NO133491B - - Google Patents

Description

The present invention relates to a method for producing refractory magnesium oxide from a magnesium hydroxide sludge.

Magnesium oxide is used as a difficult-to-melt material or

- for the production of..refractory forms such as bricks or blocks, and are usually produced from naturally occurring magnesium-containing ores, mainly magnesite, or from magnesium salts, especially those in the form of magnesium-containing brines or seawater. In the production of magnesium oxide from magnesium-containing brines and seawater and in the production from certain ores where the magnesium content must be taken up in solution, mag-

the nesium content obtained by it being precipitated as a hydroxide which has a very low solubility in water. This is achieved by treating the magnesium-containing solution, e.g. be it a brine or sea water, with an alkali, e.g. either limestone or slaked lime, dolomitic lime or slaked dolomitic lime or caustic soda.

The precipitation is achieved by adding a suspension or a solution of a suitable base under controlled conditions to the magnesium-containing solution, as e.g. is described in the U.K. Patent Nos. 482,339, 482,340, 539,027, 540,139, 571,276, 615,782 and 635,781. The precipitated hydroxide was then separated by means of sedimentation, after which the sedimented or settled sludge of magnesium hydroxide can be separated and recovered, while the spent brine or sea water is discarded. This sedimentation is usually carried out in large tanks known as thickening tanks and which are designed so that they can receive a continuous stream of suspension at the same time that the sludge of magnesium hydroxide can be taken out separately for further processing in addition to a separate outlet for used brine or sea water.

The content of the magnesium hydroxide sludge is usually recovered at concentrations of from 150 to 400 grams of magnesium hydroxide per litres, all depending on the conditions under which the magnesium hydroxide was precipitated, and the operating technique used in the tank.

In the said reaction, a control is carried out, not only for the purpose of achieving combined physical properties of the precipitated hydroxide, but also to achieve a combined level of other compounds, such as calcium, boron and sulfur compounds. The cursed level may be the lowest possible, but this need not always be the case.

The thickened sludge is usually dewatered by filtration. When producing magnesium oxide on a large scale, as is necessary for refractory purposes, continuous vacuum filters of the drum, plate or disk type are usually used. This dewatering results in a cake with a consistency that can be left

a stiff paste and the paste will usually contain from 40 to 60% solids. If the reaction has been carried out carefully and with selected raw materials to obtain pure magnesium hydroxide, then the solids in the cake will essentially consist of magnesium

hydroxide, and as this magnesium compound contains approx.

31% bound water, the total water content (bound plus free) in the cake will thus vary from approx. 72% (with a content

of 40% solids) to approx. 59% (at approx. 60% solids).

During further processing of the magnesium hydroxide sludge,

be it to caustic calcined magnesium oxide or to dbd-burnt magnesium oxide, the sludge is quickly sent to suitable furnaces, e.g. multistage furnaces or rotary furnaces and subjected to a suitable heat treatment. For caustic calcination, a suitable heat treatment will include maximum temperatures up to 1200°C, more commonly up to approx. 1000°C. In the case of dbd burning, the treatment will necessitate temperatures up to 1600°C or even hbya~ up to e.g. 2000<Q>C.

When -one submits a paste with a normal concentration, e.g. from 40 to 60% solids, such a heat treatment, the resulting magnesium oxide product will be in the form of a mixture of fine granules and powder particles, and very few of the granules will have an average diameter of more than 6.35 mm.

When burning a paste, it is possible to obtain coarser granules by adding fluxes and similar compounds such as silicon dioxide or combinations of three or more compounds such as silicon dioxide, lime, iron oxide and aluminum oxide, but in cases where it is desirable to produce a burnt magnesium oxide product with a low content of these impurities, and in particular with a content of less than approx. 2% silicon dioxide,

it is very difficult to obtain granules that are coarser than 6.35 mm in average diameter.

When producing calcined magnesium oxide for refractory use, it is also desirable to obtain granules with maximum density, i.e. an apparent density or volume density that exceeds 3.20 g/cm ■ x, and this is again very difficult with direct calcination of cakes containing from 40 to 60% solids. The reason seems to be that cakes of this type, when baked or, more particularly, when calcined in the form of weak and brittle bodies which are then subjected to wear and tear (such as occurs in a rotary kiln), easily break down into small granules ,

and that it is very difficult, despite the fact that these granules are very small, to burn them to a sufficiently high density.

Refractory db-burnt products produced by direct dead burning of magnesium hydroxide cakes containing from 40 - 60% solids and produced from brine or seawater, typically have the following composition, density and particle size distribution:

The problem of how to produce dead-burnt magnesium oxide with a large grain size and high density from high-purity magnesium hydroxide has been solved in a way by means of a so-called "pelletisation process". In this process, the magnesium hydroxide is first caustic calcined to a chemically reactive oxide, usually in a three-stage furnace, after which the oxide is briquetted in rollers under high pressure, after which the briquettes are dead-burned, usually in rotary kilns or special gas-fired shaft kilns. It is obvious that this double firing with an intermediate briquetting is a very expensive process, although it gives a good sized product with up to 50% retention on a 6.35 mm sieve and with a bulk density of 3.3 - 3, 4 g/cm-'..

