Method of making purified precipitated calcium carbonate from lime mud

Abstract

To provide a method of making purified precipitated calcium carbonate from lime mud.SOLUTION: A method of making a purified precipitated calcium carbonate from lime mud includes: admixing a lime mud cake with water and sodium carbonate to form a first slurry; heating the first slurry under conditions to age the slurry and form one or more of pirssonite, shortite, and gaylussite; separating a solid portion from the aged slurry; washing the solid portion under conditions sufficient to decompose the one or more of pirssonite, shortite, and gaylussite to a CaCO3 solid fraction and Na2CO3 solid fraction and to remove sodium salts; and admixing the CaCO3 solid fraction with water and a dispersant to disperse the CaCO3 solid fraction in water and form a dispersed slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm, thereby producing a dispersed slurry containing the purified precipitated calcium carbonate.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates generally to a method for treating lime mud cake produced as a waste product to produce precipitated calcium carbonate, and more particularly to a method for treating lime mud cake produced as a waste product to produce precipitated calcium carbonate suitable for use as a filler and / or pigment in paper and paperboard.

[0002] Brief description of the related art The primary component of paper and paperboard is cellulosic pulp fibers produced from wood or other plant sources by various mechanical and / or chemical pulping processes. The primary chemical pulping process used in the paper industry is the alkaline "Kraft" process, which uses sodium hydroxide (caustic soda) and sodium sulfide in a digestion process step to extract and separate non-cellulosic materials from the cellulosic pulp fibers. Notably, another common variation in pulping is to eliminate the use of sodium sulfide, known as caustic pulping.

[0003] To maximize the operational and economic efficiency of the pulping process, chemicals are recovered and reused whenever possible. In this chemical recovery process, some pulping operations include lime kilns. The lime kilns function to produce lime (CaO) which is combined (deactivated) with "green liquor" in a process called causticization. Green liquor comes from the pulping process between pulping and causticization. In the absence of lime kilns, commercially available lime (CaO) is purchased and used in a single pass of the causticization circuit.

[0004] The main component of green liquor is sodium carbonate, and the reaction between lime and sodium carbonate produces sodium hydroxide (caustic soda) and calcium carbonate (CaCO3) through the following reaction steps: CaO+H2O→Ca(OH)2(1) Ca(OH)2+Na2CO3→CaCO3+2NaOH (2)

[0005] The calcium carbonate produced in step 2 is referred to in the industry as "lime mud" and the sodium hydroxide (caustic soda) solution is referred to as "white liquor". The reaction conditions in step 2 cause the lime mud to precipitate as relatively large particles that can be quickly separated from the white liquor stream. After washing and filtering, the lime mud is either recycled to the lime kilns, if present, or disposed of in landfills. In practice, systems often contain both recycled lime mud and some landfill purged lime mud.

[0006] Paper and paperboard products often contain calcium carbonate as a fine particle produced from ground limestone or synthetically precipitated as a filler and / or as a coating pigment due to its inherently superior whiteness and brightness compared to other mineral pigments. Despite containing calcium carbonate, lime mud is generally not suitable for use as a filler and / or coating pigment in paper and paperboard. Lime mud has relatively low whiteness and brightness. Furthermore, lime mud generally has a large particle size and high pH due to residual caustic soda. Lime mud produced by pulping non-wood species such as reeds and straw often contains elevated levels of high surface area siliceous minerals, which increase the specific surface area of ​​the lime mud particles, making them unsuitable for use as a filler in conventional papermaking.

[0007] Another drawback of lime mud is that black particles of char can become mixed with the lime mud particles. Black char can originate from combustion in the recovery boiler, which can be carried into the green liquor and, ultimately, into the lime mud produced from the recausticization of the green liquor. Black char, which can produce black specks, is highly undesirable in white pigments intended for use in paper. For these reasons, lime mud is disposed of in landfills if it is not reburned in lime kilns to produce calcium oxide (CaO) for reuse in the recausticization process. Even when lime mud is recycled, it may be advantageous to purge more lime mud to (i) increase pulping capacity or (ii) clean up the system of non-process elements to improve process efficiency. Summary of the Invention

[0008] It would be beneficial if the lime mud could be processed and made suitable for use as a filler and / or coating pigment in paper and board, as opposed to being landfilled. This would also improve the overall utilization of purchased lime at paper mills and reduce consumption of purchased mineral fillers and / or coating pigments. Furthermore, refining this lime mud into a viable new product would support global efforts by companies to minimize waste and support a circular economy where resources are used more efficiently by reusing, recycling or repurposing materials.

[0009] According to an embodiment of the present disclosure, a method of making purified precipitated calcium carbonate from lime mud may include mixing lime mud cake with water and sodium carbonate to form a first slurry, heating the first slurry under conditions to mature the slurry and form one or more of pirsonite, shortite, and geylasite, separating a solid portion from the aged slurry, washing the solid portion under conditions sufficient to decompose the one or more of pirsonite, shortite, and geylasite into a CaC0 solid fraction and a NaC0 solid fraction and to remove sodium salts, mixing the CaC0 solid fraction with water and a dispersing agent to disperse the CaC0 solid fraction in the water and form a dispersed slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm, thereby producing a dispersed slurry containing purified precipitated calcium carbonate.

[0010] According to an embodiment of the present disclosure, a method of making purified precipitated calcium carbonate from lime mud may include mixing lime mud cake with water and a dispersant to form a first slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm; milling the first slurry to a median particle size of about 0.4 microns to about 5 microns; phase separating the milled slurry under conditions sufficient to obtain a centrifuged slurry containing impurity particles and a paste containing purified calcium carbonate; and diluting the paste to a target solids content in water, thereby producing a dispersed slurry containing the purified precipitated calcium carbonate.

[0011] According to an embodiment of the present disclosure, a method of making purified precipitated calcium carbonate from lime mud may include mixing lime mud cake with water to form a first slurry; adjusting the pH of the first slurry to be about 10 to about 11; centrifuging the first slurry under conditions sufficient to achieve a g-force of about 500 to about 2000 g for a residence time of about 1 to about 10 minutes to obtain a centrifuged slurry comprising a paste including impurity particles and purified calcium carbonate; and mixing the paste with water and a dispersant to form a dispersed slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm and containing the purified precipitated calcium carbonate.

[0012] According to an embodiment of the present disclosure, a method of making purified precipitated calcium carbonate from lime mud includes mixing lime mud cake with water to form a first slurry, adjusting the pH of the first slurry to be about 8 to about 11, mixing the first slurry with a silicate flotation collector compound to form a second slurry, processing the second slurry through a flotation cell system under conditions sufficient to form a tails slurry containing concentrated foam containing silicate particulate impurities and CaC0, washing and separating the tails slurry into a liquid phase containing excess soluble salts and a paste containing purified CaC0, and mixing the paste with water to form a dispersed slurry having a viscosity of less than about 100 cps at 100 rpm and containing purified precipitated calcium carbonate.

