Chenzhou, Bayan Nur, Yong’an, and Panshi) from different geographical origins were collected for the purification study. These origins cover major natural graphite mines across China. It is worth mentioning that the HTR-10 and HTR-PM under construction used natural flake graphite from the Beishu Graphite Mine [11,34], which is not included in this study.

Before purification, the FG and MG ores were treated

Purity

The ash content and total EBC values of purified FG and MG are summarized in Fig. 3. The residual ash content after thermal purification differed widely depending on the source of the natural graphite, but was of the order of several hundred ppm (100 ppm ash content corresponds to 99.99% purity), meaning that the purity of natural graphite generally reaches 99.9% after thermal purification. This purity level is still far from that required for the nuclear grade, since the matrix graphite powder

Concluding remarks

The major impurities in natural graphite include Al, Si, Fe, K, and Ca. Thermal purification effectively reduces the impurity level of natural graphite. When natural graphite ores that have undergone beneficiation are used as feedstock, a purity of at least 99.9% can be achieved through thermal purification at 3000 °C for both FG and MG. This purity level can meet the requirements of Li-ion battery anode materials. The residual impurities are mainly elements that form carbides with

CRediT authorship contribution statement

Ke Shen: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft, Writing - review & editing, Funding acquisition. Xiaotong Chen: Investigation. Wanci Shen: Resources. Zheng-Hong Huang: Resources, Funding acquisition. Bing Liu: Resources, Supervision. Feiyu Kang: Resources, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Fundamental Research Funds for the Central Universities. The authors also thank the Shaanxi Joint Laboratory of Graphene (Northwestern Polytechnical University).

References (42)

Cited by (25)

View all citing articles on Scopus
View full text