CA2081831A1 - Stabilization of precipitated calcium carbonate - Google Patents

Abstract

STABILIZATION OF PRECIPITATED CALCIUM CARBONATE

Abstract of the Disclosure Method of preventing destabilization of an aqueous slurry of crystalline precipitated calcium carbonate (PCC), as made by the carbonation process. The slurry is treated after crystallization by adding a stabilizing factor as one or more agents selected from the group consisting of dry hydroxy ethyl cellulose of wet and dry types, hydropho-bically modified hydroxy ethyl cellulose of wet and dry types, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydrophobically modified hydroxy ethyl cellulose, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydroxypropyl methylcellulose, urethane associative stabilizers, hydroxybutyl methylcellulose, hydroxypropyl cellulose, guar and natural gum derivatives, carbon dioxide in gaseous form, and mixtures of the foregoing, in amount as determined by viscosity adjustment to be sufficient to prevent destabilization of the slurry in bulk quantity without agitation or further treatment for a long period, providing a stabilized new commercial PCC
slurry product with PCC content of 20-75%/w, capable of storage or shipment without stratification or gelation for at least 30 days.

Description

STABILIZATION OF PRECIPITATED CAICIUM CARBONATE

Background of the Invention This invention relates to aqueous precipitated calcium carbonate slurries. It relates particularly to methodology for stabilizing such slurries by treatment to permit their shipment or storage over substantial periods of time or at elevated temperatures, or both, by preventing destabiliza-tion of the slurry in bulk quantity.
Precipitated calcium carbonate ("PCC") is sold commercially in the form of a slurry to make available calcium carbonate as a fiber extender in paper-making processes and, because of the optical characteristics of such material (e.g., opacity), as an additive in paper coating and for use in paint compositions, such as an extender for titanium dioxide pigment.
Such slurries, as intended for making paints and for paper-making applications, are typically alkaline, with pH
extending typically to moderately alkaline levels, e.g., 13.5-11.0 for shipment. Initially, pH levels as low as 7 are possible in the process of product manufacture, but pH
is expected to rise with increased storage intervals to as high as 11.5. With increase of temperature, the rate of pH
rise is increased.
A known problem for the shipment and/or storage of such slurries is that, over relatively long shipment periods, and at possibly high ambient temperatures to which such products may be exposed during shipment, for example, by rail car, the slurry may gel and~or stratify with consequent syneresis so as to be incapable of flow. In either case, whether as a gel or settled mass, there will be difficulty, if not relative impossibility, of removing the slurry or storage from a shipment vessel or vehicle such as a railroad tank car, barge or other shipment container.
As such a destabilized PCC slurry is of diminished utility and value, the customer may reject slurry so shipped and received in such deteriorated condition.
Such difficulties may be expected normally to be encountered in the shipment, over even reasonable periods of time, and with exposure to expectable variations in ambient temperatures, of PCC slurries ranging from relatively low concentration, e.g., 20 weight percent to more common concentrations of about 50 weight percent of solids per weight of aqueous medium. Stratification and gelling problems are greatly exacerbated for PCC slurry concentrations with weight percentages substantially between about 20 and 65%/w. Such is a matter of increasing concern because of economic and commercial pressures for ever-increasing percentages of PCC slurries, including concentrations to 75%/w of solids.

The problem of stabilizing slurries of materials used for pigments is known. For example, Fitton U.S. Pat.
3,790,396 described a method of stabilizing aqueous slurries of various inorganic paint pigments, including silicas, by adding acidic material such as aluminum sulfate to control p~ and thereby stabilize the slurry to avoid settling.
Eberts U.S. Pat. 3,245,819 proposed stabilizing calcium carbonate slurries by using an alkaline agent to prevent inability to flow where such calcium carbonate was formed by ~o a process which results in a tendency of the pH to decrease with passage of time causing the slurry to form a thick pasty mass. Such patent is directed to the solution of a problem relating from a tendency of the material to become more acidic after manufacture, specifically proposing to use an alkaline agent, including calcium hydroxide, sodium hydroxide, barium hydroxide, magnesium hydroxide and potassium hydroxide, as well as sodium hexametaphosphate as a dispersant. PCC slurries manufactured by the carbonation method have a different problem of an increase of pH with time.
Shurling et al, U.S. Pat. 3,291,626 proposed a method of stabilizing aqueous slurries of inorganic pigments by adding alum or sulfuric acid to reduce the pH to a range preventing the slurry from fractionating but not with respect to slurries of PCC. The alum or sulfuric acid was said to be used alone or combined with carboxymethyl cellulose (CMC), starches, protein, gums and polyphosphates (e.g., TSPP). It is believed that the addition of sulfuric acid to PCC slurries would be counterproductive and the use of alum apparently would not be predictably efficaceous.

