Lime causticization product brightness improvement via pre-slaking

Abstract

A method for obtaining particulate calcium carbonate exhibiting improved brightness and color and having uniformity of size such that when the particles are formed in the causticization process in a kraft pulp mill, the time required to separate the particles from liquors in which they are suspended is minimized and the amount of liquor recovered is maximized, with minimal dilution by water used for washing. The method includes the steps of a) slaking calcium oxide in water or an alkaline liquor containing as dissolved species predominantly sodium hydroxide; b) mixing the slaked lime with green liquor from a kraft pulping process to complete a causticization reaction that produces white liquor and lime mud; c) separating the lime mud from the white liquor; and d) milling a portion of the lime mud for use as a white mineral pigment in applications where such pigments are typically used.

Claims

1. A method for obtaining particulate calcium carbonate product comprising: providing green liquor from a kraft pulping process; adding calcium oxide to a fluid to form a reaction product of calcium oxide and the fluid; agitating the reaction product of calcium oxide and the fluid; after agitation, optionally adding white liquor from a kraft pulping process to the reaction product of calcium oxide and the fluid; after the agitation and the optional addition of white liquor, reacting the reaction product of calcium oxide and the fluid, optionally comprising added white liquor, with the green liquor to complete a causticization reaction which produces white liquor and unwashed lime mud comprising calcium carbonate; separating the unwashed lime mud from the white liquor; washing the unwashed lime mud with water to provide washed lime mud and weak wash; and reacting the washed lime mud with carbon dioxide vapor; wherein the fluid to which the calcium oxide is added is weak wash or a combination of water and weak wash; wherein the weak wash is weak wash from washing the lime mud with water; and wherein a portion of the white liquor from the causticization reaction is the white liquor which is optionally added after the agitation.

2. The method of claim 1, wherein after agitation, white liquor is added, such that a portion of the white liquor from the causticization reaction is the white liquor added after the agitation.

3. The method of claim 1, wherein separating the lime mud from the white liquor comprises use of a Whatman no. 4 filter.

4. The method of claim 1, wherein the agitation occurs for at least 15 minutes.

5. A method for obtaining particulate calcium carbonate product comprising: providing green liquor from a kraft pulping process; slaking calcium oxide in a fluid comprising water to form a milk of lime; reacting the milk of lime with the green liquor to complete a causticization reaction which produces white liquor and unwashed lime mud comprising calcium carbonate; and separating the lime mud from the white liquor, washing the lime mud with water to provide washed lime mud, and reacting the washed lime mud with carbon dioxide vapor; wherein the fluid in which the calcium oxide is slaked is weak wash or a combination of water and weak wash, the weak wash being weak wash from washing lime mud with water.

6. The method of claim 5, in which slaked calcium oxide is combined with white liquor from a kraft pulping process and the combination is reacted with the green liquor from a kraft pulping process, wherein a portion of the white liquor from the causticization reaction is the white liquor which is combined with the slaked calcium oxide.

7. The method of claim 5, comprising combining the slaked calcium oxide with white liquor from a kraft pulping process; reacting the combination of slaked calcium oxide and white liquor with the green liquor from a kraft pulping process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagrammatic representation of the pulping, black liquor evaporation and causticization processes.

(2) FIG. 2 is a diagrammatic representation of the kraft causticization processes in a mill that does not use a mud kiln.

(3) FIG. 3 is a diagrammatic representation of the kraft causticization of green liquor according to a first embodiment of the present invention.

(4) FIG. 4 is a diagrammatic representation of the kraft causticization of green liquor according to a second embodiment of the present invention.

(5) FIG. 5 is a diagrammatic representation of the kraft causticization of green liquor according to a third embodiment of the present invention.

(6) FIG. 6 is a diagrammatic representation of the kraft causticization of green liquor according to a fourth embodiment of the present invention.

(7) FIG. 7 is a diagrammatic representation of the kraft causticization of green liquor according to a fifth embodiment of the present invention.

(8) FIG. 8 is a diagrammatic representation of the kraft causticization of green liquor according to a sixth embodiment of the present invention.

(9) FIG. 9 is a diagrammatic representation of the kraft causticization of green liquor according to a seventh embodiment of the present invention.

(10) FIG. 10 is a diagrammatic representation of the kraft causticization of green liquor according to an eighth embodiment of the present invention.

(11) FIG. 11 is an illustration showing the correlation between the surface area and filtration time of a control sample of ALBACAR precipitated calcium carbonate, treated lime mud of the present invention and lime mud from a typical kraft pulp mill.

