Micronized CaCO3 slurry injection system for the remineralization of desalinated and fresh water

Abstract

The present invention concerns a process for treating water and the use of calcium carbonate in such a process. In particular, the present invention is directed to a process for remineralization of water comprising the steps of (a) providing feed water having a concentration of carbon dioxide of at least 20 mg/l, preferably in a range of 25 to 100 mg/l, and more preferably in a range of 30 to 60 mg/l, (b) providing an aqueous slurry comprising micronized calcium carbonate, and (c) combining the feed water of step (a) and the aqueous slurry of step (b) in order to obtain remineralized water.

Claims

1. A process for remineralization of feed water comprising the steps of: a) providing feed water in need of remineralization, wherein the feed water has a concentration of free carbon dioxide of 20 to 60 mg/l that is not a result of addition of gaseous carbon dioxide to the feed water, b) providing an aqueous slurry comprising 2 to 40 wt.-% of micronized calcium carbonate based on the total weight of the slurry, wherein the micronized calcium carbonate has a particle size from 0.5 to 15 m, and a HCl insoluble content from 0.02 to 2.5 wt.-% based on the total weight of the micronized calcium carbonate, and c) combining the feed water of step a) and the aqueous slurry of step b) in order to obtain remineralized water.

2. The process according to claim 1, wherein the feed water has a concentration of carbon dioxide of 30 to 60 mg/l.

3. The process of claim 1, wherein the concentration of calcium carbonate in the slurry is from 2 to 25 wt.-%, based on the total weight of the slurry.

4. The process of claim 1, wherein the concentration of calcium carbonate in the slurry is from 2 to 20 wt.-%, based on the total weight of the slurry.

5. The process of claim 1, wherein the concentration of calcium carbonate in the slurry is from 3 to 15 wt.-%, based on the total weight of the slurry.

6. The process of claim 1, wherein the calcium carbonate has a particle size from 2 to 10 m.

7. The process of claim 1, wherein the calcium carbonate has a particle size from 3 to 5 m.

8. The process of claim 1, wherein the calcium carbonate has a HCl insoluble content from 0.05 to 1.5 wt.-%, based on the total weight of the micronized calcium carbonate.

9. The process of claim 1, wherein the calcium carbonate has a HCl insoluble content from 0.1 to 0.6 wt.-%, based on the total weight of the micronized calcium carbonate.

10. The process of claim 1, wherein the calcium carbonate is a ground calcium carbonate, modified calcium carbonate, or precipitated calcium carbonate, or any mixture thereof.

11. The process of claim 1, wherein the slurry comprises further minerals containing magnesium, potassium, sodium, magnesium carbonate, calcium magnesium carbonate, dolomitic limestone, calcareous dolomite, dolomite, half-burnt dolomite, magnesium oxide, burnt dolomite, magnesium sulfate, potassium hydrogen carbonate, or sodium hydrogen carbonate.

12. The process of claim 1, wherein the time period between the preparation of the slurry in step b) and combining the slurry with the feed water in step c is less than 24 hours.

13. The process of claim 1, wherein the time period between the preparation of the slurry in step b) and combining the slurry with the feed water in step c is less than 2 hours.

14. The process of claim 1, wherein the obtained remineralized water has a calcium concentration as calcium carbonate from 15 to 200 mg/l.

15. The process of claim 1, wherein the obtained remineralized water has a calcium concentration as calcium carbonate from 50 to 150 mg/l.

16. The process of claim 1, wherein the obtained remineralized water has a calcium concentration as calcium carbonate from 100 to 125 mg/l.

17. The process of claim 1, wherein the obtained remineralized water has a calcium concentration as calcium carbonate from 15 to 100 mg/l.

18. The process of claim 1, wherein the obtained remineralized water has a calcium concentration as calcium carbonate from 20 to 80 mg/l.

19. The process of claim 1, wherein the obtained remineralized water has a calcium concentration as calcium carbonate from 40 to 60 mg/l.

20. The process of claim 1, wherein the slurry comprises further minerals containing magnesium and the obtained remineralized water has a magnesium concentration from 5 to 25 mg/l.

21. The process of claim 1, wherein the remineralized water has a turbidity value of lower than 5.0 NTU.

22. The process of claim 1, wherein the remineralized water has a turbidity value of lower than 1.0 NTU.

23. The process of claim 1, wherein the remineralized water has a turbidity value of lower than 0.5 NTU.

24. The process of claim 1, wherein the remineralized water has a turbidity value of lower than 0.3 NTU.

25. The process of claim 1, wherein the remineralized water has one or more of a Langlier Saturation Index from 2 to 1, a Slit Density Index SDI.sub.15 below 5, and a Membrane Fouling Index MFI.sub.0.45 below 4.

26. The process of claim 1, wherein the remineralized water has one or more of a Langlier Saturation Index from 1.9 to 0.9, a Slit Density Index SDI.sub.15 below 4, and a Membrane Fouling Index MFI.sub.0.45 below 2.5.

27. The process of claim 1, wherein the remineralized water has one or more of a Langlier Saturation Index from 0.9 to 0, a Slit Density Index SDI.sub.15 below 3, and a Membrane Fouling Index MFI.sub.0.45 below 2.