2o8l83l Grishaberl described stabilization of pigment slurries by adding additional dispersant to clay and precipitated slurries using, e.g., sodium hexametaphosphate as a dispersant to maintain viscosity.
Windle U.S. Pat. 3,797,610 proposed stabilization of natural calcium carbonate pigment concentrations of 70-85%/w by the addition of specific anionic polymers in the range of 0.05 to 0.5%/w based on the weight of dry natural calcium carbonate.
Calcium carbonates are classifiable2 according to whether made from natural products, such being referred to as natural calcium carbonate, or instead from chemical reaction of raw materials, namely precipitated calcium carbonate (PCC). Crystalline polymorphic variation of calcium carbonate as between calcite and aragonite types is among various of chemical and physical factors3 characteristic of the material to be considered in determining or affecting usage of PCC. The myriad possibilitie~ and permutations resulting from such crystalline and other physical and unique chemical characteristics, including crystallography, not only complicate processing but also make long-term stabilization of PCC slurries highly problematic. Hagemeyer4 describes acute time-temperature sensitivity and solidification expectancy for calcium carbonate slurries.

1. Grishaber, R.J., Prolonqed Storage of Piament Slurries, ~APPI S3~ 465 (March 1970).

2. Hagemeyer, R.W. (ed.), Paper Coatina Piqments, TAPPI
Monograph 38, (4):37 (1976).

3. Hagemeyer, supra, at 38.

4. Hagemeyer, supra, at 56-57.

, 20,~1~31 In the process of manufacturing a precipitated calcium carbonate (PCC), the type of crystals produced (calcite, aragonite, or a mixture of both) and the particle size distribution of said crystals is governed primarily by the conditions under which the precipitation occurs. For supplying this product for use in the plastics, paint, ink and paper industries, it is important to maintain the integrity of the crystalline structure. When the material is subjected to mechanical shearing forces necessary to produce a high solids slurry (65-80%/w solids) or in the drying process, many of the physical and optical properties can be compromised. Therefore, an ability to provide PCC's at lower solids concentrations, e.g., at 50%/w or less, is advantageous. However, at such lower concentrations, the solids and/or viscosity in a PCC suspension are insufficient to enable storage for extended periods of time without some form of mechanical agitation As a further aspect of classifying the present disclosure relative to the prior art, one must consider the process by which carbonate is precipitated in the formation of PCC. According to said Eberts U.S. Pat. 3,245,819, there is used the so-called Wyandotte method of precipitation wherein milk of lime is reacted with amonium chloride to form a solution of calcium chloride. The calcium chloride is then reacted with a soda ash solution, producing calcium carbonate and salt. Any residual sodium or chloride ions in the carbonate tend to lower the pH of the carbonate slurry (especially at elevated temperatures) necessitating the addition of an alkaline agent to raise the pH level.
In comparison, when manufacturing PCC using the l'carbonation process~ with which the present disclosure is concerned, the problem of slurry storage will be manifested not by a lowering of pH but rather by a shift of pH in the ' ,~ .

2~81831 alkaline direction as hydroxyl ions go into solution thus raising the pH of the carbonate slurry. It is for this reason that such carbonate slurries are time-temperature sensitive. At elevated temperatures, such slurries will thicken much more rapidly than if the same material were stored at temperatures less than 100F.

, 2o8l83~

Summary of the Invention Among the several objects of the invention may be noted the provision of novel methodology which provides long-term stabilization of calcium carbonate slurries; and which is commercially viable and immediately useful in commercial production, storage and shipment of bulk quantities of PCC
slurries.
Among still other objects of the invention may be noted the provision of highly effective and commercially economic methods for producing PCC slurries with PCC made by the carbonation process wherein crystalline calcium carbonate is precipitated with aragonitic or calcitic morphology, or a combination of both, to provide stabilized PCC slurry products which may be shipped or stored for periods of at least 30 days, including under conditions of high ambient temperatures, as by shipment methods including, without limitation, rail car, tank truck, barge and pipeline, with assurance against destabilization such as by stratification or gelation.
Among further objects of the invention are to provide not only effective and highly economic processes for stabilizing PCC slurries but also to provide new stabilized PCC slurry products having superior physical characteris-tics, including viscosity stability, heat stability and improved texture for facilitating loading, shipment, unloading and customer handling, all without fear of degradation of product by destabilization prior to use, even under adverse handling and temperature conditions as well as unexpected holding delays, as during rail shipment (such as may result in temperature increases or permit change in pH
levels); and where the solids concentration of PCC in such stabilized, viscosity-stable slurries may range from about 20% to about 75~-by weight.