(12) FIG. 12 is a graph showing the correlation between brightness and size of particulate calcium carbonate of the present invention and from a typical kraft pulp mill.

(13) FIG. 13 is a graph showing the correlation between filtration time and size of particulate calcium carbonate of the present invention and from a typical kraft pulp mill.

DESCRIPTION OF THE INVENTION

(14) The present invention comprises a method of obtaining particulate calcium carbonate from the causticization process in a kraft pulp mill. The method comprises the steps of a) slaking calcium oxide (CaO, lime) in water or an alkaline liquor containing as dissolved species predominantly sodium hydroxide; b) mixing the slaked lime with green liquor from a kraft pulping process to complete a causticization reaction that produces white liquor and lime mud; c) separating the lime mud from the white liquor; and d) milling a portion of the lime mud for use as a white mineral pigment in applications where such pigments are typically used.

(15) In one embodiment of the present invention, the calcium oxide of step a) is first slaked in water to form milk of lime. The milk of lime is then fed into a reactor containing kraft mill green liquor to complete a causticization reaction that produces white liquor and lime mud. The lime mud and white liquor are separated, and the mud is washed with water before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 170 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

(16) In one embodiment of the present invention, shown in FIG. 3, the calcium oxide of step a) is first slaked in water to form milk of lime. The milk of lime is then fed into a reactor containing kraft mill green liquor to complete a causticization reaction that produces white liquor and lime mud. The lime mud and white liquor are separated, and the mud is washed with water before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 170 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

(17) In another embodiment of the present invention as seen in FIG. 5, the calcium oxide which can be stored in lime silo 310 of step a) is slaked at slaker 330 in weak wash generated when lime mud that has been separated such as by filtration by a filtration means, here mud wash filter 360 from white liquor is further washed with water to remove residual white liquor from the mud cake. The slaked lime slurry is then fed into a reactor containing kraft mill green liquor which can be filtered at green liquor filter 320 to complete a causticization reaction that produces white liquor and lime mud. The causticization reaction can occur in causticization tanks 340. The lime mud and white liquor are separated such as by filtration by a filtration means, here filter 350, and the mud is washed at mud wash filter 360 with water before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 370 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

(18) In another embodiment of the present invention as seen in FIG. 6, the calcium oxide which can be stored in lime silo 410 of step a) is slaked at slaker 430 in white liquor produced by the causticization reaction of a kraft pulping process.

(19) The slaked lime slurry is then fed into a reactor containing kraft mill green liquor which can be filtered at green liquor filter 420 to complete a causticization reaction that produces white liquor and lime mud. The causticization reaction can occur in causticization tanks 440. The lime mud and white liquor are separated such as by filtration by a filtration means, here filter 450, and the mud is washed at mud wash filter 460 with water before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 470 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

(20) In another embodiment of the present invention, the causticization reaction between kraft green liquor and milk of lime or lime hydrate or alkaline slurry of lime in white liquor or weak wash is done continuously.

(21) In another embodiment of the present invention, the causticization reaction between kraft green liquor and milk of lime or lime hydrate or alkaline slurry of lime in white liquor or weak wash is done in a batch reactor.

(22) In another embodiment of the present invention as seen in FIG. 7, the calcium oxide which can be stored in lime silo 510 of step a) is slaked at slaker 530 in white liquor produced by the causticization reaction of a kraft pulping process. The slaked lime slurry is then fed into a stream or volume of white liquor before being subsequently fed into a reactor containing kraft mill green liquor which can be filtered at green liquor filter 520 to complete a causticization reaction that produces white liquor and lime mud. The causticization reaction can occur in causticization tanks 540. The lime mud and white liquor are separated such as by filtration by a filtration means, here filter 550. A portion of the white liquor from filter 550 can be the white liquor into which slaked lime slurry is fed. The mud is washed at mud wash filter 560 with water before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 570 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

(23) In another embodiment of the present invention as seen in FIG. 8, lime hydrate powder which can be stored in lime hydrate storage tank 675 is combined with a stream or volume of white liquor before being subsequently fed into a reactor containing kraft mill green liquor which can be filtered at green liquor filter 620 to complete a causticization reaction that produces white liquor and lime mud. The causticization reaction can occur in causticization tanks 640. The lime mud and white liquor are separated which can occur by means of a filter 650. A portion of the white liquor from filter 650 can be the white liquor into which slaked lime slurry is fed. The mud is washed at mud wash filter 660 with water before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 670 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

