WO1998024725A1

WO1998024725A1 - Process for preparing calcium carbonate which contains low concentrations of non-calcium metals

Info

Publication number: WO1998024725A1
Application number: PCT/US1997/022249
Authority: WO
Inventors: R. Keith Frank; Jaime SIMÓN; Ted J. Hanson; John E. Pavlick
Original assignee: The Dow Chemical Company
Priority date: 1996-12-05
Filing date: 1997-12-04
Publication date: 1998-06-11
Also published as: PA8442301A1; AU5593298A

Abstract

The present invention concerns a process for preparing calcium carbonate where a calcium compound in an aqueous media is carbonated in the presence of a chelating agent, with the reaction run under controlled pH. The resulting calcium carbonate product contains less non-calcium metal concentration than the starting calcium compound.

Description

PROCESS FOR PREPARING CALCIUM CARBONATE WHICH CONTAINS LOW CONCENTRATIONS OF NON-CALCIUM METALS

Generally calcium carbonate is presently produced commercially by one of two processes. In one process fine-ground calcium carbonate is prepared by grinding limestone (which consists largely of calcium carbonate,) such that the 95% of the product passes through a 325 mesh sieve (US Standard). The second process for the preparation of calcium carbonate produces a material that is referred to as precipitated calcium carbonate (PCC). In this process limestone is mined and calcined to produce calcium oxide and carbon dioxide. Addition of water to the oxide yields a calcium hydroxide slurry that is converted to calcium carbonate by the addition of carbon dioxide. This process is described below in Scheme 1.

- Scheme 1

Precipitated Calcium Carbonate Process

CaC0₃ ^Heat > CaO + C0₂

CaO ) Ca(OH)₂

Ca{OH)₂ + C0₂ → CaC0₃ + H₂0

Many impurities are removed during this PCC process. Thus PCC products contain less impurities than fine-ground calcium carbonate products. However, this process does not remove all of the impurities and significant amounts of metal ions are still present in PCC products.

Some of the references available to prepare calcium carbonate are the following:

WO 96/15985 reports precipitated calcium carbonate containing 715 parts per million (ppm) of iron (reported as ppm Fe₂O₃). In addition, significant amounts of Pb are present in PCC samples. Thus even though the precipitation process reduces the concentration of metal ions in the calcium carbonate, PCC product contains significant amounts of non-calcium metals. WO 96/15985 describes a process for purification of calcium carbonate wherein calcium carbonate, as an aqueous slurry, is first treated with a chelating agent, then the slurry is heated and treated with carbon dioxide. Results using this process are reported to reduce the iron content of the calcium carbonate from about 700 ppm to about 300 ppm (reported as ppm Fe₂0₃); and U.S. Patent 4,824,653 teaches that treating a slurry of ground limestone with a chelating agent, such as ethylenediaminetetraacetic acid (EDTA), and a bleaching agent, such as sodium hydrosulfite, improves the color of the calcium carbonate produced to a whiter product.

In contrast to the prior knowledge, it has now been found by using the present process that the carbonation of calcium compounds in the presence of certain chelating agents produces calcium carbonate with significantly lower non-calcium metal content. This present process for producing calcium carbonate comprises carbonating a calcium compound in an aqueous media in the presence of a chelating agent, running the reaction at a controlled pH, and having the resulting calcium carbonate product contain reduced non- calcium metal concentrations compared to the starting calcium carbonate prepared by existing methods. As an example of the present process, reacting aqueous calcium chloride with sodium hydroxide and C0₂ in the presence of diethylenetriaminepentaacetic acid (DTPA) produced calcium carbonate with a substantially lower Pb content than CaC0₃ prepared by the same process without DTPA. Substantially lower Pb means less than 200 ppb, preferably less than 100 ppb, and more preferably less than 60 ppb of Pb. The pH at which the carbonation reaction is done is important towards controlling the metal content of the final product. The pH of some of the reactions can be controlled by the rate of addition of carbon dioxide.

The calcium carbonate produced by the present process is particularly useful in pharmaceutical applications and in food grade products where the calcium carbonate is ingested. In addition, the material produced has applications in a variety of other uses such as, for example, paper products, plastic products, coatings, paints, adhesives and sealants, or other applications where reduction of non-calcium metals is desired.

