Purification Technologies for Graphite Materials

Overview

Carbon-graphite materials, a category of inorganic non-metallic materials, exhibit diverse physical and chemical properties. For instance, natural graphite is an essential raw material for lithium battery anodes due to its lubricity and excellent layered structure. Synthetic specialty graphite, known for its high-temperature mechanical properties and chemical stability, plays a vital role in basic industries, scientific research, and national defense. Additionally, emerging C/C composite materials combine outstanding mechanical properties with flexible forming methods, expanding the application fields of carbon-graphite materials.

With the extensive research and gradual application of new materials like carbon nanotubes and graphene, carbon-graphite materials have become fundamental raw materials in modern high-temperature, high-pressure, high-speed industries, as well as in modern biology, information, and energy sectors. Consequently, many countries, including the United States, have designated carbon-graphite materials as strategic resources. Purity is a critical performance metric for these materials, often limiting their industrial applications. As such, purification technology has become a fundamental production technology for carbon-graphite materials.

This document reviews the development of purification technologies for carbon-graphite materials, explains the processes and technical features of various purification methods, and evaluates their advantages and disadvantages based on practical production scenarios at Sinosteel New Materials (Zhejiang) Co., Ltd. Additionally, it introduces commonly used purity characterization methods.

Historical Development and Current Status

Early purification techniques focused on natural graphite ore, developing flotation methods to enrich and purify the material. Flotation achieves a natural graphite purity of 85-95%. Chemical purification methods such as alkaline-acid, hydrofluoric acid, and chlorination roasting can further purify natural graphite to 99-99.9%, though they pose environmental pollution risks requiring accompanying environmental protection equipment.

China’s synthetic graphite industry, including graphite electrodes, specialty graphite, and synthetic graphite powder, began and rapidly developed during the natural graphite mining and purification process. Synthetic graphite generally uses coke and asphalt with fewer impurities as raw materials, with high-temperature graphitization heat treatment being the final crucial production step. This process achieves a purity of 99.95%, and halogen gas purification can further increase purity to 99.995%.

In the 21st century, with the rapid advancement of global industrial levels, carbon-graphite materials have become widely used in various industries as basic materials, driving higher purity requirements. For example, graphite used as anode material in lithium-ion batteries must have a purity of over 99.98%. Natural graphite used to synthesize synthetic diamonds must have a purity of over 99.999%, with boron content less than 0.01×10⁻⁶. Graphite powder used for synthesizing third-generation semiconductor SiC wafers requires a purity of over 99.9995%. High-temperature vacuum furnace-based purification technology has been widely applied to achieve higher purity levels.

Simultaneously, significant research and development have focused on new carbon-graphite materials such as carbon fiber materials, carbon nanotubes, and graphene. Traditional purification techniques have been continuously innovated and developed, producing high-performance, high-purity new carbon-graphite materials.

Principles, Features, and Applications of Purification Technologies

Flotation Method

2. 1. Principle:Utilizes the natural floatability of graphite to separate and enrich flake or microcrystalline graphite from coexisting minerals such as kaolinite, quartz, and mica.
   2. Application:Generally, the first step in purifying natural graphite ore, preparing it for further purification.
   3. Purity:Achieves 95% purity from natural graphite ore.

Alkaline-Acid Method

4. 1. Principle:Consists of molten alkaline and acid leaching reactions to remove silicon-containing impurities and metal oxides, respectively.
   2. Application:Further purifies natural graphite to 99.9% purity.
   3. Advancements:Microwave-assisted acid leaching and mixed acid purification have improved efficiency and effectiveness.

Chlorination Roasting Method

6. 1. Principle:Introduces chlorine gas at around 1000°C to oxidize metal oxides into volatile chlorides.
   2. Application:Supplementary purification process, particularly effective for removing metal impurities.

Hydrofluoric Acid Method

8. 1. Principle:Hydrofluoric acid reacts with impurities to form water-soluble or volatile substances, removed through washing and drying.
   2. Application:Achieves up to 99.98% purity.
   3. Advancements:Combined with other strong acids and high-temperature treatments for even higher purity levels.

High-Temperature Purification Method

10. 1. Principle:Elevates temperature to 2700°C or higher, exceeding the boiling points of most impurities, which volatilize and are expelled from the graphite.
    2. Application:Used for further purifying already high-purity carbon-graphite materials.
    3. Purity:Achieves 99.9-99.99% purity.

Halogen Gas Purification Method

12. 1. Principle:Uses halogen gases (mainly chlorine and halogenated hydrocarbons) at high temperatures to oxidize and volatilize metal impurities.
    2. Application:Achieves purity levels of 99.999% and above, crucial for high-end industrial applications like semiconductors.
    3. Advantages:Targeted reduction of specific harmful impurity elements.

Characterization Methods for Purity

Ash Content Method

2. 1. Principle:Measures residual ash after burning the graphite material at 900°C.
   2. Application:Simple and convenient, widely used in production.
   3. Accuracy:Limited in detecting specific impurity elements.

Inductively Coupled Plasma (ICP)

4. 1. Principle:Dissolves ash in strong acids, uses plasma to excite solution components, and quantitatively analyzes elements via mass or optical spectrometry.
   2. Application:High accuracy, with detection limits down to the 10⁻⁹ level.
   3. Limitations:Complex procedure and high operational costs.

Glow Discharge Mass Spectrometry (GDMS)

6. 1. Principle:Uses a glow discharge ion source to bombard the sample, analyzing secondary ions to determine elemental composition.
   2. Application:Directly detects carbon-graphite material purity with high sensitivity.
   3. Limitations:Point detection only, requiring expensive equipment.

Secondary Ion Mass Spectrometry (SIMS)

8. 1. Principle:Uses high-energy primary ions to bombard the sample surface, analyzing secondary ions to determine elemental composition.
   2. Application:Extremely sensitive, detecting surface or shallow layer elements.
   3. Limitations:Limited by complex sample preparation and high costs.