Refractories

Refractories 101

Refractories are the unsung heroes of the high-temperature process industries.

April 1, 2013
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Refractories are materials that resist high temperatures, liquid slags and aggressive atmospheres, and do not change form (e.g., burn or melt) in service. Typically used at temperatures above about 500°C, these materials maintain their critical physical and chemical properties at the temperatures required to make other high-melting-point products.

These processes operate at such temperatures and conditions that steel alone is not sufficient to contain the process and/or the reaction products. Refractories are therefore “heat containment” materials. Figure 1 details the various process temperatures found in relevant industries.

Although steel melts above 1550°C, it loses most of its mechanical strength above 500°C. Therefore, the steel shells of reaction vessels have to be protected with a refractory lining and are usually designed so that their temperature remains below 300°C. Obviously, refractories must not melt at the temperature of use, so a high melting point is important (1200-1700°C, depending on the application). They must also be able to support themselves and tolerate a certain amount of additional mechanical stress due to the weight of surrounding material and process vessel contents. Most refractories have compressive strengths above 20 MPa. In liquid contact, they must not react (or react very slowly) with the process melt, giving a reasonable lining life.

Refractories are generally made from naturally occurring minerals, sometimes with little or no beneficiation, only calcination. Most are made from metal oxides (e.g., alumina, silica, magnesia, doloma, etc.) or mixtures of oxides. The most common are made from calcined clays. Under certain conditions, it may be appropriate to use non-oxide materials like carbon or silicon carbide/nitride. If these meet the criteria regarding the maintenance of properties and temperature of use, then we can also class them as refractories.

Applications

The type of refractory used depends entirely on the conditions of each application. Factors to be considered include:

•  Operating temperature/temperature variations

•  Gas conditions (oxidizing or reducing)

•  Chemistry of any liquid present

•  Mechanical stresses within the lining

•  Impact, abrasion and erosion factors

The majority of refractories fall within what is technically called the alumino-silicate range (or clay-alumina in the U.S.). This means they are mixtures of alumina and silica, ranging from the lower alumina, acidic clays (< 30% alumina) through the fireclay range (35-45%), to more neutral high-alumina products containing andalusite, bauxite, etc. (> 55% alumina content). Materials in this range can generally be tailored to suit most conditions by varying the alumina content and by the appropriate choice and/or blending of the raw materials.

For some applications where a liquid phase is present, such as with a slag containing chemically basic oxides, it is necessary to move to basic refractories, with magnesia and doloma (MgO, CaO) being the major materials. Most liquid slag phases produced in cement, steel and non-ferrous metal production are basic in character and will aggressively attack alumino-silicates. To enhance the magnesia-based products, additions are often made to the base mineral, such as:

•  Chrome ore (chromite), to reduce basicity for the non-ferrous industry and to provide more ductility for the cement industry

•  Artificial spinel and/or zirconia, where chromite may not be allowed for health reasons

Categorization

Refractory products are mainly categorized by their chemistry (basic, acid or neutral), but other factors include form, density, bonding and method of placement.

Chemistry

A refractory’s ability to resist corrosion/erosion to certain environments is due to its chemical makeup, which depends on the predominant metal oxide used. Basic refractories are made from magnesia and doloma, where the active chemical compounds are MgO and CaO. Also often considered in the basic range (mildly) are forsterite (magnesium silicate) and chromite (chromic oxide). In general, divalent metal oxides produce basic refractory products.

Acid refractories contain crystalline silica at operating temperatures (e.g., crystobolite). With reference to the Al2O3-SiO2 phase diagram (see Figure 2), any material whose composition falls to the left of the mullite line will have some acid characteristics. The higher the silica content, the more acid characteristics will be shown. As the composition moves to the right toward the mullite line, the materials show less acid character. Refractories based on zircon (zirconium silicate) also exhibit mild acid characteristics.

The neutral category contains all materials that cannot be categorized as either basic or acid, and generally comprises the oxides of tri-valent metals. For alumino-silicates, which make up the largest component, it refers to the compositions that are higher in alumina than mullite (70%), where free silica phases are not stable. Other refractory materials in this group include carbon, silicon carbide, zirconia and chromic oxide.

Form

Another common way of categorizing materials is to describe them as shaped or monolithic. Shaped generally refers to preformed/pressed bricks or pre-cast shapes. These come ready to use for installation and can generally be installed without auxiliary placement systems (e.g., anchors).

Monolithics is an old term for materials that may be delivered in bags or boxes, to which water is sometimes added. The term means literally “one piece.” These materials are initially without shape and can be installed to almost any desired contour with the right formwork and anchoring system. Refractory concretes fit in this category, regardless of chemical character.

Density

Refractory density is categorized as dense, medium or lightweight. The first refers to “normal” hot-face refractories with an installed density of > 1.8 t/m3; medium weight refers to semi-insulating products with a density of between 1.2-1.8 t/m3; lightweight materials are manufactured specifically to enhance their insulating properties (e.g., low thermal conductivity) and have densities between 0.5-1.2 t/m3.

Bonding

This form of categorization depends on the bonding mechanism employed to keep the refractory in one piece. Most bricks, for example, are fired in kilns at high temperatures that cause inter-granular reactions to produce a ceramic bond.

This (expensive) process is often not required, and the material’s components are held together by other chemicals. These bonding agents deliver strength at lower temperatures (400-1000°C) and often also imbue the material with some added resistance to aggressive species, such as alkalis. These refractories are termed chemically bonded and are of great use in medium-temperature applications or where dimensional accuracy is an issue for installation.

