Essential Machinery for Sand and Aggregate Processing

Challenges of Aggregate Processing

While the geological meaning of the word “aggregate” quite literally means “(a material) made up of a mixture of mineral fragments, crystals, or similar materials,” aggregate for construction is made by separating particles by size and often by shape as well. This is done using screening equipment to classify finer and coarser particles, with the former type of aggregate separated into different grades of sand. Dryers are used in aggregate processing facilities to remove naturally occurring moisture from cleaning, while feeders and conveying machinery move raw and finished product.

Sand is particularly important in the construction sector, as it makes asphalt, concrete, and mortar stronger and more stable by adding mass. To produce sufficiently high-quality finer aggregates, the equipment must also be of a high standard to avoid common aggregate handling issues. Aggregation manufacturers must address challenges involving inconsistent feeding, level control, overflow, and other potential processing problems to operate smoothly and keep downtime to a minimum. When looking for equipment used to process aggregate,

Challenges inherent in aggregate processing include:

• Segregation: When larger and denser particles settle first, it disrupts proper particle size distribution, disrupting feed flowrates as improper blending can cause the material to separate based on size and density; though the risk of segregation depends on the process, it can result in clogging upstream processes with the buildup of denser particles and is a major reason why screening for tight particle size distributions is necessary in aggregate processing.
• Ratholing: Occurring primarily when particles are irregularly shaped, ratholing happens when material in the center empties from a bin, yet some still clings to the sides, resulting in an erratic flow of material known as a “rathole”; it’s a pervasive problem, as even modern sensors used to detect aberrations during aggregate processing can be fooled into thinking a hopper is fuller than it is.
• Erosion: Aggregate handling equipment sometimes experiences problems with feeding, partly because erosion wears away parts of the machinery; this can be resolved by lowering particle velocities and making turning angles for the pipeline less tight.
• Collapse: This is a potential hazard posed by certain types of aggregates, where a void within a hopper can collapse should a worker attempt to walk on its surface, which can then pull in and bury the worker under the material.
• Clogs: Feed ports and other cavities in equipment for processing aggregate are prone to clogging, which can lead to inconsistencies like varying shapes and sizes; proper screening helps ensure consistent particle size distribution.
• Bridging: Also known as “arching,” this occurs when a bridge appears over the hole’s top as material empties from a hopper, causing flowrates to vary. Agitating and tapping materials can generally mitigate such bridging issues, as can adjusting the speed at which material goes through a system.

Processing aggregates like crushed stone, gravel, and sand requires high-quality equipment, which is particularly important when processing sand. Dryers, devices for screening, conveyors, bins, and other specialized machinery are commonly used for aggregates like sand. Dryers are especially critical, as moisture must be removed from most types of sand.

Processing Sand: Dryers & Other Equipment Used 

Sand is sourced from many different areas, including near or within bodies of water. Sand found by or in lakes, oceans, rivers, or seas will inevitably have high moisture content. For it to be useful for any construction, it’s important to dry the sand. Dryers in industrial sand processing facilities remove moisture from this aggregate through heating that vaporizes the moisture contained within the sand.

These sand dryers, along with other aggregate processing equipment, are important in the construction industry’s production of sand. Like many other bulk materials, sand production occurs along a production line that includes temperature control systems, storage bins for wet and dry sand, sand dryers, mixers, fans, dust collectors, drive units, conveyors, and other machinery. This equipment works together to prepare sand for its end uses. Now, let’s look at how these various pieces of equipment are used to process sand.  

Conveyors

Initially, a conveyor carries raw material into a production facility and moves it to an area set aside to prepare the sand. Dryers are often fed by belt conveyors that evenly distribute the sand across the conveyor’s surface to ensure greater efficiency and regulate the speed at which sand is transported. Increasingly, sand processing companies use vibrating conveyors instead of belt conveyors to transport finer aggregate material, as these are less prone to clogging when dealing with heavy loads of sand. Dryers are only one point along the production line connected to these conveyors, and they are used throughout the facility to transport sand along the production line.

Screeners

Used widely in the processing of aggregates, like conveyors, these screening devices can be used at various points throughout a sand processing system. They play a critical role in classifying and separating sand based on grain size and shape, though the placement depends on the application they serve. Once extracted, sand is first screened to rid it of unwanted debris like rocks and to ensure sand grains fall within a certain desired range. Rotary drums and vibrating screeners are commonly used in each configuration to aid in scalping, screening, separation operations, and removing moisture from the sand.

Sand can also be screened during the washing process to remove clay, finer particles, and other contaminants to ensure purity and quality. Dryers often have screeners positioned after them and before packaging the final product. Specialized screening is often done to classify and grade sand with specific particle size distributions for filtration systems, foundries, and the glassmaking industry.

Mixer

The first processing stop for wet sand is the mixer, which breaks up moisture-infused clumps of sand. Dryers come later, and the mixer is used to break down sand to facilitate drying later in the process. Once properly dispersed, wet sand is again transported via the conveyor to the next processing stage.

Wet Sand Storage Bin

After the mixer, the loosened wet sand is temporarily collected in a storage bin, which accepts upstream sand. As it leaves storage, the wet sand storage bin distributes the sand evenly to upstream processes. A vibratory feeder receives the wet sand from this storage bin at a controlled flow rate from another conveyor.

