Kraft mills

Modijications to the Recope Cycle. The recovery system is a principal capital cost in a kraft mill. Consequently, any recovery process that is less expensive to build can improve pulping economics. There have been numerous attempts to improve the kraft recovery process. Two examples are the direct alkaline recovery scheme (DARS) and the autocausticizing scheme using sodium borates (37). Both schemes eliminate the lime loop of the conventional kraft mill. As of 1996, neither is commercially used. 

S oda—Anthraquinone. A few mills worldwide use soda pulping of hardwoods. In such cases, the addition of anthraquinone is immediately justifiable in terms of increased yield and upgraded pulp quaHty. The conversion of existing kraft mills is not as simple because AQ contributes no alkalinity to the process as sulfide does, and most kraft causticizing systems would have to be expanded by about 33%. This conversion is probably not justifiable in terms of the yield gain. The greatest benefit from AQ is for new mills in which expenditures for air pollution abatement devices can be reduced. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

Closed-Mill Concept. The closed-mill concept, or water circuit closure, has been studied by the pulp and paper industry for many years. In some parts of the paper manufacturing process, up to 98% of the water is recycled within the process, eg, the wet end of the paper machine. However, in the pulp mill, especiaUy kraft mills, effluents are produced owing to the need to purge from the system various metals that come in with the wood, as weU as organic by-products from the pulping process, additives, and especially chloride ions that originate in the bleach plant. 

M. Paleologou, R. M. Berry, and B. I. Fleming, "Bipolar Membrane Electro dialysis A New Solution to the Problems of Chemical Imbalance iu Kraft Mills," 78th Finnual Meeting, Technical Section, Canadian Tulp and Taper Association, Preprints A,Jan. 28—29, 1992, pp. KM—KSl. 

M. Fredette, "CIO2 Generators And Kraft Mill Chemical Balance," Tappi 1991 Bleach Plant Operations Short Course, June 23—28, 1991, pp. 175—188. 

A major tac tic that was adopted in the Clean Water Act was to estabhsh uniform technology standards, by class of pollutant and specific industiy type, which applied nationwide to aU dischargers. Thus, a kraft mill in Oregon would have to meet essentially the same discharge standards as a kraft mill in New York. In establishing these... 

Environmental antiandrogens Vinclozolin Procymidone Kraft mill effluent... 

Complex mixtures Tannery and Kraft mill effluent Sediment extract Sewage effluent... 

However, not all EDs with a high log possess or require the ability to bioaccnmulate in order to be biologically active. For example, phthalate plasticisers, chlorophenols from Kraft mill effluents and natural or synthetic hormones can influence an organism s hormone profile and affect reproductive function and immune response without exhibiting bioaccnmiilation. ... 

Merriman, J.C., Anthony, D.H.J., and Kraft, J.A. et al. (1991). Rainy River water-quality in the vicinity of bleached Kraft mills. Chemosphere 23, 1605-1615. 

Kutney JP, E Dimitriadis, GM Hewitt, PJ Salisbury, M Singh (1982) Studies related to biological detoxification of kraft mill effluent IV—the biodegradation of 14-chlorodehydroabietic acid with Mortierella... 

Saski EK, JK Jokela, MS Salkinoja-Salonen (1996a) Biodegradability of different size classes of bleached kraft mill effluent organic halogens during wastewater treatment and in lake environments. In Environmental Fate and Effects of Pulp and Paper Mill Effluents (Eds MR Servos, KR Munlittrick, JH Carey, and GJ van der Kraak), pp. 179-193. St Lucie Press, Delray Beach, FL. 

This section describes the applicable national regulatory requirements for bleached, unbleached, and dissolving kraft mills. Potential pollutants of concern for kraft pulp mills as reflected in the effluent limitations guidelines and standards promulgated by U.S. EPA and in a sampling of NPDES permits are summarized in Table 21.12. 

Prior to the Cluster Rules, direct discharge kraft mills were regulated as shown in 11.4.4.1. 

Indirect discharge kraft mills were subject to performance standards for existing sources or new sources (PSES or PSNS, as applicable) for the control of pentachlorophenol and trichlorophenol. 

For kraft pulp mills, the Cluster Rules add toxic and nonconventional pollutants to the list of regulated pollutants only for bleached papergrade kraft mills. Effluent limitations guidelines and standards were added for the following BAT and PSES pollutants (and NSPS/PSNS for new sources) chloroform, 2,3,7,8-TCDD, 2,3,7,8-TCDF, 12 chlorinated phenolic compounds, and AOX. All of the... 

Type of Kraft Mill Discharger BPT Rules BAT BAT Rules PSES PSES... 

Bleached kraft mills Direct discharger Indirect discharger T T T T T... 

In the previous regulations, bleached kraft mills were divided into four subparts and unbleached kraft mills were divided into three subparts. As a result, the remaining preCluster Rules limits (i.e., BPT for BOD5, TSS, and pH, and BAT and PSES for pentachlorophenol and trichlorophenol) for the four previous bleached kraft mill subparts now exist as four segments. Likewise, the remaining preCluster Rules limits for the three previous unbleached kraft subparts now exist as three segments. 

Kraft pulp mills treat wastewater using primary (physical) and secondary (biological) treatment to reduce pollutant discharges to receiving waters. Kraft mills typically collect and treat the following wastewaters36 ... 

After conversion to the use of sodium-based chemicals, spent liquor could be incinerated, and sulfur dioxide, sodium sulfate, carbonate, or sulfide could be recovered. These compounds could be sold for use at nearby kraft mills or for other industrial uses. 

Figure 21.8 shows a typical sequence of the major equipment systems in a wastewater treatment plant.36 The function of primary treatment is to remove suspended solids from the wastewater, and then to remove organic materials by biological secondary treatment. Primary treatment processes used by kraft mills typically involve screening followed by either sedimentation or flotation.40... 

About 75% of U.S. kraft mills use aerated stabilization basins.36 These basins are equipped with continuous mechanical aerators or diffusers to introduce air into the wastewater. By aerating the... 

Some kraft mills use basins without mechanical aerators. Known as stabilization basins, this is the simplest form of aerobic treatment. This process uses shallow basins that cover very large areas and relies on natural diffusion of air into the wastewater and algae to create aerobic conditions. At depths greater than 1.2 m (4 ft), anaerobic microorganisms will become active in lower depths thus, stabilization basins are shallow. Typically, the basin is earthen although some are lined with compacted clay. Wastewater retention time may last up to 30 d to achieve up to 90% BOD5 removal. 

Some kraft mills use both aerated and nonaerated basins. The stabilization basin, which may precede or follow the aerated stabilization basin, serves as a polishing or holding pond to remove additional organic materials, including biological solids, or to reduce final effluent discharges to receiving waters. 

Many kraft mills use screw presses that can achieve up to 55% biosolids concentration when dewatering primary biosolids. This operation does not require preconditioning to achieve high concentrations. 

Subsequent to biosolids handling processes, kraft mills dispose of biosolids by land application, landfill, or combustion. 

This is the most common disposal method. Kraft mills may use on-site landfills or off-site commercial landfills.63... 

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