~.~$~

It is also an object of the invention to provide such processes and viscosity-stable PCC products permitting higher concentrations of PCC in PCC slurries than previously practical without stabilization as herein proposed.
Briefly, a method according to the present disclosures prevents the destabilization of a crystalline aqueous slurry of precipitated calcium carbonate, as made by the carbonation process, resulting in deleterious change in consistency thereof, the weight percent of said precipitated calcium carbonate being from 20 to 75, comprising or consisting essentially of treating the slurry after crystallization by adding a stabilizing factor one or more agents selected from the group consisting of dry hydroxy ethyl cellulose, wet hydroxy ethyl cellulose, dry hydrophobically modified hydroxy ethyl cellulose, wet hydrophobically modified hydroxy ethyl cellulose, mixtures, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydroxypropyl methylcellulose, urethane associative stabilizers, hydroxybutyl methylcellulose, hydroxypropyl cellulose, guar and natural gum derivatives, carbon dioxide in gaseous form, and mixtures of the foregoing, in amount sufficient to prevent destabilization of said slurry in bulk quantity without agitation or further treatment for a period of not less than 30 days.
Other objects and features of the invention will be apparent or are described hereinbelow.

Brief Description of the Drawings The single figure is a graph showing viscosity data from examples of the invention wherein final viscosity is plotted as a function of start viscosity for various concentrations of stabilizers in a final PCC slurry product.

Description of the Preferred Embodiments In the process of forming PCC according to the carbonation process, calcium hydroxide slurry or solution, or a combination of calcium hydroxide and water are introduced into a precipitator. Carbon dioxide is then bubbled through the precipitator to prepare calcium carbonate in water ranging from 10-20%/w for example. Such material may then be screened and/or filtered to provide higher concentrations for fur~her treatment or for shipment. In accordance with the present invention, such increased concentration material, as a slurry, is treated by the addition of a stabilizing agent to prevent destabiliza-tion such as stratification or gelation.
The stabilizing factor may be one or more agents selected from a group consisting of wet hydroxy ethyl cellulose ("wet HEC"), dry hydroxy ethyl cellulose ("dry HEC"), hydrophobically modified hydroxy ethyl cellulose of wet or dry types ("wet HM-HEC~ or "dry HM-HEC"), and mixtures of the foregoing, alkali-soluble acrylic polymers, alkali swellable associate thickeners, other associative thickeners, attapulgite clays, algenates r sodium algenate, - starches, carbon dioxide in gaseous form, and mixtures thereof. Other members of the group from which the stabilizing agent may be selected include hydroxypropyl methylcellulose, urethane associative stabilizers, hydroxybutyl methylcellulose, hydroxypropyl cellulose, guar and natural gum derivatives, and mixtures thereof.
Preferably, the stabilizing factor consists essentially of hydroxethyl ethyl cellulose selected from dry, wet and hydrophobically modified types, the latter being most preferred, or the combination of such a factor with the additional stabilizing agent gaseous carbon dioxide.

2081~3~

By way of illustrative example indicative of the various possibilities for a preferred aspect of the invention, a typical PCC slurry, e.g. of 50% PCC by weight, with dispersant (as described hereinbelow) added, is treated by the addition in predetermined amount of HM-HEC, in organic liquid form as available under the trade name NATROSOL PLUS ~FPS~ (fluidized polymer suspension) type from Aqualon Company. The amount of HM-HEC or other stabilizer is determined according to desired predetermined starting (before addition) and desired predetermined ending (after addition and stirring) viscosities.
More specifically, the HM-HEC (hereinafter sometimes referred to generically as HEC) so added is broadly in the range of about 0.1-0.9%/w, and more preferably, 0.2-0.6%/w, but with most specifically preferred concentration of 0.5~/w in a slurry mixing and/or handling stage of the process, by addition of the HEC during stirring of a batch quantity of the slurry. As the HEC is added, stirring is carried out using known stirrers under conditions known to be suitable for minimizing crystalline damage. Mixing of the slurry with HEC is then carried out by the stirrer for sufficient time, over several minutes, to build viscosity from about 2000 cps to about 4000-5000 cps, as measured using a Brookfield viscometer with a No. 4 spindle at 20 rpm. Such weights of stabilizing factor are based on weight of PCC in the slurry.
The bulk quantity of mixed, stabilized slurry is then transferred to bulk storage or shipment container, e.g., a truck, rail car or truck trailer for shipment or storage.
Prior to or during such transfer, or in the storage tank, carbon dioxide in gaseous state may be introduced by bubbling into the slurry, as additional to the HEC, thus lowering the pH of the slurry to further increase the viscosity. The rate of introducing the CO2 is controlled so as to bring about a creamy consistency characterized by a frothy appearance from which it will be evident that bubbles of C2 are entrained for subsequent reaction in situ.
A suitable commercially available biocide is preferably added at about the same point in time as the HEC in an amount to ensure biocidal protection of the product for a period of not less than 30 days.
If the slurry will not be exposed to high ambient temperatures during shipment or storage, the addition of CO2 may be omitted. By relatively high ambient temperatures is meant over about 80F and typically 80-100F may be regarded for shipment purposes as high.
Particularly where during shipment high ambient temperatures are expected with consequent risk of pH
increase in the slurry, CO2 will be introduced during mixing or thereafter in sufficient amount to reduce the pH without greatly increasing the viscosity.
In adding CO2 as a stabilizing agent, it is preferred broadly to reduce pH to about 8.0-9.0, and viscosity in the range of about S000-10,000 cps may be achieved. A preferred range of pH after CO2 addition is 8.0-8.5, assuming a starting pH of typically 9.5-10Ø
For slurries of high PCC concentration, e.g., broadly at least in the range of 65-75%~w but preferably all those of 70%/w PCC or more, the addition of HEC may be omitted without degradation of stability, and where accordingly carbon dioxide is the only stabilizing agent.
At the other extreme, where the slurry may have PCC
concentrations of broadly about 20-30%/w or more, and more specifically those of concentration more than 25-30%/w, and most specifically preferably those of concentration of more than 30%/w, a dispersant is added during the point in the process after filtration and before HEC addition.

