IE43262B1 - Improvements in the manufacture of magnesia - Google Patents

Description

This invention relates to an improved process for the production of magnesia grain, such as that used in the manufacture of such basic refractory materials as refractory bricks and blocks and gunning compositions for steel furnace repair.

Magnesia grain for such purposes should be of relatively high density in order to be of low porosity and, consequently, resistant to hydration. Furthermore, the grain should be of angular shape to overcome certain disadvantages of rounded or spherical grain, such as the tendency for refractory bodies made from spherical grains to crumble at the edges and for spherical grains to rebound when used in gunning operations.

It is therefore art objective of this invention to provide a process which will make it possible to manufacture magnesia grain of good characteristics in a reliable and economical manner.

Magnesia for refractory grain is commonly made from naturally !5 occurring magnesium-bearing ores such as magnesite or from magnesium salts, especially when these salts are in the form of aqueous solutions such as sea water. The present invention is particularly applicable to the production of magnesia from dolomite and sea water. In such a process, mined dolomite, the common name for magnesium calcium carbonate, is calcined to yield magnesium calcium oxide, commonly known as dolime: MgCOj.CaCOj MgO.CaO + 2C02 The dolime is treated with sea water to hydrate the dolime, and the hydrated lime which forms reacts with magnesium ions present in the sea water to yield additional magnesium hydroxide; MgO.CaO + 2H20 -*· Mg(0H)2 + Ca(0H)2 Ca(0H2) + Mg++ —- Mg(0H)2 + Ca++ The magnesium hydroxide suspension so obtained is subjected to successive steps of sedimentation and washing to reduce soluble salts present in the suspension. The resulting slurry is then commonly filtered on a continuous vacuum filter to yield a paste, or mud, with a free water content of 45 to m <3863 weight percent. The mud is the.: dead burned to magnesia in a rotary kiln: Mg(0H)2 -* MgO -i H„i) Such a process produces a dead burned magnesia of low density, high porosity and small, non-uniform grain size. In addition, the high water content of the filter cake drastically limits the kiln capacity.

Certain variations on this procedure to produce an improved magnesia grain have been proposed. In s so-called pelletizing process, described by H.C. Gilpin in The Refractories Ji.urrial .March,1969, page 68, the magnesium hydroxide is caustic calcined to chemically reactive oxide, the oxide is compacted into briquettes and the briquettes are then dead burned. Because of the double burning with intermediate briquetting, this process is costly, but is reported to produce a product of good size and of bulk density of 3.3 to 3.4 g/cc.

In another process, described in a product brochure issued in Japan by Ube Chemical Industries Co. Ltd., filter cake from vacuum filtration is dried in a rotary dryer, pelletized ar.c finally dead burned at over 1900°C. in a rotary kiln, reportedly producing a magnesia of high refractoriness. However, vacuum filtration necessarily introduces a very heavy load to the subsequent drying operations.

In a further process, which forms the subject of Patent Specification No. 37428, magnesium hydroxide slurry is simultaneously dewatered and compacted, preferably at pressures greater than 400 psi (27 atm), to a filter cake with a water content of not more than 32.5 weight percent and a green bulk density of at least 1.15 g/cc, and the filter cake is converted by a suitable heat treatment to magnesia. To produce dead burned magnesia, Specification No. 37428 recommends that the filter cake be reduced to a free water content of 5 to 20 percent, by drying if necessary, then broken into granules, pelletized and dead burned at 1600 to 2000°C.; the product from such a ρ/ccess reportedly has a bulk density of about 3.4 g/cc. Expensive high pressure filtration equipment is required in such a process. « 3 43262 ’5 It has now been found that magnesia grain of particularly high quality can be obtained in a reliable and effective manner if magnesium hydroxide slurry is filtered to yield a filter cake with a free moisture content of 35 to 40 weight percent, the filter cake being partially dried and compacted into briquettes prior to dead burning. Specifically, the present invention provides a process for the production of magnesia grain comprising the steps of: (a) filtering magnesium hydroxide slurry to filter cake having a free water content of 35 to 40 weight percent; (b) drying the filter cake to a free water content of 2 to 10 weight percent; (c) compacting the dried filter cake into briquettes; and (d) dead burning the briquettes at a temperature of 1650 to 2000°C.

In a preferred embodiment of the process, heat required for the drying step is supplied by exhaust gases from the dead burning step, thereby effecting considerable savings in energy consumption; preferably the cake and exhaust gases flow co-currently through the drying zone.

