Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0057—Oxidation of liquors, e.g. in order to reduce the losses of sulfur compounds, followed by evaporation or combustion if the liquor in question is a black liquor

Definitions

• the present invention relates to an improvement in any process which generates polysulphide by the oxidation of white liquor ; the invention also relates to a method of increasing the yield of pulp in Kraft pulping with an oxidized white liquor.
• This conversion is achieved by either oxidizing the aldehyde to its corresponding carboxylic acid (Alfredsson et ah, 1963), (Holton, 1977) or, alternatively, reducing it to its alcohol form (Hartler, 1959), (Pettersson et al, 1961).
• the two methods that are applied in the pulp and paper industry involve the oxidation process and use anthraquinone (Holton, 1977), or polysulphide (Clayton et al, 1967), (Landmark et al, 1965), (Sanyer et al, 1964), (Teder, 1969), or both as oxidizing agents.
• Anthraquinone is a catalytic additive while polysulphide is generated from white liquor by oxidation of sodium sulfide in one of several processes (Dorris, 1992), (Smith et al, 1977).
• the present invention seeks to increase the concentration of PST JV measured at 285 or 286 nm or PS vis measured at 416 nm and the PS UV PSQ R or PS VIS /PSQ R ratio of polysulphide liquors generated by the oxidation of white liquor without substantial loss of polysulphide charge.
• the present invention also seeks to increase the yield of pulp from wood particles by cooking the wood particles in a polysulphide liquor having a high concentration of PSuv measured at 285 or 286 nm or PS vis measured at 416 nm and the PS UV /PSQ R or PS VIS /PSQ R ratio, generated by the oxidation of white liquor.
• the invention seeks to increase the content of active polysulphide in a polysulphide liquor generated by the oxidation of white liquor.
• the present invention also seeks to increase the yield of pulp from wood particles by cooking wood particles in a polysulphide liquor generated by the oxidation of white liquors and which have been thermally treated.
• a method which comprises exposing an oxidized white liquor to a temperature effective to increase the concentration of PS v measured at 285 or 286 nm or PS V ⁇ s measured at 416 nm and the PS V /PSQ R or PS VIS /PS GR ratio of polysulphide in the oxidized white liquor.
• a method of increasing the concentration of PSuv measured at 285 or 286 nm or PS V ⁇ s measured at 416 nm and the PS UV /PSQ R or PS VIS /PS GR ratio of a polysulphide liquor comprising heating or cooling if necessary and then storing said liquor at a temperature between 20°C and 95°C for a time of up to 72 hours.
• a method of producing an oxidized white liquor containing polysulphide comprising: oxidizing a white liquor to produce an oxidized white liquor containing polysulphide and having a first PS UV /PSQ R or PS VIS /PSQ R ratio, and heat said oxidized white liquor to produce an oxidized white liquor having a second PS UV /PSQ R or PS VIS /PS GR ratio, wherein said second ratio is greater than said first ratio.
• a method of increasing the yield of pulp in Kraft pulping with a white liquor containing polysulphide comprising: oxidizing a white liquor to produce an oxidized white liquor containing polysulphide, ii) heating or cooling if necessary and then storing said oxidized white liquor to increase the concentration of PSuv measured at 285 or 286 nm or PS V ⁇ s measured at 416 nm and the PS UV /PSQ R or PS VIS /PS GR ratio of polysulphide in the oxidized white liquor, and in a subsequent step: iii) delignifying pulp with the oxidized white liquor from step ii).
• the invention relates to the heat treatment of a polysulphide liquor generated by the oxidation of white liquor .
• This heat treatment is preferably at a temperature below 95°C, more preferably between 20°C and 95°C, and most preferably between 50 and 95°C for a time up to 72 hours, preferably 1 to 48 hours, more preferably 6 to 30 hours, even more preferably 12 to 24 hours.
• a method which comprises exposing an oxidized white liquor produced by oxidation of white liquor in the presence of lime mud, or Mn0 2 or both lime and Mn0 2 to a temperature effective to increase the concentration of PSuv measured a t 285 or 286 nm or PS vis measured at 416 nm and the PS UV /PSQ or PS VIS /P GR ratio of polysulphide in the oxidized white liquor.