The dispersant may be selected copolymers and homopolymers of acrylic acid and methacrylic acid, inorganic polyphosphates, AMP (namely 2-amino-2-methyl-1-propanol), and mixtures thereof.
Broadly, efficacity of stabilization of PCC slurries according to the invention is demonstrated for concentration of PCC in such slurries ranging from about 20 to about 75~/w.
By so treating the aqueous solution in this manner with a stabilizing agent described hereinabove in sufficient amount, destabilizing of the slurry in bulk quantity without agitation or further treatment is prevented for a period of at least 30 days, being sufficient to ensure of the capability of shipment as well as storage under typically adverse conditions, with shipment possible over long distances, whether by truck or rail car, including delays sometimes associated with such modes of shipment.
The addition of C02 as a stabilizer is indicated, a noted above, when exposure to high ambient temperatures is expected for the stabilized slurry. The mechanism by which C02 provides a stabilizer is believed to go beyond merely the initial lowering of pH upon introduction of such gas by reaction then occurring. That is, the entrained bubbles of carbon dioxide resulting from gasification of a slurry will remain for in situ availability to control pH at times well beyond completion of the process and measurement of pH and viscosity at the time of loading of product for storage or shipment~ The mechanism by which the in situ reaction of bubbles of carbon dioxide control pH is believed to come about as follows: When crystals of PCC are grown in the carbonation process, whether of aragonitic or calcitic morphology, or both, starting with calcium hydroxide, some calcium hydroxide is surrounded by calcium carbonate crystals in an occluded condition, and cannot be carbonated. Such occluded calcium hydroxide structures remain as a core, available for reaction. With time and temperature, such core leaches from the calcium carbonate matrix, and goes into solution with leaching increasing in rate with increasing temperature. As the leached calcium oxide becomes part of the solution, it affects pH, driving it upward. There is availability of entrained CO2 bubbles for reaction with the occluded calcium hydroxide as it leaches into solution, it being likely that calcium hydroxide reacts with CO2, producing additional calcium carbonate. Excess bicarbonate ions in solution continually provide effective neutralization over periods to 30 days or more and buffer the slurry so as to prevent or inhibit rise of pH. Broadly, the entrainment of C02 according to the constraints discussed in the disclosure will ensure that a PCC slurry will maintain its pH broadly in a range of about 7.5 to about 11 to provide a viscosity-stable product. The addition of C02 as an economic stabilizing agent effectively also frees up dispersants which are objectionable because of relative expense.
The physical characteristic of frothiness provided by the entrained C02 bubbles provides a useful augmentation of smoothness and consistency of the PCC slurry as a synergistic effect going beyond mere improvement in viscosity. Thus not only does entrainment of CO2 bubbles provide means for neutralization for reaction with calcium hydroxide as it leaches from occluded sites but also provide textural augmentation for better handling and consistency.
The following examples illustrate aspects of the invention without being intended to limit the scope or utility of the invention.

