Installation Methods For Castable .1 Installation Temperature

Installing castables at sub-zero temperatures requires pre-heating of the job site internally and externally, pre-heating of the water and of the monolithic materials to at least 60° F. Failure to do so may cause the following problems:

- poor material densification

- incomplete hydration of the binder yielding a product without strength - no setting or delayed setting

- formation of heat-sensitive phases that cause stream explosion during heatup

The risk of explosion increases with the amount of water required for casting, and with the lining thickness. Insulating castables present the additional risk of being trapped between the steel shell and the dense lining.

Plastic refractories are also sensitive to sub-zero temperatures. The hardening reaction between the phosphate phase and alumina requires a minimum initial temperature to start. If the reaction only takes place during the kiln heatup, the lining may result severely damaged or even collapse upon form removal.

Installation at temperatures above 100° F also present risks to material performance. Similar to what happens at sub-zero temperatures, poly-hydrated calcium aluminate phases may form.

During heat-up severe steam explosions may occur when large volumes of chemically bonded water is released at moderate to high temperatures. Another risk is rapid waer evaporation before the binder is fully hydrated, during the curing period. An underhydrated castable will be weak and easily destroyed by chemical attack. The remedy in such cases is to keep the bags and the mixing water in a cool place before the installation starts, and to cure the cast lining under a membrane or moisture saturated air.

6.10.2 Amount of Water

Every product in the market requires a specific amount of water for proper installation and strength development. Low cement and ultra-low cement castables are particularly sensitive to the amount of water used during the grain porosity and the quantity of cement used in the product formulation.

The symptoms of too much water are quite evident:

- no abrasion resistance

- smooth surface finish, without voids or open pores - visible wood-grain pattern from the plywood forms - sensitivity to thermal shocks

- unreacted, dry dust inside the pores - reduced bulk density

- considerably reduced modulus of rupture

The solution to this problem is simple: control the amount of water added to the mixer to the nearest hundredth of one percent. If the installation is done by gunning, then control the amount of pre-mixing water and the amount of water added to the nozzle.

Rarely a castable fails for lack of water, and the reason is understandable: it is very difficult to install a “dry” castable.

The same concepts applied to moisture, also apply to the binder in chemically bonded products.

Binders are usually phosphoric acid or phosphate salts dissolved in water. Too much binder generates excess heat that may induce expansion cracks in the lining, besides impairing material workability during vibration. It also promotes grain segregation and further material spalling.

Insufficient binder or weak binder solutions lead to unacceptably long setting time, or no setting at all.

6.10.3 Water Quality

The best advice here is: if you cannot drink it, do not use it. Brackish waters, with high concentrations of dissolved salts or organic matter cause serious setting problems, besides introducing fluxes into the castable. This is particularly true for low-, ultra low- and no-cement castables with reduced amounts of calcium aluminate cement, in high temperature applications:

flash-calciner, riser duct, hood, back wall in the cooler, nosering, bullnose and burner pipe.

6.10.4 Curing

After the product is mixed with waer or liquid binders, a considerable amount of time is required for full strength development. The strength comes from chemical reactions and new-phase formation in the product. Failure to observe the curing time leads to premature material wear by disintegration, abrasion, thermal shock or chemical attack. No matter how quick the installation is performed, the curing time cannot be changed. For most castables the binder is calcium aluminate cement that requires a minimum of 12 hours for complete hydratation. Plastics and some no-cement castables can be cured in shorter times without any problem. Material hardening or setting must not be confused with material curing. Hardening occurs in a few hours or minutes after mixing, whereas curing requires a minimum of 6 hours. Beware of “magic” products that can be “cast and fired in minutes”. This is only possible with hot-gunning materials used in the steel industry, but the binding mechanism for those products is totally different from castables.

6.10.5 Dryout

Problems with product dryout are easy to identify because they usually occur in an explosive way, exposing lives, lining and equipment to serious risk.

Monolithic products made with hydraulic binders contain three different types of water:

chemically combined, absorbed and adsorbed.

Adsorbed or free water is released at room temperature, through normal evaporation. It rarely presents any problem if it leaves the castable.

Absorbed water is retained inside mini-pores and crevices of the refractory grains. The porous grains act like a sponge. Since the water is retained by capillary forces, it requires more energy (and time) to be released than free waer, and could cause material cracking or fracturing if the temperature raise is too abrupt.

Chemically combined water, a product of hydration reactions, is chemically bonded into the structure of the compounds, and requires high amounts of bonded into the structure of the

compounds, and requires high amounts of energy to be released. The quantity of chemically combined water varies with the amount of hydraulic binder and the curing temperature, as previously discussed. If the material is installed outside the recommended temperature range, the amount of chemically combined water can increase to large proportions, relative to the total amount of water. During dryout, the chemically combined waer is released at temperatures above 500° F, when the castable surface is already at a much higher temperature. That generates tremendous volumes of stream in a short period of time. If the dryout is not slowed down at that moment, the internal pressure will blow the castable out with complete destruction of lining. The best remedy to avoid such accidents is to contract the dryout from a specialized company. Their burners are capable of releasing large volumes of moderate temperature gases and vapor that promote steady steam release at critical temperatures, without creating considerable thermal gradients in the lining. The price charged for a professional dryout more than compensates for the cost of an accident. More and more plants are moving in that direction.

6.10.6 Anchor Material

No castabe will be stronger than the anchor that holds it in place. Experience has shown that most castable failures are attributable to anchor failure.

Based solely on metal scaling resistance:

up to 1550° F ASI 304 up to 1850° F ASI 309 up to 1900° F ASI 310 up to 2000° F ASI 330

up to 2000° F INCONEL or, preferably, CERAMIC ANCHORS

The problem with temperature tables is their failure to address the fatigue properties, structural transformations and load bearing capacity of metals at kiln temperatures. Tensile, elongation and flexural strength must also be taken into account when selecting an anchor material.

