Continuous blender increases production, diminishes heat buildup

Reprinted from Powder and Bulk Engineering, January 2002

CJB Industries, Valdosta, Ga., custom-formulates, processes, and packages chemicals and provides research, development, and engineering services. CJB's newest production line is for a high-loading solid that combines 50 percent liquid acid with 50 percent dry clay product, resulting in an 8- to 16-mesh dry corrosive granular material that resembles kitty litter. The customer for this material formerly had it produced by another custom processor that used a batch tumbling mixer in its processing line. But the original custom processor couldn't meet the customer's needs because the material's long residence time in the batch mixer caused heat buildup, often ruining the material. As a result, the original custom processor wasn't producing the quantity of material that the customer needed. To meet the customer's needs, CJB set up a new production line and installed a rotary continuous blender to properly produce the material.

Long residence time, low production

To produce the high-loading solid, the original custom processor used a batch tumbling mixer set on a load cell to weigh the amount of material in the mixer. An auger fed the dry material into the mixer while a pump metered the liquid acid into the mixing chamber. The mixer blended the ingredients for 15 minutes, a residence time that caused excessive heat buildup and frequently ruined the material. When this happened, the operators were forced to stop the process, wait for the batch mixer to cool down, and then discard the material. If the material was produced at the appropriate consistency without overheating, it was dumped from the mixer into a packaging hopper and then bagged — a process that took between a half hour and an hour. Then, the batch-by-batch process would begin again. The custom processor was only producing 1,000 tons of material per year, not enough to meet the customer's needs.

After a year of inconsistent quality and inadequate quantity, the customer approached CJB to do the job. CJB contacted Munson Machinery, Utica, N.Y., a mixer and blender supplier that had worked with CJB on previous mixing and blending projects. CJB knew the supplier could provide a mixer that would get the results CJB needed. The supplier recommended a 16-inch-diameter by 4-foot-long mild steel rotary continuous blender with a throughput capacity of up to 100 ft3/h .the smallest blender the supplier manufactures. The blender can be manufactured up to 16 inches in diameter by 15-feet-long with a 3,000 ft3/h throughput capacity and can be made of stainless steel or abrasion-resistant steel, and can be epoxy- or Teflon-coated.

Because of the high-loading solid's hazardous nature, CJB decided to test the material at its plant in Georgia instead of following the usual practice of testing at the mixer supplier's plant. CJB leased the recommended blender from the supplier, and over 2 weeks tested the material in the unit. The results were good. CJB found that the blender's mixing action created a gentle cascading motion and didn't cause material degradation; the material's 2-minute residence time in the blender's mixing drum prevented heat buildup; and the blender combined the liquid acid and dry clay product to the customer's requirements. Consequently, CJB purchased the blender.

The rotary continuous blender

The rotary continuous blender's mixing vessel is a long, narrow, enclosed rotating drum. The drum's internal mixing element's four mixing flights tumble the material as the drum rotates. The mixing flights are creased in a Z pattern, which causes half of the material that drops off the flights to move toward the drum's front end, while the other half moves toward the drum's back end. As a result, material is consistently and gently pushed forward and backward a number of times during each of the drum's revolutions. The blender thoroughly mixes the ingredients in one quick pass (the amount of time needed for the material to move from the blender's inlet to its discharge). In CJB's case, one pass takes less than 2 minutes.

In a ribbon blender or paddle mixer, the mixing elements plow through the material, forcibly pushing it around the mixing chamber. But in the rotary continuous blender, the mixing drum's rotation and mixing flights' design allows gravity to do the work. This keeps the blender's horsepower requirements extremely low. And, because the material isn't compressing against itself as it does in a ribbon blender or paddle mixer, the rotary continuous blender produces less heat buildup.

Doughnut-shaped end plates (or weirs) on the blender's discharge end also help reduce heat buildup. The weirs allow the material to overflow through the doughnut hole, discharging to the next processing step. The weirs' wall height determines the material's residence time in the blender's mixing drum, preventing material from remaining there too long, which can cause heat buildup.

CJB's high-loading solid production line

To produce the high-loading solid, CJB uses a weighbelt feeder to feed the dry material to a bucket elevator. The bucket elevator dumps the material into the rotary continuous blender through a low-profile, stationary gravity-feed inlet chute. As the material is gently mixed in the blender's mixing drum, spray nozzles located inside the mixing drum spray the liquid acid onto the material. The material discharges from the blender onto a sloping inclined belt conveyor that moves the material to a hopper. The hopper drops the material onto a screener where it's classified prior to being bagged.

Meeting the customer's needs

Since installing the rotary continuous blender in its high-loading solid production line, CJB has experienced increased productivity and reduced heat buildup compared to the original custom processor. With the rotary continuous blender, CJB is able to continually produce the high-loading solid at 10,000 lb/h, and it only takes 2 1/2 minutes to get the material from the blender to the bag. Clinton Beeland, CJB's president, says, "The blender's continuous motion, reduced residence time, and gentle cascading action have eliminated the heat buildup that batch mixing used to cause the original custom processor. We're now producing more than 6,000 tons per year — that's 5,000 more tons than the original custom processor did."

When the mixing is completed, a sloping inclined belt conveyor moves the high-loading solid to a hopper that drops it onto a screener where it's classified prior to bagging.

The rotary continuous blender's mixing vessel is a long, narrow, enclosed rotating drum that uses gravity to mix the material.