Process for producing high grade hydromagnesite and magnesium oxide

Abstract

The present invention provides a process for producing high purity hydromagnesite from a source of magnesium chloride. The process involves preparation of a magnesium chloride brine of a specific concentration, which is ammoniated at a specific temperature range, followed by carbonation, while maintaining the reaction at a specific temperature range to form a hydromagnesite precipitate. The product can be calcined to generate high purity magnesium oxide compounds.

Claims

1. A method of preparing hydromagnesite from a source of magnesium chloride, comprising: a) preparing a first brine solution from said source of magnesium chloride, wherein said first brine solution comprises magnesium chloride and calcium chloride; b) mixing a sulfate salt into said first brine solution capable of converting said calcium chloride into a calcium sulfate precipitate thereby forming said calcium sulfate precipitate and a second brine solution comprising magnesium chloride; c) removing said calcium sulfate precipitate from said second brine solution; d) ammoniating said second brine solution, after step c), at a temperature range of about 20 C. to about 60 C. to convert magnesium chloride at least partially into magnesium hydroxide and to form ammonium chloride; and e) carbonating said magnesium hydroxide while maintaining the reaction temperature at about 20 C. to about 120 C. to form a hydromagnesite precipitate, f) wherein said first brine solution contains from about 25% to about 35% by weight magnesium chloride in water, g) wherein said mixing of said sulfate salt is carried out at a temperature of about 60 C. to about 90 C.

2. The method of claim 1, wherein said first brine solution has a specific gravity from about 1.2 to about 1.35.

3. The method of claim 1, further comprising adjusting a magnesium chloride concentration of said second brine solution to be in a range of about 10% to about 20% by weight magnesium chloride in water, after removing calcium sulfate.

4. The method of claim 1, wherein said carbonation is carried out in multiple steps.

5. The method of claim 1, wherein said sulfate salt is magnesium sulfate or sodium sulfate, wherein said salt is provided as a solid or solution in water.

6. The method of claim 1, wherein said step of mixing said sulfate salt is carried out at a temperature of about 80 C.

7. The method of claim 1, wherein carbon dioxide for said step of carbonating is obtained from calcination of limestone, wherein CaO is produced along with said carbon dioxide.

8. The method of claim 7, further comprising treating said CaO with said ammonium chloride to form ammonia gas and recycling said ammonia gas to the step of ammoniating.

9. The method of claim 1, wherein said second brine solution further comprises water soluble impurities, said method further comprising the step of washing said hydromagnesite precipitate to remove said water soluble impurities.

10. The method of claim 9, wherein said water soluble impurities comprise at least one of sodium chloride or potassium chloride.

11. The method of claim 9, wherein said step of washing includes the step re-slurring said hydromagnesite precipitate to form an about 50% slurry in water.

12. The method of claim 11, wherein said step of re-slurring is conducted at a temperature of between about 50 C. and about 100 C.

13. The method of claim 9, further comprising the step of drying said hydromagnesite precipitate after washing to produce a dried hydromagnesite precipitate.

14. The method of claim 13, wherein said step of drying is carried out at a temperature of about 100 C. to about 150 C.

15. The method of claim 14, wherein said hydromagnesite precipitate has a purity of at least about 99%.

16. The method of claim 1, wherein said hydromagnesite precipitate comprises particles having an average particle size from about 20 microns to about 50 microns.

17. The method of claim 1, wherein said first brine solution contains from about 27% to about 35% by weight magnesium chloride in water.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 depicts a simplified flow-chart illustrating an embodiment of the process according to the present invention.

(2) FIG. 2 shows the XRD analysis for the hydromagnesite product obtained by an embodiment of the process of the present invention.

(3) FIG. 3 shows the XRD analysis for the hydromagnesite product obtained by another embodiment of the process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Definitions

(4) Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term Hydromagnesite refers to magnesium carbonate with the chemical formula Mg.sub.5(CO.sub.3).sub.4(OH).sub.2.4H.sub.2O.

