Pollution prevention and abatement handbook pulp and paper mills

Chips that exceed the maximum acceptable thickness can be diverted to a chip slicer, that cuts them radially and reintroduce them to the screening system see Figure Mills may control storm water discharges by installing curbs, dikes, and drainage collection systems around wood and chip piles and wood processing areas.

Collected stormwater can be collected and transported to the wastewater treatment facility which should effectively remove the pollutants of concern. Costs for stormwater collection and treatment are variable and quite site-specific. They depend more on the current configuration of the mill woodyard and location of treatment facility than on the particular type of controls installed.

Prepared by Beak Consultants, Ltd. EPS 3-WP March, Smook, , Gary A. Stakes and Bielgus, George Strakes and Joe Bielgus. Tikka et al. This section and the rest of the report retains the traditional distinction between pulping and bleaching, which is to classify stages that occur prior to the application of chlorine-based bleaching agents as pulping stages, and those tfiat occur following application of these agents as bleaching stages. With the increased use of non chlorine- based delignification agents, however, these distinctions are becoming less and less meaningful.

For optimum pollution prevention potential, many experts would recommend viewing pulping and bleaching as integrated processes. Results in greater delignification, leading to reduced requirements for chlorine-based bleaching chemicals and their associated environmental impacts. Mow it works In conventional kraft pulping, the digester is filled with chips and given a one-time charge of cooking chemicals.

The alkali concentration in the reactor is initially high, but then falls as the cook progresses and the cooking chemicals are consumed. Normal reaction times are between one and three hours. Longer cooks will further reduce pulp lignin content but wiE also begin to degrade the cellulose, as the reactions become less selective towards lignin. By applying principles developed by the Swedish Forest Products Research Institute STFI , it has become possible to extend pulp cooking times without impacting pulp quality or yield.

These principles are embodied in technology offered by the major equipment suppliers. Source: Kamyr, Inc. Equipment diagram for EMCC extended cooking, showing multiple liquor addition points and liquor addition to wash zone. RDH was originally developed by the Beloit Corp. Beloit, Wisconsin. The chemical delivery demands of an RDH-type batch system require additional equipment and a sophisticated process control system.

This is especially true where the mill may run as many as 20 batch digesters at a time, sharing common process equipment accumulator tanks, pumps, piping. Production scheduling can be thrown off if there are disruptions due to equipment malfunction or operator error. Much of the work done since the discovery of the RDH principles has involved refinement of the "tank farm" configuration and improvement to the distributed control system that oversees the process.

Installations Presently, world capacity for extended cooking is near 11 million tons per year tpy , representing 20 percent of bleached kraft capacity see Table Twenty-five of these projects are in the United States, Including 16 new installations and 9 retrofits. Combined U. The average capacity of these installations is over 1, tpd. Installations of extended batch cooking are also shown in Table All four U. Note that some mills may retrofit their existing pulping equipment without the input of the vendor companies discussed above.

The retrofit potential for existing batch systems in the United States is somewhat limited, however, by Ihe age of the digesters. Most U. Space requirements for the RDH tank farm may. Hardwood kappas can be reduced from around 20 to a range of 8 to 10, with current targets generally around 12 to The impact of brownstock kappa number reductions on bleaching chemical demands and formation of many chlorinated organics is now well understood. AOX and polychlorinated phenols will be reduced in approximate proportion to reductions in the brownstock kappa number.

Compatibility When used in combination with oxygen delignification, extended delignification has produced softwood pulps in the range of kappa number 6 to These pulps are extremely bleachable and could be brought to full brightness using elemental chlorine-free ECF or totally chlorine-free TCP sequences. Extended delignification increases the amount of lignin and organic solids removed during the cooking process, and it is beneficial for the mill to burn these in the recovery boiler.

Many boilers currently operate at or near capacity, thus it is often a challenge to find ways to accommodate the increase in solids load. Many mills already practice some of these techniques: Additional evaporator - Additional evaporation stages will increase the concentration of the black liquor, resulting in improved combustion reduced gas flow as well as lower sulfur emissions.

Although this is a common upgrade option, the higher consistency solids are more difficult to handle and will necessitate improved pumping and firing equipment; Transport black liquor solids orfsite for disposal - Where other kraft mills with excess recovery boiler capacity are within miles of the mill, it may be feasible to ship additional solids off site for firing.

This has become common practice in some areas of North America and Europe; Reduce boiler load per ton of solids - Although boiler load is discussed in terms of pounds of solids burned, in practice the capacity ultimately depends on the heat content of the black liquor solids; 2 ECF pulps are produced using percent cMorine dioxide substitution for chlorine; no elemental chlorine or hypocMorite are used.

In TCP sequences, chlorine dioxide is also eliminated. Note that no bleached kraft mills in the U. Louisiana-Pacific Corp.

