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A very beneficial and cost effective way to use ozone in the delignification of pulp is in conjunction with chlorine dioxide and no intermediate washing. The process is referred to as D/Z or Z/D for production of ECF pulp. Ozone can replace between 2 to 4 kg ClO2 per kg of ozone applied in the D0 stage.

In TCF or, totally chlorine free bleaching, ozone is a key stage to achieve a high brightness, high quality pulp without the use of chorine bleaching compounds.

 

 

 

Process Description:

Ozone can be added to pulp at either high consistency (HC), medium consistency (M.C.) or low consistency. All are operated under acidic conditions. HC systems require a fluffer and a press prior to the stage to thicken the pulp to 40-45% consistency. 

Medium consistency ozonation is used for those mills already under 11-14% consistency. Ozone at 10-14% weight concentration is injected into the pulp slurry at high pressures in high shear mixers. Higher ozone concentrations are required in M.C. systems to minimize the gas fraction in the mixer and maximize ozone usage. A small transition tube with a few seconds retention time follows the last mixer for separation of gas from pulp. and some additional kappa reduction. The same M.C. system is considered for low consistency D0 stages, but with the possibility of using only one mixer.

Following the rapid reaction of ozone with pulp, the ozone stage vent gas (mainly oxygen) is separated from the pulp and sent to an ozone destruct unit to destroy any residual ozone. 

The table below summarizes the operating conditions for each system.

 

Operating Parameters:

 

High Consistency

Medium Consistency

Low
Consistency

Temperature

< 150oF

< 150oF

<150oF

Pressure

atmospheric

150 psig

85-150 psig

pH

2.5

2.5

2.5

Ozone, wt.%

6-14% O3

10-14% O3

6-14% O3

 

• Mill Implementation

With a high consistency reactor, lower ozone concentration and atmospheric pressure leads to lower cost ozone since there is no need for ozone compression. Mill modifications include a pulp press, a fluffer and a high consistency reactor which can lead to high capital expenses estimated at >US$15,000,000.

With medium or low consistency systems, a higher ozone concentration is recommended and an ozone compressor is needed. The ozone cost is slightly higher, but the mill modifications cost in terms of piping, high shear mixer, transition tube, scrubber and M.C. pump are reduced to around US$3,500,000 installed.

For a low consistency system, the capital cost installed is around US$2,500,000.

 

• Existing Mill Installations

As of December 2000, there were 25 ozone installations in operation worldwide and one under construction. Of these, three mills in North America utilize ozone for bleaching of virgin pulps (Union Camp in Franklin, VA,  Consolidated Papers in Wisconsin Rapids, WI,  Domtar [formerly E.B. Eddy Forest Products] in Espanola, ON).

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VCP, Jacarei

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EB EDDY Forest Products
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• New Developments

Recently there have been new start-ups at Votorantim in Brazil and Domtar (formerly E.B.Eddy Forest Products) in Canada. The systems are described below.

Recent comments made by Domtar at the CPPA Bleaching committee meeting were:

"Ozone has operated since April 1999 and is a very effective bleaching agent. The replacement ratios of kg of ClO2 replaced by kgs of ozone have been up to 4 kg/kg."

 

• Advantages

For a while many mills looked at ozone bleaching to produce TCF pulps because of tightening environmental regulations. Ozone today is seen as a complement to produce ECF at a lower cost than standard ECF pulp without ozone. With replacement ratios of ClO2 by O3 in the range of 2.0 to 3.5, savings of 6-8 US$/mT of pulp net can be achieved.

• Environmental Benefits

Whenever chlorine bleaching chemicals are replaced by non-chlorine chemicals, a reduction in AOX is seen. When ozone completely substitutes the first chlorination stage (C/D), then the extract from the washers following the ozone stage can be recycled to the chemical recovery system leading them closer to closing the mill water loop. If an extraction stage follows the ozone stage the extract from this washer can also be recycled. This is greatly desired since most of the COD and BOD, are contained in this extract.





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