(5) The term precipitation refers to the formation of a solid material in a solution during a chemical reaction.

(6) A suspension or slurry comprises insoluble solids and water and optionally further additives and usually contains large amounts of solids and, thus, is more viscous and generally of higher density than the liquid from which it is formed.

(7) The term calcining refers to a thermal treatment process applied to solid materials causing loss of moisture, reduction or oxidation, and the decomposition of carbonates and other compounds resulting in an oxide of the corresponding solid material.

(8) The term carbonation in the meaning of the present invention refers to addition of CO.sub.2.

(9) As used herein, the term about refers to a +/10% variation from the nominal value. It is to be understood that such a variation is always included in a given value provided herein, whether or not it is specifically referred to.

(10) The present invention provides a process of preparing high purity hydromagnesite and magnesium oxide. The process of the present invention allows for the efficient and controlled production of hydromagnesite of a very high purity. According to the process of the present invention hydromagnesite having a purity of at least 99%, and varying particle sizes can be provided or prepared directly.

(11) In one embodiment, the process of the present invention involves the preparation of a magnesium chloride brine solution from at least one source of magnesium chloride, ammoniating the resulting brine solution, followed by carbonation at a concentration and a temperature sufficient to form a hydromagnesite precipitate.

(12) In accordance with the present invention, the magnesium chloride source can be provided in the form of magnesium chloride salt. It can also be provided in the form of a source of carnallite, a source of sylvinite, or any other mineral which can be processed to provide magnesium chloride. In one embodiment, the source of magnesium chloride comprises a magnesium chloride brine solution obtained from evaporation of a brine solution obtained by dissolving a source of carnallite in water.

(13) The magnesium chloride brine (as a source of magnesium chloride) can be a byproduct of other processes, for example, a process of obtaining potassium chloride from a source of carnallite as described in U.S. Pat. No. 8,282,898, the disclosure of which is incorporated herein by reference.

(14) The concentration of the brine solution before the ammoniation step can be up to 35% by weight of magnesium chloride. In one embodiment, the concentration of the brine solution before ammoniation is from about 10% to 20% by weight of magnesium chloride, preferably from about 15% by weight.

(15) The ammoniation step can be carried out by adding ammonia gas and/or ammonia solution (NH.sub.4OH).

(16) In one embodiment, the reaction temperature during the ammoniation step is maintained at about 20 C. to about 60 C., preferably about 30 C. to about 40 C., more preferably about 20 C. to about 30 C.

(17) The ammoniated reaction mixture is then carbonated by adding gaseous carbon dioxide. In one embodiment, the reaction temperature during the carbonation step is maintained at about 20 C. to about 120 C., preferably about 50 C. to about 100 C., more preferably about 60 C. to about 90 C., most preferably about 80 C., to form a hydromagnesite precipitate.

(18) In one embodiment, the feedstock magnesium chloride brine solution also comprises calcium chloride, for example when, the feedstock brine solution is formed by dissolving in water a magnesium chloride source which comprises calcium chloride impurities.

(19) In one embodiment, when calcium chloride is present, the feedstock magnesium chloride brine can comprise calcium chloride up to about 5% by weight. In one embodiment, the feed stock magnesium chloride brine comprises calcium chloride in the range of about 1.6% to about 2.0% by weight,

(20) In one embodiment, the feedstock magnesium chloride brine also comprising calcium chloride is mixed with a sulfate salt. This mixing step results in the conversion of calcium chloride component of the feedstock brine into calcium sulfate precipitate (gypsum), which is removed from the remaining brine solution. In one embodiment, the mixing of the sulfate salt is carried out at a temperature of about 50 C. to about 100 C., preferably about 60 C. to about 90 C., more preferably about 80 C.