Soap removal can reduce the heat value of the liquor solids by 4 to 8 percent, thereby enabling further capacity increases; Increase black liquor storage capacity - Some mills may lack sufficient black liquor storage capacity, resulting in insufficient supply to keep the boiler operating steadily at capacity essential for efficient operation. By constructing additional supply capacity the mill may be able to obtain a higher liquor solids throughput from the existing boiler; Black liquor gasification - A relatively recent technology, this proprietary process involves gasification of strong black liquor in a closed vessel.

This produces a smelt similar to that from a conventional recovery furnace. In the four U. While this may seem exceedingly costly, for an older mill a modem boiler will bring substantial additional benefits in the form of greater efficiency, easier maintenance, and reduced air pollution; Production penalty - Mills may choose to decrease production to accommodate the additional solids per ton of pulp that results from oxygen or extended delignification.

This operational penalty, however, is generally considered excessive. In addition, it is always possible for the mill to discharge the effluent containing any additional non- chlorinated solids that cannot be accommodated in the recovery boiler.

These will be treated in the existing wastewater treatment system. The modern recovery furnace is equipped with sophisticated Mr emissions control equipment such as electrostatic precipitation ESP. Precipitates primarily sodium sulfate and sodium carbonate are returned to the liquor makeup system, hence the only material losses are the dregs and grits.

These are usually landfflled. A higher degree of deligniflcation would probably result in a minor i. Extended delignifieation will cause an increase in steam demand die to the longer cooking period , though more energy will be recovered from the additional lignin solids. The greatest energy impacts may be observed indirectly though, through reductions in bleaching chemical demands.

Most bleaching agents are manufactured by applying energy to raw inorganic minerals e. McCubbin has modeled the onsite and offsite energy impacts of a variety of pollution prevention measures. Oxygen a powerful bleaching agent is mixed with the pulp and allowed to react.

Lignin content in the pulp is further reduced prior to the bleaching stages, leading to decreased bleaching chemical demands and associated environmental impacts. How it works The brownstock pulp from the digester is first washed and then mixed with oxygen as it enters the reactor see Figure , In high consistency systems, a press is required to remove excess water from the pulp prior to reaction.

In the reactor, the pulp undergoes oxidative delignifieation. The pulp is then washed again to remove dissolved lignin solids before proceeding to the bleaching line.

Oxygen systems are available from all of the major pulp equipment vendors, Installations Until recently, oxygen delignifieation had been more widely adopted outside of North America. In recent years the adoption rate within the U. Presently, there are close to mills worldwide that operate oxygen delignifieation systems, representing 26 million tons of annual production. Oxygen is currently installed or planned for 27 U. Of these, 16 will have come online since only. Process flow for high-consistency HC oxygen delignification.

Union Camp Location of Mil W. At the low end of this range, the savings in chemical costs, plus savings on current or future effluent treatment requirements, may favor the switch to oxygen.

Installation of an oxygen delignification stage may also help mills avoid the costs of installing a new chlorine dioxide generating system or replacing an aging C-stage in their bleaching line. At other mills considerably greater investment may be required, particularly to upgrade pulp washing equipment. Successful implementation of oxygen delignification requires effective pulp washing both in front of and following the oxygen stage.

Most mills today using oxygen for bleaching will generate it onsite using non-cryogenic systems provided by equipment vendors. Typically, the mill enters into a to year "over the fence" supply contract with the vendor.

The mill supplies the land for construction of the plant, while the vendor installs the equipment and in some cases operates the plant as well. The technologies for onsite generation are much smaller in scale and produce slightly lower purity oxygen compared to cryogenic methods, although this does not appear to affect pulp properties or mill operations to any significant degree.

Advances in pressure swing adsorption PSA and vacuum swing adsorption VSA technologies have enhanced the attractiveness of onsite generation. Pollution prevention Since effluent from the oxygen stage can be recycled to the raill's recovery system, oxygen delignification will reduce discharges of pollutants such as BOD5 color, and organochlorines. Oxygen and extended delignification can also reduce the overall wastewater flows from the mill by up to 25 percent each, putting it on the track towards zero effluent pulping and chlorine- free bleaching.

Compatibility Oxygen delignification is compatible with most conventional bleaching sequences. In addition, since oxygen delignification has the potential to lower the pre-bleach stage kappa number by as much as 50 percent, a variety of innovative bleaching methods may also be applied. In atypical application, the mill follows the oxygen stage with a mixture of chlorine and chlorine dioxide, followed by a caustic extraction stage, another C1O2 bleaching and extraction stage, and a final bleaching stage, i.

