GB2076788A - Grate Calcination of Mg(OH)2 - Google Patents
Grate Calcination of Mg(OH)2 Download PDFInfo
- Publication number
- GB2076788A GB2076788A GB8017825A GB8017825A GB2076788A GB 2076788 A GB2076788 A GB 2076788A GB 8017825 A GB8017825 A GB 8017825A GB 8017825 A GB8017825 A GB 8017825A GB 2076788 A GB2076788 A GB 2076788A
- Authority
- GB
- United Kingdom
- Prior art keywords
- magnesium hydroxide
- cake
- magnesia
- grate
- kiln
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- VTHJTEIRLNZDEV-UHFFFAOYSA-L magnesium dihydroxide Chemical compound [OH-].[OH-].[Mg+2] VTHJTEIRLNZDEV-UHFFFAOYSA-L 0.000 title claims abstract description 34
- 239000000347 magnesium hydroxide Substances 0.000 title claims abstract description 34
- 229910001862 magnesium hydroxide Inorganic materials 0.000 title claims abstract description 34
- 238000001354 calcination Methods 0.000 title claims description 12
- 235000012254 magnesium hydroxide Nutrition 0.000 title 1
- 239000007789 gas Substances 0.000 claims abstract description 25
- 239000007787 solid Substances 0.000 claims abstract description 22
- 238000010438 heat treatment Methods 0.000 claims abstract description 13
- CPLXHLVBOLITMK-UHFFFAOYSA-N Magnesium oxide Chemical compound [Mg]=O CPLXHLVBOLITMK-UHFFFAOYSA-N 0.000 claims description 45
- 238000000034 method Methods 0.000 claims description 39
- 239000000395 magnesium oxide Substances 0.000 claims description 17
- 239000012065 filter cake Substances 0.000 claims description 14
- 239000000047 product Substances 0.000 claims description 13
- 238000001035 drying Methods 0.000 claims description 9
- 239000002002 slurry Substances 0.000 claims description 6
- 238000011085 pressure filtration Methods 0.000 claims description 2
- 238000004519 manufacturing process Methods 0.000 claims 1
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 8
- 239000000446 fuel Substances 0.000 description 8
- 239000000463 material Substances 0.000 description 6
- 239000004484 Briquette Substances 0.000 description 4
- 229910052681 coesite Inorganic materials 0.000 description 4
- 229910052906 cristobalite Inorganic materials 0.000 description 4
- 239000013535 sea water Substances 0.000 description 4
- 239000000377 silicon dioxide Substances 0.000 description 4
- 235000012239 silicon dioxide Nutrition 0.000 description 4
- 229910052682 stishovite Inorganic materials 0.000 description 4
- 229910052905 tridymite Inorganic materials 0.000 description 4
- 235000019738 Limestone Nutrition 0.000 description 3
- 239000006028 limestone Substances 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 239000003518 caustics Substances 0.000 description 2
- 239000010459 dolomite Substances 0.000 description 2
- 229910000514 dolomite Inorganic materials 0.000 description 2
- 238000001914 filtration Methods 0.000 description 2
- 238000003825 pressing Methods 0.000 description 2
- 238000005245 sintering Methods 0.000 description 2
- 239000002912 waste gas Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 239000000654 additive Substances 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 239000000567 combustion gas Substances 0.000 description 1
- 238000010790 dilution Methods 0.000 description 1
- 239000012895 dilution Substances 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 239000000295 fuel oil Substances 0.000 description 1
- 239000008187 granular material Substances 0.000 description 1
- 230000005484 gravity Effects 0.000 description 1
- 239000010763 heavy fuel oil Substances 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 238000002156 mixing Methods 0.000 description 1
- 238000012856 packing Methods 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 239000008188 pellet Substances 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 239000002699 waste material Substances 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J6/00—Heat treatments such as Calcining; Fusing ; Pyrolysis
- B01J6/001—Calcining
- B01J6/002—Calcining using rotating drums
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F5/00—Compounds of magnesium
- C01F5/02—Magnesia
- C01F5/06—Magnesia by thermal decomposition of magnesium compounds
- C01F5/08—Magnesia by thermal decomposition of magnesium compounds by calcining magnesium hydroxide
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F5/00—Compounds of magnesium
- C01F5/14—Magnesium hydroxide
Landscapes
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Geology (AREA)
- Inorganic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Manufacture And Refinement Of Metals (AREA)
- Muffle Furnaces And Rotary Kilns (AREA)
Abstract
A cake of magnesium hydroxide containing at least 58% by weight of solids is dried and/or calcined by heating in a travelling grate furnace (6) preferably utilising hot gases derived from a dead-burning kiln (3). <IMAGE>
Description
SPECIFICATION
Grate Calcination
The present invention relates to a method of heat-treating magnesium hydroxide.
