PROCESS FOR MANUFACTURING A SOLUTION OF AN EARTH ALKALI HYDROGEN CARBONATE

Abstract

The present invention relates to a process for the preparation of an aqueous solution comprising at least one earth alkali hydrogen carbonate, a process for the mineralization of water as well as the use of an aqueous solution comprising at least one earth alkali hydrogen carbonate obtained by the process for the mineralization of water.

Claims

1. A process for the preparation of an aqueous solution comprising at least one earth alkali hydrogen carbonate, comprising the steps of: a) providing water in a main process flow (1) and in at least one side process flow (2); b) adding at least one earth alkali carbonate-comprising material to the water provided in a first portion (4) of the side process flow (2) to obtain a suspension comprising the at least one earth alkali carbonate-comprising material; c) adding carbon dioxide or an acid having a pK.sub.a-value <5 to the water provided in a second portion (5) of the side process flow (2) and adjusting the pH-value to a range from 2.5 to 7.5 to obtain acidified water; d) combining the suspension obtained in step b) with the acidified water obtained in step c) to obtain a solution or suspension of at least one earth alkali hydrogen carbonate; e) dosing the solution of at least one earth alkali hydrogen carbonate obtained in step d) into the water provided in the main process flow (1) to obtain an aqueous solution of at least one earth alkali hydrogen carbonate; and f) adding an earth alkali hydroxide provided in water to the aqueous solution of at least one earth alkali hydrogen carbonate obtained in the main process flow (1) of step e) to adjust the pH-value of the aqueous solution of at least one earth alkali hydrogen carbonate to a range from 7.0 to 9.0 and to form an aqueous solution of at least one earth alkali hydrogen carbonate having an earth alkali concentration as earth alkali hydrogen carbonate in the range from 10 to 300 mg/l; wherein the molar ratio between the at least one earth alkali carbonate-comprising material in step b) and the earth alkali hydroxide in step f) is in the range from 50:1 to 1:10.

2. The process according to claim 1, characterized in that the at least one earth alkali carbonate-comprising material in step b) is selected from the group consisting of precipitated calcium carbonate, modified calcium carbonate, ground calcium carbonate and mixtures thereof, preferably the at least one earth alkali carbonate-comprising material in step b) is ground calcium carbonate.

3. The process according to claim 1, characterized in that the at least one earth alkali carbonate-comprising material in step b) is selected from the group consisting of marble, limestone, chalk and mixtures thereof.

4. The process according to claim 1, characterized in that the at least one earth alkali carbonate-comprising material in step b) is added in dry form or in aqueous form and/or the at least one earth alkali hydrogen carbonate obtained in step d) comprises calcium hydrogen carbonate and preferably consists of calcium hydrogen carbonate and/or the earth alkali hydroxide added in step f) comprises calcium hydroxide and preferably consists of calcium hydroxide and/or the acid in step c) has a pK.sub.a-value <4 and/or the acid in step c) is selected from the group consisting of sulphuric acid, hydrochloric acid, nitric acid or citric acid and mixtures thereof.

5. The process according to claim 1, characterized in that the second portion (5) of the side process flow (2) is located before the first portion (4) of the side process flow (2).

6. The process according to claim 1, characterized in that combining step d) is carried out in a reactor unit (6), preferably a tank or a tube reactor, and/or the solution or suspension of at least one earth alkali hydrogen carbonate obtained in step d) is subjected to a separation step g) in a separation unit (7), preferably the separation step g) is carried out by passing the solution or suspension of at least one earth alkali hydrogen carbonate through a filter, a membrane or a filter bed or by centrifuging the solution or suspension of at least one earth alkali hydrogen carbonate.

7. The process according to claim 1, characterized in that the pH-value in step c) is adjusted to the range from 3.0 to 7.0 and preferably to the range from 4.0 to 5.0 and/or the pH-value in step e) is adjusted to the range from 5.5 to 7.5 and preferably to the range from 6.0 to 7.0.

8. The process according to claim 1, characterized in that the aqueous solution of at least one earth alkali hydrogen carbonate obtained in step f) has an earth alkali concentration as earth alkali hydrogen carbonate in the range from 20 to 200 mg/l and preferably from 50 to 120 mg/l.

9. The process according to claim 1, characterized in that the molar ratio between the at least one earth alkali carbonate-comprising material in step b) and the earth alkali hydroxide in step f) is in the range from 50:1 to 1:3, preferably from 3:1 to 1:1.5.

10. The process according to claim 1, characterized in that the suspension comprising the at least one earth alkali carbonate-comprising material obtained in step b) has a solids content in the range from 0.01 to 20.0 wt.-%, preferably in the range from 1.0 to 15.0 wt.-% and more preferably in the range from 5.0 to 10.0 wt.-%, based on the total weight of the suspension.

