FI57392C - FOERFARANDE FOER FRAMSTAELLNING AV ELDFAST MAGNESIUMOXID - Google Patents

Description

Tl rft1 ADVERTISEMENT cnrao V§Ta [b] (11) utläggningsskrift (4s) Pr.tont E'Jdrlit ^ v ^ (51) Kv.ik.3 / Int.a.3 c 01 F 5/08 FINLAND —FINLAND ( 21) P »t * nttlh« k «mu» - Pat «ntam6knlnf 3400/72 (22) HakemitpUvf —Araeknlnpdtg 30.11.72 (23) Alkupllvft — GiHI | h * t» da | 30.11.72 (41) Tullut juiklMkil - Bllvlt offantllg 02.06.73

Patent and Registration Office (44) Date of issue and date of registration. -

Patent- och registrstyrelsen Arnttkan utlagd och uti.tkrift «n pubiicmd 30.0i + .80 (32) (33) (31) Pyydetty stuolksu * —Begird prloritet 01.12.71 04.04.72 United Kingdom-Storbritannien (GB) 55820/71, I5I + 82/72 "(71) Steetley (Mfg.) Limited, Gateford Hill, Worksop, Nottinghamshire,

England-England (GB) (72) William Cecil Gilpin, Welbeck Woodhouse, Nr. Worksop, Nottinghamshire,

Noel Heasman, The Grove, no. Hartlepool, Durham, John Williamson,

Hart, no. Hartlepool, County Durham, England-England (GB) (7 * 0 Berggren Oy Ab (54) Method for the production of refractory magnesia - Förfarande för framställning av eldfast magnesiumoxid

The present invention relates to a process for producing refractory magnesium oxide by filtering a magnesium hydroxide slurry containing 150 to 400 g of magnesium hydroxide per liter, forming a clump of magnesium hydroxide and heating the clump to convert the magnesium hydroxide into magnesium oxide.

Magnesium oxide used in refractory aggregates, "or molds, such as refractory bricks and ingots, is made from naturally occurring magnesium-containing ores, mainly magnesite, or magnesium salts, especially magnesium-containing salt solutions or seawater. the magnesium obtained has to be reconstituted, the magnesium is recovered by precipitation as a hydroxide because it is poorly soluble.This recovery is carried out by treating magnesium-containing solutions, brine or seawater with an alkali, usually alkali hydroxide lye and most often either quicklime or slaked lime, or sodium hydroxide.

Precipitation is carried out by adding a suspension or solution of a suitable base to the magnesium-containing solution under controlled conditions, as mentioned e.g. UK Patent Nos. 482,339; 482 340; 539,027; 540,139; 571,274; 615,782; and 635,781. The precipitated hydroxide is then separated by settling, after which 2,579,392 of precipitated magnesium hydroxide slurry can be separated and recovered, and spent or spent brine or seawater is discarded. Precipitation is usually performed in large tanks, called thickeners, which are constructed to operate continuously.

The content of magnesium hydroxide slurry obtained from thickeners is usually 150 to 400 grams of magnesium hydroxide per liter, depending on the conditions under which the magnesium hydroxide is precipitated and the efficiency and mode of action of the thickeners.

The precipitation process is controlled so that the physical properties of the precipitated hydroxide are as desired and that other ingredients, such as calcium, boron and sulfur compounds, are obtained in the desired amount. This desired amount is usually as small as possible, but not always.

More water is removed from the thickened slurry, most often by filtration. In the large-scale production of magnesium oxide for refractory purposes, continuous vacuum drum, plate or disc filters are most commonly used for filtration. The product obtained after this water separation is a rigid paste with a dry matter content of 40-60%. It should be noted that if precipitation is performed skilfully and selected raw materials are obtained from high purity magnesium hydroxide, the pulp contains mainly magnesium hydroxide, and since this magnesium hydroxide contains about 31% chemically bound water, the total water content of the pulp varies (bound + free water) from about 72 5i (40% solids) to about 59 5S (60% solids).

Upon further processing of the magnesium hydroxide slurry into either basic calcined magnesium oxide or dead-burned magnesium oxide, it is fed to suitable furnaces (e.g. multi-grate or rotary kilns) for incineration. A maximum temperature suitable for alkaline calcination is 1200 ° C, but most usually about 1000 ° C. The temperature used for overburning must be above 1600 ° C up to 2000 ° C.