(24) In another embodiment of the present invention as seen in FIG. 9, the calcium oxide which can be stored in lime silo 710 of step a) is slaked at slaker 730 in water. The slaked lime is then fed into a stream or volume of white liquor before being subsequently fed into a reactor containing kraft mill green liquor which can be filtered at green liquor filter 720 to complete a causticization reaction that produces white liquor and lime mud. The causticization reaction can occur in causticization tanks 740. The lime mud and white liquor are separated which can occur by means of a filter 750. A portion of the white liquor from filter 750 can be the white liquor into which slaked lime slurry is fed. The mud is washed with water at mud wash filter 760 before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 770 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3). In another embodiment of the present invention shown in FIG. 10, the calcium oxide which can be stored in lime silo 810 of step a) is slaked at slaker 830 in weak wash generated when lime mud that has been separated at filter 850 from white liquor is further washed with water to remove residual white liquor from the mud cake. The slake is then fed into a stream or volume of white liquor before being subsequently fed into a reactor containing kraft mill green liquor which can be filtered at green liquor filter 820 to complete a causticization reaction that produces white liquor and lime mud. The causticization reaction can occur in causticization tanks 840. The lime mud and white liquor are separated at filter 850. A portion of the white liquor from filter 850 can be the white liquor into which lime slurry which was slaked in weak wash is fed. The mud is washed with water at mud wash filter 860 before a portion of the lime mud is milled for use as a white mineral pigment in applications where such pigments are typically used. The washed lime mud can be treated at treatment means 870 by passing a stream of vaporized carbon dioxide (CO.sub.2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na.sub.2CO.sub.3/NaHCO.sub.3).

EXAMPLES

Example 1

(25) This Example is intended to be a comparative example carried out according to conventional means and not according to the method of the current invention.

(26) To a 4-liter SS reactor were added 1850 ml of kraft green liquor filtered through No. 4 Whatman filter paper and comprised of, as dissolved species, 12.54 wt. % sodium carbonate (Na.sub.2CO.sub.3), 2.36 wt. % sodium sulfide (Na.sub.2S) and 1.59 wt. % sodium hydroxide (NaOH), and having a specific gravity of 1.21. The contents of the reactor were then heated to 95 C. and held at that temperature by means of a circulating bath containing a 1:1 mixture of ethylene glycol and water. Agitation was accomplished by means of an impeller rotating at 1000 rpm.

(27) 100 g of granular lime having an active CaO content of about 95% was added to the reactor over about 15 seconds, and the maximum temperature (T.sub.max) in the reactor was recorded as 102 C.

(28) The mixture in the reactor was held at 95 C. under agitation for 180 minutes before passing the contents over a 100 mesh screen followed by filtration through No. 4 Whatman filter paper to separate the white liquor from the lime mud. The lime mud filter cake was washed with tap water until the conductivity of the effluent stream was measured as <1 mS. The ISO (R.sub.457) dry brightness of the washed cake was 85.8, CIE b* was 0.91 and BET Specific Surface Area (SSA) was 4.1 m.sup.2/g.

Example 2

(29) This Example was carried out according to the process of the current invention. To a 4-liter SS reactor were added 1850 ml of the filtered kraft green liquor of Example 1. The contents of the reactor were then heated to 95 C. and held at that temperature by means of a circulating bath containing a 1:1 mixture of ethylene glycol and water. Agitation was accomplished by means of an impeller rotating at 1000 rpm.

(30) To a 1-liter SS beaker containing 500 g of water at an initial temperature of 76 C. were added 100 g of the granular lime of Example 1. Slaking was carried out for 15 minutes under agitation by means of an impeller rotating at 1000 rpm. T.sub.max was recorded as 100 C. after about 1 minute.

(31) The contents of the SS beaker were then poured into the SS reactor over about 1 minute at which time the temperature in the reactor dropped to about 92 C. before rising to 95 C. over about 5 minutes. The contents of the reactor were then agitated at temperature for 180 minutes before passing the contents over a 100 mesh screen followed by filtration through No. 4 Whatman filter paper to separate the white liquor from the lime mud. The lime mud filter cake was washed with tap water until the conductivity of the effluent stream was measured as <1 mS. The ISO (R.sub.457) dry brightness of the washed cake was 91.8, CIE b* was 1.13 and BET SSA was 4.3 m.sup.2/g.

Example 3

(32) This is another Example carried out according to the process of the current invention.