[0013] According to an embodiment of the present disclosure, a method for removing black char from a starting slurry containing calcium carbonate and black char includes flowing the starting slurry through a first hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion to form a first overflow and the calcium carbonate to be retained at a lower portion to form a first underflow, flowing the first underflow through a second hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion to form a second overflow and the calcium carbonate to be retained at a lower portion to form a second underflow, and flowing the first underflow through a second hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion to form a third overflow and the calcium carbonate to be retained at a lower portion to form a third underflow. flowing the first overflow to a third hydrocyclone under conditions sufficient to retain the first overflow at a lower portion and form a third underflow; flowing the second underflow to a collection chamber; flowing the second overflow to the first hydrocyclone; flowing the third underflow to the first hydrocyclone; flowing the third overflow to a fourth hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion and form a fourth overflow and the calcium carbonate to retain at a lower portion and form a fourth underflow; flowing the fourth overflow to a waste container; and flowing the fourth underflow to the third hydrocyclone.

[0014] According to an embodiment of the present disclosure, a method for removing black char from a starting slurry containing calcium carbonate and black char may include flowing the starting slurry through a hydrocyclone under conditions sufficient to cause the black char particles to rise to the top forming an overflow and the calcium carbonate to remain at the bottom forming an underflow, flowing the overflow to waste, and flowing the underflow to a collection chamber.

[0015] According to an embodiment of the present disclosure, a method for removing black char from a starting slurry containing calcium carbonate and black char may include flowing the starting slurry to a first hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion to form a first overflow and the calcium carbonate to remain at a lower portion to form a first underflow, flowing the first underflow to a second hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion to form a second overflow and the calcium carbonate to remain at a lower portion to form a second underflow, flowing the first and second overflows to waste, and flowing the second underflow to a collection chamber.

[0016] According to an embodiment of the present disclosure, a method of removing black char from a starting slurry containing calcium carbonate and black char may include loading the starting slurry into the trap tank using a continuous inflow at or near the center of the trap tank, agitating the starting slurry in the trap tank using an agitator under conditions sufficient to cause the black char particles to rise to the top of the trap tank and the calcium carbonate to settle to the bottom of the trap tank for a residence time of about 4 to about 10 minutes at a tip speed of about 0.1 m / sec to about 1.5 m / sec, and pumping the calcium carbonate from the bottom of the trap tank to a collection vessel.

[0017] According to an embodiment of the present disclosure, a method for removing black char from a starting slurry containing calcium carbonate and black char includes flowing an ozone-containing gas through the starting slurry at a flow rate of about 0.1 liters per minute to about 2 liters per minute per liter of starting slurry with an agitator at a tip speed of about 1 m / sec to about 5 m / sec, wherein the black char is oxidized to carbon dioxide gas by the ozone and removed with the gas stream.

[0018] According to an embodiment of the present disclosure, a method of removing black char from a starting slurry containing calcium carbonate and black char may include mixing the starting slurry with a foaming agent and a flotation collector compound to form a second slurry, processing the second slurry through a flotation device under airflow of about 1 slpm to about 3 slpm per liter of second slurry at a tip speed of about 150 m / min to about 500 m / min for about 1 minute to about 10 minutes with an agitator, together with the foam overflowing from the flotation device and the tail slurry remaining in the flotation device, the foam containing black char and the tail slurry containing calcium carbonate, collecting the tail slurry and dispersing the tail slurry in water to form a dispersed slurry containing calcium carbonate.

[0019] In any of the foregoing embodiments of the method for removing black char, the starting slurry can be a slurry of purified precipitated carbonate product. For example, the starting slurry can be a slurry of purified precipitated carbonate product resulting from any of the methods disclosed herein. [Brief description of the drawings]

[0020] [Figure 1A] FIG. 2 is a schematic diagram of a trap tank according to an embodiment of the present disclosure. [Figure 1B] FIG. 2 is a schematic diagram of a trap tank according to an embodiment of the present disclosure. [Diagram 2] FIG. 1 is a simplified flow scheme showing four hydrocyclones for removing black char in the waste stream. [Diagram 3] Field emission scanning electron microscope images of a lime mud sample are shown at two magnifications. The higher magnification shows how impurities (platelets and very fine particles) are embedded within the calcium carbonate aggregates. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Methods for treating lime mud according to embodiments of the present disclosure may include a series of chemical and mechanical treatments. The methods according to the present disclosure process the lime mud waste product into particles of suitable size and purity for use as a filler and / or pigment in paper and paperboard products, and remove undesirable inorganic species such as mineral silicates and black carbonaceous materials. Precipitated calcium carbonates generally suitable for use as fillers or pigments have a median particle size distribution of about 0.5 μm to about 5 μm, a mean particle size distribution of about 3 μm, and a mean particle size distribution of about 3 μm. 2 / g~about 20m 2 / g specific surface area, ISO color value of greater than about 80, and no visible black char. Unless otherwise noted herein, the median particle size distribution of the resulting precipitated calcium carbonate is measured using a laser light scattering instrument such as a Horiba LA-950. Sample preparation for particle size measurement includes mixing with a dispersant polymer such as poly(acrylic acid) followed by sonication.

[0022] The method of making precipitated calcium carbonate from lime mud according to the present disclosure may include one or both of the removal of impurities with high specific surface area and the removal of visible black char. The method of making precipitated calcium carbonate by the removal or reduction of impurities with high specific surface area and the removal of visible black char may include a two-step process. In an embodiment, the method of the present disclosure may remove high specific surface area material to yield purified precipitated calcium carbonate. In an embodiment, the method may further process the resulting purified precipitated calcium carbonate to remove visible black char for further purification of the purified precipitated calcium carbonate. In an embodiment, the method of the present disclosure may remove black char to yield purified precipitated calcium carbonate.

[0023] The high specific surface area impurities may include one or more of silicates, calcium silicate hydrates, hydrotalcite-like compounds, calcium aluminates, calcium phosphates, and amorphous silicates. High specific surface area is generally defined as impurities having a surface area of ​​20 m 2 / g. For example, a high specific surface area material is an impurity particle with a specific surface area of ​​more than 20 m 2 / g over ~100m 2 For example, a typical surface area of ​​such a high surface area material may be about 40 m 2 / g~about 60m 2 / g.

[0024] Black char is a black carbonaceous material that may be present in lime mud. Depending on the source of the lime mud, it may be necessary to treat the lime mud to remove both the high surface area impurities and the black char. Alternatively, only the removal of the black char may be necessary. When used in combination with a method for removing high surface area materials, the removal of the black char may occur after the removal of the high surface area materials. Generally, the removal of the black char is carried out to the point where no visible black char remains.

[0025] The embodiments of the present disclosure include removal of high surface area impurities by one or more of flotation cell methods, thermal aging methods, and phase separation methods. The embodiments of the present disclosure may separately or additionally include removal of visible black char by one or more of hydrocyclone methods, trap tank methods, ozone methods, and flotation methods. Any suitable combination of any one or more of the high surface area impurity removal methods and any one or more of the visible black char removal methods may be utilized.

