The mining process of phosphate
The mining process of phosphate ore deposits includes prospecting, exploration, mine development and ore exploitation. The ore extraction could be underground or open pit which is the most common method. The main phosphate rock extraction stages in open pit are drilling, blasting, excavating, loading, haulage and dumping.
The overburden, is dug with draglines or dozers, after blasting and placed into a pit. A large variety of techniques and types of equipment are used to extract phosphate ore which is in the form of layers. After stripping the overburden, the phosphate ore is stripped by dozers and transported by tracks, belts or water carrying to the beneficiation area.
Apatite is the main mineral of phosphate ore. The apatite group includes Fluorapatite, Chlorapatite, and Hydroxyapatite. But, the major mineral, especially in sedimentary phosphate rock is fluorapatite. Beside apatite, there are many other minerals and impurities. Many impurities are inside apatite mineral, but some of them are outside the apatite mineral. The major part of exogangue impurities are composed of silicates, carbonates and clays.
To concentrate phosphate rock in apatite (BPL or P2O5 content) we have to separate impurities from apatite using a beneficiation process.
The beneficiation process could include a large variety of operations depending on the rock impurities content, the particles size and their hardness. The main operations are: classification, electrostatic separation, magnetic separation, crushing, grinding, dry enrichment, cyclonning and calcination and wet processes like chemical leaching, washing, hydrocyclonning and flotation.
The main beneficiation method which is worldwide used is froth flotation. The process of froth flotation involves crushing, grinding to reduce the particles size to a fine size; and attrition, scrubbing & classification.
Crushing: In primary crushing, ore particle sizes are reduced to about 10 cm. The secondary crushing aims to reduce particle sizes from 10cm to smaller sizes between 1cm to 2cm. The final crushing stage reduces particles sizes to less than 50 mm. but, in general these figures depend on the ore composition, grain sizes and hardness.
Grinding is carried out either to increase the surface area of mineral particles, reduce the size of a rock particles to a desired grain size, or to liberate the mineral of value from the gangue impurities prior to concentration.
Attrition & classification: this technique is used when most of the gangue minerals are mostly clay minerals. Because clay grains are fine and packed, they can easily be separated from the phosphate by adding water. This operation breaks and disperse the matrix in water. So, it can be classified easily.
This technique could also be used when the exogangue impurities are mainly coarse silica. The phosphate ore is treated by a wet screening where most of the fine phosphate fractions are washed off and separated from the coarse silica. Sometimes the situation could be opposite: the apatite could be coarse and the gangue minerals could be fine. In this case, the oversize phosphate fraction can be separated by scrubbing with water prior to flotation.
These operations liberate the apatite mineral particles from impurities and consequently increase the surface area of the apatite minerals to react with reagents and additives which are involved in flotation processes.
The main beneficiation method which is worldwide used is froth flotation. Froth flotation is a method commonly used to concentrate phosphate ore by separating minerals. It is a wet physical separation method which use air bubbles and chemical reagents. It allows to separate particles by improving the ability of air bubbles to selectively adhere to specific mineral surfaces from the slurry. The particles attached to air bubbles are carried to the surface and removed while the particles that remain completely wetted remain in bulk in the slurry. To foster the particles attachment to air bubbles a variety of surfactants are used while depressants are also used to retain the other mineral suspended in bulk.
There are two types of flotation processes which are used in phosphate ore beneficiation: direct and reverse flotation. We talk about direct flotation when phosphate minerals are floated and reverse flotation when they are mineral impurities which are floated. Some processes are combination of both direct and reverse flotation. In terms of technology froth flotation can be carried out using cells or columns. The process efficiency and economics generally pilot the technology choice.
After concentrating and separating the phosphate ore the wastes which contain essentially mineral impurities and remaining phosphate rock, diluted in water, undergo a sedimentation process in static decanters to recover water and concentrate sludge. The decantation underflow which contains mineral impurities phosphate rock and water is carried by pipes to be stored in dumps.
The concentrations of phosphate ores deposits are going to be low and phosphate ore beneficiation is becoming a real challenge for phosphate industry. The froth flotation could be considered up to now as a good solution but it consumes a lot of water and chemical reagents and leads to huge quantities of sludge (tailings). The R&D should explore other dry beneficiation pathways beside working on enhancing the efficiency of flotation processes (in term of yields, water, energy and reagents consumption) and reusing or valorizing the tailings.