It has been found that a particularly suitable method of drying or calcining a wet cake of magnesium hydroxide is to use a travelling grate furnace always provided that the magnesium hydroxide cake contains more than 58% by weight of solids and that the cake is in the form of lumps capable of being retained on the travelling grate.
Accordingly the present invention provides a method for drying or calcining a cake comprising magnesium hydroxide containing at least 58% by weight of solids involving feeding the cake to a travelling grate furnace, heating the cake in the furnace to give a product comprising a substantially dry magnesium hydroxide, a partially calcined magnesia or a calcined magnesia.
It is entirely unexpected that one would be able to use a travelling grate furnace for drying or calcining lump magnesium hydroxide since such lumps when heated to remove free water or combined water (to form magnesia) usually break down into a granular material and powder which, due to its particle size and packing characteristics would not be amenable to treatment on a travelling grate.
The product containing magnesium hydroxide and/or magnesia obtained by the above method may be converted to refractory or dead burned magnesia by heating in, for example, a rotary kiln to a temperature in the range 1 6000C to 2 100aC. By using the waste gases from the kiln to heat the material in the travelling grate furnace, a very fuel-efficient process is realised.
Accordingly, therefore, the present invention also provides a method for making sintered or dead burned magnesia which comprises feeding the product from the method described above to, for example, a rotary kiln, there heating the product to a temperature above 1 6000C, preferably to a temperature in the range 1 7000C to 21 000C., and recovering sintered or dead burned magnesia. The hot waste gases resulting from the heating of the rotary kiln, optionally with supplementary hot gases, are used to heat the material in the travelling grate furnace.
The residence time in the kiln will depend on the chemical composition of the feed to the kiln, the characteristics of the kiln and the temperature at which the kiln is operated. Usually however the residence time will not be less than one hour or more than 10 hours.
Optionally the product obtained from the travelling grate furnace is briquetted before it is fed to the kiln.
Briquetting may be accomplished by feeding the product, desirably while still hot, from the travelling grate furnace to high pressure rolls which may operate, for example, at a pressure of 100--500 MN/m2. The size of the briquettes is desirably in the range 1 to 10 cms.
In a travelling grate furnace the heating gas passes through a bed of solids supported on a grate or grid in the form of a continuous conveyor contained within a refractory-lined housing. The bed of solids, which in this case comprises a cake of filtered magnesium hydroxide, may be 10 cms, to 100 cms. thick. The heating gases enter the bed from above and exit through the grate. The gases on entry are preferably at a temperature in the range 5000C to 1200 C. Due to the method of heating a temperature gradient exists within the bed of solids being higher at the upper surface of the solids than at the point where the solids contact the grate.
The magnesium hydroxide used in the present method may be precipitated by reacting calcined dolomite or limestone with seawater. The purity of the resulting magnesium hydroxide depends on the raw material used, processing skills and any additives made to modify the refractory qualities of any magnesia that might be made from the magnesium hydroxide.
In order to obtain a magnesium hydroxide cake containing more than 58% by weight of solids from the precipitated magnesium hydroxide as required by the process of the present invention, one of several methods may be adopted.
For instance one method of obtaining a cake containing more than 58% by weight of solids comprises vacuum filtering the precipitated magnesium hydroxide using a vacuum filter, mixing the resulting cake with previously dried magnesium hydroxide and/or magnesia in quantities such that the solids content of the resulting mixture is greater than 58% and pressing or extruding the mixture.
Another method of obtaining a cake containing more than 58% by weight of solids comprises subjecting the precipitated magnesium hydroxide slurry to filtration on a plate and frame filter to the desired solids content.
A further method of obtaining a cake containing more than 58% by weight of solids, for example more than 67.5% by weight of solids, is described in British Patent No. 14007199 wherein a magnesium hydroxide slurry is simultaneously dewatered and compacted using a filter of the type described in British Patent No.
1 240466. This method is especially preferred.
If the latter method is used to make a cake containing more than 67.5% by weight of solids one method of carrying out the invention comprises feeding a magnesium hydroxide slurry to a filter, applying pressure thereto to give a cake containing more than 67.5% by weight of solids, feeding the cake in the form of lumps to the grate of a travelling grate furnace, heating the cake in the furnace to such a temperature and for such a time that the magnesium hydroxide is converted to calcined magnesia which is discharged from the grate, briquetting the calcined magnesia, sintering the briquetted magnesia in a rotary kiln and recovering sintered magnesia therefrom, the waste heating gases from the kiln being used to heat the cake in the travelling grate furnace. If necessary or desired, supplementary heat may be provided by feeding hot gases into the furnace from another source.
The apparatus used in the process of the present invention is illustrated in the accompanying drawings.
In these drawings there is illustrated a travelling grate pre-heater 1, a briquetting machine 9 and a rotary kiln 3.
