WO2015122772A1

WO2015122772A1 - Method for producing dead burned magnesia and products obtainable thereby

Info

Publication number: WO2015122772A1
Application number: PCT/NL2015/050094
Authority: WO
Inventors: Frans Hendrikus Johannes GOORMAN; Marten Robert Pieter Van Vliet; Christos Aneziris
Original assignee: Nedmag Industries Mining & Manufacturing B.V.
Priority date: 2014-02-14
Filing date: 2015-02-13
Publication date: 2015-08-20
Also published as: EP3105184A1; NL2012271C2

Abstract

Method for producing dead burned magnesia and products obtainable thereby. The invention relates to magnesium oxides and methods for producing them. Provided is a method for producing a sintered, high density, dead burned magnesia (DBM) from an aqueous slurry of magnesium hydroxide or a mixed magnesium hydroxide / magnesium oxide slurry,characterized inthat Ti0₂and graphite are added to the aqueous slurry each in amount of 0.05 to 0.5 percent, preferably 0.1 to 0.3 percent, by weight of the magnesium hydroxide. Also provided is a sintered, high density, dead burned magnesia having a MgO content of 97-99 wt%, a density of at least 3.50 g/cm³and a crystal size of at least 90 microns.

Description

Title: Method for producing dead burned magnesia and products obtainable thereby.

The invention relates to magnesium oxides and methods for producing them. It also relates to shaped and unshaped refractory

compositions. Magnesium oxide (MgO, also called periclase) is one of the most important materials used in the production of high temperature- resistant ceramics. Due to its high refractory properties (MgO melts at 2823 ± 40°C), MgO ceramic is non-toxic and chemically inert in basic

environments at elevated temperatures, resistant to the effect of metal infiltration , alkali slag, neutral salts, and react with carbon only above 1800°C. Today, in large-scale technical processes, magnesia (MgO) for refractories is produced from two sources: natural and synthetic. Magnesia from natural sources constitutes 82 % of the world's magnesia installed capacity. The dominant source is magnesite (MgCOe) which occurs in both a macro and a cryptocrystalline forms. Less significant are dolomite

(CaCO₃ MgCO₃), hydromagnesite (3MgCO₃ Mg(OH)₂ -3H₂O), brucite

(Mg(OH)2) and serpentine (Mg3(Si2O5)(OH)4). Synthetic materials are manufactured either from seawater or from magnesia rich brines. The process involves the extraction of dissolved magnesium, which has a concentration of around 1.3 g / dm³ in seawater, and 3 to 40 times this values for brines, and the reaction of magnesium chloride salts with lime or dolomitic lime to produce a magnesium hydroxide precipitate. The precipitate is washed and calcined to form caustic magnesia (CCM). An other production method is the thermal decomposition of magnesium chloride into magnesium oxide and hydrochloric acid (Amman process).

Dead burned magnesium oxide is typically used as refractory bricks inside high temperature steel and cement kilns to prevent the outside part of the kiln structure against melting. Refractory quality MgO is typically prepared in multiple steps. It is well known that dead burned refractory magnesia can be prepared from an aqueous magnesium hydroxide slurry by heating the slurry to achieve decomposition of magnesium hydroxide to magnesium oxide, followed by the densification of the magnesium oxide to the refractory grain material. One conventional method of producing dead burned refractory magnesia is known as the "double burning" or "two-stage" process. This process involves: (1) heating (calcining) the magnesium hydroxide slurry at a temperature of from about 900°C to about 1200°C to produce a chemically still somewhat reactive oxide of magnesia; (2) compacting the thus calcined magnesium in a high pressure briquetting roller; and (3) dead burning (sintering) the briquettes in a rotary or shaft kiln at a temperature of from about 1500°C to about 2000°C. WO82/02190 relates to the production of dead burned magnesia directly from an aqueous magnesium hydroxide slurry in a single burning step. US4,330,525 discloses a method for producing a high density dead burned magnesia from an aqueous slurry of magnesium hydroxide. In example 1 a washed filter cake contains 50 per cent by weight magnesium hydroxide and the final sintering temperature is 1700°C.

After the calcination the white powder undergoes a physical transformation via briquetting, the predensification step. At high pressure the powder is pressed into so called green briquettes. The quahty of the final MgO also depends on the way the briquettes are prepared. To obtain a correct crystal size and density, the briquettes need to be dense so that neighboring MgO crystals could melt and merge into larger crystals during the sintering process.

During the last step of the MgO production process, the sintering step, the briquettes are sintered at high temperatures kiln (typically from 1600-2000°C) to form a highly sintered magnesium oxide, called DBM (Dead Burned Magnesium). Important quality parameters for this DBM material are, depending on the application, crystal size, density and purity. Besides DBM there is also a material called FM (fused Magnesia), which is a form of MgO produced via a large scale electro-fusing process. In this process two large electrodes are placed in a large quantity of MgO, and via a very high voltage electric arc process (in so called electric arc furnaces, EAF), the new large crystal FM is being formed at very high temperatures (> 2750°C). Refractory applications for FM typically differ from DBM.

Refractory bricks made with high purity synthetic DBM are mainly used in cement kilns, while FM material typically has applications in steel producing kilns.

