Overview
Aside from having a higher density, often between 30 and 50 lbs/ft3, densified silica fume has similar technical performance characteristics to undensified silica fume. The higher density and flow characteristics allow for better optimization of freight and storage costs for better cost-effectiveness.
Conventional cement silos house and move silica fume between storage and bulk pneumatic transportation. Standard practices utilize bucket elevators, cam line belt feeders, or pneumatic lines to transport between the silo and bulk tanker. Being less prone to agglomerate, nearly dustless, and easily transported, densified silica fume is ideally suited for efficient transport and loading.
With the proper loading and unloading procedures, clean silos, and correct transport pipes and equipment, bulk pneumatic truckloads of up to 26 net tons can be loaded or unloaded in between 1.5 to 3.5 hours. This guide aims to help these results at your particular facility.
Transportation
Because silica fume is amorphous and over 100 times smaller than cement particles, silica fume cannot be pumped as quickly as cement, allowing for greater duration times in preparing for transportation and shipping. Transporting dry densified silica fume in bulk pneumatic tankers is typical, with sizes ranging from approximately 600 – 3000 cubic feet. The material in the bulk tankers can then be transferred to silos specific for silica fume storage [1].
Storage
Ideal storage silo capacity should be at least 80m3 or 2,825.17 cubic feet. Since silica fume’s bulk density is approximately half the bulk density of cement, this recommended silo capacity allows for sufficient on-hand materials to be present for projects and allow tankers to completely discharge a load of dry densified silica fume without compacting material in the pipes, and the silo. Storage silos should be in good condition overall and free from both internal and external leaks. Pipe location for filling the silo must also be clearly identified. The silo must be vented utilizing a dust collection system that is properly sized with the volume of the silo. The dust collection system must be cleaned at the time of delivery to minimize any back pressure during the discharging process.
To eliminate or reduce the static charge that may occur during the discharge process, each silo should have a grounding cable affixed to the silo leg and able to attach to the bulk tanker (Fig.1). If the silo has any metallic piping, the static charge can cause a buildup of fume on the sides of the pipe. Attaching the grounding cable will eliminate or reduce the charge minimizing the buildup of the fume [1].
Transloading Procedure
The fill pipe for filling the silo from the bulk tanker is critical for ensuring an efficient and practical process. The ideal hose for efficient transfer is a 150 mm, or 6 inch, diameter, smooth-lined rubber hose. Intervals between hose fasteners should be between 3 m to 4 ½ m, or 9 to 15 feet, for ideal conditions to allow the hose to vibrate and perform a self-clean function. Piping should not have any sharp or 90-degree bends but should have a 1.5 m, or approximately 5-foot, radial bend to eliminate choking as well as minimization of horizontal piping line sections [1].
To remove any existing moisture and condensation from lines, clean air should be blown into the silo up-pipe. A backpressure reading should be taken, any reading over 5 to 7 psi, will indicate a problem in either the system or with the baghouse.
While still in the truck trailer, the silica fume should be aerated and fluffed using a pneumatic pressure of approximately 7 psi for nearly 8 to 10 minutes. Do not use the air from top pressure inlets. Rather use air from air pads or vibra cones.
Starting with the furthest compartment from the truck cab, open the material valve and begin discharging the silica fume into the silo. When the fume is moving efficiently into the silo while at low pressure, after approximately 5 minutes, increase the tank pressure to between 8 to 10 psi. This can be done by gradually closing the line pressure valve.
Resume discharging the compartment until the line pressure drops to zero indicating that the compartment is empty. Quickly close the value and move to the next compartment. Open the next compartment and choke the line pressure back until the pressure is between 5 to 7 psi. While the pressure will continue to build due to the flow of material, continue opening the line pressure valve until the tank pressure remains steady between 8 to 10 psi. Pressure can be controlled by opening or closing the line pressure valve, or by opening the blowdown valve. Pumping should continue at 8 to 10 psi until the truck discharge is complete.
Steel piping should be eliminated if at all possible, except for the connection to the top of the silo. However, if steel pipe is present, a vibrator should be affixed to the top bend radius to produce cycling vibrations enough to prevent clogging and chocking of the fume in the pipe.
Ideally, the rubber piping transporting the silica fume from the tanker to the silo should attach to a short piece of 150 mm, or 6 inches, connecting steel pipe at the top of the silo, as close to the center as possible. If a short piece of steel pipe is not affixed to the top of the silo, a rubber hose can be used, however, this may make weatherproofing the connection pipe difficult [1].
Considerations
Transloading dry densified silica fume is typically a process that takes time. Depending on silo conditions, it can take 90 minutes or more to complete the discharge process.
Maintenance of the silo can determine the rate of the discharge of the load. Clean lines and routine maintenance should be regularly completed.
Fume will move more quickly and efficiently through a shorter length of hose. This will allow the fume to travel the shortest distance possible and can reduce the time that it takes to discharge. – As fume may clog lines, having a rubber-coated mallet to strike lines and shake clogs loose is considered a best practice.
Filling the Weigh Hopper
Controlling the flow of fume into the weigh hopper is a critical aspect of proper transference from the silo. Silica fume flows downward at a faster rate than it pumps vertically. Aerators should be installed in the cone section of the silo, and it is suggested that they are run for 15-20 minutes before transfer into the weigh hopper to help fluff the silica fume.
If the silo utilizes a cement screw conveyor, consider using a shorter length of a screw as clogging will be less likely to occur and will be considerably easier to clean. Feed rates to both the cement screw conveyor and weigh hopper must be observed to ensure that the required amount is filling the hopper as the silica fume must be discharged at a controlled rate. A controlled flow rate can be obtained by varying between altering the gate size or adjusting the opening as needed. If the silo has a rotary feeder at the bottom, the flow can be adjusted through the feeder to meet the required rate [1].
Reference:
[1] (2005) Silica Fume User's Manual. Silica Fume Association. PDF version, April, 2005.