28. The process of claim 1, wherein the feed water is desalinated seawater, brackish water, brine, treated wastewater, natural water, ground water, surface water or rainfall.

29. The process according to claim 1, wherein the remineralized water is blended with feed water.

30. The process according to claim 1, wherein the process further comprises a particle removal step.

31. The process according to claim 1, wherein one or more of a alkalinity, conductivity, calcium concentration, pH, total dissolved solids and turbidity is measured and compared to a predetermined parameter value, and the amount of slurry is adjusted based on the difference between the measured and the predetermined parameter value.

32. The process of claim 31, wherein the predetermined parameter value is a pH value, wherein the pH value is from 5.5 to 9.

33. The process of claim 31, wherein the predetermined parameter value is a pH value, wherein the pH value is from 7 to 8.5.

Description

EXAMPLES

Measurement Methods

(1) CO.sub.2 Measurement

(2) The concentration of carbon dioxide contained in the feed water samples used was determined by using a titrimetric method. The principle of this method consists in the fact that CO.sub.2 reacts with sodium carbonate or sodium hydroxide to form sodium bicarbonate (NaHCO.sub.3). The completion of the reaction is indicated potentiometrically or by the development of the pink colour characteristic of phenolphthalein indicator at the equivalence pH of 8.3

(3) The titration of the feed water was conducted at 25 C. using a Mettler Toledo M 416.

(4) A three point calibration (according to the segment method) of the instrument was first made using commercially available buffer solutions (from Mettler Toledo) having pH values of 4.01, 7.00 and 9.21.

(5) Then the pH of a 100 ml sample of the feed water was measured in function of the amount of titrant used until the end-point of pH 8 was reached. In the present measurement the titrant was a 0.01 mol/l sodium hydroxide solution.

(6) From the amount of titrant that was necessary to reach the end-point of pH 8.3, and using the following equation (I), the CO.sub.2 content can be easily calculated.

(7) $\begin{array}{cc}\mathrm{mg}{\mathrm{CO}}_2\text{/}L=\frac{AN44000}{b}& \left(I\right)\end{array}$
where:
A=ml titrant, N=normality of NaOH, and b=ml sample.

(8) Formula (I) is described in Chapter 4500-CO.sub.2 Carbon Dioxide on pages 4-28 to 4-34 of Standard Methods for the Examination of Water & Wastewater, 21.sup.st Edition, 2005, prepared and published jointly by the American Public Health Association, American Water Works Association, Water Environment Federation, publication office American Public Health Association 800 I Street, NW, Washington, D.C. 20001-3710, Centennial Edition. Therefrom it can be derived that the CO.sub.2 contents given in the present invention refer to the content of free CO.sub.2 in the water.

(9) BET Specific Surface Area

(10) The BET specific surface area (also designated as SSA) was determined according to ISO 9277 using a Tristar II 3020 sold by the company MICROMERITICS.

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

(12) Sedigraph 5100

(13) The weight median particle diameter and the particle diameter mass distribution of a particulate material were determined via the sedimentation method, i.e. an analysis of sedimentation behavior in a gravimetric field. The measurement is made with a Sedigraph 5100 sold by the company MICROMERITICS.

(14) The method and the instrument are known to the skilled person and are commonly used to determine particle size of fillers and pigments. Samples were prepared by adding an amount of the product corresponding to 4 g dry PCC to 60 ml of an aqueous solution of 0.1% by weight of Na.sub.4P.sub.2O.sub.7. The samples were dispersed for 3 minutes using a high speed stirrer (Polytron PT 3000/3100 at 15,000 rpm). Then it was submitted to ultrasound using an ultrasonic bath for 15 minutes and thereafter added to the mixing chamber of the Sedigraph.

(15) Weight Solids (% by Weight) of a Material in Suspension

(16) The weight solids (also called solids content of a material) was determined by dividing the weight of the solid material by the total weight of the aqueous suspension.

(17) The weight of the solid material was determined by weighing the solid material obtained by evaporating the aqueous phase of the suspension and drying the obtained material to a constant weight.

(18) The micronized products used to prepare the slurries of the present invention consisted of several micronized carbonate rocks: a marble calcium carbonate with an HCl insoluble content of 1.5 wt.-% from Bathurst, Australia, with d.sub.50=2.8 m (sample A), a marble calcium carbonate with an HCl insoluble content of 0.1 wt.-% from Salses, France, with two different particle sizes d.sub.50=5.5 m (sample D), and d.sub.50=3.5 m (sample E), a limestone calcium carbonate with an HCl insoluble content of 0.7 wt.-% from Superior, Ariz. (sample F: d.sub.50=3.5 m), a marble calcium carbonate with an HCl insoluble content of 1.0 wt.-% from Lucerne Valley, Calif. (sample J: d.sub.50=2.0 m) a limestone calcium carbonate with an HCl insoluble content of 0.1 wt.-% from Orgon, France (sample K: d.sub.50=3.0 m)

(19) Table 1 summaries the different products used during the remineralization tests