• the temperature in this latter preferred embodiment, is below 95°C and the period of exposure is for a time up to 72 hours; more preferably the temperature is between 50°C and 95°C, for an exposure time up to 48 hours.
• a method of increasing the PS UV /PSQ or PS VIS /PS GR ratio of a polysulphide liquor generated by oxidation of white liquor in the presence of lime mud or Mn0 2 or both lime mud and Mn0 2 comprises heat treating the oxidized white liquor at a temperature between 50°C and 95°C for a time of up to 72 hours, and preferably up to 48 hours.
• a method of increasing the yield of pulp in Kraft pulping with a white liquor containing polysulphide comprising: i) oxidizing a white liquor in the presence of lime mud, Mn0 2 or both lime mud and Mn0 2 to produce an oxidized white liquor containing polysulphide, ii) heat treating the oxidized white liquor to increase the concentration of PSuv measured at 285 or 286 nm or PS vis measured at 416 nm and the PS UV PSQ R or PS VIS /PS GR ratio of polysulphide in the oxidized white liquor, and iii) cooking wood chips with the oxidized white liquor from step ii) to produce pulp.
• FIG. 1 illustrates graphically the yield benefit in pulp delignification with increase in PS UV /PSQ R ratio
• FIG. 2 illustrates graphically the relationship between polysulphide decomposition and temperature
• FIGS. 3 and 4 show the relationship between pulp yield and permanganate number for polysulphide liquors of the invention at different temperatures; and conventional white liquors for different pulps.
• the oxidized white liquor in this invention is one produced by oxidizing sodium sulphide in the white liquor to sodium polysulphide.
• the invention is not confined to any particular oxidation procedure for producing the polysulphide or oxidized white liquor.
• the invention thus extends to oxidized white liquors in which the oxidation is carried out with oxygen or oxygen-containing gases such as air, in the presence of a catalyst, for example, wet-proofed activated carbon in the MOXY (trademark of The Mead Corporation) process; lime mud in the PAPRILOX (trademark of Pulp and Paper Research Institute of Canada) process, lime mud spiked with manganese dioxide; or with oxygen or oxygen containing gas in the presence of a metal oxide, such as oxides of manganese, iron, cobalt, zinc, aluminum, nickel or chromium, which metal oxide functions as a catalyst for polysulphide formation.
• a catalyst for example, wet-proofed activated carbon in the MOXY (trademark of The Mead Corporation) process; lime mud in the PAPRILOX (trademark of Pulp and Paper Research Institute of Canada) process, lime mud spiked
• An especially preferred or advantageous oxidized white liquor for use in the invention is that produced by oxidation of a white liquor produced by causticizing green liquor and containing the lime mud generated in the causticization process.
• Green liquor is produced from the smelt derived from black liquor in chemical recovery of a conventional Kraft liquor cycle.
• the green liquor comprises sodium carbonate and sodium sulphide and the causticization involves addition of lime, calcium oxide, to the green liquid.
• the lime reacts with the sodium carbonate to produce sodium hydroxide with precipitation of insoluble calcium carbonate.
• the suspended solids comprising the calcium carbonate, unreacted calcium oxide and other insoluble solids present in the smelt, is referred to as lime mud.
• This lime mud is thus a by-product of the white liquor formation.
• the white liquor suspension containing lime mud can be employed directly in the production of the oxidized white liquor, as outlined in U.S. Patent 5,082,526 incorporated herein by reference.
• a catalytic amount of manganese dioxide is added to the white liquor suspension to further enhance the oxidation.
• Suitable catalytic amounts of manganese dioxide comprise 0.1 to 2.0 g/1 of a white liquor.
• the resulting oxidized white liquor contains the lime mud and, where applicable, the added manganese dioxide catalyst, as suspended solids.