When pulps with a dry matter content of the conventional range, i.e. 40-60%, are fired at the above-mentioned temperatures, a mixture is obtained in which the magnesium oxide is distributed from the granules to the fine flour with a maximum average granule size of up to 6.35 mm.

When burning such a pulp, larger granules can be obtained if a flux or the like is added to the pulp before firing, such as quartz or a mixture which may contain quartz, lime, iron oxide and alumina, but if it is desired to produce superfired magnesium oxide with only low impurities and specifically less than 2% quartz, it is very difficult to obtain granules larger than 6.35 mm in length.

In addition, in the production of dead-burned magnesia for talc-resistant purposes, it is desired to obtain granules with the highest possible density, e.g. a bulk density of more than 3.20 g / cc, which in turn is very difficult to achieve when burning as such a pulp with a dry matter content of 40-60%. The reason for this seems to be that such a mass, when dried and even more so when weakly calcined, forms weak and easily breakable bodies which, when subjected to abrasion in a rotary kiln, break into small granules which, despite their small size, are difficult to burn to a higher density.

When dead-burned magnesium is produced by direct-burning 40-60% of a mass of magnesium hydroxide prepared by precipitation from brine or seawater, the composition of the finished product, the size distribution of the granules and the density are:

k BCD

SiO 2% 0.9 2.0 3.0 0.8 Al 2 O 3 "0.6 0.6 0.2 0.5

Pe203 "1.4 1.4 0.2 1.4

CaO "2.0 2.2 1.0 0.8

MgO "(approximate value) 94/95 93/94 94/95 96/97

The bulk density of the granules was g / cm 2 3.0 3.1 3.2 2.9

Granule size: +6.35 mm 2% 10% 5% 0 + 2.41 mm 15% 30% 30% 10% + 1.68 mm 50% 60% 55% 40%

The difficulty of producing sufficiently large-grained and dense overburned magnesium oxide from high-purity magnesium hydroxide has been solved by the so-called pallosintrausmenetelmällä. According to this method, magnesium hydroxide is first calcined to a basic, chemically reactive oxide, usually in a multi-kiln furnace, after which the oxide is compressed into briquettes, which are then generally overburned in rotary kilns or shaft kilns. It is clear that such two-part incineration with intermediate briquetting makes the process expensive, although the finished product obtained with it is large granular (perhaps 50% remains on a 6.35 mm sieve) and has an apparent bulk density of 57392 3.3-3.6 g / cm3.

According to the present invention, it has surprisingly been found that if water is removed from a magnesium hydroxide slurry as described above by pressing the slurry under high pressure, above 28 atm and up to 1400 atm, a lump with a dry matter content of at least 67.5% is obtained which is already tough and strong and which, after drying and calcination, withstands rubbing and abrasion. Since this finding, it has further been found that magnesium hydroxide lumps with a dry matter content of at least 67.5% can be calcined or incinerated as such, most commonly in a blast furnace, resulting in lump-burned magnesum oxide with a large grain size of more than 6.35 mm.

A further finding is that when magnesium hydroxide lumps containing at least 67.5% dry matter are cross-burned in a certain manner, a product with a density higher than that obtained from conventional lumps containing 40-60% dry matter is obtained. By means of the present invention, it is now possible to obtain a product of the same value as the so-called spherical sintered dead-burned magnesia, which significantly reduces production costs.

In addition, it has been found that the magnesium content of the magnesia produced by this method is lower than that of ordinary filtered magnesia.

According to the present invention, there is provided a process for the production of magnesium oxide, characterized mainly by filtering the slurry at a pressure of 28-700 kg / cm 2, simultaneously removing water and condensing to a lump with a dry matter content of at least 67.5% by weight and a crude volume of at least 1% by weight. , 15 g / cm 3.

Magnesium hydroxide slurry can be prepared from a variety of starting materials. The slurry can be prepared, e.g., by treating solutions containing magnesium ions, such as, for example, brine, seawater, and the like, with alkali, and by separating the magnesium hydroxide, e.g., by precipitation. The slurry can also be prepared by treating magnesium-containing minerals with an acid, such as hydrochloric acid, and by precipitating magnesium from the solutions thus obtained. An alternative to preparing the magnesium hydroxide slurry is to thermally decompose the magnesium chloride to magnesium oxide, which is then converted to the hydroxide and pressure filtered in accordance with the present invention.