(21) The sulfate salt can be magnesium sulfate or sodium sulfate, which can be added as a solid or as a concentrated solution in water. In one embodiment, the sulfate salt is magnesium sulfate septahydrate. In one embodiment, the calcium chloride impurities can be removed by adding approximately between 60 and 100 grams of magnesium sulfate per liter of brine.

(22) In one embodiment, the brine solution remaining after the removal of calcium sulfate has a calcium chloride component of less than about 0.2% by weight, preferably less than about 0.1% by weight.

(23) The addition of ammonia to the brine solution obtained after removal of calcium sulfate, converts magnesium chloride at least partly into magnesium hydroxide, and forms ammonium chloride, which readily dissolves in water. The ammoniated reaction mixture is then carbonated by adding carbon dioxide to form a hydromagnesite precipitate.

(24) In one embodiment, the concentration of the brine solution obtained after removal of calcium sulfate is adjusted to be about 10% to 20%, preferably about 15% by weight of magnesium chloride, before mixing with sodium carbonate.

(25) In one embodiment, the brine solution obtained from the magnesium chloride source can further comprise water soluble impurities. In such a case, the process of the present invention further comprises the step of washing the hydromagnesite precipitate to remove the water soluble impurities. In one embodiment, the water soluble impurities comprise unreacted magnesium chloride, sodium chloride and/or potassium chloride. In one embodiment, sodium chloride and/or potassium chloride is present in an amount up to about 3% by weight. In one embodiment, sodium chloride is present in an amount about 1.0% to about 2.5% by weight. In one embodiment, potassium chloride is present in an amount of about 1.0% to about 2.5% by weight.

(26) In one embodiment, the washing step further includes forming a slurry with the filtered cake of hydromagnesite precipitate in water, and separating the precipitates via solid/liquid separation. The formation of slurry is conducted at a temperature of about 50 C. to 100 C. The washed hydromagnesite precipitate is then dried to form a dried hydromagnesite precipitate. The drying step is carried out at a temperature of about 100 C. to about 150 C., preferably about 110 C. to about 130 C., more preferably about 115 C.

(27) Washing step removes any residual sodium chloride and potassium chloride and ammonium chloride formed during the carbonation step, from the hydromagnesite precipitate.

(28) Carbon dioxide for the carbonation step can be obtained by calcining a source of limestone in a kiln. Other limestone alternative include Magnesian limestone and dolomite.

(29) The calcination of limestone/magnesium limestone/dolomite produces CO.sub.2 and CaO, wherein CO.sub.2, as discussed above, is used in the carbonation step. In one embodiment, CaO from the kiln is mixed with NH.sub.4Cl solution obtained after separation of hydromagnesite precipitate, to produce NH.sub.3 gas and CaCl.sub.2 brine solution. In one embodiment, the so formed NH.sub.3 gas is collected and used in the ammoniation step. The recycling of the reclaimed ammonia gas improves the efficiency of the overall process by 98%.

(30) The CaCl.sub.2 brine obtained in this reclamation step is separated for disposal, for example, disposal in a deep formation from the reclamation process. Calcium chloride brine may be disposed of in a deep formation through an injection well.

(31) The carbonation step can be carried out in as a batch process or as a continuous process.

(32) FIG. 1 depicts one embodiment of the process in accordance with the present invention, wherein a feedstock brine solution comprising magnesium chloride and calcium chloride, and optionally sodium chloride, and potassium chloride is provided in a reactor. In one embodiment, in the feedstock brine solution, magnesium chloride is present in a concentration of about 25% to 35% and the calcium chloride is present in a concentration of about 0.1% to 2.0%. The sodium chloride concentration can be up to about 2.5% and the potassium chloride can be up to about 2.7%, all percentages being by weight.

(33) Magnesium sulfate septahydrate is added to the feedstock magnesium chloride brine solution in the reactor. The reaction temperature is maintained from about 50 C. to about 100 C., preferably about 60 C. to about 90, more preferably at about 80 C. The precipitated calcium sulfate (gypsum) is separated from the mother liquor via solid/liquid separation.

