METHOD OF MAKING PURIFIED PRECIPITATED CALCIUM CARBONATE FROM LIME MUD

1 . A method for making a purified precipitated calcium carbonate from lime mud, comprising:

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 CaCO 3 solid fraction and Na 2 CO 3 solid fraction and to remove sodium salts; and

admixing the CaCO 3 solid fraction with water and a dispersant to disperse the CaCO 3 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.

2 . The method of claim 1 , further comprising milling the dispersed slurry to a median particle size of about 0.4 microns to about 5 microns.

3 . The method of claim 2 , further comprising adjusting the pH of the milled slurry to a pH of about 9 to about 10.5.

4 . The method of claim 1 or 2 , further comprising adjusting the pH of the dispersed slurry to a pH of about 9 to about 10.5.

5 . The method of any one of the preceding claims, comprising heating the slurry to a temperature of about 80° C. to about 100° C. for about 2 to about 8 hours.

6 . The method of any one of the preceding claims, wherein the first slurry comprises about 20 wt % to about 40 wt % sodium carbonate and about 5 wt % to about 40 wt % lime mud cake based on the total weight of the first slurry.

7 . The method of any one of claims 1 to 3 , further comprising:

collecting the sodium carbonate solution fraction;

removing dissolved silica from the collected sodium carbonate solution fraction to form a treated sodium carbonate, by:

lowering the pH to about 9.5 to precipitate silica and filtering out the precipitated silica, or

admixing the removed sodium salts with a sodium aluminate solution to precipitate aluminosilicate and filtering out the precipitated aluminosilicate,

where the treated sodium carbonate is recycled for use in a subsequent method of making a precipitated calcium carbonate from lime mud.

8 . A method for making a purified precipitated calcium carbonate from lime mud, comprising:

admixing a 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 centrate slurry comprising impurity particles and a paste comprising a purified calcium carbonate; and

diluting the paste in water to a target solids content to thereby produce a dispersed slurry containing the purified precipitated calcium carbonate.

9 . The method of claim 8 , 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.

10 . The method of claim 8 or claim 9 , wherein phase separating 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.

11 . The method of claim 8 or claim 9 , wherein phase separating comprises gravity settling the milled slurry.

12 . The method of claim 11 , wherein the gravity settling is performed for a settling time of about 2 to 8 hours to a settling depth of about 2 cm to about 3000 cm.

13 . The method of claim 8 , further comprising, prior to forming the dispersed slurry, admixing the lime mud cake with water to form a first slurry and washing first slurry to remove caustic soda present in the lime mud cake.

14 . The method of any one of claims 8 to 13 , wherein the first slurry has a solids content of about 25 wt % to about 50 wt %.

15 . A method for making a purified precipitated calcium carbonate from lime mud, comprising:

admixing a 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 centrate slurry comprising impurity particles and a paste comprising a purified calcium carbonate; and

admixing 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.

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

17 . The method of claim 16 , further comprising adjusting the pH of the milled slurry to be about 9 to about 10.5.

18 . The method of any one of claims 8 to 17 , further comprising adjusting the pH of the dispersed slurry to be about 9 to about 10.5.

19 . The method of any one of claims 15 to 18 , wherein the first slurry has a solids content of about 10% to about 25%.

20 . A method for making a purified precipitated calcium carbonate from lime mud, comprising:

admixing a lime mud cake with water to form a first slurry;

adjusting the pH of the first slurry to be about 8 to about 11;

admixing 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 concentrate foam containing silicate particle impurities and a tail slurry containing CaCO 3 ;

washing and separating the tail slurry into a liquid phase containing excess soluble salts and a paste containing purified CaCO 3 ; and

admixing the paste with water to form a dispersed slurry having a viscosity of less than about 100 cps at 100 rpm and containing the purified precipitated calcium carbonate.

21 . The method of claim 20 , wherein either 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% before being processed through the flotation cell system.

22 . The method of claims 20 to 21 , further comprising milling the dispersed slurry to a median particle size of about 0.4 microns to about 5 microns.

23 . The method of claim 22 , further comprising adjusting the pH of the milled slurry to be about 9 to about 10.5.

24 . The method of any one of claims 20 to 23 , further comprising adjusting the pH of the dispersed slurry to be about 9 to about 10.5.

25 . The method of any one of claims 20 to 24 , wherein the first slurry has a solids content of about 20% to about 25%.

26 . The method of any one of claims 20 to 25 , wherein the silicate flotation collector compound is provided in an amount of about 0.1 wt % to about 0.3 wt % based on the dry mass of the lime mud cake.

27 . The method of any one of claims 20 to 26 , wherein the silicate flotation collector compound is amine-based.

28 . The method of claim 27 wherein the silicate flotation collector compound is one or more of primary amines, dialkyl amines, tertiary amines, and quaternary amines.

29 . The method of claim 28 , wherein the silicate flotation collector is one or more of tallow, coco amines, hydroxyethyl alkyl imidazoline, lauryl amine, long chain alkyl pyridinium, and n-alkyl trimethyl ammonium.