During the refractory brick making process the DBM material is typically crushed into various particle sizes, and as main component of the bricks used at 50-98% and -depending on the type of brick- mixed with about 2-50% of other metal oxides or mixed metal oxides like spinel clinker MgO.A Oe materials and small amounts 0-5% of additives and other processing aids to facilitate the formation of a refractory brick or other refractory materials. Subsequently these bricks are pressed into the right form and again heated in a tunnel kiln or treated at low temperature to produce the final refractory brick or refractory material. The economic value of the refractory bricks and other refractory articles, and hence of refractory quality MgO, depends on their life expectancy inside these ovens and in other refractory applications. The resistance and strength of the refractory materials depend on many factors like the specific temperatures inside these ovens, the type of slag used, the type of fuels being used (with alternative fuels creating more decomposition pathways for the bricks due to the presence of higher amounts of corrosive materials), the type of mechanical forces the bricks are exposed to, the speed of warming up and cooling down (temperature shock), and the resistance against water vapor . It is well known that the size of the primary magnesium oxide crystals is a key parameter for the stability as a refractory material in steel applications. Large crystals have a less vulnerable total surface area than smaller crystals and are less susceptible to chemical breakdown. Another important parameter for strong refractory brick is the density of the MgO starting material. More dense MgO will result in better bricks. Yet another important parameter for the quality of the MgO starting material is the purity. Because of their natural background (minerals) they typically carry small quantities of related mineral materials like calcium, boron, silicium, iron, and manganese. During the high temperature production process of the magnesium oxide these small quantities of additional materials typically could form new mixed oxides with Mg. These mixed oxides are typically present as amorphous materials, a so called "second phase", at the edges of the pure MgO crystals. Unfortunately, these mixed oxides typically have significantly lower melting points than pure MgO, and create pathways in the refractory brick for corrosive gasses to enter the inside of the brick, giving rise to a faster breakdown of these bricks.

Depending on the starting raw materials and process conditions the currently known DBM production processes produce a quality of DBM with a crystal size of 50 -150 microns and a density of about 3,38 - 3,44 g / cm³.

The amounts of impurities are fixed due to the quality of the starting raw materials. The physical process conditions of the sintering process can be optimized to improve the DBM quahty. In particular, longer sintering times at increased temperatures typically result in larger crystal size. However, this approach is economically unattractive for large scale production of refractory magnesium.

A further serious problem in the application of magnesia is the tendency of the magnesia to hydrate to magnesiumhydroxide. This has hampered one of the growth areas: the steel making ladle. Mainly formed products (bricks) are used in this application. Lining these ladles with bricks is time consuming and labour intensive and thus expensive. It could be

advantageous to use magnesia based castables instead of magnesia based bricks. Installation of a monolithic lining by casting or vibro casting is less time consuming and less labour intensive than the installation of a brick lining. But drying and curing of a monolithic lining is more troublesome. One of the major problems is hydration of magnesia during drying of the installed lining. The formation of magnesiumhydroxide from

magnesiumoxide is accompanied by a volume expansion. Owing to this volume expansion large cracks can be formed in the installed lining. In most cases the presence of such large cracks is detrimental for the lifetime of the installed lining. It is known that for magnesia based castables in which the bonding system contains boric acid, the sensitivity of the material towards hydration is reduced. The boric acid content (analysed as B₂O₃) of such materials can be up to 3% (m/m). However the presence of boronoxide reduces the refractoriness of the castable to a large extent. Therefore such castables are not attractive materials for applications at temperatures over 1400°C. For high quality magnesia based materials, which can be applied at temperatures over 1600°C, the boron content should not exceed 0.05% (m/m). Castables on the basis of such high quality magnesia grades are not yet available, due to the problem of hydration.

The present inventors therefore set out to develop a method for improving the quality of DBM in a commercially attractive manner. In particular, they sought to produce DBM having a crystal size of more than 70 microns and having a density of at least 3.45 g/cm³ using relatively low sintering temperatures. Moreover, the DBM thus produced preferably has at least equal properties, more preferably superior properties, as compared to known DBMs and FMs. More particularly, it is an object of the invention to provide a process for preparing magnesia having reduced hydration tendency, thus having advantageous application properties when included in a refractory product or castable..

It was surprisingly found that the addition of very small amounts of T1O2 and graphite (carbon) in a wet stage of the process dramatically improves the quality of the resulting DBM. The crystal size doubles and density goes from 3.40 g/cm³ to around 3.50 g/cm³, thus approaching the theoretical maximum of 3.58 g/cm³. The amounts of Ti and C to be added are very small and are preferably added in a early stage (pre-calcination and in the wet stage) of the process to achieve a homogeneous distribution through the product. Advantages of the Ti/C addition are:

A larger crystal size at similar T and t profiles

- A higher density product at similar T and t profiles

Possibility to calcine and sinter more economically, e.g. at a temperature below 1700°C, while maintaining a high quality

Besides crystal size and density, for the first time ever, a new crystalline 2^nd phase has been observed, consisting of a mixed Mg Ca T1O3. - The vapor resistance of refractory products prepared from the

DBM show increased (up to 40%) resistance to vapor, evidenced for example by the modulus of rupture (MOR, a 3-point bending strength method) after a one week room temperature exposure to regular humidity air. This advantage is not observed when the process is performed in the absence of adding TiO2 and graphite in the small amounts defined above, for example using the method of US4,330,525.

