Method for producing magnesium aluminate spinels

Abstract

A process for producing a magnesium aluminate spinel comprising the steps of: i) preparing a magnesium suspension containing a magnesium compound; ii) preparing an aluminum suspension containing an aluminum compound; iii) feeding the magnesium suspension and aluminum suspension independently into a spray dryer nozzle to form a mixed magnesium, aluminum suspension; iv) feeding the mixed magnesium, aluminium suspension from the spray dryer nozzle into a spray dryer to form a mixed magnesium and aluminum compound; and v) calcining the mixed magnesium and aluminum compound to generate a magnesium aluminate spinel.

Claims

1. A process for producing a magnesium aluminate spinel comprising the following steps: i) preparing a magnesium suspension containing a magnesium compound; ii) preparing an aluminum suspension containing an aluminum compound; iii) feeding the magnesium suspension and aluminum suspension independently into a spray dryer nozzle to form a mixed magnesium, aluminum suspension; iv) feeding the mixed magnesium, aluminium suspension from the spray dryer nozzle into a spray dryer to form a mixed magnesium and aluminum compound; and v) calcining the mixed magnesium and aluminum compound to generate a magnesium aluminate spinel.

2. The process according to claim 1, wherein the magnesium suspension and the aluminum suspension are fed into the spray dryer by a spray dryer nozzle comprising at least two inlets that allows the magnesium suspension and the aluminum suspension to be fed independently into the spray dryer nozzle where they are combined and fed out into the spray dryer as a mixed magnesium aluminum suspension.

3. The process according to claim 1 wherein a pump system is used to feed the magnesium and aluminum suspensions independently into the spray dryer nozzle.

4. The process according to claim 1, wherein the aluminum compound comprises aluminum oxyhydroxide, aluminum oxide, aluminum hydroxide, or mixtures thereof.

5. The process according to claim 1, wherein the aluminum compound is milled prior to preparing the suspension in step ii).

6. The process according to claim 1, wherein an acidic solution is added to the alumina suspension in step ii) such that the alumina suspension has a pH of 3 to 5.

7. The process according to claim 1, wherein the magnesium compound includes magnesium oxides and magnesium salts.

8. The process according to claim 7, wherein the magnesium compound is magnesium oxide or magnesium hydroxide.

9. The process according to claim 1, wherein the magnesium suspension in step i) has a pH of about 8 to 9.

10. The process according to claim 1, wherein a base is added to the alumina suspension in step ii) and to the magnesium suspension in step i) after about 30 minutes of the formation of the respective suspensions to increase the pH of the respective suspensions to a pH of 8 to 10.

11. The process according to claim 1, wherein the magnesium suspension is dispersed for a period of 20 to 50 min.

12. The process according to claim 1, wherein the magnesium suspension is dispersed for a period of 25 to 35 min.

13. The process according to claim 1, wherein weight loadings of the mixed suspensions of higher than 10% are achieved when spray drying in step iv).

14. The process according to claim 13, wherein the conversion rate to spinel is equal to or greater than 95%.

15. The process according to claim 3, wherein the pumping rates of the magnesium suspension and the alumina suspension can be adjusted.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) The invention is related to a process of making a magnesium aluminate spinel which includes an aluminum suspension and a magnesium suspension. The process begins with the preparation of the aluminum suspension and the magnesium suspension.

(2) The production of the aluminum containing suspension includes dispersing an aluminum compound in an aqueous solution to form an aluminum suspension.

(3) The aluminum compound can be milled prior to use to improve dispersibility.

(4) An acidic solution including formic acid, acetic acid, citric acid, nitric acid, hydrochloric acid, oxalic acid, butanoic acid, or sulfuric acid may be added to the aqueous solution in amounts of 0.001-10 wt. %. It is preferable that the acidic solution is added as a dilute solution rather than at high concentrations. It is further preferable to add the acidic solution to the aqueous solution before dispersing the aluminum. The aluminum suspension has a pH of about 3-5.

(5) Preparing the magnesium suspension includes for example an aqueous suspension of Mg(OH).sub.2. The magnesium suspension has a pH of about 8-9.

(6) A base may be added to the aluminum suspension and to the magnesium suspension to increase their pH. The base preferably includes ammonium hydroxide. The ammonium hydroxide is added to the aluminum suspension and the magnesium suspension after about 30 minutes to increase the pH of the suspensions to a pH of 8 to 10. The base is added in amounts of 0.001 to 0.5 wt. % of the suspensions. It is preferable if the base is added as a dilute solution rather than at high concentrations.

(7) The magnesium suspension is to be dispersed for a specified time. The magnesium suspension may be dispersed for between 20 to 50 min, preferably 25 to 35 min and most preferably for 30 min.

(8) As is well known by those skilled in the art, typical processes of producing spinels involve a drying step. The drying can be by means of direct or indirect heating methods. These methods may include spray dryer, contact dryer, or pan dryer. A preferred method is spray drying.

(9) Drying takes place at various temperatures depending on the drying technique used. Larger dryers are operated at inlet temperatures of between 350 C. to 400 C. and outlet temperatures of 100-105 C., whilst smaller dryers are operated at inlet temperatures of about 275 C. and outlet temperatures of 100-105 C. outlet temperatures.

(10) The magnesium suspension and the aluminum suspension are fed into a spray dryer nozzle separately by use of a spray dryer nozzle including at least two feed lines. Examples of nozzles that can be used are a 3-fluid nozzle or a Y-feed inlet line. The mixing of the two suspensions occurs in short time following their contact in the spray dryer nozzle or in the atomizer body in the nozzle. The resulting mixed magnesium, aluminum suspension is then fed into the spray dryer.