[0026] In any of the methods of the present disclosure, lime mud cake may be produced as a waste product in many processes such as paper kraft pulp mills, sugar beet production, and as a by-product of acetylene production. The lime mud may be a reaction product of reacting lime and green liquor from a recovery boiler refinery. The refinery may be obtained from the combustion of Kraft Process Black Liquor or Soda Process (NSSC) black liquor, or from chemi-mechanical pulping liquor (CMP, CTMP, APMP).

[0027] In any of the processes disclosed herein, the lime mud cake may be washed by mixing the lime mud cake with water to form a first slurry and washing the first slurry to remove the caustic soda present in the lime mud cake.

[0028] In any of the methods of the present disclosure where pH adjustment is performed, the pH may be reduced by flowing carbon dioxide gas and / or a carbon dioxide-containing gas through the slurry. For example, flue gas may be used. Other suitable gases include pure carbon dioxide gas, such as from liquefaction sources, lean flue gas, ethanol or carbon dioxide by-products from petrochemical sources. The flue gas may be from a boiler or kiln with a carbon dioxide content of about 8% to about 30%.

[0029] In any of the methods of the present disclosure where grinding is performed, the grinding may be accomplished by any known suitable method for fine grinding. For example, the slurry may be ground by a variety of known grinding techniques, such as ball mills, sand mills, and media mills. For example, the slurry may be ground using a vertically or horizontally agitated media mill that uses glass, sand, and / or ceramic media. The media may have a median diameter of about 0.5 mm to about 3 mm. The slurry is ground to a particle size suitable for the anticipated end use. For example, when processing lime mud to form precipitated calcium carbonate, the methods of the present disclosure may include grinding to a median particle size of about 0.5 microns to about 5 microns, about 1 micron to about 2 microns, about 0.5 microns to about 1 micron, about 3 microns to about 5 microns, or about 2 microns to about 3 microns. Suitable sizes include median particle sizes of about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 microns.

[0030] Flotation Cell Method In an embodiment, a method of making precipitated calcium carbonate from lime mud may include the use of a flotation cell to remove high surface area impurities from the lime mud. Flotation cell techniques are commonly used to remove larger particle size impurities, with an average particle size of about 50-500 microns. It has been advantageously found that a method according to the present disclosure may enable the use of flotation cell techniques to remove smaller particle size impurities, such as those with a particle size of 4-20 microns. With reference to FIG. 3, the high surface area material is generally associated with the calcium carbonate crystals as an aggregate. Surprisingly, it has been found that despite this aggregate, separation of such impurities may be achieved by the flotation cell method of the present disclosure. One skilled in the art would not have expected the ability to separate such aggregates without removing or adversely affecting the calcium carbonate using conventional flotation cell techniques or even centrifugation.

[0031] The method may include mixing the lime mud cake with water to form a first slurry and adjusting the pH to about 8 to about 11, or about 9 to about 10.5, or about 8 to about 10, or about 8 to about 11. Other suitable pH values ​​include about 8, 8.5, 9, 9.5, 10, 10.5, and 11. The first slurry may then be mixed with a silicate flotation collector compound to form a second slurry, and the second slurry may be diluted with water to a solids content of about 5% to about 15% by weight based on the total weight of the second slurry. Alternatively, dilution may be performed when mixing the lime mud cake with water to form the first slurry to achieve a solids content of about 5% to about 15% by weight based on the total weight of the slurry.

[0032] The second slurry may then be processed through a flotation cell system under conditions sufficient to form a concentrated foam containing silicate particulate impurities and a tail slurry containing CaC03.

[0033] The tail slurry can be washed and separated into a liquid phase containing excess soluble salts and a paste containing purified CaCO3. 。The paste may be mixed with water and a dispersant to form a dispersed slurry having a viscosity of less than about 1000 cps at 100 rpm.

[0034] The purified precipitated calcium carbonate product produces a dispersed slurry which, if required for a particular application, can be milled to a median particle size of about 0.5 microns to 5 microns and / or the pH can be adjusted to about 9 to about 10.5 if the pH is outside this range after milling.

[0035] In embodiments, the first slurry having a silicate flotation collector compound may be mixed to a solids content of about 5% to about 40% by weight based on the total weight of the slurry. Other suitable ranges include about 5% to about 20% by weight, about 5% to about 10% by weight, about 20% to about 25% by weight, and about 30% to about 40% by weight. Other suitable values ​​include about 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 37, 38, and 40% by weight based on the total weight of the first slurry.

[0036] The silicate flotation collector compound can be an amine-based compound. For example, the silicate flotation collector can be one or more of a primary amine, a dialkylamine, a tertiary amine, and a quaternary amine. Examples of specific compounds include, but are not limited to, tallow, cocoamine, hydroxyethyl alkyl imidazoline, laurylamine, long chain alkyl pyridinium, and n-alkyl trimethyl ammonium. Silicate flotation collector compounds are commercially available and may include one or more of Flotigam® 3135 (Clariant), Flotigam® K2C (Clariant), Armak® 1019 (Akzo Nobel Surface Chemistry, LLC), MD 20763 (Akzo Nobel), CustAmine® 1208, (ArrMaz) and Tomamine® DA-17 (Evonik Industries), Ethomeen® (Akzo Nobel), and Duomac-T® (Akzo Nobel).

[0037] The silicate flotation collector compound may be provided in the first slurry in an amount of about 0.1% to about 0.3% by weight based on the dry weight of the lime mud cake.

[0038] Heat maturation method According to an embodiment, a method for making precipitated calcium carbonate from lime mud may include thermally aging the lime mud in sodium carbonate to reduce high surface area impurities. In an embodiment, the method may include mixing the lime mud cake with water and sodium carbonate to form a first slurry, and heating the first slurry under conditions to age the slurry and form one or more of pirsonite (NaCa(CO), 2H0, shortite (NaCa(CO), and gaylasite (NaCa(CO).5H0). The method may further include separating a solid portion from the aged slurry and washing the solid portion under conditions to decompose one or more of pirsonite, shortite, and gaylasite into a CaCO solid fraction and a NaCO solution fraction, and to remove sodium salts. The CaCO3 solid fraction can then be mixed with water and a dispersant to disperse the CaCO3 solid fraction in water and form a dispersed slurry having a Brookfield viscosity of less than about 100 cps at 100 rpm. Unless otherwise specified, viscosity values ​​reported herein are Brookfield viscosity values. The CaCO3 solid fraction is a purified precipitated calcium carbonate with a specific surface area suitable for use in, for example, papermaking applications.

[0039] If necessary for the desired application, the dispersed slurry may then be milled to a median particle size of about 0.5 microns to about 5 microns and / or the pH may be adjusted to a pH of about 8 to about 11. For example, the pH may be about 9 to about 10, or about 9 to about 10.5, or about 8 to about 10, or about 8 to about 11. Other suitable pH values ​​include about 8, 8.5, 9, 9.5, 10, 10.5, and 11.