It has now surprisingly been discovered that if magnesium hydroxide sludge of the type described above is dewatered under high pressure of more than 28 kg/cm p, e.g. with pressure up to 1430 kg/cm to a cake containing more than 67.5% solids, these cakes will have the remarkable property of not only being tough and strong in themselves, but giving tough and strong bodies by drying and calcining and which are resistant to wear and tear. Furthermore, it has been discovered that such magnesium hydroxide cakes containing a minimum of 67.5% solids can be calcined or dddburned directly, usually in a rotary kiln, whereby a lumpy dead-burnt magnesite is obtained where a substantial part of the granules have a diameter of more than 6 .35 mm.

Another surprising feature is that magnesium hydroxide cakes with a content of 67.5% solids or more, when subjected to a dead burn treatment, give a product with a significantly higher density than when using cakes with a content of from ^0 - 60 % solids. With this discovery, a method has thus been found which corresponds to the one used in the production of pelletized dead burnt magnesium oxide, but where only a single combustion is used and no briquetting, thereby producing a greater cost reduction.

It has also been found that magnesium oxide produced by this method has a lower content of boron than ordinary filtered magnesium oxide.

According to the present invention, a method for the production of magnesium oxide is thus provided, which consists in that a sludge containing 150 - U00 g of magnesium hydroxide per liters simultaneously dewater.s and compressed by filtration at pressure in the range.28 - 700 kg/cm 2 to form a cake with a solids content of at least 67.5% by weight and with a volume density in the "green" state -at least- 1.15 g/cm^, and in that the cake thus obtained is heated to convert the magnesium hydroxide into magnesium oxide.

The magnesium hydroxide sludge can be derived from a number of different sources. The sludge can e.g. be obtained by treating solutions containing magnesium ions, e.g. brine, seawater, etc. with alkali, after which the precipitate of magnesium hydroxide is separated, e.g. by sedimentation. Furthermore, the sludge can also be obtained by treating minerals containing magnesium with acid, such as hydrochloric acid, and then precipitating the magnesium hydroxide from the solution resulting from such treatment. Another alternative is that the magnesium hydroxide sludge can be obtained by thermal decomposition of magnesium chloride to magnesium oxide, after which the oxide is converted to the hydroxide which can then be pressure filtered using the present method.

The slurry of magnesium hydroxide is preferably dewatered under conditions which include the application of a pressure of more than 28.3 kg/cm ? , e.g. up to 710 kg/cm 2. The pressure used during dewatering should preferably lie in the range from 53.5 - 535 kg/cm<2>.

A method for simultaneous dewatering and compaction involves feeding the sludge to a filter and to this above a pressure, preferably of more than 28.3 kg/cm . A suitable apparatus for doing this includes a cylindrical filter cloth around a hollow cylindrical perforated filter core, and where the pressure is applied hydraulically to the material to be filtered, usually via a flexible sleeve that surrounds the filter. Examples of this type of filtering apparatus are described in British patents no. 907,485 and 1,240,466. A thicker cake, which is advantageous when the magnesium oxide is to be used for the production of bricks, can be obtained by a filtration where this method is used in two stages, a first stage which includes filtration at relatively low pressures, e.g. in the range from 28.3 - 70 kg/cm , and a second step which includes adding a further amount of sludge to the filter cake obtained in the first step, after which a relatively high pressure is applied, e.g. of more than 70 kg/cm .

Another method where dewatering and compaction is achieved at the same time includes feeding the sludge into a perforated mold lined with a filter cloth and applying pressure to the sludge via a piston which is adapted to be fed into the mould, whereby a stdpt block is obtained of magnesium hydroxide which can then be cut into suitable pieces if this is desirable.

The thawing and compaction can be carried out at temperatures above room temperature, e.g. at temperatures from approx. 25 - 100°C. It is also desirable in certain cases to use a surfactant in the water to carry out the compaction and dewatering.

The cake of magnesium hydroxide, which preferably has a density of more than 1.26 g/cirr^, in the "green" state can be converted to magnesium oxide by suitable heat treatment. The heat treatment can be such that an active or caustic magnesium oxide is obtained by calcination at maximum temperatures of up to 1200°C, but more commonly up to 1000°C in a multi-stage furnace. The heat treatment can also be such that a burnt magnesium oxide is obtained by using temperatures above approx. 1600°C, even up to 2000°C, e.g.

in a rotary kiln. For dbd burning, the cake can be pelletized, and in such a case it is advantageous to pelletize a cake that

contains at least 74%, preferably from 80 - 95% solids by weight. Such a content of solids can be achieved either by dewatering the cake at a pressure of more than 141 kg/cm 2 , or by dewatering at -a pressure between 28.3 or 53.5 kg/cm -and 141 kg/cm 2 , and then partially dry the material that has been unwatered under pressure, and this drying can be achieved by e.g. heat and can reduce the water content from 5-20 percent by weight.