(11) With the production of spinels the homogeneity is critical, and inadequate mixing usually results in poor conversion to spinel upon calcination. What is surprising with this process is that comparable results are achieved in only the short amount of time from when the suspensions are contacted in the spray dryer atomizer to when they exit the spray drying nozzle/atomizer within the spray drying nozzle, i.e., on the order of minutes rather than the hour(s) required using a batch process.

(12) The magnesium aluminum mixed compound can be calcined to spinel by heating in a furnace at 1200 C. for 4 hours or 1375 C. for 2 hrs. Commercially the spinel is calcined over 8 hours to a max temperature of 1200 C.

(13) The invention will now be exemplified according to the following non-limiting example(s).

Example 1

(14) A magnesium suspension of 40.6 g Mg(OH).sub.2 in 239.9 g DI water was prepared and stirred. The magnesium suspension was then dispersed for 30 min. Simultaneously an aluminum suspension of Pural UHPA SB1 was prepared using 97.8 g of boehmite and 239.9 g of DI water and stirred. After stirring for ca. 30 minutes, the boehmite was filtered through a 40 m screen to eliminate any large aggregates. Each suspension was fed independently through a Buchi 3-fluid nozzle via a peristaltic pump. The mixed magnesium and aluminum suspension was then fed into a spray dryer. The process permitted drying at 22 wt. % solids and produced mixed oxides which achieved high conversion greater than or equal to 95% upon calcination.

Example 2

(15) A magnesium suspension of 121.8 g Mg(OH).sub.2 in 719.8 g DI water was prepared and stirred. The magnesium suspension was then dispersed for 30 min. Simultaneously, an aluminum suspension of Pural UHPA SB1 was prepared using 293.4 g of boehmite and 719.8 g of DI water and stirred. The pH of both suspensions was adjusted to 10 using an ammonium hydroxide solution. Each suspension was fed independently through a Y feed line into the dryer via a peristaltic pump. The process permitted drying at 21 wt. % solids and produced materials which achieved high conversion greater than or equal to 95% upon calcination.

Comparative Example 1

(16) A suspension of 406 g of Mg(OH).sub.2 in 2000 g of DI water was brought to a pH of 10 using ammonium hydroxide and stirred on the bench top for 45 minutes. Simultaneously a suspension of Pural UHPA SB1 was prepared using 926 g of boehmite and 17000 g of water. The pH of the aluminum suspension was adjusted to 4 using formic acid and stirred on the bench top for 30 minutes. The pH of the aluminum suspension was adjusted to 10 using 5 wt % ammonium hydroxide. Once the pH was reached, the aluminum suspension was stirred for 15 minutes. The suspensions were then combined by pouring the pH 10 boehmite suspension into the pH 10 Mg(OH).sub.2 suspension. The two were blended together well and pumped into the atomizer of the spray dryer, via a peristalic pump. When the boehmite is rehydrated and introduced to the Mg(OH).sub.2 suspension, a high conversion greater than or equal to 96% can be achieved upon calcination, however the process only permitted 7% weight loading.

Comparative Example 2

(17) A magnesium suspension of 40.6 g Mg(OH).sub.2 in 239.9 g DI water was prepared and stirred. The magnesium suspension was then dispersed for 30 min. Simultaneously an aluminum suspension of Pural UHPA SB1 was prepared using 97.8 g of boehmite and 239.9 g of DI water, pH adjusted to 4 using formic acid, and stirred. After stirring for ca. 30 minutes, the boehmite suspension was filtered through a 40 m screen to eliminate any large aggregates. The Mg(OH).sub.2 suspension was then added to the stirring boehmite suspension. The mixed magnesium and aluminum suspension was then fed into a spray dryer. While the process permitted drying at 22 wt. % solids, the mixed oxides produced achieved poor conversion of less than or equal to 23% upon calcination. The low conversion rate is due to improper blending of the magnesium and aluminum suspensions.

(18) The results of the above Examples are included in Table 2 hereunder:

(19) TABLE-US-00002 TABLE 2 Al Mg Containing Containing Weight Conver- Preparation suspension suspension loading sion % Example 1 Buchi UHPA SB1 Mg(OH).sub.2 22.3 96.6 nozzle Example 2 Y feed line UHPA SB1 Mg(OH).sub.2 21.2 96 Compar- Mixed UHPA SB1 Mg(OH).sub.2 7 96 ative suspension Example 1 Compar- Mixed UHPA SB1 Mg(OH).sub.2 22.3 23 ative Suspension Example 2

(20) As can be seen from the results, by utilizing the process of the invention higher loadings are possible for the spray dryer, whilst maintain the required high conversion percentages.

(21) The Comparative Examples show that when the magnesium suspension is properly blended with the aluminum suspension before being pumped into an atomizer of a spray dryer that a low weight loading occurs with a high conversion rate (Comparative Example 1). If on the other hand, one increased the weight loading by lessening the time that the magnesium suspension and the aluminum suspensions are blended, the weight loading increases but the percentage conversion decreases drastically. (Comparative Example 2). The inventors have found that by feeding the magnesium suspension and the aluminum suspension independently into a spray dryer nozzle to form a mixed magnesium, aluminum suspension in the spray dryer nozzle leads to the advantages shown in Examples 1 and 2.

(22) Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined. Those skilled in the art will understand that the embodiments shown and described are exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.