A significant benefit of oxygen delignification is that it allows the mill to increase the substitution rate of chlorine dioxide for chlorine in the CD stage without requiring additional chlorine dioxide generating capacity. A typical mill-currently operating at 30 percent C substitution, for example, could operate at 70 percent by increasing its dioxide generating capacity, or by adding an oxygen delignification stage and using existing dioxide capacity at 70 percent.

HC 14 11,5 40 1. Other options include "short sequence" bleaching, in which oxygen delignification is followed by two rather than three bleaching stages i. While short sequence bleaching has the benefits of lower capital costs and reduced effluent flow, its feasibility for an individual mill will depend on the pulp quality requirements, including the target brightness level With fewer bleaching stages there is less opportunity for "fine tuning" the pulp characteristics and probably more variability in pulp quality.

Integrated mills are more likely to consider short-sequence bleaching than market pulp producers. Finally, when combined with extended delignification, mills may be able to produce oxygen delignified pulps with kappa numbers below 8 to 10 and without loss of strength or yield.

As with extended cooking, the addition of an oxygen stage increases the amount of recovered black liquor solids available for heat recovery.

The recovery of 80 percent of the oxygen stage solids will generally increase the solids going to the boiler by 50 to 55 kg per metric ton approximately 3 percent for hardwoods and by 30 to 35 kg per metric ton approximately 2 percent for softwoods. Additional post- oxygen washing stages can increase this further still. At older mills where pre-chlorination washing is currently inefficient, the increase could be as much as 10 percent Section 4. Ozone an extremely powerful bleaching agent is mixed with the pulp which undergoes a rapid exothermic reaction, Ligrtin content in the pulp is greatly reduced, Since mis occurs prior to any chlorine-based bleaching stages, dramatic decreases in bleaching chemical demands and associated environmental impacts can result.

Peak ozone delignification efficiency has been found to occur at low pH, thus pulp is normally pretreated with sulftaic acid prior to ozonation. As with oxygen, both medium and high consistency systems are available. The acidified pulp is fluffed and deposited in me ozone reactor. Ozone gas generated onsite is delivered to the pulp in an oxygen carrier gas. During reaction, the ozone is consumed and the carrier gas is recovered and either returned to the ozone generator or used elsewhere in.

Equipment for Mgji-consistency HC ozone delignificatian. Source: Miller, The successful "taming" of ozone has been greatly anticipated, however, since it opens the door to elimination of chlorine compounds in bleaching and raises the possibility of complete closure of the mill's bleach plant. As a result of a considerable research effort, the past year has seen the startup of the first two full-scale mills 1, tpd or more operating ozone bleaching lines. This technology consists of a series of tubes through which the feed oxygen or air flows.

As high voltage is applied across the discharge gap, free electrons in the corona collide with the diatomic oxygen and cause disassociation of the O2 molecules, which recombine to form ozone. Ozone is unstable with a half life of only 15 minutes and will decompose to regular oxygen, thus ozone must be generated onsite and fed immediately to the pulp reactor.

Installations Table shows the chronology of ozone pilot plant and full-scale installations worldwide. Based on their pilot experiences, the company just recently September completed the startup of a full-scale ozone bleaching line at its 1, tpd mill in Franklin, Virginia -- the first in the world to operate at that scale.

The mill uses oxygen, ozone, and chlorine dioxide to produce elemental chlorine-free ECF pulp from southern pine. All of the pulp will be used for onsite production of bleached uncoated free sheet and coated and uncoated bleached board. The mill is reported to be using 30 kilos of peroxide per ton of pulp to produce totally chlorine-free market pulp TCF at 88 to 89 ISO brightness for sale in Germany; « The Lenzing mill in Austria has installed medium consistency ozone in an EOPZP configuration at its tpd dissolving pulp mill.

The mill is reportedly converting a separate qsd line at this time; 4 Note that chlorine dioxide generation requires approximately the same amount of energy as ozone, but that ozone is twice as powerful a bleaching agent. Eddy mill in Espanola, Ontario is considering installation of a full-scale ozone plant following successful operation of their pilot plant. The technology is based on use of oxygen delignification, gas phase ozone, and a small amount of chlorine dioxide, which enables the licensee to produce ECF pulp at full market brightness.

Economics Capital costs for the ozone delignification equipment will depend on type of system selected i. Others using ozone as a "polishing stage" for pulp brightening may favor medium consistency.

In comparison with chlorination stage bleaching, ozone requires additional process equipment pulp press, high shear mixers, acid handling system but benefits from the ability to use cheaper construction materials corrosion problems are less severe compared to chlorine bleaching.