Example 1. 1500g of aqueous PCC slurry produced by the carbonation process, having pH of 9.8 as manufactured and containing 50% by weight PCC solids having mixed calcitic and aragonitic morphology, was adjusted to a starting viscosity 1000 cps through the addition of sodium polyacrylate as a dispersant according to the previously described process methodology. To this slurry 0.2% of HM-HEC was added as a stabilizer, along with 0.3g biocide, and mixed at 3100 rpm for 8 minutes using a Cowles blade.
The viscosity increased to 1600 cps initially. However, the slurry settled out before 30 days. Viscosities in this example, as in all data herein se~ forth, were measured by Brookfield Viscometer using a No. 4 spindle at 20 rpm. CO2 was not added. Normal ambient temperatures existed.
Example 2. The procedure of Example 1 was repeated except the starting viscosity was adjusted to 1500 cps by dispersant addition. The viscosity increased to a final value of 2650 cps by addition of HM-HEC during stirring.
The sample remained stably fluid, i.e., without gelation or evident stratification, to 30 days.
Example 3. The procedure of Example 2 was repeated except that the starting viscosity was adjusted to 2000 cps by dispersant addition. The viscosity was increased by HM-HEC addition to 3400 cps. The slurry remained stably fluid for 30 days.
Exam~le 4. The procedure of Example 3 was repeated except that the viscosity was adjusted to 2500 cps. The viscosity was increased to 4500 cps by HM-HEC addition.
The slurry remained stably fluid for 30 days.
Example 5. The procedure of Example 4 was repeated except that the viscosity was adjusted to 3000 cps. The viscosity was increased to 4700 cps by HM-HEC addition. The slurry remained stably fluid for 30 days.

_ 16 -Example 6. The procedure of Example 5 was repeated except that the viscosity was adjusted to 1000 cps. The addition of 0.3% by weight HM-HEC increased the viscosity to 2050 cps by HM-HEC addition. The slurry remained stably fluid for 30 days.
Example ~. The procedure of Example 6 was repeated except that the viscosity was adjusted to 1500 cps. The viscosity was increased to 3800 cps by HM-HEC addition. The slurry remained stably fluid for 30 days.
Exam~le 8. The procedure of Example 7 was repeated except that the viscosity was adjusted to 2000 cps. The viscosity was increased to 4200 cps by HM-HEC addition. The slurry remained stably fluid for 30 days.
Example 9. The procedure of Example 8 was repeated except that the viscosity was adjusted to 2500 cps. The viscosity was increased to 5650 cps by HM-HEC addition. The slurry remained stably fluid for 30 days.
Example 10. The procedure of Example 9 was repeated except that the viscosity was adjusted to 3000 cps. The viscosity was increased to 5800 cps by HM-HEC addition. The slurry remained stably fluid for 30 days.
Example 11. The procedure of Example 10 was repeated except that the viscosity was adjusted to 1000 cps. The addition of 0.4% by weight HN-HEC increased the viscosity to 2700 cps. The slurry remained stably fluid for 30 days.
Example 12. The procedure of Example 11 was repeated except that the viscosity was adjusted to 1500 cps. The viscosity increased to 5500 cps. The slurry remained stably fluid for 30 days.
Example 13. The procedure of Example 12 was repeated except that the viscosity was adjusted to 2000 cps. The viscosity increased to 5650 cps. The slurry remained stably fluid for 30 days.

2081~31 ExamPle 14. The procedure of Example 13 was repeated except that the viscosity was adjusted to 2500 cps. The viscosity increased to 6950 cps. The slurry remained stably fluid for 30 days.
Example 15. The procedure of Example 14 was repeated except that the viscosity was adjusted to 3000 cps. The viscosity was increased to 8100 cps. The slurry remained stably fluid for 30 days.
Example 16. The procedure of Example 15 was repeated except that the viscosity was adjusted to 1000 cps. The addition of 0.5% by weight HN-HEC increased the viscosity to 3450 cps. The slurry remained stably fluid for 30 days.
Example 17. The procedure of Example 16 was repeated except that the viscosity was adjusted to 1500 cps. The viscosity was increased to 5750 cps. The slurry remained stably fluid for 30 days.
Example 18. The procedure of Example 17 was repeated except that the viscosity was adjusted to 2000 cps. The viscosity was increased to 6400 cps. The slurry remained stably fluid for 30 days.
Example 19. The procedure of Example 18 was repeated except that the viscosity was adjusted to 2500 cps. The viscosity was increased to 8100 cps. The slurry remained stably fluid for 30 days.
Example 20. The procedure of Example 19 was repeated except that the viscosity was adjusted to 3000 cps. The viscosity was increased to 9200 cps. The slurry remained stably fluid for 30 days.
ExamDle 21. The procedure of Example 20 was repeated except that the viscosity was adjusted to 1000 cps. The addition of 0.6% by weight HM-HEC increased the viscosity to 4200 cps. The slurry remained stably fluid for 30 days.