(34) The resulting magnesium chloride brine is diluted to about 10% to about 20% by weight and cooled to a temperature of about 20 C. to 60 C., preferably about 30 C. to about 40 C., more preferably about 20 C. to about 30 C.

(35) The resulting solution is then treated with ammonia at about 1:1 to 1:1.2 stoichiometric ratio. The ammoniated reaction mixture is then treated with carbon dioxide, while the temperature is maintained from about 20 C. to about 120 C., preferably about 50 C. to about 100 C., more preferably about 60 C. to about 90 C., and further more preferably about 80 C., to form hydromagnesite precipitate.

(36) In one embodiment, the ammonia off gas from the ammoniation/carbonation step is recovered for reuse.

(37) The formed hydromagnesite precipitate is then separated via solid/liquid separation, and washed.

(38) The washed cake is then re-slurried to approximately 50% of the density, filtered and centrifuged. The re-slurrying operation is conducted at about 50 C. to 100 C., preferably about 80 C. to about 100 C., more preferably about 80 C. The centrifuged product is then dried and transported for bagging. The product is dried at a temperature from about 100 C. to about 150 C. to produce a greater than 99% pure hydromagnesite.

(39) In one embodiment, ammonium chloride solution obtained after separation of hydromagnesite precipitate is collected and reacted with CaO obtained from the source of carbon dioxide, to form ammonia which is recycled to the ammoniation step.

(40) The desired particle size and purity of hydromagnesite being obtained by the process of the present invention can be achieved and improved by specifically controlling or adjusting the process conditions during the preparation of the hydromagnesite.

(41) Conveniently, the process of the present invention does not require extensive manipulation attributed to the processes used in the prior art; in the instant protocol, solubilities are conveniently manipulated to synthesize magnesium chloride brines which are diluted with progressive precipitation of unwanted compounds. This progressive precipitation results in very effective removal of calcium contamination from the magnesium chloride brine and hydromagnesite which is contributory to the remarkable purity achievable by practicing the technology of the instant invention.

(42) The process of the present invention can result in hydromagnesite having particle size in the range of 3 to 100 microns. The temperature, residence time, rate of addition of Na.sub.2CO.sub.3 addition etc. is adjusted to precipitate as large of a particle size as possible (for example 20-50) microns. This particle size allows efficient settling, and increased ability to filter the particles from the mother liquor efficiently. This also allows for thorough cake washing with water to completely remove all of the soluble impurities. The wet cake contains much less water than a filter cake of a smaller particle size, and is efficiently dried. Large concrete like lumps are not formed with this larger particle size, and any small lumps are friable and turn into a free flowing powder when touched.

(43) The large particles are actually composed of agglomerations of 2-3 micron hydromagnesite particles. Dry milling and size classifying easily breaks these agglomerations down to the desired sizes used in pigments, fillers, etc. The product does not have to be shipped as liquid slurry, dramatically cutting shipping costs, and process issues.

(44) The removal of the impurities such as calcium, sodium, potassium, etc., with the process of the present invention results in an extremely high purity product. As a result of this purity, brightness is increased significantly to 100.2 TAPPI, with almost no yellowness (no bleaching step is required). This brightness typically exceeds hi-grade pigment TiO.sub.2. Replacing a portion of TiO.sub.2 with this hydromagnesite actually increases the brightness overall, rather than the significant decrease found when using other fillers.

(45) The very high purity of the hydromagnesite obtained via the process of the present invention (i.e. over 99%) makes the hydromagnesite an ideal feedstock for producing hi-grade magnesia (MgO), which in turn can be used as a feedstock for producing Mg(OH).sub.2 with the addition of H.sub.2O.

(46) In one embodiment, the process of the present invention therefore further includes calcining the dried hydromagnesite precipitate obtained according to the process of the present invention to form a magnesium oxide product.