While magnesium hydroxide slurry suitable for the practice of this invention can be obtained by precipitation of magnesium hydroxide from any magnesium-bearing solution by controlled addition of alkali, solution or suspension, the preferred source is from treatment of calcined dolomite with sea water as herein-before described. The hydroxide so obtained is allowed to settle in suitable sedimentation equipment, such as the so-called Dorr thickener, and the settled solids are rediluted with water and resettled, generally several times, to reduce the soluble salt content of the slurry to a desired level.

The resulting slurry, preferably at a solids content of from 150 to 450 g/liter, is then filtered in suitable filter apparatus so as to produce a filter cake with a free water content of from 35 to 40 weight percent.

This range of water content is required for optimal heat utilization during - 4 43262 the subsequent drying and dead burning of the filter cake as hereinafter described, and it may be achieved with the use of conventional filtration equipment. Water contents greater than 40 percent necessitate burning of additional fuel during the drying step. Reduction of the water content to levels substantially below 35 percent requires the use of more expensive high pressure filters, and, in the case where exhaust gases from the dead burning are used to dry the cake, the heat available in these gases is greater than that required to completely dry filter cake of such low water content. The preferred free water content of the filter cake is about 35 to 37 weight percent.

Filtration to the required filter cake free water content of 35 to 40 weight percent is readily accomplished with pressure filters operating at gauge pressures of from 50 to 365 psi (3.4 to 24.8 atm). Suitable low pressure apparatus includes such filter presses as, for example, the filter press having a maximum operating pressure of 25 atm. supplied by Rittershaus & Blecher GMBH, Wuppertal, West Germany, and membrane filters such as, for example, the diaphragm filter press and the automatic filter supplied by Eberhard Hoesch & SShne, DUren, West Germany, and the V.C. cylindrical diaphragm filter supplied by L.B. Holliday & Co. Ltd,, Huddersfield, England.

If desired, a granular oxide such as zirconium oxide, titanium dioxide, calcium oxide, ferric oxide or, preferably, silica is added to the slurry in an amount of up to about 0.02 g/g magnesium hydroxide prior to the slurry filtration to aid sintering during the subsequent dead burning of the filter cake. The oxide may, for example, be added to the precipitated hydroxide before its settling and washing.

The wet filter cake is then dried to a free moisture content of from about 2 to 10 weight percent, preferably about 5 weight percent, by passing through a drying zone operating at a temperature of from about 100 to 750°C., preferably from about 200 to 750°C. The preferred apparatus for the drying is the rotary drum dryer, although alternative units such as flash dryers, S · spray dryers and tray dryers may also be used.

The dried filter cake is compacted into briquettes, preferably between rollers operating at a pressing force of from about 4 to 9 tons/cm of roll width (3600 to 8200 kg/cm). The briquette shape may be, for example, almond or pillow, and the briquette size may be from about 3 to 120 cc. Almond shaped briquettes of 5 cc. are preferred. Suitable compactors are such as those supplied by KBppern, Hattingen, West Germany;Sahut-Conreur & Co., Raisroes, France; Hutt GMBH, Heilbronn, West Germany and K-G Industries, Inc., Rosemont, Illinois.

The briquettes are then dead burned at from 1650 to 2000°C., preferably from 1700 to 1850°C., to yield the desired magnesia grain. Preferred apparatus for the dead burning is a rotary kiln, although alternative units such as a shaft kiln may also be used.

In an important optional aspect of the process, the heat required for the drying step is supplied by the exhaust gases from the dead burning step. Preferably the drying is effected by co-current flow of the filter cake and exhaust gases through the drying zone; that is to say, the filter cake and the hot exhaust gases flow through the dryer in the same direction. The advantage of feeding the filter cake and hot gases in co-current flow is that the cake in contact with the gases at their highest temperature still has a relatively high free water content; the cake thus remains relatively Cool throughout the drying. This condition permits better control of the drying operation and also minimizes the possible formation of magnesium sulphate by reaction of the product with sulphur dioxide present in the j exhaust gases, which is accelerated by increased temperature.

The product from the above process is substantially superior to the conventional product obtained by the direct dead burning of vacuum filter mud, as demonstrated by higher bulk density, lower total porosity, improved grain size distribution (the grain being uniformly coarse) and angularity of grain shape, and greater resistance to hydration. In addition, the process results in a considerable reduction in energy costs. Reduction of the free water content of the cake charged to the dead burning from, for example, 50 to 5 weight percent reduces the moisture fed to the unit from 1.45 to a mere 0.08 g/g magnesia produced, with a resultant increase of at least one third in the unit's capacity; and the use of the exhaust gases from the dead burning unit to dry the filter cake saves valuable energy lost in prior operations.

The following Examples are merely illustrative and are not to be construed as limiting the invention, the scope of which is defined by the appended claims.