Accordingly, the invention provides a method for producing a sintered, high density, dead burned, refractory magnesia from an aqueous slurry of magnesium hydroxide or a mixed magnesium hydroxide / magnesium oxide slurry, characterized in that T1O2 and graphite are added to the aqueous slurry each in amount of 0.05 to 0.5, preferably 0.1 to 0.3 percent by weight of the magnesium hydroxide.

The addition of T1O2 to MgO materials is known in the art.

Xu, Y. et al (Refractory Cement Glass Ceramic industry, 6.

International Symposium 2011) processed samples with either 0, 1, 5 or 10 wt% T1O2 and sintered at 1600 °C. It was found that the grains of sintered specimens without T1O2 were small (about 30 pm) and not strongly bonded together. Plenty of pores could still were found. Whereas, when T1O2 was added, grain size of MgO increased obviously and became more uniform. The grain size of sample with 1 % T1O2 was about 50 pm.

Rada et al. (Keramische Zeitschrift 48 (1996)) used 1-5 % T1O2 and 1-5 % ZrO2 and found the best results with 1-2 % T1O2 and sintering at 1700 °C. The holding time made big crystals. The optimum was 6 hours.

Lee, Y., B. et al (Journal of Materials Science 33 (1998) 4321-4325) dissolved magnesium sulphate (MgSO₄ 7 H2O) and titanyl sulphate

(TiOSO₄ · 2H₂O) in distilled water. The amount of T1O2 addition was 0.5, 1, 2, 4, 6, 8 and 10 %, calculated as T1O2. The mixed solution was freeze dried at -50 °C and 0.6 Pa. The dry powder was calcined in air at 1200 °C for 2h. Discs 12 mm diameter by 50 mm were pressed at 15 MPa, followed by isostatically pressing at 150 MPa and sintered in air in an electric furnace at 1400- 1600 °C for 2h with a heating rate of 4 °C/min. At 1500 °C , 76 % density is achieved by the 0 % TiO₂-MgO, and 98 % by the 2 % TiO₂-MgO. - At higher temperature than 1300 °C the T1O2 content >0,2 % forms

Mg₂TiO₄.

Thus, the prior art studies on T1O2 work in the region of 1- 10 wt% addition because the solid solubility limit for T1O2 is 0.3 wt%. In contrast, the present invention uses an upper value of about 0.5wt%, preferably 0.3 wt%, which is well below those known or suggested in the art.

The use of graphite in combination with MgO is known in the art to produce MgO-C refractories. Graphite contents of typical bricks range from 1 - 35% natural flake graphite. For example, WO2010/080336 discloses a refractory material for lining a ladle used in steel making, said refractory comprised of: about 45% to about 95% by weight MgO, about 1% to about 20% by weight carbon; and about 4% to about 45% by weight aluminous chamotte. The material may optionally further comprise about 1% to about 20% by weight graphite.

Aneziris et al. (J. European Ceramic Society 27 (2007) 73-78) describe the effect of T1O2- and Al-additions on the oxidation resistance and the mechanical properties of MgO-C refractories. Test compositions comprising MgO, about 12 wt% graphite and 0.4 wt% T1O2 are disclosed. However, the art is silent about the combined use of small amounts of graphite, or any other carbon source, and T1O2 in a method for producing a sintered refractory MgO.

The T1O2 for use in the present invention can be commercially obtained, typically as a powdered composition. Preferably, the average particle size (d50) is in the range of 0.1- 10 μιη, more preferably 0.5-5 pm. Very good results were obtained with T1O2 having a d50 of about 1.6 pm, which is available from Crenox GmbH, Germany.

The graphite for use in the present invention is also commercially available from various sources, but other crystalline and amorphous carbon sources might also be used. Natural graphite with a very fine grinding is preferred. Very good results were achieved with graphite AF 96/97 from Graphit Kropfmiihl AG, Germany, having a particle size distribution 99.7%<40 μΜ, d50: 8.5- 11 pm, d90≤25 pm.

For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. The invention is characterized by the addition of T1O2 (Ti) and graphite (C) to the aqueous slurry each in amount of 0.05-0.5, preferably 0.1-0.3 percent by weight. The use of Ti or C as single additive at this low level was not found to give the desired product quality. Ti and C can be added in different amounts or in equal amounts. Typically, they are used in a relative weight ratio of between 1:3 and 3 : 1, preferably between 1:2 and 2: 1. In one embodiment, Ti is used in excess of C. For example, Ti is added at at least 0.2wt% and C is added up to 0.2wt%. In another embodiment, C is used in excess of Ti. For example, C is added in an amount of at least 0.2wt% and Ti is added up to 0.2wt%. In another preferred embodiment, T1O2 and graphite are used in essentially equal amounts, for example in a relative weight ration of between 1: 1.2 and 1.2 to 1. Very good results are obtained wherein Ti and C are used in the range of about 0.15-0.25wt% each. In a specific aspect, a combination of 0.2wt% Ti and 0.2wt% C is used. The skilled person will appreciate that the invention is advantageously used in any method for producing dead burned refractory magnesia involving an aqueous mixed magnesium hydroxide / magnesium oxide slurry, either involving a multiple (double) stage or a single stage burning process. For instance, it is suitably applied to a process described in US 3,221,082 comprising a calcination step, a briquetting step and a shaft kiln sintering step.

