According to Daniel Hammar, Development Engineer PM 10 at the Stora Enso Grycksbo mill, there were two goals when the charge control loop in the broke treatment system at the fine paper mill was installed: One goal was stabilizing the demand for retention aid on PM 10 and the other was reducing fixation agent and retention-aid costs. Both goals were achieved by controlling the charge in the broke reject with a TSS Thick Stock Sampler and PCT 20 Charge Titrator. Daniel Hammar, Development Engineer PM 10 at the Stora Enso Grycksbo mill.
Broke the major source of charge variation
Broke containing coating, and therefore negatively charged anionic trash, is the major source of charge variation in the furnishes for the PMs at Grycksbo. Otherwise the furnishes contain very clean, bleached hardwood and softwood pulps for the production of high-quality fine papers.
Broke from all three PMs is mixed and treated in a single system before it is added to the paper machine furnishes. See Figure 1.
The amount of coating, and therefore anionic trash, on the broke entering the system can vary greatly, depending on where the broke comes from before, during or after coating. Obviously, the amount of coating applied to the broke also affects variation.
As shown in Figure 1, the TSS and PCT are positioned where the broke is distributed to the three PMs after leaving the broke treatment system. The fixative is added earlier in the system. The job of the PCT is to control and optimize the amount of negatively charged anionic trash into the PMs, the result of which is optimized consumption of fixative and retention aid. See Figure 2.
More fixative, less retention aid equals lower costs
According to Daniel Hammar, Development Engineer PM 10 at Grycksbo, a lot of trial and error took place during the first nine months of operation of the TSS and PCT: "Using these instruments in conjunction with the retention-aid control loop on PM 10 allowed us to test our ideas objectively.
The retention-aid control loop was not the only reason the study focused on PM 10. At a machine speed of 1100 m/min, it runs two to three times faster than the other two PMs and produces papers at lower basis weights, factors that increase retention-aid consumption and thereby costs.
The trials amounted to trying different setpoints for the charge of the broke to determine the affect on retention-aid consumption. It was found that a lower charge setpoint resulted in lower levels of retentionaid consumption on PM 10.
"Thats what its all about. Our fixative is much cheaper than retention aid," says Mr Hammar, adding, "the saving from use of less retention-aid should permit us to pay for the costs of the TSS and PCT in six to 12 months or less, depending on the charge setpoint."
Rented the equipment before purchasing
Grycksbo ultimately purchased the TSS and PCT but was able to rent it in order to test its efficiency. "BTG was the only supplier that would work with us under these circumstances," says Mr Hammar.
According to Mr Hammar, the reliability and availability of the equipment is better than expected. In terms of maintenance, he notes that it is necessary to pay attention to the operational requirements of the instruments for them to work properly.
Mr Hammar notes that the PCT can affect quality, runnability and machine speed, but in his eyes charge control is only one among a number of items that can influence such factors.
THE STORA ENSO GRYCKSBO MILL
in Sweden produces 240,000 tpy of matt-coated fine paper in basis weights ranging from 80 to 250 g/m2 on three paper machines, PM 10 PM 9 and PM 7. The raw material for production is bleached hardwood and softwood sulfate pulp.