• the process of this invention for increasing the concentration of PSuv measured at 285 or 286 nm or PS vis measured at 416 nm and the PS UV /PSQR or PS VIS /PS GR ratio of polysulphide liquors generated by the oxidation of white liquor is a process in which the polysulphide liquor is heat treated within a range of temperatures and times without substantial loss of polysulphide charge.
• oxidized white- liquor is separated from the oxidation catalysts such as lime mud and manganese oxide prior to the heat treatment.
• the liquors generated by the oxidation of white liquor in the presence of Mn0 2 'lime mud or both Mn0 2 and lime mud are heat treated at a temperature below 95°C for a time up to 72 hours to increase the concentration of PSuv measured at 285 or 286 nm or PS V ⁇ s measured at 416 nm and the PS UV /PSQ R or PS VIS /PS GR ratio .
• the temperature of the polysulphide liquor for the heat treatment can be adjusted using a heat exchanger.
• the temperature of the polysulphide liquor can also be adjusted by evaporative cooling with an oxygen-containing gas. Normal practice is to remove the heat generated by the reactions between sodium sulphide and oxygen, to prevent the oxidized liquor temperature from rising to or above the liquor boiling point (Uloth et al., 1997, Tench et al., 1999).
• Storage may be provided by existing tankage provided both for liquor clarification and flow buffering or by new tankage.
• the target storage temperature and storage time can be optimized to ensure that the maximum charge of active polysulphide in the polysulphide liquor, is delivered to the pulp digester.
• the heat treatment is preferably carried out by maintaining the oxidized liquor at a temperature of 50°C to 90°C for a time of 1 to 48 hours.
• Polysulphide can be generated from sodium sulphide in a white liquor by various methods including the direct addition of sulphur to the white liquor. However, this method cannot be used industrially without a bleed of sulphur from the Kraft recovery cycle, which is expensive to provide. Having different methods of polysulphide generation, however, allows comparisons of the form of the polysulphide that is generated by each of the different methods. These comparisons have shown that there are differences in what is measured as polysulphide when polysulphide liquor is generated by the direct addition of sulphur to the white liquor and when it is generated by the oxidation of white liquor.
• Polysulphide can be measured in many ways but two of the most simple and effective are measurement by gravimetry (PS GR ) and by UV or VIS absorption (PSuv or PSyis)- The two methods can be used to give a PS UV /PSQ R or PS VIS /PS GR ratio.
• Polysulphide generated by direct sulphur addition has a PS UV /PSQ R or PS VIS /PS GR ratio very close to 1, but polysulphide generated by oxidation of white liquor has a ratio that varies depending on the way that it has been made.
• polysulphide in the white liquor changes the degree to which the yield of pulp is increased by the application of a given polysulphide charge.
• Polysulphide liquors that have a PS U /PSQ R or PS VIS /PSQ R ratio of 1 deliver the full yield expected from the application of a given polysulphide charge in the Kraft pulping process.
• the polysulphide content may thus be considered to be active polysulphide i.e. polysulphide which oxidizes aldehyde groups on wood polys accharides to inhibit carbohydrate degradation during delignification of pulp.
• Oxidized liquors that have lower ratios are found to deliver diminishing amounts of the expected yield ( Figure 1).
• the concentration of PSuv measured at 285 or 286 nm or PS vis measured at 416 nm and the PS UV /PSQ R or PS VIS /PS GR ratio of the polysulphide liquor be as high as possible, or as close to 1 as possible.
• the heat treatment of the invention does result in some loss in the total polysulphide content, determined as both active and inactive. The loss depends on the treatment temperature and time.
• the heat treatment parameters are desirably selected to establish a satisfactory content of active polysulphide for the protective oxidation of the carbohydrate aldehyde groups.
• a balance is to be achieved between PS UV /PSQ R or PSV I S/PSQ R ratio and the actual concentration of active polysulphide.
• a PST JV /PSQ R or PS VIS /PS GR ratio close to 1 will not be beneficial if the heat treatment has decreased the total polysulphide content to a level where the active polysulphide content is inadequate for the protective oxidation reaction.