In one embodiment, in the first (gass oven calcination) step the fine magnesium hydroxide (or a mixture of magnesium hydroxide and

magnesium oxide as a concentrated slurry), is being transformed into an oxide. This chemical decomposition reaction starts at about 350°C.

Subsequently and in the same calcination step the resulting MgO is heated till about 1000°C to shrink the surface of the MgO crystals, going from an initially very porous material to a hardly porous material. The end product of this step is called CCM, caustic calcined magnesia. In another

embodiment, the Ti/C additive according to the invention is applied to a single step burning process, for instance to a method based on the teaching of WO82/02190.

The aqueous slurry of magnesium hydroxide to be supplied with T1O2 and C according to the present method can be derived from a number of sources. For instance, the slurry may be obtained by treating solutions containing magnesium ions, for example, brine, sea water, and the like, with an alkali, such as dolime, and separating the precipitate of magnesium hydroxide. The present method is particularly suited for use with slurries of mixed magnesium hydroxide / magnesium oxide obtained from brine.

Mixed magnesium hydroxide / magnesium oxide slurries obtained from brine generally have a water content of from about 25 to about 50 percent by weight and a magnesium hydroxide /magnesium oxide solids content of from about 50 to about 75 percent by weight.

In one embodiment, a method of the invention comprises, or essentially consists of, (i) reacting a magnesium salt brine or seawater with a source of calcium oxide, preferably dolime (CaO.MgO), to precipitate magnesium in hydroxide form, followed by (ii) washing of the precipitate (iii) preparing an aqueous slurry of the magnesium hydroxide and magnesium oxide (Mg(OH)2 / MgO) comprising the additives T1O2 and graphite; (iv) calcining the slurry to transform the (Mg(OH)2 into magnesium oxide (MgO) crystals; at a temperature of 500- 1000°C to produce caustic calcined magnesia; and (v) sintering the caustic calcined magnesia at a temperature of 1600 -2000°C, to obtain dead burned magnesia. As is shown herein below, the additives of the invention allow the production of a high quality DBM at sintering

temperatures that are significantly lower than what is needed in the prior art to obtain the desired quality. In one embodiment, the sintering is performed at a temperature in the range of 1600- 1900°C. For economical reasons, a sintering temperature up to about 1800°C, preferably 1750°C, more preferably up to 1700°C is preferred. In a specific aspect, the sintering temperature is between 1600 and 1700°C.

It may be preferred to at least partially dewater the slurry before employing the present method. This initial dewatering step may involve feeding the slurry to a conventional filtering means, e.g., a vacuum type filter, or a rotating drum filter, to remove at least a portion of the free water content of the magnesium hydroxide. Generally, the magnesium hydroxide / magnesium oxide slurry after filtration contains about 40 to 70 percent solids, and about 30 to 60 percent free water. The ratio between magnesium hydroxide / magnesium oxide depends very much on the production process If an oxide has to be formed, the magnesium oxide levels are generally as high as possible, typically around 25%. The mixture of magnesium

hydroxide / magnesium oxide can either be hydrated in a separate process step to completely magnesium hydroxide or directly calcined.

Hence, in one embodiment the invention relates to the production of sintered magnesia from magnesium hydroxide in which a magnesium hydroxide suspension is first freed from water e.g. by filtration, the filter cake thus obtained is calcined. The caustic MgO is briquetted under pressure and the briquettes thus obtained are sintered wherein T1O2 (Ti) and graphite (C) are added to the filter cake in amount of 0.05-0.5, preferably 0.1-0.3 percent by weight. The invention also provides a sintered, high density, dead burned

magnesium oxide obtainable by a method according to the invention and any use thereof, e.g. in the manufacture of a refractory material. The material typically has a MgO content of 97-99wt%, a density of at least 3.45 g/cm³ and/or a crystal size of at least 70, preferably at least 80, more preferably at least 90 microns or even larger, like 100 microns or more.

Provided is a sintered, high density, dead burned magnesia having at least two, preferably all, of the following properties: a) a MgO content of 97-99 wt%; b) a density of at least 3.50 g/cm³ ; and c) a crystal size of at least 90 microns. In one embodiment, it has a density of at least 3.46 g/cm³, preferably at least 3.48 g/cm³, more preferably at least 3.50 g/cm³ and a crystal size of at least 90 microns. In addition, as is shown herein below, a DBM of the present invention it is characterized by the presence of a crystalline 2^nd phase consisting of a mixed Mg Ca T1O3. Hence, in one embodiment the invention provides a DBM, which can but does not have to be obtained by a method of the invention, comprising a crystalline phase comprising or consisting of a mixed MgO:CaO:Ti02 composition, preferably in a relative molar ratio of from 1: 1:1 to 1:2:2.

A further embodiment of the invention relates to a refractory composition for forming shaped and unshaped refractories, the composition comprising a sintered, high density, dead burned, refractory magnesium oxide according to the invention, and possibly other mixed magnesium oxides, or other metal oxides or additives. Preferably, the refractory composition of the shaped and unshaped refractories has an MgO content of about 50-98%. The

composition may contain, typically in an amount of from 2-50%, one or more other additives in addition to Ti/C. For example, MgO.A Oe or MgO.Fe2O3, or Carbon/resin/Graphite, or other metal oxides like CaO, and sometimes small quantities of other processing aids.

