New chemicals, white water management, and retention control improve efficiency for machine-finished specialty and newsprint producer

UPM-Kymmene Mill Reaps Benefit Of Stabilized Wet End Chemistry

Papermakers are constantly searching for stability in their papermaking operations. Once stability is achieved, they can then focus their attention on process efficiency and quality optimization rather than chasing unpredictable upsets.

This elusive search for stability is particularly relevant to mechanical pulp grades where, in recent years, many mills have turned to specialty products with a variety of basis weights and customized properties. Fillers are now commonly used in these specialty grades and in so-called "improved newsprint."

But the transition from yesterday's news is often hard for papermakers to handle. With changing grade structures and frequent grade changes, the control of wet end chemistry, retention, and white water quality has been troublesome in many cases.

ESTABLISHING STABILITY. UPM-Kymmene in Kajaani, Finland, is a good example of a mill that has established stability in its wet end operations and has experienced the benefits. Paper machines No. 3 and No. 4 at the Kajaani mill are now running with improved efficiency through better management of their white water recirculation systems, improved knowledge and control of wet end chemistry, and new retention aid products.

UPM's PM 3 is a 7.65-m trim Sym-Former R machine that produces 210,000 tpy of machine-finished specialty grades, with grammages from 45 to 55 gsm. The furnish for PM 3 is thermomechanical pulp (TMP), pressurized groundwood, and a small kraft component. PM 4 is an 8.4-m trim Sym-Former machine that produces 220,000 tpy of newsprint with grammages from 42.5 to 48.8 gsm. The furnish is mainly TMP with some pressurized groundwood.

The control of wet end chemistry on PM 3 was a particular challenge since the machine runs multiple grades with varying brightness levels up to 72 ISO. The furnish type-either TMP or groundwood-and the degree of bleaching varied between these grades.

In 1991, PM 3 was producing directory grades ranging from 36 to 55 gsm, which were "difficult to run," according to Kari Isokäänta, production superintendent of the MF Specialty mill. To increase opacity, the mill introduced Hydrex, a synthetic filler, into the papermaking system. At the same time they also implemented a retention aid program supplied by Nalco to optimize retention of the filler and promote its opacifying properties. Although the mill achieved its goals of retaining fillers, the stabilization of the wet end operations would require some reengineering of their white water handling systems.

The forming section of paper machine No. 3 at UPM-Kymmene's Kajanni mill is a Valmet Sym-Former R.

TURNAROUND IN 1995. In 1995 and 1996, several interrelated changes to white water handling, chemical formulation, and the automated control of retention of PM 3 and PM 4 would lead to significant improvements in the stability of retention and white water quality. After these changes, production efficiencies showed a marked improvement.

First, the white water systems of the two machines were separated. Previous to the installation of a new disk filter in May 1995, the white waters of the two machines were mixed, causing instabilities in the white water fines content and upsetting retention. Chemical feed rates were difficult to control in this environment.

Now, all the white water is filtered. Clear filtrate with consistent quality is returned to the TMP and groundwood mills. Filtrate is also used in wire showers equipped with new nozzles that consume less water. The white water systems are now more closed. Water use on PM 3, including dilution water, has been reduced from about 11 to 9.5 m3/metric ton.

NEW COAGULANT, BETTER CONTROL. Second, Nalco introduced a new hybrid coagulant product to the mill in early 1996. PM 3 and PM 4 use a dual-polymer system that consists of a high molecular weight polyacrylamide flocculent and a coagulant polymer. New hybrid coagulant polymers are the result of research to improve the retention of fillers, pitch particles, and fine colloidal material in the sheet, thereby producing a cleaner recirculated white water and avoiding machine deposits.

Hybrid coagulants are customized copoylmers that strike a balance between cationic charge density and molecular weight in order to retain fine and colloidal particles without sacrificing formation or drainage. Published studies show that the hybrid coagulants can produce better filler and pitch particle retention at a lower dosage than a reference coagulant based on polyethylene imine.

In March 1997, the mill commissioned a Valmet Automation/Kajaani RM-200 retention measurement and control system. On control, the RM-200 adjusts the chemical feed rate based on a continuous measurement of the retention on the bottom wire. The instrument calculates both total and filler retention.

According to Isokäänta, the combination of these changes has resulted in significantly better stability on PM 3. "Before, with mixed white water systems, the white water consistency and chemical feed rates were hard to control," he says.
"This variable white water consistency also caused difficulties in the TMP and groundwood mills. Now, our white water consistency is at the same level even when running different grades. With the RM-200 on control, retention targets are achieved quickly after startups. This helped us a lot."

CLEANER SYSTEM, IMPROVED EFFICIENCIES. Isokäänta also reports that since the new hybrid coagulant was added for pitch control and retention of fine colloidal materials, turbidity levels of the white water were lower and more even, indicating a cleaner white water system and better control over it. He also says that the new coagulant is "more efficient," achieving better results with less chemical addition.

Before the introduction of the hybrid coagulant, chemical use was about 1 kg/metric ton. After the coagulant was introduced, the feed rates were between 300 to 600 g/metric ton. There is a corresponding reduction in the chemical cost per metric ton-a win-win situation for the mill.

Figure 1: Turbidity of the white water on UPM's PM 3 machine has been reduced significantly and is now much more stable.

Figure 1 shows the significant improvements in white water turbidity during the four year period from 1994 to 1997. Since mid-1997, the turbidity levels have been exceptionally low and consistent. The RM-200 retention control, which was commissioned in March 1997, produces a more stable white water consistency, which allows higher retention levels and a corresponding drop in white water consistency.

Figure 2 shows that pitch count levels in the white water were reduced significantly during 1997. This indicates that the pitch is being retained in the sheet and removed from the white water circulation system. During this period the mill changed the chemical formulation to retain more pitch.

Figure 2: Following a change in wet end chemical formulation on PM 3, pitch count in the white water has been lowered, indicating that more fine pitch particles are being retained in the sheet.

The speed of PM 3 has increased significantly. Prior to 1995, speeds were at 1,200 mpm or less. Now PM 3 is running at a top speed of 1,330 mpm. Machine efficiencies have increased several points since early 1996 when the new coagulant was added (Figure 3). The mill also made changes in the dryer section to improve runability. The new equipment includes vacuum rolls in the dryer section, twin-run blow boxes, and ropeless sheet threading.

Figure 3: PM 3 efficiencies have been increased since the new coagulant was introduced in early 1996. The mill also made dryer section improvements.

At the same time, the PM 4 newsprint machine has experienced significant improvements in the cleanliness of the white water, according to Erkki Rauhala, development engineer, newsprint mill. He also reports decreased levels of pitch, better runability, and improved drainage on PM 4.

UNDERSTANDING WET END CHEMISTRY. The UPM-Kajaani mill values the increased knowledge of wet end chemistry and the control it gives them, so they feel it is important to keep track of key process variables. Important process and furnish measurements such as retention, turbidity, charge, and pitch count are made daily by the onsite Nalco representative. A periodic cross-check is provided by the mill laboratory. With the representative's reports, the mill keeps track of performance and chemical consumption on a day-by-day basis. They can also diagnose process or equipment problems.

The mill is a firm believer in wet end chemistry control and the stability that it can introduce. Isokäänta says, "Understanding wet end chemistry is more important today. With the proper control, it is a competitive advantage. And, when you are running better it is easier to make good quality."