208183~

Example 22. The procedure of Example 21 was repeated except that the viscosity was adjusted to 1500 cps. The viscosity was increased to 8600 cps. The slurry remained stably fluid for 30 days.
Example 23. The procedure of Example 22 was repeated except that the viscosity was adjusted to 2000 cps. The viscosity was increased to 9600 cps. The slurry remained stably fluid for 30 days.
Example 24. The procedure of Example 23 was repeated except that the viscosity was adjusted to 2500 cps. The viscosity increased to 11900 cps. The slurry remained stably fluid for 30 days.
Example 25. The procedure of Example 24 was repeated except that the viscosity was adjusted to 3000 cps. The viscosity was increased to 12500 cps. The slurry remained stably fluid for 30 days.
The data from Examples 1 through 25 are summarized by the graph labeled Figure 1. This data indicates that a 50%
PCC slurry may be kept flowable for up to 30 days by adding HM-HEC in the range of 0.2-0.6~ by weight of PCC. The starting viscosities should be in the range of 1000-3000 cps.
Example 26. A 1500g sample of 40~ solids PCC paste was adjusted to 1900 cps with a sodium polyacrylate dispersing agent. To this slurry lOg of HM-HEC was added, producing a viscosity of 3500 cps. After 5 weeks the slurry was still stable and flowable. Mixing time and viscosity measurement were identical to Example No. 1.
Example 27. The procedure used in Example 26 was followed except a 30% solids PCC slurry was dispersed down to 2000 cps. To this mixture 11.5g of HM-HEC was added producing a viscosity of 2900 cps. This sample was still stably fluid after 5 weeks.

~081831 ExamPle 28. The procedure used in Example 27 was followed except a 20% solids PCC slurry was dispersed down to 700 cps. To this slurry 14 grams of HM-HEC was added producing a viscosity of 1400 cps. This sample was still stably fluid after 5 weeks.
The results from Examples 26, 27, and 28 are shown in Table 1.

ExAMæLE SOLIDS PASTE WATER STAB. START VISC. STA~3. VISC.
26 40% 1500g 0g 10.0g 1900 3500 27 30% 1125g 375g 11.5g 2000 2900 28 20% 750g 750g 14.0g 700 1400 Example 29. The viscosity of a 50% solids PCC slurry was reduced to 2200 cps. 7.5g HN-HEC was added. Biocide and mixing conditions were identical to Example 1. The pH was recorded at 9.2. One-half of the sample was then placed in an oven at 50C and the pH measured weekly for 4 weeks.
Example 30. The procedure of Example 29 was followed except gaseous CO2 was bubbled into the slurry until a pH of 8.0 was reached. One-half of the sample was also placed in an oven at 50C with the pH being measured weekly.
Exam~le 31. One-half of the slurry produced in Example 28 was monitored weekly under room temperature (25C) storage conditions.
Example 32. One-half of the slurry produced in Example 30 was monitored weekly under room temperature (25C) storage conditions.
The results from Examples 29 through 32 are shown in Table 2. This data in Table 2 indicates the CO2 ~ddition made the slurry heat stable for a period of up to 4 weeks.
No CO2 is needed for room temperature stability.

~o --STARTING pH pH pH pH FLOWABILITY
EXAMPLE pH WEEX 1 WEEK 2 WEEK 3 WEEK 4 AFTER_4 WEEKS
29 9.2 10.0 10.1 10.8 11.4 Not Flowable 8.0 9.0 9.2 9.4 9.6 Flowable 31 9.2 9.3 9.3 9.4 9.4 Flowable 32 8.0 8.1 8.3 8.4 8.7 Flowable Example 33. A production sample of 70% PCC slurry without the addition of stabilizing agent was split with one-half being stored in oven at 50F and monitored weekly for 4 weeks for pH and viscosity.
Example 34. The second half of slurry drawn in Example 33 was gassed with CO2 for a sufficient time to lower the pH
to 8.7. It was then placed in the oven as in Example 33.

Start Week 1 Week 2 Week 3 Week 4 ~H Visc. pH Visc. pH Visc. PH Visc. pH Visc.
Ex. 33 10.1 800 10.4 1200 10.6 2000 11.3 2200 12.1 2650 Ex. 34 8.7 1200 9.2 900 9.7 800 10.0 850 10.5 800 Example 35. A rail tank car, approximately 16,000 gallons, of 50.8% PCC was produced using the formulation outlined in Example 18. (2000 cps start viscosity, biocide and 0.5% HM-HEC addition with an 8 minute mix time.) The final viscosity was measured at 3500 cps with a pH of 10.5.
The car was unloaded successfully 30 days later with no evidence of stratification or gelation.
Example 36. A rail tank car of 50.5% PCC was produced as described in Example 35. The viscosity was 4300 cps with a pH of 10.3. This, however, was shipped during a period of high ambient conditions and possibly extreme heat and proved not to be heat stable as evidenced by a very thick non-flowable mass experienced when car was opened 3 weeks later. Samples of car held at room temperature remained stably fluid for 4 weeks. A sample, however, placed in oven at elevated temperatures thickened after about a week.
Example 37. A tank car of 48.75% PCC was produced as described in Example 36. As this car was to be shipped during the hot season this slurry was treated with gaseous C2 as outlined in Example 30 to lower the pH from 10.3 to 8.8. The viscosity was 5400 cps. The car was opened 23 days later and found to be very fluid. It was pumped off with no evidence of stratification, i.e., settling, and processed as a TiO2 extender in a paint formulation.
Although Example 35 was successful with HEC only (No C2 addition) it was shipped during cool temperatures.
Examples 36 and 37 indicate that CO2 stabilization addition along with an additional stabilizing agent, e.g., HEC, is necessary to produce adequate stability if high temperatures are expected.
Exam~le 38. Four 800-gallon batches of 52.3% PCC
slurry were produced using 0.6% dry HEC. These four batches were transferred to a rail tank car. After 3 weeks the slurry was gravity drained from the car with no noticeable stratification (settling).