Unshaped refractory materials have the ability to form a joint -less lining, and are often referred to as monolithic. These materials are useful for example as lining for cupolas hearth and siphon, blast furnaces, main, secondary and tilting runners, and more generally vessels or vessel spouts, ladles, tundishes, reaction chambers and troughs that contain, direct the flow or are suitable to facilitate the industrial treatment of liquid metals and slags, or any other high temperature liquids, solids or gases. Unshaped refractories are typically manufactured in powdered form and mixed with water prior to application. The wet material may be applied as a lining using techniques such as casting, spraying and gunning followed by setting and drying, prior to firing. In one embodiment, the invention provides a castable refractory composition comprising a sintered, high density, dead burned, refractory magnesia prepared from an aqueous slurry of magnesium hydroxide or a mixed magnesium hydroxide / magnesium oxide slurry, wherein Ti02 and graphite are added to the aqueous slurry each in amount of 0.05 to 0.5, preferably 0.1 to 0.3 percent by weight of the magnesium hydroxide. By castable is meant that the refractory composition is formable into a joint -less or unshaped product upon addition of water, setting and drying to remove excess water. In one embodiment, the castable refractory contains 5 to 50 mass % of a DBM according to the invention, a balanced amount of a material mainly comprised of alumina, a bonding agent and a dispersant.

In accordance with a further aspect, there is provided an installable refractory lining obtained by mixing the castable refractory composition with 2 % to 40 % by weight water. Also provided a method of installing the installable refractory lining of the present invention using a technique selected from casting, self flowing, shotcreeting, rodding, cast-vibrating, spraying, conventional dry gunning or high density gunning, followed by setting and drying. Hence, also encompassed is an installed refractory lining obtainable by the method of the third aspect. The lining may be a lining for cupolas hearth and siphon, blast furnaces, main, secondary and tilting runners, vessels or vessel spouts, ladles, tundishes, reaction chambers and troughs that contain, direct the flow or are suitable to facilitate the industrial treatment of liquid metals and slags, or any other high

temperature liquids, solids or gases. Still further, there is provided a method of installing a refractory comprising: mixing the castable refractory composition according to the invention with water, forming the mixture into an article, allowing the article to set, and drying the article to remove excess water.

Also provided is a method for providing a shaped refractory article using a sintered, high density, dead burned, refractory magnesium oxide of the invention as (main) source of MgO, as well as a refractory article based on MgO. Due to the properties of the DBM sinter used in the refractory brick, the brick will have an improved properties, in particular enhanced life time, increased corrosion resistance and/or improved hydration resistance e.g. in Mag-Carbon bricks. A refractory brick of the invention can withstand high temperature, and also usually has a low thermal conductivity for greater energy efficiency. Usually dense fired bricks (Mag-spinel or Mag-Carbon bricks) are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a kiln using alternative fuels or furnaces for metal and steel production, which are subject to abrasion from fuels, fluxing from ash or slag, and high temperatures.

Accordingly, the invention also provides a furnace comprising a lining /wall comprising a refractory article, preferably a brick, according to the invention. Also provided is the use of a refractory article for building or restoring the lining of a furnace. The furnace is for instance a refuse incinerator, a cement burning furnace, a steel ladle, an annealing furnace, a heating furnace or a steel making furnace.

In view of their superior thermo mechanical properties, a high density large crystal refractory article of the invention is advantageously used in the steel making industry. Two of the key drivers in steelmaking operations are operational security and cost reduction. Steelmaking processes rely on refractory linings to contain the hot liquids and gases involved in producing steel. Modern processes increasingly use higher temperatures and techniques such as bottom bubbling and injection that greatly increase the flow of fluids within the vessel. These conditions are highly beneficial to productivity and can increase product quality. However, they also tend to significantly accelerate the rate of degradation of the refractory lining. Higher costs associated with more frequent replacement of the refractories and the associated down time are counterproductive, and some of the benefits of the new processing methods can be lost. Accelerated wear can also reduce product quality through an increase in the number of nonmetallic inclusions in the metal. Therefore, the refractory performance due to using the improved refractory bricks of the invention has a

significant impact on both operational security and costs.

EXPERIMENTAL SECTION Materials:

AH Magnesium oxide samples are commercial Nedmag materials. The commercial grade nedMag 99 caustic powder was used for powdered CCM (with a typical d50 particle size around 4-10 micron). So called green briquettes were used as a commercially available pressed reference. Regular Nedmag 'nedMag 99 DBM' Magnesium oxide briquettes were used as reference representing a burned commercial DBM quality.

The magnesium hydroxide filtercake is a well-known intermediate product in the production of synthetic magnesia (CCM or DBM). The production of magnesium hydroxide from brine or seawater entails direct precipitation with quicklime (CaO) or more preferable with dolime or dolomitic lime. The resulting thickened slurry is vacuum-filtered to produce a filter cake, typically with about 65 weight % solids and 35% water, the solids containing about 75% of magnesium hydroxide and 25% of remaining magnesium oxide coming from the dolime. Because nearly all the

magnesium hydroxide produced in this way is presently calcined to MgO, the characteristics of the mixed precipitate are optimized for solid liquid separation and for MgO content. However, for the present invention samples with 24.0, 15.8, 14.0, 9.4 and 0 % of magnesium oxide respectively, were used without a difference in end-result.

