Examining the use of Fluid Bed Dryers for Silica Sand.
Silica sand is of great importance to a wide variety of industries. These industries include glass, foundry, ceramics, sports and leisure, paints, plastics, sealants and adhesives, oil and gas, horticulture and agricultural sectors.
The Extraction Process
Industrial sand is usually extracted from naturally occurring open sand quarries.
Sand is extracted from the quarry normally by wheel loaders. In some cases, sand is below the water table and a floating dredge is used to extract the sand.
The dredge delivers slurry of sand and water to the processing plant via a pipeline, while the wheel loader simply scoops the loose sand up and transports onto conveyor belts for delivery to the processing plant.
In the processing plant, the sand can then be sorted through a series of screens. The sand is washed, and sent to a sand classification plant, where the particles are further separated. If required, the sand can be further processed using magnetic separation and crushed if smaller particles are needed, to ensure that the sand meets the end user specifications.
At the end of wet processing, the sand can be sold as moist sand or can be sent to a dryer for producing dry sand.
The dried sand can then be delivered to customers by trucks or sent to an automated bagging plant to pack the sand into bags.
Our customer is one of the largest silica sand producers in Turkey and has been in business since 1982. The company has been delivering high quality silica sand to various industries and had been supplying 500.000 tonnes of moist sand to its clients annually. Increasing pressure to supply dry, high quality silica sand from foundries persuaded our client to invest into a rotary drum dryer in 2002.
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 as a dry product 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.
Although this is 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, because it is challenging to control what is happening inside a rotating tube.
Vibrating Fluid Bed Dryers
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 be used to control the retention time of product inside dryer and is variable 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
Side view of a 50 t/h silica sand dryer with a wet scrubber unit
Top view of a 50 t/h silica sand dryer with a wet scrubber unit
Minerals and raw materials such as sand, quartz, silica and feldspar are commodities and subject to global competition. This means that extraction plants must be highly efficient. Furthermore, the demand for an increase in product quality and the decrease in the carbon emissions of the production facility, have been the main drivers in the move to a more energy efficient thermal drying technology.
The financial evaluation for this customer showed that changing to a vibrating fluid bed dryer would substantially reduce operating costs. In 2015, the customer decided to invest in a new drying plant with a capacity of 50 tonnes per hour. The scope of the supply was a vibrating fluid bed dryer unit complete with wet product feeding units, wet scrubber gas cleaning system, a product cooling system and storage silos. This is shown in the image of the plant below.
The design values of the purchased fluid bed dryers were as follows:
- Feed capacity: 50 tonnes/h moist silica sand
- Particle size: <5 mm
- Wet product water content: 6% (w/w)
- Dry product water content : <0.2%(w/w)
The fluid bed dryer has been handed over to production for over one and a half years and the operating cost of the drying plant has been reduced by 29% compared to the rotary drum dryer. So, not only has the technology improved the efficiency, reliability and control of the plant it has also delivered substantial cost savings.
We are keen to help and we will be pleased to discuss any issues you may have:
Centri-Force Engineering Co. Ltd.
1-7 Montrose Avenue, Hillington Park, Glasgow G52 4LA.
Scottish Registered Company SC70659
VAT Reg. No. GB 334 3138 81.
Tel: +44 (0)141 882 3351