Graphite is an essential non-metallic mineral. After purification, it can applicate in many fields, such as optics, energy, aerospace, environment, and new materials.
Graphite ore often associates with quartz, illite, kaolinite, andalusite, sericite, pyrite, limonite, tourmaline, calcite, etc. It needs to be purified and removed before use. Standard graphite purification methods include:
- Selective flocculation
- Alkali-acid method
- Acid leaching
- Chlorination roasting
- High-temperature roasting
1. Selective Flocculation
To purify graphite ore by selective flocculation, adding a polymer flocculant to a suspension containing two or more components is necessary so that the flocculant can selectively adsorb specific elements in the suspension and pass the bridge chain. The effect produces flocculation and precipitation to achieve the purpose of component separation.
The main flocculants are sodium silicate, sodium hexametaphosphate, and lignin starch. Carboxymethyl cellulose and water glass are used as dispersants. Tapioca powder, acorn powder, sodium alginate, and polyacrylamide are used as flocculants to purify graphite ore.
The selective flocculation purification method and equipment of graphite ore are relatively simple, and the cost is low, but the recovery rate of fixed carbon is low, only about 40%.
2. Graphite Ore Flotation
Flotation is a standard method for graphite ore purification, mainly by adding a series of flotation reagents to enrich the selected target minerals in the gas-liquid interface to separate them from impurity minerals to achieve the purpose of purification.
Graphite has good natural floatability and hydrophobicity, so conventional graphite ore can be purified by flotation. In the purification process, to protect the large flakes of graphite, the procedure generally adopts the technique of multi-stage grinding, multiple separations, and coarse concentrate regrinding and selection.
The commonly used collectors in graphite flotation are mostly coal tar, the foaming agents are mostly terpineol oil or butyl ether oil, and the inhibitors are mostly water glass and sodium fluorosilicate.
Graphite ore flotation purification can make the grade of graphite reach 80%~90% or even about 95%. And the method has low drug consumption, low energy consumption, and low cost. Graphite ore contains very fine silicate minerals and compounds of potassium, calcium, sodium, magnesium, aluminum, and other elements. The monomer dissociation can’t achieve in the grinding stage, and it needs to purify again by other processes after flotation.
3. Alkaline Acid
The acid-base purification of graphite ore is a relatively mature process, which can be divided into alkali melting and acid leaching.
Alkali melting process: Under high-temperature conditions, the alkali in the molten state and the acidic impurities (silicate, aluminosilicate, quartz) in the graphite are chemically reacted to generate soluble salts, and then the impurities are removed by washing.
Acid leaching process: Use acid to react with metal oxide impurities, convert some of the unreacted impurities in the alkali melting process into soluble salts, and then remove impurities by washing to separate them from graphite to improve the purity of graphite.
The alkali-acid purification of graphite ore can make the graphite taste up to 99.5%, the equipment is simple, the energy consumption is low, and the one-time investment is small. However, the acid and alkali of this method will have strong corrosiveness to the equipment, and the purified wastewater will be seriously polluted. In addition, there is also the disadvantage of graphite loss.
4. Acid Leaching
Acid leaching can use to purify graphite ore by sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, and other methods.
- Sulfuric acid and hydrochloric acid are more suitable for leaching, with strong leaching ability. Still, it can use some sulfuric acid leaching methods to purify graphite ore by sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, etc.,
- Sulfuric acid and hydrochloric acid are more suitable for leaching, with strong leaching ability. But some sulfates have low solubility, and hydrochloric acid is more expensive.
- Nitric acid is highly oxidative and volatile, easily decomposed when exposed to light, and easily produces highly toxic phosgene, which can easily explode at high temperatures.
- Hydrofluoric acid cannot undergo an oxidation reaction nor experience a reduction reaction. The critical point is that it can effectively dissolve sio2 and silicate. After the graphite and hydrofluoric acid are fully mixed, the hydrofluoric acid will react with the impurities in the graphite to form water-soluble compounds and volatiles, and then remove the soluble impurities by washing with water to obtain high-purity graphite.
Graphite ore hydrofluoric acid purification can effectively remove impurities in minerals with low energy consumption. However, this method is highly toxic and causes great environmental pollution.
5. Chlorination Roasting
The purification of graphite ore by chlorination roasting method is mainly to add an appropriate amount of reducing agent to the graphite ore. And roast it at a high temperature under a specific atmosphere and equipment so that the valuable metal in the mineral is combined with chlorine and transformed into a gas phase or accumulation with a low melting point and boiling point. Then precipitate and achieve effective separation from other components to obtain high-purity graphite.
The chlorination roasting method has high efficiency, low energy consumption, and low cost for the purification of graphite ore. However, in this method, chlorine gas has strong corrosiveness and toxicity, and the external pollution is relatively large.
6. High-temperature Roasting
The melting point of graphite ore is very high (melting point is 3652℃, the boiling point is 4250℃), much higher than other impurity minerals. High-temperature roasting method to purify graphite uses the difference in melting point of graphite to select. When heating the graphite ore to 2700~3000℃, most of the impurities have been gasified, so it can effectively separate the graphite from the impurities.
The high-temperature roasting method can purify graphite ore with a grade of 99.99% or even higher. Still, the energy consumption is large, the requirements for equipment are very high, and there are specific requirements for the purity of graphite ore.
The above is the standard six kinds of graphite ore purification methods. According to the ore properties of the graphite ore, the conditions of the processing plant, and the investment budget, choose which method to use. When selecting a process, carry out the graphite ore beneficiation test first. Select the purification process in a targeted manner through the test report to strive for excellent technical and economic benefits.