Titanium dioxide powder: the TR-HP-2 material from Crenox GmbH in Krefeld Germany with a d50 particle size of about 1,6 micron, also known as Sachtleben TRHP2, was used. Other commercial qualities will perform well too.

Zirconium dioxide : the 99.0-99.5% pure powder from Saint Gobain with a typical particle size of 0.3 micron was used. As a carbon source we used graphite from Kropfmuhl AF 96/97 with a d50 of

8,5- 11 micron, however other graphite qualities as well as carbon forming substances are equally suitable.

All dosing quantities of titanium dioxide and graphite are expressed as weight percentage calculated versus 100% Mg(OH)2

suspension. The commercial grades of graphite and T1O2 are close enough to

100% purity that no corrections were applied and the materials were dosed as is.

Typical magnesium hydroxide slurries used contain or were diluted to about 55.5% Mg(OH>2 (as expressed as MgO it would be 38.3% based on the differences in molecular weight, i.e., 58.3 for Mg(OH>2 and 40.3 for

MgO). By way of example, one must therefore add 3 x 0.555 = 1.665 grams of T1O2 to 1 kg of an aqueous slurry of Mg(OH)2 (containing 55.5% of

Magnesium hydroxide and 44.5% of water) in order to obtain 0.3% (w/w) of TiO₂

Mechanical and Physical Preparation and Characterization:

Particle size measurements were carried out with Beckman Coulter equipment. EDAX microanalysis was used to identify the elemental composition of the matrix and in the wedges of the crystals. The structure in the wedges were analysed with EBSD to look whether amorphous or crystalline structures exist. Bulk density (g/cc) and MgO crystal size (in microns) measurements were carried out with methods known to those skilled in the art and for instance described in ISO 8840: Mercury Method with vacuum (for BD), while a microscopic measurement method was used to determine the grain size of the crystals.

Dry powder mixing/homogenizing of the CCM and the additives was carried out in an Eirich EL 01 mixer for 5- 10 minutes, in batches of typically 350 gram with a stirrer rpm of 5500 and a container counter flow of 170 rpm. The filter cake samples were diluted with some water up to a solid content of about 30-50% , and homogenized with the additives for 10 minutes with a paddle mill scrabber at 600 rpm. Typical batches amount to 300 gram. The batches were dried in plastic forms at 60°C for 24 hr.

Calcining: The commercial samples CCM were calcined at the Nedmag production facilities in Veendam, The Netherlands. The other samples were calcined in the electro kilns from the TU Bergakademie Freiberg IKGB Germany with a typical calcination regime for the heating up till 1050°C a 3.3-3.5K/min and a calcination time at 1050°C of 3 hours. Densification: In the commercial process professional industrial briquetting equipment is being used, operating at 50 - 200 bar. For the lab and pilot samples an uniaxial press with a forming pressure of 150MPa (RUCKS KV270, with an aperture of 50mm, and a height of 15mm) and an isostatic press at 300 MPa (EPSI S.O. 5-8359-0, aperture 25mm, and height of 60mm) were used.

Sintering: The industrial Nedmag briquette is sintered by Nedmag during commercial production. The other samples are sintered in the electro kilns from the TU Bergakademie Freiberg IKGB Germany or in a commercially available pilot scale gas kiln from Polysius Germany. A typical sintering cycle is characterized by a heating up rate of 10 K/min until 1200°C and a subsequent heating up rate of 5 K/min until final sintering temperature has been reached (typically from 1500- 1900°C). The holding time at the final sintering temperature typically amounts to 2-3 hrs.

Example 1

In this example the effect of adding either T1O2 and Zr02 to dry CCM, uniaxial pressed at 100 MPa and sintered at 1600°C is compared.

Table 1

From Table 1 it is clear that the addition of larger quantities of Titanium dioxide to dry CCM do has a positive effect on the density of DBM, while an equivalent large addition of Zirconium dioxide has no positive effect at all. Example 2

In example 2 the addition of either T1O2 Zr02 or graphite as separate additives to dry CCM, uniaxial pressed at 150MPa, and sintered at 1600°C or 1700°C is compared. Table 2

Product Density 1600 Density 1700

CCM, no additions 3,04 3,20

CCM, 0,6% Ti 3, 18 3,29

CCM, 0.6% Zr 3, 10 3, 16

CCM, 1,2% Zr n.a. 3,20

CCM 1,0% Graphite n.a. 3, 18 The results show that, in comparison with table 1, densification at higher pressure (150 vs 100) gives a significantly higher density, even for the pure CCM powder (3,04 vs 2,86 g/cm³). We also observe that higher sintering temperature increases the density. We also observe that the addition of 0,6% Titanium could lower the sintering temperature with about 100°C. Addition of similar or even double the amounts of Zr did not lead to higher densities. The addition of 1% of Graphite does not improve the density either. Example 3

This example compares the different processing routes that can be followed to add the Titanium dioxide, i.e. addition in the dry stage via dry powder mixing after calcining with CCM, or mixing in the wet stage pre-calcining with the filtercake (an aqueous based slurry with a magnesium hydroxide and magnesium oxide mixture, subsequently calcined at 1050°C).