(33) To a 4-liter SS reactor were added 1850 ml of kraft green liquor filtered through No. 4 Watman filter paper and comprised of, as dissolved species, 12.54 wt. % sodium carbonate (Na.sub.2CO.sub.3), 2.36 wt. % sodium sulfide (Na.sub.2S) and 1.59 wt. % sodium hydroxide (NaOH). The contents of the reactor were then heated to 95 C. and held at that temperature by means of a circulating bath containing a 1:1 mixture of ethylene glycol and water. Agitation was accomplished by means of an impeller rotating at 1000 rpm.

(34) To a 1-liter SS beaker containing 500 g of kraft mill white liquor comprised of, as dissolved species, 8.04 wt. % sodium hydroxide (NaOH), 1.93 wt. % sodium carbonate (Na.sub.2CO.sub.3) and 1.89 wt. % sodium sulfide (Na.sub.2S), and having a specific gravity of 1.15 at an initial temperature of 77 C. were added 100 g of the granular lime of Example 1. Slaking was carried out for 15 minutes under agitation by means of an impeller rotating at 1000 rpm. T.sub.max was recorded as 104 C. after about 2 minutes.

(35) The contents of the SS beaker were then poured into the SS reactor over about 1 minute at which time the temperature in the reactor dropped to about 92 C. before rising to 96 C. over about 5 minutes. The contents of the reactor were then agitated at temperature for 180 minutes before passing the contents over a 100 mesh screen followed by filtration through No. 4 Whatman filter paper to separate the white liquor from the lime mud. The lime mud filter cake was washed with tap water until the conductivity of the effluent stream was measured as <1 mS. The ISO (R.sub.457) dry brightness of the washed cake was 92.3, CIE b* was 1.31 and BET SSA was 3.1 m.sup.2/g

Example 4

(36) This is another Example carried out according to the process of the current invention.

(37) To a 4-liter SS reactor were added 500 g of the kraft white liquor of Example 3 which were heated to 95 C. and held at that temperature by means of a circulating bath containing a 1:1 mixture of ethylene glycol and water. Agitation was accomplished by means of an impeller rotating at 1000 rpm.

(38) To a glass 4-liter reactor were added 2238 g of the green liquor of Example 1 which were heated to 75 C. using an electric heating mantle.

(39) To a glass 2-liter reactor were added 500 g of the kraft white liquor of Example 3 which were heated to 70 C. using an electric heating mantle. The heating mantle was turned off and 100 g of the lime from Example 1 were added to the heated white liquor under agitation provided by an impeller rotating at 700 rpm. Tmax was 98 C. after about 3 minutes. After about 20 minutes, the electric mantle was again turned on. At this time, alkaline slurry of lime in white liquor had cooled to about 85 C.

(40) The alkaline slurry of lime in white liquor was pumped at a rate of 17.7 ml/min into the 4-liter SS reactor containing the heated white liquor. Simultaneously, green liquor was pumped at a rate of 62.0 ml/min into the 4-liter SS reactor. As the slurry and green liquor were added, agitation was accomplished by means of an impeller rotating at 1000 rpm. Addition of the slurry and green liquor was complete after about 27 minutes during which time the temperature of the reactor contents rose from about 92 to about 94 C. Mixing was continued for about 20 minutes before passing the contents of the SS reactor over a 100 mesh screen followed by filtration through No. 4 Whatman filter paper to separate the white liquor from the lime mud. The lime mud filter cake was washed with tap water until the conductivity of the effluent stream was measured as <1 mS. The ISO (R.sub.457) dry brightness of the washed cake was 90.9, CIE b* was 1.22 and BET SSA was 1.2 m.sup.2/g.

(41) As can be seen in the above Examples 2, 3 and 4 made according to the present invention, the ISO(R.sub.457) dry brightness of the washed lime mud filter cake or particulate calcium carbonate was above 90 in each case. Further in each of these three above-mentioned examples, a CIE b* value was achieved for the precipitated calcium carbonate which is acceptable for papermaking.

Examples 5-10

(42) These Examples were carried out according to conventional means and not according to the method of the current invention. A series of reactions were carried out, each reaction done according to the same process that is described below. The results are recorded in Table 1.

(43) To a 4-liter reactor were added 1850 ml of kraft green liquor filtered through No. 4 Whatman filter paper and comprised of, as dissolved species, 12.54 wt. % sodium carbonate (Na.sub.2CO.sub.3), 3.36 wt. % sodium sulfide (Na.sub.2S) and 1.59 wt. % sodium hydroxide (NaOH), and having a specific gravity of 1.21. The contents of the reactor were then heated to 95 C. and held at that temperature by means of a circulating bath containing a 1:1 mixture of ethylene glycol and water. Agitation was accomplished by means of an impeller rotating at 1000 rpm.