• a low PS V PSQ R or PS VIS /PS GR ratio will be beneficial where the total polysulphide content remains high such that the ratio signifies an adequate active polysulphide content for the protective oxidation reaction.
• an active polysulphide concentration of at least 4 g/1, and preferably at least 6 g/1, in the oxidized white liquor, is required for effective oxidation of the carbohydrate aldehyde groups in the wood chips.
• the PSuv can be measured at, for example, at 285nm, 286 nm or a 416 nm wavelength.
• the liquors used to generate the results in Figure 1 were obtained as follows.
• White liquors of varying concentrations and compositions 800 mL; preheated to 70 °C in a microwave oven) were brought to 90 °C (in an oil bath) in a stainless steel reactor (1 L) equipped with a condenser (5 °C) and ports for adding gas and Mn0 2 and for withdrawing samples.
• the liquor was stirred mechanically (600 rpm; Eurostar Power Digi- Vise-trademark) under a nitrogen flow (50 mL/min; 2 ⁇ m stainless steel sparger (Supelco-trademark)).
• the impeller type used was a Rushton disk turbine with 6 flat blades (48 mm diameter).
• Finely powdered Mn0 2 (1 g/L; Brickox 6807- trademark of Prince Manufacturing Company) was added to the reactor when the liquor reached 90 °C.
• PS 0WL was generated by bubbling air (450 mL/min) into the liquor at a constant stirring rate of 1000 rpm.
• the generation of polysulphide was monitored with a UV spectrometer at 286 nm (HP Vectra QS/165- trademark; 1 mm path-length cells; oxygen-free NaOH (IN) used for dilution and blanks).
• the PSuv measurement for the ratios shown in Figure 1 were obtained at 286 nm, a wavelength at which all polysulphide species absorb with the same absorptivity.
• the PS measurement could also have been made at 416 nm with a similar relationship between PS UV /PSQ R or PS VIS /PS GR ratio and yield being found.
• Figure 2 presents the % decomposition of polysulphide determined by gravimetry and by UV spectrophotometry (286 nm) of a typical liquor produced by catalytic oxidation with manganese dioxide as it is heated at 1.8°C per minute; this corresponds to the conventional rise to temperature used in Kraft cooking of 90 min to 170°C. It is very clear from this figure that, at temperatures above 100°C, polysulphide decomposes rapidly.
• Oxidized white liquor was produced by causticizing 0.75 L of green liquor with 45 g reburned lime, spiked with 0.6 g Mn0 2 .
• the manganese content of clarified green liquor samples is typically 0.3 to 6.0 mg/L.
• the amount of manganese added in the Mn0 2 in this example (504 mg/L) is about a hundred times that normally found in green liquor.
• oxygen was sparged into the causticized slurry at a rate of 0.1 L/min for 30 min. After oxidation, the resulting CaC0 3 lime mud with added Mn0 2 , was separated from the oxidized white liquor.
• Example 1 illustrates the increase in the PSU V /P Q R ratio when a freshly oxidized white liquor is treated at 73°C for up to 48 hours.
• Table I shows that by heat treatment at 73°C for 48 hours, the PS UV /PSQ R ratio was changed from 0.45 to 0.86 while the polysulphide concentration (PS G )was only decreased from 9.3 g/L to 7.1g/L. Over 48 hours at 73 C, the PSuv concentration increased by 44 %, from 4.23 to 6.08 gpl as sulphur. As can be seen from Figure 1, such an increase in ratio will allow the yield increase from a given concentration of polysulphide to be increased from zero to almost the full potential of that concentration. Table 1-Stability at 73°C
• Example 2 illustrates the change in the ratio when the same liquor is heat treated at 95°C. At this temperature the activation of the liquor, as measured by the change in ratio, is very rapid. Within an hour the ratio has increased to a useful 0.74. Long times of treatment (>3 hours) are less useful at this temperature because of the increasing loss of polysulphide concentration measured either by UV spectrometry or gravimetry.
• Example 3 illustrates the change in the ratio and polysulphide concentration at an intermediate temperature of 85°C. At this temperature, it takes between 2 and 3 hours for the activation of the liquor. Again longer times of treatment are less useful because of the increasing loss of polysulphide charge.