START FINAL
BATCH NO. MIX TIME HEC ADDITION VISCOSITY VISCOSITY
1 15 min. 0.6% 1900 3200 2 15 min. 0.6% 2200 3800 3 20 min. 0.6% 2300 3300 4 20 min. 0.6% 2200 3500 20~31 In view of the foregoing, it will be seen that the several objects of the invention are achieved and other advantages are attained.
Although the foregoing includes a description of the best mode contemplated for carrying out the invention, various modifications are contemplated.
As various modifications could be made in the methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting.

Claims (28)

1. A method of preventing the destabilization of an aqueous PCC slurry of precipitated calcium carbonate resulting in deleterious change in consistency thereof, comprising treating the slurry after crystallization by adding a stabilizing factor consisting essentially of at least one agent selected from the group consisting of hydroxy ethyl cellulose of wet and dry types, hydrophobically modified hydroxy ethyl cellulose of wet and dry types, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydroxypropyl methylcellulose, urethane associative stabilizers, hydroxybutyl methylcellulose, hydroxypropyl cellulose, guar and natural gum derivatives, carbon dioxide in gaseous form, and mixtures thereof, in amount sufficient to prevent destabilization of said slurry in bulk quantity without agitation or further treatment for a period of not less than 30 days.

2. A method according to claim 1 wherein the weight percent of said precipitated calcium carbonate is from 20 to 75 in the slurry.

3. A method according to claim 1 wherein the PCC
slurry is made from the carbonation process and the crystalline calcium carbonate is of aragonitic or calcitic morphology or mixtures thereof.

4. A method according to claim 1 wherein the stabilizing factor consists essentially of hydroxethyl ethyl cellulose selected from dry, wet and hydrophobically modified types.

5. A method according to claim 1 wherein the stabilizing factor consists essentially of hydrophobically modified hydroxy ethyl cellulose in amount, according to the weight of precipiated calcium carbonate, of 0.1-0.9%/w .

6. A method according to claim 1 wherein the stabilizing factor consists essentially of hydrophobically modified hydroxy ethyl cellulose in amount, according to the weight of precipiated calcium carbonate, of 0.2-0.6%/w.

7. A method according to claim 1 wherein the stabilizing factor consists essentially of wet hydrophobically modified hydroxy ethyl cellulose in amount, according to the weight of precipiated calcium carbonate, of 0.5%/w.

8. A method according to claim 5 and wherein the stabilizing factor further consists essentially of carbon dioxide bubbled into the slurry after adding the hydroxy ethyl cellulose, the slurry having with said carbon dioxide therein a pH of from about 7.5 to about 11.

9. A method according to claim 1 wherein the stabilizing factor consists essentially of carbon dioxide bubbled into the slurry to provide entrained bubbles therein for subsequent reaction in situ and such that the slurry has a frothy texture and maintains a pH of from about 7.5 to about 11 for a period of at least 30 days.

10. A method according to claim 1 wherein before adding a stabilizing factor, the slurry is treated with a dispersant to provide a predetermined starting viscosity, and during adding the stabilizing factor, the slurry is adjusted to a predetermined final viscosity.

11. A method according to claim 7 wherein the starting viscosity is about 2000 cps and the final viscosity is about 4000-5000 cps, as measured using a Brookfield viscometer with a No. 4 spindle at 20 rpm.

12. A method of preventing the destabilization of an aqueous PCC slurry of crystalline precipitated calcium carbonate resulting in deleterious change in consistency thereof, wherein the PCC slurry is made from the carbonation process and the crystalline calcium carbonate is of aragonitic or calcitic morphology or mixtures thereof, comprising treating the slurry after crystallization with a dispersant to provide a predetermined starting viscosity, and thereafter adding to a batch quantity of the slurry while stirring a stabilizing factor to adjust the slurry to a predetermined ending viscosity, the stabilizing factor consisting essentially of at least one agent selected from the group consisting of hydroxy ethyl cellulose of wet and dry types, hydrophobically modified hydroxy ethyl cellulose of wet and dry types, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydroxypropyl methylcellulose, urethane associative stabilizers, hydroxybutyl methylcellulose, hydroxypropyl cellulose, guar and natural gum derivatives, and carbon dioxide in gaseous form, and mixtures thereof, in amount sufficient to prevent destabilization of said slurry in bulk quantity without agitation or further treatment for a period of not less than 30 days, the viscosities being measured using a Brookfield viscometer with a No. 4 spindle at 20 rpm.