(20) TABLE-US-00001 TABLE 1 Samples Calcium carbonate rock d.sub.50 (m) HCl insoluble (%) A Marble 2.8 1.5 D Marble 5.5 0.2 E Marble 3.5 0.2 F Limestone 3.5 0.7 J Marble 2.0 1.0 K Limestone 3.0 0.1
Membrane Fouling Index (MFI) and Langelier Saturation Index (LSI) During Remineralization of the RO Water:

(21) Permeate produced by desalination processes is corrosive to concrete and metal because of its low pH and LSI value. If the permeate is not stabilized it leaches calcium from unprotected concrete in the storage tanks, wells and corrodes the cement-mortar lined ductile iron pipe commonly used for water distribution. At the majority of advanced water and wastewater treatment facilities permeate is stabilized by the addition of chemicals such as lime.

(22) However, the dosing of chemicals for post-treatment may result in high turbidity (>0.2 NTU) and elevated particulate levels (high Modified Fouling Index, e.g. in the 2-15 units range) in the final treated water, thereby increasing the potential for fouling of the injection wells.

(23) For indirect potable use, injection to barrier wells for seawater intrusion control it is specified that the permeate water turbidity shall be <0.2 NTU units and the modified fouling index (MFI) shall be <2.0 units.

(24) The feed water used for the remineralization tests of the present examples was obtained from a reverse osmosis desalination process of two different sewage plants (Plant 1 and Plant 2) and had the following parameters:

(25) TABLE-US-00002 Parameter Plant 1 Plant 2 pH 5.58 5.54 Alkalinity (mg/L as 21.0 9.0 CaCO.sub.3) Ca.sup.2+ Hardness (mg/L) 0.8 0.8 CO.sub.2 (mg/L) 35.0 45.0 Turbidity (NTU) 0.3 0.2 TDS (mg/L) 12.6 6.7 LSI 5.0 5.3 MFI 0.1 0.3

(26) The RO permeate remineralization tests were performed using 2-liter cubic jars with the aim of increasing the hardness of the RO water, e.g. 0.8 mg/L as CaCO.sub.3, up to the target of around 50 mg/L as CaCO.sub.3.

(27) Different types of micronized calcium carbonate (samples A, D, E, F, J and K) were tested for MFI and LSI analyses. The solid content of the CaCO.sub.3 slurries was 3.5 wt %, based on the weight of the micronized calcium carbonate. An appropriate dose of the CaCO.sub.3 slurries was added to achieve the desired water quality. The stabilized finished water should reach the following quality requirements:

(28) TABLE-US-00003 Parameter Value pH 6.5 to 8.5 Alkalinity, mg/L as CaCO.sub.3 40 to 80 Calcium, mg/L 10 to 50 LSI 0.5 to 0.0 Turbidity, NTU <0.2 MFI <2.0

(29) After adding the CaCO.sub.3 slurry, the samples were allowed to mix for 4 hours and samples collected at 10, 20, 30, 60, 120, and 240 minutes. Turbidity, pH, total alkalinity, and calcium hardness was measured at the individual sampling times. Equilibrium time was determined as the time when the turbidity stabilized. After the equilibrium time was reached, LSI was calculated and MFI measured.

(30) Table 2 shows the different results obtained for the remineralization of two different RO waters after the addition of approximately 50 mg/L as CaCO.sub.3 using 3.5 wt % CaCO.sub.3 slurries, based on the weight of the micronized calcium carbonate.

(31) TABLE-US-00004 TABLE 2 RO water Equilibrium Alkalinity Turbidity LSI MFI CaCO.sub.3 slurries supply time (min) pH (mg/L CaCO.sub.3) (NTU) (units) (units) Sample J Plant 1 120 7.6 49 1.3 1.2 1.6 (marble, d.sub.50 = 2.0 m) Sample F Plant 1 120 7.5 49 0.7 0.88 1.9 (limestone, d.sub.50 = 3.5 m) Sample J Plant 2 120 7.8 41 1.3 0.88 1.3 (marble, d.sub.50 = 2.0 m) Sample F Plant 2 120 7.8 43 1.7 1.02 1.7 (limestone, d.sub.50 = 3.5 m) Sample K Plant 2 120 7.9 44 0.5 1.61 0.1 (limestone, d.sub.50 = 3.0 m) Sample E Plant 2 120 7.9 43 0.9 1.8 0.62 (marble, d.sub.50 = 3.5 m) Sample D Plant 2 120 7.9 41 1.2 1.7 0.5 (marble, d.sub.50 = 5.5 m) Sample A Plant 2 120 8.0 48 1.4 1.85 1.9 (marble, d.sub.50 = 2.8 m)

(32) As it can be taken from Table 2, the use of micronized calcium carbonate products for the remineralization of RO water met the water quality requirements for pH, total alkalinity, calcium hardness, and MFI for all performed tests. The micronized calcium carbonate products presented turbidity level between 0.5 and 1.7 NTU, and LSI values between 1.85 and 0.88. Based on turbidity measurements with respect to time, equilibrium time required for dissolution of calcium carbonate products was approximately 120 minutes.