5 57392

The water is preferably removed from the magnesium hydroxide at a pressure of more than 28 atm, for example 700 atm. The most preferred pressure for dewatering is between 52-520 atm.

Dewatering with simultaneous compaction requires feeding the slurry to a filter that is subjected to a pressure greater than 28 atm. The filter used for this purpose has a cylindrical, perforated tube with a filter cloth on top. The outer hose is a flexible hose that allows the pressure to be applied to the sludge to be filtered. Filter devices of this type are described in English Patents Nos. 907,485 and 1,240,466. A thicker lump, which is an advantage when magnesium oxide is used in the construction of bricks, is obtained by carrying out the above-described filtration in two steps, the first step using a relatively low pressure, e.g. -70 atm, after which slurry is added to the filter and refiltered, this time under relatively high pressure, e.g. 70 atm.

Another method of simultaneous dewatering and compaction is to use a mold lined with a filter cloth into which the slurry is poured. The filter device also has a piston suitable for the mold, with which the slurry is compressed. The result is a magnesium hydroxide lump which can be reduced to the desired small pieces.

Dewatering and compaction can also be performed at temperatures above ambient temperature, e.g. in the temperature range of 25-100 ° C. In certain cases, it is preferable to add a surfactant to the slurry prior to dewatering and compaction.

The magnesium hydroxide pack, which should have a crude density of at least 1.20 g / cm 2, is converted to magnesium oxide by a suitable heat treatment. The result is active or basic magnesium oxide if the calcination is carried out at a temperature of at most 1200 ° C, most usually 1000 ° C, for example in a multi-kiln furnace. The heat treatment results in dead-burned magnesia if the temperature is 1600 ° C up to 2000 ° C in the blast furnace. The lump can be sintered into spheres before perch firing, and in this case it is preferable to sinter into spheres a lump with a dry matter content of at least 74%, most preferably 80-95% by weight. Thus, dry lumps are obtained if pressures of more than 140 atm are used for dewatering or if the dewatering first takes place at a pressure of 28-52 atm, after which the compacted lump is dried, e.g. by heat, until its water content drops to 5-20% by weight.

If the filter cluster is to be sintered before the ball is burned, it is desirable that it be broken or crushed into granules 6 57392, e.g. in a disintegrator. The granules are then compressed into spheres or briquettes in a rolling mill, where compressive forces of 1.5 to 0.5 tons per square centimeter can be provided.

Refractory magnesium oxide mixtures sometimes contain refractory materials in addition to magnesium oxide for refractory purposes. Surprisingly, it has been found that if these alloys or their ingredients are mixed with a magnesium hydroxide slurry, the present process produces superfired magnesium alloys having significantly better properties than alloys prepared by conventional methods. If, for example, fine chromium ore is added to the slurry, the density of the finished product obtained from this is extremely high. According to the invention, a fine refractory mixture or its fine components are also added to the magnesium hydroxide slurry.

Suitable refractory mixtures or substances are therefore calcareous substances, chromium-containing substances, zirconium, zirconia and zirconium salts, silica, iron oxide (Pe 2 O 2) and alumina. Lime-containing substances include e.g. dolomite, limestone and other substances containing calcium carbonate, whether or not calcined at a temperature of 800 to 1300 ° C for the decomposition of carbonate. The calcined calcareous substances can then be hydrated. As the chromium-containing substance, it is best to use a chromium ore having a silica content of less than 2.5% by weight or, alternatively, chromium sesquioxide (Cr2O2) which can be added, for example, 0.1 to 0.5% by weight. Suitable zirconium salts include, for example, zirconium carbonate, zirconium hydroxide, zirconium sulfate and zirconium nitrate.

The amount of refractory mixture to be mixed with the magnesium hydroxide slurry depends on the quality of the mixture and the requirements for the finished product. In the case of calcareous substances, lime or calcined dolomite is used to such an extent that the amount of magnesium oxide in the finished calcined product is ^ 0-95 #, i.e. 55-85 by weight .- #. In the case of chromium-containing substances, chromium ore is used in an amount of 2 to 50% by weight, based on the amount of magnesium oxide in the magnesium hydroxide slurry. When zirconium-containing substances are used, the mixture ratio should be such that when the zirconium reacts with the calcium oxide in the magnesium oxide, there will be 0.2-5% by weight of zirconium in the finished overburned magnesium oxide.