13. A method according to claim 12 wherein the stabilizing factor consists essentially of wet hydro-phobically modified hydroxy ethyl cellulose in amount, according to the weight of precipiated calcium carbonate, of 0-.1-0.9%/w

14. A method according to claim 12 wherein the stabilizing factor consists essentially of wet hydro-phobically modified hydroxy ethyl cellulose in amount, according to the weight of precipiated calcium carbonate, of 0.2-0.6%/w.

15. A method according to claim 12 wherein the stabilizing factor consists essentially of wet hydro-phobically modified hydroxy ethyl cellulose in amount, according to the weight of precipiated calcium carbonate, of 0.5%/w.

16. A method of preventing the destabilization of an aqueous PCC slurry of crystalline precipitated calcium carbonate resulting in deleterious change in consistency thereof, wherein the PCC slurry is made from the carbonation process and the crystalline calcium carbonate, comprising treating the slurry after crystallization with a stabilizing factor consisting essentially of carbon dioxide in gaseous form, in amount sufficient to prevent destabilization of said slurry in bulk quantity without agitation or further treatment for a period of not less than 30 days, and the weight percent of said precipitated calcium carbonate in the slurry is from about 20 to about 75.

17. A method according to claim 16 wherein the weight percent of said precipitated calcium carbonate in the slurry is from about 65 to about 75.

18. Stabilized PCC slurry according to claim 16 wherein the PCC slurry is made from the carbonation process and the crystalline calcium carbonate is of aragonitic or calcitic morphology or mixtures thereof.

19. Stabilized PCC slurry according to claim 16 wherein the carbon dioxide is bubbled into a batch quantity of the slurry, the slurry having with said carbon dioxide therein a pH of from about 7.5 to about 11.

20. Stabilized PCC slurry according to claim 19 wherein the carbon dioxide bubbled into the slurry provides entrained bubbles therein for subsequent reaction in situ and such that the slurry has a frothy texture and maintains pH in said range for a period of at least 30 days.

21. Stabilized PCC slurry consisting essentially of precipitated crystalline calcium carbonate and water wherein the weight percent of precipitated calcium carbonate therein is from 20 to 75, and a stabilizing factor selected from the group consisting of hydroxy ethyl cellulose of wet and dry types, hydrophobically modified hydroxy ethyl cellulose of wet and dry types, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydrophobically modified hydroxy ethyl cellulose, alkali-soluble acrylic polymers, alkali swellable associate thickeners, attapulgite clays, algenates, sodium algenate, starches, hydroxypropyl methylcellulose, urethane associative stabilizers, hydroxybutyl methylcellulose, hydroxypropyl cellulose, guar and natural gum derivatives, and carbon dioxide in gaseous form, and mixtures thereof, in amount sufficient to prevent destabilization of said slurry in bulk quantity without agitation or further treatment for a period of not less than 30 days.

22. Stabilized PCC slurry according to claim 21 wherein the PCC slurry is made from the carbonation process and the crystalline calcium carbonate is of mixed aragonitic or calcitic morphology or mixtures thereof.

23. Stabilized PCC slurry according to claim 21 wherein the stabilizing factor consists essentially of hydroxethyl ethyl cellulose selected from dry, wet and hydrophobically modified types.

24. Stabilized PCC slurry according to claim 21 wherein the stabilizing factor consists essentially of a combination of hydrophobically modified hydroxy ethyl cellulose in amount of 0.1-0.9%/w according to the weight of precipiated calcium carbonate and carbon dioxide bubbled into the slurry, the slurry having with said carbon dioxide therein a pH of from about 7.5 to about 11.

25. Stabilized PCC slurry according to claim 21 wherein the stabilizing factor consists essentially of carbon dioxide bubbled into the slurry to provide entrained bubbles therein for subsequent reaction in situ and such that the slurry has a frothy texture and maintains a pH of from about 7.5 to about 11 for a period of at least 30 days.

26. Stabilized PCC slurry consisting essentially of precipitated crystalline calcium carbonate and water wherein the weight percent of precipitated calcium carbonate therein is from about 20 to about 75, and carbon dioxide in gaseous form, as a stabilizing factor in amount sufficient to prevent destabilization of said slurry in bulk quantity without agitation or further treatment for a period of not less than 30 days.

27. Stabilized PCC slurry according to claim 26 the carbon dioxide is bubbled into the slurry to provide entrained bubbles therein for subsequent reaction in situ and such that the slurry has a frothy texture and maintains a pH of from about 7.5 to about 11 for a period of at least 30 days.

28. Stabilized PCC slurry according to claim 26 wherein the weight percent of precipitated calcium carbonate therein is from about 65 to about 75.