[0040] In an embodiment, the process may include washing the first slurry prior to heat aging. The first slurry may be washed, for example, using a filter press, a clarifier, and / or a rotary vacuum filter. In an embodiment, washing may be accomplished using water. For example, an amount of water equal to about 1 to about 5 times the dry weight of the solids of the first slurry may be used.

[0041] In embodiments, heat aging can be carried out by heating the slurry to a temperature of about 80° C. to about 130° C., about 90° C. to about 100° C., about 85° C. to about 95° C., or about 80° C. to about 90° C. Other suitable temperatures include about 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, and 130° C.

[0042] In embodiments, the slurry may be aged for about 2 hours to about 8 hours, about 2 hours to about 4 hours, about 3 hours to about 7 hours, about 5 hours to about 8 hours, or about 3 hours to about 6 hours. Other suitable times include about 2, 3, 4, 5, 6, 7, or 8 hours.

[0043] In embodiments, the first slurry may include about 20% to about 40% by weight of sodium carbonate, based on the total weight of the first slurry. Other suitable amounts of sodium carbonate include about 25% to about 40% by weight, about 30% to about 35% by weight, about 20% to about 30% by weight, or about 25% to about 35% by weight. For example, the first slurry may include about 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40% by weight, based on the total weight of the first slurry.

[0044] In embodiments, the first slurry may include about 5% to about 30% by weight of lime mud, based on the total weight of the first slurry. Other suitable amounts of lime mud include about 5% to about 15% by weight, about 10% to about 20% by weight, and about 15% to about 30% by weight. For example, the first slurry may include about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30% by weight.

[0045] In an embodiment, the solid portion is separated from the aged slurry and then washed with water. Washing may be accomplished by passing water through the cake or paste using known equipment. For example, a filter press may be used. Alternatively, the solid portion may be resuspended in water and the separation process repeated. Other known washing techniques and equipment may be used.

[0046] In an embodiment, the sodium carbonate used in the process can be recycled for subsequent use. However, before recycling in the process, the dissolved silica present in the sodium carbonate must be removed from the process. This can be done, for example, by lowering the pH of the recovered sodium carbonate to about 9.5 to precipitate the silica, and filtering off the precipitated silica using any suitable filtering method. In an embodiment, the dissolved silica can be removed by mixing the recovered sodium carbonate with a sodium aluminate solution to precipitate aluminosilicates, and then filtering off the aluminosilicates using any suitable filtering method known in the art.

[0047] phase separation method According to an embodiment, the method of making purified precipitated calcium carbonate from lime mud may include the use of phase separation to remove high surface area impurities. It has been advantageously found that high surface area impurities may be phase separated from the calcium carbonate and removed to produce a purified precipitated calcium carbonate product.

[0048] In an embodiment, the method may include mixing the lime mud cake with water and a dispersant to form a dispersed slurry having a viscosity of less than about 1000 cps at 100 rpm, and adjusting the pH of the dispersed slurry to a pH of about 8 to about 10.5. In an embodiment, the dispersed slurry may have a viscosity of less than about 100 cps at 100 rpm. The method may further include milling the dispersed slurry to a median particle size of about 0.5 microns to about 5 microns. After milling, the pH may be readjusted to about 8 to about 10.5, if necessary. The method may then include inducing phase separation in the milled slurry using conditions sufficient to obtain a slurry containing impurity particles and a paste containing purified calcium carbonate. The paste may be separated from the centrifugal product and dispersed in water to a desired solids content to produce precipitated calcium carbonate.

[0049] In an embodiment, the method may include mixing the lime mud cake with water to form a first slurry, where the pH of the first slurry may be adjusted to about 10 to about 11, and then inducing phase separation to separate the impurities and produce a paste containing the purified calcium carbonate. The paste may then be separated and mixed with water and a dispersing agent to form a dispersed slurry having a viscosity of less than about 1000 cps at 100 rpm. In an embodiment, the dispersed slurry may have a viscosity of less than about 100 cps at 100 rpm. The dispersed slurry may be milled to a particle size of about 0.5 microns to about 5 microns, and the pH may be adjusted to about 9 to about 10.5. In such embodiments where milling is performed after the phase separation step, it has been found that a dispersing agent is not required in the first slurry, but may instead be added when forming the dispersed slurry of the purified precipitated calcium carbonate.

[0050] In any of the phase separation methods disclosed herein, centrifugation may be used as the phase separation method. For example, the method may include centrifuging the milled slurry for a residence time of about 1 to about 10 minutes under conditions sufficient to achieve a g-force of about 500 to about 2000 g to obtain a centrifuged slurry containing impurity particles and a paste containing purified calcium carbonate. The paste may be separated from the centrate and dispersed in water to a target solids content depending on the end use of the purified precipitated carbonate product.

[0051] In any of the phase separation methods disclosed herein, gravity separation or other known phase separation techniques may be used to phase separate impurities from the calcium carbonate. Gravity settling parameters such as settling time and vertical liquid depth may be adjusted for a given slurry.

[0052] The first slurry can have a solids content of about 10% to about 35% by weight, based on the total weight of the slurry. Other suitable amounts include about 15% to about 30% by weight, about 10% to about 20% by weight, about 20% to about 25% by weight, or about 10% to about 30% by weight. For example, the first slurry can have a solids content of about 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, or 35% by weight.

[0053] In any of the foregoing embodiments, when the paste or slurry is diluted to form a dispersed slurry of purified precipitated calcium carbonate, the dispersed slurry may have a solids content of about 25% to about 50% by weight, based on the total weight of the dispersed slurry. Other suitable solids contents by weight based on the total weight of the dispersed slurry include about 25% to about 40%, about 30% to about 45%, about 30% to about 50%, or about 40% to about 50%. For example, the dispersed slurry may have a solids content of about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, and 50% by weight, based on the total weight of the dispersed slurry.

[0054] In any of the foregoing embodiments, the dispersing agent may be one or more of sodium poly(acrylate), polycarbomo- or copolymers of monomeric units including acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, hydroxyacrylic acid, and maleic anhydride.

[0055] Hydrocyclone treatment to remove black char Hydrocyclone treatment may be used alone or in combination with any method for removing high surface area material and black char from lime mud. When used in combination with a method for removing high surface area material, removal of the black char may occur after removal of the high surface area material.

[0056] As noted above, the black char removal process may be performed on precipitated calcium carbonate or lime mud cake resulting from treatment to remove high surface area impurities. When used with purified precipitated calcium carbonate after removal of high surface area impurities, the resulting product is generally a slurry of purified precipitated calcium carbonate with water and a dispersing agent. When used with lime mud cake, the lime mud cake may be dispersed in water, optionally with a dispersing agent, to form a slurry for treatment by the black char removal process. Any of the dispersing agents described above may be used. For ease of reference, the term "black char removal starting slurry" is used herein and is understood to refer to a slurry formed from the pre-treated precipitated calcium carbonate slurry or lime mud cake from which high surface area impurities have been removed.