11. The process according to claim 1, characterized in that the solution or suspension of at least one earth alkali hydrogen carbonate obtained in step d) has an earth alkali metal concentration as earth alkali hydrogen carbonate in the range from 100 to 1 000 mg/l and preferably in the range from 300 to 600 mg/l, and/or the concentration of the earth alkali hydroxide in the water of step f) is in the range from 800 to 1 700 mg/l and preferably in the range from 1 000 to 1 300 mg/l and/or the earth alkali hydroxide as provided in step f) is in form of a solution generated on site, preferably the solution has an earth alkali hydroxide content from 0.05 wt.-% to 0.4 wt.-%, preferably about 0.1 wt.-%, based on the total weight of the solution, or the earth alkali hydroxide as provided in step f) is in form of a suspension having an earth alkali hydroxide content from 15.0 to 25.0 wt.-%, preferably about 20.0 wt.-%, based on the total weight of the suspension.

12. The process according to claim 1, characterized in that the earth alkali hydroxide in the form of a solution is provided in the water of a second side process flow (3).

13. The process according to claim 1, characterized in that the carbon dioxide concentration in the solution or suspension of at least one earth alkali hydrogen carbonate obtained in step d) is in the range from 50 to 2800 mg/l, preferably in the range from 200 to 750 mg/l and/or the carbon dioxide concentration in the aqueous solution of at least one earth alkali hydrogen carbonate obtained in step e) is in the range from 10 to 2 400 mg/l, preferably in the range from 100 to 550 mg/l and/or the carbon dioxide concentration in the aqueous solution of at least one earth alkali hydrogen carbonate obtained in step f) is in the range from 0.001 to 20 mg/l, preferably in the range from 0.1 to 5 mg/l.

14. The process according to claim 1, characterized in that the aqueous solution of at least one earth alkali hydrogen carbonate obtained in step f) has a pH-value in the range from 7.2 to 8.9 and preferably in the range from 7.8 to 8.4.

15. A process for the mineralization of water, the process comprises the steps of i) providing water to be mineralized, ii) providing an aqueous solution comprising at least one earth alkali hydrogen carbonate obtained by the process, as defined in claim 1, iii) combining the water to be mineralized of step i) and the aqueous solution comprising at least one earth alkali hydrogen carbonate of step ii) in order to obtain mineralized water, and iv) adding at least one earth alkali hydroxide to the mineralized water obtained in step iii).

16. Use of an aqueous solution comprising at least one earth alkali hydrogen carbonate obtained by the process according to claim 1 for the mineralization of water.

17. The use according to claim 16, wherein the water is desalinated or naturally soft water.

Description

EXPERIMENTAL SECTION

1. Measurement Methods

[0169] In the following the measurement methods implemented in the examples are described.

pH of an Aqueous Suspension or Solution

[0170] The pH of a suspension is measured at 25° C. using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab® Expert Pro pH electrode. A three point calibration (according to the segment method) of the instrument is first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20° C. (from Aldrich). The reported pH values are the endpoint values detected by the instrument (the endpoint is when the measured signal differs by less than 0.1 mV from the average over the last 6 seconds).

Solids Content of an Aqueous Suspension

Moisture Analyser

[0171] The solids content (also known as “dry weight”) was determined using a Moisture Analyser HR73 from the company Mettler-Toledo, Switzerland, with the following settings: temperature of 120° C., automatic switch off 3, standard drying, 5 to 20 g of product.

Particle Size Distribution (Mass % Particles with a Diameter <X) and Weight Median Diameter (d.sub.50) of a Particulate Material

[0172] The particle size distribution of the products was measured using a Malvem Mastersizer 2000 Laser Diffraction System (Malvem Instruments Plc., Great Britain) using the Fraunhofer light scattering approximation. The method and instrument are known to the skilled person are commonly used to determine particle sizes of fillers and other particulate materials.

[0173] The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and in the presence of supersonics.

Turbidity of an Aqueous Suspension of Solution

[0174] The turbidity was measured with a Hach Lange 2100AN IS Laboratory Turbidimeter and the calibration was performed using StabCal turbidity standards (formazine standards) of <0.1, 20, 200, 1 000, 4 000 and 7 500 NTU.

Alkalinity of the Aqueous Solution and Hardness

[0175] The alkalinity of the aqueous solution has been determined by titration of a sample with a 0.1 M solution of hydrochloric acid. The end point of the titration is reached at a constant pH of 4.3. The amount of the alkalinity has been calculated by the following equation:

Alkalinity=Volume of acid (ml)×0.1×100.08×1000/(2×Volume of sample (ml))

[0176] The hardness refers to the German hardness and is expressed in “degree German hardness, ° dH”. Hardness and alkalinity have been determined by the same method in one titration.

Acidity of Aqueous Solution

[0177] The acidity of the aqueous solution has been determined by titration of the free CO.sub.2 with a 0.01 M solution of sodium hydroxide. The end point of the titration is reached at a constant pH of 8.3. The amount of free CO.sub.2 has been calculated by the following equation:

Free CO.sub.2=Volume of NaOH (ml)×0.01×44.01×1000/Volume of sample (ml)

BET Specific Surface Area of a Material

[0178] Throughout the present document, the specific surface area (in m.sup.2/g) of the mineral filler is determined using the BET method (using nitrogen as adsorbing gas), which is well known to the skilled man (ISO 9277:1995). The total surface area (in m.sup.2) of the mineral filler is then obtained by multiplication of the specific surface area and the mass (in g) of the mineral filler prior to treatment.

