Anti-strip hydrated lime
Hydrated lime tends to coagulate when allowed to come in contact with moisture. Fiber tends to bridge when dispersed. Fly ash becomes cementitious when exposed to water.
These ingredients are conveyed in enclosed elevators, augers, chutes, pipes and hoses. Sometimes they need to be aerated and vibrated in order to flow. Hydrated lime requires at least a 60-degree slope on the discharge cone. Aerating and pneumatic conveying requires conditioned dry air. Air drying and pneumatic conveying is expensive. Compared to augers and elevators, pneumatic conveying requires higher horsepower, can take up plant baghouse and fan capacity (which could lower plant production capacity) and wears out equipment.
Hydrated lime likes to bridge especially when exposed to humidity. Aeration becomes necessary and requires about 5 psi at 60 cfm per silo. When using plant air, the air must be dried. Drying this compressed air is expensive.
A better alternative is using 5-psi compressed air from a positive displacement blower. The type of aeration produced is a very important item for maintaining product fluidity while reducing energy and equipment costs. The energy it takes to operate a 10-hp compressor to do the work can be done with a 5-hp blower. An air compressor is much more expensive to purchase and maintain than a blower. Plus, the air compressor needs an expensive air dryer. When advised of the benefits, many companies are turning to positive displacement blower for this task.
Hydrated lime has been used in the asphalt mix as a mineral filler, anti-strip and modifier. As an anti-strip, hydrated lime is usually added into a continuous pugmill with water. Water is required to moisten aggregate before the aggregate is to be dried. This process promotes better adhesion to the aggregate. Since hydrated lime is added to the aggregate before the aggregate is to be dried, both batch and continuous mix plants should mix the hydrated lime in a continuous pugmill or belt plow before the dryer.
The aggregate drying process inherently separates the fines from the aggregate. When air velocity changes due to production, moisture, temperature variations of the dryer, so does the size and amount of fines separated. Consequently, the loading of the baghouse changes and so does the amount of dust returned.
Complicating the issue further is the baghouse discharging the dust in sags and surges (plus or minus 21 percent over a three-minute period of time). Adding insult to injury is the Hot Stops that allows the bags to relax when the fan is turned off, allowing most of dust to fall and fill the auger below.
When the plant is started again, the dryer initially sees the full auger surge (59 percent). Since the bags released most of their dust, the augers run empty until the bags build up a dust cake again. This situation then causes the dust to go from a surge to sag (59 percent) until it slowly builds up a dust cake over a 5-minute period.
The solution to this problem is the right equipment and correct operation procedure: slowly change from one production rate or temperature to another, keep aggregate moistures consistent, slowly change exhaust damper settings, and invest in a surge bin with flow measuring and control.
Continuous mix plants
Automatic set point – virgin aggregate belt scale required to provide caontinuously changing set point.
Two choices are available when returning dust to a batch plant.
1) Batch plants with a dust bin in the tower can automatically add dust to the weigh hopper in its normal cycle as a separate pull.
2) Batch plants without a dust bin in the tower can automatically add dust by blending the aggregate as it is being introduced to the hot leg. Batch plants that introduce the baghouse dust with the sand should have the dust automatically proportioned to the aggregate, or the plant operator will have to adjust the dust flow rate as the production rate changes.
Mineral filler as minus 200 makeup, such as dry lime dust, fly ash, calcium carbonate and hydrated lime is often added to achieve desired mix specifications.
In order to make up the difference with mineral filler, especially when producing SMA mixes, the baghouse dust must be accounted for at all times. Controlling the baghouse sags and surges is just as important when adding mineral filler. This can be accomplished by one of two ways:
1) Weigh all the material through one flow scale or weigh pod and vary the speed of the added mineral filler to the proper proportion
2) Divert all dust from the baghouse into the mineral filler silo and weigh the silo output with weigh pod or flow scale. This method reduces the amount of mineral filler storage. This system works well when hydrated lime is being used as mineral filler because the dust dilutes the hydrated lime and makes hydrated lime more flowable.
Many plants do not have equipment to smooth out dust sags and surges. This investment can be absorbed in all the mixes processed by using the mineral filler silo to smooth out dust sags and surges. Dust can be controlled by diverting the baghouse dust through the mineral filler silo for just the cost of transferring the dust into the silo.
The schematic below illustrates how easy it might be to retrofit an existing facility to control dust in all applications, add mineral filler and compensate for baghouse dust differences required for most SMA mixes. Add a silo and hot leg next to the dust incline auger to control downstream flow of both.
SMA dust makeup control
When used to make SMA, a 10-percent total minus 200 is needed in the mix — dust is augured directly to the flow scale and into the drum, the flow scale reports not enough minus 200 is being fed into the drum, the controller increases the feed from the mineral filler silo into the incline auger, the flow scale reports the increased flow, the controller adjusts until satisfied.