AISI 300-series stainless steels, when held between 800-1500° F for long periods of time,

where the lining thickness is reduced and the service temperature is relatively high: hose-ring, bullnose and burner pipe. Like for any other metal, its limit of rupture drops sharply with temperature:

INCONEL 601 INCONEL 700

1200° F for 1,000 hours 57,000 psi 86,000 psi

1500° F for 1,000 hours 24,000 psi 32,000 psi

1800° F for 1,000 hours 17,000 psi 3,500 psi

Whenever possible, ceramic anchors should be used because of their many advantages over metal anchors:

- higher refractoriness - no scaling

- no elongation - higher hot strength - lower thermal expansion - higher holding capacity - higher load-bearing properties - not sensitive to welding

When selecting ceramic anchors attention must be given to material behavior towards chemical attack and thermal cycling.

6.10.7 Anchor Dimensioning

When it comes to metal anchors, there are two different schools of thought: those who prefer many thinner anchors, and those who prefer a few thicker anchors. Since anchor failures occur in both cases, the problem with anchor dimensioning must be somewhere else.

For a given anchor spacing, the anchor dimension is a function of the load it will bear (lining thickness and position), the scaling resistance of the metal and the working temperature which in turn affects the mechanical properties of the metal. Of particular interest are the tensile strength and the flexural strength.

Common sense dictates that less metal is needed in floor positions, more metal is needed in walls and a lot more in suspended roofs. It also tells us that more metal is required in hotter areas than in colder areas.

The problem with too many anchors or with massive anchors is the expansion damage they can cause. Since the expansion problem can be easily addressed, more metal should be the way to go, especially in highly corrosive environments. Too many thinner anchors have the disadvantage of offering too much surface for corrosion and scaling, and making lining removal for repairs very difficult. None of these difficulties exist with ceramic anchors.

Another important point in anchor dimensioning is the anchor height. Ceramic anchors are always flush with the dense lining, whereas metallic anchors are always embedded in the lining. Here, again, there are two lines of thought: some people try to keep the metal anchor as far away from the hot face as possible, to protect it from heat. Others prefer to expose the anchor tips to direct heat contact, claiming that it holds the material better and the anchor wears out with the castable.

Common sense tells us that if the anchor is to hold the castable in place, then the 2/3 or 3/4 rule is wrong: 1/3 to 1/4 of the lining is not anchored.

The most common mistake is to bury the anchor in the castable, 25% to 33% far from the hot face, without allowing for longitudinal, lateral and diametral thermal expansion. Anchors with bent or reversed tips, waves or any other curve that could hinder free movements should be avoided, in favor of smooth V-clips that can at least over up and down. In addition, no matter how thick or tall the anchor is, it must be fully coated with mastic, wax, masking tape, plastic or any other combustible material. The space created around the metal keeps the anchor from spalling the castable off, or cracking it radically or diagonally. If these rules are observed, the anchor height then becomes of secondary importance.

6.10.8 Anchor Spacing

Anchor spacing is function of the lining weight, and its relative position: overhead, wall or floor.

Here are some guidelines for a typical 9in. lining done in 60% alumina castable:

Ceramic Anchor Overhead 12 in. centers

Ceramic Anchor Wall 16 in. centers

Ceramic Anchor Floor 18 in. centers

Ceramic Anchor Rotary Kiln 14 in. axial, 16 in. radial Ceramic Mini-Anchor Burner Pipe 6 in. axial, 40 degrees radial

Metallic Anchor Overhead 6 in. centers Metallic Anchor Wall 12 in. centers Metallic Anchor Floor 16 in. centers

Metallic Anchor Rotary Kiln 9 in. axial, 12 in. radial Metallic Anchor Burner Pipe 3.5 in. axial, 3 in. radial

These recommended spacings can be changed to accommodate different installation procedures and materials. Installers prefer less anchors per square foot, because it is easier to cast and gun with less obstacles. Refractory suppliers prefer more anchors per square foot to maximize lining stability.

6.10.9 Anchor Welding

Improper welding is a very common reason for anchor failure. In most situations, low carbon, high chromium, high nickel alloys are welded to a low-alloy, carbon steel plate. If the welders are not properly instructed about the purpose of those anchors, chances are that they will not pay much attention to details. They will just weld it as quick as possible.

Some stainless steels have a strong tendency to undergo structural changes welding. That tendency increases with the welding current and the with the rod diameter. Metallic anchors must be fully welded to the shell, using a rod diameter no bigger than 1/8 in. Spot welding will not work in any application. If the welding was properly done, the welding should not exhibit any magnetic attraction at all.

6.10.10 Anchor Expansion

Statistically, this is the most frequent cause of monolithic lining failures: hindered expansion.

During heatup, cooling and in normal operation, there is a sequence of relative movements between the steel shell, the insulating material, the dense castable and the ceramic or metal anchor.

As the shell expands, the anchors welded to it move away from each other. Since the castable does not move, either the anchor yields or the castable breaks, if not the anchor itself. If the heatup is too fast, the castable expands towards the hot face and sideways. If the anchors or the shell do not follow the movement (and they don’t), tension will develop inside the lining. Again, either the castable spalls off or the anchor breaks. That is why it is so important to use flexible anchors and to always coat it with a burnable material.

During cooling, while the lining is still soaked in heat, the anchors are already cold and pushing the lining backwards. If it resists, either the anchor or the castable will break, if not the welding.

Similar differential expansion occurs during the diametral expansion of the anchor, but it can be promptly addressed with plastic coated anchor tips.

SECTION 7