In Finland, mechanical pulps are manufactured for use in paper and board either by grinding roundwood (SGW, PGW and SPGW) or by refining chips (RMP, PRMP, TMP and CTMP). Their manufacture is integrated with that of paper and board, and so water circulates from the grinding department to the headbox, bringing along both heat energy but also all the components dissolved from the wood into the forming section of the paper or board.
In closed water circulation, the content of disruptive substances rises when less surplus circulation water is produced. If the dissolved solids are removed from the surplus circulation water and the clean filtrate, the need for fresh water in the system is reduced.
"A number of studies show that pulp contains a great many substances that disrupt the process. The sugar content of spruce varies, depending on the season, from one to five kg per ton. The extractives in spruce are mainly fatty acids, pitch, sterols and lignins, which amount to around 10-15 kg per ton", says Göran Lindholm.
It is relatively simple to separate all the groups except the triglycerines and sterols. The alkaline conditions used in connection with peroxide bleaching of pulp contribute many additional disruptive substances such as carboxyl acids.
Water circulation can be closed by using ultrafiltration, flotation, evaporation and biological treatment. It's easiest to circulate the water from the coating plant, which can be treated by ultrafiltration. A few nearly "clean" plants employing this method are already operational.
The main problem at paper mills is the paper machine itself, which needs 50 liters of clean shower water per second, or 4000 cubic meters per day, in the press section. Most of the impurities can be eliminated by ultrafiltration and flotation but the rising salt content becomes a problem.
"The main problem is caused by organic compounds with small molecules such as methanol, ethanol, formic and acetic acids and the terpenes. Treatment by evaporation is hindered by the fact that the boiling points of many substances are below that of water," says Esko Jantunen of PI-Consulting.
A greenfield newsprint mill using mechanical pulp as raw material with no process effluent has been designed by PI-Consulting Ltd. A mill like this normally consumes five to 10 cubic meters of fresh water per ton of paper and discharges the water to a nearby recipient after use. The mill produces 725 tons of newsprint per day.
The mass and energy balances, including those for the wood room, TMP and PGW plants, and the paper machine, were calculated with the company's RAMI® simulation program. If the intake of fresh water is reduced, organic material dissolved from the wood tends to build up in the process water, causing problems in the papermaking process. In the simulation, the amount of water treatment was seven cubic meters per ton of paper. Besides the normal water treatment methods such as flotation, two internal methods were employed: evaporation and ultrafiltration.
Effluent from the wood room together with condensates from the pulp lines totaling 2,600 cubic meters per day are treated by evaporation. Other water (2,400 cubic meters per day) from the pulp mills, paper machine press section and the sewers is treated by ultrafiltration.
Flue gases from the bark-fired boiler are first filtered and then washed in an alkali scrubber. The water can be further treated by evaporation and then fed into the boiler.
A membrane system was installed in Finland by Metsa-Serla Kirkniemi Mills for treating presswater from its magazine mill operations. Previously, the company had used sedimentation, or the Kratzer clarification process. Based on the first machine's online performance, Metsa-Serla Kirkniemi decided to extend its ultrafiltration plant with 15 more CR membrane filtration units.
Saint Laurent Paper
Saint Laurent Paper Products Corporation's (SLPPC) West Point, VA facility (formally Chesapeake Paper Products Company) is a turn of the century pulp and paper mill. The mill produces both hard wood pulp, soft wood pulp, and secondary fiber. The power plant provides the steam required for the process from two black liquor recovery boilers and four power boilers. As is typical in a pulp and paper mill, about 60 percent of the steam condensate is returned for use as boiler feedwater. The remainder of the boiler feedwater is produced by processing water from the mill operated wells. Historically, the treatment of the water consisted of strong acid cations, vacuum decarbonation, strong base anions, polishers (mixed bed and split bed demineralizers). The average rate of production of boiler feedwater is approximately 230 cubic meters per hour (1000 gallons per minute).
In 1993, the process of developing and evaluating options was initiated by the Power Plant supervision. At the time, the major issue was the need to increase the ability to effectively neutralize the feedwater plant's effluent. In the process of identifying the potential solutions, two different courses of action were identified. The first was to increase the size of the neutralization system. The second was to reduce the amount of acid and caustic regenerant used so that the neutralization requirements were reduced.
There were proformas developed on a new reverse osmosis system located in front of the present demineralizers. By installing this equipment, the number of regenerations would decrease and thus the total amount of neutralization needed would decrease. The option to increase the neutralization area was approximately $1.75 million above the reverse osmosis options. Therefore, installing the reverse osmosis system in front of the present demineralizer system was the most economical choice.