30 . The method of any one of the preceding claims wherein the dispersed slurry has a solids content of about 25% to about 50%.

31 . The method of any one of the preceding claims, wherein the dispersant is one or more of sodium poly(acrylic acid), polycarboxylate homo or co-polymer of monomer units including acrylic, methacrylic, itaconic, maleic, fumaric, crotonic, hydroxyacrylic acids, and maleic anhydride.

32 . The method of any one of the preceding claims, wherein the lime mud cake is generated as a pulp mill waste byproduct, sugar beet production byproduct, or the byproduct of acetylene production.

33 . The method of any one of the preceding claims, further comprising adjusting pH with carbon dioxide gas and/or carbon dioxide containing gas.

34 . A method of removing black char from a starting slurry containing calcium carbonate and black char, comprising:

flowing a starting slurry into a first hydrocyclone under conditions sufficient to cause black char particles to rise to a top portion of and form a first overflow and the calcium carbonate to remain at a bottom portion to form a first underflow;

flowing the first underflow to a second hydrocyclone under conditions sufficient to cause black char particles to rise to a top portion and form a second overflow and the calcium carbonate to remain at a bottom portion to form a second underflow;

flowing the first overflow to a third hydrocyclone under conditions sufficient to cause black char particles to rise to a top portion and form a third overflow and the calcium carbonate to remain at a bottom portion to form a third underflow;

flowing the second underflow to a recovery 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 black char particles to rise to a top portion and form a fourth overflow and the calcium carbonate to remain at a bottom portion to form a fourth underflow;

flowing the fourth overflow to a waste container; and

flowing the fourth underflow to the third hydrocyclone.

35 . A method of removing black char from a starting slurry containing calcium carbonate and black char, comprising:

flowing a starting slurry into a hydrocyclone under conditions sufficient to cause black char particles to rise to a top portion of and form an overflow and the calcium carbonate to remain at a bottom portion to form an underflow;

flowing the overflow to waste; and

flowing the underflow to a recovery chamber.

36 . A method of removing black char from a starting slurry containing calcium carbonate and black char, comprising:

flowing a starting slurry into a first hydrocyclone under conditions sufficient to cause black char particles to rise to a top portion of and form a first overflow and the calcium carbonate to remain at a bottom portion to form a first underflow;

flowing the first underflow to a second hydrocyclone under conditions sufficient to cause black char particles to rise to a top portion and form a second overflow and the calcium carbonate to remain at a bottom portion to form a second underflow;

flowing the first and second overflows to waste; and

flowing the second underflow to a recovery chamber.

37 . A method of removing black char from a starting slurry containing calcium carbonate and black char, comprising:

loading the starting slurry into a trap tank using a continuous inflow at or near a center of the trap tank;

agitating the starting slurry in the trap tank using an agitator 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 min and under conditions sufficient to cause black char particles to rise to a top portion of the trap tank and calcium carbonate to settle to a bottom portion of the trap tank; and

pumping the calcium carbonate from the bottom portion of the trap tank to a recovery container.

38 . The method of claim 37 , wherein the residence time is about 6 min to about 10 min.

39 . The method of claim 38 , wherein the residence time is about 8 min to about 10 min.

40 . The method of any one of claims 37 to 39 , wherein the tip speed is about 0.1 m/sec to about 0.2 m/sec.

41 . A method of removing black char from a starting slurry containing calcium carbonate and black char, comprising:

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

42 . The method of claim 41 , wherein the concentration of ozone in the ozone-containing gas is about 2 g/m 3 to about 50 g/m 3 .

43 . The method of claim 41 or 42 , wherein the starting slurry is heated to elevated temperature of about 40° C. to about 60° C. during flowing of the ozone containing gas through the starting slurry.

44 . The method of any one of claims 41 to 43 , wherein a pH of the starting slurry is adjusted to a pH above 10 during flowing of the ozone containing gas through the starting slurry.

45 . A method of removing black char from a starting slurry containing calcium carbonate and black char, comprising:

admixing the starting slurry with a frother and a flotation collector compound to form a second slurry;

processing the second slurry through a flotation apparatus under an air flow per liter of second slurry of about 1 slpm to about 3 slpm and with an agitator at tip speeds of about 150 m/min to about 500 m/min for about 1 min to about 10 min, a foam which overflows from the flotation apparatus and a tail slurry which remains in the flotation apparatus, the foam comprising the black char and the tail slurry comprising the calcium carbonate; and

collecting the tail slurry and dispersing the tail slurry in water to form a dispersed slurry containing the calcium carbonate.

46 . The method of claim 45 , wherein the flotation collector is present in an amount of about 100 to about 1000 ppm.

47 . The method of claim 45 or 46 , wherein the flotation collector compound is one or more of kerosene or diesel oil.

48 . The method of any one of claims 45 to 47 , wherein the starting slurry is a lime mud cake dispersed in water and optionally a dispersant.

49 . The method of any one of claims 34 to 48 , wherein the starting slurry is purified slurry containing calcium carbonate after processing for removal of high specific area impurities.