Example 1 About 900 gal (3410 liters) of a magnesium hydroxide slurry obtained from the treatment of calcined dolomite with sea water and with a solids content of about 280 g/1iter was filtered on a membrane filter (Hoesch diaphragm recessed plate filter press) to a final operating pressure of 90 psi (6atm). The resulting filter cake had a green density of 1.58 g/cc and a free moisture content of 37 weight percent.

The filter cake was dried in a rotary drum dryer in which the cake was fed in co-current with the drying gases; the gases entered the dryer at 62O°C. and exited at 100°C., while the exit temperature of the dried cake, which had a free moisture content of about 5 weight percent, was 86°C. The dried filter cake was then compacted (Koppern 60/10 B400 K4) at a pressing force of 5 tons/cm (4500 kg/cm) into 5 cc pillow-shaped briquettes. Average dry bulk density of the briquetted cake was 1.72 g/cc.

The briquettes were fed to a rotary kiln operating at a hot zone temperature of about 1750°C. Samples of the dead burned product after about 5 to 6 nours of operation had a bulk density of 3.27 to 3.29 g/cc, a total porosity of 8.1 to 8.7 volume percent and an open porosity of 1.0 to 1.6 volume percent. Following is a typical chemical analysis of the product (in weight percent): MgO 93.94 CaO 2.24 Si02 1.55Fe2°3 1.72 ai2o3 0.55 Example 2 The process of Example 1 can he repeated using a magnesium hydroxide slurry with a solids content of about 450 g/liter in which the slurry is filtered on a conventional filter press to a final operating pressure of about 50 psi (3.4 atm) to give a filter cake with a free moisture content of about 40 weight percent, the filter cake dried to a free water content of about 2 weight percent, the dried cake compacted into briquettes at a pressing force of about 9 tons/cm (8200 kg/cm) and the briquettes dead burned at about 2000°C, to yield a product of comparable quality.

Example 3 The process of Example 1 can be repeated using a magnesium hydroxide slurry with a solids content of about 150 g/liter in which the slurry is filtered on a conventional filter press to a final operating pressure of about 355 psi (24.8 atm) to give a filter cake with a free moisture content of about 35 weight percent, the filter cake dried to a free moisture content of about 10 weight percent, the dried cake compacted into briquettes at a pressing force of about 4 tons/cm (3600 kg/cm) and the briquettes dead burned at about 1650°C., to yield a product of comparable quality.

Exampl e 4 The process of Example 1 can be repeated using a flux in the form of granular silica equivalent to about 2 g/100 g of magnesium hydroxide in the slurry, which is added with mixing to the slurry prior to the filtration.

A product of comparable density and porosity, but with a proportionate SO increase in silica content, may thus be obtained.

Claims (15)

1. CLAIMS:1. A process for the production of magnesia grain comprising the steps ofs(a) filtering magnesium hydroxide slurry to a filter cake having a free water content of 35 to 40 weight percent; (b) drying the filter cake to a free water content of 2. To 10 weight percent; (c) compacting the dried filter cake into briquettes; and (d) dead burning the briquettes at a temperature of 1650° to 2000°C.

2. A process according to claim 1, wherein the magnesium hydroxide slurry is obtained from the treatment of calcined dolomite with sea water.

3. A process according to claim 1 or claim 2, wherein the solids content of the magnesium hydroxide slurry is 150 to 450 grams/litre.

4. A process according to any one of the preceding claims, wherein up to 0.02 gram of granular silica is added to the slurry per gram of magnesium hydroxide prior to the filtration step.

5. A process according to any one of the preceding claims, wherein the operating pressure for the filtration is from 50 to 365 psi gauge.

6. A process according to any one of the preceding claims, wherein the filter cake is dried to a free moisture content of 5 weight percent.

7. A process according to any one of the preceding claims, wherein the drying step is effected by rotary drum drying.

8. · A process according to any one of the preceding claims, wherein the compacting is effected at a pressure of from 4 to 9 tons/cm. of roll width.

9. A process according to any one of the preceding claims, wherein the dead burning step is effected in a rotary kiln.

10. A process according to any one of the preceding claims, 5 wherein the dead burning step is effected at a temperature of 1700° to 185O°C.

11. A process according to any one of the preceding claims, wherein the heat required for the drying step is supplied by exhaust gases from the dead burning step. LO

12. A process according to claim 11 wherein the drying Step is effected by co-current flow of the filter cake and the exhaust gases through a drying zone.

13. A process according to claim 12, wherein the drying zone is maintained at a temperature of from 100 to 75O°C. :5

14. A process for the production of magnesia grain according to claim 1 and substantially as hereinbefore described with reference to the Examples.

15. Magnesia grain whenever prepared by a process according to any one of the preceding claims.