• Example 4 illustrates the change in the ratio and polysulphide charge at 78°C but with the liquor having been pre-activated over 4 days at ambient temperature.
• the ambient temperature treatment increased the ratio from 0.46 to 0.55 without any loss of polysulphide concentration.
• Oxidized white liquor was produced by causticizing 0.75 L of green liquor with 53 g reburned lime, spiked with 1.5 g Mn0 2 . After 60 min causticizing time at 95°C, air was sparged into the causticized slurry at a rate of 0.55 L/min for 58 min. After oxidation, the resulting CaC0 3 lime mud with added Mn0 2 , was separated from the oxidized white liquor. Samples of the clarified white liquor were then stored in a thermostated bath held at a desired temperature. At regular time intervals, small samples of liquor were withdrawn for determination of polysulphide concentration by UV spectrometry (PSuv) and by gravimetry (PSGRAV)-
• Example 5 illustrates the increase in the concentration of PSuv measured at 285 or 286 nm and PS V ⁇ s measured at 416 nm and the PS UV /PSQ R and PS VIS /PS GR ratio when a freshly oxidized white liquor is treated at 60°C for up to 20 hours.
• the data in the table in this example shows that by heat treatment at 60°C for 20 hours, the PS UV /PSQ R or PS 285 /PS GR ratio was changed from 0.44 to 0.60, and the PS VIS /PS G or PS 416 /Ps GR ratio increased from 0.18 to 0.40, while the gravimetric polysulphide concentration was only decreased from 8.4 g/L to 7.5 g/L.
• the PS 416 concentration in the oxidized liquor doubled from 1.5 to 3.0 gpl (as sulphur) during the 20 hours of storage at 60 C.
• Example 6 illustrates the change in the ratio when the same liquor is heat treated at 80°C. At this temperature the activation of the liquor, as measured by the change in ratio, is more rapid. Within 16 hours the ratio has increased to a useful 0.89.
• the data in the table in this example shows that by heat treatment at 80°C for 20 hours, the PS UV /PSQ R or PS 285 /PS GR ratio was changed from 0.44 to 0.93, and the PS VIS /PS GR or PS 16 /PS GR ratio increased from 0.18 to 0.86, while the gravimetric polysulphide concentration was decreased from 8.4 g/L to 4.6 g/L.
• such an increase in PS UV /PSQ R or PS 285 /PSQ R A V ratio will allow the yield increase from a given concentration of polysulphide to be increased from zero to almost the full potential of that concentration.
• Example 7 illustrates the change in the ratio when a similar liquor is heat treated at 70°C. At this temperature the activation of the liquor, as measured by the change in ratio, is less rapid than at 80°C, but more rapid than 60°C.
• the PS UV /PSQ R or PS 285 /PS GR ratio has increased to a useful 0.72 and the PS VIS /PSQ R or PS 416 /PS GR ratio increased from 0.21 to 0.57.
• the PS 41 concentration was more than doubled from 1.8 to 3.8 gpl (as sulphur).
• Example 8 illustrates the change in the ratio when the same liquor is heat treated at 90°C. At this temperature, the activation of the liquor, as measured by the change in ratio, is very rapid. Within 5 hours, the PS UV /PSQ R or PS 285 /PS GR ratio has increased to a useful 0.84, and the PS VIS /PS G or PS 16 /PS GR ratio increased from 0.21 to 0.74, while the gravimetric polysulphide concentration was decreased from 8.5 g/L to 4.3 g/L. Longer times at this temperature resulted in a lower PS GR , PSuv and PS V ⁇ s concentrations with only a small gain in the ratios.
• Unclarified mill white liquor- containing 100 g/L of lime mud was oxidized with air in the presence of 2.0 g/L Mn0 2 at 85-90°C for 60 minutes, cooled quickly to room temperature (20°C) using a water bath and filtered to remove the lime mud and to give a clarified oxidized white liquor.