Vibratory Feeder

The vibratory feeder breaks down sand further, so dryers can more effectively remove moisture from the material later in the process. Sand is then distributed evenly onto the next conveyor, which moves material into the sand dryer via the sand dryer’s inlet.

Fans

Hot air blown from a fan speeds the vaporization of moisture within the wet sand. Dryers receive the hot air as it’s blown into their interior to remove the moisture. The fan adjusts airflow, transporting materials within and from the sand dryer. The moving air also dissipates any waste gases from the drying process.

This regulated airflow entering the sand dryer controls temperatures and humidity, which is done by regulating the speed or adding more openings to the sand dryer through which air can blow. As the material dries, the fan’s airflow also helps convey material throughout the sand dryer, ensuring more consistent heating of the sand.

Temperature Control System

A control system tracks and adjusts the heat source as needed to keep the sand dryer within the required temperature range. This helps prevent the sand from overheating or underheating. Dryer processes controlled by the system also keep temperatures stable and within a specific range while managing energy use and making the environment safer for workers.

The temperature control system ensures the sand dryer functions reliably during drying. Its safety features monitor the sand dryer, shutting down automatically should it become too hot or exhibit any temperature-related anomalies.

Drive Unit

One of the more important elements of many sand dryers is the drive unit, which transmits power to revolve a rotary dryer’s drum to dry the wet sand evenly. Dryers that instead use conventional fluid bed technology don’t require drive units, however. Drive units control the conveying equipment that transports sand into the drum while controlling rotational speed to provide reliable power and powering automated systems associated with the rotary dryer.

Sand Dryer

When processing sand, dryers are generally rotary or fluid bed dryers. With a rotating drum at a nearly horizontal position, rotary dryers can dry sand continuously, entering from the higher end and exiting from the lower part of the drum. Rotary dryers heat via indirect or direct heat transfer, with heated surfaces heating the sand through radiation and conduction as the drum rotates until it reaches a certain dryness level. 

With fluid bed dryers, hot air or gas surrounds the material to fluidize the sand. Dryers using this technology augment the sand’s natural flowability, keeping it almost liquid as the heated gas moves through the sand. Dryers with fluid beds handle sand grains more gently and involve a lower capital investment. Additionally, fluid bed dryers are less expensive to operate, reducing the costs of dust collection and boosting productivity. Since fluid bed dryers have no internal moving parts, the maintenance cost is also considerably lower.

 Whether of the rotary or fluid bed variety, sand dryers regulate: 

• Humidity: Sand dryers decrease moisture content by heating and ventilating materials.
• Quality: Removing moisture lowers the chance of mold developing within the sand, enhancing its quality.
• Stability: Removing moisture also helps keep other microbial life from developing, which can affect its quality over time, thus extending the sand’s shelf life.
• Transportation costs: Removing moisture also lightens the weight of the sand, lowering transportation costs.

Rotary dryers are more expensive but require less vigilance as they tend to be more forgiving when faced with processing issues. Fluid bed dryers require minimal maintenance due to their fewer moving components, cost less upfront, and offer better thermal efficiency. When considering which option is best for an application, sand processing companies should consider moisture content, desired throughput, and particle size

Dust Collector 

From the end that discharges the sand, dryers connect to dust collectors that remove any dust produced by the drying process while also ensuring any discharged gas remains free of dust and solid pollutants to meet emissions standards. The dust collector also keeps solid particles from the sand dryer and its discharge system, reducing wear and preventing blockages. This, in turn, extends the sand dryer’s lifespan while also eliminating impurities so that the sand has the correct texture, properties, and look. After drying, a conveyor transports the material to the dry sand storage bin.

Dry Sand Storage Bin

Once dry, processed sand is temporarily stored in the dry storage bin before being packaged, transferred to vehicles, and shipped to customers.

Carrier Sand Dryers & Other Aggregate Processing Equipment 

The abrasiveness of aggregates creates difficulties during processing, particularly for sand. Dryers need to be able to handle these small, dense, and weighty particles. Carrier Vibrating Equipment Inc. makes vibrating conveyors, screeners, feeders, and fluid bed dryers for processing sand and other aggregates.

Carrier’s aggregate and sand processing solutions include: 

• Carrier vibrating sand conveyors: These units are available as heavy-duty and medium-duty conveyors, offering multiple functions that include accumulating, conveying, cooling, dewatering, drying, orienting, quenching, screening, and separating.
• Carrier vibrating feeders: With multiple robust designs, Carrier’s vibrating feeders can be customized for set amounts of bulk materials. Drive systems include brute force, natural frequency, tuned, and two-mass direct drive systems.
• Carrier sand aggregate screeners: Designed particularly for sand and other aggregates, these can conduct wet or dry screening while efficiently handling as much as 1500 tons hourly and utilizing screening bars that avert clogging.  
• Carrier fluid bed sand dryers: Can handle rocks, pebbles, and fine sand, Carrier dryers vibrate to fluidize material without clogging and ensure even drying. They also effectively break down clumps.

Carrier designs machinery to meet specific requirements for our customers in the aggregate processing industry. With decades of experience in engineering, all manner of systems for handling sand and other aggregates Carrier Vibrating Equipment can help you build a customized system to meet your needs.