New mixer reduces energy costs, chemical consumption at pulp facility, Solutions!, Online Exclusives, May 2003

By David Lazare

In a drive to reduce manufacturing costs, the Alabama River Pulp Company, the largest market pulp mill in the United States, took a look at its bleaching process. Replacing two older mixers with one new Optimix mixer from ITT Industries’ Goulds Pumps unit increased mixing efficiencies, lowered chemical consumption and greatly reduced the energy required to run the bleaching process.

The Alabama River Pulp plant is a large pulping complex located along the Alabama River in Perdue, Alabama, USA. It consists of two mills, pulping hardwood and softwood respectively. Each mill produces about 400,000 tons per year of bleached kraft pulp. The mill’s major processes consist of continuous digesters, bleach plants, drying machines, chemical recovery plants, and associated washers, screens and holding tanks.

In 2000, the Alabama River Pulp plant began to look at improving manufacturing processes in an effort to reduce overall manufacturing costs. When the plant switched to 100 percent chlorine dioxide bleaching from using both chlorine and chlorine dioxide, a number of changes were made in the entire bleaching process-including the mixers at the front end of the process. According to Bob Amend, fiber line manager at the plant, "Our bleaching process was using two shear mixers. We were having maintenance and metallurgical problems with the mixers." Ilkka Seppa, an engineering consultant for medium consistency pulp manufacturing, suggested that the plant provide a trial for a new mixer from Goulds for the bleaching process.

As Tony Chastain, sales engineer for Goulds Pumps explains, "In producing a white fiber, the pulp is put through a multi-stage bleaching process. The more effectively you can mix the chemicals you are adding, the less you have to use and the more consistent product you can produce. The mixers that Alabama River Pulp was using were old and used a lot of horsepower." Chastain goes on to say that their consulting engineer knew of the efficiencies that could be achieved by using the Goulds Optimix mixer.

To avoid chemical waste, a pulp mill removes water from the pulp prior to bleaching. The resulting thick pulp, known as medium consistency pulp, resists any attempt to mix with chemicals due to the nature of the dense fiber matrix. To promote chemical mixing, mechanical mixers are used to disperse the chemicals. Conventional large blade agitator mixers are not effective in medium consistency pulp. The blade speed is too slow to generate the energy needed to separate fibers within the flocs of pulp to allow contact with the chemicals. Unless the fibers are separated allowing chemical contact, mixing will be ineffective. Ineffective mixers require pulp mills to add excessive chemicals leading to residual chlorides and reduced pulp brightness.

"The Optimix mixer introduces a lot of shear to the fluid," said Chastain. "You may have several thousands of gallons of stock per minute going through the mixer and you have a fairly small amount of chlorine dioxide molecules that you are trying to distribute widely and evenly throughout the stock." In addition to thorough mixing, one of the characteristics of chlorine dioxide is that it vaporizes at 55 degrees F. Because the stock is at about 170 degrees F. "They want to get the fibers and the chlorine dioxide next to each other to start the bleaching process before the chlorine dioxide flashes to a gas," said Chastain.

The "double shear" method
In the development of the Optimix mixer, Goulds Pumps launched an exhaustive development program to produce the most effective medium consistency mixer. It was felt that, to be highly effective, mixing must take place in three planes simultaneously. The Goulds design creates pulp turbulence in a horizontal, axial and vertical plane through a unique method trademarked as DoubleShear™.

David Peschell, global marketing manager for the pulp and paper industry for Goulds Pumps, explains. "This is accomplished with a special rotor spinning within a fixed casing with internal baffles or blades, aided by a unique premixing injection of chemical into the intake flow of the pulp."

The mixer’s stationary casing uses angled internal vanes that move pulp from front to back inside the casing, helping to promote turbulence and mixing. The casing design was developed as a straight-through shape that is bolted into the pulp line, allowing simple pulp mill installation with the least amount of pressure drop across the mixer.

Within the casing is a special rotor, shaped with angles that move pulp laterally and axially as the rotor turns. The holes in the rotor promote pulp flow through and around the rotor, creating a turbulence to mix chemicals at a highly effective level.

Higher efficiency, lower costs
As at most pulp mills, Alabama River Pulp makes its chlorine dioxide on site. "Clorine dioxide is pretty expensive-about 35 cents per pound. Alabama River Pulp was able to reduce the use of chlorine dioxide by several pounds per ton," said Seppa. In a plant that produces in excess of 2,600 metric tons a day of pulp, 365 days a year, the chemical savings are significant. As Bob Amend, fiber line manager for Alabama River Pulp, noted, "We greatly reduced our chemical costs on the front end of the bleach plant. We had big chemical savings right off the bat."

The reduction in chemical usage also creates savings after the manufacturing process. According to Seppa, with the Optimix mixer, the plant is reducing the chemical loading in the wastewater streams going into the water treatment plant.

In replacing the two older mixers with one Optimix mixer, the plant reduced its horsepower requirements by approximately 70%. The two original mixers used in the bleaching process were each powered by 350 horsepower motors. The Optimix mixer is powered by a single 250 horsepower motor. According to Seppa, at a typical pulp plant, per horsepower costs average about $240 per year. The energy savings alone from the Optimix mixer is over $100,000 per year.
Maintenance was not an issue with the Optimix, said Amend. "We basically installed the Goulds mixer, turned it on, and have not had any problems with it," he reported. "We’ve had problems with our all our other mixers including spinning the nuts off the shafts and losing impellers. The Optimix has saved a tremendous amount of money in its both mixing efficiency and in its reliability. It is heavy duty."

Seppa also notes that the original mixers had a number of maintenance problems associated with them. "Since the installation of the Optimix mixer, there have been no maintenance issues at all, no vibrations or any other problems."

Improvement in mixing efficiencies
The mill installed the Optimix mixer at the end of 2000, and is "now producing whiter pulp with greater efficiencies and less chlorine dioxide added," said Chastain.

"We measured the mixing efficiency of a new Goulds mixer in a normal mill operation at 96.5 percent," says Seppa. "The older mixers were at 90%. When we originally looked at replacing the mixers, we would have considered keeping the same level of mixing effectiveness as a success. But in this case, we even improved on that efficiency-and that surprised us."