The present invention involves a process for the preparation of calcium carbonate with significantly reduced concentrations of non-calcium metal contamination in the final product. Surprisingly, carbonating calcium in the presence of certain chelating agents produces calcium carbonate with a substantially lowered non-calcium metal content than if the chelating agent was not present at the time of carbonation.

The present invention relates to a process for the production of calcium carbonate that has low concentrations of non-calcium metals. The process involves the use of certain chelating agents added at specific times during the process. Calcium carbonate produced by this process has lower concentrations of metal ions such as Pb, Fe and others. The calcium carbonate produced using this process can be used in a variety of applications including, for example, as medicaments and additives in the pharmaceutical industry, paints, coatings, adhesives and sealants, as additives for foods and containers or packaging for the food industry, as additives for plastics, as additives in the paper making industry, and other applications.

Carbonation of calcium is done by addition of carbon dioxide gas to an aqueous slurry of calcium hydroxide produced by adding water to calcium oxide. There are many methods of preparing calcium hydroxide slurries. For example, calcium carbonate can be calcined, then water added to produce calcium hydroxide slurry as described below in Scheme 2.

Scheme 2

Process For Preparation of Calcium Hydroxide Slurry From Calcium Carbonate

CaC0₃ ^HeUt > CaO + C0₂

H O CaO ² ) Ca{OH)₂

Another method for preparing a calcium hydroxide slurry is starting from a calcium halide. For example treating calcium chloride with sodium hydroxide also yields a calcium hydroxide slurry as described below in Scheme 3.

Scheme 3 Preparation of Calcium Hydroxide from a Calcium Halide

CaCl₂ + 2NaOH ^→ Ca(OH)₂ + 2NaCl

Other calcium salts, such as the nitrate, can be used to prepare calcium hydroxide. The method used to prepare the calcium hydroxide is not critical to the invention.

Converting calcium hydroxide to calcium carbonate is accomplished by bubbling carbon dioxide gas into the slurry as described below in Scheme 4. Scheme 4 Conversion of Calcium Hydroxide to Calcium Carbonate

Ca ( OH ) ₂ + C0₂ → CaC0 + H₂0

Monitoring pH of the reaction, by any suitable means, can be used to determine the endpoint of the reaction; namely, a pH drop to about 8 is an indication that substantial conversion of the calcium hydroxide to the carbonate has occurred. It is during this carbonation step that presence of chelating agents have been found to reduce the non- calcium metal concentration.

Another process for carbonating calcium is by combining a soluble calcium salt, such as a calcium nitrate or halide (such as calcium chloride or bromide), with magnesium hydroxide, then bubbling carbon dioxide into the reaction until the reaction is complete. The solid calcium carbonate is isolated by filtering it from solution. This process can be done in multiple steps or in a single step as described below in Scheme 5.

Scheme 5 Preparation of Calcium Carbonate from Calcium Chloride, Magnesium Chloride and Carbon Dioxide

CO

CaCl₂ + Mg(OH)₂ ²→ CaC0₃ + Mg(Cl)₂

A critical requirement for this invention is that the chelating agent is present during the carbonation step, regardless whether the reaction is done in one step or multiple steps.

A further method of carbonating calcium is by reacting a water soluble calcium salt with a carbonate salt. An example of such a reaction is to react calcium chloride with sodium carbonate. Addition of chelating agents to a solution of each reagent prior to combining them is another aspect of this invention.

Yet another method of this invention is to add a calcium containing salt to an aqueous solution containing carbonic acid. For example, a slurry of calcium hydroxide containing dissolved chelating agent is added slowly to water that also contains dissolved chelating agent, which mixture is continuously bubbled with carbon dioxide. The pH of the solution can be controlled by the rate of addition of calcium hydroxide slurry to the carbonic acid solution. Chelating agents used in this invention include any chelant that has a higher stability constant for the metal ions to be removed than for calcium [i.e., a difference of 1 log K (or 10 K) between the stability constant, K, for the complex formed between the chelating agent and the non-calcium metal compared to the stability constant for the complex formed between the chelator and calcium]. Examples of such suitable chelating agents include aminocarboxylic acids made from non-cyclic amines, such as for example nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminetetraacetic acid (DTPA), and triethylenetetraaminehexaacetic acid (TTHA). Of these chelating agents EDTA and DTPA are preferred. Aminocarboxylic acid chelating agents derived from cyclic amines are also included as suitable chelating agents for use in this invention. Examples of this class of chelating agent include 1 ,4,7,10-tetraazacyclododecane-1 ,4,7,10-tetraacetic acid (DOTA) and 1 ,5,8,12-tetraaza-cyclotetradecane-1 ,5,8,12-tetraacetic acid (TDTA).