If the filter cake is pelletized before it is incinerated, it is desirable to break the filter cake down into granules before carrying out the pelletization, and this breakdown can, for example, be carried out with the help of a crushing machine. The granules can then be pelletised or briquetted by using low pressure rollers which are capable of exerting a pressure of 140 - 420 kg/cm.

For refractory purposes it is often necessary and desirable

to produce dead-burned magnesium oxide compositions containing other refractory compounds in addition to the magnesium oxide. It has surprisingly been found that if these compounds or their precursors are mixed with the sludge of magnesium hydroxide in the present method, a burnt magnesium oxide composition is obtained which has better properties than corresponding compositions produced by usual known methods. If e.g. a finely divided chrome ore is added in this way, a product of excellent density is obtained.

The present invention thus provides a method in which the slurry of magnesium hydroxide is mixed with a finely divided refractory compound or a precursor for such a compound.

Suitable refractory compounds or precursors thereof include calcareous materials, chromium-containing materials, zircon, zirconium oxide and zirconium salts, silicon dioxide, iron oxide (Fe^ Oj ) and aluminum oxide. The calcareous materials may be dolomite, limestone or other calcium carbonate containing materials, or such materials after they have been subjected to calcination to decompose the carbonate, e.g. calcination at temperatures from 800 - 1300°C. The calcined, calcareous material can suitably be hydrated. The chromium-containing material should preferably and preferably be a chromium ore with a silicon dioxide content of less than 2.5% by weight, or can alternatively be chromium sesquioxide (Cr^O-^) such as e.g. can be used in amounts from 0.1-0.5 weight-5?. Suitable zirconium salts are zirconium carbonate, zirconium hydroxide, zirconium sulphate and zirconium nitrate.

The amount of refractory compounds that are mixed with the magnesium hydroxide sludge will depend on the type of refractory compound used and the phoronic product. In compounds with calcareous materials, the amount of lime or burnt dolomite will be such that the amount of magnesium oxide in the burnt product will be from 40 - 95%, or from 55 - 85% by weight. In compounds with chromium-containing materials, the amount of chromium ore will be in the amount range from 2-50 percent by weight based on the weight of MgO in the magnesium hydroxide sludge. In the case of zirconium materials, the amount should be such that calcium zirconate is formed together with the lime in the magnesium oxide, so that, for example, 0.2-5% by weight of zirconium oxide is obtained in the dead-burnt magnesium oxide.

In order to achieve a proper mixture, it is desirable that the particle size of the unhydrated refractory material is suitably small, e.g. so that 100% passes through a 72 B.S.S, sieve, preferably so that from 50 - 80% passes through a 200- B.S.S. term.

The mixing itself can be carried out in any known apparatus, which is designed to homogenize liquid/solid components, such as in rapid mixers, pan mixers, ball mixers, etc. If the refractory material is not sufficiently small in size, the necessary size reduction can be carried out during mixing by using a mixer of the mdlle type.

When producing certain refractory compositions based on magnesium oxide and chromium, it is necessary to use some chromium ore with a relatively large particle size. Under such circumstances, it may be desirable to mix the large-particle material with the filter cake obtained in the present method, after which the two materials can be briquetted together. The present invention thus provides a method where finely divided chrome ore, known as chrome flour, is mixed with a filter cake consisting of a slurry of magnesium hydroxide and chrome ore with a relatively large particle size, e.g. with a diameter of between 0.21 and 3.18 mm.

although the boron content in the magnesium oxide obtained by the present method is unexpectedly low, this can be further lowered by adding sodium carbonate or the like (see British patent no. 1,115,386) to the sludge or cake before dead burning. It is noteworthy that the density of the resulting magnesium oxide is not lowered to the extent that might be expected.

The following examples illustrate the invention.

Example 1

In accordance with the present method, magnesium hydroxide sludge was produced from hydrated lime and seawater and which contained 300 g of magnesium hydroxide per litres, subjected to pressure filtration using an ECLP Alfa Laval tube-2

press at a pressure of 107 kg/cm . The resulting cakes had a magnesium hydroxide content of 75%, and on trial drying gave dried hydroxide of acceptable density. The cakes, which were in the form of irregular pieces with a length of up to 50.8 mm and a width of 25.4 mm with a thickness of 12.7 mm, were dead-burned by being directly fed into an oven which was fired so that a heat treatment corresponding to 1 hour at 1800°C was obtained. During the firing, there was little degradation of the cakes. The chemical analysis, the volume density and the particle size distribution of the various products are indicated in the following table where the oxide content is expressed as a percentage by weight.

