Continuously mixing one batch at a time

Ready-mix company improves mixing efficiency and ingredient accuracy by switching to a rotary batch mixer.

When a concrete and ready-mix producer moved to a new plant, the mixing process was redesigned to include batching for greater control over the final product 's ingredients.

Rich-Mix Products makes concrete and ready-mix products and packages them for sale in home improvement stores across the country. Headquartered in Wichita, Kans., the company also operates plants near Fort Worth, TX, Oklahoma City, OK, Kansas City, KS, Tulsa, OK, and Springfield, MT.

At the plant just south of Fort Worth in Alvarado, Tex., the company makes Quickrete, a line of packaged ready-mix products for retail sale. The plant also produces a grout mix for bulk distribution. Rich Mix's executive vice-president, Gary Kraisinger, said the majority of the plant's 175 different products are ready-mix.

"We are a Quickrete producer; that's our claim to fame," Kraisinger said. "That's primarily why the [Alvarado ] plant was built. The plant's main products are bagged concrete mix, sand mix, and mortar mix. They comprise a large portion of what we do on a day-to-day basis. We sell the bags to retailers like Home Depot and Lowes, who in turn sell directly to the public."

Raw materials for the various products include concrete sand, masonry sand, rock, portland cement, masonry cement, lime, and fly ash. The materials vary in particle size and bulk density.

The rock and sand aggregates are wet when received at the plant. They are dried and stored in overhead bins. The powder ingredients are brought in by tanker and are pneumatically blown into overhead storage silos. The plant is equipped with two processing lines.

According to product specifications, a percentage of each ingredient is fed from the storage silos and bins into a hopper in either processing line for mixing. Powders move to the hopper via an auger while the aggregates are gravity fed.

The ingredients are mixed, and the mixture is then sent to one of two surge hoppers. From the surge hoppers, the ready-mix is packaged in 60-or 80-pound bags, while grout product is packaged in 3,000-pound bulk bags.

Not in the mix

In April 1998, Rich-Mix moved its Texas plant from Irving to the Alvarado site. In designing the new plant, Kraisinger said the company wanted room to expand for future growth. But with the move, the company also wanted to resolve any processing line problems. One such problem was the mixing operation.

To mix the products at the Irving plant, the various ingredients were fed into an end of a product screw. Operating continuously, the screw mixed the ingredients and delivered the blended product to a holding hopper, from which the product was bagged directly.

Dave Smarsh, electrical and mechanical systems supervisor, said the feed-and-mix process didn't allow any ingredient accuracy checks on the final product. For example, if an ingredient was left out or added in the wrong proportion, operators couldn't detect mixture accuracy unless they checked the bagged products.

"There was no way in that process to define whether [ingredient proportions were right] until you took an inventory," he said. "And when we had a problem, we had to reclaim all of the product made because one ingredient percentage wasn't correct."

Off-spec product wasn't scrapped. It could be put back into the system and reclaimed, Smarsh said, but this took extra time. "I wouldn't call it downtime, but I would call it extra hours to redo a product that you've already made once," he said. "The reclaimed product had to be metered in and that meant additional labor cost."

The ingredients were very abrasive, which caused considerable wear on the product screw during mixing. As the screw continued to wear, the amount it could move and mix diminished. "The smaller the screw, the less product you made," Smarsh said. "We didn't get more than 2 to 3 years out of an auger before we had to change it."

Without measuring volume or weighing the ingredients before they were mixed and packaged, it was difficult to determine if screw wear was diminishing product output and if mixture proportions were correct.

"You had no way of stopping this [continuous feeding and mixing process ]," Kraisinger said, "With a batch method you could obviously stop a 10,000-pound batch [to check the ingredients ] before discharging it. But on our continuous system, to stop it and start back up didn't make us as comfortable about accuracy. We decided to go to a batching system that feeds a continuous process, but we needed a way to mix the ingredients. That's how we started looking into different mixing equipment."

Looking for a way to batch and mix in a continuous process

The company began to examine different mixing systems used in the concrete and ready-mix industry. Rich-Mix revisited a process used several years before when the company bought a ready-mix plant in Tulsa, Okla. The plant's processing line included a rotary batch mixer, which received product from a weigh-batch hopper. Kraisinger said the mixer had stood up to considerable wear.

"We were impressed that the mixer had been in there since 1972," he said, "and we changed the [lifters and baffles] on it only 3 years ago. Before that, the thing had never been touched."

Five years ago, Rich-Mix installed similar rotary batch mixers when building a new plant in Springfield, Mo. Again, Kraisinger said the mixer has stood up very well against wear and abrasion.

While the company had success with rotary batch mixers in these plants, Kraisinger said Rich-Mix tried to find other equipment that would work just as well or better at the new Alvarado plant. The mixers the company considered had subtle differences, but unlike the rotary batch mixers, each had to reverse motion to discharge material — a feature Rich-Mix wanted to avoid.

"We looked at pan mixers and some other drum-type mixers, but they had to stop and back up to discharge and we didn't like that," Kraisinger said. "So the continuous discharge action of [the rotary batch mixer ] was a real big selling point. The mixer doesn't stop. That was one of the features we liked."

Pan mixers are used in the cement industry, especially for grouts and special mixes. Yet, while pan mixers blend and discharge product rapidly, Kraisinger said their wear plates are also prone to wear rapidly.

