Today 95 % of the EU’s graphite demand is imported from China who currently accounts for almost three quarters of world graphite production. Only one of the reasons why the EU raw materials initiative identified graphite as a critical high tech raw material. China’s raw material export restrictions and a graphite industry that experiences high growth rates with the lithium battery industry and electromobility being the main drivers in need have fostered the development of numerous graphite projects outside China.
Graphite consists of a stack of individual carbon layers in which the carbon atoms are arranged in a honeycomb structure, individual layers being weakly held together. This structure results in very good heat and electrical conductance within the layers, flexibility, high resistance to chemical attack and its highly refractory character.
These unique properties make natural graphite the material of choice for a wide variety of applications including steel manufacturing, refractories, lubricants, automotive parts, carbon brushes, batteries and a variety of other applications.
Besides these rather traditional uses there is a wealth of emerging applications including lithium ion batteries, fuel cells, pebble bed nuclear reactors, ceramic armour tiles and a variety of special applications of graphene in the high-tech industry which will lead to a greatly increased demand.
Graphite is traded in amorphous (70-85% C), flake (85-90% C), vein (90-96% C) and synthetic (97-99% C) grade. Prices achieved for the grades are dependent on carbon content, flake size, ash levels, impurity levels and impurity types.
Flake or vein graphite can be processed to the high value expandable graphite or spherical graphite qualities which are obtained by a sequence of processing steps ending up with a high purity product. Flake graphite is playing a key role in the green energy revolution since electromobility and energy storage solutions rely on spherical graphite as anode material in Li ion battery technology offering higher power densities, being lighter and more compact than conventional batteries. Graphite consumption in Li ion batteries is more than 20 times that of Li. Amorphous graphite is used in applications such as brake linings, refractories and steelmaking.
Graphene, a material consisting of just one single carbon layer, is of considerable interest and holds tremendous potential for many emerging and highly advanced technical applications since it combines the favorable physical properties of graphite such as superior strength and flexibility compared to steel and extremely good heat and electrical conducting properties with transparence making it a superior material.
There are three forms of naturally occurring graphite: amorphous graphite, flake graphite and hydrothermal vein graphite with large flake and vein graphite deposits being most valuable.
Most important factors that determine the value of a graphite deposit include: carbon content, flake size, type and levels of impurities, deposit size and amenability.
Flake graphite is a naturally occurring form of graphite with graphite crystals present in the form of discrete flakes. Individual flakes can be easily recognized by naked eye with typical sizes ranging from fine (<150 µm or 100 mesh) to coarse (>150 µm). Typically graphite occurs as disseminated flakes in metamorphic rocks (e.g gneisses) displaying carbon grades ranging from 5 to 30 wt.-%. Such can be concentrated by physical processing and purified by advanced thermal technology and chemical purification techniques into high purity products +99.9 wt.-%. Specifically large flake sizes are sought after since they are needed for high purity, technology grade graphite applications such as the production of spherical graphite used in Li-ion batteries. Graphite flakes are made up of parallel sheets of carbon atoms in a hexagonal arrangement. It is possible to insert other chemical species between the sheets, a process termed intercalation, thereby modifying its structure and tuning its physical and chemical properties. Graphite can be intercalated with sulfuric and nitric acids which will serve as a feed material for production of expanded graphite from which foils are formed that are used in seals, gaskets, and fuel cells.
Large flake grades make just over 20% of total flake graphite output of 375,000 tonnes in 2013, and with competition for these grades from other traditional markets (i.e. the refractories sector), new projects are likely to be required to meet the battery market demand.
Flake graphite deposits are generally found at or near surface and are therefore amenable to open-pit mining. There are significant flake graphite deposits in China, India, Brazil, Germany, Canada and North Korea with recent production dominated by China (60%) and Brazil (23%).
Hydrothermal vein graphite is the rarest and most pure type of naturally occurring graphite. As opposed to flake and amorphous graphite it has formed by deposition from high temperature (hydrothermal) carbonaceous fluids, filling steeply inclined veins and fissures in the surrounding host rock. Vein graphite is present in the form of coarse (exceeding 4 mm), platy or needle-like crystals which are recovered in the form of lumps from cm to meter scaled graphite veins. Typically vein graphite deposits show natural purities in excess of 90 wt.-% graphitic carbon. Veins are mined using conventional shaft or surface methods. Typical carbon contents in hydrothermal vein graphite products are in the range 98 to 99.9 wt.-% graphitic carbon. Due to its exceptional purity and highly perfect crystals vein graphite products show superior thermal, electrical and mechanical properties compared to amorphous and flake graphite in certain applications. Vein graphite deposits can be found in Sri Lanka, Great Britain, Canada and in other places; sole commercial source of vein graphite to date is Sri Lanka.
