Bleaching

Certain types of paper and paperboard require that the pulp be bleached. Chemicals used for this purpose dissolve and further eliminate the lignin, the natural glue that binds wood fibers. The resulting product is not only whiter but also more resistant and absorbent. Without bleaching, the paper would become brittle and yellow with time, like old newspapers for instance.

In terms of its bleaching practices, the pulp and paper industry has been called upon to clearly demonstrate its commitment to protecting the environment. In the mid 1980s, traces of toxic substances were detected in the effluent of certain mills. An investigation revealed that the source of these contaminants was the chlorine used in the bleaching process. Following intensive research and massive investments, the industry succeeded in considerably reducing discharges of these unwanted substances. This turnaround was made possible by replacing chlorine with other bleaching agents, like chlorine dioxide and hydrogen peroxide, by reducing the use of chemicals, by improving pulp cleaning systems, and by implementing process water secondary treatment systems in several mills.

ECF

Elemental Chlorine-Free (ECF) pulp, bleached with chlorine dioxide, continues to dominate the world bleached chemical pulp market. By the end of 1999, ECF production topping 48.5 million tons, totaling more than 62% of the world market share. Market data show an ever-widening gap between ECF and TCF (Totally Chlorine-Free) production. Over the last five years, ECF grew at an annual rate approaching 15% per year, with an additional 6.5 million tons entering the market in 1999. Over the same period, TCF remained steady at 6% of world bleached chemical pulp production.

Amid growing public and scientific debate about bleaching technologies, consensus is emerging around three important developments. First, chlorine dioxide, the cornerstone of ECF, is an environmentally superior bleaching agent. Second, chlorine dioxide is not a barrier to closing a mill's process water loop. Third, market growth is being driven by ECF. This consensus suggests that chlorine dioxide is not an interim technology, nor is it a transitional phase in the industry's quest for the minimum-impact mill.

The clear and urgent need to eliminate persistent, bio-accumulative, toxic substances in pulp and paper mill waste water has pushed the conversion to chlorine dioxide bleaching. But some feel this conversion is simply not enough. Two divergent approaches shape the public debate: ECF and Totally Chlorine-Free (TCF). The U.S. EPA's proposed Cluster Rule for the pulp and paper industry, which has ECF as one of its core Best Available Technology (BAT) elements, has sharpened the focus. This paper discusses why ECF is the consensus choice for pollution prevention technology for the bleached pulp and paper industry.

ECF bleaching, while producing high quality pulp:

The chlorine dioxide molecule consists of one chlorine atom and two oxygen atoms. The oxidation level of the chlorine atom in chlorine dioxide is +4, while the oxidation level of the chlorine atom in molecular chlorine is 0. Based on reduction to chloride at an oxidation level of -1, there is a change of five oxidation levels with chlorine dioxide compared to one for elemental chlorine. Chlorine dioxide therefore has five times the oxidation potential.

These differences make chlorine dioxide not only a superior bleaching agent, but one with distinct environmental benefits. During the pulp bleaching process, both chlorine and chlorine dioxide can form chlorinated organics. However, whereas chlorine tends to substitute and combine with lignin -- the substance that holds the wood fibers together -- chlorine dioxide typically oxidizes the lignin, opening up the aromatic structure. Any remaining chlorinated organics formed by ClO2 bleaching are water soluble and do not bio-accumulate. Instead, they are very similar to chemical substances occurring naturally in the environment.

Complementing its oxidizing strength is its selectivity. ClO2 attacks lignin and other substances such as resins, while preserving the wood's cellulose fibers -- those that provide the strength in the final paper products.

Mills implementing ECF have eliminated dioxin and furan in mill effluent to non-detect levels. Consequently, downstream of North American pulp and paper mills, dioxin levels in fish tissue have significantly declined.

Having developed and implemented a strategy for virtual elimination of persistent, bio-accumulative, and toxic compounds, research on the environmental responses to pulp mill effluent has shifted focus. Subtle responses such as the induction of liver enzymes in exposed fish are now the focus of attention in the scientific and technical communities. The ecological significance of such responses has yet to be determined.

Initially thought to be due to exposure to bleached pulp mill effluent, subsequent research has shown such subtle responses to be found both at bleached and unbleached pulp mills. Current research suggests the source may be natural wood extracts that are released into the environment during the processes that prepare the cellulose fibers prior to bleaching. Issues such as these subtle responses and industry's commitment to minimizing the environmental impact of papermaking are driving the development of minimum-impact mills.

One aspect of minimum-impact mills is minimizing wastewater discharge. Taking inspiration from successful application of closed cycle processes for mechanical and semi-chemical pulp mills, current research and development is focusing on technologies to "close the loop" to an even greater degree than is presently practiced at bleached chemical pulp mills.

Pilot plant and demonstration processes for elimination of bleaching effluent are in operation at many facilities throughout the world. Processes are being developed for both ECF-based bleaching and TCF-based bleaching.