The concentration of chelating agent preferred depends on the chelator used and the amount of non-calcium metal contained in the reaction mixture (e.g., water, reagents, starting calcium carbonate, magnesium hydroxide, sodium hydroxide). The amount of chelating agent used ranges from about an equal molar amount (compared to the amount of non-calcium metal) to about 1x10⁹ molar excess compared to the non-calcium metal present. More preferred is a concentration of about 1x10² to about 1x10⁸ in excess to the concentration of non-calcium metal, and most preferred is a concentration of about 1x10³ to about 1x10⁷ in excess of the amount of non-calcium metal.

Figure 6 Structures of DOTA and TDTA

DOTA TDTA

Aminophosphonic acids prepared from linear amines are also chelants suitable for use in this invention. Examples of these chelating agents include ethylenediaminetetramentylenephosphonic acid (EDTMP) and diethylenetriaminepentamethylenephosphonic acid (DTPMP) and others. Phosphonic acids prepared from cyclic amines are also included as chelating agents in this invention, such as 1 ,4,7,10-phosphonomethyl-1 ,4,7,10-tetraazacyclododecane (DOTMP). In addition, some cyclic and linear amines also have a higher stability constant for other metal ions than for calcium and are included in this invention, for example diethylenetriamine.

The rate of addition of carbon dioxide has been found to make a difference in the metal content of the final product. For example, reactions run with a higher rate of carbon dioxide addition yielded lower metal contents than reactions run at slower carbon dioxide addition rates. The rate of carbon dioxide addition affects the pH of the reaction. Thus another aspect of this invention is running the carbonation reaction at a controlled pH by controlling the rate of addition of carbon dioxide. The preferred pH is from about 4 to about 8, more preferred is about 5 to about 7, and most preferred is from about 5.5 to about 6.0.

The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the present invention.

Examples

Example A

Samples of calcium carbonate were analyzed for Pb by inductively coupled plasma optical emission spectroscopy (ICP) before and after the precipitation reactions described in Scheme 1. The results showed Pb concentrations of about 1 ,500 parts per billion (ppb) before the precipitation process and 300 to 600 ppb after the precipitation process. This shows that although the precipitation process decreases significantly the amount of Pb in the sample, there are still significant amounts of Pb in the PCC product.

Example B

A quantity of 20 g of CaC0₃ was weighed into each of nine individual 250 mL Erlenmeyer flasks and 150 mL deionized (Dl) water was added to each flask. To each flask was added chelating agents as described in Table 1 below. The flasks were placed on hot plates and boiled for 1 hour while continuously stirring the reaction mixtures. Calcium carbonate was recovered by vacuum filtration from all flasks separately and then each of the samples were washed with 500 mL of Di water. The samples were then dried for 48 hours in a vacuum oven at 60°C. The samples were then analyzed for Pb using ICP.

Table 1 Results of Example B

Structures given below.

Cyclam Cycien

It is evident from the results in Table 1 above that treatment of a CaC0₃ slurry with chelating agents does not significantly reduce the concentration of the lead in the final calcium carbonate product.

Example 1

Calcium carbonate was prepared by reaction of CaCI₂ with Na₂C0₃ or NaOH and C0₂ both with and without chelating agent (DTPA) added. The results are described in Table 2 below.

Sample 1 : To a 1 L plastic wide-mouth bottle was added 172 g of a 38% CaCI₂ aqueous solution and 400 mL of Dl water. In a second bottle, sodium carbonate (53 g, analytical reagent (AR) grade) was dissolved in 400 mL Dl water. The Na₂C0₃ solution was added rapidly to the CaCI₂ solution, the 1 L bottle was capped, and the mixture was shaken vigorously. Sample 2: Same as Sample 1 , but with DTPA (1 mL of a 40% Na₅DTPA aqueous solution) added to each solution prior to mixing.

Sample 3: To a 1 L plastic wide-mouth bottle was added 172 g of a 38% CaCI₂ aqueous solution and 400 mL of Dl water. In another bottle, sodium hydroxide (80 g of a 50% solution) was diluted to 400 mL with Dl water. The NaOH solution was added rapidly to the CaCI₂ solution, the 1 L bottle was capped, and the mixture was shaken vigorously. Gaseous C0₂ was bubbled into the solution until the pH dropped to 8.

