Key Project No. CF-2023-I-719

Main researcher: Dr. Guadalupe Alan Castillo Rodríguez

This project is funded by the National Council of Humanities, Science and Technology of Mexico, within the Frontier Science Fund in its 2023 call. Project with code CF-2023-I-719.

Refractories are used to conserve heat, are suitable for applications that handle high temperatures, and must be able to withstand high temperatures, sudden changes in temperature (thermal shock), compressive, bending, and tensile stresses, resist vibrations, abrasion and erosion, resist molten product, slag, gases and vapors, acidic or basic environments.

In addition, it is known that refractories with lower density are weaker to this type of severe environment, generating in these cracks, microcracks, spalling, penetration of slag and molten product; In other words, the service life of the refractory material is shortened.

Therefore, this project aims to obtain a dense sintered magnesia, by incorporating doubly calcined microparticles in rough (with cations of oxidation state 1+, 2+, 3+ and 4+), determining the effect of the microparticles in the refractory matrix on its microstructure, its physical and mechanical properties.

By virtue of the above, it is intended to modify the way sintered magnesia is obtained.

Refractories are used in industrial processes that operate at high temperatures, such as the steel, metallurgical, cement, ceramics, petrochemical, and glass industries, among many others.

Thanks to refractories we can obtain iron, steel, cement, glass, ceramics, non-ferrous metals (nickel, palladium, platinum, titanium, aluminium, tin, lead, zinc), etc.

Magnesia-based refractories are used as coatings in various industrial processes. They are exposed to mechanical, thermal, and chemical loads; that cause: chipping, micro-cracks, cracks, corrosion, etc. Although, magnesia refractories are known to exhibit good high temperature resistance, basic slag resistance, corrosion resistance, and abrasion resistance; With continuous use and for a short period of time (in some cases a maximum of two weeks), cracks, micro-cracks, metal penetration and molten slag occur in the refractories, causing the material to collapse.

This causes a partial or total change of the furnace coating that is reflected in the short lifetime of the material, in addition to presenting economic expenses in industries that could be reduced.

An important property of a refractory is density, refractories with lower density are more prone to the effects.

And to obtain high-performance MgO-based refractories (better mechanical properties, corrosion resistance, etc.), it is necessary to use magnesia with high purity (greater than 98.0% by weight) and high density.

To this end, this study aims to obtain dense MgO raw material to produce MgO-based refractories.

This will be done by modifying the process of obtaining sintered magnesia, by incorporating microparticles of cations with oxidation states 1+, 2+, 3+ and 4+ in the brucite matrix, modifying the way of obtaining sintered magnesia, aiming to obtain dense raw material.

The aim of this study is to elucidate the effect of particles on doubly calcined brucite to obtain dense sintered magnesia. One goal is to reduce porosity and increase density, by substituting cations at some sites corresponding to the Mg2+ ion.

The hypothesis is that with the incorporation of microparticles of cations with oxidation states 1+, 2+, 3+ and 4+ in brucite and their subsequent calcination, it is considered that there will be a substitution of cations in the MgO matrix, which will modify the crystal lattice, favoring atomic diffusion and decreasing porosity and increasing the density of the sintered MgO.

The general objective of the project is to obtain dense sintered magnesia from brucite as a precursor, with the addition of cation particles with oxidation states 1+, 2+, 3+ and 4+, to analyze their physical and mechanical properties, as well as their morphology, microstructure and oxidation states of the phases formed in the mixtures obtained.

Likewise, the possible phases formed will be determined by means of a thermodynamic analysis of brucite and the addition of microparticles with oxidation state 1+, 2+, 3+ and 4+.

In addition, X-ray emission photo spectrometry (XPS) analysis will be performed to determine oxidation states and chemical analysis of the raw material (brucite and cation particles with oxidation states 1+, 2+, 3+ and 4+).

On the other hand, caustic magnesium oxide powders will be obtained by heat treatment at 960° C, from brucite mixed with different percentages of cation microparticles with oxidation state 1+, 2+, 3+ and 4+.

In the same way, green specimens of caustic magnesia powders will be made by uniaxial pressing.

In the same vein, the specimens will be sintered in green at 1600 °C, using a conventional oven.

Once sintered, the phases obtained from the compounds will be determined by X-ray diffraction.

The morphology and microstructure of the sintered compounds will be analyzed by scanning electron microscopy.

To complement the study, X-ray emission photo spectrometry (XPS) analyses will be performed to determine oxidation states of the sintered compounds.

In the same way, the physical properties in terms of density and porosity to sintered composites will be determined by means of Archimedes' principle test.

Finally, the mechanical properties will be determined by testing cold compressive strength to sintered specimens.

According to the state of the art of refractories and their general processing. A refractory material is a non-metallic compound that retains its physical, chemical, and mechanical properties at high temperatures, above 1500°C.

These are used to line boilers and all types of furnaces, blast furnaces, stoves, pig iron mixers, electric furnaces, converters, and ladles, among many others.

The purpose of refractories is to conserve heat inside the furnace, or the unit lined with this material.