(44) 100 g of granular lime having an active CaO content of about 95% was added to the reactor over about 15 seconds.

(45) The mixture in the reactor was held at 95 C. under agitation for 180 minutes before passing the contents over a 100 mesh screen followed by filtration through No. 4 Whatman filter paper to separate the white liquor from the lime mud. The lime mud filter cake was washed with tap water until the conductivity of the effluent stream was measured as <1 mS. The particle size distribution of the lime mud particles was measured using a Micromeritics Sedigraph 5100. The Hunter brightness and color of the washed, wet cake were recorded as was the ISO (R.sub.457) dry brightness and BET Specific Surface Area (SSA).

(46) TABLE-US-00001 TABLE 1 WASHED SLURRY TESTING OVEN DRY Hun- Filtration TESTING d.sub.90, d.sub.50, Hunter ter rate SSA, ISO CIE m m Rd b (sec) m.sup.2/g R.sub.457 b* Example 5 16.7 6.0 27.4 1.3 29 2.9 70.8 2.4 Example 6 12.7 4.3 34.9 1.0 35 4.1 77.6 1.7 Example 7 13.3 4.7 38.2 1.2 35 3.7 76.7 1.3 Example 8 13.6 4.8 33.7 1.3 30 4.0 74.6 1.9 Example 9 12.3 4.4 38.9 1.3 31 4.2 79.7 1.2 Example 10 14.4 4.7 36.0 1.0 25 5.7 77.3 1.2

Examples 11-15

(47) These Examples were carried out by the method of the current invention. A series of reactions were carried out, each reaction done according to the conditions described in Example 3, but using a green liquor containing the same dissolved species in the same concentrations as used in Examples 5-10. The results are recorded in Table 2.

(48) TABLE-US-00002 TABLE 2 WASHED SLURRY TESTING OVEN DRY Filtration TESTING d.sub.90, d.sub.50, Hunter Hunter rate SSA, ISO CIE m m Rd b (sec) m.sup.2/g R.sub.457 b* Example 11 4.5 2.9 40.7 -0.6 32 3.1 88.0 2.6 Example 12 4.7 3.1 37.4 -0.4 23 3.1 87.1 2.7 Example 13 5.3 3.4 33.9 -0.5 22 3.4 81.0 1.9 Example 14 4.0 2.5 58.8 0.1 29 3.9 92.3 1.4 Example 15 4.8 3.1 49.2 -0.1 30 3.3 89.8 1.5

(49) The data contained in Tables 1 and 2 are compared in FIGS. 12 and 13, and serve to illustrate the differences among lime mud particles produced via conventional means and lime mud particles produced according to the method of the current invention.

(50) In FIG. 12 it is evident that lime mud particles arising from the method of the current invention exhibit sizes about one-half that of particles arising from conventional causticization. This in turn gives rise to higher brightness. It is both counterintuitive and surprising to observe the data shown in FIG. 13.

(51) The filtration data shown in FIG. 13 was obtained from a laboratory filtration test designed to standardize and measure the time required to separate small amounts of lime mud from the free liquid in which the particles of lime mud are suspended. The test is done as follows: 1. A 5.5 cm Buchner funnel is connected to a sidearm flask under water aspiration and fitted with w/Whatman No. 5 filter paper. 2. 50 g of 201% solids of slurry that has first been water-washed to 1 mS conductance in the effluent is well-shaken and instantly poured into the Buchner funnel. 3. The time (in seconds) until there is no visible liquid on top of the cake is recorded.

(52) FIG. 13 clearly shows that although the particles produced by the method of the current invention are smaller than those produced via conventional means, they also are more easily separated from the liquid in which they are suspended, as evidenced by filtration times that are equal or lower than for particles produced by conventional processes.

(53) Accordingly, by the method of the present invention a particulate calcium carbonate suitable for paper can be made. Paper can be made comprising the particulate calcium carbonate made by the method of the present invention.

(54) Paper can be made comprising pulp made by chemical pulping such as the Kraft process and also comprising particulate calcium carbonate made by the method of the present invention.

(55) Accordingly, it is understood that the above description of the present invention is susceptible to considerable modifications, changes and adaptations by those skilled in the art, and that such modifications, changes and adaptations are intended to be considered within the scope of the present invention.