Depending on how close the mill comes to closing its effluent cycle and whether it maintains one or more D stages, there should be reduced costs for effluent treatment and bleach plant scrubbing systems. Bleaching costs, including ozone generation, are lower than for conventional sequences. Union Camp reports that bleaching costs at their Franklin mill are approximately 68 and 43 percent lower man the costs of a modern mill's DEDED sequence running at percent chlorine dioxide substitution see Table Union Camp's decision to retain a chlorine dioxide stage was based on the expectation that elimination of elemental chlorine would be sufficient to guarantee satisfactory effluent levels.

The use of ozone in the bleach line, however, introduces the possibility of eliminating all chlorine-containing compounds to produce totally cMorine-free TCP pulp.

Hydrogen peroxide would be used as a replacement for chlorine dioxide in a TCP sequence. Since either oxygen delignification or extended cooking or both are considered prerequisites for successful ozone bleaching, the more widespread adoption of these technologies will undoubtedly increase interest in ozone. Also, ozone's cost vis-a-vis conventional bleaching sequences has improved as it is now more likely to replace more expensive chlorine dioxide rather than less expensive chlorine.

Assumptions for costing purposes are shown below. At Union Camp, the bleaching sequence has been simplified to OZED, eliminating all elemental chlorine and retaining just one stage of chlorine dioxide bleaching. Emissions from the process are extremely low because of the ability to recycle all of the O, Z, and E stage effluents see Table Total organic halides TOX are below 0.

Note: Om refers to an oxygen stage using a wash of ozone stage effluent and Y is sodium hydrosulfite. The absence of elemental chlorine in these sequences and the elimination of all chlorine-based compounds in some indicates that effluents from future mills using ozone will be extremely low in pollutants of current concern.

Compatibility One concern raised by some in the industry is that ozone-bleached pulps tend to be of lower strength and hence lower quality than those produced by conventional bleaching processes.

Most of these concerns center around observed decreases in the viscosity of pulps bleached using ozone. Viscosity has traditionally been used as an indicator of pulp strength, and ozone-bleached pulps have in fact been found to have lower viscosities than conventionally-bleached pulps of similar kappa number.

However, numerous researchers have found that the viscosity-strength relationship is different for non-conventionally bleached pulps, so that despite the lower viscosities ozone bleaching does not impair pulp strength properties. Ozone generators are equipped with sensors that will shut off power to the unit if any leaks are detected. Ozone production stops as soon as the power is cut.

Under pressure, ozone may also present explosion hazards. For this reason, high consistency systems that operate at or near atmospheric pressures may be considered safer than medium consistency systems that operate under pressure. One distinct safety advantage of ozone over chlorine is that the ozone is generated onsite. Chlorine is generally shipped to the mill in ton tanker cars; this gas must then be transferred and stored onsite.

Since ozone is produced on demand there is no onsite storage, hence only the small quantities contained in the pipeline several kilograms would pose a danger. Source: Nutt et al. Alternatively, the technique can be used to reduce the amount of lignin solids generated per ton of pulp, thereby facilitating adoption of extended cooking or oxygen delignification.

How it works The AQ is added to pulp during cooking using an application rate of around 0. The AQ catalyzes or accelerates the fragmentation of lignin, rendering it more vulnerable to attack and dissolution by the cooking chemicals. Installations AQ pulping is used in an estimated 60 percent of Japanese mills to improve yield from what are relatively expensive wood sources, and in at least two mills in Canada. The trade journal Paper Age has reported that over mills worldwide use anmraquinone. There is reported to be increasing interest in AQ as a means of achieving extended delignification and overcoming boiler capacity bottlenecks.

Economics By increasing yield or reducing chemical requirements, anthraquinohe catalysis offers a potential means to offset the tendency of oxygen delignification or extended delignification to overload the kraft recovery system.

It has been estimated that AQ could compensate for an increase in chemical recovery load of up to 7 or 8 percent.

An AQ charge of 0,04 percent on wood in a 1, adt per day mill would have the following effects: 1 raise yield by 0. In some mills, a 4 to 5 percent reduction in boiler load will be sufficient to enable the mill to accommodate the additional solids load that results from oxygen delignification. The additional cost of AQ would, of course, offset the cost savings that would otherwise result from using oxygen. Pollution prevention Anthraquinone addition has the potential to alternatively decrease bleaching chemical requirements or facilitate adoption of oxygen delignification.

In either role, anthraquinone will have a positive i. By producing brownstock pulp with lower lignin content, the mill can decrease overall bleaching chemical requirements. And by improving the feasibility of oxygen delignification, the mill can recycle additional effluent as well as cut back on bleaching chemical usage.

As with other delignification modifications mat lower the pre- ddorination kappa number, chlorinated organics formation should decrease in approximate proportion to the drop in lignin content. AQ is not detectable in pulps subject to further chemical bleaching. Anthraquinone is produced from coal tar generated in the coking process at steel mills. The chemical is used in several industries, including textiles where it serves as an intermediary in dyestuff manufacture.

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