Castables are high-temperature concretes incorporating sized aggregates mixed with bonding agents. The most common bonding agent is calcium aluminate cement (CAC), which provides a cold set and low-temperature strength. There are many types of cement-based castables with varying levels of cement, many with additives to enhance rheology, or specific properties for differing applications and conditions. Other castables (often termed “cement-free”) do not rely on CAC for bonding, but on chemical agents such as alumina gels and colloidal silica.

Method of Placement

Commonly used methods of placement include:

•  Bricking—the traditional method of lining construction using a pre-shaped piece with mortar

•  Casting—placed by pouring pre-mixed concrete into shuttering or formwork

•  Gunning—a fine castable-type mix is pneumatically conveyed to a nozzle where water is added; the mixture is sprayed onto the target surface by air pressure

•  Pumping—placement of castable with appropriate rheology so that, after mixing, it can be pumped long distances (up to 100 m); similar to civil concrete

•  Shotcreting—spraying of a pre-mixed, wet material that is generally of the pumpable type and includes additional additives to ensure safe placement

Lining Wear

While it is accepted that linings do not last forever, the wear/loss mechanisms are often not understood. Spalling results from not a single cause, but a variety of factors (mechanical, thermal and chemical) that result in similar damage to a lining. Spalling refers to the slabbing of the front face, a loss of maybe up to 35 mm of lining at once. It might seem like this occurs as a single event, but spalling is generally the end result of a series of imperceptible minor events.

Small (micro-) cracks are generally induced behind the front face over a period of time, increasingly weakening the lining at each stage. Eventually, these cracks will join up and the lining will reach a stage where little mechanical strength remains. The next “process” event will then cause the failure of the entire front face to a level where there are few or no existing micro-cracks.

Thermal factors (i.e., rapid and/or repeated temperature cycling) are some of the most common causes of spalling. Rapidly changing thermal conditions cause subcutaneous micro-cracks as the material tries to accommodate the changes. Continued cycling eventually causes spalling loss. The amount of spalling and its timing depends on the degree and frequency of the thermal cycling, as well as the material’s properties.

A secondary but important cause is the lining being heated above its specified design range, or at the limit of the refractory capability. This is usually the result of poor operational control.

Pinch spalling is a purely mechanical effect caused by stress concentrations at the hot face of the lining. It is most common in bricked arches and cylinders, such as rotary kilns, where the expansion at the “sharp” end of an arch or wedge brick causes too much compression. In effect, it is the result of inadequate expansion allowance (or overheating). The end of the brick breaks off, often by as much as 20-30 mm, to a level that relieves the compressive stress.

Impregnation/infiltration can also cause a thermal disturbance due to the prior densification of the refractory hot face by subcutaneous saturation of liquid or condensed gas phases. This is most common in steel, non-ferrous and cement processes. The infiltration of liquid metal or slag via the material’s inherent pores changes the physical properties of part of the lining so that the affected portion and the unaffected portion act as separate entities, and the affected zone is easily dislodged.

Refractories are largely ceramic bodies, and thus very brittle, so impact damage is sustained easily. They may shatter on initial impact, although any impact is likely to cause the initiation of micro-cracks. Erosion is the long-term effect of mechanical and chemical attack of the solid or liquid contents of a vessel on the lining, while abrasion is the similar effect of gas-born particles.

Note that, mechanically, refractories are strong only in compression, as they have poor tensile or bending strength and should not be expected to perform in those circumstances. Their inter-granular bonds are generally of a ceramic nature and can be easily ruptured in tension.

Chemical reactions between the vessel’s contents and the refractory can cause corrosion, which progressively eats away at the lining. This causes part of the refractory to be dissolved into the melt, or loosened by this chemical action, and swept away. Most chemical-based effects are caused by a mismatch between the reactive phases and the refractory (e.g., acid vs. basic). Figure 3 shows a guide to the chemistry of the slags produced by various metallurgical processes, along with the types of refractories generally available.

A classic chemical-based failure occurs in the aluminum industry when silica reacts with molten aluminum to form corundum and silicon. This has two main effects: it can cause dislocation of the refractory’s structure by removing the silica from the refractory’s matrix; and then the formation of corundum, accompanied by a volume change, again physically disrupts the refractory structure and encroaches into the working volume of the furnace.

Gaseous attack by carbon monoxide or hydrogen is also common. High iron content in the refractory catalyses the breakdown of CO within the material. This reaction deposits carbon within the refractory structure, with a commensurate volume change (expansion). This is known as carbon bursting. If CO is present in quantity and at temperature, a low iron content in the refractory material is necessary.

Hydrogen can attack the silica in refractories at high temperatures, reducing it to the gaseous monoxide and causing subsequent structural damage to the existing refractories. The potential to re-form silica downstream, often in boiler tubes, also arises in these instances.

The last real issue is alkali attack, which manifests itself as a combination of physical and chemical damage to alumino-silicate refractories. Alkali oxides and chlorides generally volatilize above 800°C. They react chemically, forming reaction products with greater volumes on a crystalline level. The mullite phase reacts this way with potassium ions, while corundum reacts with sodium ions. Both reactions result in a significant expansion of the refractory (around 30%).

Failures

For this discussion, a failure is defined as unexplained loss of lining, significantly reducing the lining campaign life. Such a failure indicates a major deficiency in design, materials or operations. The factors that typically lead to such failures can include:

•  Inadequate mechanical or thermal design

•  Incorrect material choice(s)

•  Incorrect process specification

•  Poor process control

•  Poor material quality

•  Poor installation quality

Vital Materials

Refractories are the unsung heroes of the high-temperature process industries. Most of the metals and industrial minerals used today could not be produced without them. Over the last 100 years, refractories engineering has evolved from a “black art” into an engineering discipline. Refractories development has historically been achieved by a successful multi-discipline approach, which will hopefully continue to lead to improved materials and practices.


 For additional information, contact the author at refmet@iprimus.com.au or visit www.refmet.net.

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