• One portion of the oxidized white liquor was treated at 70 °C for 20 hours.
• Another portion was stored at room temperature (20 °C) for 20 hours.
• the PSuv / PS GR of the oxidized white liquor treated at 70 °C for 20 hours was thus 0.80 and the PSuv / PSQ R of the oxidized white liquor stored at 20 °C for 20 hours (OWL-20 °C) was 0.49.
• the same amounts of these two oxidized white liquors were then used for the pulping of mixed softwood chips (50/50 black spruce and pine) in a micro-digester using 50 g (OD weight) of the wood chips in each of four stainless steel laboratory bombs.
• a control Kraft cook using the white liquor (WL) was also carried out.
• the liquor to wood ratio and the maximum cooking temperature were 4.5 to 1 and 170 °C, respectively.
• the PSQ R charges were 1.3 and 1.5% (on wood) for the cook using OWL-70 °C and the cook using OWL- 20 °C, respectively.
• Each bomb was cooked to a certain H-factor.
• the pulp from each bomb was well washed and screened through a laboratory flat screen plate (0.2 mm or 0.008" slot).
• the screened pulp yields were measured by weighing the oven-dried screened pulps and the permanganate numbers determined according to PAPTAC, Standard G. 17H.
• Unclarified mill white liquor containing 100 g/L of lime mud was oxidized with air in the presence of 2.0 g/L Mn0 2 at 85-90°C for 60 minutes, cooled quickly to room temperature (20°C) using a water-bath and filtered to remove the lime mud and to give a clarified oxidized white liquor.
• One portion of the oxidized white liquor was treated at 70 °C for 20 hours.
• Another portion was stored at room temperature (20 °C) for 20 hours.
• This example summarizes the optimum storage time needed to maximize the PSuv content of a polysulphide liquor generated by the oxidation of white liquor.
• the active polysulphide concentration (PS 16 ) at a given storage time is described by the curve which increases with time.
• the temperature at which the liquor is held in storage is described by the curve that decreases with time.
• Figure 3 shows that, at the lowest temperature evaluated (60°C), a storage time of 60 hours is needed to produce 6 g/L of active polysulphide from a liquor initially having a PSQ R concentration of 8.5 g/L. At the highest temperature evaluated (103°C) a storage time of 2 hours is needed to produce 2.3 g/L of active polysulphide from the same liquor.
• a polysulphide liquor was made with Mn0 2 but without lime mud.
• a synthetic white liquor was prepared from sodium hydroxide and sodium sulphide.
• a sample (750 mL) of this white liquor was oxidized using air at 450 ml/min with 0.4 grams of a commercial grade Mn0 2 (0.53 g/L Mn0 2 ).
• the composition of the synthetic white liquor and the product oxidized liquor are illustrated in Table 9.
• Table 10 shows that heat treatment at 77°C for 16.5 hours of this type of oxidized liquor is effective in increasing the PS UV /P GR ratio from 0.46 to 0.97 and increasing the PS 416 concentration in the oxidized liquor from 1.4 to 3.3 gpl (as sulphur).
• Table 9 Composition of liquors used in this example.
• a polysulphide liquor was made with a wet-proofed activated carbon catalyst.
• Wet proofing was done by spraying a dry film lubricant (TFE in Freon-trademark) on activated carbon (50-200 mesh from Fisher Scientific).
• the resulting paste was dried in the fume hood under a flow of nitrogen.
• Table 11 shows that heat treatment at 65°C, or aging at 25°C, over 60 hours of this type of oxidized liquor increases the active polysulphide from 2.15 g/L to
• the oxidation was done at room temperature by adding 5 g of wet-proofed carbon to about 300mL of artificial white liquor pre-heated to 85°C in a 500mL beaker. Oxidation was done for about 15h by letting air diffuse through the floating carbon into the white liquor. Samples of the oxidized liquor were then analyzed by gravimetry and by UV spectrophotometry, just after production (fresh) and then after storage for 60 h at 25°C. Another aliquot of the fresh oxidized liquor was also stored at 65°C for 60 h and then analyzed again.

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