The travelling grate furnace 1 consists of a continuous conveyor 4 made up of a series of metal grate panels hinge-linked laterally and driven by chains or rollers 5 and contained within a refractory-lined house 6. The apertures in the grate were approximately 0.5 by 3 inches in size.
Magnesium hydroxide filter cake is fed on to the conveyor 4 by gravity from the hopper 1 5 to form a bed 16 on conveyor 4. The heat treated cake falls from conveyor 4 into a duct 7 and from there into a hopper 8 which feeds the heat-treated material to a pair of briquetting rolls 9.
Briquettes from the briquetting rolls are transferred to an elevator 10 which conveys the briquettes to a screen 11 where undersize material is returned to briquetting machine 9 via duct 7 and oversize material is fed to duct 12.
Duct 12 feeds material into rotary kiln 3 heated by combustion gases from burner 13 whence sintered magnesia is passed to a cooler (not shown) via duct 14.
Hot gases are drawn from rotary kiln 3 heated through the bed 16 on conveyor 4 by means of a suction chamber 17 located beneath the bed 1 6.
The ducting for the gas flow is not shown but may be so arranged that hot gases from the rotary kiln pass through only a first zone of the bed 16, the gases from this first zone then being passed through a second zone of the bed 16. Any supplementary heat may be supplied by burner 18 located at the discharge end of the housing 6.
If desired the heat-treated cake can be transferred direct to the rotary kiln rather than via the briquetting circuit involving briquetting machine 2.
Example 1
A slurry containing magnesium hydroxide was made by reacting sea water with calcined limestone and this slurry was filtered at a pressure of 1 5 MN/m2 to give a cake containing 75% by weight of solids as described in our British Patent
No. 1407199. The filter cake was in the form of irregular sized pieces some 12 mm. thick and 100 to 250 sq. cms. in area.
This filter cake was fed into a travelling grate furnace. The furnace was divided into two zones, the first of which used exhaust gases from the second zone. The gases entering the first zone passed through the filter cake on the grate, drying the cake in the process. The exhaust gases from this chamber were discharged to atmosphere via a gas cleaning device and were at 2000C when they left the furnace. The dried filter cake from the first zone (the drying zone) passed into the second zone where exhaust gases from a rotary kiln at
1 2000C and supplementary heat from the grate
burner were passed through the bed of filter cake
calcining the dried filter cake in the process. The
under grate temperature was controlled to
6500C. The gases from under the grate in this zone were passed into the first zone.The passage through the pre-heater was continuous subjecting the filter cake to gradual drying and then calcination. The calcination was controlled to give
a magnesia with a loss on ignition of less than 1%. The hot gases entering the grate furnace from the rotary kiln were at 1 2000C and were produced by burning heavy fuel oil in the rotary kiln to sinter the calcined magnesia at temperatures of 1850 C. The supplementary hot gases from the grate burner were also produced by burning heating fuel oil.
The calcined magnesia from the grate furnace was conveyed to the briquetting rolls and briquetted at a temperature 600-7000C.
Briquetting was carried out using high pressure (250 MN/m2) briquette rolls. The resulting briquettes were extremely strong and had densities of 2.0 g/ml. These briquettes were transported to the rotary kiln via a 10 mm.
aperture screen. The rejects from this screen were fed back to the briquette rolls. The pellets passing over the screen were fed to the rotary kiln where they were sintered at a temperature of 1 850cC.
This process had a fuel consumption of 2200 kcals/kg. of product which is much lower than conventional caustic calcining-briquetting sintering processes which have typical fuel consumption of 3500-5000 kcals/kg.
The quality of the product produced in the example outlined above was as that produced by the conventional caustic calcining-briquettingsintering route e.g. for a high purity seawater magnesia.
CaO 2.0%
SiO2 0.6%
Al203 0.2%
Fe203 0.1% B203 0.04%
B.D. 3.40 g/ml
Grading 75%+5.0 mm.
10%-0.5 mm.
Example 2
This example is the same as Example 1 up to and including the drying and calcination on the grate furnace except that calcined dolomite was used in place of calcined limestone. The calcined magnesia from the grate in this case was not briquetted but fed directly to the kiln from the grate preheater. This process had a fuel consumption of 2100 kcals/kg. compared to a fuel consumption of 2500 kcals/kg. when the high pressure filter cake was fed directly to a rotary kiln which was not equipped with a furnace.
The quality of the product via. the grate furnace route was the same as that by the direct feed to rotary kiln route, e.g. for seawater magnesia:
CaO 1.0%
SiO2 1.0%
Al203 0.5%
Fe203 1.3%
B203 0.15% B.D. 3.20 g/ml.
Grading 60%+4.0 mm.
10%-0.5 mm.
Example 3
The example is the same as Example 2 up to
and including the high pressure filtration.