[0057] The black char removal starting slurry having water and dispersant and black char impurities may be passed through an apparatus including one to four hydrocyclones in a series and / or parallel configuration. As the slurry passes through the apparatus, the black char impurities float to the top of the slurry and may be removed. With reference to FIG. 2, for example, in an embodiment having four hydrocyclones, the process may include passing the slurry through a first hydrocyclone, where the overflow (lighter particles) moves to a third hydrocyclone and the underflow (heavier particles) moves to a second hydrocyclone. In the second hydrocyclone, the overflow moves to the first hydrocyclone while the underflow moves to a collection chamber. In the third hydrocyclone, the overflow moves to a fourth hydrocyclone and the underflow moves to the first hydrocyclone. In the fourth hydrocyclone, the overflow moves to waste and the underflow moves to the third hydrocyclone.

[0058] Trap tank treatment for black char removal Instead of hydrocyclone processing, a trap tank process may be utilized to remove the black char. Referring to Figures 1A and 1B, schematic images of a trap tank are shown. The trap tank may have a generally conical shape. In the embodiment shown in Figures 1A and 1B, the trap tank has an upper cylindrical portion disposed at a frustoconical bottom. The trap tank may further include an overflow valve, a rotating blade, and an outlet. The overflow valve may be disposed at the upper region of the trap tank where the slurry is fed into the trap tank to collect overflow that may result from having a higher feed rate than the output rate, etc. Without intending to be bound by theory, it is believed that the trap tank design balances the tendency of the black char slurry to move upwards toward the surface against the slower downward flow of the slurry. Agitation near the surface of the slurry promotes separation of the black char from the lime mud particles.

[0059] In an embodiment as shown in Figures 1A and 1B, the cylindrical section may include a feed line for feeding the black char removal starting slurry from a feed tank or other receptacle, and a rotating blade disposed at the bottom of the cylindrical section for agitating the slurry as it is fed into the trap tank. As the slurry in the trap tank is agitated, the black char impurities rise to the surface, leaving the purified sample at the bottom of the tank. An outlet pump may be incorporated at the bottom of the trap tank to collect the purified sample. In an embodiment, the system may be operated to maintain a constant or substantially constant flow of feed to the trap tank and purified sample from the trap tank. In an embodiment, the method may include passing the collected sample through the trap tank one or more times, two or more times, three or more times, four or more times, or five or more times. For example, about 1 to about 3 times, about 2 to 5 times, about 1 to 4 times, and about 1 to 5 times. Once the feed / collected sample has been passed through the trap tank the desired number of times, it may be collected in a collection chamber. If desired, the recovered product can be further treated to make it more suitable for use as a filler or pigment, such as by adjusting the pH and / or adjusting the solids content.

[0060] The agitator may be a flat disc blade, a dispersing blade, and / or a sawtooth impeller. The agitator may be any blade capable of achieving laminar flow in the slurry. In an embodiment, the agitator may be a Cowles blade having a diameter equal to about 0.4 to about 0.95 times the diameter of the cylindrical section of the trap tank.

[0061] The agitator may rotate at a tip speed of about 0.1 to about 1.5 m / sec while maintaining laminar flow.

[0062] In embodiments, the slurry may have a residence time in the tank of about 2 to about 10 minutes, about 5 to about 8 minutes, about 4 to about 6 minutes, about 3 to about 7 minutes, or about 2 to about 9 minutes. Other suitable residence times include about 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes.

[0063] Ozone method for removing black char In an embodiment, the black char may be removed by exposing the black char removal starting slurry to ozone treatment. The black char removal starting slurry may be or may include a refined form of precipitated calcium carbonate resulting from any of the methods described above. The exposure to ozone may be performed while the black char removal starting slurry is stirred. Without intending to be bound by theory, it is believed that the ozone oxidizes the black char to carbon dioxide, which is then removed in the gas stream.

[0064] Any suitable device for generating and flowing ozone through the black char removal starting slurry may be used. The generator may use ultraviolet light, electrical discharge or electrolysis. The ozone may be generated in flowing air or pure oxygen, or a mixture of the two. The air or oxygen may be flowed at a flow rate of about 0.1 to about 2 liters / minute of ozone-containing gas per liter of lime mud slurry. For example, a dry air flow of about 1 liter per minute may be used to treat 675 mL of lime mud slurry at 10% solids. The concentration of ozone in the air or oxygen stream is about 2 g / m 3 ~50g / m 3 , about 25g / m 3 ~about 50g / m 3 , about 5g / m 3 ~Approx. 12g / m 3 , about 2g / m 3 ~about 15g / m 3 , about 8g / m 3 ~about 15g / m 3 , or about 35 g / m 3 ~about 45g / m 3 Other suitable concentrations include about 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, and 50 g / m 3 Examples include:

[0065] The process may be carried out using any agitator design suitable for maximizing gas contact with the liquid. For example, radial flow or Rushton impellers may be used with tip speeds of about 1 m / sec to about 5 m / sec. The ozone reaction may be carried out at high temperatures and high pH. For example, a temperature of 40°C may be used. For example, temperatures of about 40°C to about 80°C, about 50°C to about 60°C, about 40°C to about 60°C, about 50°C to about 70°C, or about 45°C to about 65°C may be used. For example, a pH greater than pH 10 or greater than pH 11 may be used. Higher reaction rates with ozone are achieved at these conditions of higher temperature and pH, thus allowing for shorter reaction times in the vessel.

[0066] Flotation method for removing black char In an embodiment, the black char may be removed using flotation. The black char removed starting slurry may be mixed with a foaming agent and a flotation collector compound. The mixture may be mixed at a tip speed of about 150 m / min to about 500 m / min, about 200 m / min to about 350 m / min, about 300 m / min to about 400 m / min, or about 150 m / min to about 250 m / min under airflow for 1 to 10 minutes, collecting the foam as it overflows. The black char is removed with the foam. The remaining tail slurry contained purified precipitated calcium carbonate.

[0067] Suitable blowing agents include standard alcohols having a hydrocarbon chain with 5 to 10 carbon atoms, such as methyl isobutyl carbinol (MIBC), amyl alcohol, cresol, and terpineol. Other blowing agent types, such as polyalkoxy ethers and polyglycol ethers, may also be used. Any combination of one or more blowing agents may be used. The blowing agents may be used at levels of 10 to 250 ppm, or from about 10 ppm to about 100 ppm, from about 100 ppm to about 250 ppm of blowing agent per dry weight of lime mud. Other suitable values ​​include about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70 75, 80, 85, 90, 95, 100, 125, 150, 175, 200, 225, and 250 pp of blowing agent per dry weight of lime mud.

[0068] Suitable flotation collector compounds include kerosene, or other oily, water immiscible compounds such as diesel fuel. Collectors may be used at 100 to 1000 ppm, 200 to 500 ppm, about 150 to about 350 ppm, or about 700 ppm to about 1000 ppm of collector per dry weight of lime mud. Other suitable values ​​include about 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, and 1000 ppm of collector.