The basic or frequently used procedure for the elaboration of a ceramic is the obtaining of the material, crushing, screening, mixing, forming, drying, and sintering.

It should be noted that sintering occurs only when the temperature of the product exceeds one-half to two-thirds of its melting temperature.

High-temperature heat treatment is necessary to cause atomic diffusion and solid solution in the sintering stage.

In addition, in some cases a viscous flow occurs when a liquid phase is present or is caused by a chemical reaction of the components.

During the processing of these ceramic materials, it is possible to modify and predict some properties that are obtained after sintering; And as for modifying its properties, it can be done by adding other substances.

As for the fundamental requirements of a refractory material, we must remember that these are used to conserve heat, with the ability to handle at high temperatures. In addition, they must be able to withstand sudden changes in temperature (thermal shock), compressive, bending, and tensile stresses, resist vibrations, abrasion and erosion, resist molten products, slag, gases and vapors, acidic or basic environments. In general, they must resist high temperatures and have high physical, chemical and mechanical properties due to their exposure to aggressive environments.

On the other hand, refractories consist of 1. Grain or aggregate: corresponds to 70% of the refractory product and different sizes of grain are used to manufacture more compact refractory bodies. 2. Matrix or filling materials: these are materials used to fill the empty areas between the grains and their size is less than 150 micrometers. 3. Binder: This is used to bind the aggregates and matrix together to form a product that has green strength or dry strength. 4. Pores: these are the open hollow areas that are always present in refractories.

The aggregates provide mechanical and chemical resistance to the refractory compound, while the matrix is the weakest area mechanically and to attack by slag and gases. Due to its significant reactivity, high porosity, and specific surface area, it facilitates impregnation and corrosion in the liquid phase.

At present, the properties of the matrix are reinforced by introducing ultrafine particles. There are references based on empirical knowledge which alludes to the fact that, if micro and nano particles are introduced into a ceramic matrix, they benefit the mechanical, chemical, and thermal properties of the material.

For example, in the magnesia-carbon refractory system, when graphite is incorporated into magnesia, its in-service performance improves its toughness, resistance to thermal shock and slag corrosion. In addition, antioxidants can be added to reduce the oxidation of graphite and create new bonds by its reaction at high temperature, which reduce porosity and increase mechanical strength.

In basic magnesium oxide (MgO) refractories, materials with a chemical composition ranging from 75% of MgO and a maximum of 3% of the sum of the components of calcium oxide, silicon oxide and iron oxide are considered. In these materials, the oxygen anions of MgO form a cubic structure centered on the faces, these anions form octahedral and tetrahedral interstices, in which Mg2+ cations fully occupy the octahedral sites.

Previously, calcined magnesium carbonate and magnesium hydroxide recovered from brines were used as raw material to produce refractory products, which have been slowly displaced by high-temperature calcined magnesia (sintered magnesia) and electro fused magnesia.

Electro fused magnesia is obtained by fusion and has better properties compared to sintered magnesia, as it is more compact, more mechanically and chemically resistant. But its production is expensive, so its selling cost is very high compared to refractory systems that are not made from this material.

Magnesia refractories include sintered magnesia, electro fused magnesia, magnesia-carbon, magnesia-carbon with antioxidants, magnesia-zirconia, and magnesia-alumina. These refractories withstand high temperatures and are used as coatings in basic ambient areas, used in industrial processes that require high temperatures in their processes. Normally used in metallurgical furnaces, in non-ferrous furnaces and in the regenerator of glass furnaces.

Sintered magnesium oxide of synthetic origin in Mexico is obtained from the addition of brines with the addition of calcined dolomite in a gas rotary kiln. This methodology to obtain sintered magnesia is used by the Peñoles Corporation located in Torreón, Coahuila. In this process, dolomite is calcined and mixed with magnesium chloride in aqueous solution, from which calcium hydroxide and magnesium hydroxide are obtained. In the reactor, calcium hydroxide is combined with calcium chloride to precipitate magnesium hydroxide. The magnesium hydroxide obtained from dolomite and through the crystallization of brines is sent to a furnace for calcination at one thousand degrees Celsius, from where caustic magnesium oxide is obtained.

It should be noted that in this work we will be adding particles with valence cations 1+, 2+, 3+ and 4+ in the brucite before obtaining caustic magnesia, in this way we will modify the obtaining of sintered magnesia and analyze the effects caused by the additives in the refractory. The caustic magnesium oxide powders are then compacted and sent to a vertical furnace for sintering at temperatures ranging from 1500°C to 2000°C, resulting in sintered magnesia. In the sintering stage, necks are formed at the contact points of the particles, the compact is shrinked, and finally, the pores present in the material are reduced and isolated. This neck growth between particles is due to the mass diffusion mechanism that occurs across the surfaces of adjacent particles within the same ceramic body.

The following podcasts will explain the advances and findings in this research work. Which is funded by the National Council of Humanities, Science and Technology of Mexico, within the Border Science Fund in the 2023 call.

Introduction to the effect of the addition of cations 1+, 2+, 3+ and 4+ in doubly calcined Mg(OH)2