In order for the mixing to be efficient enough, the particle size of the non-hydrated refractories should be at least small enough that 100 # passes through a 72 B.S.S. sieve, but preferably so fine that 5Ο-8Ο% passes through a 200 B.S.S. sieve.

The actual mixing can take place in any device designed to homogenize a mixture of liquids and solids, such as, for example, a friction mixer, a cholera mill, a screw mixer or a ball mill. If the refractory is not fine enough, mixing must be carried out in a mill-based mixer.

In the manufacture of certain refractory alloys based on magnesium oxide and chromium, it is necessary to use somewhat coarser chromium ore. In this case, it may be most advantageous to mix the coarse material into a filter pack prepared according to the present method, e.g.

by briquetting both ingredients together.

Thus, the present invention includes a method of mixing finely divided chromium ore, also referred to as chromium flour, into a magnesium hydroxide slurry and mixing relatively coarse-grained chromium ore having a grain size of 0.21 to 3.18 mm into a filter pack.

Although the boron content of the magnesium oxide produced by the present process is unexpectedly low, it can be further reduced by adding sodium carbonate or the like (see British Patent No. 1,115,386) to the slurry or agglomerate prior to percolation. It should be noted that the density of the finished magnesia does not decrease as much as might be expected.

It is, of course, to be understood that the present invention also encompasses basic and superfired magnesia, if prepared by the methods of the invention, as well as refractory aggregates and moldings, such as bricks and ingots, if made from such superfired magnesia. The invention further includes furnace walls or liners if they ✓ are made in whole or in part of the refractory moldings according to the invention.

The invention will now be described in more detail by means of the following examples.

Example 1

Various magnesium hydroxide slurries prepared according to the process of the present invention, based on slaked lime and seawater and containing 300 g of magnesium hydroxide per liter, were filtered in an ECLP Alfa Laval filter press at 105 atm. The magnesium hydroxide content of the filter packs was 75% and they resulted in dry hydroxide of S7392 which had been experimentally dried and had a density of good quality. The lumps were irregular, about 50.8 mm long, 25 μm wide and 12.7 mm thick, and were fired for 1 hour at 1800 ° C. During incineration, there was very little crumbling of the lumps. The following table shows the chemical analysis, bulk density and grain size distribution of the various batches, as well as the oxide content in% by weight: 9 „57392

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Example 2

According to a further development of Example I, the pressure-filtered mass was disintegrated into granules in a disintegrator, after which the granules were dried until they had a dry matter content of 87%. The semi-dried granules were briquetted under high pressure (462 MN / m) in a briquetting roll. The briquettes thus prepared were very strong and, after experimental drying, gave a dry hydroxide having a density of 3.38 g / cm 2. This method, although it involves briquetting, is much cheaper than alkaline calcination - briquetting - overburning because calcination is eliminated. Example 3 10 parts by weight of a magnesium hydroxide slurry prepared from seawater by precipitation, precipitation and washing were wet mixed with 5.46 parts by weight of chromium flour.

The composition of the magnesium oxide slurry was as follows: Magnesium hydroxide 280 g / liter

Quartz (SiO2) 1.3 "

Alumina (A 2 O-j) 0.7 "

Iron oxide (Pe20 ^) 2.6 "

Lime (CaO) 1.8

Free water 890 "

The particle size of the chromium flour was such that at least 90% passed through a 300 B.S.S. sieve. The chromium flour was made of chromium ore with the following composition:

Cr203 «5.7%

Fe 2 O 3 25.6%

SiO 2 1.2% Al 2 O 3 16.6%

MgO 10.9%

The mixture was filtered on an ECLP Alfa Laval pressure filter at a pressure of 105 atm. The resulting filtrate cube was about 12.7 mm thick, 25.4 mm wide and 50.8 mm long. This was mixed with 2.71 parts by weight of coarse chromium ore having a particle size such that 100% passed through 8 B.S.S. screens and had the same chemical composition as chromium flour. The mixture was fired for 2 hours at 1800 ° C and the resulting product had a density of 3.55 and a grain composition of 60% + 4 mm and 1% - 0.5 mm.