[0069] The air flow can be about 1 standard liters per minute (SLPM) to about 6 SLPM, about 3 SLPM to about 5 SLPM, about 1 SLPM to about 2 SLPM, or about 4 SLPM to about 6 SLPM. Other suitable flow rates include about 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8, 5, 5.2, 5.4, 5.8, and 6 SLPM.

[0070] In any of the embodiments herein, the precipitated calcium carbonate product and / or slurry, whether or not further processed for removal of black char, may be mixed with optional additives to formulate the precipitated calcium carbonate into a suitable formulation for use as a filler and / or pigment for a given papermaking application. EXAMPLES

[0071] Example 1: Thermal Aging of Sodium Carbonate (HA) Lime mud was treated to obtain a precipitated calcium carbonate material with a reduced specific surface area and reduced impurity content compared to the starting material. The lime mud cake was obtained as a waste product from a pulp mill. The lime mud cake was slurried in a 25-30% sodium carbonate solution with a calcium carbonate solids content of 10-20%. In particular, in 1 kg of slurry, 200 g CaCO3 (20% CaCO3), 240 g Na2CO 3、 560g H2O(30% Na2CO3).

[0072] The slurry was then heat aged at 100°C for 8 hours to form the mixed salt pirsonite (Na2Ca(CO3)2·5H2O). A comparison of the process without the heat aging step was performed and the results are outlined in the table below.

[0073] The slurry was then filtered or separated into solid and liquid phases by vacuum filtration using a Buchner funnel and paper. In the Buchner funnel, the cake was washed with three portions of water, each portion equal to the solid mass of the original lime mud cake. The solids were washed with the removed water to remove sodium salts, which could optionally be recycled to the start of the process. This washing also decomposed the pirsonite back to CaCO3 and Na2CO3.

[0074] The CaCO3 solids were separated and dispersed in water at 35% solids using sodium poyl (acrylic acid) as a chemical dispersant. The dispersant was used to reduce the viscosity of the slurry to a Brookfield viscosity of less than about 100 cps at 100 rpm.

[0075] The pH of the milled slurry was adjusted to a pH of about 9.

[0076] To recycle the sodium carbonate solution, the dissolved silica must first be removed and the wash water separated. The silica can be removed by either 1) lowering the pH to about 9.5 with carbon dioxide gas to form precipitated silica, which is then filtered off, or 2) precipitation as an aluminosilicate with a sodium aluminate solution, which is also filtered off. The water can be removed by evaporation or a membrane process such as reverse osmosis.

[0077] The resulting precipitated calcium carbonate formed by thermal aging showed a significant reduction in the specific surface area and the amount of silicon dioxide present compared to the treatment without the aging step. [Table 1]

[0078] Example 2: Removal of impurities using phase separation Lime mud cake obtained as a waste product from a pulp mill was slurried in water at 20% solids. A filter press with a wash cycle was used to process the slurry to remove excess white liquor from the pulp mill. A filter press was used to first form a cake from the 20% solids slurry, then water was passed through the cake. A wash cycle was performed using water in an amount four times the mass of the dry solids.

[0079] The washed cake was dispersed in water at 20% solids using sodium poly(acrylate) as a chemical dispersant. The resulting dispersed slurry had a viscosity of about 10 cps at 100 rpm.

[0080] The dispersed slurry was then gassed with flue gas having a carbon dioxide content of about 15% to reduce the pH to a pH of about 10.5.

[0081] The pH adjusted slurry was then milled to the desired particle size suitable for use as a paper filler or coating pigment. In this example, the pH adjusted slurry was milled using a vertical media mill operated continuously and in a single pass therethrough. The resulting milled median particle size was about 3.5 microns.

[0082] The pH of the milled slurry was adjusted again by gassing with carbon dioxide gas to a pH of 9.5. The slurry was then continuously centrifuged at 1400-1500 rpm (see table) to obtain a g-force of 713 or 819 g with a residence time of 4.8-8.4 minutes. The centrate containing particulate impurities was removed and the paste containing the purified product was separated for further processing.

[0083] Four samples were prepared by the above method and the paste obtained from each sample had the solids content shown in the table below. The resulting precipitated calcium carbonate had a significant reduction in impurity elements such as silicon, aluminum, magnesium, and iron. The reduction in specific surface area and SiO2 content compared to the starting kiln lime mud is shown below. [Table 2]

[0084] Example 3: Removal of impurities using phase separation of lime mud before grinding Lime mud cake obtained as a waste product from a pulp mill was slurried in water at 20% solids. A horizontal filter press with a wash cycle was used to process the slurry to remove excess white liquor in the liquid phase. The filter press was used to first form a cake from the 20% solids slurry, thereby obtaining a cake with 65% solids, and then water was passed through the cake. A wash cycle was performed using water in an amount four times the mass of the dry solids.

[0085] The resulting washed cake was slurried in water at 23% solids and then treated to reduce the pH to 10.9 by gassing with carbon dioxide containing flue gas.

[0086] The pH adjusted slurry was then continuously centrifuged at 1300 rpm to obtain a g-force of 615 with varying residence times as indicated in the table. The centrate containing particulate impurities was removed and the paste containing the purified product was separated for further processing.

[0087] The resulting centrifuged paste was dispersed in water at 30% solids using sodium poly(acrylate) as the dispersant. The Brookfield viscosity of the dispersed slurry was 13 cps at 100 rpm. Four samples were prepared using the above method as shown in the table below.

[0088] The resulting product had less impurity elements such as silicon, aluminum, magnesium and iron, and also had reduced impurity phases such as calcium silicate, hydrotalcite and black char, as well as a lower specific surface area. [Table 3]

[0089] Example 4: Removal of impurities using gravity settling The process of Example 2 was repeated, except that after grinding, a gravity settling process was utilized instead of centrifugation. In particular, the centrifugation step of Example 2 was replaced by allowing the mixture to settle in a test tube cylindrical container at the indicated liquid height and settling time specified below. After the specified settling time, the top layer was poured off and removed, and the bottom layer was tested.

[0090] Gravity settling resulted in some reduction in the specific surface area, which was believed to be due to the reduction of impurities such as silicates. [Table 4]

[0091] Example 4: Flotation to remove impurities from lime mud Lime mud cake obtained as a waste product from a pulp mill was slurried in water at 20% solids. The slurry was then gassed with pure carbon dioxide to reduce the pH to the pH value shown in the table.

[0092] For silicate specific flotation collection, Flotigam® 3135 (CLARIANT) or CustAmine® 1208 (ARRMAZ) were added to the pH adjusted slurry. The table below shows the dosage based on the dry weight of the lime mud.

[0093] The slurry was then diluted with water to 8 wt. % solids and processed through a flotation cell system to produce a concentrated foam containing silicate particle impurities and a tail slurry containing the purified product.

