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BOWATER THUNDER BAY, ONTARIO, KRAFT MILL REDUCES DEFOAMER USAGE WITH NEW SILOXANE TECHNOLOGY

Glenn Mudaly, R&D Specialist - Buckman Laboratories of Canada and Mike Stranges, Process Engineer - Bowater Thunder Bay

(Presented at PAPTAC 92nd Annual Meeting 2006)

Problem Identification

Bowater Thunder Bay, an integrated Kraft mill located in Canada, producing 1700 t/day pulp, used oil-based defoamers for many years in the pulping process to combat foam. High defoamer usage (1.40- 3.00kg/t) created deposit build-up in the cleaning system and high dirt count excursions which required boil outs to the cleaner system in the bleach plant periodically. The mill produces both HWD and SWD. The driver for the conversion from an oil based defoamer to Buckman's water based siloxane technology was cost and reduced deposit potential. The goals were to reduce the defoamer programme cost and reduce pitch control dispersant chemistry cost without compromising productivity and quality.

Problem analysis

The mill identified there was a need to reduce the overall cost for the defoamer programme and improve system cleanliness but at the same time maintain productivity.

Oil based defoamers are dispersions of waxes and/or silicas, with dispersants, emulsifiers and a variety of materials to modify the surface activity of the defoamers and/or the process liquors in which they may function. The unfortunate side effects of these chemicals become apparent as they are pushed to their maximum. "Pitch" or dirt deposits in paper are found to contain large amounts of high molecular weight fractions of the oils, or agglomerated particulates of the insoluble amide waxes, as are compounds fouling wires and felts in subsequent paper making operations. The products are found to contain greater or lesser amounts of dioxin and furan precursors contributing to the safety concerns surrounding goods manufactured from bleached pulps containing residual defoamers. Buckman Laboratories has now addressed these and many other mill specific concerns and process chemical requirements. The result is New Siloxane Technology; highly proprietary foam control agents for the pulp industry of today, based on the "cutting" edge of organosiloxane chemistry.

Problem resolution

The New Siloxane Technology proved to be the solution. Buckman approached this problem by developing a "custom blend" based on the mill's specific requirements. This product was developed to not only control foaming during the pulping process but improve drainage on the washers and to lead to profitable pulping. The New Siloxane Technology balances polydimethylsiloxane chemistry, silica chemistry, and organomodified siloxane chemistry with the chemistry and process conditions of the black liquors in brownstock washing. Unlike oil based defoamers,and conventional silicone emulsion defoamers, the New Siloxane Technology products are formulated to function at much lower dosages, and to possess optimal foam and drainage control for specific liquors and processes. This is to eliminate any potential impact on product or process, and to utilize water as the carrier medium for the active ingredients rather than potentially detrimental mineral oils.

Polydimethylsiloxane chemistry and its derivatives are unique in that they possess the following properties that are advantageous in pulp production:

  • very low surface tension
  • excellent spreading characteristics
  • non-toxic and chemically inert
  • resistant to high temperatures
  • highly effective
  • long term efficiency in alkaline medium
  • low environmental hazard
  • organomodified siloxanes exhibit low dynamic and equilibrium surface tensions.

The combination of the mentioned physical properties resulted in the development of highly efficient foam control agents and drainage aids.

These organomodified siloxanes are modified polydimethyl siloxanes that make them compatible with, or soluble in, aqueous and/or organic systems. This was achieved by incorporating various organic moieties. Furthermore, this type of modification makes it possible to produce specific physicochemical interactions or chemical reactivity with, or in, a substrate phase. In addition to their pronounced surfactant properties in aqueous and organic systems, these organomodified siloxanes are primarily characterized by their multi -functionality. These products reduce surface tension much more than organic surfactants.

The general chemical structure of these organomodified siloxanes is shown below:

Polydimethyl siloxane Viscosity: 50 - 100,000cps

Bowater fig a

Bowater fig b

Based on the results of the laboratory evaluations, the mill proceeded to evaluate the programme in the first quarter 2003. Before starting the evaluation, a Buckman Transition Workshop took place with the people involved in the process. The mill defined their requirements and success criteria. A joint trial protocol and action plan was formulated prior to start-up.

Defoamer Evaluations and Results

The Bowater Mill in Thunder Bay has two fibrelines. The B fibreline has a Kamyr Digester followed by a pressure diffuser, 2-stage atmospheric diffuser and 2 high-submergence Ingersol Rand vacuum washers. This fibreline swings from hardwood to softwood production. The A fibreline also has a Kamyr Digester followed by a pressure diffuser and 2 Canron rotary vacuum washers. After screening, the pulp is further washed on a Sherbrooke decker. It was on this line where several excursions of high dirt off-quality pulp were experienced in late 2002. The dirt was confirmed to be mainly defoamer components that originated from build-ups in the cleaning system after the bleach plant. Only softwood is made on this line. After boiling out the A-side cleaner cones in early 2003, the Mill ran trials using siloxane defoamers from two different suppliers. The Mill was aware that the oil-based defoamers were favoured in high temperature applications and that the water-based emulsions had a shorter active life for defoaming. Nonetheless, each supplier had free reign to select defoamer addition points and work with the process operators over a 2- week time frame. The trial criterion for success included:

  • the ability of the defoamer to be used on both fibre lines whether on hardwood or softwood
  • a measured reduction in defoamer carryover to the A-side cleaning system
  • a reduction in overall defoamer costs .