"The wear plates in those have to be replaced every 10 to 18 months," he said, "and that's quite a cost, not to mention the downtime. However, we considered the pan mixer, because it will do the work. The problem was that for our business, those wear plates would wear out too fast."

The design and operation of the rotary batch mixer, where product randomly falls on top of product in a fluidized zone, appealed to Rich-Mix Products. With this operation, the material causes less wear on the mixer's lifters and baffles. Also, Kraisinger said having seen the life span on the mixer in Tulsa "was really exciting to us."

Yet, the Tulsa plant's mixer had a smaller capacity than what was needed at the new Texas plant. Rich-Mix managers traveled to the mixer manufacturer's facility near Utica, N.Y., to see larger capacity mixers and determine whether the mixing action and the wear longevity were the same as those of the smaller mixer.

"We were concerned whether we would have all of the same advantages in a bigger mixer," Kraisinger said. With positive answers to their questions, Rich-Mix managers decided to install two Munson Machinery rotary batch mixers at the new Texas plant.

"From our experience in Tulsa and Springfield, we pretty well had our minds made up that these were most likely the mixers we would buy if we ever did build a new plant," Kraisinger said. "I don't mean to say we didn't look [at other mixers]. We flew around the country looking at other systems, but in every case we kept coming back to what we felt had already proven itself."

Two rotary batch mixers are installed

When the new Texas plant began operating April 22, 1998, it was equipped with two rotary batch mixers, one for each production line.

"We'd run out of space at the other plant," Kraisinger said. "So not only did we want to grow, but we wanted to have the ability to add production later on. We also wanted to build a dual-line system. That's the reason for the two mixers."

Systems supervisor Smarsh said there were many processing considerations when they built the plant "from the ground up." But he said the installation went very smoothly, including the mixers. "The cement weigh batchers, the overhead storage, the entire system just fit together very well," he said. "And the [rotary batch mixer ] fit into that scheme."

Rather than mix via a screw to a hopper, ingredients at the new plant are first loaded and measured in weigh-batch hoppers. There are two weigh hoppers for each production line; one hopper holds powder ingredients and the other aggregate ingredients. Aggregate ingredients are fed by gravity to the weigh hoppers, while screw conveyors feed the powdered ingredients. The material is weighed to determine ingredient proportions. Once enough ingredient for one batch is loaded into the weigh hoppers, it's discharged to the rotary batch mixer.

The mixer's components include a product inlet with a plug gate, a rotary drum equipped with lifters and baffles, and a discharge gate and housing. The mixer has a single self-adjusting face seal at the inlet to allow dust-free operation. The discharge gate uses a seal-less rotating urethane disc that mates with a machined seat. Each mixer has a batch volume of 110 cubic feet, with a total volume of 218 cubic feet. The mixer can handle a maximum batch weight of 11,000 pounds.

"We're batching from 9,000 to 10,000 pounds," Smarsh said. "The reason for that number is that our grout mix is packaged in 3,000-pound bulk bags. So every time you make a batch you have enough to fill three bags."

The rotary mixer operates using what Smarsh calls a "continuous batch" process. A 20-horsepower motor rotates the drum, causing the baffles and lifters to move material on top of itself. The mixer continues to rotate during fill, mix, and discharge steps.

"The one big thing I like is that the mixer never shuts off," Smarsh said. "It takes a batch from [the weigh hopper ], mixes it, and discharges it. But the second the material is discharged, another batch is coming in."

While it batch mixes, the rotary mixer continually operates without reversing direction to discharge material. The internal lifter design ensures complete emptying of the drum during each cycle.

The right stuff

Since the rotary batch mixers were installed at the Texas plant, the company has improved the mixture accuracy of its final products.

"By going to a batching system — which loads precise amounts of ingredients in the weigh hopper and consequently into the [rotary batch mixer] we feel we have a better handle on the entire process," Smarsh said. "We have better accuracy."

With better control on the ingredients before they are mixed and packaged, the company has also benefited by no longer having to reclaim improperly mixed products, improving overall plant efficiency.

The rotary batch mixer uses less power than that required to mix a similar load with other equipment. Kraisinger said part of the power savings results from using gravity to load the mixers.

"Gravity is obviously free," he said. "So if you have to convey products from different points to a mixer or a product screw, it will require more power. With this mixer, we can use gravity to feed some ingredients. You require power for only the mixer's 20-horsepower motor."

Kraisinger said the mixers also operate efficiently. By continually rotating, without starting and stopping to discharge, the mixer operates without power surges. "I think that helps not only the efficiency, but the operating cost as well," he said.

The rotary batch mixers handle a variety of materials with varying degrees of abrasiveness. Thus far, the mixers have experienced less wear than the previous design, resulting in less maintenance downtime and allowing for better batch accuracy.

"This unit tends to be more expensive than anything else we looked at," Kraisinger said. "But we feel the benefits outweigh the cost. We just opened the doors [at the new plant ] in April, but I would say, predicated around the mixer's performance to this point, there would be no question in my mind that if we built another plant tomorrow the mixer would be central to the operation."

Ready-mix ingredients are loaded into the rotary mixer from a weigh hopper. A 20-horsepower motor rotates the mixer's drum.

Mixing one batch at a time, the rotary mixer continually operates without reversing direction to discharge material.