Sample 4: Same as Sample 3, but with DTPA (1 mL of a 40% Na₅DTPA aqueous solution) added to each solution prior to mixing.

All samples were vacuum filtered and washed well with Dl water (about 1.5 L). The samples were then dried in a vacuum oven and analyzed for lead by ICP.

Table 2 Results of Example 1

These results show that CaC0₃ with significantly reduced Pb level can be produced by having a chelating agent present during the carbonation step compared to the same reaction without a chelating agent when starting with a soluble form of calcium. It is also evident from these examples that carbonate can be successfully introduced in various forms (e.g. as a carbonate salt, or as C0₂).

Example 2

Calcium carbonate was prepared by reaction of CaCI₂ with Mg(OH)₂ and C0₂ both with and without chelating agent (DTPA) added. The results are described in Table 3 below.

Sample 1 : Calcium chloride (61.3 g of a 38% aqueous solution, 0.21 mole) and Mg(OH)₂

(11.7 g, 0.20 mole) were combined in a 2 L 3-neck round bottom flask equipped with a mechanical stirrer, thermometer, and gas dispersion tube. Deionized water (Dl) (1500 mL) was added. Sample 2: Same as Sample 1 , but with DTPA (2 mL of a 40% Na₅DTPA aqueous solution).

Carbon dioxide was introduced into both samples through the gas dispersion tube as the samples were rapidly stirred. The temperature was maintained at 50°C using heat lamps. Both reactions were allowed to continue for four days, then the samples were each vacuum filtered, washed well with Dl water (about 1 L) and dried in vacuum oven. Sample 1 was a yellowish powder (18.2 g) and Sample 2 was a white powder (17.3 g).

Table 3 Results of Example 2.

These results clearly show that the presence of chelant reduces the amount of Pb.

Example 3

Into each of two 2 L 3-neck round bottom flasks was added 122.6 g of 38% CaCI₂ (0.42 mole Ca), 23.4 g Mg(OH)₂ (0.40 mole Mg), and 1500 mL Dl water. The flasks were equipped with a stirrer, thermometer, and gas dispersion tube. A volume of 4.0 mL of 40% pentasodiumdiethylenetetraaminepentaacetic acid was added to each flask. The slurries were rapidly stirred. The temperature was maintained at 50°C using heat lamps controlled by a Therm-O-Watch™. Carbon dioxide gas was introduced very slowly (about 5 cc/min) into Flask 1 and rapidly (about 1500 cc/min) into Flask 2.

The reaction was allowed to continue for 22 hours. Periodic sampling of the solution was done and each sample was analyzed for concentrations of Mg and Ca. The pH of the solution was also measured periodically. The concentrations of Mg and Ca in solution, plus the pH was used to determine that the reaction in both flasks had gone to completion. The solids from each flask were isolated by vacuum filtration, washed with 500 mL of deionized water, washed with acetone, and dried at 70°C in a vacuum oven overnight. Flask 1 yielded 38.2 g of a white powder. Flask 2 yielded 35.9 g of white powder. The material obtained from each of the above reactions was analyzed for Pb by ICP. The material from Flask 1 contained 247 ppb of Pb. The material from Flask 2 contained 54 ppb of Pb. Example 4

Approximately 150 g of Ca(OH)₂ slurry (50% calcium hydroxide in water) was placed in a 1 L beaker containing a magnetic stir bar. A volume of 2.0 mL of 40% aqueous pentasodiumdiethylenetetraaminepentaacetic acid (DTPA solution) and Dl water (as much as necessary to 1 L) was added. The mixture was stirred to disperse. The diluted slurry was decanted into another 1 L beaker to leave behind heavier sandy material.