Example 4 9 parts by weight of a magnesium hydroxide slurry having the same composition as in Example 3 was wet mixed with 1 part by weight of chromium ore having the same composition as that of the chromium ore mentioned in Example 3. The particle size of the chromium ore was as follows:

Size (B.S.S. sieve)% + 30 0 -30 + 72 0.9 -72 + 100 2.5 -100 + 200 27.1 -200 + 300 16.1 - 300 53.4

The mixture was filtered on an ECLP Alfa Laval pressure filter at 105 "atm and the resulting filter pack was fired for 2 hours at 1800 ° C. The density of the finished product was 3.41. Example 5 - Magnesium hydroxide slurry containing 30.6 parts by weight

Mg (0H) 2 and whose composition was as follows:

Magnesium hydroxide 295 g / liter

Quartz (SiO2) 1.4 "

Alumina (Al 2 O 2) 0.8 "

Iron oxide (Pe20 ^) 2.5 "

Lime (CaO) 1.6 "

875 ”of free water was wet mixed with 35 parts by weight of hydrated calcined dolomite. The particle size of both magnesium hydroxide and dolomite was such that 100% passed through a 300 B.S.S. sieve. The mixture was filtered on an ECLP Alfa Laval pressure filter at 105 atm and the filter pack was incinerated for one hour at 1500 ° C. The bulk density of the product obtained was 3.56 g / cm 2.

Example 6

The aqueous magnesium hydroxide slurry was subjected to various pressures and the filter cake thus obtained was incinerated at 1800 ° C. The deficient composition of magnesium hydroxide was as follows:

MgO 96.9 weight?

SiO 2 0.45 "Al 2 O 3 0.2"

Fe 2 O 3 0.2 "

CaO 1.6 "b2o3 0.1

Here. the table shows the effect of the above measures on the amount of free water contained in the curd (column 1), on the raw bulk density (column 2), on the volume weight of the product burned 12 57392 (column 3) and on the boron content (column 4).

Pressure 1 2, 3, 4 atm weight-? g / cnr g / cnr weight? 0.7 50 - 2.95 0.05 35 31 1.16 3.28 0.035 70 26.5 1.28 3.33 0.038 140 26.0 1.34 3.35 0.032 210 23.0 1.39 3 .37 0.028 230 22.0 1.44 3.40 0.035

The values shown in the table for a pressure of 0.7 atm are included for comparison only.

Example 7

The magnesium hydroxide slurry was filtered under various pressures and the resulting filter cake was overburned at 1800 ° C. The deficient composition of magnesium hydroxide was as follows:

MgO 95.7?

Quartz 1.0%

Alumina 0.5%

Fe 2 O 3 1.4%

Lime 0.9%

The details of the filtration and the results obtained are shown in the following table:

Pressure Free water Raw bulk density atm% g / cnr of the burnt mixture g / cnr 70 27.5 1.24 3.26 140 26.0 1.31 3.28 230 22.6 1.42 3.31 420 21.2 1.44 3.34 700 19.4 1.54 3.36 1400 17.0 1.62 3.39

Claims (9)

A process for producing refractory magnesium oxide by dewatering a magnesium hydroxide slurry containing 150-400 g of magnesium hydroxide per liter to form a cake of magnesium hydroxide and by heating the cake to convert the magnesium hydroxide to magnesium oxide, characterized by heating the slurry under a pressure of 28-700 kg / cm, at the same time it releases water from the slurry and this is concentrated to a cake whose dry matter content is at least 67.5% by weight and red volume weight is at least 1.15 g / cm 2. 2. A method according to claim 1, characterized in that the dewatering and concentration are carried out by feeding the magnesium hydroxide slurry into such a filtration device where the material is to be filtered into a space between a cylindrical filter and a centrally flexible membrane which is pressed against the material. which is to be filtered, thereby forcing the liquid component through the filter. Method according to Claim 1 or 2, characterized in that the dewatering and concentration are carried out by feeding magnesium hydroxide slurry into a filter-lined, perforated mold and by directing a pressure on it by means of a piston moving in the mold. Process according to any one of claims 1-3, characterized in that the heating comprises calcining the magnesium hydroxide cake at about 1200 ° C to form active or caustic magnesium oxide. Method according to any one of claims 1-3, characterized in that the heating comprises deadening the magnesium hydroxide cake by heating it to a temperature above 1600 ° C.