The conversion on both fibrelines to the New Siloxane Technology despite a variety of particularly harsh process conditions underwent successfully. Although different addition points were tried, the previously established feed points used for the oil-based defoamer proved to be the best. Criterion 1 had been met.

The main area of focus was the A-screen room (A decker) defoamer use since this fibre-line was the one which experienced the frequent off-quality pulp due to defoamer dirt. The defoamer usage reductions on the critical A-line were 35 - 40% and the A-screen room defoamer use, closest to the bleach plant, was reduced 40 - 50%. The B-side reduction on hardwood runs was almost 50% and little reduction was seen on softwood runs due to the short time span available during the trials. The initial reductions were encouraging and indicated further reductions were possible.

The Mill had taken pulp samples from various points on each fibreline for defoamer carryover tests before the trials on oil-based defoamer and during the siloxane trials. This was an attempt to quantify any reduction in potential defoamer deposit build-up in the cleaners from the shearing off of defoamer from the fibre. Table 1 outlines the results.

Bowater table 1

The defoamer carryover tests show a dramatic decrease in the defoamer load exiting the bleach plant to the cleaner system with the new siloxane technology defoamer. The oil -based carryover also varied with feed rate. Interestingly, the defoamer carryover increased on the B-side across the bleach plant and was quite high. Unfortunately, no B-side base-line samples were collected but this fibreline uses talc for deposit control and some defoamer is also used in the bleach plant proper which explains the increase in carryover.

A pitch box containing 4 plates samples the A decker feed stock and the defoamer deposits were averaged over a daily period before, during and after the siloxane defoamer trials. The results confirmed the defoamer carry-over tests given above. Figure 1 shows the pitch box trend.

Bowater fig 1

The results of the pitch box catches and the defoamer carryover tests suggested a reduction in defoamer carryover into the bleach plant. However an inspection of the A-side cleaner cones showed a slight deposit which was identified as siloxane defoamer. Consultation with Paprican provided us with some guidelines for defoamer use targets with respect to deposit potential as indicated in Table 2.

Bowater table 2

TABLE 2: Guidelines for feed rates of defoamers. (Total flow if multiple points of addition)

Oil-based defoamer use on the A-side was over 1.5 kg per tonne at the time of the high dirt excursions and probably well over 2 kg per tonne. Although defoamer usage was reduced during the trials by 40 -50%, it was clear that for criteria 2 and 3 to be met, further reductions in defoamer were needed. The Mill target for the A-side was 0.25 - 0.4 kg per tonne and overall less than 0.2 kg per tonne.

Over the course of the last 18 months, optimization of defoamer use continued to meet the Mill targets. It was evident to the Mill that the critical defoamer feed points had to be measured and controlled. A feed system complete with variable speed pumps, interchangeable heads and magmeters for flow measurement was installed. The magmeters frequently read incorrectly or not at all and another measuring element was needed to get "closed-loop" control of the defoamer feed. In the interim the pump speed and defoamer flows were constantly measured and targets based on pump speed were noted. These were edited every time smaller pump heads were installed as defoamer flows were reduced. This measurement made it easier to determine kg per tonne targets for future use.

 

In mid 2004, the Mill trialled Coriolis meters to measure defoamer flows. The unit trialled had a totalizer function which allowed a comparison between measured flows over time to the volume collected in a container over that same time period. The results were consistently within 5% and the Mill replaced the magmeters for Coriolis meters on all critical defoamer feeds. This allowed closed loop control of the defoamer such that defoamer use targets in kg per tonne were used for each critical addition point. The defoamer flow now varied with production rate. The operator has the option to run his own flow set-point in the event of upset plant conditions but the defoamer flow controllers will track the target flowrate when not in remote control.

The result of closed loop control using the New Technology Siloxane defoamer is shown in the graphs below.

Bowater graph 1

Bowater graph 2

The Mill finally met all criterions for the switchover to the New Siloxane Technology defoamer and where high quality bleached pulp is produced at any given time without unnecessary downtime because of boil-outs to cleaners etc. This technology was developed to meet the demands of the pulp industry and to offer economic advantages and embraces all aspects of the environment. The overall defoamer dosage has been reduced to less than 0,30 kg/tonne of pulp produced. The return on investment should be focused on defoamer and pitch control programme reductions that were only possible because of this leading defoamer/pulp washing technology. Overall return on investment for the mill was approximately $800K in defoamer and pitch dispersant savings, while improving system cleanliness.