A volume of 500 mL Dl water and 2.0 mL DTPA solution were placed in a 2 L 5-neck round bottom flask equipped with stirrer, thermometer, gas dispersion tube, and pH electrode. Carbon dioxide gas was introduced rapidly (2000 mL/min) into the stirred solution and the temperature was maintained at 50°C using Therm-O-Watch™ controlled heat lamp. The lime slurry from above was transferred to the reaction flask at such a rate to maintain the pH at or below 6.6. This took about 2.5 hours and the slurry turned a yellow color. The mixture was allowed to stir at 50°C with continued addition of carbon dioxide

(1000 mL/min) for an additional 7 hours. The CaC0₃ was recovered by vacuum filtration. It was washed with 500 mL Dl water then with 150 ml of acetone. The solid was dried at 70°C in vacuum oven overnight. A white solid weighing 80 grams was recovered. The solid was found to have 186 ppb of Pb. This compares to 300-400 ppb of Pb for calcium carbonate produced by the PCC process.

Although the invention has been described with reference to its preferred embodiments, those of ordinary skill in the art may, upon reading and understanding this disclosure, appreciate changes and modifications which may be made which do not depart from the scope and spirit of the invention as described above or claimed hereafter.

Claims

1. A process for preparing calcium carbonate comprising carbonating a calcium compound in an aqueous media in the presence of a chelating agent, running the reaction at a controlled pH, and having the resulting calcium carbonate product contain reduced non-calcium metal concentration compared to calcium carbonate prepared by existing processes.

2. The process of Claim 1 wherein the non-calcium metal is lead.

3. The process of Claim 2 wherein the calcium carbonate product has a lead concentration less than 60 ppb.

4. The process of Claim 1 wherein the carbonation reaction is run at pH 4 to 8.

5. The process of Claim 1 wherein the carbonation reaction is run at pH 5 to 7.

6. The process of Claim 1 wherein the carbonation reaction is run at pH 5.5 to 6.

7. The process of Claim 1 wherein the calcium carbonate product is prepared by reacting calcium hydroxide with carbon dioxide.

8. The process of Claim 1 wherein the calcium carbonate product is prepared by reacting calcium chloride with sodium hydroxide or potassium hydroxide and carbon dioxide.

9. The process of Claim 1 wherein the calcium carbonate product is prepared by reacting calcium chloride with magnesium hydroxide and carbon dioxide.

10. The process of Claim 1 wherein the calcium carbonate product is prepared by reacting a water-soluble calcium salt with a water-soluble carbonate salt.

1 1. The process of Claim 10 wherein the water-soluble calcium salt is a calcium halide.

12. The process of Claim 10 wherein the water-soluble carbonate salt is sodium carbonate.

13. The process of Claim 1 , 7, 8, 9 or 10 wherein the chelating agent is an aminocarboxylic acid, or a metal or amine salt thereof, or a mixed salt thereof.

14. The process of Claim 13 wherein the aminocarboxylic acid is nitrilotriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, tnethylenetetraaminehexaacetic acid, or a metal or amine salt thereof, or a mixed salt thereof.

15. The process of Claim 14 wherein the aminocarboxylic acid is ethylenediaminetetraacetic acid, or diethylenetriaminepentaacetic acid, or a metal or amine salt thereof, or a mixed salt thereof.

16. The process of Claim 1 or 3 wherein the chelating agent is an aminophosphonic acid, or a metal or amine salt thereof, or a mixed salt thereof.

17. The process of Claim 1 or 3 wherein the chelating agent is an amine.

18. The process of Claim 13 wherein the aminocarboxylic acid is prepared from a cyclic amine.

19. The process of Claim 18 wherein the cyclic amine is 1 ,4,7,10- tetraazacyclododecane-1 ,4,7,10-tetraacetic acid or 1 ,5,8,12- tetraazacyclotetradecane-1 ,5,8,12-tetraacetic acid.

20. The process of Claim 15 wherein the carbonation reaction is run at pH 5 to 8.

21. The process of Claim 15 wherein the carbonation reaction is run at pH 5.5 to 6.

22. The process of Claim 1 wherein the a slurry of calcium hydroxide is added to a container that has water into which carbon dioxide is continuously bubbled.

23. The process of Claim 22 wherein the carbonation reaction is run at pH 4 to 8.

24. The process of Claim 22 wherein the carbonation reaction is run at pH 5.5 to 6.

25. The process of Claim 11 wherein the carbonation reaction is run at pH 4 to 8.

26. The process of Claim 11 wherein the carbonation reaction is run at pH 5.5 to 6.

27. A composition comprising calcium carbonate containing less than 200 ppb of lead.

28. The composition of Claim 27 comprising calcium carbonate containing less than 100 ppb of lead.

29. The composition of Claim 28 comprising calcium carbonate containing less than 60 ppb of lead.