The filter cake was fed to the travelling grate
furnace. Exhaust gases from the rotary kiln were
passed to the grate chamber and the temperature
of these gases were controlled to give a maximum
grate temperature of 3000 C. This was achieved
by reducing the rotary kiln exhaust gas
temperature by air dilution or by use of a by-pass
system. In this way drying of the magnesium
hydroxide occurred. The maximum temperature of
3000C minimized calcination to magnesia. The
dried magnesium hydroxide from the grate was
briquetted at temperature using high pressure
briquette rolls (about 250 MN/m2) and gave
briquettes with a density of 1.75 g/ml. The
briquettes were transported to the rotary kiln via a
10 mm aperture screen.The rejects from this kiln were fed back to the briquette rolls and the oversize was fed to the rotary kiln and sintered at a temperature of 1 8500C.
The fuel consumption of this process was 2250 kcals/kg. The quality of the product from this process was as follows:
CaO 1.0%
SiO2 1.0%
Al203 0.5%
Fe203 1.3%
B203 0.15% B.D. 3.25g/ml.
Grading 60%+4.0 mm.
10%0.5 mm.
Example 4
The example is the same as Example 3 but in this case the dried magnesium hydroxide from the grate furnace was not briquetted being fed directly to the rotary kiln where it was sintered at 18500C.
The fuel consumption of this process was 2200 kcals/kg. The quality of the product was as follows:
CaO 1.0%
SiO2 1.0% Awl203 0.5%
Fe203 1.3%
B203 0.15% B.D. 3.20 g/ml.
Grading 60%+4.0 mm.
10%0.5 mm.
Example 5
A filter cake containing a minimum of 60% solids was substituted for the high pressure filter cake used in Examples 1 to 4. This cake was made using a plate and frame filter at a pressure of 1.0-1.5 MN/m2. The 58% minimum solids filter cake was fed to the travelling grate furnace.
The conditions of the grate, at the briquetting stage and in the kiln were the same as in Example 1. and the final product quality was similar. The fuel consumption of this process was inferior due to the lower solids content of the cake (and, therefore, higher free moisture content). The fuel consumption was 2700 kcals/kg.
Claims (9)
1. A method for drying or calcining a cake comprising magnesium hydroxide containing at least 58% by weight of solids which method comprises feeding of the cake to a travelling grate furnace, heating the cake in the furnace to give a product comprising a substantially dry magnesium hydroxide, partially calcined magnesia or a calcined magnesia.
2. A method as claimed in claim 1 wherein the cake of magnesium hydroxide is obtained by pressure filtration of a slurry of magnesium hydroxide.
3. A method as claimed in claim 1 or claim 2 wherein the cake in the travelling grate furnace is subjected to hot gases at a temperature of from 500 to 12000C.
4. A method as claimed in any one of the preceding claims wherein the product of the travelling grate furnace is subjected to a briquetting process.
5. A method as claimed in any preceding claim further comprising dead-burning the dried
magnesium hydroxide or wholly or partially calcined magnesia in a kiln.
6. A method as claimed in claim 5 wherein hot gases from the kiln are used to heat the travelling grate furnace optionally in combination with hot gases from another source.
7. A method for calcining magnesium
hydroxide substantially as hereinbefore described
in any one of the examples.
8. A method for producing dead-burned
magnesia from a magnesium hydroxide filter cake substantially as hereinbefore described with reference to any one of the accompanying drawings.
9. A method for making sintered or dead
burned magnesia which method comprises feeding dried magnesium hydroxide or wholly or
partially calcined magnesia produced by a method
as claimed in any one of claims 1 to 7 to a kiln and heating to a temperature above 16000 C. to
produce cintered or dead-burned magnesia.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8017825A GB2076788A (en) | 1980-05-30 | 1980-05-30 | Grate Calcination of Mg(OH)2 |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8017825A GB2076788A (en) | 1980-05-30 | 1980-05-30 | Grate Calcination of Mg(OH)2 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2076788A true GB2076788A (en) | 1981-12-09 |
Family
ID=10513734
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8017825A Withdrawn GB2076788A (en) | 1980-05-30 | 1980-05-30 | Grate Calcination of Mg(OH)2 |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2076788A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1009717A1 (en) * | 1997-08-20 | 2000-06-21 | Martin Marietta Magnesia Specialties Inc. | WET MILLING OF Mg(OH) 2? SLURRY |
-
1980
- 1980-05-30 GB GB8017825A patent/GB2076788A/en not_active Withdrawn
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1009717A1 (en) * | 1997-08-20 | 2000-06-21 | Martin Marietta Magnesia Specialties Inc. | WET MILLING OF Mg(OH) 2? SLURRY |
| EP1009717A4 (en) * | 1997-08-20 | 2002-06-19 | Martin Marietta Magnesia | WET MILLING OF Mg(OH) 2? SLURRY |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |