Kaolin is the deposit of naturally occurring minerals containing hydrated aluminum silicates. Widely known as white clay or china clay, these minerals have desirable properties crucial for industrial and commercial applications. For that reason, the kaolin mining and processing steps thoroughly put the nature of the crude deposits and the end-use of the products into consideration.
China Clay Mining and Processing Steps
1. The Mining Stage
The mining stage is a process of extracting deposits from their first or original mineral environments. It usually includes the drilling procedure performed in types of open-pit or underground mining. The mining practice uses electro to diesel shovels and also large draglines.
Trucks will load the extracted mineral deposits and transport them to the processing plants. Or else, the draglines will drop them directly to the blunger (portable blunger).
2. The Processing Stages
After the mining stage, the kaolin will go through further processing stages, either dry or wet.
a. Dry Processing
Kaolin undergoes the dry processes of crushing, drying, pulverizing, and classifying to purify the deposits. Roll crushers will crush big chunks of raw kaolin to the preferred sizes. These crushed kaolins go to the rotary driers for further drying, pulverizing, and air floating procedures. The purpose is to eliminate coarse grits. Only fine particles will move to the collecting chambers, while the coarse grits return to the pulverizer.
Although air floating has a limitation of ability in enhancing the clay brightness, the dry process results in relatively low-cost products. Rubber industries are the largest user of dry-processed kaolin.
b. Wet Processing
Be ready since the wet processing can be lengthy and meticulous handling.
This process also starts from the mining stage, for example, by using water jet mining to extract the crude kaolin. The blunger will grind the white clay mineral deposits into smaller lumps and mix them with water. There is also an addition of a chemical dissolving agent to create a slurry or a clay slip.
Centrifugal or high-pressure pumps will push the slurry from the blunger to settling boxes to begin the degritting process. Screens, hydrocyclones, and hydro-separator (classifier) or vats are also present to eliminate the coarse substances called “grit”.
Centrifuge separators then divide the slurry into fine fractions and coarse. The fine-particle kaolin goes through the most challenging and innovative processing treatments. In other words, centrifugal sizers fraction the degritted clay of minus 325 mesh to yield fine (ultra-fine) fractions as needed by different markets.
The largest user of fine fractions is the industries for coated paper, high-gloss paint, special-made ceramics, rubber, and ink. Meanwhile, the coarse fraction is highly applicable for filler in paints, plastics, paper coatings, and adhesives.
This phase also uses the chemically bleaching method in refining the substance and eliminating some ferruginous tints or coloring due to iron impurities. Sulfuric acid acidifies the clay slip to an approximately pH 3.0 for solubilizing the iron components. The addition of strong reducing agents of sodium hydrosulfite or zinc turns the iron into a soluble sulfate.
After the leaching (whitening and brightening) process, filtering and dewatering come after employing centrifuges, a filter press, tube filters, or rotary vacuum filters. The filtered-dewatered slurry materials undertake the drying stage performed in rotary dryers, spray dryers, or aprons.
Centrifugal dewatering process by adding chemical dispersing agent and the help of spray dryer will produce a pre-dispersed clay. The rotary filter will extrude kaolin noodles that undergo the apron drying to yield a lump clay and a pulverized clay.
As for the filter press, there will be the removal of water and dissolved impurities, the formation of kaolin cakes and noodles, and the apron drying to create pulverized clay. Kaolin noodles will become kaolin powder through natural drying and disc milling machines. If the filter-pressed cakes are put into dispersion and drum dryer, it produces a pre-dispersed clay.
What rigorously long yet advanced kaolin mining and processing, isn’t it?
As previously stated, kaolin slurry passes the ‘degritting‘ process through particular devices such as screens, hydrocyclones, and hydro-classifiers. These screens here can be the 100 mesh screen, 200 mesh screen, and 325 mesh screen. Meanwhile, the classifiers consist of centrifuge hydro-separator and centrifugal seizures. So, what about hydrocyclones?
Hydrocyclones (or shortened simply as cyclone or hydrocyclone) is a specific mechanical unit useful for classifying, separating or sorting particles in a liquid suspension. For the kaolin case, the liquid suspension here is the kaolin slip or slurry.
How Does it Look Like?
Hydrocyclones have a conical-cylindrical shape. It has a tangential inlet to feed the liquid flow into a cylinder-shaped section. At each axis of the cylindrical segment, an outlet called the vortex finder extends to the cyclone. It is in charge of reducing direct short-circuit flow from the inlet. There is also a spigot, namely the second outlet at the conical end, more precisely at the lower end.
Both outlets are generally in the open state in the separation process by size, exposed to the open air. Since hydro clones have a vertical operation thus the ultra-fine product is ‘the overflow’ while the coarse product is ‘the underflow’. This device offers various diameter sizes (Ø75, Ø50, and Ø25) for multiple separation processes.
How Does it Work?
The sorting process occurs utilizing the ratio of the centripetal force (a power that has a body following a curvaceous pathway) to the fluid resistance. The margin is high for coarse particles (separate by size) and dense particles (separate by density). On the contrary, it is low for light and fine particles.
The conical vortex-generating chamber plays a significant role during the operation. The process starts when liquid suspension (or slurry for kaolin case) goes into the cyclone’s conical wall via a vortex finder. It will create a tangential flow and hence a robust vortex in the hydrocyclone.
The slurry then will spin swiftly in the high-velocity vortex. The fine (or ultra-fine) particles able to remain in the liquid suspension will go out the top central pipe, therefore the overflow. Meanwhile, the coarse particles will spiral down as the underflow at the narrow vortex bottom-end.
To sum up, the above hydrocyclones imply that kaolin mining and processing take into account technical and other aspects. The goal is to get the finest and the most desired products. They need to consider the nature of the mineral deposits and the economical end-use of the finished products, even before starting the processes.