(47) In one embodiment, the calcining of hydromagnesite precipitate is carried out at a temperature of about 475 C. to about 1000 C. In one embodiment, the calcining of hydromagnesite precipitate is carried out at a temperature of about 1000 C. to about 1500 C. In one embodiment, the calcining of hydromagnesite precipitate is carried out at a temperature of about 1500 C. to about 2800 C. In one embodiment, the calcining of hydromagnesite precipitate is carried out at a temperature over 2800 C.

(48) As an option, depending upon the final use of the product, the hydromagnesite obtained via the process discussed above is calcined at a predetermined temperature to produce a host of magnesium oxide products. The calcination in the temperature ranges of about 475 C. to about 1000 C. produces Reactive Magnesia. The calcination in the temperature ranges of about 1000 C. to about 1500 C. produces Hard Burned Magnesia. The calcination in the temperature ranges of about 1500 C. to about 2800 C. produces Dead Burned Magnesia, and the calcination over 2800 C. produces Fused Magnesia.

(49) The invention will now be described with reference to specific examples. It will be understood that the following examples are intended to describe embodiments of the invention and are not intended to limit the invention in any way.

EXAMPLES

Example 1

(50) One liter of feedstock brine solution comprising 27% to 35% magnesium chloride, 1.6% to 2.0%, calcium chloride, 1.0% to 2.5% sodium chloride, and 1-2.7%, potassium chloride was provided in a reactor/vessel. Magnesium sulfate septahydrate was added to the feed stock magnesium chloride brine solution in an approximately 1:1.1 to 1.2 stoichiometric ratio relative to calcium chloride. The temperature of the mixture was maintained at about 80 C. The resulting calcium sulfate precipitate was removed via the solid/liquid separation.

(51) The resulting magnesium chloride brine was diluted to about 15% by weight, and cooled to about room temperature or 22-27 C. At this point, ammonia was added slowly to the diluted brine solution for up to an hour, followed by addition of CO.sub.2 over the course of approximately up to 2 hours with stirring being continuous and while the temperature was maintained at about 80 C. to form the hydromagnesite precipitate.

(52) The mixture was retained in the mixing vessel for approximately 2 hours after ceasing the stirring. The solid precipitate was allowed to settle to approximately one-third of the original volume and at least some of the liquid was then decanted from the reaction vessel.

(53) The hydromagnesite precipitate was filtered and re-slurried to approximately 50% of the density, and subsequently centrifuged. The re-slurrying was conducted at about 80 C. The centrifuged product was then dried in at a temperature from about 100 C. to about 150 C. to produce a greater than 99% pure hydromagnesite with no complex hydration complexes.

(54) The reaction was also conducted starting from 5, liters, 10 liters and 15 liters of feedstock brine solution.

(55) The hydromagnesite obtained via the process discussed above was calcined at about 600 C. to about 1000 C. produce magnesium oxide product of different grades having a purity of greater than 99%.

(56) Analytical Tests:

(57) Purity of the products was determined by ICP-OES and ICP-MS analysis. This confirmed the level of impurities in the final product as well as the ratio of magnesium to sample weight.

(58) The determination of hydromagnesite vs other magnesium compounds was determined by XRD analysis. This confirmed that the product was hydromagnesite and had the proper amount of waters of hydration.

(59) Thermo Gravimetric Analysis was performed in order to determine the temperatures where the hydromagnesite would begin to decompose, and when waters of hydration were lost. Calcination temperatures were also determined using this test.

(60) Differential thermal analysis was also performed using aluminum oxide as a reference in order to gain further information for the dryer.

(61) Scanning electron microscope pictures and analysis determined the particle sizes and shapes and confirmed the crystal form and ability to mill into pigment sizes.

(62) It will be appreciated by those skilled in the art that the numerical representations noted herein are exemplary.

(63) All documents cited in the Detailed Description of Embodiments of the Invention are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention.

(64) Although embodiments of the invention have been described above, it is not limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the spirit, nature and scope of the claimed and described invention.