Case Study to show how replacing a Rotary Drum Dryer with a Fluid Bed Dryer in a mineral extraction process reduced operating costs by more than twenty-five percent per annum.
The majority of the rocks that make up the earth’s crust contain an abundance of minerals that are suitable for extraction commercially. Collectively these mineral containing rocks are known as feldspar, and the main minerals of most interest are sodium and potassium.
Many of the products that are used daily are derived from feldspar and this includes glass, fibreglass, ceramics, paint, plastics and many others used in industrial processes.
The Extraction Refining Process
Feldspar is usually extracted by conventional open-cast mining. The ore is transported by trucks to a central processing facility. Following a flotation process to remove unwanted waste material the ore is crushed and ground until it is less than 20 microns.
Depending on the mica content the ore will then undergo a two or three stage acid flotation process to separate out impurities such as the mica, iron bearing minerals and quartz. Then follows a de-watering step where the moisture content is reduced to 5-10% by volume.
Magnetic separators are used to as a back-up process to remove any iron minerals present in dry product.
Classification and Drying
After extraction the minerals are then classified to produce a uniform product depending on the use for which it is destined. This is either transported by trucks or packaged into various sizes of plastic and paper bags for commercial use.
In most cases the water content within the extracted minerals needs to be carefully controlled. The technical specification of the required product will determine the amount of water that is acceptable. In addition, there is the final packaging to be considered because of the various sizes from 1 kg plastic bottles to 1,000 kg big bags. These are the main drivers that underpin the requirement for thermal drying plants.
Our customer is one of the largest mineral producers in Turkey. For the last ten years the drying process was based around a 25 tonnes/hour rotary drum dryer.
Typically, a rotary drum dryer is made up of a large, rotating cylindrical tube. A hot gas stream, normally a mixture of fresh air and combustion gases from an exhaust, is fed into the rotating tube to introduce heat energy to the wet material. Wet material is fed from one end of the tube and discharged from the other end.
The gas stream can be flowing in the same direction of material feed or on opposite direction. Normally a rotary drum dryer is equipped with a series of internal fins which lift up the wet product. As the material gets high enough to roll back off the fins, it falls back down to the bottom of the dryer, passing through the hot gas stream as it falls. This physical displacement of material provides heat exchange and thus the evaporation of water.
As one of the oldest drying technologies, rotary dryers have some certain disadvantages.
- are thermally less efficient as heat exchange performance is very low.
- are typically larger when compared to other technologies and require a larger footprint.
- require more maintenance as they are made of many moving and heavy parts.
- need long retention time for proper drying
- are more sensitive to abrasion. Internal fins need to be replaced as they are worn out by time.
- provide poor process control features since to monitor inside a rotating tube is challenging.
In contrast, a vibrating fluid bed dryers combines two processes into one: vibration and fluidization.
Basically, a fluid bed dryer is a combination of a perforated plate and a drying chamber. The wet material is fed onto the perforated plate at a controlled bed depth. Hot air is used as drying medium and this is blown into the dryer from the underside of the perforated plate, passing through precision holes at high speed which has the effect of fluidizing the material bed. In the turbulence inside the dryer, particles of wet material are thoroughly mixed and a very high level of heat exchange is achieved.
The vibrating fluid bed dryer is equipped with a high frequency vibration motor. This motor keeps the whole drying chamber vibrating continuously which improves the efficiency of heat exchange between the hot air and wet product and transports the product along the dryer to the outlet. This vibration can also control the retention time of product inside dryer and is flexible depending on the physical properties of wet feed that is being dried.
When compared to a rotary drum dryer, fluid bed dryers demonstrate some outstanding advantages.
- Very high thermal efficiency due to fluidized bed and large heat exchange surface
- Smaller footprint
- No friction between dryer parts and product. No abrasion.
- Easy to maintain thanks to low number of parts that need maintenance
- Effective process control
Minerals are commodities and subject to global competition. This means that extraction plants must be highly efficient and the drive to increase quality and to decrease carbon emissions of the production facility, have been the major motivations to investigate a more energy efficient thermal drying technology.
The financial evaluation showed that changing to a vibrating fluid bed dryer would substantially reduce operating costs but to prove this a pilot scale trial was conducted on site at 500 kg per hour.
The trials demonstrated that the theoretical thermal efficiency could be delivered at small production scale and this was enough to justify the decision to invest in a plant with a capacity of 25 tonnes/hour capacity. The scope of the supply was a vibrating fluid bed dryer unit complete with wet product feeding units and exhaust gas cleaning system.
Design values of the purchased fluid bed dryers are as follows:
- Feed capacity: 25 tonnes/h wet feldspar
- Particle size: <10 mm
- Wet product water content: 8.5% (w/w)
- Dry product water content : <0.5%(w/w)
The fluid bed dryer has been handed over to production for over a year and the operating cost of the drying plant has been reduced by 25% compared to the rotary drum dryer. The decision has recently been taken to invest in a second vibrating fluid bed dryer of a similar size which is a resounding vote of confidence in the technology.