2. Examples

Inventive Installation

[0179] A general process flow sheet of one installation according to the present invention is shown in FIG. 1. The installation comprises an earth alkali carbonate storage silo (8) with dosing screw feeder and a vessel for preparing a suspension of the earth alkali carbonate (9), a reactor tank (6), static mixers for dosing both carbon dioxide (5) and the earth alkali carbonate suspension (4), concentrate dosing system and filtration system (7) and an earth alkali hydroxide dosing system.

[0180] An earth alkali hydrogen carbonate solution is produced and dosed in a first side process flow (2). An earth alkali hydroxide is dosed in a second side process flow (3). Both side process flows are merged in the main process flow (1).

[0181] In a full sized installation, a saturated solution of earth alkali hydroxide will normally be generated first of 0.1 to 0.15 wt.-% concentration. For carrying out trials on a pilot scale, as in Example 1 below, it is more useful to use a commercially available but highly reactive earth alkali hydroxide suspension of high purity, which can be dosed via a second side process flow (see FIG. 2) or directly to the main process flow. As the concentrations of the suspension are much higher (approx. 20 wt.-%), the flow rates for this process stream (3) are much slower.

[0182] Feed water is provided in all process flows, the feed water was obtained from reverse osmosis system, producing water of the following water specification:

TABLE-US-00001 Sodium: <1 mg/l Chloride: <2 mg/l Calcium: 1.2 mg/l  Magnesium: <1 mg/l °dH: <2 pH 6.3 Conductivity: 10.5 μS/cm

[0183] An earth alkali carbonate solution can be produced in the first side stream using the above mentioned equipment in the following manner: An earth alkali carbonate-comprising material is added to the vessel for preparing the suspension of earth alkali carbonate (9) from the storage silo (8) using a loss-in-weight screw feeder to accurately measure the quantity of material added. Water is also added to the tank and the mixer used to create a homogeneous suspension of known solids content. Carbon dioxide is added to the remainder of the first side stream process via a static mixer to form an acidic solution. The suspension is then transferred to a process flow stream a dosing pump where it is mixed via a static mixer. The combined side stream is then added to the reactor tank (6). The reaction between the earth alkali carbonate material and the acidic solution (carbon dioxide or otherwise) allows the formation of an earth alkali hydrogen carbonate solution within the reactor tank (6). From the reactor tank (6), a dosing pump is used to add the earth alkali hydrogen carbonate solution to the main process flow (2) through a set of filters (7) which remove any unreacted earth alkali carbonate material which appear as undissolved solids.

[0184] Within the second process side stream, a 20 wt.-% suspension of earth alkali hydroxide is stored within a vessel (11). A dosing pump is used to add this suspension to the main process flow (3).

Example 1

[0185] Natural calcium carbonate powder (Millicarb from Omya International, Orgon, France, d.sub.50=3 μm, according to technical data sheet) and a calcium hydroxide suspension (Schaferkalk, Precal 72, 20 wt.-% concentration in water) have been used as starting materials in a pilot plant. The Schaferkalk product (Precal 72) is a highly reactive 20 wt.-%% calcium hydroxide suspension and has been directly dosed into the main stream.

[0186] The reaction and operation conditions are given in Table 1 below.

TABLE-US-00002 TABLE 1 Reaction and operation conditions Example 1. First side process Main process flow Ca(OH).sub.2 dosing flow Flow rate (1/hr) 200       0.36 4000 Calcium carbonate 600 N/A N/A dosage (mg/l) CO.sub.2 dosage [mg/l] 925 N/A N/A Water quality: Alkalinity (mg/l as .sup.  540 .sup.c .sup.  270 000 .sup.d 50 CaCO.sub.3) Hardness (mg/l as .sup.  540 .sup.c .sup.  270 000 .sup.d 50 CaCO.sub.3) pH    6.4      11.7 8.2 Temperature [° C.]  16    16 16 Turbidity [NTU]  0 N/A (suspension) 0 Solid content (%) 0.06 .sup.b/0.006 .sup.a/0 .sup.c    20 0 .sup.a After reaction between CO.sub.2 and calcium carbonate; .sup.b Before reaction between CO2 and calcium carbonate; .sup.c After filtration; .sup.d Concentration expressed as mg/l CaCO.sub.3; (N/A = not applicable).

LIST OF REFERENCE SIGNS

[0187] (1) Main process flow [0188] (2) First side process flow [0189] (3) Second side process flow [0190] (4) Portion of (2) where the at least one earth alkali carbonate-comprising material is provided [0191] (5) Portion of (2) where carbon dioxide or acid having a pK.sub.a-value <5 is provided [0192] (6) Reactor unit [0193] (7) Separation unit [0194] (8) Earth alkali carbonate-comprising material storage silo [0195] (9) Vessel for preparing suspension of earth alkali carbonate-comprising material [0196] (10) Silo for earth alkali hydroxide [0197] (11) Vessel for preparing suspension of earth alkali hydroxide [0198] (12) Vessel for preparing saturated solution of earth alkali hydroxide