[0094] The tail slurry was then filtered to remove excess soluble salts in the liquid phase using a vacuum filter and paper. The processing resulted in a paste containing purified CaCO3.

[0095] The resulting product contained fewer impurity elements such as silicon, aluminum, magnesium and iron. It also had a reduced amount of impurity phases such as calcium silicate and hydrotalcite, and a lower specific surface area. The table below shows the SiO2 reduction achieved with this method. [Table 5]

[0096] Example 5: Removal of black char by hydrocyclone treatment The final product slurry, consisting of 30% calcium carbonate in water, sodium polyl(acrylic acid) as a dispersant, and black char impurities, was further processed to remove the black char impurities. The viscosity of the slurry was 40 cps. The slurry was processed through an apparatus containing four hydrocyclones (HCs). As the slurry passed through, the black char impurities floated to the top of the slurry and could be processed or separated from the refined recovered product (see Figure 2). The overflow (light particles) and underflow (heavy particles) of each HC flow to the next HC as follows: [Table 6]

[0097] Qualitative observations indicated that the HC2 underflow contained less black char than the feed product.

[0098] Example 6: Removal of black char using a trap tank The trap tank depicted in Figures 1A and 1B was used to separate the black char from the final product slurry, which contained 34.8% calcium carbonate in water, sodium poyl (acrylic acid) as a dispersant, and black char impurities. The slurry had a Brookfield viscosity of 20 cps at 100 rpm.

[0099] The feed lines in the trap tank were positioned just above the slurry level near the center of the agitator to maximize the retention time of the slurry on the blades. The feed and product flow pumps were adjusted to maintain a constant level in the trap tank. Dual head peristaltic pumps were used for the feed and product flow streams.

[0100] The feed stream was started to fill the trap tank to the overflow port, and a double head pump was used to recycle the product line to the trap tank during filling. The agitator was turned on and set to rpm as outlined in the table below. An airflow was provided in some runs, as identified in the table below. Airflow was not found to aid in separation, as illustrated in the table below. The table also indicates the run time. In each run, the feed and product streams ran for approximately three retention times. After the final pass, the product was collected in a clean beaker. The product in the beaker was observed to qualitatively determine the amount of black char present in the product compared to the trap tank and overflow tank.

[0101] All runs use a 1.65 liter trap tank with an internal diameter of 4.9 inches and either a 4.8 inch diameter flat blade or a 4.0 inch diameter Cowles blade agitator. These runs demonstrate the need for low rpm agitation and slow feed rates. [Table 7]

[0102] Example 6: Removing black char with ozone treatment A lime mud slurry of 10% solids was prepared from the lime mud by dilution with water.

[0103] Approximately 625 ml of the lime mud slurry was poured into a 1 liter bottle with a hole in the lid for an R100 agitator and another hole in the top for supplying ozone via SS tubing. The slurry was mixed at 730 rpm. The valve on the dry air cylinder was opened to supply the ozone generator at 10 psi pressure. The dry air stream was supplied at 1 lpm. The ozone generator (Model 1KNT purchased from Oxidation Technologies) was operated at 100% ozone level. Without intending to be bound by theory, it is believed that the ozone reacts with the black char and oxidizes it to carbon dioxide, which is then removed in the gas stream.

[0104] After processing, Hunter and ISO dry brightness were tested, and pictures of the slurry surface were taken with a camera and the % black spot area was determined using ImageJ software. TGA-DSC analysis was performed. [Table 8]

[0105] Example 7: Removing black char using flotation The filter cake, which was about 70% solids, was diluted with water to make a lime mud slurry with 10% solids.

[0106] 200 or 500 ppm of kerosene collector was added to the slurry and mixed for 2 minutes.

[0107] Then 100 or 250 ppm of MIBC (methyl isobutyl carbinol) blowing agent was added and the mixture was mixed for 0.5 minutes.

[0108] The mixture was then mixed at 2000 or 1500 rpm under an air flow of 4 standard liters per minute. Mixing and aeration continued for 5 or 10 minutes, collecting the foam as it overflowed.

[0109] The collected foam was tested for brightness as an indicator of black char content. The remaining tail slurry contained purified CaCO3 with reduced black char. [Table 9]

[0110] The use of "a" or "an" is employed to describe elements and components of the embodiments herein. This is done simply for convenience and to give a general sense of the description. This description should be read to include one or at least one, and the singular also includes the plural unless it is clear otherwise.

[0111] Still further, these figures depict preferred embodiments for illustrative purposes only: those skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein.

[0112] Thus, while specific embodiments and applications have been illustrated and described, it should be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those skilled in the art, may be made in the arrangement, operation and details of the methods and apparatus disclosed herein without departing from the spirit and scope, as defined by the appended claims.

Claims

1. 1. A process for producing purified precipitated calcium carbonate from lime mud comprising the steps of: mixing the lime mud cake with water and a dispersant to form a first slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm; milling the first slurry to a median particle size of about 0.4 microns to about 5 microns; allowing the milled slurry to phase separate under conditions sufficient to obtain a centrifuged slurry containing impurity particles and a paste containing purified calcium carbonate; and diluting the paste in water to a target solids content, thereby producing a dispersed slurry containing the refined precipitated calcium carbonate.

2. 10. The method of claim 1, further comprising adjusting the pH of the dispersed slurry to a pH of about 8 to about 10 and / or adjusting the pH of the milled slurry to a pH of about 8 to about 10.

3. 3. The method of claim 1 or 2, wherein phase separation comprises centrifuging the milled slurry at a g-force of about 500 g to about 2000 g for a residence time of about 1 minute to about 10 minutes.

4. 3. The method of claim 1 or 2, wherein phase separation comprises allowing the milled slurry to settle by gravity.

5. 5. The method of claim 4, wherein the gravity settling is carried out for a settling time of about 2 to 8 hours to a settling depth of about 2 cm to about 3000 cm.

6. 2. The method of claim 1, further comprising mixing the lime mud cake with water to form a first slurry prior to forming the dispersed slurry, and washing the first slurry to remove caustic soda present in the lime mud cake.

7. The method of any one of claims 1 to 6, wherein the first slurry has a solids content of about 25% to about 50% by weight.

8. 1. A process for producing purified precipitated calcium carbonate from lime mud comprising the steps of: mixing the lime mud cake with water to form a first slurry; adjusting the pH of the first slurry to about 10 to about 11; centrifuging the first slurry under conditions sufficient to achieve a g-force of about 500 to about 2000 g for a residence time of about 1 to about 10 minutes to obtain a centrifuged slurry comprising a paste including impurity particles and refined calcium carbonate; mixing said paste with water and a dispersant to form a dispersed slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm and containing said refined precipitated calcium carbonate.

9. The method of claim 8, further comprising milling the dispersed slurry to a median particle size of about 0.4 microns to about 5 microns.

10. 10. The method of claim 9, further comprising adjusting the pH of the milled slurry to be from about 9 to about 10.

5.

