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Bleached kraft mills cannot be closed loop

Northern Pulp in Abercrombie. File
Northern Pulp in Abercrombie. File

In a perfect world, all manufacturing processes would have no appreciable impact on the environment. We all want this, but it is unrealistic to expect no impact. As with other industries, the Pulp and Paper sector has worked to improve its environmental footprint.

At Northern Pulp, the impact of effluent (wastewater) has been reduced by significant ongoing improvements dating back to the 1990s. Since taking over the Abercrombie mill in 2011, Northern Pulp has been making efforts to be as eco-conscious as present technology allows. The disconnect for the public, however, seems to be the perception that simply creating a closed-loop (zero-effluent) system in the proposed new effluent treatment facility will solve all environmental issues, and that Northern Pulp is not willing to create such a system because of its hefty price tag. The reality is bleached Kraft mills (which is what Northern Pulp is) cannot be zero-effluent. It is not a question of money, rather a result of current technology limitations.

There is a substantial difference between bleached and unbleached Kraft processes. Northern Pulp is a bleached Elemental Chlorine Free (ECF) Kraft system, one that produces effluent streams that cannot be recycled in the process. The majority of effluent produced by the process comes from the bleach plant, which is why an open system is required.

Some unbleached Kraft mills can operate in a zero-effluent configuration. An example is the Visy Tumut Kraft mill, New South Wales, Australia. This mill operates a closed-loop system where all of the wastewater is land-applied through irrigation for agriculture. In Canada, unbleached Kraft production represents less than 10 per cent of bleached Kraft production.

“In the mid-70s, the Great Lakes Paper Company in Thunder Bay, Ontario, attempted to install a closed-loop system in its bleached Kraft pulp mill,” Guy Martin, KSH Solutions explains. “After a number of issues, the process was abandoned.”

 

Effluent options for pulp processes. - Sponsored
Effluent options for pulp processes. - Sponsored

 

Other bleached Kraft mills around the world have attempted, and failed, to implement closed-loop systems since then. The difficulty stems from the buildup of chlorides and minerals in the pulping process. Chlorides originating from the bleaching process cause equipment corrosion, just as the same chlorides naturally occurring in seawater corrode metals.  Calcium salts and other minerals are naturally present in both wood and water and lead to significant equipment scaling issues, as homeowners with hard water can attest.

“There are a few mills, such as Paper Excellence’s Meadow Lake Pulp Mill in Saskatchewan, that operate a Bleached Chemi-Thermo Mechanical Pulp (BCTMP) mill with a closed-loop effluent system. BCTMP is an entirely different pulping process, which provides a completely different product than that of a Kraft pulp mill,” explains Martin.

BCTMP employs heat, mechanical action and a pre-treatment with chemicals to separate cellulose fibers rather than chemically dissolving the lignin holding fibers in the wood matrix, such as Northern Pulp’s Kraft process. Because lignin is still present in mechanical pulp, it produces a weaker, lower grade of paper that yellows over time. Mechanical pulp bleaching is accomplished with hydrogen peroxide and/or sodium hydrosulfite. Two mechanical pulp processes were permanently closed in Nova Scotia in part due to the decline in newsprint markets (Bowater Liverpool and Port Hawkesbury PM1).

The addition of Oxygen Delignification to reduce bleaching chemicals by 30 to 40 per cent will have a very positive impact. The Canadian Council of Ministers of the Environment (CCME) guidelines stipulate that all effluent parameters should meet background concentrations of the receiving water in less than 100 metres from the outfall. As effluent has been flowing into the Northumberland Strait for 50 years without diffusion, the new treatment facility and diffused outfall will reduce impact on the Strait, according to Martin.

“The proposed new treatment system for Northern Pulp will be a modern Activated Sludge (AST) system that mills, industries and municipal facilities throughout the world have in place to treat wastewater,” Martin says. This system will replace the current Aerated Stabilization Basin (ASB) technology.

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