Subsequently, both products are pressed at 150MPa and sintered at 1600°C or 1700°C, respectively.

Table 3

From the data it is clear that homogeneous incorporation in the wet stage before calcining is preferred over incorporation in the calcined CCM powder stage. The results also demonstrate that, when Titanium is incorporated in the wet stage the maximum sintering benefit is already obtained at 1600°C, while for regular CCM the difference in density between 1600 and 1700°C is still very significant. Those skilled in the art will appreciate that this lowering of the maximum sintering temperature translates in a significant commercial benefit.

Example 4

In this experiment, the Ti, Zr or Graphite additives are added in the wet stage, followed by calcination at 1050°C, isostatically press at 300 MPa, and sinter at 1700°C.

Table 4

The density data show that addition of lower levels of Titanium, nor single levels of Zirconium nor low or medium levels of added Graphite can improve the density of the formed DBM.

Example 5

The next series of examples summarizes the effect of the addition of combined low levels of Titanium dioxide and low levels of Graphite according to the present invention. Example 5 specifically focusses at the dry processing route. After addition of the ingredients to CCM, and homogenizing for 10 minutes, the samples were isostatically pressed at 300MPa, and sintered at 1600 or 1700°C. Table 5

In the dry processing route, addition of small amounts of Titanium and Carbon (Graphite) does not have a positive effect on the density of sintered DBM, not at 1600°C nor at 1700°C. However, a small positive effect was observed after the addition of 0,6% of both ingredients and sintering at 1600°C. At 1700°C this effect is relatively small.

Example 6

Example 6 summarizes the effect of the addition of combined low levels of Titanium dioxide and low levels of Graphite, but now during the wet filtercake processing route. After addition of the ingredients to the filtercake, and homogenizing for 10 minutes, the samples were dried and calcined at 1050°C, and subsequently isostatically pressed at 300MPa, and sintered in an electro kiln at 1600°C or 1700°C.

Table 6

Product Density 1600 Density 1700

Filtercake, no addition 3,35 3,42

Filtercake, 0,2% Ti and 0,2% C 3,40 3,44

Filtercake, 0,2% Ti and 0,3% C 3,41 3,43

Filtercake, 0,3% Ti and 0,2% C 3,41 3,43

Filtercake, 0,3% Ti and 0,3% C 3,46 3,44

Filtercake, 0,3% Ti and 0,4% C 3,41 3,43

Filtercake, 0,6% Ti and 0,6% C 3,43 3,45 These series of experiments nicely illustrate that higher levels of density can be achieved via the wet processing route (compare with example 5). Mixtures of relative small amounts of Titanium dioxide and Graphite will increase the density after sintering both at 1600°C and at 1700°C. However, the difference is more pronounced at 1600°C. Additions of more than 0,3% did not yield higher densities than what is achieved with 0,3% Ti and 0,3% Graphite. Example 7

The samples of Example 5 and Example 6 were subjected to scanning electron microscopy (SEM) and subsequent Energy Dispersive X-ray

Spectroscopic (from EDAX) microanalysis to analyze the composition of the material on the wedges between the crystals.

Product from Example 5: Dry processing route, with 0,6% Ti and 0,6% Graphite, Sintering at 1700°C and with a resulting a BD of 3,37

Table 7A

Product from Example 6: Wet processing route, with 0,3% Ti and 0,3% Graphite, Calcination at 1050°C, and sintering at 1600°C and with a resulting BD of 3,46 Table 7B

From the data above it can be concluded that the composition of the material outside the MgO crystals, and in particular in the triangular wedges between the crystals, has a mixed MgO:CaO:TiO2 composition with ratio's from 1: 1: 1 to 1:2:2, probably in the form of mixed Calcium

Magnesium Titanates (CMTs like CaTiO₃ and Mg₂TiO₄ etc).

Example 8

After calcination and sintering in an electro kiln, the products were analyzed, specifically the structure in the wedges, with electron backscatter diffraction (EBSD) phase analysis to look whether amorphous or crystalline structures exist. Table 8

This example shows that the new method of the invention to produce DBM gives rise to not only a higher density DBM, but also to a DBM material with distinct crystalline properties of the second phase, i.e. of materials that fill the wedges between the periclase crystals. Example 9

In a pilot plant scale gas kiln commercial Nedmag green briquettes

(commercially calcined and subsequently pressed) were sintered (at 1800°C with a final sintering time at the high temperature for 3 hrs) and compared on BD and crystal size with pressed (isostatic at 300 MPa) and sintered experimental materials containing (a) no Titanium and no graphite; (b) with 0,2% Ti02 / 0,2% graphite or (c) 0,3TiO2 / 0,3% graphite. All three were prepared from the filtercake route. Table 9A

Subsequently, the above samples were subjected to a destructive crystal size microscopic determination. The results are shown in Table 9B.