Amorphous graphite is a term used for microcrystalline graphite occurring in masses that consist of individual very fine graphite crystals at the µm-scale that can’t be resolved by naked eye or optical microscopy. It is the most abundant form of graphite. It is typically formed from anthracite, i.e. thermally metamorphosed coal seams during a metamorphic event, i.e. the action of temperature or pressure due to intrusion of a magmatic body or a tectonic event. Amorphous graphite deposits typically show total graphitic carbon contents (TGC) ranging from 20 to 40 wt.-%, while amorphous graphite products will be in the range 70 to 85 wt.-%. Therefore, amorphous graphite is typically lower in purity than other natural graphite. This is due to an intimate contact between graphite micro crystals and the mineral impurity phases with which it is associated. This close graphite/impurities association makes flotation and other density and chemical based separation techniques inefficient if not impossible.
Due to its limited purity and flake size amorphous graphite is mainly used in standard commodities (lubricants, brake linings, refractories, steelmaking where higher ash contents are acceptable) and is the lowest priced graphite. Major deposits of amorphous graphite are found in China, Mexico; there are also deposits in the US and in Europe.
Each graphite ore has unique characteristics which need to be addressed in the processing. Therefore it is of crucial importance to determine graphite flake morphology, primary flake size and liberation size. Typical flake graphite deposits are gneissic, i.e. they consist mainly of feldspar, quartz and mica.
The size of the graphite flake is a very important commercial consideration. Therefore, it is in the best interest of a flake graphite producer to maximize the amount of large flake removed from the deposit. This means that any processing which will tend to grind or reduce the size of constituent flake must be minimized. Based on aforementioned ore specifics the appropriate degree of grinding and liberating is defined. Typically graphitic ore is crushed in cone crushers or vertical shaft impactors (in large scale), ground in roller mills and classified. Grinding is carried out in a most gentle manner trying to avoid breaking valuable coarse graphite flakes when comminuting granular side minerals such as quartz and feldspar. Preferably shearing grinding techniques such as ball or rod mills are used to achieve this target.
The crushed and ground graphitic rock is then subjected to froth flotation. Froth flotation takes place in a water-mineral suspension. It is used for selectively separating minerals by taking advantage of differences in their hydrophobicity. The surfaces of the graphite particles are hydrophobic (water-repellent) and it is thus very amenable to flotation in water using suitable/selective conditioning reagents. Hydrophobic graphite particles become attached to air bubbles that are introduced into the suspension and are carried to a froth layer above the liquid, thereby being separated from the hydrophilic (wetted) particles. In contrast the flotation reagents do not stick to the country rock; therefore these particles sink to the bottom of the cell and are removed from the process. Flotation beneficiated flakes may be re-floated to increase its purity.
Flotation process designs vary in complexity depending on degree of liberation and the desired purity of the product but will typically include several rougher, liberation and cleaner flotation steps with intermediate regrinding steps.
Flake which is in the purity range of 80-98% typically represent materials which have been beneficiated using only froth flotation. Although surface chemistry provides the mechanism by which flotation is affected, the process cannot change the purity of the discrete graphite particle. Such remaining impurities finely intergrown with the graphite can then be removed by applying chemical or thermal treatment steps to achieve highest purity grades.
Application of graphite in high tech products such as lithium ion batteries or fuel cells demands higher purity than is typically achieved by flotation. For further purification of a graphite concentrate to natural high purity graphite products with carbon contents of up to 99,99 wt.-% advanced chemical and thermal processes have to be applied.
Several procedures for purification of graphite concentrates are available. A basic process is thermal treatment in the presence of caustic reagents to dissolve siliceous impurities such as quartz, feldspar or mica. The graphite flotation concentrate is mixed with caustic reagent and baked at elevated temperatures. After baking the graphite is leached with water to wash away dissolved impurities.
In case ultimate purity levels are targeted, the more finely intergrown mineral phases residing in between the graphite layers, have to be removed. Typically an additional one or multi-stage acid washing with different acids or combinations thereof to remove impurities that are insoluble at alkaline conditions, is applied.
Optimal reaction conditions and reagent uses have to be matched by considering type and level of impurities present in the respective graphite concentrate, targeted product quality, markets and finally economics of the process.
GPE uses its in-house technical center and innovative machinery manufacturer’s equipment for tailor-made process development including downstream processes (spherical graphite), providing large scale samples for customer tests and end user approval. GPE bench scale and pilot plant processing facilities cover the full value chain of graphite processing starting from liberation and physical concentration (flotation) of graphite to its purification and refinement by means of chemical and thermal processing steps – including conventional and alternative processing routes. Both caustic and/or acid treatment combinations including baking at elevated temperatures, final washing and drying can be offered.
Besides various conventional comminution technologies, GPE offers innovative processing concepts in flake size optimization such as selective liberation of large flakes via electrodynamic comminution.
Pilot plant equipment is accessible in house and will be arranged in response to the specific process requirements developed in lab and bench scale in order to approve process design and product quality in semi technical conditions. At the same time GPE engineers gain scale up parameters needed for advanced plant design (Basic Engineering).
Read More >>
- 75 Edridge Rd, Croydon, Greater London, CR0 1EJ
- 0745-230-4089
- Mon - Fri: 9:00 - 18:00