Paper360 - November/December 2009 - (Page 22)

LOW ALKALI DEINKING SMARTMILL Low alkali system for ONP/OMG recycling Changes in environmental regulations and lower brightness targets for North American newsprint have caused some ONP/OMG deinking plants to rethink their deinking recipes DAVID R. JONES M ost North American recycled newsprint mills use alkaline floatation deinking. The main chemicals added to the pulper are caustic (sodium hydroxide), peroxide (hydrogen peroxide) and silicate (sodium silicate), and play an interrelated role. Caustic helps detach ink from the fiber but can cause alkaline darkening of mechanical fiber. Peroxide can offset this darkening. Silicate helps stabilize the peroxide and prevent reattachment of the ink. Other chemicals that may be added include synthetic deinking surfactants, fatty acids, calcium chloride and chelants. Caustic also provides alkalinity to saponify the fatty acid in the system to form a soap. Yet, at most ONP deinking plants, this is done in a separate make-down system and then added to the system. the pulper and fatty acid to the flotation feed. Surfactants can also be used without fatty acid as is the case in a number of North American mills. Synthetic deinking surfactants vary widely in their foam characteristics. They do not need the alkalinity or hardness that fatty acid requires, and are not pH dependent. The low alkali system uses well planned step-by-step process changes. It should begin with a clear goal, address all concerns, and help put everyone on the same page as relates to monitoring, communication and decision making. Such a system also provides flexibility to meet changing requirements. If a mill needs to increase brightness or produce a new grade, then caustic and peroxide can be increased to more traditional levels. LOW ALKALAI SYSTEM RESULTS Lab studies done to investigate the viability of the low alkali system, have shown that it is possible to reduce caustic and still maintain brightness and ERIC . Mill A was running a standard alkaline deinking chemistry and needed to reduce sodium in the effluent. Caustic is one of the main sources of sodium. The mill was adding surfactant in the pulper and fatty acid at floatation feed. Both were replaced with a new synthetic surfactant added in the pulper, and only small changes were made with other chemicals such as peroxide and silicate. As a result, caustic was reduced in steps and completely eliminated, THE LOW ALKALI SYSTEM In a low alkali system, caustic can be replaced with a synthetic surfactant to detach ink. This results in less alkaline darkening which can reduce or eliminate the need for peroxide. Peroxide is more effective at higher pH, so as the pulper pH decreases, the bleaching efficiency of peroxide is reduced—but not to zero. In addition silicate provides some alkalinity. Synthetic deinking surfactants also produce foam in the floatation cells and in many cases are used in combination with fatty acid; surfactant is usually added to with no change to recycled pulp quality. Mill B needed to reduce COD (chemical oxygen demand) in the effluent, which would require an overall reduction in deinking chemistry. This mill had a dual loop deinking plant and was using alkaline deinking chemistry. No surfactant was used; fatty acid was added in the pulper and to the feed to the pre- and postfloatation. A synthetic surfactant replaced the fatty acid in the pulper. Step changes resulted in reduction of more than 50 percent of the caustic and all of the peroxide added to the pulper. Fatty acid was no longer added to the pulper, although it was still added to the first loop floatation feed. Peroxide was added at the dispurger between the two loops. Again, the goal was met in that pulp quality was maintained while total chemical usage was reduced and COD decreased. The lowering of brightness targets for newsprint in North America offered Mill C an opportunity to investigate a reduction in deinking chemistry. The mill was using standard alkaline deinking chemistry with a synthetic surfactant added to the pulper and fatty acid to the floatation feed. In this case, no changes were made in the chemicals being used, just the addition rates. Both caustic and peroxide were reduced by about 50 percent, and recycled pulp quality was maintained throughout the transition. David R. Jones is Industry Specialist with Buckman Canada. Contact him at Paper360º November/December 2009

Table of Contents for the Digital Edition of Paper360 - November/December 2009

Paper360 - November/December 2009
Over the Wire ... News Summary
Effluent Clean Enough to Drink
A Klass Act
Verso Awakens Maine
Twenty-Three Years of Bringing Students to the Industry
Marcal Rises From the Ashes
Buckeye Turns Lemon to Lemonade
Forty Years and "Still Doing It"
There's No Business Like Paper Business
Saving Energy at the Paper Machine
Low Alkali System Meets the Challenges of ONP/OMG Recycling
Around the Industry
Conquering Innovation Fatigue
Give Yourselves a Hand
Index of Advertisers

Paper360 - November/December 2009