Table 9B

This data set shows that a very significant crystal growth enlargement occurs when a regular filtercake mixed Mg(OH)2/MgO product is homogeneously mixed with small quantities of Titanium Dioxide and

Graphite. The crystal size increases from about 45 to about 90- 100 microns, giving rise to a refractory material with dramatically improved qualities. Example 10

A finished DBM product according to example 9 (with 0,2 Ti / 0,2 C and a density of 3,51 g/cm³ and crystal size of 95 microns and a second phase consisting of a calcium magnesium titanates) was used to prepare a castable, and subsequently compared to a similar castable prepared with a commercial sample of Fused Magnesium (FM with a density of 3,52 g/cm³ and a very typical crystal size of around 800 microns).

In order to prepare a castable, an amount of MgO (crushed and milled to similar grain size fractions: 25% 2-4 mm, 25% 1-2 mm, 20% 20% 0-1 mm and 20% < 0, 1 mm and subsequently mixed with 5% Carbores P from

Rutgers Chemicals and 5% Thermax powder N991 from Cancarb

corporation) is mixed with a binder (WhiteCast R 11 SE 18 ex

Momentive/Hexion/BAKELITE AG, about 2% on solids), a dispersing agent (VP 95 L ex Castament series from BASF, about 0,25% on solids) and a hardener (White Cast H 9 ex BAKELITE AG, about 0,25% on solids) and some water (typically about 6 %).

Subsequently the masses are made homogeneously and poured into 20/20/150 mm pieces. The pieces are air dried for 6 days, and

subsequently cooked at 10 hours in the heating up phase and subsequently hold at 1000°C for 3 hours.

The finalized castables are tested on the modulus of rupture (MOR, a 3-point bending strength method) after a one week room temperature exposure to regular humidity air. Modulus of rupture, also known as flexural strength, bend strength, or fracture strength, is a mechanical parameter for brittle material, is defined as a material's ability to resist deformation under load. Equipment used TIRAtest, 28100 series with5 kN. Table 10

Product Strength after 1 week exposure

FM l,5 +- 0, l MPa

DBM 2,4 +- 0,2 MPa

Conclusion: The new DBM quality shows > 37% increase in strength in comparison with a castable prepared with fused magnesium.

Claims

1. A method for producing a sintered, high density, dead burned magnesia (DBM) from an aqueous slurry of magnesium hydroxide or a mixed magnesium hydroxide / magnesium oxide slurry, characterized in that T1O2 and graphite are added to the aqueous slurry each in amount of 0.05 to 0.5 percent, preferably 0.1 to 0.3 percent, by weight of the magnesium

hydroxide.

2. Method according to claim 1, wherein the aqueous slurry contains from about 40 to about 70 percent by weight magnesium hydroxide solids.

3. Method according to claim 1 or 2, wherein T1O2 and graphite are used in a relative weight ratio of between 1:3 and 3:1, preferably between 1:2 and 2: 1.

4. Method according to any one of claims 1-3, wherein T1O2 and graphite are used in a relative weight ration of between 1:1.2 and 1.2 to 1.

5. Method according to any one of the preceding claims, wherein T1O2 and graphite are used in the range of about 0.1-0.3, preferably 0.15-0.3 wt%, more preferably in an amount of around 0.2 wt% each.

6. Method according to any one of the preceding claims, comprising the steps of

(i) reacting a magnesium salt brine or seawater with a source of calcium oxide, preferably dolime CaO.MgO), to precipitate magnesium in hydroxide form, followed by

(ii) preparing an aqueous slurry of magnesium hydroxide and magnesium oxide (Mg(OH) / MgO) comprising the additives T1O2 and graphite (iii) calcining the magnesium hydroxide at a temperature of 500- 1000°C to produce caustic calcined magnesia; and

(iv) sintering the caustic calcined magnesia at a temperature of 1600- 2000°C, preferably 1700-1900°C, to obtain dead burned magnesia.

7. A sintered, high density, dead burned magnesia (DBM) obtainable by a method according to any one of claims 1-6.

8. A sintered, high density, dead burned magnesia having the following properties:

a) a MgO content of 97-99 wt%

b) a density of at least 3.50 g/cm³

c) a crystal size of at least 90 microns

d) comprising a crystalline phase comprising or consisting of a mixed MgO:CaO:Ti02 composition, preferably in a relative molar ratio of from

1: 1: 1 to 1:2:2.

9. A refractory composition for forming shaped or unshaped refractories, the composition comprising a sintered, high density, dead burned magnesia according to claim 7 or 8.

10. The refractory composition of claim 9, wherein the composition has a MgO content of about 50-95%.

11. The refractory composition of claim 9 or 10, including a binder, further mixed metal oxides and/or processing aids.

12. The refractory composition according to any one of claims 9-11, in the form of crushed particles.

13. A shaped refractory article comprising a refractory composition according to any one of claims 9-12.

14. Refractory article according to claim 13, being a refractory brick.

15. A castable refractory composition comprising a sintered, high density, dead burned magnesia according to any one of claims 7 to 9.

16. A furnace comprising a lining comprising a refractory article according to claim 13 or 14 or a castable according to claim 15.

17. Use of a refractory article according to claim 13 or 14 or a castable according to claim 15 for building or restoring the lining /wall of a furnace.

18. Furnace according to claim 16 or use according to claim 17, wherein said furnace is selected from a group consisting of a refuse incinerator, a cement burning furnace, a steel ladle, an annealing furnace, a heating furnace and a steel making furnace.