Example 2

In a further development of the method described in example 1, the high-pressure filtered paste was broken down into granules by means of a crushing machine, after which the granules were semi-dried to a solids content of 87%.

The semi-dried granules were then briquetted using a high-pressure briquetting roller (462 MN/m 2 ). The resulting briquettes were very strong and gave a dried hydroxide with a density of 1.75 g/cm 2 upon prewetting. By burning the briquettes directly in an oven, i.e. without drying, a product was obtained with an analysis of the type indicated earlier and a density of 3.38 g/cm^. Although this method introduces a briquetting step, it is far less expensive than the method where caustic calcination - briquetting - ddd burning is used, because it eliminates the calcination step.

Example 3

10 parts by weight of a magnesium hydroxide slurry obtained from Waste water from precipitation, sedimentation and washing was wet-mixed with 5.46 parts by weight of chrome flour.

The magnesium hydroxide sludge had the following analysis:

The chromium had a particle size such that at least 90% passed a 300 B.S.S, sieve and was obtained from a chromium ore with the following analysis:

The mixture was subjected to pressure filtration using an ECLP AlfaLaval rotary press at a pressure of 107 kg/cm 2 . The resulting filter cake was approx. 12.7mm thick, 25.4mm wide and 50.8mm long. It was mixed with 2.71 parts by weight of coarse chrome with a particle size such that 100% passed an 8 B.S.S, sieve and had the same chemical analysis as the chrome flour. The mixture was fired at 1800 C for 2 hours and gave a product with a density of 3.55 _g per crn^ and a particle size distribution of 60% + 4 mm and 1% - 0.55 mm.

Example 4

9 parts by weight of magnesium hydroxide sludge with the same analysis as stated in example 3 were wet mixed with 1 part by weight of chrome ore with the same analysis as stated in example 3. The particle size of the chrome ore was as follows:

The mixture was subjected to pressure filtration using an ECLP Alfa Laval rotary press at a pressure of 107 kg/cm, and the resulting filter cake was fired at 1800°C for 2 hours, yielding a product with a density of 3.41 g/cm ^.

Example 5

Magnesium hydroxide sludge containing 30.6 parts by weight Mg(OH)2

■and with the following analysis:

was wet mixed with 35 parts by weight of hydrated calcined dolomite. Both the magnesium hydroxide and the dolomite had a particle size such that 100% passed a 300 B.S.S. term. The mixture was subjected to pressure filtration using an ECLP Alfa Laval rbr press at a pressure of 107 kg/cm 2 and the resulting filter cake was fired at 1500°C for 1 hour. The resulting product had

a bulk density of 3.36 g/cm .

Example 6

A magnesium hydroxide-greyed oil was filtered at different pressures, and the filter cakes obtained were dead-burned at 1800°C. The magnesium hydroxide had the following lossless analysis:

The following table shows the effect of the above-mentioned operation on the content of free water in the cake (column 1), the "green" volume density (column 2), the volume density obtained after firing (column 3) and the boron content (column 4).

Mentioned data in the table for the method where a pressure of 0.7 kg/cm was used is only given for comparative purposes.

Example 7

Magnesium hydroxide sludge was filtered at various pressures, and the resulting filter cake was calcined at 1800°C. Magne- . The sodium hydroxide had the following analysis:

e

Details regarding filtering and the results obtained are as follows:

Claims (6)

1. Process for producing refractory magnesium oxide from a magnesium hydroxide sludge, characterized in that a sludge containing 150 - 400 g of magnesium hydroxide ._pr. liters are simultaneously dewatered and compressed by filtration at a pressure in the range of 28 - 700 kg/cm 2 to form a cake with a solids content of at least 67.5% by weight and with a volume density in the "green" state of at least 1.15 g/ cm^, and by the cake thus obtained being heated to convert the magnesium hydroxide into magnesium oxide. 2. Method according to claim 1, whereby the magnesium hydroxide cake is dead-burned at a temperature above 1600°C, characterized in that the cake is pelletized before dead-burning. 3. 'Procedure according to claim 2, characterized in that the cake to be pelletized is dewatered and compressed to a solids content of at least 74% by weight. 4. Method according to claim 3, characterized in that the dewatering and compression is carried out at a pressure between 28.3 and 142 kg/cm 2 , after which the cake is partially dried. 5. Method according to claims 1-4, characterized in that the slurry of magnesium hydroxide is mixed with a finely divided refractory compound or a precursor for such. 6. Method according to claim 5, characterized in that finely divided chrome ore, known as chrome flour, is mixed with the slurry of magnesium hydroxide, after which chrome ore with a relatively large particle size is mixed with the filter cake.