11. The method of any one of claims 1 to 10, further comprising adjusting the pH of the dispersed slurry to be from about 9 to about 10.

5.

12. The method of any one of claims 8 to 11, wherein the first slurry has a solids content of about 10% to about 25%.

13. 1. A process for producing purified precipitated calcium carbonate from lime mud comprising the steps of: mixing the lime mud cake with water to form a first slurry; adjusting the pH of the first slurry to about 8 to about 11; mixing the first slurry with a silicate flotation collector compound to form a second slurry; Concentrated foam containing silicate particle impurities and CaCO ３ treating said second slurry through a flotation cell system under conditions sufficient to form a tails slurry containing The tail slurry is washed and separated to separate the liquid phase containing excess soluble salts and purified CaCO ３ and forming a paste containing mixing said paste with water to form a dispersed slurry having a viscosity of less than about 100 cps at 100 rpm and containing said refined precipitated calcium carbonate.

14. 14. The method of claim 13, wherein the first slurry is formed to a solids content of about 5% to about 15% or the second slurry is diluted with water to a solids content of about 5% to about 15% prior to being processed by the flotation cell system.

15. 15. The method of claim 13 or 14, further comprising milling the dispersed slurry to a median particle size of about 0.4 microns to about 5 microns.

16. 16. The method of claim 15, further comprising adjusting the pH of the milled slurry to be from about 9 to about 10.

5.

17. The method of any one of claims 13 to 16, further comprising adjusting the pH of the dispersed slurry to be from about 9 to about 10.

5.

18. The method of any one of claims 13 to 17, wherein the first slurry has a solids content of about 20% to about 25%.

19. 19. The method of any one of claims 13 to 18, wherein the silicate flotation collector compound is provided in an amount of about 0.1% to about 0.3% by weight, based on the dry weight of the lime mud cake.

20. A method according to any one of claims 13 to 19, wherein the silicate flotation collector compound is amine-based.

21. 21. The method of claim 20, wherein the silicate flotation collector compound is one or more of a primary amine, a dialkyl amine, a tertiary amine, and a quaternary amine.

22. 22. The method of claim 21, wherein the silicate flotation collector is one or more of tallow, cocoamine, hydroxyethyl alkyl imidazoline, laurylamine, long chain alkyl pyridinium, and n-alkyl trimethyl ammonium.

23. A method for removing black char from a starting slurry containing calcium carbonate and black char, comprising the steps of: flowing the starting slurry through a first hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion thereof to form a first overflow and the calcium carbonate to remain at a lower portion thereof to form a first underflow; passing said first underflow through a second hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion thereof to form a second overflow and to cause said calcium carbonate to remain at a lower portion thereof to form a second underflow; passing said first overflow through a third hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion thereof to form a third overflow and to cause said calcium carbonate to remain at a lower portion thereof to form a third underflow; directing the second underflow into a collection chamber; directing the second overflow into the first hydrocyclone; directing the third underflow into the first hydrocyclone; passing said third overflow through a fourth hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion thereof to form a fourth overflow and to cause said calcium carbonate to remain at a lower portion thereof to form a fourth underflow; draining the fourth overflow into a waste container; and and directing the fourth underflow to the third hydrocyclone.

24. 1. A method for removing black char from a starting slurry containing calcium carbonate and black char, comprising the steps of: flowing the starting slurry through a hydrocyclone under conditions sufficient to cause the black char particles to rise to the top forming an overflow and to cause the calcium carbonate to remain at the bottom forming an underflow; draining said overflow to waste; and flowing the underflow into a collection chamber.

25. 1. A method for removing black char from a starting slurry containing calcium carbonate and black char, comprising the steps of: flowing the starting slurry through a first hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion thereof to form a first overflow and the calcium carbonate to remain at a lower portion thereof to form a first underflow; passing said first underflow through a second hydrocyclone under conditions sufficient to cause the black char particles to rise to an upper portion thereof to form a second overflow and to cause said calcium carbonate to remain at a lower portion thereof to form a second underflow; draining the first and second overflows to waste; and flowing the second underflow into a collection chamber.

26. 1. A method for removing black char from a starting slurry containing calcium carbonate and black char, comprising the steps of: loading the starting slurry into the trap tank using a continuous inflow at or near the center of the trap tank; agitating the starting slurry in the trap tank using an agitator under conditions sufficient to cause black char particles to rise to an upper portion of the trap tank and calcium carbonate to settle to a lower portion of the trap tank at a tip speed of about 0.1 m / sec to about 1.5 m / sec for a residence time of about 4 to about 10 minutes; and pumping the calcium carbonate from the bottom of the trap tank to a collection vessel.

27. 27. The method of claim 26, wherein the residence time is from about 6 minutes to about 10 minutes.

28. 28. The method of claim 27, wherein the residence time is from about 8 minutes to about 10 minutes.

29. 29. The method of any one of claims 26 to 28, wherein the tip speed is from about 0.1 m / sec to about 0.2 m / sec.

30. 1. A method for removing black char from a starting slurry containing calcium carbonate and black char, comprising the steps of:

1. A method comprising flowing an ozone-containing gas through a starting slurry at a flow rate of about 0.1 liters per minute to about 2 liters per minute per liter of starting slurry with an agitator at a tip speed of about 1 m / sec to about 5 m / sec, wherein the black char is oxidized to carbon dioxide gas by the ozone and removed with the gas stream.

31. The concentration of ozone in the ozone-containing gas is about 2 g / m ３ ~Approx. 50g / m ３ The method of claim 30, wherein

32. 32. The method of claim 30 or 31, wherein the starting slurry is heated to an elevated temperature of about 40°C to about 60°C while the ozone-containing gas flows through the starting slurry.

33. 33. The method of any one of claims 30 to 32, wherein the pH of the starting slurry is adjusted to a pH greater than 10 while the ozone-containing gas flows through the starting slurry.

34. 1. A method for removing black char from a starting slurry containing calcium carbonate and black char, comprising the steps of: mixing the starting slurry with a foaming agent and a flotation collector compound to form a second slurry; treating the second slurry through a flotation device under an air flow of about 1 slpm to about 3 slpm per liter of second slurry with an agitator at a tip speed of about 150 m / min to about 500 m / min for about 1 minute to about 10 minutes, together with foam overflowing from the flotation device and tail slurry remaining in the flotation device, the foam containing the black char, and the tail slurry containing the calcium carbonate; collecting the tail slurry and dispersing the tail slurry in water to form a dispersed slurry containing the calcium carbonate.

35. 35. The method of claim 34, wherein the flotation collecting agent is present in an amount of about 100 to about 1000 ppm.

36. 36. The method of claim 34 or 35, wherein the flotation collector compound is one or more of kerosene or diesel oil.

37. A method according to any one of claims 34 to 36, wherein the starting slurry is lime mud cake dispersed in water and optionally a dispersing agent.

38. 38. The method of any one of claims 23 to 37, wherein the starting slurry is a refined slurry containing calcium carbonate after treatment for removal of high surface area impurities.

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